metal foam : tomorrow’s bumper?
The dictionary defines a foam as a thick mass of bubbles. Sponges, honeycombs or meringues come to mind. Metals do not. However, metal foams have some very interesting properties, particularly when their density is factored into the equation.
Metal foams are good at dissipating energy. We defined a foam as a thick mass of bubbles. Each of these bubbles may be considered a “cell”, where a closed cell system will have walls between the cells. Each of these walls can deform, attenuating energy. If one adds the energy attenuated by every cell wall the foam can dissipate considerable energy. Now, considering a foam is typically over 80% air, the energy attenuated per mass of metal is high.
What type of energy? It might be mechanical energy, as may be associated with an impact or blast pressure wave, or it might be acoustic energy i.e. sound waves, to give two examples. It may also be electromagnetic energy as these are metal-based systems.
Fabricating a metal foam requires a source of gas to create the bubbles and a metal system that will retain the foam structure during solidification. The gas is often provided by either a gas liberating powder or direct injection of a gas e.g. air. The ability of the metal to maintain the foam structure is not straight forward as the low viscosity of most metal melts allows the cells to collapse before solidification.
a few patents
Early development looked for metal alloys having the required viscosity. Suitable metal systems often included low melting point metals such as cadmium and mercury, making them unfeasible for broad application. In a newer approach, a viscosity enhancer is added to a common, low viscosity metal melt. 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.
These early patents around aspects of the basic foam structure and fabrication processes have long since expired. That said, refinements in both gas liberating or gas injection methods are being patented. One can also envision fabrication methods associated with specific structures or specific structures for given applications. It could all become quite interesting should this “niche” material system solve large scale, commercial problems.