24Gasreleasing particle decomposition in semisolids

Foaming agents can be introduced into metals in the solid state by mixing and consolidating powders. Titanium hydride, a widely used foaming agent, begins to decompose at about 465°C, which is well below the melting point of pure aluminum (660°C)i/and of its alloys. This raises the possibility of creating a foam by dispersing the foaming agent in solid aluminum using powder metallurgy processes and then raising the temperature sufficiently to cause gas release and partial or full melting of the metal, allowing bubble growth. Cooling then stabilizes the foam. Several groups, notably IFAM in Bremen, Germany, LKR in Randshofen, Austria, and Neuman-Alu in Marktl, Austria, have developed this approach.

A schematic diagram of the manufacturing sequence is shown in Figure 2.4. It begins by combining particles of a foaming agent (typically titanium hydride) with an aluminum alloy powder. After the ingredients are thoroughly mixed, the powder is cold compacted and then extruded into a bar or plate of near theoretical density. This 'precursor' material is chopped into small pieces, placed inside a sealed split mold, and heated to a little above the solidus temperature of the alloy. The titanium hydride then decomposes, creating voids with a high internal pressure. These expand by semi-solid flow and the aluminum swells, creating a foam that fills the mold. The process results in components with the same shape as the container and relative densities as low as 0.08. The foam has closed cells with diameters that range from 1 to 5 mm in diameter.

IFAM, Bremen, have developed a variant of the process, which has considerable potential for innovative structural use. Panel structures are made by first roll-bonding aluminum or steel face-sheets onto a core-sheet of unex-panded precursor. The unexpanded sandwich structure is then pressed or deep-drawn to shape and placed in a furnace to expand the core, giving a shaped, metal-foam cored sandwich-panel. Only foamed aluminum is commercially available today, but other alloy foams are being developed using different foaming agents.

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