Abstract: Coupled thermo-mechanical model is used to simulate the material flow in friction stir welding(FSW) The flow traces are studied for the determination of the stirring zone(SZ) and the thermo-mechanical affected zone(TMAZ). The histories of the true strain rate and the temperature on the boundaries of the SZ and the TMAZ are given to calculate the Zener-Holloman parameter and the grain sizes. The comparison to the data in published literature shows the validity of the current model. Results indicate that uniform temperature can lead to uniform lgZ in the SZ, although the distribution of the strain rate is not uniform. This means that the effect of temperature on the grain sizes is much more important than the effect of the strain rate. The strain rates of the material particles in rotation with pin can reach 22 s–1. The boundaries of the SZ and TMAZ can be determined by the different material flow rules of different traced particles. The SZ and the grain sizes are both increased with the increase of the shoulder diameter. Smaller shoulder can lead to smaller SZ.

Key words: coupled thermo-meohanical model, flow trace, friciton stir welding, grain size