New High Strength Aluminium Alloy

THE highest strength wrought aluminium alloys currently available are based on the aluminium‐zinc‐magnesium‐copper system, and such alloys offer considerable potential for weight savings in airframe structures. However, these alloys have presented problems in service, arising from deficiencies in fracture toughness and fatigue crack propagation resistance together with a susceptibility to exfoliation corrosion and stress‐corrosion, which have led to restrictions being placed on their use by individual aircraft companies and by procurement authorities in a number of countries. This situation has led to the wide‐spread use in the UK and continental Europe of lower strength alloys of the aluminium‐copper‐magnesium‐silicon type, even though significant weight penalties are incurred in the process. There has been a more general acceptance of the high strength aluminium‐zinc‐magnesium‐copper alloys in the USA, where problems associated with their use have been partially alleviated by a willingness to replace components at short intervals, but even so during recent years a trend has developed there towards the use of lower strength versions of these alloys in attempts to improve airframe durability and reliability.