Molecular dynamics simulation of dissimilar friction stir spot welding: Al[sbnd]Al and Al[sbnd]Mg joints

This work reveals novel molecular dynamics results to explain the thermomechanical differences in the nanoscale friction stir spot welding (FSSW) of Al[sbnd]Al and Al[sbnd]Mg joints. The maximum Tresca stress during Al[sbnd]Al joining (similar material joining) reached 1.65 GPa, higher than the peak Tresca stress of 1.12 GPa in the Al[sbnd]Mg joining (dissimilar material joining), which was attributed to strain localisation effects. The Al[sbnd]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[sbnd]Mg (750 K at 1.5 nm penetration vs 2.4 nm in Al[sbnd]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.