Mechanical, microstructural and fractographic characterization of friction stir welded AA6061-T651 joints produced by square and tri-flat tools

Friction stir welding (FSW) is widely employed for joining AA6061-T651. However, weld quality under industrially relevant 0° tool tilt, where spindle inclination is restricted, remains unexplored because of reduced forging pressure and increased susceptibility to material consolidation defects. This study systematically investigates the influence of square (SQ4) and tri-flat (TF6) tool pin profiles on weld formation, microstructural evolution and mechanical performance under 0° tool tilt. Weld integrity was evaluated using radiographic, tensile testing, optical microscopy, microhardness, SEM, EDS and fractography. The radiographic examinations exhibited void and tunnel defects for both welds and led to UTS of 132 MPa (SQ4) and 139 MPa (TF6). Dynamic recrystallization produced refined stir zone grain sizes of 1.4 μm (SQ4) and 1.5 μm (TF6); however, hardness recovery remained incomplete in SZ of 57 HV for SQ4, whereas TF6 exhibited improved stir-zone hardness of 70.2 HV. SEM-EDS analysis confirmed Mg2Si precipitate evolution and thermo-mechanically driven redistribution of intermetallic particles, while fractography revealed predominantly ductile microvoid coalescence with localised strain concentration. The results demonstrate that investigated tool and processing conditions were insufficient to fully compensate for the absence of tilt-induced forging action, providing experimentally validated guidance for tool and process variable selection in industrial FSW applications.