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Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.1, pp.79-88, 2010
jmmce.org Printed in the USA. All rights reserved
EDM Studies on Aluminum Alloy-Silicon Carbide Composites Developed by Vortex
Technique and Pressure Die Casting
M. Kathiresan and T. Sornakumar*
Department of Mechanical Engineering, Thiagarajar College of Engineering,
* Corresponding Author: firstname.lastname@example.org
Aluminum based metal matrix composites (MMCs) offer potential for advanced structural
applications when high specific strength and modulus, as well as good elevated temperature
resistance, are important. In the present work, aluminum alloy-silicon carbide composites
were developed using a new combination of vortex method and pressure die casting
technique. Electrical Discharge Machining (EDM) studies were conducted on the aluminum
alloy-silicon carbide composite work piece using a copper electrode in an Electrical
Discharge Machine. The Material Removal Rate (MRR) and surface roughness of the work
piece increases with an increase in the current. The MRR decreases with increase in the
percent weight of silicon carbide. The surface finish of the machined work piece improves
with percent weight of silicon carbide.
Keywords: Aluminum Alloy, Silicon carbide, Die-casting, Electric Discharge Machining,
Discontinuously reinforced metal matrix composites (MMCs) have proven to be promising as
hi-tech structural and general engineering materials because of their higher specific modulus,
strength, thermal stability, and improved tribological properties over their conventional
monolithic counterparts. Aluminum based matrix composites with ceramic particle
reinforcements combine the high ductility and toughness of aluminum with the high modulus
and tensile strength ceramics. These MMCs are primarily particulate reinforced aluminum
alloys; for cast products, the composite is typically an aluminum casting alloy reinforced with
SiC or Al2O3 . The aluminum alloy reinforced with discontinuous ceramic reinforcements
80 M. Kathiresan and T. Sornakumar Vol.9, No.1
is rapidly replacing conventional materials in various automotive, aerospace and automobile
Electrical discharge machining (EDM) is a non-traditional manufacturing process where the
material is removed by a succession of electrical discharges, which occur between the
electrode and the work piece. These are submersed in a dielectric liquid such as kerosene or
deionised water. The electrical discharge machining process is widely used in the aerospace,
automobile and moulds industries to machine hard metals and its alloys. During the electrical
discharge, a discharge channel is created where the temperature reaches approximately
12,000 0C, removing material by evaporation and melting from both the electrode and the
work piece. When the discharge ceases there is a high cooling on the surface of the work
piece creating a zone affected by the heat that contains the white layer. Electrical discharge
machining is governed by a thermal phenomenon therefore not only removes material from
the work piece but also changes the metallurgical constituents in the zone affected by the heat
Metal matrix composites (MMCs) contain a certain amount of secondary hard and abrasive
reinforcements to give high strength, hardness and stiffness. However the machining of
MMCs using conventional tool materials is very difficult due to the presence of the abrasive
reinforcing phases, which cause severe tool wear. The greater the reinforcement in a
composite the faster is the tool wear. Thus non-traditional machining like electric discharge
machining (EDM) can be used to perform the precision machining of MMCs . Metal
matrix composites (MMCs) are well known for their superior mechanical properties over un-
reinforced alloys . These composite material s are composed of a metallic base material called
matrix, which is reinforced with ceramic fiber, whisker or particulates that impart a
combination of properties not achievable in either of the constituents individually. A full
scale application of these advanced materials however has been hindered due to their high
cost of machining. They can be machined with either electroplated diamond-grinding wheel
or with carbide / poly crystalline diamond cutting tools. In view of difficulties encountered
e.g. high tool wear and high tooling cost, during conventional machining, non-contact
material removal processes such as the electric discharge machining (EDM) offer an effective
Lloyd (1994) reported that vortex mixing technique was suitable for the preparation of
ceramic particle dispersed aluminum composite . The vortex mixing technique for the
preparation of ceramic particle dispersed aluminum matrix composites was developed by
Surappa and Rohatgi . The stir casting involves incorporation of ceramic particulate into
liquid aluminum melt and allowing the mixture to solidify. Here, the crucial thing is to create
good wetting between the particulate reinforcement and the liquid aluminum alloy melt. The
simplest and most commercially used technique is known as vortex technique or stir casting
technique. The vortex technique involves the introduction of pre-treated ceramic particles
into the vortex of molten alloy created by the rotating impeller . Bronze–alumina
composite was developed using stir casting method .
Vol.9, No.1 EDM Studies on Aluminum Alloy-Silicon Carbide Composites 81
The development of the pressure die casting process should have a priority over other metal
casting technologies since it ensures the production of thin walled cast articles of a
complicated shape with a high yield and high dimensional precision. Such products
effectively do not need further mechanical treatment. A distinctive feature of the process is
structural inhomogeneity of metal due to the formation of a fine grained layer 0.3–0.8 mm
thick on the surface of die castings. It is formed during the near instantaneous (0.01–0.05 s)
solidification in a water cooled die under high pressure . Pressure die casting is a process
ideally suited to manufacture mass produced metallic parts of complex shapes requiring
precise dimensions. In this process, molten metal is forced into a cold empty cavity of a
desired shape and is then allowed to solidify under a high holding pressure. The entire cycle
can be divided into three stages, the first or slow speed stage, the second or high speed stage
and the third or intensification stage, where hydraulic pressure is exerted to avoid shrinkage
or gas problems. The high pressure die comprises two basic parts, the fixed half and the
moving or ejector half. When the die is opened, the casting is retained in the moving half,
from which it is ejected by pins activated either hydraulically or mechanically .
The attractiveness of die casting is its ability to make near net shape parts with tight
tolerances and requiring little or no machining. The more applied of the different die casting
processes is high pressure die casting (HPDC) with high rates of production. The automotive
industry uses an extensive range of aluminum HPDC parts including transmission housings,
cylinder heads, inlet manifolds, engine sumps as well as decorative trim. This trend is
increasing as replacement of steel parts with lighter aluminum HPDC parts grows
(Tharumarajah, 2008) .
2. EXPERIMENTAL PROCEDURE
In the present work, aluminum alloy and different weight compositions of silicon carbide
were die casted, using LM24 aluminum alloy as the matrix material and silicon carbide
particles of average particle size of 16 microns as a reinforcement material. The aluminum
alloy was melted in a graphite crucible at a controlled temperature protected with an argon
gas atmosphere. The graphite stirrer was introduced into the crucible to perform mixing
process when the molten temperature reached 850 0C. The stirring was carried out for 45
minutes at the rate of 200 rpm. Silicon carbide particles were preheated to 200 0C and
introduced into the vortex created in the molten alloy. The internal surface of the die was
applied with a water based die coat before each casting which acts as a lubricant between the
molten metal and die, and also prevents the adhesion between the die cast metal and die. A
420 ton cold chamber hydraulic type die casting machine was used for making the castings.
The pouring temperature of molten mixture was 850 0C and molten metal was injected into
the runner of the closed die with the initial velocity of 0.23 m/sec up to runner gate. Then the
ram movement is given with 1.8 m/sec for injection and simultaneously shot in the die. The
molten mixture was poured into the plunger sleeve and forced into the die cavity with
pressure of 100 MPa. The shot accumulation force of 420 tons is applied at the end of
injection and the die is simultaneously cooled with demineralized water. Then the MMC was
82 M. Kathiresan and T. Sornakumar Vol.9, No.1
ejected from the die at a temperature of 150 0C and it is allowed to cool in air. The hardness
of the specimen was determined using a Brinell hardness testing machine. The actual density
of the specimen was determined using the Archimedes’ principle.
2.1. EDM Experiments
The EDM studies were conducted in an electrical discharge machine. The rectangular slots
were machined. The tool electrode material used is copper. The pulse current used is 1.5, 3
and 4.5 A. The pulse-on duration is 200 µs and the pulse-off duration is 30 µs. The voltage
used is 80V dc straight polarity. The dielectric used is commercial grade EDM oil. The
flushing pressure is 1.5 kg/cm2. The material removed was measured using an electronic
balance and the material removal rate is calculated. The surface roughness of the machined
surface was observed using a stylus type surface roughness tester.
3. RESULTS AND DISCUSSION
The microstructure of the plain LM24 aluminum alloy is presented in Figure 1. The
microstructure shows the interdendritic particles of euctectic silicon and CuAl2 in a matrix of
aluminum solid solution. The X-ray diffraction pattern of the plain LM24 aluminum alloy is
given in Figure 2. The hardness and density of the plain LM24 aluminum alloy and the
aluminum alloy-silicon carbide composites are given in Table 1. The hardness of the
aluminum alloy-silicon carbide composite increases with amount of silicon carbide
reinforcement and is higher than the plain LM24 aluminum alloy due to particulate hardening
and higher hardness of the silicon carbide. The density of the aluminum alloy-silicon carbide
composite increases with amount of silicon carbide reinforcement and is higher than the plain
LM24 aluminum alloy due to higher density of silicon carbide.
Figure 1. The microstructure of the plain LM24 aluminum alloy.
Vol.9, No.1 EDM Studies on Aluminum Alloy-Silicon Carbide Composites 83
10 2030 40 50 6070 80
2 Theta, Degrees
Figure 2. XRD pattern of the plain LM 24 aluminum alloy.
Table 1. The hardness and density of plain LM24 aluminum alloy and the aluminum alloy-
silicon carbide composites.
Material Hardness, BHN Density, g/cc
LM24 96 2.790
LM24 + 1 % wt SiC 104 2.794
LM24 + 3 % wt SiC 107 2.803
LM24 + 5 % wt SiC 110 2.812
3.1. Effect of Current and Particle Reinforcement on Material Removal Rate
The variation of material removal rate (MRR) with current is presented in Figure 3. The
increase in MRR with the increase in discharge current is due to the fact that the spark
discharge energy is increased to facilitate the action of melting and vaporization, and
advancing the large impulsive force in the spark gap, there by increasing the MRR. Higher
current results in a higher thermal loading on both the cathode and anode, followed by a
higher amount of material being ejected. This results in a larger crater size and thus the
surface finish becomes rougher.
84 M. Kathiresan and T. Sornakumar Vol.9, No.1
Material removal rate, mg/min.
LM24 LM24 + 1%SiC
LM24 + 3%SiCLM24 + 5%SiC
Figure 3. Material removal rate vs Current.
The material removal rate for the aluminum alloy–silicon carbide composite is lower than the
plain aluminum alloy. This is attributed to a number of factors. The electrical conductivity of
the aluminum matrix decreases due to the presence of the ceramic reinforcement. Also,
because of the low thermal conductivity, and the much higher thermal resistance of the
silicon carbide, the aluminum alloy between the ceramic particles is preferentially removed.
The material removal of the aluminum alloy–silicon carbide composite occurs through the
process of melting and vaporizing the matrix material around the ceramic particle and at
some point the entire silicon carbide particle becomes detached. The material removal rate for
the aluminum alloy-silicon carbide material is lower as the silicon carbide particle
reinforcement increases, due to the more shielding effect of the silicon carbide ceramic
3.2 Effect of Current and Particle Reinforcement on Surface Roughness
The surface roughness and surface integrity are important performance measures which
indicate the performance level attained by using the particular work material-tool material
combination and the corresponding voltage, current, pulse-on duration and the pulse-off
duration. The importance of considering surface roughness and surface integrity when
machining aluminum metal matrix composites arises due to the high thermal conductivity and
low melting temperatures. The effect of percentage weight silicon carbide on surface
roughness is presented in Figure 4. High current results in higher thermal loading on both
electrodes (tool and work piece) followed by higher amount of material being removed from
both electrodes and hence lead to high material removal rate and tool wear rate. This would
also result in high crater size and hence rougher the surface.
Vol.9, No.1 EDM Studies on Aluminum Alloy-Silicon Carbide Composites 85
% Weight SiC
Surface roughnes, Micron
Current, 1.5ACurrent, 3ACurrent, 4.5A
Figure 4. Surface roughness Ra vs Percentage weight silicon carbide.
The surfaces obtained for the plain aluminum alloy are rougher than those observed for the
aluminum–silicon carbide composite materials machined under the same conditions. The
silicon carbide particles shield and protect the aluminum matrix from being removed. This
results in less material becoming superheated or molten during the discharge phase.
3.3. Optical Microscopy
The optical micrograph of the EDMed surface of the plain LM24 aluminum alloy work piece
is presented in Figure 5, 6 and 7. As the pulse current is increased, the deeper craters were
most evident and rougher surfaces were more pronounced. This is due to the fact that when
pulse current is increased, more intensely discharges strike the surfaces, a great quantity of
molten and floating metal suspended in the electrical discharge gap during EDM and
resulting in deterioration of the surface roughness.
86 M. Kathiresan and T. Sornakumar Vol.9, No.1
Figure 5. The EDMed surface of plain LM24 aluminum alloy at current of 1.5A.
Figure 6. The EDMed surface of plain LM24 aluminum alloy at current of 3A.
Vol.9, No.1 EDM Studies on Aluminum Alloy-Silicon Carbide Composites 87
Figure 7. The EDMed surface of plain LM24 aluminum alloy at current of 4.5A.
Aluminum alloy-silicon carbide composites were developed using vortex method and
pressure die casting technique. The EDM studies showed that the MRR and the surface
roughness are greatly influenced by the current and percent weight silicon carbide. The MRR
increases with an i ncrease in the current and decrease in th e percent weight of s ilico n carbide.
The surface finish improves with decrease in the current and increase in the percent weight of
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