Wire Electrochemical Grinding of Tungsten Micro-rod with Electrostatic Induction Feeding Method

Available online at www.sciencedirect.com

ScienceDirect Procedia CIRP 68 (2018) 699 – 703

19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain

Wire electrochemical grinding of tungsten micro-rod with electrostatic induction feeding method Wei Hana, Masanori Kuniedaa b

Department of Precision Engineering, the University of Tokyo, Tokyo 113-8656, Japan

* Corresponding author. Tel.: +81-080-4335-2199; fax: +81-080-4335-2199. E-mail address: [email protected]

Abstract In this study, tungsten was machined successfully with the neutral electrolyte NaNO3 aqueous solution and bipolar current because the nonconductive oxide layer on the surface, which interrupts the electrochemical dissolution process, was removed by the NaOH generated when the polarity of the tungsten is negative. However, the tool electrode was worn due to the bipolar current. Although the tool wear ratio was decreased when the platinum sheet was used as tool electrode than that with the stainless steel plate used as tool electrode, the platinum tool was also slightly worn. Hence, to avoid the influence of the tool wear, a new method, WECG (wire electrochemical grinding), for tungsten machining with neutral electrolyte and bipolar current is proposed, and the running brass wire is used as the tool electrode. Thereby, the problem of tool wear inevitable due to the bipolar current was eliminated because the running wire tool was used in this method. 2018The The Authors. Published by Elsevier B.V. © 2018 © 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 scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining

Keywords: Wire electrochemical grinding, tungsten micro-rod; ultra-short pulse; electrostatic induction feeding method; micro-machining;

1. Introduction The accurate and precise micro-tools are widely used for the micromachining of complex micro features in a wide range of engineering materials. Since the tungsten material has the advantages of high erosion resistance, high electrical and thermal conductivity, and high stiffness [1], tungsten is often fabricated as micro-tools for micro-machining process. Microtools have been successfully fabricated by micro electrical discharge machining (micro EDM). However, drawbacks such as tool wear and heat-affected zones below the machined surface exist due to the thermal effect of this process. Sundaram and Rajurkar [2] analyzed the causes of tool electrode surface cracks and material exfoliation after machining of micro tools by WEDG. It is necessary to find alternative processes that can avoid the above drawbacks to fabricate micro tools. Electrochemical machining (ECM) is an anodic electrochemical dissolution process [3]. It has such advantages as no generation of burrs, cracks, nor heat-affected zones on the machined surface. Hence, ECM is a potential method for fabricating precision micro tools. In recent years, Schuster et al. [4] found that the use of ultra-short pulse voltage in the order of

several tens of ns realizes micro interelectrode gap which is equivalent to micro EDM. Feeding capacitance C1

Tool electrode

Pulse voltage

Workpiece

Fig. 1 Electrostatic induction feeding method

On the other hand, Kunieda et al. [5] developed the electrostatic induction feeding method used for micro EDM, as shown in Fig. 1. Using this method, discharge craters of submicrometer diameters can be obtained by eliminating the influence of the stray capacitance in the electric feeders. Then, Koyano and Kunieda [6] newly applied the principle of the electrostatic induction feeding method to micro-ECM. Current flows through the working gap only at the instance when the pulse voltage changes to high or low because the pulse power supply is coupled to the tool electrode by a feeding capacitance

2212-8271 © 2018 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 scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.140

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C1. Hence, the pulse current duration is nearly equal to the rise and fall time regardless of the pulse on-time of the pulse voltage. With this method, short current pulse of several tens of ns can easily be obtained, without the use of an expensive ultra-short pulse generator.

determines the rod length. Two gaps are formed in the machining: axial gap and radial gap, as shown in Fig. 3, and the electrolyte is jetted into the axial gap using a nozzle, as shown in Fig. 3. Feeding direction Rotation Nozzle Axial gap

2. Principle of electrostatic induction feeding ECM The electrostatic induction feeding ECM can be modelled as shown in Fig. 2(a). When the pulse voltage is supplied across the working gap, the electric double layer is formed on the surface of the electrodes, which can be expressed as Cdl as shown in the equivalent circuit of Fig. 2(a). Then, the working gap can be modeled as the Faraday impedance Rf, resistance of electrolyte in the machining gap Rg and capacitance of electric double layer Cdl formed on the electrode [7]. A pulse power supply is coupled to the working gap by a feeding capacitance C1. Hence, the current can flow through the working gap only when the pulse voltage changes. For this reason, the pulse duration of the gap current equals the rise and fall times of the pulse voltage as shown in Fig. 2(b). At the rise time of the pulse voltage, current flows through the working gap with a positive polarity. The pulse current is cut off within the pulse duration because of C1. At the fall time, the current flows through the working gap with the opposite polarity. This cycle is repeated. Hence, the current pulse duration is nearly equal to the rise and fall times regardless of the pulse-on time of the pulse voltage. In addition, with the electrostatic induction feeding method, the electric charge q per each pulse is the same for both polarities, and can be expressed using the amplitude of pulse voltage E0 shown in Fig. 2(b), and feeding capacitance C1 as q=C1E0 [6].

Cdl

Rf g

Rf

(a) Equivalent circuit

Gap voltage

Cdl Pulse voltage R

Pulse Gap current voltage

Feeding capacitance C1

Pulse period T

Workpiece: Tungsten Cut depth

Tool electrode: SUS304/Platinum Radial gap

Feed distance

Fig. 3 Machining method of tungsten micro-rods

There is a non-conductive oxide layer formed on the surface of cemented tungsten carbide, which interrupts the electrochemical dissolution. Using the bipolar current and neutral electrolyte of sodium nitrate (NaNO3) aqueous solution, Maeda et al. [8] found that the following electrochemical reaction occurs when the tungsten carbide electrode was in negative polarity. ʹܰܽା ൅ ʹ‫ܪ‬ଶ ܱ ൅ ʹ݁ ՜ ʹܱܰܽ‫ ܪ‬൅ ‫ܪ‬ଶ

(1)

Because of the NaOH, the tungsten oxide (WO3) would be dissolved as ܱଷ ൅ ʹܱܰܽ‫ ܪ‬՜ ܰܽଶ ܹܱସ ൅ ‫ܪ‬ଶ ܱ

(2)

Hence, tungsten carbide is machined using neutral NaNO3 aqueous solution as electrolyte. It is considered that tungsten can be machined due to the same mechanism.

E0 Time 0

Fall time

0

3.2. Influence of current pulse duration Time Rise timeTime

0

(b) Gap current and voltage waveforms

Fig. 2 Principle of electrostatic induction feeding ECM

To demonstrate the influence of the current pulse duration, tungsten micro-rods were machined by a bipolar current which was generated by a bipolar power source (NF Corporation, HSA4101) without using the electrostatic induction feeding method. Table 1. Experimental conditions used for long pulse current.

3. Principle of tungsten machining Pulse voltage

A micro-EDM machine (Panasonic, MG-ED72W) with a positioning resolution of 0.1 μm for x, y and z axis was exploited for experiments. The tool electrode was fixed in a mandrel supported by V-shape ceramic guides and rotated up to 3000 rpm with sufficiently small run-out under 0.5 μm. A silver mica capacitor was used for the feeding capacitance C1. 3.1. Tungsten machining with bipolar current The micro-rod machining method is shown in Fig. 3. The tungsten workpiece is positioned over the plate tool electrode before machining. The cut depth in radial direction determines the rod diameter and the feed distance in axial direction

Amplitude [V]

30

Frequency [Hz]

500k, 1M, 3M

Duty factor [%]

50

Electrolyte

NaNO3 aq. 6wt%

Workpiece rotation [rpm]

3000

Feed speed [μm/s]

0.5

Tool electrode

SUS304 plate

The materials of tungsten and stainless steel (SUS304) were used as workpiece and tool electrode materials, respectively. The initial diameter of workpiece rod was 200 μm. Table 1 shows the experimental conditions. The voltage was 30 V in amplitude and frequencies were 500 kHz, 1 MHz and 3 MHz. The feed speed was 0.5 μm/s. The workpiece was positioned

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50μm

A servo feed control system was developed to control the feeding of workpiece. The equivalent circuit of the machining gap in Fig. 2 (a) can be simplified as a resistance Rg and T is the period of the pulse voltage. When the gap voltage is halfwave rectified, according to the average working gap voltage Vave can be expressed as [6]

50μm

+

50μm

Workpiece

4 Micro-rods machined with different frequencies

72000

+

Reference servo voltage

㼃㻹㼍

80000

Analog switch

V/F circuit

Z-axis controller

Z-axis

Amplifier

(c) 3MHz

(3)

With the electrostatic induction feeding method, the electric charge per each pulse q=C1E0 is constant regardless of the gap width. Since Rg is proportional to the gap width, the above equation indicates that Vave increases with increasing the gap width. Fig. 7 shows the the control system.

Tool

(b) 1MHz

ܴ௚ ் ܴ௚ ‫ݍ‬ න ݅݀‫ ݐ‬ൌ ܶ ܶ ଴

ܸ௔௩௘ ൌ

Amplifier

(a) 500kHz

4.1. Servo feed system for electrostatic induction feeding method

Average circuit

Unknown material

4. Tungsten machining with platinum tool electrode

Half-wave rectifier circuit

over the top surface of tool electrode with an initial gap width of 5 μm. The cut depth in radial direction and feed distance in axial direction were set as 50 μm and 100 μm, respectively. The neutral NaNO3 aqueous solution of 6 wt% in concentration was used as electrolyte. Fig. 4 shows the micro-rods machined with different frequencies of pulse voltage. The tungsten micro-rod was shortened with the low frequency of 500 kHz because the electrochemical dissolution occurred in a large working gap width with a long pulse current duration. The electrochemical dissolution can be localized in a small working gap with short pulse duration [4], hence, the machining accuracy increased with increasing the frequency of pulse voltage. It is considered that an ultra-short pulse current is necessary to improve the machining accuracy, therefore, the electrostatic induction feeding method was used in the following sections to generate an ultra-short pulse current in the range of several tens of ns. In addition, it is noted that there was an unknown material precipitated on the surface of machined micro-rods, as shown in Fig. 4. Fig. 5 shows the spectrum analysis of the material. Since some oxygen element was detected, it is thought that the surface of machined micro-rods were covered by a layer of tungsten oxide.

64000

Fig. 7 Schematic of improved servo feed control system

48000 40000

㼃㻸㼎㻞 㼃㻸㼎

㼃㻸㼍

㼃㻸㼘

㼃㻹㼦

16000

㼃㻹㼞

㻯㻷㼍

32000 24000

Table 2. Experimental conditions used for platinum tool electrode.

㻻㻷㼍

Counts

56000

8000 0 0.00

Pulse voltage 1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

keV

Fig. 5 EDX analysis of the material on the micro-rod surface

Fig. 6 shows the tool electrode after machining with the frequency of 1 MHz. It was worn severely due to the bipolar current, because the electrochemical dissolution occurred on the tool electrode when it was in positive polarity. Frontal surface

Amplitude [V]

90

Frequency [kHz]

500

Duty factor [%]

50

Rise/fall time [ns]

40

Feeding capacitance C1 [pF]

350

Electrolyte

NaNO3 aq. 6wt%

Workpiece rotation [rpm]

3000

Reference voltage [V]

3.9

Tool electrode

Platinum sheet

Side surface

4.2. Machining experiments

50μm

Fig. 6 Tool electrode after machining

50μm

The platinum sheet of 20 μm in thickness was used as tool electrode, because it is well-known that the platinum is difficult to be machined by ECM. Since it was reported that the influence of stray current in the axial gap could be decreased with a thin thickness of tool electrode [9], the thickness of platinum tool electrode was as small as 20 μm. Table 2 shows the experimental conditions. The reference voltage was 3.9 V.

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The cut depths in radial direction were 90 μm and 92.5 μm. The feed distance in axial direction was 100 μm. Fig. 8 shows the feed distances of workpieces. The difference between the material removal rates was slight with the cut depths of 90 μm and 92.5 μm in the radial direction. Fig. 9 shows the machined micro-rods. The micro-rod was shortened when the cut depth in radial direction was 92.5 μm because of the stray current flowing through the end of microrod during machining. The rod diameter of 16 μm with the aspect ratio of 5.9 was machined successfully when the cut depth in radial direction was 90 μm. Fig. 10 shows that the platinum tool electrode was slightly worn after machining because of the bipolar current. However, the tool wear ratio was decreased with the platinum used as the tool material than that with the tool electrode made of stainless steel, as shown in Fig. 6.

Fig. 11 shows the WECG method. This method is the same as the wire electro grinding (WEDG) method developed by Masuzawa et al. [10] for micro-EDM. The commercial copper wire of 100 μm in diameter was used as the tool electrode and kept running during machining. The wire tool is held by a wire guide. Since the wire tool electrode is running during machining, the influence of the tool wear can be eliminated when the bipolar current is used. The rod workpiece is fed toward the wire tool electrode during machining and rotates with a speed of 3000 rpm. The feed distance in axial direction determines the rod length and the cut depth in radial direction determines the rod diameter. The electrolyte is jetted into the working gap by a nozzle of 210 μm in inner diameter near the working gap. The bipolar current is supplied by the electrostatic induction feeding method. Wire electrode

Feed distance [μm]

120

Cut depth in radial direction

Cut depth in radial direction of 90 μm

100

Axial gap

80

Feed distance in radial direction

60 40

0 0

50

100 150 Time [s]

200

250

Fig. 8 Feed distances of workpieces with different cut depths in radial direction (a) 90μm

16μm

Wire guide

Wire guide Wire electrode Radial gap Rod workpiece

Cut depth in radial direction of 92.5 μm

20

Rod electrode

Fig. 11 Wire electrochemical grinding (WECG) using wire electro discharge grinding (WEDG) method developed by Masuzawa et al. [10] Table 3. Experimental conditions used for WECG method.

Pulse voltage

(b) 92.5μm

50μm

50μm

Fig. 9 Micro-rods machined with different cut depths in radial direction

Amplitude [V]

90

Frequency [kHz]

500

Duty factor [%]

50

Rise/fall time [ns]

40

Feeding capacitance C1 [pF]

470

Electrolyte

NaNO3 aq. 6wt%

Workpiece rotation [rpm]

3000

Feed speed [μm/s]

1.0

Tool electrode

Brass Φ100 μm

90V

Pulse voltage

0V

Gap current 626.2mA

20μm

0A 1μs Fig. 10 Platinum tool electrode after machining

0V

55.02V

Gap voltage

5. Wire electrochemical grinding of tungsten rod Tungsten micro-rods were machined with neutral electrolyte and bipolar current, however, the tool was worn when the polarity of the tool electrode was in positive. Even if the platinum was used as the tool electrode material, it was also slightly worn as shown in Fig. 10. Hence, the wire electrochemical grinding machining (WECG) method was proposed in this study, in which the running wire is used as the tool electrode. Therefore, the influence of the tool wear due to the bipolar current can be eliminated.

Fig. 12 Waveforms of gap current and voltage

Table 3 shows the experimental conditions. The feeding capacitances was 470 pF. The cut depth in radial direction was 50 μm, which was kept constant during machining. The feed distance in axial direction was 150 μm which was larger than the wire electrode diameter of 100 μm. The feed speed of workpiece was 1.0 μm/s.

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Fig. 12 shows the waveforms of gap current and voltage, in which the bipolar current was generated by the electrostatic induction feeding method. Since the wire tool electrode was kept running during machining, the tool wear due to bipolar current did not influence the machining. Fig. 13 shows the micro-rod successfully machined by the WECG method. The micro-rod was not straight sufficiently because of the stray current in the radial gap shown in Fig. 11 resulted in material dissolution from the side surface of micro-rod.

50μm

Fig. 13 Micro-rod machined with WECG method

6. Conclusions Tungsten micro-rods were machined successfully using neutral electrolyte and bipolar current in this research. The machining accuracy was increased with decreasing the pulse current duration because the electrochemical dissolution was localized in a smaller working gap. It is noted the tool electrode was worn due to the bipolar current even if platinum was used as tool material. To overcome the problem of the tool wear due to the bipolar current, the novel WECG method was proposed. Micro-rods were machined successfully without the influence of tool wear because the wire electrode was kept running during machining.

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Acknowledgements This work was supported by the Cross-Ministerial Strategic Innovation Promotion Program (SIP): Innovative Design/Manufacturing Technologies, funded by NEDO. References [1] Bhattacharyya B, Munda J. Experimental investigation on the influent of electrochemical machining parameters on machining rate and accuracy in micromachining domain. International Journal of Machine Tools and Manufacture 2003; 43(13): 1301-1310. [2] Sundaram MM, Rajurkar KP. Study on the surface integrity of machined tool in micro EDM. Proceedings of the 16th International Symposium on Electromachining, Shanghai, China, 2010; pp: 685–689. [3] Rajurkar KP, Zhu D, McGeough JA, Kozak J, De Silva A. New Developments in Eletro-Chemical Machining. CIRP AnnalsManufacturing Technology 1999; 48(2): 567–579. [4] Schuster R, Kirchner V, Allongue P, Ertl G. Electrochemical Micromachining. Science 2000; 289(5476): 98–101. [5] Kunieda M, Hayasaka A, Yang XD, Sano S, Araie I. Study on nano EDM using capacity coupled pulse generator. CIRP Annals-Manufacturing Technology 2007; 56(1): 213–216. [6] Koyano T, Kunieda M. Micro electrochemical machining using electrostatic induction feeding method. CIRP Annals-Manufacturing Technology 2013; 62(1): 175-178. [7] Bard AJ, Faulkner LR. Electrochemical Methods: Fundamentals and Applications, second edition. John Wiley & Sons, New York, 2001; pp: 376-377. [8] Maeda S, Saito N, Haishi Y. Principle and characteristics of electrochemical machining. Technical report of Mitsubishi Electric 1967; 41(10): 1267-1279. [9] Han W, Kunieda M. Fabrication of micro-rods with electrostatic induction feeding ECM. Journal of Materials Processing Technology 2016; 235: 92104. [10] Masuzawa T, Fujino M, Kobayashi K, Suzuki T, Kinoshita N. Wire electro-discharge Grinding for micro machining. CIRP AnnalsManufacturing Technology 1985 ; 34(1) : 431-434.