*3.2. The Heat Treatment Solution*

Alloy A356 is a heat-treatable hypoeutectic Al–Si alloy. Figure 3a–b shows optical micrographs of the both the primary α-Al phase and the eutectic Si particles in both alloy samples, before the heat treatment process. As for the primary α-Al phase (bright phase), grains with a globular structure could be observed in the cooling slope-cast alloy sample, while large grain, surrounded by coarse eutectic constituents (dark phase), appeared in the as-cast alloy sample. The flake and acicular morphology of Si particles in as-cast and in cooling slope samples were transformed into a spheroidized shape after T6 heat treatment, as shown in Figure 3c–d. In a solution heat treatment sample, the eutectic Si particles were fragmented and spheroidized during the coarsening processes [35,36]. It was observed that the eutectic Si particles were refined and had fewer sharp angles edges, as in the as-cast alloy before heat treatment. The same behavior was reported by previous studies, which found that the T6 heat treatment process initiated the spheroidization of Si particles [37,38], but a considerable amount of acicular shapes remained after the T6 heat treatment process.

**Figure 3.** Microstructure of eutectic phase: (**a**) as-cast, (**b**) cooling slope, (**c**) as-cast-T6 and (**d**) cooling slope-T6.
