Metal Matrix Composites

1. I.Karthikeyan ,
Assistant Professor,
Department of Mechanical Engineering.
SRI RAMAKRISHNA INSTITUTE OF TECHNOLOGY,
An Autonomous Institution
(Approved by AICTE, New Delhi – Affiliated to Anna University, Chennai,
Accredited by NAAC with ‘A’ Grade and All UG Engineering Programmes are Accredited by NBA)
UMEC202 Composite Materials (Professional
Core Course)

2. Thermosetting and thermoplastics

3. Thermo and thermosetting plastics
1. A thermoplastic, or thermosoftening plastic, is a plastic polymer material that
becomes pliable or moldable at a certain elevated temperature and solidifies upon
cooling.
2. Most thermoplastics have a high molecular weight.
3. The polymer chains associate by intermolecular forces, which weaken rapidly with
increased temperature, yielding a viscous liquid.
4. In this state, thermoplastics may be reshaped and are typically used to produce
parts by various polymer processing techniques such as injection
molding, compression molding, calendering, and extrusion.

4. Let’s get to know
 The molecular mass (m) or molecular weight is the mass of a
given molecule: it is measured in daltons (Da or u).
 Different molecules of the same compound may have different
molecular masses because they contain different isotopes of an
element.
 The dalton or unified atomic mass unit (symbols: Da or u) is
a unit of mass widely used in physics and chemistry.
 It is defined precisely as 1/12 of the mass of an unbound neutral atom
of carbon-12 in its nuclear and electronic ground state and at rest.

5.  Isotopes are variants of a particular chemical element which
differ in neutron number, and consequently in nucleon number.
 All isotopes of a given element have the same number of protons
but different numbers of neutrons in each atom.
 Carbon (6C) has 15 known isotopes, from 8C to 22C, of
which 12C and 13C are stable.The longest-lived radioisotope
is 14C, with a half-life of 5,700 years

6. Let’s get to know
 Carbon-12 (12C) is the more abundant of the
two stable isotopes of carbon (carbon-13 being the other),
amounting to 98.93% of the element carbon;
 its abundance is due to the triple-alpha process by which it
is created in stars.
 Carbon-12 is of particular importance in its use as the standard
from which atomic masses of all nuclides are measured, thus, its
atomic mass is exactly 12 daltons by definition.
 Carbon-12 is composed of 6 protons, 6 neutrons, and 6 electrons.

7.  The triple-alpha process is a set of nuclear
fusion reactions by which three helium-4 nuclei (alpha
particles) are transformed into carbon

8. For at least a portion of its life, a star shines due
to thermonuclear fusion of hydrogen into helium in
its core, releasing energy that traverses the star's
interior and then radiates into outer space
A star is an astronomical object consisting of a
luminous spheroid of plasma held together by its
own gravity.The nearest star to Earth is the Sun

9. Acrylic
 Acrylic, a polymer called poly(methyl methacrylate) (PMMA), is
also known by trade names such as Lucite, Perspex and Plexiglas.
 It serves as a sturdy substitute for glass for items such as
aquariums, motorcycle helmet visors, aircraft windows, viewing
ports of submersibles, and lenses of exterior lights of
automobiles.
 It is extensively used to make signs, including lettering and logos.
 In medicine, it is used in bone cement and to replace eye lenses.
Acrylic paint consists of PMMA particles suspended in wate

10. PMMA is an economical alternative
to polycarbonate (PC) when tensile strength, flexural
strength, transparency, polishability, and UV
tolerance are more important than impact strength,
chemical resistance, and heat resistance

11. Let’s get to know
 Flexural strength, also known as modulus of rupture, or bend strength,
or transverse rupture strength is a material property, defined as
the stress in a material just before it yields in a flexure test.
 The transverse bending test is most frequently employed, in which a
specimen having either a circular or rectangular cross-section is bent
until fracture or yielding using a three point flexural test technique.
 The flexural strength represents the highest stress experienced within
the material at its moment of yield. It is measured in terms of stress,
here given the symbol {displaystyle sigma }.

12.  When an object formed of a single material, like a wooden beam or a steel rod,
is bent (Fig. 1), it experiences a range of stresses across its depth (Fig. 2).
 At the edge of the object on the inside of the bend (concave face) the stress will
be at its maximum compressive stress value.At the outside of the bend (convex
face) the stress will be at its maximum tensile value.
 These inner and outer edges of the beam or rod are known as the 'extreme
fibers'.
 Most materials generally fail under tensile stress before they fail under
compressive stress, so the maximum tensile stress value that can be sustained
before the beam or rod fails is its flexural strength

13. 2 . Acrylonitrile butadiene styrene (ABS) is a terpolymer synthesized
from styrene and acrylonitrile in the presence of polybutadiene.
 ABS is a light-weight material that exhibits high impact resistance and mechanical
toughness.
 It poses few risks to human health under normal handling. It is used in many
consumer products, such as toys, appliances, and telephones.
3.Nylon
 Nylon belongs to a class of polymers called polyamides. It has served as
a substitute mainly for hemp, cotton and silk, in products such as
parachutes, cords, sails, flak vests and clothing.
 Nylon fibers are useful in making fabrics, rope, carpets and musical
strings, whereas in bulk form, nylon is used for mechanical parts
including machine screws, gears and power tool casings.
 In addition, it is used in the manufacture of heat-resistant composite
materials.

14. Thermosetting polymers
 A thermosetting polymer, resin, or plastic, often called a thermoset, is
a polymer that is irreversibly hardened by curing from a soft solid or
viscous liquid prepolymer or resin.Curing is induced by heat or
suitable radiation and may be promoted by high pressure, or mixing with
a catalyst.
 It results in chemical reactions that create extensive cross-
linking between polymer chains to produce
an infusible and insoluble polymer network.

15. Examples of thermo-sets
 Polyester resin fiberglass systems: sheet molding compounds and bulk molding
compounds; filament winding; wet lay-up lamination; repair compounds and protective
coatings.
 Polyurethanes: insulating foams, mattresses, coatings, adhesives, car parts, print rollers,
shoe soles, flooring, synthetic fibers, etc. Polyurethane polymers are formed by combining
two bi- or higher functional monomers/oligomers.
 Polyurea/polyurethane hybrids used for abrasion resistant waterproofing coatings.
 Vulcanized rubber.
 Bakelite, a phenol-formaldehyde resin used in electrical insulators and plasticware.
 Duroplast, light but strong material, similar to Bakelite used for making car parts.
 Urea-formaldehyde foam used in plywood, particleboard and medium-density
fibreboard.

17. Metal Matrix Composites

18. Metal Matrix Composites
 Metal matrix composites, as the name suggests, consist of fibres or particles
surrounded by a matrix of metal.
 The use of a metal matrix offers the potential of producing a composite with
very high stiffness and strength as well as very high temperature
resistance.
 The temperature resistance is not only superior to polymer matrix composites
but also to the pure metal itself.

19.  While metal matrix composites enjoy other advantages over polymer matrix
composites such as better abrasion resistance, creep resistance, resistance to
degradation by fluids, dimensional stability, and non-flammability, they are limited in
application due to their much higher weight and cost of production.
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
 Better wear resistance

20.  Metal matrix composites can be classed as having either continuous or
discontinuous fibre reinforcement.
 Discontinuous reinforced MMCs appear to offer more potential due to
their ease of manufacture.And canexist in the form of short fibres,
whiskers, platelets, or particles.

21. The advantages of MMCs over polymer matrix composites are:
 Higher temperature capability
 Fire resistance
 Higher transverse stiffness and strength
 No moisture absorption
 Higher electrical and thermal conductivities
 Better radiation resistance
 Fabric-ability of whisker and particulate-reinforced MMCs with conventional
metalworking equipment.

22.  Some of the disadvantages of MMCs compared to monolithic metals and
polymer matrix composites are:
 Higher cost of some material systems
 Relatively immature technology
 Complex fabrication methods for fiber-reinforced systems (except for
casting)
 Limited service experience

23. Current Application and Market Oppotunities
 Current markets for MMCs are primarily in military and aerospace
applications.
 Experimental MMC components have been developed for use in
aircraft, satellites, jet engines, missiles, and the
 NationalAeronautics and Space Administration (NASA) space shuttle.
The first production application of a particulate-reinforced MMC in the
United States is a set of covers for a missile guidance system.

24.  The most important commercial application to date is the MMC
diesel engine piston made byToyota.
 This composite piston offers better wear resistance and high-
temperature strength than the cast iron piston it replaced.
 It is estimated that 300,000 such pistons are produced and sold in
Japan annually.
 This development is very important because it demonstrates that
MMCs are at least not prohibitively expensive for a very cost sensitive
application.
 Other commercial applications include cutting tools and circuit-
breaker contact.

25.  MMCs appear attractive: high-temperature fighter aircraft engines and
structures; high-temperature missile structures; and spacecraft
structures.

26.  Some of the disadvantages of MMCs compared to
monolithic metals and polymer matrix composites are:
 Higher cost of some material systems.
 Relatively immature technology.
 Complex fabrication methods for fiber-reinforced systems (except for casting)
 Limited service experience.

28. An Overview
 Metal matrix composites (MMCs) comprise a relatively wide range of materials
defined by the metal matrix, reinforcement type, and reinforcement geometry.
 In the area of the matrix, most metallic systems have been explored for use in metal
matrix composites, including Al, Be, Mg,Ti, Fe, Ni, Co, andAg. By far the largest
usage is in Aluminum matrix composites.
 From a reinforcement perspective, the materials used are typically ceramics since they
provide a very desirable combination of stiffness, strength, and relatively low density.

29.  Candidate reinforcement materials include SiC,Al2O3, B4C,TiC,TiB2,
graphite, and a number of other ceramics. In addition, there has been work on
metallic materials as reinforcements, notablyW(Wolfram) and steel fibers.

30. In Space
The extreme environment in space presents
both a challenge and opportunity for material
scientists. In the near-earth orbit, typical
spacecraft encounter naturally occurring
phenomena such as vacuum, thermal radiation,
atomic oxygen, ionizing radiation, and plasma,
along with factors such as micrometeoroids
and human-made debris.
 For example, the International Space Station,
during its 30-year life, will undergo about
175,000 thermal cycles from +125°C to –
125°C as it moves in and out of the Earth’s
shadow. Re-entry vehicles for Earth and Mars
missions may encounter temperatures that
exceed 1,500°C.

31.  Critical spacecraft missions, therefore, demand lightweight space structures with high
pointing accuracy and dimensional stability in the presence of dynamic and thermal
disturbances.
 Composite materials, with their high specific stiffness and low coefficient of thermal
expansion (CTE), provide the necessary characteristics to produce lightweight and
dimensionally stable structures.
 Therefore, both organic-matrix and metal-matrix composites (MMCs) have been
developed for space applications.

32.  Superalloy composites reinforced with tungsten alloy fibers are
being developed for components in jet turbine engines that operate
temperatures above 1,830 °F.
 Graphite/copper composites have tailorable properties, are useful to high
temperatures in air, and provide excellent mechanical characteristics, as well
as high electrical and thermal conductivity.
 They offer easier processing as compared with titanium, and lower density
compared with steel.

33. Limitations
 Despite the successful production of MMCs such as continuous-fiber
reinforced boron/aluminum (B/Al), graphite/ aluminum (Gr/Al), and graphite/
magnesium (Gr/Mg),the technology insertion was limited by the concerns related
to ease of manufacturing and inspection, scale-up, and cost.
 Organic-matrix composites continued to successfully address the system-level
concerns related to microcracking during thermal cycling and radiation exposure,
and electromagnetic interference (EMI) shielding;
 MMCs are inherently resistant to those factors. Concurrently, discontinuously
reinforced MMCs such as silicon-carbide particulate (p) reinforced aluminum
(SiCp/Al) and Gp/Al composites were developed cost effectively both for
aerospace applications (e.g., electronic packaging) and commercial applications

34. Materials and Reinforcements
 The main matrix materials employed in MMCs are aluminium,
titanium, magnesium, and copper.
 Also includesThin-walled (hollow) steel sections employed with fibre-
reinforced polymer (FRP) composites.
 Production of steel laminates etc.,

35.  The most important MMC systems are:
 Aluminum matrix
 Continuous fibers: boron, silicon carbide, alumina, graphite
 Discontinuous fibers: alumina, alumina-silica
 Whiskers: silicon carbide
 Particulates: silicon carbide, boron carbide
 Magnesium matrix
 Continuous fibers: graphite, alumina
 Whiskers: silicon carbide
 Particulates: silicon carbide, boron carbide

36.  Titanium matrix
 Continuous fibers: silicon carbide, coated boron
 Particulates: titanium carbide
 Copper matrix
 Continuous fibers: graphite, silicon carbide
 Wires: niobium-titanium, niobium-tin
 Particulates: silicon carbide, boron carbide, titanium carbide.
 Superalloy matrices
 Wires: tungsten

37. Let’s get to know
 Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a
shiny gray solid which bears a close physical resemblance to the other five elements in the
second column (group 2, or alkaline earth metals) of the periodic table: all group 2
elements have the same electron configuration in the outer electron shell and a similar
crystal structure.
 Magnesium is the ninth most abundant element in the universe. It is produced in large,
aging stars from the sequential addition of three helium nuclei to a carbon nucleus.
 Magnesium is the eighth most abundant element in the Earth's crust and the fourth most
common element in the Earth (after iron, oxygen and silicon), making up 13% of the
planet's mass and a large fraction of the planet's mantle. It is the third most abundant
element dissolved in seawater, after sodium and chlorine

38.  Aluminum MMCs are produced by casting, powder metallurgy, in situ development of
reinforcements, and foil-and-fiber pressing techniques. Consistently high-quality products
are now available in large quantities, with major producers scaling up production and
reducing prices.
 They are applied in brake rotors, pistons, and other automotive components, as well as
golf clubs, bicycles, machinery components, electronic substrates, extruded angles and
channels, and a wide variety of other structural and electronic applications.
 It is a chemical element with the symbol Al and atomic number 13. It is a silvery-
white, soft, non-magnetic and ductile metal in the boron group. By mass, aluminium
makes up about 8% of the Earth's crust, where it is the third most abundant element
(after oxygen and silicon) and also the most abundant metal. Occurrence of
aluminium decreases in the Earth's mantle below, however.The chief ore of
aluminium is bauxite

39.  Titanium is a chemical element with the symbolTi and atomic number 22. It is a
lustrous transition metal with a silver color, low density, and high strength.Titanium is
resistant to corrosion in sea water, aqua regia, and chlorine.
 Titanium was discovered in Cornwall, Great Britain, byWilliam Gregor in 1791 and
was named by Martin Heinrich Klaproth after theTitans of Greek mythology.
 The element occurs within a number of mineral deposits,
principally rutile and ilmenite, which are widely distributed in the Earth's
crust and lithosphere; it is found in almost all living things, as well as bodies of water,
rocks, and soils

40.  In the commercial aerospace industry, magnesium was generally
restricted to engine-related components, due to fire and corrosion
hazards.
 Currently, magnesium alloy use in aerospace is increasing, driven by the
importance of fuel economy.
 Development and testing of new magnesium alloys continues, notably
Elektron 21, which (in test) has proved suitable for aerospace engine,
internal, and airframe components.
 The European Community runs three R&D magnesium projects in the
Aerospace priority of Six Framework Program.

41. Reinforcements
 MMC reinforcements can be divided into five major categories: continuous
fibers, discontinuous fibers, whiskers, particulates, and wires.
 With the exception of wires, which are metals, reinforcements generally are
ceramics.
 Key continuous fibers include boron, graphite (carbon), alumina, and silicon
carbide. Boron fibers are made by chemical vapor deposition (CVD) of this
material on a tungsten core.
 Carbon cores have also been used.These relatively thick monofilaments are
available in 4.0, 5.6, and 8.0-mil diameters.
 To retard reactions that can take place between boron and metals at high
temperature, fiber coatings of materials such as silicon carbide or boron
carbide are sometimes used.

42.  Silicon carbide monofilaments are also made by a CVD process, using a
tungsten or carbon core.
 A Japanese multifilament yarn, designated as silicon carbide by its
manufacturer, is also commercially available.
 This material, however, made by pyrolysis of organometallic precursor
fibers, is far from pure silicon carbide and its properties differ significantly
from those of monofilament silicon carbide.

43.  Ductile superconductors have been fabricated with a matrix of copper and
superconducting filaments of niobium-titanium. Copper reinforced with tungsten
particles or aluminum oxide particles is used in heat sinks and electronic packaging.
 Titanium reinforced with silicon carbide fibers is under development as skin
material for the NationalAerospace Plane. Stainless steels, tool steels, and
 Inconel (austenitic nickel-chromium-based superalloys. are oxidation-
corrosion-resistant materials well suited for service in extreme
environments subjected to pressure and heat.)
 are among the matrix materials reinforced with titanium carbide particles and
fabricated into draw-rings and other high-temperature, corrosion-resistant
components.

44.  Ceramics and ceramic reinforced metal matrix composites (MMCs) are
widely used in severe working conditions and have been applied in
biomedical, aerospace, electronic, and other high-end engineering industries owing to
their superior properties of high wear resistance, outstanding chemical inertness, and
excellent properties at elevated temperatures.
 These superior properties, on the other hand, make it difficult to process these materials
with conventional manufacturing methods, posing problems of high cost and energy
consumptions.
 In response to this problem, direct additive manufacturing (AM), which is equipped
with a high-power-density laser beam as heat source, has been developed and extensively
employed for processing ceramics and ceramic reinforced MMCs.

45.  Compared with other direct AM processes, laser deposition-additive
manufacturing (LD-AM) process excels in several aspects, such as lower
labor intensity, higher fabrication efficiency, and capabilities of parts
remanufacturing and functionally gradient composite materials
fabrication. Besides these benefits, problems of poor bonding, cracking,
lowered toughness, etc. still exist in LD-AM fabricated parts.

46.  Continuous alumina fibers are available from several suppliers.
Chemical compositions and properties of the various fibers are significantly
different. Graphite fibers are made from two precursor materials,
polyacrilonitrile (PAN) and petroleum pitch.
 Efforts to make graphite fibers from coal-based pitch are under way.
Graphite fibers with a wide range of strengths and moduli are available.
 The leading discontinuous fiber reinforcements at this time are alumina and
alumina-silica. Both originally were developed as insulating materials.The
major whisker material is silicon carbide.

47.  Silicon carbide and boron carbide, the key particulate reinforcements, are
obtained from the commercial abrasives industry. Silicon carbide
particulates are also produced as a by-product of the process used to make
whiskers of this material.
 A number of metal wires including tungsten, beryllium, titanium, and
molybdenum have been used to reinforce metal matrices. Currently, the
most important wire reinforcements are tungsten wire in superalloys and
superconducting materials incorporating niobium-titanium and niobium-
tin in a copper matrix.
 The reinforcements cited above are the most important at this time. Many
others have been tried over the last few decades, and still others
undoubtedly will be developed in the future.

48. 

49. Sintering or frittage is the process of compacting and forming a solid mass
of material by heat or pressure without melting it to the point of
liquefaction.
 Sintering happens naturally in mineral deposits or as a manufacturing
process used with metals, ceramics, plastics, and other materials.
 The atoms in the materials diffuse across the boundaries of the particles,
fusing the particles together and creating one solid piece.
 Because the sintering temperature does not have to reach the melting
point of the material, sintering is often chosen as the shaping process for
materials with extremely high melting points such
as tungsten and molybdenum.
 The study of sintering in metallurgy powder-related processes is known
as powder metallurgy

50.  Powder metallurgy or ceramics it is possible to fabricate components which
otherwise would decompose or disintegrate.
 All considerations of solid-liquid phase changes can be ignored, so powder processes
are more flexible than casting, extrusion, or forging techniques.

51.  LIQUID metal infiltration of ceramic preform is apparently the best
suited fabrication method to produce metal matrix composite components
with variety of complex shapes having high volume fraction of reinforcement.
 Infiltration is a liquid-state fabrication method, in which a porous preform
(reinforcement) such as ceramic particles, fibers, woven etc. are impregnated
in a molten matrix metal, which fills the pores between the dispersed-phase
inclusions.
 Synthesis of porous ceramic preform with sufficient mechanical strength,
uniform pore distribution, pore size, and porosity level is one of the crucial
steps involved in the infiltration processing of composites.
 The captivating properties of these ceramic foams such as thermal resistance,
low density, controlled permeability, low thermal conductivity, high surface
area, and high structural uniformity make them potential candidates for
multiple engineering applications

52.  Thermal spraying techniques are coating processes in which melted (or heated) materials
are sprayed onto a surface.The "feedstock" (coating precursor) is heated by electrical (plasma
or arc) or chemical means (combustion flame).
 Thermal spraying can provide thick coatings (approx. thickness range is 20 microns to several
mm, depending on the process and feedstock), over a large area at high deposition rate as
compared to other coating processes such as electroplating, physical and chemical vapor
deposition.
 Coating materials available for thermal spraying include metals, alloys, ceramics, plastics and
composites.They are fed in powder or wire form, heated to a molten or semimolten state and
accelerated towards substrates in the form of micrometer-size particles.
 Combustion or electrical arc discharge is usually used as the source of energy for thermal
spraying. Resulting coatings are made by the accumulation of numerous sprayed particles.The
surface may not heat up significantly, allowing the coating of flammable substances.

53. HVOF
 A mixture of gaseous or liquid fuel and oxygen is fed into a combustion chamber,
where they are ignited and combusted continuously.
 The resultant hot gas at a pressure close to 1 MPa emanates through a converging–
diverging nozzle and travels through a straight section.The fuels can be gases
(hydrogen, methane, propane, propylene, acetylene, natural gas, etc.) or liquids
(kerosene, etc.).
 The jet velocity at the exit of the barrel (>1000 m/s) exceeds the speed of sound.
A powder feed stock is injected into the gas stream, which accelerates the powder
up to 800 m/s.The stream of hot gas and powder is directed towards the surface to
be coated.The powder partially melts in the stream, and deposits upon the
substrate.The resulting coating has low porosity and high bond strength

54. ]

55. Syllabus Check
 Characteristics of MMC,Various types of Metal matrix composites Alloy vs.
MMC,Advantages of MMC, Limitations of MMC, Metal Matrix -
Reinforcements – particles – fibres. Effect of reinforcement -Volume fraction –
Rule of mixtures. Processing of MMC - Powder metallurgy process –
diffusion bonding – stir casting – squeeze casting.

56.  Magnesium is the third-most-commonly-used structural metal,
following iron and aluminium.
 The main applications of magnesium are, in order: aluminium alloys, die-
casting (alloyed with zinc),removing sulfur in the production of iron and steel,
and the production of titanium in the Kroll process.Magnesium is used in
super-strong, lightweight materials and alloys. For example, when infused with
silicon carbide nanoparticles, it has extremely high specific strength.
 Historically, magnesium was one of the main aerospace construction metals and
was used for German military aircraft as early asWorldWar I and extensively
for German aircraft inWorldWar II.The Germans coined the name "Elektron"
for magnesium alloy, a term which is still used today.

57. Processing of MMC- (Fabrication
techniques)
 Squeeze Casting
Squeeze casting is a process that combines gravity and pressurized casting.
In general, molten metal is poured into a pre-heated die.When filling is
complete, a ram is used to slowly apply high pressure to the molten
metal head.
This pressurization helps ensure that metal flows throughout the solidifying
casting, minimizing shrinkage and microshrinkage porosity.
When combined with a controlled cooling step, this process can produce a
very fine microstructure. In certain cases, mechanical properties
approaching those of forged parts can be obtained.

59.  In the squeeze casting process, liquid metal is introduced into a die cavity, either
by direct pour or via a simple running system. High pressures (up to 250 MPa)
are applied to the liquid metal, so that solidification takes place under the
applied pressure.
 There are two types of process. Dies are usually relatively simple and sand
cores are not usually used.There is usually no extra metal needed to supply
feeding requirements, so theoretically the yield can be 100%.

60.  Squeeze casting is a hybrid of low pressure casting and high pressure casting,
and it has the potential to completely eliminate the gas defects associated with
high pressure die casting, and to enable heat treatment of the castings.
 In squeeze casting, the die is filled slowly with metal to maintain laminar flow.
Once the cavity is full, the pressure on the melt is increased to over 100 MPa
and maintained to feed the casting to compensate for shrinkage until the casting
has solidified.
 Die design for squeeze casting is different from that for die casting, and
includes thick gates and a large shot end biscuit to ensure that the gates do not
freeze before the casting, in the cavity has solidified and to ensure feeding the
shrink during solidification.

61.  It is important to prepare sufficiently rigid preform which is usually reached by
adding an organic binder to the fibres. Normally, the organic binder contains
small quantities of silica which remain after the binder is burnt off during
preheating the preform. Homogeneity of fibre packing effects essentially the
maximum strength of the composite. So preform should be sufficiently
homogeneous.To achieve it, Ju et al. [290] used ultrasonic dispersion of
short carbon fibres in water containing 10% of sodium silicate.The preform
should be preheated to make conditions of the infiltration more favorable.

62.  To make fibre distribution in the preform more homogeneous and, at the same
time, to control fibre volume fraction more accurately, some authors use the so
called hybridization [80], that means adding particles or whiskers to the fibrous
preform.This is done, for example, by impregnating fibres into aqueous
suspension of particles (or whiskers).
 A polymer is used as a binding agent and an organicmetallic compound serves as
a dispersing agent. Mechanical tests show that an optimal content of the
particulate exists that certainly depends on the fibre volume fraction.

63.  As well as potentially giving a 100% yield in poured metal, the process
enables solidification shrinkage to be eliminated. Cooling rates are high, as there
is intimate contact between the casting and the die throughout solidification.
Consequently, fine grain sizes and dendrite arm spacings are achieved even in
thick sections.The main drawbacks of the process are the long cycle times,
which can be as much as 5 min for castings containing thick sections, and the
inability to incorporate cores into the geometry.

64. Stir Casting
 stir casting is a liquid state method for the fabrication of composite
materials, in which a dispersed phase is mixed with a molten
matrix metal by means of mechanical stirring.
 Stir Casting is the simplest and the most cost effective method of
liquid state fabrication.
 Stir casting is an economical process for the fabrication of
aluminum matrix composites.
 There are many parameters in this process, which affect the final
microstructure and mechanical properties of the composites

65.  Aluminum matrix composites (AMCs) and hybrid aluminum matrix
composites (HAMCs) becomes choice for automobile and aerospace
industries due to its tunable mechanical properties such as very high strength
to weight ratio, superior wear resistance, greater stiffness, better fatigue
resistance, controlled co-efficient of thermal expansion and good stability at
elevated temperature.
 Stir casting is an appropriate method for composite fabrication and widely
used industrial fabrication of AMCs and HAMCs due to
flexibility,
cost-effectiveness and
best suitable for mass production.

66.  Stir casting is a suitable processing technique to fabricate
aluminum matrix composites and hybrid aluminum matrix
composites as it is an economical process and preferred for mass
production.The first step of stir casting involves melting of
aluminum.
 During melting, aluminum melt reacts with the atmosphere and
moisture and forms a layer of aluminum oxide (Al2O3) as given
by Eq. (1).This layer shields the surface of the melt from further
reaction with atmosphere

67.  2Al+3H2O⇔Al2O3+6H E1
 Stir casting process involves stirring of melt, in which the melt is stirred
continuously which exposes the melt surface to the atmosphere which
tend to continuous oxidation of aluminum melt.
 As a result of continuous oxidation, the wettability of the aluminum
reduces and the reinforcement particles remain unmixed.
 Al2O3 is a very stable chemical compound, which cannot be reduced
under normal conditions and the wettability of melt remains unchanged.
 To stop the oxidation completely, an inert environment has to be created,
which involves lots of complications.Therefore adding wetting agents
such as borax and magnesium in the melt is an alternate solution of this
problem and widely used for the fabrication of AMCs

68. Set up
 Stir casting setup as shown in Figure 1, consist of a furnace,
reinforcement feeder and mechanical stirrer.
 The furnace is used to heating and melting of the materials.The bottom
poring furnace is more suitable for the stir casting as after stirring of the
mixed slurry instant poring is required to avoid the settling of the solid
particles in the bottom the crucible.
 The mechanical stirrer is used to form the vortex which leads the
mixing of the reinforcement material which are introduced in the melt.
Stirrer consist of the stirring rod and the impeller blade.
 The impeller blade may be of, various geometry and various number of
blades.

69.  Flat blade with three number are the preferred as it leads to axial
flow pattern in the crucible with less power consumption.
 This stirrer is connected to the variable speed motors, the rotation
speed of the stirrer is controlled by the regulator attached with the
motor.
 Further, the feeder is attached with the furnace and used to feed
the reinforcement powder in the melt.A permanent mold, sand
mold or a lost-wax mold can be used for pouring the mixed
slurry.

71.  Various steps involved in stir casting process is shown in Figure 2. In this
process, the matrix material are kept in the bottom pouring furnace for
melting.
 Simultaneously, reinforcements are preheated in a different furnace at
certain temperature to remove moisture, impurities etc.
 After melting the matrix material at certain temperature the mechanical
stirring is started to form vortex for certain time period then
reinforcements particles are poured by the feeder provided in the setup
at constant feed rate at the center of the vortex, the stirring process is
continued for certain time period after complete feeding of
reinforcements particles.

72.  The molten mixture is then poured in preheated mold and
kept for natural cooling and solidification.
 Further, post casting process such as heat treatment,
machining, testing, inspection etc. has been done.
 There are various impeller blade geometry are available.
Melting of the matrix material is very first step that has been
done during this process