## 3 Results and Discussion  
### Microstructure and Reinforcement Dispersion  
The microstructures of the AA2024 alloy and SiC/AA2024 composite are shown in Figure 1a-h. SEM and EDS analyses revealed that black-colored SiC particles were distributed in the Al matrix. In the case of AA2024 (Figure 1a), after the stir-casting (SC) process, small pores (marked as blue-dot circles) were observed throughout the matrix. With the reinforcement-particle addition, relatively large micropores could be observed. These are marked with yellow arrows in Figure 1b-d. Porosity increases with particle addition are attributed to the generation of particle agglomeration and clustering during solidification [25; 26]. The microstructures of the matrix and of the AMCs that were fabricated by the die-casting (DC) process are shown in Figure 1e-h. In Figure 1d, pores and cracks are not observed in the matrix alloy. In addition, it was confirmed that the dispersion of the SiC reinforcement was uniform, and micropores were almost completely removed near the agglomerated particles (Figure 1f-h). Microstructural images also show that the grain size of the AMCs is smaller than that of the base alloy because the added reinforcements act as heterogeneous nucleation sites during solidification [27]. Moreover, for the same volume fraction, the grain size of AMCs produced by the DC process was much smaller than that of AMCs fabricated by SC because of the different cooling rates of the two processes. To better understand the composition of the grain boundary precipitates, field-emission electron probe microanalyzer (FE-EPMA) microstructure-mapping observation was conducted on the 9 vol.% SiC/AA2024 composite before (Figure 2a) and after the DC process (Figure 2b). In all microstructural images, a thin white band is observed, similar to that in the scanning electron microscope (SEM) images of the matrix; this band confirms the presence of Al-Cu and Al-Cu-Mg intermetallic compounds and precipitation trapped in the grain boundary. As can be seen from the EPMA analysis, after the DC process, the specimen had more uniform particle dispersion without agglomeration than that of the SC composite. As the size of the grains decreased due to the DC process, the size of the intermetallic phases decreased, and the dispersion increased. Figure 1: Microstructure of AA2024 alloy and aluminum matrix composites (AMCs) formed by the stir-casting (SC) process: (**a**) AA2024, (**b**) 3 vol.%, (**c**) 6 vol.%, (**d**) 9 vol.%, and after the die-casting (DC) process, (**e**) AA2024, (**f**) 3 vol.%, (**g**) 6 vol.%, (**h**) 9 vol.% (same magnification). To evaluate the pore amounts in the AA2024 alloy and the composite after SC and DC processes, the actual and theoretical density were measured, and the results are shown in Table 2. The porosity was confirmed to be 3.8%, 4%, 5%, and 5.4% for the 0, 3, 6, and 9 vol.% SiC-reinforced stir-cast specimens, respectively. Previous studies already showed the approximate quadratic polynomial equation correlation between the volume fraction of the reinforcement and the porosity; the equation is as follows:  
\[P=2.19+0.36\times(S)-0.006\times(S)\]  
where \(P\) is the percent of porosity and \(S\) is the volume fraction of particles in the casting. The calculation implies that the porosity continues to increase to a 30 volume percent of reinforcement and then decreases after this critical point. Therefore, the relative density slightly decreased (porosity increased) as the volume fraction increased in these volume fraction ranges. These effects are thought to result from the amount of air sucked inside the liquid metal vortex during the stir process. In addition, the generation of particle clusters is more active at low volume fraction than at high volume fraction. In the case of high volume fraction, there is a relatively low fluidity of the melt, and the large number of particles tend to physically restrict their own movement. Thus, the generation and growth of clusters are suppressed, and there are fewer clusters.