3.3.1. Material Flow inside the Tool Pin

Before the tool pin is not plunged, as shown in Figure 10, three sets of tracking particles are arranged inside the tool pin, where P1–P8 particles are arranged in the center area of the tool pin, P1 is set on the surface of the workpiece, and the minimum diameter of the tool pin end is 4.6 mm. P9–P16 is set in the retreating side area 2 mm away from the axis, and the rest, P17–P24, is set in the symmetrical advancing side area. Where the plunging direction is the negative direction of the *Z*-axis, the spindle does a rotation movement around the *Z* axis, and the welding direction is the negative direction of the *X*-axis. Therefore, the top view, front view and side view are the positive *Z* direction, positive *X* direction and negative *Y* direction, respectively.

**Figure 10.** Setting of tracking particles inside the tool pin.

Figure 11 shows that during the pressing process, the particles (P1–P8) at the axis do not rotate around the tool pin but are squeezed to the bottom of the keyhole created by the tool pin. The reason for this phenomenon is that the linear velocity generated by the tool pin at the axial position is small, and the material fails to participate in the stirring action. After plunging onto the given amount, some of the particles inside the tool pin rotate to the top of the material, and some of the particles are distributed below the material.

**Figure 11.** Tool needle internal particle plunging process.

Figure 12a,b shows that in the welding stage, the plastic material flows with the pin rotation motion. The particles undergo multiple rotational movements and are finally deposited behind the tool pin. A part of the material stays in the outer edge of the weld seam under the forging of the tool shoulder. A small number of particles follow the shoulder of the tool, and most of the material is deposited behind the weld zone, with a material distribution on both the retreating side and the advancing side. For example, both P10 and P18 particles flow around the tool shoulder. The P10 particles eventually stay on the retreating side of the weld, while the P18 particles stay on the advancing side of the weld. At the same time, due to the extrusion of the tool pin, a part of the material is deposited on the bottom of the tool pin.

**Figure 12.** Material flow inside the tool needle during the welding phase. (**a**) Particle flow during welding; and (**b**) particle deposition behind the tool needle.
