Metallic foams mimic natural cellular materials like cork, bone and coral which, as a result of their structure, tend to have a very high compression strength and stiffness by comparison to their low weight. This makes them promising materials for the automotive and aerospace industries, where they protect against heat, radiation, noise and vibration, and have good energy absorption characteristics which could be very useful in the event of a crash or hard landing. Metal foams have also been used as an experimental prosthetic in animals.
Metallic foams, which are basically a liquid metal and gas mixture that is then solidified, are a relatively new class of materials, produced using a variety of methods that were first proposed in the 1950s. The first method involves pouring molten metal into an open-celled polyurethane foam skeleton. The second method involves adding ceramic powder or an alloying element to a molten metal to make it more viscous, so that bubbles of gas don’t immediately rise to the surface, and then injecting very fine bubbles of gas (air, nitrogen, argon) into it. A third method also relies on viscous molten metal with a gas-releasing ‘blowing agent’ (e.g. titanium hydride) added to the mix that produces bubbles of gas in-situ as it decomposes. Foamed metals can also be produced from metal powders: the powder is mixed with a blowing agent and compacted, then heat treated so that metal sinters together and the blowing agent decomposes and produces gases that give the finished product a porous structure.
Composite metal foams (CMFs) are another variety of metal foam where the size of the gas-filled pores, and therefore the material’s mechanical and structural properties, can be controlled. In composite foams, uniform hollow spheres of one metal (e.g. stainless steel) are distributed within a matrix made of another metal (e.g. aluminium), and combined by casting or a powdered metallurgy technique.