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Projektinfo – Detailed information on energy research

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Open-cell metal foam
© Fraunhofer IFAM Dresden
Metallic materials
Projektinfo 11/2016

Latent heat storage device with aluminium foam-paraffin composite
© Fraunhofer IFAM Dresden

In addition to metal foams, selected fibrous structures were also produced as sandwich samples for the heat transfer measurements
© Fraunhofer IFAM Dresden

Metal foam replaces the lamella structure in a model cooler.
© Fraunhofer IFAM Dresden
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Metal foam – a material for heat engineering

Metal foams are increasingly developing into materials with diverse uses. While metal foams with closed pores have already become established as rigid and strong lightweight materials, the open-cell variant is suitable for thermal engineering applications. Until now, the material has been rarely used in heat exchangers or coolers because the production is expensive and its application little tested. Researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM in Dresden now want to change this. Together with industry partners, they are further developing the production method, are characterising different metal foams and are testing them in practice.

Metal foams provide an ideal prerequisite for constructing heat exchangers, coolers and convectors. Their porosity of up to 95 per cent makes it easy for gases or liquids to flow through them, whereby the large surface area of the foam together with the good conductivity of the metal enables the transmission of large amounts of heat. Open-cell metal foams can be made with cell widths between 0.3 and 5 mm. However, the thermal and fluid dynamic behaviour of the different metal foams had not been sufficiently researched in the past. This was, together with the high production costs, the biggest hurdle for using the material in power engineering.

m.pore GmbH is one of the few companies capable of producing open-pore metal foams in a precision casting process: open-cell polyurethane foams that are available in different pore sizes are used as the model for the casting. The size of the pores and the thickness of the webs between the pores dictate the subsequent properties of the metal foam. In a first step, the technicians stabilise the thin webs with wax and bring them to the desired thickness. They then cast the plastic with a liquid ceramic suspension. Its high water content enables it to penetrate into the fine pores and fill them. It dries and solidifies at a temperature of 120 °C. A further increase in temperature to 600 °C decomposes the shaping plastic and cures the resulting refractory mould. The still hot mould can now be filled with any castable metal alloy. Aluminium alloy is used for most applications. After cooling, the speciality ceramic decomposes and can be washed out.

The researchers have optimised each individual process step for a new business location. Partial automation has enabled them to more than double the furnace throughput and reduce the specific energy consumption. In the future it is intended to treat and reuse both the water and the ceramic. The manufacturing process is used to produce foam metal sheets 450 x 250 x 40 millimetres in size. Their inner surface is about the size of a football field.

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Powder metallurgical process

If metal foams are required with very small cell diameters, the precision casting process is pushed to its limits. The metal no longer flows completely into the cavities when the pore diameters are less than about one millimetre in size. However, scientists at Fraunhofer IFAM in Dresden can create such delicate structures using a powder metallurgy manufacturing technology that was developed several years ago. They also use wax-stabilised PU foam as the shaping model. They coat this with a metal powder-binder suspension. They then decompose the resin and the binder by applying high temperatures. What remains is a powder metal skeleton that is sintered to form a solid structure at about 80% of the melting temperature of the metal alloy used. The trick is to avoid cracking and pore defects. Researchers have improved the method with new powders and binders. In particular, they have recently succeeded in producing small-cell copper foams.

Systematically measuring metal foams

The pore size, web thickness and shape as well as the choice of metal alloy substantially determine the thermal and fluidic properties of metal foams. However, it has not yet been possible to precisely predict these properties using mathematical models. The scientists have therefore developed different test rigs to systematically measure the effective thermal conductivity of the metal foams and metal fibre samples, the heat transfer to a gas flow and the pressure loss generated by the metal structures. They have summarised the measurement results in a database. This also serves as a basis for improving the modelling of metal foams mathematically.

Can metal foams and metal fibres also exploit their physical advantages relative to conventional coolers, heat exchangers or convectors in practice? Experimental investigations conducted on various technical systems have tried to answer this question.

Projektinfo 11/2016:
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Addresses

Systematic characterisation of open-cell metal structures, project management
Fraunhofer IFAM

Development of prototype components for heat transfer and heat storage
WätaS GmbH

Casting-based manufacturing of open-cell metal foams
m.Pore GmbH

Links

IFAM
Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM

IWU
Fraunhofer Institute for Machine Tools and Forming Technology IWU