*2.4. Mechanical Properties*

The hardness tests were performed using the Vicker hardness method, using a load of 500 g applied for 10 s. Three lines were chosen on the transverse section of the welds with a distance of 1 mm between neighboring measured points, as shown in Figure 1. The specimens were cut to 3-mm slices to release the residual stress, which minimized the influence of residual stress on the hardness test.

The tensile tests on welded specimens were performed on the transverse, i.e., perpendicular to the welding direction. Eight samples were cut using EDM from the single-sided FSW and double-sided FSW. The transverse cross-section of the tensile samples is shown in Figure 3. These tensile samples were 25 mm thick, 6 mm wide and 200 mm long, with an initial gauge length of 100 mm. The tensile test was carried out on a universal electronic tensile testing machine (MTS Landmark) with a crosshead moving speed of 2 mm/min. The range of load cell is ±100 KN.

**Figure 3.** The layout of a tensile specimen.
