*3.4. Prediction of Welding Defects*

Friction stir welding is a solid phase joining technique. Welding defects will also occur if improper welding parameters lead to an abnormal material flow. Two kinds of welding defects of the joint appeared in many simulation attempts.

The formation of hole defects is mainly due to insufficient heat input in the welding process, insufficient materials reaching the plastic softening state and an insufficient material flow, resulting in an incomplete closure in the welding interior [40]. When the spindle speed is low or the welding speed is too fast, the joint is prone to holes and crack defects. This type of weld defect is usually located in the middle and lower parts of the advancing side and near the weld surface. In the simulation process, the holes that were found after the welding speed increased by 2 times were formed in the lower parts of the advancing side, as shown in Figure 18b. When the rotation speed of the main shaft is further reduced by 0.5 times, a crack defect occurs after the welding, as shown in Figure 18a.

**Figure 18.** Simulation of the formation of weld defects. (**a**) Crack defects; and (**b**) hole defects.

According to the above research on the material flow, during the welding process the material flows from the advancing side to the retreating side of the material, and the material on the retreating

#### *Metals* **2019**, *9*, 621

side flows less to the advancing side. However, when the material flow is abnormal, the material on the rear side of the tool is difficult to fill in time, which will lead to the generation of holes. Therefore, in order to avoid the appearance of holes and cracks in the joint during the welding, it is very important to optimize the welding process parameters. A finite element analysis can effectively predict the welding defects. In this paper, the formation of welding defects is predicted by the law of material flow. Welding defects themselves are not expected to occur. This paper mainly studies the law of material flow, so the detailed study of welding defects will be carried out in the following work.
