Cyclic deformation and residual stress characteristics of ultra-thin aluminium lap joints by pinless friction stir welding

Lightweight thin aluminium sheets are increasingly used in transportation and packaging industries, but their reliable joining remains challenging due high residual stresses, and poor fatigue resistance in conventional fusion welding. This study addresses these limitations by employing pinless friction stir welding to fabricate 0.5 mm thin commercially pure aluminium lap joints, and systematically investigating their microstructural, mechanical, residual stress, and fatigue behaviour. Defect-free welds were obtained without hook or cold lap formation, though limited flash and thickness reduction were observed. X-ray diffraction residual stress analysis revealed predominantly compressive stresses, limited to −3.8 ± 8 MPa. Low-cycle fatigue tests demonstrated stable cyclic response at 0.2% strain amplitude, while higher amplitudes (0.3–0.7%) caused strong cyclic softening linked to grain coarsening. Hysteresis loop analysis confirmed strain-dependent energy dissipation, with widening and rotation at higher amplitudes. Overall, the study establishes that pinless FSW enables sound joints in ultra-thin aluminium sheets with low residual stresses, reasonable lap shear strength, and cyclic deformation response, making it a promising technique for lightweight structural applications.