PM2012 World Congress: Powder metallurgy process for high temperature alloy metal foam commercialised
Walther stated in his presentation that the novel Powder Metallurgy process developed by IFAM Dresden was started to be used commercially by Alantum Corp. in 2010 in a plant constructed in Onsan, Korea. The plant is currently producing around 500,000 m2 of PM high temperature foam material annually.
The new foam alloy materials produced by Alantum are said to have unique properties which allow their use for a wide variety of applications. For example from tests done on the materials it was found that FeCrAl foam is stable in oxidizing environment up to 1000°C, 30h, and that NiFeCrAl and NiCrAl foams are stable in oxidizing environment up to 1000°C, 100h.
However, NiCrAl foam has proved to be the most durable alloy foam so far because of the high amounts of Al that can be incorporated. These alloys are known to form the impervious aluminium oxide layer in the Ni-base superalloy and chromium oxide layer in the Fe-base superalloy to ensure oxidation resistance at high temperatures. Under oxidation conditions, the alloy surface concentration of chromium and aluminium decreases triggering the diffusion of Cr and Al to the surface.
Fig. 1 Metal alloy foam surfaces with pore size of 580 µm (left) and 3000 µm (right). (From paper: ‘Alloy Metal Foam Process and Applications for Filtration and Catalysts Support’ by G. Walther, B. Klöden, B. Kieback, R. Poss, J.S. Bae, M.J. Jang, M.N. Park. Presented at the PM2012 World PM Congress, Yokohama, October 15-18, and published in the Congress Proceedings by JPMA/JSP&PM, Japan)
Walther reported that the patented manufacturing process for the nickel- and iron-based, temperature stable superalloy foam involved transforming a pure metal foam substrate into a highly alloyed foam material. This was achieved by continuously unwinding pure nickel or iron foam and coating it first with a binder solution using a spraying technique and then coating with a high alloy powder.
The coated foam can be cut into sheets of the desired size which are then debound and sintered during which step the elements from the alloy powder diffuse into the foam struts and ensure a homogeneous alloy foam composition. The most important material parameters, namely the final composition, the specific surface area and the pore size of the foam can be varied in order to achieve optimum flexibility for the respective applications.
Table 1 Properties of available foam qualities and alloys. (From paper: ‘Alloy Metal Foam Process and Applications for Filtration and Catalysts Support’ by G. Walther, B. Klöden, B. Kieback, R. Poss, J.S. Bae, M.J. Jang, M.N. Park. Presented at the PM2012 World PM Congress, Yokohama, October 15-18, and published in the Congress Proceedings by JPMA/JSP&PM, Japan)
Table 1 shows the available alloys and foam qualities currently produced by Alantum. As mentioned, the foam sheets can be cut to any length, and can also be rolled, stacked, bent and shaped for the respect requirements. The thickness can be increased substantially by sintering a stack of foams together, allowing the manufacturing of geometries like cylinders or cuboids. The foam is characterized by a density < 1 g/cm³ and a porosity > 90%. There is a wide range of pore sizes and alloys available, allowing filtration characteristics like pressure drop or filtration efficiency to be adapted to the respective applications.
In terms of mechanical properties, Walther compared bending test results for the currently used NiFeCrAl alloy with the newly developed NiCrAl alloy. The NiCrAl foam was found to give the highest values and because both alloys have multi-phase microstructures, the most likely reason for the difference is the y’-phase in the NiCrAl foam. This phase is well-known for its strengthening effect in Ni-base superalloys but which decreases foam ductility. However, NiCrAl is said to offer promising possibilities to reduce the y’-phase in order to increase the ductility whilst retaining the high oxidation resistance.
Fig. 2 Schematic diagram of the foam filter (integrated with DOC and DPF function for Korean retrofit programme (left) and an installed system in a light duty diesel truck (right). (From paper: ‘Alloy Metal Foam Process and Applications for Filtration and Catalysts Support’ by G. Walther, B. Klöden, B. Kieback, R. Poss, J.S. Bae, M.J. Jang, M.N. Park. Presented at the PM2012 World PM Congress, Yokohama, October 15-18, and published in the Congress Proceedings by JPMA/JSP&PM, Japan)
Walther said that the unique combination of properties of the Alantum foam allows it to be used for a wide range of applications. One application cited was in the retrofit market for high temperature filter and catalyst substrates (i.e. as diesel particulate filter (DPF) or diesel oxidation catalyst (DOC)) in the exhaust system of commercial vehicles. In Europe filter systems are certified for light and heavy duty trucks for Euro IV.
Further development is ongoing for the Korean and Chinese markets with a so called integrated design which combines the DOC and the particle oxidation catalyst (POC) in different foam layers. Because of the close connection of the DOC and the filter the deposited soot can be removed by a continuous oxidation at low exhaust temperatures without an active regeneration system. The advantage of the irregular foam structure causes higher turbulent flow characteristics and therefore more intensive gas reaction with the catalyst. Therefore the reduction of filter volume and saving of precious metal compared to competitive substrates reduces the component cost considerably.
This innovative superalloy foam material not only has high potential for automotive exhaust after-treatment systems, but is also used for catalyst applications in reforming processes, as electrode material in fuel cells and supercapacitors, and noise absorption in automotive and motor cycle silencers. Sulphur-resistant oxidation foam catalysts are used for the after-treatment of waste gases in the desulphurisation of biogas
PM2012 World Congress Proceedings
Papers presented at the PM2012 World Congress will be published by the JPMA/JSP&PM in the Congress Proceedings, available in early 2013. Further information will be posted on the conference website.
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