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Int. J. Mech. Eng. & Rob. Res. 2012 Prashant S N et al., 2012

PREPARATION AND EVALUATION OFMECHANICAL AND WEAR PROPERTIES OF Al6061

REINFORCED WITH GRAPHITE AND SiCPARTICULATE METAL MATRIX COMPOSITES

Prashant S N1*, Madev Nagaral1 and V Auradi1

*Corresponding Author: Prashant S N,prashant.064@gmail.com

Aluminium alloy materials found to the best alternative with its unique capacity of designing thematerials to give required properties. Aluminium alloy Metal Matrix Composites (MMCs) aregaining wide spread acceptance for automobile, industrial, and aerospace applications becauseof their low density, high strength and good structural rigidity. In the present work, an attempt ismade to prepare and compare the mechanical and wear properties of Al6061-Graphite andAl6061-SiC composites. The composites were prepared using stir casting method in whichamount of reinforcement is varied from 6-12% in steps of 3wt%. The prepared composites ofAl6061-Graphite and Al6061-SiC are characterized by microstructural studies and hardness,density, and mechanical, tribological properties were evaluated as per the standards. Themicrophotographs of the composites revealed fairly uniform distribution of the particles in bothAl6061-Graphite and Al6061-SiC composites with clustering at few places. The experimentaldensities were found to be lower than theoretical densities in all the composites. Micro-hardnessof the Al6061-SiC composite found increased with increased filler content and where as Al6061-Graphite composite found decreased with increased filler content are found to be 98-151VHNand 98-76VHN respectively. The dispersed Graphite and SiC in Al6061 alloy contributed inenhancing the tensile strength of the composites. The wear resistance of the Al6061-SiCcomposite found decreased with increased filler content where as the wear resistance of theAl6061-Graphite composite found to decrease up to 6wt% but thereafter tends to increase.

Keywords: Al6061, SiC, Graphite, Metal matrix composites, Mechanical properties

ISSN 2278 – 0149 www.ijmerr.comVol. 1, No. 3, October 2012

© 2012 IJMERR. All Rights Reserved

Int. J. Mech. Eng. & Rob. Res. 2012

1 Department of Mechanical Engineering, Siddaganga Institute of Technology, Tumkur 560010, India.

Research Paper

INTRODUCTIONMetal Matrix Composites (MMCs) areincreasingly becoming attractive materials for

advanced aerospace applications becausetheir properties can be tailored through theaddition of selected reinforcements (Christy

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Int. J. Mech. Eng. & Rob. Res. 2012 Prashant S N et al., 2012

et al., 2010; and Swamy et al., 2011). Inparticular, particulate reinforced MMCs haverecently found special interest because of theirspecific strength and specific stiffness at roomor elevated temperatures (Ramesh et al.,2010). The various reinforcements that havebeen tried out to develop aluminium matrixcomposites(AMCs) are graphite, siliconcarbide, titanium carbide, tungsten, boron,Al203, flyash, Zr, TiB2. Addition of hardreinforcements such as silicon carbide,alumina, and titanium carbide improveshardness, strength and wear resistance of thecomposites (Christy et al., 2010; and Rameshet al., 2010). Aluminium-based Metal MatrixComposites (MMCs) have received increasingattention in recent decades as engineeringmaterials. The introduction of a ceramicmaterial into a metal matrix produces acomposite material that results in an attractivecombination of physical and mechanicalproperties which cannot be obtained withmonolithic alloys (Manocha and Bunsell, 1980).The particulate reinforced MMCs is mainly useddue to easy availability of particles andeconomic processing technique adopted forproducing the particulate-reinforced MMCs.Aluminium alloy-based particulate-reinforcedcomposites have a large potential for a numberof engineering applications. Interest inreinforcing Al alloy matrices with ceramicparticles is mainly due to the low density, lowcoefficient of thermal expansion and highstrength of the reinforcements and also due totheir wide availability. Among the various usefulaluminium alloys, Aluminium alloy 6061 istypically characterized by properties such asfluidity, castability, corrosion resistance andhigh strength-weight ratio. This alloy has beencommonly used as a base metal for MMCs

reinforced with a variety of fibres, particles andwhiskers (Berghezan, 1966; Pandey, 2004; andKarthigeyan et al., 2012). Amongst differentkinds of the recently developed composites,particle-reinforced metal matrix compositesand, in particular, aluminium base materialshave already emerged as candidates forindustrial applications.

In the present work attempt has been madeto study the influence of Graphite

p/SiC

p addition

on the microstructure, micro-hardness, andmechanical and wear behavior of 6061Al-alloy.For this purpose 6 wt%, 9 wt% and 12 wt% ofGraphite

p/SiC

p addition was used. Mechanical

properties were evaluated as per the standardsusing computerized universal testing machineand wear properties were evaluated using pin-on-disc wear testing machine.

EXPERIMENTAL DETAILSThe following section highlights the materialsused its properties and method of compositepreparation and evaluation of mechanical andwear properties.

Materials Used

The matrix material for present study is Al6061.Table 1 gives the chemical composition ofAl6061. Table 2 gives the details of the physicaland mechanical properties of Al6061. Thereinforcing material selected was Graphite andSiC of particle size 125 µm. Table 3 gives thedetails of the physical and mechanicalproperties of graphite.

Preparation and Testing ofComposites

The 6061 Aluminium alloy is used in thisexperiment as the matrix. The conventionalcasting technique was used for this compositematerial. Stir casting method has been

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Int. J. Mech. Eng. & Rob. Res. 2012 Prashant S N et al., 2012

The prepared Al6061-Graphite, Al6061-SiCcomposites and the castings of the base alloysare characterized by microstructural studies.Hardness measurement of the base alloy andcomposites were carried out using Z-wickMicro-Vickers hardness testing machine.Hardness is measured at 30 different locationson each specimen and value reported isaverage of 100 readings. Further, themechanical, wear properties were evaluatedas per the standards. For each test, three testspecimens were used and the value reportedis average of three tests.

RESULTS AND DISCUSSIONThe test results of Al6061 and its compositescontaining Graphite and SiC particulates withvarious weight percentages are presented inthese sections.

Microstructure

Figure 1a shows the microstructure of Al6061base alloy, which consists of -Al dendrites andeutectic Si which are distributed at grainboundaries. Figures 1b-1f reveals themicrophotographs of Al6061 reinforced withGraphite and SiC particulates. From figure it isclear that, the distribution of reinforcingparticulates in both the composites is fairlyuniform in all the compositions studied whileclustering of the particles were seen at fewlocations.

Density Measurements

Table 4 shows the comparison between thetheoretical and experimental densitymeasurements of the prepared composites.Theoretical density is found using rule of mixturewhile experimental density is evaluated basedon Archimedes principle. From the table it can

Si Cu Fe Mn Mg

0.62 0.23 0.22 0.03 0.84

Zn Pb Ti Sn Al

0.22 0.10 0.01 0.01 BAL

Table 1: Shows the Chemical Compositionof Al6061 Alloy Used in the Present Study

70-80 2.7 0.33 30 115

Table 2: Shows the Physical andMechanical Properties of Al6061 Alloy

Used in the Present Study

ElasticModulus

(GPa)

Density(g/cc)

Poisson’sRatio

Hardness(HB500)

TensileStrength

(MPa)

Graphite 8-15 2.09 0.14 1.7 mohsscale

SiC 410 3.10 0.14 2800

Table 3: Shows the Physical andMechanical Properties of Graphite and

Silicon Carbide Particulates

Rein-forcement

ElasticModulus

(GPa)

Density(g/cc)

Poisson’sRatio

Hardness(HB500)

adopted to prepare Al6061-Graphite andAl6061-SiC composites. Preheated Graphiteor SiC powdered particulates of laboratorygrade purity of particle size 125 µm wereintroduced into the vortex of the molten alloyafter effective degassing. Mechanical stirringof the molten alloy for duration of 10 min wasachieved by using ceramic-coated steelimpeller. Speed of the steel impeller wasmaintained at 400 rpm. A pouring temperatureof 730 °C was adopted and the moltencomposite was poured into permanent castiron mould. The extent of incorporation ofGraphite

p or SiC

p in the matrix alloy was varied

from 6 to 12 wt% in steps of 3 wt%. Thuscomposites containing particles wereobtained in the form of cylinders of diameter12.5 mm and length of 125 mm.

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Int. J. Mech. Eng. & Rob. Res. 2012 Prashant S N et al., 2012

Hardness

Hardness measurements were carried out onboth the composites as well as on the basematrix using Zwick hardness testing machineat a load of 2N for a time of 10 seconds. Thevalue reported is average of 100 readingstaken at 30 different locations and are presentedin Figure 2. It is observed that the hardness ofAl6061-SiC composite increases withincreased wt% of reinforcement while in Al6061-Graphite composite the hardness decreaseswith increasing wt% of reinforcement.

The increase in hardness of Al6061-SiCcomposite with increasing SiC content could bedue to the fact that the reinforcement material ismuch harder than that of the matrix material forSiC and also could be due to better wettability ofSiC by 6061 matrix which leads to good bondingbetween the matrix and reinforcement. However,decrease in hardness of Al6061-graphitecomposites is possibly due to poor wettingcharacteristics of Graphite by 6061Al.

Tensile Properties

Tensile properties of the both SiC/Graphitereinforced alloy and unreinforced Al6061 alloy

Figure 1: Showing the OpticalMicrophotographs of (a) 6061Al Alloy and

(b-f) with Different wt% of SiC andGraphite Particulates Taken at 100X

(a) 100X A16061

(b) 100X 6% Sic (c) 100X 6% Gr

(d) 100X 9% Sic (e) 100X 9% Gr

(f) 100X 12% Sic (g) 100X 12% Gr

100X 100X

100X100X

100X100X

100X

be concluded that the theoretical density valuesare slightly higher than experimental densitiesfor all compositions and can be attributed tothe presence of porosities. Further the densityincreases with increased percentage of fillercontent in the composites. The higher densityof Al6061-SiC composites over Al6061-Graphite and can reasoned for the higherdensity values of SiC.

Al6061-SiC TheoreticalDensity 2.70 2.72 2.73 2.75

ExperimentalDensity 2.65 2.67 2.68 2.74

Al6061- TheoreticalGraphite Density 2.70 2.67 2.66 2.64

ExperimentalDensity 2.65 2.58 2.53 2.58

Table 4: Comparison of Theorecticaland Experiment Densities of Al6061-SiC

and Al6061-Graphite Composites

MaterialDensity(g/cc)

Weight %Reinforcement

0 6 9 12

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Int. J. Mech. Eng. & Rob. Res. 2012 Prashant S N et al., 2012

were evaluated using computerized universaltesting machine. For each composition threespecimens were tested and the value reportedis average of three readings which is shownin Figure 3. From the Figure 3 it can be seenthat the tensile strength of composites is higherthan the unreinforced matrix. However, withincreasing wt% of Graphite/SiC particulatestensile strength of the composite increases.Also from the figure it can be observed thatthe tensile strength of the Al6061-Graphitecomposites is higher than that of thecomposites of Al6061-SiC. This improvementin tensile strength of the composites may beattributed to the fact that the fuller graphitepossesses higher strength when compared toSiC. Further, at 12wt% the tensile strengthvalue of Al6061-Graphite composite remainidentical to that of Al6061-SiC compositeswhich could be attributed poor wettingcharacteristics of Graphite by the matrix.

Wear TestWear is a process of material removalphenomenon. The as cast Al6061 alloy and

Al6061alloy with varying weight percentage ofGraphite and SiC particles are subjected towear test under dry sliding conditions usingpin-on-disc wear test machine (Model ED-201). The test were conducted on 8 mmdiameter, 30 mm long cylindrical specimensagainst a rotating EN-32 steel disc (count face)having hardness 63Rc. The tangential frictionalforce and wear rate were monitored with helpof electronic sensors. Both these parameterswere measured as a function of load andsliding distance. For each type of material,tests were conducted at three different nominalloads (10N, 30N and 50N) keeping the slidingdistance at 1.6 m/sec fixed at 400 rpm. Weartests were carried out at room temperaturewithout lubrication for 15 min.

From the Figures 4, 5 and 6 it can beconcluded that the wear rate of SiC compositesdecreases with increasing content of SiC in thematrix for all three loads and at constant slidingspeed of 1.6 m/sec. However, in case of Al6061-Graphite composites a decrease in wear ratewas observed only with 6 wt% of Graphite, while

Figure 2: Comparisons of Hardnesswith Different wt% of Reinforcement

wt% of Reinforcement

Ha

rdn

ess

(V

HN

)

Figure 3: Comparision of Tensile Strengthof Al6061-Graphite and Al6061-SiC with

Different wt% of Reinforcement

wt% of Reinforcement

Te

ns

ile

Str

en

gth

(N

/mm

2 )

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Int. J. Mech. Eng. & Rob. Res. 2012 Prashant S N et al., 2012

better wear resistance properties than that ofGraphite reinforced composites.

CONCLUSIONThe conclusions based on the present workon Al6061-SiC and Al6061-Graphite metalmatrix composites are as follows.

• Al6061-SiC and Al6061-Graphitecomposites are successfully preparedusing stir casting method with filler contentsup to 12 wt%.

• The microstructural studies revealed thefairly uniform distribution of the particles inboth Al6061-SiC and Al6061-Graphitecomposites.

• Higher value of Hardness was observed incomposites when compared to matrixalone. Further, hardness of the Al6061-SiCcomposite increases with increasingamount of reinforcement and where as inAl6061-Graphite composite increasingamount of Graphite has resulted indecrease of hardness.

increasing Graphite wt% has resulted increasein wear rate. Probably due to poor bondingbetween particles and matrix and during wearthis has resulted in dislodging of graphiteparticles hence, more wear rate. Therefore,evaluating SiC and Graphite composites it canbe seen that SiC reinforced composites exhibits

Figure 4: Comparison of Wear RateResults of Graphite and SiC for Load

of 10N-400 rpm

wt% of Reinforcement

We

ar

Ra

te (

mm

3 /N

-m)

Figure 5: Comparison of Wear RateResults of Graphite and SiC for Load

of 30N-400 rpm

wt% of Reinforcement

We

ar

Ra

te (

mm

3 /N

-m)

Figure 6: Comparison of Wear RateResults of Graphite and SiC for Load

of 50N-400 rpm

wt% of Reinforcement

We

ar

Ra

te (

mm

3 /N

-m)

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Int. J. Mech. Eng. & Rob. Res. 2012 Prashant S N et al., 2012

• The addition of SiC/Graphite has resultedin increase in tensile strength of Al6061alloy when compared to unreinforced alloy.The tensile strength is a function of volumefraction of reinforcement. As volume fractionof reinforcement increases tensile strengthof composite increases.

• The wear rate of the Al6061-SiC compositefound decreased with increasing SiCcontent where as the wear rate of theAl6061-Graphite composite found todecrease up to 6 wt% but thereafter tendsto increase.

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7. Ramesh D, Swamy R P andChandrashekar T K (2010), “Effect ofWeight Percentage on MechanicalProperties of Frit Particulate ReinforcedAl6061 Composite”, ARPN Journal ofEngineering and Applied Sciences,Vol. 5, No. 1, pp. 32-36.

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9. Swamy A R K, Ramesha A, Veeresh KumarG B and Prakash J N (2011), “Effect ofParticulate Reinforcements on theMechanical Properties of Al6061-WC andAl6061-Gr MMCs”, Journal of Minerals &Materials Characterization & Engineering,Vol. 10, No. 12, pp. 1141-1152.

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