Fabrication of metal matrix composites

Fabrication techniques of metal matrix composites

Due to demand of high strength, high toughness and low-density, the metal matrix composite are extensively used over monolithic metals. Metal matrix composites have better physical and mechanical properties such high specific modulus, strength and thermal stability as compared with pure metallic counterpart. Selection of materials together with the processing parameter (e.g. sintering temperature) plays vital role in metal matrix composite prepared via powder metallurgy route. Aluminum is widely used as matrix phase in metal matrix composite due to its light weight and high ductility. Selection of fabrication process is very important for the uniform distribution and dispersion of reinforcement phase into matrix phase. The MMC fabrication process can be divided into three categories as follows:

1.      Solid phase processes

2.      Liquid phase processes

3.      Two- phase (solid- liquid) processes

Solid phase process

Solid state phase process of metal matrix composites is a method, in which the metal matrix composites are produced as a result of closeness of matrix metal and reinforcement phase due to mutual dispersion occurring between them in solid state phase at under pressure and elevated temperature. The low temperature of solid state fabrication process reduces undesirable reactions on the boundary between the matrixes and dispersed isolated phases. The properties of metal matrix composites may be changed also upon the sintering operation by pressing, rolling, forging and drawing or extrusion. The deformation operation may be two types – one is hot working (above the recrystallization temperature) and another is cold working (below recrystallization temperature). The MMC fabrication methods belongs to this category are: (a) powder metallurgy and (b) diffusion bonding.

Liquid phase process

In liquid phase processes, the fabrication of metal matrix composites involves acombination of discrete

 phase into a molten metal matrix, followed by its solidifications. Good interfacial bondingbetween the 

dispersed phase and the liquid matrix phase is required in order to achieve the high level of mechanical 

properties of the composites. Proper coating of reinforcement phase not only prevents chemical 

interaction but also reduces the interfacial energy between the dispersed phase and the matrix. The

 simplest method of liquid state fabrication is stir casting, while, other method of liquid state fabrication

 of MMC is infiltration method. In stir casting process at first, we have to make die cavity then molten 

metal is poured into die cavity at slow speed, a high pressure is applied during solidification. In this 

technique, all the absorbed gasses expelled by force of infiltration.

Two phase process

The two phase processof MMC fabrication involves the mixing of ceramic (as reinforcement phase) and metal (as matrix phase) in a region of the phase diagram where the matrix contains both liquid and solid phases. Two phase methods are three types, such as: (a) spray deposition, (b) compo casting and (c) variable code position of multiphasiccomposite materials.

1      Deposition technique

Deposition technique can also be employed for fabrication of metal matrix composites. In this process coating of individual fibres in a drag of matrix materials is required to fabricate the composite followed by diffusion bonding for the formation of combined composite plate or structural shape.  The microstructures of the composite prepared by this process are similar to that of the casting process. Three popular deposition techniques of MMC fabrication methods are: (i) electroplating, (ii) immersion plating, and (iii) spray deposition method.

In-situ processes

There are several techniques to prepare reinforcement materials, including the formation of compounds and their decompositions, phase change, nucleation and growth of recrystallization. The reinforced phase is formed in situ during the composite fabrication process. In this process particles are formed in solvent (which can exist in all the three form of states of matter) due to chemical reaction or diffusion which is normally occur under constant temperature (isothermal). This process generally produces periodic microstructure. In-situ process provides excellent reinforcement of dispersed phase in metal matrix for MMCs.

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