OBRABOTKAMETALLOV Vol. 25 No. 4 2023 technology excess gas can escape from the molten metal [15]. Compared to the GI method, less closed porosity and more micropores in the cell walls are obtained. In this case, aluminum foam is produced both by foaming and by gas injection methods [16]. Solid-Gas Eutectic Reaction Method This method is also known as GASAR. It was developed by the Ukrainian scientist Shapovalov V. in 1993. Its advantages over the powder metallurgy processes and foaming method are: no losses of raw materials, no chemical treatment, ease of control of pore size and orientation, and, in addition, compared to other processes, it is cost-effective [17]. The metal is melted in an autoclave under high pressure, which allows large amounts of hydrogen to be introduced into it. After reducing the temperature and pressure, the alloy is in “liquid + gas” state; upon subsequent cooling below the eutectic temperature, the liquid crystallizes and a “solid + gas” is obtained. The formation of bubbles occurs due to the release of hydrogen during the solidification of the metal due to the decrease in solubility of hydrogen gas when the solidification of the liquid metal begins under controlled gas pressure. In this method, axial or radial pore orientation can be achieved by controlling the heat dissipation direction. Porosity, pore size, morphology, pore orientation can be easily controlled during the solidification. These things are controlled by gas pressure, solidification rate, pouring temperature, total solidification gas pressure, solidification cooling rate, and solidification cooling direction. Among these controllable parameters, gas pressure plays important role in deciding the pore size in the metallic foam. The researcher implemented two conditions: 1) only pure hydrogen gas was used. In this case, a decrease in porosity occurred with an increase in gas partial pressure (Pair); 2) the total gas pressure (Ptotal) remained constant, while the porosity increased with increasing partial pressure of hydrogen gas (PH) [18]. An increase in solidification pressure has a negative effect on the pore size – its diameter decreases. The pore diameter varies from 10 µm to 10 mm, and the porosity ranges from 5 to 75 %. Due to the fact that the material contains small and large pores, the distribution is uneven [19]. Producing metal foam by investment casting method Investment casting is also used to produce metallic foams. In this case, no foaming agents are required; instead, open-cell polymer foam is used. Usually, polyurethane is used to fabricate a model. Polyurethane is a linear covalently bonded polymer with relatively long, flexible, soft chain segments joined at the ends. Open cells are filled with the heat resistant resin, such as plaster slurry, a mixture of mullite and calcium carbonate, which is dissolved in water after the polymer foam model is filled with plaster slurry and left to dry. Plaster slurry act as space holder in the polymer foam. During subsequent heating, the polymer foam burns out, leaving a porous cavity in the mold [20, 21]. Further molten metal is poured into the mold cavity under pressure to fill all areas of the mold. The mold is additionally vacuumed. After crystallization of the melt, the mold is irrigated with water; the plaster is soaked and washed off. The porosity in the metallic foam obtained in this way ranges from 80 to 97 %, and the pore size varies from 4 to 0.5 mm [22, 23]. The mechanical properties of metal foam produced by modified investment casting are higher than those of reticulated foam; the permissible compressive load is significantly higher than that of reticulated metallic foam [24]. Various chemical compositions, high compressive strength with high strain hardening are possible. Mechanical properties are improved by struts [25]. Alfredo suggested that the external cell size is greater than 0.05 mm, so the resulting foam should be less defective. He used logistic regression to optimize the process parameters and obtain the best aluminum foam [26]. Producing metal foam by deposition technique The technology of solid phase deposition is similar to the technology of investment casting. In both cases, neither foaming agents nor gas injection into the molten metal are required. The polymer foam is electroplated with dissolved metal ions, and then the polymer foam is replaced with molten metal. Galvanic deposition on polymer foam requires some electrical conductivity of the initial polymer foam. To make
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