-
W.
Pr arC urTe m
A con cfi Homr n ofm der ms s solvm andt coma n weo examo uresc
ach Ss Thesa both8 ainin
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AluaerostreprouseThecifiproducducOthprochefacitiontageof mis
abidinhcau
thesurface [5]. However, the aluminum-silicon carbide
composites
Mpubl
1LEgy
2PandFacumrb
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
Powitespowmecumusewittiontivereceparthethe
CabsoxidSiCtribreinarea mtionin alowbetwtion458for
pacplacareincrSamfereforinertheAl2dizanacandnatuimmageweitheforemp
on of
8
7
FIGcal m
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
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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.
-
weiagintheof ttiongrerelaalumlatitioncomreacslig
carbmattioncrea
gethplacrouticlHowpac
FIGfor t
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
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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
from
700
gre
har
tere
tere
tem
beh
crea
dec
incr
minused hardness compared with the green state for samples
that
orized.
-
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
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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
Fmenturesamtemdidpheterecommelshoposit ca
a dmansurfshoandplacstre11.behComcracin tcirc
FIGseve
FIGsam
FIGbide
FIGbide
FIG
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
Twei25in aat 9therspeon t
Tspespe
evecracthespeposcom
Di
Prostredredistminforccombe
tpromatparconmouseincrevediststrecespropowsintof
aformandtualSiCbetwgooliqualumboustro[13ertimaj
promatpoioxipromenandturecomforcces
sionunr
FIGbide
FIGbide
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
FIG taininat 6
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
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10
[11
[12
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.
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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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KHAIRALDIEN ET AL. ON ALUMINUM-SILICON CARBIDE COMPOSITES 13
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EST 2014Downloaded/printed byYildiz Teknik Universitesi pursuant to
License Agreement. No further reproductions authorized.