Microstructure and mechanical properties of refill friction stir spot welded joints: Effects of tool size and welding parameters

A novel refill friction stir spot welding (RFSSW) technique employing large-sized tools is proposed. The microstructure and mechanical properties of joints produced with a large-sized welding tool and a conventional tool are compared. The results show that the exit line resulting from the sleeve becomes longer with increasing the plunge depth, and the diameter of nugget increases with higher rotational speed for the joints produced by both conventional and novel tools. The plunge depth increases from 2.0 mm to 2.2 mm, then to 2.4 mm, which affects the hook defect to bend upwards, almost parallel to the lap interface, to bend downwards, respectively. The joints produced with the novel tool have a flat hook compared to the conventional tool. The microstructure evolution of the conventional and novel joints is similar. The tensile-shear and tearing forces measured of the novel joints are higher than those of the conventional joints for the same welding parameters. For conventional joints, the maximum tensile-shear and tearing forces are 8.6 ± 0.1 kN and 4.4 ± 0.2 kN, respectively. The maximum tensile-shear and tearing forces for novel joints are 10.9 ± 0.1 kN and 5.6 ± 0.1 kN, respectively. After the tensile-shear test, there are three modes of fracture, the upper-mixed, the lower-mixed, and the shear fracture one. The plunge depth has a pronounced effect on the fracture mode of joints.