U. ÇAYDAª, M. AY: WEDM CUTTING OF INCONEL 718 NICKEL-BASED SUPERALLOY: EFFECTS ...
117–125
WEDM CUTTING OF INCONEL 718 NICKEL-BASED
SUPERALLOY: EFFECTS OF CUTTING PARAMETERS ON THE
CUTTING QUALITY
WEDM REZANJE NIKLJEVE SUPERZLITINE INCONEL 718:
VPLIV PARAMETROV REZANJA NA KVALITETO REZANJA
Ulaº Çaydaº, Mustafa Ay
University of Firat, Technology Faculty, Department of Mechanical Engineering, 23119 Elazið, Turkey
ucaydas@firat.edu.tr, ucaydas@gmail.com
Prejem rokopisa – received: 2015-01-26; sprejem za objavo – accepted for publication: 2015-02-19
doi:10.17222/mit.2015.026
Investigations of the effects of machining parameters on the cutting quality of wire-electrical-discharge-machining (WEDM)
cutting of an annealed Inconel 718 nickel-based superalloy are described in this paper. The cutting-quality characteristics
considered are the kerf width, the recast-layer thickness and the surface roughness of the cut specimens. The essential process
input parameters were identified as the pulse-on time, the pulse-peak current, and the injection pressure. The analysis of
variance (ANOVA) technique was used to find the parameters affecting the cut quality. The regression analysis was used for the
development of empirical models able to describe the effects of the process parameters on the quality of the WEDM cutting.
The ANOVA results show that the pulse-on time and the pulse-peak current are significant variables affecting the surface
roughness of wire-EDMed Inconel 718. The surface roughness, the kerf width and the recast layer of the test specimens
increased as these two variables increased. The measured and modelled results were in good agreement with respect to the
correlation coefficients Ra, RLT and Kü.
Keywords: wire-electrical-discharge machining (WEDM), Inconel 718, recast layer, surface morphology, analysis of variance
V ~lanku je opisana raziskava vplivov parametrov obdelave na kvaliteto reza, z `i~no elektroerozijo (WEDM) rezane `arjene
nikljeve superzlitine Inconel 718. Upo{tevane so bile karakteristike reza, kot je {irina reza, debelina nataljenega sloja in
hrapavost povr{ine odrezanega vzorca. Ugotovljeno je, da so bistveni vhodni parametri procesa impulzi v ~asu, tok vrha impulza
in tlak vbrizgavanja. Za iskanje parametrov, ki vplivajo na kvaliteto reza je bila uporabljena tehnika analiza variance (ANOVA).
Regresijska analiza je bila uporabljena za razvoj empiri~nega modela, ki lahko opi{e vpliv procesnih parametrov na kvaliteto
WEDM rezanja. Rezultati iz ANOVE ka`ejo, da sta impulz v ~asu in tok vrha impulza pomembni spremenljivki pri rezanju
Inconela 718 z `i~no elektroerozijo. Hrapavost povr{ine, {irina zareze in debelina pretaljene plasti so nara{~ali pri preizkusnih
vzorcih, ~e sta nara{~ali ti dve spremenljivki. Izmerjeni in modelirani rezultati so se dobro ujemali s korelacijskimi koeficienti
Ra, RLT in Kü.
Klju~ne besede: `i~na elektroerozija (WEDM), Inconel 718, pretaljena plast, morfologija povr{ine, analiza variance
1 INTRODUCTION
Among nickel-based alloys, the superalloy Inconel
718 is one of the most important ones. Due to its high
corrosion and high temperature resistance, it is
commonly used in the space industry, in particular for
the hot parts of plane, marine and industrial gas-turbine
engines, rocket engines, nuclear reactors, submarines,
pressure tanks, steam-turbine generators and other
high-temperature applications.1–5 Despite this widespread
use, it is classified as a difficult-to-machine material due
to its unique characteristics such as low thermal
conductivity, hardness, work hardening and the presence
of abrasive carbide particles in its microstructure.3,6
Due to its thermal machining ability, wire-electri-
cal-discharge machining (WEDM) has been an important
manufacturing method as it provides effective solutions
for machining difficult-to-machine materials, such as
zirconium, nimonic, titanium, nickel, etc., that cannot be
machined with traditional methods.7 Nonetheless, there
are problems that remain to be solved such as the
stresses that occur on the top layer where the material
solidifies and in the formation of the heat-affected zone,
micro-cracks, porosity, local hardness/softness, grain
growth and small alloys that get transferred via dielectric
fluids or the tool.8,9 For a complete and efficient WEDM,
the machining parameters should be selected in
accordance with the nature of the material.
Effects of the EDM-process parameters have been
investigated. Kanlayasiri and Boonmung10 studied the
effects of the pulse-on time, pulse-off time, pulse-peak
current and wire tension (a wire electrode made of Cu-35 %
of mass fractions of Zn; the wire was KH Sodick with
0.25 mm in diameter, tolerating a tension of up to 900
MPa) on the surface roughness of the DC53 die steel.
Kuriakose and Shunmugam11 investigated the characte-
ristics of a WEDMed Ti6Al4V surface. Gökler and
Ozanözgü12 experimentally investigated the effects of the
cutting parameters on the surface roughness in the
WEDM process for the 1040, 2379 and 2738 steels.
Miller et al.13 investigated the effects of the EDM process
parameters, particularly the spark cycle time and
Materiali in tehnologije / Materials and technology 50 (2016) 1, 117–125 117
UDK 669.245:621.9.048:519.61/.64 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(1)117(2016)
spark-on time on thin cross-section cuts of the Nd–Fe–B
magnetic material. Ramakrishnan and Karunamoorty14
developed a neural-network model in order to predict
process outputs and to establish the optimum process
parameters. Kang and Kim15 investigated the EDM
characteristics of nickel-based heat resistant alloys;
Hastelloy-X. Bai et al.16 developed electrical discharge
surface alloying of superalloy Haynes 230 with Al and
Mo. Aspinwall et al.17 developed 3D topographic maps
of workpiece surfaces, microstructural and microhard-
ness depth-profile data of Inconel 718.
So far, the most common workpiece materials used in
the research of WEDM have been tool or die steels. Very
little research was done to investigate the machinability
of Inconel 718 with respect to WEDM. As a result, the
effects of the machining parameters on the formation of
the recast layer and the surface integrity during the
WEDM of Inconel 718 remain to be elucidated in detail.
2 EXPERIMENTAL WORK
In this study, an annealed Inconel 718 nickel alloy
with the AMD 5596 standard number was used. The
chemical composition of this material is given in Table
1. WEDM cutting experiments were performed using an
Accutex CNC WEDM machine. In the conducted experi-
ments, a CuZn37 wire with a tensile strength of 900 N/m2
and a diameter of 0.25 mm was used. In the experimental
studies, the pulse current (A), pulse duration (μs) and
dielectric flushing circulation pressure (MPa) were cho-
sen as variable parameters. With these cutting parameters
and their levels, the Taguchi L9 experimental-design
method was chosen as the basis and a total of 9 experi-
ments were performed. The experimental parameters and
their levels are provided in Table 2 and the sequence of
the experiments is provided in Table 3.
Table 3: Experimental layout using an L9 orthogonal array
Tabela 3: Eksperimentalna postavitev z uporabo L9 ortogonalne
matrike
Exp. No Pulsed current(A)
Pulse-on
duration (ìs)
Flushing
pressure (MPa)
1 8 5 1.27
2 8 7 1.47
3 8 9 1.76
4 10 5 1.47
5 10 7 1.76
6 10 9 1.27
7 12 5 1.76
8 12 7 1.27
9 12 9 1.47
At the end of the experiments, the width of each cut
was measured at five points along the cutting channel in
order to determine the kerf widths. The measurements
were performed using a profile-measuring microscope
with a sensitivity of 0.002 mm (Model 98-0001,
SCHERR-TUMICO, USA). In order to obtain the sur-
face images, determining the microstructures of the
surfaces and the heat-affected zones, the surface that was
perpendicular and adjacent to the surface that was being
machined was chosen and these surfaces were polished
using 200–1200 mesh abrasive paper and a 3-μm
diamond paste. The polished surfaces were etched with
the electrolytic-etching method in a 50 % HCl and 50 %
methanol solution and at a 4.5 V voltage.1 The micro-
structural analysis of the specimens was done with a
scanning electron microscope (SEM) and a three-dimen-
sional atomic-force microscope (AFM). In order to
determine the elements and phases on the machined
surfaces, EDS (energy dispersive spectrograph) and
XRD (X-ray diffraction) analyses were performed.
Microhardness tests were conducted with a Leica
Q500/L hardness tester using the Vickers scale. The
surface roughness was obtained using a Mitutoyo
Surftest SJ-201 portable device.
3 RESULTS AND DISCUSSION
3.1 Effect of the cutting parameters on the surface
structure
In Figure 1, an image obtained after the cutting of
the Inconel 718 nickel alloy with WEDM is shown. The
surface is composed of spherical grains detached from
the material that cannot be removed from the plain space
with the liquid pressure, debris that melt and stick to the
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118 Materiali in tehnologije / Materials and technology 50 (2016) 1, 117–125
Table 1: Chemical composition of Inconel 718
Tabela 1: Kemijska sestava Inconel 718
Element Composition (w/%)
Ni 53.60
Cr 18.20
Nb 5.06
Mo 3.04
Ti 0.97
Al 0.44
C 0.052
B 0.003
Si 0.10
S <0.002
P <0.005
Fe Balance
Table 2: Machining parameters of WEDM in this study
Tabela 2: Parametri obdelave z WEDM v tej {tudiji
Cutting conditions Settings
Level 1 Level 2 Level 3
Pulsed current (A) 8 10 12
Gap voltage (V) 39 – –
Pulse-on duration (μs) 5 7 9
Pulse-off duration (μs) 9 – –
Wire tension (N) 10 – –
Wire speed (mm/s) 15 – –
Flushing-circulation pressure (MPa) 1.27 1.47 1.76
Table-feed ratio (mm/min) 10
surface in drops, cracks, residues and randomly dis-
persed craters of various sizes. These craters are charac-
terized as cavities formed by the spherical chips
detached from the surface due to the effect of the sparks
between the wire and the workpiece during the ma-
chining. In terms of the surface appearance, it was
determined that the densities of the machined surfaces
are similar, whereas the densities of the craters and hills
on the surface are different, depending on the variations
in the current and pulse duration.
Surface images of the specimens cut under different
currents, pulse durations and fluid-circulation pressures
are seen on Figure 2. As can be seen, when the strength
of the current is constant, the surface gets rougher as the
pulse duration increases. The pulse duration is the time
duration of the discharge applied to the wire electrode. In
other words, it is the time of the spark discharge that
forms between the wire and the workpiece. Therefore, an
increase in the pulse duration leads to an increased trans-
fer of heat onto the surface of the workpiece and a for-
mation of larger craters on the surface of the workpiece.
Similarly, the surface was observed to deteriorate also
with an increased current.
An increase in the current intensity leads to a more
intense energy discharge to the workpiece surface; thus,
more chips detach from the surface. The increase in the
amount of detached chips from the surface, at the same
time, deteriorates the surface roughness.18 The results of
the experiments show that the liquid-circulation pressure
had little effect on the surface structure. Similarly, pre-
vious studies indicate that while the surface roughness
increases with the increasing current and pulse duration,
and wire fractures are prevented using an appropriate
liquid-circulation pressure, the effect of the liquid-circu-
lation pressure on the surface roughness, the thickness of
the recast layer and the microstructure is very low.9,19
3.2 Effect of the cutting parameters on the recast-layer
thicknesses
The basic metallurgical effects that occur during the
cutting with WEDM are the formation and width of the
recast layer and/or the heat-affected region. Figure 3
shows a SEM photo obtained from the surface adjacent
to a machined surface. When the image is examined, it is
seen that three different regions formed after the cutting
operation. At the very top, there is the recast layer. This
layer developed because of the rapid cooling and
re-solidification of the molten material with an auxiliary
liquid-circulation pressure, without being removed from
the cutting channel during the cutting with WEDM. The
heat-affected region is the region where no melting
occurs during the cutting, but the base metal undergoes
microstructural changes under the influence of the result-
ing heat. At the very bottom, there is the base material.
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Materiali in tehnologije / Materials and technology 50 (2016) 1, 117–125 119
Figure 1: SEM micrograph of a WEDM-cut surface of Inconel 718
Slika 1: SEM-posnetek povr{ine reza Inconel 718 po rezanju z
WEDM
Figure 2: SEM micrographs of a surface WEDM-processed under
different machining conditions
Slika 2: SEM-posnetki z WEDM obdelane povr{ine pri razli~nih po-
gojih obdelave
Figure 3: SEM of a kerf cross-section showing the RLT and HAZ
Slika 3: SEM-prikaz pre~nega preseka zareze, ki ka`e RLT in HAZ
EDS and XRD results for the recast layer are pro-
vided in Figures 4 and 5. When the analyses are eva-
luated, the results show that the recast layer is enriched
with oxygen and carbon. This phenomenon was
explained by T. R. Newton et al.20 to be the result of the
electrolysis taking place in the dielectric liquid. In addi-
tion, the presence of copper and zinc compounds in the
recast layer is evident. While Inconel 718 contains
0.08 % of mass fractions of Cu, the EDS results for the
recast layer indicate that the density of Cu increases.
Even though there is no Zn in the structure, the presence
of this element in the recast layer indicates that these
elements diffuse from the wire electrode into the
workpiece, as reported in the previous studies.21
Table 4: Microhardness distribution through the cut surface section
Tabela 4: Razporeditev mikrotrdote skozi prerezano podro~je na
povr{ini
Measurement
region
Experiment number
1 2 3 4 5 6 7 8 9
Base material 418 410 408 416 420 416 413 406 410
Recast layer 387 392 400 375 402 402 396 400 402
Heat-affected
zone 415 474 422 424 426 412 418 410 418
Table 4 gives the average microhardness values for
the three regions defined. While the hardness of the
heat-affected region shows similarity to the base mate-
rial, the microhardness of the recast layer shows a slight
decrease when compared to the base material. The
reason for this is assumed to be a decrease in the hard-
ness of the recast layer, being a result of the dissolution
of the 
’ (N3Al,Ti) secondary phases and carbides back
into the main matrix, after the heat treatment of the
specimens. In their study, T. R. Newton et al.20 indicated
that the hardness of the recast layer reduces due to the
wire-electrical-discharge machining of the age-hardened
Inconel 718 alloy. In their study, a reduced chromium
density in the recast layer and the existence of Cu and Zn
in this region cause the properties of the region to
change.
Since the outermost recast layer is in direct contact
with the environment, the determination of the thickness
of this layer plays an important role when assessing the
finish cut. Figure 6 shows the effects of the cutting para-
meters on the recast-layer thickness (RLT). In addition,
in Figure 7, SEM images show a variation in the RLT
depending on the experimental parameters. When these
figures are analyzed, it is seen that while the effect of the
circulation pressure on the recast layer is small, the
effects of the current and the pulse duration are large.
Under constant current conditions, the average RLT in-
creases with the increasing pulse duration. Since the
amount of heat transferred onto the surface of the work-
piece increases with the increasing pulse duration, the
amount of the material to be melted from the surface
increases as well.
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120 Materiali in tehnologije / Materials and technology 50 (2016) 1, 117–125
Figure 5: X-ray analysis of the recast layer
Slika 5: Rentgenska analiza pretaljene plasti
Figure 6: Main-effect plots of WEDM cutting factors for the RLT
Slika 6: Diagrami glavnih vplivnih faktorjev pri WEDM rezanju na
RLT
Figure 4: EDS analysis of the recast layer
Slika 4: EDS-analiza pretaljene plasti
As the pulse duration increases, the isothermal curves
of the material spread more into the sublayers and the
amount of the heat spreading under the surface covers a
large area. This, in turn, causes a large region to be in-
fluenced by the heat and the RLT to increase.22 More-
over, the average RLT increases considerably with
increased current values under constant pulse-duration
conditions. As the current increases, the discharge
energy gets more intense. High discharge energy causes
more material to melt. In parallel with the thickness of
the melted material, the RLT increases as well. In their
study, Kanlayasiri and Boonmung10 mentioned that in the
cases where the current and pulse duration were high, the
machined surface was rougher and thermal damages
were deeper since the discharge energy acting on the
workpiece surface was more intense. For this reason,
they indicated that the current and pulse duration should
be kept at low levels, but also that in the cases where
these two parameters are kept at low levels, the duration
and cost of the machining process increase.
3.3 Effect of the cutting parameters on the surface
roughness
During wire-electrical-discharge machining, the dis-
charge energy released with each discharge produces a
very high amount of heat at the point where the spark
hits the workpiece. This heat causes small pieces on the
surface to melt and detach via vaporization. Spherical
chips removed from the surface with each discharge
change the machined surface into a crater structure. The
surface morphology depends on the sizes of the craters
that form due to the contact between the sparks and the
surface. Therefore, the surface profile is a function of the
current and pulse duration, determining the intensity of
the applied sparks.23
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Materiali in tehnologije / Materials and technology 50 (2016) 1, 117–125 121
Figure 7: Cross-sectional SEM micrographs of a surface WEDM-processed under different machining conditions
Slika 7: SEM-posnetki presekov z WEDM obdelane povr{ine pri razli~nih pogojih obdelave
Figure 8: Main-effect plots of WEDM cutting factors for the surface
roughness
Slika 8: Diagrami glavnih vplivnih faktorjev pri WEDM rezanju na
hrapavost povr{ine
Figure 8 demonstrates the effects of the variables
such as the current intensity, the pulse duration and the
dielectric flushing pressure on the surface roughness, Ra.
The fact that the current intensity and pulse duration
have large effects on the surface roughness is clearly
seen on the graphs. If the current is low, the intensity of
the sparks that hit the surface of the piece with each
discharge is also lower. This leads to a smoother surface
due to a better erosion effect. Additionally, the amount of
heat transferred to the surface of a piece decreases with a
shortening pulse duration; therefore, less metal is melted.
Thus, with the craters being more superficial, the surface
roughness decreases.23,24
The lowest surface roughness of 2.08 μm was
measured at the end of experiment no. 1, while the
highest surface roughness of 3.01 μm was measured at
the end of experiment no. 9. In Figure 9, SEM and 3D
AFM images of specimens 1 and 9 are shown. On the
AFM image, it is possible to see the maximum and mini-
mum heights of the surface profile and hillocks and
valleys. A higher roughness can be assessed on the basis
of the values for the Z depth of the AFM profile. When
both figures are taken into account, it is observed that the
surface-roughness values of the specimens show a ten-
dency to decrease while all the cutting parameters
decrease.
3.4 Effect of the WEDM parameters on the kerf width
Figure 10 shows a schematic representation of a kerf
formed during wire-electrical-discharging. During
wire-electrical-discharge machining, pieces are removed
from the workpiece via melting as a result of the high
temperature caused by spark discharges, occurring bet-
ween the wire and the workpiece and the kerf occurs
when these pieces are removed from the intermediate
region with the liquid-circulation pressure. The kerf
width varies depending on the parameters used during
the machining. Alias et al.25 proved that as the feed ratio
increases, the kerf width decreases and, as a result of the
experiments, low feed ratios increased the kerf width.
Somashekhar and Ramachandran26 did not recommend
machining at high feed ratios as high feed rates increase
the distortions in the kerf width. Tosun et al.27 demon-
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122 Materiali in tehnologije / Materials and technology 50 (2016) 1, 117–125
Figure 9: SEM and three-dimensional AFM images of the first and
ninth experiments
Slika 9: SEM- in tridimenzionalni AFM-sliki prvega in devetega
preizkusa
Figure 11: Main-effect plots of WEDM cutting factors for the kerf
width
Slika 11: Diagrami glavnih vplivnih faktorjev pri WEDM rezanju na
{irino zareze
Figure 10: Schematic representation of the kerf width
Slika 10: Shemati~en prikaz {irine zareze
strated experimentally and mathematically that the
open-circuit voltage and the pulse duration have a high
influence on the kerf width and the material-lift ratio.
They indicated that, for the kerf-width control, the volt-
age is three times more effective than the pulse duration.
Graphs showing the effects of the experimental para-
meters on the kerf width are given in Figure 11. As it
can be seen, the average top kerf width varies dramati-
cally, depending on the variations in the current intensity
and pulse duration. An increase in the pulse duration
causes a longer-term spark shock and, accordingly, the
removal of more chips from the surface. As a result,
depending on the amount of the chips removed from the
cutting zone, the kerf width increases. Similarly, an
increase in the kerf width was observed with the
increasing current intensity. If the current is low, the
intensity of the sparks that hit the surface of the piece
with each discharge is also low. High-intensity sparks
lead to the widening of the kerf by creating a deeper
corrosion effect on the surface.
Figure 12 gives images of specimens 1 and 9 where
the lowest and highest kerf widths were obtained, respec-
tively. Under the conditions of experiment 1 where the
current intensity (8 A), pulse duration and liquid-circu-
lation pressure (5 μs, 1.27 MPa) were at their lowest
values, the kerf width was measured as 0.335 mm. Under
the conditions of experiment 9 where the current inten-
sity, pulse duration and liquid-circulation pressure (12 A,
9 μs, 1.47 MPa) were at their highest values, the kerf
width was measured as 0.375 mm.
4 STATISTICAL ANALYSIS OF THE
EXPERIMENTAL RESULTS
In this study, an analysis of the WEDM experimental
results was performed using the analysis of variance
(ANOVA) method. This method is used to statistically
examine the impacts of the machining parameters on the
operational performance. The experiment numbers and
the measurements obtained at the end of the experiments
are collectively presented in Table 5. The ANOVA
results for the RLT, surface roughness and kerf width in
the case of WEDM can be seen in Tables 6 to 8, respec-
tively. According to the results in Table 6, the current
intensity has a major effect of as much as 84 % on the
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Materiali in tehnologije / Materials and technology 50 (2016) 1, 117–125 123
Figure 12: Light micrographs of the kerf width obtained at different
cutting conditions: a) pulsed current = 8 A, pulse-on duration = 5 μs,
flushing pressure (kg/cm2) = 13, b) pulsed current = 12 A, pulse-on
duration = 9 μs, flushing pressure (kg/cm2) = 15
Slika 12: Svetlobna posnetka {irine zareze, dobljena pri razli~nih
pogojih rezanja: a) tok pulza = 8 A, trajanje pulza = 5 μs, tlak
splakovanja (kg/cm2) = 13, b) tok pulza = 12 A, trajanje pulza = 9 μs,
tlak splakovanja (kg/cm2) = 15
Table 7: Results of ANOVA for the surface roughness
Tabela 7: Rezultati ANOVA za hrapavost povr{ine
WEDM cutting
parameter
Degree of
freedom
(df)
Sum of
square
(SSA)
Mean
square F P (%)
Pulsed current
(A) 2 8.154 4.077 13.83 82.17
Pulse-on
duration (μs) 2 1.21 0.60 0.42 12.18
Flushing
pressure (MPa) 2 0.16 0.08 0.05 1.57
Error – – – – 4.08
Total 6 9.524 – – 100
Table 8: Results of ANOVA for the kerf width
Tabela 8: Rezultati ANOVA za {irino reza
WEDM cutting
parameter
Degree of
freedom
(df)
Sum of
square
(SSA)
Mean
square F P (%)
Pulsed current
(A) 2 0.000636 0.000318 3.65 54.87
Pulse-on
duration (μs) 2 0.000508 0.000254 2.34 43.83
Flushing
pressure (MPa) 2 0.000014 0.0000074 0.04 1.17
Error – – – – 0.13
Total 6 0.05939 – – 100
Table 5: Experimental results
Tabela 5: Rezultati eksperimentov
Exp. No RLT (μm)
Surface
roughness
(μm)
Kerf width
(mm)
1 2.61 2.08 0.335
2 2.65 2.31 0.348
3 2.69 2.51 0.351
4 2.73 2.61 0.349
5 2.76 2.69 0.359
6 2.80 2.89 0.365
7 2.78 2.95 0.353
8 2.84 3.00 0.367
9 2.87 3.01 0.375
Table 6: Results of ANOVA for the recast-layer thickness
Tabela 6: Rezultati ANOVA za debelino pretaljene plasti
WEDM cutting
parameter
Degree of
freedom
(df)
Sum of
square
(SSA)
Mean
square F P (%)
Pulsed current
(A) 2 0.04969 0.02484 15.11 83.65
Pulse-on
duration (μs) 2 0.00962 0.00481 0.58 16.16
Flushing
pressure (MPa) 2 0.00008 0.00004 0.00 0.15
Error – – – – 0.04
Total 6 0.05939 – – 100
RLT. The effect of the pulse duration on the RLT is 16 %,
whereas the liquid-circulation pressure has no effect on
the RLT. Similarly, Tables 7 and 8 clearly show that the
current intensity has a much larger effect on the surface
roughness and kerf width than the pulse duration.
5 MATHEMATICAL MODELING OF THE
EXPERIMENTAL RESULTS
In this study, mathematical modeling of the RLT,
surface roughness and kerf width were performed by
employing the multiple-linear-regression method on the
results of the cutting experiments with WEDM. In
addition, the appropriateness of the models obtained with
the method of regression was investigated with ANOVA.
Regression models with dependent parameters of the
current intensity (Ip), pulse duration (Ton) and liquid-cir-
culation pressure (Ps), and control parameters of the
recast-layer thickness (RLT), surface roughness (Ra) and
kerf width (Kw) are expressed as follows:
Recast-layer thickness
RLT = a0 + a1 × IP + a2 × Ton – a3 × Ps (1)
Surface roughness
Ra = a0 + a1 ×IP + a2 × Ton + a3 × Ps (2)
Kerf width
Kw = a0 + a1 × IP + a2 × Ton – a3 × Ps (3)
The regression and correlation coefficients obtained
are presented collectively in Table 9. When these coeffi-
cients are substituted into the equations, the mathema-
tical models of the recast-layer thickness (RLT), surface
roughness (Ra) and kerf width (Kw) are obtained as:
RLT = 2.18 + 0.0450 × IP + 0.0200 × Ton –
– 0.00140 × Ps (4)
Ra = 0.307 + 0.172 × IP + 0.0642 × Ton +
+ 0.0130 × Ps (5)
Kw = 0.278 + 0.00508 × IP + 0.00450 × Ton –
– 0.000325 × Ps (6)
The correlation coefficients of the models from Table
9 show that the models that are quite appropriate.
Experimental values of the recast-layer thickness (RLT),
surface roughness (Ra) and kerf width (Kw) are given in
Table 10 along with their predicted values calculated
with the obtained mathematical models.
Table 9: Regression and correlation coefficients
Tabela 9: Regresijski in korelacijski koeficienti
Regression
coefficients Correlation coefficients (r)
a0 a1 a2 a3
Recast-layer
thickness 2.18 0.0450 0.0200 0.00140 98.1 %
Surface
roughness 0.307 0.172 0.0642 0.0130 97.8 %
Kerf width 0.278 0.00508 0.00450 0.000325 95.9 %
Figures 13 to 15 show graphs of these values in their
respective order. As it is seen on the figures, the expe-
rimental and numerical values show distributions quite
close to the regression lines.
U. ÇAYDAª, M. AY: WEDM CUTTING OF INCONEL 718 NICKEL-BASED SUPERALLOY: EFFECTS ...
124 Materiali in tehnologije / Materials and technology 50 (2016) 1, 117–125
Figure 15: Comparison of experimental and predicted kerf-width
values
Slika 15: Primerjava eksperimentalno dolo~ene in napovedane {irine
zareze
Figure 13: Comparison of experimental and predicted recast-layer
thickness values
Slika 13: Primerjava eksperimentalno dolo~ene in napovedane debe-
line pretaljenega sloja
Figure 14: Comparison of experimental and predicted surface-rough-
ness values
Slika 14: Primerjava eksperimentalno dolo~ene in napovedane hrapa-
vosti povr{ine
6 CONCLUSIONS
Based on the conducted research and investigations,
the following conclusions can be drawn:
1. In the WEDM cutting process, the liquid-circulation
pressure has little effect on the recast-layer thickness,
surface roughness and kerf width, whereas the
current and pulse duration are the most important
parameters determining the quality of a cut. The
highest quality cut was obtained with experiment 1
where the current and pulse duration were at their
lowest values.
2. Considering the results of ANOVA, the intensity of
the current was statistically found to have a larger
effect on the surface roughness, kerf width and RLT
than the pulse duration.
3. Taking the correlation coefficients into account, the
obtained linear-regression models yield estimates
with appropriate error rates. These results, in turn,
show that the obtained models are appropriate.
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U. ÇAYDAª, M. AY: WEDM CUTTING OF INCONEL 718 NICKEL-BASED SUPERALLOY: EFFECTS ...
Materiali in tehnologije / Materials and technology 50 (2016) 1, 117–125 125
Table 10: Estimated values and calculations obtained with mathematical models
Tabela 10: Dolo~ene in z dobljenim matemati~nim modelom izra~unane vrednosti
Exp. No
Recast-layer
thickness Surface roughness Kerf width
Experimental
RLT
Estimated
RLT
Experimental
Ra
Estimated
Ra
Experimental
Kt
Estimated
Kt
1 2.61 2.61111 2.08 2.15889 0.335 0.334444
2 2.65 2.65444 2.31 2.26556 0.348 0.348444
3 2.69 2.68444 2.51 2.47556 0.351 0.351111
4 2.73 2.72444 2.61 2.57556 0.349 0.349111
5 2.76 2.76111 2.69 2.76889 0.359 0.358444
6 2.80 2.80444 2.89 2.84556 0.365 0.365444
7 2.78 2.78444 2.95 2.90556 0.353 0.353444
8 2.84 2.83444 3.00 2.96556 0.367 0.367111
9 2.87 2.87111 3.01 3.08889 0.375 0.374444