## 4 Discussion
The percentage decrease was around 13% in AA6082 and 25% in AA7075 parallel to the rolling direction. The percentage increase perpendicularly to the rolling direction was about 28% in AA6082 and 7% in AA7075 (Tables 6 and 7). These dimensional variations make the grains present in water-cooled welds smaller than those present in the same position for air-cooled welds. On the contrary, in AA2024, the average grain size increases by 15% in the direction parallel to the rolling direction and decreases by 30% in the perpendicular direction (Table 5). As a general observation, it can be seen that in each alloy, the grain size in the TMAZ is much closer to that of the base material in the direction perpendicular to the rolling direction in water-cooled welds. In the case of AA6082, perpendicularly to the rolling direction, the water-cooled TMAZ grains are characterized by an average size even greater than those of the base alloy. This is further proof of how the material responds differently to the cooling system in microstructural terms and, consequently, in the mechanical characteristics. In fact, from the mechanical tests data, it is possible to observe a behavior common to all the precipitation-hardening alloys (AA2024, AA6082, and AA7075). In particular, for all the welded conditions, the specimens have specific zones characterized by hardness lower than the one of the base materials. This characteristic trend of the hardness values in precipitation-hardening alloys is mainly due to the thermo-mechanical action of the pin that acts on the alloy modifying its microstructural characteristics. In fact, for these alloys, it is possible to identify a clear microstructural change caused by the welding process (Figures 6-8). The heat generation and the mechanical action of the stirring generate a dynamic recrystallization that particularly affects the nugget. In TMAZs, the dynamic action of the pin and the heat generated are not sufficient to modify the microstructure of the base material. Therefore, in these zones, the grains are similar to the original ones (Figure 5) maintaining an elongated shape along the rolling direction (Figure 6c,d, Figure 7c,d and Figure 8c,d). Even the precipitates are influenced by the FSW process in two ways. In fact, depending on the action of the pin and the heat input, it is possible to have both the crushing of large precipitates by the mechanical action and the coalescence of the precipitates in sub-micrometric dimensions. These are the reasons why the lower hardness areas are concentrated in the TMAZs, where the grain recrystallization is not complete and where the alloy-hardening-precipitates are partially crushed. On the other hand, in the nugget, the dynamic recrystallization is complete and the precipitates are fully crushed reaching the sub-micrometric dimensions suitable for strengthening the matrix formed by the micrometric grains (Figure 6a,b, Figure 7a,b and Figure 8a,b). Besides, AA5754 presents a completely different behavior and a different hardness map. In fact, since AA5754 is a work-hardening alloy, the thermo-mechanical action of the pin has led to a hardness increase in the nugget area, where the solid-state mixing has completely taken place. By comparing the hardness maps measured on air-cooled and water-cooled welds, it is possible to observe how the effect of water-cooling is twofold on each alloy (Figures 11 and 13). The first effect can be traced in the numerical value of the hardness achieved in the TMAZs, as regards the three precipitation-hardening alloys and in the nugget for the work-hardening alloy. In these areas, for all the alloys and for all the considered conditions, the hardness is higher than those measured in air-cooled welds. This effect is less noticeable on AA2024 joint. Moreover, AA6082 Rockwell tests showed an increase in the hardness values at the nugget in water-cooled welds. The second effect is the narrowing of the area characterized by the lower hardness values.