## 3 Results and Discussion  
The LM and SEM images of S1, S1-HT, S2, and S2-HT are shown in Figure 4. The LM image of the S1 cast without reinforcement and heat treatment shows the preservation of the base cast aluminum structure with minimal changes and rare pores, typical of gravity casting. In the SEM image, the microstructure exhibits a smooth surface without a significant number of artifacts. Minimal defects, such as micropores or small particles, were observed. The dispersion level of inclusions or contaminants is extremely low, which is typical for pure aluminum without reinforcement. The LM and SEM images of S1-HT were similar to those of S1. The number of visible pores and defects is minimal. The pores in the specimen are attributed to the following factors related to the T6 heat treatment. During the T6 heat treatment, especially during the heating step up to the quenching temperature (typically 500-550 \({}^{\circ}\)C), dissolved gases (e.g., hydrogen) may be released from the aluminum matrix. These gases accumulate in localized areas and form pores, particularly if their initial concentration in cast aluminum is significant. During heating to high temperatures, existing pores or microcavities in a material can increase in size owing to thermal expansion and the increased pressure of gases inside them [43], a characteristic phenomenon for materials with microdefects. Rapid cooling (immersion in water after heating) can contribute to the formation of additional stresses and increase pore volume. Quenching does not provide sufficient time to eliminate defects formed during heating. Nevertheless, the T6 heat treatment had a positive effect on the homogeneity of the structure and reduced the porosity and agglomeration of inclusions visible in the LM and SEM images of S2-HT. The inclusion of microsilica resulted in significant changes in the grain structure, visible in the LM and SEM images of S2 and S2-HT. The LM images of these specimens show that the microsilica is relatively uniformly distributed with visible small dark dots (inclusions). In the SEM images, finely dispersed microsilica particles are visible as light dots. The grain boundaries were less pronounced, indicating interaction of the inclusions with the \(\alpha\)-Al matrix. The presence of pores (dark recesses in SEM images) is visible owing to the difficulty of the uniform distribution of reinforcing microsilica particles during casting; however, their number is minimal, which suggests the effectiveness of ultrasonic-assisted stir casting. Some microsilica particles accumulate as agglomerates, which is a characteristic problem of stir-casting processes in general. Microcracks around the microsilica agglomerates were observed on the surface because of the stresses at the matrix-particle interface, which can weaken the material. In general, the number of pores was minimal. Compared with S1 after T6 heat treatment, the reinforced particles were more evenly distributed in S2-HT. The grain boundaries became clearer compared to those in S2 owing to recrystallization. The number of visible pores and defects decreased because of the improved homogeneity of the structure. Microsilica particles were better integrated into the matrix with less-pronounced agglomerates, making the surface smoother; microcracks were reduced owing to the heat treatment. The microstructural homogeneity and mechanical performance,including ductility, were significantly improved. Defects became less pronounced at the interface between the matrix and microsilica particles, indicating better adhesion and reduced localized stresses. Some microsilica particles in both specimens (S2 and S2-HT) formed agglomerates (clusters), which were observed as larger light-colored zones caused by insufficient efficiency of ultrasonic mixing, especially at low concentrations of the reinforcing material. The XRD patterns of the specimens are shown in Figure 5. The diffraction peaks corresponding to the main phases present in the specimens were successfully identified.