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1. MICRO ELECTRO DISCHARGE MACHINING OF CNT-BASED NANOCOMPOSITE
MATERIALS
Yi Wan, Dave (Dae-Wook) Kim, Jae-Soon Jang
Mechanical Engineering
School of Engineering and Computer Science
Washington State University Vancouver
Vancouver, WA 98686
Young-Bin Park
High-Performance Materials Institute (HPMI)
Department of Industrial and Manufacturing Engineering
Florida A&M University-Florida State University College of Engineering
Tallahassee, FL 32310-6046
ABSTRACT
Since the discovery of carbon nanotubes (CNTs), there has
been great interest in CNT-based composites. Well-developed
micromachining processes are necessary to realize micron-size
CNT-based composite products. Micro electro discharge
machining (micro-EDM) has been applied into many
challenge-to-cut materials such as ceramic composites. In this
study, micro-EDM is used to machine CNT-reinforced polymer
composites in the micro scale. CNT-based polymer composites
were fabricated using solution casting, in which CNTs were
dispersed in the polymer-solvent solution via high energy
sonication, followed by precipitation and hot pressing. The
investigation uses design of experiments (DOE) approach to
screen of influential input factors for process measures. A 2
level fractional factorial design was used with four input
factors; CNT loading on the workpiece, µ-EDM supply
voltage, pulse on-time duration, and pulse on-time ratio. With
16 µ-EDM experiments, supply voltage was found to be most
influential to the material-removal-rate (MRR). Scanning
electron microscope (SEM) was used to investigate
characteristics of the machined CNT-based nanocomposite
surfaces.
INTRODUCTION
Electrical discharge micromachining (micro-EDM) is a
recently developed novel technique for machining materials on
micro- and nano-scales and an ideal process for obtaining burr-
free machined micron-size apertures in many materials [1-4].
The method is based on localizing an electro discharge reaction
in the immediate vicinity of the tool by applying voltage pulses
that are ultra-short, i.e. a few nanoseconds long. Electrical
discharge machining technique is widely applied for machining
of composite materials including Al matrix composites with
SiCp or alumina reinforcements, graphite fiber reinforced
polymer composites, and conductive ceramic composites with
accuracies and finishes down to submicron ranges [5-9]. Due to
its versatility of cutting conductive engineering materials,
electrical discharge machining is also feasible to machine CNT-
based composite materials [10]. In this short technical note, the
experiments with a 2 level fractional factorial design were
conducted to investigate EDM machinability of the CNT-
reinforced composites. Four input factors were chosen for the
experiments; CNT loading on the workpiece, µ-EDM supply
voltage, pulse on-time duration, and pulse on-time ratio. Micro
size holes were created on the CNT-based nanocomposite, and
MRR and hole size were chosen for the output responses.
1 Copyright © 2007 by ASME
Proceedings of IMECE2007
2007 ASME International Mechanical Engineering Congress and Exposition
November 11-15, 2007, Seattle, Washington, USA
IMECE2007-41961
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2. EXPERIMENTAL PROCEDURE
CNT based nanocomposite preparation: Multi-walled
carbon nanotubes (MWCNTs) having a purity rating of 95%
were obtained from Sigma Aldrich (St. Louis, MO). The
MWCNTs had the dimension ranges of 0.5-500 µm in length,
5-10 nm in ID, and 60-100 nm in OD. The polymer matrix used
was molding-grade polymethylmethacrylate (PMMA) (Acrylite
S10/8N) obtained from Cyro Industries (Rockaway, NJ).
PMMA pellets were dissolved in chloroform using a magnetic
stirrer. MWCNTs were added to the PMMA/chloroform
solution (at 1, 10, and 35 wt% with respect to polymer), and the
mixture was sonicated for 2 hours. In this study, the “CNT
loading” is the weight percentage of CNTs with respect to the
composite. A desired CNT loading is achieved by carefully
weighing the CNTs before adding them to the polymer matrix
solution. The uniform PMMA/MWCNT/chloroform solution
was film cast in a Petri dish and was allowed to dry in air for 4
hours at room temperature. Upon solvent evaporation, the
mixture was subsequently dried in a vacuum oven at 60°C for 3
hours. The cast nanocomposites were hot pressed into 127 µm
(0.005 in) thick films using a 10-ton hydraulic Carver press
(Wabash, IN). The films measured 50.8 mm by 76.2 mm (2 in
by 3 in). After the first hot pressing process, the film was
broken into small pieces, put back in the die, and hot pressed
again. This step was repeated twice for each film to enhance
CNT dispersion. The detailed description about sample
preparation procedures and CNT dispersion on the sample was
presented elsewhere [10, 12].
Micro-EDM process: For this investigation, the
Optimation Profile 42 micro-EDM machine was used to
drilling micro holes. Holes were created in the CNT-based
nanocomposite workpiece using a Tungsten tool-electrode with
a diameter of 500 µm. The low conductivity and high
brittleness of the composite property make the direct machining
very challenging, in order to avoid the deflection of the flexible
thin film composite, a 5 mm-thick, flat aluminum carrier was
put under the material as the base of the material. Micro-EDM
experiments were performed according to the input parameters
set by the experimental design as shown in Table 1. These
levels are chosen according to the previous study in [10].
Machining was performed until a complete hole was created in
all cases. Figure 1 shows the system used in this study.
Figure 1. The micro EDM system
Experimental design: Micro-EDMed holes were created on
the CNT based nanocomposite workpieces as determined from
a design of experiments (DOE) approach. Through the
preliminary study, EDM process parameters have been chosen
[10]. Table 1 presents the results of the preliminary study that
show at least 55 V is required to machine the CNT-based
nanocomposite workpiece. With 0 wt% or 1wt% CNT, EDM
was not possible at any machining conditions. The micro EDM
experiments were done according to a 2-level fractional
factorial experimental design. It has four input factors which
varied the CNT weight, input voltage, pulse on-time duration,
and pulse on-time ratio. The factors and levels chosen for the
experiments matrix are shown in Table 2. After each
experiment, the change in volume of the workpiece, and the
machining time were recorded. The value of the material
removal rate was evaluated for each condition by dividing the
measured amount of material removal by the machining time.
Hole size was measured using the optical microscope.
Table 1. Feasibility of machining PMMA/CNT nanocomposites
with various micro-EDM voltages at 2 µsec on-time and 6 µsec
off-time (M: micro-EDMed to create a crater, F: failed to create
a crater)
100 M M M
70 M M M
60 M M M
55 M M M
45 F M M
Micro-EDM
Voltage(V)
40 F F F
10 20 35
MWNT wt%
Hole size measurement: The hole size is determined by the
numerical diameter. Images size of 320*256 pixels were taken
by the Nikon MM-40 Optical Microscopy, an irregular micro
EDM hole was shown in the fig 2. In order to determine the
actual average diameter, these images were imported to
software to calculate pixel number of the holes.
By using the following equation, the diameter can be found
2
2
2
lN
D
⋅=⎟
⎠
⎞
⎜
⎝
⎛
⋅π
where D is the numerical diameter, N is the number of
hole pixel, l is the pixel length.
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3. Figure 2. the hole size measurement.
Table 2. Factors and levels investigated.
Levels
Code Factors
-1 +1
A CNT wt (%) 10 35
B Voltage (V) 55 70
C Pulse on-time duration (µs) 1 4
D Pulse on-time ratio (%) 10 50
RESULTS AND DISCUSSIONS
CNT loading, supply voltage, pulse on-time duration, and
pulse on-time ratio were varied in order to generate predictive
models for micro-EDM machinability parameters. Since the
present work was performed as a preliminary study of micro-
EDM machinability for the CNT-based nanocomposite, a one-
half fractional factorial design was carried out to obtain
information on the effects of the investigated factors. The data
from the sixteen runs were subsequently used in assessing the
main effects and interactions using analysis of variance
(ANOVA).
Material removal rate (MRR): MRR defines the amount of
workpiece material removed in a given machining time and it is
a good measure for process productivity in micro-EDM. Table
3 shows the ANOVA for MRR. Figure 3 shows the main effects
vs. MRR.
Table 3. ANOVA for MRR
Effect
Sum of
Square
d.f
.
Mean
Square
F-
Value
A 0.005379 1 0.005379 16.1
B 0.056039 1 0.056039 167.3
C 0.001989 1 0.001989 5.9
D 0.002985 1 0.002985 8.9
AB 0.006673 1 0.006673 19.9
AC 0.004024 1 0.004024 12.0
AD 0.002362 1 0.002362 7.1
Pure Error 0.002679 8 0.000335
For main factor effects, all input parameters positively
affect the MRR. Supply voltage (B) is the most influential input
factor for MRR, while pulse on-time (C) is the least, as shown
in Table 3 and Figure 3. MRR increased with increasing supply
voltage. Increasing the input voltage results in an increase in
the power of the individual sparks. More powerful sparks will
intuitively remove more material. CNT loading (A) is the
second most influential input factor. As the CNT loading
increases on the material system, the MRR increases. This is
because the resistivities of the CNT based nanocomposites
decreased with increasing CNT loading on the material system
[SAMPE]. Both pulse on-time duration (C) and pulse on-time
ratio (D) positively affect the MRR but these effects are not one
of the most important in relation to the output response. The
product of voltage and CNT loading shows the highest effect
on MRR among interactions of input factors.
Figure 3. Trend of the main effects for the MRR. (Error
bars indicate the standard deviations of each data point.)
Hole size: Micro-EDMed hole sizes were measured to
investigate the effects of micro-EDM process parameters and
CNT loading on dimensional accuracy. Table 4 shows the
ANOVA for hole size. Measured hole sizes were not varied
with different input parameters. This causes increasing the
noise and decreasing F-Values, as ratio of the mean square with
respect to the variance of the experimental errors. For the main
effects, pulse on-time ratio (D) is the most influential input
factor for hole size. CNT loading (A) is the least influential
input factor, so hole size is mostly dependent on micro-EDM
process parameters. Interaction effects were not very
significant for hole size.
Table 4. ANOVA for hole size
Effect
Sum of
Square
d.f
.
Mean
Square
F-Value
A 13.74703 1 13.74703 0.04
B 103.2255 1 103.2255 0.32
C 662.6129 1 662.6129 2.08
D 782.7636 1 782.7636 2.46
AB 30.81027 1 30.81027 0.096
AC 4.432012 1 4.432012 0.014
AD 8.400719 1 8.400719 0.026
Pure Error 2549.757 8 318.7197
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4. Figure 4 shows the main effects vs. hole size. Average hole
size is close to 576 µm which is approximately 15% larger than
the electrode diameter (500 µm). In the EDM process, micron-
size gaps are always found between a workpiece and a tool
electrode, which provide a room for discharge to take place. All
input parameters except pulse on-time ratio (D) negatively
affect the hole size. Input parameters such as CNT loading (A),
supply voltage (B), and pulse on-time ratio (D) are most
influential factors on the MRR and they positively affect the
MRR. Smaller holes were produced when the MRR became
higher or the EDM process took longer. It can be concluded
that longer micro-EDM electrode engagement enlarged the hole
sizes. This may be due to the secondary discharge sparking that
occurred in the gap in the low voltage configuration. At lower
voltage, the gap between the electrode and workpiece becomes
smaller to generate the spark. Due to the small gap size and low
debris removal rate, the debris is accumulated in the gap, which
results in the second discharge sparking and widening of the
drilled hole [11].
Figure 4. Trend of the main effects for hole sizes.
SEM observations: Figure 5 shows the SEM images of a micro-
EDMed hole fabricated in the CNT based nanocomposite
sample with 35% CNT loading. Micro-EDMed nanocomposite
surface showed a typical EDMed surface which revealed a
recast layer and heat affected zone (HAZ). As shown in Figure
3 (b), solid spheres or debris, ranged from several nm to 3 µm
in diameter, were formed by electrical discharge during the
process and left on the workpiece surface. In addition,
accumulation of small debris is found on the machined surface.
Due to polymer’s low solidification temperature, a small
amount of molten material is resolidified during the pulse
offtime or cooling period and left onto the surface. In Figure 3
(c), CNTs were found on the machined surface at a high
magnification. At the micro scale, the quality of the product
and process is important. On going research topics include
Further research is focus on the investigation the quality feature
such as hole roundness, hole surface defects, and heat affected
zone.
(a)
(b) (c)
Figure 5. SEM images of micro-EDMed hole surface.
(sample with 35 wt% CNT loading, Micro-EDM process
parameters: voltage = 70 V, pulse on-time duration = 1 µsec,
pulse on-time ratio = 10 %)
CONCLUSIONS
Experimental investigation on the effect of micro-EDM
machining conditions on machining performance was
conducted on CNT based nanocomposite materials. A two-
level fractional factorial design was planned to study the effect
of four input factors: CNT loading on the workpiece, µ-EDM
supply voltage, pulse on-time duration, and pulse on-time ratio
were taken in consideration in the experimental tests. Based on
the results the following conclusions were made:
• All input parameters positively affect the MRR.
Micro-EDM supply voltage is the most influential input
factor for MRR. Also, the MRR increases with the increase
of MWNT loading as conductivity increases. The pulse on-
time duration is the least influential input factor for MRR.
• Hole sizes were mainly dependent on the micro-EDM
process parameters. Hole sizes increase when input
parameters result lower MRR or longer process time. This
may be due to the secondary discharge sparking.
• The morphology of the micro-EDMed surfaces
revealed that the polymer matrix underwent the liquefying
solidification cycle due to the heat generated during the
EDM process. CNTs were exposed to the machined
surface.
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5. • Further research is required in investigating the
material removal mechanisms and machined feature
quality of CNT based nanocomposites in the micro-EDM
process.
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