Numerical simulation is regarded as a powerful tool to understand the physical mechanisms taking place during friction welding, provided that the numerical approach accurately models the complex interface phenomena and as long as the related model parameters are identified from experimental data acutely measured close to the interface. After describing the utilized finite element formulation, the friction and heat and friction interface models are calibrated from thermal measurements, both for the FSW welding of aluminum under a lap joint configuration and for the LFW of titanium alloys.