*2.3. Macrostructural Investigation*

Cross sections of welded joints were prepared for metallographic analysis using standard metallographic procedure [32]. The samples were etched using Keller's reagent for a period of 40–50 s. at room temperature to reveal the macrostructure of the welded samples and then washed with water and acetone, and then air-dried.

**Figure 1.** Friction stir welding (FSW) tool used in the FSW experiments (**a**) Computer aided design (CAD) drawings with detailed dimensions in mm and (**b**) image of the used FSW tool.

**Figure 2.** (**a**) The clamping of the plates on the FSW machine table with the advancing side (AS) and retreating side (RS) indicated and the tool rotation direction indicated also by the red arrow and (**b**) Aluminum alloys butt joint on the table after FSW.


**Table 3.** FSW welding parameters and position of alloys at the AS and RS.
