chapter 2 case study-i comparative analysis of edm...
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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.