The new science of metallic foams is growing rapidly, in both the scientific research community and in industrial applications. Several methods now exist for foaming metals. One of these was invented a few years ago at the Fraunhofer-Institute in Bremen [1, 2]. The foam is fabricated from a metal powder, often aluminium, which is mixed with a blowing agent that is chosen to release gas close to the melting point of the metal, e.g. 99.5% aluminium powder and 0.5% titanium hydride powder. This powder mixture is processed to give a dense precursor material which is then heated up to the melting point of the metal. As the metal starts to melt, the blowing agent releases gas and the mixture expands. The resulting foam is then cooled to freeze the structure, resulting in a solid foam. Figure 1 shows an example of such a foam, which can easily be fabricated inside a mould, leading to the possibility of reduced post-processing. After the expansion phase therefore, the foamed liquid metal undergoes simultaneous liquid drainage and cooling. The liquid drainage, due to gravity, introduces inhomogeneity into the structure, which is generally undesirable in view of the uniform properties required in the solidified structure. If it continues for too long, rupture and collapse of the bubbles will occur. These mechanisms are prevented if the freezing process is rapid enough. Freezing fronts move inwards through the sample, arresting the drainage process. In the model described below we estimate their velocity in relation to the velocity of drainage. Figure 1 shows that uniform foams can currently be fabricated. We wish to define the physical and material parameters which will allow other materials to be foamed with this process.
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