## 4 Discussion
Regarding the precipitation-hardening alloys, the extent of the zone characterized by the lower hardness is considerably reduced in the water-cooled specimens. On the contrary, in AA5754, the extent of the area characterized by a greater hardness is almost constant for air-cooled and water-cooled welds (Figures 11 and 13). These effects are mainly due to the presence of the local water-cooling during the welding process that leads to a better dimensional control of the grains in the joints, which is closely related to the mechanical properties of the piece. Also, the hardness is closely related to the microstructure of the materials. As a consequence, it is possible to correlate the increase in hardness in the TMAZ of AA6082 and AA7075 welded with the water-cooling system to the smaller dimensions of the grain present in these areas. On the contrary, it has already been noted that AA2024 has mitigated the effect in increasing the hardness in the TMAZ of water-cooled welds. This characteristic of AA2024 finds its explanation in the further elongation that the grains undergo in the TMAZ of water-cooled welds, as reported in Section 3.1. By observing the Vickers hardness maps (Figure 13), it is possible to draw the same considerations made for the Rockwell hardness maps (Figure 11). In fact, it is possible to notice, also in the cross-section of the specimens, an increase in the hardness in the central areas of the specimens, except for AA6082, and a reduction in the extent of the area characterized by lower hardness values for the hardening alloys. Concerning the AA6082, the decrease in Vickers hardness recorded in the cross-section could be related to the considerable increase in the average grain size at the nugget of the water-cooled welds. In fact, this alloy has proved to have the highest average percentage growth rate of the grains present in the water-cooled weld nugget, as already reported in Table 6. Finally, it is possible to observe, for AA5754, a significant increase in the hardness values in correspondence of the nugget. Also, in the tensile tests, it is possible to observe different behaviors between the studied alloys (Figures 14 and 15). In particular, for the AA2024, the values of ultimate tensile strength and percentage of elongation are almost comparable for both cooling methods and are much lower than those of the base metal. Both cooling conditions led to breakages in the nugget during the tensile tests. In regard to the tensile tests of the AA6082, the average values of UTS and percentage elongation are comparable between air-cooled and water-cooled welds. But the cooling system seems to affect the fracture locations of the tested joints. In fact, the air-cooled joints showed fractures at the interface between the nugget and TMAZ, while the water-cooled joints broke especially at the nugget. Moreover, the AA7075 has shown a benefit in water-cooling in the UTS but both cooling conditions led to breakages in the interface between the nugget and TMAZ during the tensile tests. However, it is possible to recognize the welding obtained from the AA5754 joints as the welding that has reached the value of UTS and of percentage elongation closer to those of the base alloy both in the air-cooled and water-cooled process. Finally, for this work-hardening alloy, the percentage elongation was found to be on average higher for water-cooled welds than those air-cooled and, in particular, it was possible to observe that in the water-cooled joints the point of failure is placed in the base material, away from the welded area. On the contrary, in the air-cooled joints, the breakages were found in the nugget. This peculiarity of AA5754 could be explained by the benefit that a water-cooling performed during the friction stir processing would lead to a work-hardening aluminum alloy.