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Improvement of surface integrity of electro-chemical discharge machining process using powder-mixed electrolyte
Journal of Materials Processing Technology 191 (2007) 224–227
Improvement of surface integrity of electro-chemical discharge machining process using powder-mixed electrolyte Min-Seop Han, Byung-Kwon Min ∗ , Sang Jo Lee School of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea
Abstract A new method has been investigated to improve the surface integrity of electro-chemical discharge machining (ECDM) process by use of conductive particles in the electrolyte. In conventional ECDM processes the generation of fine sparks with uniform energy has been most desired technique to improve the machining efficiency and the surface quality. However, precise control of the spark generations in ECDM process has been a challenging problem. In electrical discharge machining (EDM) processes, which is thermal erosion machining process using spark energy similar to the ECDM, powder-mixed EDM (PM-EDM) fluids have been used to improve machining quality. Although the exact role of the conductive particles in the EDM process could not be clearly explained yet, it has been reported that the powder stabilizes discharge current as a result of discharge energy dispersion. Considering the similarity of the ECDM process compared to EDM where electrical sparks are utilized, powder-mixed electrolyte was introduced to create similar effects. In this paper fine graphite powder (which has good thermal and electrical conductivity) mixed with electrolyte has been applied to the ECDM process. Borosilicate glass, which is frequently used as a material for micro structures, was used as a workpiece. To investigate effectiveness of the proposed method experiments were conducted. The experiment results demonstrated that the breakdown voltage was reduced and the peak current during the process was decreased by ten percents. Discharging pattern was modified such that a single discharge pulse was branched into two or three. As a result, the surface quality was improved compared to that from the conventional process. Various experiment results of product quality with respect to powders volume ratio are also presented. © 2007 Elsevier B.V. All rights reserved. Keywords: Conductive particle; Discharge energy
1. Introduction For microsystem applications, the demand of precise glass fabrication process is being increased. However, not many fabrication methods are effective in glass machining. ECDM is an attractive micro machining method for fabricating micro-hole, micro-channel, and microstructure because it can be applied to non-conducting materials such as silicon and glasses [1]. However, the spark generation of ECDM is irregular due to the random formation of hydrogen film in the vicinity of the tool tip. This characteristic of the ECDM results in poor surface integrity and repeatability. In order to overcome these drawbacks of the conventional ECDM process, a new method has been investigated by use of conductive particles in the electrolyte. The electrolyte has important functions in the ECDM process: cooling electrodes, flushing debris away from the gap, and determining the critical voltage for discharge ignition. When
∗
Corresponding author. Tel.: +82 2 2123 5813; fax: +82 2 312 2159. E-mail address: [email protected] (B.-K. Min).
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.03.004
conductive particles are added to the electrolyte the spark initiation voltage is decreased because of the local electric field intensification between the tool and workpiece. In this study, graphite particle, which is widely used for its good electrical and thermal conductivity in the PM-EDM process [2], with the average diameter of 10 m are selected to be mixed with the electrolyte. Various experiment results of product quality with respect to powders volume ratio are presented. 2. Electrostatic model of powder-mixed ECDM process The role of the particles in the electrolyte which decreases spark ignition voltage can be explained using two models: hydrogen film model and single conductive particle charge model [3–6]. 2.1. Hydrogen film model Fig. 1 shows the model of a power-mixed ECDM (PMECDM) process. To apply the hydrogen film model to the ECDM process, an idea of imaginary electrode, which was proposed by
M.-S. Han et al. / Journal of Materials Processing Technology 191 (2007) 224–227
225
Fig. 1. Model of electric field between plate electrodes: (a) without particle and (b) with one conductive particle.
Jain et al. [3], has been applied. Because in an ECDM process, the discharge is not generated between two solid electrodes but inside the hydrogen film formed from bubbles in the electrolyte, it is assumed that an imaginary electrode exists at the boundary of a hydrogen bubble. Jain et al. calculated approximate hydrogen film thickness by treating it as the distance between two parallel electrodes. Using the proposed model, the hydrogen film is insulating gas located between two parallel electrodes charged by DC voltage. The gap distance becomes the theoretical film thickness when the spark ignites. The presence of conductive particles reduces the dielectric strength of the film [4]. The reduction of spark ignition voltage can be explained using Dascalescu et al. [5]’s model. Dascalescu et al. developed a model to calculate electric field between plate electrodes with the presence of conductive particles. The dielectric strength (E0 ) and electric field intensification (E/Eh ) with respect to the particle conditions, such as number, size, and chain shape of the particles, can be evaluated using the computational model. Dascalescu et al. also proposed that the size and formation of particle chains were the dominant parameters affecting the breakdown conditions of insulating gas system. 2.2. Single conductive particle charge According to [6], conductive particles motion in electrically stressed dielectric promotes the charge transfer between the electrodes. The potential between the tool and the imaginary electrodes proposed in Section 2.1 is decreased when the particles exist inside the hydrogen film. The potential Vc of the
particle, when it is assumes as a sphere, is [6]: Vc =
q q = c 4πεR
(1)
where q, c, ε, are charge (C), capacitance (F), and permittivity (F/m), respectively. Therefore, if a charged particle attached to or moves between the electrodes the potential between the electrodes decreases. Important behaviour of the particle considered in the model is the particle detachment from the tool electrode by the electrostatic and gravitational force. When a particle is subjected to an electric force, it tends to detach itself from the electrode [7]. A micro discharge is caused if the detached particle approaches to the opposite electrode. Successive micro discharges in the hydrogen film results in the electrical breakdown. Fig. 2(a) illustrates the schematic diagram of series discharge suggested by Zhao et al. [8] which explains the electric field intensifying due Table 1 Experimental condition for tests Micro ECDM device Electrolyte Tool electrode
Axes resolution: 1.0 m NaOH solution Material: tungsten carbide Tip diameter: 200 m
Digital oscilloscope
Model: LC334AM (LeCroy) Sampling rate: 50 M/s
Workpiece
Borosilicate glass (thickness: 1 mm)
Machining conditions
Applied voltage: DC 35 V Peak current: 1.1 A Electrolyte concentration: 30% Graphite powder concentration: 0.5–2 wt.%
Fig. 2. Electric field deformation due to micro particles: (a) schematic diagram of series discharge [8] and (b) contour graphics of potential distribution between particle and electrode [9].
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M.-S. Han et al. / Journal of Materials Processing Technology 191 (2007) 224–227
Fig. 3. Schematic diagram of micro ECDM device.
to the conductive particles in PM-EDM process. Under the DC voltage separation of positive and negative charges within the particle occur respectively at the top and bottom surface. As a result local electric intensification is generated between the particle-to-particle and particle-to-electrode gap. First discharge breakdown easily occurs where the electric field density is the highest. Tatsushi Matsuyama et al. [9] proposed a charge relaxation model for a particle charging mechanism due to impact or contact with electrode. Fig. 2(b) shows contour graphics of the potential distribution around a particle of ε = 5.0 and the particle radius-contact gap ratio is 0.05.
Fig. 4. Comparison of surface roughness with respect to powder concentration: (a) without powder, Ra : 4.86 m; (b) powder: 0.5 wt.%, Ra : 1.63 m; (c) powder: 1 wt.%, Ra : 1.44 m and (d) powder: 2 wt.%, Ra : 5.26 m.
3. Experimental results Experimental conditions and the prototype of micro ECDM device are presented in Table 1 and Fig. 3, respectively. Experiment was conducted using cylindrical tungsten carbide (WC) electrode with the diameter of 0.2 mm, 30% NaOH as electrolyte, and 35 VDC as the applied voltage. In a conventional ECDM process, the locally concentrated spark energy causes irregularity of machined surface due to the microcracks, local fracture, and breakage of the workpiece. By use of the conductive particles, there were evident decreases of microcracks and fractures on the machined surface of the workpiece. Optical microscope images of the machined surface are presented in Fig. 4. At 0.5 and 1.0 wt.% powder concentration, the surface becomes fine and smooth. However, when the concentration is above 2.0 wt.%, microcracks were developed. Fig. 5 shows the surface roughness with respect to machining feedrate and powder concentrations. As can be seen in the figure, powder concentration clearly influences the machined surface. At the low powder con-
Fig. 5. Feedrate vs. Ra according to powder concentration.
centration of 0.5–1.0 wt.%, surface roughness (Ra ) was reduced as small as 0.95 m. Particle charging induces different discharge waveforms. Using the powdermixed electrolyte a multiple discharging effect suggested by Chow et al. [10] in PM-EDM process is created in PM-ECDM process. As a result, the peak value of current is considerably reduced from 1.1 A down to 0.9 A with 1 wt.% powder concentration (See Fig. 6). This implies that the dispersion of the dis-
Fig. 6. Comparison of current peak and pulse width: (a) without powder and (b) with powder (1 wt.%).
M.-S. Han et al. / Journal of Materials Processing Technology 191 (2007) 224–227 charge energy per single discharge pulse results in the improvement of surface integrity.
4. Conclusion In this study, a power-mixed electro-chemical discharge machining method to fabricate glass micro surface has been proposed, and the experiments were conducted. The presence of conductive particles within the hydrogen film reduces the critical breakdown strength resulting in the decreased spark energy per single discharge pulse. This breakdown strength reduction is obtained mainly by two kinds of the particle behaviours: (1) attachment on the tool electrode which causes the local electric field intensification results in easier and stable discharge, and (2) dynamic particle movements due to the electrostatic or gravitational force. The hydrogen film at the moment of discharge was modelled as breakdown of the insulating gas between two parallel plates which distance of the gap becomes the theoretical thickness of the film. The presence of conductive particles reduces the dielectric strength of the film. Experiment results displaying product quality with respect to powders volume ratio are also presented. Experiment results showed that the use of electrolyte mixed with conductive particle improved the surface integrity of ECDM process. By use of 1.0 wt.% graphite powder concentration in 30% NaOH, the number of microcracks was significantly reduced and the surface roughness was improved from 4.86 to 1.44 m.
227
References [1] R. W¨uthrich, V. Fascio, Machining of non-conducting materials using electrochemical discharge phenomenon-an overview, Int. J. Mach. Tools Manuf. 37 (2005) 1095–1108. [2] Y.S. Wong, L.C. Lim, I. Rahuman, W.M. Tee, Near-mirror-finish phenomenon in EDM using powder-mixed dielectric, J. Mater. Process. Technol. 79 (1998) 30–40. [3] V.K. Jain, P.M. Dixit, P.M. Pandey, On the analysis of the electrochemical spark machining process, Int. J. Mach. Tools Manuf. 39 (1999) 165– 186. [4] L. Dascalescu, R. Tobazeon, P. Atten, Behaviour of conducting particles in corona-dominated electric fields, J. Phys. D: Appl. Phys. 28 (1995) 1611–1618. [5] L. Dascalescu, A. Samuila, R. Tobaz´eon, Size of solid contaminants and formation of particle chains: two factors affecting the dielectric strength of insulating gases, J. Electrostatics 40–41 (1997) 419–424. [6] P. Felsenthal, B. Vonnegut, Enhanced charge transfer in dielectric fluids containing conducting particles, Br. J. Appl. Phys. 18 (1967) 1801– 1806. [7] L. Dascalescu, A. Samuila, R. Tobaz´eon, Dielectric behaviour of particlecontaminated air-gaps in the presence of corona, J. Electrostatics 36 (1996) 253–275. [8] W.S. Zhao, Q.G. Meng, Z.L. Wang, The application of research on powder mixed EDM in rough machining, J. Mater. Process. Technol. 129 (2002) 30–33. [9] T. Matsuyama, H. Yamamoto, Charge relaxation process dominates contact charging of a particle in atmospheric conditions, J. Phys. D: Appl. Phys. 28 (1997) 2418–2423. [10] H.-M. Chow, B.-H. Yan, F.-Y. Huang, Study of added powder in kerosene for the micro-slit machining of titanium alloy using electrodischarge machining, J. Mater. Process. Technol. 101 (2000) 95– 103.
Improvement of surface integrity of electro-chemical discharge machining process using powder-mixed electrolyte Min-Seop Han, Byung-Kwon Min ∗ , Sang Jo Lee School of Mechanical Engineering, Yonsei University, Seoul 120-749, South Korea
Abstract A new method has been investigated to improve the surface integrity of electro-chemical discharge machining (ECDM) process by use of conductive particles in the electrolyte. In conventional ECDM processes the generation of fine sparks with uniform energy has been most desired technique to improve the machining efficiency and the surface quality. However, precise control of the spark generations in ECDM process has been a challenging problem. In electrical discharge machining (EDM) processes, which is thermal erosion machining process using spark energy similar to the ECDM, powder-mixed EDM (PM-EDM) fluids have been used to improve machining quality. Although the exact role of the conductive particles in the EDM process could not be clearly explained yet, it has been reported that the powder stabilizes discharge current as a result of discharge energy dispersion. Considering the similarity of the ECDM process compared to EDM where electrical sparks are utilized, powder-mixed electrolyte was introduced to create similar effects. In this paper fine graphite powder (which has good thermal and electrical conductivity) mixed with electrolyte has been applied to the ECDM process. Borosilicate glass, which is frequently used as a material for micro structures, was used as a workpiece. To investigate effectiveness of the proposed method experiments were conducted. The experiment results demonstrated that the breakdown voltage was reduced and the peak current during the process was decreased by ten percents. Discharging pattern was modified such that a single discharge pulse was branched into two or three. As a result, the surface quality was improved compared to that from the conventional process. Various experiment results of product quality with respect to powders volume ratio are also presented. © 2007 Elsevier B.V. All rights reserved. Keywords: Conductive particle; Discharge energy
1. Introduction For microsystem applications, the demand of precise glass fabrication process is being increased. However, not many fabrication methods are effective in glass machining. ECDM is an attractive micro machining method for fabricating micro-hole, micro-channel, and microstructure because it can be applied to non-conducting materials such as silicon and glasses [1]. However, the spark generation of ECDM is irregular due to the random formation of hydrogen film in the vicinity of the tool tip. This characteristic of the ECDM results in poor surface integrity and repeatability. In order to overcome these drawbacks of the conventional ECDM process, a new method has been investigated by use of conductive particles in the electrolyte. The electrolyte has important functions in the ECDM process: cooling electrodes, flushing debris away from the gap, and determining the critical voltage for discharge ignition. When
∗
Corresponding author. Tel.: +82 2 2123 5813; fax: +82 2 312 2159. E-mail address: [email protected] (B.-K. Min).
0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.03.004
conductive particles are added to the electrolyte the spark initiation voltage is decreased because of the local electric field intensification between the tool and workpiece. In this study, graphite particle, which is widely used for its good electrical and thermal conductivity in the PM-EDM process [2], with the average diameter of 10 m are selected to be mixed with the electrolyte. Various experiment results of product quality with respect to powders volume ratio are presented. 2. Electrostatic model of powder-mixed ECDM process The role of the particles in the electrolyte which decreases spark ignition voltage can be explained using two models: hydrogen film model and single conductive particle charge model [3–6]. 2.1. Hydrogen film model Fig. 1 shows the model of a power-mixed ECDM (PMECDM) process. To apply the hydrogen film model to the ECDM process, an idea of imaginary electrode, which was proposed by
M.-S. Han et al. / Journal of Materials Processing Technology 191 (2007) 224–227
225
Fig. 1. Model of electric field between plate electrodes: (a) without particle and (b) with one conductive particle.
Jain et al. [3], has been applied. Because in an ECDM process, the discharge is not generated between two solid electrodes but inside the hydrogen film formed from bubbles in the electrolyte, it is assumed that an imaginary electrode exists at the boundary of a hydrogen bubble. Jain et al. calculated approximate hydrogen film thickness by treating it as the distance between two parallel electrodes. Using the proposed model, the hydrogen film is insulating gas located between two parallel electrodes charged by DC voltage. The gap distance becomes the theoretical film thickness when the spark ignites. The presence of conductive particles reduces the dielectric strength of the film [4]. The reduction of spark ignition voltage can be explained using Dascalescu et al. [5]’s model. Dascalescu et al. developed a model to calculate electric field between plate electrodes with the presence of conductive particles. The dielectric strength (E0 ) and electric field intensification (E/Eh ) with respect to the particle conditions, such as number, size, and chain shape of the particles, can be evaluated using the computational model. Dascalescu et al. also proposed that the size and formation of particle chains were the dominant parameters affecting the breakdown conditions of insulating gas system. 2.2. Single conductive particle charge According to [6], conductive particles motion in electrically stressed dielectric promotes the charge transfer between the electrodes. The potential between the tool and the imaginary electrodes proposed in Section 2.1 is decreased when the particles exist inside the hydrogen film. The potential Vc of the
particle, when it is assumes as a sphere, is [6]: Vc =
q q = c 4πεR
(1)
where q, c, ε, are charge (C), capacitance (F), and permittivity (F/m), respectively. Therefore, if a charged particle attached to or moves between the electrodes the potential between the electrodes decreases. Important behaviour of the particle considered in the model is the particle detachment from the tool electrode by the electrostatic and gravitational force. When a particle is subjected to an electric force, it tends to detach itself from the electrode [7]. A micro discharge is caused if the detached particle approaches to the opposite electrode. Successive micro discharges in the hydrogen film results in the electrical breakdown. Fig. 2(a) illustrates the schematic diagram of series discharge suggested by Zhao et al. [8] which explains the electric field intensifying due Table 1 Experimental condition for tests Micro ECDM device Electrolyte Tool electrode
Axes resolution: 1.0 m NaOH solution Material: tungsten carbide Tip diameter: 200 m
Digital oscilloscope
Model: LC334AM (LeCroy) Sampling rate: 50 M/s
Workpiece
Borosilicate glass (thickness: 1 mm)
Machining conditions
Applied voltage: DC 35 V Peak current: 1.1 A Electrolyte concentration: 30% Graphite powder concentration: 0.5–2 wt.%
Fig. 2. Electric field deformation due to micro particles: (a) schematic diagram of series discharge [8] and (b) contour graphics of potential distribution between particle and electrode [9].
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M.-S. Han et al. / Journal of Materials Processing Technology 191 (2007) 224–227
Fig. 3. Schematic diagram of micro ECDM device.
to the conductive particles in PM-EDM process. Under the DC voltage separation of positive and negative charges within the particle occur respectively at the top and bottom surface. As a result local electric intensification is generated between the particle-to-particle and particle-to-electrode gap. First discharge breakdown easily occurs where the electric field density is the highest. Tatsushi Matsuyama et al. [9] proposed a charge relaxation model for a particle charging mechanism due to impact or contact with electrode. Fig. 2(b) shows contour graphics of the potential distribution around a particle of ε = 5.0 and the particle radius-contact gap ratio is 0.05.
Fig. 4. Comparison of surface roughness with respect to powder concentration: (a) without powder, Ra : 4.86 m; (b) powder: 0.5 wt.%, Ra : 1.63 m; (c) powder: 1 wt.%, Ra : 1.44 m and (d) powder: 2 wt.%, Ra : 5.26 m.
3. Experimental results Experimental conditions and the prototype of micro ECDM device are presented in Table 1 and Fig. 3, respectively. Experiment was conducted using cylindrical tungsten carbide (WC) electrode with the diameter of 0.2 mm, 30% NaOH as electrolyte, and 35 VDC as the applied voltage. In a conventional ECDM process, the locally concentrated spark energy causes irregularity of machined surface due to the microcracks, local fracture, and breakage of the workpiece. By use of the conductive particles, there were evident decreases of microcracks and fractures on the machined surface of the workpiece. Optical microscope images of the machined surface are presented in Fig. 4. At 0.5 and 1.0 wt.% powder concentration, the surface becomes fine and smooth. However, when the concentration is above 2.0 wt.%, microcracks were developed. Fig. 5 shows the surface roughness with respect to machining feedrate and powder concentrations. As can be seen in the figure, powder concentration clearly influences the machined surface. At the low powder con-
Fig. 5. Feedrate vs. Ra according to powder concentration.
centration of 0.5–1.0 wt.%, surface roughness (Ra ) was reduced as small as 0.95 m. Particle charging induces different discharge waveforms. Using the powdermixed electrolyte a multiple discharging effect suggested by Chow et al. [10] in PM-EDM process is created in PM-ECDM process. As a result, the peak value of current is considerably reduced from 1.1 A down to 0.9 A with 1 wt.% powder concentration (See Fig. 6). This implies that the dispersion of the dis-
Fig. 6. Comparison of current peak and pulse width: (a) without powder and (b) with powder (1 wt.%).
M.-S. Han et al. / Journal of Materials Processing Technology 191 (2007) 224–227 charge energy per single discharge pulse results in the improvement of surface integrity.
4. Conclusion In this study, a power-mixed electro-chemical discharge machining method to fabricate glass micro surface has been proposed, and the experiments were conducted. The presence of conductive particles within the hydrogen film reduces the critical breakdown strength resulting in the decreased spark energy per single discharge pulse. This breakdown strength reduction is obtained mainly by two kinds of the particle behaviours: (1) attachment on the tool electrode which causes the local electric field intensification results in easier and stable discharge, and (2) dynamic particle movements due to the electrostatic or gravitational force. The hydrogen film at the moment of discharge was modelled as breakdown of the insulating gas between two parallel plates which distance of the gap becomes the theoretical thickness of the film. The presence of conductive particles reduces the dielectric strength of the film. Experiment results displaying product quality with respect to powders volume ratio are also presented. Experiment results showed that the use of electrolyte mixed with conductive particle improved the surface integrity of ECDM process. By use of 1.0 wt.% graphite powder concentration in 30% NaOH, the number of microcracks was significantly reduced and the surface roughness was improved from 4.86 to 1.44 m.
227
References [1] R. W¨uthrich, V. Fascio, Machining of non-conducting materials using electrochemical discharge phenomenon-an overview, Int. J. Mach. Tools Manuf. 37 (2005) 1095–1108. [2] Y.S. Wong, L.C. Lim, I. Rahuman, W.M. Tee, Near-mirror-finish phenomenon in EDM using powder-mixed dielectric, J. Mater. Process. Technol. 79 (1998) 30–40. [3] V.K. Jain, P.M. Dixit, P.M. Pandey, On the analysis of the electrochemical spark machining process, Int. J. Mach. Tools Manuf. 39 (1999) 165– 186. [4] L. Dascalescu, R. Tobazeon, P. Atten, Behaviour of conducting particles in corona-dominated electric fields, J. Phys. D: Appl. Phys. 28 (1995) 1611–1618. [5] L. Dascalescu, A. Samuila, R. Tobaz´eon, Size of solid contaminants and formation of particle chains: two factors affecting the dielectric strength of insulating gases, J. Electrostatics 40–41 (1997) 419–424. [6] P. Felsenthal, B. Vonnegut, Enhanced charge transfer in dielectric fluids containing conducting particles, Br. J. Appl. Phys. 18 (1967) 1801– 1806. [7] L. Dascalescu, A. Samuila, R. Tobaz´eon, Dielectric behaviour of particlecontaminated air-gaps in the presence of corona, J. Electrostatics 36 (1996) 253–275. [8] W.S. Zhao, Q.G. Meng, Z.L. Wang, The application of research on powder mixed EDM in rough machining, J. Mater. Process. Technol. 129 (2002) 30–33. [9] T. Matsuyama, H. Yamamoto, Charge relaxation process dominates contact charging of a particle in atmospheric conditions, J. Phys. D: Appl. Phys. 28 (1997) 2418–2423. [10] H.-M. Chow, B.-H. Yan, F.-Y. Huang, Study of added powder in kerosene for the micro-slit machining of titanium alloy using electrodischarge machining, J. Mater. Process. Technol. 101 (2000) 95– 103.