Effects of step-quench and aging on mechanical properties and resistance to stress corrosion cracking of 7050 aluminum alloy

The influence of microstructural characteristics on the mechanical properties and resistance to stress corrosion cracking (SCC) of the 7050 aluminum alloy was investigated. The higher fraction of recrystallization would cause a larger amount of incoherent Al3Zr dispersoids and high-angle grain boundaries, which increases quench sensitivity and SCC susceptibility of the alloy. Therefore, both strength and SCC resistance of the cold-rolled alloy are low. On the other hand, the hot-lolled alloy has the highest strength and superior SCC resistance, as a result of the lowest fraction of recrystallization. The SCC resistance can be significantly enhanced owing to the fine grain structure obtained with the intermediate thermomechanical treatment (ITMT); however, the strength cannot be effectively improved. The proposed treatment, step-quench and aging (SQA), can significantly improve SCC resistance and attain optimum strength by controlling the grain boundary and matrix microstructures. Higher SCC resistance and strength together with much reduced aging time are obtained simultaneously with the proposed SQA(220 degrees C/10 s or 200 degrees C/30 s) after hot-rolling, as compared to those with the conventional T73 treatment.