Electro-Micromachining and Microfabrication: Principles and Research Advances [1 ed.] 9781774913796, 9781774913819, 9781003397793

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ELECTRO-MICROMACHINING AND MICROFABRICATION Principles and Research Advances

Frontiers of Mechanical and Industrial Engineering

ELECTRO-MICROMACHINING AND MICROFABRICATION Principles and Research Advances

Edited by Sandip Kunar, PhD Golam Kibria, PhD Prasenjit Chatterjee, PhD

First edition published 2024 Apple Academic Press Inc. 1265 Goldenrod Circle, NE, Palm Bay, FL 32905 USA

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© 2024 by Apple Academic Press, Inc. Apple Academic Press exclusively co-publishes with CRC Press, an imprint of Taylor & Francis Group, LLC Reasonable efforts have been made to publish reliable data and information, but the authors, editors, and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors are solely responsible for all the chapter content, figures, tables, data etc. provided by them. The authors, editors, and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged, please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www.copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. Library and Archives Canada Cataloguing in Publication Title: Electro-micromachining and microfabrication : principles and research advances / edited by Sandip Kunar, PhD, Golam Kibria, PhD, Prasenjit Chatterjee, PhD. Names: Kunar, Sandip, editor. | Kibria, Golam, editor. | Chatterjee, Prasenjit, 1982- editor. Description: First edition. | Series statement: Frontiers of mechanical and industrial engineering | Includes bibliographical references and index. Identifiers: Canadiana (print) 20230556051 | Canadiana (ebook) 2023055606X | ISBN 9781774913796 (hardcover) | ISBN 9781774913819 (softcover) | ISBN 9781003397793 (ebook) Subjects: LCSH: Micro-electro discharge machining. | LCSH: Micromachining. | LCSH: Microfabrication. Classification: LCC TJ1191.5 .E44 2024 | DDC 671.3/5—dc23

ISBN: 978-1-77491-379-6 (hbk) ISBN: 978-1-77491-381-9 (pbk) ISBN: 978-1-00339-779-3 (ebk)

ABOUT THE FRONTIERS OF MECHANICAL AND INDUSTRIAL ENGINEERING BOOK SERIES

EDITORS-IN-CHIEF Dr. Prasenjit Chatterjee

Department of Mechanical Engineering, MCKV Institute of Engineering, West Bengal, India

Dr. Dilbagh Panchal

Department of Industrial and Production Engineering, Dr. B. R. Ambedkar National Institute of Technology (NIT) Jalandhar, Punjab, India

Description of the Series Mechanical engineering applies the principles of mechanics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems, whereas industrial engineering involves figuring out how to make or do things better with minimum resource consumption. Mechanical and industrial engineering utilize a deep foundational understanding of mathematics, physics, and analysis to develop machines and systems. The former specializes in thermodynamics, combustion, and electricity to make complex machines. The latter focuses on designing workflow and making production efficient. This series reports on the cutting-edge developments to present novel innovations in the field of mechanical and industrial engineering. Volumes published in this series aim to embrace all aspects, subfields, and new challenges of mechanical and industrial engineering and to be of interest and use to all those concerned with research in various related domains of mechanical and industrial engineering disciplines. The series aims to provide comprehensive value-added resources to the body of the literature to serve as essential references for years ahead. Features of the volumes under this series explore recent trends, model extensions, developments, solutions to real-time problems, and case studies. The coverage of the series is committed to provide a diverse yet rigorous research experience that builds on a solid knowledge of science and

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About the Frontiers of Mechanical and Industrial Engineering Book Series

engineering fundamentals and application-oriented knowledge of mechanical and industrial engineering practices for efficient functioning in today’s global multidisciplinary rapidly changing work environment. It includes all new theoretical and experimental findings in mechanical, industrial engineering, and allied engineering fields. Topics and Themes: The primary endeavor of this series is to introduce and explore contemporary research developments in a variety of rapidly growing research areas. The volumes will deal with the following topics but not limited to: • • • • • • • • • • • • • • • • • • • • • • • • •

Automotive engineering Aerospace technology Big data in mechanical / manufacturing / industrial engineering Computational methods for optimizing manufacturing technology, robotics, mechatronics Decision science applications in mechanical, manufacturing, and industrial engineering Design and optimization of mechanical components Dynamical systems, control Engineering design Engineering thermodynamics, heat, and mass transfer Evolutionary computation Finite element method (FEM) modeling/simulation Fluid mechanics Fuzzy logic and neuro-fuzzy systems for relevant engineering applications Green and sustainable engineering techniques for modern manufacturing Innovative approaches for modeling relevant applications and real-life problems Life cycle engineering Machinery and machine elements Manufacturing Mathematical concepts and applications in mechanical and industrial engineering Materials engineering Mechanical structures and stress analysis MEMS Modeling in engineering applications Nanotechnology and microengineering Non-conventional machining in modern manufacturing systems

About the Frontiers of Mechanical and Industrial Engineering Book Series

• • • • • •

vii

Precision engineering, instrumentation and measurement Numerical simulations Soft computing techniques Sustainability in mechanical, manufacturing, and industrial engineering Theoretical and applied mechanics Tribology and surface technology

The series aims to serve as a valuable resource for undergraduate, postgraduate, doctoral students, researchers, academicians, and industry professionals in mechanical, manufacturing, industrial engineering, product design, management, and more. BOOKS IN THE SERIES Optimization Methods for Engineering Problems Editors: Dilbagh Panchal, PhD, Prasenjit Chatterjee, PhD, Mohit Tyagi, PhD, and Ravi Pratap Singh, PhD Modern Manufacturing Systems: Trends and Developments Editors: Rajiv Kumar Garg, PhD, Ravi Pratap Singh, PhD, Rajeev Trehan, PhD, and Ramesh Singh, PhD Multi-Criteria Decision-Making Methods in Manufacturing Environments: Models and Applications Shankar Chakraborty, PhD, Prasenjit Chatterjee, PhD, and Partha Protim Das, PhD Supply Chain Performance Measurement in Textile Enterprises Editors: Pranav G. Charkha, PhD, Santosh B. Jaju, PhD, and Prasenjit Chatterjee, PhD Electro-Micromachining and Microfabrication: Principles and Research Advances Editors: Sandip Kunar PhD, Golam Kibria, PhD, and Prasenjit Chatterjee, PhD Robotics and Automation in Healthcare: Advanced Applications Editors: R. Thanigaivelan, PhD, Sanjay Singh, PhD, and Clement Christy Deepak C., PhD Optimization of Advanced Manufacturing Processes Editors: Sandip Kunar, PhD, Prasenjit Chatterjee, PhD, and M. Sreenivasa Reddy, PhD

ABOUT THE EDITORS Sandip Kunar, PhD Assistant Professor, Department of Mechanical Engineering, Aditya Engineering College, Andhra Pradesh, India Sandip Kunar, PhD, is working as an Assistant Professor in the Department of Mechanical Engineering at the Aditya Engineering College, A.P., India. He has completed ME and PhD from Jadavpur University, Kolkata, India. His research interests include non-conventional machining processes, micromachining processes, advanced manufacturing technology, and industrial engineering. He has carried out his research work in Saha Institute of Nuclear of Physics, Kolkata, and RRCAT, Indore, BARC Unit, India. He has five years of research experience and three years six months of teaching experience. He has published 19 research papers in various reputed international journals, three research papers in reputed national conference proceedings and 24 research papers in international conference proceedings, eight book chapters and six books with reputed international book publishers. He has published one international webinar publication in USA and has filed two patents. He is a life member of the Indian Society for Technical Education, New Delhi, India. He has about 10 of professional memberships from various renowned international professional bodies like the Institution of Mechanical Engineers, UK, ISSMO, Denmark, International Association of Engineers, Hong Kong, IACSIT, Singapore, SCIEI, Hong Kong, ICST, Belgium, ESRInnov, France, CSTA, IAENG, UAMAE, USA, IRED, USA, International Technology and Engineering Educators Association, USA, etc. He has received the Best Innovative Paper Award two times at renowned international conferences and several scholarships. He is a reviewer of 21 international journals such as the Journal of Material Science and Technology, Elsevier; IEEE/ASME Transactions on Mechatronics, Materials, and Manufacturing Processes, Taylor and Francis; Journal of Physics D: Applied Physics, IOP Science, etc. He has also delivered lectures in FDP in faculty development programs, short-term courses, workshops, and seminars.

x

About the Authors

Golam Kibria, PhD Assistant Professor, Department of Mechanical Engineering in Aliah University, Kolkata, India Golam Kibria, PhD, is an Assistant Professor in the Department of Mechanical Engineering at Aliah University, Kolkata, India. He completed his MTech in Production Engineering from Jadavpur University, Kolkata, in 2008 and PhD from Jadavpur University, Kolkata, in 2014. He has worked as Senior Research Fellow (SRF) in a Council of Scientific and Industrial Research (CSIR) sponsored project from 2008 to 2011. His research interests include non-conventional machining processes, micromachining, and advanced manufacturing and forming technology. He is a life member of The Institution of Engineers (IEI), India. He is the author of several book chapters with internationally recognized book publishers such as Elsevier, Springer, and Nova Publishers. He has also published 26 international and national research papers in various reputed journals and 37 research papers in reputed national and international conference proceedings. He is an editorial board member as well as a reviewer of a number of reputed international journals, namely Optics and Laser Technology, International Journal of Advanced Manufacturing Technology, Manufacturing Review, International Journal of Physical Sciences, etc. He received I.S.T.E. National Award for Best MTech Thesis in Mechanical Engineering in 2008 and the Institution Prize (Gold Medal) of 2008–2009 from The Institution of Engineers (India) for the best paper. Prasenjit Chatterjee, PhD Dean (Research and Consultancy), MCKV Institute of Engineering, West Bengal, India Prasenjit Chatterjee, PhD, is currently the Dean (Research and Consultancy) at MCKV Institute of Engineering, West Bengal, India. He has over 100 research papers in various international journals and peer-reviewed conferences. He has authored and edited more than 15 books on intelligent decision-making, supply chain management, optimization techniques, risk, and sustainability modeling. He has received numerous awards, including Best Track Paper Award, Outstanding Reviewer Award, Best Paper Award, Outstanding Researcher Award, and University Gold Medal. Dr. Chatterjee is the Editorin-Chief of the Journal of Decision Analytics and Intelligent Computing.

About the Editors

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He has also been the Guest Editor of several special issues in different SCIE/Scopus/ESCI (Clarivate Analytics) indexed journals. He is also the Lead Series Editor of the book series Smart and Intelligent Computing in Engineering; Founder and Lead Series Editor of several book series: Concise Introductions to AI and Data Science; AAP Research Notes on Optimization and Decision-Making Theories; Frontiers of Mechanical and Industrial Engineering; and River Publishers Series in Industrial Manufacturing and Systems Engineering. Dr. Chatterjee is one of the developers of two multiplecriteria decision-making methods called Measurement of Alternatives and Ranking according to COmpromise Solution (MARCOS) and Ranking of Alternatives through Functional mapping of criterion sub-intervals into a Single Interval (RAFSI).

CONTENTS

Contributors............................................................................................................ xv Abbreviations ......................................................................................................... xix Acknowledgment ..................................................................................................xxiii Preface .................................................................................................................. xxv 1.

Introduction to Micro-Electro Machining and Fabrication........................1 Golam Kibria, Sandip Kunar, T. Jagadeesha, M. S. Reddy, and Prasenjit Chatterjee

2.

Advances in Ultrasonic Micromachining and Assisted Electro-Machining for Microfabrication ....................................................21 Sayan Doloi, Amlana Panda, Ramanuj Kumar, and Ashok Kumar Sahoo

3.

Emerging Trends in High-Speed Micro-Machining of Dental Ceramics Through CAD-CAM Systems.........................................47 Sivaranjani Gali and R. Suresh

4.

Micro-Machining Performances by ECDM Process for Non-Conducting Materials...........................................................................71 Bijan Mallick, B. Doloi, B. R. Sarkar, and B. Bhattacharyya

5.

Advancement of Electrochemical Discharge Micromachining: Processing Micro-Features in Non-Conducting Materials........................85 Maneetkumar R. Dhanvijay, Bijan Mallick, Sadashiv Bellubbi, and N. Sathisha

6.

Simulation Analysis During Laser Microgrooving of Alumina Ceramic ........................................................................................ 111 Sudhansu Ranjan Das and Debabrata Dhupal

7.

Laser Beam Micromachining and Fabrication ........................................139 Ravindra Nath Yadav, Sanjay Mishra, and Ajay Suryavanshi

8.

Micro-Wire Electric Discharge Grinding as a Future Technology in Micromachining .................................................................173 Parthiban Madhavadev and Harinath Marimuthu

9.

Nano-Additives Assisted MQL and Optimization in Micro-Machining Processes .......................................................................193 T. Jagadeesha, Sandip Kunar, Golam Kibria, and Manoj Nikam

xiv

Contents

10. Breakthrough of Powder Additives in Powder Mixed Micro-Electric Discharge Machining ........................................................205 T. Jagadeesha, Sandip Kunar, Golam Kibria, and Manoj Nikam

11. Micro-Electro Discharge Machining: Principles and Applications........227 Sumanta Banerjee

12. An Insight on Micro-End Milling Process ................................................263 Chetan Devendra Varma and K. Vipindas

13. Recent Advancement in Microtexturing Using Electrochemical Micromachining ..........................................................................................293 Sandip Kunar, Golam Kibria, Prasenjit Chatterjee, T. Jagadeesha, Bh. V. Prasad, S. Rama Sree, and M. S. Reddy

14. Surface Modification Through Micro-EDM Process...............................321 Asma Perveen and Samet Akar

15. A Review on Micromachining of Ti-6Al-4V Using Micro-EDM.............337 Priyanshu Ghosh, Disha Mondal, Debolina Dutta, and Manish Mukhopadhyay

16. A Review on Electrical Micromachining Using Silicon Electrodes ........355 Hritrisha Naskar, Debolina Dutta, and Manish Mukhopadhyay

17. Performance Enhancement of Micro-Machined Surfaces Using Powder Mixed EDM.........................................................................371 S. Tripathy, Smrutiranjan Biswal, and D. K. Tripathy

Index .....................................................................................................................385

CONTRIBUTORS Samet Akar

Department of Mechanical Engineering, Çankaya University, Ankara, Turkey

Sumanta Banerjee

Department of Mechanical Engineering, Heritage Institute of Technology, Kolkata, West Bengal, India

Sadashiv Bellubbi

Department of Mechanical Engineering, Alva’s Institute of Engineering and Technology, Moodabidri, Mangalore, Karnataka, India

B. Bhattacharyya

Production Engineering Department, Jadavpur University, Kolkata, West Bengal, India

Smrutiranjan Biswal

Mechanical Engineering Department, ITER, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India

Prasenjit Chatterjee

Department of Mechanical Engineering, MCKV Institute of Engineering, Howrah, West Bengal, India

Sudhansu Ranjan Das

Department of Production Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, India

Maneetkumar R. Dhanvijay

Department of Manufacturing Engineering and Industrial Management, College of Engineering, Shivajinagar, Pune, Maharashtra, India

Debabrata Dhupal

Department of Production Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, India

B. Doloi

Production Engineering Department, Jadavpur University, Kolkata, West Bengal, India

Sayan Doloi

School of Mechanical Engineering, Kalinga Institute of Industrial Technology, Bhubaneswar, Odisha, India

Debolina Dutta

Mechanical Engineering Department, Meghnad Saha Institute of Technology, Kolkata, West Bengal, India

Sivaranjani Gali

Department of Prosthodontics, Faculty of Dental Sciences, M.S. Ramaiah University of Applied Sciences, Bangalore, Karnataka, India

Priyanshu Ghosh

Mechanical Engineering Department, Meghnad Saha Institute of Technology, Kolkata, West Bengal, India

T. Jagadeesha

Department of Mechanical Engineering, National Institute of Technology, Kozhikode, Kerala, India

Golam Kibria

Department of Mechanical Engineering, Aliah University, Kolkata, West Bengal, India

xvi

Contributors

Ramanuj Kumar

School of Mechanical Engineering, Kalinga Institute of Industrial Technology, Bhubaneswar, Odisha, India

Sandip Kunar

Department of Mechanical Engineering, Aditya Engineering College, Surampalem, Andhra Pradesh, India

Parthiban Madhavadev

Department of Mechanical Engineering, PSG College of Technology, Coimbatore, Tamil Nadu, India

Bijan Mallick

Department of Mechanical Engineering, Global Institute of Management and Technology, Krishnanagar, West Bengal, India

Harinath Marimuthu

Department of Mechanical Engineering, PSG College of Technology, Coimbatore, Tamil Nadu, India

Sanjay Mishra

Department of Mechanical Engineering, Madan Mohan Malviya University of Technology, Gorakhpur, Uttar Pradesh, India

Disha Mondal

Mechanical Engineering Department, Meghnad Saha Institute of Technology, Kolkata, West Bengal, India

Manish Mukhopadhyay

Mechanical Engineering Department, Meghnad Saha Institute of Technology, Kolkata, West Bengal, India

Hritrisha Naskar

Mechanical Engineering Department, Meghnad Saha Institute of Technology, Kolkata, West Bengal, India

Manoj Nikam

Department of Mechanical Engineering, Bharati Vidyapeeth College of Engineering, Mumbai, Maharashtra, India

Amlana Panda

School of Mechanical Engineering, Kalinga Institute of Industrial Technology, Bhubaneswar, Odisha, India

Asma Perveen

Mechanical and Aerospace Engineering Department, School of Engineering and Digital Sciences, Nazarbayev University, Republic of Kazakhstan

Bh. V. Prasad

Department of Mechanical Engineering, Aditya Engineering College, Surampalem, Andhra Pradesh, India

M. S. Reddy

Department of Mechanical Engineering, Aditya Engineering College, Surampalem, Andhra Pradesh, India

Ashok Kumar Sahoo

School of Mechanical Engineering, Kalinga Institute of Industrial Technology, Bhubaneswar, Odisha, India

B. R. Sarkar

Production Engineering Department, Jadavpur University, Kolkata, West Bengal, India

N. Sathisha

Department of Mechanical Engineering, Yenepoya Institute of Technology, Moodabidri, Mangalore, Karnataka, India

S. Rama Sree

Department of Computer Science and Engineering, Aditya Engineering College, Surampalem, Andhra Pradesh, India

Contributors

xvii

R. Suresh

Department of Mechanical and Manufacturing Engineering, M.S. Ramaiah University of Applied Science, Bangalore, Karnataka, India

Ajay Suryavanshi

Department of Mechanical Engineering, Bundelkhand Institute of Engineering and Technology, Jhansi, Uttar Pradesh, India

D. K. Tripathy

Ex-Professor Emeritus, IIT Kharagpur, West Bengal, India

S. Tripathy

Mechanical Engineering Department, ITER, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India

Chetan Devendra Varma

Department of Mechanical Engineering, Indian Institute of Information Technology, Design and Manufacturing Kurnool, Andhra Pradesh, India

K. Vipindas

Department of Mechanical Engineering, Indian Institute of Information Technology, Design and Manufacturing Kurnool, Andhra Pradesh, India

Ravindra Nath Yadav

Department of Mechanical Engineering, BBD Institute of Technology and Management, Lucknow, Uttar Pradesh, India

ABBREVIATIONS µ-EDM AE AI Al2O3 AMPs ANN ANOVA ANSYS AS-WEDG BCs CAD CAD-CAM CAM CCD CNC CNT CO2 CPTi CRM CVD CW DOC DOP DR EC ECDM ECDT ECM ED EDG EDM EDS EMM f

micro-electro discharge machining acoustic emissions artificial intelligence alumina advanced machining processes artificial neural network analysis of variance analysis software active suppling wire-electro discharge grinding boundary conditions computer aided design computer aided machining and designing computer aided machining charge-coupled device computer numerical control carbon nanotubes carbon dioxide commercially pure titanium chip removal mechanism chemical vapor deposition continuous wave diametric overcut depth of penetration duty ratio electrochemical electrochemical discharge machining electrochemical discharge trepanning electrochemical machining electrical discharge electro discharge grinding electric discharge machining electro-dispersive x-ray spectroscopy electrochemical micromachining frequency

xx

FEM FLC GA GNP GRG HAZ He-Ne HSS IEG IPG IR LB LBM LBMM LS-WEDT MD MEM MEMS ML MMC MQL MR MRR MRUM MUCT NB Nd-YAG OC PCD PF PFE PMEDM PSO PVD RA RC circuit RSM RUM SAMMA

Abbreviations

finite element method fuzzy logic control genetic algorithm graphene nanoparticles added grey relational grade heat affected zone helium-neon high speed steel inter electrode gap implantable pulse generator infrared larger-the-better laser beam machining laser beam micromachining low speed wire electric discharge turning machining depth micro-electro-mechanical micro-electromechanical systems machine learning metal matrix composite minimum quality lubrication machining rate material removal rate micro-rotary ultrasonic machining minimum uncut chip thickness nominal-the-better neodymium doped yttrium-aluminum-garnet over cut polycrystalline diamond pulse frequency plasma flushing efficiency powder mixed electric discharge machining particle swarm optimization physical vapor deposition regression analysis resistor-capacitor circuit response surface methodology rotary ultrasonic machining self-aligned multilayer machining and assembly

Abbreviations

SB SD SE SLEMM SOD SPD SQ STL TEW TMEMM TWR TWR UAECDM UA-ECDT UAECM UAEDM USM UV V VR-ECDM WECDM WEDG WEDM WOC XRD

xxi

smaller-the-better sintered diamond spark energy sandwich-like electrochemical micromachining stand-off distance severe plastic deformed surface quality standard tessellation language tool electrode wear through-mask electrochemical micromachining tool wear ratio total wear rate ultrasonic assisted electro-chemical discharge machining ultrasonic Assisted ECDT ultrasonically assisted electrochemical machining ultrasonic-assisted electro discharge machining ultrasonic machining ultraviolet voltage vibro-rotary electrochemical discharge machining wire electrochemical discharge micro-machining wire electric discharge grinding wire electric discharge machine width of cut X-ray diffraction

ACKNOWLEDGMENT

The successful completion of this book is undoubtedly due to the contributions of various individuals through their persistent directions, helpful suggestions, ongoing association, support, encouragement, and excellent advice at every level of the book’s development, right from its initial stages down to its completion. This work would not have been completed as effortlessly without the constructive and timely suggestions provided over the course of its creation. The editors are indebted to everyone for the considerable time and effort they invested in this publication. The editors would like to express their gratitude to all the authors for their important contributions to the book. Words are not sufficient to express the editors’ gratitude to the entire editing and production team of Apple Academic Press, particularly Sandra Sickels, Ashish Kumar, and Rakesh Kumar, for their continuous support, inspiration, and guidance throughout the publishing process. This book would not have been possible without their great support. The editors would like to express their gratitude to the reviewers who generously invested their time and skills to make such a high-quality book on such a contemporary topic. The editors would also like to thank their family members for their love, understanding, and support during the book’s preparation. Finally, the editors would like to express their thanks to all the readers for their support. We hope that this book will continue to inspire and guide them in their future endeavors. —Editors

PREFACE

Fabrication of micro-parts featuring precise micro-features has posed a significant challenge within global micromachining research. This challenge is mostly due to the increasing demand for micro-parts in healthcare, avionics, defence, and other industries. These micro-parts find applications in a wide array of contexts, thus necessitating the advancement of improved micro-manufacturing techniques capable of delivering enhanced precision and higher material removal rates with greater environmental sustainability. The progress in metallurgy calls for the development of materials that exhibit exceptional machinability properties. Manufacturing engineers are confronted with the task of identifying a range of microfabrication procedures including both conventional and unconventional, that can cater to the requirements of micro-parts. As of now, there are only a limited number of books in the field of electro micromachining and microfabrication, which often focus solely on statistics and investigative researches without offering comprehensive, in-depth, and crucial insights on the subject matter. Electro-Micromachining and Microfabrication: Principles and Research Advances aims to bridge the gap between the need for microelements and commercial microfabrication of goods. This book begins with an introduction of a variety of micromachining technologies, with a focus on novel nontraditional approaches and current developments in each. Ultrasonic micromachining processes as well as assisted hybrid micromachining with ultrasonic vibration of the tool or workpiece for fabricating a variety of micro-shapes such as micro-holes, micro-slots, and micro-walls all contribute to increase precision and furthering research. The history of computer-aided design (CAD) and computer-aided manufacturing (CAM) as well as currently available CAD-CAM dental micromachining technologies are also covered. Micro-electrical discharge machining (EDM), laser micro-grooving, and laser micromachining are the cutting-edge micro-manufacturing methods. It also discusses the use of electrochemical micromachining (EMM) technique to enhance micro-texturing as well as the use of nano-additives to improve MQL and micromachining process optimization. The following is a chapterby-chapter summary of the topics covered in the book. Chapter 1 covers fundamental discussions about micro-EDM, wire EDM (WEDM), and laser micro-grooving processes and their applications. Chapter

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Preface

2 provides valuable insights about ultrasonic micromachining process assisted with electro-machining for microfabrication. It also provides a comprehensive explanation of the input parameters, machining setup, and current state of progress in micro-manufacturing of complex components with high level of surface quality. Chapter 3 provides comprehensive exploration on material considerations associated with CAD-CAM dental ceramics and presently available CAD-CAM dental micromachining systems. Chapter 4 deliberates the effects of process parameters such as pulse frequency, duty ratio, applied voltage, and electrolyte concentration on machining criteria such as material removal rate, surface roughness, diametric or width of overcut, machining depth, and heat affected zone during micromachining operations using electro-chemical discharge machining (ECDM) procedure. Chapter 5 covers micromachining of glasses and ceramic materials using ECDM process variants such as gravity feed, traveling wire, vibro-rotary, and magnetic field effect to investigate the output responses such as surface finish, diametric overcut (DOC), heat-affected zone, and other qualitative parameters during the generation of micro-features. In Chapter 6, ANSYS is used to explore the influence of input parameters such as laser beam temperature, air pressure, and laser beam pulse width on the thermal and structural properties of micro-grooved alumina ceramic. Chapter 7 explores the basic mechanism of ultrafast lasers and their interaction with materials and during the laser micro-grooving process, thermal parameters include temperature distribution and heat flux generation. The distinctive characteristics of short and ultra-short laser pulses as well as their potentiality for micromachining and microfabrication have been explained to comprehend the fundamental principles of the laser micro-grooving process. Chapter 8 investigates different micromachining process parameters for hard materials such as tungsten and HSS materials. Wire electric discharge grinding technique is used to fabricate micro-components such as micro-electrodes, microtools, and micro-probes to enhance the manufacturing process. Chapter 9 explains nano-additives-assisted MQL and process parameter optimization to lower costs and exploit advantageous decision factors like MRR. The significance of different nanoparticles in MQL is focused along with the synthesis technique. In Chapter 10, an in-depth description of the coating technique applied to workpiece using powder-mixed electric discharge machining technique is found. The chapter also explores response surface methodology (RSM) and Taguchi method for optimizing input parameters. Chapter 11 provides modeling methods for micro-EDM as well as detailed discussions about theoretical temperature, crater size, and other calculations.

Preface

xxvii

Chapter 12 is focussed on understanding micro-end milling method and the impact of the cutting-edge radius effect on removal processes. It also explains the mechanics of burr creation, cutting forces encountered and their dependencies on removal mechanisms, and tool wears elements for process monitoring as well as the contemporary technologies used and the surface finish aspect to address the quality of the work surface. Chapter 13 provides an exhaustive study of micro-texturing and the creation of microelliptical patterns using electrochemical micromachining (EMM) process. Chapter 14 thoroughly discusses about surface modification techniques employed to improve functional attributes of surfaces, machined by micro-EDM. Chapter 15 provides a comprehensive and up-to-date review of plentiful new research discoveries, as well as the development and enhancement of micromachining processes in Titanium Alloy using EDM. Chapter 16 contemplates micro-EDM method and its evolution in surface modification, involving upcoming technology with long-term potential, electrode use, and electrode selection based on various materials. The effect of adding powders, types of powders, particle size, surface alterations, and geometrical accuracy produced by the process are discussed in Chapter 17. This book provides an ample overview of several nontraditional microfabrication procedures and future perspectives, serving as a valuable resource for micromachining researchers and engineers. It will be useful for the students as a research and reference book for the subject of microfabrication and micromachining procedures. It will encourage academicians, researchers, and engineers in the field of microfabrication as well as micromachining. Finally, this book will encourage and motivate further research in the advanced field of micromachining.

CHAPTER 1

INTRODUCTION TO MICRO-ELECTRO MACHINING AND FABRICATION GOLAM KIBRIA,1 SANDIP KUNAR,2 T. JAGADEESHA,3 M. S. REDDY,4 and PRASENJIT CHATTERJEE5 Department of Mechanical Engineering, Aliah University, Kolkata, West Bengal, India 1

Department of Mechanical Engineering, Aditya Engineering College, Surampalem, Andhra Pradesh, India

2

Department of Mechanical Engineering, NIT Calicut, Kozhikode, Kerala, India

3

Department of Mechanical Engineering, Aditya Engineering College, Surampalem, Andhra Pradesh, India

4

Department of Mechanical Engineering, MCKV Institute of Engineering, Howrah, West Bengal, India

5

ABSTRACT The application of advanced micromachining processes increases in emerging areas of microengineering for various difficult-to-machine materials, metals, ceramics, etc. Currently, non-traditional micromachining procedures have extended their pertinence in the field of micromanufacturing and recommended improved prospects with numerous essential benefits that make these methods improved as well as more economical than traditional ones. Advanced micromachining procedures involve micro-wire EDM, microEDM, LBM, EMM, ECDM, micro-WEDG, vibration-assisted micro-EDM, Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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Electro-Micromachining and Microfabrication

etc. Non-traditional micromachining methods have been applied effectively to produce micro-features with greater accuracy. Hybrid micromachining is also employed successfully for producing intricate parts. In this study, various untraditional micromachining systems and hybrid processes are discussed properly. 1.1 INTRODUCTION Micromachining has already entered the machining world, revolutionizing the whole machining field. The need for improvement and use of the micromachining process is taking control in the industrial sector and is being used in various industries and are growing in recent years [1–5]. Figure 1.1 shows the classification of micromanufacturing processes schematically. Micromachining can create micropatterns and reduce all the unnecessary effects caused by traditional machining with so many factors under consideration. In Figure 1.2, the detailed classification of micromachining processes with hybrid processes and surface finishing techniques is shown schematically. Electro machining is the type of manufacturing process where removing material from the workpiece takes place with the application of electrical energy. The real application of electro-machining processes has grown up to a great level in the last two decades due to its several extraordinary advantages. In electro-machining processes, there may be the use of a physical tool that is applied based on the dimensions and features of machining cavities. These processes use hard materials as workpieces which are very tough to the machine by any other machining approach, whether conventional or nonconventional strategy. Electro machining at the micro-level is the approach where scale-down levels of process parameters are applied for the generation of micro-features or geometries. There are several advantages of microelectro machining for the fabrication of micro-features like minimization of applied energies, greater material use, reduction in power consumption, faster devices, improved process accuracies and selectivity, and integration with MEMS for simplifying the process or system. 1.2 NEED FOR MICRO-ELECTRO MACHINING AND FABRICATION Since the approach of micro-electro machining is widely demanded in the fabrication of several micro-parts, obviously, there are several basic needs for

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immediate implementation of these machining technologies in the domain of micro-manufacturing sectors. These advantages are as follows: •

• • • • • •

Some micro-electro machining procedures do not require any contact between the substrate and the microtool. However, some micro-electro machining processes need physical micro-tool. No material damage and vibration of machine elements are occurred in non-contact type machining. Complex micro-profiles with greater accuracy are effortlessly machined in micro-electro machining processes. The material removal relies on optical and thermal properties of the job material. Some micro-electro machining processes create small HAZ effect which extends the durability of the machined part. In these processes, microfeatures with greater aspect ratio are generated with satisfactory accuracy. Electro micromachining lowers material wastage. Hybridization of two or more processes can be applied to take the advantages of each of the processes.

FIGURE 1.1

Detailed classification of micromanufacturing processes.

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FIGURE 1.2 Classifications of micromachining and nano-finishing processes based on type of energy used.

There are several basic needs of application of micro-electro machining processes. With the extraordinary inventions and research findings, the metallurgy scientists discovered several materials with extraordinary properties of materials like high hardness, corrosion resistant, high strength, high heat resistant, etc. To manufacture macro- and micro-parts with highly intricate shapes or profiles, it is not sufficient to apply traditional material removal methods due to complicacy in designing the equipment or machine tool and high tooling cost involvement. Furthermore, it is complicated to manufacture complex three-dimensional components like turbine blades using traditional processes. If it is possible, then it requires complicated machine tools with high tooling cost. In addition, it is not always flexible to manufacture miniaturized features in the traditional machining processes. For example, it is difficult to produce 1,000 numbers of micro-holes (of size few tens of micrometer) in filters used in food processing and textile

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applications. Additionally, nano-level surface roughness is not possible to generate on wide variety of materials (conducting and non-conducting) of having complex geometries and features. Thus, for manufacturing of micro-features in micro-sized products, it is obviously needed to implement one or combined micro-electro machining processes which effectively and efficiently can produce these features. 1.3 TYPES OF MICRO-ELECTRO MACHINING AND FABRICATION The electro micro-machining is classified into five main categories, i.e., micro-EDM, micro-wire EDM, micro-LBM, EMM, and hybrid micromachining. The hybrid type micro-electro machining includes several processes which are combined for the aid of taking advantages of the processes. In the following sub-sections, all these five types of micro-electro machining processes are discussed briefly with process mechanisms and illustrations. 1.3.1 MICRO-EDM The eroding impact of successive electrical discharges (EDs) because of melting and evaporation of the substrate is used to remove material in this method. In Figure 1.3, the schematic representation of working principle is depicted. For higher intense of electric field, electrons begin to flow towards the workpiece, resulting in breakdown induction. An electron avalanche arises when faster electrons collide with natural species in the dielectric [6]. According to Lee [7], the cathode tool’s field emissions and thermionic emissions are considered for most of the electron emission. In accordance with Thomas-Fermi theory, the radiation process is highly dependent on strong temperature and electrical field [8]. Because there are some debris accumulated in the dielectric fluid with diameters on the demand of the slight gap and surface asymmetrical of the workpiece, the electric field will convince these accumulated debris to essence at the field’s greatest point, in addition to the discharge fact. Consequently, a plasma channel, a highly conductive bridge, emerges across the gap. The plasma channel exhibits a uniform development pattern at first, followed by a quicker growth pattern. As a result, a substantial amount of energy will be accumulated in plasma column that connects the tool and job. A spark is created when the pressure and temperature in the plasma channel suddenly increase. Less quantity of material dissolves and evaporates off the electrode and job at the site of spark

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contact. The gas bubbles quickly grow away from the spark path. When the expanding bubble’s pressure falls below that of the surrounding atmosphere, the bubble collapses and liquid jets invade the bubbles. This liquid jet ejects molten material from the molten crater that has been created [9].

FIGURE 1.3 Schematic representation of micro-EDM setup details and mechanism of material removal phases.

1.3.2 MICRO-WIRE EDM Micro-WEDM refers to a part that is made in a wire-EDM machine with dimensions of less than 1 mm (1 to 999 µm). An endless wire electrode (50 to 250 µm) is incessantly pushed through the component by a wire feeding approach without doing any contact with the component. Direct contact between the tool and job does not cut the part, as is the case with standard mechanical micromachining methods. Instead, the pulse (ED) formed between the electrode and the part is used to manufacture it. The power system generates a sequence of fast, repetitive, and distinctive spark discharges inside the dielectric fluid between the component and wire [10]. Pulses are another name for the spark discharges. A DC power unit is utilized for this operation. When the power is switched on, the wire and

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job are attached to the cathode and anode, respectively, resulting higher voltage among electrodes. The voltage reaches a particular point (breakdown voltage) in a relatively short amount of time, causing an electric discharge. A little amount of material from both the wire and component vaporizes during the pulse discharges, leaving tiny craters on both the wire electrode and part. Machining rate (MR) from the wire and job is controlled by shifting the polarity of electrodes. The basic schematic representation of the microWEDM is shown in Figure 1.4.

FIGURE 1.4

Schematic view of micro-wire EDM setup indicating various sub-systems.

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1.3.3 LASER MICROMACHINING In the field of micromachining procedures, lasers are becoming increasingly popular. The schematic diagrams of several types of laser beam machining (LBM) technologies are shown in Figure 1.5. Depending on the shape of the material erosion from the job, the procedures can be categorized. The beam is static with respect to the job to be treated as one type of laser technique. The laser beam and the workpiece surface do not move in relation to each other. The laser micro-drilling method is a good example of this form of machining. To remove material, the erosion is placed at the tail end of the machined micro-hole and transmitted in the path of the line source. Another form of laser micromachining technique involves the laser beam moving in one direction relative to the workpiece. At the leading side of laser beam, the erosion front is positioned. As a result, material removal is accomplished by repeatedly moving the erosion front in the direction of the workpiece’s depth. Laser micro-cutting, and laser micro-grooving are typical examples of this type of LBM. One or two laser beams are utilized in the third form of laser micromachining. Each of the beams creates a cutting front by allowing the workpiece to move relative to the laser beams. At the leading side of every laser beam, the erosion front for each cutting operation can be found. Laser micro-turning is an illustration of this form of LBM. In Figure 1.6, the setup details of laser micromachining processes are shown schematically.

FIGURE 1.5 Schematic view of three types of laser micromachining approaches – (a) one dimensional; (b) two dimensional; and (c) three dimensional.

1.3.4 ELECTROCHEMICAL MICROMACHINING (EMM) Chemical energy-based micromachining procedures remove material with acidic chemicals, whereas electrochemical micromachining (EMM) uses the principle of electrolysis. It connects the tool to the cathode and a job to the

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anode. According to Faraday’s principle [11], material is removed from the anode (workpiece) when a sufficient electrolyte streams through the inter electrode gap (IEG) between the job and tool with an acceptable applied voltage. This method provides several advantages over other procedures, including a superior SQ, the lack of residual stresses, the theoretical absence of tool wear, the absence of burr formation, and feature distortion. Milling, multiple hole drilling, and surface structuring have been done with an EMM and a CNC machine [12–14]. Controlling the region on which material removal occurs, is critical in EMM. To achieve great dimensional precision, tool surface insulation and workpiece surface masking are required. This approach improves to decrease undercut [15]. The machining gap through which electrolyte flows is known as the IEG. In Figure 1.7, the schematic view of EMM is shown.

FIGURE 1.6

Representation of a typical laser micromachining setup.

1.3.5 VIBRATION ASSISTED ELECTRO-MACHINING The implementation of ultrasonic vibration during micro-electro machining is one of the emerging machining strategies to augment the machining

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precision. Several researchers have used this novel ultrasonic vibration approach in the machining of wide-ranging materials all over the world.

FIGURE 1.7

Schematic of typical electrochemical micromachining arrangement.

1. Vibration-Assisted Micro-EDM: The electrode is vibrated using ultrasonic vibrations in this procedure [16]. Because of the vibrating and sucking action, ultrasonic vibration promotes dielectric circulation and debris removal [17]. Significant vibrations of the spinning electrode, on the other hand, degrade machining quality in micro-EDM, especially when the electrode is very delicate. As a result, electrode vibration machining precision in a deep microhole is extremely difficult to achieve. Instead of vibrating the tool, an ultrasonic transducer vibrates the workpiece to fix the problem. The work is immediately connected to a transducer to prevent vibration. Workpiece vibration, rather than tool vibration, allows for more flexibility in tool design because it does not require the use of a transducer, horn, or cone. Additional benefit is that it is far

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simpler and more efficient than conventional systems [18]. The use of an ultrasonic aided method boosts material removal by 66.48% and output efficiency more than 60 times [19, 20]. In Figure 1.8, the schematic view of vibration assisted micro-EDM setup is shown.

FIGURE 1.8

Schematic of ultrasonic vibration assisted micro-EDM system.

2. Vibration-Assisted Micro-EDM Milling: Scanning micro-EDM is ideal for cutting high aspect ratio 3D microstructures. Tool electrode wear (TEW) becomes an extrusive concern in the scanning microEDM process, and it has a direct impact on the size and shape to be created [21]. Intermittent feed or specific estimate approaches are commonly used to adjust tool wear [22]. To begin with, the discharge area is smaller because to the thin tool-electrode wire’s short crosssectional area compared to the comparatively large volume to be evacuated. Maintaining a consistent discharge area is difficult due to the scanning velocity between electrodes. The machining process is optimized using a workpiece vibration-assisted servo scanning 3D micro-EDM approach. When compared to conventional microEDM milling, the researchers claim that machining efficiency has

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increased by 6.5 times [23]. In Figure 1.9, the schematic view of vibration assisted micro-EDM milling is depicted.

FIGURE 1.9

Schematic view of vibration assisted micro-EDM milling (scanning tool strategy).

3. Vibration-Assisted Reverse EDM: Reverse micro-EDM is a reversible micro-EDM technology [24]. Between the micro-EDM and R-MEDM methods, there is no fundamental change in material erosion principle. Micro-EDM uses electrodes with changing cross-sections to machine duplicate micro-cavities. In the R-MEDM technique, however, micro-cavities are employed to create microrods [25]. Micro-cavity patterns are created by micro-milling, microdrilling, etc., on thin metallic foils. As a cathode, this thin plate electrode is used. On the cathode, the workpiece surface is matched across micro-cavities. Sparking occurs whenever two electrodes come into contact. As a result, the pattern of micro-rods generated on the workpiece is a perfect match for the pattern of micro-cavities. Each of the micro-cavities present on a cathode produces a microrod. A produced micro-rod’s cross-section is an exact reproduction of a micro-cavity’s cross-section [26]. The overall feed delivered to the moving electrode controls the aspect ratio of the micro-rod. In Figure 1.10, the schematic representation of vibration assisted reverse EDM for the generation of batch micro-tool is depicted.

Introduction to Micro-Electro Machining and Fabrication

FIGURE 1.10

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Scheme of vibration assisted reverse EDM for batch tool fabrication.

4. Vibration-Assisted WEDM: There are two types of vibration assisted wire-EDM: those that use ultrasonic vibration on the moving wire and those that use ultrasonic vibration on the stationary workpiece. The wire system contains wire guide and a wire electrode, is coupled to the piezoelectric actuator [27]. Because the wire electrode and the piezoelectric actuator are connected via the wire holder, the wire electrode is stimulated in a forced vibration situation when the piezoelectric actuator is turned on [28]. By increasing liquid pressure variation and dielectric flow circulation, the displacement of antinodes will increase the flushing of melted materials. When a piezoelectric actuator is triggered using an insulating adapter to deliver ultrasonic vibration to a workpiece, the workpiece will shift in a regular manner. Short circuits can arise because of workpiece vibration. The discharge procedure will begin as soon as the workpiece is drawn back [29]. The new fresh dielectric will be sucked out by the high-pressure variation, allowing the discharge to quickly breakdown. When the workpiece is pressed close to the wire, the debris is washed away under high pressure [30]. The schematic representation of mechanism of vibration assisted WEDM is depicted in Figure 1.11. 5. Vibration-Assisted ECM: Vibrations inside the machining zone during EMM at the stationary electrolyte have a substantial impact on the diffusion and convection of dissolved metal ions due to hydrodynamic effects on bubble behavior. The pressure in the inter-electrode gap will diminish as the micro-tool moves upward. This pressure drop separates the forces that hold the liquid molecules together, resulting

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in tiny electrolyte vapor bubbles or transitory cavitation [31]. The pressure builds as the anodic micro-tool descends with the production of warmth and pressure, causing the rapid collapse of micro-bubbles [32]. Because the anode is pulsated, the bubble comes quite close to the anodic instrument. The potential energy of the expanding cavity is transferred to kinetic energy of the liquid jet velocity during energetic collapses. Increased convective mass movement of dissolved ions, breach of the diffusion layer, and replenishment of fresh electrolyte are all caused by the impact of the micro-jets on the anodic surface [33]. In addition to pulling hydrogen bubbles out of the IEG, increasing pressure enhances conductivity. In the case of continual cavitation from the solution due to mass convection, periodic size oscillations of bubbles are possible, depending on the amplitude and frequency of vibration. Vibration improves conductivity and boosts current density by increasing convective mass movement, diffusion rate, and conductivity [34].

FIGURE 1.11 The scheme of vibration assisted wire EDM for both case (vibration of traveling wire and vibration of workpiece).

1.3.6 MICROMACHINING BY ECDM This method is a hybrid and advanced machining technique, where two methods, i.e., ECM and EDM are combined to attain high quality characteristics specifically on non-conducting materials. In this method, electrochemical and

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electro-discharge action that assist in the production of gas bubbles. Between the tool and auxiliary electrode, which are submerged in the electrolyte solution, pulsed DC or DC power is used. Consequently, O2 bubbles creates on the counter electrode and H2 bubbles develop on the tool surface due to electrolysis. Due to higher potential difference, the greater current density increases the creation of air bubbles near the tool. These created bubbles steadily mix to form a gas layer that acts as a shielding media and generates adequate voltage between two electrodes. ED happens when the voltage between the tool and the electrolyte exceeds a critical point. Due to the heat generated by the ED, machining happens in the form of melting, evaporation, and thermal erosion if the work is positioned in the discharge zone. Electrolysis and electro-discharging between electrodes are used to remove the substance [35]. The combined methods help to eliminate high amount of material in micro-ECDM than other one. The diagram is revealed in Figure 1.12. It is generally utilized for micromachining on difficult to machine materials. This procedure is effectively applied in nonconductive materials, i.e., glass, ceramics, etc., for micro-drilling and micro-slots proficiently. Generation of intricate micro-structures is very difficult by ECDM process with high degree of accuracy on advanced materials. Because of their importance in the MEMS field, thorough research is essential for machining of non-conductive materials. For instance, semiconductor materials are stuck with quartz owing to their translucent property. In addition, the method is used to machine ceramic materials used in high-tech sectors [36]. It has many benefits such that it can drill glass, ceramics, etc. Other several applications comprise production of deep drilling, micro-dies, etc. 1.3.7

MICRO-WIRE ELECTRIC DISCHARGE GRINDING (WEDG)

The generation of microfeatures on curved surfaces is the most challenging issue since the pressure is exerted by the grinding wheel on the required components; for instance, the micro-electrodes can produce the components to deflect, creating it challenging to attain the required accuracy. Truly, in some cases, it is difficult to generate tiny parts utilizing this method. Different conventional cutting and grinding methods depend on the force produced by the abrasive materials or harder tools to eradicate the weaker job material. EDM method applies electrical sparks to wear down the unwanted material and creates the required shape. Another advancement is to rotate pieces in a spindle installed on a wire EDM unit’s table and allow the wire to “grind” the parts to shape and size. The diagram is shown in Figure 1.13. This is advantageous in micromachining situations where the material is removed

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electrically rather than mechanically through spark erosion. Because the job and wire never come into contact, there is no risk of deflection. Furthermore, the wire EDM unit may easily be programmed to generate contoured features, whereas grinding operations would necessitate lengthy setup periods and contoured wheels. Wire guides are employed by WEDG to keep the wire tight in the discharge area between the wire and the rod to reduce wire rattling. One of WEDG’s advantages is its ability to produce a variety of intricate shapes, i.e., stepped and tapered shapes, at different segments [37].

FIGURE 1.12

Representation of micro-ECDM setup arrangement.

1.4 SUMMARY Recent development in microfabrication methods is prominent in the production of distinctive characteristics in micro/nanoscale for their purpose in various microengineering sectors. Micromachining techniques have several advantages and numerous functions for society. The requirements of microparts are growing incessantly, with accurate features varying from a few nanometers to a few microns. Microsystems are established for the generation of microparts which have different applications in our everyday lives. Untraditional micromachining approaches are applied for the generation of intricate features where traditional machining methods are not reasonable because of several inevitable causes. The summary of these micromachining

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procedures will aid the intangible concept where chemistry and physics are concerned. The significance of each method applied and numerous procedures of untraditional micromachining, as well as hybrid machining, are deliberated in the framework of microfeature production and improvement of the quality of the surface. These methods can be enhanced further by growing applications in MEMS where efficient necessities of several devices require very close dimensions and wide applications of advanced materials.

FIGURE 1.13

Schematic view of arrangement of wire electro discharge grinding process.

KEYWORDS • • • • • •

electrochemical micromachining fabrication hybrid machining laser micromachining micro-electro machining microfabrication

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REFERENCES 1. Rajurkar, K. P., & Yu, Z. Y., (2000). 3D micro-EDM using CAD/ CAM. CIRP Annals, 49(1), 127–130. 2. Yu, Z. Y., Masuzawa, T., & Fujino, M., (1998). 3D micro-EDM with simple shape electrode: Part 1. International Journal of Electrical Machining, 3, 7–12. 3. Yu, Z. Y., Masuzawa, T., & Fujino, M., (1998). 3D micro-EDM with simple shape electrode: Part 2. International Journal of Electrical Machining, 3, 71–79. 4. Mohri, N., Takezawa, H., Furutani, K., Ito, Y., & Sata, T., (2000). A new process of additive and removal machining by EDM with a thin electrode. CIRP Annals, 49(1), 123–126. 5. Yeo, S. H., & Nachiappan, R., (2001). Investigation of micro-EDM controllable factors on machined silicon surface quality. Journal of Engineering Manufacture, 215(B6), 811–817. 6. Meek, J. M., & Craggs, J. D., (1978). Electrical Breakdown of Gases. Wiley, New York. 7. Lee, T. H., (1959). T-F theory of electron emission in high-current arcs. Journal of Applied Physics, 30(2), 166–171. 8. Kunieda, M., Lauwers, B., Rajurkar, M. P., & Schumacher, B. M., (2005). Advancing EDM through fundamental insight into the process. CIRP Annals-Manufacturing Technology, 54(2), 64–87. 9. Hockenberry, T. O., & Williams, E. M., (1967). Dynamic evolution of events accompanying the low-voltage discharges employed in EDM. IEEE Transactions on Industry and General Applications, 4, 302–309. 10. Puri, A. B., (2017). Advancements in micro wire-cut electrical discharge machining. In: Kibria, G., Bhattacharyya, B., & Davim, J. P., (eds.), Non-Traditional Micromachining Processes-Fundamentals and Applications (pp. 145–178). Springer International Publishing AG, Switzerland. 11. Datta, M., & Harris, D., (1997). Electrochemical micromachining: An environmentally friendly, high speed processing technology. Electrochimica Acta, 42(20), 3007–3013. 12. Ghoshal, B., & Bhattacharyya, B., (2013). Micro electrochemical sinking and milling method for generation of micro features. Journal of Engineering Manufacture, 227, 1651–1663. 13. Sen, M., & Shan, H. S., (2005). A review of electrochemical macro-to micro-hole drilling processes. International Journal of Machine Tools and Manufacture, 45(2), 137–152. 14. Landolt, D., Chauvy, P. F., & Zinger, O., (2003). Electrochemical micromachining, polishing and surface structuring of metals: Fundamental aspects and new developments. Electrochimica Acta, 48(20), 3185–3201. 15. Jain, V. K., Sidpara, A., Ravisankar, M., et al., (2012). Micromanufacturing: An introduction. In: Jain, V. K., (ed.), Micromanufacturing Processes (pp. 3–38). CRC Press, New York. 16. Kremwer, D., et al., (1989). Effects of ultrasonic vibration on the performances in EDM. CIRP Annals, 38(1), 199–202. 17. Zhixin, J., Jianhua, Z., & Xing, A., (1997). Study on a new kind of combined machining technology of ultrasonic machining and electrical discharge machining. International Journal of Machine Tools and Manufacture, 37, 193–199. 18. Gao, C., & Liu, Z., (2003). A study of ultrasonically aided micro-electrical-discharge machining by the application of workpiece vibration. Journal of Materials Processing Technology, 139, 226–228.

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19. Mahardika, M., Prihandana, G. S., Endo, T., Tsujimoto, T., Matsumoto, N., Arifvianto, B., & Mitsui, K., (2012). The parameters evaluation and optimization of polycrystalline diamond micro-electrodischarge machining assisted by electrode tool vibration. International Journal of Advanced Manufacturing Technology, 60, 985–993. 20. Huang, H., Zhang, H., Zhou, L., & Zheng, H. Y., (2003). Ultrasonic vibration assisted electro discharge machining of microholes in nitinol. Journal of Micromechanics and Microengineering, 13, 693–700. 21. Bleys, P., Kruth, J. P., Lauwers, B., Zryd, A., Delpretti, R., & Tricaricol, C., (2002). Real-time tool wear compensation in milling EDM. CIRP Annals, 51, 157–160. 22. Yu, Z. Y., Masuzawa, T., & Fujino, M., (1998). Micro-EDM for three-dimensional cavities—development of uniform wear method. CIRP Annals, 47, 169–172. 23. Tong, H., Li, Y., & Wang, Y., (2008). Vibration-assisted servo scanning 3D micro EDM. Journal of Micromechanics and Microengineering, 18, 025011. 24. Singh, A. K., Patowari, P. K., & Deshpande, N. V., (2016). Experimental analysis of reverse micro-EDM for machining microtool. Materials and Manufacturing Processes, 31, 530–540. 25. Mastud, S., Garg, M., Singh, R., Samuel, J., & Joshi, S., (2012). Experimental characterization of vibration-assisted reverse micro electrical discharge machining (EDM) for surface texturing. Proceedings of the ASME 2012 International Manufacturing Science and Engineering Conference (MSEC2012). Notre Dame, Indiana, USA. 26. Mastud, S. A., Singh, R. K., & Joshi, S. S., (2012). Analysis of fabrication of arrayed micro-rods on tungsten carbide using reverse micro-EDM. International Journal of Manufacturing Technology and Management, 26(1–4), 176–195. 27. Guo, Z. N., Lee, T. C., Yue, T. M., & Lau, W. S., (1997). Study on the machining mechanism of WEDM with ultrasonic vibration of the wire. Journal of Materials Processing Technology, 69, 212–221. 28. Guo, Z. N., Lee, T. C., Yue, T. M., & Lau, W. S., (1997). A study of ultrasonic-aided wire electrical discharge machining. Journal of Materials Processing Technology, 63, 823–828. 29. Hsue, A. W. J., Wang, J. J., & Chang, C. H., (2012). Milling tool of micro-EDM by ultrasonic assisted multi-axial wire electrical discharge grinding processes. Proceedings of the ASME 2012 International Manufacturing Science and Engineering Conference (MSEC2012). Notre Dame, Indiana, USA. 30. Hoang, K. T., & Yang, S. H., (2013). A study on the effect of different vibrationassisted methods in micro-WEDM. Journal of Materials Processing Technology, 213, 1616–1622. 31. Banks, C., & Compton, R. G., (2003). Voltammetric exploration and applications of ultrasonic cavitation. Chem Phys Chem, 4, 169–178. 32. Skoczypiec, S., & Ruszaj, A., (2005). Discussion of cavitation phenomena influence on electrochemical machining process. International Journal of Manufacturing Science and Technology, 7, 27–32. 33. Ghoshal, B., & Bhattacharyya, B., (2013). Influence of vibration on micro-tool fabrication by electrochemical machining. International Journal of Machine Tools & Manufacture, 64, 49–59. 34. Bhattacharyya, B., Malapati, M., Munda, J., & Sarkar, A., (2007). Influence of tool vibration on machining performance in electrochemical micro-machining of copper. International Journal of Machine Tools & Manufacture, 47, 335–342.

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35. Bhattacharyya, B., Doloi, B., & Sorkhel, S. K., (1999). Experimental investigations into electrochemical discharge machining (ECDM) of non-conductive ceramic material. Journal of Materials Processing Technology, 95, 145–154. 36. Jain, V. K., & Adhikary, S., (2008). On the mechanism of material removal in electrochemical spark machining of quartz under different polarity conditions. Journal of Materials Processing Technology, 200, 460–470. 37. Masuzawaa, T., Fujino, M., Kobayashi, K., Suzuki, T., & Kinoshita, N., (1985). Wire electro-discharge grinding for micro-machining. CIRP Annals, 34, 431–434.

CHAPTER 2

ADVANCES IN ULTRASONIC MICROMACHINING AND ASSISTED ELECTRO-MACHINING FOR MICROFABRICATION SAYAN DOLOI, AMLANA PANDA, RAMANUJ KUMAR, and ASHOK KUMAR SAHOO School of Mechanical Engineering, Kalinga Institute of Industrial Technology, Bhubaneswar, Odisha, India

ABSTRACT Micromachining of brittle and hard materials can be performed for microfabrication using the principle of ultrasonic micro-machining (USM) as well as the ultrasonic-assisted advanced electro-micromachining process. The present chapter includes the ultrasonic micro-machining processes for generating various micro-features such as micro-holes, micro-slots, micro-walls, etc. In ultrasonic micro-machining, the ultrasonic vibration of micro-tool is transmitted to the abrasive particles of micron size present in the slurry, and the repetitive impact of abrasive particles with ultrasonic frequency utilized for material removal due to mechanical failure such as brittle fracture of brittle and workpiece materials. In assisted hybrid micromachining ultrasonic vibration of a tool or workpiece plays an important role in increasing the MR and accuracy. In assisted electro-machining, ultrasonic vibration of micro-tool assists the material during electromachining, such as micro-electric discharge machining (EDM), microelectrochemical machining (ECM), and micro-electrochemical discharge machining (ECDM). In ultrasonic-assisted micro-EDM, material removal is Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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enhanced due to better flushing and cavitation effect. For micro-drilling and micro-fabrication of slots and grooves, ultrasonic-assisted micro-EDM is applied. In ultrasonic-assisted ECM, higher MR is obtained due to the efficient removal of precipitates of chemical compounds. The present chapter also includes a discussion on ultrasonic-assisted micro-EDM, ultrasonicassisted micro-ECM, and ultrasonic-assisted micro-ECDM utilized for micro-fabrication. 2.1 INTRODUCTION Ultrasonic micromachining is a mechanical based advanced micromachining process and has gained popularity in recent years, where various miniaturized components are produced due to its special machining characteristics. Requirement of various intricate and accurate parts of hard and brittle materials in different industries like electronics, aerospace, opto-electrical, automobile, biotechnology, communication, etc., have pushed researchers to carry out experimental investigation into ultrasonic micro-machining process. Utilization of ultrasonic machining (USM) process for micromachining have provided the path to fabrication of micro-sized holes and 3D structures on hard, brittle, and non-conductive materials such as ceramics, glass, quartz, composites, and stronger and tougher materials like nickel, titanium, tungsten alloy, etc. Many micro-machining processes like electric discharge micromachining, electro chemical micromachining, laser beam micromachining (LBMM), etc., are commercially available in the market but none of them has used to machine hard, brittle, and nonconductive material due to various issues like development of residual stress, thermal damage to the surface to perform desire operation on these work materials. This is a state-of-the-art machining technology that creates breakthroughs in the fabrication of almost any 3 D microstructure with high aspect ratio on most materials, especially hard, brittle, and non-conductive materials like borosilicate glass, quartz, silicon nitride, and ceramic. Non-thermal, non-chemical machining behavior along with no thermal stress, no residual stress and no metallurgical property alteration take place during machining of work-parts which provides a significant characteristic to ultrasonic micromachining that enhances its values in modern competitive market. Ultrasonic micro-machining with advanced controlling parameters is capable to drill a through hole as low as 5 μm with high aspect ratio, précised and accurate dimension. Some basic applications of this micromachining are in producing different components such as biomedical filter, inkjet printer nozzle, fuel

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nozzle, grids, optical apparatus, orifice, sensors, micro-pipettes, manipulator, etc. Ultrasonic micro-machining is only suitable machining process for creation of intricate and miniaturized parts on hard and brittle material. The ceramic material which has attractive features, and it is also very useable material in the modern developed industries. These materials have properties like high thermal resistance, high hardness, higher ratio of strength to weight, chemical inertness, higher life expectancy, etc. Alumina based ceramic has been used in electrical insulator, grinding media, thermocouple tube, ballistic armor, laser tubes, electronic devices, and medical prostheses, etc. USM process is non-electrical, nonchemical non-thermal and so that physical and chemical properties of material don’t alter. There is no significant effect on material microstructure of material takes place during USM machining. Different intricate parts, round, and complex shaped holes, blind holes, taper holes, micro-cavity, and other three-dimensional machining can be done with high precision and accuracy in dimension of components and less surface damage. It can be used to make multiple features simultaneously and time taken of machining is also significantly lower so that it validates the economy of machining process by using USM. USM machining process generates very low residual stress and almost zero thermal stress on work piece material. Electrical conductivity of work piece doesn’t affect by this machining process, so it is useful for electrical conducting as well as non-conducting materials along with hard and brittle material. Alumina, zirconia, boron carbide, silicon carbide, boron nitrides, silicon nitride, are few of advanced and newest developed engineering materials, these can also be machined by using USM. Tool materials like tungsten carbide, stainless steel, and Monel are generally used in USM and these tool materials are capable for higher transmission of energy to abrasive grains, and these have high resistant to wear. Modern micro-USM machining machines has more feathers such has rotary ultrasonic machining (RUM) enhances the material removal rates (MRRs) and decreases the overcut, circularity error, surface damage, roundness error during machining. RUM uses tool bonded with abrasive for deeper holes drilling instead of abrasive slurry for stationary micro-USM. Ultrasonic vibration of workpiece or tool in electro-physical machining such as ECM, EDM, and ECDM assists materials removal with high accuracy. Ultrasonic-assisted EDM decreases surface roughness and increases sparking intensity, and it assists molten globule to get removed easily, form spherical shape of smaller size. It produces narrower heat affected zone (HAZ) and lesser micro-cracks. But insufficient debris removal from the inter electrode gap (IEG) caused some problems influencing process conditions

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and surface quality (SQ). Applications of ultrasonic vibrations solved these problems. Ultrasonic-assisted EDM enhances machining performance of difficult-to-machine materials. For producing 3-D cavities with simple geometric features with sharp corners ultrasonic-assisted electro discharge machining (UAEDM) is applied. Ultrasonic-assisted electro-discharge milling is effective for better flushing, uniform removal of debris in dielectric. The use of ultrasonic vibrations in powder mixed dielectric ensures uniform mixing and restrict conglomeration for better sparking condition. The assistance of Ultrasonic vibration in dry EDM solves the problem of poor debris removal, debris reattachment, and non-uniform machining rates (MRs), etc. Electrochemical machining (ECM) is an anodic dissolution process to produce free-form surfaces by eliminating thermal damage, residual stress, and tool wear. But ECM has problem of generating passive oxide layer which decreases the MRR. Ultrasonically assisted electrochemical machining (UAECM) can control the generation of passive layer by providing the vibration to workpiece or tool which results in causes a chaotic, turbulent flow in electrolyte. In UAECM, ultrasonic vibration favors for generating micro-bubbles adjacent to the workpiece and electrode surface. Ultrasonic vibration affects the electrolytic conditions favorable for the enhanced rate of chemical reactions and electrochemical dissolution. Electrochemical discharge machining (ECDM) is a process which combines the principle of EDM and ECM process, but ECDM process is useful for machining electrically non-conducting materials such as glass, ceramics, and composite materials. ECDM overcomes the limitation of ECM and EDM as both are limited to electrically materials. ECDM with gravity feeding arrangement has also limitations such as low depth of penetration (DOP) and low MRR. The assistance of ultrasonic vibration in ECDM improves MRR at higher depth as molten materials are removed easily due to ultrasonic vibration of tool or workpiece. During ECDM micro-machining process, ultrasonic vibration assists for formation of small size bubbles, and it increases the availability of electrolyte in the tool interface for more spark generation. As a result, the machining performance of Ultrasonic-assisted ECDM is enhanced. 2.2 ULTRASONIC MICRO-MACHINING The ultrasonic micro-machining works on the principle of ultrasonic vibration of tool with help of horn. Power supply generates a kHz signal that is

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applied to piezoelectric convertor. Piezoelectric converter converts electrical energy into mechanical motion. Amplification of the amplitude of vibration is done by horn and it transfers to the tool. Tool vibrates at this high frequency and transfers momentum to the abrasive particles. A small gap is present between the tip of tool and work piece to continuous flow of abrasive particles. Abrasive particles gain momentum by mechanical vibration of tool and impact on the targeted workpiece. A localized stress is developed on the work piece material and removal of material takes place by micro-chipping. The main characteristics feature of USM is that there is no contact between tool and work piece whereas it is mandatory in traditional machining process. The term USM is used for vibration wave having frequency more than 20 KHZ. The highly frequent vibration driven abrasive slurry is used to perform the machining operation on various material. Frequency, working gap, slurry flow rate, abrasive slurry concentration, abrasive particle size, slurry medium, slurry medium viscosity, applied static load, applied power rating, tool material plays as independent variables in USM where MRR, tool wear rate, surface roughness quality, out of roundness, taper ratio, etc., are response during machining. Power supply unit, abrasive slurry supply unit, tool holder, horn, work holding magnetic base table, pneumatic based system for tool feeding, ultrasonic frequency generator are main parts of USM machine. The sketch of ultrasonic micro-machining system is represented in Figure 2.1.

FIGURE 2.1

Sketch of ultrasonic micro-machining system.

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Micro-USM is a useful process to machining brittle and hard materials such as ceramics, glass, silicon, and quartz. Micro-USM can remove materials from both conducting and non-conducting workpiece. Furthermore, micro-USM does not cause any thermal or chemical deviation to the workpiece. The electrical energy of low frequency is converted into an electrical signal of high frequency. The electrical energy is converted into mechanical vibrations in ultrasonic transducer. The horn amplifies the vibration. The micro-tool with tool holder relates to horn for micromachining operation. During micro-USM, tool vibrates at ultrasonic frequency with amplitude 25 µm over work piece. Ultrasonic vibration is transmitted to abrasive particle from tool. Large number of abrasive particles strikes 20,000 or more times per second on workpiece. There is fatigue failure of workpiece materials due to repetitive impacts. Due to striking of sharp abrasive particle, there will be initiation of cracks as brittle fracture of materials takes place. 2.3 HISTORY OF ULTRASONIC MICRO-MACHINING Ultrasonic micro-machining is advanced version of USM process having at least one of the machined dimensions in range of 1 μm to 999 μm. USM had been developed in 1927 and L. Balamuth patented this in 1945, at that time it was also named as “impact grinding,” “ultrasonic machining” or slurry drilling. It depends on cutting action of abrasive grain in slurry flowing in the gap between the tip of tool and work piece. The first USM tool mounting on body of drilling machines has fabricated during 1953–1954. Due to its special features for machining of hard and brittle material it went commercialized in market by 1960 and used for various material machining in different industries such as opto-electrical, aerospace, electronics, mechanical electrical system. In 1990, it was first attempt to descaling the tool from macro-dimension to micro-level by Masuzawa of Tokyo University. Miniaturization of components and fabrication of intricate and complex shape on hard, brittle, and non-conducting material, unavailability of alternate machining process has boosted its application and enhance further research for getting better result. At the present time, it has been used for fabricating blind hole as low as 5 μm diameter with high aspect ratio and micro-channel, groove, and micro-cavity, etc., for miniaturization of components.

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2.4 TYPES OF ULTRASONIC MICRO-MACHINING USM machine is mainly of two types, one of them is stationary ultrasonic machine and other is rotary ultrasonic machine. Rotary USM is advanced version of stationary USM. Stationary USM is done either by vibrating work piece or tool, in the same ways RUM can be done with the help of vibrating and rotating work piece or tool. The modern advanced ultrasonic machine is hybrid, which is combination of ultrasonic and other machining process such as combination of USM and EDM, USM, and ECM, USM assisted drilling and turning, etc. 2.4.1 MACHINING BY LONGITUDINAL VIBRATION ONLY 1. Machining by Longitudinal Vibration of Tool: In this method tool is formed with help of WEDG/EDM and amplified mechanical vibration is provided to the tool. As result of Piezo-electric effect tool vibrates along longitudinal direction and transfer energy to the abrasive particles which strikes to the available work piece and machining takes place. It is conventional way of machining by using ultrasonic frequency. 2. Machining by Longitudinal Vibration of Work Piece: There is further development in micro-USM to control the accuracy and precision at required level. Vibration is provided to the work piece instead of tool. The process provides a better tool holding and application of high precision tool spindle. This advancement needs a vibration mechanism for work piece that vibrates it at ultrasonic frequency. Longitudinal vibration of tool gives dimensions with high precision and accuracy. 2.4.2

ROTARY USM (RUM)

It is an advanced USM in which simultaneous rotation and translation of machine tool take place. It is much faster and efficient process as compared to stationary micro-USM. A rotating core drill is bonded with diamond abrasive. Stationary micro-USM process has high tool wear rate and low MR whereas rotary micro-USM eliminate this drawback. It is an integration of USM

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process and conventional diamond grinding. RUM is capable to perform machining operations like grinding, milling, finishing, surface texturing and fabrication of different intricate shape on different brittle and hard materials such as titanium, titanium alloy, ceramic, carbon fiber reinforced polymer, zirconia, glass ceramics, etc. This machining process is used for fabrication of various useful component in field of electronic, computer, aircraft, medical, automotive industries. Machining setup of RUM consists of power providing setup, data acquisition system and rotary vibrated tool system, a force and feed providing mechanism and unit of coolant supply. Figure 2.2 represents a sketch of the micro-rotary ultrasonic machining (MRUM) setup. RUM comprises of ultrasonic tool rotation system, a coolant supply system, and motion control system. Ultrasonic tool rotation unit has various subsystems such as a power supply unit, an electric motor, ultrasonic spindle, and a control system. Power supply supplies high frequency (20 kHz) converting from the low-frequency (50 Hz) electrical energy. Piezoelectric transducer fitted in the spindle generates mechanical vibrations. The coolant supply system has various components such as a pump, coolant tank, pressure gages, and valves, pressure regulator, etc. The motion control system has various components such as a dynamometer, a charge amplifier, an analog to digital converter, and computer, etc. 2.5 ADVANTAGES OF ULTRASONIC MICRO-MACHINING Micro-USM is a modern advanced process, specially used for brittle and hard materials. Micro-USM is an advanced non-conventional machining process. It has many advantages as compared to other machining processes which are discussed below: • • • • • •

There is absence of contact, and it doesn’t generate thermal or residual stress in work piece. It is non-electrical, non-thermal, and non-chemical machining process so it doesn’t alter metallurgical properties of work piece. It is useful for machining of electrically non-conductive, conductive, brittle, and hard materials. Better surface finish, good accuracy level and less surface damage of machined parts. It generates burr free surface. Different types of machining such as deep holes, simultaneous creation of multiple holes, steeped holes and blind holes can be produced.

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• • • •

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Three dimensional cavities, complex microstructure with good surface finish can be produced. Rotary ultrasonic micro-machining enhances the MRR as compared to stationary micro-USM and decreases the circularity error, taper, overcut, surface damages, etc. Less power has consumed in USM process. It is non-chemical process; machining of chemically active and inactive materials can be done.

FIGURE 2.2

Sketch of the rotary ultrasonic micromachining setup.

2.6 LIMITATIONS OF ULTRASONIC MICRO-MACHINING Although ultrasonic micro-machining is advanced version of machining process, but it has also few limitations those are mentioned below: •

Tool wear in lateral and longitudinal direction may affect the roundness of workpiece.

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• • • • •

•

Creation of deep micro-holes is difficult due to problem of tool wear. Proper selection of weight applied, feed rate, tool holder attachment, horn profile is necessary. A frequent tuning of USM vibration unit should be done. Tool fabrication is difficult. Controlled supply of abrasive particles and outgoing of debris particles is necessary to avoid any thermal problem. Experiments show that tool rotation enhances the performance of micro-USM but during rotation tool gets deflected due to unbalance forces. To get very highly improve results it is necessary to maintain balance between all parameters. In ultrasonic micro-machining the same level of slurry concentration is difficult to maintain because the evaporation of slurry medium occurs during machining.

2.7 APPLICATIONS OF ULTRASONIC MICRO-MACHINING Micro-USM has broad applications in industry for fabrication of micro-level machining at hard and brittle materials. It has many applications in many industries like aerospace, electronics, bio-medical devices, opto-electrical communication, etc. Some important applications are mentioned below: • • • • •

This process is mainly useful for drilling brittle and hard material such as ceramic, glass, and advanced materials. It is used in different industries such as pump and valve industries, aerospace, automotive industries, tool mold and machine industries. Micro-USM process could successfully machine hybrid composite stack at micro-level with good dimensional finishing and accuracy. Micro-hole drilling of glass, the drilled diameter lies in order of 10 micrometer. Preparation of three-dimensional microstructures for air turbine fabrication.

2.8 RESEARCH ON ADVANCES OF ULTRASONIC MICROMACHINING A comprehensive review of literature on diverse aspects of USM has been presented here.

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Sun et al. [1] conducted experiments to compare different machining parameters between conventional and ultrasonic micro-machining. For achieving 3D microstructure by using micro-USM a method which was combination of WEDG and EDM to fabricate the micro-tool was proposed. These combinations were used to generate co-axial micro-tool and microUSM is carried out with help of this tool. Experiments were performed with Tungsten Carbide as abrasive material (average diameter 0.58 μm), vibration amplitude from 1.0 μm to 3.5 μm, working load 0.1 mgf/μm2 and rotational speed is more than 300 rpm. Tungsten Carbide was preferred over conventional tool material such as stainless steel, Tungsten was used as tool material to avoid large tool wear. Side wall roughness, Ra about 0.2 μm and out of roundness 1.0 μm, taper ratio 5% were achieved. Apart from these the smallest hole of 15 μm diameter and 32 μm depths on silicon and glass material were achieved successfully. The focus in this experiment was to develop a self-aligned multilayer machining and assembly (SAMMA). Initially a 3D micro-chamber was made by micro-USM on a 300 μm thick wafer. WEDG/ EDM was used to make micro-parts which were subsequently inserted into central hole with self-alignment and made micro-center pin air turbine. Sun et al. [2] developed a methodology which gave accurate and highly précised machining result on brittle and hard materials such as Silicon, Glass, Silicon nitride, Quartz, Ceramic, etc. Electro discharge machining (EDM) and wire electro discharge grinding (WEDG) and were combined in the USM machine to make co-axial micro-tool system for USM. The machining conditions were working load 30–50 mg, vibration amplitude 0.58 μm, 16 rotational speed 300 rpm, abrasive size 0.58 μm of tungsten carbide with 50% slurry concentration. A micro-hole of 50 μm diameter and 150 μm depths on silicon wafer material was successfully made. The side wall surface roughness, taper ratio and out of roundness were the criteria for evaluating micro-holes quality. Taper ratio is the ratio of difference between the holes entrance and exit diameters to the hole depth. It is affected by wearing of tool in its transverse direction. It is an important factor to be controlled as tool wear increases with increase in tool diameters. Tungsten Carbide used as tool material and micro-machining speed are in the range of 2.0–6.0 μm min–1. Egashira et al. [3] developed a technique of ultrasonic micro-tool fabrication with the help of WEDG and EDM. To solve micro-tool fixing problem a technique was proposed in which tool was fixed to the horn of machine after that micro-tool was fabricated with the help of WEDG while rotating the machining head. Different machining characteristics with varying

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parameters had been studied and concluded many points as: smaller size of abrasive increases surface roughness and suppress chipping but resulted in low MR, Higher machining load and vibration amplitude increase the MR and produce high tool wear ratio (TWR) (ratio of length of tool wear to depth of hole). Tool rotation improves MR, but it affects the TWR. Abrasive grain size less than 1 μm, machining load 1–3 mN, vibration amplitude about 1 μm and rotational speed 200–500 rpm was recommended. Egashira et al. [4] conducted the experiments on micro-USM with vibration of work piece instead of tool vibration. A conventional tool such as stainless steel was worn out faster, so sintered diamond (SD) was tested as a tool material. Micro-hole of 5 μm diameter was successfully drilled on quartz glass and silicon with depth of 9 μm and 37 μm, respectively. A comparison between WC and SD was made for machining on soda glass and it was found that tool wear rate was decreased, and MR was increased in case of SD tool. Tool material, SD was recommended for USM of multiple holes to maintain accuracy and precision in range of μm. Kuriyagava et al. [5] developed a force controlling mechanism during micro-USM. As breakage of tool is a common problem occurred due to application of compressive load. In this experiment machining force was constantly monitored using a dynamometer and the amount of in feed of the ultrasonic vibration head was controlled using the NC fine in feed mechanism to obtain constant machining force. In the present study, the tool rotates to stir the slurry between the tool and work piece simultaneously eliminating bubbles and facilitating the supply of new abrasive particles. The system was designed to withdraw the tool automatically if any sudden increment in applied force was detected. The effect of diameter of tool on machining characteristics was examined. Effect of tool diameter on machining speed and tool wear was also studied. As the diameter of tool becomes smaller the machining speed becomes lower and tool wear increases. Sundaram et al. [6] studied the effects of oil based abrasive slurry on MRR and surface roughness. Oil based abrasive slurry gave smooth and more quality surface finish whereas MRR decreases as compared to water based abrasive slurry. Increment in abrasive size increases MRR but decreases surface finish and smaller abrasive particles shows just opposite trend of it. Yu et al. [7] carried out experiment and derived the theoretical model to calculate tool wear rate in micro-USM in terms of its dependent parameters such as fatigue life, amplitude, static load, concentration of abrasive particles. The result obtained from model is satisfied by Tungsten and SS 316L. Tool rotation has no effect on tool wear suing large diameter tool in micro-USM.

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Cheema et al. [8] studied the influences of tool wear on form accuracy of micro-channel produced by micro-USM. Borosilicate was used as work piece and stainless, tungsten carbide as tool material. Width of micro-channel was more when larger abrasive particles were utilized. Abrasive size and step feed and were observed as two main parameters to control form accuracy. Stainless steel tool showed more wear because of ductile in nature. Strain hardening of SS also put effect on form accuracy of micro-channel whereas there is no such effect in WC as a tool. Agarwal [9] studied the material removal mechanism in USM and developed an expression for shocking force. In present study an analytical model on MRR was developed in terms of various machining characteristics such as slurry concentration, abrasive size, tool material hardness, static load, etc. Comparison of different previous model was done by the author and noticed that all model showed increasing tendency of MRR with grit of abrasive grain, frequency, and amplitude of vibration, static load applied, abrasive grain size. Yu et al. [10] conducted the experiment to determine the influences of tool size and static load on MRR, tool wear and gap. The developed tool wear method coupled with CAD; CAM software was used to generate microcavity. Investigation showed when static load was increased, tool wear was also increased, gap was decreased, and it suppressed lateral stress vibration. Wang et al. [11] conducted experiment and developed a simulation model to show the effect of tool wear. Stainless steel 304, carbon steel, 1045 and tungsten carbide were utilized to verify the result of developed simulation model. Finite element method (FEM) was applied to analyze tool deformation and fracture of work piece. Observations showed that abrasive particles size was decreased with machining time and fastest rate of decrement in case of WC tool among stainless steel, carbon steel and WC. Stainless steel showed less wear because of strain hardening. Too hard tool material decreased MRR drastically. Popli et al. [12] conducted an experiment using rotary ultrasonic machine of sonic mill and tried to overcome the problem of chipping with finite element analysis during machining of advanced ceramics (Al2O3). It was noticed that chipping size was inversely related to support length, if the support length increases then chipping thickness decreases in case of advanced ceramic and it was verified by changing the support length during experiment. Pratap Singh et al. [13] made comparison of different types of horn profile by using finite element based ANSYS software. Tungsten and Aluminum horn profile were made in exponential and stepped profile. It was concluded

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that Aluminum horn with exponential profile has high frequency as compared to its stepped profile for all nodes, thus design shapes contribute greatly on machining performance and SQ of work parts. Wang et al. [14] investigated MRR, abrasive particles wear and machining tool wear, etc. Smooth particle hydrodynamics simulation was used, and experiments were conducted to verify of simulation result. MRR was calculated by scanning the area by using scanning electron microscope (model SU1510) and measuring the hole depth using laser probe profile-meter and it was found that MRR decreases with increasing in fracture toughness and hardness of workpiece materials. Kuriyagawa et al. [15] developed a micro-USM for fabrication of microelectrical device or micro-electromechanical device components. In the present method an ultrasonic vibration spindle with an aerostatic bearing supporting an ultrasonic vibration unit was developed. Optical non-contact displacement meter was used to measure the vibration characteristics at tip of horn. To apply controlled force during machining a double strain gauge dynamometer with a resolution of 0.1 mN, a maximum available load of 1.2 was used. Tungsten carbide material was fixed to tip of ultrasonic horn and fabricate the micro-tool by using side of diamond wheel. Machining test was conducted to access the effect of tool rotation and change in protrusion height for different rotational speed using 370 μm diameter tools. The points were concluded as, the protrusion height increases with machining time, tool rotation decreases the severity of protrusion along machining path and improves the machining speed. Singh et al. [16] studied the influences of power rating, abrasive particle size, abrasive material on MRR and TWR with stainless steel tool by using ultrasonic micro-machining. Power rating of 150 W, 300 W, 450 W, alumina grain of grit size 100 and 320 with 25% concentration, distilled water as slurry media were used for the experiment. Pertho-meter was used to check the roughness of machined surface. The photo micrograph of machined surface suggested that hole drilling could be done on titanium alloy without excessive surrounding damage. Increment in grit size of abrasive particles showed decrement in TWR and MRR, better SQ was observed with fine abrasive particle. Singh et al. [17] developed a mathematical model for MRR evaluation of titanium alloy based on Taguchi technique. The influencing parameters were divided into two parts noise factor and control factor. Input variable were tool, slurry type, slurry concentration, slurry grit size, slurry temperature and power rating, etc. ExpertTM software was used for analysis purpose.

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Titanium alloy was machined by using boron carbide slurry and StainlessSteel tool at 90% power rating. It was concluded that ultrasonic power rating, type of tool, slurry types has contributed 28%, 24.6% and 13.3% respectively in MRR, where effect of slurry concentration, slurry temperature and abrasive grit size were insignificant. The recommended parameters were SS tool, boron carbide slurry and 90% power rating and this improved the MRR by 34.46%. Singh et al. [18] studied the influences of slurry on machining criteria of titanium alloy with various power rating during USM micro-machining. High speed tool and titanium and were used as tool. Boron carbide, silicon carbide, and alumina were used as abrasives. Power rating were 150 W, 300 W, 450 W, 500 W. Various graphs were plotted to analyze the influences of various process parameters on TWR and MRR and compare the SQ in conventional and advanced micro-USM. It was experimentally proved that MRR was increased with increment in power rating due to more momentum of abrasive particles. Stainless steel, Titanium, high speed steel (HSS) alloy tools were successfully used to drill holes on titanium alloy material without excessive surface damage. High MRR and low TWR were achieved in higher tougher material also, by selecting appropriate tool, abrasive slurry and power rating. The recommended parameters were 300 W power rating, stainless steel tool, B4C slurry and it was observed that MRR was increased by 34.46%. 2.9 ULTRASONIC-ASSISTED ELECTRICAL DISCHARGE MACHINING (UAEDM) Electrical discharge machining (EDM) is a thermoelectric process applied for die making because the generating surface comprising of overlapping craters has self-lubricating properties as the lubricating oil is retained on the craters while die is in action. EDM is used to produce 3-D complex cavities on variety of electrically conducting materials [19]. In the late 1940s EDM technology has been developed [20] where the ED occurred in dielectric medium to remove the material due to melting and vaporization. The interelectrode gap should be maintained small for occurrence of spark discharge. The applied voltage should be high enough so that ionization of dielectric fluid occurs. Short duration electrical spark discharges are produced in an interelectrode gap. The temperature in the sparking zone is very high which is higher than the melting point of material. The material is removed due

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to thermal energy of the spark discharges from the tool as well as workpiece. There are chances of accumulation of molten material in the form of re-solidified layer. And hence it results in the formation of abnormal spark discharges, which causes more tool wear, and the MRR is also reduced [21]. Therefore, proper, and effective removal of debris from the machining gap plays a significant role in EDM process. Development of proper flushing techniques to reduce the chances of debris accumulation in sparking gap is one of the important areas of research in EDM. Ultrasonic-assisted electrical discharge machining (UAEDM) is an assisted hybrid machining process in which vibration with small amplitude and ultrasonic frequency is applied to either workpiece or tool to enhance the machining efficiency and efficient flushing [22]. Ultrasonic vibration of tool or workpiece changes the IEG rapidly and high pressure is generated which assists the removal of molten material and it results in higher MRR [23]. There are various ways of applying ultrasonic vibration in EDM process. The most common method is to provide vibration to workpiece in UAEDM. Piezoelectric or Magnetostrictive transducer is used to vibrate the workpiece in die sinking EDM, Wire-cut EDM as well as micro-EDM operation [24, 25]. Figures 2.3 and 2.4 show the schematic diagram of UAEDM with tool electrode vibration and workpiece vibration system, respectively. In ultrasonic-assisted Wire-Cut EDM, the ultrasonic vibration is given to the wire which results lower surface roughness and higher cutting rate [26]. The concurrent vibration of workpiece and tool is a research area in UAEDM proposed by some researchers [27]. In ultrasonic-assisted micro-EDM, deeper micro-hole is produced by combining planetary movement of tool electrode with ultrasonic vibration [28]. Ultrasonic transducer is kept in dielectric tank to impart ultrasonic vibration in the dielectric fluid. The ultrasonic activated dielectric fluid results in increase in kinetic energy of debris to remove from the machining zone. The MR as well as surface finish increases due to assistance of ultrasonic vibration in UAEDM [29, 30]. 2.10 ULTRASONIC-ASSISTED ELECTROCHEMICAL MACHINING (UAECM) ECM has wide application in defense, space, and aircraft, tool making industry and in domestic applications. ECM process is useful for generating micro-features on components with better surface finish. During ECM process, there is formation of passive oxide layer on the machining zone which results lesser MRR. Ultrasonic vibration of workpiece or tool can be

Advances in Ultrasonic Micromachining

FIGURE 2.3

Schematic diagram of ultrasonic-assisted EDM with tool vibration.

FIGURE 2.4

Schematic diagram of ultrasonic-assisted EDM with workpiece vibration.

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applied to break and remove the passive oxide layer and then, MRR increases. Ultrasonic vibration also assists to remove the products of chemical reaction in ECM process. Ultrasonic tool vibration in ECM increases the electrochemical dissolution rate as electrolyte temperature increases to some extent due to vibration. During ultrasonic-assisted ECM, generated micro-bubbles are

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collapsed in the area adjacent to electrodes that causes concentration of mass and transportation of electric charge and finally it increases the temperature of electrolyte and the rate of electrochemical dissolution. Ultrasonic vibration created large pressure in the electrolyte between electrodes which also leads to efficient removal of reaction products. The amplitude of vibration plays a vital role in ultrasonic-assisted ECM. Low amplitude of vibration has no effect, but high amplitude of vibration affects the dimensional accuracy during producing micro-features. The frequency of ultrasonic vibration varies from 19 to 23 kHz and the amplitude of vibration varies from 20 to 30 micron. The UAECM system consists of a DC pulsed power supply, ultrasonic head with a transducer, ultrasonic generator, horn, coupler, and tool, motion control unit and electrolyte circulation unit. Figure 2.5 represents the sketch of UAECM with the application of ultrasonic vibration at tool.

FIGURE 2.5

Sketch of ultrasonic-assisted ECM (UAECM).

Classical ECM process and ECM process with electrode ultrasonic vibration were compared by Ruszaj et al. [31]. It is mentioned that the energy of ultrasonic vibration is utilized for supporting the basic ECM process by intensification of ions and as a result the rate of diffusion of metal ion is enhanced. High quality machined surface was achieved by optimal condition of electrolyte flow, heat exchange and sludge removal from machining area during UAECM process. Various aspects of UAECM process were

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discussed by Skoczypiec [32] and it was concluded that the ultrasonic vibration facilitates the reaction byproduct removal and heat removal, passivation minimization, optimal hydrodynamic conditions generation and electrolytic reactions rate enhancement. Ultrasonic vibrations generate higher pressure in the interelectrode gap, and this causes increased diffusion of electrolyte and elimination of bubble. ECM process variables are voltage, current density, pulse parameters, and electrolyte concentration. In addition of ECM process variable, amplitude of ultrasonic vibration plays a vital role in UAECM process. The experimental investigation into chemical assisted USM of glass was performed by Choi et al. [33]. Due to chemical effect of low concentration of hydrofluoric acid added with alumina slurry, MRR was increased up to 200%, depth of hole increases and superiority of machined surface also increased as compared to conventional USM. It was recommended that to use HF with 3–5% concentration to avoid holes enlargement due to chemical erosion. 2.11 ULTRASONIC-ASSISTED ELECTRO-CHEMICAL DISCHARGE MACHINING (UAECDM) Electro-chemical discharge machining (ECDM) is hybrid advanced machining process for micro-fabrication of electrically non-conductive materials. ECDM is applied to produce micro-holes, channels, and grooves on non-conductive materials. A high voltage is applied in the gap filled in electrolyte solution between auxiliary electrode and tool and to form an insulating gas bubble layer and to generate spark discharge in the layer. Generally, two electrodes are used in ECDM, one of which is tool electrode that is comparatively very smaller than auxiliary electrode. Machining is done on a machining chamber where the auxiliary electrode and tool electrode are dipped into electrolyte and kept at a certain distance. Tool electrode is touched with negative terminal and auxiliary electrode is joined with positive terminal of pulsed DC power supply. Evolution of oxygen gas and hydrogen occurs at auxiliary electrode and tool electrode, respectively when potential difference is maintained between these two electrodes. The generated hydrogen gas forms bubbles and accumulate around the tool electrode and after a certain time they collapse to form gas film around the exposed area of tool electrode. After a certain voltage, this gas film breaks down and discharge occurs between tool electrode and electrolyte. If a material is placed at the vicinity of the discharge area, then due to thermal energy the material will be removed in the form of melting and vaporizing. This gas film formation and its shape is the major factor in ECDM which

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decide the discharge energy, machined SQ, MRR depending on its thickness. In normal way the formation of gas bubbles and its size cannot be controlled, as a result thickness of gas film varied during machining. The addition of ultrasonic vibration to the tool can solve this problem. ECDM process has limitations like low DOP and large overcut. The limitations can be overcome to some extent with rotation the tool and designing the tool tip geometry. The vibration at ultrasonic frequency can be imparted on either workpiece or tool to enhance the efficiency of ECDM process. In ultrasonic-assisted ECDM ultrasonic vibration with low amplitude of the tool or workpiece improves the electrolyte circulation and changes the spark discharge behavior so that the machining performance can be improved. System of an ultrasonic-assisted ECDM is shown in Figure 2.6. The system has a XYZ stage which can give motion in three directions controlled by a computer. An ultrasonic vibrator is added with the tool electrode and attached with Z axis of the stage. The addition of ultrasonic vibration to the tool electrode can reduce gas film thickness and control the gas film thickness throughout the machining which provides better quality of machining surface. Generally, gas bubbles which stick with the tool electrode are used to form gas films. So, if larger gas bubbles are present during forming of gas film, then thicker gas bubbles will form due to coalescence of those larger gas bubbles. The ultrasonic vibration of the tool electrode force to detach larger gas bubble from the tool surface and allow the smaller gas bubbles to present at the tool electrode surface. These smaller gas bubbles form thinner gas film which confined the discharge at a desired location of the workpiece and produce better quality machining surface. Besides the formation of thinner gas film, the ultrasonic vibration also allows the machined product to come out of the machining zone and create a path for fresh electrolyte entry. 2.12

RESEARCH ON ADVANCES IN UAECDM

Research work in the different aspects of ultrasonic-assisted ECDM has been carried out to explore the effects of ultrasonic vibration of workpiece or tool for improving the machining performance. Some results are highlighted in this section. Chen et al. [34] demonstrated ultrasonic-assisted electrochemical discharge micro-drilling operation. With the application of ultrasonic vibration to the tool electrode, the required machining voltage becomes less. As a result, accuracy

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of array of hole fabricated by this process was improved and thermal crack area around the hole was also reduced.

FIGURE 2.6

Sketch of UAECDM setup.

Meifal & Furutani [35] investigated the influences of ultrasonic vibration and electrolyte level on drilling on glass plate by ECDM using tungsten rod as tool. It was observed that ultrasonic vibration at high amplitude caused lower MRR. Therefore, for higher MRR, low amplitude ultrasonic vibration is preferred. It was also observed that lower electrolyte level caused higher temperature of workpiece and tool with ultrasonic vibration of tool electrode during ECDM operation. Singh et al. [36] analyzed the energy channelization behavior during ultrasonic-assisted electrochemical discharge machining (UA-ECDM). A low amplitude and high frequency (35 kHz) ultrasonic vibration was applied to the tool. In UA-ECDM process, high-pressure evacuation and cavitation actions assist more material removal along with melting and vaporization phenomenon. Ultrasonic vibration in ECDM decreases heat-affected zone Due to ultrasonic vibration, less energy was consumed than the ECDM process per unit removal of material. It was reported that there is five-time increment of energy channelization index due to ultrasonic assistance. Higher MRR and depth to diameter ratio were obtained as there was better electrolyte replenishment and effective ejection of sludge/debris from the machining zone with the assistance of ultrasonic vibration.

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Rathore & Dvivedi [37] compared ECDM process with ultrasonicassisted electrochemical discharge machining (UAECDM). It was observed that MRR was enhanced by 11.13% due to ultrasonic vibration of tool electrode and DOP by 27.17% and hole overcut was reduced by 23.10%. The ultrasonic vibration controls the radius of gas bubble, and it regulates gas film thickness. Efficient sludge removal and better electrolyte replenishment were also reported. Elhami & Razfar [38] showed that ultrasonic vibration in ECDM modified the current signal pattern to enhance the numbers of spark discharge. In this study, ultrasonic vibration is applied using special configuration and equipment and single discharge is generated. Due to ultrasonic vibration MRR was enhanced up to 35% and tool wear was decreased up to 14%. Min-Seop et al. [39] mentioned that ECDM is an electrolytic dischargebased micromachining process applied for the generation of microstructures on ceramics and glass. Electrolyte vibration at ultrasonic frequency was applied to increase the depth of machining during ECDM drilling by ensuring a sufficient flow of electrolyte and it assists to maintain uniform spark. Quality of surface at the hole at entry, was increased with a side-insulated electrode with the aid of pulse-power generator. The stray electrolysis was prevented by side-insulated electrode, and it concentrates the spark discharge at the tool tip. Periodic pulse-off time was introduced to reduced thermal damage to workpiece surface. Micro-holes with less overcut and high MD were fabricated with ultrasonic vibration assistance. Li et al. [40] developed ultrasonic-assisted micro-electrochemical discharge machining (UAECDM) setup and performed drilling and milling operation for generation of microstructures with high machining quality. A micro-hole of inlet diameter, 133.2 μm was produced and array of microholes was also produced on the glass workpiece with 300 μm thickness. On the glass workpiece microgroove width of 119.2 μm was successfully milled. Microstructures on non-conductive brittle and hard materials like ceramics and glass were fabricated by UAECDM with better geometric accuracy and SQ. Kapil et al. [41] mentioned about the utilization of ultrasonic energy to improve the machining efficiency of electrochemical discharge trepanning (ECDT). The influences of ECDT process variables such as power rating, pulse on time, applied voltage and tool rotation rate on machining criteria such as DOP and over cut (OC) had been experimentally investigated. The application of ultrasonic vibrations in ECDT facilitated the sludge evacuation from the machining region. Stable and thin gas film over the tool electrode

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was generated. Ultrasonic-assisted ECDT (UA-ECDT) increases the energy channelization index (ECI) by 10 times compared to simple ECDT process. The parametric combination of power rating of 17%, applied voltage of 60 V, tool rotation rate of 42 rpm and pulse on time of 3 ms was recommended for the UA-ECDT process. Chen et al. [42] discussed about the application of ultrasonic vibration into the wire electrochemical discharge micro-machining (WECDM). Microhelical electrode with ultrasonic vibration was proposed for achieving high quality high aspect ratio microstructures on glass. Micro-helical electrode of 100 μm diameter was used to carry out experiments. The effects of amplitude of vibration, pulse frequency (PF), duty factor, machining voltage and feed rate on width of slit were studied. The machining quality was significantly enhanced by assistance of ultrasonic vibration. When the amplitude of ultrasonic was 5.25 μm, the average width of slit was decreased to 128.63 μm with a reduction of 20.78%. Micro-cantilever structure and micro-planar coil structure were produced successfully on glass plate. Ultrasonic vibration of micro-helical electrode achieved the capacity of fabricating microstructure having high aspect ratio made up of brittle and hard materials. 2.13

CONCLUSIONS

Ultrasonic vibration is used in various fields, such as testing, measurement, and machining. Ultrasonically vibrated abrasive particles remove materials at the micron level from hard and brittle materials. The ultrasonic micromachining process is widely applied for producing micro-features for microfabrication. The rotary ultrasonic micromachining process is well suited for micromachining applications, and it is a faster process than the stationary USM process. The ultrasonic vibration of a workpiece or tool assists in electro-machining processes such as EDM, ECM, and ECDM for material removal in micromachining applications. In ultrasonic-assisted EDM, there is a better surface finish and higher material rate due to the efficient removal of material from the machining region. During ultrasonic-assisted ECM, the anodic dissolution rate is faster than ECM as the higher temperature of the electrolyte is achieved due to ultrasonic vibration. During ultrasonic-assisted ECDM, a higher depth of machining is achieved due to better circulation of electrolytes in the machining region caused by ultrasonic vibration. Further research is needed to explore the influences of ultrasonic vibration in improving the machining performance of the electro-micro-machining process for micro-fabrication.

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KEYWORDS • • • • • • •

assisted electro-micromachining electro-micro-machining microfabrication rotary USM ultrasonic-assisted electrochemical machining ultrasonic micromachining ultrasonic vibration

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30. Schubert, A., Zeidler, H., & Hackert-Oschätzchen, M., (2013). Enhancing micro-EDM using ultrasonic vibration and approaches for machining of non-conducting ceramics. Journal of Mechanical Engineering, 59(3), 156–164. 31. Ruszaj, A., Zybura-Skrabalak, M., Skoczypiec, S., & Úrek, R., (2001). Electrochemical machining supported by electrode ultrasonic vibrations. In: Proceedings of the 13th International Symposium for Electromachining ISEM XIII (Vol. II, pp. 953–964). Fundacion Tekniker, Bilbao, Spain. 32. Skoczypiec, S., (2011). Research on ultrasonically assisted electrochemical machining process. Int. J. Adv. Manuf. Technol., 52(5–8) 565–574. 33. Choi, J. P., Jeon, B. H., & Kim, B., (2007). Chemical-assisted ultrasonic machining of glass. Journal of Materials Processing Technology, 191, 153–156. 10.1016/j.jmatprotec. 2007.03.017. 34. Chen, H., Liu, Y., & Wei, Z., (2019). Ultrasonic vibration assisted micro electrochemical discharge drilling technology. In: 2019 IEEE International Conference on Robotics and Biomimetics (ROBIO) (pp. 2839–2843). doi: 10.1109/ROBIO49542.2019.8961642. 35. Rusli, M., & Katsushi, F., (2012). Performance of micro-hole drilling by ultrasonicassisted electro-chemical discharge machining. Advanced Materials Research (Vol. 445, pp. 865–870). Trans Tech Publications, Ltd. doi: 10.4028/www.scientific.net/amr.445.865. 36. Tarlochan, S., Akshay, D., Anurag, S., & Pradeep, D., (2021). Experimental investigations of energy channelization behavior in ultrasonic-assisted electrochemical discharge machining. Journal of Materials Processing Technology, 117084. 37. Ranjeet, S. R., & Akshay, D., (2020). Sonication of tool electrode for utilizing high discharge energy during ECDM. Materials and Manufacturing Processes, 35(4), 415–429. doi: 10.1080/10426914.2020.1718699. 38. Elhami, S., & Razfar, M. R., (2018). Effect of ultrasonic vibration on the single discharge of electrochemical discharge machining. Materials and Manufacturing Processes, 33(4), 444–451. doi: 10.1080/10426914.2017.1328113. 39. Min-Seop, H., Byung-Kwon, M., & Sang Jo, L., (2009). Geometric improvement of electrochemical discharge micro-drilling using an ultrasonic-vibrated electrolyte. Journal of Micromechanics and Microengineering, 19(6), 065004. 40. Li, X., Ren, Y., Wei, Z., & Liu, Y., (2019). Development of ultrasonic vibration assisted micro electrochemical discharge machining tool, Source: Recent Patents on Mechanical Engineering, 12(4), 313–325. https://doi.org/10.2174/2212797612666190808101736. 41. Kapil, P., Akshay, D., & Tarlochan, S., (2019). On performance enhancement of electrochemical discharge trepanning (ECDT) process by sonication of tool electrode. Precision Engineering, 56, 8–19. ISSN 0141-6359.https://doi.org/10.1016/j.precisioneng. 2018.08.016. 42. Chen, Y., Feng, X., & Xin, G., (2021). Experimental study on ultrasonic vibrationassisted WECDM of glass microstructures with a high aspect ratio. Micromachines, 12, 125. https://doi.org/10.3390/mi12020125.

CHAPTER 3

EMERGING TRENDS IN HIGH-SPEED MICRO-MACHINING OF DENTAL CERAMICS THROUGH CAD-CAM SYSTEMS SIVARANJANI GALI1 and R. SURESH2 Department of Prosthodontics, Faculty of Dental Sciences, M.S. Ramaiah University of Applied Sciences, Bangalore, Karnataka, India 1

Department of Mechanical and Manufacturing Engineering, M.S. Ramaiah University of Applied Science, Bangalore, Karnataka, India

2

ABSTRACT Dental ceramic restorations are typically fabricated using current popular ceramic processing techniques such as the heat press and computer-aided machining and designing (CAD-CAM) technology. The popular CAD-CAM ceramics are leucite and lithium disilicate glass-ceramics, feldspathic porcelains, and pre- and post-sintered zirconia. As the traditional method of lost-wax technique for metal restorations consume more time with an increased possibility of human errors, alternative technologies such as CAD-CAM in dentistry have been explored in dentistry. CAD-CAM ceramic blanks have a good advantage of homogeneity with an occurrence of minimum flaws. The review shall provide background on material perspectives of CAD-CAM dental ceramics and the currently available CAD-CAM dental micromachining systems. The chapter shall provide deep insight into the emerging trends in the micro-machining of dental materials and advancements in the machinability process. Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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3.1 EVOLUTION OF CAD/CAM TECHNOLOGY IN DENTISTRY CAD/CAM technology in dentistry began in the 1970’s, when Francois Duret introduced the Sopha system consisting of optical impressions of prepared teeth, designing, and milling dental restorations using computer-controlled machines. Further, Mormann and Brandestini developed a prototype that would capture 3D images of the patient cavities, design inlay restorations using a 3D software and fabricate prostheses through computer-controlled machining. Grinding trials were initially performed on feldspathic ceramics, with a spindle and a grinding wheel, called the CEREC 1 system. CEREC 1 system had developments in hardware and software systems with an improved engine power, with smaller grain size of the grinding disks and a rigid machining system [1, 2]. The machining set up of CEREC 1 system was further improved with CEREC 2, with an optimized scanning system and the addition of a cylindrical diamond. CEREC 3 was improved with a two-bur system and CEREC 4 with a step bur system and a reduced diamond tip for better precision. The emergence of CEREC system had various stages of development of image capturing system with an oral camera, hardware, and software systems [3]. CAD/CAM technology was also applied for base metal alloys for fabricating composite veneers for titanium copings, which led to the development of the Procera system [4]. 3.2 CURRENT STATUS OF CAD/CAM DENTAL CERAMIC SYSTEMS Primary machining approach of fabricating dental restorations is through the typical CAD/CAM milling. As the traditional method of lost wax technique has limitations of being time-consuming with increased likelihood of human errors, alternative technologies such as CAD/CAM were explored in dentistry [5]. Technological developments such as computer aided design (CAD) and computer aided machining (CAM) have made a significant impact in dentistry, particularly used to produce crown and bridge prostheses. Systems of CAD/CAM essentially consists of a scanner, software, and a production technology to transform the digital data to a desired restoration. The system generally uses power driven Computer Numerically Controlled (CNC) machines with advanced cutting tools to shape the preferred dental material to a desired geometry [6]. A typical CNC machine is built with adjustable parameters of feed rate, depth of cut and angulation, to accommodate milling of a desired product [7]. Computer aided designing software virtually designs the corresponding restoration for the prepared teeth and converts the

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digital data to standard tessellation language (STL) file. Surface of virtual models is used to calculate the tool path [8, 9]. The STL file imported to the CAM software, which issues a set of orders to the CAM unit to produce the G-codes, the computer control numerical language [10, 11]. The machining tool path calculation generated by the G-code are further decoded by the CNC machines to mill the restorations [8, 12, 13]. A typical CAD/CAM workflow is presented in Figure 3.1.

FIGURE 3.1

Schematic illustration of CAD/CAM workflow.

Scanners work on either optical or mechanical scanning mechanism. Optical scanners work based on the premise of triangulation. Based on angle between the sources of light such as laser with detector, the threedimensional image of the object is captured. Mechanical scanner works on the accuracy of a ruby ball that reads master cast line by line. Based on the digital intra oral impression or a scanned image of the die, computer assisted designing of the restoration followed by customized milling are done [14]. Dental materials used for CAD/CAM technology range from feldspathic ceramics to mica, leucite, lithium-disilicate glass-ceramics and polycrystalline oxide ceramics such as zirconia and alumina. Methods of hard and soft milling have been used for dental glass-ceramics and pre-sintered or green zirconia blanks, respectively [15, 16]. Dental CAD/CAM systems have the prime advantage of milling dense homogenous ceramic blanks with reduced flaws and facilitate faster milling, greater material removal rate (MRR) and reduced tool wear with smooth crown margins [17–19]. It also minimizes cross-contamination from clinics to laboratories [20]. Current challenges of CAD/CAM systems are obtaining good quality of margins, minimizing digitization errors, surface modeling errors and improving the milling tool accuracy [21]. Further, existing CAD/CAM dental materials such as glass-ceramics have limitations of low strength for load bearing restorations and zirconia has limitations of lack of adhesive bonding, low temperature degradation and susceptibility to veneer fractures [22–31].

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3.3 DESCRIPTION OF CAD/CAM MILLING MACHINES CAD/CAM milling machines can be broadly classified depending on the number of axes in the milling machines such as 4 or 5 axes milling, on the type of milling such as hard or soft milling or in-office or laboratory milling. Hard milling is typically performed for dental workpieces with high hardness such as metals, compactly sintered zirconia, and dental composite resins. The milling system therefore requires substantial cutting forces and power for an effective material removal. High cutting power and low thermal conductivity of hard zirconia and titanium can result in heat generation, reduce tool life, and further can damage the machining system [21]. Hence, continuous chilling is vital to preclude rise in temperature of the machined material [32]. This system can also be called wet-milling, often used for milling dental glass-ceramics. Some of the demerits of hard machining is phase transformation of tetragonal zirconia to monoclinic and time-consuming [33–36]. Such phase transformation will further worsen the micro-cracking process and lead to low thermal degradation. Soft-machining or dry-milling on the other hand simplifies milling procedure and improves its time efficiency. An oversized pre-sintered zirconia workpiece is typically milled and sintered. The composition of the milled pre-sintered soft zirconia varies from the sintered hard machined zirconia. The pre-sintered zirconia has reduced hardness with improved machinability and has about 25–30% shrinkage [33]. Soft milling has advantages of fast machining, reduced cutting forces, with longer tool life and improved surface features. With dry milling, there is less chance of moisture contamination of zirconia blanks and can therefore avoid the drying process of zirconia blanks required before sintering. Further, the milled pre-sintered zirconia has retained tetragonal phase than hard-milled zirconia. The main drawback of this procedure is dimensional discrepancy when it compared to hard machined zirconia [37, 38]. Nevertheless, developments in soft machining of zirconia show dependable shrinkage compensation process [39]. Accuracy of a machinable dental crown depends on the quality of digital impressions, the type of the milling tools, axes milling machines, the complexity of the restorations and its associated shrinkage [40]. Existing CAD/CAM systems are fabricated either at the chairside, at the laboratory or at the centralized production. In the production of clinical chairside or direct CAD/CAM restorations, processes of intra-oral scanning, designing, and milling are performed at the dental office. Indirect CAD/CAM dental restorations are outsourced to either dental laboratory or production

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center. In semi-CAD/CAM restorations, intra-oral digitized images are sent to the laboratory, which further outsource to production centers for fabrication of CAD/CAM prostheses [6]. Production equipment in an in-office chairside CAD/CAM can only mill softer dental materials and are equipped with one cutting bur or an abrasive such as Lyra, GACD. A typical dental laboratory CAD/CAM set-up can mill a broad range of dental materials and are equipped with advanced machining strategies such as DWX-50, Roland. Impressions sent from the dental office are poured into master casts or models, which are further scanned or digitized, designed, and milled. A dental production center can mill almost all dental materials including sintered zirconia, are more rigid and equipped with several abrasive cutting tools such as Ultrasonic 20 series [13]. Dental ceramics, composites, and resins can be milled in chairside milling systems. Milling machines were exclusively designed for dental prostheses such as Cerec InLab MCXL and Kavo Artica Engine. Kavo Arctica Engine and Roland DWX-50 are 5-axis milling machines consisting of 2 translations with a rotation for the workpiece and 1 translation and a rotation for the spindle. The machines accommodate grinding and cutting tools, with an automated tool changer and tool stations. Charly Dental CD04-S milling center is a 4-axis milling machine with 12 tool stations [13]. Dental production milling machines can mill prostheses with undercuts, have better tool surface contacts to machine thin-walled restorations with a good surface finish. They can mill a wide range of dental materials from metallic alloys to waxes. 3-axis milling machines are typically defined by degrees of movement of spindle in x-y-z axes directions. Though 3-axis milling machines are simple and cost-effective, they have limitations of milling complex structures involving axis convergences, divergences, and subsections. 4-axis machines are designed to house an additional tension bridge, known as a rotational axis A axis. This additional axis accommodates vertical dimension in 3-unit simple bridges, thus saving the material consumed and the required milling time. The complex 5-axis machines have rotational vertical spindle called the B axes, other than the typical axes components of 4-axis machines. This enables milling of complex geometries such as convergence and sub-sections [13]. A schematic illustration of all axes of rotation are presented in Figure 3.2. ISO 841 specifies that milling machines must direct processing tools and track the coordinate axes. Machine tool kinematics dictates the surface integrity and the ease of fabricating intricate structures of dental restorations.

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FIGURE 3.2

Electro-Micromachining and Microfabrication

Illustration showing the different axes of rotation in a CNC machine.

The contact surface of the material and the cutting tool must correspond to the geometrical shape of the prosthetic shape to avoid cracks and roughness during milling. Productivity of milling machines can be greatly improved with reduced milling time and accurate positioning of spindles. Milling time can be reduced by high power, high spindle speed with an optimal torque and feed rate [6]. Quality of dental restorations depends on scanning accuracy, data-processing, and manufacture methods more than on the number of processing axes [6, 41]. Working space of the milling machines and their kinematics determine the number of prostheses and materials to be machined. The number of axes further determines the ability to machine the complex shapes with undercutting. Contact surfaces between tool and the workpiece provide better milling conditions and surface integrity of the prostheses. Milling machines are equipped to monitor the trajectory of the machining axes through embedded technologies such as brushless motor and stepped motor axis technology. Position monitoring of the axes and simultaneous corrections influence the accuracy [13]. Tool health are monitored with laser measurement. Spindle speeds in a typical laboratory milling machine is in the range of 42,000 to 60,000 rpm. Chairside in-office milling cannot accommodate such high-end spindle speeds. Some milling machines can adapt their spindle speed to the material being milled. Spindle power of dental CAD/CAM milling machines ranges from 100 to 500 W and in a production center, spindle power is 9 kW. Power typically is a product of torque and cutting speed. Milling process consists of a series of steps of

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grinding, semi-finishing, and finishing, requiring change in tools. This leads to loss of productivity and milling time. High spindle power is compensated in the milling machines to overcome such productivity loss and time. Milling feed rate in lab milling set-up range from 1.5 mm/min to 3,000 mm/min. In large scale production centers, feed rate ranges to 10 m/min. Feed rate determines the MRR. Feed rates are generally higher in laboratory milling machines than chairside in-office machines though their milling times are the same [13]. 3.4 DESIGN AND REQUIREMENTS OF CAD/CAM MILLING TOOLS CAD/CAM milling machines are typically equipped with pairs of diamond burs operating at 40,000 to 45,000 revolutions per minute through two fouraxis stepped up motor with a milling accuracy of ±25 µm [42]. Machining of ceramic blocks are performed with a pair of cylindrical diamond burs for initial coarse machining and fine finishing [43]. Milling tools of CAD/CAM are designed for optimum machining of dental materials such as zirconia, polymethyl methacrylate resins and metal alloys such as titanium and cobalt chromium. Milling tools are designed in the form of diamond grits, adhered, and bound to the shank of the tool. Milling tools are available as conical, cylinder pointed and stepped shaped burs [44, 45]. Examples of zirconia milling tools of various lengths and diameters are presented in Figure 3.3.

FIGURE 3.3

A dental zirconia CAD/CAM milling bur.

Tungsten carbide burs are commonly used for zirconia crowns and Diamond burs are widely applied for finishing feldspathic dental crowns. Milling tools are expected to demonstrate good abrasion, fracture resistance, with longer durability. The tools are often coated with nickel-chrome for milling poly-methyl methacrylate resin blocks, diamond like carbon coating for milling zirconia blocks. The bur blank of the milling tool is much susceptible to fracture due to the high machining loads ranging from 1.5–2 kgf [43]. Essentially, the diamond grit with binding to the shank must

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avoid micro-cracks and reduce the sensitivity to thermal impact. Geometry, quantity, and dimensions of diamond particles in the milling burs influence mechanical properties of the machined component. As these burs are frequently used, the diamond particles are gradually worn out [44, 46, 47]. Coarse grit size is used for shaping and fine grit size are used for finishing and polishing the prosthesis. Feldspathic ceramics showed higher roughness than zirconia [48]. Milling machine with 4-axis unit (in Lab MC XL; Sirona Dental Systems) use stepped burs for milling the internal surface and pointed burs for milling the restoration of the external surface. The stepped burs were of diameters of 1.2 mm and 1.0 mm are used for peripheral finish and end-milling modes for occlusal finishing of a dental prosthesis [48]. In endmilling mode, the tool tip is used to milling of the prosthesis. Flank-milling involves forward and backward milling using a ball-end tool tip. The tool-track also consists of solid-carbide ball-nose and torus end-mills for machining dental ceramics, composites, waxes, and alloys. They have a ±0.004 mm radius tolerance with a 0.01 mm diameter tolerance. Solidcarbide ball-nose end-mills are suitable for coarsening and final finishing, as their geometry provides better stability. Tapered neck ball-nose end-mills provide better sturdiness against rebound and are suited for finishing. TiAlN coated torus end mill with four flutes, protect tools from early wear and from adhering to the tool, indicated for machining non-precious metal alloys such nickel-chromium, polymethyl methacrylate resins and wax. Diamond coating warrants prolonged tool-life and is indicated for machining zirconium oxide [44, 45, 49]. Dental CAD/CAM systems with diamond burs were evaluated for their durability using laser measuring device and a contact probe. The milling systems were accommodated with initial and final cutting burs of lengths ranging from 40–45 mm and diameters ranging from 2–4 mm. Number of diamond particles and surface roughness depends on the hardness of ceramic blocks, harder ceramic block resulting in more wear of the diamond particles of the milling tool [43]. Diamond burs while milling CAD/CAM restorations naturally generate surface modifications (increased surface roughness), and serious defects on the surface of the ceramic [44, 49]. Imperfections on the cementation of milled restorations act as stress concentrators, foremost to crack spread [51, 52]. Life cycle of diamond burs while CAD/CAM machining can influence the fatigue behavior of dental glass-ceramics and mechanical properties. Frequent milling can result in loss of diamond particles after a definite sum of milling procedures [47] and can vary the ceramics’ surface pattern and affect ceramic fatigue behavior [44, 51]. Dental CAD/CAM burs

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can be used for machining up to 15 crowns consecutively [42] According to makers of two dental CAD/CAM systems (GN-I, GC Corp., Tokyo Japan; and Cadim, Advance, Tokyo, Japan), more than 10 ceramic crowns can be produced by diamond burrs. However, a study on the influence of new and old burs on fatigue strength of lithium-disilicate dental ceramics did not find any significant influence on its strength [53]. 3.5 SUBSURFACE DAMAGE IN MACHINED DENTAL CERAMICS Surface damage in dental ceramics during machining refers to the damage on surface and subsurface alterations on the machined components. Although subsurface damage denotes to the incidence of edge-chipping [54–57]. Dental ceramics due to their brittle nature, are prone to sub-critical ceramic surface damage during machining as micro-fractures and chipping defects. Such defects can be a source of stress for crack propagation under masticatory forces. Shaping of a ceramic blank occurs with micro-fractures during the contact between the milling tool and the ceramic during the milling procedure. Material removal mechanism essentially depends on the CAD/ CAM material. For example, in zirconia, marginal removal occurs with micro-cracks. In dental glass-ceramics, mechanism of crack propagation during machining is dependent on its microstructure and fracture toughness. Micro-cracks in leucite glass-ceramics occur through both amorphous and crystalline structures and through glass matrix in lithium-disilicate glassceramics. Machining of dental ceramics induces cracks that can potentially reduce the strength of the ceramics. It also induces localized residual stresses with plastic deformation around the machined zone. Median cracks occur parallel to the machining groove that are directed towards the bulk of the material and are more detrimental. Lateral cracks occur normal to the machining direction and have much less influence on strength of the ceramic. The intensity or the number of cracks depend on the dynamics between residual stress and crack blunting mechanism adjacent to the groove [58]. The emergence of median and lateral cracks in the ceramic during machining is shown in Figure 3.4. Essentially, when the milling tool contacts the dental ceramic block, it induces elastic deformation followed by plastic deformation. Plastic deformation further results in crack induction and residual stress accumulation. The chronic damage results in crack growth resulting in tensile stresses, leading to micro-chipping and fracture with residual subsurface damage [53].

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FIGURE 3.4

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Schematic illustration of emergence of cracks during machining.

Machined ceramics tend to have altered surface roughness with reduced strength Polishing of feldspathic porcelains has been shown to reduce surface defects with improved strength [59–61]. To reduce strength limiting problems of machined ceramics, fine milling is achieved at a higher feed-rate with lower cutting forces and coarse grinding is through at a lower feed-rate and increased cutting forces [33, 34]. Fine milling will minimize the surface roughness and reduce chip thickness of the machined ceramics [59–61]. Finer polishing grits and smooth surface finish are recommended. Grits of less than 64 um and ceramics with higher hardness were shown to induce less damage [62]. Micro-slurry of alumina particle jet erosion on lithiumdisilicate glass-ceramics showed surface and subsurface damage reduced after 50 cycle processing. Micro-slurry jet erosion can be an effective method to obtain fracture free surface quality (SQ) of diamond machined dental glass-ceramics as an alternative to manual polishing [61]. 3.6 EMERGING TRENDS IN MICROMACHINING DENTAL CERAMICS Micro-machining is one of the key technologies that deals with dimensions in the order of microns. Micro-machining is an emerging technique to produce micro-parts with micro-features. Recent trends in micro-machining are ultrasonic machining (USM), electrical-discharge machining (EDM),

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electro-chemical-discharge machining (ECDM), laser-beam machining (LBM), electro-chemical machining (ECM), micro-milling, and micro-grinding. Laser-beam machining (LBM) uses thermal energy of directed Laserbeam for material removal from metals or non-metals directed from a Laserbeam. High frequency monochromatic photonic light heat and melt the surface. Laser-beam machining is recommended for brittle materials with low conductivity. Effect of micro-machined geometry and surface roughness of laser machined zirconia ceramics reveals challenges in creating microchannels through optimization of laser parameters [63, 64]. Ultra-short, pulsed laser machining was used for fabricating high precision alumina toughened zirconia dental implants with retained tetragonal zirconia phase and machining error of 5 µm [65]. Pulsed laser micromachining zirconia ceramics using millisecond Nd-YAG laser demonstrates a process for rough machining with high MRRs [66]. Laser ablation on surfaces of natural models of cobalt chromium alloys with a nanosecond pulsed Yb laser showed improved bonding to veneering ceramics [67]. Laser assisted machining of yttria stabilized zirconia proved to be an effective grinding process with reduced tool damage and grinding forces [68]. USM is a subtractive machining process. It works through the principle of high frequency and low amplitude vibrations. The vibrations of the milling tool against the material surface takes place with a slurry of abrasive particles. The milling at amplitudes ranging from 0.05 to 0.125 mm. Abrasive grain sizes range from 100 to 1,000 micrometers. Small grains with a high grain concentration yield smoother finishes [69]. Ultrasonic assisted machining techniques are used for machining ceramics such as alumina-zirconia, silicon nitride, alumina, and [70–76]. This machining technique is a fusion of applications of traditional grinding and ultrasonic vibration. USM increases MRRs with reduced grinding forces and improved SQ [73, 74, 76, 77]. Ultrasonic assisted grinding on a machinable mica containing glass-ceramic showed initial reduction and increase in cutting forces with the ultrasonic vibration power. Further, surface roughness of the machinable ceramic had an inverse relation with power [78]. Normally, cavity preparation and removal of plaque and calculus through non-rotary ultrasonic assisted techniques [79–85]. However, they yield lower MRRs and low cutting efficiencies compared to high-speed rotary instruments [53, 84, 86–88]. New ultrasonic assisted high-speed rotary abrasive machining techniques must be explored to machine dental ceramics with increased removal efficiencies. Electrical-discharge machining (EDM) is a machining process of obtaining a desired shape using electrical discharges (EDs) [90]. Quickly frequent

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current releases between two electrodes, subject to an electric voltage remove material from the workpiece. Electric discharge machining (EDM) was earlier explored in dentistry for producing precision attachments, telescopic crowns, and implant fixed superstructures, improving the fit of cast prostheses and correcting their defects [91–93]. Electric-discharge machining is typically indicated for electrically conductive materials with a resistivity less than 100 Ωm [94, 95]. Zirconia being non-conductive, graphite powder was mixed with dielectric liquid, and machined with an optimum MRR using electric discharge method [97]. Considering the oxide nature and inability to etch zirconia, EDM machined dental zirconia was investigated for improved shear bond strength to resin cements [96]. 3.7 MACHINABILITY TESTS OF DENTAL CERAMICS A dental material must be machinable under a specific set of cutting conditions. The material must offer a reasonable surface finish, abrasiveness, an acceptable tool life with low power consumption [97, 98]. In a typical in-office chair side milling set up, the software program instructs the path of the machine tool at a specific feed rate and spindle speed resulting in variable cutting forces of the tool on the material. Generally, high feed rate delivering low torque or spindle speed is essential for easy machining [13, 17]. Machinability has been evaluated through Quantitative evaluation of parameters like, cutting force, power tool wear and surface roughness [99]. Tool wear is an important criterion for machinability, as it directly affects the tool’s surface integrity and the associated machining cost. In general, a longer life of the machining tool demands high tool hardness with minimal wear. Microstructure, hardness, and SQ of the substrate determine the appearance of the cutting tool [100, 101]. Surface roughness of the substrate can be related to tool set-up and its associated properties [101]. Cutting force of the tool, provides an approximation of the power requirements and estimates its impact on a tool’s endurance [101]. Machinability of dental ceramics has also been influenced by their mechanical properties, other than the cutting tools [46]. To be machinable, the material demands equilibrium between hardness (resistance to cutting) and toughness (resistance to failure) of the material. Machinability can be estimated by ‘brittleness index,’ which is the ratio of hardness to fracture toughness of a material [102]. A brittleness index of 4.3 μm–1/2 is normally used for machinability of the materials [103]. Studies on machining dental ceramics have been conducted with dental handpieces and diamond burs under certain loads [104–106]. Rotary diamond

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tools have been used at the chair side clinic, for dental ceramic prosthesis adjustment. Studies on the effect of diamond cutting tools on micro-damage of dental ceramics were investigated [107–112]. Specific cutting energy rates, cutting speed, MRRs, diamond tool wear, chipping, machining forces and surface roughness were evaluated for predicting machinability of dental ceramics to determine the material’s restorative application and restorative quality [104–106, 114, 115]. Various studies have performed machinability tests on glass-ceramics and proposed machinability indices to predict the ease of machining of a dental material. Chavali et al. customized a computer-controlled milling machine for evaluating machinability of CAD/CAM dental materials [17]. Machinability of aluminosilicates glass-ceramics is analyzed by the volume of mica, for example, more than two-thirds to one-third of mica by volume is required to be machinable. Taruta et al. qualitatively evaluated machinability on calcium-potassium micas with a high-speed drilling machine of rotational frequency of around 620 rpm. Due to the interlocking microstructure, calcium micas were easily machined [116]. Taruta et al. continued the experiments with the above-mentioned drilling machine at 620 rpm on zirconia mica composites. The ease of machining a high strength mica glassceramic was performed using the drill tool of 1.5 mm diameter [117]. Qin et al. synthesized mica glass-ceramics at varying temperatures and time and evaluated machinability using cutting depth with high-speed steel twist drill bits. The study found calcium mica showing higher machinability due to the nature of mica crystals [118]. Magnetic tachometer was used to evaluate machinability [104, 119]. Volume loss derived from density was used for evaluation of machinability and weight loss for uniformly dense materials [120]. Boccaccini & Baik et al. proposed machinability indices such as the brittleness index (B = H/KIC), which is the ratio of hardness to fracture toughness, used for predicting the material’s machinability. Mica glassceramics with their good machinability tend to demonstrate low brittleness indices [105, 107]. The values of hardness and toughness are sensitive to the technique or methods used to test them [121, 122]. Machinability is influenced by mechanical properties such as hardness, thermal conductivity, plasticity, and chemical composition of dental ceramics [123]. Various abrasive drilling tests were designed for characterizing machinability of dental ceramics. In simulated clinical conditions, machining of lithium-disilicate glass-ceramics was done to investigate tool wear, chipping control, machining forces and surface finish [106]. In studies on intra-oral abrasive finishing of dental ceramic restorations, machining parameters such as MRR, surface finish and the amount of surface damage were evaluated. In such process of clinically

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simulating the cutting conditions, mechanical controlled and CAM apparatus were proposed [124]. Cutting test machines with air turbine hand pieces and weights were designed to rank the machinability of ceramics [89]. 3.8 FUTURE DIRECTIONS Though CAD/CAM technology has hugely contributed to the technological advancement in the fabrication of dental restorations, having overcome most technical liabilities of the traditional lost-wax technique, it still has the drawbacks of creating subsurface damages on the machined surfaces of dental ceramic restorations that could result in catastrophic fractures of restorations. Such damages can be sufficiently reduced by strictly following the manufacturer’s instructions, polishing and glazing exposed machined ceramic surfaces and proper designing of ceramic prostheses with explicit communication between dental professionals and dental technicians. KEYWORDS • • • • • • • •

CAD-CAM in dentistry CAM milling tools ceramic restorations dental ceramics dentistry high speed micromilling machinability micromachining

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CHAPTER 4

MICRO-MACHINING PERFORMANCES BY ECDM PROCESS FOR NONCONDUCTING MATERIALS BIJAN MALLICK,1 B. DOLOI,2 B. R. SARKAR,2 and B. BHATTACHARYYA2 Faculty, Global Institute of Management and Technology, Krishnanagar, West Bengal, India

1

Faculty, Production Engineering Department, Jadavpur University, Kolkata, West Bengal, India

2

ABSTRACT Advanced ECDM process tries to take part in the industrial field for the production of micro-features on non-conducting materials, and increasing machining depth (MD) is a challenging task during micro-machining performances. The basic characteristics, fundamentals, and advanced microECDM setup development, as well as feeding system and tool holding device, have been developed and analyzed in this chapter for increasing MD during micro-features generation in hard materials like glass, silicon-wafer, and various ceramics. The chapter also consists of the effects of process parameters like pulse frequency (PF), duty ratio (DR), applied voltage (V), and concentration of electrolyte (EC) on machining criteria such as rate of material removal, surface roughness, diametric or width of overcut, MD and heat affected zone (HAZ) during micro-machining operation by ECDM process. This chapter also addresses some merits and demerits as well as the utility of the micro-ECDM process.

Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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4.1 INTRODUCTION The fabrication of miniature parts attracts major concentration from the industrial field to fabricate small or tiny products, which have growing demands in modern society. So, to meet the terms demand, scientists and technologists are facing more and more exigent problems in the field of manufacturing industries. Electrochemical discharge micro-machining is formed by the hybridization of two consecutive processes one is ECM, and another is EDM [1–3]. Micro-ECDM is a mirrored influential manner, and the tool is contemplated at the assignment specimen [4, 5]. Two electrodes are utilized in ECDM: one is the micro-tool which acts as a cathode and the other is an auxiliary electrode (graphite) which is used as anode in the ECDM cell. The activity specimen is positioned simply under the micro-tool and in conjunction with the anode, is wrapped up in NaOH alkaline in the Pyrex chamber. The thermal erosive outcome of ED and electrochemical (EC) reaction paralleling happens in micro-ECDM. These electrochemical actions form positive H2 bubbles which gathers at the tool. The electric discharge (ED) motion takes vicinity among throughout the H2 bubble layers. Sparking is begun due to cause applied voltage finally to cross the breakdown voltage. The depth of sparking and electricity of the discharge amplifies with the growth of the implemented voltage (V) between microtool and graphite plate [6]. If a non-carrying out process specimen is positioned in the closed locality of the ED, the fabric of the process specimen is melted, vaporized, and windswept due to the diffusion of a portion of spark energy (SE) to the activity specimen. Suppose the temperature of the machining area becomes high and takes part in removing materials because mechanical shock suddenly provokes phase exchange. Additional cloth is eliminated because of thermal spalling at the job specimen [7, 8]. In this bankruptcy, a newly designed micro-ECDM setup with its feeding device, tool preserving systems, fundamentals, merits-demerits, parametric impacts, and micro-ECDM application has been illustrated. 4.2 FUNDAMENTALS OF µ-ECDM PROCESS After exhaustive assessment on previous studies works, its miles cleared that electrochemical discharge micro-machining (µ-ECDM) is comes from combine effects of ECM and EDM. MRR mechanism in μ-ECDM method is similar with that of traditional ECDM method and by the mixed outcomes of EC and ESD action can achieve desired shapes by removing excess materials.

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The spark discharge is initiated if the implemented voltage is greater than the critical voltage and the cloth is eliminated from the task specimen because of the melting and vaporization when an electrically non-carrying out fabric is stored on the closed area of spark among micro-device and electrolyte across the gas bubble layer [1, 2]. An arrangement of the electrolyte cell in the μ-ECDM technique is proven in Figure 4.1.

FIGURE 4.1

Electrochemical cell of the µ-ECDM process.

The electrochemical reaction happens on the steel-electrolyte interface and the switch of ions inside the NaOH solution takes area with the aid of migration in an electrical discipline. The cathode and anode reactions take region when the capacity within the inter-electrode gap of the machining zone is reached to a threshold price. 4.3

MERITS AND DEMERITS OF µ-ECDM PROCESS

Electrically non-conductive substances, which might be very tough to device, can be without problems machined by μ-ECDM method. It is a hybrid era includes electrochemical response and electric discharge enables in machining of ceramics like aluminum oxide, silicon carbide, silicon nitride, superior ceramics, and glass and so on. Its precise function of the use of thermal electricity to machine non-conductive elements irrespective of their

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inflexibility has been its wonderful benefit inside the production of mildew, die, and car, aerospace engineering applications and surgical and medical apparatus. Also, there are some greater advantages of μ-ECDM technique are said under: • • • • • • •

No external pressure is needed to eliminate fabric by using this method. Low machining set up fee. Using ECDM warmth treatment may be executed within the surface of the task specimen. No remote chamber is required for the micro-ECDM installation. No professional worker is needed for micro-machining operations. Good repeatability. Good accuracy and precision.

Some negative aspects are–thermal results are observed, activity specimen break down at better voltage, conductive cloth isn’t suitable for this machining. 4.4 APPLICATIONS OF µ-ECDM Micro-electrochemical discharge machining (ECDM) is applicable in aerospace engineering, medical science and instruments, automobile industries, glass industries, research purpose, glass and ceramic research field and electrical and electronics industry, etc. In the field of micro-fluidic device μ-ECDM process has the ability to generate micro-channel and micro-slots on hard materials (e.g., ceramics, silicon-wafer, glass, and composites, etc.), which are generally used to pass fluid to the required position as per requirement. There are so many applications of micro-ECDM process like as follows: • • • •

Welding can be done by using ECDM process; Surface hardness can be improved using this process; Nano-deposition of material can be done; Truing and dressing of grinding wheel can be done, etc.

4.5 DETAILS OF MICRO-ECDM To accomplish the objective of the present, look at and the expertise base via the part of the experimental base studies work on µ-ECDM system, an advanced experimental installation became used to carry out machining operation. The micro-machining characteristics mainly depends on machining

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voltage, machining current, the feeding movement of job specimen and inter electrode gap (IEG), pulse frequency (f), duty ratio (DR%), pulse on time, pulse off time, rate of tool movement, etc., µ-ECDM experimental set up is publicized in Figure 4.2.

FIGURE 4.2

3D CAD solid model of micro-ECDM setup.

4.6 ADVANCEMENT OF FEEDING SYSTEM AND TOOL HOLDING UNIT FOR MICRO-ECDM PROCESS The chrome steel bars steer the spring in this type of manner that the job conserving plate can move wavering as per requirement by the side of the axis of the bars, i.e., most effective vertical movement is feasible for the job keeping plate in addition to the task specimen. Figures 4.3 and 4.4 show 3D CAD version of the activity feeding unit and device maintaining unit. The tool holder is made from chrome steel. It was made to serve the cause

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for containing the circular tool, that’s used to generate micro-channel on the process specimen. With the help of recently evolved set-up and the use of cylindrical micro-tool, the micro-channel of various natures is viable to generate on glass, quartz, and ceramics. A template is made and outfitted with the device holding unit which can move within the same profile supported by way of the template. The device holder beside with tool electrode may be rotated. The tool conserving unit is equipped with the cover plate by tighten screw-nut, as a result placement of the micro-tool can be accustomed in keeping with the requirement. This mechanism will assist in upward and downward motion of the device keeping unit. Automated spring-feed mechanism is used, and tool is moved the usage of CAM-follower mechanism to increase machining depth (MD) at some point of micro-machining performances.

FIGURE 4.3

3D CAD solid model of feeding system.

4.7 PARAMETRIC INFLUENCES ON MICRO-MACHINING PERFORMANCES 4.7.1 PARAMETRIC EFFECTS ON MRR Effects of implemented voltage, electrolyte concentration (EC), pulse frequency (f) and DR on MRR for fixed (IEG) inter-electrode gap (40 mm)

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during fabrication of micro-channel on hard glass and NaOH is used during machining as an electrolyte, are proven in Figures 4.5(a)–(d) correspondingly to analyze the parametric effects. These figures show that rate of material removal for NaOH electrolyte solution will raise with the enhance of carried out voltage, electrolyte conductivity as increase of concentration and DR however decreases with swell of pulse frequency (f). Figures 4.5(a) and (b) display that MRR is excessive for 55 V and 30 wt.% electrolyte concentration, respectively. Also, MRR will increase with growth of length of On-time, which will increase with duty factor [8, 9]. MRR cascade with augment of PF due to the fact the period of the release will boost as the frequency (f) dwindles, even though the intact time that voltage is implemented stays the alike. Figure 4.5(c) shows that material removal rate (MRR) will be near about equal for the PF of 400, 600, and 800 Hz. At 200 Hz frequency and 65% DR, rate of material removal becomes higher as shown in Figures 4.5(c) and (d), respectively.

FIGURE 4.4

3D CAD solid model of tool holding unit.

4.7.2 PARAMETRIC EFFECTS ON OC Overcut is an undesirable characteristic in case of micro-machining operations. In every machining operation, the desirable condition is least overcut or zero overcut if possible. In the present work, overcut has also been

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observed with varying the input process parameters. Figures 4.6(a) and (b) demonstrate the parametric effects on overcut. Basically, rate of discharge amplifies with the raise of voltage and weight concentration of electrolyte and therefore width of cut (WOC) becomes higher due to side scattering of discharge and overcut becomes enlarge. Figure 4.6(c) and (d) exhibit the parametric effects on overcut such as pulse frequency (f) (Hz) and DR (%), respectively. DR amplifies the OC if higher DR is applied and OC reduces when applied PF becomes lower [10, 11]. Because violent sparks take place due to higher applied voltage which causes side discharge.

(a)

(b)

(c)

(d)

FIGURE 4.5 Parametric effects on MRR for (a) applied voltage (V); (b) electrolyte concentration (EC) (wt.%); (c) duty ratio (DR) (%); and (d) pulse frequency (f) (Hz).

4.7.3 PARAMETRIC EFFECTS ON HAZ Figures 4.7(a) and (b) demonstrate the effects of applied voltage and electrolyte concentration on HAZ area. During ECDM whole process heat is produce and directly effects on glass surface. Some heat is radiated to ambiance, a few is gone astray into electrolyte with the aid of convection and the rest is conducted to the task specimen; job specimen. The principal

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purpose at the back of the HAZ across the machined profile is because of the heat conducted to the job specimen. It is determined from Figures 4.7(a) and (b) that HAZ vicinity almost regularly will increase with boom of applied voltage (V) and electrolyte concentration (wt.%) (EC) and at 55 Volt and 30 wt.% of NaOH HAZ reaches at higher level during micro-fabrication. Figures 4.7(c) and (d), respectively represents the actual effects of DR (%) and pulse frequency (f) (Hz). It is obvious from the figures that to start with HAZ location increases after which it decreases with frequency while it will increase with DR after 55%. As a result, HAZ spreads around large area. The experimental outcomes monitor that HAZ region can be decreased with the aid of lowering the DR and escalating the PF (Hz) of the carried-out voltage (V), so as MRR reduces.

(a)

(b)

(c)

(d)

FIGURE 4.6 Parametric effects on OC for (a) applied voltage (V); (b) electrolyte concentration (EC) (wt.%); (c) duty ratio (DR) (%); and (d) pulse frequency (f) (Hz).

4.7.4 PARAMETRIC EFFECTS ON MD Figures 4.8(a)–(d) exhibits the effects of applied voltage (V), electrolyte concentration (wt.%), and DR (%) and PF (Hz) on MD, respectively. From the Figures 4.8(a)–(d), it is revealed that by means of the increase of voltage,

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DR and electrolyte concentration (EC), MD will increase up to 50 volts at 60% DR and 25 wt.% of NaOH, respectively but after that MD reduces. Because current density at the tool’s top rises but lack of electrolyte on the tip of tool is discovered and additionally side sparking occurred because hydrogen bubble gathered on the side wall of electrode. At high voltage and DR, MD becomes higher when positive hydrogen bubbles are available in the micro-machining zone, but opposite cases occur for PF. The feasible cause is that if pulse on-time is reduced, PF falls down [12, 13] for the fixed DR and it reduces the sparking charge on the micro-machining zone.

(a)

(b)

(c)

(d)

FIGURE 4.7 Parametric effects on HAZ for (a) applied voltage (V); (b) electrolyte concentration (EC) (wt.%); (c) duty ratio (DR) (%); and (d) pulse frequency (f) (Hz).

4.7.5

PARAMETRIC EFFECTS ON RA

The effects of carried out voltage (V), weight percentage of electrolyte concentration (EC), DR (%) and pulse frequency (f) (Hz) on surface roughness (Ra) for fixed (IEG) inter-electrode gap 40 mm are shown in Figures 4.9(a)–(d), respectively when micro-channel is fabricated on glass. From the Figures 4.9(a)–(c) it is obvious to declared that Ra will enhance if

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voltage, electrolyte concentration and DR are amplified because of increase in sparking rate however better surface roughness creates at 50 V and 25 wt.% and 55% DR. Heat is formed into the machining zone as a result HAZ location amplifies and irregularities forms at the floor of the task specimen that increases roughness (Ra) of the µ-channel. If the strike sparking is in regular form and stray sparking is very less, that assist to achieve better surface finish. From Figure 4.9(d) it is propounded that if PF is expanded, at the beginning Ra is reduced because pulse on-time is decreased. If high PF (Hz) is applied, it is very hard to manage the uniform discharge so as secondary irregularities creates on the µ-channel [14–17]. So, after that (Ra) surface roughness becomes higher.

(a)

(b)

(c)

(d)

FIGURE 4.8 Parametric effects on MD for (a) applied voltage (V); (b) electrolyte concentration (EC) (wt.%); (c) duty ratio (DR) (%); and (d) pulse frequency (f) (Hz).

4.8 UTILITY OF MICRO-ECDM Micro-channels, micro-complex profile, micro-holes, round, square or taper marking, groove cutting, contour machining, micro-structure of glass wafers preparation and micro-reactor applications are highly needed in

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micro-products and micro-fluidic components or in MEMs and sometimes can be utilized as lab-on chips, which can be generated by developed µ-ECDM process on silica glass. This analysis on ECDMing on hard and brittle non-conducting material has wide scope of applications in electrical, bio-medical, electronics, and mechanical engineering. Figure 4.10 shows the µ-channel of Silica Glass which is produced by µ-ECDM.

(a)

(b)

(c)

(d)

FIGURE 4.9 Parametric effects on Ra for (a) applied voltage (V); (b) electrolyte concentration (EC) (wt.%); (c) duty ratio (DR) (%); and (d) pulse frequency (f) (Hz).

FIGURE 4.10

µ-channel cutting on silica glass by µ-ECDM.

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4.9 CONCLUSIONS After fulfilling the desired objectives of the present research work, the author thinks that further investigation and development of µ-ECDM is highly required for utilizing the process in the modern industrial field and to generate different complex shapes on other advanced ceramic materials as well as on quartz. The following conclusions are drawn out: 1. From the fundamentals of µ-ECDM process it is observed that Sparking contributes to erosion of material melting and chemical etching of job specimen during micro-machining. 2. Automated spring feed improves MD as well as other machining performances like MR, surface roughness. 3. PF, DR, applied voltage (V), electrolyte concentration (EC) influences on MRR, MD, OC, Ra, and HAZ. 4. µ-channel can be used as a micro-fluidic appliance and in MEMS. KEYWORDS • • • • • • •

feeding system glass micro-ECDM micro-machining non-conducting material parametric effects tool holding unit

REFERENCES 1. Basak, I., & Ghosh, A., (1996). Mechanism of spark generation during electrochemicaldischarge machining: A theoretical model and experimental verification. Journal of Materials Processing Technology, 62, 46–53. 2. Bhattacharyya, B., Doloi, B. N., & Sorkhel, S. K., (1999). Experimental investigations into electro-chemical discharge machining (ECDM) of non-conductive ceramic materials. Journal of Materials Processing Technology, 95, 145–154.

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3. Sarkar, B. R., Doloi, B., & Bhattacharyya, B., (2017). Non-traditional micromachining processes fundamentals and applications-10. Electrochemical Discharge Micro-machining of Engineering Materials. Springer International Publishing. ISBN: 978-3-319-52008-7. 4. Mochimaru, Y., Ota, M., & Yamaguchi, K., (2012). Micro hole processing using electrochemical discharge machining. Journal of Advanced Mechanical Design, Systems and Manufacturing, 6, 949–957. 5. Jana, D., Ziki, A., Didar, T. F., & Wuthrich, R., (2012). Micro-texturing channel surfaces on glass with spark assisted chemical engraving. International Journal of Machine Tools & Manufacture, 57, 66–72. 6. Mallick, B., Biswas, S., Sarkar, B. R., Doloi, B., & Bhattacharyya, B., (2019). On performances of electrochemical discharge micro-machining process using different electrolytes and tool shapes. International Journal of Manufacturing, Materials, and Mechanical Engineering (IJMMME), 10, 2. 7. Mallick, B., Sarkar, B. R., Doloi, B., & Bhattacharyya, B., (2018). Analysis on the effect of ECDM process parameters during micro-machining of glass using genetic algorithm. Journal of Mechanical Engineering and Sciences, 12(3), 3942–3960. 8. Mallick, B., Sarkar, B. R., Doloi, B., & Bhattacharyya, B., (2017). Analysis of electrochemical discharge machining during micro-channel cutting on glass. International Journal of Precision Technology, 7(1), 32–50. 9. Mallick, B., Sarkar, B. R., Doloi, B., & Bhattacharyya, B., (2019). Modeling and analysis on performance of ECDM process for the fabrication of µ-channels on glass through response surface methodology. Manufacturing Technology Today (MTT), 18, 9. 10. Mallick, B., Sarkar, B. R., Doloi, B., & Bhattacharyya, B., (2014). Multi criteria optimization of electrochemical discharge micro-machining process during microchannel generation on glass. Applied Mechanics and Materials, 592–594, 525–529. 11. Mallick, B., Hameed, A. S., Sarkar, B. R., Doloi, B., & Bhattacharyya, B., (2019). Experimental investigation for improvement of micro-machining performances of µ-ECDM process. Elsevier’s Materials Today. 12. Bellubbi, S., Sathisha, N., Mallick, B., (2021). Multi response optimization of ECDM process parameters for machining of micro channel in silica glass using Taguchi–GRA technique. Silicon, 14, 4249–4263. 13. Antil, P., (2020). Modeling and multi-objective optimization during ECDM of silicon carbide reinforced epoxy composites. Silicon, 12, 275–288. 14. Paul, L., & Hiremath, S. S., (2021). Model prediction and experimental study of material removal rate in micro ECDM process on borosilicate glass. Silicon. https://doi.org/10.1007/ s12633-021-00948-1. 15. Rajput, V., Goud, M., & Suri, N. M., (2021). Three-dimensional finite element modeling and response surface based multi-response optimization during silica drilling with closed-loop ECDM. Silicon, 13, 3583–3609. 16. Rajput, V., Goud, M., & Suri, N. M., (2021). Finite element modeling for comparing the machining performance of different electrolytes in ECDM. Arab J Sci Eng, 46, 2097–2119. 17. Mallick, B., Sarkar, B. R., Doloi, B., & Bhattacharyya, B., (2022). Improvement of surface quality and machining depth of μ-ECDM performances using mixed electrolyte at different polarity. Silicon. https://doi.org/10.1007/s12633-021-01587-2.

CHAPTER 5

ADVANCEMENT OF ELECTROCHEMICAL DISCHARGE MICROMACHINING: PROCESSING MICRO-FEATURES IN NONCONDUCTING MATERIALS MANEETKUMAR R. DHANVIJAY,1 BIJAN MALLICK,2 SADASHIV BELLUBBI,3 and N. SATHISHA4 Department of Manufacturing Engineering and Industrial Management, College of Engineering, Shivajinagar, Pune, Maharashtra, India

1

Department of Mechanical Engineering, Global Institute of Management and Technology, Krishnanagar, West Bengal, India

2

Department of Mechanical Engineering, Alva’s Institute of Engineering and Technology, Moodabidri, Mangalore, Karnataka, India

3

Department of Mechanical Engineering, Yenepoya Institute of Technology, Moodabidri, Mangalore, Karnataka, India

4

ABSTRACT Recent developments in materials research have opened a plethora of challenges for their fabrication in various areas, such as the medical, aerospace, defense, and automotive fields. Machining micro-features in non-conducting materials is a very challenging task using traditional machining processes owing to its high hardness property. Micro-Electrochemical-dischargemachining (µ-ECDM) is a hopeful way to overcome these challenges Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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since a few unconventional micro-machining processes are unsuitable for non-conducting materials. µ-ECDM process involves spark generation in the electrolyte between the auxiliary electrode and tool electrode. These discharges contribute to material erosion, melting, and chemical etching of the workpiece, which helps in improving material removal. This chapter includes micromachining of glasses and ceramic materials which have been subjected to µ-ECDM process variants such as gravity feed, traveling wire, Vibro-rotary, and magnetic field effect to investigate the responses like material removal rate (MRR) primarily, surface finish, tool wear rate, diametric overcut (DOC), heat-affected zone, and other qualitative parameters during micro-features generation on non-conducting materials. 5.1 INTRODUCTION The significance of materials processing has been rapidly increasing in recent years. Low aspect ratios, material removal rate (MRR), and quality are all disadvantages of the available machining processes. Given the variety of micro-machining technologies available, such as micro-EDM, micro-ECM, Laser assisted micro-machining, water jet machining, and so on. ECDM is a hybrid machining method that is used to manufacture a variety of non-conducting materials. It is commonly used in the automotive, aerospace, and medical industries, to mention a few. The non-conducting materials are hard and have high strength and hence, difficult to process by various unconventional machining techniques. In the ECDM process, the tool electrode is immersed in an electrolyte, and it is connected to the positive potential and the anode is kept at a suitable distance, called the interelectrode gap (IEG) [1], which is found out from trail experimental studies. Anode is several times bigger compared to that of tool electrode size [2]. Materials that may not be conductors, such as glasses, composite materials, ceramics have superior features such as a high strength-to-weight ratio and a high stiffness-to-weight ratio, impact resistance and abrasion resistance. However, the challenge is to machine them as per the design requirements which create the need of novel machining technologies such as ECDM process although MRR is found to be quite high while machining a variety of glasses. Paul and Hiremath [3] have done the thermal modeling of the ECDM process SE discharge for MRR and experimented with borosilicate glass, authors have observed 0.3730 mg/min by modeling and 0.4140 mg/min experimentally; it is not so while machining Al2O3 or Si3N4 ceramics or FRP material. For FRP machining with the electrolyte circulation Dhanvijay et al.

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[4] observed that maximum MRR is 0.007950 mg/min. Mixed Electrolyte (NaNO3 + NaOH and NaCl + NaOH) with pressurization is supplied to the machining zone to observe its influence on MRR and DOC. Kharche et al. [5] varied the electrolyte flow rate in the range of 1–5 ml/min. It is seen that MRR is 2.160 g/min at 3 ml/min using NaCl + NaOH electrolyte and minimum DOC is 1.2213 mm with 4 ml/min flow rate and using NaNO3 + NaOH. With the objective of further increasing the MRR on different materials, vibration, rotation, and the combined effect of vibration and rotation has been implemented by several researchers. It is critical to reduce or combine some process to control production time and cost by implementing new methods [6]. Cheng et al. [7] reported that the tool rotations have the least effect in the gas film formation and the same was concluded from the constant current signals. Studies using tool rotations of 10, 100, 500, and 1,000 rpm and 40 V with 5 M KOH were adopted for the experimental work. Yang et al. [8] worked with cylindrical and spherical tool electrodes for machining of quartz. It was observed that the machining depth (MD) was least when no tool rotations were given as compared to the tool rotation of 500 rpm for which the MD was maximum (330 µm). Subsequent increase in tool rotations decreased the MD which is due to the strong centrifugal forces disrupting the formation of the gaseous film around the tool-electrode. On the contrary MD remained constant in the studies conducted by Zheng et al. [9] for machining Pyrex glass. They imparted tool rotations of 200, 500, 1,000, 1,500, 2,000 rpm using 5 M KOH and observed that the groove width decreased with increasing tool rotations. This is related to the production of a thin gaseous film and there by lower rate of sparking contributing to lower MRR. Effect of destabilization of the sparking process is reported by Wuthrich & Fascio [10]. It is observed that the sharpness of the machined grooves improved when tool rotations are imparted. Gautam & Jain [11] As cutting borosilicate glass and quartz, tool rotations improved machining performance in terms of depth and machining rate (MR) when compared to a stationary tool. They divided the sparking region in two zones and report that higher tool rotations in zone II caused destabilization of the gas film. Chak & Rao [12] carried out experimentation using diamond abrasive bonded rotary electrode for machining of inserts of sintered aluminum oxide. Authors reported the formation of electrochemical discharges at the electrode bottom due to the rotational movement. In comparison to the stationery tool, the volumetric material removal (80 mg) was higher for the abrasive rotary tool when the applied parameters were set to duty factor of 0.96, DC voltage 120 V and Electrolyte Conductivity of 380 mmho/cm.

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Implementation of the magnetic field effect is also an important parameter towards improvement in MR and accuracy. With the magnetic field in the range of 0.26 ± 0.01 T Cheng et al. [13] MHD currents are set up which increases the surface coverage of the gas bubbles on the tungsten carbide tool electrode having 0.2 mm diameter. The geometric accuracy and machining efficiency was 23.8% and 57.4%. Razfar et al. [14] conducted experiments using longitudinal tool vibrations (6–500 Hz, 1.5–27 µm) of sinusoidal and square waveforms for machining of soda lime glass using cylindrical and drill tools. For sinusoidal waveforms, for the first 120s the MD was 250 µm. Thereafter it is seen that the longitudinal tool vibration effects the circulation of the electrolyte as well as the debris removal from the machining zone. Significant improvement in MRR resulted while using square waveform. Tool vibrations are another prominent effect used in the ECDM process. Goud et al. [15] It was concluded that the electrolyte reaches the machining zone because of various tool kinematics such as tool rotation and vibrations, which help in enhancing electrochemical reactions and thus etching. Singh & Dwiwedi [16] emphasized the role of tool vibration, rotation, and the magnetic field for improvement of ECDM machining performance namely MRR, surface integrity and quality of the workpiece machined. Elhami & Razfar [17] In the ECDM process, ultrasonic vibrations were used (UAECDM). They found that while machining soda lime glass, the thickness of the gas-film around the tool was reduced by 65%. Current signals were also observed to note the gas film stability during the machining process. It is seen that with a 10 µm vibration amplitude, the current signals were most uniform and hence enhancing the MRR. Harugade et al. [18]. The effect of different electrolytic solutions on material removal in ECDM was investigated. The results showed that the applied voltage was the most important parameter for material removal, and KOH has a higher MRR than the other proposed electrolyte concentrations. Jui et al. [19] The possibility of machining high aspect ratio micro-holes in glass material with low concentrations was investigated and discovered. Lower concentration reduced 22.0% of overcut, while lowering the concentration reduced tool wear and hole taper by 39.0% and 18.0%, respectively. Krishankant et al. [20] discussed the Taguchi method’s applications for optimizing turning processes based on the effect of machining factors. Somashekhar et al. [21] created an ECDM setup for machining a variety of profiles in non-conducting materials such as borosilicate glass, Pyrex glass, and composites. They also discovered that side sparks affect the periphery of

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machined holes on glass pieces. Bellubbi et al. [22] investigated the effects of various input parameters by machining holes in borosilicate glass using the ECDM process and concluded that applied voltage is the most important factor. According to the authors, the best combination of process factors to maximize MRR based on Taguchi technique included 50 V voltage, 25% concentration, and an 8-minute machining time (A3–B3–C3). Han et al. [23] used tungsten carbide as a work tool to machine micro-channels on a glass substrate at 35 V and 20 wt.% NaOH concentration. A ceramic tube was used to cover the insulated WC tool. The authors discovered that the electrodes without coating produced uneven spark pulses as well as broken width and linearity of the machined grooves due to variations in thickness. Mallick et al. [24] carried out the experiments with varying concentration and voltages. The authors discovered that the tapered side wall and flat front tool tip shape is most seen when making circular holes. Wuthrich & Fascio [10] done experiments with various sized electrodes, with the anode electrode being much larger than the cathode electrode. Furthermore, the authors conveyed that the current-voltage appearances of an electrochemical cell were dependent on generation of bubble during the machining. Arab et al. [25] compared the physical contact method and a specific gap between the cathode tool and work sample to achieve significant material removal while decreasing tool wear. Depending on the type of electrolyte and electrolyte concentrations, the average tool and sample gap. Bellubbi et al. [26] investigated the effect of input parameters on MRR, MD, and overcut in an ECDM process using Taguchi L9OA, and then optimized using analysis of means (ANOM). According to the ANOM results, the difference between the experimental and predicted values of MRR, MD, and OC is 4.195%, 1.919%, and 3.458%, respectively. Datsiou et al. [27] demonstrated the optimization of a laser powder bed fusion method for glasses and evaluated the physical and mechanical properties of the produced parts. Two soda-lime-silica-glass feed stock materials with varying particle sizes were given process maps. Innumerable studies have been carried out on several materials using the novel electrochemical discharge machining (ECDM) by varying a number of parameters such as IEG, tool rotations and vibrations, different electrolytes and their combinations, etc. These aspects were aimed towards improvement in the process efficiency and quality and researchers were able to achieve very good results as discussed in this section. The subsequent sections focus on the principle of ECDM process, experimental setup for Vibro-rotary machining of alumina 95 ceramics and the experimental results and discussion. Finally, conclusions are presented based on the experimental work.

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5.2 WORKING PRINCIPLE OF ECDM The tool electrode is preferred very smaller compared to that of auxiliary electrode [1]. As shown in Figure 5.1(a), the work piece is placed beneath the tool and immersed in electrolyte solution. Electrochemical reactions begin when the applied voltage exceeds the threshold voltage value as the voltage is applied across the electrodes. With a constant supply of voltage, more electrochemical reactions occur, resulting in the formation of gas bubbles around the tool electrode, as shown in Figure 5.1(b). As shown in Figure 5.1(c), the high density of gas bubbles forms a thin film around the tool, which acts as a dielectric medium. This layer serves as a dielectric between tool and electrolyte concentration; when the applied voltage exceeds the gas film dielectric strength, spark discharges originate all around the tool material, as showed in Figure 5.1(d). As a result of the SE, thermal erosion, and chemical etching, the work sample from under the tool electrode softens, and electrical discharges (EDs) promise a continuous micro-eruption from the sample layer, hence micro-quantities of material are continuously taken from the work.

FIGURE 5.1 Working principle of ECDM process. (a) Basic ECDM setup; (b) generation of hydrogen bubbles around tool vicinity; (c) gas film formation around tool; and (d) spark initiation.

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5.3 NEED OF µ-ECDM Because of their advantageous properties, progressive engineering materials such as quartz, ceramics, alumina, and glasses are increasingly used in modern manufacturing. However, a few forward-thinking machining processes are essential to manufacture products with advanced materials. The cutting ability in the Abrasive Jet Machining process is dangerous owing to its cross-cutting speed and involves a high initial investment and maintenance. Furthermore, the product quality and surface quality (SQ) of the machined product are poor. Ultrasonic machining (USM) is commonly processed on electrically non-conducting materials, however, it has a few inherent drawbacks, such as increased tool wear and a high initial investment. In laser beam machining (LBM), a high intensity of monochromatic light is utilized while processing of work samples, and this process involves the formation of a larger uninvited heat affected zone (HAZ), which reduces product quality and also requires a high investment for process setup. ECM is based on chemical etching and requires skilled workers to operate the setup. The main disadvantage of the ECM process is the disposal of utilized electrolyte and the effects of stray current. Electro-discharge machining is mainly process on electrically conductive material and it have some negatives like difficult to produce different shapes by using this process, it requires long machining time to produce micro-features, also the initial investment cost is also slight high compared to other processes. So, the alternative machining process is tried to bring within the minimum investment for cutting non-conducting materials like glass. According to result, it is necessary to develop a special machining process that will allow for the production of products made of electrically non-conductive materials while also addressing the drawbacks of the previous machining processes. The ECDM process can machine electrically non-conductive materials like Pyrex glass and ceramics in a variety of ways. Formation of HAZ due to thermal effects are minimal compared to the above machining techniques. Because it is simple in construction and independent of the chemical and physical properties of the work material, this ECDM process does not require skilled workers to operate. The ECDM process required less capital and construction space.

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5.4 EXPERIMENTAL SETUP Vibro-rotary electrochemical discharge machining (VR-ECDM) the process can be used for processing various materials such as metals, composite, glasses, and ceramics. Alumina 95 ceramics is generally used while producing microchips in the electronics industry as well tool and die making industry. Alumina 95 ceramics is available with various shapes and sizes such as square, round, rectangular, and with various thicknesses. Due to its high strength, this material has poor machinability, making it a difficult task for investigators and manufacturing industries. Considering the possibility of increasing the material removal and reducing diametric overcut (DOC), machine setup was proposed to process with 95% Al2O3 ceramics having zero porosity. Further, VR-ECDM process had developed to meet all the requirements for experimental studies. VR-ECDM system consists of various subsystems like Mechanical subsystem, electric power supply unit and electrolyte supply unit. The developed machining setup of VR-ECDM machine along with the control unit is shown in Figure 5.2. DC power supply is connected between the auxiliary electrode (+ve) and the tool electrode (–ve). Permanent magnet DC motor is used to vary the tool speed from 0–3,000 rpm in clockwise and counter-clockwise manner.

FIGURE 5.2

VR-ECDM machine with control unit.

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5.5 MATERIALS USED Any experimentation requires several materials such as tools, workpieces, auxiliary materials such as electrolytes which directly or indirectly affect the machining performance. The workpiece material is subjected to the combined effects of tool rotation and vibration. NaOH with varying concentrations was used for the vibro-rotary experimentation on alumina ceramics. Table 5.1 lists the properties of alumina 95 ceramic having 0% porosity. TABLE 5.1

Properties of Alumina 95 Ceramics

SL. No. 1. 2. 3.

Particular Density Nominal alumina percentage Crystal size

Specification 3.95 g/cc 95% 5 µm

4. 5. 6. 7. 8. 9. 10. 11.

Porosity Flexural strength Color Fracture toughness Melting temperature Tensile strength Hardness R45N Elastic modulus

0% 258 MPa White 4 MPa 2,051°C 220 MPa 78 295 GPa

5.5.1 STANDARD TOOLS FOR EXPERIMENTATION Pilot experiments were conducted using various sizes of tools and materials such as stainless steel, tungsten, copper. Tool diameters of 1 mm, 0.8 mm, 1.55 mm, 1.60 mm and 1.61 mm were chosen based on cost and availability. Electrolytic copper resulted in less tool wear compared to SS and copper for processing of 20 minutes, hence electrolytic copper is utilized for further work, and it is shown in Figure 5.3. Table 5.2 represents the properties of electrolytic copper material. 5.5.2 MATHEMATICAL MODELING AND OPTIMIZATION TECHNIQUE FOR ECDM PROCESS To improve the performance of process parameters of ECDM process, various types of traditional metaheuristic optimization algorithms and “Evolutionary

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Algorithms” are used to form mathematical models. Response surface methodology (RSM) as a desirability function analysis, Taguchi-GRA, particle swarm optimization (PSO), artificial neural network (ANN), genetic algorithm (GA), ant bee colony, differential evaluation, ANFIS, fuzzy logic control (FLC), grey wolf optimizer, cat swarm optimization (CSO), cuckoo search optimization and various types of hybrid optimization also can be used, such as HPSOCSO.

Electrolytic copper tool.

FIGURE 5.3 TABLE 5.2

Properties of Electrolytic-Copper Material

Properties

Value

Thermal conductivity

391.1 W/m–°K at 20°C

Density

8.91 gm/cm3 at 20°C

Co-efficient of thermal expansion

17.6 × 10–6/°C

Melting point

1,083°C

Specific heat capacity

393.5 J/kg–°K at 293 K

5.6 RESULTS AND DISCUSSIONS Based on experimentation planning, the performance characteristics of vibrorotary ECDM while machining alumina-95 ceramics were measured from each experimental run. The output characteristics were considered as MRR, tool wear rate (TWR) and DOC. The machining responses have been analyzed using ANOVA, regression analysis (RA) and plots. Further, characterization of machined surface carried out through FESEM. Figures 5.4–5.6 depict the

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FE-SEM images of the machined small holes in alumina 95 ceramic through VR-ECDM process.

FIGURE 5.4

Micrograph of the machined hole at trial no. 31 (Mag: 59X).

FIGURE 5.5

Micrograph of the machined hole at trial no. 31 (Mag:141X).

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FIGURE 5.6

Electro-Micromachining and Microfabrication

Micrograph of the machined hole at trial no.: 33 (Mag: 51 X).

5.6.1 TAGUCHI ANALYSIS Based on the pilot experimentation, further, experiments were conducted using Taguchi L27OA, orthogonal array provides optimal level of each factor and design of experiments helps to reduce in number of experiments which saves time and cost of investigators [6, 28]. The intention of objective of these experiments was to investigate the effects of the input factors on output characteristics. Table 5.3 represents the experimental design for 27 runs which is obtained through MINITAB 17 statistical software based on randomization of the experimental runs. Smaller-the-better (SB), larger-the-better (LB), and nominal-the-better (NB) are the three types of quality characteristics in the analysis of S/N ratio. 5.6.1.1 SMALLER THE BETTER In this case, the objective function is to reduce the occurrences of undesirable characteristics such as thrust force, torque, overcut quality, and so on. The S/N ratio must be calculated using the Eqn. (1): 1

n



η = –10 log  ∑ yi   n i =1  2

(1)

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5.6.1.2 LARGER THE BETTER In this case, the requirement is to maximize gains such as automobile mileage, shielding material resistance, composite strength, and so on. The S/N ratio is as follows: 1 n 1  ∑ 2  n i −1 yi 

η = –10 log 

(2)

5.6.1.3 NOMINAL THE BETTER This case is used when the ideal quality of responses is computed to a specific nominal value. In an automobile, for example, the size of the piston rings must be nominal to ensure good quality and precision. Such type S/N ratios can be computed using Eqn. 3.  1 n yi 2  ∑ 2  n i =1 s 

η = –10 log 

(3)

where, η is the resultant S/N ratio; yi is the ith value of the measured response; s2 is the variance of the output responses; n is the number of observations. Experiments were carried out under constant environmental conditions, with specific focus placed on various noise factors that can affect the performance of VR-ECDM machining of alumina ceramics. Estimating the sensitivity in a suitable manner for any combination of process parameters is critical. Proper selection of testing conditions was taken care to minimize the sensitivity to noise factors. Controlling the effects of whole noise factors is impracticable, some of the factors associated with the VR-ECDM machining of alumina ceramics are listed below: • • • • • •

Variation in tool travel distance while machining; Discrepancies in material thickness; Ambient conditions during the experimentation; Change in operator’s behavior during the machining; Variation in the control factors over the inbuilt machine inaccuracies; Variation in controlling the performance of the machine.

The experimental outcomes were measured and obtained using a variety of standard measuring instruments and devices. Experiments were conducted based on design and responses were measured carefully using measuring instruments. MRR, TWR, and DOC are considered as response

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Electro-Micromachining and Microfabrication

characteristics. MRR need to be maximized hence the LB characteristics is applied. Tool wear and DOC need to be minimum hence, SB characteristics is used. TABLE 5.3

Taguchi Experiment Results

Run Frequency Voltage

Duty Electrolyte RPM MRR TWR DOC Factor Conc. (mg/min) (mg/min) (mm)

1

18

88

77

25

220

0.071

1.191

0.3799

2

18

88

77

25

240

0.041

1.315

0.2650

3

18

88

77

25

300

0.052

1.182

0.2484

4

18

90

80

30

220

0.060

1.511

0.2775

5

18

90

80

30

240

0.081

2.856

0.1391

6

18

90

80

30

300

0.091

2.481

0.1780

7

18

95

83

35

220

0.089

2.496

0.3594

8

18

95

83

35

240

0.122

3.112

0.3311

9

18

95

83

35

300

0.116

3.221

0.1998

10

19

88

80

35

220

0.100

3.456

0.3272

11

19

88

80

35

240

0.060

3.622

0.1891

12

19

88

80

35

300

0.101

3.421

0.2001

13

19

90

83

25

220

0.112

2.096

0.1234

14

19

90

83

25

240

0.093

2.041

0.0754

15

19

90

83

25

300

0.092

2.000

0.1468

16

19

95

77

30

220

0.091

2.334

0.2028

17

19

95

77

30

240

0.066

2.616

0.1895

18

19

95

77

30

300

0.066

2.136

0.1394

19

20

88

83

30

220

0.114

1.271

0.3990

20

20

88

83

30

240

0.089

1.001

0.3001

21

20

88

83

30

300

0.133

1.726

0.1890

22

20

90

77

35

220

0.040

1.561

0.0486

23

20

90

77

35

240

0.047

0.892

0.0120

24

20

90

77

35

300

0.046

2.093

0.0593

25

20

95

80

25

220

0.089

1.191

0.1782

26

20

95

80

25

240

0.112

1.566

0.1619

27

20

95

80

25

300

0.111

1.571

0.1101

In the Taguchi method, the factor with the highest S/N ratio is more robust and less susceptible to noise, so the process involves the maximization of

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S/N ratios. The statistical software MINITAB 17 was used for the analysis, and the results are shown in Table 5.4. TABLE 5.4

S/N Ratio of Response Variables

Run

MRR (dB)

TWR (dB)

DOC (dB)

1

–23.0981

–1.5108

8.4088

2

–27.9589

–2.3786

11.5385

3

–25.6429

–1.4377

12.1006

4

–24.4371

–3.5861

11.1378

5

–21.9383

–9.1122

17.1398

6

–20.9152

–7.8891

14.9966

7

–20.9152

–7.9415

8.8908

8

–18.4165

–9.8584

9.6035

9

–18.7861

–10.1572

13.9834

10

–19.9998

–10.7704

9.7062

11

–24.4372

–11.1767

14.4704

12

–19.9992

–10.6806

13.9795

13

–19.0837

–6.4238

18.1808

14

–20.9152

–6.1927

22.4642

15

–20.9152

–6.0207

16.6691

16

–20.9152

–7.3608

13.8628

17

–23.7418

–8.3495

14.4525

18

–23.7419

–6.5926

17.1211

19

–18.7861

–2.0762

7.9828

20

–20.9157

0.0001

10.4577

21

–17.7212

–4.7386

14.4775

22

–27.9591

–3.8625

26.2853

23

–26.7449

1.0010

38.4892

24

–26.9358

–6.4112

24.5537

25

–21.1103

–1.5109

14.9868

26

–19.1721

–3.8903

15.8205

27

–19.1721

–3.9180

19.1722

The analysis of VR-ECDM process conducted for evaluating the effects of voltage, frequency, electrolyte concentration, duty factor and tool rotations on the output responses of MRR, TWR, and DOC. Experimental results were

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Electro-Micromachining and Microfabrication

analyzed through the ANOVA technique to identifying the most significant process factors. 5.6.2 ANALYSIS OF INPUT FACTORS BASED ON TAGUCHI METHOD 5.6.2.1 ANALYSIS OF MATERIAL REMOVAL RATE (MRR) ANOVA is a useful tool for identifying the most important process variables in the machining process. Table 5.5 shows the ANOVA results for MRR. The p-values for voltage and duty factors are less than 0.05, indicating that both parameters are significant at the 95% confidence level. TABLE 5.5

Analysis of Variance for MRR

Source

DF

Seq SS

Adj SS

Adj MS

F-Value

P-Value

Freq

2

3.907

3.907

1.954

0.70

0.512

Voltage

2

31.695

31.695

15.847

5.66

0.014

Duty factor

2

148.502

148.502

74.251

26.52

0.000

Concentration

2

7.008

7.008

3.504

1.25

0.313

Rotation

2

6.574

6.574

3.287

1.17

0.334

44.790

2.799

–

–

Pure error

6

44.790

Total

16

242.476

Note: S = 1.6729; R-Sq = 81.50%; R-Sq (adj) = 70.00%

Figure 5.7 depicts a main effects plot for MRR of input factors, with the points in the plots representing the means of responses at the individual levels of each factor. A reference line drawn at the grand mean of values for response date. It is noticed from the plot that MRR increased with the increase in frequency from 18 to 19 kHz and further it is almost constant. This may initially be due to the fact that resonant frequency is 18 kHz at which the amplitude of vibrations is maximum. The high amplitude may disturb the bubbles formation around the tool. As the amplitude of vibrations reduces the electrochemical reactions are more stabilized hence, it tends to increase in MRR. Applied voltage is second major affected in the improvement of MRR. At low voltages (88–90 V) less electrochemical reactions taking place and less sparks generated which in turn causes low etching effect and hence the MRR

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Mean of MRR (mg/min)

is less. At voltage above 90 V the MRR increased and is found to be 0.095 mg/min. From the plot it is noticed that duty factor played a very prominent role in increasing of MRR. As the duty factor increased the sparking period is delayed which leads to very fast reactions and hence increased in MRR.

FIGURE 5.7

Main effects plots for means of material removal rate.

Mean of SN ratios

Table 5.3 shows the experimental design and the corresponding responses. As a result, the mean S/N ratios for each level and response were calculated and are shown in Table 5.4. Figure 5.8 shows the main effects plot for the S/N ratio for MRR.

FIGURE 5.8

Main effects plots for S/N ratio of MRR.

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Electro-Micromachining and Microfabrication

The mean to standard deviation ratio is represented by S/N ratios, and the higher the S/N ratio, the better. The variance of the frequency around the desired mean is smaller. The comparative importance of the process factors affecting the MRR determined by computing the delta value for each parameter. The first rank was assigned to the highest delta value, and so on. Table 5.6 shows that the main factor affecting MRR is duty factor, followed by voltage. Based on the results of the S/N ratios, the optimal values of the process parameters for the quantified range. TABLE 5.6

Response Table for Signal to Noise Ratios MRR (Larger is Better)

Level Frequency Voltage

Duty Factor Electrolyte Concentration Tool Rotation

1

–22.461

–22.061

–25.189

–21.899

–21.81

2

–21.53

–23.32

–21.24

–21.46

–22.69

3

–22.06

–20.66

–19.61

–22.69

–21.54

Delta 0.930

2.651

5.591

1.230

1.161

Rank 5

2

1

3

4

The optimal factors for MRR are found to be frequency level 2 (19 kHz), Voltage level 3 (95 V), Duty factor level 3 (83%), concentration level 2 (30 wt.%) and tool rotation level 3 (300 rpm). 5.6.2.2 ANALYSIS OF TOOL WEAR RATE Analysis of tool wear is necessary to understand the effects of process variables on tool. Table 5.7 shows that ANOVA is significant for frequency, duty factor, and electrolyte concentration. Main effects plots of tool wear rate are shown in Figure 5.9. TWR varies with the change in frequency, it is due to intense frictional heat produced between the tool electrode and the auxiliary electrolyte. Beyond 18 kHz the amplitude of vibrations reduces however the increase in tool vibration frequency contributes to tool wear because of increased frictional heat. The optimal parameters for tool wear rate are found to be frequency level 3 (20 kHz), Voltage level 1 (88 V), Duty factor level 1 (77%), electrolyte concentration level 1 (25 wt.%), and tool rotation level 1 (220 rpm). Always TWR must be as low as possible, hence for analyzing TWR by Taguchi method, SB criteria is applied (Table 5.8). Figure 5.10 demonstrates the impact of the process variables on TWR during the machining alumina 95 ceramics in VR-ECDM process.

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Analysis of Variance for TWR

Source

DF

Seq SS

Adj SS

Adj MS

F-Value

P-Value

Freq

2

130.281

130.281

65.140

18.53

0.000

Voltage

2

13.1386

13.1386

6.593

1.88

0.186

Duty factor

2

37.507

37.507

18.753

5.33

0.017

Concentration

2

74.571

74.571

37.286

10.60

0.001

Rotation

2

9.270

9.270

4.635

1.32

0.295

Pure error

6

56.259

56.259

3.516

–

–

Total

16

321.0266

Mean of TWR (mg/min)

Note: S = 1.875; R-Sq = 82.50%; R-Sq(adj) = 71.50%.

Main effects plots for means TWR (mg/min).

FIGURE 5.9

TABLE 5.8

Response Table for Signal to Noise Ratios TWR (Smaller is Better)

Level

Frequency Voltage

Duty Factor Electrolyte Concentration Tool Rotation

1

–5.987

–4.974

–4.100

–3.697

–5.005

2

–8.174

–5.389

–6.948

–5.523

–5.551

3

–2.823

–6.620

–5.934

–7.762

–6.427

Delta

5.350

1.646

2.849

4.065

1.424

Rank

1

4

3

2

5

Electro-Micromachining and Microfabrication

Mean of SN ratios

104

FIGURE 5.10

Main effects plots for S/N ratio of TWR (dB).

5.6.2.3 ANALYSIS OF DIAMETRIC OVERCUT (DOC)

Mean of DOC (mm)

Figure 5.11 demonstrates the main effects plot, and it is noticed that all the input factors influence the overcut of machining holes on alumina 95 ceramics through VR-ECDM. DOC is observed to be reducing with the increase in tool frequency and tool rotation however, it increased when the duty factor increased.

FIGURE 5.11

Main effects plots for means of DOC (mm).

Table 5.9 represents the ANOVA table for DOC. The tool frequency and rotation cause the decrease in the overcut due to high frequency quickens the sparking effect which leads to increase in the sparks at the tool bottom and less

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sparking at the tool rim. Also, as the tool feed growths the time of frictional contact with the electrolyte decreases which causes lessening in overcut. The R2 value of 86.40% designates that the predictors measured in the model elucidates 86.4% of the variations in the overcut. However, the adjusted R2 value of 77.80% indicates that the number of indicators considered in the model can be increased to improve the predictability of the model. TABLE 5.9

Analysis of Variance for DOC

Source Freq Voltage Duty factor Concentration Rotation Pure error Total

DF 2 2 2 2 2 16 26

Seq SS 230.65 444.21 121.26 81.79 75.61 150.53 1104.05

Adj SS 230.65 444.21 121.26 81.79 75.61 150.53

Adj MS 115.325 222.106 60.631 40.896 37.804 9.408

F-Value 12.26 23.61 6.44 4.35 4.02 –

P-Value 0.001 0.000 0.009 0.031 0.039 –

Note: S = 1.875; R-Sq = 86.40%; R-Sq (adj) = 77.80%.

Referring to the S/N ratios given in Table 5.10 and Figure 5.11 the most influential parameter for DOC is discovered to be frequency (Level 3, 20 kHz), voltage (Level 2, 90 V), duty factor (Level 1, 77%), electrolyte concentration (Level 3, 35 wt.%) and tool rotation (Level 2, 240 rpm). Figure 5.12 depicts the impact of the process variables on DOC during the machining alumina 95 ceramics in VR-ECDM process. TABLE 5.10 Response Table for S/N Ratio DOC Level 1 2 3 Delta Rank

Frequency 11.9811 15.66 19.14 7.1611 2

Voltage 11.46 21.10 14.21 9.641 1

Duty Factor 18.53 14.60 13.63 4.901 3

Electrolyte Concentration 15.48 13.51 17.77 4.261 4

Tool Rotation 13.27 17.16 16.34 3.891 5

5.7 CONCLUSIONS In this work, the machining of alumina 95 ceramic was carried out using VR-ECDM. The Taguchi design was used to conduct experiments by

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Mean of SN ratios

Electro-Micromachining and Microfabrication

FIGURE 5.12 Main effects plots for S/N ratio of DOC (dB).

considering five input factors with three levels each. Output responses were selected as MRR, TWR, and DOC. Single objective optimization of all responses was studied with the help of Taguchi analysis. The impact of the process variables on the output product was determined using ANOVA and ranked based on the importance of the variables on the parameters by calculating the delta values. Surface characterization of a few samples was studied. The following conclusions are ascertained: • • •

High voltage (above 90 V) resulted into an MRR of 0.095 mg/min, lowest tool wear rate as well as minimum DOC was observed for 20 kHz. From ANOVA table it is observed that only tool rotations are significant. Better hole circularity was observed due to the combined effects of vibro-rotary tool mechanism for machining alumina 95 ceramics.

KEYWORDS • • • • •

µ-ECDM diameter of overcut magnetic feld material removal rate micro-features

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• • • • •

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optimization technique Taguchi analysis tool wear rate tool-rotation vibro-rotary

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CHAPTER 6

SIMULATION ANALYSIS DURING LASER MICROGROOVING OF ALUMINA CERAMIC SUDHANSU RANJAN DAS and DEBABRATA DHUPAL Department of Production Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, India

ABSTRACT In the world of micro-manufacturing, machining micron-size features with much accuracy, excellent quality, and high precision within less time is a challenge that is being successfully faced by laser technology. Nanosecond micromachining assisted by laser faces a drawback in the form of HAZ on the micro-machined material, which becomes a hindrance in achieving good precision and accuracy, which is an adequate requirement of micro-machined materials. The objective of this research is to determine the effect of the input parameters such as laser beam temperature, air pressure, and laser beam pulse width on the thermal properties and structural properties of the micro-grooved on alumina ceramic using ANSYS. The thermal properties include spatial distribution of temperature and generation of heat flux during the laser microgrooving process. The structural properties include the generation of normal stress, stress intensity, equivalent stress, strain, and deformation that the material undergoes during the microgrooving process. Also, an attempt has been made to determine whether these responses are in any way contributing to the deviation of groove width and depth during the microgrooving operation. In different parametric settings, the developed different thermal models showed certain changes in the structural properties, Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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which has affected greatly the dimensional deviations of the microgroove. The analysis shows that the heat-affected zone (HAZ) can increase with an increase in the laser beam temperature, and it also increases with increasing pulse width. However, the air pressures employed in this analysis have no effect on the generated heat flux or on the temperature distribution but have little effect on the structural properties of the micro-machined material. The proposed model will be useful in the field of laser microgrooving to optimize pre-cited machining parameters. 6.1 INTRODUCTION Non-traditional machining processes paved its way into the manufacturing industry due to many reasons. One of them was the drawbacks of the traditional machining processes such as machining hard and vulnerably delicate materials. The need for machining complex shapes without damaging the work piece as well as preventing unnecessary material removal was a tough challenge faced by many designers and manufacturers. Introduction of laser marked the beginning of a new era in the field of manufacturing. Machining by lasers can procure variety of products depending on the size of the work piece used and the level of precision and fineness it requires. The wear of the tool and the work piece diminishes as there is no contact in this process. Recent studies have also shown how damage can be detected over a large area such as an aircraft or over an aircraft’s body with the help of “Lamb Waves,” that are emitted during laser ablation, with the help of this technology, damage area can be inspected and detected, and also it is a non-contact damage detection method. This has been proposed by Hosoya et al. [1] and also a three-dimensional molecular dynamics model has been presented for simulation of a thin micro-hole by laser ablation [2]. The present work generally digs deeper into the analysis of the ceramic material that is being laser micro-machined and how it affects the structural properties of the material such as equivalent strain, normal strain, deformation, and thermal properties such as heat flux and spatial temperature distribution. Micromachining has always been an active area of research and investigations; however very little research has been done on materials which are difficult to be machined. These materials include ceramics, glasses, hardened steels, stainless steel and so on. Experimental investigations may include problems like the unpredictable tool failure, excessive tool wear, surface, and sub-surface cracks, so these kinds of investigations can be carried by a theoretical approach such as using simulation techniques [3, 4] or using

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mechanical analysis software such as ANSYS (analysis software). Another fact that cannot be overlooked is the growing industrial demands for these kinds of materials due to their versatility in use and easy availability, which makes it a significant area of research. Dhupal et al. [5] have experimentally created microgrooves on an alumina ceramic by using an Nd:YAG laser and has shown how the groove width and depth deviation depend on laser turning parameters. This study included experimental as well as computational techniques for the analysis and optimization of the process involved. Knowles et al. [6]; and Gamage et al. [7] have presented growing needs of miniaturization, the applications of laser micromachining and detailed study of the process of laser ablation in metals, ceramics, and polymers. The benefit of high laser intensity for efficient laser micro-fabrication has been discussed. Sheng et al. [8] have utilized the concept of element life and death theory of finite element analysis. Simulative investigations on a plastic joint created via. Laser ablation has been studied by Engelmann et al. [9] that presented the applications of lasers in using metals and plastics together where a laser beam first irradiates a metallic plate creating microstructure, and then molten plastic is interlocked into the microstructures after curing. This kind of application would solve a lot of problems faced in the automotive industry. Zheng et al. [10] has developed an analytical solution to non-Fourier heat conduction, assumed to be in a local surface of a semi-infinite medium. Wan et al. [11] has recently carried out an experimental investigation using femtosecond laser, in order to obtain the image frame of a microscale structure which is very small, they have utilized a multi-image mosaic. Swain et al. [12] has recently analyzed the machining characteristics by using microend mills, the material that is under investigation is a nickel-based alloy known as Nimonic 75. To find an optimized combination of the machining characteristics, a design of experiments has also been done. Xu et al. [13] has performed a study of damage features of polycrystalline silicon based on nanosecond pulse laser irradiation under varying wavelengths. Very few research has been done in the field of laser micromachining using the numerical approaches such as finite element modeling and ANSYS. Modest et al. [14] has studied the laser drilling and the laser scribing process and has utilized the finite element method (FEM) to assess where and what kind of stresses occur during both the processes. It predicts the temporal temperature fields and the receding surface of the ceramic. The analysis allows the prediction of stresses during continuous wave (CW) and pulsed laser operation under the influence of a variety of cutting speeds. Kingrey et al. [15] have considered the problem of thermal stresses developed from both

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the theoretical and experimental points of view. They have made an attempt to measure a material property known as resistance to thermal stresses and also a detailed study of factors that contribute to thermal stress resistance. Illiyama et al. [16] has investigated the effect of thermal spalling on a refractory product. The main objective of their investigation was to provide a method which can improve the spalling resistance of the ceramic products that is to be used in the refractory furnaces. While investigating different ways of analytical and numerical approaches to describe different laser micromachining process, in-depth knowledge of the underlying principles of the laser beam, the material in contact with the laser and the beam-material interaction is required. Hsieh et al. [17] has created micro-fluidic channels on biodegradable materials using the process of direct laser micromachining and studying the process of laser ablation in this process. Some recent investigations DAddona et al. [18] have shown the prediction of the process characteristics of laser milling if poly-methyl-methacrylate based on the application of a computing technique such as artificial neural network (ANN) and fuzzy data to predict the depth and surface roughness of the material. In the present work, a laser micro-grooving process has been considered. The micro-grooving is done on the surface of an alumina ceramic. The main objective is to determine the effect of the input parameters such as the laser beam temperature, the air pressure and the pulse width of the laser beam on the thermal properties and the structural properties of the material. The thermal properties include the spatial distribution of the temperature and the generation of heat flux after the micro-grooving process. The structural properties include the generation of normal stress, stress intensity, maximum principal stress, strain, and deformation that the material undergoes during the micro-grooving process. We have also attempted to determine whether these responses are in any way contributing to the deviation of groove width and depth during the micro-grooving process. Response surface methodology (RSM) has been used here to find out all the possible combinations of the three different input parameters, which can be implemented in the analysis that is to be done by using the ANSYS software, in order to obtain the main objective of this work. 6.2 MATERIALS AND METHOD The workpiece material, considered in the present study is alumina (Al2O3) ceramic. The main properties of alumina (Al2O3) ceramic are shown in Table 6.1. The design of experiments is a technique which has been in use in the manufacturing industry to improve the quality of the experiments and

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analysis performed. There are certain design methodologies used for the testing purposes. Computers solve complex statistical analyzes. Therefore, the user or the operator must always have an in-depth understanding of the underlying principles and appropriateness of the diverse methods. DoE is a powerful statistical tool. Design methodology basically studies a unique condition and develops a method specifically for that condition. It encourages innovative and collaborative thinking to work through a proposed method to achieve best results. It is commonly used in technical fields. There are three input parameters taken into consideration in this process. The three input parameters with their different values are given in Table 6.2. Table 6.3 lists all the possible combinations of the input parameters using RSM. TABLE 6.1

Mechanical and Thermal Properties of Alumina

Properties

Values

Melting temperature

2,050°C

Density

3,984 kg/m3

Specific heat

755 J/kg-K

Thermal conductivity

33 W/mK

Tensile strength

267 MPa

Bulk modulus

257 MPa

Shear modulus

167 MPa

Vaporization range

2,977°C to 3,530°C

TABLE 6.2

Different Input Parameters and Their Values

Parameters

Levels –2

–1

0

1

2

2

4

6

8

10

Air pressure (kgf/cm )

0.5

0.9

1.3

1.7

2.1

Laser beam temperature (°C)

2,977

3,115

3,253

3,391

3,530

Pulse width (ns) 2

As per the standard specified by CIRP, the machining in the range of dimensions less than 0.5 mm is called micromachining. For performing quality micromachining operations in accordance with pertaining standards on advanced engineering material, a high-power pulsed Nd:YAG laser setup (Sahajanand Laser Technology, SLT-SP-2000) has been utilized with required modifications for experimentation. The specifications of laser machining setup are given in Table 6.4. A microprocessor-based fixture was developed

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indigenously to generate desired profiles on the work surface. The CNC pulsed Nd:YAG laser machining system comprises of various subsystems, i.e., laser beam generation unit, CCD camera, cooling system CCTV, X-Y-Z controller, power supply system, air supply system and developed fixture, etc., shown in Figure 6.1. The detailed of laser generation process is shown in Figure 6.2. TABLE 6.3

Combinations of Input Parameters Using Response Surface Methodology

No. of Coded Values Possible Air Laser Beam Combinations Pressure Temperature 1 –1 –1 2 1 –1 3 –1 1 4 1 1 5 –1 –1 6 1 –1 7 –1 1 8 1 1 9 –2 0 10 2 0 11 0 –2 12 0 2 13 0 0 14 0 0 15 0 0 TABLE 6.4

Pulse Width –1 –1 –1 –1 –1 1 1 1 0 0 0 0 –2 2 0

Air Pressure (kgf/cm2) 0.9 1.7 0.9 1.7 0.9 1.7 0.9 1.7 0.5 2.1 1.3 1.3 1.3 1.3 1.3

Actual Settings Laser Beam Temperature (°C) 3,115 3,115 3,391 3,391 3,115 3,115 3,391 3,391 3,253 3,253 2,977 3,530 3,253 3,253 3,253

Pulse Width (ns) 4 4 4 4 4 8 8 8 6 6 6 6 2 10 6

Specifications of the CNC Pulsed Nd:YAG LMS

Specification Type of laser Operation mode Wavelength Laser beam mode Q-switch (type) used Mirror reflectivity Laser beam spot diameter Beam diameter 1/e2 PW

Description Nd:YAG laser Q-switched (pulsed) 1,064 nm Fundamental mode (TEM00) Acoustics optic Q-switched Rear mirror 100%, and front mirror 80% 1 mm 1 mm 120 ns–150 ns

Simulation Analysis During Laser Microgrooving

FIGURE 6.1

117

Laser microgrooving machine tool setup.

6.3 MATHEMATICAL MODELLING A model is developed by using mathematical concepts and mathematical languages. A model basically helps to explain a system and to study the effects of different parameters on the components of the system and to make predictions based on that behavior. The accuracy of the model can be determined based up to the degree the system behaves as predicted by the model. The lack of agreement encourages advanced research. A mathematical model represents a set of variables and a set of equations. These equations explain the relationship between the set of variables, these variables represent main properties of the system which are important to define the system’s behavior. For nanosecond pulses, the classical heat conduction theory based on the assumption micro-machined groove are of a single temperature apart from the room temperature is valid. Following assumptions will be used in the present model: (i) the model is assumed to be a steady-state process; (ii)

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the micromachining is assumed to be a 3-D material removal process as it is a grooving process; (iii) the alumina ceramic material is considered to be homogeneous and isotropic; (iv) the process is assumed to be linear; and (v) the model is assumed to be a deterministic type of model.

FIGURE 6.2

Schematic diagram of Nd:YAG laser machining system.

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1. Boundary and Initial Conditions: The temperature distribution in a medium depends on the conditions at the boundaries of the medium as well as the heat transfer mechanism within the medium. To describe a heat transfer problem completely, two boundary conditions (BCs) must be given for each direction of the co-ordinate system along which the heat transfer is significant [19]. Therefore, we need to specify two BCs for a one-dimensional problem, four BCs for two dimensional problems and six BCs for three dimensional problems. Figure 6.3 shows the alumina ceramic workpiece of 20 x 20 mm2 cross-sectional area on which a square groove of 200 µm width and depth is achieved by laser micromachining. Figure 6.4 shows the modified sketch of the groove to be micro-machined.

FIGURE 6.3 surface.

Pictorial view of square microgroove formed on flat alumina ceramic workpiece

Here, the laser beam temperature is uniform over the whole of the square groove. The heat conduction equation is first order in time; thus, the initial condition cannot involve any derivative, i.e., it is limited to a specified temperature which in this case is the laser beam temperature. However, the heat conduction equation is second order in space co-ordinates and thus the BCs may involve first order derivatives at the boundary as well as specified values of temperature. i.

Boundary Conditions in the x-Direction: T (0, t) = T1 (30°C, room temperature) T (Δx, t) = Tx (corresponding laser beam temperature)

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ii. Boundary Conditions in the y-Direction: T (0, t) = T1 (30°C, room temperature) T (Δy, t) = Ty (corresponding laser beam temperature) iii. Boundary Conditions in the z-Direction: T (0, t) = T1 (30°C, room temperature) T (Δz, t) = Tz (corresponding laser beam temperature)

FIGURE 6.4

Modified sketch of the square groove to be micro-machined on the workpiece.

Note: Where; Δx, Δy, Δz specifies the material removal in the x, y, and z directions, respectively. The material removal is considered to be a 3D process. Ḣx, Ḣy, Ḣz are the rate of heat conduction at x, y, and z, respectively and Ḣx+Δx, Ḣy+Δy, Ḣz+Δz is the rate of heat conduction at x+Δx, y+Δy and z+Δz, respectively. A condition that is usually specified at time t = 0 is called the initial condition. But under steady condition, the heat conduction equation does not involve any time derivatives, and thus we do not specify an ideal condition.

2. Heat Generation: A medium through which heat is conducted may involve the conversion of electrical, nuclear or chemical energy into heat or thermal energy. In case of laser machining techniques, the material is irradiated by a laser beam, this laser beam is monochromatic, less diffracted, and more focused, coherent, and electromagnetic in nature. This electromagnetic wave is absorbed by the material and is

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converted into thermal energy which results in material removal. In heat conduction analysis, such conversion processes are characterized as heat generation. Heat generation is a volumetric phenomenon. That is, it occurs throughout the body of a medium. Therefore, the rate of heat generation in a medium is usually specified per unit volume, whose unit is given by W/m3.  = ∫ g dV Watts G V

(1)

The rate of heat generation in a medium may vary with time as well as position within the medium. When the variation of heat generation with position is known, the total heat generation in a medium of a volume, V can be determined. 3. Final Governing Equation: Pertaining to Figure 6.2, the governing equation is, {Rate of heat conduction at x, y, z}–{Rate of heat conduction at x+Δx, y+Δy, z+Δz} + {Rate of heat generation inside the groove} = {Rate of change of energy content of the element}, i.e. (Ḣx + Ḣy + Ḣz) – (Ḣx+Δx – Ḣy+Δy – Ḣz+Δz) + Ġelement = ΔEelement/Δt

(2)

So, it comes down to: ∂ ∂T ∂ ∂T ∂ ∂T g =0 ∂x (∂x) + ∂y (∂y ) + ∂z (∂z ) + k

(3)

For a steady state 3-D heat conduction equation also called the Poisson’s equation. The convective and radiation heat losses are neglected here as it is quite negligible when machining at such small micron level. 6.4

RESULTS AND DISCUSSION

6.4.1 SIMULATION RESULTS OF THERMAL AND STRUCTURAL PROPERTIES OF LASER MICROGROOVE In the present work, a micron sized groove of depth and width 200 µm is being machined by an Nd:YAG laser of wavelength 1,064 nm. This work enlists the theoretical findings and predictions that had been done based on finite element analysis done with the help of the mechanical ANSYS. After setting the input parameters accordingly using RSM and constructing the finite element model, the model is solved, and various results of the thermal properties and structural properties taken into consideration for analysis. The images given below shows the different views of the temperature distribution

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and heat flux of a microgroove analyzed under a certain combination of process parameters. The temperature of the groove has been set according to different laser beam temperature stated in the possible combinations. The temperature distribution images give an idea as to how the temperature is spatially distributed along the groove at the end of 10 ns. As the time is set in terms of nanoseconds, all the thermal and structural properties are viewed within the 10 ns time scale. It provides us an idea and gives us the value of a specific temperature at specific time duration up till 10 ns duration. The view of temperature distribution, as shown in Figure 6.5(a) gives an insight of the spatial temperature distribution along the walls of the groove and the base of the groove, respectively. Figure 6.5(b) shows the heat flux generated during laser microgrooving for a particular set parameter of the laser beam temperature and air pressure. The heat flux generated after each pulse width can be known, as the value alongside the views shows the different values of the heat flux starting from 1 ns to 10 ns. The more the heat flux is, the more is the material removal taking place from that portion of the square groove. The more the heat flux is, the more is the HAZ is a hindrance to laser micro-machined features thereby these regions will somewhat affect the deviation in the width and depth of the groove. Normal stress is the stress that occurs when the axial force is applied to an element. The value of the normal force for each prismatic section is the force divided by cross-sectional area. Stress is a measure of the inner strength of a continuous particle of matter. Significant stress can occur even if deformity is minor or non-existent. Stress can exist by change in temperature which is the main case in the present work or chemical composition or by external electromagnetic field. Figure 6.5(c) shows the normal stress distribution along the groove and the particular values of the normal stresses at different points and at different time intervals. Stress intensity provides a stress near the tip of the crack caused by a remote load or residual stress. This is theoretical design that is commonly applied to homogeneous, linear, elastic, and useful materials and represents the failure criterion for brittle material and critical technology in the field of damage resistance. This can also be applied to low yield materials in the upper part of the crack. Figure 6.5(d) gives an insight at stress intensity at different points of the groove, if the maximum stress for crack formation is known, then the stress intensity values at different points on the groove can give an idea as to where the crack will generate first and how it will propagate. This crack formation can be referred to thermal spalling found in ceramics. Figure 6.5(e) depicts the distribution of all the Von-Mises stress. The overall equivalent stress is often used by designers to ensure that designs

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are tolerant of certain load condition. Von-Mises stress is taken as safe place for designers. Based on this, the engineer can determine whether the project has filed or not. The Von-Mises stress is maximum if the material does not exceed the strength of the material. It fits in most cases, especially suitable for ductile material.

FIGURE 6.5 View of FE simulation for thermal properties ((a) temperature distribution; (b) heat flux), and structural properties ((c) normal stress; (d) stress intensity; (e) equivalent stress; (f) equivalent strain; and (g) deformation) along the base of laser micro-grooved surface on alumina ceramic at temperature of 3388.3 K and pressure of 0.5 kgf/cm2.

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The equivalent elastic strain or most commonly known as the equivalent Von-Mises strain is nothing but the equivalent or Von-Mises stress divided by the Young’s modulus. The Von-Mises strain gives an idea if there is any change or any deviation in the formation of the groove due to the equivalent or overall strain induced during laser micromachining. The values of the strain induced will be quite less as the equivalent stress induced or the Von-Mises stress induced is also small, but close analysis of these values determine the very small deviation that the groove had undergone, and it has also affected its dimensional accuracy. Figure 6.5(f) shows the images of the equivalent or Von-Mises strain induced and distributed along the groove during laser micromachining. In material science, deformation represents all changes in temperature that is deformation energy. In this study it is carried out by heat. The most important factor determining temperature are the mobility of structural defects point vacancies, grain boundaries, line, and screw dislocation, twins, and staking faults in both non crystalline and crystalline solids. The transfer or displacement of such mobile defects in thermally activated and limited by diffusion rate of atoms. Depending on material type, geometry and size of the object, and the applied forces, various types of deformation may result. The deformation in this case is very as it is a micron size feature that we are dealing with. The deformation can be related to strain produced. Theses may also contribute to the deviation produced in the groove depth and groove width. Figure 6.5(g) shows the little deformation that the groove undergoes during the laser microgrooving process. Figures 6.6(a) and (b) shows the normal stress distribution along the groove and its corresponding normal strain that is induced along the groove, respectively. As it is known when the laser beam irradiates the pulse width then compressive stresses are generated and when it is cooled down tensile stresses are generated. The analysis of the groove has been done after it has undergone the laser micromachining process. Additionally, it can be seen from Figure 6.6(a) how the tensile and compressive stresses are distributed, the tensile stresses can be seen along the groove where deformation is seen, the deformation is mostly seen in the region where tensile stresses are generated and as we move away from the laser beam irradiated surface, distribution of compressive stresses can be seen. There is a sign reversal that can be seen from the pulse width of 8 ns to 1 ns in this analysis. Figure 6.6(b) shows the corresponding normal elastic strain generated due to the normal tensile and compressive stresses induced. The normal strain is the deviation of the original dimension, and it is the ratio between the change in dimension and deviation in the dimension to the original dimension. The corresponding

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strain that is generated shows that the tensile stresses do not generate much of strain, therefore the strain generated due to tensile stresses is quite less and they do not contribute to the deviation in the depth of the groove, the strain distribution along the width of the groove can be seen, but the distribution of the strain for the corresponding tensile stresses is quite less, and from Figure 6.6(e) strain distribution along the depth is only sideways and quite less. The strain is measured per meter of the material. The maximum normal strain that it will induce is 1.4151e-15 (m/m) per meter which is 1.4151e-12 per millimeter (mm/mm). Figure 6.6(c) shows the distribution of the equivalent stress or overall stress or Von-Mises stress. When a finite element analysis is done, there are various stress components acting on element, it is indeed difficult to analyze all the stress components simultaneously at every point. A method of combining various stress components into a single figure is needed; so that comparison can be drawn with yield strength of material obtain from simple tension test. Von-Mises proposed that stress which have any influence on the required dimension shall only for checked for yielding. Von-Mises stress is a scalar quantity with magnitude only. This equivalent stress gives an idea about its distribution along the depth and width of the groove, and Figure 6.6(d) shows the corresponding equivalent strain clearly along the groove. Distribution of the equivalent strain shows that the overall strain during the micromachining process affects the depth as well as the width of the groove. As can be seen from Figure 6.6(a), the strain that is generated during 2 ns, 3 ns, and 4 ns pulse width affects the depth as well as the width of the groove, the shape of the distribution of strain during these pulse widths shows us the way the deviation from the depth and width may occur. The strain that is induced during these pulse times is very small, as the micromachining is being done on a micron level, the strain values give an approximate idea of deviation from the original depth and width of 20 µms. Similarly, as can be seen from the same figure, the strain that is generated during 10 ns, 9 ns and 8 ns pulse width affects the width of the groove but their contribution towards deviation is quite small as can be seen from their respective distribution along the width of the groove. Figure 6.6(e) depicts the total deformation that the groove has undergone during the laser microgrooving process. The deformation that the groove has undergone is 3.3437e-15 per mm. The deformation that is seen here is the plastic deformation that had occurred due to the generation of compressive and tensile stresses due to the laser beam irradiation. The deformation also gives the value of contribution to the deviation in the depth as well as the width of the groove.

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FIGURE 6.6 View of FE simulation for (a) normal stress; (b) normal strain, stress intensity; (c) equivalent stress; (d) equivalent strain; and (e) deformation on the surface of groove at temperature of 3526.5 K and 0.5 kgf/cm2.

6.4.2 COMPARISON OF THERMAL AND STRUCTURAL PROPERTIES BASED ON INCREASE IN THE LASER BEAM TEMPERATURE Table 6.5 shows the temperature distribution values and the heat flux generated as well as seven other structural properties such as equivalent elastic strain, stress intensity, normal elastic strain, normal stress, deformation, and

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equivalent elastic stress based on two optimal conditions on which the groove is analyzed. The values stated in the table are taken for two cases. In the first case, the laser beam temperature is set at 3388.3 K, 10 ns PW and AP of 0.5 kgf/cm2, the values for all the thermal and structural properties for this particular combination are stated in the first column. In the second case, the laser beam temperature is set at 3526.1 K, keeping the AP and the PW same as the first case, the values for this case are stated in the second column. The spatial temperature distribution along the groove will always remain same for all the possible combinations of MG, but it is set according to the maximum temperature along the groove and that is the laser beam temperature. The temperature distribution along the groove will be set according to the temperature at which laser beam irradiates the WP surface and removes the material. It can clearly find from values stated in the table, that more heat flux is generated for the second case with increased laser beam temperature, which shows laser beam temperature increases, the heat flux generated will be more, therefore the material removal will also be more as well as the HAZ will be more [20]. However, the behavior in which the heat flux is generated and distributed along the groove will remain the same in both the cases. TABLE 6.5 Comparison of Thermal and Structural Properties between Two Parametric Conditions based on Increase in Temperature Thermal and Structural Properties

MG at 3388.3 K, 0.5 kgf/cm2 and 10 ns

MG at 3526.1 K, 0.5 kgf/cm2 and 10 ns

Behavior

Temperature

3388.3 K

3526.1 K

Increases

Heat flux

1.5106e9

1.6459e9

Increases

Deformation

3.3437e-18

3.3437e-18

Remains the same

Normal elastic strain

1.4151e-15

1.4151e-15

Remains the same

Equivalent elastic strain

9.873e-15

9.873e-15

Remains the same

Equivalent stress

3.7036

3.7036

Remains the same

Normal stress

0.22968

0.22968

Remains the same

Stress intensity

3.9273

3.9273

Remains the same

As can be seen from the values, the increase in the laser beam temperature does not quite affect the structural properties though there are three kinds of strain, and three kinds of stresses analyzed during the laser MG process. The strain induced during the process is very less and it is mostly seen to remain the same during the increase in the temperature, the same is the case of the stresses, but the stresses induced are not very small, but it is also seen to

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remain the same for most of the combinations during the laser MM process. The above comparisons are an example where the structural properties are quite unaffected by the increase in the laser beam temperature. The spatial temperature profile along the groove will always remain same for all the possible combination of MG, but it is set according to the maximum temperature along the groove and that is the laser beam temperature. The temperature distribution along the groove will be set according to the temperature at which the laser beam irradiates the WP surface and material removal takes place similarly like the first case. But there is a change seen in the variation of the structural properties taken into consideration. In the previous comparison there was no change seen in the structural properties with the increase in temperature, but in the present comparison even though there is very negligible increase in the strain related properties, there is an increase seen in the stress related properties. The table clearly indicates the increase in normal stress, stress intensity and Von-mises stress because of rise in the laser beam temperature. The deformation however is quite negligible, and it will remain the same for both the comparisons. When a laser beam strikes ceramic WP surface, there is a sudden rise in temperature, causing in thermal activation of the atoms, resulting in changes in the structural properties. Abrupt rise in temperature also induced thermal stress. In the present work, the groove is analyzed after the MM has been done, and it can be seen that the tensile stress is observed in the deformity zone and compressive stress is distributed outside it, the area in the red color signifies the tensile zone, and the negative values of stresses have begun from 8 ns PW, these negative values of stresses can be considered as compressive normal stresses, this is so because when laser beam irradiates the WP surface then compressive stresses are induced in the beginning but then tensile stresses are generated during cooling down of the groove, these kind of behavior has been seen for laser drilling and laser scribing of ceramics as investigated by Rihakova & Chmelickova [21]. This is so because when the peak power generation of laser beam density is greater than the plasma threshold of the WP, the topmost surface layer of the WP evaporates instantly to form plasma, due to this plasma formation scattering of the laser beam may also occur which may cause abnormal stress distribution along the groove and may affect the dimensional accuracy of the DP and width of the groove. The high-pressure plasma thus generated instantaneously compresses WP surface and the surface displacement caused by the compression generates a shock wave that propagates in the DP direction of the WP. The shock wave, when its pressure exceeds the yield stress of the WP which cause local plastic deformation.

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In the previous research, it has been found that normal tensile stresses had been induced parallel to the surface as well as a thick layer below which can be considered as the cause of sub-surface cracks, as in cases of laser drilling and laser scribing of materials using a focused laser beam. It is suggested that this tensile stress can be reduced to avoid stress damage by eliminating the sharpest temperature gradient by using a secondary semi-focal laser beam to heat the surrounding material. The sharp temperature gradient is the reason for induced of compressive and tensile stresses caused by large localized thermal expansion in the ceramic material. In solid materials where temperature is greater than zero Kelvin, atoms are in continuous motion. Oscillation of atoms is also affected by vibrations of neighboring atoms through bonding which may result in an increased normal stress. As the temperature rises atomic movements become vigorous resulting in higher induced stresses. Generally, ceramics have strongly bonded light atoms with high frequency of vibration in the crystal lattice. Therefore, ceramic materials undergo small deformation, but have high heat capacities and melting points. Figure 6.7 shows the effect laser beam temperature on various structural properties and the heat flux of produced microgroove. In the graphical analysis, the laser beam temperature is increased keeping the AP and PW constant. It can be seen that the graph shows an increasing trend in almost every property that has been considered, it increases the stress-related properties as well as the strain that is induced, as the material is brittle the strain and deformation that it undergoes is small, but it still increases with when laser beam temperature increases. 6.4.3 COMPARISON OF THERMAL AND STRUCTURAL PROPERTIES BASED ON INCREASE IN AIR PRESSURE The values stated in Table 6.6 are taken for two cases. In the first case, the laser beam temperature is set at 3526.25 K, PW of 10 ns and AP at 0.5 kgf/ cm2, the values for all the thermal and structural properties for this particular combination are stated in the first column. In the second case, the AP is set at 2.1 kgf/cm2, keeping the laser beam temperature and the PW same as the first case, the value for this case is stated in the second column. As can be seen from the values stated in the table, heat flux generated with AP increases due to no change in the temperature, therefore the thermal properties and are remaining unaffected but the increase in the structural properties can be remaining unaffected by the increase in the AP used, but a considerable

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increase is seen in stress related properties, i.e., the normal stress, von-mises or equivalent stress, and the stress intensity. There is increase seen in normal and equivalent elastic strain too, but as these values are quite negligible and very small, the increase is not significant. There is also increase in the deformation that the groove had undergone during MM with the increase in AP.

FIGURE 6.7 Effect of laser beam temperature on: (a) heat flux; (b) normal stress; (c) stress intensity; (d) equivalent stress; (e) equivalent strain; (f) normal elastic strain; and (g) deformation of microgrooves.

Simulation Analysis During Laser Microgrooving

131

AP is mostly introduced as a means of obtaining a clean groove. During the laser MM process, the presence of HAZ due to heat flux generation would create re-solidification and de-burring, the presence of which will affect the precision and dimensional accuracy of the groove. The presence of air would decrease the HAZ thereby enabling the formation of a clean groove. But using AP in this case has not yielded particularly to the formation of a clean groove, because it can be well visible that when air is applied in this process, the heat flux generated will remain unaffected, neither it has any changes induced in it, therefore it remains unaffected. There are five different values of AP taken into consideration in this analysis, results in values of different kinds of strain that is obtained as well as deformation is quite small as the process is being performed at such a micron level, so there is not much changes seen by the application of AP, even though there is a slight increase in these properties, it can be considered as quite negligible due to AP increases. The stress properties such as the normal stress, equivalent stress, and stress intensity can be seen to be affected by AP increase. They increase up to some extent with the AP increase. TABLE 6.6 Comparison of Thermal and Structural Properties with the Increase in Air Pressure Thermal and Structural MG at 3526.5 K, MG at 3526.5 K, Behaviors Properties 0.5 kgf/cm2 and 2.1 kgf/cm2 and 10 ns 10 ns Temperature

3526.25

3526.25

Remains the same

Heat flux

1.5782e9

1.5782e9

Increases

Deformation

1.8115e-18

3.7032e-18

Increases but the changes are quite negligible

Normal elastic strain

7.6667e-16

1.5673e-15

Increases but the changes are quite negligible

Equivalent elastic strain

5.9947e-15

1.0935e-14

Increases but the changes are quite negligible

Equivalent stress

2.0064

4.1017

Increases

Maximum principal stress 0.65373

1.3365

Increases

Normal stress

0.12443

0.25438

Increases

Stress intensity

2.1276

4.3496

Increases

Figure 6.8 shows the influence of air pressure on various structural properties and the heat flux. In the graphical analysis, the AP is increased keeping the temperature and PW constant. It can be seen that the graph shows an increasing trend in almost every property that has been considered

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if the AP is being increases. Though the main aim of using air is to obtain a clean groove but if it is affecting the structural properties in any way then this fact cannot be ignored, and proper measures should be taken so as not to increase it to such levels which can be damageable to the surface of the ceramic. It can be concluded that even though the pressure of the air is not enough to create a clean groove still it does affect the structural properties.

FIGURE 6.8 Effect of air pressure on: (a) heat flux; (b) normal stress; (c) stress intensity; (d) equivalent stress; (e) equivalent strain; (f) normal elastic strain; and (g) deformation of microgrooves produced by Nd:YAG laser treatment.

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133

6.4.4 COMPARISON OF THERMAL AND STRUCTURAL PROPERTIES BASED ON INCREASE IN PULSE WIDTH Table 6.7 shows the heat flux generated as well as five other structural properties such as equivalent elastic strain, stress intensity, normal elastic strain, normal stress, and equivalent elastic stress based on varying conditions of pulse width on which the groove is analyzed. In this analysis, the time step is set in terms of nanosecond values. TABLE 6.7 The Values of Various Thermal and Structural Properties at Different Values of PWs Pulse Heat Flux Normal Width Stress

Stress Intensity

Equivalent Equivalent Normal Elastic Stress Strain Strain

10

1.5106e9

0.13946

2.3846

2.0064

5.9947e-15

1.4151e-15

9

1.3428e9

0.056301

2.1197

1.7835

5.3287e-15

1.259e-15

8

1.1749e9

–0.026855

1.8547

1.5606

7.6626e-15

1.0886e-15

7

1.0071e9

–0.11001

1.5898

1.3376

3.9966e-15

9.2528e-15

6

8.3924e8

–0.19317

1.3248

1.1147

3.3305e-15

7.6199e-16

5

6.7139e8

–0.27632

1.0599

0.89173

2.6645e-15

6.9871e-16

4

5.0354e8

–0.35948

0.79494

0.66886

1.9984e-15

4.3542e-16

3

3.3569e8

–0.44264

0.54999

0.44593

1.3324e-15

3.7213e-16

2

1.6785e8

–0.52579

0.26604

0.22301

6.663e-15

6.4444e-17

1

0.24795e8 –0.60895

2.4298e-19

6.3256e-17

9.5946e-5 7.8823e-5

The values of the various structural and thermal properties in Table 6.5 shows that when PW increases, all the properties show an increasing behavior. As, the PW increases the more is the heat flux generated and the more time the laser beam interacts with the WP the more are the stresses generated. The structural property related to stress such as stress intensity and equivalent stress gives the different values and different aspects of the stress generated on each small finite element, combining each of these small units is the component made of. The deformation values are not stated here because this analysis had only achieved the minimum and maximum value of deformation rather than a range of values ranging from 10 ns to 1 ns. The strain values also increase with increase in pulse rate. The strain induced during the MM process also increases the deviation in the desired dimension. Therefore, the more is

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the strain induced due to more laser beam interaction time the more it may contribute to the deviation from the desired dimensions. Figure 6.9 shows the effect of laser pulse width on various structural properties and the heat flux. In the graphical analysis, the PW is increased keeping the temperature and AP constant. It can be seen that the graph shows an increasing trend in almost every property that has been considered if the PW is being increased. The reasons have been stated above. PW is quite an important parameter while analyzing any type of MM process. It plays an important part in the mechanism of ablation and material removal as well as it also affects the structural and thermal properties of the material. This interaction time has interesting effects on the properties. 6.5 CONCLUSIONS From the finite element analysis that has been done on a laser micro-machined groove using ANSYS, the following conclusions are drawn: •

•

•

With the increase in the laser beam temperature, the heat flux generation also increases which can prove to be disadvantageous, it also increases the stress generated during the laser micromachining process. There are two kinds of normal stress generated, i.e., tensile and compressive. In such a case when the tensile stresses are dominant then shear stress is not to be considered is based on a previous experimental investigation. The maximum stress that can be induced in the principal plane and the distribution of the maximum principal stresses is also shown. The strain and deformation values are quite small as alumina is a crystalline ceramic material and it does not deform easily. The thermal expansion that the groove undergoes under the effect of the thermal stresses induced as well as the strain that is induced gives an idea as to how they affect the deviation obtained in the groove depth and width. Scattering of the laser beam can also be considered as a reason contributing to the deviation from the required dimensions. The equivalent strain also increases with the increase in the various input parameters, i.e., the laser beam temperature, the pulse width and the air pressure, as well as the normal strain, but the normal strain does not contribute much to the generation of the overall strain along the groove. This can be concluded from the way normal strain is distributed and the quite small values of normal strain induced.

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135

FIGURE 6.9 Effect of pulse width on: (a) heat flux; (b) normal stress; (c) stress intensity; (d) equivalent stress; (e) equivalent strain; (f) normal elastic strain; and (g) deformation of microgrooves generated by Nd:YAG laser treatment.

•

The overall stress induced and the overall strain that is undergone by the groove can be known by considering the values of the VonMises stress and strain, and the way it is distributed along the groove

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Electro-Micromachining and Microfabrication

•

•

is also shown. The design engineer always has a specified value of stress and strain, surpassing which will cause thermal spalling and cracking in ceramics, knowing theses values if Von-Mises stress and strain beforehand will provide them with a better insight as to how to avoid cracks and reduce thermal spalling along the groove on a micro-scale. This is a tensile stress dominated groove as tensile stresses are mostly distributed along the groove after the machining is over, and the maximum principal stress that is obtained is also tensile in nature as positive values of maximum principal stress has been obtained. Not only dimensionally accurate and precise but a defect-less groove is also required while micromachining the groove, as a single mistake or mishap may fully damage the ceramic components. Grooving is one of the few processes along with laser marking, engraving, etc., which is done on the surface of the ceramic materials that are mostly used as MEMS devices or in the electronics industry. These ceramic materials are too small in structure, but if there is any manufacturing defect or problem in installation, a whole device or machine won’t be able to function properly. The applicability makes the designer understand how to apply this analysis while mostly checking for cracks, brittle fracture, spalling or any other such effect that can be a disadvantage in the usage of the ceramic materials. This increases the scope and field of application of such an analysis that has been done at a micron level.

KEYWORDS • • • • • • •

air pressure ANSYS heat affected zone (HAZ) laser microgroove micromachining pulse square grooves

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137

REFERENCES 1. Hosoya, N., Umino, R., Kanda, A., Kajiwara, I., & Yosinaga, A., (2018). Lamb wave generation using nanosecond laser ablation to detect damage. Journal of Vibration and Control, 24, 5842–5853. 2. Markopoulous, A. P., & Manolakos, D. E., (2017). Ablation study of laser micromachining process with molecular dynamics simulation. Journal of Engineering Manufacture: Part B, 231, 415–426. 3. Mohammed, Z., & Saghafifar, H., (2014). Optimization of strongly pumped Yb-double doped double-clad fiber lasers using a wide-scope approximate analytical model. Optics and Lasers Technology, 55, 50–57. 4. Partes, K., (2009). Analytical model of the catchment efficiency in high speed laser cladding. Surface & Coatings Technology, 204, 366–371. 5. Dixit, S. R., Das, S. R., & Dhupal, D., (2019). Parametric optimization of Nd: YAG laser microgrooving on aluminum oxide using integrated RSM-ANN-GA approach. Journal of Industrial Engineering International, 15, 333–349. 6. Knowles, M. R. H., Rutterford, G., & Kamakis, D., (2007). A Ferguson micro-machining of metals, ceramics and polymers using nanosecond lasers. International Journal of Advanced Manufacturing Technologies, 33, 95–102. 7. Gamage, J. R., & DeSilva, A. K. M., (2015). Assessment of research needs for sustainability of unconventional machining processes. Procedia CIRP, 26, 385–390. 8. Ri-Sheng, L., Wei-jun, L., Fei, X., & Hua-Bing, W., (2008). Numerical simulation of thermal behavior during laser metal deposition shaping. Transactions of Nonferrous Metals Society of China, 18, 691–699. 9. Engelmann, C., Eckstaedt, J., Olowinsky, A., Aden, M., & Mamuschkin, V., (2016). Experimental and simulative investigations of laser assisted plastic metal-joints considering different load directions. Physics Procedia, 83, 1118–1129. 10. Zhang, L., & Shang, X., (2015). Analytical solution to non-Fourier heat conduction as a laser beam irradiating on local surface of a semi-infinite medium. International Journal of Heat and Mass Transfer, 85, 772–780. 11. Wang, F. B., Tu, P., Wu, C., Chen, L., & Feng, D., (2017). Multi-image mosaic with SIFT and vision measurement for microscale structures processed by femtosecond laser. Optics and Lasers in Engineering, 100, 124–130. 12. Swain, N., Venkatesh, V., Kumar, P., Srinivas, G., Ravishankar, S., & Barshilia, H. C., (2016). An experimental investigation on the machining characteristics of Nimonic 75 using uncoated and TiAlN coated tungsten carbide micro-end mills. CIRP Journal of Manufacturing Science and Technology, 16, 34–42. 13. Xu, J., Chen, C., Zhang, T., & Han, Z., (2017). A study of polycrystalline silicon damage features based on nanosecond pulse laser irradiation with different wavelength effects. Multidisciplinary Digital Publishing Institute: Materials, 10, 260. 14. Modest, M. F., (2015). Transient elastic and viscoelastic thermal stresses during laser scribing of ceramics. Journal of Heat Transfer, 123, 171–177. 15. Kingrey, W. D., & Bowen, H. K., (1995). Factors affecting thermal stress resistance of ceramic materials. Journal of the American Ceramic Society, 38, 3–15. 16. Iiyama, M., & Kayama, Y., (1982). US Patent on Prevention of Thermal Spalling in the Ceramic Products. US4334858A, United States.

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17. Hsieh, Y., Chen, S. C., Huang, W. L., Hsu, K. P., Gorday, K. A. V., Wang, T., & Wang, J., (2017). Direct micromachining of microfluidic channels on biodegradable materials using laser ablation. Multidisciplinary Digital Publishing Institute: Polymers, 9, 242. 18. DAddona, D. M., Genna, R. S., Leone, C., & Matarazzo, D., (2016). Prediction of polymethyl-methacrylate laser milling process characteristics based on neural networks and fuzzy data. Procedia CIRP, 41, 981–986. 19. Chryssolouris, G., (1991). Laser Machining: Theory and Practice. Springer-Verlag, New York. 20. Dixit, S. R., Panigrahi, D., Rout, S., Panda, S., & Dhupal, D., (2021). A tribological parametric analysis of laser textured microgrooves on Si3N4. Lasers in Engineering, 49, 319–339. 21. Rihakova, L., & Chmelickova, H., (2017). Laser drilling of alumina ceramics using solid state Nd: YAG laser and QCW fiber laser: Effect of process parameters on the hole geometry. Advances in Production Engineering & Management, 12, 412–420.

CHAPTER 7

LASER BEAM MICROMACHINING AND FABRICATION RAVINDRA NATH YADAV,1 SANJAY MISHRA,2 and AJAY SURYAVANSHI3 Department of Mechanical Engineering, BBD Institute of Technology and Management, Lucknow, Uttar Pradesh, India 1

Department of Mechanical Engineering, Madan Mohan Malviya University of Technology, Gorakhpur, Uttar Pradesh, India

2

Department of Mechanical Engineering, Bundelkhand Institute of Engineering and Technology, Jhansi, Uttar Pradesh, India

3

ABSTRACT Micromachining of advanced materials with high resolution at the submicron level requires an extremely precise processing tool that can be effectively integrated and automated to produce micro-components of complex profiles. The developments of ultrafast lasers have opened a new avenue for laser beam micromachining (LBMM) in terms of 3D micro-structuring and surface modification. The ultrafast lasers with pulse duration of femtosecond to picosecond can be effectively used to tailor the electrical, mechanical, tribological, and optical properties of different materials through surface structuring. Due to nonlinear absorption at a wavelength corresponding to ultrashort, laser micromachining can be conveniently used to fabricate photonic devices. Since the absorption process of lasers having ultrafast pulse duration is independent of mechanical and tribological properties, so, the optical devices can be easily fabricated by the LBMM process. In the present chapter, the basic mechanism of ultrafast lasers and their interaction Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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with materials have been discussed. Instead of this, the unique capabilities of short and ultra-short laser pulses with their potential for micromachining and microfabrication have been presented in such a way that it becomes easier for readers to understand the basic concept of the LBMM process. 7.1

INTRODUCTION

Advanced materials offer unique properties like excellence strength and durability, high resistance to oxidation and corrosion, negligible deformation under high pressure and temperature. Such materials become the first choice of advanced industries like aerospace, automobile, marine, oil, and gas, petrochemical, thermal, and power generation due to superior properties over conventional materials. The unusual properties of such materials make them difficult to machine and fabricate by existing traditional processes. The processing becomes much more challenges for industries when microprocessing requires removal of material at micro-level under controlled conditions. However, various methods are broadly used to achieve miniature accordingly needs by selection of appropriate microfabrication techniques such as material deposition and material removal [1, 2]. In material removal, the micro-components are fabricated by controlled removal of undesired material from workpiece surface at micro-scale (1 µm–999 µm) in the form of debris without any constraint of size [2]. Generally, some of the traditional material removal processes can be utilized for microfabrication but these have several technological constrains such as stringent requirement of design, unusual size, complex, and unusual profiles [3, 4]. Instead of this, the hardness of tool more than workpiece hardness also limits the application of traditional processes for microfabrication [3]. On other hand, the material deposition technique suffers with long production time with poor surface quality (SQ) [1]. To overcome these situations, advanced machining processes (AMPs) have been found more appropriate methods for microfabrication by removal of material at micro/nanoscales. Mostly, AMPs can be employed for macro- as well as micro-machining of difficult to machine materials. The basic purposes of AMPs are to shaping and sizing of parts with better SQ. Nowadays, various AMPs are used for micromachining such as thermal energy based (electro discharge micromachining, laser/electron beam micromachining, plasma/ion beam micromachining), mechanical energy based (water jet micromachining, abrasive jet micromachining, ultrasonic micromachining) or chemical electrochemical energy based (electrochemical

Laser Beam Micromachining and Fabrication

141

spark micromachining, chemical micromachining) [2]. Such AMPs are broadly accepted by industries for microfabrication of advanced materials. Even though, these AMPS experiences with several limitations regarding material, profile, and complexity [4, 5]. In different AMPs, laser beam machining (LBM) is found more beneficial for micromachining because it can be focused accurately on microscopic area [6]. Such process is referred as laser micromachining or laser beam micromachining (LBMM). 7.2 LASER BEAM MICROMACHINING (LBMM) Laser beam micromachining (LBMM) refers as non-contact material removal process where control amount of material is removed due to melting and vaporization with application of high intensity laser beam [6–8]. Mostly, LBMM process is used to create micro-features while tolerance in nanometer (nm) scale by controlled focusing of laser beam on selective part of material. For this, short or ultrashort lasers are employed in LBMM process where nanosecond (ns) to microsecond (µs) time duration for heat diffusion and picoseconds (ps) to nanosecond (ns) duration for coupling time of electronphoton [8]. Since laser energy deposition rate is much shorter than electronphoton coupling and heat transfer duration as a result no collateral damages occurs [8]. Hence, heat affected zone (HAZ) is negligible which makes it suitable for micro/nano-processing. In LBMM, the material removal takes place through ablation where material under laser radiation and absorbs the laser energy to transform the material either liquid or gas [2]. Initially, the material is heated up to melting temperature in focal region and subsequently to vaporization temperature depending upon pulse duration, wavelength, laser intensity and material properties [2, 8]. The melting and subsequently to vaporization of material depending upon laser intensity, pulse duration, wavelength, and material properties. The melted material is expelled for focal region (melt zone) by recoil action while vapor itself removes from focal area [2]. Such phenomena responsible for material removal from workpiece surface to get the desired profiles. Generally, LBMM process offers miniatures with high resolution and precise geometrical control because of wide rages of pulse duration (ns to fs), wavelength and frequency (single pulse to MHz). It is effectively used to create 3D miniatures ranges micrometer (µm)to nanometer (nm) by heating, melting, and vaporization [8–10]. For this, short and ultra-short (ultrafast) pulse lasers are preferred due to minimize the thermal effects and burr

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Electro-Micromachining and Microfabrication

formation resulting elimination of post processing. Due to this, it is preferred in automobiles and medical industries [2, 7]. Now, it is used for fabrication of micro/nano components related to solar cells and semiconductors [7]. It is also used in variety of areas such as micro-electronics, micro-sensor, micro-optics, micro-biology, and micro-chemistry. It can be employed to manufacture submicron-components, chips of read only memory, optical memory, and chips for biological applications [8]. It can be employed for machining of traditionally extremely hard materials like diamond, glass, ceramics, super alloys, graphite with capability to machine soft materials like plastic and polymers. 7.3 LASER AND ITS TYPES Laser is a form of electromagnetic radiation and stands as light amplification by stimulated emission of radiation [10]. It is coherent (same phase), monochromatic (same wavelength) in nature having high power density in order to 1 MW/cm2 and easier to focus using optical lenses in wavelength ranges from 0.50 μm–70 μm [4, 11]. It consists photons of same wavelength and frequency and also high-power density [10]. The amplification of light achieved by stimulated emission due to incident of high energy. The photons are the key factor of laser which makes it practical applications. It can travel long distance without scattering and focus upon a very tiny as well as inaccessible area [7]. It is possible to easily change the configuration of laser based on industrial needs as cutting, drilling, milling, grooving or turning purposes. In general, the high flexibility, high lateral resolution and low heat input makes laser more suitable for micromachining. In practical, a variety of lasers are used such as solid laser, gas laser liquid laser, fiber laser and diode (semiconductor) laser. In solid state laser, the optically transparent (crystals or glasses) acts as laser medium which is doped by one of the doping materials such as neodymium ytterbium, holmium, and thulium. The solid lasers are mostly used for cutting, marking, and welding applications due to high power density. The neodymium doped yttrium-aluminum-garnet (Nd-YAG), neodymium doped glass (Nd-glass), neodymium doped yttrium lithium fluoride (Nd-YLF) and titanium-doped sapphire (Ti-sapphire), etc., are the several common solid lasers [4, 11, 12]. The ruby laser is the first solid state laser that used in different industrial applications [4]. For solid laser, the light energy (flash lamp, flash tube or arc lamp) is used to achieve pumping source to get laser (Figure 7.1).

Laser Beam Micromachining and Fabrication

FIGURE 7.1

143

Schematic of solid laser beam system.

In gas laser, the laser medium is in gaseous form and electrical discharge (ED) is employed to generate laser. The most common gas lasers are carbon dioxide (CO2) laser, carbon monoxide (CO) laser, Helium-Neon (He-Ne) laser, excimer laser, nitrogen laser, hydrogen laser and argon ion laser [11, 12]. Fiber laser is one type of solid laser where laser media is optics fiber (silica glass). The ytterbium and erbium doped lasers are example of fiber laser. It is mostly used for texturing, marking, cutting, and welding applications. In liquid laser, the liquid die is considered as laser medium, and it referred as die laser [4]. It is used in medical applications, removal of birth marks, separation of isotopes and spectroscopy. In semiconductor laser, the P-N junction of semiconductor diode is used to form the laser medium or active medium and optics gain is produced by semiconductor material when electrical energy employed as heat source. Such type of laser uses as barcode reader and laser-based pointer, printer, scanner, and other similar applications. In some cases, it is also used as energy source to pump the other lasers. In various existing lasers, the Nd-YAG, and CO2 lasers are widely employed for industrial application [1, 12]. The CO2 laser shows better beam quality with high efficiency than Nd-YAG and most suitable for metal cutting applications. On other ways, the absorptivity of Nd-YAG laser is higher than CO2 laser with low beam power [1, 4]. The characteristics and applications of several lasers used in industrial applications are summarized in Table 7.1 [1, 12–19]. Based on the mode of operation, two types of lasers are used in practical application as continuous wave (CW) laser and pulse laser [16–18]. In CW

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Electro-Micromachining and Microfabrication

laser, the uninterrupted flow of energy with single shoot of laser for various applications like cutting, welding, and heat treatment. The common applications of CW laser are as laser pointers, printers, and scanners. In pulse laser, the energy allows to flow in an interrupted way at regular interval and reach to high peak than CW laser. It is used in many industrial applications such as cutting, spot welding, grooving, and micro-processing of materials. Generally, pulse laser can be easily generated by using pulsed pumping device such flash lamp/flash tube. However, several techniques are used to generate pulse laser as Q-switching, gain switching, mode locking technique [2]. The Q-switching is suitable for Nd-YAG laser to produce pulse width in few ns while Nd-glass laser produces pulse width in ps with application of mode locking technique. In recent, pulse laser generates the pulse width of femtosecond (fs) for micro- and nano-processing of materials. Such pulse mode laser can be achieved by mode-lock of broadband laser sources. TABLE 7.1

Lasers for Materials Processing [1, 12–19]

Laser

Wavelength Overall Output (µm) Efficiency Power (in (%) CW) Mode

Applications

Nd: YAG laser 1.06

1–3

Up to 16 kW Cutting, drilling, machining, marking, milling, and rapid prototyping.

CO2 laser

10.6

5–10

Up to 20 kW Cutting, laser ablation, cladding, alloying, glazing, surface treatment and rapid prototyping.

Excimer laser

0.125–0.351 1–4

300 W

Scribing, precision machining like micro-drilling, etching, micromachining, and optical stereo-lithography. Laser printer, scanner, jet ink, optical communication devices, bar codes scanner, 3D scanner, photocopiers, laser ablation, cutting, pollution detection, ozone layer detection.

Semiconductor 0.7–1.0 laser (diode laser)

30–50

Up to 4 kW

Fiber laser

10–30

Up to 10 kW Laser cleaning of aircraft, print stripping, microelectronic devices, integrated circuit chips, medical applications.

1.07

Source: Adapted from Ref. [1, 12-19]

Laser Beam Micromachining and Fabrication

145

The laser can be categorized into long pulse laser, short pulse laser and ultrashort pulse laser based on pulse duration. The millisecond (ms) and microsecond (µs) lasers refer as long pulse laser where pulse duration is longer than heat diffusion time. The ms laser is suitable for removal of hairs while microsecond laser applicable for spectroscopy applications. The nanosecond (ns) laser comes in categories of short laser while picoseconds (ps) laser and femtosecond (fs) laser refer as ultrashort or ultrafast lasers. The short and ultrashort lasers are more suitable for processing of materials at micro- and nano scales. It is because the heat generated by laser into material does not have sufficient time to move away from focal spot between two successive pulses [2, 8, 9]. As a result, high temperature is generated instantly, and material melted beyond the melting temperature. Finally, temperature goes beyond the evaporation temperature. Hence, the ultrafast laser is effectively used to machine very hard materials (molybdenum, rhenium, super alloy, etc.) having high melting temperature at micro- and nanoscales. 7.3.1

LASER FOR MICROMACHINING

Generally, the long pulse lasers suffer with low heat diffusion at focal spot due to longer pulse duration than heat diffusion time. Further, the molten material ejects from melt zone in form of droplets and forms recast layer [2]. Due to this, the long pulse lasers are not suitable for micromachining. On other hand, the short/ultrashort lasers generated by solid, or gas lasers are found more suitable for micromachining by selective removal of material from focal spot/melt zone [8]. For LBMM process, variety of lasers are used likes deep-ultraviolet (UV) and medium-infrared (IR). Simply, conversion of wavelength of IR (700 nm–1 nm) laser can be achieved by passing of light through non-linear optical crystals (beta barium borate or lithium niobate). The other lasers as Nd-YAG, Nd-YVO4 and Nd-YLF are producing UV (10 nm–400 nm) laser radiations. The several lasers and their characteristics used for micromachining applications are summarized in Table 7.2 [8]. The generation of laser (short/ultrashort) for micromachining is schematically presented in Figure 7.2. It consists of laser material, pulse generator (Q-switch), pumping source and mirrors. The laser material is used to generate stimulated emission of laser while Q-switch/mode-lock generates the pulsed laser. The pumping system uses to pump the energy into laser medium by optical pumping or electrical pumping system. The mirrors (fully reflecting and partially reflecting) are used for optical resonators while laser medium is placed between properly aligned mirrors. Generally, low

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Electro-Micromachining and Microfabrication

density (gaseous) or high density (solid or semiconductor) are used as laser media for laser generation. The generated laser may be CW laser, or pulsed (short/ultrashort) laser depend upon laser media, pumping device/source and Q-switching/mode locked. The Q-switching is suitable for ns pulse laser while mode locked is used for ps and fs pulse laser generation. A variety of lasers are used for micromachining like Nd-YAG laser, CO2 laser, excimer laser, diode laser and Ti-sapphire laser. TABLE 7.2

Lasers for Micromachining and their Characteristics [8]

Type of Laser Laser material

Wavelength

Pulse length

Frequency

Solid laser

Gas laser

Nd-YAG

266-532 nm

100-10 ns

50 Hz

Nd-Glass

155 nm

7 ps

—

Nd-YLF

1047-1053 nm

15-37 ps

76 MHz-2.85 GHz

Yb-YAG

1030 nm

340-730 fs

35-81 MHz

Yb-Glass

1025-1082 nm

58-61 fs

112 MHz

Cr-YAG

1.52 µm

44-120 fs

81 MHz-1.2 GHz

Fiber Laser

164 nm

100 ns

20-50 Hz

Diode Laser

0.8 µm

—

—

ArF

193

5-25 ns

1-1000 Hz

KrF

248

2-60 ns

1-500 Hz

XeCl

308

1-250 ns

1-500 Hz

XeF

353

0.30-35

1-1000 Hz

CO2

10600

200 µm

5 Hz

Source: Reprinted w ith permission from Ref. [8]. Copyright © 2015. Elsevier Ltd.

FIGURE 7.2

Schematic of laser generation for LBMM process.

Laser Beam Micromachining and Fabrication

147

7.4 LASER MATERIAL INTERACTION In laser machining, radiation of laser is absorbed by material when laser is focused on surface of target metal and finally converted into heat. The absorption coefficient of material depends upon wavelength and temperature and also responsible for decay of layer intensity with depth into metal. The laser radiation absorbed into material is expressed by Beer-Lambert’ law [8, 20]. Iz = I0 exp–az

(1)

where; I0 is the incident intensity of laser beam; Iz is the intensity of beam at z depth; and α is the coefficient of absorption. In general, material absorbed the laser radiation by exciting free electron or by vibration/transition to ions, molecules, or atom. Maximum depth where penetration takes place is known as penetration (δ) or attenuation depth and leads to conduction of heat into material. The penetration depth depends upon absorption coefficient (α) of material on which laser beam is focused and determined as [8]. δ = 1/α

(2)

The laser beam is convergence and divergence in nature (Figure 7.3) and heat absorbed into workpiece depends upon intensity of beam. Theoretically, the distribution of beam intensity follows the Gaussian distribution. Hence, the intensity (I) of beam along the cross section is varied as [2]: −2r 2

I = I max exp

ω2

(3)

where; r is the beam radius; ω is the waist of beam where 85% of energy is concentrated; and I is the beam intensity. The characteristics of laser beam are significantly affected the Gaussian profiles and heat transfer into the material. Let, λ is the wavelength and ω0 is minimum spot/focal diameter of a convergence beam (Figure 7.3). If, the beam is considered as perfect beam where M2 = 1 (M2 = beam quality number). Then, half angle (ϴ) can be determined as [20].

θ=

λ πω0

(4)

In general, the realistic laser beam is consisted with higher order modes as which the divergence half angle (ϴ) of beam is higher than ideal laser beam and determined as:

θ=

λ πω0

(5)

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FIGURE 7.3

Divergence/convergence nature of laser.

In case of laser machining, the minimum spot size governs the size of features and minimum spot size is only possible with perfect beam where M2 = 1. The beam quality number as M2 is an important parameter that influences the minimum spot size of features. Hence, prefect beam (M2 = 1), small wavelength and short length of focal lens are preferred for micromachining by laser. The beam waist (ω0) is a significant parameter that solely responsible for the profile of Gaussian of a laser beam having wavelength as λ. It is used to measure beam size at its focal (z = 0) where beam waist is smallest and largest beam intensity on axis (r = 0). From this, the other parameters of beam geometry such as Rayleigh range (zr) or half of focusing depth can be determined [21]. zr =

πω02 λ

(6)

In simple, Rayleigh range (half of focusing depth) can be defined as distance from beam waist (ω0) to transverse plane where w(z) = √2 ω0 or equivalent where area of beam becomes approximately doubled. 7.5

LASER ABLATION

A typical diagram of laser ablation is shown in Figure 7.4 [17]. In laser machining, light energy of laser is absorbed by workpiece material when laser focuses on it. Due to laser energy, the material is melted and subsequently

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vaporized. The melting and vaporization of material is completed in various phases like heating of material, melting, vaporization, and ablation as presented in Figure 7.5.

FIGURE 7.4

Laser ablation process with pulsed laser [17].

Source: Reprinted w ith permission from Ref. [17]. Copyright © 2007 Elsevier B.V.

FIGURE 7.5

Phases of material interaction with laser.

Initially, the laser beam is focused on target metal as which heating of material occurs due to conversion of absorbed energy into heat energy. The heating of material depends upon magnitude of absorbed laser energy and focal area. The material in focal region reaches from heating to melting

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temperature and subsequently reaches the vaporization temperature. The vaporization of material occurs itself while molten liquid is expelled from focus region and finally get a cavity known as ablation [2]. Sometimes, material is partially vaporized and expelled from depth as which the remaining materials is cooled quickly and formed recast layer. In simple, laser ablation can be defined as process of removal of material from target by pulse laser. It can also be possible to ablate material by CW laser when high intensity laser is utilized. The ablation of metal occurs due to absorption of laser radiation that involves absorption of photon by electron (10–15 s) and transfer the energy to lattice (10–12 s) [2]. The ablation with long pulsed lasers may causes of thermally alter or damage the target material (especially solid) with higher HAZ while ultrashort pulsed laser shows minimum thermal damages. Hence, such laser pulses are suitable for micromachining and micro-processing of materials [8, 22]. In the same way, the low intensity of laser beam is heated the target material by absorbed laser radiations and removal of material occurs by vaporization (transition of material from solid to liquid and then gas) or sublimation (phase change directly from solid to gas) [22]. On other way, the plasma formation leads important role for material removal in the case of high intensity laser beam which is collapsed during pulse interval (off time) of laser pulses. The ablation depends upon optical properties of material, wavelength of laser and pulse width. The total volume/amount of material removed from target is known as ablation rate [23]. In case of metals and glasses, the ablation occurs due to vaporization of molten metal for target material while photochemical changes, photo-physical or photo-thermal are responsible for laser ablation [2, 8]. In case of photochemical, the electrons are excited with UV light energy and bond will break when sufficient energy absorbs. The electronic ablation plays important role for bond breaking during photothermal ablation while thermal and non-thermal processes responsible for photo-physical ablation. 7.5.1 MECHANISM OF LASER ABLATION Material interaction plays significant role for laser ablation where laser energy firstly absorbed by free electrons when focused on material. Such absorbed energy propagates to the subsystem of electron and subsequently transmitted to the lattices [18]. Due to this, the heating and subsequently melting and vaporization of material because of conversion of laser light into heat energy. The energy absorbed by material depends upon cooling time of

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electron (Tec), heating time of lattice (Tlh) and laser pulse duration (Tpl). In general, heat capacity of electron is lower than lattice. Hence, Tec 1 ms>> Tlh >>Tec. It means pulse duration in ms or infinite (in case CW laser). In this case, typical time scale is longer than electron-lattice coupling time. The basic mechanism of ablation is the melting and ejection of molten metal by jet of assisting gas. The laser cutting is an example of such time scale and applicable for cutting of ferrous and non-ferrous metals, steel, and non-metals [18]. The CO2 laser (wavelength = 10.60 µm) with power capacity of few KW with intensity 1–2 WM/cm2 is preferred for this time scale laser processing. In case II, where Tpl in ns, i.e., Tpl >1 ns>> Tlh >>Tec. Here, the electron absorbed energy has sufficient time to transfer to lattice and finally electron and lattice gain thermal equilibrium. The ablation of material occurs due to melting and vaporization from melt zone when high intensity beam is applied. The heat loss occurs due to conduction of heat into material (target metal). The HAZ is comparatively lesser than CW laser. Generally, Nd-YAG laser (frequency = 355 ns) and intensity as 100–200 J/cm2 is preferred for processing of materials such as copper, aluminum, and it alloys due to better absorptivity of UV laser [18]. The laser marking, drilling, grooving, and scribing are some examples of laser material processing in this time scale regime. In case III, where Tpl in fs, i.e., Tpl >1fs>> Tlh >>Tec. In this case, pulse time is smaller than cooling time of electrons. Here, instant heating of electron occurs and subsequently electron transmits energy to lattice ions. The intensity of energy is so high to break the bond of lattice ions. In this time scale, the sublimation (directly from solid to gas) leads in material ablation by laser. In this mechanism, the HAZ is negligible because lattice breaks suddenly without any transfer of energy to neighboring lattice. For this, ultrafast laser with high pulse energy is utilized in materials processing. Mostly, Ti-sapphire laser (wavelength = 780 nm and power intensity = 0.10–10 J/cm2) is preferred [18]. Almost, all types of materials (metals, polymers, ceramics, and alloys) can be processed within such time scale. It is applicable for marking, drilling, cutting, grooving, and scrubbing or any other material processing application. 7.5.2 ABLATION WITH NS LASER In ns laser ablation, pulse duration is higher than time requires for transfer of energy from electron to lattice (10–12 s). Here, the ns laser is focused

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on the target metal and incident photons are instantly absorbed by lattice. The ablation occurs due to photo thermal process causes of metal ejection and generation of nanoparticles [8, 24, 25]. In ns ablation, the target metal is heated to its melting point and then gets the vaporization temperature. In case of lower intensity, the thermal evaporation dominates the ablation and ionization of vapor occurs due to direct heating of laser radiations. On other way, gas ionization occurs due to higher laser intensity than ionization threshold. The intensity of laser and pulse duration (ns) play significant role for the laser ablation. The instant heating of metal beyond vaporization temperature leads for laser ablation because of higher intensity (109 W/cm2) and shorter pulse duration (ns) [2]. In this situation, the surface temperature reaches to thermodynamic critical point and explosion occurs. Due to explosion, the target metal is changed their phases as overheating liquid to vapor and liquid droplets [8]. In same way, the plasma (high pressure and temperature ionized gas column) is formed at end of laser pulse resulting molten droplets are ejected with supersonic velocity [7]. The temperature of plasma exceeds of 104 K at plasma-metal interaction which is so high than short pulse laser [2]. The interaction of plasma with ejected liquid droplets is responsible for deposition of metal at kerf and leads to formation of recast layer surrounding ablated zone. Such phenomenon changes the topology of machined area and needs further processing like super finishing process. The ablation depth significantly affects the crater profile forms by ns laser. The ablation depth (Za) per pulse is determined as [2]: F  za = Dt In  a   Fth 

(7)

where; Fa is the absorbed fluence; Fth is the threshold fluence; D is the thermal diffusion coefficient; and t is the pulse duration. The term Dt refers as thermal diffusion depth of laser radiation. The thermal diffusion coefficient depends upon thermal conductivity (k) and specific heat capacity per unit volume (Ci) and can be determined as [2]: D=

k Ci

(8)

Generally, HAZ is formed during processing of material by ns laser is very small and can be determined by given equation [2]: HAZ = (Dt)1/2

(9)

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153

The ns laser ablation uses for micromachining of variety of materials including plastics, metals, non-metals, and glass. It is used for scribing, cleaning of Si-wafers, drilling of micro-holes of inkjet for printers, microablation of glass material, micro-structuring, microfabrication, micro-molds and dies manufacturing and other micro-processing of materials for engineering applications. 7.5.3 ABLATION WITH PS LASER In ps laser ablation, the pulses are same time scale as it requires time to transmit the energy from electron to lattice of metal. In this case, smaller amount of heat conduction with higher heat flow causes by free electron. However, some metals don’t have free electron thus multi-photon absorption and electron impact absorption or interband transition play roles for energy deposition [2]. Due to this, extremely hard and transparent materials like diamond, glass, and ceramics can be effectively machined by it. In ultrashort laser, ablation occurs due to critical point phase separation mechanism where no appropriate change of density of metal occurs until maximum temperature reaches. At maximum temperature, ablation takes place for the metal whose expansion trajectory becomes unstable region. The free electron absorbed energy and transmitted to the lattice due to electron photon coupling. At focal region, the direct solid to gas or solid to plasma transition lead for ablation [8]. Even though, the liquid phase presents inside molten metal. Similar to ns laser ablation, the Za for ps laser can be determined as [2]: F  za = α −1 In  a   Fth 

(10)

where; a is the absorption depth. In ps laser ablation, the undesired thermal effects are negligible because of pulse duration is less than electron photon coupling time. Several materials like iron, steel, copper, and aluminum have thermalization time around 10 ps. Hence, ps laser machining is suited for such materials to create various operations like cutting, drilling, milling, structuring, pattern marking and micro-processing. It has very high-power intensity (terawatt per square centimeter) and most suitable for machining of diamond and glass like hard materials with high precision and accuracy. It is capable to produce better hole geometry without any thermal ablation and cracks.

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7.5.4 ABLATION WITH FS LASER In fs laser ablation, the energy deposition occurs within limited time scale which is very short as compared to other processes. The intensity of fs laser is so high to drive highly non-linear absorption process in metals that doesn’t absorb the laser radiations. There is no liquid phase transition occurs during fs laser ablation. Mostly, electromagnetic wave interacts with particle and multi-photon absorption take place during fs laser ablation. The bonded electrons can be directly ionized by multi-photon absorption. In case of multi-photon absorption, there is not any dependency on electron to initiate the process and any electron can be ionized during ionization phenomenon. Here, vaporization takes place rapidly and HAZ is much little or negligible. In fs laser ablation, no energy is transmitted to lattice and all energy stores into thin surface layer of material. The stored energy is higher than specific heat of evaporation then vigorous evaporation occurs after each pulse [2]. Here, plasma formation also takes place. The plasma formed is expanded quickly and finally expelled from metal surface. It is a very precise ablation process because there is no limit of heat transfer. It is significantly machined the surfaces with minimum HAZ. In this process, ablation depth can be determined by using Eqn. (10) [2]. The multi-photon absorption doesn’t depend upon free electrons of target material and linear absorption capability of laser radiation by metal. Due to this, the fs laser can be able to process the any types of materials. It also shows their ability to machine 3D structuring inside the transparent materials with complex geometry. It is used for micro-structuring and texturing, machining of trenches in glass, micromachining of absorptive materials for drilling, removal of surface defects, photolithographic mask repair, microfabrication of chips, actuators, and actuators [26–30]. 7.6

LBMM SYSTEM AND METHODS

A typical 4-axis micromachining system for excimer laser is shown in Figure 7.6 [17]. Generally, LBMM system consists with laser source, galvanometer scanner, beam expansion telescope, focusing lenses, monitoring system and positioning system. The laser source is used to emit the short (ns pulse) laser or ultrashort (ps/fs pulse) laser for micro-processing of material. The purpose of galvanometer scanner is to guide the high-speed beam with

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high accuracy at target metal. It is mostly programmable and easily controls the beam through monitoring system while galvo-shutter is employed to switch the laser beam.

FIGURE 7.6

Four axis laser micromaching system [17].

Source: Reprinted with permission f rom Ref. [17]. Copyright © 2007 Elsevier B.V.

The diameter of emitted beam is increased by beam expansion telescope and guided by quarter wave plate to get circular polarized on target material. The basic purpose of circular beam is to achieve equal absorption of laser radiation around the machining zone. The beam is focused by focusing lens on target and circular opening is positioned before lens to remove the low intensity of spatial distributed laser beam. The LBMM system has a central computer system for control and monitoring of target material under the focused laser beam. It is also used to maintain the adequate positioning of metal to maintain the focal. A highspeed CCD camera is used to monitor the machining process. The purpose of high-speed camera is to observe the laser ablation during machining operation. The X, Y, and Z movement of target material is control by positioning system during the micromachining process. A schematic setup for ns laser ablation is exposed in Figure 7.7 [28]. Here, Nd-YAG laser (wavelength = 1,064 nm and maximum energy = 2.50 J/shot)

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Electro-Micromachining and Microfabrication

is used as laser source to emit ns pulse laser while a quartz lens (focus lens = 660 mm) is used to focus the incident beam at target. The laser pulses are generated by Q-switched mechanism. A digital pulse generator is employed to generate pulsed laser as per requirement. The distance between lens and target (metallic glass) is kept as 580 mm and laser ablation process is observed by high-speed camera (Phantom v2512) with the help of computer system.

FIGURE 7.7

Schematic of ns pulse laser experimental setup [28].

Source: Reprinted with permission from Ref. [28]. Copyright © 2019 Elsevier B.V.

7.6.1 METHODS FOR LBMM PROCESS Laser ablation is primary mechanism of material removal of LBMM process to achieve micro- and submicron level structuring. There are three different mechanisms (direct writing, mask projection and interference technique) are used in the laser ablation process [29, 30]. In direct writing technique, the laser beam is focused on surface of substrate as shown in Figure 7.8 [30]. The micromachining can be achieved by rather translating the substrate with respect to fixed beam or scanning the laser beam with respect to substrate. The flat-field mirror is employed to maintain the focal condition at different angle of deflection. In some cases, the galvanometer-controlled scanning mirror is employed to continuously deflect the beam at desired pattern. It is a simple technique and used for drilling, cutting, and scribing of target metals [8]. Several microstructures made by direct writing technique are presented in Figures 7.9(a) and (b) [29, 30].

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FIGURE 7.8

157

Direct write laser machining system [29].

Source: Reprinted with permission from Ref. [29]. Copyright © 2002 Elsevier Science B.V.

FIGURE 7.9 Micro-components made by direct write technique – (a) electrical interconnect structures for panel devices [30]; and (b) micro-fluidic channels [29]. Source: (a) R eprinted with permission from Ref. [30]. Copyright © 2005 Elsevier B.V. (b) Reprinted with permission from Ref. [29]. Copyright © 2002 Elsevier Science B.V.

In mask projection method, laser is employed to illuminate the desired pattern on mask. The mask is generally made of quartz and formed with thin layer of chrome [2]. The pattern on mask is then projected and shrunk on substrate by a projecting lens. Such technique is used to generate large pattern on the surface of substrate with application of single or multiple laser

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shots. The excimer laser is mostly preferred for mask technique due to their optical properties as projection technique is effectively utilized for various micromachining. Such technique includes high resolution of features, excellent depth control and reproductively with ability to cover large area. The micro-components made by mask projection technique are summarized in Figures 7.10(a) and (b) [30].

FIGURE 7.10 Microstructures made by mask projection technique–(a) polyimide structure; and (b) inkjet printer nozzle [30]. Source: Reprinted with permission from Ref. [30]. Copyright © 2005 Elsevier B.V.

Laser interference method involves splitting of laser beam using beam splitter followed by super position of beam to create interference pattern. In general, primary beam is spitted into number of single laser beam. Such laser beam is superimposed to generate the interference pattern which is projected on substrate. The interference pattern shows unique variation in laser intensity which can be utilized for micromachining. The interference pattern is achieved with structure size of 50–300 nm based on intensity [31]. 7.6.2 VARIANTS OF LBMM PROCESS LBMM is a highly flexible microfabrication process where removal of material occurs at micron and submicron level. Due to flexibility in nature, LBMM can be developed in different variants such as drilling-LBMM, cutting-LBMM, turning-LBMM, milling-LBMM, and grooving-LBMM as shown in Figure 7.11. Drilling-LBMM process is employed to create 1D profile especially holes (through or blind) by laser ablation process. In LBMM process, holes may be created by either percussion drilling or trepanning drilling. In percussion,

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there is no relative movement of either workpiece or laser. In this, the laser beam is directly applied to workpiece surface where hole to be created. On other hand, laser ablation involves around circumference of hole which can be generated by trepan drilling-LBMM. It is much advantageous process for drilling of holes with higher productivity within the smaller processing time.

FIGURE 7.11

Variants of LBMM process.

The cutting-LBMM process is used to produce 2D profiles with application of laser beam either straight or curve manner. It is used for cut-off, marking or parting of metals, alloys, ceramics, and plastics [31, 32]. A typical laser cutting process is shown in Figure 7.12 [33]. Due to flexibility nature of laser, the cutting-LBMM can be applied for manufacturing of highly complex profiles which are difficult by tradition micromachining processes. The grooving-LBMM process is suitable for making 2D complex profiles with curve cutting of materials. Milling-LBMM and turning-LBMM processes are required two simultaneous lasers to achieve desired 3D profiles. Mostly, the fiber optics is utilized to focus the laser beams at desired locations. Milling-LBMM prefers for machining/shaping of complex profiles like micro-pockets, micro-grooves, micro-trenches micro-gears or other similar objects. In the Turing-LBMM, the object is rotating in nature like traditional turning and laser beams are employed to remove the undesired material from rotating surface.

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FIGURE 7.12

Electro-Micromachining and Microfabrication

A typical laser cutting system based on fiber laser [33].

Source: Reprinted with permission from Ref. [33]. Copyright © 2008 Elsevier Ltd.

7.7 PROCESS AND PERFORMANCE MEASURES LBMM is complex process and varieties of factors affected the performances as presented in Figure 7.13. These parameters are either controllable or uncontrollable (humidity, machine efficiency, temperature, work environment, etc.) factors. The common process parameters are as wavelength, beam profile, pulse width, pulse frequency (PF), focal lens, nozzle diameter and stand-off distance (SOD). Instead of this, laser system parameters (laser power, beam quality, beam intensity), material parameters (material properties and material thickness) and other parameters such as interaction time, spot diameter and length of focal spot are also affected the performances of LBMM process. The wavelength of laser leads in ablation process and control the feature size. The selection of wavelength with minimum absorption depth ensures the rapid ablation because of high energy deposition within small region. The pulse duration is one of the most significant parameters that effectively controls the SQ. For precise machining, higher PF means more efficient ablation and lesser amount of energy goes loss to surrounding. It also affects the etch rate and depth of machined object. The other parameters as beam energy and pulse width lead to formation of HAZ and higher energy means larger HAZ. The width of HAZ is significantly affected by heat power and beam travel rate.

Laser Beam Micromachining and Fabrication

FIGURE 7.13

161

Process and performance parameters of LBMM.

The heat input depends upon beam power and significantly affects the material to be cut and their thickness. Generally, the heat input of 1,000 W requires for cutting of 1 mm thick sheet of aluminum and steel while mild steel and titanium sheet of same thickness requires heat input about 400 W [12]. The beam profiles (Gaussian or square) responsible for feature size and uniformity in micromachining. Focus and focus depth leads to maintain the aspect ratio while inert gas (helium) provides the protective environment for machining and plays important role in ejection of material from melt zone. The pressure of inert gas also assists to minimize the recast layer and dross. Normally, argon is selected for machining of Titanium and its alloys while Nitrogen is preferred for Stainless Steel and Nickel based alloys. For rough cutting, mostly O2 is preferred as assisting gas and recommended for higher productivity while N2 recommended for fine cut [32]. The focal diameter and focal length are influencing parameters related to the lens. The minimum diameter of beam where it is focused after passing through lens refers as focal/spot diameter (Figure 7.3). The maximum temperature occurs at focal spot which significantly affects the melt zone. The larger focal length uses for thick sheet cutting while thin plate cutting is done by lower focal length [12]. The nozzle plays their role to control the flow of assist gas in desired direction for quality cutting. The distance between nozzle and workpiece surface is known as SOD and is kept an appropriate (0.5 to 1.50 mm) to avoid the turbulence.

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In general, the performance measures indicate the machinability of process as etching rate, SQ, kerf, taper, HAZ, and re-cast layer. The etch rate indicates the removal of material per unit time and related to productivity. It means, higher etching rate means productivity is higher but declines in the SQ of machined parts. On other hand, SQ represents the quality of machined objects and measured in term of surface roughness. The higher surface finish indicates the precise quality of machined parts, and such characteristic of products is always recognized by manufacturing industries to reflect the quality. The kerf width and taper are other parameters that indicate the quality of cut and represent the geometrical errors. The kerf width varies according to length of cut and higher at top surface while taper indicates the gradually smaller dimension from start to finish end. The HAZ is also considered as output parameter and related to heat transfer into the target metal while short and ultrashort laser pulses are used to minimize it. The effects of several factors of LBMM are summarized in Figures 7.14–7.17 [33–36]. The effect of cutting speed and frequency on kerf width is shown in Figures 7.14(a) and (b) [33]. Figure 7.14(a) shows kerf width size increases with increase of laser power due to high power density while decreases with cutting speed because of lower power density and low assist gas (O2) pressure (0.30 MPa). In Figure 7.14(b), the size of kerf width is increased with increase of pulse length and frequency due to longer time for laser heat with workpiece interaction as pulse length increased. On other way, the increase in frequency means more energy goes into workpiece as a result more heat interaction with metal that leads the kerf width.

FIGURE 7.14 Effect of parameters on kerf width – (a) kerf width vs. cutting speed; and (b) kerf width vs. repeat frequency [33]. Source: Reprinted with permission from Ref. [33]. Copyright © 2008 Elsevier Ltd.

The effect of nozzle fluence and nozzle throat diameter on each rate and entrance hole diameter for ns percussion drilling of SS (316L) is shown in

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Figures 7.15(a)–(d) [34]. Figure 7.15(a) indicates that each rate per pulse increases with increase the laser fluence at different condition of assist gas. It also shows that average each rate per pulse is higher with ambient condition than other obtained by assist gas jets. Each rate also decreases with increase of nozzle diameter for the same nozzle diameter and laser fluence. Figure 7.15(b) shows that hole entrance diameter increases with increase of laser fluence due to more laser radiation. It also indicates that lower diameter of hole is achieved with drilling in ambient conditions than assist gas environment. The etch rate decreases with increase of nozzle throat diameter as shown in Figure 7.15(c). This is due to ineffective ejection of molted metal at workpiece surface with larger diameter of assist gas nozzle. Figure 7.15(d) shows the drilled hole by nozzle throat diameter of 200 µm with assist gas (O2) pressure of 8 bar.

FIGURE 7.15 Effect of nozzle fluence and diameter on responses: (a) laser fluence vs. average each rate; (b) laser fluence vs. hole entrance diameter; (c) nozzle through diameter vs. average each rate at saturation threshold; and (d) SEM image of drilled hole [34]. Source: Reprinted with permission from Ref. [34]. Copyright © 2006 Elsevier Ltd.

Figures 7.16(a)–(d) show effect of laser fluence and number of shots on micro/nano-height of ultrafast laser machined surface [35]. Figure

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7.16(a) shows linear relation between laser fluence, and micro/nano-height as increases with increase in laser influence. The spacing between conical microstructure or inter-cone distance also depends upon laser fluence as shown in Figure 7.16(b). Figure 7.16(c) indicates that number of laser shots directly influence the height of microstructures and peaks. Figure 7.16(d) shows the micro/nanostructure generated by ultrafast laser (laser fluence = 0.16 J/cm2) on aluminum surface.

FIGURE 7.16 Effect of laser fluence (J/cm2) on micro/nano-height – (a) laser fluence vs. microstructure height (µm); (b) laser fluence vs. inter-cone distance; (c) number of laser shots vs. microstructure height; and (d) SEM image of microstructure [35]. Source: Reprinted with permission from Ref. [35]. Copyright © 2010 Elsevier Ltd.

Figures 7.17(a) and (b) reflect the effect of laser fluence and spot diameter of laser drilled holes into aluminum sheet [36]. Figure 7.17(a) indicates that variation in entrance diameter of drilled holes with respect to average etch depth as entrance diameter is always higher than spot diameter of beam. Figure 7.17(b) indicates the average etch depth is the function of laser fluence and wavelength. It means etch depth increases with increase of laser fluence or wavelength due to higher laser radiation into surface of target material.

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FIGURE 7.17 Effect of laser fluence on etch depth and spot diameter – (a) etch depth per pulse vs. spot diameter; and (b) laser fluence vs. spot diameter [36]. Source: Reprinted with permission from Ref. [36]. Copyright © 2005 Elsevier Ltd.

7.8

PROCESS CAPABILITIES AND SHORTCOMINGS

Laser is able for micromachining of all kinds of materials including soft as well hard and brittle in nature including polymer to diamond and gaining acceptability for micro- and nano-engineering applications [8]. It can be effectively machined the silicon and silicon wafers for microfabrication and micromachining [37]. The miniatures made of optical materials (lithium niobate, lithium tantalite, Gallium Arsenide, indium phosphide, etc.) can be precisely machined by it. The detectors, sensors, and thermal equipment made of artificial CVD diamond can be effectively processed by ultrashort pulse lasers [38, 39]. It can be employed to create 1D, 2D, and 3D miniatures profiles with application of appropriate configuration (Figure 7.11) of LBMM process. The application of short/ultrashort pulse laser significantly eliminates the HAZ and recast layer formation as which no requirement of post processing operations. The miniatures for solar cells, semiconductors, electronic, and optics applications, biomedical, and micro-electro-mechanical (MEM) devices are made by LBMM process. The micro- and nano components like chips, optical data memory and chip for read only memory and other micron and submicron items can manufacture with laser ablation of ultrafast lasers [8]. The highresolution elections devices and circuits in metal layers can be made by it. LBMM process is widely used for microfabrication of MEM devices for implantable pressure sensor and drugs delivery systems. The hollow tube for medical applications especially cardiovascular stent (Figure 7.18) can be made by LBMM process [33]. It shows potential for micro-processing of polyester for micro-fluidic channels and for integrated optical, electrical, and chemical sensitive devices that able to sense the biological molecules

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flowing through micro-channels. Various micro/nanostructures made by LBMM process are summarized in Figures 7.19(a)–(f) and 7.20(a)–(f).

FIGURE 7.18 Cardiovascular stent made by LBMM process – (a) cut with fiber laser; (b) cut with YAG laser; and (c) ichnography of 316L stainless steel cardiovascular stent [33]. Source: Reprinted with permission from Ref. [33]. Copyright © 2008 Elsevier Ltd.

FIGURE 7.19 SEM image of different microstructures created by LBMM – (a) micro-dilled hole [36]; (b), (c) laser scanned tracks (2D profile) [36]; (d) micro-channel like structures (3D profile) [30]; and (e), (f) multilevel micro-structuring [30]. Source: Reprinted with permission from Ref. [30]. Copyright © 2005 Elsevier B.V.

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FIGURE 7.20 Micro/nanostructure made by LBMM process – (a), (b) high aspect ratio silicon micro-structure by laser cutting [37]; (c) array of micro/nano-pores formed in thin Ti foil [35]; (d) enlarge view of pores [35]; and (e), (f) column like structure on side wall [37]. Source: (a) Reprinted with permission from Ref. [37]. Copyright © 2005 Elsevier B.V. (b) R eprinted with permission from Ref. [35]. Copyright © 2010 Elsevier Ltd. (c) R eprinted with permission from Ref. [37]. Copyright © 2005 Elsevier B.V.

Off course, LBMM is effective method for micro- and nanofabrication, but it suffers with taper cut because of convergence and divergence nature. Due to this, straight cut profile is difficult to achieve. In case of drilled holes, the wall of holes is not straight or parallel as which hole diameters at entry and exit are differed. Instead of this, heat transfer and accumulation of heat into target material also presents challenge for LBMM process [2]. The diameter

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of focal spot leads role in formation of micro-craters and their profiles. Hence, the machining of high aspect ratio is difficult by LBMM process. In case of milling-LBMM, the re-positioning of workpiece is a big challenge [40]. 7.9 SUMMARY LBMM process shows their capability for precise manufacturing for micromachining and microfabrication. It is a highly flexible micro-processing process that can be employed for micron and submicron-level cutting, drilling, grooving, milling, and scribing operations on a variety of materials like plastics, metals, and ceramics. Extremely hard materials like diamond, glass, and graphite can be effectively machined by the LBMM process. Off course, the LBMM process significantly plays their roles in microfabrication by micromachining of material due to laser ablation. For this, the ultrafast (ps and fs) lasers offer excellent properties with high beam power to machine any type of material. The present chapter covers all the fundamentals related to the LBMM process, which are necessary for micromachining. Overall, the present documentation becomes a unique platform for readers to enhance their knowledge and know the roles and applicability of the LBMM process for microfabrication and micromachining areas. ACKNOWLEDGMENTS Authors gratefully acknowledged to Elsevier publishers and Journals for their kind permission for reprint/reuse of figures and Table of the previously published papers. The authors are also thankful to copyright center for their support to get the reprint/reuse permission easily from the desired location. KEYWORDS • • • • •

ablation absorption exotic laser laser beam micromachining

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• • • • • •

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laser material interaction materials melting microfabrication micromachining vaporization

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14. Asibu, Jr. E. K., (2009). Principles of Laser Materials Processing. John Wiley and Sons, New Jersey. 15. Nath, A. K., (2013). High power lasers in material processing applications: An overview of recent developments. Laser-Assisted Fabrication of Materials, 161, 69–111. 16. Becker, B., Enikeev, D., Glybochko, P., Rapoport, L., Taratkin, M., Gross, A. J., Vinnichenko, V., Herrmann, T. R. W., & Netsch, C., (2020). Effect of optical fiber diameter and laser emission mode (CW vs. pulse) on tissue damage profile using 1.94 µm Tm: Fiber lasers in a porcine kidney model. World Journal of Urology, 38, 1563–1568. 17. Desbiens, J. P., & Masson, P., (2007). ArF excimer laser micromachining of Pyrex, SiC and PZT for rapid prototyping of MEMS components. Sensors and Actuators A: Physical, 136, 554–563. 18. Yao, Y., Chen, H., & Zhang, W., (2005). Time scale effects in laser material removal: A review. International Journal of Advanced Manufacturing Technology, 26, 598–608. 19. Steen, W. M., (2001). Laser Materials Processing. Springer Nature, London, UK. 20. Duarte, F. J., (2015). Tunable Laser Optics. CRC Press, New York. 21. Bandres, M. A., & Vega, J. C. J., (2004). Ince–Gaussian beams. Optics Letters, 29, 144–146. 22. Chichkov, B. N., Momma, C., Nolte, S., Alvensleben, F. V., & Tünnermann, A., (1996). Femtosecond, picosecond and nanosecond laser ablation of solids. Applied Physics A, 63(2), 109–115. 23. Veiko, V. P., Skvortsov, A. M., Tu, H. C., & Petrov, A., (2015). Laser ablation of monocrystalline silicon under pulsed-frequency fiber laser. Scientific and Technical Journal of Information Technologies, Mechanics and Optics, 15, 426–434. 24. Kim, B., Iida, R., Doan, D. H., & Fushinobu, K., (2017). Mechanism of nanosecond laser drilling process of 4H-SiC for through substrate vias. Applied. Physics A, 123, 392–401. 25. Farrokhi, H., Gruzdev, V., Zheng, H., & Zhou, W., (2019). Fundamental mechanisms of nanosecond-laser-ablation enhancement by an axial magnetic field. Journal of the Optical Society of America B, 36, 1091–1100. 26. Zhang, H. Z., Huang, T., Liu, Z., Zhang, X., Lu, J. L., & Xiao, R. S., (2018). High fluence nanosecond laser machining of SiCp/AA2024 composite with high pressure assistant gas. Journal of Manufacturing Processes, 31, 560–567. 27. Silfvast, W. C., (2012). Laser Fundamentals. Cambridge University Press, Cambridge, UK. 28. Song, X., Xiaoc, K. L., Wu, X. Q., Wilde, G., & Jiang, M. Q., (2019). Nanoparticles produced by nanosecond pulse laser ablation of a metallic glass in water. Journal of Non-Crystalline Solids, 517, 119–126. 29. Cheng, J. Y., Wei, C. W., Hsu, K. H., & Young, T. H., (2004). Direct-write laser micromachining and universal surface modification of PMMA for device development. Sensors and Actuators B, 99, 186–196. 30. Rizvi, N. H., & Apte, P., (2002). Developments in laser micro-machining techniques. Journal of Materials Processing Technology, 127(2), 206–210. 31. Chen, T. C., & Darling, R. B., (2005). Parametric studies on pulsed near ultraviolet frequency tripled Nd: YAG laser micromachining of sapphire and silicon. Journal of Materials Processing Technology, 169(2), 214–218. 32. Klancnik, S., Begic–Hajdarevic, D., Paulic, M., Flick, M., Cekic, M., & Husic, M. C., (2015). Prediction of laser cut quality of tungsten alloy using the neural network method. Journal of Mechanical Engineering, 61, 714–720.

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33. Meng, H., Liao, J., Zhou, Y., & Zhang, Q., (2009). Laser micro-processing of cardiovascular stent with fiber laser cutting system. Optics and Laser Technology, 41, 300–302. 34. Khan, A. H., Celotto, S., Tunna, L., O’Neill, W., & Sutcliffe, C. J., (2007). Influence of microsupersonic gas jets on nanosecond laser percussion drilling. Optics and Lasers in Engineering, 45, 709–718. 35. Nayak, B. K., & Gupta, M. C., (2010). Self-organized micro/nano structures in metal surfaces by ultrafast laser irradiation. Optics and Lasers in Engineering, 48, 940–949. 36. Tunna, L., O’Neilla, W., Khana, A., & Sutcliffe, C., (2005). Analysis of laser micro drilled holes through aluminum for micro-manufacturing applications. Optics and Lasers in Engineering, 43, 937–950. 37. Pan, C. T., Hwang, Y. M., & Hsieh, C. W., (2005). Dynamic characterization of siliconbased microstructure of high aspect ratio by dual-prism UV laser system. Sensors and Actuators A, 122, 45–54. 38. Ali, B., Litvinyuk, I. V., & Rybachuk, M., (2021). Femtosecond laser micromachining of diamond: Current research status, applications and challenges. Carbon, 179, 209–226. 39. Gower, M. C., (2000). Industrial applications of laser micromachining. Optics Express, 7, 56–67. 40. Pham, D. T., Dimov, S. S., & Petkov, P. T., (2007). Laser milling of ceramic components. International Journal of Machine Tools and Manufacture, 47, 618–626.

CHAPTER 8

MICRO-WIRE ELECTRIC DISCHARGE GRINDING AS A FUTURE TECHNOLOGY IN MICROMACHINING PARTHIBAN MADHAVADEV and HARINATH MARIMUTHU Department of Mechanical Engineering, PSG College of Technology, Coimbatore, Tamil Nadu, India

ABSTRACT The need for micro-fabrication techniques has significantly increased in today’s manufacturing industries which include automobiles, aerospace, electronics, healthcare implants, biomedicine, AI, MEMS, etc. With advantages like small, lightweight, stability, functionality, and good performance of versatile micro-products make, it has wide applications almost in all areas globally. Many innovative hybrid manufacturing techniques are used in various industries to produce complex shapes with very good accuracy. The authors in this chapter designed a hybrid machining setup called wire electric discharge grinding (WEDG) to study the various micro-machining process parameters for hard materials like tungsten and HSS materials. WEDG provides high geometrical flexibility such that micro-parts with high aspect ratios can be manufactured. To enhance the manufacturing process, micro-components such as microelectrodes, micro-tools, and micro-probes were manufactured by the WEDG process. Furthermore, modeling and optimization techniques were investigated to obtain the best combination of machining process parameters to achieve optimal material removal rate (MRR), surface roughness (Ra), and diameter accuracy. Confirmation tests have been performed, and a maximum deviation of 1.5% was observed. A Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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maximum of 400 µm was achieved with this process, which is difficult to achieve in the conventional machining process. 8.1 INTRODUCTION A recent trend towards miniaturization has increased the demand for microparts in different applications from electronics to medical implants. With reduced size, weight, interest in the microscopic scale, and the rapid growth of micro-electromechanical systems (MEMS) the micromachining become important in manufacturing micro-components with different manufacturing techniques. To produce high precision micro-parts as well as productivity, now a day’s industries implement latest manufacturing technologies to overcome the difficulties during machining process. As a result, various micro-machining techniques such as laser micro-machining techniques, micro-electro discharge machining (µ-EDM), and micro-ultrasonic machining (USM) techniques are widely used in different industries. To increase the Performance of the above micro-machining techniques, various researchers are working with new techniques to fabricate micro-components. Among all microfabrication techniques, Micro-EDM is now becoming one of the promising micromachining techniques to fabricate complex three-dimensional microstructures with high accuracy and precision components of various shapes [1, 2]. Because of its distinctive advantages, it is suitable for machining micro-holes, micro-drilling for miniature parts. Micro-wire electric discharge machining (µ-WEDM) is a kind of EDM technology, normally used to produce components with complex-shaped micro-parts in a wide range of industries. In WEDM, instead of a rigid tool, a traveling wire is used to remove the material. However, to machine cylindrical Components with varying diameters additionally, a rotary mechanism is required in WEDM machine. Masuzawa et al. [3–5] (1985) introduced the new concept wire electric discharge grinding (WEDG) process to produce small cylindrical parts in WEDM by introducing additional rotary axes setup in WEDM machine and used the same traveling wire as the electrode to machine the materials of micron scale. The principle of WEDG machining is depicted in Figure 8.1. Micro-pins of size up to diameter 50 µm and surface roughness of about 18 nm Ra was achieved for a tungsten carbide material. The machining process involved two stages. In the first stage, the material was removed by the WEDG process under rough machining conditions, in the second and final step lapping processes were used to achieve the surface roughness. With further investigations, micro-pins and micro-tools with various diameters and types ranging several 100 micrometers were fabricated in the WEDG

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process. The fabricated micro-tools were applied as the tool for machining three dimensional shapes in electric discharge machining (EDM) process. Fleischer et al. [6] developed WEDG setup and suggested that it may be used to fabricate very small electrodes along with products like ejection pins and mold inserts. Using tungsten carbide as the work material micro-milling tools of size up to 100 µm were fabricated using this process.

pulse generator

dielectric

wire guide

wire electrode FIGURE 8.1

Principle of WEDG (Fleischer, 2004).

Source: Reprinted with permission from Ref. [6]. Copyright © 2004 Elsevier B.V.

Hence, the WEDG process uses a simple geometry, it is compared with the ion beam machining and the laser beam machining (LBM) process. Among these machining processes, the time duration to produce a single milling tool is much shorter in WEDG than other two machining processes. Dong-Yea Sheu [7, 8] by integrating wire EDM technique with OPED method, multiple spherical probes were manufactured. Multi-spherical probes of size around 40 µm were achieved and it is not possible to achieve the same results in any conventional machining processes. The special mechanisms are introduced in EDM to achieve different process variants by controlling the process variables in machines. The researchers compared the relationship between the

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hybrid turning and grinding process in diesinker EDM and Wire-cut EDM processes, and results reveal that the WEDG process can provide high quality and precision components [9]. In recent times the need for machining of precision micromachining technologies has increased because of the application of commercial products. Hence, to increase the machining efficiency and productivity various studies are implemented by the researchers. One such development is the implementation of Hybrid circuits and the Twin-Wire method in the WEDM process. To perform rough and finish machining in EDM process (DONG–YEA SHEU) [10], implemented the hybrid circuit system combining the transistor electro discharge circuit and RC electro pulse generator in Twin-Wire WEDG technology. By the newly developed hybrid system, the fabrication of micro-electrode tools was performed. Another method active suppling wire-electro discharge grinding (AS-WEDG) was developed and implemented to improve the aspect ratio of a micro-electrode and wire fluctuation during the machining process [11]. By combining numerical control technology (Yao-Sun) [12, 13], fabricated ultra-small electrodes utilizing low speed wire electric discharge turning process (LS-WEDT). The LS-WEDT method is illustrated schematically in Figure 8.2.

FIGURE 8.2

Schematic diagram of LS-WEDT (Yao-Sun, 2017).

Source: Reprinted with permission from Ref. [12]. Copyright © 2017 Elsevier B.V.

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With the help of the developed method, three spiral micro-cutting tools and a D-shaped micro-cutting tool were developed. Further to study the WEDG process, the authors [14–17] fabricated the micro-tools for micro-EDM with materials such as HSS, Tungsten carbide, polycrystalline diamond (PCD) micro-grinding tools and the fabricated micro-tools are further used to study the micro-hole drilling on micro-EDM. Extensive research is carried out in WEDM operation, to enhance the machining parameters with a new and hybrid optimization technique. During the last two decades, researchers had focused on more improved and machine learning (ML) algorithm techniques for machining different materials. Phate et al. [18] investigated the Aluminum silicate metal matrix composite (MMC) material in WEDM. The Taguchi-Based Grey-Fuzzy Approach is used to analyze the machining parameter and the most affecting parameter was shown to be the pulse on time. The influence of stress resulting on WEDM during machining of aluminum alloy was examined by Rao et al. [19] using the Taguchi technique. Kumar et al. [20] proposed response surface methodology (RSM) with GRA to optimize the response variables of Inconel 825 material on WEDM. Using additional tungsten powder mixed in the dielectric fluid, Kumar & Batra [21] investigated surface modification in the EDM process utilizing the Taguchi technique. Goswami et al. [22] used Nimonic 80A alloy as the work material to analyze the surface integrity and trim cut in WEDM. To investigate the process parameters, Taguchi parametric analysis was employed, with analysis of variance (ANOVA) as a confirmation test. Somashekhar et al. [23] to improve and optimize MRR, overcut, and SR in WEDM. A simulated annealing model was used to study the output responses. Similarly, to investigate the WEDM input process parameters, Sarkar et al. [24] suggested an integrated artificial neural network (ANN) technique, Bayesian regularization and early halting based on modeling. This technique was used to evaluate surface roughness in WEDM. Pasam et al. [25] investigated input factors to investigate surface finish during machining process. Applying genetic algorithm (GA), association between the input parameters and the surface finish were determined by the authors. In this chapter, an investigation is carried out to analyze the machining process of WEDG. To execute the micro-grinding operation, the WEDM machine is equipped with a rotating submersible axis. High speed steel (HSS) material is used as work material in this study. Achieving aspect ratio in micro-diameters is very tough. This chapter focuses on achieving a higher aspect ratio in machining HSS. The micro-electrode of size up to ø 400 µm were fabricated by the WEDG process. Modeling and optimization

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approaches were also examined to achieve optimal solutions for the selected response parameters such as material removal rate (MRR) and diametrical accuracy. 8.2 EXPERIMENTAL METHODS AND MATERIALS 8.2.1 EXPERIMENTAL SETUP OF WEDG MITSUBISHI WIRE CUT EDM (MODEL-FA 10S) machine was used for the experiments. Figure 8.3 represents the block diagram of the rotary axis mechanism used to conduct the machining operation on the WEDM machine.

FIGURE 8.3

Block diagram of rotary accessory for WEDM.

The rotary axis setup accessory is shown in Figure 8.4 was installed on the machine and the necessary settings were made to perform the experimentation by inspecting the run-out error of the spindle. Distilled water is used as the dielectric medium and pilot experimental runs were carried to find the suitable process parameter. HSS was used as the workpiece, which is a kind of material widely used in making taps and dies of various sizes with thermal stability and hardenability. Table 8.1 lists the physical parameters of HSS.

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TABLE 8.1 Material Property of HSS Material Property

Values

Density (Kg/m )

7.6

Poisson’s ratio

1.28

Modulus of elasticity (Gpa)

233

3

Thermal conductivity (W/(m-k))

20.2

Thermal expansion coefficient (K–1)

12.6×10–6

Specific heat (J/(Kg-K))

0.42

FIGURE 8.4

Rotary spindle set up on the WEDM.

For machining the workpiece material, the process constants values are obtained directly from the machine library. Using the design matrix as a guide, the input process parameters are selected, and experimentation is carried out using the WEDG machining setup. Table 8.2 gives the constant process parameters that are selected for machining HSS material. 8.2.2

EXPERIMENTAL DESIGN

To improve the machining process, statistical design techniques are implemented in the engineering field. The Taguchi approach for designing experiments aids

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in the planning of experimental runs with a goal of desired data in a constrained manner. The product quality can be improved by performing experimental runs. And the performance data can also be obtained by the specified process. So, in this study the use of Taguchi’s technique is applied to determine the relationship between the impacts of process factors in the WEDG process during machining HSS material. It quantifies the variance in studies using a general S/N Ratio. And the most influencing parameters for designed factors are determined using ANOVA. TABLE 8.2

Constant Parameters

Parameters

Unit

Values

Wire speed

(m/min)

1

Diameter of the specimen

(mm)

2

Machining length

(mm)

10

Feed rate

(mm/min)

3

Wire tension

(N)

10

Current

(A)

7

Table 8.3 lists the factors and levels used in WEDG investigations. Factor’s interplay is not taken into consideration in this study. The experimental plan consists of 9 runs focused on the input parameters of voltage, spark gap, and spindle speed. The diametrical accuracy and MRR are the output responses that were investigated. Three factors are chosen for the experimental work from the L9 orthogonal array, which has three levels. Table 8.4 shows the experimental table for machining process parameters employing the L9 orthogonal array. TABLE 8.3

Experimental Plan

Parameters

Level 1

Level 2

Level 3

Spark Gap (mm)

0.18

0.2

0.22

Spindle speed (rpm)

300

500

700

Voltage (V)

50

60

70

The levels of the orthogonal array of performing the experimental experiments were assigned to the experimental runs according on the factors. The compatibility values were acquired in each of the nine sets of trials, with three replications each trial, and the average values are calculated. Before and after

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the machining process, the stereo microscope is utilized to measure the actual size of the specimens. Table 8.5 shows the intended parameter measurements and computed records, as well as their responses. The MRR and diametrical change on the material were observed from experimental findings. TABLE 8.4 Exp. No. 1 2 3 4 5 6 7 8 9

Design Matrix-L9 Orthogonal Array Spark Gap in mm 0.18 0.18 0.18 0.20 0.20 0.20 0.22 0.22 0.22

Spindle Speed in rpm 300 500 700 300 500 700 300 500 700

Voltage in V 50 60 70 60 70 50 70 50 60

8.3 RESULTS AND DISCUSSIONS 8.3.1 DESIRED PARAMETERS AND THEIR RESPONSE The S/N ratio measurement is used to correlate the level of machining attributes. Process variable performance and S/N ratio values are determined based on the better machining character type, such as lower-the-better, nominal-the-better (NB), and higher-the-better. While comparing two sets of samples, it is the most significant and useful parameter in considering target and variation by comparing the mean values alone. Taguchi’s approach of experimental design implements the signal-to-noise ratio in ANOVA calculations. Finally, the result was measured in terms of the respective product’s response to noise and signal factors. Each level of the cutting parameter’s mean value of S/N ratio summarizes the MRR and diametrical accuracy. The standard relation given in Eqn. (1) has been used to calculate rate of material removal in wire-electrical discharge (ED) grinding process: π 4

 

Material removal rate (MRR) = × (D02 − D 2 ) 1/ T (mm3/min.)

(1)

where; ‘D0’ is the workpiece initial diameter in (mm); ‘D’ is the workpiece final diameter in (mm); ‘l’ is the workpiece length; T is the time (min).

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TABLE 8.5

Designed Parameters and Their Response

Trial. Spark No. Gap (mm)

Spindle Speed (rpm)

Voltage (V)

Final Diameter Machining MRR Diameter Deviation Time (µs) (mm3/min) D (mm) (µm)

1.

0.18

300

50

0.575

–25

212

8.645

2.

0.18

500

60

0.585

–15

302

8.618

3.

0.18

700

70

0.592

–8

704

8.599

4.

0.20

300

60

0.609

9

340

8.551

5.

0.20

500

70

0.580

–20

691

8.632

6.

0.20

700

50

0.599

–1

252

8.579

7.

0.22

300

70

0.626

26

682

8.501

8.

0.22

500

50

0.620

20

261

8.519

9.

0.22

700

60

0.631

31

342

8.487

From Figure 8.5, the spark gap of 0.22 mm, spindle speed of 700 rpm, and voltage of 50 V are the peak values in the graph and clearly show the optimum parameters for the best diametric accuracy. This is because that the amount of thermal energy produced at high spindle speed, makes the material removal in a consistent manner, which could result in lesser diameter deviation during machining of the HSS material.

FIGURE 8.5

S/N ratio for diametric accuracy.

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The S/N ratio values for the maximum rate of material removal is shown in Figure 8.6. So, Large the better characteristics is selected for the optimum parameter. For a spark gap of 0.18 mm, a spindle speed of 500 rpm, and a voltage of 50 V, the graph’s maximum peak values were observed. It is observed that a smaller spindle speed facilitates more thermal energy at the cutting zone, which removes more material and results in higher MRR.

FIGURE 8.6

S/N ratio for MRR.

The ANOVA results are analyzed with the help of statistical software as shown in Tables 8.6 and 8.7. Table 8.6 of ANOVA for MRR reveals that the most significant impact on the MRR was spark gap, with a percentage contribution of 815. Similarly, the spark gap with a percentage of 50% influences on diametrical accuracy, followed by spindle speed and voltage, which have a 24% and 17% contribution, respectively, and have a lower impact on diametric accuracy. This shows that at a lower spark gap the higher MRR will be obtained. As more thermal energy takes place during a short pulse duration. Which might remove more material whereas the spindle speed and voltage do not have much effect on the material removal.

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TABLE 8.6

ANOVA – MRR

Source

DF

Adj. SS

Adj. MS

Spark gap (mm) Voltage (V) Spindle speed (rpm) Error Total

2 2 2 2 8

0.03002 0.00296 0.00296 0.00096 0.03689

0.01501 0.00148 0.00148 0.00048 –

TABLE 8.7

F-Value P-Value Percentage Contribution 31.42 0.031 81 3.09 0.244 8 3.09 0.244 8 – – 3 – – 100

ANOVA – Diameter Accuracy

Source

DF

Adj. SS

Adj. MS

Spark gap (mm) Voltage (V) Spindle speed (rpm) Error Total

2 2 2 2 8

374.9 128.8 184.8 75.1 –

187.4 34.4 92.4 37.6 –

F-Value P-Value Percentage Contribution 1.25 0.445 50 0.05 0.949 17 0.24 0.806 24 – – 9 – – 100

8.3.1.1 IMAGES OF THE MACHINED COMPONENT After the machining process, a polarized light optical microscope is used to take measurements on all the samples. An optical microscope image of the component measured for sample number 5 is shown in Figure 8.7. 8.3.1.2 OPTIMAL SOLUTIONS The output response values for each experimental trial are gathered and recorded to calculate the maximum MRR and the nominal diametrical accuracy for the machining operation. After obtaining the optimal values from the L9 experiments, the experiments are repeated with the optimal values to validate optimal results. The optimal results for the output responses are shown in Table 8.8. 8.3.2

GREY RELATIONAL ANALYSIS

Several researchers implemented Grey relational analysis technique to find the best optimal parameters. It is an effective way for determining similarity

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among the factors. With the degree of proximity multiple performance characteristics and their interrelationship can be effectively determined. The performance of the multiple responses is normalized and turned into a number between 0 and 1 based on the gray relational grade. Based on the grade results, the best machining parameters are then identified.

FIGURE 8.7 TABLE 8.8

Machined workpiece. Optimum Machining Parameters

Parameters

Spark Gap (mm)

Spindle Speed (rpm) Voltage (V)

Diametrical accuracy

0.20

700

50

MRR

0.18

300

50

The steps following outline the GRA analysis approach that was utilized in this study to determine the best parameter for the WEDG process. 8.3.2.1 PREPROCESSING THE DATA In data pre-processing, the S/N ratio value of the experimental data set was converted into normalized values. The data values converted were a range of 0–1. The normalization has been done using the formula in Eqns. (2) and (3):

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 For “Higher-the-Better” Condition: Xi =

yi− min .yi max .yi − min .yi

(2)

 For Lower-the-Better Condition: Xi =

max .yi − yi max .yi − min .yi

(3)

where; Xi is the normalized value; yi is the S/N ratio value where, i = 1, 2, 3, …, n. 8.3.2.2 GREY RELATIONAL CO-EFFICIENT The computation of gray relational coefficient is necessary to evaluate the actual and ideal relationship between the normalized values. It is calculated by using Eqn. (4). γi =

∆ min − £.∆ max ∆ i + £.∆ max

(4)

where; Δi the deviation sequence of the reference sequence; the reference sequence value considered as “1;” £ = 1, 0 it is considered as 0.5. 8.3.2.3 GREY RELATIONAL GRADE (GRG) The gray relational grade gives the optimal solution. The corresponding parameter that has higher GRG value is considered as the optimal value. The weighted sum of the gray relational coefficient is the gray relational grade (GRG). Eqn. (5) is used to calculate it. = GRC

∑

n i =1

Wi . Γi

(5)

8.3.2.4 GREY RELATIONAL GRADE (GRG) CALCULATION The above steps were used to analyze the grey relational grade (GRG) matrix. Individual quality targets can be determined by the degree of effect for each controllable process factor. Table 8.9 shows the calculated response values and S/N ratio for each experiment.

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TABLE 8.9 Response Value and S/N Ratio Values Ex. No.

ACC.

S/N

MRR

S/N

1

–25

–27.9588

8.645

18.7353

2

–15

–23.5218

8.618

18.7081

3

–8

–18.0618

8.599

18.689

4

9

–19.0849

8.551

18.6403

5

–20

–26.0206

8.632

18.7222

6

–1

0

8.579

18.6687

7

26

–28.2995

8.501

18.5894

8

20

–26.0206

8.519

18.6078

9

31

–29.8272

8.487

18.5751

Table 8.10 displays the normalized S/N ratio values as well as the deviation values. Table 8.11 displays the computed gray relational coefficient value as well as the gray relational grade values. The Ranks were assigned to all the experimental runs based on the gray relational grade values. TABLE 8.10 Normalized S/N Ratio Values and the Deviation Values Ex. No.

Accuracy (X1)

Normalized S/N Ratio Values MRR (X2)

Accuracy (Δ1)

Deviation Values MRR (Δ2)

1

0.937359

1

0.062641

0

2

0.788602

0.830212

0.211398

0.169788

3

0.605548

0.710986

0.394452

0.289014

4

0.639849

0.406991

0.360151

0.593009

5

0.872378

0.918227

0.127622

0.081773

6

0

0.58427

1

0.41573

7

0.948782

0.089263

0.051218

0.910737

8

0.872378

0.20412

0.127622

0.79588

9

1

0

0

1

8.3.3 TRIALS FOR ASPECT RATIO MACHINING Using the optimal parameters given in Table 8.8, machining process were done to achieve a good aspect ratio, i.e., L/D ratio. With current experimental setup, the aspect ratio of 80 [L/D 32/∅0.4 mm] and 104 [L/D 26/∅0.25 mm] were achieved. Figure 8.8 shows the aspect ratio of the machined sample.

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TABLE 8.11 Grey Relational Coefficient Value and the Grey Relational Grade Values Ex. No.

Grey Relational Coefficients

GRG

RANK

γ1

γ2

1

0.888666

1

0.944333

1

2

0.702842

0.746505

0.724673

3

3

0.559001

0.633703

0.596352

6

4

0.581293

0.457453

0.519373

8

5

0.796658

0.859442

0.82805

2

6

0.333333

0.546012

0.439673

9

7

0.907082

0.354425

0.630753

5

8

0.796658

0.385838

0.591248

7

9

1

0.333333

0.666667

4

FIGURE 8.8

Micro-electrode with different aspect ratio (80 and 104).

8.4 CONCLUSIONS Investigation of Hard material has been studied to increase the wire cut EDM’s machining performance. To accomplish the needed micromachining processes, the designed and developed rotary axis setup was placed on the WEDM machine. Using this WEDG technology, micro-tools and parts were fabricated with a high aspect ratio. In this chapter, the best diametrical accuracy and MRR parameters for micromachining of HSS material as the workpiece were discovered and compared with contour and surface plots.

Micro-Wire Electric Discharge Grinding as a Future Technology

189

Grey relation analysis was used to investigate the effects of various process factors and their interactions. The most significant parameter is determined using ANOVA. From the analysis, the percentage contribution of the spark gap of 50% and 81%, respectively, reveals that they have a significant impact on diametric accuracy and MRR during micro-machining process. The gray relational also reveals that the values for optimal parameters are better through its rankings from the results. Maximum aspect ratios up to 80 and 104 were achieved with the current experimental model. 8.5 SCOPE FOR FUTURE WORK • • •

Limited materials like HSS, Tungsten, and Tungsten carbide were investigated in this process. Hard and light materials which are used in aerospace, defense, MEMS, biomedical applications can be studied. Machining parameters can be analyzed by varying the Wire diameters in WEDM. Micro-tools for EDM and other micromachining applications can be manufactured.

KEYWORDS • • • • • • •

grey relation analysis grey relational grade micro-electrodes micro-machining optimal solutions wire electric discharge grinding wire electric discharge machine

REFERENCES 1. Rahman, M., Asad, A. B. M. A., Masaki, T., Saleh, T., Wong, Y. S., & Senthil, K. A., (2010). A multiprocess machine tool for compound micromachining. International Journal of Machine Tools and Manufacture, 50, 344–356.

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2. Hourmand, M., Sarhan, A. A., & Sayuti, M., (2016). Micro-electrode fabrication processes for micro-EDM drilling and milling: A state-of-the-art review. The International Journal of Advanced Manufacturing Technology, 91, 1023–1056. 3. Masuzawa, T., Fujino, M., & Kobayashi, K., (1985). Wire electro-discharge grinding for micro-machining. CIRP Annals—Manufacturing Technology, 34, 431–434. 4. Yu, Z., Masuzawa, T., & Fujino, M., (1998). Micro-EDM for three-dimensional cavities–development of uniform wear method. CIRP Annals, 47, 169–172. 5. Masuzawa, T., (2000). State of the art of micromachining. CIRP Annals, 49, 473–488. 6. Fleischer, J., Masuzawa, T., Schmidt, J., & Knoll, M., (2004). New applications for micro-EDM. Journal of Materials Processing Technology, 149, 246–249. 7. Sheu, D. Y., (2004). Micro-spherical probes machining by EDM. Journal of Micromechanics and Microengineering, 15, 185–189. 8. Sheu, D. Y., (2004). Multi-spherical probe machining by EDM combining WEDG technology with one-pulse electro-discharge. Journal of Materials Processing Technology, 149, 597–603. 9. Uhlmann, E., Piltz, S., & Jerzembeck, S., (2005). Micro-machining of cylindrical parts by electrical discharge grinding. Journal of Materials Processing Technology, 160, 15–23. 10. Sheu, D., (2010). Microelectrode tools manufacturing by hybrid circuits twin-wire electro discharge grinding. Materials and Manufacturing Processes, 25, 1142–1147. 11. Li, Z., Bai, J., Cao, Y., Wang, Y., & Zhu, G., (2019). Fabrication of microelectrode with large aspect ratio and precision machining of micro-hole array by micro-EDM. Journal of Materials Processing Technology, 268, 70–79. 12. Sun, Y., & Gong, Y., (2017). Experimental study on the microelectrodes fabrication using low speed wire electrical discharge turning (LS-WEDT) combined with multiple cutting strategy. Journal of Materials Processing Technology, 250, 121–131. 13. Sun, Y., Ma, X., & Gong, Y., (2018). An experimental study for evaluating speed parameters on surface roughness of LS-WEDT and its application in fabricating microelectrodes. International Journal of Abrasive Technology, 8, 261–277. 14. Parthiban, M., Krishnaraj, V., & Naveen, A. R., (2014). Optimization of parameters for diameter accuracy in wire electric discharge grinding for micro machining of tungsten rods. Applied Mechanics and Materials, 592–594, 625–629. 15. Pratap, A., & Patra, K., (2019). Effects of electric discharge dressing parameters on polycrystalline diamond micro-tool surface topography and their micro-grinding performances. International Journal of Refractory Metals and Hard Materials, 82, 297–309. 16. Huang, T., & Sheu, D., (2020). High aspect ratio of micro hole drilling by micro-EDM with different cross-section shape micro tools for flushing process. Procedia CIRP, 95, 550–553. 17. Parthiban, M., Krishnaraj, V., Kanchana, J., & Kandha, B. M., (2018). Optimization of material removal rate in wire electric discharge grinding for micro machining of tungsten electrodes. Indian Journal of Engineering and Material Sciences, 25, 307–314. 18. Mangesh, R. P., Shraddha, B. T., & Vikas, R. Ph., (2019). Analysis of machining parameters in WEDM of Al/SiCp20 MMC using Taguchi-based grey-fuzzy approach. Modeling and Simulation in Engineering, 2019. 19. Rao, P. S., Ramji, K., & Satyanarayana, B., (2016). Effect of wire EDM conditions on generation of residual stresses in machining of aluminum 2014 T6 alloy. Alexandria Engineering Journal, 55, 1077–1084.

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20. Kumar, P., Gupta, M., & Kumar, M., (2018). Optimization of process parameters for WEDM of Inconel 825 using grey relational analysis. Decision Science Letters, 7, 405–416. 21. Kumar, S., & Batra, U., (2012). Surface modification of die steel materials by EDM method using tungsten powder-mixed dielectric. Journal of Manufacturing Processes, 14, 35–40. 22. Goswami, A., & Kumar, J., (2017). Trim cut machining and surface integrity analysis of Nimonic 80A alloy using wire cut EDM. Engineering Science and Technology, an International Journal, 20, 175–186. 23. Somashekhar, K. P., Mathew, J., & Ramachandran, N., (2012). A feasibility approach by simulated annealing on optimization of micro wire electric discharge machining parameters. International Journal of Advanced Manufacturing Technology, 61, 1209–1213. 24. Sarkar, S., Ghosh, K., Mitra, S., & Bhattacharya, B., (2010). An integral approach to optimization of WEDM combining single pass and multi pass cutting operations. Materials and Manufacturing Processes, 25, 799–807. 25. Pasam, V. K., Battula, S. B., Valli, P. M., & Swapna, M., (2010). Optimizing surface finish in WEDM using the Taguchi parameter design method. Journal of the Brazilian Society of Mechanical Sciences and Engineering, 32, 107–113.

CHAPTER 9

NANO-ADDITIVES ASSISTED MQL AND OPTIMIZATION IN MICROMACHINING PROCESSES T. JAGADEESHA,1 SANDIP KUNAR,2 GOLAM KIBRIA,3 and MANOJ NIKAM4 Department of Mechanical Engineering, National Institute of Technology, Kozhikode, Kerala, India

1

Department of Mechanical Engineering, Aditya Engineering Collage, Andhra Pradesh, India

2

Department of Mechanical Engineering, Aliah University, West Bengal, India

3

Department of Mechanical Engineering, Bharati Vidyapeeth College of Engineering, Mumbai, Maharashtra, India

4

ABSTRACTS Metal cutting is a subtractive manufacturing process used to give size, shape, and surface finish to the finished components. Metal cutting is a high-shear, high energy-consuming, and heat-generation process with a considerable amount of tool wear. It is important to reduce the cutting force and improve the metal removal rate to increase productivity and to produce quality products meeting customer specifications. Cutting fluids can reduce the cutting force, reduce heat generation and improve surface finish. Wet machining gives a better surface finish than dry machining but at the expense of extra cost. Minimum quality lubrication (MQL) is an impressive alternative to conventional dry machining processes. Rather than spraying Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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lubricants as jets, MQL small amount of coolant or mist of coolant is sprayed at the cutting zone. This causes a significant reduction in cutting forces, cutting temperature, and material wear by reducing the friction coefficient significantly. This chapter deals with nano-additives assisted MQL and optimization of the process parameter to minimize the cost and maximize the beneficial decision variables such as MRR, etc. The experimental procedure is discussed in detail. The importance of the various nanoparticles in MQL is discussed in the synthesis procedure. Finally, Detailed optimization of the nano-assisted MQL procedure is presented for effective cutting processes. 9.1 INTRODUCTION Metal Cutting is one of the most important manufacturing processes which deals with removal of undesired material from the workpiece. Metal Cutting finds its application in production of many of crucial and complex parts. The process is susceptible to lower material removal, heat generation and pollution due to coolants. This led to emergence of hard machining technique especially for materials with hardness value 45–75 HRC like hardened steel. Despite its benefits like adaptability to complex contours, high productivity and better tool life courtesy the tools with carbon insert, the idea of coolant or lubricating agents influencing not only thermal shock but also avoid breakage arose lot of questions. At the same many studies came up that emphasized on comparison between untextured and textured tools and the influence of reduced contact length influencing machining parameters [1]. Use of textured tools in machining of Ti-6Al-4V have better tool chip tribological property compared to untextured tools [2]. Combination of microtextured particle along with lubricant can significantly reduce cutting force, cutting temperature and improve anti-adhesion and anti-wear [3]. Thrust force imparts higher value and feed rate greatly influences roughness value. The heat generated in hard machining is comparatively really high and hence leads to thermal distortion. Thus, necessitates bringing down of cutting zone temperature [4]. It was then minimum quality lubrication (MQL) developed as an alternative to dry machining processes. In MQL, small amount of emulsion or lubricant is sprayed at the primary cutting zone so that there is a reduction in heat carried away by the chip and tool. This also reduces the cutting forces as well as strain energy loss due to high friction [5]. Application of MQL system can save up to 50% of coolant for the machining operation there by enabling the system to be more sustainable and ecofriendly [6]. Tough to cut

Nano-Additives Assisted MQL and Optimization

195

material can use MQL technique by adding nano-additives to the base fluid. Vegetable oil for Minimum Quantity Lubrication (MQL) improved wear behavior and surface quality (SQ) while machining difficult-to-cut materials [7]. Adding nanoparticle additives enhance the lubrication performance and important MQL machining techniques uses different mineral oil, vegetable oil and nanofluids [8]. The tool wear quality can be significantly improved by 37.2% on addition of 4 wt.% Al2O3 nanoparticles when we machine difficult to cut materials with MQL technique [9]. The SQ of the machined surface is a function of feed rate and volume fraction of nano-additives [10, 11]. Using graphene’s nanofluid to machine Inconel 718 cutting force, tool life, and surface smoothness can be improved as well as tribological and thermal characteristics [12–15]. 9.2 METHODOLOGY This study divided into two experimental set up, where first one is focused on the comparison of MQL with Nano-additives with other lubricant-coolant techniques, by taking Inconel 718 material and nano-additives as graphene (5 μm and 15 μm) mixer in the vegetable oil. Second experimental is focused on determining how the MQL parameters influences the significant machining parameter like surface integrity, power consumption and tool life. Here the performance of MQL parameters is evaluated by observing the hard turning of difficult to cut materials. 9.2.1 EXPERIMENTAL SETUP 1 For this experiment Inconel 718 material was used, which is difficult to cut alloy material, of dimension 41 mm of diameter and 1,000 mm of length. Using Kalling’s reagent the workpiece was polished so as to look at microstructure. The grain size is shown in Figure 9.1. The CVD coated cutting tool used in this experiment. A conventional flood is a process where the lubricant is poured on the workpiece continuously without any regulation. Whereas MQL is that the method where lubricants utilized are used in a minimal quantity with a mixture of combination of compressed air. During this experiment, the pressure of the air is taken as 70 MPa, and also the rate at which the lubricant can flow within the process is 1.320 liter per hour. Nozzle is used to make use of lubricants effectively on the cutting zone (Figure 9.2).

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Electro-Micromachining and Microfabrication

FIGURE 9.1

(a) Microstructure of Inconel 718 material; and (b) distribution of grain size.

FIGURE 9.2

The experimental setup of MQL with nano-additives.

For cutting fluid base oil LB 2000 is taken, which is biodegradable made from natural ingredients. Two different sizes graphene were considered of size 5 μm and 15 μm, thickness is almost same for both within the range 6–8 nanometer, surface area for 5 μm size graphene is 50–80 m2/g and for 15 μm is 120–150 m2/g as shown in Figure 9.3(a) and (b). The Nanofluid was prepared in two steps. The first graphene was weighed up to 0.1 mg and mixed with base oil for 30 min using a magnetic stirrer and placed in Elmasonic ultrasonic vibration bath for 1 hr. The fully prepared fluid is shown in Figure 9.3(c). Manual visualization of sedimentation was being done after 24 hrs. of stay since there is no other alternatives available.

Nano-Additives Assisted MQL and Optimization

197

FIGURE 9.3 SEM image of (a) graphene 5 μm platelets; (b) graphene 15 μm platelets; and (c) final solution of nanofluids.

As per ISO-251782:2012 standard, areal surface texture parameter analyzes were done. Chips were collected and scanned under an SME microscope and readings were taken. 9.2.2 EXPERIMENTAL SETUP 2 9.2.2.1 DEVICES 1. Set Up: CS460×1000 lathe, tungsten carbide inserts coated with Al2O3/TiCN by chemical vapor deposition (CVD), tool holder with higher precision. 2. MQL System: NOGA minicool, pressure stabilizer, soybean oil, emulsion with 5 Al2O3 (average size of 30 nm) and MoS2 (size 30 nm) particles and water based. 3. Measuring Devices: dynamometer, SJ-210, data acquisition system, DASY lab software.

198

Electro-Micromachining and Microfabrication

4. Material-90CrSi: 0.85–0.95% C, 1.2–1.6% Si, 0.3–0.6% Mn, max. 0.4% Ni, max. 0.3% S. 9.2.2.2 AL2O3 AND MOS2 NANO-FLUID PREPARATION Weight percent concentration =

Solute weight (g) ×100 Solution weight (g)

(1)

The nanoparticles are found to have a non-homogenous mixture due the fact that Al2O3 and MoS2 are insoluble in the vegetable oil and water-based emulsions. This leads to sedimentation of these particles as waste in the bottom and efficient MQL properties. Hence, they are stored in 3000868 ultrasonicator and are directly passed on to MQL system. 9.2.2.3 DESIGN OF EXPERIMENT A full factorial design is conducted to study the effects of various MQL parameters like Base fluid, nanoparticles, nano-concentration, and cutting speed against cutting forces and surface roughness. A 2k-p analysis with k = 4 variables is conducted. N = 24 –1 = 8. The analysis is conducted using Minitab software considering spindle speeds as 650 and 950 rpm and depth of cut = 81.7 mm/rev and feed = 0.1 mm/rev (Table 9.1). TABLE 9.1

Various Control Factors and Their Levels for ANOVA Analysis

Control Factor Primary fluid Secondary nanoparticles Concentration (wt.%) Cutting speed (m/min)

Notation X1 X2 X3 X4

Two Levels Emulsion Aluminum oxide 1.1 81

Soyabean oil Molybdenum disulfide 3.1 119

9.3 RESULTS AND DISCUSSION 9.3.1 RESULT OBTAINED FROM COMPARISON OF FIVE DIFFERENT LUBRICANT-COOLANT CONDITION 9.3.1.1 NANOFLUIDS PROPERTIES The developed nanofluids were found to be firm and stable and no significant sedimentations were found to be observed, even after the duration of 24

Nano-Additives Assisted MQL and Optimization

199

hours. The density of MQL_5 and MQL_15 was calculated, and it is found out to be 918.29 kg/m3. This calculated density is found to be moderately higher than that of the pure oil. Figure 9.4 gives the graphical view of cutting nanofluid’s dynamic viscosity as a shear rate function at the temperature of 40°C and 100C. From the graph drawn a usual trend is found out, i.e., at the lower shear rate all the cutting fluids are found to show shear thinning behavior and as the shear rates is found to increase it is found to shown Newtonian behavior for both the temperatures. Furthermore, it is observed that as the temperature raises, the viscosity was found to increase as expected. The addition of only few micro- (15–17) nanoplatelets changes viscosity value and increases up to of 7–8% and 1–1.5% at 40°C and 100°C, respectively, while adding in base oil. Whereas by adding 5 μm size nanoplatelets graphene it is observed that at testing temperature, decrease in the viscosity. Lowest viscosity displaced in the conventional flood condition since it was mainly made of water. Due to the boiling of water, it is not possible to draw curve at 100°C during flood time.

FIGURE 9.4

Dynamic viscosity vs shear rate of cutting fluids at 40 and 100°.

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Electro-Micromachining and Microfabrication

Figure 9.5 displays cutting fluid specific heat as a function of temperature. With the irrespective of the coolant condition, small increase in the specific heat capacity is totally acceptable. Nevertheless, it is clearly visible in the graph is that MQL_5 has more specific heat when compared to MQL_15.

FIGURE 9.5

Relation between temperature with specific heat capacity.

9.3.1.2 SURFACE FINISH For all of the lubricating cooling settings studied, the surface roughness increases as the cutting speed increases as shown in Figure 9.6. Furthermore, independent of the cutting speed, the influence of the lubricating-cooling condition is observed. Dry cutting, as expected, produced the roughest surfaces at all cutting speeds. Dry cutting, it was supposed, resulted in the development of a rough surface at any speed. It has been proven that using MQL 5 always aids in achieving the best SQ. As it is observed from Figure 9.7, in the dry conditions the helix angle is small. As also other than dry conditions there is no significant change in the helix angle in the other conditions. No matter the utilization of nano-additives there is no change in the mechanism of the formation of the chip.

Nano-Additives Assisted MQL and Optimization

FIGURE 9.6

201

Different lubricating-cooling conditions in surface roughness vs. cutting speed.

FIGURE 9.7 Chip morphology at (a) dry lubrication condition, (b) flood lubrication, (c) MQL-pure vegetable oil, (d) MQL with 5 µm graphene, and (e) MQL with 15 µm graphene. Figures (f), (g), (h), (j), and (k) are magnified images of the chips of (a), (b), (c), (d), and (e), respectively.

9.3.1.3 EFFECT OF DIFFERENT COOLANT CONDITION ON CHIP FORMATION Chip comparison ratio λ is given in Table 9.2. Figures 9.8(a) and (b) show the increase in grain size from the machined surface, gradually attaining 120 μm, and also it shows the severe plastic deformed (SPD) in which it

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Electro-Micromachining and Microfabrication

is observed that in cutting direction grain size are deformed and reduced. During dry cut high-speed cutting leads to thermal softening which reduces the degree of deformation of grain. TABLE 9.2 Relation between Cutting Speed and Lubricating-Cooling Condition on Compression of Chip 70 m/min

100 m/min

130 m/min

No lubrication

2.7

2.5

2.7

Flooded lubrication

1.9

2.1

2.1

MQL (pure)

2.0

2.0

2.3

MQL (5)

1.8

1.8

2.0

MQL (15)

2.3

2.0

2.2

FIGURE 9.8 (a) Microstructure; (b) grain size; (c) dry; and (d) MQL-5 condition microhardness Vs distance from machine surface at 130 m/min.

Nano-Additives Assisted MQL and Optimization

203

The heat dissipation will be higher when there is high specific heat, and hence cooling takes place, and also poor machinability character, with high viscosity which is the consequence of reduction in loss of pressure at a specific rate of flow [4]. Low viscosity, less friction force between tool and workpiece. Figures 9.4 and 9.5 shows that MQL-5 gives the better working fluid result when compared with rest of the condition. In case of MQL-15 shows high viscosity when compared with pure oil, no significant raise in the specific heat hence it gives poor performances when compared to MQL-Pure. 9.4

CONCLUSIONS

After studying the performance of various lubricating techniques, it was concluded that MQL-5 (5 μm graphene added into the vegetable oil) increases the specific heat without increasing the viscosity, whereas we got the opposite result in adding 15 μm graphene. It was also observed that MQL-5 gave better surface roughness, as a result of increasing the best working fluid. Whereas using MQL-15 there was no improvement in the surface finish, even it resulted in worsen the case when compared to conventional flood condition. Thin chip was obtained in the presence of MQL-5 in the vegetable oil and got highest when MQL-15 is used. The presence of Al2O3 and MoS2 significantly improved the tribological and thermal properties of the MQL lubricant. Furthermore, it was emphasized that for hard turning since the thrust force contributes more to the dimensional accuracy Al2O3 nanofluid is utilized. For minimizing the surface roughness value Al2O3 with soybean oil is found to be the better alternative. In further investigations main priority should be given to the thermal and tribology impacts on nanoparticle concentration and nanoparticle morphology along with optimizing Al2O3 and MoS2 parameter in nanofluid. KEYWORDS • • • • • •

cutting speed lubricant-coolant condition minimum quality lubrication nano-additives nano-fuid nanoparticle

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REFERENCES 1. Bouacha, K., Yallese, M. A., Mabrouki, T., & Rigal, J. F., (2010). Statistical analysis of surface roughness and cutting forces using response surface methodology in hard turning of AISI 52100 bearing steel with CBN tool. International Journal of Refractory Metals and Hard Materials, 21, 349–361. 2. Duc, T. M., Long, T. T., & Chein, T. Q., (2019). Performance evaluation of MQL parameters using Al2O3 and MoS2 nanofluids in hard machining of 90CrSi steel. Lubricants, 1, 40–47. 3. Zhang, K., Deng, J., Meng, R., Gao, P., & Yue, H., (2015). Effect of nano-scale textures on cutting performance of WC/Co-based Ti55Al45N coated tools in dry cutting. International Journal of Refractory Metals and Hard Materials, 51, 35–49. 4. Singh, R. K., Sharma, A. K., Dixit, A. R., Tiwari, A. K., Pramanik, A., & Mandal, A., (2017). Performance evaluation of alumina-graphene hybrid nano-cutting fluid in hard turning. Journal of Cleaner Production, 161, 830–845. 5. Sharma, A. K., Tiwari, A. K., & Dixit, A. R., (2016). Effect of minimum quality lubrication (MQL) in machining processes using conventional and nanofluid based cutting fluids: A comprehensive review. Journal of Cleaner Production, 121, 1–18. 6. Hegab, H., Salem, A., Rahnamayan, S., & Kishawy, H. A., (2021). Analysis, modeling, and multi-objective optimization of machining Inconel 718 with nano-additives based minimum quantity coolant. Applied Soft Computing, 101, 107416. 7. Sakthi, S., Hariharan, R., Mahendran, S., & Azhagunambi, R., (2021). Effect of nano additives on magnesium alloy during turning operation with minimum quality lubrication. Materials Today: Proceeding. (In press). 8. Anandan, V., Babu, M. N., Sezhian, M. V., Yildirim, C. V., & Babu, M. D., (2021). Influence of graphene nanofluid on various environmental factors during turning of M42 steel. Journal of Manufacturing Processes, 61, 90–103. 9. Yi, S., Mo, J., & Ding, S., (2019). Experimental investigation on the performance and mechanism of graphene oxide nano fluids in turning Ti-6Al-4V. Journal of Manufacturing Processes, 41, 164–174. 10. Le, G., Rachele, B., Andrea, G., Ning, H., & Stefania, B., (2020). Sustainable turning of Inconel 718 nickel alloy using MQL strategy based on graphene nanofluids. The International Journal of Advanced Manufacturing Technology, 101, 3159–3174. 11. Hegab, H., Darras, B., & Kishawy, H. A., (2018). Sustainability assessment of machining with nano-cutting fluids. Procedia Manufacturing, 21, 245–254. 12. Yongsheng, S., Zhen, L., Liang, L., Jianbin, W., Hong, G., & Gang, W., (2017). Cutting performance of micro-textured polycrystalline diamond tool in dry cutting. Journal of Manufacturing Processes, 21, 1–7. 13. Luka, S., Dinesh, M., Peter, K., & Franci, P., (2020). The influence of single channel liquid CO2 and MQL delivery on surface integrity in machining of Inconel 718. Procedia CIRP, 81, 164–169. 14. Murat, T. O., (2015). Surface roughness during the turning process of a 50CrV4 (SAE 6150) steel and ANN based modeling. Materials Testing, 51, 10–17. 15. Kishan, Z., Prassan, S., Alborz, S., & Navneet, K., (2021). Recent advancements in nanolubrication strategies for machining processes considering their health and environmental impacts. Journal of Manufacturing Processes, 61, 481–511.

CHAPTER 10

BREAKTHROUGH OF POWDER ADDITIVES IN POWDER MIXED MICRO-ELECTRIC DISCHARGE MACHINING T. JAGADEESHA,1 SANDIP KUNAR,2 GOLAM KIBRIA,3 and MANOJ NIKAM4 Department of Mechanical Engineering, National Institute of Technology, Kozhikode, Kerala, India

1

Department of Mechanical Engineering, Aditya Engineering Collage, Andhra Pradesh, India

2

Department of Mechanical Engineering, Aliah University, West Bengal, India

3

Department of Mechanical Engineering, Bharati Vidyapeeth College of Engineering, Navi Mumbai, Maharashtra, India

4

ABSTRACT As there is rapid growth in the development of hard and difficult-to-machine materials like ceramics, stainless steel, high-speed steels, and many other alloys, unconventional machining is more significant these days. Electric discharge machining (EDM) is one of the important unconventional machining processes in which material is removed by erosion caused by repetitive sparks produced between the workpiece and tool submerged in a dielectric medium without any contact between them. Despite the advantages of EDM, we need to advance the EDM technique to further optimize the process and workpiece Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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characteristics. Powder mixed electric discharge machining (PMEDM) is one of the advancements in the EDM process, which can coat simultaneously on the workpiece. In this method, the powder is added to the dielectric, which changes the properties of the gap, thereby enhancing the characteristic of the process and workpiece. Different optimization techniques like response surface methodology (RSM) Taguchi method for different input parameters were discussed in the chapter. Response variables like material removal rate (MRR), tool wear resistance, surface roughness, crater depth and width, the thickness of the white layer, and surface material accretion of the PMEDM process were discussed when different powder additives were added to the dielectric. In this EDM, different types of powders like graphite, Aluminum, Silicon carbide, chromium, and Titanium carbide are used. From various studies, it was concluded that powder addition could increase discharge current and discharge gap. When powders are used, machining becomes more stable. Studies have shown that Silicon carbide powder gives minimum MRR, and graphite powder gives maximum MRR. Powders like silicon give a good, polished surface. 10.1

INTRODUCTION

The electrical discharge machining (EDM) was initially recognized by the material removal due to passage of electric discharges between electrodes. From then numerous extensive research were done to improve and enhance the potential of the process around the globe. EDM implementation has increased widely. Its applications include manufacturing of aerospace parts, automotive components, nuclear components and also the surgical instruments. It is required mainly for the post finishing operations. Mostly used application is for molds, dies, and for cavities. The technique followed by EDM is thermal processing and it is an extensively used non-contact process. It is used for hardened materials. It replaces many common traditional machining processes like drilling, polishing, milling, grinding, and many more. As both the workpiece and tool are separated without any contact, only a considerable amount of residual stresses are induced in this process. The advantage of using this process is that we can machine any electrically conductive material even the hardened materials and we can arrive at the desired accuracy and surface roughness by maintaining the variables of the process. Work material is machined by electrical spark discharges. The selection of cathode and anode electrodes depends on whether the material to be machined

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is fine or coarse aggregate. If it is fine then tool is considered as cathode and workpiece as anode, if it is coarse then tool is anode and workpiece is cathode for the electrolysis process in liquid dielectric medium of the EDM setup. The electrodes are immersed in a dielectric medium. It takes place within a very narrow space between the workpiece and tool electrodes in a very less time in the order of microseconds. For every discharge, a minute quantity of material will be separated from workpiece. However, several electrical discharges (EDs) take place simultaneously resulting in removal of material in desired amount. The servo control mechanism is responsible for maintaining the spark gap to be unchanged. 10.2 10.2.1

MECHANISM OF ELECTRO-DISCHARGE MACHINING MECHANISM OF CONVENTIONAL EDM

Here the electrical energy is being converted to thermal energy. When electrons are moving fast on the surface due to their high energy, anode melting takes place during single discharge. This material solidifies as the plasma channel radius increases which causes the decrement of heat intensity at the anode. This intense heating at the cathode is responsible for larger material removal than at the cathode so workpiece is considered as the anode for fine material machining operations. When the voltage (V) and current (I) applied is more than the withstand value of the dielectric, it breaks down and the material is removed by erosion as shown in Figure 10.1. As an extreme plasma pressure is generated during this time which creates disintegration of dielectric and generation of severe heat the course of spark formation and hence erosion by melting and vaporization is initiated. The dielectric fluid confines the plasma radius expansion on the surface of the electrode which increases the intensity of heat flux which helps the process of evaporation and melting of the workpiece. Dielectrics prevent the deposition of debris particles on the workpiece by flushing them away as they form layer causing the ED overlapping, acts as a coolant. Also insulates the electrodes during sparking. The requirement of desired accuracy, MRR and tool wear led to the development of various EDM techniques. One of the emerged categories of EDM machining is Additive mixed ED machining. It is developed to increase the efficiency of the electric discharge processing for improved surfaces. It is used for both surface enhancement and for machining. Schematic setup of EDM is shown in Figure 10.2.

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FIGURE 10.1

The mechanism of electro-discharge machining.

FIGURE 10.2

The schematic setup of EDM.

10.2.2

COMPARISON OF EDM AND PMEDM

The additives added in dielectric medium can be categorized as two types. The metallic or conductive particles are solid additives and the surfactants,

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209

urea solution, calcium aqueous solution are liquid additives. When the solid additives are implemented in EDM processing it is called PMEDM. The inclusion of the suspended particle in dielectric fluid stabilizes the process. The conductive particles ensure the secondary discharges which lowers the breaking resistance of dielectric which in turn increases the machining rate (MR). The surface integrity and material removal rate (MRR) are improved by using nano-sized particles rather than the micro-particle in dielectric fluid during EDM. This size of the added powder particles contributes to achieving the desired outcomes of the machining process such as MRR, tool wear rate (TRR), surface roughness (Ra) and the surface topography features such as corrosion resistance, surface defects and thickness of the recast layer. For medical applications, during PMEDM reduces the machining time, improves the process stability and ensures distribution of the EDs by the particle scattering. Nano-structured highly hardened porous layers are formed due to the re-solidification, melting, deposition of dispersed additives which enhances the internal bone growth and enhancement of cell adhesion. Comparison of EDM and PMEDM is shown in Figure 10.3.

FIGURE 10.3

10.2.3

The comparison of EDM and PMEDM.

PMEDM MECHANISM

In PMEDM, multi-directional branching of the discharge passage occurs due to the involvement of suspended conductive particulates of powder in the dielectric medium. The Interelectrode gap (IEG) is developed due to

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Electro-Micromachining and Microfabrication

high pressure and temperature. Schematic setup of PMEDM is shown in Figure 10.4.

Mixer

Tool Electrode Powder Mixed Dielectric

Workpiece FIGURE 10.4

Reservoir

The schematic setup of PMEDM.

As the discharge stops, the electrode material melts and disperses into the dielectric fluid this occurs at regular intervals of time. Hence heated particles which are near to discharge region gets accelerated in molten metal region. The particles get polarized and form a chain like structures as shown in Figure 10.5, which enables improved migration of electrons and causes the possible secondary discharges. This molten material is solidified and formed as debris on the surface and in medium is further carried away by the dielectric medium to avoid layer formation and overlapping of electric discharges. The dispersed powder particles are responsible for ED occurrences and breakdown of dielectric. This breakdown will be reduced by the DSE which causes the discharge at larger electrode gap. The outcome of the process also depends on the

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211

movement of suspended particles. There are various categories of particle motions such as cohesion to the electrodes, reciprocating, cluster, and chain stagnation. The mechanical activity of these dispersed particles controls the crack deepening into the material as shown in Figure 10.6.

FIGURE 10.5

10.2.4

Powder movement in interelectrode gap.

SPECIFIC ASPECTS OF PMEDM

There are different specified aspects linked with the properties of the PMEDM which are discussed in subsections. 10.2.4.1 ENLARGED IEG The efficiency of the process depends on the properly dispersed conductive particle of powder added in the dielectric. The electric field formed due to the dispersion of particles is the most common property. The powder particles attain polarity due to the high voltage. When a group of such particles adjusts themselves in such a way that the electric field density reaches maximum and decreases the resistance of the dielectric. Finally, this causes the enlargement of IEG because of dispersion of positive and negative charges due to the short circuit.

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FIGURE 10.6 Control mechanism of crack penetration in PMEDM process.

10.2.4.2 WIDENING OF PLASMA PASSAGE The electrons and ions get accelerated due to induced voltage across the electrodes. They collide with the neutral atoms and powders present in the dielectric liquid and exchanges their polarities among themselves as a result gets scattered. And this phenomenon is extreme in PMEDM leads to the widening of the plasma passage. 10.2.4.3 RADIALLY EXPANDED AND AXIALLY CONTRACTED CAVITIES The cavities formed in this process cannot penetrate deep in the material because the material removal is high radially rather than axially. Hence

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213

radially widened cavities can be seen in this process, and they are uniformly distributed. The property ensures the improved efficiencies which is required in machining of rough surfaces. 10.2.4.4 NEAR-MIRROR-FINISH PHENOMENON By conventional EDM process we get matte finish surfaces, to get more finefinish or glossy finish surfaces we should use low current pulses. By taking low current pulses, it takes lot of time for machining, dispersion, and also there is uncertainty in the final finish. So different methods are evolved to increase reflectiveness of surfaces efficiently like to divide the electrode itself into subdivision to reduce the capacitance in the gap, to distribute the charge on working surface by coating high resistance material. Also, by powder mixed dielectric medium in electric discharge machining (EDM), we can get high surface finish without compromising much on time and efficiency. In Powder mixed dielectric medium, graphite is more used for roughing than finishing and it increases rate of material removal which increases speed of process and decreases rate of tool wear. When it comes to silicon-copper electrodes, it gives a very fine-finishing by controlled process. But making silicon-copper electrodes is difficulty, so soon silicon powders were replaced by copper powder. By using these copper and graphite powders in dielectric medium, a very good glossy finish on the machined surface is obtained even at a very high current pulse, under controlled conditions. The powders in dielectric medium like silicon and graphite powders basically distribute charge over the spark gap which creates glossy finish machined surfaces. The powders present in the dielectric medium generally reduces the decrease of breakdown voltage such that distribution of charge occurs more widely in spark gap increases flushing and making the machining very stable, which in turn creates more reflective surfaces in less time. Comparison of microstructural surface for EDM and PMD-EDM are shown in Figure 10.7. 10.2.4.5 SURFACE MODIFICATION BY PMEDM It is implemented using PMEDM for surface modifications to enhance diverse properties of the material. During the material removal, debris formed, and it deposits as a layer on the workpiece which actually helps to change the surface of the workpiece by using different kinds of composite electrodes and dielectric and their combinations.

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FIGURE 10.7 Comparing microstructural surface for (a) EDM; and (b) PMD-EDM for graphite powder in dielectric medium.

10.2.4.6 BIOCOMPATIBILITY Implants can be machined using surface modification. Due to the bone degradation and other failures the joints are replaces with artificial joints as implants. These implant surfaces require a certain set of properties as they are inside the human body so the surface and material must be biocompatible. 10.3

LITERATURE REVIEW

Al-Khazraji et al. [1] explained about the effect of kerosene as the dielectric fluid with the graphite micro-powder. It is found out that best performance is obtained if the input parameters given are 22 A of pulse current, 120 μs pulse duration and 82.84 mm3/min of flow rate of dielectric. It is given that the MRR is increased by 274%. The best surface roughness of 2.77 μm is obtained when pulse current is 8 A and 40 μ s pulse duration. The best tool wear ratio (TWR) of 0.31% is obtained when pulse current is 8 A and 120 μs pulse duration. Mahendra & Deepak [2] Mane explained about the influence of dielectric with metal powder in EDM of Inconel 718 work material. It is concluded that the MRR is maximum when the peak current is 8 A, 5 μ s pulse duration and powder is graphite. There is low TWR when the peak current is 12 A, 20 μ s pulse duration and powder is SiC. Cyril et al. [3] made an attempt to explain about the drilling performance on 316L stainless steel when dielectric fluid is mixed with additives in microEDM. It is concluded that by adding additives to the dielectric the MRR is enhanced, and wear of tool is reduced. There is an increase in MRR because of the enhanced dielectric strength which decreased the length of sparking.

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215

Wong et al. [4] used Near-mirror-finish phenomenon by taking powder suspended in dielectric medium in EDM. The necessary condition for the Mirror finish phenomenon is usage of negative electrode polarity. Powders like silicon and graphite are found to generate glossy finish surfaces due to distribution of discharges in spark gap. Correct combination of powder and workpiece to be selected. Tzeng & Lee [5] studied the efficiency of Electro discharge Machining by using powder characteristics. Addition of powders increases MRR and decreases TWR. Powder concentration, Particle size have great effect on EDM performance. The MRR is greatest powders of Cr then Al, Cr, SiC, and Cu. And TWR has reverse trend. Zhao et al. [6] explained about the discharge parameters influence on the machining efficiency on Powders mixed discharge fluids. By choosing optimal discharge parameters efficiency of the machining increases and surface roughness decreases. Pecas & Henriques [7] studied about the silicon influence on the dielectric fluid in EDM. It is explained that increase electrode area due to the less distance between the workpiece and electrode may lead to disturbance of discharge process. Min-Seop Han et al. [8] described about the surface integrity improvement by mixing powder in the electrolyte in the process of machining which is involves electro chemical discharge. It is shown that the peak current and the voltage breakdown decreased by 10%. By using powder there is considerable improvement in surface quality (SQ). Furutani et al. [9] studied the effect of powder in electrolyte fluid on the Titanium carbide. For the better accretion, thin electrode with the high concentration of Titanium carbide powder or rotating disc with the rotating disc of gear shape. Han-Ming et al. [10] studied about the effect of Al and SiC powder mixed with electrolyte for the Titanium workpiece in EDM. The larger gaps occur due to addition of powder in dielectric medium results in the surface roughness, material removal depth and debris removal rate. Powder addition also leads to surface carbon nuclides adherence which intern leads to increase of electrode wear rate giving expansion of slit. Quan et al. [11] described about the influence of adding different powders to dielectric fluid. Added additives results in decrease of the wear rate of tool, increasing value of MRR, and even leads to reduce in roughness of the surface. Addition of powder to the electrolyte fluid can increase discharge current, increase discharge gap and lower discharge voltage. Ojha et al. [12] explained PMEDM of En-8 steel in chromium powder suspended dielectric medium under different input parameters to get MRR and wear rate of tool considered using response surface methodology (RSM) and concluded that concentration of chromium powder results in and increase in peak current leads to increasing MRR

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Electro-Micromachining and Microfabrication

and decreasing tool wear rate. Hourmand et al. [13] investigated about Mg2Si metal matrix composite (MMC) and performed RSM, which results in voltage, current, and two-level interaction of voltage and current and two-level interaction of voltage and pulse ON time are important factors influencing high MRR. 10.4 PARAMETERS IN PMEDM 10.4.1 RESPONSE VARIABLES/OUTPUT PARAMETERS IN PMEDM 1. Material Removal Rate (MRR): It is defined as material removed by machining process in unit time. By noting the weights before and after each experiment MRR can be calculated. The formula is shown in Eqn. (1): MRR =

weight loss of workpeicebefore and after machining Machining Time

(1)

2. Total Wear Rate (TWR): It is determined by using Total weight loss as shown in Eqn. (2). For the better performance in EDM process this should be low. TWR =

Total weight loss of workpeice Machining Time

(2)

3. Relative Wear Ratio (RWR): It is for workpiece relative to tool. RWR =

Total wear Rate Material Removal rate

(3)

4. Surface Roughness (SR): It is described as the real surfaces with shorter frequencies with respect to the troughs. This is commonly affected by the microscopic asperity of the surface of each part. Surface Roughness is generally measured using profilometer. Surface Roughness can also be expressed in various ways like in terms of average peak to height of valley (Rz), arithmetic average (Ra) peak roughness (Rt), etc. According to ISO 4987:1999 it is generally expressed as arithmetic average (Ra). We need to reduce surface roughness to increase the performance of machining [14]. Ra =

1 L | h ( x ) dx | L ∫0

(4)

Breakthrough of Powder Additives

10.4.2

217

INPUT PARAMETERS IN PMEDM

10.4.2.1 ELECTRIC PARAMETERS 1. Pulse Current/Discharge Current (Ip ): When preset level is reached current increases this current is known as Discharge current. This parameter is related to power consumption in EDM process so, it is very important parameter. Pulse current is the one of the inevitable machining parameters in EDM, as it directly influences power consumption during machining process [15]. 2. Pulse Off Time (Toff ): Pulse off time if generally defined as the time taken between two successive sparks expressed in micro-seconds. This parameter effects the stability of cut on material and speed of the EDM process. This should not be small as it will make the spark unstable cut. Hence, the spark will become more unstable if the pulse off time is very less in value. This is also called the spark off time. 3. Pulse on Time (Ton ): It is defined as the time between the flow of current in two successive cycles. As the energy applied in this time increases MRR also increases. Peak current and length of spark controls this energy. This is also called the spark on time. 4. Supply Voltage (V): It is the potential supplied in volt. MRR can be affected by supply voltage. 5. Duty Cycle (τ): It is determined by using Eqn. (7.5) and it is described as the percentage calculated between on-time and total cycle time which is the sum of off-time and on-time. τ=

Ton Ton + Toff

(5)

10.4.2.2 NON-ELECTRIC PARAMETERS 1. Nozzle Flushing: Foe the supplying and cleaning the dielectric fluid in the chamber this process is very important. When the cavity is deep then the flushing becomes very difficult. Usually flushing range 0.1 to 0.2 Kgf /cm2. 2. Electrode Lift Time: When the tool lifts by flushing the fresh dielectric fluid enters in the electrode chamber this time after lifting to flushing is known as electrode lift time. 3. Normal Working Time: It is time taken for the machining when the EDM performance is as maximum as possible [16].

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10.4.2.3 POWDER RELATED PARAMETERS 1. Powder Type: Different Types of powder like graphite, Silicon carbide, etc., can be used which shows different effects on response parameters and performance of EDM. 2. Powder Concentration: The amount or quantity of powder nanoparticles mixed to the dielectric fluid relative to the di dielectric fluid alone without powder particles. 3. Powder Shape: The shape of nano-powder added to the dielectric fluid. 4. Grit Size: This indicates the grain size of the abrasive side of the nano-powder. 10.4.2.4 ELECTRODE-BASED PARAMETERS 1. Electrode Material: The material on anode and cathode side of the electrode. 2. Size of Electrode: Commonly diameter of electrode is considered. 10.4.2.5 OTHER PARAMETERS 1. Over Cut (OC): It is the clearance between workpiece and electrode after the completion of the machining operation. 2. Inter Electrode Gap (IEG)/Arc Gap: During the machining time the arc gap between the workpiece and electrode is known as Arc gap. Commonly mentioned by servo system mechanism is responsible for this gap. It is also called spark gap. This is important for proper flushing and spark stability. 10.5 10.5.1

OPTIMIZATION TECHNIQUES IN PMEDM PMEDM TAGUCHI METHOD IN PMEDM

Taguchi method is a collection of analogies for optimizing process or product. There are three steps involved in this process like system, parameter, and tolerance design. In first step it is not effected by quality and cost, the second step is more important than other two steps, as the parameters are pulse

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219

on time, flushing pressure and pulse current. S/N ratios are calculated for different levels of process parameters through computation. It is very simple that the better performance characteristic is for high S/N ratio. Also, ANOVA analysis is performed for statistical analysis and better understanding of parameters. By combined results of statistical and S/N ratios we can get optimal values [17]. 10.5.2

RESPONSE SURFACE METHODOLOGY (RSM) IN PMEDM

In numerous fields of research and industry, experimental design is critical. Experimentation is the process of applying treatments to experimental units and then measuring one or more responses as part of a scientific approach. It is important to keep a close eye on the process and the system’s operation. As a result, an experimenter must get a result in order to obtain a final result. RSM is one of the most widely used experimental strategies for optimization. It is a valuable approach because it allows for the evaluation of the effects of numerous factors and their interactions on one or more response variables. Recent research has been gathered with the goal of extracting plant material with high yield and quality, as well as determining the best conditions for this extraction process [18–22]. RSM is a collection of different approaches which includes mathematical approach and statistical approaches to find the optimal parameters which influences response variables by modeling and analyzing the change in response parameters with input parameters which is affected by more than one variable and the ultimate result is to maximize the response. It is often used to optimization based on models developed in the analysis using experiment values in particular conditions. By using regression analysis (RA), response model can be created with different input parameters which are independent. Optimum results can be obtained using this RA model of response. Independent parameters quantitatively are given in Eqn. (6). = F g ( X 1 , X 2 , X 3 ,…..X n ) ± ε

(6)

where; g is the function of response and F is the response and X 1 , X 2 , X 3 ,…..X n are independent parameters, modeling by linear response with independent parameters as Eqn. (7). The various steps involved in RSM are shown in Figure 10.8. F = C0 + C1 X 1 + C2 X 2 + C3 X 3 +…+ Cn X n ± ε

(7)

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Electro-Micromachining and Microfabrication

FIGURE 10.8 Comparing microstructural surface for (a) EDM; and (b) PMD-EDM for graphite powder in dielectric medium.

It allows us to analyze response variable with different independent input parameters, also determines the optimal structure in particular region of input variables. The mix of elements that produces the optimum response may then be determined by carefully analyzing the response surface model. Different optimization techniques, RSM, Taguchi method, also by taking one variable at a time are used for different input parameters like pulse current, discharge gap, powder concentration and so on to find relation for response different response variables MRR, TWR, SR for different materials is shown in Table 10.1.

Authors Ahmed et al.

Work Powder Material AISI D2 die Graphite steel

Mahendra Inconel 718 Graphite et al. Silicon Carbide Aluminum oxide Cyril et al. 316L Graphite stainless Silicon Carbide steel sheet Aluminum oxide Zhao et al. 45 steels

Wong et al.

Tzeng & Lee

Aluminum

SKH 51 steel, SKH 54 steel

Graphite powder, silicon powder, carbide, aluminum, and molybdenum powders SKD 11 steel Copper powder, aluminum powder, silicon carbide and chromium powders.

Zhao et al. 45 steels

Aluminum

Input Parameter

Response Variables Type of electrode, MRR concentration of graphite TWR powder, pulse current, pulse on-time. Duty cycle, input MRR current, pulse on-time, TWR concentration of powder Powder concentration, MRR voltage, spark pulse on TWR time, inter-electrode gap. Discharge gap, discharge MRR passage length SR Nozzle flushing, MRR inter-electrode gap, pulse TWR current, pulse on-time. SR MRR TWR

MRR SR

Improvement in MRR is 274% and TWR by 0.31%.

Graphite as powder gives maximum MRR, with SiC gives minimum MRR. With the powder in dielectric TWR decreases due to heat dissipation and MRR increases due to the enhanced dielectric strength and spark frequency stability. Proper selection of discharge parameters increases the efficiency and better surface finish for PEDM. Machining is more stable during the presence of powder. For superior surface finish combination of powder and workpiece to be selected. 70–80 nm and 0.5 cm3 / l powders produced greater MRR. MRR is greater in order of Cr, Al, SiC, Cu, and reverse trend for TWR.

Proper selection of discharge parameters increases the efficiency and better surface finish for PEDM.

221

Powder concentration, electrical resistivity, particle size, particle density, electrical conductivity, pulse current. Discharge gap, discharge passage length.

Summary

Breakthrough of Powder Additives

TABLE 10.1 Review on Optimization on Process Parameters for Different Parameters

Authors Pecas & Henriques

(Continued) Work Material AISI H13

Powder

Input Parameter

Silicon

Peak current, flushing, electrode area.

Min-Seop Borosilicate Graphite Han glass Furutani et al.

AISI 1049

Aluminum, Silicon carbide Silicon carbide

Ojha et al. EN-8

Chromium

Hansal et al.

Silicon

EN-31

Summary Use of silicon in discharge fluid increases the polishing process of work material.

The peak current and the voltage breakdown decreased by 10%.

Surface material Appropriate shapes for accretion are thin accretion. electrode and gear shape rotating electrode.

MRR TWR SR Pulse width, pulse MRR interval, powder TWR concentration. SR Current duty cycle, angle MRR of electrode, chromium TWR powder concentration Silicon powder concen- MRR tration, peak current, SR pulse on time, duty cycle

With PEDM there is increase surface roughness and in material removal depth. Powder addition can increase discharge current, increase discharge gap and lower discharge voltage. Response surface analysis is performed, and the factors angle of electrode which increases current and concentration of powder. Factors, peak current and concentration of silicon powder interact with each other, has more responsible for change in high MRR.

Electro-Micromachining and Microfabrication

Han-Ming Titanium Chow alloy et al. Quan Yan Ti-6Al-4V Ming et al.

Titanium carbide

Electrode diameter, voltage applied, electrode gap. Arc gap, powder concentration, angle of electrode, electrode shape. Angle of electrode, Arc gap

Response Variables Crater depth, SR, crater width, thickness of white layer. SR

222

TABLE 10.1

Breakthrough of Powder Additives

10.6

223

CONCLUSIONS

The application of materials with high strength and good material properties is increasing day by day in industries and daily applications. Powder additives in electronic discharge machining are developed as manufacturing of those hard-strength materials is not suitable through conventional processes. By changing different parameters and optimized techniques, the required element is manufactured in stipulated times. Different powders are added to the dielectric medium for corresponding work materials to achieve better results. By depending on different input parameters, different response surface variables can be increased or decreased based on requirements. All input parameters, electric parameters, non-electrical parameters, powderrelated parameters, and electrode-based parameters had significant effects on MRR, surface roughness, and other parameters. By using RSM, the Taguchi method, and other optimization techniques, a significant effect of each input parameter can be found. This helps in controlling different parameters based on requirements. ANOVA analysis can also be performed for better statistical results and to get optimized values. KEYWORDS • • • • • • • •

biocompatibility electric parameters electro-discharge machining near-mirror-fnish phenomenon optimization techniques PMEDM response surface methodology Taguchi method

REFERENCES 1. Al-Khazraji, A. N., Amin, S. A., & Ali, S. M., (2015). Studying and modeling the effect of graphite powder mixing electrical discharge machining on the main process characteristics. Al-Khwarizmi Engineering Journal, 11, 20–36.

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2. Mahendra, R. G., & Deepak, M. V., (2014). Study on effect of powder mixed dielectric in EDM of Inconel 718. International Journal of Scientific and Research Publications, 4, 11–18. 3. Cyril, J., Paravasu, A., Jerald, J., Sumit, K., & Kanagaraj, G., (2017). Experimental investigation on performance of additive mixed dielectric during micro-electric discharge drilling on 316L stainless steel. Materials and Manufacturing Processes, 32, 638–644. 4. Wong, Y. S., Lim, L. C., Rahuman, I., & Tee, W. M., (1998). Near-mirror-finish phenomenon in EDM using powder-mixed dielectric. Journal of Materials Processing Technology, 79, 30–40. 5. Tzeng, Y. F., & Lee, C. N., (2001). Effects of powder characteristics on electrodischarge machining efficiency. International Journal of Advanced Manufacturing Technology, 15, 586–592. 6. Zhao, W. S., Meng, Q. G., & Wang, Z. L., (2002). The application of research on powder mixed EDM in rough machining. Journal of Material Processing Technology, 129, 30–33. 7. Pecas, P., & Henriques, E., (2003). Influence of silicon powder-mixed dielectric on conventional electrical discharge machining. International Journal of Machine Tools & Manufacture, 43, 1465–1471. 8. Han, M. S., Min, B. K., & Lee, S. J., (2007). Improvement of surface integrity of electrochemical discharge machining process (ECDMP) using powder-mixed electrolyte (PME). Journal of Materials Processing Technology, 191, 224–227. 9. Furutani, K., Sanetoa, A., Takezawaa, H., Mohria, N., & Miyakeb, H., (2001). Accretion of titanium carbide by electrical discharge machining with powder suspended in working fluid. Journal of the International Societies for Precision Engineering and Nanotechnology, 25, 138–144. 10. Chow, H. M., Yan, B. H., Huang, F. Y., & Hung, J. C., (2000). Study of added powder in kerosene for the micro-slit machining of titanium alloy using electro-discharge machining. Journal of Materials Processing Technology, 101, 95–103. 11. Ming, Q. Y., & He, L. Y., (1995). Powder-suspension dielectric fluid for EDM. Journal of Materials Processing Technology, 52, 44–54. 12. Ojha, K., Gard, R. K., & Singh, K. K., (2011). Parametric optimization of PMEDM process using chromium powder mixed dielectric and triangular shape electrodes. Journal of Minerals & Materials Characterization and Engineering, 10, 1087–1102. 13. Hourmand, M., Farahany, S., Ahmed, A. D., & Noordin, M. Y., (2015). Investigating the electrical discharge machining (EDM) parameter effects on AlMg2Si metal matrix composite for high material removal rate and less EWR–RSM approach. International Journal of Advanced Manufacturing Technology, 77, 831–838. 14. Kansal, H. K., Singh, S., & Kumar, P., (2005). Parametric optimization of powder mixed electrical discharge machining by response surface methodology. Journal of Materials Processing Technology, 169, 427–436. 15. Sonawane, H., & Pawade, R., (2012). Effects of powder mixed dielectric on electro discharge machining (PMEDM) of HSS tool steel. International Journal of Mechatronics and Manufacturing Systems, 5, 431–454. 16. Ibrahim, A. N., Amin, S. A., & Ali, S. M., (2015). Prediction of surface roughness, material removal rate and tool wear ratio models for sic powder mixing EDM. Journal of Engineering and Development, 19, 73–93.

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17. Jibin, P. T., Mathew, J., & Kuriachen, B., (2021). Transition from EDM to PMEDM – impact of suspended particulates in the dielectric on Ti6Al4V and other distinct material surfaces: A review. Journal of Manufacturing Sciences, 64, 1105–1142. 18. Shabgard, M. R., Gholipoor, A., & Baseri, H., (2016). A review on recent developments in machining methods based on electric discharge phenomenon. International Journal of Advanced Manufacturing Technology, 87, 2081–2097. 19. Ramana, P. V., Kharub, M., Singh, J., & Singh, J., (2020). On material removal and tool wear rate in powder contained electric discharge machining of die steels. Materials Today: Proceedings, 38, 2411–2416. 20. Mathivanan, N. R., Jerald, J., & Behera, P., (2011). Analysis of factors influencing deflection in sandwich panels subjected to low-velocity impact. International Journal of Manufacturing Technology, 52, 433–441. 21. Gangil, M., & Pradhan, M. K., (2017). Modeling and optimization of electrical discharge machining process using RSM: A review. Materials Today: Proceedings, 4, 1752–1761. 22. Tan, P. C., & Yeo, S. H., (2011). Investigation of recast layers generated by powdermixed dielectric micro electrical discharge machining process. Journal of Engineering Manufacture, 225, 1051–1062. 23. Singh, A., & Singh, R., (2015). Effect of powder mixed electric discharge machining (PMEDM) on various materials with different powders: A review. International Journal for Innovative Research in Science & Technology, 2, 164–169.

CHAPTER 11

MICRO-ELECTRO DISCHARGE MACHINING: PRINCIPLES AND APPLICATIONS SUMANTA BANERJEE Department of Mechanical Engineering, Heritage Institute of Technology, Kolkata, West Bengal, India

ABSTRACT Electric discharge machining (EDM) is presently considered a primary process as much as other conventional machining processes. Irrespective of hardness, it offers the ability to cut any electrically conductive material in the absence of cutting forces. This makes EDM the preferred choice in the manufacturing sector over other advanced machining technologies. In keeping with the progressive demand for miniaturized devices for technological applications, machining at small length scales has warranted increased attention of industrial R&D efforts. Consequently, different variants of the microscale version of EDM (micro-EDM, or µ-EDM) extend their application domains to all sectors that make use of micro-components or micro-features. Some of the potential application sectors in this regard are the biomedical, aerospace, automobile, microelectronics, jewelry, and precision equipment industries. The present chapter provides an introductory overview of micro-EDM technology and its variant processes. Tracing its history up to the present-day advancements, the chapter discusses the working principle of μ-EDM, and summarizes the crucial roles played by its principal machine components, and discusses their most important features. An overview of the varied forms of micro-EDM techniques are presented. The features Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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and potential application domains of die-sinking micro-EDM and µ-EDM milling are briefly discussed. The working principle and machining behavior of micro-EDM drilling and other variants of micro-EDM are also addressed. The salient process parameters and their effects on the surface quality (SQ) of the machined faces are discussed with respect to the multi-physics nature of the length and time scales involved, as well as the geometric error and surface features. In addition, the chapter outlines the modeling techniques for the micro-EDM and the methodologies for theoretical estimation of temperature, crater-size distribution, etc. An exhaustive list of published works is included as References for an in-depth study of this novel art of machining. 11.1 INTRODUCTION 11.1.1 NEED FOR MICROMACHINING TECHNOLOGIES Owing to advancements in microfabrication techniques at the global scale, the last couple of decades have witnessed a technological upsurge in the domain of miniaturization of instruments, parts, and machinery. Keeping up with the rising concurrent demand for progressive miniaturization and enhanced features, induction of new technologies is now regular. These novel applications aim to reduce the dimensions of machine components, while retaining their functional accuracy. The potential of miniaturization lies in the possibility of creating an immensely diverse range of technologies with products and devices that are small in size and, yet, ultra-precise in technological capability. Some of the major benefits of miniaturization are as follows [1]: • • • •

The inertial effects of miniaturized components are significantly low during motion, which facilitate precision-movements of the constituent parts. Tiny, lightweight components that demand little space, get packed within small volumes, and find large-scale use in electronics packaging, aerospace, and biomedical applications. Requirement of lesser quantity of raw material for production of components. Possibility of compact packaging of components, which lead to higher component-density in assemblies.

In addition, the batch-production of mini and micro-scaled parts reduce the unit cost of products significantly. With the rise in affordability, the

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demand for miniaturized systems is also increasing day by day. Moreover, the multi-functional ability of devices can be enhanced as more and more miniaturized components of different functional features are coupled. 11.1.2 MICROFABRICATION TECHNIQUES: MAJOR APPLICATION AREAS Most of the microfabrication technologies may be thought of as scaled-down versions of existing (macroscale) manufacturing processes. The underlying principles and implementation challenges are, however, not the same. Factors which play significant roles in determining the accuracy of micromachined parts do not exert much influence in macromachining [1, 2]. At the µ-scale, even small variations in the processing parameters, or slightest of deviations in the manufacturing environment can affect the accuracy of the final product. Factors such as: (a) micro-defects in the raw material; (b) cutting tool geometry; (c) influence of temperature on tool movements along the axes; and (d) presence of chatter (vibrations) cast significant influence on the replicability and accuracy of µ-manufacturing processes. Bio-medical components (e.g., µ-robots for drug delivery), µ-electromechanical system (MEMS) devices (e.g., micromotors fabricated using surface micromachining processes), µ-molded parts, µ-check valves (for use with corrosive fluids and biological samples), micro-electronics devices (e.g., µ-supercapacitors), or lab-on-a-chip components (manufactured using micro-injection molding) in microfluidic applications are some primary application areas of µ-fabrication techniques. Microfabrication techniques also offer significant opportunities in the field of medicine [3–8]. Untethered µ-robots (comparable to size domain of bacteria), employed for in-situ drugdelivery systems, are outcomes of advancements in μ-fabrication techniques of actuators and sensors. These advancements have enabled μ-scaled devices to reach unprecedented tiny gaps and crevices inside the human body [9–11]. In neuro-stimulation methods, an implantable pulse generator (IPG) is used for stimulating the brain cells. The IPG comprises of electrodes and the power source unit. Whereas classic technology can only enable a couple of connections, advancements in molding, stamping, forming, and assembly techniques at the micro-scale have helped incorporate up to about 32 connections in an IPG unit [12–14]. Similarly, microfabrication techniques can massproduce thermopneumatic/piezoelectric/ferrofluidic magnetic micropumps, thermopneumatic/shape-memory alloy μ-valves, and electrokinetically driven/recirculation-flow/droplet μ-mixers [15–17]. Microfluidic devices

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are now a reality and promise affordable medical diagnosis and health care. Microfabrication systems have successfully fabricated lab-on-a-chip devices for microfluidic applications and droplet-based microfluidic circuits. By enclosing the molecular processes to droplet volume, the reagent volumes may be reduced significantly [17–19, 21]. Over and above, silicon-based μ-manufacturing systems have facilitated the fabrication of smartphones and personal computers, with advanced technical specifications, at affordable rates. 11.2 CLASSIFICATION OF MICROFABRICATION PROCESSES Microfabrication processes are conventionally classified as follows: 1. Subtractive Processes: These which are designed to remove the designed part features from workpiece, utilize mechanical, thermal, chemical, or high-intensity photon energy sources [22–29]. The classification of these processes is based on the energy source utilized for material removal. Conventional µ-machining processes (e.g., turning, milling, drilling) use cutting tools (with greater material hardness) to chip out excess material. For contact-type machining process, the minimum-achievable part dimension is determined by factors like: (a) tool rigidity; (b) amount of contact force exerted on the microfeatures; and (c) tool dimensions [22]. Examples of other subtractive processes are laser/electron beam machining. Here, material removal is facilitated by a high-intensity laser/electron beam [25, 30]. In the photolithographic process, a light source/plasma beam enables to shape out a photosensitive material [32, 33]. Etching processes carry out disintegration of a material into its required form by using chemical agents [27, 34]. The electrochemical machining (ECM) process is used to machine electrically conductive materials. It uses the phenomena of ion transfer between conductive electrodes, subjected to a potential difference, and electrolyte in the system [28, 35]. Similarly, use of electrical spark as a means to melt and evaporate workpiece material is employed in electro-discharge machining (EDM) [29, 36]. 2. Additive Processes: Wherein layer wise addition of material enable fabrication of the µ-parts [37–40]. It can be classified into scalable additive manufacturing processes (e.g., stereolithography, inkjet printing, selective laser sintering, and fused deposition modeling),

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3D-direct writing (e.g., laser-induced forward transfer, beam-based deposition, precision pump and syringe-based methods), as well as hybrid processes, which includes shape deposition modeling and electrochemical fabrication. In selective laser sintering, the final part is produced by using a guided laser beam to melt the layers of metal powder [37]. Electrochemical fabrication processes utilize (a) structural material deposition; (b) sacrificial material deposition; and (c) planarization techniques to prepare the microparts [37]. 3D-microstructures can also be fabricated using laser chemical vapor deposition (CVD) methods, although direct writing techniques are principally used in 2D-structuring applications. 3. Deforming Processes: These which offer the advantages of (a) less material wastage; and (b) negligible thermal effects [41, 42]. Examples are deep drawing, microstamping, forging, extrusion, and incremental forming. However, the time and feasibility of fabrication is principally determined by material formability. Complex and intricate microfeatures can be fabricated by μ-injection molding and casting. These are microfabrication processes where the molten material solidifies inside a mold [43, 44]. Some deformation-based microfabrication processes are micro-/nanoimprinting, hot embossing, and microforming, sub-classified further as extrusion type, deep-drawing type, and stamping type. The common variants of casting/moldingbased microfabrication techniques are investment casting, vacuum pressure casting, injection molding, and centrifugal casting. 4. Joining Processes: This which allow for the assembly of different μ-parts without incurring damaging to either the individual parts or the entire assembly [45, 46]. Examples are μ-solid state bonding, μ-soldering, μ-fusion welding, and μ-adhesive bonding. 11.3 MICROFABRICATION TECHNOLOGIES: ADVANTAGES AND CHALLENGES In all the afore-mentioned process categorizations, the machining parameters, machine tool movement, workpiece, and tool holding fixtures, workpiece materials and/or cutting tools will cast a cumulative influence on the output. As the dimensions reduce to progressively smaller length scales, the term “accuracy” acquires altered significance. While machining a part of 10 mm × 10 mm cross section, a tolerance of ±50 μm will suffice; the same magnitude

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of tolerance amounts to unacceptable dimensional error for a part with crosssectional dimensions 100μm × 100 μm. Handling of miniaturized parts poses considerable challenges, as the rigidity of μ-assemblies and their intricate features diminishes significantly. Rigidity issues give rise to handling-based form distortions of the products. Also, reduction in the part size demands intricate tooling and fixture arrangements. In short, machining at significantly small length scales warrants multiple areas of concern. The successful production of microparts depends on the – (a) machining environment; (b) reproducibility of the process; and (c) repeatability of the machine tool. Parameters such as vibrations (induced during machining), precision maneuvering of µ-parts before/after machining, resolution/precision of movements of the machine tool, and management of cutting fluids/lubricants play crucial roles in this context [47]. Lithographic machining processes remove material in layer-by-layer fashion in a 2D-plane, and this feature often results in inaccurate fabrication of intricate 3D-surfaces and structures with high aspect ratios. Moreover, the prospects for mass production are rendered limited by low values of material removal rate (MRR). Fabrication of μ-features with high resolution are rendered possible by photolithographic processes for a limited range of materials [47–49]. Implementing these technologies, however, requires maintenance of clean room environment, and also incur high capital investments, owing to expensive machine tools and supporting technologies [47]. Laser beam machining (LBM) enables higher values of MRR; however, it lacks SQ and dimensional accuracy [50]. The conventional machining processes (e.g., milling, drilling, and turning) enable fabrication of 3D-structures for a wider range of materials with lesser processing time [22]. But unavailability of adequately rigid cutting tools limits their use in certain processes. When the size scales over µ-dimensions, rigidity of cutting tool is seriously compromised, which leads to their early breakage. Thus, while machining difficult-to-cut materials, frequent tool changes are common. The cutting tool cost escalates as the tools fabrication process warrants more intricacies (especially when the dimensions go below ~200 μm). Frequent tool breakage also adds up the total machining cost. Contrary to subtractive processes, additive manufacturing techniques enable rapid production of µ-parts without raw material wastage. However, any additive process is very slow and has limited usable material range [47]. Also, the surface of the final product needs some amount of processing to attain end-use compatibility. Electrophysical and chemical micromachining put forward novel, stateof-art solutions for most of the discussed challenges. Micro-ECM and µ-EDM

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can fabricate 3D-components with higher precision and accuracy by using CNC (computer numerical control) axes movements [51]. As the tool (for material removal) and the workpiece are not in direct contact, tool breakage is absent. With µ-ECM and µ-EDM, the material removal mechanisms do not depend on workpiece material hardness. However, the machining time in micro-ECM is (undesirably) increased as the MRR is very low. Therefore, µ-ECM is popular in manufacturing domains where intricate 3D-structures of superior surface finish are fabricated over (relatively) longer durations of machining times. 11.4 MICRO-EDM: OVERVIEW AS A PROMISING MICROMACHINING TECHNIQUE Micro-EDM is an electrothermal process which involves material removal through melting and vaporization. It is a thermal abrasion process that utilizes electrical discharges (EDs) between the tool and the workpiece. The advantages of µ-EDM in producing microfabricated structures of complex 3D-topologies are pronounced for ceramic machining, as the process is of non-contact nature and the abrasion rate is independent of material properties (like brittleness and hardness). Additional advantages are low impacts on the material, and the achievable accuracy of µ-structures. The MRR is also comparatively high. One of the main features of the µ-EDM technique is its “force-free” nature; a certain distance, called “spark gap,” always separate the tool and the workpiece. This non-contact mode of machining facilitates fabrication of small parts without rendering shape distortions. Micro-EDM gives higher design autonomy at relatively low capital investment, as it demands simpler fixtures and smaller setup times [47]. The latter feature enables to minimize chances of chatter or vibrations during machining [29, 36]. The effects of thermal stress, vibration, and heat-affected zone are also minimal for μ-EDM. This machining technique can also be extended to materials with medium or poor values of electrical conductivity [52]. 11.5 EVOLUTION FROM MACRO- TO MICRO-EDM TECHNIQUES: BRIEF HISTORY Joseph Priestley, the English physicist, first noted the erosive effect of EDs in the year 1770. He observed that sparks erode metals as it impinges on their

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surface. Roughly circular eroded contours (Priestley’s rings) are created on the surface around the central crater, and the crater depth is observed to depend on the electrode material. Priestley made use of Leyden jar batteries to generate high voltages, which resulted in intermittent discharges over shorter time durations. This apparent disadvantage eventually formed the technological basis of different processes known today. Among these are explosive emissions, formation of erosion craters, possibility of µ-particles generation, various material coating techniques, and so on [47, 53]. Although the applicability of ED-enabled machining came to be realized by mid-20th century, the unpredictability and non-uniform behavior of the discharges proved as hindrances in using EDs. As opposed to arc discharges used in welding, the discharges employed for machining metals need to be more uniform to ascertain precise amount of material removal per spark. Also, during machining, the melt pool must be removed from the site of the cavities; in welding, however, the melt pool must be retained on the surface of the workpiece to prevent oxidation and secure a sound weld [47]. A major development towards discharge-guided controlled machining took place in the USSR. During the 2nd World War, in 1943, two scientists, Dr. Boris and Dr. Natalya Lazarenko, were entrusted to investigate ways of preventing erosion of tungsten electrical contacts induced by sparking, which led to recurrent breakdowns of military vehicles. Owing to high material hardness, the contact points in their distributor circuits were made of tungsten. The quality of performance was, however, adversely affected owing to material erosion from discharges, as sparking produced small pits at the contact points. As a result, the contact points needed to be replaced frequently. In addition to inconvenience, frequent changing of tungsten contact points was expensive. The Lazarenko scientist couple, in a bid to overcome the hurdle, desperately tried many techniques. Among these, the contact points were immersed in mineral oils (dielectrics). When the electrodes were immersed in dielectric fluids, the sparks were rendered more uniform and predictable, although they could not douse the sparks between the contact points. The (uniform) sparks now produced more uniform pitting over the material surface. In other words, the erosion was more precisely controlled. Realizing the potential of this discovery, the scientists continued to work on this idea, which eventually led to the invention of the earliest version of the EDM machine capable of working difficult-to-machine materials (e.g., tungsten). The Lazarenkos’ machine is also known as an R-C-type machine, as the resistor-capacitor circuit (RC circuit) was used to charge the

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electrodes [47, 53, 54]. Owing to sizeable time delays attributed to charging of capacitor, the RC circuit suffered from low discharge rates. In addition, the pulse ON–OFF time could not be effectively regulated in the pulse generator. Therefore, the RC circuit was replaced with the RCL circuit for higher discharge rates. But, the presence of an inductor enhanced the stochastic nature of discharges, with the adverse consequence that the discharge energy became more non-uniform in each cycle. The RC-circuit based system was further developed by introducing a servo control mechanism. This mechanism enabled to keep the spark gap constant during machining. Simultaneously, yet independently, an American team of engineers, H. Stark, V. Harding, and J. Beaver, working for an American valve-making company, developed an EDM machine to remove broken drills and taps from aluminum castings. It is well known in the machining industry that breakage of drill bits and taps are common. Electrical spark-assisted disintegration and removal of these broken pieces of tools could now be achieved. This process gradually replaced the (conventional) tedious removal techniques of the jammed taps and drills from the holes. Initial designs of their machines did not meet with much success, as the process, relying on under-powered electricetching tools, was too slow to present an industrial solution. More powerful sparking units, however, enabled to reduce the erosion time substantially. An increase in spark intensity was combined with automatic spark repetition with an electromagnetic interrupter arrangement. These improvisations were able to produce practical machines. These improved versions could produce ~60 sparks per second. Subsequently, vacuum tube circuits were employed for these machines, which could generate thousands of sparks per second and thereby increased machining speed. However, the molten layers of accumulated materials affected SQ of the product [47]. During machining, water was introduced as a coolant to wash away the molten metal. In order to increase the spark frequency several folds, a vacuum tube-based pulse generating system was used as the power supply. Eventually, precisioncontrol of the spark-gap was achieved by an electronic servo control system. Thus, vacuum tube-based EDM machines were invented, which eventually evolved to transistor-based control circuits [47, 54]. Thereafter, technologies related to EDM have witnessed an upward trend. The previous versions were replaced by CNC-based axes control systems. They rendered greater precision in part dimensions. Shape distortions in part geometry were reduced by tool erosion compensation systems. Newer dielectric fluids were also introduced. The pulse generators have also been

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frequently upgraded for better performance. Gradually, EDM evolved into an advanced machining technique with the advent of static pulse generators, and introduction of servomechanisms operable on integrated circuits. Transistor-based pulse generator systems became popular, as they offered higher frequencies for discharge and offered flexibility to control pulse characteristics. The latter was achieved by changing pulse ON–OFF times. The scenario evolved to new-generation technology with the introduction of the microprocessor (Intel, 1971), which eventually led to the development of modern-day CNC systems. However, the theoretical models on metal removal mechanism by EDM process continued to remain ambiguous and weak, and warranted fundamental understanding of the process. Consequently, more and more hypotheses were put forward in the 1950s and 1960s, in order to seek in-depth explanations for (a) spark formation; (b) mechanism of spark gap control; (c) material removal mechanism; and (d) thermal distortions of electrodes. Although their future was in doubt, EDM machines driven by RC pulse generators continued to thrive, as experimental trials (involving drilling µ-holes) indicated that RC circuits offered a couple of advantages. They are capable of discharges with very small pulse duration, which reduce the discharge energy level to significantly low values. The capacitor discharges very quickly, which enables discharge durations ~ns. A capacitor with very low rating (~pF) enables an ultrasmall discharge energy to be realized. All of these conducive factors enable precision-removal of the material from the workpiece. These experiments furthered the use of RC circuits, eventually leading to the novel idea of ED-assisted precision micromachining or the present-day μ-EDM technique. Although the concept (of micro-EDM) was proposed way back in 1968, considerable R&D efforts over the decades enabled (a) understanding of its capabilities; (b) optimization of process parameters; and (c) extension of its areas of application. For a detailed discussion on the evolution of micro-EDM technology, the reader is referred to an authoritative review on the topic [47]. 11.6 MODELING ASPECTS OF MICRO-EDM 11.6.1 INTRODUCTION The overall performance of any machining process can only be assessed through a fundamental understanding and prediction of the process behavior.

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An in-depth knowledge of the basic physics behind the material removal mechanism is essential for process modeling. Compared to conventional machining methods, modeling of the EDM process is significantly more complex, owing to its multiphysics and multi-time scale nature [55]. In EDM, the μm-scale machining region is comprised of solid, liquid, gas, and the plasma states, and all the material states may exist together for a short period in the range of microseconds (μs) to nanoseconds (ns). The basic mechanism of material removal can only be explained by incorporating together the principles of thermodynamics, plasma physics, magnetohydrodynamics, and fluid dynamics. To incorporate these principles in modeling, various assumptions and simplifications have to be incorporated by research groups world-wide [47]. 11.6.2 MICRO-(µ-) EDM MODELING VERSUS CONVENTIONAL EDM MODELING The pulse duration in micro-EDM, as opposed to traditional EDM, is very short (~ns). Additionally, the energy density of plasma channel in µ-EDM is much higher than its conventional variant [56]. While determining the plasma characteristics, factors that are neglected in the macro-regime (e.g., surface tension, viscous force, and the pinch effect) become important in scaled-down µ-regime. In order to calculate the pulse power, the current and voltage (in µ-EDM) cannot be time-averaged (as in conventional EDM), as they are functions of time in an RC pulse generator (popularly used this technique). Moreover, the theories that are used to explain the breakdown phenomena and plasma expansion phenomena in macro-EDM are rendered ineffective. For example, the theory of dielectric breakdown in macro-EDM (bubble mechanism) cannot be directly applied in µ-EDM. For extremely short pulse durations, the slow process of bubble expansion fails to explain the rapid breakdown phenomena in µ-EDM [47, 57]. Most of the models available for conventional EDM are related to cathode erosion. Although the machining process starts with rapid erosion rates as accelerated electrons bombard the anode surface, the MRR slows down afterward due to expansion of plasma channel radius [58], which reduces the local heat flux near the anode surface. Moreover, as the plasma radius is small at or near to the cathode surface, the power density gets higher, which enables to create a deeper crater. So, the workpiece is connected to the cathode during conventional EDM.

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However, the plasma radius expansion at the anode is comparatively less significant in µ-EDM, as the duration of discharge is very short. The rapid rates of material removal (due to the impact of high-velocity electrons) will be prominent under such conditions. So, the workpiece in µ-EDM is connected to the anode; consequently, applying the cathode erosion model in µ-EDM would result in serious prediction errors [47, 59]. Micro-EDM plasma generates much higher power density (≈ 30 times) than the conventional EDM plasma [56]. Augmentation in power density leads to higher material removal efficiency and lesser heat loss to the surrounding fluid medium. In addition, the heat carried away by the particle-debris is significant in modeling any µ-EDM process. The modeling is rendered more complex owing to debris particle-reattachment to the tool surface. The modeling approaches in micro-EDM can be classified as follows: 1. Analytical Models: These models are theoretically developed, from first principles, by analyzing the material removal mechanism. The process is disintegrated into simpler, mathematically interpretable forms by selecting appropriate assumptions. The accuracy of any analytical model depends on the chosen assumptions, and demand in-depth knowhow. Whereas oversimplification of process parameters results in lesser accurate models, realistic modeling is rendered more complex and increases solution times [47]. 2. Empirical Models: In order to develop empirical models, extensive experimentation is carried out to correlate the controllable parameters to the machining performance characteristics. During experimentation, the machining variables are changed to analyze the trend in machining output parameters, primarily with respect to MRR, tool wear, surface finish, etc. The input variables are then fitted into certain empirical relations (viz. polynomial, power-law, or exponential) that can predict the variations in output parameters. However, the universality of empirical models is rendered limited, as it largely depends on the machining environment and the machine tools [47]. 3. Numerical Models: These are developed to solve the pertinent governing equations with least time and adequate accuracy. Analytical and empirical relations are used to determine the boundary conditions (BCs) necessary for solution. Literature survey indicates that the finite element method (FEM) analysis of the plasma-material interaction is popularly used in µ-EDM modeling to generate the thermal distribution profile [47].

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Micro-EDM modeling can be classified into following stages: single-spark modeling, and multispark modeling. For calculating the tool wear and MRRs in real time, a reliable singlespark analytical model must be developed which predicts the crater dimensions accurately. The craters are generated by material ablation, due to the action of ED. To calculate the dimensions of the crater produced by individual discharges, the energy transferred to the workpiece or tool has to be known. The heat from plasma zone dissipates into the cathode, anode, and the dielectric fluid in different proportions. The energy fractions transferred, respectively to the anode and the cathode, as well the energy dissipated to the dielectric fluid must all be estimated. To generate the temperature distribution on the tool and the workpiece, certain assumptions must be considered, and some BCs need to be assigned. To simulate the heat flux affected area, the intensity of energy, distribution of energy density, and the plasma radius must be defined. The molten pool is mapped by generating the temperature profile, recognizing the points where the (local) temperature exceeds the melting temperature [47]. This intricate modeling process can be further subdivided under two heads: plasma channel modeling and melt pool modeling. 11.6.3.1 MODELING OF PLASMA CHANNEL The formation of plasma channel in µ-EDM is primarily explained by the theory of bubble mechanism [60] or electron impact ionization mechanism [61, 62]. From the early stages of development of EDM technology, analytical models have been put forward to define the nature of EDM plasma. As the process of plasma channel formation is complex, it has been subdivided into the following stages for better understanding: breakdown phase, heating phase, and material removal phase [47, 63, 64]. In the first stage (breakdown phase), the potential barrier between the two electrodes is diminished by various factors. These include the presence of positive ions in the gap between the electrodes, and the high density of the charged particles on the micropeaks of the cathode. The local electron density is increased as the potential barrier weakens. Bombardment of the electrons with the dielectric fluid particles leads to heating of liquid and the formation of vapor bubbles at the micropeaks of the cathode. The bubble is further expanded, and the bubble pressure reaches a threshold value, leading to a discharge [65].

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The bubble mechanism model suggests that the local instability due to liquid inhomogeneity and electrode surface irregularity are the main reasons behind the breakdown of the dielectric medium. According to the electron impact ionization model, the ionization of molecules due to accelerating electrons causes a dielectric breakdown [47, 61, 62]. The bubble mechanism can be effectively used to explain the discharge phenomena, where the discharge time is large. However, for short pulses (~ns) in µ-EDM, the bubble expansion theory is rendered ineffective to explain the physics behind rapid discharges. In order to explain the rapid breakdown phenomena, the theory of gas breakdown (streamer model) can be utilized [66], which cites that breakdown takes place when the growing electron stream transforms into a torrent [67]. A modified breakdown model may be proposed in order to adapt the streamer model to the liquid environment of the dielectric fluid [64]. According to this model, the breakdown process begins with the nucleation of bubbles at the cathode. The latter expands eventually due to increased density of the charged particles. When the bubble characteristics meet the electron impact criteria, instantaneous ionization of dielectric fluid column takes place, and eventually breakdown occurs. The modified model combines the bubble mechanism and the streamer model to explain the µ-EDM plasma more efficiently [68]. The characterization of the discharge plasma is done by calculating the temperature in the plasma column and the electron density. These plasma parameters can be calculated using analytical models [69], optical spectroscopy [70], photomultipliers [71], and so on. Furthermore, understanding the mechanism behind plasma formation helps to explain some unique phenomena related to µ-EDM, including the cooling effect produced by elevated iron content in the plasma [72]. 11.6.3.2 MODELING OF PLASMA-ELECTRODE INTERACTION IN µ-EDM A brief survey of published technical literature on this topic is presented here. In the model proposed in Ref. [73], one of the early approaches to model the interaction of plasma with electrode material in µ-EDM, the discharge current and voltage, dielectric permittivity, duration of spark, and electrode diameter have been grouped as a single dimensionless variable. The leastsquare regression method is used to calculate µ-crater area, electrode current density, rate of expansion of plasma channel, and channel power dissipation.

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Pioneering work in modeling and interaction of the micro-EDM plasma, by considering different stages of discharge formation, is carried out in Ref. [57]. As discussed earlier, the theory of bubble mechanism fails to address all the aspects of the plasma generation process in µ-EDM. The speed of plasma channel propagation, as found from experiments, is found to be ~105 m/s, which cannot be described by the slow dynamics of the vapor bubble expansion. Hence, a combination of bubble mechanism (bubble initiation) and electron impact ionization (bubble expansion and dielectric breakdown) mechanism has been suggested in Ref. [57] to explain the rapid breakdown phenomena in micro-EDM. In this work, the plasma model uses a cylindrical geometry in water as a dielectric medium. Inside the spark gap, charged particles are accelerated towards opposite polarity, which is followed by an increase in plasma radius. However, the expansion is restricted by body forces at the plasma boundary. The general steps for modeling of µ-EDM plasma in Ref. [57] has been outlined with great detail in Ref. [47]. Experimental investigation of plasma temperature and electron density with optical emission spectroscopy diagnostics has also been reported [74]. Assuming single-pulse discharge between parallel plates, a global plasma model approach has been analyzed in Ref. [69]. In this model, the species densities in plasma are volume averaged, and the Maxwellian energy distribution is applied. By coupling the particle balance equations, plasma dynamics, and energy balance, the plasma temperature and pressure are calculated. The average electron temperature, as predicted by the model, is around ~6817 K, and the electron density is ~8.9 × 1023 m–3. Even though the above-mentioned models could incorporate the physics of the micro-EDM plasma to some extent, certain fundamental aspects of the process had still not been addressed. Consequently, Chu et al. [68] have worked on a comprehensive model that predicts the plasma radius, pressure, and temperature. This model includes the combined effects of surface tension, viscous force, and the magnetic pinch force. The plasma radius, plasma temperature, and the plasma pressure are used to model the heat source and to provide the BCs in the modeling of the crater surface [47]. 11.6.3.3 ENERGY PARTITION, HEAT FLUX, SPARK RADIUS: PERTINENT PLASMA PARAMETERS To simulate the erosion of material due to interaction with plasma, the BCs have to be defined accurately. The plasma radius, heat flux distribution,

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energy partition factor, discharge intensity, electron density, plasma pressure, and plasma temperature are defined in the BCs for solving the governing equations in the model. 1. Energy Partition Factor: The discharge energy is partitioned between electrodes, dielectric fluid, and debris particles. Determination of the energy fraction transferred to the electrodes enables to calculate the temperature field in the electrodes. The energy distribution ratio also depends on the electrode materials, dielectric fluid, discharge energy, and fluid pressure. The transport of energy by the debris particles is considered in µ-EDM, as the short-pulse durations result in high power density and high material removal efficiency. So, a considerable part of the liberated heat is also dissipated to the debris particles [47, 56]. 2. Heat Flux: It is defined as a spatial distribution of heat intensity in the plasma channel. The type of heat flux chosen for an analysis makes a significant difference in the simulation results. As plasma is defined as a heat source with different distribution-modes of thermal energy, the following types of heat flux models have been used for analysis: (i) Point heat source, where the thermal load is taken to be applied at a single point; (ii) steady (i.e., time-invariant), uniformly distributed heat flux of constant magnitude, where the flux density is uniform over the cross-sectional area and the energy intensity independent of time; (iii) uniform, unsteady (i.e., time-dependent) heat flux distribution, where the magnitude of energy density is even on the plasma column at any given instant, but varies with time; (iv) Gaussian heat flux, with the magnitude of energy density at a point assumed to be constant during discharge. The heat flux density is taken to be maximum at the center and minimum at the periphery of the plasma channel in accordance with the Gaussian function; and (v) transient Gaussian heat flux with time-dependent magnitudes of energy density at a point. Of all the schemes, the Gaussian heat flux with time-varying magnitude is considered a more realistic approach to model the heat source, as the plasma is observed to expand during the discharge period [47, 75]. 3. Spark Radius: Once the heat flux model is selected, the spark radius is estimated from the plasma models and used as an input to calculate the magnitude of the heat flux at different locations. The spark radius can also be found out using empirical relationships formulated via extensive experimentation. However, selecting the empirical formula for the

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plasma radius is challenging, as the spark radius is affected by the type of electrodes, dielectric medium, and flow pressure. Error in the estimation of spark radius will affect the accuracy of heat flux magnitude, and eventually affect the accuracy of the predicted crater dimensions. For a detailed exposition of the spark radius equations used by different research groups, the interested reader is referred to [47]. In this context, the concept of plasma flushing efficiency (PFE) deserves mention. The PFE is defined as the ratio of the actual volume of material removed from the crater to the volume of the molten pool. The boundary of the crater is often defined at the isothermal line above the melting temperature of workpiece material. However, all the molten material may not be flushed out from the machining zone, which brings significant errors in the calculation of crater dimensions. To realize an accurate model, PFE should be incorporated into the model [47]. 11.6.4 SIMULATION OF CRATER FORMATION: ASSUMPTIONS AND MODELING ASPECTS The electrothermal model or the electromechanical model can be used with reasonable efficacy to explain material removal in µ-EDM. The melting and superheating of the electrode material are crucial parameters for material removal in the electrothermal model. The partial differential equations governing the different heat transfer modes are solved to obtain the surface temperature distribution. On the other hand, analysis of the electric field strength in the discharge plasma helps to calculate the surface stress due to particle bombardment. To obtain the dimensions of the discharge crater, material rupture on the surface that occurs due to impact of charged particles can also be simulated. To model the crater formation, most of the published results in open literature are based upon electrothermal models. The major assumptions in electrothermal modeling of µ-EDM are as follows [47]: • • • •

The tool and the workpiece material are homogenous. The material properties of tool and workpiece can be simulated as temperature-dependent or temperature-independent, depending on design requirements. Single plasma channel formation per pulse is considered for simplicity. Energy loss due to radiation is neglected in most of the models (owing to very low length and/or time scales).

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The assumptions are related to the heat flux shape (circular/pointed), its type (uniform/Gaussian), and the plasma radius (constant/time dependent). The assumptions are also related to heat transfer modes (convection and radiation to the dielectric fluid, and conduction to the electrodes). The plasma flushing efficiency is to be mentioned. While conducting a 2D-analysis of the plasma–electrode interaction, half of the interaction zone is modeled, and the axis-symmetric assumption is considered.

For a detailed review on the governing field equations of heat transfer modes, thermal BCs on the surface of electrodes, generation of temperature profile, and the forms of crater geometry, the reader is referred to Ref. [47]. 11.6.5 MULTISPARK MODELING Once the dimensions of the crater are found by solving the governing equations (analytically or numerically), it can be used to calculate the topography of the surface machined by µ-EDM. For that, multiple sparks have to be generated on the workpiece surface. The general steps in the multispark modeling of µ-EDM process are summarized as follows. As the first step, simulation of the μ-crater geometry is done using the single spark model. In this step, the flow dynamics of bubbles, debris particles, and the dielectric fluid are considered. The body forces exerted on the electrode are also taken into account in the modeling. The discharge location is determined next, which takes into account the randomness in space-energy-time, as well as the position of craters with respect to each other. Appropriate analytical and/or numerical methods are then employed to simulate the machined surface. Finally, one then calculates the surface roughness. It must be noted that determination of the discharge location, simulation of the machined surface, and calculation of the surface roughness together constitutes the second step. Some literature review in this direction is conducted next. Modeling the surface roughness in µ-EDM with the help of non-overlapping multiple sparks has been reported by Kurnia et al. [76]. The craters are assumed to be densely packed in line without any bulging on the rim. The reattachment of debris is neglected along with the effect of microcracks. The surface roughness values are calculated from mathematical analysis of the geometry of the craters aligned without overlap. Another multispark model, developed by

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Kiran & Joshi [77], discusses the effect of debris particles on surface roughness. The micropeaks in the workpiece surface are assumed to be normally distributed and the spark impinges on random points, keeping the distance between the tool and the workpiece a minimum. The distribution of debris particles in dielectric medium is included. As the presence of debris particles affects the electrode potential and the plasma composition, the radius of the melt pool for a single spark is changed accordingly. These calculations are used in the multispark model to alter the surface roughness model for getting more realistic results. Tan & Yeo [78] have developed an FEM model for overlapping craters. The machined surface topography is extracted, based on the maximum asperity condition (deepest crater profile) and the minimum asperity condition (shallowest crater profile). The distance between two craters is taken same as the heat source diameter, and the 2D-model is solved for calculating the surface roughness. Somashekhar et al. [79] used the finite volume method, with implicit flux discretization on a uniform grid, to solve the multispark model. However, most of the multispark models for micro-EDM is oversimplified, and therefore fails to give the real picture of the random multisparks. Jithin et al. [80] have developed a more sophisticated 3D-multispark model for conventional EDM, where the 3D surface roughness is calculated by solving a single discharge model. The stochastic nature of the EDM process is well represented by including randomness of space, energy, and time of the discharges. The PFE is also considered for the crater geometry calculations. Interactions of the simultaneously formed craters are simulated, which includes craters with/without overlap and craters touching boundaries. 11.6.6 SOFT COMPUTATION AND STATISTICAL MODELS EMPLOYED FOR MICRO-EDM To model the micro-EDM process, several soft computing techniques have been utilized by different research groups. Majority of published research works in this applied soft-computing domain focus on optimizing the performance parameters and the machining conditions to achieve better machining attributes. Among the soft computing and statistical techniques used in µ-EDM, the salient tools are genetic algorithm (GA), grey rational analysis, particle swarm optimization (PSO), adaptive neuro-fuzzy inference system, artificial neural network (ANN), supporting vector machine, fuzzy modeling, response surface methodology (RSM), Taguchi based approach, Elitist teaching-learning-based optimization, and so on [47].

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11.6.7 SOME OTHER MICRO-EDM MODELS: BRIEF LITERATURE SURVEY In addition to the modeling and simulation of crater geometry and surface topography, some other models are also available in published technical literature that relate to µ-EDM. Roy et al. [81] have numerically modeled the changes in feature geometry (in reverse micro-EDM) due to presence of debris particles and secondary discharges. Taking into consideration the reattachment of the debris particles on the tool surface, the single-spark model has been dynamically modified. Modeling of vibration assisted micro-EDM process has been reported in Ref. [82]. The simulation studies concerning motion of eroded material in vibration-assisted μ-EDM and in reverse micro-EDM have been reported [83, 84]. Wang et al. [85] have simulated the micro-EDM deposition process by using an electrothermal model. A model-based analysis of energy consumption in μ-EDM drilling have been carried out in Ref. [86]. A mathematical model that enables to predict manufacturing cost in μ-EDM drilling has been developed by D’Urso et al. [87]. 11.6.8 SALIENT CONCLUSIONS ON MICRO-EDM MODELING Modeling of micro-EDM process helps to develop deeper understanding of the machining behavior. The primary step is to model the plasma channel in order to obtain the plasma radius, as well as the temperature and pressure fields. These parameters are used to calculate the energy dissipated to the electrode surface. Using the principles of heat transfer, the temperature profile can be simulated. The crater geometry is then obtained by analyzing those zones of the electrode material where the local temperature exceeds the melting temperature. Assumptions incorporated in different models ascertain the accuracy of the calculation of crater dimensions. To increase the accuracy of the models, fluid dynamic equations can be incorporated. Apart from thermal modeling, researchers have also attempted to model the process using electro-mechanical models, where the theory of electron–ion impact mechanism is employed. The crater dimensions obtained through simulations can be used to estimate the machined surface using multispark modeling. In addition to analytical and numerical modeling, soft computing models have also been developed to correlate the machining parameters and the performance parameters [47].

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11.7 MICRO-EDM TECHNIQUE: OVERVIEW OF GENERAL PRINCIPLE The micro-EDM technology makes use of the phenomena of spark generation when two electrically conducting electrodes are brought together under a potential difference. When greater number of sparks are generated, considerable amounts of material are removed from both the electrodes. When the two electrodes are connected across a potential difference, the electrons drift from the cathode towards the anode. On overcoming the work potential of the material, cold emission of electrons occurs from the cathode surface. These electrons travel towards the positively charged electrode (anode); however, they encounter with the dielectric fluid in between. These collisions eventually lead to the dielectric breakdown, i.e., the dielectric (fluid) molecules split into electrons and ions, which reduce the effective resistance and increase the conductivity of the dielectric fluid. This transition from an insulating environment to conducting environment between the two electrodes causes a continuous flow of electrons. This avalanche of electrons collides further with more and more molecules of the dielectric fluid, which then leads to the generation of greater numbers of electrons and ions. Vaporization of the dielectric fluid leads to the formation and expansion of bubbles, which covers the region and further reduces the effective resistance [54]. All these events facilitate the formation of a plasma channel, which is a material state comprised of electrons and ions. As the air resistance is minimal at the point where the spark gap is minimum, the ED channelizes to this ‘minimum gap’ point [88, 89]. The bombardment of electrons with the surface of the workpiece results in the irreversible transformation of kinetic energy to thermal energy. This sudden transfer of thermal energy within a very short interval of time melts the workpiece material locally, while some parts evaporate instantly. The ions that travel towards the cathode surface constitute the erosion of material from the tool surface. This aspect is popularly known as tool wear. As the mass of ions is greater than the mass of electrons, ions are expected to remove more material. If the tool and workpiece in EDM employ the same material, then the electrode that connects to the positive polarity (anode) experiences more material removal, as compared to the cathode. This odd phenomenon can be explained by the following facts [47]: •

As the mass of the particle increases, the inertial effects will be dominant, and the acceleration experienced by the ions towards cathode will be comparatively low. Lesser numbers of ions therefore reach the cathode surface with lower kinetic energy, as compared to

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•

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the large numbers of high-velocity electrons hitting the anode surface within the specific pulse duration. The hydrocarbon oil deposits a thin layer of carbon over the cathode surface.

During μ-EDM, the tool electrode is connected to cathode and the workpiece electrode to the anode. As the supply turns off after pulse ON time, the spark disappears, and the plasma channel is destroyed. The vapor covering then explodes, creating a vacuum wherein the new dielectric fluid molecules rush into the spark gap. Fluid in the spark gap regains its dielectric strength. The pressurized molten pool of material splashes to the dielectric fluid and washes away. The new cycle of events starts with the starting of the next pulse ON time, which leads to the generation of next spark. Owing to removal of material from the tool electrode and the workpiece, the location of “minimum gap point” continuously changes. The plasma channel formed at this new location develops a fresh crater on the specific area. Compared to the arc discharges, the pulsed power supply ensures uniform distribution of discharges over the surface, which enables uniform material removal from the workpiece surface. 11.8 MICRO-EDM VS. MACRO-EDM: SALIENT PARAMETERS The µ-EDM can be considered as the scaled-down version of (conventional) macro-EDM. In both the processes, material removal occurs due to the ablation of material by melting and vaporization. However, to remove material in a precise and controlled manner, one must design the system to restrict the energy intensity to a lower range. Therefore, the differences in plasma radius and energy intensity are recognized as the principal differences between the micro- and macro-versions of EDM technology. Furthermore, the material available to dissipate the thermal energy is substantially less in the case of microparts. Excessive spark energy (SE) may burn the tool electrode, and excessive flushing pressure will cause tool breakage or deflection. The electrode wear rate is also proportionally higher in micro-EDM, which therefore demands highly sophisticated tool wear compensation methods [51]. Empirical relations that predict the trend of MRR or tool wear rate in macro-EDM are also not valid in the μ —regime. Considering these facts, micro-EDM has to be treated as a separate machining strategy in order to comprehend the mechanisms behind material removal and factors affecting the machining quality. Table 11.1 makes comparison of the conventional versus μ-EDM with reference to salient process parameters.

Machining Parameters Macro-EDM Power supply • The main aim is to obtain a higher MRR with acceptable dimensional accuracy and surface finish.

Micro-EDM • The main aim is to obtain precise microfeatures with a great deal of dimensional accuracy.

• The discharge energy requirement is ~μJ and the range • Theoretically, there is no limitation to the discharge of pulse duration ~50ns – 100μs. For this, the power energy that can be used for machining in EDM. supply has to be modified so as to generate very low However, to obtain adequate surface quality, extreme discharge energy for ultra-small durations of time. voltage and current conditions are not preferred. • Transistor-based pulse generators are popularly used to generate sparks because of the higher frequency of sparks and flexibility in setting the pulse duration levels.

Tool size/spark gap/ plasma radius

• Tools of dimensions ~mm are used for machining.

• Delay in transistor-based circuits makes them unsuitable to achieve small and uniform pulse durations. RC-based circuits with very low capacitor ratings can be used to produce small discharge energy. As the capacitor discharging time is very small, pulse durations ~ns can be achieved. The upper limit of discharge energy must be carefully chosen so that the tool electrode does not get burned. The frequency of pulses has to be high for good surface finish and surface uniformity. • Microelectrodes of dimensions < 1 mm are chosen as the tool.

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TABLE 11.1 Comparison between Macro-EDM and Micro-EDM [47, 90–94]

• The tool electrode is more rigid and less susceptible to thermal distortions or burning. The tool electrode • These tools are less rigid and susceptible to bending during transportation, handling, or due to excessive is less affected by the flushing pressure, and internal flushing pressure. Because of this, the flushing pressure flushing can be easily employed because of the higher has to be moderated according to the tool material. In dimensions of the tool. addition, on-machine fabrication of the tool electrode • The plasma radius in macro-EDM is much less is recommended to avoid bending, breakage, and tool than the tool diameter, and the amount of erosion as mounting-related problems. compared to the total volume of the tool is low. The

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• The plasma radius is considered to be comparable to the spark gap is in the range of ~10 – 500 μm. The tool electrodes can be fabricated using any other machining tool size. Spark gap has to be maintained < 10 μm. process and transported to the EDM machine tool.

(Continued)

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Machining Parameters Macro-EDM Micro-EDM Crater size/surface • As the discharge energy is higher in macro-EDM, it • Micro-EDM uses very small discharge energy, which produces larger craters. roughness/MRR/ produces crater volume in the order ~μm3. minimum achievable • As the crater size increases, the surface roughness is • This reflects on the surface quality, as the roughness is size of the part increased, and the MRR will be higher. The surface in the range of ~0.05 – 0.5 μm. The discharge frequency created by EDM has a roughness Ra ~3 – 30 μm. The of RC circuits is comparatively lower than that of MRR is in the range of ~0.2 – 0.8 mm3/s. Minimum transistor circuits. This also makes the machining process slow (MRR ~200 – 1,000 μm3/s). Even though achievable dimensions are ~mm. the minimum achievable size is different for different variants of micro-EDM, all processes are capable of fabricating features of dimension < 50 μm. Microwire EDG can machine microcutting tools of dimension down to ~3 μm.

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TABLE 11.1

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11.9

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GENERAL COMPONENTS OF MICRO-EDM

The micro-EDM machine tool consists of the power supply, pulse generator, tool electrode, spindle system, worktable, CNC axes with independent and precise axis control, servo control system, dielectric circulation unit, and measurement systems. Table 11.2 shows an overview of the μ-EDM machine tool components. 11.10 GENERAL COMPONENTS OF MICRO-EDM A suitable choice of the machining parameters controls the performance of µ-EDM. The latter include electrical parameters like peak current, voltage, and capacitance (for RC circuits), pulse ON/OFF time, and pulse interval (for transistor-based circuits), and also non-electrical parameters like the tool feed rate, the spindle speed, the dielectric flow rate and flushing type, and the type of dielectric. Performance of µ-EDM can also be assessed using tool wear ratio (TWR), MRR, surface roughness, recast layer, and geometrical distortions due to tool wear. Each of the machining parameters has a unique relationship with the performance parameters. A judicious choice of the optimum machining parameters helps to lower the machining time and increase the SQ [47]. 11.11 MICRO-EDM VARIANTS The term micro-EDM refers to a collection of different types of machining strategies, with unique structure and capabilities, and is not just a particular machining technique. The broad categorizations of micro-EDM are now discussed. 11.11.1 MICRO-DIE-SINKING EDM In this µ-EDM technique, the tool electrode (with a particular geometry) is plunged to the workpiece without tool rotation. The microelectrode is plunged to the workpiece to produce the corresponding mirror images. With the help of EDs, the form and geometry of the tool is printed as a mirror image on the workpiece. This is considered to be one of the simplest and quickest forms of micro-EDM, without considering the tool fabrication time. However, the lack of tool rotation results in a greater number of arc discharges, and

Component

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TABLE 11.2 Brief Description of the Micro-EDM Machine Tool Components Brief Description

Spark generator/ Pulse generators are responsible for making pulsed discharges for EDM. Pulse generators are of two kinds: transistor based, pulse generator and RC based. Among this, RC circuit is the most popular spark generation system. in which very short pulses ~ns duration can be realized. The discharge energy can be brought to ultrasmall levels using capacitors of very low rating. Transistor pulse generators use a field-effect transistor for a pulse ON and OFF control. The pulse duration can be easily controlled using a transistor-based circuit [47]. The spark gap in micro-EDM has to be continuously monitored and maintained. A servo control system is, therefore, an important part of micro-EDM. This enables to avoid the occurrences of short circuits and arc formations by pulling back the tool electrode, time to time, to the optimum spark gap. Servo control system makes use of the ignition delay time to control the spark gap. A microcontroller takes control over the servomotor with the help of an analog to digital (A/D) converter, a PWM module, and a feedback system. It monitors the discharge pulses continuously to predict the optimum spark gap, based on certain algorithms for minimum arc formation and short circuits [47, 95].

Dielectric fluid supply system

The dielectric fluid is the essential part of micro-EDM responsible for predictable and uniform sparks, debris removal, and cooling. The dielectric fluids chosen for the operations also vary and reported experiments have been conducted using deionized water [96], kerosene [97], mixed powder fluids [98], plasma jet [99], and cryogenic coolants [100]. The dielectric fluid delivery system consists of a tank, pump, filters, pipes, and nozzle. The flow rate can be adjusted with valves, and the fluid is constantly filtered and recirculated. The presence of debris affects the effective resistance of the dielectric fluid. So, filters and dielectric fluid have to be changed frequently.

Positioning system

While fabricating features of µ-dimensions, the resolution and accuracy of the axes have to be very high. The axis control system in µ-EDM comprises of servomotors for movement and linear encoders (with nanometer resolution) for feedback. The slide straightness has to be maintained near to ~2μm/100 mm. The base must have high stiffness and damping abilities for vibration isolation [47].

Measurement system

An online measurement system has to be incorporated in the machine tool for monitoring and measuring the µ-features. It enables to find out the irregularities in the fabricated part, without removing it from the spindle or vice. A charge-coupled device (CCD) camera is used for measuring the µ-dimensions of on-machine fabricated microtools and microparts [47].

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Servo control system

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the tool is susceptible to localized welding with the workpiece surfaces at a higher voltage, capacitance, or current rating [47, 101]. The applications are in fabrication of microgears, microinjection molds, embossing molds, etc. 11.11.2 MICRO-EDM MILLING AND DRILLING In μ-EDM milling, the tool electrodes of cylindrical geometry are used to trace out complex shapes by scanning over the workpiece surface along programed tool path. In other words, rotating microelectrodes with simple geometries follow a predetermined tool path to produce complex microfeatures. This eradicates the need for complex tool fabrication processes. The effect of tool wear is dominant, as the linear wear will result in depth difference and corner wear will result in shape difference. A tool wear compensation system has to be implemented to avoid the errors due to tool wear [29, 47]. The applications are in fabrication of microfluidic channels, micropillars, other microfeatures, etc. In micro-EDM Drilling, a rotating microelectrode with simple geometry is plunged to the workpiece to produce shallow or deep microholes. Applications are in the fabrication of microholes for inkjet nozzles, reverse EDM tool electrodes, etc. In reverse μ-EDM process, the tool electrode with microholes (circular or non-circular in cross-section) is plunged to produce micropillars. The applications are in fabrication of micropillars for biomedical applications, fabrication of EDM or ECM tool electrodes for microdrilling of a series of holes, etc. In planetary micro-EDM process, planetary motion is given to the microtool electrode to cut high-aspect-ratio microholes of a circular or noncircular geometry. The applications are in fabrication of injection nozzles, microfluid systems, starting holes for μ-wire EDM, etc. 11.11.3 MICRO-WIRE EDM In microwire EDM, a microwire is used to slice out the microfeatures. That is, a continuously moving microwire cuts microfeatures by traveling along a programed tool path. The continuously traveling wire reduces the effects of tool wear. Sharp corners can be made using microwire EDM. The applications are in fabrication of forming tools, stamping tools, spinning nozzles, etc. However, frequent wire rupture, wire vibrations, and the need of starting holes make it challenging to use in all machining requirements [47].

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MICRO-ELECTRO-DISCHARGE GRINDING

In micro-electro-discharge grinding, the polarity of the tool is reversed to positive during machining. This process is mainly used for fabrication of microtool electrodes. Four types of micro-electro-discharge grinding (EDG) processes are presently employed μ, namely Block µ-EDGs (stationary and moving), Wire µ-EDG, and Disc µ-EDG [20, 29, 31, 47]. Stationary or Moving block µ-EDG is differentiated on the basis of fabrication of microelectrodes using a stationary or moving sacrificial block. Both of these find applications in on-machine fabrication of microelectrodes for micro-EDM drilling and milling. The Wire µ-EDG concerns with fabrication of microelectrodes using a continuously moving microwire. The applications lie in creating cavities for microinjection molding, embossing or coining tools, pin electrodes, rolling tools, etc. The Disc µ-EDG is a process concerned with fabrication of microelectrodes using a rotating disc. The application deals with on-machine fabrication of microelectrodes for micro-EDM drilling and milling [92]. Table 11.3 shows the machining characteristics and capabilities of different variants of micro-EDM technology. TABLE 11.3

Different Micro-EDM Variants and Their Capabilities [91, 92]

Micro-EDM Variant

Max. Aspect Min. Feature Surface Quality Machining Capability Ratio Size (μm) Ra(μm)

WEDM

~100

~30

~0.07 – 0.2

1 2 D Tapered surfaces; 2

Die-sinking EDM

~15

~20

~0.05 – 0.3

3D Free-form surfaces; undercut by planetary erosion; delimited by electrode manufacturing

maximum angles of 150°.

EDM milling

~10

~20

~0.2 – 1.0

3D free-form surfaces

EDM drilling

~25

~5

~0.05 – 0.3

Deep holes

WEDG

~30

~3

~0.5

Axi-symmetrical structures, minimum possible electrode diameter.

Source: Adapted from Ref. [91,92]

11.12 ADVANTAGES AND CHALLENGES Some salient advantages of micro-EDM techniques are listed as follows [47]:

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• • • • • • • • • • •

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Lower capital costs, as compared to lithographic processes. Higher flexibility in product design. Practically no need to fabricate special fixtures. Advanced CNC control systems enable easy machining of free-form surfaces. Any conductive material can be machined, regardless of the workpiece hardness. Ultra-small heat-affected zones. Significant reduction in tool-handling errors, owing to the possibility of on-machine tool fabrication. Absence of burr formation. Absence of tool breakage due to tool-workpiece contact. Possibility of machining thin walls without distortion. Fabrication of high-aspect-ratio structures without tool breakage.

Some technical challenges with respect to the implementation of microEDM are also listed [47]: • • • • • • •

Tool wear creates dimensional inaccuracies in the machined microfeatures. The workpiece must be electrically conductive. Formation of the recast layer will lead to surface irregularities. Surface integrity affected by microcracks and micropores due to thermal stress. The MRR is low as compared to other machining techniques. Debris accumulation in the machining zone has to be controlled to reduce the number of harmful discharges. The spark gap has to be kept constant for stable machining.

KEYWORDS • • • • •

dielectric fuid electric discharge machining micro-EDM microfabrication plasma

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CHAPTER 12

AN INSIGHT ON MICRO-END MILLING PROCESS CHETAN DEVENDRA VARMA and K. VIPINDAS Department of Mechanical Engineering, Indian Institute of Information Technology, Design and Manufacturing Kurnool, Andhra Pradesh, India

ABSTRACT The colossal requirement of miniaturization due to technological advancements has had a great impact on manufacturing industries. The advent of non-conventional manufacturing techniques like EDM, LBM, IBM, ECM, etc., has made manufacturing procedures more precise and contactless. The complexity, machining time, and cost of machining using these nontraditional methods is something that is to be justified in the machining of micro-components. Thus, the need of the hour is to understand the processes which serve the production rates as well as the aforementioned business aspects to encapsulate the industrial demands and comprehend about the miniaturization of the macro-conventional machining methods terming it as micromachining processes. Micro-machining has been applied to various industrial regimes like automobiles, aerospace, electronics, and bio-medical fields to manufacturing various components like turbines, pumps, etc., for their micro-scale applications. The micro-size ranges from 1–999 µm, which is either considered over the whole workpiece or some micro-features on the macro-components. This instantiates the prerequisite of a thorough comprehension of side effects. The cutting-edge radius effect is negligible in the conventional methods as the uncut chip thickness is quite large, and tool can be considered acicular. The various effects like the grain size, micro-structure effect, cutting edge radius, etc., drive these micro-machining methods. The Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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type of material deformation mechanism becomes a critical characteristic co-related to the surface quality (SQ). The micro-end milling process is found to be the scaled-down version of the conventional method, similar in the operational aspect but different in terms of the cutting phenomena and material removal mechanisms. This portion of the book will give emphasis on comprehension of micro-end milling process and the impact of the cutting-edge radius effect on the removal mechanisms. It will also enlighten on the burr formation technicalities, cutting forces that are encountered and its interdependence on the removal mechanisms, tool wear aspects for process monitoring alongside the modern methods that are incorporated, and surface finish aspect to address the quality of the work surface. 12.1

INTRODUCTION

Micromachining has become the face of the contemporary subtractive machining world given the scale of technological advancements that the world is witnessing. The rapid enhancements in the industrial sectors like in the medical applications, aircraft industry, automobile sector, etc., has been the backbone to miniaturization. Even though various non-traditional manufacturing methods have been employed to produce miniaturized components with precise geometrical efficacy, these methods pose various problems of their own. Operation time and cost of manufactured component becomes unavoidably very high. Considering the advancements and the demand in the various industrial sector the mass production rates are to be enhanced which gives the down scaled version of the traditional machining methods like micro-milling, micro-turning micro-grinding, etc., an edge over these nontraditional methods [1–4]. Camara et al. [1] has shown a few miniaturized components and their operational time to illustrate this aspect. The intriguing perspective in these traditional methods is the fact that when the cutting edge of the tool becomes comparable to the size of the machining operation the conventional machining technicalities (considering it as down-scaled approach of the traditional method) cannot be sustained [1]. The maximum range of any micro-feature or micro-component is in the scale of a few hundreds of micro-meters. The researchers consider the machining of 1–999 µm range in the micro-manufacturing categorization [5]. This engulfs the idea of comprehending the scale down aspects of micro-machining which is assumed to be pre-requisite. Knowing the scaling down effects would help to grasp the chapter with much better perspective.

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Micro-Milling has been found to be very effective process to address the production rates with relatively high MRR to produce complex 3D miniaturized components [6]. Though micro-end milling is like scale down version of the conventional method in the operational aspect, it is very different in terms of tool-workpiece interaction and chip formation mechanism [3]. The material deformation mechanism has significant impact on the quality of the product machined [4]. The impact after scaling down in end milling process are the following: • • •

Feed per tooth becoming comparable to the cutting-edge radius – edge radius effect; Uncut chip thickness is significant to cutting edge radius – grain size effect; The burr size becomes significant to machining feature [2, 7].

The effect of cutting forces is very significant in the material removal mechanism. Barnabas et al. [4] analyzed the cutting force effects on 303 hardened steels with AlTiN alloy coated micro-end mill and developed a model to optimize them. Micro-burr formation is very difficult to remove from the work material. Vipindas et al. [7] also contributed to this field and depicted the modeling of burr thickness on Ti-6Al-4V which is often applied in wide range of sectors like communication, optics, biomedical devices. It has been observed that for extremely high spindle speeds (of about 4,50,000 rpm), cutting speeds are found to be very effective. Such speeds are still not feasible and the relative runout if any existing while machining will be catastrophic and may lead to tool being broken [2, 3]. Thus, this also gives an indication of tool monitoring and understanding tool wear aspects in micro-end milling [2]. This chapter of the book will walk you through the fundamentals of micro-end milling, the importance of cutting-edge radius, mechanism of material removal, cutting force analysis, burr formation, micro-end milling tool aspects, surface finish aspect for end milling components and at last the recent advancements in these methods. 12.2 CHIP REMOVAL MECHANISM (CRM) Chip removal mechanisms (CRMs) have been very key to decode the machinability of various subtractive manufacturing methods. This CRM in the macro-machining domain has been explained by different mechanical effects the work material is undergoing. These include shearing, bending,

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crushing, etc., which also depends on the type of material under operation [2]. In the macro-machining domain, the cutting edge of the tool is always considered so sharp that its effects are quite insignificant relatively to the size of work material. In micro-machining, the cutting-edge radius effect becomes very significant as the cutting-edge radius (re) of the micro-tool becomes relatively comparable to the size of work material thereby affecting the quality of the product. This difference between macro- and micromachining considering the cutting-edge radius is shown in Figure 12.1.

FIGURE 12.1 Differentiating between the cutting-edge radius insignificance in (a) macromachining; and significance in (b) micro-machining. Source: Reprinted with permission from Ref. [6]. Copyright © 2018, Springer-Verlag London Ltd., part of Springer Nature

The most important characteristic with which the cutting edge is radius is dominantly compared is minimum uncut chip thickness (MUCT or hmin) which drives the CRM. At any instantaneous point in micro-end milling if chip thickness (h) becomes less than or equal to the MUCT (h ≤ hmin) plowing phenomena dominates. When h becomes greater than hmin (h > hmin) then the shearing phenomena similar to macro-machining dominates and aids in machining. This relationship of cutting-edge radius effect to the CRM is depicted in Figure 12.2. The plowing phenomena occurring at the tool-workpiece interaction is explained by the virtue of negative effective rake angle formed at the cutting zone [2, 6]. The effective rake angle is defined to be angle between the tangent to the normal at the contact point of the tool-workpiece interaction [6]. Effective Rake angle can be mathematically addressed as [6]:

An Insight on Micro-End Milling Process

γ eff = sin −1 (

267

re − t ) re

(1)

where; γeff corresponds to effective rake angle; t corresponds to uncut chip thickness; re corresponds to cutting edge radius of micro-end mill tool.

FIGURE 12.2 Depicting the chip removal mechanism for cutting edge radius effect for case (a) h smaller than hmin, case; (b) h almost equal to hmin and case; and (c) h greater than hmin. Note: where; Vc corresponds to the cutting velocity; rβ corresponds to the cutting-edge radius; γeff corresponds to effective rake angle; and h corresponds to uncut chip thickness. Source: Reprinted with permission from Ref. [2]. Copyright © 2020, The Author(s). http:// creativecommons.org/licenses/by/4.0/.

12.2.1

MUCT ESTIMATION

MUCT is defined to be a point of chip thickness below which the plowing mechanism dominates. There have been efforts taken to consider the estimation of MUCT using various methods. Analytical, numerical, and experimental methods have been disguised to encounter sophisticated estimation of MUCT. Liu et al. [8] in his study used normalized MUCT and created analytical model with experimental validation on 1040 steel and Al6082-T6 materials. An Experimental investigation was also conducted on AISI 1045 steel which found MUCT to be in the range of 0.23 to 0.38 times of cuttingedge radius of the tool (hmin = 0.23 to 0.38 re) for three different experimental test cases [9]. Another similar approach was applied to decode the MUCT using stagnation point which is defined as point above which chip formation occurs. The MUCT was analytically proposed as:

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h= re (1− cos θ m ) min

(2)

where; Ɵm is the stagnant angle which is found to be equal to friction angle using Infinite shear strain or minimum energy principle [10]. The stagnant angle is correlated to the effective negative rake angle as follows: θ m=

π 2

− αe

(3)

Woon et al. [11] performed FEM simulation using Arbitrary LagrangianEulerian Method for AISI 4340 steel and confirmed the plowing effect being stemmed from the negative effective rake angle. The plowing effect was formulated in correlation to the uncut chip thickness and cutting-edge radius as follows: h/re ≤ 0.2625

(4)

In micro-milling the big difference lies in the fact that the CRM will vary instantaneously along the whole machining process and is dependent upon the uncut chip thickness (h) and MUCT (hmin) relationship at that instant. Moges et al. [3]. has explicitly explained this using the engagement angle to enhance geometry model for micro-milling. This instantaneous nature between the tool and the work surface is depicted in Figure 12.3 which explains the instantaneous cutting zones during the machining operation. The shearing zone or plowing zone will machine material based on the uncut chip thickness and MUCT relationship at that particular instant.

FIGURE 12.3

Showing the instantaneous chip removal zones in micro-end milling.

Source: Adapted with permission from Ref. [6].

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The chip deformation will significantly vary as there is variable crosssectional area induced on the micro-end mill tool as shown in Figure 12.3. Schneider et al. [12] also confirmed the influence of higher effective rake angle increasing the deformation in micromilling by using the quick stop method to analyze the chip formation and its surface topographical characteristics. De Oliveira et al. [13] also analyzed chip geometrical attributes on Inconel 718 using micro-end milling and described three different forms of chip formation under SEM characterization which were: 1. Helicoidal Chip: It is formed because of shearing dominant machining and the chip takes the helical shape as it passes through the flutes of the tool. The SEM image of the same is shown in Figure 12.4.

FIGURE 12.4

Depicting the helicoidal chip formation in micro-end milling.

Source: Reprinted with permission from Ref. [5]. Copyright © 2021 Elsevier B.V.

2. Ribbon Chip: This type of chip gets formed when the chip is formed due to combination of plowing and shearing mechanisms, and it was found that the chip volume was much more than the theoretical volume and was explained by the fact that the chip from preceding revolution adhered to the successive chip. The ribbon type of chip is shown in Figure 12.5.

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FIGURE 12.5

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Showing the ribbon chip formation in micro-milling.

Source: Reprinted with permission from Ref. [5]. Copyright © 2021 Elsevier B.V.

3. Spheroidal Chip: This type of chip is formed when high specific cutting energy is consumed which leads to high heated chip coming in contact with oxygen leading to an exothermic reaction which leads to sudden melting of the material. The follow up is high quenching rate which generates the spheroidal shape to the chip. This chip was not directly encountered in the machining process but was found adhered to the bottom surface of the slot micro-milled when observed under SEM [13]. This type of chip is shown in Figure 12.6. 12.3 CUTTING FORCE ANALYSIS One of the crucial characteristics which illustrates the feature-based information in subtractive machining domain is the comprehension of the cutting force at the tool-workpiece interaction. The intriguing difference in force as compared to the up scaled version is the enhanced impact of the thrust force to the cutting force. This is again aided by the negative effective rake angles which are inducing a lot of thrust force which may also lead to the tool chatter effect and eventually the tool wear. This can be shown by the following schematic as shown in Figure 12.7.

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FIGURE 12.6

271

Depicting the spheroidal shaped chip adhered to the slot.

Source: Reprinted with permission from Ref. [13]. Copyright © 2019 Elsevier B.V.

FIGURE 12.7 Depicting the cutting forces in the micro-manufacturing process. Note: Where; ff corresponds to the feed force; fc corresponds to the tangential force; and ft corresponds to the thrust force.

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The ratio of uncut chip thickness to the cutting-edge radius is correlated to the cutting forces induced in the process. At lower h/re values the magnitude of cutting force becomes very high duly corresponding to the higher thrust force as compared to the tangential forces. This also indicates the transition of CRM from shearing to the plowing zone. In micro-end milling, cutting force analysis plays crucial part in describing the information about the material removal mechanism, chip formation, tool deformation, vibration. The important aspect with which this analysis is carried out is the submissive consideration of the elastic recovery in this machining process. This elastic deformation alongside plastic deformation leading to material removal contributes to the force requirements and thus the specific cutting energy requirements. The force consideration in this machining process will be in a few Newtons (N) due to the size effect [2]. Modeling of cutting force is required to understand the cumulative characteristic information to get a better perception about the process beforehand. There are other realistic manufacturing aspects which are also hampering the material machined using micromilling which are assumed to be ideal when the process is designed. Alignments errors, runout, spindle tilt, etc., also impact the chip formation which in turn induces load variation on each flute of the tool and thus the cutting forces [14]. There have been many cutting force models based on the different characteristics like runout, feed per tooth, chip thickness, etc. All these account for predicting the cutting forces with precision. 12.3.1 12.3.1.1

CUTTING FORCE MODEL FEED/TOOTH MODEL

This model was an analytical model which was experimentally validated where the forces were estimated based on tool rotation. Following were the assumptions considered during the model consideration: •

•

The 3D micro-milling process under the consideration of very small depth of cut and the effect of helix angle being neglected can be considered as a 2D process. This assumption of a 2D process is shown in Figure 12.8. The deformed area can be considered as the orthogonal machining process as diameter of the tool is very much larger than machining deformed area. This deformed area and the orthogonal machining consideration is supported by the schematic shown in Figure 12.9.

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FIGURE 12.8 depth of cut.

273

2D consideration of the micromilling process because of very less value of

Source: Reprinted with permission from Ref. [6]. Copyright © 2018, Springer-Verlag London Ltd., part of Springer Nature

FIGURE 12.9 assumption.

Depicting the very small deformation area aiding to the orthogonal machining

Source: Reprinted with permission from Ref. [6]. Copyright © 2018, Springer-Verlag London Ltd., part of Springer Nature

This model construction was a 2D cutting force model based on the schematic given in Figure 12.10.

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FIGURE 12.10

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Tool rotation schematic for cutting forces empirical relationship.

Note: Where; fR is the radial force; fC is cutting force; ϕ is tool rotation angle; γ is helix angle; and fX, fY are X, Y force components. Source: Adapted with permission from Ref. [6]. Copyright © 2018, Springer-Verlag London Ltd., part of Springer Nature

The forces were modeled empirically based on Figure 12.10 and later on validated using experimental method. The empirical relationship of the forces is depicted below [6]: = f X f R sin φ + f C cos γ cos φ = fY f R cos φ − f C cos γ sin φ

(5) (6)

The experimental method consisted of KISTLER 9256C2 dynamometer on Ti-6Al-4V work material upon which the micro-end milling operation was performed. The micro-end mill tool was a two-flute tool, so the experimental process was only done for 180° to carry out the validation for single flute tool. Cutting speed and DOC were kept constant and the feed/tooth (f/z) used were 0.2, 0.4, 0.6, µm. The experimental characteristics can be tabulated in Table 12.1. The validated model results were precisely visualized and force for varying feed/tooth (f/z) were compared with the experimental outcome. The

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predicted force values in accordance with the Experimental results were quite precise. This visualization at different feed/tooth are shown in Figure 12.11. TABLE 12.1 Showing the Experimental Process Parameter for Cutting Force Model Validation Work Material

Cutting Speed (m/min)

Feed/Tooth (f/z) (µm) Depth of Cut (µm)

Ti6Al4V

15.7

0.2, 0.4, 0.6

FIGURE 12.11 and f/z.

100

The model vs. experimental force values for various tool rotational angle

Source: Reprinted with permission from Ref. [6]. Copyright © 2018, Springer-Verlag London Ltd., part of Springer Nature

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This subtle variation between the predicted vs actual was explained by tool wear that was encountered in the experimental results. The tool wear was assumed to be none during model consideration. It was found that the cuttingedge radius was increased from 6.5 µm to 12 µm after the experimental completion which was signature of the tool wear effect which accounted for the variation in predicted force values to that of the experimental data [6]. 12.3.1.2 SHEARING AND PLOUGHING FORCE MODEL This model constituted considering the force components based on the CRMs that we have learned before [15]. The model predicts the differential forces in the feed and cross feed directions considering the shearing and plowing together with the fact that in micromilling, the contact type of material removal procedure varies based on the instantaneous uncut chip thickness and MUCT correlation at that particular instant. The differential shearing force components are given as below [15]:  dFxs   cosα sinα 0    K ts h (θ ) a   K te a   +   =   ×   dFys   −sinα cosα 0    K rs h (θ ) a   K re a  

(7)

where; dFxs, dFys are the shearing components in X, Y direction, respectively; α is the transformation angle; a is depth of cut in the axial direction; Kts, Krs are the shearing coefficients; Kte, Kre are the edge coefficients; and h(θ) is the chip thickness at a particular tool rotation angle. The differential Ploughing force components are given as below [15]:  dFxp   cosα sinα 0    K tp Ap a   K te a   +  =   ×   dFyp   −sinα cosα 0    K rp Ap a   K re a  

(8)

where; dFxp, dFyp are the plowing components in X, Y direction, respectively; α is transformation angle; a is depth of cut in axial direction; Ktp, Krp, are plowing coefficients; Kte, Kre are edge coefficients; and Ap is the plowing area depending upon height of the elastic recovered material. The overall force components are considered without taking into account the tool wear aspects and are given by the following [15]: Fx = Σ dFxs + dFxp

(9)

FY = Σ dFys + dFyp

(10)

The experimental validation was done on Al-7050 material with a depth of cut of 300 µm at 20,000 rpm spindle speed with a feed per tooth of 20 µm

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and results found very close to the predictions from the model as shown in Figures 12.12(a) and 12.12(b), respectively.

FIGURE 12.12(a) model.

Depicting the X-directional force validation for the shearing and plowing

Source: Reprinted with permission from Ref. [15]. Copyright © 2013 The Authors. Published by Elsevier B.V.

FIGURE 12.12(b) model.

Depicting the Y-directional force validation for the shearing and plowing

Source: Reprinted with permission from Ref. [15]. Copyright © 2013 The Authors. Published by Elsevier B.V.

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BURR FORMATION IN MICROMILLING

Burr formation has been in study for decades especially for macro-machining components. Burr formation and its removal is an unvalued add on procedure in machining domain considering the business aspect and the time boundedness in production. For fundamentals reconsideration, Burr is the part of the workpiece which does not get completely detached from the base material but yet is deformed plastically due to the force exertion from the cutting tool. The shape, size of the burr is correlated mainly with tool geometry and process parameters. Its formation is not completely avoidable but yet the mitigation can be persuaded. It is far easier to deburr in the macro-subtractive domain but very strenuous procedure in the micro-domain [7, 16, 17]. The strenuous nature of deburring in micro-milling is synonymous to the fact that the burr size is comparable to the micro-feature of the work material itself and these micro-burrs may hamper the work piece material characteristics drastically if neglected. The ISO standard that is benchmarked for defining the dimensionality of the undefined edges (which includes Burr Formation) is ISO 13715:2017. Yet there are no specific standards especially accounting for the micromachining domain and therefore these traditional machining standards are benchmarked [17]. With regards to metric associated with burr formation, burr thickness therefore becomes very critical parameter, and its pre-emption would address the deburring issue with better comprehension [7]. Kiswanto et al. [18] in their study interrogated the effect of process parameters on burr formation for AA 1100 Al alloy and depicted the various types of burrs that are found in the component after a simple milling operation as shown in Figure 12.13. These various types of burrs that were disguised are as follows: • • • •

Entrance side burr; Top burr; Exit side burr; Bottom burr.

The subdue ideology followed in the aforementioned study was the fact that plowing effect will eventually give rise to higher degree of burr formation and higher surface roughness thus giving a good experimental metric to assess the optimal cutting parameters for better quality of final product [18]. These types of burrs were also encountered in one of the studies which studied the burr formation in UNS S32205 duplex stainless steel [16].

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FIGURE 12.13

279

Depicting the types of burrs found in the milling operation.

Source: Reprinted with permission from Ref. [18]. Copyright © 2014 The Society of Manufacturing Engineers. Published by Elsevier Ltd.

12.4.1

MODELING OF BURR THICKNESS

As mentioned earlier burr formation is unavoidable but can be mitigated. Burr thickness as metric would help in taking critical steps to ensure the final product of the desired quality. Comprehending the modeling of burr thickness would explicitly provide glimpse of the uncertainty hovering around the nonvalued deburring. Vipindas et al. [7] explored the burr thickness in their study to come up with model to simulate and predict the burr thickness and validate with the experimental results on Ti6Al4V material using micro-milling operation. This analytical model was constructed based on the consideration that the work done required for chip formation is equal to the work done for burr formation under the continuity of the cutting forces. This was given by [7]: Δwb = Δwc

(11)

where; ∆wb is the work done required for burr formation during machining; and ∆wc is the work done required for chip formation during machining.

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These were given as follows [7]: = ∆wb

Bt σ σ Rθ a p [ e cos 2 β 0 + e tan β 0 ] tan β 0 4 2 3

Δwc = Ft Rθ

(12) (13)

where; Bt is Burr thickness; ap is the depth of cut; σe is the effective flow stress; Rθ is the total tool movement distance; Ft is the tangential force. Based on the above consideration and as per Eqn. (11) the Burr thickness was found to be as [7]: Bt =

ap[

σe 2 3

Ft tan β 0 cos 2 β 0 +

σe 4

tan β 0 ]

(14)

This model was evaluated using the experimental method wherein initial phase was adopted to come up with appropriate material constitutive model for a good estimate of the effective flow stress which was correlated to the cutting temperature and indirectly also contribute to the burr thickness estimation [7]. The initial consideration in this phase was cutting temperature was considered constant throughout. The next phase also took into account the cutting temperature as a variable into the burr thickness estimation. Based on their experimental validation the result explicitly depicts that when temperature was considered as the variable factor the reduction in the prediction error in model was better as compared to when temperature was considered as constant factor in the burr thickness estimate. Thus, predicting the burr thickness would increase the chances of avoiding the deburring to a greater extend enhancing the surface quality (SQ). 12.5 TOOL WEAR IN MICROMILLING Tool wear has been one of the detrimental factors in the production houses. With the fast-moving technological requirements, the manufacturing industrial community would desire to enhance productivity, mitigate the time lapses with high level of flexibility. Around 20% of the total time lapses though are recurred due to the tool failures [19]. Micro-milling relative to its macro-counterpart depicts differences in tool wear characteristics and the wear recurrences occur corresponding to the wear mechanisms and wear modes. The various tool wear mechanisms in micro-milling are as follows [20]:

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1. Adhesive Wear: It is caused by metronomic layer formation and layer removal (Adhesive layer forms during the current pass of operation and gets removed in the subsequent pass during machining) eventually leading to flaking of the tool material. Temperature gradients and the pressure induced on the tool substrate during machining can be the major underlying causes for adhesion. The critical aspect with regards to the CRMs affecting the tool wear is the f/re ratio. As the f/ re ratio decreases the plowing phenomena dominates. This leads to increase in the rubbing of the tool substrate and work surface thus leading to increase in the adhesive wear. Vipindas et al. [20] study on Ti alloy depicts, f/re 1, Shearing phenomena dominates and thus the adhesive layer formation is relatively slower. 2. Abrasion Wear: It is primarily caused due to the excessive rubbing of tool substrate and work surface especially during the initial and steady wear stage. Though it is not as severe as adhesive phenomena as the adhesive layer itself forms a protective layer and abrasion marks are found on this abrasive layer. The glimpse of wear mechanisms is shown in Figure 12.14.

FIGURE 12.14

Adhesive layer, abrasion marks, and flaking of tool material.

Source: Reprinted with permission from Ref. [20]. Copyright © 2019 Elsevier B.V.

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These mechanisms affect the chip morphology formed under cutting conditions. For shear dominant regime helical chips are observed and for plowing dominant regime due to elastic recovery ribbon or long strip chips were observed [13, 20]. The wear modes are undermined by these tool wear mechanisms to characterize the tool wear aspect in micromilling. The tool wear modes include [20]: i.

Tool Radius Enlargement: The excessive rubbing to tool flank due to increase in the tool radius causes increase in surface roughness and machining temperature indicating the tool enlargement. ii Coating Delamination: For micro-end mill tools, the coating elements may react with oxygen to form the sacrificial layer providing resistance to the tool substrate which is referred to as coating delamination. Thermal properties of the coating matrix and the tool substrate has direct impact on the coating delamination. Variation in coefficient of thermal expansion induces crack propagation at the substrate. iii Crater Wear: It is found to be negligible for plowing dominant regimes as the chip flows around the tool radius rather than rake face whereas in shearing dominant regime it is meagerly affecting the rake face only during the initial stage of operation as the rake face is rubbed against the chip flow. iv Flank Wear: The most significant form of wear mode through which material failure is characterized is Flank wear. The criticality in micromilling lies in the fact that the mitigated depth of cut incorporated leads to the wear patterns being unrecognizable on the major flank face. Thus, minor flank face is considered for quantifying the tool wear aspect. The schematic of this consideration is given in Figure 12.15 [20]. Flank wear is critically being addressed in three different stages initial wear or primary wear, steady wear or secondary wear, and severe wear or tertiary wear. The major flank face, the minor flank face and rake face of the tool are schematically mentioned as M, F, R, respectively as shown in Figure 12.16 and depicted the fresh tool and worn tool geometry [20]. This literature suggested to take into account the maximum flank wear width on the intersecting surface of the major and minor flank face given as OY in Figure 12.16 and found it to be around 20 µm when cutting Ti6Al4V with TiAlN coating. Tool wear reduction is based on reducing the cutting forces that are induced on the tool during the tool and workpiece interaction. There are methods to delay the onset of tool wear like edge preparation using polishing or polish blasting methods, edge preparation using the immersed tumbling,

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etc. [2]. Coatings have also been found to be very effective in increasing the tool life and reducing the cutting forces. Alloys like TiAlN and CrTiAlN as coatings are found to be very useful in reducing the flank wear in micromilling [2]. Alhadeff et al. [21] depicts the coating performances on Brass, Titanium, and Ni-Mo alloy using the steady state wear criteria to confirm the tool life enhancement. The study also differentiated the macro- and the micro-domain wear mechanisms. Though coatings have good outcomes with regards to enhancing tool life characteristics, it comes with the negative aspects of increasing the edge radius which is not ideal for the final product quality in micromilling.

FIGURE 12.15 and rake face.

The geometrical consideration of the end mill tool with regards to flank

Source: Reprinted with permission from Ref. [20]. Copyright © 2019 Elsevier B.V.

The rapid wear rate in micromilling needs attention and benchmarks to control especially in accordance with production at higher rates. The ISO standard that is specified for the tool life and tool deterioration criteria in conventional end milling is ISO-8688-2 [19]. It cannot be directly considered

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for micromilling, but reference can be surely inferred. It conceptualizes two important inferences: • •

Tool life criteria (This specifies a predetermined tool deterioration metric value); and Tool life (Total cutting time required for the tool to reach this predetermined value).

FIGURE 12.16

The general process flow of the tool condition monitoring.

There are various metrics that can be employed in the measurement of the extent of the tool wear. Area of the wear land, radial wear, flank wear and diameter wear are some of them that can be considered in the tool wear characterization [22]. The deterioration metric that is usually employed is VB (The variation of the average flank wear or the flank wear land). This metric corresponds to the average loss of tool material from the tool flank after the cutting operation [19]. Measurement of tool wear itself is a challenging aspect in regard to the size effect. 12.5.1 TOOL CONDITION MONITORING The aforementioned insights lead us to a very important aspect that effectively could help us predict the tool wear in much precise manner namely

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tool condition monitoring or tool wear monitoring. There are various contemporary methods that have been employed in conventional manufacturing domain for tool condition monitoring. These methods are divided into two types as follows: • •

Indirect method; and Direct method.

Indirect methods usually include applying sensor signals using force analysis, vibration analysis and acoustic emissions (AE) which transform the physical process to characterization procedures like the time frequency characterization, dynamic characterization, frequency spectrum characterization, etc. These procedures are duly transformed to come up with conditioning outcomes and relate that to the tool life [23]. There have been studies which have also the incorporated the contemporary techniques like ML methods, neural networks or fuzzy logic to predict the tool wear based on the data driven modeling to come up with meaningful inferences with regards to the historic data of machining process. Direct methods include the digital image processing methods being constructed on the images taken from the high-speed cameras amidst machining process being carried out. These digital image analysis methods include image segmentation, edge detection, boundary detection, area detection, texture analysis, successive image analysis, etc. [23]. These methods eventually extract the tool wear area estimation or tool wear width estimation based on which the tool condition is disguised. These methods are incorporated for micromilling but yet demands for more research and exploration in which the methods could be super imposed in micromilling with their specifications to come up with hybrid monitoring methods for better estimation of tool wear. The process flow of the tool wear monitoring method in general in micromilling is shown in Figure 12.17. With the ever-increasing demands of Industry 4.0 in the foresight we believe this exploration would be very beneficial for micromilling process. 12.6

SURFACE ROUGHNESS IN MICROMILLING

SQ is of prime importance regarding any manufacturing method. It is one of the very good measures of how subtle the method is regarding final product quality. The CRMs have drastic impact on SQ in micromilling. The kind of interaction between workpiece and tool defines the SQ. Ploughing dominant regime will suffer a poor surface roughness owing to elastic recovery and

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rubbing action deteriorating the surface. Shearing dominant regime will have superior surface owing to chip formation like macro-domain. Vipindas et al. [6] also confirmed the similar aspect. In the Tool wear section, we noted that coatings often are very helpful in enhancing the tool life for micro-end mill tool but there will always be a trade-off between tool life and surface roughness as the coatings increases the tool edge radius making the process enter plowing dominant regime faster.

Chip Tool Elastically recovered section

1 FIGURE 12.17

3

6 5

The surface quality variation based on the chip removal mechanisms.

Source: Reprinted with permission from Ref. [6]. Copyright © 2020 Elsevier Ltd.

Thus, SQ is always questionable in trying to enhance the tool life due high economical aspect of the tool. As depicted in Figure 12.17 for comprehension case, the region 1-3-5 was plowing dominant region wherein the material elastically recovered hampering the surface roughness in that region. The region 3-5-6 was under shear dominant region thus producing chip and better-quality work surface at that instant. In micromilling though, the workpiece and tool material interaction are quite instantaneous thus final SQ is quite a complex aspect and needs serious exploration. The various other parameters like tool radius, spindle speed, feed rate, depth of cut also has impact on the final surface roughness. One of the studies proposed that impact of tool tip radius was higher as compared to cutting edge radius for shorter cutting lengths but quite opposite for larger cutting lengths during micromilling of Ti6Al4V [24]. They also found that surface roughness was less for up milling as compared to down milling. Wang et al. [25] in his

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study experimentally showed the impact of these machining parameters on SQ by micromilling on brass using ANOVA method and found that the tool radius had the highest impact owing to the size effect followed by Spindle speed owing to the stiffness of the cutting tool and then followed by feed rate drastically varying aspect relative to the macro-domain. Depth of cut had mediocre impact on the final surface roughness quite similar to the macro-domain counterpart. In contrast to this Vipindas et al. [26] in his study found that depth of cut and spindle speed were most influencing factors on SQ followed by feed when performing micro-slot on Ti6Al4V using the similar ANOVA method. Aurich et al. [27] depicted in his study that the spindle tilt angle also impacted the surface roughness in micromilling in contrast to macro-domain. They also recommended that tilt angle along the feed direction mitigates the surface roughness in micromilling. This way the influence of various factor is quite contrast to various studies and their respective approaches and thus more exploration is very critical to explicit understanding of process parameters on SQ. The practical importance of SQ in various applications is very much domain oriented. Comprehending the practical importance would impart emphasis on SQ being so critical to any manufacturing method. In this regard a study was conducted to understand the importance of surface roughness on the thrust performance of micro-nozzles which were machined using micro-milling [28]. The study depicted that large surface roughness values degraded the gas exit velocity and therefore the total thrust produced by the divergent section of the micro-Laval nozzle which would result in large fuel consumption in space application using micro-space components. It also depicted that the large surface roughness imparted large boundary layer effects on the gas-surface interaction resulting in degradation of exit velocity. Such domain applications involving micromilling indicate the trade-off between tool wear and SQ in micromilling is quite complex and strenuous work to be executed. 12.7

CONCLUSIONS

With the technological advancements and miniaturization, the demands to produce micro-sized products is the currently increasing. Thus, the inclination in the micro-manufacturing domain to satisfy the technological demands are increasing. Micro-milling in that aspect is quite satisfying the drawback of operational time of Non-traditional methods. It is able to manufacture various complex 3D geometries with ease but comes up with

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its own complexities and differences relative to the conventional milling method. The miniaturization in the process brings various scaling down effects like cutting edge radius effect, size effect, etc., which are important to be known. The mechanism of material removal is quite complex and needs attention. This is based on cutting edge radius which becomes comparable to the work material dimensions. The criteria which emphasis this aspect is MUCT. If the uncut chip thickness is less than or equal to MUCT, plowing phenomena occurs and if uncut chip thickness is greater than MUCT then shearing phenomena occurs. For, h ≤ MUCT (hmin) plowing phenomena occurs. For, h > MUCT (hmin) shearing phenomena occurs. The plowing phenomena is supported by the negative rake angles that occurs during machining. There are methods employed to come up with the models to predict MUCT using analytical methods, simulation methods and experimental methods. Cutting Force analysis will let you understand various machining parameter requirements like the specific energy needs, tool wear aspects, vibrational aspect, etc. Thus, understanding the cutting force incurred and its modeling is also very important in regard to differences relative to the conventional method. Micro-burrs encountered in the process are critical as it is comparable to the size of the work material. Modeling of burr thickness would help in mitigating the strenuous deburring and maintain better SQ. Tool wear in micro-milling is quite complex relative to the macro-milling. The conceptualization of tool wear mechanism and wear modes is critical to deep diving into the tool wear aspects. Flank wear among all modes is most important form of wear encountered and studied in micromilling. Unlike in macro-domain, tool life is not backed by any specific ISO standard and thus needs to be thoroughly explored. With industry 4.0 in the foresight more exploration of various tool condition monitoring techniques is also needed to predict the tool wear aspects with much precise manner. SQ is also quite intriguing aspect in micro-milling. Impact of various process parameters on the surface roughness is key to optimize and choose the best ones for SQ. Surface roughness is very key in applications in different sectors. Thus, the trade-off between the enhancing the tool life and enhancing the SQ is quite domain-oriented aspect and needs more attention in regard to micro-milling and to explore its complete potential in terms of application in various sectors.

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KEYWORDS • • • • • • • • • •

burr formation burr thickness cutting force analysis edge radius effect micro-end milling micromilling size effect surface fnish surface roughness tool wear

REFERENCES 1. Camara, M. A., Campos, R. J. C., Abrao, A. M., & Davim, J. P., (2012). State of the art on micromilling of materials, a review. Journal of Materials Science & Technology, 28, 673–685. 2. Barnabás, Z. B., Norbert, G., Márton, T., & Paulo, D. J., (2021). A review on micromilling: Recent advances and future trends. The International Journal of Advanced Manufacturing Technology, 112, 655–684. 3. Tesfaye, M. M., Desai, K. A., & Rao, P. V. M., (2016). Improved process geometry model with cutter runout and elastic recovery in micro-end milling. Procedia Manufacturing, 5, 478–494. 4. Barnabás, Z. B., Norbert, G., Csongor, P., Dániel, I. P., & Márton, T., (2021). Analysis of cutting force and vibration at micro-milling of a hardened steel. Procedia CIRP, 99, 177–182. 5. Deborah De, O., Milla, C. G., Gustavo, V. D. O., Aline, G. D. S., & Marcio, B. D. S., (2021). Experimental and computational contribution to chip geometry evaluation when micromilling Inconel 718. Wear, 476, 203658. 6. Vipindas, K., Anand, K. N., & Jose, M., (2018). Effect of cutting edge radius on micro end milling: Force analysis, surface roughness, and chip formation. The International Journal of Advanced Manufacturing Technology, 97, 711–722. 7. Vipindas, K., & Jose, M., (2019). Modeling of burr thickness in micro-end milling of Ti6Al4V. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 233, 1087–1102. 8. Liu, X., DeVor, R. E., & Kapoor, S. G., (2006). An analytical model for the prediction of minimum chip thickness in micromachining. The Journal of Manufacturing Science and Engineering, 128, 474–481.

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9. Reginaldo, T. C., Anselmo, E. D., & Tatiany M. Da. S., (2017). An experimental method to determine the minimum uncut chip thickness (hmin) in orthogonal cutting. Procedia Manufacturing, 10, 194–207. 10. Malekian, M., Mostofa, M. G., Park, S. S., & Jun, M. B. G., (2012). Modelling of minimum uncut chip thickness in micro machining of aluminum. Journal of Materials Processing Technology, 212, 553–559. 11. Woon, K. S., Rahmana, M., Fang, F. Z., Neo, K. S., & Liu, K., (2008). Investigations of tool edge radius effect in micromachining: A FEM simulation approach. Journal of Materials Processing Technology, 195, 204–211. 12. Schneider, F., Bischof, R., Kirsch, B., Kuhn, C., Müller, R., & Aurich, J. C., (2016). Investigation of chip formation and surface integrity when micro-cutting cp-titanium with ultra-fine grain cemented carbide. Procedia CIRP, 45, 115–118. 13. Déborah De, O., Milla, C. G., & Márcio, B. Da. S., (2019). Spheroidal chip in micromilling. Wear, 426, 427, 1672–1682. 14. Martin B. G. J., Xinyu, L., Richard, E. De. V., & Shiv, G. K., (2006). Investigation of the dynamics of micro end milling—Part I: Model development. Journal of Manufacturing Science and Engineering, 128, 893–900. 15. Ali, M., Ehsan, L. K. S., & Ismail, L., (2013). Machining forces and tool deflections in micro milling. Procedia CIRP, 8, 147–151. 16. Leticia, C. S., & Marcio, B. Da. S., (2019). Investigation of burr formation and tool wear in micromilling operation of duplex stainless steel. Precision Engineering, 60, 178–188. 17. Suman, S., Sravan, K. A., Sankha, D., & Partha, P. B., (2020). An investigation on the top burr formation during minimum quantity lubrication (MQL) assisted micromilling of copper. Materials Today: Proceedings, 2, 1809–1814. 18. Kiswanto, G., Zariatin, D. L., & Ko, T. J., (2014). The effect of spindle speed, feed-rate and machining time to the surface roughness and burr formation of aluminum alloy 1100 in micro-milling operation. Journal of Manufacturing Processes, 16, 435–450. 19. Alessandro, C., Antonio, F., Elisabetta, C., & Aldo, A., (2019). Tool wear analysis in micromilling of titanium alloy; Precision Engineering, 57, 83–94. 20. Vipindas, K., & Jose, M., (2019). Wear behavior of TiAlN coated WC tool during micro end milling of Ti-6Al-4V and analysis of surface roughness. Wear, 424, 425, 165–182. 21. Alhadeff, L., Marshall, M., & Slatter, T., (2019). The influence of tool coating on the length of the normal operating region (steady-state wear) for micro end mills. Precision Engineering, 60, 306–319. 22. Yiquan, D., & Kunpeng, Z., (2017). A machine vision system for micro-milling tool condition monitoring. Precision Engineering, 52, 183–191. 23. Kunpeng, Z., & Xiaolong, Y., (2017). The monitoring of micro milling tool wear conditions by wear area estimation. Mechanical Systems and Signal Processing, 93, 80–91. 24. Yishun, W., Bin, Z., Juncheng, W., You, W., & Chuanzhen, H., (2020). Effect of the progressive tool wear on surface topography and chip formation in micro-milling of Ti–6Al–4V using Ti(C7N3)-based cermet micro-mill. Tribology International, 141, 105900. 25. Wang, W., Kweon, S. H., & Yang, S. H., (2005). A study on roughness of the micro-endmilled surface produced by a miniatured machine tool. Journal of Materials Processing Technology, 162, 163, 702–708.

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CHAPTER 13

RECENT ADVANCEMENT IN MICROTEXTURING USING ELECTROCHEMICAL MICROMACHINING SANDIP KUNAR,1 GOLAM KIBRIA,2 PRASENJIT CHATTERJEE,3 T. JAGADEESHA,4 BH. V. PRASAD,1 S. RAMA SREE,5 and M. S. REDDY1 Department of Mechanical Engineering, Aditya Engineering College, Surampalem, Andhra Pradesh, India

1

Department of Mechanical Engineering, Aliah University, Kolkata, West Bengal, India

2

Department of Mechanical Engineering, MCKV Institute of Engineering, Howrah, West Bengal, India

3

Department of Mechanical Engineering, NIT Calicut, Kozhikode, Kerala, India

4

Department of Computer Science and Engineering, Aditya Engineering College, Surampalem, Andhra Pradesh, India

5

ABSTRACT Microtexturing is the art of miniaturization of patterned surfaces. Depending on the applied texturing method and surface functionalization for a particular application, microtextures can be produced as micro-pillars and micro-pits. Surface microtexturing is the method of generating précised arrays of indistinguishable micro/nano-features on the surface to change its surface Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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topographies of their functionality and product life. Hence, patterned substrates are implicitly microtextured engineered surfaces, regardless of the pattern type. The term ‘microtextured surfaces’ characterize the exteriors that comprise several features, such as micro-holes, micro-dimples, micro-square pattern, etc., of significant geometrical parameters. The categorization of microtextures is required by the sum of their prime symmetrical features such as shape, size, density, dimensions (dependent on the geometric shape), distribution on the surface, etc. In current times, microtextures are used in several fields, such as metrology, electronics, optics, tribology, biomedicine, etc. Primarily, the practical properties of microtextured surfaces are from the environment. For a better insight into micro-texturing using the electrochemical micromachining (EMM) process, fundamental concepts such as electrochemistry, limitations, and advantages are discussed. The investigation into the generation of micro-ellipse patterns is carried out in this chapter. 13.1

INTRODUCTION

Surface phenomena especially micropatterned surfaces perform a significant role in the performance of industrial parts. Their perceptive and control of microtextures are vital to the progress of several innovative fields, such as computer technology, biomimetics, energy, etc. Microtextured surfaces depend on its characteristics to attain an anticipated efficient performance of engineering components. Because of enormous progression of generation of micropatterns, many opportunities in technology use to make new kinds of micropatterns/microstructures or to reconstitute existing micropatterns in down-sized version. These fabricated micropatterns have become a key issue in advanced manufacturing fields such as microelectronics, automotive, aerospace, optics, biomedical, communications, and avionics, etc. Micropatterns also provide the opportunity to study fundamental scientific phenomena that occur at small dimensions. This highly demanded area is attracting present researchers and engineers to endorse suitable procedures in producing microtextures to maintain quickness with the research requirements and recent market trends. Continuous changes in present societies’ requirements have concentrated to introduce numerous microtextures in different industrial applications. For instance, inspection of human body is generally preferred without pain. The microparts of medical tools can fulfill the requirements of human body. The micropatterned surface improves friction and tribological performance of sliding components. Micropatterned surface acts as lubricant reservoir and debris collector during movement of mechanical components.

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Emerging micromachining techniques have great potentiality to the present and future societies for various types of applications that help machines and people intermingle with the real world. The microtextured devices are linked with various fields including medicine, defense, electronics, etc., with produced features in a few micro-sizes. Functionalization and life of many microparts and microdevices require very close dimensions and the usage of a different advanced materials like stainless steel, titanium, etc. As per prerequisite of product microtexturing, the significance will be provided on sophisticated microfabrication techniques as well as the trend of miniaturized equipment and metrology systems. So microtexturing is the crucial technology in MEMS. Micropatterns may have overall size of microns, but these have many features that fall in micro-range from 1 µm to 999 µm and latest definition as per CIRP is from 1 μm to 500 μm. Achievable of better machining accuracy and other performance criteria of microtextures is the major challenging issues in advanced manufacturing processes. Owing to the advancements of microtextured parts, its components currently can be evident in almost every manufacturing field. The outline of micropatterning is to provide an elementary understanding of micropattern generation without having to know too much about the physics and chemistry of it. As the prerequisite of microtextures is exponentially growing, the demand of microtextured surfaces from sophisticated engineering materials becomes more importance. The miniaturization of components started from 1900 and developed rapidly with time. The development of machining accuracy is shown under the generalized classification of precision machining, normal machining, and ultra-precision machining with machine tools and measuring instruments [1]. Current variations in public demands have compelled researchers to create different microtextures into various industrial products. Microtexturing technology portrays an intensifying pivotal role in the microtextured components varying from microsensors to biomedical applications. Microtexturing is one of the vital technologies to achieve the miniaturization needs for micropatterned fields, which are quickly growing. The word “micromachining” implies to the material removal of small dimension that varies from 1 μm to 999 μm. Advanced microfabrication techniques may comprise of different ultra-precision activities to be carried out on very thin workpieces. Various simple and complex microtextures are required to be manufactured in huge quantities by advanced micromachining methods. Sometimes, when these are accomplished with traditional manufacturing techniques, the created difficulties are heat generation at the job-tool interface and tool wear. Besides, it is problematic to create

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3D micro-shapes. At present, non-traditional techniques is becoming more prominence because of some certain benefits, which can be used during the microtexturing operation. But LBM, EBM, and EDM are thermal oriented processes, which may affect thermal distortion to the machined structures. Electrochemical machining (ECM) and chemical machining are thermalfree procedures, but chemical machining cannot be operated correctly in the micro-domain and furthermore, it is difficult to create microtextures on chemically unaffected materials. ECM procedure is employed to the micromachining products for fabrication of micro-products and microtextures; it is known as electrochemical micromachining (EMM). EMM acts to be a very prominent microtexturing technology because of its many benefits that comprise high MRR, better surface quality (SQ), flexible, and reliable and it also performs the microtexturing on chemically unaffected materials like Iridium, Tantalum, Copper alloys, etc., which are extensively utilized in electronic, MEMS, and NEMS applications. EMM is favorably engaged in most microtexturing of metallic parts owing to its cost efficiency and attainable greater accuracy, which are formerly produced by chemical micromachining. EMM is the main microfabrication technique for the creation of distinct microtextures in the fields of optics, avionics, automotive, etc. Experimentation is carried out to produce the micro-ellipse pattern using maskless EMM method and the influence of input variables are studied on dimpled characteristics. An exploration has been made to analyze the micrographs for achieving the best parameter setting. 13.2 LITERATURE REVIEW The material removal mechanism during microtexturing using EMM based on electrolysis was identified in the earlier era. The purpose for the fabrication of EMM system is principally for the expanding use of extremely advanced materials, which are not machined reasonably by traditional technique. In latest years, EMM process is successfully utilized for précised machining owing to its many benefits such as greater MRR and accuracy, generation of 3D structures, free from tool wear, burr formation, crack formation, HAZ zone, etc. EMM performance is controlled by electrolysis principle in every electrolyte. Many input factors during texturing makes this process difficult to regulate the attainable accuracy. Few study makes have previously been made employing the EMM process principle in microtexturing. However, to explore full capability of EMM in microtexturing, study is still required to

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acquire better knowledge. Review of microtexturing using EMM is carried out in this area as stated here under. Chen et al. [2] used through-mask electrochemical micromachining (TMEMM) approach to create microtextures using an environment friendly electrolyte of NaNO3 and a recyclable PDMS mask. The influence of important parameters is explored on etching factor, machining accuracy, and depth of micro-dimples. Kunar & Bhattacharyya [3] utilized maskless EMM for producing micro-ellipse patterns using controlled flow electrolyte system with controlled microtextured features. Natsu et al. [4] developed the electrolyte jet machining method for fabricating three dimensional complex surfaces such as arrays of machined pits and grooves using NaNO3 aqueous solution. Chen et al. [5] proposed a feasible complementary technique of TMEMM for manufacturing micro-dimples on stainless steel (SS-304) workpiece. Kunar et al. [6] demonstrated the generation of varactor micropattern by EMM. The produced varactor is applied in many products, i.e., parametric amplifiers, radio frequency, etc. The effect of input parameters is revealed on output characteristics, i.e., textured depth and MRR. Zhang et al. [7] used a revised micropatterned transfer process without photolithography method, utilizing portable dry-film mask in EMM to create micro-impressions. Investigational analysis revealed that the combined effect of lower texturing time and greater current density was suggested for higher dimensional accuracy and uniform depth generation of micro-dimples. Chen et al. [8] applied an alternative method of through-mask EMM for producing microtextures and eliminating islands by using thick mask of 200 μm from the micro-dimples. Kunar & Bhattacharyya [9] revealed that maskless EMM is a distinctive approach for the creation of microtextures using several patterned tools with better geometrical shape. Qian et al. [10] applied TMEMM method for producing micro-circular patterns on hard chrome coated surface using NaNO3 electrolyte and direct current. Winkelmann & Lang [11] introduced a unique method of EMM using a structured a counter-electrode incorporated with microfluidic channel. It has capability to create numerous structures in parallel. Mahata et al. [12] exploited TMEMM for preparing micro-dimple arrays. Investigation is conducted to explore the influence of duty cycle on the dimensional accuracy and other output characteristics of the produced micro-dimples. Sjöström & Su [13] used EMM for micropatterning of bulk titanium (Ti) surfaces for biomedical and microengineering applications. Kern et al. [14] used through-mask EMM method for surface micromachining of metallic and Ti surfaces. Kunar & Bhattacharyya [15] applied maskless EMM method to produce the microtextures inexpensively. The outcome of important process parameters on surface characteristics has been studied.

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Zhao et al. [16] anticipated a TMEMM approach with a foamed cathode for creation of microstructures with a high geometric uniformity. Hao et al. [17] conceived large-area microstructures on curved objects using EMM approach. These methods prepare various shapes of microstructures over a large area. Kunar & Bhattacharyya [18] used maskless EMM technique to produce microsurface textures on conductive substances using various pulse waveforms of pulsed DC power unit on SS-304. Hou et al. [19] applied TMEMM process for creating micropatterns over large area. To explore the dimensional accuracy of microtextures over a large and rounded machining area of diameter 40 mm, an experiment is carried out with 25 V voltage, 20% duty cycle and 24s machining time. Ming et al. [20] studied the TMEMM procedure to create micro-dimples with substantially identical symmetrical shapes on the large planer and curved surfaces. Kunar & Bhattacharyya [21] utilized EMM process for generating many good micro-square patterns compared to TMEMM, which is costly and timetaking process for mass production. The generated all impressions are consistent in geometric shape because of regulated etching. The generated depth is nearly undistinguishable because of uniform machining for the distribution of current flux. Patel et al. [22] utilized a porous and flexible electrode based TMEMM technique for microtexturing on any curved surface. Sun et al. [23] utilized TMEMM to produce micro-pillar arrays on aluminum workpiece. Kim et al. [24] projected that reverse electrical discharge (ED) machining is applied to produce micro-electrodes of different shapes and then used for EMM to improve productivity over separate micro-electrodes and different microtextures are fabricated. Kunar & Bhattacharyya [25] utilized maskless EMM approach to produce the microslot arrays. Kunar & Bhattacharyya [26] used maskless EMM for manufacturing of cascade micropatterns on SS-304. The generated microtextures has appropriate geometrical shape due to précised machining. Kunar et al. [27] applied maskless EMM for creating micro-circular impressions. Wang et al. [28] intended TMEMM for creating hole array on metal parts. To expand the microtexturing area, a serpentine flow approach with several curves is applied for TMEMM. 13.3 ELECTROCHEMICAL MICROMACHINING (EMM): FOCUSING AREA ON MICROTEXTURING Because of the multiple advantages of microtextures, such as a smaller amount of space, less raw material, low cost, less energy, and light weight,

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product microtextured surfaces with integrated functionality has become a current trend in advanced manufacturing industries. Microtextured surfaces must accomplish many purposes in undesirable circumstances, which requires the microtextured surfaces to be fabricated from advanced materials like iridium, nickel alloy, etc. With the advancement in microtextured surfaces, microtextures are extensively utilized in different areas, especially optical, computer technology, etc., for production of microtextured products. Diverse microtexturing approaches are existing to produce the microtextures on different hard materials, which can be chosen depending upon the number of output responses such as SQ, manufacturing quantity, and manufacturing cost [29]. 13.3.1

ECM AND EMM IN MICROTEXTURING

The word “microtexturing” describes to the removal of material of lesser dimensions varying from 1 to 999 µm. When ECM is employed for fabricating of microtextures in micro-level, it is described as EMM [29]. EMM is an electrolysis method in which pulsated DC with high frequency and low voltage is applied between the workpiece as anode and microtextured tool as cathode, submerged in liquid solution with narrow interelectrode gap. The anodic material disperses by the electrolysis producing H2 gas bubbles on the tool surface. Microtextured topographies of the tool are precisely transferred to the workpiece, hence microtextuured tool performs a crucial role during manufacturing of microtextures by EMM. 13.3.2 ADVANTAGES AND LIMITATIONS OF EMM IN MICROTEXTURING The procedure has several advantages and disadvantages during generation of microtexturing. This approach has the following benefits on the microtextured product: i. The generated microtextured surfaces are burrs free; ii. No thermal effect during microtexturing; iii. By implementing the innovative monitoring capabilities, high machining accuracy and lower surface roughness can be achieved; iv. It has high microtexturing speed and comparatively low cost.

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Although EMM process has following disadvantages: •

• • •

Previously, EMM is labeled as a procedure that is harmful to the environment. The procedure has become less hazardous to the environment because of extensive improvements in the treatment of electrolytes. EMM, on the other hand, requires a very small amount of electrolyte. Each microtextured material and product needs the latest investigation. So, productivity will be higher for the cost effectiveness of the procedure. In situ manufacturing necessitates careful manipulation of microtextured tool. EMM’s operation necessitates a somewhat large knowledge base.

13.3.3 ROLE OF EMM IN MICROTEXTURING Microtexturing is the process of creating microtextures on microtextured parts with great accuracy. The microtexturing approach will continue to be a crucial technique in the upcoming applications since people will require more effective space use with more précised products. As a result, more emphasis must be concentrated to future improvements in conventional and untraditional microtexturing processes to enhance microtexturing precision. Microtexturing of advanced engineering materials requires suitable microtexturing techniques to produce précised and accurate microtextures on products. Micromachining procedures including micro-grinding, micro-milling, and micro-turning have cutting tool and fixture precision constraints, poor SQ, and significant tool wear. Unconventional microtexturing technologies such as LBM and EDM, on the other hand, are thermal in nature and result in a heat impacted zone and tool wear, correspondingly. LIGA procedure is restricted to the production of 2D microstructures and chemical etching procedure cannot be regulated appropriately in micro-area [30]. As a result, EMM is the best microtexturing method on HSTR materials owing to its numerous benefits, including improved accuracy, enhanced SQ, etc. [31]. 13.4

FUNDAMENTALS OF ELECTROCHEMICAL MICROTEXTURING

EMM is employed for micro-scale field in microtexturing purposes. It is essentially an anodic dissolving technique in which a microtextured tool serves as the cathode and the job serves as the anode, both of which are

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immersed in the liquid solution with a tiny interelectrode gap, and a low potential difference with pulsed DC is supplied between the electrodes. The job dissolves into metallic ions through electrochemical processes. The electrolyte carries sludges from the constricted micromachining zone. The machined microtextures’ ultimate shape and size are nearly a negative mirror copy of the microtextured tool. Different microtexturing conditions in the narrow machining zone affect microtexturing accuracy. Input parameters of EMM such as electrolyte concentration, pulse frequency (PF), machining current, and tool feed rate, among others, affect microtexturing criteria such as machined surface properties, material removal rate (MRR), and machining accuracy [32]. 13.4.1 ELECTROCHEMISTRY OF ELECTROCHEMICAL MICROTEXTURING Electrolysis is an electrochemical reaction that happens when a power supply is provided between two electrodes immersed in an electrolytic cell. ‘Electrodes’ are conductive substances submerged in a liquid solution. Anode refers to the electrode with positive polarity, while cathode refers to the electrode with negative polarity. Electrodes use electron mobility to conduct electrical current. The ‘electrochemical cell’ is a system of electrodes and electrolytes. Electrode reactions occurs at the anode and cathode, respectively. Electrolytes conduct electricity by moving atoms that have either acquired or lost electrons, causing in a negative or positive charge. These particles are known as ions. Anions drift towards the anode Cations have positive charges and move through an electrolyte towards the cathode [33]. The electron transfer between electrodes completes the electrical circuit when a small amount of current is provided between the job and tool, immerged in liquid solution with tiny inter-electrode gap. The electric current is maintained by removing electrons from the job because electrons cannot travel through the electrolyte. The metal substance takes anodic dissolution from the job surface, and the positive ions enter the electrolyte. As demonstrated in Figure 13.1, and negative ions move towards the anode and positive ions drift towards the cathode through the liquid solution. Outside the cell, the flow of ions is conducted by the movement of electrons in the opposite direction, and both activities are caused by the utilized voltage. The electrons that are least firmly bonded and detected on the workpiece’s surface and pass into the electrolyte solution. These electrons separate from the job and move into the electrical circuit. Dispersed metal emerges as a precipitated solid in

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EMM. Chemical processes take place in the liquid solution at the cathode and anode. The reaction with the slightest oxidation potential occurs first at the cathode, which is the microtextured tool, and the reaction with the biggest oxidation potential occurs first at the anode, which is the workpiece. As a result, parameters like (i) distribution of current flux; (ii) the nature of metal; (iii) the form of electrolyte; and (iv) the electrolyte temperature affects the potential of oxidation and thus the kind of reaction occurs. The machining rate (MR) is controlled by: (a) mass transport factors; (b) electrical variables; (c) variables of electrolyte; (d) external variables such as temperature and pressure; and (e) variables of electrode. During machining, the following electrochemical reactions occurs at the electrodes [34].

FIGURE 13.1

Principle of anodic dissolution of metal.

13.4.1.1 CATHODE REACTIONS Two potential reactions happen to the cathode: (i) creation of hydrogen gas; and (ii) neutralization of positively ions. The creation of hydrogen gas is occurred by the following reactions:

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2H + + 2e − → H 2 ↑ (for acidic electrolyte)

(1)

2H 2 O + 2e − → H 2 ↑ (for alkaline electrolyte)

(2)

The reaction causes the neutralization of a positively charged metal ion: M + + e − → M (Metal)

(3)

When the workpiece is iron, for example, the cathode reactions are: Fe − 2e → 2Fe 2+

(4)

2Fe 2 + + 2OH − → Fe(OH) 2

(5)

When neutral electrolytes are utilized, metal ions create metal hydroxides. Because these are unsolvable in water, these are appeared as solid precipitates. The electrochemical process is unaffected by these precipitates. 2H 2 O + Fe → Fe(OH) 2 + H 2 ↑

(6)

Ferric hydroxide is formed when ferrous oxide reacts with oxygen and water to form: 4Fe(OH) 2 + O 2 + 2H 2 O → 4Fe(OH)3 (7) 13.4.1.2 ANODE REACTIONS Two potential reactions take place at the anode: (i) the dissolution of metal ions; and (ii) the generation of oxygen gas. The followings are the reactions that lead to the generation of oxygen gas: 2H 2 O → O 2 + 2H + + 2e − (for acidic electrolyte)

(8)

4(OH) − → 2H 2 O + O 2 + 4e − (for alkaline electrolyte)

(9)

The chemical reaction that causes the metal to dissolve. M → M + + e−

(10)

M + OH → M(OH)

(11)

+

−

The following are the overall reactions that occur during iron ECM: Anode : Fe → Fe ++ + 2e −

(12)

Cathode : 2H 2 O + 2e → 2 ( OH ) + H 2 ↑

(13)

−

−

Metal dissolution has been seen to be the predominant that happens at the anode with the electrolyte acting only as a current carrier. The current

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efficiency is described as the ratio between the actual MR to the theoretical MR for a given time and current, is commonly less than 100%. This is because, in addition to metal dissolution, other anode processes can take place, such as the emission of oxygen gas and water oxidation: 2H 2 O → 4H + + O 2 ↑ + 4e −

(14)

The amount that this reaction reduces current efficiency is highly reliant on the workpiece material, the distribution of current flux, and the electrolyte. In addition to the oxidation of water, metal ions can also be oxidized at the anode. 13.4.2

FARADAY’S LAW OF ELECTROLYSIS

In 1833, Michael Faraday invented two basic rules for electrolysis that controlled the occurrence. The most prevalent formulations of Faraday’s law are as follows: i.

During electrolysis, the mass of material removal or deposition is proportional to the amount of electricity passing through the electrodes. ii. Chemical equivalent weights are directly proportional to the amounts of dissimilar materials removed or deposited by the same amount of electricity at the electrodes. These two rules are amalgamated mathematically to give mass (m) removed from or deposited on the electrode, as follows:  Q  P  m =     F  z 

(15)

where; ‘Q’ signifies the total electrical charge delivered through the metallic material; ‘F’ characterizes Faraday’s constant (96,485 C/mol), the atomic weight is represented by ‘P,’ and the valency number is ‘z.’ The total charge transmitted through the material during constant current electrolysis can be calculated as follows: Q = It

(16)

where; ‘I’ denotes the current flowing through the electrodes over time ‘t.’ As a result, the quantity of mass lost (m) is stated as:  It  P  m =     F  z 

(17)

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The material removal (P/z.F) is described as the “electrochemical equivalent” when one coulomb charge passes through the electrodes. According to the preceding equations, the rate of anodic dissolution is determined by the atomic valency, weight, current, and machining time. The rate of disintegration of metal is unaffected by its hardness or any other mechanical qualities. Because at the cathode surface, only hydrogen gas is produced during the electrochemical reaction and no tool wear occurs. This functionality is extremely beneficial in EMM applications when cutting intricate micro-features using micron-sized tools. 13.5

DIFFERENT TYPES OF EMM

EMM can fabricate 3D and 2D microtextures and surface characteristics on micro- and macro-items. Controlled metallic dissolution in the microscopic realm can be performed in EMM using pulse DC power supply with high-frequency and accurate movement of the electrodes. One of the most important characteristics to consider when choosing an EMM type is the flexibility for electrolysis using the electrochemical approach. EMM is generally categorized into two forms, as illustrated in Figure 13.2, based on the degree of localization effect.

FIGURE 13.2

Types of EMM.

13.5.1 THROUGH-MASK EMM The installation of a masked pattern on the conductive surface limits material removal, enabling dissolution only from the appropriate parts of the metal

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surface. For fabricating 2D microfeatures and microtextures, EMM with a through-mask is superior. The mask is applied to a conductive workpiece in the proper pattern for EMM and utilized as an anode, allowing only the uncovered areas of the job to be removed by electrochemical reactions. In this method, dissolution occurs at the job surface, the cavity is created at the bottom of TMEMM [35]. The production of mask selection, and preparation of job are all part of the TMEMM approach. Due to the isotropic nature of metal removal, undercutting occurs below the mask. When producing a photoresist mask, it is crucial to understand the MR and undercutting in TMEMM at various electrolytes. The undercut of microtextured surface is influenced by a variety of elements such as mask thickness, spacing to opening ratio, and aspect ratio, etc. [36]. TMEMM provides more control and flexibility for micro-production than chemical etching. The purpose of a less corrosive electrolyte and quicker material removal are two advantages of TMEMM. This method can also handle different materials including HSTR alloys. 13.5.2

MASKLESS EMM

Photoresist masking does not limit MR from the job surface; instead, a localized material dissolving process controls it. In 2D or 3D scale, the greater localized from the job surface can produce the required microtextures [37]. In a maskless EMM, anodic dissolution is regulated by the distribution of current flux, which is affected by different input parameters. The narrow gap between electrodes is kept to a minimum to reduce stray current effect. Because of its potential to generate oxide layers and produces oxygen in the stray current region, passivating electrolyte is appropriate for this method. This approach needs microtexturing setup for precision tool and workpiece movements to create highly localized anodic dissolution. To keep up and control the tiny IEG during microtexturing, the system requires very précised controlling devices that should use closed loop control techniques. One of the major issues in maskless EMM is the distribution of electrolyte in the limited microtexturing zone. Another critical aspect that effects the regulated MR at various electrolyte–metal combinations is the selection of an appropriate electrolyte during micromachining of various materials. The elimination of machining by-products and produced heat at narrow IEG is another key issue. This technique requires a DC pulsed electrical unit with higher frequency for effective metal removal localization. It is classified into three types: 3D EMM, micro-drilling, and Jet EMM, for instance.

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13.6 INVESTIGATION INTO FABRICATION OF MICRO-ELLIPSE PATTERN USING MASKLESS EMM Maskless EMM can fabricate a microtexture containing micro-ellipse pattern, which is one of the key microtextures generated by this method. Machining of 3D micro-features is one of the key advantages of this method. Maskless EMM can produce a variety of microtextures utilizing its MR mechanism’s versatility. It can be used to create microstructures ranging in depth from extremely shallow to very deep. This technique can be chosen depending on the depth and geometrical complexity of the microtextures. This process is utilized to create the high-quality micro-ellipse pattern with good SQ. 13.6.1

INTRODUCTION

Microtexturing has been obtained a substantial enhancement in interfacial science as it is confirmed to be attractive in enhancing the surface properties. Modern micromachining technique assists it conceivable to enhance the interfacial function by quite governing the size and shape of surface microtextures. Presently, micropatterning/microtexturing has been concerned in dropping friction, anti-wear, lowering vibration, anti-creeping, etc. For instance, micro-ellipse pattern is broadly utilized for enhancing tribo-characteristics. Various micromanufacturing techniques have been advanced for generating micro-ellipse pattern, such as EDM, LBM, micromilling, and EMM. EMM is an advanced microtexturing method to eradicate the materials from the confined area by anodic dissolution from the job in the electrochemical unit with a suitable parameter setting. Because of irrespective of material hardness, absence of thermal zone, crack, and burr, high MR, etc. This method is a protuberant for producing micropatterns [38]. Microcircular pattern is fruitfully generated with diameter of 300 μm and depth of 5 μm using EMM with 275 μm diameter tool electrode [39]. Microdimples are produced with depth of 40 μm and diameter of 400 μm on Al–Si cylinder material using jet EMM [40]. These techniques have low productivity due to individual fabrication of micropattern. For producing the good micro-dimples, TMEMM is applied. Hemispherical micropattern are fabricated on Ti applying this method [41]. Microtextures are prepared on curved surfaces with diameter of 40 µm [42]. A new development of TMEMM, in which an insulation is formed on Ti for acting as a mask and for creating patterns laser is applied [43]. A flexible PDMS mask is presented

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in TMEMM for the creation of microtextures [44]. A modified TMEMM method with an insulated sheet of through-holes wrapped with conductive layer, is applied as a mask for generating microtextures [45]. This method is not appropriate for mass production due to specific masking before every machining. Sandwich-like electrochemical micromachining (SLEMM) is employed for boosting the dimensional accuracy of micropattern, in which the tool is kept close the dry film mask bonded with the workpiece and the electrochemical reaction occurs in the confined unit [46]. However, the machining depth (MD) of micro-dimples is very less due to accumulation of sludges in the confined machining unit. So, a permeable metal cathode of an open electrochemical unit in SLEMM removes the electrochemical products and produces deeper micro-dimples [47]. Though, the dimensional accuracy of micropattern is less in SLEMM due to still electrolyte. The dimensional accuracy of micropattern has the great effect on the tribological performance and enhancing the accuracy of micropattern is a significant influence for augmenting the properties of tribology [48]. Many research works have directed that the electrolyte flow method has a great effect on the dimensional accuracy of micropattern. The greater dimensional precision is accomplished by developing the electrolyte flow dispersal, in which the liquid solution is delivered upright from down to upward directions through the thin gap between electrodes. This vertical cross flow electrolyte is proposed to improve the consistency of flow of electrolyte and to eradicate the distraction in the flow field. This developed flow electrolyte approach is utilized for producing higher SQ of micropatterns. Many experimentations are demonstrated that pulsed EMM is a promising technique for improving the SQ of micropatterns. This research work concentrates on the improvement of the SQ and dimensional accuracy of micro-ellipse patterns created by maskless EMM. The pulsed current is employed to enhance the geometric uniformity of micropattern, and the developed electrolyte flow approach is applied to explore their impact on the geometric accuracy. Higher dimensional accuracy of micropattern is prepared with greater and regular electrolyte supply. Hence, experiments are investigated on the developed electrolyte flow approach to attain more uniform electrolyte flow distribution. The outcome of influencing parameters on substantial micropatterned features, viz. minor axis overcut, depth, major axis overcut, and surface roughness (Ra) are revealed throughout manufacturing of microtextures. A confirmation experiment and analysis are conducted using the optimal parameter setting for fabricating the précised micropattern.

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EXPERIMENTAL INVESTIGATION

For conducting the experiments for production of micropatterns, the setup is manufactured as shown in Figure 13.3. This setup has investigational cell, electrical connection facility and electrolyte flow scheme. The cell comprises electrode fixtures, power supply facility, and electrolyte circulation path. Perspex material is employed to make the cell to evade the oxidation from atmosphere. The outlet and inlet ports are produced by SS 304 to avoid from rusting for flow of electrolyte. The perpendicular cross flow electrolyte technique is mostly the noteworthy characteristics, combined within the prepared investigational cell. The fabricated flow procedure creates the supplementary back pressure, which is the most important characteristics inside the developed flow technique. The generated back pressure confiscates the sludges from the narrow gap. This current enhances the quality of micropattern with higher precision. The pulsed DC electrical unit provides the electrical facility. The electrical unit is integrated with protection function and function generator. For the preparation of an ellipse micropatterned tool with minor axis length of 434 µm and major axis length of 597 μm and on SS-304 sheet utilizing SU-8 2150 insulation, the lithography process is applied. The insulation thickness of a tool is 217 μm. The indispensable processing attributes for creation of essential micropatterns are applied for good industrial application of this system. The significant attributes on the creation of micropatterned features, i.e., geometric accuracy, depth, and surface roughness (Ra) are presented, viz. inter electrode gap (IEG), flow rate, voltage, duty ratio (DR) and frequency on SS 304 substrate. The investigation is conducted out with changing one factor at a time with keeping other factors fixed. The essential attributes, viz. IEG, flow rate, voltage, DR and frequency are varying from 50 to 200 μm, 3.45 to 6.45 m3/hr., 7 to 13 V, 30 to 60%, and 2 to 5 kHz, respectively. The intermingled electrolyte of NaNO3 (0.19 M/L) and NaCl (0.27 M/L) is applied for investigation. with an optical microscope and a 3D Profilometer, the micropatterned characteristics are examined. 13.6.3

RESULTS AND DISCUSSION

To study the consequence of process attributes, viz. IEG, flow rate, voltage, DR and frequency during production of micropatterns, experimentations are conducted to appraise the micropatterned criterion, viz. dimensional accuracy, depth, and Ra of micropatterns.

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FIGURE 13.3

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Experimental setup.

Experiments are performed to sightsee the consequence of voltage on dimensional precision, depth, and Ra as exhibited in Figure 13.4. The greater major and minor axis overcuts are revealed with greater voltage because of higher machining current and stray current distribution. The higher depth is observed at higher voltage because of regular machining. Higher machining current is observed at greater voltage resulting higher material removal from the machining zone. The SQ deteriorates with higher voltage owing to irregular machining. In addition to it, bubbles formation and sludges rise in roughness at higher voltage. Hence, for producing the précised ellipse micropattern, lower voltage is recommended. Figure 13.5 presents the effect of IEG on the geometric accuracy, depth, and Ra. The IEG is the greatest persuading attribute for producing the ellipse micropattern utilizing this method. The greater overcuts are observed with growing IEG because of higher ohmic resistance and irregular dispersal of current flux. Lesser IEG enhances the geometric accuracy throughout the pattern due to homogeneous etching. The depth diminishes with higher IEG owing to the heterogeneous etching throughout ellipse micropattern and irregular spreading of current flux. The SQ deteriorates with higher IEG owing to unregulated etching for the scattering of current flux. For précised ellipse micropattern, lower IEG is proposed. Ellipse micropattern is produced using maskless EMM to study the outcome of flow rate on major axis overcut, minor axis overcut, depth, and Ra as shown in Figure 13.6. The electrolysis products are separated from the

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narrow gap at higher flow rate and the etching zone remains clean. Consequently, the regular etching occurs throughout the micropatterned zone. As a result, the geometric accuracy improves with higher flow rate. The greater depth is observed at higher flow rate owing to regulated machining for good flushing in controlled electrolyte system. The SQ improves because of controlled etching throughout the etching area. For good quality microtextured product, higher flow rate is proposed.

FIGURE 13.4

Variation of overcut, depth, and Ra with voltage.

FIGURE 13.5

Variation of overcut, depth, and Ra with IEG.

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FIGURE 13.6

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Variation of overcut, depth, and Ra with electrolyte flow rate.

Microtextures are produced to study the outcome of DR on micropatterned characteristics as shown in Figure 13.7. The dimensional accuracy lowers with increasing DR due to increasing in machining time which leads to more stray current influence. The textured depth rises with boosting DR because of the availability of greater machining time which advances to regulated machining influence throughout the etching zone. The SQ reduces with the greater DR owing to regular etching throughout the machining zone. For good quality micropatterns, lower DR is indorsed. Experimentation is conducted to investigate the impact of frequency on dimensional accuracy, depth, and Ra as shown in Figure 13.8. The overcuts of minor and major axes lower with higher frequency because of the accessibility of lower machining time in higher frequency. The lower texturing time starts to lower stray current impact and the dispersion of current flux resulting higher machining accuracy. Lower MD is seen in greater frequency since lower machining time leads lower regulated machining. The SQ improves in higher frequency due to regular machining in lower machining time. For controlled ellipse micropattern, greater frequency is recommended. 13.6.4 ANALYSIS BASED ON MICROGRAPHS The micrograph of produced proper ellipse micropattern has been demonstrated in Figure 13.9. It is fabricated at the parametric machining conditions,

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i.e., IEG of 50 μm, flow rate of 6.45 m3/hr., voltage of 7 V, DR of 30%, and frequency of 5 kHz. The microtexture holds its approximately regular shape and size owing to homogeneous machining for controlled spreading of current flux. The textured depth of microtexture is also uniform throughout the micropattern because of regular etching. The SQ is also improved because of regulated etching. Figure 13.10 represents 3D view of a small-machined portion with a 2D depth profile of 24.5 μm. Figure 13.11 depicts the roughness profile with a value of 10.3 nm.

FIGURE 13.7

Variation of overcut, depth, and Ra with duty ratio.

FIGURE 13.8

Variation of overcut, depth, and Ra with frequency.

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FIGURE 13.9

Regular micro-ellipse pattern.

FIGURE 13.10

3D view with depth profile.

FIGURE 13.11

Roughness profile.

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315

CONCLUSIONS

Microtextures are produced using the micropatterned tool using reusable mask. The influencing parameters, i.e., IEG, flow rate, voltage, DR and frequency are applied to show the effect on micropatterned characteristics. The ellipse micropattern is manufactured by this method economically. Experimentation is conducted out using fabricated EMM cell and developed electrolyte flow system. One reusable tool fabricates many textured workpieces economically. Microtexturing with lower DR, IEG, voltage, higher flow rate and frequency is proposed for précised machining. From the study of micrographs for micro-ellipse pattern, it can be achieved that the parametric setting, i.e., flow rate of 6.45 m3/hr., IEG of 50 μm, DR of 30%, voltage of 7 V, and frequency of 5 kHz generates the best array of micropatterns. 13.7

SUMMARY

Because of its multiple advantages, electrochemical micromachining (EMM) has become one of the most essential microtexturing processes. The vital perceptions in EMM, EMM types, setup development, and the material removal technique as well as current improvements, are covered in this chapter. Array of microslots, 3D microstructures, and micropins with higher aspect ratio are benefitted from the EMM process. Applications with higher SQ such as edge finishing of surgical blades and print bands can gain from EMM. For effective use of EMM in diverse sectors of application, massive research endeavors and continual improvement are needed. To improve the applications of this method, developments in microtexturing setup, removal of micro-sparks generation, controlling of IEG, control of machining accuracy, design, and development of microtextured tool, and selection of electrolyte may be made. The design of the microtextured tool, insulation of the tool, supply of clean electrolyte, and avoiding micro-sparks at IEG during microtexturing are all key elements to consider for greater control over material removal. EMM opens a slew of previously untapped possibilities. Further study in EMM will throw up a slew of new challenges for using EMM effectively in the microscopic environment. Because of its advantages, such as productivity, quality, and eventually cost efficacy, EMM will be more promising and capable to perform a crucial function in microand nanotexture fabrication.

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13.8

FUTURE SCOPE

EMM is a cost-effective and environmentally friendly method of cutting tiny forms. Furthermore, the method provides increased better accuracy, burr-free machining and consistency, flexibility, and machining of etching of advanced materials such as tantalum, iridium, nickel alloy, and many more. Additionally, as EMM is not a thermal-oriented method, it has no negative thermal effects, such as the creation of a heat-affected zone (HAZ) in the microtextures, tool wear, distortion, crack formation, etc. This process is a flexible procedure that uses a variety of ways to generate required microtextures, including TMEMM, 3D EMM, and so on. Different types of précised microtextured tools for EMM can be manufactured according to product requirements and then used for microtexturing. In maskless EMM one textured tool can generate numerous microtextured samples efficiently with low time. However, this material removal procedure presents several opportunities that have yet to be explored and require additional research, inquiry, and analysis. As a result, researchers are concentrating on suitable electrolyte, suitable design and development of different microtextured tools, proper control of IEG, enhancement of tool movement, and eradication of micro-park created in IEG, and so on. Electrochemical for the manufacture of tiny features, the micromachining method requires a longer processing time. Furthermore, because of the fundamental nature of this micromachining method, it can only be used on electrically conductive materials. Other study areas that require special attention include improving microtexturing operation, generating 3D microtextures as well as nanotextures, improving operating parameter optimization, and obtaining optimal control and stability during microtexturing. KEYWORDS • • • • • • • •

electrochemistry EMM machining accuracy micro-ellipse micrographs microtexturing surface roughness textured depth

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CHAPTER 14

SURFACE MODIFICATION THROUGH MICRO-EDM PROCESS ASMA PERVEEN1 and SAMET AKAR2 Mechanical and Aerospace Engineering Department, School of Engineering and Digital Sciences, Nazarbayev University, Republic of Kazakhstan 1

Department of Mechanical Engineering, Çankaya University, Ankara, Turkey

2

ABSTRACT Electro-discharge machining, among other nonconventional machining processes, is known for its capability to machine materials regardless of hardness. In addition to its contactless and zero-force nature of machining, this process is reported to contribute to surface alteration and modification. Literature reveals surface modification through powder mixed EDM; modification through electrode materials, modification through electrical discharge (ED) coating process as some of the imperative surface modification techniques that can alter surface microstructure and therefore, enhances its properties such as microhardness, surface finish, wear, and corrosion resistance. This chapter will present a comprehensive discussion on these surface modification practices implemented to improve the functional characteristics of the surface machined by micro-EDM. 14.1 INTRODUCTION Enhancing materials properties such as strength, biocompatibility, resistance to corrosion, erosion, and abrasive wear is very imperative from the Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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product design and development perspective. Surface modification is known to enhance mechanical, physical, and biochemical properties that helps in withstanding the extreme and hostile environment such as high temperature and pressure situation [1–4]. Surface modification has impacted biomedical industries immensely, particularly in the dental and orthopedic application where enhanced biochemical and morphological compatibilities are crucial to promote osteointegration [5]. Plasma treatment can contribute to surface modification by enhancing surface adhesion and wettability, removing contaminants as well as reducing surface friction [6, 7]. Surface modification can be conducted by both with materials addition and with no material addition. Electro discharge machining [8], quenching [9], shot peening [10], sand blasting [11], laser surface modification and surface texturing are under the categories of surface modification without materials addition. On the other hand, electrical discharge (ED) coating [12], physical vapor deposition (PVD) [13], chemical vapor deposition (CVD) [14], electrodeposition [15], thermal spraying [16] are done by materials addition techniques. As reported in the literature, some of the surface modification techniques deployed widely in the industries involves electroplating, carburizing, as well as plasma spraying [17]. Electro discharge machining is considered as an electrothermal process that helps improve surface functionality by necessary modification. Over the years, EDM process has been developed and modified to serve different purposes. One of such important modification is known as electrical discharge coating (EDC), where electrical polarity is changed. EDC works similarly as EDM where electrode tool material is melted and accumulated on the workpiece surface using electrothermal energy and it is applicable for all kind of conductive materials regardless of hardness [18, 19]. In addition, powder mixed EDM is reported to improvise surface integrity, corrosion resistance and abrasion resistance due to the recast layer formed [20]. Size, concentration, and conductivity of the added powder does influence the machined surface [21, 22]. Many mechanical parts with low friction surface can benefit from powder mixed micro-EDM (PMEDM). In addition, electrode materials used during micro/EDM process has certain influence on the machined surface characteristics due to material migration occurrence from tool electrode to workpiece. Other than the electrode made of copper, steel, tungsten, graphite, copper, brass, copper tungsten, silver tungsten, graphite, recent research on powder metallurgy produced electrode for EDM also demonstrated its influence on surface modification [23–26]. Recent developments in the micro-EDM have led to renewed interest in its contribution on surface modification. In this chapter,

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surface modification using, powder mixed EDM, ECD, electrode materials will be discussed. 14.2

SURFACE MODIFICATION BY POWDER MIXED MICRO-EDM

The use of powder-mixed µ-EDM not only increases the material removal rate (MRR) of the process but can effectively improve the surface integrity characteristics of the machined surfaces by coating/alloying the surface. Strictly speaking, one of the main motivations behind using PMEDM was to enhance the material removal without sacrificing the surface quality (SQ) of the machined components [27]. Its effect on surface roughness largely depends on the change in the material removal mechanism. The existence of conductive particles in the gap reduces the dielectric’s insulative capacity and changes the discharge regime from single discharge to multiple discharges. Consequently, the energy will be distributed, which results in a shallow crater on the surface. In µ-EDM, many parameters affect the process performance which are narrated in the cause-and-effect illustration of Figure 14.1.

FIGURE 14.1

Cause-and-effect diagram for powder-mixed µ-EDM.

Among various parameters of the process, the concentration of the powder, capacitance, and voltage are shown to be the most important parameters affecting the process performance.

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EFFECT OF VOLTAGE

Various researchers investigate the effect of voltage on the performance characteristics of the powder-mixed µ-EDM process. Kuriachen & Mathew [28] have studied µ-EDM milling of Ti6Al4V using silicon carbide (SiC) mixed dielectric. The authors reported that SiC particles on the machined surface had altered the resolidified layer. Their SEM observations revealed fewer microcracks in the center of machined micro-channels instead of the end positions. It was mainly attributed to the time duration that the tool electrode spent at the end locations, which is higher than that of the center. In this situation, more powder and melted material from tool electrode are deposited on the surface. Electron dispersion spectroscopy (EDS) confirmed the presence of silicon on the machined surface. In a study conducted by Ni et al. [27] the open circuit voltage is shown to bear a considerable influence on the surface roughness of machined parts using a nanoparticle surfactant. The addition of 50% TiO2 reduces the surface roughness effectively. Figure 14.2 shows the effect of voltage on surface roughness and demonstrates the increasing value of surface roughness with increased value of voltage irrespective of the type of nanoparticles added to the dielectric.

FIGURE 14.2

Effect of voltage using different kind of nanoparticles.

Source: Reprinted from Ref. [27]; http://creativecommons.org/licenses/by/4.0.

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EFFECT OF POWDER CONCENTRATION

Machining of through-holes of 300 µm diameter on Ti6Al4V using deionized water dielectric mixed with different concentrations of copper, nickel, and cobalt powders has been investigated by Tiwary et al. [29]. They have considered three different concentrations of 2, 4, and 6 g/L. Their results demonstrated a significant rise in term of MRR when adding metal powders to the dielectric. They achieved the best MRR when using copper at a concentration of 4 g/L. Nevertheless, the tool wear rate is shown to be increased due to secondary discharges. When the process’s overall performance has been considered, the cobalt powder of 4 g/L is shown to be the optimal setting. In this case, the white layer thickness is found to be reduced. In the PMEDM process of H11 die steel using aluminum oxide powder, Tripathy & Tripathy [30] achieved a significant improvement in the SQ of the machined surfaces by increasing the concentration of the particles. They also showed that the recast layer also reduces substantially with increasing powder concentration. 14.2.3

EFFECT OF POWDER TYPE

Various powders have been attempted in the PMEDM process, and their effect on different process performance outputs are investigated. The use of aluminum and silicon powders has been investigated by Klocke et al. [31] during powder mixed micro-EDM of Inconel 718 superalloy. When aluminum is added to the dielectric, the plasma channel expands. As a result, the discharge energy is distributed over a larger area resulting in a thinner rim zone on the machined surface. The usage of silicon powder results in a gray zone under the white layer. The existence of the gray zone is attributed to the high heat of the fusion of silicon particles. The use of boron carbide as a powder for PMEDM has been reported by Kibria et al. [32] during µ-EDM hole drilling on Ti6Al4V. Their results revealed an increase in the MRR. SEM micrographs exhibited improved SQ when employing B4C mixed deionized water. Furthermore, the white layer thickness is reported to be reduced. Presence of substantial amount of tungsten and boron element on the machined surface are confirmed by EDS analyzes where tungsten infusion is due to the migration from tool electrode and boron infusion is due to the migration from dielectric mixed with carbide powder. Similar results have also been reported when using dielectric mixed with maghemite (γ-Fe2O3) nano-powder during machining of CoCrMo alloy [33].

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14.2.4 ULTRASONIC VIBRATION-AIDED POWDER MIXED MICRO-EDM Chen et al. [34] explored ultrasonic vibration aided powder mixed micro/ EDM process on Al–Zn–Mg alloy and reported enhanced machining performance along with wear resistance and hardness properties. Prihandana et al. [35] explained the reason behind improved surface performance which is basically due to the avoidance of powder deposition which otherwise tends to settle down on the bottom of the dielectric tank. As a result, debris particles are given enhanced kinetic velocity and experimental results shows increased MRR as well as enhanced surface integrity when ultrasonic vibration is applied to MoS2 powder mixed EDM process. 14.2.5 MATERIALS DEPOSITION PHENOMENA Migration of materials either from the tool electrode or from the particles suspended in the dielectric profoundly influences the SQ characteristics of the PMEDMed surfaces. Upon successful deposition/migration of the mixed powder particles to the machined surface, wear, corrosion, and other machined surfaces can be engineered by introducing suitable alloying elements to the surface. Although there are many investigations related to the phenomenon of material deposition from the tool electrode to the workpiece, few researchers report this material migration phenomena from the suspended powder associated with the PMEDM process. The microstructure evaluations generally involve elemental analysis using energy dispersive spectroscopy (EDS) and microhardness measurements. In a study of Bhattacharya et al. [36] a 50% increase in the microhardness has been reported which is due to the carbide formation on the machined surface. The authors have worked on three different alloys of H11, AISI 1045 and HCHCr. Three different powders are silicon, graphite, and tungsten, with a maximum concentration of 10 g/L. Their EDS results showed material migration from the powder mixed dielectric to the workpiece machined surface; eventually a thicker white layer has also been reported. For the case of graphite powder, iron carbide has been observed due to the migration of carbon from graphite, increasing the microhardness of the surface. When silicon is used as the powder, a thinner white layer is formed, and the waviness of the surface has been reduced substantially. The migration of boron from boron carbide powder used in powder mixed µ-EDM hole drilling of Ti6Al4V is reported by Kibria et al. [32].

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14.3 SURFACE MODIFICATION BY MICRO-ELECTRICAL DISCHARGE COATING Electrical discharge coating is an extension of die sinking EDM process where the electrode is linked to the positive terminal and workpiece is linked with the negative terminal. This type of polarity allows more material removal from the tool. Micro-EDC is considered as material additive process contrary to micro/EDM which is considered as subtractive method. Therefore, Micro/EDC can deposit high performance coating materials that is useful for surface enhancement using high current electrical spark in the presence of dielectric [37, 38]. Micro/EDC is also known as electrical discharge (ED) alloying due to its involvement in alloying substrate surface [39]. Micro/EDC follows similar sparking mechanism as micro/EDM to coat surface substrate [40] which alters surface characteristics, hardness as well as resistance to corrosion and wear. Process schematic is illustrated using Figure 14.3. Several factors influencing this surface modification includes machining parameters (current, pulse on time), electrode materials, electrode fabrication, powder suspension in the dielectric [19, 41, 42]. Reverse (positive) polarity aids in surface modification during micro/EDC where electrode connected with anodic terminal experiences faster melting than cathode. Bai & Koo [4] suggested effectiveness of positive polarity in surface modification. Although energy density in negative terminal is higher than positive terminal, however higher discharge spot of the positive terminal can generate wider melting zone on the anode contrary to the generation of deep molten zone in the cathode due to higher current. That is how decomposed and melted materials get deposited on the surface. Krishna & Patowari [44] corroborate this observation by revealing the generation of W and Cu rich layer using reverse polarity. Similar observation was reported by other researchers as well [45–47]. Thermal decomposition of both electrodes, dielectric, and subsequent quenching of dielectric medium during Micro/ EDC result in deposition of material which introduces enhanced properties on worksurface [43]. Jahan et al. reported marginal changes in the mechanical properties and surface composition of micro-EDMed Ti alloy surface due to small discharge energy which causes the generation of a thinner recast layer compared to traditional EDM [48]. In another study, Jahan et al. reported on the micro/EDM induced porous and thick titanium oxide (TiO2) layer which can be exploited for promoting bony ingrowth [49]. Material migration from electrode, dielectric, and atmosphere can alter the surface topography, crater size as well as microhardness.

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FIGURE 14.3 Schematic of EDC process (a) spark generation at the closest point; (b) discharge gap increase in powder-mixed dielectric; and (c) material melting, and deposition of molten material. Source: Reprinted with permission from Ref. [43]; ©Springer.

14.3.1

SURFACE MODIFICATION BY DIELECTRIC FLUID

Dielectric influences the EDC process significantly as it helps transferring and depositing suspended particles or molten materials to the workpiece surface [50]. Kerosene is one of the widely used dielectric which generates carbon element from decomposition at high temperature. If this carbon atom combines with Ti particles originated from electrode, it can create coating with increased surface hardness and reduced microcrack as well as surface roughness [51]. Ti Powder mixed EDC also enhances surface hardness of WC-Co and reduces generation of microcracks as well as voids [37]. Research also reported about deionized water as dielectric fluid which can enhance biocompatibility and corrosion resistance of the implant surfaces. In another study, Chen et al. [52] suggested enhanced hydrophilicity during Ti powder mixed deionized water EDC. Bai & Khoo [4] observed higher

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coating hardness on superalloy Haynes 230 generated with distilled water compared to kerosene oil due to outstanding solidification rate. However, coating generated with kerosine possess finer morphology. Kiran et al. investigated surface medication of Ti alloy using different dielectrics such as, EDM oil, sunflower oil, powder mixed EDM oil, powder mixed sunflower oil and reported different value of recast layer thickness. Both Sunflower oil and powder mixed (MoS2) sunflower oil provides larger recast layer thickness compared to EDM oil owing to the higher thermal conductivity. In addition, higher viscosity of sunflower oil aids in retention of deposited materials in the discharge gap during the cooling process of workpiece materials. They also observed increasing trends of recast layer thickness with the increase of duty factor and voltage due to presence of more molten materials because of more discharge energy availability. Microhardness value for powder mixed dielectric seems to be higher than just dielectric which is due to the formation of hard and self-lubricant Ti-MoS2 layer, and its value increases along with the increasing voltage. Sunflower oil results in finer surface roughness compared to EDM Oil. In addition, low surface roughness is observed for powder mixed dielectric since retention of powder particles in the viscous fluid can cause better heat energy distribution on the substrate surface. Moreover, increasing voltage as well as duty factor, both can increase the surface roughness [53]. 14.3.2 SURFACE MODIFICATION BY ELECTRODE MATERIALS Diesinker EDM with powder metallurgically produced electrode can be used to perform EDC which is useful for depositing coating materials on substrates. During the EDC, heat energy of plasma channel created between workpiece and tool due to spark melts and vaporizes some part of workpiece and tool materials. Then melted material is solidified with the help of dielectric and eventually gets deposited on the workpiece surface. In the process, material of the tool electrode is transported to the negative workpiece [54–57]. Mohanty et al. conducted micro/EDM of Ti alloy using two different micro-tools made of brass and tungsten where dielectric used was deionized water mixed with W2S. Their study reported significant improvement of the microhardness value after the micro-EDM process for both tool electrode. In case of brass tool, microhardness decreased with increasing value of powder concentration at constant duty factor. Other than soft lubricating W2S powder that reduces the microhardness of the surface, self-lubricating materials brass also introduces similar properties to the surface by generating intermetallic

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compound. Tungsten tool reveals opposite result for increasing of powder concentration in the same range. The reason behind that is the generation of hard tungsten carbide phase owing to the reaction of carbide from dielectric with the tungsten from tool and powder. In addition, this microhardness increases with the increasing duty factor which facilitates more hard tungsten carbide material deposition due to longer pulse on time. It is also reported that surface roughness for tungsten tool is more than brass tool. The reason behind this is the higher thermal conductivity as well as melting point of Tungsten which results in bigger crater size due to more heat transfer to the workpiece. Moreover, higher recast layer thickness is also observed for tungsten tool compared to brass tool for the same reason. Compositional analysis confirms the formation of tungsten carbide and titanium carbide for both electrode, thus validating the theory of material migrations [58]. 14.4

RESEARCH CHALLENGES

Considering the effectiveness of the coating technology and its inexpensive nature, surface modification using micro/EDM can provide the upper hand over the existing coating techniques and, therefore can be eligible to be applied in the field of automotive, aerospace as well as maritime industries in addition to biomedical industries. Applications can also be found in engine pistons, Jet turbines, and marine propellers. Research about antibacterial surface generation with the help of powder mixed micro/EDM has been striking huge interest lately. It is reported that nano-silver particles powder mixed EDM process generate silver coating which aids in reduced Aureus bacterial clusters [59]. Antibacterial coating appears to have great application in the biomedical applications for bone implantation. Cell attachment and proliferation of implant surface can be greatly improved using EDC. Since micro/EDM comes with pollution issue due to the usage of dielectric fluid and production of by-product, further research needs to direct towards the environmental aspects of micro/EDM process if it is intended to use for surface modification purpose. Therefore, investigation related to bio/dielectric can be of another area of interest to deal with health and environmental issue. Another research area that would be worth exploring will of nano-power supply, nano-tools, nano-EDM technology. Significant research efforts must be channeled for optimizing the process parameters so that defect-free coating can be produced in an eco-friendly fashion. It is also imperative to develop a process control algorithm to generate surface topography and roughness in a

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controlled manner [60]. Mathematical modeling and simulation of micro- and nano-EDM for geometric surface prediction should be another research area of interest [61]. KEYWORDS • • • • • • • •

carbide phase coating dielectric fuid electrode powder recast layer surface modifcation ultrasonic vibration

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42. Chaudhury, P., Samantaray, S., & Sahu, S., (2017). Multi response optimization of powder additive mixed electrical discharge machining by Taguchi analysis. Materials Today: Proceedings, 4, 2231–2241. 43. Mohanty, S., Bhushan, B., Das, A. K., & Dixit, A. R., (2019). Production of hard and lubricating surfaces on miniature components through micro-EDM process. The International Journal of Advanced Manufacturing Technology, 105, 1983–2000. 44. Eswara, K. M., & Patowari, P., (2013). Parametric optimization of electric discharge coating process with powder metallurgy tools using Taguchi analysis. Surface Engineering, 29, 703–711. 45. Vijayakumar, S., Mohan, N., Dineshbabu, C., & Karthikeyan, G., (2016). Reduce the mass losses for coated Al 7075 by using powder mixed electric discharge coating. Int. J. Mod. Trends Eng. Sci., 3, 184–186. 46. Gill, A. S., & Kumar, S., (2016). Surface roughness and microhardness evaluation for EDM with Cu–Mn powder metallurgy tool. Materials and Manufacturing Processes, 31, 514–521. 47. Das, A., & Misra, J. P., (2012). Experimental investigation on surface modification of aluminum by electric discharge coating process using TiC/Cu green compact toolelectrode. Machining Science and Technology, 16, 601–623. 48. Jahan, M. P., & Alavi, F., (2019). A study on the surface composition and migration of materials and their effect on surface microhardness during micro-EDM of Ti-6Al-4V. Journal of Materials Engineering and Performance, 28, 3517–3530. 49. Jahan, M., Alavi, F., Kirwin, R., & Mahbub, R., (2018). Micro-EDM induced surface modification of titanium alloy for biocompatibility. International Journal of Machining and Machinability of Materials, 20, 274–298. 50. Prakash, C., Kansal, H., Pabla, B., & Puri, S., (2017). Experimental investigations in powder mixed electric discharge machining of Ti–35Nb–7Ta–5Zrβ-titanium alloy. Materials and Manufacturing Processes, 32, 274–285. 51. Hwang, Y. L., Kuo, C. L., & Hwang, S. F., (2010). The coating of TiC layer on the surface of nickel by electric discharge coating (EDC) with a multi-layer electrode. Journal of Materials Processing Technology, 210, 642–652. 52. Chen, S. L., Lin, M. H., Huang, G. X., & Wang, C. C., (2014). Research of the recast layer on implant surface modified by micro-current electrical discharge machining using deionized water mixed with titanium powder as dielectric solvent. Applied Surface Science, 311, 47–53. 53. Kiran, P., Mohanty, S., & Das, A. K., (2021). Surface modification through sustainable micro-EDM process using powder mixed bio-dielectrics. Materials and Manufacturing Processes, 1–12. 54. Mussada, E., & Patowari, P., (2015). Characterization of layer deposited by electric discharge coating process. Surface Engineering, 31, 796–802. 55. Algodi, S., Murray, J., Clare, A., & Brown, P., (2015). Characterization of TiC layers deposited using an electrical discharge coating process. In: Journal of Physics: Conference Series (p. 012008). IOP Publishing. 56. Mohri, N., Saito, N., Tsunekawa, Y., & Kinoshita, N., (1993). Metal surface modification by electrical discharge machining with composite electrode. CIRP Annals, 42, 219–222. 57. Singh, P., Kumar, A., Beri, N., & Kumar, V., (2010). Some experimental investigation on aluminum powder mixed EDM on machining performance of Hastelloy steel. International Journal of Advanced Engineering Technology, 1, 28–45.

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CHAPTER 15

A REVIEW ON MICROMACHINING OF TI-6AL-4V USING MICRO-EDM PRIYANSHU GHOSH, DISHA MONDAL, DEBOLINA DUTTA, and MANISH MUKHOPADHYAY Mechanical Engineering Department, Meghnad Saha Institute of Technology, Kolkata, West Bengal, India

ABSTRACT Micro-machining is a mechanical machining process, which involves the use of several micro-tools with defined cutting edges or sometimes involves the use of laser technology, micro-EDM, etc., and having devices or features with a micrometer range (1–999 µm) of dimension in the workpiece or tool. Selecting the appropriate machining technique is the key to achieving the fineness of the machined product. EDM is a popular method for micromachining because it can machine electrically conductive materials with various hardness, strength, and temperature resistant as well as complex shapes with accurate dimensions and fine surface roughness. Moreover, this method is widely used to make micro-scale products such as micro-actuators and micro-fabricated electronic sensors. We present a comprehensive and state-of-the-art review of various new research findings in the following paper as well as the development, advancement of Micro-machining processes using EDM in Titanium Alloy starting from the various kinds of micro-electrode, work-piece materials, and dielectrics (flushing included) that have been used by previous researchers. Although electrical discharge machining (EDM) is suitable for machining titanium alloys, it is difficult to find higher machining parameters for high rates and accuracy in machining. Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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15.1 INTRODUCTION Titanium and its alloys are abundant in the earth’s crust, especially magnesium alloys, iron alloys, and aluminum alloys. These are utilized in various industries, including automotive, aerospace, chemical, medical, and electronics, to make components and equipment. Ti-6Al-4V titanium has a high specific strength and strong corrosion resistance. It is also known as TC4, Ti64, or ASTM Grade 5 titanium [1]. It’s commonly employed in both industrial and commercial settings. It’s one of the most widely used titanium alloys, and it’s employed in a variety of applications that demand low density and good corrosion resistance, such as aerospace and biomechanical applications (implants and prostheses). Titanium alloys are becoming more popular as biomaterials because they have a lower modulus, are more biocompatible, and have greater corrosion resistance than stainless steel and cobalt-based alloys. Owing to these properties, commercially pure titanium grade 1 (CPTi) and Ti-6Al-4V, also other innovative titanium alloy compositions including orthopedic metastable B titanium alloys, were developed as early as the 1920s. They’re also biocompatible, have a low elastic modulus, and can withstand notch and strain-controlled fatigue making these electric machining a sought-after choice for cutting these difficult to machine materials [2–5]. One such method of cutting electrically conductive mat erials with precision inside a dielectric fluid, between an electrode and a workpiece, named as electric or electro discharge machining (EDM). The electrode might be regarded as the cutting tool in this case. On die-sinking EDMs, electrodes must be manufactured in the precise alternate facing and fitting profile as the workpiece [6–8]. The electrode in wire cut EDMs is constant fed wire. From the electrode wire to the workpiece, at the point of least distance, a spark is formed. When material removal takes place using EDM, the electrode circumvents physical contact with the workpiece. As the electrodes steer clear of contact with the workpiece, there is no tool force associated with EDM. Spark gaps, also known as electrode separations, must always be maintained between the electrode and the WP. The sparking stops when the electrode comes into contact with the WP, and no material is removed from the electrode. Some EDM machines let the electrode make touch with the WP. They are usually used to remove broken taps and drills. Another important aspect of the process states there is single spark at any given time. Generation of spart occurs at a frequency of 2,000 to 5,00,000 per second, giving the impression that numerous sparks are occurring at the same time. In normal EDM, as sparking occurs, the sparks migrate from one location on the electrode to another [9].

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Classification of this electric machining can be done based on variation of dielectrics for dedicated operations to be performed. Die sink EDM requires the use of hydrocarbon oil, while wire-EDM and micro-EDM, as well as deep hole drilling, demand the use of deionized water [10]. Researchers vouched for the use of water-based dielectric for micro-EDM process in place of hydrocarbon oils such as kerosene, which harms health and the environment. Dielectrics are responsible for squeezing the produced bubbles at discharge zone, cooling the debris, and flushing it out of the interelectrode gap [9]. Primarily material is evacuated by the end of the sparking phenomenon because the bubble pressure prevents the bubble from expanding due to the inertia and viscosity of the surrounding dielectric liquid during the discharge. However, recent research showed that material removal occurs during the discharge process instead of at the end of discharge Electrodes or tool is the most important part of the EDM setup. In EDM, wear rate of electrodes varies with the material type and property of the tool [10]. Lei et al. [11] used a combination of Cu, Si, and Cu-Si alloy disc foils to produce a laminated disc electrode to take advantage of these features, which they subsequently employed in EDM to process microgrooves on titanium alloy workpieces [12] used tungsten, copper tungsten and silver tungsten as tool material. Among these the latter as the choice of tool material performed most satisfactorily under die sinking micro-EDM setup. 15.2

PRINCIPLE OF MICRO-EDM

The working principle of EDM is based on the use of electric spark discharge from electrode to remove workpiece material (electrically conductive) under the presence of a dielectric fluid with pulsed voltage. The spawned thermoelectric energy in form of sparks/arc between work and tool end, melts the workpiece, and thus, material removal takes place. Due to the high intensity thermoelectric emission the workpiece gets evaporated and the formed debris after are removed by the flowing dielectric [11–13]. WP material is vaporized and melted with the thermal energy from each discharge. The discharge channel collapses when the electric power is turned off, and the materials are washed away. Upon repetition and uninterrupted forwarding of the electrode, the tool geometry is transferred [14]. Accelerated electrons in the spark gap zone, interacts with the dielectric fluid molecules causing the creation of an electron avalanche [13].

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The spark’s energy can be controlled by differing process parameters such as frequency, duty cycle, current, voltage, etc. In µ-EDM, the pulsed voltage at discharge energies ranging from a few microjoules produces continuous material removal. It is possible to fabricate micro-components and even MEMS devices using µ-EDM [15]. Since the dielectric fluid always contains microscopic contaminants (debris) wi th a diameter similar to the gap size and the surface irregularity of the WP, as well as electron emission, these contaminants affect the formation and concentration of electric fields in the spark gap zone, leading to the formation of a high conductive bridge of plasma [16]. 15.2.1

EXPERIMENTAL SETUP AND PROCESSES OF µ-EDM

The basic components of a μ-EDM setup are Electrode/Tool Material, pulse generator, dielectric, and recycling pump, workpiece, and finally the feeding mechanism. The details of the setup and their functions and processes are elaborated below and is shown in Figure 15.1:

FIGURE 15.1

Micro-EDM setup.

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1. Electrodes/Tool Materials: Tool selection is crucial because the tool determines the accuracy and shape of the microfeature produced. The primary tool materials in micro-EDM are tungsten electrodes and tungsten carbide electrodes. Cost-effective materials such as brass, copper, graphite, and stainless steel have also been attempted as tool electrodes to complement this commonly used pricey material. Unlike traditional chip-making machining operations, the electrode does not make direct contact with the WP for material removal. Because the electrode does not make contact with the WP, EDM produces no tool force. The sparking gap, or the distance required for sparking, must always be present between the electrode and the WP. The sparking stops when the electrode makes contact with the WP, and no material is removed [17, 18]. 2. Pulse Generators and Sparking: In µ-EDM, RC type, and transistor type iso-pulse generators are the most common pulse generators. An RC generator has a capacitor that stores energy, which is then released during machining. Rather than using a capacitor to generate rectangular pulses, transistor-type pulse generators use transistors to switch the pulses between 0 V and 60 V. The high discharge frequency and low discharge energy of RC-type generators make them superior to transistor-type generators. In most cases, the discharge energy has been supplied by a transistor-type generator, which is suitable for traditional EDM. RC-type circuits, according to Jahan, may be more suitable for creating microstructures in tungsten carbide, where precision and surface polish are critical. EDM is a thermal process in which heat is used to remove the material. A spark of electricity generates heat between the electrode and the workpiece. Whenever a spark originates and concludes between the electrode and the workpiece, the material is heated until it vaporizes at the closest points between the electrode and the workpiece [17, 18]. 3. Dielectric Fluids: During EDM, the electrode and workpiece should never be hot to the touch, but the area where each spark happens is extremely hot. Because each spark only heats a limited area, the vaporized material, as well as the electrode and workpiece surfaces, are swiftly cooled by the dielectric fluid. However, metallurgical changes may occur as a result of the spark heating the workpiece surface. To keep the sparking gap between the electrode and the workpiece open, a dielectric substance is necessary. Normally, this dielectric substance is fluid. Hydrocarbon oil is typically used

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in diesinker EDM machines, while deionized water is typically used in wire cut EDM machines. Dielectric fluid is an electrical insulator until enough electrical energy is given for it to convert into an electrical conductor. Except at the closest places between the electrode and the workpiece, the dielectric fluids utilized for EDM machining can remain electrical insulators. Deionized water and kerosene are the most often utilized dielectric fluids. Total EDM 3 oil, hydro carbide dielectric liquids, and other dielectric fluids are used in the EDM process. During EDM operations, researchers mixed micro-molybdenum disulfide (–MoS2) powder with dielectric fluid and used ultrasonic vibration to increase surface quality (SQ) and MRR. At these locations, sparking voltage causes the dielectric fluid to change from an insulator to a conductor, resulting in a spark. The ionization point is the point at which a fluid transforms into an electrical conductor. The dielectric fluid deionizes when the spark has been extinguished, and it becomes an electrical insulator again. When a spark occurs, the dielectric fluid changes from being an insulator to a conductor, and back again. The dielectric fluid used in EDM machines provides important functions in the EDM process. These are: • Spark gap control; • Cooling of debris formed; • Effective elimination of debris. Sparks cause a modest quantity of electrode and workpiece material to vaporize. Essentially, a cloud of evaporated material forms in the sparking gap. This cloud solidifies when the spark is stopped. After each spark, an EDM chip or a very small hollow sphere is formed from the electrode and the workpiece material. These images illustrate the cloud of vapor that produces sparks, the cloud in suspension, and the vaporized cloud that forms into the EDM chip [17, 18]. 4. Flushing: The removal of particles from the work-tool gap is crucial to successful EDM work. Good flushing is crucial for good machining and surface finishes. Of all the functions performed by the cutting fluid, the removal of particles, or swarf, is the most important. If the swart is not properly flushed from the gap, the gap becomes overly conductive Efficiency drops off sharply, surface finish is poor, and shorting conditions occur more frequently. The flushing operation is as follows: clean dielectric is forced, either around the area where the electrode enters the

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workpiece (known as jet flushing) or through a hole drilled in the electrode (known as pressure flushing) that exits into the area gap; the dielectric is then removed from the work tank area and filtered. The simplest and least effective (though most often used by homebuilders in particular) flushing method is jet flushing. If a shallow through, or blind hole, is burned in the workpiece, jet flushing may be used. It is flushed by forcing dielectric from a nozzle into the area around the electrode workpiece interface, see drawing below. It’s advantageous to extract the electrode occasionally to permit clearing of dirty dielectric fluid, this improves stability, speed, and finish [19, 20]. 5. Tool Feed Mechanisms: During intermittent material removal, the tool-work distance should be kept constant. The feed mechanism of tool detects voltage variations in the interelectrode gap and raises or lowers the tool to sustain a coherent interelectrode spacing. The majority of the systems use a servo feed control method. While some studies used microactuators based on technology to impart tool movements. Typical servo-controlled tool feed systems have been outperformed. Makenzi et al. [21] introduced a novel servo driven tool feeding arrangement that monitors the mean gap voltage and uses it as a feedback signal to tweak the feed in order to maintain the spark gap. They improved their machining method by creating an electrode-servo system that kept the electrode-to-workpiece sparking gap constant during the EDM machining cycle. To maintain a constant distance between the electrodes, An EDM system with a piezo-actuated tool feed mechanism was developed by Raju & Hiremath [22]. 6. Workpiece Material: µ-EDM can be used to machine a variety of metals and alloys. In the literature, Cu-Ti alloy Ti-6Al-4V, SK3 carbon tool steel, SS304, high nickel alloys, tungsten carbide, and ceramics are among the various materials machined with µ-EDM [19]. 15.2.2 HISTORY OF EDM DEVELOPMENTAL PROCESS Today’s technique of machining, electrical discharge machining offers precision even with difficult materials. By supplying EDM with a massive function in facilitating the improvement of superior applied sciences in creating industries such as aerospace, many before not possible matters are now rather easy. All technical fields made developments in the 20th century,

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which led to the improvement of EDM. EDM, however, used to be anticipated almost two centuries in the past by using Joseph Priestly, a philosopher, and scientist [23]. Joseph Priestley’s significant accomplishments replicate his dedication to philosophy, the Church, and scientific inquiry for the duration of his lifetime. His contributions to science encompass the invention of soda water and the founding of Unitarianism. The bodily essence of electrical discharge machining used to be observed by way of him. Until the 20th century, EDM ought to no longer be developed due to the lack of expertise about its relevant importance [23]. As a result of the development of numerous important technologies during the 20th century, an era of unprecedented technological advancements emerged. As the Second World War came to an end, the rise of the Soviet Union and the West spurred each side to invest heavily in research and development. The Soviet Union and the US developed EDM independently during this period. In 1943, Lazarenkos discovered that we could control erosion by immersing electrodes in a dielectric fluid in an attempt to reduce the erosion of tungsten electrodes. From this grew one of the first EDM machines. As a result of Jack Beaver, Harold Stark, and Victor Harding’s invention, the EDM machine was invented in the United States to remove broken tools from cast aluminum [16, 24]. Since then, EDM has primarily focused on refining existing techniques and exploring new applications. EDM is used the most in the aerospace industry today, with small holes being drilled in almost every gas turbine engine and liquid fuel rocket nozzle. In every industry that requires high precision and materials that have a high resistance to wear, EDM is used. The economies of these industries are among the most dynamic in the world, with a profound impact on daily life. The future is bright for EDM [24, 25]. As per their use, micro-EDM can be categorized as: i.

Micro-EDM Drilling: Micro-holes of various dimensions can be formed on the workpiece through both blinds and through holes. ii. Micro-EDM Milling (Trajectory EDM): Micro-electrodes are used in this process to be produced 3D microfeatures by adopting a path-strategy. iii. Die-Sinking Micro-EDM: As part of the process, microelectrodes of different sizes are used to create 3D microfeatures and microcavities. iv. Micro-WEDM: The process employs the use of a fine wire (10 µm dia.) to cut material.

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345

FACTORS IN EDM

Below mentioned are the principal factors affecting EDM process: 1. Discharge Voltage (V): During machining, it is the mean operating voltage in spark gap. Size and overcut are directly influenced by voltage. Low voltage is preferred for materials with elevated electrical conductivity. If open-circuit voltage is increased then spark gap also increases improving flushing conditions [26, 27]. 2. Peak Current (Ip): It is the value of the most amperage accessible for individual pulse from the power-supply generator. For duration of pulse on, the current increases gradually and attains a maximum value. When the peak current is higher, more material will be removed, but the surface finish and tool life will be decreased [26, 28]. 3. Pulse on Time (Ton): It is the duration of discharge. During Ton, energy generated determines the rate of material removal (MRR) [27]. 4. Pulse Off Time (Toff): Discharges are not applied during this period. In this, the debris are flushed away from the interaction zone. Selecting the pulse off time correctly will ensure stable machining [27]. 5. Polarity (P): A charge may be positive or negative. An oppositepolarity charge is applied to one of the two electrodes and the workpiece. The electrode is normally charged positively [28, 29]. 6. Discharge Gap (G): It is the work-tool gap. In micro-EDM, G is usually between 0.1 and 0.1 mm [28]. 7. Flushing: The flow speed of the fluid towering the machining area is one of the non-electrical parameters. There are various types of dielectric fill-drain systems in EDM machines. Die-sinker machines normally submerge the workpiece, so dielectric fluid must be drained away from the work area during setup and inspection. By flushing dielectric fluid into the machining area, temperatures are reduced and filter out the debris. That is why proper flushing is so vital [29]. 15.3.1 PERFORMANCE MEASURE PARAMETERS [29] EDM process performance is affected by several factors, including MRR, EWR, and exterior quality. MRR is identified as the material removal volume per unit of time. MRR techniques and methods are continually improved by manufacturers. In comparison with other non-conventional machining

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processes, EDM has a low MRR. Therefore, it is imperative to enhance the MRR of the process. Similar to MRR, the EWR (erosion rate of the tool electrode) can be calculated based on the amount of material removed from the electrode per unit time. Since electrode wear affects the electrode profile and the accuracy of the electrode, previous studies focused on reducing EWR. The SQ of a machined surface is a measure of its condition. SQ can be assessed by measuring surface roughness, heat-affected zones, recast layers, and microcrack density. In electrical-discharge machining, the material removal rate (MRR) depends on the current and melting point of the workpiece material, but other factors, such as temperature and frequency, also play a role. EDM typically removes metal at a rate of 2 to 400 mm3/min. As a result of high rates, the surface is not smooth, and the presence of recast layer may be observed. Hence, finishing cuts are usually performed at lower rate of material removal, and the recast layer is removed later via finishing operations. In a more recent technique, oscillating electrodes are used which produce a highly polished surface, requiring significantly after EDM processing. 15.4 ABOUT TITANIUM AND ITS ALLOYS (TI-6AL-4V) Titanium is a less dense metal that can be considerably enhanced through proper alloying. Titanium and its alloys have a unique mix of qualities, including high strength, excellent toughness, biocompatibility, low density, and enhanced resistance to corrosion at temperatures ranging from low to high. Weight reduction in aeronautical constructions and other high-utility applications is possible thanks to their exceptional qualities [30–32]. Titanium and its alloys offer enticing high strength-to-density properties as well as excellent corrosion resistance due to the protective oxide covering. They are extensively used in several fields such as biomedical and aerospace industries, petrochemical production, and many other areas. Combined with β-stabilizers, titanium alloys containing α-stabilizers are known as α-β alloys [33–37]. Figure 15.2 shows some titanium ingots. 15.5

ROLE OF DIELECTRIC

The dielectric serves the purpose of squeezing the formed bubble at the discharge zone, cooling the debris, and flushing it from the spark gap zone. It

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was previously thought that the bubble pressure generated during discharge would restrict bubbling by the inertia and viscosity of the surrounding dielectric liquid and the removal of materials occur at the end process. However, Hayakawa et al. [37, 38] validated that the removal takes place while the discharge process and not at the end process. The researchers of Yoshida & Kunieda [39] found no difference between the way debris formed and was distributed in liquids and air in the presence of pulsed on-times that exceeded 90 microseconds. In such case, the arc columns in liquid and air are comparable, as the diameter of the bubble increases. Therefore, it can be said that the dielectric is not responsible for material removal, but it serves the purpose of cooling and flushing debris [32].

FIGURE 15.2

Titanium ingots.

15.5.1 MICRO-EDM PERFORMANCE USING DIFFERENT DIELECTRICS According to Prakash et al. [40], hydrocarbon liquid dielectrics such as kerosene are much better as compared to deionized/distilled in die sink EDM. During the finishing and roughing process mixing deionized water with organic compounds generates more satisfactory result. Similar or sometimes

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better results are achievable using water-based dielectrics, rather than hydrocarbons. Another evolution in micro-EDM is the use of dielectrics mixed with powdered compounds, using this method in place of pure dielectric HC oils and deionized water produces higher MRR and smoother surfaces. Also, using less viscous dielectrics gives better performance of micro-EDM. Due to high viscosity in hydrocarbon oils, the total machining time is not affected as much as effected by less viscous dielectrics. Further studies have shown that high MRR can be achieved by using gas assisted micro-EDM in specific machining conditions. 15.5.2 COMPARATIVE STUDY OF VARIOUS METAL POWDER MIXED DIELECTRIC According to a study reported by Tiwary et al. [41], MRR, TWR, OC, and taper were the micro-EDM performance measures used in the experimental study. Due to the uncertainties associated with micro-EDM each experimental run was replicated twice and the result of each experimental run was evaluated based on the average machining response. By difference in weight method, MRR and TWR are estimated, i.e., the weight difference of each of the electrodes before and after machining per unit machine time, correspondingly [30]. Thus, MRR = (Wb −Wa ) / T

(1)

TWR = (Tb − Ta ) / T

(2)

where; Wb is the weight of the workpiece prior to machining; Wa is the weight of the workpiece after completion of machining; Tb is the tool before machining; Ta is the tool after machining; and T is the machining time. The measurement of the weight of the workpieces and micro-tools was done with precision. Diametrical overcut was measured as the remaining of the initial diameters of hole and tool. 15.6 ROLE OF TOOL ELECTRODE ON EDM PERFORMANCE OF TI-6AL-4V Some common electrode materials of EDM include, graphite, brass, copper, copper-tungsten alloy, etc. Various techniques are employed to shape the

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tools used for EDMing, viz, forming, casting, powder metallurgy, etc. Tools having diameter in range of few microns may be used for the process. Tool wear is an extremely significant factor because it adversely affects dimensional accuracy and the shape produced. In no-wear EDM, it is possible to reduce tool electrode wear (TEW) by reversing the wire polarity and by using copper tools [42]. According to the report published by Chen et al. [42] they studied the impact of electrode variation on the machining characteristics of the Ti-6Al-4V alloy. In this experiment electrodes used are brass electrode, Cu-W electrode, and W electrode. 15.7 EXPERIMENTAL STUDIES AND OPTIMIZATION TECHNIQUES μ-EDM is an electro-thermal process in which electrical power is used to create a thermal effect that allows the material to be removed. As a result, the amount of electrical power input is critical. Most of the publications have focused on fine-tuning these process parameters to attain optimal performance measures such as MRR and TWR. To investigate the effects of process factors on machining, they are divided into electric and non-electric categories. Voltage, frequency, Pon, Poff, discharge energy, duty cycle, and other electrical constraints are among them [25]. Non-electrical parameters include, among other things, dielectric fluid type, flushing characteristics, properties of induced vibrations, and the insertion of some micro- and nano-sized particles to increase the rate of machining. MRR and TWR are two performance indicators that help determine how efficient the μ-EDM process is (TWR). To assess the quality of machined characteristics, such as holes and channels, SQ, dimensional compactness, taper, overcut (OC), and other performance measurements are used. To analyze and optimize, various evaluatory studies such as gray relational analysis, Taguchi technique, multi-objective optimization, ANOVA, and genetic algorithm (GA) are used [7]. 15.8 APPLICATION AREAS A wide range of applications for µ-EDM are found in nozzles of inkjet printers, turbine blades cooling holes, microchannels in microfluidic analysis, micromolds, honey-comb fabrications, etc. Inkjet nozzles were machined by

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Allen et al. [43] using the µ-EDM process. Diesel injector nozzle spray holes were drilled on diesel injector nozzles employing µ-EDM process by Tong et al. [44] Using the µ-EDM process, Shabgard et al. [45] manufactured WC and polycrystalline diamond (PCD) ball end mills. According to Sahu & Mandal [46], nanoparticles with a mean diameter of less than 10 nm were developed with the utilization of micro-EDM as an efficient coolant for automobiles. There is a unique experimental approach based on Ultrasonic technology for determining copper nanofluid concentrations [47]. Using a SS microcompressor and a ceramic miniature gas turbine as examples, Liu et al. [13] explored two- and three-dimensional microstructures and their applications. Another application is the machining of MEMS parts which can be done easily using µ-EDM. 15.9

CONCLUSIONS

Apart from conventional machining, micro-EDM is an effective nonconventional method that is utilized in wide-ranging applications. The following review reveals the mathematical, theoretical, and practical understanding of micro-EDM in Ti-6Al-4V alloy using several parameters like MMR and TWR, graphical representations, and intuitive distinctions using image representations. Also, for better understanding, we have used several conventional examples, which are more feasible methods than critical ones. Finally, the advancement of micro-EDM depends more on powdered mixed dielectric methods, various other hybrid forms of machining, optimization of various parameters, and automation of monitoring and various process controls. KEYWORDS • • • • • •

alloys dielectric electrical micromachining micro-EDM optimization techniques titanium

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CHAPTER 16

A REVIEW ON ELECTRICAL MICROMACHINING USING SILICON ELECTRODES HRITRISHA NASKAR, DEBOLINA DUTTA, and MANISH MUKHOPADHYAY Mechanical Engineering Department, Meghnad Saha Institute of Technology, Kolkata, West Bengal, India

ABSTRACT EDM, or what is known as electric discharge machining (EDM), is one of the non-contact type machining processes that is performed at a very profound level of innovation, at the microscopic level, which in turn has evolved as a major part of the manufacturing industry. This process is a material removal method to achieve high accuracy in geometrical and dimensional parameters. The process has a principal phenomenon which has subjective based on thermal and electrical, which is concatenated as a thermoelectric phenomenon in which there is a gap created between the electrode and the workpiece, which in turn induces the desired output in the form of a material removal process from the surface. This chapter reviews the process of micro-EDM and its evolution in a surface modification that has upcoming technology-sustainable potential. The use of electrodes and their selection based on different materials is one of the most vigilant parts. Therefore one such electrode material, like the use of composite electrode Silicon (Si) and its use in the machining, is also covered as an intrinsic aspect.

Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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Electro-Micromachining and Microfabrication

INTRODUCTION

EDM or electric discharge machining is one of a modern age technological concept of machining which is being extensively used to manufacture components of/entire dies and molds. Also made to be used for final products having high quality output for aerospace, automotive industry and medicalbio-medical industries [1]. The process was conceptualized and initiated around 1940s where uninterrupted electric discharges were employed for material removal under the presence of a dielectric [2, 3]. Proximity of the electrode (typically the tool) is reduced to ionize the dielectric in between to produce a stabilized arc [4]. Electric discharges create further separation between work and tool. Due to the obtained continuous electric discharges, material gets removed from both end, however, more prominently from the work end [5]. As the process is devoid of tangible contact, thus, the approach minimizes mechanical stresses, vibrations, and other issues during machining [1]. Materials which can conduct electricity are suitable for cutting and that can be of any hardness [6]. This micro-machining of EDM has thus evolved as a trend in machining and will evolve in future as long as there are keen interests within the researchers and there is sound availability of technology [7]. For establishing micro-finished features, current microEDM can be synchronized into several ways which are: 1. Micro-Wire EDM: Through conductive workpiece, a wired tube of dia. falling to 0.02 mm is cut. 2. Micro-EDM Die-Sinking: A minor picture of an electrode with micro-efficient characteristics is employed to machine the work. 3. Micro-EDM Drilling: Here, microelectrodes (of dia. 5–10 m) are used to create micro-holes on the work. 4. Micro-EDM Milling: Microelectrodes (dia. 5–10 m) are employed to form craters, similar to that of a conventional setup. 16.2 EDM PROCESS Feature sizes and tolerance are very small and sensitive, therefore the process of implementing the micro-EDM should be done very effectively. During the preparing stage and machining process there are chances of certain inevitable as well as accidental errors that should be avoided so as that might lead to undesired deliverables. These errors vary due to the probabilistic character of the sparking procedure in one hand and the

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equipment imperfection on the other hand. Optimization of EDM performance may be done analyzing the effect of variations of the rate of material removal and the rate of tool wear (MRR and TWR), and quality surface produced (SQ) [1]. Process parameters and their importance affecting the micro-EDM process are still under research and that is being constantly trying to improve by analyzing the effects of variations of these parameters. A significant hinderance for understanding these effects is the stochastic thermal nature of the process. The lack of insight in the heat distribution is a significant reason for the obstruction in understanding the complete science behind the micro-EDM process. Another challenge to understand the characteristics is the unavailability of proper micro-tools available for the process that can be supported on a CNC controlled micro-EDM setup. For instance, it is a potent problem to generate tool paths through existing CAM systems for milling of 3D cavities using micro-EDM. The existing process is unable to include compensation for electrode wear; also, it is not adapted to support variation of slice thickness or compensate for directional changes during the slicing operation [9]. However, the research community is constantly working towards overcoming these limitation [9]. The EDM process is categorized and established in a Figure 16.1 to present a clear representation.

FIGURE 16.1

EDM process parameters.

The mechanism for material removal is principally responsible for thermal conversion by generating a sequence of discrete electrical release stuck n midst

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of the work and tool, submerged in a dielectric fluid gap [12]. Thermal energy is employed in this case to form a plasma channel between both the anode and the cathode [13], occurring between 8,000 and 12,000°C [14]. Generated plasma plume disintegrates at a rate approximately 15,000 to 30,000 Hz when DC pulsating supply is used [15]. The sudden disintegration of the plasma plume, helps in voltage drop, thus ensuring properly flushed dielectric fluids to sweep up the debris from the spark gap zone. A variation in voltage pulses causing dielectric breakdown is described in McGeough’s [16] interpretations of the segments of EDM. The strength of dielectric breakdown in between the gap has to be overcome to discharge with a high voltage current. By accelerating electrons toward the anode, this plasma channel breaks down. The displacing electrons and ions, hiving high kinetic energy, are subsequently driven towards the terminals while colliding with neutral atoms. Owing to the colliding interactions happening at higher frequencies, the magnitude of material removal is low. Electrical discharge (ED) occurs at a particular location on both the tool electrode and the work piece when the electrode materials melt and evaporate, then are ejected as debris in the molten phase (as part of a pulse). Upon cooling and re-solidifying, the unwanted material forms debris particles within the dielectric fluid that will be washed out from the gap by dielectric pressure flow. At the completion of the discharge, a quick temperature drops with in plasma channel and on the surface of the electrode that comes into contact with the plasma leads in rapid recombination of ions and electrons. To ensure stability, the subsequent discharge takes place at a location enough far from the previous discharge position for each consecutive subsequent pulse discharge to take place. Reported studies indicated that for complete deionization of the previously formed plasma channel an optimized interval of five seconds or more is preferred. If it is too short, then the dielectric breakdown strength around the preceding discharge will substantially diminish. It is demonstrated in Figure 16.2 that the proper staging of different EDM phases must be done so that the next time voltage charge is supplied, the position may be retrieved. Because of surface roughness and machining instability created by short pulse interval time, short pulse interval time should not be used [17]. 16.2.1

EDM CIRCUITS

EDM circuit facilitates the conversion of electrical to heat energy in order to maintain the machining gap, which facilitates the formation of a sequence of consistent electric discharge as sparks. Principle of capacitance is adopted to

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store the electrical energy prior to conversion and subsequent spark generation [18]. The generally utilized arrangements of powers supply are RC generators, rotary impulse generator circuit, and controlled pulse arrangement. Among these, RC generators are preferred conventionally, owing to these being economic and operation friendly. The most widely utilized power supply circuits in the EDM process are the resistance–capacitance (R-C) circuit, the rotary impulse generator circuit, and the controlled pulse circuit. RC generators are generally simple and inexpensive to operate. The generated voltage follows a sawtooth waveform, involving a resistor and condenser. In an R-C circuit, a current source charges the condenser of capacitance ‘c’ via a resistance ‘Rc.’ Interrupted charging takes place till attainment of condenser voltage to breakdown point before generation of spark in the spark gap. Dielectric fluid is used to fill the gap. The resulting spark after ionization is initiated, contributing to an oscillatory flow of current that reaches a highest value [19]. A rotary impulse generator produces a voltage of around 110 volts. Because the voltage waveform is uncontrolled, arcing is a frequent phenomenon, resulting in a high tool wear rate. To achieve the pulsing switch effect, vacuum tubes or transistors are used in a pulse circuit. Resistance also has the potential to be utilized in these arrangements to reduce the time necessary to charge the capacitor.

FIGURE 16.2

16.2.2

Phases of an EDM process.

DIELECTRIC UNIT

The system comprises of a dielectric tank, a pumping unit, and a filtering unit to handle various forms of dielectric. Hydrocarbon (petroleum) are generally used as dielectric fluid. Kerosene, liquid paraffin, and silicon oils are also used as dielectric fluids. The circuiting dielectric fluid often plays

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the role of a quench on media by extracting the heat and solidifying the debris generated from the operation [20, 21]. 16.2.3

SERVO CONTROL UNIT

It requires to maintain the right distance of the spark gap during the machine operation to maintain uninterrupted generation of spark. This drive system controls the spark gap for continuous operation with the desired tool feed. The servo feed control maintains the right width of the working gap [22]. Figure 16.3 depicts the overall EDM machine and its components to help clarify the picture [23].

FIGURE 16.3

16.3

Schematic of EDM process.

EDM TRENDS IN SURFACE MODERATION

Use of hybrid energy source like thermoelectric, has helped to innovate the conventional material removal theories. The have extended an economical advantage in processing of advanced materials possessing poor machinability

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characteristics [24]. The technique involves the role of the energy displace by the movement of electrons, forming spark, and eventually arcs while being submerged in dielectric medium [3]. The plasma flux is created when the HA region of a work melts beside a little amount of electrode material existing in the dielectric medium [26]. The plasma plume was constituting particles originating from dielectric medium breakdown and electrode erosion due to the presence of powder in the dielectric medium [27]. Due to this occurrence some changes noticed chemical composition wise and also the layer forms on the workpiece surface, which is known as a recast layer after solidification, changes. The particular thermoelectric machining type finds is application in the manufacture of intricate geometries of products [28]. Mahajan and Sidhu presented a review study on the use of EDM in the design of bioimplants, emphasizing its resilience and adaptability [29]. The chemical change generated by EDM treatment validates the rise in bioactivity and also the machined surface’s favorable structure [30]. Meanwhile, various EDM procedure capabilities in surface modification are described in Table 16.1. In terms of biocompatibility, EDM may provide biocompatible surfaces with extremely fine polish within the acceptable Ra values of 0.40 m [39] and up to a value of 2 m (Ra) for a suitable textured surface [40]. Ekmekei et al. [41] applied the processing technique to cast a layer of HAp on alloys of titanium, with satisfactory results. In spite of this, a novel porous nanoceramic surface was achieved using PMEDM on the unique titanium-based implant. Recently, Peng et al. [42] studied the unique idea of producing nanostructured bioactive recast layers on Titanium by an electro discharge machining method. A nanostructured oxide layer was formed during the process which helps to achieve an improved biocompatibility. It is composed of both the phase and the Ti-H phase. The production of carbides, and a variety of intermetallic compounds on the substrates increased the microhardness and fatigue strength of PMEDM-treated surfaces considerably [43]. By the use of Ti nanopowder dielectrics, it is possible to improve the nano-scale morphology and material removal rate (MRR) of AlSl D2 steel along with the average surface roughness through this process [44]. The corrosion resistance and biocompatibility of EDM-machined surfaces were investigated in a subsequent investigation. As per the results, treating a surface with a W-Cu electrode tool at enhanced energy discharge of spark improves biological characteristics significantly [45]. Likewise, EDM was done on a Cr-co alloy substrate, and biocompatibility testing demonstrated encouraging results in comparison to an untreated surface [46].

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TABLE 16.1 Surface Modification with EDM Process

Work Material

Tool Material

Dielectric Medium

Observation

References

Wang et al.

Elec. Dis. Coat

C-Steel

Green titanium powder

Kerosene

Thick layer of TiC having greater hardness.

[31]

Yan et al.

EDM

Alloy of Titanium

Elec-Cu

Urea and distilled water

Prominent wear of TiN

[32]

Chen & Lin

Hybrid USM-EDM

Aluminum–Zinc– Elec-Cu Magnesium

TiC mixed in Kerosene

Enhanced machinability

[33]

Janmanee & Muttamara

Elec. Dis. Coat

WC-Co

Hydrocarbon oil

Enhanced hardness

[34]

Khan et al.

Powder mixed EDM

Low carbon steel Cu-Ti

TiC and Alumina mixed in Kerosene

Enhanced recast layer formation

[35]

Harcuba et al.

EDM

Titanium grade 5 Graphite

Hydrocarbon oil

Improved strength and hardness.

[36]

Kumar & Batra

EDM

Die steel

Elec-Cu

Ti powder mixed with Kerosene

Enhanced microhardness.

[37]

Selvarajan et al.

EDM

Si3N4-TiN ceramic

Electrolytic Cu

EDM oil

Enhanced machinability

[38]

Elec-Cu

Electro-Micromachining and Microfabrication

Authors

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ELECTRODE MATERIALS

The EDM process alters the machined surface in ways that were first observed in the early 1960s. Formed layer of recast material was observed on the machined substrate following EDM with a medium which is paraffin. They reported that the filing process was difficult to machine [56]. The decomposition of oil in combination with electrode material creates this coating on the machined surface [47]. Thermal stability, hardness, and wear resistance have all enhanced as a result of the created surface [48]. The EDM testing was performed on a variety of steel materials with the use of brass and copper electrode materials, kerosene, and distilled water. The results indicated that high-speed steel deposition is superior to other materials. Additionally, the EDM process in deionized water medium resulted in less surface alteration with depth because of less diffusion than that of the EDM process in kerosene fluid media [49]. Roethel & Garbajs [50] made similar observations. Soni & Chakraverti [51] studied surface characteristics while using a rotating tool electrode to machine HCHCr dies steel material. Material changes, as well as the existence of the layer of recast and micro-hardness, were noted. Tsukahara & Sone [52] applied the method for surface hardening of alloys of titanium and reported the existence of a prominent coating of TiC with few fractures. There were additional advances in corrosion, rheological, and mechanical characteristics. The existence of coating can also be observed on tool materials as reported by Mohori et al. [53], where they reported the formation of layer of carbon materials on W-Cu tool while machining D2 tool steel. A copper electrode used by Yan et al. [54] to try a combined EDM/ball burnishing technique on Al alloy. The negative polarization of copper and copper–tungsten electrodes create a high hard surface on the materials, as well as the microstructure and machined surface of OHNS steel [11]. 16.5 SUITABILITY OF SILICON COMPOSITES AS ELECTRODE MATERIAL Ramulu et al. [56], reported the processing of metal matrix composites (MMCs) under EDM, employing SiCw/Al. The study involved the use of Cu and brass as the tool material, and critically assessed the performance under varying parametric variations. MRR and tool wear resistance were used to

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assess EDM performance. Reported results indicated the ease of machining using the used cutting tool. Brass electrodes are found to wear out faster than copper electrodes. For deeper understanding Dhupal et al. [57] applied multi-objective optimization of process variations while processing A-SiC composites with Cu electrodes. The surface stability of Al–SiC and Al–SiC–B4C composites studied by Kumar et al. [58]. It is his observation that when electro discharge machining performed on Al–SiC–B4C the depth of the heat-affected zone was larger than Al–SiC composites. The roughness of the surface crater diameter was also noticeable on Al–SiC–B4C. Copper electrodes were also used to assess the feasibility of EDM machining Al2O3/6061Al composites. Using the Taguchi technique Yan et al. [59] optimized the magnitude of material removal and wear rate of tool observing surface finish as response parameter. When compared to other machining processes, this method reports that the process is feasible. Parametric evaluation of machinability reported by Liu [60] was performed on TiN/Si3N4 using Cu and brass tools under varying Discharge current and duration. SEM revealed the presence of layer of solidified material on Cu tool, indicating consistent by slick material transfer, thus generated a smoother work surface. In a reported study by Kumar et al. [61] the investigated the effect of AlN, Si3N4 and ZrB2 while processing Al2618 MMC using different reinforcement ratios while varying the process parameters. The reports suggested that the reinforcement ratio grave affected the MRR and has an inversely proportional relationship with the same. According to Rengasamy et al. [62], while machining Al 4032–TiB2–ZrB2 MMCs formed as a static reaction. The impact of reinforcement content on process variables is investigated. It influences both MRR and TWR greatly. In an experimental investigation involving the use of Cu tools for processing Al–Mg2Si, Hourmand et al. [63] reported that discharge current and open voltage heavily perturbs the MRR of the MMC. Also, the discharge time had significant influence over TWR. Similar inferences were made by Puertas et al. [55] employing die sinking EDMing process. Selvarajan et al. [38] examined electro discharge machining of Silicon Nitride composites, employing the Taguchi technique to optimize different process parameters utilizing a variety of response parameters. In a research, Yongfeng et al. [25] investigated the significance of electrode polarity. The study involved EDM of ZrB2-20 vol.%SiC composites. Results indicated the formation of considerable size of molten droplets on the substrate, whereas formation of polygonal material layers were found using negative polarity. The discharge energy was discovered to have a substantial impact on MRR.

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Hanaoka et al. [10] employed the electrode assistant to conduct EDM studies on silicon nitride with carbon nanotubes (CNTs) and graphene nanoparticles added (GNP). Analysis was carried out under varying ratios of reinforcements of the MMC and were compared with the pure one. CNT and GNP reinforcements were reported to significantly reduce tool wear and resulting generation of smoother surfaces. However, it achieved lower MRR than the pure one. 16.6 CONCLUSIONS Apart EDM has become an intrinsic part of material removal process in the present as well as upcoming eras. To sum it up in a summary as the following conclusions drawn are: • Because the feature sizes and tolerances of the machined surfaces are so small, micro-EDM process design must be done with great care. • Choosing of electrode material plays an integral part of the machinability. • Silicon composites being visualized as an electrode. KEYWORDS • • • • • • •

dielectric unit EDM circuits electrical micromachining electrode materials micro-EDM silicon electrode surface moderation

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56. Ramulu, M., & Taya, M., (1989). EDM machinability of SiCw/Al composites. Journal of Materials Science, 24(3), 1103–1108. 57. Dhupal, D., Naik, S., & Das, S. R., (2018). Modeling and optimization of Al–SiC MMC through EDM process using copper and brass electrodes. Materials Today: Proceedings, 5(5), 11295–11303. 58. Kumar, S. S., Uthayakumar, M., Kumaran, S. T., Varol, T., & Canakci, A., (2019). Investigating the surface integrity of aluminum based composites machined by EDM. Defense Technology, 15(3), 338–343. 59. Yan, B. H., Wang, C. C., Liu, W. D., & Huang, F. Y., (2000). Machining characteristics of Al2O3/6061Al composite using rotary EDM with a disklike electrode. The International Journal of Advanced Manufacturing Technology, 16(5), 322–333. 60. Liu, C. C., (2003). Microstructure and tool electrode erosion in EDMed of TiN/Si3N4 composites. Materials Science and Engineering: A, 363(1, 2), 221–227. 61. Kumar, N. M., Kumaran, S. S., & Kumaraswamidhas, L. A., (2015). An investigation of mechanical properties and material removal rate, tool wear rate in EDM machining process of AL2618 alloy reinforced with Si3N4, AlN and ZrB2 composites. Journal of Alloys and Compounds, 650, 318–327. 62. Rengasamy, N. V., Rajkumar, M., & Kumaran, S. S., (2016). An analysis of mechanical properties and optimization of EDM process parameters of Al 4032 alloy reinforced with Zrb2 and Tib2 in-situ composites. Journal of Alloys and Compounds, 662, 325–338. 63. Hourmand, M., Farahany, S., Sarhan, A. A., & Noordin, M. Y., (2015). Investigating the electrical discharge machining (EDM) parameter effects on Al-Mg2Si metal matrix composite (MMC) for high material removal rate (MRR) and less EWR–RSM approach. The International Journal of Advanced Manufacturing Technology, 77(5), 831–838.

CHAPTER 17

PERFORMANCE ENHANCEMENT OF MICRO-MACHINED SURFACES USING POWDER MIXED EDM S. TRIPATHY,1 SMRUTIRANJAN BISWAL,1 and D. K. TRIPATHY2 Mechanical Engineering Department, ITER, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India 1

Ex-Professor Emeritus, IIT Kharagpur, West Bengal, India

2

ABSTRACT Electro-discharge machining (EDM) is among the most widely used nonconventional processes used in die-making, aerospace, sports, medicine, and other industries for removing material and producing micro-machined surfaces with intricate shapes. The process has gained popularity for its ability to deform difficult-to-machine materials with ease resulting in superfinished surfaces. The process possesses demerits like less machining rate (MR) with more tool wear. Improving the machining capabilities of the process has gained much focus from researchers recently. Hybrid machining approaches can overcome the inherent drawbacks in the process. There has been continuous development in the process of EDM, some of which include wire EDM, powder mixed EDM (PMEDM), micro-EDM, nanoEDM, magnetic field assisted EDM, machining using composite tools made using powder metallurgy route. The present chapter focuses on increasing the manufacturing efficiency, geometrical accuracy, and surface properties of micro-machined surfaces generated using powder-mixed micro-EDM. In PMEDM, an appropriate powder is added to the dielectric fluid causing Electro-Micromachining and Microfabrication: Principles and Research Advances. Sandip Kunar, Golam Kibria, & Prasenjit Chatterjee (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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rapid sparks and erosion from the desired surface. A review will be presented on the application of PMEDM for micromachining. The study will focus on the influence of adding powders, types of powders, size of powders, surface modifications, and geometrical accuracy achieved by the process. 17.1

INTRODUCTION

To miniaturize products and parts within a dimension between 1 and 999 µm, micro-machining is used. The present-day industries use miniaturized parts in all industries which majorly include medical, electrical devices, information technology, robotics, aerospace, biotechnology, environmental testing devices, etc. Micro-machining is gaining popularity day by day in industries which demand the application of miniaturized parts with sufficient dimensional accuracy and intricate profiles made up of exotic materials bearing more strength. The micro-machining processes can be conventional, non-conventional or hybrid. Conventional processes use shear force to remove the material wherein a physical contact is developed between the electrodes. Examples of such processes are micro-milling, micro-drilling, grinding, etc. The non-conventional machining processes use other forms of energy like sound, light, electrical, mechanical, chemical, electrons, ions, etc., for material removal. Such processes have some restrictions like limited work piece material, use of special facilities for proper functioning and small thickness of material removed. Electro-discharge Machining (EDM) is a non-traditional material removal method that has found wide application in various industries due to its tremendous capabilities of machining extremely hard materials possessing high electrical conductivity. Materials possessing high strength to weight ratio can be easily shaped using EDM. Hybridized processes are a combination of conventional and non-conventional machining processes. They are used to make micro-components with all kinds of materials like metals, plastics, and semi-conductors. Such processes include micro-turning, micro-milling, micro-EDM, etc. The principle of EDM lies in transforming electrical to thermal energy during generation of a series of sparks amidst a set of conductive electrodes submerged in the dielectric fluid. The bombing of ions and electrons results in generation of a plasma channel which has a temperature of 8,000°C–12,000°C in the discharge region. The material from the tool and workpiece surface thus gets eroded as a result of melting and evaporation. The principle of micro-EDM is comparable to EDM apart from the size of electrode, energy of discharge,

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resolution of movement in the axis which is at microns level. The process is efficient of fabricating micro-sized parts without causing any distortion to the workpiece due to stress or vibration. The micro-EDM technology is divided into five types as follows: i.

Die sinking µ-EDM involves the application of a micro-electrode to replicate the image of the tool on the machined surface; ii. µ-WEDM uses conductive wire of very small diameter to generate the required profile on the workpiece surface; iii. µ-EDM drilling for drilling µ-holes using µ-electrodes; iv. µ-EDM milling produces 3D craters using µ-electrodes; and v. µ-electro-discharge grinding is used to fabricate µ-electrodes. The electrode materials for µ-EDM possess higher electrical conductivity, melting point and boiling point. The electrodes with higher electrical conductivity exhibit lower temperatures on surfaces. Materials like W, AgW, CuW, etc., are suitable electrode materials for micro-EDM. Powder mixed EDM is another variation to EDM, which has increased the machining capabilities of EDM in tremendous ways since 1980s. It produces mirror like surfaces with less surface cracks. The process involves adding an abrasive fine powder to the dielectric which increases the interelectrode gap and lowers the strength for insulation of the dielectric. The removal of the debris becomes easier. Voltage of 80–320 V leads to the formation of electric field in the range of 105–107 V/m. This phenomenon generates positive-negative charge on the powder particles. On being energized, the abrasive particles move in a zig-zag fashion forming clusters in the sparking region. Several discharges are formed on one pulse as a result of bridging effect occurring under the region under spark. This phenomenon results in causing faster sparks and the workpiece material get worn out with ease. The machining rate (MR) of the process is thus improved due to the easy short circuit. As the plasma channel gets widened, stable, and uniform sparks are produced. The craters formed are shallower having higher surface quality (SQ). The debris on getting mixed with the abrasive powder, form a coating on the machined surface, thus enhancing the SQ. Al, Cr, graphite, silicon, titanium powders may be added. The set up used for machining is shown in Figure 17.1. A similar principle may be applied to powder mixed micro-EDM as the mechanism is same for both processes. The difference lies in the electrode size and the energy of discharge. Different powders exhibit different optimum results when applied to both the processes.

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FIGURE 17.1

17.2

Principle of PMEDM.

LITERATURE SURVEY

Jeswani [1] performed PMEDM using graphite powder added to kerosene. There was an increase of 60% in the MRR, 15% decrease in the TWR. The wear ratio was found to decrease by 28%. Pecas & Henriques [2] added silicon powder to study the SQ and measure the process times for different processing areas and found that adding silicon powder lowers the machining time and achieve enhanced SQ, reduces the SR by generating mirror-like surfaces. Kansal et al. [3] presented a broad history of research work for PMEDM by addressing the mechanism, issues, applications, and observations. Singh et al. [4] used H13 material as workpiece to investigate the effect of input parameters and powder concentration on surface roughness during EDM. Kansal et al. [5] optimized the process condition for PMEDM of AlSiCp MMC by conducting experiments with RSM. Chow et al. [6] used pure water mixed with SiC powder for micro-slit EDM along with low discharge energy as a form of green manufacturing. The MRR was increased, and a minor crater was generated. Jabbaripour et

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al. [7, 8] conducted experiments using PMEDM of γ-TiAl with aluminum, graphite, chrome, silicon carbide, and iron powders to explore its effect on SQ, topography, MRR and electrochemical corrosion resistance. Singh et al. [9] investigated the influence of input process parameters on MRR, TWR, percent wear ratio and SR for EDM of Hastelloy using Cu tool. Long et al. [10] added titanium powder to the dielectric and machined using Cu and graphite electrodes. The MRR was found to increase whereas the TWR and SR reduced with the generation of a thin temperature affected machined area. Furutani et al. [11] performed investigations on AISI 1049 steel surface with PMEDM and observed formation of TiC coating with 2000 HV hardness. Govindharajan et al. [12] mixed Gr and Ni powders during PMEDM which enhanced the productivity by raising the MRR, lowering the TWR with superior geometrical accuracy of the machined surface. Gurtej Singh et al. [13] suspended Al powder to the dielectric in varying concentrations for EDM and examined modifications in the surface roughness of SKD61 steel. Khedkar et al. [14] observed that mixing tungsten powder to dielectric while performing EDM on the OHNS steel, increases the micro-hardness by 100% due to migration of tungsten powder to the machined surface. Vipin Kumar et al. [15] found that the powder size and type bear a strong impact on MRR and Ra in EDM machining of EN31 steel. The productivity and quality of surface increased with powder particles and the machining time got lowered. Kumar et al. [16] mixed aluminum and silicone powdered particles to distilled water and compared machining without powders for rough and trim cut during WEDM of Nimonic-90 and evaluated the MR, SR, RLT, and microhardness. Past work demonstrates contribution of several input process parameters for controlling the machined component quality during PMEDM. The relative significance of the process parameters on output responses thus needs investigation. The present chapter presents a comprehensive review on powder mixed micro-EDM. The influence of adding powders on MRR, TWR, and improvement on SQ has been presented. The current status of PMEDM, its application for micro-EDM, potential, and future perspectives has been discussed. 17.3 RESEARCH AND DEVELOPMENT IN POWDER MIXED MICRO-EDM 17.3.1 APPLICATIONS OF PM-µEDM 1. Difficult-to-machine metals and alloys which find wide application in aerospace industries need an effective way of material removal.

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EDM has also found huge application in automotive and nuclear industries where high temperature and stress conditions prevail. It is a capable method to meet up rising demands of complex miniaturized components such as parts used in micro-electronics. Application prospective of EDM may be improved by enhancing its machining capabilities and by proper estimation and control of the surface damages caused by the process. PMEDM has grown out of the need to overcome the poor surface integrity produced by EDM and to improve the safety and life of the component. The precise applications of PMEDM comprise manufacturing of engine blocks, piston heads, carburettors, and cylinder liners. It also finds extensive use in surgical equipment, dental instruments and medical implants.

17.3.2 PMEDM PROCESS PARAMETERS The process of PMEDM has various influencing process parameters like powder-based parameters which include powder type, conductivity, size, density, concentration apart from the electrical, non-electrical, and electrodebased parameters. Figure 17.2 shows the detailed flowchart of machining parameters, materials, and powders used in micro-PMEDM. Some of the important parameters are discussed in subsections.

FIGURE 17.2

Flow chart of Micro-EDM parameters.

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17.3.2.1 ELECTRODE MATERIAL The EDM electrodes must help in achieving maximum MRR and minimum TWR. The various PMEDM electrodes widely employed are brass, copper, copper tungsten, cast iron, zinc-based alloys, etc. 17.3.2.2 TYPES OF POWDERS The different types of powders used are aluminum, chromium, silicon carbide, silicon, titanium, graphite, alumina, CNTs and many more. Depending upon the type of powder used the performance characteristics of the process tend to improve. With silicon carbide powder added to the dielectric, the material removal rate (MRR) increases thereby increasing the tool wear rate and the surface roughness. The addition of aluminum powder improves the SQ while reducing the tool wear rate. Chromium powder is observed to increase the machining efficiency by decreasing the electrode wear rate and increasing the MRR. Tungsten increases the microhardness of the surface while titanium minimizes the micro-cracks formed. Adding silicon powders reduce the surface roughness whereas graphite powder improves the MRR along with the lubrication properties. Present day researchers have used CNTs which decrease the surface roughness, surface crack formation while reducing the recast layer formation. Many other studies have also been performed by adding other powders to the dielectric fluid which improves the SQ produced. 17.3.2.3 INFLUENCE OF POWDER CONCENTRATION AND POWDER SIZE It has been observed from the past research that the increase in powder concentration increases the MRR. Researchers have performed various studies using different powder materials by varying the powder size and powder concentration. Powders particles of size