Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.12, pp.1141-1152, 2011
jmmce.org Printed in the USA. All rights reserved
1141
Effect of Particulate Reinforcements on the Mechanical Properties of
Al6061-WC and Al6061-Gr MMCs
A. R. K. Swamy
1
, A. Ramesha
2
, G.B. Veeresh Kumar
3*
, J. N. Prakash
4
1
Research Scholar, Dr. M.G.R Educational & Research Institute, Chennai & Department
Mechatronics Engineering, Acharya Institute of Technology, Bangalore, India.
2
GATES Institute of Technology, Gooty, Andhra Pradesh, India.
3
Department of Mechanical Engineering, Amrita Vishwa Vidyapeetham, Bangalore, India.
4
Department of Mechanical Engineering, Alpha College of Engineering, Bangalore, India.
*Corresponding author: [e-mail]
ABSTRACT
This paper presents the comparative study of the mechanical properties of Al6061-Tungsten
carbide composites containing Tungsten carbide (WC) particulate, and Al6061-graphite
particulate composites containing graphite particles. The reinforcing particulates in the
MMCs vary from 0% to 4% by weight. The 'vortex method' of production was employed to
fabricate the composites, in which the reinforcements were poured into the vortex created by
stirring the molten metal by means of a mechanical agitator. The composites so produced
were subjected to a series of tests.
The results of this study revealed that as the Tungsten carbide particle content was increased,
there were significant increases in the ultimate tensile strength, hardness and Young's
modulus, accompanied by a reduction in its ductility. There was, however, only a very
marginal increase in the compressive strength, where as in graphite reinforced composites as
the graphite content was increased, there were significant reduction in hardness and
monotonic increases in the ductility, ultimate tensile strength (UTS), compressive strength
and Young's modulus of the composite, An attempt is made in the paper to provide
explanations for these phenomena.
Keywords: Aluminum alloys, Tungsten carbide, Graphite, Composites, Mechanical
properties.
1. INTRODUCTION
Metal matrix composites (MMCs) are increasingly becoming attractive materials for
advanced aerospace applications because their properties can be tailored through the addition
of selected reinforcements [1-2]. In particular, particulate reinforced MMCs have recently
found special interest because of their specific strength and specific stiffness at room or
elevated temperatures [3]. It is well known that the elastic properties of metal matrix
composites are strongly influenced by micro structural parameters of the reinforcement such
as shape, size, orientation, distribution and volume fraction [4].

1142 A. R. K. Swamy, A. Ramesha, G.B. Veeresh Kumar, J. N. Prakash Vol.10, No.12
Aluminium-based MMCs have received increasing attention in recent decades as engineering
materials. The introduction of a ceramic material into a metal matrix produces a composite
material that results in an attractive combination of physical and mechanical properties which
cannot be obtained with monolithic alloys. There is an increasing need for knowledge about
the processing techniques and mechanical behaviour of particulate MMCs in view of their
rising production volumes and their wider commercial applications. Interest in particulate
reinforced MMCs is mainly due to easy availability of particles and economic processing
technique adopted for producing the particulate-reinforced MMCs. Aluminium alloy-based
particulate-reinforced composites have a large potential for a number of engineering
applications. Interest in reinforcing Al alloy matrices with ceramic particles is mainly due to
the low density, low coefficient of thermal expansion and high strength of the reinforcements
and also due to their wide availability. Among the various useful aluminium alloys,
aluminium alloy 6061 is typically characterized by properties such as fluidity, castability,
corrosion resistance and high strength-weight ratio. This alloy has been commonly used as a
base metal for MMCs reinforced with a variety of fibres, particles and whiskers [5-7].
In recent years, considerable work has been done on graphite reinforced metal matrix
composites which exhibit low friction, low wear rate and excellent antiseizing properties. The
graphite in these composites presumably imparts improved tribological properties to the
composites through the formation of a graphite-rich film on the tribo-surface which provides
solid lubrication. Journal bearings made of graphite particle dispersed composites perform
much better than conventional bearing alloys [8]. Graphite particles of size ranging from 50
to 200 µm yield the best results [9]. Manufacturing automotive pistons out of graphite
reinforced composites instead of other conventional materials resulted in a saving of 5-7% on
fuel and lubricating oil [10]. The presence of graphite in the matrix improves its oil
spreadability over the contact surface, thus reducing the tendency to score or seize. Graphite,
which consists of carbon atoms arranged in a layer-like structure, displays a very low
coefficient of friction while sliding on another clean surface, thus suggesting that it can be
used as solid lubricants [11]. Because of this solid lubricative property, graphite in the form
of particles has a wide range of applications in composite materials which are used to make
components requiring great wear resistance such as engine bearings, pistons, piston rings and
cylinder liners [12]. Although fibre reinforcements lead to marked enhancement in its
properties, composites using particulate or discontinuous reinforcement for high-volume
applications are being increasingly sought. The most widely used reinforcements have been
SiC, alumina, graphite, B
4
C and TiC. These have been used to achieve improvements in
selected properties. Most of the published data pertain to the mechanical properties of
particulate-reinforced MMCs deal with tensile properties while only a relatively small
amount of data has been obtained dealing with compression properties, although it is
generally known that the compressive strength of an MMC is invariably higher than its UTS.
Such MMCs are quite brittle compared with monolithic materials and have values of
percentage elongation typically less than 5 per cent. For unreinforced aluminium alloy 6061,
Awerbuch et al. [13] have shown that the deformation in compression could be greater than
50 per cent for specimens having a length-diameter ratio L/D equal to unity. Such effects
could also be responsible for the apparently high value of the compressive strength. Hence, in
the present investigation, importance is also given to the compressive properties of the
MMCs, together with the tensile properties such as the UTS, ductility, hardness and Young's
modulus.
In the present investigation, aluminium alloy 6061 was used as the matrix material. Al6061
alloy has the highest strength and ductility of the aluminium alloys with excellent

Vol.10, No.12 Effect of Particulate Reinforcements 1143
machinability and good bearing and wear properties [14]. Most of the particulate reinforced
metal matrix composites are produced by liquid metallurgy, sometimes known as the 'vortex
method' [15], although many different processes for fabricating these cast composites are also
available which have been reported by various researchers. In the present work, the 'vortex
method' of producing AMC’s , in which Tungsten carbide and graphite particulates have
been used as the candidate reinforcements of particulate sizes ranging from 5 to 15 µm and
added to the vortex formed in the Al6061 melt above its liquidus temperature. Since the
hardness, ultimate tensile strength (UTS), compressive strength, Young's modulus and
ductility of the composite material are all vital properties of a structural material, the present
investigation aims at studying these properties in the Al6061 alloy-particulate composites.
2. EXPERIMENTAL DETAILS
The properties of materials adopted and methods followed for the fabrication and testing of
MMCs in the present studies are presented in the following sections.
2.1 Matrix and Reinforcement Materials Details
The matrix for the present studies selected was Al6061 alloy and were procured from Fenfee
Metallurgicals, Bangalore, in the form of ingots. The chemical composition of Al6061 alloy
is given in Table 1. The reinforcing materials selected were tungsten carbide (WC) of 5 µm
size and graphite (Gr) of 15 µm particle size and the properties of the matrix and
reinforcement materials used are presented in the Table 2.
Table 1. Chemical Composition of Al6061 by Weight percentage.
Chemical Composition Si Fe Cu Mn Mg Cr Zn Ti Al
Al6061 0.62 0.23 0.22 0.03 0.84 0.22 0.10 0.1 Bal
Table 2. Properties of matrix and reinforcement materials.
Material Elastic Modulus
(GPa)
Density
(g/cc)
Hardness
(HB500)
Tensile Strength
(MPa)
Al6061 70-80 2.7 30 115
WC 627 14.9 1630** 5000
Gr 8-15 2.09 1.7* 20 – 200
*Mohs scale; ** Kg/mm
2
2.2 Preparation of Composites
The Al6061-graphite composites were prepared by the vortex method [9]. The graphite
contents used for the preparation of the composites were 0%, 1%, 2% and 4%. This is
because graphite compositions of 7% and above would lead to rejection from the melt [16].
Addition of graphite into the molten aluminium alloy melt above its liquidus temperature of
500 °C was carried out by creating a vortex in the melt using a mechanical stainless steel
stirrer coated with aluminite (to prevent migration of ferrous ions from the stirrer material
into the aluminium alloy melt). The melt was rotated at a speed of 500 rpm in order to create
the necessary vortex. The graphite particles were preheated to 400 °C and added to the melt
through the vortex. The graphite particles were added, and the melt was thoroughly stirred
and subsequently degassed by passing hexachloroethane (C
2
Cl
6
) solid degasser. The molten
metal was then poured into permanent moulds for casting. The above vortex method was

1144 A. R. K. Swamy, A. Ramesha, G.B. Veeresh Kumar, J. N. Prakash Vol.10, No.12
followed to prepare Al6061-Tungsten Carbide composites; the size of WC was 5 µm. The
preheated particles were introduced into the vortex and stirred for eight minutes at a speed of
500 rpm. A pouring temperature of 720 °C was adopted and the molten composite was
poured into preheated cast iron moulds. The extents of incorporation of particles into the
matrix alloy were varied from 1 to 4 wt % in the steps of 1wt% increment. The after effective
degassing the molten metal was then poured into permanent moulds for casting. The
cylinders of 22 X 210 mm
2
cast MMCs of Al6061-Gr and Al6061-WC were obtained.
2.2 Testing of Specimens
Carefully polished and mirror finished specimens were examined under NIKHON – Japan
make, ECLIPSE 150 model up right metallurgical microscope to obtain microphotographs.
Tensile tests were conducted at room temperature using a universal testing machine (UTM)
in accordance with ASTM Standard E 8-82. The tensile specimens of diameter 12.5 mm and
gauge length 60 mm were machined from the cast composites with the gauge length of the
specimens parallel to the longitudinal axis of the castings.
For each composite, four tensile test specimens were tested and the average values of the
UTS, Young's modulus and ductility were measured. The hardness tests were conducted in
accordance with ASTM Standard E 10 using a Brinell hardness tester with a ball indenter of
10 mm diameter and a load of 500 kg. The load was applied for 30 sec. Eight hardness
readings were taken for each specimen at different locations to circumvent the possible
effects of particle segregation. Compression tests were conducted on a UTM in accordance
with ASTM Standard E 9 at room temperature. In this test the compression loads were
gradually increased and the corresponding strain was measured until the specimen failed.
Each result is an average of four readings.
3. RESULTS AND DISCUSSIONS
3.1 Microstructure Studies
Figures 1 and 2 show the optical micrographs of Al6061 alloy, Al6061-Gr and Al6061–WC
composites. Micrographs reveal that there is fairly uniform distribution of WC/Gr particulates
throughout the matrix alloy and the porosity is lower. It is reported that higher hardness is
always associated with lower porosity of the MMCs [17]. Also, it can be observed that there
is good bonding between the matrix and the reinforcement particulates resulting in better load
transfer from the matrix to reinforcement material.
3.2 Hardness
Hardness, is the measure of a material's resistance to surface indentation, also it is a function
of the stress required to produce some specific types of surface deformation. Figure 3 is a
graph showing the effect of graphite and WC reinforcements on the hardness of cast Al6061-
graphite and Al6061-WC particulate composites. Each value represented is an average of six
measurements. The results are repeatable in the sense that each individual result did not vary
more than 5% from the mean value.

Vol.10, No.12 Effect of Particulate Reinforcements 1145
a) Al6061 alloy a) Al6061 alloy
b) Al6061-1 wt% Graphite
c) Al6061-2 wt% Graphite
c) Al6061-2 wt% Tungsten Carbide
d) Al6061-3 wt% Graphite
d) Al6061-3 wt% Tungsten Carbide
e) Al6061-4 wt% Graphite
e) Al6061-4 wt% Tungsten Carbide
Figure 1. Micrographs of Al6061 and Al6061-Gr
MMCs at 100X magnification for 50µm.
Figure 2. Micrographs of Al6061 and Al6061-WC
MMCs at 100X magnification for 50µm.

1146 A. R. K. Swamy, A. Ramesha, G.B. Veeresh Kumar, J. N. Prakash Vol.10, No.12
Figure 3. Effect of the WC and graphite content on Hardness.
It can be seen that as the graphite content increases, the hardness of the composite material
decreases monotonically by significant amounts. In fact, as the graphite content is increased
from 0% to 4% the hardness decreases by about 23%. In case of metals, as the Ultimate
Tensile Strength (UTS) of a material increases, so would the hardness but the opposite is seen
in this particular composite, whereby the hardness drops as the UTS increases [18]. There is a
good reason for this phenomenon, though, since graphite, being a soft dispersoid, does not
contribute positively to the hardness of the composite. K.H.W Seah et al., [19] have reported
a reduction in hardness from 107 BHN to 77 BHN (about 28% differences) on addition of
similar weight percentages of graphite to ZA-27 (Zinc Aluminium) alloy. Such a monotonic
decrease in the hardness of the composite as graphite content is increased poses a limit to
how much graphite may be added to enhance its other mechanical properties, since hardness
is directly related to wear resistance, a compromise is necessary when deciding how much
graphite should be added to enhance the ductility, UTS, compressive strength, and Young's
modulus of the composite without sacrificing too much of its hardness.
Further the hardness of the Al6061-WC particulate composite is increasing with the increase
of reinforcing particulate content. The hardness value of the composite is higher than that of
its matrix alloy. It was observed that the hardness value improved up to 3 wt % of WC, the
hardness increased by about 14% and again the hardness is decreased for 4 wt. % of WC.
Better hardness was found at 3 wt. % of WC. Hence from the studies it can be concluded that
up to 3 wt. % of WC can be successfully incorporated into the matrix.
3.3 Ultimate Tensile Strength
Figure 4 is a graph showing the effect of reinforcement content on the Ultimate Tensile
Strength (UTS) of cast Al6061-graphite and Al6061-WC particulate composites. Each value
represented is an average of six measurements. The results are repeatable in the sense that
each individual result did not vary more than 5% from the mean value. It can be seen that as
the graphite content increases, the UTS of the composite material increases monotonically by
significant amounts. In fact, as the graphite content is increased from 0% to 4%, the UTS
increases by about 38%. These results are in accordance with those obtained by Pillai [20]

Vol.10, No.12 Effect of Particulate Reinforcements 1147
who reported similar findings. This increase in UTS may be due to the graphite particulates
acting as barriers to dislocations in the microstructure [21].
Figure 4. Effect of the WC and graphite on Ultimate Tensile Strength.
It was observed that as the wt. % of WC is increased from 0% to 4% it was observed that the
tensile strength value improved up to 3 wt. % WC and the tensile strength decreased for 4 wt.
% WC. Better tensile strength was found at 3 wt % WC. As the WC content is increased from
0% to 3%, the UTS increases by about 55%.The decrease in tensile strength of the
composites beyond 3 wt. % WC is attributed to improper bonding between the matrix and
reinforcement materials. Finer the grain size better is the hardness and strength of composites
leading to lowering of wear rates. The increase in UTS can be attributed to the presence of
hard WC particulates that impart strength to the matrix alloy, thereby providing enhanced
resistance to tensile stresses. There is a reduction in the interspatial distance between the hard
WC particulates, which causes an increase in the dislocation pile-up as the particulate content
is increased. This leads to a restriction in the plastic flow due to the random distribution of
the particulates in the matrix, thereby providing enhanced tensile strength to the composites.
Similar observations were made by Ghosh and Ray [22], who fabricated Al
2
O
3
particulate-
reinforced aluminium alloy composites using the compo-casting method and by McCoy et al.
[23] who produced TiB
2
-particulate-reinforced aluminium alloy composites with particulate
contents ranging from 10 to 25 vol %. An increase in the UTS was observed as the TiB
2
content was increased from 10 to 18 vol %.
3.4 YOUNGS MODULUS
Figure 5 is a graph showing the percentage variation in Young’s modulus provides a
comparative insight in to the actual experimental results recorded for the Al6061-WC and
Al6061-graphite particulate reinforced composites. The effect of graphite content on the
Young's modulus of cast Al6061-graphite particulate composites. As in the cases of UTS
described above, it can be seen that as the graphite content increases, the Young's modulus of
the composite material increases monotonically by significant amounts. In fact, as the
graphite content is increased from 0% to 4%, the Young's modulus increases by about 9%.
Similar results have been obtained in aluminium matrix composites where the Young's

1148 A. R. K. Swamy, A. Ramesha, G.B. Veeresh Kumar, J. N. Prakash Vol.10, No.12
modulus has been reported to increase with increase in the content of the reinforcing material,
regardless of the type of reinforcement used [24]. Nevertheless, it must be emphasized at this
point that the values of Young's modulus presented in this paper are only useful for the
purpose of comparison among themselves.
Figure 5. Effect of the WC and graphite content on Young’s Modulus.
It can be seen that as the WC content increases, the Young's modulus of the composite
material increases monotonically by significant amounts. In fact, as the WC content is
increased from 0% to 3%, the Young's modulus increases by about 70% and when increased
from 3% to 4% the Young’s modulus decreases monotonically as shown in the Figure 5. This
increase in Young's modulus is broadly in line with the rule-of-mixtures prediction.
McDanels [24] who obtained similar results for particle-reinforced aluminium composites
reported that Young's modulus increases with increase in reinforcement content regardless of
the type of reinforcement used.
3.6 Ductility
Figure 6 is a graph showing the reinforcement content on the ductility of cast Al6061-
graphite and Al6061-WC particulate composites (measured in terms of percentage
elongation). As the graphite content increases, the ductility of the composite material
increases monotonically by significant amounts, i.e., when the graphite content is increased
from 0% to 4% the ductility was found to increases by about 165%. The effect of graphite is
expected to be mechanical in nature since the particles are unreactive with the matrix phase
[20]. This considerable increase in ductility is due to the graphite additions, being an effective
solid lubricant [25, 8-11], eases the movement of grains along the slip planes.

Vol.10, No.12 Effect of Particulate Reinforcements 1149
Figure 6. Effect of the WC and graphite content on Ductility
The elongation of the composite material Al6061-WC is less as compared to that of the cast
Al6061 alloy. It is clear that the composite material exhibits higher brittleness with increased
filler content (WC), in other words the matrix material suffers with ductility due to the
influence of the reinforcement material. In fact, as the WC content is increased from 0% to
4% the ductility drops by about 100%. These results tally with those obtained by other
researchers [26, 27] who also observed that the ductility of the composites decreases with
increase in the reinforcement content. This decrease in ductility in comparison with the
matrix alloy is a most commonly encountered disadvantage in discontinuously reinforced
MMCs [28]. The reduction in ductility can be attributed to the presence of a hard ceramic
phase that is prone to localized crack initiation and increased embrittlement effect due to
local stress concentration sites at the reinforcement-matrix interface. Hence, the introduction
of this hard secondary ceramic phase creates slip regions. Moreover, the reinforcing
particulates resist the passage of dislocations either by creating stress fields in the matrix or
by inducing large differences in the elastic behaviour between the matrix and the dispersoid.
Mummery et al. [28] are of the opinion that this loss in ductility is probably due to the voids
which nucleate during the plastic straining of the reinforcement.
3.7 Compressive Strength
Figure 7 is a graph showing the effect of graphite content on the compressive strength of cast
Al6061-graphite particulate composites. It can be seen that as the graphite content increases,
the compressive strength of the composite material increases monotonically by significant
amounts. In fact, as the graphite content is increased from 0% to 4%, the compressive
strength increases by about 56% and this increase in compressive strength may be due to the
graphite particles acting as barriers to dislocations in the microstructure.

1150 A. R. K. Swamy, A. Ramesha, G.B. Veeresh Kumar, J. N. Prakash Vol.10, No.12
Figure 7. Effect of the WC and graphite content on compression strength.
Figure 7 is a graph showing the effect of the WC particulate content on the compressive
strength of the composites. From Figure 7 it can be observed that the compressive strength of
the composites are higher than that of the base alloy because the WC is having very high
compressive strength then the matrix material and further it can be observed that with
increase in WC content the composites compressive strength also increases. As the wt % of
WC is increased from 1 % to 4 wt %, the compressive strength value improved up to 3 wt %
WC, the compressive strength increased by about 48% and the compressive strength
decreased for 4 wt % WC. Better compressive strength was found at 3 wt % WC.
Similar results were observed by earlier researchers such as Webster [29] and Awerbuch et
al. [13] when they conducted tests on whisker-reinforced composites. Towle and Fried [30]
compared the compressive and tensile properties of magnesium-based MMCs and observed
similar trends. The increase in compressive strength is chiefly due to the decrease in the inter-
particle spacing between the WC particles, since WC is much harder than aluminium alloy
6061. The presence of the WC particles resists deforming stresses, thus enhancing the
compressive strength of the composite material. The increase in compressive strength is not
as spectacular as those seen in the UTS, hardness and Young's modulus because the
compressive strength of the unreinforced matrix material itself is already very high, in fact
several times the UTS. There is only a very marginal increase in the compressive strength
when a secondary ceramic phase is introduced. Nevertheless, the addition of hard ceramic
particulates has caused the MMCs to behave as brittle rather than ductile materials, as is
evident from the above results.
4. CONCLUSIONS
The significant conclusions of the studies on Al6061-WC and Al6061-Gr metal matrix
composites are as follows.
1. Al6061-WC and Al6061-graphite composites were prepared successfully using liquid
metallurgy techniques by incorporating the reinforcing particulates up to 4 wt %.

Vol.10, No.12 Effect of Particulate Reinforcements 1151
2. It was found that increasing the graphite content within the aluminum matrix results
in significant increases in the ductility, UTS, compressive strength and Young's
modulus, but a decrease in the hardness.
3. A compromise is necessary when deciding how much graphite should be added to
enhance the mechanical properties of the composite without sacrificing too much of
its hardness and hence its wear resistance.
4. The properties of the cast Al6061-WC composites are significantly improved by
varying the amount of WC. It was found that increasing the WC content within the
matrix material, resulted in significant improvement in mechanical properties like
hardness, tensile strength, and compressive strength at the cost of reduced ductility.
5. Highest values of mechanical properties like hardness, tensile strength and
compressive strength were found at 3 wt% WC.
ACKNOWLEDGEMENTS
The authors express their thanks to Shri B. M. Reddy, President, Mr. B. Premnath Reddy,
Chairman, Dr. G. P. Prabukumar, principal, Dr. K. G. Satyanarayan, Research Director,
Acharya Institute of Technology, JMJ Education Society, Bangalore, Karnataka, India for the
support and encouragement during this research studies.
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