Mechanical and Industrial Engineering: Historical Aspects and Future Directions (Materials Forming, Machining and Tribology) 3030904865, 9783030904869

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Materials Forming, Machining and Tribology

J. Paulo Davim   Editor

Mechanical and Industrial Engineering Historical Aspects and Future Directions

Materials Forming, Machining and Tribology Series Editor J. Paulo Davim, Department of Mechanical Engineering, University of Aveiro, Aveiro, Portugal

This series fosters information exchange and discussion on all aspects of materials forming, machining and tribology. This series focuses on materials forming and machining processes, namely, metal casting, rolling, forging, extrusion, drawing, sheet metal forming, microforming, hydroforming, thermoforming, incremental forming, joining, powder metallurgy and ceramics processing, shaping processes for plastics/composites, traditional machining (turning, drilling, miling, broaching, etc.), non-traditional machining (EDM, ECM, USM, LAM, etc.), grinding and others abrasive processes, hard part machining, high speed machining, high efficiency machining, micro and nanomachining, among others. The formability and machinability of all materials will be considered, including metals, polymers, ceramics, composites, biomaterials, nanomaterials, special materials, etc. The series covers the full range of tribological aspects such as surface integrity, friction and wear, lubrication and multiscale tribology including biomedical systems and manufacturing processes. It also covers modelling and optimization techniques applied in materials forming, machining and tribology. Contributions to this book series are welcome on all subjects of “green” materials forming, machining and tribology. To submit a proposal or request further information, please contact Dr. Mayra Castro, Publishing Editor Applied Sciences, via [email protected] or Professor J. Paulo Davim, Book Series Editor, via [email protected]

More information about this series at https://link.springer.com/bookseries/11181

J. Paulo Davim Editor

Mechanical and Industrial Engineering Historical Aspects and Future Directions

Editor J. Paulo Davim Department of Mechanical Engineering University of Aveiro Aveiro, Portugal

ISSN 2195-0911 ISSN 2195-092X (electronic) Materials Forming, Machining and Tribology ISBN 978-3-030-90486-9 ISBN 978-3-030-90487-6 (eBook) https://doi.org/10.1007/978-3-030-90487-6 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

We can broadly define mechanical engineering as a discipline “which includes the application of principles of physics, design, manufacture and maintenance of mechanical systems.” The subareas of classical mechanical engineering include: thermal and fluids, applied mechanics and design, materials and manufacturing processes, automation and control, etc. Industrial engineering can be considered an offshoot of mechanical engineering that is concerned with “increasing productivity through people management, business organization methods and technology” or, in other words, “industrial engineering is the engineering of system efficiency.” Depending on the subareas involved, industrial engineering covers management science, operations management, systems engineering, manufacturing engineering, safety engineering, etc. This book brings together contributions on historical aspects and future directions of mechanical and industrial engineering. Chapter 1 of the book is dedicated to tribology (a tool for mechanical and industrial engineering). Chapter 2 describes cutting force modeling (genesis, state of the art and development). Chapter 3 contains information evolution of additive manufacturing processes (from the background to hybrid printers). Chapter 4 is dedicated to busbars for e-mobility (state-of-the-art review and a new joining by forming technology). Chapter 5 describes autofrettage: from development of guns to strengthening of pressure vessels. Chapter 6 contains information on machining of fibrous composites (recent advances and future perspectives). Finally, Chapter 7 is dedicated to management of industrial technologies. This book can be recommended for a final undergraduate degree in engineering or even at the postgraduate level. In addition, the book can serve as a useful reference for academics, researchers, mechanical and industrial engineers, as well as for professionals in related industries. Scientific interest in this book is evident to many universities, colleges and institutes, as well as to industry. Therefore, it is hoped that this book will inspire the conduct of research in mechanical and industrial engineering.

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Preface

The editor thanks Springer for this opportunity to publish this book. Finally, the editor would like to thank all chapter authors for their willingness to participate in this work. Aveiro, Portugal December 2021

J. Paulo Davim

Contents

1 Tribology—A Tool for Mechanical and Industrial Engineering . . . . . Prasanta Sahoo and Suman Kalyan Das

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2 Cutting Force Modeling: Genesis, State of the Art, and Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Viktor P. Astakhov

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3 Evolution of Additive Manufacturing Processes: From the Background to Hybrid Printers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I. Buj-Corral, A. Tejo-Otero, and F. Fenollosa-Artés

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4 Busbars for e-mobility: State-of-the-Art Review and a New Joining by Forming Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Rui F. V. Sampaio, Maximilian F. R. Zwicker, João P. M. Pragana, Ivo M. F. Bragança, Carlos M. A. Silva, Chris V. Nielsen, and Paulo A. F. Martins 5 Autofrettage: From Development of Guns to Strengthening of Pressure Vessels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Rajkumar Shufen and Uday S. Dixit 6 Machining of Fibrous Composites: Recent Advances and Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Jinyang Xu and J. Paulo Davim 7 Management of Industrial Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Marius Gabriel Petrescu, Costin Ilinc˘a, Maria T˘anase, and Hailong Fu Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

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About the Editor

J. Paulo Davim is Full Professor at the University of Aveiro, Portugal. He is also distinguished as Honorary Professor in several universities/colleges/institutes in China, India and Spain. He received his Ph.D. degree in mechanical engineering in 1997, M.Sc. degree in mechanical engineering (materials and manufacturing processes) in 1991, mechanical engineering degree (5 years) in 1986, from the University of Porto (FEUP), the aggregate title (Full Habilitation) from the University of Coimbra in 2005 and the D.Sc. (Higher Doctorate) from London Metropolitan University in 2013. He is Senior Chartered Engineer by the Portuguese Institution of Engineers with an MBA and specialist titles in engineering and industrial management as well as in metrology. He is also Eur Ing by FEANI, Brussels, and Fellow (FIET) of IET London. He has more than 35 years of teaching and research experience in manufacturing, materials, mechanical and industrial engineering, with special emphasis in machining and tribology. He has also interest in management, engineering education and higher education for sustainability. He has guided large numbers of postdoc, Ph.D. and master’s students as well as has coordinated and participated in several financed research projects. He has received several scientific awards and honors. He has worked as evaluator of projects for European Research Council (ERC) and other international research agencies as well as examiner of Ph.D. thesis for many universities in different countries. He is Editor in Chief of several international journals, Guest Editor of journals, Editor of books, Series Editor of book and Scientific Advisory for many international journals and conferences. Presently, he is Editorial Board Member of 30 international journals and acts as reviewer for more than 100 prestigious Web of Science journals. In addition, he has also published as Editor (and Co-editor) more than 200 books and as Author (and Co-author) more than 15 books, 100 chapters and 500 articles in journals and conferences (more than 300 articles in journals indexed in Web of Science core collection/h-index 61+/12500+ citations, Scopus/h-index 66+/15500+ citations, Google Scholar/h-index 84+/25000+ citations). He has been listed in World’s Top 2% Scientists by Stanford University study.

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Chapter 1

Tribology—A Tool for Mechanical and Industrial Engineering Prasanta Sahoo and Suman Kalyan Das

Abstract Human civilisation has employed the concepts of tribology from the very beginning if not in a formal way. It started with solving problems related to friction and lubrication in the activities of day-to-day life. Gradually with the interests of some bright minds, tribology began to take the form of a specific subject and humankind began to appreciate its potential of transforming their lives. Industrial revolutions definitely played a part in the development of tribology and benefits of same has been reciprocated back to the industries. The knowledge of tribology has now got an additional facet due to the present problems of energy conservation and climate change. Obviously, tribology has yet to offer lot more considering these aspects and the true potential of it can only be revealed by proper and wide application of it.

1.1 Introduction The word ‘Tribology’ was first coined by the British commission guided by Peter Jost in his historic report in 1966. This marked the beginning of a unified approach to the studies and research related to the interaction between moving surfaces in contact. This led to other popular bodies adopt this terminology viz. the American Society of Mechanical Engineers (ASME) started a new division in 1983 by the name Tribology Division and the American Society of Lubrication Engineers renamed itself to the Society of Tribologists and Lubrication Engineers in 1985 [1]. Tribology is not a topic which is domain specific. Rather it is an interdisciplinary science encompassing the knowledge from physics, chemistry, material science, engineering, biology and so on to understand the phenomena occurring during contact of a pair of bodies in motion. Tribology as a subject is very relevant for mechanical and industrial engineering. Friction which is one of major hindrances towards energy conservation nowadays, can be effectively managed with proper application of the knowledge of tribology. Wear which shortens the device service life and increases the down time of machineries can also be controlled to a great extent by knowing the proper tribological tools. P. Sahoo (B) · S. K. Das Department of Mechanical Engineering, Jadavpur University, Kolkata 700032, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 J. P. Davim (ed.), Mechanical and Industrial Engineering, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-90487-6_1

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Besides, other aspects like lubrication, surface engineering and corrosion which are almost a part and parcel of human society belong to the field of tribology. Hence, the significance of the tribological applications are evident more and more. But still, Tribology as a subject doesn’t enjoy a very high visibility among the engineering and scientific community. Hence, proper awareness must be created about the subject and the present work is a small attempt in that direction.

1.2 Main Aspects of Tribology Tribology is currently a matured subject of science and technology. As already mentioned it is related to interaction between moving surfaces in contact. The tribological interactions between two bodies give rise to various physics which can be explained by different theories and models (refer Fig. 1.1). Hence, tribology as a subject is highly multiphysical in nature. The types of phenomena that can take place in an interface and its local surroundings are as follows: mechanical (solid and fluid), thermal, electro-magnetic, metallurgical, quantum and others [2]. Among these phenomena, the significant ones are discussed in the following text.

1.2.1 Friction Friction is the resistance faced by a body (Fig. 1.2) when it slides tangentially on another body. Obviously, the friction force acts opposite to the direction of motion and exists even though the body is pushed but not in motion. The critical friction force which initiates the motion of the body is the static friction force which is normally

Fig. 1.1 Multiphysical nature of tribology: two bodies make contact when exposed to various loads: mechanical, thermal, electric, and environmental [2]

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Fig. 1.2 Friction (courtesy Vishakha.malhan under CC BY-SA 4.0 licence)

higher than the kinetic friction force i.e. the friction force experienced by the body once the motion starts. The significance of friction can mainly be categorised as [3]: • Sufficiently higher friction for actions like walking, gripping, etc. • Minimising friction in case of machinery. • Maintaining a constant friction in case of precision devices, rolling industries, etc. Experimental observation on friction over time has led to the following three empirical laws on sliding friction: 1. 2. 3.

Friction force is proportional to the normal load acting between the two bodies in contact. Friction force is independent of the apparent area of contact between the two bodies. Friction force is independent of the sliding speed.

Apart from sliding friction, friction can also be like rolling friction which is experienced when a circular body rolls on a flat surface. Rolling friction is normally lower than sliding friction and is a popular configuration employed to lower friction in rotary parts of machines.

1.2.2 Wear Wear is the damage and gradual removal of material from one or both the contacting surfaces in relative motion. It is a system response and not a material property. In general wear is related to hardness of a material and higher hardness usually mean lower wear. The traditional means of measuring wear is by the weight loss suffered by the component. However, with advancement in instrumentation, wear is also represented in terms of depth. Wear is normally undesired as it leads to shortening of

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the service life of components. However, in some cases viz. machining, polishing, shearing, etc. wear is deliberately inflicted in a controlled manner. Wear takes place either by a mechanical or a chemical process or by a combination of both. Wear is found to be accelerated in case of enhanced thermal conditions (high temperature conditions). Wear can be broadly classified as: • Adhesive wear: occurs when two flat surfaces are in sliding contact and adhesion takes place at the interface particularly at the asperities. • Abrasive wear: occurs when a softer surface is scratched by a harder surface. • Corrosive wear: occurs when sliding between two surfaces takes place in corrosive environment. • Fatigue wear: occurs during repeated sliding and rolling contacts. • Erosive wear: occurs when material from a target surface is removed due to repeated impacts of solid particles.

1.2.3 Lubrication The idea of lubrication was initiated to reduce friction at first. But later it was found effective in limiting wear as well. Hence, much attention was given towards development of effective lubrication scheme. Lubrication is mainly achieved by including a medium separating the two surfaces intending to make contact. The medium is usually a liquid but, in some cases, it can be a suitable gas. Friction now depends on the resistance to shear deformation of the liquid which is nothing but the viscosity of the liquid. As the viscous forces are much lesser compared to the resisting force faced by the surface when making actual contact, friction is substantially reduced. Besides, as physical contact is avoided, wear is also limited to a great extent. Lubrication has seen tremendous evolution with time and based on applications there are now a variety of lubrication techniques which can be categorised as follows: • Fluid lubrication • Liquid lubrication: Lubrication carried out by liquid • Gas lubrication: Lubrication carried out by gas • Boundary lubrication: Sliding surfaces are separated by a very thin molecular film of lubricant, so that the chemical and physical natures of the surfaces and the lubricant play significant role in the lubrication. • Solid lubrication: Lubrication is carried out by inserting solid particles/ layer in between the two surfaces. The characteristics of these solid particles help in reducing friction and wear.

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1.3 The Development of Tribology as a Science 1.3.1 Pre-history and Ancient Works (up to 1600 AD): Solving Simple Problems of Friction and Lubrication Ancient people obviously didn’t have an organised knowledge in Tribology. They used common sense to solve problems related to friction, wear and lubrication. In fact, fire was believed to be discovered by the frictional heating between two stones. Gradually with time, human civilisations set up and there are definite proofs available regarding deliberate use of tribological techniques in order to overcome the issue of friction and wear. Early history of tribology is compiled and written in an organised way by Dowson [2]. One of the earliest evidences of the usage of tribological concepts can be found in the Egyptian civilisation. While constructing the pyramids, the rolling elements in the form of logs are known to have been used to transport heavy stones. This indicated that the people at that time were able to appreciate that rolling friction was lesser than sliding friction and required lesser energy. Besides, the popular painting about the transportation of a huge colossus aided by liquid lubrication also hints towards the basic knowledge of lubrication being known to them (refer Fig. 1.3). Water when used in appropriate amounts, made the sand stiff and reduced the sliding friction over sand by about 50% compared to when moving over dry sand [4]. The invention of wheel is considered to be an important milestone in the history of humankind. The same can also be considered to be a great proof about the tribological knowledge prevailing during that time. Sumerians and Mesopotamians are in fact believed to

Fig. 1.3 Transporting an Egyptian colossus from the tomb of Djehutihotep, El-Bersheh, (c.1880 B.C.) (courtesy Youssef Grace under CC BY-SA 4.0 licence) https://upload.wikimedia.org/wikipe dia/commons/9/9b/Djhutyhotep_Deir_El-Barsha_Youth_Union.jpg

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Fig. 1.4 Ljubljana marshes wheel (courtesy - Petar Miloševi´c under CC BY-SA 4.0 licence) https://upload.wikimedia.org/wikipedia/commons/8/85/Ljubljana_Marshes_Wheel_with_axle_% 28oldest_wooden_wheel_yet_discovered%29.jpg

have used chariots for war. The oldest wheel and axle known as Ljubljana Marshes Wheel (Fig. 1.4) has been discovered in Ljubljana (Slovenia) and is dated to Copper Age (about 3000 BC). It is made of wood and indicates that wheels appeared almost simultaneously in Mesopotamia and Europe. Miniaturized wheel cart and toys have been unearthed from the sites of Harappan Civilisation. In fact, the people used bearings in wheel and axle system to concentrate wear in a single part. Chariots have been found to be used by Ramesses II in various contemporary paintings. Ashurbanipal, the Assyrian king is believed to have used anti-wear protection for the wheel surface. In fact, the great literature of India, Mahabharata mentions the use of chariots in its text. The Greek and the Romans have contributed significantly in the field of tribology. Various mechanical systems viz. lathes, wheeled transport, pulleys, gears, mills, cranes, etc. were used by them [5]. Leonardo Da Vinci the great scientist has contributed significantly to the early development of tribology. He had great knowledge acquired from observations, logical reasoning and intuition which he employed to conduct scientific experiments in an organised way. From the various sketches obtained from his book, it is proved that he designed various devices based on tribological applications to solve problems of day-to-day life. Leonardo Da Vinci is the earliest person to conduct experiments on friction. In fact, he got the idea that friction is independent of contact area and also initiated the idea of lubrication in order to reduce friction. The sketches on ball

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bearings as proposed by Leonardo Da Vinci actually forms the basis of modern day ball bearings [6].

1.3.2 Classical Works (1600–1950): Transition of Tribology from Art Towards a Science The works undertaken during this period can be considered as classical works on tribology. They helped in establishing to some extent ‘Tribology’ as a science and technology of interacting surfaces. These works were undertaken mainly during the epochs of first and second industrial revolution and are characterized by improved designs of machine elements [7]. Significant portion of research undertaken during this period mainly concentrate on solving the problems related to friction and lubrication. However, there have been a few significant developments in understanding and modelling wear. Some of the important works during this period are discussed in the following sections: • Amontons (1699) Amonton carried out experiments on tribology particularly friction in a systematic manner. His works led to the establishment of two important laws of friction: • Friction force is proportional to the normal load. • Friction is independent of the apparent area of contact. • J.T. Desaguliers (1725) He carried out works related to cohesion of lead and suggested that adhesion might be relevant to friction. • Leonhard Euler (1748) He proposed a mathematical relationship for the coefficient of friction. He was the first to distinguish between the static and kinetic friction coefficient and stated that static friction is greater than kinetic friction. He believed in the rigid interlocking between asperities during sliding cotact. • Charles Coulomb (1785) Coulomb proposed the third law of friction which states that “Friction is independent of sliding speed.” Coulomb in fact stated that friction happens due to the interlocking of asperities between the two contacting surfaces. His work verified the propositions made by Leonardo da Vinci and Amontons [7]. • Gustav Adolf Hirn (1847–49) He was the first person to conduct experiments with lubricated contacts. He found that viscosity of the fluid played a significant role in lubricated sliding contact. Hirn

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further established a linear relationship for friction between a journal and bearing and the speed. As mineral oil was discovered around that period, Hirn used oil and found it to be an excellent lubricant. • Rudolf Hertz (1881) Hert basically investigated rolling friction [7]. He analytically determined the area between two solid surfaces in the elastic domain with geometries defined by quadratic surfaces [3]. • N. P. Petrov (1883) and Tower (1883) Both Petrov and Tower conducted lubrication based studies in journal bearings. Petrov proposed that the friction in an adequately lubricated bearing is due to the viscous shearing of the fluid present in between the surface while Tower introduced the concept of hydrodynamic pressure in journal bearings. • Osborne Reynolds (1886) Reynolds gave the equation for the hydrodynamic pressure which is the basis of hydrodynamic lubrication theory [8]. This theory is the basis for the calculation of bearings. In fact, Reynolds stated that wear can be reduced to almost nil in case of sufficiently lubricated bearings. • Prantl (1928) and Tomlinson (1929) Both of them attempted independently to explain friction based on atomic theory [9]. Their theory laid the stepping stone towards the development of modern nano tribology. Prantl worked in the area of mechanics of plastic deformation. They proposed that during contact between two surfaces, the atoms slide over each other which affect their positions. They didn’t consider wear in their model. Hence, according to them atoms would interact with each other within the intermolecular or interatomic range and won’t move out of their position. During this interaction, the lattice goes into vibration and subsequently generate phonons as well as heat. This dissipated energy is reflected as friction. • Bowden and Tabor (1950) They gave the Junction Growth model to explain friction. The stated that friction force is contributed by two phenomena: • Bearing of the normal load which results in mechanical interaction between the asperities (cohesive forces) • Overcoming the attractive intermolecular forces (adhesive forces) Now, during contact, the pressure at the tip of the contacting asperities makes the asperities deform plastically resulting in the real contact area to enlarge and get cold welded. The body needs to break these contacts in order to move forward which is reflected as the friction force [3].

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1.3.3 Modern Works (1950–1990) Establishment of Tribology as a Science The modern period comprise the works carried out around the period of third industrial revolution. Based on the work done during the previous period, a clear understanding of friction and lubrication was obtained. However, due to lack of sophisticated experimental facilities, many of these theories could not be experimentally validated. Many of the works done validate these theories. Besides, the concept of wear was developed during this period. • Archard (1953) Archard was the first to deal with modelling of wear. He investigated wear problems which produced the classical equation for the volume of worn material [10]. • Bailey and Courtney-Pratt (1955) They made pioneering work in the development of adhesion theory. They were able to measure the adhesion between two surfaces using two mica crossed cylinder method using Newton’s interference fringes [11]. • Dowson and GR Higginson (1959) They provided the numerical solution to the problems of elasto-hydrodynamic lubrication (EHL) which satisfied both elastic deformation of the contacting surfaces as well as hydrodynamic lubrication. • Greenwood and Williamson (1966) Surface roughness is a significant factor in the friction behaviour of a material as well as determining the transition between one lubrication regime to the other. Greenwood and Williamson proposed the first contact model considering roughness [12]. Surface roughness was recognized to play a major role in forming friction and transition from one lubrication regime to another [1]. Greenwood pioneered with first rough contact model in 1966 [12]. • Jost Report (1966) This famous report formally gave rise to the term “Tribology” and highlighted the importance of studying various surface interaction phenomena particularly friction and wear so that machines with higher efficiency could be developed. • Tabor and Winterton (1968) They developed the surface force apparatus to get the first direct measurement of normal and retarded Vander Wall’s forces between two mica sheets for separations less than 100 Å and 200 Å respectively [13]. • Johnson, Kendall and Roberts (JKR model) (1971)

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They combined elasticity with adhesion to model the contact between two surfaces. Their theory is more suitable for contact between a soft surface with a harder one [3]. • Suh (1973) Suh first put forward the delamination theory of wear [3]. • Derjaguin, Muller and Toporon (DMT model) (1975) In addition to the normal load, they also considered the forces outside the actual contact area. This means the inter atomic forces outside the contact zone was considered along with the Hertzian contact model. This theory was more relevant for harder materials in contact [3]. • D Tabor (1977) Tabor proposed the Tabor’s coefficient, a parameter to determine the applicability of JKR or DMT model to define a particular contact between two bodies ending uncertainty of choosing between the two models [14]. • Briscoe and Evans (1982) They performed the first experiments on the shear properties of mono layers of aliphatic carboxylic acids. They introduced the lubricant in between two smooth mica sheets [15]. Their work is significant w.r.t. nano-lubrication and nano-tribology. Apart from the works already discussed, this period also saw application of numerical simulation towards solving or validation of various proposed models in tribology. Numerical scheme was applied to problems of elastohydrodynamics upto point contacts [16]. Besides, the concept of vanishingly low friction theory was proposed [17]. As the importance of energy conservation gradually cropped, developments were made in very low friction bearings [18] using air. Magnetic bearings also were developed. The concepts of surface engineering for customizing surface properties came into existence. As a result, various types of coatings were also developed. Apart from these works, some reputed bodies were also formed during the second and third industrial revolution which carried out some of the outstanding works in tribology and even continuing to do so. Some of these bodies are [5]: • • • • •

Institution of Mechanical Engineers (IMechE, founded in London in 1847), American Society of Mechanical Engineers (ASME, founded in 1880) American Society for Testing Materials (ASTM, founded in 1898) American Gear Manufacturers Association (AGMA, founded in 1916) Society of Tribologists and Lubrication Engineers (STLE, founded in 1944 by ASLE, the American society of Lubrication engineers) • Japanese Society of Tribologist (JAST, founded in1956).

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1.4 Recent Progress in Tribological Research Tribology has developed into a matured subject presently. From what started as solutions to simple problems of friction has evolved into understanding the same in atomistic and molecular level. Researches is this area today has grown both by scope and depth. There are now a wide range of academic journals available covering the multi-faceted aspects of tribology where articles on experimental as well as theoretical works are regularly published. These publications cover physics, chemistry, surface science, nanotechnology, materials science and engineering, biomedical engineering, as well as mechanical and manufacturing engineering [19].

1.4.1 Lubrication Related Advancements Lubrication being an important part of tribology, continuous effort has been ongoing in the development of more effective ways of lubrication. It has been reported that almost 1.0–1.4% of country’s GDP may be saved by undergoing proper research and development in the field of lubrication [20]. This type of report has propelled the advancement in lubrication so that higher efficiency and durability of components could be achieved. In case of lubricants, the research is mainly in two avenues the first one being developing better base oils and second one to find effective additives. After all a commercial lubricant is mixture of a suitable base well with additives in it viz. antioxidants, detergents, dispersants, friction modifiers, antiwear and/or extremepressure additives, and viscosity modifiers [21]. However, there have been continual efforts to find new schemes and types of lubricants. Some of the contemporary works towards the development of lubricants and advanced lubrication schemes are presented in the following text:

1.4.1.1

Additives

In case of liquid lubricants, about 70–90% is the base oil while the rest is additives [22]. Additives when included into the base oil imparts some specific properties into the oil. Sometimes new properties are brought into the oil by additives. While sometimes additives are employed to enhance a particular property already present in the oil. Some additives also help in avoiding/reducing undesirable changes in the oil during its service life. Based on properties induced, some of the additives are discussed as follows: Pour Point Depressant Additives This additive helps the oil retain its fluidity even at lower temperatures. At lower temperatures, there is a tendency of wax formation by the paraffin molecules present

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in oil. This affects the viscosity of the oil. Alkylaromatic polymers and polymethacrylates are commonly used additives in this class [23]. They help in reducing the pour point in modern oils by as low as 30 °C. Anti-Wear Additives As the name indicates these additives prevent wear and scuffing of the surfaces. They help particularly in boundary lubrication regime where there are chances of asperity to asperity contact between the bodies. As the additives are polar in nature, they get attached easily to the metallic surfaces. Subsequently during action, tribo and mechano-chemical reactions occur in this layer forming an anti-wear film on the component surface. This film protects the underlying metallic surface. Phosphorus compounds are normally used as anti-wear additives. However, the most popular additive used for a long time is Zinc dialkyldithiophosphate (ZDDP). However, due to its toxic nature, other additives viz. molybdenum-based additives act as replacements. Antioxidant Additives Oxidation prevention is required to enhance the life of lubricants particularly the components of the base oil. Oxidation is accelerated at higher temperatures. Moreover, the presence of wear debris and other contaminants also promote oxidation in the lubricant. Oxidation may lead to the formation of certain acids and sludge. The acids may corrode the surface while sludge tends to increase the viscosity of the oil. Some of the common antioxidant additives include Zinc dialkyl dithiophosphates, hindered phenols, sulphurized phenols, and aromatic amines. The formation of free radical reaction is hindered by these compounds as well as these compounds decomposes peroxides. Extreme Pressure Additives In some applications where severe sliding takes place, the lubricant may be subjected to higher temperature as well as higher loads. In these cases, due to elastohydrodynamic lubrication or even metal to metal contact, surface damage may occur. Here extreme pressure additives are added to the oil which help in reducing friction and wear. As they prevent surface damage, these additive are also sometimes referred to as anti-scuffing agents [22]. These additives have a chemical reaction with the surfaces in action and form a layer over the surfaces which is also insoluble to the oil. Moreover, the reaction is dependent on the localized temperatures generated by rubbing of the surfaces. Extreme pressure additives generally are sulphur and phosphorous compounds as well as chlorine and boron compounds. Ashless additives viz. dithiocarbamates, dithiophospates, thiolesters, phosphorothioates, thiadiazoles, aminephosphates, phosphites may be preferred in some applications where chlorine may cause corrosion. Apart from these, the lubricant also contains additives which promotes dispersion, prevents foaming and also prevents rust and corrosion.

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Nano Additives

As part of continuous development in liquid lubrication by the industry as well as the research community, the idea of using nano materials as additives in lubricating base oil came up. Nanomaterials due to their small atomic sizes are able to enter in the actual contact zones and create a protective layer which prevents the material from further wear and tear. Moreover, due to their higher surface activity, these materials can be adsorbed on the friction surface thereby resulting in a stable film. Due to these unique physical and chemical characteristics, there has been lot of interest in the use of these materials in the field of tribology. In fact, nano materials as additives have found to enhance the anti-wear characteristic of the oil together with reduction in friction and energy consumption. The nano-additives may be broadly classified into three types [25]: i. Nano-metal based additives

Includes pure metals, metal oxides, metal sulfides, metal hydroxides, and metal salts e.g. Cu, Ag, Fe, Pd, Ni, CuO, ZnO, Al2 O3 , TiO2 , ZrO2 , WS2 , MoS2 , CuS, ZnS, CaCO3, LaF3

ii. Nano-carbon based additives

Pure carbon: Nano Diamond, Fullerenes, Carbon Nanotubes, Graphene Polymer: PTFE, PSS, PVP

iii. Nano-composite based additives

Cu-SiO2, Al2O3 -TiO2, Cu -MoS2, G -MoS2, α-Fe2O3 -GO, FeS2 -G, Ag -G, Cu -GO, Mn3O4 -G, La2O3 -PI, Alumina -MWCNT

Some researchers have investigated the efficacy of using 2D nano additives in the lubricating oil. Nano materials having layered structure are frequently used as solid lubricants viz. graphite and MoS2 . In these materials, the shear strength between two layers is low due to weak Van der Waals force [26]. Hence, the layers can easily slide over each other thus providing the lubricating effect. In the same layer, atoms are bonded by covalent bonds rendering high modulus and strength to the structure [24]. Among the nano-additives, 2D nanomaterials have relatively higher specific area. This is an advantage as these materials can cover a larger area when adsorbed on the component surface. This way they can give higher protection to the surface and reduce the probability of metal to metal contact during sliding [27]. Figure 1.5 illustrates the application of 2D nano additives in liquid lubrication and its probable mechanisms. The performance comparison of a 2D nano additive (liquid like graphene) with pure water and graphene oxide (GO) is presented in Fig. 1.6. It is observed that liquid like graphene can achieve about 91% reduction in wear rate compared to water.

1.4.1.3

Ionic Liquids

Lubrication scheme using Ionic Liquids (IL) was initially explored in 2001 [29]. ILs are salts which remain in molten state below 100 °C and particularly at ambient

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Fig. 1.5 2D nanosheets as lubricant additive and possible mechanisms for reducing friction and wear [24]

Fig. 1.6 Performance comparison of liquid like graphene as additive with pure water and graphene oxide (GO) [28]

temperatures. They are called ionic as being a salt, the liquid consists of cations and anions. Hence, ILs have special properties viz. inherent polarity for strong surface adsorption, low volatility, higher thermal stability and lower flammability. Besides, unlike conventional liquid lubricants, ILs have low sensitivity (in terms of rheological behaviour) towards environmental variation. However, ILs alone are not economically viable to be used as lubricants. Again, ILs aren’t readily soluble in commonly available nonpolar hydrocarbon lubricating oils. However, with advancement in research, some oil soluble ILs were reported around 2012 [30]. Those ILs also showed potential anti-wear capabilities. These resulted in new direction in research for lubricants where ILs were used as additives in oil. Figure 1.7 shows the structure of some of these additives.

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Fig. 1.7 Structures and abbreviations of cations and anions of some ILs used as lubricant additives [21]

1.4.2 Super Lubricity Efforts have been continuously underway to reduce friction to save energy and increase the efficiency of various mechanical systems. The research on super lubricity is a step in this direction. Super lubricity is the condition in which friction is very low and is almost imperceptible. Actually, super lubricity occurs in a sliding regime in which physical and/or chemical interactions are so small that frictional resistance essentially is absent. This would yield tremendous energy savings as well as wear resistance. Further this would result in high efficiency machines with longer device

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service life. Super lubricity is one of the fastest developing fields in the recent years and if developed may be an important milestone in the history of tribology. Several theories have been proposed to explain the phenomenon of super lubricity. Atomistic theory is one of them. Super lubricity can be categorised broadly into two types:

1.4.2.1

Solid Super Lubricity

Diamond like carbon (DLC) film is already known for their solid lubricating capabilities. They have been further researched for lowering the friction significantly and for application in for super lubricity. In case of DLC, research have extended mainly in two avenues viz. DLC-based emerging lubricants and DLC-related lubricity mechanisms [19]. The research on DLC films have opened up scopes for developments of several lubricants. Besides, various two-dimensional materials viz. graphene, carbon nanotubes, etc. seem to have potential for super-lubricity related research [19]. These materials have weak inter layer connection leading to very low shear resistance. Super lubricity can be achieved in many different systems and length scales. This has opened several new avenues for design of new mechanisms for this phenomenon.

1.4.2.2

Liquid Super Lubricity

The mechanism for super-lubrication in case of liquids is dependent on mechanisms viz. hydration effect, chemical reaction layer, hydrodynamic effect, double electric layer interaction, etc. as well as a mixture of these mechanisms [19]. Commonly available liquids viz. water, acids, alcohols, acids, oils, etc. can act as base for the lubricant.

1.4.3 Surface Engineering Surface engineering is modifying the surface so that primarily a suitable friction and wear behaviour as needed in particular application can be obtained. However, surface engineering also involves changing the surface characteristics viz surface texture and imparting other properties viz. physical, chemical, electrical, electronic, magnetic, and corrosion-resistant properties depending on the application. Naturally, these types of requirements are faced by wide range of industries viz. automotive, aerospace, missile, power, electronic, biomedical, textile, petroleum, chemical, steel, cement, machine tools and construction industries. Surface engineering methods may be classified as follows [31]: • Modification of surface with no compositional change

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– Transformation hardening – Surface melting – Surface texturing • Modification of surface involving compositional change – Solid solution and precipitation modification via diffusional processing – Formation of surface layers by thermochemical reactions with component material – Formation of surface layers by electrochemical reactions with the component material • Coatings deposited on component surface – Coatings deposited from a solution of ions – Coatings deposited in the liquid state – Coatings deposited in the solid state

1.4.4 Advanced Surface Engineering Techniques Surface engineering has enormous potential to satisfy the requirements of various tribological applications. It can be highly customized for applications on case to case basis. It is found to be highly relevant in the domain of materials technology especially in aerospace, automotive, bio-medical and engineering applications recently. Although a lot of superior techniques have been developed viz. physical vapour deposition (PVD), chemical vapour deposition (CVD), diffusion processes, thermal spray, sol–gel, etc., still scope exists to establish relationship between the process parameters and the properties of the surface obtained. Besides, the increasing demands from industry require evolution of the surface engineering techniques so that components with high functional density could be achieved. This has given rise to multilayer and nano-structured coatings/nano-surfaces.

1.4.5 High Temperature Tribology Studying the tribological behaviour of materials under high temperature conditions are becoming important with respect to applications viz. automotive, aerospace, power generation, metal working, etc. Under high temperature conditions, especially when the ambient temperature is above the recrystallization temperature of the materials participating in tribological interaction, microstructural changes may be induced in the materials. This may lead to completely different tribological behaviour from the participating materials compared to normal room temperature conditions. Besides, a moderate high temperature condition may lead to grain softening and other phenomena which may further result in modified friction and wear behaviour of the

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Fig. 1.8 Test setup for high temperature reciprocating friction and wear tester [32] (under CC BY licence)

materials. Lubrication also poses a challenge in case of high temperature conditions. Based on temperatures generated, conventional liquid lubrication isn’t possible in some applications which make the situation more challenging. However, the reliable functioning of the components is still expected and that too for an extended period of time. The tribological phenomena are greatly affected by high temperature. High temperatures may generate frictional stresses which can lower the efficiency of a system. It is also to be kept in mind that high temperature situation can also be created by frictional heating without a sufficient heat dissipation rate. Elevated temperature can also aggravate the wear from materials as the material may soften in high temperature conditions. Various materials are researched which can withstand higher levels of temperatures. Moreover, suitable incorporations viz. refractory materials, etc. are made into surface coatings as part of surface engineering so that parts can display a stable friction and wear behaviour. Also, there is challenge to conduct friction and wear tests under high temperatures. Special tribo-testing apparatus have been developed to conduct these tests under laboratory scales (refer Fig. 1.8). Many studies have been conducted in the area of high temperature tribology based on the needs of the industries. For automotive parts, lightweight materials are preferred from the point of fuel consumption. For this, materials viz. boron steel and ultra-high-strength aluminium alloys have been employed. The new generation coatings based on AlTiN are highly wear and abrasion resistant in high temperature conditions also.

1.4.5.1

Lubrication for High Temperature Tribology

Selection of lubricant for high temperature application depends on many factors. Significant factors are viscosity, thermal stability and resistance towards oxidation. The lubricant ideally should have a very high viscosity but at the same time can be

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pumpable at normal temperature conditions. When exposed to elevated temperatures, the lubricant should be stable and should not leave residue on the hot surfaces. Moreover, it should not oxidize at higher temperature which implies hydrocarbonbased lubricant can’t be the choice for high temperature applications. Even most of the popular base oils have thermal decomposition temperatures below 400 °C. A few lubricants viz. polyphenol ethers and silicates can be used at temperatures above 400 °C but they are solids at room temperature. The dropping point for most of the greases are also reported to be well below 400 °C. The dropping point is the temperature above which the grease loses its gel like consistency and behaves like a liquid. Hence, solid lubricants are the only choice for high temperature lubrication. Service temperature of some solid lubricants may reach up to 1000 °C. In the modern day tribological systems, the component surfaces and even cores which intend to be in tribological interaction during the service conditions make use of solid-lubricating materials for high performance, efficiency, and durability. This makes them usable in a variety of tribological conditions over a wide range of temperature without suffering much wear and tear. The solid lubricating materials suitable for high temperature applications mainly comprise of a high temperature matrix material, high temperature solid lubricants as well as some supplementary components [33]. There are a variety of fabrication techniques to prepare the solid lubricant. Now, there are some challenges with respect to these techniques as the materials are sensitive to normal environmental conditions. Hence, research has advanced towards finding a novel material which can be used for making solid lubricants. In general, the lubrication mechanism of solid lubricants may be of the following types [33]: a. b. c.

Materials in the form of layered structure with very low interlayer force (e.g. graphite, MoS2 ) Softer metals with multiple slip planes (e.g. Ag, Au) Fluorides and oxides of metals with thermal softening (e.g. CaF2 , PbO, AgMoO4 ).

Materials with layered structure are found to be effective at service temperatures below 400 °C. Above this temperature oxidation occurs in the materials. Fluoride, graphite and WS2 can withstand temperatures up to 500 °C. Hexagonal Boron Nitride (hBN) can further withstand temperatures reaching up to 1000 °C. Fluorides and oxides of metals although quite effective in controlling friction and wear at elevated temperature, but fail to do so at room temperature or below. Figure 1.9 presents the working temperature for a variety of solid lubricants. Ceramic materials have been included among solid lubricants for enhancing the thermal stability of the lubricants for temperatures touching 1000 °C. Ceramic materials have already proven themselves for controlling friction and wear and hence used in the new generation bearings. Besides, ceramics can effectively withstand corrosion as well as oxidation. The hardness of structural ceramic is found to be around 15–30GPa which is found to remain effective for higher levels of temperature as well [33]. Hence, this type of material can be suitable candidate for making solid lubricants for elevated temperature applications. However, ceramics are prone to micro fracture which is a major challenge as far as wear resistance and longevity

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Fig. 1.9 Working temperature range for solid lubricants [33]

of the component is considered. One of the ways to achieve these properties is to have a ceramic matrix with suitable incorporations in the form of composites. These matrices are mainly ZrO2 , Al2 O3 and SiC based [33].

1.4.6 Computer Simulations of Tribology Phenomena Traditionally the tribological behaviour of a material is analysed through experimentation. The friction and wear behaviour can be obtained by using tribological testing devices which can have various contact configurations as desired. The most common of them is the pin-on-disc setup although various other setups like block on ring, ring on ring, ball on disc, etc. are available. Based on the configuration, the motion generated could be sliding or rolling. These setups allow the determination of mainly the coefficient of friction for a particular material pair as well as the wear rate. Some setups are also available with special arrangement and chambers so that the tests can be conducted at a specific temperature or environment. The wear mechanism can also be determined by observing the tested samples under high resolution microscopes. However, with the advancement in computer systems and increase in the computation power, simulation based techniques have gained popularity. Computer simulation allows to analyse a particular system without actually realizing it physically. Computer simulations have found broad applications in tribology viz. predicting wear of various systems, including cutting tools, bearings and artificial joints. Besides, various contact mechanics problems can be conveniently modelled and solved using these techniques. Nowadays, due to the introduction of nano tribology, tribological problems which were already multi physical in

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Fig. 1.10 A time-versus length-scales map of models developed in tribology [2]

nature now vary a lot in scales. This micro and nano scale studies along with multi scale problems can quite efficiently be handled by these simulation tools which the researchers are increasingly embarking upon. Figure 1.10 illustrates various tribological models built across the scales. There are many simulation tools which can analyse the tribological scenarios quite effectively. The most commonly used ones are the finite elements and boundary elements method and molecular dynamics simulation.

1.4.6.1

Finite Elements and Boundary Elements Method

The finite element method (FEM) and boundary element method (BEM) are popular tools used worldwide to solve various problems of mechanics and design. In FEM, an explicit relationship between stress and strain is considered with a finite strain formulation. Instead, in case of BEM, the relationship between force and pressure is considered with displacements in two orthogonal directions [41]. Several researchers have used this technique to investigate various problems in tribology a few of which are listed in Table 1.1.

1.4.6.2

Molecular Dynamics Simulation

Molecular dynamics simulation (MD simulation) is a method of simulating and analysing the movements of atoms and molecules in a computer system. This is an atomistic approach towards understanding and explaining the behaviour of a system. In this method, the atoms and molecules are allowed to interact among themselves as they would in the real system but for a fixed period of time. This gives a view of

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Table 1.1 Application of FEM/BEM for studying tribological problems S. No.

Tribological issues

Simulation tool employed

Researcher/Research group

1

Effect of tool geometry on Ti6Al4V tool wear

FEM

Ducobu et al. [34]

2

Prediction of abrasive wear on steel against various types of copper ore

BEM

Perazzo et al. [35]

3

Effect of friction on relative wear on mining hopper

BEM

Rojas et al. [36]

4

Prediction of fretting fatigue FEM and wear

Zhang et al. [37]

5

Simulation of scratching on polycarbonates polymer composite

FEM

Krop et al. [38]

6

Modelling fatigue life and wear on railway tracks

FEM

Lian et al. [39]

7

Prediction of the damage on fiber-reinforced polymers caused by adhesive wear

FEM

Din et al. [40]

the dynamic evolution of the system. The path traced by the atoms and molecules are evaluated numerically using classical equations of motion. The interaction between the atoms and molecules are studied by considering interatomic potentials or molecular mechanics force fields. The MD methods have been successfully employed in fields of chemistry, materials science and biophysics. In case of tribological research MD methods have well complemented the laboratory work. This is particularly due to the information about individual atomic interaction provided by this method and which is very much relevant to tribological interactions between two surfaces. Even though numerical methods have got a huge boost due to increase in the computational power, they sometimes face challenges due to time scale and system sizes. In a model system, there are typically thousands of atoms and system sizes in the range of tens of thousands of atoms. These restricts the typical size of the simulated tip as well as the sample below tens of nano meters in any direction [42]. Lately, MD simulation have become a popular tool for simulation of tribological system particularly through atomistic approach. Along with MD method, the first-principle calculations (computing atomic relationships via quantum mechanics) have also gained popularity in case of tribological simulations. These methods have been used to determine the molecular interactions at surface interfaces. These, simulation methods have been found to be particularly useful in explaining the actual dynamics occurring during friction and wear. However, these methods are also useful in studying the lubrication behaviour viz. boundary slippage occurring in lubricant rheology.

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Fig. 1.11 Rolling and deformation of diamond and SiO2 nano particles. (upper block hidden for better visibility) [43]

Tribological behaviour of hard nano particles viz. diamond and silicon di-oxide (SiO2 ) are simulated using MD technique (Fig. 1.11) [43]. The nano-particles are pressed between two iron blocks under a load of 1000 MPa. Apart from the naturally evident atoms, blue atoms are used to visualize deformation. Some part of the nanoparticles is colour coded in yellow to mark the rotation of the nano particles. The nano-particles are found to separate the two blocks and undergo minimal plastic deformation. However, the diamond particle maintained its spherical shape owing to its higher hardness. On the other hand, SiO2 particles are found to be deformed and crushed under the applied load. The yellow coded atoms reveal that even after application of load, diamond particle is still able to roll and acts as ball bearing between the two plates. But, the high deformation of SiO2 particle prevents it from rolling. In order to give rise to physically realistic models, researchers have presented multifactorial models where many factors can be analysed at the same time. However, these types of analysis require a large amount of computing resources which may increase the associated costs.

1.4.7 Biotribology As the research in tribology went intense, the various tribological interactions within the human body caught the attention of the researchers. This included, the phenomena occurring at the various joints of the human body, the teeth, the interaction due to the rubbing of the eyelids with the eyeball and so on. This encounter of tribology with the medical domain has given birth to the topic of bio-tribology. Bio tribology is one of the growing fields in the area of tribology. As already mentioned it is devoted towards understanding the natural processes and how they work and function. How, diseases are developed and what medical solutions may

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Table 1.2 List of major topics in bio tribology research [19] (under CC BY 4.0 licence) Classification type

Major Investigations

Joint tribology

Natural synovial joints, articular cartilage, synovial fluid, mucin, and artificial replacement, etc

Skin tribology

Skin friction behavior, moisturiser and cosmetics, skin pathology, textile material, prosthesis, and tactile perception, etc

Oral tribology

Natural teeth, tongue, saliva, implant teeth, and dental restorative materials, etc

Tribology of other biological system

Tribology of other human bodies, medical device, animal tribology, and plant tribology, etc

be suitable. Bio tribology encompasses a wide variety of areas. However, the main areas of research currently can be classified as presented in Table 1.2.

1.4.8 Biomimetics Tribology 1.4.8.1

Hydrophobic Coatings—from Lotus Leaf

The surfaces which repel water are known as hydrophobic surfaces. In nature, lotus leaf is a very common example of having this type of surface. These surfaces normally provide the necessary roughness along with low surface energy which provides it the ability to repel water [44]. In case of superhydrophobic coatings, water can fully bounce upon hitting the surface. Tulip poplar leaf surface possess super hydrophobicity as can be seen in Fig. 1.12a. Figure 1.12b on the other hand shows a water droplet on a super hydrophobic coated surface. These coatings are made of composite

(a)

(b)

Fig. 1.12 Water droplet on (a) Tulip poplar leaf and (b) super hydrophobic coated surface [44]

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materials which can provide the optimal combination of surface roughness and surface energy. Some of the common bases for these coatings include: • • • •

Manganese oxide polystyrene (MnO2/PS) nano-composite Zinc oxide polystyrene (ZnO/PS) nano-composite Carbon nano-tube structures Silica nano-coating

Silica based coatings are found to be amongst the cost-effective options available. The gel form of the coatings is easy to use as they can be applied to the surface of the object easily by dipping the object in the gel or through spraying. There ae many utilities for these types of coatings. In the form of paints, they help in making various surfaces water repellent viz. umbrella, shoes, building materials, etc. In fact, various clothing can be made breathable and water repellent. This would eliminate the problem of sweating or getting the clothes drenched in rainy conditions. These coatings are further used in case of self-cleaning windows and lenses. This prevents moisture formation which is a problem in case of windshield of vehicles or mere glasses. Further, the glass can be cleaned by simply spraying water over them. The technique of self-cleaning is also useful for solar panels which many times are covered with dust over time. Hydrophobic coatings are also found useful in de-icing as the ice doesn’t stick to the surface. This is particularly useful in cold countries where icing is a common problem. The cost involved in the de-icing procedure can be saved along with time and effort. As these coatings repel water, they prevent the formation of bacteria colony on surface and hence prevent bio-fouling as well. Besides, barnacles and mussels also can’t get attached to these surfaces.

1.4.8.2

Riblet Effect—from Shark Skin

The shark skin has microscopic scale like structures (Fig. 1.13) which lets water pass through them without forming vortices and hence reduces drag while swimming [46]. This technique has been effectively used in many air crafts sea vessels and even

Fig. 1.13 Scales on the skin of various shark species [45]

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by the automotive industry. Airbus implemented riblet design for their aircraft wings and reported 6% lesser air drag which leads to significant savings in its fuel. Some swimsuit companies have taken inspiration from the riblet design for designing their range of swimsuits.

1.4.8.3

Adhesive Surface—Gecko Effect

The remarkable adhesive strength displayed by the geckos in climbing walls has been the subject of interest for many researchers. Researchers have in fact investigated at the microstructural level and have found that the feet of geckos contain a complex hierarchical structure of lamellae, setae (microscale hairs), branches, and spatulae [48]. In each of the toes of the gecko, around 1.5 million setae are present. These setae further branch off into 100 – 1000 nanoscales spatulae (refer Fig. 1.14). Considering the surface area of each of the spatula, the cumulative area of contact becomes very high which leads to higher adhesion on various surfaces.

1.5 Spin-Offs from Research on Tribology The research in tribology till date has already resulted in many spin-offs which has been very useful to the society at large. A few of them are listed in the following texts.

Fig. 1.14 Gecko feet in detail (a) Rows of setae (ST) and (b) branches (BR) and (c) terminal spatulae (SP) from each seta [47]

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Fig. 1.15 Hard disk drive (courtesy Evan-Amos under CC BY-SA 4.0 licence) https://upload.wikimedia. org/wikipedia/commons/f/f8/ Laptop-hard-drive-exposed. jpg

1.5.1 Hard Disk Drive Technology The advancement of computers has revolutionized the human civilization in the last few decades. With this advancement, the requirement of storing up of digital data came up. Research and knowledge in tribology proved to be very significant towards development of this technology. In a typical hard drive there are many discs which are mounted on the same spindle (Refer Fig. 1.15). A slider mounted on an actuator arm slides on the discs when the drive is in operation. The slider is a magnetic head and its contact with the disc results in reading and writing of data. Failure at the contact interface may result in erasure of the date. Moreover, generation of wear particles at the slider and disc interface may enter into some critical part leading to catastrophic failure of the whole system. Research in tribology has led to the development of protective carbon overcoats and boundary lubrication which maintain at least a monolayer of lubricants on the disc surface. Moreover, the topology of the disc can also now be precisely controlled. All these resulted in holding the recording head at a minimal gap from the disc surface thus reducing wear and tear considerably [49]. This has led to the prolonged life of the storage media as well as their increased reliability.

1.5.2 Ceramic Bearings Bearing technology is highly dependent on the knowledge in tribology. In fact, due to the progress in tribology, bearing technology has improved and given rise to next

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Fig. 1.16 Ceramic bearing [50] (CC BY 3.0 licence)

generation of bearings. Ceramic bearing (refer Fig. 1.16) is one of them and they have the following advantages: Ability to Handle High Speed Compared to metallic bearings, ceramic bearings are advantageous as they can handle higher speed efficiently. This is because ceramic bearings present lower rolling resistance due to smoothness. This smoothness is due to the precision manufacturing which is possible in case of ceramic parts. They have higher dimensional accuracy over metallic, particularly steel bearings. This aids in distributing the service loads uniformly over all the rolling elements of the system. Moreover, ceramic material possesses lower coefficient of friction (about 20–30 times lower) than steel ball bearings under standard conditions of lubrication. Lighter Than Steel Bearings As a material, ceramic is much lighter compared to steel. Typically, they weigh about 40% lesser compared to steel counterparts. This lighter weight translates to lower centrifugal forces on the outer race when the bearing is in operation. Due to these lower forces, ceramic bearings are able to 20–40% faster than the conventional steel bearings. Or in other words, they consume relatively lower energy to maintain the same speed. Higher Stiffness Than Steel Bearings Ceramic bearings are also found to be stiffer compared to steel bearings and hence are more durable. Their service life is in fact roughly about 5–20 times longer than similar steel bearings. Smooth surface of ceramics reduces the risk of bearing seizure in case of limited lubrication. Moreover, ceramics bearings are known to be inert to most of the chemicals and hence can operate in harsh environment. Electrical insulation of ceramics also makes these bearings resistant against electrical erosion and pitting. The development of ceramic bearings has improved various mechanical systems operating at high speed and at extremely high precision viz. ultracentrifuges, machine

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tool spindles, turbomolecular pumps, etc. NASA is reported to have increased the longevity of fuel feed pumps for their space missions by replacing them with ceramic bearings [49].

1.5.3 Durable Implants The knowledge of tribology isn’t limited to machinery and mechanical equipment only. The knowledge of tribology has been successfully implemented in case of biomedical applications. This has led to the development of high quality and durable implants. According to www.nature.com, “Biomedical materials are biomaterials that are manufactured or processed to be suitable for use as medical devices (or components thereof) and that are usually intended to be in long-term contact with biological materials.” Thus, along with bio-compatibility the implants particularly those used in joints should have high resistance to wear and tear. This would enable higher life of the implant along with safety to the patient receiving it. Knowledge of tribology enabled the selection of proper materials for biomedical implants. The suitable materials for joint replacement include Ultra High Molecular Weight Polyethylene (UHMWPE), Ceramics, Titanium and its alloys, etc. (Fig. 1.17). Ceramic on ceramic has been found to be very suitable in case of hip arthroplasty [51]. Ceramic has the perfect combination of hardness and durability and is highly resistant to wear. Besides, it doesn’t have toxic effects on the body. Ceramics on plastics (UHMWPE) has also been recognised as good combination of materials for

Fig. 1.17 Implants for hip arthroplasty (courtesy Science Museum Group, UK under CC BY-SA 4.0 licence) (https://upload.wikimedia.org/wikipedia/commons/4/47/Hip_joint_replacement%2C_ United_States%2C_1998_Wellcome_L0060175.jpg)

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arthroplasty. This combination has a potential wear at a rate of about 0.05 mm each year, which is 50% lesser than metal on polyethylene [51].

1.5.4 Development in Micro-electromechanical Systems (MEMS) Miniaturization and multifunctionality are the two keywords of modern-day engineering applications. Development and application of MEMS have seriously contributed towards these objectives. However, proper application of MEMS requires the understanding of friction and wear behaviour of surfaces at nano/micro levels. In fact, the extensive application of MEMS is often limited by poor tribological performance and adhesion issues. This is due to the high surface-to-volume ratio in MEMS and is typically a problem in case of actuator-based MEMS applications where inconsistent or high friction restrict the smooth movement of the components. Moreover, surface forces viz. meniscus force, surface tension, viscous drag and adhesive forces are quite significant comparing the size scale of the devices and components. These forces can have a large influence on the integrity of the devices thereby decreasing its performance and durability. Besides, the conventional lubrication techniques can’t be used in these types of systems. An additional challenge is, silicon, which is a popular material in this type of applications and has poor tribological properties. Based on these challenges, research has been pursued. It was clear that surface forces need to be minimized in order to have improved performance and life span of MEMS devices. Hence, investigation on modification of surface either by change in topography or chemical processing is carried out. Topography modification include the change in the surface roughness by texturing or similar so that friction behaviour could be altered. Chemical modification of the surfaces through coatings is also seen to yield promising results. Some of the suitable thin film coatings found suitable in case of MEMS include ionic liquids (IL), diamond-like carbon coatings (DLC) and self-assembled monolayers (SAM). Hence, tribology has contributed significantly towards the development of MEMS based devices.

1.6 New Paradigms in Tribology and Its Future 1.6.1 Trends in Lubrication 1.6.1.1

Liquid Lubrication

At present liquid lubrication are developed and improved keeping in mind the internal combustion engines (ICE) and drive trains. However, as environment friendly Electric Vehicles (EV) are gradually coming into the focus, the lubrication technology also

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needs a paradigm shift. This type of vehicles is driven by electric motors and due to the higher power density of the smaller gear boxes, there are heating issues. Hence, the lubricant should be able to efficiently cool the engine. Besides, right now the design variation of these engines is considerable and hence targeted use of lubricants based on a particular engine is required. The lubricant required in EVs have greater technical requirements compared to that of ICEs. Apart from acting as coolants, the lubricants should also possess good anti-wear capabilities, lowering friction, having electrical combability and insulation. EVs require lubricants in vital electrical components such as coolants for the car battery, gear oils for differentials, chassis, gear reducer, and wheels, brake fluids, and grease for other components of the EV. Electrical combability of the lubricant in the rolling element bearing of EVs help in mitigating the problem of electro-corrosion resulting from high frequency and high energy discharges. Ionic liquids have shown potential in preventing this type of charge accumulation [23].

1.6.1.2

Solid Lubricants

Although great strides have been taken for the development of lubrication schemes for high temperature tribology, there are challenges faced by the industry in regards to their fabrication and implementation. The fundamentals of tribological mechanisms occurring at high temperature is yet to be understood fully. The basics are still majorly based on the understanding of solid lubrication theory. There are possibilities of further reduction in friction and wear and future materials should be directed towards this direction. In general, a friction coefficient < 0.2 and wear rate 80 20

Brass 15–30

(2.31)

which does not include any reference to the shape of the uncut chip cross section. Although Hippler regarded this formula as correct one, he used other formula in his further considerations C σ =√ 4 Ac

(2.32)

As such, he did not explain in any of his publications why and how he found it possible to use his formula (Eq. (2.32)) instead of that by Friedrich (Eq. (2.31)). Table 2.3 shows the values of constant C according Hippler. No conditions or test details under which these values were obtained were given by Hippler, who was extremely brief with explanations of his formulas and results. Therefore, according to Hippler, the cutting pressure is calculated as C Ac = C A3c / 4 P = σ Ac = √ 4 Ac

(2.33)

Even less explanation was provided by Hippler for the cutting speed. It is worth to mention that he, for the first time, suggested that the equations for the cutting pressure and the cutting speed have the same structure. He wrote: “Professor Friedrich gave the following equation for the cutting speed that we can use v=

e √ ω1 + k Ac

(2.34)

or in a simplified form k v=√ 4 Ac

(2.35)

The values of k are given in Table 2.4.” That is all his explanation on the cutting speed. The values of k shown in Table 2.4 are given by Hippler with no explanations how they were obtained.

2 Cutting Force Modeling: Genesis, State of the Art …

57

Table 2.4 Values of k according to Hippler Work material

Cast iron

Iron

Steel

Bronze

Brass

Tensile strength 8–10

10–20

38–40

40–50

50–60

60–70

70–80

>80

20

15–30

Soft

50

33

70

62

–

–

–

–

–

100

Hard

39

15

–

45

40

32

22

15

100

80

It needs to point out that many publications by Hippler contain contradictions and incoherent/ illogical statements so that it’s very difficult to follow the way of his thinking. Nevertheless, Hippler’s idea that that the equations for the cutting pressure and the cutting speed have the same structure was adopted in the above mentioned AWF tables and by Kronenberg.

2.4.3 Kronenberg Considerations The Kronenberg considerations are discussed in his fundamental book [14]. His considerations for the equation for the cutting speed differ from that of Friedrich. He developed an equation for the cutting speed using Taylor experimental data and, the same as Hippler, suggested that the equation for the cutting force has the same structure as that for the cutting speed. To derive his equation for the cutting speed, Kronenberg uses the experimental data presented by Taylor in his classical work “On the art of metal cutting”, 1907 [9]. In this work, Taylor presented extensive tables for practical cutting speeds (the cutting speed corresponding to a given tool life). As such, a separate table is given for each work material and the size of the turning tool. Needless to say that the uncut chip thicknesses and widths are separated in these tables. For a given depth of cut, series of the cutting feeds is indicated and the cutting speed corresponding to 90-min tool life is given for each cutting feed. Using these tables and multiplying the depths of cut by the cutting feeds, Kronenberg obtained the uncut chip areas and then assembled the same areas in groups. Obviously, he obtained different cutting speeds for the same uncut chip area as can be seen in Table 2.5 for a 7/8 cutter for the depth of cut, cutting feed, chip cross-sectional area (in modern designations). The obtained results did not make Kronenberg to re-consider his approach so he continued his derivation. He plotted the obtained data in double logarithmic coordinates where the chip cross-sectional area was in the x-direction and the cutting speed was in the y-direction. He found that the obtained scattered points were located on inclined straight lines. For the cutting speed, Kronenberg adopted the equation for the cutting speed suggested by Hippler (Eq. (2.35)) with some modification, which changes the root square of Ac into the root of certain power ε, i.e.

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V. P. Astakhov

Table 2.5 Example of Kronenberg calculations using the Taylor experimental data Depth of cut

Feed

Chip cross-sectional area

Cutting speed for cast iron

ap

f

Ac

soft

moderate

hard

2.38

4.79

}1.9

51.5

25.8

15.1

4.76

0.4

53.4

27.1

15.9

2.38

1.59

37.2

18.7

10.9

4.76

0.79

41.8

20.9

12.2

9.52

0.4

43.9

21.9

12.8

2.38

3.18

26.3

13.2

7.7

3.18

2.38

28

14

8.2

4.76

1.59

30.3

15.2

8.8

9.52

0.79

33.8

16.9

9.8

}3.8

}7.6

C v=√ ε Ac

(2.36)

Takin the log of both sides of this equation, one can obtain lg v = lg C −

1 lg Ac ε

(2.37)

For the considered graph construction, lgv = y, lgAc = x and designating constants -1/ε = a and lgC = b, one can obtain y = ax + b

(2.38)

i.e., an equation of a straight line. The first stage of his task was completed. Having obtained this equation for the cutting speed, Kronenberg tried to find the values of C and ε using the experimental data by Taylor, AWF, and the calculated results using equation by Friedrich and Hippler. As a result, a collection of various values of C and ε was obtained. Table 2.6 shows an example for steels of intermediate hardness. Table 2.6 Results for ε and C obtained by Kronenberg Using Taylor data Carbon tool

High-speed Steel tool

Using AWF data

Using Friedrich data

Using Hippler data

ε

C

ε

C

ε

C

ε

C

ε

C

1.76

5.4

1.94

80

2.44

35

2.3

46

4

23

–

–

1.94

64

–

–

–

–

–

2 Cutting Force Modeling: Genesis, State of the Art …

59

Analyzing the obtained results, Kronenberg wrote “Some values of C and ε show satisfactory agreement with each other but still significant differences exist for others.” In reality, however, there is no one “satisfactory agreement” for any work material analyzed in his tables. The equation for the cutting pressure (force) used by Kronenberg is the same as above-discussed equation proposed by Wiebe, and then used by Friedrich and Hippler, i.e., this pressure is proportional to the (uncut) chip cross-sectional area, Ac , i.e. P = σ Ac

(2.39)

As per the unit cutting pressure, Kronenberg (the same as Hippler) thought that it can be expressed by an equation having the same structure as that for the cutting speed, i.e. C1 σ = √ ε1 Ac

(2.40)

As before, C 1 and ε1 are constants to be determined experimentally. To do that, Kronenberg used the test results obtained by Taylor, Klopstock [22], AWF, and others and the equations by Friedrich and Hippler. It is of particular interest how he fit the experimental results by Taylor. Taylor obtained his equations for the cutting pressure (force) through a very extensive experimental program carried out for more than 25 years and come out with the following results: For hard cast iron f 3/4 P = 138a 14/15 p

(2.41)

f 3/4 P = 88a 14/15 p

(2.42)

P = 200a p f 14/15

(2.43)

For soft cast iron

For steel

These equations show that the depth of cut, ap and cutting feed, f have different influence on the cutting pressure (force)2 so they did not in agreement with the Kronenberg approach according to which they should have the same influence so that their product, known as the (uncut) chip cross-sectional area, Ac can be used 2

Arguing with Nicolson about the influence of the (uncut) chip thickness, Taylor wrote “We have gone to great length in the paper to make it clear that it is the thickness of the chip which is the main factor, in allowing high cutting speeds for tools with broad cutting edges. And yet, Mr. Nicolson claims theta neither the thickness of the chip not the shape of the cutting edge of the tool need to be particularly considered in the problem”.

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V. P. Astakhov

in calculations of the cutting pressure (force). This did not stop Kronenberg to use these data. He proceeded as follows. All cross sections of the chip in the experimental results by Taylor, were separated into three groups: first is the square chips having the ratio ap /f = 1, thin chips having the ratio ap /f = 1/10, and thick chips having the ratio ap /f = 10/1. After this, the following manipulation was carried out with each of the Taylor equations. For hard cast iron f 3/4 P = 138a 14/15 p

(2.44)

When the chip is square f = ap , so that f 3/4 = 138 f 1.683 P = 138a 14/15 p

(2.45)

On the other hand, f · ap = Ac so that a 2p = Ac or ap =



Ac = A1/2 c

(2.46)

and then = A1/2·1.683 = A0.842 a 1.683 p c c

(2.47)

Substituting this result into Eq. (2.45), Kronenberg obtained P = 138A0.842 c

(2.48)

The unit cutting pressure is then calculated as σ =

138 P = 138A0.842−1 = 138A−0.158 i.e. σ = 6.34 √ c c Ac Ac

(2.49)

For thick chips (the second group) f = ap /10 so P = 138a 14/15 p

 a 3/4 p

=

138a 1.683 p

(2.50) 10 100.75

On the other hand, f ·ap = Ac so that a 2p 10 = Ac f 2 /10 = Ac or a p = (10 Ac )1/2 . Substituting these results into Eq. (2.50), Kronenberg obtained P=

138 · 100.842 A0.842 c = 170 A0.842 c 100.75

and the corresponding unit cutting pressure is calculated as

(2.51)

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61

Table 2.7 Coefficients C 1 and ε1 for the steel of moderated hardness Work material

Using Taylor data C1

Steel 50–60 kg/mm2

C1

C1

Using kurrein data ε1

C1

216 200 185 28.5 260

σ =

Using Using AWF data friedrich data

Using hippler data

ε1

C1

ε1

C1

ε1

C1

ε1

6

160

7.8

198

15

240

4

P 170 = 6.34 √ Ac Ac

(2.52)

For thin chips (the third group) f = 10ap . Using the same way, Kronenberg obtained σ =

112 √ Ac

(2.53)

6.34

As can be seen, Kronenberg obtained three equations for the same work material. For the results of the meticulous Taylor’s tests where each feed and depth of cut are accounted for, a rough averaging is used, i.e., the Taylor and other researchers’ approach on the different influence of the depth of cut and cutting feed on the cutting pressure (force) are completely extinguished. The same way was used by Kronenberg to manipulate with the experimental results obtained by Taylor, Kurren [23, 24], AWF and the calculated data using the equations by Friedrich and Hippler. As a result, Kronenberg obtained a table of coefficients C 1 and ε1 . Table 2.7 shows these coefficients for the steel of moderated hardness. The results shown in Table 2.7 speak for themselves. Kronenberg only noted “A comparison of the values of coefficients C 1 and ε1 shows that they differ from each other.” Kronenberg was not fully satisfied with his equation for the cutting speed so he attempted to develop a more detailed equation which should include the properties of the work material and tool geometry. Because these data were available in the test conditions of other investigators, he chose the Brinell hardness, H to represent the properties of the work material and the tool wedge angle, β (the angle between the rake and flank faces in the normal section plane) as the tool geometry parameter. Kronenberg, using the same (as above-discussed) way of manipulating with the experimental data obtained by other researchers, developed the cutting pressure (force) formula for the above-discussed work materials. For example, for the steel of moderate hardness he gave the following equation P=

A0.803 c

· 29.9

√

2.2

H·

1.68

β 50

(2.54)

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V. P. Astakhov

An example is given here to show how he manipulated with the results by other researches to develop this equation. In incorporating the tool wedge angle in his Eq. (2.54), Kronenberg referring to a paper by De Leeuw and Plainfield [25], presented a graph shown in Fig. 2.6a which depicts the influence of the tool wedge angle on the cutting pressure(force). Using this graph, he concluded that for wedge angles less that 50°, the cutting pressure (force) changes significantly with β whereas for wedge angle greater that 50°, this change is small. Everything looks logically till one actually reads the referred paper by De Leeuw and Plainfield [25]. The caption of the original graph presented by De Leeuw and Plainfield [25] is “Probable relation between contained angle of cutting tool and power.” The corresponding text reads as follows: “In The Art of Cutting Metals Mr. Taylor stated that his experiment showed no perceptible difference in power consumption for various contained angles of the cutting tool. The writer thought that this conclusion would probably be correct only for the range of cutting angles tried by Mr. Taylor. He imagined that the relation between contained angle and power consumption would probably be a curve of the nature of Fig. 2.5 (of their paper, auth.), and that all experiments made by Mr. Taylor were within the horizontal line of the curve.” As can be seen a probable relation between the power consumption and contained angle drawn in qualitative manner became a proven fact of the cutting pressure (force) and the wedge angle retaliation in the Kronenberg derivations of the cutting pressure (force) equation. Analyzing Eq. (2.54) and, particular the way it was developed, and comparing the results with his experimental results and results by others, Cheluskin [26] called this equation as pure fantasy. The foregoing analysis of Kortenberg’s consideration relates to his book published in 1923 [14]. Since then, a number of editions of this book have been published. What Fig. 2.5 The sense of the cutting perimeter in the case of the curved chip as a function of the cutting feed f , depth of cut, ap , and cutting edge curvature, ρ

2 Cutting Force Modeling: Genesis, State of the Art …

63

Fig. 2.6 The curves of the influence of the tool wedge angle: a as presented by Kronenberg referring to De Leeuw and Plainfield, and b original curve by De Leeuw and Plainfield

has not been changed is his believe in the sole influence of the uncut chip crosssectional area Ac . In the last edition of his book published in 1966 [15], Kronenberg pointed out the following (Chap. 6, page 99): “The chip cross-sectional area A and the cutting speed v are the two main quantities that can be controlled by the lathe operator by shifting some gears in the headstock, setting the feed and the depth of cut. The product these to variables (A·v) is the volume of the chip removed per minute…For this reason chip cross sectional area A and the cutting speed v are also taken as the two principal quantities in applied metal cutting research. We shall take A as an independent and v as the dependent variable. Their relationship shell be termed the A–v relationship.” Then, he incorporated this relationship in Taylor’s formula for tool life in the manner as he did with the cutting pressure (force). As can be seen, the results of more than 40 years research in metal cutting, according to which the influence of the undeformed chip thickness and its width on the cutting force and tool life, did not affect the Kronenberg thinking, and thus approach. Inspiring by this approach, the milestone conference on machinability at the Institution of Mechanical Engineers (IMechI, London) held on 24th May 1946 concluded that “The interdependence of the tangential force T for material, and the cross-sectional area of chip A, given by its quotient I= T/A, should be called the machinability index” [27]. In conclusion, the author has to point out that although the ideas of Friedrich, Hippler, and Kronenberg were widely recognized in Germany, not everyone in Germany shared these ideas. For example, professor Wallichs in a series of his publications presented the multiple experimental results done in the Aachen Polytechnic (now known as RWTH Aachen University) showing that the results of his experimental studies proved the validity of the Taylor’s approach, and thus Taylor’s results. In his most noticeable work [28], Wallichs harshly criticized approaches and results by Friedrich, Hippler, and Kronenberg. Considering their formulas for the

64

V. P. Astakhov

cutting speed, he pointed out that there is no enough data to obtain the formula for the cutting speed, which accuracy would much the experimental data within 20%. Comparing the formulas obtained by Friedrich and Hippler, he showed that the calculated cutting speeds according to these formulas can differ up to 150%. Moreover, he drew attention to a fact that all three analyzed formulas do not account on the shape of the chip cross section although Friedrich, Hippler, and Kronenberg stated that this shape affects the cutting speed.

2.5 Modern State of the Art 2.5.1 General Reading the above analysis, one may wonder how relevant it is for the presence, where the theory and practice of metal cutting reportedly advanced to the stage where the above-listed old works seems to have only a historical/sentimental value. Unfortunately, it is not so as discussed in this section. The essence of Eq. (2.1), according to which the cutting force is calculated as the product of the constant (presently known as the specific force coefficient, power coefficient, specific energy and so on) and the uncut chip thickness area is not just alive but is in the core of many students’ textbook, monograph, scientific papers (more than 10,000 and counting), and trade literature. Moreover, major cutting tool manufacturers use it in their catalogs/training guides to help end users to calculate cutting force(s) and power. The second issue one can have is with a necessity to know the cutting force/power. Taylor believed that knowing the cutting force is not important as it has no corrections with tool life (the cutting speed corresponding to 20/60/90 min tool life) (par. 96 in [9]). To support this statement, Taylor explained that the cutting pressure (force) in machining of cast irons is generally greater than that in machining of steels whereas the cutting speed for a given duration of cut is smaller for steels. Although he put forward a great idea about the prime (by far) influence of the temperature on tool life, Taylor mistakenly believed that only friction between the tool and chip causes this temperature. The further developments showed that the cutting power, considered as the product of the cutting pressure (force) and the cutting speed, is the prime source of the thermal energy in cutting as the most of the cutting power converts into this energy. The flows of this energy as heats in the cutting system (the chip, tool, workpiece, coolant etc.) create a particular tool temperature field directly affecting tool life. Obviously, other factors of the cutting process can change the influence of the cutting pressure (force) and, moreover, sometimes, can even “overpower” this influence so that the considered end result (tool life, for example) can just opposite to that expected when only the cutting force is considered. In other words, the cutting force should be considered together with other factors of the process because they are tightly intertwined.

2 Cutting Force Modeling: Genesis, State of the Art …

65

Besides the outcomes of the cutting process, as tool life, quality of machined surfaces, productivity and efficiency of the process, the exact knowledge of the cutting force and cutting power is needed for the design of the efficient cutting tools (e.g. for high-speed machining where the tool is pushed to its limits in terms of the force and temperature), design of part holding fixtures (particularly when numerical modeling is used), coolant type, brand, and its application technique selection as the coolant should remove some part of the cutting power converted to the thermal energy, and thus reduce the temperature loads on the tool and workpiece, assessment of the part accuracy etc. The foregoing consideration suggest that it is important to know the exact cutting force and power for the efficient design the whole cutting operation. The following section discusses how the modern literature and electronic sources can assist one in relation to this matter.

2.5.2 Text Books, Monographs, and Trade Literature As an example of general textbooks on manufacturing processes, consider the fourth edition of a book “Fundamentals of Modern Manufacturing. Materials, Processes, and Systems” [29]. The notion of the unit power is also known as the specific energy U (page 498) (note that the designations and units are kept the same as in the book) U = Pu =

Fc Fc v = vto w to w

(2.55)

where F c is the cutting force, v is the cutting speed, t o is the chip thickness before cut, w is the width of cut. It is noted that the units for the specific energy are typically N·m/mm3 (in·lb/in3 ). However, the last expression in Eq. (2.55) suggests that the units can be reduced to N/mm2 (lb/in2 ). According to the author of this book, it is more meaningful to retain the units as N·m/mm3 or J/mm3 (in·lb/in3 ). The book gives a table for U(Pu ) for t o = 0.25 mm. It is pointed out that the chip thickness before the cut also affects the specific energy values. As t o is reduced, unit power requirements increase. This relationship is referred to as the size effect. To account for this fact, the book introduced a correction factor (a multiplier) to determine U and Pu for any other chip thickness. It follows from Eq. (2.55) that the cutting force is calculated as Fc = U to w

(2.56)

This equation is the almost the same as the equation suggested by Wiebe in 1858, i.e., Eq. (2.1). The only difference is that the uncut chip thickness affects U through the introduced correction factor. As an example of more specialized textbooks, consider some most common textbooks. The first one is a textbook by Boothroyd and Knight “Fundamentals of

66

V. P. Astakhov

Machining and Machine Tools”, 2nd edition [30]. The following equation for specific cutting power, ps can be found on page of 82 of this book ps =

Pm Fc = Zw Ac

(2.57)

where Pm is the rate of energy consumption during machining, Z w is the metal removal rate, F c is the cutting force, Ac is the cross-sectional area of the uncut chip. From Eq. (2.57), one can calculate the cutting force as Fc = ps As

(2.58)

This equation is the same as the equation suggested by Wiebe in 1858, i.e., Eq. (2.1). In other words, it is recognized that the uncut chip thickness and chip width have the same influence as believed by Wiebe, Friedrich, Hippler, Kronenberg and many others. The irrefutable fact the uncut chip thickness and its width have different influence on the cutting force is shifted into the constant ps . The book, however, points out that “The specific cutting energy can vary considerably for a given material and is affected by changes in cutting speed, feed, tool rake, and so on. However, for a given tool rake at high cutting speeds and large feeds the specific cutting energy tends to become constant as shown in Fig. 2.7. This constant value can be a useful guide, in practice, to the forces required to cut a given

Fig. 2.7 Effect of the cutting speed and undeformed chip thickness on specific cutting energy, where ac is the undeformed chip thickness and the material is mild steel, the normal rake angle is 10 deg, and the width of the chip is 1.25 mm

2 Cutting Force Modeling: Genesis, State of the Art … Table 2.8 Approximate values of specific energy for different materials cut with rake angle γ = 0° and cutting feed hD = 0.25 mm) for cutting with continuous chip and no built-up edge (BUE)

67

Work material

u0 , J/m3

Aluminum alloy

7.2 × 10−3

Gray cast iron

10.53 × 10−3

Free-machining brass

10.53 × 10−3

Free-machining steel (AISI 1213)

17.55 × 10−3

Mild steel (AISI 1018)

21.06 × 10−3

Titanium alloys

35.10 × 10−3

Stainless steel (18–8)

49.14 × 10−3

High-temperature alloys (Ni and Co base materials)

49.14 × 10−3

work material at large speeds and feeds, and for the results presented in Fig. 2.7 for a low-carbon steel, ps approaches 1 GN/m3 .” According to the book, plowing force and “size effect” are responsible the dependence of the specific cutting energy on the uncut chip thickness. Shaw in his book “Metal Cutting Principles”, 2nd edition [31] in the section “Estimation of cutting forces” (page 35) pointed out that it is frequently important to estimate cutting force(s). This may best be done in terms of total specific energy (u) since this tends to remain approximately constant for a given work material operating under different cutting conditions. For example, the power component of the cutting force can be estimated as Fp =

uvbh d = ubh D v

(2.59)

The values of u’s for various work materials in metric units are shown in Table 2.8. Shaw pointed out that: • The specific cutting energy is essentially independent of cutting speed (v) over a wide range of values, provided a large built-up edge is not obtained. • The values of the unit energy shown in Table 2.8 for rake angle γ = 0° and decrease about 1% per degree increase in the rake angle. • The specific cutting energy varies approximately with the undeformed chip thickness, hD in the usual range of chip thickness as

u ∼ 1 h 0.2 D

(2.60)

Reading this in page 35, one may wonder why he or she needs to read the remaining approx. 600 pages of the book if Eq. (2.59) gives the result acceptable in practice. In other words, the power component of the cutting force can be estimated as F p = ubh D

(2.61)

68

V. P. Astakhov

If one substitutes Eq. (2.60) into Eq. (2.61), he or she obtains that the cutting force F p is proportional to h 0.8 D , i.e. the result published in 1906 by Taylor [9]. As such, the existence of the specific cutting energy for a given work material is considered as a fact. In other words, the whole business of the tool and process optimizations including the tool geometry (besides the rake angle), tool materials, coolant and so on becomes insignificant. Yet another textbook, mostly used in Germany, is that by Klocke “Manufacturing Processes 1. Cutting” [32]. In the consideration of the cutting force, Klocke used the approach developed in 1952 by Kienzle [33]. According to this approach, the specific cutting force (as related to the power component of the cutting force) is the ratio of the cutting force, F c and the removed chip cross-sectional area, A, so the cutting force is calculated as Fc = kc A

(2.62)

As the specific cutting force depends strongly on the chip thickness as shown in Fig. 2.8 [34], Kienzle and Victor [34] proposed to determine the specific cutting force as kc =

kc1.1 h qc

(2.63)

and thus the cutting force as Fc = kc · A =

kc1.1 · b · h = kc1.1 · h 1−qc · b h qc

Fig. 2.8 Determining of k c from the known k c1.1

(2.64)

2 Cutting Force Modeling: Genesis, State of the Art …

69

where k c1.1 is the specific cutting force when the chip width b = 1 mm; uncut chip thickness h = 1 mm; so that the uncut chip cross-sectional area A = 1 mm2 , qc = tanρ (see Fig. 2.8). As follows from Eq. (2.64), the specific cutting force varies only with the uncut chip thickness. This is the same as in the previous-analyzed books. As stated above, the influence of the tool and process parameters including the tool geometry (besides the rake angle), tool materials, coolant and so on becomes insignificant. As the specialized trade books meant for the use in manufacturing shops, consider the book “Engineering Formulas for Metalcutting” [35] because it compiles the results presented in most common books as, for example, Machining Data Handbook [36], “Machinery’s Handbook” [37]. The designations and units are kept the same as in the book. In this book for turning (page 108), the tangential cutting force F t (the proper term is the power component of the cutting force) is calculated as Ft = 60000d f K p

(2.65)

where 60,000 is the unit conversion factor to metric units, d is the depth of cut, f is the cutting feed (wrongly termed as the feed rate in the book), K p is the power constant. The book points out that the unit for K p is hp/in3 /min or kW/cm3 /min. It is stated that “For many years it was considered to be good practice to use the value of the power constant K p = 1.0 hp/in3 /min to estimate power consumption when cutting steels (the authors believe that many machine shops still use the same power constant value).” The book points out that the use of this value may lead to wrong results so it gives many tables for the values of the power constant K p for various work materials and their harnesses. As such, only the hardness and work material grade are considered. It is pointed out that a particular value of the power constant chosen from the tables should be then multiplied by the feed factor, C and by the tool wear factor, C w .

2.5.3 Tool Manufacturers’ Technical Guides In this section, a few examples of how the leading cutting tool manufacturers provide some help for end users in calculating the cutting force/cutting power. The technical guide by Sandvik Coromant [38] gives an equation for calculating the tangential force, F t as  Ft = kC,D,A ×

0.4 fn

0.29 × fn × A P

(2.66)

where k C,D,A is not shown in the nomenclature and then is provided at the top of the same page. Instead, it is shown k c is the specific cutting force, N/mm2 , f n is

70

V. P. Astakhov

feed per revolution, mm/r (as shown although it should be mm/rev), AP not shown in the nomenclature, is provided at the top of the same page. Instead, it is shown ap is cutting depth, mm. Under this equation, it is shown mc as a “constant, depending on material. Use 0.29 as general value” although it can’t be found in this equation. After this “Tangential force, simplified formula” is presented as  Ft = kC,D,A ×

0.4 f n × sin K A P R

m c × fn × A P

(2.67)

where KAPR is “entering angle, degree” as indicated the nomenclature. The it is written “When the entering angle (KAPR) is 75 degrees or longer, sin KAPR ~ 1. Then this simplified formula can be used.” Our analysis of these equations showed that Eq. (2.66) is actually the simplified equation so that Eq. (2.67) should be shown in its place. KAPR is actually the tool cutting edge angle according to standard ISO 3002–1: “Basic quantities in cutting and grinding —Part 1: Geometry of the active part of cutting tools —General terms, reference systems, tool and working angles, chip breakers.” The standard designation is κr so that it became KAPR in the Guide. Sandvik Coromant discussed the specific cutting force as follows [39]: “For power, torque and cutting force calculations, the specific cutting force, or k c1 , is used. It can be explained as the force, F c in the cutting direction, needed to cut a chip area of 1 mm2 that has a thickness of 1 mm. The k c1 value is different for the six material groups, and also varies within each group.” The values of for the standard ISO groups of work materials (DIN ISO 513(2001)) are given as shown in Fig. 2.9a (ISO P—color code is blue. Steels ranging from unalloyed to high-alloyed materials including steel castings and ferritic and martensitic stainless steels; ISO M—color code is yellow. Stainless steels. ISO K—color code is red. Cast irons. ISO N—color code green.

Fig. 2.9 The values of k c1 for the six material groups (a) and determination of k c (b)

2 Cutting Force Modeling: Genesis, State of the Art …

71

Non-ferrous metals as aluminum, copper, brass, etc. ISO S—Heat resistant super alloys. ISO H—color code is gray. Hardened materials.). It is pointed out that the values of k c1 shown in Fig. 2.9a are valid for a neutral insert with a rake angle, γ0 , = 0°; other values must be considered to compensate for this. For example, if the rake angle is more positive than 0 degrees, the actual k c value will decrease. Therefore, according to Sandvik Coromant, the actual value of k c to be used should be determined knowing k c1 and actual uncut chip thickness hm (actual standard designation is hD ) as  γo c × 1− kc = kc1 × h −m m 100

(2.68)

where mc is determined as shown in Fig. 2.9b. This picture also shows a qualitative example of determination of k c1 for the uncut chip thickness equal to 0.3 mm. Our analysis of Eq. (2.68) shows that: 1. 2.

3.

Sandvik Coromant uses the Kienzle and Victor [34] (Fig. 2.8) approach although no reference is given. There is a mistake, namely instead of γ0 which is, as stated above, equal to 0°, the actual rake angle γ should be used to account on the influence of the rake angle (which is generally in line with the above-discussed findings by Zvorykin and suggestion by Shaw on the influence of the rake angle [31]). Although Sandvik Coromant and some other cutting tool manufacturers suggest accounting for the rake angle in the calculation of the actual values of the specific cutting force, it is not clear how to define/determine this angle in many practical applications where cutting inserts with sculpture shapes of the rake face are used and their rake angles are not indicated in tool catalogs.

Our analysis of the whole procedure of the cutting force determination according to Sandvik Coromant indicates that it is impossible for even the most advanced users to determine this force using the information presented including errors. The problem starts with the selection of k c1 from the data shown in Fig. 2.9a for a particular work material. As indicated by Sandvik in its “Workpiece material” page” [40], “ISO P – Steel is the largest material group, ranging from unalloyed to high-alloyed material and including steel castings and ferritic and martensitic stainless steels. Machinability is usually good, but differs a lot depending on material hardness, carbon content, etc.” so that how come one should make the proper selection of this unit force from Fig. 2.9a. Then is not clear how to find mc for a given work material? The statement “Use 0.29 as general value” is not acceptable for a wide variety of the work materials and their properties. Sumitomo, one of the major cutting tool manufacturers, in its Technical Guidance Reference [41], points out that the cutting force, P is calculated as P=

kc × a p × f kc × q = 1, 000 1, 000

(2.69)

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Fig. 2.10 Graphs found in the sumitomo technical guidance reference

where k c is the specific cutting force, MPa, q is the chip area, mm2 , ap is the depth of cut, mm, f is the feed rate, mm/rev (it is actually the cutting feed as the feed rate is measured in mm/min, auth.). It is stated, that “rough value of k c : Aluminum: 800 MPa, general steel: 2500 to 3000 MPa, cast iron: 1500 MPa.” As can be seen, this equation is exactly the same as that by Wiebe (Eq. (2.1)), so that the uncut chip thickness and its width have the same influence on the cutting force. Further, after introducing the equation for the cutting force calculation (Eq. (2.69)), Sumitomo Technical Guidance Reference [41] presents three graphs shown in Fig. 2.10. According to these graphs, the cutting force depends on the cutting speed and rake angle although these data are not reflected in the equation for the cutting force. Moreover, the specific cutting force also depends on the cutting feed (mistakenly shown as the feed rate in the Technical Guidance Reference) and on the work material transverse rupture strength (mistakenly shown as Traverse rupture strength the in the Technical Guidance Reference). One may wonder what the transverse rupture strength (TRS) also known as "modulus of rupture", "bend strength", or "flexural strength" has to do with the specific cutting force and how to use its dependence on the cutting feed in cutting force calculations. Tangaloy in its User’s Guide – Technical Reference (page G032 in [42]) presents the following equation to calculate the cutting force F = kC × a p × f (N)

(2.70)

where k C is the specific cutting force, N/mm2 , ap is the depth of cut, mm, f is feed, mm/rev.

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Table 2.9 Values of the specific cutting force (k C ) for some work materials as given by Tangaloy Workpiece material (JIS)

Tensile strength (MPa)

Harness (HB)

Value of specific cutting force on feed k C (N/mm2 ) 0.04 (mm/rev)

0.1 (mm/rev)

0.2 (mm/rev)

0.4 (mm/rev)

1.0 (mm/rev)

SS400,S15C

390

100

3430

2840

2450

2080

1700

S50C, SCr430

785

230

4900

4020

3430

2940

2400

Aluminum alloy

(89HB)

89

1350

1130

950

810

670

As can be seen, this equation is exactly the same as that by Wiebe (Eq. (2.1)), so that the uncut chip thickness and its width have the same influence on the cutting force. The values of k C is given for a limited number (13) of work materials grades in the manner shown in Table 2.9, i.e. the specific cutting force is the function of the cutting feed. The same approach is generally followed by many other cutting tool manufacturers in their on-line cutting force/power calculators with some difference. For example ISKAR [43] additionally includes the cutting speed and effective rake angle, and the tool cutting edge angle in the input data. Mitsubishi in its cutting power calculation additionally includes the cutting speed and machine coefficient, which actually machining efficiency (0.8–0.9) which one needs to know. Reading these materials by the leading tool manufacturers, one may wonder if the exact determination of the cutting force, and thus the cutting power is important for modern users. It was discussed above that the knowledge of the forces acting on the tool is of high importance as the cutting force affects the design of the efficient cutting tools, tool holders, and part fixtures. The cutting power, which eventually convers into the thermal energy, which directly affects tool life through the temperature over the contact interfaces. To demonstrate the price to pay for not knowing the exact amount of the cutting power, let’s consider the following. Taylor divided all metallic work material into 41 classes according to their standard cutting speeds (the cutting speed corresponding to 20-min tool life (par. 140 in [9])), which vary from one another with the common ratio of 1.1, namely: Class No.1 corresponds to that metal which will give us the highest cutting speed which we likely ever to use in a machine shop; Class No. 2 represents a metal whose cutting speed is that of Class No. 1 divided by 1.1., or Cutting speed of Class 1 1.1

and so on, the cutting speed of each class being connected with the one preceding it by the ratio of 1.1.(par.1134 in [9]). In other words, the standard cutting speed for a given work material was determined with much higher that 10% accuracy. In the author’s opinion, this assured great popularity of the Taylor’s works becoming

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known as Taylor’s scientific management theory. His book on the subject [44] is classical and still actual till now. Unfortunately, the Taylor’s accuracy in determining the cutting speed is lost in modern manufacturing environment. For example, ISCAR recommends [45] for turning of low alloy and cast steels (less than 5% of all alloyed elements) having hardness HB 276, tensile strength 930 MPa (work material group 7) the following cutting speeds: 70–130 m/min for tool material IC3028/830, 160–280 m/min for IC8250, 110–190 m/min for IC9025, 200–320 m/min for IC8150, and 220–340 m/min for IC5005/428. Walter recommends [46] for turning of medium-carbon steels having hardness HB 150, tensile strength 500 MPa (ISO material group P2) the following cutting speeds: 200 – 340 m/min for tool material WPV10, and 130– 200 m/min for WRV20. Kennametal recommends [47] for turning of low-carbon (0.3% C) steel the following cutting speeds: 180–495 m/min for tool material KCP05B/KCP05/KCPK05, and 180–460 m/min for KT315/KTP10. Mitsubishi in its selecting standards [48] recommends for turning of ISO P10 steel the cutting speeds 200–400 m/min for tool material UE6105 and 150–400 m/min for MC6015. As can be seen, one can select the cutting speeds for the same combination of work/tool materials that differ two times. Moreover, the cutting feed and depth of cut are not accounted for. As a result, trial-and-error testing is needed to establish the cutting speed for the required tool life. Moreover, the development and design of modern manufacturing plans and/or production line require knowing the productivity, which directly depends on the cutting speed and which define plant/line capacity, i.e., how many production machines are actually needed for the required yearly program. In the author’s experience, billions of dollars are simply wasted due to not knowing the exact machining regime with the Taylor’s accuracy. The blame, unfortunately, goes to the end users of cutting tools because they do not request the exact cutting data needed to calculate productivity of machining operations.

2.5.4 Mechanistic Approach There are two kinds of mechanistic approaches used, namely mechanistic approach 1 (MA 1) and 2 (MA 2). The major difference between MA 1 and MA 2 is the structure of the equation(s) for the unit pressure (force)/power although both are mainly used in modeling of the cutting forces in milling where the colorful kinematic and dynamic equations completely mask the metal cutting foundations of these approaches. The latter are normally just cited or mentioned by a few words in the text keeping readers in dark on the matter. The metal cutting foundations of MA 1 is extensively covered in the above-mentioned literature souses and mostly used by the leading tool manufactures so it is discussed only briefly in Sect. 5.4.1. As this is not nearly the case with MA 2, the complete analysis of this approach is discussed in this Sect. 5.4.2.

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75

Mechanistic Approach 1 (MA 1)

Our search shows that the mechanistic approach in metal cutting, which is actually practical application of the above discussed approached to determine the cutting pressure (force)/energy, is used as early as in 1928 by Salomon [49]. He presented the following equation of the energy spent over one revolution of the milling tool An =

2k+1 k+2 k · λ1 · b · t 2 · Snk+1 · z −k · D − 2 k+2

(2.71)

where D is the diameters of the milling cutter, λ1 is a constant dependent on the work material and tool rake angle, b is the width of milling, t is the depth of cut, S n is the feed per one revolution of the milling cutter, z is the number of teeth of the cutter, k is the constant for a given work material. Salomon was probably the first who used variable cutting pressure (force) as a function of the uncut chip thickness, i.e. K S = λ1 Sek = λ1 Szk sink ψ

(2.72)

where K S is unit cutting pressure, kg/mm2 , S e is the instant uncut chip thickness, mm, k is a work material constant, S z is the feed per tooth, mm/z, ψ is the location angle of the considered tooth. In his work, Salomon offered the following formula for K S for steels of intermediate hardness KS =

170 Se0.28

(2.73)

so that k = 0.28. To understand the essence of Eq. (2.73), the equation for the power component of the cutting force, Pc obtained by Cheluskin [26] can be re-called Pc = C p−c bh nD

(2.74)

where C p-c is the constant, which depends on the work material, b is the width of cut, hD is the uncut chip thickness, n is an exponent. The test results by Taylor and Cheluskin conclusively proved that n = 0.75 for steels of intermediate hardness. Expressing the undeformed chip width and its thickness through the depth of cut and cutting feed as b = a p / sin κr and h D = f sin κr (where ap is the depth of cut and κr is the cutting edge angle in the modern standard designations) and then substituting these results into Eq. (2.74), one can obtain Pc =

C p−c a p f 0.75 sin κr0.25

(2.75)

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Table 2.10 The values of C p-c for common steels Steels, the ultimate tensile strength, kg/mm2

C p-c kg/mm2

C p-p kg/mm2

C p-t kg/mm2

35

140

19

27

55

165

42

67

75

200

67

125

When κr = 90°, Eq. (2.75) becomes Pc = C p−c a p f 0.75

(2.76)

Therefore, the unit cutting pressure is kc =

C p−c C p−c a p f 0.75 Pc = 0.25 = Ac ap f f

(2.77)

The values of C p-c for common steels are shown in Table 2.10 [50]. Comparing Eqs. (2.73) and (2.77) (and also accounting the data shown in Table 2.10), one can see their practically full resembles. In other words, Eq. (2.73) is valid only when κr = 90° and the depth of cut ap is in power 1. Although it is true for the power component of the cutting force, Pc , it is not quite so for other two components of the cutting force, namely for the back force, Pp and the feed force, Pt [50] 0.55 Pp = C p− p a 1.2 p f

(2.78)

0.75 Pt = C p−t a 0.9 p f

(2.79)

The values of C p-p and C p-t for common steels are shown in Table 2.10 [50]. Some modern researchers think that “Mechanistic models enable quick cutting force computation. These models are based on Martellotti’s idea [51, 52]that the cutting force is proportional to the uncut chip thickness and the specific cutting force (also called the cutting force coefficient).” [53]. It is not true even to the first approximation. Boston in the discussion of Martellotti’s paper (page 695 in [51]) pointed out that the kinematic aspect of the milling, i.e. the analysis of the actual curvature of travel by the tool point, was previously made in the papers by Salomon [49] and Sawin [54]. The use of the average thickness of (uncut, auth.) chip (known as the average thickness of cut or A.T.C.) for energy determination was previously done by Persons [55]. Boston discussed that the use of A.T.C. is not a good criterion for energy calculations. Rather, a formula for energy, E in milling should have the following structure E = Cw f x d y

(2.80)

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where C is a constant, w is the width of cut, f is the feed per chip (feed per tooth, auth.), d is the depth of cut. In his paper [56], values for C, x, and y were determined for a wide range of the work materials for a milling cutter with the rake angle of 15°. For example, for unleaded brass (50-A) E = Cw f 0.77 d 0.96

(2.81)

C = 6040 when cutting up and C = 5171 when cutting down. Note that w and d are in inches, f is in inch/chip. Boston pointed out that Eq. (2.80) holds only for energy in milling, but for the torque and thrust in drilling, and cutting force in turning (page 697 in [51]) Note that the structure of this formula fully resembles Eqs. (2.76), (2.78), and (2.79). Unfortunately, the subsequent researches on the mechanistic approach did not follow the discussed Boston finding about the A.T.C. and structure of the formulas for specific cutting forces/energies. For example, in the work by Fu, DeVor and Kapoor [57], the specific cutting force is still related to the average A.T.C. as −0.1452

K T = 42433.8C t KR =

(psi)

−0.2977 0.0974C t

(2.82)

where K T is the specific cutting pressure, K r is an empirical constant relating radial forces to tangential forces, C t average undeformed chip thickness (average A.T.C.)

2.5.4.2

Mechanistic Approach 2 (MA 2)—The Fundamental Idea and Modern Studies

The father of MA 2 is Armarego, who in 1967 [58] presented the basic of MA 2. He provided better explanations for the foundations of this approach 33 years later [59]. Armarego suggested that, in order to reflect the real situation in cutting, the equation/formula for the cutting force should include two terms. The first term should be responsible for the force(s) on the tool rake face while the second one should reflect the force on the tool flank face due to the processes on the flank face of the cutting edge. Using this consideration, Armarego suggested that the power component, F p and the radial component, F Q (the back force, F r is the modern ISO designation) of the total cutting force are represented as F p = K cp hb + Cep b = K cp A + Cep b

(2.83)

FQ = K cQ hb + CeQ b = K cQ A + CeQ b

(2.84)

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V. P. Astakhov

where K cp and K cQ are corresponding unit forces, h is the uncut chip thickness, b is the uncut chip width, A is the uncut chip cross-sectional area, C ep and C eQ are the edge coefficients for the given tool-workpiece material combination. Analyzing the structure of these equations, one can see that their first term is the cutting coefficient multiplied by the uncut chip cross-sectional area, i.e., fully resembles the above-discussed equation by Wiebe (Eq. (2.1)). The second term is considered by some as controversial (e.g. in [60]). According to Armarego, the ‘edge’ forces on the flank face, C ep and C eQ are constants for a given work material so that the force on the flank face can be accounted for multiplying these constants by the (active, auth.) length of the cutting edge. According to Armarego, C ep b and C eQ b are “concentrated edge forces due to rubbing and ploughing which were proportional to the width of cut b but did not affect the deformation φn and associated shearing and friction processes where both the friction angle β and shear stress τ were also independent of the cut thickness h and width of cut.” These forces were called by Armarego as ‘additional intercept forces.” He did not explain in any of his publication how to obtain these forces. As “ the intercept forces” exist only when “both the friction angle β and shear stress τ were also independent of the cut thickness h and width of cut,” Armarego for years presented the same supporting results of his experiment done in early 1960th shown in Figure 2.11 [58]. Note that in year 2000, the caption of this figure became as “Effect of h and b on the forces and shear angle in ‘classical’ orthogonal cutting operations,” [59] i.e. was generalized with no indication of conditions under which it is possible.

(a)

(b)

Fig. 2.11 Influence of the undeformed chip thickness on: (a) force per unit width for single-edge orthogonal cutting, (b) the shear angle. Tool: 18–1-1 H.S.S. with a 15° rake angle. Work material: aluminum alloy 655T6. Dry cutting

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Looking on the data presented in Fig. 2.11, a specialist in metal cutting should ask a reasonable question about the independence of the shear angle on the uncut chip thickness as it is in the direct contradiction with the basic equations of the single shear plane model used by Armarego and the modern followers of his approach. The essence of this contradiction is explained as follows. According to this model, the shear angle φ is calculated as tan φ =

cos γ ζ − sin γ

(2.85)

where γ is the rake angle and ζ is the chip compression ratio, i.e., the ratio of the chip thickness, hC to the uncut chip thickens, hD . It follows from this equation that the shear angle directly depends on the chip compression ratio. Multiple known experimental results, e.g. by Zorev [18], Astakhov [61], confirm this dependence. For example, Fig. 2.12 shows examples for two steels [61]. As can be seen, the chip compression ratio depends on the cutting feed so, because the undeformed chip thickness is calculated as h D = f sin κr where f is the cutting feed, mm/rev, and κr is the cutting edge angle, it directly depends on the uncut chip thickness. Therefore, according to Eq. (2.85), for a given rake angle, the shear angle depends on the uncut chip thickness as it follows from the theory and practice of metal cutting. It implies that the data shown in Fig. 2.11 are in direct contradiction to the commonly known results. The revealed contradiction can be explained based upon the theory of metal cutting and available experimental results. The conditions for which the cutting force on the flank face is independent on that on the rake face, and thus the uncut chip thickness does not affect the shear angle were first discussed by Rozenberg [62] and later by Zorev [18]. Rozenber and Eremin on page 54–55 of their book [62] discussed that the graph as shown in Fig. 2.11a can be obtained only when the temperature on the tool rake face is kept unchanged. This condition was maintained by changing the

Fig. 2.12 The chip compression ratio vs. cutting speed for different feeds a work material—steel AISI 1030, tool material—carbide P20, rake angle γn = 10°, cutting edge angle κr = 60°, depth of cut ap = 2 mm, b work material—tool steel H13, tool material—carbide M10, rake angle γn = -10°, cutting edge angle κr = 60°, depth of cut ap = 2 mm

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V. P. Astakhov

Fig. 2.13 The dependence of the chip compression ratio on the uncut chip thickness when the temperature of the rake face is kept invariable. In this picture δ = 90°—γ is the cutting angle (vide Sects. 3.2.2 and 3.2.3). Work material—steel 5120, cutting edge angle κr = 90°, cuting speed is not constant, the rake face temperature is constant

cutting speed for each experimental point. Artamonov et al. [63] discussed that the cutting speed and uncut chip thickness affect the chip compression ratio only to the extent to which they affect the temperature over the rake face. Figure 2.13 shows an example of their experimental results. To keep the temperature of the rake face invariable, the cutting speed when the cutting angle δ = 91° (90° – γ, where γ is the tool rake angle) was varied from 23 m/min when hD = 0.51 mm to 120 m/min when hD = 0.04 mm. The next issue to discuss it “the intesept force” as termed by Armarego (page 31 in [59]) and considered as the invariable flank forces. Rozender and Eremin [62] discussed that when the temperature of the rake face is kept constant, the dependance of the cutting and back forces on the uncut chip thinness are expressed by straight lines. They argued that if there were no forces on the flank face, these lines would start from the coordinate origin. As their test results showed that they are not, Rozenberg and Eremin assumed that by the conditionally continuing (i.e., by extrapolation) these lines to the vertical axis where hD = 0, one can obtain the corresponding forces F c-0 and F p-0 on the tool flank plane in the manner shown in Fig. 2.14. The word ‘extrapolation’ is actually used by the proponents of the Arnarego force consideration, e.g. by Altintas “edge forces can be identified by extrapolating the measured forces at zero cut thickness (h = 0) intercept” [64], because these lines never actually intersect the vertical axis in reality so there is no ‘intersepts” of these lines with vertical axis. As can be seen, forces F c-0 and F p-0 do not depend on the uncut chip thickness so Rozenberg and Eremin suggested that these forces do not depend on the conditions on the tool rake face, including the uncut chip thickness under the discussed condition of the invariable temperature on the tool rake face. Moreover, Rosenberg and Eremin found that these forces very small compared to other forces. They suggested that when ratio of the depth of cut, ap to the cutting feed, f is greater than 5, i.e. ap /f ≥ 5 (which is rather normal for common machining operations), these forces can be safely ignored. The later condition was totally unnoticed by further reseaches. In real cutting, the force on the flank face does depend on the uncut chip thickness. This can be easily demonstrated as follows. If one assumes that this force is constant

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81

Fig. 2.14 Forces F c-0 and F p-0 on the tool flank plane obtained by extrapolations

for a given work material and rake angle (as suggested by Armarego), i.e., it does not depend on the uncut chip thickness then, under some invariable cutting speed, the tool flank wear should not depend on the uncut chip thickness as the force and speed are constants. It is not nearly the case in reality. Figure 2.15 shows a typical example. As can be seen, the flank wear does depend on the uncut chip thickness. Moreover, this dependence is highly nonlinear. The reason for that is explained with multiple examples by Astakhov (Sect. 4.3.2 page 243 in [61]). Continuing with the Armarego’s approach, Altintas developed this approach for years. The methodology and obtained results are summarized in [64, 65]. Altintas stated that the cutting coefficients to be used in Eqs. (2.83) and (2.84) can be found from orthogonal cutting tests. According to Altintas: “When these coefficients are

Fig. 2.15 Influence of the cutting feed, f on the flank wear land, VBB . Work material—AISI steel 4320, cutting speed v = 200 m/min, depth of cut ap = 2 mm, tool material—carbide P20, cutting time – 10 min

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V. P. Astakhov

found from the slope or trend of the force measurements, the method is called mechanistic modeling. When the cutting force coefficients are evaluated from the shear stress, shear angle, and friction angle, the method is based on the micro-mechanics of orthogonal cutting. The mechanics approach relates the basic material property, friction and tool geometry directly to the magnitudes of cutting forces. Depending on the material behavior during machining, the three orthogonal cutting parameters (τ s , φ c , μa ) may vary with the cut thickness (h), cutting speed (V) and rake angle (α r ). In order to cover wide range of cutting conditions, the orthogonal cutting tests must be conducted at a range of cutting speeds, feeds and rake angles. The orthogonal parameters can be curve fitted to empirical expressions to cover the range of cutting conditions used in experiments.” There are two problems with the suggested way of the determination of the cutting coefficients. The first one is a great number of orthogonal cutting test is needed, special equipment and high qualification of an experimentalist are required. For example, Altintas pointed out that that “180 orthogonal cutting tests conducted using tungsten carbide (WC) cutting tools and Ti6Al4V titanium alloy work material.”[65]. In each test, the evaluation of the shear angle, average friction angle, and shear yield stress should be carries out using the results of the force and chip geometry measurements. Then, unjustified assumption should be adopted: the orthogonal shear angle is equal to the normal shear angle in oblique cutting; the normal rake angle is equal to the rake angle in orthogonal cutting; the chip flow angle is equal to the oblique angle by adopting Stabler’s chip flow rule; the friction coefficient and shear stress are the same in both orthogonal and oblique cutting operations for a given speed, uncut chip thickness, and tool–work material pair. The second problem is more serious: calculations of the cutting coefficients using the experimental data obtained in orthogonal test are carried out using the singleshear plane model which is inadequate to any real cutting process even to the first approximation [66]. This model suffers from two major flaws described as follows. The model, considered by Altintas, assumes that the shear force considered as the product of the shear stress and the area of the shear plane is the prime work material resistance characteristic in cutting. Therefore, the process of metal cutting is considered as a process of plastic deformation of the work material totally ignoring the classical studies as by Time [4], Reuleaux [67], Taylor [9], Nicolson [11], Okoshi and Fukui [12] etc. and modern publications by Atkins [68–70], Astakhov [61, 71, 72], Sidjanin and Kovac [73] Williams et.al. [74, 75] etc. which defined metal cutting as the purposeful fracture of the work material. In modern modeling of metal cutting, material representation is carried out using the so-called constitutive models with a degradation/damage part considering the influence of strain hardening, strain rate, temperature and state of stress, which includes influence of the stress triaxiality and Lode parameter in the material plasticity. For example, instead of 180 orthogonal cutting tests conducted using tungsten carbide (WC) cutting tools and Ti6Al4V carried out by Altintas for the characterization of titanium alloy TiaAl4V in metal cutting, a much smaller number of tests are needed for this material and no unreasonable assumptions needed when test to determine the coefficients constitutive model using specially designed test specimens are carried out for this work material. Using

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such a model, one can carry FEM simulations of practical machining operations within a wide range of cutting parameters obtaining the needed outcomes [76, 77]. In the single-shear plane mode used by Altintas in his equations for the calculations of the cutting coefficients, the constant friction coefficient over the tool-chip interface is used. He suggested that this coefficient, μ can be calculated using the measured cutting F c and back, F p forces in an orthogonal cutting test as follows (Eq. (2.4) in [64]) βa = γ + arctan

Fp , Fc

μ = tan βa

(2.86)

As pointed by Kronenberg [15] with relation to Eq. (2.86), “…the coefficient of friction increases with increasing true rake. Such relationship is – in my opinion – just the opposite of what would be expected.” Further, Kronenberg pointed out: “For this and other reasons discussed later I do not agree with the commonly accepted concept of coefficient of friction in metal cutting…” Then he explained why the determination of the friction coefficient through the ratio of the cutting forces in the manner suggested by Eq. (2.86), and thus used by Altintas and many others is incorrect. As presented and explained by the author earlier [61], in most engineering and physical situations, friction effects at a tribological interface are described by a constant coefficient of Coulomb friction μf , μf =

F N

(2.87)

where N is the normal force acting at the contact interface and F is the frictional force at this interface. Although it is well established that contact between the two bodies is limited to only a few microscopic high points (asperities), it is customary to calculate the stresses by assuming that the forces are distributed over the total (apparent) contact area. Such an approximation, however, is not far from reality in metal cutting, where the actual and apparent contact areas are practically the same due to extremely high contact pressures [61]. If it is so, the numerator and denominator of Eq. (2.87) can be divided over the tool–chip contact area, Ac and then, recalling that the mean normal stress at the interface is σc = N/Ac and the mean shear (frictional) stress at the interface is τc = F/Ac , one can obtain μf =

τc σc

(2.88)

Equation (2.88) reveals that if the friction coefficient at the tool–chip interface is constant as assumed in the single-shear plane model, and thus used by Altintas then the ratio of the shear and normal stresses should be the same along the entire contact length. In reality it is not so even to the first approximation. As discussed in

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V. P. Astakhov

Fig. 2.16 Distributions of the normal and shear stresses over the tool-chip contact surface (interface): a Qualitative curves after Zorev, and b Example of experimental results

ANY textbook on metal cutting including the Altintas book (Fig. 2.2 in [65]), the distributions of the normal and shear stress at the tool chip interface are as shown in Fig. 2.16a (knows after Zorev [18]). The problem is that the shown qualitative distributions of the normal and shear stress are often drawn in books and paper using imaginations of various authors so that the envelope of shear stress distribution often crosses that of the normal stress (Fig. 2.2 in [65]), which is impossible. To clarify the issue, Fig. 2.16b shows an example of experimental study where the stress distribution was actually measured using a split-tool design. As can be seen, the value of the normal stress is always higher than that of the shear stress over the whole contact length l c . It is interesting to mention that a similar result was obtained first in 1933 by Okoshi and Fukui [12] using photoelastic tests.

2.6 Discussion and Conclusion Although many years have passed since Wiebe introduces his formula for the cutting force calculation (Eq. 2.1) and billions of dollars worldwide spent on metal cutting research, the current state of the art is not that far from this formula according to which there is a certain material property constant to be used in such calculations. The above considerations show that this constant is sometimes modified by the actual

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uncut chip thickness, rake angle, and sometimes the cutting speed whereas the other vitally parameters of particular cutting operations (i.e., the tool material, cutting edge and its inclination angles) are ignored. Armarego believed that his approach resulted in the development of new cutting models, “capable of predicting all the force components, torque and power for all the machining operations as well as the chip flow angle, components geometrical deviations on errors and “theoretical” surface roughness…” [59]. He suggested that further research should be focused on the development of the databases for the wide variety of tool-workpiece materials used in practice. These databases should be then used in relabel predictive models for tool life. He concluded that “This ‘Unified-Generalized Mechanics of Cutting Approach’ can therefore be seen as a first step towards the development of a ‘House of Predictive Models’.” In other words, he essentially proposed to run a worldwide program similar to that carried out by Taylor. One should recall that Taylor spent more than 25 years only for rough turning operations using handful of the developed cutting tools and rather limited number of work materials (compared to many-thousand types/grades of work materials used today). The major problem with the above-discussed approaches is that there are too many variables involved. Denkena and Tonshoff in their book “Machining Basics” [78] pointed out that the general functional relationship between the specific cutting force and the influencing variables is written down to kc = kc0 · 0 (kc0 ; h; b; γ ; vc ; μ....)

(2.89)

where k c0 is the specific internal force, which is valid for fixed reference values of the influence parameters h, b, γ, vc , μ (coefficient of friction) etc.  0 is a function that describes the influences of these parameters and the interactions between them.  0 is dimensionless. It follows from Eq. (2.89) that, for accurate determination of the cutting force, millions tests should be carried out for thousand work materials and cutting tools to determine function  0 to be used in practice. It is clearly unfeasible so that, in the author’s opinion, this activity should be abolished. In the author’s opinion, one of the most feasible ways to go with modeling of metal cutting is the proper utilization of the final element method (FEM). With the evolution of computer science, numerical methods such as FEMs are used in many complex engineering applications. These FEMs are often used as an alternative to very costly experimental methods. As pointed out in [79], FEM is already used in fields such as mechanical and civil engineering, crash analysis and biomechanics, allowing an investigation of local strain and stress. In car crash analysis where large stresses, great deformations, high strain rates, fracture, complex interfacial interactions and so on are involved, FEM became an engineering design tool, which saved a lot of time and billions of dollars in the development/design new vehicles [80]. Numerical modeling of using FEM seems to be an attractive alternative to actual testing in manufacturing. Until ten years ago, the design of metal-forming tools and processes was mostly based on knowledge gained through experience, and designing

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of optimum tools often required a protracted and expensive trial-and-error testing. Today, even in the earlier phases, simulations of the forming process are carried out using FEM analyses. The most important goals of using FEMs are the verification of manufacturability of the sheet metal parts and obtaining vital information on the optimal tool design. As a result, great savings have been achieved due to the introduction of process simulation in metal forming. These savings originate from rapid development of tools and from dramatic shortening of trial-and-error testing. Tool development and production time have been reduced by about 50% due to the usage of simulations. The simulation of forming tool has already reached the stage where its results can be fed directly into the press tool digital planning and validation process. Thus, today, starting from the design model and through practically all process steps (as far as the actual design of the press tool), the production of a component can be fully simulated before a first prototype is built [81]. It is clear that FEM simulation in metal cutting is not nearly as readily utilized as it is in metal forming although metal cutting process is often thought of as a metaldeforming process [30, 31]. It appears as a surprise for many specialists because the two most essential inputs to any FEM simulation of metal shaping processes, namely the work materials’ behavior model under deformation (known as the constitutive model) and friction model to describe contact conditions over the workpiece– tool interfaces, are the same for both forming and metal cutting, whereas practical significances of the modeling results are drastically different. The latter is particularly surprising because the commercial FEM packages designed especially for metal cutting modeling/simulations, for example, Abaqus, Deform2D, Thirdwave AdvantEdge, and others are readily available in the marketplace. One may wonder why these are not implemented in the numerous computer-aided manufacturing (CAM) commercial packages similar to, for example, MasterCAM software, to visualize the material removal sequence, so the kinematic and physical modeling of the machining operation can be carried out in planning of metal cutting operations as it is carried out in metal forming. Moreover, thousands (and still counting) of papers and many books on FEM in metal cutting are published in various countries do not seem improve the practical use of FEM in this area. One may wonder what seems to be the problem(s)? The major problem, as the author sees it now, is that any FEM is a formal computational method which results fully depend on the authenticity of the information one puts in it in the form of input data. A deceiving feature of FEM modeling is that no matter how improper is the input information, one always has the result often represented in colorful, seems to be a ‘realistic’, picture. This is popularly expressed with the acronym “GIGO,” or “garbage is, garbage out.” Therefore, the development of relevant FEM to be used in practice, specialists in metal cutting and in numerical analysis should work together. Then, the developed product is adjusted for the practical use by tool design/process engineers in various manufacturing companies. Astakhov [82] and Astakhov and Outeiro [2] provided more information on the matter. To make the use of FEM in metal cutting at the same level as in the design of forming tools and forming operations, the following four equally important items should be addressed, developed, and used.

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The first one is a clear practical objective(s) of the results. It is to say that the obtained results should be at least of the same level of usefulness as those in the design of metal forming tools and operations. Obtaining the stress, temperature, strain and other outputs that also often set as the objective of the study or even of simulations cannot be considered as the final result because no one really knows what to do with the data obtained [82]. Tool life optimization, increased productivity, improved quality including surface integrity and so on should be considered as the proper objective of FEM modeling if this modeling is considered as an engineering tool. The second is a fracture model of metal cutting. Model of metal cutting as the purposeful fracture of the layer being cut occurring due to complex interactions of the tool, workpiece, and chip [4, 9, 11, 12, 69, 71] should be considered. In other words, the realistic model of metal cutting accounting for the particularities of this process should be always put ahead of any computational model so that the latter should be developed to accommodate the former, not wise-versa as it is today. As a result, the fracture properties of the work material obtained under the state of stress, temperatures, and strain rate particular to metal cutting should be considered in the testing and modeling of this process. Numerical interpretation of the crack formation and propagation should be addressed. Till recently, it was a great computational issue. Nowadays, different options are proposed to numerically model crack occurrence and propagation in ductile fracture. The most noticeable are the phase field models. A comprehensive review of such models of ductile fracture can be found in [83]. For example, Ambati et. al. [84] introduced a new degradation function that contains the equivalent plastic strain in an exponential form. According to this approach, the material is degraded in strain-localized regions so that the crack propagation is directly affected by plasticity. Borden et al. [85] added the plastic strain energy along with a plastic degradation function that depends only on the phase field variable in the crack driving force. Therefore, this computation issue is practically resolved with practical implementations in the FORGE® package which is a finite element software specialized in material forming simulation [86]. The third is a proper model of work material behavior. One should realize that the metal cutting is not unique process where some unknown properties, and thus behavior of the work material involved. For example, the amount of plastic deformation is the same as in deep drawing, the severity of the interfacial conditions and temperatures are at the same level as in hot extrusion, and the strain rate is of the same order as in die punching, etc. In other words, there is nothing unique about metal cutting in terms of the properties of the work material that can’t be accounted for. Figure 2.17 depicts two typical true stress (σ)–strain (E) responses of a ductile metal in a uniaxial test ductile metals (Abacus 6.11 manual [87]). The dashed line is a hypothetical undamaged stress–strain curve, known as the flow curve. It is commonly used to represent the work material behavior in metal forming. To restrict the strain, i.e. to set the right limit for strain growth, the so-called forming limit diagrams are used to assure save forming regions [88]. Note that in the vast majority of FEM modeling of metal cutting use the same curve described by the Johnson–Cook model with no restriction on strain growth. That is why the obtained strains in such a

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Fig. 2.17 Typical true stress (σ)–strain (E) response of a ductile metal in a uniaxial test

modeling far exceed the strains found in any material testing. Therefore, the first step towards the proper modeling in metal cutting is to abolish completely the use of this incorrect model. When fracture of the material is accounted for, the stress–strain curve with progressive damage degradation (the damage curve) shown by the solid line Fig. 2.17 is used (i.e. Abacus, vide Sect. 23.2.2 Damage initiation for ductile metals in Abacus 6.11 manual [87]). As shown, the damage curve follows the flow curve from point a to b. At point b, the experimental yield surface starts to depart from the virtual undamaged pl yield surface. The plastic strain at point b is ε 0 marks the damage (D) initiation. From this point on, both the elastic modulus, E and the plastic flow resistance degrade with increasing strain (in Fig. 2.17, an example is shown for point c). The macroscopic measured response from point b to point d corresponds to microscopic damage pl evolution till final fracture, ε f , which is the plastic strain at fracture. Although the use of damage curve instead of the flow curve in metal cutting modeling (e.g. in [89]) is step forward, it is only a half way to the proper destination. It is true that the area under the damage curve represents the work done in plastic deformation and fracture of a unit volume of the work material. As such, the fracture energy is εd Gf =

σ dε

(2.90)

εa

The problem is that this final fracture strain is assumed to be independent of the stress state so here is no feasible way to reduce strain at fracture and thus amount of plastic deformation is metal cutting. In reality, it is not so. As discussed in [90], this strain, and thus energy of plastic deformation are functions of the stress triaxiality,

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Fig. 2.18 Fracture locus for steel AISI 1045 obtained using the developed double-notched specimen

η defined as η=

σm σ

(2.91)

where σm is the mean stress, and σ is the equivalent stress. Figure 2.18 shows the material (AISI steel 1045) fracture locus obtained from pl the double-notched specimen experiment. The fracture strain ε f scalar values were calculated from the DIC (digital image correlation) measurements for a number of specimens used in the material characterization experiment. As expected, the amount of the material plasticity is proportional to the stress triaxiality state. It can be clearly seen that the amount of plastic deformation varies 5–8 times with the stress triaxiality. This result clearly demonstrates the importance of the stress triaxiality in ductile fracture modeling, and thus the way to minimize the plastic deformation in metal cutting in the manner discussed in [90]. Moreover, the real significance of the cutting tool geometry parameters can be revealed [91]. For the proper determination of the discussed damage curve and fracture locus for a given work material, special test methodology should be used in the determination of the model coefficients [76, 92]. The specially-designed test specimens and appropriate testing procedure should be used should also be used [93]. The fourth one is a physics-based model of contact conditions over the contact interfaces. This issue requires further development. A mentioned above, the severity of the interfacial conditions and temperatures are at the same level as in hot extrusion. The same can be said about stamping, rolling, etc. Even in metal forming where FEM modeling advances are obvious and of great practical significance, the issue with the

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proper representation of the contact conditions is highly debatable [94–96]. Some evidence-base models to approximate these contact conditions are developed. The accuracy of the FEM modeling in forming process does not suffer significantly as the impact of the contact conditions is relatively small. The author’s analysis of the evaluable experimental results summarized in [61] shows that the contact processes at and contact conditions at the chip-rake face and workpiece-flank face interfaces contribute up to 15% to energy consumption in the cutting process although some exceptions is possible when improper cutting tool microgeometry is used. To address the issue associated with tool wear/tool life using the FEM modeling, a considerably different approach to the determination of the cutting speed based upon the energy passing through the cutting wedge is developed [97]. According to this approach, for a given tool material/geometry, there is a limited amount of such energy, taken as the area under the above-discussed damage curve, that the cutting wedge can sustain before reaching the selected criterion of tool life. This limit is considered as the technical resource of the cutting tool.

References 1. Astakhov VP (2014) Drills: science and technology of advanced operations. CRC Press, Boca Raton, FL 2. Astakhov VP, Outeiro J (2019) Importance of temperature in metal cutting and its proper measurement/modeling. In: Davim PJ (ed) Measurement in machining and tribology. Springer, London, pp 1–47 3. Wiebe FKH (1885) Die Maschinen-Baumaterialien Und Deren Bearbeitung. Kessinger Publishing, LLC, Berlin (reprint 2010) 4. Time I (1870) Resistance of metals and wood to cutting. Dermacow Press House, St. Petersbourg, Russia (in Russian) 5. Zvorykin KA (1893) On the force and energy needed to separate the chip from the workpiece (in Russian). Russian Typo-Litography, Moscow, pp 57–96 6. Merchant ME (1944) Basic mechanics of metal cutting process. J Appl Mech 11:A168–A175 7. Merchant ME (1945) Mechanics of the metal cutting process. I. Orthogonal cutting and a type 2 chip. J Appl Phys 16:267–275 8. Backer WR, Marshall ER, Shaw MC (1952) Size effect in metal cutting. Trans Am Soc Mech Eng 74:61–72 9. Taylor FW (1907) On the art of cutting metals. Trans ASME 28:70–350 10. Boston OW (1926) The elements of metal cutting. Ann Arbor, MI: University of Michigan Press 11. Nicolson JT (1904) Experiments with lathe-tool dynamometer. Trans ASME 23:883–935 12. Okoshi M, Fukui S (1933) Researshes on the cutting action of planning tool, by microkinetographic, photoelastic and piezoelectric methods. Sci Papaes Inst Phys Chem Res 22:97–166 13. Bricks AA (1896) Metal cutting (planing). MM Stasucevich Publ. House, St.Petersburg 14. Kronenberg M (1923) Grundzüge der Zerspanungslehre. Springer, Berlin 15. Kronenberg M (1966) Machining science and application: theory and practice for operation and development of machining processes. Pergamon Press, Oxford, New York 16. Friedrich H (1909) Ueber den Schnittwiderstand bei der Bearbeitung der Metalle durch Abheben von Spänen (in German). Z VDI 58(23):860–866 17. Friedrich H (1930) Ueber die Zerspanungstheorie (in German). Maschinenbau 2:47–51 18. Zorev NN (1966) Metal cutting mechanics. Pergamon Press, Oxford

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81. Roll K (2008) Simulation of sheet metal forming—necessary developments in the future. In: The 7th international conference and workshop on numerical simulation of 3d sheet metal forming processes (NUMISHEET). Interlaken, Switzerland 82. Astakhov VP (2011) Authentication of FEM in metal cutting, chapter 1. In: Davim JP (ed) Finite element method in manufacturing processes. Wiley, pp 1–43 83. Ambati M, Gerasimov T, De Lorenzis L (2015) Phase-field modeling of ductile fracture. Comput Mech 55:383–405 84. Ambati M, Gerasimov T, De Lorenzis L (2015) Phase-field modeling of ductile fracture. Comput Mech 55:1017–1040 85. Borden MJ, Hughes TJ, Landis CM, Anvari A, Lee IJ (2016) A phase-field formulation for fracture in ductile materials: Finite deformation balance law derivation, plastic degradation, and stress triaxiality effects. Comput Methods Appl Mech Eng 312:130–166 86. Eldahshan H, Bouchard P-O, Alves J, Perchat E, Munoz DP (2021) Phase field modeling of ductile fracture at large plastic strains using adaptive isotropic remeshing. Comput Mech 67:763–783 87. Abacus 6.11 manual 88. Astakhov VP (2018) Mechanical properties of engineering materials: relevance in design and manufacturing. In: Davim P (ed) Introduction to mechanical engineering. Springer International Publishing AG, Cham, Switzerland, pp 3–41 89. Liu J, Bai Y, Xu C (2013) Evaluation of ductile fracture models in finite element simulation of metal cutting processes. J Manuf Sci Eng 136:011010; Evaluation of ductile fracture models in finite element simulation of metal cutting processes. J Manuf Sci Eng 136:011010 90. Astakhov VP, Xiao X (2016) The principle of minimum strain energy to fracture of the work material and its application in modern cutting technologies. In: Davim P (ed) Metal Cutting Technology. De Gruyter Publishers, Boston, MA, pp 1–35 91. Abushawashi Y, Xiao X, Astakhov V (2017) Practical applications of the “energy–triaxiality” state relationship in metal cutting. Mach Sci Technol Int J 21(1):1–18 92. Abushawashi Y, Xiao X, Astakhov VP (2013) A novel approach for determining material constitutive parameters for a wide range of triaxiality under plane strain loading conditions. Int J Mech Sci 74:133–142 93. Wang B, Xiao X, Astakhov VP, Liu Z (2019) The effects of stress triaxiality and strain rate on the fracture strain of Ti6Al4V. Eng Fract Mech 219:106627 94. Klocke F (2013) Manufacturing processes 4 forming. Springer Heidelberg, New York 95. Joun MS, MG, Moonc HG, NG., Choi, I.S., Lee MC, Jun BY (2009) Effects of friction laws on metal forming processes. Tribology Int 42:311–319 96. Kim H, Kardes N, Chapter 7: friction and lusbrication. In: Altan T, Tekkaya AE (eds) Sheet Metal forming—fundamentals. ASM International: Novelty, OH 97. Astakhov VP, Shavets SV (2020) Technical resource of the cutting wedge is the foundation of the machining regime determination. Int J Manuf Mater Mech Eng 10:1–17

Chapter 3

Evolution of Additive Manufacturing Processes: From the Background to Hybrid Printers I. Buj-Corral, A. Tejo-Otero, and F. Fenollosa-Artés

Abstract The Additive Manufacturing (AM) field is revolutionizing the industrial sector in different areas such as automotive, aeronautics, medicine, etc. Many patents about AM processes were granted at the end of the XXth century. However, until their release, the use of AM was very limited, mainly because of the high cost of the equipment. From that moment on, many 3D printing technologies started to bloom and, along with it, the commercialization of new 3D printers, including hybrid 3D printers. They are defined as a combination of AM and subtractive technologies within the same machine, but also as a merge of different AM technologies. With all this in mind, the present chapter first presents an overview of the different AM technologies, as well as the history of AM, including recent advances. Then, the description of the possible future trends with the use of hybrid 3D printers is discussed.

3.1 Introduction The AM field is blooming and revolutionizing both the research and the industrial sector in different areas, such as automotive or robotics. This development has led to an improvement in the manufacture of new specialised products and innovative devices, as well as an enhancement in the quality of life. In the early years of additive manufacturing, it was not even possible to imagine the possibility of 3D printing objects but only prototypes. However, in recent years, this has not only changed with the development of low-cost machines, but also because of the customization of new 3D printing machines. These avantgarde 3D printers are known as hybrid printers, which are defined as a combination of additive and subtractive technologies within the same machine. They perform hybrid additive-subtractive manufacturing operations (HASM) [1]. Additive and subtractive technologies have remarkable complimentary I. Buj-Corral (B) · A. Tejo-Otero · F. Fenollosa-Artés Department of Mechanical Engineering. School of Engineering of Barcelona (ETSEIB), Universitat Politècnica de Catalunya, Barcelona, Spain e-mail: [email protected] F. Fenollosa-Artés CIM-UPC, Llorens i Artigas, 12, 08028 Barcelona, Spain © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 J. P. Davim (ed.), Mechanical and Industrial Engineering, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-90487-6_3

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capabilities that, when combined, open up a new whole level of both design and manufacturing. The first approach of HASM consisted of combining additive technologies with subtractive technologies; however, the future implies the combination of different AM technologies within the same machine, and leaving apart subtractive technologies for specific applications. For example, some machines combine two different material extrusion technologies such as FFF (Fused Filament Fabrication) with DIW (Direct Ink Writing) or vat photopolymerization (VPP) with material jetting (MJT) techniques. In fact, hybridization is one of the resources available in order to increase the flexibility and efficiency of the manufacturing systems. In general, it includes conventional and non-conventional processes, according to the combination of different process mechanisms and/or energy sources or tools with a positive effect on the properties of the obtained parts [2]. In the machining field, hybridization integrates different machining processes, for example in the milling and turning opreations in the multi-tasking machines. The first HASM system was released in Japan in the late 90s of last century, combining the powder bed fusion (PBF) AM technology with subtractive manufacturing in a vertical machining centre. Currently, there is a commercial version of the hybrid machine, the LUMEX Avance-25 by Matsuura, which employs a high-power laser for the PBF printing and a high speed machining centre to finish the parts [3]. Another example of a hybrid additive/subtractive machine is the Mazak VC-500 AM hybrid multi-tasking machine, which combines a 5-axis machining centre with the direct metal deposition technique [4]. The main objective of the present chapter is to review the background and history of AM processes, and to show the recent developments about the relevant research carried out by the authors in the manufacturing area, as well as its use in different fields: automotive, electronics or medicine. Firstly, an overview of the AM technologies is outlined as well as an explanation of each of them. Next, the background and first patents in this field are introduced. Then, the different hybrid 3D printers commissioned are discussed: (1) explanation of the machine, (2) outcomes, (3) advantages and (4) case studies. Finally, the forecast of possible future trends in this field is presented.

3.2 Classification of Additive Manufacturing Processes ISO/ASTM 52,900 Standard [5] divides AM technologies into seven categories: binder jetting (BJT), directed energy deposition (DED), material extrusion-MEX(which includes FFF and DIW), material jetting (MJT), powder bed fusion -PBF(which includes SLS-Selective Laser Sintering- and SLM-Selective Laser Melting-) , sheet lamination (SHL) and vat photopolymerisation (VPP) (which includes SLAstereolithography). A brief definition and application examples of each category are presented next:

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• BJT is an AM technology process in which a binding agent is deposited onto the powder material so as to bind the particles. It is applied, for example, to manufacture sand moulds and cores in the sand-casting industry [6]. • DED uses an energy source (electron, laser beam or electric arc) that is directed to the material either in the form of powder or wire with the aim of melting the materials while they are being deposited. It is employed, for instance, to repair metallic automotive and aerospace components [7]. • MEX by DIW, designated as MEX-DIW, is a 3D printing technique in which 3D structures are manufactured by forcing an ink through a nozzle, using pneumatic, piston or screw dispensing, in layer-upon-layer stages. MEX-FFF, is a process that uses a continuous filament of a thermoplastic material such as PLA (PolyLactic Acid), ABS (Acrylonitrile Butadiene Stirene) or PVA (Polyvinyl Alcohol) among other, which is heated and dispensed through a nozzle. Material extrusion processes have several biomedical applications, for example the manufacture of surgical guides or medical devices in plastic material [8]. • MJT is an AM technology in which droplts are jetted onto a build platform. There are three main types of material jetting processes, namely the Polyjet technology, the NanoParticle Jetting (NP) and the Drop-On Demand (DOD) process. As an example, the Polyjet technology uses a liquid photopolymer that is jetted onto a base and immediately cured with UV light. The MJT technique is used, for example, to manufacture circuit boards [9]. • PBF can manufacture both plastic parts through SLS or metallic parts by means of SLM. On the one hand, SLS uses a heat source in order to manufacture 3D objects layer-by-layer from plastic powder. Once a layer is deposited the heat source sinters the particles. On the other hand, SLM uses a scanning laser or an electron beam with the aim of melting different layers of metallic particles to produce the 3D printed part. These techniques allow printing, for example, titanium alloy implants [10]. Another common PBF technique is Electro Beam Melting (EBM), in which metallic particles are fused with an electron beam instead of a laser source. • Sheet lamination (SHL), also known as Laminated Object Manufacturing (LOM) consists of the manufacture of 3D objects by stacking and laminating thin sheets of material. The lamination can be carried out with the help of different processes such as bonding, ultrasonic welding or brazing. It is used, for instance, to manufacture glass ceramic substrates [11]. • Vat photopolymerization (VPP) provides 3D printed parts using a laser technology solidification process, through targeted light-activated polymerization, of a liquid resin. Stereolithography (SLA) is employed, for example, to manufacture injection moulds [12].

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3.3 First Patents 3.3.1 The Background of AM The base of additive manufacturing processes comes from both photo-sculpture and topography [13]. As for photo-sculpture, in 1860, the sculptor François Willème developed and obtained a French patent about a system with 24 equispaced cameras around an object. The obtained photographs allowed to trace 24 profiles of the object on wood, by means of a cutter attached to a pantograph. This system is known as photo-sculpture. In 1864, he filed US patent 43822A about a similar idea [14]. In 1901, Carlo Baese di Castelvecchio [15] applied graduated light to a photosensitive gelatin. Then gelatin rings were joined to obtain an object. Regarding topography, in 1892, Joseph E. Blanther invented a method to obtain topographical maps by means of stacking of wax plates, each one of them cut along the contour lines with a different shape [16]. Later, in 1940, Víctor Perera used a similar method to obtain reliefs by cutting contours on cardboard sheets, and then stacking and pasting them [17]. Eugene E. Zang refined the approach using transparent plates with topographical data on them instead of cardboard [18]. Theodore A. Gaskin developed a topographical teaching device for a land mass in 1973 [19]. In 1976, K. Matsubara used photo-hardening materials to obtain the layers of the topographical map [20]. The same year, Paul L. Dimatteo produced pieces with difficult shapes by means of stacking layers. The contours were milled from metallic plates. The layers were joined by means of adhesion, bolts or tapered rods [21]. In 1979, Mizugaki et al. used the lamination technique to obtain a blanking tool [22]. He later developed different kinds of tools with the same technique.

3.3.2 The 50s of the XXth Century Regarding additive manufacturing techniques, in 1951, Otto John Munz [23] invented a system that is comparable to modern stereolithography, in order to photo-record phenomena in a recording space filled with a photosensitive material. The system was called photo-glyph recording, and consisted of a container filled with a photosensitive material, in which a shape was recorded by photographic means.

3.3.3 The 60s of the XXth Century In the 60s of the XXth century there was another early attempt to create something similar to the stereolithography technology. The idea was to solidify a photopolymer resin by intersecting two laser beams. The resin had been developed by DuPont in the 50s. This was first mentioned in 1960 at the Battelle Memorial Institute [24].

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In 1968, Wyn Kelly Swainson patented a system to produce a 3D figure with holography. The system also used two laser beams [25]. Later, he launched the company Formigraphic Engine Co. and developed the technology, although it was never commercialized.

3.3.4 The 70s of the XXth Century On 3rd October 1974, David E. H. Jones wrote in his column named “Ariadne”, in the magazine New Scientist, about the idea that lots of liquid monomers could be polymerized by using UV light to create a solid object, introducing in this way the concept of 3D-model and polymeric resin tank. In 1977, the Dynell Electronics Corp launched some patents about solid photography. It consisted of a laser that was controlled by a computer and was capable to create a 3D object [26]. In the following years the company merged with others and changed its name. Its activity stopped in 1989. The same year, the company Formigraphic Engine Co. patented an apparatus to obtain three-dimensional objects from the application of two lasers on a photosensitive medium [27].

3.3.5 The 80s of the XXth Century In 1981, what is considered to be the first layer-by-layer approach to manufacture was proposed by H. Kodama in the Nagoya Municipal Industrial Research Institute of Japan. It was a single-beam laser curing approach, in order to polymerize a photosensitive resin with UV light. However, he did not patent his work. In 1982, Alan. Herbert, who worked in the 3M Graphic Technologies Sector Laboratory, [28] described a technology that applies an argon ion laser beam to a resin vat using a system of mirrors to create small basic objects [28]. The company decided not to commercialize the design. In 1984, Willam. E. Masters, an American entrepreneur, filed a patent under the name “Computer Automated Manufacturing Process and System” [29], in which he described that a part could be designed on a computer, which would generate a data file of the coordinate information of the part. The same year, a group of French inventors formed by Alain Le Méhaute et al. prepared another patent explaining the stereolithography process. However, because of lack of funds they abandoned the project and did not formalize the patent. Finally, the same year Charles W. Hull filed the patent “Apparatus for production of three-dimensional objects by stereolithography” [30], in which he explained the system operation about a computer-controlled beam of light that photo-hardens a series of cross sections. In 1986, Hull co-founded the company 3D Systems Inc. with Raymond Freed. The company released the SLA-1 printer, which was the first commercial one.

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In 1988, Carl Deckard and Joe Beaman from the Texas University patented the Selective Laser Sintering (SLS) technology [31]. They started up their own company under the name DTM to commercialize SLS machines. In 2001, 3D Systems acquired DTM. In 1989, Scott and Lisa Crump founded Stratasys in order to commercialize the fused deposition modeling (FDM) process, also known as fused filament fabrication (FFF) [32]. Later, Stratasys purchased IBM’s rapid prototyping intellectual property. Also, Stratasys had a partnership with Hewlett Packard (HP), acquired Solidscape and has subsidiary companies like MakerBot and GrabCAD among others. The same year, Langen founded the company Electro Optical Systems (EOS). Some years later, the company delivered its first printer called STEREOS 400, which used the SLA technology.

3.3.6 The 90s of the XXth Century In 1994, EOS GmbH (Electro Optical Systems), founded by Hans J. Langer and Hans Steinbichler, launched the first European SLS machine, EOSINT P 350, for plastic prototypes. Thus, it became the first company in the world to develop both SLS and SLA machines. In 1995, the EOSINT M 250 was introduced in order to manufacture rapid tooling with the DMLS (direct metal laser sintering) technology [33]. In 1995, the selective laser melting (SLM) process was developed by the Fraunhofer ILT Society together with Dieter Schwarze and Matthias Fockele. A patent was filed about this process in 1996 [34]. In 1999, the first organ was obtained in a laboratory, specifically urothelial and muscle cells were seeded on a bladder-shaped scaffold by the Wake Forest Institute of Regenerative Medicine. The scaffold was covered by the patient’s own cells, in order to avoid rejection, and then implanted. The work was published in 2006 [35].

3.3.7 The 2000s In 2003, the first functional solid organ was printed by the Wake Forest Institute, a small kidney, which filtered blood and produced urine in an animal model [36]. The same year, Arcam launched the first commercial EBM machine. In 2005, Dr. Adrian Bowyer from the University of Bath (United Kingdom) founded the RepRap project, in order to obtain printers which allowed printing many of their components. During this year, Z Corporation launched the Spectrum Z510, which is considered the first high resolution 3D printer capable of 3D printing pieces of different colours [37].

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In 2006, the SLS patent expired. That year, the Fab@Home project from the Cornell University in New York (United States) allowed the massive use of a multimaterial printer. In 2008, the first 3D printed prosthetic leg was additively manufactured. Although before this year prosthetic legs had already been 3D printed, they were obtained by assembling different parts. However, by 2008 the first full prosthesis was developed in a single piece. This became a great innovation in the medical sector. Additionally, the quality of these prostheses increased rapidly due to the use of 3D scanners. In 2009, the FDM patent expired. That allowed both people and companies to use this technology without having to pay for the rights. This fact enabled the FDM process to expand while the prices of the machine dropped [37].

3.3.8 The 2010 and 2020s In 2010, Organovo, using the NovoGen technology developed by Dr. Gabor Forgacs from the University of Missouri, produced the first fully bio-printed blood vessels [37]. In 2011, the first car with a 3D printed body, Urbee, was reported to be printed by Kor Ecologic. The same year the first 3D printed plane was obtained at the University of Southampton. The same year, a project by the University of Exeter and the Bruel Univeristy adapted an inkjet style 3D printer to obtain chocolate parts [37]. In 2012, the first 3D printed jaw implant was obtained in titanium, which was designed by Jules Poukens of the BIOMED Research Institute of Belgium and the company LayerWise [38]. In 2019, 3Dirigo manufactured a 3D printed boat of 7.62 m long and 2.2 tons in weight. In 2014, NASA 3D printed the first object in the space. Due to space limitations, during space travel is not possible to bring everything that might be needed at any given time. In this way, 3D printing technology allows the printing material to be transformed into the pieces that are needed at that specific moment. The same year, Hewlett Packard entered in the 3DP market with his new printer, the Multi Jet Fusion (MJF). Around 2015 the first 3D printed houses were manufactured in Europe, using big printing machines. Some experts claim that they are not really 3D printers but big robotic arms with a material extruder. In 2017, Adidas launched the first sneakers of its brand that incorporate 3D printing technology. Specifically, they associated with the company Carbon 3D and used printers based on the photopolymerization technology. Finally, due to the sanitary crisis that took place in 2020, 3D printing appeared allowed to fight against Covid-19. With AM technologies, different medical devices were manufactured for protecting both hospital workers and the population. For example, face masks or door openers. Additionally, it was used also for 3D printing parts of a respirator.

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3.4 Towards Hybrid 3D Printers The development of the different AM processes towards hybrid 3D printers came in hand with the necessity of society, industry, and research of finding more suitable objects. Hybrid 3D printer solves the problem of manufacturing only monomaterial objects for certain applications that require multi-materiality, because of the complexity of the parts and in order to enhance their functionality [39]. Additionally, another problem is also addressed, which is the possibility of developing 3D pieces with different properties, for example parts being softer or harder within a surgical model [40]. Hybridization of AM processes is expected to become more important in the future, together hybridization with post-processing subtractive processes, as well as nanofabrication and large-scale manufacture [41]. The present section is divided into two subsections: (1) combination of AM technologies; (2) combination of AM and subtractive technologies.

3.4.1 Hybrid 3D Printers: Combining AM Technologies This section is mainly focused on the manufacture of certain types of materials like resins, polymers and ceramics. Two main types of machines are addressed: those combining either two extrusion processes such as FFF and DIW, photopolymerization and extrusion, powder bed fusion and extrusion, multijet and extrusion, or photopolymerization and inkjet. An example about the combination of 4 technologies in 1 is also presented.

3.4.1.1

Material Extrusion: FFF and DIW

Within material extrusion, as was previously mentioned, two different 3D printing techniques can be highlighted: FFF and DIW. Combining these two can make an important contribution to the state-of-the-art of 3D printers. Until now, FFF was normally used alone, which has important limitations: (1) the need to use relatively hard materials; and (2) the manufacture of mono-material 3D printed pieces. The latter problem could be solved using several printing heads as was previously described in Buj-Corral et al. [42]. The combination of FFF and DIW can be appropriate in certain applications. For example, in recent years, in the medical field new requirements have arisen. One of them is the use of real prototypes for preoperative surgical planning. These prototypes could be a solution to improve the rehearsal experience of surgeons. Usually, doctors need to carry out complex technical tasks during a surgical procedure in a short period of time. This may lead to damaging organs or spending more time in the intervention. This implies a higher likelihood of damaging any organ since the doctor can get exhausted. In this way, the manufacture of 3D physical models can be solved

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Fig. 3.1 Multi-material FFF/DIW Additive Manufacturing system developed a CIM UPC within the ERDF QuirofAM project (RisCAT) program

as well as using materials able to mimic the corresponding tissue such as the tumour or organ. In order to do that, within the QuirofAM project, a hybrid multi-material 3D printer was developed by CIM UPC engineering team, in which most of the anatomical structures are 3D printed using different filaments (PLA, TPU or PVA) as well as viscosity pump for the softer parts such as, for instance, the liver (Fig. 3.1). Another example about the combination of FFF and DIW technologies is the TRANSPORT project, in which metallic parts were replaced by 3D printed parts, in order to improve the performance of the current components and to entail cost reduction of the raw material and the production process, while preserving the physicochemical properties of the coating (transmission bars, fasteners, shock absorber shafts, etc.). For 3D printed samples, a filament was used with the FFF technology while for the paste-based 3D printing a conductive ink screen printing was required (see Fig. 3.2).

3.4.1.2

Photopolimerization and Extrusion

Peng et al. [43] developed a hybrid platform with a top-down digital light processing (DLP) system that is used when high-resolution is required and a DIW head to print functional materials. This system integrates high-resolution and low-cost technologies. Stereolithography (SLA) has also been combined with direct print, in order to manufacture structures with electronic devices inside it [44].

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b

a

Fig. 3.2 a Final Demonstrator of Transport project 8 embedded contact sensors (black ink) in a FFF structure (white filament). b Visualization of the internal structure manufactured by a hybrid 3D printer (DIW + FFF) commissioned at CIM UPC

3.4.1.3

Powder Bed Fusion and Extrusion

As an example, the powder bed fusion (PBF) technology was hybridized with direct print processes, in order to improve the strength of the parts, to obtain multi-material parts, to increase the conductivity of the part, etc. [45].

3.4.1.4

Multijet and Extrusion

In order to print multi-material parts, the multijet (MJT) technology (Polyjet) has been combined with the direct writing (DIW) technology [46]. The main purpose of this system is to print different types of plastic materials with MJT, including rigid and elastomeric materials, and embedding conductive materials with the DIW technology.

3.4.1.5

Photopolymerization and Inkjet

The photopolymerization technique can be used together with the inkjet technique. For instance, the project Nhibrid, developed at CIM UPC facilities, aims to integrate a three-dimensional printing system by digital light processing (DLP) technology with a material deposition system by inkjet technology and specifically drop on demand (DOD) type. The idea is to 3D print devices with electronic circuitry embedded within the structure of the object, allowing them to obtain more efficient devices, smaller in size and with more complex geometries adapted to the society needs. For example, Muguruza et al. [47] manufactured conductive tracks of 200 µm on substrates previously photopolymerized with layers between 25 and 50 µm thick and it has been verified the possibility of connecting two conductive tracks vertically separated by a slim photopolymerized layer between them.

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Table 3.1 Examples of 3D printed parts [48] IJ Race car wheel

FFF

✕

DIW ✕

Soft pneumatic actuators

✕

✕

Stretchable, multi-material light ribbon

✕

✕

Vertical interconnect access

✕

✕

Digital LED light

✕

✕

IJ inkjet, FFF fused filament fabrication, DIW direct ink writing

3.4.1.6

Four Technologies in One

One of the latest developments within the manufacturing engineering field was achieved by Roach et al. [48] who developed a multi-material multi-method additive manufacturing platform. In it four AM technologies (material jetting, FFF, DIW and aerosol jetting -AJ-) are combined with robotic arms for pick-and-place (PnP) and photonic curing for intense pulsed light (IPL) sintering. The curing system can be either photonic or UV. This new machine opens the paradigm of manufacturing devices that traditionally are difficult to be 3D printed or even impossible. In this way, with the current approach of combining so many technologies it is possible to overcome that limitation. Additionally, the higher the number of technologies, the more novel functionalities can be added to the devices. With the current development in this field, different demonstrators could be built: embedded electronics, sensors, robots and medical devices, for different areas such as medicine or aeronautics. Table 3.1 summarizes the different 3D printed parts manufacturing in Roach et al. [48], as well as the technologies used.

3.4.2 Hybrid 3D Printers: Combining AM with Subtractive Technologies The combination of AM and subtractive technologies into a hybrid system is mostly focused in the metal sector, although there are examples that are not included within this group, like concrete 3D printing. This kind of hybrid machine can be used to print a near-net shape that substitutes a cast or forged part, and then, subtractive processes are applied to finish the shape of the part and/or to improve its surface finish.

3.4.2.1

DIW + CNC Machining (Milling)

One of the projects that changed the road in the research and development of concrete 3D printing is Hindcon, founded in a H2020 project. The aim of this work was to solve different important problems in the construction sector such as the lack of

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Fig. 3.3 Demonstrator of funded EU H2020 project HINDCON, with the participation of CIM UPC, introducing Additive and Subtractive Manufacturing in construction

flexibility in the manufacturing processes and the low productivity rate as well as the long manufacturing times of unique products. The commissioned hybrid 3D printer is based on the 3D printing of the paste by means of DIW and subsequent milling operation in order to smooth the surface finish in the lateral walls of the printed part (Fig. 3.3). This new approach in concrete manufacturing will lead to an important improvement in construction.

3.4.2.2

DED + CNC Machining (Milling)

Usually, hybrid systems are equipped with a DED head for metal deposition and machine tools such as lathe or mill. Bonaiti et al. [49] investigated the micro-milling machinability of Ti-6Al-4 V alloy produced by a Laser Engineered Net Shaping (LENS) additive manufacturing (AM) process. It was confirmed that increased trend towards burr formation in case of down milling of AM samples compared to wrought titanium samples. Additionally, in recent years, a Hybrid Manufacturing Simulation software has been developed by MachineWorks Ltd. which offers a full-machine tool simulation, including DED and CNC machining capability [50]. Other authors have addressed the problem of using cutting fluid when machining is combined with laser metal deposition (LMD) [51].

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FFF + Machining (Grinding)

Another example of hybrid system consists of the hybridization of a grinding machine with a desktop FFF printer for educational use [52]. The controller hardware was NI myRio, and the LabVIEW program was employed for the graphical user interface.

3.5 Future Trends The possible future trends regarding hybrid machines can go in different directions, but specially towards the incorporation of new 3D printing techniques. For example, the FRESH (Freeform Reversible Embedding of Suspended Hydrogels) technique, an Embedded 3D printing (EMB3D) technology, was first mentioned in 2011 [53] (see Fig. 3.4). This technique offers two advantages: (1) there is no need for support since the gel acts at the supporting material, and (2) the gel can be re-used for several times. Regarding FRESH, amazing developments have taken place such as the manufacture of implantable epidermal devices [54] or patient-specific pulse oximeters [55] as well as phantoms [56]. These medical devices are required to have different mechanical properties and, therefore, the combination of this technology with DLP could be an option. However, for that, it would be necessary to have a gel not only transparent, but also photo sensible to UV light. Continuing with this line of future trends, in 2019, but the first effort of this team began by the end 2017, Kelly et al. [57] developed the computed axial lithography (CAL) volumetric fabrication which is based on the rotation of a photopolymer in a dynamical evolving light field. The UV light is sent to the material volume as 2D images, in a way that each image projection arrives from a different angle, thanks to the rotation of the recipient that contains the photopolymer. This allows solidifying the material in the desired geometry. This new technique has three advantages over

Fig. 3.4 FRESH 3D printing of an object

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traditional AM techniques in the medical field: (1) it is possible to print around pre-existing objects, (2) it is faster, and (3) there is no need of support material.

3.6 Conclusions The first AM patents were issued in the 80s of the last century. However, previous attempts to obtain physical three-dimensional parts started yet in the XIXth century with two main inventions: photo-sculpture and topography. Current trends of AM processes include the combination of two or more processes in the same machine. The development of hybrid 3D printers opens a wide range of possibilities both to industry and to research. In this way, it is possible to create 3D printed objects, combining different technologies and materials, that before were not able to be manufactured. This new approach in the industrial revolution will lead to bettering the outcomes of researchers and engineers. Nowadays, these machines are still not usual among many sectors, for instance, bioprinting, but it is possible to imagine that, once it arrives to a sector, it will bloom up. Additionally, this chapter shows that it is not only possible to merge two AM technologies into a single machine, but also there is the possibility for combining AM and subtractive technologies. The latter are usually required to improve the shape of the part and/or to perform finish operations of the previously printed parts. Acknowledgements The authors thank the CIM UPC for the technical support regarding the development of the hybrid printers. The present chapter was co-financed by the European Union Regional Development Fund within the framework of the ERDF Operational Program of Catalonia 2014-2020, with a grant of 50% of total cost eligible, project BASE3D, grant number 001-P-001646.

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41. Gao W, Zhang Y, Ramanujan D et al (2015) The status, challenges, and future of additive manufacturing in engineering. CAD Comput Aided Des 69:65–89. https://doi.org/10.1016/j. cad.2015.04.001 42. Buj-Corral I, Tejo-Otero A, Fenollosa-Artés F (2021) Use of FDM technology in healthcare applications: recent advances 43. Peng X, Kuang X, Roach DJ et al (2021) Integrating digital light processing with direct ink writing for hybrid 3D printing of functional structures and devices. Addit Manuf 40.https:// doi.org/10.1016/j.addma.2021.101911 44. Lopes AJ, MacDonald E, Wicker RB (2012) Integrating stereolithography and direct print technologies for 3D structural electronics fabrication. Rapid Prototyp J 18:129–143. https:// doi.org/10.1108/13552541211212113 45. Folgar CE, Folgar LN, Cormier D (2013) Multifunctional material direct printing for laser sintering systems. In: 24th annual international solid freeform fabrication symposium—an additive manufacturing conference SFF, pp 282–296 46. Perez KB (2013) Hybridization of PolyJet and direct write for the direct manufacture of functional electronics in additively manufactured components mechanical engineering 47. Muguruza A, Bo JB, Gómez A et al (2017) Development of a multi-material additive manufacturing process for electronic devices. Procedia Manuf. https://doi.org/10.1016/j.promfg.2017. 09.180 48. Roach DJ, Hamel CM, Dunn CK et al (2019) The m4 3D printer: aA multi-material multimethod additive manufacturing platform for future 3D printed structures. Addit Manuf. https:// doi.org/10.1016/j.addma.2019.100819 49. Bonaiti G, Parenti P, Annoni M, Kapoor S (2017) Micro-milling machinability of DED additive titanium Ti-6Al-4V. Procedia Manuf. https://doi.org/10.1016/j.promfg.2017.07.104 50. Flynn JM, Shokrani A, Newman ST, Dhokia V (2016) Hybrid additive and subtractive machine tools—research and industrial developments. Int J Mach Tools Manuf 51. Cortina M, Arrizubieta JI, Ruiz JE, et al (2018) Study of the porosity generated by the use of cutting fluid in hybrid processes combining machining and Laser Metal Deposition (LMD). Procedia CIRP 733–737 52. Ghadamli F, Linke B (2016) Development of a desktop hybrid multipurpose grinding and 3D printing machine for educational purposes. Procedia Manuf 5:1143–1153. https://doi.org/10. 1016/j.promfg.2016.08.090 53. Wu W, Deconinck A, Lewis JA (2011) Omnidirectional printing of 3D microvascular networks. Adv Mater 23.https://doi.org/10.1002/adma.201004625 54. Tan WS, Bin JMA, Shi Q et al (2020) Development of a new additive manufacturing platform for direct freeform 3D printing of intrinsically curved flexible membranes. Addit Manuf. https:// doi.org/10.1016/j.addma.2020.101563 55. Abdollahi S, Markvicka EJ, Majidi C, Feinberg AW (2020) 3D printing silicone elastomer for patient-specific wearable pulse oximeter. Adv Healthc Mater 9.https://doi.org/10.1002/adhm. 201901735 56. Lee A, Hudson AR, Shiwarski DJ, et al (2019) 3D bioprinting of collagen to rebuild components of the human heart. Science (80-). https://doi.org/10.1126/science.aav9051 57. Kelly BE, Bhattacharya I, Heidari H, et al (2019) Volumetric additive manufacturing via tomographic reconstruction. Science (80-). https://doi.org/10.1126/science.aau7114

Chapter 4

Busbars for e-mobility: State-of-the-Art Review and a New Joining by Forming Technology Rui F. V. Sampaio, Maximilian F. R. Zwicker, João P. M. Pragana, Ivo M. F. Bragança, Carlos M. A. Silva, Chris V. Nielsen, and Paulo A. F. Martins Abstract The changes in the automotive market and their effects on industry are nowadays hot topics in metal forming seminars and conferences around the world. The rise in the number of electric vehicles will inevitably lead to a decrease in the demand of components for combustion engines and power drive trains. Typical forming components such as pistons, connecting rods, valves, camshafts, crankshafts, multi-speed gear boxes and others that exist in diesel or petrol vehicles, will no longer be required. However, the lightweight construction requirements for the body-inwhite of electric vehicles, the production of components for asynchronous motors and the fabrication of battery components, namely busbars, are bringing new challenges and opportunities for the metal forming industry. This chapter is focused on busbars, which are metallic strips or sheets that are utilized to distribute electric power to multiple equipment such as the electric motor, the electric power R. F. V. Sampaio · J. P. M. Pragana · C. M. A. Silva · P. A. F. Martins (B) IDMEC, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal e-mail: [email protected] R. F. V. Sampaio e-mail: [email protected] J. P. M. Pragana e-mail: [email protected] C. M. A. Silva e-mail: [email protected] M. F. R. Zwicker · C. V. Nielsen Department of Mechanical Engineering, Technical University of Denmark, Produktionstorvet, 2800 Kgs. Lyngby, Denmark e-mail: [email protected] C. V. Nielsen e-mail: [email protected] I. M. F. Bragança CIMOSM, Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, Lisbon, Portugal e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 J. P. Davim (ed.), Mechanical and Industrial Engineering, Materials Forming, Machining and Tribology, https://doi.org/10.1007/978-3-030-90487-6_4

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steering unit, and the AC/DC converters. In particular, the chapter addresses the challenge of replacing copper busbars by hybrid busbars made from copper and aluminium, due to the expected savings in weight and cost. For this purpose, the authors discuss the challenge of connecting copper to aluminium in hybrid busbars by means of existing joining technologies and introduce a new joining by forming process aimed at connecting hybrid busbars at room temperature without giving rise to material protrusions above and below the sheet surfaces. The effectiveness of the new process is compared against fastening by measuring the electric resistivities in both types of hybrid busbar joints. Finite element analysis gives support to the presentation and proves to be suitable for the electro-thermo-mechanical analysis of busbar connections.

4.1 Introduction E-mobility, or electromobility, is defined as a road transport system in which vehicles are moved by electricity. It is believed to play a key role in the increase of flexibility in transportation because electric vehicles may use different types of energy sources, as electricity can be obtained from nuclear power, fossil fuels, or renewable resources. This gives electric vehicles some advantages over internal combustion engine (ICE) vehicles while contributing to lower CO2 emissions, especially if electricity is produced by nuclear power or renewable sources. Electric vehicles appeared at the end of the nineteenth century and the first commercially available electric vehicle was developed in 1897 by the Electric Carriage and Wagon Company [1]. Although technological development is usually motivated by costumer preference, this has not been the case in electromobility. In fact, costumers are mostly pleased with their ICE vehicles, and fossil fuels are not expensive enough to stimulate a move into electric vehicles. The consistent underpricing of fossil fuels (Fig. 4.1) is also an obstacle to the transition to electromobility [2]. However, electromobility is nowadays a route for automakers to be ahead of their competitors in terms of green thinking and environmental compliance. Lower taxes on electric cars are in many countries stimulating consumers to move from ICE to electric vehicles. It is worth noticing that the importance of hybrid busbars is not limited to electric vehicles because alternative mobility solutions based on hydrogen is heavily dependent on the installation of water electrolysis plants [3] in which electricity running through busbar systems will be used to decompose water into oxygen and hydrogen [4]. Hydrogen produced in water electrolysis plants can also contribute to zero carbon emission objectives in e-mobility if the electricity is produced from renewable sources.

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Fig. 4.1 Underpricing of fossil fuels in the G20 economies in 2015. Adapted from [2]

4.1.1 Busbars and Busbar Systems The distribution of electric power is carried out by wires, cables, and busbars. Busbars are generally preferred in low-voltage (LV—up to 1 kV) systems with high electric currents because of their advantages regarding ease of installation and maintenance (flexibility), safety, cost, and limitations in space. In fact, the utilization of a high number of cables in parallel for high current applications is not a good solution due to difficulties in installation and maintenance and to troubles in diagnosing and locating problems in the distribution of electric power. Especially in power plants, six different types of busbar systems can be identified (Fig. 4.2): (i) non-segregated busbars, (ii) segregated busbars, (iii) isolated phase busbars, (iv) rising mains (vertical busbar systems), (v) overhead busbars (horizontal busbar systems), and (vi) non-conventional busbars (like sandwich or gas insulated) [5]. Some of these (e.g., isolated phase busbars) are applied in electric vehicles while others are included for broadening the presentation. In non-segregated systems (Fig. 4.2a), the busbars (corresponding to the different phases) are stored in a single metallic enclosure, where insulating supports maintain a certain distance between the busbars and to the enclosure. There are no barriers between them. These systems are simple, economic and are the most widely used in LV systems up to electric currents of approximately 6 kA. In segregated systems (Fig. 4.2b), the busbars are also stored in a single enclosure, but there is an additional metallic barrier between each busbar. These barriers are constructed from the same material as the enclosure (usually aluminum) and provide magnetic shielding, isolating each busbar from the others and protecting the phases against short-circuits. They behave somewhat like a Faraday cage, so that parasitic currents are drawn to the aluminum barrier rather than to the other phases. Although

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Fig. 4.2 Different types of busbar systems where each phase has only one busbar. a Non-segregated, b segregated, c isolated phase, d overhead, e rising mains, and f non-conventional

being preferred for high voltage (HV—from 35 to 230 kV) rather than for LV, this type of busbar system is used for high electric currents (3–6 kA) on all voltage systems. In isolated phase systems (Fig. 4.2c), each busbar is kept in an individual nonmagnetic metallic container to prevent phase-to-phase faults, eliminate proximity effects (e.g., heating), facilitate installation and maintenance, and protect operators from high voltages across the enclosure and metallic structures that arise from parasitic electro-magnetic currents. Isolated phase systems are utilized for extremely large electric currents (above 10 kA) in HV systems. Rising main systems (Fig. 4.2d) are vertically running busbar systems used for power distribution in multistorey high-rise buildings, allowing for power supply to the multiple floors. This type of system usually operates with very low electric currents and, therefore, is not the best choice from an economical point of view. Overhead busbar systems (Fig. 4.2e) run horizontally, usually below ceilings, and are used to distribute power through a single floor. Large rooms with machine tools may largely benefit from overhead busbar systems, as cables become unwieldy. Non-conventional systems (Fig. 4.2f) try to replicate the compactness advantages of cables in busbars by using proper insulation techniques, in which heat transfer occurs by conduction rather than convection through air, as in case of conventional

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systems (Figs. 4.2a–e). In fact, the poor dissipation of heat by natural air convection is one of the reasons why conventional busbar systems rely on the use of busbars and enclosures with large cross-sections. Thus, non-conventional systems are not only more compact, but also more energyefficient for both LV and HV applications. In sandwich busbars, insulation is achieved by means of epoxy or thin films of polyester. By also coating the inside of the enclosure, busbars can be placed touching each other and the enclosure. Gas insulated busbars allow electric power distribution in HV systems with electric currents up to 8 kA. They are used in gas insulated switchgear substations to interconnect the switchgear with the transformer. These systems are modularly constructed and the flanged geometry at the end of every segment allows for rapid and easy installation and replacement [6]. Busbars can have various cross-sectional shapes (Fig. 4.3): (i) circular, (ii) tubular, (iii) rectangular, and (iv) complex geometries such as the U or H-shapes [7] and the tunnel shape [8]. Guidelines for estimating the heat dissipation by natural and forced convection, and by radiation in each cross-section shape of Fig. 4.3 are provided in the above-mentioned references [7, 8]. Busbars are preferentially made of copper due to its high electric conductivity. However, because aluminium is a good electrical conductor that is both lighter and cheaper than copper, there is a growing interest in utilizing aluminium busbars. However, the switch from copper to aluminium comes at the cost of diminishing the current carrying capacity and increasing the overall impedance of the busbars. This

Fig. 4.3 Schematic representation of several busbar geometries: a circular, b tubular, c rectangular, d H-shaped, e U-shaped, and f tunnel-shaped

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means that the cross section of the aluminium busbars must be increased to obtain an electric power performance like that of copper. The greater hardness, lower coefficient of linear thermal expansion and higher melting point of copper are also advantageous because the busbars can be made more resistant to mechanical damage during installation and service, and less sensitive to thermal damage caused by localized hot spots or possible flashovers during operation. Still, both copper and aluminium have high affinity to oxygen and therefore, naturally create oxide films in contact with air. While the copper oxide film is still conductive, the aluminium oxides have insulating properties, which may cause long term problems in the distribution of electric power [7]. One solution to combine the technical advantages of copper with the lightweight and economic advantages of aluminium is by using copper-clad aluminium (CCA). Copper-clad aluminium was developed for wires in the late 1960’s [9] and later applied to busbars [10]. CCA is named as Cuponal when used in busbars and consists of a metal composite bar in which the core is made of aluminium and the skin is made of copper. When compared to copper, CCA has a lower density and cost, while maintaining excellent electric and thermal conduction properties and giving rise to oxide layers that are irrelevant for the electric conductivity in busbar systems. Table 4.1 presents a comparison of physical, mechanical, and economic data for copper, aluminium and CCA/Cuponal. Another solution to combine the technical advantages of copper with the lightweight and economic advantages of aluminium is by using hybrid busbars, in which the thinner and costlier parts made of copper are only used in specific key locations. In current state-of-technology, this requires connecting copper to aluminium by means of overlapped joints produced by conventional joining processes such as welding and fastening, which is not always possible or effective to achieve. Moreover, the electrical resistance of the overlapped joints increases with the reduction of the overlap length, due to streamline distortion of the electric current distribution, and temperature also increases due to a smaller area available for heat dissipation [14].

4.1.2 Batteries and Electric Power Distribution in e-mobility Besides the energy applications, busbars are essential components of battery packs for electrified vehicles. Electric, plug-in hybrid-electric, and hybrid-electric-vehicle’s battery packs are modularly designed and consist of several cells. Most electric vehicles (EVs) and plug-in hybrid-electric vehicles (PHEVs) use lithium-ion battery cells, while most hybrid-electric vehicles (HEVs) use nickel-metal hydride battery cells [15]. The battery cells can be cylindrical, prismatic, or pouch-shaped (Fig. 4.4) and are interconnected (either in series or in parallel) by means of busbars joined to the cell terminals to create battery modules (Fig. 4.5) [16]. These busbars will be referred to

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Table 4.1 Physical, electrical, thermal, mechanical, and economic data for copper, aluminium, and CCA/Cuponal Coppera

Aluminiumb

Typical grades used in busbars

C11XXX

AA 1XXX AA 6XXX

Density (kg/m3 )

~8900

2680–2920

Melting point (°C)

~1080

~660

Electrical resistivity (µ·m)

0.0168–0.0172

0.0267–0.047

Slightly lower than aluminium

Oxide’s electrical Resistance

Negligible for busbar applications

High

Similar to copper

Thermal conductivity (W/(m·K))

385–388

167–234

Slightly higher than aluminium

Coefficient of thermal expansion (1/K)

~17 × 10–6

~24 × 10–6

Yield strength (MPa)

69–365

28–324

Ultimate tensile strength (MPa)

220–455

51–414

Elongation (%)

4–50

4–19

CCA/Cuponal — Slightly higher than aluminium —

— — — —

Hardness (HB)