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Procedía Materials Science 5 (2014) 2004 - 2013
International Conference on Advances in Manufacturing and Materials Engineering,
AMME 2014
Statistical modeling and material removal mechanism of electrical discharge machining process with cryogenically cooled electrode
Vineet Srivastavaa'* and Pulak M. Pandeyb
a Department of Mechanical Engineering, SRM Institute of Management & Technology, Modinagar, Uttar Pradesh, India-201204
*email: vineetsrivastava.psit@gmail.com b Department of Mechanical Engineering, Indian Institute of Technology Delhi, New Delhi, India-110016
Abstract
In this work, effect of discharge current, pulse on time, duty cycle and gap voltage has been studied on electrode wear ratio (EWR), material removal rate (MRR) and surface roughness (SR) in EDM process using cryogenically cooled electrode (CEDM). M2 grade HSS is the work piece material. Response surface methodology has been used to plan the experiments. The analysis reveals that discharge current, pulse on time and duty cycle significantly affect EWR and MRR. Discharge current and pulse on time are significant in affecting SR. The mechanism of material removal in CEDM process has also been explained.
© 2014ElsevierLtd.Thisisanopenaccessarticleunder the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).
Selection and peer-review under responsibility of Organizing Committee of AMME 2014
Keywords: Electrical Discharge Machining; cryogenic cooling; electrode wear ratio; material removal rate; surface roughness
1. Introduction
Electrical discharge machining (EDM) is a non-traditional manufacturing process based on removing material from a workpiece by means of a series of repeated electrical discharges between a tool, called electrode, and the workpiece being machined in the presence of a dielectric fluid. When the electrodes are submerged in liquid dielectric bath and the pulses of voltage are applied, it causes a sequence of breakdowns and recoveries of the in-between dielectric (Ho and Newman, 2003). In EDM, the electrode wear problem is very critical since the tool shape degeneration directly affects the final shape of the die cavity. The machinability of a material is a factor of its thermal and electrical properties in EDM. Material's electrical resistivity is dependent on its temperature. Copper as electrode has a low electrical resistance, resulting in a more efficient energy transfer to the work piece. In addition,
2211-8128 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
Selection and peer-review under responsibility of Organizing Committee of AMME 2014 doi: 10.1016/j.mspro.2014.07.533
the cost of a part manufactured by the EDM is determined mainly by the tool cost, which consists of the raw material cost of the tool, the tool production cost and the number of tools required for operation. In most of the EDM operations, the contribution of the tool cost to the total operation cost is more than 70% (Ozgedik and Cogun, 2006). It is known that during the cut by EDM, MRR decreases, which is due to process instability. However, the decrease of MRR is due to the change of métallurgie constituent in the zone affected by the heat. The quality of the surface machined plays an important factor in evaluating the productivity. Surface roughness is a significant design factor which has considerable influence on properties such as fatigue, strength and wears resistance. It is one of the most important measures in finishing operation. It is imperative to achieve a good surface finish (Fonda et al., 2007).
The effect of cryogenic cooling during conventional and non-conventional machining has been evaluated by many researchers. Hong and Broomer (2000) presented improved results by using an economical cryogenic cooling approach designed for CNC turning of AISI 304 after studying the cryogenic properties of the stainless steel material. They observed that by injecting a small amount of liquid nitrogen to the chip-tool interface, their approach yielded a 67% tool-life improvement when compared with conventional emulsion cooling. They also compared different cryogenic machining approaches in the machining test using commercial carbide inserts. The results showed that the cooling approach was crucial in attaining the benefits of cryogenic machining in cutting stainless steel. Venugopal et al. (2007) investigated the effects of cryogenic cooling on growth and nature of tool wear while turning Ti-6A1-4V alloy bars with microcrystalline uncoated carbide inserts under dry, wet and cryogenic cooling environments. Cryogenic cooling by liquid nitrogen jets enabled substantial improvement in tool life through reduction in adhesion-dissolution-diffusion tool wear through control of machining temperature desirably at the cutting zone during turning of Ti-6A1-4V alloy. Stewart (2003) applied cryogenic treatment to C2 tungsten carbide (WC -6% Co) and compared it with untreated carbide to determine if tool wear could be reduced during turning tests with medium density fiberboard (MDF). Both the tool force data and observation of the cutting edges indicated that tool wear was reduced. The cryogenic treatment had an effect upon the cobalt binder by changing phase or crystal structure so that more cobalt binder was retained during cutting.
Paul and Chattopadhyay (1995) employed liquid nitrogen in the form of jet in the surface grinding of various steel specimens and compared it with the surface ground under dry conditions and with soluble oil. There was found appreciable improvement in the chip formation mechanism and reduction in specific energy requirement, grinding temperature and residual stress in cryo-grinding when compared with grinding dry and with soluble oil. Wang and Rajurkar (1997) examined the wear characteristics of a CBN tool while turning silicon nitride under dry cutting and cryogenic cooling conditions. They showed that the tool wear significantly reduces with cryogenic cooling. With the help of finite element analysis, they found that the maximum temperature on the tool falls from 1153°C to 829°C. Abdulkareem et al. (2009) studied the cooling effect of copper electrode on the die-sinking of electrical discharge machining of titanium alloy (Ti-6A1-4V). Analysis of the influence of cooling on the responses has been carried out and presented in their work. It was found that electrode wear ratio reduced up to 27% by electrode cooling. Surface roughness was also reduced while machining with electrode cooling.
From the review of literature, it was found that although attempts have been made to use cryogenic liquid in conventional machining processes, only Abdulkareem et al. (2009) have attempted it in EDM, where they introduced fluid into the electrode (tool) as coolant. However it has been found that there is no study related to the development of models predicting EWR, MRR and surface roughness in cryogenic assisted EDM process. Further it was found that there is no literature regarding the material removal mechanism of cryogenic assisted EDM process Therefore in this work the effect of process parameters, namely discharge current, pulse on time, duty cycle and gap voltage, has been studied on EWR, MRR and SR in EDM process using cryogenically cooled electrode (CEDM). M2 grade high speed steel has been chosen as work piece material and response surface methodology has been used to plan the experiments. ANOVA and regression analysis have been used to model EWR, MRR, and SR. Further, with the help of EDX performed on the EDMed surface layer, material removal mechanism of CEDM process has been explained.
2. Planning of Experiments
2.1. Details of work piece, tool assembly and selection ofprocessparameters
M2 grade high speed steel work pieces have been spark eroded using copper as electrode material. The workpiece used for this study was high speed steel having the dimension of 15x15x15 mm. The hardness of the work piece was 108 HRB. Energy Dispersive X-ray spectroscopy (EDX) was done on non-machined work piece to determine its composition. The percentage composition is presented in Table 1. Copper has been chosen as the electrode material because of its lower electrical and thermal resistance. The electrode tip diameter was chosen as 7 mm. The liquid nitrogen was stored in a Dewar and passed into the container, which has an attached electrode, through an attachment. The electrode setup mounted on the EDM machine has been shown in figure 1.
Table 1: Chemical composition (wt.%) of High Speed Steel
C V Cr Mo W Fe 0.99 2.06 4.21 5.03 6.10 Rest
Figure 1: The electrode setup fixed on the EDM machine
The performance of EDM of steel is governed by a large number of interactive variables. However to facilitate the experimental work, only four controllable variables are considered namely discharge current, pulse-on time, duty cycle and gap voltage. The experiments have been conducted keeping these factors at various levels. The range of each factor has been selected based on the capabilities of the machine and preliminary experiments conducted. When the current was kept below 3A, it was observed that MRR was insignificant and when current more than 7A was selected, it resulted in poor surface finish necessitating the selection of the values at an intermediate level. The range selected for the pulse-on time is commonly used for the EDM based on literature survey. The range selected for the duty cycle covers a wide range of duty cycle. Whereas the range of gap voltage selected is in accordance to that available on the machine used for the experimentation (Srivastava and Pandey, 2011). The process parameters with levels have been given in Table 2.
Table 2: Process parameters with levels.
Units Levels
Factors
-2-1012
Discharge current A 3 4 5 6 7
Pulse-on time 100 200 300 400 500
Duty cycle - 0.24 0.40 0.56 0.72 0.88
Gap voltage (V) 50 55 60 65 70
2.2. Planning of experiments
Die sinking EDM experiments with cryogenically cooled electrode were carried out on EDM machine (Model PS
LEADER ZNC, Electrónica, India). In the experiments, kerosene was used as dielectric medium. The machining time was kept 25 minutes for all the experiments. Total 31 experiments have been carried out in this study. Electrode wear ratio has been defined, as the ratio of the wear weight of electrode to the wear weight of work piece after machining and is given below
EWR(%) = w^M-Weam x 100 (1)
WwBM- WwAM
Similarly material removal rate has been defined as the ratio of the wear weight of workpiece to machining time, MRR(mg/min) = w^m~wwam x 1qo (2)
Where WEBM is weight of electrode before machining, WEAM is weight of electrode after machining, WWBM is weight of work piece before machining, WWAM is weight of work piece after machining and T is the total Utor. during which machining was performed. The surface finish after machining was measured and the centre line average value of the surface roughness (Ra), was selected in this study (Kiran et al., 1998). Each sample was measured three times and average was taken as the response. The measured values of MRR, EWR and surface roughness for each of the experiment have been presented in Table 3.
Table 3: Measured responses corresponding to each trial
Expt. No. Discharge Current (Ip )(A) Pulse On Time (Ton )(MS) Duty Cycle (DC) Gap Voltage (Vs )(V) EWR (%) MRR (mg/min) Surface Roughness (^m)
1 4 200 0.40 55 1.15 7.33 1.94
2 5 300 0.88 60 1.73 32.40 1.98
3 7 300 0.56 60 1.75 16.30 2.57
4 6 400 0.40 55 1.40 15.79 2.68
5 5 500 0.56 60 1.09 29.03 3.18
6 4 400 0.72 55 1.21 23.98 2.35
7 5 300 0.56 60 1.53 14.94 2.47
8 4 400 0.40 55 0.95 13.58 2.43
9 5 300 0.56 50 1.23 18.82 2.27
10 4 200 0.40 65 1.28 8.43 1.94
11 5 100 0.56 60 1.60 12.30 2.32
12 4 200 0.72 65 1.48 14.86 1.95
13 6 200 0.72 65 1.77 21.31 2.18
14 5 300 0.24 60 1.24 11.01 2.16
15 4 200 0.72 55 1.38 15.15 1.97
16 6 200 0.40 65 1.61 10.81 2.18
17 5 300 0.56 60 1.55 14.83 2.55
18 6 200 0.40 55 1.47 10.84 2.11
19 3 300 0.56 60 1.09 7.88 1.80
20 6 400 0.40 65 1.36 15.39 2.55
21 6 400 0.72 55 1.37 34.99 2.86
22 5 300 0.56 60 1.47 13.26 2.44
23 5 300 0.56 70 1.43 16.53 2.33
24 6 400 0.72 65 1.48 29.54 2.56
25 4 400 0.72 65 1.19 20.30 2.29
26 4 400 0.40 65 1.09 12.64 2.46
27 5 300 0.56 60 1.53 14.06 2.59
28 5 300 0.56 60 1.45 14.14 2.43
29 5 300 0.56 60 1.47 13.64 2.56
30 5 300 0.56 60 1.51 13.53 2.47
31 6 200 0.72 55 1.68 24.93 2.30
3. Analysis ofthe experimental data
3.1. Electrode Wear Ratio
A model for the EWR was obtained by analyzing the data presented in table 3, and is given below as equation (3) after eliminating all the insignificant parameters
EWR = -7.49 + (0.397 X Ip) + (0.00141 X Ton) + (0.581 X DC) + (0.232 X Vg) - (0.0241 X ij) -(4 X IQ"6 X T02n) - (0.00186 x Vq2)
1.70 -1.55 -
I 1.40
1.25 -1.10 -
# # # # / / # ^ / * 60
Figure 2: (a) Percentage contribution of the factors on EWR (b) Main effects plots for EWR
ANOVA was used to check the adequacy ofthe developed model. The F-ratio ofthe predictive model (35.37) was compared with the standard tabulated value ofthe F-ratio (3.45) for a specific confidence interval. The analysis revealed that the model is adequate and lack of fit was insignificant. Percentage contributions for each term of the model are shown in figure 2(a). The figure shows that discharge current, pulse on time and duty cycle are the most influential parameters affecting EWR. Figure 2(b) displays the main effect plots for EWR. It can be seen that EWR increases with increase in discharge current. It is observed that EWR reduces with increase in pulse on time. EWR showed an increase with duty cycle. It can be seen from figure 2 (a) & (b) that the gap voltage does not influence the EWR.
Figure 3(a) shows the effect of discharge current on the value of EWR. The surface plot reveals that EWR increases with discharge current. This may be due to the formation of the electrical discharge column in the machining gap, which not only removes the unwanted workpiece material but it also wears out the electrode. Increase in the discharge current causes more electrical discharge energy to be conducted into the machining gap, thereby increasing the EWR (Rahman et al., 2011). The effect of duty cycle on the value of EWR is presented in figure 3 (b). It shows that the value of EWR increases with an increase ofthe duty cycle. An increase in duty cycle leads to generation of higher spark energy, which causes an increase of electrode wear (Patel et al., 2011). This increase of electrode wear eventually resulted into higher EWR. The effect of pulse on time on the value of EWR is presented in figure 3(c). It is observed that an increase in pulse-on time decreases the values of electrode wear ratio. This is due to the fact that the diameter ofthe discharge column increases with the pulse duration which reduces the energy density ofthe electrical discharge on the discharge spot. It has also been reported that at longer pulse-on time, the carbon from the decomposition of hydrocarbon-based dielectric liquid deposits on the surface of the electrode (Chen and Mahdavian, 1999). This deposited layer increases the wear resistance ofthe electrode and reduces EWR.
Pulse On Time(microsec)
Discharge Current(Ampere)
Discharge Current (Amp ere)
Pulse On Time(Microsec)
Figure 3: Response surfaces for EWR
3.2. Material RemovalRate
A model for the material removal rate was obtained by analyzing the data presented in table 3, and is given below as equation (4) after eliminating all the insignificant parameters
MRR = 121 + (2.89 x 1P) - (0.0854 x Ton) - (50 x DC) - (3.5 xVg)~ (0.582 x Jj) + (0.000156 x T2n) +
(71.2 x DC2) + (0.0326 x Vg2) + (10 x IP x DC) + (0.0491 x Ton x DC) - (0.998 x DC X Vg)
ANOVA was used to check the adequacy of the developed model. The F-ratio of the predictive model (99.60) was compared with the standard tabulated value of the F-ratio (3.45) for a specific confidence interval. The analysis revealed that the model is adequate and lack of fit was insignificant. Percentage contributions for each term of the model are shown in figure 4(a). The figure shows that discharge current, pulse on time and duty cycle are the most influential parameters affecting MRR. Figure 4(b) displays the main effect plots for MRR. It can be seen that MRR increases with discharge current. It is also observed that MRR increases with pulse on time. Further it can also be observed that increase in the duty cycle increases MRR. It can be seen from figure 4 (a) & (b) that the gap voltage is not a significant parameter to influence MRR.
Figure 5(a) shows the effect of discharge current on MRR. MRR increases with the increase in discharge current. This could be due to an increase in both diameter and the depth of the craters as well as discharge energy at the discharge point to improve the rate of melting and evaporation. The effect of duty cycle on the value of MRR is presented in figure 5(b). It can be seen that an increase in the duty cycle leads to increase of the MRR. The increase of duty cycle effectively means applying the discharge for a longer duration and this causes an increase in MRR.
The effect of pulse on time on the value of MRR is presented in figure 5(c). It can be seen that MRR decreases with the increase in pulse-on time initially but after a certain value of pulse-on time, it increases. Due to lower initial discharge energy, some of the melt material re-solidifies on the work piece, which leads to decrease in MRR. However, after a certain value of pulse-on time, increase in the discharge energy conducted into the machining gap within a single discharge causes the MRR to increase (Chiang, 2008).
IpxOC 0CxV8
Ton x DC _1%
1 * 6 1 ' i # # # # / / / / / <? i <? * t
Figure 4: (a) Percentage contribution of the factors on MRR (b) Main effects plots for MRR
fiiiJseiOn Time(mfas£ee)
Figure 5: Response surfaces for MRR
3.3. Surface Roughness
A model for the surface roughness was obtained by regression analysis of the data presented in table 3, and is given below as equation (5) after eliminating all the insignificant parameters
Ra = -10.6 + (1.13 x Ip) - (0.00082 x Ton) + (4.22 x DC) + (0.285 xVg)~ (0.0969 x ij) + (5 x 10"6 x T2n) - (3.81 X DC2) - (0.0024 X V2) (5)
ANOVA was used to check the adequacy of the developed model. The F-ratio of the predictive model (19.02) was compared with the standard tabulated value of the F-ratio (3.45) for a specific confidence interval. The analysis revealed that the model is adequate and lack of fit was insignificant. Percentage contributions for each term of the model are shown in figure 6(a). The figure shows that discharge current and pulse on time are the most influential parameters affecting SR. Figure 6(b) displays the main effect plots for SR. It can be seen that SR increases with discharge current. It is also observed that SR increases with pulse on time. It can be seen from figure 6 (a) & (b) that duty cycle and gap voltage does not affect the SR.
Figure 7 shows the effect of pulse-on time and discharge current on surface roughness. The surface plot reveals that surface roughness increases with discharge current. As the discharge current increases, the discharge energy density and the impulsive force increase and results in the formation of deeper and larger discharge craters, which in turn increases surface roughness. It can be further observed that increase in pulse-on time with increase in discharge current increases the surface roughness. This is due to the expansion of plasma channel with an increase in pulse-on time (Pandey and Jilani, 1986), which reduces both energy density and the impulsive force. As a result, the melted debris cannot be removed completely leading to increase in the surface roughness.
p I / ! 1 ß *
T » 'I -
Mi^ $ of i * * * * *
Figure 6: (a) Percentage contribution of the factors on SR (b) Main effects plots for SR
Figure 7: Response surface for SR
4. Mechanism of material removal in EDM with cryogenically cooled electrode
M2 grade of high speed steel has good electrical conductivity and so is easily machinable by EDM. Roethel et al. (1976) reported that during electric discharge machining, the material can be transferred between the electrodes in solid, molten or gaseous state simultaneously. EDX was performed on the EDMed surface layer for the M2 grade high speed steel machined by cryogenically cooled electrode. It is evident that Fe, C, W, Mo, Cr and V are the main contents in the resolidified layer as shown in table 4. The presence of Cu has not been observed in the resolidified layer. This implies that there is absence of any visible electrode material transfer to the work piece surface during the electrical discharge machining process.
Table 4: Chemical composition (wt. %) of M2 grade High Speed Steel after machining C V Cr Mo W Fe 15.37 1.56 3.29 3.37 5.38 Rest
A comparison of the percentage constituents, of work piece before machining given in table 1 and machined work piece given in table 4, shows that there is a large enrichment in carbon content in the white layer with respect to the base material. The drastic increase of carbon over the machined surface was due to the deposition of carbon separated from the consumed kerosene dielectric medium mainly as a result of dielectric cracking (Mamalis et al., 1987). There are three theories of material removal mechanism have been proposed on the basis of experimental results: high temperature theory (thermal, electro-thermal mechanism), static field theory (electromechanical mechanism) and high pressure theory (Mishra, 1997). However, the high temperature theory is widely acceptable which can take care to explain the major portion of the stock removal phenomenon in EDM. This indicates that material removal is mainly due to melting and evaporation. Cryogenic cooling in the present experimental work has reduced MRR and EWR (figure 8) which again supports the applicability of high temperature theory in EDM. There was minor amount (< 1%) of oxygen found in the samples after machining. Therefore it can be concluded that to some extent material removal is also due to oxidization and decomposition. Similar observations have been recorded by Mamalis et al. (1987) for EDM of steel and Patel et al. (2009) for EDM of ceramics.
I EWR of CEDM process ] EWR of EDM process
) 15 20 25 30
Experiment Number
Experiment Number
Figure 8: Comparison between conventional EDM process and CEDM process for a) EWR b) MRR
In CEDM process, it can also be concluded that the mechanism of sparking and material removal does not change, but due to cryogenic cooling of the electrode, the temperature of the electrode decreases. Due to this reduction in temperature, more time is taken up for ignition of the discharge to be initiated as compared to EDM. As the ignition of the discharge starts, high amount of heat is generated in the ionized zone. However some amount of heat generated in ionized zone is utilized to maintain the temperature of the electrode at elevated level. As a result, the effective heat that is transferred to the work piece is reduced resulting smaller formation of the crater. This resulted in reduction of MRR as compared to EDM.
5. Conclusions
In this work, EDM with cryogenically cooled electrode has been successfully performed on M2 HSS workpiece material. Statistical models have been developed for predicting MRR, EWR, and surface roughness in CEDM by correlating the input parameters, namely, discharge current, pulse-on time, duty cycle, and gap voltage. In CEDM process, the experimental results revealed that discharge current, pulse-on time, and duty cycle significantly affect the MRR and EWR, while discharge current and pulse-on time affect the surface roughness. It can be concluded that the mechanism of sparking and material removal does not change, but due to cryogenic cooling of the electrode, the temperature of the electrode decreases, resulting in smaller formation of the crater. This results in reduction of MRR as compared to EDM.
Acknowledgements
The authors gratefully acknowledge the financial support provided by Department of Science & Technology (DST), New Delhi, India for carrying out this work.
References
Abdulkareem, S., Khan, A.A., Konneh, M., 2009. Reducing electrode wear ratio using cryogenic cooling during electrical discharge machining.
International Journal of Advanced Manufacturing Technology 45, 1146-1151. Chen, Y., Mahdavian, S.M., 1999. Parametric Study into Erosion Wear in a Computer Numerical Controlled Electro Discharge Machining Process. Wear 236, 350-354.
Chiang, K., 2008. Modeling and Analysis of the Effects of Machining Parameters on the Performance Characteristics in the EDM Process of
Al203+TiC Mixed Ceramic. International Journal of Advanced Manufacturing Technology 37, 523-533. Fonda, P., Wang, Z., Yamazaki, K., Akutsu, Y., 2007. A fundamental study on Ti- 6Al-4V's thermal and electrical properties and their relation to
EDM productivity. Journal of Materials Processing Technology 202 (1- 3), 583-589. Ho, K.H., Newman, S.T., 2003. State of the art electrical discharge machining (EDM). International Journal of Machine Tools & Manufacture 43, 1287-1300.
Hong, S.Y., Broomer, M., 2000. Economical and ecological cryogenic machining of AISI 304 austenitic stainless steel. Clean Production & Processing 2, 157-166.
Kiran, M.B., Ramamoorthy, B., Radhakrishnan, V., 1998. Evaluation of surface roughness by vision system. International Journal of Machine Tools & Manufacture 38, 685-690.
Mamalis, A.G., Vosniakos, G.C., Vaxevanidis, N.M., 1987. Macroscopic and microscopic phenomena of electro-discharge machined steel
surfaces: An experimental investigation. Journal of Mechanical Working Technology 15 (3), 335-356. Mishra, P.K., 1997. Nonconventional Machining. Narosa Publishing House, London.
Ozgedik, A., Cogun, C., 2006. An experimental investigation of tool wear in electric discharge machining. International Journal of Advanced
Manufacturing Technology 27, 488-500. Pandey, P.C., Jilani, S.T., 1986. Plasma channel growth and the resolidified layer in EDM. Precision Engineering 8 (2), 104-110. Patel, K.M., Pandey, P.M., Rao, P.V., 2009. Determination of an Optimum Parametric Combination Using a Surface Roughness Prediction
Model for EDM of Al203/SiCw/TiC Ceramic Composite. Materials & Manufacturing Processes 24, 675-682. Patel, K.M., Pandey, P.M., Rao, P.V., 2011. Study on Machinability of A1203 Ceramic Composite in EDM Using Response Surface
Methodology. Journal of EngineeringMaterials Technology 133, 021004-1-021004-10. Paul, S., Chattopadhyay, A.B., 1995. A study of effects of cryo-cooling in grinding. International Journal of Machine Tools & Manufacture 35 (1), 109-117.
Rahman, M.M., Khan, M.A.R., Kadirgama, K., Noor, M.M., Bakar, R.A., 2011. Experimental Investigations into Electrical Discharge Machining
of Stainless Steel 304. Journal of Applied Science 11 (3), 549-554. Roethel, F., Kosec, L., Garbajs, V., Peklenik, J., 1976. Contribution to the micro-analysis of the spark eroded surfaces. Annals of CIRP 25 (1), 135-140.
Srivastava, V., Pandey, P.M., 2011. Performance evaluation of EDM process using cryogenically cooled electrode. Materials & Manufacturing Processes 27 (6), 683-688.
Stewart, H.A., 2003. Cryogenic treatment of tungsten carbide reduces tool wear when machining medium density fibreboard. Forest Production Journal 54 (2), 53-56.
Venugopal, K.A., Paul, S., Chattopadhyay, A.B., 2007. Tool wear in cryogenic turning of Ti-6A1-4V alloy. Cryogenics 47, 12-18. Wang, Z.Y., Rajurkar, K.P., 1997. Wear of CBN tool in turning of silicon nitride with Cryogenic cooling. International Journal of Machine Tools & Manufacture 37 (3), 319-326.
