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Materials Science and Engineering A 480 (2008) 117–124

Synthesis of fly ash particle reinforced A356 Alcomposites and their characterization

Sudarshan, M.K. Surappa ∗Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India

Received 7 July 2006; received in revised form 27 June 2007; accepted 27 June 2007

bstract

A356 Al–fly ash particle composites were fabricated using stir-cast technique and hot extrusion. Composites containing 6 and 12 vol.% flysh particles were processed. Narrow size range (53–106 �m) and wide size range (0.5–400 �m) fly ash particles were used. Hardness, tensiletrength, compressive strength and damping characteristics of the unreinforced alloy and composites have been measured. Bulk hardness, matrixicrohardness, 0.2% proof stress of A356 Al–fly ash composites are higher compared to that of the unreinforced alloy. Additions of fly ash lead

o increase in hardness, elastic modulus and 0.2% proof stress. Composites reinforced with narrow size range fly ash particle exhibit superiorechanical properties compared to composites with wide size range particles. A356 Al–fly ash MMCs were found to exhibit improved damping

apacity when compared to unreinforced alloy at ambient temperature. 2007 Elsevier B.V. All rights reserved.

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eywords: Fly ash; Stir-cast technique; Damping capacity

. Introduction

Metal matrix composites (MMCs) possess significantlymproved properties including high strength, high stiffness andamping capacity, compared to unreinforced alloy. There haseen an increasing interest in composites containing low den-ity and low cost reinforcements. Among various discontinuousispersoids used, fly ash is one of the most inexpensive and low-ensity reinforcement available in large quantities as solid wastey-product during combustion of coal in thermal power plants.ence, composites with fly ash as reinforcement are likely tover come the cost barrier for wide spread applications in auto-otive and small engine applications. It is therefore expected

hat the incorporation of fly ash particles in aluminium alloyill promote yet another use of this low-cost waste by-product

nd, at the same time, has the potential for conserving energy-ntensive aluminium and thereby, reducing the cost of aluminiumroducts [1–3].

In general aluminium-based MMCs offer substantial increasen elastic modulus and strength over the unreinforced alloysnd often accompanied by large reduction in percent elonga-

∗ Corresponding author. Tel.: +91 80 22932697; fax: +91 80 23600472.E-mail address: mirle@materials.iisc.ernet.in (M.K. Surappa).

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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2007.06.068

ion. However, it has been shown that secondary processing suchs extrusion, rolling, forging, etc. can significantly improve theuctility while retaining the strength and stiffness. The staticechanical properties of aluminium alloys and its composites

ave been documented by many researchers [1,4–6].Mechanical properties of composites are affected by the size,

hape and volume fraction of the reinforcement, matrix materialnd reaction at the interface. These aspects have been discussedy many researchers. Rohatgi [1] reports that with the increase inolume percentages of fly ash, hardness value increases in Al–flysh (precipitator type) composites. He also reports that the ten-ile elastic modulus of the ash alloy increases with increase inolume percent (3–10) of fly ash. Aghajanian et al. [7] havetudied the Al2O3 particle reinforced Al MMCs, with varyingarticulate volume percentages (25, 36, 46, 52 and 56) and reportmprovement in elastic modulus, tensile strength, compressivetrength and fracture properties with an increase in the reinforce-ent content. Composites behave normally up to the yield point

nder both tensile and compressive loads. However, compres-ion samples (52 vol.% Al2O3 reinforced Al–10Mg MMC) wereble to accommodate far more strain before failing than tensile

amples; exhibit strength of 1035 MPa in compression in con-rast to 531 MPa in tension. Although MMCs exhibit enhancedtrength and modulus, percentage elongation reduces drasticallyue to the presence of ceramic reinforcement. Generally, frac-

118 Sudarshan, M.K. Surappa / Materials Science and Engineering A 480 (2008) 117–124

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Table 1Chemical composition of A356 Al–Si alloy (in weight percent)

Si 7.083Mg 0.409Cu 0.059Fe 0.097Ti 0.132Mn 0.003CA

p(pafrawns

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2

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Fig. 1. Particle size distribution curve for as-received fly ash.

ure of particle and localization of the matrix deformation areonsidered as the main factors for reduced ductility of particleeinforced MMCs. It has been shown that ductility of parti-le reinforced MMCs decreases with increasing particle volumeraction [8,9]. Very large size particles reduce ductility whereashe particle size has little influence on ductility when particlesre smaller in size [8]. Percentage elongation is reduced by theresence of particle clusters [10–12].

The interface between the matrix and reinforcement playscritical role in determining the properties of MMCs. Stiffen-

ng and strengthening rely on load transfer across the interface.oughness is influenced by the crack deflection at the interfacend ductility is affected by the relaxation of peak stress nearhe interface [13–15]. Generally compressive yield strength isigher than the tensile yield strength and compressive ductilitys greater than the tensile ductility.

More recently, discontinuously reinforced MMCs have beennvestigated by a number of researchers as a means of efficientlyncreasing the damping capacity [16–20]. Particulate reinforced

MCs are of particular interest as a result of their feasibil-ty for mass production, improved mechanical properties, andigh damping capacity [19]. Among the materials available asarticulate reinforcements, SiC, Al2O3, and graphite (Gr) par-iculates are most frequently used in MMCs. Enhancement as

ell as decrease in damping capacity with the addition of SiCarticulates have been reported [21,22].

Zhang et al. [19,23,24] experimentally investigated the influ-nce of different factors on the damping capacity of Al matrix

Fig. 2. Particle size distribution curve for sieved fly ash.

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articulate-reinforced MMCs, including types of particulateSiC and graphite), volume fraction of second phase, tem-erature, frequency, strain amplitude, and so on. Rohatgi etl. [25] concluded that the cast particulate composites wereound to be have better damping capacity than the fibereinforced MMCs, and could be potential candidate materi-ls in vibration–insulation application in consumer industries,hereas the expensive fibre-reinforced composites may be eco-omically useful only for applications in advanced aerospacetructures.

In the present work, A356 Al–fly ash composites were fab-icated using a simple and cost effective experimental route i.e.tir-casting technique and subsequently hot extruded. Further,ttributes of fly ash particle dispersed A356 Al–fly ash compos-tes have been studied.

. Materials and experimental procedure

Materials used in this study were A356 Al–fly ash particlesomposites containing 6 and 12 vol.% fly ash particles. Narrowize range (53–106 �m) and wide size range (0.5–400 �m) flysh particles were used. Fly ash from Raichur thermal powerlant (India) had a wide particle size distribution. Particle sizef fly ash, in the as-received condition (Fig. 1), lies in the rangerom 0.5 to 400 �m and had a density 2.09 g/cm3. Hence, aseceived particles were sieved and those particles, which passhrough 140 mesh and retained on 270 mesh were chosen. It hashe density of 2.10 g/cm3. Particle size analysis was done using aomputerized particle size analyzer (Malvern® make laser lightarticle size analyzer). Fifty percent of as received particles wereess than 60 �m and 80% of the particles were less than 152 �m.n the case of sieved particles (−140# + 270#), 50% of the par-

icles were less than 76 �m and 80% of the particles were lesshan 104 �m (Fig. 2). Chemical composition of the matrix alloynd fly ash particles are shown in Tables 1 and 2. Fly ash from

able 2hemical composition of fly ash (in weight percent)

iO2 64.80l2O3 24.01e2O3 5.23aO 2.76gO 0.90

iO2 0.50OIa 0.87–1.33

a Loss on ignition.

Sudarshan, M.K. Surappa / Materials Science and Engineering A 480 (2008) 117–124 119

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Table 3Silicon content in A356 Al alloy and its composites

Material Silicon (wt.%)

A356 Al 7.08C6(S) 8.33CC

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Fig. 3. SEM micrograph of fly ash particles.

aichur thermal power plant (India) was used in this study andheir scanning electron microscopy (SEM) image is shown inig. 3.

A356 Al–fly ash (particle of as received (D50 ∼ 60 �m) andieved (D50 ∼ 76 �m)) composites were fabricated using stir castechnique. Fly ash particles were preheated to 800 ◦C for 2 h andhen dispersed into the vortex (created by mechanical stirring) ofhe A356 Al alloy melt held at 770 ◦C and subsequently pourednto 60 mm diameter cast iron moulds. A356 Al–12 vol.% flysh (as-received) composite (C12(AR)), A356 Al–6 vol.% flysh (sieved) composite (C6(S)) and A356 Al–12 vol.% fly ash

sieved) composites (C12(S)) were prepared. The unreinforcedlloy was also processed under similar conditions for the purposef comparison. Cast ingots were machined to 55 mm diame-er. Later homogenized at 530 ◦C for 2 h before extruding at an

t

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Fig. 4. Optical microstructure of as cast: (a) A356 Al alloy, (b) C6(S)

12(S) 15.8312(AR) 15.31

xtrusion ratio of 21:1. The test specimens were made from thextruded rods.

In the present work GABO EPLEXOR® dynamic mechani-al thermal analyzer (DMTA) [26] was used to measure dampingapacity of composites. Specimens for damping tests were rect-ngular bars and had a dimensions 2 mm × 10 mm × 50 mm.amples were made from the as extruded rods. Symmetrical

hree point bending technique was used to measure the dynamicodulus and damping capacity. The test was done under a

ynamic load of 10 N and static load of 20 N at 10 Hz.

. Results and discussion

.1. Microstructure and interface

Fly ash particles were spherical in shape. The fly ash mainlyonsisted of solid particles although smaller quantities of par-ially solid or hollow spherical particles were seen. Fly asharticles, which are in the form of solid spheres, are knowns precipitator fly ash. Those in the form of hollow spheres are

ermed as cenosphere fly ash.

Microstructures of as cast composites reveal relatively uni-orm distribution of fly ash particles in the matrix (Fig. 4).owever, at some locations clusters of smaller fly ash parti-

composite (c) C12(S) composite and (d) C12(AR) composite.

120 Sudarshan, M.K. Surappa / Materials Science and Engineering A 480 (2008) 117–124

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aaotof mixtures (ROM). This could be attributed to the presence ofporosity in the composites. In the extruded condition the porescontent in matrix is low and as a result the density values are veryclose to the predicted values based on ROM. Porosity values

Table 4Density values and porosity content of the A356 Al alloy and its composites inthe as cast and extruded conditions

Material Theoreticaldensity (g/cm3)

Measured density(g/cm3)

Porosity (vol.%)

As-cast Extruded As-cast Extruded

ig. 5. Optical micrographs of C6(S) composite in the extruded condition (a) trection and C12(AR) composite (e) transverse and (f) longitudinal section.

les and pores were observed. Hot extrusion leads to substantialmprovement in the integrity of microstructure of compositesFig. 5). Particles and eutectic silicon were aligned in the extru-ion direction (Fig. 5b, d and f). Aluminium from the matrixould react with SiO2 and Fe2O3; magnesium reacts with SiO2

nd Al2O3 constituents of the fly ash [2,27].Thermodynamic analysis indicates that there is a possibility

f chemical reaction between aluminium melt and fly ash par-icles. As these fly ash particles consist of alumina, silica andron oxide, they are likely to undergo chemical reduction duringheir contact with the melt, as follows: [28].

Al(l) + 32 SiO2(s) = 3

2 Si(s) + Al2O3(s) (1)

Al(l) + Fe2O3(s) = 2Fe(s) + Al2O3(s) (2)

he elements (Si and Fe) formed by reduction reaction wouldlloy with the matrix. Gibbs free energy and the heats of reac-

ions are highly exothermic in nature. As a result of this reactionEq. (1)) greater amount of eutectic silicon is seen in the compos-tes. The chemical analysis (Table 3) also indicates an increasen silicon level in the matrix as compared to unreinforced alloy.

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rse, (b) longitudinal section; C12(S) composite (c) transverse, (d) longitudinal

.2. Density and porosity

Table 4 compares the bulk densities of aluminium alloy–flysh composites with that of aluminium alloy matrix in the as castnd extruded conditions. In the as cast condition measured valuesf the densities of aluminium alloy–fly ash composites appearso be slightly lower than the values predicted based on the rule

356 Al 2.680 2.665 2.676 0.560 0.1496(S) 2.645 2.616 2.640 1.096 0.18912(S) 2.609 2.442 2.591 6.401 0.69012(AR) 2.611 2.458 2.602 5.860 0.345

Sudarshan, M.K. Surappa / Materials Scienc

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ig. 6. Hardness ((a) macro and (b) micro) of A356 Al alloy and its compositesn as cast and as extruded conditions.

ere obtained from the difference between ROM and mea-ured density values. Volume percent porosity in the compositeseduces substantially after extrusion. 12-vol.% fly ash reinforcedomposites C12(AR) has a density of 2.60 g/cm3, which is theighest and very close to the ROM value (2.61 g/cm3). Com-osite with 6-vol.% fly ash shows less porosity compared toomposites with 12-vol.% fly ash in both as-cast and as-extrudedonditions. Obviously increase in volume percentage of the rein-

orcement gives rise to more porosity. This may be attributed dueo long processing time, which consequently leads to increasen pick up of hydrogen from the atmosphere.

mm

able 5ensile and compression properties of the unreinforced alloy and its composites

aterial 0.2% proof stress (MPa) Ul

Tension Compression Te

356 Al 83 90 166(S) 101 104 1912(S) 108 109 1412(AR) 91 107 14

e and Engineering A 480 (2008) 117–124 121

.3. Hardness

Macrohardness and microhardness values of the alloy andomposites are shown in Fig. 6. Vickers microhardness mea-urements were made on the aluminium matrix, far away fromy ash particles to provide insight into the effect of fly ash on theatrix hardness. Results show considerable increase in both the

ulk hardness and micro hardness due to the addition of fly ash.icrohardness of composites and unreinforced alloy in the as

ast condition is higher when compared to the as extruded con-ition. This is due to the fact that hard dendritic structure of asast materials, undergo recrystallization during extrusion. How-ver, macrohardness of 12-vol.% fly ash reinforced compositeshows lower hardness as compared to unreinforced alloy and6(S) composite in the as cast condition due to higher porosity.6(S) composite shows a higher (both micro and macro) hard-ess in the as extruded condition compared to 12-vol.% fly asheinforced composites and the unreinforced alloy.

.4. Tensile and compressive properties

Composites (C12(AR), C6(S) and C12(S)) show higher 0.2%roof stress than the unreinforced alloy (Table 5). This indi-ates that the fly ash addition leads to improvement in 0.2%roof stress. The C6(S) composite shows highest ultimate tensiletrength (UTS), which followed by unreinforced alloy and 12-ol.% fly ash reinforced composites. 12-vol.% fly ash reinforcedomposites exhibits lowest UTS. This low UTS in 12-vol.%y ash reinforced composites is due to the presence of poros-

ty and the formation of second phase particles in the matrixnd at particle/matrix interfaces [2,27]. C12(S) composite showsigher 0.2% proof stress and ultimate tensile strength comparedo C12(AR) composite. The percent elongation decreases withncrease in volume fraction of fly ash particles. The steep reduc-ion in the percent elongation of 12-vol.% fly ash reinforcedomposites could be attributed to the presence of pores, parti-le clusters and other defects. Percent elongation of compositess affected by presence of pores and volume fraction of secondhase particles. Both these factors contribute to decrease in vol-me fraction of the ductile phase (volume fraction of the matrixhase) and there by leads to decrease in percent elongation. Evenn a defect free composite, the percent elongation of compositess affected by various factors including particle size, shape and

Fracture surfaces of tensile specimens of composites showixed mode (ductile and brittle) fracture (Fig. 7). Dimples comeainly from the fracture of �-aluminium, whereas interface as

timate strength (MPa) % elongation in tension

nsion Compression

5 458 244 548 215 427 132 417 10

122 Sudarshan, M.K. Surappa / Materials Science

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The damping capacity (tan φ) and dynamic modulus (E) ofunreinforced alloy and their composites at room temperatureare shown in Fig. 9. Tests were done at a frequency of 10 Hz.

Fig. 7. SEM micrographs showing the fracture surfaces of composites.

ell as eutectic Si regions exhibit brittle fracture. Hollow (Ceno-phere) fly ash particles fracture (Fig. 7b) show good interfacialonding, whereas fly ash particles, which have been partially

onded with the matrix show an interfacial gap resulting inebonding from the matrix (Fig. 7c). At macro level, the fractureurfaces of the composites appeared to be flat, confirming loweructility of composites. Brittle nature of the fracture surfaces

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and Engineering A 480 (2008) 117–124

s consistent with low elongation values obtained from the ten-ile tests. On the microscopic level, it is worth noting that the flysh particles exhibit characteristic brittle fracture, surrounded byhe dimples in the adjoining matrix (Fig. 7a), i.e. mixed mode ofracture. Matrix voiding and particle cracking appear to be theechanisms of damage (Fig. 7). Presence of cracked particle

n composites also confirm excellent bonding between fly asharticle and the matrix.

Compressive properties of alloy and its composites are pre-ented in Table 5. It is clear that the compressive strength valuesf 12-vol.% fly ash reinforced composites are lower than thenreinforced alloy. It could also be due to presence of poros-ty in the matrix. On the other hand, compressive strengthf the C6(S) composite is higher than the unreinforced alloy.able 5 compares the compressive and tensile 0.2% proof stressf unreinforced alloy and their composites. Under compressiveoading condition, 0.2% proof stress of composites are higherhan the unreinforced alloy. Rohatgi et al. [29] showed thatn A356–fly ash (cenosphere) composites as the volume frac-ion of the particle increases from 20 to 35% the correspondingompressive yield strength decrease from 75 to 64 MPa. Theyttribute this to the hollow nature of the particles (instead ofolid particles). In our case most of the fly ash particles areolid in nature, hence composites show higher 0.2% proof stress.ower compressive strength of high volume fraction compos-

tes is attributed to higher porosity content. Alloy as well asomposites possess higher 0.2% proof stress in compressionompared to that in tension. Typical SEM micrographs of lon-itudinal sections of compression test samples are shown inig. 8. The arrow indicates the loading axis. It is clear that par-

icle fracture takes place due to the application of compressiveoad.

.5. Damping behaviour

ig. 8. SEM micrograph of Al–fly ash composites showing fractured fly asharticle in the compression.

Sudarshan, M.K. Surappa / Materials Scienc

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ig. 9. (a) Damping capacity and (b) modulus of A356 Al alloy and its com-osites.

odulus and the damping capacity values are higher in fly asheinforced composites compared to unreinforced alloy. Dampingapacity of 6- and 12-vol.% fly ash reinforced composites were.2- and 1.5-times the unreinforced alloy, respectively. As theeinforcement volume fraction increases from 6 to 12, dampingapacity increases from 0.03 to 0.04. Damping capacity of 12-ol.% fly ash reinforced composites is higher than 6-vol.% flysh reinforced composite by 18%. Fig. 9b shows increase in theodulus of Al–fly ash composites containing different volume

ercentages of fly ash.Improved damping capacity of the composites could be

ttributed to the presence of fly ash particle and the con-omitant modification in the microstructure of the aluminiumlloy matrix. The modified microstructural characteristicsnclude: finer grain size, thermal mismatch induced dislo-ations and particle/matrix interfaces. Coefficient of thermalxpansions (CTEs) of fly ash (hollow) ranges from 3.7 to.2 × 10−6 ◦C−1, where as A356 Al alloy is 22 × 10−6 ◦C−1

30,31]. CTE mismatch leads to dislocation generation at thenterface during cooling and solidification of MMCs. Con-equently, this becomes a possible source of high internalriction due to the motion of the dislocations under cyclicoading. In MMCs, point defect damping is relatively smallompared to other sources of defects damping. Dislocations

ontribute to damping by the motion of vibrating disloca-ion lines, viscous sliding of grain boundaries and mobility ofnterfaces.

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e and Engineering A 480 (2008) 117–124 123

. Conclusions

. About 6 and 12 vol.% fly ash reinforced A356 Al compositeshave been fabricated by casting route.

. Narrow size range (53–106 �m) fly ash particle reinforcedcomposites show better properties compared to compositeswith the wider size range (0.5–400 �m) particles.

. About 0.2% proof stress of A356 Al–fly ash composites ishigher compared to that of unreinforced alloy.

. Alloy as well as composites possesses higher 0.2% proofstress in compression compared to that in tension.

. Fracture surface of composites show mixed mode (ductileand brittle) fracture. Matrix voiding and particle fracture arethe fracture mechanisms in composites. Presence of defectsand the porosity are the additional factors responsible for thefailure of composites. Both particle fracture and interfacialdebonding are observed.

. C6(S) composite shows a higher compressive strength com-pared to 12-vol.% fly ash reinforced composites and theunreinforced alloy.

. A356 Al–fly ash MMCs exhibit superior damping character-istics compared to unreinforced alloy at ambient temperature.

. Damping capacity of fly ash reinforced Al-based compositeincreases with the increase in volume fraction of fly ash.

. The C12(AR) composite exhibits best damping capacitywhere as C12(S) composite shows highest modulus at roomtemperature.

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