Effect of tool diameter on the performance of 6061-T6 aluminum alloy in refill friction stir spot welding zone

In this study, stirring tools, with pin diameters of 7 mm and 8 mm and sleeve diameters of 10 mm and 12 mm, were designed and manufactured to perform refill friction stir spot welding (RFSSW) on 3-mm thick 6061-T6 aluminum alloy. The spot welding was conducted under process parameters with rotation speeds of 1400 rpm and 1600 rpm and welding speeds ranging from 20 to 50 mm/min. We achieved well-formed spot welding zones and conducted performance tests on these zones. The test results indicate that the grains within the nugget zone are sufficiently refined and remain consistent regardless of changes in the diameter of the stirring pin. The grains in the thermal and mechanical affected zone (TMAZ) are smaller, reaching their smallest size with a stirring pin diameter of 8 mm. The presence of a "Hook" defect was observed, which serves as a crack initiation point for tension-shear fractures. The maximum tension-shear force was measured at 14.79 kN for an 8-mm stirring pin and 12.81 kN for a 7-mm pin, indicating a significant increase with the larger diameter. Thus, the increase in the diameter of the stirring tool not only impacts the maximum tensile and shear force in the spot welding zone but also contributes to considerable annealing.