OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 polymer foam electrically conductive, a thin conductive layer is applied to its surface using electrolytic deposition. This process is suitable for a limited number of materials. The low deposition rate and nonuniform pattern of deposition are the reason for the low mechanical properties of the resulting material. For this reason, another method uses chemical vapor deposition to produce metal foam [23]. Producing metal foam by powder metallurgy Powder metallurgy processes also used to manufacture the metallic foam. In this case, instead of molten metal, metal powder is used and other processes are involved. These include: Fraunhofer process, gas entrapment, foaming of slurries. Metal powder used to produce the metallic foam whereas we have seen that molten metal has been used in the above technique. In this several other techniques are used to manufacture the metal foam. These techniques are Fraunhofer processes, Gas entrapment method, Foaming of slurries. Molten route method is commonly used method and more popular one, but it also dominates in terms of quality. Sandwich panels can be manufactured using this method. All methods are characterized by three stages: mixing the metal powder with a foaming agent or space holder, compacting the metal powder to provide shape and strength, and sintering the compact shape at its recrystallization temperature. Fraunhofer processes This method is not as popular as the melt route one. Advantages of the Fraunhofer method over the melt route method/processes are: the ability to produce a product of complex shape and size, as well as better control of the porous structure. Metallic foam manufacturing begins with mixing metal powder with the suitable foaming agent. The preparation of the metal powder makes the Fraunhofer method expensive, and its storage requires certain conditions. When mixing metal powder and foaming, it is necessary to achieve its uniform distribution. The resulting mixture is compacted by pressing. The compaction process can be realized by hot isostatic pressing, hot compaction, extrusion and powder mixture rolling. The choice of compaction method depends on the desired final shape. The material should be compacted before plastic deformation of the powder particles begins. Compactedmetal powder contains small pores and cracks, which can hinder foaming processes [27]. Heat treatment is carried out at a temperature below the temperature of the metal powder melting point. When choosing a foaming agent, it is important to ensure that its melting point is lower than the melting point of the metal powder. During heat treatment, the foaming agent, which is uniformly distributed throughout the melt, decomposes. The released gas forces the source material to expand, forming a highly porous structure. In a semi-solid metal, expansion occurs quickly, but the bubbles collapse, so rapid cooling is required to fix the foam structure. The stability of aluminum alloy foam can be improved by adding Mg to the powder mixture. After the Mg addition, Al2O3 particles at the interface are more fully incorporated into the cell walls [28]. The controlled process parameters are: the foaming agent content, temperature and heating rates. These are common in the production of metal foam. Further, the percentage of porosity and relative density required will depend on the process parameter. TiH2 is the most popular foaming agent because its melting point is close to that of aluminum alloy. Titanium hydride is expensive and dangerous to handle due to the risk of fire due to the release of hydrogen gas during the decomposition of TiH2, so another foaming agent, calcium carbonate, was proposed [29]. The characteristics of CaCO3 and TiH2 as foaming agents were determined. It was found that metal foam using CaCO3 has a finer and more homogeneous pocellre structure [30]. CaCO3 is used as a foaming agent in the production of foams from magnesium alloys. However, it is impossible to produce foam metal with adequate structure using only Mg and CaCO3 alone, because CaCO3 may decompose to release CO2 before Mg melts, which will lead to two consequences: the starting material may crack and a reaction may occur between the released CO2 and Mg. Therefore, it is necessary to reduce the melting temperature of the starting material by adding Al and Zn so that CaCO3 reacts with molten Mg and cracking of the material does not occur [31].
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