Two contact models are used to simulate the thermo-mechanical interaction process in friction stir welding. Comparison shows that the classical Coulomb friction model can be accurate enough for the simulation of friction stir welding in lower angular velocity. But in higher angular velocity, the classical Coulomb friction model fails to work due to the increase of the dynamic effect of the welding tool. Because the shear failure of material is considered in modified Coulomb friction model, the increase of the frictional stress on the tool-plate interface is limited by the shear failure. So, this model can keep valid even when the angular velocity of the welding tool is increased to a high level.