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Electrochemistry-informed electrochemical machining (ECM) and microstructure-determined flattening mechanism of Inconel 738 superalloy

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For nickel (Ni) superalloys, including Inconel 738, electrochemical machining (ECM) is commonly used. However, the mechanisms behind ECM surface flattening and improved surface quality have not been fully elucidated, and the relationship between corrosion and ECM is weakly established without microstructural correlation. To bridge this knowledge gap, we have systematically investigated the electrochemical dissolution behavior of wrought Inconel 738 superalloy in the selected NaNO3 solution and narrowed down the optimal ECM parameters (i.e., 12 V, 0.8 mm/min, and 0.8 MPa). The results show that the precipitates and secondary phases of the Inconel 738 superalloy dominate the final surface quality and the ECM efficiency. Importantly, we have confirmed that a core–shell precipitate phase with shell-layer Ti and Mo enrichment remains on the surface even after the 12 V ECM, limiting the final roughness. This novel finding clarifies why the morphology differs under two representative conditions (i.e., anodic dissolution at 1.3 V, 1500 s and optimal ECM with 12 V, 0.8 mm/min, and 0.8 MPa), and how the core–shell precipitates evolve during the non-equilibrium 12-V ECM condition leads to a totally different flattening process. This ECM analysis method, with solid electrochemical theory and detailed microstructural investigation, reveals the microstructure-dependent ECM flattening mechanisms at precipitate scale and demonstrates how the microstructures influence the ECM processing quality. This novel finding plays a guiding role in improving the quality of ECM processing, shows great value for various industrial applications, and helps strategically break the current surface roughness limit.
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The International Journal of Advanced Manufacturing Technology
https://doi.org/10.1007/s00170-024-13758-7
ORIGINAL ARTICLE
Electrochemistry‑informed electrochemical machining (ECM)
andmicrostructure‑determined flattening mechanism ofInconel 738
superalloy
YuhanXing1,2· YingyueYin3,4,5· FulanWei5· XiaoniMa5· ShuoZang1,2· JianhuaZhang1,2 · ShuaihangPan6·
XiaomingYue1,2
Received: 29 March 2024 / Accepted: 6 May 2024
© The Author(s), under exclusive licence to Springer-Verlag London Ltd., part of Springer Nature 2024
Abstract
For nickel (Ni) superalloys, including Inconel 738, electrochemical machining (ECM) is commonly used. However, the
mechanisms behind ECM surface flattening and improved surface quality have not been fully elucidated, and the relationship
between corrosion and ECM is weakly established without microstructural correlation. To bridge this knowledge gap, we
have systematically investigated the electrochemical dissolution behavior of wrought Inconel 738 superalloy in the selected
NaNO3 solution and narrowed down the optimal ECM parameters (i.e., 12V, 0.8mm/min, and 0.8MPa). The results show
that the precipitates and secondary phases of the Inconel 738 superalloy dominate the final surface quality and the ECM
efficiency. Importantly, we have confirmed that a core–shell precipitate phase with shell-layer Ti and Mo enrichment remains
on the surface even after the 12V ECM, limiting the final roughness. This novel finding clarifies why the morphology dif-
fers under two representative conditions (i.e., anodic dissolution at 1.3V, 1500s and optimal ECM with 12V, 0.8mm/min,
and 0.8MPa), and how the core–shell precipitates evolve during the non-equilibrium 12-V ECM condition leads to a totally
different flattening process. This ECM analysis method, with solid electrochemical theory and detailed microstructural
investigation, reveals the microstructure-dependent ECM flattening mechanisms at precipitate scale and demonstrates how
the microstructures influence the ECM processing quality. This novel finding plays a guiding role in improving the quality
of ECM processing, shows great value for various industrial applications, and helps strategically break the current surface
roughness limit.
Keywords Electrochemical machining· Ni superalloy· Microstructure· Precipitates· Corrosion
Yuhan Xing and Yingyue Yin contributed equally to this study.
* Jianhua Zhang
jhzhang@sdu.edu.cn
* Shuaihang Pan
Shuaihang.Pan@utah.edu
* Xiaoming Yue
xmyue@sdu.edu.cn
1 Key Laboratory ofHigh Efficiency andClean Mechanical
Manufacture, Ministry ofEducation ofChina, School
ofMechanical Engineering, Shandong University,
Jinan250061, China
2 School ofMechanical Engineering, National Demonstration
Center forExperimental Mechanical Engineering Education,
Shandong University, Jinan250061, China
3 Shandong Institute ofMechanical Design andResearch,
Jinan250031, China
4 Department ofMechanical Engineering, Qilu University
ofTechnology (Shandong Academy ofSciences),
Jinan250353, China
5 Department ofOrthodontics, School andHospital
ofStomatology, Cheeloo College ofMedicine, Shandong
University & Shandong Key Laboratory ofOral Tissue
Regeneration & Shandong Engineering Research Center
ofDental Materials andOral Tissue Regeneration &
Shandong Provincial Clinical Research Center forOral
Diseases, Jinan250012, China
6 Lab ofAdvanced Manufacturing (LoAM), Department
ofMechanical Engineering, University ofUtah,
SaltLakeCity, UT841142, USA
The International Journal of Advanced Manufacturing Technology
1 Introduction
Wrought nickel (Ni)-based high-temperature alloys play a
dominant role in aero-engines and industrial gas turbines
[13], because the Ni austenite can withstand a wide range
of high temperatures and can be alloyed with a variety of
elements to form γ matrix phase and γ precipitates. This
facilitates the material strengthening, including solid solu-
tion or precipitation strengthening, and can easily obtain
the desired mechanical properties by tuning the morphol-
ogy, distribution, and volume fraction of the γ phase [4].
Secondly, Ni-based alloys have excellent oxidation resist-
ance and high-temperature stability. For example, Ni–Co-
based high-temperature alloys containing Co and Ti can
increase the service temperature by almost 50°C [5].
Specifically, the widely used wrought Inconel 738
superalloy is a precipitation-hardened Ni–Cr–Co alloy
for high-temperature applications. The alloy has solid
solution strengthening by Co, Cr, and Mo, can be further
strengthened by Al and Ti addition to form the γ phase,
and depends on B and Zr to purify and strengthen the
grain boundaries (GBs) [6]. Due to its good yield strength,
fatigue resistance, oxidation resistance, and corrosion
resistance at high temperature, it is now adopted in gas
turbines, turbine disks, working blades, high-temperature
fasteners, flame cylinders, shafts, and turbine magazines
[7].
However, as the wrought Inconel 738 superalloy ben-
efits from these characteristics, its strength has been
enhanced to the extent that further conventional machin-
ing and processing are not capable of [8]. Therefore, elec-
trochemical machining (ECM) is chosen and designed
for this alloy. ECM is a method of machining workpieces
with complex shapes through electrochemical anodic dis-
solution reaction and is commonly used in the machin-
ing of high-strength alloys. Compared with the traditional
machining technology, ECM has the advantages of high
machining efficiency, less tool wear (theoretically down
to 0 mm3/min), minimal thermal influence, no residual
stress, and no constraints by the materials’ hardness and
toughness, which attracts wide attention from industrial
production [9, 10].
In recent years, many scholars have done research on
electrochemical processing of Inconel 738 and related
Ni-based alloys. Liu etal. [11] have optimized the ECM
parameters with the material removal modeling to suc-
cessfully predict and improve its material removal rates.
Singh etal. [12] have investigated the ECM of Inconel
series by response surface methodology and combined
several metrics such as surface roughness but without suf-
ficient corrosion design (e.g., going directly with NaCl
solution). Guo etal. [13] have found that machining gap
modifies the ECM response of Inconel alloys, and the insu-
lating treatment can effectively inhibit the stray corrosion,
thus improving the surface quality (Sa, from 1.759μm to
0.055μm) and corrosion zone (from 0.035 to 0.010mm).
Except for the ECM processing study, many studies
focus on microstructural tuning to improve ECM qual-
ity. For example, Zhu etal. [14] have confirmed that dif-
ferent heat treatments (solid solution state and solution-
aging state) change the ECM surface roughness of Inconel
alloys. These studies clearly indicate that the microscopic
insights are needed for ECM understanding and design.
Therefore, people have been exploring the electrochemi-
cal dissolution behavior of Ni-based alloys with micro-
structural correlation. Wang etal. [15] have investigated
the corrosion behavior of laser additive manufactured Ni-
based alloy and found that the pitting corrosion caused by
the galvanic cell between the γ-Ni phase and inclusion is
dominating its dissolution process. Qiao etal. [16] further
prove that this galvanic tendency is derived from in the
large electron density-enabled cohesive energy difference
between the intergranular compound and the matrix phase.
In addition to studying how microstructures induce cor-
rosion, the influence from corrosion products and related
outcomes (e.g., corrosion cracking) is key to understand-
ing the electrochemical dissolution and designing ECM
for Ni-based alloys. Yin etal. [17] have investigated their
electrochemical dissolution behavior and found that the
densities of passivation films containing Cr2O3, Cr(OH)3,
Fe2O3, FeOOH, and Ni(OH)2 are related to the heat treat-
ment conditions. Except for the surface tuning to change
the electrochemical dissolution behavior, it is confirmed
that M23C6 carbides at GBs or inside the grains form
micro-corrosion cracks and interfere with the dissolution
preference locations [18].
However, little attention has been paid to form the inte-
grated microstructure, electrochemistry (for processing),
and final ECM quality relationships in Ni-based alloys,
and the question remains when/if any phase (e.g., precipi-
tation phases) is dominantly changing electrochemical dis-
solution behavior and ECM. What microstructures should
be the target of electrochemistry and process parameters
tuning is not clear to achieve a flatter surface. In particular,
the execution of ECM relies on a non-steady-state electro-
chemical environment (e.g., with a high voltage of > 10V,
higher than the water dissociation and oxygen-involved
reaction potentials of 1.23V [39]). This may change the
traditional roles of different microstructures in electro-
chemical and ECM responses. Therefore, it is important
to systematically study the wrought Inconel 738 superalloy
microstructure, electrochemical dissolution behavior, and
ECM. Bridging these knowledges will provide theoretical
and practical guidance for ECM optimization and ECM
quality improvement.
The International Journal of Advanced Manufacturing Technology
In this paper, electrochemical measurements of the
wrought Inconel 738 superalloy have been gauged with the
open-circuit potentials, kinetic potential polarization curves,
and electrochemical impedance spectroscopy to explore its
electrochemical dissolution behavior in neutral 10 wt.%
NaNO3 solution. Then scanning electron microscopy (SEM),
energy dispersive spectrometer (EDS), X-ray photoelectron
spectroscopy (XPS), and other characterization methods are
combined to elucidate the differences in the alloy surface
morphology and phases under different polarization volt-
ages. Finally, the surface morphology of wrought Inconel
738 superalloy under constant potential polarization is
compared and analyzed with the optimal ECM parameters,
and we aim to answer which microstructures dominate and
should be targeted, if we are to improve the ECM surface
quality, by incorporating the united microstructure, electro-
chemistry, and processing understanding. This will provide
theoretical guidance for future ECM quality improvement
through microstructural and electrochemical behavior.
2 Experimental procedures
2.1 Material preparation
In this study, we used the wrought Inconel 738 superalloy
with a chemical composition (wt.%) of Cr 16.0, Co 8.5, Al
3.4, Ti 3.4, Mo 1.75, Ta 1.7, Nb 0.9, C 0.11, Zr 0.06, and
Ni as balance [19]. The wrought counterpart was commer-
cially fabricated via hot forging at 1160°C for 120min and
air cooled for 24h [20]. A cylindrical sample as working
electrode (14.8mm in diameter and 2mm in height) was
obtained from the wrought Inconel 738 superalloy samples
by molybdenum wire electrical discharge machining (EDM)
(with an average voltage of 30V, an average DC current of
16 A, and a feed rate of 4mm/min). These samples were
mainly for phase identification, microstructure analysis, and
electrochemical measurements.
2.2 Microstructure andcomposition
characterization
All the samples were ground using SiC abrasive papers from
300- to 1500-grit roughness and vibrationally polished with
a SiO2 polishing suspension (0.04µm) for texture observa-
tion. The polished samples were electrochemically etched
in 10 wt.% oxalic acid at 3.3V for 15s to further reveal the
morphology [21, 22]. The microstructural morphology and
the information on the grain/GB orientation were both char-
acterized on a scanning electron microscope (on a model of
JEOL, JSM-7800F) equipped with an electron backscatter
diffraction system (Oxford Nordlys Max3). Local mapping
and line-scanning analysis of elemental distributions were
done on an electron probe microanalyzer (JEOL, EPMA-
JXA-8530F plus).
The chemical composition of the surface of the speci-
men (here, for those after potentiostatic polarization) was
analyzed by X-ray photoelectron spectroscopy (XPS,
ESCALAB 250XI, Thermofischer, with the emission exci-
tation up to 1486.6eV with a power up to 300 W) at 50kV
and 150mA. All peaks were calibrated with a binding
energy of 284.6eV C1s. The calibrated XPS spectra were
fitted to the Thermo Advantage software data, using Shirley
Background correction mode.
2.3 Electrochemical measurements
The standard three-electrode electrolytic cell device con-
sisted of the wrought Inconel 738 superalloy sample (the
working electrode), a saturated calomel electrode (the refer-
ence electrode), and a platinum electrode (the counter elec-
trode), and electrolytes of 5 wt.%, 10 wt.%, and 20 wt.%
NaNO3 and NaCl solutions were used. In order to ensure
that electrochemical measurements matched the results of
the subsequent electrochemical processing, the tempera-
ture of the electrolyte at room temperature (RT, with a real
measurement up to 40°C) was chosen as the temperature
for electrochemical measurements. Oxygen dissolved in the
electrolyte was removed by degassing with nitrogen. The
working electrode was submerged in the solution for 2h
to ensure the stability of the open circuit potential (OCP).
The anodic polarization test was performed at a scan rate of
1mV/s. The different stages of the potentiostatic polariza-
tion for the wrought Inconel 738 superalloy were determined
by the potentiodynamic polarization curve. The potentials
of the four different stages (0.34, 0.55, 0.8, and 1.3V vs.
VSCE) were then selected for the potentiostatic polarization
test for 1500s to study the electrochemical behavior. The
electrochemical impedance scanning (EIS) tests were per-
formed at the corresponding potentiostatic potentials in the
frequency range of 10−2Hz to 105Hz with a sinusoidal volt-
age signal amplitude of 10mV (mean). The collected EIS
data were analyzed and fitted using the ZSimpWin software.
Three sets of electrochemical experimental tests were per-
formed for each electrochemical condition to ensure data
reproducibility.
2.4 ECM setup andtests
The surface of the wrought Inconel 738 superalloy was
ground to obtain the finish using SiC abrasive papers from
300- to 1500-grit roughness and vibrationally polished
with a SiO2 polishing suspension (about 0.04µm rough-
ness) before all the ECM tests. The surface impurities of
the sample were cleaned using an ultrasonic cleaner. An
internally sprayed cylindrical brass tool electrode (with a
The International Journal of Advanced Manufacturing Technology
diameter of 0.8mm and a length of 400mm) was selected
for electrochemical drilling of wrought Inconel 738 superal-
loy. To reduce the effect of stray corrosion on the machined
surface, the outer wall of the tool electrode is insulated using
electrophoresis deposition, and only the bottom (perpendic-
ular to the feed direction) is conductive, which facilitates
the study of each processing condition on the dimension
accuracy and surface quality. The ECM experiments of the
wrought Inconel 738 superalloy were performed on a micro-
pore electrochemical machining setup (BaoMa-DB703A,
Shandong, China), as shown in Fig.1. The high processing
accuracy and surface quality in the wrought Inconel 738
superalloy holes were achieved by optimizing the processing
parameter sets of voltage, electrolyte type, flow pressure, and
feed rate. In addition, the roughness of the holes was meas-
ured using an optical profilometer (Veeco NT9300, Jinan,
China), and the taper calculation was obtained by Eq.1:
where D is the inlet diameter, d is the outlet diameter, and L
is the depth of the small hole.
3 Results anddiscussion
3.1 Microstructure characterization
It can be seen from Fig.2a that the wrought Inconel 738
superalloy consists of equiaxed grains of an average size
of 50.1µm. As summarized in Fig.2b, the fractions of the
high angle grain boundaries (HAGB) and low-angle grain
boundary (LAGB) are 90% and 10%, respectively.
Figure3 shows a typical microstructure SEM image
of wrought Inconel 738 superalloy. There are large-size
(1)
Taper
=
(Dd)
L
Fig. 1 ECM set-up for drilling process
Fig. 2 EBSD map of wrought
Inconel 738 superalloy: a
inverse pole figure for grain size
statistics and b grain boundary
(GB) misorientation distribution
The International Journal of Advanced Manufacturing Technology
precipitated secondary phases with a length of about 10µm
in Fig.3a, consistent with the results reported by Chen
etal. and Montazeri etal.[23, 24]. In addition, a large num-
ber of finer granular intermetallic phases with a diameter
of ~ 0.3µm are visible in the grains (Fig.3b).
To further distinguish if these morphology-different
phases have a different composition, the elemental profile
of Fig.4 and the line scanning in Figs.5 and 6 are added.
Figure5 shows that the large-size precipitates on the GBs
have a core–shell structure. The outer layer is primarily Mo
rich with depletion in Ni and Cr, while the center part has
a higher concentration of Ti. The enrichment of Ti and Mo
within the large-size precipitates (though at different layers)
has also been observed by Bagoury etal. [2527]. In com-
parison, for the inner-grain intermetallic phases (Fig.6), the
elements Cr, Ti, and Co are all rich.
3.2 Electrochemical behavior analysis
With the microstructural understanding, we proceed with
electrochemical analysis to guide our targeted ECM design.
The OCP curves of the wrought Inconel 738 superalloy in
the designed electrolytes are plotted in Fig.7. Figure7a
shows the curves of OCP of wrought Inconel 738 superalloy
in various concentrations of NaCl at RT as a function of
immersion time (as the comparison baseline). The results
show that the OCP of the wrought Inconel 738 superalloy is
only stable after 1500s, because the dynamic passive film
formation equilibrium is achieved after then. The overall
trend is that the OCP decreases with the increasing concen-
tration of both electrolytes, indicating that the electrochemi-
cal activity of wrought Inconel 738 superalloy increases with
a higher concentration of Cl and NO3
to trigger a larger
corrosion tendency.
Meanwhile, several differences are noticeable by Fig.7.
First, the OCP in NaCl solution shows a downward trend,
whereas the OCP in NaNO3 solution shows an upward trend
over time. Second, the OCP in NaCl solution exhibits a cer-
tain oscillation, whereas NaNO3 solution varies gradually
and stably. Additionally, the OCP of NaNO3 solution is
higher than that of NaCl solution at the same concentra-
tion. This result is consistent with the findings reported by
Wang etal. [28, 29] These previously discussed OCP differ-
ences between Cl and NO3
conditions are understandable:
NO3
ions have stronger antioxidant properties (i.e., it can
only be reduced) than Cl ions due to their higher affinity
to metallic surfaces and smaller atomic radius [3032]. The
formation rate of oxide film is impeded by Cl, requiring a
Fig. 3 Different SEM morphol-
ogy of phases after electro-
chemical etching: a precipitates;
b intermetallic phases within
the grains
Fig. 4 Elemental distribution on the surface of wrought Inconel 738 superalloy
The International Journal of Advanced Manufacturing Technology
higher formation and rupture equilibrium of oxide film and
resulting in a low OCP and a trend of decline and oscillation
[33, 34]. Comparatively, the NaNO3 solution yields the quick
reduction of NO3
or the reduction products of NO3
as a
cathode reaction, which gives it a stable and high OCP, and
the passivation film in NaNO3 solution becomes denser with
this quicker formation, further increasing the passive film’s
breakdown resistance. NO3
can even prevent the hydrogen
evolution reaction at the early stage of corrosion [30, 35].
With this, these different characteristics between Cl and
NO3
explain the potential difference.
After the quasi-static analysis, Fig.8 shows the anodic
polarization curves of the wrought Inconel 738 superalloy
in different electrolytes. Overall, the corrosion resistance
of the wrought Inconel 738 superalloy in NaNO3 solution
is higher than that of the NaCl solution at the same con-
centration. The results are consistent with the change trend
of the OCP in various electrolytes (Fig.7). The wrought
Fig. 5 Line-scanning EDS elemental analysis of large-size precipitates on the surface of wrought Inconel 738 superalloy (all the x-axis for EDS
scanning distance, and all the y-axis for EDS scanned intensity in a.u.)
Fig. 6 Line scanning EDS elemental analysis of fine inner-grain intermetallic phases on the surface of wrought Inconel 738 superalloy (all the
x-axis for EDS scanning distance, and all the y-axis for EDS scanned intensity in a.u.)
The International Journal of Advanced Manufacturing Technology
Inconel 738 superalloy’s current density and potential still
satisfy a nearly linear relationship in the transpassive region
for NaNO3 and NaCl solution (see Fig.8a and b). That is,
the current density increases with increasing electrolyte
concentration in NaNO3 and NaCl solution only when the
potential surpasses approximately 1V vs. VSCE because of
passivation.
With the passivation consideration for corrosion uniform-
ity and the moderate controllable dissolution speed, the ideal
electrolyte concentration for ECM of wrought Inconel 738
superalloy is hence 10 wt. % NaNO3 to lower the electrolyte
cost.
Knowing this electrolyte selection, Fig.9a summarizes
the potentiodynamic polarization curves of wrought Inconel
738 superalloy in 10 wt.% NaNO3 solution. The current den-
sity is generally stable when the voltage is between 0.5V
vs. VSCE and 0.2V vs. VSCE, when the first passivation phe-
nomenon occurs to have oxides on the sample surface. When
the voltage rises above 0.2V vs. VSCE, the oxide layer is
gradually broken down to expose the substrate, leading to an
increased current density. Luckily, the secondary passivation
soon takes place when the voltage reaches 0.45V vs. VSCE,
after which the current density stops increasing. This can be
attributed to the fact that the secondary phases (previously
as localized anodes at a lower biased voltage) start partici-
pating in corrosion to form more nucleation sites for the
passivation film and densifying the passivation layer [36].
When the voltage is more than 0.6V vs. VSCE, the dense
oxide layer (associated with the secondary phases) by the
secondary passivation dissolves again with pitting and other
localized corrosion phenomena existent, leading to a rapidly
increasing current density [37]. When the voltage exceeds
the breakdown potential of 0.8V (Eb), oxygen-involved reac-
tion occurs at the anode, and the current density begins to
stabilize and saturate.
After gauging the different polarization response ranges,
Fig.9b shows the potentiostatic results of wrought Inconel
738 superalloy at different constant potentials in 10 wt.%
NaNO3 solution for 1500s. All curves at < 1.3V decrease
sharply in the early stages due to the rapid nucleation of the
passivation film (even though the simultaneous dissolution
is expected at > 0.55V). After this stage, all curves remain
stable (i.e., the current density is maintained at a fixed value
and barely fluctuates), implying that a stabilized corrosion,
passivation, and dissolution condition is achieved. Compara-
tively, the 1.3V results show a constant fluctuation with
a much higher current density, indicative of a progressive
oxygen-involved corrosion reaction.
The EIS fitting results of wrought Inconel 738 superal-
loy at different potentiostatic polarization potentials are
shown in Fig.10. We have used the different fitting models
for 0.34V and 0.55V (with the R(QR) model) and 0.8V
and 1.3V (with the R(QR(QR)) model) to accommodate the
passivation difference (consistent with Fig.9a). It is worth
Fig. 7 Time dependence of the
Eocp of wrought Inconel 738
superalloy in a NaCl solution
and b NaNO3 solution with dif-
ferent concentrations at 40°C
Fig. 8 The anodic polarization
curves of wrought Inconel 738
superalloy in a NaCl solution
and b NaNO3 solution with dif-
ferent electrolyte concentrations
The International Journal of Advanced Manufacturing Technology
mentioning that the R2, representative of the initial passi-
vation film intactness, decrease gradually with the increas-
ing potential, implying the continuous breakdown of this
first passivation-induced passivation film. Besides, the EIS
results from Fig.10 confirms this corrosion trend from Fig.8
because a smaller Nyquist loop indicates a quicker overall
corrosion rate and a smaller corrosion resistance, and the
oxide-film-related resistance (R2 in Table1) leads to the
same conclusion.
To correlate the electrochemical understanding and pre-
pare the microstructural investigations for ECM design,
Fig.11 shows the SEM image of the polarization 1500s of
the wrought Inconel 738 superalloy at different potentials
(corresponding to Figs.9 and 10). As shown in Fig.11, the
surface is relatively flat due to the formation of passivation
film on the surface of the substrate at the potential of 0.34
and 0.55V. When the voltage reaches 0.8V, the passivation
film on the surface begins to rupture and gradually exposes
the large-size precipitated phase. When the voltage reaches
1.3V, the passivation film is completely ruptured, and the
GB precipitates are exposed and corroded away (with the
strong oxygen-involved corrosion disturbance). Due to this,
the GBs are preferentially corroded.
As ECM usually operates under a higher voltage and
1.3V indeed shows a different morphology, it is necessary to
zoom in to see the surface microstructures after 1.3V polari-
zation. Clearly, as shown in Fig.12, the passivation film is
completely dissolved and removed at 1.3V, and the metal
matrix is continuously exposed and dissolves together with
the intermetallic phases. The EDS analysis result shows that
the residual exposed intergranular precipitates are rich of
Ti-, Mo-, and C-rich compounds (similar to the inner-grain
precipitate’s composition enrichment in Fig.6), while the
element of Ni and Cr is poor, which is consistent with the
previous EDS mapping on the pre-corrosion surfaces and
confirms that these inner-grain precipitates have not been
affected by or affecting the corrosion process after exposure.
To support the morphology observation, we proceed with
more elemental and chemical analysis on the post-corrosion
surfaces, as summarized in the XPS results of Fig.13.
The fitting results find only metal and hydroxide signals
for Ni2p, and only metal and oxide component signals show
up for Ti2p and Mo3d. In comparison, metal, oxide, and
hydroxide components all exist for Cr2p.
To be more specific, at OCP (which we use for the XPS
comparison baseline), the Ni peaks are fitted with metallic
Ni (at 852.5eV) and Ni(OH)2 (at 856.04eV). The Ti peaks
confirm metallic Ti (at 454.26eV) and TiO2 (at 458.35eV)
components. Interestingly, at this OCP bias, Mo only
shows metallic components, and the Cr lacks its hydroxide
component.
At 0.34V, the Ni, Mo, and Cr peaks have the same
components as the OCP condition. However, the Ti can
only find TiO2 (at 458.84eV) component. At 0.55V, the
Ni still has the same components. The Ti2p3/2 ioniza-
tion is fitted with two peaks, namely TiO (454.53eV)
and TiO2 (458.56eV). However, the MoO3 component (at
232.64eV) starts existing for the Mo peaks, and the Cr
peaks now transfer from Cr2O3 to Cr(OH)3 (at 576.97eV),
proving its hydration process under higher voltage. At
0.8V where localized corrosion and breakdown happen,
the Ni still shows no change, whereas the Ti is fully TiO2
(at 459.5eV) again. Now, the Mo is totally composed
of MoO3 (at 233.05 eV), and the Cr also finishes the
Fig. 9 a Potentiodynamic polarization curve of wrought Inconel 738 superalloy in 10 wt.% NaNO3 solution. b Time-dependent current density
evolution of wrought Inconel 738 superalloy at the different biased voltages
The International Journal of Advanced Manufacturing Technology
Fig. 10 EIS map of wrought Inconel 738 superalloy at different polarization potentials of a 0.34V, b 0.55V, c 0.8V, and d 1.3V
The International Journal of Advanced Manufacturing Technology
oxidation and hydration transition to only have Cr(OH)3
(at 577.62eV). In the further transpassive state at 1.3V,
the Ni, Ti, Mo, and Cr components do not change any-
more and are all in their stabilized oxidation or hydroxide
forms.
It is easy to understand this oxidation and corrosion
process, if we call back on the previous EDS mapping
for the representative precipitates in grains and near GBs.
When the polarization voltage is lower than 0.55V, the Ti
element in the shell layer of the large-size GB precipitates
(Fig.5) is gradually oxidized from Timet to TiO2, while
the Mo element in the shell layer remains unoxidized.
When the voltage is greater than 0.55V, Momet starts to
participate in the reaction and is gradually oxidized to
MoO3, opening the opportunities for Ti in the core layer
to get oxidized. That is, the oxidation of Mo changes the
“shell” crystal structure and loses the corrosion-protec-
tion role from pure Ti oxides, which exposes the “core”
part (mainly, the Ti in the core part) to further proceed
oxidation and corrosion. With this, the Ti element (from
the core layer) reappears in the form of a lower valence
TiO and is again completely oxidized to TiO2 after a
higher voltage.
This understanding will be further used to interpret the
ECM and anodic dissolution surface results later from a
microstructural point of view.
3.3 Wrought Inconel 738 superalloy
electrochemical processing experiments
After systematically probing the electrochemical and anodic
dissolution behavior via Sect.3.2, we could safely use the
understanding as the starting point for the ECM processing
optimization by considering more processing parameters,
including processing voltage, flow pressure, and feed rate,
and bridge the low-voltage and ECM processing conditions
with a unified microstructural picture.
3.3.1 ECM optimization andanalysis
Effect of processing voltage on hole quality According to
Fig.14, the pore size increases as the ECM machining volt-
age increases, widening the machining gap and expanding
the ECM-influenced zone, because the machining voltage is
directly proportional to the value of the current density. With
this, the taper will be accordingly increased (see the posi-
tive correlation trend of taper and voltage in Fig.14b), and
the roughness is less controllable (Fig.14). Therefore, the
machining voltage should be kept to a minimum to ensure
the precision of hole machining. However, it is known that
when the processing voltage is less than 12V (more spe-
cifically, 10V), poor processing occurs more frequently
because of short circuit, and the machining removal rate is
Table 1 Electrochemical
impedance parameters obtained
at different potentials on the
surfaces for wrought Inconel
738 superalloy in 10 wt.%
NaNO3 solution at RT
Electrochemical
parameters
Surface
0.34 V 0.55 V0.8
V1
.3 V
R1(ohm·cm2) 13.37 13.81 17.10 18.17
R2(ohm·cm2) 1.041073.067×1067.647×1042.64104
R3(ohm·cm2)üü
0.01375 1.01104
Q1(ohm·cm2·secn) 1.426×10-5 2.0010-5 1.832×10-4 3.606×10-4
n10.8387 0.8297 0.786
20
.8340
Q2(ohm·cm2·secn)üü
1.940×10-5 5.138×10-5
n2üü
10
.8324
Model
The International Journal of Advanced Manufacturing Technology
Fig. 11 SEM image of corro-
sion morphology of wrought
Inconel 738 superalloy after
being corroded under the dif-
ferent polarization potentials of
a 0.34V, b 0.55V, c 0.8V, and
d 1.3V
Fig. 12 EDS analysis of wrought Inconel 738 superalloy after 1.3V polarization for 1500s
The International Journal of Advanced Manufacturing Technology
largely reduced [3840]. Therefore, 12V is the minimum
voltage for typical processing. Figure15 shows the typical
hole sidewall morphology and roughness. The roughness
of Ra = 676.94nm is also the lowest at a machining voltage
of 12V (Figs.14 and 15) with the smallest hole diameter
(850nm) and taper (0.003), further confirming the optimal
choice of 12V as the processing voltage.
Effect of pressure of NaNO3 electrolyte on hole surface qual
ity With the similar comparison methodology, we optimize
the electrolyte pressure for ECM in Figs.16 and 17. The
effect of the electrolyte pressure on the hole diameter, rough-
ness, and taper is less noticeable (or less linearly predictable)
than the voltage effect. These trivial changes when the flow
pressure exceeds 0.4–0.8MPa are expected, because the
electrolyte could readily flush the debris and pollutants away.
Figsure 16 and 17 similarly compare the resultant mor-
phology, dimension, and roughness at different NaNO3
electrolyte pressures. Though the roughness at 0.4MPa
(Fig.17a) is the lowest, the hole diameter (900nm) and
taper (0.018) at this condition are very large, which will
affect the machining accuracy and quality of the hole, so it
is not suitable for ECM. As mentioned previously, when the
pressure of NaNO3 electrolyte falls in 0.4–0.8MPa, the hole
diameter and taper change are less than 5%, and the rough-
ness increase becomes significant (~ 15%) with > 0.8MPa
Fig. 13 XPS analysis results of wrought Inconel 738 superalloy at different potentiostatic polarization potentials: a 0V, b 0.34V, c 0.55 V, d
0.8V, and e 1.3V (“sat” indicates “satellite peaks”)
The International Journal of Advanced Manufacturing Technology
Fig. 14 The processing voltage’s effects on a inlet and outlet diameters and b hole taper and roughness
Fig. 15 Effect of processing voltage on hole quality: a 12V, b 16V, and c 20V
Fig. 16 Regularity plot of the pressure of NaNO3 electrolyte’s effect on a inlet and outlet diameters and b hole taper and roughness
The International Journal of Advanced Manufacturing Technology
pressure. Therefore, considering the repeatable diameter
and taper quality and the minimized roughness, 0.8MPa is
considered ideal, given energy conservation and an overall
higher processing quality.
Effect of feed speed on hole quality Feed rate is also an
important indicator of the ECM capacity. As seen in Figs.18
and 19, when the feed rate is slow, the diameter of the small
hole increases significantly, because feeding slowly increases
processing time and causes more electrochemical dissolution
and a potential stray corrosion near the ECM entrance [41].
Therefore, the taper also increases. In comparison, as the
feed rate increases, the machined roughness increases sig-
nificantly. This roughness dilemma is also related with the
processing time: High feed rate gives a shorter processing
time (i.e., a shorter corrosion process to flatten the surface),
the electrochemical dissolution reaction will be insufficient,
and the processing impurities and debris are unable to flow
or dissolve away.
Generally speaking, Figs.18a and 19 indicate that the
hole diameter concerns want a feed rate > 0.6–0.8mm/min.
Meanwhile, Figs.18b and 19 stand in favor for a feed rate
around 0.8mm/min, if we consider the balanced dimen-
sional taper and surface roughness. Since we confirm that
Fig. 17 Effect of pressure of NaNO3 electrolyte on surface quality of pores: a 0.4MPa, b 1.2MPa, c 2.0MPa
Fig. 18 Regularity plot of the feed speed’s effect on a inlet and outlet diameters and b hole taper and roughness
The International Journal of Advanced Manufacturing Technology
the small hole diameter of 850nm, the small taper of only
0.003 and the low roughness of Ra = 676.94nm are possible
with the feed rate at 0.8mm/min: choosing 0.8mm/min feed
rate will be safer to ensure a high surface quality by ECM
for wrought Inconel 738 superalloy.
3.3.2 Characterization ofsurface quality ofholes
underoptimal parameters
When it comes to the real ECM case (at the optimized 12V,
0.8mm/min, and 0.8MPa), the surface observation is differ-
ent, due to the ultra-high voltage bias leading to highly non-
equilibrium dissolution process of matrix and intermetallic
phases. There are remaining intergranular precipitates on the
post-ECM surface with the optimal parameters, as shown
in Fig.20 (similar to Fig.12 at 1.3V). The EDS mapping
results confirm that the residual intermetallic compounds
are similarly rich of the element of Ti, Mo, and C. However,
the size of the intergranular precipitates (of ~ 5μm) after
the ECM processing is much bigger than that under 1.3-V
processing.
This resultant surface intermetallic phase’s size differ-
ence is understandable, because the electrochemical dis-
solution behavior of the wrought Inconel 738 superalloy
by ECM condition is rapid and non-equilibrium with high
voltage (12V) and short time (300s). At 1.3V 1500s con-
dition, the electrochemical behavior is a quasi-equilibrium,
which allows for slower but more uniform dissolution of
the substrate and intermetallic phases simultaneously. In
contrast, the wrought Inconel 738 superalloy at 12V 300s
is subjected to the synergistic high electrolyte flushing
and non-equilibrium electrochemical dissolution, reduc-
ing the timeframe to allow matrix-precipitate interactions.
This is consistent with the findings of Abioye etal. [42]
Therefore, the matrix and intermetallic phases will dis-
solve individually, and the exfoliation rate of the precipi-
tated phases is much slower than that of the surrounding
matrix, leading to a large precipitate stick-out as shown in
Fig.21. Furthermore, the quick selective matrix corrosion
around these carbides adds to this corrosion difference
by the formation of a large number of localized primary
cells between the precipitates and the substrate. The pre-
cipitates’ interfaces may provide rapid diffusion paths for
the Cr and Mo elements at GBs, further accelerating the
expansion of the locally corroded regions.
This deduction is valid, as Fig.20 shows that there are
obvious microcracks and sharp edges on the large inter-
granular precipitates (in contrast to the spherical interme-
tallic phases in Figs.11 and 12). This means that the larger
precipitates are newly exposed from the matrix and in the
process of dissolution, dealloying, and corrosion cleavage
(which cannot catch up with the rapid matrix dissolution
under a huge biased voltage). This large intergranular pre-
cipitate generation by non-equilibrium matrix quicker dis-
solution will be one of the important factors to determine
the roughness of the wrought Inconel 738 superalloy after
Fig. 19 Effect of feed speed on hole quality: a 0.4mm/min, b 0.8mm/min, and c 1.2mm/min
The International Journal of Advanced Manufacturing Technology
ECM, and the GB precipitates should be the target, if we
want to achieve an even better surface quality.
Clearly, the surface intermetallic phases unable to fol-
low the high-voltage non-equilibrium electrochemical
process for short periods are the primary influence fac-
tors of the surface roughness for the wrought Inconel 738
superalloys during the ECM process. Excitingly, with this
knowledge, we can foresee two directions to enhance sur-
face quality in ECM by targeting GB precipitates. First,
hybrid processing by techniques like ultrasonic vibration-
assisted particle removal can target the GB precipitates,
then excite them away in electrolytes, and thus reduce the
surface roughness. Second, as precipitates are the head-
ache, mitigation of their formation during materials fab-
rication is also a feasible way. Rapid solidification such
as laser powder bed fusion of Inconel 738 can promote
grain refinement, introduce a large solidification tempera-
ture gradient, and thus reduce the size of the resulting
GB precipitates, all of which will help with the targeted
precipitate ECM removal.
4 Conclusions
The quantitative electrochemical comparison results con-
firm that NaNO3 solution is more suitable than the NaCl
solution for the electrochemical dissolution process of
wrought Inconel 738 superalloy, after comparing five
different electrochemical reaction stages at low current
densities (i.e., primary passivation, primary passivation
breakdown, secondary passivation, secondary passiva-
tion dissolution, and further transpassive stage), and an
appropriate concentration of 10 wt.% is needed. After the
direct ECM-related parameters’ tuning, excellent surface
quality (Ra = 676.94 nm) and taper of the small holes
(taper = 0.003) have been obtained at the optimized con-
dition of a pulse voltage of 12V, feed rate of 0.8mm/min,
and electrolyte pressure of 0.8MPa by the ECM.
To understand this ECM outcome, two different pre-
cipitation phases (the large-size precipitates and the inner-
grain intermetallic phases) have been identified in the
Fig. 20 SEM morphology imaging of the ECM processed surface of wrought Inconel 738 superalloy: a the overall view of the ECM quality; b
the zoomed-in zone showing residual intermetallic phases sticking out; and c the corresponding EDS mapping
The International Journal of Advanced Manufacturing Technology
wrought Inconel 738 superalloy. The large-size precipi-
tates on the GBs have a core–shell structure, with a size
of about 10µm and Mo- and Ti-enrichment at different
layers. Mo elements in the outer layer will protect the inner
layer to delay its oxidation and corrosion. For the inner-
grain intermetallic phases, Cr, Ti, and Co enrichment is
clear, but without the signature core–shell structure. These
phase differences of the wrought Inconel 738 superalloy
change the surface quality outcomes at high voltage for a
short period of time (12V 300s) and low voltage for a
long period of time (1.3V 1500s). Specifically, the inter-
metallic phase is large and tall at high voltages for short
periods of time and small and uniform at low voltages for
long periods of time, due to the different electrochemical
dissolution behaviors.
This phase-determined dissolution mechanism is impor-
tant to designing high-quality high-efficiency ECM pro-
cessing route of the wrought Inconel 738 superalloy and
other alloys containing Mo and Ti elements. Moreover,
with the novel microstructural insights, electrochemical
phenomena can be effectively integrated as the basis for
ECM machining, which makes our analysis and methodol-
ogy more universal and better guides the design of preci-
sion machining to achieve better surface roughness.
Acknowledgements This work was supported by the National Key
Research and Development Program of China (No. 2021YFF0501700)
and the Fundamental Research Funds for the Central Universities (No.
2022JC017). The authors thank Shandong Institute of Mechanical
Design and Research and the technical support of Shandong Univer-
sity Testing.
Author contribution Yuhan Xing: writing—original draft, investiga-
tion, data curation, and writing—review and editing. Yingyue Yin:
conceptualization, methodology, investigation, data curation, and
writing—review and editing. Fulan Wei: resources, and writing—
review and editing. Xiaoni Ma: writing—review and editing. Shuo
Zang: investigation. Jianhua Zhang: supervision, conceptualization,
methodology, investigation, formal analysis, and writing—review and
editing. Shuaihang Pan: supervision, conceptualization, methodology,
investigation, visualization, and writing—review and editing. Xiaom-
ing Yue: supervision, project administration, and resources.
Declarations
Conflict of interest The authors declare no competing interests.
Fig. 21 Mechanism diagram: a
initial state, b1 1.3V polariza-
tion, and b2 12-V electrochemi-
cal machining
The International Journal of Advanced Manufacturing Technology
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... Meanwhile, we have: Subscript 1 indicates "matrix' , while subscript 2 indicates "carbides". This non-uniform dissolution is also observed in the authors' recent study [23] and has been reported in many ECM anodic dissolution studies [12,24]. As shown in Fig. 3, we could expect carbide-remained or matrixintact condition during ECM process with different voltages and processing time length, both supporting our simplified dissolution model setup in Fig. 2. ...
... With the two case studies here, we can see our theoretical model fits the general quantification purpose of post-ECM surface roughness (see the reported real ECM surface roughness in Fig. 8) by building a clear relationship between the applied ECM voltage bias (as the . 8 The reported surface roughness after ECM and unexpected surface roughness dependence on machining time during ECM processing (all the data points are extracted from continuous ECM and pulse-ECM with clearly defined voltage exertion period for Ni based alloys) [16,17,19,23,[32][33][34]. All the roughness is obtained at the ECM voltage above the rapid anodic dissolution range Pan et al. ...
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Electrochemical machining (ECM) represents a prominent electrochemistry-driven technique for surface flattening, post-processing, and (ultra-)precision machining, attracting considerable research interests recently. The method exhibits advantages in the machining of hard-to-machine nickel (Ni) superalloys, particularly those created via additive manufacturing approaches such as laser powder bed fusion (LPBF), in which enhanced microstructural features and mechanical properties are achieved with compromised surface quality. This study explores the intricate relationship between Ni alloy-specific microstructures, such as carbide precipitates, and the principles of electrochemistry integral to ECM. It further emphasizes the emerging requirement to re-examine the surface quality outcomes of ECM. We present a concise overview of the inherent paradoxes in ECM, encompassing the prediction of surface roughness range, the quantification of charge transfer coefficients, the efficiency of material removal, and the temporal dependence of the ECM process. These paradoxes necessitate systematic experimental and theoretical research to advance our understanding, and we wish to welcome, stimulate, and urge more raised awareness and attention to this matter about ECM surface quality control and prediction.
... Meanwhile, we have: Subscript 1 indicates "matrix' , while subscript 2 indicates "carbides". This non-uniform dissolution is also observed in the authors' recent study [23] and has been reported in many ECM anodic dissolution studies [12,24]. As shown in Fig. 3, we could expect carbide-remained or matrixintact condition during ECM process with different voltages and processing time length, both supporting our simplified dissolution model setup in Fig. 2. ...
... With the two case studies here, we can see our theoretical model fits the general quantification purpose of post-ECM surface roughness (see the reported real ECM surface roughness in Fig. 8) by building a clear relationship between the applied ECM voltage bias (as the . 8 The reported surface roughness after ECM and unexpected surface roughness dependence on machining time during ECM processing (all the data points are extracted from continuous ECM and pulse-ECM with clearly defined voltage exertion period for Ni based alloys) [16,17,19,23,[32][33][34]. All the roughness is obtained at the ECM voltage above the rapid anodic dissolution range Pan et al. ...
Article
Full-text available
Electrochemical machining (ECM) represents a prominent electrochemistry-driven technique for surface flattening, post-processing, and (ultra-)precision machining, attracting considerable research interests recently. The method exhibits advantages in the machining of hard-to-machine nickel (Ni) superalloys, particularly those created via additive manufacturing approaches such as laser powder bed fusion (LPBF), in which enhanced microstructural features and mechanical properties are achieved with compromised surface quality. This study explores the intricate relationship between Ni alloy-specific microstructures, such as carbide precipitates, and the principles of electrochemistry integral to ECM. It further emphasizes the emerging requirement to reexamine the surface quality outcomes of ECM. We present a concise overview of the inherent paradoxes in ECM, encompassing the prediction of surface roughness range, the quantification of charge transfer coefficients, the efficiency of material removal, and the temporal dependence of the ECM process. These paradoxes necessitate systematic experimental and theoretical research to advance our understanding, and we wish to welcome, stimulate, and urge more raised awareness and attention to this matter about ECM surface quality control and prediction.
... According to Faraday's law of electrolysis, material removal occurs when a stable voltage is put between the workpiece and the tool electrode. (17)(18)(19) Moreover, ECM is an economical and effective technique for cutting highstrength, heat-resistant materials with convoluted shapes without causing mechanical distortion, compressive stresses, cracks, or thermal distortion. (20)(21)(22) However, this technique is affected by the hydrodynamic instability of the anode boundary layer due to the surface roughness. ...
... This dissolution can be inhibited at passivation region by the formation of a thin oxide film on metallic electrode at a higher potential. However, at an even higher potential, such inhibitive film gets soluble and may break down at the transpassive region [36][37][38][39]. In the test case, for the unprocessed GRCop-42 sample, two passivation regions were observed (starting around -0.25 V and ending around 0.05 V) before the transpassive region (starting around 0.36 V). ...
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