For years, materials scientists and engineers have been trying to create porous metals and metal foams based on in an attempt to emulate naturally porous materials, such as bone, coral and cork.
Metal foam is a cellular structure made up of a solid metal containing a large volume fraction of gas-filled pores. These pores can either be sealed (closed-cell foam), or they can be an interconnected network (open-cell foam). The closed-cell foam is referred to as metal foams, while the open-cell foam is referred to simply as porous metal.
Metals that Can Be Used
The metal that is commonly used to make metal foams is aluminium. However, other varieties of metals can be used to make the foam, such as titanium and tantalum.
Properties of Metal Foam
The key properties of metal foam are as follows:
- Ultralight material (75–95% of the volume consists of void spaces)
- Very high porosity
- High compression strengths combined with good energy absorption characteristics
- Thermal conductivity is low
- High strength
Metallic melts can be foamed by creating gas bubbles in the liquid. These gas bubbles in the metallic melt tend to rise to the surface due to the high buoyancy forces in the high-density liquid. In order to prevent this from happening, the viscosity of the molten metal has to be increased. This can be done by adding fine ceramic powders or alloying elements to form stabilizing particles in the melt.
Three ways of foaming metallic melts are listed below:
- Injecting gas into the liquid metal from an external source
- Causing the precipitation of gas that had just been dissolved in the liquid
- Causing an in-situ gas formation in the liquid by admixing gas-releasing blowing agents to the melt
Foaming of Metallic Melts using Gas Injection
Foaming aluminium and aluminium alloys is used by Cymat Aluminium Corporation in Canada and Hydro Aluminium in Norway. Silicon carbide, aluminium oxide, or magnesium oxide particles can be used to enhance the viscosity of the melt.
The mixing techniques should be consistent to ensure uniform distribution of particles throughout the melt. The melt is then foamed by injecting gases, namely, air, nitrogen, and argon into it using rotating impellers or vibrating nozzles.
Foaming of Melts with Blowing Agents
A second method for foaming melts directly is to add a blowing agent to the melt instead of injecting gas into it. Gas is introduced into the melt by using compounds such as hydrides or carbonates.
The compound tends to decompose and forms gas bubbles when heated in a liquid metal or semi-solid pellet. The resulting foam has to be stable so that the porous metals will have uniform pore sizes and densities.
Shinko Wire Company, Amagasaki, Japan, has been producing foams using this method. About 1.5 wt.% calcium metal is added to an aluminium melt at 680°C (1256F°). The melt is mixed well and the viscosity starts to increase due to the formation of calcium oxide, calcium aluminum oxide, or Al4Ca intermetallics. This aids in thickening of the liquid metal.
Once the viscosity reaches the desired value, titanium hydride as a blowing agent is added serving to release hydrogen gas in the hot viscous liquid. The melt then begins to expand slowly and gradually, filling the foaming vessel. The foaming has to take place at constant pressure. When the vessel is cooled below the melting point of the alloy, the liquid foam turns into solid aluminium foam and can be taken out of the mold for further processing.
Solid-Gas Eutectic Solidification
Porous materials formed by solid-gas eutectic solidification are called ‘gasar’, meaning ‘gas-reinforced.’ It has been a known fact that certain liquid metals form a eutectic system with hydrogen gas.
When one of these metals is melted in a hydrogen atmosphere under high pressure of up to 50 atm, the resultant melt is homogeneous and charged with hydrogen. The melt will have a eutectic transition to a heterogeneous solid+gas system when the temperature is lowered. The solid+gas system should have a eutectic concentration, and then a segregation reaction will occur at a specific temperature.
The melt begins to solidify causing gas pores to precipitate and become entrapped in the metal. Mostly elongated pores oriented in the direction of solidification are formed.
The main applications of metal foams and porous metals are listed below:
- The closed variety is used for structural applications requiring load-bearing features, and for weight-saving and impact-absorbing structures in vehicles
- The open variety is ideal for vibration and sound absorption, filtration and catalysis at high temperatures, for heat exchange and in medical devices.
- The open variety is also useful in functional applications such as filtration and damping.
- Foam metal is being used as an experimental prosthetic in animals.
- Metal foams with high strengths can act as high-capacity impact-energy absorbers.
- Automotive industry - the foams reduce the number of parts in the car frame, facilitate assembly, thereby reducing costs and improving performance.