## 2 Materials and Methods  
The chemical composition and the mechanical properties of the base aluminum alloys used in the experimental procedure are reported in Tables 1 and 2. Both air-cooled and water-cooled FSW tests were carried out for each material on \(200\times 80\times 4\) mm sheets using a FAMUP MC60 CNC machine equipped with a suitable tool for the Friction Stir Welding. The tool was characterized by a smooth flat shoulder (16 mm diameter) and a frustum of cone pin (maximum and minimum diameters equal to 6 and 4 mm, height equal to 3.8 mm) (Figure 1). Due to the considerable forces generated during an FSW welding, it was necessary to fix the sheets to be welded on a support plate through 6 grips, as reported in Figure 2.  
The welding parameters have been defined through a detailed bibliographic analysis, to obtain the best combination of rotational speed and feed rate for each alloy maximizing the mechanical characteristics of the welded joints [6; 11; 13; 29; 30; 31; 32; 33]. Table 3 shows the parameters used in the experimental campaign. To realize the water-cooled welded joints, a cooling system was appropriately designed and implemented, which allows a continuous and controlled flow of water at room temperature in the welding area. To ensure that no particles of water remain blocked in the joint during the welding process, the water has been kept away from the not yet welded part of the sheets by the presence of a bulkhead fixed near the pin (Figure 3). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Alloy** & **Al** & **Si** & **Fe** & **Cu** & **Mn** & **Mg** & **Zn** & **Ti** & **Cr** \\ \hline
**AA2024–T3** & bulk & 0.06 & 0.09 & 4.50 & 0.46 & 1.40 & 0.08 & 0.04 & 0.03 \\ \hline
**AA6082–T6** & bulk & 0.9 & 0.36 & 0.07 & 0.42 & 0.8 & 0.05 & 0.03 & 0.02 \\ \hline
**AA7075–T6** & bulk & 0.05 & 0.10 & 1.53 & 0.008 & 2.54 & 5.72 & 0.04 & 0.20 \\ \hline
**AA5754–H111** & bulk & 0.103 & 0.34 & 0.059 & 0.318 & 2.808 & 0.019 & 0.009 & 0.019 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition (% weight) of the base materials. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Alloy** & **Yield Strength (YS)** & **Ultimate Tensile** & **Max strain [\%]** \\  & **[MPa]** & **Strength (UTS) [MPa]** & **Max strain [\%]** \\ \hline
**AA2024–T3** & 344 & 464 & 16 \\ \hline
**AA6082–T6** & 281 & 326 & 16 \\ \hline
**AA7075–T6** & 512 & 576 & 13.5 \\ \hline
**AA5754–H111** & 84 & 212 & 25.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanical properties of the base materials. The samples were obtained by cutting the welded sheet in the rectangular area underlined in Figure 4, obtaining a \(160\times 20\) mm sample, as shown in Figure 5.  
The mechanical characteristics of both air-cooled and water-cooled joints were evaluated by means of hardness and tensile tests of the joints. The authors chose to perform the Vickers hardness tests on the cross-section of the specimen and the Rockwell hardness on the top surface of the specimens mainly due to the surface preparation requirements of the ASTM E92-17 standard for performing the Vickers test. In fact, in order to execute Vickers hardness tests, it is necessary to achieve a surface preparation that was not possible to obtain  
Figure 4: Part of the welded sheet on which the samples were cut out. Figure 5: Appearance and morphology of a longitudinally cut Friction Stir Welded joint. Figure 3: Water-cooling system (**a**) and detail of the bulkhead (**b**). on the whole extension of the top of the samples (\(160\times 20\) mm). Therefore, the authors preferred to perform Rockwell hardness tests on the entire length of the specimens without losing the data relating to the outermost parts of the sample.