Investigating the role of tool modification on microstructure and mechanical properties of dissimilar friction stir welding of AA6061 to AA5083 aluminum alloys

The present work aims to investigate the influence of tool modification on the microstructural evolution and mechanical properties of dissimilar AA6061-T6 and AA5083-H112 aluminum alloy joints fabricated using conventional Friction Stir Welding (FSW) and Vortex-Friction Stir Welding (VFSW) processes. The incorporation of a stir bar into the VFSW tool generates an additional vortex-driven material flow, which leads to altered material deformation behavior and mixing characteristics in the stir zone. Visual inspection reveals that the material distribution becomes more symmetrical in VFSW joints, and no keyhole formation is observed at the end of the joint. Moreover, enhanced material filling during VFSW reduces the size and number of defects compared with the conventional FSW process. Microstructurally, the average grain size decreases from ∼10–15 μm (FSW) to ∼3–8 μm (VFSW), which corresponds to 40–60% grain refinement due to intensified dynamic recrystallization. In addition, KAM values become lower in VFSW, which indicates reduced dislocation density and lower residual strain levels in VFSW joints. Regarding hardness, there is a noticeable difference between the two techniques in terms of stir zone homogeneity. The hardness value increases to ∼105–115 HV in VFSW joints, while remaining stable at ∼95–105 HV in conventional FSW samples. Additionally, VFSW joints exhibit a lower degree of softening in the heat-affected zone. Mechanical tests demonstrate a significant improvement in strength and ductility of VFSW joints. The average tensile strength rises from 154 ± 4 MPa for FSW to 196 ± 3 MPa for VFSW (increase 27.3%), while the mean yield strength increases from 46 ± 5 MPa to 103 ± 2 MPa. Elongation also improves from ∼9% in FSW to ∼11% in VFSW samples. Finally, the fracture surface analysis reveals the formation of smaller and more densely distributed dimples.