Metal foam


In materials science, a metal foam is a material or structure consisting of a solid metal with gas-filled pores comprising a large portion of the volume. The pores can be sealed or interconnected. 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.

Definitions

Open-cell

Open-celled metal foam, also called metal sponge, can be used in 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

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 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.

Stochastic foam

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 metal
  • Vapor deposition
  • Direct or indirect random casting of a mold containing beads or matrix

    Regular foam

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 selective laser melting.
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 Plateau's laws and has conducting pores of the shape of a truncated octahedron Kelvin cell.
Image:Truncatedoctahedron.gif|thumb|right|Kelvin cell

Hybrid foam

Hybrid metal foams typically have a thin film on the underlying porous substrate. Coating metal foams with a different material has been shown to improve the mechanical properties of the metal foam, especially because they are prone to bending deformation mechanisms due to their cellular structure. The addition of a thin film can also improve other properties such as corrosion resistance and enable surface functionalization for catalytic flow processes.
To fabricate hybrid metal foams, thin films are deposited onto a foam substrate with electrodeposition at room temperature. A two-electrode cell setup in a Watt's bath can be used. Recent studies have demonstrated issues with the uniformity of the thin-film due to the complex geometry of metal foams. Issues with uniformity have been addressed in more recent studies through the implementation of nanoparticle thin films, leading to improved mechanical and corrosion resistance properties.
Recent studies on hybrid foams have also been used to address non-renewable energy resources. Transition metal hybrid foams have previously been fabricated through a combination of electrodeposition and hydrogen bubbling processes to enhance the diffusivity of fluids through the porous material and improve the electrical properties for enhanced charge transfer. Thus, such foams can be used to make electrocatalytic water splitting processes more efficient.
Hybrid metal foams may have favorable conductive properties for flexible devices. Through the application of a thin layer of metal onto a porous polymer substrate via gas-phase deposition, researchers have been able to achieve high conductivity while maintaining the flexibility of the polymer matrix. Through cycling testing, it has been shown that hybrid foams are capable of surface deformation sensing. Future efforts seek to characterize the change in cross-linking and porosity of materials as deposition occurs. Additionally, the interaction or compatibility between different polymers and metals in foam ligands can be explored in order to get an improved understanding of their sensitivity to external forces. This would help improve resistance to compressive forces.

Manufacturing

Open-cell

Open cell foams are manufactured by foundry or powder metallurgy. 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 polyurethane foam skeleton.

Closed-cell

Foams are commonly made by injecting a gas or mixing a foaming agent into molten metal. Molten metal 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 molten metal, or by other means. Molten metal can be foamed in one of three ways:
  • 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.
To stabilize the molten metal bubbles, high temperature foaming agents 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 with the powdered metal before it is melted. This is the so-called "powder route" of foaming, and it is probably the most established. After metal powders and foaming agent 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, extrusion and flat rolling.

Composite metal foam

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", and CMF is currently proprietary technology owned by the company Advanced Materials Manufacturing.

High-speed impact/blast/ballistics testing

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 transfer and thermal insulation for space vehicle atmospheric re-entry, with many times the resistance to fire and heat as the plain metals. Another study testing CMF's resistance to.50 caliber rounds found that CMF could stop such rounds at less than half the weight of rolled homogeneous armour.

HEI/fragment testing

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. CMF was tested against blasts and fragments. The panels were tested against 23 × 152 mm high explosive incendiary rounds 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 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 rounds that detonate at 18 inches away. Steel CMF plates 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 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.