Multi-scale modelling of the microstructure evolution during friction stir welding of 2195 Al-Li alloy

Al-Li alloy has a wide range of application prospects in aerospace and rail transportation fields. Friction stir welding (FSW) is suitable for joining of Al-Li alloy. The microstructure evolution in FSW of Al-Li alloy determines the final joint properties. However, it is difficult to directly observe the microstructure evolution in FSW of Al-Li alloy only by experimental methods. Therefore, a Monte Carlo model in cooperation with a thermomechanical finite element model was proposed to study the microstructure evolution in FSW of 2195 Al-Li alloy. The validated thermomechanical finite element model was first used to calculate the temperature field, strain rate and strain evolution during the FSW process. Then, the achieved thermomechanical history was used in the Monte Carlo model to predict the transient microstructure evolution and the final morphology of the grain structure. It is found that the plastic materials on the advancing side experience a higher strain compared with that on the retreating side with the same distance from the joint line. The plastic material in the middle of the weld nugget zone (WNZ) experiences a relatively high temperature with more severe plastic deformation. Therefore, fully dynamic recrystallization takes place for the material at this region as it flows across the tool during FSW, which results in a fine recrystallization grain in the middle of WNZ. However, the material on the advancing side at 2 mm away from the joint line experiences a lower strain rate, which results in partial recrystallization with some relatively large grains of the base materials. The simulated average grain size and grain morphology were in good agreement with the measured results by EBSD method.