Page 229 - Aircraft Stuctures for Engineering Student

7.1 Materials of aircraft construction 213

Fig. 7.1 Typical spar sections fabricated from thin steel sheet.

In 1909 Alfred Wilm, in Germany, accidentally discovered that an aluminium 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 possessing mechan-
ical properties of 0.1 per cent proof stress not less than 230 N/mm’, tensile strength
not less than 390N/mm2 and an elongation at fracture not less than 15 per cent.
However, 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. Artificially aged duralumin has a 0.1 per cent proof stress of not less
than 370N/mm2, a tensile strength not less than 460N/mm2 and an elongation of 8
per cent.
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 aluminium alloy. These were a better understanding of the process of heat
treatnent, the introduction of extrusions in a wide range of sections and the use of
pure aluminium cladding to provide greater resistance to corrosion. By 1938, three
groups of aluminium 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 strengthen-
ing under heat treatment.
The first group is contained under the general name duralumin having a typical
composition of: 4 per cent copper, 0.5 per cent magnesium, 0.5 per cent manganese,
0.3 per cent silicon, 0.2 per cent iron, with the remainder aluminium. The naturally
aged version was covered by Air Ministry Specification DTD 18 issued in 1924,
while artificially aged duralumin came under Specification DTD 11 1 in 1929. Typical
properties of the two types have been quoted above although DTD 11 1 provided for
slight reductions in 0.1 per cent proof stress and tensile strength.
The second group of aluminium alloys differs from duralumin chiefly by the
introduction of 1 to 2 per cent of nickel, a high content of magnesium and possible
variations in the amounts of copper, silicon and iron. Y‘ alloy, the oldest member