## 3 Results and Discussion
### Microstructure and Reinforcement Dispersion
On the other hand, the physical suppression of cluster generation is different between cases of high volume fraction and low volume fraction, and high fluidity of the melt can make particles move easily. For this reason, moving particles gathered into specific areas and formed clusters [28]. Figure 2: Results of electron probe microanalyzer (EPMA) elemental mapping at 9 vol.% SiC/AA2024: (**a**) SC process, (**b**) DC process. Reinforcement was uniformly distributed after the DC process, and micropores are not found in the microstructure images. The porosity considerably decreased compared to that of AMCs produced by SC process. The porosity values of the DC composites were confirmed to be 0.8%, 0.2%, 0.9%, and 1.1%. The dispersion of particles improved and the porosity decreased after the DC process, largely because of the two factors of fast cooling rate and fluid shear force. Conventional high-pressure die-casting (HPDC) has attractive advantages in that the process can induce fluid shear and high cooling rate. Aggregated particles are dispersed by the fluid shear force at high pressure, and the movement time of reinforced particles is minimized by the fast cooling rate. In addition, grains are critically refined with the increase of nucleation sites and rapid solidification. Thus, pores generated by agglomerated particles decrease because of the elimination of clusters and the rearrangement of particles. More details of both mechanisms of solidification are shown in Figure 3. Considering the schematic diagram, particles that are well-dispersed by fluid shear force can create many heterogeneous nucleation sites in the early stage of solidification, and primary \(\alpha\)-Al phases are generated and grown. In middle of the solidification, the fast cooling rate of the DC process can lead to grain refinement. A number of liquid-solid surfaces are gradually enlarged by refined primary \(\alpha\)-Al dendrite, and this can push particles to the last solidification zone (grain boundary). Therefore, coarse grains with a slow cooling rate push particles in a small specific area, and this causes agglomeration of the particles; however, fine grains with fast solidification speed push the particles in various directions to large areas (grain boundary or last freezing zone) [29; 30; 31]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Density**} & \multicolumn{4}{c}{**Stir-Casting**} & \multicolumn{4}{c}{**Die-Casting**} \\ \cline{2-9}  & **AA2024** & **3 vol.\%** & **6 vol.\%** & **9 vol.\%** & **AA2024** & **3 vol.\%** & **6 vol.\%** & **9 vol.\%** \\ \hline Actual density (g/cm\({}^{3}\)) & 2.674 & 2.680 & 2.665 & 2.666 & 2.757 & 2.788 & 2.782 & 2.786 \\ Theoretical density (g/cm\({}^{3}\)) & 2.780 & 2.793 & 2.806 & 2.818 & 2.780 & 2.793 & 2.806 & 2.818 \\ Relative density (\%) & 96.2 & 96.0 & 95.0 & 94.6 & 99.2 & 99.8 & 99.1 & 98.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Density of AA2024 and aluminium matrix composites (AMCs) in stir-casting (SC) and die-casting (DC) processes. Figure 3: Schematic diagram of solidification behavior of SiC/AA2024 AMCs. The grain size was observed by electron backscatter diffraction (EBSD) analysis (Figure 4) to determine the average grain size of SC- and DC-processed 9 vol.% SiC-reinforced specimens. In Figure 4, the black color indicates SiC particles; the average grain sizes are 68.06 \(\upmu\)m (SC, Figure 4a) and 17.67 \(\upmu\)m (DC, Figure 4b), respectively. It is confirmed that the average grain size decreases by about 75% due to grain refinement from the fast cooling rate of the DC process. The addition of SiC particles and the fast cooling rate during the DC process increase the grain-refinement effect of the SiC/ AA2024 composite, which might contribute to strengthening of the composite. As mentioned, we believe that the DC process improves SiC dispersion in SiC/ AA2024 AMCs, and this improvement is qualitatively presented with microscopic images and EBSD-analysis results.