In this study, a coupled thermomechanical model has been developed for the plunge stage of friction-stir welding (FSW) for joining dissimilar Al 6061 to TRIP steel. Governing equations of mass, momentum, and energy have been formulated for the bulk material and the interface, respectively. Generalized material properties defined with the field variable are introduced for material identification at different regions. Local instant formulation based on binary phase flow theories has been proposed for developing conservation equations at the interface. These analytical derivations are then implemented into a finite-element model for numerical simulations. In the early stage of plunging, the estimated axial force correlates well with the experimental results, where a short plateau can be observed before the final peak occurs. Discrepancies at the later stage can be attributed to different experimental configurations and related simplified model assumptions.