The welding of aluminium and its alloys

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.

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

1.1 A Richardson and Associates (Australia) Ocean Viewer all­aluminium vessel. The hull is 5mm thick A5083. Courtesy TWI Ltd.

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.