Metal Foams: A Design Guide

Chapter 4
Properties of metal foams
The characteristics of a foam are best summarized by describing the material
from which it is made, its relative density, / s (the foam density, , divided
by that of the solid material of the cell wall, s), and stating whether it has
open or closed cells. Beyond this, foam properties are influenced by structure,
particularly by anisotropy and by defects – by which we mean wiggly, buckled
or broken cell walls, and cells of exceptional size or shape.
Metal foams are still inadequately characterized, but the picture is changing
rapidly. An overview of the range spanned by their properties is given by
Table 4.1 and the property charts of Section 4.3. The primary links between
properties, density and structure are captured in scaling relations, listed in
Section 4.4. They allow foam properties to be estimated, at an approximate
level, when solid properties are known.
The producers of metal foams have aggressive development programs for
their materials. The properties described here are those of the currently available
generation of foams, and should be regarded as a basis for initial, scoping,
calculations and designs. The next generation of foams will certainly be better.
Final design calculations must be based on data provided by the supplier.
4.1 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 / s D 0.05, and an average cell size approaching 5 mm; foams
from this source are available in the range 0.02 < / s < 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 < / s < 0.2; that shown here has a value of