Effect of Pre-aging Temperature in Retrogression and Re-aging Treatment on Stress Corrosion Resistance of 7050 Aluminum Alloy

The effect of the pre-aging temperature in retrogression and re-aging(RRA)treatment on stress corrosion resistance of 7050 aluminum alloy was investigated by means of hardness tests,conductivity tests,slow strain rate tensile(SSRT)tests,transmission electron microscopy(TEM)and scanning electron microscopy(SEM).The results show that when the pre-aging temperature increases,the main intragranular precipitates in 7050 aluminum alloy transform from GP zones toη′phases after RRA.The grain boundary precipitates become coarser and consequently the grain boundary becomes discontinuous,which leads to the reduction of sensitivity of stress corrosion.At the meantime, the precipitate free zone(PFZ)becomes wider,whose width goes up to 140nm when pre-aging temperature rises to 120℃,causing stress concentration and anode dissolution which are harmful to the stress corrosion resistance of the alloy.When the pre-aging temperature is 80℃,as in under-aged condition at the pre-aging stage of RRA,the alloy shows better stress corrosion resistance.The tensile strength,elongation and stress corrosion index(ISSRT)of the alloy are 473.5MPa,10.67% and 0.05824 with slow strain rate(10-6s-1)and corrosion medium of 3.5% NaCl solution.