Effects of plunge depth on macro-/microstructure and mechanical properties of refill friction stir spot welded 2195-T6 Al–Li alloy

Refill friction stir spot welding (RFSSW) was employed to join 2195-T6 Al–Li alloy thin plates, and the effects of plunge depth on the macro-/microstructure and mechanical properties of the joints were systematically investigated. Different from the high-plasticity Al alloys, all 2195-T6 RFSSW joints displayed a slight upward hook at the edges of the lap interfaces, with the hook height remaining relatively constant. Conversely, the hook angle decreased significantly from approximately 147.9°–86.7° as the plunge depth increased from 2.1 mm to 2.9 mm. Material softening occurred in the stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ) compared to the base material (BM) in all the joints primarily due to the dissolution and coarsening of T 1 precipitates and a reduction in dislocation density. The SZ exhibited higher hardness values than TMAZ and HAZ due to the significant grain refinement resulted from the DRX within the SZ. The tensile shear properties were closely dependent on the hook morphologies, particularly the hook angle, and the weak bonding areas, including bonding ligament and SZ/TAMZ boundary. The tensile shear loads initially increased and then decreased as the plunge depth increased, with the fracture mode transitioning from shear mode to shear-plug mode, and finally to plug mode. The optimized RFSSW joint, with a maximum tensile shear load of ∼10947 N, was achieved at the plunge depth of 2.5 mm, exhibiting a shear-plug mode fracture, where one crack propagation path through the SZ and the other along the SZ/TMAZ boundary.