(E

Mad

omposites were processed by stir casting technique with commercially puremabjepoECAgraite gcon

reinforacceps alumlar atteength

been d

cause it has potential to produce large samples [1214]. Mostinvestigations on ECAP have concentrated on pure metals andmetallic alloys. Some limited reports are available on applicationof ECAP to metal matrix composites [1518]. However, reinforce-ments in the composites used in these studies are primarilyAl2O3 and no reports are found on Al metal matrix composites rein-forced with SiC particles. Further, the ECAP process in these studies

heated at 850 C for 4 h. This treatment was given to ensure goodwettability between SiCp and liquid aluminium [9]. Once Al wasmelted, temperature was measured with the help of a thermocou-ple and degassing was carried out with argon gas for 5 min. Afterdegassing, a preheated graphite impeller was lowered in to themolten metal and rotated by a shaft connected to a motor(Fig. 1). Preheated SiC particles were added by an addition chutethrough the periphery of the vortex, which was created by the stir-ring action of the rotating mechanical impeller. During stirringsmall pieces of magnesium was added to the melt to improve thewettability of SiC particles with Al melt [9]. Once all of the powder

* Corresponding author. Tel.: +91 44 22574778; fax: +91 44 22574752.

Materials and Design 30 (2009) 35543559

Contents lists availab

an

elsE-mail address: rbauri@iitm.ac.in (R. Bauri).facture of particle/whisker/short ber-reinforced composites [2,69]. Melt stirring or stir casting technique is currently one of thesimplest and most economical fabrication routes for manufactur-ing particle-reinforced metal matrix composites. A careful controlof process variables yields high degree of microstructural integrityand uniformity in this technique [10,11].

Severe plastic deformation is a useful processing tool to renethe grain size to the submicron or even nanometer size [1214].Although several severe plastic deformation techniques are avail-able, equal channel angular pressing is an attractive process be-

2.1. Material selection and composite processing

The composites used here were processed by stir casting. Com-mercial pure Al was used as a matrix material in the present study.Silicon carbide particles of average particle size 30 lm were usedas reinforcement. Fig. 1 shows the schematic of the stir castingset up used in this study. Weighed quantity of aluminium wastaken for melting and corresponding SiC powder was weighedaccording to the required volume fraction. SiC powder was pre-1. Introduction

In recent years ceramic particle-posites (MMCs) have gained wideattractive properties. Amongst MMCites (AMMCs) have received particudecades due to their high specic strrior wear resistance [15].

Number of processing routes has0261-3069/$ - see front matter 2009 Elsevier Ltd. Adoi:10.1016/j.matdes.2009.03.001ced metal matrix com-tance because of theirinium matrix compos-ntion in the past threeand stiffness and supe-

eveloped for the manu-

was carried out at elevated temperature. The aim of the presentwork is to fabricate AlSiC composites by equal channel angularpressing at room temperature and evaluate the effect of SiC parti-cles on the deformation behavior during ECAP. The effect of ECAPon the microstructure and mechanical properties of AlSiCp com-posites will also be evaluated.

2. Experimental detailsMechanical properties (E)composites. Both the composites exhibit improvement in mechanical properties after ECAP.

2009 Elsevier Ltd. All rights reserved.Effect of equal channel angular pressingand properties of AlSiCp composites

G. Ramu a,b, Ranjit Bauri a,*aDepartment of Metallurgical and Materials Engineering, Indian Institute of TechnologybResearch Centre Imarat (RCI), Vignayanakancha, Hyderabad 500 069, India

a r t i c l e i n f o

Article history:Received 13 January 2009Accepted 2 March 2009Available online 9 March 2009

Keywords:Metal matrix composites (A)Stir casting (C)

a b s t r a c t

Aluminium metal matrix caluminium as the matrixAlSiCp composites were su(ECAP). As cast AlSiCp comparticle distribution afterAl5 vol.% SiCp compositein Al10 vol.% SiCp composand compression test were

Materials

journal homepage: www.ll rights reserved.terial and 5, 10 vol.% of SiC (30 lm) particulates as the reinforcement.cted to severe plastic deformation through equal channel angular pressingsites shows uniform distribution of particles and there is no change in theP. It was found that ECAP renes the grain size of matrix material. Inin size was reduced to 8 lm from 45 lm after two ECAP passes whereasrain size was rened to 16 lm from 45 lm after rst ECAP pass. Hardnessducted at room temperature to evaluate the mechanical properties of theCAP) on microstructure

ras, Chennai 600 036, India

le at ScienceDirect

d Design

evier .com/locate /matdes

Samples of required diameter were machined from the castcomposite billets for ECAP. The composite billets were encapsu-

cast, as annealed and as ECAPed condition. Fracture surface of

nd Dlated in pure aluminium hollow cylinder with a wall thickness of2 mm to avoid die damage by the hard composite surface duringECAP and also to prevent instabilities at the surface of the compos-ite that may occur during ECAP [19]. The size of the nal samplewas 19.3 mm in diameter and 90 mm in length. Press-t encapsu-lated samples were annealed at 400 C for 4 h to homogenizeis added, the motor is stopped and the impeller is taken out fromthe melt. Finally, the composite melt was poured in preheatedcylindrical cast iron molds. Two composites having 5 vol.% and10 vol.% of SiCp were processed.

2.2. Equal channel angular pressing of AlSiCp composites

2.2.1. Sample preparation for equal channel angular pressing (ECAP)

Fig. 1. Schematic of the stir casting set-up used for the fabrication of Al/SiCpcomposite.

G. Ramu, R. Bauri /Materials amicrostructure and relieve stresses introduced due to press tting.

2.2.2. Equal channel angular pressing (ECAP)Annealed composite billets were subjected to ECAP using a

channel angle of 120. MoS2 was used as lubricant to minimize fric-tion between die wall and the sample. Bc route, where the sampleis rotated by 90 in one direction either clockwise or anti clockwiseafter each successive pass, was adopted in the present work. TheECAP was carried out at room temperature. Samples were takenafter each pass for characterization. The ECAP experiments wererepeated once to verify the reproducibility of the results. The pureAl layer was removed by machining for making samples for micro-structural and mechanical behavior studies.

2.3. Characterization

2.3.1. Density and porosityDensity of AlSiCp composite was measured using Archimedes

principle. Distilled water was used as immersion uid. Theoreticaldensity was calculated by rule of mixture and compared with mea-sured densities. Percent porosity was calculated from the differ-ence between measured and theoretical densities.

2.3.2. Volume fractionVolume fraction of as cast composites was measured by chem-

ical dissolution method. Small pieces of composite were cut fromcracks formed after second ECAP pass in Al/10SiCp was also ob-served under SEM.

2.3.4. MicrohardnessMicrohardness of the composites was measured in as cast, as

annealed conditions and after each ECAP pass using a Vickersmicohardness tester (WILSON WOLPERT). A load of 1 kg and adwell time of 9 s was used. Hardness was measured on the matrixof the composites. The values reported are average of at least vereadings.

2.3.5. Compressive testRoom temperature compressive tests were carried out using

standard ASTM samples on an Instron machine in as cast and as an-nealed condition and after each ECAP pass. Cylindrical sampleswith an aspect ratio of 1.2 were used for the compressive tests. Astrain rate of 1.8 103 s1 was used. At least three samples weretested for each material.

3. Results and discussion

3.1. Density and porosity

Table 1 shows the results of density and porosity measure-ments. Almost 100% densication was achieved in the unrein-forced metal. However, some amount of porosity was observedin the composites in as cast condition. Porosity in Al/5SiCp wasfound less compared to the Al/10SiCp composite. The porosity ismainly due to gas entrapment during stirring of melt in openatmosphere. Longer stirring time and higher SiC content in Al/10SiCp composite can be attributed to comparatively higher poros-ity in this composite.

3.2. Microstructural characterization

SEM micrographs in Fig. 2a and b shows the distribution of SiCparticles in as cast Al/5SiCp and Al/10SiCp composites, respectively.The distribution is fairly uniform and there was no segregation orclustering of the particles. The interfacial bonding between SiC par-ticle and matrix was also observed. The bonding between particledifferent portions. Weight of composite pieces was measured usingan electronic balance having an accuracy of 0.1 mg. Diluted hydro-chloric acid was used for dissolution. Aluminium was dissolved inHCl and SiC particles were settled at the bottom of the beaker as aresidue. Undissolved SiC powder was carefully ltered and dried inan oven at 120 C. Weight of dried powder was measured usingelectronic balance and the corresponding volume fractions weremeasured using the following relation.

Vp mpqp

!,mpqp

mmqm

!1

where Vp volume fraction of particles. mp, qp mass and density ofthe particles, mm, qm mass and density of matrix.

2.3.3. Microstructural characterizationSliced samples of composites were polished with emery paper

up to 400 grit size, followed by polishing with Al2O3 suspensionon velvet cloth. Finally samples were polished with 0.5 l diamondpaste. Microstructural characterization was done by scanning elec-tron microscope (SEM) and optical microscope. Particle distribu-tion, interface features, particle orientation were observed in as

esign 30 (2009) 35543559 3555and matrix is good and there was no indication of any interfacialreactions. Fig. 3a shows the particle matrix interface in the Al/10SiCp composite. The corresponding EDAX (Fig. 3b) taken on the

interface conrms this to be interface between the Al matrix andSiC particles. SEM micrograph in Fig. 2c shows particle distributionafter ECAP. No change in particle distribution or orientation wasobserved after ECAP. There was no change in particle size whichmeans particles were not fractured during ECAP. Optical micro-graphs of the etched samples before and after ECAP are shown inFig. 4. Fig. 4a and b shows the grain size of Al/5SiCp composite inas annealed and ECAPed (2nd pass) conditions, respectively. A con-siderable amount of grain renement was observed after ECAP.Similar observations were made for Al/10SiCp composites as canbe seen from optical micrographs in Fig. 4c and d (as annealedand after 1st ECAP pass, respectively). The grain size of both thecomposites in as annealed condition was observed to be 45 lm.It is interesting to note that the grain size of both the compositeafter annealing is equal. After second ECAP pass grain size of Al/5SiCp is reduced to 8 lm. In Al/10SiCp grain size was reduced to16 lm after rst ECAP pass. In case of the Al/10SiCp composite sur-face cracks starts appearing after the second ECAP pass (inset pic-ture in Fig. 5a). Semiatin et al. [20] investigated the deformationbehavior during ECAP for materials having markedly different

strain rate sensitivity (m) values. Metal matrix composites (MMCs)generally exhibits higher strain rate sensitivity than the base metal[2124]. The authors of Ref. [20] report that deformation duringECAP process in strain-hardenable materials with higher m valueis non-uniform. They have shown that for such a material thedeformation takes place in a relatively diffuse zone with higherstrain rates near the front leg radius and the bottom corner ofthe die. They have also shown that the stress distribution acrossthe billet is also non-uniform. The central portion of the billetexperiences compressive stress while top surface is subjected totensile stress. This leads to formation of cracks propagating verti-cally from the top surface of the billet. Since similar kind of surfacecracks were observed (inset picture in Fig. 5a) on the Al/10SiCpcomposite, formation of these cracks can be attributed to non-uni-form stress distribution and deformation in the billet from top sur-face to center of the billet. The fracture surface was observed usingSEM. Fig. 5 shows the SEM micrographs of fracture surface of Al/10SiCp after second ECAP pass. Particulate reinforced compositesfail by a mixed mode [25] and gives rise to a bimodal distributionof dimples. As can be seen from Fig. 5a larger dimples are associ-ated with SiC particles whereas smaller dimples are associatedwith ductile failure of the matrix. Fig. 5b shows a magnied imageof Fig. 5a. The micrograph in Fig. 5b clearly shows that particledebonding has taken place at particlematrix interface. In a com-posite the load is transferred from the matrix to the stronger rein-forcement via the reinforcementmatrix interface [26]. The highshear strains generated in the ECAP process might have led tothe separation at the interface. Using Cockcroft-and-Latham [27]approach Semiatin et al. [20] have also shown that the number

Table 1Densities and porosity of pure Al and its composites.

Material Measured density(gm/cm3)

Theoretical density(gm/cm3)

Porosity(%)

As cast aluminium 2.690 2.70 0.37As cast Al/5SiCp 2.683 2.72 1.36As cast Al/10SiCp 2.685 2.75 2.63

3556 G. Ramu, R. Bauri /Materials and Design 30 (2009) 35543559Fig. 2. SEM micrograph of composites (a) Al/SiCp as cast, (b) Al/10SiCp as cast, and (c) Al/10SiCp after 1st ECAP pass.

of ECAP passes without fracture is proportional to the tensile frac-ture strain (ef) of the material. Since the fracture strain decreaseswith increasing reinforcement content in MMCs [28,29], numberof passes without fracture is one in the composites containinghigher volume fraction of SiC (Al/10SiC) compared to two for Al/5SiC composite. The strain rate sensitivity increases with increas-ing SiC content [24] and this may lead to more non-uniform defor-

i.e. Al/5SiCp these effects were less prominent and therefore twoECAP passes without fracture was possible in this composite.

3.3. Hardness

Table 2 shows the results of the Vickers hardness measure-ments. The microhardness of both the composites are higher than

Fig. 3. (a) Particlematrix interface and (b) the corresponding EDAX taken at the interface in Al/10SiCp composite.

G. Ramu, R. Bauri /Materials and Design 30 (2009) 35543559 3557mation and early failure of Al/10SiC composite during ECAP. Highervolume fraction of SiCp also causes more strain hardening duringplastic deformation [30,31] and as a result the ductility comesdown after rst pass in the Al/10SiCp composite. This may also in-crease the chance of cracking of the billet during the 2nd ECAPpass. In the composite containing lower volume fraction of SiCpFig. 4. Optical micrographs of (a) Al/5SiCp as annealed, (b) Al/5SiCp after secondthe unreinforced pure metal in as cast condition. The hardness in-creases with increasing SiC content. A considerable enhancementin the hardness is observed after ECAP. It can also be observed thathardness has decreased after annealing compared to as cast condi-tion. This is attributed to annihilation of dislocations due to recov-ery process that takes place during annealing.ECAP pass, (c) Al/10SiCp as annealed, and (d) Al/10SiCp after rst ECAP pass.

due to mismatch in thermal expansion coefcient between the ma-

3558 G. Ramu, R. Bauri /Materials and D3.4. Compressive test

Table 3 shows the 0.2% proof stress of the composites in as castand annealed conditions and after each ECAP pass. As expected Al/

Fig. 5. Fracture surface of Al/10SiCp after 2nd pass. Inset picture in (a) shows thecracks on the billet. High magnication micrograph in (b) shows the particlematrix debonding at the interface.

Table 2Vickers hardness values (HV1)a.

Material As cast Annealed After ECAP(1st pass)

After ECAP(2nd pass)

Pure aluminium 24.5 Al/5SiCp 29.4 26.8 48.5 61.2Al/10SiCp 44.3 38.0 50.2

a Corresponding values for pure Al as reported in the literature [34] are 18.6, 40and 44 in as annealed condition, after 1st ECAP pass and after 2nd pass,respectively.

Table 30.2% proof stress of Al/SiCp composites before and after ECAPa.

Material 0.2% proof stress (MPa)

As cast Annealed After(1st ECAP pass)

After(2nd ECAP pass)

Al/5SiCp 89 85 121 149Al/10SiCp 98 90 125

a Flow stress of pure Al after eight ECAP passes: 132 MPa [35]; / = 90, w = 20.126 MPa after 2nd pass, / = 90, w = 90 [36].0.2% proof stress was increased by 75% in Al/5SiCp composite. Asdescribed above ECAP leads to signicant grain renement in thecomposites. This in turn leads to increment in the strength accord-ing to the HallPetch relation. Grain renement also leads to in-creased hardness after ECAP as described above. The measuredproperties (hardness and proof stress) of the composites werecompared with those reported in the literature [3438] for ECAPedpure Al in order to asses the effect of SiC particles. It was found thathardness and 0.2% proof stress of both the composites after ECAPare higher than ECAPed pure Al. e.g. ow stress of pure Al aftereight ECAP passes is reported to be 132 MPa [35] and 126 MPaafter two ECAP passes [36]. As can be seen from Table 3, 0.2% proofstress of Al/5SiCp after 2nd ECAP pass is considerably higher thanthese values. Similarly the hardness values (Table 2) are also signif-icantly higher than that reported in the literature [34] for pure Al.Although the Al/10SiCp composite cracks after the second pass, itshould be noted that there is a considerable improvement in hard-ness and strength even after one pass. It should also be noted thatall the ECAP experiments were carried out at room temperature.This opens up possibilities of further studies on high temperatureECAP of AlSiC composites as has been done previously for someother Al alloy composite containing a relatively softer reinforce-ment [1518].

4. Conclusions

AlSiCp composites with 5 and 10 volume fractions of SiCp weresuccessfully produced by stir casting route. Castings were freefrom porosity and segregation or clustering with fairly uniformdistribution of SiC particles in aluminium matrix.

ECAP of the AlSiCp composites were done in Bc route with dieangle of 120. ECAP of Al/5SiCp composite was successfully car-ried upto two passes. Whereas in case of Al/10SiCp compositeonly one pass was possible. In the second pass cracks appearsat the surface of Al/10SiCp composite. Further studies are neededto see if the number of passes can be increased by high temper-ature ECAP.

Hardness and 0.2% proof stress of AlSiCp composites with 5 and10 vol.% SiCp were increased signicantly by EACP.

Considerable grain size renement was observed in Al/5SiCp andAl/10SiCp composites after ECAP.

No particle breakage was observed after ECAP. There was nochange in SiC particle orientation or distribution due to ECAP.

Acknowledgements

The authors would like to thank Dr. Uday Chakkingal, AssociateProfessor, Dept. of Metallurgical and Materials Engineering, IIT Ma-dras, India, for providing the ECAP facility used in this study.

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Effect of equal channel angular pressing (ECAP) on microstructure and properties of AlSiCp compositesIntroductionExperimental detailsMaterial selection and composite processingEqual channel angular pressing of AlSiCp compositesSample preparation for equal channel angular pressing (ECAP)Equal channel angular pressing (ECAP)

CharacterizationDensity and porosityVolume fractionMicrostructural characterizationMicrohardnessCompressive test

Results and discussionDensity and porosityMicrostructural characterizationHardnessCompressive test

ConclusionsAcknowledgementsReferences