**2. Materials and Methods**

The materials under investigation are the aluminum alloys AA5083-O (AlMg4.5Mn0.6) and AA5754-H14 (AlMg3.1). More details on the full chemistry of both AA5083-O and AA5754-H14 are found in our previous work [11]. The temper designated "O" in AA5083- O state means in the annealed condition, which is applied to increase subsequent alloy workability. While "H14" in AA5754-H14 states that strain hardened-1/2 hard condition. The hardness values of AA5083-O and AA5754-H14 are 68 and 74 HV, respectively; the hardness values were measured using 1 Kg load and averaged out of 10 measurements. Vickers hardness tester machine (HWDV-75, TTS Unlimited, Osaka, Japan) was used. The alloys were supplied in the form of rolled plates of 10 mm thick. The FSW butt joints were designed to be 200 mm × 110 mm on each side. Figure 1 represents a schematic for the FSW process, showing all the basic elements and the movement direction. Friction stir welding was carried to produce three different joints between the two aluminum alloys with FSW rotation rates and welding traverse speeds combination as follows: J1 (600 rpm-60 mm/min), J2 (400 rpm-60 mm/min), and J3 (400 rpm-20 mm/min). The FSW tool used was made from the H13 tool steel that was heat treated and tempered to 54 HRC (hardness Rockwell C) hardness. The joints after production were section perpendicular to the welding direction (WD) and prepared to read the optical macrographs. For the EBSD investigation, samples from the top and bottom of the weld Nugget were cut. These samples were then mechanically polished and subsequently electropolished with a solution of 30% nitric acid in methanol for 60 s at 14 V and −15 ◦C. FEI Quanta FEG 250 Field Emission Gun Scanning Electron Microscope (FEGSEM), FEI company (Hillsboro, OR, USA), equipped with a Hikari EBSD camera controlled by EDAX-OIM7.3 (EDAX Inc. Mahwah, NJ, USA) analysis software, was used for EBSD data acquisition and post processing. To evaluate the changes in the mechanical properties due to the FSW process, the base alloys, as well as the FSWed joints, were tested using tensile and hardness testing. Vickers macro-hardness tests were performed on the transverse cross-sections with an interspacing distance of 2 mm using a test load of 1 kg force and a dwell time of 15 s. The tensile test properties of the welded stir zone and transverse flat tensile specimens were used. Tensile samples were machined perpendicular to the FSW direction to the dimensions, as shown in Figure 2. The specimen's dimensions agree with the DIN EN10002-1 2001(D) standards. Tensile tests were carried out at room temperature and at a quasi-static strain rate of ε 0.001 s−<sup>1</sup> using the tensile testing machine Instron Type 4210, Norwood, MA, USA.

**Figure 1.** Schematic representation of the friction stir welding process.

**Figure 2.** Tensile test specimen dimensions.
