Definitions
Open-cell
Open-celled metal foam, also called metal sponge, can be used in heat exchangers (compactClosed-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 appliedStochastic 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 (powder) metal * Vapor deposition (CVD on a random matrix ) * Direct or indirect random casting of a mold containing beads or matrixRegular foam
image:Fabrication des mousses fonderie.jpg, Manufacturing process of a regular metal foam by direct molding, CTIF processRecherche sur la production de pièces de fonderie en mousse métallique – Recherche en fonderie : les mousses métalliquesHybrid 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 intoComposite 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 atHigh speed impact/blast/ballistics testing
A plate less than one inch thick has enough resistance to turn aHEI/fragment testing
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 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 separateSmall arms testing
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 armor system. This composite armor 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 armor system and allowed the composite armor 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 armor for Type III and Type IV threats..50 Cal AP testing
CMF has been tested against large-caliber armor-piercing rounds.Rabiei, Marx, Portanova. (2019). Ballistic performance of composite metal foam against large caliber threats. Composite Structures 224 (2019) 111032. S-S CMF panels were manufactured and paired with a ceramic faceplate and aluminium backplate. The layered hard armors were tested against 12.7 × 99 mm 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 armor or was able to fully penetrate and exit the backing plate. The experimental results were compared to commercially available armor 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. At impact velocities above 800 m/s, the CMF layer consistently absorbed up to 79% of the impact energy. As the impact velocity increased, so did the effective strength of the CMF layer due to the strain rate sensitivity of the material. The mass efficiency ratio of the armors, when compared to RHA, was calculated to be 2.1. The CMF hard armors can effectively stop an incoming round at less than half the weight of the required rolled homogeneous armor. The weight savings afforded by using such novel armor can improve the fuel efficiency of military vehicles without sacrificing the protection of the personnel or the equipment inside.Puncture testing
Composite Metal Foam has been tested in a puncture test. Puncture tests were conducted on S-S CMF-CSP with different thicknesses of stainless steel face sheets and CMF core. The bonding of the S-S CMF core and face sheets was done via adhesive bonding and diffusion bonding. Various thicknesses of the CMF core and face sheets created a variety of target areal densities from about 6.7 to about 11.7 kg per each tile of 30 x 30 cm. Targets were impacted using 2.54 and 3.175 cm diameter steel balls fired at velocities ranging from 120 to 470 m per second, resulting in puncture energies from 488 to 14 500 J over a 5.06–7.91 cm2 impact area for the two size sphere balls. None of the panels, even those with the lowest areal densities, showed complete penetration/puncture through their thickness. This was mostly due to the energy absorption capacity of the S-S CMF core in compression, whereas the face sheets strengthen the CMF core to better handle tensile stresses. Sandwich panels with thicker face sheets show less effectiveness, and a thin face sheet seemed to be sufficient to support the S-S CMF core for absorbing such puncture energies. Panels assembled using adhesive bonding showed debonding of the face sheets from the CMF core upon the impact of the projectile while the diffusion bonded panels showed more flexibility at the interface and better accommodated the stresses. Most diffusion bonded panels did not show a debonding of face sheets from the S-S CMF core. This study proved CMF's energy absorption abilities, indicating that CMF can be used to simultaneously increase protections and decrease weight.Fire/extreme heat testing
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. It is worth mentioning that the same CMF panel prior to the above-mentioned jet fire testing was subjected to a pool-fire test. In this test, the panel was exposed to 827 °C temperatures for 100 minutes. The panel withstood the extreme temperature for 100 minutes with ease, reaching a maximum backface temperature of 379 °C, far below the 427 °C failure temperature. For reference, the test was calibrated using an equal-sized piece of solid steel that failed the test in approximately 13 minutes. These studies indicate the extraordinary performance of CMF against fire and extreme heat. 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.Other abilities
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.Regular foams gallery
Applications
Design
Metal foam can be used in product or architectural composition.Design gallery
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. 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 vascularAutomotive
The primary functions of metallic foams in vehicles are to increaseElectrocatalysis
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.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
Heat conduction in regular metal foam structure Heat transfer in regular metal foam structure 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. Kisitu et al. pioneered the experimental investigation of using compressed copper foam for advanced two-phase cooling for high heat flux electronics. The metallic foam samples are designed and manufactured by a US-based company, ERG Aerospace Corporation. Heat fluxes as high as 174 W/cm2 were tested/handled. Data reveal that compressing the foam by four times in the streamwise direction (4X) enhanced thermal performance by more than 3 times, compared to the uncompressed metal foam. This was attributed to the fact that compressing foam proportionally reduces the effective hydraulic diameter and increases both the surface area per unit volume and foam bulk thermal conductivity, which all improve two-phase cooling performance. In addition, results show that compressed foam has a potential to increase the critical heat flux (CHF), which is pivotal in the safe operation of two-phase cooling at high heat densities. Preliminarly results show that compressed metallic foams can solve several issues faced with microchannels, including clogging, flow instabilities, low CHF, and others. As such, compressed foams are being proposed as new powerful alternatives to microchannels in pumped two-phase cooling for high heat flux electronics cooling/thermal management, including high performance computers, aerospace, military and defence, and power electronics.See also
*References
External links