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Metallic foams

Tin, lead and bismuth powders are used as compacted metal powders in the production of metallic foams. The metal powders are mixed with foaming agents to give a completely homogeneous mixture. The result of the subsequent densification step is a foamable material that, upon heating to temperatures within the range of the melting point, expands into a highly porous cellular solid with a closed pore structure. The typical structure of lead foam is shown in figure below.

Figure 1. Optical micrograph of cross-section of lead foam. [1]

More details on production of metallic foam ^[3]

In the past, metal foams were obtained by adding a foaming agent to a molten metal when the viscosity of the melt is appropriately established. The foaming agent is usually a powder of metal hydride such as TiH2, which decomposed forming hydrogen when heated to a temperature above 400°C. As the foaming agent comes into contact with the molten metal, the latter instantly decomposes so that there is insufficient time to accomplish a homogeneous distribution of the gas-forming particles. Therefore, this process is difficult to control.

The advanced production method for production of non-ferrous foams uses conventional foaming agents and metal powders. Commercially available powders (such as aluminum, zinc, tin, or lead) are mixed with the foaming agent by conventional means, such as a drum-type mixer. Subsequent to mixing, the powder blend is compacted to give a dense, virtually non-porous body near to TMD. Several compaction methods can be employed such as uniaxial pressing, powder extrusion and even roll compaction. When this compact is heated to a temperature around the melting point, it expands into a highly porous cellular body with a closed-pore structure. This implies that each particle of the foaming agents is embedded in a gas-tight metallic matrix so that the formed gas cannot escape via interconnected pores. By heating this material to the melting point, the foaming process is initiated so that it is alsopossible to obtain complex-shaped foamed articles.

Various foaming agents have been shown to yield good results for this purpose. Among the foaming agents used are metal hydrides such as titanium, zirconium and magnesium hydrides, which have decomposition temperatures between 280 and 600°C.

Several compaction methods can be employed that range from uniaxial compaction to isostatic pressing even to injection molding and extrusion.

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Manufacture of Bismuth

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