Metal Foams: A Design Guide

14 Metal Foams: A Design Guide
cooling, the mold materials are removed leaving behind the metal equivalent
of the original polymer foam. Metal powder slurries can also be used instead
of liquid metals. These are subsequently sintered. The method gives open-cell
foams with pore sizes of 1–5 mm and relative densities as low as 0.05. The
process can be used to manufacture foams from almost any metal that can be
investment cast.
In a variant of the process, the precursor structure is assembled from
injection-molded polymeric or wax lattices. The lattice structure is coated with
a casting slurry and fired, burning it out and leaving a negative image mold.
Metal is cast or pressure-cast into the mold using conventional investment
casting techniques.
2.6 Metal deposition on cellular preforms
Open-cell polymer foams can serve as templates upon which metals
are deposited by chemical vapor decomposition (CVD), by evaporation
or by electrodeposition. In the INCO process, nickel is deposited by
the decomposition of nickel carbonyl, Ni⊲CO⊳4. Figure 2.6 schematically
illustrates one approach in which an open-cell polymer is placed in a CVD
reactor and nickel carbonyl is introduced. This gas decomposes to nickel and
carbon monoxide at a temperature of about 100°C and coats all the exposed
heated surfaces within the reactor. Infrared or RF heating can be used to
heat only the polymer foam. After several tens of micrometers of the metal
have been deposited, the metal-coated polymer foam is removed from the
CVD reactor and the polymer is burnt out by heating in air. This results in a
cellular metal structure with hollow ligaments. A subsequent sintering step is
used to densify the ligaments.
Nickel carbonyl gas is highly toxic and requires costly environmental
controls before it can be used for manufacturing nickel foams. Some countries,
such as the United States, have effectively banned its use and others make it
prohibitively expensive to implement industrial processes that utilize nickel
carbonyl gas. Electro- or electroless deposition methods have also been used to
coat the preforms, but the nickel deposited by the CVD technique has a lower
electrical resistance than that created by other methods. The pore size can be
varied over a wide range. Foams with open pore sizes in the 100–300 µm
diameter range are available. The method is restricted to pure elements such
as nickel or titanium because of the difficulty of CVD or electrodeposition of
alloys. It gives the lowest relative density (0.02–0.05) foams available today.
2.7 Entrapped gas expansion
The solubility in metals of inert gases like argon is very low. Powder metallurgy
techniques have been developed to manufacture materials with a dispersion
of small pores containing an inert gas at a high pressure. When these