Aluminium Foam

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, to protect against heat, radiation, noise and vibration. They have good energy absorption characteristics which could be very useful in the event of a crash or hard landing. Metal foams have also been tested as an experimental prosthetic in animals, as their porous structure enables good connection and bonding with bone.

Metallic foams are a relatively new class of materials, first conceived in the 1950s. In open-cell foams, the pores are interconnected, allowing fluid to flow through the material. In closed-cell foams, pores don’t touch and the material is effectively sealed shut. 

Open-cell metallic foams are usually made by replicating a piece of polyurethane foam through evaporative-pattern casting, a form of investment casting. The soft polyurethane foam is coated in ceramic to create a negative mould then vaporised, and liquid metal is then poured into the empty gaps. 

Closed-cell metallic foams are made by introducing gas bubbles to liquid metals. Because they are much less dense than the liquid metal, gas bubbles are extremely buoyant, so the trick isn’t really in creating the bubbles in the metal, but in keeping them inside of it. This can be achieved by adding ceramic powder or an alloying element to the molten metal: by making it more viscous, the rise of the bubbles is slowed down. To create the gas bubbles, you can inject gas (air, nitrogen, argon) directly from an external source, or add a gas-releasing ‘blowing agent’ (e.g. titanium hydride) to the metal, which produces bubbles 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.

This sample is a 30 pores per inch, or 11.8 pores per cm, open-celled aluminium foam.