Abstract: Friction stir welding process is simulated by using a fully coupled thermo-mechanical model.The material flow patterns in the friction stir welding process are analyzed.The comparison with experimental results can validate the current model and shows that the material flows and temperature distributions can be predicted precisely.From the research on material flows around the welding tool, the formation of weld fash in friction stir welding is explained.The 3D material flow patterns on different thicknesses are studied, which is compared with the 2D case to show that the material flows obtained in 2D numerical simulation correspond to the ones near the bottom surface obtained in 3D simulation.The equivalent plastic strain distribution also shows that the shoulder can affect the material behaviors near the top surface and the effect of the shoulder becomes weaker near the bottom surface, which demonstrates that the 2D case correspond to the part near the bottom surface in 3D case.