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. • MD simulation of dissimilar welding using friction stir spot welding was done • Single crytal Al Al joining was compared to the Al Mg joining • Al Mg joining leads to delamination of Mg and substitution of all Mg atoms by the Al atoms • Al Al joint showed highest peak stress of deformation than the Al Mg joining