OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 1 2025 Fig. 2. Flowchart and schematic representation detailing the fabrication sequence of the present study reinforced with TiO2 particles was determined using the Archimedes method. The theoretical density was calculated using the rule of mixtures, based on the mass fraction of TiO2 particles. The porosity of the composites was then calculated by comparing the experimental and theoretical densities for each specimen. The hardness of the composites evaluated using Vickers method. Results and Discussion Fig. 3 presents the results of SEM and EDS analysis of the Al- 7Si alloy in the as-cast state. It is evident from Figs. 3, a and 3, c that the microstructure of the matrix alloy consists of primary α-Al dendrites and interdendritic areas, which are either shrinkage pores or a Si-rich eutectic phase. The abovementioned phases in the microstructure are reflected in the elemental spectra and mass fractions of the elements at various regions displayed in Fig. 3. The microstructure of the Al- 7Si alloy matrix with varying mold wall thicknesses and cooling rates is shown in Figs. 4 to 6. It is evident from these figures that as the cooling rate increases, both α-Al and eutectic silicon get finer. A stepped mold with wall thicknesses of 29 mm, 12 mm, and 8 mm was used to achieve variable cooling rates during solidification. Mold wall thickness significantly affected the microstructure of the as-cast Al-7Si alloy. As can be seen from the presented micrographs, increasing the mold wall thickness leads to a refinement of the alloy’s microstructure. This is likely due to the fact that increasing the thickness of the metal mold wall increases the rate of heat extraction and, consequently, the cooling rate. Cooling conditions are known to influence the degree of grain refinement. Rapid solidification promotes the formation of a finegrained structure with uniform grain distribution, while slow solidification leads to the formation of coarse grains [27]. The effect of mold wall thickness on the grain size in the cast alloy is depicted in Fig. 7. As the mold wall thickness increases from 8 mm to 29 mm, the average grain size at a distance of 9 mm from the outer surface decreases from 63 μm to 34 μm. At a distance of 4 mm from the outer surface, the average grain size decreases from 48 μm to 28 μm. Thus, the solidification rate has a decisive effect on the degree of grain refinement, which explains the effect of mold wall thickness on microstructure and grain size [28–30]. Kang
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