3.2.2. Effect of Pin Length on the Heat Input

Figure 13 shows the temperature of the top and bottom surface of the weld under different pin lengths. It is obvious from the figure that the welding heat input is basically unchanged when the pin length changes in a small range (0.2 mm). Therefore, the temperature of the plastic metal at the root of the weld is basically unchanged when welding with different lengths of the pin.

We continually increased the length of the pin by a step of 0.01 mm and studied the plastic metal flow at the bottom of the weld with the length of the pin at 5.96 mm, 5.97 mm, 5.98 mm, and 5.99 mm. Figure 14 shows the temperature field distribution of the weld cross-section when the pin length exceeds 5.95 mm. As can be seen from the Figure 14, when the pin length is 5.95 mm, the highest temperature of the weld (423 ◦C) appears on the upper surface of the weld, as shown in Figure 14a. However, when the pin length is greater than 5.95 mm, the maximum temperature of the weld appears at the center of the bottom of the weld. When the pin length increases from 5.96 mm to 5.99 mm,

the maximum temperature increases from 439 to 481 ◦C, as shown in Figure 14b–e [38]. As aluminum alloy is a low-melting alloy, if the weld temperature is too high, the recrystallization of the weld will be coarsened. According to the Hall–Petch principle, the coarsening of the weld will reduce the joint strength.

**Figure 12.** Flow velocity curves of plastic metal at the root of joint with different pin lengths (1000 rpm, 120 mm/min). (**a**) 0.05 mm per step from 5.8 mm to 5.95 mm; (**b**) 0.01 mm per step from 5.91 mm to 5.95 mm.

**Figure 13.** Influence of pin length on temperature field of the top and bottom surface of the weld (1000 rpm, 120 mm/min).

Although the temperature at the bottom of the weld increases with the increase of the length of the pin, when the length of the pin exceeds 5.95 mm, the flow velocity of the plastic metal at the bottom of the weld does not increase accordingly, as shown in Figure 15 (within the two black dashed lines). At the center of the weld, the flow rate of the plastic metal is basically the same, about 5 mm/s. Therefore, when the length of the pin exceeds 5.95 mm, further increasing the length of the pin will not improve the flow capacity of the plastic metal at the bottom of the joint, but it will sharply increase the temperature at the bottom of the weld, leading to the coarsening of the recrystallization tissue and the reduction of the joint strength. On the other hand, if the pin is too long, it will pass through the whole thickness of the workpieces and directly contact with the hard backing plate or the FSW machine substrate. In the friction stir welding process, the two will produce intense friction and wear, resulting in the rapid damage of the pin and the FSW machine substrate. At the same time, the pin itself also changes significantly in length, shape, and structure due to rapid friction and wear, and the stable FSW welding process becomes difficult to control, which leads to the deterioration of welding forming and affects the quality of welded joints greatly. Therefore, for 6 mm thick 2024 aluminum

alloy, a 5.95 mm pin is most suitable for welding, which can eliminate the "S line" and non-penetration defects at the bottom of the welding seam.

**Figure 14.** Temperature field of weld cross-section at different lengths of pin (1000 rpm, 120 mm/min, 6 mm thick workpiece) (**a**) 5.95 mm; (**b**) 5.96 mm; (**c**) 5.97 mm; (**d**) 5.98 mm; (**e**) 5.99 mm.

**Figure 15.** Distribution curve of flow velocity of plastic metal at the bottom of weld seams with different lengths of pin (1000 rpm, 120 mm/min).

In fact, studying the different pin lengths is not only necessary for the engineering design and use of the stirring pin; more importantly, the pin length itself is varied in the actual FSW process owing to the wear of the stirring pin. Hence, the material flow behavior under the pin bottom is also influenced by the change of pin length. Therefore, studying the different pin lengths is to consider the inevitable wear of the stirring pin in the actual FSW process and its effect on the formation of the FSW joint, which is of great significance. The issues related to the wear of the stirring pin will be discussed in our research work and papers in the future.
