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2010 Elsevier Ltd. All rights reserved.

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wear behavior of Al-MMCs. They also comprehensively studiedthe effects of normal load, sliding distance, sliding velocity, sizeand amount of reinforcement on the sliding wear characteristics.Wilson and Alpas [8,9] constructed wear mechanism maps forA356 Al20%SiC. They investigated that addition of SiC particlesenhances the wear resistance as well as shifts the transition frommild to severe wear under higher load and higher sliding velocity.The critical temperature, at which the transition from mild wear to

There are several methods, such as powder metallurgy, squeezecasting, stir casting [1517], to fabricate the particle reinforcedmetal matrix composites. However, these conventional techniques,which need expensive and dedicated tools such as mould or dies,are not suitable for small volume production and complex shapes.Direct metal laser sintering (DMLS) process is another potentialmethod to develop such composites. Main advantage of this pro-cess is that the three dimensional (3D) parts can directly be fabri-cated by bonding powdered materials using laser energy [18,19]. Inthis method powdered materials are selectively fused by focusedlaser beam in layer-by-layer fashion to obtain a near-net-shape

* Corresponding author. Tel.: +91 3222281926; fax: +91 3222282278.

Materials and Design 32 (2011) 139145

Contents lists availab

an

elsE-mail address: psaha@mech.iitkgp.ernet.in (P. Saha).nation of properties such as improved stiffness, reduced density,good corrosion resistance, improved high temperature properties,controlled thermal co-efcient of expansion and enhanced electri-cal performance. In automobile sectors, the Al-MMCs are used tomanufacture brake drums, cylinder liners, cylinder blocks, driveshaft, etc. They are also widely used to fabricate structural parts,rotor vanes, drive shaft, rotor plates, etc. in aerospace industry[36]. In these applications, several parts are used in tribologicalsystems that require improved friction and wear performance ofthe Al-MMCs [7]. Several researchers [2,4,714] reported sliding

of wear resistance is highly dependent on the kind of reinforce-ment as well as its volume fraction. The authors also concludedthat the particles are more benecial as compared to whiskersand bers for improving the wear resistance of the MMC. Maet al. [14] studied dry sliding wear behavior of cast SiC reinforcedAl-MMCs. The 20 wt.% and 50 wt.% of SiC were added in the twocast specimens. They examined the wear behavior of the MMCsas function of load and sliding distance. Natarajan et al. [12] alsoperformed sliding wear testing of in situ TiB2 reinforced Al-MMCsat different levels of elevated temperatures.1. Introduction

Discontinuously reinforced metemerged as important materials dustrength and low density [1,2]. SiC rtrix composites (Al-MMCs) are onewhich are widely used in automobildefense and other related sectors sin0261-3069/$ - see front matter 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.06.020trix composites haveeir high stiffness, highed Al-based metal ma-above stated materialsing, mineral, aerospace,have excellent combi-

severe wear occurred, was increased by the addition of SiC particlein the alloy. Hassan et al. [4] studied friction and wear behavior ofAlCuMg alloy and SiC particle reinforced AlCuMg basedcomposite. They reported that rate of wear volume loss forcomposite was lesser than that of the alloy. Miyajima et al. [10]conducted dry sliding wear tests of aluminium matrix compositereinforced with SiC-whiskers, Al2O3-bers and SiC particles usinga pin-on-disc wear tester. They investigated that the improvementE. Wearrevealed that during the wear test SiCp fragments into small pieces which act as abrasives to result inabrasive wear in the specimen.Crack and wear behavior of SiC particulamatrix composite fabricated by direct m

Subrata Kumar Ghosh, Partha Saha *

Department of Mechanical Engineering, IIT Kharagpur, Kharagpur 721302, India

a r t i c l e i n f o

Article history:Received 25 March 2010Accepted 10 June 2010Available online 16 June 2010

Keywords:A. Al-based metal matrix compositeC. Direct metal laser sintering

a b s t r a c t

In this investigation, crackmetal matrix composite (Astudied. Mainly, size and vof the composite. The studcentage (vol.%) of SiCp. Thwear resistance of the com300 mesh the specic wearresistance after 20 vol.% of

Materials

journal homepage: www.ll rights reserved.reinforced aluminium based metall laser sintering process

nsity and wear performance of SiC particulate (SiCp) reinforced Al-basedMC) fabricated by direct metal laser sintering (DMLS) process have beene fraction of SiCp have been varied to analyze the crack and wear behaviors suggested that crack density increases signicantly after 15 volume per-per has also suggested that when size (mesh) of reinforcement increases,ite drops. Three hundred mesh of SiCp offers better wear resistance; abovee increases signicantly. Similarly, there has been no improvement of wearforcement. The scanning electron micrographs of the worn surfaces have

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product without the help of any binding material. Any arbitrarygeometry with variations in size and complexity can be producedto a high degree of accuracy. Wide varieties of powders such aspolymers, ceramics, and metals can be sintered by this technique[20,21].

Till now very few studies were reported on the fabrication ofMMCs by DMLS process as well as wear characteristic of the com-posites. Gu et al. [18] prepared submicron WC10%Co reinforcedCu matrix composite by DMLS process. The effects of processingparameters on microstructure and properties of laser sinteredspecimens were investigated. Gaard et al. [22] developed In-var36TiC composite by the same process. They conducted thesliding wear testing using a block-on-cylinder tribometer whereblock was the composite material and 0.12% C steel was used for

increases up to 5 vol.% of SiC, then it decreases abruptly.

placed in two opposite sides of the substrate and the height of slipgauges was adjusted depending upon the layer thickness and thesubstrate height. After placing powder on the substrate, a scrapperwas moved on the two stacks of slip gauges. Once the powder layerwas compacted and required height was attained, the substrateholder was moved inside the chamber and placed below the quartzwindow. The door of the chamber was closed. Argon gas was con-tinuously supplied inside through the nozzle at the rate of 5 l/minto provide an inert environment.

After ring a laser pulse at a particular spot, CNC table wasmoved to a new position in the horizontal plane as per the hatch-

140 S.K. Ghosh, P. Saha /Materials andThe existing literature survey reveals that Al-based metal ma-trix composites can be applied to components needing improvedwear resistance. However there is no such investigation on wearcharacteristic of Al-MMCs fabricated by DMLS process.

Onat et al. [16] developed an Al-MMC through squeeze castingroute where Al4.5Cu3Mg and SiC particulate (SiCp) were used asmatrix alloy and reinforcement respectively. They studied the ef-fects of volume fraction of SiCp on microstructure, hardness, den-sity of the composite.

In this present investigation, the above metal matrix compositewas fabricated by DMLS technique. Wear behavior was studiedwith respect to size and volume fraction of SiC particulate. Forma-tion of crack poses another hurdle in laser sintering process. There-fore, crack density under different parametric combinations wasalso studied.

2. Experimental details

2.1. Set-up

The set-up consists of a pulsed Nd-YAG laser system (maximumpulse energy 20 J) and an inert gas chamber (Fig. 1). The inert gaschamber was made of perspex. It was placed on the CNC table ofthe machine. Substrate was xed in a holder. The holder could becylinder. The authors concluded that abrasive wear was the dom-inant wear mechanism. Ramesh et al. [23] fabricated the ironSiCcomposite by DMLS process and characterized its abrasive wearbehavior using a pin-on-disc wear tester. They showed that thecomposite has excellent abrasive wear resistance which increaseswith increase of SiC content in the iron matrix. Simchi et al. [24]prepared SiC particle reinforced Al0.7SiMg composite andinvestigated inuence of SiC particle on densication rate of thecomposite. They showed that the densication rate constantFig. 1. Schematic of inert gas chamber.moved inside the chamber by sliding. A door was provided to loadand unload the substrate holder. Laser was focused by a lens of fo-cal length 116 mm through a quartz window which was placed onthe top surface of the chamber. A brass nozzle was mounted on oneof the side walls of the chamber. Some tiny holes were provided onthe opposite side wall so that the gas present inside the chambercould escape out and an inert atmosphere could be maintained.

2.2. Parameter settings

For pulsed laser sintering using Nd-YAG laser, Chaterjee et al.[25] and Murali et al. [26] suggested a number of controllable inputparameters to obtain a sintered specimen with desired qualities.Some of the parameters such as layer thickness, hatching distance,laser pulse energy, pulse width, distance of the powder layer belowthe focal plane and powder composition were considered for thepresent investigation. The specimens were fabricated based ontwo parameter settings which are given in Table 1.

For both these two parameter settings, size (mesh) and volumepercentage (vol.%) of silicon carbide particulates (SiCp) were variedto fabricate the specimens in this present investigation. At rst,particulate size was varied at 300, 600, 800, 1000 and 1200 meshof SiCp at the constant amount of 15 vol.%. Secondly, volume per-centage of SiCp particulates was chosen at 10, 15, 20, 25 and 30%respectively at constant mesh size of 300.

2.3. Laser sintering procedure

Five homogeneous powder mixtures of 92.5 wt.% of Al powder(average particle size 44 lm), 4.5 wt.% of Cu powder (average par-ticle size 44 lm) and 3 wt.% Mg (average particle size 44 lm) wereprepared with introduction of 300, 600, 800, 1000, 1200 mesh ofSiC particulates respectively. Similarly, another ve homogeneouspowder mixtures were arranged with addition of 10, 15, 20, 25,30 vol.% of 300 mesh of SiC particulates respectively. The mixtureswere prepared from commercially pure aluminium (99.5% Al)powder, electrolytic copper (99.5% Cu) powder, 99% pure magne-sium powder and SiC particulates. Aluminum substrate of15 mm 15 mm 7 mm was taken to deposit powder layers. Arequired thickness of powder layer was applied on the substratewith the help of a scrapper and slip gauges. The slip gauges were

Table 1Parameter settings.

Parametersetting

Parameters

Layerthickness(lm)

Pulseenergy(J)

Pulsewidth(ms)

Vol.%ofSiC

Distance ofpowder layerfrom focalplane (lm)

Hatchingdistance(lm)

I 300 9 18 15 450 450II 400 10 16 15 500 450

Design 32 (2011) 139145ing distance (450 lm). The operation was repeated to completethe entire cross section. After consolidation of one layer, pow-der/substrate holder was taken out and a new layer of powder

s andFig. 2. Scheme of laser scanning for sintering on powder bed.

S.K. Ghosh, P. Saha /Materialwas applied. Vertical distance was adjusted according to theheight of new powder layer by lowering down the table. The pro-cess was repeated to obtain a sintered specimen of desired height.In this investigation, specimens of size 10 mm (length) 10 mm(breadth) 2.5 mm (height) were fabricated. The scheme of laserscanning is shown in Fig. 2. In this way, two experiments werecarried out for each mesh of SiC particulates based on the twoparameter settings as mentioned in Table 1. Similarly experimentswere also carried out for vol.% of reinforcement. One fabricatedspecimen is shown in Fig. 3.

2.4. Surface crack length measurement

At rst, all the specimens were cold mounted and polished withpolishing papers. These samples were further mirror polishedusing diamond pastes of grade 1 and 1=4. Then the specimens wereetched with Kellers solution. Photographs of the top surface werecaptured using a scanning electron microscope (make ZEISS,model EVO-60). Thereafter, lengths of the surface cracks weremeasured from these photographs using Image Tool Software.

2.5. Wear testing

The dry sliding wear test was conducted using a ball-on-discwear testing machine (make DUCOM, model TR-201-M3)where the ball was a cemented WCCo ball (Make: SALEM, WC:93.594.5%, Co: 5.56.5%) and the disc was the test specimen.The schematic representation of the wear testing set-up is shownin Fig. 4. At rst, all the specimens were ground on 800 grid emery

Fig. 3. Laser sintered specimen.Fig. 4. Schematic representation of ball-on-disc wear testing.

Table 2Operating condition for wear test.

WCCo ball diameter 5 mmTrack diameter 4 mmNormal load 4.9 NSliding distance 150.72 mSliding speed 0.063 m/s

Design 32 (2011) 139145 141paper to have uniform standard surface since surface nish of thespecimens would inuence friction and wear characteristics [13].After that, the specimens were cleaned in ultrasonic bath with ace-tone. The ball was placed perpendicularly on the disc which wasrotated at 300 rpm. The total sliding distance, sliding speed andnormal load were kept constant at 150.72 m, 0.063 m/s and 4.9 Nrespectively for all the tests. The tests were carried out at ambientcondition. Therefore, the test specimens were again cleaned inultrasonic bath with acetone. Before and after the wear test,weights of the test specimen were measured using an electronicbalance (Make- Mettler Toledo, Model AB265-S/FACT) havingleast count of 0.03 mg. The specic wear rate [= volume abraded/(sliding distance load applied)] was measured for the all speci-mens. The operating conditions are noted in Table 2.

3. Results and discussion

3.1. Surface crack behavior

Among the surface defects, cracking is one of the most impor-tant criteria because it leads to a reduction in fatigue, wear andcorrosion resistance of the material. Since it is not easy to quantifycracking in terms of the width, length or depth of the crack, this

Fig. 5. Graphical representation of crack density vs. vol.% of SiCp variation.

and142 S.K. Ghosh, P. Saha /Materialsstudy denes a term called surface crack density, which is the totallength of cracks (lm) per unit area, to evaluate the severity ofcracking [27]. Cracks were found in all the fabricated compositespecimens. The basic reason for which the metal matrix compos-ites were susceptible to crack was the existence of residual stressin the composites. If the residual stress is higher than the strength,fracture takes place. The residual stress is composed of thermalstress and contraction stress [28,29]. In pulsed Nd-YAG laser, theintensity of energy is higher at the centre of the beam and thenit decreases radially towards the periphery. So the temperaturein the powder material within the irradiated area rises in a similarfashion during the laser-powder interaction and maximum tem-perature is attained at the centre of the spot. Similarly, laser energyis absorbed at the top portion of the powder layer and then it isconducted into the bottom of the layer. Consequently, the top por-tion attains a higher temperature. The non-uniform nature of tem-perature distribution in the affected area during heating, meltingand solidication leads to temperature gradient, which eventuallyinduces the thermal stress in the composites. Zhou et al. [28] stud-ied the contraction stress that leads to crack sensitivity during

Fig. 6. SEM micrographs of worn surface of the specimens fabricated wDesign 32 (2011) 139145laser cladding. In this study it is presumed that similar kind of con-traction stress was generated in the composite due to rapid heating

ith: (a) 300, (b) 600, (c) 800, (d) 1000 and (e) 1200 mesh of SiCp.

Fig. 7. SEM micrograph of one fragmented SiCp in back scatter electron mode.

s andS.K. Ghosh, P. Saha /Materialand rapid solidication during laser sintering process. The contrac-tion stress in the sintering process was of two types. The rst onewas originated by the volume contraction of the matrix materialfrom liquidus to solidus curve, which was mainly generated fromphase transformation. The second one was caused by volume con-traction from solidus curve to room temperature. Sometime, crackswere also found through SiCp and along the particulate-matrixboundary. This was due to the generation of temperature gradientbetween matrix material and reinforcement.

Although cracks existed in all the specimens, there was no par-ticular trend of variation of crack density for the fabricated speci-mens with the variation of mesh of SiCp. Fig. 5 shows how thesurface crack density varied with volume percentage of SiCp. It isobserved that the surface crack density increased slightly from10 vol.% to 15 vol.% of SiCp. Then onwards it increased signi-cantly. Sahin et al. [5] reported that if the porosity in the compositespecimens increases, then the strength decreases. The amount ofSiCp was increased in this investigation. Therefore, the chancesof the clustering effect, which resisted the ow of molten material,

Fig. 8. SEM micrographs of worn surface of the specimens fabricaDesign 32 (2011) 139145 143increased. This incident led to the formation of pores in the speci-mens. This is the reason for increase of crack density with higheramount of reinforcement.

3.2. Wear behavior

3.2.1. Analysis of worn surfaces using SEMThe SEM micrographs of the worn out surface of the specimens,

sliding at room temperature, are shown in Fig. 6. From the micro-graph, it is evident that grooves running parallel to each other inthe sliding direction were formed distinctly. The deeper and widergrooves were formed in case of specimens fabricated with 1000and 1200 mesh of SiCp. This is because of smaller size of SiCp inthe composite specimens. For the specimens fabricated with rela-tively bigger size of reinforcement such 300, 600 and 800 mesh,SiCp restricted the plastic deformation or ow of matrix alloymaterial during sliding until they themselves were fragmentedinto small pieces [13]. High hardness of the smaller SiCp fragments,

ted with: (a) 10, (b) 15, (c) 20, (d) 25 and (e) 30 vol.% of SiCp.

and144 S.K. Ghosh, P. Saha /Materialsso created, resulted in deeper and wider grooves in the specimen.Fig. 7 illustrates how SiCp got fragmented into small pieces.

Some scratches were also found in the worn surfaces. This sug-gests that smaller pieces of SiC particulates which were ploughedfrom the surface material, acted as abrasive. Consequently, abra-sive wear also took place. The SEM micrograph of the worn surfaceof the specimens fabricated with varying volume percentage ofSiCp is shown in Fig. 8. It is seen from the Figs. 6 and 7 that theworn surfaces were, somewhere, covered with compacted weardebris. The cleaning of the specimen in the ultrasonic bath withacetone could remove some of the loose debris only. The Fig. 8 also

Fig. 9. (a) SEM micrograph of the worn surface and c

Fig. 10. Graphical representation of specic wear rate vs. mesh of SiCp variation.Design 32 (2011) 139145reveals that the number of scratches and the amount of compactedwear debris increased with the addition of more SiCp. The Al andO2 mapping shows (in Fig. 9) that the compacted debris was richwith Al and O2. Many researchers [12,30] stated the occurrenceof oxidative wear at ambient temperature. This suggests that com-pacted debris were aluminum oxide. A large number of microcracks were observed on the attached debris and sliding surfaces.

3.2.2. Specic wear rateThe change of specic wear rate with respect to mesh of rein-

forcement is shown in Fig. 10. It is clear from the gure that the

orresponding elemental map of (b) Al and (c) O2.

Fig. 11. Graphical representation of specic wear rate vs. vol.% of SiCp variation.

specic wear rate increases with the decrease of SiCp size. Kumaret al. [13] also investigated same trend, though size of the rein-forcement and manufacturing method were different. They haveexplained that the particlematrix interfacial area is larger for nerSiCp; as a result chance to pull out the particulates from the matrixincreases for smaller SiCp. But in case of bigger size of the rein-forcement, SiC particulates are expected to be embedded with

[2] Shipway PH, Kennedy AR, Wilkes AJ. Sliding wear behaviour of aluminium-based metal matrix composites produced by a novel liquid route. Wear1998;216:16071.

[3] Sawla S, Das S. Combined effect of reinforcement and heat treatment on thetwo body abrasive wear of aluminum alloy and aluminum particle composites.Wear 2004;257:55561.

[4] Hassan AM, Alrashdan A, Hayajneh MT, Mayyas AT. Wear behavior of AlMgCubased composites containing SiC particles. Tribol Int 2009;42:12308.

[5] Sahin Y, zdin K. A model for the abrasive wear behaviour of aluminium basedcomposites. Mater Des 2008;29:72833.

S.K. Ghosh, P. Saha /Materials and Design 32 (2011) 139145 145the matrix alloy until they themselves get fragmented into smallpieces. This phenomenon restricts the plastic deformation of thesurface material. As a result, wear resistance drops with the in-crease of mesh of SiCp.

The behavior of specic wear rate with respect to variation ofvolume percentage of SiCp was studied for 300 mesh of reinforce-ment. The change of specic wear rate is shown in Fig. 11. It is ob-served from the gure that specic wear rate keeps on decreasingup to 20 vol.% of SiCp. The reason may be attributed to the fact thatwith increase in volume fraction of reinforcement ductility of thecomposite drops [31] and hardness increases [13,4]. Hassan et al.[4] suggested that SiC particulates carry major portion of the ap-plied load and prevent plastic deformation of the surface material.This may increase the wear resistance of the composite. It is alsofound from the gure that beyond 20 vol.%, the wear rate becomesalmost stable. Although it may be thought that specic wear rateshould have reduced further, the experimental trend indicates aninsignicant change of specic wear rate. This behavior is causedby resultant effect of abrasive wear and cracks. SiCp resists theplastic deformation, tries to decrease the wear of the surface mate-rial, but the fragmented SiCp introduces abrasive wear. The surfacematerial is also worn out along the cracks. Since the number ofboth abrasives as well as crack density increases with higheramount of SiCp, wear resistance does not improve further. Supple-mentary addition of SiCp may result in more wear.

4. Conclusion

Through the investigation, it can be concluded that the MMCspecimens, so fabricated, were susceptible to cracks. These cracksare formedmainly due to two types of residual stresses, one is ther-mal stress and the other one is contraction stress. There is no par-ticular trend in the change of crack density for a xed volumepercentage of SiC (15 vol.%). But the crack density increases withthe increase of the amount of reinforcement. It is found that after15 vol.%, crack density changes notably. Therefore, addition of SiCpshould be restricted to 15 vol.%. The specic wear rate increaseswith the decrease of reinforcement size for a certain volume per-centage of SiCp. Wear resistance initially improves with the in-crease of the content of SiCp, but there is no further enhancementin wear resistance after 20 vol.%. Apart from sliding wear abrasivewear also takes place. Fragmented SiC particulates act as abrasives.Compacted wear debris increases with the increase in the amountof SiCp. This was basically Al2O3.

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Crack and wear behavior of SiC particulate reinforced aluminium based metal matrix composite fabricated by direct metal laser sintering processIntroductionExperimental detailsSet-upParameter settingsLaser sintering procedureSurface crack length measurementWear testing

Results and discussionSurface crack behaviorWear behaviorAnalysis of worn surfaces using SEMSpecific wear rate

ConclusionReferences