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thesurface [5]. However, the aluminum-silicon carbide composites

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Journal of Testing and Evaluation, Vol. 35, No. 6Paper ID JTE100677

Cop

minum is used widely as a structural material, especially in thespace industry, because of its light weight. However, the lowngth and the low melting point of aluminum have always been ablem.An inexpensive method for solving these problems was toa reinforced element such as SiC particles and whiskers [1].ceramic particle additions make it possible to increase the spe-

c elastic modulus of aluminum as well as improve its thermalperties [2,3]. Using the powder metallurgy (PM) method to pro-e aluminum composites reinforced with SiC particulates pro-es a homogenous distribution of reinforcements in the matrix.er methods of production like casting and thixoforming haveblems of reinforcement segregation and clustering, interfacialmical reactions, high localized residual porosity, and poor inter-al bonding.Yet other production methods such as spray deposi-are very expensive [4]. Powder metallurgy also has the advan-of producing net-shape components, thus reducing the amountachining needed to produce the final product. Final machininggreat problem in the case of aluminum reinforced silicon car-e particles SiCp composite due to high tool wear caused by theerent abrasiveness of the hard SiC particles. Machining alsoses cracking of SiC particles and produces debonding between

produced by PM have low relative strength. This low strength is dueto the presence of the oxide layer surrounding the aluminum par-ticle that prevents welding of the particles during the sintering pro-cess. The oxide film also prevents grain growth and movement ofdislocations at or through the boundary and produces a highstrength, brittle, and high temperature resistant material. As amethod of overcoming this drawback the composite can be ex-truded after sintering to break the oxide layer and produce weldingbetween the aluminum particles. However, this method eliminatesthe advantage of net-shape products. The composite after extrusionalso exhibits a nonuniform distribution of the reinforcement in thematrix [6].

In this work the problem of poor sintering of an aluminum com-posite was solved by increasing the sintering temperature above themelting temperature of aluminum 660C. The high temperaturesintering process causes the aluminum particulate surrounded bythe oxide layer to expand in volume and rupture the oxide envelope.Then, contact with melted aluminum from nearby particles causeswelding to take place. The oxide layer breaks into small shell frag-ments scattered within the aluminum matrix, restricting the move-ment of dislocation and increasing the composites strength. Thealuminum powder used has a high percentage of aluminum oxideAl2O3 in the form of a thick layer surrounding the particles. Nocanning or degassing processes were used before mixing the pow-der to reduce cost. Seven different compositions were prepared andtested containing 0, 5, 10, 15, 20, 25, and 30 weight percent siliconcarbide. Compression, microhardness, and microstructure samples

anuscript received June 3, 2006; accepted for publication May 29, 2007;ished online August 2007.ecturer, Mechanical Engineering Department, Assiut University, Assiut,pt.rofessor of Mechanical Design and Professor of Production EngineeringMechanical Design, respectively, Mechanical Engineering Department,M. Khairaldien,1 A. A. Khalil,2 and M. R. Bayoumi2

oduction of Aluminum-Silicon Composites Using Powder Metallmperatures Above the Aluminu

BSTRACT: The extensive utilization of aluminum reinforced with silind a cost effective technological production method for these composites.epresent the significant problems pertaining to these composites. Productioade from aluminum-silicon carbide composites can be achieved using powtrength of the aluminum-silicon carbide produced by powder metallurgy ielting temperature of the aluminum. This method produces a local fusing

hick oxide layer surrounding the particles prevents the total melting of thend 30 wt % silicon carbide were prepared. Samples from each compositiother specimens were left without sintering for comparison. Microstructureut for each of the 49 combinations of SiC contents and sintering temperatomposite properties and to detect the optimum sintering temperature for etrength and ductility to increase upon increase in the sintering temperature.luminum with no silicon carbide content, 700C for composites containing00C for composites containing 20 wt % SiC, 850C for composites cont

EYWORDS: aluminum silicon carbide composites; powder metallurg

roductionwertere

lty of Engineering, Assiut University, Assiut, Egypt, e-mail:@acc.anu.edu.eg

yright 2007 by ASTM International, 100 Barr Harbor Drive, PO Box C700, WestCopyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authbidegy at SinteringMelting Point

arbide in different structural applications has motivated the need toogeneity, machinability, and interfacial reaction of the constituentsa homogenous, high strength, and net-shape structural componentsetallurgy (PM) technology. In the present work the problem of lowed by raising the sintering temperature of the composite above thewelding of the aluminum particles. Using aluminum powder with aposite. Green compacted specimens containing 0, 5, 10, 15, 20, 25,re sintered at 650, 700, 750, 800, 850, and 900C separately, whileination, a microhardness test, and a compression test were carried

to study the effect of sintering temperature and SiC contents on theiC weight percent. Generally the results show the tendency for bothe specific sintering temperature levels are found to be 650C for the5 and 10 wt % SiC, 750C for composites containing 15 wt % SiC,

g 25 wt % SiC, and 900C for composites containing 30 wt % SiC.

echanical properties; sintering temperature

matrix and reinforcement interfaces underneath the machined

Available online at: www.astm.orge prepared and examined at the green state as well as those sin-d at temperatures of 650, 700, 750, 800, 850, and 900C.

Conshohocken, PA 19428-2959. 1orized.

Ma

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TAB

2 JOURNAL OF TESTING AND EVALUATION terial Preparation

der metallurgy methods were used for producing the compos-with seven different mixtures of aluminum and silicon carbideders to study the effect of silicon carbide weight percent on thehanical properties of the composite. The weight percent, vol-e percent, apparent density, and tapped density of the mixturesd are listed in Table 1. The mixtures were weighted and blendedhout any preprocessing operation to decrease the cost of produc-. The used aluminum powder was a flake-type having a rela-ly large amount of aluminum oxide surrounding it. The as-ived aluminum and silicon carbide commercial powderticles used in the operation are shown in Fig. 1. It can be seen infigure that the aluminum particles have irregular shapes whilesilicon carbide particles have sharp edges and flat surfaces.hemical analysis of the aluminum powder using both atomic

orption and XRF methods is given in Table 2 along with thees distribution in the sample. The aluminum powder and thepowder used in the production have almost the same size dis-ution that makes it easy to get a homogenous distribution of theforcement in the matrix. The sieve analyses of the two powderslisted in Table 3. These mixtures were weighted and blended inechanical blender for 15 min to reach a homogenous distribu-of the reinforcement in the mixture. Mixtures were then placedfloating type compaction die. This die type (the upper and

er punches move separately while the die floats on springs ineen) was selected to enhance the composite density distribu-. The mixtures were then cold compressed under a pressure ofMPa to produce the maximum possible compact green densityall mixtures. The pressure was applied using a 500-ton high ca-

TABLE 1Powder mixtures used for producti

Mixture 1 2 3

Al powder weightpercentage

100 95 90

SiC powder weightpercentage

0 5 10

Al powder volumepercentage

100 95.75 91.43

SiC powder volumepercentage

0 4.25 8.57

Apparent density inKg/m3

1083.7 1126.79 1146.4

Tapped density inKg/m3

1207.07 1228.21 1275.7

. 1The as-received aluminum and silicon carbide powder under the opti-

icroscope.

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authity compression testing machine. Changes of compaction dis-ement versus compaction pressure for the seven compositionsshown in Fig. 2. As shown, as the silicon carbide percentageeases more pressure is needed to achieve maximum density.ples from each of the seven mixtures were sintered at six dif-nt temperatures, namely 650, 700, 750, 800, 850, and 900C,1 h in an air atmosphere and left to cool in the furnace. Using ant gas or vacuum atmosphere furnace was not necessary becausealuminum powder used already contained large amounts ofO3, and as such, this will decrease the chance of aluminum oxi-tion during the sintering process using an air atmosphere fur-e. The samples were then solution heat treated at 515C for 1 hthen quenched in ice water. In order to prevent the initiation ofral aging after this quench, all samples were artificially agedediately after the solution heat treatment. All samples wered at 200C and the time at this temperature depended on theght percent of the silicon carbide. A 2-h aging was applied to30 wt % SiCp composite with an additional 2 h of aging timeeach decrease of 5 wt % SiCp. This variation of aging time wasloyed in order to approach the T6 peak aged condition for all

aluminum silicon carbide composites.

4 5 6 7

85 80 75 70

15 20 25 30

87.04 82.58 78.05 73.44

12.96 17.42 21.95 26.56

1163.47 1227.94 1228.42 1230.33

1302.72 1308.53 1314.56 1343.8

LE 2The elementary analysis of aluminum powder and oxidedistribution in it.

Composition Analysis ofPure Aluminum inWeight Percent

Oxygen Distribution BetweenPhases in Aluminum

Powder

O2 10.8 Al2O3 99.5

Mg 0.18 MgO 0.2

Al 88.6 SiO2 0.18

Si 0.2 Fe2O3 0.09

Fe 0.2

Zn 0.015

Ti 0.006

TABLE 3Particle size distribution % retained on sieve analysis.

Property Aluminum Powder Black SiC Powder

pparent Density 1083.7 Kg/m3 1305.1 Kg/m3

icle size distributionretained on sieve

analysis

180 microns 0 180 microns 0.063

150 microns 16.67 125 microns 0.063

90 microns 40.04 90 microns 0.2

75 microns 21.1 63 microns 5.053

45 microns 11.43 45 microns 36.52

Fines 10.76 max Fines 58.1 maxA

Part%orized.

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KHAIRALDIEN ET AL. ON ALUMINUM-SILICON CARBIDE COMPOSITES 3 ght percent combinations by taking account of the acceleratedg experienced by the matrix phase caused by the presence ofSiC [3]. The change in density relative to the theoretical densityhe composites calculated using rule of mixtures versus compac-pressure during compaction is shown in Fig. 3(a). Compact

en density increases as the weight percent of SiCp increases. Thetively high density is due to the presence of a large amount ofinum oxide in the composite that prevents the accurate calcu-

on of the porosity in the composite. Figure 3(b) shows the varia-of density with sintering temperature for the seven types ofposites. It can be seen that the compact with no silicon carbidehes maximum density at 750C and the density decreaseshtly as the sintering temperature increases. The 5, 10, 15, and

. 2The changes of compaction displacement versus compaction pressurehe seven compositions.FIG. 3The change in relative density versus (a) compaction pressure, an

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authwt % SiCp composites show similar behavior only at high sin-ng temperatures. The 25 and 30 wt % SiCp composites show aerent behavior as there density ratio drops slightly beneath otherbinations and rises a slight amount at 900C. The greatest den-ratio was reached with the 15 and 20 wt % SiCp combinations.

crostructure Examination

ples for microstructure examination from all of the 49 combi-ons of SiC weight percent and sintering temperatures were pre-ed to investigate grain size, morphology, distribution of the sili-carbide particles in the composite, and interfacial integrityeen the matrix and reinforcement. The samples were polishedg silicon carbide paper (320, 400, 800, 1000, and 1500 grit) andlly using a short-nap cloth with fine alumina powder as thery. Kerosene was used as a coolant during polishing to preventedding of the abrasive particles in the sample. The samplese then etched using the modified Kellers reagent [2 mL HF%), 3 mL HCL (concentrated), 20 mL HNO3 (concentrated),mL water] [7] for 140 s. The long etch time was due to the high

de content of the aluminum powder. Figure 4 shows the opticalroscope photographs for composite samples containing sevenerent weight percent of SiCp. The figure shows that the siliconide particles are homogenously distributed in the aluminumrix although some SiCp clusters rise in the matrix for combina-s containing more than 15 wt % SiCp and the clustering in-ses as the SiCp weight percent increases.Aluminum particles at the green state were found to be held to-er by mechanical interlocking due to the deformation that takese during the compaction process. Some of the oxide film sur-nding the particle had been broken and the silicon carbide par-es trapped between the aluminum particles as shown in Fig. 5.ever, the oxide layer damage from breaking during the com-

tion process was not enough to facilitate the adhesion of alumi-20teridiffcomsity

Mi

Samnatiparconbetwusinfinaslurembwer(48175oximicdiffd (b) sintering temperature for the seven types of composites.

orized.

4 JOURNAL OF TESTING AND EVALUATION FIG. 4Optical microscope photographs for the seven combinations show the distribution of the reinforcement in the matrix.Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authorized.

numcrogcomefficreathenumsintpos

Mi

VictesttestmatnesminSiCterm

KHAIRALDIEN ET AL. ON ALUMINUM-SILICON CARBIDE COMPOSITES 5 particles during the sintering process. Scanning electron mi-raphs showing the aluminum grain for the 15 wt % SiCpposite are shown in Fig. 6. The figure shows an increase in theciency of the sintering process as the sintering temperature in-ses. Poor adhesion between aluminum particles was found atsintering temperature of 650C. The adhesion between alumi-particles increases and the number of voids decreases as the

ering temperature increases; however, voids reappear for com-ites sintered at 900C.

crohardness Test

kers microhardness tests were preformed using a MICROMETer diamond indenter and 200 g indenting force. Microhardnesss are nondestructive and the measurements can be correlated toerial ultimate strength [8,9]. These reasons make microhard-s testing quite suitable for studying mechanical behavior of alu-um reinforced with SiCp. The 49 composite combinations of

p weight percents and sintering temperatures were tested to de-

FIG. 5Scanning electronic micrographs showing the aluine the effect of these two parameters. From four to six inden- crea

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authons were made for each sample according to the repeatability ofreadings and an average of these reading was then calculated.position of indentation in the sample surface was chosen ran-ly due to the presence of two distinct materials in the compos-

The results in Fig. 7 illustrate that pure aluminum samples show

e increase in hardness with an increasing sintering temperature

the green samples as well as samples that were sintered at

C and then the hardness decreased for samples sintered at the

ater temperature. For composites containing 5 wt % SiCp the

dness value increased slightly from the green state to those sin-

d at 700C. Then the hardness increased abruptly for those sin-

d at 750C and decreased for samples sintered at the higher

perature. The 10 wt % SiCp composite samples show a similar

avior to the 5 wt % SiCp composite except for the abrupt in-

se in hardness for samples sintered at 800C and the hardness

reasing for the rest of the sample as the sintering temperature

eases. Composites containing 15 and 20 wt % SiCp show in-

m and silicon carbide particles for the green state.tatitheThedom

ite.

littl

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700

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minused hardness compared with the green state for samples that

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werharoccnes

6 JOURNAL OF TESTING AND EVALUATION e sintered at 800C then reach a maximum value at 850C.Thedness then decreases for samples sintered at 900C. The sameurred for composites containing 25 and 30 wt % SiCp as hard-

FIG. 6Scanning electronic micrographs showing the as increases with increased sintering temperature with a maxi- ing

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authm value for samples sintered at 900C The microhardness re-s were slightly greater than those obtained for composites with1050 (99.5 wt aluminum) alloy matrix produced by stir cast-

num particle at different sintering temperatures.musultthe

lumi[8], but somewhat less than those for composites with a matrix

orized.

con

[10

duc

FIGpera

KHAIRALDIEN ET AL. ON ALUMINUM-SILICON CARBIDE COMPOSITES 7 taining 5 % Cu produced by the vacuum infiltration process

] as well as composites with 6061 aluminum alloy matrix pro-

ed by the compocasting technique.

. 7The variations of microhardness average values with sintering tem-ture for the 49 combinations of AlSiCp composites.FIG. 8The compression specimen and a

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authmpression Test

pite the fact that microhardness measurements have providedd and representative results, a confirmation is still necessary be-se the composite contains more than one constituent. This canse the microhardness test result to be inaccurate and misleading.example, silicon carbide or aluminum particles may be greaterize than the indenter, or the indenter may land on a hidden voidcluster of silicon carbide particles. These drawbacks were cir-vented by taking many readings for the same sample and anrage of the readings was taken to be the measured and reporteddness value. Compression testing was thought to be the bestice for conformation since only a small specimen is neededh very little machining.A flat-bottomed, lipped-recess geometrycimen with dimensions shown in Fig. 8(a) was used to eliminateend effect [11]. PTFE sheets were attached to the ends of thecimen using a grease to fill in the end recess. A computerizedion testing machine was used for these tests. However, becausespecimens were quite small, a special fixture was made to mea-the force displacement curve during the test. The arrangement

hown in Fig. 8(b) and consists of a force transducer equippedh a strain gauge bridge and two steel plate cantilevers welded onh sides of the arrangement with strain gauges attached to each of

to work as a displacement transducer. The displacement wasn as the average of two displacements measured from the dis-Co

DesgoocaucauForin sor acumaveharchowitspethespetensthesureis switbotthemtakerrangement used in the test.

orized.

plactranof tmemthesubthrecomsiliccracwitsenphofail

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8 JOURNAL OF TESTING AND EVALUATION ement transducer. The force transducer and the displacementsducer output signals were attached to an amplifier. The outputhe amplifier was connected to an oscilloscope with a digitalory to acquire and store the signals. The data are taken fromoscilloscope onto a disk in the form of spreadsheet files to besequently processed to obtain the stress-strain curve. Frome to four compression specimens were tested for each of the 49binations. While some of the samples (those with zero or lowon carbide content) withstood a great amount of stress withoutking and showed completely ductile behavior, others (thoseh high silicon carbide content) shattered in a brittle manner es-tially at the beginning of loading. The stress-strain curves andtographs for some of the compression samples illustrating theure type are shown in Figs. 915.igure 9 shows the stress-strain curve for the aluminum speci-s with no silicon carbide content at different sintering tempera-s. It can be seen from the figure, except for the case of the greenple, the specimens show a clear ductile behavior at all sinteringperatures. However, the composite specimen sintered at 650Cshow some brittle cracking after compression. These crackingnomena decreased in the composite compression samples sin-d at 700C and totally disappeared for rest of the samples. Theposites samples that were sintered at 900C were partiallyted during sintering and therefore no stress-strain curve iswn in the figure. The strengthened effect of the SiCp for com-ite containing 5 wt % SiCp is shown in Fig. 10. From the figuren be seen that even composite samples sintered at 650C show

. 9The stress-strain curves for the pure aluminum samples sintered atn different temperatures.

. 10The stress-strain curves for the aluminum with 5 wt % silicon carbide

ples sintered at seven different temperatures. bide

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authuctile behavior to some extent. However, it finally cracked in aner similar to the green sample with vertical cracks on its outerace. This indicates the sintering process was not complete aswn in the photos attached to this figure. Samples sintered at 800900C attained a relatively large strain before cracking tooke. The 10 wt % SiCp composite samples show an increase inngth compared to the 5 wt % SiCp composite as shown in Fig.The composite sample that was sintered at 650C shows ductileavior and fails by longitudional cracking as shown in the photo.posite samples that were sintered at a higher temperature alsoked. However, they attained a greater strain and cracked mainlyhe recess at the end of the specimen in a circular manner. Thisumferential cracking may be due to the flat-bottomed, lipped-

. 11The stress-strain curves for the aluminum with 10 wt % silicon car-samples sintered at seven different temperatures.

. 12The stress-strain curves for the aluminum with 15 wt % silicon car-samples sintered at seven different temperatures.

. 13The stress-strain curves for the aluminum with 20 wt % silicon car-

samples sintered at seven different temperatures.

orized.

receconshotheterecracsamduc

Tmengitusint13.in acreaHowfore

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KHAIRALDIEN ET AL. ON ALUMINUM-SILICON CARBIDE COMPOSITES 9 ss geometry, sharp internal corners of the recess acting as stresscentration regions. The 15 wt % SiCp composite sampleswed increased strength compared to the composite containinglower weight percent SiCp. Composites samples that were sin-d at 650 and 700C failed in a brittle manner by longitudinalking after only a small strain as shown in Fig. 12. Compositeples that were sintered at higher temperatures showed moretility and experienced greater strains before they failed.he 20 wt % SiCp composite shows brittle behavior for speci-s that were sintered below 750C where they cracked in a lon-dinal manner in more than one position. This illustrates that theering process was not successfully completed as shown in Fig.The samples that were sintered at higher temperatures behavedmore ductile manner indicating good sintering. Ductility in-ses as the composite sample sintering temperature increases.ever, the maximum strength that these composites reached be-cracking was below that of the 15 wt % SiCp composite.he specimens show ever increasing brittle behavior as theght percent of silicon carbide increases as shown in Fig. 14 forwt % SiCp. It is obvious from the figure that most specimens failbrittle manner except for composite samples that were sintered00 and 850C. The cracks were severe and deep indicatinge was little adhesion between the particles. Even for compositecimens that were sintered at 900 and 850C, longitudinal crackshe exterior surface appeared after a moderate amount of strain.he 30 wt % SiCp composite shows a clearly brittle behavior for

cimens sintered at temperatures from 650to 850C. Compositecimens that were sintered at 900C show some ductility. How-

. 14The stress-strain curves for the aluminum with 25 wt % silicon car-samples sintered at seven different temperatures.

. 15The stress-strain curves for the aluminum with 30 wt % silicon car-

sintsamples sintered at seven different temperatures.

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authr, they finally failed in a brittle manner with small longitudinalks on their outer surface as shown in Fig. 15. The strength of30 wt % SiCp specimens were below that of the 25 wt % SiCpcimens sintered at the low temperature shattered right after ex-ure to a small stress indicating the sintering process was notpleted successfully.

scussion

ducing aluminum reinforced with SiCp composites with highngth, light weight, free from defects, and inexpensively is theam of engineers. However, poor machinability, inhomogeneousribution of the reinforcement, oxide layer surrounding the alu-um powders, and interactive reactions between matrix and rein-ement are problems that need to be solved to make this dreame true. Powder metallurgy was thought by many researchers tohe solution as it provides the capability of producing net-shapeducts with homogenous distribution of the reinforcement in therix. However, the thick oxide layer surrounding the aluminumticle obliges the usage of aluminum powder with minimal oxidetent and special sintering furnaces with vacuum or inert gas at-sphere. Extrusion, forging, rolling, or hot compaction were alsod to eliminate the effect of oxide layers by breaking them andeasing the possibility of aluminum particles welding [6]. How-r, nonuniform dislocation density distribution, inhomogeneousribution of the reinforcement in the matrix, and decrease inngth resulting from microcracking occurs during these postpro-sing operations [12]. Also, the advantage of making net-shapeducts is eliminated when using these processes subsequent toder metallurgy. Another procedure that employs liquid phaseering (LPS) to overcome the oxide layer problem uses additionsn alloying element (usually copper) to the matrix. This additions a low melting point transient eutectic phase during sinteringpenetrates the oxide layer surrounding the particles and even-ly causes them to break down. Another procedure is to coatp particles with another metal to enhance wetting and cohesioneen the matrix and reinforcements. Copper has proven to be ad choice for coating SiC particles because it forms an Al-Cuid eutectic at temperatures below the sintering temperature ofinum powder. The eutectic flows into the porous area and the

ndary between aluminum particles and SiC particles provides ang, ductile bond between the matrix and the reinforcement,14]. However, the low melting eutectic phase affects the prop-es of the composite at a high temperature which is one of theor disadvantages of this type of composite.The basic idea of our work is to use the liquid phase sinteringcess, but instead of adding alloying elements to the aluminumrix, the sintering temperature was raised above the meltingnt of pure aluminum.The aluminummatrix melts and breaks thede layer to mix with aluminum leaking from other particles toduce good adhesion. The alumina layer shatters into small frag-ts thereby impeding the deformation of the aluminum matrixincreasing its strength as well as improving its high tempera-properties. So, instead of aluminum reinforced with an SiCpposite, the process produces an inexpensive composite rein-ed with both SiCp and Al2O3 without the need for any prepro-sing or postprocessing operation.Microstructural examination, microhardness test, and compres-test results showed that good sintering was achieved for an

einforced matrix even at 650C sintering temperature and the

ering efficiency increased as the sintering temperature in-

orized.

creapermelmelpoiuntThethetheperas meveeratgrebenexasint650at 8Mic700

Idraparthesstremaiof tof tmenHowonenalmalcommis

10onlto t10clusalumnumreinSiCingnumthe

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10 JOURNAL OF TESTING AND EVALUATION sed. The aluminum particles expand more and more as the tem-ature increases until the temperature reaches the aluminumting point causing the aluminum inside the alumina layer tot. As the temperature increases above the aluminum meltingnt the liquid aluminum pressure inside the alumina increasesil it finally becomes great enough to burst the alumina envelope.leaking liquid aluminum from the broken alumina mixes withmelted aluminum leaking from neighboring particles, closingvoids between them and connecting the particles.A higher tem-ature is needed for aluminum particles with thicker oxide layersore liquid pressure is needed to burst these thick layers. How-

r, for the sintering temperature above 700C this process accel-es and the aluminum leak rate from the particles becomes soat that voids begin to form inside the particles that eliminates theefit of this process. This can be seen from the microstructuremination shown in Fig. 6 that shows the voids decreasing as theering temperature of the composite increases fromto 750C. The voids nearly disappear for composites sintered00 and 850C, then reappear for composites sintered at 900C.rohardness values also increase for composites sintered atC and decrease afterward, mainly due to the presence of voids.n the 5 and 10 wt % SiCp composites, the SiCp becomes a hin-nce for the flow of melted aluminum leaking from aluminumticles and increases the sintering temperature required to bypasse obstacles. The 10 wt % SiCp composite also shows higherngth and microhardness values than the 5 wt % SiCp compositenly because it contains double the content of SiCp.The presencehe SiCp also increases the composite strength as they carry parthe force applied to the material and hinder dislocation move-t in the matrix, thereby constraining the matrix plastic flow.ever, the increase in strength is very small compared with thethat can be calculated using the simple rule of mixtures. Inter-stress also rises in the composite due to the difference in ther-contractions at the Al/SiCp interfaces during the cooling of theposite after sintering that produces misfit strains and resultant

. 16Typical X-ray diffraction (XRD) patterns for aluminum composite: (a) con50C.fit stresses. The mechanism of sintering in the presence of 5 and stoo

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authwt % SiCp reinforcement is similar to that of pure aluminum,y a higher temperature is needed to produce good sintering duehe presence of the SiCp. The amount of SiCp in the 5 andwt % SiCp composites is relatively minimal to produce particleters. The interaction in aluminum reinforced with SiCp is eitherinum with aluminum particles that adhere properly, or alumi-with SiCp. Interfacial bonds between the aluminummatrix andforcement at an elevated temperature is critically important asis thermodynamically unstable at temperatures above the melt-point of aluminum 660C and reacts with the liquid alumi-to form aluminum carbide and silicon [1517] according to

following reactions:

4Alliquid + 3SiCsolid Al4C3solid + 3Siliquid (1)Aluminum carbide is a deleterious phase and can act as a dam-nucleation site in the material. The possibility of aluminum car-e formation is greater for composites containing 10 wt % SiCpause they have a greater amount of silicon carbide than 5 wt %p composites, thereby increasing the possibility of siliconide/silicon carbide particle contacts [18]. X-ray diffraction pat-s for two samples of 10 and 5 wt % SiCp composites sintered atC are shown in Fig. 16. From the figure the presence of Al,, Al2O3, and Al4C3 phases can be identified in the XRD pat-s, thereby confirming the formation of aluminum carbide.ever, using the as-received SiC powder hinders this reaction

ause the as-received SiC particles have a thin layer of siliconde (silica). This layer serves as a barrier to block the SiC par-es from being attacked by molten aluminum and decreases thence of producing aluminum carbide [1922] according to theowing reaction:

3SiO2solid + 4Alliquid 2Al2O3solid + 3Siliquid (2)Also, the introduction of free Si as a result of the second reac-of molten aluminum leaking from the particles also decreasespossibility of producing aluminum carbide. This can be under-

g 10 wt % SiCp sintered at 650C, and (b) containing 5 wt % SiCp sinteredagebidbecSiCcarbtern650SiCternHowbecoxiticlchafoll

tionthed from the Al-Si-C phase diagram [15,22] shown in Fig. 17.

orized.

Theformthe

FIG677

KHAIRALDIEN ET AL. ON ALUMINUM-SILICON CARBIDE COMPOSITES 11 se reactions decrease the amount of aluminum carbide thats during the manufacturing of the composite, especially whenSiCp weight percent is minimal, as in the case of composites

. 17The calculated isothermal section of the Al-Si-C phase diagram atC with -SiC [8].FIG. 18Scanning electronic micrograph showing two d

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authtaining 5 and 10 wt %, and even for composites containing asch as 15 wt % SiCp. However, as the silicon carbide content in-ses, conditions become more suitable for the formation of alu-um carbide. Other factors that must be taken into considerationn studying aluminum reinforced with SiCp composites are par-es size and distribution. Selecting a suitable SiCp reinforcementticle size is very important because using large particle sizesduces composites with low strength that fail by initiation andakage of the SiC reinforcement particles. Smaller particle sizesduce composites with high yield and ultimate tensile strengthThe main reason for this is that the large SiC particles are oftend with stacking faults and cracks that can easily initiate com-ite cracking upon initial loading. However, using very small

p particle sizes increases the surface area of contact between theinum matrix and SiCp reinforcement. This increases the

nce of formation of aluminum carbide and interfacial failure ofSiC/Al interface becomes the main source of composite failureiation [23].Particle size distribution is also of great importance. Using ae range of SiC particle sizes produces inhomegenity and in-ses the possibility of SiC particles clustering [24]. Because ofconmucreaminwheticlparprobrepro[3].filleposSiCalumchatheinit

widcreaefects that may take place in the composite.

orized.

thiswas

Tcomcomperto tthedicathatcesas tdefmecworandturebe18(droite tstrecomat pdiscvaluanlandducsitySiCleakTheless850speterelesscomhib900

Co

Re

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[2]

[3]

[4]

[5]

[6]

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12 JOURNAL OF TESTING AND EVALUATION the particle size used to produce the composites in this workselected to be in a narrow range as shown in Table 3.he increase of silicon carbide content in the 15 wt % SiCpposite also increases the strength and hardness. However, thepression specimens failure tends to be more brittle. The tem-ature needed to achieve good sintering increases to 750C duehe increase in SiCp content. This tends to hinder more and morealuminum particles adhesion. Compression results obtained in-te this composite has lower strength but greater ductility thanproduced by powder metallurgy followed by the extrusion pro-s for a composite with a matrix containing 5 % Cu [3]. However,he silicon carbide content increases the chance of formation ofects, also increases such as with reinforcement clustering. Thishanism can also trap voids between the clustered particles andk as damage nucleation sites thereby weakening the compositedecreasing its strength as shown in Fig. 18(a). Cracked or frac-d particles produce a microdefect in the composite that can nothealed in the subsequent sintering process as shown in Fig.b). This becomes clearer in the 20 wt % SiCp where the strengthps below that of the 15 wt % SiCp. In the 20 wt % SiCp compos-he SiCp content becomes great enough for defects to affect thength of the composite that is less than that of the 15 wt % SiCpposite. Also, the possibility of formation of aluminum carbidearticle interfaces increases with increasing the SiCp content asussed previously. However, the 20 wt % SiCp microhardnesse is still greater than that of the 15 wt % SiCp because there isincreased chance that the indenter of the hardness tester mayon a silicon carbide particle. The temperature required to pro-

e good sintering also increases to 800C. But the relative den-is less than that of the 15 wt % SiCp because increasing thep contents in the aluminum matrix prevents melted aluminuming from the particles from filling the voids between particles.25 wt % SiCp exhibits brittle behavior in which the strength isthan that of the 20 wt % SiCp and sintering temperatures up toC were needed to achieve good sintering. The compressioncimen sintered at lower temperatures cracked and even shat-d at relatively low stresses compared with the composite withSiCp content. The hardness was still greater than that of theposites with less silicon carbide content. The 30 wt % SiC ex-ited a brittle behavior for all cases in specimens sintered up toC.

nclusions

The problem of aluminum-silicon carbide composites poormachinability can be solved by producing net-shape prod-ucts using the powder metallurgy method that produces ho-mogenous distribution of the reinforcement in the matrix.The low strength of the composite produced by PM can besolved by increasing the sintering temperature of the com-posite above the melting temperature of the aluminum pow-der, thereby producing a local fusion and welding of the alu-minum particles. The temperature required to achieve asuccessful sintering process increases as the silicon carbidecontent increases.Microstructural examination illustrated that the compositehas a homogenous distribution of the reinforcement particlesand the bond between the constituents is enhanced as the sin-tering temperature increases up to a certain limit dependingon the silicon carbide content. The microhardness test results

and the compression test results confirmed this observation.

Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authThe interfacial reaction between SiC and aluminum wasminimized by using SiCp with a silicon oxide layer.

Sintering temperature of 650C was great enough to pro-duce a successful sintering for the unreinforced aluminumcompacts while a slight increase in the sintering temperaturewas needed to produce good sintering for the composite con-taining 5 wt % SiCp. The composite containing 10 wt %SiCp required a sintering temperature of 700C to producegood sintering. The 15 wt % composite required a tempera-ture above 750C to produce good sintering while the20 wt % composite showed good sintering at temperaturesabove 800C. The 25 wt % SiC composite needed a tem-perature of 850C to exhibit some sign of good sintering,while the 30 wt % SiC composite produced good sintering ata temperature of 900C.

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Taya, M., and Arsenault, R. J.,Metal Matrix Composites, Per-gamon Press, Oxford, UK, 1989.Karnezis, P. A., Durrant, G., and Cantor, B., Characteristicsof Reinforcement Distribution in Cast Al-Alloy/SiCp Com-posites, Mater. Charact., Vol. 40, 1998, p. 97.Lu, Y. X., Meng, X. M., Lee, C. S., Li, R. K.Y., Huang, C. G.,and Lai, J. K. L., Microstructure andMechanical Behavior ofa SiC Particles Reinforced Al-5Cu Composite Under Dy-namic Loading, J. Mater. Process. Technol., Vol. 94, 1999, p.175.ODonnell, G., and Looney, L., Production of AluminumMatrix Composite Components Using Conventional PMTechnology, Mater. Sci. Eng., A, Vol. 303, 2001, p. 292.Hung, N. P., Boey, F. Y. C., Khor, K. A., OH, C. A., and Lee,H. F., Machinability of Cast and Powder-Formed AluminumAlloys Reinforced with SiC Particles, J. Mater. Process.Technol., Vol. 48, 1995, p. 291.Cocen, U., and Onel, K., Ductility and Strength of ExtrudedSiCp/Aluminum-Alloy Composites, Compos. Sci. Technol.,Vol. 62, 2002, p. 275.ASM Handbook Committee,Metals HandbookVol. 7 PowderMetallurgy, American Society for Metals, Metals Park, Ohio,1984.Gupta, M. and Srivatsan, T. S., Interrelationship BetweenMatrix Microhardness and Ultimate Tensile Strength of Dis-continuous Particle-Reinforced Aluminum Alloy Compos-ites, J. Mater. Sci. Lett., Vol. 51, 2001, p. 255.Bekheet, N. E., Gadelrab, R. M., Salah, M. F., and Abd El-Azim, A. N., The Effect of Aging on the Hardness and Fa-tigue Behavior of 2024 Al Alloy/SiC Composites, Mater.Des., Vol. 23, 2004, p. 153.

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] Lovato, M. L., and Stout, M. G., Compression Testing Tech-nique to Determine the Stress/Strain Behavior of Metal Sub-ject to Finite Deformation,Metall. Trans. A, Vol. 23A, 1992,p. 937.

] Kanetake, N., Ozaki, M., and Choh, T., Effects of ParticleCharacters on Flow Stress of Particle Reinforced AluminumMatrix Composites, J. Jpn. Soc. Technol. Plast., Vol. 34,

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[15] Yaghmaee, M. S., and Kapta, G., On the Stability Range ofSiC in Ternary Liquid Al-Si-Mg Alloy, Materials Worlds,Proceedings of Hungarian Materials Science Society, II, Year3, No. ISSN 1586-0140, 2001.

[16] Shorowordi, K. M., Laoui, T., Haseeb, A. S. M.A., Celis, J. P.,and Froyen, L., Microstructure and Interface Characteristicsof B4C, SiC and Al2O3 Reinforced Al Matrix Composites: AComparative Study, J. Mater. Process. Technol., Vol. 142,2003, p. 738.

[17] Zhao, Z., Zhijian, S., and Yingkum, X., Effect of Microstruc-ture on the Mechanical Properties of an Al Alloy 6061-SiCParticle Composite, Mater. Sci. Eng., A, Vol. 132, 1991, p.83.

[18] Tham, L. M., Gupta, M., and Cheng, L., Predicting the Fail-ure Strains of Al/SiC Composites with Reacted Matrix/Reinforcement Interfaces, Mater. Sci. Eng., A, Vol. 354,

2003, p. 369.[19] Davidson, A. M., and Regener, D., A Comparison of the

Aluminum-Based Metal-Matrix Composites Reinforced withCoated and Uncoated Particulate Silicon Carbide, Compos.Sci. Technol., Vol. 60, 2000, p. 865.

[20] Salvo, L., LEsperance, G., Suery, M., and Legoux, J. G., In-terfacial Reactions and Age Hardening in Al-Mg-Si MetalMatrix Composites Reinforced with SiC Particles, Mater.Sci. Eng., A, Vol. A177, 1994, p. 173.

[21] Gu, M., Jin, Y., Mei, Z., Wu, Z., and Wu, R., Effects of Re-inforcement Oxidation on the Mechanical Properties of SiCParticulate Reinforced Aluminum Composites, Mater. Sci.Eng., A, Vol. 252, 1998, p. 188.

[22] Miserez, A., Mller, R., Rossoll, A., Weber, L., andMortensen, A., Particle Reinforced Metals of High CeramicContent, Mater. Sci. Eng., A, Vol. 387, 2004, pp. 822831.

[23] Varma, V. K., Kamat, S. V., and Kutumbarao, V. V., TensileBehavior of Powder Metallurgy Processed (Al-Cu-Mg)/SiCpComposites, Mater. Sci. Technol., Vol. 17, 2001.

[24] Srivatsan, T. S., and Al-Hajri, M., The Fatigue and FinalFracture of SiC Particle Reinforced 7034 Aluminum MatrixComposites, Composites, Part B, Vol. 33, 2002, p. 391.

KHAIRALDIEN ET AL. ON ALUMINUM-SILICON CARBIDE COMPOSITES 13 Copyright by ASTM Int'l (all rights reserved); Sat Feb 22 14:26:58 EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to License Agreement. No further reproductions authorized.