*3.2. Uniaxial Tensile Properties*

After the uniaxial tensile tests, it was found that the fracture occurred at the HAZ of the AA6061 side for all the FSW joints. This means that the ultimate tensile strength of AA6061 was weakened after welding, and the HAZ on the AA6061 side became the weakest area. Figure 4 shows the ultimate tensile strength of the base AA6061 and the FSW joints welded at various R/T ratios and under different cooling conditions. For FSW with NC, the joint welded at an R/T ratio of 600/200 r/mm had the maximum ultimate tensile strength of 198.27 MPa, which was 29% lower than that of the base AA6061. As the R/T ratio increased from 3 (600/200) r/mm to 12 (1200/100) r/mm, the ultimate tensile strength continued to decrease slowly, and the decrease was within 10%. It is evident from Figure 4 that there were two levels of decrease for the tensile strength of the FSW joints.

**Figure 4.** The variation in ultimate tensile strength with the ratio of rotational speed to welding speed. There were two levels of decrease for the tensile strength of the FSW joints. The ultimate tensile strength of the joints with FAC was generally 10% higher than for those with NC.

For the first-level decrease, the tensile strength of the FSW joint was largely weakened once the AA6061 was friction stir welded. As Peel et al. [28] pointed out, this is because the rolled AA6061 was kept in an extremely work-hardened state and had highly unstable microstructures. The recrystallisation caused by the temperature rising during FSW can readily destroy the hardened state and significantly weaken the mechanical properties. In addition, precipitation hardening is one of the strengthening mechanisms for heat-treatable AA6061 [29,30], and grain size also plays an important role according to the Hall-Petch relationship [31]. For AA6061, the main strengthening phase is the needle-shaped β" phase. With the increase of the heat input, the fine needle-shaped β" phase dissolved and grew to the coarsened rod-shaped β phase and equilibrium β phase, which weakened the mechanical properties of the joint [32,33]. A comparison of Figure 3b,g reveals that the grains became coarser after welding. To sum up, the first-level decrease in tensile strength was attributed to the destruction of the hardened state (the primary reason), the dissolution and coarsening of the strengthening phases, and the coarsening of the grains. For the second-level decrease, the tensile strength of the FSW joint was slightly weakened with the increase of the R/T ratio. This was mainly because the higher R/T ratio generated a higher temperature, which led to more strengthening phases to be coarsened and causing grains to grow larger. The further dissolution of the precipitates and further coarsening of the grains could weaken the tensile strength with the increase of the R/T ratio, as shown in Figure 4.

With the aid of FAC, the cooling process was accelerated, and the affecting time of the high temperature was reduced. This was conducive to suppressing the coarsening of the grains and the dissolution of the precipitates in the HAZ of 6061, and thus improved the mechanical properties. As shown in Figure 4, the ultimate tensile strength of the joints with FAC was commonly improved by 10% compared to that with NC.
