25Casting using a polymer or wax precursor as template

Open-cell polymer foams with low relative densities and a wide range of cell sizes of great uniformity are available from numerous sources. They can be used as templates to create investment-casting molds into which a variety of metals and their alloys can be cast. It is thought that the ERG DUOCEL range of foams are made in this way. The method is schematically illustrated in Figure 2.5.

PARTICLE DECOMPOSITION IN SEMI-SOLID

a) Select Ingredients

b) Consolidation & Extrusion

Extrusion die

Extrusion die

Dense bar or plate Preconsolidated billet —^

c) Shaped mold

Shaped container

Extruded alloy bar or plate (containing foaming agent)

d) Foaming

Foamed component

Shaped mold

Furnace _I

Figure 2.4 The sequence of powder metallurgy steps used to manufacture metal foams by gas-releasing particles in semi-solids (the Fraunhofer and the Alulight processes)

Shaped mold

Furnace _I

Figure 2.4 The sequence of powder metallurgy steps used to manufacture metal foams by gas-releasing particles in semi-solids (the Fraunhofer and the Alulight processes)

a) Preform

Polymer ligaments

Polymer ligaments

d) Remove mold material

SOLIDIFICATION IN OPEN CELL MOLD b) Burnout

Casting slurry (sand)

Open channels

H2O CO

c) Infiltrate

H2O CO

Open channels

Mold material

Mold material

Pressure

Molten metal o

Pressure

Molten metal d) Remove mold material

Figure 2.5 Investment casting method used to manufacture open cell foams (DUOCEL process)

An open-cell polymer foam mold template with the desired cell size and relative density is first selected. This can be coated with a mold casting (ceramic powder) slurry which is then dried and embedded in casting sand. The mold is then baked both to harden the casting material and to decompose (and evaporate) the polymer template, leaving behind a negative image of the foam. This mold is subsequently filled with a metal alloy and allowed to cool. The use of a moderate pressure during melt infiltration can overcome the resistance to flow of some liquid alloys. After directional solidification and 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.

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