Inertia Friction Welding (IFW) is a solid-state joining process where one rotating (connected to an inertia) and one stationary part are brought together under an axial load, causing frictional heat generation and plastic deformation at the interface; upon cooling a weld is formed between the components. There is evidence in welds between dissimilar materials which show a flow regime that may keep impurities at the weld interface and may have implications for weld strength and fatigue life. Numerical modelling of IFW using Finite Element Analysis (FEA) has allowed the successful prediction of temperature profile, upset (length loss) and flash shape and process parameters such as flywheel slowdown. However, due to the lack of knowledge of the behaviour of the severely plasticised zone (shear zone) and the fluid-like nature of the material near the interface, the use of Computational Fluid Dynamics (CFD) has been considered. This paper presents a method to utilise both FEA and CFD modelling techniques to provide a better modelling strategy for the IFW processes. By using the results of an FEA model as the boundary/initial conditions for the CFD, simple models have allowed comparison between the two numerical approaches and have validated the implementation and consistency of material properties and modelling methodology for both. A model of the interface has been produced with CFD with this method which illustrates the possible material behaviour and material flow in that zone.