Effect of Cryogenic Treatment on 7A09 Aluminum Alloy and Optimization of Process Parameters

7A09 aluminum alloy is prone to produce significant uneven residual stresses during solid solution treatment. It is inefficient to eliminate the residual stresses via traditional heat treatment processes. Therefore, this study investigates the effects of cryogenic treatment combining the traditional heat treatment on the residual stress and microstructure of 7A09 aluminum alloy. The orthogonal test and range analysis method were conducted to optimize process parameters. The optimized cryogenic treatment parameters are: holding temperature 77 K, holding time 24 h, cryogenic cycles 2 times, and cooling rate 3 K/min. The results show that combining cryogenic treatment with traditional heat treatment can significantly eliminate residual stress. The optimal residual stress relief rate can reach 71.16%. OM, XRD, SEM, and TEM were used to reveal the microscopic mechanisms of residual stress evolution during the heat treatment of 7A09 aluminum alloy. The reduction in dislocation density and the dispersion of nano-scale precipitates are the main factors in the elimination of residual stress.