Effects of tool design on the microstructure and mechanical properties of refill friction stir spot welding of dissimilar Al alloys

Al6022-T4/Al7075-T6 welds were fabricated by refill friction stir spot welding using different tool designs with identical welding parameters. Cracks and voids were formed in the welds manufactured using a standard tool. When tool penetration depth is low, metallurgical bonding was not formed completely through the weld center despite the presence of mechanical inter-locking. When tool penetration was increased to 1.5 mm, more intermixing and metallurgical bonding was formed in the weld center with some voids at the weld boundary. Defect-free welds were fabricated using a modified tool with three grooves or notches in the tool sleeve, which improved metallurgy bonding, material intermixing and mechanical inter-locking at the weld interface. The intermixed structure resulted from incorporation of the top and bottom sheet materials promoted by the grooves on the bottom of the tool sleeve, and this suppressed formation of cracks. Thus, the welds exhibit higher overlap shear strength with less sensitivity to welding parameters.