Friction stir rivet welding of AA6061-T6 sheets: A hybrid bonding approach with thermo-mechanically regulated microstructure and enhanced mechanical properties

Lightweight alloy joints produced by conventional friction stir spot welding (FSSW) are typically marred by the keyhole and hook defects, which impair mechanical performance and viability. This study applies Friction stir rivet welding (FSRW) to AA6061-T6 aluminum alloy sheets, producing joints with metallurgical joining across the upper-lower sheet interface and mechanical interlocking at the rivet–sheet interface. The microstructure evolution, dynamic recrystallization mechanism, and welding parameter effects on mechanical properties were examined. Microstructural characterization reveals that the stir zone undergoes continuous dynamic recrystallization, with grains refined to 2.08 μm and a recrystallized fraction of 75.9%. The peak temperature at the rivet–sheet interface remains at 70–90% of the AA6061-T6 solidus, suppressing Fe–Al intermetallic compounds formation. Rotation speed, feed rate, and dwell time define the thermal history, which in turn governs the effective joining dimensions and associated microstructure characteristics. The mechanical response is governed by the combined effects of thermally induced microstructure evolution and changes in the effective joining dimensions. Under optimized conditions, the FSRWed joint achieves maximum shear tension and cross tension strengths of 11.09 kN and 8.12 kN, which are 82.7% and 235.5% higher than those of the conventional FSSWed joint, respectively. Interrupted fracture observations reveal that crack initiation preferentially occurs at the migrated upper–lower sheet interface, while elevated heat input promotes a transition from ductile dimple-dominated failure to shear-dominated peeling or pull-out. These findings demonstrate that FSRW is a feasible high-strength joining technology for lightweight structures, provided that the process parameters are appropriately controlled.