Drilling of CFRP by EDM (1)

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DRILLINGOFCARBON/EPOXYCOMPOSITESBYELECTRICALDISCHARGEMACHININGCONFERENCEPAPERNOVEMBER2014DOI:10.13140/2.1.2515.4244

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The 1st

International Conference on Industrial, Systems

and Manufacturing Engineering (ISME14) -----------------------------------------------------------------------

DRILLING OF CARBON/EPOXY COMPOSITES BY ELECTRICAL DISCHARGE

MACHINING

Jamal Y. Sheikh-Ahmad, [email protected]. Mechanical Engineering Department, The Petroleum

Institute, Abu Dhabi, UAE

Sandeep R. Shinde, [email protected]. Department of Industrial & Manufacturing Engineering,

Wichita State University, Wichita, Kansas, USA

ABSTRACT

This study investigates the feasibility of electrical discharge machining process (EDM) for drilling carbon fiber

reinforced composites (CFRP). EDM drilling of CFRP was conducted on a small hole machining system using

dionized water as the dielectric fluid. Copper and graphite rods were used as the electrode materials. The effect of

gap current, pulse-on time and electrode material on material removal rate, tool wear and delamination was

investigated. It was found that the highest material removal rates were obtained with graphite electrode at the

highest current and highest pulse-on time (highest energy input). On the other hand, these conditions also

corresponded to the highest electrode wear rate and most delamination damage. Because if this, an optimum set of

conditions for EDM drilling could not be found within the ranges of parameters tested. However, due to the

interaction between process parameters it is found that intermediate levels of pulse-on time cause a slight reduction

in delamination damage.

Keywords: EDM, CFRP, Drilling, Material removal rate, Delamination, Wear

1. INTRODUCTION

Drilling of carbon fiber reinforced polymer composites (CFRPs) is an important secondary machining process in the

aerospace industry and it almost accounts for 50% of the total cost of machining composites. This is due to the fact

that even a smaller private jet has up to 250,000 to 400,000 holes and a bomber or a transport aircraft has 1,000,000

to 2,000,000 holes [1]. CFRPs are difficult to machine by conventional processes because of their inhomogeneous

structure. The defects observed in drilling these materials include hole taper, hole roundness error, surface

roughness, splintering, fiber pullout, matrix cracking and delamination. The existence of these defects poses a threat

to aircraft damage tolerance, survivability and reliability. Therefore there is a need for better machining processes

that can reduce or eliminate machining damage.

Electrical discharge machining (EDM) is a non-traditional machining process typically used to cut hard-to-machine

materials with high accuracy [2]. This is due to the fact that the workpiece is never in contact with the tool and

hence there are no contact forces acting between the tool and the workpiece. Machining is done by the effect of

spark erosion which is generated in a gap between an electrode and the workpiece material. The gap, and sparking

action, are stabilized by a servo control mechanism. A few studies in the literature have addressed the feasibility of

EDM of CFRPs. These studies confirmed the feasibility of this process and pointed out potential problems arising

from its application, specifically thermal damage to the workpiece.

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Strong et al. [3] performed EDM drilling of AS4/3501-6 CFRP using a graphite electrode at different levels of gap

voltage, pulse-on time and pulse-off time. Material removal rate (MRR) was evaluated as a measure of

machinability. It was reported that MRR is highest at highest energy levels. However, the size of heat affected zone

was twice as large as the hole diameter. Guu et al. [4] performed drilling of plain weave carbon/phenolic composite

with copper electrode. Delamination damage and surface roughness were evaluated. It was pointed out that

delamination has its highest levels at highest current setting. The experiments carried out by George et al. [5] and

Lau et al. [6] addressed the issue of electrode wear and the effects of current and polarity. Wang et al. [7]

investigated the effect of carbon fiber direction on material removal rate, electrode wear and surface roughness when

copper and graphite electrodes were used. This study was further clarified by Habib et al. [8] who concluded that the

material removal mechanism depends on the direction of the carbon fibers.

The intent of this study is to investigate the use of electrical discharge machining for machining small holes in

aircraft carbon fiber composites and to analyze the effect of process parameters like pulse-on time, interval time, gap

current and tool material on tool wear rate, material removal rate and hole quality.

2. MATERIALS AND METHODS

2.1 Workpiece

The workpiece material used in this work was a rectangular multidirectional CFRP coupon measuring

46mmx23.5mmx1.8mm. The coupons were cut with a diamond saw from a laminate with the stacking sequence

[(PW/XW)2PW]s, where PW represents a single ply with the fiber directions aligned coincident with the coupon

axes and XW is a single ply aligned at 45o with the coupon axes. A layer of nonconductive epoxy resin was

removed from the top surface of the coupon with 220 grit sand paper to expose the carbon fibers and allow for

current to flow in the workpiece. The resistivity of the sanded surface ranged for 2.5 to 4.5 cm. Figure 1 shows an

optical image of the coupon with top surface shown after sanding.

Figure 1. Top surface of CFRP sample.

2.2 EDM Electrodes

Copper and graphite were used as electrode materials in this experiment. The electrodes were cylindrical in shape

with a diameter of 6mm, which is the nominal hole diameter. The electrodes were received in long rods, which were

cut to proper size with a diamond saw. The ends of the electrodes were squared by face grinding. Sets of four

electrodes of each graphite and copper were used for the machining. The end of each electrode was ground flat after

EDM machining of one hole so that it can be reused for further experiments.

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2.3 Machine Setup

The machine used for this experiment was a 5-axes RAYCON small hole EDM system. The system consisted of an

SH10 small hole EDM generator to provide the necessary DC power cycle for machining, Fanuc GN 6 series

numeric controller to control the machine axes, Barnstead 210 distilled water generator to produce deionized water

for the dielectric medium and a Lauda RE106 series refrigerating circulator of 16-liter capacity to circulate and cool

the dielectric medium throughout the system. Figure 2 shows a schematic of the EDM system and machining setup.

Initial setup of the SH10 generator consisted of adjusting pulse-on time, interval time, gap voltage and DC current.

The pulse-on time was adjusted to produce various degrees of surfaces finish as well as to achieve regular and

efficient machining conditions. The interval time was adjusted to provide maximum efficiency with stable

machining. Interval time also provided a little time between arcs to remove the debris from the machined hole.

The electrode was attached to the Z-axis of the machine and the gap between the workpiece and electrode was

controlled by a servomechanism, which maintained the required gap by comparing reference or set voltage value

and online voltage value. Spark erosion took place during the pulse-on time, which removed the material from the

workpiece and electrode. During pulse-off time the dielectric fluid cooled the electrode and workpiece and flushed

away debris from the gap. In this experiment the electrode was attached to the positive pole of the power generator

and the target gap voltage was set at 65V.

Figure 2. Schematic of the EDM machining setup.

2.4 Response Measurement

The responses measured in each experiment were material removal rate (MRR), tool wear rate (TWR) and

delamination factor (DF). Material removed was calculated by weighing the workpiece before and after machining.

An ACCULAB L-series weighing scale is used to measure the weight in grams. The difference in the weight of

workpiece before and after the machining is divided by machining time to give material removal rate (MRR) in

grams/min.

(1)

The machining time was recorded by using a stopwatch.

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A similar procedure was used to calculate the tool wear rate (TWR). The electrodes are weighed before and after

machining and the difference was used to give electrode wear in grams. This difference in weight was divided by the

machining time to give the electrode wear rate in grams/min. The graphite electrodes were dried for sufficient time

before weighing them. This is done to avoid any misinterpretation in weighing the graphite electrodes, as they

tended to absorb small amounts of dielectric fluid during machining

Delamination factor gives the extent of hole edge damage and it is defined by

d

dDF max (2)

where, DF is delamination factor, dmax is maximum damaged diameter d is the hole diameter as shown in the Figure

3.

2.5 Parametric Study

Factors that were taken into consideration while designing the experiments were pulse-on time, pulse-off time, gap

current, dielectric temperature, dielectric flow rate, gap between electrode and work piece, and electrode material.

An initial screening experiment determined that gap current and pulse-on time were the two most significant

parameters affecting machining performance. Further experiments were conducted only with these two parameters

and electrode material as a categorical factor.

A face centered central composite design was selected to run at three levels of gap current and pulse-on time and

two levels of categorical factor electrode material. A central composite design commences with a factorial or

fraction factorial design with center points and added axial runs also called as star runs. Addition of star runs

along with center points allows estimation of curvature in the model. This is due to that fact that addition of axial

points basically includes the quadratic terms in the model [9]. This design produced a set of 26 runs with 5

repetitions of the center point. The entire experimental design was repeated twice to ensure repeatability of the

results. Experiments were conducted in random order to eliminate the effects of unknown factors. Pulse-off time is

the time between two sparks, also called as interval time was held constant at 100sec. Gap between electrode and

work piece was maintained constant by servomechanism and the gap voltage was maintained at 65V. Table 1 shows

the different parameters of the experiment and their levels.

Table 1. Experimental parameters and their levels

Parameter Values

Gap Current 0.4, 1.2, 2.0 A

Gap Voltage 65 V

Pulse-on Time 20, 105, 190 sec

Pulse-off Time 100 sec

Electrode material Copper, Graphite

Dielectric Temperature 25 oC

Dielectric Flow Rate 17 l/min

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3. RESULTS AND DISCUSSION

3.1 Hole Characteristics

It was observed during EDM machining of CFRP that material is removed in solid and gaseous states. Carbon fibers

are electrically conductive but the bonding epoxy resin is not electrically conductive. Therefore, carbon fibers were

removed by spark erosion while bonding material was decomposed due to extreme high temperature and presence of

high intensity sparks during machining. Eroded carbon fibers were removed by flushing with the dielectric fluid in

the gap between electrode and workpiece whereas, decomposition of resin layers caused release of gases in form of

intermediate puffs.

The appearance of the machined hole was studied for presence of delamination, fiber pullout and surface finish.

Figure 3 shows an optical image of the entry (top) and exit (bottom) of the machined hole. It can be seen from these

images and other similar images of other pieces that entry and exit of the hole do not show any fiber pullout. Also,

the walls of the hole were clean and no fiber pullout was observed on the cross-section of the hole. However,

significant delamination was observed on the hole entry (top) which resulted in the loss of one or more of the top

plies as shown in Figure 3. Very little or no delamination was observed at hole exit (bottom).

Figure 3. Optical images of the EDMed hole top surface (left) and bottom surface (right).

Delamination on top surface of the workpiece was observed due to high localized temperature and high spark

intensity. During machining, base surface of the workpiece was not exposed to these high intensity sparks. Also,

temperature at the base surface was very low as compared to the top surface. Delamination on base surface was

hence very minute and in some cases not observed. Quality of the machined hole was quantified by delamination

factor as defined by equation (2).

3.2 Electrode Wear

In EDM machining material removal takes place due to spark erosion of both workpiece and electrode materials.

During each spark the electrode is bombarded with either electrons or positive ions depending upon electrode

polarity. This bombardment causes localized heat generation at the surface of the electrode which causes localized

dmax

d

Delamination

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melting and/or vaporization of the electrode material. This material removal, which occurs at sparking surface of the

electrode, is called as electrode wear.

Figure 4 shows before and after machining optical images of a graphite electrode. It can be seen from these pictures

that the electrode shows higher amounts of wear on the corner of the circular electrode. This is observed because of

the higher concentration of sparks at the corner. The number of sparks required to produce the 3D shape or cavity in

a workpiece is higher when compared to number of sparks require to produce a flat surface. Since each spark

removes some material from electrode, edge of the circular electrode shows more wear than end of the electrode. In

EDM, low electrode wear is desired. As the machined surface is an exact replica of the electrode, excessive

electrode wear results in poor dimensional accuracy of the hole and poor surface finish.

Figure 4. Optical images of graphite electrode (a) side view of new electrode, (b) end view of worn electrode, (c)

side view of worn electrode

3.3 Analysis of Variance (ANOVA)

ANOVA of the experimental results was performed using Design-Expert software. Face centered central composite

design was used with two numerical factors and one categorical factor. These are namely gap current, pulse-on time

and type of electrode, which are noted as A, B, and C respectively in terms of coded factors.

The results of ANOVA showed that all three factors significantly affect the outcomes of MRR and TWR whereas

delamination factor was mainly affected by current and pulse-on time. Second order polynomial was fitted to the

data and the resulting models are shown in Table 2. The table also shows the goodness of fit in terms of adjusted R2.

It can be seen that the models for MRR and TWR are of good fit while the model for DF is of moderate fit. The

model equations obtained from ANOVA were used to generate graphs that show the nature of interaction between

all the process parameters. The experimental results were also plotted on these graphs to understand effect of

current, pulse-on time and tool material on all the three responses.

Table 2. Regression models for responses (A is current and B is pulse-on time).

Electrode Model R2 (adj)

Graphite MRR = 2.712E-03 + 0.011 A + 2.938E-05 B - 1.926E-07 B2 + 6.265E-05 AB 0.983

Copper MRR = - 8.264E-04 + 0.013 A + 4.723E-05 B - 1.923E-07 B2 + 1.528E-05 AB 0.983

Graphite TWR = 4.762E-04 + 4.492E-03 A - 3.263E-06 B - 8.480E-04 A2 + 9.851E-06 AB 0.925

Copper TWR = 1.417E-03 + 5.263E-04 A - 1.241E-05 B + 8.248E-04 A2 + 9.851E-06 AB 0.925

Graphite &

Copper

DF = 1.16474 + 0.072151 A - 6.84265E-004 B + 3.83894E-006 B2

0.780

(a) (b) (c)

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Material removal rate (MRR): Figure 5 shows the effects of current and pulse-on time on MRR for copper and

graphite electrodes. These graphs are plotted for low, medium and high levels of pulse-on time. It can be concluded

form these plots that both electrodes show high MRR at high settings of current and low MRR at low settings of

current. It is also interesting to notice that at low current settings there is no significant increase in the MRR for

corresponding increase in pulse-on time for both copper and graphite electrodes. As current setting increases, any

increase in pulse-on time shows significant increase in MRR for both types of electrodes. This is evidenced by the

presence of the interaction term AB in the regression models for MRR. It can also be inferred from the plots that as

compared to copper electrodes, graphite electrode shows higher MRR and are considerably affected by pulse-on

time at higher current settings.

Figure 5. Effect of current on material removal rate.

Delamination factor: Figure 6 shows the effect of current and pulse-on time on delamination factor for copper and

graphite electrodes. It is clear from these graphs that, for copper as well as for graphite, an increase in the gap

current results in a proportional increase in delamination factor. The effect of pulse-on time on delamination factor

is not significant. DF appears to be slightly higher for low and high pulse-on time and slightly lower for medium

pulse-on time. The highest DF is caused by the highest current and highest pulse-on time (highest discharge energy

level) and the lowest delamination is caused by the lowest current and the intermediate pulse-on time. The graphs

also show that different current and pulse-on time settings give almost same amount of delamination factor for

copper and graphite electrodes. This means that DF is independent of electrode material and the same regression

model applies for both electrodes.

Tool wear rate (TWR): Figure 7 shows the effects of current and pulse-on time on tool wear rate for copper and

graphite electrodes. It is clearly seen from this Figure that the response of TWR to process parameters is different

for copper and graphite electrodes. The differences are in concavity of curvature of the curves and the interaction

between current and pulse-on time. Furthermore, wear rates for the copper electrodes are generally lower than those

for graphite. For copper electrodes TWR increases with an increase in the current with an upward concavity. For

low level of current, an increase in pulse-on time corresponds to a decrease in TWR. This trend is inverted for the

highest current setting. The effect of pulse-on time on TWR at intermediate current setting is not significant.

Current (Amp)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

MR

R (

g/m

in)

0.00

0.01

0.02

0.03

0.04

0.05

20 sec Model

105 sec Model

190 sec Model

20 sec Exp

105 sec Exp

190 sec Exp

Current (Amp)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

MR

R (

g/m

in)

0.00

0.01

0.02

0.03

0.04

0.05

20 sec Model

105 sec Model

190 sec Model

20 sec Exp

105 sec Exp

190 sec Exp

Copper Electrode Graphite Electrode

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The graphite electrode TWR also increases with an increase in current. However, the concavity of the curve is down

and which suggests that there exists an optimum value (outside the range of parameters of these experiments) for

TWR after which any increase in current will result in decrease of the TWR. The effect of pulse-on time on TWR is

not significant for the lowest current setting, and it is the highest for the highest current setting. At the intermediate

and high current settings an increase in pulse-on time leads to an increase in TWR.

Figure 6. Effect of current on delamination factor.

Figure 7. Effect of current on tool wear rate.

4. CONCLUSIONS

EDM drilling of plain weave carbon fiber reinforced polymer composite was conducted using copper and graphite

electrodes under different levels of gap current and pulse-on time and machinability was evaluated by material

Current (Amp)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

DF

1.0

1.1

1.2

1.3

1.4

1.5

20 sec Model

105 sec Model

190 sec Model

20 sec Exp

105 sec Exp

190 sec Exp

Current (Amp)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

DF

1.0

1.1

1.2

1.3

1.4

1.5

20 sec Model

105 sec Model

190 sec Model

20 sec Exp

105 sec Exp

190 sec Exp

Current (Amp)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

TW

R (

g/m

in)

0.000

0.002

0.004

0.006

0.008

0.010

20 sec Model

105 sec Model

190 sec Model

20 sec Exp

105 sec Exp

190 sec Exp

Current (Amp)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

TW

R (

g/m

in)

0.000

0.002

0.004

0.006

0.008

0.010

20 sec Model

105 sec Model

190 sec Model

20 sec Exp

105 sec Exp

190 sec Exp Copper Electrode Graphite Electrode

Copper Electrode Graphite Electrode

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removal rate, electrode wear rate and delamination damage. The following conclusions can be drawn from this

study:

1. Material removal rate is influenced by all the three factors namely gap current, pulse-on time and electrode

material. Both copper as well as graphite electrodes show high material removal rates for higher settings of

current and pulse-on time (high discharge energy level).

2. As compared to copper, graphite electrodes are significantly influenced by any changes in pulse-on time

and current. Also at different settings graphite electrodes give higher MRR as compared to copper.

3. Tool wear rate is also influenced by gap current, pulse-on time and type of electrode material. For copper

and graphite electrodes, decrease in gap current and pulse-on time (lower energy level) shows significant

decrease in tool wear rate. As compared to graphite, copper electrodes show less amount of tool wear at

different settings of pulse-on time and current.

4. Delamination factor is only influenced by gap current and pulse-on time. High current and high pulse-on

time (high energy level) result in more delamination rate. Low delamination can be achieved by keeping

current at low level and pulse-on in the rage of 80s to 105s.

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Drilling of CFRP by EDM (1)