A rapid, low-cost method of producing metal foams involves direct injection of gas in to molten metal. It is necessary to increase the viscosity of the metal: if gas was directly added to molten aluminium, it would simply bubble through the melt and escape. One approach to this problem, developed by Alcan International and CYMAT Corporation, is to add a dispersion of fine particles of a refractory material to the molten metal. Addition of 5-15% by volume of particles smaller than 20µm makes the molten metal sufficiently viscous for molten foams to be stable against collapse. It appears that this is partly due to enhanced viscosity of the melt, and partly due to the particles acting as surfactants on the metal/gas interface.

The choice of material is restricted - many non-metallic reinforcements will react with molten metals. Alumina, boron carbide, silicon nitride and boron nitride are all suggested as possible materials, but silicon carbide is the only material used in practice. Although silicon carbide will react with molten aluminium to form aluminium carbide and silicon, it has been established that the rate of reaction can be reduced to an acceptable level by holding the melt at a relatively low temperature during mixing, coating the particles, and inhibiting the reaction by raising the Si content of the aluminium. A rotating dispersing impeller is used to shear the melt and the particles past each other, ensuring wetting between the particles and the metal without creating a vortex (which would draw gas into the melt).

The production process is shown in Figure 1. In a preliminary stage (not shown), the ceramic particles are dispersed within the melt. Gas is then added under the surface of the melt using a rotating impeller designed to produce small bubbles. The foam which forms on the surface is drawn off, rolled slightly to form flat sheets, and cooled.

Figure 1: Continuous production of metal foam sheet by direct addition of gas to a melt with enhanced viscosity.

This is the cheapest of all the methods of foam production, and the only one to have been developed as a continuous process (by CYMAT™ corporation). Foam panels can be produced at rates of up to 900 kg/hour. The main disadvantage of this process is the poor quality of the foams produced is poor. The cell size is large and often irregular, and the foams tend to have a marked density gradient. Although various methods have been developed to improve the drawing off of the foam, including vertical solidification to avoid settling of the foam towards the base, the shape of the foam and the size distribution of the pores is still difficult to control (as shown in Figure 2). The cell structure is influenced by the gas flow rate, the viscosity and temperature of the melt, the mechanism used to draw off the foam, the compression required to form the semi-liquid foam into a useful shape, and the design and rotation speed of the impeller.

Attempts have been made to use alloys which do not require the addition of solid particles. In one case, a 91% Mg-9% Al alloy was stirred rapidly at a temperature between the solidus and liquidus temperatures, producing a viscous dendritic melt, to which argon gas was added. In general, such attempts have been unsuccessful, only producing metals with up to 20% porosity. The main difficulty lies in maintaining a uniform temperature within a very narrow range. If the temperature is too low the melt is too viscous to process, and if the temperature is too high the dendrites in the melt do not grow sufficiently quickly, and the melt is not sufficiently viscous to trap the gas. It can also be difficult to produce dendrites which are sufficiently fine to generate a slurry on the scale of the cell wall thickness.