المحاضرة 7

Lec.7
Processing of metal matrix composites
Fibers and particles can be processed to porous preforms with the addition of binders in the following ways:
• processing of long fibers for reinforcement and fabrics
• pressing and burning/sintering
• casting of an aqueous suspension, profiling and subsequent water removal
• gel-cast freeze drying.
Figure 1 shows examples of preforms of different geometry and materials. Since the production of fiber preforms and hybrid preforms (fibers + particles) is already described in detail in other places in this book, only the production of particle preforms is described here.

Fig. 1 Examples of different preforms.
II-cylinder crankcases in series; for the achievement of a high porosity of 70–75% the gel-casting freeze-drying method is used (Fig. 2). In this procedure the pores are introduced by the place holder, water. A homogeneous slurry, consisting of silicon powder, water and binders is produced first and cast into a metallic negative form of the desired geometry. After cooling the filled form, at approximately 60°C, to temperatures below the freezing point, the parts are freeze dried in the frozen condition. That means the water is sublimed at approximately 6 mbar pressure by a supply of energy. Thus, in the places where the ice crystals were previously, the desired uniform cavities are produced, and these are then infiltrated with aluminum during the casting process.