**2. Experimental Procedure**

Plates of the dissimilar aluminum alloys of AA2024-T4 and AA7075-T6 were friction stir welded in T-butt joints. The dimensions for FSW samples were 4 mm × 50 mm × 200 mm for AA2024-T4 and 5 mm × 100 mm × 200 mm for AA7075-T6 alloys. The T-butt joint setup was designed to be two plates of AA2024-T4 alloy as a skin and one plate of AA7075- T6 as a stringer. The chemical composition of the aluminum alloys was analyzed using Q2 ION-OES -Optical Emission Spectrometry (Bruker, Billerica, MA, USA). The chemical composition of the investigated alloys is listed in Table 1.

**Table 1.** Chemical analysis (wt. %) of the AA2024-T4 and AA7075-T6 alloys.


378

The T-butt joints were friction stir welded using gantry type FSW (EG-FSW-M1, made by Suez University, Suez, Egypt) machine [2,6]. Adaptive fixture was designed [15] and fabricated from steel for T-butt joint configuration setup, Figure 1a,b. A constant travel speed of 50 mm/min and three various rotational speeds of 400, 600 and 800 rpm were used in this work. The shoulder tilting angle and the shoulder plunge depth were 3◦ and 0.20 mm, respectively. The other welding conditions were kept constant. A concave tool shoulder with a diameter of 32 mm and a threaded taper pin with a length of 3.8 mm was used (Figure 1c). The bottom and top diameters of the pin were 12 mm and 10 mm, respectively. The tool was machined from H13 tool steel and heat treated to obtain a hardness value of 58 HRC. Figure 1c shows top view appearance of a friction stir welded joint.

**Figure 1.** (**a**,**b**) FSW fixture setup configuration of AA2024 and AA7075 T-butt joint, (**c**) Engineering drawing of the used tool (**d**) top view appearance of a friction stir welded joint.

In order to investigate the developed T-joints microstructure and mechanical properties, the samples were cross-sectioned perpendicular to the welding direction (WD). For microstructural analysis, the sectioned samples were ground using SiC papers with different grit up to 2400 and polished on felt cloth with alumina 0.05 μm paste and then etched with Keller's reagent (6 mL hydrofluoric acid, 6 mL hydrochloric acid, 5 mL nitric acid and 150 mL water). Stereo microscope (Optica SZR 10, Optica, Ponteranica (BG), Itaty) was used to examine macrostructures of the joints. Microstructural analyses were carried out using optical microscope (OM) Olympus -BX41M-LED, Olympus, Tokyo, Japan. And scanning electron microscopy (SEM) Type Quanta FEG-250 (FEI company, Hillsboro, OR, USA) equipped with electron back scatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) were also used to examine the grain structure, texture and precipitates composition.

Vickers hardness tester machine (HWDV-75, TTS Unlimited, Osaka, Japan) was used to evaluate the hardness profiles along the width of the weld samples using a load of 2.0 kg

and dwell time of 15 s, where the free space between any two indentations was 0.5 mm, which represents at least 2.5 times the maximum indentation diameter. The hardness maps were also analyzed and drawn by collecting three lines measurements across the stir zone (SZ) and the stringer. Two tensile tests were carried out along skin and stringer axes using tensile testing machine (Instron-4208-300 kN capacity, Norwood, MA, USA) at a room temperature and a cross-head speed of 1.0 mm/min, Figures 2 and 3, respectively. At least three tensile samples were prepared from each T-butt joint and the average value of tensile strength was considered. The fracture surfaces of tensile test samples were investigated using the SEM. For EBSD investigation, the polished samples were subjected to electropolish for 60 sec at −15 ◦C and 14 V. The electro-polishing electrolyte consists of 30 vol.% nitric acid in 70 vol.% methanol. EBSD was performed for the base materials (BMs) and stir zone (SZ) of T-butt joints at 20 kV and 0.5 μm step size. The collected EBSD data were analyzed by OIM DC 7.3 software, developed by EDAX, AMETEK, Draper, UT, USA.

**Figure 2.** Photographs of tensile testing along skin direction (**a**) T-butt joint tensile test specimen and (**b**) after tensile test.

**Figure 3.** Photographs of tensile testing along stringer direction (**a**) T-butt joint tensile test specimen and (**b**) tensile test clamping system.
