Improving the mechanical properties and reducing the residual stresses of AA7075 by combination of cyclic close die forging and precipitation hardening

Abstract

As one of the major strengthening mechanisms, the precipitation hardening is used to enhance the mechanical properties of aluminum alloys. Based on the results of the residual stress measurement, after fast quenching, the core of the material is under tensile stress while the surfaces are under compressive residual stress. Distortion commonly happens during the machining of precipitation-hardened aluminum alloys due to the residual stresses created during the quenching step in the heat treatment process. In this study, the combination of cyclic close die forging and precipitation hardening was used to improve the mechanical properties and control the residual stresses of AA 7075, simultaneously. According to the results, a considerable level of residual stresses was developed in the sample after the quenching step. Performing the cyclic close die forging process immediately after the quenching step changed the pattern of the residual stresses and reduced them significantly. The reduction of the residual stresses after the first pass of cyclic close die forging was about 50%, while after two passes, the sample was almost fully stress relieved. Besides, the results of the microhardness and uniaxial tensile tests demonstrated the improvement of the mechanical properties of the processed samples when compared to the T6 condition. Also, in comparison to AA 7075-T6, the yield stress, ultimate tensile stress, and microhardness were increased by about 24%, 22%, and 48%, respectively.