Current technologies for aluminum castings and their machinability

Metal-Matrix Composites

Deborah D.L. Chung, in Carbon Composites (Second Edition), 2017

9.2.3 Liquid Metal Infiltration and Squeeze Casting

In traditional metal casting, metal liquid is poured into a mold, which has a hollow cavity of the desired shape and dimensions, and then the liquid is allowed to cool and solidify. However, the pouring operation can be replaced by other methods of introducing the liquid metal into a mold.
In case of the metal in the liquid state, liquid metal infiltration is dominant. This method involves the squeezing of the liquid metal into a preform of the reinforcement. The low viscosity of liquid metals compared to resins or glasses makes infiltration very appropriate for metal-matrix composites. Nevertheless, pressure is usually required because of the difficulty for the liquid metal to wet the carbon fibers.
The preform typically contains a small amount of a binder, which is necessary to provide a degree of mechanical integrity to the preform. Otherwise, the preform may be deformed during the liquid metal infiltration. The deformation would mean that the process is not near-net-shape. The binder amount must be low, so that the binder would not block the channels in the preform for the liquid metal to infiltrate. When the reinforcement volume fraction is high, the channels are small in width and the binder needs to be effective even at a very low binder amount. The squeezing of the liquid metal can be achieved by using a piston (a variation of the method known as squeeze casting, Fig. 9.3) or a gas pressure (a variation of the method known as pressure casting). Squeeze casting is faster than pressure casting. However, as infiltration using a gas pressure involves a smaller rate of pressure increase compared to using a piston, infiltration is more suitable than squeeze casting for near-net shape processing.
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Instead of treating the carbon fibers, the wetting of the carbon fibers by molten metals can be improved by the addition of alloying elements into the molten metals. For aluminum as the matrix, effective alloying elements include Mg, Cu, and Fe.
Even though the surfaces of a fiber bundle are wet well with the molten metal, infiltration of the metal into the fiber bundle is limited. This problem can be alleviated by ultrasonic vibration of the molten metal (Katzman, 1990).