S.-H. KIM et al.: SURFACE CHARACTERISTICS OF INVAR ALLOY ACCORDING TO MICRO-PULSE
745–749
SURFACE CHARACTERISTICS OF INVAR ALLOY ACCORDING
TO MICRO-PULSE ELECTROCHEMICAL MACHINING
KARAKTERISTIKE POVR[INE INVAR ZLITINE GLEDE NA
MIKROPULZNO ELEKTROKEMI^NO OBDELAVO
Seong-Hyun Kim, Seung-Geon Choi, Woong-Kirl Choi, Eun-Sang Lee
Inha University, Department of Mechanical Engineering, 253 Yonghyun-Dong, Nam-Gu, Incheon 402-751, Republic of Korea
kimseonghyun@inha.edu, leees@inha.ac.kr
Prejem rokopisa – received: 2016-07-15; sprejem za objavo – accepted for publication: 2017-01-24
doi:10.17222/mit.2016.187
Invar is a 36 % nickel iron alloy that has a low thermal expansion, compared to other metals and alloys, at temperatures ranging
from room temperature up to approximately 230 °C. Invar alloy is ductile, easily weldable and its machinability is similar to that
of austenitic stainless steel. Due to the low thermal expansion of Invar, it is used for shadow masks for display devices such as
UHDTV – organic light-emitting diode. In this study, micro-pulse electrochemical machining (MPECM), which is a non-contact
ultra-precision machining method, was developed to manufacture Invar sheets; optimum parameters of MPECM were defined
and the basic MPECM experiments were carried out on an Invar sheet. The optimum parameters were determined with pulse-on
time and duty-ratio analysis. The experimental results show that MPECM is hard to control. Therefore, using ultrashort charging
times and very high pulses, it is possible to achieve a successful anodic dissolution at a very small electrode gap. Hence, a
longer pulse-on time and a small electrode gap may provide the scope for further improvement of the machining accuracy by
controlling the localization effect. Furthermore, the machining depth and MPECM efficiency were investigated with respect to
various parameters and pulse-on time, considering different duty-ratio conditions.
Keywords: Invar alloy, electrochemical machining, surface characteristics
Invar je 36 % zlitina niklja in `eleza, ki ima v primerjavi z vsemi kovinami in zlitinami, nizek koeficient toplotnega raztezka v
obmo~ju sobne temperature pa do okoli 230 ° C. Invar zlitina je duktilna, primerna za enostavno varjenje in njena obdelovalnost
je podobna avstenitnemu nerjavnemu jeklu. Zaradi nizkega koeficienta termi~nega raztezka, se zlitina Invar uporablja v maskah
za sen~enje displejev v OLED-UHD TV (angl.: Organic Light Emitting Diode – Ultra High Definition TeleVision) napravah. V
tej {tudiji je bila razvita metoda mikropulzne elektrokemi~ne obdelave kovin (angl. MPECM), ki je brezkontaktna ultraprecizna
tehnologija mehanske obdelave. Izvedeni so bili osnovni poskusi na zlitini Invar. Dolo~eni so bili optimalni parametri postopka.
Eksperimentalni rezultati ka`ejo, da je obdelovalnost z MPECM te`ko nadzorovati. Izjemno kratki ~asi, z zelo visokimi
napetostnimi impulzi, omogo~ajo nastanek pogojev za uspe{no anodno raztapljanje kovine pri zelo majhni re`i elektrode. Zato
se pri zelo visokem {tevilov impulzov in manj{em razmiku elektrod, lahko zagotovi nadaljnje izbolj{anje to~nosti obdelave z
MPECM. Nadalje je bila raziskana obdelovalna globina in obdelovalnost z MPECM pri razli~nih procesnih parametrih
(napetost-~as-storilnost).
Klju~ne besede: Invar zlitina, elektrokemi~na obdelava, povr{inske karakteristike
1 INTRODUCTION
Invar is an alloy of 64 % iron and 36 % nickel. Its
coefficient of expansion is very low, namely 1.8 × 10–6
cm/°C, at temperatures ranging from a cryogenic tem-
perature of –196 °C to a Currie temperature of 260 °C.
This value is 10 times lower than that of stainless steel
and 100 times lower than that of iron, which makes Invar
appropriate for applications in various fields of study.
Invar is used in ultra-high-definition televisions, display
shadow masks, organic light-emitting diodes, controllers
for quality, colors, and for the definition of images in
mobile devices and other instruments that require precise
measurements.1–3 Invar is also used in electron-gun
electrodes (devices that eject an electron beam into the
hole in a shadow mask), lead frames (to protect semicon-
ductor chips from external damage) and other industrial
devices.
Invar is currently manufactured with mechanical pro-
cesses or electroforming; other methodologies have also
been suggested. To maintain the specific characteristics
of Invar alloys, advanced machining methods are being
studied, such as laser machining, supersonic-wave ma-
chining, micro-end mills/drilling, electropolishing and
electrochemical machining.4–6 Moreover, the effects of
hydrodynamics and temperature on the electrode po-
sition of Fe–Ni (Invar) alloys has been investigated with
DC, pulse, and pulse-reverse electrodeposition tech-
niques.7,8 MPECM has seen a resurgence of industrial
interest within the last decade, due to its many
advantages, such as no tool wear, stress-free and smooth
surfaces, and the ability to machine complexly shaped
products made from different materials, regardless of
their hardness and high strength, high tension, or
whether they are heat-resistant materials.9 In this study,
the possibility of pulse electrochemical machining is
investigated by analyzing the machinability characteris-
tics of processed Invar prior to pulse electrochemical
machining.
Materiali in tehnologije / Materials and technology 51 (2017) 5, 745–749 745
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 62-4-023.7:669.018.47:621.039.333 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(5)745(2017)
2 EXPERIMENTAL PART
2.1 Theory of the MPECM system
Figure 1 shows a schematic diagram of the combined
electrochemical process and the MPECM set-up with a
previously developed microprobe. A pulsed voltage from
a high-voltage pulse generator (Hewlett-Packard
HP8116A) is applied to the gap between the anode and
cathode, and the electrolyte solution is supplied through
a hole in the cathode or an external electrolyte supplier.
This homogenizes the electrochemical reaction and
removes the gas and heat generated. The anode is the
Invar alloy with a thickness of 30 μm and the cathode is
stainless steel with a 1-mm diameter.
The fully programmable 50 MHz pulse generator has
a variable pulse width (10 ns min) and the maximum
output amplitude of the maximum voltage of 32 V p-p
(in an open circuit). The high-speed data-acquisition sys-
tem was provided by a TDS 2002C digital oscilloscope
(Tektronix) with a bandwidth of 200 MHz and a
sampling rate of 500 MS/s. Typical experimental condi-
tions are shown in Table 1.
Table 1: Experimental conditions
Parameter Value
Pulse generator
spec.
HP8116A (Hewlett-Packard), 50MHz,
32 V p-p, various (10 ns) pulse widths
Workpiece (anode) Invar (Fe 64 %, Ni 36 %) 30-μmthickness
Electrode (cathode)
Stainless steel 304 (ø 1 mm)
Non-insulation, Insulation
Digital oscilloscope Tektronix TDS 2002C
MPECM time 40 min (pulse-on time: 500 ns-2 μs,Duty factor: 33–66 %)
Applied voltage (V) 16
Electrode gap (μm) 50
Electrolyte Sodium nitrite + sodium tartrate + DIwater
Measurement
Optical microscopic analysis, Scanning
electron microscopy (SEM),
Non-contact 3D profile
2.2 Electrode type
The concentration of the current at the edges and
irregular parts of an electrode can lead to localized
sedimentation on the anode surface. A specially designed
electrode is recommended for electrochemical micro-
machining to control the dimensions of the Invar surface.
Figure 2 compares the surfaces that were electrochemi-
cally micro-machined using two different electrodes.
Electrode 1, with the traditional design, had a concen-
trated current at the edge, which is marked as a 2.
Electrode 2 has a non-conducting part that is marked as a
3. The current distribution for electrode 2 is uniform on
the surface, which is marked as a 4. Localized sedimen-
tation was identified on the anode workpiece surface in
the shape profile created by electrode 1, but none was
identified for electrode 2. This results from non-conduct-
ing part 3 on the side of the electrode. The cathode
design should be taken into account to precisely control
the shape and prevent sedimentation.
When the electrodes are far apart, the voltage flows
in the regions other than the processed area, even when
no machining is intended. To reduce the unwanted
voltage flowing in these regions, an electrode that is
coated on the top can be used as the tool. To minimize
this voltage and observe the behavior of the machining
current, the sides of the tool need to be completely
insulated.
3 RESULTS AND DISCUSSION
3.1 Machining characteristics versus the pulse-on time
using a non-insulated electrode
The machining characteristics were studied using a
voltage of 16 V, a gap of 50 μm a pulse-off time of 1 μs
and a machining time of 40 min. An insulated electrode
was used, and the pulse-on time was changed from
500 ns to 2 μs. Figure 3 shows that the machining radius
was larger as the pulse-on time increased. The processed
radius was 559.04 μm at the pulse-on time of 500 ns,
620.76 μm at 600 ns, 675.21 μm at 800 ns, 689.73 μm at
1 μs, 693.36 μm at 1.5 μs, and 715.14 μm at 2 μs. When
the pulse-on time was 1 μs or higher, the machining
radius drastically increased. Figure 4 shows the results
after the machining with respect to the pulse-on time.
S.-H. KIM et al.: SURFACE CHARACTERISTICS OF INVAR ALLOY ACCORDING TO MICRO-PULSE
746 Materiali in tehnologije / Materials and technology 51 (2017) 5, 745–749
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Comparison between two different electrodes
Figure 1: Micro-pulse electrochemical machining system
When the pulse-on time was 500–800 ns, the machinabi-
lity declined due to a low current density, but at 1–2 μs,
the machinability improved due to a high current density.
The precision and the resulting image became better and
better.
3.2 Machining characteristics versus the pulse-on time
using an insulated electrode
The machining characteristics were also examined
using an insulated electrode, with the other conditions
kept the same as in the previous experiment. Figure 5
shows that the machining radius was consistent with the
increase in the pulse-on time. The processed radius was
642.54 μm at 500 ns, 656.81 μm at 600 ns, 644.76 μm at
800 ns, 638.72 μm at 1 μs, 647.77 μm at 1.5 μs, and
656.81 μm at 2 μs.
Figure 6 shows the results after the machining. The
machining may occur on the sides of the electrode with-
out the insulation, but the insulated electrode prevents
this and makes the machining consistent with a very fine
outcome.
3.3 Surface analysis versus the pulse-on time using
insulated and uninsulated electrodes
Figure 7a shows a SEM image of the Invar film
surface after the machining with the uninsulated elec-
trode. The processed area is clearly displayed. Figure 7b
shows the non-contact 3D profile measured. The tapering
remained on the side of the Invar film surface after the
experiment. Figure 7c shows the results for the pulse-on
time of 2 μs, which allowed the best machinability with
both types of electrode. Figure 8 shows the equivalent
results obtained with the insulated electrode. The voltage
in Figure 8c is slightly increased compared to Figure
7c.
With the uninsulated electrode, the processed radius
is increased, with almost no influence on the depth. This
is attributable to the parts other than the intended ma-
chining area with the repeated charging and discharging
in the dual layer of electricity because the voltage pulse
is exhausted on the other sides of the uninsulated elec-
trode. However, with the insulated electrode, the voltage
is concentrated only in the machining area, so the radius
is consistently maintained and the machining depth
increases. Figures 7c and 8c show that the voltage
permitted during the actual machining is lower with the
uninsulated electrode than with the insulated one.
S.-H. KIM et al.: SURFACE CHARACTERISTICS OF INVAR ALLOY ACCORDING TO MICRO-PULSE
Materiali in tehnologije / Materials and technology 51 (2017) 5, 745–749 747
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: Crater-shape patterns with pulse-on time / duty factor and
insulated electrode: a) 500 ns / 33 %, b) 600 ns / 37 %, c) 800 ns / 44
%, d) 1 μs / 50 %, e) 1.5 μs / 60 %, f) 2 μs / 66 %
Figure 5: Increase of hole-diameter size versus pulse-on time
Figure 3: Increase of hole-diameter size versus pulse-on time
Figure 4: Crater-shape patterns with pulse-on time / duty factor and
non-insulated electrode: a) 500 ns / 33 %, b) 600 ns / 37 %, c) 800 ns /
40 %, d) 1 μs / 50 %, e) 1.5 μs / 60 %, f) 2 μs / 66 %
2 EXPERIMENTAL PART
2.1 Theory of the MPECM system
Figure 1 shows a schematic diagram of the combined
electrochemical process and the MPECM set-up with a
previously developed microprobe. A pulsed voltage from
a high-voltage pulse generator (Hewlett-Packard
HP8116A) is applied to the gap between the anode and
cathode, and the electrolyte solution is supplied through
a hole in the cathode or an external electrolyte supplier.
This homogenizes the electrochemical reaction and
removes the gas and heat generated. The anode is the
Invar alloy with a thickness of 30 μm and the cathode is
stainless steel with a 1-mm diameter.
The fully programmable 50 MHz pulse generator has
a variable pulse width (10 ns min) and the maximum
output amplitude of the maximum voltage of 32 V p-p
(in an open circuit). The high-speed data-acquisition sys-
tem was provided by a TDS 2002C digital oscilloscope
(Tektronix) with a bandwidth of 200 MHz and a
sampling rate of 500 MS/s. Typical experimental condi-
tions are shown in Table 1.
Table 1: Experimental conditions
Parameter Value
Pulse generator
spec.
HP8116A (Hewlett-Packard), 50MHz,
32 V p-p, various (10 ns) pulse widths
Workpiece (anode) Invar (Fe 64 %, Ni 36 %) 30-μmthickness
Electrode (cathode)
Stainless steel 304 (ø 1 mm)
Non-insulation, Insulation
Digital oscilloscope Tektronix TDS 2002C
MPECM time 40 min (pulse-on time: 500 ns-2 μs,Duty factor: 33–66 %)
Applied voltage (V) 16
Electrode gap (μm) 50
Electrolyte Sodium nitrite + sodium tartrate + DIwater
Measurement
Optical microscopic analysis, Scanning
electron microscopy (SEM),
Non-contact 3D profile
2.2 Electrode type
The concentration of the current at the edges and
irregular parts of an electrode can lead to localized
sedimentation on the anode surface. A specially designed
electrode is recommended for electrochemical micro-
machining to control the dimensions of the Invar surface.
Figure 2 compares the surfaces that were electrochemi-
cally micro-machined using two different electrodes.
Electrode 1, with the traditional design, had a concen-
trated current at the edge, which is marked as a 2.
Electrode 2 has a non-conducting part that is marked as a
3. The current distribution for electrode 2 is uniform on
the surface, which is marked as a 4. Localized sedimen-
tation was identified on the anode workpiece surface in
the shape profile created by electrode 1, but none was
identified for electrode 2. This results from non-conduct-
ing part 3 on the side of the electrode. The cathode
design should be taken into account to precisely control
the shape and prevent sedimentation.
When the electrodes are far apart, the voltage flows
in the regions other than the processed area, even when
no machining is intended. To reduce the unwanted
voltage flowing in these regions, an electrode that is
coated on the top can be used as the tool. To minimize
this voltage and observe the behavior of the machining
current, the sides of the tool need to be completely
insulated.
3 RESULTS AND DISCUSSION
3.1 Machining characteristics versus the pulse-on time
using a non-insulated electrode
The machining characteristics were studied using a
voltage of 16 V, a gap of 50 μm a pulse-off time of 1 μs
and a machining time of 40 min. An insulated electrode
was used, and the pulse-on time was changed from
500 ns to 2 μs. Figure 3 shows that the machining radius
was larger as the pulse-on time increased. The processed
radius was 559.04 μm at the pulse-on time of 500 ns,
620.76 μm at 600 ns, 675.21 μm at 800 ns, 689.73 μm at
1 μs, 693.36 μm at 1.5 μs, and 715.14 μm at 2 μs. When
the pulse-on time was 1 μs or higher, the machining
radius drastically increased. Figure 4 shows the results
after the machining with respect to the pulse-on time.
S.-H. KIM et al.: SURFACE CHARACTERISTICS OF INVAR ALLOY ACCORDING TO MICRO-PULSE
746 Materiali in tehnologije / Materials and technology 51 (2017) 5, 745–749
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Comparison between two different electrodes
Figure 1: Micro-pulse electrochemical machining system
When the pulse-on time was 500–800 ns, the machinabi-
lity declined due to a low current density, but at 1–2 μs,
the machinability improved due to a high current density.
The precision and the resulting image became better and
better.
3.2 Machining characteristics versus the pulse-on time
using an insulated electrode
The machining characteristics were also examined
using an insulated electrode, with the other conditions
kept the same as in the previous experiment. Figure 5
shows that the machining radius was consistent with the
increase in the pulse-on time. The processed radius was
642.54 μm at 500 ns, 656.81 μm at 600 ns, 644.76 μm at
800 ns, 638.72 μm at 1 μs, 647.77 μm at 1.5 μs, and
656.81 μm at 2 μs.
Figure 6 shows the results after the machining. The
machining may occur on the sides of the electrode with-
out the insulation, but the insulated electrode prevents
this and makes the machining consistent with a very fine
outcome.
3.3 Surface analysis versus the pulse-on time using
insulated and uninsulated electrodes
Figure 7a shows a SEM image of the Invar film
surface after the machining with the uninsulated elec-
trode. The processed area is clearly displayed. Figure 7b
shows the non-contact 3D profile measured. The tapering
remained on the side of the Invar film surface after the
experiment. Figure 7c shows the results for the pulse-on
time of 2 μs, which allowed the best machinability with
both types of electrode. Figure 8 shows the equivalent
results obtained with the insulated electrode. The voltage
in Figure 8c is slightly increased compared to Figure
7c.
With the uninsulated electrode, the processed radius
is increased, with almost no influence on the depth. This
is attributable to the parts other than the intended ma-
chining area with the repeated charging and discharging
in the dual layer of electricity because the voltage pulse
is exhausted on the other sides of the uninsulated elec-
trode. However, with the insulated electrode, the voltage
is concentrated only in the machining area, so the radius
is consistently maintained and the machining depth
increases. Figures 7c and 8c show that the voltage
permitted during the actual machining is lower with the
uninsulated electrode than with the insulated one.
S.-H. KIM et al.: SURFACE CHARACTERISTICS OF INVAR ALLOY ACCORDING TO MICRO-PULSE
Materiali in tehnologije / Materials and technology 51 (2017) 5, 745–749 747
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: Crater-shape patterns with pulse-on time / duty factor and
insulated electrode: a) 500 ns / 33 %, b) 600 ns / 37 %, c) 800 ns / 44
%, d) 1 μs / 50 %, e) 1.5 μs / 60 %, f) 2 μs / 66 %
Figure 5: Increase of hole-diameter size versus pulse-on time
Figure 3: Increase of hole-diameter size versus pulse-on time
Figure 4: Crater-shape patterns with pulse-on time / duty factor and
non-insulated electrode: a) 500 ns / 33 %, b) 600 ns / 37 %, c) 800 ns /
40 %, d) 1 μs / 50 %, e) 1.5 μs / 60 %, f) 2 μs / 66 %
4 CONCLUSIONS
This study demonstrated that MPECM has suitable
process characteristics for machining Invar-alloy sur-
faces. The machined shape and surface qualities, such as
the surface condition and the machining defect, were
observed. The important factors influencing the hole-
shape quality of the Invar alloy are the pulse-on time, the
duty factor, the shape of the cathode and the electrolytic
process parameters. During the application of MPECM
to a fine sheet metal, the electrochemical machining
characteristics of the surface of the alloy were revealed
and analyzed with respect to different electrode shapes,
pulse-on times and applied voltages.
When the uninsulated electrode was used, the
machining speed differed depending on the area of the
tool electrode that was submerged in the liquid. With a
greater machining depth, the machining speed was re-
duced and there was almost no machining after reaching
a certain limit. This is attributable to the current being
exhausted because of the repeated charging and dis-
charging in the dual layer of electricity by the pulse, for
any part other than the intended machining part.
The insulated electrode prevents the permitted volt-
age from being exhausted and improves the machin-
ability with the increasing machining depth. In addition,
since the voltage flows only into the machining area, a
taper does not occur since the sides of the electrode are
well insulated. Accordingly, if insulated electrodes are
used, it is possible to increase the precision of the form
and the processing depth.
This study indicates that the electrode shape and
suitable MPECM conditions affect the machinability,
and that there is a close interrelation between the electric
field on the Invar-alloy surface and the polishing rate. A
comparison of the insulated and uninsulated electrodes
was made, and the effect of the pulse-on time on the
electrochemical machinability was determined. MPECM
can be recommended to practitioners in different fields
to improve the conditions for making holes in the Invar
alloy.
The electrolyte of sodium nitrite + sodium tartrate +
DI water was used and holes with a ø1 mm diameter
were made into the 30-μm-thick Invar alloy. The future
work, therefore, can be made when more engineering
data has been gathered. In addition, more research needs
to be done on the related factors, such as the surface
roughness and the shape accuracy, using controlled
micro-pulses.
Acknowledgment
This work was supported by the Inha University
Research Foundation, Korea.
5 REFERENCES
1 D. Grimmentt, M. Schwartz, K. Node, A comparison of DC and
pulsed Fe–Ni alloy deposits, Journal of The Electrochemical Society,
140 (1993) 4, 973–978, doi:10.1149/1.2056238
2 A. Stupnik, M. Leisch, Study on the surface topology of
vacuum-fired stainless steel by scanning tunnelling microscopy,
Vacuum, 81 (2007) 6, 748–751, doi:10.1016/j.vacuum.2005.11.061
3 L. S. Andrade, S. C. Xavier, R. C. Rocha-Filho, N. Bocchi, S. R.
Biaggio, Electropolishing of AISI-304 stainless steel using an
oxidizing solution originally used for electrochemical coloration,
Electrochimica Acta, 50 (2005) 13, 2623–2627, doi:10.1016/
j.electacta.2004.11.007
4 Y. N. Hu, H. Zhou, L. P. Liao, H. B. Deng, Surface quality analysis
of the electropolishing of cemented carbide, Journal of Materials
Processing Technology, 139 (2003) 1–3, 253–256, doi:10.1016/
S0924-0136(03)00230-9
S.-H. KIM et al.: SURFACE CHARACTERISTICS OF INVAR ALLOY ACCORDING TO MICRO-PULSE
748 Materiali in tehnologije / Materials and technology 51 (2017) 5, 745–749
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 7: SEM images and non-contact 3D profiles of crater-shaped
patterns at different pulse-on times using the uninsulated electrode
Figure 8: SEM images and non-contact 3D profiles of crater-shaped
patterns at different pulse-on times using the insulated electrode
5 D. Grimmentt, M. Schwartz, K. Node, A Comparison of DC and
Pulsed Fe-Ni Alloy Deposits, Journal of The Electrochemical
Society, 140 (1993) 4, 973–978, doi:10.1149/1.2056238
6 S. Fujimoto, K. Tsujino, T. Shibata, Growth and properties of Cr-rich
thick and porous oxide films on Type 304 stainless steel formed by
square wave potential pulse polarisation, Electrochimica Acta, 47
(2001) 47, 543–551, doi:10.1016/S0013-4686(01)00782-4
7 T. Koyano, M. Kunieda, Ultra-Short Pulse ECM Using Electrostatic
Induction Feeding Method, Procedia CIRP, 6 (2013), 390–394,
doi:10.1016/j.procir.2013.03.066
8 M. Boxhammer, S. Altmannshofer, Model Predictive Control in
Pulsed Electrochemical Machining, Journal of Process Control, 24
(2014) 1, 296–303, doi:10.1016/j.jprocont.2013.11.003
9 E. Lee, T. Shin, B. Kim, S. Baek, Investigation of short pulse elec-
trochemical machining for groove process on Ni-Ti shape memory
alloy, International Journal of Precision Engineering and Manu-
facturing, 11 (2010) 1, 113–118, doi:10.1007/s12541-010-0014-3
S.-H. KIM et al.: SURFACE CHARACTERISTICS OF INVAR ALLOY ACCORDING TO MICRO-PULSE
Materiali in tehnologije / Materials and technology 51 (2017) 5, 745–749 749
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
4 CONCLUSIONS
This study demonstrated that MPECM has suitable
process characteristics for machining Invar-alloy sur-
faces. The machined shape and surface qualities, such as
the surface condition and the machining defect, were
observed. The important factors influencing the hole-
shape quality of the Invar alloy are the pulse-on time, the
duty factor, the shape of the cathode and the electrolytic
process parameters. During the application of MPECM
to a fine sheet metal, the electrochemical machining
characteristics of the surface of the alloy were revealed
and analyzed with respect to different electrode shapes,
pulse-on times and applied voltages.
When the uninsulated electrode was used, the
machining speed differed depending on the area of the
tool electrode that was submerged in the liquid. With a
greater machining depth, the machining speed was re-
duced and there was almost no machining after reaching
a certain limit. This is attributable to the current being
exhausted because of the repeated charging and dis-
charging in the dual layer of electricity by the pulse, for
any part other than the intended machining part.
The insulated electrode prevents the permitted volt-
age from being exhausted and improves the machin-
ability with the increasing machining depth. In addition,
since the voltage flows only into the machining area, a
taper does not occur since the sides of the electrode are
well insulated. Accordingly, if insulated electrodes are
used, it is possible to increase the precision of the form
and the processing depth.
This study indicates that the electrode shape and
suitable MPECM conditions affect the machinability,
and that there is a close interrelation between the electric
field on the Invar-alloy surface and the polishing rate. A
comparison of the insulated and uninsulated electrodes
was made, and the effect of the pulse-on time on the
electrochemical machinability was determined. MPECM
can be recommended to practitioners in different fields
to improve the conditions for making holes in the Invar
alloy.
The electrolyte of sodium nitrite + sodium tartrate +
DI water was used and holes with a ø1 mm diameter
were made into the 30-μm-thick Invar alloy. The future
work, therefore, can be made when more engineering
data has been gathered. In addition, more research needs
to be done on the related factors, such as the surface
roughness and the shape accuracy, using controlled
micro-pulses.
Acknowledgment
This work was supported by the Inha University
Research Foundation, Korea.
5 REFERENCES
1 D. Grimmentt, M. Schwartz, K. Node, A comparison of DC and
pulsed Fe–Ni alloy deposits, Journal of The Electrochemical Society,
140 (1993) 4, 973–978, doi:10.1149/1.2056238
2 A. Stupnik, M. Leisch, Study on the surface topology of
vacuum-fired stainless steel by scanning tunnelling microscopy,
Vacuum, 81 (2007) 6, 748–751, doi:10.1016/j.vacuum.2005.11.061
3 L. S. Andrade, S. C. Xavier, R. C. Rocha-Filho, N. Bocchi, S. R.
Biaggio, Electropolishing of AISI-304 stainless steel using an
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS