Materials Today: Proceedings 38 (2021) 1861–1865

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Materials Today: Proceedings

journal homepage: www.elsevier.com/locate/matpr

Effects of wire-EDM parameters on the surface integrity and mechanical

characteristics of additively manufactured Inconel 939
Ozan Can Ozaner ⇑, Gokhan Dursun, Guray Akbulut
TUSAS Engine Industries, Inc., Eskisehir, Turkey

a r t i c l e i n f o a b s t r a c t

Article history: Inconel 939 is nickel-based superalloy which offers remarkable material characteristics at high temper-
Received 7 May 2020 ature applications. It is therefore mainly used in hot section of gas turbine engines parts that require high
Accepted 19 August 2020 temperature corrosion resistance and creep performance. In this research, the W-EDM parameters such
Available online 25 September 2020
as pulse off time and pulse on time are investigated to examine their effects on surface integrity and
mechanical characteristics of additively manufactured Inconel 939 with different building direction.
Keywords: Experimental study is performed to reveal the surface integrity properties including recast layer and sur-
Wire EDM
face roughness in addition to their impacts on the tensile properties of Inconel 939.
Selective laser melting
Surface integrity
Ó 2020 Elsevier Ltd. All rights reserved.
Selection and peer-review under responsibility of the scientific committee of the International Confer-
ence on Modern Trends in Manufacturing Technologies and Equipment 2020.

1. Introduction methods such as electrolyzing, erosion or fusion techniques are

considered to meet required tolerances in preference to conven-
The operating conditions of gas turbine technology require tional techniques. The electrical discharge machining (EDM) is
exceptional material properties at extreme loading environments based on the erosion of part surface with non-contact material
of high temperature applications. Therefore, advanced properties subtraction of engineering components [2]. With the application
under severe service conditions are considered as critical aspects of Wire EDM (W-EDM), it had been made possible to utilize mate-
for aerospace industry so as to endure elevated operating require- rials with high thermo-mechanical stress that otherwise would be
ments. In this sense, Nickel-based superalloys are excellent prefer- challenging to process [3].
ences to be used in hot sections of engines exposing to high Wire EDM offers numerous advantages in manufacturing appli-
mechanical and thermal stresses through excellent corrosion and cations, such as producing of sharp corners and turbine disk root
wear resistance to achieve intended demands. In contrast to the slots [3]. In the meantime, Wire EDM can be preferably used for
conventional manufacturing techniques, the current study processing the holes rather than milling since the vibrations gener-
employs the Laser Bed Powder Fusion (LBPF) technology to process ated during milling may cause the instability in surface profile. In
Inconel 939 nickel superalloy. The LBPF is an innovative manufac- contrast, there are disadvantages to be considered since EDM is a
turing technique that gains significant attention from industry by thermal process and operated at high temperatures, yielding the
allowing the influential freedom of design and manufacturing iter- differences on the surface topography, microstructure, residual
ations. Accordingly, the time spent for product development with stress, and micro-hardness of parts being processed [4] as well as
intricate geometries does not take as much of the conventional the micro cracks induced during the process cycle [5].
techniques. The Inconel 939 utilized in this research is a Ni- It is of great importance that surface properties of parts are crit-
based super alloy that is considered to be used at service temper- ical factors for the reason that surface-initiated crack mechanism
atures up to 850 °C owing to outstanding corrosion resistance and deteriorates the overall mechanical properties and shortens the
high temperature material properties [1]. service-life [6,7]. Meanwhile, the surfaces of engineering compo-
Many different applications are employed for the production of nents machined by Wire EDM may result in considerably different
parts in aerospace industry that have narrow tolerance limits. The characteristics depending on the varying process parameters and
eventually leads the variation in mechanical properties [8]. It has
been identified that the pulse off, pulse on and voltage have been
⇑ Corresponding author.
regarded as major parameters of Wire EDM [4,9]. As for the
E-mail address: ozancan.ozaner@tei.com.tr (O.C. Ozaner).

https://doi.org/10.1016/j.matpr.2020.08.486
2214-7853/Ó 2020 Elsevier Ltd. All rights reserved.
Selection and peer-review under responsibility of the scientific committee of the International Conference on Modern Trends in Manufacturing Technologies and Equipment
2020.
O.C. Ozaner et al. Materials Today: Proceedings 38 (2021) 1861–1865

Table 1
The Nominal Chemical Composition of Inconel 939 in wt%

Cr Co C Fe W Mo Nb Ta Ti Al Zr B Ni
22.5 19.0 0.15 – 2.0 – 1.0 1.4 3.7 1.9 0.1 0.01 Bal.

material removal rate, the pulse on and pulse off are considered dimension of 40
16
9 mm are produced and machined by W-
more significant parameters whereas the voltage and pulse on EDM. The experimental studies were performed on Agie Charmilles
have more effects on surface quality [9]. In literature, pulse on CUT300Sp Wire-EDM machine. Thermo Tex copper wire with
and pulse off have been investigated extensively since they are diameter of 0.30 mm was used in the experiments. Afterwards,
considered as substantial parameters affecting the discharged the specimens are etched with Schanz prior to the investigation
energy [4,5,9]. through optical microscope so as to reveal the effects of varying
To the best of author’s knowledge, no studies have been parameters on the thickness recast layer. The recast measurements
reported associated with surface integrity of Inconel 939 processed is conducted based on the continuous thickness identified on the
by means of W-EDM. Therefore, the current research attempts to surface of each sample at multiple locations and average value is
reveal the detailed analysis of surface properties of Inconel 939 calculated. Similarly, the final geometry of dog bone shaped tensile
specimens produced by Laser Bed Powder Fusion and Wire EDM. samples are extracted from 125
26
3 mm prism based on the
Furthermore, the tensile properties of test samples are revealed varying processing parameters and all surfaces are machined by
with respect to Wire EDM parameters. The design of experiment Wire EDM, as seen both in Fig. 1.
is generated to extract the influence of process parameters of Wire Based on the different number of passes, the experimental
EDM on the surface characteristics and the tensile strength of design and process parameters are chosen for sample processing
specimens. in a single pass for Inconel 939. In this way, different energy dis-
charges are achieved in a single pass with the parameters used.
The surface morphology of specimens was examined through Poly-
1.1. Materials and methods
tec TMS-500 profilometer by extracting the three dimensional
topography with a cut-off length of 0.8 mm at multiple locations.
The fabrication of Inconel 939 specimens was conducted using
The area used for 3D surface analysis on gauge length of tensile
EOS M290 Direct Metal Laser Sintering (DMLS) machine equipped
has dimensional area of 22 mm
6 mm. The surface analysis was
with 400 W Yb-fiber laser. The dog bone shaped tensile specimens
conducted prior to tensile tests in order to facilitate the under-
based on the ASTM E8 standard were designed in CAD model and
standing of results with respect to surface properties. Accordingly,
placed on the build plate in both parallel and perpendicular,
tensile tests were conducted using servo-hydraulic machine in
referred as XY and Z samples, respectively. The build platform tem-
ambient air.
perature was pre-heated 80 °C and fabrication was performed
under argon gas atmosphere in order to prevent oxidation. After-
wards, they were cut off the platform using Wire Electrical Dis-
charge Machining. Accordingly, the heat treatment of solution
annealing followed by aging were applied to samples, 4 h at
1160 °C and 16 h at 850 °C, respectively. The chemical composition
of Inconel 939 is demonstrated in Table 1 [10].
The design of experiments is carried out based on Taguchi L18
orthogonal array of W-EDM process parameters as detailed in
Table 2. Four factors with three levels of input parameters, namely
pulse on, pulse off and open voltage are correlated in experimental
design with two levels of building directions, and remaining Fig. 1. The recast measurement samples and tensile geometries processed by W-
parameters are kept constant. The rectangular prism samples with EDM.

Table 2
Experimental results.

Sample No Pulse on (ms) Pulse off (ms) Voltage (V) Recast layer (mm) Surface roughness Sa (mm) Tensile strength (MPa)
and direction
1 XY 0.1 3 80 3,88 2,72 1470
2 XY 0.1 6 120 2,54 3,62 1460
3 XY 0.1 12 200 2,58 3,60 1420
4 XY 0.4 3 80 4,03 4,11 1360
5 XY 0.4 6 120 2,55 4,43 1420
6 XY 0.4 12 200 3,34 3,65 1430
7 XY 0.6 3 120 5,34 8,12 1440
8 XY 0.6 6 200 5,00 4,75 1420
9 XY 0.6 12 80 3,00 4,29 1460
10 Z 0.1 3 200 3,27 4,22 1420
11 Z 0.1 6 80 2,55 2,50 1450
12 Z 0.1 12 120 1,67 2,90 1420
13 Z 0.4 3 120 2,64 3,54 1430
14 Z 0.4 6 200 3,87 4,46 1340
15 Z 0.4 12 80 4,56 3,28 1420
16 Z 0.6 3 200 3,38 5,07 1420
17 Z 0.6 6 80 3,33 4,03 1420
18 Z 0.6 12 120 4,25 4,18 1410

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O.C. Ozaner et al. Materials Today: Proceedings 38 (2021) 1861–1865

2.1. Recast layer thickness

Fig. 2 shows the optical image of recast layers for some of spec-
imens processed with pre-defined parameters. Additionally, Table 2
presents that varying recast thickness values have been attained
based on the different combination of parameters. The results of
the current work indicate that the variation in recast thickness
on Inconel 939 specimens considerably depends on the machining
parameters. As it can be seen in Fig. 5, mean value of recast thick-
ness noticeably increases with higher pulse on and shows constant
growth in contrast with remaining parameters. Considering the
pulse on, similar results have been obtained previously in litera-
ture [5]. The increasing trend of recast layer with respect to pulse
on can be associated with excessive heat input and eventually the
increased discharge energy, which is calculated using Eq. (1) as
follows;
Z te
1
Ee ¼ V e ðt ÞIe ðt Þdt ¼ V e Ie t on ð1Þ
0 ton þ t off

where Ve stands for discharge voltage, Ie is discharge current, te rep-

resents discharge duration, ton is pulse on time and toff is pulse off
time. Eq. (1) shows discharge energy highly depends on above-
mentioned parameters and in particular, pulse on and voltage are
expected to be more effective rather than pulse off.
As for the pulse off, higher values led a decrease in continuous
recast layer thickness, as seen in Fig. 5. During Wire EDM process,
continuous flushing is performed from upper and lower heads to
move away the material that removed from workpiece in pulse
off time. Therefore, longer pulse time leads molten material to be
removed without re-solidification and eventually improved recast
characteristic. In the meantime, the ratio of pulse off and pulse on
is found to be effective on recast growth, as it is seen in Fig. 4
increasing pulse off/pulse on results in reduced recast layer
Fig. 2. Optical image of recast layers for some of specimens. thickness.
In general, increasing pulse off at constant pulse on and voltage
leads reduced recast layer thickness. The overall trend in reducing
recast thickness has been illustrated in Fig. 3, revealing that almost
2. Results and discussion all samples have smaller recast with increasing pulse off except
two samples. However, sample 8 and sample 15 show different
The results of recast layer thickness, surface roughness and ten- trend in comparison with remaining specimens, which is attribu-
sile strength of specimens based on the experimental design of ted to local molten material solidification in certain locations of
machining parameters are summarized in Table 2. deep craters as a result of insufficient material removal as seen
in Fig. 2. The craters generally form owing to unsteady discharges,

Fig. 3. Pulse Off/Pulse On ratio vs Recast layer thickness.

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O.C. Ozaner et al. Materials Today: Proceedings 38 (2021) 1861–1865

roughness owing to high pulse off allowing additional time for

flushing during processing, as seen in Fig. 2 for sample 12 and sam-
ple 16. Additionally, a correlation between surface roughness and
recast thickness has been noticed that the surface roughness
increases in the same way with increasing recast thickness as
depicted.
Furthermore, the surface roughness can be assumed to be pro-
portional to the thickness of recast layer. However, sample 8 and
15 illustrate the opposite situation. The reason is that formed cra-
ter is filled with molten material as show in Fig. 2. In contrast, in
sample 9, although crater was formed, surface roughness increased
even though recast layer thickness decreased. Because the molten
material has gone away without filling the crater.

2.3. Tensile strength

Fig. 4. Pulse Off vs recast layer thickness. The ultimate tensile strength of samples are presented in
Table 2. Fig. 5c demonstrates the tensile strength behavior of
Inconel 939 specimens based on building directions along with dif-
and crater diameter can be explained as function of discharge cur-
rent and pulse off time. During EDM process discharge current can
be changed and this may lead unstable cases resulting in formation
of craters.
On the other hand, it has been detected that voltage effects on
recast formation has no consist change as the increasing voltage
leads a slight decrease in recast thickness with 120 V, and eventu-
ally shows an increase with 200 V. The current work utilizes open
voltage, which is a function of distance between the workpiece and
wire. As the open voltage value increases, the distance between
part and wire expands proportionally resulting in larger zone for
the removal of raw material. As a result, reduced recast layer thick-
ness is expected to be achieved with increasing voltage; however,
the recast growth also depends on interaction of parameters each
other. Even if the high voltage is provided for adequate distance,
insufficient pulse off time yields stacking of molten material
between workpiece and wire.
It is noteworthy that EDM has many inputs and the optimum
machining parameters should be determined by considering the
interaction of remaining parameters such as flushing, current and
distance between nozzles. Accordingly, the optimum values are
crucial for these parameters to achieve improved recast layer. As
a result, it is shown that pulse on is more influential input param-
eter as regards the recast thickness since the fluctuations are
noticeably more than that of pulse off and voltage.

2.2. Surface roughness

Based on the changing machining parameters, different surface

roughness results have been attained and presented in Table 2. The
results of roughness values exhibit that surface quality of Inconel
939 is affected by varying input parameters same as the recast
layer growth. Correspondingly, similar results of process parame-
ters have been noticed along with recast layer development. For
instance, the effect of building direction indicates gradually a
decrease with Z direction similar to that of recast thickness. On
the other hand, an increase in surface roughness has been observed
with higher pulse on, changing mean value of surface roughness
from 3.26 mm to 5.07 mm which is in good agreement with litera-
ture [5,11] as the high discharged energy induced by increased
pulse on leads rough surface textures. Eventually, high discharged
energy resulted in larger craters as shown in sample 7 (Fig. 2) due
to removal of material on part surface as a result of vaporizing dur-
ing process. On the contrary, increasing pulse off resulted in notice-
ably decrease in roughness. This is because of fact that the craters Fig. 5. Main effects plots for Recast Layer, Surface Roughness and Ultimate Tensile
depth reduces with decreased recast thickness and surface Strength.

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ferent Wire EDM input parameters. Due to the fact that additive  Different building direction of Inconel 939 specimens led varia-
manufactured parts indicate anisotropy in grain growth mecha- tion in tensile strength attributing the fact that elongated grains
nism in a dissimilar manner based on building direction, tensile generated during fabrication yields anisotropy in microstruc-
strength of specimens shows noticeable variation as the Z samples ture and mechanical characteristics.
have mean of 1414 MPa whilst the XY samples higher mean value
of 1431 MPa. This variance is primarily attributed to inconsistency
of microstructure generated by elongated grains as a result of dif- Declaration of Competing Interest
ferent building direction [12]. As for the pulse on and pulse off, no
consist change has been observed considering tensile strength of The authors declare that they have no known competing finan-
Inconel 939 specimens, however, the largest variation is noticed cial interests or personal relationships that could have appeared
in varying pulse on value since its range is more than remaining to influence the work reported in this paper.
parameters. On the other hand, increasing voltage up to 200 V
resulted in apparent decrease of mean tensile strength from Acknowledgement
1430 MPa to 1408 MPa. From the Fig. 5b, it is observed that higher
voltage leads the rougher surface texture and eventually resulted Authors would like to express sincere gratitude to TUSAS
in detrimental effect on tensile strength. In the meantime, the Engine Industry (TEI) for generous support to current study.
kinetic energy of ions and electrons, which are essential factors
of erosion between part and wire, increases as a result of more References
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