Metal & Metal Parts: Foamy Future
March 1, 2009
The term metal foam is self-evident as a description; less obvious is the reason for making it. As the name suggests, metal foam is metal material that has been created with a porous, cellular structure, making it sort of a metallic sponge. The point of making such a material is to provide designers with the characteristic benefits of metal, such as strength, rigidity, thermal conductivity, and electromagnetic shielding, with the uncharacteristic benefits of lighter weight and increased surface area.
Only about a dozen companies around the world produce the material, which is now being used in a range of applications such as medical implants, burners, filters, electrodes, catalysts, and heat exchangers.
The foams have a cellular structure that contains a large amount of pores. They can be made out of just about any metal, but the vast majority of such foam is aluminum. Nickel, however, is a popular material for nickel-metal-hydride (NiMh) and nickel-cadmium (NiCd) batteries. Other materials are selected for various properties such as corrosion resistance, high-temperature resistance, and shape-memory capabilities (see Sidebar). They are light, typically between 5 percent and 25 percent of the density of the metal from which they are made. In fact, some metal foam materials have densities that are one tenth that of water. They typically retain many of the physical properties of their base material.
Thomas Hipke of the Germany-based Fraunhofer Institute, which is a leading metal foam research organization, says that metal foams can be selected by the attainable density, desired strength, modulus of elasticity, and other properties, and can be adjusted to the targeted application.
There are two general classes of metal foams: those with open pores, which are known as open-cell foams; and those with closed pores, which are known as closed-cell foams. Both open-cell and closed-cell metal foams are filled with voids that reduce the weight of material in relation to volume. The open cells are connected, allowing a path for migration, whereas closed cells do not connect. So closed cell foam does not provide access or a migration path for gases and liquids. The two different structures provide different mechanical properties that make them suitable for different applications.
Closed-cell metal foams are typically used in structural, load-bearing applications, or as an impact-absorbing material, similar to the polymer foams found in a bicycle helmet because they provide higher strength, says David Dunand, professor at Northwestern University, Evanston, and an expert on metal-foam architecture.
Dunand says that metal foams, especially open-cell foams, feature the mechanical strength, stiffness, and energy absorption that are much greater than those of polymer foams. As compared to polymers, they are more thermally and electrically conductive and they maintain their properties at much higher temperatures than polymers.
The open-cell foams do have a structure similar to open-celled polyurethane foams, he adds. This structure provides metal foams with very high specific surface areas, which are good for fluid-flow applications. Their sponge-like architecture is linked by ligaments, called struts, and it is these struts that create the additional surface area. In one application, for instance, a solid metal panel was replaced with a metal foam panel in a computer case and the computer’s internal temperature decreased by approximately 20 DegC.
The cellular structure of metallic foams can be created through a number of techniques, including casting methods. Some of the more obvious methods include bubbling a gas through the molten metal, or mixing in a foaming agent. Another technique is to inject gas into a liquid metal under pressure, then slowly relieving the pressure, allowing the gas to create cells in a controlled manner. Mixing metal powder with a foaming agent then heating the mixture into a slurry is another option. With the casting method, an open-cell ceramic mold can be created by mixing ceramic material with a heat-soluble material such as wax or polymer. Molten metal infiltrating the mold would then burn out the heat-soluble material, and then the ceramic material would also be removed. Casting is slower than other methods, but offers more control and customization. M-pore, Dresden, Germany, a manufacturer of metal foams, casts their metal foams to customer specifications, providing foam with specified sizes for the struts and cells.
Dieter Girlich, founder of M-pore, says that the uniqueness of the material requires not just an understanding of foam’s mechanical properties, but also a completely different way of thinking about design and construction using the material. For example, in a traditional heat-exchanger application, the cabinet is usually built around the exchanger and the fins so that the fins are oriented in line with airflow. With metal foam, fluid flow direction is not as important because air flows through the cells from many different directions. Because of this increased flow, a heat exchanger can be used to achieve increased heat transfer levels, or, conversely, a smaller heat exchanger can be used to achieve the same heat transfer levels.
The struts and pore sizes can influence fluid flow, Girlich says. Smaller struts, referred to as fine struts, are often specified for filtering applications, while thicker struts are used to obtain greater surface area and aid in heat transfer efforts. Of course, these decisions may require compromises. Thicker struts aid heat transfer, but can cause a high-pressure drop. Conversely, thinner struts, with big pores and lots of space between the struts, can cause a low-pressure drop. This might require a more powerful pressure pump to push fluid through the foam, says Girlich.
Choices must also be made between materials when they each have their own benefits. Bekaert, a Belgian manufacturer of heat exchangers and other industrial products, conducted internal studies about the best metal foam materials for use in compact heat exchangers. The company compared copper and aluminum foam. They found that although copper has a higher thermal conductivity, for some applications it is too expensive, too heavy, and, in some environments, too prone to corrosion. For them, cast open-cell aluminum foam is the better material for compact heat exchangers, provided the application does not require entrance temperatures higher than 400 DegC. Aiming for the lowest possible weight and dimensions, Bekaert found that the use of open cell aluminum foam resulted in an increase in heat transfer of 25 percent.
The thermal capabilities of open-cell foams are a key benefit of metal foams, says Mark Heamon, market manager for Porvair Advanced Materials, Hendersonville, N.C. The company produces MetFlame metal foam that is used in a gas, infrared burner for barbeque grills, as well as in commercial applications such as a high-volume production of tortilla chips, and in textile and paper drying applications, says Heamon.
MetFlame is made from an open-cell reticulated, iron-based high-temperature alloy (FeCrAlY) foam. In testing, the average surface temperature of the burner plate for the MetFlame emitters operated at as much as 50 DegC hotter than ceramic tiles at all gas input rates above 160 kW/m2. One reason for this was that the materials captured and held onto the waste heat from the escaping exhaust gases, which helped it achieve higher operating temperatures from the same gas input. Compared to metal-fiber burners, the MetFlame burners achieved up to a 200 DegC higher surface temperature than knitted metal fibers, and 100 DegC higher surface temperatures than sintered metal fibers.
It is not just the increased temperature range that metal foams offer, but also the speed to which the burners get up to temperature. According to Heamon, a ceramic burner might take up to 4 minutes to get up to temperature, whereas the metal foam burner reaches that temperature in about 30 seconds and cools down equally as quick, he says.
In a different, and more novel flow-through application, German manufacturer FPE Fischer used metal foam produced by M-pore to make a computer cabinet. They found that the metal foam was highly permeable to streaming air, while still providing a tight barrier for EMI. Temperature measurements have shown that the cabinet’s temperature was reduced about 20 DegC without any other changes to the cabinet, says Ulrich Fischer, president of the company.
Metal foam capabilities, and aesthetics, can be increased with some post processing. The material can be machined into various forms and shapes, painted in all colors, or coated. For instance, FPE Fischer was able to coat the computer cabinet with a metallic coating that allowed the cabinet to act as an air purifier by de-ionizing the air in the office in which it operates.
In another case, Hipke says that a stylish effect was added to an open-cell metal foam to create a lamp. The lamp, which measured 35 centimeters in height and consisted of 90 percent air, was coated with a transparent synthetic resin and colored plastic.
Metal foams can be mounted by traditional joining methods such as brazing, soldering, sintering, welding, and melting. The researchers at Bekaert found that sintering yielded the best heat transfer results because the foams could be used at higher temperatures and did not include an intermediate layer that could act as a heat transfer barrier or increase the corrosion rate at the foam/carrier interface.
The material is available in a number of standard shapes and forms. They can come as precut sheets, blanks, 3D components, blocks, and sandwich constructions with solid metal panels surrounding a metal foam core. Some are available preassembled. Porvair’s MetFlame burner emitters, for instance, can be purchased cut-to-size and assembled in a stainless steel frame that can be dropped into place. Metal foams can also be fabricated into custom shapes.
While the benefits of metal foam have established it as a viable choice for designers, the suppliers say that the material cost is greater, in some cases twice the cost, and this discourages use of the material.
A lack of knowledge was also cited as a reason that metal foams have not yet found a wider audience, but the suppliers point out that it is still a relatively young industry. As the benefits of metal foam become wider known, and suppliers work to improve these benefits and bring down costs, more applications for a lightweight metal material are expected.
For more information, email:
FPE Fischer: firstname.lastname@example.org
Porvair Advanced Materials: email@example.com
Sidebar: Shape Memory FoamsA new metal alloy with shape memory attributes has been developed by researchers at Boise State University, Boise, Idaho, and Northwestern University, Evanston, Ill., that may lead to smaller, lighter, and more precise pumps and other devices.
Shape memory alloys change shape when subjected to an external stimulus such as temperature or magnetic fields. While the material changes shape, it still has the memory of its original shape and will return to that form when it is exposed to a different stimulus or if the original stimulus is removed. Temperature is considered too slow for many uses. Magnetic shape change, on the other hand, can change shapes almost as soon as the field is engaged.
One material that is especially appropriate for shape memory applications in magnetic fields is a nickel-manganese-gallium alloy (Ni-Mn-Ga), says Peter Mullner, a professor and alloy expert at Boise State. The alloy is especially good for devices that require very precise, repeatable, and rapid positioning such as pumps, sonar devices, microscopes, and medical devices. For relatively large samples such as a 6 mm length, a length change of 10 percent (0.6 mm) could be achieved within less than a millisecond.
In pumps, for example, the magnetic shape memory alloy (MSMA) would control the position (on/off) of a valve. The advantage is that with MSMA, the on/off positions can be attained very precisely and in very short time. Compared to devices using solenoid technology, the frequency may see an improvement by a factor of 10 (given the same amount of stroke).
A single crystal of the Ni-Mn-Ga alloy deforms by 10 percent when subjected to the magnetic field. It will retain its new shape when the field is turned off and return to its original shape when the field is rotated 90 Deg. Shape change is more pronounced with a single crystal because its internal structure is oriented in the same direction throughout the entire piece and will move predictably. However, single crystals are expensive and time consuming to make.
Because of cost considerations, most of the alloys used are polycrystals, which contain many smaller crystals. Polycrystals have a grain structure pointing in every direction and as they move in various directions, the individual movements mutual obstruct each other. By foaming the polycrystalline alloy, crystals have more room to grow and expand, says Mullner.
To create the foam, molten Ni-Mn-Ga is poured into a mold of porous sodium aluminate salt. After cooling, the salt is leached out with acid. In testing, samples of the foam were exposed to a 1-Tesla magnetic field that rotated at 12,000 rpm, while a laser system measured the induced strain.
The shape change was only about 0.12 percent, which was significantly less than a single crystal, but better than polycrystals, and a cause for optimism. Since then, Mullner says they have gotten significantly better results, but declined to give actual numbers until new results are published in a scientific journal later this year.
Mullner says that the foam structure may not get to 10 percent deformation as with a single crystal, but because of cost tradeoffs, he believes the material will find many applications. Any device such as an actuator, pump, valve, positioning tool, shutter, etc., that requires a fast and precise motion is a potential application. If long stroke and lightweight is important, the advantages of magnetic shape memory foam become even more relevant.