Material of the month – metal foams
They have been capturing headlines over the past decade and finding applications from the automotive industry to biomedical implants and hydrogen-based technologies. This month, Anna Ploszajski investigates metal foams.
Metal foams are solid materials made from metal filled with pores such that up to 95% of the volume consists of gas-filled voids. The pores themselves can adopt different structures, either to form an interconnected network, called open-cell foam, or distinct sealed bubbles, called closed-cell foam.
The first published work on metal foams fell into the latter category, reported in a 1926 French patent by Alexandre de Meller, where he suggested that light metals may be foamed using gas injection or by incorporating gas-releasing blowing agents into the melt. Many patents followed, after a small hiatus during the 1950s and 60s, with variants on foaming processes and materials. However, little was published in the academic setting, and many of the finer details were forgotten, necessitating a re-discovery of these original techniques when research into metal foams re-emerged in the late 1980s, efforts which still continue today. During the 1990s, American and German research programmes brought the R&D of the fundamental concepts of metal foaming processes into the academic arena, allowing the science to start catching up with existing industrial production technology.
The basic principles of the two methods for creating closed-cell metal foams from the melt described in the early patent literature are still widely used. The first method involves the direct foaming of metallic melts by gas injection. Gas is bubbled through the molten metal by special rotating or vibrating injectors, forming a foam, which is removed by, for example, a conveyor belt, and allowed to cool. The second method incorporates a chemical blowing agent, traditionally titanium hydride (TiH2), which decomposes in the melt, producing gas bubbles.
To prevent the gas bubbles coalescing or rupturing before solidification, small (10um) stabilising ceramic particles such as silicon carbide or alumina are added in quantities up to 20% by volume, adhering to the metal-gas interface. This produces a regular and highly porous foam, although the additives render the final material extremely brittle and difficult to machine. This unfortunate side-effect can be mitigated by reducing the size and number of the ceramic particles. A similar stable foam, however, can be formed by adding just 5% of SiC particles 70nm in diameter. Alternatively, stabilising phases may be formed in situ by adding calcium metal to an aluminium melt to form calcium oxide (CaO), calcium-aluminium oxide (CaAl2O4) or even Al4Ca intermetallics, increasing the viscosity of the melt by a factor of five. To get away from additives altogether, melts can be blown with inert gases at temperatures very close to the melting point to achieve sufficiently high viscosity.
Metal foams can also be prepared from powders. The powdered metal, alloy, or blends thereof, are mixed with a blowing agent and compacted. Conventional powder processing methods may be used at this point to press, extrude or flat-roll into the desired shape. Next, this precursor is heated closely to the melting point of the metal matrix, and the blowing agent decomposes to release gas and form uniformly distributed pores. The most common metal for all metal foaming processes is aluminium, used since the early research because of its low melting point, simple processing, and favourable density for weight-sensitive applications. The advantage of powder-based methods, aside from the pre-forming capabilities, is its flexibility regarding the metal itself, allowing for not just aluminium, but other metals and alloys of tin, zinc, brass, lead and gold to be made into foams, too.
Owing to its excellent specific strength and vibration dampening properties, one application of closed-cell aluminium foam is as a light-weight construction material. Usually, the metal foam is sandwiched between two dense aluminium sheets in order to achieve the required stiffness, by roll-cladding the outer sheets onto a foamable precursor layer made from powders containing the metal with a blowing agent. The final heat treatment in which the foam core expands leads to the desired sandwich composite structure. These composites have resulted in significant weight reductions in automobile car body parts, and have already been applied by Audi, Bentley and Ferrari. The ever-increasing pressure for greater fuel economy may see more widespread adoption of these composites in the coming years.
Like the expanded polystyrene foam in my bicycle helmet that keeps me safe in the London traffic, aluminium closed-cell foams can absorb a large amount of mechanical energy when deformed, and are therefore an attractive material for impact energy absorbers on vehicles. Aluminium foams convert the energy of impact into plastic deformation with relative ease, very effectively decelerating the vehicle in the event of a crash. The fact that it doesn’t spring back like a polymer foam can prevent secondary damage. Aluminium foams can be used in heavy-duty vehicles such as lorries and trucks as part of their under-run protection, to absorb energy on impact and prevent smaller cars being run over in the event of a collision.
Although polymer foams are far superior in absorbing sound to metal foams, there are instances where polymers cannot be used. For example, automobile components made from aluminium foam cast inside a dense aluminium shell have higher stiffness and greater damping compared to hollow parts, being only slightly heavier, but able to withstand harsh temperature and chemical environments. Mechanical vibrations from the car engine are dissipated into thermal energy by the metal foam core, making for a quieter and smoother ride. This technology can be rolled out to all sorts of machinery, providing noise damping of frequencies up to 370Hz by up to 60%.
Open-cell foams have interconnecting pores, and their processing routes usually require space-holders or templates, around which the metal may be solidified, either by powder routes or casting.
Alternatively, another method exists, developed and patented in 1993 by Shapovalov, a Ukranian scientist, who called it the Gasar process. It exploits the fact that some liquid metals will form a eutectic system with hydrogen gas and become charged with hydrogen when the melt is put under a high-pressure hydrogen atmosphere. Lowering the temperature through the eutectic point causes a phase transition into a heterogeneous two-phase system – the metal solidifies and the hydrogen gas precipitates out to become entrapped gaseous pores. The pore morphology of the final material is largely determined by the hydrogen content, overpressure, and direction and rate of cooling. Because of this latter relationship, pores tend to be elongated, oriented in the direction of solidification.
The interconnected pores in an open-cell metal foam, together with their stability at elevated temperatures makes such foams useful for emission control applications, such as diesel particulate filters and catalytic converters. For example, NiCrAl alloy metal foams are very effective at trapping large particulates due to their tortuous porous structure, large internal surface area, and high temperature stability. These benefits, combined with the ability to deposit catalytic coatings onto the surface makes metal foams integral to our struggle against vehicle emissions.
The combination of the high thermal conductivity of metals with the high surface area brought by the foam microstructure makes metal foams excellent in heat transfer settings. Other benefits include resistance to thermal shock, high temperature and pressure, corrosive environments and thermal cycling. Using metal foams allows for the physical size of the heat exchangers to be substantially reduced, bringing down manufacturing costs.
Most of the applications mentioned so far have been linked to automobiles of some kind or another, and their future in this area may be guaranteed for years to come in hydrogen-based technologies. Metal foams can be used to improve fuel cell stacks in many areas, such as thermal management, flow distribution for improved mass transport, membrane support and current collection. There has been mounting interest around hydrogen-based portable technologies, with the first hydrogen fuel cell vehicles brought to market by Toyota and Hyundai in 2015, so manufacturers will welcome the efficiency savings offered by new metal foams in the coming years. This is also fortunate because electrodes for nickel metal hydride and nickel cadmium batteries are probably the largest industrial application of metallic foams today, and these battery technologies are being overtaken by Li-ion.
Cutting edge research
Titanium and tantalum alloys have long been favoured for bone implant materials thanks to their biocompatibility and suitable physical properties. However, much of the long-term success of such an implant depends on the strength of the bond at the interface between the bone and the implant. Using highly porous metal foam coatings on metallic bone implants allows for increased osteo-integration (growth of new bone into surface of the implant). In fact, the pore size and shape can be engineered to share microscopic characteristics of cancellous bone.
Metal foams are an exciting and growing area. New and cutting-edge materials are regularly being reported. One example, in November 2015, came from researchers at ETH Zurich, Switzerland, who created ultra-light foam made from gold. Although gold metal foams are not new, this one, technically an aero-gel made using milk proteins, is the lightest ever created and the method could pave the way to other record-breaking lightweight metal foams.
Despite showing much promise in many diverse areas, metal foams are struggling to find mainstream applications outside of niche markets. One reason dates back to their beginnings, when the basic mechanisms of metal foaming were under-researched. There is still a widespread lack of understanding of some of the fundamental processes and variables surrounding metal foam production, and this has resulted in difficulties obtaining consistency between batches, and of quality control of the structure and morphology of the materials. The science behind these materials is developing, but needs to catch up with industry before metal foams become an attractive investment.