Severe Plastic Deformation Technology Edited by Andrzej Rosochowski Reader in Light Metals Advanced Technology, University of Strathclyde, UK Whittles Publishing Prelims.indd 1 23/11/16 11:45 am Published by Whittles Publishing, Dunbeath, Caithness KW6 6EG, Scotland, UK www.whittlespublishing.com © 2017 A. Rosochowski ISBN 978-184995-091-6 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, recording or otherwise without prior permission of the publishers. The publisher and authors have used their best efforts in preparing this book, but assume no responsibility for any injury and/or damage to persons or property from the use or implementation of any methods, instructions, ideas or materials contained within this book. All operations should be undertaken in accordance with existing legislation, recognized codes and standards and trade practice. Whilst the information and advice in this book is believed to be true and accurate at the time of going to press, the authors and publisher accept no legal responsibility or liability for errors or omissions that may have been made. Printed by Prelims.indd 2 23/11/16 11:45 am Contents Preface vii The Authors ix 1 Equal channel angular extrusion (ECAE) 1 Vladimir M. Segal 1.1 Introduction 1 1.2 Materials processing for properties 2 1.2.1 Processing mechanics 2 1.2.2 Structure evolution and deformation mode 3 1.3 SPD processes 4 1.4 Concept and history of ECAE 7 1.4.1 Concept of ECAE 7 1.4.2 History of ECAE 8 1.5 Plastic zone during ECAE 11 1.5.1 Frictionless conditions 11 1.5.2 Uniform friction in channels 12 1.5.3 Non-uniform friction in channels 13 1.5.4 Round corner channels 14 1.5.5 Additional effects 14 1.6 Outlet channel 16 1.6.1 Contact friction in outlet channel 16 1.6.2 Material flow 16 1.6.3 Movable channel walls 17 1.7 Inlet channel 20 1.7.1 Friction in the inlet channel 21 1.7.2 Stress distribution 21 1.7.3 Movable channel walls 23 1.8 ECAE of batch billets 25 1.8.1 Shapes of billets 25 1.8.2 Multi-pass processing 27 1.8.3 Processing parameters 28 1.8.4 Defects associated with ECAE 29 1.9 Continuous ECAE 30 1.9.1 Friction conditions 31 1.9.2 Plastic zone 33 1.9.3 Analysis of CECAE 33 1.10 Modifications of ECAE 35 1.11 Applications 36 iii Prelims.indd 3 23/11/16 11:45 am iv | Contents References 37 Notes 40 2 Incremental ECAP 41 Andrzej Rosochowski 2.1 Concept of incremental ECAP (I-ECAP) 41 2.1.1 First idea 41 2.1.2 Process definition 42 2.1.3 Relation to ECAP 43 2.1.4 Place of I-ECAP in the metal-forming discipline 44 2.2 I-ECAP of square bars 45 2.2.1 FE simulation 46 2.2.2 Tooling 49 2.2.3 Machine 51 2.2.4 Controls 52 2.2.5 Results 53 2.3 I-ECAP of plates 58 2.4 I-ECAP of sheets 66 2.5 ECAP/I-ECAP with converging billets 70 2.6 I-ECAP of tubes 75 2.7 Incremental angular splitting 80 2.8 Summary 84 References 84 3 Tooling for ECAP 87 Lech Olejnik 3.1 General recommendations for process design 87 3.2 Configuration of ECAP channel 91 3.2.1 Inclination angle 91 3.2.2 Cross-section 97 3.2.3 Number of turns along channel 101 3.3 Reduction of friction 111 3.3.1 Movable die parts 111 3.3.2 Common face pressing 111 3.3.3 Lubrication 112 3.4 Die design 114 3.4.1 Bolted dies 114 3.4.2 Split dies 115 3.4.3 Prestressed dies 116 3.5 Punch design 121 3.6 Tool materials for die and punch 122 3.7 Working conditions 123 3.8 Monitoring of ECAP 126 Prelims.indd 4 23/11/16 11:45 am Contents | v References 130 Note 134 4 High pressure torsion (HPT) 135 Reinhard Pippan and Anton Hohenwarter 4.1 Introduction 135 4.1.1 Unlimited strain 136 4.1.2 Efficiency of the technique 136 4.1.3 Simplicity of the process 136 4.1.4 Reliability of the technique 136 4.1.5 Flexibility of testing parameters 136 4.1.6 Versatility of high pressure torsion 136 4.1.7 Sufficient sample dimensions 137 4.2 Some characteristics of HPT microstructures 137 4.3 Principles of HPT: description of different setups 141 4.3.1 The idealised HPT version 141 4.3.2 Fully constrained HPT 142 4.3.3 Quasi-constrained HPT 143 4.4 Design criteria for building an HPT device 143 4.4.1 The test rig 143 4.4.2 Applied pressure 145 4.4.3 Applied torque 150 4.4.4 Anvil design and material selection 152 4.4.5 Tool failures 154 4.4.6 Attachments for HPT experiments 155 4.4.7 Upscaling of HPT 157 4.5 Outlook 162 References 163 5 Cyclic extrusion–compression (CEC) 165 Jan Richert 5.1 Initial CEC device for unlimited deformation 165 5.2 Microstructural evolution during CEC 166 5.3 Force parameters of CEC processes 168 5.4 Effect of tool geometry on formation of shear bands 171 5.5 Special hydraulic press for CEC processes 178 5.6 Grain refinement in Al6082 alloy 179 5.6.1 Evolution of shear bands under varying backpressure 180 5.6.2 Evolution of microstructure of Al6082 alloy 184 5.6.3 Finite element simulation of CEC processes 185 5.6.4 Stress path analysis in CEC processes 188 5.6.5 Stress state in the deformation zone 191 Prelims.indd 5 23/11/16 11:45 am vi | Contents 5.7 Plastic consolidation of metallic powder materials 195 References 199 6 Twist extrusion (TE) 202 Y an Beygelzimer, Victor Varyukhin, Roman Kulagin and Dmytro Orlov 6.1 Introduction and historical retrospective 202 6.2 Mechanics of plastic flow in TE 206 6.2.1 Characteristic properties of deformation in TE 206 6.2.2 Stress–strain state in TE 209 6.3 TE as a processing technique 215 6.3.1 Principal equations for estimating processing characteristics 215 6.3.2 Industrial prototype of TE machine 217 6.4 Formation of structure and properties for different applications 219 6.4.1 Major effects of TE 219 6.4.2 Applications of TE 221 6.5 Recent developments in TE and its derivatives 225 6.5.1 Planar TE 225 6.5.2 Laboratory horizontal tool set for TE 228 Summary 230 References 230 7 Accumulative roll-bonding (ARB) 235 Nobuhiro Tsuji 7.1 Introduction 235 7.2 Background of development and principles of ARB process 235 7.3 Nanostructures obtained by ARB 245 7.4 Summary 248 References 249 Index 253 Prelims.indd 6 23/11/16 11:45 am Preface In the past, improvements in the properties of metallic materials were achieved mainly by adding more alloying elements and/or applying complex thermo- mechanical treatments. The former is a costly solution and may involve rare or strategic elements while the latter is usually limited in terms of the achiev- able improvements. Recently, a more unconventional approach has been tried, which is based on refining grain structure of metals below the level normally achievable by traditional techniques, that is below an average grain size of 1µm. Such metals are referred to as ultrafine grained (UFG) metals. The most feasible method of producing UFG metals is based on so-called severe plastic deformation (SPD), a new branch of metal forming technology, in which very large plastic deformation of the material is not accompanied by any substantial change of its shape and dimensions. UFG metals are characterised by improved mechanical, physical and techno- logical properties. The most prominent one is much higher yield strength, which enables the design of substantially lighter structures. Ultimate tensile strength and high cycle fatigue are also improved. Typically, ductility is reduced but for some naturally brittle metals such as magnesium alloys ductility at room temperature can be increased. This opens up an opportunity to form these materials without heating. UFG metals exhibit increased fracture toughness at low temperatures, which makes them ideal candidates for cryogenic and outer space applications. There are claims of more uniform corrosion and improved biocompatibility through faster integration of biological tissue with UFG metal surfaces. The dif- fusion rate is increased due to the larger surface of grain boundaries, which is beneficial in technological processes such as superplastic forming and diffusion bonding since they can be performed at a lower temperature and/or higher strain rate. For micro-components, the UFG structure helps avoiding so-called scale effects. The above benefits of UFG metals should make them very popular in a variety of applications spread over many industrial sectors such as transport (aerospace, automotive, rail), energy (traditional, nuclear, renewable), medical (implants and devices) and micro-manufacturing. However, progress in practical applications of UFG metals is very slow due to three main reasons: limited avail- ability of UFG metals, few examples of industrial applications which could be followed and lack of awareness among designers and engineers. The fundamental knowledge of UFG metals is well developed because of the work of numerous groups of researchers, thousands of publications and many dedicated conferences. The problem is that only a small percentage of this research effort refers to methods of producing UFG metals and when it does, it only briefly describes laboratory equipment used to produce small size UFG vii Prelims.indd 7 23/11/16 11:45 am viii | Preface samples for structural investigations. The challenge still remains to convert the laboratory SPD processes to scaled-up processes, which are industrially viable. One element of this challenge is the lack of detailed knowledge regarding engi- neering aspects of different SPD processes. This book aims to provide this sort of insight into the most popular SPD processes of ECAE (equal channel angular extrusion)/ECAP (equal channel angular pressing), I-ECAP (incremental equal channel angular pressing), HPT (high pressure torsion), CEC (cyclic extrusion– compression), TE (twist extrusion) and ARB (accumulative roll-bonding). The historical background of these processes is followed by explaining their prin- ciples, engineering implementations in terms of machines, tooling and pro- cess parameters, some structural results and material properties as well as new process developments. We hope the book, written by a team of international experts, will be useful to researchers trying to understand the background of the main SPD processes and practical challenges in their implementation as well as to engineers interested in transferring these processes from laboratory to industry. Andrzej Rosochowski Prelims.indd 8 23/11/16 11:45 am The Authors Yan Beygelzimer is Dr.Sc., professor and principal research scientist of Donetsk Institute for Physics and Engineering named after O. O. Galkin of the Ukrainian National Academy of Sciences. In 1999 he suggested the idea of twist extrusion and subsequently was a research leader for the development of this process. His research interests lie in the area of severe plastic deformation, metal forming, materials sciences and mathematical modelling. He proposed a continuum model of grain refinement and damage of polycrystalline materials during severe plastic deformation and developed a continuum theory of plastic deformation in structurally-inhomogeneous porous bodies. Based on this the- ory, he proposed a mathematical model of metal forming of porous and powder materials and developed a new model for predicting the ductility of materials under deformation. Anton Hohenwarter studied materials science at the Montanuniversität of Leoben and undertook his PhD at the Erich Schmid Institute of Materials Science of the Austrian Academy of Sciences. Currently, he is a group leader at the Department of Materials Physics of the Montanuniversität Leoben. His research interests concern mainly the fracture and fatigue behaviour of ultrafine- grained and nanocrystalline metals and alloys. He has been working with Prof. Reinhard Pippan for many years and has been strongly involved in the further development of high pressure torsion as a severe plastic deformation method. Roman Kulagin has a Ph.D. degree in materials science and engineering. For over 10 years, he has been working on industrially relevant problems in metal forming and the theory of plasticity. Between 2005 and 2014, he worked at the Donetsk Institute for Physics and Engineering, Ukraine. He is presently a postdoctoral research fellow at the Karlsruhe Institute of Technology, Germany. His work has led to many improvements in metal forming processes already in use as well as to the development of novel forming techniques (e.g., hot section rolling on a continuous rolling mill, continuous extrusion of copper (CONFORM), drawing of high precision copper profiles, direct extrusion of aluminium profiles, and forging of titanium alloys for gas-turbine engine applications). Lech Olejnik is professor at the Department of Metal Forming and Casting in the Faculty of Production Engineering at Warsaw University of Technology, the biggest technical university in Poland.  Since 2005, he has been chair of the UFGbySPD Group which carries out both research work and production of ix Prelims.indd 9 23/11/16 11:45 am