Fabrication of metal matrix composites

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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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