41 Foam structure

Figures 4.1(a)-(c) show the structure of metal foams from three different suppliers: Cymat, Mepura (Alulight) and Shinko (Alporas). The structures are very like those of soap films: polyhedral cells with thin cell faces bordered by thicker cell edges ('Plateau borders'). Some of the features appear to be governed by surface energy, as they are in soap films: the Plateau borders are an example. But others are not: many faces have non-uniform curvature or are corrugated, and have occasional broken walls that still hang in place.

The three figures are ordered such that the relative density increases from the top to the bottom. The Cymat (Al-SiC) foam in Figure 4.4(a) has a relative density p/ps = 0.05, and an average cell size approaching 5 mm; foams from this source are available in the range 0.02 < p/ps < 0.2. The Alporas (Al-Ca) foam in Figure 4.4(b) has smaller cells and comes in a narrower range of relative density: 0.08 < p/ps < 0.2; that shown here has a value of

Figure 4.1 (a) A Cymat foam of relative density p/ps = 0.04 (density 108 kg/m3 or 6.7lb ft3). (b) An Alporas foam of relative density p/ps = 0.09 (density 240 kg/m or 15 lb ft3). (c) An Alulight foam of relative density p/ps = 0.25 (density 435 kg/m3 or 270 lb ft3)

Figure 4.1 (a) A Cymat foam of relative density p/ps = 0.04 (density 108 kg/m3 or 6.7lb ft3). (b) An Alporas foam of relative density p/ps = 0.09 (density 240 kg/m or 15 lb ft3). (c) An Alulight foam of relative density p/ps = 0.25 (density 435 kg/m3 or 270 lb ft3)

0.09. The Alulight foam (Al-TiH) in Figure 4.4(c) has a relative density of 0.25, which lies at the upper end of the range in which this material is made (0.1 < p/ps < 0.35).

The properties of metal foams depend most directly on those of the material from which they are made and on their relative density; but they are influenced by structure too. This influence, imperfectly understood at present, is a topic of intense current study. Better understanding will lead to greater process control and improved properties. But for now we must accept the structures as they are, and explore how best to design with the present generation of foams. With this in mind, we document here the properties of currently available, commercial, metal foams.

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