Page 9 - Welding of Aluminium and its Alloys
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                         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 labora-
            tory curiosity for the next 30 years when some limited commercial pro-
            duction 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, Al 2 O 3 (alumina),
            from the ore, the second the electrolytic reduction of the alumina at
            between 950°C to 1000°C in cryolite (Na 3AlF 6 ). 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

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