Foams are commonly made by injecting a gas or mixing a foaming agent into molten metal. Molten metal can be foamed by creating gas bubbles in the material. Normally, bubbles in molten metal are highly buoyant in the high-density liquid and rise quickly to the surface. This rise can be slowed by increasing the viscosity of the molten metal by adding ceramic powders or alloying elements to form stabilizing particles in the molten metal, or by other means. Molten metal can be foamed in one of three ways:
* by injecting gas into the liquid metal from an external source;
* by causing gas formation in the liquid by admixing gas-releasing blowing agents with the molten metal;
* by causing the precipitation of gas that was previously dissolved in the molten metal.
To stabilize the molten metal bubbles, high temperature foaming agents (nano- or micrometer- sized solid particles) are required. The size of the pores, or cells, is usually 1 to 8 mm. When foaming or blowing agents are used, they are mixed with the powdered metal before it is melted. This is the so-called "powder route" of foaming, and it is probably the most established (from an industrial standpoint). After metal (e.g. aluminium) powders and foaming agent (e.g. TiH) have been mixed, they are compressed into a compact, solid precursor, which can be available in the form of a billet, a sheet, or a wire. Production of precursors can be done by a combination of materials forming processes, such as powder pressing, extrusion (direct or conform) and flat rolling.