*3.1. Microstructures*

Defect-free joints were obtained for all welding parameters and conditions. Figure 2a shows a representative cross section of the joint, obtained at the ratio of rotational speed to transverse speed (R/T ratio) of 1200/100 r/mm with FAC. As shown in Figure 2, "onion rings" and interlaced ripple structures, which indicate a good mixing of materials, were observed in the NZ. The formation of an "onion ring" is attributed to the thermal softening of the materials, and the stirring action, extrusion, and transverse movement of the threaded tool [26]. Comparison of the "onion rings" in Figure 2b,c revealed that the spacing between the ripples decreases with an increase in the rotational speed of the tool, which agrees with Rodriguez's report [27]. There are two main reasons for this: (1) the higher heat input generated by the higher rotational speed makes the materials softer and easier to flow under mechanical stirring, and (2) higher rotational speed provides a higher stirring force, which mixes these two materials sufficiently to form narrower ripples.

**Figure 2.** Macroscopic features of the joints with forced air cooling: (**a**) cross-section of a joint welded at a rotational speed to transverse speed (R/T) ratio of 1200/100 r/mm, (**b**) NZ of a joint welded at an R/T ratio of 600/100 r/mm, and (**c**) NZ of a joint welded at an R/T ratio of 1200/100 r/mm.

Figure 3 shows typical microstructures of different regions of the FSW AA5A06-AA6061 joints welded at a rotational speed of 1200 rpm and a welding speed of 100 mm/min with NC and FAC. Rolled prolate grains were observed in the BM of both AA5A06 and AA6061, and their average grain size was 49 μm and 50 μm, respectively (Figure 3a,b). For both cooling conditions, the NZ exhibits dynamically recrystallised microstructures with fine equiaxed grains (Figure 3c,d), which are the result of thermal softening and large plastic deformation during the FSW process. The average grain sizes in the NZ were 14 μm and 8 μm for FSW with NC and FAC, respectively. Figure 3e,f shows the grain structures of the HAZ of the 5A06 side for FSW with NC and FAC, and their average grain sizes, which were 52 μm and 51 μm, respectively. It was found that the average grain size in the HAZ and the BM of AA5A06 were almost the same and independent of the cooling conditions. The reason for this was that AA5A06 is a non-heat treatable aluminium alloy, and the heat generated by the FSW had little effect on the grain structures. On the 6061 side, however, the average grain size of the HAZ for FSW with NC is 57 μm (Figure 3g), an increase of about 14% compared to the average grain size of the BM. This was due to the fact that AA6061 is a heat-treatable alloy, and is therefore sensitive to temperature variations. The elevated temperature generated during the FSW process induced the growing and coarsening of grains in the HAZ. With the aid of the FAC, the average grain size of the HAZ on the 6061 side was reduced to 53 μm (Figure 3h). The FSW with FAC treatment could accelerate the cooling process

and reduce the affecting time of high temperatures, thus suppressing the coarsening and growth of the grains.

**Figure 3.** Microstructures of different regions of the FSW joints welded at an R/T ratio of 1200/100 r/mm with natural cooling (NC) and forced air cooling (FAC): (**a**) base material (BM) of 5A06, (**b**) BM of 6061, (**c**) nugget zone (NZ) (with NC), (**d**) NZ (with FAC), (**e**) heat affected zone (HAZ) of 5A06 (with NC), (**f**) HAZ of 5A06 (with FAC), (**g**) HAZ of 6061 (with NC), and (**h**) HAZ of 6061 (with FAC).
