FABRICATION AND CHARACTERISATION OF ALUMINIUM …composite materials fabrication, in which a...

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Int. J. Mech. Eng. & Rob. Res. 2014 Yellappa M et al., 2014

ISSN 2278 – 0149 www.ijmerr.comVol. 3, No. 2, April, 2014

© 2014 IJMERR. All Rights Reserved

Research Paper

FABRICATION AND CHARACTERISATION OFALUMINIUM-FLY ASH COMPOSITE USING

STIR CASTING METHOD

Yellappa M1*, Puneet U3, G V Krishnareddy4, Giriswamy B G4, Satyamurthy N2

*Corresponding Author: Yellappa M, � [email protected]

Metal matrix composites (MMCs) possess significantly improved properties including high specificstrength; specific modulus, damping capacity and good wear resistance compared to unreinforcedalloys. There has been an increasing interest in composites containing low density and low costreinforcements. Among various discontinuous dispersoids used, fly ash is one of the mostinexpensive and low density reinforcement available in large quantities as solid waste byproductduring combustion of coal in thermal power plants. Hence, composites with fly ash asreinforcement are likely to overcome the cost barrier for wide spread applications in automotiveand small engine applications. It is therefore expected that the incorporation of fly ash particlesin aluminium alloy will promote yet another use of this low-cost waste by-product and, at thesame time, has the potential for conserving energy intensive aluminium and thereby, reducingthe cost of aluminium products. Now a days the particulate reinforced aluminium matrix compositeare gaining importance because of their low cost with advantages like isotropic properties andthe possibility of secondary processing facilitating fabrication of secondary components. Thepresent investigation has been focused on the utilization of abundantly available industrial wastefly-ash in useful manner by dispersing it into aluminium to produce composites by stir castingmethod.

Keywords: Particulate composites, Industrial waste, Applied load and sliding velocity

INTRODUCTION

Conventional monolithic materials havelimitations in achieving good combination ofstrength, stiffness, toughness and density. Toovercome these shortcomings and to meetthe ever increasing demand of modern day

1 Assistant Professor, Mechanical Department, SJBIT, Bangalore-60.2 Associate Professor, Mechanical Department, SJBIT, Bangalore-60.3 Assistant Professor, Mechanical Department, SJBIT, Bangalore-60.4 Lecturer, Mechanical Department, Govt Polytechnic, Bangalore-60.

technology, composites are most promisingmaterials of recent interest. Metal matrixcomposites (MMCs) possess significantlyimproved properties including high specificstrength; specific modulus, damping capacityand good wear resistance compared tounreinforced alloys. There has been an

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increasing interest in composites containinglow density and low cost reinforcements.Among various discontinuous dispersoidsused, fly ash is one of the most inexpensiveand low density reinforcement available inlarge quantities as solid waste by-productduring combustion of coal in thermal powerplants. Hence, composites with fly ash asreinforcement are likely to overcome the costbarrier for wide spread applications inautomotive and small engine applications. Itis therefore expected that the incorporationof fly ash particles in aluminium alloy willpromote yet another use of this low-costwaste by-product and, at the same time, hasthe potential for conserving energy intensivealuminium and thereby, reducing the cost ofaluminium products (P K Rohatgi, 1994); (TP D Rajan et al., 2001); P K Rohatgi et al.,1997).

Now a days the particulate reinforcedaluminium matrix composite are gainingimportance because of their low cost withadvantages like isotropic properties and thepossibility of secondary processing facilitatingfabrication of secondary components. Castaluminium matrix particle reinforcedcomposites have higher specific strength,specific modulus and good wear resistanceas compared to unreinforced alloys.

Composite material is a materialcomposed of two or more distinct phases(matrix phase and reinforcing phase) andhaving bulk properties significantly differentfrom those of any of the constituents. Manyof common materials (metals, alloys, dopedceramics and polymers mixed with additives)also have a small amount of dispersedphases in their structures, however they arenot considered as composite materials sincetheir properties are similar to those of theirbase constituents (physical property of steelare similar to those of pure iron). Favorable

properties of composites materials are highstiffness and high strength, low density, hightemperature stability, high electrical andthermal conductivity, adjustable coefficient ofthermal expansion, corrosion resistance,improved wear resistance etc.

MATRIX PHASE

1. The primary phase, having acontinuous character,

2. Usually more ductile and less hardphase,

3. Holds the reinforcing phase andshares a load with it.

REINFORCING PHASE

1. Second phase (or phases) isimbedded in the matrix in adiscontinuous form,

2. Usually stronger than the matrix,therefore it is sometimes calledreinforcing phase.

Composites as engineering materialsnormally refer to the material with thefollowing characteristics:

1. These are artificially made (thus,excluding natural material such aswood).

2. These consist of at least two differentspecies with a well-defined interface.

3. Their properties are influenced by thevolume percentage of ingredients.

4. These have at least one property notpossessed by the individualconstituents.

CLASSIFICATION OF COMPOSITES

a) On the basis of Matrix

• Metal Matrix Composites (MMC)Metal Matrix Composites are

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composed of a metallic matrix(aluminium, magnesium, iron, cobalt,copper) and a dispersed ceramic(oxides, carbides) or metallic (lead,tungsten, molybdenum) phase.

• Ceramic Matrix Composites (CMC)Ceramic Matrix Composites arecomposed of a ceramic matrix andimbedded fibers of other ceramicmaterial (dispersed phase).

• Polymer Matrix Composites (PMC)Polymer Matrix Composites arecomposed 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

• Particulate Composites: ParticulateComposites consist of a matrixreinforced by a dispersed phase inform of particles.

• Composites with random orientation ofparticles.

• Composites with preferred orientationof particles.

Dispersed phase of these materials consistsof two-dimensional flat platelets (flakes), laidparallel to each other.

Fibrous CompositesShort-fiber reinforced composites: Short-fiber reinforced composites consist of a matrixreinforced by a dispersed phase in form ofdiscontinuous fibers (length <100*diameter).

• Composites with random orientation offibers.

• Composites with preferred orientationof fibers

Long-fiber reinforced composites: Long-fiber reinforced composites consist of a matrixreinforced by a dispersed phase in form ofcontinuous fibers.

• Unidirectional orientation of fibers.

• Bidirectional orientation of fibers(woven).

Laminate Composites

When a fiber reinforced composite consistsof several layers with different fiberorientations, it is called multilayer (angle-ply)composite.

RULE OF MIXTURES

Rule of Mixtures is a method of approach toapproximate estimation of composite materialproperties, based on an assumption that acomposite property is the volume weighedaverage of the phases (matrix and dispersedphase) properties.

According to Rule of Mixtures properties ofcomposite materials are estimated as follows:

Density dc = dm*Vm + df*Vf

Where,

dc,dm,df- densities of the composite,matrix and dispersed phaserespectively;

Vm,Vf- volume fraction of the matrixand dispersed phase respectively.

Coefficient of Thermal Expansion• Coefficient of Thermal Expansion

(CTE) in longitudinal direction (alongthe fibers)

α cl= (

α

m*Em*Vm+

α

f*Ef*Vf)/ (Em*Vm +Ef*Vf)

Where,

α

cl,

α

m,

α

f - CTE of composite in longitudinaldirection, matrix and dispersed phase (fiber)respectively;

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Em,Ef- modulus of elasticity of matrix anddispersed phase (fiber) respectively.

• Coefficient of Thermal Expansion (CTE)in transverse direction (perpendicular tothe fibers)

α ct = (1+Pm) m *Vm+ f* Vf

Pm - Poisson ratio of matrix.

Poisson's ratio is the ratio of transversecontraction strain to longitudinal extensionstrain in the direction of applied force.

Modulus of Elasticity

• Modulus of Elasticity in longitudinaldirection (Ecl)

Ecl = Em*Vm + Ef*Vf

• Modulus of Elasticity in transversedirection (Ect)

1/Ect = Vm/Em + Vf/Ef

Tensile Strength

• Tensile strength of long-fiber reinforcedcomposite in longitudinal direction

c= m*Vm+ f*Vf

Where,c, m, f- tensile strength of the composite,

matrix and dispersed phase (fiber)respectively.

• Tensile strength of short-fiber compositein longitudinal direction (Fiber length isless than critical value Lc)

Lc= f*d/ c

Where,

d - Diameter of the fiber;c-shear strength of the bond between

the matrix and dispersed phase (fiber).c= m*Vm+ ?f*Vf*(1 - Lc/2L)

Where,L - Length of the fiber

• Tensile strength of short-fiber compositein longitudinal direction(fiber length isgreater than critical value Lc)

c= m*Vm+ L* c*Vf/d

METAL MATRIX COMPOSITES (MMCs)

Metal Matrix Composites are composed of ametallic matrix (Al,Mg,Fe,Cuetc) and adispersed ceramic (oxide, carbides) ormetallic phase( Pb,Mo,Wetc). Ceramicreinforcement may be silicon carbide, boron,alumina, silicon nitride, boron carbide, boronnitride etc. whereas Metallic Reinforcementmay be tungsten, beryllium etc. MMCs areused for Space Shuttle, commercial airliners,electronic substrates, bicycles, automobiles,golf clubs and a variety of other applications.From a material point of view, whencompared to polymer matrix composites, theadvantages of MMCs lie in their retention ofstrength and stiffness at elevatedtemperature, good abrasion and creepresistance properties. Most MMCs are still inthe development stage or the early stages ofproduction and are not so widely establishedas polymer matrix composites. The biggestdisadvantages of MMCs are their high costsof fabrication, which has placed limitationson their actual applications. There are alsoadvantages in some of the physical attributesof MMCs such as no significant moistureabsorption properties, non-inflammability, lowelectrical and thermal conductivities andresistance to most radiations. MMCs haveexisted for the past 30 years and a wide rangeof MMCs have been studied.

Compared to monolithic metals, MMCshave• Higher strength-to-density ratios

• Higher stiffness-to-density ratios

• Better fatigue resistance

• Better elevated temperature properties

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• Higher strength

• Lower creep rate

• Lower coefficients of thermal expansion

• Better wear resistance

The advantages of MMCs over polymermatrix composites are:

• Higher temperature capability

• Fire resistance

• Higher transverse stiffness and strength

• No moisture absorption

• Higher electrical and thermalconductivities

• Better radiation resistance

• No out gassing

• Fabric ability of whisker and particulatereinforced

MMCs with conventional metalworkingequipment.

Some of the disadvantages of MMCscompared to monolithic metals and polymermatrix composites are:

• Higher cost of some material systems

• Relatively immature technology

• Complex fabrication methods for fiber-reinforced systems (except forcasting)

• Limited service experience

MATERIALS AND METHODS

Stir Casting Method Of Fabrication

Of Mmcs

Liquid state fabrication of Metal MatrixComposites involves incorporationofdispersed phase into a molten matrix metal,followed by its Solidification. In order toprovidehigh level of mechanical properties ofthe composite, good interfacial bonding(wetting) betweenthe dispersed phase andthe liquid matrix should be obtained.

Wetting improvement may be achieved bycoating the dispersed phase particles (fibers).Proper coating not only reduces interfacialenergy, but also prevents chemical interactionbetween the dispersed phase and the matrix.

The simplest and the most cost effectivemethod of liquid state fabrication is StirCasting.

Stir Casting

Stir Castingis a liquid state method ofcomposite materials fabrication, in which adispersed phase (ceramic particles, shortfibers) is mixed with a molten matrix metalby means of mechanical stirring. The liquidcomposite material is then cast byconventional casting methods and may alsobe processed by conventional Metal formingtechnologies.

Figure 1: Stir Casting Setup

Stir Casting is characterized by thefollowing features

• Content of dispersed phase is limited(usually not more than 30 vol. %).• Distribution of dispersed phase throughoutthe matrix is not perfectly homogeneous:• There are local clouds (clusters) of thedispersed particles (fibers);• There may be gravity segregation of thedispersed phase due to a difference in thedensities of the dispersed and matrix phase.

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• The technology is relatively simple and lowcost.

Strengthening Mechanism OfComposites

The strengthening mechanisms of thecomposites are different with different kindofreinforcing agent morphology such as fibers,particulate or dispersed type ofreinforcingelements.

Dispersion Strengthening Mechanism of

Strengthened Composite

In the dispersion strengthened composite thesecond phase reinforcing agents arefinelydispersed in the soft ductile matrix. Thestrong particles restrict the motion ofdislocationsand strengthen the matrix. Herethe main reinforcing philosophy is by thestrengtheningof the matrix by the dislocationloop formation around the dispersedparticles. Thus thefurther movement ofdislocations around the particles is difficult.Degree of strengtheningdepend upon theseveral factors like volume % of dispersedphase, degree of dispersion,size and shapeof the dispersed phase, inter particle spacingetc. In this kind of composite the load ismainly carried out by the matrix materials.

Strengthening Mechanism of Particulate

Composite

In the particulate reinforced composite thesize of the particulate is more than 1 ?m, soit strengthens the composite in two ways.First one is the particulate carry the load alongwith the matrix materials and another way isby formation of incoherent interface betweenthe particles and the matrix. So a largernumber of dislocations are generated at theinterface, thus material gets strengthened.The degree of strengthening depends on theamount of particulate (volume fraction),

distribution, size and shape of the particulateetc.

Fly Ash

Fly ash is one of the residues generated inthe combustion of coal. It is an industrial byproduct recovered from the flue gas of coalburning electric power plants. Dependingupon the source and makeup of the coalbeing burned, the components of the fly ashproduced vary considerably, but all fly ashincludes substantial amounts of silica (silicondioxide, SiO2) (both amorphous andcrystalline) and lime (calcium oxide, CaO).In general, fly ash consists of SiO2,

Al2O3, Fe2O3 as major constituents andoxides of Mg, Ca, Na, K etc. as minorconstituent. Fly ash particles are mostlyspherical in shape and range from less than1 µm to 100 µm with a specific surface area,typically between 250 and 600m2/kg. Thespecific gravity of fly ash vary in the range of0.6-2.8 gm/cc. Coal fly ash has many usesincluding as a cement additive, in masonryblocks, as a concrete admixture, as a materialin lightweight alloys, as a concrete aggregate,in flowable fill materials, in roadway/runwayconstruction, in structural fill materials,asroofing granules, and in grouting. The largestapplication of fly ash is in the cement andconcrete industry, though, creative new usesfor fly ash are being actively sought like useof fly ash for the fabrication of MMCs.

Classification of Fly Ash on the Basis of

Chemical Composition

i. Class F fly ash

The burning of harder, older anthracite andbituminous coal typically produces Class Ffly ash. This fly ash is pozzolanic in nature,and contains less than 10% lime (CaO).Possessing pozzolanic properties, the glassysilica and alumina of Class F fly ash requires

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a cementing agent, such as Portland cement,quicklime, or hydrated lime, with the presenceof water in order to react and producecementitious compounds. Alternatively, theaddition of a chemical activator such assodium silicate (water glass) to a Class F ashcan leads to the formation of a geopolymer.

ii. Class C fly ashFly ash produced from the burning of youngerlignite or sub bituminous coal, in addition tohaving pozzolanic properties, also has someself-cementing properties. In the presence ofwater, Class C fly ash will harden and gainstrength over time. Class C fly ash generallycontains more than 20% lime (CaO). UnlikeClass F, self-cementing Class C fly ash doesnot require an activator. Alkali and sulfate(SO4) contents are generally higher in ClassC fly ashes.

On the basis of size, shape and structure

i. Precipitator fly ashIt is spherical in nature, the spheres are solidand the density is in the range of 2.0-2.5 g/cm3.

ii. Cenosphere fly ashIt is also spherical in shape but these spheresare hollow, so the density of this kind of flyash is very less as compared to theprecipitator fly ash. Here density is less than1 gm/cm3(0.3-0.6 gm/cm3)

WHY FLY ASH

1. The preference to use fly ash as a filleror reinforcement in metal and polymermatrices is that fly ash is a byproduct ofcoal combustion, available in very largequantities (80 million tons per year) atvery low costs since much of this iscurrently land filled. Currently, the use ofmanufactured glass microspheres haslimited applications due mainly to their

high cost of production. Therefore, thematerial costs of composites can bereduced significantly by incorporating flyash into the matrices of polymers andmetallic alloys. However, very littleinformation is available on to aid in thedesign of composite materials, eventhough attempts have been made toincorporate fly ash in both polymer andmetal matrices. Cenosphere fly ash hasa lower density than talc and calciumcarbonate, but slightly higher than hollowglass. The cost of cenosphere is likely tobe much lower than hollow glass.Cenosphere may turn out to be one ofthe lowest cost fillers in terms of the costper volume.

2. The high electrical resistivity, low thermalconductivity and low density of fly-ashmay be helpful for making a light weightinsulating composites.

3. Fly ash as a filler in Al casting reducescost, decreases density and increasehardness, stiffness, wear and abrasionresistance. It also improves themachinability, damping capacity,coefficient of friction etc. which areneeded in various industries likeautomotive etc.

4. As the production of Al is reduced by theutilization of fly ash. This reduces thegeneration of greenhouse gases as theyare produced during the bauxiteprocessing and alumina reduction.

Chemical Reaction Between AlAnd Fly Ash

The thermodynamic analysis indicates thatthere is possibility between the reaction of Almelt and the fly ash particles. The particlescontain alumina, silica and iron oxide whichduring solidification process of Al fly ashcomposites or during holding such

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composites at temperature above 8500?C,are likely to undergo chemical reactions,reported by P.K.Rohatagiand Guo. Theexperiments indicate that there is aprogressive reduction between SiO2,Fe2O3and mullite by Al and formation ofAl2O3, Fe and Si. The wall of cenosphere flyash particles progressive disintegrates intodiscrete particles into the reaction progress.

EXPERIMENTAL PROCEDURE

First of all, 400 gm of commercially purealuminium was melted in a resistance heatedmuffle furnace and casted in a clay graphitecrucible. For this the melt temperature wasraised to 993K and it was degassed bypurging hexachloro-ethane tablets. Then thealuminium-fly ash (5%, 10%, 15%, and 20%)composites were prepared by stir castingroute. For this we took 400 gm ofcommercially pure aluminum and then (5, 10,15, 20) wt% of fly ash were added to the Almelt for production of four differentcomposites. The fly ash particles werepreheated to 373K for two hours to removethe moisture. Commercially pure aluminiumwas melted by raising its temperature to 993Kand it was degassed by purging hexachloroethane tablets. Then the melt was stirredusing a mild steel stirrer. Fly-ash particleswere added to the melt at the time offormation of vortex in the melt due to stirring.The melt temperature was maintained at953K- 993K during the addition of theparticles. Then the melt was casted in a claygraphite crucible. The particle size analysisand chemical composition analysis was donefor fly ash. The hardness testing and densitymeasurement was carried out Al-(5, 10, 15,and 20) wt% fly ash composites. Thehardness of the samples was determined byBrinell hardness testing machine with 750 kgload and 5 mm diameter steel ball indenter.

The detention time for the hardnessmeasurement was 15 seconds.

The wear characteristics of Al-fly ashcomposites were evaluated using weartesting machine. For this, cylindricalspecimens of 1.1 cm diameter and 2.1 cmlength were prepared from the castAl- fly ashcomposites. Test was performed at underdifferent loads and rpm for 10 minutes. TheSEM was done for all the samples.

WORKS DONE

1. Commercially pure Al was melted andcasted.

2. Al-(5%, 10%, 15%, and 20%) fly ashcomposite was fabricated by stir castingmethod.

3. Particle size analysis was done for fly ashused.

4. Hardness measurement was carried outfor Al-fly ash composite samples.

5. The wear characteristics of Al-fly ashcomposite samples were evaluated andcompared.

6. SEM analysis was done for all the Al-flyash composite samples, Fly-ash andworn surfaces.

RESULTS AND DISCUSSION

Chemical Analysis of Fly Ash

Table 1: Analysis of Fly Ash

Compounds

SiO2

A12O3

Fe2O3

CaO

MgO

Percentage (%)

67.2

29.6

0.1

1.4

1.7

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Figure 2: Particle Size Analysis

Particle Size Analysis of Fly Ash

The size, density, type of reinforcingparticles and its distribution have apronounced effect on the properties ofparticulate composite. Size range of fly ashparticles is reported in the above figure. Thesize range of the particles is very wide i.e.0.1 micron to 100 micron. The size ranges ofthe fly ash particles indicate that thecomposite prepared can be considered asdispersion strengthened as well as particlereinforced composite.

As is seen from the particle size distributionthere are very fine particles as well as coarseones (1-100 ?m). Thus the strengthening ofcomposite can be due to dispersionstrengthening as well as due to particlereinforcement. Dispersion strengthening isdue to the incorporation of very fine particles,which help to restrict the movement ofdislocations, whereas in particlestrengthening, load sharing is themechanism. Strengthening of matrix mayoccur because of solid solution strengthening.

Hardness Measurement

The above table shows that incorporation offly ash particles in Aluminium matrix causesreasonable increase in hardness. Thestrengthening of the composite can be dueto dispersion strengthening as well as due toparticle reinforcement. Thus, fly ash as fillerin Al casting reduces cost, decreases densityand increase hardness which are needed invarious industries like automotive etc.

Figure 3: Hardness

Wear Behaviour

In the graphs below the colour coding is asfollows:-

1. Blue for Al-5% Fly ash2. Red for Al-10 % Fly ash3. Black for Al-15% Fly ash4. Green for Al-20% Fly ash

Figure 4(a): Wear Behavior of MMCS withDifferent % of Fly Ash at 20N Load and 240rpm

Figure 4(b): Variation of Frictional Force withDifferent % of Fly Ash at 20N Load and 240 rpm

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Figure 4(c): Wear Behavior of MMCS withDifferent % of Fly Ash at 50N Load and 240rpm

Figure 4(d): Variation of Frictional Force withDifferent % of Fly Ash at 50N Load and 240 rpm

Figure 4(e): Wear Behavior of Al-(20%)Fly Ash composite at 20N Load at 240rpm(red line) and 500rpm(blue line)

Figure 4(f): Variation of Frictional Force forAl-(20%) Fly Ash Composite at 20N Load at240 rpm(red line) and 500 rpm(blue line).

Figure 4(g): Wear Behavior of for Al-(5%) Fly Ash Composite at 240 rpm andat 20N (red line) and 50N(blue line) load

Figure 4(h): Variation of Frictional forcefor Al-(5%) Fly Ash at 240 rpm and at20N (red line) and 50N(blue line) Load

The figures [4(a), 4(b)] shown above,clearly indicates that the wear rate hasimproved significantly and frictional force hasdecreased with the increase in thepercentage of fly-ash.

The figure [4(c), 4(d)] shows the variationof wear rate and frictional force with time atdifferent percentage of fly ash at 240 rpm. Itshows that the wear rate has increased andthe frictional force has decreased withincrease in percentage of fly ash.

The figure [4(e), 4(f)] shows the variationof wear rate and frictional force with time ofAl-20% fly ash at 20N load at 240 and 500rpm. It shows that the wear rate hasincreased with the increase in rpm at constantload. The reason being the oxide particlesare removed giving rise to three body wearand wear rate increases with increase insliding velocity.

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The figure [4(g), 4(h)] shows the variationof wear rate and frictional force with time ofAl-5% fly ash at 20N and 50N load at 240rpm. It shows that the wear rate hasincreased with the increase in load atconstant rpm. Al-fly ash composites also havean increasing trend of wear with applied loaddue to deformation generation of crackswithin the oxide film that may act as a threebody wear on removal of the particlesincreasing the wear rate drastically at highloads. The addition of fly ash acts as a barrierto the movement of dislocations and therebyincreases hardness of the composite. Thusaddition of fly ash particles to the aluminiummelt significantly increases its abrasive wearresistance. The improvement in wearresistance is due to the hard alumino-silicateconstituent present in fly ash particles.

From the view of material, influencingfactors on friction force are mechanicalproperties of the matrix, hardness, chemicalstability of the particles, composition andstrength of the interface. Interaction betweenthese and tribological parameters (such asload and speed, environment and theproperties of the counter faces materials) areresponsible for the overall response.

SEM ANALYSIS

SEM photographs were taken to analyze thefly ash particles and surfaces of Al-(5%, 20%)fly ash composites.

Figure 5(a): SEM Micrographof Fly Asash Particles

Figure 5(b): SEM Micrographof Al-5% Fly Ash Composite

Figure 5(c): SEM Micrographof Al-20% Fly Ash Composite

The figure 4.5(a) shows that the fly ashparticles consist of precipitator particles. Thefigure [4.5(b),(c)] shows that with increase inpercentage of fly ash there is increase inincorporation of fly ash in the composites.

Thermodynamic analysis indicates thatthere is a possibility of chemical reactionbetween aluminium melt and fly ash particles.As these fly ash particles consist of alumina,silica and iron oxide, they are likely to undergochemical reduction during their contact withthe 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 byreduction reaction would alloy with the matrix.Gibbs free energy and the heats of reactionsare highly exothermic in nature. As a resultof this reaction (Eq.(1)) greater amount ofeutectic silicon is seen in the composites.

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CONCLUSION

1. From the study it is concluded that wecan use fly ash for the production ofcomposites and can turn industrial wasteinto industrial wealth. This can also solvethe problem of storage and disposal offly ash.

2. Fly ash upto 20% by weight can besuccessfully added to Al by stir castingroute to produce composites.

3. The hardness of Al-fly ash compositeshas increased with increase in additionof fly ash.

4. Both the frictional forces and the wearrates has decreased significantly with theincorporation of fly ash in Al melt.

5. Strengthening of composite is due todispersion strengthening, particlereinforcement and solid solutionstrengthening.

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