Impact of shoulder morphology on macrostructural forming and the texture development during probeless friction stir spot welding

The effect of tool morphology on the joint forming during probeless friction stir spot welding (P-FSSW) of an Al–Li alloy was evaluated with the assistance of experiments and numerical simulation. Compared to the flat tool, the surface temperature not only increased rapidly at the inner side of the shoulder periphery, but also locally appeared high value at the groove features using the tools with annular and involute profiles. In addition, the primary texture that developed during welding was determined to be a simple shear texture component of A/A¯, followed by B/B¯ and A1∗/A2∗, with the occurrence of continuous dynamic recrystallization. Based on the texture development, the circumferential material flow remained predominant due to the shear acting of the rotation shoulder. Meanwhile, the radial inward flow also existed to a certain extent, promoted by the involute feature, which was beneficial to improve the material circulation to obtain an enlarged welding area. Nevertheless, higher rotation speed for a long time should be avoided in P-FSSW as the surface material was prone to be excessively squeezed out to form flash due to the excavation effect.