OBRABOTKAMETALLOV Vol. 25 No. 4 2023 technology Cylinder-shaped metal foam is manufactured by hot powder extrusion (press machine) and foaming, which attempts to determine the relative density and deformation energy of the foam. Aluminum alloy powder with a foaming agent is heated above its melting point in a foaming mold and a relative density of 0.22 is obtained. The density of metal foam decreases as the molding speed increases [32]. Aluminum foam manufactured by this method has a homogeneous cell structure and a relative density 20 % less than solid aluminum, while the relative density of steel foam is 40 % less than solid steel. This porosity helps during deformation, that is, it absorbs mechanical energy by collapsing the cell pores [33]. Gas entrapment This concept was developed by Martine. The process is rarely used because of the complex processing involved. The mixture of base material is enclosed in a shell, the air is replaced with α-stabilizing gas, such as argon, and it is sealed. The filled shell is then placed in a gasostat and subjected to hot isostatic compression, thereby achieving a density of 95 %. At the next stage, the semi-finished product is rolled and, at the final stage, heated in a furnace. Since the shell is sealed, the argon contained in the space between the powder particles begins to expand and further consolidate the particles of the base metal from the inside, forming partitions of the future foam. The porosity of the powder obtained in this way reaches 50 %, and the pore diameter varies from 6 to 10 µm. Low porosity and various cell size are significant disadvantages of this method [34]. Space holder method In powder metallurgy, space holder method gives maximum control over the shape, pore size, porosity distribution. In this method metal powder is mixed with space holder material along with binder, which provides strength to green powder during compaction. Mixing time can range from 1 to 4 hours; actually it depends on how long it takes to mix to a homogeneous state. It is necessary to ensure that the metal powder is mixed until homogeneous, otherwise the cell size and percentage of porosity will be less. It is necessary to select a space holder in such a way that it can easily evaporate during sintering (1), not react with the metal (2), is easy to process (3), and there should be no residue left after processing. To manufacture aluminum foam, sintering in an electric furnace and plasma-spark sintering are used, and for the production of copper foam, carbonate-free sintering is used [35]. In biomedical implants, space holder residues in metal foam are a serious problem. Therefore, sodium chloride is widely used as a space holder in the manufacture of titanium biomedical implants, since it is easily removed when dissolved in water. According to literature data, the maximum porosity in steel foam is 60%. D.P. Mondal tried to increase the porosity of stainless steel foam to 80 % using ammonium bicarbonate as a space holder. Pore size, porosity and relative density depend significantly on the sintering temperature; at 1,100 °C the cell size is the same as the size of the space holder particles, if sintering occurs at a higher temperature than this, the pore walls will become permeable, and hence the porosity can be reduced. Other space holders (carbide, sodium chloride, tapioca starch, magnesium) were used to produce open-cell titanium foam [36]. Nidhi Jha used NaCl powder as a space holder with a particle size seven times larger than the particle size of titanium powder, this ensured that the titanium powder would be completely dispersed around the NaCl powder, making it possible to obtain uniform porosity throughout the titanium foam. The thickness of the cell wall increased as the amount of titanium in the powder mixture increased. Cell size and porosity varied depending on NaCl size and the ratio of components in the mixture [37]. The researcher identified some parameters that affect the cell pore size, porosity and strength, etc. These parameters were the mixture composition, sintering temperature and pressing pressure. The small pore size, its uniform distribution and spherical shape give metal foam better mechanical properties. However, it is difficult to control these parameters during processing [38]. Aluminum fractions affect the relative density and compressive strength. Mechanical properties can be improved by increasing the ratio of cell wall thickness to cell wall length by reducing the cell size. The density of the foam can also be controlled by the amount of NaCl. Sazegaran studied the effect of the chromium amount on the density of
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