machinability performance of powder mixed dielectric in ... · additive in dielectric fluid when...

International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:15 No:04 37

153804-0707-IJMME-IJENS © August 2015 IJENS I J E N S

Machinability Performance of Powder Mixed

Dielectric in Electrical Discharge Machining (EDM)

of Inconel 718 With Copper Electrode

M.A. Lajis *,

, S. Ahmad Sustainable Manufacturing and Recycling Technology (SMART), Advanced Manufacturing and Materials Center (AMMC),

Universiti Tun Hussein Onn Malaysia (UTHM),

86400 Parit Raja, Batu Pahat, Johor, Malaysia *Corresponding author, email: [email protected]

Abstract-- This study mainly explores the effects of powder

additive in dielectric fluid when electrical discharge machining of

Inconel 718 by employing high Peak current and Pulse duration.

Copper was selected as a tool. Peak current, Pulse duration, and

Concentration of the nano Alumina powder were chosen as a

variable parameter to study the EDM performance in terms of

Material removal rate (MRR), Electrode wear rate (EWR) and

Surface roughness (Ra). The experiment results show that, the

MRR has improved significantly compared to without powder

concentration at a high level of Peak current and Powder

concentration for both electrodes. When EDM machining at 4g/l

of powder concentration, the MRR is improved about 32% in

comparison to the highest MRR value obtained without powder

suspension dielectric. The maximum MRR value 45.70mm3/min

was obtained at 40A of Peak current, 200µs of Pulse duration

and 4g/l of powder concentration. In conventional EDM, the

EWR is increased at high peak current and shortest pulse

duration. But when powder suspension was applied, higher peak

current and longer pulse duration was decreased the EWR. The

lowest EWR value -0.244mm3/min was achieved at the highest

powder concentration 4g/l with the highest value of Peak current

of 40A and the longest Pulse duration of 400µs. The negative

value for EWR is indicated that the deposition effect has

occurred on the electrode surface. The value of Ra also increased

by increasing of peak current but decreased with longer pulse

duration. The Ra value is worst when powder concentration was

applied. The result suggested that, lower peak current with

longer pulse duration and without powder additive in a dielectric

is better for Ra. The lowest Ra value 8.98µm is obtained at 20A of

Peak current and 400µs of Pulse duration without powder

suspension dielectric. Index Term-- Electrical discharge machining (EDM); Inconel

718; Aerospace material; Powder suspension dielectric;

Machinability; Copper electrode 1. INTRODUCTION

Up to now, EDM has been an important manufacturing

process for the tool and die industry. It has proved for the

machining of high toughness aerospace material alloys such

Inconel 718 that are difficult to cut by conventional methods.

However, compared with traditional machining processes,

especially the high-speed machining (HSM), the low

efficiency of EDM limits its application. The application of

EDM is confined to conditions where traditional machining

processes cannot be resorted to. It seems that in order to

achieve a high material removal rate (MRR), the pulse off-

time or pulse interval, which interrupts the material removal

process, should be decreased as much as possible.

Unfortunately, both of them are so indispensable that

neglecting them will make EDM impossible (Fenggou and

Dayong, 2004; Han et al., 2009). Since the invention of EDM

in the 1940s, many efforts have been made to enhance the

machining performance and stability of EDM process. It is

because that the EDM process in the common dielectric oils is

very unstable owing to arcing or short-circuiting. To fulfil this

requirement, a relatively new method by introducing an

additive of powder into the dielectric fluid of EDM and

currently known as powder mixed dielectric EDM (PMEDM).

This method was often reported to be effective in improving

EDM performance. The results show that the PMEDM can

improve the machining rate and surface quality, and decreased

the tool wear (Kansal et al., 2007; Tzeng, 2008; Bhattacharya

et al., 2013).

Suspended powders increase the spark gap distance

due to their presence between tool and workpiece. It has two

outcomes: firstly, increased the spark gap is useful in effective

removal of debris from the gap; secondly, it makes the powder

EDM process highly stable with effective discharge dispersion

throughout the gap. An increase in the distance decreases the

electrostatic capacitance of the gap. Efficient discharge

dispersion not only produce uniform work surface, but also

prevents the occurrence of concentrated arc discharge and

hence reduces finishing time (Jahan et al., 2011; Kumar et al.,

2011). Suspension powder in the dielectric of EDM reduces

machining time significantly and improving surface quality of

work material compared to conventional EDM methods. This

statement supported by a few researchers doing an experiment

regarding powder suspended dielectric in EDM machining.

Based on analysis done by Kumar et al., (2010) on

the study of potential of graphite powder in AEDM of Nickel

Based Super alloy 718 with the three type powder

concentration which is 0, 6, and 12g/l, they observed that an

increase in peak current and the concentration of graphite fine

powder in dielectric fluid increase MRR. The observation

suggests that the suspension of an appropriate amount of

powder into the dielectric fluid causes greater erosion of the

material. The reason for the enhancement of MRR is mainly

attributed to a reduction in the breakdown strength of the



dielectric fluid leading to early spark, and increase in

frequency of sparking within the discharge. 12g/l

concentration of graphite powder produced the maximum

MRR in Inconel 718 with the improvement approximately

27%. Other research done by Kumar et al., (2011), regarding

EDM machining of Inconel 718 with powder mixed in

dielectric. This time the researcher use Aluminium as powder

suspension in dielectric with three different particle size,

which is fine (400 mesh = 38µm), medium (325 mesh =

44µm) and coarse (200 mesh = 74µm) and with 5 different

concentrations from 0g/l to 12g/l. Medium size of aluminium

additive powder enhances machining rate significantly by

almost 90% of improvement and at the same time reduces

wear rate (WR) by 80% and Ra by 17%. Certain powder

concentration in dielectric also improves machining rates. 6g/l

of Aluminium with medium size additive powder in dielectric

produces maximum MRR and minimum Ra. However, higher

contamination leads to unstable machining conditions. 4 g/l

powder concentration produces minimum WR and thereafter,

it increases at higher concentrations. The author also remarked

that powder mixed EDM is an effective option for machining

Inconel 718.

According to an experiment done by Singh et al.,

(2010) when machining hastelloy with Cu electrode and Al

powder mixed as a suspension in dielectric with three types of

mesh size (fine, medium and coarse) and five levels of powder

concentrations up to 12g/l, the experiment shows that MRR

yielded by conventional EDM is low. Thereafter, with the

addition of more powder in dielectric MRR starts increasing at

a higher rate. However, too low and too high concentration of

powder may reduce MRR. Highest MRR is produced at 6g/l

concentration. MRR is higher by suspending medium grain

size aluminium powder in EDM oil at equal concentrations.

The highest MRR value is achieved at 6 g/l concentrations

with medium size of powder particles. For TWR, electrode

erodes at a much higher rate in a pure dielectric fluid. EWR

decreases by adding 3g/l concentration aluminium powder in

the dielectric fluid and increases slightly by adding more

powder in dielectric fluid up to 6g/l concentration. Then,

surface produced without an additive of powder in dielectric

fluid has a large surface roughness (Ra) value. Ra lowers down

by suspending aluminium powder in a dielectric fluid. So, the

researchers have made a conclusion, aluminium powder

suspended in the dielectric fluid affected MRR, EWR, and Ra

and too low and too high concentration and grain size of

aluminium powder in EDM oil reduces MRR. EWR can be

lowered down by reducing the size of suspended aluminium

powder particles in EDM oil and the surface finish can be

enhanced by reducing the size of aluminium powder up to a

certain particle size. Too small powder particles produce

rough surfaces.

Chromium powder is a choice made by Ojha et

al.,(2011) for additive suspension in the dielectric of EDM

machining EN-8 steel. He reported, current, powder

concentration and electrode diameter are significant factors

affecting both MRR and EWR. Both the performance

measures were observed an increasing trend by increase in

current for all parameter settings. MRR was increased with

increasing in powder concentration. The trend shows that

MRR will increase further with further increase in

concentration. The highest MRR value is obtained at the

highest peak current and powder concentration 8A and 6g/l

respectively. EWR increases with a lower range of powder

concentration, but then decrease. The authors also

recommended for more workpiece, powder, electrode

materials and experimental settings combinations are used to

investigate further for much understanding of the process.

Ming and He (1995) have reported on their research

that MRR clearly increase by all additives used in the tests,

especially in the middle-finish machining and the finish

machining. In the condition of middle-rough machining, the

MRR can be increased by about 50% due to adding additives.

In the middle-finish machining the MRR can be doubled.

During the finishing the MRR is even 2 to 3 times as fast as it

is for pure kerosene. It has been discovered that when two

kinds additive agents which is solid powder and liquid are

added at the same time the MRR is higher. However, there is

an optimum value of the quantity of the additive. They also

found that EWR can be lower by almost all kinds of additive,

especially in middle-rough machining. The essential difficulty

in EDM is that the SR is not so good. One purpose of adding

the additive to kerosene is to improve the surface quality.

Generally the more the additive in the kerosene, the surface

quality is better, but when excessive of additive used it

becomes worse. The researchers also stated that the condition

of fatigue stress, micro cracks, recast layer and hardness

during the EDM process can be improved by adding an

addition to the working fluid. The recast layer will be thinner

and so the cracks will be less. For the hardness in the top

layer, it was greater when adding additive.

Jahan et al., (2011) has done an experiment regarding

the effect of graphite nano-powder on the EDM of WC-Co

and they claimed the spark gap increases significantly with the

increase of powder concentration for graphite-mixed

dielectric. Although the spark gap is increased, very high

concentration of powder particles in the dielectric can result in

series of discharging and arcing thus causing surface defects.

With the increase of powder concentration also the MRR

increases due to the stability of machining process at increased

spark gap after adding nano-powder in dielectric. The addition

of powder particles can reduce the electrical discharge power

density and gap explosive pressure for a single pulse, which

result in smaller craters with uniform distribution. Moreover,

due to much effective flushing of debris in higher spark gap

and reduced size craters, the overall MRR increases. The

highest MRR is obtained at 0.8g/l of powder concentration.

For EWR, an optimum range of 0.1–0.4 g/l was found to

provide lower EWR. Then, for the average surface roughness

(Ra), it was decreases first with the increase of powder

concentration. Then again tend to increase at higher

concentration of powder particles. The authors also found that

an addition of graphite nano-powder in dielectric oil provides

smooth and defect-free surface.



Powder mixed dielectric in EDM process is still not

widely used in industry. Many fundamental issues of these

new processes, including machining mechanism with various

additives, are still not well understood. Previous researchers

also tend to use low peak current in the range 0.5A< Ip < 20A

and low pulse duration in range 10µs < ton < 150µs to control

the tool wear and surface quality. As a consequence, the

machining rate becomes slow and lead in the lower

productivity. There is a researcher (Kumar et al., 2011) had

used the pulse duration up to 750µs when EDM machining of

Inconel 718 with powder mixed dielectric fluid but the highest

peak current selected in their research is just 9A. Thus, for this

research, the complexity of this process, especially from the

effects of the powder mixed dielectric in relation with higher

peak current and pulse duration to the EDM performance

requires further investigations.

2. EXPERIMENTAL SET-UP AND PROCEDURES

The EDM experiments were conducted on the CNC Sodick

High Speed EDM die sink AQ55L (3 Axis Linear). The

maximum travel range of the machine is 550 mm×400

mm×350 mm with the step resolution of 0.1 μm in X, Y and Z

directions. All Inconel 718 specimens were standardized with

a size of 40 mm × 30 mm × 10 mm by using an Okamoto

grinding machine (ACC52DX) with a diamond-grain resin-

bond grinding wheel. Table 1 shows the alloy composition of

Inconel 718. The tool electrode used to be cylinder-type

copper with a diameter of 10 mm each. The EDM depth of cut

is 3 mm was evaluated in all experiments. Kerosene was

selected as a liquid dielectric with three conditions, without

powder concentration, with 2g/l of powder concentration and

with 4g/l of powder concentration. 99.5% purity of the nano

alumina powder with an average particle size of 45nm was

selected as a powder suspended in a dielectric fluid. The

experimental process variables and settings are summarized in

Table 2. For experiments involving powder suspension, an

external tank device called high performance electrical

discharge machining device (HPEDM) as shown on Figure 1

are attached on the CNC Sodick High Speed EDM. The device

has its own controller and functioning as „plug and play‟ to the

conventional EDM machine. Figure 2 shows the schematic

diagram of HPEDM. The experiment was conducted in full

factorial by using one trial for one variable approach.

Table I

Alloy compositions of Inconel 718

Alloy composition %

Nickel (plus Cobalt) 50.00-55.00

Chromium 17.00-21.00

Iron Balance

Niobium (plus Tantalum) 4.75-5.50

Molybdenum 2.80-3.30

Titanium 0.65-1.15

Aluminium 0.20-0.80

Cobalt 1.00 max

Carbon 0.08 max

Manganese 0.35 max

Silicon 0.35 max

Phosphorus 0.015 max

Sulfur 0.015 max

Boron 0.006 max

Copper 0.30 max

Table II

Machining conditions and parameters

Parameters Levels

Work piece material

Tool electrode

Powder suspension

Peak current, Ip (A)

Pulse duration, ton (µs)

Powder concentration, Cp (g/l)

Pulse interval, toff (µs)

Voltage, V

Dielectric fluid

Electrode polarity

Depth of cut

Inconel 718

Copper

Nano alumina powder, 45nm

20, 30, 40

200, 300, 400

0, 2, 4

Based on 80% duty factor

120

Kerosene

Positive

3mm



Fig. 1. HPEDM machining setup

Fig. 2. HPEDM schematic diagram

3. RESPONSES

The selected response variables MRR, EWR and Ra are

defined as follows:

The material removal rate was calculated from the difference

of weight of work-piece before and after the machining

process.

MRR = (Wb - Wa / ρ718 .t) mm3/ min (1)

Where, Wb is the initial weight of workpiece in g; Wa is the

weight of the workpiece after machining in g; t is the

machining time in minutes and ρ718 is the density of Inconel

718 (8.19 x 10-3

g/mm3).

The wear of copper electrode was calculated from the weight

difference of electrode before and after the machining and is

expressed as:

TWR = (Eb – Ea / ρe t) mm3/ min (2)

Where, Eb is the initial weight of electrode in g; Ea is the

weight of electrode after machining in g; t is the machining

time in minutes and ρe is the density of Cu electrode 8.96 x 10-



3 g/mm

3. The weight of the electrodes and workpiece before

and after machining needs to be measured in order to obtain

MRR and EWR. The changes in weight from the tool electrode

or workpiece are suspected to be small. Thus, the more

decimal points are better to eliminate the possibilities of large

error. For this analysis Shimadzu weight balance measurement

was used. Maximum weight can be measured is 210g until five

decimal point accuracy. However, for this study the decimal

point of the weight balance is to set to 4 decimal. Mitutoyo SJ-

400 Surface Roughness Tester is used to measure the average

surface roughness (Ra) of the machining surface. When

measuring surface roughness, the only parameter to be

evaluated was Ra as this is the most widely used parameter in

industrial applications (Baraskar et al 2011). A scanning

electron microscope (SEM) JSM−6380 was used to evaluate

the surface topography of machined surface after EDM

machining.

4. RESULTS AND DISCUSSION

The focus of the experiments is to determine the optimum

parameters corresponding to different Peak current (Ip), Pulse

duration (ton) and Powder concentration (Cp). This various

parameters have significant influence on the quality of

machining of Inconel 718. It affects the Material removal rate

(MRR), Electrode wear rate (EWR) and Surface roughness

(Ra). These results were extracted from a series of full

factorial experiment which overall trials of 27. Then the

comparison of performance was made between the

conventional EDM and the HPEDM with powder suspension

dielectric. The experimental results for MRR, EWR and Ra

after EDM machining of Inconel 718 by using Copper (Cu) is

shown in Table III.

Table III

Result of MRR, EWR, and Ra

Trial Ip, A ton, µs Cp, g/l MRR EWR Ra, µm

1 20 200

0

18.67 -0.0028 10.02

2 20 300 16.74 -0.0043 9.62

3 20 400 14.96 -0.0098 8.98

4 30 200 32.05 0.0375 15.06

5 30 300 30.30 0.0106 14.21

6 30 400 29.99 -0.0030 14.13

7 40 200 34.57 0.0598 16.87

8 40 300 30.84 0.0046 16.19

9 40 400 30.82 0.0015 15.80

10 20 200

2

26.97 -0.0013 14.45

11 20 300 23.25 -0.0082 14.59

12 20 400 18.54 -0.0109 14.36

13 30 200 41.03 0.0181 19.78

14 30 300 39.85 -0.0114 18.27

15 30 400 37.52 -0.0163 16.41

16 40 200 43.68 0.0616 21.03

17 40 300 41.28 -0.0160 19.59

18 40 400 38.40 -0.0223 18.51

19 20 200

4

26.29 -0.0064 14.74

20 20 300 21.20 -0.0082 14.71

21 20 400 17.57 -0.0156 14.31

22 30 200 40.71 0.0274 17.93

23 30 300 40.40 -0.0140 18.94

24 30 400 36.94 -0.0165 17.60

25 40 200 45.70 0.0427 21.00

26 40 300 43.32 -0.0191 18.23

27 40 400 40.78 -0.0244 19.79



4.1 Material removal rate

Material removal rate (MRR) represents the average volume

of material removed from the workpiece per unit time

(mm³/min). There are several factors need to be considered to

ensure the results gained are useful in increasing the

productivity in EDM operation. The most important factor to

increase the speed of the machining is due to how much the

volume of the material can be removed per time taken. The

MRR is a standard value that is calculated to determine the

rate of production in machining. The effect of Peak current

(Ip) and Pulse durations (ton) on the MRR at a different Powder

concentration (Cp) of Cu electrode is shown in Figure 3. From

Figure 3(a) it is revealed that the Ip affects the MRR

significantly when EDM machining of Inconel 718 without

powder suspension dielectric. At high Ip=40A, the intensity of

energy release during sparking is proportionally increased

whereby higher temperature produced by the spark, melts

more material and removes from the workpiece. Therefore, by

increasing the Ip, MRR will increase (Ghewade and Ninapikar,

2011; Rajesha et al., 2011; Sudhakara et al, 2012).

Conversely, higher ton has decreased MRR for all conditions

of Ip when Cu was used as an electrode. The reason is with a

constant setting of 80% duty factor, the pulse interval will

increase with the increment of ton. This high ignition delay due

to high pulse interval reduces the machining rate, thus MRR is

decreased. (Kumar et, al. 2011; Sudhakara et al, 2012). The

similar trend also can be observed as shown in Figure 3(b) and

3(c).

By comparing the effect of powder concentration

(Cp) on MRR, the MRR was increased by increasing the Cp

into the dielectric fluid. At Cp=2g/l, Ip=40A and ton=200µs the

MRR enhanced significantly from 34.57mm3/min (Figure

3(a)) to 43.68mm3/min as shown in Figure 3(b). Then, as

indicated in Figure 3(c), further increment in MRR value up to

45.72mm3/min is observed when Cp=4g/l was used in the

same parameter setting. This observation suggests that the

addition of an appropriate amount of additives into the

dielectric fluid of EDM causes greater erosion of the material.

The reason for the enhancement of MRR at higher powder

concentration is mainly attributed to a reduction in the

breakdown strength of the dielectric fluid leading to early

spark, and increase in frequency of sparking within the

discharge (Kumar et al, 2010). However, effect of ton on MRR

is inversely proportional. MRR is decreased by the increment

value of ton. During the machining period, in addition to the

expansion of plasma channel, at high pulse duration levels the

localized temperature is increased and as a consequence the

decomposed carbon from dielectric fluid stacked to the

electrode surfaces. Thus, the discharge efficiency is reduced

and become unstable, thus, the MRR is decreased (Hascalik

and Caydas, 2007).

(a) Cp = 0g/l (without powder suspension)

14.96

34.57

0

10

20

30

40

50

200 300 400

MR

R,

mm

3/m

in

Pulse duration (ton), µs

Material Removal Rate (MRR)

Ip=20A

Ip=30A

Ip=40A

18.54

43.68

0

10

20

30

40

50

200 300 400

MR

R, m

m3/m

in



Ip=20A

Ip=30A

Ip=40A



(b) Cp = 2g/l

(c) Cp = 4g/l

Fig. 3. Effect of Peak current (Ip) and Pulse duration (ton) on MRR at different Powder concentration (Cp) [(a) Cp=0g/l, (b) Cp=2g/l and (c) Cp=4g/l]

Within a selected parameter, the highest value for MRR is 45.70mm3/min obtained when Cp=4g/l was suspended in the

dielectric fluid at 40A and 200µs of Ip and ton, respectively. The improvement is about 32% as shown in Figure 4 in compared to

without powder concentration at the same parameter setting Ip=40A and ton=200µs.

Fig. 4. The improvement of MRR when EDM machining of Inconel 718 employing powder suspension dielectric

On EDM applications, the usage amount of current is

depending by the surface area of the cut and the process

requirement. Higher peak current generally used in roughing

operations with large surface areas and the lowest Peak

current used for the finishing process. High Peak current (Ip)

improved the MRR but it will affect the severe conditions of

the machined surface topography of the workpiece. The

surface topography of the material that has been machined

closely related to the Ip supplied. During the EDM process, the

high temperature in every spark causes material melt and

evaporation, and then leaves a crater on the machined surfaces

(Li et al., 2013). From the Figure 5(a-1) shows that, at lower

Ip=20A, longest ton=400µs, and without powder concentration

(Cp) of the dielectric, the conditions of craters are shallow and

flatten, whereas at high Ip=40A as indicated in Figure 5(a-2),

the severe surface conditions such as the large crater

formation and more nodules were appeared. This is due when

increase of current intensity, the working energy increased, so

that discharge craters become deeper and wider, thus

contributing to a more noticeable surface topography (Theisen

and Schuerman, 2004). The existence of micro-voids is due

the stress released from the underneath of machined surface at

high temperatures. The temperature at machined surface is

raised when the high Ip is applied and the hot bubble air

trapped on the machine surface is exploding, thus created the

micro-void (Li et al., 2013). There are also nodules on the

machined surface produced from reattachment of molten

metal during an improper flushing condition. The effect of

powder suspension dielectric on the surface topography of

Inconel 718 also more profound.

17.57

45.70

0

10

20

30

40

50

200 300 400

MR

R,

mm

3/m

in



Ip=20A

Ip=30A

Ip=40A

32%

Cp=0g/l Cp=4g/l



a-1) MRR=14.96mm

3/min; Ip=20A; ton=400µs;

Cp=0g/l - [Trial 3]

a-2) MRR=34.57mm


Cp=0g/l - [Trial 7]

b-1) MRR=18.54mm


Cp=2 g/l - [Trial 12]

b-2) MRR=43.68mm



c-1) MRR=17.57mm



c-2) MRR=45.70mm



Fig. 5. The EDM machined surface topography of Inconel 718 at a different powder concentration (Cp) [a) Cp=0g/l, b) Cp=2g/l, and c) Cp=4g/l]

Based on Figure 5(b-1), at low Ip=20A and Cp=2g/l the surface

looks rough and the size of nodules is bigger compared to

Cp=0g/l at low Ip (Figure 5(a-1)), and the condition is better

when Cp=4g/l was used as shown in Figure 5(c-1). This is due

to the suspension of powder particles in the dielectric fluid

enlarged the plasma channel, caused an electric density

decrease and hence uniform distribution of the sparking takes

place, thus shallow craters was produced (Sharma et al.,

2010). When the highest Ip=40A was used, the topography

looks worst. This is because powder settling is a common

problem at higher powder concentration due to the dielectric

loses its ability to distribute uniformly all the powder particles

and because of that, the bridging of powder particles may

occur, which results in short-circuiting and arcing more

frequently. This bridging effect can result in more

concentrated discharge energy (Sharma et al, 2010; Kumar et

al, 2010; Jahan et al., 2011). Due to increase in frequency of

discharging and high Ip, faster erosion takes place from the

workpiece thus damaged the machined surface. The

formations of nodules are bigger and existence of micro-voids

are clearly visible at higher MRR as indicated in Figure 5(b-2)

and 5(c-2).

Low MRR High MRR

Low MRR High MRR

Low MRR High MRR

Craters

s

Nodules

Globules

Micro-voids



4.2 Electrode wear rate

In EDM applications, higher amperage is used in roughing

operations with large surface areas and the lower current used

for the finishing process. The higher current improve the MRR

but the electrode wear and the quality of surface finish will be

decreased. The high rate of electrode wear is one of the main

problems in EDM. Electrode wear must be effectively

compensated in order to achieve the required accuracy of the

machined features. The effect of Peak current (Ip), Pulse

durations (ton) and Powder concentration (Cp) on the Electrode

wear rate (EWR) of Cu is shown in Figure 6. Higher Peak

current, Ip is resulting an increasing in EWR for Cu electrodes

at the constant Pulse duration (ton) when EDM machined

without powder concentration, Cp=0g/l as indicated in Figure

6(a). The reason is due to high discharge current promoted to

high spark energy eroded more material from the workpiece

and the electrode which in effect increases the EWR.

However, EWR was decreased when increasing of ton for each

of the Ip used respectively. The reason is that, because of the

deposition effect of decomposed carbon from dielectric oil on

the tool electrode at a high temperature for the longer pulse

duration (Kang and Kim, 2003; Hascalik and Caydas, 2007).

(a) Cp = 0g/l

(b) Cp = 2g/l

-0.0098

0.0598

-0.05

0.00

0.05

0.10

0.15

200 300 400

EW

R,

mm

3/m

in


Electrode Wear Rate (EWR)

Ip=20A

Ip=30A

Ip=40A

0.0616

-0.0223 -0.05

0.00

0.05

0.10

0.15

200 300 400

EW

R,

mm

3/m

in



Ip=20A

Ip=30A

Ip=40A



(c) Cp = 4g/l

Fig. 6. Effect of Peak current (Ip) and Pulse duration (ton) on EWR of Cu electrode at a different Powder concentration (Cp) [(a) Cp=0g/l, (b) Cp=2g/l, and (c)

Cp=4g/l]

As observed in Figure 6 (b), the effect of powder

concentration is significant on the EWR of Cu electrode. The

EWR is slightly increased when Cp=2g/l was used at the

highest Ip and the lowest ton compared to Cp=0g/l, but then

decreased with the increment of ton. The similar trend also can

be observed when 4g/l of Cp was supplied in the dielectric

fluid as shown in Figure 6(c), the EWR is increased at the

highest Ip and the lowest ton, but further decreased when high

Ip with high ton were promoted. The reason behind is that by

suspended nano aluminium powder into dielectric fluid the

machining process become more reactive and generated more

heat, as a consequence the decomposed of Carbon from

dielectric embedded on the electrode surfaces which functions

as a wear resistant layer for electrode and helps to decrease the

electrode wear (Murray et al., 2012). In overall, ton and Cp

were identified as the two factors that improved EWR. EWR

decreases with an increase in ton and further decrease with

suspension of powder in the dielectric fluid. Previous works

have found that the wear of tool electrodes is a dynamic

process that is influenced by two opposite factors; electrical

discharges erode materials from both the tool electrode and

the workpiece and cracked carbon from the dielectric oil may

be deposited on the surface of the electrode (Han et al 2009).

The lowest EWR produced from this experiment is

approximately -0.0244mm3/min obtained at Ip=40A, ton=400µs

and Cp=4g/l. A dissolved metal from the workpiece also

revealed deposited on the electrode surface as indicated in

Figure 7, 8 and 9.

a-1) Low EWR [Ip=20A; ton=400µs; Cp=0g/l]- Trial 3 a-2) EDX test [Ip=20A; ton=400µs; Cp=0g/l]- Trial 3

0.0427

-0.0244 -0.05

0.00

0.05

0.10

0.15

200 300 400

EW

R,

mm

3/m

in



Ip=20A

Ip=30A

Ip=40A

Carbon

deposited



b-1) High EWR [Ip=40A; ton=200µs; Cp=0g/l]- Trial 7 b-2) EDX test [Ip=40A; ton=200µs; Cp=0g/l]- Trial 7

Fig. 7. Surface morphology and EDX testing of the Cu electrode at a powder concentration, Cp=0g/l [a) Low EWR and b) High EWR]


b-1) High EWR [Ip=40A; ton=200µs; Cp=2g/l]-Trial 16 b-2) EDX test [Ip=40A; ton=200µs; Cp=2g/l]- Trial 16



Material

deposited



b-1) High EWR [Ip=40A; ton=200µs; Cp=4g/l]-Trial 25 b-2) EDX test [Ip=40A; ton=200µs; Cp=4g/l]- Trial 25


Based on the Figure 7-9, the deposited material was analyzed

according to the lowest and the highest EWR value condition

at a different Cp conditions. It is observed that, the distribution

of the deposited material is wider and more at the low EWR

condition compared to at the high EWR at Cp=0g/l as shown

in Figure 7. The similar condition also can be observed at

Cp=2g/l and Cp=4g/l as indicated in Figure 8 and 9,

respectively. The EDX testing has been performed

accordingly in order to clarify the elements of the deposited

material on electrode surfaces. Based on the graph on the

Figure 7(a-2) and 7(b-2), Carbon and the alloy elements of the

material of Inconel 718 were deposited on the electrode

surface. The counts of Carbon on the electrode surface were

increased when powder concentration was used for EDM of

Inconel 718 as shown in Figure 8(a-2) and 9(a-2). The

negative value for the lowest EWR is indicating that the

electrode was deposited by the carbon and material from the

workpiece is more than the wear of electrode. Thus, the

increment in the electrode mass after machining can be

explained by this deposition effect.

4.3 Surface roughness

Figure 10 shows the effect of peak current, pulse durations

and powder concentration on Surface roughness (Ra) of

Inconel 718. Without powder concentration which is Cp=0g/l

the Ra value is increased when peak current is increased. By

increasing the Ip, the amount of energy in the EDM process

will increase as shown in Figure 10(a). This can be attributed

to the fact that the high Ip may cause massive expulsion due to

high discharge density leading to the formation of deeper and

larger craters on the surface of the workpiece, thus the Ra

value is increased (Theisen and Schuermann, 2004; Patel, et

al., 2009; Li et al., 2013). The result also showed that with

increasing of the ton the Ra was decreased at all of Ip

conditions. This is due to the fact that an increase in ton was

equivalent to a decrease in the frequency of the pulse, which is

lead in reducing of sparking intensity on the machined surface

and produced a shallower crater as shown in Figure 11. This

result was contrasted with the previous researcher (Bharti et

al., 2010). According to Bharti et al., the Ra will increase at

the higher level of ton. Then, at Cp=2g/l, it is observed that, the

Ra value was increased with high level peak current and but

decreased when longest ton was applied.

(a) Cp = 0g/l

8.98

16.87

0

5

10

15

20

25

200 300 400

Ra,

µm


Surface Roughness (Ra)

Ip=20A

Ip=30A

Ip=40A



(b) Cp = 2g/l

(c) Cp = 4g/l

Fig. 10. Effect of Peak current (Ip) and Pulse duration (ton) on Surface roughness at a different Powder concentration (Cp) [(a) Cp=0g/l, (b) Cp=2g/l and (c) Cp=4g/l]

The similar condition also can be observed when the highest Cp=4g/l was applied. At Cp=4g/l the Ra was increased as the Ip and

ton increases as shown in Figure 10(c). At very high concentrations, the dielectric loses its ability to distribute uniformly all the

powder materials. Therefore, powder settling is a common problem at higher concentration, although spark gap increases. In

addition, at higher concentration of alumina nano-powder, the bridging of powder particles may occur, which results in arcing and

short-circuiting more frequently (Jahan et al, 2011). The bridging effect can result in more concentrated discharge energy and,

finally, deteriorating the Ra as indicated in Figure 12. The lowest Ra value of 8.98µm is achieved at Ip=20A and ton=400μs and

Cp=0g/l. Then, the highest Ra value is 21µm was obtained when the highest level of Ip=40A, shortest ton=200µs with Cp=4g/l.

Thus, for this study the highest variable parameter setting is not suggested when good surface finish is desirable and the existence

of powder suspension in the dielectric did not improve the Ra.

14.36

20.16

0

5

10

15

20

25

200 300 400

Ra,

µm



Ip=20A

Ip=30A

Ip=40A

14.31

21.00

0

5

10

15

20

25

200 300 400

Ra,

µm



Ip=20A

Ip=30A

Ip=40A



a-1) Low Ra [Ip=20A; ton=400µs; Cp=0g/l]- Trial 3 a-2) Low Ra [Ip=20A; ton=400µs; Cp=0g/l]- Trial 3

b-1) High Ra [Ip=40A; ton=200µs; Cp=0g/l]- Trial 7 b-2) High Ra [Ip=40A; ton=200µs; Cp=0g/l]- Trial 7

Fig. 11. Surface topography of machined surface at Powder concentration, Cp=0g/l at [a) low Ra, and b) High Ra]

a-1) Low Ra [Ip=20A; ton=400µs; Cp=4g/l]- Trial 21 a-2) Low Ra [Ip=20A; ton=400µs; Cp=4g/l]- Trial 21

b-1) High Ra [Ip=40A; ton=200µs; Cp=4g/l]- Trial 25

b-2) High Ra [Ip=40A; ton=200µs; Cp=4g/l]- Trial 25 Fig. 12. Surface topography of machined surface at Powder concentration, Cp=4g/l at [a) low Ra, and b) High Ra]

Low magnification High magnification


Craters

Craters

Nodules



Craters

Craters



5 CONCLUSIONS

By employing high peak current and pulse duration, the EDM

machinability of Inconel 718 at a different powder

concentration was studied. Based on the experimental result,

the following conclusion can be made:

i) The peak current and powder concentration is

the most contributing factor that improves the

MRR. The metal removal rate will increase as

the peak current and powder concentration

increase. The result shows the highest of MRR

as Cu as electrode is 45.7mm3/min at peak

current 40A. With 4g/l of powder concentration

The improvement is almost 32% in comparison

to without powder concentration at the same

parameter setting. The result shows that the

introduction of powder concentration in

dielectric fluid will helps to enhance the

machining efficiency. It also found that within

selected parameters, 4g/l is the best powder

concentration to achieve high MRR.

ii) The electrode wear rate (EWR) is increased

when peak current is increased, but inversely

proportional with pulse duration. However, the

EWR is decreased when high concentrations of

powder additive with the combination of high

peak current, and longer pulse duration. The

lowest EWR of Cu is -0.0244mm3/min obtained

at Ip=40A, ton=400µs and Cp=4g/l.

iii) Through the EDX analysis, it was found that the

carbon from dielectric and the workpiece

material has been deposited on the electrode

surface. The negative value for the EWR is due

to the this deposited effect on the electrode after

machining.

iv) High Ip is not recommended for surface

roughness. The surface roughness will increase

when the increase of the Ip. The result also shows

that an increasing in the ton value the surface

roughness will decrease and there is no

improvement in surface roughness when powder

additive was suspended in a dielectric fluid. The

lowest Ra value 8.98µm was obtained at Ip=20A,

ton=400µs, Cp=0g/l and then the highest Ra value

21µm achieved at Ip=40A, ton=200µs, and

Cp=2g/l.

ACKNOWLEDGEMENTS

This research was supported by Exploitary Research

Grant (ERGS) and Fundamental Research Grants (FRGS)

under Ministry of Education, Malaysia. The authors would like

also to thank to the Sustainable Manufacturing and Recycling

Technology (SMART) research cluster, Advanced

Manufacturing and Materials Center (AMMC), and Advanced

Machining Laboratory (AML), Universiti Tun Hussein Onn

(UTHM) for providing the facilities.

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