Electron Beam Wire Deposition Technology and Its Application (Additive Manufacturing Technology) 9811907587, 9789811907586

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Additive Manufacturing Technology

Shuili Gong Jianrong Liu Guang Yang Haiying Xu

Electron Beam Wire Deposition Technology and Its Application

Additive Manufacturing Technology

This series systematically summarizes the technology developments in the additive manufacturing field in China in recent years, introducing the technical development status in terms of additive manufacturing processes, materials, technologies, and applications. This series is one of the national key publishing projects in China, and has been listed in the national key book projects of China’s “13th Five-Year Plan”, supported by the National Publishing Fund.

More information about this series at https://link.springer.com/bookseries/16896

Shuili Gong · Jianrong Liu · Guang Yang · Haiying Xu

Electron Beam Wire Deposition Technology and Its Application

Shuili Gong AVIC MTI (Manufacturing Technology Institute of China) Beijing, China

Jianrong Liu Institute of Metal Research Chinese Academy of Sciences Shenyang, Liaoning, China

Guang Yang AVIC MTI (Manufacturing Technology Institute of China) Beijing, China

Haiying Xu AVIC MTI (Manufacturing Technology Institute of China) Beijing, China

ISSN 2731-6114 ISSN 2731-6122 (electronic) Additive Manufacturing Technology ISBN 978-981-19-0758-6 ISBN 978-981-19-0759-3 (eBook) https://doi.org/10.1007/978-981-19-0759-3 Jointly published with National Defense Industry Press, Beijing, China The print edition is not for sale in China (Mainland). Customers from China (Mainland) please order the print book from: National Defense Industry Press © National Defense Industry Press 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 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 publishers, 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 publishers nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Foreword

The method of heating and bonding materials with laser beam, electron beam, plasma, or ion beam as the heat source to directly manufacture parts is called the high-energy beam additive manufacturing technology. It is an important technology branch of additive manufacturing and the most common technology in the industrial sector. Regarding the additive manufacturing technology in the aerospace industry, the additive manufacturing of metals, non-metals, or metal matrix composites with highenergy beams is currently the most progressive research direction, while the Electron Beam Wire Deposition (EBWD) technology is one of the most representative additive manufacturing technologies. The EBWD technology is a digital manufacturing technology that uses electron beams as the heat source to melt added materials layer by layer for the direct manufacturing of three-dimensional metal parts. The EBWD technology has some unique features, such as high prototyping efficiency, high material utilization, sound prototyping quality, etc. It can save a considerable cost for expensive metal materials in the aerospace field, such as titanium alloys, aluminum alloys, and nickel-based alloys. Also, it features high internal compactness and low defect rate. The ultrasonic flaw detection of titanium alloys can reach the AA standard. The Electron Beam Wire Deposition technology is one of the hottest research topics in the global aviation manufacturing industry. If the titanium alloy structures with unusually complex shapes on an aircraft are manufactured by forgings, the production cycle will be lengthy on the one hand, and the thickness of forging blanks vary greatly on the other hand, making it difficult to guarantee the uniformity of internal quality and mechanical properties. Some parts require multiple structure adjustment during the design stage, which is hardly met with traditional methods. With the rapid development of aviation manufacturing technologies, increasingly higher requirements have been raised for part manufacturing cycle and cost. Using the EBWD technology to manufacture complexly structured titanium alloy parts can greatly accelerate the iterative loop of design verification, shorten the development cycle, and reduce the development cost. Relying on National Key Laboratory for Power Beam Processing and Aeronautical Key Laboratory for Additive Manufacturing, Professor Shuili Gong and his team v

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from AVIC Manufacturing Technology Institute was among the very first to launch R&D on EBWD in China. In collaboration with Shenyang Institute of Metal Research under Chinese Academy of Sciences and other domestically leading research institutions while considering the actual industrial needs, the team has conducted technical research into materials, processes, equipment, and application evaluation. Particularly, it has achieved fruitful results, including the proprietary material, equipment, process, and quality testing and evaluations, from the research of the EBWD technology and its application in aerospace-used titanium alloys, high-strength steel, and other key structures. This book is a systematic summary of their research outcomes gained in the past few years, providing a systematic and comprehensive introduction to the technical principles, materials, processes, and equipment of the EBWD technology. It mainly introduces the conceptual connotation, principle, and characteristics of the EBWD technology; its position and function in the additive manufacturing technology system; the direction and trend of technological development at home and abroad; and the fundamentals and application results of the EBWD technology, including technical principles, equipment technology, special materials, manufacturing technology, quality testing, and application practices, marking a systematic and all-covered monograph at home and abroad. I believe this book, after being published, will definitely provide important references for the scientific research and study of a broad range of technology practitioners and university teachers and students who are engaged in relevant technologies and areas. Meanwhile, it will boost the in-depth research, application, and development of related professional technologies.

October 2020

Prof. Bingheng Lu Academician of the Chinese Academy of Engineering Xi’an Jiaotong University Xi’an, China

Preface

Additive manufacturing refers to a scientific and technological system that directly manufactures parts based on the principle of discrete-stacking and driven by the three-dimensional data of parts. Based on different classification principles and ways of understanding, the additive manufacturing technology is also called rapid prototyping, rapid manufacturing, and 3D printing. Its definition is being furthered and expanded, not only inwards, but also outwards. It is also classified into metal forming, non-metal forming, biomaterial forming, and so on by the type and method of processed materials. The method of heating and bonding materials with laser beam, electron beam, plasma, or ion beam as the heat source to directly manufacture parts is called high-energy beam rapid manufacturing. It is an important branch of additive manufacturing and the most common technology in the industrial sector. Regarding the additive manufacturing technology in the aerospace industry, the additive manufacturing of metals, non-metals, or metal matrix composites with high-energy beams is currently the most progressive research direction. The additive manufacturing technology for metal parts is a digital manufacturing technology that uses high-energy beams (laser beams, electron beams) or other heat sources to melt added materials layer by layer for the direct manufacturing of threedimensional metal parts. While organically integrating material preparation and part manufacturing technologies, it will pose an important impact on the equipment manufacturing industry. The characteristics and advantages of metal additive manufacturing are mainly manifested in ➀ directness: materials are directly transformed from a simple and primitive form (powder, wire, foil tape) into any complex form while bypassing the traditional melting, rolling, extrusion, forging, blank forming (casting, forging, welding), rough machining, and finish machining processes; ➁ fastness: it greatly reduces logistics and manufacturing cycles; ➂ green: it reduces energy and material consumption; ➃ flexibility: it can manufacture any complex structure that cannot be machined by traditional processes, and even manufacture functionally graded materials to provide greater freedom for design; and ➄ promotion of the transformation of production mode: it provides possibilities for the transformation of manufacturing industry when traditional assembly lines and large-scale factories are challenged by the networked production mode. vii

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The existing high-energy beam additive manufacturing technologies for metal parts mainly include the following four ones: ➀ Laser Melting Deposition (LMD). This technology uses high-power laser beams as the heat source to achieve highefficiency melting and deposition, and is therefore suitable for manufacturing largescale structures with complex shapes. It is the earliest and the most developed technology at present. ➁ Selective Laser Melting (SLM) and ➂ Electron Beam Selective Melting (EBSM). This technology uses low-power and small laser beams or electron beams as the heat source to quickly scan and melt the powder bed layer by layer. The thickness of the single deposition layer is as small as 20 µm and the spot diameter is 0.1 mm, making this technology suitable for the formation of small-sized, fine, and complex structures. After 10 years of efforts, this technology has been partly applied in the industry. ➃ Electron Beam Wire Deposition (EBWD). It applies high-power electron beams to melt and stack wire materials in a vacuum space, forming a molten pool on the metal surface. Metal wires are fed into the molten pool through a wire feeder and melted. The molten pool moves along a pre-designed path. Metal materials are solidified and stacked layer by layer to form a dense metallurgical bond and produce metal parts or blanks. The electron beam additive manufacturing technology of metal parts mainly refers to EBSM and EBWD. The principle is to mesh and layer the CAD model of parts to obtain the two-dimensional profile information of each cross section and generate the processing path. It melts the filling materials (titanium alloy wire or powder) in the vacuum chamber along the pre-designed processing path by applying high-energydensity electron beams as the heat source, then stacks the filling materials layer by layer on the matrix, and eventually manufactures high-performance metal parts in near net shape. Electron beam additive manufacturing is the latest development direction of the high-energy beam processing technology. Based on the mature highenergy beam surfacing and cladding technology, it also combines the rapid prototyping, the computer-aided design and manufacturing (CAD and CAM), and the flexible automation technologies to directly manufacture high-performance, complex, and compact metal parts in near net shapes. It is one of the latest development directions of the advanced manufacturing technology and the additive manufacturing technology. The Electron Beam Wire Deposition technology is uniquely advantageous in fast development speed, short development cycle, low cost, and sound part performance. An electron beam is prone to a power output of tens of kilowatts. The maximum forming speed of titanium alloys and aluminum alloys can reach 15kg/h. The rapid forming of electron beams is carried out in a vacuum environment of less than 5×10-2 Pa, which can better protect metal materials at high temperature and is very suitable for the processing of active metals such as titanium and aluminum. Compared with forging/casting and machining technologies, the EBWD technology does not require large casting and forging dies, and directly converts parts from CAD models into near-net-shaped blanks without intermediate state heat treatment, rough machining, and other procedures. It can save the amount of materials by 80–90%, reduce the workload of mechanical processing by 80%, and shorten the production cycle by more than 80%. It can reduce costs effectively, especially for expensive

Preface

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metal materials in the aerospace industry, such as titanium alloys, aluminum alloys, and nickel-based alloys. The parts are internally compact with a low defect rate, and the ultrasonic flaw detection of titanium alloy can reach the AA standard. The Electron Beam Wire Deposition technology is one of the hottest research topics in the global aviation manufacturing industry. If the titanium alloy structures with unusually complex shapes on an aircraft is manufactured by forgings, the production cycle will be lengthy on the one hand, and the thickness of forging blanks vary greatly on the other hand, making it difficult to guarantee the uniformity of internal quality and mechanical properties. Some parts require multiple structure adjustment during the design stage, which is hardly met with traditional methods. With the rapid development of aviation manufacturing technologies, increasingly higher requirements have been raised for part manufacturing cycle and cost. Using the EBWD technology to manufacture complexly structured titanium alloy parts can greatly accelerate the iterative loop of design verification, shorten the development cycle, and reduce the development cost. The study on the EBWD technology began in other countries in the 1990s. The Massachusetts Institute of Technology and Pratt & Whitney conducted trial production of superalloy turbine disks. After 2000, this technology achieved rapid progress in the aircraft structure manufacturing. NASA, Boeing, Lockheed Martin, and other organizations have participated in the testing of related technologies and planned to apply the technology to space stations, naval drones, F35 fighter jets, new-generation transport aircraft, and other applications to reduce the manufacturing cost and shorten the development cycle. The National Key Laboratory of High Energy Beam Processing Technology of AVIC Manufacturing Technology Institute (formerly Beijing Aeronautical Manufacturing Technology Research Institute) has taken the initiative to study this technology in China since 2003. After years of efforts and in cooperation with Shenyang Institute of Metal Research under Chinese Academy of Sciences, and AVIC aircraft design institutes in Shenyang, Xi’an, and Chengdu, breakthroughs have been made in the high-speed and stable fusing technology of wires, the path optimization technology of complex parts, the deformation control technology of large structures, the mechanical property control technology, the special material development technology, and many other technologies. The study of the EBWD technology has been furthered on and on and it is now industrialized from a technical concept. From small prototypes to the world’s leading electron beam forming equipment, from process research to raw material development, the leaping development is witnessed in materials, equipment, and technical services. Titanium alloy parts rapidly prototyped with electron beams have been applied in aircraft construction. In order to promote the popularization and application of the EBWD technology, the author has combined the research results over the past few years to provide a systematic and comprehensive introduction to the technical principles, materials, processes, and equipment of the EBWD technology. This book is divided into eight chapters. The first chapter is the introduction, which introduces the conceptual connotation, principle, and characteristics of the electron beam additive manufacturing technology; its position and function in the additive manufacturing technology

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system; the direction and trend of technological development at home and abroad. The second to the eighth chapters introduce the fundamentals and application results of the EBWD technology, including technical principles, equipment technology, special materials, manufacturing technology, quality testing, and application practices. The first chapter was written by Shuili Gong; the second chapter was written by Haiying Xu and Guang Yang; the third chapter was written by Jianrong Liu, Shuili Gong, and Guang Yang; the fourth and the sixth chapters were written by Shuili Gong, Guang Yang, and Jianrong Liu; the fifth and the seventh chapters were written by Shuili Gong and Guang Yang; and the eighth chapter was written by Guang Yang and Shuili Gong. The whole book was conceived, designed, and proofread by Shuili Gong. In the process of formulating this book, the author has received supports from AVIC Manufacturing Technology Institute, Shenyang Institute of Metal Research, Chinese Academy of Sciences, Key Laboratory of High Energy Beam Processing Technology, Key Laboratory of Additive Manufacturing Aviation Technology, Beijing Key Laboratory of Additive Manufacturing, AVIC Shenyang Aircraft Design Institute, AECC Beijing Institute of Aeronautical Materials, Huazhong University of Science and Technology, and many other organizations. The author extends sincere gratitude for those supports. The author sincerely appreciates the Professor Qingjiang Wang from Shenyang Institute of Metal Research, Chinese Academy of Sciences for providing many valuable comments and suggestions for the writing of this book. Also, gratitudes are extended to the author’s colleagues, Professor Congjin Zuo, Dr. Hongbo Suo, Engineer Fan Yang, as well as Dr. Shengyong Pang and Professor Yiwei Shi from the project cooperation team. Their full support and assistance helped the author complete this book. This book may contain some deficiencies and errors due to the limited knowledge of the author, and criticisms and corrections by readers are welcomed. Beijing, China May 2020

Shuili Gong

Introduction

This book comprehensively introduces the progress of the electron beam wire deposition technology (EBWD), an additive manufacturing technology, at home and abroad in recent years, while focusing on the research results of the author’s scientific research team engaged in this technology in China. The book provides a systematic and comprehensive introduction to the technical principles, materials, processes, and equipment of the EBWD technology. It mainly introduces the conceptual connotation, principle, and characteristics of the EBWD technology; its position and function in the additive manufacturing technology system; the direction and trend of technological development at home and abroad; and the fundamentals and application results of the EBWD technology, including technical principles, equipment technology, special materials, manufacturing technology, quality testing, and application practices. This book can serve as a reference book for teachers, students, engineers, and scientific researchers in both scientific research and application institutions who are engaged in relevant studies.

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Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Concept and Connotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Technical Features and Advantages . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Principles and Characteristics of the Electron Beam Wire Deposition Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Role of the EBWD Technology in the National Defense and National Economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Current Status and Development Trends at Home and Abroad . . . . . 1.5.1 Technological Status of Foreign Researches . . . . . . . . . . . . . . 1.5.2 Status of Domestic Researches . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 4

9 10 10 20 22

2 Electron Beam Wire Deposition Equipment . . . . . . . . . . . . . . . . . . . . . . . 2.1 Structure and Principle of the Equipment . . . . . . . . . . . . . . . . . . . . . . 2.2 Overview of Equipment at Home and Abroad . . . . . . . . . . . . . . . . . . . 2.2.1 Overview of Equipment Abroad . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Overall Development in China . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Introduction to Homemade Equipment . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Inverter Power Supply for Electron Beam Machining . . . . . . 2.3.2 Electron Beam Gun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Vacuum System and Its Control . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Wire Feeding System and Its Control . . . . . . . . . . . . . . . . . . . 2.3.5 Three-Dimensional Workbench . . . . . . . . . . . . . . . . . . . . . . . . 2.3.6 Image Monitoring and Acquisition System . . . . . . . . . . . . . . 2.3.7 Electrical Control System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.8 Data Processing Software—Electron Beam RP . . . . . . . . . . .

25 25 27 27 29 31 32 36 37 39 42 43 44 44

6 6 7

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3 Typical Materials for Electron Beam Wire Deposition . . . . . . . . . . . . . 3.1 Manufacturing Technology of Metal Wire for EBWD Use . . . . . . . . 3.1.1 Technical Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 Fabrication Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Characteristics of EBWD Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Burning Loss of Composition During EBWD Process . . . . . 3.2.2 Characteristic Microstructure and Formation Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Anisotropy of Tensile Properties of EBWD Deposit . . . . . . . 3.3 Mechanical Properties Control of Titanium Alloy Fabricated by EBWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Composition Control—Relationship Between Composition and Properties of Ti–6Al–4V Deposit . . . . . . . 3.3.2 Deposition Process Control—Relationship Between Deposition Processes, Microstructure and Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3 Heat Treatment Control—Relationship Between Microstructure and Mechanical Properties . . . . . . . . . . . . . . . 3.4 Damage and Fracture Mode in Tensile Tests . . . . . . . . . . . . . . . . . . . . 3.5 Several Feeding Wires and Mechanical Properties of Their Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 TC4EM Alloy Wire and Mechanical Properties of Its Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 TC4EH Alloy Wire and Mechanical Properties of Its Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.3 A-100 Steel Wire and Mechanical Properties of Its Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Fundamentals of Electron Beam Wire Deposition Technology . . . . . . 4.1 Research on the Behavior of the Molten Pool During the EBWD Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Numerical Simulation of the EBWD Process . . . . . . . . . . . . . 4.1.2 Temperature Field Characteristics of the Molten Pool Under the Wire-Free Technology . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Impact of Wire Deposition on the Temperature Field of the Molten Pool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4 Flow Field Distribution of the Molten Pool Under the Wire-Free Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.5 Impact of Wire Deposition on the Flow Field of the Molten Pool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Fundamentals of the EBWD Technology . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Prototyping Process Control . . . . . . . . . . . . . . . . . . . . . . . . . . .

71 75 75 76 77 78 79 89 94 94

96 101 108 113 113 115 118 121 123 123 123 126 130 131 133 134 134 142

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4.3 Typical Defects and Their Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Types of Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Defect Control Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Deformation Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Principle of Deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Methods for Controlling Deformation . . . . . . . . . . . . . . . . . . . 4.4.3 Deformation Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.4 Partitioned and Fractal Machining . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

145 145 160 163 163 164 166 177 183

5 Non-destructive Inspection of EBWD-Fabricated Parts . . . . . . . . . . . . 5.1 Ultrasonic Inspection Technology for EBWD-Fabricated TC4 Titanium Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 Technical Solutions to Non-Destructive Inspection of TC4 Titanium Alloy Fabricated by EBWD . . . . . . . . . . . . 5.1.2 Acoustic Characteristics and Defect Characteristics of TC4 Titanium Alloy Fabricated by EBWD . . . . . . . . . . . . 5.1.3 Comparison of the Contrast Deposit and the Forged TC4 Deposit in Different Prototyping Technologies . . . . . . . 5.1.4 Comparison of Inspection Sensitivity of Different Non-Destructive Inspection Methods for EBWD . . . . . . . . . . 5.1.5 Inspection of Actual Parts Fabricated by EBWD . . . . . . . . . . 5.2 Non-Destructive Inspection Method and Defect Determination of A-100 Steel Fabricated by EBWD . . . . . . . . . . . . . 5.2.1 Research on the Ultrasonic Inspection of A-100 Steel Fabricated by EBWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Research on the Magnetic Particle Testing of EBWD-Fabricated A-100 Steel . . . . . . . . . . . . . . . . . . . . . . 5.2.3 X-Ray Inspection of A-100 Steel Fabricated by EBWD . . . . 5.2.4 Ultrasonic Automatic Scanning and Evaluation Technology for Typical Parts Fabricated by EBWD . . . . . . .

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6 Fundamentals of EBWD Manufacturing of TC18 Titanium Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Typical Microstructure Characteristics of TC18 Titanium Alloy Fabricated by EBWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 Microstructure Characteristics of Deposited State . . . . . . . . . 6.1.2 Characteristics of the Heat-Treated Microstructure . . . . . . . . 6.2 Properties Control of TC18 Titanium Alloy Fabricated by EBWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Test of Typical Mechanical Properties of TC18 Standard Parts Fabricated by EBWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Static Property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 High Cycle Fatigue Property . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.3 Corrosion Fatigue Property . . . . . . . . . . . . . . . . . . . . . . . . . . . .

185 185 186 197 199 205 210 210 213 216 219 221 223 223 223 227 233 233 234 234

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6.4 Static and Fatigue Tests on Typical Element Parts of EBWD-Fabricated TC18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Experimental Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 Static Test Results of Typical Element Parts . . . . . . . . . . . . . . 6.4.3 Results of Fatigue Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Fundamentals of Electron Beam Wire Deposition Technology for A-100 Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Microstructure Characteristics of A-100 Material Fabricated by EBWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.1 Microstructure Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.2 Microstructure Evolution During Heat Treatment . . . . . . . . . 7.1.3 Influence of Pre-heat Treatment Parameters on the Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Properties Control Methods of A-100 Fabricated by EBWD . . . . . . 7.2.1 Static Property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Fracture Toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Typical Defects and Control Methods of A-100 Material Fabricated by EBWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Metallographic Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 Correlation Between Discontinuity and Inspection Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.3 Correlation Between Internal Structure and Acoustic Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Static Property of Typical A-100 Components Fabricated by EBWD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Lug Specimen of Axial Load . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.2 Lug Specimen of Lateral Load . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.3 Constraint Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.4 Test Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.5 Test Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.6 Test Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.7 Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Fundamentals of Electron Beam Wire Deposition Hybrid Prototyping Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Microstructure Characteristics of TC4-DT Titanium Alloy Fabricated by Electron Beam Wire Deposition Hybrid Prototyping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.1 Microstructure Characteristics of the Forging Matrix Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.2 Microstructure Characteristics of the Transition Zone . . . . . 8.1.3 Microstructure Characteristics of the Wire Deposition Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

245 245 256 267 273 275 276 276 277 279 282 282 289 292 295 297 300 304 304 304 304 306 306 306 308 310 311

312 313 313 315

Contents

8.2 Characteristics of Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Tensile Properties at Room Temperature . . . . . . . . . . . . . . . . . 8.2.2 Impact Property at Room Temperature . . . . . . . . . . . . . . . . . . 8.3 Defect Control of Forging—EBWD Structures . . . . . . . . . . . . . . . . . . 8.4 Batch Stability of the Mechanical Properties of Electron Beam Wire Deposition Hybrid Prototyping . . . . . . . . . . . . . . . . . . . . . 8.4.1 Tensile Properties at Room Temperature . . . . . . . . . . . . . . . . . 8.4.2 Fracture Toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Research on the Mechanical Properties of Hybrid-Fabricated Titanium Alloy Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Static Property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.2 Fatigue Property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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315 316 320 320 322 325 325 326 328 329 331

Chapter 1

Introduction

Abstract This chapter introduces the concept of the electron beam wire deposition technology. At the same time, the technical features and advantages of the technology have been introduced. Also in this chapter, the principles and characteristics of this technology have been introduced. At the last, the current status and development trends at home and abroad have been introduced.

1.1 Concept and Connotation Currently, a new round of digital and intelligent manufacturing is raging around the world. Facing the declining manufacturing competitiveness in recent years, the United States, EU member states and other developed countries have vigorously advocated the strategy of “re-industrialization and re-manufacturing”. They proposed that intelligent robots, artificial intelligence and additive manufacturing are key technologies for digital manufacturing, expecting to secure and escalate their leadership in the manufacturing industry by making breakthroughs in those three major digital manufacturing technologies. Meanwhile, China is now facing a strategic opportunity to turning from a big to a powerful manufacturing country. The additive manufacturing technology, as an emerging technology, is to produce physical objects by stacking materials layer by layer based on digital models. It embodies the close combination of the information network technology, the advanced material technology, and the digital manufacturing technology, and therefore is an important part of advanced manufacturing. Additive Manufacturing refers to a scientific and technological system that directly manufactures parts based on the principle of discrete-stacking and driven by the threedimensional data of parts, as shown in Fig. 1.1. As a new technological concept, the additive manufacturing technology is still weak after having developed for more than 30 years, but it has already posed a significant impact on the manufacturing industry. Its impact on people’s mindset is particularly significant, enabling people to remove the constraints of structural shapes on thinking, and then transform from two-dimensional abstract graphics into three-dimensional concrete physical parts

© National Defense Industry Press 2022 S. Gong et al., Electron Beam Wire Deposition Technology and Its Application, Additive Manufacturing Technology, https://doi.org/10.1007/978-981-19-0759-3_1

1

2

1 Introduction

Fig. 1.1 Schematic diagram of the additive manufacturing technology principal

during design/production. Instead of being a subversion and replacement of traditional manufacturing methods, the additive manufacturing technology opens up a whole new chapter, allowing people to use a new technical means in manufacturing. Based on different classification principles and ways of understanding, the additive manufacturing technology is also called rapid prototyping technology, rapid manufacturing technology, and 3D printing technology. Its definition is being furthered and expanded, not only inwards, but also outwards. The “additive manufacturing” mentioned here shares the same meaning with “rapid prototyping” and “rapid manufacturing”. Professor Guan Qiao, an academician of the Chinese Academy of Engineering, put forward the concepts of “broad” and “narrow” additive manufacturing [1], as shown in Fig. 1.2. The “narrow sense” of additive manufacturing refers to a technology system in which different energy sources are combined with computeraided design/computer-aided manufacturing (CAD/CAM) technology to stack materials layer by layer; the “broad sense” of additive manufacturing is a large scale of technical clusters that basically feature material increase and aim at directly manufacturing parts. If classified according to the type and method of processed materials, it can be divided into metal forming, non-metal forming, and biomaterial forming, as shown in Fig. 1.3. The method of heating and bonding materials with laser beams, electron beams, plasma or ion beams as the heat source to directly manufacture parts

Fig. 1.2 Schematic diagram of broadly defined additive manufacturing technology clusters

1.1 Concept and Connotation

3

Rapid manufacturing technology clusters

Direct manufacturing

Direct manufacturing

Direct manufacturing

of non-metal parts

of biological structures

of metal parts

Melting

Vapor

Block

Liquid

Melting

Optical

Frustrated

deposition

deposition

welding

deposition

solidification

solidification

assembly

Laser

Electron

beam

beam

Mechanical heat source for welding

Electroch

Other light

Other forms

emistry

sources

of power

Fig. 1.3 Schematic diagram of additive manufacturing technology clusters

is called high energy beam rapid manufacturing. It is an important branch of additive manufacturing and the most common technology in the industrial sector. Regarding the additive manufacturing technology in the aerospace industry, the rapid manufacturing of metals, non-metals or metal matrix composites with high energy beams is currently the most progressive research direction. The existing high-energy beam additive manufacturing technologies for metal parts mainly include the following four ones. The first one is Laser Melting Deposition (LMD). This technology uses high-power laser beams as the heat source to achieve high-efficiency melting and deposition, and is therefore suitable for manufacturing large-scale structures with complex shapes. It is the earliest and the most developed technology at present. The second one is Selective Laser Melting (SLM) and the third one is Electron Beam Selective Melting (EBSM). This technology uses low-power and small laser beams or electron beams as the heat source to quickly scan and melt the powder bed layer by layer. The thickness of the single deposition layer is as small as 20 μm and the spot diameter is 0.1 mm, making this technology suitable for the forming of small-sized, fine and complex structures. After 10 years of efforts, this technology has been partly applied in the industry. The fourth one is Electron Beam Wire Deposition (EBWD). It applies high-power electron beams to melt and stack wire materials in a vacuum space, forming a molten pool on the metal surface. Metal wires are fed into the molten pool through a wire feeder and melted. The molten pool moves along a pre-designed path. Metal materials are solidified and stacked layer by layer to form a dense metallurgical bond and produce metal parts or blanks (Fig. 1.4). The comparison with high energy beam additive manufacturing technology is shown in Table 1.1.

4

1 Introduction

Fig. 1.4 Schematic diagram of the additive manufacturing process of metal parts

Table 1.1 Comparison with high energy beam additive manufacturing technology Item

Coaxial laser powder Selective laser feeding melting

Electron beam wire deposition

Electron beam selective melting

Energy source

Laser

Laser

Electron beam

Electron beam

Dimensional accuracy

1 ~ 3 mm (margin)

0.05 mm (margin)

3 ~ 5 mm (margin)

0.4 mm (margin)

Maximum size Unlimited

350 mm × 350 mm × 500 mm

Unlimited

¢350 × 380 mm

Forming speed/ Kg/H

0.2 ~ 2

0.05 ~ 0.15

2 ~ 15

0.2 ~ 0.35

Protection effect

Protection by inert gas

Protection by inert gas

Vacuum environment

Vacuum environment

Materials used Powder

Powder

Wire

Powder

Material utilization/%

70 ~ 80

100

70 ~ 80

Operation cost Consumption of inert Consumption gas of inert gas

Consumption of filament

Consumption of filament

Scope of application

Large-sized integral structure

Small-sized complex structure

50 ~ 70

Small-and-medium integral structure

Small-sized fine structure

1.2 Technical Features and Advantages Electron beams and lasers are both heat sources with high energy density. Their energy density is in the same order of magnitudes, making them very suitable for the rapid prototyping of metal parts. The laser rapid prototyping has now been widely applied, and the examples of engineering applications have been seen. Compared

1.2 Technical Features and Advantages

5

with laser rapid prototyping, the EBWD technology shares some unique advantages, mainly in the following aspects: (1)

(2)

(3)

(4)

(5)

(6)

(7)

Fast prototyping. Most solid-state lasers used for rapid prototyping have a power of 1 to 5 kW and a prototyping rate of 2 to 15 cm3 /h. Although such a rate is acceptable for small and medium-sized parts, it is not conducive to the engineering applications of large parts. Compared with solid-state lasers, CO2 lasers have higher power, but cannot be transmitted by optical fibers, which will be restricted when being used for prototyping complex parts. An electron beam is prone to a power output of tens of kilowatts. According to the statistics made by the NSSA Laney Research Center, the maximum deposition rate of laser rapid prototyping is 0.5-9 lb/h, while that of electron beam wire deposition is 30 lb/h, which is several to dozens of times faster than the former, presenting obvious advantages for the prototyping of large titanium alloy frames and beams. Sound protection effect. Laser rapid prototyping protects parts with inert gas. So it is difficult to protect the active aluminum alloys and titanium alloys, especially large parts or the parts with complex cavities. The electron beam wire deposition prototyping is performed in a vacuum environment below 10–2 Pa, which can effectively avoid metal oxidation at high temperatures. It is very suitable for the processing of active metals such as titanium and aluminum, and can effectively protect parts. Lower possibility of defects. The surface area of powder is much larger than that of wire under the same quality condition, and the possibility of surface oxidation and impurities is greater than that of wire. In addition, large impurities will break the wire easily during drawing, which can indirectly ensure the quality of wire [2]. Therefore, the electron beam wire deposition prototyping technology using filler wires is less likely to produce defects than the laser rapid forming using powders. High efficiency in material melting. At present, the laser rapid prototyping applies powdery materials, and the material melting efficiency is 5–85%. The EBWD technology applies wire materials, and the melting efficiency is up to 100%. High energy conversion efficiency. The electro-optical conversion rate of YAG laser is 2–3%; that of CO2 laser is 20%; and the energy conversion rate of electron beam is above 95%. A wide range of machinable materials. The energy utilization rate of laser rapid prototyping is low when it is machining some materials with strong light reflection, such as aluminum and aluminum alloys. However, electron beams are not affected by the light reflectivity of materials, so the absorption rate is high and the energy loss is small for all kinds of metal materials. Efficient and flexible beam control. The control of laser can only be done by the mechanical movement of mirrors or lens; while the electromagnetic coil can accurately control the focus of beams and the movement of beam spots, which

6

(8)

1 Introduction

lessens the dependence on mechanical movement and enable more flexible and high-speed movement. Low operating cost. Laser rapid prototyping requires protection by inert gas, such as N2 , CO2 , He, etc., which means the gas consumption will be considerable when processing large parts due to large protection areas. However, the EBWD technology does not need to consume protective gas. Instead, it only consumes electric energy and inexpensive filament.

However, compared with laser rapid prototyping, the EBWD technology also has some shortcomings: (1) (2) (3)

Since the EBWD technology must rely on the vacuum chamber, the size of the workpiece is subject to certain restrictions. The vacuum system increases the operating complexity to a certain extent. The surface quality and positioning accuracy of the parts stacked with filler wires are not as high as that of the parts filled with powder in laser rapid prototyping.

The electron beam wire deposition and the laser rapid prototyping have their respective advantages and disadvantages. Since the EBWD features faster deposition and better protection compared with laser rapid prototyping, and is an ideal choice for the direct prototyping of metal parts, the United States and other developed still carried out researches on EBWD and has made significant progress when the laser rapid prototyping technology was already rather developed and applied. In China, the study on the electron beam wire deposition technology using filler wires has not yet been systematically started. Since the next-generation aerial warfare weapons choose titanium alloys as the classical material, the study on the EBWD technology plays a significant role in rapidly filling the gap with international competitors in this new sector, developing new weapons by the country, and boosting industrial manufacturing technologies for the national economy.

1.3 Principles and Characteristics of the Electron Beam Wire Deposition Technology 1.3.1 Principle Electron Beam Wire Deposition is a direct energy deposition process. It uses an electron beam to melt the simultaneously fed metal wires in a vacuum environment, stack the wires layer by layer along a pre-designed path, and directly manufacture the parts or blanks required. It can also be used for part repair. The Electron Beam Wire Deposition (EBWD) technology is a direct manufacturing technology for metal structures developed based on the “discrete-stacking” principle. The computer hierarchically slices the three-dimensional CAD models of parts and plans the processing path at each level. Electron beams melt the fed metal

1.3 Principles and Characteristics of the Electron Beam Wire Deposition Technology

(a) Principle of forming process

7

(b) Principle of equipment

Fig. 1.5 Schematic diagram of electron beam wire deposition technology [2, 3]

wires in a vacuum environment, solidify and stack them in layers along the predesigned path to form dense metallurgical bonding, thereby directly manufacturing metal parts or near-net-shaped blanks. The principle of the EBWD technology is shown in Fig. 1.5.

1.3.2 Features Compared with other rapid prototyping technologies for metal structures, the EBWD technology features the following advantages: (1)

Fast prototyping speed, large processing size, suitable for the high-efficiency manufacturing of large metal structures. The laser selective melting/electron beam selective melting technologies based on powder coating feature a small beam source power and a small effective processing area. At present, the effective processing area of Arcam’s largest A2XX EBM equipment is only 350 × 380 mm. In contrast, the effective processing are of the EBWD equipment of Sciaky Inc. can reach 5.8 × 1.2 × 1.2 m [4]. The maximum prototyping speed of electron beam selective melting (EBSM) of titanium alloys can reach 60 cm3 /h (266 g/h), while that of the laser selective melting is even slower, which is only about 50 g/h.

The most typical rapid prototyping technology by laser cladding deposition is the laser selective near-net-shape prototyping technology developed by Sandia National Laboratories in the United States. This method usually uses the Nd:YAG laser with a low-power (750 W). The deposition rate is generally 50 ~ 200 g/h [5]. Low deposition rate is ideal in the manufacturing small and fine parts. However, the deposition time

8

1 Introduction

is too long for the manufacturing of large parts. Even if a high-power CO2 laser or fiber laser is used, the deposition rate can hardly reach several kilograms per hour due to the low energy conversion efficiency. In contrast, electron beams can easily achieve a high power output of tens of kilowatts, leading to a material deposition rate up to 18 kg/h (40 lb/h), which is several to dozens of times faster than other rapid prototyping technologies. The EBWD technology is very advantageous in terms of speed for the prototyping of large metal structures. (2)

(3)

Sound protection, low possibility of impurity mixture, and outstanding internal quality. The working environment of EBWD is generally vacuum under a pressure of 10–3 Pa or higher, which can effectively prevent harmful impurities or elements (0, N, H, etc.) in the air from being mixed with metal parts during the forming process. The laser rapid prototyping and arc surfacing rapid prototyping are generally carried out in an inert gas environment, where the protection effect depends on the purity of the inert gas. Since it is easier to control the degree of vacuum, the EBWD technology can deliver better protection effects for active aluminum alloys and titanium alloys, especially on large parts, and is therefore conducive to acquiring better internal quality [5]. High efficiency, easy cleaning, and safe storage and transportation of wire melting. Compared with powder, wires are characterized by a small specific surface area, low possibility of absorbing impurities and water vapor in the air, and zero risk of combustion or explosion. Therefore they present no potential harm to human health, and are easy to be stored and transported. The wire-making process is also a process when the internal quality of material is examined. The materials with impurities are more likely to be broken, and it is easier to ensure the purity of materials by using wires as the raw material for prototyping. However, as restricted by the wire making process, materials must be ductile.

Due to the outstanding advantages mentioned above, this technology has been more and more valued across the globe. Promising prospects are seen in traditional manufacturing scenarios of complex metal structures that are costly and lengthy [6–8]. (4)

Flexible process and method control to enable synergized and optimized multiprocess design and manufacturing of large complex structures. Boasting high power, electron beams can realize movement and focus control through electromagnetic field and realize high-frequency complex scanning movement. The surface scanning technology allows it to achieve large-area preheating and slow cooling, while the multi-beam splitting processing technology enables the coordination among multiple beams. In this case, one beam is used for prototyping while other electron beams are used to apply surface scanning and generate a temperature field around the path. This technology is of great significance for controlling the stress and deformation during the prototyping of large structures. Both the wire deposition and the deep penetration welding

1.3 Principles and Characteristics of the Electron Beam Wire Deposition Technology

(5)

9

technologies can be realized on one piece of equipment. A variety of processing technology combinations can be adopted According to the structural form and the service performance requirements of parts to minimize the cost and optimize the performance and process. By utilizing the multi-functional processing capability of electron beams, this technology can realize the multi-process and collaborative optimization, design and manufacturing of large and complex structures, which can be reflected in the large titanium alloy universal joint produced by Sciaky USA and Beaver Aerospace and Defense in 2002. The main part of the universal joint was manufactured by rapid prototyping. The specially required trunnion was processed by forgings and welded on the main part by the electron beam welding technology. This dual-process capability is an important feature of the electron beam processing technology, allowing users to manufacture parts in the most cost-efficient way. It is a promising green manufacturing technology with low consumption, low pollution, high efficiency, energy saving, and environmental protection. The energy conversion efficiency of electron beams is high, and the power consumption is low; the prototyping process does not require inert gas, and the prototyping equipment suffers nearly zero loss except a small amount of tungsten wires as the cathode; the electron gun features high reliability and long service life, even in a high-power state for a long term; the prototyping process is carried out in a seal vacuum chamber, generating no waste liquid, waste gas or other pollution. Thus, it is a green manufacturing technology.

Compared with the laser deposition technology based on synchronous powder feeding, the electron beam wire deposition technology also has some disadvantages, such as lower dimensional accuracy, expensive equipment, complicated process control, and difficult heat dissipation in vacuum environment. However, with outstanding advantages in terms of prototyping efficiency and internal quality, the EBWD technology has received more and more attention in the weaponry development and manufacturing industry at home and abroad. Promising prospects are seen in traditional manufacturing scenarios of complex metal structures that are costly and lengthy.

1.4 Role of the EBWD Technology in the National Defense and National Economy The EBWD technology has important strategic significance for the design, manufacturing, innovation, and development of weaponry, which is mainly embodied in four aspects. First, it provides strong technical support for structural innovation. The emergence of the additive manufacturing technology means that any structure can be manufactured if it is designed. Challenges of complex structure manufacturing that can never be realized by traditional manufacturing technologies are now solved. This manufacturing technology also lays a solid foundation for advanced design

10

1 Introduction

methods such as stress design and optimized design by the integration of structures and functions. Second, it allows for rapid response and agile manufacturing. With no need for molds and fewer processes, the additive manufacturing technology can boost the rapid development of weaponry. Third, the near-net shape can save a lot of material and machining workload and cost for complex structures. Fourth, weapons can now be re-manufactured and repaired, which can greatly extend the service life of weaponry and reduce the full-life cost. The EBWD technology is not only a new digital manufacturing technology, but also an innovative concept that better binds up the design, manufacturing, materials, application and other resources. It is a powerful driving force to promote the upgrading of manufacturing technologies, accelerate the development of weaponry and civilian equipment, and boost the development of advanced manufacturing technologies.

1.5 Current Status and Development Trends at Home and Abroad 1.5.1 Technological Status of Foreign Researches The EBWD technology is the latest development direction of the high-energy beam processing technology. Based on the mature high-energy beam surfacing and cladding technology, it also combines the rapid prototyping, the computer-aided design and manufacturing, and the flexible automation technologies to directly manufacture high-performance, complex, and compact metal parts in near net shapes. It is the latest development direction of the advanced manufacturing technology and the additive manufacturing technology. The EBWD technology based on wire deposition can be back to 1995 when Dave from the Massachusetts Institute of Technology (MIT) proposed the concept of Electron Beam Solid Freeform Fabrication (EBSFF). Subsequently, Matz and Eargar established the EBSFF device. The workbench has two shafts, and the workbench in contact with the workpiece is cooled by water. The wire feeding mechanism has three shafts, and the stacking is realized by shaft elevation. Then the In718 turbine disc was trial-produced by using the device they built. The result showed that the positioning accuracy was better than that of spray forming, but the surface quality was not high, and subsequent processing was still needed. The main global technological developers of electron beam wire deposition and fabrication technologies for metal structures were NSSA Laney Research Center and Sciaky USA. Those who involved in the research, testing and evaluation of the EBWD technology included Lockheed Martin, Boeing, Air Force and Navy departments, and the Canadian Space Manufacturing Technology Center. The NSSA Laney Research Center has studied the Electron Beam Freeform Fabrication (EBF3 ) technology. The EBF3 system developed by it consists of a high-power

1.5 Current Status and Development Trends at Home and Abroad

11

Fig. 1.6 EBF3 equipment developed by NASA langley research center

electron gun, a dual wire feeding system that can be independently controlled respectively, and a six-axis mechanical system. The movement range of the workbench is 72 inches × 24 inches × 24 inches. The equipment is shown in Fig. 1.6. In 2002, NASA Langley Research Center and Mashall Space Center announced the EBF3 technology, and continued to carry out application research in the space microgravity environment. The purposes of the technology were initially the fabrication of super-large complex metal structures in space and the manufacturing of metal parts in space environment (shown in Figs. 1.7 and 1.8), and the technological accumulations for space exploration. The main research subjects were structural titanium alloys, aviation aluminum alloys, aluminum–lithium alloys, etc., and the technology could be used to produce very complex parts (as shown in Figs. 1.9 and 1.10). As technologies are growing more and more developed, NASA has progressively carried out the low-cost manufacturing of large structures like spacecraft and large engines since 2007, and formulated a development plan for the electron beam wire deposition technologies for the next few years, as shown in Fig. 1.11. Together with partners such as Boeing and Lockheed Martin, NASA has accelerated the development of specifications and standards, and has developed ground applications such as the cabin panels for next-generation heavy-duty transport aircraft and engine structures, as shown in Figs. 1.12 and 1.13. Sciaky USA began the development of electron beam wire deposition process and equipment in 2000. In 2002, it cooperated with Lockheed Martin to carry out the research of EBF3 technology. Figure 1.14 shows the EBF3 equipment developed by Sciaky. The electron gun is 60 kW, 60 kV. The materials fabricated mainly include titanium alloy (Ti-6Al-4 V, Ti-8Al-1Er), aluminum alloy (Al 2319, Al 2195) and so

12

Fig. 1.7 Zero-gravity fabrication test carried out by NASA

Fig. 1.8 Schematic diagram of the large spacecraft fabricated by electron beams

Fig. 1.9 Complex titanium alloy structure formed by NASA

1 Introduction

1.5 Current Status and Development Trends at Home and Abroad

(A) Blank

13

(B) After machining

Fig. 1.10 Al2219 aluminum alloy propeller frame made by NASA using EBF3 technology

Fig. 1.11 NASA’s EBWD technology development plan

on. As for aluminum alloys and titanium alloys, the maximum deposition rate can reach 3500 cm3 /h, and the performance can rival that of forgings. The test and evaluation results of EBWD show that the maximum deposition speed can reach 3500 cm3 /h when depositing titanium alloys and aluminum alloys with the strength and plasticity superior to that of forgings. In addition to commonly used aerospace materials such as Al2319, Al2195, aluminum–lithium alloys, copper alloys, and Ti-6Al-4 V, the United States has also developed Ti-8Al-1Er, a titanium material specifically for the electron beam forming process. Sciaky USA has conducted a performance evaluation for the Ti-6Al-4 V titanium alloy AMAD support on an F-22 fighter (as shown in Fig. 1.15). They ran two full-life broad-spectrum fatigue tests and a maximum load test according to the quality requirements for parts at the time, but no permanent deformation was found. Since 2002, Sciaky USA has been committed to the study on the manufacturing of large-scale aerial metal structures and the EBWD equipment and technology. It has cooperated with Beaver Aerospace and Defense to manufacture large-scaled Ti-6Al-4 V metal universal joints (as shown in Fig. 1.16) by applying the

14

1 Introduction

Fig. 1.12 NASA’s research efforts on the fabrication of cabin panels on heavy-duty transport aircraft

Fig. 1.13 Aircraft engine casing manufactured by NASA

1.5 Current Status and Development Trends at Home and Abroad

15

Fig. 1.14 EBF3 equipment of Sciaky USA

Fig. 1.15 Ti-6Al-4 V F-22 AMAD support

electron beam wire deposition and the electron beam welding combination methods. This dual-process capability is one of the characteristics of the EBWD technology, allowing users to manufacture parts in the most cost-efficient way. During the production of universal joints, a total of 108 kg of Ti-6Al-4 V wires at ϕ2.4 mm are used, and the deposition rate is 500 cm3 /h (2.3 kg/h). The final dimension of the part is ϕ432 × 297 mm and the wall thickness is 76 mm. It takes about 5 weeks to complete all processing steps, instead of at least 12 weeks needed by traditional methods, speaking

16

1 Introduction

Fig. 1.16 Ti-6Al-4 V metal universal joint

Table 1.2 Room-temperature tensile properties of Ti-6Al-4 V manufactured by the EBF3 technology

UTS/ MPa

Yield/ MPa

RA/%

X

993

881

17.8

Y

1014

899

20.0

Z

967

848

23.3

AMS4928

896

827

20.0

to the maturity of this technology. The comparison data of Ti-6Al-4 V parts manufactured by the EBF3 technology and the AMS4928 in room-temperature tensile properties are shown in Table 1.2. The tensile strength and yield strength exceed standard requirements in all the aspects of fabrication. The cross-sectional scanning electron microscope image shows it is an α-β biphasic organization (Fig. 1.17). Fig. 1.17 Cross-sectional scanning electron microscope image of Ti-6Al-4 V manufactured by the EBF3 technology [3]

1.5 Current Status and Development Trends at Home and Abroad

17

Brochu et al. from Canada used Sciaky’s equipment to study the internal structure and properties of Ni–Cr-Si-B solder and Hastalloy X wire after being formed by electron beam wire deposition. The results showed that the solder produced more holes after being formed, and the number of holes could be reduced by multiple times of re-melting; wires could be used to manufacture hole-less structures, making it suitable for the forming and repair of aerospace components [2]. In addition to NASA and Sciaky USA, the University of Virginia, the University of Pennsylvania, Boeing, Lockheed Martin, Pratt and Whitney Aircraft Engines, U.S. Naval Research Laboratory (ONR), Oak Ridge National Laboratory (ORNL) and many other organizations have also participated extensively in the studies on basic theory, application foundation and engineering applications. The materials they research include aluminum alloys, titanium alloys, nickel-based alloys, stainless steel, etc. The main research topics include the performance control mechanism and performance reliability evaluation, the online beam quality measurement, the molten pool temperature simulation, the stress and deformation laws, the active control of fusing process and intelligent optimization of process parameters, the nondestructive testing methods, the structural optimization and design, and the law of element evaporation of complex alloy systems, etc. [2, 9–12] AMS4999A, a standard for the direct prototyping of metal parts, has been established. More specifications and standards are being formulated. Performance tests and assessments are being carried out. Technologies are actively applied and promoted. In terms of electron beam heat source, molten pool behavior and temperature field research, the United States has conducted a lot of basic research, and the research methods mainly include numerical simulation, online detection, and thermal imaging. The research emphasis is placed on the shape and temperature of the molten pool to reveal the laws of heat/mass transfer and fusing in the EBRM process. The purpose is to control the fusing process of this technology in an online manner. The study of molten pool behaviors mainly focuses on the transmission and effects of matter, force, and energy between temperature field, flow field, wires, and the molten pool as well as the evaporation and dissipation of alloy elements. The studies and reports on the EBWD technology are fewer than that of laser. In 1995, Dave et al. established a mathematical model for the rapid prototyping of electron beam wire deposition on the basis of comparing electron beam, laser and arc as heat sources [13, 14]. According to the theory, the author believes the electron beam is a “body heat source” while the laser and the arc are “surface heat source” based on the principle of thermal energy conversion. This theory explains the big difference in the efficiency of power conversion and heat absorption. Thereby the author proposed the mathematical model of a new heat source for EBRM prototyping, which can obtain the electron beam power required by a specific material at a specific deposition rate. After a test on stainless steel materials, the mathematical model is aligned with the test result. In 2005, Hofmeister et at. studied the shape change, temperature distribution, and flow behavior of the molten pool during the EBRM prototyping process with an infrared thermometer. They found that the temperature gradient of the molten pool was large, and the temperature of the molten pool was directly related to the energy density of beams [15]. The study found that, under the circular scanning by the electron

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beams, the temperature at the center of the circular area acted by the beam spot with the highest energy density reached above 1900 °C, while the temperature at the tail of the molten pool dropped sharply to about 1500 °C. Through the study of the dynamic temperature field of the molten pool, the contours of the molten pool, the end of wires and the liquid transition metal can be calibrated, so as to establish the relationship between the shape of the molten pool, the stacking process, and process parameters, along with the position of the end of wires relative to the molten pool (critical to the stability of the fusing process), thus providing data support for dynamically adjusting heat source parameters and actively controlling the transition behavior of liquid metal. In 2005, with the support of NASA, Chandra et al. [16] started to study the finite elements of electron beam wire deposition, including the simulations of heat sources and molten pools, stress and deformation. Remarkable progress has been achieved in grain size prediction of solidified structures, molten pool flow behavior and influence, optimization of process parameters, and online control of the prototyping process. In order to control the fusing process, with the support of the SBIR project undertaken by U.S. Air Force (USAF), Sciaky USA has joined hands with multiple organizations to carry out the research on the real-time monitoring of electron beam energy density and the online measurement of molten pool temperature and topography. There is a genetic effect between the orientation of solidified structures formed by electron beam wire deposition and the matrix structure. Compton et al. [17] explored the possibility of using electron beam wire deposition to produce single crystals. The results showed that monocrystalline accumulations at a certain height can be obtained from monocrystalline Nb alloys. Wallace et al. [18] studied the effects of various parameters and parameter combinations of electron beam wire deposition on the shape of solidified structures and the molten pool through orthogonal experiments. Kelly et al. [19, 20] systematically studied the growth characteristics of columnar crystals during the laser multilayer stacking process through experiments and numerical simulations, and revealed the formation mechanism of periodic layer band-like gradient structures that appear along the height direction. Zalameda et al. [21] studied the thermal distribution image of parts manufactured by EBWD; Fox et al. [22] studied the image calibration method of the molten pool of EBWD; Soylemez et al. [23] studied the method of controlling the area of the molten pool of EBWD by coupling multiple parameters such as power and speed. The abovementioned studies are of great significance to realize the closed-loop control of the prototyping process by controlling the area and temperature of the molten pool. In terms of the material structure and mechanical properties of EBWD, a great number of studies have been conducted abroad on the structure and mechanical properties of titanium alloys and aviation aluminum alloys prototyped by the EBWD technology. Those researches mainly focus on the influence of process parameters on prototyping and the influence of heat treatment on structural properties. Also, efforts have been made to explore the significant role of chemical composition on mechanical properties [24–26]. For example, when Barnes et al. [27] were studying the mechanical properties of TC4 titanium alloy prototyped by electron beam wire deposition, they found that the strength of titanium alloy materials made of common ingredients

1.5 Current Status and Development Trends at Home and Abroad

19

was significantly lower than that of forgings. The analysis indicated that the prototyping process was completed in vacuum environment where the alloying elements (such as Al, etc.) evaporate violently at a low melting point, leading to more losses during the prototyping process. Lach et al. [28] systematically studied the influence of beam power, moving speed, wire feeding speed and parameter combinations on the evaporation of Al element through orthogonal experiments. They found that wire feeding speed significantly acted on element evaporation. Brice et al. [2, 29] studied the structure and properties of different titanium alloys, and specially developed the Ti-8Al-1Er alloy for electron beam wire deposition. Karen et al. [30, 31] studied the influence of process parameters (such as moving speed and wire feeding speed) on the deposition morphology of Al2219 and Al2319 aluminum alloys as well as the evolution law of the internal structure of materials. They sought for ways to control the internal structure via the prototyping process. The scarcity of massive and comprehensive performance data is the biggest challenge faced when the electron beam wire deposition technology is applied in the aerospace field. For this reason, the United States has stepped up the efforts of performance testing and assessment. Boeing has cooperated with Sciaky USA [32] to conduct systematic tests on the Ti-6Al-4 V material prototyped by electron beam wire deposition according to AMS4999A and company standards. The results showed that the fatigue life and fracture toughness were much higher than the rated value, and the Z-axis tensile strength was also higher than the rated value. But the performance on the X and Y axes was slightly lower than the standard (with the maximum difference of 26 MPa). According to the composition test, it was found that after Ti6Al-4 V was prototyped by EBF3 , the Al element suffered a great loss and dropped below the standard value. They believed that the lower vapor pressure in the vacuum environment was the possible reason for the excessive evaporation of the Al element. However, due to the easy adjustment of the electron beam power, it was very possible to find a suitable process, but only with more follow-up researches. In the research on the stress and deformation control of prototyped structures, the rapid prototyping technology requires both the filling of paths in the stacking layer and the superposition of paths between layers. After going through complex multiple thermal cycles, materials become very complicated in their internal stress state. Alexander [33] compared the thermal stress phenomena in the prototyping process of various 3D additive manufacturing technologies, and studied the relationship between prototyping paths, layer parameters and deformation behaviors. Mulani et al. [34–36] conducted an in-depth study on the stress and deformation laws of the flat rib structure prototyped by electron beam wire deposition, and found that the anti-deformation effect of the curved rib is significantly better than that of the linear rib. Then they optimized the shape and direction of the curved rib and its position relative to the flat rib, and achieved fine results. Bird et al. [37] applied the experimental measurement method to study the deformation law of In718 alloy thin plate rib structure prototyped by electron beams. The experiment compared the deformations of the substrate when it’s water-cooled, pre-heated, and neither watercooled nor pre-heated. Lin et al. [38] also studied the residual stress and deformation

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1 Introduction

through finite element analysis, and found that most of the residual stress appeared in the first few layers, and the stress was concentrated in the T-shaped joint. In terms of prototyping quality control, Stecker et al. [39] carried out the “Research on Closed-loop Control Technology for Electron Beam Direct Manufacturing” under the support of the SBIR project. In the first stage (2009), the manufacturing and implementation of a closed-loop process control system were completed to realize the closed-loop process control for the fusing process and improve the repeatability and consistency of products. In the second stage (2011), the fully-closed-loop control system for titanium with high deposition rate was demonstrated. The control parameters coming from the feedback of the molten pool improved the material quality, reduced the dependence on the operator, and lowered the cost. Under the support of the Air Force’s SBIR project, Sciaky USA has carried out the research on the EBWD technology for titanium alloys and its performance reliability. Since 2009, improvements have been made on the data processing system of sensors and process control to provide high-quality and repeatable parts for supporting the applications in military and commercial entities, reducing costs, shortening delivery time, and providing designers with more flexible room for design. Sciaky produced titanium alloy aerospace structures, reduced cost and delivery time, and improved the quality and reliability of parts produced by the electron beam direct manufacturing (EBDM) technology to meet the plan and requirements of the Air Force. In 2008, Slattery et al. [40] systematically studied the imaging characteristics of titanium alloy prototyped by electron beam wire deposition under ultrasonic, X-ray, penetration, and eddy detection. In the next stage, they furthered their research to the non-contact non-destructive testing with raw surface and the real-time testing technology during deposition. But there is still a lack of research on the abnormal noise signals in ultrasonic testing and uneven shadows in X-ray testing. In summary, other countries are aggressively developing the wire deposition prototyping technology to manufacture large-scale monolithic titanium alloy structures. This technology is also regarded as one of the key technologies for the development of new-generation weaponry.

1.5.2 Status of Domestic Researches China began the research on the additive manufacturing technology of electron beam wire deposition in 2003. The Key Laboratory of High Energy Beam Processing Technology of AVIC Manufacturing Technology Institute (formerly Beijing Aeronautical Manufacturing Technology Research Institute) has taken initiative to study this technology in China. After years of efforts and in cooperation with Shenyang Institute of Metal Research under Chinese Academy of Sciences, and AVIC aircraft design institutes in Shenyang, Xi’an, and Chengdu, breakthroughs have been made in the high-speed and stable fusing technology of wires, the path optimization technology of complex parts, the deformation control technology of large structures, the mechanical property control technology, the special material development technology, and

1.5 Current Status and Development Trends at Home and Abroad

(a) First EBWD equipment in China

21

(b) Largest EBWD equipment in China

Fig. 1.18 Electron beam wire additive manufacturing equipment developed by AVIC manufacturing technology institute

many other technologies. The study of the EBWD technology has been furthered on and on and it is now industrialized from a technical concept. From small prototypes to the world’s leading electron beam forming equipment, from process research to raw material development, the leaping development is witnessed in materials, equipment, and technical services. The secondary and primary titanium alloy bearing structures prototyped by the EBWD technology were installed and applied for the first time in 2012 and 2016 respectively. In terms of equipment development, China has developed the first and currently the largest electron beam wire melting prototyping equipment in China, as shown in Fig. 1.18. Meanwhile, a large number of test pieces prototyped by electron beam wire deposition have been provided for the development of new aircraft. The materials involved include titanium alloys such as TC4, TC4-DT, TA15, TC18, TC21, TC11, TC17, and A-100 ultra-high-strength steel, GH4169G alloy, etc. AVIC Manufacturing Technology Institute collaborated with Shenyang Institute of Metal Research, Chinese Academy of Sciences to jointly study the performance control on titanium alloy materials, ultra-high-strength steels and superalloys fabricated by the EBWD technology. Suo Hongbo et al. [41, 42] found that, as for mediumstrength titanium alloys, their strength is low but the plasticity is sound after being prototyped by electron beams. As for high-strength and high-temperature titanium alloys prototyped, their strength is high but the plastic toughness is significantly lower. The alloying degree of high-strength and high-temperature titanium alloys is high, and the mechanical properties such as plasticity and toughness are very susceptible to structural changes. Although the A-100 ultra-high-strength steel features sound plasticity and strength, its fracture toughness is low, therefore the in-depth study on its strengthening and toughening mechanism is urgently needed. Pang Shengyong et al. [43, 44] carried out numerical simulation studies on the dynamic behavior of the molten pool and defect formation mechanism. AECC

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Beijing Institute of Aeronautical Materials, AVIC Manufacturing Technology Institute, and other organizations jointly conducted research on the non-destructive testing characteristics of prototyped materials. In summary, China has seen fast progress in the application study, and EBRMfabricated parts have been applied on aircraft. However, we still don’t have sufficient knowledge about this new technology, especially the microstructure evolution and performance control mechanism of materials.

References 1. Qiao G (2018) Guan Qiao collection [G]. Aviation Industry Press, Beijing, pp 506–512 2. Hongbo S (2014) Research on the microstructure and mechanical properties of TC4 titanium alloy fabricated by electron beam wire deposition [D]. Huazhong University of Science and Technology, Wuhan 3. Gong Shuili, Li Huaixue, Suo Hongbo (2009) Application and development of high-energy beam processing technology [J]. Aeronaut Manufact Technol (14):34–39 4. Morring F (2013) Additive advances 3-D printing moves on to the factory floor[R]. Aviat Week Space Technol, 18 5. Brice CA, Henn DS (2002) Rapid prototyping and freeform fabrication via electron beam welding deposition[C]. Proceeding of Welding conference 6. Walker BH, Walker CM (2003) Material presented on Canadian aviation expo[R]. Montreal, Canada 7. Concurrent technologies corporation[R] (2007) Aircraft Application. http://www.cte.com 8. Wagner J (2010) Near net shape forming of a metallic orion shell[C]. 2010 National Space and Missile Materials Symposium (NSMMS2010). Scottsdale, Arizona, USA 9. Cooper KP (2012) ONR progroms in direct part manufacturig[R]. http://www.midwestsampe. org/content/files/events/dpmworkshop2012/Cooper%20ONR%20AM.pdf 10. Yarrapareddy E, Carbone F, Valant M (2005) Mechanical and microstructural characterization of inconel 718 3D direct metal depositions by electron beam[J]. SAMPE, pp 1–14 11. Bird RK, Hibberd J (2009) Tensile properties and microstructure of inconel 718 fabricated with electron beam freeformfabrication (EBF3) [R]. NASA Langley Research Center, Hampton, VA 23692:1–19 12. Wang L, Felicelli SD, Coleman J et al (2011) Microstructure and mechanical properties of electron beam deposits of AISI 316L stainless steel[C]. ASME 2011 International Mechanical Engineering Congress and Exposition.Denver, Colorado, USA, 3, pp 15–21 13. Dave VR (1995) Electron Beam (EB)-Assisted Materials Fabrication[D]. Massachusetts Institute of Technology 14. Dave VR, Matz JE, Eagar TW (1995) High energy electron beam (HEEB)-solid interaction model for EB deposition and shock processing[C]. Proceedings of International Conference on Beam Processing of Advanced Materials, pp 255–260 15. Hofmeister WH (2005) MS&T.Pittsburgh, PA 16. Chandra U, Barot G (2007) Finite element models for the electron beam freeform fabrication process[C]. AeroMat Conference and Exposition. Baltimore, Maryland, USA 17. Compton C, Baars D, Bieler T (2007) Studies of ternative techniques for niobium cavity fabrication[R]. Material presented at the 13th International Workshop on RF Superconductivity, Bejing, China, WEP: Poster Session II, pp 429–433 18. Wallace TA, Bey KS, Taminger KMB et al (2004) A design of experiments approach defining the relationships between processing and microstructure for Ti-6Al-4V[C]. Proceedings of 15th SFF Symposium. Austin, TX(US), pp 104–115

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19. Kelly SM, Kampe SL (2004) Microstructural evolution in laser-deposited multilayer Ti-6Al-4V builds. Part I. Microstructural characterization[J]. Metall Mater Trans A 35(6):1861–1867 20. Kelly S, Kampe S (2004) Microstructural evolution in laser-deposited multilayer Ti-6Al-4V builds. Part II. Thermal modeling [J]. Metall Mater Trans A 35(6):1869–1879 21. Stockton GR, Zalameda JN, Burke ER et al. (2013) Thermal imaging for assessment of electron-beam freeform fabrication (EBF3) additive manufacturing deposits[C]. Proceedings of SPIE 8705, Thermosense: Thermal Infrared Applications XXXV. Baltimore, Maryland, USA, 87050M-87050M-8 22. Fox J, Beuth J (2013) Process mapping of transient melt pool response in wire feed E-beam additive manufacturing of Ti-6Al-4V[C]. In Proceedings of Solid Freeform Fabrication Symposium. Pittsburgh, Austin, Texas, USA, pp 675–683 23. Soylemez E, Beuth JL, Taminger K (2010) Controlling melt pool dimensions over a wide range of material deposition rates in electron beam additive manufacturing[C]. Proceedings of Solid Freeform Fabrication, pp 571–582 24. Taminger KMB, Hafley RA, Fahringer DT et al (2004) Effect of surface treatments on electron beam freeform fabricated aluminium structures[C]. Proceedings of 15th SFF Symposium. Austin, TX(US), pp 460–470 25. Henry C (2006) Devlopment of laser fabricated Ti-6Al-4V[R]. National Aeronautics and Space Administration, Glenn Research Center, pp 1–21 26. Taminger KM, Hafley RA (2006) Electron beam freeform fabrication (EBF3) for cost effective near-net shape manufacturing[R]. National Aeronautics and Space Administration, Langley Research Center, AVT-139-unassigned: 1-unassigned 9 27. Branes JE, Brice CA, Taminger KM et al. (2005) Fabrication of titanium aerospace components via electron beam freeform fabrication[C]. 2005 AeroMat Conference and Exposition.Orlando, Florida, USA 28. Lach CL, Taminger KM, Schuszler AB, et al (2007) Effect of electron beam freeform fabrication (EBF3) Processing parameters on composition of Ti-6–4[C]. 2007 AeroMat Conference and Exposition. Baltimore, Maryland, pp 1–19 29. Bush RW, Brice CA (2012) Elevated temperature characterization of electron beam freeform fabricated Ti–6Al–4V and dispersion strengthened Ti–8Al–1Er[J]. Mater Sci Eng 544:13 30. Domack MS, Taminger KMB, Begley M (2006) Metallurgical mechanisms controlling mechanical properties of aluminum alloy 2219 produced by electron beam freeform fabrication[J]. Mater Sci Forum 519:1291–1296 31. Taminger KM, Hafley RA, Domack MS (2006) Evolution and control of 2219 aluminum microstructural features through electron beam freeform fabrication[J]. Mater Sci Forum 519:1297–1302 32. Heck D, Slattery K, Salo R et al (2007) Electron beam deposition of Ti 6-4 for aerospace structures[C]. AIAA SPACE 2007 Conference & Exposition, Long Beach, California, USA, 6195, pp 1–7 33. Nickel AH (1999) Analysis of thermal stresses in shape deposition manufacturing metal parts[D]. Stanford University, Ph.D.Thesis 34. Kapania RK, Li J, Mulani SB, et al (2007) Design and optimization of structure using additie manufacturing processes. http://nia-cms.nianet.org/getattachment/resources/Education/Contin uining-Education/Seminars-and-Colloquia/Seminars-2007/Kapania_080307.pdf.aspx 35. Mulani SB, Li J, Joshi P, et al (2007) Optimization of stiffened electron beam freeform fabrication (EBF3) panels using response surface approaches[M]. A collection of technical papers, Waikiki, Hawaii, pp 2429–2437 36. Gaillard J, Locatelli D, Mulani SB (2008) Residual stresses in a panel manufactured using EBF3 process[C]. Comsol Conference Boston, USA 37. Brice CA, Taminger KM (2011) Additive manufacturing workshop. http://amcrc.com.au/wpcontent/uploads/2013/03/CSIRO-NASA-additive-manufacturing-workshop.pdf 38. Lin SY, Hoffman EK, Domack MS (2007) Distortion and residual stress control in integrally stiffened structure produced by direct metal deposition. http://ntrs.nasa.gov/search.jsp?R=200 80013388

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39. Stecker S, Lachenberg KW, Wang H, et al. (2006) Advanced electron beam free form fabrication method &technology[C]. Soc Manufact Eng Techn Conf 35–46 40. Chen Zheyuan, Suo Hongbo, Li Jinwei (2010) Electron beam wire deposition rapid manufacturing technology and microstructure characteristics [J]. Aeron Manufact Technol (1):36–39 41. Jun L, Hongbo S, Jianrong L et al (2012) Tensile properties of the columnar crystal structures of TC18 titanium alloy fabricated by electron beam wire deposition [J]. Trans Mat Heat Treat 33(6):110–115 42. Pang SY, Chen BB, Suo HB, et al. (2012) A preliminary study on the heat transfer and fluid flow of weld pool in EBF3 process[C]. 2012 International Conference on Power Beam Processing Technologies. Qingdao, Shandong, China 43. Binbin C, Shengyong P, Jianxin Z et al (2013) Numerical simulation of scanning electron beam welding temperature field of TC4 titanium alloy [J]. Trans China Weld Inst 7:33–37 44. Tang Qun, Pang Shengyong, Chen Binbin, et al. (2013) Numerical simulation for heat transfer and flow in the molten pool formed by the electron beam wire deposition of titanium alloy [C]. The 18th National Welding Conference, Nanchang, Jiangxi

Chapter 2

Electron Beam Wire Deposition Equipment

Abstract This chapter introduces structure and principle of Electron Beam Wire Deposition (EBWD) Equipment. At the same time, an overview of equipment at home and abroad has been done. The homemade equipment has been introduced in detailed such as Inverter Power Supply, Electron Beam Gun, Vacuum System and Its Control, Wire Feeding System and Its Control, Three-Dimensional Workbench, Image Monitoring and Acquisition System, Electrical Control System and Data Processing Software—Electron Beam RP, etc.

2.1 Structure and Principle of the Equipment The electron beam wire deposition (EBWD) equipment evolves from the electron beam welding equipment. Compared with the latter, it requires higher equipment stability and beam accuracy and better adaptability to long-term operation in terms of hardware. Also, the equipment adds the wire feeding system to fit for the prototyping of complex parts. The freedom of translation is higher than the welding equipment, making it capable of prototyping parts of greater complexity. The equipment usually has two layouts, the fixed-gun type and the moving-gun type, as shown in Figs. 2.1 and 2.2. A typical unit of EBWD equipment mainly includes: (1)

(2)

(3)

An electron beam generation system: It is mainly composed of an electron gun and a power supply. Its function is to generate a stable and controllable electron beam with high energy density, which is used as the heat source of the equipment to melt the wire feeding. A vacuum system: It is mainly composed of a vacuum pump set, a working vacuum chamber and corresponding sensors. Its function is to provide a vacuum environment suitable for generating electron beam. A translation and wire feeding control system: It is mainly composed of a multiaxis translation platform, a wire feeding mechanism and a control system. The minimum requirement for the degree of freedom (DOF) on the translation platform is three DOFs, while more DOFs can be added for manufacturing more complex parts. The wire feeding mechanism can feed wires through one

© National Defense Industry Press 2022 S. Gong et al., Electron Beam Wire Deposition Technology and Its Application, Additive Manufacturing Technology, https://doi.org/10.1007/978-981-19-0759-3_2

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2 Electron Beam Wire Deposition Equipment

Fig. 2.1 Schematic diagram of the fixed-gun EBWD equipment 1 High-voltage power supply; 2 Vacuum pump set; 3 Multi-DOF workbench; 4 Working substrate; 5 Prototyped parts; 6 Working vacuum chamber; 7 Control system; 8 Wire feeding system; 9 Electron beam gun; 10 High voltage cable

Fig. 2.2 Schematic diagram of the moving-gun EBWD equipment 1 High-voltage power supply; 2 Vacuum pump set; 3 Multi-DOF workbench; 4 Working substrate; 5 Prototyped parts; 6 Working vacuum chamber; 7 Control system; 8 Wire feeding system; 9 Electron beam gun; 10 High voltage cable; 11 Electron gun translation mechanism

2.1 Structure and Principle of the Equipment

(4)

(5)

27

channel or multiple channels. The control system controls the wire feeding mechanism to send the wire to the designated position for melting and prototyping, and controls the multi-axis translation platform to move along the preset path. A monitoring system: It is mainly composed of an image acquisition system (including cameras, sensors, etc.) and a data processing system. Its main function is to monitor the prototyping process, including component morphology, molten pool characteristics, temperature field distribution, etc. An integrated control system: The main function is to comprehensively control the electron beam source generating system, the vacuum system, the cooling system, the translation and wire feeding control systems, the monitoring system, etc.

In addition, the EBWD equipment is also accompanied by some auxiliary devices and software, such as: (1)

(2)

(3)

Water cooling device: It is used to cool the hardware that generates a lot of heat during the working process, such as electron beam gun, high voltage oil tank and vacuum pump set. Air supply system: A pneumatic device is usually needed for the opening and closing of vacuum valves, and the pneumatic device is generally equipped with an air supply system. Processing software: It’s the software required for the wire deposition. The software enables the planning of layers and path required according to the pre-set machining procedure.

2.2 Overview of Equipment at Home and Abroad 2.2.1 Overview of Equipment Abroad At present, foreign companies engaged in the research and development of EBWD equipment mainly include the NASA Langley Research Center of the United States, the Sciaky USA and the Nuclear Technology Advanced Manufacturing Research Center of the University of Sheffield in the United Kingdom. The NASA Langley Research Center began to develop the EBWD additive manufacturing technology in 2001, and has discovered the field of “EBWD additive manufacturing in microgravity space environment”. It cooperated with Sciaky USA to develop a prototype used for the microgravity environment, which is shown in Fig. 2.3. With the support of the U.S. Department of Defense and other national funds, Sciaky USA, in conjunction with the University of Virginia and other research institutions, conducted some basic research on the dynamic measurement of electron beam power density during the fabrication process and the real-time monitoring of

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Fig. 2.3 NASA EBF3 equipment for zero-gravity test

molten pool temperature and size, and part temperature. It can simulate the prototyping process beforehand according to the material and structure of parts and automatically generate an optimized machining plan. The developed interlayer real-time imaging and sensing system (IRISS) can sense and digitally adjust metal cladding, with excellent accuracy and repeatability. It is currently the only system in the field of metal additive manufacturing that can achieve real-time monitoring and controlling. Sciaky’s EBAM metal additive manufacturing equipment is categorized into EBAM 300, EBAM 150, EBAM 110 (large), EBAM 88 and EBAM 68 (medium). The length of prototyped parts can reach 5.79 m. Its have been delivered to NASA Langley Research Center (Fig. 2.4) and Lockheed Martin Company (Fig. 2.5). In addition, the equipment developed by Sciaky USA can realize five-coordinate (Fig. 2.6) and six-coordinate (Fig. 2.7) translation. A unit of EBAM 110 EBWD manufacturing equipment was delivered to Airbus in December 2016. Being capable of prototyping parts sized as large as 1778 mm (width) × 1194 mm (length) × 1600 mm (height), the equipment is used to manufacture large aerospace metal parts to reduce costs and time of manufacture. The Nuclear Technology Advanced Manufacturing Research Center currently has the world’s largest EBWD equipment K2000, as shown in Fig. 2.8a. The maximum fabricable size can reach 6.4 m. The typical prototyping materials mainly include TC4 titanium alloys, 316L stainless steel, etc. The maximum deposition rate is 400cm3 /h, and the prototyping accuracy is high. In 2017, NVO Chervona Hvilya of Ukraine successfully developed the world’s first electron beam coaxial wire deposition additive manufacturing equipment, as shown in Fig. 2.8b. Figure 2.8c is the state diagram of the “X-EBAM” annular

2.2 Overview of Equipment at Home and Abroad

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Fig. 2.4 Sciaky’s equipment drawing used by Langley Research Center

electron beam wire produced by this equipment. The equipment has a maximum prototyping efficiency of 2000cm3 /h and adopts the coaxial melting method, which has greater freedom of prototyping; the annular electron beam focuses on heating the coaxially fed wire, and the input heat is concentrated on the wire to avoid unnecessary heat input on the substrate of prototyped parts, which makes for improving the prototyping quality (Figs. 2.6 and 2.7).

2.2.2 Overall Development in China China began the research on the additive manufacturing technology of electron beam wire deposition in 2003. Relying on the National Key Laboratory of High Energy Beam Processing Technology, the former Beijing Aeronautical Manufacturing Technology Research Institute of China Aviation Industry Corporation (currently AVIC Manufacturing Technology Institute) took the lead in China in establishing the professional research direction of electron beam wire deposition additive manufacturing, with focus on the research of electron beam wire deposition additive manufacturing equipment, prototyping technologies of typical metal structure, performance control

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Fig. 2.5 Sciaky’s equipment used by Lockheed Martin

Fig. 2.6 Five-coordinate EBAM equipment of Sciaky

2.2 Overview of Equipment at Home and Abroad

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Fig. 2.7 Six-coordinate EBAM equipment of Sciaky

and reliability. In terms of equipment development, it developed China’s first fixedgun EBWD equipment (as shown in Fig. 2.9a), the first moving-gun EBWD equipment (as shown in Fig. 2.9b), and the largest vertical EBWD equipment (shown in Fig. 2.9c). In 2019, it successfully developed a large-scale mobile horizontal EBWD equipment with double guns and double wires that leads both China and the globe (as shown in Fig. 2.9d), with a volume of 53 cubic meters and a prototyping capacity of 6 × 1.8 × 1.2 m. Now, it has formed a full family of EBWD additive manufacturing equipment products, including the hot cathode fixed-gun type, the moving-gun type, and the cold cathode coaxial type (as shown in Fig. 2.9e).

2.3 Introduction to Homemade Equipment China developed the first EBWD equipment (ZD60-10A) in 2009. The equipment is divided into the software part and the hardware part. The software includes the data processing software Electron Beam RP and the comprehensive control software EBAM. The hardware includes high-voltage power supply, electron beam gun, vacuum system, wire feeding system, translation system, image monitoring and acquisition system, electrical control system, etc. The software system functions to control the hardware system and process the data related to the CAD models of

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(a) EBWD equipment K2000 of the Nuclear Technology Advanced Manufacturing Research Center

(b) Cold cathode annular electron beam melting additive manufacturing equipment of Chervona Hvilya

(c) Annular electron beam melting status of Chervona Hvilya’s equipment

Fig. 2.8 Most advanced electron beam melting additive manufacturing equipment across the globe in recent years

parts; coordinate all the functional units of the EBWD platform to realize their functions; promote the orderly machining through man–machine dialogue, information processing and feedback.

2.3.1 Inverter Power Supply for Electron Beam Machining The inverter power supply for electron beam machining is mainly composed of three parts: the high-voltage acceleration power supply, the filament power supply and the grid power supply, as shown in Fig. 2.10. In Fig. 2.10, the three-pole vacuum electron gun is installed on the vacuum chamber, and the workpiece is placed in the vacuum chamber. The positive pole of the high-voltage acceleration power supply is grounded to the anode of the electron beam gun. The negative pole is connected to the positive and negative poles of the filament power supply after passing through the resistors R1, R2 and R3, and then connected to the cathode (filament), thus providing a –60 kV electron acceleration

2.3 Introduction to Homemade Equipment

(a) Small electron beam prototyping equipment

33

(b) Large moving-gun electron beam prototyping equipment (2m×0.8m×0.6m)

(d) Large horizontal electron beam prototyping equipment (6m×1.8m×1.2m)

(c) Large vertical electron beam prototyping equipment (1.5m×0.8m×3m)

(e) Coaxial melting prototyping equipment

Fig. 2.9 Electron beam wire deposition prototyping equipment developed by AVIC Manufacturing Technology Institute

voltage between the cathode and the anode of the electron beam gun; after the grid power supply is connected in series with the high-voltage acceleration power supply, its negative pole is connected to the grid of the electron beam gun, and a bias voltage of 0 ~ –2500 V is formed between the grid and the filament to control the electron beam current. The acceleration voltage adjustment range is 0 ~ –60 kV; the output

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2 Electron Beam Wire Deposition Equipment R2 R1

+ power Filament supply -

12V/ 50A

Filament EB gun

R3

Grid 2500V/300mA +

-60kV

High voltage acceleration power supply +

Electron beam

Grid power supply

Anode Focus coil Vacuum chamber Workpiece

Fig. 2.10 Composition of the inverter power supply for electron beam machining

voltage range of the grid power supply is 0 ~ –2500 V, and the maximum current is 300 mA. The filament power supply is used to heat the filament to generate electrons. The output current is adjustable from 0 to 20A, and the output voltage is 12 V. Since the acceleration voltage of the high-voltage power supply can reach -60 kV, the metallic vapor in the electron gun and the impurities in the fuel tank can easily cause high-voltage discharge. The high-voltage discharge not only makes the highvoltage power supply unstable, but produces voltage or current spikes that cause strong electromagnetic interference or even damage to other devices on the power grid (such as PLC systems, servo control systems, etc.). Therefore, in order to effectively prevent the voltage and current spikes generated by high-voltage discharge from being fed back to the power grid, an inverter DC power supply (with an isolation transformer) is applied to the high-voltage acceleration power supply, the grid bias power supply, and the filament heating power supply to isolate from the 380 V power frequency grid and effectively reduce the interference to other devices on the power grid. The low-voltage parts of the high-voltage acceleration power supply, the grid power supply, and the filament power supply all adopt the series connection of an inverter DC power supply and a bridge inverter circuit, as shown in Fig. 2.11. The appearance of the power supply and its internal inverter bridge are shown in Fig. 2.12. The high-voltage acceleration power supply is immersed in a fuel tank filled with transformer oil by a step-up voltage doubler rectifier circuit. Since the filament rectifier circuit and the grid rectifier circuit are suspended on -60 kV, in order to obtain sound insulation, the filament power transformer and the grid power transformer are used to separate the filament rectifier circuit and the grid rectifier circuit from their low voltage ends, and all the circuits are placed in the fuel tank. The appearance of the fuel tank is shown in Fig. 2.13a. Figure 2.13b shows the internal voltage equalization circuit, the voltage doubler rectifier circuit and the step-up transformer bank inside the fuel tank.

High voltage acceleration power supply transformer Grid power supply transformer Filament power supply transformer

Half-bridge inverter cricuit

Half-bridge inverter DC power supply

P3AC2 P3-

Half-bridge inverter DC power supply

Fig. 2.12 Inverter power supply

250V

P2AC2 P2-

(b) Main circuit of power supply

(a) Appearance of power supply

P3+

Half-bridge inverter cricuit

Full-bridge rectifier circuit W

250V

Full-bridge inverter cricuit

N

P3AC1

P-

P2+

P1AC2 P1-

P2AC1

500V V

High voltage oil ful tank Three phase 380V input

P1AC1 P1+ P+

Full-bridge inverter DC power supply

U

35 2.3 Introduction to Homemade Equipment

Fig. 2.11 Block diagram of the high-voltage power supply

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2 Electron Beam Wire Deposition Equipment

(a) Appearance of fuel tank

(b) Voltage doubler rectifier circuit and step-up transformers

Fig. 2.13 High pressure fuel tank for voltage doubling and rectifying

2.3.2 Electron Beam Gun The ZD60-10A EBWD equipment is equipped with a directly-heated electron gun developed by Beijing Aeronautical Manufacturing Technology Research Institute (currently AVIC Manufacturing Technology Institute) (Fig. 2.14), with a rated power of 10 kW and a maximum acceleration voltage of -60 kV. The core devices include cathode components, grid and anode components. After electron beam current pass

Fig. 2.14 Directly-heated electron gun

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37

Fig. 2.15 Electron beam current

through the magnetic field generated by the coaxial coil, the focus coil, and the scanning coil, they will become suitable for different machining requirements (as shown in Fig. 2.15). looseness-1The beam spot diameter at the focal point of the electron beam is usually not more than 1 mm. The filament heating current generated by the filament power supply and the bias voltage generated by the grid power supply are respectively applied to the both ends of the filament and the grid of the electron gun through the high-voltage cable. The filament power supply and the grid power supply are connected with –60 kV, so that the filament and the grid are in the negative high voltage potential, the anode is grounded. Therefore, an accelerating electric field is generated between the filament, the grid and the anode. When the grid voltage is low, the electrons thermally emitted from the cathode (tungsten filament) are accelerated to about 0.4c by a high-voltage electric field, and then are focused by the electrostatic convergence and electromagnetic focusing system to form a high-energy electron beam current. The power of the electron beam current can be adjusted by changing the bias voltage on the grid.

2.3.3 Vacuum System and Its Control The vacuum system includes a chamber vacuum system and a gun vacuum system. The chamber vacuum system consists of a vacuum chamber and a high-power vacuum pump group (as shown in Fig. 2.16). The gun vacuum system (as shown in Fig. 2.17) consists of the field space of the electron gun, a molecular pump, a low-power mechanical pump, a compound vacuum gauge, a cooling water circuit, a pneumatic pipeline, etc. The interface composition of the vacuum system integrated in the control system is shown in Fig. 2.18.

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Fig. 2.16 Chamber vacuum system

Fig. 2.17 Gun vacuum system

The logic sequence of the actions of the vacuum unit is controlled by the PLC program. After the PLC program accepts the “start up” instruction issued by the EBAM integrated control system in the industrial computer, it will implement actions in sequence according to the predetermined logic sequence. After the EBAM system issues a “shut down” instruction, the PLC program will shut down the machine

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Fig. 2.18 Schematic diagram of vacuum system compositions P1 Mechanical pump; P2 Roots pump; P3 Diffusion pump; P4 Mechanical pump; P5 Molecular pump; V1 ~ V5 valve; VG1 ~ VG4 vacuum gauge. Note The unspecified valves are in “closed” state

according to the predetermined logical sequence. The logical relationship is shown in Fig. 2.19.

2.3.4 Wire Feeding System and Its Control The wire feeding system (Fig. 2.20) consists of a wire feeder (Fig. 2.21), a threedimensional alignment device (Fig. 2.22), a wire storage wheel, a wire feeding hose, and a wire guide nozzle. The wire feeder, like the three-dimensional workbench, is controlled by an industrial computer. In order to ensure the versatility of various types of equipment, an independent control system is designed for the three-dimensional alignment device. The wire feeder features four-roller for four-drive, adjustable and dispersed clamping force, invulnerable wire, and more stable translation. The large-power servo motor (750 W) is not sensitive to load fluctuations and can meet the requirements for precise and complex control. The Serialized grooved rollers and wire guide nozzles can be adapted to four types of wire with diameters ranging from 1.0 mm, 1.2 mm, 1.6 mm, to 2.0 mm respectively. The gap between the bore of the wire guide nozzle and the wire is about 0.1 mm.

40

Fig. 2.19 Working flow chart of vacuum system Fig. 2.20 Wire feeding system of ZD60-10A electron beam prototyping equipment

2 Electron Beam Wire Deposition Equipment

2.3 Introduction to Homemade Equipment Fig. 2.21 Wire feeder

Fig. 2.22 Three-dimensional alignment device

41

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The three-dimensional alignment device is one of the key devices to ensure the stable implementation of the wire deposition process. Its function is to adjust the displacement of the wire end and the center of the molten pool, the height from the surface of the workpiece, the angle between the feeding direction of the wire and the plane of the workpiece, and the length of the wire end extending from the wire guide nozzle. Its main structure is three lead screw guide rail linear translation modules which are arranged perpendicular to each other, with a stroke of 60 mm. Driven by a stepper motor, the slide table of the module moves 0.1 mm upon one inching.

2.3.5 Three-Dimensional Workbench The higher degree of freedom the EBWD equipment has, the higher complexity the machinable parts have. The EBWD equipment requires at lease three degrees of freedom. Because the machining is completed layer by layer, it must have a high degree of freedom in direction. The ZD60-10A EBWD equipment has three degrees of freedom to realize the trajectory movement of plane and geometric figures with the help of the interpolation movement along two horizontal axes. Three-axis linkage can be realized under the control of the NC system. The workbench controls the thickness of the deposition layer in the vertical direction, and achieves the translation accuracy of 0.05 mm. The 3D workbench is shown in Fig. 2.23. Fig. 2.23 Three-dimensional workbench

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43

The control of the 3D workbench is executed by the 8-axis NC translation card installed in the industrial computer, and its parameters can be set in the setup interface of the EBAM system. The size of the workbench surface is 320 × 300 mm; the vertical movement range is 0~200 mm, and the machining range of parts is 240 × 160 × 160 mm.

2.3.6 Image Monitoring and Acquisition System The image monitoring and acquisition system functions to firstly observe and adjust the machining benchmarks (Fig. 2.24); secondly monitor the prototyping process on a real-time (Fig. 2.25); thirdly record and preserve the machining phenomena. The ZD60-10A EBWD system is equipped with an industrial CCD. The optical part is composed of a light source, a reflecting mirror, a reflecting prism and an optical lens barrel. The focal length range is 300 ~ 750 mm below the prism, equipped with an adjustable light transmission device. The aperture range is 1 ~ 20 mm. The zoom lens can magnify up to 8 times. When equipped with a suitable filter lenses, it can Fig. 2.24 Observation and adjustment of machining benchmark

Fig. 2.25 Real-time monitoring and recording

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adjust the brightness of the field of view on site and observe the high-brightness molten pool by adjusting the diameter of the aperture. The image information is extracted by the CCD, and the video distributor divides the signal into two channels: one channel is displayed in real time on the monitor, and the other is connected to the image acquisition card in the industrial computer, and is processed and stored in the industrial computer. Further research will define the relative position of the wire end and the molten pool through image analysis and processing technologies, compare it with the predetermined criterion, and send a feedback signal to correct the translation trajectory or other control parameters to make the prototyping process a closed-loop monitoring process.

2.3.7 Electrical Control System The high-voltage power control circuit is installed in the cage in the middle of the electrical cabinet. The control circuit board in the cage slot contains the filament, the coaxial control circuit board, the scanning control board, the focus coil power supply board, the beam current control circuit board, the bias power amplifier circuit board, and the high voltage control circuit board. In addition to the above-mentioned control circuit boards, the electrical cabinet also contains motor servo systems for various translation axes, power supplies of various specifications, etc. The main functions of the industrial computer are as follows: (1) (2) (3) (4) (5)

Set the parameter, detection and display of machining parameters; Realize the control of the translation system; Dynamically display the working status of the equipment; Realize human–computer interaction; Realize the acquisition and processing of image information. The main functions of PLC are as follows:

(1) (2) (3)

Realize the control of the vacuum system; Realize the measurement and control of machining parameters; Communicate and transfer data with the industrial computer through serial ports. The electrical control principle is shown in Fig. 2.26.

2.3.8 Data Processing Software—Electron Beam RP At present, the widely used 3D modeling software includes ProE, UG, Catia, AutoCAD, SolidWorks, etc. While they have similar functions and all support the STL format, they focus on different aspects. Figure 2.27 describes almost all the data flow of rapid prototyping. The data

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45

Fig. 2.26 Electrical control principle

Fig. 2.27 Data flow in prototyping

of rapid prototyping comes from a wide range of sources, mainly including 3D CAD models, 3D models after triangle meshing, data from reverse engineering, mathematical and geometrical data, medical/voxel data, sliced data, etc. STL is a file format for data conversion between CAD models and rapid prototyping (RP) equipment proposed by 3Dsystems of the United States. It has now been accepted by most CAD system and RP equipment manufacturers and has become an actual benchmark in the RP technology sector. At present, most commercial CAD software can perform triangle meshing for CAD models, and export “*.stl” files. A STL file is obtained by using small triangular facets to approximate the free surface, and then triangulating the model surface. The accuracy of the approximation is usually controlled by the distance of the curved triangle planes or the chord height difference between the curved surface and the triangle side. The more irregular the surface, the more triangles are needed, and the larger the STL file will be. Each triangular facet is represented by three vertices and a normal vector to the outside of the model. The STL file features simple generation, easily segmented model, and easy slicing algorithm. The common ways for a CAD system to export STL files are as follows: (1)

AutoCAD from Autodesk: The exported model must be a three-dimensional entity with positive coordinates. Enter the command “FACETERS” on the

46

(2)

(3) (1)

2 Electron Beam Wire Deposition Equipment

command line, set FACETRES to a value between 1 and 10 (1 represents low precision, 10 represents high precision), enter the command “STLOUT” on the command line, select an entity, select “Y”, output a binary file, enter the file name; I-DEAS of SDRC: File > Export > Rapid Prototype File > select the exported model > Select Prototype Device > SLA500.dat > set absolute facet deviation to 0.000395 > select Binary; Pro/Engineer of PTC: File > Export > Model, or File > Save a Copy > select *.stl > set the Chord Height to 0 and set the Angle Control to 1. Overall framework of Electron Beam RP system

The Electron Beam RP system is a rapid prototyping software system based on the principle of metal wire fused deposition manufacturing. Based on the STL files exported by other CAD systems (models described by triangular facets), this system slices the part models and generate slice files based on the requirements of the fused deposition prototyping technology, while planning the machining path of each layer and eventually generating the NC codes aligned with the needs of the machining equipment. The system not only boasts the functions of common rapid prototyping software systems, but also combines the electron beam wire melting rapid prototyping technology to generate a reasonable, accurate and efficient numerically controlled machining program. The system is mainly composed of STL file data inspection and slicing, machining path planning and NC program generation. The data inspection and slicing of the system enables STL file inspection, establishment of data structure, model layering, generation of silhouette lines of each layer, and rationality processing. There are two ways to plan for the machining path, namely the grid method and the offset method. The grid method means that the machining path of each layer is composed of parallel lines in the specified direction. The position of these parallel lines is determined by the silhouette line of the current layer. The machining directions of two adjacent layers form a certain angle.The offset method means that the padding path of each layer is the equidistant offset line of the silhouette. The NC program generation refers to the export of NC files of layers that meet the machining conditions according to the NC system of the machining equipment and related instructions. The Electron Beam RP system is developed under Visual C++ 6.0, and its graphic display and operation apply OpenGL graphics library. The software runs in Windows XP, and the functional modules can be selected from the menu and toolbar, and can be implemented in accordance with the requirements of the EBWD technology. In the process of realizing these functions, the data files of different stages are stored, such as layered files, padding path files, etc. The exported NC program files conform to the format required by Siemens 840D NC system. The system structure is shown in Fig. 2.28. The data flow of the system is shown in Fig. 2.29.

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Fig. 2.28 System structure

Fig. 2.29 System data flow

(2)

Installation and operating environment of Electron Beam RP

Copy ElectronBeamRP.exe to the computer via removable storage (such as U disk, removable hard disk) or CDs, and double-click ElectronBeamRP.exe to run the program. The initial system interface is shown in Fig. 2.30. The operating environment of the system is as follows: (1) (2) (3)

Hardware environment: Microcomputer of Pentium IV and above; hard drive space above 10G; RAM above 512 M. Software platform: operating system: Windows XP; software development environment: Visual C + + 6.0; browsing tool of data file: notepad or wordpad. Main functions and technical characteristics of Electron Beam RP system

The functional modules provided by Electron Beam RP system include STL file data extraction, slicing, machining path planning, NC file generation, graphic display and operation. Each module has the following functions as shown in Fig. 2.31. (1)

STL file data processing

In the test, we used Pro/E to build a three-dimensional model of a typical part, as shown in Fig. 2.32, and convert it into the STL format, as shown in Fig. 2.33. The

48

Fig. 2.30 Initial interface of the system

Fig. 2.31 Main functional structure of the system

2 Electron Beam Wire Deposition Equipment

2.3 Introduction to Homemade Equipment

49

Fig. 2.32 Read three-dimensional entity

part is a cuboid with a hole in the middle. Select the submenu “Open” under the main menu “File” or directly select the toolbar button to open the file dialog box; select the STL file, and click the “Open” button to open the specified STL file. When an error occurs while reading the STL file, the system can display the error message, then you can return to the CAD model for modification. (2)

Graphic display and graphic operation function

The menu option “View” houses options such as “zoom in, zoom out, translation, rotation, appropriate size” that the user can use to observe the model. Use the options in “View Selection” to observe the bottom view, top view, front view, back view, left view, right view, and side view of the model. (1)

(2)

Wireframe display: Display the STL file by the silhouette line of the triangular facets. Click the submenu “Wireframe Display” under the main menu “Display”. Illumination display: Display the STL file by the illumination mode of the triangular facets. Click the submenu “Illumination Display” under the main menu “Display”.

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Fig. 2.33 Three-dimensional entity in STL format

(3)

(4)

(5)

(6)

(7)

Layered silhouette display: Display the slice file, and all layers of the model. Click the submenu “Layered Silhouette Display” under the main menu . “Display” or click the toolbar button Padding path display: Display the padding file, and the padding path of all layers of the model. Click the submenu “Padding Path Display” under the . main menu “Display” or click the toolbar button Single-layer silhouette display: Display the silhouette data of the specified layer. Click the toolbar button , and the layer selection dialog box will pop up, prompting you to select the number of the layer whose silhouette will be displayed. Single-layer padding path display: Display the padding path of the specified and the layer selection dialog box will pop layer. Click the toolbar button up, prompting you to select the number of the layer whose padding path will be displayed. Graph zoom-in: Click the submenu “Zoom in” under the main menu “View” or click the toolbar button , and left-click to confirm the area to zoom in the graphics in the selected area.

2.3 Introduction to Homemade Equipment

(8)

(9)

(10)

(11)

(12)

51

Undo the previous zoom in: Click the submenu “Zoom out” under the main to return to the graph size before menu “View” or click the toolbar button the zoom-in. Graph translation: Click the submenu “Translation” under the main menu “View” or click the toolbar button , and then left-click and drag the mouse to move the graph. Graph rotation: Click the submenu “Rotate” under the main menu “View” or click the toolbar button , then left-click and drag the mouse to rotate the graph in the direction of the cursor. Display in appropriate size: Click the submenu “Appropriate Size” under the main menu “View” or click the toolbar button to display the graph in its initial state, that is, the zoomed-in and translated graphics will restore the original state; View setting: The system provides side view, front view, rear view, left view, right view, top view, bottom view on the corresponding toolbar .

To change the model display effect, open the drop-down menu from the menu bar option “Display”, and switch “Wireframe Model” over “Illumination Model”. Model change can only be operated when the wireframe model is displayed or the illumination model is displayed. That is because the display of these two states will display the STL file. The change of model will delete the existing slice file (the sli file corresponding to the current model). Therefore, the model change cannot be performed when the slice file (*.sli) or the padding file (*.net) is displayed. In addition, the model change function only converts the data in the STL file read by the current system. It will not alter the data inside the STL file. Therefore, when the model is changed and the STL file is reopened, its data is different from that in the slice file (including the padding file). (3)

Slicing function

The sliced data file exported by the system not only records the layer type, height range, thickness and other information, but also the silhouette data of each layer and the inner and outer silhouette marks. The sliced data file is the basis of machining path planning. Therefore, each part model must be sliced before the machining path planning can be carried out. The slicing algorithm is an important part in rapid prototyping. The slicing algorithm in the rapid prototyping technology can be divided into the direct slicing of CAD models and the slicing based on STL models by the data format, or can be divided into the equal-thickness slicing and the adaptive slicing by the slicing method. Direct slicing of CAD models has the advantages of small data size, high accuracy, short data processing time, and no errors in the model. But it also has obvious shortcomings, such as the reliance on special CAD software and difficulty in automatically adding support to the model. The slicing based on STL models has the shortcomings of error-prone models, large data size, and low accuracy. However, it

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does not rely on the CAD software, and the accuracy level can be set according to the complexity of the part. Therefore, it is still a mainstream subject of research. The STL model is the result of the discretization of triangles on the CAD model, so it is actually a polyhedron model. The slicing produces a series of polygonal silhouettes. The flow of slicing algorithm is shown in Fig. 2.34. EBWD is a layered manufacturing technology, so it will cause a step effect on the surface of parts. In order to improve the accuracy and reduce the step effect, it is necessary to reduce the layer thickness (Fig. 2.35), but this will greatly reduce the manufacturing efficiency and increase the manufacturing cost of parts. In order to maintain a balance between manufacturing accuracy and manufacturing cost, the more advanced adaptive slicing method is currently applied in rapid prototyping. The principle is that the software can automatically determine the layer thickness according to curved surface of the three-dimensional model to guarantee the surface accuracy of parts specified by the user and therefore enables fast prototyping speed and high accuracy. However, during the adaptive slicing, the technological parameters must be greatly changed according to the thickness of the layers. It poses high control flexibility for technological parameters, and requires the support of the technology library. However, the electron beam wire deposition technology is infrequently carried out, and does not have the technology library needed for supporting the adaptive slicing. The traditional slicing method is equal-thickness slicing, which means the thickness of each layer is identical along the height of the model. As for the EBWD technology, all the technological parameters of different layers remain unchanged except the path, which is conducive to simplifying the technology and improving the stability and reliability of machining. However, the equal-thickness slicing for parts with complex silhouettes cannot guarantee both high accuracy and high efficiency. The EBWD process can neither apply the adaptive slicing nor the equal-thickness slicing. Electron Beam RP proposes a segmented slicing method by combining the advantages of the above-mentioned two slicing methods and the characteristics of electron beam wire melting process. This method is a tradeoff solution between the adaptive slicing and the equal-thickness slicing. Its characteristic is that it will set 5 segments along the height direction of the 3D model. The height of each segment or the coordinates of the starting layer along the height direction can be manually set according to the complexity of the part, and the thickness of layers in each segment can also be set separately. That means a part can be divided into at most 5 segments of equal thickness to achieve both high machining efficiency and high prototyping accuracy. The layer thickness can be larger in the equal section or approximately equal section to increase the prototyping speed. The layer thickness in the non-equal section can be smaller to reduce the deposition steps and improve the silhouette accuracy. To realize the slicing function, Electron Beam RP has to do the slicing along the height direction based on the layer thickness, and automatically distinguish the inner and outer silhouette lines during the slicing. As for the incorrect data models in STL files, the system will prompt when the silhouette lines are not closed, or an overlapping or other abnormal phenomena occur.

2.3 Introduction to Homemade Equipment

Fig. 2.34 Flow of slicing algorithm

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Fig. 2.35 Slicing step effect

Open the menu bar option “Prototyping Process” and select the “Slicing” option in the drop-down menu. Set the type of layer thickness and the starting coordinates in the “Parameter Setting” dialog box (Fig. 2.36). The equal-thickness slicing is shown in Fig. 2.37. The effect of segmented slicing is shown in Figs. 2.38 and 2.39. (4)

Path planning function

The polygonal section silhouette is obtained after the STL model of the part is sliced. These polygons are formed by a chain of connected vertices in sequence. The process of generating the path is the process of padding the silhouette of the polygonal section. Path planning not only affects the performance and accuracy of parts, but also poses an important impact on the process and cost of machining. (1)

Padding method. The basic methods to generate a path include silhouette offset (also called parallel silhouette padding), grid padding (also called parallel straight line padding), and fractal scanning. Since the path generated by fractal

Fig. 2.36 Slicing parameter setting dialog box

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Fig. 2.37 Result of equal-thickness slicing

scanning is relatively short and tortuous, which is not suitable for the rapid prototyping technology of electron beam wire deposition, so Electron Beam RP only adopts the first two methods. Click the submenu “Offset Padding” under the main menu “Prototyping Process” or click the toolbar button , and a dialog box will pop up to set the padding parameters, as shown in Fig. 2.40; enter the parameters and confirm the padding method; click the “OK” button. If the slice file of the current model does not exist, this function cannot be completed, in which case slicing needs to be performed first. If the slice file already exists, then calculate the padding trajectory according to the set parameters, and generate the padding file (*.net), as shown in Fig. 2.41; click the “Cancel” button to cancel this function. There are two types of grid padding: the two-way padding and the one-way padding. The difference lies in that the parallel lines in the two-way padding connect end to end with each other to form polylines (Fig. 2.42) while the lines in the one-way padding are parallel. The offset padding is shown in Fig. 2.43.

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Fig. 2.38 Result of slicing into 5 segments

Fig. 2.39 Side view of the result of slicing into 5 segments

(2)

➀ ➁

Padding angle. In the Electron Beam RP system, users can set the padding direction of different layers to obtain parts with balanced performance in all directions. There are four optional modes in terms of the padding direction between layers. 0° ~ 90°: The direction of the padding line in the first layer forms a 0° angle with the axis, and the direction of the padding line in the second layer forms a 90° angle with the X axis, and so on. 0°: The direction of the padding line in each layer is parallel to the axis.

2.3 Introduction to Homemade Equipment

Fig. 2.40 Padding parameter setting dialog box

Fig. 2.41 Effect of path padding

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Fig. 2.42 Two-way grid padding

Fig. 2.43 Offset padding

➂ ➃

90°: The direction of the padding line in each layer forms a 90° angle with the axis. 0° ~ 45° ~ 90°: The direction of the padding line in the first layer forms a 0° angle with the axis; the direction of the padding line in the second layer forms a 45° angle with the X axis; the direction of the padding line in the third layer forms a 90° angle with the axis, and so on.

The padding direction only needs to be considered for grid padding, but not for offset padding. The padding effect of 0° ~ 90° padding mode is shown in Fig. 2.44. (3)

Processing of silhouette lines. The option “Number of Offset Loops in Silhouette” is provided in the padding parameter setting window. If it is set to = 0, there is no silhouette line; if it is set to > 0, the silhouette is offset according to the given number of loops while the grid padding is still performed internally. Therefore it is actually a mixed padding method. This method leads the machining from the inside to the outside and finally reaches the silhouette line during the prototyping process. Adjusting the offset between the center line of the path and the silhouette line of the model allows the user to control the prototyping accuracy, as shown in Fig. 2.45.

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Fig. 2.44 Effect of 0° ~ 90° padding

Fig. 2.45 Schematic diagram of silhouette offset

(4)

Path spacing. The distance between deposition paths is the same as the distance between parallel lines during prototyping; adjusting this distance can change the flatness of the surface of the deposition layer. Generally, the empirical formula “spacing = half width of deposition path + half width of wire” is used.

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Fig. 2.46 Schematic diagram of corner radius

Fig. 2.47 Corner transition diagram of path

(5)

Corner transition. In the Electron Beam RP system, a smooth transition is adopted for the path with an angle smaller than 150° between two adjacent straight lines. The corner radius can be set through the dialog box, and the default value is less than or equal to half of the line spacing (Fig. 2.46); calculate the arc tangent point and the path after arc transition according to the transition arc radius provided by the system. In actual machining, the moving speed of workpieces is accelerated at the corner to avoid excessive deposition of heat and mass transfer. The corner arc transition in the process of generating the path is shown in Fig. 2.47.

(6)

Export NC program. After the path planning is completed, enter the technological parameters through the machining parameter dialog interface (Fig. 2.48), such as wire feeding advance displacement, wire feeding lag displacement, beam rising displacement, beam falling displacement, reverse wire drawing amount, wire feeding rate, linear translation speed, arc translation speed, Z-axis translation speed, acceleration voltage, beam current, focus current, etc. Each layer is saved as an independent NC machining program in consideration of machining safety and equipment capabilities. Call the machining program of next layer after the machining of one layer is successfully completed; start the machining of next layer after troubleshooting if the process is interrupted due to an abnormality. Therefore, corresponding action statements should be added to the beginning and the end of each machining program to ensure a sound connection between machining processes and avoid interference between the workpieces and the equipment, as shown in Fig. 2.49.

2.3 Introduction to Homemade Equipment

Fig. 2.48 Diagram of the machining parameter dialog interface

Fig. 2.49 NC machining program

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(4)

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EBAM integrated control system and software

The EBAM system is a special software developed upon major upgrades on the control system of the electron beam welding machine. It works as a bridge to collect and display system information, man–machine dialog, modify or set parameters, call and edit NC programs, and issue control instructions. (1)

Operation interface:

The EBAM system has three display interfaces (respectively for displaying the equipment composition, the working status of vacuum system, and machining parameters, etc.): (1)

System status interface (Fig. 2.50): The interface displays the working conditions of the equipment, including circulating water, compressed air, emergency stop status, vacuum chamber door status, equipment preheating and cooling status, etc. The flashing system icon means the conditions are not met, and

Fig. 2.50 System status interface

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Fig. 2.51 Statuses of the vacuum system

(2)

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users can grasp the basic working conditions of the equipment from the interface. There are operation function menu buttons at the bottom of the interface, from which users can access to specific modules. Vacuum system status interface (Fig. 2.51): The interface indicates vacuum pump, solenoid valve and vacuum gauge, and can also dynamically display the working status of each component of the vacuum system. The red icon indicates that the corresponding pump or valve is in the STOP state, the green icon indicates the START state, and the icon flashing in red and yellow indicates a fault. Machining parameter display interface (Fig. 2.52): The interface displays the set value and measured value of some machining parameters, the part machining method, the displacement of each translation axis, translation mode, translation speed, etc. Also, the file name of the machining program, the program machining mode, and the program execution coordinate system are displayed at the bottom of the screen. Operation menu

There are 12 function selection menus at the bottom of the display interface. Each menu includes 7 buttons with function names and corresponding hot keys indicated. The hot keys correspond to the buttons on the keyboard. To operate, users need to use the mouse or keyboard to press the hot key and select the function corresponding to the name or enter the next-level menu step by step. Users can press the “ESC” button

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Fig. 2.52 Machining parameters

to exit the menu step by step (or click continuously the “ESC” button in the boot menu to continuously switch the display interface on the screen). The hierarchical relationship of these 12 menus is shown in Fig. 2.53. The menu buttons have the following functions: (1)

➀ ➁

(2)

Basic operation: By pressing this button, users can start and close the vacuum system, electron gun, and filament of the equipment. At the same time, the screen will automatically switch to the working interface of the vacuum system. Start vacuum: It is the start button of the vacuum system; press this button to warm up the vacuum system (only need to be executed once a day). Close vacuum: It is the close button of the vacuum system; after the vacuum system finishes its work of the day, the operator must press this button to start to cool the vacuum system. When the diffusion pump temperature drops to the set cooling temperature, the vacuum system will automatically shut down and the operator can shut down the equipment. Servo enable: Before the translation system works, users need to press this button to enable the servo system, or need to disable the servo system if the translation system needs not to work or does not work for a long time. After the servo system is enabled, an icon will appear on the screen to inform the user of the enabling status of the equipment’s servo system.

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Fig. 2.53 Hierarchy diagram of menu

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Gun deflation: It is used to deflate the electron gun (identical to the “Gun Deflation” button on the console). Chamber deflation: It is used to vent the vacuum chamber (identical to the “Chamber Deflation” button on the console). Chamber vacuum: It is used to pump out the vacuum chamber (identical to the “Chamber Vacuum” button on the console).

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Manual operation: If the user needs to manually operate the translation system, he can press this button to select the operation mode and set the speed. Zero return mode: Press this button to set the NC system in the zero return mode. The system will return to the zero position according to the coordinate system (relative coordinate, absolute coordinate) and translation axis selected by the user. Absolute coordinates: Press this button to return the translation system to the zero point of the absolute coordinates. Meanwhile, “Absolute Coordinates” will be displayed in the “Coordinate System” column on the interface. The absolute zero returns of X-axis and Y-axis are both in the negative direction, which means, the translation axis must be in the positive direction of the mechanical zero point to perform absolute zero return. Relative coordinates: Press this button and the translation system will return to the zero position, which is the current position of each axis, in a motionless manner. Meanwhile, “Relative Coordinates” will be displayed in the “Coordinate System” column on the interface. The translation axis return to zero at any position. Coordinate axis X–Y-A: When the turntable must be removed from the workbench due to the need for welding parts, the user should remove the two aviation plugs of the axis servo motor from the connector of the workbench when the equipment is powered off, and turn the coordinate axis switch button in the electrical cabinet to the upper position. After the equipment is powered on, the user must press this button once to remove the axis from the coordinate system of the translation system. JOG mode: Press this button to have the NC system in the JOG mode. The user can control the translation of each axis through the buttons X + , X-, Y + , Y-, A + , A-, STOP on the console. Inching mode: This function is used to control the inching of the translation axis, in which the system will perform fixed-length inching according to the inching distance selected by the user. 10 mm: Press this button, and the linear translation axis will move in steps of 10 mm each time. The rotation axis will rotate by 10° each time. 1 mm: Press this button, and the linear translation axis will move in steps of 1 mm each time. The rotation axis will rotate by 1° each time. 0.1 mm: Press this button, and the linear translation axis will move in steps of 0.1 mm each time. The rotation axis will rotate by 0.1° each time. 0.01 mm: Press this button, and the linear translation axis will move in steps of 0.01 mm each time. The rotation axis will rotate by 0.01° each time. Handwheel mode: Press this button to control the translation of each translation axis by shaking the handwheel on the console. Part diameter: After pressing this button, the user can set the diameter parameter according to the diameter of the welded part.

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Translation speed: The button for setting the linear speed of the translation axis. This button can only be pressed when each translation axis is at a standstill, and the set speed value will only take effect when the setting status is canceled after the setting is completed. Automatic operation: This button allows a user to select, start, and stop a program if he needs to machine parts by programming them. Select program: Press this button and the program editing dialog box will pop up on the screen. In the program editing dialog box, a user can input programs, read in, save, and download machining programs, and so on. The machining program edited by the user must be downloaded before it can be executed. Start program: After this button is pressed, a dialog box will pop up for the user to make a decision. When the user selects OK, the system will start the most recently downloaded machining program of the user. If the button is pressed but the machining program does not start, it may be because an unterminated program is running, which stops a new machining program from starting. In this case, the user can click “Stop Program”, and press the “Start Program” button. Stop program: Terminate the machining program in progress. Program machining mode: There are three modes for machining parts through programming. The user can press this button to enter the machining mode selection menu. Machining beam mode: The output value of beams is subject to the set value of the program during programming and machining. No beam current mode: The output value of beam current is zero during programming and machining. The system only executes the displacement instructions of each translation axis. Small beam current mode: The output value of beam current is manually set (IB) during programming and machining. Parameter adjustment: Press this button to enter the machining parameter setting menu, and the interface will automatically switch to the machining interface. When the user is setting the parameters, the interface will display the value of selected parameters in red, and the user can modify the parameters by rotating the handwheel. After the setting is completed, the user may once again press this button or press other buttons of the menu to cancel the selection state of the pressed button, and the red figures on the screen will return to their original color. Set high voltage: A button used to set the high voltage; press this button to modify the high voltage parameter. Set filament: A button used to set filament current. Set focus: A button used to set focus current. Set beam current: Press this button and the next-level menu of beam current parameters will pop up. Beam current size: Press this button to set the beam current size (mA) when the machining mode is not automatic.

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Rise time: Press this button to set the rise time of beam current (s) when the machining mode is not automatic. Fall time: Press this button to set the fall time of beam current (s) when the machining mode is not automatic. Machining time: The system will determine the output time (s) of beams according to this value when the machining mode is “timing”. Machining mode: When the user selects the welding mode and continuously presses this button, the machining mode will be switched between “manual, “automatic”, “inching” and “timing”. Base value of beam current: Set the voltage value of the bias dead zone. DC deflection: Press this button, and the menu for setting DC deflection parameters will pop up on the screen. DCX: Set the amplitude of beam deflection along the X axis. DCY: Set the amplitude of beam deflection along the Y axis. IJX: Set the coaxial amplitude along the X axis. IJY: Set the coaxial amplitude along the Y axis. AC scanning: Press this button, and the menu for setting beam scanning parameters will pop up on the screen. Scanning waveform: Press this button continuously to switch between different waveforms of “straight line, “circle”, “upper semicircle”, “lower semicircle” and “no waveform”. ACX: Set the scanning amplitude of beam current along the X axis. ACY: Set the scanning amplitude of beam current along the Y axis. Scanning frequency: Set the frequency of the scanning waveform within a range of 1 ~ 1000 Hz. Scanning direction: Select the direction of the scanning waveform, including horizontal and vertical directions. Program editing: Press this button, and the program editing dialog box will pop up on the screen. The dialog box includes the file name of the machining program, the program editing area, and buttons to open, save, clear, and download files. A user can use this dialog box to open, save, edit, and download machining program files. Software failure

When the equipment is working, if the control program is terminated and the system crashes, you can press "Ctrl+Alt+Del” and meanwhile select the machining program file that has no response, end the running process and then re-run the working program. Since the vacuum system is controlled by PLC, the reset of the industrial computer will not affect the normal operation of the vacuum system. (4)

Calling and editing of machining program

The EBAM system can both edit the NC machining program, and call the edited NC machining program files. The editing and calling interface of machining program is shown in Fig. 2.54. This function is the interface between EBW and Electron Beam

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Fig. 2.54 Editing window of machining program

RP. The instructions sent by the NC program include the M instruction and the G instruction. The commonly used M instructions of the EBWD prototyping system are shown in Table 2.1. The control process is not only a complete data flow (from 3D modeling to NC program) process, but also a parametric machining program generation and implementation process. This process is completed jointly by the data processing software Electron Beam RP and the integrated control software EBW. The prototyping control software is responsible for controlling the coordination of all hardware systems, setting process conditions, such as vacuum degree, focus current, acceleration voltage, etc., and leaving a window for calling the machining program. In the data processing software, it is also responsible for setting the layer thickness and machining path, and generating path information; introducing the control parameters required according to the machining technology to generate the parameterized Table 2.1 Commonly used M instructions Address

Meaning

Programming

M02

End of program

M02…

M03

Wire feeder runs forward

M03S15

M04

Wire feeder runs inversely

M04S25

M05

Wire feeder stops

M05

M06

Instruction to define beam current rising or falling rate

M06 X 10 W 20

M13

Beam current rising enable

M13

M14

Beam current falling enable

M14

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Fig. 2.55 Generation and implementation of parameterized machining program

numerical control machining program, and inputting the actual machining parameter values through the man–machine dialog; completing the secondary programming to generate the NC program that meets the format requirements of the prototyping control software. When the environmental requirements (vacuum degree, workpiece position, etc.) are met, the prototyping control system will call the NC machining program generated by the data processing software to control the vacuum unit, the electron gun and the workbench to work coordinatively and implement machining, as shown in Fig. 2.55.

Chapter 3

Typical Materials for Electron Beam Wire Deposition

Abstract This chapter mainly deals with the issues concerning materials used in EBWD technology. A brief introduction on the present status of materials used for EBWD investigations was given at the beginning of this chapter. Following that, characteristics, technical requirements and manufacturing methods of feeding wires used for EBWD technology are proposed and discussed. After that, the main technical features of typical EBWD deposits, namely burn loss of alloying elements, anisotropy of mechanical properties, macrostructure characterized by columnar crystals and “three zones and two lines”, are introduced separately in different sections. For the aim of properly matching the different mechanical properties of EBWD parts, researching results on effects of alloying element content, depositing beam current and heat treatment schedules on microstructure and mechanical properties of EBWD Ti–6Al–4V deposits are introduced and discussed. Based on which, methods for tailoring of mechanical properties of EBWD products similar to Ti–6Al–4V alloy are proposed. Near the end of this chapter, new findings on fracture modes and deformation mechanism in tensile tests of EBWD Ti–6Al–4V are illustrated. In the end of this chapter, several materials developed and certified to be suitable for EBWD use are introduced and typical mechanical properties of their EBWD deposits are listed. The primary intention of the present chapter is to give a preliminary understanding of the EBWD technology and its typical products for the beginners and potential users interested in this technical field.

Materials are the foundation for manufacturing industry, and there is no exception in additive manufacturing technologies. In recent years, a large number of studies have been conducted on the macro/microstructure and mechanical properties of deposits of titanium alloys, aviation aluminum alloys, and high-strength steels, which were built by electron beam wire deposition (EBWD) method. Most researches have focused their research work on the influence of processing parameters on shape-controlling of parts and the influence of heat treatment on macro/microstructure and mechanical properties for a given deposit. Also, efforts have been made to explore the significant impact of chemical composition on mechanical properties. Je Matz et al. [1] studied the macro/microstructure and mechanical properties of a 718 nickel alloy fabricated by EBWD method and found that the EBWD 718 nickel © National Defense Industry Press 2022 S. Gong et al., Electron Beam Wire Deposition Technology and Its Application, Additive Manufacturing Technology, https://doi.org/10.1007/978-981-19-0759-3_3

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alloy can obtain fine primary carbides at a higher cooling rate, and the dispersion of carbides can increase some mechanical properties of this alloy. R. Keith Bird [2] studied the macro/microstructure and room-temperature tensile properties of EBWD IN 718 alloy. The result shows that the strength of the IN 718 deposit is higher than that of cast 718 with the same chemical composition, but lower than that of forged 718 alloy. The ductility value is comparable to that of forged and cast 718 alloy, but the elastic modulus of EBWD IN 718 alloy is lower than the counterparts fabricated by traditional technologies. They attributed the low modulus characteristics of EBWD in 718 alloy to the texture formed in the deposit. After solution treatment, grain growth occurred and elastic modulus increased. Tayon, Shenoy et al. [3] also used the a high temperature superalloy IN 718 alloy for EBWD inveatigation. They studied the macro/microstructure, texture evolution and the mechanical properties of of a block of IN 718 fabricated via the EBF3 process. The result shows that mechanical properties of the EBF3 -processed IN 718 block are strongly affected by texture, and the properties of as-deposited EBF3 being significantly lower than the wrought in 718; significant improvement in both strength and modulus of the EBF3 product to levels nearly equal to those for wrought in 718 may be achieved through heat treatment. Domack, Karen, Hafley et al. [4, 5] studied the influence of depositing parameters (such as translation speed, wire feeding rate, etc.) on the morphology and microstructural evolution of EBWD deposits of 2219 and 2319 aluminum alloys. They sought for ways to control the microstructure via the depositing processes. Many studies have been conducted on titanium alloys fabricated by the EBWD technology and Ti–6Al–4V alloy is on top of the list. Barnes and Brice from Lockheed Martin and Karen and Hafley et al. [6] from the Langley Research Center of NASA studied the mechanical properties of the EBWD Ti–6Al–4V alloy. It was found that during the depositing process, the high vacuum environment caused violent evaporation of Al element with a low melting point, resulting in lower strength as compared to the forged material; Lach et al. [7] systematically studied the effect of the depositing parameters on the evaporation of Al element. They found that the the most effective parameter on evaporation of Al element is wire feeding rate. Studies have also been conducted on the macro/microstructure, residual stress, static mechanical strength and elongation, fracture toughness, crack growth performance of the Ti–6Al–4V alloy fabricated by EBWD method, as well as the impact of the post-processing technology (mechanical machining and shot peening, etc.) on the fatigue properties [8]. The result shows that the fatigue property of EBWD Ti–6Al– 4V is lower than the forged material of the same composition due to the metallurgical defects formed during depositing. The fatigue property after shot peening was not improved apparently. However, the actual structural parts went through the fatigue loading testing without undergoing a premature failure. Heck et al. also systematically studied the mechanical properties of EBWD Ti–6Al–4V. The result shows that the fatigue life is much longer and fracture toughness are much higher than the specified value, and the Z-direction tensile strength is also higher than the specified value. But the strength in the X and Y directions is slightly lower than the standard value (the maximum difference is 26 MPa). Composition analysis shows that the EBWD

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Ti–6Al–4V alloy suffers a significant burning loss of Al elements, leading to reduced strength which fail to meet the standard requirement [9]. R.W. Bush from the U.S. Air Force Academy and CA Brice from Texas State University studied the room-temperature and high-temperature tensile properties, creep properties and oxidation resistance of Ti–6Al–4V and Ti–8Al–1Er alloys fabricated by the EBWD technology [10, 11]. Compared with the Ti–8Al–1Er alloy fabricated by the laser additive manufacturing technology and the forged Ti–6Al– 4V alloy, the high temperature properties of the Ti–8Al–1Er alloy fabricated by the EBWD technology is equivalent to that of the forged Ti–6Al–4 V alloy. Its creep resistance is better than the forged Ti–6Al–4V alloy, but lower than the Ti–8Al–1Er alloy fabricated by laser additive manufacturing technology. The oxidation resistance of the Ti–8Al–1Er alloy fabricated by the EBWD technology is better than the forged Ti–6Al–4 V. In summary, EBWD investigations have been carried out on steel, superalloy, aluminum alloy and titanium alloys. Much research work focused on titanium alloys, emphasis was placed on the effect of depositing processes on macro/microstructure and mechanical properties. However, more detaied work is needed for EBWD materials, especially basic research work related to the formation of the characteristic structure, deformation mechanism and failure modes under various loading conditions. In China, a research team comprised of researchers from AVIC Manufacturing Technology Institute and Institute of Metal Research, Chinese Academy of Sciences (IMR CAS) carried out a systematic research work on matching different mechanical properties of EBWD titanium alloys and ultra-high-strength steels to a level comparable to the correspongding forging counterparts. The results show that for titanium alloys with medium-strength, their EBWD deposits tends to exibit lower strength but higher ductility due to elemental burn loss. For high-strength and high-temperature titanium alloys which are sensitive to microstructural changes, the EBWD deposits exhibit comparable strength but low ductility and fracture toughness as compared with the forging counterparts due to coarsened and directionally grown columnar grains. Ductility and strength of the ultra-high-strength steel A-100 fabricated by EBWD can meet the requirements but the fracture toughness fails to meet the requirements of the material standard for A-100 alloy. Chen Zheyuan et al. [12] studied the deposition processes for thin-wall and bulk samples by EBWD technology using Ti–6Al–4V alloy feeding wire. The processing characteristics for building the two kinds of samples were analyzed; the corresponding macro/microstructural characteristics and the principle behind it were discussed. Lou Jun et al. [13–15] studied the effect of heat treatment parameters on microstructure and mechanical properties of TC18 alloy (Ti–5Al–5Mo–5V–1Cr– 1Fe) fabricated by EBWD. Suo Hongbo et al. [16] systematically studied the microstructure and mechanical properties of Ti–6Al–4V fabricated by EBWD. His work summarized the microstructure characteristics of EBWD Ti–6Al–4V as directionally growing columnar crystals and “three zones and two lines”. Formation and evolution mechanism of the

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microstructure during the EBWD process with superimposition of multiple alternating thermal cycles were analyzed. Suo’s work gave a clear illustration and interpretation of this concept and threw light on the macro/microstructure of titanium alloys formed by EBWD method. They tested the basic mechanical properties and found that the microhardness was highly correlated with the microstructure, and the tensile properties exhibited obvious anisotropy. Also, they found that the stress relieving annealing had no significant influence on the mechanical properties, and the hot isostatic pressing plays a significant role on eliminating microscopic defects and improving fatigue properties. They studied the effects of Al, Fe, B, Y and other chemical elements on the microstructure and mechanical properties. The changes of Al and Fe have no obvious influence on the microstructure and macrostructure of EBWD Ti–6Al–4V, and addition of B or Y, especially both, will significantly reduce the width of columnar grains. The strength increases significantly with the increase of the Al content when the Al content is below 5.7% in the deposit. At Al content above 5.7%, increase of the strength are no longer significant, and even a slightly decrease was found. When the Fe content is below 0.28%, the tensile strength and yield strength in three directions increase significantly with the increase of the Fe content. When the Fe content exceeds 0.28%, the strength increase is not obvious. Compounds are prone to form and precipitate on the grain boundary in B element containing titanium alloys, leading to significantly reduced tensile ductility due to intergranular fracture. A nearly dispersed compounds form in Y-containing Ti–6Al–4V, which hinder the growth of columnar crystals, resulting in a better match between the strength and ductility. They studied the effect of heat treatment temperature and cooling methods on the morphology, size and content of α phase in the EBWD Ti–6Al–4V alloy, and established a rough relationship between heat treatment parameters and mechanical properties. Cai Yusheng, Liu Jianrong et al. [17, 18] studied the relationship among the microstructure, hardness and tensile deformation behavior of EBWD TC18. The result shows that under a single annealing condition, the microstructure is composed of primary α phase and metastable β phase. As the temperature rises, the volume fraction of the primary α phase decreases, and the microhardness of the β matrix changes slightly. Under a dual annealing condition, fine, basket-weave, strip-like α phases formed on metastable β phase during the second low temperature annealing, which significantly increased the hardness of the matrix. As the temperature of the second annealing treatment rises, the size of the precipitated α phases coarsened and the amount decreased, resulting in a decrease of hardness of the matrix. Under a triple annealing condition, coarse bamboo-leaf-like primary α phases produced during the controlled cooling process and intermediate-temperature annealing, and the amount of the primary α phases decrease with the increase of the intermediate annealing temperature, which has a slight impact on the microhardness. The angle between the growth direction of columnar crystals and the direction of tensile stress has a critical impact on the tensile deformation behavior: highest ductility was found when the tensile direction was parallel to the growth direction of columnar crystals, and the fracture mode is a ductile one; when the angle is around 45°, well matched strength

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and ductility of the deposit are found; when the angle is around 90°, brittle fracture occurred with an intercrystalline fracture mode.

3.1 Manufacturing Technology of Metal Wire for EBWD Use 3.1.1 Technical Requirements At present, two types of additive manufacturing technologies use metal wire as the raw material, namely the arc deposition technology and the electron beam wire deposition technology. In the early stage of technological development, the technical standards and specifications for additive manufacturing (AM) parts were not well developed. Technical standards of casting or forging are used in the early stage for developing of AM parts. As the raw material for arc deposition and electron beam wire deposition additive technologies, feeding wires undoubtedly have a key influence on the depositing process and mechanical properties. Technical characteristics and requirements for metal wires used in EBWD technology are as follows: (1)

(2)

Wire composition. Chemical composition, fabrication methods and heat treatment are the three factors that determine material properties. The additive manufacturing technology is far different from the traditional casting or forging. The present research results show that, in most cases, the mechanical properties of titanium alloy parts fabricated by the EBWD technology is higher than the casting of the same material. However, it depends on the alloy type and the fabrication methods to reach the technical standards of forging. Firstly, burning loss of elements exists more or less during the EBWD process which leads to decreased content of elements with lower vapour pressure. Secondly, the microstructure of EBWD parts exhibit a columnar crystal structure with directional solidification characteristics which leading to anisotropy of mechanical properties. Therefore, difference both from chemical composition and macro/microstructure lead to different mechanical properties between EBWD parts and forged ones. Therefore, according to the microstructural characteristic of the part fabricated by additive manufacturing and the change of the composition before and after fabrication, it is generally necessary to modify the composition of the feeding material and the heat treatment technology of the parts, for the aim to meet the demands of technical specifications of forging parts. Therefore, composition is one of the main key technical demands for wire materials for EBWD use. Wire morphology and diameter tolerance. A small moving molten pool forms during EBWD process, it is necessary to require the feeding wire to feed into the center of the molten pool accurately and stably to ensure a stable deposition operation. Improving the dimensional accuracy enables smaller tolerant gap between the wire and feeding nozzle, which may help to avoid excessive

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vibration of the wire tip, increase size accuracy of the parts and decrease occurrence of defects. For this reason, the dimensional tolerance of wires needs to be strictly controlled. Therefore, the wire morphology and dimensional tolerance is the second main technical demand for EBWD wires. The wire morphology generally includes curvature radius, warping, twist, etc. Cleanliness. Cleanliness of the wire surface is an important factor that affects splashing of molten metal, stability of electric current and voltage in depositing process, and the metallurgical quality of parts. Therefore, it is the third main demand for EBWD wires. Cleanliness mainly covers various forms of pollution on the surface of the wire, including oxygen-rich layer, nitrogen-rich layer, carburized layer, oil, dust, etc.

Undoubtedly not all metals are suitable for EBWD manufacturing. In china, a team comprised of researchers from AVIC Manufacturing Technology Institute and Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS) carried out a research work on suitability of titanium alloys and high-strength steel wires for EBWD technique since 2010. They made various feeding wires and used them to build EBWD deposits for evaluation of processing window and match of mechanical properties. The materials investigated include TA15 (Ti–6.5Al–2Zr–1Mo–1V), Ti– 6Al–4V, TC11 (Ti–6.5Al–3.5Mo–1.5Zr–0.35Si), TC17 (Ti–5Al–2Sn–2Zr–4Mo– 4Cr), TC18 (Ti–5Al–5Mo–5V–1Cr–1Fe), A-100 ultrahigh-strength steel and so on. On this basis, some titanium alloy wires with modified chemical compositions and modified heat treatment schedule for high-strength steel are developed to meet the demands of EBWD technology. Particularly, several parts fabricated by two titanium wires with chemical compositions modified based on Ti–6Al–4V alloy have passed assessment for usage in a new developing airplane.

3.1.2 Fabrication Technology In terms of the fabrication methods, titanium alloy feeding wires can be classified into traditional drawn wire and rolled wire. In terms of the source of bars for wiremaking, traditional melting method and non-smelting method are both used. The routes for feeding wire making are roughly illuastrated in Fig. 3.1. Route 1 represents the traditional process, which includes steps of arc melting by consumable electrode vaccum furnace, forging, rolling, unwinding of wire coil, drawing, annealing, scale removing, morphorlogy shaping, winding and packaging, etc. The advantage of route 1 is wide adaptability and suitable for production of almost all wires ranging from pure titanium to its alloys. The disadvantages include lengthy process, low efficiency, and slight pollution. Route 2 represents the rolling process. Feeding wire are produced by cold rolling using a specially designed rolling equipment materials using billets with diameters of 8–10 mm. The advantages of route 2 are high efficiency and low pollution. The disadvantage is narrower adaptability, not suitable for fabrication of highly alloyed brittle alloys. Route 3 represents a new technology

3.1 Manufacturing Technology of Metal Wire for EBWD Use

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Fig. 3.1 Routes for metal wire manufacturing

developed by the MORSKTITANIUM company from Norway. It is characterized by mechanically mixing titanium sponge and master alloy, sintering and extruding to form wire billets under high temperature and high pressure, and then using wire drawing or wire rolling machines to produce finished wires. The advantages of this technology include shortened process, increased efficiency, and reduced cost. The disadvantage lies in the limited applications. The materials suitable to this process may be restricted by such factors as difference of melting points between different alloying elements, diffusion abilities of alloying elements, etc. For highly alloyed alloys containing elements with super-high or super-low melting points, uniformity of chemical composition is extremely difficult to control by high temperature sintering. If compositional nonuniformity occurs, breaking of wires may frequently occur in the subsequent drawing process, making stable production impossible. Therefore, this method may be only limited to production low-alloyed metal wires with high efficiency.

3.2 Characteristics of EBWD Materials The inherent technical characteristics of additive manufacturing such as layer-bylayer deposition may be reflected in the microstructure of the deposited materials. Columnar crystal structure growing upwardly and outwardly from the matrix are prone to form for titanium alloys during EBWD depositing process. The α + β two-phase structure forms in the columnar crystal for many titanium alloys suitable for EBWD. The morphology, size, quantity and arrangement of the α phase in the columnar crystal depend on the alloy type and deposition process. For Al alloys [4, 5], if the electron beam power is constant, increase of deposition rate and cooling rate of the molten pool is helpful for formation of finer equiaxed structure. Inversely, decrease of the deposition rate and the cooling rate of the molten pool may lead to coarsened grain size and tends to form a dendritic microstructure. As for most α + β two-phase titanium alloys, band-like zones may form inside the deposited samples perpendicular to the grain growth direction.

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The burning loss of alloying elements is apparent during the EBWD process, and the extent of loss is related to the types of material and the deposition processes. Therefore, the material compositions before and after EBWD are usually inconsistent.

3.2.1 Burning Loss of Composition During EBWD Process Due to the high energy density and vacuum environment inherent to electron beam wire deposition method, burning loss of alloying elements is inevitable and easiness of burning loss is related closely to the vapor pressure of each alloying elements. According to the result of thermodynamic calculations, the order of burning loss of commonly used alloying elements in titanium alloys (from being easy to being difficult) is Mn > Al > Sn > Cr > Fe > Ti > Mo > Nb > V > W. According to the results of titanium alloys and A-100 steel, Al and Cr exhibit the most obvious burning loss. Figure 3.2 shows the comparison of Al and V contents in Ti–6Al–4V feeding wires and their EBWD deposits. It can be seen that the Al content in EBWD deposit is significantly lower than that in the feeding wire, see Fig. 3.2a; while V element is slightly higher as compared to the feeding wire, see Fig. 3.2b. This phenomenon indicates that V element is more resistant to burning loss than Al and Ti, resulting in the phenomenon of higher V content in EBSD deposits than that in feeding wires. The effect of deposition parameters on the burn loss of Al element in Ti–6Al–4V alloy was also systematically investigated [6, 7]. The results show that the degree of Al element loss is related to the deposition process, and the range is between 0.27 and 1.42%. Under condition of constant voltage, focus beam, wire diameter and the wire feeding rate exhibits the largest impact, the beam power has a medium impact, while the translation speed has the slightest impact. Tables 3.1 and 3.2 are the results of chemical composition analysis of EBWD A100 and TC17 alloys, respectively. From Table 3.1 we can see that Co, Ni, Mo contents increase by 5%, 2.8% and 9%, respectively; while Cr content decreases by

Fig. 3.2 Burn loss during EBRM process of Ti–6Al–4V alloy: a Al element; b V element

3.2 Characteristics of EBWD Materials

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Table 3.1 Chemical composition of A-100 steel before and after EBWD (weight percent, %) Types of A-100 steel

Elements Co

Ni

Feedingwire

13.0

11.1

3.00

Deposit

13.71

11.32

2.79

1.33

−7.0%

10.7%

Rate of burn loss, %

5.5%

Cr

2.0%

Mo 1.22

Table 3.2 Chemical composition of TC17 alloy before and after EBWD (weight percent, %) Types of TC17 alloy

Elements Al

Sn

Zr

Mo

Cr

Wire

5.23

1.97

1.98

3.93

3.95

F 2.0 double wire feeding deposit

3.38

1.89

2.03

3.97

3.25

F 2.0 single wire feeding deposit

3.54

1.97

1.97

3.95

3.31

F 1.2 single wire feeding deposit

3.46

1.90

1.93

3.95

3.20

Average of deposit

3.46

1.92

1.98

3.96

3.25

−13.72

−2.54

0.00

0.68

−17.64

Rate of burn loss, %

7% compared with the feeding wires of A-100 steel. Table 3.2 shows the comparison of elements of TC17 alloy (Ti–5Al–2Sn–2Zr–4Mo–4Cr) before and after EBWD. No apparent difference of chemical composition is found among the three deposits built with defferent building processes, indicating that wire diameter and feeding modes (single or double feeding) having little impact on burn loss of elements. Averaging the composition analysis results of the three deposits and compare with composition of feeding wire, we can see that the burn loss rates of Al, Sn, and Cr elements are 13.72%, 2.54% and 17.64%, respectively. The burn loss rate of Zr element was zero, and Mo content increased by 0.68%. Apparently, in the two alloy systems of Ti and Fe, basic principle of burning loss of elements is consistent, but the amount of burning loss for the same element is apparently related to the alloy system. For example, the burn loss of Cr in A-100 is 7% while in TC17 the value is 17%.

3.2.2 Characteristic Microstructure and Formation Mechanism The EBWD process is a dynamic non-uniform melting/solidification process characterized by wire deposition under the action of linear scanning of the heat source of electron beam. During the depositing process, any position in the deposit has been subjected to complex multiple thermal effects. Under the superimposition of multiple alternating thermal cycles during the depositing process, a unique microstructure

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that is different from both cast and forged titanium alloys is formed. Macroscopically, a coarse columnar crystal structure growing along the direction of deposition height, and band-like lines parallel or nearly parallel to the deposition layer are produced, exhibiting characteriatics of obvious anisotropy, locally gradient changes, and interally periodic changes. (1)

Columnar crystal structure

The most significant feature of microstructure of titanium alloys fabricated by EBWD is columnar crystals. Suo Hongbo et al. [16] applied a single-pass and 5-layer deposit to study the formation of columnar crystal in Ti–6Al–4V, as shown in Fig. 3.3.

(a) Single pass, single layer

(b) Single pass, two layers

(c) Single pass, three Layers

(d) Single pass, four layers

(e) Single pass, five layers

Fig. 3.3 Growing process of columnar grains during EBRM process

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81

In Fig. 3.3a, we can see that epitaxially grown columnar crystals are formed in the melting zone even in the condition of single-pass single-layer deposition. The transition area between the melting zone and the base metal zone is the heat-affected zone, which is similar to the microstructure of the welding joint of titanium alloys. From Fig. 3.3b, c, we can see that the length and width of the columnar crystals increase with the increasing number of deposition layers, penetrating through the bottom of the molten pool to the top. From Fig. 3.3d, we can see that when the sample height keeps increasing, some columnar crystals may break, and a small amount of equiaxed crystals appear in the middle of the deposition zone. As the number of deposited layer increases, temperature rises due to heat accumulating in the deposit, which causes slow-down of solidification rate. The columnar grains are easier to grow and show a tendency of growing wider from the bottom to the top. The directional growth mechanism of columnar crystals can be explained as follows. Under the action of electron beams, a molten pool is formed on the surface layer of the solid materials along with strong elemental evaporation; melted wire is continuously fed in, impacting the molten pool. Violent motion of molten pool occurred, leading to a quite uneven temperature field. Violent motion and rapid solidification conditions make it difficult for the molten pool to reach a critical degree of supercooling required for spontaneous nucleation. For most titanium alloys, impurity elements and oxides are strictly controlled, and hence no heterogeneous particles can serve as crystal nucleus for non-spontaneous nucleation. On the contrary, the Gibbs energy required for direct growth from partially melted β grains is lower, resulting in a structure growing epitaxially direct on the unmelted β grains at the bottom of the molten pool. As the deposition proceeds, β grains continue to grow along the direction of the largest temperature gradient. According to Fig. 3.3e, after the fifth layer deposition, black lines that are slightly concave down in the horizontal direction appeared. The number of black lines is identical to the number of deposition layers. Apparently, the black lines are not superposing with the fusion lines that is the boundary between the solid and liquid phases indicated by arrows in Fig. 3.3e. Also in Fig. 3.3e, another main feature of EBWD titanium alloys characterized by “three zones and two lines” appeared. The “three zones” refer to heat-affected zone, macrostructure uniform zone and stripband zone, respectively. The heat-affected zone is in the substrate which hasn’t been melted by electron beam but suffered from the heat conducted from the molten pool, characterized by gradual microstructural changes from the substrate to melting zone; the strip-band zone refers to the depositing region where horizontal black lines exist; and the macrostructure uniform zone refers to the top region of deposit which is above the strip-band zone where no horizontal black lines exist. The “two lines” refer to the black lines in the strip-band zone, which is named as band interface line, and the indistinguishable boundary between solid and liquid phases formed during depositing process, which is named as the fusion line [16]. (2)

The “three zone and two line” structure

Figure 3.3 gives a rough illustration of the formation of “three zone and two line” structure. However, in Fig. 3.3 the fusion line is not clear enough. Figure 3.4 shows

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Fig. 3.4 “Two lines” formed in EBRM Ti–6Al–4V

the macrostructure of a single-pass multi-layer deposit of Ti–6Al–4V. The dents on both sides of the sample can be regarded as the sign of overlaping between layers. The faintly-visible and upward-curved lines connecting the dents on both sides of the sample is called the fusion line, which can be regarded as the boundary between solid and liquid phases during the deposition process, as indicated by the arrow in Fig. 3.4. The horizontal black lines which seems to be the interface of the bands in the strip band zone is named as band-interface line. The band-interface line and the fusion line are called “two lines” for short. The band-interface line is actually a indication of the α + β/β phase transition temperature of titanium alloy. During the deposition process, the temperature of the area above the last band-interface line is in the β phase field, the closer to the top, the higher the temperature. Each band interface line suffers from a transient heating at a temperature equal to α + β/β phase transition point (Tβ ). Temperature above this line is higher than Tβ while below this line, temperature is lower than Tβ . For a given site, the temperature drops once with depositiong of a new layer because of increase of distance from the heat source. A new band-interface line also forms after depositiong of a new layer. In this sense, layer thickness is identical to the spacing between the two adjacent band-interface lines leaving behind. (3)

Microstructure characteristics of “three zone and two line” structure

Macrostructure uniform zone: In Fig. 3.4, macrostructure uniform zone corresponding to the area above the latest band-interface line. We can see that this zone contains three deposition layers as indicated by A, B, and C. When the layer C is being deposited, the layer A and B are heated by the liquid metal of layer C to temperature above Tβ , leading to α + β → β phase transformation throughout layer A−C

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83

Fig. 3.5 Microstructure of macrostructure uniform zone

and uniform β phase produced. When the electron beam moves away from layer C, β → α + β phase transition occurs in layer A–C in the subsequent cooling process, forming a uniform β-transformed microstructure. It is apparent that the height of the macrostructure uniform zone is related to factors such as the thermal conductivity of materials, the input power, the translation speed of the molten pool, etc. The more heat gains per unit time, the deeper the molten pool, the larger the area below the fusion line that reaches Tβ , and the larger the macrostructure uniform zone will be. The microstructure of the macrostructure uniform zone in Fig. 3.4 is shown in Fig. 3.5. It is characterized by clusters composed of fine-needle-like phases, and different clusters are arranged in a woven form. The needle-like phase are transformed α produced in colling stage by transformation of high temperature β phase to α and retained β phase. Fusion line: Fig. 3.4 shows the commonly indistinguishable fusion line, which is the boundary between the solid and liquid phases during the deposition process. It is characterized by a discontinuous thin line bowed upwardly. In Fig. 3.4, dents can be observed on both sides of the sample, which is seemingly a clear indicator of the boundary between layers. The macroscopic fusion lines seem to connect the dents on both sides. However, when observed in high magnification, microstructure of the dents is characterized by clonies composed of parallel α laths, and no trace of linear characteristic is found, as shown in Fig. 3.6. However, an arc-shaped boundary was found at 50–80 μm above the dent, and the orientations of α colonies on both sides of the boundary are different. It can be seen from Fig. 3.6a that the arched boundary is not continuous and may be interrupted by large α colonies. Apparently, the arc-shaped “fusion line” observed in macrostructure is actually the interface of α colonies with different orientation. The dents on the two sides of the sample are most likely formed by the liquid metal flowing down 50–80 μm due to gravity, while the actual fusion line is 50–80 μm above the dents, as illustrated by the dotted line in Fig. 3.6. Band interface line and strip band zone: The band interface line is acturally a special isotherm of the temperature field generated during the deposition of each layer which corresponding to the α + β/β phase transition point (Tβ ) for titanium

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Fig. 3.6 Microstructure near the fusion line

alloys. Formation of a new band interface line is undoubtedly companied by addition of a new layer. The area in depositing zone where interface lines exsits is named as the strip band zone. After the N th layer boundary is formed, the temperature field generated during deposition of the (N + 1) th layer will still affects the N th band interface line and the area below it, but the temperature is lower than Tβ due to the increased distance from the molten pool. When a new layer deposited, the temperature imposed on the same location decreases again, but the microstructure will change until the temperature is too low to affect the microstructure. Assuming that after depositing of m layers above the N th layer, the microstructure of the area below the N th layer will no longer change, but different extent of microstructural changes still occur among the (N + 1) th to the (N + m) th layers, causing a microstructural characteristics similar to that of the heat-affected zone of the weld. The microstructure below the N th layer reaches a stable state, and the microstructure difference between the band interface line and its adjacent areas is no longer obvious. Figures 3.7, 3.8, 3.9, 3.10 show the microstructures at the first and the fourth band interface line as well as their adjacent areas in Fig. 3.4. Here, the first and the fourth band interface line represents two typical places located in mocrostructure unstable and stable zone, respectively. Figure 3.7 shows the microstructure of the macrostructure uniform zone. It is composed of fine-needle-like α-phase arranged in

Fig. 3.7 Microstructure above the first band interface line in Fig. 3.4

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85

Fig. 3.8 Microstructure near the first band interface line in Fig. 3.4

Fig. 3.9 Microstructure under the first band interface line in Fig. 3.4

a woven form, with residual β-phase in between. Figure 3.8 shows the microstructure at the first band interface line at the top of Fig. 3.4. At high magnification, the band interface line exhibits a band with a zigzag shape as shown in Fig. 3.8a. Inside the band α platelets are arranged in arrays. The aspect ratio of the α platelets in each array is smaller than that in Fig. 3.7b. Figure 3.9 shows the microstructure below the first band interface line, which is characterized by small aspect ratio α phases arranged in a woven form, around the α phases is retained β phase. The microstructure in Fig. 3.7 undergoes a short-term heat treatment in the α + β two-phase region, original thin and high-aspect-ratio α phases is broken by penetrating of new-born β phase produced by α → β phase transition, shortened α phase embedded in high temperature β phase formed at this process. Then the β → α phase transformation occurs in the subsequent cooling process, during which the shortened α phase is coarsened and the microstructure shown in Fig. 3.9 formed. Figure 3.10 shows the microstructure at the 4th band interface line and its adjacent area [16]. Due to the long distance from the latest layer, the heat effect from the new deposited layer is not prominent and the microstructure changes are not obvious. Microstructure at the line and in its upper and lower sides tends to be consistent. The microstructure of the 4th band interface line is not as clear as that of the first one. The main microstructural difference of the

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Fig. 3.10 Microstructure of near the fourth band interface line in Fig. 3.4

4th band interface line from its adjacent area is that the platelet α phases in ”Line” area are aligned in colonies and the α platelets are relatively thiner, while the α phases in both sides of the ”line” area are a little thicker and mainly exist in a woven form. Heat-affected zone: The heat-affected zone refers to the area below the fusion line of the first layer. Due to the effect from the temperature field produced in depositing of the initial several layers, significant change of contrast in macrostructure is found as compared to the substrate, Fig. 3.3e. The area of the heat-affected zone is large, and its depth is about three times of the thickness of a single-pass layer. Since strip bands also exists in this area, the heat affected zone also exhibits the characteristics similar to that of the strip-band zone found in Fig. 3.4 due to effect of temperature fields in depositing of the first several layers until the isotherm of the temperature

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87

field corresponding to the α + β/β phase transition point (Tβ ) moves out of this area. Figure 3.11 shows the macrostructure of a single-pass layer deposited on a Ti–6Al–4V substrate which is rolled in the α + β phase region. Figure 3.11 exhibits an integral figure of EBWD deposit including deposition zone, heat-affected zone and substrate. It can be seen that the heat-affected zone exhibits a gradual changing macro/microstructure from the substrate to the deposit. Coarsening of grain size was found from the substrate to the deposition zone. In Fig. 3.11, the arched white bright line where C located is the first band interface line. Figure 3.12 shows the microstructures at five positions from A to F in Fig. 3.11. Figure 3.12a shows the microstructure of the original substrate, which is a deformed duplex microstructure. During the deposition process, the substrate close to the molten pool is subjected to an instantaneous heating, producing a temperature field which covers a certain area. At sites where the temperature is below the α/α + β phase transformation point, no obvious microstructural changes can be found due to short heating time. At sites where the temperature is above the α/α + β but below the α + β/β phase transformation point, microstructural changes may be detectable due to α → β phase transformation. For example, Fig. 3.12b is corresponding to site B, the basic characteristics of the duplex microstructure retains due to short time heating while the boundary of the equiaxed α phase becomes blurred. At sites close to the first band interface line, the temperature is approaching the α + β/β phase transformation point, the volume fraction of the equiaxed α phase decreases and traces of the equiaxed α phase retained due to insufficient time for elemental diffusion, Fig. 3.12c. Across the band interface line, the temperature is above the α + β/β phase transformation point and most equiaxed α phase transformed to fine equiaxed high temperature β phase in the heating stage and then transformed to a needle-like microstructure in the cooling stage. However, a small amount of equiaxed α phase still remains, also due to insufficient time for elemental diffusion, as shown in Fig. 3.12d. From D to E in Fig. 3.11, the temperature is high above the α + β/β phase transformation point and the equiaxed α phase in substrate disappears completely and apparent β grain growth happens due to complete α → β phase transformation. During the cooling stage the coarsened β grains transformed into a fine needle-like microstructure, Fig. 3.12e.

Fig. 3.11 Typical macrostructure of the heat affected zone (HAZ)

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3 Typical Materials for Electron Beam Wire Deposition

Fig. 3.12 Microstructure at different positions from A to F in Fig. 3.11

β grains grow significantly near the fusion line, and some coarse equiaxed β grains grow into columnar crystal embryos. They grow epitaxially to form a columnar crystal structure, Fig. 3.12f. The grain size of the original substrate has a significant impact on the transition characteristics of the microstructure in the heat-affected zone. If the grains of matrix A are coarse, the transition from fine to coarse grains (as shown from A to E in Fig. 3.12) will be not obvious. As for multi-pass and multi-layer deposition, the “three zone and two line” structure also exists. After undergoing a heat treatment at high temperature, the “three zone and two line” feature will be weakened or completely disappear, as shown in Fig. 3.13. When the “three zone and two line” feature disappears, the macrostructure

3.2 Characteristics of EBWD Materials

(a) as-deposited;

89

(b) annealed at 550°C; (c) annealed at 945°C

Fig. 3.13 Microstructures of deposits in different states a as-deposited; b annealed at 550 °C; c annealed at 945 °C

of the deposit can be divided into three zones according to the grain size and shape: the base metal zone, heat-affected zone and the deposition zone. Microhardness in different positions is tested for Ti–6Al–4V deposits, as shown in Fig. 3.14a, b. For the as-deposited sample, the results show that highest microhardness is found in the strip band zone, the second high microhardness is in the macrostructure uniform zone, while the lowest microhardness is in the substrate. The microhardness in the heat-affected zone exhibit an increasing trend from the substrate to the deposition zone, and the microhardness at the band interface line is a little lower than those tested in their adjacent area, see Fig. 3.14a. After heat treatment, the “three zone and two line” feature disappear, and the large difference of microhardness among different sites also disappear, Fig. 3.14b.

3.2.3 Anisotropy of Tensile Properties of EBWD Deposit Different degree of anisotropy was found for EBWD materials. As for titanium alloys, low strength and high ductility was found along the direction of deposition height. The tensile properties along the other two directions are equivalent, exhibiting high strength and low ductility. The macrostructure of the titanium alloy fabricated by EBWD shows obvious directional growth characteristics. In most cases, it shows a

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3 Typical Materials for Electron Beam Wire Deposition

(a) as-deposited;

(b) 945ºC annealed

Fig. 3.14 Micro-hardness fo EBWD Ti–6Al–4V along the growth direction of deposit a asdeposited; b 945 ºC annealed

columnar crystal structure along the deposition height direction, which means that the high-temperature β phase exists in the form of columnar crystals and hasn’t been interrupted by the layer-to-layer depositing mode. As for two-phase titanium alloys, when the temperature drops below the β/α + β transition point, the β → α + β phase transformation occurs, and the columnar β phase transforms into strip-like or needlelike α phase and interfacial residual β phases, making it difficult to directly determine the crystallographic orientation of the original columnar β phase. However, since the new-formed α phase and its β parent-phase follows Burgers orientation relationship, namely {0001}α {110}β , α β , determination of crystallographic orientation of the β phase is practicable by inverse calculation using the crystallographic orientation results of the α phase obtained by EBSD methods. Figures 3.15, 3.16, and 3.17 show the tensile properties of Ti–6Al–4V deposits fabricated by EBWD in different directions after hot isostatic pressing [19]. Figure 3.15 shows tensile properties along five directions on the X–Z plane.

(a) Yield (TYS) and tensile strength (UTS); (b) Elongation (EL) and reduction in area (RA)

Fig. 3.15 Tensile properties of EBRM Ti–6Al–4V alloy in different directions on the X–Z plane: a Yield (TYS) and tensile strength (UTS); b Elongation (EL) and reduction in area (RA)

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91

(a) Yield (TYS) and tensile strength (UTS); (b) Elongation (EL) and reduction in area (RA)

Fig. 3.16 Tensile properties of EBRM Ti–6Al–4V alloy in different directions on the Y–Z plane: a Yield (TYS) and tensile strength (UTS); b Elongation (EL) and reduction in area (RA)

(a) Yield (TYS) and tensile strength (UTS);

(b) Elongation (EL) and reduction in area (RA)

Fig. 3.17 Tensile properties of EBRM Ti–6Al–4V alloy in different directions on the X–Y plane: a Yield (TYS) and tensile strength (UTS); b Elongation (EL) and reduction in area (RA)

Figure 3.16 shows tensile properties along five directionson the Y–Z plane. According to Figs. 3.15 and 3.16, Ti–6Al–4V alloy fabricated by EBWD exhibits obvious anisotropy in tensile properties. From Figs. 3.15a and 3.16a we can see that with increase of the angle between the tensile direction and the X or Y direction, both the tensile and yield strength exhibit a trend of first increase and then decrease, and accordingly, the ductility exhibit a first decrease and then increase trend. Both the strength and ductility reach their maximum and minimum values in the range of 22.5–45°. When the tensile direction is parallel to the Z direction, minimum strength and maximum ductility are found. Figure 3.17 shows tensile properties along five directions on the X–Y plane. No obvious changes of the tensile and tield strength are found with increase of the angle between the tensile direction and the X direction. Maximum tensile and yiled strength is found when the tensile direction is parallel to the X direction and the minimum is

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found when the angle is close to 45°. However, the range between the maximum and minimum value is only about 37 MPa. The elongation shows a slight decrease with increase of the angle, while the reduction in area experienced a first slight decrease, then followed by a moderate increase with increase angle on the X–Y plane. The variation of ductility is greater when the angle is within 45°, while above 45° no obvious change of the ductility values is found, Fig. 3.17. Other titanium alloys fabricated by EBWD, such as TA15 (Ti–6.5Al–2Zr–1Mo– 1V), TC11 (Ti–6.5Al –3.5Mo–1.5Zr–0.35Si), TC18 (Ti–5Al–5Mo–5V–1Cr–1Fe), etc., show the same or similar phenomena of anisotropy of tensile properties as Ti– 6Al–4V alloy. As shown in Fig. 3.18, the strength shows a parabolic opening down trend on the X–Z plane with the increase of the angle between the tensile axis and the X direction, while the ductility shows an opposite trend, similar to that found in Fig. 3.15. It should be noted that the aforementioned anisotropic phenomenon has nothing to do with hot isostatic pressing (HIP). In this chapter, selecting the deposit after HIP as experimental material is for the sake of avoiding data dispersion caused by deposition defects, which may affect the correct demonstration of the anisotropy phenomenon. The aforementioned anisotropy is mainly attributed to the special microstructure and crystal orientation formed during the EBWD process. Figure 3.19 shows the

(a) Yield (TYS) and tensile strength (UTS);

(b) Elongation (EL) and reduction in area (RA)

Fig. 3.18 Tensile properties of EBWD TC18 alloy in different directions on the X–Z plane: a Yield (TYS) and tensile strength (UTS); b Elongation (EL) and reduction in area (RA)

Fig. 3.19 Crystal orientation distribution of α phases in columnar grains

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93

Fig. 3.20 Pole figure of α phases in columnar crystals

grain orientation distribution of the α phase in columnar crystals of the Ti–6Al–4V alloy fabricated by EBWD, and Fig. 3.20 is the pole figure corresponding to Fig. 3.19. We can see that the basal plane of the α phase in the columnar crystal (crystal c axis) is concentrated in the 45° and 90° directions to the plane of substrate (X–Y plane). According to the Burgers orientation relationship between the α phase and the β phase, the pole figure of the crystallographic orientation distribution map of the original columnar crystal (β parent phase) is calculated based the EBSD data of α phase and illustrated in Fig. 3.21. Figure 3.22 is the orientation distribution map of the columnar β grains corresponding to Fig. 3.21. According to Figs. 3.21 and 3.22, the {100} β planes of the matrix β phase are concentrated in two directions that are

Fig. 3.21 Pole figure of the parent columnar β phase in Fig. 3.19

Fig. 3.22 EBSD IPF map of parent β phase on the Y–Z plane corresponding to Fig. 3.21

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nearly parallel or perpendicular to the X–Y plane, and therefore we can determine that the growth direction of β columnar crystals is direction. From Fig. 3.21 we can also see that the densely packed surface (111) is concentrated on directions 25–55° to the matrix plane (X–Y plane), which gives a reasonable interpretation of the experimental phenomenon that the tensile strength is low and the ductility is high when tested along the direction of columnar crystal growth. Though the findings can partly explain the experimental phenomenon of the lower tensile strength when tested along the Z direction[20]. However, since anisotropy involves some basic issues such as the complex compatible deformation between the β parent phase and the lamellar α phase, influence of the columnar crystal boundary α phase and its orientation, the accurate deformation mechanism needs to be further studied.

3.3 Mechanical Properties Control of Titanium Alloy Fabricated by EBWD According to the current investigations, EBWD titanium alloys exhibit such problems as difficulty in matching of strength and ductility, elemental burning loss and obvious anisotropy. Therefore, optimization of chemical compositions and heat treatment processes is necessary to tailor the mechanical properties to meet the customer requirments according to characteristics of EBWD technology.

3.3.1 Composition Control—Relationship Between Composition and Properties of Ti–6Al–4V Deposit Figures 3.23 and 3.24 show the influence of chemical composition on the tensile properties of Ti–6Al–4V deposits fabricated by EBWD. The numbers on the X axis in the figure represent the code of each deposit. The Al content of the deposits numbered 1, 2, and 3 is 5.3 wt%, 5. 7wt% and 6.4 wt% respectively, while the contents of the remaining alloy elements are identical. The Al content is 5.3wt.% for the deposits numbered 1, 4, and 5, but the content of Fe is 0.04 wt%, 0.25 wt% and 0.5 wt%, respectively. The Al and Fe content of the deposit numbered 6 is 6.4 wt% and 0.25 wt%, respectively. Three different heat treatment processes were used with solution temperature of 930 °C, 950 °C and 970 °C, respectively. According to the figure, we can see that, regardless of heat treatment processes, similar influences of Al and Fe are found on tensile properties of Ti–6Al–4V fabricated by EBWD. With increase of Al content from 5.3 to 5.7%, the strength increases significantly and data points of the reduction in area (RA) is much scatterand and the trend is not clear; Al content increases from 5.7 to 6.4%, both the strength and RA decreases slightly. According to the trend of the tensile strength and RA of the materials numbered 1, 4, and 5 in the figure, it can be seen that when the Fe content increases from 0.04 to

3.3 Mechanical Properties Control of Titanium Alloy …

(a) Tensile strength along Z direction

(c) Tensile strength along Y direction

95

(b) Reduction in area (RA) along Z direction

(d) Reduction in area (RA) along Y direction

Fig. 3.23 Relationship between Al/Fe contents and tensile properties of EBWD Ti–6Al–4V in Y and Z directions

(a) Tensile strength along X direction

(b) Reduction in area (RA) along X direction

Fig. 3.24 Relationship between Al/Fe contents and tensile properties of EBWD Ti–6Al–4V in X direction

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0.25%, moderate increase of tensile strength is found and no obvious change of RA is found. Fe content increases from 0.25 to 0.5%, insignificant increase of tensile strength is found followed by an enlarged scatter of RA data. Comparison of Figs. 3.23 and 3.24 we can also see that obvious anisotropies of the strength and ductility are found in X, Y, and Z directions. High strength and low ductility are found along the X and Y directions; low strength and high ductility along the Z direction; no obvious differences of strength and ductility was found along the X and Y directions. Comparison the data of the materials numbered 4 and 5 in Figs. 3.23a, c and 3.24a, it can be seen that, when the Fe content increases from 0.25 to 0.5%, strength along the Y and Z directions slightly increases while remains unchanged along the X direction, indicating that increase of Fe content may reduce the difference of strength along the X and Z directions. Increase of both Al and Fe contents may increase the strength along the three directions significantly, while the ductility decreases slightly, as shown by alloy numbered 6 in Figs. 3.23 and 3.24. From Figs. 3.23 and 3.24 we can see that no significant effect of heat treatment temperature on the strength was found in the temperature range of 930–970 °C, and the ductility data exhibits large fluctuations.

3.3.2 Deposition Process Control—Relationship Between Deposition Processes, Microstructure and Mechanical Properties (1)

Influence of depositing process on microstructure

Figure 3.25 shows the macrostructure of Ti–6Al–4V alloy fabricated by EBWD at different beam currents. It can be seen that the macrostructure still exhibits the characteristics of “three zones and two lines”. With decreasing of beam current, the width of the columnar grains decereased, too. At small beam current of 20 mA,

Fig. 3.25 Micrographs of deposits built at different beam currents

3.3 Mechanical Properties Control of Titanium Alloy …

97

the columnar grains became uncontinuous, the band surface line and fish scale-like fusion line in the macrostructure uniform zone became clearly visible. The macrostructure uniform zone is composed of two parts. The first part is the newly deposited metal at the top layer. The liquid metal in the molten pool solidifies to form a high-temperature β phase by L → β phase transition. The second part is formed by α + β → β phase transition of the solid metal near the molten pool which is heated to the β phase region by heat conduction. Because of the same parental β phase, same phase transition products are expected which forms the macrostructure uniform zone. The height of the macrostructure uniform zone is related to such factors as the thermal conductivity of materials, the input power, the translation speed of the molten pool, and temperature of the matrix. The higher the heat imposed on the material per unit time, the deeper the molten pool, and the larger the region which reaches the β transition temperature adjacent to the molten pool, and the larger the macrostructure uniform zone will be. Therefore, the largest size of the macrostructure uniform zone is found at the largest beam current of 130 mA. This conclusion is consistent with the results discussed in Sect. 3.2.2. The size of the columnar crystal is also related to the heat imposed on the material per unit time. The largest width of columnar crystal is found at beam current of 130 mA, with the columnar crystal grows throughout the entire deposit. At small beam current of 20 mA, the columnar crystals become discontinous and exhibit the smallest width. At the medium beam current of 35 mA, the columnar crystals are growing straight upwards but the width is much smaller than that at beam current of 130 mA. (2)

The effect of beam current on burn loss of elements

The chemical compositions of deposits fabricated at different beam currents are inveatigated and the results are shown in Table 3.3. Al elements are found to be lower than the lower limit as regulated by the CBQB903-002 standard due to burn loss at all three beam currents. The burn loss of the Al element become more obvious as the beam current gets larger. The EBWD process is similar to a vacuum melting process during which the Al element is prone to evaporation due to its low melting point. Burn loss of Al will lead to decrease of the tensile strength of the deposit due to weakened solution strengthening from Al. In order to ensure Al content in the Table 3.3 Chemical compositions of Ti–6Al–4V deposits with different depositing beam currents (wt%) Materials for chemical composition analysis

Elements Al

V

Fe

C

N

H

O

Ti

Wire

5.92

4.0

0.035

0.019

0.012

0.0047

0.13

Balance

No. 1 deposit/130 mA

5.28

4.06

0.04

0.0094

0.010

0.0022

0.12

Balance

No. 2 deposit/35 mA

5.48

4.01

0.034

0.013

0.010

0.0028

0.12

Balance

No.3 deposit/20 mA

5.49

4.00

0.025

0.013

0.010

0.0030

0.19

Balance

98

3 Typical Materials for Electron Beam Wire Deposition

range as regulated by the CBQB903-002 standard, compensation of Al element in feeding wire is necessary. From Table 3.3 we can see that a noticable increase of O content is found in No. 3 deposit. Two sources of O element can be considered in an EBWD deposit: inherited O from feeding wires and absorbed O during depositing process. The O element in feeding wire exists in two forms: thin oxide film on the surface of the wire and O atoms in solid solution state inside the wire. Assuming the thickness of the oxide film is a constant regardless of the diameter of feeding wire. If the thickness of the oxide film is d, the radius of the wire is R, the oxygen content of the oxide film per unit volume is δ, the mass of wires used for depositing part of the same size is identical, and its volume is also constant. We command the volume as V and the increased content of oxygen as  O , then:  O = [π R 2 − π(r − d)2 ] × V /(π R 2 )δ = V dδ(2/R − d/R 2 ) here d is on the order of macro-nano magnitude, so −d/R 2 can be ignored, and the simplified formula is  O = 2V dδ/R. It implies that for deposits with the same volume, the increase of oxygen from feeding wire is only related to its diameter. The smaller the diameter is, the larger the increase of oxygen content will be. Considering the vacuum chamber environment is related to the depositing process, we hereby establish the following simplified model: (a) (b)

(c)

Assuming the vacuum degree in the vacuum chamber is constant during depositing, the content of O element is also constant; The cross-section of the deposit is simplified to be an arc shape, as shown in Fig. 3.26. The pick-up of O element only occurs on the surface of the singlepass deposit that is in contact with the environment instantaneusly, as shown by the red boundary in the figure. Supposing the increase of O element per unit area is δ; Suppose the width of a single-pass deposit is b, the overlaping amount is b/3, the layer thickness is h, the arc radius is R, and the size of the deposit is L× W×H, and the total oxygen increase for this volume of deposit is  O ;Then the relationship between R and b, h is as follows: (R−h)2+ (b/2)2= R2 , that is, R=h/2+b2 /(8h); the arc length of the single-pass deposit is: 2 Rarcsin(b/2R).

Fig. 3.26 Schematic diagram of pick-up of O content in the EBWD process

3.3 Mechanical Properties Control of Titanium Alloy …

99

When a single layer is deposited, the surface area on which oxygen increase is: 2πRarcsin(b/2R)×L×W/(b − b/6), then the total oxygen increase is: oxygen = 2 Rarcsin(b/2R) × L × W/(b − b/6) × H/h × δ Deposition experiments were carried out with different processes. The process for No. l deposit is: beam current 130 mA, F 2.0 mm feeding wire, double wire feeding, single pass width b = 10 mm, and layer thickness h = 1.5 mm; the process for No. 2 deposit is: beam current 35 mA, F 2.0 mm feeding wire, single pass width 6 mm, and layer thickness 1.2 mm; the process for No. 3 deposit is: beam current 20 mA, F 1.2 mm feeding wire, single pass width 3 mm, and layer thickness 0.3 mm. For the convenience of calculation, the size of the deposit is now set as L(250 mm) × W(60 mm) × H(120 mm). The results are as follows: The O element increase of No. 1 deposit is 1.52 × 106δ (Unit: millimeters); The O element increase of No. 2 deposit is 1.99 × 106δ; The O element increase of No. 3 deposit is 7.39 × 106δ. It can be seen from the above mentioned calculation that, for the same volume of deposit in the same vacuum environment, the difference of oxygen increase between No. 1 and No. 2 processes is minor, but the oxygen increase in No. 3 process is three times larger than that in No. 2 deposit indicating the larger the deposit volume is, the greater the difference of O element will be. In order to control the O element in the deposit, the vacuum degree in the vacuum chamber must be controlled during the deposition process. The smaller the width of the single-pass deposit, the more necessary to increase the vacuum degree. Considering the two factors, the smaller the diameter of the feeding wire and the width of the single-pass deposit are, the more the oxygen pich-up in the deposit will be. (3)

The influence of deposition processes on microhardness

Figure 3.27 shows the microhardness changes from the base metal to the deposit 400 HAZ at small beam current

Micro-hardness (HV)

Fig. 3.27 Microhardness along the height direction of deposits built at three beam currents

Substrate

350

Deposit zone

300

HAZ at large beam current

250

No.1 deposit, 130mA No.2 deposit, 35mA No.3 deposit, 20mA

200 -2

0

2

4

6

8

10

12

Distance, mm

14

16

18

100

3 Typical Materials for Electron Beam Wire Deposition

zone for three deposits with different beam currents. At large beam current of 35 mA and 130 mA the microhardness values keep stable in the substrate, and then follows a slight decrease trend in the heat affected zone and deposit zone. An opposite trend is found at small beam current of 20 mA. A quick increase of microhardness is found in the transition zone from substrate to deposite and then keeps constant in the deposit zone. The microhardness is also related to the composition and microstructure. Both the composition and microstructures are similar for deposits built at beam current of 35 and 130 mA, and hence the microhardness is nearly the same and follows the same trend that the microhardness in the deposit zone is slightly lower than that in the substrate. For No.3 deposit built at beam current of 35 mA, the content of O element is significantly higher than that in the other two deposits. Since the O element is an α-phase stable and interstitial element, high content of O element may lead to higher microhardness due to solution strengthening. The other reason for the higher microhardness in the deposition zone of 3# deposit is due to its much finer microstructure. Finer microstructure implies higher intensity of interface. According to Hall–Petch principle, a higher microhardness is expectable in 3# deposit. (4)

Influence of deposition processess on tensile properties

Table 3.4 is the room temperature tensile properties in the Z and Y directions of deposits built under three beam currents of 130 mA, 35 mA and 20 mA in the as-built state (without heat treatment). It can be seen that, at large beam current of 130 mA, the tensile strength in Z direction is only about 780 MPa, the tensile strength in Y direction is about 810 MPa, the elongation is more than 13%, exhibiting a feature of Table 3.4 Tensile properties for Ti–6Al–4V deposits built at three beam currents Deposition condiitons

Specimen orientation

σ0.2 /MPa

σb /MPa

δ/ %

ψ/%

No. 1 deposit 130 mA

Z

735

781

13.3

50.4

745

796

14.2

50.3

Y

No. 2 deposit 35 mA

Z

Y

No. 3 deposit 20 mA

Z

730

780

15.2

48.9

750

809

13.8

45.2

757

810

13.2

43.1

755

816

14.1

45.2

777

829

13.3

44.8

796

831

14.6

46.1

779

827

13.8

48.4

781

846

16.9

40.5

797

831

16.4

44.9

796

840

16.2

44.3

910

1025

9.5

46.9

941

1016

10.6

26.9

917

1000

7.1

18.1

3.3 Mechanical Properties Control of Titanium Alloy …

101

low strength and high ductility; For No.2 deposit built at the medium beam current of 35 mA, tensile strengths in the Z direction and Y direction are about 830 MPa and 840 MPa, respectively, and the elongation is above 12%, exhibiting a better balance between strength and ductility; For No.3 deposit built at small beam current of 20 mA, the tensile strength in Z direction is more than 1000 MPa, while the elongation is more than 7%, exhibiting a high strength and relatively low ductility.

3.3.3 Heat Treatment Control—Relationship Between Microstructure and Mechanical Properties Ti–6Al–4V is a typical α + β two-phase titanium alloy mainly composed of α and β phase in equilibrium or near equilibrium state. The α phases may distribute on the β matrix with needle-like, strip-like, rod-like, or equiaxed shapes. The ratio of α phase in different forms can be tailored by heat treatments. Figure 3.28 shows microstructures of the as-built and HIP states of Ti–6Al–4V alloy fabricated by EBWD method. It can be seen from Fig. 3.28a that a fine strip-like structure was found for the as-built state, and the strip phase is α phase. The morphology and arrangement of the strip-like α phases at different positions are slightly different. The strip-like α phase is obviously coarsened after HIP according to Fig. 3.28b. The α-phase laths in most areas are arranged in a woven manner, while only a small part of area shows a parallel arrangement of α phase. If defining the α phase existing before heat treatment as the primary α phase, and the α phase produced by β → α + β transformation after heat treatment as the secondary α phase. Obviously, the morphology, quantity and volume fraction of the primary α phase can be tailored by heat treatment in the α + β phase region. The sum of the volume fraction of the primary and secondary α phase is close to 100%, so the two phases are just offset from each other in equilibrium state. The morphology, size and arrangement of the secondary α phase are mainly tailored by the cooling methods after heat treatment in the α + β phase region. Figures 3.29, 3.30, 3.31, and 3.32 shows the air-cooled

Fig. 3.28 Microstructures of EBWD Ti–6Al–4V alloy prior heat treatment: a As-built, b HIP

102

3 Typical Materials for Electron Beam Wire Deposition

Fig. 3.29 Microstructure of EBWD Ti–6Al–4V after heat treatment of 930 °C/2 h, AC

Fig. 3.30 Microstructure of EBWD Ti–6Al–4V after heat treatment of 950 °C/2 h, AC

Fig. 3.31 Microstructure of EBWD Ti–6Al–4V after heat treatment of 965 °C/2 h, AC

(AC) microstructures after heat treatment at 930 °C/2 h, 950 °C /2 h, 965 °C /2 h, and 985 °C/2 h, respectively. It can be seen that α → β phase transition occurs in all the above-mentioned temperatures. As the temperature rises, the white strip-like primary α phase partially or completely transforms into high temperature β phase. In the

3.3 Mechanical Properties Control of Titanium Alloy …

103

Fig. 3.32 Microstructure of EBWD Ti–6Al–4V after heat treatment of 985 °C /2 h, AC

subsequent cooling process, the high temperature β phase transforms into secondary α phase and residual β phase. The transition products are also referred to as β-phase transition structures. The primary α phase co-existing with high-temperature β phase during the heat treatment process is embedded on the matrix of the β-phase transition structure. Heat treatment temperature increases, the quantity and volume fraction of the primary α phase decreases, and the quantity and volume fraction of the secondary α phase increases accordingly. When the temperature exceeds 985 °C, the primary α phase is completely transformed into high temperature β phase, forming a complete Widmanstatten structure composed of woven secondary α phase and residual β phase (Figs. 3.30, 3.31). Figure 3.33 shows the effect of solution temperature on the tensile and impact properties of EBWD Ti–6Al–4V alloy under air-cooled conditions. It can be seen from Fig. 3.33a that, when the temperature is in the α + β two-phase region, no obvious change of the tensile strength and ductility are found with increase of temperature; the strength increases slightly and the reduction of area decreases significantly at around the phase transition point. It can be seen from Fig. 3.33b that the impact toughness shows a first increase then decrease trend as the solution temperature

Fig. 3.33 Mechanical properties of EBWD Ti–6Al–4V at different heating temperatures (a) tensile properties; (b) tensile strength and impact work

104

3 Typical Materials for Electron Beam Wire Deposition

increases. The peak of impact toughness appears around 950 °C, indicating that the appropriate temperature for a good match of strength and toughness is around 950 °C. It can be seen from Figs. 3.29, 3.30, 3.31, and 3.32 that, when the volume fraction, morphology and size of the primary α phase are tailored by heat treatment temperature, the volume fraction and morphology of the secondary α phase change accordingly. Apparently, controlling of the heat treatment temperature only can not gain the aim of changing one variable while keep the other constant for primary and secondary α phase. However, by controlling the β → α + β phase transition process, it is possible to change the morphology and quantity of the secondary α phase while keeping the volume fraction and morphology of the primary α phase constant. Cooling rate after solution treatment is the key parameter that can influence the β → α + β phase transition process. Figures 3.34, 3.35, 3.36, and 3.37 are the microstructures of EBWD Ti–6Al– 4V alloy after heat treatment at 930 °C/2 h followed by different cooling rates. Figure 3.34a, b are water-cooled (WQ) microstructures after 930 °C with 2 h holding. The white rod-like α phase is uniformly distributed on the matrix of the dark β-phase transition structure. The dark β-phase transition structure may be the product of β → α + β or β → α phase transition. The water-quenched microstructure is closest to

Fig. 3.34 Microstructure of EBWD Ti–6Al–4V after heat treatment of 930 °C /2 h, WQ

Fig. 3.35 Microstructure of EBWD Ti–6Al–4V after heat treatment of 930 °C /2 h, OQ

3.3 Mechanical Properties Control of Titanium Alloy …

105

Fig. 3.36 Microstructure of EBWD Ti–6Al–4V after heat treatment of 930 °C /2 h, FANC

Fig. 3.37 Microstructure of EBWD Ti–6Al–4V after heat treatment of 930 °C /2 h, AC

that at 930 °C because of the “frozen” microstructrural features due to quick cooling. The needle-like secondary phase inside the β-phase transition structure, as shown in Fig. 3.34b, which is the martensitic phase produced by β→ α phase transition. As the cooling rate decreases, both the volume fraction and thickness of the white rod-like α phase increased. Under the condition of oil quenching (OQ), there are parallelly arranged stripe-like phases in the β-phase transition structure, which can be regarded as the product of β → α + β phase transition. The strip-like phase is the secondary α phase, as shown in Fig. 3.35a, b. Under the fan-cooling (FANC) and air-cooling (AC) conditions, obvious coarsening of α phase is found as a result of slow β → α + β phase transition. The primary α phase and coarsened secondary α phase can not be clearly distinguished, given a false appearance that the volume fraction of primary α phase increased and secondary α phase decreased, as shown in Figs. 3.36 and 3.37. Figure 3.38 shows the microstruture of the EBWD Ti–6Al–4V after heat treatment of 965 °Cholding for 2 h followed by different cooling rates. Because of more α stabilizing element solubilized in the high temperature β phase, the driving force of β → α + β or β → α phase transition increased and the details of β phase transition structure becomes more recognizable. In the water quenched microstructure, primary

106

3 Typical Materials for Electron Beam Wire Deposition

Fig. 3.38 Microstructure of EBWD Ti–6Al–4V heat treated at 965°C/2 h with different cooling rates a water quenching (WQ); b oil quenching (OQ); c fan cooling (FANC); d air cooling (AC)

α phases with large and small aspect ratio coexist and fine needle-like martensitic phases can be observed in the the β-transformed structure, as shown in Fig. 3.38a. As the cooling rate decreases, the primary α phases undergo a coarsening process, but the change of the β-phase transition structure is more significant: the martensitic phase in the water-queched β-phase transition structure is difficult to accurately distiguish at optical microscopy condition, Fig. 3.38a; the secondary α phase is faintly visible in the oil-quenched β-phase transition structure, as shown in Fig. 3.38b; under the conditions of fan and air cooling, coarsened secondary α-phase laths aligned in parallel are clearly seen, as shown in Fig. 3.38c, d. Figure 3.39 shows the effect of cooling rate after heat treatments of 930 °C/2 h and 965 °C/2 h on the room-temperature tensile properties of EBWD Ti–6Al–4V alloy. With increase of the cooling rate, an obvious increase of tensile strength and a decreasing trend of the room-temperature ductility are found. For the same cooling rate, heat treatment temperatures of 930 and 965 °C have little effect on the strength of EBWD Ti–6Al–4V, but the ductility after heat treatment at 930 °C is higher than that heat treated at 965 °C. This phenomenon once again verifies that the solution temperature in the α + β phase region has little effect on the tensile strength when other parameters are identical. However, with the increase of cooling rate, the impact properties show different trends: the impact energy of the material after heat treatment at 930 °C shows a slight increase trend, while the

3.3 Mechanical Properties Control of Titanium Alloy …

107

Fig. 3.39 Effect of cooling rate after solution treatment on tensile properties: a tensile and yield strength; b elongation and reduction of area

impact energy of the material heat treatment at 965 °C shows a first increase then decrease trend as shown in Fig. 3.40. The impact property of titanium alloy is closely related to the type of microstructure and the morphology of α phase. Generally, the impact property of an equiaxed or dual-state structure is lower than that of lamellar structure, and the impact property of a fine-grained structure is lower than that of coarse-grained one. The impact property of lamellar structure is greatly influenced by the thickness of lath α phase: both microstructures with too thin and too thick α-phase laths are not favorable to obtain high impact properties. After the heat treatment at 965 °C, as the cooling rate increases, both primary and secondary α phases become finer. The fan-cooled structure exhbits the highest impact energy, indicating that the thickness of the lath α-phase under the fan-cooled condition is optimal for higher impact energy. Figure 3.41 shows the influence of the heat treatment parameters on the hardness (HRC) of EBWD Ti–6Al–4V alloy. The  symbol in the figure is the hardness data Fig. 3.40 Effect of cooling rate on impact properties of EBWD Ti–6Al–4V

108

3 Typical Materials for Electron Beam Wire Deposition

Fig. 3.41 Effect of heat treatment parameters on the HRC of EBWD Ti–6Al–4V

of the fan-cooled samples heat treated in the range of 930–985 °C. It can be seen that the annealing temperature has little impact on HRC, and the difference of the HRC data with different solution temperatures are within the error range. The  and  symbles in the figure are HRC results of samples solution treated at 930 ºC and 965 ºC respectively followed by different cooling rates. It can be seen that the HRC tends to increase as the cooling rate inceases under the same solution temperature and similar trend was found for tensile strength as shown in Fig. 3.39.

3.4 Damage and Fracture Mode in Tensile Tests The following typical tensile fracture modes are found for EBWD Ti–6Al–4V alloy sample: ductile dominant transgranular fracture, brittle dominant transgranular fracture and fracture near the grain boundary α phase as shown in Fig. 3.42, Fig. 3.43 and Fig. 3.44, respectively. In Fig. 3.42, obvious plastic deformation characteristics are found near the fracture surface and the strip-shaped α phase is elongated or bent obviously which indicates occurrence of high degree of ductile deformation. Figure 3.43 shows characteristics of brittle dominant transgranular fracture. A flat and less wavy fracture surface is found, showing relatively lower ductility. Figure 3.44 shows that the tensile fracture position is near the grain boundary α phase, indicating that obvious plastic deformation occurred inside the prior β grains and been impeded on one or both sides of a grain boundary α phase. The plastic deformation near the grain boundary α phase is constrained and incompatible deformation occurrs which leads to fracture. Due to the different degree of intragranular deformation, the fracture near the grain boundary α phase does not mean low ductility. Liu Zheng et al. [19] from IMR CAS conducted a systematic study on the incompatible deformation on two sides of a grain boundary α phase. Figure 3.45 shows slip bands appeared on the surface of a EBWD Ti–6Al–4V sample after 2% tensile

3.4 Damage and Fracture Mode in Tensile Tests

109

Fig. 3.42 Transgranular ductile fracture

Fig. 3.43 Transgranular brittle fracture

deformation at room temperature. The tensile direction is in the direction of wire translation during the deposition process, namely X direction which is vertical to the growth direction of columnar crystals. Figures 3.45a, b show the slip bands appeared in the original β grains. In Fig. 3.45a, the slip bands appear at the α-platelet boundary. In Fig. 3.45b, the slip bands appear inside α platelets. The arrow in Fig. 3.45c, d points to a grain boundary α phase (GB α). It can be seen that different degree of deformation occurs on both sides of the grain boundary α phase. However far more slip bands

110

3 Typical Materials for Electron Beam Wire Deposition

Fig. 3.44 Fracture near the grain boundary α phase

Slip bands at α phase boundaries (a) and inside of α platelets (b) (2% strain)

Slip bands at two sides of GB α at 2% strain (c) and 5% strain(d)

Fig. 3.45 Slip bands in tensile specimen perpendicular to the columnar crystal direction

are found in the right grain than in the left one, indicating deformation unconsistency in different grains. As the deformation proceeds, the number of slip bands in the right grain increases significantly, as shown in Fig. 3.45d. The slip trace lines close to the grain boundary

3.4 Damage and Fracture Mode in Tensile Tests

111

shows a curved morphology, indicating that it is difficult for dislocations to pass through the grain boundary in the plastic deformation process, and a large amount of dislocation piled up at the grain boundary and caused stress concentration, leading to prominent cross slip of dislocation. According to the analysis of the crystallographic orientation in the gauge length section of the tensile specimen, it can be found that when the tensile direction is perpendicular to the growth direction of the columnar crystals, the easiness for deformation of the β columnar crystals is related to the angle ϕ between the (001) plane and the tensile axis, as shown in Fig. 3.46. The black thin line in the figure indicates the slip trace, and the tensile direction is horizontal. When ϕ is within 16°, the columnar crystals appear as deformation-easy grains; when ϕ is above 36°, the columnar crystals appear as deformation-difficult grains. The theoretically calculated critical ϕ value is about 24°, which can be taken as criteria for the easiness of deformation for columnar grains. Figure 3.47 shows the morphology of slip bands found in the tensile specimen tested parallel to the columnar crystal direction after 2.0% plastic deformation. Both the columnar crystal direction and the tensile direction are horizontal in the figure, and the black thin lines indicate the slip trace lines. The horizontal thin line indicated by the arrow in the middle part of Fig. 3.47a is a grain boundary α phase. Apparently, slips are formed inside the two adjacent columnar crystal β grains. However, the spatial orientations of the slip traces in the two grains are different: the slip trace lines in the upper grain are shorter and those in the lower grain are much longer and at least two slip systems activated. This difference in slip band morphology is mainly

Fig. 3.46 Columnar crystal orientation in gauge section of specimen after tensile deformation

112

3 Typical Materials for Electron Beam Wire Deposition

(a) Slip bands at two sides of a GB α phase

Slip bands above the GB α phase (b) and below the GB α phase (c)

Fig. 3.47 Slip bands on the surface of the tensile specimen after 2.0% deformation parallel to the columnar crystal direction

due to the difference in orientation of the original β grains and the different spatial orientations of the α platelet formed inside the columnar β grains. In Figs. 3.45 and 3.47, mainly prismatic and basal slip systems initiated, together with a small portion of pyramidal slip. Slips are more easily formed in α phases with favorable schmidt factors. Since most adjacent α platelets have different orientations, the slips are liable to be constrained inside the α platelets at small deformation, and cross slips are liable to form near the boundary of α platelets.

3.5 Several Feeding Wires and Mechanical Properties of Their Deposits

113

3.5 Several Feeding Wires and Mechanical Properties of Their Deposits 3.5.1 TC4EM Alloy Wire and Mechanical Properties of Its Deposit TC4EM is a titanium wire developed by IMR CAS for fabrication of medium-strength and high-toughness titanium alloy parts by EBWD methods. Its nominal composition is Ti–6.5Al–3.5V–0.15O. Deposits built using TC4EM feeding wire exhibit good match between strength and toughness. The room-temperature density of the TC4EM deposit is 4.4 kg/m3 , and the mechanical properties are close to that of Ti–6Al– 4VELI. The mechanical properties of the deposit with a cross section dimension of 100 mm × 100 mm fabricated using TC4EM wires are as follows: (1)

Tensile Porpeties (Table 3.5)

(2)

Notched tensile properties (Table 3.6)

(3)

Impact properties (Table 3.7)

Table 3.5 Tensile properties of TC4EM deposits (Kt = 1) Test temp. −60 °C

23 °C

100 °C

200 °C

300 °C

400 °C

σb

σ0.2

δ

ψ

MPa

MPa

%

%

L

1010

960

11.0

25.0

T

1020

955

10.5

24.0

Specimen orientation

ST

965

865

14.5

42.5

L

875

790

14.0

37.0

T

875

790

13.5

38.0

ST

820

745

15.5

53.0

L

765

660

16.5

47.0

T

790

675

16.0

48.0

ST

735

610

19.0

57.5

L

685

560

18.0

51.0

T

685

555

19.5

52.5

ST

625

500

20.0

60.5

L

590

460

16.5

56.0

T

605

470

17.0

56.0

ST

555

420

19.5

63.5

L

540

415

15.0

58.0

T

545

415

16.0

58.5

ST

510

370

19.0

63.0

114

3 Typical Materials for Electron Beam Wire Deposition

Table 3.6 Notched tensile strength of TC4EM deposits, σbH /MPa

Stress concentration factor

Specimen orientation L

ST

Kt = 2

1338

1337

1340

1337

1359

1321

1343

1326

Kt = 3

Table 3.7 Impact properties of TC4EM deposits at different temperatures, AKU2 /J

Test temp. −60 °C

23 °C

(4)

1351

1318

1343

1333

1366

1302

1364

1338

1352

1331

1358

1329

Specimen orientation L

T

ST

42

48

37

48

45

33

37

35

36

38

40

39

39

41

37

45

54

42

55

51

42

43

50

45

39

50

39

45

51

43

High cycle fatigue properties

The room-temperature smooth (Kt = 1) and notched (Kt = 3) high-cycle fatigue S–N curves of the deposits fabricated with TC4EM alloy wire are shown in Fig. 3.48. The S–N curve of deposit after hot isostatic pressing (HIP) at Kt = 1 exhibits an upward convex shape, and the fatigue limit can reach 600 MPa, which is higher than the forgings with similar chemical composition. The notched fatigue S–N curve exhibits a downward convex shape, with a fatigue limit of about 200 MPa. (5)

Fracture toughness

For deposits fabricated with TC4EM wires, the plane strain fracture toughness KIC tested using CT specimens at room temperature is about 110MPm1/2 , and the difference of KIC between the L–T and T-L directions is within 5MPm1/2 . (6)

Fatigue crack growth rate

3.5 Several Feeding Wires and Mechanical Properties of Their Deposits

115

Fig. 3.48 Smooth and notched fatigue S–N curves of TC4EM deposits: a Kt = 1; b Kt = 3

(a) L-T directions

(b) T-L directions

Fig. 3.49 da/dN ~ K FCP curves in the L–T and T–L directions of the TC4EM deposit

The fatigue crack growth curves of deposits fabricated with TC4EM wires at room temperature is shown in Fig. 3.49. The threshold of fatigue crack growth in the L–T direction is slightly higher than that in the T–L direction, and the C and n values of Paris Formula are roughly equivalent in the two directions.

3.5.2 TC4EH Alloy Wire and Mechanical Properties of Its Deposit TC4EH is another titanium alloy designed by IMR CAS for feeding wire used for fabricating of EBWD parts with medium to high tensile strength. Its nominal composition is Ti–7.0Al–3.5V–0.2O–0.5Fe. The deposit built using TC4EH wire has sound match between strength and toughness. The density of EBWD TC4EH sample at room temperature is 4.4 kg/m3 . The mechanical properties are close to Ti–6Al–4V forgings.

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3 Typical Materials for Electron Beam Wire Deposition

The physical and mechanical properties of samples fabricated with TC4EH wire are as follows. (1)

Elastic properties

The Young’s modulus, shear modulus and Poisson’s ratio of TC4EH deposits at different temperatures are shown in Fig. 3.50. (2)

Tensile properties of smooth specimen

Smooth specimen tensile properties at different temperatures from −60 to 400 °C of TC4EH deposits are given in Table 3.8. (3)

Impact properties

Impact properties at −60 °C and room temperatures of TC4EH deposits are given in Table 3.9. (4)

High cycle fatigue properties

The smooth (Kt = 1) and notched (Kt = 3) high-cycle fatigue S–N curves of the TC4EH deposits at room temperature are shown in Fig. 3.51. The smooth fatigue S–N curve of TC4EH deposits after hot isostatic pressing is slightly concave to the lower left, and the fatigue limit is about 625 MPa, which is higher than that of forgings with similar chemical composition. The notch fatigue S–N curve is obviously concave downward, with a fatigue limit of about 220 MPa. (5)

Low cycle fatigue properties

The strain-controlled low-cycle fatigue ε–N curves of the TC4EH deposits at room temperature and two strain ratios of R = 0.1 and R = −1 are shown in Fig. 3.52. At R = −1, the low-cycle fatigue ε–N curve of TC4EH deposits is comparable to that of Ti–6Al–4V forgings but higher than that of Ti–6Al–4V castings as shown in Fig. 3.52a. At R = 0.1, the low-cycle fatigue ε–N curve of the EBWD TC4EH deposits coincides with the ε–N curve of TC4 forgings, as shown in Fig. 3.52b. In

Fig. 3.50 Young’s modulus, shear modulus and Poisson’s ratio of TC4EH deposits at different temperatures

3.5 Several Feeding Wires and Mechanical Properties of Their Deposits

117

Table 3.8 Smooth Specimen Tensile Properties of TC4EH deposits Test temp. −60 °C

Room temperature

100 °C

200 °C

300 °C

400 °C

σb

σ0.2

δ

ψ

MPa

MPa

%

%

L

1102

1044

10.8

21.7

T

1110

1050

10.3

22.2

ST

1019

926

17.3

41.9

L

958

865

11.1

29

T

965

867

12.5

27.6

ST

886

773

17.4

49.4

L

875

761

15.4

43.1

T

868

746

15.3

45.1

ST

795

669

19.4

56

L

760

619

15.9

51.7

T

768

613

18

48.5

ST

688

553

21.6

58.8

L

667

529

16.3

60.9

T

691

528

17.2

52.1

ST

609

463

23.4

65.6

L

624

488

15.9

61.3

T

635

483

17.1

59.7

ST

565

416

19.8

63.7

Specimen orientation

Table 3.9 Impact properties of TC4EH deposits at −60 °C and room temperature, AKU2 /J

Test temp. −60 °C

Room temperature

Specimen orientation L direction

T direction

ST direction

47

39

47

21

38

51

29

45

50

39

47

47

38

45

46

57

53

57

62

59

59

67

56

60

57

57

57

61

58

57

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3 Typical Materials for Electron Beam Wire Deposition

Fig. 3.51 Room-temperature fatigue S–N curves of TC4EH deposits: a Kt = 1; b Kt = 3

Fig. 3.52 Low-cycle fatigue ε–N curves of TC4EH deposits: a R = −1; b R = 0.1

general, the low-cycle fatigue property of EBWD TC4EH deposit is comparable to Ti–6Al–4V forgings and higher than Ti–6Al–4V castings.

3.5.3 A-100 Steel Wire and Mechanical Properties of Its Deposit The AerMet series alloys are new type ultra-high-strength steel developed to be used as the landing gear of the US Navy’s F/A18E/F fighter. It is strengthened by precipitation of (Mo,Cr)2 C carbide. The recommended heat treatment process is: hardening or solid solution treatment at 885–968 °C, aging treatment at 441–496 °C for 3–8 h; cryogenic treatment at −73 °C for 1 h, recovered to room temperature in a natural air atmosphere. Now AerMet family alloys include AerMet100, AerMet310 and AerMet340. Among them, the standard required minimum ultimate tensile strength of AerMet100 is 1930 MPa and the required minimum fracture toughness is 110 MPa·m1/2 ; the required minimum ultimate tensile strength of AerMet310 is 2137 MPa; the required

3.5 Several Feeding Wires and Mechanical Properties of Their Deposits

119

minimum ultimate tensile strength of AerMet340 is 2344 MPa. Compared with the high-strength 18Ni martensitic steel as Marage250, 300, 350 and titanium alloys as Ti–6Al–4V and Ti–10V–2Fe–3Al commonly used in the aviation industry, the AerMet alloys exhibit not only high strength but superior toughness and long fatigue life, which make it more attractive and competitive than the other high strength alloys. At present, the AerMet100 steel has been successfully used in the landing gear of F/A-18 and F-22 fighters. In China, Fushun Special Steel has established a technical standard for AerMet100 bar, with standar No. of QJ/DT01.53039-2008. Deposits with size of 155 mm in length, 81 mm in width, and 80 mm in height were built using feeding wire of A-100 steel with diameter of 1.6 mm by EBWD method. The following three heat treatment schedules are used: The base heat treatment schedule is selected according to that recommended by the QJ/DT01.53039-2008 standard, namely: normalizing treatment at 890 °C holding for 1 h, followed by air cooling; tempering at 650 °C for no less than 8 h followed by air cooling; quenching at 885 ± 15 °C for 60 ± 15 min followed by oil quenching; cold treatment at −73 ± 8 °C, holding for 60 ± 5 min followed by restoring to room temperature naturally; tempering at 482 °C ± 5 °C, holding for 5–8 h, followed by air cooling. The first four steps of the second and third heat treatment schedules are the same as the first one, but the fifth step is different from the base one. Details of the three heat treatment schedules are summarized in Table 3.10. The mechanical properties are listed in Tables 3.11, 3.12, 3.13 and 3.14. Table 3.10 Heat treatment schedules used for EBWD A-100 deposits No.

Normalizing

Tempering

Quenching

Cold treatment

Tempering

1

890 °C/1 h, AC

650 °C/ ≥8 h, AC

885 ± 15 °C/60 ± 15 min, OQ

−73 ± 8 °C /60 ± 5 min, AC

482 ± 5 °C/5–8 h, AC

2

890 °C/1 h, AC

650 °C / ≥8 h, AC

885 ± 15 °C/60 ± 15 min, OQ

−73 ± 8 °C /60 ± 5 min, AC

465 ± 5 °C /5–8 h, AC

3

890 °C/1 h, AC

650 °C / ≥8 h, AC

885 ± 15°C/60 ± 15 min, OQ

−73 ± 8 °C /60 ± 5 min, AC

450 ± 5 °C /5–8 h, AC

Table 3.11 Tensile properties of EBWD A-100 deposit with No.1 heat treatment schedule Heat treatment

Direction

σb /MPa

σ0.2 /MPa

δ/%

ψ/%

482 °C ± 5 °C/5–8 h, AC

X

1735

1485

12.3

61.5

Y

1780

1470

12.0

61.5

Z

1760

1465

12.2

63.5

Vertical/L

≥1930

≥1620

≥10

≥55

Horizontal/T

≥1930

≥1620

≥8

≥45

Specification

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3 Typical Materials for Electron Beam Wire Deposition

Table 3.12 Tensile properties of EBWD A-100 deposit with No.2 heat treatment schedule Heat treatment

Direction

σb /MPa

σ0.2 / MPa

δ/%

ψ/%

465 ± 5 °C /5–8 h, AC

X

1970

1753

11.8

60.0

Y

1980

1782

11.0

59.0

Specification

Vertical/L

≥1930

≥1620

≥10

≥55

Horizontal/T

≥1930

≥1620

≥8

≥45

Table 3.13 Tensile properties of EBWD A-100 deposit with No.3 heat treatment schedule Heat treatment

Direction

σb /MPa

σ0.2 /MPa

δ/%

ψ/%

450 °C ± 5°C/5–8 h, AC

X

2095

1830

11.5

53.5

Y

2080

1790

11.5

53.0

Vertical/L

≥1930

≥1620

≥10

≥55

Horizontal/T

≥1930

≥1620

≥8

≥45

Specification

Table 3.11 shows the tensile properties of the EBWD A-100 with No.1 heat treatment schedule. It can be seen that the ductility data in three orientations meets the requirement of QJ/DT01.53039-2008. However, the yield and tensile strength data are about 200 MPa lower than that required in QJ/DT01.53039-2008. No apparent difference of tensile data in the three directions of X, Y and Z is found, indicationg deposits of EBWD A-100 exhibit low degree of anisotropy. Tables 3.12 and 3.13 show the room-temperature tensile properties of A-100 deposits with No.2 and No.3 heat treatment schedules. It can be seen that the tensile properties of deposit with the No. 2 heat treatment schedule meet the requirements of QJ/DT01.53 039-2008. The tensile strength, yield strength and elongation of deposit with the No. 3 heat treatment schedule meet requirements of QJ/DT01.53 039-2008, while the reduction in area can only meet the requirement of horizontal direction but fail to meet that of the vertical direction. It can be seen that the tempering temperature in the final state of the heat treatment schedule has a significant effect on the tensile properties of A-100 steel fabricated by EBWD. As the final tempering temperature decreases, the strength increases, but the ductility tends to decrease. Table 3.14 shows the HRC and impact properties of the EBWD AerMet100 with Table 3.14 HRC and impact properties of EBWD A-100 deposit with No.1 heat treatment schedule Direction

AKU2 /J 59

73.5

49.0

63

78

X Y Z Specification

αKU2 /J/cm2

HRC

48.0

73

90

Vertical/L

≥ 53

_

_

Horizontal/T

≥ 53

_

_

3.5 Several Feeding Wires and Mechanical Properties of Their Deposits

121

No.1 heat treatment schedule. The samples used for HRC testing is the same as that for impact test in X-direction, and the direction of HRC is defined to be the normal direction of the tested plane. The hardness values in the Y and Z directions are 49HRC and 48HRC, respectively, failing to meet the requirements of QJ/DT01.53039-2008. The impact energy is between 65J and 118J, exhibiting large scatter of test data. Impact toughness in the X and Y directions is nearly identical and lower than that in the Z direction.

References 1. Matz JE, Eagar TW (2002) Carbide formation in alloy 718 during electron-beam solid freeform fabrication. Metall and Mater Trans A 33(8):2559–2567 2. Bird R K, Hibberd J (2009) Tensile properties and microstructure of inconel 718 fabricated with electron beam freeform fabrication (EBF3). NASA Langley Research Center, Hampton, VA 23692:1–19 3. Tayon WA, Shenoy RN, Redding MR, Keith BR, Hafley RA (2014) J Manuf Sci Eng 136. 061005-061005-061007 4. Domack MS, Taminger KMB, Begley M (2006) Metallurgical mechanisms controlling mechanical properties of aluminum alloy 2219 produced by electron beam freeform fabrication. Mater Sci Forum 519:1291–1296 5. Taminger KM, Hafley RA, Domack MS (2006) Evolution and control of 2219 aluminum microstructural features through electron beam freeform fabrication. Mater Sci Forum 519:1297–1302 6. Branes J E, Brice CA, Taminger KM, et al (2005) Fabrication of titanium aerospace components via electron beam freeform fabrication. In: 2005 AeroMat conference and exposition.Orlando, Florida, USA 7. Lach C L,Taminger K M, Schuszler A B, et al (2007) Effect of electron beam freeform fabrication (EBF3) processing parameters on composition of Ti-6–4[C]. 2007 AeroMat conference and exposition. Baltimore, Maryland, Jun 27, 2007, pp 1–19 8. Edwards P, O’Conner A, Ramulu M (2013) Electron beam additive manufacturing of titanium components: properties and performance. J Manuf Sci Eng 135:061016-1~7 9. Heck D, Slattery K, Salo R, et al (2007) Electron beam deposition of Ti 6-4 for aerospace structures. In: AIAA SPACE 2007 conference & exposition, long beach, California, USA, vol 6195, pp 1–7 10. Brice CA, Henn DS (2002) Rapid prototyping and freeform fabrication via electron beam welding deposition. Proceeding of Welding conference 11. Bush RW, Brice CA (2012) Elevated temperature characterization of electron beam freeform fabricated Ti–6Al–4V and dispersion strengthened Ti–8Al–1Er. Mater Sci Eng 544:13 12. Chen Z, Suo H, Li J (2010) Electron beam wire deposition rapid manufacturing technology and microstructure characteristics. Aeronaut Manuf Technol (1):36–39 13. Jun L, Hongbo S, Jianrong L et al (2012) Tensile properties of the columnar crystal structures of TC18 titanium alloy fabricated by electron beam rapid prototyping. Trans Mater Heat Treat 33(6):110–115 14. Guang Y, Shuili G, Hongbo S (2012) Research on the microstructure characteristics of multiple depositions of TC18 alloy fabricated by electron beam rapid prototyping. Aeronaut Manuf Technol 8:71–73 15. Zhitao H, Hongbo S, Guang Y et al (2015) The effect of heat treatment technologies on the microstructure properties of TC18 titanium alloy fabricated by electron beam wire deposition. Trans Mater Heat Treat 36:50–53

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16. Hongbo S (2013) Research on the microstructure and mechanical properties of TC4 titanium alloy fabricated by electron beam rapid prototyping. Huazhong University of Science and Technology, Wuhan 17. Yusheng C (2013) Study on the tensile deformation behavior and deformation mechanism of TC18 fabricated by electron beam rapid prototyping. Shenyang Ligong University, Shenyang 18. Yusheng C, Guang J, Hongbo S, Jianrong L (2014) The relationship between microstructure and hardness of TC18 titanium alloy fabricated by electron beam rapid prototyping. Aeronaut Manuf Technol 463:81–85 19. Zheng L (2019) Study on the microstructure and tensile mechanical behavior of TC4 alloy fabricated by electron beam wire deposition. University of Science and Technology of China, Anhui 20. Compton C, Baars D, Bieler T (2007) Studies of ternative techniques for niobium cavity fabrication. Material presented at the 13th International Workshop on RF Superconductivity, Bejing, China, WEP: Poster Session II:429–433

Chapter 4

Fundamentals of Electron Beam Wire Deposition Technology

Abstract This chapter introduces the fundamentals of electron beam wire deposition technology, such as the behavior of the molten pool during the deposition process, numerical simulation of the EBWD process, temperature field characteristics of the molten pool under the wire-free technology and impact of wire deposition on the temperature field of the molten pool, etc. The fundamentals of the process technique of the EBWD, such as prototyping process control, types of defects and control methods,deformation control, etc. have been introduced.

4.1 Research on the Behavior of the Molten Pool During the EBWD Process The electron beam wire deposition process is essentially the process of the liquidto-solid transition of the molten pool generated by the transition of the liquid metal droplets to the deposition plane. Its behavior change is closely related to the technological parameters, environmental conditions and working conditions of the prototyping process. Conducting research on the behavior of the molten pool and mastering its behavior rules are important for the stable and smooth completion of the prototyping process. In this section, we apply the combination of the numerical simulation analysis and the experimental verification to study the behavior of the molten pool based on the need of EBWD for optimization [1].

4.1.1 Numerical Simulation of the EBWD Process The geometric model of the matrix is 100 mm in length, 30 mm in width, and 15 mm in thickness. We use uniform grids for division with a grid step of 0.5 mm. It is agreed that the moving direction of the electron beam relative to the matrix is the positive X axis and the depth direction of the molten pool is the positive Z axis, as shown in Fig. 4.1.

© National Defense Industry Press 2022 S. Gong et al., Electron Beam Wire Deposition Technology and Its Application, Additive Manufacturing Technology, https://doi.org/10.1007/978-981-19-0759-3_4

123

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.1 Geometric model

In order to verify the reliability of the mathematical model, we conducted the single-pass wire-free EBWD experiment and the numerical simulation, and compared the simulation results with the experimental results. The experimental results and simulation results of the comparative examples on the width and depth of the molten pool are shown in Fig. 4.2. As shown in Fig. 4.2, the simulation results are in good agreement with the experimental results in terms of the value size and changing trend of the depth and width of the molten pool. In Fig. 4.3, we compare the metallographic figures of the cross-sectional morphology of the molten pool obtained from the experiment with the numerical simulation results. We can see that the cross-sectional shape of the molten pool is also in good agreement. In order to study the heat transfer and flow behavior of the molten pool during EBWD process, we studied the heat transfer and flow behavior of the electron beam single-pass wire-free and wire deposition prototyping.

Fig. 4.2 Comparison of experimental results and simulation results of the depth and width of the molten pool

4.1 Research on the Behavior of the Molten Pool …

125

Fig. 4.3 Comparison of experimental results and simulation results of the cross-sectional morphology of the molten pool

Figure 4.4 shows the evolution process of the molten pool during the single-pass wire-free deposition process with simulated technological parameters (beam current 130 mA, translation speed 15 mm·s−1 , elliptical scanning, wire free). As shown in Fig. 4.4 that at 0.2 s, the starting position of the machining is rapidly melted under the continuous heating of the electron beam, forming an inverted hump-shaped molten pool. At 1.0 s, as the matrix moves, the electron beam moves in the positive direction of the X axis relative to the matrix, and the molten pool also moves. Meanwhile, the depth, the width and the length of the molten pool continue to increase, reaching 6.5 mm, 14.5 mm and 21 mm respectively. As shown in the side view of the molten pool in Fig. 4.4(c), a small step is produced at the front of the molten pool. This is because the scanning effect of the electron beam will first form a level 1 molten pool in the scanning area. Then, as the electron beam moves forward, the part of the first-level molten pool where the electron bean passes will continue to be heated to absorb energy, so that the width and depth will further increase to form a second-level molten pool. The step is the dividing point of the two levels of molten pools. At this time, the depth and width of the molten pool are stable and no longer change, but the length of the molten pool is still increasing. Due to the backlash pressure and thermal capillary force at the front end of the molten pool, the liquid surface of the molten pool will sink downward. The depression will always be located at the front end of the molten pool and move forward synchronously with the molten pool. The shape of the front end of the molten pool remains basically unchanged as the prototyping continues, and the length will continue to increase. This is because the heat transfer and heat dissipation of the matrix makes the tail of the molten pool narrower and shallower. This process will last until the molten pool is steady.

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.4 Evolution process and temperature field distribution of the molten pool

4.1.2 Temperature Field Characteristics of the Molten Pool Under the Wire-Free Technology In order to help study the temperature field characteristics of the molten pool under the wire-free technology, we take 10 points along the central axis of the machining direction 1 mm from the surface of the molten pool and along the Y axis 15 mm from the initial machining position. The spacing between two points is 1.5 mm. Marks are shown in Fig. 4.5.

4.1 Research on the Behavior of the Molten Pool …

127

Fig. 4.5 Schematic diagram of marked points

Figure 4.6 shows the thermal cycle curve of different marked points. We can see that when the electron beam starts to act on the marked area, the temperature of the marked points 1 to 6 near the energy center of the electron beam will quickly rise to the highest value. The farther from the center, the lower the maximum temperature will be. The maximum temperatures of marked points 1–6 are 3127 K, 3103 K, 3049 K, 2762 K, 2123 K, and 1606 K respectively. After reaching the maximum temperature, points 1 to 5 will decrease to about 2000 K at a faster rate. At this point, the cooling rate begins to decrease until it is cooled to the temperature 1878 K of the solid phase mark line, at which the cooling rate increases slightly. But as the cooling proceeds, the cooling rate begins to slowly decrease again. The temperature of the marked points 7 to 10 far from the center slowly increases, and then join with the Fig. 4.6 Thermal cycle curve of marked points 1–10

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4 Fundamentals of Electron Beam Wire Deposition Technology

points 1 to 6 in the cooling state at about 1200 K, and finally decreases at a slower rate. (1)

Heating rate

The average heating rate of marked points 1 to 6 from bottom to top is shown in Fig. 4.7. As shown in Fig. 4.7, the marked point 1 in the center of the molten pool sees the fastest heating rate, reaching 3127 K/s. As the distance away from the center of the molten pool (Y-direction) increases, the heating rates of the remaining marked points gradually decrease. The heating rates of marked points 2–6 are 3121 K/s, 3021 K/s, 2850 K/s, 2014 K/s, and 1236 K/s respectively, and the reduction amplitude is increasing. (2)

Temperature gradient of the molten pool surface in the vertical machining direction

The temperature values of different marked points at 1.0 s are shown in Fig. 4.8. As you can see, the temperature trend shows that the marked point 1 on the central axis indicates the highest temperature. The temperature continues to decrease as it gets farther away from the center. The temperature values are 3098 K, 3078 K, 3038 K, 2745 K, 2114 K, 1548 K, 1053 K, 653 K, 427 K, 340 K respectively. The temperature differences between two adjacent positions are 20 K, 40 K, 293 K, 631 K, 566 K, 495 K, 400 K, 226 K, 87 K. We can see that the temperature gradient of the molten pool surface in the vertical machining direction first shows a trend of increase as it gets farther from the center, and the temperature gradient begins to gradually decrease after passing the marked point 5. (3)

Residence time in high temperature zone

During the EBWD process, how long different positions of the matrix are exposed to high temperature (β phase transition temperature of 1248 K or above) will affect the growth of the original β-phase columnar crystals and the degree of transition from Fig. 4.7 Heating rate of marked points 1–6

4.1 Research on the Behavior of the Molten Pool …

129

Fig. 4.8 Temperature of marked points 1–10 at 1.0 s

α phase to β phase. Figure 4.9 shows the residence time of different marked points in the high temperature zone. As shown in the figure, the center of the molten pool has the longest residence time in the high temperature zone. The residence time in the high temperature zone gradually decreases as the distance from the center of the molten pool increases. (4)

Cooling rate

The average cooling rate of the marked points 1–6 from the maximum temperature to the β phase transition temperature is shown in Fig. 4.10. As shown in Fig. 4.10, the farther it is from the center of the molten pool, the lower the average cooling rate will be, and the cooling rate values are within the range of 20–410 K/s, which is consistent with the known results of experimental analysis. Fig. 4.9 Residence time of marked points 1–10 in the high-temperature zone

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.10 Cooling rate of marked points 1–6

4.1.3 Impact of Wire Deposition on the Temperature Field of the Molten Pool In order to study the influence of droplet deposition on the heat transfer of the molten pool, the EBWD process is dynamically simulated based on the technological parameters (beam current 130 mA, translation speed 15 mm·s−1 , elliptical scanning, double wire feeding, single-pass wire feeding rate 35 mm·s−1 ). The time for the droplet to start dropping is 6.0 s; the lower end is 1 mm from the surface of the molten pool, which lags behind the center of the heat source by 5 mm in the machining direction; the intrinsic temperature is 2000 K. The droplet deposition process is shown in Fig. 4.11. As shown in the figure, before the droplet deposition, the temperature field distribution of the molten pool is the same as (e) and (f) in Fig. 4.11. The droplet just completely enters the molten pool at 6.09 s. The deposition of droplets changes the morphology of the gas–liquid boundary because a part of droplets fill the back of the recessed liquid surface. After the droplets with intrinsic temperature are deposited into the molten pool, the temperature of the contact area rapidly shifts towards the self-contained temperature through conduction heat transfer and convection heat transfer. Compared with the temperature field distribution at 6.00 s, the temperature range of around 2000 K in the molten pool is significantly increased at this time, which almost covers the middle and back parts of the entire molten pool. At 6.16 s, the surface height of the molten pool is increased to a certain extent, and a deposition layer will be formed after solidification. This is when the effect of droplet deposition begins to slowly weaken, and the distribution of the temperature field of the molten pool is restored to the state before the droplet deposition. Therefore, as the prototyping process proceeds, the temperature field of the molten pool will be completely restored to the state before the droplet deposition, and then start the next cycle of the above-mentioned temperature field change as new droplets deposit.

4.1 Research on the Behavior of the Molten Pool …

131

Fig. 4.11 Temperature field distribution during wire deposition

4.1.4 Flow Field Distribution of the Molten Pool Under the Wire-Free Technology Figure 4.12 shows the flow field distribution of the molten pool during the electron beam deposition process with simulated technological parameters (beam current 130 mA, translation speed 15 mm·s−1 , elliptical scanning, wire free). It can be seen from the figure that the molten pool is very shallow at 0.2 s, so there is basically no velocity along the depth direction. On the surface of the molten pool, since the movement of the matrix is slow, the electron beam continues to act on the initial area, causing the velocity to diverge from the center of the molten pool, as shown in Fig. 4.12b. At 1.0 s, the velocity field changes significantly along with the simultaneous growth of the molten pool in the depth, width, and length directions. According to Fig. 4.12c, d, the flow process in the molten pool is divided into two parts by the concave liquid surface. The first part is the velocity that the front end of the molten pool bypasses the concave liquid surface and flows backward along the two sides. This is because the melt here is squeezed by the electron beams and

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.12 Flow field distribution during the evolution of the molten pool

the concave liquid surface. The second part is the upward velocity along the back wall of the recessed liquid surface that starts from the bottom of the back wall of the recessed liquid surface. After reaching the surface of the molten pool, there will be a great backward flow velocity, and a downward backflow generated at the tail of the molten pool that flows from the bottom of the molten pool back to the bottom of the back wall of the recessed liquid surface, converges with the melt flowing from the foremost end of the molten pool, and then continues to flow upwards along the back wall of the recessed liquid surface. After passing through the surface of the molten, the melt will enter the next flow cycle. The flow trend remains unchanged at 2 s.

4.1 Research on the Behavior of the Molten Pool …

133

However, as the molten pool grows in the length direction, the length of the flow circulation route becomes longer after the recessed liquid surface of the molten pool. On the surface of the molten pool, we can see a clear trend of bypassing the recessed liquid surface at the front end of the molten pool, the backward flowing of electron beams, and a large backward flow velocity in the middle of the molten pool. At 3 s, although the length of the molten pool continues to increase, the flow circulation route just follows the molten pool to move forward since the tail of the molten pool is shallowed and narrowed. The length is basically stable with no increase, and the tail of the molten pool basically sees no velocity. Reviewing the flow field distribution of the entire molten pool, we can find out that the second part of the flow field, which is the cyclic flow in the depth direction, features strong regularity and stability. The length of the route and the relative position to the front of the molten pool after 6.0 s are basically unchanged.

4.1.5 Impact of Wire Deposition on the Flow Field of the Molten Pool In order to study the influence of droplet deposition on the flow of the molten pool, the EBWD process is dynamically simulated based on the technological parameters (beam current 130 mA, translation speed 15 mm·s−1 , elliptical scanning, double wire feeding, single-pass wire feeding rate 35 mm·s−1 ). The flow field distribution of the droplet deposition process is shown in Fig. 4.13. As shown in Fig. 4.13, before the droplet deposition, the flow field distribution of the molten pool is the same as (e) and (f) in Fig. 4.13. The droplet just completely enters the molten pool at 6.09 s. Apparently, the deposition of droplets changes the flow field distribution after the position of the recessed liquid surface in the molten pool. After the droplet enters the molten pool, it will flow toward the tail of the molten pool, and meanwhile the entire surface and interior part of the molten pool will have a large backward velocity. When the melt flows to the end of the molten pool, the impact with the matrix causes it to turn back and flow toward the front of the molten pool. However, it will encounter the melt flowing to the tail of the molten pool during the backflow process, and backand-forth oscillating flow will occur along the length direction of the molten pool on the surface and from the inside of the molten pool. The speed of oscillation will gradually decrease as the effect of droplet deposition on the flow of the molten pool weakens. At 6.16 s, the flow at the bottom of the molten pool has been restored to the state before the droplet deposition, and the velocity on the surface of the molten pool is still turbulent, but the value has been significantly reduced. As the prototyping process proceeds, the velocity distribution on the surface of the molten pool will gradually restore to the state before the droplet deposition, and then start the next cycle of the above-mentioned flow field change as new droplets deposit.

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.13 Flow field distribution during droplet deposition

4.2 Fundamentals of the EBWD Technology 4.2.1 Data Processing (1)

Model reconstruction

The EBWD is usually used to manufacture precise blanks of large metal parts. Since the finishing is required to ensure the accuracy of the parts after being prototyped, the final numerical model of the part needs to be rebuilt as the technological numerical model for prototyping. The following notes are necessary for the creation of technological numerical models: ➀

Ensure machining allowance. Since the surface of the workpiece after being fabricated by EBWD is uneven, the higher the prototyping speed, the lower the dimensional accuracy and flatness, and deformation is easily caused during the prototyping process. Therefore, it is required to reserve machining allowance before the prototyping according to the prototyping technologies and the shape

4.2 Fundamentals of the EBWD Technology

➁

➂

➃ ➄ ➅

➆

(2)

135

of parts. Generally, an allowance of 1–5 mm is planned for one side of an easy plane structure; Add technological support. Since the EBWD is a sliced additive manufacturing technology, the previous layer need to offer support for the subsequent layers. When a cantilever structure is machined, if there is no multi-axis linkage mechanism to ensure that the beam current is in the normal direction of the machined surface, it is necessary add a support to the cantilever of the part. Fill the micropores. The accuracy of the EBWD technology is generally in the millimeter level. The dimensional accuracy of some micropores cannot be guaranteed, and follow-up finishing is required. Therefore, some micropores are usually filled as entities during the numerical model reconstruction, which not only reduces the complexity of the prototyping path, but also improves the efficiency of prototyping. Depending on the size of parts and the technological parameters, the through holes or blind holes with a diameter of less than 8 mm are generally filled. Add physical and chemical test materials. In order to verify the structures and properties of parts, it is necessary to manufacture physical and chemical materials for testing while manufacturing parts. Add deposits. Some parts are complicated in shape and are difficult to be clamped and inspected during NC machining. Therefore we can produce the deposits for clamping and positioning during prototyping. Adapt to the need for non-destructive testing. The ultrasonic testing method has certain requirements on the shape of workpieces. In order to reduce the blind area of test and improve the test effect, it is generally preferable to have regular structures with minimal thickness changes, steps, and large curvatures. Adapt to the need for the directionality of mechanical properties. The mechanical properties of titanium alloy parts fabricated by EBWD are directional. Therefore it is necessary to plan suitable machining directions for them according to the characteristics of the load, and the numerical model has to be modified accordingly. Slicing of the numerical model

The slicing of the numerical model is the basis of machining path planning. Therefore, each part model must be sliced before the machining path planning can be carried out. The slicing algorithm is an important part of additive manufacturing. The slicing algorithm in additive manufacturing technology can be divided into the direct slicing of CAD models and the slicing based on STL models by the data format. It can also be divided into the equal-thickness slicing and the adaptive slicing by the slicing method. Direct slicing of CAD models has the advantages of small data size, high accuracy, short data processing time, and no errors in the model. But it also has obvious shortcomings, such as the reliance on special CAD software and difficulty in automatically adding support to the model. The slicing based on STL models has the

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shortcomings of error-prone models, large data size, and low accuracy. However, it does not rely on the CAD software, and the accuracy level can be set according to the complexity of the part. Therefore, it is still a mainstream subject of research. The STL model is the result of the discretization of triangles on the CAD model, so it is actually a polyhedron model. The slicing produces a series of polygonal silhouettes. Additive manufacturing is a layered manufacturing technology, so it will cause a step effect on the surface of parts. In order to improve the accuracy and reduce the step effect, it is necessary to reduce the layer thickness (such as Fig. 4.14), but this will greatly reduce the manufacturing efficiency and increase the manufacturing cost of parts. In order to maintain a balance between manufacturing accuracy and cost, the more advanced adaptive slicing method is currently applied in additive manufacturing. The principle is that the software can automatically determine the layer thickness according to curved surface of the three-dimensional model to guarantee the surface accuracy of parts specified by the user and therefore enables fast prototyping speed and high accuracy. However, during the adaptive slicing, the technological parameters must be greatly changed according to the thickness of the layers. It poses high control flexibility for technological parameters, and requires the support of the technology library. The traditional slicing method is equal-thickness slicing, which means the thickness of each layer is identical along the height of the model. As for the EBWD technology, all the technological parameters of different layers remain unchanged except the path, which is conducive to simplifying the technology and improving the stability and reliability of machining. However, the equal-thickness slicing for parts with complex silhouettes cannot guarantee both high accuracy and high efficiency. The EBWD process can neither apply the adaptive slicing nor the equal-thickness slicing. Considering the advantages of the above-mentioned two slicing methods and the characteristics of the electron beam wire machining technology, we proposed a segmented slicing method that is a tradeoff solution between the adaptive slicing and the equal-thickness slicing. Its characteristic is that it will set several segments along the height direction of the 3D model. The height of each segment or the coordinates of the starting layer along the Z direction can be manually set according

Fig. 4.14 Slicing step effect

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to the complexity of the part, and the thickness of layers in each segment can also be set separately. That means a part can be divided into multiple segments of equal thickness to achieve both high machining efficiency and high prototyping accuracy. The layer thickness can be larger in the equal section or approximately equal section to increase the prototyping speed. The layer thickness in the non-equal section can be smaller to reduce the deposition steps and improve the silhouette accuracy. (3)

Path planning

The polygonal section silhouette is obtained after the STL model of the part is sliced. These polygons are formed by a chain of connected vertices in sequence. The process of generating the path is the process of padding the silhouette of the polygonal section. Path planning is the key to ensuring the quality of prototyping. The reasonableness of path planning is directly related to the internal quality, deformation and properties of parts. ➀

Path padding affects the internal quality and stress deformation of parts. Padding method and path spacing are considered in path padding. (a)

Padding method

The basic padding methods include silhouette offset and grid padding. The silhouette offset is usually used for the parts with regular cross-sectional shape and small area, while the grid padding is used for the parts with complex cross-sectional shape and large area. As for the plate-rib structure for example, the grid padding is usually applied for webs and the silhouette offset is applied for ribs, as shown in Fig. 4.15. Different path padding methods have a remarkable impact on the stress and deformation of workpieces. In this section, we mainly discuss the influence of different padding paths on residual stress and residual deformation. The path padding methods are shown in Fig. 4.16.

Fig. 4.15 Selection of the padding methods for the plate-rib structure

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(a) Spiral from inside to outside

(c) Zigzag

(e) Interlace

(b) Spiral from outside to inside

(d) Line by line

(f) Hilbert

Fig. 4.16 Six different padding methods

The different path padding methods shown in Fig. 4.16 are used to form square plates. After three layers of deposits, the cloud charts of residual stress are shown in Tables 4.1 and 4.2. As shown laterally in Figs. 4.15 and 4.16, the in-plane residual stress gradually decreases, and the in-plane stress gradient decreases as the number of deposition layers increases. This is because the previously filled and solidified titanium alloy can

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Table 4.1 Cloud charts of XX residual stress in prototyping with different paths (unit: Pa) Padding method

Layer one

Layer two

Layer three

Zigzag

Line by line

Spiral from outside to inside

Spiral from inside to outside

Interlace

Hilbert

quickly transfer heat as the number of deposition layers increases, thereby effectively reducing the in-plane temperature and temperature gradient. Another noteworthy feature is the interlayer stress gradient, that is, the difference of residual stress between layers. According to Figs. 4.15 and 4.16, we can see that the interlayer stress gradient of the zigzag padding and the line-by-line padding is the largest, followed by the

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Table 4.2 Cloud charts of YY residual stress in prototyping with different paths (unit: Pa) Padding method

Layer one

Layer two

Layer three

Zigzag

Line by line

Spiral from outside to inside

Spiral from inside to outside

Interlace

Hilbert

Hilbert padding, and then the spiral padding. The smallest interlayer stress gradient occurs in the interlaced padding. For the convenience of comparison, Fig. 4.17 shows the residual stress comparison of 6 different padding methods, including the maximum in-plane stress σxx , σYY , and the maximum equivalent stress Mises. Padding methods are sorted by the magnitude of stress as follows:

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Fig. 4.17 Comparison of residual stress of different padding methods

σZigzag ≈σLine by line > σHilbert > σSpiral from inside to outside > σSpiral from outside to inside > σInterlaced The maximum warpage displacement of different padding methods is shown in Fig. 4.18. Padding methods are sorted by the maximum warpage displacement as follows:

Fig. 4.18 Comparison of maximum warpage of different padding methods

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Fig. 4.19 Schematic diagram of selecting a reasonable deposition spacing

UZigzag > ULine by line > UHilbert > USpiral from outside to inside > USpiral from inside to outside > UInterlaced As shown in Fig. 4.18 the warpage displacement is the smallest when the interlaced padding is used, which is consistent with the previous result of residual stress analysis. (b)

Path spacing

The setting of the path spacing is related to the technological parameters and the characteristics of the section. It is necessary to consider the morphology of the singlepass deposit under the current technological parameters and the special treatment of the edge of irregular sections. The spacing between single-pass deposits (related to the lap ratio) directly affects the prototyping stability and also affects internal defects. The prototyping stability factors are mainly considered in the study of technological parameters. Generally, the empirical formula “spacing = half width of deposition path + half width of wire” is used, as shown in Fig. 4.19. It is usually not easy to ensure the spacing between the padding line and the silhouette line is aligned with the set value at the edge of the section. In this case, special treatment is required. The principle is to ensure that the path spacing is not greater than the set value, while trying to limit the spacing within 80% of the set value.

4.2.2 Prototyping Process Control The key technical problem of EBWD is about how to realize the high-speed and stable fusion of wires without scorching, sticking, wire-workpiece interference, unevenness, and cumulative effects of head and tail dimensions. Any problem unsolved will interrupt the prototyping process or lead to failure. There are usually two ways to

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control the parameters. The first one is based on displacement (or coordinates), and the other is based on time. (1)

Parameter control scheme based on displacement

The prototyping technological parameters that need to be controlled mainly include the acceleration voltage U, the focus current If , the beam current I b , the workbench walking speed v, the arc interpolation speed vxy , the wire feeding rate vf , the beam rising displacement a, the beam falling displacement b, the wire feeding advance displacement c, the wire feeding lag displacement d, the reverse wire drawing amount e, the wire feeding angle θ, the wire end extension distance ls , the scanning waveform, the scanning frequency, etc. The actual technological process logic is shown in Fig. 4.20. On path AB, the technological process is described as follows: ➀ ➁ ➂

The beam starts at the place where the distance from the starting point A is − a; The beam increases from 0 to the set value Ib during the movement of the workbench from −a to A; Wire feeding starts at a distance of -c from the starting point A, and the speed is the set value vf The beam starts to decay from the end B of the path; the beam decays to 0 in the process from B to −b;

Fig. 4.20 Logistic relationship and control parameters of technological process

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Table 4.3 Some variables and their relationships based on time

➃

(T)

(Z)

(F)

t1

Z1

F1

(W)Ib

t2

Z2

F2

(F)Vw

t3

Z3

F3

t4

Z4

F4 =

t5

Z 5 = 15 − Z 1 − Z2 − Z3 − Z4

(S)Vs1 z 4 vs2 (t1 −t2 )vs1

(S)Vs2 h (layer thickness is provided)

Wire feeding stops immediately and the reverse wire drawing starts after passing through B by a distance of d, the reserve drawing amount is e.

Comprehensively control the x, y, z axes of the workbench, the beam current Ib , the focus current If , and six degrees of freedom for the wire feeding axis to ensure the synchronized action of each degree of freedom. (2)

Parameter control scheme based on time

The prototyping technological parameters that need to be controlled mainly include the acceleration voltage U, the focus current I f , the beam current I b , the workbench walking speed v, the art interpolation speed vxy , the wire feeding rate vf , the beam rising time t 1 , the starting time of wire feeding t 2 , the leading motion time of wire feeding t 3 , the advance stop time of motion t 4 , the beam decay time t 5 , the motion displacement Z 1 ~ Z 4 and speed F 1 ~ F 4 of all stages along Z axis, the forward and reverse wire feeding rate S 1 and S 2 , the wire feeding angle θ, the wire end extension distance ls , the scanning waveform, the scanning frequency, etc. See Table 4.3 for some variables and their relationships based on time. The actual technological process logic is shown in Fig. 4.21. Take the machining of a path as an example, the process is described as follows on the time axis: ➀ ➁ ➂ ➃ ➄ ➅ ➆ ➇

The beam current starts and rises at time 0, and reaches the set value Ib after time t 1 ; Wire feeding starts at t 2 after time 0, when the beam current is still rising; The workbench starts to move after time t 3 , when the beam current has stayed in the maximum value for a period; After the motion stops, the beam current starts to decay after time t 4 , and the Z axis is moving accordingly; The wire feeding continues for some time and then the reverse wire drawing starts, and wires leave the molten pool; Wires leave the molten pool after being reversely drawn for a short period of time; The beam current decays to 0 after time t 5 , and the path machining is completed. The Z axis descends rapidly and the workbench moves to the starting point of the next path, and ascends to the machining surface.

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Fig. 4.21 Logical relationship of the process based on time

4.3 Typical Defects and Their Control 4.3.1 Types of Defects The EBWD-fabricated workpieces have typical defects such as pores, incomplete fusion and microcracks, etc. The common defects in titanium alloys include pores and incomplete fusion. In addition to above, microcracks are also found on ultrahigh-strength steels. (1) ➀

Pores Macroscopic morphology of pores

Figure 4.22 shows the optical photograph of the macrostructure of the cross section of an EBWD-fabricated titanium alloy specimen. As shown in the picture, we can see that pores are easily generated during the EBWD process. They are usually fusion

Fig. 4.22 Macrostructure of the cross-section of single-pass deposit

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line pores according to the prototyping position, and most pores are in the spherical shape [2]. ➁

Microstructure of pores

Figure 4.23 shows the SEM picture of the macroscopic pores and their internal morphology.

Fig. 4.23 Structure of the pores of single-pass deposit

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The pores are mainly divided into the following three types according to their morphological characteristics obtained from experiments: type I pores with smooth inner wall; type II pores with spherical structures on the inner wall; type III pores with irregular structures on the inner wall. After analyzing the experimental results, we know that titanium alloys are facing severe pore problems during the electron beam wire deposition additive manufacturing. Most pores have smooth inner wall, while a small part of pores have irregular shapes. However, all the three types of pores have a continuous spherical contour, and are caused by gas. Since there is no specific answer about how much the formation of pores is affected by hydrogen, oxygen, nitrogen, carbon, etc. during the prototyping process, the following study and analysis is mainly based on hydrogen. Many studies have raised a conclusion that the prototyping technology will absorb hydrogen, oxygen, nitrogen and other gases from the protective gas. However, the EBWD fabrication of titanium alloys is performed in the vacuum environment and thus involving no protective gas. However, gas still exists during the process of electron beam wire deposition additive manufacturing. Therefore, pores mainly come from the hydrogen inside the materials, the remaining contaminants or the oxide film on the surface of the matrix and the moisture it absorbs. Some other pores come from the gas invading the molten pool during the prototyping process. The hydrocarbons contained in the material and the moisture adsorbed by the oxide film will decompose at high temperature or react with the components in liquid alloys to produce carbon, hydrogen, oxygen, nitrogen and other gases. Meanwhile, the titanium alloy matrix itself also contains a certain amount of carbon, hydrogen, oxygen, and nitrogen. During the wire deposition process, when the electron beam bombards the matrix, the matrix melts to form a high-temperature molten pool, as shown in Fig. 4.24. Carbon, hydrogen, oxygen, and nitrogen disperse into the molten pool. Due to the different solubility of gas in the liquid and solid alloy liquids, a large amount of gas separates out as the high-temperature liquid molten pool cools down and solidifies, therefore forming local supersaturation in the molten pool. When the partial pressure of the gas is greater than the bubble formation pressure, the bubbles will adhere to the columnar crystals, inclusions and the cracks, holes or grooves of the specimen

Fig. 4.24 Morphology of joints and the molten pool

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Fig. 4.25 Solubility curve of hydrogen in titanium changed by temperature

to nucleate. The bubbles develop in the subsequent process, and will remain in the deposit in the form of pores if they cannot float up and escape. (a)

Type I pores with smooth inner wall

As for titanium and titanium alloys, hydrogen is one of the main factors affecting the number of pores, and the metallurgical pores of titanium are mainly caused by hydrogen. During the electron beam additive manufacturing of titanium alloys, there are two sources of hydrogen in the molten pool. One is the original hydrogen content in the base metal (intrinsic hydrogen), and the other is the hydrogen introduced from the external environment during the prototyping process (exotic hydrogen). From the perspective of hydrogen solubility, that is, the solubility curve of hydrogen in high-temperature titanium changed by temperature, hydrogen diffuses from the molten pool and the base metal to the fusion line, and the hydrogen in the molten pool escapes the atmosphere. This depends on their solubility in the titanium and the temperature field during prototyping. Figure 4.25 shows the solubility curve of hydrogen in titanium. According to the figure, from the formation to the solidification of the molten pool, the solubility of hydrogen in the liquid-phase and solid-phase titanium will decrease with the increase of temperature, This not lead to the precipitation of hydrogen during the solidification process, and thus forming pores. However, in the solidification temperature range, when the liquid-phase titanium solidifies, the solubility of hydrogen encounters a sudden drop, and the solubility of hydrogen is reduced by 1.3 times. This indicates that the solidification process of titanium alloy is a process of releasing hydrogen. Therefore, bubbles are easily formed during the solidification process. However, we can also learn that the solubility of hydrogen in the low temperature range (approximately 1200 °C) is much greater in the solid phase than that in the liquid phase. Therefore, the content of hydrogen exceeds its solubility at a certain point during the solidification of the liquid metal, and the hydrogen will gradually diffuse and separate out from joint center to the fusion line and its vicinity, forming bubbles. When bubbles are formed in the molten pool, hydrogen will diffuse from the hydrogen-rich area to the bubbles and cause the bubbles to grow. Figure 4.26 shows the upper cross-sectional view of the molten pool. When the matrix is melted,

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Fig. 4.26 Macrostructure of the upper section of the molten pool

hydrogen will diffuse from the center A of the joint of specimen to the fusion line C and its vicinity. Therefore, hydrogen will become supersaturated and separate out to form bubbles near the final fusion line. Due to the fast prototyping process and short joint solidification time, most pores formed by the gathering of bubbles are captured by the solidification front and solidified into internal pores before they can float to the joint surface. At the edge of the molten pool (i.e. inside the freshly melted metal) including the fusion lines on both sides of the joint and the bottom of the molten pool that is not penetrated, the solubility of hydrogen is quite high, and the liquid metal at the crystallization front is supersaturated by hydrogen. Therefore, the fusion line is supersaturated by hydrogen, creating a key condition for generating pores. In terms of the existence time of the molten pool, it can be seen from Fig. 4.26 that the existence time of the liquid metal at positions A, B and C is: TA > TB > TC . That means the existence time of the liquid metal near the fusion line is very short, which is very disadvantageous for the escape of bubbles. If given sufficient time, the fusion line bubbles can also grow up and escape, so that the number of pores can be slightly reduced. Therefore, increasing the temperature and prolonging the solidification time of the molten pool, especially the temperature of the fusion zone, can theoretically effectively reduce or eliminate pores. However, under normal prototyping conditions, due to the limited existence time of the molten pool, it is unlikely that the pores of the fusion line will decrease as the molten pool exists longer. Many factors cause the pores to precipitate at the fusion line. From another perspective of bubble nucleation, the fusion line is located at the solidification boundary in the molten pool, where the energy required for bubble nucleation is much lower than the energy required for the bubbles in the center of the molten pool, so bubbles are easily nucleated at the fusion line. The pores with smooth inner wall are formed in the mushy area at the solidification front of the specimen during the solidification process. The schematic diagram of the pore formation process is shown in Fig. 4.27. Figure 4.27a shows the depression area fabricated by the electron beam and the distribution of the molten pool. Figure 4.27b shows the process of bubble formation, and Fig. 4.27c shows the process of interdendritic pore formation. According to the schematic diagram of the pore formation process shown in the figure, it is speculated

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Fig. 4.27 Schematic diagram of hydrogen pore formation process

that the pore formation process of titanium alloys in EBWD additive manufacturing is as follows: the hydrogen in the liquid metal enters the molten pool to form gas during the melting of the base metal. Meanwhile, the hydrogen stored in the partial defects of the base metal will diffuse into the solidification front through the grain boundary, form hydrogen gas, and enter the molten pool. When the molten pool starts to solidify, the hydrogen is precipitated due to a sudden change in the solubility. The gas dissolved in the liquid metal is continuously repelled to the front of the liquid/solid boundary. The precipitated hydrogen gathers at the solidification front. As the solidification proceeds, the solidification front will definitely become a hydrogen-rich region, which will increase the concentration of gas, and also the partial pressure in equilibrium with the gas, until it reaches a supersaturated state and is eventually separated out the form of bubbles. The formation of bubbles has to undergo nucleation and growth. Research results show that bubbles tend to nucleate at the roots of dendrites. During the solidification of the metal in the molten pool, interdendritic bubbles will also grow by absorbing the supersaturated gas in the surrounding liquid phases through diffusion as the dendrites grow, so as to reduce the free energy of the boundary and slowly separate outward under the action of buoyancy and external convection.

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When multiple dendrites grow together and coincide, they will form dispersive microscopic pores between the dendrites if the bubbles do not have time to float out of the molten pool surface. Due to the fast speed of EBWD, the pores formed by the gas precipitation mechanism are generally in a regular spherical or ellipsoidal shape. Comparing the graphs shown in Fig. 4.27, we can find that the schematic diagram of the pore formation process in the figure can be used to explain the formation process hydrogen-induced pores. (b)

type II pores with spherical structures on the inner wall

Figure 4.28 shows a partial enlargement of the type II pores with spherical structures on the inner wall. Figure 4.28a shows a partially enlarged image of the pore A area shown in Fig. 4.23c, while Fig. 4.28b, c show the 11,000-times and 20,000-times enlarged images of the corresponding positions B and C in Fig. 4.28a. As shown in Fig. 4.28a, there are granular spherical structures attached to the inner wall of type II pores, and the spherical structures vary in size and are unevenly distributed. Figure 4.28b shows the surface microstructure of the spherical structure. It can be seen that the surface of the spherical structure is regular and slightly smooth. Figure 4.28c shows the microstructure around the spherical structure. The high-magnification

Fig. 4.28 Spherical structure inside the pore

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image shows that this area shows an irregular structure and many small granular substances are distributed on the surface. We further analyzed the type II pores with spherical structures on the inner wall shown in Fig. 4.23, and performed energy spectrum scanning analysis on the matrix area around the pores, the inner wall of the pores, and the spherical structure surface in the pores. The location of the energy spectrum analysis and the analysis results are shown in Fig. 4.29. Figure 4.29a–f show the energy spectrum analysis results of the pores in Fig. 4.23c, d.

Fig. 4.29 Energy spectrum analysis on the spherical structure of pores

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Figure 4.29a shows the result of energy spectrum analysis on the matrix around the pores. Figure 4.29b shows the weight percentage of the base elements around the pores. The chemical compositions of the matrix are consistent with that of TC4 titanium alloy wires, and the result of the energy spectrum analysis on the base metal area is also consistent with the chemical compositions of the material. It shows that the base metal is TC4 alloy, and the alloy composition is Ti–6Al–4V, in which the atomic weight percentage of Al is about 6%, the atomic weight percentage of V is about 4%, and the compositions of the matrix and the base metal around the pores are roughly the same. This is because the matrix used is a TC4 titanium alloy plate, and the wire used in the deposition process has the same compositions as the matrix, so the compositions of the prototyped joint are not much different from that of the matrix. Figure 4.29c, d show the energy spectrum analysis results of the element content distribution on the spherical structure. The results show that the Al content is greatly reduced relative to the component content of the matrix. The atomic weight percentage is reduced to 3%, and the atomic content is reduced by half. Figure 4.29e, d show the results of element distribution on the surface of the pore wall of type II pores with spherical structures on the inner wall. According to the results of energy spectrum analysis, it can be seen that the average content of Al on the inner wall of the spherical structures of pores is also greatly reduced to about 3.8%, which is about 36% less than the 6% Al content in the matrix. The analysis result tells us that the element content at the joints around the pores with spherical structures is basically unchanged, and stays consistent with the compositions of the base metal. Inside the pores, the content of Al element is greatly reduced, whether on the pore wall or on the surface of spherical structures. According to the conservation of element mass, it can be inferred that the Al element exists in the spherical structure of the pores. In the atmospheric environment, the basic physical properties of the three elements Ti, Al and V are shown in Table 4.4. It can be seen that the melting point and boiling point of Al, an easy-to-evaporate element, are much lower than those of Ti and V. Al has a boiling point of 2519 °C, while Ti has a boiling point of 3287 °C, and V has a boiling point of 3407 °C. The boiling point differences between Al and the latter two elements are more than 700°C. In a vacuum environment, the boiling point of the material is greatly lower than that in the atmosphere. For the TC4 titanium alloy, when the Ti element starts to melt, the Al element has already begun to evaporate and become a metallic vapor state. Table 4.4 Physical properties of titanium, aluminum and vanadium

Element

Density ρ/(g·cm−3 )

Melting point Tm /°C

Boiling point Tb /°C

Titanium/Ti

4.5

1678

3287

Al

2.7

660.3

2519

Vanadium/V

6.11

1910

3407

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4 Fundamentals of Electron Beam Wire Deposition Technology

Therefore, Al, Ti, and V will evaporate in order in the TC4 titanium alloy and then condense in the reverse order. Al element is most likely to evaporate and escape from the high temperature molten pool, while V is the least likely to evaporate and escape. Therefore, the mass fraction of Al in the metallic vapor will increase while the mass fraction of V will decrease. This phenomenon explains why the content of Al element is reduced to 3.00% on the inner wall of the pores, and to 3.79% on the spherical structure. In type II pores, the V content does not increase uniformly; the V content on the inner wall of the pores increases to 12.06%, while the V content in the spherical structure decreases to 2.76%. This indicates that V is the first to deposit on the inner wall of the pores before the formation of the spherical structure. At the same time, the V element that first condenses on the inner wall of the pores provides a large number of heterogeneous nucleation cores. The EBWD additive manufacturing technology features a fast cooling rate and a large degree of undercooling, which provides a strong driving force for the core to grow at the solidification front. The action from both sides leads to the production of type II pores. Gas bubbles and metallic vapor in the high temperature molten pool all satisfy the following pressure balance formula: PG = PL + PE N V + 2σ/r

(4.1)

PG , PL and PENV pores represent bubbles, metallic vapor pressure, liquid metal pressure and environmental pressure respectively. σ represents the surface tension of the interface, and r represents the radius of the bubble. Since the EBWD additive manufacturing is carried out in a vacuum environment, the environmental pressure is zero. As the rapid prototyping proceeds, the molten pool begins to solidify when the electron beam moves forward, so the pressure of the liquid metal gradually tends to zero. As the temperature drops, the molecular forces between the liquid metal and the metallic vapor tend to consistent, so the surface tension between the liquid metal and the metallic vapor tends to zero, while the surface tension between the bubble and the liquid metal is not zero. According to the equation, this means that the pressure may suddenly drop to zero during the condensation of metallic vapor. During the solidification of bubbles, such changes will not occur due to the presence of surface tension. This indicates that type II pores cannot be formed from pure metallic vapor that does not contain gas. Also, the size of a type II pore is generally larger than that of a type I pore of metallurgical gas, which can also explain that type II pores are formed from the mixture of gas bubbles and metallic vapor. Based on the aforesaid analysis, we propose the formation mechanism of the new type II pores in the EBWD process. (1)

Under the bombardment of the electron beam, the matrix or the deposit forms a high-temperature molten pool, in which the metal begins to evaporate to form metallic vapor. The volatile elements Al and Ti in the metallic vapor may evaporate and then escape the molten pool. The molten pool forms gas bubbles due to the precipitation of dissolved gas. As shown in Fig. 4.30a, metallic vapor

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Fig. 4.30 Schematic diagram of the formation of type II pores with spherical structures a metallic vapor and bubbles are produced; b Bubbles and metallic vapor in the molten pool approach each other; c metallic vapor and bubbles merge; d V element is deposited on the surface of the pores; e heterogeneous nucleation generates microspheres; f grow up to form pores during rapid cooling

(2)

(3)

(4)

(c)

and gas bubbles move under the action of liquid and buoyancy in the molten pool. The moving metallic vapor encounters gas bubbles. They collide with each other to form larger mixed bubbles as shown in Fig. 4.30c. The mixed bubbles contain Al, Ti, V, and various gas atoms, which are represented in red, green, blue, and purple for easy observation. As the prototyping proceeds, the electron beam moves forward and the molten pool begins to cool and solidify. Inside the mixed bubbles, the V element with the highest boiling point is the first to condense and deposit inside the bubbles. As shown in Fig. 4.30d, this also explains the increase in the content of V on the inner wall of the pores according to the energy spectrum analysis. The V element that is the first to deposit also provides a large number of heterogeneous nucleation cores. Meanwhile the large solidification speed in the molten pool provides a strong driving force for the core to grow at the solidification front, as shown in Fig. 4.30e. Subsequently, all the cores grow into spherical structures of various sizes, thus forming type II pores. Type III pores with irregular structures on the inner wall

Figure 4.31 shows the partial enlarged image of the microstructure of the irregular structures inside the type III pores in Fig. 4.23f. It can be seen from Fig. 4.23f that

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(a) 3500-times magnification at position A

(c) 4000-times magnification at position B

(b) 12000-times magnification at position A

(d) 25000-times magnification at position C

Fig. 4.31 Irregular structures inside the pore

this type of pore has a roughly spherical outer silhouette and discontinuous edges. The inside, as shown in Fig. 4.31, has tear-like irregular organization. According to the spherical silhouette of the pores, it is inferred that the bubbles formed at the initial stage are still gas pores, and then hydrogen bubbles are formed by the enrichment and precipitation of hydrogen. As the solidification proceeds, hydrogen bubbles enter the dendrite arms at the solidification front, as shown in Fig. 4.32. (1) ➀

Not fused Analysis of causes of non-fusion defects

Due to the layered deposition 3D prototyping of EBWD, the dense metallurgical bond may not be formed between different deposition layers or adjacent deposition layers during the prototyping process for the reason of many factors such as improper control of technological parameters or non-complying operations, which will lead to poor fusion or non-fusion defect. During the prototyping process of a simple single-pass metal part, when the deposit of the upper layer is gradually cooled, the electron beam will act to keep

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Fig. 4.32 Schematic diagram of irregular pore formation process

heating the deposit of the upper layer when the lower layer is prototyped. Meanwhile, wires are heated and melted to form droplets. The deposit of the previous layer melts again to form a molten pool, and the droplets gradually cool and solidify after entering the molten pool to form a metallurgical bond. However, since the liquid droplets flow around and form depressions, making this layer thinner than the average layer thickness, as shown in Fig. 4.33; or the dripping droplets form protrusions under the action of surface tension, as shown in Fig. 4.34, and the prototyping process continues. When a certain layer of deposit is formed, these depressions or

Fig. 4.33 Depressions formed between the deposits of the upper and the lower layers

Fig. 4.34 Protrusions formed between the deposits of the upper and the lower layers

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4 Fundamentals of Electron Beam Wire Deposition Technology

protrusions will cause the temperature nearby to fail to reach the level that is enough to form a metallurgical bond. The next layer of deposit is only covered on the upper layer, and then a partial bond, or only a discontinuous partial bond, will be formed at these positions. Most areas are not fused, thus resulting in defects. Also, as for the parts of greater complexity, the liquid metal melts to form a molten pool during the prototyping process, and then the metal is deposited on the upper layer of deposit. As for different prototyping paths, the shrinking of liquid metal during the solidification process will cause gaps between two adjacent deposition layers after shrinking. If the temperature of the liquid metal in between is low, metallurgical bonds cannot be formed, and non-fusion defects are likely to appear. As shown in Fig. 4.35, the dashed line represents the scanning path, that is, the line where the electron beam or the center of the molten droplet moves. The solid line represents the silhouette of the deposit fabricated by cooling and solidification. The figure shows two horizontal deposition layers and one deposition layer not parallel to this horizontal direction. If the prototyping proceeds along this path, the liquid metal will form a gap after cooling at the junction of the three path. When the next layer of deposits continues to form, the liquid metal here will quickly cool down, leading to a low temperature. A non-fusion defect will appear if the droplet cannot drip into the gap or the temperature of the dripping droplet does not meet the condition to form a bond with the next deposition layer. As for different prototyping paths, the liquid metal may also flow to the adjacent deposition layers during the process of solidification and shrinkage to form a deposit, resulting in excess metal in this part and thus overlapping parts. Therefore, in the subsequent deposition and prototyping process, protrusions will be generated where the liquid metal flows and overlaps. The liquid metal near the protrusions fails to reach the expected temperature, leading to excessively low local temperature that will cause non-fusion. As shown in Fig. 4.36, the dashed line represents the scanning path, that is, the line where the electron beam or the center of the molten droplet moves. The solid line represents the silhouette of the deposit fabricated by cooling Fig. 4.35 Gap between adjacent deposits

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Fig. 4.36 Overlap between adjacent deposition layers

and solidification. The figure still shows two horizontal deposition layers and one deposition layer not parallel to this horizontal direction. Similar to the aforesaid case, if the prototyping proceeds along this path, deposition layers will overlap after being cooled to form protrusions at the junction of the three paths. When the next layer of deposits continues to form, the protrusions will cause the liquid metal to flow around and cool down rapidly, therefore creating non-fusion due to excessively low temperature. (2)

Microcracks

When depositing materials with high strength and poor ductility, it is easy to produce microcracks during the deposition process. Figure 4.37 is a metallographic figure of the typical micro-cracks A-100 ultra-high-strength steel fabricated by EBWD. According to the figure, we find out that the crack is small in size and distributed along the dendrite grain boundaries. The liquid metal in the molten pool goes through the liquid phase, liquid–solid coexistence, and solid phase during the cooling process. When the melt temperature drops to the liquidus, a solid phase (ie, solid solution dendrite) separates out of the main liquid phase, and the melt enters the liquid–solid state. A large amount of liquid phase partitions and surrounds the solid phase and can flow between different solid phases. Therefore, the weld metal will exhibit a certain degree of ductility and can be deformed, while the solid phase can only be displaced accordingly without being deformed. When the temperature is further reduced till the solid phase dendrites begin to interweave and grow together to form a dendritic backbone, the melt enters the brittle temperature range and the solid–liquid phase. The dentritic backbone begins to deform at the beginning of the solid–liquid phase. When the dentritic backbone deforms and cracks, for example, the liquid phase may still flow into and fill the cracks of the dentritic backbone through its flowing due to the volume and quantity. This is the phenomenon of crack healing. But in the late solid–liquid phase, the solid-phase dentritic backbone has grown into a rigid network, the strain is highly developed, the number of residual liquid phase is sharply reduced, the fluidity is weakened and is squeezed between the dendrites and stays in place. Therefore a liquid film is formed

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.37 Metallographic figure of crack defect of the specimen of EBWD-fabricated A-100 steel

which has low strength and poor ductility, and is prone to fracture, and crystalline cracks are formed under the action of shrinkage stress. The layer-by-layer deposition process of EBWD is very similar to the process of multi-layer and multi-pass welding of filler wires. The deposited metal undergoes thermal cycles of heating, cooling, re-heating, re-cooling. When the cooling rate is too high, the growth of the dentritic backbone also becomes very fast, and the liquid film is easier to form. The faster the melt is cooled down, the greater the shrinkage stress will be, and the more likely crystalline cracks will be formed.

4.3.2 Defect Control Methods (1)

Methods for controlling pore defects

The main methods to prevent pore defects include the followings: strictly clean the raw materials; select the appropriate heat input and parameter matching; increase the stability of the wire feeding; if the metal surface is uneven and there are many nodules

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or deep depressions, remelting should be performed to eliminate hidden dangers; hot isostatic pressing can effectively eliminate pore defects inside the parts. (2)

Methods for controlling non-fusion defects

Non-fusion is a kind of area defect. It reduces the effective thickness of the structure, and the unfused edge is prone to stress concentration, which then expands into cracks and affects the quality of the entire specimen. Therefore, certain measures must be taken to avoid the occurrence of non-fusion defects. The main factors affecting non-fusion are: ➀ ➁ ➂ ➃ ➄

Electron beam power, namely the acceleration voltage, beam current, and focus current; Prototyping speed. The stability of electron beams and wires; Prototyping parameters, wire diameter and wire feeding rate; Prototyping path.

Therefore, here we raise corresponding measures to avoid non-fusion defects corresponding to the influencing factors of non-fusion discussed above. It aims to help the future prototyping process design and the theoretical research of non-fusion, so as to avoid non-fusion and optimize prototyping technologies. The specific measures are: (1)

Properly choosing the size of the electron beam current

The size of the electron beam current should be appropriate, so that it can prevent the formation of non-fusion at the edge of the molten pool due to the excessively small electron beam current, and prevent depressions or protrusions during the prototyping process when the electron beam current is too large. Therefore, properly selecting the size of electron beam current can effectively avoid the formation of non-fusion defects. (2)

Choosing the right focus current

The focus current is closely related to the beam spot diameter, and the beam spot diameter directly affects the area on the deposit or the matrix affected by the electron beam, thus causing different distribution of energy density. Therefore, we should select the appropriate focus current by considering the factors such as material properties, equipment conditions, and prototyping requirements. (3)

Appropriate selecting the prototyping speed

The prototyping speed is affected by many factors, such as the moving speed of the electron beam, the movement of the matrix, etc. During the EBWD process, we should select the appropriate prototyping speed and adjust the motion of the electron beam, the wire, the matrix, or the deposit, so that the entire prototyping process proceeds smoothly. Also, we should ensure that the temperature distribution of the molten pool is balanced during the deposition process to avoid the occurrence of non-fusion defects.

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4 Fundamentals of Electron Beam Wire Deposition Technology

Controlling the stability of electron beams and wires

The small diameter of the electron beam spot may lead to a small irradiation range. Since wires are controlled by the wire feeding system during the prototyping process, it may lead to wire swing. Therefore, it is necessary to improve the sensitivity of the electron beam translation system and the accuracy of the wire feeding device, minimize the swing amplitude of the wire, and ensure that the wire can be fed stably and aligned accurately with the center of the beam spot. We can apply a highlyconfigured prototyping device and flexibly adjust the wire height according to the actual situation, so as to meet the need of customers. (5)

Choosing the right wire feeding rate

During the EBWD process, the wire feeding rate directly affects how many droplets drip down into the molten pool and how much electron beam energy is absorbed by the molten pool. When the wire feeding rate is low and the number of droplets is small, the electron beam will act upon the molten pool for a long time and the molten pool will absorb excessive energy, causing flow inside the molten pool. If the wire feeding rate is too high, the wires will not be completely melted before they enter the molten pool, resulting in low heat in some areas of the molten pool. Both situations will result in non-fusion defects. In the case of layered deposition during prototyping, every layer is deposited on the previous layer that plays a role of positioner and supporter for the current layer. With the deposition of the previous layer, the height of the deposit will increase continuously, and the area and shape of each deposition layer will change. When the shape changes greatly, the deposit will be tilted or bent, and the shape of boundary will change accordingly, affecting the prototyping accuracy of parts. Therefore, we have to fully consider the thickness of a single layer of deposits, and choose the appropriate wire diameter and wire feeding rate. We can only guarantee the required prototyping of the entire deposit if every layer is prototyped accurately, and this is the only way to smoothen the bottom-to-top “growing” of parts. Therefore, in the actual prototyping process, we should select the appropriate wire feeding rate and wire diameter according to the prototyping requirements. (6)

Selection of prototyping path: Round-trip linear scanning, path adjustment, silhouette scanning and padding, scanning partition refinement;

Reduce the acceleration and deceleration process, which means reducing the jump of wires; a better temperature distribution should be in place during prototyping, which is beneficial to enhance the effective fusion of layers so as to avoid the formation of non-fusion defects; optimize the operating state of the scanning mechanism to reduce the noise and vibration and shorten the prototyping time. A sound scanning path should ensure shorter prototyping time and higher prototyping quality, and should effectively avoid the formation of non-fusion defects. (7)

Material quality: During the EBWD, the quality of wires and the matrix is closely related to the formation of non-fusion defects in the prototyping

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163

process. Therefore, appropriate prototyping materials and wires should be selected according to actual requirements. (3)

Methods for controlling microcracks

(1)

Control of electron beam parameters

The smaller the prototyping coefficient ψ (ψ = B/H, B is the deposition width and H is the height) of the single-pass deposit, the narrower and deeper the molten pool will be, which is unfavorable to the crystallization direction and tends to produce cracks in the center of the deposit. From the theory of metal crystallization, it is known that during the crystallization process, most impurities are concentrated around the grain boundaries, causing the eutectic “liquid films” with lower melting points to form at these positions, which will crack along the grain boundaries when the metal cools and shrinks. Therefore, it is necessary to adjust the focus current and the scanning waveform by controlling the electron beam parameters reasonably, to obtain a molten pool as shallow and wide as possible. The single-pass deposit should be as flat as possible. Also, the electron beam scanning pattern with complex shapes and reasonable scanning parameters can intensify the stirring of the molten pool, which helps to refine the crystal grains of the deposit, and effectively improves the crack resistance and mechanical properties of the specimen. (2)

Control of translation speed

A low translation speed can prolong the staying time of the electron beam in the fusion zone. On the one hand, it can improve the prototyping coefficient. On the other hand, as the translation speed decreases, the rise of heat input will remarkably increase the pool width if other conditions remain unchanged, and the tendency of verticality between the growth directions of the grain’s main axis and the central interface of the deposit will be slowed down, making it difficult to form a fragile joint surface, and enhancing the crack resistance. (3)

Control prototyping process

Appropriate preheating is suggested before prototyping. We can preheat the deposited part during the prototyping process by means of the scanning with electron beams of large beam spots, so that the parts stay in high temperature to reduce the possibility of cracks.

4.4 Deformation Control 4.4.1 Principle of Deformation During the EBWD process, the electron beam will preheat the wire and the workpiece. As a result, the surface of the workpiece will be heated to melt to form a molten pool,

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4 Fundamentals of Electron Beam Wire Deposition Technology

(a) Cracking of matrix

( b) Deformation of parts

Fig. 4.38 Cracking and deformation of parts fabricated by EBWD

and the wire will be heated to melt and form droplets that will deposit into the molten pool under the action of gravity. As the electron beam heat source moves, the metal at the end of the molten pool gradually solidifies and gradually deposits into the shape of a workpiece. In this physical process, apart from the fusion zone, a workpiece undergoes repeated circles of rapid heating and rapid cooling. The uneven thermal expansion occurs on the titanium alloy workpiece, and the thermal stress generated by this thermal expansion is sufficient to make the workpiece reach the yield point, thus leading to permanent ductile strain and residual stress after the deposition is completed, which will affect the properties of the workpiece. Deformation and cracking are the technical challenges of EBWD to fabricate parts. This problem will become more noticeable as the structure gets bigger. The technical contradiction lies in that, when a large part is machined, if a rigid fixture is used to fix the matrix and reduce the deformation, the matrix of the part may crack; if the matrix is left in a free state, the part may be greatly deformed, leading to insufficient local dimensions of the part and making the prototyping hard to proceed. In addition, cracks are more likely to occur in a stress concentration area, which is the place where the size of a part changes suddenly. The root cause of stress deformation is the large temperature difference inside the part during the prototyping process and the large temperature gradient. The uneven shrinkage during the cooling process forms internal stress. After repeated thermal cycles, the stress accumulates to a certain extent and causes deformation or cracking. The deformation and cracking of large structures are shown in Fig. 4.38.

4.4.2 Methods for Controlling Deformation Table 4.5 shows the common measures to control the stress deformation of large structures, along with their advantages and limitations.

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165

Table 4.5 Common measures of deformation control No. Measure

Advantage

Limitation

1.

Rigid fixture

Easy and convenient

It may cause cracks in the stress concentration area of the part

2.

Multiple times of stress relieving and annealing

Stress relieving has better effect

Interrupt the prototyping process, increase technological complexity, increase cost and cycle time

3.

Partitioned prototyping and spliced into a whole

Effective, especially suitable Increase technological for industrial applications complexity, weaken the integrity of parts, increase cost and cycle time

4.

Simulation, optimization of Convenient, low cost, will prototyping process, not interrupt the prototyping including parameters, process machining sequence, etc

5.

Partitioned prototyping, change the direction and size of the padding path

Convenient, low cost, will Technologically challenging not interrupt the prototyping and low prototyping process efficiency Although this can be improved, the problem of stress accumulation cannot be solved fundamentally

6.

Auxiliary rolling, impacting, etc

Effective

The machine structure is special and can only apply to a limited range of materials

7.

Heat treatment sizing

–

Only effective for some low-rigidity structures, increasing cost and cycle time

Very technologically challenging. Although this can be improved, the problem of stress accumulation cannot be solved fundamentally

The methods mentioned above can reduce the stress to a certain extent, but they also have great limitations, and none of them alone can solve the problems of stress and deformation fundamentally. In real life scenario, we often use more than one methods. Stress deformation control is still a major technical challenge in rapid prototyping, but there have been some successful cases in industrial applications. Beihang University manufactured a large titanium alloy frame with a projection area of 5 m2 by means of partitioned prototyping and rapid splicing. Sciaky from America used the simulation technology to predict stress and deformation, and then achieve active adjustment of the prototyping technology. They performed many times of stress relieving and annealing and harvested positive results. Cranfield University used the rolling method to process the aluminum alloy siding fabricated by arc deposition, and achieved positive results.

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Compared with other prototyping technologies, the EBWD technologies has both common points and differentiated features. It has its unique advantages in the control of stress deformation. Firstly, the EBWD process is carried out in a vacuum environment, which means the heat conduction method is different from that in an inert gas environment. It mainly depends on the heat conduction to the matrix and the overall thermal radiation of the part. As a result, the cooling of the part is slower than that in inert gas, so the temperature of the part is more easily maintained at a higher level. Secondly, the electron beam has a large energy density and can control the electron beam spots by using the magnetic field. Therefore it is possible to locally heat the part by using the electron beam to change the temperature distribution of the part, which is also beneficial to controlling the stress and deformation. The deformation control method of EBWD is usually predicted by numerical simulation before prototyping, and then the technological parameters and prototyping path are adjusted. In addition, the large electron beam spot is used to scan the heated part during the prototyping process to control the temperature distribution of the entire part for conformal annealing and reducing the tendency of stress and deformation. The deformation control method for specific parts will be introduced below.

4.4.3 Deformation Prediction (1)

Basic assumptions

In this part, we simulate the temperature field and stress–strain field of the large plate-rib structure to predicts its deformation behavior and study the deformation rules during the prototyping process, in a bid to provide an important theoretical reference and basis for the EBWD fabrication of typical structures. The following assumptions are made for technological characteristics and component structures: The metal material has always been in a quasi-solid state and the entire deformation process is small deformation, which ignores the physical phenomena such as the flow of metal droplets, and does not consider the microscopic properties of the metal material. The heat exchange phenomenon only considers heat conduction, and considers both convection and heat radiation based on equivalent heat conduction. The temperature field calculation is a transient calculation, and the stress field calculation is a quasi-static calculation. The finite element analysis needs to construct the governing equations first, and then evolve the problem to be solved into a system of algebraic equations by the Galerkin method. (1)

Thermal conductivity differential equation

The differential equation for non-steady-state heat conduction of isotropic materials is:

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167

  ∂ ∂ T (x, y, z, t) ∂ T (x, y, z, t) − λ ρc(T ) ∂t ∂x ∂x     ∂ T (x, y, z, t) ∂ ∂ T (x, y, z, t) ∂ λ − λ =0 − ∂y ∂y ∂z ∂z

(4.2)

wherein x, y, z t T (x, y, z, t) λ = λ(t) ρ c(T ) Q(x, y, z, t)

is the space coordinate; is the time; is the temperature field; is the thermal conductivity of the material; is the material density; is the specific heat capacity of the material; is the internal heat source (such as latent heat) density.

The definite boundary conditions of this governing equation are classified into three categories: The first type of boundary condition (1 ) is the boundary condition of given temperature: T (x, y, z, t) = T (x, y, z, t)

(4.3)

where T (x, y, z, t) is the given boundary temperature. The second type of boundary condition (2 ) is the boundary condition of given heat flux density ∂ T (x, y, z, t) ∂ T (x, y, z, t) ∂ T (x, y, z, t) q nx + ny + nz = ∂x ∂y ∂z λ

(4.4)

  where q is the heat flux density given on the boundary, and n x , n y , n z is the normal unit vector outside the boundary. The third type of boundary condition (3 ) is the boundary condition of given convective heat transfer. ∂ T (x, y, z, t) ∂ T (x, y, z, t) ∂ T (x, y, z, t) h nx + ny + n z = (Ta − T ) ∂x ∂y ∂z λ

(4.5)

where Ta is the ambient temperature, and h is the convective heat transfer coefficient. Metal will release a mass of crystallization heat, also called latent heat, during the solidification process. When calculating the latent heat, we use equivalent specific heat capacity, as shown in Fig. 4.39. Due to the heavy calculation workload for convective heat transfer and radiation heat transfer, the equivalent conductivity is often used to replace convective heat

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Fig. 4.39 Numerical calculation processing of crystallization latent heat

transfer and radiation heat transfer in engineering calculations. This approximation is also applied in this study. The finite element method discretizes the differential Eq. (4.2) into a grid in space while using the shape function N as the basis, and discretizes it into an ordinary differential equation in the time domain: C T˙ + K T = P.

(4.6)

where C is the heat capacity matrix, K is the heat conduction matrix, P is the temperature load vector, T is the nodal temperature vector, and T˙ is the derivative of the temperature vector with respect to time. C K and P are integrated by the element heat capacity matrix C e , the element heat conduction matrix K e , and the element temperature vector and load vector P e .  Ciej =



ρcNi N j dΩ

(4.7)

4.4 Deformation Control

K iej =

169

    ∂ Ni ∂ N j ∂ Ni ∂ N j ∂ Ni ∂ N j λ +λ +λ d + h Ni N j dΓ ∂x ∂x ∂y ∂y ∂z ∂z  3    P ie = ρ Q Ni d + q Ni N j dΓ2 + hTa Ni dΓ3 

Γ2

(4.8) (4.9)

Γ3

The most commonly used method for numerical calculation of Eq. (4.6) is the twopoint loop formula of immediate integration. In order to ensure the stable solution and avoid oscillation, the finite method generally adopts the backward difference. The default integration scheme of ABAQUS® is the backward difference method. (2)

Finite element simulation of stress field

The relationship between strain and displacement field under small deformation conditions is:   εi j = u i, j + u j,i /2

(4.10)

where i j is the tensor indicator. Strain ε consists of three parts - the elastic strain εel , the ductile strain εpl , and the thermal strain εth : ε = εel + εpl + εth

(4.11)

The component of thermal strain εth caused by temperature change is εthij = α(T − T0 )δij

(4.12)

where α is the thermal expansion coefficient of the material. The force balance conditions are: σi j, j + f i = 0

(4.13)

where σ is the stress tensor and is f the physical strength. The boundary conditions that need to be met on the force boundary are: σi j, j n i = pi

(4.14)

where pi is the given boundary force. The boundary conditions that need to be met on the displacement boundary are: u=u u is the given boundary displacement.

(4.15)

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4 Fundamentals of Electron Beam Wire Deposition Technology

The stress–strain relationship is also called the constitutive equation. The elastic stress rate and elastic strain rate satisfy the Hooke’s law: σ˙ el = D : ε˙ el

(4.16)

where D is the elasticity tensor. As for isotropic materials, D only has two independent parameters, Young’s modulus E and Poisson’s ratio ν. (3)

Finite element model

(1)

Model description

Aiming at the structural form and basic prototyping technology of typical specimens, we established a finite element model in this study: The type of the rib is double-sided, symmetrical rib, which means ribs are deposited on both the front and back sides of the plate. The ribs on the two sides are defined as front ribs and reverse ribs, as shown in Fig. 4.40. According to the deposition sequence, front ribs are deposited first, and then reverse ribs are deposited based on the corresponding deformation amount. We investigated the deformation of the rib deposition process, the ribs are deposited on the plate. We assume that the plate is in a stress-free and flat state. The size of the experimental plate is 1500 mm × 500 mm × 30 mm. The rib height is 30 mm and the width is 23 mm. The schematic diagram is shown in Fig. 4.41a. In the figure, ribs are shown in green and the plate is shown in white. The deposition height of ribs is 30 mm, with totally 15 layers deposited. The width of each rib is 23 mm, which is deposited by four passes. The spacing between each pass is 4.5 mm, as shown in Fig. 4.41b. The first pass, which is the outer silhouette edge, is 3.2 mm away from the actual outer silhouette. The other 3 passes are positioned on the basis of the first pass, and the spacing between each pass is 4.5 mm. According to the deposition sequence of the four passes, the outermost pass is deposited first, then the innermost square pass is deposited, followed by the passes in the middle. The 6 blocks are edged one by one in a random order.

Fig. 4.40 Schematic diagram of deposited ribs

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171

(a) Schematic diagram of rib structure

(b) Schematic diagram of path planning

Fig. 4.41 Schematic diagram of the prototyping of the plate-rib structure (Ribs are shown in green, 30 mm-thick plates are shown in white, the prototyping path is shown in red)

Reverse ribs are deposited on front ribs. They remain the deformation state before the deposition with the internal stress state removed. The deposition method and sequence are the same as the deposition of front ribs. (2)

Physical parameters

The material selected is TC4, with a melting temperature range of 1630–1650 °C and a density of 4.44 g/cm3 . Other physical parameters are shown in Tables 4.6, 4.7, 4.8, 4.9 and 4.10. (3)

Meshing

The finite element meshing of the model is shown in Fig. 4.42. The DC3D8 element and the C3D8R element are used in the thermal analysis and the mechanical analysis stages. Table 4.6 Elastic modulus of TC4 titanium alloy

Temperature θ/°C

Young’s ModulusE /Gpa

Shear Modulus G /Gpa

Poisson’s Ratio μ

25

119

44.5

0.34

100

115

42.8

0.34

200

110

40.6

0.35

300

105

38.7

0.36

400

101

36.9

0.36

Note heating rate: 400–3°C /min; test atmosphere: high vacuum

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4 Fundamentals of Electron Beam Wire Deposition Technology

Table 4.7 Tensile properties of TC4 titanium alloy

θ/°C

σb /MPa

σ0.2 /MPa

20

967

860

100

846

736

200

741

613

300

690

543

350

665

532

400

645

508

500

583

401

600

413

212

700

245

89

Table 4.8 Thermal conductivity of TC4 titanium alloy θ/°C

100

200

300

400

500

600

W/(m·o C)

7.86

8.64

9.42

10.1

10.8

11.5

Table 4.9 Specific heat capacity of TC4 titanium alloy θ/°C

100

200

300

400

500

600

c/(J/(kg o C))

577

577

578

579

580

580

Table 4.10 Expansion coefficient of TC4 titanium alloy θ/°C

100

200

300

400

500

600

α × 10−6 /°C

8.47

9.12

9.60

9.93

10.2

10.4

(4)

Simulation process

We used the commercial finite element software ABAQUS for thermal decoupling numerical simulation. The simulation process is carried out in two stages:

Fig. 4.42 Schematic diagram of finite element meshing

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173

The first stage is thermal analysis. In the first analysis step, the plate element is killed, and only the matrix element is retained. Use the element activation function in ABAQUS to activate elements one by one. Set an analysis step each time an element is activated, and give a certain unit heat flow. The unit heat flow is the heat passing through a unit volume and within a unit period of time. Through trial calculations, a reasonable unit heat flow value is determined so that the temperature is slightly higher than the melting point of the material when the element is activated. Set the cooling step each time a layer is activated, and cool down for 30 min. Cool down to the room temperature after all elements are activated. The second stage is mechanical analysis. Import the temperature field of each analysis step in the thermal analysis result file one by one as the initial condition for the corresponding mechanical analysis step, so as to obtain the stress field and the strain field. (4)

Calculation results

(1)

Transient temperature field

Figure 4.43 shows the transient temperature field when the front and reverse ribs are deposited. According to the figure the temperature of the ribs is significantly higher than the temperature of the plate. The high temperature areas of on the plate are mainly distributed near the ribs, and the low temperature areas are distributed in a diamond shape around the grid fabricated by the ribs. In addition, when the reverse ribs are deposited, the transient temperature of the plate is higher than the transient temperature upon the completion of front rib deposition. (2)

Residual stress

After the front ribs are deposited and cooled to the room temperature, the distribution of the primary stress in the two directions is shown in Fig. 4.44. It can be seen that xx is mainly distributed along the longitudinal rib, and yy is mainly distributed along the transverse rib. A certain amount of residual stress is created on the plate near the root of the rib, but is small in size and distribution area. After the reverse ribs are deposited and cooled to the room temperature, the distribution of the primary stress in the front and reverse directions is shown in Fig. 4.45. According to the figure the residual stresses xx and yy on the plate are distributed in strips along the longitudinal and transverse ribs respectively, and are parallel to the direction of the ribs. Compared with the stress on the plate when the single-sided ribs are deposited, the residual stress on the plate after the deposition of double-sided ribs is greater in magnitude and distribution range, and the stress on the reverse side is greater than the stress on the front side of the plate. This is because the plate continues to receive heat from reverse ribs when they are deposited, thereby increasing the internal stress of the plate. Figure 4.46 shows the residual stress distribution on the plate upon the deposition of front ribs v.s. reverse ribs. The inspection path is shown in the figure. According to the previous calculation results in Figs. 4.44 and 4.45, we can see that the plate

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.43 Transient temperature field after the deposition and prototyping of double-sided ribs (unit: °C)

stress is mainly concentrated at the root of the ribs, so here we only present the plate stress distribution on the typical path. As shown in Fig. 4.46, when the reverse ribs are deposited, the plate stress increases compared to that upon the deposition of front ribs. The stress concentrates at the intersection of ribs. (1)

Residual deformation

Front ribs are cooled down to the room temperature after being deposited to obtain the single-sided rib wall. The distribution of residual deformation on the single-sided rib deposition wall is shown in Fig. 4.47. According to the figure, the deformation on the single-sided rib wall is basically the same as that on the plate. The direction of the plate warping is opposite to the direction of the rib deposition thickness. The maximum deformation is U 3 = 3.36 × 10−4 m, which occurs at the center of the wall. The high deformation zone is distributed along the length of the rectangular wall in a strip shape. After the single-sided rib wall is cooled to the room temperature, symmetrical reverse ribs continue to be deposited on the reverse side of the plate. After reverse ribs

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Fig. 4.44 Distribution of residual stress after the front ribs are deposited and cooled to the room temperature (unit: Pa)

are deposited and cooled down to the room temperature, the double-sided symmetrical rib wall is obtained. The final deformation is shown in Fig. 4.48. It can be seen that the maximum deformation still occurs at the center of the double-sided rib wall, and the high deformation area is distributed along the length of the wall in a approximately elliptical shape. It is noteworthy that the deformation direction of the front ribs is the same as that of the wall, while the deformation direction of the reverse ribs is opposite to that of the wall. This is because front and reverse ribs cause plate deformation in opposite directions, which will offset each other. Figure 4.49 shows the comparison of plate deformation in the thickness direction between single-sided rib deposition and double-sided rib deposition. As shown in the figure, the plate deformation is significant reduced after the double-sided rib deposition. The maximum plate deformation is 3.36 × 10−4 m in the case of singlesided rib deposition, and 1.24 × 10−4 m in the case of double-sided rib deposition. This is because a deformation in the opposite direction to the front rib deposition is generated on the plate after the reverse ribs are deposited, therefore it offsets a part of deformation caused by single-sided deposition. The maximum plate deformation of double-sided rib deposition is one third that of single-sided rib deposition.

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.45 Distribution of residual stress after the reverse ribs are deposited and cooled to the room temperature (unit: Pa)

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177

Fig. 4.46 Distribution of residual stress on the plate

4.4.4 Partitioned and Fractal Machining Reduce the accumulation of stress and deformation through partitioned and fractal machining. Partitioning is to decompose a large structure into several small structures according to structural characteristics. Fractal refers to the planning of multiple machining areas on a plane and machining them separately. The path and direction of each machining area must be specially planned to facilitate the neutralization of stress. The current partitioning principle is to locate partitions at the least stressed positions as much as possible according to the stress characteristics and dimensions of the part, and also enlarge the maximum size of each partition. The fractal machining is mainly for the purpose of dispersing the stress to improve deformation. It needs to comprehensively consider the prototyping efficiency and cross section characteristics. In order to investigate the influence of partitioning on deformation, we have designed the partitioned prototyping test and carried out simulations.

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.47 Residual deformation after front ribs are deposited and cooled down to the room temperature (unit: m)

The fractal deposition is to divide the prototyped plane into several partitions of the same size according to the size of the plate structure (length and width). Each partition is padded with a single path, and the paths of adjacent partitions are orthogonal. The paths of adjacent layers at the same position are also orthogonal. In order to simplify the physical model, the starting point of each partition is fixed: the deposition starts from the bottom left corner and proceeds upward when the path is vertical; the deposition starts from the top left corner and proceeds rightward when the path is horizontal. The planning of partition paths is shown in Fig. 4.50. The deposition moves from one partition to another, and the sequence is decided by a random function. The experimental matrix is the TC4 titanium alloy sheet with a dimension of 1660 mm × 620 mm × 6 mm; the dimension of the prototyped plate structure is 1500 mm × 500 mm × 30 mm. The partitioned deposition is used to divide the prototyped plane into several partitions of the same size according to the size of the plate structure (length and width). Each partition is padded with a single path, and the paths of adjacent partitions are orthogonal. The paths of adjacent layers at the same position are also orthogonal. In order to simplify the physical model, the starting point of each partition is fixed: the deposition starts from the bottom left corner and proceeds upward when the path is vertical; the deposition starts from the top left corner and proceeds rightward when the path is horizontal. The planning of

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179

Fig. 4.48 Residual deformation after reverse ribs are deposited and cooled down to the room temperature (unit: m)

Fig. 4.49 Comparison of plate deformation between single-sided rib deposition and double-sided rib deposition

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4 Fundamentals of Electron Beam Wire Deposition Technology

Fig. 4.50 Path planning of partitioned deposition

(a)

Schematic diagram of partition path

1

2

3

4 (b)

Numbers of 4 partitions

1

2

3

4

5

6

7

8

(c)

Numbers of 8 partitions

4 partitions

8 partitions

Level 1

1, 3, 2, 4

6, 1, 8, 3, 2, 5, 4, 7

Level 2

1, 4, 3, 2

1, 7, 4, 3, 2, 5, 8, 6

Level 3

3, 4, 2, 1

3, 5, 8, 7, 6, 4, 2, 1

Level 4

3, 4, 1, 2

5, 6, 8, 3, 4, 1, 7, 2

(d) Randomly generated padding order of partitions

partition paths is shown in Fig. 4.50a. The deposition moves from one partition to another. The sequence is decided by a random function. Two finite element models are established after considering the number of different partitions, and the plate is divided into 4 small partitions and 8 small partitions, and each small partition is divided into 3 × 3 elements. Due to the complexity of the padding path, the model is created manually and activates the elements one by one according. Figure 4.51 shows the transient temperature distribution of the plate in different padding stages. After the first layer is padded, cool it down for 30 min, and the temperature drops to about 454 °C. After the last layer is padded, cool it down to the room temperature of 20 °C. As shown in Fig. 4.51, the transient temperature increases as the number of padded partitions increases. The distribution of residual stress on the rectangular plate of partition padding is shown in Fig. 4.52. It can be seen that the lengthwise stress σ xx is concentrated on the lateral edge of the matrix, and the widthwise stress σ yy is concentrated on the longitudinal edge of the matrix. The lengthwise stress σ xx in the plate is higher than the widthwise stress σ yy . Figure 4.53 shows the residual deformation on the rectangular plate of partition

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181

First Layer

Last Layer

First partition

Fourth partition

Padding completed

Cooling (the first layer is cooled for 30 minutes, the last layer is cooled to the room temperature)

Fig. 4.51 Transient temperature field of partition padding on rectangular plate with Substrate (Unit: °C)

padding. It can be seen that the deformation is concentrated near the centerline along the length direction of the plate. The maximum deformation is 1.524 × 10−3 m. Consider three plate padding methods: single-path padding, four-partition padding and eight-partition padding. Figure 4.54 shows the change of the plate deformation along the centerline of the length direction after using the above-mentioned three padding methods. It can be seen that the plate warping is reduced after the partition padding method is used. The more partitions there are, the less warping the plate has. The maximum warping is 1.68 × 10−3 m in single path padding; the maximum warping is 1.52 × 10−3 m in four-partition padding; the maximum warping is 1.19 × 10−3 m in eight-partition padding.

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4 Fundamentals of Electron Beam Wire Deposition Technology

Substrate

First layer

Last layer

Fig. 4.52 Residual stress distribution of partition padding on rectangular plate with Substrate (Unit: Pa)

Fig. 4.53 Residual deformation of partition padding on rectangular plate with Substrate (Unit: m)

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Fig. 4.54 Impact of partition padding on deformation

References 1. Tang Q, Pang S, Chen B, Suo H, Zhou J (2014) A three dimensional transient model for heat transfer and fluid flow of weld pool during electron beam freeform fabrication of Ti-6-Al-4Valloy[J]. Int J Heat Mass Transf 78:203–215 2. Chen T, Pang S, Tang Q, Suo H, Gong S (2016) Evaporation Ripped Metallurgical Pore in ElectronBeam Freeform Fabrication of Ti-6-Al-4-V. Mater Manuf Processes 31(15):1995–2000

Chapter 5

Non-destructive Inspection of EBWD-Fabricated Parts

Abstract This chapter introduces the non-destructive inspection technology of EBWD-fabricated typical metal parts, such as Titanium alloy and A100 steel. Acoustic characteristics and defect characteristics of TC4 Titanium Alloy and A100 steel Fabricated by EBWD have studied. At the same time, the different methods such as the ultrasonic inspection, the magnetic particle testing and X-ray inspection have been used for non-destructive inspection of A100 steel.

5.1 Ultrasonic Inspection Technology for EBWD-Fabricated TC4 Titanium Alloy 5.1.1 Technical Solutions to Non-Destructive Inspection of TC4 Titanium Alloy Fabricated by EBWD For the specimen of TC4 titanium alloy fabricated by EBWD, two sound beam incident directions parallel with and vertical to the deposition direction were selected for ultrasonic C-scan inspection test to study the impact of the incident direction of the sound beam on the defect inspection result of EBWD-fabricated parts. While using the TC4 electron beam deposition specimen (as shown in Fig. 5.1), we applied the zone-focused C-scan ultrasonic inspection method to allow for sound beam incidence from different directions. We also evaluated the noise level of the material and measured the sound velocity and attenuation in different areas of the specimen.

© National Defense Industry Press 2022 S. Gong et al., Electron Beam Wire Deposition Technology and Its Application, Additive Manufacturing Technology, https://doi.org/10.1007/978-981-19-0759-3_5

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Fig. 5.1 Size of the EBWD-fabricated TC4 specimen

5.1.2 Acoustic Characteristics and Defect Characteristics of TC4 Titanium Alloy Fabricated by EBWD (1)

Acoustic characteristics of TC4 material

The EBWD technology uses high-energy electron beams to melt the wire material layer by layer, and its structure is quite different from that of the deformed titanium alloy. Therefore, the research on the acoustic characteristics of the EBWD technology begins with the structure of the material. (2)

Sound velocity measurement

The sound velocity of the TC4 material along the directions parallel with and perpendicular to the deposition direction are measured respectively. The results are shown in Table 5.1 and Fig. 5.2. Table 5.1 Measurement results of the sound velocity of the EBWD-fabricated TC4 material Measuring Point

The sound beam is incident along the deposition direction (parallel) Thickness/ mm

Sound velocity/ (m/s)

1

130.18

6144

2

130.04

3

130.00

4

Measuring Point

The sound beam is incident perpendicular to the deposition direction (vertical) Thickness/mm

Sound velocity/ (m/s)

1

99.34

6149

6139

2

99.40

6144

6141

3

99.42

6144

129.92

6134

4

99.40

6144

5

129.86

6134

5

99.34

6146

6

129.92

6139

6

99.32

6162

7

129.96

6141

7

99.36

6167

8

129.94

6139

8

99.50

6159

Mean

/

6138.9

Mean

/

6151.9

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Fig. 5.2 The measurement results of the sound velocity of the TC4 specimen fabricated by EBWD

As shown in Table 5.1 and Fig. 5.2, as for the EBWD-fabricated TC4 material, the largest difference of the sound velocity measured in different random areas on the same incident direction of sound beam is 23 m/s, and the relative deviation does not exceed 0.4%; when sound beams are incident from two directions parallel with and perpendicular to the deposition direction, the difference of sound velocity is 13 m/s, and the relative deviation does not exceed 0.2%. Therefore, the difference in sound velocity of the EBWD-fabricated TC4 material measured in different areas and deposition directions is very small and ignorable. (3)

Measurement results of attenuation

Measure the gain value when the bottom wave in different directions of the specimen reaches 80% of the full-screen wave height, and compare the gain value of bottom wave with a deposit of the same thickness. The corresponding forging deposit is Ti6Al4V, and the measurement results are shown in Table 5.2. As shown in Table 5.2, the attenuation of the TC4 material along the deposition direction is large as for different incident directions of sound beams. In addition, compared with the contrast deposits used, the attenuations of the TC4 specimen in two directions are greater than that of the Ti6Al4V deposit for forgings, which are 5 dB and 8 dB respectively. Apparently there is a certain difference between the acoustic properties of the currently used Ti6Al4V deposit for forgings and the EBWD-fabricated material. Such a difference may have a certain impact on the accuracy of the test results. It is very necessary to design and produce a contrast deposit of the same material. Since the incident directions of sound beams are different, Table 5.2 Measurement results of the attenuation of EBWD-fabricated materials Specimen material

TC4

Contrast deposit material

Sound beam along the deposition direction

Sound beam perpendicular to the deposition direction

Specimen

Deposit of the same thickness

ΔdB

Specimen

Deposit of the same thickness

ΔdB

Ti6Al4V

45 dB

37 dB

8

41 dB

36 dB

5

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Along the deposition direction (Depth 4 27mm, scanning sensitivity Φ0.8+18dB)

Along the deposition direction (Depth 25 51mm, scanning sensitivity Φ0.8+12dB)

Perpendicular to the deposition direction (Depth 4

27mm, scanning sensitivity Φ0.8+18dB)

Perpendicular to the deposition direction (Depth 25 51mm, scanning sensitivity Φ0.8+12dB)

Fig. 5.3 Noise level of the EBWD-fabricated TC4 material

the acoustic characteristics of different grades of materials also present a significant difference. It is necessary to make deposits for different grades and incident directions of sound beams. (4)

Measurement results of noise level

We compared the noise levels of the TC4 materials at different depths and different incident directions of sound beams. The measurement results are shown in Fig. 5.3. It can be seen from Fig. 5.3 that for the EBWD-fabricated TC4 material, the noise level is not higher than the equivalent of 0.4-24 dB flat-bottomed hole no matter parallel with or perpendicular to the deposition direction, and the noise level vertical to the deposition direction is slightly higher. (5)

Research on defect characteristics of EBWD-fabricated materials in different deposition directions

We used the Ti6Al4V-0.8 mm contrast deposit to produce the DAC curve for the ultrasonic inspection of the EBWD-fabricated TC4 material. The distance-amplitude curve of the probe is shown in Fig. 5.4. The C-scan results in different incident directions of sound beams are shown in Figs. 5.5 and 5.6. It can be seen from Fig. 5.6 that when the sound beam is incident perpendicular to the deposition direction under the same scanning sensitivity of 0.8-18 dB, a large

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Fig. 5.4 Distance-amplitude curve of the probe (Ti6Al4V deposit)

Fig. 5.5 C-scan image of TC4 along the deposition direction

Fig. 5.6 C-scan image of TC4 perpendicular to the deposition direction

number of dots and chains will be displayed on the C-scan image, and the equivalent size is between 0.8–19.5 dB and F0.8-25 dB. These displays may be caused by structural abnormalities. The typical A-scan waveform is shown in Fig. 5.7. Due to the complexity in the wire deposition prototyping technology for titanium alloys and the structural characteristics, the defect display cannot be ascertained only based on

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Display 1 (buried depth 5.5mm, equivalent Φ0.8-23.5dB)

Display 2 (buried depth 9mm, equivalent Φ0.8-23.5dB)

Fig. 5.7 Scan waveform A of typical abnormality display

the C-scan image and the A-scan waveform. We need to further analyze the results of the metallographic experiment. No obvious abnormality display is seen on the C-scan image (as shown in Fig. 5.5) on which the sound beam is incident along the deposition direction. According to the result, the EBWD-fabricated TC4 material has obvious directivity, which will have an impact on the ultrasonic inspection. A 5 MHz flat probe is used to monitor the bottom wave of the specimen. The Cscan results are shown in Figs. 5.8 and 5.9. According to the results, the amplitude of

Fig. 5.8 Result of bottom wave monitoring of TC4 along the deposition direction

Fig. 5.9 Result of bottom wave monitoring of TC4 perpendicular to the deposition direction

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191

the bottom wave changes obviously no matter from which direction the sound beam is incident, indicating the uneven material structure and considerable attenuation changes. Among them, the areas with greater bottom wave loss along the deposition direction are mostly point-shaped and flat-shaped. The maximum difference of amplitude can reach 13 dB. The areas with significant reduction of bottom wave perpendicular to the deposition direction are mainly chain-shaped, and show an obvious layered distribution. The maximum amplitude difference of the bottom wave can reach 20 dB. (6)

Inspection test of flat-bottomed holes on EBWD-fabricated TC4 material in different directions

We produced two pieces of EBWD-fabricated TC4 material with a size of 65 × 65 × 65 mm and flat-bottomed holes in different directions, and then tested them by the water immersion method and the contact method. Tables 5.3 and 5.4 show the C-scan image of the flat-bottomed hole, the waveform of the flat-bottomed hole in the water immersion method and the contact method, and the metallographic photos of different deposition directions. Table 5.5 summarizes the sensitivity levels of the flat-bottomed holes in different directions and at different attenuation positions. It can be seen from Tables 5.3, 5.4 and 5.5 that: (1) (2)

(3)

(4)

The signal-to-noise ratio of the EBWD-fabricated TC4 material is better when being tested by the water immersion method than by the contact method; When the water immersion method is used, if the incident directions are different, the sensitivity of flat-bottomed hole differs by up to 5.5 dB at small attenuation parts (No. 1) and by up to 4.5 dB at large attenuation parts (No. 2). In both cases the Z-direction sensitivity is the smallest. If the incident directions are identical, the sensitivity of small attenuation parts is maximally 9 dB higher than that of large attenuation parts (Y direction); When the contact method is used, there is almost no difference in sensitivity in different directions and at similar attenuation parts; the sensitivity of small attenuation parts (No. 1) is maximally 5 dB higher than that of large attenuation parts (No. 2) The structure of the EBWD-fabricated TC4 material shows obvious nonuniformity. Coarse columnar crystals are seen on the cross-sections along the X and Y directions, which pass through multiple deposition layers, and even penetrate the entire cross-section. Coarse and fine crystals are unevenly distributed along the Z direction, showing no obvious regularity. The material structure corresponds to the result of the bottom wave loss scanning.

(4)

Ultrasonic inspection of the contrast deposit

(1)

Overview of the deposit

The schematic diagram of the contrast deposit of the EBWD-fabricated TC4 is shown in Fig. 5.10.

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Table 5.3 Result of ultrasonic inspection of flat-bottomed holes on No. 1 specimen in different directions Test surface

Y–Z surface X–Z surface (Y-direction X–Y surface (X-direction inspection) inspection) (Z-direction inspection)

Punching Position of Flat-Bottomed Holes on No. 1 Specimen

61 dB (56-83 dB/80%)

61 dB (53-84 dB/80%)

59 dB (55-81 dB/80%)

Sensitivity: 61.5 dB

Sensitivity: 59.5 dB

Sensitivity: 65 dB

Sensitivity: 53 dB

Sensitivity: 52 dB

Sensitivity: 52 dB

C-Scan Image of Flat-Bottomed Holes on No. 1 Specimen

Waveform of Water Immersion Method for Flat-Bottomed Holes on No. 1 Specimen Flat-Bottomed Holes on No. 1 Specimen Waveform of Contact Method Macroscopic Metallographic Photos (1:1)

Macrostructure Photos (7.8:1)

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Table 5.4 Result of ultrasonic inspection of flat-bottomed holes on No. 2 specimen in different directions Test surface

Y–Z surface (X-direction X–Z surface (Y-direction X–Y surface (Z-direction inspection) inspection) inspection)

Punching Position of Flat-Bottomed Holes on No. 2 Specimen 61 dB (59-77 dB/80%)

61 dB (53-84 dB/80%)

59 dB (59-81 dB/80%)

Sensitivity: 66 dB

Sensitivity: 68.5 dB

Sensitivity: 70.5 dB

Sensitivity: 56 dB

Sensitivity: 57 dB

Sensitivity: 57 dB

C-Scan Image of Flat-Bottomed Holes on No. 2 Specimen

Waveform of Water Immersion Method for Flat-Bottomed Holes on No. 2 Specimen Flat-Bottomed Holes on No. 2 Specimen Waveform of Contact Method

Table 5.5 Sensitivity of flat-bottomed holes in different directions Inspection method

Specimen no

X-Direction inspection

Y-Direction inspection

Z-Direction inspection

Water immersion

1#

61.5 dB

59.5 dB

65 dB

2#

66 dB

68.5 dB

70.5 dB

1#

53 dB

52 dB

52 dB

2#

56 dB

57 dB

57 dB

Contact method

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Fig. 5.10 Contrast deposit of TC4

Fig. 5.11 Distance-amplitude curve of Ti6Al4V-0.8 mm

(2)

Inspection result of the deposit

First, we performed an ultrasonic inspection on the deposit, and the parameters tested are as follows: The testing of the contrast deposit was carried out using the USIP40 ultrasonic flaw detector, the HGE5827A probe (10 MHz focusing probe), and the 5 MHz waterimmersed flat probe. When the HGE5827A probe is used, the water distance is 50 mm, the inspection sensitivity is 0.8 mm, and the Ti6Al4V-0.8 mm contrast deposit is used to configure the inspection sensitivity. See Table 5.6 for sensitivity adjustment parameters. The result of the ultrasonic C-scan is shown in Fig. 5.12. The waveform of typical defects is shown in Fig. 5.13. The scanning result of bottom wave loss is shown in Table 5.6 TCG curve data of the Ti6Al4V-0.8 forging

No

Buried depth /mm

Gain /dB

1

3.8

67.8

2

6.8

60.8

3

9.7

57.3

4

16.8

55.5

5

16

55.4

6

19

56.2

7

22

57.5

8

25

59

9

31.5

61.5

10

44

66.9

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Fig. 5.12 Result of ultrasonic C-scan

Fig. 5.13 Graphic of typical defect waveform (Buried depth: 30 mm, equivalent size: 0.8-2 dB)

Fig. 5.14. It can be seen from Figs. 5.12, 5.13 and 5.14 that no obvious abnormalities are found on the deposit except a few areas, and the bottom wave loss is uniform. The defect area should be avoided when the deposit is manufactured. (3)

Design and manufacturing of the contrast deposit

We designed the corresponding contrast deposits according to the thickness of typical titanium alloy parts, and manufactured totally two sets of deposits with flat-bottomed steps and a hole diameter of 0.8 mm. The completed deposits with flat-bottomed holes are shown in Fig. 5.15.

Fig. 5.14 Result of bottom wave loss scanning

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Fig. 5.15 Photo of the independently developed contrast deposits with flat-bottomed holes fabricated by wire deposition

Fig. 5.16 Overlay pattern of typical flat-bottom holes

The flat-bottomed holes of the finished contrast deposit are overlaid with silicone rubber for testing. The hole diameter and chamfer of the deposit with flat-bottomed holes are conforming. The test results are shown in Fig. 5.16. Through the ultrasonic inspection on the deposit, a contrast deposit for TC4 ultrasonic inspection has been designed and manufactured. The deposit meets the inspection requirements and can be used for actual inspection of parts. A 5 MHz flat probe and a 10 MHz focusing probe were used to carry out sensitivity tests on the contrast deposit. The distance-amplitude curve of the probe is shown in Fig. 5.17. According to the A-scan waveform in Fig. 5.18, when the flat-bottomed hole with a buried depth of 35 mm adopts the 5 MHz flat probe, the device gain is 83.5 dB when the amplitude reaches 80%, and the signal-to-noise ratio is 0.8-18 dB. The device gain is 72 dB when the 10 MHz probe is used, and the signal-to-noise ratio is higher than 0.8-18 dB. The sensitivity and signal-to-noise ratio of the 10 MHz focusing probe are better than those of the 5 MHz flat probe, and both can meet the inspection requirements of 0.8 mm equivalent flat-bottomed holes.

5.1 Ultrasonic Inspection Technology for EBWD-Fabricated …

Distance-amplitude curve of the 10MHz focusing probe

197

Distance-amplitude curve of the 5MHz flat probe

Fig. 5.17 Distance-amplitude curves of different probes

(a) Ultrasonic A-scan waveform of 35mm buried depth (5MHz)

(b) Ultrasonic A-scan waveform of 35mm buried depth (10MHz)

Fig. 5.18 Contrast waveform of A-scan SNR of different probes

5.1.3 Comparison of the Contrast Deposit and the Forged TC4 Deposit in Different Prototyping Technologies (1)

Comparison of the deposits in different prototyping technologies by the contact method

During the inspection of fabrication parts, in order to clarify the difference in the inspection sensitivity of the deposit used in the EBWD technology, the deformed titanium alloy TC4 deposit, and the TC4 deposit used by the laser wire deposition prototyping technology, we applied the contact method to compare and analyze the distance-amplitude curves of the three deposits. The test used the 5 MHz probe for contact method. The test results are shown in Fig. 5.19. It can be seen from the figure that the overall attenuation law of the three deposits tested by the contact method is consistent, but the reflection amplitude of the flatbottomed hole on the forging TC4 deposit is lower than that of the deposits of the two wire deposition prototyping technologies. The contact method is in the range

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Fig. 5.19 Distance-amplitude curves of deposits in different ultrasonic inspection technologies

of test depth. The maximum difference between the deposit used by the laser wire deposition prototyping technology and the forging deposit is 4 dB, and the maximum difference between TC4 used by the EBWD technology and the forging deposit is 5 dB. The inspection sensitivity of the two wire deposition prototyping technologies is basically the same in the depth range of 30 mm parallel to the deposition direction. (2)

Comparison test of the three deposits by water immersion method

HGE, V322-200 and V322-250 probes were used for the ultrasonic zone-focusing test. The distance-amplitude curve in the test result is shown in Fig. 5.20. According to Fig. 5.20, the general trend shows that the reflection amplitude of the flat-bottomed holes on the TC4 deposit is lower than the deposits of the two wire deposition prototyping technologies, but the individual data still has a certain degree of dispersion. In the depth range of 25 mm, the maximum difference between TC4 and the EBWD deposit is 11 dB, and the maximum difference with the deposit of laser wire deposition prototyping technology is 7.5 dB. This result is consistent with the trend of the contact method, but the difference in the results of the water immersion method is larger.

Fig. 5.20 Distance-amplitude curves of the three deposits in the ultrasonic water immersion zonefocusing test

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199

As shown in the test results of the contact method and the water immersion method, generally speaking, the TC4 titanium alloy deposits of the two wire deposition prototyping technologies differ from the deformed titanium alloy TC4 deposit in the sensitivity. However, according to the conclusions of previous tests, we know that the attenuation in different areas of the material used in wire deposition prototyping technologies varies greatly. If holes are drilled at positions with different attenuation, their sensitivity will differ. If we fully consider the attenuation difference of the punching positions, the sensitivity difference between laser wire deposition and TC4 deposit is relatively small, and the sensitivity difference between EBWD and TC4 deposit is slightly greater. The use of TC4 deposit is an alternative measure when the deposit for the same technology is not available. In order to ensure the accuracy of the test results, a deposit whose material, technology used, and deposition direction is identical with the tested part shall be manufactured when the parts of the two wire deposition prototyping technologies are tested.

5.1.4 Comparison of Inspection Sensitivity of Different Non-Destructive Inspection Methods for EBWD Aiming at the TC4 specimen of EBWD, we selected the specimens of typical defects, carried out tests by using different methods, and analyzed the defects, then we compared and analyzed the sensitivity of different inspection methods. (1)

Ultrasonic inspection

For the anatomical TC4 specimen of EBWD, the sound beam is incident from the Z, X, and Y directions for ultrasonic C-scanning and inspection. The C-scan diagrams and defect equivalents of the same defect in different directions are shown in Table 5.7. It can be seen from Table 5.7 that the ultrasonic display of the same defect in different incident directions of the sound beam is very different. In all anatomical specimens, when the sound beam is incident along the deposition direction, the signal-to-noise ratio of the ultrasonic inspection is the highest, and the equivalent size of the defect is larger than that of the sound beam incident in the direction vertical to the deposition direction, and the difference is more than ten decibels. (2)

X-ray inspection

Perform an X-ray inspection on the above specimens, and the results are shown in Table 5.8 and Fig. 5.21. According to Table 5.8, when the X-ray inspection is used, the material thickness has a significant impact on the inspection sensitivity. When the thickness is 2021 mm, the X-ray cannot find the defect display with the quantitative ultrasound of 1.3 mm ~ 1.5 mm; when the thickness is reduced to less than 13 mm, the smallest

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Table 5.7 Result of the ultrasonic C-scanning of anatomical TC4 specimens fabricated by EBWD Specimen number

Buried depth in Z direction/mm

2#

19.93

3#

4#

5#

6#

Incident in Z direction

Incident in X direction

Incident in Y direction

Equivalent 0.8 + 9 dB

Equivalent 0.8–7.5 dB

Equivalent 0.8-7 dB

Equivalent 0.8 + 6 dB

Equivalent 0.8-8 dB

Equivalent 0.8-3 dB

Equivalent 0.8 + 7.5 dB

Equivalent 0.8-10 dB

Equivalent 0.8-10 dB

Equivalent 0.8 + 9.5 dB

Equivalent 0.8-13 dB

Equivalent 0.8-16 dB

Equivalent 0.8 + 9.5 dB

Equivalent 0.8-15 dB

Equivalent 0.8-14 dB

8.24

16.18

8.9

10.8

(continued)

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201

Table 5.7 (continued) Specimen number

Buried depth in Z direction/mm

No. 7

12

Incident in Z direction

Incident in X direction

Incident in Y direction

Equivalent 0.8 + 5 dB

Equivalent 0.8-10 dB

Equivalent 0.8-12 dB

Table 5.8 Comparison of results between ultrasonic inspection and X-ray inspection Specimen number

Specimen thickness /mm

Ultrasonic equivalent size (Z Direction/mm)

X-Ray inspection

Nature of defect

1#

21

2#

20

1.5 mm

Not detected

/

1.34 mm

Not detected

/

3#

8.5

1.13 mm

Detected, with a size of 1.2 mm

Pore

4#

13

1.2 mm

Detected, with a size of 1mm

Poor pores and poor fusion

5#

10

1.38 mm

Detected, with a size of 0.4 mm

Abnormal pores and microstructure

6#

12

1.38 mm

Detected, with a size of 0.5 mm

Abnormal pores and microstructure

No. 7

16.5

1.06 mm

Detected, with a size of 1.2 mm

Pore

NO.3 specimen

No.4 specimen

Fig. 5.21 Result of the X-ray inspection of typical defects

No.7 specimen

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Fig. 5.22 Fluorescence inspection display

defect size that can be detected by X-ray is 0.4 mm (the quantitative ultrasound is 1.4 mm). (3)

Fluorescence penetrant inspection

A fluorescence penetrant inspection was performed on the above specimens. The inspection results are shown in Fig. 5.22. The fluorescence inspection can clearly show pore defects on the surface. (4)

CT inspection

Select No. 2, No. 3, and No .5 specimens in Table 5.7 for CT inspection. The test results are shown in Figs. 5.23, 5.24 and 5.25. The CT inspection method allows to “slice” materials for imaging inspection. It can detect more defects and show their positions more accurately. In Fig. 5.23, we can see that the No. 2 specimen is detected by CT and two non-fusion defects and multiple pore defects are found. In Fig. 5.24, a large pore defect is detected on the No. 3 specimen, and the pore sizes are around 1 mm when measured from three directions. In Fig. 5.25, multiple pore defects are detected on the No. 5 specimen. (5)

Anatomical metallographic observation

After cutting and polishing the defective specimen to the depth of the defect, we analyzed the shape, feature, size, and other features of the defect with a scanning microscope. The morphology of typical defects is shown in Fig. 5.26. As shown in the scan photos of the above-mentioned defective specimens, the shape of the defect on No. 1 specimen is irregular, which may be a non-fusion defect; No. 2 specimen has many small pores on the surface; No. 3 specimen has a pore defect.

5.1 Ultrasonic Inspection Technology for EBWD-Fabricated …

203

(a) Non-fusion

(b) Strip-like non-fusion defects

(c) Dense pores

Fig. 5.23 No. 2 specimen

(6)

Comparison of different inspection results

We used the above-mentioned inspection methods to test the specimens in Table 5.8, and the detected defects are shown in Table 5.9. According to the analysis in Table 5.9, the ultrasonic detection method has higher inspection sensitivity for EBWD-fabricated materials, especially in the inspection of thick parts, but it is manifested as an overall defect at a certain depth, and it cannot indicate the nature of the defect. When there are defects at different depths of the same position, the defects at different depths cannot be clearly distinguished. The X-ray inspection is suitable for parts with a small thickness. In this test, it can properly detect the pore defects on the specimens with a thickness smaller than 13 mm, but the inspection result is not well for the specimens with a thickness larger than 20 mm. In addition, this method cannot properly detect non-fusion defects with a smaller thickness along the transillumination direction. The fluorescence penetrant inspection can well detect the surface defects of EBWD-fabricated materials. The CT inspection has high sensitivity and can accurately detect the positions and sizes of defects, and manifest the nature of defects to a certain extent. However, the CT inspection is time-consuming and costly, making it not suitable for large parts. To sum up, when selecting the inspection methods, we shall comprehensively analyze the advantages and disadvantages of various non-destructive inspection

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

(a) Vertical to Z direction (1.15mm)

(b) Parallel with Z direction, vertical to X direction (1.15mm)

(c) Parallel with Z direction, vertical to Y direction (1.08mm)

Fig. 5.24 No. 3 specimen

Fig. 5.25 No. 5 specimen

methods, fully consider the size, deposition direction, defect nature, defect location and inspection sensitivity requirements of the inspected part, and use all non-destructive inspection methods in different combinations.

5.1 Ultrasonic Inspection Technology for EBWD-Fabricated …

205

No.1 specimen

No.2 specimen

No.3 specimen

Fig. 5.26 Morphology of defects on different specimens

5.1.5 Inspection of Actual Parts Fabricated by EBWD (1)

Before hot isostatic pressing

The ultrasonic inspection is performed on typical parts. The inspection surface and the sound beam incident direction are shown in Fig. 5.27. The inspection results are shown in Figs. 5.28, 5.29, 5.30 and 5.31 and Tables 5.10, 5.11, 5.12 and 5.13. As shown in the inspection result mentioned above, the number of detected defects along the deposition direction is much greater than that vertical to the deposition

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Table 5.9 Comparison of defects detected by different inspection methods Specimen Specimen Ultrasonic Result of Result of Result of number thickness equivalent size X-ray fluorescence CT /mm inspection penetrant inspection inspection

Result of metallographic inspection

1

21

Non-fusion 7.5 mm × 4.3 mm

2

20

Not detected

Non-fusion on the surface

–

Not detected

Surface pores

Pore, Multiple pores, Non-fusion Non-fusion

1.5 mm

1.34 mm 3

8.5

Pore, with Surface a size of pores 1.2 mm

Pore

A single large hole of 1.17 mm

Pore, with Surface a size of pores 1.0 mm Poor fusion

–

–

Pore, with Surface a size of pores 0.4 mm

Pore

–

Pore, with Surface a size of pores 0.5 mm

–

–

Pore, with Surface a size of pores 1.2 mm

–

–

1.13 mm 4

13

1.38 mm 5

10

6

12

7

16.5

1.38 mm

1.06 mm

1.34 mm

Fig. 5.27 Schematic diagram of the specimen (thickness: 54 mm)

5.1 Ultrasonic Inspection Technology for EBWD-Fabricated …

207

Fig. 5.28 Inspection result when the sound beam is vertical to the deposition direction (front)

Fig. 5.29 Inspection result when the sound beam is vertical to the deposition direction (reverse)

Fig. 5.30 Inspection result when the sound beam is parallel with the deposition direction (front)

Fig. 5.31 Inspection result when the sound beam is parallel with the deposition direction (reverse)

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Table 5.10 Inspection result when the sound beam is vertical to the deposition direction (front)

Table 5.11 Inspection result when the sound beam is vertical to the deposition direction (reverse)

Table 5.12 Inspection result when the sound beam is parallel with the deposition direction (front)

Table 5.13 Inspection result when the sound beam is parallel with the deposition direction (reverse)

No

Buried depth of defect

Equivalent of defect

1

25

0.8–4 dB

2

20

0.8–9 dB

3

38

0.8–8.5 dB

4

9

0.8–1.5 dB

5

5

0.8–1.5 dB

No

Buried depth of defect

Equivalent of defect

1

12

0.8 + 4 dB

2

26

0.8 + 4 dB

3

6

0.8–3.5 dB

4

33

0.8

5

7

0.8–6.5 dB

6

5

0.8 + 4.5 dB

7

5

0.8 + 0.5 dB

No

Buried depth of defect

Equivalent of defect

1

28

0.8 + 5.5 dB

2

29

0.8 + 5.5 dB

3

28

0.8 + 6 dB

4

29

0.8 + 9.5 dB

5

9

0.8 + 7.5 dB

No

Buried depth of defect

Equivalent of defect

1

34

0.8 + 5 dB

2

31

0.8 + 3 dB

3

31

0.8 + 16.5 dB

4

30

0.8 + 5.5 dB

5

30

0.8 + 4.5 dB

direction, so is the defect size. This also proves that the optimal inspection direction is the direction along the deposition direction. (2)

After hot isostatic pressing

The inspection result of parts along the deposition direction after hot isostatic pressing is shown in Fig. 5.32.

5.1 Ultrasonic Inspection Technology for EBWD-Fabricated …

209

Inspection result on the front side

Inspection result on the back side

Fig. 5.32 Inspection result after hot isostatic pressing when the sound beam is parallel with the deposition direction

After comparison, it was found that after hot isostatic pressing, no obvious abnormality was found inside the parts under the same sensitivity (an equivalent of 0.8 mm). It proves that hot isostatic pressing can greatly improve the internal quality of parts. We reach the following conclusions: (1)

(2)

(3)

The EBWD-fabricated TC4 material has obvious directionality. The sound velocity, attenuation and metallographic structure of different deposition directions and different positions in the same deposition direction may vary significantly. The inspection sensitivity of 0.8 mm required on current typical parts can be met in three directions. Among them, the inspection sensitivity and signal-tonoise ratio are the highest when the sound beam is incident along the deposition direction. In actual inspection, the water immersion and focusing methods is preferred, while the contact method is considered for the inspection of special parts where the water immersion method is not working. As for the TC4 material fabricated by EBWD, the internal defects are mainly pores and non-fusion defects that can be detected to a certain extent by ultrasonic, X-ray, CT, and fluorescence penetrant inspection methods. The ultrasonic inspection method is less limited by the thickness of materials, but has limitations in determining the nature of defects and the resolution of longitudinal inspection. The X-ray inspection method is limited by the thickness of the tested part, and the inspection sensitivity is lowered as the thickness increases, and it cannot properly detect the non-fusion defects with small openings. The fluorescence penetrant inspection method has high sensitivity and is suitable

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

for detecting surface defects. The CT inspection method also features high sensitivity, sound resolution, and the ability to determine defect nature, but it is time-consuming and costly, and is only suitable for small parts.

5.2 Non-Destructive Inspection Method and Defect Determination of A-100 Steel Fabricated by EBWD 5.2.1 Research on the Ultrasonic Inspection of A-100 Steel Fabricated by EBWD (1)

Research Technology Scheme of A-100 Steel Fabricated by EBWD

We performed the ultrasonic inspection test on the A-100 steel specimens fabricated by EBWD to determine the ultrasonic detectability of A-100 steel. We used the 5 MHz flat probe (MATEC) and the USIP40 equipment to measure the gain value of the specimens when the bottom wave in different directions reaches 80% of the full-screen wave height, and compare the gain value of bottom wave with a deposit of the same thickness, so as to evaluate the sound attenuation characteristics of the material. Since the size of the specimen is 150 × 150 × 15 mm, the sound beam can only be incident parallel to the deposition direction. The inspection uses the HGE probe for detecting defects and the 5 MHz water-immersed flat probe for detecting bottom wave loss. The test results are shown in Fig. 5.33. Fig. 5.33 Ultrasonic C-scan image of A-100 steel

5.2 Non-Destructive Inspection Method and Defect Determination …

211

The maximum equivalent of abnormal display in the figure is 0.8 + 8 dB, and the buried depth is randomly distributed within the depth range. The noise level of A-100 steel and the waveform of typical abnormality display are shown in Fig. 5.34. As shown in the test results above, the noise level of A-100 steel does not exceed 5% of the full screen of the flaw detector (as shown in Fig. 5.34a) when the sensitivity is 0.8 mm, and the scanning shows even bottom wave loss (as shown in Fig. 5.35). The 10 MHz focusing probe can effectively find abnormality display in the material (as shown in Fig. 5.34b), proving that this material features sound ultrasonic detectability. (2)

Manufacturing of A-100 steel contrast deposit fabricated by EBWD

We designed the corresponding contrast deposits according to the thickness of typical A-100 steel parts, and manufactured deposits with flat-bottomed steps and a hole diameter of 0.8 mm, as shown in Fig. 5.36.

(a) Noise level of A-100 steel

(b) Waveform of typical abnormality display

Fig. 5.34 Noise level of A-100 steel and waveform of typical abnormality display

Fig. 5.35 Result of bottom wave loss scanning of A-100 steel

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Fig. 5.36 A-100 deposits with flat-bottomed holes fabricated by EBWD (Z-direction inspection)

(a) Buried depth: 3mm;

(b) Buried depth: 10mm; (c) Buried depth: 20mm; (d) A buried depth of 30mm

Fig. 5.37 Overlay photo of flat-bottomed holes

We performed the overlay inspection on the deposit. The result (as shown in Fig. 5.37) shows that all flat-bottomed holes have smooth walls with no obvious chamfers at the hole bottom. The pore diameter basically meets the requirements. The distance-amplitude curve of the manufactured deposit is shown in Fig. 5.38. According to the distance-amplitude curve shown in Fig. 5.38, we can see that the amplitude distribution of the flat-bottomed hole does not completely conform to the sound field distribution of the flat probe. Considering the test result mentioned

Fig. 5.38 Distance-amplitude curve of EBWD-fabricated A-100 deposit

5.2 Non-Destructive Inspection Method and Defect Determination …

213

above and after analysis, we believe that this is caused by the uneven microstructure of the A-100 material fabricated by EBWD.

5.2.2 Research on the Magnetic Particle Testing of EBWD-Fabricated A-100 Steel (1)

Magnetic particle testing technology

The inspection equipment is the class 2000 AC and DC magnetic particle flaw detector; the inspection method is the continuous wetting; the magnetization method is the DC longitudinal magnetization; the inspection material is the fluorescent oil suspension; and the magnetization current value is 10A. The display is clear by using the sensitivity 30/100 specimen, as shown in Fig. 5.39. A bar-shape penetrating display was found when the specimen is inspected, as shown in Fig. 5.40. The above test results show that magnetic particle inspection can effectively find surface defects of parts. (2)

Measurement of magnetic characteristics

Under normal circumstances, the magnetic particle inspection technology depends on the magnetic characteristics of the material (maximum magnetic induction intensity Fig. 5.39 Display of sensitivity specimen

Fig. 5.40 Display of penetrating magnetic traces

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Bs, residual magnetic field intensity Br, coercive force Hc, maximum permeability µm). Therefore, it is very important to master the magnetic characteristics of A100 steel in the final heat treatment state, and it is particularly important for the formulation of test parameters and processes. According to the requirements of Q/6S 2059–2005 Measuring Method of Hysteresis Loop for Magnetic Particle Inspection, we processed the A-100 steel in different heat treatment states into circular specimens. Figure 5.41 shows the circular specimen of A-100 steel. The test equipment is shown in Fig. 5.42, which is a hysteresis loop test system. The research team measured the magnetic properties of the A-100 steel in the final heat treatment state, and the magnetization curve is shown in Fig. 5.43.

Fig. 5.41 Circular specimen of A-100 steel

Fig. 5.42 Hysteresis loop test system

5.2 Non-Destructive Inspection Method and Defect Determination …

215

Fig. 5.43 Magnetization curve of A-100 steel fabricated by EBWD in the final heat treatment state

After the experiments and research, we finally established the inspection method and technological parameters. Regarding other conventional specifications and technologies of magnetic particle inspection, we refer to the content in HB/Z 72 to ensure the completeness and comprehensiveness of this standard. (3)

Formulation of standard technological specifications

(1)

Specifications of circumferential magnetization The basic calculation formula of the electromagnetic field is: H=

I 2πr

Its magnetizing current is: I = 2πr H = π D H where: I means current, and the unit is A; r means workpiece radius, and the unit is mm; D means workpiece diameter, and the unit is mm; H means tangential magnetic field strength, and the unit is kA/m.

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

According to related textbooks, the standard magnetization specification adopts the magnetic induction intensity to be close to saturation, which is about 80% to 90% of the saturated magnetic induction intensity. The strictly regulated magnetization field is selected in the basic saturation region, which is about 90% or more of the saturated magnetic induction intensity. Based on this, we can calculate the circumferential magnetization specification, namely: Standard magnetization specification: I = (22 ~ 25)D; Strict magnetization specification: I = (25 ~ 32)D. (2)

Longitudinal magnetization specification

The longitudinal magnetization specification is subject to the regulations in HB/Z 72. By analyzing the magnetic characteristics of the final heat treatment state of A-100 steel parts fabricated by EBWD, we finally compiled the magnetic particle inspection method of A-100 steel parts fabricated by EBWD. The standard is more pertinent, providing a basis for the magnetic particle inspection of massively produced A100 steel parts fabricated by EBWD in the future, and offering a reference for the evaluation of abnormal magnetic traces identified in the magnetic particle inspection of other structural steel.

5.2.3 X-Ray Inspection of A-100 Steel Fabricated by EBWD To study the X-ray inspection, we selected four A-100 steel specimens as the test object, and a 0.8 mm × 5 mm flat-bottomed hole was added on each specimen, as shown in Fig. 5.44. We performed the X-ray inspection on the specimens according to HB/Z60 X-ray Radiographic Inspection. The flat-bottomed holes are

Fig. 5.44 Appearance of A-100 steel specimen

5.2 Non-Destructive Inspection Method and Defect Determination …

217

Fig. 5.45 Schematic diagram of transillumination layout and method

clearly visible on the film, and the blackness and sensitivity of the film meet the relevant requirements of HB/Z60. (1)

Transillumination of specimen

The four specimens were transilluminated by an X-ray machine. The transillumination layout and method are shown in Fig. 5.45. The specific transillumination parameters are shown in Table 5.14. (2)

Inspection results

The transilluminated film is processed to obtain a qualified film with the sensitivity and blackness meeting the relevant requirements of HB/Z60-96 as measured by the image quality indicator, as seen in Table 5.15. After observing the film, we see that the flat-bottomed holes in the four specimens are all clearly visible on the image, as shown in Fig. 5.46. From the above test results, it can be concluded that the contrast sensitivity of X-ray inspection can reach 1.6% in the range of 25 mm, which is in line with the level-B (advanced) image quality sensitivity in HB/Z60-96. Table 5.14 Transillumination parameters Specimen

Focal Length /m

Transillumination angle

Voltage/V

Exposure

25 mm thick

1.5

⊥

250

30 mA·min

20 mm thick

1.5

⊥

220

30 mA·min

15 mm thick

1.5

⊥

190

30 mA·min

8 mm thick

1.5

⊥

155

30 mA·min

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Table 5.15 Film quality parameters Specimen

Requirements on film blackness

Film blackness

Target wire number of image quality indicator

Actual wire number of image quality indicator

Wire diameter of image quality indicator (Mm)

Contrast sensitivity

25 mm thick

1.5 ~ 4.0

1.57

12

12

0.25

1%

20 mm thick

1.5 ~ 4.0

1.69

12

13

0.20

1%

15 mm thick

1.5 ~ 4.0

1.90

13

13

0.20

1.3%

8 mm thick 1.5 ~ 4.0

6.01

15

15

0.125

1.6%

Fig. 5.46 Transillumination film of specimen

5.2 Non-Destructive Inspection Method and Defect Determination …

219

Fig. 5.47 Design diagram of the special scanning device for EBWD-fabricated parts

5.2.4 Ultrasonic Automatic Scanning and Evaluation Technology for Typical Parts Fabricated by EBWD In order to realize the ultrasonic automatic scanning and evaluation of EBWDfabricated parts, first, we designed a special scanning device for EBWD-fabricated parts, which consists of a water tank, a movable supporting frame, and a three-axis scanner, based on the shape and dimension of the typical parts. The design diagram of the device is shown in Fig. 5.47. Its single scanning range can reach 500 × 500 × 300 mm (X × Y × Z). The movable supporting frame allows the three-axis scanner to move in longitudinal and transverse directions of the water tank, so as to realize the selection and switch among different parts of the inspected part and avoid invalid scanning areas for fast and integral inspection. In view of the commonly seen electrical noise interference of the ultrasonic inspection system, we embarked on the selection of motor drivers, and developed an effective solution to ensure the inspection sensitivity via the reasonable selection of motor drives and control devices. We programmed CScan Pro, a special control software for C-scan, which can realize the functions such as workpiece inspection, data access, image processing, etc. Also, the system also integrates the defect parameter measurement module based on the C-scan image, which can measure the linear size and area of the defect, and complete summarization, query, and other works, thereby satisfying the requirements for defect evaluation and realizing automatic ultrasound scanning imaging and evaluation. The main interface of the system software is shown in Fig. 5.48.

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5 Non-Destructive Inspection of EBWD-Fabricated Parts

Fig. 5.48 Main interface of the scanning control software

Based on this, we developed an ultrasonic automatic scanning and evaluation system for large-sized integral structure fabricated by high energy beam wire deposition prototyping technology (as shown in Fig. 5.49). We also conducted verification tests including plane scanning, artificial injury inspection, natural defect detection, etc. The result shows that the ultrasonic automatic scanning and evaluation system we designed and produced has reliable imaging results, high positioning and measurement accuracy, and satisfies the technical indicators of scanning resolution of 0.1 mm and 256-level color display. At present, this scanning and evaluation system has been initially applied in the ultrasonic inspection of many EBWD products to meet the requirements for the inspection sensitivity.

Fig. 5.49 Ultrasonic automatic scanning and evaluation system for EBWD-fabricated parts

Chapter 6

Fundamentals of EBWD Manufacturing of TC18 Titanium Alloy

Abstract This chapter introduces the fundamentals of EBWD manufacturing of TC18 titanium alloy. It’s consisted of microstructure characteristics of both deposited state and heat-treated state and properties control. At the same time, typical mechanical properties such as the static property, high cycle fatigue property, corrosion fatigue property, etc. have been introduced in this chapter.

TC18 is a high-alloyed high-strength titanium alloy. The corresponding alloy grade on the globe is BT22. It is a high-strength titanium alloy developed by the former Soviet Union in the late 1960s, and its nominal composition is Ti-5Al-5Mo-5 V-1Cr1Fe. As for the chemical compositions specified in 51–2002 the Technical Conditions for Aviation TC18 Titanium Alloy Forgings, please refer to Table 6.1 Chemical compositions specified in 51–2002 the Technical Conditions for Aviation TC18 Titanium Alloy Forgings. TC18 is a transitional α + β titanium alloy with critical concentration components. Its β-stabilizing element content is between the α + β two-phase alloy and the β alloy. It can also be called metastable near-β titanium alloy. The conditional stability coefficient of β phase Kβ = 1.173. That is to say, the martensitic microstructure cannot be obtained by quenching above the temperature of β → α + β phase transition. Therefore, this alloy has a better heat treatment strengthening effect and greater hardenability. The hardening depth can reach 250 mm. Since the TC18 titanium alloy has both the property characteristics of α + β titanium alloy and β titanium alloy, it features the advantages of sound thermal process ductility and high hardenability, which makes it especially suitable for manufacturing large load-bearing structural parts such as aircraft fuselage and landing gear. If we use the TC18 titanium alloy to replace TC4 or high-strength steel in the aircraft structure, it can reduce the weight of the structure by 15–20%.

© National Defense Industry Press 2022 S. Gong et al., Electron Beam Wire Deposition Technology and Its Application, Additive Manufacturing Technology, https://doi.org/10.1007/978-981-19-0759-3_6

221

TC18

Grade

Mo

4.0 ~ 5.5

V 4.0 ~ 5.5

Cr 0.5 ~ 1.5

Fe 0.5 ~ 1.5

0.1

0.15

Si 0.3

Zr

0.18

O

0.05

N

C

4.4 ~ 5.7

Al

Ti

Base

Impurity elements, not greater than

Main compositions

Chemical composition, wt./%

Table 6.1 Chemical compositions specified in 51–2002 the Technical Conditions for Aviation TC18 Titanium Alloy Forgings

H 0.015

0.1

Single

0.3

Sum

Other impurities ≤

222 6 Fundamentals of EBWD Manufacturing …

6.1 Typical Microstructure Characteristics of TC18 …

Continuous prototyping

223

Discontinuous prototyping

Fig. 6.1 Macrostructure of two deposited states of TC18 titanium alloy fabricated by EBWD

6.1 Typical Microstructure Characteristics of TC18 Titanium Alloy Fabricated by EBWD 6.1.1 Microstructure Characteristics of Deposited State Figure 6.1 shows the macrostructure of two deposited states (continuous prototyping and discontinuous prototyping) of TC18 titanium alloy fabricated by EBWD. The macrostructure of deposited state consists of fine equiaxed crystals in the heataffected zone of the matrix, fine columnar crystals and coarse columnar crystals in the initial deposition stage. The “three zone and two line” structure in TC4 alloy is not found in the macrostructure of TC18 fabricated by EBWD, but the regional transition characteristics caused by discontinuity can be identified. This may be related to the low α + β/β transition point of TC18 titanium alloy and the small change of the α phase under the instantaneous thermal shock during the deposition. Figure 6.2 shows the microstructure of the deposited TC18 titanium alloy fabricated by EBWD. It can be seen that the fine needle-like α phases are dispersed on the β matrix of TC18 titanium alloy [1].

6.1.2 Characteristics of the Heat-Treated Microstructure Figure 6.3 shows the microstructure of the HIP state after heat preservation at 900 °C for 3 h (900 °C/3 h/HIP). The macrostructure shows no significant change after hot isostatic pressing. When the temperature drops below the β/α + β transition point,

224

6 Fundamentals of EBWD Manufacturing …

Fig. 6.2 Microstructure of the deposited TC18 titanium alloy fabricated by EBWD

Macrostructure

Microstructure

Fig. 6.3 Microstructure of heat-treated EBWD-fabricated TC18 at 900°C/3 h/HIP

the α phase separates out of the original β grains. The α phase precipitated at high temperature grows during temperature dropping, forming mixed structures consisting of rod-like, strip-like, and needle-like α phases and residual β phases on the matrix. Figure 6.4 shows the microstructure of EBWD-fabricated TC18 titanium alloy in different heat treatment states after going through the 900°C/3 h/120 MPa hot isostatic pressing. Figures 6.4a ~ d show the heat-treated structures at 740 °C, 770 °C, 800 °C, and 830 °C respectively. It can be seen that the microstructure is composed of the β matrix and the rod-shaped α phase embedded on it. When the temperature is low, the aspect ratio of the rod-shaped α phase is large, and the volume fraction is high. As the temperature increases, the aspect ratio of the rod-shaped α phase decreases, and the number decreases. The rod-shaped α phase is obviously coarsened, the aspect ratio is further reduced, but the volume fraction increases after heat preservation at 830 °C for 1 h, furnace cooling to 750 °C and heat preservation for 2 h, and then air-cooling (AC) to the room temperature, as shown in Fig. 6.4d, f. Figure 6.5 Impact of different heat treatment temperature on the microstructure of EBWD-fabricated TC18 titanium alloy (temperature of the first heat treatment is 830°C) shows the influence of the temperature change of the second heat treatment

6.1 Typical Microstructure Characteristics of TC18 …

225

Fig. 6.4 Microstructure of different heat treatment states after 900°C/3 h/HIP

on the microstructure of the TC18 titanium alloy fabricated by EBWD under the condition of triple heat treatments. Figure 6.5a-d shows the heat-treated microstructure under one-hour heat preservation at 725 °C, 740 °C, 755 °C, and 770 °C. It can be seen that there are two forms of α phases on the β matrix, which are the coarse primary α phase and the fine, dispersed secondary α phase. During the process of heat preservation at 830°C for 1 h and furnace-cooling to 725 °C ~ 770 °C, theoretically, as the temperature of the second heat treatment decreases, the β → α

226

6 Fundamentals of EBWD Manufacturing …

Fig. 6.5 Impact of different heat treatment temperature on the microstructure of EBWD-fabricated TC18 titanium alloy (temperature of the first heat treatment is 830 °C)

phase transition becomes more complete, the volume fraction of the primary α phase increases, the force that drives the transition of the fine and dispersed secondary α phase produced by the third heat treatment decreases and the number is reduced. As shown in Fig. 6.5 Impact of different heat treatment temperature on the microstructure of EBWD-fabricated TC18 titanium alloy (temperature of the first heat treatment is 830°C)(a)-(d), the secondary α phase increases in both size and number as the temperature of the second heat treatment increases. Figure 6.6 shows the impact of low-temperature heat treatment of EBWDfabricated TC18 titanium alloy on its microstructure. Figure 6.6 (a)-(d) show the heat-treated microstructures at the β/α + β transition point obtained after going through heat preservation for 40 min, then water quenching, then heat preservation at 500°C, 550°C, 600°C, 650°C for 4 h and then air cooling. It can be seen that there is only one state of α phase existing on the β matrix, that is the fine and dispersed strip-like α phase. As the heat treatment temperature increases, the size of the α phase increases, and the volume fraction increases. The shape, dimension and size of this α phase are similar to those of the secondary α phase produced by the last low temperature annealing out of the triple heat treatments recommended for TC18. The difference is that there is no primary α phase in Fig. 6.6, and the metastable β phase

6.1 Typical Microstructure Characteristics of TC18 …

227

is completely transited into the low temperature α phase, resulting in a larger aspect ratio, a larger quantity, and a higher volume fraction.

6.2 Properties Control of TC18 Titanium Alloy Fabricated by EBWD Figure 6.7a shows the microhardness of the β matrix in the TC18 fabricated by EBWD after single annealing, and the corresponding microstructure is shown in Fig. 6.6a-d. It can be seen that the microhardness of the β matrix shows a decreasing trend as the temperature rises within the heat treatment temperature range of 700 to 830°C. After measuring the indentation size of the microhardness tester, we found that the diagonal distance of the indentation is about 30 μm. After annealing at a lower temperature, a large number of primary α phases will exist in the β matrix, so that the primary α phase will participate in or constrain the deformation of the β matrix

Fig. 6.6 Impact of low-temperature heat treatment on the microstructure of EBWD-fabricated TC18 titanium alloy

228

6 Fundamentals of EBWD Manufacturing …

when the gap between primary α phases (i.e. the area where metastable β phases exist) is close to or smaller than the indentation size, resulting in the high measured value of microhardness. Therefore, the microhardness of the specimen annealed at the temperature below 800°C as shown in Fig. 6.7a should be close to the apparent hardness of the experimental material rather than the hardness of the β matrix. The change of actual hardness is close to the dotted line shown in Fig. 6.7a. Since the difference between the maximum microhardness and the minimum microhardness as shown in Fig. 6.7a is within 15 and the measurement error is taken into account, it is basically believed that the volume fraction of the primary α phase can barely affect the microhardness of the metastable β matrix under the single annealing condition. Figure 6.7b shows the low aging temperature of 830°C/1 h water quenching on the microhardness of the original β matrix region on the EBWD-fabricated TC18. The corresponding microstructure is shown in Fig. 6.5. Impact of different heat treatment temperature on the microstructure of EBWD-fabricated TC18 titanium alloy (temperature of the first heat treatment is 830°C). Under this solid solution and aging condition, a large number of fine low-temperature α phases are precipitated from the original metastable β grains. It can be seen from the figure that as the annealing temperature increases, the microhardness decreases. Under the condition of recommended triple heat treatments, the temperature of the second intermediate-temperature annealing has little impact on the microhardness of the original β matrix region on the EBWD-fabricated TC18; the microhardness is low before the third low temperature annealing, and increases significantly after the low temperature annealing; a singularity appears near 755°C, which may be caused by experimental error, as shown in Fig. 6.7c. In general, it is considered that the temperature of the second intermediate temperature annealing under the condition of recommended triple heat treatment has a slight impact on the microhardness of the original β matrix region on the EBWD-fabricated TC18. The heat treatment temperature recommended for TC18 titanium alloy is triple heat treatments. The heat treatment technology recommended by the technical standard is 820 to 850°C, heat preservation for 1 to 3 h, furnace cooling (FC) to 740 to 760°C, heat preservation for 1 to 3 h, AC + 500 to 600°C/2 to 6 h, AC. They are called high temperature annealing, intermediate temperature annealing and low temperature annealing as per the temperature. In view of the material characteristics of TC18 titanium alloy, the influence of heat treatment technologies on the mechanical properties fall into the following two situations: (1)

The temperature of the high temperature annealing is above the α + β/β transition point. If the effect of annealing temperature on the growth of β grains is not considered, the temperature of high temperature annealing has no significant impact on the mechanical properties of the EBWD-fabricated TC18. If other conditions remain unchanged, the rising temperature of intermediate temperature annealing will present an increasing trend of strength and a decreasing trend of ductility. The increasing temperature of low-temperature annealing will present a significantly decreasing trend of strength and a significantly increasing trend of ductility.

6.2 Properties Control of TC18 Titanium Alloy Fabricated by EBWD

229

Fig. 6.7 Impact of the heat treatment technology on the microhardness of the EBWD-fabricated TC18, where the first heat treatment temperature is 830 °C

(2)

The temperature of high temperature annealing is in the α + β two-phase region. If other conditions are not changed, the rising temperature of high temperature annealing will present an increasing trend of strength and a decreasing trend of ductility; the rising temperature of intermediate temperature annealing will present an increasing trend of strength and a decreasing trend of ductility; the rising temperature of low temperature annealing will present a decreasing trend of strength and an increasing trend of ductility. The range of change in strength and ductility depends on the matching of high, intermediate, and low temperatures. When the temperatures of both the high-temperature and the intermediate-temperature annealings reach their upper limit, the strength and ductility show the greatest change. When the temperatures of both the hightemperature and the intermediate-temperature annealings reach their lower limit, the strength and ductility show the slightest change. Table 6.2 shows how the tensile strength of the EBWD-fabricated TC18 titanium

230

6 Fundamentals of EBWD Manufacturing …

Table 6.2 Tensile strength of TC18 titanium alloy fabricated by EBWD under several typical triple annealing temperature combinations, MPa Combination of high and intermediate annealing temperature/°C

Temperature of low-temperature annealing/°C 550

600

620

635

820 → 750

1100 1100

–

1030 1050

–

830 → 760

–

1110 1120

–

–

830 → 790

–

1140 1130

–

–

850 → 800

–

–

1200 1181

1095 1112

850 → 790

–

–

–

1064 1080

850 → 780

–

–

–

1029 1027

850 → 740

–

987 1007

–

–

alloy is changed with different heat treatment parameters under the triple annealing conditions. As shown in the table, when the high-temperature and the intermediatetemperature annealings are at the same temperature, the rising temperature of the low-temperature annealing reduces the strength. When the temperatures of hightemperature and low-temperature annealings remain unchanged, the temperature of the intermediate-temperature annealing shows the same trend as the strength. When the temperature of the low-temperature annealing is unchanged, the increase of both temperatures in high-temperature and low-temperature annealings can significantly increase the material strength. Figure 6.8 shows the relationship between the tensile and yield strength of the EBWD-fabricated TC18 titanium alloy and the first annealing temperature under dual annealing conditions. The first heat treatment temperature is 740 °C/800 °C/840 °C, heat preservation for 1 h, followed by air cooling; the second heat treatment temperature is 550°C, heat preservation for 4 h, followed by air cooling. If the second annealing temperature is unchanged, the tensile and yield strength will increase significantly with the increase of the first annealing temperature. The strength anisotropy is not obvious under the dual annealing conditions, and the tensile and yield strength is virtually the same in two directions (Y and Z directions, Z direction is the direction of the deposition height) perpendicular to the direction of wire translation. Figure 6.9 shows the relationship between the tensile and yield strength of the EBWD-fabricated TC18 titanium alloy and the second annealing temperature under

6.2 Properties Control of TC18 Titanium Alloy Fabricated by EBWD

231

Fig. 6.8 Influence of the first annealing temperature on the tensile properties under dual annealing conditions

Fig. 6.9 Influence of the second annealing temperature on the tensile properties

dual annealing conditions. The dual annealing system: the first heat treatment temperature is 830 °C, heat preservation for 1 h, and air cooling remains unchanged; the second heat treatment temperature is 550 °C, 600 °C, and 650 °C, heat preservation for 4 h, air cooling remains unchanged. It can be seen that if other conditions remain unchanged, the tensile and yield strength will decrease and the ductility will increase as the second annealing temperature increases.

232

6 Fundamentals of EBWD Manufacturing …

Fig. 6.10 Influence of the heat preservation time of second annealing on the tensile properties

Figure 6.10 shows the relationship between the tensile and yield strength of the EBWD-fabricated TC18 titanium alloy and the heat preservation time of the second annealing under dual annealing conditions. The dual annealing system: the first annealing temperature is 830 °C, heat preservation for 1 h, followed by air cooling; the second annealing temperature is 600 °C, heat preservation for 3 h, 24 h and 100 h, followed by air cooling. It can be seen that if other conditions remain unchanged, the tensile and yield strength shows a slowly decreasing trend as the second annealing time is extended. Figure 6.11 Influence of cooling rate on tensile properties after the first annealing shows the relationship between the tensile and yield strength of the EBWD-fabricated TC18 titanium alloy and the cooling rate after the first annealing under dual annealing conditions. The dual annealing system: the first annealing temperature is 830 °C, heat preservation for 1 h, followed by air cooling (AC), oil quenching (OQ), and wind quenching (WQ); the second annealing temperature is Fig. 6.11 Influence of cooling rate on tensile properties after the first annealing

6.2 Properties Control of TC18 Titanium Alloy Fabricated by EBWD

233

600 °C, heat preservation for 4 h, followed by air cooling. It can be seen from the figure that the influence of the cooling rate on the tensile and yield strength after the first annealing is negligible.

6.3 Test of Typical Mechanical Properties of TC18 Standard Parts Fabricated by EBWD 6.3.1 Static Property The mean values of tensile, compression, impact, torsion and durability properties of TC18 titanium alloy material fabricated by EBWD after heat treatment are shown in Table 6.3. Table 6.9 Tensile properties of TC18 titanium alloy fabricated by EBWD after heat exposure (mean). Table 6.3 Room temperature tensile properties of TC18 titanium alloy fabricated by EBWD (mean) Material model

Sample direction

Section Test Tensile Extension Elongation Reduction size temperature/°C strength strength after break of area /mm (Rm/ MPa) (RP0.2 / A/% Z/% MPa)

TC18 X ϕ5 Deposition direction Prototyping Y ϕ5 orientation

Room temperature

1050

982

4.3

12.5

Room temperature

1112

1048

4.8

13.3

Z direction ϕ5

Room temperature

1068

983

10.5

38.3

Table 6.4 Room temperature compression properties of TC18 titanium alloy fabricated by EBWD (mean) Material model

Sample direction

Section size/mm

Sample length L/mm

Test temperature/°C

Elasticity modulus of compression Ec/Gpa

Compression strength RP0.2 /MPa

TC18 Deposition forming

X

ϕ5

50

Room temperature

118

1024

Y

ϕ5

50

Room temperature

121

1073

Z

ϕ5

50

Room temperature

114

1011

234

6 Fundamentals of EBWD Manufacturing …

Table 6.5 Results of room temperature impact test of TC18 titanium alloy fabricated by EBWD (mean) Material Model

Section size of U-Notch /mm

Test temperature /°C

Initial potential Energy energy of tester absorbed by (mean)/ J impact (mean) /J,

Impact toughness Ak /(J/cm2) ), (mean)

TC18 deposition prototyping

10.0 × 8.0

Room temperature

150

26.5

22

6.3.2 High Cycle Fatigue Property The reference standards for high cycle fatigue property testing are specified in HB 5287–96. The fatigue limit is tested by the up-and-down method; the fatigue S–N curve is formulated by the grouping method; the stress ratio is R = -–1, R = 0.06 and R = 0.5; the experimental waveform is a sine wave; the environment is room temperature atmosphere, and the specimen orientation is X direction. The fatigue limit is measured by the paired up-and-down method. We also compiled the axial loading fatigue S–N curve along the X direction in 9 combinations of three stress concentration factors of Kt = 1, Kt = 3, and Kt = 5 and three stress ratios of R = -1, R = 0.06, and R = 0.5, and obtain the fatigue limit based on 107 cycles, as seen in Table 6.10. It can be seen from the table that the fatigue limit of the smooth specimen is far better than the measured value of the forging, but the fatigue property of the notch is lower than that of the forging, reflecting the high notch sensitivity of the EBWD-fabricated TC18 titanium alloy. The fatigue S–N curves of EBWD-fabricated TC18 titanium alloy under various conditions are shown in Fig. 6.12 through Fig. 6.12.

6.3.3 Corrosion Fatigue Property The author studied the fatigue crack growth threshold Kth and the fatigue crack growth rate of EBWD-fabricated TC18 titanium alloy in a salt spray environment. The test adopts the modified WOL specimen whose form and size are shown in (Fig. 6.21). Wherein: v—crack opening displacement, a—crack length, B—specimen thickness, W —specimen width. We fabricated the crack on the specimen before the test whose a0 > 0.2 W according to the standards. After opening the notch with a wire, in order to ensure the point effect of the crack, we fabricated the fatigue crack on the high-frequency fatigue tester to extend the initial crack to the specified length. Stress ratio: R = 0.06, R = 0.3, R = 0.5. Loading form: axial loading, trapezoidal wave. Temperature and environment: room temperature, 3.5% NaCl solution. For the X, Y, and Z directions,

specimen no

X

Material model

TC18 deposition prototyping

Gauge length/mm

50

Nominal size of specimen section /mm

ϕ10 45.2

Shear modulus (G /GPa)

Table 6.6 Result of the tensile torsion test on TC18 titanium alloy fabricated by EBWD

876.0

Torsional strength (τ m/ Mpa)

648.8

Specified non-proportional torsional strength (τ P0.015 / MPa)

806.5

Specified non-proportional torsional strength (τ P0.3 / Mpa)

6.3 Test of Typical Mechanical Properties of TC18 … 235

236

6 Fundamentals of EBWD Manufacturing …

Table 6.7 Three-direction durability of TC18 titanium alloy fabricated by EBWD Load (MPa)

Number of Duration /h samples /pieces

X-direction durability 780

3

>100, >100, >100

800

6

>100, >100, >100, 0.07, 0.05, 0.05

820

5

0.03, 0.03, 0.05, 0.05, >100

840

1

0.03

700

1

>100

720

2

0, >100

740

4

>100, 0, >100, >100

760

3

>100, 0, 0.03

780

4

>100, >100, 0.03, 0.02

800

4

>100, >100, 0, 0.02,

820

3

>100, >100, 0

840

1

>100

700

1

>100

760

5

>100, >100, 0.02, >100, >100

780

3

>100, >100, 0.02

800

7

0.05, >100, >100, >100, 0, >100, 0

820

4

0.05, 0.03, 0, 0.03

Y-direction endurance limit

Z-direction endurance limit

Table 6.8 Fracture toughness of TC18 titanium alloy fabricated by EBWD (mean) Heat treatment

K IC / (MPa · m1/2 ) X–Z direction

Y–Z direction

830°C/1 h, WQ + 635°C/4 h, AC

76.3

55.92

84.7

830°C/1 h, AC + 650°C/4 h, AC

82.3

90.9

126.5

Z-Y direction

and the three stress ratios of 0.06, 0.3, 0.5, the paired up-and-down method is used to measure the threshold value of fatigue crack growth. After the threshold value is determined, we continued with the experiment of measuring the fatigue crack growth rate without disassembling the specimen. The test results of the fatigue crack growth threshold Kth are shown in Table 6.11 Threshold results. Under the conditions of a loading frequency of 0.01 Hz and a stress ratio R of 0.06, the threshold results for the three specimens in X, Y, and Z directions are shown in Table 6.12. As we can see from the data in the table, under the same experimental

6.3 Test of Typical Mechanical Properties of TC18 …

237

Table 6.9 Tensile properties of TC18 titanium alloy fabricated by EBWD after heat exposure (mean) Specimen direction

300°C/500 h Rm /MPa

Rp0.2 /MPa

A/%

Z/%

X direction

946

878

12.5

34.9

Y orientation

949

901

10.5

28.1

Z direction

904

835

19.8

60

X direction

945

889

11.1

43.2

Y orientation

941

897

10.2

28.4

Z direction

905

823

19.2

56.0

300°C/1000 h

Table 6.10 High cycle fatigue limit under various stress levels and stress ratios (MPa)

Fatigue Limit

R = 0.5

R = 0.06

R = -1

Kt = 1

889.82

746.08

528.33

Kt = 3

342

220

115

Kt = 5

197

98

65

Fig. 6.12 Fatigue life curve when Kt = 1, R = 0.5

conditions, the crack growth thresholds in the three specimen orientations are: Xorientation > Z-orientation > Y-orientation. The threshold value of the X-orientation specimen is about 34.5% higher than that of the Y-orientation specimen, and about 23.3% higher than that of the Z-orientation specimen. Considering the influence of different specimen orientations on the crack growth rate, we picked specimens X02, Y01, Z01 for research under the conditions of a

238 Fig. 6.13 Fatigue life curve when Kt = 1, R = 0.06

Fig. 6.14 Fatigue life curve when Kt = 1, R = -1

Fig. 6.15 Fatigue S–N curve when Kt = 3, R = -1

6 Fundamentals of EBWD Manufacturing …

6.3 Test of Typical Mechanical Properties of TC18 … Fig. 6.16 Fatigue S–N curve when Kt = 3, R = 0.06

Fig. 6.17 Fatigue S–N curve when Kt = 3, R = 0.5

Fig. 6.18 Fatigue S–N curve when Kt = 5, R = -1

239

240

6 Fundamentals of EBWD Manufacturing …

Fig. 6.19 Fatigue S–N curve when Kt = 5, R = 0.06

Fig. 6.20 Fatigue S–N curve when Kt = 5, R = 0.5

loading frequency of 0.01 Hz and a stress ratio of 0.06, and the crack growth rate curve and Paris fitting results obtained are shown in Figs. 6.22 and 6.23. It can be seen from the figure that the crack growth rate in the X direction is the fastest at low K ; and the crack growth rate in the Z direction is the√fastest at high K . The stable growth stage of specimen X02 √ is about 31–44 M Pa m; the stable m, and the stable growth stage of growth stage of specimen Y01 is 32–43 M Pa √ specimen Z01 is 35–41 M Pa m. However, the figure shows that the stable growth stage of the Z-orientation specimen is relatively short. When combining with the a-N curve of the specimen Z01, we can find that this is due to the large local instability that occurred in the initial stage of crack growth. √ The curves of specimens X02 and Y01 intersect at K √ = 36M Pa m, as shown growth rate of by point A in the figure. In the event of K < 36M Pa m, the crack√ specimen X02 is greater than Y02; in the event of K > 35.1M Pa m, the crack growth rate of specimen X02 is less than Y02. Similarly, the curves of specimens

6.3 Test of Typical Mechanical Properties of TC18 …

241

atigue S-N curve when Kt=5, R=0.5

Fig. 6.21 Form and size of WOL specimen for corrosion fatigue test Table 6.11 Threshold results Specimen no

Stress ratio R

Threshold K th √ /M Pa m

Subsample standard √ deviation/M Pa m

Number of pairs

Coefficient of variation

X01

0.06

12.14

0.6732

8

0.05546 0.04794

X02

0.06

14.38

0.6894

8

Y01

0.06

9.81

0.2066

6

0.02106

Y02

0.06

9.91

0.2246

6

0.02266 0.02407

Z01

0.06

10.75

0.2588

8

X05

0.3

15.79

0.6409

6

0.04059

X03

0.5

11.66

0.3615

6

0.03100

X04

0.5

13.03

0.3615

6

0.02774

Table 6.12 Threshold results of different specimen orientations √ Specimen no Threshold/M Pa m X direction

X01 X02

14.38

Y orientation

Y01-12

9.81

Y02

9.91

Z01

10.75

Z direction

12.14

√ Mean/M Pa m 13.26 9.86 10.75

242

6 Fundamentals of EBWD Manufacturing …

Fig. 6.22 Original da/dN-K curve

Fig. 6.23 Fitting curve of Paris equation

√ X02 and Z01 intersect at K = 37.7M Pa m, as shown by point B√in the figure. The curves of specimens Y01 and Z01 intersect at K = 40.7M Pa m, as shown by Point C in the figure. In order to study the influence of stress ratio on the threshold of fatigue crack growth, we selected the X-orientation specimen and a frequency of 0.01 Hz, and studied the specimens under three stress ratios R = 0.06, R = 0.3, and R = 0.5. The threshold values of the crack growth are shown in (Table 6.13). According to the data in the table, the threshold value reaches the top, up to 15.79 √ M Pa m, when the stress ratio is 0.3. Overly low and high stress ratios will reduce the threshold value to a certain extent, thereby reducing the crack growth life of the material. In order to study the influence of stress ratio on the crack growth rate, we selected three X-orientation specimens in this research under three different stress ratios of R = 0.06, R = 0.3, and R = 0.5, and obtained the da/dN-K curve of the specimen. We fitted the curve under each stress ratio to the Paris equation to get the curve shown in Fig. 6.26. In the 3.5% NaCl solution, the overall trend of the fitting curve

6.3 Test of Typical Mechanical Properties of TC18 … Table 6.13 Threshold values under different stress ratios

243

Stress ratio (R)

Specimen no

Threshold value/ √ (M Pa m)

Mean/ √ (M Pa m)

0.06

X01

12.14

13.26

X02

14.38

0.3

X03

15.79

15.79

0.5

X04

11.66

12.35

X05

13.03

of the crack growth rate shifts upward as the stress ratio R increases, which means the crack growth rate increases at a higher stress ratio R. The conclusion is basically consistent with general material properties. Meanwhile, as the stress ratio increases, the slope of the da/dN-K curve in the figure is gradually enlarged, which means the fatigue crack growth rate da/dN increases faster at a higher stress ratio as the stress intensity factor range K increases. Apparently, an increase in the stress ratio will significantly accelerate the crack growth rate and reduce the life of parts. In order to reflect the influence of the stress ratio R on the crack growth rate, we fit the curve with the following Walker equation. n  n da n 1− n = k n0 C0 0 (1 − R)m−1 K dN

In the equation n = n 0 + a R b , a, b, k, m are undetermined parameters; C 0 and n0 are the parameters of Paris equation when R = 0. The Walker equation of this experimental material according to the above formula is fitted as: Fig. 6.24 Original da/dN-K curve

244

6 Fundamentals of EBWD Manufacturing …

 1− n   n da = 2.34 × 10−3 1.42 7.97 × 10−6 1.42 [(1 − R)K ]n dN In the equation, n is a function of R: n = 1.42 + 15.04R 1.53 . We drew the curve for the modified Walker equation, as shown by the green line in Fig. 6.25. The curve obtained truly expresses the original data curve. Also, the Wakler formula obtained can be used to predict the crack growth rate curve under other stress ratios. The red line in Fig. 6.26 shows the prediction results of the crack growth rate curve at R = 0.4, R = 0.6, and R = 0.7. The research results above indicate that: (1)

The EBWD-fabricated TC18 titanium alloy can have sound tensile strength after proper heat treatment. The tensile strength of the material is anisotropic.

Fig. 6.25 Curve of the modified Walker equation

Fig. 6.26 Prediction result of the modified Walker equation

6.3 Test of Typical Mechanical Properties of TC18 …

(2)

(3)

245

The room-temperature tensile strength along the deposition directions (X, Y) is close, and the tensile strength along the deposition height direction (Z) is slightly low. The Z-direction ductility is high, but the isotropic ductility and impact toughness are generally lower than those of TC18 forgings. The smooth specimen (Kt = 1) has excellent high-cycle fatigue property, and its fatigue limit is much higher than the measured value of forgings, but the fatigue limit of the notch is significantly lower than that of forgings, reflecting the high notch sensitivity and poor ductility and toughness of EBWD-fabricated materials. Under the condition of salt spray corrosion, the crack growth thresholds in the three specimen orientations are: X-orientation > Z-orientation > Y-orientation. The threshold value of the X-orientation specimen is about 34.5% higher than that of the Y-orientation specimen, and about 23.3% higher than that of the Z-orientation specimen.

6.4 Static and Fatigue Tests on Typical Element Parts of EBWD-Fabricated TC18 6.4.1 Experimental Design (1)

Selection of Specimens

There are two kinds of structures in this specimen. One is the single-hole lug, which mainly bears tensile load and whose stress concentration factor is generally considered to be approximately K t = 3; the other is the I-shaped short beam, which mainly bears bending load and whose stress concentration factor is generally considered to be approximately K t = 1. There are 6 lug specimens, all of which are for static tests. There are 15 short beam specimens, including 6 for static tests and 9 for fatigue tests. The theoretical structure dimensions of the two kinds of specimens are shown in Fig. 6.27. The specimen number and test items are shown in Table 6.14 and 6.15. (2)

Development of typical element parts

The main EBWD manufacturing technologies of short beams and lugs are shown in Fig. 6.31. There are two types of EBWD technologies for typical element parts. One is the double-wire deposition technology, the other is the improve single-wire prototyping technology (Fig. 6.28). The deposition path is shown in (Fig. 6.29): The deposited blanks are processed by hot isostatic pressing (910°C, 150Mpa, 2 h). The heat treatment process for element parts of double-wire high-speed deposition is 830°C/2 h, air-cooled, aging at 620 ~ 650°C/4 h; parts of single-wire prototyping are treated with optimized triple heat treatment. The processing flow of specimens is seen in Fig. 6.30.

246

6 Fundamentals of EBWD Manufacturing …

Table 6.14 Number of lug specimens Specimen category

Specimen number Test

TC18 forging lug

EP-601D-01, 02

Loading form Quantity Structure

Static Stretch

2

EBWD-fabricated EP-601 K-01, 02 lugs (Double wire prototyping technology)

Static

2

EBWD-fabricated EP - No.1, No.2 lugs (Single-wire prototyping technology)

Static

2

Fig. 6.27 Schematic diagram on the structure of typical element parts

Single hole lug 20 mm × 60 mm × 190 mm

6.4 Static and Fatigue Tests on Typical Element Parts of EBWD-Fabricated TC18

247

Table 6.15 Number of short beam specimens Specimen category

Specimen number

Test

Loading form

Quantity

Structure

Forging short beam

DL-601D-01, 05

Static

DL-601D-02, 03, 04

Fatigue

Four-point bending

2

I-shaped short beam 60 mm × 50 mm × 520 mm

Short beam fabricated by EBWD (Double wire prototyping technology)

DL-601 K-01, 03

Static

2

DL-601 K-02, 04, 05

Fatigue

3

Short beam fabricated by EBWD (Single wire prototyping technology)

DL - No.2, No.3

Static

2

DL - No.1, No.4, No.5

Fatigue

3

3

Fig. 6.28 Manufacturing flow of short beams and lugs

Fig. 6.29 Prototyping path and direction

(3)

Sampling performance of specimens

Two deposition prototyping technologies are used to make one short beam anatomical part respectively. The properties of anatomical parts are seen in Table 6.16. The room temperature tensile strength of short beam anatomical parts in double-wire high-speed deposition technology is 1097 ~ 1118Mpa, and the elongation is low; the strength of single-wire high-speed deposition technology is 1067 ~ 1095 Mpa, which is equivalent to that of forgings (≥1080Mpa). The ductility is significantly improved, and the impact toughness reaches above 25 J/cm2 (forging standard ≥ 20).

248

6 Fundamentals of EBWD Manufacturing …

(a) EBWD fabrication of short beam and lug blanks

(b) Drawing of rough-machined blanks used for non-destructive flaw detection

(c) Drawing of air cooling after heat treatment

(d) Short beams and lugs after being machined

(e) Short beams after surface treatment

(f) Lugs after surface treatment

Fig. 6.30 Manufacturing process of short beam and lug specimens

6.4 Static and Fatigue Tests on Typical Element Parts of EBWD-Fabricated TC18

249

Fig. 6.31 Supporting and loading form of single-lug specimens

Table 6.16 Properties of short beam anatomical parts fabricated by two deposition technologies Deposition Technology

Rm / Mpa Rp0.2 /Mpa A/% Z /% aku2 /(J/cm2)

Properties of Short Beam Anatomical Parts 1097 Fabricated by the Double Wire High-Speed 1091 Deposition Technology 1118 (3.5 kg/h) 1101

1059

1.0

4.0

1045

4.0

12.0

1062

1.0

3.0

1063

0.5

3.0

1097

1060

8.0

26.0

1114

1066

1.0

3.0

Properties of Short Beam Anatomical Parts 1086 Fabricated by the Single-Wire 1087 Medium-Speed Prototyping Technology 1095 (0.75 kg/h) 1082

1024

5.5

13.5

1024

5. 5

8.0

1034

6.5

24.0

1023

6.5

33.0

1080

1012

8.5

30

26.5

1067

1005

8.5

32

33.7

1077

1013

6.0

22

26.5

(4)

Loading form

The supporting and loading form of single-lug specimens are shown in Fig. 6.31. The patch position and number of specimens are shown in Fig. 6.32. The fixture used for the static test of the lug specimen is shown in Fig. 6.33. During the test, a 30 bolt is used to match the lug hole to ensure that the entire lug is evenly loaded on the connecting part. It is connected to the bolt of the specimen through the holes in the lug fixture. The head is fixed to the other end of the lug specimen and loads a certain pre-tightening force to ensure that the rigidity center of the specimen in the vertical direction coincides with the loading center of the tester (Fig. 6.33). The supporting and loading form of the four-point bending test of the short beam specimen is shown in Fig. 6.34. The patch position and number of the specimens are shown in Fig. 6.35. A total of 10 strain gauges are attached to the short beam fatigue specimen. The specific locations of the strain gauges are shown in Fig. 6.36.

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Fig. 6.32 Patch position of single-lug specimens

Fig. 6.33 Lug test fixture

Fig. 6.34 Loading of four-point bending test of short beams

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Fig. 6.35 Patch positions of short beam specimens (32 patches per specimen)

The short beam specimen is placed on a specially designed support, and the static load is applied through the loading fixture. The loading is shown in Fig. 6.37. After the finite element analysis, we know there is a friction effect between the loading point and the short beam, and friction has a greater impact on the loading. Therefore, during the loading process, we applied graphite grease on the fixture loading point and the contact point of the specimen to effectively eliminate the impact of friction on loading.

Fig. 6.36 Positions of strain gauges for fatigue specimens

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Fig. 6.37 Loading of four-point bending

Since a certain degree of friction will be produced between the specimen and the loading point during the fatigue loading process, the grease fails to work in the continuous friction process. We make some improvements to the test fixture on the basis of the static test fixture. The 4 fixed loading points are remade into rotatory loading points, and the graphite grease is applied on loading points to reduce friction, as shown in Fig. 6.38. In the constant-amplitude spectrum test, the two ends of the specimen will warp up and down under the action of fatigue load, and may deviate to one side. In order to prevent deflection, restraint plates are installed on both sides of the test fixture. The distance between the restraint plate and the end of the short beam is 5 mm. The finite element analysis shows that the short beam will not contact the restraint plate. If the short beam deviates to one side and contacts the restraint plate during the test, the test shall be suspended immediately, the short beam shall be repositioned before the test is resumed. (5) •

Static test load and loading requirements Static test load of lug:

Static failure load calculated when σb = 1080 MPa: Psj = 233280 N. Before the test, the Psj of the EBWD-fabricated part should be determined according to the actually measured σb value of the part. • Procedures of the static test for the lug: (1) (2) (3)

Install the specimen and ensure that the rigidity center is on the axis of symmetry, and the load is applied to the rigidity center; The theoretical static failure load of the specimen is Psj = 200KN; Pre-loading of static test: load step by step by 5% Psj (24KN) load increase, while measuring and recording the strain and displacement step by step, load to 40% Psj (96KN), and then unload and check the equipment and the force on the specimen.

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Fig. 6.38 Fixture for fatigue failure test

(4)

Formal test: loading: (a)

(b)

The loading sequence is 10, 20, 30, 40, 50, 60, 67%; measure the displacement and strain step by step, check whether there is residual deformation or data repeatability after each unloading; The second loading sequence is 10, 20, 30, 40, 50, 60, 67%; measure the displacement and strain step by step. The continuous loading method is adopted until the fracture failure occurs on the specimen and the loading rate is 3 mm/min.

• Static test load of short beam: Static failure load calculated when σb = 1080 MPa: Psj = 201,389.7 N. Before the test, the Psj of the EBWD-fabricated part should be determined according to the actually measured σb value of the part. The designed load Psj of the short beam specimen is shown in Table 6.17. • Procedures of the static test for the short beam: (1) (2)

Install the specimen as shown in and ensure that the rigidity center is on the axis of symmetry, and the load is applied on the rigidity center; The theoretical static failure load of the specimen is Psj = 201,389.7 N;

Table 6.17 Theoretical designed load of short beam specimens Specimen

Cross-sectional moment of inertia, I/mm4

Cross-sectional flexural modulus, W/mm3

Designed load, Psj / N

Ductility correction factor, k

Designed load considering ductility correction, Psj/ /N

Short beam

379,008

12,633.6

181,924

1.107

201,389.7

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6 Fundamentals of EBWD Manufacturing …

(3)

Pre-loading of static test: load step by step by 5% Psj (10KN) load increase, while measuring and recording the strain and displacement step by step, load to 40% Psj (80KN), and then unload and check the equipment and the force on the specimen; Formal test loading:

(4)

Load step by step by 10% Psj (20KN) load increase, then load to 67% Psj (135KN), and load 7% Psj (15KN) in the last step, measure the displacement and strain step by step, and repeat it twice; check whether there is residual deformation or data repeatability after each unloading; (5)

(6) •

In the third loading, load step by step by 5% Psj (10KN) load increase. After loading to 67% Psj (135KN), directly switch to continuous loading at a rate of 3 mm/min to track and measure the strain and displacement (strain gauges 1, 2, 5, 6, 7 ~ 18, 19, 20, 23, 24) until the specimen is damaged, and then record the failure load; Fatigue test load of the short beam and loading requirements Load spectrum of fatigue test:

The load spectrum used in this fatigue test is shown in Fig. 6.39 Fatigue load spectrum of the short beam. Each spectrum partition performs 315 cycles at the first and second levels, which is equivalent to 100 flight hours for that spectrum partition. When P = Fi × Psj , the peak value of the first level is -8.05KN, the valley value of the first level is -36.13KN; the peak value of the second level is -14.33KN, and the valley value of the second level is -66.23KN. The load of weighted fatigue is shown in Fig. 6.40. Each spectrum partition performs 315 cycles at the first and second levels. The applied load is P = 1.2 × Fi × Psj . The peak value of the first level is -9.67KN, the valley value of the first level is -44.56KN; the peak value of the second level is -16.20KN, and the valley value of the second level is -79.47KN. Fig. 6.39 Fatigue load spectrum of the short beam

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Fig. 6.40 Load spectrum of weighted fatigue

• Procedures of general fatigue loading: (1) (2) (3) (4) a.

b.

c.

d.

Install the specimen as shown in Fig. 6.34 and ensure that the rigidity center is on the axis of symmetry, and the load is applied on the rigidity center; The load spectrum is as shown in Fig. 6.39. Test and debug, and the load above level 1 is not allowed during the debugging (absolute value); Formal test: The fatigue test adopts the load spectrum shown in Fig. 6.39. The test frequency is 3 Hz, and the strain is measured once every 5 spectrum partitions when P reaches the valley value in level 2 and repeated it 5 times; Switch to strain tracking and measurement (strain gauges 4, 5, 7, 8); use the DH3817 dynamic strain gauge, and the sampling frequency is 50 Hz (the sampling frequency is not an integer multiple of the loading frequency, also it is more than 10 times the loading frequency to ensure the accuracy and reliability of data collection); After every 10 spectrum partitions are tested, conduct a comprehensive visual inspection on the specimen, especially the welding area of the test section (within 15 mm near the weld); If no damage occurs, run a total of 150 spectral partitions;

• Procedures of weighted spectrum fatigue (1)

(2)

After completing the 5-times life cycle fatigue load spectrum (150 spectrum partitions) by applying the load spectrum shown in Fig. 6.40, conduct a comprehensive visual inspection on the specimen, perform fluorescent inspection on the weld area in the test section, and record the inspection results; After the inspection, if it is confirmed that the specimen has no cracks, then increase the load spectrum and continue the test according to the fatigue load spectrum shown in Fig. 6.40 with a loading frequency of 3 Hz;

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6 Fundamentals of EBWD Manufacturing …

(3)

After every 10 spectrum partitions are tested, conduct a comprehensive visual inspection on the specimen, with the focus on the weld area; A total of 5-times life cycle (150 spectral partitions) is completed;

(4)

• Procedures of fatigue failure test (1)

(2)

After testing the 5-times life cycle (150 spectrum partitions) of the weighted fatigue load spectrum, conduct a comprehensive visual inspection on the specimen, and perform a fluorescent inspection on the weld area in the test section; After the inspection, if it is confirmed that the specimen has no cracks, then continue to increase the load and change to a constant-amplitude load spectrum. The load Pmax = 0.5Psj = -130.025KN (Psj is the actual static failure load), R = 0.06, Pmin = -6.80KN, frequency is 3 Hz, run for 100,000 times. Perform the residual strength test if no damage occurs.

• Procedures of residual strength test (1) (2) (3)

The residual strength test is performed on the undamaged specimens if no damage occurs after the fatigue failure test; Install the specimen and ensure that the rigidity center is on the axis of symmetry, and the load is applied to the rigidity center; Continuously load (at a loading rate of 3 mm/min) until the specimen is damaged, and record the failure load.

6.4.2 Static Test Results of Typical Element Parts (1) (a)

Finite element analysis of typical parts under static load Finite element model of lugs

A hexahedral mesh is used to discretize the lug model, and the upper end of the lug is fixed and restrained. A round rod in contact with the lug hole is established to simulate the actual loading situation, and the load acts on the lug through the round rod. (b)

Finite element analysis results of lugs

A static analysis is carried out on a given failure load applied on the lug according to material parameters. When the load is 200KN, the stress of the lug is shown in Fig. 6.41. It can be seen from the figure that the maximum stress of the lug in the load direction appears near the edge of the hole, and the maximum value is 684.6 MPa. The failure strength of the material has not been reached. On this basis, we calculated the stress of the lug when the load reaches 400KN. The result is shown in Fig. 6.42. It can be seen from the figure that the maximum stress at the edge of the hole is 1272 MPa. The direction where the maximum stress occurs is about 45 degrees to both sides of the axis. According to the figure, we can ascertain that the lug structure will not be damaged under the originally given load of Psj = 200KN.

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Fig. 6.41 Cloud chart of lug stress under a 200KN load

Fig. 6.42 Cloud chart of lug stress under a 400KN load

(c)

Finite element model of the short beam

A finite element analysis model was established according to the three-dimensional numerical model of the short beam. Considering the symmetry of the specimen structure and the loading form, we selected half of the model for calculation. A hexahedral mesh is used to discretize half of the short beam model. The four-point bending is loaded symmetrically. The displacement along the length direction (Y direction), and the rotation along the height direction (Z direction) and the width direction (X direction) are restricted on the symmetry plane of the model. The load acts on the specimen through the rigid body, which corresponds to the actual load. The rigid body at the loading point on the specimen model is restrained by a fixed support, and only the displacement along the loading direction is retained for the lower rigid body, while other degree of freedom is restrained. Considering that there is friction between the fixture and the specimen in the actual loading process, we carried out the contact calculation, and attached the tangential friction properties to the rigid body loading point and the specimen model. (d)

Results of finite element analysis

We calculated the finite element model of the short beam with f = 0, f = 0.1, and f = 0.3 friction coefficient under a load of F = 120KN, as shown in Fig. 6.43. From the calculation, we can see that during the four-point bending process of the short beam,

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the central area presents pure bending, and the upper and lower rafter see the same absolute strain values, which are caused by pressing and tensioning respectively. The web is mainly subjected to shear loads. After the friction coefficient increases, the strain in the center area decreases accordingly, indicating that the friction between the loading point of the fixture and the short beam has a certain effect on the applied load. Table 6.18 shows the strain in the center area under different friction coefficients. (2) (a)

Results of static test of the lug Static test curve

Figure 6.44 shows how the measurement results of each strain gauge are changed with the load when the EP-601 K-01 specimen reaches 67% of the designed load. It can be seen that the distribution law of strain gauges during the lug loading is consistent with the actual situation. The strain value of the No.6 strain gauge and No.1 strain gauge pasted on the edge of the hole is very small. Among the No.2, No.3, and No.4 strain gauges pasted on the same section, the measured value of the No.3 strain gauge. In the middle is lower than that of the No.2 and No.4 strain gauges. It can also be seen from the figure that the strain result shows fine linearity when loaded to 67% of the designed load, and the material is in the linear elastic response stage. Figure 6.45 shows how the measurement results of each strain gauge are changed with the load when the No.1 specimen reaches 67% of the designed load. We can see Table 6.18 Results of static load finite element analysis for the short beam

No

Friction coefficient

Load,F/KN

1

0

120

6585

2

0.1

120

6450

3

0.3

120

6069

Fig. 6.43 Cloud chart of different friction coefficient pairs when F = 120KN

Strain

6.4 Static and Fatigue Tests on Typical Element Parts of EBWD-Fabricated TC18

Fig. 6.44 Strain-load curve of EP-601 K-01

Fig. 6.45 Strain-load curve of EP-No.1 lug

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Table 6.19 Results of static test of the lug Specimen type

Specimen number

Failure load/KN

Average failure load/KN

Average failure Load/Psj

Forging lugs

EP-601D-01

526.0

529.3

227%

EP-601D-02

532.6

EBRM-fabricated lugs (double wire prototyping)

EP-601 K-01

509.0

510.4

219%

EP-601 K-02

511.7

EBRM-fabricated lugs (improved technology, single wire)

1#

518

520.3

223%

2#

522.6

that the strain changes little before and after the technological improvement. Also, the distribution law of each strain gauge during the lug loading is consistent with the actual situation. The strain value of the No.6 strain gauge and No.1 strain gauge pasted on the edge of the hole is very small. Among the No.2, No.3, and No.4 strain gauges pasted on the same section, the measured value of the No.3 strain gauge in the middle is lower than that of the No.2 and No.4 strain gauges. After comparing the strain data of sample No.1 and EP-601 K-01, we can see that the strain data of the two lugs is basically the same. It can also be seen from the figure that the strain result shows fine linearity when loaded to 67% of the designed load, and the material is in the linear elastic response stage. (b)

Failure load and failure form of static test of lug

Before and after the improvement of the EBWD lug fabrication technology and upon the static failure of forging lugs, all specimens are fractured, and the fracture load is shown in Table 6.19. According to the data in the table, the failure load of the EBWD-fabricated lug is slightly increased after the technological improvement. The lugs before and after the technological improvement show a significant turn in the load-strain curve at the positions of No.8, No.9, No.17, and No.18 strain gauges after being loaded to 120KN, indicating that the lug is partially in the ductile yield state. The failure load of each lug specimen is greater than the theoretical load of 233.2KN (1080 MPa), and the failure loads of the forging, the double-wire prototyped lug, and the single-wire prototyped lug reach 227%, 219%, and 223% of the designed failure load respectively. Compared with the former double-wire prototyped lug, the improved single-wire prototyped lug has a 4% higher failure load and better ductility. The fracture of the specimen is shown in Fig. 6.46. (3) (a)

Results of short beam test Static test curve

6.4 Static and Fatigue Tests on Typical Element Parts of EBWD-Fabricated TC18

(a) Failure of forging lugs EP-601K-01

261

(b) Failure of double-wire prototyped lugs EP-601K-01

Fig. 6.46 Diagram of lug failure

Figure 6.47 shows how No.1—No.6 strain pasted on the upper and lower rafters on the same section of the DL-601 K-1 short beam is changed with the load. It can be seen from the figure that curves show fine linearity when loaded to 67% of the designed load, indicating that the short beam is in the linear elastic deformation stage at this load level. The No.1, No.2, and No.3 strains of the upper rafter all turn negative, indicating that the upper rafter is under compression, and the No.1 and No.2 strains on the outer surface are higher than the No.3 strain on the inner surface. The No.4, No.5, and No.6 strains of the lower rafter all turn positive, indicating that the lower rafter is in tension, and the No.5 and No.6 strains on the outer surface are higher than the No.4 strain on the inner surface (Fig. 6.47). Figure 6.48 shows the strain on the outer surface of the upper rafter in the pure bending area of the short beam. It can be seen from the figure that the curves are basically overlapped, which indicates that the strain responses on the same surface in the pure bending area of the short beam are consistent and aligned with the actual situation. Figure 6.49 shows how the midpoint displacement of the short beam is changed with the load when reaching 67% of the designed load. It can be seen from the figure that the midpoint displacement curve also shows sound linearity, indicating that the

Fig. 6.47 Sectional strain curve of DL-601 K-01

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Fig. 6.48 Strain curve of DL-601 K-01 upper rafter

Fig. 6.49 Midpoint displacement curve of DL-601 K-01

short beam is in the linear elastic deformation stage at this load level (Figs. 6.48 and 6.49). Figure 6.50 shows the curve of how the strain at the cross-sectional measurement point is changed with the load when reaching 100% Psj . It can be seen that the change is basically linear under 100% Psj load. Figure 6.51 shows the load–displacement curve of the DL-601D-01 forged short beam. Compared with the load–displacement curve of DL-601 K-01, the difference in the failure form is clearly seen. The short beam fabricated by EBWD suffers brittle fracture failure, while the forged short beam suffers buckling failure.

Fig. 6.50 Failure strain curve of DL-601 K-01

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Fig. 6.51 Load–displacement curve of DL-601D-01 in the static test

Fig. 6.52 Sectional strain curve

Figure 6.52 shows how No.1—No.6 strain pasted on the upper and lower rafters on the same section of the DL-No.2 short beam is changed with the load. It can be seen from the figure that curves show fine linearity when loaded to 67% of the designed load, indicating that the short beam is in the linear elastic deformation stage at this load level. The No.1, No.2, and No.3 strains of the upper rafter all turn negative, indicating that the upper rafter is under compression, and the No.1 and No.2 strains on the outer surface are higher than the No.3 strain on the inner surface. The No.4, No.5, and No.6 strains of the lower rafter all turn positive, indicating that the lower rafter is in tension, and the No.5 and No.6 strains on the outer surface are higher than the No.4 strain on the inner surface. Figure 6.53 shows the strain on the outer surface of the upper rafter in the pure bending area of the short beam. It can be seen from the figure that the curves are basically overlapped, which indicates that the strain responses on the same surface in the pure bending area of the short beam are consistent and aligned with the actual situation. The curve of the eighth strain gauge on DL-No.2 shows an obvious turn after being loaded to 60KN. This might be caused by the poor quality of the strain gauge that leads to abnormal data. No damage has occurred on the specimen. Figure 6.54 shows how the midpoint displacement of the short beam is changed with the load when reaching 67% of the designed load. It can be seen from the figure that the midpoint displacement curve also shows sound linearity, indicating that the

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Fig. 6.53 a Load-strain curve of DL-601K-01 upper surface. b Load-strain curve of DL-No.2 upper surface

Fig. 6.54 Strain curve of upper rafter

Fig. 6.55 Midpoint load–displacement curve

short beam is in the linear elastic deformation stage at this load level. In addition, the deformation of the fracture specimens made by these two technologies after being loaded to 144KN is basically the same, and the bending stiffness is the same. Figure 6.55 shows the curve of how the strain at the cross-sectional measurement point is changed with the load when reaching 100% Psj . It can be seen that the short beam fabricated by the improved technology under the load of 100% Psj presents linear changes, and the strain of the improved EBWD-fabricated short beam is basically changed linearly. This is because of the poor attachment of the strain gauge and the No.5 strain gauge on the inner side of the rafter falls off due to large deformations.

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Fig. 6.56 Midpoint load–displacement curve

Fig. 6.57 Load-strain curve of theoretical failure

Figure 6.56 shows the load–displacement curve during the loading process of the tester. It can be seen that the curve is basically linear at the beginning stage. After the load reaches 200KN, the EBRM-fabricated short beam enters the buckling stage. At this time, the displacement of the tester is about 11 mm. Afterwards, the improved short-beam specimen partially enters the ductile yield state, and the displacement changes greatly with the slow increase of the load. When the load reaches 240KN, the specimen suddenly undergoes brittle failure, and the final failure displacement exceeds 20 mm. Figure 6.57 shows the load–displacement curve of the DL-601 K-01 short beam. From the curve in the figure, we can see that the load–displacement curve shows

Fig. 6.58 a Load-displacement curve of DL-No.2 static test. b Load-displacement curve of DLNo.3 static test

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a good linear trend in the initial stage of loading, and the load–displacement curve shows a slight turn after being loaded to 200KN. After the load reaches 225KN, the specimen suddenly undergoes fracture failure, and the final fracture displacement is about 11 mm. After comparing the load–displacement curves, we can see that the improved EBWD-fabricated short team shows significant yield, and the final failure displacement is significantly increased, indicating that the toughness of the material is significantly improved, the fracture strain is increased, and the load-bearing capacity is also enhanced after the technology is improved. (b)

Failure load and failure form of the short beam

After the failure test, the failure form and failure load of the short beam is shown in Table 6.20. The failure load of each specimen is greater than the theoretical failure load of 201.3KN. The mean values of the failure load of the forging, the double-wire prototyped short beam, and the single-wire prototyped short beam reach 129, 114, and 119% of the designed load respectively. The improved single-wire prototyped short beam has higher load-bearing capacity and ductility than the double-wire prototyped short beam. The short beam of the forging undergoes buckling failure and large deformation that cannot be recovered. The specific failure form is shown in Fig. 6.58. The double-wire prototyped short beam undergoes brittle failure. The failure occurs in the area with the largest deformation in the center. The web at the center fractures, the upper rafter is separated from the beam, and undergoes a large residual deformation. See Fig. 6.59 Overall failure diagram of DL-601 K-01 for specific failure forms. The single-wire prototyped short beam undergoes brittle failure. The failure occurs in the area with the largest deformation in the center. The web at the center fractures, the upper rafter undergoes a large residual deformation. See Fig. 6.60 for specific failure forms.

Fig. 6.59 Load-displacement curve of loaded short beam

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Table 6.20 Failure load and failure form Specimen number Failure load/KN Mean/KN Mean value of failure Failure form load/Psj DL-601D-01

262

DL-601D-05

258.09

DL-601 K-01

228.5

DL-601 K-03

228

DL-2#

234.9

DL-No.3

243.1

260.05

129%

Buckling failure

228.25

114%

Brittle failure, fracture of the central web, deformation and separation of the upper rafter

Buckling failure

Brittle failure, fracture of the central web and plastic deformation of the upper rafter 239

119%

Brittle failure, fracture of the central web, deformation and separation of the upper rafter Brittle failure, fracture of the central web and plastic deformation of the upper rafter

Fig. 6.60 Load-displacement curve of DL-601K-01 static test

6.4.3 Results of Fatigue Test (1)

Results of general fatigue test (94,500 cycles)

After the general load spectrum fatigue test, we ran fluorescence inspections on all the fatigue tests, and the results show that no cracks occur in the dangerous area of the specimen after the general load spectrum fatigue test. Figure 6.61 shows the strain change of DL-601D-2013–02, DL-601 K-2013–02 and DL-No.4 specimens. From the figure, we can see that the strains of No.1, No.3,

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Fig. 6.61 Failure form of DL-601D-01-2013-01

Fig. 6.62 Overall failure diagram of DL-601K-01

No.6 and No.9 specimens are on the same level, and the strains of No.2, No.4, No.5, No.7, No.8, No.10 specimens are on the same level. The error in the mean value measured in each channel is within 100 and the error is less than 5%. The strain value of the EBWD-fabricated short beam is about 100 higher than that of the forging short beam. The strain value of the improved EBRM-fabricated short beam is about 50 higher than that before the improvement, but they are still on the same level, indicating that the stiffness of the structure after improvement does not change much. Figure 6.62 shows the results of the dynamic tracking measurement of DL-601D2013–02, DL-601 K-2013–02 and DL-No.4 in the 50th cycle. It can be seen from the figure that when the small load spectrum is changed to a large load spectrum, the strain

Fig. 6.63 Static failure of DL-No.2

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269

changes accordingly, and the strain consistency of all channels is outstanding. Among them, the average peak value of the DL-601D-2013–02-No.4 strain gauge is 3450, the DL-601 K-2013–02-No.4 is 3650, and the DL-No.4-No.4 is 3600. Compared to the static strain value, the error of the dynamic tracking measurement is less than 5%. (2)

Results of weighted spectrum fatigue test (94,500 cycles)

After the general load spectrum fatigue test, no crack or failure is seen on any short beam, and then the weighted spectrum fatigue test is carried out instead. After the weighted spectrum fatigue test, the EBRM-fabricated short beam sees no crack or failure. (3)

Results of fatigue failure test

No crack occurs in the forged short beam and the double-wire prototyped short beams after the fatigue failure test is carried out 100,000 times. When the DL-No.1 fatigue test of the single-wire EBRM-fabricated short beam proceeds to the 63,957 times, a fracture failure occurs on the right side of the upper loading point, and another fracture failure occurs on the right side of the upper loading point when the DL-No.5 constant-amplitude spectrum test proceeds to the 4518 times. No failure or crack is seen on the DL-No.4 EBRM-fabricated short beam and the improved short beam when the constant-amplitude spectrum test proceeds to the 100,000 times. Therefore, a residual strength test is carried out. The results of fatigue test on all specimens are shown in Table 6.21. (4)

Results of residual strength test

Fig. 6.64 Static strain measurement of the short beam

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Fig. 6.65 Dynamic strain measurement of the short beam

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271

Fig. 6.65 (continued)

Table 6.21 Results of fatigue test on typical element parts Category

Number

General Load Weighted Load Fatigue Failure Pmax Spectrum P = F i × Spectrum P = 1.2 × = 0.5PSJ F i × Psj Psj 1,000,000 Cycles 94,500 Cycles 94,500 Cycles

Forging

DL-601D-02

Pass

Pass

Pass

DL-601D-03

Pass

Pass

Pass

DL-601D-04

Pass

Pass

Pass

Double wire DL-601 K-02 Pass

Pass

Pass

DL-601 K-04 Pass

Pass

Pass

DL-601 K-05 Pass

Pass

Pass

DL-No.1

Pass

Pass

63,957

DL-No.4

Pass

Pass

Pass

DL-No.5

Pass

Pass

4518

Single wire

We ran residual strength tests on the 6 short beams that were not damaged in the fatigue test. The tests were carried out by the continuous loading method and adopted a loading speed of 3 mm/min. The results are shown in Table 6.22. We compared the results of the residual strength test with the result of the static test, as shown in Table 6.23. It can be seen that the strength of the forged short beam after the fatigue test shows a certain change, and the strength is reduced by about 6%. After

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Table 6.22 Results of residual strength tests Specimen number

Failure form

Buckling value/KN

Final load/KN

Mean/KN

601 K-02

No obvious buckling change, brittle fracture at the center

No

228.4

230.5

Buckling failure

220

601 K-04 601 K-05 601D-2

231.4 231.7 245.1

601D-3

250

601D-4 DL-No.4

246.9

245.5 Brittle fracture from the center after yielding

230

249.4

249.4

Table 6.23 Comparison of residual strength and static strength Item

Test type

Average load

Failure form Test type

Average load

Failure form

EBRM-fabricated short beam

Static test

228KN

Fracture in the center

230KN

Fracture in the center

Residual strength

Fracture in the center

Fracture in the center Fracture in the center

Forging short beam Static test

260KN

Buckling failure

Residual strength

246.8KN

Buckling failure

Buckling failure Buckling failure Buckling failure

EBRM-fabricated short beam (improved)

Static test

239KN

Fracture in the center

Residual strength

249KN

Fracture in the center

the fatigue test, the residual strength and static strength of the double-wire EBWDfabricated short beam show no significant changes, and the failure forms are all fracture failure. The residual strength of the improved single-wire EBWD-fabricated short beam after the fatigue test is increased by 4%. (1)

Static test

The failure loads of forgings, double-wire prototyped lugs and single-wire prototyped lugs, and short beams are all much higher than the designed failure load (1080 MPa). According to the static test results, that the load-bearing capacities of doublewire prototyped lugs and forged lugs differ by about 4%, and that of double-wire prototyped short beams and forged short beams differ by about 3%. Apparent there is

6.4 Static and Fatigue Tests on Typical Element Parts of EBWD-Fabricated TC18

273

no significant difference in the bearing capacities of double-wire EBRM-fabricated specimens and forged specimens. After technological improvement, the load-bearing capacity of single-wire prototyped lugs is increased by about 3%, that of single-wire prototyped short beams is increased by about 5%, which is on a par with forged parts. It can be seen from the load–displacement curve that the short beam shows an obvious yielding after the technological improvement, and the load-bearing capacity has been improved to a certain extent. (2)

Fatigue test

Both the forged short beam and the double-wire prototyped short beam pass the fatigue failure test after the weighted load spectrum test. The single-wire prototyped short beam passes the weighted load spectrum test, and shows two points of failure in the subsequent failure load spectrum test. Through the fatigue test of the short beam, it is found that the double-wire prototyped short beam is better than that of the forged short beam in fatigue resistance. Both short beams show no failure in the fatigue test. However, according to the result of the residual strength test after the fatigue test, the strength of the double-wire prototyped short beam is not changed, but that of the forged short beam is decreased. After technological improvement, we found that two single-wire EBWDfabricated short beams were damaged in the fatigue failure test, and the remaining short beams were not damaged in the residual strength test. According to the results of the residual strength test after the fatigue test, the strength of short beams does not change before and after the technological improvement.

Reference 1. Yang Guang (2014) Research on the microstructure and mechanical properties of TC18 fabricated by electron beam rapid prototyping in the state of multiple depositions [D]. Beijing: China Academy of Aeronautics and Astronautics

Chapter 7

Fundamentals of Electron Beam Wire Deposition Technology for A-100 Steel

Abstract This chapter introduces the fundamentals of EBWD manufacturing of A-100 steel. It’s consisted of microstructure characteristics of both deposited state and heat-treated state, properties control, typical defects and control methods. At the same time, typical mechanical properties such as the static property, fatigue property, etc. have been introduced in this chapter.

The AerMet100 steel is a kind of Co–Ni ultra-high-strength steel, and is the steel with the best comprehensive performance on the market. After proper heat treatment, the forged part can realize a tensile strength of 1900 MPa and a fracture toughness of 110 MPa m1/2 . Its excellent mechanical properties make it widely used in aerospace, weaponry, automobile construction and other industries. The strengthening mechanism of the secondarily-hardened Co–Ni ultra-highstrength steel is similar. They all improve the performance by obtaining the martensitic structure of the fine laths base on improving the hardenability of the steel. The alloy carbide precipitates at the lath boundary, the subgrain boundary, and the grain boundary, and undergoes secondary hardening to further improve the strength of the steel [1] As for the AerMet100 steel, the toughness is improved by double vacuum melting, strictly controlling the content of impurity elements such as S and P, obtaining ultra-pure structures, and using the film-like reversed austenite generated during tempering. The AerMet100 steel is sensitive to the tempering temperature. Slight temperature disturbances will cause rapid changes in properties in the vicinity of the optimal tempering temperature of 482 °C. The data released by Carpenter is as follows: After 200 °C, the tensile strength and yield strength show a trend of increase followed by a decrease with the increase of the tempering temperature. The peak tensile strength 2112 MPa shows at 452 °C, and the peak yield strength moves slightly to the right at 469 °C, reaching 1666 MPa. The elongation is not sensitive to the tempering temperature and is almost a horizontal straight line. The reduction of area presents an inflection point at 482 °C and then starts to rise. This should be related to the dispersion of the second phase particles and the appearance of reversed austenite at the lath boundary. After 149 °C, the impact energy begins to decrease, and reaches the minimum of 37 J at 386 °C. After that, it proceeds to 463 °C, and the impact energy © National Defense Industry Press 2022 S. Gong et al., Electron Beam Wire Deposition Technology and Its Application, Additive Manufacturing Technology, https://doi.org/10.1007/978-981-19-0759-3_7

275

276

7 Fundamentals of Electron Beam Wire …

begins to recover, returning to 56 J at 506 °C. The fracture toughness rises linearly from 66 MPa m1/2 at 454 °C to 165 MPa m1/2 at 510 °C. Speich [14][60] summarized the HY180 steel and believes that the fine alloy carbides make a great contribution to the strength when aging at 482 °C. The maximum toughness value can only be obtained when all the coarse lath martensitic cementite particles are replaced by fine M2 C. Coarse Fe3 C particles precipitate when the AerMet100 steel is tempered near 420 °C, and the impact energy and fracture toughness are affected, reaching a low point. As the tempering temperature rises, Fe3 C particles dissolve and are replaced by more finely dispersed Mo2 C that maintains a coherent relationship with the matrix, which improves the impact resistance and toughness of the steel while increasing the strength. The reversed austenite is formed at 482 °C and reaches a peak value at 590 °C, up to 20%. The content of reversed austenite has an inverse relationship with its hardness [2–3].

7.1 Microstructure Characteristics of A-100 Material Fabricated by EBWD The thermal cycle of the EBWD technology is complex and changeable, and the heat dissipation conditions are poor in the vacuum environment. Compared with forged parts, the prototyped A-100 ultra-high-strength steel parts have a big difference in the structure. The structure fabricated by EBWD has significant anisotropy in the prototyping plane direction and the prototyping height direction, and shows typical dendritic features in the prototyping height direction [4].

7.1.1 Microstructure Analysis Figure 7.1 shows the metallographic image of the original state after EBWD. Although the electron beam prototyping process is different from the heat treatment process, but their effects are very similar. When prototyping, the thermal cycle

Fig. 7.1 Original state of the EBWD-fabricated A-100

7.1 Microstructure Characteristics of A-100 …

277

of the material consists of multiple rounds of rapid cooling and heating, which is similar to the process of quenching followed by heat preservation and then rapid cooling. It can be seen from the figure that the original structure after prototyping is a martensitic structure with light and dark phases. The heat dissipation conditions are poor in the vacuum environment, and the prototyped workpiece amounts to undergoing a low temperature heat preservation treatment, which is similar to the effect of aging, so there are white precipitated phases between the martensitic laths. This is very similar to the AerMet100 steel structure after the final heat treatment. After undergoing complex thermal cycles in the prototyping process, the structure of the material is not uniform enough, and there are some pores and micro-cracks defects. Although the prototyped workpiece is somewhat similar to the finalized material in terms of the structure, it cannot be used directly, and still requires further heat treatment control.

7.1.2 Microstructure Evolution During Heat Treatment The prototyped material undergoes 7 heat treatment steps including homogenized annealing, hot isostatic pressing, normalizing, annealing, quenching, cryogenic cooling, and aging. There will be some changes on the structure in every step of heat treatment. The metallographic images of each step are shown in Fig. 7.2. Under the action of high temperature, the diffusion behavior of alloying elements accelerates, the element segregation gradually disappears, and the material at the micro-pores and micro-cracks softens and deforms. Under the action of high pressure, the surface edge of the defect begins to contact. As the contact area increases, the atoms on the boundary diffuse and penetrate each other to form a bonding layer that is strengthened as the atoms on the contact area diffuse more completely. When the strength of the bonding layer is consistent with the surrounding structures, the pores and cracks disappear and the defect density decreases. After the material has been kept insulated for a long time, the precipitated phase will grow from the dispersed small particles, break away from the coherent relationship with the matrix, mature and grow up, and finally dissolve and disappear, leaving the martensitic structure with light and dark phases. Since the hot isostatic pressing requires furnace cooling to release the pressure, the structure also retains some feathery lower bainite structures. Electron beam additive manufacturing is a typical layer-by-layer deposition manufacturing process. We can see clear layer transition structures from the metallographic image (Fig. 7.2a). On both sides of the layer, dendrites grow in the same direction, with slight angle changes occurring occasionally. The dendrite diameter changes significantly before and after the layer. The average diameter of dendrites is 16um in the upper layer of the deposition direction. Compared with the structure in the lower layer whose average diameter is 24um, the upper structure is obviously smaller than the lower structure. This phenomenon is especially obvious at the junction of two passes. This may be related to the “reheating” effect of the current heating layer on

278

7 Fundamentals of Electron Beam Wire …

a) After homogenized annealing and hot isostatic pressing

b) After normalizing

c) After annealing

d) After quenching

e) After cryogenic cooling

f) After aging

Fig. 7.2 Metallographic structure images of different stages of EBWD-fabricated A-100 (heat treatment technology parameters: 930 °C homogenized annealing and hot isostatic pressing)

the upper layer during the prototyping process. This phenomenon is more noticeable when the grains formed in the upper layer continue to grow under the heating effect, causing the heat to gather at the junction of two passes. After normalizing, strain-free crystalline cores are formed in the grain boundary and subgrain boundary, surrounded by large-angle grain boundaries and are separated

7.1 Microstructure Characteristics of A-100 …

279

from the matrix. The crystalline core grows when the large-angle grain boundaries migrate. There is no major change in the structural form, and the size of the dendrites is reduced. Before normalizing, the dendrite diameter is about 24um. After normalizing, the dendrite diameter is about 21um, indicating that the grains are refined to a certain extent (as shown in Fig. 7.2b). After tempering and softening, the dendritic grain boundaries and layered structures disappear, and the strength of the material decreases, which is conducive to mechanical machining (as shown in Fig. 7.2c). After heat preservation at 899 °C, air cooling, and quenching, the structure is the lath martensite and retained austenite. The martensitic laths are precipitated from inside the original austenitic grains, and the proportion of retained austenite is still large (as shown in Fig. 7.2d). After cryogenic cooling, the martensitic structure continues to precipitate (as shown in Fig. 7.2e), and the retained austenite is reduced from 42.19% after quenching to 29.44% (limited to the area shown in the metallographic image), which is less than the ideal content. This may be caused by the long interval between quenching and cryogenic cooling. The structure undergoes mechanical and thermal stabilization, which delays the further transformation of austenite to martensite. The content of retained austenite is high, and it is not distributed at the boundary of the martensitic structure in the ideal form of film. Also, it affects the strength and toughness of the material. After aging, the carbides (Mo, Cr) 2C are dispersed and precipitated from the boundary of the martensitic structure, and basically maintain a coherent relationship with the matrix in terms of orientation. The second phase is dispersed and precipitated, and continues to strengthen the AerMet100 steel with less impact on ductility (as shown in Fig. 7.2f).

7.1.3 Influence of Pre-heat Treatment Parameters on the Structure After homogenization and hot isostatic pressing (as shown in Fig. 7.3), we can clearly see the dendrite stripes with light and dark phases. The average diameter of the grains in the 1000 °C treatment group (as shown in Fig. 7.3b) is larger than that of the 930 °C treatment group (as shown in Fig. 7.3a). The martensite first nucleates from the position where the original austenitic grains meet. The degree of martensitic formation in the 930 °C treatment group is higher than that in the 1000 °C treatment group. The control group (as shown in Fig. 7.3c) is a structure that has not undergone any heat treatment after prototyping, but it is similar to the annealed structure due to the complex thermal cycle and heat dissipation conditions of the prototyping process. We do not see clear dendritic grains with light and dark phases on the image. There

280

7 Fundamentals of Electron Beam Wire …

a) 930ć test group

b) 1000ć test group

c) Control group

Fig. 7.3 Metallographic structure of EBWD-fabricated A-100 after homogenized annealing and hot isostatic pressing

a) 930ć test group

b) 1000ć test group

c) Control group

Fig. 7.4 Metallographic structure of EBWD-fabricated A-100 after normalizing

are strips or massive phases of different sizes in it, and the orientation law is not obvious. This is the impact of the uneven distribution of chemical components. After normalizing (as shown in Fig. 7.4), the two test groups undergo recrystallization treatment to regenerate nucleation cores at the grain boundary and sub-grain boundary, and control the grain size at a temperature much lower than the temperature of the homogenization treatment. As seen from the actual effect, the heredity of the recrystallized structure is relatively obvious, the grains have been refined to a certain extent, but the effect of the refinement is not outstanding. There are still some precipitated phases appearing at the position of the grain boundaries, which are the same kind of structure as the untreated precipitated phases just formed. The grain size of the control group is significantly smaller than that of the test group, and dendrites with obvious light and dark phases appear, the diameter of which is about 0.5 times that of the test group, and even the precipitated phases are refined. After annealing (as shown in Fig. 7.5), the precipitated phase begins to dissolve and disappear. According to the structure distribution, the precipitated phase at 1000 °C (as shown in Fig. 7.5b) dissolves most thoroughly, and the remaining structures are more evenly distributed. Although the control group has the finest structures, there are still some precipitated phase and the structure distribution is less even than the test group. After quenching (as shown in Fig. 7.6), martensitic laths are formed in the original austenitic grains, and the long axis of the laths is distributed along the radial direction

7.1 Microstructure Characteristics of A-100 …

a) 930ć test group

b) 1000ć test group

281

c) Control group

Fig. 7.5 Metallographic structure of EBWD-fabricated A-100 after annealing

a) 930ć test group

b) 1000ć test group

c) Control group

Fig. 7.6 Metallographic structure of EBWD-fabricated A-100 after quenching

a) 930ć test group

b) 1000ć test group

c) Control group

Fig. 7.7 Metallographic structure of EBWD-fabricated A-100 after cryogenic cooling

of the original grains. In the control group (as shown in Fig. 7.6c), precipitated phases are precipitated apart from the martensitic nucleation. After cryogenic cooling (as shown in Fig. 7.7), the martensite content increases and the retained austenite decreases. The 1000 °C test group (as shown in Fig. 7.7b) precipitates secondary phases from now on, and the 930 °C test group (as shown in Fig. 7.7a) shows no signs of secondary phase precipitation after cryogenic cooling. After aging (as shown in Fig. 7.8), the 930 °C test group (as shown in Fig. 7.8a) just started to precipitate secondary phases. So far, all the secondary phases have been precipitated in the three metallographic phases of this step. The control group (as shown in Fig. 7.8c) has the most precipitated phases, the 930 °C test group has the least precipitated phases, and the 1000 °C test group (as shown in Fig. 7.8b) has the largest precipitated phases.

282

7 Fundamentals of Electron Beam Wire …

a) 930ć test group

b) 1000ć test group

c) Control group

Fig. 7.8 Metallographic structure of EBWD-fabricated A-100 after normalizing

After comparison, we see that the structure of the control group is the smallest. But after each step of heat treatment, the secondary precipitates are present in the structure. This may be related to the uneven distribution of materials in this group, the large fluctuation of element contents, and the presence of small defects, making the secondary phases more likely to precipitate at these boundaries. The structure forms the two test groups are basically the same, and the structure of the 1000 °C test group is slightly larger. At the initial stage of the martensitic formation in the 1000 °C test group, since the content of martensite transformed from austenite is low, the martensitic content of the two test groups is nothing different after cryogenic cooling and aging. Secondary phases are precipitated in the 1000 °C test group after cryogenic cooling, and precipitated phases are coarser than those in the 930 °C test group.

7.2 Properties Control Methods of A-100 Fabricated by EBWD 7.2.1 Static Property (1)

Tensile strength

In order to investigate the effect of homogenized annealing and hot isostatic pressing on the strength property of AerMet100 steel deposited by EBWD [5], we tested 4 sets of specimens with a Z050 tester. The results of tensile strength tests are shown in Table 7.1. As shown in Table 7.1, the average tensile strength of the specimen in the X direction without homogenized annealing and hot isostatic pressing is only 1775 MPa, and the tensile strength of the material is significantly improved after homogenized annealing and hot isostatic pressing. After 930 °C homogenization and hot isostatic pressing, the tensile strength of the No. 2 test group increases to 1947.3 MPa, which is 173.3 MPa higher than that of the control group, and 1931 MPa higher than the forging standard. After 1000 °C homogenization and hot isostatic pressing, the tensile strength of the No. 3 test group increases to 1911.3 MPa, which is 136.3 MPa higher than the control group and slightly lower than the forging standard.

7.2 Properties Control Methods of A-100 Fabricated by EBWD

283

Table 7.1 Influence of pretreatment parameters on tensile strength No. 1 (control group)

No. 2 (930 °C treatment)

No. 3 (1000 °C treatment)

X direction

Z direction

X direction

Z direction

X direction

Z direction

Data 1

1861

1864

1940

1952

1939

1908

Data 2

1689

1867

1954

1901

1884

1900

Data 3

–

–

1951

1943

1911

1908

Mean

1775

1865.5

1947.3

1932

1911.3

1905.3

Remarks: Forging standard is ≥1931 MPa

The tensile strength of the control group is 1865.5MPa in the Z direction of the specimen, which is higher than the X-direction data in the same state. After homogenization and hot isostatic pressing, the tensile strength of the No. 2 test group reaches 1932 MPa, which is 66.5 MPa higher than that of the control group, but 16.3 MPa lower than that of the X-direction data in the same state. After treatment, the tensile strength of the No. 3 test group is increased to 1905.3 MPa which is 39.8 MPa higher than that of the control group. Similar to the No. 2 test group, the tensile strength of the No. 3 test group is reduced by 6 MPa compared with the X-direction data in the same state. Compared with the forging standard, the tensile strength of No. 2 test group at 930 °C is higher than the standard, while that of the No. 3 test group at 1000 °C needs to be improved. (2)

Yield strength

The measured result of the yield strength of AerMet100 steel deposited by EBWD is shown in Table 7.2 Influence of pretreatment parameters on yield strength. The results show that the yield strength in X direction of the control group without pretreatment reaches 1616 MPa. After homogenized annealing and hot isostatic pressing, the yield strength of the No. 2 test group reaches 1635.5 MPa, and the yield strength of the No. 3 test group reaches 1647 MPa, increased by 19.5 MPa and 31 MPa, or 1.2% and 1.9% respectively. The yield strength of the control group in the Z direction without pretreatment is 1556.5 MPa. After treatment, the yield strength of No. 2 test group reaches 1649.7 MPa, and No. 3 test group reaches 1665.7 MPa, increased by 93.2 MPa and 109.2 MPa, or 6.0% and 7.0% respectively. The yield Table 7.2 Influence of pretreatment parameters on yield strength No. 1 (Control group)

No. 2 (930 °C treatment)

No. 3 (1000 °C treatment)

X direction

Z direction

X direction

Z direction

X direction

Z direction

Data 1

1634

1533

1649

1635

1675

1667

Data 2

1598

1580

1606

1650

1632

1659

Data 3

–

–

1651

1664

1634

1671

Mean

1616

1556.5

1635.3

1649.7

1647

1665.7

Remarks: Forging standard is ≥1620 MPa

284

7 Fundamentals of Electron Beam Wire …

Table 7.3 Influence of pretreatment parameters on elongation No. 1 (control group)

No. 2 (930 °C treatment)

No. 3 (1000 °C treatment)

X direction

Z direction

X direction

Z direction

X direction

Z direction

Data 1

11.5

11.5

13.0

12.0

13.5

14.0

Data 2

–

12.0

13.0

12.5

10.0

12.5

Data 3

–

–

12.5

–

13.0

13.0

Mean

11.5

11.75

12.83

12.25

12.17

13.17

Remarks: Forging standard is ≥10%

strength increases after treatment. But different from the tensile strength, the yield strength in the X direction is better than the yield strength in the Z direction before treatment, and the data in the Z direction is better than the data in the X direction after treatment. Obviously, the yield strength increment in the Z direction is higher than that in the X direction after pretreatment. This should be related to the yield mechanism and the original microstructure of AerMet100 steel. The main role of the heat treatment process is to adjust the microstructure of the material. But this can also explain the reasons for the difference in variation between the tensile strength and the yield strength. In the process of homogenized annealing and hot isostatic pressing, alloying elements diffuse among the grains, gradually reducing the adverse effects brought by element segregation. Also, small pore defects are also welded under the action of pressure. But meanwhile the grain size will also grow. The 1000 °C test is theoretically better than the 930 °C test to homogenize and eliminate micro-defects, but also grains grow more vigorously. Although this is conducive to the increase of its yield strength, the large grains are not convenient for coordinated deformation, and slip is not easy to occur, so that the strength value increased by deformation is lower than the test group at 930 °C. Therefore, the tensile strength of the 930 °C test group is higher; the yield strength of the 1000 °C test group is higher. This principle can also be used to explain the difference between the X direction and the Z direction. Due to the characteristics of prototyping, the fast-growing AerMet100 steel presents a typical dendritic structure, with the long axis pointing to the Z direction and the short axis distributed along the X direction. Although it has undergone a line of complicated heat treatment processes, the orientation of the original grains still exists according to the hereditary character. This causes the grains to slide in the Z direction more easily than in the X direction. For this reason, the X-direction tensile strength is better than that in the Z direction, and the Z direction yield strength is better than that in the X direction. (3)

Elongation and reduction of area

The elongation and the reduction of area reflect the ductility of the material and manifest its deformation ability. According to the results of the tensile test (as shown in Table 7.3 and Table 7.4), the data of the three groups differ slightly, and the difference is basically within the measurement error, indicating that the homogenization

7.2 Properties Control Methods of A-100 Fabricated by EBWD

285

Table 7.4 Influence of pretreatment parameters on the reduction of area No. 1 (control group)

No. 2 (930 °C treatment)

No. 3 (1000 °C treatment)

X direction

Z direction

X direction

Z direction

X direction

Z direction

Data 1

60

59

57

60

58

58

Data 2

–

50

58

59

61

62

Data 3

–

–

59

58

59

61

Mean

60

54.5

58

59

59.3

60.3

Remarks: Forging standard is ≥55%

treatment and hot isostatic pressing treatment, along with their temperature, have little impact on the ductility of the material. The elongation and the reduction of area of the materials, no matter it is pre-treated or not, can meet the forging standard. Before and after homogenized annealing and hot isostatic pressing, the anisotropy of the material is improved, and the gap between X and Z directions is reduced after pre-treatment. (4)

Mechanical properties of different heat treatment conditions and different specimen orientations

Figure 7.9 and 7.10 summarize and compare the tensile properties from the perspective of three test parameters and the angle of anisotropy (X direction, Z direction). The test group treated at 930 °C gives the best results, followed by the test group treated at 1000 °C, and finally the control group. Before and after homogenized annealing and hot isostatic pressing, the anisotropy of the material is improved, and the gap between X and Z directions is reduced after pre-treatment. Heat treatment has a significant impact on the performance of A-100 steel. In order to further improve the static mechanical properties of A-100 steel fabricated by EBWD, we applied three heat treatment technologies for regulation. We ran the tensile, compression, and shear tests on the specimens aged at 487 °C. The specimen for the tensile test is designed according to ASTM E8, as shown in Fig. 7.11. The specimen for the compression test is designed according to ASTM E9, as shown in Fig. 7.12. The specimen for the shear test is designed according to ASTMB769, as shown in Fig. 7.13. The tensile test is carried out on a 1000 kN static/fatigue tester. The support state of the specimen and the installation position of the extensometer are shown in Fig. 7.14. The specimen is clamped in two semicircular fixtures, which are connected and fixed by bolts. The fixtures are clamped by the tester during the tensile test. The compression test is carried out on a 1000 kN static/fatigue tester. The support state of the specimen and the installation position of the extensometer are shown in Fig. 7.15. The shear test is carried out on a 1000 kN static/fatigue tester. The support state of the specimen is shown in Fig. 7.16. The fixture base is fixed in the lower chuck of the tester. The shear specimen is placed in the fixture, and the upper chuck of the tester applies ballast to perform the shear test.

286

7 Fundamentals of Electron Beam Wire …

a) X direction

b) Z direction

Fig. 7.9 Comparison of tensile properties of different test groups

The load-deformation curve of the tensile test is shown in Fig. 7.17. The failure photo in the test is shown in Fig. 7.18. The result of the tensile test is shown in Table 7.5. During the compression test, the extensometer measures the deformation of the specimen within the gauge length, and the tester records the test load. The loaddeformation curve of EBWD-fabricated A-100 in the compression test is shown in Fig. 7.19 and the yield photo is shown in Fig. 7.20. The failure photo of EBWD-fabricated A-100 in the shear test is shown in Fig. 7.21, and the result of the shear test is shown in Table 7.6.

7.2 Properties Control Methods of A-100 Fabricated by EBWD Fig. 7.10 Comparison of tensile properties in X and Z directions

a) 930ć test group

b) 1000ć test group

c) Control group

287

288 Fig. 7.11 Specimen for the tensile test

Fig. 7.12 Specimen for the compression test

Fig. 7.13 Specimen for the shear test

Fig. 7.14 Tensile test

7 Fundamentals of Electron Beam Wire …

7.2 Properties Control Methods of A-100 Fabricated by EBWD

289

Fig. 7.15 Compression test

Fig. 7.16 Shear test

Fig. 7.17 Load-deformation curve of EBWD-fabricated A-100 in the tensile test

7.2.2 Fracture Toughness The standard C (T) specimen of the EBWD-fabricated A-100 is placed on MTS-810 specimens to test the fracture toughness of three specimens. The results are shown in Table 7.7. As shown in the table, the fracture toughness of the control group is the highest, but its tensile strength and yield strength are significantly lower than the

290

7 Fundamentals of Electron Beam Wire …

Fig. 7.18 Failure photo of EBWD-fabricated A-100 in the tensile test

two test groups. In the test group, the fracture toughness of the 930 °C test group is slightly higher than that of the 1000 °C test group, so are the tensile strength and the yield strength. The factors influencing fracture toughness are very complex. They macroscopically include loading method, workpiece state, environmental temperature, etc., and microscopically include grain size, intergranular strength, strain distribution, etc. The main difference between the test group and the control group is different grain sizes caused by the long-lasting high temperature and high pressure pre-heat treatment. Although the grains are re-refined through the normalizing process, the grain size of the test group is still much larger than that of the control group. The fracture toughness is inversely proportional to the square root of the grain size, and the inter-grain stress intensity factor has a −1/4 correlation with the grain size [4]. This is consistent with the experimental results that the fracture toughness of the control group is higher than that of the test group, and the fracture toughness at 930 °C is higher than that at 1000 °C. Compared with the average fracture toughness of forgings, there is a big gap between the control group and the test group, which is related to the technological state of EBWD. During the thermal deformation of forged parts, their internal structure changes, the excessively long grains are broken, re-nucleate and grow, and the grains are refined. The loose structure becomes compact under the action of the applied load. The defects in the billet gradually lessen [5]. During the EBWD process of AerMet100 steel, the thermal cycle lasts long and is complex, The structure maintains a larger grain size after prototyping. The element segregation caused by the distribution coefficient and pores and microcracks generated during the prototyping process are also preserved. After analyzing the test results, we know the grain size has a greater effect on thermal fracture than microscopic defects. As for the phenomenon that the fracture toughness of AerMet100 steel is not significantly improved by the pre-heat treatment, the existing data analysis tells us that we should ensure homogenization and hot isostatic pressing to reduce defects and improve the strength while applying the normalizing treatment, so as to further refine the coarse grains due to long-term heat preservation, improve the distribution of structure forms, and obtain the AerMet100 steel parts of guaranteed strength and improved fracture toughness.

7.2 Properties Control Methods of A-100 Fabricated by EBWD

291

Table 7.5 Result of tensile test of EBWD-fabricated A-100 Number

Specimen Diameter/mm

Area/mm2

Failure load/N

D1

D2

D3

D mean

K1121A001

9.03

9.03

9.03

9.03

64.01

126,266

K1121A002

9.03

9.03

9.04

9.03

64.06

126,315

K1121A003

9.03

9.03

9.03

9.03

64.01

126,225

K1121A004

9.02

9.03

9.02

9.02

63.92

126,358

K1121A005

9.03

9.03

9.03

9.03

64.01

126,613

K1121A006

9.03

9.03

9.03

9.03

64.01

126,931

K1121A007

9.02

9.02

9.02

9.02

63.87

126,234

K1121A008

9.02

9.02

9.03

9.02

63.92

126,785

K1121A009

9.03

9.02

9.02

9.02

63.92

126,830

K1121A010

9.02

9.02

9.03

9.02

63.92

126,294

K1121A011

9.03

9.03

9.03

9.03

64.01

126,916

K1121A012

9.01

9.01

9.02

9.01

63.77

126,355

K1121B101

9.02

9.02

9.02

9.02

63.87

126,212

K1121B102

9.02

9.03

9.02

9.02

63.92

126,165

K1121B103

9.02

9.02

9.02

9.02

63.87

126,558

K1121B104

9.03

9.03

9.03

9.03

64.01

126,250

K1121B105

9.03

9.03

9.03

9.03

64.01

126,313

K1121B106

9.03

9.03

9.04

9.03

64.06

126,549

K1121B107

9.02

9.03

9.02

9.02

63.92

126,357

K1121B108

9.01

9.01

9.02

9.01

63.77

126,572

K1121B109

9.03

9.03

9.04

9.03

64.06

126,436

K1121B110

9.02

9.02

9.02

9.02

63.87

126,970

K1121B111

9.02

9.02

9.02

9.02

63.87

126,288

K1121B112

9.02

9.01

9.02

9.02

63.82

126,029

K1121C201

9.03

9.03

9.04

9.03

64.06

128,081

K1121C202

9.03

9.03

9.03

9.03

64.01

127,331

K1121C203

9.02

9.02

9.03

9.02

63.92

127,488

K1121C204

9.03

9.02

9.02

9.02

63.92

126,963

K1121C205

9.03

9.02

9.03

9.03

63.96

126,974

K1121C206

9.03

9.03

9.03

9.03

64.01

126,959

K1121C207

9.00

9.01

9.00

9.00

63.63

127,605

K1121C208

9.03

9.03

9.03

9.03

64.01

126,981

K1121C209

9.02

9.02

9.02

9.02

63.87

126,966

K1121C210

9.03

9.03

9.03

9.03

64.01

127,383

K1121C211

9.03

9.03

9.03

9.03

64.01

127,141 (continued)

292

7 Fundamentals of Electron Beam Wire …

Table 7.5 (continued) Number

Specimen Diameter/mm

Area/mm2

Failure load/N

D1

D2

D3

D mean

K1121C212

9.03

9.04

9.04

9.04

64.10

126,925

K1121A001

9.03

9.03

9.03

9.03

64.01

126,266

K1121A002

9.03

9.03

9.04

9.03

64.06

126,315

K1121A003

9.03

9.03

9.03

9.03

64.01

126,225

K1121A004

9.02

9.03

9.02

9.02

63.92

126,358

K1121A005

9.03

9.03

9.03

9.03

64.01

126,613

K1121A006

9.03

9.03

9.03

9.03

64.01

126,931

Fig. 7.19 Load-deformation curve of EBWD-fabricated A-100 in the compression test

7.3 Typical Defects and Control Methods of A-100 Material Fabricated by EBWD The main defects in the EBWD process of A-100 include pores and microcracks. Han Liheng et al. [8] conducted a preliminary study on defects in prototyping through ultrasonic inspections combined with X-ray inspections. The test results are shown in Fig. 7.22. According to the preliminary analysis of X-ray inspection results, the density distribution of A-100 steel fabricated by EBWD is more uniform and consistent than that of A-100 steel forgings of the same thickness and same specifications. No obvious defects are found inside. The X-ray inspection only detects a 0.3 mm pore defect, which is a far cry from the multiple suspected signal areas calibrated by the ultrasonic inspection. During the ultrasonic inspection, some suspected inspection signals produce stronger echo signals when the ultrasonic incident angle is inclined than when the ultrasonic incident angle is perpendicular. It can be preliminarily determined that the defects causing suspected signals, as detected by the ultrasound, are distributed in parallel with or similar to the inspection surface, indicating that the defects causing suspected signals should be parallel with or form a small angle with the inspection surface. This is well aligned with the theoretically easy failure of X-ray

7.3 Typical Defects and Control Methods of A-100 …

293

Table 7.6 Result of shear test of EBWD-fabricated A-100 Number

Diameter/mm

Area/mm2

Shear load/N

Shear Strength/MPa

D1

D2

D3

D mean

K3221A001

10.00

10.0

10.00

10.00

77.50

194,987

1241.96

K3221A002

10.00

9.99

10.00

10.00

77.45

194,291

1237.52

K3221A003

9.99

10.00

10.00

10.00

77.45

194,005

1235.70

K3221A004

10.00

10.0

10.00

10.00

77.50

191,536

1219.97

K3221A005

10.00

10.0

10.00

10.00

77.50

194,892

1241.35

K3221A006

10.00

10.0

10.00

10.00

77.50

196,592

1252.18

K3221A007

10.00

10.0

9.99

10.00

77.45

193,356

1231.57

K3221A008

10.00

10.0

10.00

10.00

77.50

196,460

1251.34

K3221A009

10.00

10.00

10.00

77.45

196,676

1252.71

K3221A010

10.00

10.0

10.00

10.00

77.50

194,871

1241.22

K322LB101

10.00

10.0

10.00

10.00

77.50

188,697

1201.89

K322LB102

10.00

10.0

10.00

10.00

77.50

187,152

1192.05

K322LB103

10.00

10.0

10.00

10.00

77.50

186,732

1189.38

K322LB104

10.00

10.0

10.00

10.00

77.55

186,973

1190.91

K322LB105

10.00

10.0

9.99

10.00

77.45

186,357

1186.99

K322LB106

10.00

10.00

10.00

77.45

185,120

1179.11

K322LB107

10.00

10.0

10.00

10.00

77.50

185,351

1180.58

K322LB108

10.00

10.0

10.00

10.00

77.50

184,920

1177.83

K322LB109

10.00

10.0

10.01

10.00

77.55

185,012

1177.42

K322LB110

10.00

10.0

10.00

10.00

77.50

185,623

1182.31

K322LC201

10.00

10.0

10.00

10.00

77.55

182,901

1164.97

K322LC202

9.99

9.99

9.99

9.99

77.34

183,717

1170.17

K322LC203

9.99

9.98

9.99

9.99

77.29

183,185

1166.78

K322LC204

10.00

10.0

9.99

10.00

77.45

183,911

1171.41

K322LC205

10.00

10.0

10.00

10.00

77.50

183,800

1170.70

K322LC206

9.99

9.99

9.99

9.99

77.34

183,087

1166.16

K322LC207

9.99

9.99

10.00

9.99

77.40

183,251

1167.20

K322LC208

9.98

9.99

9.99

9.99

77.29

183,536

1169.02

K322LC209

9.99

10.00

9.99

9.99

77.40

183,545

1169.08

K322LC210

9.99

9.99

9.99

9.99

77.34

184,012

1172.05

9.99

9.99

inspections. In addition, the smallness of defects is possibly a reason for the failure of X-ray inspections. The reasons why the X-ray inspection fails to identify defect signals also include the special microstructure of EBWD-fabricated A-100 steel parts, the suspected ultrasonic signals caused by the discontinuity of the microstructure, and the worse inspection capability of the X-ray inspection to detect microstructure

294

7 Fundamentals of Electron Beam Wire …

Fig. 7.20 Yield photo of EBWD-fabricated A-100 in the compression test

Fig. 7.21 Failure photo of EBWD-fabricated A-100 in the shear test

Table 7.7 Influence of pretreatment parameters on fracture toughness

KIC /MPam1/2

Rm /MPa

Rp0.2 /MPa

930 °C test group

79.7

1930

1650

1000 °C test group

76.2

1910

1630

Control group

83.3

1860

1550

differences than the ultrasonic inspection. The metallographic inspection will be used for validation and analysis.

Fig. 7.22 X-ray inspection of A-100 steel specimen

7.3 Typical Defects and Control Methods of A-100 …

295

Fig. 7.23 Macrostructure photo of the A surface of the EBWD-fabricated A-100 steel specimen

However, considering the theoretical analysis, the following three factors are considered: First, the X-ray inspection has poor penetrating ability and is not suitable for inspecting ultra-thick steel parts, and its capability of detecting small defects is significantly weakened as the inspection thickness increases. Second, the defects of EBWD-fabricated A-100 steel are small. They are parallel with or form a small angle with the inspection surface, which is not conducive to the X-ray inspection that is only sensitive to volumetric defects. Third, the ultrasonic inspection is more sensitive than the X-ray inspection considering the uneven discontinuity of the microstructure. It can be preliminarily considered that X-ray inspection has certain limitations in the defect inspection of A-100 steel fabricated by EBWD.

7.3.1 Metallographic Inspection (1)

Metallographic inspection of surface macrostructure

The specimens for the ultrasonic inspection are those with machined and uniformly rough surfaces, as shown in Fig. 7.23a. After corrosion of the A surface of the specimen, we found that the surface of the A-100 steel part fabricated by EBWD has obvious differences in layer structure and dendrites along the growth direction of the layer (see the framed area in Fig. 7.23b). Suspected micro-crack defects can be visually seen on the surface (see the elliptically marked area in Fig. 7.23b), and the main direction of the suspected defects is the same or tends to be the same as the direction of dendrite growth. After observing the suspected crack areas with the scanning electron microscope, we further confirmed that the defects found are microcracks, and the defect direction is consistent with the direction of dendrite growth, as shown in Fig. 7.24. It can be seen that the micro-crack defects of A-100 steel parts fabricated by EBWD have a tendency to distribute along the direction of dendrite growth. This can explain

296

7 Fundamentals of Electron Beam Wire …

Fig. 7.24 SEM photo of the A surface of EBWD-fabricated A-100 steel

the messy inspection signals and the failure of distinguishing crack defect signals from structure signals when the ultrasonic incident direction is perpendicular to the direction of dendrite growth. (2) ➀

➁

Metallographic anatomy and inspection inside the abnormal area The attenuation of the bottom wave of the incident ultrasonic wave after passing through the workpiece shows an obvious strip-like display, that is, the attenuation of the ultrasonic wave in different layers of the EBWD-fabricated A-100 steel is different. After studying the variation trend of the ultrasonic propagation velocity and the attenuation coefficient, we found that the ultrasonic propagation velocity and the attenuation coefficient of different layers vary significantly. The strip-like area with large bottom wave attenuation has a larger attenuation coefficient, but the ultrasonic propagation velocity in this area is greater than that in the area with less attenuation, and the change of sound velocity is more sensitive. The change of sound velocity in the areas with small attenuation also varies to a certain extent, but is somewhat consistent in the strip direction. During the ultrasonic inspection, echo signals similar to defects are found in some areas of the workpiece. The C-scan image of the defect shows a flocculent shape, and the echo depth at different positions of the flocculent varies from 1 to 4 mm. As for the inspection surface A, when the ultrasonic incident angle is different, the definition of the C-scan image varies significantly. Some display is clear when the incident angle is perpendicular and some is clear when the incident angle is 5° or 10°. When the incident angle is larger, the defect signal disappears. In order to preliminarily analyze the causes of the changes in the ultrasonic propagation signal, we calibrated the areas on the specimen where the above-mentioned changes occur, and ran metallographic anatomies for validation. The sampling position is shown in Fig. 7.25 (forging control specimens are sampled at the equivalent position of No. 6 forging of the same specification, marked 6-No. 1), and the red mark indicates the observation surface of the corresponding numbered specimen. In the figure, No. 5, No. 7 and No. 9 indicate the locations where defect signals are found in ultrasonic inspection; No. 3, No. 4 indicate the locations where suspected defect signals are found; No. 6, No. 7, No. 8 indicate the measurement area of sound velocity and ultrasonic

7.3 Typical Defects and Control Methods of A-100 …

297

attenuation; No. 1, No. 2 indicate the areas with obvious strip-like ultrasonic attenuation (consistent with the attenuation characteristics at No. 6, No. 7, No. 8 sampling positions). When we performed the ultrasonic inspection on the specimen perpendicular to the No. 5 and No. 10 observation surfaces, the bottom wave attenuation also shows a strip-like shape. Then we added more specimens to the No. 10 observation surface as a supplement to No. 5 observation surface. Considering the characteristics of ultrasonic inspection, we believe the ultrasonic inspection of EBWD-fabricated A-100 steel has two main characteristics:

7.3.2 Correlation Between Discontinuity and Inspection Signals We ran the metallographic inspection on the No. 3, No. 4, No. 5, No. 7, and No. 9 positions where defect signals are found by the ultrasonic inspection. According to the results of the ultrasonic inspection, the No. 3 position shows the transverse wave inspection on the A surface; the No. 5 and No. 7 positions show the inspection of vertical incidence of longitudinal waves on the A surface; the No. 9 position clearly shows the line-shaped phased array detection (including the reflection image of endless angle on the whole strip when the No. 9 strip-like area undergoes the oblique incidence inspection by the focusing probe, and an end angle reflection clearly shows at the same position of the forging); the No. 4 position shows the vertical incident

Fig. 7.25 Schematic diagram of the metallographic sampling position and observation surfaces of EBWD-fabricated A-100 steel

298

7 Fundamentals of Electron Beam Wire …

Fig. 7.26 Metallographic image of crack defects of EBWD-fabricated A-100 steel (No. 5 specimen) (the triangle mark is the side of the ultrasonic inspection surface)

inspection of longitudinal waves (no defect is displayed on this inspection surface when the ring-shaped phased array detection is performed by the dynamic in-depth focusing probe). The metallographic inspection finds crack defects at No. 5, No. 7 and No. 9 positions, as shown in Figs. 7.26 and 7.27, while more obvious defects are found at No. 3 position. Structural differences (the absence of defects may be related to the deviation of the cutting position) are shown in Fig. 7.28. No defect display is found at No. 4 position, but there is also structural nonuniformity, as shown in Fig. 7.29. We preliminarily presume that the crescent image may be the false display caused by the proximity of the probe to the side wall, resulting in interference from the side wall. It may also be caused by the cutting error that leads to the failure of identifying the defect. The inspection characteristics of this inspection surface are to be further studied by testing the specimen with large planes. As shown in the metallographic image of the above crack defects, the crack defects inside the EBWD-fabricated A-100 steel have a small inclination or parallel state according to the aforementioned theoretical analysis. The crack depth is small compared with the inspection surface A parallel to the growth direction of dendrites, and there are small deep cracks at a certain angle to the inspection surface in the crack direction, and there are also microcracks parallel with the inspection surface. Apparently, the vertical longitudinal wave incidence method is used on the inspection surface A. When being used in combination with the small-angle transverse wave

7.3 Typical Defects and Control Methods of A-100 …

a) No.7 specimen

299

b) No.9 specimen

Fig. 7.27 Metallographic image of crack defects of EBWD-fabricated A-100 steel (No. 7 specimen and No. 9 specimen, the triangle mark is the side of the inspection surface)

Fig. 7.28 Structural nonuniformity of EBWD-fabricated A-100 steel specimen (No. 3 specimen)

method, it can better and more comprehensively detect the small cracks in the A-100 steel fabricated by EBWD.

300

7 Fundamentals of Electron Beam Wire …

Fig. 7.29 Structural nonuniformity of EBWD-fabricated A-100 steel specimen (No. 4 specimen)

7.3.3 Correlation Between Internal Structure and Acoustic Parameters We performed metallographic observations on two mutually vertical structures at No. 6 (forgings 6-No. 1), No. 7, No. 8, No. 5, and No. 10 specimens shown in Fig. 7.25 for the areas showing uneven changes of the acoustic parameters of the ultrasonic propagation of EBWD-fabricated A-100 steel. We also performed metallographic observations on No. 6 (forgings 6-No. 1), No. 7, and No. 8 specimens perpendicular to the end face of the ultrasonic attenuation distribution strip (i.e. the width direction section of the measurement area of acoustic characteristic parameters), as shown in Figs. 7.30 and 7.31. The ultrasonic attenuation of the No. 1 and No. 2 specimens is similar to that of the No. 6–8 areas, therefore we took samples and ran metallographic observation on equivalent end faces, as shown in Figures 7.32 through 7.36. No. 5 and No. 10 specimens are the metallographic observation of the end surface parallel with the ultrasonic attenuation strip, as shown in Fig. 7.37.

7.3 Typical Defects and Control Methods of A-100 …

301

Fig. 7.30 Metallographic image of A-100 steel forging (No. 6-1 specimen)

Fig. 7.31 Structural nonuniformity of EBWD-fabricated A-100 steel specimen (No. 6 specimen)

After comparing the metallographic images, we found that the forging structure presents a small unit cell. The EBWD-fabricated A-100 steel has obvious characteristics of directionally growing dendrites. Due to the different deposition sequence of the upper and lower parts, the width of the dendrites presents obvious inconformity, forming different strips. This is related to the complex repeated thermal cycles in the EBWD process. In the strip-like areas where the attenuation is large,

302

7 Fundamentals of Electron Beam Wire …

Fig. 7.32 Structural nonuniformity of EBWD-fabricated A-100 steel specimen (No. 7 specimen)

Fig. 7.33 Structural nonuniformity of EBWD-fabricated A-100 steel specimen (No. 8 specimen)

Fig. 7.34 Structural nonuniformity of EBWD-fabricated A-100 steel specimen (continuous metallographic images of No. 6–8 specimens—10 times)

the grains are coarser than those in other areas. The factors that change the ultrasonic propagation characteristics are complex and diverse, such as the grain size, the shape of crystal grains, the grain boundary, the various phases of the structure and the size of the phases. All these factors can cause variations in propagation characteristics. However, by utilizing the characteristics of the ultrasonic propagation of

7.3 Typical Defects and Control Methods of A-100 …

303

Fig. 7.35 Structural nonuniformity of EBWD-fabricated A-100 steel specimen (No. 2 specimen)

Fig. 7.36 Structural nonuniformity of EBWD-fabricated A-100 steel specimen (No. 1 specimen)

specific microstructures, we can conduct non-destructive evaluations of materials, such as determining material porosity, distinguishing different heat treatment states, and microstructure uniformity. It is feasible to further utilize the ultrasonic propagation characteristic parameters of the EBWD-fabricated A-100 steel to evaluate the internal microstructure uniformity or different heat treatment states. Since the EBWD process is carried out in a vacuum environment, the formation of pores and microcracks are not subjected to oxidation or nitridation, and the pores are

304

7 Fundamentals of Electron Beam Wire …

Fig. 7.37 Structural nonuniformity of EBWD-fabricated A-100 steel specimen

vacuum. We can use the hot isostatic pressing technology to eliminate the defects. Hot isostatic pressing mainly works to compact the pores and microcracks formed during the prototyping.

7.4 Static Property of Typical A-100 Components Fabricated by EBWD 7.4.1 Lug Specimen of Axial Load The lug specimen of axial load is designed according to the shear-extrusion failure mode of the lug. The manufacturing technology is EBWD manufacturing. There are 4 specimens, numbered KA2201, KA2202, KA2203, KA2204. The schematic diagram of the specimens is shown in Fig. 7.38.

7.4.2 Lug Specimen of Lateral Load The lug specimen of lateral load was designed according to the shear-fracture failure mode of the lug. The manufacturing process is electron beam additive manufacturing. There are 4 specimens, numbered KT2201, KT22012, KT2203, KT2204. The schematic diagram of the specimens is shown in Fig. 7.39.

7.4.3 Constraint Method The boundary constraint conditions of specimens are shown in Table 7.8.

7.4 Static Property of Typical A-100 Components Fabricated by EBWD

Fig. 7.38 Structure diagram of axial load specimens of A-100 steel fabricated by EBWD

Fig. 7.39 Schematic diagram of lateral load specimens of A-100 steel fabricated by EBWD

305

306

7 Fundamentals of Electron Beam Wire …

Table 7.8 Boundary constraints of specimens Specimen

Test form

Test load

Boundary constraints

Axial lug

Axial tension

Axial load

Simple support at both ends

Lateral lug Lateral tension Lateral load Simple support for single lug, fixed support for base

Table 7.9 Test load No.

Loading direction

Specimen number

Test load (N)

1 2

Axial load

KA2201, KA2201, KA2203, KA2204

642,850

Lateral load

KT2201, KT2201, KT2203, KT2204

708,746

Table 7.10 List of test equipment No.

Name of device or instrument

Quantity

Technical requirements

1

1500 kN actuator

1 unit

Range: 1500 kN Accuracy: ±0.5%

2

Coordinated loading system FlexTest200

1 set

–

3

Temperature and humidity meter

1 unit

0.1 °C /0.1%

7.4.4 Test Load The test load parameters are shown in Table 7.9.

7.4.5 Test Equipment The models and parameters of the test equipment are shown in Table 7.10.

7.4.6 Test Installation Installation of the Lug Specimen of Axial Load The specimen is loaded with a 1500 kN actuator (installed on the overall steel frame). The lower end of the specimen is restrained by the double lugs (made of 30CrMnSiNi2A) installed on the base. The upper end is connected to the actuator through a single/double-lug connecting plate (made of 30CrMnSiNi2A). The upper and lower ends of the specimen are connected to the fixture with pins (made of A100). The schematic diagram of the specimen installation is shown in Figs. 7.40 and 7.41.

7.4 Static Property of Typical A-100 Components Fabricated by EBWD

307

Fig. 7.40 Schematic diagram of the specimen installation (axial)

Fig. 7.41 Specimen installation (axial)

Installation of the lug specimen of lateral load The specimen is loaded with a 1500 kN actuator (installed on the overall steel frame). The specimen is installed to the side of the base through 6 M22 bolts (made of 30CrMnSiNi2A), and the upper end is connected to the actuator through a single/double-lug connecting plate (made of 30CrMnSiNi2A). All specimens are connected with the single/double-lug connecting plate using pins (made of A-100). The schematic diagram of the specimen installation is shown in Figs. 7.42 and 7.43.

308

7 Fundamentals of Electron Beam Wire …

Fig. 7.42 Schematic diagram of the specimen installation (lateral)

Fig. 7.43 Specimen installation (lateral)

7.4.7 Test Results The results of the axial test are shown in Table 7.11, and the failure images of the tests are shown in Figs. 7.44 and 7.45.

7.4 Static Property of Typical A-100 Components Fabricated by EBWD

309

Table 7.11 Test results No.

Load

Specimen number

Test load/N

Failure load/N

Failure mode

1

Axial load

KA2202

642,850

620,586

Tension-shear compound failure

2

KA2203

635,679

Tension-shear compound failure

3

KA2204

626,815

Tension-shear compound failure

800,289

Shear failure

8

Lateral loading

KT2201

708,746

9

KT2202

815,757

Shear failure

10

KT2204

817,555

Shear failure

Fig. 7.44 Test images after failure (axial)

Fig. 7.45 Test images after failure (lateral)

310

7 Fundamentals of Electron Beam Wire …

References 1. Cao R, Chen J, Yan Y, Du W, Peng Y, Tian Z (2009) Research on the bending fracture mechanism of a new type of 980MPa high-strength steel [J/OL]. Sciencepaper Online 2. Li W, Li JCM (1991) The effect of grain size on fracture toughness. J Wuhan Tech Univ Surv Mapp 16(2):79–90 3. Xiaohong Y, Shihong Z, Zhongtang W (2007) Thermal deformation behavior of AerMet100 ultra-high-strength steel. J Plast Eng 14(6):121–126 4. Yang Fan (2014) The effect of homogenized heat treatment and hot isostatic pressing on the properties of AerMet100 steel fabricated by electron beam. Beijing: China academy of aeronautics and astronautics 5. Fan Y, Shuili G, Hongbo S, Zhitao H, Guang Y (2014) The effect of hot isostatic pressing on the properties of AerMet100 steel fabricated by electron beam prototyping. Aeronaut Manuf Technol 2015(15):90–93 6. Brice CA, Henn DS (2002) Rapid prototyping and freeform fabrication via electron beam welding deposition[C]. In: Proceeding of welding conference 7. Walker BH, Walker CM (2003) Material presented on Canadian aviation expo[R]. Montreal, Canada 8. Han Liheng (2015) Research on ultrasonic inspection characteristics of A-100 steel fabricated by electron beam wire deposition. China Academy of aeronautics and astronautics, Beijing 9. Fang Weiping, et al. Journal of Materials Engineering[J]. 2010(9): 95 10. Zhu Zhishou, Wang Xinnan, Tong Lu, et al. Research on new titanium alloy used by China’s flight structures [J]. Titanium, 2007, 24(6): 28-32. 11. Suo Hongbo,ChenZheyuan, et al. Microstructure and Mechanical Properties of Ti6Al-4V by Electron Beam Rapid Manufacturing [J]. Rare Metal Materials and Engineering,2014,43(4):0780-0785. 12. Zan Lin, Chen Jing, Lin Xin, et al. Research on the deposited structure of TC21 titanium alloy fabricated by laser melting deposition [J]. Rare Metal Materials and Engineering, 2007, 36(4): 612-616. 13. Lu Wei, Shi Yaowu, et al. Microstructure characteristics of the electron beam welding joints of thick-walled TC4-DT titanium alloy [J]. Rare Metal Materials and Engineering, 2013, 42 (4): 54~57. 14. Speich G S. Innovations in Ultrahigh-strength steel Technology [C]. 34th Sagamore Army Materials Research Conference, 1990

Chapter 8

Fundamentals of Electron Beam Wire Deposition Hybrid Prototyping Technology

Abstract This chapter introduces fundamentals of electron beam wire deposition hybrid prototyping technology. Microstructure characteristics of TC4-DT titanium alloy fabricated by electron beam wire deposition hybrid prototyping have been studied in this chapter. At the same time, the mechanical properties of hybridfabricated titanium alloy components and the defect control of fording-EBWD structures have been investigated.

One of the main requirements for the structure of a modern aircraft is a lower structural weight coefficient while meeting functional requirements. The structure made of integral titanium alloy forgings boasts sound rigidity, less parts and less structural weight. However, during the development of the demonstration engine, we found that the properties of the super-sized titanium alloy forging are uneven. Especially, the conventional tensile properties of different parts on the forgings with large differences in section thickness may vary significantly, and even their microstructures vary greatly. We had to reduce the allowable value of the material during the design process, which restrained the property advantages of the material. In addition, due to the complexity of the structure, there are many undetectable areas and residual closed corners remained, resulting in increased weight of the structure. For connection or functional requirements, the overall part needs to be thickened in some positions, which requires the use of super-sized billets or to be mechanically connected in segments. But these two methods will increase the manufacturing cost and weight [1–3]. The use of the additive manufacturing technology can quickly, directly and accurately transform design ideas into models or parts with certain properties, effectively shorten the development cycle, reduce costs, and solve the machining problems faced by some complexly-shaped parts. Therefore this technology gains wide attention nationwide. The electron beam wire deposition technology allows us to manufacture new structures based on titanium alloy forgings/castings. When using the electron beam wire deposition hybrid prototyping technology to manufacture titanium alloy structures, we can simplify the shape of original forgings, reduce the forging difficulty of super-thick forgings, reduce the connections caused by new parts, reduce structural weight, improve material utilization rate, and reduce the machining workload © National Defense Industry Press 2022 S. Gong et al., Electron Beam Wire Deposition Technology and Its Application, Additive Manufacturing Technology, https://doi.org/10.1007/978-981-19-0759-3_8

311

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8 Fundamentals of Electron Beam Wire …

of parts. The microstructure of titanium alloy parts in the interface zone fabricated by the electron beam wire deposition hybrid prototyping technology shows very obvious gradient characteristics. The process of wire deposition poses a very evident thermal effect on the matrix of the original forging. Therefore the interface zone is the weak point of the whole structure. The key of the hybrid manufacturing technology is how to effectively control the structural and mechanical properties of this zone. The research team of the author has studied the electron beam hybrid manufacturing technology for complex titanium alloy parts, achieved breakthroughs in key technical challenges including the structural and mechanical properties optimization for hybrid manufacturing interfaces, achieved the electron beam hybrid manufacturing of complex titanium alloy beams, obtained a new method for manufacturing complex titanium alloy structures, and laid a solid technological foundation for the development and manufacturing of frame and beam structures on next-generation aircraft.

8.1 Microstructure Characteristics of TC4-DT Titanium Alloy Fabricated by Electron Beam Wire Deposition Hybrid Prototyping Figure 8.1 is the macro morphology photo of the electron beam hybrid manufacturing joints of TC4-DT titanium alloy. It can be divided into three zones according to the different characteristics of the structure: the forging matrix zone, the transition zone,

Fig. 8.1 Macro morphology of TC4-DT electron beam hybrid manufacturing joint

8.1 Microstructure Characteristics of TC4-DT Titanium Alloy …

313

Fig. 8.2 Microstructure morphology of the forging matrix zone

and the wire deposition prototyping zone. The characteristics of the deposition layer can be clearly distinguished in the wire deposition prototyping area, and the width of the transition zone is 3 mm.

8.1.1 Microstructure Characteristics of the Forging Matrix Zone Figure 8.2 is a photo of the microstructure of forging matrix zone. As shown in the photo, this zone is a typical equiaxed structure where the grain size is about 10 μm. It is composed of equiaxed α phases and intercrystalline β phases. The content of α phases is above 90%.

8.1.2 Microstructure Characteristics of the Transition Zone The macroscopic morphology of the transition zone of hybrid manufacturing is shown in Fig. 8.3a. Small equiaxed crystals are mainly seen at the bottom of the transition zone. The size of the equiaxed crystals is 40–90 μm, and the grain size grows bigger when getting closer to the wire deposition zone. The size of the equiaxed crystals grows to 100–300 μm at the top of the transition zone near the wire deposition zone, and gradually transits to columnar crystals. The structural morphology of the transition zone may vary as the distance with the wire deposition zone changes. There is little difference in structural morphology between the bottom of the transition zone and the forging matrix zone, as shown in Fig. 8.3b, both of which are equiaxed α phase and intercrystalline β phase. The content of β phase is increased when compared with that of the forging matrix. The

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8 Fundamentals of Electron Beam Wire …

Fig. 8.3 Microstructure morphology of the transition zone (a Macroscopic morphology of the transition zone, b bottom, c middle, d top)

middle part of the transition zone is mainly composed of fine needle-like α-phase and β-transition structures with blurred grain boundaries, as shown in Fig. 8.3c. The structural characteristics at the top of the transition zone are shown in Fig. 8.3d, which is mainly composed of needle-shaped α phase and residual β phase. The solidification rate of the molten pool in the electron beam hybrid manufacturing process is very fast. Since multiple thermal cycles will be generated due to layer-by-layer deposition, the bottom of the transition zone is less affected by the heat of the molten pool, and the temperature is low, which cannot reach the transition temperature of martensitic, but only reach the temperature for the increase of the intercrystalline β phase content. The temperature is high in the middle of the transition zone, and can reach the upper edge of the α + β phase region, so the nearby intercrystalline β phase transforms into a fine needle-like α phase, penetrates into the interior of the α phase, making the grain boundary blurred. The temperature at the top of the transition zone can top the β phase transition point. Due to the rapid heating and cooling rate, the α-to-β phase transition may be incomplete. The α phase is not transited during fast cooling, and the β is transited into the needle-like martensitic α phase. After multiple thermal cycles that are equivalent to annealing, the α phase is gradually transformed into the α phase [4, 5].

8.1 Microstructure Characteristics of TC4-DT Titanium Alloy …

315

8.1.3 Microstructure Characteristics of the Wire Deposition Zone The morphology of the wire deposition zone is shown in Fig. 8.4. It is mainly composed of coarse columnar crystals, with a width ranging from 200 μm to 2000 μm. They grow outwards along the deposition height direction based on the equiaxed crystals in the transition zone. The inside of the columnar crystals is a basket-like structure. The length of the α phase ranges from about 30 to 100 μm, and the width is about 1–2 μm, which is smaller than the size of the needle-shaped α phase in the transition zone. This is because the wire deposition zone is directly and rapidly solidified from the liquid metal, which will form the needle-like martensitic α phase. After many thermal cycles, the unstable martensitic α phase will transform into the basket-like α phase, and finally form the basket-like α phase and residual β phase.

8.2 Characteristics of Mechanical Properties The properties of EBWD-fabricated specimens are closely related to the prototyping technology, and the properties under different technological conditions may vary significantly. In this article, we analyzed the mechanical property characteristics under several different technological conditions. The test conditions are shown in Table 8.1.

(a) Macrostructure characteristics

(b) Microstructure characteristics

Fig. 8.4 Microstructure morphology of the wire deposition zone

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8 Fundamentals of Electron Beam Wire …

Table 8.1 Main parameters of the four prototyping technologies

Electron Wire beam current

Matrix state

Specimen no. 1

Large beam current (130 mA)

Thick wire (F 2.0), double wire

Matrix is not heated

Specimen no. 2

Small beam current (35 mA)

Thick wire (F 2.0), single wire

Matrix is heated

Specimen no. 3

Small beam current (35 mA)

Thick wire (F 2.0), single wire

Matrix is not heated

Specimen no. 4

Small beam current (20 mA)

Thin wire (F 1.2), single wire

Matrix is heated

8.2.1 Tensile Properties at Room Temperature We tested the room temperature tensile properties of the Z direction of the hybrid manufacturing joint, the Y direction of the EBWD subject, and the Z direction of the forging under four technological conditions. The test results are shown in Table 8.2. The test results show that the fracture position of the hybrid manufacturing joint is located in the EBWD subject in the case of large beam currents (130 and 35 mA). The room temperature strength of the EBWD-fabricated part is lower than that of the forging, and the strength decreases as the beam current increases. When the beam current is small (20 mA), the fracture position of the hybrid manufacturing joint is located in the forging, and the room temperature strength of wire deposition is higher than that of the forging, and the ductility is equivalent. As we can see from the tensile fracture, the fractures of the TC4-DT alloy fabricated by EBWD are ductile fractures. There are two typical macroscopic forms of the fracture. The type I fracture (as shown in Fig. 8.5) features a flat surface and it forms a 50–60° angle with the specimen axis. The type II fracture has obvious tear characteristics. In the test results, most fractures are type II fractures and only a few of them are type I. Type I fractures appear in various directions and technological conditions, but the number is very small, and the specimen necking is not obvious. When comparing it with the mechanical properties, we found that its tensile strength and yield strength are on a par with the normal fracture, and the elongation is slightly lower. The reduction of area is significantly reduced, but is still over 20%. We did not find obvious fiber areas and shear lips on the electron microscope image (as shown in Fig. 8.6). The radial texture is clearly displayed along the direction of the fracture from top to bottom, and equiaxed dimples are seen in all parts of the fracture. Most specimens in the EBWD tensile test have cut-off type II fractures (as shown in Fig. 8.7), but the fracture characteristics are different from the smooth specimens of general homogeneous materials. The surface of the EBWD-fabricated TC4-DT alloy’s tensile fracture is staggered and undulating, with sharp protrusions and depressions covering the fracture. There is no continuous fiber zone plane, radiation zone,

8.2 Characteristics of Mechanical Properties

317

Table 8.2 Tensile properties of joints between the deposit and the matrix at room temperature Specimen no.

Rp0.2 /(N/mm2 )

1Z-1

735.4

1Z-2

Rm /(N/mm2 )

A/%

Z/%

Fracture location

Sampling location

780.8

13.3

50.4

Through one side of the deposit

Through the interface

745.2

795.7

14.2

50.3

Through one side of the deposit

Through the interface

1Z-3

730.2

778.8

15.2

48.9

Through one side of the deposit

Through the interface

1Y-1

750.1

808.2

13.8

45.2

Random

EBWD subject

1Y-2

752.9

810.4

13.2

43.1

Random

EBWD subject

1Y-3

755.4

815.5

14.1

45.2

Random

EBWD subject

2Z-1

761.9

805.1

12.5

50.7

Through one side of the deposit

Through the interface

2Z-2

796.6

831.1

13.6

44.3

Through one side of the deposit

Through the interface

2Z-3

788.4

828.0

13.3

47

Through one side of the deposit

Through the interface

2Y-1

788.2

826.9

13.6

46.1

Random

EBWD subject

2Y-2

778.3

835.6

12.3

43.4

Random

EBWD subject

2Y-3

800.9

835.6

14.7

47.0

Random

EBWD subject

3Z-1

776.7

828.1

13.3

44.8

Through one side of the deposit

Through the interface

3Z-2

792.2

831.1

14.6

46.1

Through one side of the deposit

Through the interface

3Z-3

778.3

826.9

13.8

48.4

Through one side of the deposit

Through the interface

3Y-1

781.2

842.2

12.9

40.5

Random

EBWD subject (continued)

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8 Fundamentals of Electron Beam Wire …

Table 8.2 (continued) Specimen no.

Rp0.2 /(N/mm2 )

3Y-3

796.6

3Y-3

Rm /(N/mm2 )

A/%

Z/%

Fracture location

Sampling location

831.1

12.4

44.9

Random

EBWD subject

792.2

838.8

12.2

44.3

Random

EBWD subject

4Z-1

883.38

948.45

11.04

42.46

Through one side of the forging

Through the interface

4Z-2

874.04

928.05

8.80

32.61

Through one side of the forging

Through the interface

4Z-3

871.95

927.46

11.40

42.76

Through one side of the forging

Through the interface

4Z-4

910.32

1024.67

8.48

42.91

Random

EBWD subject

4Z-5

940.62

1016.46

10.56

26.92

Random

EBWD subject

4Z-6

912.97

1000.32

7.08

18.06

Random

EBWD subject

D-1

885.2

940.4

10.4

23.0

Random

Forging

D-2

880.1

932.2

11.5

18.2

Random

Forging

Fig. 8.5 Tensile fracture of 1Y-2 specimen (Type I fracture)

or shear lip, and it is difficult to be distinguished visually. But as seen in the electron microscope image of section scanning (as shown in Fig. 8.8), it is possible to distinguish the irregular fiber zone and the broken shear lip in most fractures, but the radiation zone is not obvious.

8.2 Characteristics of Mechanical Properties Fig. 8.6 Microstructure SEM image of the tensile fracture of 1Y-2 specimen

Fig. 8.7 Tensile fracture of 3Z-1 specimen (Type II fracture)

Fig. 8.8 Microstructure SEM image of the tensile fracture of 3Z-1 specimen

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8.2.2 Impact Property at Room Temperature We tested the impact properties of the four technologies at room temperature, and the test results are shown in Table 8.3. The results show that the room temperature impact properties of the TC4-DT EBWD joint under four technological conditions are higher than the standard value of the forging (35 J/cm2 ). When the beam current is 35 mA, the impact toughness of the hybrid manufacturing joint is above 60 J/cm2 and remains very stable. When the beam current is 20 mA, the impact toughness of the hybrid manufacturing joint is about 50 J/cm2 , while that of the EBWD joint is about 60 J /cm2 . The typical sheet layer and the basket structure of EBWD-fabricated parts are beneficial to improve the impact toughness of the material.

8.3 Defect Control of Forging—EBWD Structures The inside of the EBWD hybrid prototyping joint is mainly composed of round micropores whose size is related to the prototyping technology. We pre-treated the matrix to ensure the surface roughness of the deposited matrix is Ra1.6 and used a wire brush to polish the deposition surface to remove the oxide scale before prototyping, and then preheated the matrix with a large beam spot to effectively minimize the defects at the boundary. During the prototyping process, we enhanced the vacuum degree of the vacuum chamber, repeatedly remelted the uneven positions on the deposited layer to reduce the unevenness of the deposited surface and improve the overlap rate and the internal quality. Figure 8.9 shows the X-ray flaw detection films of the hybrid manufacturing joint under a beam current of 20 mA and after multiple times of remelting. Figure 8.10 and Table 8.4 show the results of ultrasonic flaw detection under the same technological conditions, indicating that the EBWD-fabricated structure has unique acoustic reflection characteristics. Currently there is no standard for the ultrasonic inspection of electron beam wire deposition. According to the test results of Beijing Institute of Aeronautical Materials, no abnormal display signal is found at the forging part, and the equivalent of the abnormal display signal at the electron beam position does not exceed  0.8 mm (the contract specimens are made of different materials so the evaluation result is for reference only). When being inspected from two directions, the equivalent of the abnormal display signal of the two surfaces combined does not exceed  0.8 mm. According to the evaluation standard for forgings, the ultrasonic test result of the titanium alloy deposit in this state has reached Level AAA.

8.3 Defect Control of Forging—EBWD Structures

321

Table 8.3 Impact property of hybrid manufacturing joints at room temperature Specimen no.

Specimen orientation

Forging

Z–X direction

No. 1 room temperature impact in Z direction-01

Z–X direction

No. 1 room temperature impact in Z direction-02

Sampling location

Room temperature

68

53

Z–X direction

67

Through the interface

No. 2 room temperature impact in Z direction-03

64

62

Z–X direction

No. 3 room temperature impact in Z direction-02

62

Through the interface

No. 3 room temperature impact in Z direction-03 No. 4 room temperature impact in Z direction-01

≥35 56

Through the interface

No. 2 room temperature impact in Z direction-02

No. 3 room temperature impact in Z direction-01

Impact a kU2 /(J/cm2 )

No. 1 room temperature impact in Z direction-03 No. 2 room temperature impact in Z direction-01

Test temperature

64

63

Z–X direction

50

(continued)

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8 Fundamentals of Electron Beam Wire …

Table 8.3 (continued) Specimen no.

Specimen orientation

No. 4 room temperature impact in Z direction-02

Sampling location

Test temperature

Impact a kU2 /(J/cm2 )

Through the interface

48

No. 4 room temperature impact in Z direction-03

53

No. 4 room temperature impact in Z direction-04

56

No. 4 room temperature impact in Z direction-05 No. 4 room temperature impact in Z direction-06

Z–X direction

EBWD-fabricated part

57

64

Fig. 8.9 Films of X-ray flaw detection of the EBWD hybrid manufacturing joint

8.4 Batch Stability of the Mechanical Properties of Electron Beam Wire Deposition Hybrid Prototyping With a beam current of 35 mA and the feeding of thick single wires, this technology features sound tensile and impact properties, a room-temperature tensile strength

8.4 Batch Stability of the Mechanical Properties of Electron Beam Wire …

323

Fig. 8.10 Results of ultrasonic flaw detection of the electron beam hybrid manufacturing joint

Table 8.4 Results of ultrasonic flaw detection of the electron beam hybrid manufacturing joint

Defect number

Defect buried depth mm

Defect equivalent dB

Remarks

F1

6.5

 0.4–0.5

Defects at the interface

F2

22

 0.4–2.5

Defects at the interface

F3

4

 0.4 + 1.5

F4

4

 0.4 + 5

F5

4

 0.4 + 5

F6

4

 0.4 + 1.5

of about 830 Mpa, and a high prototyping rate (1 kg/h). In order to investigate the batch stability, 3 specimens were prototyped in batches. The specimens are shown in Fig. 8.11. The test items are shown in Table 8.5. The third specimen underwent the high-cycle axial load fatigue test (Kt = 1, R = 0.06). Samples were taken in the transition zone to test the fatigue limit and the SN curve. The test standard is HB5287.

Fig. 8.11 Specimens for mechanical properties

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Table 8.5 Multi-batch uniformity of tensile properties at room temperature σb /MPa

Mean/MPa

Deviation/MPa

Deviation percentage/%

1-H-01

878.47

852.671

26.799

0.031429

1-H-02

883.83

852.671

31.159

0.036543

1-H-03

873.91

852.671

21.239

0.024909

1-H-04

892.45

852.671

38.779

0.046652

1-H-05

858.11

852.671

6.439

0.007552

1-H-06

898.08

852.671

45.409

0.053255

1-H-07

900.61

852.671

47.939

0.056222

1-H-08

883.71

852.671

31.039

0.036402

1-K-01

830.17

852.671

22.501

0.026389

1-K-02

821.75

852.671

30.921

0.036264

1-K-03

848.15

852.671

3.521

0.004129

1-K-04

814.14

852.671

38.531

0.045189

1-K-05

803.85

852.671

48.821

0.057257

1-K-06

815.80

852.671

36.871

0.043242

1-K-07

808.87

852.671

43.801

0.051369

1-K-08

827.87

852.671

24.801

0.029086

2H-01

904.53

865.2894

38.2406

0.04535

2H-02

896.20

865.2894

30.9106

0.035723

2H-03

896.37

865.2894

31.0806

0.035919

2H-04

881.51

865.2894

16.2206

0.018746

2H-05

910.67

865.2894

45.3806

0.052446

2H-06

894.10

865.2894

28.8106

0.033296

2H-07

901.18

865.2894

35.8906

0.041478

2H-08

923.08

865.2894

57.7906

0.066788

2K-01

822.98

865.2894

42.3094

0.048896

2K-02

850.91

865.2894

14.3794

0.016618

2K-03

842.73

865.2894

22.5594

0.026072

2K-04

851.36

865.2894

13.9294

0.016098

2K-05

828.43

865.2894

35.8594

0.041442

2K-06

833.66

865.2894

31.6294

0.036554

2K-07

838.24

865.2894

26.0494

0.030105

2K-08

837.32

865.2894

27.9694

0.032324

2K-09

822.77

865.2894

42.5194

0.049139

2K-10

837.17

865.2894

28.1194

0.032497

Specimen no. First batch

Second batch

(continued)

8.4 Batch Stability of the Mechanical Properties of Electron Beam Wire …

325

Table 8.5 (continued) σb /MPa

Mean/MPa

Deviation/MPa

Deviation percentage/%

3-H-01

865

841.375

23.625

0.028079

3-H-02

858

841.375

16.625

0.019759

3-H-03

867

841.375

25.625

0.030456

3-H-04

846

841.375

4.625

0.005497

3-K-01

820

841.375

21.375

0.025405

3-K-02

820

841.375

21.375

0.025405

3-K-03

830

841.375

11.375

0.01352

3-K-04

825

841.375

16.375

0.019462

Specimen no. Third batch

8.4.1 Tensile Properties at Room Temperature We totally ran 3 batches of tests for the tensile property at room temperature. The number of specimens was 42. The standard round bar specimens were used, and the gauge lengths of the specimens were located in the EBWD subject and the transition zone. The test results show that the tensile strength in the transition zone is slightly higher than that of the EBWD subject. This is because heat dissipation is easier when it is closer to the matrix, therefore the cooling rate is faster, and the crystal grains formed are smaller, which improves its tensile strength. The room-temperature Zdirection tensile strength of the hybrid manufacturing joint and the EBWD subject is higher than 780 MPa. The average value of each batch calculated according to GJB/Z 18A and the deviation of each specimen from the average value of this batch are shown in Table 8.5. After calculation, the deviation, from the average value, of the room-temperature tensile strength σb of single-batch joints along the same specimen orientation in the transition zone and the wire deposition zone is 6.68% at maximum, which is less than 7%.

8.4.2 Fracture Toughness We ran two batches of tests for fracture toughness. The number of samples was 12, and samples were taken from the EBWD subject and the hybrid manufacturing joint. The KIC value could not be measured due to the small sample size, but the effectively measured KQ value was greater than 1.1KIC (standard value), and a > 1.326(KQ/ Rp0.2)2, wherein a is the crack length. It is estimated that the fracture toughness KIC is about 100 MPam1/2 and greater than 75 Mpam1/2 . The KQ average value and deviation of each batch along the same direction calculated according to GJB/Z 18A are shown in Table 8.6. After calculation, the deviation, from the average value, of

326

8 Fundamentals of Electron Beam Wire …

Table 8.6 Multi-batch uniformity of fracture toughness K Q / MPa.m1/2

Mean/ MPa.m1/2

Deviation/ MPa.m1/2

Deviation percentage/%

1-DL-H-01

124

117.8

6.2

0.052632

1-DL-H-02

121

117.8

3.2

0.027165

1-DL-H-03

122

117.8

4.2

0.035654

1-DL-K-01

113.6

117.8

4.2

0.035654

1-DL-K-02

118.4

117.8

0.6

0.005093

1-DL-K-03

107.8

117.8

10

0.08489

2-DL-H-01

123

117.95

5.05

0.042815

2-DL-H-02

125

117.95

7.05

0.059771

2-DL-H-03

116

117.95

1.95

0.016532

2-DL-K-01

116.8

117.95

1.15

0.00975

2-DL-K-02

112.7

117.95

5.25

0.04451

2-DL-K-03

114.2

117.95

3.75

0.031793

Specimen no. First batch

Second batch

the fracture toughness of single-batch joints along the same specimen orientation in the transition zone and the wire deposition zone is 8.4% at maximum, which is less than 12%.

8.5 Research on the Mechanical Properties of Hybrid-Fabricated Titanium Alloy Components Based on the evaluation of the mechanical properties of materials, we ran tests for the mechanical properties of typical components (as shown in Figs. 8.12, 8.13 and 8.14). By comparing the tensile-bearing capacity and fatigue resistance of the forging subject and the EBWD-fabricated part, we obtained the load-bearing characteristics of typical components (lugs). The specimens for the static test are three sizes (d/e = 1.5, 2 and 2.5), and the specimens for the fatigue test are in one size (d/e = 2). During the static test, the specimen is loaded until the lug fails. The load for the fatigue test is 70, 50, and 20% of the maximum failure load used in the static test.

8.5 Research on the Mechanical Properties …

Fig. 8.12 Lug specimen and patch

Fig. 8.13 Lug test

Fig. 8.14 Part of the lug test where the specimen is loaded until it fails

327

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8 Fundamentals of Electron Beam Wire …

8.5.1 Static Property Part of the static tensile curve of the forging lug and the EBWD-fabricated lug is shown in Figs. 8.15 and 8.16. The result of the static test is shown in Table 8.7. The research result shows that when the size of the specimen is the same as the d/e, the average failure load of the forging subject is 4.18 and 2.06% higher than that of the EBWD-fabricated specimen, both of which are higher than the designed load.

Fig. 8.15 Tensile curve of the forging lug

Fig. 8.16 Tensile curve of the EBWD-fabricated lug

8.5 Research on the Mechanical Properties …

329

Table 8.7 Result of the tensile test of the lug Specimen number

Length/mm

d/e

Maximum failure load/KN

Mean/KN

Maximum failure stress/MPa

Mean/ MPa

EP-DJ-01-01

96

2

108.89

108.63

678.9

633.1

EP-DJ-01-02

96

EP-DJ-01-03

96

EP-DJ-02-01

72

46.24

578

EP-DJ-02-02

72

EP-DJ-02-03

72

EP-DJ-03-01

120

154.2

642.5

EP-DJ-03-02

120

EP-DJ-03-03

120

EP-EBRM-01-01

96

104.09

650.6

EP-EBRM-01-02

96

EP-EBRM-01-03

96

EP-EBRM-02-01

72

41.63

520

EP-EBRM-02-02

72

EP-EBRM-02-03

72

EP-EBRM-03-02

120

151.92

633

107.95 108.07 1.5

46.42 46.15 46.16

2.5

157.73 151.13 153.70

2

104.66

601.2

103.21 104.40 1.5

42.08 41.10 41.70

2.5

151.92

8.5.2 Fatigue Property The results of the fatigue test for the forging lug and the EBWD-fabricated lug are shown in Table 8.8. The fatigue curve is shown in Fig. 8.17. The comparison of fatigue life under different stress levels is shown in Fig. 8.18. The research result shows that the fatigue life of forgings is slightly higher than that of EBWD-fabricated parts under a high stress load. The fatigue life of EBWD-fabricated parts is 55.6% higher than that of forgings under a moderate stress load, and more than 500% higher under a low stress load.

330

8 Fundamentals of Electron Beam Wire …

Table 8.8 Results of the fatigue test for lugs Specimen number

Width/mm

Thickness/mm

Maximum Stress/MPa

Cycles/Times

Average times

EP-DJ-01-04

32.01

10.02

475

2847

3791

EP-DJ-01-07

32.04

8.98

475

4326

EP-DJ-01-15

31.98

8.99

475

4566

EP-DJ-01-18

32.00

10.02

475

EP-DJ-01-05

32.04

10.00

340

50%

8357

8357

EP-DJ-01-08

32.00

10.00

270

40%

18,189

16,532

EP-DJ-01-10

31.97

8.99

270

14,291

EP-DJ-01-14

32.01

10.00

270

18,352

EP-DJ-01-17

32.03

10.01

270

15,295

EP-DJ-01-06

31.99

10.02

203

30%

36,626

36,626

EP-DJ-01-12

32.00

10.00

170

25%

49,488

49,488

EP-DJ-01-13

31.99

8.99

135

20%

188,921

206,390

EP-DJ-01-16

31.97

10.03

135

EP-DJ-01-11

32.00

8.98

135

EP-DJ-01-09

32.00

10.01

122

18%

1,214,555

1,214,555

EP-EBRM-01-04

32.07

10.05

455

70%

6354

5438

EP-EBRM-01-12

32.08

10.06

455

4759

EP-EBRM-01-09

32.12

10.01

455

5202

EP-EBRM-01-13

32.09

10.01

260

EP-EBRM-01-05

32.09

10.00

260

EP-EBRM-01-10

32.08

10.00

260

EP-EBRM-01-07

32.11

10.03

143

22%

75,986

75,986

EP-EBRM-01-06

32.09

10.02

130

20%

1,115,688

1,041,424

EP-EBRM-01-08

32.08

10.07

130

1,007,786

EP-EBRM-01-11

32.11

8.98

130

1,000,798

70%

3672

212,256 217,994

40%

10,369

33,527

36,433 53,780

Note EP-DJ-01-06, -06, -12 are the specimens for low loads determined in the pre-test

8.5 Research on the Mechanical Properties …

331

Fig. 8.17 Results of the fatigue test for lugs

Fig. 8.18 Comparison of fatigue life between forging lugs versus rapidly-prototyped lugs under different stress levels

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