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CFD Simulation of Dielectric Fluid Flow in Micro Electro Discharge Milling Process
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 24792–24798
www.materialstoday.com/proceedings
IConAMMA_2017
CFD Simulation of Dielectric Fluid Flow in Micro Electro Discharge Milling Process S A Mullyaa*, G Karthikeyanb a
PhD Scholar, bAssistant Professor, Department of Mechanical Engineering, BITS Pilani K K Birla Goa Campus, NH-17B Zuarinagar, Goa 403725, India.
Abstract The complex phenomenon of sparking and melting in micro electro discharge milling process occurs at an interelectrode gap of dimension less than 50 microns. High-temperature plasma can melt practically any material and the material is removed from the gap in the form of debris particles. The behavior of fluid properties at interelectrode gap will be useful to elaborate material removal and effect of tool rotation in the micro electro discharge milling process. Based on previous reports it was observed that tool rotation is an inherent part of spark micromachining which directly influences the debris flushing and redeposition. For stable machining performance, removal of debris from the gap is important. Dielectric flow plays an important role in flushing away debris from the gap and cooling the electrode. This work investigates the fluid flow in the interelectrode gap. The fluid flow at interelectrode gap for different machining conditions is analyzed by computational fluid dynamics simulation. The simulation results were compared with scanning electron micrographs. The influence of inlet nozzle velocity, tool rotation and electrode gap on the dielectric fluid flow at the gap is investigated and the results are presented © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017]. Keywords: micro electro discharge milling; debris; dielectric; simulation; computational fluid dynamics.
1. Introduction Today’s world is approaching towards miniaturization. Mechanical and Electronic products are in great demand having small size and light in weight. This necessitates fine precision machining with advanced technology. During World War II physicists B.R. and N.I. Lazarenko in Moscow started the development of Electrical Discharge Machining (EDM), using controlled discharge conditions, for achieving precision machining. Since then, EDM technology has developed rapidly and become indispensable in manufacturing applications such as die and mold * Corresponding author. Tel.: +91 9545100231 E-mail address: [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017].
S A Mullya, G Karthikeyan / Materials Today: Proceedings 5 (2018) 24792–24798
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making, micro-machining, prototyping, etc. However, the phenomenon occurring at Inter-Electrode Gap (IEG) of EDM is very complex attracting researchers all over the world. Electrical discharge phenomena in EDM occur over a very short time period, in a very narrow space of few micrometers filled with a liquid dielectric, bubbles, debris thus making both observation and theoretical analysis extremely difficult [1]. Micro Electro Discharge milling (μED-milling) is a new machining technology which is similar to μEDM except that a cylindrical tool electrode is rotating to achieve the desired shape by following a programmed path. The main advantage of μED-milling is that it avoids manufacturing of complex tools required for achieving 3D profiles. High machining aspect ratio, the capability to machine any hard-conductive material and low machining cost are the advantages of μED-milling. Currently, μED-milling is mostly used for the production of microcavities with high aspect ratio and tools such as micro molds for microinjection molding [2]. In order to understand the physics of the process, it is very important to understand material removal (debris), a crater formed and dielectric fluid flow with molten metal. Accumulation of debris in discharge gaps usually causes a poor discharge, which not only causes a low material removal rate but also severely damages the machined surface [3]. Material removal occurs intermittently during or just after the discharge duration. Material removal occurs while the generated bubble is expanding, whereas no debris particle is removed while the bubble is contracting [4]. The effect of pressure drops, such as cavitation of the molten metal and degassing of solution gas in the molten metal, is one of the possible causes of the material removal [5]. Throughout the discharge duration, debris spatters at IEG. The plasma diameter expands within a few microseconds after dielectric breakdown and the plasma diameter is much larger than the discharge crater. It was found that the heat source diameter is smaller than the plasma diameter but larger than the crater diameter [6-7]. Due to resolidification of the molten material, recast layer or white layer is formed on the machined surface. The thickness of the recast layer depends upon different parameters such as peak current, pulse on time, dielectric flushing. Presence of cracks within the recast layer is due to the use of hydrocarbon-based dielectric which is rich in carbon content [8]. It is agreed that the physics of the EDM process is complex. It is a classic example of the multi-physics problem. Most of the researchers used CFD method to simulate and study the EDM process. Stefan et al. [9] used CFD simulation to study the fluid flow in grinding process and obtained the distribution of temperature, pressure, velocity and liquid volume fraction and determine the flow patterns, including useful and wasted flows. Okada et al. [10] investigated the fluid flow from the machined kerf of wire EDM. The flow field, debris motion and better jet flushing conditions of working fluid from the nozzles were analyzed by CFD simulation. Haas et al. [11] designed and analyzed dielectric injection nozzles of wire EDM process by CFD simulation for improving the cleaning process in the gap. When the spark frequency and power are high, the machining speed is governed mainly by hydrodynamics. Pontelandolfo et al. [12] investigated the dynamics of the dielectric fluid in the die sinking EDM process. Researchers have studied the use of different dielectric in μEDM and their effect on its performance. But very few studies have been reported in the literature on the fluid flow pattern over a narrow IEG of μED-milling. The pattern of fluid flow, fluid velocity determines the size and shape of debris particles, its movement and subsequent redeposition of molten metal in the gap. So, it becomes important to study the fluid behavior at IEG. The objective of this paper is to study the dielectric fluid flow pattern of μED-milling process on machining of microchannels. The fluid flow patterns and velocity profiles in the discharge gap were analyzed by computational fluid dynamics (CFD) analysis. The CFD analysis results were compared with the observed images of machining obtained using scanning electron microscope (SEM). The results are presented considering different machining conditions. Microscopic observation has shown that unlike μEDM process, μED-milling experience different flow pattern due to rotation of the tool electrode. Rotation of the electrode causes a stirring action in molten metal flow with the corresponding flushing and non-uniform deposition. 2. Dielectric Fluid Dielectric, the working fluid in μEDM plays an important role in material removal rate and the properties of the machined surface. The dielectric fluid serves various functions such as insulation, ionization, cooling of electrodes and removal of debris particles. The effectiveness depends upon the flow patterns generated, [13] and regions within
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the gap which are starved of the dielectric and/or a region where the dielectric forms eddies. In μEDM, the dielectric fluid with lower viscosity improves the machining efficiency. The low viscosity dielectric fluid influences the machining cycle time more than the hydrocarbon oils [14]. The μEDM process can be classified according to the type of dielectric fluid used. Die sink EDM generally operates with a hydrocarbon oil, while wire, micro EDM, and fast hole drilling usually use deionized water [15]. Pure kerosene, which is mostly used in μEDM, creates several problems, such as degradation of dielectric properties, pollution of air, and adhesion of carbon particles on the work surface. Deionized water can be used efficiently as an alternative to kerosene (hydrocarbon oil) [16-17]. 3. Problem Formulation 3.1. Model description Top view of a 2D model used for CFD analysis of dielectric fluid flow is shown in Fig. 1(a). Tool electrode of diameter 500 μm is rotating in a clockwise direction and fed from right to left direction. The constant interelectrode gap of 50 μm is maintained between the tool and the workpiece. The width and length of the microchannel are 600 μm and 1300 μm respectively. In conventional μEDM, electrodes are submerged under dielectric and additional dielectric enters through the nozzle. For simulation, inlet and outlet of fluid flow are considered on either side of the wall as shown in Fig. 1(a). For ease of simulation wall (partition) of 200 μm is located at the center to distinguish between inlet and outlet. The presence of wall does not affect the flow pattern near the tool electrode. Rotating tool electrode, which is solid domain meshed with quadrilateral elements and the fluid domain that is microchannel gap meshed with triangular elements as shown in Fig. 1(b). As the dimensions are in micrometers the entire domain was finely meshed to improve the accuracy of results. The fluid flow simulation of 2D geometry was done by Finite Volume method. Realizable K-Ɛ model with standard wall functions was used for simulation. The main algorithm was SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) for pressure-velocity coupling. Moving Reference Frame is used to provide rotation speed to tool electrode. Table 1. shows CFD parameters used for simulation. The effect of gravity was neglected as in the actual μED milling process; tool and workpiece are submerged under dielectric fluid. The boundary conditions used were velocity inlet, pressure outlet, and no-slip condition. The dielectric fluid used for simulation was kerosene; the properties are given in Table 2. 3.2. Analysis points Due to the rotation of the tool, the velocity of the fluid is varying along the gap. Since the fluid is continuous, selected few points are considered for analysis. Analysis points are chosen as shown in Fig. 2. The position I represent the point near to workpiece, position II at the center of the gap and position III near to tool electrode. The electrode gap is divided into an equal number of points and average values are taken. The velocity of the fluid is
(a) (b) Fig. 1. (a) Schematic diagram of 2D model of micro channel used in CFD analysis; (b) Discrete meshing of solid and fluid domain of 2D model.
S A Mullya, G Karthikeyan / Materials Today: Proceedings 5 (2018) 24792–24798
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Table 1. CFD parameters
Solver type Simulation type Velocity formulation Gradient option Turbulence model Momentum, turbulent & dissipation rate and energy discretization method Pressure-velocity coupling Residual absolute criteria Grid type and size
Pressure based 2D double precision, no gravity effect Steady-state calculations Absolute Least Squares Cell - based Realizable k- Ɛ turbulence model with standard wall functions Second order upwinding SIMPLE 10-3 for continuity, velocities, k & epsilon, 10-6 for energy 4433 quadrilateral and triangle elements.
constant along the semi-circular circumference, whereas it is varying across the gap. The CFD simulations are performed with commercial ANSYS Fluent software. The governing differential equations are Navier – Stokes equations of the flow physics solved numerically on a computational mesh. Navier – Stokes momentum equation is as follow
= . where is the density, u, v & w are the velocity components along x, y & z direction respectively, g is the gravitational acceleration, p is pressure and is the dynamic viscosity of the fluid. 4. Results and Discussion 4.1. Effect of inter electrode gap In μED-milling, the interelectrode gap (IEG) is the order of less than 50μm. As the IEG is very small, it becomes important to remove debris from the gap. Among different conventional techniques, jet flushing is effective. But when the depth of profile is high and tool diameter is small then jet pressure has to be reduced which makes it difficult to remove the debris. This makes the tool rotation as a critical parameter of machining. Rotation creates centrifugal force and agitation in the fluid. As the viscosity of the molten metal is more than the dielectric fluid it helps in flushing debris from the gap and lets fresh fluid into the gap. Molten metal removed from the gap is quenched by a dielectric to form spherical particles, called as globules. The spherical shape is due to surface tension and decrease in surface energy [18-19]. The size and shape of the globules depend upon discharge energy, crater size and amount of molten metal expelled. The molten metal removed is quenched by dielectric and creates two scenarios; in the first case when the quantity of expelled metal is more it gets deposited to the nearby surface forming a redeposited layer. In the second case when the quantity of metal is less, then stirring action caused by rotating tool breaks globules into small particles. Here for CFD simulation study, liquid kerosene is used as a dielectric medium with inlet velocity of 0.01 cm/s. The width of the microchannel is kept constant and the diameter of the electrode is changed to change the gap width. The effect of variation in the gap on the velocity of fluid was studied by considering 30, 40 and 50μm gap sizes. The speed values considered are 100, 500 and 800 rpm. The minimum value of 100 rpm is required to initiate normal discharge and 800 rpm is the limiting value above which Table 2. Properties of kerosene
Thermal Conductivity Specific Heat Viscosity Density Composition
0.149 W/m K 2090 J/kg oC 2.4 X 10-6 Pas 780 kg/m3 100% hydrocarbon Fig. 2. Analysis Points
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Fig. 3. Effect of gap size on the average velocity of the dielectric at IEG for electrode speed of (a) 100 rpm (b) 500 rpm and (c) 800 rpm.
speed does not contribute noticeably to material removal rate (MRR) and tool wear rate (TWR). A CFD simulation result shows that as the gap size decreases, the average velocity of the fluid in the gap increases. This is due to the fact that a tool is rotating; the fluid in contact with the tool will rotate at high speed. But the velocity of fluid across the gap goes on decreasing towards the work. The velocity of the fluid near to work is very small. As tool rpm increases the average velocity across the gap increases. Fig. 3 shows the effect of IEG on dielectric velocity. The significant increase in velocity happens with higher rpm as expected. But for a given position of IEG, the velocity does not seem to be affected by the variation in the gap. Thus, the fluctuation in voltage or irregularity in surface peaks which causes small variation in the gap does not affect the dielectric movement along the IEG. As the velocity of dielectric fluid near the tool electrode is high, the debris particle will be restricted to deposit on the tool surface, whereas the velocity of fluid near to work is very low due to which debris particles get deposited to the machined surface forming recast layer. Also, the velocity of the fluid and the centrifugal force acting on the globules will push them towards the surface of the stationary workpiece. This results in rough work surface and relatively smooth tool surface as shown by SEM images of work and tool surface in Fig. 4. 4.2. Effect of inlet velocity of dielectric fluid Increase in the speed of the electrode increases velocity and centrifugal force in the gap. Due to this flushing will be effective and spark position keeps on changing which will create smaller craters on the surface. The amount of metal removed depends on input energy and electrode speed. At lower rpm and lower energy level, the quantity of molten metal expelled is small, flushing is poor and the recast layer consists of the thin rod-like structure. But as energy level increases the quantity of metal expelled increases and with an increase in speed, same metal is separated to form smaller globules. The effect of inlet velocity of dielectric fluid on the velocity of the fluid in the gap is studied by using a constant gap of 50μm. The inlet velocity of 0.001, 0.01 and 0.1 cm/s were used for analysis. It was found
(a) (b) Fig. 4. SEM micrograph of (a) tool surface; (b) work surface.
Fig. 5. Average velocity of fluid at IEG for inlet velocity of 0.001, 0.01, 0.1cm/s.
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Fig. 6. Velocity path lines at different inlet conditions (a) 100 rpm, 0.1cm/s (b) 100 rpm, 0.01cm/s (c) 500 rpm, 0.001cm/s.
that there was no any considerable change in the fluid velocity in the gap due to change in inlet fluid velocity. It is due to the fact that, the tool is rotating at high speed, due to this the inlet velocity is obstructed by rotating tool and thus, the effect of inlet velocity is nullified. Fig. 5 shows the average velocity at different inlet conditions. The change in velocity from section I to section III for 800 rpm is much larger than 100 rpm. The fluid flows faster along the IEG for higher rpm ensuring faster removal of debris and globules. Rotating tool forms eddies in the fluid which acts as a stirrer in the process. It supplies fresh dielectric in the gap and removes debris particles from the gap. The size and shape of the eddy are changing due to the change in inlet velocity. Velocity path lines at different inlet conditions are shown in Fig. 6. It is clear from the figure that fluid flow pattern changes with the change in inlet velocity and tool speed. The simulation results showed that the velocity at section I is slightly less as compared to section III. Also due to rotation of tool and formation of eddies, there is an insufficient amount of dielectric flow at work surface near to rotating tool reducing the flushing effect. It is observed that there are some areas where fluid flow is not adequate. The formation of eddies similar to the one obtained through simulation is found in SEM micrograph. This indicates the common behavior of simulation and experiment. As the tool is rotating clockwise, static pressure at entry (bottom) is high as compared to exit (top). Due to this expelled molten metal will redeposit more at the bottom than at the top creating taper in the microchannel. 4.3. Effect of change in dimension of micro channel and tool electrode diameter The dimension of microchannel and electrode is changed without changing the gap width. The size of microchannel considered is 600, 500 and 400μm with corresponding tool diameter of 500, 400 and 300μm respectively. This can definitely happen in real time while machining different dimension channels with same energy conditions. For the present study, the gap width of 50μm was kept constant for all combinations. It is observed that as the size of microchannel and tool diameter reduces, average velocity in the gap decreases as shown in Fig. 7. The reason is that as the size of microchannel decreases, very small volume of fluid can enter into the channel and fill the gap. And as the gap is small it will restrict the flow of fluid in the gap. This will affect flushing of debris from the gap. Shortcircuiting or arching occurs due to inadequate removal of debris from the gap.
(a) 100 rpm
(b) 500 rpm
(c) 800 rpm
Fig. 7. Effect of tool diameter on average velocity of the fluid at IEG for electrode speed of (a) 100 (b) 500 and (c) 800 rpm.
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5. Conclusion Removal of debris particles from the interelectrode gap plays an important role in μED-milling. Flushing of the debris particles from the gap is done by the dielectric fluid. The size and shape of debris, surface texture formed depends upon fluid velocity in the gap. In this study, dielectric fluid flow pattern in the electrode gap is investigated by using CFD tool. The results of the simulation were compared with the SEM images of the machined surface. The effect of a change in electrode gap, inlet velocity of fluid, change in dimension of microchannel and tool electrode diameter on fluid behavior in the gap was investigated. The simulation results showed that as the gap size decreases, the average velocity of the fluid in the gap increases. In μED-milling, as the electrode is rotating at high speed the fluid in contact with the electrode will rotate at high speed and the speed reduces across the gap towards the stationary workpiece. The average velocity of the fluid in the gap is not affected by the change in inlet velocity, but there is a change in a fluid pattern which creates eddies. Eddies are acting as a stirrer in the process. It supplies fresh dielectric in the gap and removes debris particles from the gap. The nonuniform redeposition is observed on the milled surface which is due to the rotation of electrode; variation of fluid velocity in the gap and formation of eddies. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
M. Kunieda, B. Lauwers, K. P. Rajurkar, B. M. Schumacher, Advancing EDM through Fundamental Insight into the Process, CIRP Annals – Mfg. Tech. 54 (2005) 64 – 87. G. Karthikeyan, J. Ramkumar, S. Dhamodaran, S. Aravindan, Micro ED milling process performance: An experimental investigation, Int. J. of Machine Tools & Manufacture 50 (2010) 718-727. J. Wang, F. Han,G. Cheng, F. Zhao, Debris and bubble movements during EDM, Int. J. of Machine Tools & Manufacture 58 (2012) 11–18. J. Wang, F. Han, Simulation model of debris and bubble movement in consecutive-pulse discharge of EDM, Int. J. of Machine Tools & Manufacture 77 (2014) 56-65. S. Hayakawaa, Y. Sasakia, F. Itoigawaa, T. Nakamuraa, Relationship between occurrence of material removal and bubble expansion in EDM, Procedia CIRP 6 (2013)174-179. T. Kitamura, M. Kunieda, Clarification of EDM gap phenomena using transparent electrodes, CIRP Annals – Mfg. Tech. 63 (2014) 213216. T. Kitamura, M. Kunieda, K. Abe, High-speed imaging of EDM gap phenomena using transparent electrodes, Procedia CIRP 6 (2013) 314319. P. C. Tan, S. H. Yeo, Modeling of Recast Layer in micro EDM, J. of Mfg. Sci. and Engg.132 (2010) 031001-1. S. D. Mihic, S. Cioc, Detailed Study of Fluid and heat transfer in abrasive grinding contact using CFD method, J. of Mfg. Sci. and Engg. 135 (2013) 041002-1. A. Okada, Y. Uno, S. Onoda, S. Habib, CFD analysis of working fluid flow and debris movement in wire EDMed kerf, CIRP Annals – Mfg. Tech.58 (2009) 209-212. P. Haas, P. Pontelandolfo, R. Perez, Particle hydrodynamics of the electrical discharge machining process. Part 1: Physical considerations and wire EDM process improvement, Procedia CIRP 6 (2013) 41-46. P. Pontelandolfo, P. Haas, R. Perez, Particle hydrodynamics of the electrical discharge machining process. Part 2: Die sinking process, Procedia CIRP 6 (2013) 47-52. S. Hinduja, M. Kunieda, Modeling of ECM and EDM processes, CIRP Annals – Mfg. Tech. 62 (2013) 775-797. S. Chakraborty, V. Deya, S. K. Ghosh, A review on the use of dielectric fluids and their effects in EDM characteristics, Precision Engg. 40 (2015) 1-6. F. N. Leão, I. R. Pashby, A review on the use of environmentally-friendly dielectric fluids in EDM, J. of Materials Processing Tech.149 (2004) 341-346. Y. Zhang, Y. Liu, Y. Shen, R. Ji, Z. Li, et al., Investigation on the influence of the dielectrics on the material removal characteristics of EDM, J. of Materials Processing Tech. 214 (2014) 1052-1061. G. Kibria, B. R. Sarkar, B. B. Pradhan, B. Bhattacharyya, Comparative study of different dielectrics for micro-EDM performance during microhole machining of Ti-6Al-4V alloy, Int J of Adv. Mfg. Tech. 48 (2010) 557-570. G. Karthikeyan, A. K. Garg, J. Ramkumar, S. Dhamodaran, A microscopic investigation of machining behavior in micro ED-milling process, J. of Mfg. Processes. 14 (2012) 297-306. S. Vidya, Vijay, S. Barman, A. Chebolu, Nagahanumaiah, et al., Effects of Different Cavity Geometries on Machining performance of micro ED milling, J. of Micro- and Nano-Mfg. 3 (2015) 011007-1.
ScienceDirect Materials Today: Proceedings 5 (2018) 24792–24798
www.materialstoday.com/proceedings
IConAMMA_2017
CFD Simulation of Dielectric Fluid Flow in Micro Electro Discharge Milling Process S A Mullyaa*, G Karthikeyanb a
PhD Scholar, bAssistant Professor, Department of Mechanical Engineering, BITS Pilani K K Birla Goa Campus, NH-17B Zuarinagar, Goa 403725, India.
Abstract The complex phenomenon of sparking and melting in micro electro discharge milling process occurs at an interelectrode gap of dimension less than 50 microns. High-temperature plasma can melt practically any material and the material is removed from the gap in the form of debris particles. The behavior of fluid properties at interelectrode gap will be useful to elaborate material removal and effect of tool rotation in the micro electro discharge milling process. Based on previous reports it was observed that tool rotation is an inherent part of spark micromachining which directly influences the debris flushing and redeposition. For stable machining performance, removal of debris from the gap is important. Dielectric flow plays an important role in flushing away debris from the gap and cooling the electrode. This work investigates the fluid flow in the interelectrode gap. The fluid flow at interelectrode gap for different machining conditions is analyzed by computational fluid dynamics simulation. The simulation results were compared with scanning electron micrographs. The influence of inlet nozzle velocity, tool rotation and electrode gap on the dielectric fluid flow at the gap is investigated and the results are presented © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017]. Keywords: micro electro discharge milling; debris; dielectric; simulation; computational fluid dynamics.
1. Introduction Today’s world is approaching towards miniaturization. Mechanical and Electronic products are in great demand having small size and light in weight. This necessitates fine precision machining with advanced technology. During World War II physicists B.R. and N.I. Lazarenko in Moscow started the development of Electrical Discharge Machining (EDM), using controlled discharge conditions, for achieving precision machining. Since then, EDM technology has developed rapidly and become indispensable in manufacturing applications such as die and mold * Corresponding author. Tel.: +91 9545100231 E-mail address: [email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA 2017].
S A Mullya, G Karthikeyan / Materials Today: Proceedings 5 (2018) 24792–24798
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making, micro-machining, prototyping, etc. However, the phenomenon occurring at Inter-Electrode Gap (IEG) of EDM is very complex attracting researchers all over the world. Electrical discharge phenomena in EDM occur over a very short time period, in a very narrow space of few micrometers filled with a liquid dielectric, bubbles, debris thus making both observation and theoretical analysis extremely difficult [1]. Micro Electro Discharge milling (μED-milling) is a new machining technology which is similar to μEDM except that a cylindrical tool electrode is rotating to achieve the desired shape by following a programmed path. The main advantage of μED-milling is that it avoids manufacturing of complex tools required for achieving 3D profiles. High machining aspect ratio, the capability to machine any hard-conductive material and low machining cost are the advantages of μED-milling. Currently, μED-milling is mostly used for the production of microcavities with high aspect ratio and tools such as micro molds for microinjection molding [2]. In order to understand the physics of the process, it is very important to understand material removal (debris), a crater formed and dielectric fluid flow with molten metal. Accumulation of debris in discharge gaps usually causes a poor discharge, which not only causes a low material removal rate but also severely damages the machined surface [3]. Material removal occurs intermittently during or just after the discharge duration. Material removal occurs while the generated bubble is expanding, whereas no debris particle is removed while the bubble is contracting [4]. The effect of pressure drops, such as cavitation of the molten metal and degassing of solution gas in the molten metal, is one of the possible causes of the material removal [5]. Throughout the discharge duration, debris spatters at IEG. The plasma diameter expands within a few microseconds after dielectric breakdown and the plasma diameter is much larger than the discharge crater. It was found that the heat source diameter is smaller than the plasma diameter but larger than the crater diameter [6-7]. Due to resolidification of the molten material, recast layer or white layer is formed on the machined surface. The thickness of the recast layer depends upon different parameters such as peak current, pulse on time, dielectric flushing. Presence of cracks within the recast layer is due to the use of hydrocarbon-based dielectric which is rich in carbon content [8]. It is agreed that the physics of the EDM process is complex. It is a classic example of the multi-physics problem. Most of the researchers used CFD method to simulate and study the EDM process. Stefan et al. [9] used CFD simulation to study the fluid flow in grinding process and obtained the distribution of temperature, pressure, velocity and liquid volume fraction and determine the flow patterns, including useful and wasted flows. Okada et al. [10] investigated the fluid flow from the machined kerf of wire EDM. The flow field, debris motion and better jet flushing conditions of working fluid from the nozzles were analyzed by CFD simulation. Haas et al. [11] designed and analyzed dielectric injection nozzles of wire EDM process by CFD simulation for improving the cleaning process in the gap. When the spark frequency and power are high, the machining speed is governed mainly by hydrodynamics. Pontelandolfo et al. [12] investigated the dynamics of the dielectric fluid in the die sinking EDM process. Researchers have studied the use of different dielectric in μEDM and their effect on its performance. But very few studies have been reported in the literature on the fluid flow pattern over a narrow IEG of μED-milling. The pattern of fluid flow, fluid velocity determines the size and shape of debris particles, its movement and subsequent redeposition of molten metal in the gap. So, it becomes important to study the fluid behavior at IEG. The objective of this paper is to study the dielectric fluid flow pattern of μED-milling process on machining of microchannels. The fluid flow patterns and velocity profiles in the discharge gap were analyzed by computational fluid dynamics (CFD) analysis. The CFD analysis results were compared with the observed images of machining obtained using scanning electron microscope (SEM). The results are presented considering different machining conditions. Microscopic observation has shown that unlike μEDM process, μED-milling experience different flow pattern due to rotation of the tool electrode. Rotation of the electrode causes a stirring action in molten metal flow with the corresponding flushing and non-uniform deposition. 2. Dielectric Fluid Dielectric, the working fluid in μEDM plays an important role in material removal rate and the properties of the machined surface. The dielectric fluid serves various functions such as insulation, ionization, cooling of electrodes and removal of debris particles. The effectiveness depends upon the flow patterns generated, [13] and regions within
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the gap which are starved of the dielectric and/or a region where the dielectric forms eddies. In μEDM, the dielectric fluid with lower viscosity improves the machining efficiency. The low viscosity dielectric fluid influences the machining cycle time more than the hydrocarbon oils [14]. The μEDM process can be classified according to the type of dielectric fluid used. Die sink EDM generally operates with a hydrocarbon oil, while wire, micro EDM, and fast hole drilling usually use deionized water [15]. Pure kerosene, which is mostly used in μEDM, creates several problems, such as degradation of dielectric properties, pollution of air, and adhesion of carbon particles on the work surface. Deionized water can be used efficiently as an alternative to kerosene (hydrocarbon oil) [16-17]. 3. Problem Formulation 3.1. Model description Top view of a 2D model used for CFD analysis of dielectric fluid flow is shown in Fig. 1(a). Tool electrode of diameter 500 μm is rotating in a clockwise direction and fed from right to left direction. The constant interelectrode gap of 50 μm is maintained between the tool and the workpiece. The width and length of the microchannel are 600 μm and 1300 μm respectively. In conventional μEDM, electrodes are submerged under dielectric and additional dielectric enters through the nozzle. For simulation, inlet and outlet of fluid flow are considered on either side of the wall as shown in Fig. 1(a). For ease of simulation wall (partition) of 200 μm is located at the center to distinguish between inlet and outlet. The presence of wall does not affect the flow pattern near the tool electrode. Rotating tool electrode, which is solid domain meshed with quadrilateral elements and the fluid domain that is microchannel gap meshed with triangular elements as shown in Fig. 1(b). As the dimensions are in micrometers the entire domain was finely meshed to improve the accuracy of results. The fluid flow simulation of 2D geometry was done by Finite Volume method. Realizable K-Ɛ model with standard wall functions was used for simulation. The main algorithm was SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) for pressure-velocity coupling. Moving Reference Frame is used to provide rotation speed to tool electrode. Table 1. shows CFD parameters used for simulation. The effect of gravity was neglected as in the actual μED milling process; tool and workpiece are submerged under dielectric fluid. The boundary conditions used were velocity inlet, pressure outlet, and no-slip condition. The dielectric fluid used for simulation was kerosene; the properties are given in Table 2. 3.2. Analysis points Due to the rotation of the tool, the velocity of the fluid is varying along the gap. Since the fluid is continuous, selected few points are considered for analysis. Analysis points are chosen as shown in Fig. 2. The position I represent the point near to workpiece, position II at the center of the gap and position III near to tool electrode. The electrode gap is divided into an equal number of points and average values are taken. The velocity of the fluid is
(a) (b) Fig. 1. (a) Schematic diagram of 2D model of micro channel used in CFD analysis; (b) Discrete meshing of solid and fluid domain of 2D model.
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Table 1. CFD parameters
Solver type Simulation type Velocity formulation Gradient option Turbulence model Momentum, turbulent & dissipation rate and energy discretization method Pressure-velocity coupling Residual absolute criteria Grid type and size
Pressure based 2D double precision, no gravity effect Steady-state calculations Absolute Least Squares Cell - based Realizable k- Ɛ turbulence model with standard wall functions Second order upwinding SIMPLE 10-3 for continuity, velocities, k & epsilon, 10-6 for energy 4433 quadrilateral and triangle elements.
constant along the semi-circular circumference, whereas it is varying across the gap. The CFD simulations are performed with commercial ANSYS Fluent software. The governing differential equations are Navier – Stokes equations of the flow physics solved numerically on a computational mesh. Navier – Stokes momentum equation is as follow
= . where is the density, u, v & w are the velocity components along x, y & z direction respectively, g is the gravitational acceleration, p is pressure and is the dynamic viscosity of the fluid. 4. Results and Discussion 4.1. Effect of inter electrode gap In μED-milling, the interelectrode gap (IEG) is the order of less than 50μm. As the IEG is very small, it becomes important to remove debris from the gap. Among different conventional techniques, jet flushing is effective. But when the depth of profile is high and tool diameter is small then jet pressure has to be reduced which makes it difficult to remove the debris. This makes the tool rotation as a critical parameter of machining. Rotation creates centrifugal force and agitation in the fluid. As the viscosity of the molten metal is more than the dielectric fluid it helps in flushing debris from the gap and lets fresh fluid into the gap. Molten metal removed from the gap is quenched by a dielectric to form spherical particles, called as globules. The spherical shape is due to surface tension and decrease in surface energy [18-19]. The size and shape of the globules depend upon discharge energy, crater size and amount of molten metal expelled. The molten metal removed is quenched by dielectric and creates two scenarios; in the first case when the quantity of expelled metal is more it gets deposited to the nearby surface forming a redeposited layer. In the second case when the quantity of metal is less, then stirring action caused by rotating tool breaks globules into small particles. Here for CFD simulation study, liquid kerosene is used as a dielectric medium with inlet velocity of 0.01 cm/s. The width of the microchannel is kept constant and the diameter of the electrode is changed to change the gap width. The effect of variation in the gap on the velocity of fluid was studied by considering 30, 40 and 50μm gap sizes. The speed values considered are 100, 500 and 800 rpm. The minimum value of 100 rpm is required to initiate normal discharge and 800 rpm is the limiting value above which Table 2. Properties of kerosene
Thermal Conductivity Specific Heat Viscosity Density Composition
0.149 W/m K 2090 J/kg oC 2.4 X 10-6 Pas 780 kg/m3 100% hydrocarbon Fig. 2. Analysis Points
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Fig. 3. Effect of gap size on the average velocity of the dielectric at IEG for electrode speed of (a) 100 rpm (b) 500 rpm and (c) 800 rpm.
speed does not contribute noticeably to material removal rate (MRR) and tool wear rate (TWR). A CFD simulation result shows that as the gap size decreases, the average velocity of the fluid in the gap increases. This is due to the fact that a tool is rotating; the fluid in contact with the tool will rotate at high speed. But the velocity of fluid across the gap goes on decreasing towards the work. The velocity of the fluid near to work is very small. As tool rpm increases the average velocity across the gap increases. Fig. 3 shows the effect of IEG on dielectric velocity. The significant increase in velocity happens with higher rpm as expected. But for a given position of IEG, the velocity does not seem to be affected by the variation in the gap. Thus, the fluctuation in voltage or irregularity in surface peaks which causes small variation in the gap does not affect the dielectric movement along the IEG. As the velocity of dielectric fluid near the tool electrode is high, the debris particle will be restricted to deposit on the tool surface, whereas the velocity of fluid near to work is very low due to which debris particles get deposited to the machined surface forming recast layer. Also, the velocity of the fluid and the centrifugal force acting on the globules will push them towards the surface of the stationary workpiece. This results in rough work surface and relatively smooth tool surface as shown by SEM images of work and tool surface in Fig. 4. 4.2. Effect of inlet velocity of dielectric fluid Increase in the speed of the electrode increases velocity and centrifugal force in the gap. Due to this flushing will be effective and spark position keeps on changing which will create smaller craters on the surface. The amount of metal removed depends on input energy and electrode speed. At lower rpm and lower energy level, the quantity of molten metal expelled is small, flushing is poor and the recast layer consists of the thin rod-like structure. But as energy level increases the quantity of metal expelled increases and with an increase in speed, same metal is separated to form smaller globules. The effect of inlet velocity of dielectric fluid on the velocity of the fluid in the gap is studied by using a constant gap of 50μm. The inlet velocity of 0.001, 0.01 and 0.1 cm/s were used for analysis. It was found
(a) (b) Fig. 4. SEM micrograph of (a) tool surface; (b) work surface.
Fig. 5. Average velocity of fluid at IEG for inlet velocity of 0.001, 0.01, 0.1cm/s.
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Fig. 6. Velocity path lines at different inlet conditions (a) 100 rpm, 0.1cm/s (b) 100 rpm, 0.01cm/s (c) 500 rpm, 0.001cm/s.
that there was no any considerable change in the fluid velocity in the gap due to change in inlet fluid velocity. It is due to the fact that, the tool is rotating at high speed, due to this the inlet velocity is obstructed by rotating tool and thus, the effect of inlet velocity is nullified. Fig. 5 shows the average velocity at different inlet conditions. The change in velocity from section I to section III for 800 rpm is much larger than 100 rpm. The fluid flows faster along the IEG for higher rpm ensuring faster removal of debris and globules. Rotating tool forms eddies in the fluid which acts as a stirrer in the process. It supplies fresh dielectric in the gap and removes debris particles from the gap. The size and shape of the eddy are changing due to the change in inlet velocity. Velocity path lines at different inlet conditions are shown in Fig. 6. It is clear from the figure that fluid flow pattern changes with the change in inlet velocity and tool speed. The simulation results showed that the velocity at section I is slightly less as compared to section III. Also due to rotation of tool and formation of eddies, there is an insufficient amount of dielectric flow at work surface near to rotating tool reducing the flushing effect. It is observed that there are some areas where fluid flow is not adequate. The formation of eddies similar to the one obtained through simulation is found in SEM micrograph. This indicates the common behavior of simulation and experiment. As the tool is rotating clockwise, static pressure at entry (bottom) is high as compared to exit (top). Due to this expelled molten metal will redeposit more at the bottom than at the top creating taper in the microchannel. 4.3. Effect of change in dimension of micro channel and tool electrode diameter The dimension of microchannel and electrode is changed without changing the gap width. The size of microchannel considered is 600, 500 and 400μm with corresponding tool diameter of 500, 400 and 300μm respectively. This can definitely happen in real time while machining different dimension channels with same energy conditions. For the present study, the gap width of 50μm was kept constant for all combinations. It is observed that as the size of microchannel and tool diameter reduces, average velocity in the gap decreases as shown in Fig. 7. The reason is that as the size of microchannel decreases, very small volume of fluid can enter into the channel and fill the gap. And as the gap is small it will restrict the flow of fluid in the gap. This will affect flushing of debris from the gap. Shortcircuiting or arching occurs due to inadequate removal of debris from the gap.
(a) 100 rpm
(b) 500 rpm
(c) 800 rpm
Fig. 7. Effect of tool diameter on average velocity of the fluid at IEG for electrode speed of (a) 100 (b) 500 and (c) 800 rpm.
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5. Conclusion Removal of debris particles from the interelectrode gap plays an important role in μED-milling. Flushing of the debris particles from the gap is done by the dielectric fluid. The size and shape of debris, surface texture formed depends upon fluid velocity in the gap. In this study, dielectric fluid flow pattern in the electrode gap is investigated by using CFD tool. The results of the simulation were compared with the SEM images of the machined surface. The effect of a change in electrode gap, inlet velocity of fluid, change in dimension of microchannel and tool electrode diameter on fluid behavior in the gap was investigated. The simulation results showed that as the gap size decreases, the average velocity of the fluid in the gap increases. In μED-milling, as the electrode is rotating at high speed the fluid in contact with the electrode will rotate at high speed and the speed reduces across the gap towards the stationary workpiece. The average velocity of the fluid in the gap is not affected by the change in inlet velocity, but there is a change in a fluid pattern which creates eddies. Eddies are acting as a stirrer in the process. It supplies fresh dielectric in the gap and removes debris particles from the gap. The nonuniform redeposition is observed on the milled surface which is due to the rotation of electrode; variation of fluid velocity in the gap and formation of eddies. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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