Weld formation under different tool shoulder morphologies and microstructural characteristics of the optimized large-involute friction stir welded 2195-T87 Al–Li alloy joint

This study investigates the effect of tool shoulder morphology on weld formation and tensile response in friction stir welded 2195-T87 Al–Li alloy, followed by microstructural characterization of the optimized joint. Three shoulder designs, namely large involute, small involute and concentric circles, were compared under identical welding parameters. The large-involute shoulder produced the smoothest weld surface and the lowest visible surface-defect area fraction (0.04%), whereas the small-involute and concentric-circle shoulders showed higher values of 5.58% and 14.16%, respectively. Their ultimate tensile strengths were statistically similar: 421 ± 8, 418 ± 7 and 419 ± 7 MPa. Therefore, the large-involute joint was selected for OM, EBSD, XRD, microhardness and fracture analyses. EBSD revealed fine equiaxed recrystallized grains in the weld nugget zone, with an average diameter of 6.9 μm and a high-angle grain-boundary fraction of 86.7%. XRD patterns were dominated by Al matrix peaks, and peak broadening was attributed to combined effects of grain refinement, microstrain/residual stress and precipitation changes. The joint showed a W-shaped hardness profile with a minimum of about 104 HV in the retreating-side TMAZ, where ductile fracture initiated.