3.1.1. Velocity Distribution

Figure 5 shows the "S line" and non-penetration defect formed at the root of the weld due to insufficient plastic material flow. The material at the root of the weld can be divided into two different zones according to the flow behavior. As can be seen from Figure 5, at Zone I, the materials flow parallel to the bottom of the pin, and the materials on the advancing side (AS) and retreating side (RS) flow in the opposite direction and mix on the RS side. However, materials on both sides fail to be fully mixed, resulting in the appearance of a weak connection "S line" at the root of the weld. As for Zone II, near the weld center, all materials flow in the same direction (RS→AS), while almost all materials stop at the weld center, leading to the "vertical line" shape defect, i.e., non-penetration defect. In fact, the area under the pin bottom is called SWZ (the Swirl Zone) in Zeng's work [37], which refers to Zone I + Zone II in this paper. In their experimental research of the root flaws in a 6 mm 2014Al-T6 FSW joint under a pin length of 5.73 mm, the "S line" defects appear at the weld root and extend to the stirring zone under a wide range of process parameters. The shape of the "S line" defects obtained in this paper in Figure 5 is similar with that observed in Figure 3 of Zeng's work. A similar root flaw was also observed in a 6 mm DH36 steel FSW joint under a pin length of 5.7 mm in Al-Moussawi's work [21].

**Figure 5.** Simulation result of the plastic flow at the root of the weld.

To explore the fluidity of plastic metal at the root of the FSW joint, the velocity distribution of the plastic material in Figure 5 was measured under different depths (0.05, 0.1, and 0.15 mm under the pin). Figure 6a–c shows the measurement results for the velocity distribution in the x, y, and z-directions, respectively. The horizontal axis represents the distance to the center of the weld, in which the positive axis is AS, the negative axis is RS, and the vertical axis represents the partial velocity.

**Figure 6.** Speed curves of different distances from the bottom of the pin (1000 rpm, 120 mm/min, 5.8 mm pin length). (**a**) Velocity in the x-direction; (**b**) velocity in the y-direction; (**c**) velocity in the z-direction.

In Figure 6a, it can be seen from the figure that the velocity of the plastic metal in the x-direction takes the center of the weld (y = 0) as the boundary line, advancing side (AS) plastic metal flows in the welding direction (v < 0), and retreating side (RS) plastic metal flows in the opposite direction of the welding (v > 0). The closer it is to the end face of the pin (0.05 mm), the better the fluidity of the plastic metal in the x-direction and the faster the flow speed.

As can be seen in Figure 6b, near 0.25 mm from the weld center on the RS side, the transverse flow velocity at different depths is close to 0, indicating that the plastic flow in this area is poor. As a result, weak connection defects can easily form near the center line of the weld due to insufficient material fluidity. However, at the middle position between the end of the pin and the bottom of the workpiece (0.1 mm), the fluidity is relatively good. AS and RS plastic metal move in the opposite direction toward the center of the weld line and meet at RS (x = −0.25).

In addition, the maximum velocity of the plastic metal at the root of the weld is only 0.45 mm/s in the depth direction (z direction), as shown in Figure 6c. Therefore, the plastic metal at the root of the weld basically has almost no flow in the depth direction. It can also be found that the velocity distribution in Zone II is always the smallest in three directions compared with zone I, indicating the poorer fluidity of material in Zone II.
