Influence of Post-Weld Heat Treatment on Mechanical Properties and Microstructure of Friction Stir Welded Rheocast AA6082 Joints

This study investigates the mechanical properties, microstructural changes, and fracture characteristics of friction stir welded (FSW) rheocast AA6082 aluminum alloy joints that underwent second-stage post-weld heat treatment (PWHT). All welds were executed utilizing optimal welding parameters of 910 rpm rotation speed and 113 mm/min welding speed to guarantee steady heat input and defect-free connections. Two post-weld heat treatment (PWHT) methods were assessed: artificial aging at 190°C for 6 to 18 hours and solution treatment at 550°C for 2 to 8 hours, succeeded by quenching and artificial aging. Microstructural analysis indicated that extended solution treatment increasingly destabilizes the dynamically recrystallized stir zone, resulting in grain coarsening and the emergence of micro-defects when the holding duration surpasses roughly 3 hours. Mechanical testing revealed that, whereas the base metal demonstrates a conventional strength–ductility trade-off, the welded joints undergo concurrent decreases in ultimate tensile strength from 251 MPa to 211 MPa and elongation from 3.89% to 2.80% as the period of solution treatment increases. As a result, joint efficiency diminishes from around 82% at 2 hours to 72% at 8 hours. Hardness tests reveal a rise in softening and heterogeneity inside the weld core with prolonged holding time. Fracture analysis indicated that ductile failure was mostly governed by microvoid coalescence under both ideal artificial aging (190°C for 12 hours) and solution treatment (550°C for 2 hours). A solution treatment at 550°C for 2 hours yields the optimal equilibrium among precipitation regulation, grain stability, hardness consistency, and joint efficacy.