Post-weld heat treatment of AA7075 friction stir welded joints manufactured under various tool rotation speeds and cooling conditions

Abstract This study investigated the effect of post-weld heat treatment (PWHT) on the microstructure, microhardness, tensile properties, and fracture behavior of friction stir welded AA7075 joints produced under different cooling conditions (air, water, quenching oil, cutting fluid) and tool rotation speeds (600–1400 rpm). PWHT increased and homogenized microhardness, with the stir zone generally exhibiting higher hardness than the surrounding regions. Abnormal grain growth (AGG) occurred in the shoulder-driven zone of all specimens, irrespective of cooling condition or tool rotation speed, and was also observed in the pin-driven zone of most joints. For defect-free specimens, PWHT increased yield strength by more than 70% and ultimate tensile strength by approximately 35%, with the highest UTS values exceeding that of the base material. The air-welded joint also showed nearly doubled ductility and was the only heat-treated joint to fracture outside the weld region. Defective joints exhibited limited ductility. Fractography revealed intergranular cracking, dimpled transgranular fracture, particle-assisted microvoid formation, and strain localization. Overall, PWHT improved hardness and strength but did not prevent AGG, particularly in the shoulder-driven zone.