3.3.3. Material Flow on the Retreating Side

Two sets of tracking particles are arranged on the retreating side, and the position is symmetrical to the advancing side. P1–P8 is close to the leading edge of the tool, and P9–P16 is close to the trailing edge of the tool. Since the rotation direction of the tool is counterclockwise, the particles in the initial stage of pressing extend outward under the extrusion action of the tool pin, as shown in Figure 16. As the pressing temperature gradually increases, the plastic softening layer in the weld zone becomes thicker, and the material plastically flows. The particles in the P1 group move backwards to the rear of the weld with the rotation of the tool pin.

**Figure 16.** Particle migration during the plunging stage.

Figure 17a,b shows that the material flows under the action of the tool pin, whereas the material flow velocity on the retreating side is less than the advancing side, which also results in less material accumulating on the advancing side, which confirms the results of the literature [39].The trajectory of the marker material particles on the return side has two main parts. A part of the material is rotated around the tool pin for about half a week and then deposited onto the area near the advancing side of the tool. The other part of the material hardly rotates around the tool pin, but is gradually pushed down by the tool onto the area between the center line and the retreating side of the rear weld. A small number of particles on the tool shoulder is deposited in front of the retreating side after rotating around the tool for several weeks.

**Figure 17.** Material flow on the retreating side. (**a**) The tool is pressed down onto the given amount; and (**b**) the particles are deposited behind the tool.
