Đang tải... (xem toàn văn)
welding of aluminum
The welding of aluminium and its alloys The welding of aluminium and its alloys Gene Mathers Cambridge England Published by Woodhead Publishing Limited, Abington Hall, Abington Cambridge CB1 6AH, England www.woodhead-publishing.com Published in North America by CRC Press LLC, 2000 Corporate Blvd,NW Boca Raton FL 33431, USA First published 2002, Woodhead Publishing Ltd and CRC Press LLC © 2002, Woodhead Publishing Ltd The author has asserted his moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the author and the publishers cannot assume responsibility for the validity of all materials. Neither the author nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from the publishers. The consent of Woodhead Publishing and CRC Press does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing or CRC Press for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN 1 85573 567 9 CRC Press ISBN 0-8493-1551-4 CRC Press order number: WP1551 Typeset by SNP Best-set Typesetter Ltd., Hong Kong Printed by TJ International, Padstow, Cornwall, England Contents Preface ix 1 Introduction to the welding of aluminium 1 1.1 Introduction 1 1.2 Characteristics of aluminium 4 1.3 Product forms 6 1.4 Welding: a few definitions 6 2 Welding metallurgy 10 2.1 Introduction 10 2.2 Strengthening mechanisms 10 2.3 Aluminium weldability problems 18 2.4 Strength loss due to welding 31 3 Material standards, designations and alloys 35 3.1 Designation criteria 35 3.2 Alloying elements 35 3.3 CEN designation system 36 3.4 Specific alloy metallurgy 40 3.5 Filler metal selection 46 4 Preparation for welding 51 4.1 Introduction 51 4.2 Storage and handling 51 4.3 Plasma-arc cutting 52 4.4 Laser beam cutting 58 4.5 Water jet cutting 63 4.6 Mechanical cutting 64 4.7 Cleaning and degreasing 66 v 5 Welding design 69 5.1 Introduction 69 5.2 Access for welding 70 5.3 Welding speed 71 5.4 Welding position 72 5.5 Edge preparation and joint design 72 5.6 Distortion 84 5.7 Rectification of distortion 88 5.8 Fatigue strength of welded joints 89 6 TIG welding 97 6.1 Introduction 97 6.2 Process principles 97 6.3 Mechanised/automatic welding 114 6.4 TIG spot and plug welding 115 7 MIG welding 116 7.1 Introduction 116 7.2 Process principles 116 7.3 Welding consumables 130 7.4 Welding procedures and techniques 135 7.5 Mechanised and robotic welding 141 7.6 Mechanised electro-gas welding 143 7.7 MIG spot welding 144 8 Other welding processes 147 8.1 Introduction 147 8.2 Plasma-arc welding 147 8.3 Laser welding 150 8.4 Electron beam welding 155 8.5 Friction welding 160 9 Resistance welding processes 166 9.1 Introduction 166 9.2 Power sources 167 9.3 Surface condition and preparation 169 9.4 Spot welding 171 9.5 Seam welding 175 9.6 Flash butt welding 176 vi Contents 10 Welding procedure and welder approval 181 10.1 Introduction 181 10.2 Welding procedures 181 10.3 Welder approval 191 11 Weld defects and quality control 199 11.1 Introduction 199 11.2 Defects in arc welding 199 11.3 Non-destructive testing methods 205 Appendix A British and ISO standards related to welding and aluminium 216 Appendix B Physical, mechanical and chemical properties at 20°C 226 Appendix C Principal alloy designations: cast products 227 Appendix D Alloy designations: wrought products 228 Bibliography 230 Index 235 Contents vii Preface Engineering is not an exact science and, of the many disciplines within engi- neering, welding is probably one of the most inexact – rather more of an art than a science. Much of the decision-making is based on experience and a ‘gut feel’ for what is or is not acceptable. When the difficulties of shop floor or site control are taken into account and the occasional vagaries of the welder and the sometimes inadequate knowledge of supervisory staff are added, the problems of the practising shop floor engineer can appear overwhelming. I hope that some of this uncertainty can be dispelled in this book, which is aimed at those engineers with little or no knowledge of metallurgy and perhaps only the briefest acquaintance with the welding processes. It does not purport to be a metallurgical or processes textbook and I make no apology for this. Having lectured fairly extensively on welding technology, I have come to realise that most engineers think of metals as being composed of a large number of small billiard balls held together by some form of glue. I have attempted to describe the metallur- gical aspects of the aluminium alloys in these terms. I have therefore kept the contents descriptive and qualitative and have avoided the use of mathematical expressions to describe the effects of welding. The book provides a basic understanding of the metallurgical principles involved in how alloys achieve their strength and how welding can affect these properties.I have included sections on parent metal storage and prepa- ration prior to welding and have also described the more frequently encoun- tered processes. There are recommendations on welding parameters that may be used as a starting point for the development of a viable welding pro- cedure. Also included are what I hope will be useful hints and tips to avoid some of the pitfalls of welding these sometimes problematic materials. I would like to thank my colleagues at TWI, particularly Bob Spiller, Derek Patten and Mike Gittos, for their help and encouragement during the writing of this book – encouragement that mostly took the form of ‘Haven’t you finished it yet?’. Well, here it is. Any errors, inaccuracies or omissions are mine and mine alone. Gene Mathers ix [...]... (Mg), silicon (Si) and zinc (Zn) One of the first alloys to be produced was aluminium–copper It was around 1910 that the phenomenon of age or precipitation hardening in this family of alloys was discovered, with many of these early age-hardening 1 2 The welding of aluminium and its alloys alloys finding a ready use in the fledgling aeronautical industry Since that time a large range of alloys has been developed... This wide range of uses gives some indication of the extensive number of alloys now available to the designer It also gives an indication of the difficulties facing the welding engineer With the ever-increasing sophistication of processes, materials and specifications the welding engineer must have a broad, comprehensive knowledge of metallurgy and welding Introduction to the welding of aluminium 1.1... – the formation of an oxide film of a controlled thickness The coefficient of thermal expansion of aluminium is approximately twice that of steel which can mean unacceptable buckling and distortion during welding The coefficient of thermal conductivity of aluminium is six times that of steel The result of this is that the heat source for welding aluminium needs to be far more intense and concentrated... aluminium–silicon alloys although heat-treatable (age-hardening) alloys are often used for sand, lost wax and permanent mould castings Lost wax and die-casting give products with smooth surfaces to close tolerances and are processes used extensively for aerospace products A number of alloys, their product forms and applications are listed in Table 1.1 1.4 Welding: a few definitions Before dealing with the problems of. .. parent material should be welded within a short period of time, a period of four hours frequently being regarded as acceptable Further details of mechanical cleaning, degreasing and workshop conditions are given in Chapter 4 22 The welding of aluminium and its alloys Table 2.3 Summary of causes and prevention of porosity Mechanism of Potential causes porosity formation Hydrogen entrapment Gas/air entrapment... Table 2.3 summarises the causes and prevention of porosity 2.3.2 Oxide film removal during welding The need to remove the oxide film prior to welding to reduce the risk of porosity has been covered above It is also necessary to disperse this film Welding metallurgy 23 2.9 Oxide entrapment in fillet weld Courtesy of Roland Andrews during welding if defects such as lack of fusion and oxide film entrapment are... size 2.3 General relationship of grain size with strength, ductility and toughness where d is the average grain diameter, and sI and ky are constants for the metal Typical results of this relationship are illustrated in Fig 2.3 The practical consequence of this is that a loss of strength is often encountered in the HAZ of weldments due to grain growth during welding A loss of strength may also be found... porosity when the rate of gas evolution from the weld exceeds the rate of absorption – slowing the rate at which the weld freezes allows the 20 The welding of aluminium and its alloys hydrogen to bubble out of the weld A similar effect can be achieved by reducing the travel speed Increasing arc voltage and/ or arc length increases the exposure of the molten metal to contamination, and porosity will thereby... of aluminium and its alloys 1.3 Product forms Aluminium is available in both wrought and cast forms The wrought forms comprise hot and cold rolled sheet, plate, rod, wire and foil The ductility and workability of aluminium mean that extrusion is a simple method of producing complex shapes, particularly for long, structural members such as I and H beams, angles, channels, T-sections, pipes and tubes Forging,... bulk of the material The atoms, stripped of their outer electrons, become positively charged ions immersed in a ‘cloud’ of negatively charged electrons It is the magnetic attraction between the positively charged ions and the cloud of mobile, negatively charged electrons that binds the metal together These atomic scale events give metals their high thermal and 12 The welding of aluminium and its alloys . The welding of aluminium and its alloys The welding of aluminium and its alloys Gene Mathers Cambridge England Published by Woodhead. addition 6 The welding of aluminium and its alloys Introduction to the welding of aluminium 7 Table 1.1 Typical forms and uses of aluminium alloys Aluminium