P 431 - Development of technologies for the production of steel foams
The research project was carried out by the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research (IFAM) and the Institute of Ferrous Metallurgy at RWTH Aachen University (IEHK) as part of a …
Content
- Publication
- 2003
- Authors
- Dr.-Ing. Gerald Rausch, Prof. Dr.-Ing. Wolfgang Bleck, Dipl.-Ing. Ulrike Mohr
The research project was carried out by the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research (IFAM) and the Institute of Ferrous Metallurgy at RWTH Aachen University (IEHK) as part of a joint institute project of the Federal Ministry of Education and Research (BMBF). The aim was to develop and evaluate suitable processes for the production of porous steel materials based on powder and fusion metallurgy. The project was co-funded by the Steel Application Research Foundation, Essen, through the Studiengesellschaft Stahlanwendung e.V.. The overall project is divided into two sub-projects, which were carried out at IFAM Bremen and IEHK Aachen.
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In the IFAM sub-project, two different powder metallurgical technologies were investigated, the blowing agent process and the spacer process. In the blowing agent process, a model material is produced from a mixture of iron powder and carbon (2.5 %) and the blowing agent strontium carbonate (0 to 0.5 %). By extruding the powder mixture, compaction to 97 to 99 % of the theoretical density is achieved. Forming the foamable semi-finished product by hot rolling is possible and has no significant effect on the properties of the foams produced from it. Inductive heating of the semi-finished product produced in this way up to the semi-liquid phase range causes a decomposition reaction of the blowing agent with the release of carbon dioxide CO2. The released gas leads to the formation of pores, and after cooling, the materials have porosities of up to 64%. The blowing agent process is suitable for the production of near-net-shape components, although there is still a need for further development in the production of suitable foaming moulds. In the placeholder process, a stainless steel powder is mixed with a plastic granulate ("placeholder") and compacted, whereby the placeholders are removed again in a thermal and/or chemical expulsion process. This produces open-pored steel foams with a porosity of up to 90 %. Various processes can be used for compaction and moulding, e.g. axial powder pressing, hot pressing, cold isostatic pressing and even a modified metal powder injection moulding process. The selection of the appropriate process depends on the component geometry, the porosity of the component and the quantity to be produced. For example, filter cartridges with a diameter of 62 mm and a length of 65 mm with an open porosity of 60 % were produced by cold isostatic pressing and subsequent heat treatment. In the IEHK sub-project, a melting metallurgical production route for open-pored steel foams was initially investigated. The selected process is based on the recasting of mineral spacers. In the process, foundry sand is pelletised with an organic binder to form spherical spacers, then heated to 500 °C in a casting mould and encapsulated with a superheated molten metal. The casting heat causes the organic binder to break down so that the foundry sand can be mechanically removed from the casting after solidification. Open-pored metal foams with a pore size of 2 to 5 mm and densities between 2.8 and 3.4 g/cm3 can be produced using this process. In the course of the investigations, a number of fundamental process limitations emerged that make this method unsuitable for the production of fine-pored steel foams. For this reason, an alternative manufacturing process was evaluated to achieve the formulated objectives: the slurry reaction foam sintering (SRSS) process. This process can be used to produce open-pored metal foams from a variety of different iron and steel powders as well as refractory metals and alloys as a base material. The green foams are produced at room temperature with little equipment. A slurry is produced from a fine metal powder (particle size between 50 and 150 μm) and a solvent, for example water or alcohol, possibly with the addition of a suspension-stabilising dispersant. Concentrated phosphoric acid is added to this slurry as a binding agent. Two parallel reactions then take place in the suspension, which lead to the formation of a stable foam green compact. Firstly, the chemical reaction between the metal and the acid produces hydrogen, which causes the slurry to foam up. Secondly, a metal phosphate is formed, which acts as a binding agent and leads to the "freezing" of the foam structure. As the green compact dries at room temperature, the initially closed-pored structure is transformed into an open-pored structure as the solvent evaporates. The green compact is sintered in an oxygen-free atmosphere to form an open-pored metal foam. Foams with absolute densities r between 1.0 and 2.5 g/cm³ and pore diameters between 0.01 and 5.0 mm can be produced. The SRSS steel foams have good structural and functional properties. Applications are particularly conceivable in areas where good flowability and a high specific surface area are required, such as in filter elements, catalyst carriers or components for sweat cooling. The SRSS process can be used for a wide range of matrix materials such as pure iron, various unalloyed and high-alloy steels, nickel, copper and zinc.
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P 431 - Development of technologies for the production of steel foams—
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