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ScienceDirect Procedia CIRP 42 (2016) 362 – 366

18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)

Characteristics of different transparent and conductive materials applied for observation of ECM gap phenomena Fuchen Chua, Tomoyuki Shimasakia, Masanori Kuniedaa * a The University of Tokyo, Hongo 7-3-1, Bunkyo-ku Tokyo 113-8656, JAPAN * Corresponding author. Tel.: +81-3-5841-6462; fax: +81-3-5841-1952. E-mail address: [email protected]

Abstract

In electrochemical machining (ECM) processes, bubbles and sludge of metal oxides are generated in the gap. These products may influence the conductivity of electrolyte and the current distribution on the surface of workpiece, which directly influence the machining accuracy of ECM. In order to improve the machining accuracy, it is necessary to elucidate the gap phenomena of ECM. In this study, to investigate the mechanism of the change in the current density distribution, the gap phenomena were directly observed from the direction normal to the machining surface by a high speed video camera through the electrode made of transparent material such as SiC and ITO glass, which are optically transparent and electrically conductive. The influence of characteristics of transparent electrodes on the observation phenomena were compared. Moreover, the similarity of observed phenomena using different transparent electrodes to the actual ECM process was discussed. © 2016 The Authors. Published © Published by by Elsevier ElsevierB.V. B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM (http://creativecommons.org/licenses/by-nc-nd/4.0/). XVIII). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)

Keywords: Electrochemical Machining, Transparent Electrode, ITO, Bubbles, current distribution

1. Introduction Electrochemical machining (ECM) is a non-traditional machining process. Material is removed by the mechanism of anodic electrochemical dissolution. During this process, in the gap, hydrogen bubbles are generated from cathode, while metallic sludge is mainly generated from anode. These products will influence current distribution in the gap and on the surface of the electrodes. To discharge these products, pulse power source and flushing of electrolyte is usually introduced during machining. All these factors make the gap phenomena complicated. It is difficult to control current density distribution on the surface of workpiece, thereby the accuracy of ECM is decreased. It is the leading reason which prevents the development of ECM. In the previous researches, Sakai et al. [1] observed the gap phenomena from the side direction and found that in ECM process after machining current was applied, gap was immediately filled with products such as bubbles and sludge. By observation from side direction, excellent accordance

between simulation and experimental results was indicated in the research of Klocke F. et al [2]. In PECM, based on the direct observation results, the link between electrochemical inprocess and geometric shaping of different and complex geometries was investigated by Rebschläger A. [3]. Zhang et al. [4] have succeeded in observing such gap phenomena from the normal direction using transparent electrode made of SiC (silicon carbide) single crystal wafer. Zhang et al. [5][6] also utilized transparent electrode to observe the gap phenomena to investigate the influence of flushing and jumping on ECM process. Moreover, Shimasaki et al. [7] elucidated the relationship between the bubble generation rate and concentration of electrolyte and measured the time until the boiling of electrolyte in the gap using the same transparent wafer. However, since the transparent electrode of SiC is semiconductor, its special material characteristics might influence the gap phenomena even in the processes of ECM. Hence, to elucidate the influence of characteristics of transparent conductive materials on ECM processes, it is extremely necessary to observe the gap phenomena of ECM

2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.199

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Fuchen Chu et al. / Procedia CIRP 42 (2016) 362 – 366

utilizing different kinds of optically transparent and electrically conductive material as electrode.

Electrolyte

NaNO3 aq (5%)

Resolution [pixel]

960×960

Frame rate [fps]

10,000

2. Transparent materials In this research, SiC (silicon carbide) single crystal wafer and ITO (Indium Tin Oxide) coated glass which is one of the most widely used transparent conducting oxides in various fields were utilized as electrode in ECM. Specifications of transparent materials are shown in Table 1 and Table 2. SiC is usually produced into bulk or wafer by crystallization. In the occasion of ITO, PVD (physical vapor deposition) is often utilized as coating method, hence, it is usually utilized as thin film, and sheet resistance defined as ܴ௦ ൌ ߩΤ‫ݐ‬ǡ where ߩ is bulk resistivity and t is the sheet thickness, is used to characterize the resistance of ITO. In this report, ITO is attempted to be utilized in ECM process to observe gap phenomena while machining for the first time.

4. Comparison between SiC and ITO In this section, transparent materials (SiC and ITO), which are utilized for the observation, are compared from several aspects. Some aspects, such as transparency; and thermostability, are compared based on data mainly from the previous researches and suppliers. The other aspects, such as, electrical conductivity, available polarity for observation, wear and the similarity of observed phenomena to the actual ECM process, are compared based on our experiments.

Flushing flow

Cell SiC, ITO coated glass or SUS304

3. Experimental method Experimental setup is shown in Fig. 1. The cylindrical pipe electrode is made of carbon steel (S45C). The transparent electrode is embedded into a metal plate which has the same thickness with the transparent electrode. This metal plate is sandwiched by other two metal plates, which have holes at the center for observation. This combination of three plates is fixed onto the machining cell. Through transparent electrode, gap phenomena of ECM process were observed using a high-speed video camera. Furthermore, electrolyte flushing can be conducted through the hole at the center of the cylindrical pipe electrode. When ECM is conducted, voltages applied between electrodes including the voltage drop in electrolyte and voltage drop in electrode itself are recorded by oscilloscope. The highspeed camera used is MEMERECAM HX-3 (made by NAC IMAGE TECHNOLOGY). Pulse signal is generated by a function generator and amplified by a bipolar power supply. Experimental conditions and photographing conditions are shown in Table 3.

Carbon steel (S45C)

High speed video camera

Fig. 1 Experimental setup

4.1 Transparency of original materials Based on the research conducted by Kitamura. et al [8], transmittance of SiC single crystal wafer is approximately 3040%. It depends on the wavelength of light, and near the area of blue light (465nm), transmittance is nearly zero. In the occasion of ITO, which has a thickness of 0.33μm in this research, the transmittance is over 70% and independent of wavelength. Hence, by using ITO, observation is easier and clearer. 4.2 Thermostability

Table 1. Specifications of SiC Spectral transmission factor

40%

Resistivity [Ωcm]

0.013 ~ 0.025

Thickness [μm]

380

Decomposition temperature [Υ]

2830

Table 2. Specifications of ITO Spectral transmission factor

70%

Sheet resistance [Ω/sq]

5

Thickness [μm]

0.33

Heatproof temperature [Υ]

200

Table 3. ECM machining and photographing conditions Electrode material

SiC, ITO, S45C

Gap width [μm]

100,200

Machining current [A]

0.8, 1, 2

In ECM, because of high machining current, Joule heating generated in the gap cannot be ignored. The thermostability of transparent materials is necessary. SiC single crystal wafer has a high decomposition temperature of 2830 Υ , and thermostability is good. On the other hand, since the thin ITO film utilized in this research cannot tolerate a temperature over 200Υ, the thermostability of ITO film is not good enough. Hence, ITO can be used for the observation of ECM only in a relatively low range of machining current. 4.3 Electrical conductivity Compared to normal metallic materials, SiC and ITO both have high electrical resistivity. To investigate the electrical conductivity of the transparent electrodes, under the same electric current, the voltage applied between a steel electrode and transparent electrodes including voltage drop in the


Voltages applied between electrodes [V]

60 50

Anode Cathode

50

40

30 20 10 0

11 11

10

6

SiC

ITO

6

S45C

Fig. 2 Voltages applied between electrodes using different materials

4.4 Change in transparency during machining 4.4.1 Observation using SiC In the case that SiC was used as cathode, the gap phenomena were observed successfully. However, metal was electroplated on the surface of SiC wafer as shown in the microscopic image observed by SEM in Fig. 3. After several times of machining, the attached metal influenced the observation. However, attachment could be removed by polishing. Hence, the repeated usage of SiC for observation is possible. When SiC was used as anode, under the same experimental conditions, the voltage applied between electrodes was significantly higher than that when SiC was cathode. The diode oxide film mentioned above is considered to be the reason of this phenomenon. Hence, SiC is only available for the observation as cathode.

4.4.2 Observation using ITO When ITO was used as anode, it keeps its transparency while machining. The gap phenomena (shown later in Fig. 8(a)) were observed successfully. However, when ITO was used as cathode, it turned non-transparent immediately after machining began. Thus, the gap phenomena could not be observed. Fig. 4 shows the SEM image of the surface of ITO before and after electrolysis. The surface roughness became bad. It is considered that ITO film was deteriorated by chemical ion exchange reaction. Hence, ITO is only available for observation as anode. 4.5 Wear It is known that SiC cannot be machined by ECM neither as anode nor as cathode. However, ITO could be machined in both polarities. To exclude the influence of temperature, flushing of electrolyte was conducted, and the cross section of ITO after machining using steel pipe electrode was measured. Fig. 5 shows the cross section of the ring shape machined area on ITO (machining time was 200ms and ITOwas anode). When ITO was used as anode, the depth of ring shape machined area became deeper and deeper until 120ms with a depth of around 150nm as shown in Fig. 6(a). After this time point, since the film thickness in the machined area was too thin, the high resistance in this area was too high for current to flow through. Thus, machining current started to flow through the outer area making the diameter of the machined area larger and larger as shown in Fig. 6(b), thereby no more bubbles were generated within the machined area after 120ms. Hence, within a relatively short time of 120ms, ITO film of 0.33μm in thickness can be utilized for the observation as anode. When ITO was used as cathode, the wear rate was quicker than the removal rate when used as anode. Considering that the transparency is lost immediately as described in the previous section, use of ITO as cathode is difficult. 0.5μm

0.4mm

Fig. 5 Cross section of machined area on ITO 300 250 200 150

100

10

240 150

Cathode

Diameter of machined area [mm]

electrolyte and voltage drop in the electrodes was recorded as shown in Fig. 2. As reference, the voltage drop between the electrodes which are both made of steel was also measured. Since the current is the same, the voltage drop in the electrolyte should be the same. Hence, the difference comes from the voltage drop in the electrode. When transparent electrode is cathode, voltage drop between electrodes is a little higher than ordinary metallic material. However, when SiC is anode, voltage drop between electrodes is significantly higher than other two kinds of material. It is considered that an oxide film generated on the surface of SiC, which has a diode characteristic, has a high resistivity when current flows with the polarity of positive SiC. It will make the gap phenomena unlike the actual ECM processes.

Wear[nm]

364

Anode

8

170

160

170

6

130 100

40 50 20 0 0 0 100

6.2 6.2 6.2 6.2

Fig. 4 Microscopic image of ITO by SEM

8

2 0

200 300 Time[ms]

400

500

0

100

200 300 Time [ms]

400

500

(b)

Fig. 6 Change of depth and diameter of machined area

4.6 Similarity to actual ECM process.

Fig. 3 Microscopic image of SiC by SEM

10μm

7.5

6.2

4

(a)

10μm

7

10μm

4.6.1 Bubbles generation mechanism when ITO is utilized as anode With the flushing of electrolyte from the steel pipe electrode, since generated bubbles can flow out of the gap, number density of bubbles per unit area on the electrode surface is low. Thus, shadow of each bubbles projected by the illumination of the high-speed camera can be observed on the surface of the steel pipe electrode. To make it easy to observe the shadow of current

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was set to be low and the gap width was set to be large. The photographing results are shown in Fig. 7. From the shadow of bubbles, it was found that bubbles were generated on both electrodes, which are oxygen from anode of ITO and hydrogen from cathode of carbon steel.

0.2mm㻌

0.2mm㻌

(a) Bubbles with shadow (b) Bubbles without shadow Fig. 7 Observation result using ITO as anode

4.6.2 Different behaviors of bubbles with different transparent electrodes The observation videos taken by high-speed camera using different transparent electrodes are shown in Fig. 8. Fig. 8(a) shows the results when ITO was used as anode, and Fig. 8(b) shows the case that SiC was used as cathode. It can be seen that under the same conditions, in the occasion of SiC, bubbles are easier to coalesce with each other and can move quickly. However, in the occasion of ITO, the diameter of bubbles was uniform. Bubbles moved independently and slowly.

6. Conclusions 1. SiC has a diode characteristic. When SiC is anode, the oxide film generated on the surface of SiC makes it difficult for current to flow. It is considered that SiC is not suitable for the observation of ECM as anode. 2. SiC wafer used as cathode is useful for observation. Although transparency is lower than ITO, SiC can be used almost indefinitely because SiC is hardly consumed thermally or chemically. 3. Since ITO film (0.33μm in thickness) cannot tolerate high temperature (over 200ć), it can be utilized at relatively low machining current. 4. ITO keeps its transparency when it is utilized as anode. However, ITO is worn. After 120ms machining, the film becomes too thin for current to flow through. As a result, ITO film can be utilized for observation in a relatively short time (about 120ms) as anode. 5. ITO film becomes non-transparent when it is used as cathode because of chemical ion exchange reaction and bad surface roughness. Furthermore, when ITO is cathode, it still can be worn and the wear speed is faster than the removal rate when it used as anode. 6. By comparing the videos taken using different transparent electrodes, the phenomena of SiC are more similar to actual ECM process than ITO.

S45C (+) H2

Ø6

S45C (-)

Electrolyte

O2

Electrolyte

SiC (-) 0ms

㻌

㻌

20ms (a) Observation result with ITO

㻌㻌

ITO (+)

Fig. 9 Bubble generation mechanism

6 0ms

Electrolyte

Ø6

㻌

㻌

20ms (b) Observation result with SiC

㻌

㻌㻌

6 0ms

Fig. 8 Observation result with different transparent electrodes under same conditions

5. Discussions The reason why behaviors of bubbles appear to be different can be explained like following. In the occasion of SiC used as cathode, most of the gas in the gap is hydrogen generated on SiC as shown in Fig. 9. However, when transparent electrode is ITO used as anode, gas contains hydrogen from cathode of carbon steel and oxygen from anode of ITO. Since ITO is oxide, oxygen bubbles are considered to be generated from ITO itself. Moreover, SiC is hydrophilic and ITO is hydrophobic. The wettability is considered to be another reason for the different behaviors of bubbles as shown in Fig. 10.

Electrolyte Gas (hydrophilic)

γgl γsl

0ms

H2

θ

γsg

Gas (hydrophobic)

γgl γsl

γsg

θ

SiC

ITO

Fig. 10 Wettability of different transparent electrodes

Acknowledgement The author are grateful to HODEN SEIMITSU KAKO KENKYUSHO CO., LTD. and Mr. Kohzo Abe of HAMADA HEAVY INDUSTRIES LTD. for their kind supports in conducting the experiments. This work was supported by the Cross-Ministerial Strategic Innovation Promotion Program (SIP): Innovative Design/Manufacturing Technologies, funded by NEDO. References [1] Shigenori Sakai, Takahisa Masuzawa, Electro-chemical Finishing of Metal Mold, vol. 42ˈNo. 6ˈpp. 39-44(1990). [2] Klocke F., Zeis, M., Herrig, T., Harst, S., Klink, A., Optical In-Situ Measurements and Interdisciplinary Modeling of the Electrochemical Sinking Process of Inconel 718, In: CIRP MIC, 2014

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[3] Rebschläger A., Kollmannsperger R., Bähre D., Video based process observations of the pulse electrochemical machining process at high current densities and small gaps, In: CIRP HPC, 2014 [4] Wenhao Zhang, Tomoo Kitamura, Masanori Kunieda, Kohzoh Abe, Observation of ECM Gap Phenomena through Transparent Electrode, IJEM, 19, 40-44 (2014). [5] Wenhao Zhang, Masanori Kunieda, Influence of Gap Width on ECM Gap Phenomena Observed Using Transparent Electrodes, Proc. of Autumn Meeting of JSPE (2013), 977-978 (in Japanese). [6] Wenhao Zhang, Masanori Kunieda, Observation of ECM Gap Supplied with Electrolyte Flow Using Transaprent Electrodes, Proc. of Spring Meeting of JSPE (2013), 497-498 (in Japanese). [7] Tomoyuki Shimasaki, Tomoo Kitamura, Masanori Kunieda, Fundamental Study of ECM Gap Phenomena Using Transparent Electrode, INSECT 2014 (2014), 135-143. [8] Tomoo Kitamra, Masanori Kunieda, Clarification of EDM Gap Phenomena Using Transparent Electrodes, CIRP Annals (2014), 63, 1, 213-216.