- Email: [email protected]
Effect of Power Source in ECDM Process with FEM Modeling
Available online at www.sciencedirect.com
ScienceDirect Procedia Technology 25 (2016) 1175 – 1181
Global Colloquium in Recent Advancement and Effectual Researches in Engineering, Science and Technology (RAEREST 2016)
Effect of Power Source in ECDM Process with FEM Modeling Lijo Paula* , Libin V Koraha Department of Mechanical Engineering. St.Joseph’s college of Engineering and Technology, Pala,Kottayam, Kerala, India 656879
Abstract Electro Chemical Discharge Machining (ECDM) is a hybrid non-conventional manufacturing process which combines the features of Electro Chemical Machining (ECM) and Electro Discharge Machining (EDM). This process is important since it can machine a variety of materials including non-conducting materials like ceramics, composites, alumina, glass, etc. This paper attempts to develop a thermal model of ECDM process for determining the Material Removal Rate (MRR) with direct current (DC) and pulse DC.Comparing both DC and pulse DC on the bases of temperature variations and material removal rate, pulse DC is found to be better than DC with lower temperature range and higher MRR. ©©2016 Published by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license 2015The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of RAEREST 2016. Peer-review under responsibility of the organizing committee of RAEREST 2016 Keywords: Electro Chemical Discharge Machining; Material Removal Rate; Pulse DC; Finite Element Modelling
1. Introduction Electrochemical discharge machining (ECDM) is first introduced by Kurafuji and Suda [1], during glass micromachining [2]. Glass micro-machining was the first application and remains so far the best mastered technique but other non-conducting materials can also be machined(e.g., granite, refractory fire-brick, aluminium oxide, plexiglas, quartz) [3]. Similar results are reported for ceramic materials [4] and quartz [5]. ECDM process consists of an electrochemical cell as shown in Fig 1. The glass sample to be machined is dipped in the electrolyte and voltage is applied between the tool electrode (cathode) and the counter electrode (anode). The two electrodes are separated from each other. The tool electrode has smaller surface are than the counter electrode.
* Corresponding author. Tel.: 09846270462; E-mail address: [email protected]
2212-0173 © 2016 The Authors. Published by Elsevier Ltd. 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 organizing committee of RAEREST 2016 doi:10.1016/j.protcy.2016.08.236
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Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
Fig .1 .ECDM machining chamber
Both electrodes are connected to a DC power supply, causing bubbles to be produced on both electrodes due to electrochemical reactions[6]. When applied voltage is higher than the critical value (typically 25 V [7]), the bubbles become dense and coalesce into gas films[8,9] on the tool electrode. The gas films allows sparking between the tool electrode and electrolyte by isolating the tool from the surrounding electrolyte. If the workpiece is brought close to the tool, material removal occurs. The material removal mechanism of ECDM is a combination of thermal melting and chemical dissolution[9,10,11]. Basak et al. [12] estimated that a typical spark in ECDM contain 2,000J/cm2 of power and lasts for 100 µs. This intensive energy input causes a sudden temperature rise in the local area of the workpiece, thus causing the material removal by thermal melting. Meanwhile the alkaline in the electolyte etches silicate in glass [10,6]. This chemical reaction is enhanced by the temperature rise as well[13]. Lijo and Somashekhar [14-15] had conducted Response Surface Modeling (RSM) of ECDM with pulsating DC to understand the effect of various process parameters on micro hole and micro channel machining for Material Removal Rate (MRR), Tool Wear Rate (TWR), Heat Affected Zone (HAZ) and Radius of Overcut (ROC). They found that that temperature of the electrolyte effect micro machining. MRR decreases at higher depth due to lower spark which was due to unavailability of electrolyte . Lijo and Somashekhar [16-17] have also carried out multi objective optimization with grey relational analysis. They have made micro channels with optimized values. They have reported that haeat affected zone in micro channles can be reduced with low duty factor during machining. Lijo et al. [18] also reported that higher frequency at low duty factor will increase MRR with reduction in HAZ. Duty factor is found to be a predominant factor affecting the machining characteristics. Ranganayakulu et al. [19]developed an ECDM setup for micromachining of acrylic plate. The soft computing technique of Artificial Neuro Fuzzy Inference System (ANFIS) was used for modeling and found that voltage had predominant effect on MRR than electrolyte concentration and tool feed rate. The heat generated by the electrochemical discharges rises the temperature in the machining zone up to 5006000C [20]. At this high temperature the electrolyte is in the form of molten NaOH. The alkaline solution attacks the glass and is hydrated and totally dissolved by breaking the Si-0-Si bond on the surface of the glass. ܱܪെ ݅ܵ ؠെ ܱ െ ܵ݅ ؠ՜ܱ݅ܵ ؠെ ܪܱ݅ܵ ؠ
(1)
Followed by this chemical etching and occurs according to the following equation. ʹ ܪܱܽܰݔ ܱ݅ܵݔଶ ՜ ܱ݅ܵʹܽܰݔଷ ܪݔଶ ܱ The product of this reaction will be removed by the electrolyte outside the machining zone.
(2)
Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
ECDM drilling is one of most common type of machining process. Drilling characterized by two regimes [21] as discharge region and hydrodynamic regime. During the discharge regime the drilling is fast (up to 100 microns/s) and controlled by the number of discharges (applied voltage). For higher depth, drilling become slower (typically around 10-15 microns) and is nearly independent of applied voltage this behavior occurs during the hydrodynamic regime. Despite the considerable amount of research, which has been reported on the theoretical aspects and laboratory experiments, there are many aspects of the process control issues that are yet to be investigated for the achievement of improved productivity of the ECDM process. From the literature survey it is found that ECDM is at research level only and most of the works are experimental. Few theoretical papers are available related to determination of MRR. Basak et al[10] have given analytical model of MRR with the experimental validation, but the basic theory of spark generation proposed by them does not match with the actual situation. Most of the researches have considered uniformly distributed heat source within a spark. This assumption is far from reality. This factor is evidenced from the actual shape of a crater found by Kulkarni et al [22]. Heat flux distribution within a spark is assumed as Gaussian. In this paper a temperature variations and material removal rate are presented with DC and pulse DC for improving the efficiency of ECDM process. Nomenclature ECDM Electro chemical discharge machining MRR Material Removal Rate FEM Finite element modelling 2. Modeling ECDM is complex machining process hence its modeling requires certain assumptions. To make the analysis of ECDM following assumptions are considered [21]. 1. Workpiece material is assumed to be homogeneous and isotropic. 2. It is a single spark phenomenon. 3. Spark location is randomly distributed on tool surface immersed in electrolyte. 4. All sparks are considered to be identical during the whole machining period. 5. Only a fraction total sparking, heat flux is dissipated in to the workpiece shape of heat flux is assumed to be Gaussian distributed. From the experimental studies of Kulkarni et al[22] for single spark, the heat affected zone is circular and the crater is dome shaped. So reflecting the shape crater, we can approximate the nature of the heat flux as Gaussian. 6. The process condition during machining is kept constant. 7. Material removal by cavitation effect is neglected. All material removal is caused by thermal melting and chemical dissolution in ECDM is dependent on the temperature rise caused by sparks. 3. Thermal modeling Commercial finite element simulation software ANSYS 13 was employed to predict the material removal by direct DC and pulse DC. Governing equation for calculation of transient temperature distribution within the workpiece which is assumed to be axisymmetric is given by ଵ
డ்
డ
݇ሾ ቀݎ
ቁ
డమ ் డ௫
ሿ ൌ ܿ
డ் డ௧
(3)
Where k, c, and p are thermal conductivity, specific heat capacity and density of the workpiece material, respectively. The initial condition of ECDM process (t=0), the workpiece is immersed in the electrolyte and the
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Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
temperature of the whole domain is assumed to be at room temperature ( ܶ ). Figure 2 shows the important boundary conditions in this thermal modeling [21] . They are Boundary ܵଶ and ܵଷ are considered to be insulated. Heating process is taken to be axisymmetric about the axis of a spark, so heat flowing from the counter part of the domain is equal to the heat flowing to the counterpart. Therefore the net heat loss or gain is absolutely zero on surfaceܵଵ . 3. On surface ܵସ , where a spark occurs (AE), heat flux boundary condition is applied. Based on shape of machined surface, in this work a Gaussian heat flux distribution is taken and heat flux calculation expression is derived 1. 2.
ݓݍሺݎሻ ൌ
ͶǤͶͷܴܿܫ ܸܿ ݓ ߨʹݎ
ݎ
exp ሼെͶǤͷሺ ሻ2} ܴ
(4)
Where r is the radial distance from the axis of the spark, R w is the spark radius, Vc is the critical voltage and Ic is the critical current at which spark occurs.
Fig 2.Thermal model of ECDM process
4. FEM Modeling Mainly three steps are carried out on this FEM modeling as preprocessing, solution and postprocessing. In preprocessing borosilicate glass workpiece is prepared. The thermal properties of borosilicate glass used in model are listed Table 1. Table 1. Thermal properties of borosilicate glass Item
Value 3
Mass density p (kg/m )
2180
Specific heat C (J/kgK)
775
Thermal conductivity k (Wm/K)
1.13
The size of the workpiece is taken as 400*200 mm2 and mesh size is taken as 5 µm. Then the equation models as following in equation (5) qw (r)= 62954620*exp(-2*108({X}-C1)2)
(5)
Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
In this modelling the analysis type is taken as a transient and uniform temperature is taken as a 305 K. The second stage of modelling is solution stage and at this stage the thermal heat flux is applied on top nodes of the workpiece. In pulse DC time for 1 pulse is 3 µs and 133 pulse is taken. In DC pulse is given for 400 µs and the vector plot for the same is shown in Fig.3. Maximum temperature obtained at the nodal points is found to be 1165 K. Figure 4 shows the vector plot for temperature variation in pulse Dc mode and maximum temperature at nodal point is found to be 1173 K which lower than DC. The temperature variation with time at different node points in pulse DC is given in Fig.5. From the graph it is found that temperature varies randomly across the nodes on the surface. This is basically due to the non conducting nature of the borosilicate glass.
Fig. 3. Temperature Plot with Direct Current
Fig. 4. Temperature plot with Pulsed Current
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Fig.5. Comparison of temperature with different nodes for pulse DC model
5. Result & Discussion In case of pulse DC the maximum temperature is generated as 1173 0 C, and the maximum temperature is generated on DC is 11650 C. By comparing both the cases we can see that the temperature lower in pulse DC than DC. This is due to the intermittent cooling due to continuous forming and breaking of the spark due to pulse DC. The Material Removal Rate (MRR) is found for DC and pulse DC with the help of MATLAB software using curve fitting tool. The volume of curve is calculated using equation (6). ோ
V= - ʹ ߨfxdx
(6)
Using equation (6) in case of DC the MRR is found to be 0.4 mg/min and in case of pulse DC the MRR is found to be 1.07 mg/min. By the analysis of this result we can find that the material removal rate in case of pulse DC is higher than DC with lower temperature in pulse DC. So in the case of pulse DC most of the heat from the tool is used for melting and vaporization of the workpiece. So pulse DC is found to be better than DC in terms of higher MRR and lower temperature in machining zone. 6. Conclusion Micro machining of borosilicate glass is modeled using ANSYS 13 for direct current and pulsed DC power source. The MRR in FEM model is quantified with curve fitting tool in MATLAB software. The MRR for DC is found to be 0.4 mg/min and pulse DC is 1.07gm/min. So same power source MRR in pulse DC is found to be higher than DC. So in pulse DC most of heat produced is used to remove the material through heating melting and vaporization. The tool life also improved with cooling in pulse DC. The heat affected zone in pulse DC lower than DC, due to short pulse duration, with intermittent cooling than DC.
Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
References [1] Kurafuji, H., Suda, K. Electrical discharge drilling of glass. Annals of the CIRP 1986;16:415-419. [2] Wuthrich, R., and Fascio, V. Machining of non-conducting materials using electrochemical discharge phenomenon an overview. International Journal of Machine Tool and Manufacture 2005;45:1095-1108 [3] Cook, N. H., Foote, G. B., Jordan, P. Experimental studies in electro-machining. Journal of Engineering for Industry –Transactions of the ASME 1973;96:945-950. [4]Tokura, H., Kondoh, I., Yoshikswa, M. Machining a Glass Rod with a Lathe –Type Electro-Chemical Discharge Machine. Journal of Materials Science1989; 24:991. [5] Jain, V.K. , Chak, S.K. Electrochemical Spark Trepanning of Alumina and Quartz,” Machining Science and Technology 2000;4:277-290 [6] Jain VK, Adhikary S. On the mechanism of material removal in electrochemical spark machining of quartz under different polarity conditions. J Mater Process Technol 2008;200:460–470. [7] Fascio V, Wüthrich R, Bleuler H. Spark assisted chemical engraving in the light of electrochemistry. Electrochim Acta 2004;49:3997–4003. [8] Wüthrich R, Hof LA. The gas film in spark assiste dchemical engraving (SACE)—a key element for micromachining applications. Int J Mach Tools Manuf 2006;46:828–835. [9] Cheng CP, Wu KL, Mai CC, Yang CK, Hsu YS, Yan BH. Study of gas film quality in electrochemical discharge machining. Int J Mach Tools Manuf 2010;50:689–697 [10] Basak I, Ghosh A. Mechanism of material removal in electrochemical discharge machining: a theoretical model and experimental verification. J Mater Process Technol 1997;71:350–359. [11] Yang CT, Ho SS, Yan BH. Micro hole machining of borosilicate glass through electrochemical discharge machining (ECDM). Key Eng Mater 2001;196:149–166. [12] Jalali M, Maillard P, Wüthrich R. Toward a better understanding of glass gravity-feed micro-hole drilling with electrochemical discharges. J Micromech Microeng 2009;19:045001. [13] Panda MC, Yadava V. Finite element prediction of material removal rate due to traveling wire electrochemical spark machining. Int J Adv Manuf Technol 2009;45:506–520. [14] Lijo, P. and Somashekhar,S.H. Response Surface Modeling of Micro Holes in Electrochemical Discharge Machining Process, Procedia Engineering 2013;64:1395 –1404. [15] Lijo, P. and Somashekhar, S.H. Characterization of Micro Channels in Electrochemical Discharge Machining Process. Applied Mechanics and Materials 2014;490-491:238-242. [16] Lijo P. and Somashekhar S. H. Evaluation of Process Parameters of ECDM using Grey Relational Analysis. Procedia Materials Science 2014;5:2273-2282. [17] Lijo P. and Somashekhar S. H. Effect of process parameters on heat affected zone in micro machining of borosilicate glass using µ-ECDM process. Applied Mechanics and Materials 2014;592:224-238. [18] Lijo, P. , Somashekhar, S.H. and Ranganayakulu, J. Experimental investigation and parametric analysis of electrochemical discharge machining, Int. J Manufacturing Technology and Management 2014;28:57ϋ79. [19] Ranganayakulu, J., Somashekhar, S.H. and Lijo, P. Parametric analysis and a soft computing approach on material removal rate in electrochemical discharge machining, International Journal of Manufacturing Technology and Management 2011;24:23-39. [20] Fascio, V., Wuthrich, R., Viquerat, D. and Langen, H. 3D Micro structuring of Glass Using Electrochemical Discharge Machining (ECDM). Proc. Of the International Symposium of Micromechatronics and Human Science, Nagoya 1999;179-183. [21] Wei, C., Xu, K., Ni, J., Brzezinski, A.J. and Hu, D. A finite element based model for electrochemical discharge machining in discharge regime. Int Journal of Advanced Manufacturing Technology 2011;54:987995. [22] A. Kulkarni, R. Sharan, G.K. Lal. An experimental study of discharge mechanism in electrochemical discharge machining. International Journal of Machine Tools and Manufacture 2002;42:1121–1127.
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ScienceDirect Procedia Technology 25 (2016) 1175 – 1181
Global Colloquium in Recent Advancement and Effectual Researches in Engineering, Science and Technology (RAEREST 2016)
Effect of Power Source in ECDM Process with FEM Modeling Lijo Paula* , Libin V Koraha Department of Mechanical Engineering. St.Joseph’s college of Engineering and Technology, Pala,Kottayam, Kerala, India 656879
Abstract Electro Chemical Discharge Machining (ECDM) is a hybrid non-conventional manufacturing process which combines the features of Electro Chemical Machining (ECM) and Electro Discharge Machining (EDM). This process is important since it can machine a variety of materials including non-conducting materials like ceramics, composites, alumina, glass, etc. This paper attempts to develop a thermal model of ECDM process for determining the Material Removal Rate (MRR) with direct current (DC) and pulse DC.Comparing both DC and pulse DC on the bases of temperature variations and material removal rate, pulse DC is found to be better than DC with lower temperature range and higher MRR. ©©2016 Published by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license 2015The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of RAEREST 2016. Peer-review under responsibility of the organizing committee of RAEREST 2016 Keywords: Electro Chemical Discharge Machining; Material Removal Rate; Pulse DC; Finite Element Modelling
1. Introduction Electrochemical discharge machining (ECDM) is first introduced by Kurafuji and Suda [1], during glass micromachining [2]. Glass micro-machining was the first application and remains so far the best mastered technique but other non-conducting materials can also be machined(e.g., granite, refractory fire-brick, aluminium oxide, plexiglas, quartz) [3]. Similar results are reported for ceramic materials [4] and quartz [5]. ECDM process consists of an electrochemical cell as shown in Fig 1. The glass sample to be machined is dipped in the electrolyte and voltage is applied between the tool electrode (cathode) and the counter electrode (anode). The two electrodes are separated from each other. The tool electrode has smaller surface are than the counter electrode.
* Corresponding author. Tel.: 09846270462; E-mail address: [email protected]
2212-0173 © 2016 The Authors. Published by Elsevier Ltd. 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 organizing committee of RAEREST 2016 doi:10.1016/j.protcy.2016.08.236
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Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
Fig .1 .ECDM machining chamber
Both electrodes are connected to a DC power supply, causing bubbles to be produced on both electrodes due to electrochemical reactions[6]. When applied voltage is higher than the critical value (typically 25 V [7]), the bubbles become dense and coalesce into gas films[8,9] on the tool electrode. The gas films allows sparking between the tool electrode and electrolyte by isolating the tool from the surrounding electrolyte. If the workpiece is brought close to the tool, material removal occurs. The material removal mechanism of ECDM is a combination of thermal melting and chemical dissolution[9,10,11]. Basak et al. [12] estimated that a typical spark in ECDM contain 2,000J/cm2 of power and lasts for 100 µs. This intensive energy input causes a sudden temperature rise in the local area of the workpiece, thus causing the material removal by thermal melting. Meanwhile the alkaline in the electolyte etches silicate in glass [10,6]. This chemical reaction is enhanced by the temperature rise as well[13]. Lijo and Somashekhar [14-15] had conducted Response Surface Modeling (RSM) of ECDM with pulsating DC to understand the effect of various process parameters on micro hole and micro channel machining for Material Removal Rate (MRR), Tool Wear Rate (TWR), Heat Affected Zone (HAZ) and Radius of Overcut (ROC). They found that that temperature of the electrolyte effect micro machining. MRR decreases at higher depth due to lower spark which was due to unavailability of electrolyte . Lijo and Somashekhar [16-17] have also carried out multi objective optimization with grey relational analysis. They have made micro channels with optimized values. They have reported that haeat affected zone in micro channles can be reduced with low duty factor during machining. Lijo et al. [18] also reported that higher frequency at low duty factor will increase MRR with reduction in HAZ. Duty factor is found to be a predominant factor affecting the machining characteristics. Ranganayakulu et al. [19]developed an ECDM setup for micromachining of acrylic plate. The soft computing technique of Artificial Neuro Fuzzy Inference System (ANFIS) was used for modeling and found that voltage had predominant effect on MRR than electrolyte concentration and tool feed rate. The heat generated by the electrochemical discharges rises the temperature in the machining zone up to 5006000C [20]. At this high temperature the electrolyte is in the form of molten NaOH. The alkaline solution attacks the glass and is hydrated and totally dissolved by breaking the Si-0-Si bond on the surface of the glass. ܱܪെ ݅ܵ ؠെ ܱ െ ܵ݅ ؠ՜ܱ݅ܵ ؠെ ܪܱ݅ܵ ؠ
(1)
Followed by this chemical etching and occurs according to the following equation. ʹ ܪܱܽܰݔ ܱ݅ܵݔଶ ՜ ܱ݅ܵʹܽܰݔଷ ܪݔଶ ܱ The product of this reaction will be removed by the electrolyte outside the machining zone.
(2)
Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
ECDM drilling is one of most common type of machining process. Drilling characterized by two regimes [21] as discharge region and hydrodynamic regime. During the discharge regime the drilling is fast (up to 100 microns/s) and controlled by the number of discharges (applied voltage). For higher depth, drilling become slower (typically around 10-15 microns) and is nearly independent of applied voltage this behavior occurs during the hydrodynamic regime. Despite the considerable amount of research, which has been reported on the theoretical aspects and laboratory experiments, there are many aspects of the process control issues that are yet to be investigated for the achievement of improved productivity of the ECDM process. From the literature survey it is found that ECDM is at research level only and most of the works are experimental. Few theoretical papers are available related to determination of MRR. Basak et al[10] have given analytical model of MRR with the experimental validation, but the basic theory of spark generation proposed by them does not match with the actual situation. Most of the researches have considered uniformly distributed heat source within a spark. This assumption is far from reality. This factor is evidenced from the actual shape of a crater found by Kulkarni et al [22]. Heat flux distribution within a spark is assumed as Gaussian. In this paper a temperature variations and material removal rate are presented with DC and pulse DC for improving the efficiency of ECDM process. Nomenclature ECDM Electro chemical discharge machining MRR Material Removal Rate FEM Finite element modelling 2. Modeling ECDM is complex machining process hence its modeling requires certain assumptions. To make the analysis of ECDM following assumptions are considered [21]. 1. Workpiece material is assumed to be homogeneous and isotropic. 2. It is a single spark phenomenon. 3. Spark location is randomly distributed on tool surface immersed in electrolyte. 4. All sparks are considered to be identical during the whole machining period. 5. Only a fraction total sparking, heat flux is dissipated in to the workpiece shape of heat flux is assumed to be Gaussian distributed. From the experimental studies of Kulkarni et al[22] for single spark, the heat affected zone is circular and the crater is dome shaped. So reflecting the shape crater, we can approximate the nature of the heat flux as Gaussian. 6. The process condition during machining is kept constant. 7. Material removal by cavitation effect is neglected. All material removal is caused by thermal melting and chemical dissolution in ECDM is dependent on the temperature rise caused by sparks. 3. Thermal modeling Commercial finite element simulation software ANSYS 13 was employed to predict the material removal by direct DC and pulse DC. Governing equation for calculation of transient temperature distribution within the workpiece which is assumed to be axisymmetric is given by ଵ
డ்
డ
݇ሾ ቀݎ
ቁ
డమ ் డ௫
ሿ ൌ ܿ
డ் డ௧
(3)
Where k, c, and p are thermal conductivity, specific heat capacity and density of the workpiece material, respectively. The initial condition of ECDM process (t=0), the workpiece is immersed in the electrolyte and the
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Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
temperature of the whole domain is assumed to be at room temperature ( ܶ ). Figure 2 shows the important boundary conditions in this thermal modeling [21] . They are Boundary ܵଶ and ܵଷ are considered to be insulated. Heating process is taken to be axisymmetric about the axis of a spark, so heat flowing from the counter part of the domain is equal to the heat flowing to the counterpart. Therefore the net heat loss or gain is absolutely zero on surfaceܵଵ . 3. On surface ܵସ , where a spark occurs (AE), heat flux boundary condition is applied. Based on shape of machined surface, in this work a Gaussian heat flux distribution is taken and heat flux calculation expression is derived 1. 2.
ݓݍሺݎሻ ൌ
ͶǤͶͷܴܿܫ ܸܿ ݓ ߨʹݎ
ݎ
exp ሼെͶǤͷሺ ሻ2} ܴ
(4)
Where r is the radial distance from the axis of the spark, R w is the spark radius, Vc is the critical voltage and Ic is the critical current at which spark occurs.
Fig 2.Thermal model of ECDM process
4. FEM Modeling Mainly three steps are carried out on this FEM modeling as preprocessing, solution and postprocessing. In preprocessing borosilicate glass workpiece is prepared. The thermal properties of borosilicate glass used in model are listed Table 1. Table 1. Thermal properties of borosilicate glass Item
Value 3
Mass density p (kg/m )
2180
Specific heat C (J/kgK)
775
Thermal conductivity k (Wm/K)
1.13
The size of the workpiece is taken as 400*200 mm2 and mesh size is taken as 5 µm. Then the equation models as following in equation (5) qw (r)= 62954620*exp(-2*108({X}-C1)2)
(5)
Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
In this modelling the analysis type is taken as a transient and uniform temperature is taken as a 305 K. The second stage of modelling is solution stage and at this stage the thermal heat flux is applied on top nodes of the workpiece. In pulse DC time for 1 pulse is 3 µs and 133 pulse is taken. In DC pulse is given for 400 µs and the vector plot for the same is shown in Fig.3. Maximum temperature obtained at the nodal points is found to be 1165 K. Figure 4 shows the vector plot for temperature variation in pulse Dc mode and maximum temperature at nodal point is found to be 1173 K which lower than DC. The temperature variation with time at different node points in pulse DC is given in Fig.5. From the graph it is found that temperature varies randomly across the nodes on the surface. This is basically due to the non conducting nature of the borosilicate glass.
Fig. 3. Temperature Plot with Direct Current
Fig. 4. Temperature plot with Pulsed Current
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Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
Fig.5. Comparison of temperature with different nodes for pulse DC model
5. Result & Discussion In case of pulse DC the maximum temperature is generated as 1173 0 C, and the maximum temperature is generated on DC is 11650 C. By comparing both the cases we can see that the temperature lower in pulse DC than DC. This is due to the intermittent cooling due to continuous forming and breaking of the spark due to pulse DC. The Material Removal Rate (MRR) is found for DC and pulse DC with the help of MATLAB software using curve fitting tool. The volume of curve is calculated using equation (6). ோ
V= - ʹ ߨfxdx
(6)
Using equation (6) in case of DC the MRR is found to be 0.4 mg/min and in case of pulse DC the MRR is found to be 1.07 mg/min. By the analysis of this result we can find that the material removal rate in case of pulse DC is higher than DC with lower temperature in pulse DC. So in the case of pulse DC most of the heat from the tool is used for melting and vaporization of the workpiece. So pulse DC is found to be better than DC in terms of higher MRR and lower temperature in machining zone. 6. Conclusion Micro machining of borosilicate glass is modeled using ANSYS 13 for direct current and pulsed DC power source. The MRR in FEM model is quantified with curve fitting tool in MATLAB software. The MRR for DC is found to be 0.4 mg/min and pulse DC is 1.07gm/min. So same power source MRR in pulse DC is found to be higher than DC. So in pulse DC most of heat produced is used to remove the material through heating melting and vaporization. The tool life also improved with cooling in pulse DC. The heat affected zone in pulse DC lower than DC, due to short pulse duration, with intermittent cooling than DC.
Lijo Paul and Libin V. Korah / Procedia Technology 25 (2016) 1175 – 1181
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