The use of chemical foaming agents gives more control of cell structure than the direct injection of a gas. However, it is still relatively difficult to control the dispersion of the foaming agent within the melt, meaning that foam components cannot easily be made with homogenous cell structures. Because access to the full melt volume is necessary in order to fully disperse the powder within the melt, the foam produced has to have a relatively simple shape. The time available for foaming, and the temperature at which it takes place, are restricted by the need to accommodate the stirring of the powder into the melt.

A number of processes have been developed which seek to improve on the process by separating the dispersion of the foaming agent within the melt from the decomposition of the agent to form a foam. This provides a number of advantages over the single-step process. As foaming is not taking place during the mixing, more time is available to mix the powdered foaming agent with the melt, meaning that finer powders can be used, and they can be more uniformly dispersed. Another significant advantage is that a foamable precursor is produced which can be stored indefinitely, and cut sections of it can be heated within complex-shaped moulds without the need for access by a propeller. Net-shape foam products can thus be produced with a solid outer skin matching the shape of the desired end product. With a two-step process it is possible to control the temperature during the foaming stage more closely than in the direct gas or powder injection methods, which in turn gives better control over the porosity and cell structure.

Foamable precursor produced by powder metallurgy techniques
One of the earliest methods developed to produce a foamable precursor was to mix powdered metal with a foaming agent and any reinforcement required to increase the melt viscosity. The key to producing a suitable precursor is to compress the mixed powders into a relatively solid block, so that when foaming takes place the gas evolved does not simply escape from the material.

This can be achieved by cold compacting a powder mixture, followed by cold extrusion. The friction between particles during the extrusion destroys the oxide coatings on the particles and bonds them together . Alternatively, the powder mixture can be hot pressed at a temperature below that which will cause the decomposition of the foaming agent . In some cases it is possible to press the powder at temperatures above the decomposition temperature of the foaming agent, as the foaming agent powder becomes trapped by the metal (which is well below its melting temperature) and decomposition is inhibited by the high pressure. In a subsequent step, the precursor is melted inside a mould, and heated above the decomposition temperature of the foaming agent.

The cell structure of foams produced with these precursors are closely dependent on the hot pressing temperature, the baking temperature and time, and the alloy used, and weakly dependent on the rate at which the precursor is heated during the second step.

Figure 1: A foam produced by a two-step process, with a foamable precursor
formed from metal powders and subsequently baked.

The hot pressing method has been used to produce foams made of aluminium, bronze and copper, using between 0.5 and 1 wt.% of titanium hydride or sodium bicarbonate as foaming agents. This method is used commercially by Alulight International to produce various aluminium alloy foams with porosities between 63% and 89%, and cell sizes of the order of millimetres, as shown in Figure 1. The technique has also been used to produce steel foams, and foamed sandwich panels and complex shaped bodies in a single step.

A wide range of alloys can be used by using appropriate mixtures of metal powders, and by choosing appropriate heat treatments during the foaming step it is possible to obtain cell structures which are markedly better than the continuously processed foam. However, some form of reinforcement is still required in order to stabilise the foam, which in turn imposes some limits on the choice of alloys. The main disadvantage of powder metallurgy techniques are their relatively high cost, due mainly to the need to produce, mix and handle fine metal powders.

Foamable precursor produced by melt-route processing
The FORMGRIP process uses melt processing to produce a foamable precursor material. By subjecting titanium hydride to heat treatment, as shown in Figure 2, it is possible to produce a layer of titanium oxide on the surface, which has only limited permeability to hydrogen. The hydride is then mixed into an aluminium melt (which has added silicon carbide particles to increase the viscosity), with the oxide layer on the hydride particles acting as a barrier to delay the decomposition. The oxide delays the foaming for a sufficiently long time to enable the titanium hydride to be thoroughly dispersed within the melt. This produces a slightly porous precursor made of metal, foaming agent, and silicon carbide.

Figure 2: The first step of the FORMGRIP process: melt-route production of a foamable precursor

In a second step, the precursor is baked inside a mould, typically at 630ºC, to melt the alloy and cause the decomposition of titanium hydride to give off hydrogen gas. By adjusting the time of baking, foams with porosities between 50% and 95% can be obtained, with cell sizes between 1 and 10 mm. Because of the improved dispersion, the foams have a uniform cell structure. A typical foam produced is shown in Figure 2.25.

Figure 3: A FORMGRIP foam

These are the most homogenous and controlled foam structures which can be produced by melt-route processes. The precursor materials can be heated inside complex moulds, to produce tailored foam components. None of the steps in the process is particularly costly. The main disadvantage of this process is the requirement to add viscosity enhancing SiC particles to the melt, which adds to the cost of the process and makes the resulting foams more brittle.