*3.1. The Effect of Cooling Slope*

Figure 2 shows the microstructure of the as-cast A356 alloy sample, before and after the cooling slope casting process. Typically, the primary α-Al phase (bright phase), which is surrounded by eutectic phase (dark phase), is formed during the initial solidification phase.

**Figure 2.** Microstructure of as-cast (**a**–**c**) and cooling slope casting samples (**d**–**f**). (**a**) coarse dendritic and rosette shape of α-Al phase at thin wall zone; (**b**) and (**c**) dendritic shape in the middle and center zone in as-cast samples; (**d**) rosette shape of α-Al phase at thin wall zone; (**e**) and (**f**) nearly globular shape at the middle and in the center zone of cooling slope samples.

The morphology of primary α-Al phase in three zones was found to be mostly dendritic for the as-cast sample, before the cooling slope casting process, as displayed in Figure 2a–c. However, the evolution of the coarsen dendritic to finer dendritic could be observed from the center zones approaching the thin wall zone. The variation in cooling rate depends on the location in the mold and affects the morphology of the primary α-Al phase. Considering that the mold itself has a relatively low initial temperature, the cooling rate is more rapid near the wall of the mold, thus leading to the nucleation of numerous grains of random orientations in the thin wall zone area [33]. During the pouring of molten alloy, the presence of forced convection detaches the dendritic arms that developed in the thin wall zone area. However, the dendrites start to form in the middle zone due to the lower cooling rates in this zone. They later grew and become ripened as coarse dendrites as a result of the lowest cooling rate in this center zone [34].

The non-dendritic microstructures of the as-cast A356 sample, after the cooling slope casting process, are shown in Figure 2d–f. However, in particular, the morphology of the primary α-Al phase in the thin wall zone appeared rosette-like Figure 2d and almost globular-like in the middle zone and center zone Figure 2e–f. In the center zone, the primary α-Al phase was found to be coarser than that in the middle zone. This may be because the temperature gradient across these three sections during solidification contributes to the variation in the morphology and size of the primary α-Al phase in these three zones. The slowest cooling rate in the center zone allows the α-Al phase to grow courser, as the cooling time is prolonged.
