Foamed Metal – Tomorrow’s Bumper?
November 15, 2007
The dictionary defines foam as a thick mass of bubbles. When thinking about foams it is common to think of sponges, honeycombs or meringues, not metals. Metal foams however have very good energy absorption characteristics with each of the very large number of cell walls being able to attenuate energy, whether it is mechanical, acoustic or electromagnetic. In the case of mechanical energy from an impact the energy is attenuated through the combined crumpling of each of the cell walls. When it is considered that the foam is typically over 80% air, the energy attenuated per mass of metal is quite high, making metal foams quite attractive in applications within automotive, blast mitigation and acoustic isolation systems.
The production of metal foams requires a source of gas for bubbles and a metal system that will retain the foam structure during solidification. The gas for bubbles can be either from a gas liberating powder or direct injection of air. The ability of the metal to maintain the foamed structure is not straight forward as the low viscosity of most metal melts promotes the collapse of the cells prior to solidification.
Early development of metal foams looked at metal systems that provided the required viscosity. However, suitable metal systems often included low melting point metals such as cadmium and mercury, making them unfeasible for broad application. A new approach was developed when a viscosity enhancer was added to a more common metal. United States Patent 3,297,431 issued to Ridgway discusses the addition of a powder to stabilize the foam during solidification. The inclusion of a powder is also discussed in United States Patent 3,300,296 issued to Hardy et al. In both of the above patents the gas required for the formation of bubbles was provided by a powder that liberates a gas at a temperature near the melting temperature of the metal. While not implying they were the first, Hardy et al. used aluminum alloys i.e. an economical metal system as the base metal.
With basic foam structure patents being long expired the current contenders in this emerging field can be considered with respect to the general method of bubble formation i.e. powder system or gas injection. Current patents consider methods of production and specific applications that incorporate metal foam, with the latter group looking to protect particular market spaces, beyond how the foam is manufactured.