Manufacturing Process - Module7.0

MANUFACTURING PROCESS

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7.0 COMPOSITES

INTRODUCTION A composite material is a material system composed two or more physically dinstinct phases whose

combination produces aggregate properties that are different from those of its constituents. The

tecnological and commercial interest in composite materials derive from the fact that their properties are

not just different from their components but are often far superior.

LEARNING OBJECTIVES At the end of this course the student will be able to:

Explain three categories of composites

Describe the composites production techniques

LEARNING OUTCOMES At the end of this course the student has the ability to:

Explain the technologies for the production of components from composite

7.1 INTRODUCTION

Composites are produced when two are more materials or phases are used together to

give a combination of properties that cannot be attained otherwise. Composites

materials may be selected to give unusual combination of stiffness, strength, weight,

high temperature performance, corrosion resistance, hardness and or conductivity.

Composites can be metal-metal, metal-ceramic, metal-polymer, ceramic-polymer,

ceramic-ceramic or polymer-polymer.

Metal-ceramic composites, for example include cemented carbide cutting tools, silicon

carbide fiber-reinforced titanium, and enameled steel.Composites can be placed into

three broad categories: particulate, fiber and laminar, based on the shape of the

materials. Concrete, a mixture of cement and gravel, is a particulate composite. Plywood

having alternating layers of wood veneer is laminar composite.


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Comparison of the three types of composite materials. a) Particulate composite, b) Fiber - reinforced composite, c) Laminar composite.

Classification of composites: three main categories;

particle-reinforced (large-particle and dispersion-strengthened)

fiber-reinforced (continuous (aligned) and short fibers (aligned or random)

structural (laminates and sandwich panels)

Figure below shows a simple idea for the classification of composite materials which

consists of three main divisions.

A classification scheme for the various composite types

Composites

Particle Reinforced Fiber Reinforced Structural

Large Particle

Dispersion

strengthened

Continuous

(aligned)

Discontinuous

(short)

Laminates

Sandwich panels

Aligned Randomly oriented

a b c


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Properties of composites depend on;

properties of phases

geometry of dispersed phase (particle size, distribution, orientation)

amount of phase

Abalone shell, wood, bone and teeth are among the examples of naturally occurring

composites. Composite materials may be divided into two categories, (i) composite

materials at macro scale and (ii) composite materials at micro scale.

Example of composite materials at macro scale is steel reinforced concrete while

composite materials at micro scale are carbon or glass-fiber reinforced plastics (CFRP

or GFRP). These composites offer significant gain in specific strengths are among

materials use in airplanes, electronic components, automotive and sporting equipment.

7.2 PARTICLE-REINFORCED COMPOSITES

Large particles and dispersion-strengthened are the two sub-classifications of particles-

reinforced composites. The distinction between these is based upon reinforcement or

strengthening mechanism.

7.2.1 LARGE PARTICLES

The term large is used to indicate that particle-matrix interactions cannot be

treated on the atomic or molecular level rather continuum mechanic is used.

For most of these composites, the particulate phase is harder and stiffer than the

matrix.

These reinforcing particles tend to restrain movement of the matrix phase in the

vicinity of each particle.

In essence, the matrix transfers some of the applied stress to the particles, which

bear a fraction of the load.

The degree of reinforcement or improvement of mechanical behavior on strong

bonding at the matrix-particle interface.

Some familiar large-particles composites are concrete, being composed of

cement (the matrix) and sand and gravel (particulates).

Particles can have quite a variety of geometries, but they should be of

approximately the same dimension in all directions.


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For effective reinforcement, the particles should be small and evenly distributed

throughout the matrix.

The volume fraction of the two phases influences the behavior; mechanical

properties are enhanced with increasing particulate content.

Particles used for reinforcing include ceramics and glasses such as small mineral

particles, metal particles such as aluminum, and amorphous

materials, including polymers and carbon black.

Particles are also used to produce inexpensive composites. Reinforcers and

matrices can be common, inexpensive materials and are easily processed.

An example of particle reinforced composites is an automobile tire which has

carbon black particles in a matrix of polyisobutylene elastomeric polymer

7.2.2 CONCRETE

Concrete is a common large-particle composite in which both matrix and

dispersed phases are ceramics materials

Concrete and cements are always incorrectly interchanged.

Concrete is cement strengthened by adding particulates. The use of different

size (stone and sand) allows better packing factor than when using particles of

similar size.

Concrete is a composite material consisting of an aggregate of particles that

bound together in a solid body by some type of binding medium known as

cement.

7.2.3 DISPERSION-STRENGTHENED COMPOSITES

Metals and metal alloys may be strengthened and hardened by uniform

dispersion of several percent of fine particles of a very hard and inert material.

The dispersion phase may be metallic and non-metallic such as oxide materials.

7.3 FIBRE-REINFORCED COMPOSITES

Technologically, the most important composites are those in which the dispersed phase

is in the form of a fiber. Most fiber-reinforced composites provide improved strength,

fatigue resistance, Youngs Modulus, and strength-to-weight ratio by incorporating

strong, stiff but brittle fibers into a softer and more ductile matrix.


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The matrix material transmits the force into fibers, which carries most of the applied

forces. The matrix also provides protection for the fiber surface and minimizes diffusion

of species such as oxygen, or moisture that can degrade the mechanical properties of

fibers. The strength of the composite may be high at room temperature and elevated

temperature.

Glass fiber in polymer matrix produce fiberglass for transportation and aerospace

applications. Fibers made of boron, carbon, polymers and ceramics provide exceptional

reinforcement in advanced composites based on matrices of polymers, metals, ceramics

7.3.1 FIBER ORIENTATION AND CONCENTRATION

The arrangement or orientation of the fibers relative to one another, the fiber

concentration and the distribution all have a significant influence on the strength

and other properties of fiber-reinforced composites. With respect to orientation,

two extremes are possible;

i. A parallel alignment of the longitudinal axis of the fibers in a single direction. ii. Totally random alignment.

Continuous fibers are normally aligned as in figure 8.7 or partially oriented. The

composite is stronger along the direction of orientation of the fibers and weakest

in a direction perpendicular to the fiber. For discontinuous, random fibers, the

properties are isotropic.


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Figure 8.7: a) Continuous and aligned fiber, b) discontinuous and aligned fiber, c)

discontinuously and randomly oriented fiber-reinforced composites.

7.3.2 THE FIBER PHASE

An important characteristic of most materials, especially brittle ones, is that a

small-diameter fiber is much stronger than the bulk material. The probability of

the presence of a critical surface flaw that can lead to fracture diminishes with

decreasing specimen volume and this feature is used to advantage in the fiber

reinforced composites.

Materials used for reinforcing fibers have high tensile strengths. On the basis of

diameter and character, fibers are grouped into three classifications;

Whiskers -Very this single crystals that have extremely large length- to-

diameter ratios.

Fibers -Materials classified are either polycrystalline or amorphous and

have small diameters.

Wires- Fine wires have relatively large diameters which utilized as a

radial reinforcement in automobile tires for example.

Fibers classifications


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7.3.3 THE MATRIX PHASE

The matrix phase of fiber composites serves several functions.

i. Binds fiber together and acts as the medium by which an externally applied

stress is transmitted and distributed to the fibers.

ii. Protect the individual fibers from surface damage as a result of mechanical

abrasion or chemical reactions with environment. Such interactions may

introduce surface flaws capable of forming cracks at low tensile stress

levels.

iii. Due to matrix softness and plasticity, it prevents the propagation of brittle

cracks from fiber to fiber, which could result in catastrophic failure (matrix

phase act as barrier to crack propagation) whereby matrix separates the

fibers.

It is essential that adhesive bonding force between fiber and matrix be high to

minimize fiber pull-out. Bonding strength is an important consideration in the

choice of the matrix-fiber combination. The ultimate strength of the composite

depends to a large degree on the magnitude of this bond. Adequate bonding is to

maximize the stress transmittance from the weak matrix to the strong fibers.

7.3.4 POLYMER MATRIX COMPOSITE

Polymer impregnated with other fiber materials, carbon being the most common

also form composites. Carbon in fiber form has a much higher specific modulus

than glass. It also has a better resistance to temperature and corrosive chemicals

but more expensive and has only limited short fiber utilization. The aircraft

industry is currently implementing carbon reinforced composites as structural

components of their new aircraft as a weight savings measure.

Various polymer impregnated with boron fibers have been utilized. E.g. helicopter

rotor blades are constructed using boron fibers in an epoxy resin.A new

generation of high strength polymeric aramid fibers is beginning to be selected for

composites used in lightweight structural components, such as aerospace,

aircraft, marine and sporting equipments.


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Other fiber materials such as silicon carbide, silicon nitride in polymer matrix are

in research. Also included, carbon-carbon composites, which are composed of

carbon fibers embedded within carbonized resin matrix which are designed

principally for high-temperature aerospace applicant.

7.3.5 METAL MATRIX COMPOSITE

Metal-matrix composites are either in use or prototyping for the Space Shuttle,

commercial airliners, electronic substrates, bicycles, automobiles, golf clubs,

and a variety of other applications. While the vast majority is aluminum matrix

composites, a growing number of applications require the matrix properties of

super-alloys, titanium, copper, magnesium, or iron.

Like all composites, aluminum-matrix composites are not a single material but a

family of materials whose stiffness, strength, density, and thermal and electrical

properties can be tailored. The matrix alloy, the reinforcement material, the

volume and shape of the reinforcement, the location of the reinforcement, and the

fabrication method can all be varied to achieve required properties.

Regardless of the variations, however, aluminum composites offer the advantage

of low cost over most other MMCs. In addition, they offer excellent thermal

conductivity, high shear strength, excellent abrasion resistance, high-temperature

operation, non-flammability, minimal attack by fuels and solvents, and the ability

to be formed and treated on conventional equipment.

Aluminum MMCs are produced by casting, powder metallurgy, in situ

development of reinforcements, and foil-and-fiber pressing techniques. 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.

Super-alloy 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 tailor-able properties, are useful to high

temperatures in air, and provide excellent mechanical characteristics, as well as high

electrical and thermal conductivity.


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They offer easier processing as compared with titanium, and lower density compared

with steel. 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 National Aerospace Plane. Stainless steels, tool steels, and Inconel are among the

matrix materials reinforced with titanium carbide particles and fabricated into draw-rings

and other high-temperature, corrosion-resistant components.

7.3.6 CERAMIC MATRIX COMPOSITE

As with metal-matrix composites, the three main types of reinforcement are

continuous fiber, discontinuous fiber, and particulate reinforced.

Advantages:

Very high operating temperatures, chemical inertness (lack of action) low

thermal expansion (0.5 to 8.5 x 10-6 oC-1) and wear and erosion

resistances.

Disadvantages:

Density (2.2 - 3.95.g / cm3), low toughness, low ductility, high fabrication

temperatures and expensive.

Common Matrices are:

Glasses (boron silicate glass, lithium aluminum silicate (LAS) glass,

aluminum silicate (AS) glass.

Engineering ceramics (alumina (Al2O3), silicon nitride (Si3N4) zirconia

(ZrO2), SIALON (Al2O3+Si3N4) - improved strength.

Common Reinforcement are:

Carbon, Boron Nitride (BN), Silicon Carbide (SiC) Alumina (Al2O3)


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Example of multilayered self-healing matrix ceramic matrix composite produced.

7.3.7 CARBON-CARBON COMPOSITE

Carbon-carbon composites consist of highly-ordered graphite fibers embedded in

a carbon matrix. C-C composites are made by gradually building up a carbon

matrix on a fiber pre-form through a series of impregnation steps or chemical

vapor deposition. C-C composites tend to be stiffer, stronger and lighter than steel

or other metals.

Carbon Carbon Composites

Processing carbon-carbon composites consists of building up of the carbon matrix

around the graphite fibers. There are two common ways to create the matrix: through

chemical vapor deposition and through the application of a resin.


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7.4 STRUCTURAL COMPOSITES

A structural composite is normally composed of both homogeneous and

composite materials the properties of which depend,

- properties of the constituent materials.

- geometrical design of the various structural elements.

-

Laminar composites and sandwich panels are two of the most common structural

composites.

7.4.1 LAMINAR COMPOSITES

Is composed of dimensional sheets or panels that have a

preferred high strength direction such as is found in wood and

continuous and aligned fiber reinforced plastics.

The layers are stacked and cemented together such that the

orientation of the high-strength direction varies with each

successive layer.

One example of a relatively complex structure is modern ski and

another example is plywood.

7.4.2 SANDWICH PANELS

Consist of two strong outer sheets which are called face sheets

and may be made of aluminum alloys, fiber reinforced plastics,

titanium alloys, steel.

Face sheets carry most of the loading and stresses.

Core may be a honeycomb structure which has less density than

the face sheets and resists perpendicular stresses and provides

shear rigidity.

Sandwich panels can be used in variety of applications which

include roofs, floors, walls of buildings and in aircraft, for wings,

fuselage and tail-plane skins.


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Structural composites for a) laminar composites and b) sandwich panels.

7.5 PRODUCING THE COMPOSITES

Composite is produced dependent on the application and the materials.

7.5.1 CONTINUOUS FIBER COMPOSITES

Continuous fiber composites are produced with more specialized

techniques:

a. Hand lay up: Tapes, mats or fabrics are placed against a form,

saturated with polymer resin, rolled to assure good contact and then

cured. Fiberglass car and truck bodies can be produced this way, but is

slow an expensive.

a) Laminar composites

b) Sandwich panels


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b. Bag molding: Tapes and fabrics are placed in a die. High pressure

gases or a vacuum are introduced to force them together to get bonding. Military

aircraft skins have been produced this way.

c. Matched die molding: Short fibers or mats are placed into a two part die,

when the die is closed, the composite shape is formed.

d. Filament winding: This method is used to produce pressure tanks and

rocket motor casting. One or more continuous fibers are wrapped around a form

or mandrel to gradually build up a solid shape. The filament can be dipped in the

polymer matrix resin prior to winding, or the resin can be impregnate around the

fiber during or winding.


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Metal matrix composites with continuous fibers are more difficult to produce.

Other methods include using PM and using closed dies to compress layers

together.

7.5.2 LAMINATES

Laminar composites include very thin coatings, thicker protective surfaces,

claddings, bimetallic laminates and a host of others. In addition, the fiber

reinforced composites produced from tapes of fabrics can also be considered as

partially laminar. Many laminar composites are designed to improve corrosion

resistance while retaining low cost, high strength or light weight. Other important

applications include superior wear or abrasion resistance, improved appearance

and unusual thermal expansion characteristics.

7.5.2.1 Production Methods

a. Rolling

Most of the metallic laminar composites, such as claddings and bimetallic

are produced by hot or cold roll bonding.


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b. Explosive bonding

An explosive charge can provide the pressure required to join metals. The

pressure produced during explosive bonding strips away surface

impurities and forces the surface together at high pressures. This process

is well suited for joining very large plates that will not fit into a rolling mill.

c. Co-extrusion

Very simple laminar composites, such as coaxial cable, are produced by co-

extruding two metals through a die in such a way that the soft material

surrounds the harder material. Similarly, the thermoplastic polymer could be

extruded around a metal conductor.


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d. Brazing

Brazing can join composite plates. The metallic sheets are separated by a very

small clearance, preferably about 0.075mm, and heated above the melting

temperature of the brazing alloy. The molten brazing alloy is driven into the thin

joint.

e. Pressing

For small components, high pressures at elevated temperatures provide welding.

Hot pressing is frequently used to cure the adhesive in laminates.


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7.5.2.2 Examples and Applications

There are a huge number of laminar composites, extremely varied in their

properties and characteristics. Description of some common types as follows:

Laminates

a. Plywood, in which an odd number of wood veneer piles are stacked so that

the grain is at right angles in each alternating ply. An adhesive such as phenolic

or amine resin glues the piles. Plywood permits wood products to be available in

large sizes yet be inexpensive and resistant to splitting and warping.

b. Safety glass is a laminate in which a plastic adhesive, such as Polyvinyl

Butyral, joins two pieces of glass; the adhesive prevents fragments of glass from

flying about when the glass is broken. Laminate are used for insulation in motors,

for gears and for decorative items such as Formica counter tops and furniture.

c. A recently developed laminate, Arral (Aramid Aluminum laminate) has been

developed as a possible skin material for aircraft. An Aramid fiber, such as

Kevlar, is woven into a fabric impregnated with an adhesive, and laminated

between layers of aluminum. The composite Iaminate has a usual combination of

strength, stiffness, corrosion resistance and light weight. In addition, fatigue

resistance is improve, cracks that begin in the outer aluminum layers are stopped

on reaching the Aramid layer. The adhesives laminates combine unusual

characteristics including light weight, flame retardance, impact strength,

corrosion resistance, easy forming and machining and good insulation

characteristics.

Schematic diagram of an Aramid Aluminum laminate, Arall


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Hard Surfacing

Hard, wear resistance surfaces can be deposited on softer, more ductile

materials by fusion welding techniques known as hard-surfacing. Hard surfacing

alloys include hardenable grades of steel, irons and steels that produce hard

carbides, cobalt base alloys, and certain non-ferrous alloys. Composite tungsten

carbide welding rods can also be used to provide tungsten carbide at the wear

surface. Similar welding procedures can improve corrosion resistance or heat

resistance at surfaces.

Clad Metals.

Clad materials provide a combination of good corrosion resistance with high

strength. Alcad is a clad composite in which commercially pure aluminum is

bonded to higher strength aluminum alloys. The pure aluminum protects the

higher strength alloy from corrosion. The thickness of the pure aluminum layer is

about 1 % to 15% of the total thickness. Alcad is used in aircraft construction,

heat exchangers, building construction, and storage tanks, where combinations

of corrosion resistance, strength, and light weight are desired.

Bimetallic

Temperature indicators and controllers take advantage of the different

coefficients of thermal expansion of the two metals in the laminar composite. If

two pieces of metal are heated, the metal with the higher coefficient of thermal

expansion becomes longer (see Fig. below ). If the two pieces of metal are rigidly

bonded together, the difference in their coefficients causes the strip to bend and

produce a curved surface. If one end of the strip is fixed, the free end moves.

The amount of movement depends on the temperature; by measuring the

curvature or deflection of the strip, we can determine the temperature.

The effect of thermal expansion coefficient on the behavior of bimetallics a) Two metals

are apart, b) Two metals are joined together


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Metals selected for bimetallics must have:

a. Very different coefficients of thermal expansion

b. Expansion characteristics that are reversible and repeatable

c. High modulus of Elasticity

Often the low expansion strip is made from Invar, an iron-nickel alloy while the high

expansion strip may be brass, Monel, manganese-nickel-copper, nickel-chromium-iron,

or pure nickel. Bimetallics can act as circuit breakers as well as thermostats; if a current

passing through the strip becomes too high, heating causes the bimetallic to deflect and

break the circuit.

Concrete

Concrete is a common construction material. It is a particulate composite in which an

aggregate, usually gravel and sand, is bonded in a matrix of cement. A cementation

reaction between water and the minerals in the cement provides the required strength.

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Manufacturing Process - Module7.0