OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 1 2025 The microhardness values increase with increasing mold wall thickness due to high cooling rate at the casting surface close to the wall. Fig. 8 shows the relationship between the microhardness values of the castings and the wall thickness of the steel mold. It is evident from this figure that the microhardness of the Al- 7Si matrix alloy with a mold wall thickness of 29 mm (close to the mold wall) was 95 HV. At a distance of 9 mm from the inner mold wall, the microhardness decreased to 89 HV. Each value is averaged over four measurements. The casting with a wall thickness of 8 mm had a microhardness of 78 HV close to the mold wall and 74 HV close to the specimen center. As can be seen in Figs. 4 and 6, an increase in average grain size may be the reason for the decrease in microhardness with decreasing wall thickness. Due to the low cooling rate, the microstructure of the alloy obtained in the mold with the minimum wall thickness is characterized by a larger grain size, while in castings obtained in molds with a greater wall thickness, the grain size is smaller due to more intensive heat transfer. Decreasing the cooling rate leads to a deterioration of the alloy’s properties and structure, an increase in the dendritic arm spacing (DAS), and a reduction in the effectiveness of grain refining and modification processes [32, 33]. 50 60 70 80 90 100 110 4 8 12 16 20 24 28 32 Microhardness value Steel mold wall thickness (mm) At distance 4mm from internal mold wall At distance 9mm from internal mold wall Fig. 8. Microhardness values of the castings versus steel mold wall thickness The cooling rate also has a significant influence on the solidification rate and structure of the alloy, including the size of the secondary phase particles [34, 35]. According to Chen He et al. [36], increasing the cooling rate leads to an increase in the mechanical properties of the alloy, a significant decrease in the average grain size, secondary dendrite arm spacing, width, and volume fraction of the eutectic phase. At high cooling rates, a reduction in grain size and an increase in grain boundary length occur. Increasing the cooling rate leads to a decrease in the size of secondary phase particles, which affects the mechanical properties of the castings [37, 38]. Kong et al. [39] investigated the influence of cooling rate on the mechanical properties (hardness and tensile strength) of an aluminum alloy. They noted that the solidification of the alloy is completed in the interdendritic space. At high cooling rates, there is a compression of the α-Al grains before the dendrites are fully formed, and the number of grain boundaries increases. In contrast to resin sand molds, this leads to the formation of more grain boundaries, a reduction in grain size, and a more uniform distribution of eutectic phases along the grain boundaries. The reduction in the amount of eutectic phases indicates an increase in the concentration of dissolved elements in the solid matrix, which contributes to improving the mechanical properties of the alloy. Figs. 9 and 10 show the measured porosity and theoretical density of the composites as a function of TiO2 nanoparticle concentration (in wt.%). Fig. 9 demonstrates the effect of TiO2 nanoparticle concentration on the density of Al- 7Si alloy-based composites. As predicted by the rule of mixtures, the theoretical
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