A metal foam is a cellular structure consisting of a solid metal, frequently aluminium, 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 making these ultralight materials. The strength of foamed metal possesses a power law relationship to its density; i.e., a 20% dense material is more than twice as strong as a 10% dense material.
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 thermal conductivity will likely be reduced.
Open-cell metal foams
Open celled metal foams are usually replicas using open-celled polyurethane foams as a skeleton 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. 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.
Metallic foams are used in the field of compact heat exchangers to increase heat transfer at the cost of an additional pressure drop. However, their use permits substantial reduction in the physical size of a heat exchanger, and so fabrication costs. To model these materials, most works uses idealized and periodic structures or averaged macroscopic properties.
Closed-cell metal foams
Closed-cell metal foam was first reported in 1926 by Meller in a French patent where foaming of light metals either by inert gas injection or by blowing agent was suggested. The next two patents on sponge-like metal were issued to Benjamin Sosnik in 1948 and 1951 who applied mercury vapor to blow liquid aluminium.
Closed-cell metal foams have been developed since about 1956 by John C. Elliott at Bjorksten Research Laboratories. Although the first prototypes were available in the 50s, commercial production was started only in the 90s by Shinko Wire company in Japan. 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 an identical non-porous alloy; commonly those of aluminium) and stiff, and are frequently proposed as a lightweight structural material. However, they have not yet been widely used for this purpose.
Closed-cell foams retain the fire resistant and recycling capability of other metallic foams but add the ability to float in water.
Manufacturing Routes for closed cell metal foams
Metal foams are commonly made by injecting a gas or mixing a foaming agent into molten metal. Under certain circumstances metallic melts can be foamed by creating gas bubbles in the liquid. Normally, gas bubbles formed in a metallic melt tend to quickly rise to its surface due to the high buoyancy forces in the high-density liquid. This rise can be hampered by increasing the viscosity of the molten metal, either by adding fine ceramic powders or alloying elements to form stabilizing particles in the melt or by other means. Metallic melts can be foamed in one of three ways:
- by injecting gas into the liquid metal from an external source;
- by causing an in-situ gas formation in the liquid by admixing gas-releasing blowing agents to the molten metal;
- by causing the precipitation of gas which was previously dissolved in the liquid.
In order to stabilize the molten metal bubbles, high temperature foaming agents (nano- or micrometer- sized solid particles) are required. The size of the pores, or cells, is usually 1 to 8 mm. When foaming or blowing agents are used, they are frequently mixed to the metal in the solid state at a powder form. This is the so-called "powder route" of foaming and it is probably the most established from an industrial point of view. After metal (e.g. aluminium) powders and foaming agent (e.g.TiH2) have been mixed, they are compressed into a compact, solid precursor, which can be available on the form of a billet, a sheet or a wire. Production of precursors can be done by a combination of materials forming processes, such as powder pressing,extrusion (direct or conform), flat rolling.
Foam metal has also begun to be used as an experimental prosthetic in animals. In this application, a hole is drilled into the bone and the metal foam inserted letting the bone grow into the metal for a permanent connection. For orthopedic uses, foams from metals such as tantalum or titanium are often used, as these metals exhibit high tensile strength, corrosion resistance with excellent biocompatibility.
Clinical studies on mammals
A notable example of clinical use of metallic foams is a Siberian Husky named Triumph whose both back legs received foam metal prostheses. Studies on mammals have shown that porous metals, such as titanium foam, may allow the formation of vascular systems within the porous area.
Orthopedic use in humans
More recently, orthopedic device manufacturers have started producing devices that use either foam construction or metal foam coatings to achieve the desired levels of osseointegration.
Metallic foams are currently being looked at as a new material for automobiles. The main goal of the use of metallic foams in vehicles is to increase sound dampening, reduce the weight of the automobile, and increase energy absorption in case of crashes, or in military applications, to combat the concussive force of IEDs. As an example, foam filled tubes could be used as anti-intrusion bars.
The metallic foams that are currently being looked at are aluminium and its alloys due to their low density (0.4–0.9 g/cm3). In addition these foams have a high stiffness, are fire resistant, do not give off toxic fumes, are fully recyclable, have high energy absorbance, have low thermal conductivity, have low magnetic permeability, and are efficient at sound dampening, especially in comparison to light weight hollow parts. In addition partial addition of metallic foams in hollow parts of the car will decrease weakness points usually associated with car crashes and noisy vibrations. These foams are cheap to cast by using powder metallurgy (as compared to casting of other hollow parts).
In comparison to polymer foams (for uses in automobiles), metallic foams are stiffer, stronger, and more energy absorbent. They are more fire resistant, and have better weathering properties when considering UV light, humidity, and temperature. However, they are heavier, more expensive, and non-insulating.
Metal foam technology has also been applied in the treatment of the automotive exhaust gas. Compared to the traditional catalytic converter that uses cordierite ceramic as substrate, the metal foam substrate can offer better heat transfer and exhibits excellent mass-transport properties (high turbulence) offering possibilities for using less platinum catalyst.
- Compare Materials: Cast Aluminium and Aluminium Foam. Makeitfrom.com. Retrieved on 2011-11-19.
- F. Topin, J.-P. Bonnet, B. Madani, L. Tadrist (2006). "Experimental Analysis of Multiphase Flow in Metallic foam: Flow Laws, Heat Transfer and Convective Boiling". Advanced Engineering Materials 8 (9): 890–899. doi:10.1002/adem.200600102.
- Banhart, J. (2001). "Manufacture, Characterization and application of cellular metals and metal foams". Progress in materials Science 46 (6): 559–632. doi:10.1016/S0079-6425(00)00002-5.
- DeGroot, C.T., Straatman, A.G., and Betchen, L.J. (2009). "Modeling forced convection in finned metal foam heat sinks". J. Electron. Packag. 131 (2): 021001. doi:10.1115/1.3103934.
- M.A. De Meller, French Patent 615,147 (1926).
- B. Sosnick, U.S. Patent 2,434,775 (1948).
- B. Sosnick, U.S. Patent 2,553,016 (1951).
- Banhart, John (2000). "Manufacturing Routes for Metallic Foams". JOM (Minerals, Metals & Materials Society) 52 (12): 22–27. Retrieved 2012-01-20.
- Bonaccorsi, L.; Proverbio, E. (1 September 2006). "Powder Compaction Effect on Foaming Behavior of Uni-Axial Pressed PM Precursors". Advanced Engineering Materials 8 (9): 864–869. doi:10.1002/adem.200600082.
- Shiomi, M.; Imagama, S.; Osakada, K.; Matsumoto, R. (2010). "Fabrication of aluminium foams from powder by hot extrusion and foaming". Journal of Materials Processing Technology 210 (9): 1203–1208. doi:10.1016/j.jmatprotec.2010.03.006.
- Dunand, [editors] Louis Philippe Lefebvre, John Banhart, David C. (2008). MetFoam 2007 : porous metals and metallic foams : proceedings of the fifth International Conference on Porous Metals and Metallic Foams, September 5-7, 2007, Montreal Canada. Lancaster, Pa.: DEStech Publications Inc. pp. 7–10. ISBN 1932078282.
- Strano, M.; Pourhassan, R.; Mussi, V. (2013). "The effect of cold rolling on the foaming efficiency of aluminium precursors". Journal of Manufacturing Processes. doi:10.1016/j.jmapro.2012.12.006.
- "Triumph The Dog Ready To Run With Prostheses".
- Osseointegration with Titanium Foam in Rabbit Femur, YouTube
- Titanium coatings on Orthopedic Devices. Youtube
- Biomet Orthopedics, Regenerex® Porous Titanium Construct
- Zimmer Orthopedics, Trabeluar Metal Technology
- Zimmer CSTiTM (Cancellous-Structured Titanium TM) Porous Coating
- Strano, Matteo (2011). "A New FEM Approach for Simulation of Metal Foam Filled Tubes". Journal of Manufacturing Science and Engineering 133 (6): 061003. doi:10.1115/1.4005354.
- New Concept for Design of Lightweight Automotive Components
- Alantum Innovations in Alloy Foam: Home. Alantum.com. Retrieved on 2011-11-19.
- Development of Metal Foam Based Aftertreatment on a Diesel Passenger Car – Virtual Conference Center. Vcc-sae.org. Retrieved on 2011-11-19.