This research focuses on the computational modelling and comparative analysis of friction stir welding (FSW) and stationary shoulder friction stir welding (SSFSW) applied to AA6061-T6 aluminium alloy. SSFSW, an FSW variant, employs a stationary shoulder and a rotating pin. This study introduces a numerical model for both processes, using the innovative Smoothed Particle Hydrodynamics (SPH) technique to capture their distinct thermo-mechanical characteristics. The aim is to unravel its mechanics and multi-physics in SSFSW and compare it with conventional FSW. The temperatures predicted by the model exhibited a close agreement between the advancing side (AS) and retreating side (RS). Plastic strain patterns show that regular FSW is different from SSFSW. In SSFSW, the strain is less, and the plastic area is comparatively slightly narrower. The distinct �ironing effect� resulting from the stationary shoulder in SSFSW reduces the heat-affected zone (HAZ). Yet, it maintains efficient plasticisation and material flow within the pin-affected zone (PAZ). This research emphasises the significant impact of temperature, strain, material flow, and thermo-mechanical characteristics on the quality of joints. Future suggestions include exploring process parameters more broadly, examining dissimilar welding techniques and hybrid approaches, and comprehensively investigating the diverse effects of SSFSW under various configurations and joint angles. © 2023 by the authors.