Metal foam

A metal foam is a cellular structure consisting of a solid metal - frequently aluminum - containing a large volume fraction of gas-filled pores. The pores can be sealed (closed-cell foam), or they can form an interconnected network (open-cell foam). The defining characteristic of metal foams is a very high porosity: typically 75-95% of the volume consists of void spaces. The strength of foamed metal possesses a power law relationship to its density; i.e. 20% dense material is more than twice as strong as 10% dense material (Ceteris paribus).

Metallic foams typically retain some physical properties of their base material. Foam made from non-flammable metal will remain non-flammable and the foam is generally recyclable back to its base material. Coefficient of thermal expansion will also remain similar while conductivity will likely be reduced.

Open-cell metal foams

Open celled metal foams have a structure similar to open-celled polyurethane foams and have a wide variety of applications including heat exchangers (compact electronics cooling, cryogen tanks, PCM heat exchangers), energy absorption, flow diffusion and lightweight optics. Manufacturers include ERG Aerospace (tradename Duocel), Metal Foam Korea, M-Pore, Porvair, Metafoam and Recemat Due to the high cost of the material it is most typically used in advanced technology aerospace and manufacturing.

Extremely fine-scale open-cell foams, with cells too small to be visible to the naked eye, are used as high-temperature filters in the chemical industry. Manufacturers include Inco Ltd (trade name Incofoam).

Closed-cell metal foams

Closed-cell metal foams have been developed since about 1990, and are commonly made by injecting a gas or foaming agent into molten metal. The size of the pores - or 'cell size' - is usually between 1 mm and 8 mm.

Closed-cell metal foams are primarily used as an impact-absorbing material, similarly to the polymer foams in a bicycle helmet but for higher impact loads. Unlike many polymer foams, metal foams remain deformed after impact, and can therefore only be used once. They are light (typically 10-25% of the density of the metal they are made of, which is usually aluminum) and stiff, and are frequently proposed as a lightweight structural material. However, they have not yet been widely used for this purpose. They are currently manufactured by Cymat Corporation (Canada), the Shinko Wire Company (Japan) and Alcoa (USA).

Closed-cell foams retain the fire resistant and recycling capability of other metallic foams but add an ability to float in water (for densities less than 1g/cc).

Interestingly, naturally formed Aluminum foams have been found in the Oklo reactor, apparently formed by the combination of the reactor's extreme heat and the release of stable xenon gas.

Foam metal has also begun being used as a expermental prosthetic in animals.Where a hole is drilled into the bone and the metal foam inserted leting the bone grow into the metal for a perment connection. Most notable example is a Siberian Husky named Triumph where both back legs have prosthetics of foam metal done by Dr. Robert Taylor.


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