*3.3. Material Flow*

The material flow occurs in the weld area during the friction stir welding. However, the plunging process is an important process of the friction stir welding. The material softens and plastically deforms during this process. With the increasing temperature in the welding zone, the material flows plastically. Studying the material flow is helpful for understanding the welding seam forming rule and for preventing welding defects. In this study, a particle tracking technique was used to study the material flow.

The plastic flow of materials in the simulation analysis process easily causes an excessive deformation of the cell shape, which leads to distortion. Therefore, it is guaranteed that the material can continue to operate after a large plastic deformation. DEFORM-3D will automatically re-divide the distorted mesh after the mesh distortion reaches a certain level, and will generate a new high-quality mesh. The mesh re-division technique can be used to reconstruct the plastically deformed mesh in time, so that it can be used to study the plastic flow of the materials.
