OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 unsintered and sintered products. When chromium was added to the original powder, the product density initially decreased, but after adding additional chromium amount, the density of the unsintered and sintered material increased [39–40]. Various method and its relative density and porosity Various methods have been developed to manufacture the metallic foam. Different methods achieve different percentage level of porosity and different pore structure, such as open porous and closed porous. Table bellow discusses the various methods, indicating porosity and pore structure, as well as challenges. Porosity, pore structure and challenges of different metallic foam S.no Method Material Foaming agent/ gas/space holder Challenges Pore structure Porosity % Relative density Ref. No. 1 Gas injection method A356 Compressed air, Carbon dioxide, Nitrogen, H2, Argon Fine size bubbles formation, for its uniformly distribution and it is not suitable for oxidizing material Closed cell structure 50 to 80 0.02 to 0.2 [5–8] 2 Foaming method All grade of aluminium TiH2, CaCO3, MgCO3, Mg(OH)2, CaMg(CO3)2, (4MgCO3 ∙ Mg(OH)2 ∙ 5H2O) Bubbles entrapment. to increase the viscosity of molten some are added. Closed cell structure 60 to 86 0.12 to 0.44 [10–16] 3 Solid gas eutectic reaction Ag, Cu, Al, nickel, Chromium Hydrogen gas supplied at 50 atm pressure. It is restricted to the hydrogen gas. Closed and open cell structure 5 to 75 0.12 [17,18] 4 Fraunhofer processes Al (spherical) powder, Ti, Brass TiH2, CaCO3 Material should have high affinity towards other material Closed and open cell structure 75 to 95 0.22 [27–34] 5 Gas entrapment Titanium powder Argan gas Complicated set up required Closed and open cell structure 50 0.16 to 0.22 [34] 6 Space holder method Stainless steel, mostly used for titanium NaCl, Urea, tapioca starch Selection of space holder, which easily removed both 80 0.22 to 0.3 [35–38] Conclusion Due to its’ mechanical and thermal properties, metallic foams can be widely used in the aerospace and automotive industries. Many methods have been developed to manufacture metal foam, but the most popular is the melt route method. Aluminium alloy is mainly used to manufacture a metallic foam by melt route method. TiH2 is commonly used for foaming, but due to its high decomposition rate and cost, another foaming agent, CaCO3, has been proposed. This foaming agent is most suitable for aluminium alloys because it does not require any stabilizer. It is possible to develop another new foaming agent that does not require a stabilizer. The melt route method is not widely used in the production of stainless steel foam because the melting point of stainless steel is quite high. In powder metallurgy, space holder method is used to manufacture the biomedical implants and engineering equipment. Typically, titanium foam is made by space holder method. The advantage of this method is that pore size, porosity and relative density can be controlled by the size, shape and volume fraction of the space holder. Other foams are also manufactured using this method. This method is not limited to titanium metal only.
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