The origin of
aluminum alloys in aircraft construction started with the first practical
all-metal aircraft in 1915 made by German company, of materials said to be
`iron and steel'. Steel presented the advantages of a high modulus of
elasticity, high proof stress and high tensile strength. Unfortunately these
were accompanied by a high specific gravity, almost three times that of the
aluminum alloys and about ten times that of plywood. Aircraft designers during
the 1930s were therefore forced to use steel in its thinnest forms. To ensure
stability against buckling of the thin plate, intricate shapes for spar sections
were devised.
In 1909 Alfred
Wilm, in Germany, accidentally discovered that an aluminum alloy containing 3.5
per cent copper, 0.5 per cent magnesium and silicon and iron, as unintended
impurities, spontaneously hardened after quenching from about 480°C. The patent
rights of this material were acquired by Durener Metallwerke who marketed the
alloy under the name Duralumin. For half a century this alloy has been used in
the wrought heat-treated, naturally aged condition. The improvements in these
properties produced by artificial ageing at a raised temperature of, for
example, 175°C, were not exploited in the aircraft industry until about 1934.
In addition to
the development of duralumin (first used as a main structural material by
Junkers in 1917) three other causes contributed to the replacement of steel by
aluminum alloys. These were a better understanding of the process of heat
treatment, the introduction of extrusions in a wide range of sections and the
use of pure aluminum cladding to provide greater resistance to corrosion. By
1938, three groups of aluminum alloys dominated the field of aircraft
construction and, in fact, they retain their importance to the present day. The
groups are separated by virtue of their chemical composition, to which they owe
their capacity for strengthening under heat treatment.
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