*3.4. Fracture Characteristics*

Figure 6 shows the SEM images of the microstructures of the fracture surfaces. The dimples are observed in all fracture surfaces, indicating the ductile fracture mechanism of the joints. For FSW with NC, the fracture surface of the joint obtained at an R/T ratio of 1200/100 r/mm exhibited large and deep dimples (Figure 6a). When the R/T ratio reduced to 600/200 r/mm, the dimples became smaller (Figure 6b). For the fracture surfaces of the FSW joints with FAC, as shown in Figure 6c,d, the dimples continue to become smaller and shallower. It is understood that the size of the dimple is affected by the spacing of the precipitates, and a larger dimple leads to better plasticity and worse strength [36]. The quantity and size of the dimples can indirectly reflect the number and size of the precipitates. As mentioned above, precipitation hardening is the main strengthening mechanism for AA 6061. Typically, smaller and more uniformly distributed precipitates lead to better mechanical properties. Based on this point of view, it could be inferred from Figure 6 that the strength of joints with FAC is commonly higher than that of naturally cooled joints, which coincides well with the uniaxial tensile results.

**Figure 6.** Scanning electron microscopy (SEM) images of the fracture surface of FSW specimens obtained at: (**a**) 1200/100 r/mm with NC, (**b**) 600/200 r/mm with NC; (**c**) 1200/100 r/mm with FAC, and (**d**) 600/200 r/mm with FAC. The fracture surfaces of the FSW joints with FAC tended to have smaller and shallower dimples than those with NC.
