*2.5. FEM Computational Mesh*

In the process of FSW, large plastic deformation occurs in the welding area, and the welding temperature field and stress and strain field change violently. In order to truly simulate the stirring action of the workpiece material in the welding process and visualize the plastic flow behavior of the material, the grid size of the welding seam area must be small enough. However, in the whole welding process, the welding parts are larger, and if the whole workpiece meshes with the same grid size, the calculation will be very large, which will seriously reduce the simulation efficiency. Therefore, in this model, the workpiece grid is divided into two types of grid: free quadrilateral grids used for welding plates, and free triangular grids used for the area around the stirring tool. In order to further reduce the calculation amount while ensuring the calculation accuracy, only the grid around the stirring tool is refined. In local grid refinement, through the absolute size control grid refinement of the grid size, the minimum grid size is set to 0.004 mm, and the largest size is set to 1.4 mm, so the accuracy of the grid is sufficient to reflect the plastic flow behavior of materials, the temperature field, and stress–strain field of the weld area. The grid size of the remaining area of the workpiece is set to 2.4 mm to improve computing efficiency. In this paper, the effect of the back plate on the workpiece is considered through mechanical and thermal boundary conditions, so the backing plate is not meshed to improve computing efficiency. The mesh partitioning of the model is shown in Figure 3.

**Figure 3.** Mesh of the simulation domain.
