Molecular Dynamics Simulation of Dissimilar Friction Stir Spot Welding: Al-Al and Al-Mg Joints

This work reveals novel molecular dynamics results to explain the thermomechanical differences in the nanoscale friction stir spot welding (FSSW) of Al-Al and Al-Mg joints. The maximum Tresca stress during Al-Al joining (similar material joining) reached 1.65 GPa, higher than the peak Tresca stress of 1.12 GPa in the Al-Mg joining (dissimilar material joining), which was attributed to strain localisation effects. The Al-Mg joint exhibits 30-50% larger localised shear strain and develops asymmetric stress distribution, including elevated von Mises (≈3.5 GPa) and octahedral shear stresses (≈1.8 GPa). These differences emerge despite Al-Mg's lower Tresca stress, reflecting incompatible deformation between FCC Al and HCP Mg. Temperature profiles show faster heating in Al-Mg (750 K at 1.5 nm penetration vs 2.4 nm in Al-Al), with interfacial gradients (550 K vs 450 K) governed by Mg's poor thermal conductivity. Dislocation analysis reveals homogeneous 1/2<110> networks and intrinsic stacking faults in FCC Al versus confined 1/3<1-100> prismatic dislocations in Mg. The combined stress-strain-temperature data demonstrate how crystallographic mismatch promotes defect accumulation, explaining the 50% higher torque (9×10⁻¹⁷ N·m) and reduced weld integrity in dissimilar joints.