Mathers The Welding of Aluminium and Its Alloys

1. Auflage 2002
ISBN: 978-1-85573-763-1
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 248 Seiten

Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies

The Welding of Aluminium and its Alloys is a practical user's guide to all aspects of welding aluminium and aluminium alloys. It provides a basic understanding of the metallurgical principles involved showing how alloys achieve their strength and how the process of welding can affect these properties. The book is intended to provide engineers with perhaps little prior understanding of metallurgy and only a brief acquaintance with the welding processes involved with a concise and effective reference to the subject.It is intended as a practical guide for the Welding Engineer and covers weldability of aluminium alloys; process descriptions, advantages, limitations, proposed weld parameters, health and safety issues; preparation for welding, quality assurance and quality control issues along with problem solving.The book includes sections on parent metal storage and preparation prior to welding. It describes the more frequently encountered processes and has recommendations on welding parameters that may be used as a starting point for the development of a viable welding procedure. Included in these chapters are hints and tips to avoid some of the pitfalls of welding these sometimes-problematic materials. The content is both descriptive and qualitative. The author has avoided the use of mathematical expressions to describe the effects of welding.This book is essential reading for welding engineers, production engineers, production managers, designers and shop-floor supervisors involved in the aluminium fabrication industry. - A practical user's guide by a respected expert to all aspects of welding of aluminium - Designed to be easily understood by the non-metallurgist whilst covering the most necessary metallurgical aspects - Demonstrates best practice in fabricating aluminium structures

Gene Mathers is currently employed as a Welding Engineering Consultant at TWI. He formerly held the position of Head of the School of Welding Technology at TWI and has had more than 30 years experience in the welding industry, mostly in heavy engineering and power generation industries in welding engineering and quality management.

Autoren/Hrsg.

Mathers, G.

Inhaltsverzeichnis

1;Front Cover;1
2;The Welding of Aluminium and its Alloys;4
3;Copyright Page;5
4;Table of Contents;6
5;Preface;10
6;Chapter 1. Introduction to the welding of aluminium;12
6.1;1.1 Introduction;12
6.2;1.2 Characteristics of aluminium;15
6.3;1.3 Product forms;17
6.4;1.4 Welding: a few definitions;17
7;Chapter 2. Welding metallurgy;21
7.1;2.1 Introduction;21
7.2;2.2 Strengthening mechanisms;21
7.3;2.3 Aluminium weldability problems;29
7.4;2.4 Strength loss due to welding;42
8;Chapter 3. Material standards, designations and alloys;46
8.1;3.1 Designation criteria;46
8.2;3.2 Alloying elements;46
8.3;3.3 CEN designation system;47
8.4;3.4 Specific alloy metallurgy;51
8.5;3.5 Filler metal selection;57
9;Chapter 4. Preparation for welding;62
9.1;4.1 Introduction;62
9.2;4.2 Storage and handling;62
9.3;4.3 Plasma-arc cutting;63
9.4;4.4 Laser beam cutting;69
9.5;4.5 Water jet cutting;74
9.6;4.6 Mechanical cutting;75
9.7;4.7 Cleaning and degreasing;77
10;Chapter 5. Welding design;80
10.1;5.1 Introduction;80
10.2;5.2 Access for welding;81
10.3;5.3 Welding speed;82
10.4;5.4 Welding position;83
10.5;5.5 Edge preparation and joint design;83
10.6;5.6 Distortion;95
10.7;5.7 Rectification of distortion;99
10.8;5.8 Fatigue strength of welded joints;100
11;Chapter 6. TIG welding;108
11.1;6.1 Introduction;108
11.2;6.2 Process principles;108
11.3;6.3 Mechanised/automatic welding;125
11.4;6.4 TIG spot and plug welding;126
12;Chapter 7. MIG welding;127
12.1;7.1 Introduction;127
12.2;7.2 Process principles;127
12.3;7.3 Welding consumables;141
12.4;7.4 Welding procedures and techniques;146
12.5;7.5 Mechanised and robotic welding;152
12.6;7.6 Mechanised electro-gas welding;154
12.7;7.7 MIG spot welding;155
13;Chapter 8. Other welding processes;158
13.1;8.1 Introduction;158
13.2;8.2 Plasma-arc welding;158
13.3;8.3 Laser welding;161
13.4;8.4 Electron beam welding;166
13.5;8.5 Friction welding;171
14;Chapter 9. Resistance welding processes;177
14.1;9.1 Introduction;177
14.2;9.2 Power sources;178
14.3;9.3 Surface condition and preparation;180
14.4;9.4 Spot welding;182
14.5;9.5 Seam welding;186
14.6;9.6 Flash butt welding;187
15;Chapter 10. Welding procedure and welder approval;192
15.1;10.1 Introduction;192
15.2;10.2 Welding procedures;192
15.3;10.3 Welder approval;202
16;Chapter 11. Weld defects and quality control;210
16.1;11.1 Introduction;210
16.2;11.2 Defects in arc welding;210
16.3;11.3 Non-destructive testing methods;216
17;Appendix A: British and ISO standards related to welding and aluminium;227
18;Appendix B: Physical, mechanical and chemical properties at 20°C;237
19;Appendix C: Principal alloy designations: cast products;238
20;Appendix D: Alloy designations: wrought products;239
21;Bibliography;241
22;Index;246

Leseproben

1 Introduction to the welding of aluminium
1.1 Introduction
The existence of aluminium (Al) was postulated by Sir Humphrey Davy in the first decade of the nineteenth century and the metal was isolated in 1825 by Hans Christian Oersted. It remained as somewhat of a laboratory curiosity for the next 30 years when some limited commercial production began, but it was not until 1886 that the extraction of aluminium from its ore, bauxite, became a truly viable industrial process. The method of extraction was invented simultaneously by Paul Heroult in France and Charles M. Hall in the USA and this basic process is still in use today. Because of its reactive nature aluminium is not found in the metallic state in nature but is present in the earth’s crust in the form of different compounds, of which there are several hundreds. The most important and prolific is bauxite. The extraction process consists of two separate stages, the first being the separation of aluminium oxide, Al2O3 (alumina), from the ore, the second the electrolytic reduction of the alumina at between 950 °C to 1000 °C in cryolite (Na3AlF6). This gives an aluminium, containing some 5–10% of impurities such as silicon (Si) and iron (Fe), which is then refined either by a further electrolytic process or by a zone-melting technique to give a metal with a purity approaching 99.9%. At the close of the twentieth century a large proportion of aluminium was obtained from recovered and remelted waste and scrap, this source alone supplying almost 2 million tonnes of aluminium alloys per annum in Europe (including the UK) alone. The resulting pure metal is relatively weak and as such is rarely used, particularly in constructional applications. To increase mechanical strength, the pure aluminium is generally alloyed with metals such as copper (Cu), manganese (Mn), magnesium (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 alloys finding a ready use in the fledgling aeronautical industry. Since that time a large range of alloys has been developed with strengths which can match that of good quality carbon steel but at a third of the weight. A major impetus to the development of aluminium alloys was provided by the two World Wars, particularly the Second World War when aluminium became the metal in aircraft structural members and skins. It was also in this period that a major advance in the fabrication of aluminium and its alloys came about with the development of the inert gas shielded welding processes of MIG (metal inert gas) and TIG (tungsten inert gas). This enabled high-strength welds to be made by arc welding processes without the need for aggressive fluxes. After the end of the Second World War, however, there existed an industry that had gross over-capacity and that was searching for fresh markets into which its products could be sold. There was a need for cheap, affordable housing, resulting in the production of the ‘prefab’, a prefabricated aluminium bungalow made from the reprocessed remains of military aircraft – not quite swords into ploughshares but a close approximation! At the same time domestic utensils, road vehicles, ships and structural components were all incorporating aluminium alloys in increasing amounts. Western Europe produces over 3 million tonnes of primary aluminium (from ore) and almost 2 million tonnes of secondary or recycled aluminium per year. It also imports around 2 million tonnes of aluminium annually, resulting in a per capita consumption of approximately 17 kg per year. Aluminium now accounts for around 80% of the weight of a typical civilian aircraft (Fig. 1.1) and 40% of the weight of certain private cars. If production figures remain constant the European automotive industry is expected to be consuming some 2 million tonnes of aluminium annually by the year 2005. It is used extensively in bulk carrier and container ship superstructures and for both hulls and superstructures in smaller craft (Fig. 1.2). The new class of high-speed ferries utilises aluminium alloys for both the super-structure and the hull. It is found in railway rolling stock, roadside furniture, pipelines and pressure vessels, buildings, civil and military bridging and in the packaging industry where over 400000 tonnes per annum is used as foil. One use that seems difficult to rationalise in view of the general perception of aluminium as a relatively weak and soft metal is its use in armoured vehicles (Fig. 1.3) in both the hull and turret where a combination of light weight and ballistic performance makes it the ideal material for fast reconnaissance vehicles. 1.1 BAC 146 in flight. Courtesy of TWI Ltd. 1.2 A Richardson and Associates (Australia) Ocean Viewer all-aluminium vessel. The hull is 5 mm thick A5083. Courtesy TWI Ltd. 1.3 Warrior armoured fighting vehicle (AFV) utilising Al-Zn-Mg alloys. Courtesy of Alvis Vehicles. 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 processes. It is hoped that this book will go some way towards giving the practising shop-floor engineer an appreciation of the problems of welding the aluminium alloys and guidance on how these problems may be overcome. Although it is not intended to be a metallurgical textbook, some metallurgical theory is included to give an appreciation of the underlying mechanisms of, for instance, strengthening and cracking. 1.2 Characteristics of aluminium
Listed below are the main physical and chemical characteristics of aluminium, contrasted with those of steel, the metal with which the bulk of engineers are more familiar. As can be seen from this list there are a number of important differences between aluminium and steel which influence the welding behaviour: • The difference in melting points of the two metals and their oxides. The oxides of iron all melt close to or below the melting point of the metal; aluminium oxide melts at 2060 °C, some 1400 °C above the melting point of aluminium. This has important implications for the welding process, as will be discussed later, since it is essential to remove and disperse this oxide film before and during welding in order to achieve the required weld quality. • The oxide film on aluminium is durable, highly tenacious and self- healing. This gives the aluminium alloys excellent corrosion resistance, enabling them to be used in exposed applications without additional protection. This corrosion resistance can be improved further by anodising – 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 than that for steel. This is particularly so for thick sections, where the fusion welding processes can produce lack of fusion defects if heat is lost too rapidly. • The specific heat of aluminium – the amount of heat required to raise the temperature of a substance – is twice that of steel. • Aluminium has high electrical conductivity, only three-quarters that of copper but six times that of steel. This is a disadvantage when resistance spot welding where the heat for welding must be produced by electrical resistance. • Aluminium does not change colour as its temperature rises, unlike steel. This can make it difficult for the welder to judge when melting is about to occur, making it imperative that adequate retraining of the welder takes place when converting from steel to aluminium welding. • Aluminium is non-magnetic which means that arc blow is eliminated as a welding problem. • Aluminium has a modulus of elasticity three times that of steel which means that it deflects three times as much as steel under load but can absorb more energy on impact loading. • The fact that aluminium has a face-centred cubic crystal structure (see Fig. 2.2) means that it does not suffer from a loss of notch toughness as the temperature is reduced. In fact, some of the alloys show an improvement in tensile strength and ductility as the temperature falls, EW-5083 (Al Mg 4.5Mn) for instance showing a 60% increase in elongation after being in service at - 200 °C for a period of time. This crystal structure also means that formability is very good, enabling products to be produced by such means as extrusion, deep drawing and high energy rate...