The Welding of Aluminium and Its Alloys

G Mathers

• Publisher: Woodhead Publishing
• Year: 2002
• Print ISBN: 9781855735675
• eBook ISBN: 9781855737631
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

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

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 es...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Preface
• 1: Introduction to the welding of aluminium
• 2: Welding metallurgy
• 3: Material standards, designations and alloys
• 4: Preparation for welding
• 5: Welding design
• 6: TIG welding
• 7: MIG welding
• 8: Other welding processes
• 9: Resistance welding processes
• 10: Welding procedure and welder approval
• 11: Weld defects and quality control
• Appendix A: British and ISO standards related to welding and aluminium
• Appendix B: Physical, mechanical and chemical properties at 20°C
• Appendix C: Principal alloy designations: cast products
• Appendix D: Alloy designations: wrought products
• Bibliography
• Index