3.3.2. Material Flow on the Advancing Side

The tracking particles are arranged on the advancing side of the welding area. In order to improve the comprehensiveness of the simulation analysis, two sets of tracking particles (P1–P8 and P9–P16) are set. The distance from the axis of the tool pin is 3 mm, and the two groups of tracking particles are symmetrically distributed, as shown in Figure 13. It can be seen from the side view that they are located on both sides of the center line of the tool pin.

**Figure 13.** Advancing side tracking particle position setting.

During the plunging process, the particles on the advancing side differ from the flow of particles in the tool pin. During this stage, the particles are not rotated around the tool pin in the initial stage of pressing, but are squeezed by the tool pin to both sides of the tool pin, as shown in Figure 14. A plastic softening layer is formed, and the plastic softening zone becomes larger in the late stage of the plunging stage; the particles flow with the tool, the upper layer particles are spiraled to the upper side of the material and are finally moved to the advancing side behind the welding, and most of the other particles remain on the retreating side.

**Figure 14.** The trend of the particle flow during pressing.

Figure 15a,b shows that the particles P6, P7, and P8 are found behind the welding advancing side during the welding process. During the plunging process, the particles are plunged by the tool pin into the opposite direction of the welding, but the distance from the end of the tool pin becomes farther. The diameter of the end of the tool pin is small, and the effective influence area is smaller. Therefore, only a small part of the displacement of the three particles occurs during the welding process, and it cannot participate in the material flow.

**Figure 15.** Material flow on the advancing side. (**a**) The tool is pressed down onto the given amount; and (**b**) the particles are deposited behind the tool.

Figure 15b shows that most of the tracking particles on the advancing side are rotated by the tool pin to the vicinity of the centerline of the weld bead behind the tool, and then gradually deposited at different positions between the center of the weld and the retreating side, which has also been observed in the experiment [38]. More particles were distributed near the retrograde side, and it was also confirmed that more materials flowed to the retreating side than to the advancing side. At the same time, partial backflow occurs due to the tool shoulder material, and finally the tool shoulder is forged to the rear of the tool.
