*3.4. SEM Investigation*

As noted by optical examinations of the welded joints (Figure 8) and confirmed with the two modes SEM investigations (Figure 9) the initial coarse elongated grains of the asreceived AA2024-T4 and AA7075-T6 base alloys are thermomechanically deformed during FSW and resulted in new recrystallized equiaxed fine grain structures. The microstructures display also fine precipitates decorated the grain boundaries of the fine grains of both AA2024 and AA7075, Figure 8a–f. In addition, coarse and fragmented precipitates are detected in the SZ of the T-joints as given in VCD-mode images Figure 8b,e respectively. Moreover, the friction stir welded AA7075 alloy shows finer recrystallize grain size than that of AA2024 alloy, Figure 8d.

**Figure 8.** SEM microstructure of Friction stir welded T-joints at different rotational speeds: (**a**) ETD mode at 400 rpm, (**b**) in VCD mode, (**c**) ETD at 600 rpm, (**d**) low magnification in ETD at 800 rpm, (**e**) in VCD and (**f**) high magnification of the selected area (D) in (**d**).

The FSW and FSP lead to significant change in size and morphology of the precipitates. It can be effectively refined and redistributed both soluble and insoluble particles in precipitation strengthening non-ferrous alloys [31,32]. The precipitates in different regions of FSW zones are strongly function of the local thermo-mechanical cycles. The evolved microstructures in the SZ of the friction stir welded dissimilar T-joints at different rotational speeds are expected to be very complex. In fact, the precipitation phenomena need more research and still out of completely understood [33]. The current study is roughly an important attempt to evaluate morphologies and types of formed precipitates in the SZ of the Friction stir welded T-butt joints of AA2024-T4 and AA7075-T6. This attempt is extended to relate the precipitates influence on the recrystallization process during FSW. The precipitates display four types of morphologies: irregular, almost-spherical, spherical and rod-like and marked by symbols *I*, *A–S*, *S* and *R*, respectively. These morphologies are

shown in the SZ of all the welded joints at the different rotational speeds (400–800 rpm) and typically observed as given for example in Figure 8c.

**Figure 9.** SEM images of T-joint at 400 rpm (**a**) ETD mode and (**b**) VCD mode.

Generally, the 2024 and 7075 Al-alloys have unstable nature of precipitates. This means that, the precipitates can coarsen and transformed into more stable precipitates, and/or undergo partial or complete dissolution during suffering high temperatures and may reappear in various morphologies, amounts and crystal structures during cooling.

Precipitates may accelerate or retard the recrystallization process depending on their size and volume fraction. Coarse particles can intensify the driving force of recrystallization and act as nucleation sites, which is known as particle stimulated nucleation (PSN). In contrast, relatively finer particle can retard recrystallization by the particle pinning of the grain boundaries, which is referred to as "Zener Pinning" [34]. Figure 9 shows two examination modes SEM microstructure of Friction stir welded sample at 400 rpm, where location 1 represents fine Dynamically recrystallized (DRX) grains adjacent to coarse particle. However, location 2 denotes coarse DRX grains at a distance from the coarse particles. PSN mechanism is observed to be more effective when the precipitates are present near the grain boundaries rather than within the grain, where the local strain gradient is relatively lower [34,35].

Coarse thermal stable and non-deformable particles interact with moving dislocations producing dislocation piled-ups or loops at the particle-matrix interface, which generates a local strain mismatch at the interface. This local strain field then eases the operation of different slip systems at the interface, which causes the surrounding matrix to rotate to fit the external matrix, creating the so-called particle deformation zone (PDZ) [35,36]. Further hot deformation, of such PDZs makes them favorable nucleation sites for recrystallization. Due to the high dislocation density in a PDZ, recovery takes place by the formation of new sub-grains, which then increases the misorientation by absorbing more dislocations. When this misorientation achieves from 10 to 15◦, a likely recrystallization nuclei has evolved that may then grow into the surrounding matrix [37].
