| High strength aluminium alloys. | | | | DTD 18 issued in 1924, while artificially aged duralumin |
| The origin of aluminium alloys in aircraft construction | | | | came under Specification DTD 111 in 1929. DTD 111 |
| started with the first practical all-metal aircraft in 1915 | | | | provided for slight reductions in 0.1 per cent proof |
| made by Junkers in Germany, of materials said to be | | | | stress and tensile strength. |
| `iron and steel'. Steel presented the advantages of a | | | | The second group of aluminium alloys differs from |
| high modulus of elasticity, high proof stress and high | | | | duralumin chiefly by the introduction of 1 to 2 per |
| tensile strength. Unfortunately these were | | | | cent of nickel, a high content of magnesium and |
| accompanied by a high specific gravity, almost three | | | | possible variations in the amounts of copper, silicon |
| times that of the aluminium alloys and about ten | | | | and iron. `Y' alloy, the oldest member of the group, |
| times that of plywood. Aircraft designers during the | | | | has a typical composition of. 4 per cent copper, 2 per |
| 1930s were therefore forced to use steel in its | | | | cent nickel, 1.5 cent magnesium, the remainder being |
| thinnest forms. To ensure stability against buckling of | | | | aluminium and was covered by Specification DTD 58A |
| the thin plate, intricate shapes for spar sections were | | | | issued in 1927. Its most important property was its |
| devised. | | | | retention of strength at high temperatures, which |
| In 1909 Alfred Wilm, in Germany, accidentally | | | | meant that it was a particularly suitable material for |
| discovered that an aluminium alloy containing 3.5 per | | | | aero engine pistons. Its use in airframe construction |
| cent copper, 0.5 per cent magnesium and silicon and | | | | has been of a limited nature only. Research by |
| iron, as unintended impurities, spontaneously hardened | | | | Rolls-Royce and development by High Duty Alloys |
| after quenching from about 480°C. The patent | | | | Ltd produced the `RR' series of alloys. Based on Y |
| rights of this material were acquired by Durener | | | | alloy, the RR alloys had some of the nickel replaced |
| Metallwerke who marketed the alloy under the name | | | | by iron and the copper reduced. One of the earliest |
| Duralumin. For half a century this alloy has been used | | | | of these alloys, RR56 had approximately half of the 2 |
| in the wrought heat-treated, naturally aged condition. | | | | per cent nickel replaced by iron, the copper content |
| The improvements in these properties produced by | | | | reduced from 4 to 2 per cent, and was used for |
| artificial ageing at a raised temperature of, for | | | | forgings and extrusions in aero engines and airframes. |
| example, 175°C, were not exploited in the aircraft | | | | The third and latest group depends upon the inclusion |
| industry until about 1934. | | | | of zinc and magnesium and their high strength. |
| In addition to the development of duralumin (first | | | | Covered by Specification DTD 363 issued in 1937, |
| used as a main structural material by Junkers in 1917) | | | | these alloys had a nominal composition: 2.5 per cent |
| three other causes contributed to the replacement | | | | copper, 5 per cent zinc, 3 per cent magnesium and |
| of steel by aluminium alloys. These were a better | | | | up to 1 per cent nickel. In modern versions of this |
| understanding of the process of heat treatment, the | | | | alloy nickel has been eliminated and provision made |
| introduction of extrusions in a wide range of sections | | | | for the addition of chromium and further amounts of |
| and the use of pure aluminium cladding to provide | | | | manganese. |
| greater resistance to corrosion. By 1938, three | | | | Aircraft structural aluminium. |
| groups of aluminium alloys dominated the field of | | | | Of the three basic structural materials, namely wood, |
| aircraft construction and, in fact, they retain their | | | | steel and aluminium alloy, only wood is no longer of |
| importance to the present day. The groups are | | | | significance except in laminates for non-structural |
| separated by virtue of their chemical composition, to | | | | bulkheads, floorings and furnishings. Most modern |
| which they owe their capacity for strengthening | | | | aircraft still rely on modified forms of the high |
| under heat treatment. | | | | strength aerospace aluminium alloys which were |
| The first group is contained under the general name | | | | introduced during the early part of the 20th century. |
| duralumin having a typical composition of: 4 per cent | | | | Steels are used where high strength, high stiffness |
| copper, 0.5 per cent magnesium, 0.5 per cent | | | | and wear resistance are required. Other materials, |
| manganese, 0.3 per cent silicon, 0.2 per cent iron, | | | | such as titanium and fibre-reinforced composites first |
| with the remainder aluminium. The naturally aged | | | | used about 1950, are finding expanding uses in |
| version was covered by Air Ministry Specification | | | | airframe construction. |