FABRICATION AND CHARACTERISATION OF ALUMINIUM-FLY ASH COMPOSITE USING STIR CASTING METHOD

FABRICATION AND CHARACTERISATION OF ALUMINIUM-FLY ASH COMPOSITE USING STIR CASTING METHOD
1
FABRICATION AND CHARACTERISATION OF
ALUMINIUM-FLY ASH COMPOSITE USING STIR
CASTING METHOD
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Bachelor of Technology
In
Metallurgical & Materials Engineering
By
DEEPAK KUMAR (10504018)
SUBHAKANTA SARANGI (10504005)
Department of Metallurgical & Materials Engineering
National Institute of Technology
Rourkela
2009
2
FABRICATION AND CHARACTERISATION OF
ALUMINIUM-FLY ASH COMPOSITE USING STIR
CASTING METHOD
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
Bachelor of Technology
In
Metallurgical & Materials Engineering
By
DEEPAK KUMAR (10504018)
SUBHAKANTA SARANGI (10504005)
Under the Guidance of Dr.S.Sarkar
Department of Metallurgical & Materials Engineering
National Institute of Technology
Rourkela
2009
3
National Institute of Technology
Rourkela
CERTIFICATE
This is to certify that the thesis entitled, “Fabricaton and Characterisation of AluminiumFly ash composite using Stir Casting Method” submitted by Deepak Kumar (10504018)
and Subhakanta Sarangi (10504005) in partial fulfillment of the requirements for the
award of Bachelor of Technology Degree in Metallurgical & Materials Engineering at the
National Institute Of Technology, Rourkela (Deemed University) is an authentic work
carried out by them under my supervision and guidance.
To the best of my knowledge, the matter embodied in the thesis has not been submitted to
any other University/Institute for the award of any Degree or Diploma.
Date:
Dr. S. Sarkar
Dept. of Metallurgical & Materials Engineering,
National Institute of Technology,
Rourkela
4
ACKNOWLEDGEMENT
We welcome this opportunity to express our heartfelt gratitude and
regards to our project guide Dr.S.Sarkar, Department of Metallurgical
& Materials Engineering, National Institute of Technology, Rourkela for
his superb guidance. He always bestowed parental care upon us and
evinced keen interest in solving our problems. An erudite teacher, a
magnificent person and a strict disciplinarian, we consider ourselves
fortunate to have worked under his supervision.
We are highly grateful to Prof. B.B.Verma, Head of the department,
Metallurgical & Materials Engineering, NIT Rourkela, for providing
necessary facilities during the course of the work.
We admit thanks to Dr.A.K.Panda & Dr.M.KUMAR, Project
Coordinator, Department of Metallurgical & Materials Engineering, NIT
Rourkela for giving us such a mind stimulating and innovative project.
We wish to place our deep sense of thanks to Mr.Heymbram for his
cooperation and critical suggestions during our experimental work.
Date:
Deepak Kumar(10504018)
Subhakanta Sarangi (10504005)
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CONTENTS
PAGE Number
Abstract
………………………………………………………………
CHAPTER-1: INTRODUCTION
1.1
INTRODUCTION
7
8
8-9
CHAPTER-2: LITERATURE SURVEY
10
2.1
COMPOSITE
10
2.2
CLASSIFICATION OF COMPOSITES
11-12
2.3
RULE OF MIXTURES
12-14
2.4
METAL MATRIX COMPOSITES
14-15
2.5
STIR CASTING METHOD
16-17
2.6
STRENGTHENING MECHANISM OF COMPOSITES
17
2.7
FLY ASH
18
2.8 CLASSIFICATION OF FLY ASH
18-19
2.9 WHY FLY ASH
19-20
2.10 CHEMICAL REACTION BETWEEN Al AND FLY ASH
2.11 INTERFACIAL PARAMETERS
CHAPTER-3: EXPERIMENTAL WORK
3.1
EXPERIMENTAL PROCEDURE
20
20-23
23
23-24
6
CHAPTER-4: RESULTS AND DISCUSSION
4.1 CHEMICAL ANALYSIS OF FLY ASH
25
25
4.2 PARTICLE SIZE ANALYSIS OF FLY ASH
25-26
4.3 HARDNESS MEASUREMENT
26-27
4.4 WEAR BEHAVIOUR
27-31
4.5 SEM ANALYSIS
32-33
CHAPTER-5: CONCLUSION
34
CHAPTER-6: REFERENCES
35-36
7
ABSTRACT
Metal matrix composites (MMCs) possess significantly improved properties including high
specific strength; specific modulus, damping capacity and good wear resistance compared to
unreinforced alloys. There has been an increasing interest in composites containing low density
and low cost reinforcements. Among various discontinuous dispersoids used, fly ash is one of the
most inexpensive and low density reinforcement available in large quantities as solid waste byproduct during combustion of coal in thermal power plants. Hence, composites with fly ash as
reinforcement are likely to over come the cost barrier for wide spread applications in automotive
and small engine applications. It is therefore expected that the incorporation of fly ash particles
in aluminium alloy will promote yet another use of this low-cost waste by-product and, at the
same time, has the potential for conserving energy intensive aluminium and thereby, reducing
the cost of aluminium products. Now a days the particulate reinforced aluminium matrix
composite are gaining importance because of their low cost with advantages like isotropic
properties and the possibility of secondary processing facilitating fabrication of secondary
components. The present investigation has been focused on the utilization of abundantly
available industrial waste fly-ash in useful manner by dispersing it into aluminium to produce
composites by stir casting method.
Key words: particulate composites, industrial waste, applied load and sliding velocity
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1.0 INTRODUCTION
1.1 INTRODUCTION
Conventional monolithic materials have limitations in achieving good combination of
strength, stiffness, toughness and density. To overcome these shortcomings and to meet the ever
increasing demand of modern day technology, composites are most promising materials of recent
interest. Metal matrix composites (MMCs) possess significantly improved properties including
high specific strength; specific modulus, damping capacity and good wear resistance compared
to unreinforced alloys. There has been an increasing interest in composites containing low
density and low cost reinforcements. Among various discontinuous dispersoids used, fly ash is
one of the most inexpensive and low density reinforcement available in large quantities as solid
waste by-product during combustion of coal in thermal power plants. Hence, composites with fly
ash as reinforcement are likely to over come the cost barrier for wide spread applications in
automotive and small engine applications. It is therefore expected that the incorporation of fly
ash particles in aluminium alloy will promote yet another use of this low-cost waste by-product
and, at the same time, has the potential for conserving energy intensive aluminium and thereby,
reducing the cost of aluminium products [1-3].
Now a days the particulate reinforced aluminium matrix composite are gaining
importance because of their low cost with advantages like isotropic properties and the possibility
of secondary processing facilitating fabrication of secondary components. Cast aluminium
matrix particle reinforced composites have higher specific strength, specific modulus and good
wear resistance as compared to unreinforced alloys [4].While investigating the opportunity of
using fly-ash as reinforcing element in the aluminium melt, R.Q.Guo and P.K.Rohatagi [5-6]
observed that the high electrical resistivity, low thermal conductivity and low density of fly-ash
may be helpful for making a light weight insulating composites. The particulate composite can
9
be prepared by injecting the reinforcing particles into liquid matrix through liquid metallurgy
route by casting [7-11]. Casting route is preferred as it is less expensive and amenable to mass
production. Among the entire liquid state production routes, stir casting is the simplest and
cheapest one. The only problem associated with this process is the non uniform distribution of
the particulate due to poor wet ability and gravity regulated segregation.
Mechanical properties of composites are affected by the size, shape and volume
fraction of the reinforcement, matrix material and reaction at the interface. These aspects have
been discussed by many researchers. Rohatgi reports that with the increase in volume
percentages of fly ash, hardness value increases in Al–fly ash (precipitator type) composites. He
also reports that the tensile elastic modulus of the ash alloy increases with increase in volume
percent (3–10) of fly ash. Aghajanian et al. have studied the Al2O3 particle reinforced Al
MMCs, with varying particulate volume percentages (25, 36, 46, 52 and 56) and report
improvement in elastic modulus, tensile strength, compressive strength and fracture properties
with an increase in the reinforcement content. The interface between the matrix and
reinforcement plays a critical role in determining the properties of MMCs. Stiffening and
strengthening rely on load transfer across the interface. Toughness is influenced by the crack
deflection at the interface and ductility is affected by the relaxation of peak stress near the
interface.
Extensive studies on the tribological characteristics of Al MMCs containing
reinforcements such as SiC and Al2O3 is available in the literatures. However, reports on friction
and wear characteristics of fly ash reinforced AMCs are very limited. Rohatgi has reported that
the addition of fly ash particles to the aluminium alloy significantly increases its abrasive wear
resistance. He attributed the improvement in wear resistance to the hard aluminosilicate
constituent present in fly ash particles.
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2.0 LITERATURE SURVEY
2.1 COMPOSITE
Composite material is a material composed of two or more distinct phases (matrix phase
and reinforcing phase) and having bulk properties significantly different from those of any of the
constituents. Many of common materials (metals, alloys, doped ceramics and polymers mixed with
additives) also have a small amount of dispersed phases in their structures, however they are not
considered as composite materials since their properties are similar to those of their base constituents
(physical property of steel are similar to those of pure iron) . Favorable properties of composites materials
are high stiffness and high strength, low density, high temperature stability, high electrical and thermal
conductivity, adjustable coefficient of thermal expansion, corrosion resistance, improved wear resistance
etc.
MATRIX PHASE
1. The primary phase, having a continuous character,
2. Usually more ductile and less hard phase,
3. Holds the reinforcing phase and shares a load with it.
REINFORCING PHASE
1. Second phase (or phases) is imbedded in the matrix in a discontinuous form,
2. Usually stronger than the matrix, therefore it is sometimes called reinforcing phase.
Composites as engineering materials normally refer to the material with
the following characteristics:
1. These are artificially made (thus, excluding natural material such as wood).
2. These consist of at least two different species with a well defined interface.
3. Their properties are influenced by the volume percentage of ingredients.
4. These have at least one property not possessed by the individual constituents.
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2.2CLASSIFICATION OF COMPOSITES
(a) On the basis of Matrix:
1. Metal Matrix Composites (MMC)
Metal Matrix Composites are composed of a metallic matrix (aluminium, magnesium,
iron, cobalt, copper) and a dispersed ceramic (oxides, carbides) or metallic (lead,
tungsten, molybdenum) phase.
2. Ceramic Matrix Composites (CMC)
Ceramic Matrix Composites are composed of a ceramic matrix and imbedded fibers of
other ceramic material (dispersed phase).
3. Polymer Matrix Composites (PMC)
Polymer Matrix Composites are composed of a matrix from thermoset (Unsaturated
polyester (UP), Epoxy) or thermoplastic (PVC, Nylon, Polysterene) and embedded glass,
carbon, steel or Kevlar fibers (dispersed phase).
(b) On the basis of Material Structure:
1. Particulate Composites
Particulate Composites consist of a matrix reinforced by a dispersed phase in form of
particles.
1. Composites with random orientation of particles.
2. Composites with preferred orientation of particles. Dispersed phase of
these materials consists of two-dimensional flat platelets (flakes), laid parallel
to each other.
2. Fibrous Composites
Short-fiber reinforced composites. Short-fiber reinforced composites consist of a matrix
reinforced by a dispersed phase in form of discontinuous fibers (length < 100*diameter).
1. Composites with random orientation of fibers.
2. Composites with preferred orientation of fibers.
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Long-fiber reinforced composites. Long-fiber reinforced composites consist of a matrix
reinforced by a dispersed phase in form of continuous fibers.
3. Unidirectional orientation of fibers.
4. Bidirectional orientation of fibers (woven).
3. Laminate Composites
When a fiber reinforced composite consists of several layers with different fiber
orientations, it is called multilayer (angle-ply) composite.
2.3 RULE OF MIXTURES
Rule of Mixtures is a method of approach to approximate estimation of composite
material properties, based on an assumption that a composite property is the volume
weighed average of the phases (matrix and dispersed phase) properties.
According to Rule of Mixtures properties of composite materials are estimated as
follows:
Density
dc = dm*Vm + df*Vf
Where , dc,dm,df – densities of the composite, matrix and dispersed phase respectively;
Vm,Vf – volume fraction of the matrix and dispersed phase respectively.
Coefficient of Thermal Expansion

Coefficient of Thermal Expansion (CTE) in longitudinal direction (along the fibers)
αcl = (αm*Em*Vm + αf*Ef*Vf)/(Em*Vm + Ef*Vf)
αcl, αm, αf – CTE of composite in longitudinal direction, matrix and dispersed
phase (fiber) respectively;
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Em,Ef – modulus of elasticity of matrix and dispersed phase (fiber) respectively.

Coefficient of Thermal Expansion (CTE) in transverse direction (perpendicular to the
fibers)
αct = (1+Pm) αm *Vm + αf* Vf
Pm – Poisson ratio of matrix.
Poisson’s ratio is the ratio of transverse contraction strain to longitudinal
extension strain in the direction of applied force.
Modulus of Elasticity

Modulus of Elasticity in longitudinal direction (Ecl)
Ecl = Em*Vm + Ef*Vf

Modulus of Elasticity in transverse direction (Ect)
1/Ect = Vm/Em + Vf/Ef
Tensile Strength

Tensile strength of long-fiber reinforced composite in longitudinal direction
σc = σm*Vm + σf*Vf
Where , σc, σm, σf – tensile strength of the composite, matrix and dispersed phase (fiber)
respectively.

Tensile strength of short-fiber composite in longitudinal direction
(fiber length is less than critical value Lc)
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Lc = σf*d/τc
Where
d – diameter of the fiber;
τc –shear strength of the bond between the matrix and dispersed phase (fiber).
σc = σm*Vm + σf*Vf*(1 – Lc/2L)
Where
L – length of the fiber

Tensile strength of short-fiber composite in longitudinal direction
(fiber length is greater than critical value Lc)
σc = σm*Vm + L* τc*Vf/d
2.4 Metal Matrix Composites (MMCs)
Metal Matrix Composites are composed of a metallic matrix (Al,Mg,Fe,Cu etc) and a
dispersed ceramic (oxide, carbides) or metallic phase( Pb,Mo,W etc). Ceramic reinforcement
may be silicon carbide, boron, alumina, silicon nitride, boron carbide, boron nitride etc. whereas
Metallic Reinforcement may be tungsten, beryllium etc. MMCs are used for Space Shuttle,
commercial airliners, electronic substrates, bicycles, automobiles, golf clubs and a variety of
other applications. From a material point of view, when compared to polymer matrix composites,
the advantages of MMCs lie in their retention of strength and stiffness at elevated temperature,
good abrasion and creep resistance properties. Most MMCs are still in the development stage or
the early stages of production and are not so widely established as polymer matrix composites.
The biggest disadvantages of MMCs are their high costs of fabrication, which has placed
limitations on their actual applications. There are also advantages in some of the physical
attributes of MMCs such as no significant moisture absorption properties, non-inflammability,
15
low electrical and thermal conductivities and resistance to most radiations. MMCs have existed
for the past 30 years and a wide range of MMCs have been studied.
Compared to monolithic metals, MMCs have:




Higher strength-to-density ratios
Higher stiffness-to-density ratios
Better fatigue resistance
Better elevated temperature properties
o
o


Higher strength
Lower creep rate
Lower coefficients of thermal expansion
Better wear resistance
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
No out gassing
Fabric ability of whisker and particulate-reinforced MMCs with
conventional metalworking equipment.
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
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2.5 STIR CASTING METHOD OF FABRICATION OF MMCs
Liquid state fabrication of Metal Matrix Composites involves incorporation of
dispersed phase into a molten matrix metal, followed by its Solidification. In order to provide
high level of mechanical properties of the composite, good interfacial bonding (wetting) between
the dispersed phase and the liquid matrix should be obtained.
Wetting improvement may be achieved by coating the dispersed phase particles
(fibers). Proper coating not only reduces interfacial energy, but also prevents chemical
interaction between the dispersed phase and the matrix.
The simplest and the most cost effective method of liquid state fabrication is Stir Casting.
Stir Casting
Stir Casting is a liquid state method of composite materials fabrication, in which a
dispersed phase (ceramic particles, short fibers) is mixed with a molten matrix metal by means of
mechanical stirring.
The liquid composite material is then cast by conventional casting methods and may also be
processed by conventional Metal forming technologies.
(Laboratory stir casting set up)
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Stir Casting is characterized by the following features:

Content of dispersed phase is limited (usually not more than 30 vol. %).

Distribution of dispersed phase throughout the matrix is not perfectly homogeneous:
1. There are local clouds (clusters) of the dispersed particles (fibers);
2. There may be gravity segregation of the dispersed phase due to a difference in
the densities of the dispersed and matrix phase.

The technology is relatively simple and low cost.
2.6 STRENGTHENING MECHANISM OF COMPOSITES
The strengthening mechanisms of the composites are different with different kind
of reinforcing agent morphology such as fibres, particulate or dispersed type of reinforcing
elements.
DISPERSION STRENGTHENING MECHANISM OF STRENGTHENED
COMPOSITE
In the dispersion strengthened composite the second phase reinforcing agents are finely
dispersed in the soft ductile matrix. The strong particles restrict the motion of dislocations
and strengthen the matrix. Here the main reinforcing philosophy is by the strengthening
of the matrix by the dislocation loop formation around the dispersed particles. Thus the
further movement of dislocations around the particles is difficult. Degree of strengthening
depend upon the several factors like volume % of dispersed phase, degree of dispersion,
size and shape of the dispersed phase, inter particle spacing etc. In this kind of composite
the load is mainly carried out by the matrix materials.
STRENGTHENING MECHANISM OF PARTICULATE COMPOSITE
In the particulate reinforced composite the size of the particulate is more than 1 µm, so it
strengthens the composite in two ways. First one is the particulate carry the load along
with the matrix materials and another way is by formation of incoherent interface
between the particles and the matrix. So a larger number of dislocations are generated at
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the interface, thus material gets strengthened. The degree of strengthening depends on the
amount of particulate (volume fraction), distribution, size and shape of the particulate etc.
2.7 FLY ASH
Fly ash is one of the residues generated in the combustion of coal. It is an industrial byproduct recovered from the flue gas of coal burning electric power plants. Depending upon the
source and makeup of the coal being burned, the components of the fly ash produced vary
considerably, but all fly ash includes substantial amounts of silica (silicon dioxide, SiO2) (both
amorphous and crystalline) and lime (calcium oxide, CaO). In general, fly ash consists of SiO2,
Al2O3, Fe2O3 as major constituents and oxides of Mg, Ca, Na, K etc. as minor constituent. Fly
ash particles are mostly spherical in shape and range from less than 1 µm to 100 µm with a
specific surface area, typically between 250 and 600 m2/kg. The specific gravity of fly ash vary
in the range of 0.6-2.8 gm/cc. Coal fly ash has many uses including as a cement additive, in
masonry blocks, as a concrete admixture, as a material in lightweight alloys, as a concrete
aggregate, in flowable fill materials, in roadway/runway construction, in structural fill materials,
as roofing granules, and in grouting. The largest application of fly ash is in the cement and
concrete industry, though, creative new uses for fly ash are being actively sought like use of fly
ash for the fabrication of MMCs.
2.8Classification of fly ash
On the basis of Chemical Composition:
1. Class F fly ash
The burning of harder, older anthracite and bituminous coal typically produces Class F
fly ash. This fly ash is pozzolanic in nature, and contains less than 10% lime (CaO). Possessing
pozzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing
agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water in order
to react and produce cementitious compounds. Alternatively, the addition of a chemical activator
19
such as sodium silicate (water glass) to a Class F ash can leads to the formation of a
geopolymer.
2. Class C fly ash
Fly ash produced from the burning of younger lignite or sub bituminous coal, in
addition to having pozzolanic properties, also has some self-cementing properties. In the
presence of water, Class C fly ash will harden and gain strength over time. Class C fly ash
generally contains more than 20% lime (CaO). Unlike Class F, self-cementing Class C fly ash
does not require an activator. Alkali and sulfate (SO4) contents are generally higher in Class C
fly ashes.
On the basis of size, shape and structure:
1. Precipitator fly ash
It is spherical in nature, the spheres are solid and the density is in the range of 2.0–2.5 g cm-3.
2. Cenosphere fly ash
It is also spherical in shape but these spheres are hollow, so the density of this kind of fly ash is
very less as compared to the precipitator fly ash. Here density is less than 1 gm cm-3 (0.3-0.6
gm/cc)
2.9 WHY FLY ASH
1. The preference to use fly ash as a filler or reinforcement in metal and polymer
matrices is that fly ash is a byproduct of coal combustion, available in very large quantities (80
million tons per year) at very low costs since much of this is currently land filled. Currently, the
use of manufactured glass microspheres has limited applications due mainly to their high cost of
production. Therefore, the material costs of composites can be reduced significantly by
incorporating fly ash into the matrices of polymers and metallic alloys. However, very little
information is available on to aid in the design of composite materials, even though attempts
have been made to incorporate fly ash in both polymer and metal matrices. Cenosphere fly ash
has a lower density than talc and calcium carbonate, but slightly higher than hollow glass. The
20
cost of cenosphere is likely to be much lower than hollow glass .Cenosphere may turn out to be
one of the lowest cost fillers in terms of the cost per volume.
2. The high electrical resistivity, low thermal conductivity and low density of fly-ash
may be helpful for making a light weight insulating composites.
3. Fly ash as a filler in Al casting reduces cost, decreases density and increase hardness,
stiffness, wear and abrasion resistance. It also improves the machinability, damping capacity,
coefficient of friction etc. which are needed in various industries like automotive etc.
4. As the production of Al is reduced by the utilization of fly ash. This reduces the
generation of green house gases as they are produced during the bauxite processing and alumina
reduction.
2.10 CHEMICAL REACTION BETWEEN Al AND FLY ASH
The thermodynamic analysis indicates that there is possibility between the reaction of
Al melt and the fly ash particles. The particles contain alumina, silica and iron oxide which
during solidification process of Al fly ash composites or during holding such composites at
temperature above 8500 C, are likely to undergo chemical reactions, reported by P.K.Rohatagi
and Guo. The experiments indicate that there is a progressive reduction between SiO2, Fe2O3 and
mullite by Al and formation of Al2O3, Fe and Si. The wall of cenosphere fly ash particles
progressive disintegrates into discrete particles into the reaction progress.
2.11Interfacial parameters
a. Adsorption and Wetting
b. Inter-diffusion
c. Chemical bonding
d. Mechanical adhesion
21
a. Adsorption and WettingGood wettability is needed to generate a strong interface that will allow transfer and distribution
of load from the matrix to the dispersed phase, without premature
Thermodynamically reversible work needed to separate interface into its component parts:
Wa= Ys + YL- YSL ………………… (1)
YS = Surface energy of solid
YL= Surface energy of liquid
YSL= Interfacial energy of solid-liquid phase
Young – Dupre’s equation:
YS= YSL+ YL cosØ………………….. (2)
Ø= contact angle
Combining (1) and (2),
Wa= YL (1 + cosØ)
Thus from a knowledge of Ø and YL, the work of adhesion can be found out.
FIG 2.11 (b) LIQUID DROP ON THE SOLID SURFACE
1. For Ø=1800, the drop is spherical with only point contact with the solid and no wetting
takes place.
22
2. For Ø=00, the wetting is PERFECT.
3. For 00<Ø<1800, the degree of wetting increases as Ø decreases.
Adsorption – It is a surface reaction which is dependent on concentration, temperature, and
diffusivity. The greater the adsorption, the more the solute tends to lower the surface energy.
b. Inter-diffusion- Inter-diffusion plays only a minor role at low temperatures, but at elevated
temperatures approaching the mp of the matrix, inter-diffusion and chemical reaction can result
in the formation of brittle intermetallics which are detrimental to the mechanical properties of
MMCs.
c. Chemical bonding-For strong chemical bonding between the reinforcement and matrix a
controlled amount of chemical reaction at the interface is always desirable. However, too thick
an interfacial zones adversely the mechanical properties of the composites and leads to
premature failure.
d. Mechanical adhesion-Mechanical bonding plays a major role in load transfer by shear. Two
separate factors affect mechanical adhesion namely (1). Surface roughness, which control the
amount of mechanical interlocking that can occur, and (2). The presence of residual stresses in
the matrix as a result of fabrication.
POINTS TO REMEMBER:
1. For strong chemical bonding between the reinforcement and matrix a controlled
amount of chemical reaction at the interface is always desirable. However, too
thick an interfacial zones adversely the mechanical properties of the composites
and leads to premature failure.
2. Interfacial reactions are of concern as they can adversely affect the mechanical
performance of an MMC. So, it is clear that, as a general rule, extensive
interfacial reactions should be avoided if optimum mechanical performance is to
be achieved with MMCs.
23
3.0 EXPERIMENTAL WORK
3.1 EXPERIMENTAL PROCEDURE
First of all, 400 gm of commercially pure aluminium was melted in a resistance heated muffle
furnace and casted in a clay graphite crucible. For this the melt temperature was raised to 993K
and it was degassed by purging hexachloro ethane tablets. Then the aluminium-fly ash
(5%,10%,15%,20%) composites were prepared by stir casting route. For this we took 400 gm of
commercially pure aluminum and then (5, 10, 15, 20) wt% of fly ash were added to the Al melt
for production of four different composites. The fly ash particles were preheated to 373K for two
hours to remove the moisture. Commercially pure aluminium was melted by raising its
temperature to 993K and it was degassed by purging hexachloro ethane tablets. Then the melt
was stirred using a mild steel stirrer. Fly-ash particles were added to the melt at the time of
formation of vortex in the melt due to stirring. The melt temperature was maintained at 953K993K during the addition of the particles. Then the melt was casted in a clay graphite crucible.
The particle size analysis and chemical composition analysis was done for fly ash. The hardness
testing and density measurement was carried out Al-(5, 10, 15, 20) wt% fly ash composites. The
hardness of the samples was determined by Brinell hardness testing machine with 750 kg load
and 5 mm diameter steel ball indenter. The detention time for the hardness measurement was 15
seconds.
The wear characteristics of Al-fly ash composites were evaluated using wear testing machine.
For this, cylindrical specimens of 1.1 cm diameter and 2.1 cm length were prepared from the cast
24
Al- fly ash composites. Test was performed at under different loads and rpm for 10 minutes. The
SEM was done for all the samples.
WORKS DONE:
1. Commercially pure Al was melted and casted.
2. Al-(5%,10%,15%,20%)fly ash composite was fabricated by stir casting
method.
3. Particle size analysis was done for fly ash used.
4. Hardness measurement was carried out for Al-fly ash composite samples.
5. The wear characteristics of Al-fly ash composite samples were evaluated
and compared.
6. SEM analysis was done for all the Al-fly ash composite samples, Fly-ash
and worn surfaces.
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4.0 RESULTS & DISCUSSION
4.1 CHEMICAL ANALYSIS OF FLY ASH
COMPOUNDS
PERCENTAGE(%)
SiO2
67.2
Al2O3
29.6
Fe2O3
0.1
CaO
1.4
MgO
1.7
TABLE 4.1
4.2 PARTICLE SIZE ANALYSIS OF FLY ASH
FIG 4.2 PARTICLE SIZE ANALYSIS
The size, density, type of reinforcing particles and its distribution have a pronounced
effect on the properties of particulate composite. Size range of fly ash particles is
26
reported in the above figure. The size range of the particles is very wide i.e. 0.1 micron to
100 micron. The size ranges of the fly ash particles indicate that the composite prepared
can be considered as dispersion strengthened as well as particle reinforced composite.
As is seen from the particle size distribution there are very fine particles as well as coarse
ones (1-100 µm). Thus the strengthening of composite can be due to dispersion
strengthening as well as due to particle reinforcement. Dispersion strengthening is due to
the incorporation of very fine particles, which help to restrict the movement of
dislocations, whereas in particle strengthening, load sharing is the mechanism.
Strengthening of matrix may occur because of solid solution strengthening.
4.3 HARDNESS MEASUREMENT
Composite
Hardness(BHN)
60
Samples
Hardness(BHN)
58
56
Al-5% Fly ash
49.36
Al-10% Fly ash
52.8
Al-15% Fly ash
56.41
52
Al-20% Fly ash
58.33
50
54
48
4
6
8
10
12
14
% Fly Ash
16
18
20
22
27
The above table shows that incorporation of fly ash particles in Aluminium matrix causes
reasonable increase in hardness. The strengthening of the composite can be due to dispersion
strengthening as well as due to particle reinforcement.Thus, fly ash as filler in Al casting reduces
cost, decreases density and increase hardness which are needed in various industries like
automotive etc.
4.4 WEAR BEHAVIOUR
In the graphs below the colour coding is as follows:(1)Blue for Al-5% Fly ash
(2) Red for Al-10 % Fly ash
(3)Black for Al-15% Fly ash
(4) Green for Al-20% Fly ash
FIG 4.4 (a) –Wear behaviour of MMCS with different % of Fly ash at 20N load and 240rpm.
FIG 4.4 (b) –Variation of Frictional force with different % of Fly ash at 20N load and 240 rpm.
28
Fig.4.4(c)– Wear behaviour of MMCS with different % of Fly ash at 50N load and 240rpm.
FIG4.4(d)– Variation of Frictional force with different % of Fly ash at 50N load and 240 rpm.
.
29
FIG 4.4 (e) – Wear behavior of Al-(20%)Fly ash composite at 20N load at 240 rpm(red line) and 500
rpm(blue line).
Fig 4.4 (f)– Variation of Frictional force for Al-(20%)Fly ash composite at 20N load at 240 rpm(red line)
and 500 rpm(blue line).
30
FIG 4.4 (g) – Wear behavior of for Al-(5%)Fly ash composite at 240 rpm and at 20N(red line) and
50N(blue line) load.
FIG 4.4 (h) – Variation of Frictional force for Al-(5%)Fly ash at 240 rpm and at 20N(red line) and
50N(blue line) load.
31
The figures [4(a),4(b)] shown above, clearly indicates that the wear rate has
improved significantly and frictional force has decreased with the increase in the percentage of
fly-ash.
The figure[4(c),4(d)] shows the variation of wear rate and frictional force with time
at different percentage of fly ash at 240 rpm. It shows that the wear rate has increased and the
frictional force has decreased with increase in percentage of fly ash.
The figure [4(e),4(f)] shows the variation of wear rate and frictional force with
time of Al-20% fly ash at 20N load at 240 and 500 rpm. It shows that the wear rate has
increased with the increase in rpm at constant load. The reason being the oxide particles are
removed giving rise to three body wear and wear rate increases with increase in sliding velocity.
The figure [4(g),4(h)] shows the variation of wear rate and frictional force with
time of Al-5% fly ash at 20N and 50N load at 240 rpm. It shows that the wear rate has increased
with the increase in load at constant rpm. Al-fly ash composites also have an increasing trend of
wear with applied load due to deformation generation of cracks within the oxide film that may
act as a three body wear on removal of the particles increasing the wear rate drastically at high
loads. The addition of fly ash acts as a barrier to the movement of dislocations and thereby
increases hardness of the composite. Thus addition of fly ash particles to the aluminium melt
significantly increases its abrasive wear resistance. The improvement in wear resistance is due to
the hard aluminosilicate constituent present in fly ash particles.
From the view of material, influencing factors on friction force are mechanical
properties of the matrix, hardness, chemical stability of the particles, composition and strength of
the interface. Interaction between these and tribological parameters (such as load and speed,
environment and the properties of the counter faces materials) are responsible for the overall
response.
32
4.5 SEM ANALYSIS
SEM photographs were taken to analyze the fly ash particles and surfaces of Al-(5%,20%) fly
ash composites.
FIG 4.5 (a) SEM micrograph of Fly ash particles
FIG 4.5 (b) SEM micrograph of as Al-5%fly ash composite
33
FIG 4.5 (c) SEM micrograph of Al-20% fly ash composite
The figure 4.5(a) shows that the fly ash particles consist of precipitator particles.
The figure [4.5(b),(c)] shows that with increase in percentage of fly ash there is increase in
incorporation of fly ash in the composites.
Thermodynamic analysis indicates that there is a possibility of chemical reaction
between aluminium melt and fly ash particles. As these fly ash particles consist of alumina, silica
and iron oxide, they are likely to undergo chemical reduction during their contact with the melt,
as follows:
2Al(l) + 3/ 2SiO2(s) = 3/2 Si(s) + Al2O3(s)…… (1)
2Al(l) +Fe2O3(s) = 2Fe(s) +Al2O3(s)…… (2)
The elements (Si and Fe) formed by reduction reaction would alloy with the matrix. Gibbs free
energy and the heats of reactions are highly exothermic in nature. As a result of this reaction (Eq.
(1)) greater amount of eutectic silicon is seen in the composites.
34
5.0 CONCLUSION
CONCLUSION
1. From the study it is concluded that we can use fly ash for the production of composites and
can turn industrial waste into industrial wealth. This can also solve the problem of storage and
disposal of fly ash.
2. Fly ash upto 20% by weight can be successfully added to Al by stir casting route to produce
composites.
3. The hardness of Al-fly ash composites has increased with increase in addition of fly ash.
4. Both the frictional forces and the wear rates has decreased significantly with the incorporation
of fly ash in Al melt.
5. Strengthening of composite is due to dispersion strengthening, particle reinforcement and solid
solution strengthening.
35
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