Fabrication of Metal Matrix Composites

Fabrication Methods of MMCs
Various methods are used to manufacture MMCs, according to the temperature of the matrix during preparation of these composites, these methods can be classified to: liquid-phase processes, solid state processes, deposition techniques, in situ processes, and semi-solid state processes.
Liquid state fabrication of MMCs includes adding and mixing of dispersed phase with the molten matrix, followed by its solidification.Good interfacial bonding (wetting) between the reinforcing phase and the liquid matrix must be obtained in order to produce composites with good mechanical properties. The methods of liquid state fabrication of metal matrix composites are: stir casting, infiltration like gas pressure infiltration, squeeze casting infiltration or pressure die infiltration .
In solid state processing of MMCs,composites are formed due to bonding between reinforcement phase and matrix as a result to reciprocal diffusion which occur between them in solid state at elevated temperature and under pressure.The types of solid state method include:diffusion bonding and powder metallurgy.
Deposition Techniques of MMCs include coating of individual fibers in a tow with the matrix material required for forming of composite followed by diffusion bonding to forma consolidated composite plate or structural shape.In-situ process of MMCs includes creating of reinforcing phase in situ. The MMCs is fabricated in one step beginning with ansuitable alloy, so avoiding the difficulties in combining the separate components. Semi-solid state processes, as semi-solid powder processing, in which, powder mixture is heated up to semi-solid state and pressure is applied to form the composite.
Stir casting is considered as the simplest and most economic technique of liquid state fabrication.In this process, the reinforcement particles are mixed with molten matrix using mechanical stirring as shown in Fig. (below). The success secret of this method depend on stirring conditions of the mixture molten in the furnace. The resulted molten alloy with the reinforcing particles can then be utilized for die-casting, permanent mold casting or sand casting. Stir casting is suitable for fabricating composites up to 30% volume fractions of reinforcement .