Enhanced mechanical properties and corrosion resistance of an Al-Zn-Mg aluminum alloy through variable-rate non-isothermal aging

Abstract

A variable-rate non-isothermal aging (NIA) process that has two-stage heating and cooling procedures was proposed for a ternary Al-Zn-Mg aluminum alloy aiming to obtain the combination of high strength and excellent corrosion resistance. The mechanical properties, susceptibilities to the stress corrosion cracking (SCC), intergranular corrosion (IGC) and exfoliation corrosion (EXCO), as well as microstructural characteristics of the variable-rate NIA processed materials were investigated. They were compared with that produced by conventional constant rate NIA and isothermal aging processes (T6 and T73 temper). The results show that the new variable-rate NIA process which has different heating /cooling rates in different temperature ranges may be the solution to obtain much higher strength and excellent corrosion resistance. A much higher percentage of fully grown but not coarsened precipitates were obtained within the grains and a discontinuous distribution of GBPs was obtained on grain boundaries. The alloys produced by the new variable-rate NIA process achieved a 5–7 % higher strength than the conventional T6 temper and a higher corrosion resistance than T73 temper. The possible causes for the improvement of strength and corrosion resistance were discussed on the basis of the microstructural evolution.