## 3 Results and Discussion  
The casting structures of the Al-Mg\({}_{2}\)Si alloys were observed using an optical microscope and are shown in Figure 1. A number of crystallized phases were observed with increased Mg\({}_{2}\)Si content. One of the crystallization phases had a gray color and a needle shape, while another had a black color and a sphere shape in all of the specimens. In particular, the number of black phases increased with increasing Mg\({}_{2}\)Si content. The eutectic phase was only observed in Al-3.5 Mg\({}_{2}\)Si. The black phase was defined in the Mg\({}_{2}\)Si phase according to the component analysis. The gray needle phase was analyzed in the AlFeSi ternary intermetallic compound. This compound appears when Al contains Fe as an impurity. The AlFeSi phase was found in all of the specimens, as shown in Figure 1.  
The microstructures of the as-quenched Al-Mg\({}_{2}\)Si alloys are shown in Figure 2. The AlFeSi phase was present in all of the as-quenched samples. However, the eutectic Mg\({}_{2}\)Si phase only appeared in the Al-3.5 Mg\({}_{2}\)Si sample with a sphere shape that was larger than the as-cast sample. The Mg\({}_{2}\)Si contents of the Al-1.1 Mg\({}_{2}\)Si and Al-1.9 Mg\({}_{2}\)Si alloys were similar to or lower than the solubility of Mg\({}_{2}\)Si in the Al matrix at 590 \({}^{\circ}\)C. Therefore, the Mg\({}_{2}\)Si phase of both samples was resolved during the solid solution treatment and could not be determined, and as such are not shown in Figure 2a,b. The as-quenched Al-Mg\({}_{2}\)Si samples were investigated using DSC to identify the phase transformation of the Mg\({}_{2}\)Si, and their heat flow curves are shown in Figure 3. The heat flow curves have several exothermic peaks, and were named exothermic peaks, A, B, and C. The positions of the exothermic and endothermic peaks are at similar temperatures to those presented in previous studies [13; 20; 21]. For the Al-Mg\({}_{2}\)Si pseudo-binary system, the Mg\({}_{2}\)Si phase was precipitated as follows [22; 23; 24]: SSS \(\rightarrow\) G.P. zone \(\rightarrow\)\(\beta^{\prime\prime}\) coherent precipitates \(\rightarrow\)\(\beta^{\prime}\) semi-coherent precipitates \(\rightarrow\)\(\beta\) (Mg\({}_{2}\)Si) incoherent precipitates. When compared with earlier studies, peaks A and B are composite peaks and seem to be related to the precipitations of \(\beta^{\prime\prime}\) and \(\beta^{\prime}\), respectively, while peak C is the result of the precipitation of the \(\beta\) phase. Figure 2: Optical micrographs of as-quenched Al-Mg2 Si alloys: (**a**) Al-1.1 Mg2 Si; (**b**) Al-1.9 Mg2 Si; (**c**) Al-3.5 Mg2 Si; and (**d**) result of EDX analysis of AlFeSi phase (gray phase in (**c**)). Figure 1: Optical micrographs of as-cast Al-Mg2 Si alloys: (**a**) Al-1.1 Mg2 Si; (**b**) Al-1.9 Mg2 Si; (**c**) Al-3.5 Mg2 Si; (**d**) high magnification of Al-3.5 Mg2 Si (\(\times\)500); and (**e**) result of energy dispersive X-ray spectroscopy (EDX) analysis of Mg2 Si phase (black phase in (**d**)). The Al-1.9 Mg\({}_{2}\)Si samples were investigated using XRD to identify the presence of the Mg\({}_{2}\)Si phase before and after the DSC analysis. In Figure 4a, the diffraction pattern of the as-quenched sample only has Al peaks. This is because the Mg and Si atoms were dissolved in the Al matrix during the solid solution treatment. However, according to the DSC analysis, the sample has the Al and Mg\({}_{2}\)Si peaks in different diffraction patterns, as shown in Figure 4b. It can be determined from the Mg\({}_{2}\)Si peaks that the Mg\({}_{2}\)Si precipitation occurred during the DSC analysis sequence. The as-quenched Al-Mg\({}_{2}\)Si samples were analyzed using LFA depending on the temperature, and the results are shown in Figure 5. The Al-1.1 Mg\({}_{2}\)Si alloy had the highest thermal diffusivity at every temperature. The Al-3.5 Mg\({}_{2}\)Si alloy had lower thermal diffusivity than the others.