It would be a truism to say all aircraft are different with differing materials requirements. Military aircraft have fewer flying hours in a lifetime and fatigue and corrosion resistance are less of a consideration than in a civil aircraft with required life of 30,000 flying hours or more.
Where the properties of aluminium alloys are concerned,there is usually a trade-off to be made. Aluminium-zinc-magnesium alloys have high strength but sudden liability to crack in an unloaded condition due to retention of internal stresses after heat treatment. The Vickers Viscount replaced aluminium-zinc-manganese components with the aluminium-4 per cent copper Alloy L65 for this very reason. Since then there have been improvements in the stress and corrosion resistance of the aluminium-zinc-magnesium alloys, entirely based on varying the relative proportions of the component alloys. Improved control of casting techniques has brought further improvements in resistance to stress corrosion. The development of aluminium-zinc-magnesium-copper alloys, called the 7000 series, has delivered high strength, good fatigue crack growth resistance and a greater measure of toughness. Weight saving is also a consideration.
Duralumin alloys are used for the under-surfaces of wings with high tensile fatigue loads. Naturally aged duralumin is preferred to heat-treated for fatigue endurance and resistance to crack-propagation. A higher percentage of magnesium in the naturally aged condition, designated 2024 alloy, is the result of much development in practice.
Interest stems from affordability, and being weldable, reduced manufacturing costs. Most 2000 series aluminium alloys can be used for superplastic forming SPF. ISO 6013 alloy possesses a similar high fracture toughness and resistance to crack propagation as the 2000 series alloys.
CM001, as used on Concorde, has been found to possess better overall strength and fatigue characteristics over a wider range of temperatures than any of the other possible aluminium alloys. The latest aluminium alloys in use in the aerospace industry are the aluminium-lithium alloys. Aluminium-lithium-copper-manganese alloy, 8090, is extensively used in the main fuselage structure of EH101 helicopters. In the US, the aluminium-lithium-copper alloy, 2095 alloy, has been used in the fuselage frames of the F16 as a replacement for 2124 alloy, resulting in a fivefold increase in fatigue life and a reduction in weight. Aluminium-lithium alloys can be successfully welded. possess a high fracture toughness and exhibit a high resistance to crack propagation.