Impact of Surface State in Probeless Friction Stir Spot Welding of an Al–Li Alloy

Sheets of AA2198 alloy with various surface conditions were welded with probeless friction stir spot welding (P-FSSW). Results show that the oxide layer on the original lap-weld surface produces continuously distributed oxide impurities at the interface of the P-FSSWed joint with a large amount of voids. The visual flow at the interface provides a persuasive explanation of local preferential abrasion. Following surface grinding, local abrasion increases with surface roughness and results in the dispersion of voids and oxides, which contributes to the improvement of metallurgical connection. The corresponding mechanical strength of the P-FSSWed joints shows a relatively significant increase, while the fracture mode remains affected by the hook defect regardless of the surface state.