This general project is supported by the National Natural Science Foundation of China. The relation between the solid mechanics based model and the fluid dynamic model of friction stir welding can be established by the detailed investigations on the contact interface behaviors in friction stir welding. By the established model and the recrystallization theory, the grain size evolutions can be predicted based on the calculated deformation and temperature history. The HAZ region can be then determined in different welding parameters and the microstructure can be optimized. The mechanism of the welding defects can be studied. With the damage model and the researches on welding defects, the effect of the defects on the mechanical properties of friction stir weld can be further studied. The determination of the relation between the mechanical properties and the welding parameters can be helpful for the development of the computational method and platform of friction stir welding.