*3.3. Process of ECAP*

The microstructures of the as-cast-T6 and cooling slope-cast-T6 alloy samples, subjected to four passes of route A, were displayed in Figure 4a–b. Meanwhile, following route A, the microstructure of cooling slope-cast-T6 alloy samples, which was subjected to six passes of ECAP, is shown in Figure 4c. The primary α-Al phase and eutectic constituents were elongated into plate-like shapes for the as-cast alloy sample [39], but fibrous-like shapes, for the latter samples. As shown in Table 1, the Si particles and eutectic phase appeared to be finer in both as-cast and cooling slope-cast alloy samples, after ECAP processing. Due to ECAP processing, the distribution was observed to be more uniform in the cooling slope-cast alloy sample. The result determined agrees with our earlier study, which reported that refine Si particles were observed through the cooling slope casting of alloy [40]. Nonetheless, the spheroidization and fragmentation of Si particles in this study were acquired through the T6 heat treatment process, coupled with the ECAP process.

**Figure 4.** Microstructure of ECAPed-T6 A356 alloy (**a**) as-cast after 4 passes, (**b**) cooling slope after 4 passes, (**c**) cooling slope after 6 passes and (**d**) surface of microscopic ECAPed sample.

Based on the microstructures of both samples, following route A, the refining and the distribution of both primary the α-Al phase and eutectic constituents in the ECAPed cooling slope-cast sample became more homogeneous. Thus, the microstructure changes may lead to changes in the mechanical and electrochemical properties of the alloy [28,41–43]. This is due to the reduction and distribution of cathodic to anodic phases. The straining led to the formation of dislocated cell structures, with a high dislocation density. The dislocation remains within the cell structures, which do not affect the remaining cell boundaries or develop walls that separate these cells into smaller cells. However, persistent shear straining processes may lead to a saturated dislocation density within these cell structures, which could be significantly reduced through (i) enhanced dynamic recovery (which would stabilize the creation and annihilation of dislocations) and (ii) the conversion of cells to well-defined grains (which would cause severe movement of cell interior dislocations to cell boundaries). In fact, this suggested that the process of grain refinement depends on the level of straining or in other words, the generation of dislocations [44–46].
