*3.3. Nanoindentation Hardness Distribution*

The nanoindentation hardness distribution on the cross-section of the FSW joints is shown in Figure 5. The hardness distribution on the AA5A06 side varied slightly for all the welding parameters. The maximum hardness appeared near the NZ and TMAZ, which was similar to the FSW AA5182 reported by Tronci et al. [31,34]. The reason for this was that AA5A06 is a non-heat treatable alloy, and therefore the temperature variation does not significantly affect the hardness. Near the NZ and TMAZ, the hardness was partially improved due to the refinement of the grains as a result of the stirring action. For the AA6061 side, a dramatic drop in hardness was observed in the HAZ due to the coarsening of the grains and the dissolution of the precipitates [35]. The HAZ of AA 6061 became the weakest area in the FSW joint. This intuitively explains why the fracture occurred in the HAZ of AA6061 during tensile tests. A comparison of Figure 5a,b or Figure 5c,d reveals that the hardness in the HAZ of AA6061 was improved and the weakest area was narrowed with the help of FAC. This is consistent with the results observed in the tensile test, namely that the strength was improved via FAC. The FSW with FAC could reduce the affecting time of high temperatures and suppress the coarsening of the grains and the dissolution of the precipitate phases in the HAZ of 6061, contributing to the improvement of the joint hardness.

**Figure 5.** Nanoindentation hardness distribution on the cross-section of the FSW joint obtained in various welding conditions: (**a**) 600/200 r/mm with NC, (**b**) 600/200 r/mm with FAC, (**c**) 1200/100 r/mm with NC, and (**d**) 1200/100 r/mm with FAC. As the R/T ratio increased, the weakest area of the joint became wider. With the aid of FAC, the weakest area was narrowed.
