Abstract
A three-step aging process to reduce the susceptibility of 7475 aluminum alloy to stress corrosion cracking, and to keep high strength, was investigated. This process includes the first low temperature aging, the second retrogression at higher temperatures, and the third reaping at low temperature once again. For the condition applied to the first step the usual T6 treatment is most suitable. For the second step the higher the retrogression temperature was adopted and the longer the time is done, the lower the susceptibility is effected, although a complete recovery to the original T6 strength level is only found in the time region up to about the minimum in the retrogression curve, and in longer time the strength is gradually decreases. For the final reaging the usual T6 treatment is suitable. The maintenance of high strength and improvement of susceptibility may be attributed to different effects of the process on the precipitation structure of innergrains and of grain boundary. The fine precipitation structure of innergrains which is kept by reversion and reprecipitation may serve to hold high strength, while the coarse grain boundary structure formed by growth of precipitates may sreve to improve the susceptibility.
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