-
teeta
del-Molumy hae paposratrein
2010 Elsevier Ltd. All rights reserved.
al mae to theinforcof thee, mince they
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: [email protected] (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
le at ScienceDirect
d Design
evier .com/locate /matdes
-
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