*3.3. Mechanical Properties*

Vickers macro-hardness distribution profiles on the transverse cross-sections of the joints produced by FSW are shown in Figure 7. Figure 7a–c show the hardness maps for the three joints (J1, J2, and J3); it can be noted that the FSW-affected zones were diffusing and extended to a width of 22 mm at the bottom of the butt joint and increased to reach around 40 mm at the upper surface due to the effect of the friction and the pressure applied by the rotating shoulder to the surface of the joint.

The conical shape of the SZ and HAZ was more obvious at the joints with low *ω*/*v* values of 10 (J1: 600 rpm and 60 mm/min). The FSW nuggets showed the lowest hardness values due to the heat input concentrated in these regions, causing softening of the stirred regions of the joined materials. At both applied rotational speeds (400 and 600 rpm), the lower hardness region took place in the upper surface of the joints at the lowest travel speed (20 mm/min) and then appeared in the lower half of the cross-section at the highest travel speed (60 mm/min). This statement confirmed the softening effect of the friction and pressure of the pin shoulder on the upper surface of specimens [47].

Figure 8 represents the engineering tensile stress-strain curves of the base alloys AA5083 and AA5754 and the FSWed dissimilar joints. Flow behavior of the Al–Mg alloys of the series AA5XXX have been investigated at quasi-static and high strain rate ranges [48,49] and showed similar serration in the flow curves, which are related to the so-called Portevin-Le Chatelier effect [49–51]. This effect is due to successive pinning and unpinning of the moving dislocations by the solute atoms. The base aluminum alloys show typical stressstrain curves with moderate hardening, followed by a wide plastic strain range up to the ultimate tensile stress, followed by a slow decrease of stress value up to fracture.

Table 1 includes the tensile properties of the tested specimens of the welded joints compared with that of the base alloys. The tensile sample of the FSWed joint at the revolution of 400 rpm and travel speed of 60 mm/min (J2: 400-60) showed similar behavior to the base materials, except that the short plastic strain range was lower than the base alloys. This showed higher tensile stress than the base alloy AA5083 from the beginning of the plastic strain region till its ultimate tensile stress value (224 MPa) and decreased till fracture at a total elongation of 23%. Relating the ultimate tensile value of this joint to the ultimate tensile value of the base alloy AA5083 resulted in a welding efficiency of 96%. The other two tensile samples of the FSWed joints ((J1: 600-60) and (J3: 400-20)) were early fractured at strains of 5.5% and 4.3%, respectively, before reaching the ultimate tensile value. This behavior was due to the presence of some welding defects, such as tunnels or pores [52]. However, the yield stress of the tensile sample taken from these joints (J1: 600-60 and J3: 400-20) was comparable with the yield stress of the base alloy AA5083. The increased strength and the soundness of the sample (J2: 400-60) were related to the lowest heat input value, as shown in Table 1, where its heat index value was one third that of the sample J3: 400-20 and one half of the sample J1: 600-60.

To describe the flow behavior of the tensile stress-strain curves (*σ* − *ε*) of the materials under investigation, the engineering curves were transferred to the true stress-true strain (*σ<sup>f</sup>* − *ϕ*) up to the ultimate point by these formulas: true stress *σ<sup>f</sup>* = *σ* (1 + *ε*) and true strain *ϕ* = *ln* (1 + *ε*).

**Figure 4.** Macrostructure representing three different FSWed AA5754-AA5083 joints J1, J2, and J3 prepared by applying different combinations of rotation and travel speeds (rpm-mm/min) of 600-60, 400-60, and 400-20, respectively. EBSD measurements were performed at the centerline in the NG zone for each joint at the corresponding specified top locations (**a**–**c**) and bottom locations (**d**–**f**) for J1, J2, and J3, respectively. The inverse pole figure coloring (IPF) maps and their corresponding grain boundary (GB) maps are represented for all the denoted locations.

**Figure 5.** Distribution of grain diameters for the different dissimilar friction stir welded (FSWed) joints prepared using different rotation and travel speeds. (**a**–**c**) at the top locations and (**d**–**f**) at the bottom locations in the NG zones of J1, J2, and J3, respectively.


**Table 1.** Friction stir welding conditions and tensile properties.

There are many published models describing the flow behavior of metallic materials [53–55]. The description model can be selected depending on the suitability for the specific material and the test conditions. The model simplicity for application, represented in the low number of model parameters, is a factor helping the spread of some models. The flow curves of the tested samples were described using the empirical formula relating the flow stress (*σf*) and true strain (*ϕ*) after Ludwik [56]:

$$
\sigma\_f = \sigma\_o + k \left(\varphi\right)^n \tag{1}
$$

where initial flow stress (*σo*) is the flow stress at the plastic strain of *ϕ* = 0, *k* is a material parameter, and *n* is the material strengthening parameter.

**Figure 6.** IPF coloring maps with their corresponding (111) pole figures for the same data presented in Figure 4 after applying the required rotations to align the FSW reference frame with the shear reference frame. (**a**–**c**) are the IPF maps after rotation and their corresponding (111) pole figures for the EBSD data obtained at the top locations given in Figure 4. (**d**–**f**) are the IPF maps after rotation and their corresponding (111) pole figures for the EBSD data obtained at the bottom locations given in Figure 4.

**Figure 7.** Hardness distribution maps over the cross-section of the FSWed joints (**a**) J1: 600 rpm–60 mm/min, (**b**) J2: 400 rpm 60 mm/min, and (**c**) J3: 400 rpm and 20 mm/min.

Figure 9 shows the description of the plastic flow curves of the base alloys AA5754 and AA5083 and the FSWed joints using the Ludwik formula. It can be seen that the selected empirical model described the curves very well. The materials parameters (*k* and *n*) of the base alloys were relatively low due to the combination of the flow curve of the higher strengthening rate region at the beginning of the flow curve and the moderate hardening in the steady-state region up to the end of the flow curve. The samples of the joints welded at the conditions 400-20 and 600-60 showed higher strengthening parameter (*n*) and higher material parameter (*k*) than the base alloys due to the early fracture of the samples, leading to shortening of the flow curves, especially the lower strengthening rate region at the end of the curve. However, the FSWed joint using pin revolution of 400 rpm and a travel speed of 60 mm/min showed the highest strengthening parameter (*n* = 0.494) with a moderate *k* value of 413. The tensile flow parameters of the flow curves are summarized in Table 2. In the three joints (J1, J2, and J3), the fracture mechanism was ductile mode with very clear dimple features, and it is fully characterized in [9].

**Figure 8.** Engineering tensile stress-strain curves of the base alloys AA5083 and AA5754 and the FSWed dissimilar joints at the conditions 400 rpm/20 mm/min, 400 rpm/60 mm/min, and at 600 rpm/60 mm/min.

**Figure 9.** Description of the true tensile stress-strain curves of the base alloys AA5083 and AA5754 and the FSWed dissimilar joints using Ludwik formula.


**Table 2.** Tensile flow parameters of the flow curves.
