On the double-side probeless friction stir spot welding of AA2198 Al-Li alloy

Double-side probeless friction stir spot welding (DP-FSSW) of AA2198 alloy was conducted to investigate the microstructure and mechanical properties. Compared with common single-side probeless friction stir spot welding (P-FSSW), the plastic strain during DP-FSSW is nearly symmetrical with respect to the bondline to suppress the extension of hook defect, which is detrimental to the joint mechanical strength. With DP-FSSW, a fully metallurgically bonded region has formed due to severe plastic deformation at high temperatures. Tensile/shear tests show that the joint strength could exceed 8 kN, which is comparable to P-FSSW and refill FSSW, and all fractures happen in a shear failure mode as cracks extend along the interface of two sheets. The microhardness profile exhibits a uniform distribution along the thickness direction, in which the hook defect shows the lowest value.