## 4 Discussion  
The alloys belonging to the 2 xxx series are normally supplied in the T3 or T4 state, abbreviations indicating respectively solubilization and natural aging heat treatment. The T3 heat treatment involves a cold processing step between solubilization and aging. The cold plastic deformation allows to obtain superior mechanical properties compared to aging alone. Natural aging involves maintaining the alloy for a long time, at room temperature or at a slightly higher temperature. During this period, there is the spontaneous formation of a structure characterized by the presence of Guinner-Preston zones (G.P), which are areas of the crystalline matrix where the clusters of precipitates and solutes are aggregated. The areas thus formed oppose greater resistance to the path of the dislocations than the crystalline matrix of the base metal, generating the desired increase in mechanical strength. Natural aging appears to be an extremely slow process and, for some alloys, such as those belonging to the 6 xxx and 7 xxx series, it does not guarantee the achievement of stable conditions. For this reason, the AA6082 to AA7075 alloys are usually supplied in the T6 state, which provides solubilization, tempering and artificial aging. Artificial aging involves exposing the alloy to temperatures above room temperature for obtaining the transition phase starting from the equilibrium precipitates; this form of transition remains consistent with the matrix of the solid solution and therefore contributes to precipitation-hardening. However, as the aging temperature increases, the precipitate particles grow and reconvert to the state of equilibrium, which is generally inconsistent. These changes produce a loss of mechanical characteristics with respect to the untreated base metal. This phenomenon is called over-aging. The control of the temperatures to which these alloys are subjected is therefore of fundamental importance for the mechanical properties of both the base material and the friction stir welded one. Consequently, the presence of a water-cooling system plays a key role in controlling the thermal cycle to which these alloys are subjected during welding. Furthermore, each point of the weld, in relation to its distance from the center of the nugget, has a microstructure strongly influenced by the welding process. Therefore, in the weld region, where the material is stirred at solid-state, dissolution, coarsening of strengthening precipitates and formation of wide precipitate free zone take place. From the measurements made on the joints, a clear dependence of the average grain size from the cooling system is detectable. In particular, in the water-cooled nugget, the grains are characterized by an increase in the average size compared to the air-cooled nugget being 14% in AA2024 (Table 5), 33% in AA6082 (Table 6) and 8% in AA7075 (Table 7). It is possible to notice how the percentage growth of the average dimensions present in the nuggets of the single alloys, is significantly different from each other. This is attributable to the mechanical characteristics of the base alloys. In fact, hardness and mechanical strength of AA7075 result in greater resistance to the breakage action of the grains by the pin. This is also reflected by the average size of the grains present in its air-cooled welded nugget, which are already on average larger than those present in the other two alloys (Figure 8). On the contrary, the alloy with the highest percentage growth is the AA6082, which has the mechanical characteristics on average lower than the other two alloys and it is also characterized by grains with average lower dimensions in the air-cooled welded nugget (Figure 7). Regarding the TMAZs, AA6082 and AA7075 show grains characterized by smaller average dimensions in the rolling direction in water-cooled welds. Contrariwise, perpendicularly to the rolling direction, in water-cooled joints, the grains are slightly larger (Figures 7 and 8).