Metal foams: Production and stability

Liquid metal foam can be generated by creating gas inside a liquid or semi-liquid metallic melt which has been pre-treated in a suitable way. Such foams are self-supporting disordered structures in complete analogy to aqueous foams. The stabilisation mechanism of metallic foams is still not fully understood. In order to facilitate discussion of foam stabilisation the various methods for making such foams are classified with respect to the mode, of, gas generation and the type of melt used for foaming. Metal foams can be produced by gas injection from an external source or by in-situ nucleation of gas bubbles in, the melt for which various possibilities are known. Melts amenable to foaming range from almost pure molten alloys to melts to which particles have been added, formed by in-situ reactions or which have been already present in the solid precursor prior to melting. Some of the available experimental evidence for the action of stabilising particles in metallic foams is presented. It is found that although stabilisation seems to be based on the presence of a solid constituent in all the foaming processes, the mechanisms might vary. At present the nature of stabilisation is not yet fully understood and many questions remain open.