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Journal of Minerals & Materials Characteri zation & Enginee r ing, Vol. 8, No.6, pp 455-467, 2009
jmmce.org Printed in the USA. All rights reserved
Development of Aluminium Based Silicon Carbide Particulate
Metal Matrix Composite
Manoj Singla1, D. Deepak Dwivedi1, Lakhvir Singh2, Vikas Chawla 3*
1Department of Mechanical Engineering, R.I.E.I.T., Railmajra, Distt. Nawanshahr (Pb.)-144533,
2Department of Mechanical Engineering, BBSBEC, Fatehgarh Sahib (Pb.), India
3Department of Metallurgical & Materials Engineering, I.I.T. Roorkee (Uttaranchal), India
*Corresponding Author: email: firstname.lastname@example.org
Metal Matrix Composites (MMCs) have evoked a keen interest in recent times for potential
applications in aerospace and automotive industries owing to their superior strength to weight
ratio and high temperature resistance. The widespread adoption of particulate metal matrix
composites for engineering applications has been hindered by the high cost of producing
components. Although several technical challenges exist with casting technology yet it can be
used to overcome this problem. Achieving a uniform distribution of reinforcement within the
matrix is one such challenge, which affects directly on the properties and quality of composite
material. In the present study a modest attempt has been made to develop aluminium based
silicon carbide particulate MMCs with an objective to develop a conventional low cost method of
producing MMCs and to obtain homogenous dispersion of ceramic material. To achieve these
objectives two step-mixing method of stir casting technique has been adopted and subsequent
property analysis has been made. Aluminium (98.41% C.P) and SiC (320-grit) has been chosen
as matrix and reinforcem ent material respectively. Experiments have been conducted by varying
weight fraction of SiC (5%, 10%, 15%, 20%, 25%, and 30%), while keeping all other parameters
constant. The results indicated that the ‘developed method’ is quite successful to obtain uniform
dispersion of reinforcement in the matrix. An increasing trend of hardness and impact strength
with increase in weight percentage of SiC has been observed. The best results (maximum
hardness 45.5 BHN & maximum impact strength of 36 N-m.) have been obtained at 25% weight
fraction of SiC. The results were further justified by comparing with other investigators.
Key Words: Metal Matrix Composites MMC’s, Silicon Carbide SiC.
456 Manoj Singla, D. Deepak Dwivedi, Lakhvir Singh, Vikas Chawla Vol.8, No.6
Metal matrix composite (MMC) is engineered combination of the metal (Matrix) and hard
particle/ceramic (Reinforcement) to get tailored properties. MMC’s are either in use or
prototyping for the space shuttle, commercial airliners, electronic substrates, bicycles,
automobiles, golf clubs, and a variety of other applications.
Like all composites, aluminum-matrix composites are not a single material but a family of
materials whose stiffness, strength, density, thermal and electrical properties can be tailored. The
matrix alloy, reinforcement material, volume and shape of the reinforcement, location of the
reinforcement and fabrication method can all be varied to achieve required properties. The aim
involved in designing metal matrix composite materials is to combine the desirable attributes of
metals and ceramics. The addition of high strength, high modulus refractory particles to a ductile
metal matrix produces a material whose mechanical properties are intermediate between the
matrix alloy and the ceramic reinforcement. Metals have a useful combination of properties such
as high strength, ductility and high temperature resistance, but sometimes have low stiffness,
whereas ceramics are stiff and strong, though brittle. Aluminium and silicon carbide, for
example, have very different mechanical properties: Young's moduli of 70 and 400 GPa,
coefficients of thermal expansion of 24 × 10−6 and 4 × 10−6/°C, and yield strengths of 35 and
600 MPa, respectively. By combining these materials, e.g. A6061/SiC/17p (T6 condition), an
MMC with a Young's modulus of 96.6 GPa and a yield strength of 510 MPa can be produced
. By carefully controlling the relative amount and distribution of the ingredients of a
composite as well as the processing conditions, these properties can be further improved. The
correlation between tensile strength and indentation behavior in particle reinforced MMCs
manufactured by powder metallurgy technique . The microstructure of SiC reinforced
aluminium alloys produced by molten metal method. It was shown that stability of SiC in the
variety of manufacturing processes available for melt was found to be dependent on the matrix
alloy involved .
Among discontinuous metal matrix composites, stir casting is generally accepted as a
particularly promising route, currently practiced commercially. Its advantages lie in its
simplicity, flexibility and applicability to large quantity production. It is also attractive because,
in principle, it allows a conventional metal processing route to be used, and hence minimizes the
final cost of the product. This liquid metallurgy technique is the most economical of all the
available routes for metal matrix composite production , and allows very large sized
components to be fabricated. The cost of preparing composites material using a casting method
is about one-third to half that of competitive methods, and for high volume production, it is
projected that the cost will fall to one-tenth . In general, the solidification synthesis of metal
matrix composites involves producing a melt of the selected matrix material followed by the
introduction of a reinforcement material into the melt, obtaining a suitable dispersion.
The next step is the solidification of the melt containing suspended dispersoids under selected
conditions to obtain the desired distribution of the dispersed phase in the cast matrix. In
preparing metal matrix composites by the stir casting method, there are several factors that need
Vol.8, No.6 Development of Aluminium Based Silicon Carbide ParticulateMetal Matrix Composite 457
considerable attention, including the difficulty of achieving a uniform distribution of the
reinforcement material, wettability between the two main substances, porosity in the cast metal
matrix composites, and chemical reactions between the reinforcement material and the matrix
alloy. In order to achieve the optimum properties of the metal matrix composite, the distribution
of the reinforcement material in the matrix alloy must be uniform, and the wettability or bonding
between these substances should be optimized. The literature review reveals that the major
problem was to get homogenous dispersion of the ceramic particles by using low cost
conventional equipment for commercial applications. In the present work, a modest attempt have
been made to compare the dispersion of SiC particles in Al matrix fabricated with the help of
different processes viz. (a) without applying stirring process (b) with manual stirring process (c)
a two-step mixing method of stir casting. An effort has been made to establish a relationship
between hardness, impact strength and weight fraction of SiC in particle reinforced MMC’s
developed with the help of two - step mixing method of stir casting technique.
The melting was carried in a tilting oil-fired furnace in a range of 760 ± 100C. A schematic view
of the furnace has been shown in Figure 1.
3. Molten aluminium
5. Particle injection chamber
6. Insulation hard board
8. Graphite crucible
Fig. 1. Schematic view of setup for
Fabrication of composite
458 Manoj Singla, D. Deepak Dwivedi, Lakhvir Singh, Vikas Chawla Vol.8, No.6
In the present study, an oil fired tilting furnace has been used. The crucible material was
graphite. Diesel was used as the fuel. A forced draft fan equipped with 02 H.P 2820-rpm motor
has been used for supplying the required quantity of air.
Scraps of aluminium were preheated up to a temperature of 4500C and particles of silicon
carbide up to a temperature of 11000C in core drying oven. Crucible used for pouring of
composite slurry in the mold was also heated up to 760 0C.
In the present study, a new stir caster was developed to fabricate MMC. It has been used to
obtain an output of 600 rpm . The stir caster was mounted on the furnace with the help of four
legs. Mild steel was chosen as stirrer and impeller material. During experimental work, a four
bladed 450 angled stirrer was chosen. The stirrer position should be such that 35% of material
should be below the stirrer and 65% of material should be above the stirrer .
First of all stirring system has been developed by coupling motor with gearbox and a mild steel
stirrer. All the melting was carried out in a graphite crucible in an oil-fired furnace. Scraps of
aluminium were preheated at 4500C for 3 to 4 hours before melting and mixing the SiC particles
were preheated at 11000C for 1 to 3 hours to make their surfaces oxidized.
The furnace temperature was first raised above the liquidus to melt the alloy scraps completely
and was then cooled down just below the liquidus to keep the slurry in a semi-solid state. At this
stage the preheated SiC particles were added and mixed manually. Manual mixing was used
because it was very difficult to mix using automatic device when the alloy was in a semi-solid
After sufficient manual mixing was done, the composite slurry was reheated to a fully liquid
state and then automatic mechanical mixing was carried out for about 10 minutes at a normal
stirring rate of 600 rpm .
In the final mixing process, the furnace temperature was controlled within 760 ± 100C. Pouring
of the composite slurry has been carried out in the sand mould prepared according to the
specifications for hardness, impact and normalized displacement test specimens as shown in
Figures 2, 3 and 4 respectively.
3.1 Normalized Displacement Test
Indentation was made on hardness testing machine using a 1.587 mm ball indenter and a varying
load was applied for 30 seconds. Five different loads of 60, 100, 150, 187.5 and 250N have been
used. The penetration depth and height of model specimen has been measured by height gauge
coupled with dial indicator. The normalized displacement was calculated from following
formula. The average of four readings has been reported for the results.
Vol.8, No.6 Development of Aluminium Based Silicon Carbide ParticulateMetal Matrix Composite 459
Normalized displacement = Indentation depth
Initial height of model specimen
Fig. 2. Pictorial view of sample containing 5%
SiC by weight for hardness test
Fig. 3. Pictorial view of samples for impact test
Fig. 4. Pictorial view of sample for comparing indentation load with normalized displacement.
4. RESULTS AND DISCUSSION
Experiments have been conducted by varying weight fraction of SiC (5%, 10%, 15%, 20%, 25%,
and 30%). Hardness and impact strength were recorded and tabulated. Hardness test has been
conducted on each specimen using a load of 250 N and a steel ball of diameter 5 mm as indenter.
Diameter of impression made by indenter has been predicted by Brinnel microscope. The
corresponding values of hardness (BHN) were calculated from the standard formula.
460 Manoj Singla, D. Deepak Dwivedi, Lakhvir Singh, Vikas Chawla Vol.8, No.6
Fig. 5. Comparative bar chart (Hardness)
Fig. 6. Comparative bar chart (Impact Strength)
Calculated Load-Displacement response of Sample containing
25% SiC during Indentation (Ball dia-1.587mm)
00.010.02 0.03 0.040.05 0.060.07 0.08
Fig. 7. Normalized displacement Vs Indentation load (For 25 % SiC sample, Ball diameter –
The results as indicated in Figures 5 and 6 show the increasing trend of hardness and impact
strength with increase in weight percentage of SiC up to 25% weight fraction. Beyond this
weight fraction the hardness trend started decreasing as SiC particles interact with each other
leading to clustering of particles and consequently settling down. Eventually the density of SiC
Vol.8, No.6 Development of Aluminium Based Silicon Carbide ParticulateMetal Matrix Composite 461
particles in the melt started decreasing thereby lowering the hardness. The best value of hardness
and toughness comes out to be of sample containing 25% SiC i.e. 45.5 BHN (Hardness) and 36
N-m (Impact Strength). The resultant samples were then examined for their hardness in terms of
normalized displacement at varying loads .The Pictorial view of the sample has been shown in
The trends in the results have been almost similar and a maximum variation of 14.2 % in the
normalized displacement of indenter has been found at 250 N load in the sample containing 25%
SiC. This was illustrated in Figure 7.
Metallographic samples were sectioned from the cylindrical cast bars. A 0.5 % HF solution was
used to etch the samples wherever required. To see the difference in distribution of SiC particles
in the aluminum matrix, microstructure of samples were developed on Inverted type
Metallurgical Microscope (Make: Nikon, Range-X50 to X1500) First sample was prepared
without applying any stirring process and the second sample has been fabricated with the help of
manual stirring. All other samples were developed by using two step mixing method of stir
casting technique by taking varying weight fractions of SiC particles. The various weight
fractions were 5%, 10%, 15%, 20%, 25% and 30% of SiC particles.
Fig. 8. Micrograph of sample obtained without
Fig. 9. Micrograph of sample obtained with
manual stirring (100microns)
462 Manoj Singla, D. Deepak Dwivedi, Lakhvir Singh, Vikas Chawla Vol.8, No.6
Fig. 10. Micrograph of sample containing 5%
SiC by weight (100microns)
Fig. 11. Micrograph of sample containing 15%
SiC by weight (100microns)
Fig. 12. Micrograph of sample containing 25%
SiC by weight (100microns)
Fig. 13. Micrograph of sample containing 30%
SiC by weight (100microns)
4.2.1 Uniform distribution of reinforcement in the matrix
As observed from figure 8, when the composite has been developed without applying stirring
process, particle clustering occurred in some places, and some places were identified without SiC
inclusion. This was due to the fact that when the SiC particles were added into the molten alloys,
they were observed to be floating on the surface, though they have a large specific density than
the molten metals. This was due to high surface tension and poor wetting between the particles
and the melt. In fact, wettability between most ceramic particles and liquid metals has been poor.
A mechanical force can usually be applied to overcome surface tension to improve wettability.
Figure 9 shows the micrograph of composite developed with the help of manual stirring.
However, for the composites, manual stirring in a completely liquid state could not solve the
problem of poor wetting. Manual stirring could indeed mix the particles into the melt, but when
stirring stopped, the particles tended to return to the surface. Most of these particles still stuck to
Vol.8, No.6 Development of Aluminium Based Silicon Carbide ParticulateMetal Matrix Composite 463
one another to remain in clusters. It is not surprising for these clusters to resurface because it
might be argued that pores could exist in them to make them float. However, the fact that single
particles also tended to return to the surface strongly indicates that the particles floated mainly
because of the surface gas layers surrounding them .
The gas layers might be the main factor for the poor wettability . Firstly, gas layers can cause
the buoyant migration of particles, making it difficult to incorporate the particles into the melt.
Secondly, even the particles can be suspended in the melt by vigorous agitation; it has been still
difficult for the particles to be wetted by the molten metals because of the gas layers. The above
analysis leads to the conclusion that it was necessary to break the gas layers in order to achieve
good wettability. Single particles and particle clusters can flow easily in a completely liquid
melt, therefore, no large mechanical forces are actually applied to the particles during agitation,
making it very difficult to break the gas layers simply by stirring in the conventional way . A
two-step mixing method (as described before) was thus tried and was found to be effective.
In the first step, stirring has been carried out in a semi-solid state. In this state, primary α-Al
phase exists, so agitation can apply large forces on the SiC particles through abrasion and
collision between the primary α-Al nuclei and particles. This process can help to break the gas
layers and perhaps oxide layers as well and to spread the liquid metal onto surfaces of the
particles, thus helping to achieve good wettability. It was found that cast composites with upto
25% by weight particles could be obtained using this method. The advantages of using semisolid
slurries have been usually considered to be the increase in the apparent viscosity and the
prevention of the buoyant migration of particles.
In the present study, the breaking of particle– surface gas layers has been emphasized. When the
gas layers were broken and the particles have been wetted, the particles will tend to sink to the
bottom (due to higher specific weight) rather than float to the surface. However, this does not
ensure a uniform particle distribution.
To improve the particle distribution, the second mixing step is needed, i.e., to heat the slurry to a
temperature above the liquidus and then to stir the melts using an automatic device for 10
minutes at 600 rpm.
Figures 10, 11 and 12 depicts micrograph’s of samples containing 5%, 15%, 25% SiC by weight
respectively developed with the help of two-step mixing method of stir casting. It clearly shows
the resulting homogeneous distribution of particles in the samples.
Figure 13 shows micrograph of sample containing 30% SiC by weight. It was understood that
density of SiC particles decreases inspite of an increase in concentration. This may be attributed
to the fact that a this weight fraction, SiC particles greatly interact with each other leading to
clustering of particles and consequently settling down.
464 Manoj Singla, D. Deepak Dwivedi, Lakhvir Singh, Vikas Chawla Vol.8, No.6
As observed from figure 4, the hardness value increases up to 25% weight fraction of SiC and
beyond this weight fraction the hardness trend started decreasing. In the hardness test, severe
plastic flow has been concentrated in the localized region directly below the indentation, outside
of which material still behaves elastically. Directly below the indentation the density of the
particles increased locally, compared to regions away from the depression. This was
schematically shown in figure 13. Although plastic deformation itself has not been responsible
for volume change, the existence of very large hydrostatic pressure under the indentation can
contribute to volumetric contraction of the metal matrix.
As the indenter moves downward during the test, the pressure has been accompanied by non-
uniform matrix flow along with localized increase in particle concentration, which tends to
increase the resistance to deformation. Consequently, the hardness value increases due to local
increase in particle concentration associated with indentation up to 25% weight fraction of SiC.
Beyond this weight fraction the hardness trend started decreasing as SiC particles interact with
each other leading to clustering of particles and consequently settling down. Eventually the
density of SiC particles started decreasing locally thereby lowering the hardness.
4.2.3 Micro structural features in cast composites
Figure 8 predicts that the main micro structural features in cast composites have been in the form
of dendrites. The SiC atoms in the crystal structure of aluminum was in solid solution, and have
been distributed among the aluminum atoms in an atomic dispersion. The composites does not
freeze at a single temperature but instead over a temperature range (temperature at which the
freezing begins was called the liquidius, and the temperature at which freezing was complete is
called the solidus) .
The phenomenon of dendrite structure formation may be due to supercooling effect in which
certain preferred regions protrude as spikes into the supercooled regions and once started, grow
more rapidly then neighboring regions. This had happened because the driving force for freezing
was greater in the super cooled regions and the spikes reject the solute at their sides, thus
delaying freezing of the side regions. These spikes consequently tend to form side arms
producing a dendritic structure.
Figure 9 and 10 shows that clusters of SiC particles in the primary α- Al seemed to be finer. This
can be explained by the fact that SiC particles have a lower thermal conductivity and heat
diffusivity than those of aluminium melt and therefore, SiC particles were unable to cool down
as the melt. As a result, the temperature of the particles was somewhat higher than liquid alloy.
The hotter particles may heat up the liquid in their immediate surroundings, and thus delay
solidification of the surrounding liquid alloy.
Nucleation of α- Al phase starts in the liquid at a distance away from the particles, where the
temperature was lower. The growth of α- Al nuclei lead to enrichment of Si in the melt. The
Vol.8, No.6 Development of Aluminium Based Silicon Carbide ParticulateMetal Matrix Composite 465
enrichment of Si in the melt around the particles leads to heterogeneous nucleation. Another
effect of thermal lag was that the melt around the particles would solidify in the last stage. This
would make the particles located between dendrites. In other words, the interdendritic clusters of
SiC particles have been partly inherited from inhomogeneous distribution of particles in the
original slurries. In commercially pure aluminium (without SiC particles), dendrites were
observed to be distinctively columnar and almost randomly distributed. However in cast
composites, dendrites were found to be equiaxed and in the regions with clusters of SiC particles
in the primary α- Al seemed to be finer.
Figure 11 and 12 shows the extensive growth of the dendrites. During this growth, the freely
suspended SiC particles in the melt could either be entrapped by the dendritic front or pushed
ahead by the front, depending on the velocity of the growing front and geometrical compatability
between the dendrite arm spacing (DAS) and particle sizes. SiC particles were generally
observed to be accumulated in the inderdendritic regions and geometrical trapping by dendrites
was rarely observed. These observations suggest that the SiC particles were always pushed by
dendrite fronts during solidification regardless of the dendritic arm spacing. Pushing of particles
by dendrite fronts could almost certainly occur if they were not entrapped. The existence of SiC
particles can result in instability in the growth front .
Figure 13 predicts the effect of mass feeding in the sample containing 30% SiC by weight. The
equiaxed crystals were nucleated randomly ahead of the solidifying interface, the tendency of
these SiC particles was to sink in the liquid because of the greater density of the solid. The end
result was the decrease in the density of particles locally.
The experimental study reveals following conclusions:
(a) The results of study suggest that with increase in composition of SiC, an increase in hardness,
impact strength and normalized displacement have been observed.
(b) The best results has been obtained at 25% weight fraction of 320 grit size SiC particles.
Maximum Hardness = 45.5 BHN & Maximum Impact Strength = 36 N-m.
(b) Homogenous dispersion of SiC particles in the Al matrix shows an increasing trend in the
samples prepared by without applying stirring process, with manual stirring and with 2-Step
method of stir casting technique respectively.
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