High strength aluminium alloys.
The origin of aluminium alloys in aircraft construction started with the first practical all-metal aircraft in 1915 made by Junkers in Germany, 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 aluminium 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 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. 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 aluminium 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 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 strengthening 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 111 in 1929. DTD 111 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 of the group, has a typical composition of. 4 per cent copper, 2 per cent nickel, 1.5 cent magnesium, the remainder being aluminium and was covered by Specification DTD 58A issued in 1927. Its most important property was its retention of strength at high temperatures, which meant that it was a particularly suitable material for aero engine pistons. Its use in airframe construction has been of a limited nature only. Research by Rolls-Royce and development by High Duty Alloys Ltd produced the `RR' series of alloys. Based on Y alloy, the RR alloys had some of the nickel replaced by iron and the copper reduced. One of the earliest of these alloys, RR56 had approximately half of the 2 per cent nickel replaced by iron, the copper content reduced from 4 to 2 per cent, and was used for forgings and extrusions in aero engines and airframes.
The third and latest group depends upon the inclusion of zinc and magnesium and their high strength. Covered by Specification DTD 363 issued in 1937, these alloys had a nominal composition: 2.5 per cent copper, 5 per cent zinc, 3 per cent magnesium and up to 1 per cent nickel. In modern versions of this alloy nickel has been eliminated and provision made for the addition of chromium and further amounts of manganese.
Aircraft structural aluminium.
Of the three basic structural materials, namely wood, steel and aluminium alloy, only wood is no longer of significance except in laminates for non-structural bulkheads, floorings and furnishings. Most modern aircraft still rely on modified forms of the high strength aerospace aluminium alloys which were introduced during the early part of the 20th century. Steels are used where high strength, high stiffness and wear resistance are required. Other materials, such as titanium and fibre-reinforced composites first used about 1950, are finding expanding uses in airframe construction.