3.1.2. Driving Force Distribution

At the bottom of the pin, the pressure between the plastic metal is an important driving force for the flow of the plastic metal. Furthermore, the shear action of the tool against the plastic metal is also one of the driving forces for the flow of the plastic material. Therefore, the combined effect of pressure and shear stress on the plastic material directly affects the fluidity of the material. In order to investigate the distribution of the pressure and shear stress, a series of measurement points were set at the bottom of the pin, as shown in Figure 7. In Figure 7, the distance from the end face of the pin is 0.02 mm, 0.05 mm, 0.08 mm, 0.1 mm, 0.12 mm, 0.15 mm, and 0.18 mm, respectively.

**Figure 7.** Distribution of test points at the root of the weld.

Figure 8a shows the distribution of pressure at the root of the weld. In Figure 8, as the depth increases, the pressure reduces. Furthermore, there is a low-pressure zone near the center line on the RS side, and the lowest pressure is reached at y = −0.5. The uneven distribution of pressure near the center line causes the plastic metal to flow from both sides of AS and RS to the center of the weld, which is consistent with the velocity distribution in Figure 5. However, at the low-pressure zone, the pressure is not enough to fully mix the material from both sides of AS and RS, leading to the weak connection.

Figure 8b shows the distribution of shear stress at the root of the weld. In Figure 8b, near the weld center line (y = 0), it can be found that the shear stress is close to 0 when the distance from the bottom surface of the pin is too short (0.02–0.05 mm) or too long (0.15–0.18 mm). However, when the distance is moderate (0.08–0.12 mm), the shear stress reaches a relatively high level. Therefore, plastic metal near the bottom surface of the pin and the bottom surface of the workpiece has poor fluidity, while the plastic metal between these has strong fluidity. According to Figure 5, it can be found that the location of the left-leaning line of the "S line" is at the depth of 0.08–0.12 mm, and this is consistent with the depth range of the high shear stress, which indicates that the formation of the left-leaning line is related to the change of the shear stress. In Figure 8c, it can also be found that the difference of shear stress between y = −0.5 and y = 0 is smaller when the depth is in the range of 0.08–0.12 mm. Furthermore, combined with Figure 8a,b and Figure 5, it can be found that at the Zone II (h > 0.15 mm), the pressure and shear stress are relatively lower than that at Zone I, which is consistent with the velocity distribution at Zone II, as shown in Figure 5.

*Metals* **2020**, *10*, 913

**Figure 8.** Distribution of the driving force at a different distances from the end face of the pin (1000 rpm, 120 mm/min). (**a**) Pressure distribution; (**b**) shear stress distribution; (**c**) partial enlargement of (b).
