Material flow and performance in counter-rotational double-sided probeless friction stir spot welding of 2198 Al-Li alloy

This study develops an integrated framework combining coupled Eulerian–Lagrangian (CEL) simulation with experimental validation to systematically investigate synergistic double-sided probeless friction stir spot welding (SDP-FSSW) of 2198 aluminum–lithium alloy sheets under counter-rotational conditions. The simulation results reveal that the pronounced thermo-mechanical coupling and localized strain concentration induced by counter-rotation are the dominant driving factors governing the microstructural evolution. The tracer-particle tracking results reveal that the opposing shear actions from the upper and lower shoulders drive the material to flow toward the lower and upper sides of the interface, respectively, thereby forming a Hook morphology with opposite upward and downward deflections. This feature is fundamentally different from the single-side deflected Hook commonly observed in conventional single-sided spot welding. Mechanical testing shows that the optimal joint performance is achieved at a welding force of 5 kN and a dwell time of 3 s, yielding a peak tensile/shear strength of 14.62 kN, approximately 62 % higher than that of conventional single-sided spot welding. Fracture analysis further indicates that, under counter-rotational conditions, the deflected Hook morphology enhances the effective load-bearing area and joint strength, while an excessively deflected Hook can promote crack propagation and drive the transition from shear fracture to plug-type fracture. These findings not only elucidate the previously unclear mechanisms of material flow and fracture behavior in SDP-FSSW, but also provide theoretical insight and practical guidance for the development of high-reliability friction stir spot welded joints in Al-Li alloys.