- Email: [email protected]
Fundamental Investigation of EDM Plasmas, Part I: A Comparison between Electric Discharges in Gaseous and Liquid Dielectric Media
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 330 – 335
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, Bilbao, Spain
Fundamental Investigation of EDM Plasmas, Part I: A Comparison between Electric Discharges in Gaseous and Liquid Dielectric Media Wiessnera* M., Macedoa F. T. B., Martendala C. P., Kustera F., Wegenera K. a
Institute of machine tools and manufacturing (IWF), ETH Zurich, Switzerland
* Moritz Wiessner. Tel.: +41-0-44-632-0495; fax: +41-0-44-632-1125. E-mail address:[email protected]
Abstract Electric discharge machining (EDM) in liquid dielectric media is a well-stablished manufacturing process. Dry EDM (DEDM) has been proposed as an environmentally friendlier alternative to the EDM process in oil dielectric. The working principles of EDM and DEDM are based on interactions of electric discharge plasmas with electrodes materials, which is a poorly understood phenomenon. Therefore, optimization of EDM processes is still based on empirical methods and recipes. The investigation performed in the present work uses a collisional-radiative (CR) model for advanced optical emission spectroscopy interpretation of EDM plasmas performed in oil and air as dielectric media. This analysis is additionally supported by electrical parameter and high-speed imaging measurements. These applied plasma diagnostics allow, for the first time, a detailed comparison of EDM discharge plasmas in very different dielectric media. Emission spectra simulated by the CR model give indications on several EDM discharge properties, such as density of electrons, fraction of different ionic states and plasma composition. The emission spectra simulations indicate that EDM discharges performed in oil and air have very similar electron temperatures, while the electron densities are substantially different. In addition, the fraction of EDM plasma components is estimated quantitatively, indicating that plasmas in air are mostly composed of the electrodes materials, whereas EDM plasmas in oil are dominated by species originated from the dielectric medium. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ©2018 2018The The Authors. Published by Elsevier B.V. (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: EDM; single electrical discharges; spectroscopy; high-speed imaging
1. Introduction Electrical discharge machining (EDM) in liquid dielectric is a well-established non-conventional manufacturing process. Dry electrical discharge machining (DEDM) has been presented as an environmentally friendlier alternative to EDM processes in oil-based dielectric media. The physics of EDM electric discharges in liquid or gaseous dielectric media is not well understood yet. For this reason, optimization of these manufacturing processes is still based on empirical methods and recipes. EDM electric discharges are formed in micrometer gaps, which make their plasma diagnostics very challenging. Optical emission spectroscopy, high-speed imaging and electrical parameter measurements are some of the very few methods capable to provide information on the EDM discharges, as described by Descoeudres [1].
Optical emission spectroscopy of EDM electric discharge plasmas has been applied by several researchers for over 20 years. The EDM plasma properties normally analyzed are the electron temperature by the two-line Boltzmann method described by Griem [2], and electron density from Stark effect using the theories of Gigosos et al. [3]. Nevertheless, the outcomes of these methods do not provide a detailed description of the EDM plasmas and their interactions with the electrodes materials, such as fraction of elements or proportion of ionic species. Recently, Macedo et al. [4] presented collisional-radiative models as a successful tool for advanced interpretation of optical emission spectroscopy of EDM plasmas. This extended interpretation of optical emission spectra allows determining not only of the electron temperature and electron density, but also of the atomic and ionic compositions and population of fast electrons. An advanced investigation of
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.2017.12.073
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Wiessner M. et al. / Procedia CIRP 68 (2018) 330 – 335
EDM electric discharges in gaseous and liquid dielectric media is performed in the present work in order to compare the properties of plasmas generated under these two different processing conditions. The EDM plasmas are characterized by optical emission spectroscopy interpretation supported by emission spectra simulations. In addition measurement of electrical parameter and high-speed imaging support the optical emission spectroscopy results, allowing deeper insight into the EDM plasma physics.
ݍ ܹ ܰ
Open voltage [V] Burning voltage [V] Current [A] Pulse duration [μs] Temperature of electrons [K] Density of ions [cm-3] Density of electrons [cm-3] Population densities of excited states of the plasma atomic species [cm-3] Upper atomic state Lower atomic state Sum of rate coefficients of atomic transitions Number of energy levels
2. Materials and methods Electric discharges are performed by an Agie Charmilles Form 1000 EDM machine, which is adapted to perform single discharge pulses. Discharge voltage and current are measured by voltage and DC current probes. Voltage and current shapes of the studied transistor discharges are shown in Fig. 1.
Fig. 1a. Measured electric current
Tool
Oscilloscope
Nomenclature Uopen UB I t Te ݊ ݊ ݊௭
shown in Fig. 2. The general applied working conditions are summarized in Tab. 1. The discharge is initiated by applying an open voltage and moving the tool electrode towards the workpiece until the electrical breakdown occurs, leading to discharge gap widths that vary between 5 and 10 μm.
Fig. 1b. Measured discharge voltage
An Acton Research Spectrograph 0.275 m connected to a Vision Research Phantom V12.1 high-speed camera (one million frames/s and 300 ns exposure time) is used for optical emission spectroscopy. Plasma light emission is collected by a bundle of Ø 100 μm optical fibers, which have their aperture positioned near the erosion gap and guide the plasma light into the spectrograph. The plasma emission spectra are calculated by summing light acquired by all optical fibers. Furthermore, the referred high-speed camera is also used to record and measure the EDM plasma expansion, and in the case of oil as dielectric, also the gas bubble dimensions. Copper and aluminum are used as electrode materials in the experiments. The copper point-type electrode has Ø 1 mm diameter and an extremity with conical shape, whereas the aluminum plane-type electrode has a polished flat surface, as
Plasma Workpiece
High-speed camera
Spectrometer
Computer
Fig. 2. Optical emission spectroscopy experimental setup Table 1. Electric discharge conditions Variables of the process
Working conditions
Open voltage (Uopen)
250 V
Electric current (I)
20 A
Pulse duration (t)
316 μs
Electrodes geometry
Point-to-plane configuration
Point-type electrode polarity
Positive and negative
Dielectric media
Hydrocarbon oil IME 110; Air at atmospheric pressure
2.1. Collisional-radiative models Light emitted by an electric discharge contains important information about several of its plasma parameters. Kunze [5] explains that the population densities ݊௭ ሺሻ of the excited states of atomic species of a plasma can be calculated from measurements of its emitted spectral lines by coupled rate equations. These equations, that govern the kinetics of local populations of atomic states of ions of charge ݖinvolved in the discharges, have a large number of transitions, some of them with unknown parameters. Collisional-radiative models have been developed with a set of coupled rate equations that consider only the most relevant atomic processes and pertinent time scales. The CR commercial code PrismSPECT [6] is used in the present work to support interpretation of optical emission spectroscopy of EDM and DEDM plasmas. The set of coupled rate equations used by PrismSPECT, which depend on plasma properties such as temperature and density, can be represented by ேಽ
ேಽ
ஷ
ஷ
݀݊௭ ሺሻ ൌ െ݊௭ ሺሻ ܹ ݊௭ ሺݍሻܹ ǡ ݀ݐ
(1)
ͳ ܰ where and ݍare upper and lower atomic states respectively, ܹ is the sum of rate coefficients for upward transitions,
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Wiessner M. et al. / Procedia CIRP 68 (2018) 330 – 335
ܹ is the sum of rate coefficients for downward transitions, and ܰ is the number of energy levels included in the calculations, as proposed by Chung et al. [7]. PrismSPECT calculates collisional excitation and ionization rates of plasmas considering several important processes, such as spontaneous emission, stimulated absorption, collisional excitation, radiative recombination, collisional ionization, collisional recombination, electron capture and autoionization. PrismSPECT is also able to estimate collisional excitation and ionization rates of plasmas under non-Maxwellian electron energy distributions, such as an electron beam. EDM and DEDM plasma electron temperatures are estimated by PrismSPECT from comparisons between the height of observed and synthetic Al spectral lines, whereas ion density estimations consider height and broadening of Hα emission line. In addition, regions of the plasma with distinct properties are considered separately and summed in order to obtain the complete synthetic spectrum to be matched with the experiment, as described in detail by Macedo et al. [4]. Fig. 3 shows a fit between optical emission spectra and simulated spectra of a discharge in oil. Some of its peak clusters between 400 and 550 nm could not be identified or simulated.
dependent plasma expansion is shown in Fig. 5. High-speed imaging measurements suggest that discharge plasmas in oil have a considerably larger dimension at the beginning of the discharge. Strong light emission occurs during the start-up of these discharges due to the electrical breakdown. Descoeudres [1] performed high-speed imaging of discharges in oil normalizing its light intensity. According to his investigation, the thickness of EDM plasmas in liquid just slightly varies during the whole pulse duration. Thus, the plasma thickness of discharges in oil can be considered fairly constant during the time, around 500 μm. The dynamics of the gas bubble formed around the discharges in oil can affect the material removal and generation of craters on the workpiece material, as reported by Zhang et al. [8]. The dimension of the gas bubbles is also measured by high-speed imaging in the present work. Fig. 5 presents measurements of the electric discharge plasmas in air and oil, as well as the diameters of the observed bubbles. The gas bubble dimension reaches its maximum values between 2,500 and 4,500 μm. Bubbles that achieve maximum dimensions below 2,500 μm collapse before the discharge pulse is extinguished, as can be observed in Fig. 4c and Fig. 5. Similar phenomenon was also reported by Maradia et al. [9], who named this kind of electric discharges as “unstable sparks”.
Fig. 3. Experimental and simulated spectrum for copper anode and aluminum cathode in oil dielectric at the 145 μs after the discharge ignition
3. Experiments 3.1. High-speed imaging and discharge power High-speed imaging shows that discharges performed in oil are surrounded by a gas bubble, whereas just a plasma plume is observed for discharges in air, as presented in Fig. 4. Plasma
Tool
Plasma
1 mm
Workpiece
Fig. 4a. Discharge plasma expansion in air Plasma
Workpiece
Tool
Collapsed bubble
1 mm
Fig. 4c. Discharge plasma expansion and collapsed gas bubble in oil
Workpiece
Tool
Bubble
1 mm
Fig. 4b. Discharge plasma expansion and gas bubble in oil
A plasma thickness of 540 μm is measured for discharges in air performed with a point-type copper electrode anode, whereas a thickness of 400 μm is registered when a point-type cathode is applied. The time-
Fig. 5. High-speed imaging of electric discharge plasmas in air and oil dielectric media and gas bubble
Since electric discharge energy is consumed to create the gas inside bubble, a correlation between this discharge parameter and the gas bubble dimensions can be observed. The electric current, controlled by the EDM machine, times the burning voltage (UB) gives the power that influences the formation and the temporal behavior of the bubble. The correlation between discharge power values and high-speed imaging of gas bubbles observed in Fig. 6 suggests that unstable discharges occur when a low power is detected during the first 20 μs after the ignition. Kanemaru et al. [10] reported similar correlation for capacitive discharges. It is also
Wiessner M. et al. / Procedia CIRP 68 (2018) 330 – 335
important to state that unstable discharges are not observed in air as dielectric for the working conditions applied here. Stable discharges in oil and discharges in air as dielectric are analyzed more in detail in the following.
333
distinct regions of the plasma, it can be assumed for simplification that the different components of the plasma composition are homogeneously distributed. Despite the higher electron temperatures observed during the beginning of the discharge in oil, when compared with discharges in air, the temperatures reach similar values of around 15,000 K in the plasma plume, and about 21,000 K in the hot spot region during the development of the discharges in all studied conditions, as shown in Fig. 7.
Fig. 6. Correlation between discharge power and maximum bubble dimension
3.2. Electron temperature and densities of the plasma Optical emission spectroscopy interpretation supported by emission spectra simulations reported by Macedo et al. [4] indicates that emission spectra of EDM discharges in air as dielectric cannot be fully explained assuming a single electron temperature. This suggests the presence of a temperature profile peaking at the plasma center, or the existence of nonthermal electron components within the discharge, such as an electron beam. In this part I of the paper, an electron temperature profile of the discharge plasmas in air is assumed, whereas in the part II [11] presents results considering the formation of an electron beam within the discharge. Emission spectra simulations indicate that the plasma of electric discharges performed in oil dielectric also must have a temperature profile, since their observed spectral lines cannot be emitted by plasmas with a single electron temperature. Therefore, optical emission spectroscopy interpretation of EDM discharges in gaseous and liquid dielectric media is performed in the present work according to the methods used by [4], and assuming local thermodynamic equilibrium (LTE) of the analyzed plasmas. A simplified temperature profile is adopted here assuming two temperature regions, in order to match the observed and simulated emission spectra. The EDM plasma diameter is separated in a region of higher electron temperature at the center of the plasma, which is compatible with the formation of a hot spot, and a region of lower electron temperature, defined between the hotter center of the discharge and the measured shell diameter of the plasma. The hot spot region of the plasma in both dielectric media is assumed to have a diameter Ø 80 μm, as proposed by Macedo et al. [4] for discharges in air. The thickness of the colder region of the plasma is calculated subtracting the diameter of the hot spot region from the plasma plume diameter, measured by high-speed imaging. In addition, the ion and neutral density of the plasma is assumed to be constant over the plasma cross-section. Since the optical emission spectra are obtained by summing light emitted from
Fig. 7. Electron temperature of the plasma for different electrode polarities and dielectric media
Optical emission spectra of discharges performed in oil have spectral lines with very large broadening during the first 20 μs after the ignition, which drastically decreases afterwards. The strong continuum present in the spectrum at the beginning of the discharge also drops during the time, as presented in Fig. 8. These high broadening of spectral lines and continuum are not observed for discharges in air as dielectric. The electron density of the different analyzed regions of the plasma is calculated by PrismSPECT considering the estimated ion density and plasma mean charge, as explained by Chung et al. [7].
Fig. 8. Al II and Hα spectral lines of a discharge performed with Cu anode and Al cathode in oil dielectric
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During the first microseconds of electric discharges in oil, the electron densities of the plasma reach values up to 8 times higher than the plasma of discharges in air during a similar period, as shown in Fig. 9. These electron density calculations are in agreement with the results reported by Descoeudres [1], who infer that the EDM plasma overcomes an extreme pressure imposed by the liquid dielectric in the very beginning of the discharge, leading to very high plasma densities, around 2·1018 cm-3. The high electron density achieved immediately after the discharge ignition is the reason for the broadening and merging of Al II lines (623.1 nm and 624.3 nm) beforehand highlighted in Fig. 8. The electron densities of the cold and hot plasma regions of electric discharges in oil lie close together immediately after the ignition, demerge during 50 μs, and just slight vary afterwards.
components of the dielectric, such as nitrogen and oxygen, are not significantly present in the plasma. This plasma composition is explained by deviations of the Paschen’s law for electric discharges in micrometre gaps filled with air, which take place due to a vacuum breakdown mechanism, as described by Klas et al. [12]. Furthermore, Fig. 10 shows that DEDM plasmas have a high fraction of hydrogen, which is discussed more in detail in the part II [11] of this paper. Emission spectra simulations performed in the present work indicate that discharges in oil are mostly composed of hydrogen and carbon with some metallic contamination from the electrodes, as presented in Fig. 10. Thus, differently than EDM in air as dielectric, the electric discharge performed in oil has most of its composition originated from the dielectric medium, from the cracking of highly linked hydrocarbons.
Fig. 9. Density of electrons of the plasma for different electrodes polarities and dielectric media
The electron density drops substantially during the development of the discharges in oil, becoming up to 5 times lower than the ones observed for electric discharges in air. This density reduction is in agreement with the investigation reported by Kanemaru et al. [10], who infer that the time behavior of the dielectric pressure and dynamics of the gas bubble formed around the plasma is correlated with the electron density. Since the electric discharges in air as dielectric studied in the present work are performed in atmospheric pressure, changes in their plasma properties must be related only to the used electrode materials, electrode geometries and adopted electrical parameter set. In comparison with discharges in oil, discharges in air have much more stable electron density, as one can observe in Fig. 9. Deeper analysis of discharge plasmas in air under very different processing conditions are discussed in detail in the part II [11] of this paper. 3.3. Plasma composition Contribution of different components to the composition of EDM plasmas in air as dielectric is estimated in the present work by emission spectra simulations. DEDM discharges are dominated by metallic material from the electrodes, while
Fig. 10. Fraction of discharge plasma components for different electrodes polarities and dielectric media
Emission spectra simulations indicate that discharges in air with a point-type cathode electrode are dominated by the anode material, whereas the cathode is a passive electrode. This result agrees with the optical emission spectroscopy analysis reported by Macedo et al. [13], who infer that EDM discharges performed under the referred conditions have properties very similar to a specific type of discharge, the hot anode vacuum arcs (HAVA). Furthermore, electric discharges performed under the same conditions, but with inverted polarity, have a considerable larger proportion of the pointtype anode material in the plasma. These observations suggest that the point-type anode is not an inert electrode, as also previously proposed by Macedo et al. [13]. High variation of carbon and hydrogen is observed during the time for electric discharges in oil, whereas the proportion of metallic components slightly decreases. The new insight into the EDM discharge plasma given by emission spectra simulations contradicts other researchers [1, 9], who deduced an increase of metallic composition of the plasma in oil dielectric based on observations of rising metallic lines normalized to the Hα line.
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High fraction of aluminum is observed in the plasma probably due to the low melting and evaporation points of this material, 933 K and 2,743 K, respectively. Erosion of a workpiece with higher melting and evaporation points could lead to a lower fraction of its material in the plasma composition. This hypothesis can be properly verified by a further investigation, using optical emission spectroscopy supported by emission spectra simulations. Moreover, optical emission spectroscopy was already presented by Kunieda et al. [14] as a promising tool for investigating the wear mechanisms in EDM. The method of advanced plasma analysis proposed in the present work can be used to give even more information about the tool wear mechanisms by estimating several plasma properties.
that the metallic content of EDM plasmas in oil increase during the development for the discharge. This observation was only possible due to the new advanced plasma analysis method proposed here.
4. Summary
[3]
The investigation performed in this paper makes a comparison between EDM electric discharges in gaseous and liquid dielectric media by advanced plasma diagnostics, such as high-speed imaging, optical emission spectroscopy, emission spectra simulations and electrical parameter measurements. High-speed imaging measurements indicate that electric discharges in air as dielectric have a plasma thickness not much different from electric discharges in oil as dielectric for the working conditions applied here. Emission spectra simulations suggest the presence of a temperature profile in discharges performed with the used gaseous and liquid dielectric media and electrodes polarities. Assuming regions of the plasma with distinct properties, the simulations indicate the existence of a hotter region at the center of the plasma, compatible with the formation of a hot spot, and a colder region, which corresponds to the plasma plume. The calculated electron temperatures are very similar for all applied processing conditions, around 15,000 K in the plasma plume, and about 21,000 K in the hot spot region of the plasma. Electron densities of discharges performed in oil are up to 8 times higher in the beginning of the discharge than the ones calculated for discharges in air as dielectric. Moreover, a similar temporal behavior of the gas bubble formed around the plasma in oil and the electron densities is observed. These results are in agreement with other published investigations. Electric discharges in air are dominated by metallic species from the electrodes materials, whereas discharges in oil are mainly dominated by hydrogen and carbon originating from the dielectric medium with some metallic contamination from the electrodes. In addition, the metallic content of electric discharges in oil slightly decreases during the time. This contradicts analyzes published by other researchers, who infer
Acknowledgments We would like to thank Dr. Christoph Hollenstein for his great collaboration. References [1] [2]
[4]
[5] [6]
[7] [8]
[9]
[10]
[11]
[12]
[13]
[14]
Descoeudres, A., 2006, Characterization of electrical discharge machining plasmas: EPFL. Griem, H. R., 2005, Principles of plasma spectroscopy: Cambridge University Press. Gigosos, M. A., Cardeñoso, V., 1996, New plasma diagnosis tables of hydrogen Stark broadening including ion dynamics, Journal of Physics B: Atomic, Molecular and Optical Physics, 29/20:4795. Macedo, F. T. B., Wiessner, M., Hollenstein, C., Esteves, P. M. B., Wegener, K., 2017, Fundamental investigation of dry electrical discharge machining (DEDM) by optical emission spectroscopy and its numerical interpretation, The International Journal of Advanced Manufacturing Technology, 90/9:3697-709. Kunze, H. J., 2009, Introduction to Plasma Spectroscopy: Springer Berlin Heidelberg. MacFarlane, J. J., Golovkin, I. E., Wang, P., Woodruff, P. R., Pereyra, N. A., 2007, SPECT3D – A multi-dimensional collisional-radiative code for generating diagnostic signatures based on hydrodynamics and PIC simulation output, High Energy Density Physics, 3/1–2:181-90. Chung, H., Lee, R., Chen, M., Ralchenko, Y., 2008, The How To For FLYCHK, URL: http://nlte nist gov/fly/[cit 2010-01-10]. Zhang, Y., Liu, Y., Shen, Y., Ji, R., Li, Z., Zheng, C., 2014, Investigation on the influence of the dielectrics on the material removal characteristics of EDM, Journal of Materials Processing Technology, 214/5:1052-61. Maradia, U., Hollenstein, C., Wegener, K., 2015, Temporal characteristics of the pulsed electric discharges in small gaps filled with hydrocarbon oil, Journal of Physics D: Applied Physics, 48/5:055202. Kanemaru, M., Sorimachi, S., Ibuka, S., Ishii, S., 2011, Single bubble generated by a pulsed discharge in liquids as a plasma microreactor, Plasma Sources Science and Technology, 20/3:034007. Macedo F. T. B., W. M., Bernardelli G. C., Kuster F., Wegener K., 2017, Fundamental Investigation of EDM Plasmas, Part II: Parametric Analysis of Electric Discharges in Gaseous Dielectric Medium, Procedia CIRP Manuscript submitted for publication Klas, M., Š, M., Radjenović, B., Radmilović-Radjenović, M., 2011, Experimental and theoretical studies of the breakdown voltage characteristics at micrometre separations in air, EPL (Europhysics Letters), 95/3:35002. Macedo, F. T. B., Wiessner, M., Hollenstein, C., Kuster, F., Wegener, K., 2016, Investigation of the Fundamentals of Tool Electrode Wear in Dry EDM, Procedia CIRP, 46/:55-8. Kunieda, M., Kobayashi, T., 2004, Clarifying mechanism of determining tool electrode wear ratio in EDM using spectroscopic measurement of vapor density, Journal of Materials Processing Technology, 149/1:284-8.
ScienceDirect Procedia CIRP 68 (2018) 330 – 335
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, Bilbao, Spain
Fundamental Investigation of EDM Plasmas, Part I: A Comparison between Electric Discharges in Gaseous and Liquid Dielectric Media Wiessnera* M., Macedoa F. T. B., Martendala C. P., Kustera F., Wegenera K. a
Institute of machine tools and manufacturing (IWF), ETH Zurich, Switzerland
* Moritz Wiessner. Tel.: +41-0-44-632-0495; fax: +41-0-44-632-1125. E-mail address:[email protected]
Abstract Electric discharge machining (EDM) in liquid dielectric media is a well-stablished manufacturing process. Dry EDM (DEDM) has been proposed as an environmentally friendlier alternative to the EDM process in oil dielectric. The working principles of EDM and DEDM are based on interactions of electric discharge plasmas with electrodes materials, which is a poorly understood phenomenon. Therefore, optimization of EDM processes is still based on empirical methods and recipes. The investigation performed in the present work uses a collisional-radiative (CR) model for advanced optical emission spectroscopy interpretation of EDM plasmas performed in oil and air as dielectric media. This analysis is additionally supported by electrical parameter and high-speed imaging measurements. These applied plasma diagnostics allow, for the first time, a detailed comparison of EDM discharge plasmas in very different dielectric media. Emission spectra simulated by the CR model give indications on several EDM discharge properties, such as density of electrons, fraction of different ionic states and plasma composition. The emission spectra simulations indicate that EDM discharges performed in oil and air have very similar electron temperatures, while the electron densities are substantially different. In addition, the fraction of EDM plasma components is estimated quantitatively, indicating that plasmas in air are mostly composed of the electrodes materials, whereas EDM plasmas in oil are dominated by species originated from the dielectric medium. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ©2018 2018The The Authors. Published by Elsevier B.V. (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: EDM; single electrical discharges; spectroscopy; high-speed imaging
1. Introduction Electrical discharge machining (EDM) in liquid dielectric is a well-established non-conventional manufacturing process. Dry electrical discharge machining (DEDM) has been presented as an environmentally friendlier alternative to EDM processes in oil-based dielectric media. The physics of EDM electric discharges in liquid or gaseous dielectric media is not well understood yet. For this reason, optimization of these manufacturing processes is still based on empirical methods and recipes. EDM electric discharges are formed in micrometer gaps, which make their plasma diagnostics very challenging. Optical emission spectroscopy, high-speed imaging and electrical parameter measurements are some of the very few methods capable to provide information on the EDM discharges, as described by Descoeudres [1].
Optical emission spectroscopy of EDM electric discharge plasmas has been applied by several researchers for over 20 years. The EDM plasma properties normally analyzed are the electron temperature by the two-line Boltzmann method described by Griem [2], and electron density from Stark effect using the theories of Gigosos et al. [3]. Nevertheless, the outcomes of these methods do not provide a detailed description of the EDM plasmas and their interactions with the electrodes materials, such as fraction of elements or proportion of ionic species. Recently, Macedo et al. [4] presented collisional-radiative models as a successful tool for advanced interpretation of optical emission spectroscopy of EDM plasmas. This extended interpretation of optical emission spectra allows determining not only of the electron temperature and electron density, but also of the atomic and ionic compositions and population of fast electrons. An advanced investigation of
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.2017.12.073
331
Wiessner M. et al. / Procedia CIRP 68 (2018) 330 – 335
EDM electric discharges in gaseous and liquid dielectric media is performed in the present work in order to compare the properties of plasmas generated under these two different processing conditions. The EDM plasmas are characterized by optical emission spectroscopy interpretation supported by emission spectra simulations. In addition measurement of electrical parameter and high-speed imaging support the optical emission spectroscopy results, allowing deeper insight into the EDM plasma physics.
ݍ ܹ ܰ
Open voltage [V] Burning voltage [V] Current [A] Pulse duration [μs] Temperature of electrons [K] Density of ions [cm-3] Density of electrons [cm-3] Population densities of excited states of the plasma atomic species [cm-3] Upper atomic state Lower atomic state Sum of rate coefficients of atomic transitions Number of energy levels
2. Materials and methods Electric discharges are performed by an Agie Charmilles Form 1000 EDM machine, which is adapted to perform single discharge pulses. Discharge voltage and current are measured by voltage and DC current probes. Voltage and current shapes of the studied transistor discharges are shown in Fig. 1.
Fig. 1a. Measured electric current
Tool
Oscilloscope
Nomenclature Uopen UB I t Te ݊ ݊ ݊௭
shown in Fig. 2. The general applied working conditions are summarized in Tab. 1. The discharge is initiated by applying an open voltage and moving the tool electrode towards the workpiece until the electrical breakdown occurs, leading to discharge gap widths that vary between 5 and 10 μm.
Fig. 1b. Measured discharge voltage
An Acton Research Spectrograph 0.275 m connected to a Vision Research Phantom V12.1 high-speed camera (one million frames/s and 300 ns exposure time) is used for optical emission spectroscopy. Plasma light emission is collected by a bundle of Ø 100 μm optical fibers, which have their aperture positioned near the erosion gap and guide the plasma light into the spectrograph. The plasma emission spectra are calculated by summing light acquired by all optical fibers. Furthermore, the referred high-speed camera is also used to record and measure the EDM plasma expansion, and in the case of oil as dielectric, also the gas bubble dimensions. Copper and aluminum are used as electrode materials in the experiments. The copper point-type electrode has Ø 1 mm diameter and an extremity with conical shape, whereas the aluminum plane-type electrode has a polished flat surface, as
Plasma Workpiece
High-speed camera
Spectrometer
Computer
Fig. 2. Optical emission spectroscopy experimental setup Table 1. Electric discharge conditions Variables of the process
Working conditions
Open voltage (Uopen)
250 V
Electric current (I)
20 A
Pulse duration (t)
316 μs
Electrodes geometry
Point-to-plane configuration
Point-type electrode polarity
Positive and negative
Dielectric media
Hydrocarbon oil IME 110; Air at atmospheric pressure
2.1. Collisional-radiative models Light emitted by an electric discharge contains important information about several of its plasma parameters. Kunze [5] explains that the population densities ݊௭ ሺሻ of the excited states of atomic species of a plasma can be calculated from measurements of its emitted spectral lines by coupled rate equations. These equations, that govern the kinetics of local populations of atomic states of ions of charge ݖinvolved in the discharges, have a large number of transitions, some of them with unknown parameters. Collisional-radiative models have been developed with a set of coupled rate equations that consider only the most relevant atomic processes and pertinent time scales. The CR commercial code PrismSPECT [6] is used in the present work to support interpretation of optical emission spectroscopy of EDM and DEDM plasmas. The set of coupled rate equations used by PrismSPECT, which depend on plasma properties such as temperature and density, can be represented by ேಽ
ேಽ
ஷ
ஷ
݀݊௭ ሺሻ ൌ െ݊௭ ሺሻ ܹ ݊௭ ሺݍሻܹ ǡ ݀ݐ
(1)
ͳ ܰ where and ݍare upper and lower atomic states respectively, ܹ is the sum of rate coefficients for upward transitions,
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ܹ is the sum of rate coefficients for downward transitions, and ܰ is the number of energy levels included in the calculations, as proposed by Chung et al. [7]. PrismSPECT calculates collisional excitation and ionization rates of plasmas considering several important processes, such as spontaneous emission, stimulated absorption, collisional excitation, radiative recombination, collisional ionization, collisional recombination, electron capture and autoionization. PrismSPECT is also able to estimate collisional excitation and ionization rates of plasmas under non-Maxwellian electron energy distributions, such as an electron beam. EDM and DEDM plasma electron temperatures are estimated by PrismSPECT from comparisons between the height of observed and synthetic Al spectral lines, whereas ion density estimations consider height and broadening of Hα emission line. In addition, regions of the plasma with distinct properties are considered separately and summed in order to obtain the complete synthetic spectrum to be matched with the experiment, as described in detail by Macedo et al. [4]. Fig. 3 shows a fit between optical emission spectra and simulated spectra of a discharge in oil. Some of its peak clusters between 400 and 550 nm could not be identified or simulated.
dependent plasma expansion is shown in Fig. 5. High-speed imaging measurements suggest that discharge plasmas in oil have a considerably larger dimension at the beginning of the discharge. Strong light emission occurs during the start-up of these discharges due to the electrical breakdown. Descoeudres [1] performed high-speed imaging of discharges in oil normalizing its light intensity. According to his investigation, the thickness of EDM plasmas in liquid just slightly varies during the whole pulse duration. Thus, the plasma thickness of discharges in oil can be considered fairly constant during the time, around 500 μm. The dynamics of the gas bubble formed around the discharges in oil can affect the material removal and generation of craters on the workpiece material, as reported by Zhang et al. [8]. The dimension of the gas bubbles is also measured by high-speed imaging in the present work. Fig. 5 presents measurements of the electric discharge plasmas in air and oil, as well as the diameters of the observed bubbles. The gas bubble dimension reaches its maximum values between 2,500 and 4,500 μm. Bubbles that achieve maximum dimensions below 2,500 μm collapse before the discharge pulse is extinguished, as can be observed in Fig. 4c and Fig. 5. Similar phenomenon was also reported by Maradia et al. [9], who named this kind of electric discharges as “unstable sparks”.
Fig. 3. Experimental and simulated spectrum for copper anode and aluminum cathode in oil dielectric at the 145 μs after the discharge ignition
3. Experiments 3.1. High-speed imaging and discharge power High-speed imaging shows that discharges performed in oil are surrounded by a gas bubble, whereas just a plasma plume is observed for discharges in air, as presented in Fig. 4. Plasma
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Fig. 4a. Discharge plasma expansion in air Plasma
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Fig. 4c. Discharge plasma expansion and collapsed gas bubble in oil
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Fig. 4b. Discharge plasma expansion and gas bubble in oil
A plasma thickness of 540 μm is measured for discharges in air performed with a point-type copper electrode anode, whereas a thickness of 400 μm is registered when a point-type cathode is applied. The time-
Fig. 5. High-speed imaging of electric discharge plasmas in air and oil dielectric media and gas bubble
Since electric discharge energy is consumed to create the gas inside bubble, a correlation between this discharge parameter and the gas bubble dimensions can be observed. The electric current, controlled by the EDM machine, times the burning voltage (UB) gives the power that influences the formation and the temporal behavior of the bubble. The correlation between discharge power values and high-speed imaging of gas bubbles observed in Fig. 6 suggests that unstable discharges occur when a low power is detected during the first 20 μs after the ignition. Kanemaru et al. [10] reported similar correlation for capacitive discharges. It is also
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important to state that unstable discharges are not observed in air as dielectric for the working conditions applied here. Stable discharges in oil and discharges in air as dielectric are analyzed more in detail in the following.
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distinct regions of the plasma, it can be assumed for simplification that the different components of the plasma composition are homogeneously distributed. Despite the higher electron temperatures observed during the beginning of the discharge in oil, when compared with discharges in air, the temperatures reach similar values of around 15,000 K in the plasma plume, and about 21,000 K in the hot spot region during the development of the discharges in all studied conditions, as shown in Fig. 7.
Fig. 6. Correlation between discharge power and maximum bubble dimension
3.2. Electron temperature and densities of the plasma Optical emission spectroscopy interpretation supported by emission spectra simulations reported by Macedo et al. [4] indicates that emission spectra of EDM discharges in air as dielectric cannot be fully explained assuming a single electron temperature. This suggests the presence of a temperature profile peaking at the plasma center, or the existence of nonthermal electron components within the discharge, such as an electron beam. In this part I of the paper, an electron temperature profile of the discharge plasmas in air is assumed, whereas in the part II [11] presents results considering the formation of an electron beam within the discharge. Emission spectra simulations indicate that the plasma of electric discharges performed in oil dielectric also must have a temperature profile, since their observed spectral lines cannot be emitted by plasmas with a single electron temperature. Therefore, optical emission spectroscopy interpretation of EDM discharges in gaseous and liquid dielectric media is performed in the present work according to the methods used by [4], and assuming local thermodynamic equilibrium (LTE) of the analyzed plasmas. A simplified temperature profile is adopted here assuming two temperature regions, in order to match the observed and simulated emission spectra. The EDM plasma diameter is separated in a region of higher electron temperature at the center of the plasma, which is compatible with the formation of a hot spot, and a region of lower electron temperature, defined between the hotter center of the discharge and the measured shell diameter of the plasma. The hot spot region of the plasma in both dielectric media is assumed to have a diameter Ø 80 μm, as proposed by Macedo et al. [4] for discharges in air. The thickness of the colder region of the plasma is calculated subtracting the diameter of the hot spot region from the plasma plume diameter, measured by high-speed imaging. In addition, the ion and neutral density of the plasma is assumed to be constant over the plasma cross-section. Since the optical emission spectra are obtained by summing light emitted from
Fig. 7. Electron temperature of the plasma for different electrode polarities and dielectric media
Optical emission spectra of discharges performed in oil have spectral lines with very large broadening during the first 20 μs after the ignition, which drastically decreases afterwards. The strong continuum present in the spectrum at the beginning of the discharge also drops during the time, as presented in Fig. 8. These high broadening of spectral lines and continuum are not observed for discharges in air as dielectric. The electron density of the different analyzed regions of the plasma is calculated by PrismSPECT considering the estimated ion density and plasma mean charge, as explained by Chung et al. [7].
Fig. 8. Al II and Hα spectral lines of a discharge performed with Cu anode and Al cathode in oil dielectric
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During the first microseconds of electric discharges in oil, the electron densities of the plasma reach values up to 8 times higher than the plasma of discharges in air during a similar period, as shown in Fig. 9. These electron density calculations are in agreement with the results reported by Descoeudres [1], who infer that the EDM plasma overcomes an extreme pressure imposed by the liquid dielectric in the very beginning of the discharge, leading to very high plasma densities, around 2·1018 cm-3. The high electron density achieved immediately after the discharge ignition is the reason for the broadening and merging of Al II lines (623.1 nm and 624.3 nm) beforehand highlighted in Fig. 8. The electron densities of the cold and hot plasma regions of electric discharges in oil lie close together immediately after the ignition, demerge during 50 μs, and just slight vary afterwards.
components of the dielectric, such as nitrogen and oxygen, are not significantly present in the plasma. This plasma composition is explained by deviations of the Paschen’s law for electric discharges in micrometre gaps filled with air, which take place due to a vacuum breakdown mechanism, as described by Klas et al. [12]. Furthermore, Fig. 10 shows that DEDM plasmas have a high fraction of hydrogen, which is discussed more in detail in the part II [11] of this paper. Emission spectra simulations performed in the present work indicate that discharges in oil are mostly composed of hydrogen and carbon with some metallic contamination from the electrodes, as presented in Fig. 10. Thus, differently than EDM in air as dielectric, the electric discharge performed in oil has most of its composition originated from the dielectric medium, from the cracking of highly linked hydrocarbons.
Fig. 9. Density of electrons of the plasma for different electrodes polarities and dielectric media
The electron density drops substantially during the development of the discharges in oil, becoming up to 5 times lower than the ones observed for electric discharges in air. This density reduction is in agreement with the investigation reported by Kanemaru et al. [10], who infer that the time behavior of the dielectric pressure and dynamics of the gas bubble formed around the plasma is correlated with the electron density. Since the electric discharges in air as dielectric studied in the present work are performed in atmospheric pressure, changes in their plasma properties must be related only to the used electrode materials, electrode geometries and adopted electrical parameter set. In comparison with discharges in oil, discharges in air have much more stable electron density, as one can observe in Fig. 9. Deeper analysis of discharge plasmas in air under very different processing conditions are discussed in detail in the part II [11] of this paper. 3.3. Plasma composition Contribution of different components to the composition of EDM plasmas in air as dielectric is estimated in the present work by emission spectra simulations. DEDM discharges are dominated by metallic material from the electrodes, while
Fig. 10. Fraction of discharge plasma components for different electrodes polarities and dielectric media
Emission spectra simulations indicate that discharges in air with a point-type cathode electrode are dominated by the anode material, whereas the cathode is a passive electrode. This result agrees with the optical emission spectroscopy analysis reported by Macedo et al. [13], who infer that EDM discharges performed under the referred conditions have properties very similar to a specific type of discharge, the hot anode vacuum arcs (HAVA). Furthermore, electric discharges performed under the same conditions, but with inverted polarity, have a considerable larger proportion of the pointtype anode material in the plasma. These observations suggest that the point-type anode is not an inert electrode, as also previously proposed by Macedo et al. [13]. High variation of carbon and hydrogen is observed during the time for electric discharges in oil, whereas the proportion of metallic components slightly decreases. The new insight into the EDM discharge plasma given by emission spectra simulations contradicts other researchers [1, 9], who deduced an increase of metallic composition of the plasma in oil dielectric based on observations of rising metallic lines normalized to the Hα line.
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High fraction of aluminum is observed in the plasma probably due to the low melting and evaporation points of this material, 933 K and 2,743 K, respectively. Erosion of a workpiece with higher melting and evaporation points could lead to a lower fraction of its material in the plasma composition. This hypothesis can be properly verified by a further investigation, using optical emission spectroscopy supported by emission spectra simulations. Moreover, optical emission spectroscopy was already presented by Kunieda et al. [14] as a promising tool for investigating the wear mechanisms in EDM. The method of advanced plasma analysis proposed in the present work can be used to give even more information about the tool wear mechanisms by estimating several plasma properties.
that the metallic content of EDM plasmas in oil increase during the development for the discharge. This observation was only possible due to the new advanced plasma analysis method proposed here.
4. Summary
[3]
The investigation performed in this paper makes a comparison between EDM electric discharges in gaseous and liquid dielectric media by advanced plasma diagnostics, such as high-speed imaging, optical emission spectroscopy, emission spectra simulations and electrical parameter measurements. High-speed imaging measurements indicate that electric discharges in air as dielectric have a plasma thickness not much different from electric discharges in oil as dielectric for the working conditions applied here. Emission spectra simulations suggest the presence of a temperature profile in discharges performed with the used gaseous and liquid dielectric media and electrodes polarities. Assuming regions of the plasma with distinct properties, the simulations indicate the existence of a hotter region at the center of the plasma, compatible with the formation of a hot spot, and a colder region, which corresponds to the plasma plume. The calculated electron temperatures are very similar for all applied processing conditions, around 15,000 K in the plasma plume, and about 21,000 K in the hot spot region of the plasma. Electron densities of discharges performed in oil are up to 8 times higher in the beginning of the discharge than the ones calculated for discharges in air as dielectric. Moreover, a similar temporal behavior of the gas bubble formed around the plasma in oil and the electron densities is observed. These results are in agreement with other published investigations. Electric discharges in air are dominated by metallic species from the electrodes materials, whereas discharges in oil are mainly dominated by hydrogen and carbon originating from the dielectric medium with some metallic contamination from the electrodes. In addition, the metallic content of electric discharges in oil slightly decreases during the time. This contradicts analyzes published by other researchers, who infer
Acknowledgments We would like to thank Dr. Christoph Hollenstein for his great collaboration. References [1] [2]
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