- Email: [email protected]
Investigation of micro-EDM material removal characteristics using single RC-pulse discharges
Journal of Materials Processing Technology 140 (2003) 303–307
Investigation of micro-EDM material removal characteristics using single RC-pulse discharges Y.S. Wong a,∗ , M. Rahman a , H.S. Lim a , H. Han b , N. Ravi b a
Mechanical Engineering Department, 10 Kent Ridge Crescent, National University of Singapore, Singapore 119260, Singapore b Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore
Abstract Similar to EDM, in micro-EDM, intense heat is generated between the workpiece and tool electrode by the discharge through a dielectric medium to result in the formation of a microcrater that is much smaller in size. In this study, a single-spark generator has been developed to study the erosion characteristics from the microcrater size. Using a simple heat transfer model, the efficiency at different discharge condition is also deduced. It is found that at lower-energy (<50 J) discharges, the energy required to remove the unit volume of material, defined as the specific energy, is found to be much less than that at higher-energy discharges. Additionally, the ratio of the standard deviation to the measured microcrater size is found to be lower at lower discharge energy, indicating greater consistency in shape and size when the discharge occurs at lower energy. The fundamental erosion mechanism of material is discussed by considering melting and evaporation phenomena using theoretical modeling. The average efficiency of erosion, when estimated to be due primarily to melting or evaporation alone, is found to be up to an order of magnitude higher at lower-energy discharges than that at higher-energy discharges. © 2003 Elsevier B.V. All rights reserved. Keywords: Micro-EDM; Material removal mechanism; Spark gap; Discharge energy; Efficiency; Modeling
1. Introduction Micro-EDM is a precision machining process for micromachining of microstructures, such as high-aspect-ratio micro-holes, micro-slots and micro-molds. The basic characteristic of the micro-EDM process is essentially similar to that of the EDM process with the main difference being in the size of the tool used, the power supply of discharge energy and the resolution of the X-, Y- and Z-axes movement [1]. In order to improve the process control, it is very important to understand the effect of the critical machining parameters involved in the material removal mechanism. Several theoretical works related to the EDM process parameters, such as voltage, current, discharge energy, pulse time, pulse duration and distance of spark gap on the material removal mechanism, have been studied from the size of the crater and its surface integrity. These studies have been conducted using single-pulse discharge and multiple discharges through statistical, theoretical and heat conduction theories and models. The important characteristics associated with the removal mechanism of the formation of
∗ Corresponding author. Tel.: +65-8742221; fax: +65-7791459. E-mail address: [email protected] (Y.S. Wong).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00771-4
crater are diameter, depth, height of rim, its shape, material removal rate and volume. In the past, many studies have clearly shown that the melting action to be the predominant mechanism of material removal in EDM. Then again, when the energy level becomes lower it is claimed that vaporization phenomenon also occurs to remove the material. Guerrero-Alvarez et al. [2] have found that melting and vaporization actions are the causes of removing material through the aid of surface features examination. The increase in the spark energy has been found to decrease the net metal removal efficiency (measured volume/maximum volume that can be removed). At low energies (10–20 mJ), the net metal removal efficiency has been found to be near 1.0 and at higher energies (50–700 mJ), the efficiency is found to fall of rapidly until 0.15. Apart from melting action at low discharge energy, Willey [3] has shown that 0.2% of the molten material is removed due to vaporization action. Likewise, Lhiaubet and Meyer [4] have determined that the ratio of the vaporized metal to the total metal removed (re /a)3 is approximately 1.15–1.90% at cathode and 1.25–4.10% at anode from the computation using a linearization heat conduction model. Benzerga et al. [5] have further improvised the above model while considering the latent heat of change of state in the heat flux equation and have shown that the ratio of (re /a)3 is 0.5–2.19 and
304
Y.S. Wong et al. / Journal of Materials Processing Technology 140 (2003) 303–307
Nomenclature specific heat capacity (J/kg ◦ C) capacitance of the capacitor (F) discharge energy (J) single discharge energy supplied until the completion of discharge (J) Eg discharge energy supplied in the gap (J) Hm enthalpy of melting (J/kg) Hv enthalpy of vaporization (J/kg) Lm latent heat of melting or fusion (J/kg) Lv latent heat of boiling or vaporization (J/kg) S.D. standard deviation Sg spark gap (m) Tb temperature at boiling condition (◦ C) Tm temperature at melting condition (◦ C) T0 initial/ambient condition (◦ C) Vexp experimental volume of microcrater measured (m3 ) Vs supply voltage from the dc power supply (V) Greek letters η erosion efficiency ηm erosion efficiency due to only melting ηv erosion efficiency due to vaporization ρ density of the material (kg/m3 ) cp C DE Eds
0.5–9.42% for cathode and anode, respectively. Saha et al. [6] have extended the pointed heat source model (PHSM) by DiBitonto et al. [7] and established that the fraction of energy going to the cathode is not constant and depends on the applied energy and material pair combination. Vaseekaran and Brown [8] have studied the EDM mechanism of TiB2 and Zn using pin-type tool electrode and have found that the melting and boiling temperature of the electrode greatly influences the mechanism of material removal. In addition, the geometry of the tool electrode is found not to have any influence on the magnitude of discharge energy, irrespective of the material used. Wang et al. [9] have claimed that erosion mechanism in EDM is due to the occurrence of phenomena of melt-splashing at high energies (E > 0.03 J) and vaporization for low energies (E < 0.001 J) using SEM examination on the surface integrity of the crater produced using single-spark erosion at constant spark gaps. The erosion volume has been ascertained to correlate well with the melting temperature and enthalpy of melting per unit volume. The effect of thermal diffusivity on the volume of metal removal is found to be less. A further investigation [10] reveals that as the ratio of erosion depth (erosion volume/cross-sectional area of the electrode) and gap distance increases, the breakdown voltage level increases and causes the metal removal to happen because of vaporization to melt-ablation. The above studies clearly indicate that
melting and vaporization action to be the mechanism of material removal, when the applied energy level is low. Thus the applied energy level is found to be the critical parameter that governs the mechanism of material removal. The work presented in this paper aims to study the mechanism of material removal by investigating the formation of microcrater and its characteristics using very low single RC-pulse discharge which is approximately 1000 times lower than those in EDM, to shed a new light on this subject.
2. Experimental The experiments have been carried out in a micro-EDM system. The microcraters shown in Fig. 1 are produced using the primary micro-EDM resistance capacitance (RC) circuit. A single-discharge erosion RC circuit is developed for this study as shown in Fig. 2, between the dc voltage power supply (0–300 V). The RC relaxation circuit basically consists of a charging circuit and a discharging circuit. The main feature of the single-discharge RC power generator is that it is compact for a quicker response of charging and discharging energy. When the switch is connected to position A, energy is charged and in position B, energy is discharged between the electrodes, producing a tiny microcrater in the workpiece. The frequency of discharge (discharge repetition rate) depends upon the charging time which is decided by the resistor (R) used in the circuit. However, “R” should not be made very low because arcing phenomenon can occur instead of sparking and a critical resistance is desirable which will prevent arcing. The control of the constant spark gap distance in the Z-axis is set by the aid of a manual pulse generator to a positioning resolution of 0.25 m. The Taylor Hobson profiliometer and Talymap are used to measure the small volume of microcrater. Also, as the size of the microcrater is very small, the samples before machining are subjected to polishing until a mirror finish is obtained so that microcraters produced on the samples are easily distinguishable.
3. Results and discussion A microcrater resembling that of a quarter moon shaped structure in cross-section is produced using the single RC-pulse generator. Fig. 1(a)–(f) show some examples of microcraters produced using the single discharge on the sample. The critical problem in the single-discharge machining is the unevenness in the size of the microcraters produced. Hence, five microcraters are produced at each machining condition to determine the expected size. 3.1. Effects on the volume of microcrater Fig. 3 illustrates the volume of the microcrater which is found to increase with increase in the discharge energy (DE)
Y.S. Wong et al. / Journal of Materials Processing Technology 140 (2003) 303–307
305
Fig. 1. Microcraters at 2 m spark gap.
Pos A (-)
Pos B
R
microelectrode
Vs
C
debris
workpiece
(+) dielectric medium microcrater Fig. 2. Single-discharge RC circuit of micro-EDM.
at 100 V at various spark gaps. The effect due to increase in the spark gap on the volume of microcrater is found to be small. In Fig. 4, it can be seen that the ratio of the standard deviation (S.D.) from the mean volume to the measured volume is low and high at the respective lower and higher energies
at energy levels ranging from 20 to 11 000 J. Similarly, the ratio of the standard deviation to the mean of the diameter of the microcrater has been found to be low at lower energy. As the energy is increased, the diameter is found to have a higher deviation, as can also be seen in Fig. 4. The above phenomenon is also evident from the SEM micrographs shown in Fig. 1(a) and (b) that the shapes of the microcrater to be more uniform with better defined rim at lower energies. Fig. 1(c)–(f) show that the diameters are irregular at higher energies. These characteristics strongly indicate that the amount of the applied discharge energy is a very important machining parameter to obtain microcraters of more regular sizes. The specific energy is the energy required to remove a unit volume of material. Fig. 5 shows that at lower discharge energy, the specific energy is low (low DE zone), when compared with that at higher-energy levels.
Cavity volume of microcrater (µm3)
7.50E+04 6.00E+04 4.50E+04 3.00E+04 1.50E+04 11000 5000 2350 1100
0.00E+00 1.0
1.5
50 23.50
2.0
2.5 Spark gap (µm)
3.0
DE (µJ)
Average ratio of SD to microcrater sizes
0.25 9.00E+04
Avg SD/Avg cavity volume Avg SD/Avg diameter
0.20
0.15
0.10
0.05
0.00 23.50
50
1100
2350
5000
11000
Discharge energy (µJ)
3.5
Fig. 3. Discharge energy vs. cavity volume of microcrater.
Fig. 4. Discharge energy vs. average ratio of standard deviation (S.D.) to microcrater size.
306
Y.S. Wong et al. / Journal of Materials Processing Technology 140 (2003) 303–307
4.3. Theoretical modeling
Average specific energy (µJ/µm 3)
0.18
In a single discharge using a RC circuit, the total singledischarge energy (Eds ) gained after charging is delivered until the energy stored in the capacitor is completely discharged through the gap. Therefore, the supplied energy (Eds ) is equal to the energy delivered to the gap (Eg ), as given in Eq. (1):
0.15
0.12
0.09
0.06
Eds = Eg = 21 CV2s
low DE zone
0.03
0 23.50
50
1100
2350
5000
11000
Discharge energy (µJ)
Fig. 5. Discharge energy vs. specific energy.
4. Efficiency of micro-EDM 4.1. Assumptions The following assumptions are made to elucidate the efficiency through a simple theoretical modeling using the experimental data: (i) The maximum amount of energy stored in the capacitor is assumed to be completely discharged. (ii) The energy is transferred to the workpiece in the form of heat. (iii) The duration time of the single-discharge is approximately equal to a single RC-pulse time and is very short for micro-EDM (less than 1 s). (iv) The enthalpies of phase transition (solid–liquid and liquid–vapor interface) are neglected. (v) The workpiece material is homogenous and isotropic. (vi) The thermo-physical properties of the materials are constant and equal in all three phases from solid to liquid to vapor, which apply over the whole temperature range. 4.2. Thermo-physical properties of SUS 304 The thermo-physical properties of stainless steel (SUS 304) are as follows: • • • • • • • • • •
density, ρ = 8000 kg/m3 ; specific heat capacity, cp = 500 J/kg ◦ C; ambient temperature, T0 = 20 ◦ C; melting point temperature, Tm = 1450 ◦ C; boiling or vaporization temperature, Tb = 3000 ◦ C; latent heat of fusion or melting, Lm = 300 kJ/kg; latent heat of vaporization, Lv = 6500 kJ/kg; volumetric heat capacity, ρcp = 4 000 000 J/m3 ◦ C; ρHv = 1.20 × 1010 J/m3 ; ρHm = 5.72 × 109 J/m3 .
(1)
A part of supplied energy from the spark forms the microcrater, which determines “erosion efficiency” (η), i.e. the ratio of the actual energy (Ee ) used to erode the microcrater to the supplied energy in the gap (Eg ) as given in Eq. (2). The remaining energy supplied between the gap will be equal to the energy lost in the anode, dielectric medium, etc. η=
Ee actual erosion energy = Eg supplied spark energy
(2)
The determination of the distribution of energy from the supplied energy is very complex and it depends upon the mechanism of material removal. When the energy is supplied, the erosion of material can occur firstly by melting and/or vaporizing as the heat is being conducted to the electrode. Thus, the actual energy used to erode the material to form a microcrater is given as described in Eqs. (3a) and (3b), the product of experimental erosion volume and the respective enthalpies of melting and vaporization per unit volume: • by considering vaporization action: Ee = Vexp ρHv
(3a)
• by considering melting action: Ee = Vexp ρHm
(3b)
The minimum energy required to vaporize the cathode material per unit volume can be estimated as given in Eq. (4a): Hv =
Tm T0
cp dT + Lm +
Tb Tm
cp dT + Lv
= cp [(Tm − T0 ) + (Tb − Tm )] + Lm + Lv Hm =
Tm
T0
cp dT + Lm = cp (Tm − T0 ) + Lm
(4a) (4b)
Prior to reaching the boiling temperature of the material, the electrodes have to reach their melting temperature. Thus, the amount of material removed due to melting is found using the minimum energy required to melt the cathode material per unit volume and is as given in Eq. (4b). Thus, the efficiency of material removal is computed using Eqs. (1), (3a) and (4a) into Eq. (2) due to vaporization action to erode the material. Table 1 indicates the values of erosion efficiency due to the vaporization and melting actions, against discharge energy at various spark gap conditions.
Y.S. Wong et al. / Journal of Materials Processing Technology 140 (2003) 303–307
307
Table 1 Erosion efficiency of discharge energies at various spark gaps (Sg ) Discharge energy (J) 23.50 50 1100 2350 5000 11000
Efficiency (ηv ) at Sg
Efficiency (ηm ) at Sg
1.0 m
1.5 m
2.0 m
2.5 m
3.0 m
3.5 m
1.0 m
1.5 m
2.0 m
2.5 m
3.0 m
3.5 m
0.575 0.626 0.086 0.094 0.070 0.080
0.405 0.472 0.078 0.073 0.072 0.093
0.224 0.469 0.078 0.080 0.075 0.076
0.272 0.673 0.096 0.067 0.074 0.073
0.418 0.603 0.081 0.071 0.071 0.076
0.000 0.367 0.075 0.046 0.077 0.092
0.274 0.298 0.041 0.045 0.033 0.038
0.193 0.225 0.037 0.035 0.034 0.044
0.107 0.224 0.037 0.038 0.036 0.036
0.130 0.321 0.046 0.032 0.035 0.035
0.199 0.287 0.039 0.034 0.034 0.036
0.000 0.175 0.036 0.022 0.037 0.044
Average erosion efficiency
Table 1 shows that at lower energies (<50 J) and spark gap distances ranging from 1 to 3.5 m, the estimated eroding efficiency is from 0.3 to 0.7 and 0.1 to 0.35, while considering the erosion mechanism as due to vaporization and only melting, respectively. At higher energies (>50 J) and spark gap ranging from 1 to 3.5 m, the estimated eroding efficiency is from 0.06 to 0.1 and 0.02 to 0.04, while considering the erosion mechanisms as due to vaporization and only melting respectively. Moreover, the efficiency at lower energies is found to be seven to eight times higher than that at higher energies. More importantly, these phenomena indicate that energy is more efficiently used to form the microcrater when the applied energy is lower. Fig. 6 indicates that the effect of the discharge energy on the average erosion efficiency, while neglecting the effect of the spark gap. It shows that 20% of material is removed by vaporizing the molten metal at lower-energy levels, if vaporization action is considered as a cause for the material removal. This estimated amount of vaporization is found to be 20 and 150 times higher than that, respectively, estimated by Lhiaubet and coworkers [4,5] and Willey [3], at very high energy levels. In addition, if the energy level is low, the pulse duration will be lesser, so the possibility of the heat transfer to the surrounding medium can be less. Thus,
the temperature could rise high at the point of discharge and may constitute to vaporization of the material.
5. Conclusions The following important conclusions are drawn from the investigation of the mechanism of material removal using the single RC-pulse discharges: • The volume and size of the microcraters are found to be more consistent at lower-energy discharges than at higher-energy discharges. • The specific energy required to remove the material is found to be significantly less at lower energies (<50 J) when compared with that at higher energies. • Using a simple theoretical model, the estimated erosion efficiency of material removal at low-energy discharges is found to be seven to eight times higher than that at higher-energy discharges. The aforementioned strongly indicate that lower-energy discharges produce more consistent size of microcrater at higher efficiency.
1.20
References
1.00
[1] T. Masuzawa, CIRP Ann. 49 (2001) 473. [2] J.L. Guerrero-Alvarez, J.E. Greene, B.F. von Turkovich, J. Eng. Ind. Ser. B 95 (1973) 965. [3] P.C.T. Willey, IEE Conf. Publ. 133 (1975) 265. [4] C. Lhiaubet, R.M. Meyer, J. Appl. Phys. 52 (1981) 3929. [5] L. Benzerga, C. Lhiaubet, R.M. Meyer, J. Appl. Phys. 58 (1985) 606. [6] R. Saha, R. Kumar, M.K. Muju, SME Technical Paper MR00-208, 2000, p. 1. [7] D.D. DiBitonto, P.T. Eubank, M.R. Patel, M.A. Barrufet, J. Appl. Phys. 66 (1989) 4095. [8] S. Vaseekaran, C.A. Brown, J. Mater. Process. Technol. 58 (1996) 70. [9] B.-J. Wang, N. Saka, E. Rabinowicz, Wear 157 (1992) 31. [10] B.-J. Wang, N. Saka, E. Rabinowicz, IEEE Trans. Comp. Hyb. Manuf. Technol. 14 (1991) 37.
eff of only melt eff of vap eff of only vap (vap-melt) eff (ideal)
0.80
0.60 0.40 0.20
0.00 23.50
50
1100
2350
5000
Discharge energy (µJ)
Fig. 6. Discharge energy vs. average efficiency.
11000
Investigation of micro-EDM material removal characteristics using single RC-pulse discharges Y.S. Wong a,∗ , M. Rahman a , H.S. Lim a , H. Han b , N. Ravi b a
Mechanical Engineering Department, 10 Kent Ridge Crescent, National University of Singapore, Singapore 119260, Singapore b Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore
Abstract Similar to EDM, in micro-EDM, intense heat is generated between the workpiece and tool electrode by the discharge through a dielectric medium to result in the formation of a microcrater that is much smaller in size. In this study, a single-spark generator has been developed to study the erosion characteristics from the microcrater size. Using a simple heat transfer model, the efficiency at different discharge condition is also deduced. It is found that at lower-energy (<50 J) discharges, the energy required to remove the unit volume of material, defined as the specific energy, is found to be much less than that at higher-energy discharges. Additionally, the ratio of the standard deviation to the measured microcrater size is found to be lower at lower discharge energy, indicating greater consistency in shape and size when the discharge occurs at lower energy. The fundamental erosion mechanism of material is discussed by considering melting and evaporation phenomena using theoretical modeling. The average efficiency of erosion, when estimated to be due primarily to melting or evaporation alone, is found to be up to an order of magnitude higher at lower-energy discharges than that at higher-energy discharges. © 2003 Elsevier B.V. All rights reserved. Keywords: Micro-EDM; Material removal mechanism; Spark gap; Discharge energy; Efficiency; Modeling
1. Introduction Micro-EDM is a precision machining process for micromachining of microstructures, such as high-aspect-ratio micro-holes, micro-slots and micro-molds. The basic characteristic of the micro-EDM process is essentially similar to that of the EDM process with the main difference being in the size of the tool used, the power supply of discharge energy and the resolution of the X-, Y- and Z-axes movement [1]. In order to improve the process control, it is very important to understand the effect of the critical machining parameters involved in the material removal mechanism. Several theoretical works related to the EDM process parameters, such as voltage, current, discharge energy, pulse time, pulse duration and distance of spark gap on the material removal mechanism, have been studied from the size of the crater and its surface integrity. These studies have been conducted using single-pulse discharge and multiple discharges through statistical, theoretical and heat conduction theories and models. The important characteristics associated with the removal mechanism of the formation of
∗ Corresponding author. Tel.: +65-8742221; fax: +65-7791459. E-mail address: [email protected] (Y.S. Wong).
0924-0136/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-0136(03)00771-4
crater are diameter, depth, height of rim, its shape, material removal rate and volume. In the past, many studies have clearly shown that the melting action to be the predominant mechanism of material removal in EDM. Then again, when the energy level becomes lower it is claimed that vaporization phenomenon also occurs to remove the material. Guerrero-Alvarez et al. [2] have found that melting and vaporization actions are the causes of removing material through the aid of surface features examination. The increase in the spark energy has been found to decrease the net metal removal efficiency (measured volume/maximum volume that can be removed). At low energies (10–20 mJ), the net metal removal efficiency has been found to be near 1.0 and at higher energies (50–700 mJ), the efficiency is found to fall of rapidly until 0.15. Apart from melting action at low discharge energy, Willey [3] has shown that 0.2% of the molten material is removed due to vaporization action. Likewise, Lhiaubet and Meyer [4] have determined that the ratio of the vaporized metal to the total metal removed (re /a)3 is approximately 1.15–1.90% at cathode and 1.25–4.10% at anode from the computation using a linearization heat conduction model. Benzerga et al. [5] have further improvised the above model while considering the latent heat of change of state in the heat flux equation and have shown that the ratio of (re /a)3 is 0.5–2.19 and
304
Y.S. Wong et al. / Journal of Materials Processing Technology 140 (2003) 303–307
Nomenclature specific heat capacity (J/kg ◦ C) capacitance of the capacitor (F) discharge energy (J) single discharge energy supplied until the completion of discharge (J) Eg discharge energy supplied in the gap (J) Hm enthalpy of melting (J/kg) Hv enthalpy of vaporization (J/kg) Lm latent heat of melting or fusion (J/kg) Lv latent heat of boiling or vaporization (J/kg) S.D. standard deviation Sg spark gap (m) Tb temperature at boiling condition (◦ C) Tm temperature at melting condition (◦ C) T0 initial/ambient condition (◦ C) Vexp experimental volume of microcrater measured (m3 ) Vs supply voltage from the dc power supply (V) Greek letters η erosion efficiency ηm erosion efficiency due to only melting ηv erosion efficiency due to vaporization ρ density of the material (kg/m3 ) cp C DE Eds
0.5–9.42% for cathode and anode, respectively. Saha et al. [6] have extended the pointed heat source model (PHSM) by DiBitonto et al. [7] and established that the fraction of energy going to the cathode is not constant and depends on the applied energy and material pair combination. Vaseekaran and Brown [8] have studied the EDM mechanism of TiB2 and Zn using pin-type tool electrode and have found that the melting and boiling temperature of the electrode greatly influences the mechanism of material removal. In addition, the geometry of the tool electrode is found not to have any influence on the magnitude of discharge energy, irrespective of the material used. Wang et al. [9] have claimed that erosion mechanism in EDM is due to the occurrence of phenomena of melt-splashing at high energies (E > 0.03 J) and vaporization for low energies (E < 0.001 J) using SEM examination on the surface integrity of the crater produced using single-spark erosion at constant spark gaps. The erosion volume has been ascertained to correlate well with the melting temperature and enthalpy of melting per unit volume. The effect of thermal diffusivity on the volume of metal removal is found to be less. A further investigation [10] reveals that as the ratio of erosion depth (erosion volume/cross-sectional area of the electrode) and gap distance increases, the breakdown voltage level increases and causes the metal removal to happen because of vaporization to melt-ablation. The above studies clearly indicate that
melting and vaporization action to be the mechanism of material removal, when the applied energy level is low. Thus the applied energy level is found to be the critical parameter that governs the mechanism of material removal. The work presented in this paper aims to study the mechanism of material removal by investigating the formation of microcrater and its characteristics using very low single RC-pulse discharge which is approximately 1000 times lower than those in EDM, to shed a new light on this subject.
2. Experimental The experiments have been carried out in a micro-EDM system. The microcraters shown in Fig. 1 are produced using the primary micro-EDM resistance capacitance (RC) circuit. A single-discharge erosion RC circuit is developed for this study as shown in Fig. 2, between the dc voltage power supply (0–300 V). The RC relaxation circuit basically consists of a charging circuit and a discharging circuit. The main feature of the single-discharge RC power generator is that it is compact for a quicker response of charging and discharging energy. When the switch is connected to position A, energy is charged and in position B, energy is discharged between the electrodes, producing a tiny microcrater in the workpiece. The frequency of discharge (discharge repetition rate) depends upon the charging time which is decided by the resistor (R) used in the circuit. However, “R” should not be made very low because arcing phenomenon can occur instead of sparking and a critical resistance is desirable which will prevent arcing. The control of the constant spark gap distance in the Z-axis is set by the aid of a manual pulse generator to a positioning resolution of 0.25 m. The Taylor Hobson profiliometer and Talymap are used to measure the small volume of microcrater. Also, as the size of the microcrater is very small, the samples before machining are subjected to polishing until a mirror finish is obtained so that microcraters produced on the samples are easily distinguishable.
3. Results and discussion A microcrater resembling that of a quarter moon shaped structure in cross-section is produced using the single RC-pulse generator. Fig. 1(a)–(f) show some examples of microcraters produced using the single discharge on the sample. The critical problem in the single-discharge machining is the unevenness in the size of the microcraters produced. Hence, five microcraters are produced at each machining condition to determine the expected size. 3.1. Effects on the volume of microcrater Fig. 3 illustrates the volume of the microcrater which is found to increase with increase in the discharge energy (DE)
Y.S. Wong et al. / Journal of Materials Processing Technology 140 (2003) 303–307
305
Fig. 1. Microcraters at 2 m spark gap.
Pos A (-)
Pos B
R
microelectrode
Vs
C
debris
workpiece
(+) dielectric medium microcrater Fig. 2. Single-discharge RC circuit of micro-EDM.
at 100 V at various spark gaps. The effect due to increase in the spark gap on the volume of microcrater is found to be small. In Fig. 4, it can be seen that the ratio of the standard deviation (S.D.) from the mean volume to the measured volume is low and high at the respective lower and higher energies
at energy levels ranging from 20 to 11 000 J. Similarly, the ratio of the standard deviation to the mean of the diameter of the microcrater has been found to be low at lower energy. As the energy is increased, the diameter is found to have a higher deviation, as can also be seen in Fig. 4. The above phenomenon is also evident from the SEM micrographs shown in Fig. 1(a) and (b) that the shapes of the microcrater to be more uniform with better defined rim at lower energies. Fig. 1(c)–(f) show that the diameters are irregular at higher energies. These characteristics strongly indicate that the amount of the applied discharge energy is a very important machining parameter to obtain microcraters of more regular sizes. The specific energy is the energy required to remove a unit volume of material. Fig. 5 shows that at lower discharge energy, the specific energy is low (low DE zone), when compared with that at higher-energy levels.
Cavity volume of microcrater (µm3)
7.50E+04 6.00E+04 4.50E+04 3.00E+04 1.50E+04 11000 5000 2350 1100
0.00E+00 1.0
1.5
50 23.50
2.0
2.5 Spark gap (µm)
3.0
DE (µJ)
Average ratio of SD to microcrater sizes
0.25 9.00E+04
Avg SD/Avg cavity volume Avg SD/Avg diameter
0.20
0.15
0.10
0.05
0.00 23.50
50
1100
2350
5000
11000
Discharge energy (µJ)
3.5
Fig. 3. Discharge energy vs. cavity volume of microcrater.
Fig. 4. Discharge energy vs. average ratio of standard deviation (S.D.) to microcrater size.
306
Y.S. Wong et al. / Journal of Materials Processing Technology 140 (2003) 303–307
4.3. Theoretical modeling
Average specific energy (µJ/µm 3)
0.18
In a single discharge using a RC circuit, the total singledischarge energy (Eds ) gained after charging is delivered until the energy stored in the capacitor is completely discharged through the gap. Therefore, the supplied energy (Eds ) is equal to the energy delivered to the gap (Eg ), as given in Eq. (1):
0.15
0.12
0.09
0.06
Eds = Eg = 21 CV2s
low DE zone
0.03
0 23.50
50
1100
2350
5000
11000
Discharge energy (µJ)
Fig. 5. Discharge energy vs. specific energy.
4. Efficiency of micro-EDM 4.1. Assumptions The following assumptions are made to elucidate the efficiency through a simple theoretical modeling using the experimental data: (i) The maximum amount of energy stored in the capacitor is assumed to be completely discharged. (ii) The energy is transferred to the workpiece in the form of heat. (iii) The duration time of the single-discharge is approximately equal to a single RC-pulse time and is very short for micro-EDM (less than 1 s). (iv) The enthalpies of phase transition (solid–liquid and liquid–vapor interface) are neglected. (v) The workpiece material is homogenous and isotropic. (vi) The thermo-physical properties of the materials are constant and equal in all three phases from solid to liquid to vapor, which apply over the whole temperature range. 4.2. Thermo-physical properties of SUS 304 The thermo-physical properties of stainless steel (SUS 304) are as follows: • • • • • • • • • •
density, ρ = 8000 kg/m3 ; specific heat capacity, cp = 500 J/kg ◦ C; ambient temperature, T0 = 20 ◦ C; melting point temperature, Tm = 1450 ◦ C; boiling or vaporization temperature, Tb = 3000 ◦ C; latent heat of fusion or melting, Lm = 300 kJ/kg; latent heat of vaporization, Lv = 6500 kJ/kg; volumetric heat capacity, ρcp = 4 000 000 J/m3 ◦ C; ρHv = 1.20 × 1010 J/m3 ; ρHm = 5.72 × 109 J/m3 .
(1)
A part of supplied energy from the spark forms the microcrater, which determines “erosion efficiency” (η), i.e. the ratio of the actual energy (Ee ) used to erode the microcrater to the supplied energy in the gap (Eg ) as given in Eq. (2). The remaining energy supplied between the gap will be equal to the energy lost in the anode, dielectric medium, etc. η=
Ee actual erosion energy = Eg supplied spark energy
(2)
The determination of the distribution of energy from the supplied energy is very complex and it depends upon the mechanism of material removal. When the energy is supplied, the erosion of material can occur firstly by melting and/or vaporizing as the heat is being conducted to the electrode. Thus, the actual energy used to erode the material to form a microcrater is given as described in Eqs. (3a) and (3b), the product of experimental erosion volume and the respective enthalpies of melting and vaporization per unit volume: • by considering vaporization action: Ee = Vexp ρHv
(3a)
• by considering melting action: Ee = Vexp ρHm
(3b)
The minimum energy required to vaporize the cathode material per unit volume can be estimated as given in Eq. (4a): Hv =
Tm T0
cp dT + Lm +
Tb Tm
cp dT + Lv
= cp [(Tm − T0 ) + (Tb − Tm )] + Lm + Lv Hm =
Tm
T0
cp dT + Lm = cp (Tm − T0 ) + Lm
(4a) (4b)
Prior to reaching the boiling temperature of the material, the electrodes have to reach their melting temperature. Thus, the amount of material removed due to melting is found using the minimum energy required to melt the cathode material per unit volume and is as given in Eq. (4b). Thus, the efficiency of material removal is computed using Eqs. (1), (3a) and (4a) into Eq. (2) due to vaporization action to erode the material. Table 1 indicates the values of erosion efficiency due to the vaporization and melting actions, against discharge energy at various spark gap conditions.
Y.S. Wong et al. / Journal of Materials Processing Technology 140 (2003) 303–307
307
Table 1 Erosion efficiency of discharge energies at various spark gaps (Sg ) Discharge energy (J) 23.50 50 1100 2350 5000 11000
Efficiency (ηv ) at Sg
Efficiency (ηm ) at Sg
1.0 m
1.5 m
2.0 m
2.5 m
3.0 m
3.5 m
1.0 m
1.5 m
2.0 m
2.5 m
3.0 m
3.5 m
0.575 0.626 0.086 0.094 0.070 0.080
0.405 0.472 0.078 0.073 0.072 0.093
0.224 0.469 0.078 0.080 0.075 0.076
0.272 0.673 0.096 0.067 0.074 0.073
0.418 0.603 0.081 0.071 0.071 0.076
0.000 0.367 0.075 0.046 0.077 0.092
0.274 0.298 0.041 0.045 0.033 0.038
0.193 0.225 0.037 0.035 0.034 0.044
0.107 0.224 0.037 0.038 0.036 0.036
0.130 0.321 0.046 0.032 0.035 0.035
0.199 0.287 0.039 0.034 0.034 0.036
0.000 0.175 0.036 0.022 0.037 0.044
Average erosion efficiency
Table 1 shows that at lower energies (<50 J) and spark gap distances ranging from 1 to 3.5 m, the estimated eroding efficiency is from 0.3 to 0.7 and 0.1 to 0.35, while considering the erosion mechanism as due to vaporization and only melting, respectively. At higher energies (>50 J) and spark gap ranging from 1 to 3.5 m, the estimated eroding efficiency is from 0.06 to 0.1 and 0.02 to 0.04, while considering the erosion mechanisms as due to vaporization and only melting respectively. Moreover, the efficiency at lower energies is found to be seven to eight times higher than that at higher energies. More importantly, these phenomena indicate that energy is more efficiently used to form the microcrater when the applied energy is lower. Fig. 6 indicates that the effect of the discharge energy on the average erosion efficiency, while neglecting the effect of the spark gap. It shows that 20% of material is removed by vaporizing the molten metal at lower-energy levels, if vaporization action is considered as a cause for the material removal. This estimated amount of vaporization is found to be 20 and 150 times higher than that, respectively, estimated by Lhiaubet and coworkers [4,5] and Willey [3], at very high energy levels. In addition, if the energy level is low, the pulse duration will be lesser, so the possibility of the heat transfer to the surrounding medium can be less. Thus,
the temperature could rise high at the point of discharge and may constitute to vaporization of the material.
5. Conclusions The following important conclusions are drawn from the investigation of the mechanism of material removal using the single RC-pulse discharges: • The volume and size of the microcraters are found to be more consistent at lower-energy discharges than at higher-energy discharges. • The specific energy required to remove the material is found to be significantly less at lower energies (<50 J) when compared with that at higher energies. • Using a simple theoretical model, the estimated erosion efficiency of material removal at low-energy discharges is found to be seven to eight times higher than that at higher-energy discharges. The aforementioned strongly indicate that lower-energy discharges produce more consistent size of microcrater at higher efficiency.
1.20
References
1.00
[1] T. Masuzawa, CIRP Ann. 49 (2001) 473. [2] J.L. Guerrero-Alvarez, J.E. Greene, B.F. von Turkovich, J. Eng. Ind. Ser. B 95 (1973) 965. [3] P.C.T. Willey, IEE Conf. Publ. 133 (1975) 265. [4] C. Lhiaubet, R.M. Meyer, J. Appl. Phys. 52 (1981) 3929. [5] L. Benzerga, C. Lhiaubet, R.M. Meyer, J. Appl. Phys. 58 (1985) 606. [6] R. Saha, R. Kumar, M.K. Muju, SME Technical Paper MR00-208, 2000, p. 1. [7] D.D. DiBitonto, P.T. Eubank, M.R. Patel, M.A. Barrufet, J. Appl. Phys. 66 (1989) 4095. [8] S. Vaseekaran, C.A. Brown, J. Mater. Process. Technol. 58 (1996) 70. [9] B.-J. Wang, N. Saka, E. Rabinowicz, Wear 157 (1992) 31. [10] B.-J. Wang, N. Saka, E. Rabinowicz, IEEE Trans. Comp. Hyb. Manuf. Technol. 14 (1991) 37.
eff of only melt eff of vap eff of only vap (vap-melt) eff (ideal)
0.80
0.60 0.40 0.20
0.00 23.50
50
1100
2350
5000
Discharge energy (µJ)
Fig. 6. Discharge energy vs. average efficiency.
11000