mechanism clarification and realization of scanning

Bulletin of the JSME

Journal of Advanced Mechanical Design, Systems, and ManufacturingVol.15, No.5, 2021

© 2021 The Japan Society of Mechanical Engineers[DOI: 10.1299/jamdsm.2021jamdsm0055]Paper No.21-00143

Mechanism clarification and realization of scanning

electrochemical machining of titanium alloys

1. Introduction

Titanium alloys, due to their excellent specific strength and corrosion resistance, are widely used in various fields,

such as the medical device and aerospace industries. However, due to the active chemical properties of such alloys and

the intensive heat accumulation in the cutting tools, titanium alloys are considered to be typical difficult-to-cut materials,

corresponding to a small tool life and low machining speeds during the cutting process (Pramanik, 2014). Nevertheless,

electrochemical machining (ECM), a noncontact removal machining method based on electrolytic action, can be

effectively used to remove metallic materials, regardless of the material hardness (Saxena, et al., 2018). Due to its

desirable characteristics, the ECM process has been widely used in industrial applications to fabricate microparts for

precision instruments (Bhattacharyya, et al., 2004), parts with complicated shapes and structures (Liu, et al., 2016), and

integral blisks and diffusers for aircraft engines (Klocke, et al., 2014).

Recently, researchers have produced an array of holes in Ti-6Al-4V (Li, et al., 2016), machined the blisk sector of

the titanium alloy Ti60 through ECM (Xu, et al., 2016), and investigated the ECM characteristics of several titanium

alloys (Klocke, et al., 2013, 2016). In these studies, a specifically shaped electrode stopped or moved vertically to the

workpiece surface during machining. Although this typical die sinking ECM method has an extremely high machining

efficiency and can realize mass production, an expensive and complicated tool electrode specially designed for a specific

machining shape is needed. In contrast, scanning of a simple ECM tool electrode to generate a complicated shape is a

Saori HIZUME* and Wataru NATSU** * Lumentum Japan Inc.

4-1-55 Oyama, Chuo-ku Sagamihara, Kanagawa 252-5250, Japan ** Department of Mechanical Systems Engineering, Tokyo University of Agriculture and Technology

2-24-16, Nakacho, Koganei, Tokyo 184-8588, Japan E-mail: [email protected]

Received: 14 April 2021; Revised: 27 May 2021; Accepted: 23 June 2021

Abstract Electrochemical machining (ECM) is a noncontact removal machining method based on electrolytic action, can be effectively used to remove metallic materials, regardless of the material hardness. Due to its excellent features, ECM is usually used to realize the shape generation of difficult-to-cut metallic materials. However, when processing certain typical difficult-to-cut materials, such as titanium alloy and tungsten carbide, an oxide film is formed on the workpiece surface, which hinders the further dissolution of the material. Passivation due to oxide film formation generally occurs under a low current density. During the generation of a complicated shape through small tool electrode scanning, the low-current-density area around the peripheral area of the tool eventually covers the complete machining area. Consequently, the passivation becomes highly intense, and a suitable shape may not be generated. To solve this problem and realize the shape generation of titanium alloys in scanning ECM, the characteristics of the oxide film and its influence on material dissolution when using a suction tool were investigated based on current distribution calculations and machining experiments. A model for the scanning ECM of titanium alloys and guidelines for designing the scanning tool and determining machining conditions were proposed. The effectiveness of the proposed model and guidelines were validated through experiments with a suction tool.

Keywords : Electrochemical machining (ECM), Suction tool, Titanium alloy, Oxide film, Passivation, Scanning ECM, Electrolyte

1

2© 2021 The Japan Society of Mechanical Engineers[DOI: 10.1299/jamdsm.2021jamdsm0055]

Hizume and Natsu, Journal of Advanced Mechanical Design, Systems, and Manufacturing, Vol.15, No.5 (2021)

flexible and low-cost machining method to realize high-mix/low-volume production.

To date, several methods have been reported for shape generation by scanning a small and simple tool during ECM

processes. For example, shape generation has been realized using an electrolyte jet (Natsu, et al., 2007; Mitchell-Smith,

et al., 2018). However, in this method, the corrosive electrolyte may scatter and affect surrounding equipment. To solve

the problem of electrolyte scattering, suction tools have been proposed to confine the electrolyte and realize shape

generation (Yamamura, 2007; Endo, et al., 2014). Furthermore, an attractive cost-efficient ECM method to generate fir

tree slots with a thin wire has been realized (Klocke, et al., 2018). However, scanning ECM of titanium alloys has not

yet been successfully realized because of the passivation phenomenon (Mazzarolo, et al., 2012; Speidel, et al., 2016)

derived from tool scanning.

Several researchers have reported on the passivation phenomena in the low-current-density area in the ECM of

titanium alloys (Speidel, et al., 2016; Weinmann, et al., 2015). The passive film formed on the low-current-density area

influences material dissolution even though the current density increases when the tool is moved over the area. The

authors had investigated and discussed the uneven dissolution of titanium alloys in scanning ECM, but failed to find a

way to solve the problem (Hizume, et al., 2018). To reduce the influence of passivation, Liu et al. (2017) investigated the

travel rate parameter window, focusing on the anodic behavior in the machining of TB6 titanium alloy. Hackert et al.

(2008) proposed an air-assisted jet-ECM technique to protect the unmachined workpiece surface, and Guo et al. (2017)

introduced a scanning micro electrochemical flow cell to confine the electrolyte. Wang et al. (2019) investigated the

correlation among the jet shape, electrical parameters and edge condition in jet-EC groove milling and highlighted that

stray corrosion could be reduced by adjusting the jet shape. In the above studies, the low-current-density area was wide

because all the workpiece surfaces contacted the electrolyte. In addition, no systematic analysis and specific guidelines

for determining tool dimensions and machining conditions were given in the scanning ECM of titanium alloys. To the

best of the authors' knowledge, there are very few studies modeling the dissolution of titanium alloys in scanning ECM.

1.1 Clarification and applicability of research subjects

During shape generation with a small tool in ECM, a passive film always occurs on the area subjected to low current

density and hinders subsequent material dissolution. The schematics of the current density distribution in the three main

methods of scanning ECM are summarized in Fig. 1. One method is the conventional method (Fig. 1 (a)), in which a

small tool is scanned over the workpiece, which is submerged in an electrolyte tank. The second method is scanning with

an electrolyte jet (Fig. 1 (b) (Natsu, et al., 2007; Mitchell-Smith, et al., 2018), and the third method is scanning with a

suction tool (Fig. 1 (c)) (Yamamura, 2007; Endo, et al., 2014) without submerging the workpiece. All three methods have

a common feature in that a low-current-density area exists on the workpiece surface under the tool, although the

distribution differs for different methods, (Fig. 1 (d)). As shown in Fig. 1 (e), the identification of threshold current

density, the low-current-density area, and high-current-density area is the base to realize the scanning ECM. The present

study aimed to clarify the problem in the scanning ECM of titanium alloys, establish a material dissolution model, identify

the threshold current density and propose guidelines to design scanning tool dimensions and determine conditions suitable

for machining titanium alloys based on experimental results and theoretical analysis. The findings and measurements

obtained using a suction tool can also be applied to other scanning methods as long as the difference in current distribution

is taken into consideration.

2. Machining system and problem identification

2.1 Machining system and method

Fig. 2 shows the structure of the electrolyte suction tool used in this study, and Fig. 3 shows the configuration of the

scanning ECM system. A brass rod with a square cross-section of 1×1 mm was used as the electrode. The sidewall of the

electrode was covered with acrylic parts, and it formed an electrolyte flow path, as shown in Fig. 2. The unit composed

of the electrode and acrylic cover, which was used to circulate the electrolyte, is termed the electrolyte suction tool. In

the gap area, the suction of the pump was connected to the inlet, and the electrolyte flowed from the outlet (the electrolyte

inlet to the electrode) to the inlet (the electrolyte outlet from the electrode). The outlet was larger than the inlet to prevent

the electrolyte from overflowing the bottom of the tool. When the suction tool approached the workpiece in the normal

direction while air was sucked through the inlet hole, the sectional area of the flow channel in the gap area became

narrower, and the air flowed faster. According to Bernoulli’s principle, faster air flow between the tool electrode and

2



workpiece causes a decrease in pressure. The pressure difference caused the electrolyte to be sucked from the outlet into

the inlet and circulate beneath the tool electrode. In this way, the electrolyte was restricted and circulated in the area

between the bottom surface of the tool and the workpiece surface with the suction pump.

The distance between the surface of the workpiece and end face of the electrode before machining is defined as the

initial gap. This distance was set according to the suction pressure by considering the relationship between the gap width

and suction pressure (Endo, et al., 2014; Sato, et al., 2015).

Since the machining amount is proportional to the electric quantity (Wilson, et al., 1971) in the ECM process,

generation of a three-dimensional shape in scanning ECM can be realized by controlling the current value or scanning

speed during tool scanning. If the tool does not move and remains in a stationary state during machining, a concave shape

with a size nearly equal to the cross-section of the electrode is generated on the workpiece surface due to material

dissolution. The machining process without tool movement is termed stationary ECM in this paper. The tool was mounted

on the XYZ motorized stage of a commercially available NC milling machine. In this research, the tool was scanned only

in the ± X direction, without any movement in the Y and Z directions during scanning machining. In this manner, a

rectangular machining area was obtained.

2.2 Problem identification in the scanning ECM of titanium alloy

To clarify the characteristics of titanium alloy machining, the machining process during the scanning ECM of a

typical titanium alloy, Ti-6Al-4V, was first investigated experimentally. The machining conditions are listed in Table 1.

The maximum voltage value was set as 20 V since electrical discharges occurred at voltages larger than 20 V. Although

Tool

Electrolyte

Current

Workpiece

(a)

(b)

Tool

Workpiece

(c)

Tool

Workpiece

i

f

Area Am Area Af

Area Ae

Area Af

(e)

Curr

ent

den

sity

Width

if

Ae: Whole Area where current flows

Af: Area where only oxide film formation and gas generation occur due to low current density

Am: Area where material dissolution as well as oxide film formation and gas generation occur under high

current density

if: Threshold current density between low and high current density area

Fig. 1 Schematics of electrolyte area, current flow, density distribution and key parameters that determine machining

characteristics: (a) Electrolyte and current area in conventional scanning, (b) Electrolyte and current area in

scanning with electrolyte jet, (c) Electrolyte and current area in scanning with suction tool, (d) Schematic of

current distribution in three methods, and (e) Key parameters that determine machining characteristics.

3

Scanning with

suction tool

Scanning with

electrolyte jet Conventional

scanning

(d)



Table 1 Machining conditions.

current control instead of voltage control is ideal, our preliminary experiments showed that in constant current mode,

discharges occurred during machining. Discharges are considered to be caused by increased voltage due to the occurrence

of bubbles and sludge. Therefore, in this study, a constant voltage mode was applied to avoid electrical discharges. For

the electrolyte, a neutral sodium chloride aqueous solution was used instead of an acid or alkali solution since such neutral

solutions are effective in dissolving titanium (Weinmann, et al., 2015) and have good health and safety implications. The

tool was scanned 5 mm in the positive X direction at a speed of 0.05 mm/s. A voltage of 20 V was applied when the tool

scanned the 3 mm region in the center to avoid machining in the areas within 1 mm of both ends, where the tool scanning

speed was unstable. To enable a comparative analysis with the titanium alloy Ti-6Al-4V employed in this research, a

nickel-based superalloy, Inconel 718, was also machined.

To evaluate the shape of the machining area, as shown in Fig. 4, measurements were performed along six lines, with

three lines in the scanning direction, that is, the X direction, and the other three lines in the direction vertical to the

scanning direction, by using a contour shape measuring device (CV-3100S4, Mitutoyo Corp.). The measurement lines

were located in intervals of 0.2 mm.

Fig. 5 shows the typical machining trace shapes of the nickel and titanium alloys along the scanning direction. The

origin of the horizontal axis (X axis) is the center of the electrode at the start of machining. In addition, in subsequent

contour shape measurements, the center of the electrode at the start of machining was at 0. The results show that in the

Name Description

Electrolyte 15 wt% NaCl aq.

Flow rate 0.56 mL/s

Workpiece material Ti-6Al-4V, Inconel 718

Tool material Brass

Gap width 70 µm

Scanning speed 0.05 mm/s

Power supply Pulse constant voltage

Pulse width 0.005 s

Pulse period 0.050 s

Maximum voltage value 20 V

Φ2 mm(Outlet)

□19 mm

□1 mm

Φ3 mm(Inlet)

X

Z

Y

Electrode(Brass)

Acrylic parts

Fig. 2 Structure and dimensions of the

electrolyte suction tool: electrolyte is

confined and flows from the outlet to the

inlet on the tool bottom surface.

Fig. 3 Schematic of the scanning ECM system: the

electrolyte is circulated with a suction pump, the

pressure sensor is used to detect the gap-width, the

machining current is applied with the power supply.

4

Pressure sensor

Suction pump

Filter

Tank

Electrolyte flow

Workpiece

Tool

X

Z

Power supply

－

+

Y



case of the nickel alloy, a constant machining depth was obtained in the scanning direction. In contrast, the titanium alloy

was sparsely machined, and the machined surface was severely uneven. In addition, at some locations the height of the

surface became higher than the initial surface due to oxide film formation.

To confirm that this uneven dissolution was caused by tool scanning, stationary ECM was performed. Except for the

scanning movement, the other machining conditions were the same as those during scanning ECM, and the machining

time was set as 20 s. Fig. 6 shows the shape measurement results obtained with stationary ECM. Due to electrolyte flow,

material removal progressed on the upstream side, which involved a small amount of bubbles and sludge. On the bottom

surface of the machining trace, the entire surface of both materials was machined. Nevertheless, a difference in width

processing was observed. In the titanium alloy, machining was concentrated under the electrode, whereas in the nickel

alloy, a more extensive range was machined.

Fig. 4 Measurement lines used to evaluate the shape of the machined part: three lines along X axis and three along Y axis

in the area with oblique lines were measured with a contour shape measuring device.

Fig. 5 Shape of the machined mark in the scanning direction (scanning ECM): Ti-6Al-4V was sparsely machined, while

Inconel 718 was uniformly machined in scanning ECM.

Fig. 6 Shape of the machined mark in the electrolyte flow direction (stationary ECM): both Ti-6Al-4V and Inconel 718

were properly machined in stationary ECM.

0.2 mm0

.2 m

m

1 mm 1 mm

1 m

m

Y

Z X

Scanning distance/mm

Width /mm

5



From the above experimental results, it can be seen that during the machining of Ti-6Al-4V with a suction tool, the

machining characteristics for stationary ECM and scanning ECM are different. Specifically, the titanium alloy can be

machined through stationary ECM but not through scanning ECM under the same experimental conditions.

3. Clarification of the mechanism of scanning ECM of titanium alloy

The different machining characteristics of titanium alloy samples in stationary ECM and scanning ECM are thought

to be caused by the formation of an oxide film on the workpiece surface. As shown in Fig. 1, an area of low current

density is unavoidable for every scanning method since the current density on the workpiece gradually decreases to zero

at locations distant from the tool electrode. Because oxide films are usually formed at a low current density (Mazzarolo,

et al., 2012), the current density distribution under a suction tool and the formation of the oxide film were investigated

and discussed.

3.1 Current distribution on the workpiece surface under the suction tool

First, the current distribution under the suction tool was determined by numerical calculation because the actual

current density distribution could not be measured directly.

The electrolyte was present in an area larger than the electrode since the distance between the inlet and outlet holes

was 9 mm, centered on the electrode, as shown in Fig. 2. The current flowed from the workpiece toward the tool electrode

through the electrolyte. The two-dimensional current distribution on the workpiece surface under an applied voltage of

20 V was determined using the finite element analysis software COMSOL Multiphysics. The other conditions used in

the calculation are listed in Table 2. The current density distribution obtained by the linearized Butler-Volmer equation

(Hizume, et al., 2019) is shown in Fig. 7. The current density was the highest under the electrode and decreased as the

distance from the center increased.

Table 2 Conditions used to calculate the current distribution.

Fig. 7 Calculated current distribution on the workpiece under the suction tool: the current density decreases as the distance

from the center increased; all current is consumed for oxide film formation in the area where the current density is

lower than if1; material dissolution occurs when the current density is larger than if2 (see section 3.2).

Exchange current density 0.2 A/cm2

Anodic charge transfer coefficient 0.5

Cathodic charge transfer coefficient 0.5

Electric conductivity of electrolyte 15 S/m

Equilibrium potential 0.12 V

Workpiece potential 20 V

Tool potential 0 V

Electrolyte temperature 20 °C

if1

(4×10-5 A/cm2)

if2

102

100

10-2

10-4

10-6

10-8

10-10

10-12

6



3.2 Formation of the oxide film and its influence

In the ECM of titanium, an oxide film is formed on the workpiece surface, as expressed in equation (1), especially

under a low current density. The film thickness increases with the electric potential. It has been reported that all current

is consumed in the formation of the oxide film when the current density is lower than the threshold if1, while oxygen gas

is generated at the same time as film formation when the current density becomes higher than the threshold (Mazzarolo,

et al., 2012). The threshold if1 for titanium has been reported to be 4×10-5 A/cm2 (Mazzarolo, et al., 2012), and shown in

Fig. 7 with the red dot line. This threshold is extremely small compared to the current density used in ECM. When the

current density becomes much higher and exceeds a second threshold if2, shown with the blue dot line in Fig. 7, dissolution

of Ti in an n valence state occurs, as expressed in equation (2).

Ti + 2H2O → TiO2 + 4H+ + 4e (1)

Ti → Tin+ + ne (2)

Since the generated oxide film has a high internal stress, microcracks and holes easily occur on the film. The anodic

reaction at those cracks and holes changes from the formation of the oxide to the formation of a soluble titanium

compound with the activating anion of the electrolyte (Davydov, et al., 2017). Moreover, when using sodium chloride

aqueous solution, as in this study, the following reactions also occur (Weinmann, et al., 2015):

TiCl4 + H2O → TiOCl2 + 2HCl (3)

TiOCl2 + H2O → TiO2 + 2HCl (4)

Since the TiCl4 generated at the interface of the workpiece is unstable and short lived, it is hydrolyzed, as shown in

equation (3), when the electrolyte flows over it. The generated TiCl2 is subsequently hydrolyzed, as shown in equation

(4). In addition to these reactions, oxygen and hydrogen may be generated due to the electrolysis of water. Among these

reactions, the reactions that are more likely to occur are considerably influenced by the concentration of the electrolyte,

temperature and current density. Since reactions are very complicated in the ECM of alloys, it is difficult to clarify the

machining process just by analyzing the electrochemical equations, and experimental investigations must be performed.

3.3 Model of scanning ECM of titanium alloy

Considering the aforementioned aspects, the dissolution phenomenon in the scanning ECM of titanium alloy was

modeled and is shown in Fig. 8. As shown in Fig. 8(a), the current density is higher under the electrode and lower far

from the electrode when ECM power is applied. In addition, no current flows in the locations in which the electrolyte

does not exist. Material dissolution occurs in the area in which the current density is high, and an oxide film is formed

on the workpiece surface on which the current density is low, as shown in Fig. 8 (b). As explained in section 3.2, since

the formed oxide film has a high internal stress, microcracks and holes easily occur on the film (Fig. 8 (c)). Figs. 8 (a),

(b) and (c) correspond to the stationary ECM process. During scanning ECM, the tool is scanned over the workpiece

surface covered by the formed oxide film, as shown in Fig. 8 (d). Subsequently, the oxide film partially breaks and

Fig. 8 Model for the formation and breakdown of oxide films and the dissolution phenomenon in the scanning ECM of

titanium alloys: (a) distribution of current density when ECM power between the electrode and workpiece is applied,

(b) material dissolution under a high current density and formation of the oxide film under a low current density, (c)

occurrence of microcracks or holes on oxide film, (d) local breakdown of oxide film and occurrence of dissolution.

(a) (b)

(c) (d)

7



8

material dissolution occurs since the breakdown of the oxide film occurs gradually with increases in the anode potential

and current density (Chin, et al., 1974). Moreover, the breakdown of the oxide film occurs in a local manner under a high

material dissolution occurs since the breakdown of the oxide film occurs gradually with increases in the anode potential

anode potential and current density and requires a certain time (Datta, et al., 1977; Wang, et al., 2017).

The proposed model explains the different machining characteristics for the stationary and scanning ECM processes,

and the findings can provide guidance for the scanning ECM process.

3.4 Confirmation of electrolyte area and oxide film on workpiece surface

To verify the model, the electrolyte area and oxide film on the workpiece were investigated. The tool used for the

experiment involved an inlet and outlet at a distance of 4 mm from the electrode, and the bottom of the tool was sized 19

× 19 mm. However, the electrolyte area on the workpiece surface was unknown. To determine the area in which the

electrolyte was present under the tool, a transparent glass plate was used instead of the workpiece, and the electrolyte

flow on the tool bottom was observed. The observed image and the schematic based on the observed result in Fig. 9

shows that the electrolyte spread over a wide area compared to the electrode area. In the place where exists electrolyte

and is away from the electrode, the current density becomes low and a passivation film is likely to occur. According to

the calculation presented in section 3.1, the current density was the highest under the electrode, and it decreased with

increasing distance from the electrode. Fig. 10 shows the simplified current density distribution along the tool center in

the X direction. Although Fig. 7 indicates that the current density was nonlinear under the tool, for simplicity, a

trapezoidal current distribution was considered to qualitatively assess the influence. Herein, we considered the change in

current density at point A on the workpiece surface shown in Fig. 10 when a voltage was applied and the tool was scanned

in the scanning direction. The change in current density was in the form of a trapezoidal shape, in which a small current

started to flow once the electrolyte reached point A. Subsequently, the current gradually increased until the maximum

value was attained, and this value was maintained during the period in which the electrode passed over point A. Thereafter,

the current gradually decreased until the suction tool had completely passed through point A.

Fig. 9 Area containing the electrolyte under the suction tool and area of elemental analysis: electrolyte was confirmed to

exist in the area labeled electrolyte area, element mapping of oxygen was performed in the green rectangle: (a)

actually observed electrolyte area, (b) schematic of the bottom area based on the observation.

Fig. 10 Schematic of current density distribution on the workpiece surface: current density at point A changes in a

trapezoidal shape when the tool is scanned over it during machining.

－

＋

0

Scanning direction

Workpiece

Cu

rren

td

ensi

ty

X direction

Electrolyte electrode

A

Electrolyte

area

Electrode

Outlet (Electrolyte inlet

to electrode)

13.4

mm

X

Y

Inlet (Electrolyte

outlet from electrode)

10 mm

Electrode

Electrolyte area

(a) (b)



Subsequently, the presence of an oxide film on the workpiece surface was confirmed experimentally. The element

mapping of oxygen was performed using an electron probe microanalyzer (EPMA-1720H, Shimadzu Corp.) over a 5 ×

5 mm region contained by the green rectangle in Fig. 9, with the electrode in the center. The amount of oxygen in the

detection area was measured every 20 μm by using a sample processed through stationary ECM for 20 s with a 20 V

pulse voltage under the conditions presented in Table 1. Fig. 11 shows the analysis results. A more intense red color

corresponds to a larger amount of oxygen detected. The results indicated that oxygen was present outside the electrode

area due to existence of electrolyte, although a substantial amount of oxygen was present under the electrode.

Fig. 11 Measurement results for the elemental mapping of oxygen on the workpiece surface after ECM of the green

square part in Fig. 9.

4. Investigation of influencing factors and verification of the proposed model

4.1 Influence of time on the ECM of the titanium alloy

To clarify the influence of the duration in the low-current-density area, experiments were performed by changing

only the duration in the low-current-density area. It was considered that the time at a given current density during scanning

ECM changed in the form of a trapezoidal shape. Since a pulsed voltage was used to remove the reaction products from

the machining area during the pulse off time, the pulsed voltage was multiplied by a trapezoidal shape to realize the

change in the peak of the pulsed voltage. Under a low current, the duration was changed by changing the rise and fall

times of the trapezoidal shape. The other conditions were the same as those listed in Table 1. The applied peak voltage

waveform is shown in Fig. 12. The waveform corresponded to a trapezoidal shape in which the maximum peak voltage

was 20 V. During the actual scanning ECM process, because of the difference in the electrolyte area between the left and

right sides of the electrode, the low-current-density area was different as well. However, in this experiment, symmetrical

shapes were used to simplify the process. The rise and fall times were assigned eight values of 0, 0.1, 0.5, 1, 5, 10, 30

and 50 s. As shown in Fig. 12, because a voltage of 20 V was applied for 20 s under all conditions, the amount of

electricity increased with the rise time.

Fig. 12 Pattern of the input maximum voltage waveform: the time it takes for the voltage to rise and fall linearly from 0

V to 20 V is changed to simulate the scanning ECM with a stationary ECM; a larger gradient corresponds to a

faster the scanning speed.

X

Y

O

Electrode area

O Ka 1.5 kV

9



Fig. 13 shows the measurement results of the machined shape along the electrolyte flow direction. Figs. 13 (a) and

(b) show the results for the titanium and nickel alloys, respectively. In the case of 0 and 0.1 s, the surfaces of the machining

marks were smooth for both the titanium and nickel alloys. The left side was deeply processed owing to the larger amount

of fresh electrolyte in the region. For the nickel alloy, as shown in Fig. 13 (b), a larger rise time corresponded to deeper

machining as the electric quantity was larger. In contrast, when using the titanium alloy, in the case of 0.5 and 1 s, as

shown in Fig. 13 (a), although the whole areas were machined, the bottom of the machining marks was rough.

Furthermore, at rise times of 5, 10 and 30 s, the machining amount decreased despite an increase in the electricity amount,

and uneven dissolution occurred. At 50 s, the current exhibited its maximum value, and although the machining amount

increased, the machined surface became rough. Fig. 14 shows the relationship between the rise time and machined volume,

as obtained from the measured shapes. The machining amount in the nickel alloy increased, while the amount decreased

in the titanium alloy, even though the electric quantity increased due to an increase in the rise time.

Fig. 13 Shape of machined marks obtained through a trapezoidal form voltage of stationary ECM: (a) Ti-6Al-4V and (b)

Inconel 718.

Fig. 14 Relationship between rise time and machined amount obtained from the contour shape measurement results in

Ti-6Al-4V and Inconel 718: different from the result of Inconel 718, there exists a decrease in machined amount

in Ti-6Al-4V.

(a) (b)

10



Fig. 15 shows the current and voltage waveforms for the machining experiments conducted using the titanium alloy.

In Figs. 15(a) and (b), the rise times were 0.1 s and 1 s, respectively, and the whole area was machined. In Figs. 15(c)

and (d), the rise times were 30 and 50 s, respectively, and the machining was sparse. In the case of 30 and 50 s, the time

at which current started to flow notably deviated from the time of voltage application. Additionally, a measurable current

did not flow until the voltage reached 17 V under all conditions. Since the current was extremely low at a voltage equal

to or smaller than 17 V, a passive film was formed on the workpiece surface. Moreover, the passive film was destroyed,

and the current value increased when the voltage was higher than 17 V. As mentioned in section 3.2, since dissolution

progressed preferentially from places in which cracks and holes were generated on the film, the region that was not

machined remained, and the machining surface became rough. Under a rise time of 50 s, although a thicker film was

formed due to the large period at a low current density, the total machining time was also larger, and thus, the entire

surface was machined. In contrast, in the case of 0.1 and 1 s, the current increased rapidly since the time for the voltage

to reach 17 V was extremely small. Therefore, the time for the film to be formed was small, and no significant preventive

effect of machining was observed.

The experimental results and analyses indicated that the uneven dissolution of the titanium alloy was caused by the

movement of the electrode through the low-current-density area until the current density became sufficiently high to

dissolve the material. Under the experimental conditions, even and uneven dissolution occurred when the retention time

in the low-current-density area was less than 1 s and more than 5 s, respectively.

Fig. 15 Measured current and voltage with changed rise time to show the influence of the time under low current density:

(a) 0.1 s, (b) 1 s, (c) 30 s and (d) 50 s.

4.2 Influence of machining time on ECM of the titanium alloy

In the previous experiment, only the rise time was changed. However, during actual scanning ECM, for a given tool

shape, the time under the maximum voltage also varies with the electrode size and scanning speed. To clarify the influence

of the maximum voltage time on the machining characteristics, experiments were conducted in which the time period

was changed in the form of a trapezoidal shape. Fig. 16 shows the trapezoidal shape used in the experiment. This shape

simulated a case in which the electrolyte spread 6 mm in each direction to the left and right of an electrode sized 1 mm.

In this case, for a rise and fall time of 6 s, the time at 20 V was 1 s. From 0 to 1 in Fig. 16, which represented one cycle,

the frequency was changed to 0.0025, 0.025, 0.075, 0.15 and 0.225 Hz in five patterns. The rise times, corresponding to

the previous experiment, were 120, 12, 4, 2 and 1.2 s. Table 3 lists the parameter values for the frequency, rise time, time

under 20 V and number of repetitions. By changing the number of repetitions, the total processing time was maintained

constant in all conditions. If the uneven dissolution of the titanium alloy were primarily attributed to the rise time, the

-0.5

0

0.5

1

1.5

2

2.5

-5

0

5

10

15

20

25

0 10 20 30

Cu

rren

t/A

Vo

ltag

e /V

Time /s

-0.5

0

0.5

1

1.5

2

2.5

-5

0

5

10

15

20

25

0 50 100 150

Cu

rren

t /A

Vo

ltag

e /V

Time /s

-0.5

0

0.5

1

1.5

2

2.5

-5

0

5

10

15

20

25

0 10 20 30

Cu

rre

nt/

A

Vo

lta

ge

/V

Time /s

Voltage Current

-0.5

0

0.5

1

1.5

2

2.5

-5

0

5

10

15

20

25

0 30 60 90

Cu

rren

t /A

Vo

ltag

e /V

Time /s

(a)

(d)

(b)

(c)

11



experimentally obtained machining amount would be expected to increase as the frequency increased and the rise time

decreased. A trapezoidal shape was applied in the stationary state without scanning, and the other experimental conditions

were the same as those listed in Table 1.

Fig. 17 shows the measurement results for the machining amount under each condition. Although the machining

amount was expected to increase under a small rise time, it decreased, with a peak at 4 s. To determine the reason, the

current and voltage waveforms were investigated. Fig. 18 shows an example of the current and voltage waveforms of

one trapezoidal shape at approximately 100 s after the start of machining at 0.075 Hz (4 s) and 0.225 Hz (1.3 s). The

current started to flow later than the voltage application during the rising period. Moreover, during the falling period of

the voltage, the current decreased simultaneously with the voltage. At a low voltage, only a minimal current flowed due

to the formation of a passive film. At 0.075 Hz, at which considerable machining was performed, the maximum value of

Table 3 Frequency parameters.

Fig. 18 Voltage and current waveforms with a trapezoidal shape in the stationary ECM of Ti-6Al-4V (one trapezoidal

shape): (a) 0.075 Hz with an increase in machining amount and (b) 0.225 Hz with a decrease in machining amount

despite the short rise time.

Frequency Rise time Time at 20 V Number of repetitions

0.0025 Hz 120 s 20 s 1

0.025 Hz 12 s 2 s 10

0.075 Hz 4 s 0.67 s 30

0.15 Hz 2 s 0.33 s 60

0.225 Hz 1.2 s 0.2 s 100

/ -

-0.5

0

0.5

1

1.5

2

2.5

-5

0

5

10

15

20

25

0 4 8 12

Cu

rren

t/A

Vo

ltag

e/V

Time /s

Voltage

Current

-0.5

0

0.5

1

1.5

2

2.5

-5

0

5

10

15

20

25

0 2 4

Cu

rren

t/A

Vo

ltag

e/V

Time /s

(a) 0.075 Hz (b) 0.225 Hz

Fig. 16 Time change of voltage considered for

scanning ECM. Since the time under the tool

varies depending on the scanning speed, the

time change of voltage fluctuates.

Fig. 17 Relationship between the rising time and the

machined amount for Ti-6Al-4V. Times at any

voltage were equal because the machining times were

equal and the waveform frequency was changed.

12



the current was higher than that at 0.225 Hz, at which only a small amount of machining was performed despite the small

rise time. At 0.075 Hz, the current first increased, became constant and finally decreased. At 0.225 Hz, the current did

not become constant, but exhibited a triangular shape. Moreover, as the rise time decreased, the residence time of the

electrode decreased, and the increase in the current was insufficient; thus, the machining amount was reduced. These

results indicated that a high current and sufficient removal machining time were required to break down the passive film

formed in the low-current-density area to realize further dissolution. Since the time at 20 V with one trapezoidal shape at

0.075 Hz was 0.67 s, the time under the electrode was required to be 0.67 s or more during the scanning ECM process.

In contrast, at 0.025 Hz and 0.0025 Hz, the time at 20 V increased, likely due to the considerable time spent under a low

current density, as observed in the previous experiments.

4.3 Time at low and high current densities

Based on the experimental results and analyses, the proposed model for scanning ECM suggests the following.

When the time under a low current density in the area containing electrolyte far from the electrode is considerably larger

than 1 s, a thick passive film forms on the workpiece surface. This film prevents the occurrence of material dissolution.

Subsequently, even if the current density increases in this area when the tool is moved over it, the formed thick film

cannot be completely broken, and uniform material dissolution does not occur. In the case in which the time period under

the low current density is smaller than 1 s, only a thin and weak passive film is formed. This thin film can be completely

broken, and uniform material dissolution can occur as the current density increases. Moreover, a period longer than 0.67

s at a high current density is required to realize effective ECM processing.

5. Methodology for realizing the scanning ECM of titanium alloys

Guidelines for designing the tool and determining the machining conditions were proposed to realize the scanning

ECM of titanium alloys based on the results and analyses in sections 3 and 4.

5.1 Guidelines for determining tool dimensions and machining conditions

The relationship among the current density, formation of oxide film, generation of oxygen gas, and dissolution of

workpiece material is summarized first because the current distribution and its influence are the keys to realizing the

scanning ECM of titanium alloys. As shown in Fig. 19, three ranges of current density, If1, If2 and Im, are classified. In

range If1, where the current density is lower than the threshold if1, all current is consumed in the formation of the oxide

film; in range If2, where the current density is between thresholds if1 and if2, the applied current is consumed in the

formation of the oxide film and generation of oxygen gas; in range Im, where the current density is higher than threshold

if2, material dissolution as well as oxide film formation and gas generation occurs.

Since the current density distribution and thresholds if1 and if2 are difficult to obtain theoretically, we proposed

guidelines to identify the low- and high-current-density areas and then determine the tool dimensions and machining

conditions.

Step 1: Find the area where the current density is higher than the dissolution threshold if2 from experiments.

For any scanning method with a typical tool, the shape of the machined mark is measured after stationary machining

for 20 s. The area where the machined depth is larger than 0.001 mm is considered to be exposed to a current density

higher than the material dissolution threshold if2. This area is defined as the high-current-density area, area Am in Fig. 19.

Since current flows through all areas contacting with electrolyte, the area where electrolyte exists and other than the high-

current-density area is defined as the low-current-density area, area Af.

Step 2: Determine the tool dimension or scanning speed

It was concluded in section 4.3 that (a) in order to destroy the formed film and dissolve the base material, a time

longer than a certain Tm for the high-current-density area to pass through a certain point on the workpiec surface, and

(b) in order to avoid the formation of a thick and strong film, a time shorter than a certain Tf for the low-current-density

area to pass through the certain point are necessary to realize the scanning ECM of titanium alloy. For machining Ti-

6Al-4V, Tm and Tf are 0.67 s and 1 s, respectively.

Since the pass times are determined by the scanning speed, and the size of area Am and the size of area Af, selection

of an adequate scanning speed or adjustment of the tool sizes through tool design are the available choices.

As an application of the proposed guidelines to a suction tool, a novel tool was designed, and the effect was verified.

13



Fig. 19 Schematics of the method to identify the threshold current density and the low and high current density areas,

which are key parameters to design the tool and determine machining conditions.

5.2 Suction tool newly designed for scanning ECM

The created guideline was applied to design a suction tool for the scanning ECM of Ti-6Al-4V. As summarized in

section 4.3, to realize scanning ECM, the rise time must be less than 1 s, and the time under the electrode must be more

than 0.67 s. However, for the conventional tool described in Chapter 2, these time relationships are fixed. Therefore, a

novel tool was developed to reduce the rise time by reducing the area containing electrolyte on the bottom of the tool

without changing the electrode size. Fig. 20 shows the novel tool, which has a 1 × 1 mm rod electrode, the same as that

in the conventional tool, and a rectangular bottom surface with a size of 4 × 10 mm. Since the bottom surface of the

conventional tool has a size of 19 × 19 mm, the area in which the electrolyte is present can be restrained by reducing the

area. Fig. 21 shows the area containing the electrolyte at the tool bottom. The electrolyte is present in an approximately

elliptical shape, with a long and short diameter of 4.1 mm and 3.8 mm, respectively. If the electrolyte area other than the

area under the electrode is assumed to be in the low-current-density region, during reciprocating scanning in the X

direction, the low-current-density range is 12.4 mm when using the conventional tool. In contrast, with the novel tool,

this range decreases to 3.1 mm, which is approximately one-fourth the value for the conventional tool. Therefore, when

the scanning speed is 1.5 mm/s, the time to pass the whole electrode is 0.67 s, and the rise time is reduced to approximately

1 s. In the conventional tool, the time in the low-current-density regime is 2.6 s and 5.7 s at the inlet and outlet sides,

Fig. 20 Newly designed and fabricated tool: the inlet and outlet position were changed, and the size of the tool bottom

was reduced to narrow the low-current-density area.

Area Am Area Af

Area Ae

Area Af

if2

if1 Range If1

Range If2

Range Im T

hre

shold

cu

rren

t d

ensi

ty

Width

Ae: Whole Area where current flows, determined by the area where electrolyte exists

Am: Area where material dissolution as well as oxide film formation and gas generation occur under high current

density, determined by observing the machined mark

Af: Area where only oxide film formation and gas generation occur due to low current density, determined by (Ae-

Am)

if2: Threshold current density to dissolve the base material, determined according to the start of material dissolution

10 mm4 mm

X

ZY

0.5 mm0.5 mm

□1 mm

14



respectively. Additionally, the formation of an oxide film with the novel tool was experimentally confirmed. Fig. 22

shows the element mapping results for oxygen. The mapping surface was a titanium alloy sample subjected to the

stationary ECM process, corresponding to the 5 × 5 mm area in the green rectangle in Fig. 21. In the case of the novel

tool, oxygen was present in a smaller area than that for the conventional tool; this area corresponded to the area in which

the electrolyte was present. Thus, it was concluded that when the electrolyte area decreased, the time to form an oxide

film before machining decreased. Meanwhile, compared with Fig. 11, it can be seen that the oxidization phenomenon

become non-uniform when the novel tool was used, although the passivation region becomes smaller. Since the tool

reciprocates and has a smoothing effect in scanning ECM, the influence of the non-uniform oxidization phenomenon on

the machining characteristics is small.

5.3 Realization of scanning ECM with the newly designed suction tool

The scanning ECM of Ti-6Al-4V was conducted using the novel tool to verify the occurrence of even dissolution.

Although the electrolyte was suctioned using the same pump under the same settings for both the novel and conventional

tools, the electrolyte flow rate was different. The flow rate was measured without a power supply during machining. The

flow rate was 0.18 mL/s when using the novel tool, approximately one-third that of the conventional tool (0.56 mL/s).

The tool scanning speed was set to 1.5 mm/s. The tool was scanned for 20 mm and reciprocated 15 times while supplying

machining power. The other experimental conditions were the same as those listed in Table 1.

The results of the contour shape measurement along the scanning direction and its perpendicular direction are shown

in Fig. 23, while the three-dimensional shapes of machined grooves measured using a laser measuring instrument (KS-

1100, Keyence Corp.) are shown in Fig. 24. In the case of scanning ECM using the novel tool with a small electrolyte

area, even material dissolution was realized, and no sparse surface was observed. Thus, it was confirmed that scanning

ECM of titanium alloy could be realized with the novel tool.

-0.06

-0.03

0

-1 0 1

De

pth

/m

m

Y/mm

Conventional tool Novel tool

(b)

Width/mm

X

Y

O Ka

Electrode area

O Ka 1.5 kV

Novel toolConventional tool

(a)

Scanning distance/mm

Fig. 21 Area in which the electrolyte is present

under the novel tool: the electrolyte spread in

an elliptical shape with a width of 4.1 mm

centered on the tool.

Fig. 22 Measurement results for the elemental

mapping of oxygen within the green square

area on the workpiece surface shown in Fig.

21.

Fig. 23 Shape of a groove machined with the conventional and novel tools in Ti-6Al-4V at 90 mm/min scanning

speed: material dissolution evenly occurs in the case of the novel tool: (a) shape along the scanning direction

and (b) shape in the direction perpendicular to the scanning direction.

15



The roughness on the groove bottom surface machined with the novel tool is shown in Fig. 25. As for the surface

machined with the conventional tool, the roughness was not measured because there existed un-machined area as shown

in Fig. 23 and 24. It is found that the surface roughness along the scanning direction is quite smaller than that along its

perpendicular direction. This is because, the relative movement of the tool and the workpiece has a smoothing effect on

the surface roughness in the scanning direction.

Fig. 24 Three-dimensional shape and appearance of grooves on Ti-6Al-4V, machined with the conventional tool (a) and

the novel tool (b) at a scanning speed of 90 mm/min.

Fig. 25 Surface roughness on the bottom surface machined with the novel tool in the scanning direction X and its

perpendicular direction Y: (a) Ra and (b) Rz.

6. Conclusions

In this paper, to realize the scanning ECM of titanium alloys with a small tool, the formation of the oxide film and

its influence on the characteristics of scanning ECM were analyzed and discussed. The machining process, including the

500 μm

(a) Conventional tool

500 μm

(b) Novel tool

16



formation of an oxide film under low current density and material dissolution under high current density, was modeled.

Guidelines for designing the scanning tool and selecting the machining conditions were proposed and applied to the

scanning machining of the typical titanium alloy Ti-6Al-4V with an electrolyte suction tool, and their effectiveness was

experimentally verified. The following conclusions can be drawn:

1) The uneven dissolution of the titanium alloy is caused by the formation of an oxide film on the workpiece surface

at a low current density before the electrode passes, which leads to partial breakage of the oxide film and

dissolution of workpiece material when the electrode passes.

2) A method to specify the low-current-density and high-current-density areas based on experimental results was

proposed. The high-current-density area was determined based on the measured shape of the machined mark,

while the low-current-density area was determined from the electrolyte-containing area and the high-current-

density area.

3) The time period under the low- and high-current-density plays a key role in the scanning ECM of titanium alloy.

It is necessary to set a low-current-density time before the electrode passes that is shorter than 1 s and the removal

machining time to longer than 0.67 s to realize an even dissolution of the typical titanium alloy Ti-6Al-4V under

the experimental conditions used in this research.

Acknowledgements

This work was supported by JSPS KAKENHI Grant (Grant number JP18H01348).

References

Bhattacharyya, B., Munda, J., Malapati, M., Advancement in electrochemical micro-machining, International Journal of

Machine Tools & Manufacture, Vol.15 (2004), pp.1577-1589.

Chin, D.T., Mao, K.W., Transpassive dissolution of mild steel in NaNO3 electrolytes, Journal of Applied Electrochemistry,

Vol.4 (1974), pp.155-161.

Datta, M., Landolt, D., Film breakdown on nickel under transpassive dissolution conditions in sodium nitrate solutions,

Journal of Electrochemistry Society, Vol.124, No.4 (1977), pp.483-489.

Davydov, A.D., Kabanova, T.B., Volgin, V.M., Electrochemical machining of titanium. Review1, Russian Journal of

Electrochemistry, Vol.53, No.9 (2017), pp.941-965.

Endo, K., Natsu, W., Proposal and verification of electrolyte suction tool with function of gap-width detection,

International Journal of Electrical Machining, No.19 (2014), pp.34-39.

Guo, C., Qian, J., Reynaerts, D., Electrochemical machining with scanning micro electrochemical flow cell (SMEFC),

Journal of Matererials Processing Tech., Vol.247 (2017), pp.171-183.

Hackert, M., Meichsner, G., Schubert, A., Generating micro geometries with air assisted jet electrochemical machining,

Proceedings of the 10th Anniversary International Conference of the European Society for Precision Engineering

and Nanotechnology, (2008), pp.420-424.

Hizume, S., Natsu, W., Influence of machining conditions on ECM characteristics of titanium alloy in shape generation

by scanning tool electrode, Procedia CIRP, Vol.68 (2018), pp.746-750.

Hizume, S., Natsu, W., Goto, A., Study on film formation in electrochemical machining of titanium alloys, Proceedings

of the 2019 JSPE Spring Meeting, (2019), pp.280-281 (in Japanese) .

Klocke, F., Zeis, M., Klink, A., Veselovac, D., Experimental research on the electrochemical machining of modern

titanium- and nickel-based alloys for aero engine components, Procedia CIRP, Vol.6 (2013), pp.368-372.

Klocke, F., Klink, A., Veselovac, D., Keith, D., Leung, S., Schmidt, M., Schilp, J., Levy, G., Kruth, J., Manufacturing

technology turbomachinery component manufacture by application of electrochemical, electro-physical and

photonic processes, CIRP Annals, Vol.63, No.2 (2014), pp.703-726.

Klocke, F., Herrig, T., Zeis, M., Klink, A., Experimental research on the electrochemical machinability of selected γ-TiAl

alloys for the manufacture of future aero engine components, Procedia CIRP, Vol.35 (2016), pp.50-54.

Klocke, F., Herrig, T., Klink, A., Evaluation of wire electrochemical machining with rotating electrode for the

manufacture of fir tree slots, ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition

(2018), DOI: 10.1115/GT2018-76910.

17



Li, H., Gao, C., Wang, G., Qu, N., Zhu, D., A study of electrochemical machining of Ti-6Al-4V in NaNO3 solution,

Scientific Reports, Vol.6, 35013 (2016).

Liu, G.X., Zhang, Y.J., Jiang, S.Z., Liu, J.W., Gyimah, G.K., Luo, H.P., Investigation of pulse electrochemical sawing

machining of micro-inner annular groove on metallic tube, International Journal of Machine Tools & Manufacture

Vol.102 (2016), pp.22-34.

Liu, W., Ao, S., Li, Y., Liu, Z., Zhang, H., Manladan, S.M., Luo, Z., Wang, Z., Effect of anodic behavior on

electrochemical machining of TB6 titanium alloy. Electrochimica Acta, Vol.233 (2017), pp.190-200.

Mazzarolo, A., Curioni, M., Vicenzo, A., Skeldon, P., Thompson, G. E., Anodic growth of titanium oxide:

Electrochemical behaviour and morphological evolution, Electrochimica Acta, Vol.75 (2012), pp.288-295.

Mitchell-Smith, J., Speidel, A., Clare, A. T., Transitory electrochemical masking for precision jet processing techniques,

Journal of Manufacturing Processes, Vol. 31 (2018), pp.273-285.

Natsu, W., Ikeda, T., Kunieda, M., Generating complicated surface with electrolyte jet machining, Precision Engineering,

Vol.31 (2007), pp.33-39.

Pramanik, A., Problems and solutions in machining of titanium alloy, The International Journal of Advanced

Manufacturing Technology, Vol.70 (2014), pp.19-928.

Sato, A., Natsu, W., Proposal and verification of area-limited electroplating with suction tool, International Journal of

Electrical Machining, No.20 (2015), pp.37-43.

Saxena, K.K., Qian, J., Reynaerts, D., A review on process capabilities of electrochemical micromachining and its hybrid

variants, International Journal of Machine Tools & Manufacture, Vol.127 (2018), pp.28-56.

Speidel, A., Mitchell-Smith, J., Walsh, D.A., Hirsch, M., Clare, A., Electrolyte jet machining of titanium alloys using

novel electrolyte solutions, Procedia CIRP, Vol.42 (2016), pp.367-372.

Wang, D., Zhu, Z., He, B., Ge, Y., Zhu, D., Effect of the breakdown time of a passive film on the electrochemical

machining of rotating cylindrical electrode in NaNO3 solution, Journal of Materials Processing Technology, Vol.239

(2017), pp.251-257.

Wang, X., Qu, N., Fang, X., Reducing stray corrosion in jet electrochemical milling by adjusting the jet shape, Journal

of Materials Processing Tech., Vol.264 (2019), pp.240-248.

Weinmann, M., Stolpe, M., Weber, O., Busch, R., Natter, H., Electrochemical dissolution behaviour of Ti90Al6V4 and

Ti60Al40 used for ECM applications, Journal of Solid State Electrochemistry, Vol.19 (2015), pp.485-495.

Wilson, J.F., Practice and theory of electrochemical machining, John wiley & Sons, Inc. (1971).

Xu, Z., Chen, X., Zhou, Z., Qin, P., Zhu, D., Electrochemical machining of high-temperature titanium alloy ti60, Procedia

CIRP, Vol.42 (2016), pp.125-130.

Yamamura, K., Fabrication of ultra precision optics by numerically controlled local wet etching, CIRP Ann. - Manuf.

Technol., Vol.56, No.1 (2007), pp.541-544.

18

mechanism clarification and realization of scanning

Documents