Page 231 - Aircraft Stuctures for Engineering Student

7.1 Materials of aircraft construction 21 5
the aluminium-zinc-magnesium alloys was accompanied for many years by a sudden
liability to crack in an unloaded condition due to the retention of internal stresses in
bars, forgings and sheet after heat treatment. Although variations in composition
have eliminated this problem to a considerable extent other deficiencies showed them-
selves. Early Viscount service experience produced large numbers of stress-corrosion
failures of forgings and extrusions. The problem became so serious that in 1953 it was
decided to replace as many aluminium-zinc-manganese components as possible with
the aluminium-4 per cent copper Alloy L65 and to prohibit the use of forgings in
zinc-bearing alloy in all future designs. However, improvements in the stress-corro-
sion resistance of the aluminium-zinc-magnesium alloys have resulted in recent
years from British, American and German research. Both British and American
opinions agree on the benefits of including about 1 per cent copper but disagree on
the inclusion of chromium and manganese, while in Germany the addition of silver
has been found extremely beneficial. 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 largely
met the requirement for aluminium alloys possessing high strength, good fatigue
crack growth resistance and adequate toughness. Further development will concen-
trate on the production of materials possessing higher specific properties, bringing
benefits in relation to weight saving rather than increasing strength and stiffness.
The duralumin alloys possess a lower static strength than the above zinc-bearing
alloys, but are preferred for portions of the structure where fatigue considerations
are of primary importance such as the under-surfaces of wings where tensile fatigue
loads predominate. Experience has shown that the naturally aged version of
duralumin has important advantages over the fully heat-treated forms in fatigue
endurance and resistance to crack-propagation. Furthermore, the inclusion of a
higher percentage of magnesium was found, in America, to produce, in the naturally
aged condition, mechanical properties between those of the normal naturally aged
and artificially aged duralumin. This alloy, designated 2024 (aluminium-copper
alloys form the 2000 series) has the nominal composition: 4.5 per cent copper, 1.5
per cent magnesium, 0.6 per cent manganese, with the remainder aluminium, and
appears to be a satisfactory compromise between the various important, but some-
times conflicting, mechanical properties.
Interest in aluminium-magnesium-silicon alloys has recently increased, although
they have been in general use in the aerospace industry for decades. The reasons
for this renewed interest are that they are potentially cheaper than ahminiam-
copper alloys and, being weldable, are capable of reducing manufacturing costs. In
addition, variants, such as the IS0 6013 alloy, have improved property levels and,
generally, possess a similar high fracture toughness and resistance to crack propaga-
tion as the 2000 series alloys.
Frequently, a particular form of an alloy is developed for a particular aircraft.
An outstanding example of such a development is the use of Hiduminium RR58
as the basis for the main structural material, designated CMOO1, for Concorde.
Hiduminium RR58 is a complex aluminium-copper-magnesium-nickel-iron alloy
developed during the 1939-45 war specifically for the manufacture of forged compo-
nents in gas turbine aero engines. The chemical composition of the version used in
Concorde was decided on the basis of elevated temperature, creep, fatigue and tensile