Failure mechanisms and low-cycle fatigue behavior of double-sided friction stir welded 6061-T6 aluminum alloy joints: influence of dual-tool interaction

This study presents a comparative failure analysis of low-cycle fatigue (LCF) behavior in 6061-T6 aluminum alloy joints fabricated by synergistically double-sided friction stir welding (SDS-FSW) and conventional sequentially double-sided friction stir welding (CDS-FSW). The cyclic deformation, damage evolution, and crack propagation characteristics were systematically evaluated under strain-controlled loading. Both joints exhibited elastic response at 0.15% strain amplitude and pronounced cyclic hardening beyond 0.25%. However, the CDS-FSW joints showed consistently lower peak stress amplitudes and reduced hardening capacity, indicating inferior resistance to cyclic deformation. Increasing strain amplitude accelerated plastic strain energy accumulation and fatigue damage. Despite exhibiting higher plastic strain energy density, SDS-FSW joints achieved longer fatigue life due to their improved microstructural uniformity and reduced crack growth rate. Fractographic analysis revealed that CDS-FSW joints exhibited larger fatigue striation spacing and more rapid crack propagation, whereas SDS-FSW joints showed finer striations and more stable ductile fracture characteristics. The results establish a direct link between welding-induced thermo-mechanical conditions and fatigue failure mechanisms, providing insights for durability assessment and structural reliability of double-sided friction stir welded aluminum components.