Numerical simulation of micro refill FSSW in AA7075-T6 using smoothed particle hydrodynamics: Effect of plunge depth on thermo-mechanical evolution and material flow

• 3D SPH model developed for refill FSSW of AA7075-T6 aluminium sheets. • SPH captures thermal, strain, and flow fields without mesh distortion. • Model validated with <0.6 % error against experimental temperature data. • Higher plunge depth improves mixing but increases hook height and sheet thinning. This study develops a three-dimensional numerical framework utilizing the mesh-free Smoothed Particle Hydrodynamics (SPH) approach to examine the thermo-mechanical evolution and flow behaviour during refill Friction Stir Spot Welding (FSSW) of thin AA7075-T6 aluminium alloy sheets. A fully coupled SPH-based numerical model was used to simulate the welding process at three different plunge depths (PDs): which are 41.6 % (0.5 mm), 58.3 % (0.7 mm), and 75 % (0.9 mm) of the effective thickness of the joint. The SPH formulation facilitates precise monitoring of transient temperature gradients, plastic deformation, and interfacial mixing without mesh distortion, overcoming the constraints of conventional finite element approaches in large-deformation situations. Validation against experimental data affirmed the model's precision, with peak temperature predictions at the weld center deviating by less than 0.52%. The findings show that increasing PD increases thermal input and plastic strain in the shoulder-affected zone, which promotes material softening and stronger metallurgical bonding. Excessive PDs, however, result in geometric imperfections, including elevated hook height and reduced thickness of the bottom sheet, which compromise joint strength. These findings demonstrate the utility of SPH for high-fidelity thermo-mechanical prediction in solid-state joining, as well as practical insights into process parameter optimization for defect-free, high-integrity welds.