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A study on effects of discharge energy on geometric characteristics of high aspect ratio micro-holes on TiN-Al2O3 ceramics
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 17828–17837
www.materialstoday.com/proceedings
ICMPC_2018
A study on effects of discharge energy on geometric characteristics of high aspect ratio micro-holes on TiN-Al2O3 ceramics Rupali Baghela*, Dr. Harlal Singh Malib
a,b
Malaviya National Institute of Technology, Jaipur- 302017, INDIA
Abstract In recent years, the need for ceramics products containing micro-features has shown a pronounced and steady growth in several fields of industry application. For the development of micro-hole devices, one of the most important technologies is Micro Electro Discharge Machining (µ-EDM). Micro-EDM can be considered as an ideal process to obtain burr-free micron-size features with high aspect ratios. This work presents an innovative method for the µ-EDM of high aspect ratio (AR =25) micro holes. The investigation focuses on the influence of input parameters and discharge energy on material removal rate (MRR), electrode wear (EW) rate and more over Moreover, an analysis of the geometrical characteristics of the micro holes in terms of Taperness and diametrical overcut was carried out. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: EDM, Micro holes, Discharge energy, Taper-ness, Radial overcut, MRR, EW;
1. Introduction Miniaturization technologies can be considered as one of the most promising manufacturing sectors with application in various industries. Micromachining is useful to produce micro parts not only to enhance product functionality in less space but also to save material and energy. Micro-machining techniques are employing to generate microelectro-mechanical system (MEMS) device applicable in sensors and actuators [1]. Micro machining technologies are also useful to produce micro features like micro holes, micro-channels, and micro passage. Miniaturized products are mainly useful in information technology, medical industries, miniaturized machines, and electric devices.
* Corresponding author. Tel.: 09887134189
E-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
Rupali Baghel et al. / Materials Today: Proceedings 5 (2018) 17828–17837
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The ‘micro-perforation illumination’ technology also requires high aspect ratio micro-holes (100μm). This kind of micro-features are used in devices like smart phones, tablet computers, notebooks for displaying specific functions by illumination [2]. Micro features like micro holes, microchannels, and micro passage are widely used in MEMS, micro nozzles, micropumps, micro reactant etc. Common examples of application of high aspect ratio micro holes in industry include inkjet printer nozzles, spinnerets holes, turbine blades cooling channels, diesel fuel injection nozzles, drug delivery orifices etc. [3]. Conventional machining processes like micro turning, micromilling and micro grinding, can be used to generate micro features with the use of the single point diamond cutter and with the help of very fine grit grinding wheel. Besides conventional machining process, spark energy, light energy, vibration energy, based non-conventional methods can be used to generate micro features. Micro electrodischarge machining (μ-EDM) is one of the preferable methods to generate deep micro holes because EDM is able to machine ‘difficult to machine’ that are of high hardness like cermet and ceramics but there are still limitations regarding response parameters like material removal rate (MRR), high aspect ratio, surface finish etc. [4]. D’Ursow et al. [5] have machined stainless steel, titanium, magnesium and brass with ED Drilling process to study the effect of electrode materials and input parameters on geometric characteristics of micro holes. They experimentally claimed that tool wear rate is more influenced by the electrode materials than that of peak current and voltage level. Rengasamy et al. [6] have optimized the process parameters for Al-4032 alloy reinforced with Zrb2 and Tib2 while machining with EDM and concluded that with the increase in the reinforced particles, material removal rate (MRR) is increases and most influencing input parameter is pulse on time followed by peak current. Hu et al. [7] have successfully machined Ti3SiC2 composite material by EDM and also analyze thermal shock induced surface damage. They concluded that MRR increases with discharge current and gap voltage. Muthuramalingam et al. [8] have critically reviewed the electrical process parameter in EDM technology and concluded that response parameters i.e. MRR and surface roughness were dominated by pulse current and pulse duration. Richard et al. [9] have experimentally proved that micro- Electro- discharge milling (µ-ED Milling) has a very strong potential to generate 3D cavity with high aspect ratio. Baghel et al. [10] have drilled micro holes of diameter of 200µm in Titanium Nitride Aluminium Oxide by vibration assisted EDM process and experimentally concluded that materials removal rate is improved by 1.98 times in the influence of low-frequency vibration. Although ceramics are having excellent mechanical properties and widely used in diverse applications in numerous engineering areas including aerospace, automobile, medical, chemical but exhaustive applications of ceramics are not possible due to difficulties in machining. Therefore, machinability of ceramics is an important issue in the field of manufacturing. Many researchers have studied the simulation models of the micro heat exchanger and find that effectiveness of micro channels and micro holes is depends on the shape of these micro features. Liang Xia et al. [11] have studied improvement in the effectiveness of heat exchanger using microchannels. They also concluded that that the temperature of bulk flow in channel responded more quickly when the inlet area of the channel becomes smaller resulting in the stronger heat transfer ability for micro-sized channels. Heat transfer rate was improved due to per unit effective heat transfer area. Alam Khan et al. [12] have designed micro heat exchanger and studied flow behavior and temperature distribution. They also concluded effectiveness of the micro-heat exchanger varies with pressure difference, inlet geometry, and wall heat flux. Meiling et al. [13] have study the characteristics like roundness, roughness, hole shape facture for high aspect ratio hole in aluminium based cast alloy. They also concluded that micro bulged formed due to wettability between molten metal and periphery of hole. In this paper, an attempt has been made to study the effect of discharge energy on geometric characteristics of micro holes by creating micro through holes of 200µm diameter on 5mm thick work piece with 200µm tungsten electrode by micro electro-discharge drilling (µ-ED drilling) in Titanium Nitride-Aluminium Oxide (TiN-Al2O3). TiN-Al2O3 is a new generation advanced ceramic composite. TiN-Al2O3 is showing high resistance to various kinds of abrasions, high hardness, toughness, low friction coefficient and considerable thermal conductivity. TiN-Al2O3 ceramic-composite has applications in aerospace parts, tool industry, and heat shielding but machining of this useful ceramics is a challenge because of its mechanical properties. TiN-Al2O3 ceramic-composite is could be a potential material that can be used as chemical reactant and heat exchanger due to its properties like low density to weight ratio, chemical inertness, and considerable thermal conductivity. The electric resistivity of TiN-Al2O3 ceramic is 0.25Ωcm. 2. Planning of Experiments In the present work behavior of the EDM is studied with different discharge energy. The effect of the process parameter is also observed such as spindle speed (S), capacitance (C), voltage(V) on Material removal rate(MRR), electrode wear rate (EWR), electrode wear ratio(EW), radial overcut (ROC) and a taper angle (α). These process
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Rupali Baghel et al./ Materials Today: Proceedings 5 (2018) 17828–17837
parameter and range have been selected on the basis of existing literature, pilot experiments, and machine capability. The process parameter range and constant process parameters are shown in table1. Table 1 Input parameter Process Parameter Range Spindle speed (S) Capacitance (C) Voltage (V)
Constant Process Parameter Tool Electrode Tungsten (ø200µm) Discharge Feed 5% Machine Feed 1.5 mm/min
900, 1200, 1500 RPM 0.01, 0.1, 0.4 µF 90, 110, 130 Volt
The planning of experiments for EDM of TiN-Al2O3 ceramic composite was based on Taguchi L9 orthogonal array. A total nine holes with an aspect ratio (25) have been generated along with 6 holes to observe the effect of discharge energy on geometric characteristics. The responses MRR, EW, EW Ratio, taper angle, and ROC are measured at each setting. =
(
)=
(
( )=
ℎ −1
ℎ 2 −
( 2∗
ℎ
−
) ) ℎ
3. Experimental Setup and Procedure In this present work, all the present research work all the experiments have been carried out on Hybrid µEDM (DT-110) machine (Mikrotools Singapore). The tool electrode is of tungsten (200µm) is connected to negative polarity and ceramic workpiece connected to positive polarity. The experimental results are given in Table.3. A statistical analysis consists of calculation of signal to noise (S/N) ratio values for each response factor. The S/N ratios are calculated using equations 4 and 5. For the small characteristics (i.e. EW, taper angle, ROC), the value of S/N ratio can be calculated by ∑ = −10 log … … … … … … … … . . (1) For larger characteristics (i.e. MRR, EW ratio) can be calculated by = −10 log
∑
1 … … … … … … … … … . . (2)
3.1 Effect of Control Parameter on MRR For MRR, the calculation of S/N ratio follows larger is better and S/N graph has an increasing trend with capacitance and voltage as depicted in Fig.2 (a). Spindle speed has less significance effect on MRR. Here capacitance has rank one followed by voltage rank two and spindle speed as rank three.
Rupali Baghel et al. / Materials Today: Proceedings 5 (2018) 17828–17837
(a)
(b
Fig. 1 (a) Top view of micro les (b) Bottom view of micro holes.
(a)
(b)
(c)
(d)
(e) Fig. 2 Main effects plots of (a) MRR, (b) Electrode wear, (c) electrode wear ratio, (d) taper angle and (e) radial overcut
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MRR has found to be improving with the increase in the voltage and capacitance as they simultaneously increase the discharge energy thus causing the greater speed of bombarding electron from electrode to the workpiece, resulting in higher MRR as shown in above Fig 1 (a-b) and as shown in below Table 2. Table 2 Experimental design matrix with set of process parameters and corresponding response parameters S
C
V
Discharge Energy
Top diameter
Bottom diameter
MRR
EW
rpm
Micr o fared 0.01 0.1 0.4 0.01 0.1 0.4 0.01 0.1 0.4
volt
Micro joule
micrometer
micrometer
mm3/min
mm3/min
90 110 130 110 130 90 130 90 110
40.5 605 3380 60.5 845 1620 84.5 405 2420
341.6 352.7 360.7 355.9 351.3 358.1 380.2 323.2 341.6
296.8 336.2 325.0 312.0 298.0 302.0 321.7 270.9 311.2
0.002737 0.005162 0.006914 0.004107 0.004837 0.006425 0.004234 0.003642 0.004803
0.00030 0.00123 0.00075 0.00173 0.00252 0.00387 0.00160 0.00211 0.00294
900 900 900 1200 1200 1200 1500 1500 1500
EW Ratio
9.123333 4.196748 9.218667 2.373988 1.919444 1.660207 2.646250 1.726066 1.633673
Radial overcut
Taper angle
micrometer
radian
70.8 76.35 80.35 77.95 75.65 79.05 90.1 61.6 70.8
0.2566 0.0945 0.2062 0.2515 0.3053 0.3214 0.3351 0.2996 0.1718
3.2 Effect of Control Parameter on EW and EW Ratio Calculation of S/N ratio for electrode wear (EW) follows smaller is a better model. From Fig.2 it is observed that S/N graph for EW has a steep decreasing trend spindle speed and spindle speed also hold rank one thus most influencing parameter in case of electrode wear followed by capacitance and voltage. The S/N ratio graph for electrode wear ratio (EW Ratio) is following “larger is better” model electrode wear ratio (MRR/EW). Electrode wear ratio is high at low spindle rotation speed. For EW Ratio speed hold rank one followed by capacitance and voltage as shown in below Fig 3. 3.3 Effect of Control Parameter on taper angle and radial overcut For taper angle, the calculation of S/N ratio follows “smaller the better” model. From figure 5, it is observed that S/N graph for taper angle has a decreasing trend with speed and increasing trend with voltage. Voltage comes out to be a most influencing parameter with rank one, speed hold rank two and capacitance does not have influence significant effect on taper angle and hold rank three. The calculation of S/N ratio of the radial overcut follows “smaller the better model” and voltage is coming out to be the most influencing parameter with rank one followed by capacitance and speed. High voltage and high capacitance results in high discharge energy result large radial overcut. 4. Results and discussion After determining the influencing parameter on responses under these conditions. The effect of discharge energy is also studied on response parameters. The discharge energy is calculated by the formula E=0.5CV2. Experiments are performed at different discharge energy and at various electrode rotation speed. Shown in figure 7. It is observed from figure 7 that lower speed (i.e. =900rpm) MRR trends to increase with the increase in discharge energy and EW first increases with the discharge energy and after reaching a certain value it starts decreasing. At middle speed (i.e. =1200 rpm) MRR and EW both increases with the discharge energy value. At higher speed (i.e. =1500) both MRR and EWR increase with discharge energy after 500µJ discharge energy.
Rupali Baghel et al. / Materials Today: Proceedings 5 (2018) 17828–17837
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Fig. 3 Effect of discharge energy at MRR at various spindle speed To study the effect of discharge energy on geometric characteristics, the micro-hole of 200µm diameter have been generated at 1200 rpm speed as shown in Table 3. Tungsten electrode with 200µm has been used for these experiments. Table 3 Responses parameter at various discharge energy value
Sn.
S
V
C
Time
1 2 3 4 5 6
1200 1200 1200 1200 1200 1200
80 90 100 110 120 130
5 5 5 5 5 5
32:40 25:58 24:50 23:32 25:14 27:13
ENERGY (micro joule) 320 405 500 605 720 845
MRR (mm3/min)
EW (mm3/min)
EW Ratio
ROC (micron)
TA (radian)
0.003845 0.004837 0.005058 0.005233 0.004978 0.004615
0.000116 8.82E-05 0.000143 0.000329 0.000217 0.000212
33.11346 54.87002 35.43156 15.89724 22.96422 21.77548
64.05 59.6 64.55 67.25 67.6 66.4
0.4497 0.4377 0.3649 0.7476 0.3317 0.4354
4.1 Effect of discharge energy on material removal rate (MRR) and electrode wear (EW) From these experiments it is observed that MRR is increasing with the increase in the value of discharge energy, maximum MRR observed is 0.005233 mm3/min at discharge energy 605 µJ.
(a)
(b) Fig. 4 Effect of discharge energy on (a) MRR, (b) EW rate
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Rupali Baghel et al./ Materials Today: Proceedings 5 (2018) 17828–17837
The observed MRR value with the value of discharge energy is shown in Fig. 4 (a), it can be seen that in this working condition (s=1200rpm) MRR reached its maximum value and after that, it start decreasing with the discharge energy. MRR is good in range 500-600µJ range and after 600 µJ, MRR follows steep downfall in the value of MRR. EW is lowest at less discharge energy but at lower energy level MRR is also lowest. So we can conclude from the Fig. 4 (a) and Fig. 4 (b) that 500 µJ discharge energy is suitable for working as MRR and EW both are optimum at this point. 4.2 Effect of discharge energy on ROC and TA The effect of discharge energy on radial overcut (ROC) and a taper angle (TA) can be seen in Fig. 5 (a) and Fig. 5 (b). It can be observed that ROC is less in the range 300-500 µJ discharge energy and lowest at 400 µJ. The taper angle is less in the value of 500 and 700 µJ range.
(a)
(b)
Fig. 5 Effect of discharge energy on (a) ROC (b) Taper angle
The value of geometric characteristics should be as lower as possible. From the fig.8, we can conclude that the energy range of 400 to 500 µJ is best suited for machining of the TiN-Al2O3 ceramic composite as in this range MRR is moderate, EW rate is lowest, ROC and taper angle is lowest. 5. SEM Images of Drilled Hole Fig. 6, 7 exhibits SEM image of through holes from top and bottom side at increasing order of discharge energy. As we already discuss that for geometrical characteristics 400 µJ to 500 µJ is the best working range.
Rupali Baghel et al. / Materials Today: Proceedings 5 (2018) 17828–17837
(a)
(c)
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(b)
(d)
Fig. 6 SEM image of drilled hole from top (a) 320µJ (b) 405µJ (c) 500 µJ (d) 600 µJ
From Fig. 6 we can observed that, there is no bulging formation at the exit of the hole that generally used to observe in macro machining of ceramic-composite. The best circularity of the hole can be observed at 500 µJ discharge energy.
(a)
(b)
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Rupali Baghel et al./ Materials Today: Proceedings 5 (2018) 17828–17837
(c)
(d)
Fig. 7 SEM image of drilled hole from bottom (a) 320µJ (b) 405µJ (c) 500 µJ (d) 600 µ
It is also observed that in comparison of macro EDM hole there is a negligible recast layer form in the holes periphery. Fig. 8 shows that top and bottom image of drill hole. A bulge formation is visible in the entry/ top of the holes but in Fig. 6, there is comparative no bulge formation. This much of resolidification layer is due to pulse energy [14].
(a)
(b) Fig. 8 SEM images macro hole on TiN-Al2O3 (a) top (b) bottom [12]
6. Conclusions High aspect ratio micro holes are needed for several applications micro-electro-mechanical system (MEMS) like heat exchangers, chemical microreactors etc., have been fabricated by the micro-EDM process. Effect of discharge energy on geometric characteristics like taper angle, radial overcut and circularity is also studied. Based on this study, following conclusions can be made. 1. In this present study research, machinability study of potential ceramic composite TiN-Al2O3 has been carried out and effect of input parameter(voltage, capacitance, speed) is observed on each response parameter (i.e. MRR, EW, EW Ratio, Taper Angle, Radial overcut). 2. From the analysis, it is observed that material removal rate is mainly affected by the capacitance and voltage i.e. discharge energy and there is a steep downfall in MRR if discharge energy is beyond 600µJ. 3. EWR is also increased with the increased value of discharge energy as high discharge leads to high electrode wear. For the optimum value of MRR and EWR discharge energy is 500 µJ. 4. Voltage comes out to be the most influencing parameter with rank one, speed hold rank two and capacitance does not have influence significant effect on taper angle and hold rank three. For radial overcut also voltage is found to
Rupali Baghel et al. / Materials Today: Proceedings 5 (2018) 17828–17837
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be the most influencing parameter with rank one followed by capacitance and speed. High voltage and high capacitance results in high discharge energy result large radial overcut. 5. It is also observed that micro machining of TiN-Al2O3 ceramic composite is done more efficiently as comparison of macro machining by EDM process as there is no recast layer is observed in the case of micro – EDM. 6. The machining performance and effect of discharge energy on geometric characteristics of micro holes generated on TiN-Al2O3 ceramic-composite by the micro-EDM process is well understood by the present experimental investigation. Micro EDM is proved to have a very good potential in machining of this promising ceramic-composite. References [1]
Z. Yu, Y. Zhang, J. Li, F. Zhao and D. Guo, "High aspect ratio micro-hole drilling aided with ultrasonic vibration and planetarymovement of electrode by micro-EDM," CIRP Annals - Manufacturing Technology 58 (2009) 213–2.
[2]
Y. J. Chang, Y. Z. Hong, C. L. Kuo, J. C. Hsu and C. C. Ho, "Hybrid Stamping and Laser Micromachining Process for Micro-scale Hole Drilling," Materials and Manufacturing Processes, 2017.
[3]
M. Hourmand, A. A. Sarhan and N. M. Yusof, "Development of new fabrication and measurement techniques of micro-electrodes with high aspect ratio for micro EDM usingtypical EDM machine," Measurement 97 (2017) 64–78.
[4]
E. Ferraris, V. Castiglioni, F. Ceyssens, M. Annoni, B. Lauwers and D. Reynaerts, "EDM drilling of ultra-high aspect ratio micro holes with insulated tools," CIRP Annals - Manufacturing Technology 62 (2013) 191–194.
[5]
G. D’Urso and C. Merla, "Workpiece and electrode influence on micro-EDMdrilling performance," Precision Engineering 38 (2014) 903–914.
[6]
N. Rengasamy, M. Rajkumar and S. S. l Kumaran, "An analysis of mechanical properties and optimization of EDM process parameters of Al 4032 alloy reinforced with Zrb2 and Tib2 in-si composites," Journal of Alloys and Compounds 662 (2016) 325e3.
[7]
C. Hu, Y. Zhou and Y. Bao, "Material removal and surface damage in EDM of Ti3SiC2 ceramic," Ceramics International 34 (2008) 537–541.
[8]
T. Muthuramalingam and B. Mohan, "A review on influence of electrical process parameters in EDM process," archives of civil and mechanical engineering 15 (2015) 87-94.
[9]
J. Richard and R. Demellayer, "Development of new machining technology for micro-machining," in The Seventeenth CIRP Conference on Electro Physical and Chemical Machining (ISEM), 2013.
[10] R. Baghel, H. S. Mali and S. K. Biswas, "Study of Vibration Assisted Micro Electro-DischargeMilling of Titanium NitrideAluminium Oxide Composite," in All India Manufacturing Technology, Design and Research Conference (AIMTDR), College of Engineering, Pune, Maharashtra, INDIA, 2016. [11] L. Xia and Y. Chan, "Investigation of the enhancement effect of heat transfer using micro channel," in The 7th International Conference on Applied Energy, Energy Procedia 75 ( 2015 ) 912 – 918. [12] M. J. Alam Khan, M. R. Hasan and M. A. Hasan Mamun, "Flow behavior and temperature distribution in micro-channels for constant wall heat flux," in 5th BSME International Conference on Thermal Engineering,Procedia Engineering 56 ( 2013 ) 350 – 356. [13] J. Meiling, Q. Yingdong, Y. Junhua, L. Rongde and C. Ruirun, "Characteristics of array holes with large aspect ratio in aluminumbased cast alloy," Materials and Manufacturing Processes, 2017. [14] D. Bhaduri, A. Kaur, S. Sarkar, S. Biswas and S. Mitra, "Electro Discharge Machining of Titanium Nitride-Aluminium Oxide Composite for Optimum Process Criterial Yield," vol. 24, no. 12, 2009.
ScienceDirect Materials Today: Proceedings 5 (2018) 17828–17837
www.materialstoday.com/proceedings
ICMPC_2018
A study on effects of discharge energy on geometric characteristics of high aspect ratio micro-holes on TiN-Al2O3 ceramics Rupali Baghela*, Dr. Harlal Singh Malib
a,b
Malaviya National Institute of Technology, Jaipur- 302017, INDIA
Abstract In recent years, the need for ceramics products containing micro-features has shown a pronounced and steady growth in several fields of industry application. For the development of micro-hole devices, one of the most important technologies is Micro Electro Discharge Machining (µ-EDM). Micro-EDM can be considered as an ideal process to obtain burr-free micron-size features with high aspect ratios. This work presents an innovative method for the µ-EDM of high aspect ratio (AR =25) micro holes. The investigation focuses on the influence of input parameters and discharge energy on material removal rate (MRR), electrode wear (EW) rate and more over Moreover, an analysis of the geometrical characteristics of the micro holes in terms of Taperness and diametrical overcut was carried out. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: EDM, Micro holes, Discharge energy, Taper-ness, Radial overcut, MRR, EW;
1. Introduction Miniaturization technologies can be considered as one of the most promising manufacturing sectors with application in various industries. Micromachining is useful to produce micro parts not only to enhance product functionality in less space but also to save material and energy. Micro-machining techniques are employing to generate microelectro-mechanical system (MEMS) device applicable in sensors and actuators [1]. Micro machining technologies are also useful to produce micro features like micro holes, micro-channels, and micro passage. Miniaturized products are mainly useful in information technology, medical industries, miniaturized machines, and electric devices.
* Corresponding author. Tel.: 09887134189
E-mail address: [email protected]
2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
Rupali Baghel et al. / Materials Today: Proceedings 5 (2018) 17828–17837
17829
The ‘micro-perforation illumination’ technology also requires high aspect ratio micro-holes (100μm). This kind of micro-features are used in devices like smart phones, tablet computers, notebooks for displaying specific functions by illumination [2]. Micro features like micro holes, microchannels, and micro passage are widely used in MEMS, micro nozzles, micropumps, micro reactant etc. Common examples of application of high aspect ratio micro holes in industry include inkjet printer nozzles, spinnerets holes, turbine blades cooling channels, diesel fuel injection nozzles, drug delivery orifices etc. [3]. Conventional machining processes like micro turning, micromilling and micro grinding, can be used to generate micro features with the use of the single point diamond cutter and with the help of very fine grit grinding wheel. Besides conventional machining process, spark energy, light energy, vibration energy, based non-conventional methods can be used to generate micro features. Micro electrodischarge machining (μ-EDM) is one of the preferable methods to generate deep micro holes because EDM is able to machine ‘difficult to machine’ that are of high hardness like cermet and ceramics but there are still limitations regarding response parameters like material removal rate (MRR), high aspect ratio, surface finish etc. [4]. D’Ursow et al. [5] have machined stainless steel, titanium, magnesium and brass with ED Drilling process to study the effect of electrode materials and input parameters on geometric characteristics of micro holes. They experimentally claimed that tool wear rate is more influenced by the electrode materials than that of peak current and voltage level. Rengasamy et al. [6] have optimized the process parameters for Al-4032 alloy reinforced with Zrb2 and Tib2 while machining with EDM and concluded that with the increase in the reinforced particles, material removal rate (MRR) is increases and most influencing input parameter is pulse on time followed by peak current. Hu et al. [7] have successfully machined Ti3SiC2 composite material by EDM and also analyze thermal shock induced surface damage. They concluded that MRR increases with discharge current and gap voltage. Muthuramalingam et al. [8] have critically reviewed the electrical process parameter in EDM technology and concluded that response parameters i.e. MRR and surface roughness were dominated by pulse current and pulse duration. Richard et al. [9] have experimentally proved that micro- Electro- discharge milling (µ-ED Milling) has a very strong potential to generate 3D cavity with high aspect ratio. Baghel et al. [10] have drilled micro holes of diameter of 200µm in Titanium Nitride Aluminium Oxide by vibration assisted EDM process and experimentally concluded that materials removal rate is improved by 1.98 times in the influence of low-frequency vibration. Although ceramics are having excellent mechanical properties and widely used in diverse applications in numerous engineering areas including aerospace, automobile, medical, chemical but exhaustive applications of ceramics are not possible due to difficulties in machining. Therefore, machinability of ceramics is an important issue in the field of manufacturing. Many researchers have studied the simulation models of the micro heat exchanger and find that effectiveness of micro channels and micro holes is depends on the shape of these micro features. Liang Xia et al. [11] have studied improvement in the effectiveness of heat exchanger using microchannels. They also concluded that that the temperature of bulk flow in channel responded more quickly when the inlet area of the channel becomes smaller resulting in the stronger heat transfer ability for micro-sized channels. Heat transfer rate was improved due to per unit effective heat transfer area. Alam Khan et al. [12] have designed micro heat exchanger and studied flow behavior and temperature distribution. They also concluded effectiveness of the micro-heat exchanger varies with pressure difference, inlet geometry, and wall heat flux. Meiling et al. [13] have study the characteristics like roundness, roughness, hole shape facture for high aspect ratio hole in aluminium based cast alloy. They also concluded that micro bulged formed due to wettability between molten metal and periphery of hole. In this paper, an attempt has been made to study the effect of discharge energy on geometric characteristics of micro holes by creating micro through holes of 200µm diameter on 5mm thick work piece with 200µm tungsten electrode by micro electro-discharge drilling (µ-ED drilling) in Titanium Nitride-Aluminium Oxide (TiN-Al2O3). TiN-Al2O3 is a new generation advanced ceramic composite. TiN-Al2O3 is showing high resistance to various kinds of abrasions, high hardness, toughness, low friction coefficient and considerable thermal conductivity. TiN-Al2O3 ceramic-composite has applications in aerospace parts, tool industry, and heat shielding but machining of this useful ceramics is a challenge because of its mechanical properties. TiN-Al2O3 ceramic-composite is could be a potential material that can be used as chemical reactant and heat exchanger due to its properties like low density to weight ratio, chemical inertness, and considerable thermal conductivity. The electric resistivity of TiN-Al2O3 ceramic is 0.25Ωcm. 2. Planning of Experiments In the present work behavior of the EDM is studied with different discharge energy. The effect of the process parameter is also observed such as spindle speed (S), capacitance (C), voltage(V) on Material removal rate(MRR), electrode wear rate (EWR), electrode wear ratio(EW), radial overcut (ROC) and a taper angle (α). These process
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Rupali Baghel et al./ Materials Today: Proceedings 5 (2018) 17828–17837
parameter and range have been selected on the basis of existing literature, pilot experiments, and machine capability. The process parameter range and constant process parameters are shown in table1. Table 1 Input parameter Process Parameter Range Spindle speed (S) Capacitance (C) Voltage (V)
Constant Process Parameter Tool Electrode Tungsten (ø200µm) Discharge Feed 5% Machine Feed 1.5 mm/min
900, 1200, 1500 RPM 0.01, 0.1, 0.4 µF 90, 110, 130 Volt
The planning of experiments for EDM of TiN-Al2O3 ceramic composite was based on Taguchi L9 orthogonal array. A total nine holes with an aspect ratio (25) have been generated along with 6 holes to observe the effect of discharge energy on geometric characteristics. The responses MRR, EW, EW Ratio, taper angle, and ROC are measured at each setting. =
(
)=
(
( )=
ℎ −1
ℎ 2 −
( 2∗
ℎ
−
) ) ℎ
3. Experimental Setup and Procedure In this present work, all the present research work all the experiments have been carried out on Hybrid µEDM (DT-110) machine (Mikrotools Singapore). The tool electrode is of tungsten (200µm) is connected to negative polarity and ceramic workpiece connected to positive polarity. The experimental results are given in Table.3. A statistical analysis consists of calculation of signal to noise (S/N) ratio values for each response factor. The S/N ratios are calculated using equations 4 and 5. For the small characteristics (i.e. EW, taper angle, ROC), the value of S/N ratio can be calculated by ∑ = −10 log … … … … … … … … . . (1) For larger characteristics (i.e. MRR, EW ratio) can be calculated by = −10 log
∑
1 … … … … … … … … … . . (2)
3.1 Effect of Control Parameter on MRR For MRR, the calculation of S/N ratio follows larger is better and S/N graph has an increasing trend with capacitance and voltage as depicted in Fig.2 (a). Spindle speed has less significance effect on MRR. Here capacitance has rank one followed by voltage rank two and spindle speed as rank three.
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(a)
(b
Fig. 1 (a) Top view of micro les (b) Bottom view of micro holes.
(a)
(b)
(c)
(d)
(e) Fig. 2 Main effects plots of (a) MRR, (b) Electrode wear, (c) electrode wear ratio, (d) taper angle and (e) radial overcut
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MRR has found to be improving with the increase in the voltage and capacitance as they simultaneously increase the discharge energy thus causing the greater speed of bombarding electron from electrode to the workpiece, resulting in higher MRR as shown in above Fig 1 (a-b) and as shown in below Table 2. Table 2 Experimental design matrix with set of process parameters and corresponding response parameters S
C
V
Discharge Energy
Top diameter
Bottom diameter
MRR
EW
rpm
Micr o fared 0.01 0.1 0.4 0.01 0.1 0.4 0.01 0.1 0.4
volt
Micro joule
micrometer
micrometer
mm3/min
mm3/min
90 110 130 110 130 90 130 90 110
40.5 605 3380 60.5 845 1620 84.5 405 2420
341.6 352.7 360.7 355.9 351.3 358.1 380.2 323.2 341.6
296.8 336.2 325.0 312.0 298.0 302.0 321.7 270.9 311.2
0.002737 0.005162 0.006914 0.004107 0.004837 0.006425 0.004234 0.003642 0.004803
0.00030 0.00123 0.00075 0.00173 0.00252 0.00387 0.00160 0.00211 0.00294
900 900 900 1200 1200 1200 1500 1500 1500
EW Ratio
9.123333 4.196748 9.218667 2.373988 1.919444 1.660207 2.646250 1.726066 1.633673
Radial overcut
Taper angle
micrometer
radian
70.8 76.35 80.35 77.95 75.65 79.05 90.1 61.6 70.8
0.2566 0.0945 0.2062 0.2515 0.3053 0.3214 0.3351 0.2996 0.1718
3.2 Effect of Control Parameter on EW and EW Ratio Calculation of S/N ratio for electrode wear (EW) follows smaller is a better model. From Fig.2 it is observed that S/N graph for EW has a steep decreasing trend spindle speed and spindle speed also hold rank one thus most influencing parameter in case of electrode wear followed by capacitance and voltage. The S/N ratio graph for electrode wear ratio (EW Ratio) is following “larger is better” model electrode wear ratio (MRR/EW). Electrode wear ratio is high at low spindle rotation speed. For EW Ratio speed hold rank one followed by capacitance and voltage as shown in below Fig 3. 3.3 Effect of Control Parameter on taper angle and radial overcut For taper angle, the calculation of S/N ratio follows “smaller the better” model. From figure 5, it is observed that S/N graph for taper angle has a decreasing trend with speed and increasing trend with voltage. Voltage comes out to be a most influencing parameter with rank one, speed hold rank two and capacitance does not have influence significant effect on taper angle and hold rank three. The calculation of S/N ratio of the radial overcut follows “smaller the better model” and voltage is coming out to be the most influencing parameter with rank one followed by capacitance and speed. High voltage and high capacitance results in high discharge energy result large radial overcut. 4. Results and discussion After determining the influencing parameter on responses under these conditions. The effect of discharge energy is also studied on response parameters. The discharge energy is calculated by the formula E=0.5CV2. Experiments are performed at different discharge energy and at various electrode rotation speed. Shown in figure 7. It is observed from figure 7 that lower speed (i.e. =900rpm) MRR trends to increase with the increase in discharge energy and EW first increases with the discharge energy and after reaching a certain value it starts decreasing. At middle speed (i.e. =1200 rpm) MRR and EW both increases with the discharge energy value. At higher speed (i.e. =1500) both MRR and EWR increase with discharge energy after 500µJ discharge energy.
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Fig. 3 Effect of discharge energy at MRR at various spindle speed To study the effect of discharge energy on geometric characteristics, the micro-hole of 200µm diameter have been generated at 1200 rpm speed as shown in Table 3. Tungsten electrode with 200µm has been used for these experiments. Table 3 Responses parameter at various discharge energy value
Sn.
S
V
C
Time
1 2 3 4 5 6
1200 1200 1200 1200 1200 1200
80 90 100 110 120 130
5 5 5 5 5 5
32:40 25:58 24:50 23:32 25:14 27:13
ENERGY (micro joule) 320 405 500 605 720 845
MRR (mm3/min)
EW (mm3/min)
EW Ratio
ROC (micron)
TA (radian)
0.003845 0.004837 0.005058 0.005233 0.004978 0.004615
0.000116 8.82E-05 0.000143 0.000329 0.000217 0.000212
33.11346 54.87002 35.43156 15.89724 22.96422 21.77548
64.05 59.6 64.55 67.25 67.6 66.4
0.4497 0.4377 0.3649 0.7476 0.3317 0.4354
4.1 Effect of discharge energy on material removal rate (MRR) and electrode wear (EW) From these experiments it is observed that MRR is increasing with the increase in the value of discharge energy, maximum MRR observed is 0.005233 mm3/min at discharge energy 605 µJ.
(a)
(b) Fig. 4 Effect of discharge energy on (a) MRR, (b) EW rate
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The observed MRR value with the value of discharge energy is shown in Fig. 4 (a), it can be seen that in this working condition (s=1200rpm) MRR reached its maximum value and after that, it start decreasing with the discharge energy. MRR is good in range 500-600µJ range and after 600 µJ, MRR follows steep downfall in the value of MRR. EW is lowest at less discharge energy but at lower energy level MRR is also lowest. So we can conclude from the Fig. 4 (a) and Fig. 4 (b) that 500 µJ discharge energy is suitable for working as MRR and EW both are optimum at this point. 4.2 Effect of discharge energy on ROC and TA The effect of discharge energy on radial overcut (ROC) and a taper angle (TA) can be seen in Fig. 5 (a) and Fig. 5 (b). It can be observed that ROC is less in the range 300-500 µJ discharge energy and lowest at 400 µJ. The taper angle is less in the value of 500 and 700 µJ range.
(a)
(b)
Fig. 5 Effect of discharge energy on (a) ROC (b) Taper angle
The value of geometric characteristics should be as lower as possible. From the fig.8, we can conclude that the energy range of 400 to 500 µJ is best suited for machining of the TiN-Al2O3 ceramic composite as in this range MRR is moderate, EW rate is lowest, ROC and taper angle is lowest. 5. SEM Images of Drilled Hole Fig. 6, 7 exhibits SEM image of through holes from top and bottom side at increasing order of discharge energy. As we already discuss that for geometrical characteristics 400 µJ to 500 µJ is the best working range.
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Fig. 6 SEM image of drilled hole from top (a) 320µJ (b) 405µJ (c) 500 µJ (d) 600 µJ
From Fig. 6 we can observed that, there is no bulging formation at the exit of the hole that generally used to observe in macro machining of ceramic-composite. The best circularity of the hole can be observed at 500 µJ discharge energy.
(a)
(b)
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(c)
(d)
Fig. 7 SEM image of drilled hole from bottom (a) 320µJ (b) 405µJ (c) 500 µJ (d) 600 µ
It is also observed that in comparison of macro EDM hole there is a negligible recast layer form in the holes periphery. Fig. 8 shows that top and bottom image of drill hole. A bulge formation is visible in the entry/ top of the holes but in Fig. 6, there is comparative no bulge formation. This much of resolidification layer is due to pulse energy [14].
(a)
(b) Fig. 8 SEM images macro hole on TiN-Al2O3 (a) top (b) bottom [12]
6. Conclusions High aspect ratio micro holes are needed for several applications micro-electro-mechanical system (MEMS) like heat exchangers, chemical microreactors etc., have been fabricated by the micro-EDM process. Effect of discharge energy on geometric characteristics like taper angle, radial overcut and circularity is also studied. Based on this study, following conclusions can be made. 1. In this present study research, machinability study of potential ceramic composite TiN-Al2O3 has been carried out and effect of input parameter(voltage, capacitance, speed) is observed on each response parameter (i.e. MRR, EW, EW Ratio, Taper Angle, Radial overcut). 2. From the analysis, it is observed that material removal rate is mainly affected by the capacitance and voltage i.e. discharge energy and there is a steep downfall in MRR if discharge energy is beyond 600µJ. 3. EWR is also increased with the increased value of discharge energy as high discharge leads to high electrode wear. For the optimum value of MRR and EWR discharge energy is 500 µJ. 4. Voltage comes out to be the most influencing parameter with rank one, speed hold rank two and capacitance does not have influence significant effect on taper angle and hold rank three. For radial overcut also voltage is found to
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be the most influencing parameter with rank one followed by capacitance and speed. High voltage and high capacitance results in high discharge energy result large radial overcut. 5. It is also observed that micro machining of TiN-Al2O3 ceramic composite is done more efficiently as comparison of macro machining by EDM process as there is no recast layer is observed in the case of micro – EDM. 6. The machining performance and effect of discharge energy on geometric characteristics of micro holes generated on TiN-Al2O3 ceramic-composite by the micro-EDM process is well understood by the present experimental investigation. Micro EDM is proved to have a very good potential in machining of this promising ceramic-composite. References [1]
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