Open-cell Metal Foams

© Fraunhofer IFAM Dresden
Vertebral Body

The development of open-cellular metal foams is done in a co-operation via the bundling of the expertise of the Fraunhofer institutes IKTS and IFAM. In a powder metallurgy molding process first a PU sponge is coated in a suspension. The substrate then is removed again by a thermal treatment and the remaining structure is finally sintered.

The cell-width of the foams can be adjusted by the choice of suitable starting materials from 0,3 to 5 mm, the relative density is adjustable in a range between 9 and 30%. Due to this high variability of the structure the functional characteristics are purposefully adjustable such as firmness, sound absorption, flowing through barness and heat transfer. A further substantial advantage of the procedure is in the particularly good homogeneity of the structure, remaining the same characteristics within wide ranges guaranteed.

Applications

Applications for open-cell metal foams include the following areas:

Filter and Catalyst Carrier
Corrosion-resistant catalyst carriers for automotive engineering or cleaning of industrial exhaust gases, VCR catalysts, selective oxidation, reactors for hydrogen production (reforming), filters for soot particles, burner surfaces of exhaust gas flares

Thermal Management
Thermal insulation for high temperatures and highly corrosive environments, tube bundle heat exchangers with high volumetric power density.

Biomaterials
Permanent implants made of titanium or tantalum, spinal implants, stand-alone bone replacement implants, implants for reconversion osteotomy, implants for maxillofacial surgery, biodegradable bone replacement implants, porous coatings on joint endoprostheses or dental implants

Mechanical and Plant Engineering
Sound absorbers with high temperature resistance, dampers for vibrating liquids, local reinforcing elements for infiltration in cast components, local wear reinforcement, atomizers or evaporators, mixers for multiphase gas flows

Biomaterials

© Fraunhofer IFAM Dresden
X-Ray of a Vertebral Body

© Fraunhofer IFAM Dresden
Vertebral Body Implant, Demonstrator

An important clinical problem is the replacement of large bone defects in accident surgery, in cancer therapy or in the treatment of osteoporotic fractures. Typically, such defects are nowadays replaced by the body's own bones or by massive bone replacement material. The first method requires additional risky and painful interventions, the second one is disadvantageous due to the consequential high stiffness. These implants far exceed the stiffness of the surrounding bone and take over the load that affects the relevant body part. Since bone augmentation follows the load distribution, new bone formation in this area is disturbed.

Cellular metallic materials have a greatly reduced stiffness due to their porous structure. This value is typically in the range of the stiffness of a spongy bone. Cellular metals also allow the ingrowth of bone cells and blood vessels that are necessary for bone growth.

As permanent implants, cellular metallic materials are manufactured from a titanium alloy. This is mainly due to its extraordinarily good biocompatibility, coupled with excellent corrosion resistance. Furthermore, the good osteoconductivity of titanium is an additional argument for its use as a bone substitute. For these reasons, titanium and its alloy Ti6Al4V enjoy the highest market acceptance among metallic replacement materials.

A different concept is being pursued with the development of biodegradable materials. Here, an ideal implant initially assumes full stabilization at the beginning of healing. With increasing bone regeneration, the resorption of the implant material initiates an increased load transfer to the bone. In this ideal case, an optimal adaptation to the respective strength condition is achieved at any time through progressive osteointegration on the one hand and decomposition of the implant on the other. At the Fraunhofer IFAM cellular metals based on biodegradable magnesium are being developed.

Further information on biomaterials

Catalyst Support

© Fraunhofer IFAM Dresden
View into a Catalyst

Open-cell metal foams made of corrosion-resistant steels such as FeCrAl alloys are interesting for catalysis due to their high inner surface and high strength. Compared to conventional bulk catalysts, material savings of up to 30 % and increased durability of up to 90 % can be achieved. As catalyst carriers, open-cell steel foams can replace conventional carrier materials, such as ceramic honeycomb carriers made of cordierite or mullite, and metal honeycomb carriers. With coatings of e.g. Cu, Ni, NiO, Pt, Pd or Rh, the fabrication of catalysts for the production of hydrogen from hydrocarbons (reforming) or with coatings of platinum group metals or perovskite-like mixed metal oxides (e.g. La0.9Ag0.1MnO3) for the oxidation of various organic compound classes is possible. In practical tests in a biogas-fired combined heat and power plant, for example, it was shown that FeCrAl foams with TiO2 coating achieve the same or better results with drastically reduced platinum loads compared to commercial catalysts with about twenty times the amount of platinum.

Open-cell steels can be advantageously used both in motor vehicles and in industrial production plants for applications that require surface enlargement and increased turbulence for heat transport and mass transfer processes while at the same time offering good flowability.

Thermal Management

Open-cell metal foams can be used as compact heat exchangers due to their high surface area and good flowability. Good heat transfer requires good material bonding between the metal foams and the base body, which can be achieved by soldering, for example.

If there is no flow through open-cell metals, the material is suitable for thermal insulation due to its low thermal conductivity, especially for use under harsh conditions such as in industrial furnaces.

© Fraunhofer IFAM Dresden
Thermal Insulation Oven

© Fraunhofer IFAM Dresden
Surface Burner

Properties

MO_OZS — © Fraunhofer IFAM Dresden
MO OZS

Open-cell metal foams possess a very homogenous structure, which guarantees constant characteristics in wide-ranging areas. Open-cell metal foams can be produced in a large spectrum of pore sizes and densities. The adjustable pore sizes range from 0.3 to 5 mm, the relative density can be between 5 and 30 %. Because of the structure's high variability, the functional properties like mechanical strength, sound absorption, flow and heat transfer can be precisely adjusted.

Materials

Open-cell metal foams are produced in a powder metallurgical moulding process. This process enables the production from almost all materials available as powder. This allows the material properties to be adapted to the respective application to a large extent. At the Fraunhofer IFAM Dresden, we have experiences with open-cell metals made of the following materials:

Steels
unalloyed steels, high-alloy steels, ferritic, martensitic and austenitic steels, heat-resistant and high-temperature steels for applications in mechanical engineering and vehicle construction

Titanium, a-b Titanium Alloys, Tantalum
Biocompatible materials for medical applications or applications in highly corrosive environments

Molybdenum, Nickel-Based, Intermetallic Compounds
Materials for high temperature applications

Copper, Copper Alloys
Materials in which a good thermal conductivity of the base material is required (e.g. heat exchangers)

Technology

Open-cell metals are produced using a powder metallurgical replication technique in which reticulated polyurethane sponges are impregnated with a metal powder binder suspension. In the next step, the organic components (binder, suspension aids, carrier structure) are thermally removed and the powder skeleton is sintered. In this way, structures are created which, on the one hand, have a high degree of open-cellularity and, on the other hand, can be specifically influenced in their structure by selecting suitable preform sizes. In principle, all sinterable metals can be brought into an open-cell structure.

The process is designed for the production of near-net-shape components, in which the mold is inserted through the shape of the polyurethane sponge. For small components, it is in many cases more economical to produce larger plates and then produce the components by laser cutting or water jet cutting. Complex components can be joined to each other or to solid components both by sintering and by soldering.