## 3 Results and Discussion
This indicates that while the reinforcing material is not fully amorphous, its microcrystalline properties play a crucial role in the mechanical behavior of the composite. The results of the calculations of theoretical density, experimental density based on the masses of specimens measured in air and water using a densitometer, and the porosity values of the specimens are presented in Table 1.  
The theoretical density for all specimens remained almost unchanged: 2.680 g/cm\({}^{3}\) for S1 and S1-HT, and 2.679 g/cm\({}^{3}\) for S2 and S2-HT. This indicates an insignificant effect of 0.36 wt% microsilica reinforcement on the theoretical density of the composite, which is consistent with the similar density values of pure aluminum and microsilica. However, the experimental density was lower than the theoretical density, ranging from 2.662 g/cm\({}^{3}\) (S1) to 2.660 g/cm\({}^{3}\) (S2-HT), because of the porosity. Porosity analysis showed that adding \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Specimen** & \(\rho_{\mathbf{A1}}\) & \(\rho_{\mathbf{ms}}\) & **wt\%\({}_{\mathbf{A1}}\)** & **wt\%\({}_{\mathbf{ms}}\)** & \(\rho_{\mathbf{th}}\) & **m\({}_{\mathbf{a}}\)** & **m\({}_{\mathbf{w}}\)** & \(\rho_{\mathbf{exp}}\) & **Porosity** \\
**Designation** & **(g/cm\({}^{3}\))** & **(g/cm\({}^{3}\))** & **wt** & **(g/cm\({}^{3}\))** & **(g)** & **(g)** & **(g/cm\({}^{3}\))** & **(\%)** \\ \hline S1 & 2.680 & 0 & 100 & 0 & 2.680 & 7.445 & 4.655 & 2.662 & 0.672 \\ \hline S1-HT & 2.680 & 0 & 100 & 0 & 2.680 & 7.350 & 4.595 & 2.661 & 0.709 \\ \hline S2 & 2.680 & 2.450 & 99.640 & 0.360 & 2.679 & 7.505 & 4.695 & 2.664 & 0.560 \\ \hline S2-HT & 2.680 & 2.450 & 99.640 & 0.360 & 2.679 & 7.385 & 4.615 & 2.660 & 0.709 \\ \hline \end{tabular} Changes in the density and porosity of the specimens are shown in Figure 6.  
\end{table} 
Table 1: Theoretical and experimental densities of specimens. Figure 6: Density and porosity of S1, S1-HT, S2, and S2-HT specimens. microsilica reduced the porosity from 0.672% (S1) to 0.560% (S2). This is caused by improved degassing and reduced pore formation under the action of ultrasound, which is consistent with the results of other studies demonstrating the effectiveness of ultrasonic treatment in reducing porosity in cast AMCs [44; 45]. However, T6 heat treatment increased the porosity of both reinforced and unreinforced specimens, probably because of the thermal expansion of micropores and diffusion processes at the interface boundaries [46; 47]. Although the heat-treated specimens (S1-HT and S2-HT) slightly increased their porosity, the difference was insignificant compared to the specimens without heat treatment (S1 and S2). An increase in porosity of 0.037% was observed for S1-HT and 0.149% for S2-HT. This may indicate that in this experiment, the heat treatment conditions did not strongly affect the pore structure, although they could affect other mechanical properties of the material, such as hardness and strength. The changes in the average microhardness of the specimens are shown in Figure 7.  
The microhardness of the specimens increased with both microsilica reinforcement and T6 heat treatment. Pure aluminum (S1) showed a minimum microhardness of 21.55 HV0.1, which increased to 26.65 HV0.1 after T6 heat treatment (S1-HT), which is explained by hardening due to quenching and aging. T6 heat treatment promotes recrystallization, eliminates residual stresses, improves structural homogeneity in the aluminum matrix, and increases the strain resistance [48; 49]. Thus, the increase in microhardness from 21.55 HV0.1 for S1 to 26.65 HV0.1 for S1-HT (+23.7%) confirms the effectiveness of this heat treatment. Adding microsilica with an average dispersion of \(\sim\)128.1 nm (\(\sim\)0.36 wt%) resulted in a significant increase in microhardness compared with untreated aluminum (S1), as seen by comparing S1 and S2. The dispersed microsilica particles act as a barrier to the movement of dislocations, thereby increasing the resistance to plastic deformation [50; 51].