Metal foam

A metal foam is a cellular structure consisting of a solid (frequently ) with gas-filled  comprising a large portion of the volume. The pores can be sealed (closed-cell ) or interconnected (open-cell foam). The defining characteristic of metal foams is a high : typically only 5–25% of the volume is the base metal. The strength of the material is due to the.

Metal foams typically retain some of their base material. Foam made from non-flammable metal remains non-flammable and can generally be recycled as the base material. Its is similar while  is likely reduced.

Open-cell
Open celled metal foam, also called metal sponge, can be used in s (compact, , s), energy absorption, flow diffusion, and. The high cost of the material generally limits its use to advanced technology,, and manufacturing.

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

Metal foams are used in compact heat exchangers to increase heat transfer at the cost of reduced pressure. 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 sponge has very large surface area per unit weight and s are often formed into metal sponge, such as, , and. Metals such as and  are metaphorically called "metal sponges", but this term is in reference to their property of binding to hydrogen, rather than the physical structure.

Manufacturing
Open cell foams are manufactured by foundry or. In the powder method, "space holders" are used; as their name suggests, they occupy the pore spaces and channels. In casting processes, foam is cast with an open-celled foam 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, was suggested. Two patents on sponge-like metal were issued to Benjamin Sosnik in 1948 and 1951 who applied vapor to blow liquid aluminium.

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 foams in a  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 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 resistance and recycling potential of other metal foams, but add the property of flotation in water.

Manufacturing
Foams are commonly made by injecting a gas or mixing a into  metal. Melts can be foamed by creating gas bubbles in the material. Normally, bubbles in molten metal are highly buoyant in the high-density liquid and rise quickly to the surface. This rise can be slowed by increasing the viscosity of the molten metal 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: To stabilize the molten metal bubbles, high temperature foaming agents (nano- or micrometer- sized solid particles) are required. The size of the s, or cells, is usually 1 to 8 mm. When foaming or blowing agents are used, they are mixed with the powdered metal before it is melted. This is the so-called "powder route" of foaming, and it is probably the most established (from an industrial standpoint). After metal (e.g. ) powders and foaming agent (e.g.) 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, (direct or conform) and flat.
 * by injecting gas into the liquid metal from an external source;
 * by causing gas formation in the liquid by admixing gas-releasing blowing agents with the molten metal;
 * by causing the precipitation of gas that was previously dissolved in the molten metal.

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 more than 7 times greater energy absorption than previous metal foams.

A plate less than one inch thick has enough resistance to turn a standard-issue M2  to dust. The test plate outperformed a solid metal plate of similar thickness, while weighing far less. Other potential applications include (shielding s,  and  radiation) transfer and thermal insulation for space vehicle atmospheric re-entry, with twice the resistance to fire and heat as the plain metals.

CMF can replace rolled steel armor with the same protection for one-third the weight. It can block fragments and the shock waves that are responsible for brain injuries. CMF can block blast pressure and fragmentation at 5,000 feet per second from (HEI) rounds that detonate 18 inches from the shield. Steel CMF plates (9.5 mm or 16.75 mm thick) were placed 18 inches from the strikeplate held up against the wave of blast pressure and against the copper and steel fragments created by a 23×152 mm HEI round (as in ) as well as a 2.3mm aluminium strikeplate.

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


 * Foaming of liquid or solid (powder) metal
 * Vapor deposition (CVD on a random matrix )
 * Direct or indirect random casting of a mold containing beads or matrix

Regular
A foam is said to be regular when the structure is ordered. Direct molding is one technology that produces regular foams with open pores. Metal foams can also be produced by additive processes such as (SLM).

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

Design
Metal foam can be used in product or architectural composition.

Orthopedics
Foam metal has been used in experimental animal. In this application, a hole is drilled into the and the metal foam inserted, letting the bone grow into the metal for a permanent junction. For orthopedic applications, or  foams are common for their, corrosion resistance and.

The back legs of a named Triumph received foam metal prostheses. Mammalian studies showed that porous metals, such as foam, may allow  ization within the porous area.

Orthopedic device manufacturers use foam construction or metal foam coatings to achieve desired levels of.

Automotive
The primary functions of metallic foams in vehicles are to increase, reduce weight, increase energy absorption in case of crashes, and (in military applications) to combat the concussive force of. As an example, foam filled tubes could be used as s. Because of their low density (0.4–0.9 g/cm3), aluminium and aluminium alloy foams are under particular consideration. These foams are stiff, fire resistant, nontoxic, recyclable, energy absorbent, less thermally conductive, less magnetically permeable, and more efficiently sound dampening, especially when compared to hollow parts. Metallic foams in hollow car parts decrease weakness points usually associated with car crashes and vibration. These foams are inexpensive to cast with powder metallurgy, compared to casting other hollow parts.

Compared to polymer foams in vehicles, metallic foams are stiffer, stronger, more energy absorbent, and resistant to fire and the weather adversities of light, humidity, and temperature variation. However, they are heavier, more expensive, and non-insulating.

Metal foam technology has been applied to automotive. Compared to traditional s that use  ceramic as substrate, metal foam substrate offers better heat transfer and exhibits excellent mass-transport properties (high turbulence) and may reduce the quantity of   required.

Energy absorption
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 stress after deformation that is constant for as much as 80% of the crushing.

Thermal
Tian et al. 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.

Foams offer other thermophysical and mechanical features:


 * Very low mass (density 5–25% of the bulk solid depending on the manufacturing method)
 * Large exchange surface (250–10000 m2/m3)
 * Relatively high permeability
 * Relatively high effective thermal conductivities (5–30 W/(mK))
 * Good resistance to thermal shocks, high pressures, high temperatures, moisture, wear and thermal cycling
 * Good absorption of mechanical shock and sound
 * Pore size and porosity can be controlled by the manufacturer

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.