Metal foam

Foamed aluminium
Regular foamed aluminium

A metal foam is a cellular structure consisting of a solid metal, frequently aluminium, as well as 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 remains non-flammable and is generally recyclable back to its base material. Its coefficient of thermal expansion also remains similar while thermal conductivity is likely reduced.[1]

Although many patents describe feasible topological structures, constitutive materials and production methods, metal foams cannot be considered a commodity and relatively few commercial producers are available worldwide.

Open-cell

Open-cell metal foam
CFD (numerical simulation) of fluid flow and heat transfer on an open cell metal foam

Open celled metal foams, also called metal sponges,[2] have 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 generally used in advanced technology, aerospace and manufacturing.

Fine-scale open-cell foams, with cells smaller than can be seen unaided, are used as high-temperature filters in the chemical industry.

Metallic foams are used in compact heat exchangers to increase heat transfer at the cost of reduced pressure.[3][4][5] However, their use permits substantial reduction in physical size and fabrication costs. Most models of these materials use idealized and periodic structures or averaged macroscopic properties.

Metal sponges have very large surface area for their weight, so catalysts are often metal sponges, such as palladium black, platinum sponge, and spongy nickel. Metals such as osmium or palladium hydride are metaphorically called "metal sponges", but this is in reference to their ability to bind to hydrogen, rather than their physical structure.[6]


Manufacturing

Open cell foams are manufactured via foundry or powder metallurgy. In the powder method, "space holders" are needed; as their name suggests, they occupy the pore spaces and channels. In casting processes, foams are made using open-celled polyurethane foams used as a skeleton.

Closed-cell

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.[7] Two patents on sponge-like metal were issued to Benjamin Sosnik in 1948 and 1951 who applied mercury vapor to blow liquid aluminium.[8][9]

Closed-cell metal foams were developed in 1956 by John C. Elliott at Bjorksten Research Laboratories. Although the first prototypes were available in the 1950s, commercial production began in the 1990s 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 deformed 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 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

Foams are commonly made by injecting a gas or mixing a foaming agent into molten metal.[10] Melts can be foamed by creating gas bubbles in the material. Normally, 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 slowed by increasing the viscosity of the molten metal, either by adding 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:

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 mixed to the metal in the solid state in powder form. This is the so-called "powder route" of foaming and it is probably the most established (from an industrial standpoint). 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 in 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,[11] extrusion (direct[12] or conform[13]) and flat rolling.[14]

Composites

Composite metal foam (CMF) is formed from hollow beads of one metal within a solid matrix of another, such as steel within aluminium, show 5 to 6 times greater strength to density ratio and over 7 times higher energy absorption than previous metal foams.[15]

A less than one inch thick plate has enough resistance to turn a 7.62 x 63 mm standard-issue M2 armor piercing bullet to dust. The test plate outperformed a solid metal plate of similar thickness, while weighing far less. Other potential applications include nuclear waste (shielding X-rays, gamma rays and neutron radiation) transfer and thermal insulation for space vehicle atmospheric re-entry, handling fire and heat twice as well as plain metals.[16]

Stochastic and regular foams

Stochastic

A foam is said to be stochastic when its porosity distribution is random. Most foams are stochastic because of their method of manufacture:

Regular

Manufacturing process of a regular metal foam by direct molding, CTIF process [17][18][19]

A foam is said to be regular when its structure is ordered. One technology that produces regular foams[17][18] uses direct molding to produce a foam with open and regular pores. Alternatively, regular metal foams can be produced by additive manufacturing processes such as selective laser melting (SLM).

Plates can be used as casting cores. Their shape is customized for each application. This manufacturing method allows for "perfect" foam, so-called because it satisfies Plateau's laws and has conducting pores of the shape of a truncated octahedron Kelvin cell (body-centered cubic structure).

Kelvin cell (Similar to the Weaire–Phelan structure)

Regular foams gallery

Applications

Design

Metal foam can be used in product or architectural composition.

Design gallery

  1. ^ ALVEOTEC - Actualités - LOUPI Lighing launches his new metal foam heatsink for lighting application_66.html. Alveotec.fr. Retrieved on 2013-12-03.

Mechanical

Orthopedics

Foam metal has been used in experimental animal prosthetics. 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 junction. For orthopedic applications, tantalum or titanium foams are common for their tensile strength, corrosion resistance and biocompatibility.

A Siberian Husky named Triumph's back legs received foam metal prostheses. Mammalian studies showed that porous metals, such as titanium foam, may allow vascularizaition within the porous area.[20]

Orthopedic device manufacturers use foam construction or metal foam coatings[21] to achieve desired levels of osseointegration.[22][23][24]

Automotive

The main vehicle applications of metallic foams are to increase sound damping, reduce weight 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.[25]

Under consideration are aluminium and alloy metallic foams due to their low density (0.4–0.9 g/cm3). These foams have high stiffness, fire resistance, lack toxicity, recyclability, energy absorbance, low thermal conductivity, low magnetic permeability and efficient sound dampening, especially in comparison to lightweight hollow parts. Metallic foams in hollow car parts decrease weakness points usually associated with car crashes and vibration. These foams are cheap to cast by using powder metallurgy (as compared to casting of other hollow parts).

In comparison to polymer foams (for vehicle use), 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.[26]

Metal foam technology has been applied to automotive exhaust gas.[27] Compared to traditional catalytic converters that use cordierite ceramic as substrate, metal foam substrate can offer better heat transfer and exhibit excellent mass-transport properties (high turbulence) offering possibilities for reducing platinum catalyst.[28]

Energy absorption

Aluminium crash graph

Metal foams are used for stiffening a structure without increasing its mass. For this application, metal foams are generally closed pore and made of aluminium. Foam panels are glued to the aluminium plate to obtain a resistant composite sandwich locally (in the sheet thickness) and rigid along the length depending on the foam's thickness.

The advantage of metal foams is that the reaction is constant, regardless of the direction of the force. Foams have a plateau of constant stress after deformation. This deformation may go up to 80% of crushing.[29]

Thermal

Heat conduction in regular metal foam structure
Heat transfer in regular metal foam structure

Tian et al.[30] listed several criteria to assess a foam in a heat exchanger. The comparison of thermal-performance metal foams with materials conventionally used in the intensification of exchange (fins, coupled surfaces, bead bed) first shows that the pressure losses caused by foams are much more important than with conventional fins, yet are significantly lower than those of beads. The exchange coefficients are close to beds and ball and well above the blades.[31][32]

Foams offer other thermophysical and mechanical features:

Commercialization of foam-based compact heat exchangers, heat sinks and shock absorbers is limited due to the high cost of foam replications. Their long-term resistance to fouling, corrosion and erosion are insufficiently characterized. From a manufacturing standpoint, the transition to foam technology requires new production and assembly techniques and heat exchanger design.

See also

References

  1. ↑ Compare Materials: Cast Aluminium and Aluminium Foam. Makeitfrom.com. Retrieved on 2011-11-19.
  2. ↑ John Banhart. "What are cellular metals and metal foams?".
  3. ↑ Topin, F.; Bonnet, J. -P.; Madani, B.; Tadrist, L. (2006). "Experimental Analysis of Multiphase Flow in Metallic foam: Flow Laws, Heat Transfer and Convective Boiling". Advanced Engineering Materials 8 (9): 890. doi:10.1002/adem.200600102.
  4. ↑ 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.
  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.
  6. ↑ Ralph Wolf; Khalid Mansour. "The Amazing Metal Sponge: Soaking Up Hydrogen". 1995.
  7. ↑ De Meller, M.A. French Patent 615,147 (1926).
  8. ↑ Sosnick, B. U.S. Patent 2,434,775 (1948).
  9. ↑ Sosnick, B. U.S. Patent 2,553,016 (1951).
  10. ↑ Banhart, John (2000). "Manufacturing Routes for Metallic Foams". JOM (Minerals, Metals & Materials Society) 52 (12): 22–27. doi:10.1007/s11837-000-0062-8. Retrieved 2012-01-20.
  11. ↑ 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.
  12. ↑ 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.
  13. ↑ 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.
  14. ↑ 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.
  15. ↑ Urweb:High Performance Composite Metal Foam. . Retrieved on 2013-12-10.
  16. ↑ MICU, ALEXANDRU (April 6, 2016). "Composite metal foam better at stopping bullets than solid plates". ZME Science. Retrieved 2016-04-09.
  17. 1 2 Recherche sur la production de pièces de fonderie en mousse métallique – Recherche en fonderie : les mousses métalliques. Ctif.com. Retrieved on 2013-12-03.
  18. 1 2 ALVEOTEC – Innovation. Alveotec.fr/en. Retrieved on 2013-12-03.
  19. ↑ "ALVEOTEC - Actualités - video : making process of aluminium foam".
  20. ↑ Osseointegration with Titanium Foam in Rabbit Femur, YouTube
  21. ↑ Titanium coatings on Orthopedic Devices. Youtube
  22. ↑ Biomet Orthopedics, Regenerex® Porous Titanium Construct
  23. ↑ Zimmer Orthopedics, Trabeluar Metal Technology
  24. ↑ Zimmer CSTiTM (Cancellous-Structured Titanium TM) Porous Coating
  25. ↑ 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.
  26. ↑ New Concept for Design of Lightweight Automotive Components. (PDF) . Retrieved on 2013-12-03.
  27. ↑ Alantum Innovations in Alloy Foam: Home. Alantum.com. Retrieved on 2011-11-19.
  28. ↑ Development of Metal Foam Based Aftertreatment on a Diesel Passenger Car – Virtual Conference Center. Vcc-sae.org. Retrieved on 2011-11-19.
  29. ↑ ALVEOTEC – Actualités – Examples of metal foam applications. Alveotec.fr. Retrieved on 2013-12-03.
  30. ↑ Tian, J.; Kim, T.; Lu, T. J.; Hodson, H. P.; Queheillalt, D. T.; Sypeck, D. J.; Wadley, H. N. G. (2004). "The effects of topology upon fluid-flow and heat-transfer within cellular copper structures" (PDF). International Journal of Heat and Mass Transfer 47 (14–16): 3171. doi:10.1016/j.ijheatmasstransfer.2004.02.010.
  31. ↑ Miscevic, M. (1997). Etude de l'intensification des transferts thermiques par des structures poreuses: Application aux échangeurs compacts et au refroidissement diphasique. IUSTI. Marseille., Université de Provence
  32. ↑ Catillon, S., C. Louis, et al. (2005). Utilisation de mousses métalliques dans un réformeur catalytique du méthanol pour la production de H2. GECAT, La Rochelle.

External links

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