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The precipitation behavior and development of grain structure have been investigated to reveal their association with the microhardness and corrosion resistance of probeless friction stir spot welded (P-FSSW) joint of an Al–Li alloy. Results identified the primary strengthening phase of T1 (Al2CuLi) to be completely dissolved in the stir zone (SZ), while equiaxed grains were formed. Grain refinement was directly associated with the combined effect of continuous dynamic recrystallization and geometrized effect of imposed strain. However, as localized grain boundary bulging indicated, limited discontinuous recrystallization also participated in the microstructure evolution. The hardness dropped noticeably in the heat affected zone (HAZ) due to the weakening of precipitation and dislocation strengthening, which resulted from partial dissolution or coarsening of T1 phase and recovery effect. Grain refinement and solution strengthening were responsible for the slight increase in hardness of the SZ. In addition, the SZ had minute increased pitting resistance due to the homogenization of intermetallic particles, precipitate dissolution and refined grains.

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.