OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 4 2023 an important factor for obtaining uniform porosity on the inner surface of the cast metal. Therefore, it is necessary to choose the optimal rotation speed of the non-stirrer and the mixing time. The graphite stirrer `used in this study is equipped with an electric motor and a speed controller, as shown in fig. 2. A clay pot mould, shown in fig. 3, was used to pour the molten metal. Fig. 2. Stir casting setup Fig. 3. Clay pot mould Aluminium melt preparation Aluminium was cut into small pieces and weight calculations were carried out. The pieces of aluminium were then placed in the crucible of an induction furnace for melting. The melting process was completed in 2.5 hours. Before pouring the molten aluminium into the mold, the slag was first removed from the molten metal, since it does not allow the foaming agent to mix thoroughly. Pouring Before pouring, the mold was preheated to about 300 °C in order to prevent instant solidification. After preheating, molten aluminium is poured into the mold. A mechanized stirrer was used to form metal foam, as shown in fig. 4. Calcium carbonate powder was added in three stages. At the first stage, 40 % of calcium carbonate was dispersed into the mold. After that, molten metal was poured into the mold from the crucible. After pouring into the molten metal at a temperature of 750 °C, 30 % calcium carbonate was added again. After that, they started mixing. After a few minutes, a third part was added to the molten metal, that is, 30 % of the calcium carbonate powder. Aluminium molten metal was mixed for 4–5 minutes at 460 rpm. As a result of mixing, the calcium carbonate powder was uniformly distributed or thoroughly mixed with molten aluminium. After that, the mixture was left to decompose calcium carbonate inside the molten metal. During the decomposition of calcium carbonate, carbon dioxide is released, which should linger inside the molten metal. The metal foam covering the mouth of the pot is formed without the use of an agent that increases the viscosity, since it is formed during decomposition. The decomposition process is explained by the following reactions. CaCO3 = CaO + CO2 2Al + 3CO2 = Al2O3 + 3CO 8Al + 3CO2 = 2Al2O3 + Al4C3 Mg + CO2 = MgO + CO 2Al + Mg + 4CO2 = MgAl2O4 + 4CO
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