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CHAPTER 2

CASE STUDY-I COMPARATIVE ANALYSIS OF EDM

FOR INCONEL 718 AND 625

2.1 INTRODUCTION

Inconel 625 and 718 super alloys are extremely versatile austenitic

nickel based super alloys with excellent strength and good ductility at very

high temperature. Due to the improved mechanical properties of nickel-based

super alloy sheets, they are extensively used in aerospace applications, gas

turbines, rocket motors, spacecraft, nuclear reactors, pumps, and tooling.

However, Inconel 718 and 625 is a well known difficult-to-cut material. Its

low thermal conductivity and specific heat result in high cutting temperature.

In addition, chips are easy to weld on the tool which form build up edge

(BUE). As a result of it, cutting tool wears rapidly during machining.

Moreover, poor machinability of Inconel 718 when machining it using the

traditional mechanical cutting process leads to high tooling cost.

EDM process becomes a natural choice for machining nickel based

super alloys. Electrical Discharge Machining (EDM) is one of the most

successful, profitable, and extensively used non conventional machining

process for high degree of dimensional accuracy and economical cost of

production of any conductive material irrespective of its hardness. Electrical

discharge machining (EDM) have been explored to machine this alloy by

using some cylindrical copper and brass electrodes.

Based on the literature survey, some researchers, analyzed the

process parameters with performance measures while machining Inconel 718

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during die sinking EDM. In EDM process, material removal and its

mechanism has been one of the main concerns for several years. Since the

development of this process, researchers have explained the material removal

mechanism by developing different methods and mathematical models by

considering relationship between pulse conditions and material removal.

Traditionally, EDM is used to produce any component in any

electrical conductive material with low MRR. In order to increase MRR,

some researchers, used multi-channel electrodes, hallow electrodes, and

bunched electrode on EDM. They are used for hole drilling to improve the

performance on material removal rate, electrode wear and surface integrity for

the EDM parameters while machining Inconel 718 super alloy.

Typical applications need standard design requirement and close

form tolerances in manufactured components. The data regarding cylindricity,

circularity, perpendicularity and parallelism of the holes made by EDM are

very few or non-available. Recently, only a few researchers optimized the

parameters for micro-EDM drilling of Inconel 718 super alloy on hole taper

ratio and hole dilation by using Grey relational analysis.

From the literatures, it is observed that no credible works were

conducted on measuring cylindricity and circularity by using CMM on

machining new and advanced material, such as Inconel 718 and 625 nickel-

based super alloy in EDM process. Thus, this experimental work is attempted

to evaluate the form tolerance in EDM of Inconel 718 and 625. Taguchi

technique was used to develop Design of Experiments (DoE) to reduce the

number of trials.. Additionally, ANOVA is used to find the significant

parameter.

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2.2 DESIGN OF EXPERIMENTS

The application of design-of-experiments (DoE) requires careful

planning, prudent layout of the experiments, and expert analysis of results.

Taguchi has standardized methods for each of these DoE application steps.

This approach in finding factors that affect a product in a DoE can

dramatically reduce the number of trials required to gather necessary data.

Thus, DoE using Taguchi approach has become a much more attractive tool

to practicing engineers and scientists. In this case study, a total of four

parameters namely peak current, pulse on time, pulse off time, and flushing

pressure were chosen for the controlling factor and each parameter was

designed to have four levels denoted by 1, 2, 3 and 4 as shown in the

Table 2.1.

Table 2.1 Machining parameters and their levels

Parameter Unit Level 1 Level 2 Level 3 Level 4

A Peak current Amps 6 9 12 15

B Pulse on time µs 200 400 600 800

C Pulse off time µs 10 20 30 40

D Flu.pressure kg/cm2 0 0.25 0.5 0.75

2.2.1 Running Experiment

The chemical composition of Inconel 718 and 625 super alloys are

shown in Table A 3.1 in ‘Appendix 3’. The hardness values of the Inconel

718 and 625 super alloys is shown in Table A 3.2 in ‘Appendix 3’. The

experiments were conducted by using a die sinking SPARKONIX – Electric

Discharge machine with a capacity of 15 Amps as maximum current rating.

The die sinking EDM setup is shown in Figure 2.1. The work pieces, Inconel

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718 and 625 super alloy, which is in the form of disc and plate, are shown in

Figures 2.2 and 2.3. The work piece and electrode were connected with +ve

and –ve polarity. The electrodes as shown in Figure 2.4 to Figure 2.6 were

prepared by using CNC lathe (circular) and CNC milling (square and

hexagonal). Kerosene was used as dielectric fluid with pressure of 0-0.75

kg/cm², and side flushing technique was used to conduct all the experiments.

The weight of the electrode and work piece were measured before machining

and after machining for each trial run, by using digital weighing balance, with

an accuracy of 0.001 grams.

The Material Removal Rate (MRR) was calculated using the

formula given below

Timeremovedmateriale workpiecofWeight MRR (g / min) (2.1)

The Electrode Wear Rate (EWR) was calculated using the formula

given below

TimeremovedmaterialelectrodeofWeight EWR (g / min) (2.2)

Three trials were taken for each set of parameters and the average

roughness values were obtained. The form tolerances namely cylindricity,

circularity, perpendicularity, and parallelism are measured on electrodes and

work pieces (before and after machining) by using Co-ordinate Measuring

Machine (CMM) CHECKMASTER.

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Figure 2.1 EDM experimental setup

Figure 2.2 Inconel 718 workpiece

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Figure 2.3 Inconel 625 workpiece

Figure 2.4 Circular Electrodes

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Figure 2.5 Square Electrodes

Figure 2.6 Hexagon Electrodes

2. 3 RESULTS AND DISCUSSIONS

The 16 experimental runs were conducted in 3 trials, and the

average values of MRR, EWR, cylindricity, circularity, perpendicularity, and

parallelism for Inconel 625 and Inconel 718 are listed in Tables A 1.1 in

‘Appendix 1’and Table A 2.1 in ‘Appendix 2’ respectively.

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2.3.1 Analysis of Variance (ANOVA) - Inconel -718 and 625

Tables A 4.1 in ‘Appendix 4’ and Table A 5.1 in ‘Appendix 5’ show

the calculated F-values and P-values of the analysis of variance (ANOVA) for

MRR, EWR and form tolerance namely cylindricity, circularity,

perpendicularity and parallelism respectively to determine the relative

significances of different control factors for electrical discharge machining of

Inconel 718 and 625. It is observed that pulse on time (Ton) and peak current

have significant effect on MRR and EWR. The confidence interval of MRR,

EWR, cylindricity, circularity, perpendicularity, and parallelism for

machining of Inconel 718 are 99.3 %, 94.89%, 94.19%, 93.64%, 92.61%, and

94.65% respectively. The confidence interval of MRR, EWR, cylindricity,

circularity, perpendicularity, and parallelism for machining of Inconel 625 are

95.49%, 96.21%, 98.36%, 91.64%, 94.91%, and 98.55% respectively. The

values of “prob. > F” in Table 6.6 and 6.7 for the term of models are less then

0.05 (i.e. = 0.05 or 95% confidence) which indicates that the parameters

corresponding to these values are considered to be statistically significant to

the other parameters.

2.3.2 The Effect of Current on MRR and form Tolerance Index

The variations of MRR, perpendicularity index, and parallelism

index with respect to peak current while machining with square electrodes are

shown in Figures 2.7, 2.8, and 2.9. The variations of circularity index,

cylindricity index, and perpendicularity index with respect to peak current

while machining with circular electrodes are shown in Figures 2.10, 2.11, and

2.12. The variations of parallelism index with respect to peak current while

machining with hexagonal electrode is shown in Figure 2.13. As shown in

Figure 2.7, the MRR is increased with peak current. This result may be

attributed to the relative increase in discharge energy and impulse force as the

peak current increased.

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In general, during EDM process, the dimensions of work piece

depend upon the electrode dimensions. Similarly, the form tolerances of work

piece also depend upon the form tolerances of electrode. Moreover, these

tolerance value are again affected by the machining time due to the reason

that as the EDM machining progresses, the electrode wears out in non-

uniform manner. Therefore, it is attempted to study the performances based

on form tolerance index. The form tolerance index is separately calculated for

each form tolerance considered using the expression given below

Form tolerance of workpiece Form tolerance index *Machining timeForm tolerance of electrode

In all the Figures from Figure 2.8 to Figure 2.13 and from

Figure 2.15 to Figure 2.20 the form tolerance index values of Inconel 718 are

higher than those of to Inconel 625. This is due to the reason that the Inconel

718 has higher thermal conductivity values when compared to Inconel 625.

The circular electrodes which produce uniform spark density on its

circumference give better form tolerances where as hexagonal and square

electrode gives poor form tolerances. It is also due to the reason that the edges

of square and hexagonal profiled electrode are with higher density spark

interaction which produced irregular surfaces and also affected the form

tolerances.

As electrode advances into a work piece, the sparking area changes

and sparking is not only takes place at the bottom of the electrode but also at

lateral face. Consequently, the higher current tends to increase the EWR and it

in turn affects the form tolerance indices of the square, circular, and

hexagonal electrode.

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Influece of peak Current on MRR for various electrode (INCONEL 718)

Peak Current Amps

Influence of Peak Current on Perpendicularity Index

Peak Current Amps

Figure 2.7 Influence of peak current on MRR

Figure 2.8 Influence of peak current on perpendicularity index (Square electrode)

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Influence of Peak Current on circularity Index

Peak Current Amps

Influence of peak current on parallelism Index

Peak Current amps

Figure 2.9 Influence of peak current on parallelism index (Square electrode)

Figure 2.10 Influence of peak current on circularity index (Circular electrode)

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Influence of peak current on perpendicularity Index

Peak Current Amps

Influence of current on cylindricity Index

Peak Current amps

Figure 2.11 Influence of peak current on cylindricity index (Circular electrode)

Figure 2.12 Influence of peak current on perpendicularity index (Circular electrode)

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Influence of Ton on MRR for various electrode(Inconel 718)

Ton micro secs

Influence of Peak Current on Parallelism Index

Peak current Amps

Figure 2.13 Influence of peak current on parallelism index (Hexagonal electrode)

Figure 2.14 Influence of Ton on MRR

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Influence of Ton on parallelism Index

Ton microsecs

Influence of Ton onPerpendicularity Index

Ton micro secs

Figure 2.15 Influence of Ton on perpendicularity index (Square electrode)

Figure 2.16 Influence of Ton on parallelism index (Square electrode)

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Influence of Ton on Cylindricity Index

Ton microsecs

Influence of Ton on Circularity Index

Ton micro secs

Figure 2.17 Influence of Ton on circularity index (Circular electrode)

Figure 2.18 Influence of Ton on cylindricity index (Circular electrode)

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Influence of Ton on parallelism Index

Ton micro secs

Influence of Ton on perpendicularity Index

Ton micro secs

Figure 2.19 Influence of Ton on perpendicularity index (Circular electrode)

Figure 2.20 Influence of Ton on parallelism index (Hexagonal electrode)

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2.3.3 The Effect of Pulse on Time on the MRR and form Tolerance

Index

The variations of MRR, perpendicularity index, and parallelism

index with respect to pulse on time while machining with square electrodes is

shown in Figures 2.14, 2.15, and 2.16. The variations of circularity index,

cylindricity index, and perpendicularity index with respect to pulse on time

while machining with circular electrodes is shown in Figures 2.17, 2.18, and

2.19. The variations of parallelism index with respect to pulse on time while

machining with hexagonal electrode is shown in Figure 2.20. As shown in

Figure 2.14, the longer pulse on- time results in higher MRR up to half way in

the beginning but then starts decreasing with further increase in pulse on-time.

This event has been attributed to the increase of input energy in high pulse on

time, which results in more chopping on the gap between the work piece and

tool electrode, creating a short circuit which decreases the performance of

electrical spark erosion.

Due to this reason, MRR increased when Ton increased, and then

MRR reduced on further increase in Ton values. Consequently, the longer and

wider craters are formed as machined surface further increasing pulse on time

values. And also when increasing pulse on time there is a great quantity of

molten and floating metal suspended in the electrical discharge gap during

EDM and resulting in deterioration of both the electrode and work piece

surface. This phenomenon dominates the behavior of the electrode wear. The

wear of tool material leads to deterioration in both the size and the shape of

the machined hole in EDM drilling, producing a hole with larger roundness

error.

As a result, the form tolerances are increased when Ton increases.

On the other hand, perpendicularity index, parallelism index, cylindricity

index, and circularity index rapidly decreases with increase in pulse on time

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duration in the beginning but then starts increasing slowly and further

increase with increase in pulse on time as shown in Figure from 2.15 to 2.20.

2.4 SUMMARY OF RESULTS

Analysis of form tolerances in Electrical Discharge Machining

(EDM) of Inconel 718 and Inconel 625 were performed. An experimental

plan of Taghuchi L16 array table was employed to carry out the experimental

work. The effect of machining parameters on the performance characteristics

in the EDM process of Inconel 718 and 625 were analyzed based on the

ANOVA and second order polynomial graphs for various performances

measures yield the following conclusions:

1. The result of ANOVA used to find the most significant

parameter for MRR, EWR, cylindricity, circularity

perpendicularity and parallelism for machined Inconel 718

and 625 by using square, circular and hexagonal electrodes.

For all the performance measures the R2 values was above

92.5 % (P>0.05, ie = 0.05) confidence interval. It is found

that the pulse on time is the most significant parameter rather

than current that affects the MRR, EWR and form tolerances.

2. The value of MRR increases with increasing discharge

current. Then, it affects the form tolerance index.

3. The value of MRR first increases with an increase pulse on

time up to 400 µs, and then decrease with a further increase in

the pulse on time.

4. The value of cylindricity index, circularity index,

perpendicularity index, and parallelism index first decreases

with an increase in pulse on time up to 350 µs, and then

increase with a further increase in the pulse on time.