Metal Foams: A Design Guide

20 Metal Foams: A Design Guide
between 0.3 and 0.5. The cell size is determined by the powder particle size,
and lies in the range 10 µm to 10 mm.
In an alternative but closely related process, a bed of particles of the leachable
material is infiltrated by liquid metal under pressure, and allowed to cool.
Leaching of the particles again gives a cellular metallic structure of great
uniformity.
2.10 Gas–metal eutectic solidification
Numerous metal alloy–hydrogen binary phase diagrams exhibit a eutectic;
these include Al-, Be-, Cr-, Cu-, Fe-, Mg-, Mn- and Ni-based alloys. The
alloys are melted, saturated with hydrogen under pressure, and then directionally
solidified, progressively reducing the pressure. During solidification, solid
metal and hydrogen simultaneously form by a gas eutectic reaction, resulting
in a porous material containing hydrogen-filled pores. These materials are
referred to as GASARs (or GASERITE).
A schematic diagram of the basic approach is shown in Figure 2.10. A
furnace placed within a pressure vessel is used to melt an alloy under an
appropriate pressure of hydrogen (typically 5–10 atmospheres of hydrogen).
This melt is then poured into a mold where directional eutectic solidification is
allowed to occur. This results in an object containing a reasonably large (up to
30%) volume fraction of pores. The pore volume fraction and pore orientation
are a sensitive function of alloy chemistry, melt over-pressure, melt superheat
(which affects the hydrogen solubility of the liquid metal), the temperature
field in the liquid during solidification, and the rate of solidification. With
so many process variables, control and optimization of the pore structure are
difficult. The method poses certain safety issues, and in its present form is
a batch process. As a result, materials manufactured by this route are costly.
Though GASAR materials were among the first highly porous materials to
attract significant interest, they remain confined to the laboratory and are not
yet commercially available.
2.11 Literature on the manufacture of metal foams
General
Astro Met, Inc. Ampormat Porous Materials, Astro Met, Inc., Cinncinnati.
Davies, G.J. and Zhen, S. (1983) Metallic foams: their production, properties, and applications.
Journal of Materials Science 18 1899–1911.
Banhart, J. and Baumeister, J. (1988) Production methods for metallic foams. In Shwartz, D.S.,
Shih, D.S., Evans, A.G. and Wadley, H.N.G. (eds) (1998) Porous and Cellular Materials for
Structural Application, Materials Research Society Proceedings, Vol. 521, MRS, Warrendale,
PA, USA.