3.2.1. Effect of Pin Length on the Material Flow

In order to solve the defects of a weak connection and non-penetration at the root of FSW joints of aluminum alloy, the relationship between welding allowance and root flaws was investigated. Figure 9 shows the velocity distribution under a pin length of 5.8, 5.85, 5.9, 5.95, and 6.0 mm, respectively. Figure 10 shows the velocity distribution in the y-direction at the half depth between the bottom surface of the pin and the bottom surface of the workpiece.

When the length of the pin is 5.8 mm or 5.85 mm, the "S line" defect appears at the root of the FSW weld seam, as shown in Figure 9a,b. As the pin length increases to 5.9 mm, the "S line" shape defect changes to the "right-tilting line" shape defect (Figure 9c), and finally the "S line" defect is eliminated when the pin length is 5.95 mm (Figure 9d). In Figure 10, as the pin length increases from 5.8 to 5.95 mm, the velocity difference between RS and AS near the weld center (Figure 10 mark I, II) becomes larger, which indicates a larger driving force, leading to the material being more fully mixed near the weld center.

**Figure 9.** Effect of pin length on plastic metal flow at the root of the weld (1000 rpm, 120 mm/min). (**a**) 5.8 mm; (**b**)5.85 mm; (**c**) 5.9 mm; (**d**) 5.95 mm; (**e**) 6.0 mm.

**Figure 10.** Velocity distribution in the y-direction in Figure 5 (1000 rpm, 120 mm/min, 5.8 mm pin length).

It should be noted that the material flow formation near the weld center changes when the pin length increases. When the pin length increases from 5.8 to 5.85 mm, near the weld center, the material flow direction changes from RS→AS to AS→RS under the "S line", which causes more material transfer from AS to RS, and the "S line" further extends to the RS side. When the pin length increases to 5.9 mm, the "S line" changes to the "right-tilting line", because the material on the right side of "left-tilting line" all flows to the RS side. Finally, the material on the right side of the "right-tilting line" all flows to the RS side, and the defect-free welded joint is obtained when the pin length is 5.95 mm.

On the other hand, the material flow form on the margin of the pin also significantly changes when the pin length increases. As the pin length increases from 5.85 to 5.9 mm, the material flow on the left margin of the pin changes from vortex flow (marked red circle in Figure 9b) into the transverse flow (marked red circle in Figure 9c), and the flow direction changes from left to right, which can also be observed in Figure 10 mark III. It can also be found that the material flow form on the right margin of the pin remains in transverse flow when the pin length increases, while the velocity increases significantly as shown on the mark IV in Figure 10.

When the pin length is equal to the thickness of the workpiece, there is no welding allowance, and the welding plate does not exist in weak connections and non-penetration defects; as can be seen from Figure 9e, the plastic materials on both sides flow to the root of the weld. In this case, the pin directly contacts with the backing plate, which is easy to weld the workpiece together with the working table, and the bottom of the welding plate is poorly formed. Therefore, zero welding allowance is not recommended in practical production.

When the pin length is 5.9 mm, there is still a weak connection and non-penetration defects at the root of the weld. Therefore, the plastic metal flow behavior near the root of the weld center was investigated in detail when the pin length was 5.91–5.95 mm, as shown in Figure 11. When the pin length increases from 5.91 to 5.93 mm, the "S line" defects (red dashed line in the Figure 11) and non-penetration defects still exist. It should be noted that with the increase of the pin length, the width of the "S line" and the angle between the tangent line of the "S line" and the bottom surface of pin gradually reduces, as shown in Figure 11a–c, which may be attributed to the increased shear stress (see the Section 3.2.3) and speed difference between the RS and AS sides near the weld center (Figure 12 mark I, II). When the pin length increases to 5.94 mm (Figure 11d), the plastic metal flows through the center line under the end surface of the pin, which eliminates the "S line" defect. However, the fluidity near the bottom surface of the weld is still insufficient, so that the non-penetration defect still exists. The non-penetration defect is finally eliminated when the pin length is increased to 5.95 mm (Figure 11e).

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

**Figure 11.** Influence of root welding allowance on plastic metal flow behavior at the root of the welding seam (1000 rpm, 120 mm/min). (**a**) 0.09 mm; (**b**) 0.08 mm; (**c**) 0.07 mm; (**d**) 0.06 mm; (**e**) 0.05 mm.
