Synergistically double-sided probeless friction stir spot welding: The effect of tool rotation modes on joint formation and strengthening mechanisms

This study systematically investigates the synergistically double-sided probeless friction stir spot welding (SDP-FSSW) applied to 2198-T8 Al–Li alloy sheets. By independently controlling the rotation modes of two tool shoulders, three welding configurations—single-sided rotation, co-rotation, and counter-rotation—were explored. Macro- and microstructural analysis revealed that co-rotation produces an oscillating wavy hook, which effectively suppresses interfacial crack propagation, yielding tensile-shear strengths exceeding 10 kN—even without dwell—and reaching a peak of 13.38 kN at 3 s. In counter-rotation, opposing material flow creates upward- and downward-warping hooks that significantly increase the effective load-bearing area, with tensile-shear strength rising to a maximum of 15.10 kN at 6 s. These results demonstrate that SDP-FSSW effectively addresses the traditional trade-off between metallurgical bonding and geometric defect deterioration in spot-welded aluminium alloys, offering both improved interfacial bonding and enhanced joint stability. This study provides a novel approach to achieving high-performance spot welding, particularly for lightweight structural applications.