3.3.2. Tensile Properties Analysis

The tensile strength properties of the FSW joints are usually compared with those of the alloy having lower tensile strength [41–43]; here AA5083. For design purposes, the yield stress is used more frequently than the ultimate tensile strength so that the yield stress is determined (σ0.2%) for the tested base materials and FSW joints. Relative ultimate tensile strength (σUTS joint/σUTS 5083) and relative yield stress (σ0.2% joint/σ0.2% 5083) of the produced FSW joints were determined from the tensile stress–strain curves. These relative values (ultimate tensile strength and yield stress) were presented as a function of the welding speeds as shown in Figure 11. Figure 11a summarizes the tensile properties of FSWed joints AA5083/AA5754 at different welding speeds of 20, 40 and 60 mm/min and at rotational speeds of 400 and 600 rpm obtained from the tensile stress strain curves of the FSW samples and related to the tensile properties of the base alloy.

**Figure 9.** Hardness values along FSWed joints AA5083/AA5754 at travel speeds of 20, 40 and 60 mm/min and rotation speeds of (**a**) 400 rpm and (**b**) 600 rpm.

**Figure 10.** Hardness distribution across the joints (**a**) AA5083/AA7020 and (**b**) AA7020/AA5083 welded at travel speeds of 20, 40 and 80 mm/min showing the effect of the joint arrangement.

**Figure 11.** Relative tensile strength (σUTS joint/σUTS 5083) and relative yield stress (σ0.2% joint/σ0.2% 5083) of the produced FSW joints against the welding speeds.

As a usual trend, the tensile strength increases with increasing the travel speed and with decreasing the rotational speed, because of the lower generated heat input which permits materials softening. This statement can be supported by the relative yield stress presented in Figure 11a. The relative ultimate tensile strength of FSWed joints is greatly affected by the internal defects in some samples causing early fracture. FSW of the joints AA7020/AA5083 at rotational speed of 500 rpm and travel speed of 80 mm/min has produced joint with ultimate tensile strength comparable with the base alloy AA5083, while the yield stress is even higher than that of the base alloy Figure 11b. Regardless the defect happened at a travel speed of 40 mm/min, clamping the higher strength plate as an AS in friction stir welding of materials with high differences in strength increases the joint strength.

Figure 12 shows macrographs for the fracture positions of the tensile samples of AA5083/AA5754 joints at different welding parameters: (a) 400 rpm–20 mm/min, (b) 400 rpm–40 mm/min, (c) 400 rpm–60 mm/min. and fracture locations of AA5083/AA7020 joints at 500 rpm but different travel speeds and positions; (d) AA7020 AS-20 mm/min, (e) AA7020 AS-40 mm/min and (f) AA7020 AS-80 mm/min. It can be observed that the fracture occurred at the nugget zone in one joint of AA5083/AA5754 (Figure 12a) mainly due to the defect while the fracture occurred away from the NG zone in two joints (Figure 12b,c). In terms of the AA7020/AA5083 joints, it can be observed that the lowspeed joint fracture occurred away from the NG (Figure 12d) and the high-speed joints the fracture occurred inside the NG (Figure 12e,f) mainly due to the defects noted. The fracture surface of two samples indicated in Figure 12 are investigated using SEM and EDS analysis. Clearly, it can be observed that from Figure 13 that the fracture mechanism of the dissimilar AA AA5083/AA5754 is ductile mode with very clear dimple features as can be seen in the enlarged micrographs of Figure 13b,c. The inclusions shown in Figure 13d are mostly aluminum and magnesium oxides as detected by the EDX analysis. The lack of adherence of such oxide inclusions with the matrix has accelerated the formation of pores around the inclusions through decohesion between the inclusions and the surrounding material which make the microcracks initiation by the coalescence of the neighboring pores possible. With increasing the applied load, the microcracks grow faster and propagate until the reaching the complete fracture. Figure 14a–d shows the fracture surface SEM micrographs of the dissimilar joint AA5083/AA7020. Clearly the dimple features are dominating, which confirms the ductile fracture mechanism.

**Figure 13.** SEM micrographs of the fracture surface of tensile test sample of AA5083/AA5754 joint at 400 rpm–60 mm/min, (**a**) mixed fracture mode (**b**) brittle fracture features, (**c**) ductile fracture features, (**d**) EDX spot analysis of the points 1 and 2.

**Figure 14.** (**a**,**c**) SEM images of the fracture surface of tensile test samples of AA7020/AA5083 joint of AA7020 AS, 500 rpm–20 mm/min; (**b**,**d**) show the shape deep dimples.
