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Porous material made from a metal

In materials science, a metal foam is a material or structure consisting of a solid metal (frequently aluminium) with gas filled pores comprising a large portion of the volume. The pores can be sealed (closed cell foam) or interconnected (open cell foam). The defining characteristic of metal foams is a high porosity: typically only 5–25% of the volume is the base metal. The strength of the material is due to the square–cube law. Metal foams typically retain some physical properties of their base material. Foam made from non flammable metal remains non flammable and can generally be recycled as the base material. Its coefficient of thermal expansion is similar while thermal conductivity is likely reduced.

Open celled metal foam, also called metal sponge, can be used in heat exchangers (compact electronics cooling, cryogen tanks, PCM heat exchangers), energy absorption, flow diffusion, scrubbers, flame arrestors, and lightweight optics. The high cost of the material generally limits its use to 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. 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 catalysts are often formed into metal sponge, such as palladium black, platinum sponge, and spongy nickel. Metals such as osmium and palladium hydride are metaphorically called "metal sponges", but this term is in reference to their property of binding to hydrogen, rather than the physical structure.

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

Composite metal foam is made from a combination of homogeneous hollow metal spheres with a metallic matrix surrounding the spheres. This closed cell metal foam isolates the pockets of air within and can be made out of nearly any metal, alloy, or combination. The sphere sizes can be varied and fine tuned per application. The mixture of air filled hollow metal spheres and a metallic matrix provides both light weight and strength. The spheres are randomly arranged inside the material but most often resembles a simple cubic or body centered cubic structure. CMF is made out of about 70% air and thus, weighs 70% less than an equal volume of the solid parent material. Composite metal foam is the strongest metal foam available with a 5 6 times greater strength to density ratio and over 7 times greater energy absorption capability than previous metal foams. CMF was developed at North Carolina State University by the inventor Afsaneh Rabiei with four patents in her name, all entitled "Composite Metal Foam and Method of Preparation Thereof" (US Utility Patents 9208912, 8110143, 8105696, 7641984), and CMF is currently proprietary technology owned by the company Advanced Materials Manufacturing.

A plate less than one inch thick has enough resistance to turn a .30 06 Springfield standard issue M2 armour 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, with many times the resistance to fire and heat as the plain metals.

CMF can replace rolled steel armour with the same protection for one third the weight. It can block fragments and the shock waves that are responsible for traumatic brain injuries (TBI). CMF was tested against blasts and fragments. The panels were tested against 23 × 152 mm high explosive incendiary rounds (as in anti aircraft weapons) that release a high pressure blast wave and metal fragments at speeds up to 1524 m/s. The CMF panels were able to withstand the blast and frag impacts without bowing or cracking. The thicker sample (16.7 mm thick) was able to completely stop various sized fragments from three separate incendiary ammunition tests. It was shown that CMF is able to locally arrest the fragments and dissipate the energy of the incident blast wave and impede the spread of failure, as opposed to fully solid materials that transfers the energy across the entire plate, damaging the bulk material. In this study, stainless steel CMF blocked blast pressure and fragmentation at 5,000 feet per second from high explosive incendiary (HEI) rounds that detonate at 18 inches away. Steel CMF plates (9.5 mm or 16.75 mm thick) that were placed 18 inches from the strike plate 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 anti aircraft weapons) as well as a 2.3mm aluminium strikeplate. The performance of the steel CMF was far better than the same weight aluminium plate against the same type of blast and fragments.

Composite metal foam panels, manufactured using 2 mm steel hollow spheres embedded in a stainless steel matrix and processed using a powder metallurgy technique, were used together with boron carbide ceramic and aluminium 7075 or Kevlar back panels to fabricate a new composite armour system. This composite armour was tested against NIJ Type III and Type IV threats using NIJ 0101.06 ballistic test standard. The highly functional layer based design allowed the composite metal foam to absorb the ballistic kinetic energy effectively, where the CMF layer accounted for 60–70% of the total energy absorbed by the armour system and allowed the composite armour system to show superior ballistic performance for both Type III and IV threats. The results of this testing program suggests that CMF can be used to reduce the weight and increase the performance of armour for Type III and Type IV threats. [[File:CMF after impact. jpg|thumb|Composite metal foam after impact from a .50 BMG armour piercing round. Notice the bullet removed and placed on top of the CMF panel on the lower left. S S CMF panels were manufactured and paired with a ceramic faceplate and aluminium backplate. The layered hard armours were tested against .50 BMG ball and AP rounds at a range of impact velocities. The mild steel cores of the ball rounds penetrated one of the three samples but revealed the benefits of using multiple tiles over a single ceramic faceplate to limit the spread of damage. The hardened steel core of the AP rounds penetrated deep into the ceramic faceplate, compressing the CMF layer until the projectile was either stopped and embedded within the armour or was able to fully penetrate and exit the backing plate. The experimental results were compared to commercially available armour materials and offer improved performance with reduced weight. The CMF layer is estimated to absorb between 69 and 79% of the bullet's kinetic energy, in their unoptimized testing condition.

thumb|Composite metal foam during a torch fire test. A 12" x 12" x 0.6" thick 316L steel CMF panel with a weight of 3.545 kg was tested in a torch fire test. In this test, the panel was exposed to over 1204 °C temperatures for 30 minutes. Upon reaching the 30 minutes' time of exposure, the maximum temperature on the unexposed surface of the steel was 400 °C (752 °F) at the center of the plate directly above the jet burner. This temperature was well below the required temperature rise limit of 427 °C; therefore, this sample met the torch fire test requirements. For reference, a solid piece of equal volume steel used for calibration failed this test in about 4 minutes. These studies indicate the extraordinary performance of CMF against fire and extreme heat. thumb|CMF isolates 1,100 °C (2,000 °F) temperatures so well that it can be touched just two inches away from the white hot region of material. Composite metal foam has a very low rate of heat transfer and has proven to isolate an extreme temperature of 1,100 °C (2,000 °F) within only a few inches, leaving the material at room temperature just about two inches away from a region of white hot material. In addition, the steel CMF managed to retain most of its steel like strength at this temperature while remaining as lightweight as aluminium, a material that would melt instantly at this extreme temperature.

Composite metal foam has shown an ability to shield against x ray and neutron radiation, absorbs/mitigates shocks, sounds, and vibrations, and can withstand over 1,000,000 high load cycles, outperforming traditional solid metals in each case.

Metal foam can be used in product or architectural composition.

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. The back legs of a Siberian Husky named Triumph received foam metal prostheses. Mammalian studies showed that porous metals, such as titanium foam, may allow vascularization within the porous area. Orthopedic device manufacturers use foam construction or metal foam coatings to achieve desired levels of osseointegration.

The primary functions of metallic foams in vehicles are to increase sound damping, reduce weight, increase energy absorption in case of crashes, and (in military applications) to combat the concussive force of IEDs. As an example, foam filled tubes could be used as anti intrusion bars. 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 UV light, humidity, and temperature variation. However, they are heavier, more expensive, and non insulating. Metal foam technology has been applied to automotive exhaust gas. Compared to traditional catalytic converters that use cordierite ceramic as substrate, metal foam substrate offers better heat transfer and exhibits excellent mass transport properties (high turbulence) and may reduce the quantity of platinum catalyst required.

Metal foams are popular support for electrocatalysts due to the high surface area and stable structure. The interconnected pores also benefit the mass transport of reactants and products. However, the benchmark of electrocatalysts can be difficult due to the undetermined surface area, different foam properties, and capillary effect.