Effect of welding parameters on mechanical performance, fracture toughness, and microstructural evolution of similar and dissimilar AA6082/AZ91 friction stir welded joints

: Friction stir welding (FSW) of aluminum–magnesium systems requires careful control of heat input because excessive or insufficient thermal exposure can degrade joint integrity. Friction stir welding (FSW) of aluminum–magnesium systems requires careful control of heat input because excessive or insufficient thermal exposure can degrade joint integrity. Establishing processing conditions that preserve mechanical and fracture integrity while limiting detrimental Al–Mg intermetallic formation is therefore essential for the reliable application of lightweight AA6082/AZ91 hybrid structures. This study compares similar AA6082/AA6082, similar AZ91/AZ91, and dissimilar AA6082/AZ91 joints produced at rotational speeds of 1000, 1500, and 2000 rpm and welding speeds of 10, 16, and 31.5 mm min -1 . This study compares similar AA6082/AA6082, similar AZ91/AZ91, and dissimilar AA6082/AZ91 joints produced at rotational speeds of 1000, 1500, and 2000 rpm and welding speeds of 10, 16, and 31.5 mm min -1 . Mechanical performance was evaluated by tensile, Vickers microhardness, instrumented Charpy impact, and elastic–plastic fracture mechanics tests, while optical microscopy, SEM/EDS, and XRD were used to correlate fracture behavior with microstructural evolution. The optimum condition was 1500 rpm and 16 mm min -1 . Under these conditions, the AA6082 joint reached 255 ± 6 MPa ultimate tensile strength, 82.26% joint efficiency, and 24.5 kJ m -2 dynamic fracture toughness; corresponding ultimate tensile strengths were 205 ± 5 MPa for AZ91 and 172 ± 5 MPa for AA6082/AZ91. The dissimilar joint exhibited the highest stir-zone hardness (165 ± 4 HV), but lower ductility and fracture resistance because of brittle Al 3 Mg 2 and Al 12 Mg 17 intermetallic phases. Dynamic recrystallization and refined equiaxed grains promoted ductile fracture in AA6082, whereas the dissimilar interface showed cleavage and secondary cracking. The results identify an intermediate heat-input regime that best balances strengthening, material flow, and interfacial phase formation.