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Effects of Polarization on Machining Accuracy in Pulse Electrochemical Machining
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 493 – 498
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Effects of polarization on machining accuracy in pulse electrochemical machining Fuzhu Hana,b,*, Wei Chena,b, Weicheng Yinga,b Jin Zhanga,b a Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Beijing Key Lab of Precision/Ultra-precision Manufacturing Equipment and Control, Tsinghua University, Beijing 100084, China
b
* Corresponding author. Tel.: +86 10 62796916; fax:+86 10 62796916.E-mail address: [email protected]
Abstract Electrochemical machining (ECM) is an important processing method for conductive material with high hardness and heat-resistance. However, poor machining accuracy has always been a significant problem in ECM. In order to improve the machining accuracy of ECM, the polarization phenomenon in pulse electrochemical machining and influence of polarization on the machining accuracy were experimentally investigated in this paper. A polarization voltage was detected during the pulse-off time, and the experimental result has demonstrated that using a pulsedpower supply cannot improve the machining accuracy of ECM when there was a polarization voltage during the pulse-off time. Therefore, to reduce the influence of the polarization voltage on the accuracy of pulse electrochemical machining, a novel pulsed-power supply method, which accelerates depolarization or partial depolarization, was proposed in this paper. The novel pulsed-power supply method was verified by experiments and the experimental result showed that the machining accuracy was significantly improved. 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:Electrochemical machining;Pulsed-power supply;Polarization; Machining accuracy.
1. Introduction Electrochemical machining (ECM) is a material processing technology in which the workpiece is molded by electrochemical anodic dissolution in electrolyte. Because the material is removed from the workpiece in an ionic form, ECM has great potential use in the aerospace and microfabrication fields and has advantages such as no processing stress, no remelted surface layer and a lossless tool electrode [1]. However, because the electric field distribution between the positive and negative electrodes is difficult to control, some of the nonworking surface also corrodes, which results in starry corrosion of the workpiece and taper error in the punching process of ECM. Thus, poor machining accuracy has become a major technical bottleneck for ECM. Currently, the main method for improving the accuracy is using a highfrequency pulsed-power supply instead of direct-current (DC) power during ECM[2]. A great number of scholars have studied the influence of pulsed power on machining accuracy. Shin et al. [3] analyzed the influence of a high-frequency
pulse on the side gap between the inner surface of a hole and the tool electrode sidewall in electrochemical micro-line cutting. A small side machining gap was obtained by adjusting the applied voltage, pulse duration, and pulse period. It was found that the side gap increased with the voltage and pulseon time and decreased with the pulse period. Liu and Zeng used high-speed-rotation microelectrodes to process deep eyelets and discovered that the side gap increased with the pulse frequency [4]. Some researchers processed the microstructure [5, 6] of stainless steel using ultrashort-pulse ECM and explored the relationship between the power parameters and the accuracy of ECM. The result revealed that the slit width decreased with the pulse [7, 8]. Das and Saha conducted a series of microhole machining experiments using pulsed-power ECM to explore the effects of the pulsed-power supply parameters on the microporous taper [9]. The most well-known explanation regarding the relationship between the pulsed power and the machining accuracy of ECM was given out by Schuster et al., who proposed a physical model based on RC charging and discharging. The model was
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.2017.12.080
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Fuzhu Han et al. / Procedia CIRP 68 (2018) 493 – 498
composed of an electrical double layer and electrolyte resistor [10]. In the model, the workpiece, the end face of electrode and the electrolyte between them were equivalent to the RC circuit, and so were the workpiece, the side face of electrode, and the electrolyte between them. Because the resistance between the bottom of the electrode and the workpiece was smaller than that between the electrode-side and the workpiece, the charging time for these two directions were not the same. Thus, the anodic dissolution time between the bottom surface of the electrode and the workpiece was longer than that between the side of the electrode and the workpiece during the pulse duration of each pulse period, which resulted in a larger material removal amount at the bottom than that at the side. Hence, the secondary processing of the machined surface on the electrode side was reduced and the accuracy of electrolytic machining was improved. However, in terms of experimental results in this paper, when the pulse-off time was within about a microsecond, the polarization voltage between the workpiece and the tool can not be eliminated, and the charge on the electrode was not released. Thus, during the next pulseon time, it more specific cannot be recharged, and this meant that the different charging time constants on the different surface of the electrode can not be used to improve the ECM accuracy. Therefore in this paper, the polarization phenomenon was deeply analyzed and the influence of polarization on the machining accuracy was experimentally investigated with a pulsed-power supply. It was found that when there was a polarization voltage in the pulse-off time, the machining accuracy cannot be improved by using the pulsed-power supply. Based on the experimental result, a novel depolarization circuit was proposed in this paper. The machining accuracy was improved by depolarizing the voltage during the pulse-off time with the depolarization circuit.
to as þthe polarization of the electrodeÿ, and the deviation of the electrode potential is referred to as þthe polarization potentialÿ. 2.2. Variation of polarization voltage after removing the DC power supply A metal electrode was immersed in the electrolyte solution and connected to the DC power supply, as shown in Figure 1, to explore the formation and elimination mechanism of the polarization voltage in ECM and to assess the influence factors on the polarization voltage and depolarization time. A button in the circuit was used to control whether the voltage was applied to the anode and cathode. The voltage variation between the electrodes was observed using an oscilloscope (TDS2000). The effects of the type and concentration of electrolyte (detailed parameters as shown in Table 1) on the depolarization time were studied. The DC power supply voltage was 8V. The anode was a copper plate, and the cathode was a platinum wire. The electrolytes used in the experiments were copper sulfate, sodium chloride, and sodium nitrate, respectively. For each electrolyte, concentrations of 0.1 mol/L, 0.3 mol/L, and 0.5 mol/L were used. Oscilloscope
Cathode
S1
Electrolyte
DC power supply
Anode
2. Polarization of metal in the electrolyte solution 2.1. Polarization process According to the theory of electrical double layer, an electric double layer forms at the metal/solution interface when any metal is inserted into an aqueous solution that has its ions dissolved. Consequently, a constant potential difference exists at the metal/solution interface, which is called the metal electrode potential. If the electrode reaction is reversible, no current will flow through the metal/solution interface, and no material will dissolve or precipitate when the rate of the oxidation reaction is equal to that of the reduction reaction. As a result, a stable electric double layer is established, and the electrode potential at this moment is called the equilibrium electrode potential. In ECM, the electrode reaction does not occur under reversible equilibrium conditions with no current flowing through the metal/solution interface. Instead, it occurs under the condition of strong current flowing through the metal/solution interface with an external electric field. Under this condition, the electrode potential starts deviating from the equilibrium electrode potential, and the deviation becomes larger as the current flowing through the metal/solution interface increases. The phenomenon of the electrode potential deviating from the equilibrium potential is referred
Fig. 1. Schematic diagram of polarization experiment device. Table 1.Experimental parameters of polarization. Materials
Electrolyte/Concentration
anode Cu, cathode Pt
CuSO4 (a)0.1mol /L (b)0.3mol /L (c)0.5mol /L
anode Cu, cathode Pt
NaCl
anode Cu, cathode Pt
NaNO3 (a) 0.1mol /L (b)0.3mol /L (c)0.5mol /L
(a) 0.1mol /L (b)0.3mol /L (c)0.5mol /L
Polarization voltage
0V
Depolarization time 2V
0V
0V 2V
500ms
(a)
2V
5s
(b)
10s
(c)
Fig. 2. Diagram of voltage waveform between electrodes after removing the DC power supply (a) 0.1mol /L CuSO4; (b) 0.1mol /L NaNO3; (c) 0.1mol /L NaCl.
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DC power supply and a metal-oxide-semiconductor fieldeffect transistor (MOSFET), as shown in Figure 4. A brass electrode was inserted into a solution of sodium chloride (NaCl: 0.5 mol/L) to process the copper plate. The voltage waveform collected is shown in Figure 5. The voltage waveforms obtained with the pulse frequency of 40 kHz, 200 Hz are shown in Figure 5(a) and Figure 5(b).
MOSFET
Tool electrode
Fig. 3. Variation of depolarization time with electrolyte type and electrolyte concentration.
DC power supply
Electrolyte Workpiece
Figure 2 shows the voltage waveform obtained from the experiments. Instead of dropping to zero immediately, the voltage between the workpiece and the electrode rapidly decreased to a certain value which was the polarization voltage value after the DC power supply was removed. Thereafter, the voltage gradually fell to zero. This process was called the depolarization process. Because depolarization mainly depended on free diffusion, the polarization voltage cannot immediately fall to zero when the polarization condition disappeared after the DC power supply was removed. Instead, the polarization voltage gradually fell to zero through the electrode depolarization reaction process, and ultimately, the voltage returned back to the equilibrium electrode potential. The time of the diffusion depolarization process was different indifferent electrolytes. For example, the electrode groups composed of platinum and copper depolarized more easily in the copper sulfate solution. The polarization voltage can fall to zero very quickly. However, when the same electrode groups were inserted into sodium chloride or sodium nitrate, the elimination of polarization voltage was very slow after the DC power supply was removed. It can be found from Figure 3 that the speed of the electrode reaction accelerated and the depolarization time decreased with an increase in electrolyte concentration. 3. Polarization and depolarization process in pulse electrochemical machining Using a pulsed-power supply in ECM is equivalent to repeatedly applying and removing the power supply between the tool electrode and the workpiece; specifically, it introduces a continuous cycle of polarization and depolarization. The duration of the pulse-on time and pulseoff time affect the polarization and depolarization process. Thus, the pulse-on time and pulse-off time also affect the electrode dissolving reaction in ECM. It is important to study polarization and depolarization in the ECM process. The experiments for observing the polarization and depolarization process was conducted using a traditional pulsed-power supply circuit which was mainly made up of a
Fig.4. Schematic diagram of a traditional pulsed-power supply.
0V No change in the polarization voltage 0V Decrease in the polarization voltage 2V
2V
5μs
(a)
1ms
(b)
Fig. 5. Voltage waveforms between poles in the pulse electrochemical machining process (a) 40 kHz; (b) 200 Hz.
As seen in Figure 5, the polarization voltage between the two electrodes during the pulse-off time was not zero. Since power supply was cut off during the pulse-off time, there was no applied voltage between the tool electrode and the workpiece and the electrode should begin to depolarize. However, because depolarization mainly depended on free diffusion, the decrease of polarization voltage would be small if the pulse-off time was not long enough, which caused the voltage between the tool electrode and the workpiece remained nearly unchanged (Figure 5(a)). The decrease in polarization voltage was obvious only when the pulse-off time was long enough. However, the speed of depolarization was still very slow, and the polarization voltage was not reduced much (Figure 5(b)). Since the model (proposed by Rolf Schuster) using a pulsed-power supply to improve ECM accuracy can only be available when the polarization voltage is eliminated. Therefore, it is unreasonable to use this model to explain the relationship between pulse frequency and electrochemical machining accuracy without eliminating polarization voltage. In practice, it is impossible to set the pulse-off time long enough to completely eliminate the polarization during ECM, that will significantly reduce the ECM efficiency. Therefore, how to eliminate the polarization voltage during pulse-off
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time is a key to study the effect of polarization voltage on the ECM accuracy. 4. A method to eliminate the polarization voltage According to the analysis of polarization and depolarization, there are two methods to accelerate depolarization process and improve the machining accuracy in pulse ECM. One way is to short-circuit the anode and the cathode with a wire after the power supply is removed, so that the electrons on the cathode surface can quickly neutralize the ions on the anode surface to accelerate depolarization. The second way is to use a polarity reverse DC power supply after the power supply is removed. By this mean, the electrode potential of the anode and cathode will change rapidly which results in an acceleration of depolarization. The first method was adopted and a novel pulsed-power circuit (Figure 6) which consisted of four MOSFET was proposed. When Q1 and Q4 were conducted, the DC power was applied to the tool electrode and the workpiece. During the pulse-off time, the conduction of Q2 and Q3 can lead to a short circuit between the workpiece and tool electrode, and thus the polarization voltage can be promptly eliminated in the pulse-off time (Figure 7). Figures 8(a) and 8(b) are the voltage waveforms obtained using the traditional pulsed-power supply and the newly designed pulsed-power supply, respectively. It can be seen from the Figure 8(a) that there was a stable polarization voltage during the pulse-off time when the traditional pulsedpower supply was used, but the polarization voltage is immediately completely eliminated when the novel pulsedpower supply was used.
MOSFET:Q3
MOSFET:Q4
MOSFET:Q1
MOSFET:Q2
0V
0V Polarization voltage
Elimination of the polarization voltage 5V 50μs
5V 50μs
(a)
(b)
Fig. 8.The voltage waveform between the two poles (a) waveform obtained from traditional pulsed-power supply; (b) waveform obtained from new pulsed-power supply.
5. Effect of polarization voltage on machining accuracy For comparison with the traditional pulsed-power supply with a polarization voltage during the pulse-off time (Figure 4), the ECM experiments were carried out using the new pulsed-power supply without the polarization voltage during the pulse-off time (Figure 6). The experimental conditions are given in Table 2. The workpiece was a copper plate whose thickness was 0.1mm and the tool electrode was a copper tube with a diameter of 0.7mm. The electrolyte was a sodium chloride solution with a concentration of 0.5mol/L. A constant feed speed was adopted in the process, and the machining depth was 0.15mm. The peak value of the new pulsed-power supply was 14V, and the duty ratio was 12%. Table 2. Experimental parameters of pulse ECM. Tool/Workpiece
Electrolyte/Concentration
Cu/Cu
NaCl 0.5mol /L
Cu/Cu
NaCl 0.5mol /L
Pulsed-power Traditional pulsed-power New pulsed-power
Tool electrode
Electrolyte
Workpiece
P
P
DC power supply (a)
Fig. 6.New pulsed-power supply for depolarization. U
Fig. 9. Results of processing experiment (a) result obtained from traditional pulsed-power supply; (b) result obtained from new pulsed-power supply.
U
0
t
(b)
0
t Theoretical output waveform
Q1,Q4
Tool electrode
U
U
0
0
t
Polarization voltage
Re t
Actual waveform Q2,Q3
Workpiece
Fig. 7 Drive signal of Mosfet and power output waveform of new pulsedpower supply.
CDL
CDL
Fig. 10 Scheme of the electric double layer.
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The machined results obtained using these two power supplies are shown in Figure 9. Figure 9(a) shows the hole obtained using the traditional pulsed-power supply in which the polarization voltage was not eliminated, and Figure 9(b) shows the hole obtained using the newly designed pulsedpower supply in which the polarization voltage was eliminated through a short circuit. The side gap of the hole obtained when the polarization voltage was eliminated was significantly smaller than that obtained when the polarization voltage was not eliminated. Additionally, the roundness of the hole obtained after eliminating the polarization voltage was better than that obtained when the polarization voltage was not eliminated. The discharging of the electric double layer capacitor and the elimination of the polarization voltage were the main reasons for the improving machining accuracy using the new pulsed-power supply. Figure 10 shows the model of the electric double layer capacitor and the gap resistance (Re). The gap resistance increased with the machining gap, and this resulted in an increase in the charge time constant (Ů =2ReCDL) and a decrease in the effective pulse-on time for machining (tef=Ton –Ů). When the charge time constant exceeded the full length of the pulse-on time, electrolytic corrosion did not occur, and the machining gap no longer increased, this gap was called the maximum corrosion gap. Therefore, the machining gap can be limited within the maximum corrosion gap when the polarization voltage in the pulse-off time was eliminated, and this resulted in a higher accuracy. If the polarization voltage during pulse-off time was not eliminated, the electrolytic corrosion began directly without a charging process when the next pulse-on time began, and the machining gap got larger and larger throughout the process. Therefore, eliminating the polarization voltage using a short circuit during pulse-off time can reduce the stray corrosion in ECM. Moreover, the model (proposed by Rolf Schuster) using a pulsed-power supply to improve ECM accuracy can only be applicable when the polarization voltage during the pulse-off time was eliminated through the short circuit and a cycle for the double layer capacitor charging and discharging was formed. In other words, the pulsed-power supply did not improve the machining accuracy when a polarization voltage existed in the pulse-off time . To further verify the effects of the polarization voltage on the accuracy of pulse ECM, experiments were carried out using the new pulsed-power supply with different frequencies and compared to the experiments using the traditional pulsedpower supply. Except for the frequency, the other experimental parameters were kept the same as the former two experiments. The processing results are shown in Figure 11. It can be found from the experimental results that the side gap of the hole obtained using the new pulsed-power supply was smaller than that obtained using the traditional pulsedpower supply. In addition, with an increase in the frequency, the side gap of the hole obtained using the new pulsed-power supply became smaller, while that obtained using the traditional pulsed-power supply did not change. Because there was a polarization voltage during the pulse-off time using the traditional pulsed-power supply, the double layer capacitor
cannot be charged repeatedly, and thus, the pulsed power has no effect on the machining accuracy. The side gap of the holes did not change with the pulse frequencies. However, because the polarization voltage was eliminated through the short circuit in the new pulsed-power supply, the double layer capacitor can be charged repeatedly. Thus, the higher the frequency was, the smaller the pulse-on time and maximum corrosion gap were, and higher accuracy can be obtained.
The side gap of hole d/mm 0.6
0.501
0.497
0.4
0.333
0.490
0.266 0.153
0.2 0 10kHz
40kHz
100kHz
Traditional pulse power supply Fig. 11. Side gap of the holes obtained with different frequencies.
6. Conclusions In this paper, the generation mechanism of polarization voltage during pulse-off time and the influence of polarization on ECM accuracy were studied. A new pulsed-power supply method was proposed. Improving machining accuracy was achieved with the new pulsed-power supply method. The specific conclusions were as follows: (1) For different electrolytes, the elimination time of the polarization voltage was different after removing the external voltage, and it decreased when the electrolyte concentration increased. (2) The physical model of the RC charge and discharge that was composed of the double layer and the electrolyte resistance (proposed by Rolf Schuster) can explain the relationship between the pulsed-power supply and ECM accuracy only when the polarization voltage during pulse-off time was eliminated. (3) In pulse ECM, the polarization voltage decreased slightly but was not completely eliminated if the pulse-off time was not long enough. When there was a polarization voltage, the machining accuracy of pulse ECM cannot be improved. (4) Both dimension accuracy and shape precision of pulse ECM are significantly improved when the polarization voltage during pulse-off time was eliminated using the new pulsed-power supply method. References [1]Fang X, Qu N, Li H, Zhu D. Enhancement of insulation coating durability in electrochemical drilling. Int J Adv Manuf Technol,2013,68(9– 12):2005–2013. [2]K P Rajurkar, D Zhu, J A McGeough, J Kozak, A De Silva. New Developments in Electro-Chemical Machining. Annals of the ClRP, 1999,48(2):569-570. [3]Hong Shik Shin, Bo Hyun Kim, Chong Nam Chu. Analysis of the side gap resulting from micro electrochemical machining with a tungsten wire and
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ultrashort voltage pulses. Journal of Micromechanics and Microengineering, 2008, 18:1-6. [4]Liu Yong, Zeng Yongbin. Electrochemical drilling of deep and small holes with high speed micro electrode. Optics and precision engineering. 2014, 22(3):608-614. [5]Viola Kirchner, Laurent Cagnon, Rolf Schuster, Gerhard Ertl. Electrochemical machining of stainless steel microelements with ultrashort voltage pulses. Applied Physics Letters, 2001, 79(11):17211723. [6]A L Trimmer, J L Hudson, M Kock, R Schuster. Single-step electrochemical machining of complex nanostructures with ultrashort voltage pulses. Applied Physics Letters, 2003, 82(19):3327-3329.
[7] M Kock, V Kirchner, R Schuster, Electrochemical micromachining with ultrashort voltage pulses/a versatile method with lithographical precision. Electrochimica Acta, 2003,48 :3213-3219. [8]Viola Kirchner, Xinghua Xia, Rolf Schuster, Electrochemical Nanostructuring with ultrashort voltage pulses. Accounts Of Chemical Research, 2001, 34:371-377. [9]Alok Kumar, Das Partha Saha. Experimental investigation on microelectrochemical sinking operation for fabrication of micro-holes. The Brazilian Society of Mechanical Sciences and Engineering, 2015, 37:657– 663. [10]Rolf Schuster, Viola Kirchner, Philippe Allongue. Gerhard Ertl. Electrochemical micromachining, Science, 2000, 289(5476):98-101.
ScienceDirect Procedia CIRP 68 (2018) 493 – 498
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Effects of polarization on machining accuracy in pulse electrochemical machining Fuzhu Hana,b,*, Wei Chena,b, Weicheng Yinga,b Jin Zhanga,b a Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China Beijing Key Lab of Precision/Ultra-precision Manufacturing Equipment and Control, Tsinghua University, Beijing 100084, China
b
* Corresponding author. Tel.: +86 10 62796916; fax:+86 10 62796916.E-mail address: [email protected]
Abstract Electrochemical machining (ECM) is an important processing method for conductive material with high hardness and heat-resistance. However, poor machining accuracy has always been a significant problem in ECM. In order to improve the machining accuracy of ECM, the polarization phenomenon in pulse electrochemical machining and influence of polarization on the machining accuracy were experimentally investigated in this paper. A polarization voltage was detected during the pulse-off time, and the experimental result has demonstrated that using a pulsedpower supply cannot improve the machining accuracy of ECM when there was a polarization voltage during the pulse-off time. Therefore, to reduce the influence of the polarization voltage on the accuracy of pulse electrochemical machining, a novel pulsed-power supply method, which accelerates depolarization or partial depolarization, was proposed in this paper. The novel pulsed-power supply method was verified by experiments and the experimental result showed that the machining accuracy was significantly improved. 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:Electrochemical machining;Pulsed-power supply;Polarization; Machining accuracy.
1. Introduction Electrochemical machining (ECM) is a material processing technology in which the workpiece is molded by electrochemical anodic dissolution in electrolyte. Because the material is removed from the workpiece in an ionic form, ECM has great potential use in the aerospace and microfabrication fields and has advantages such as no processing stress, no remelted surface layer and a lossless tool electrode [1]. However, because the electric field distribution between the positive and negative electrodes is difficult to control, some of the nonworking surface also corrodes, which results in starry corrosion of the workpiece and taper error in the punching process of ECM. Thus, poor machining accuracy has become a major technical bottleneck for ECM. Currently, the main method for improving the accuracy is using a highfrequency pulsed-power supply instead of direct-current (DC) power during ECM[2]. A great number of scholars have studied the influence of pulsed power on machining accuracy. Shin et al. [3] analyzed the influence of a high-frequency
pulse on the side gap between the inner surface of a hole and the tool electrode sidewall in electrochemical micro-line cutting. A small side machining gap was obtained by adjusting the applied voltage, pulse duration, and pulse period. It was found that the side gap increased with the voltage and pulseon time and decreased with the pulse period. Liu and Zeng used high-speed-rotation microelectrodes to process deep eyelets and discovered that the side gap increased with the pulse frequency [4]. Some researchers processed the microstructure [5, 6] of stainless steel using ultrashort-pulse ECM and explored the relationship between the power parameters and the accuracy of ECM. The result revealed that the slit width decreased with the pulse [7, 8]. Das and Saha conducted a series of microhole machining experiments using pulsed-power ECM to explore the effects of the pulsed-power supply parameters on the microporous taper [9]. The most well-known explanation regarding the relationship between the pulsed power and the machining accuracy of ECM was given out by Schuster et al., who proposed a physical model based on RC charging and discharging. The model was
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.2017.12.080
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Fuzhu Han et al. / Procedia CIRP 68 (2018) 493 – 498
composed of an electrical double layer and electrolyte resistor [10]. In the model, the workpiece, the end face of electrode and the electrolyte between them were equivalent to the RC circuit, and so were the workpiece, the side face of electrode, and the electrolyte between them. Because the resistance between the bottom of the electrode and the workpiece was smaller than that between the electrode-side and the workpiece, the charging time for these two directions were not the same. Thus, the anodic dissolution time between the bottom surface of the electrode and the workpiece was longer than that between the side of the electrode and the workpiece during the pulse duration of each pulse period, which resulted in a larger material removal amount at the bottom than that at the side. Hence, the secondary processing of the machined surface on the electrode side was reduced and the accuracy of electrolytic machining was improved. However, in terms of experimental results in this paper, when the pulse-off time was within about a microsecond, the polarization voltage between the workpiece and the tool can not be eliminated, and the charge on the electrode was not released. Thus, during the next pulseon time, it more specific cannot be recharged, and this meant that the different charging time constants on the different surface of the electrode can not be used to improve the ECM accuracy. Therefore in this paper, the polarization phenomenon was deeply analyzed and the influence of polarization on the machining accuracy was experimentally investigated with a pulsed-power supply. It was found that when there was a polarization voltage in the pulse-off time, the machining accuracy cannot be improved by using the pulsed-power supply. Based on the experimental result, a novel depolarization circuit was proposed in this paper. The machining accuracy was improved by depolarizing the voltage during the pulse-off time with the depolarization circuit.
to as þthe polarization of the electrodeÿ, and the deviation of the electrode potential is referred to as þthe polarization potentialÿ. 2.2. Variation of polarization voltage after removing the DC power supply A metal electrode was immersed in the electrolyte solution and connected to the DC power supply, as shown in Figure 1, to explore the formation and elimination mechanism of the polarization voltage in ECM and to assess the influence factors on the polarization voltage and depolarization time. A button in the circuit was used to control whether the voltage was applied to the anode and cathode. The voltage variation between the electrodes was observed using an oscilloscope (TDS2000). The effects of the type and concentration of electrolyte (detailed parameters as shown in Table 1) on the depolarization time were studied. The DC power supply voltage was 8V. The anode was a copper plate, and the cathode was a platinum wire. The electrolytes used in the experiments were copper sulfate, sodium chloride, and sodium nitrate, respectively. For each electrolyte, concentrations of 0.1 mol/L, 0.3 mol/L, and 0.5 mol/L were used. Oscilloscope
Cathode
S1
Electrolyte
DC power supply
Anode
2. Polarization of metal in the electrolyte solution 2.1. Polarization process According to the theory of electrical double layer, an electric double layer forms at the metal/solution interface when any metal is inserted into an aqueous solution that has its ions dissolved. Consequently, a constant potential difference exists at the metal/solution interface, which is called the metal electrode potential. If the electrode reaction is reversible, no current will flow through the metal/solution interface, and no material will dissolve or precipitate when the rate of the oxidation reaction is equal to that of the reduction reaction. As a result, a stable electric double layer is established, and the electrode potential at this moment is called the equilibrium electrode potential. In ECM, the electrode reaction does not occur under reversible equilibrium conditions with no current flowing through the metal/solution interface. Instead, it occurs under the condition of strong current flowing through the metal/solution interface with an external electric field. Under this condition, the electrode potential starts deviating from the equilibrium electrode potential, and the deviation becomes larger as the current flowing through the metal/solution interface increases. The phenomenon of the electrode potential deviating from the equilibrium potential is referred
Fig. 1. Schematic diagram of polarization experiment device. Table 1.Experimental parameters of polarization. Materials
Electrolyte/Concentration
anode Cu, cathode Pt
CuSO4 (a)0.1mol /L (b)0.3mol /L (c)0.5mol /L
anode Cu, cathode Pt
NaCl
anode Cu, cathode Pt
NaNO3 (a) 0.1mol /L (b)0.3mol /L (c)0.5mol /L
(a) 0.1mol /L (b)0.3mol /L (c)0.5mol /L
Polarization voltage
0V
Depolarization time 2V
0V
0V 2V
500ms
(a)
2V
5s
(b)
10s
(c)
Fig. 2. Diagram of voltage waveform between electrodes after removing the DC power supply (a) 0.1mol /L CuSO4; (b) 0.1mol /L NaNO3; (c) 0.1mol /L NaCl.
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DC power supply and a metal-oxide-semiconductor fieldeffect transistor (MOSFET), as shown in Figure 4. A brass electrode was inserted into a solution of sodium chloride (NaCl: 0.5 mol/L) to process the copper plate. The voltage waveform collected is shown in Figure 5. The voltage waveforms obtained with the pulse frequency of 40 kHz, 200 Hz are shown in Figure 5(a) and Figure 5(b).
MOSFET
Tool electrode
Fig. 3. Variation of depolarization time with electrolyte type and electrolyte concentration.
DC power supply
Electrolyte Workpiece
Figure 2 shows the voltage waveform obtained from the experiments. Instead of dropping to zero immediately, the voltage between the workpiece and the electrode rapidly decreased to a certain value which was the polarization voltage value after the DC power supply was removed. Thereafter, the voltage gradually fell to zero. This process was called the depolarization process. Because depolarization mainly depended on free diffusion, the polarization voltage cannot immediately fall to zero when the polarization condition disappeared after the DC power supply was removed. Instead, the polarization voltage gradually fell to zero through the electrode depolarization reaction process, and ultimately, the voltage returned back to the equilibrium electrode potential. The time of the diffusion depolarization process was different indifferent electrolytes. For example, the electrode groups composed of platinum and copper depolarized more easily in the copper sulfate solution. The polarization voltage can fall to zero very quickly. However, when the same electrode groups were inserted into sodium chloride or sodium nitrate, the elimination of polarization voltage was very slow after the DC power supply was removed. It can be found from Figure 3 that the speed of the electrode reaction accelerated and the depolarization time decreased with an increase in electrolyte concentration. 3. Polarization and depolarization process in pulse electrochemical machining Using a pulsed-power supply in ECM is equivalent to repeatedly applying and removing the power supply between the tool electrode and the workpiece; specifically, it introduces a continuous cycle of polarization and depolarization. The duration of the pulse-on time and pulseoff time affect the polarization and depolarization process. Thus, the pulse-on time and pulse-off time also affect the electrode dissolving reaction in ECM. It is important to study polarization and depolarization in the ECM process. The experiments for observing the polarization and depolarization process was conducted using a traditional pulsed-power supply circuit which was mainly made up of a
Fig.4. Schematic diagram of a traditional pulsed-power supply.
0V No change in the polarization voltage 0V Decrease in the polarization voltage 2V
2V
5μs
(a)
1ms
(b)
Fig. 5. Voltage waveforms between poles in the pulse electrochemical machining process (a) 40 kHz; (b) 200 Hz.
As seen in Figure 5, the polarization voltage between the two electrodes during the pulse-off time was not zero. Since power supply was cut off during the pulse-off time, there was no applied voltage between the tool electrode and the workpiece and the electrode should begin to depolarize. However, because depolarization mainly depended on free diffusion, the decrease of polarization voltage would be small if the pulse-off time was not long enough, which caused the voltage between the tool electrode and the workpiece remained nearly unchanged (Figure 5(a)). The decrease in polarization voltage was obvious only when the pulse-off time was long enough. However, the speed of depolarization was still very slow, and the polarization voltage was not reduced much (Figure 5(b)). Since the model (proposed by Rolf Schuster) using a pulsed-power supply to improve ECM accuracy can only be available when the polarization voltage is eliminated. Therefore, it is unreasonable to use this model to explain the relationship between pulse frequency and electrochemical machining accuracy without eliminating polarization voltage. In practice, it is impossible to set the pulse-off time long enough to completely eliminate the polarization during ECM, that will significantly reduce the ECM efficiency. Therefore, how to eliminate the polarization voltage during pulse-off
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time is a key to study the effect of polarization voltage on the ECM accuracy. 4. A method to eliminate the polarization voltage According to the analysis of polarization and depolarization, there are two methods to accelerate depolarization process and improve the machining accuracy in pulse ECM. One way is to short-circuit the anode and the cathode with a wire after the power supply is removed, so that the electrons on the cathode surface can quickly neutralize the ions on the anode surface to accelerate depolarization. The second way is to use a polarity reverse DC power supply after the power supply is removed. By this mean, the electrode potential of the anode and cathode will change rapidly which results in an acceleration of depolarization. The first method was adopted and a novel pulsed-power circuit (Figure 6) which consisted of four MOSFET was proposed. When Q1 and Q4 were conducted, the DC power was applied to the tool electrode and the workpiece. During the pulse-off time, the conduction of Q2 and Q3 can lead to a short circuit between the workpiece and tool electrode, and thus the polarization voltage can be promptly eliminated in the pulse-off time (Figure 7). Figures 8(a) and 8(b) are the voltage waveforms obtained using the traditional pulsed-power supply and the newly designed pulsed-power supply, respectively. It can be seen from the Figure 8(a) that there was a stable polarization voltage during the pulse-off time when the traditional pulsedpower supply was used, but the polarization voltage is immediately completely eliminated when the novel pulsedpower supply was used.
MOSFET:Q3
MOSFET:Q4
MOSFET:Q1
MOSFET:Q2
0V
0V Polarization voltage
Elimination of the polarization voltage 5V 50μs
5V 50μs
(a)
(b)
Fig. 8.The voltage waveform between the two poles (a) waveform obtained from traditional pulsed-power supply; (b) waveform obtained from new pulsed-power supply.
5. Effect of polarization voltage on machining accuracy For comparison with the traditional pulsed-power supply with a polarization voltage during the pulse-off time (Figure 4), the ECM experiments were carried out using the new pulsed-power supply without the polarization voltage during the pulse-off time (Figure 6). The experimental conditions are given in Table 2. The workpiece was a copper plate whose thickness was 0.1mm and the tool electrode was a copper tube with a diameter of 0.7mm. The electrolyte was a sodium chloride solution with a concentration of 0.5mol/L. A constant feed speed was adopted in the process, and the machining depth was 0.15mm. The peak value of the new pulsed-power supply was 14V, and the duty ratio was 12%. Table 2. Experimental parameters of pulse ECM. Tool/Workpiece
Electrolyte/Concentration
Cu/Cu
NaCl 0.5mol /L
Cu/Cu
NaCl 0.5mol /L
Pulsed-power Traditional pulsed-power New pulsed-power
Tool electrode
Electrolyte
Workpiece
P
P
DC power supply (a)
Fig. 6.New pulsed-power supply for depolarization. U
Fig. 9. Results of processing experiment (a) result obtained from traditional pulsed-power supply; (b) result obtained from new pulsed-power supply.
U
0
t
(b)
0
t Theoretical output waveform
Q1,Q4
Tool electrode
U
U
0
0
t
Polarization voltage
Re t
Actual waveform Q2,Q3
Workpiece
Fig. 7 Drive signal of Mosfet and power output waveform of new pulsedpower supply.
CDL
CDL
Fig. 10 Scheme of the electric double layer.
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The machined results obtained using these two power supplies are shown in Figure 9. Figure 9(a) shows the hole obtained using the traditional pulsed-power supply in which the polarization voltage was not eliminated, and Figure 9(b) shows the hole obtained using the newly designed pulsedpower supply in which the polarization voltage was eliminated through a short circuit. The side gap of the hole obtained when the polarization voltage was eliminated was significantly smaller than that obtained when the polarization voltage was not eliminated. Additionally, the roundness of the hole obtained after eliminating the polarization voltage was better than that obtained when the polarization voltage was not eliminated. The discharging of the electric double layer capacitor and the elimination of the polarization voltage were the main reasons for the improving machining accuracy using the new pulsed-power supply. Figure 10 shows the model of the electric double layer capacitor and the gap resistance (Re). The gap resistance increased with the machining gap, and this resulted in an increase in the charge time constant (Ů =2ReCDL) and a decrease in the effective pulse-on time for machining (tef=Ton –Ů). When the charge time constant exceeded the full length of the pulse-on time, electrolytic corrosion did not occur, and the machining gap no longer increased, this gap was called the maximum corrosion gap. Therefore, the machining gap can be limited within the maximum corrosion gap when the polarization voltage in the pulse-off time was eliminated, and this resulted in a higher accuracy. If the polarization voltage during pulse-off time was not eliminated, the electrolytic corrosion began directly without a charging process when the next pulse-on time began, and the machining gap got larger and larger throughout the process. Therefore, eliminating the polarization voltage using a short circuit during pulse-off time can reduce the stray corrosion in ECM. Moreover, the model (proposed by Rolf Schuster) using a pulsed-power supply to improve ECM accuracy can only be applicable when the polarization voltage during the pulse-off time was eliminated through the short circuit and a cycle for the double layer capacitor charging and discharging was formed. In other words, the pulsed-power supply did not improve the machining accuracy when a polarization voltage existed in the pulse-off time . To further verify the effects of the polarization voltage on the accuracy of pulse ECM, experiments were carried out using the new pulsed-power supply with different frequencies and compared to the experiments using the traditional pulsedpower supply. Except for the frequency, the other experimental parameters were kept the same as the former two experiments. The processing results are shown in Figure 11. It can be found from the experimental results that the side gap of the hole obtained using the new pulsed-power supply was smaller than that obtained using the traditional pulsedpower supply. In addition, with an increase in the frequency, the side gap of the hole obtained using the new pulsed-power supply became smaller, while that obtained using the traditional pulsed-power supply did not change. Because there was a polarization voltage during the pulse-off time using the traditional pulsed-power supply, the double layer capacitor
cannot be charged repeatedly, and thus, the pulsed power has no effect on the machining accuracy. The side gap of the holes did not change with the pulse frequencies. However, because the polarization voltage was eliminated through the short circuit in the new pulsed-power supply, the double layer capacitor can be charged repeatedly. Thus, the higher the frequency was, the smaller the pulse-on time and maximum corrosion gap were, and higher accuracy can be obtained.
The side gap of hole d/mm 0.6
0.501
0.497
0.4
0.333
0.490
0.266 0.153
0.2 0 10kHz
40kHz
100kHz
Traditional pulse power supply Fig. 11. Side gap of the holes obtained with different frequencies.
6. Conclusions In this paper, the generation mechanism of polarization voltage during pulse-off time and the influence of polarization on ECM accuracy were studied. A new pulsed-power supply method was proposed. Improving machining accuracy was achieved with the new pulsed-power supply method. The specific conclusions were as follows: (1) For different electrolytes, the elimination time of the polarization voltage was different after removing the external voltage, and it decreased when the electrolyte concentration increased. (2) The physical model of the RC charge and discharge that was composed of the double layer and the electrolyte resistance (proposed by Rolf Schuster) can explain the relationship between the pulsed-power supply and ECM accuracy only when the polarization voltage during pulse-off time was eliminated. (3) In pulse ECM, the polarization voltage decreased slightly but was not completely eliminated if the pulse-off time was not long enough. When there was a polarization voltage, the machining accuracy of pulse ECM cannot be improved. (4) Both dimension accuracy and shape precision of pulse ECM are significantly improved when the polarization voltage during pulse-off time was eliminated using the new pulsed-power supply method. References [1]Fang X, Qu N, Li H, Zhu D. Enhancement of insulation coating durability in electrochemical drilling. Int J Adv Manuf Technol,2013,68(9– 12):2005–2013. [2]K P Rajurkar, D Zhu, J A McGeough, J Kozak, A De Silva. New Developments in Electro-Chemical Machining. Annals of the ClRP, 1999,48(2):569-570. [3]Hong Shik Shin, Bo Hyun Kim, Chong Nam Chu. Analysis of the side gap resulting from micro electrochemical machining with a tungsten wire and
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ultrashort voltage pulses. Journal of Micromechanics and Microengineering, 2008, 18:1-6. [4]Liu Yong, Zeng Yongbin. Electrochemical drilling of deep and small holes with high speed micro electrode. Optics and precision engineering. 2014, 22(3):608-614. [5]Viola Kirchner, Laurent Cagnon, Rolf Schuster, Gerhard Ertl. Electrochemical machining of stainless steel microelements with ultrashort voltage pulses. Applied Physics Letters, 2001, 79(11):17211723. [6]A L Trimmer, J L Hudson, M Kock, R Schuster. Single-step electrochemical machining of complex nanostructures with ultrashort voltage pulses. Applied Physics Letters, 2003, 82(19):3327-3329.
[7] M Kock, V Kirchner, R Schuster, Electrochemical micromachining with ultrashort voltage pulses/a versatile method with lithographical precision. Electrochimica Acta, 2003,48 :3213-3219. [8]Viola Kirchner, Xinghua Xia, Rolf Schuster, Electrochemical Nanostructuring with ultrashort voltage pulses. Accounts Of Chemical Research, 2001, 34:371-377. [9]Alok Kumar, Das Partha Saha. Experimental investigation on microelectrochemical sinking operation for fabrication of micro-holes. The Brazilian Society of Mechanical Sciences and Engineering, 2015, 37:657– 663. [10]Rolf Schuster, Viola Kirchner, Philippe Allongue. Gerhard Ertl. Electrochemical micromachining, Science, 2000, 289(5476):98-101.