metal matrix composites

1. Definition:
A metal matrix composite (MMCs) is composite material with at
least two constituent parts, one (matrix) being a metal. The other
material may be a different metal or another material, such as a
ceramic or organic compound
1) Up to a 20% improvement in yield strength
2) Lower Coefficient of Thermal Expansion
3) Higher Modulus of Elasticity (50%)
4) More Wear Resistant
5) High temperature resistance
6) Low cost
7) Have better electrical and thermal conductivity

2. APPLICATIONS
Automotive
Engine components,
braking systems (break discs and drums)
Aerospace
Undercarriage, guided weapons, satellites
Marine
Propellers, impellers, pressurized hulls,
marine diesel components
Sport
Rackets, cycles and frames, motor racing,
golf club
Undercarriage
Guided weapons

3. ELECTRONIC
Substrates, thermal management, racking, power sources and
storage
MARINE
Propellers, impellers, pressurized hulls, marine diesel
components
RAIL ENGINEERING
Engine and braking components

4. Industrial
Reciprocating and high speed machinery, precision equipment
Military
Gun barrel overwraps, missiles (aerofoils, fins and bodies),
military diesel components
Fins
Gun barrel overwraps Aerofoils

5. The major advantages of Aluminum Matrix Composites (AMCs)
1) Increased specific strength (σ/ρ)
2) Increased specific stiffness (E/ρ)
3) Increased elevated temperature strength
4) Improved wear resistance
5) Lower density
6) Improved damping capabilities
7) Lower thermal expansion coefficients
8) Good corrosion resistance
The principal matrix materials for MMCs are aluminum
and its alloys. To a lesser extent, magnesium and titanium
are also used, and for several specialized applications a copper,
zinc or lead matrix may be employed.

6. A metal matrix reinforced by a second phase
(continuous, and discontinues)
Reinforcing phases:
1) Particles of ceramic (these MMCs are
commonly called cermets), The most
common reinforcing materials in this category
are alumina and silicon carbide.
2) Fibers of various materials: other metals,
ceramics, carbon, and boron.

7.  Discontinuous MMCs can be isotropic, and can be
worked with standard metalworking techniques, such
as extrusion, forging or rolling.
 MMCs with discontinuous reinforcements are usually less
expensive to produce than continuous fiber reinforced
MMCs,.
 They may be machined using conventional
techniques, but commonly would need the use of
polycrystalline diamond tooling (PCD).
 Discontinuous reinforcement uses "whiskers", short
fibers, or particles.

8.  Continuous reinforcement uses monofilament wires
or fibers such as carbon fiber.
 Fibers are embedded into the matrix in a certain
direction, the result is an anisotropic structure in
which the alignment of the material affects its
strength.
 Example: boron filament as reinforcement.
Consequently, continuous fiber reinforced MMCs are
generally accepted as offering the ultimate
mechanical properties.

9.  MMC with ceramic contained in a metallic matrix
 The ceramic often dominates the mixture, sometimes up to 96% by
volume
 Bonding can be enhanced by slight improving the solubility
between phases at elevated temperatures used in processing
 Cermets can be subdivided into
(1) Cemented carbides – most common
(2) Oxide- based cermets – less common
Metal Matrix
Ceramic
Particles
(high toughness,
strength, machinability)
(high strength, stiffness
& thermal stability)

10. Cemented Carbides
One or more carbide compounds bonded in a metallic
matrix
Common examples of cemented carbides are;
 Tungsten carbide (WC),
 Titanium carbide (TiC), and
 Chromium carbide (Cr3C2)
 Tantalum carbide (TaC) and others are less
common

11. 100μm
Microstructure of an Al-SiC Metal Matrix Composite

12. Applications of Cemented Carbides
 Tungsten carbide cermets: cutting tools are most
common; other: wire drawing dies, rock drilling bits ,
dies for powder metallurgy, indenters for hardness
testers
 Titanium carbide cermets: high temperature
applications such as gas- turbine nozzle vanes,
thermocouple protection tubes, torch tips, cutting tools
for steels
 Chromium carbides cermets: valve liners, spray
nozzles, bearing seal rings

14. 2. Solid-State Processes
a) Diffusion Bonding
b) Deformation processing
c) Powder processing
d) Sinter-forging
3. In Situ Processes
4. Spray-Forming Process
1.Liquid-State Processes
a) Casting or liquid infiltration
b) Squeeze casting or pressure infiltration

15. 1. Liquid-State Processes
a) Casting or liquid
infiltration
 Liquid-phase infiltration of
MMCs has difficulties with
wetting the ceramic
reinforcement by the molten
metal.
 Fiber coatings applied prior to
infiltration, which improve
wetting and allow control of
interfacial reactions.

16.  One liquid infiltration process involving particulate
reinforcement, called the Duralcan process, has been
quite successful.
 Ceramic particles and ingot-grade aluminum are mixed
and melted. The melt is stirred slightly above the
liquidus temperature (600−700°C).
 The solidified ingot may also undergo secondary
processing by extrusion or rolling. The Duralcan
process of making particulate composites by a liquid
metal casting route involves the use of 8−12 μm
particles.

17. b) Squeeze casting or pressure infiltration
Involves forcing a liquid metal into a fibrous or particulate
preform. Squeeze casting or pressure casting are the most
common manufacturing variants for MMCs..

18.  Pressure is applied until solidification is completed. This
method obviates the requirement of good wettability of
the reinforcement by the molten metal.
 Advantages:
1) Minimal reaction between the reinforcement and molten
metal because of the short processing time involved.
2) Such composites are also typically free from common
casting defects such as porosity and shrinkage cavities.

19. 2. Solid-State Processes
a) Diffusion Bonding
(a) Apply metal foil and cut to
shape,
(b) lay up desired plies,
(c) Vacuum encapsulate and
heat to fabrication
temperature,
(d) Apply pressure and hold for
consolidation cycle, and
(e) Cool, remove, and clean part.
(a) (b)
(c)
(e)
(d)

20. This forced contact would lead to the atoms of individual
metals to diffuse into the neighboring metal. Hence, the
sheets will stick together quite tightly.
Advantages: The ability to process a wide variety of metal
matrices and control of fiber orientation and volume
fraction.
 Disadvantages: Long processing time, high processing
temperatures and pressures (which makes the process
expensive), and a limitation on the complexity of shapes
that can be produced.

21. (b) Deformation processing
 Can be used to deform the composite material. In metal−metal
composites mechanical processing (extrusion, drawing, or
rolling) of a ductile two-phase material causes the two phases to
deform, causing one of the phases to elongate and become
fibrous in nature within the other phase.
 Roll bonding is a common technique used to produce a
laminated composite consisting of different metals in layered
form.
 Such composites are called sheet laminated metal-matrix
composites. Roll bonding and hot pressing have also been used
to make laminates of Al sheets and discontinuously reinforced
MMCs

22. Powder processing, hot pressing, and extrusion process for fabricating
particulate or short fiber reinforced MMCs.
(c) Powder processing

23.  Powdered metal and dispersed metal or ceramic powder or
discontinuous fibers are mixed.
 It involves cold pressing or hot pressing to fabricate primarily
particle or whisker-reinforced MMCs. The matrix and the
reinforcement powders are mixed to produce a homogeneous
distribution
 The blending stage is followed by cold pressing .
 The cold pressed material is packaged in a closed container and
degassed to remove any absorbed moisture from the particle surfaces.
 The material is hot pressed to produce composite and extruded. The
rigid particles or fibers cause the matrix to be deformed significantly.

24. (d) Sinter-forging
Sinter-forging technique for producing near-net shape, low cost MMCs
A novel and low cost deformation processing technique.

25.  A powder mixture of reinforcement and matrix is
cold compacted, sintered, and forged.
 The main advantage of this technique is that
forging is conducted to produce a near-net shape
material, and machining operations and material
waste are minimized.
 The low cost, sinter-forged composites have
tensile and fatigue properties that are comparable to
those of materials produced by extrusion.

26. 3. In Situ Processes
It is a process in which the
reinforcing phase is formed in the
matrix as a result of precipitation
from the melt during the cooling
and solidification processes.
Types of in-situ MMCs:
–
in situ MMC
Particulate
.
1
Particulate composite reinforced by in
situ synthesized dispersed phase in form
matrix
Aluminum
g. :
e.
of particles. (
)
2
TiB
reinforced by titanium boride (
matrix reinforced
magnesium
particles,
Si particles).
2
by Mg

27. fiber
-
Short
–
reinforced in situ MMC
fiber
-
Short
.
2
composite reinforced by in situ synthesized dispersed phase
in form of short fibers or whiskers (single crystals grown
in form of short fibers).
Examples:
)
2
TiB
titanium boride (
reinforced by
Titanium matrix
-
whiskers,
)
3
TiAl
titanium aluminide (
reinforced by
Aluminum matrix
-
whiskers.
fiber
-
Long
–
reinforced in situ MMC
fiber
-
Long
.
3
composite reinforced by in situ synthesized dispersed phase
in form of continuous fibers.
Example: Nickel-aluminum (NiAl) matrix reinforced by long
continuous fibers of Mo (NiAl-9Mo alloy).

28. 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:
 Choice of the dispersed phases is limited by thermodynamic ability
of their precipitation in particular matrix;
 The size of dispersed phase particles is determined by solidification
conditions;

29. 4. Spray-Forming of Particulate MMCs
The spray-forming process
A co-spray process, uses
a spray gun to atomize
a molten aluminum alloy
matrix resulting in a fine
solid powder, into which
heated silicon carbide
particles are injected

30.  The spray process is generally automated and quite
fast.
 The formation of reaction products is generally
avoided because the time of flight of the composite
particles is extremely short.
 Silicon carbide particles of an aspect ratio
(length/diameter) between 3−4 and volume fractions
up to 20% have been incorporated into aluminum
alloys.