Porous Metals
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has proved inadequate and may mislead the designer. Powder Metallurgy (P/M) Porous metals formed by powder processing are rightly ranked as the main class of porous metals. The manufacturing process primarily consists of powder synthesis, compacting, and sintering the structural constituents, namely powder particles, chips, fibers, wire meshes or sheets, or their combinations. Powder sintering is one of the simplest methods of making porous metals. The base powder is blended with another powder that contains one or more alloys of the base metal and whose composition is such that a liquid phase forms below the melting point of the base powder. The blend is sintered in a non-oxidizing atmosphere at normal pressure. Porosity in such metals may be as high as 60%. To increase porosity, pore-forming agents like carbonyl are frequently added to the blend. Such agents decompose and are thus removed during sintering. An unsintered "green" part with the desired shape is formed without applying pressure and then heated to the eutectic point. The carbon dioxide which evolves increases the porosity in the part up to 90%. In making porous tungsten, 2-15% iodine fluoride is added to the blend as the pore-forming agent to assure a more uniform pore distribution and a greater fraction of open pores. In sintering tungsten or molybdenum, copper may serve as a pore-forming agent to be later leached by nitric acid at low pressure. Sometimes a highly soluble mineral like rock salt is added to the blend to be removed by water or acid leaching after sintering. Slip foaming consists of making a dense suspension of powder in a liquid, foaming it, and baking the foam. A variation of the method includes making air-in-water foam containing surfactants and foaming agents, charging it with the metal to 30300% of the foam weight, adding a stabilizer capable of polymerization on contact, and finally sintering. Sintering is accompanied by pyrolysis of the stabilizing agent, surfactants, and foaming agents.
Sometimes a liquid organic binder is used for making metal powder suspensions. Furthermore, a particulate organic material with an appropriate particle size is added to act as a foaming agent. The mixture is cured and heated to decompose the binder and organic material without disintegrating the molded part. As heating continues, the carbon is removed as carbon dioxide during powder sintering. Slip casting involves pouring slip into a porous mold to be dried and baked to produce a green part cast to the desired shape. This method is suited to making cellular metals with up to 90% porosity. Plastic foams, e.g., polyurethane, are commonly used for making molds. The mold is cut to dimension and impregnated with the slip by immersion. The piece is then sintered to provide the desired porosity. Fiber metallurgy offers several advantages over powder methods. At a given porosity, fiber-derived materials are superior to powder-processed analogs in strength and impact resistance. Fibers are compacted a preset amount, and green compacts are sintered i
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