21 Making metal foams

The properties of metal foam and other cellular metal structures depend upon the properties of the metal, the relative density and cell topology (e.g. open or closed cell, cell size, etc.). Metal foams are made by one of nine processes, listed below. Metals which have been foamed by a given process (or a variant of it) are listed in square brackets.

1. Bubbling gas through molten Al-SiC or Al-Al2O3 alloys. [Al, Mg]

2. By stirring a foaming agent (typically TiH2) into a molten alloy (typically an aluminum alloy) and controling the pressure while cooling. [Al]

3. Consolidation of a metal powder (aluminum alloys are the most common) with a particulate foaming agent (TiH2 again) followed by heating into the mushy state when the foaming agent releases hydrogen, expanding the material. [Al, Zn, Fe, Pb, Au]

4. Manufacture of a ceramic mold from a wax or polymer-foam precursor, followed by burning-out of the precursor and pressure infiltration with a molten metal or metal powder slurry which is then sintered. [Al, Mg, Ni-Cr, stainless steel, Cu]

5. Vapor phase deposition or electrodeposition of metal onto a polymer foam precursor which is subsequently burned out, leaving cell edges with hollow cores. [Ni, Ti]

6. The trapping of high-pressure inert gas in pores by powder hot isostatic pressing (HIPing), followed by the expansion of the gas at elevated temperature. [Ti]

7. Sintering of hollow spheres, made by a modified atomization process, or from metal-oxide or hydride spheres followed by reduction or dehydridation, or by vapor-deposition of metal onto polymer spheres. [Ni, Co, Ni-Cr alloys]

8. Co-pressing of a metal powder with a leachable powder, or pressure-infiltration of a bed of leachable particles by a liquid metal, followed by leaching to leave a metal-foam skeleton. [Al, with salt as the leachable powder]

9. Dissolution of gas (typically, hydrogen) in a liquid metal under pressure, allowing it to be released in a controled way during subsequent solidification. [Cu, Ni, Al]

Only the first five of these are in commercial production. Each method can be used with a small subset of metals to create a porous material with a limited range of relative densities and cell sizes. Figure 2.1 summarizes the ranges of cell size, cell type (open or closed), and relative densities that can be manufactured with current methods.

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Relative density

Figure 2.1 The range of cell size and relative density for the different metal foam manufacturing methods

Relative density

Figure 2.1 The range of cell size and relative density for the different metal foam manufacturing methods

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