Friction Stir WeldingFriction stir welding (FSW) is a solid-state joining technique that produces high-quality welds without melting the material. This study presents a COMSOL-based thermal analysis of friction stir weldingFriction stir welding using a bimetallic plate of thickness 12.7 mm composed of pure aluminium alloy (AA1100) and stainless steelStainless steel (AISI 304) divided along the y-axis. The coupled thermo-mechanical simulationSimulation is implemented in COMSOL MultiphysicsCOMSOL multiphysics (v 6.2) using a moving coordinate system fixed to the tool. TemperatureTemperature-dependent material propertiesProperties and a Johnson–Cook plasticity model are defined for both AA1100 and 304 steelSteel. Frictional heat generation at the tool–workpiece interfacesInterface, convective and radiative cooling, and other boundary conditions are incorporated. The simulationSimulation results reveal peak alloy welding temperaturesTemperature of ~ 664 °C for welding two aluminium plates and ~ 675 °C for two steel plates Steel, and 667 °C for aluminium and steelSteel welded together, with distinct thermal profiles. Time-dependent temperatureTemperature profiles reveal asymmetric heat flow. Aluminium rapidly conducts and spreads heat, forming broad high-temperatureTemperature plateaus near 670 °C, while steelSteel’s lower conductivityConductivity and higher heat capacity delay its temperatureTemperature rise, creating distinct troughs. Crest–trough fluctuations emerge at the interfaceInterface due to contrasting material propertiesProperties and boundary effects. These findings highlight how thermal asymmetry shapes microstructural evolution and residual stressResidual stress, offering insights for optimizing processProcess parameters in dissimilar material welding. The presented approach can reduce the need for extensive trial and error in FSW processProcess development and guide the welding of advanced alloys in aerospace and automotive applications.