Metal Matrix Composites(MMC)

• Metal matrix composites consist of a metal or an alloy as the continuous matrix and a
reinforcement that can be particle, short fiber or whisker, or continuous fiber.
• High strength, fracture toughness and stiffness are offered by metal matrices when
compared to their polymer counterparts.
• They can withstand elevated temperature than polymer composites.
• Most metals and alloys - used as matrices. Hence, require reinforcement materials -
stable over a range of temperatures and non-reactive too.
• An example is a material consisting of tungsten carbide particles embedded in a cobalt
matrix, which is used extensively in cutting tools and dies.
• MMCs are made by dispersing a reinforcing material into a metal matrix. The
reinforcement surface can be coated to prevent a chemical reaction with the matrix. For
example, carbon fibers are commonly used in aluminium matrix to synthesize composites
showing low density and high strength. However, carbon reacts with aluminium to
generate a brittle and water-soluble compound Al4C3 on the surface of the fiber. To
prevent this reaction, the carbon fibers are coated with nickel or titanium boride.
Compared to monolithic metals, MMCs have:
• Higher strength-to-density ratios
• Higher stiffness-to-density ratios
• Better fatigue resistance
• Better elevated temperature properties
• -- Higher strength
• -- Lower creep rate
• Lower coefficients of thermal expansion than metals by reinforcing with graphite.
• Better wear resistance
The advantages of MMCs over polymer matrix composites are:
• Higher temperature capability
• Higher transverse stiffness and strength
• No moisture absorption.
• Higher electrical and thermal conductivities
• Better radiation resistance
• Fire resistance.
• Higher specific strength and modulus over metals.
Disadvantages of MMCs
• Higher cost.
• Complex fabrication methods for fiber reinforced systems.
• Metal matrices are poor in chemical and mechanical compatibility with the
reinforcements.
• MMCs possess inferior insulating properties than PMCs.
• More susceptible to interface degradation at the fiber/matrix interface.
 Application of MMCs
• Metal matrix composites are used in a range of applications ranging from
aerospace, space, automobile, cutting tools, power transmission, consumer
electronics, defense, and sports
• The enhanced stiffness and strength of the MMCs make them highly suitable for
applications in military and commercial aircrafts.
• Transportation sector has been the prime consumer of MMCs and the
applications in this field include drive shafts, engine and brake components. For
example, modern sport cars built use rotor components made of carbon fiber
composites with better specific heat and thermal conductivity.
• A common example of MMCs in sports, is the lightweight bicycle frame made
using aluminium/titanium matrix composites and carbon fibers. Some other
sporting applications include fishing rods, bicycle frames, golf club heads, and
tennis/squash rackets.
• In electronics applications, MMCs are used in the new generation advanced
integrated circuits to overcome heat dissipation and thermal fatigue concerns
Types of Metal Matrix Composites
1.Particle-reinforced MMCs
2. Short fiber- or whisker-reinforced MMCs
3. Continuous fiber- or sheet-reinforced MMCs
4. Laminated or layered MMCs
Classification of Metals
Important Metallic Matrices
1.Aluminum Matrix Composites.
• This is the widest group of Metal Matrix Composites.
• Matrices of Aluminum Matrix Composites are usually based on aluminum-silicon (Al-Si)
alloys.
• Aluminum Matrix Composites (AMC) are reinforced by:
o Alumina (Al2O3) or silicon carbide (SiC) particles (particulate Composites)
o Continuous fibers of alumina, silicon carbide, Graphite (long-fiber reinforced composites);
o Discontinuous fibers of alumina (short-fiber reinforced composites)
• Aluminum Matrix Composites are manufactured by the following fabrication methods:
Powder metallurgy; Stir casting, Infiltration.
• The following properties are typical for Aluminum Matrix Composites:
o High strength
o High stiffness (modulus of elasticity);
o Low density; High thermal conductivity;
o Excellent abrasion resistance.
• Aluminum Matrix Composites (AMC) are used for manufacturing automotive parts
(pistons, pushrods, brake components), brake rotors for high speed trains, bicycles, golf
clubs, electronic substrates, cors for high voltage electrical cables.
2.Magnesium Matrix Composite
• Magnesium Matrix Composites are reinforced mainly by silicon
carbide (SiC) particles (particulate composites)
• The following properties are typical for Magnesium Matrix
Composites:
o Low density
o High stiffness (modulus of elasticity)
o High wear resistance
o Good strength even at elevated temperatures
o Good creep resistance.
• Magnesium Matrix Composites are used for manufacturing
components for racing cars, lightweight automotive brake system,
aircraft parts for: gearboxes, transmissions, compressors and engine
3. Titanium-Matrix Composites
• Titanium was selected for use as a matrix metal because of its good specific
strength at both room elevated temperatures and its excellent corrosion
resistance.
• Titanium Matrix Composites are reinforced mainly by:
o Continuous monofilament silicon carbide fiber (long-fiber reinforced composites)
o Titanium boride (TiB2) and titanium carbide (TiC) particles (particulate composites).
• The following properties are typical for Titanium Matrix Composites:
o High strength
o High stiffness (modulus of elasticity)
o High creep resistance
o High thermal stability
o High wear resistance.
• Potential applications for continuous titanium MMCs lie primarily in the
aerospace industry and include major aircraft structural components and
fan and compressor blades for advanced turbine engines.
4. Copper Matrix Composites
• Copper Matrix Composites are reinforced by:
o Continuous fibers of carbon , silicon carbon (SiC), tungsten (W), stainless steel (long-
fiber reinforced composites)
o Silicon carbide particles (particulate composites).
• Powder metallurgy and infiltration technique are used for fabrication of
Copper Matrix Composites.
• The following properties are typical for Copper Matrix Composites:
o Low coefficient of thermal expansion
o High stiffness (modulus of elasticity)
o Good electrical conductivity
o High thermal conductivity
o Good wear resistance.
• Copper Matrix Composites are used for manufacturing electronic relays,
electrically conducting springs and other electrical and electronic
components.
MANUFACTURING OF METAL MATRIX COMPOSITES
Processing of metal matrix composites (MMC) can be classified into
three main categories: -
• Solid State Processing
• Liquid State Processing
• In-Situ Processing
Liquid State Processing
• Metals with melting temperatures that are not too high, such as aluminum, can be incorporated
easily as a matrix by liquid route.
1. Casting
• Casting involves infiltration of a fiber bundle by liquid metal.
• One commercially successful liquid infiltration process involving particulate reinforcement is the
Duralcan process.
• Figure shows a schematic of this process
• Ceramic particles and ingot aluminum are mixed and melted
• The melt is stirred just above the liquidus temperature—generally
between 600 and 700 °C.
• The melt is then converted into one of the following four forms:
extrusion blank, foundry ingot, rolling bloom, or rolling ingot.
• The Duralcan process of making particulate composites by liquid
metal casting involves the use of 8–12 μm ceramic particles. Too small
particles, e.g., 2–3 μm , will result in a very large interface region and
thus a very viscous melt.
Stir Casting
• In stir casting Particulate reinforcements are mixed with liquid metal melt and the mixture then solidifies
• In this method, particulate reinforcement is gradually added to the molten matrix metal which is stirred.
(Reinforcements are preheated at certain temperature to remove moisture, impurities etc before adding to molten
metal)
• Molten metal is stirred with the help of either a mechanical stirrer or high intensity ultrasonic treatment.
• A graphite impeller is generally used for stirring. A vigorous stirring is required to ensure a good distribution of the
reinforcements.
• If the reinforcement particles are not stirred properly in molten matrix, they will tend to sink or float to the molten melt
due to the density differences between the reinforcement particles and the matrix alloy melt.
• The reinforcement-molten matrix slurry is subsequently cast by conventional methods.
• Reinforcement particles are often coated with proper wetting agents to achieve better interfacial bonding with the
matrix material and to avoid any unwanted reaction and the dissolution of reinforcement at high temperatures.
• Stirring speed and stirring time are significant parameters which affect the
distribution of the reinforcement particles within the matrix material.
• Optimum mechanical properties can be attained by the uniform distribution of
reinforcement.
• Processing of MMCs by stir mixing and casting requires special precautions, including
temperature control and design of pouring and gating systems.
Advantages of stir casting
• It is simple and economical process.
• Suitable for mass production.
• Good matrix-reinforcement interface.
• The process can be customized to meet specific size and design
requirements.
Disadvantages of stir casting
• Difficult to achieve homogeneity.
• Possibility of reaction between matrix and reinforcement and poor
wetting due to high temperature.
• The stir-casting process can sometimes produce porosity or air
pockets in the final product.
Infiltration Processes
• In the infiltration technique, liquid metal is infiltrated through the narrow
crevices between the fibers or particulate reinforcements, which are
arranged in a preform to fix them in space.
• Unlike the stir-mixing process where the reinforcements are free to float or
settle in the melt, the reinforcements do not have freedom to move in the
infiltration processes.
• As the liquid metal enters between the fibers or particles during
infiltration, it cools and then solidifies, producing a composite.
• In general, the infiltration technique is divided into three distinct
operations: the preform preparation (the reinforcement elements are
assembled together into a porous body), the infiltration process (the liquid
metal infiltrates the preform), and the solidification of liquid metal.
• In this method, a transient layer of solidified metal can form as soon as the
liquid metal comes in contact with the cold fiber.
• Melt infiltration can be achieved with the help of mechanical pressure,
inert gas pressure, or vacuum
Pressure Infiltration Casting.
• Various pressure infiltration techniques have been used to manufacture cast composites.
• In pressure infiltration casting, the mold and the preforms are kept at a temperature
close to the melting point of the metal; this gives the metal time to flow and fill the mold
without choking (refer Fig.).
• Once the metal has filled major voids in the mold and preform, the pressure can be
increased rapidly.
• After this, a uniform pressure can be maintained through the mold, and the remaining
voids can be infiltrated isostatically.
• This makes it possible to use relatively thin walled molds and to infiltrate very fragile
preforms with little damage.
• Even though the mold and preform start out at a high temperature, use of thin mold
walls allows the mold to be cooled rapidly to minimize matrix-reinforcement interfacial
reaction
Squeeze Casting
• The squeeze casting technique, shown in Fig., has been quite popular in making
composites with selective reinforcement.
• In general, the process of squeeze casting involves the following steps
• A porous fiber preform is inserted into the die.
• A pre-specified amount of molten metal is poured into the preheated die located on the bed of a
hydraulic press.
• The press is activated to close off the die cavity and to pressurize the liquid metal.
• The applied pressure (70–100 MPa) makes the molten aluminum penetrate the fiber preform and
bond the fibers.
• The pressure is held on the metal until complete solidification. This helps to eliminate
macro/micro shrinkage porosity.
• As a result, a fine-grain casting with little to no pore is produced using this method.
• Finally the punch is withdrawn, and the component is ejected.
• The parameters that affect squeeze casting have been identified as: composition of the
casting alloy, applied pressure level, die preheating temperature, pouring temperature,
die coat material (lubricant), duration of pressure application and delay time to achieve
maximum pressure.
• Aluminosilicate fiber-reinforced aluminum alloy pistons for use in heavy diesel engines
have been produced using squeeze casting
Squeeze Casting
Advantages of squeeze casting
• Squeeze casting yields components with excellent mechanical properties,
including strength and ductility.
• The process offers consistent and reproducible results.
• Dimensional precision and extraordinary grade of surface finish
• Squeeze casting results in low porosity, ensuring high-density components.
• Squeeze casting also has excellent potential for automated operations, further
increasing the production rate.
Disadvantages of squeeze casting
• Expensive due to its specialized equipment and tooling requirements.
• Common foundry defects such as porosity, cold shuts, oxides, inclusions, etc. can
be present if process parameters (pressure, lubrication, temperatures, alloy
cleanliness) are not properly controlled.
• No flexibility as tooling is dedicated to specific components.
• High costs mean high production volumes are necessary to justify equipment
investment
Spray Forming
• Spray forming technique, also known as Osprey process is an efficient method to produce near-net shaped
components.
• This process take place in two steps, first step involves atomizing liquid melt into fine droplets using inert gas
as an atomizing agent and subsequent deposition of fine droplets on a metallic substrate occurs in the
second step.
• An example is Al alloy into which heated particles (e.g., SiC) are injected.
• A spray gun is used to atomize a molten aluminum alloy matrix. A high kinetic energy inert gas is used to
impact the liquid metal to produce fine droplets. Inert gas, such as argon or nitrogen is used.
• The atomising gas mass flow rate to molten metal mass flow rate ratio is a key parameter in controlling the
droplet diameter.
• Ceramic particles, such as silicon carbide, are injected into this stream. Usually, the ceramic particles are
preheated to dry them.
• The fine droplets are deposited on a metallic substrate.
• This process is used for manufacturing semi-finished products, which can be later extruded or forged.
• Deposition rate, flight length (i.e., nozzle to substrate distance), atomizing pressure, atomizing gas, angle of
spraying etc. are some of the important processing parameters for the spray forming process
Advantages of Spray forming
• The benefit of this process is its adjustability in making different types
of composites
• Superior properties due to fine grain sizes.
Disadvantages of Spray forming
• Relatively low process yield with typical losses of ~30%. Losses occur
because of overspray (droplets missing the emerging billet) and
material 'bouncing' off the semi-solid top surface.
• Porosity resulting from gas entrapment and solidification shrinkage is
a significant problem in spray formed materials
Solid State Processing
• In the Solid-state processing method, the matrix and the reinforcement are in
solid state
1.Diffusion Bonding
• Diffusion bonding is a simple, common solid-state process for producing metal matrix composites.
• Diffusion bonding is the solid state joining of two surfaces under intimate contact and under high pressure and
temperatures resulting in an undetectable original bond line, as grains from the two original parts form common
boundaries along the original bond line
• This fabrication method is widely used for aluminum or magnesium MMCs reinforced with continuous/discontinuous
fibers.
• Diffusion Bonding is a solid state fabrication method, in which a matrix in form of foils and a dispersed phase in form of
long fibers are stacked in a particular order and then pressed at elevated temperature.
• The inter diffusion atoms at the metallic surfaces under pressure creates bonding between the metal matrix and fibers.
• The success of the bonding process depends on three parameters: (1) bonding pressure, (2) bonding temperature and (3)
holding time.
• The bonding pressure should be sufficient to make intimate contact at the faying interfaces, deform the surface asperities
and fill the gaps between the mating surfaces.
• The bonding temperature is generally kept at 0.5–0.7 times the melting temperature of the most fusible component in
the assembly. The bonding temperature influences the inter diffusion across the weld interface and decides the extent of
the diffusion zone and the nature of the reaction products formed in it.
Diffusion Bonding
Advantages of diffusion bonding
• The most notable advantage of this method is its capacity for producing a wide
range of metal matrix composites and its control over the fiber alignment and
volume fraction.
• Joints created by diffusion bonding are of high quality, and free from
discontinuity and porosity.
• Diffusion bonding allows joining similar and dissimilar materials.
• Due to a good dimension tolerance, diffusion bonding is used to produce high
precision components and complex shapes.
• The running costs are rather low, and the process is easy to approach.
Disadvantages of diffusion bonding
• Although the running costs are low, the initial setup cost is high.
• Diffusion bonding process is very time-consuming.
• The available equipment limits the size of the part.
• The outcome is highly dependent on the process parameters, such as
temperature, pressure, metal surface finish and the welding material
2. Powder metallurgy
• This method is extensively used to fabricate magnesium, aluminum , and copper-
based composites.
• The process incorporates particulate, discontinuous and continuous reinforcement.
• Powder metallurgy consists three distinct steps, i.e., (1) mixing metal and
reinforcement powders, (2) powder compaction to produce green materials, and
(3) sintering usually followed by a deformation process such as hot extrusion.
1.The matrix and the reinforcement powders are blended to produce a homogeneous
distribution.
2.Compaction
• Bulk powder can be transformed into green performs of desired geometry and density through compacting
prior to their sintering. The first step in this process is effective mixing of the multi-material powder. At this
stage, lubricant, is also added to the mixture (for reduced friction) if the powder is going to be formed in a
closed die. Lubrication should be applied in the correct quantities. Excessive lubrication will not all remain
on particle surfaces, but will also collect in the interparticle spaces, (open pores), and prevent the proper
compaction of powder.
• For cold pressing, powder is injected into a die to produce a compact structure.
• Bulk powders are compressed as the pressure is increased, the fraction of voids in the powder rapidly
diminishes and the particles deform under (first elastic and then) plastic mechanisms.
• The denser the preform is, the better are its mechanical properties and the less dimensional variation
during sintering.
3. Sintering
• Sintering is the process of fusing particles together into one solid mass by using a combination of pressure and heat without
melting the materials.
• The temperature used for sintering is below the melting point of the major constituent of the Powder Metallurgy material. The
optimum range of sintering temperatures would be between 60 % to 80 % of the melting temperature.
• Sintering is the primary method for converting loosely bonded powder into a dense body.
• Sintering involves consolidation of the powder compact by diffusion on an atomic scale.
• Moisture and organics are first burned out from the green body, and then, at the temperature range at which the diffusion process
occurs, matter moves from the particles into the void spaces between the particles, causing densification and resulting in
shrinkage of the part.
• The sintering process consists of three stages Before sintering, in the first stage called initial stage grains were contacted each
other, grain boundary areas grew and grains started to merge. In this stage, grains were smaller than merged grains formed
because of sintering; however, pores were larger before sintering process. In the second stage, necks were formed between
adjacent grains. These necks are seen in Fig. There were small numbers of large grains instead of large number of small grains.
Pores were smaller than pores before sintering. However, grain merging was not completed. Grain growing was continued in this
stage. Therefore, this stage is called intermediate stage. In the final stage merging was completed. Grains are larger and small
pores were formed.
• Its drawback lies in the need to use additives and long sintering times to achieve high densities.
• Sintering of a green compact occurs in three stages.First, the powder compact is subject to
preheating. Preheating will raise the part to a relatively low temperature, providing the burning
off of additives. Preheating will also start to strengthen bonds within the part, increasing its
integrity for the next stage. In the second stage the temperature is raised to the sintering
temperature and maintained for a specific duration necessary for the desired amount of bonding
to occur. Temperature is lowered as the part is allowed to cool during the third stage.
• Continuous furnaces provide flow through production and have three zones for the three stages
of the manufacturing process, (preheat, sinter, and cool down). A moving belt carries a
continuous supply of parts through the chambers. Heat doors can rapidly open and close to allow
parts through, while keeping heat in.
4.The material is then hot pressed, uniaxially or isostatically, to produce a fully dense
composite and extruded.
• The process incorporates particulate, discontinuous and continuous reinforcement.
Advantages of Powder Metallurgy for MMC fabrication
• No material is wasted as scrap or chips.
• Processing of carbides is possible.
• It also avoids brittle reaction products formation which normally happens in liquid state
processing of MMC.
• After development this method does not require highly skilled manpower to handle the
process.
• Tooling can be effectively and easily developed.
Demerits of P/M for MMC fabrication
• Raw material in powder form is costly.
• Capital cost may be high as for compaction high capacity press tools are required.
• Porosity can be a difficult issue to handle.
• Post processing is always required.
In-situ fabrication of Metal Matrix Composites
• In-situ composites are multiphase materials where the reinforcing phase is synthesized within the matrix
during composite fabrication. This contrasts with ex-situ composites where the reinforcing phase is
synthesized separately and then inserted into the matrix during a secondary process such as infiltration or
powder processing.
• There are two major types of in situ process: (i) reactive and (ii) non-reactive.
• The reactive type process consists of two elements, which react exothermically for the production of the
reinforcing phase. For example, TiB2 particles are formed as:

• In this process, the reinforcement phase is developed in matrix by controlled metallurgical reactions. .
• One of the reactants in the process is the molten alloy matrix.
• The other reactant may be in powder or gas form inoculated into the melt
• This method offers many advantages including the uniform particle distribution, optimal particle size, and
thermodynamic stability.
• The fineness and uniform distribution of the reinforcement particles improves the strength and other
mechanical properties of the composite.
• Moreover, the particles do not come into contact with air or contaminants, thus forming a clean matrix-
reinforcement interface which improves the strength.
• In the non-reactive type process, eutectic phases of alloys are used to form the reinforcement and matrix.
• Controlled unidirectional solidification of a eutectic alloy is a classic example this process.
• Unidirectional solidification of a eutectic alloy can result in one phase being distributed in the form of fibers or
ribbons in the other.
• One can control the fineness of distribution of the reinforcement phase by simply controlling the solidification
rate.
• A precast and homogenized rod of a eutectic composition is melted, in a vacuum or inert gas atmosphere.
• The rod is contained in a graphite crucible, which in turn is contained in a quartz tube.
• Heating is generally done by induction.
• Crucible with an eutectic alloy moves downwards (or alternatively the induction coil moves upwards)
• This movement results in melting followed by re – solidification of the alloy under controlled cooling conditions.
• Upon solidification, an alloy of eutectic composition forms a microstructure consisting of alternating layers of the
two solid phases as during the solidification atomic diffusion occurs
Advantages of in situ Metal Matrix Composites:
• In situ synthesized particles and fibers are smaller than those in materials with
separate fabrication of dispersed phase (ex-situ MMCs). Fine particles provide
better strengthening effect;
• In situ fabrication provides more homogeneous distribution of the dispersed
phase particles;
• Bonding (adhesion) between the particles of in situ formed dispersed phase and
the matrix is better than in ex-situ MMCs;
• Equipment and technologies for in situ fabrication of MMCs are less expensive.

Disadvantages of in situ Metal Matrix Composites: