As with plastics one can also process aluminium to a type of foam by adding a foaming agent (which decomposes to form a gas) to the molten metal. Metal hydrides (compounds, for example of titanium and zirconium with hydrogen) are mainly used as the foaming agent. In practice, powder of alloys of aluminium with magnesium or silicon are mixed together with a foaming agent (up to ten per cent by weight) and melted under pressure. After relieving the pressure, bubbles form, the diameters of which increase in size from some 0.4 millimetres in the centre of the melt to six millimetres at the outside. The bubble formation is accompanied by an expansion, which on reaching the desired volume is brought to a halt by quenching in water. The density of the cooled solid foam is between 200 and 650 kilograms a cubic metre (that of normal aluminium is 2700 kilograms a cubic metre). Aluminium foam floats on water. Its strength decreases with decreasing density; the material can be processed like wood but it cannot be welded. Possible applications are as the core of sandwich panels in building construction, as floating bodies, in impact-absorbing elements, such as in cars, and, in open-pored form, as filters or heat shields.
Sponge aluminium is simpler and cheaper to make, but more than twice as heavy with a density of 1200 to 1500 kilograms a cubic metre. A melt of an aluminium alloy containing magnesium or silicon is cast onto expanded clay or glass-foam balls, where it forms a sponge-like metal skeleton containing filler material. Planned applications are cheap impact-absorbing elements for cars and overhead cranes.