By making a preform of the cell shapes, which is subsequently used as a disposable mould during an infiltration stage, cell structures can be 'designed'. Because factors such as cell size and arrangement are more rigidly defined than in most other production routes, these foams have highly reproducible material properties.

Polymer preform

In one process, aluminium is rapidly injected into a mould containing polystyrene particles, and solidifies before the spheres decompose. Foams can be produced with porosity between 73 and 86%, and cell sizes between 1 and 3 mm. Because rapid high-pressure injection is required to infiltrate the spheres in ~0.2 seconds before they decompose completely, only small samples can be produced.

Water-soluble preform

In a process developed at the Swiss Federal Institute of Technology, preforms are made of a water-soluble material which is capable of withstanding the temperature of the molten metal. A mould is filled with the mould material - typically rock salt due to its low cost - which is vibrated and tapped until a stable density is achieved (1 in Figure 1). It is then sintered in air (2) and cooled. By fusing the salt grains together at the points of contact this produces a rigid block of salt in the shape of the mould, with open channels in the spaces between the original grains (3). In the simplest process, molten aluminium is simply poured onto the block to infiltrate the channels. The entire structure is then removed from the mould, cooled, and placed in water to dissolve the salt (6). This produces a foam with open-celled grains whose size and layout closely matches that of the original salt grains, and a minimum cell size of approximately 1 mm.

A more advanced version of this process uses a hot-wall pressure infiltration process, where the salt preform is held under vacuum while a block of aluminium on the preform is melted (4), and high pressure of an inert gas is applied during a subsequent infiltration step (5). The salt is then dissolved (6) to give an open-cell foam (7). This use of pressure during the infiltration step enables foams with cells as small as 50µm to be produced.

Figure 1: Production of an open-cell foam by investment casting using a water-soluble preform.

While maintaining the advantages of the powder-route process described in the powder-route mixing-and-dissolving process, this process offers the additional possibility of using salt grains of various sizes during production of the preform to produce foams with different sized cells in different regions, or with closely controlled distributions of cell sizes. The cost is also lower because metal powder is not required (which avoids production costs and also the expense of dealing with powdered metals).

Net-shape foams with a cell sizes between 50µm and 5 mm have been produced. An example is shown in Figure 2. There are no restrictions on the metals or alloys which can be used in this process, other than not reacting with the salt or water.

The quality of the foams produced is good, and a very small cell size can be achieved. The disadvantages are that the process is slow, difficult to scale up, and involves many steps - and is correspondingly costly.