Effect of cryogenically treated copper-tungsten electrode on tool wear rate during electro-discharge machining of Ti-5Al-2.5Sn alloy

Wear 386-387 (2017) 223–229

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Effect of cryogenically treated copper-tungsten electrode on tool wear rate during electro-discharge machining of Ti-5Al-2.5Sn alloy Sanjeev Kumar a, Ajay Batish b, Rupinder Singh c, Anirban Bhattacharya d,n a

Department of Mechanical Engineering, Chandigarh Engineering College, Landran, Punjab, India Department of Mechanical Engineering, Thapar University, Patiala, Punjab, India c Department of Production Engineering, GNDEC, Ludhiana, Punjab, India d Department of Mechanical Engineering, IIT Patna, Bihar, India b

art ic l e i nf o

a b s t r a c t

Article history: Received 13 September 2016 Received in revised form 6 January 2017 Accepted 18 January 2017

Titanium, due to its excellent properties with high strength to weight ratio, is increasingly being used for various applications. Electric discharge machining (EDM), due to its unique features, is extensively used for machining of titanium and its alloys as it is difficult to machine titanium with conventional machining. EDM, however is most suited for electrically conductive materials and to make up for the poor thermo-electrical properties of Ti-5Al-2.5Sn titanium alloy, deep cryogenic treated (
184 °C) coppertungsten (Cu-W) tool has been used in this study for machining. This study reports the effect of deep cryogenic treatment (DCT) on tool wear rate during electric discharge machining of Ti-5Al-2.5Sn titanium alloy by varying various process parameters namely cryogenic treatment of electrode material, peak current, pulse-on & off time and flushing pressure. Fractional factorial experimental design was used for designing the experimental study and the results were statistically analyzed to obtain optimal combination of input process parameters for minimizing tool wear. Peak current was observed to be the most significant process parameter in relative comparison to other input parameters. The results show a significant reduction in tool wear rate in case of DCT Cu-W electrode when compared to untreated Cu-W electrode. Surface characteristics of select Cu-W electrode and workpiece samples were analyzed using scanning electron microscope (SEM), energy dispersive spectrograph (EDS) and X-ray diffraction (XRD). Various chemical compounds specifically titanium carbide (TiC) were seen on both the machined and the tool surface due to material transfer during machining. & 2017 Elsevier B.V. All rights reserved.

Keywords: Electric discharge machining Titanium alloy Tool wear rate Deep cryogenic treatment Taguchi methodology ANOVA

1. Introduction Titanium is a low density element which can be strengthened greatly by alloying and deformation processing. When considering alloyed and unalloyed titanium, it is classified into as commercial pure, alpha and near alpha, alpha-beta. Ti-5Al-2.5Sn alloy comes under alpha and near alpha alloy is preferred for high temperature as well as cryogenic applications. The productivity during machining of titanium alloys is adversely affected by rapid tool wear using traditional machining processes due to its poor thermal conductivity and chemical reactivity. Rapid tool wear encountered in machining of titanium alloys is a challenge that needs to be overcome [1]. The Electrical Discharge Machining (EDM) is one of the better options in non-traditional machining processes for machining of titanium and its alloys, because there is no direct contact between the tool and workpiece. EDM is a thermo-electric process in which n

Corresponding author.

http://dx.doi.org/10.1016/j.wear.2017.01.067 0043-1648/& 2017 Elsevier B.V. All rights reserved.

metal is removed from the surface of workpiece by a repeated electric sparks generates between a work and tool immersed in a dielectric fluid. Due to generation of successive electric sparks high temperature occurs (8000–12,000 °C), resulting in removal of material from workpiece and tool surface by melting and vaporization. Metal is removed from workpiece as well as from the tool in the form of craters. To minimize the wear of tool/electrode machining condition, polarity and electrode material should be selected carefully [2]. In EDM, higher electrical conductivity, good thermal conductivity and low tool wear rate are essential requirement of tool material. Chen et al. observed low electrode wear ratio during EDM of Ti-6Al-4V in distilled water as compared to kerosene dielectric [3]. Yan et al. investigated the machining characteristics of Ti-6Al-4V alloy using two processes EDM and USM jointly and explored that MRR is larger when using distilled water as the dielectric medium than when using kerosene and the relative electrode wear ratio is smaller [4]. Hascalik and Caydas machined Ti-6Al-4V alloy using EDM process with different copper, aluminum and graphite electrodes by varying pulse current and pulse duration. Experimental results showed that electrode

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wear has tendency of increase with increasing current density and pulse duration. Graphite electrode material showed the lowest wear rate due to its higher melting point during all process conditions [2]. Pradhan et al. optimized the machining parameters for micro-EDM of Ti-6Al-4V alloy and reported that peak current has the maximum effect on TWR [5]. Yan et al. used urea solution in water as dielectric during EDM of pure titanium and concluded that electrode wear rate (EWR) increased with increase in peak current. Moreover, EWR decreased with increase in pulse duration [6]. Ferreria used three grades of copper-tungsten (Cu-W) electrodes during EDM of AISI H13 die steel to study surface finish of die helical thread cavity and reported that for Cu20W80 grade, the relative TWR decreases with increasing peak current and decreasing pulse-on duration [7]. Marafona suggested that electrode wear ratio decreases with the increase in percentage of equivalent carbon during erosion period in EDM of BS 4695 D2 die steel with Cu-W electrode [8]. Kao et al. observed that discharge current significantly affected the electrode wear ratio with almost 70% contribution in EDM of Ti-6Al-4V alloy [9]. Tang et al. used tap water in place of hydrocarbon dielectric in EDM of Ti-6Al-4V alloy. Taguchi method with grey relational analysis (GRA) was applied to optimize the multiple characteristics and observed the significant improvements in results [10]. Medellin et al. used the two electrode materials namely brass and bronze in EDM of D2 die steel using the mixture of tap water and deionized water. The experimental results revealed that by using 75% tap water and 25% deionized water mixture as dielectric, the maximum MRR and the minimum EWR can be obtained [11]. The major drawback of the conventional EDM process is low MRR, higher TWR and poor surface finish which restrict its use in manufacturing industries. To improve the machining efficiency of a process, suitable metal particles in powder form are mixed with the dielectric. The powder concentration is the most critical factor that highly affects the machining performance. These metal particles act as a conductor and are responsible for the significant reduction in the breakdown strength of the insulating liquid between the spark gap. Due to contamination, gap size is increased as well as ignition process improved, thus, stability of the machining process is also improved. Due to interlocking of metallic powders, rate of generation of sparks becomes faster, hence, the erosion process becomes faster, causing an increase in MRR and reduction in tool wear. At the same time, plasma channel is modified due to mixed powder particles, thus, uniformly distributes the sparks, which decreases the density of spark. Due to this issue, shallow craters are produced, which improve the surface finish of the machined parts [12,14,15]. Powder or additive mixed EDM is one of the famous hybrid machining process that can be used for enhancing the capabilities of EDM process. In this

process, gap between the two electrodes can be reduced resulting in more stable machining, thus, reducing the electrode wear. Researchers used various powders with dielectric and obtained significant improvement in terms of low tool wear and higher MRR with better surface finish [12–18]. Cryogenic treatment or cryogenic cooling method can improve the machining characteristic resulting in reduced tool wear. Results in the literature show tool life improvements from 92% to 817% when using the cryogenically treated HSS tools in the industry [19]. Venugopal et al. reported that cryogenic cooling with liquid nitrogen jets enables substantial reduction in tool wear in turning of Ti-6Al-4V alloy [20]. Abdulkareem et al. investigated the cooling effect of Cu electrode during EDM of Ti-6Al-4V alloy by using liquid Nitrogen (LN2) inside the special designed electrode. This improves the thermal and electrical conductivity of the electrode which results in reduction of TWR by 27% [21]. Gill and Singh investigated the effect of DCT Ti6246 titanium alloy during Electric Discharge Drilling (EDD) and reported reduction of TWR by 34.78% due to increase in thermal and electrical conductivity of the workpiece material [22]. Jafferson and Hariharan compared the experimental results of both cryogenically treated and without cryogenically treated micro electrodes (brass tube, copper tube and tungsten rod). Significant reduction of 58%, 51% and 35% in TWR for tungsten, brass and copper electrode respectively was observed [23]. Kumar et al. reported significant improvement in TWR and wear ratio in EDM of Inconel 718 workpiece when using cryogenically treated copper electrode [24]. Improvement in TWR by application of cryogenic cooling and treatment on workpiece or tool is reported by other researchers [25–29]. Cryogenic treatment aims at improving the EDM efficiency and reducing the tool wear rate. The literature review shows that efficiency of EDM process is significantly impacted by cryogenic treatment and can be enhanced because thermo-electric properties of material are improved. The present study is, therefore, aimed investigating machining efficiency during EDM of Ti-5Al2.5Sn titanium alloy with deep cryogenically treated (DCT) coppertungsten (Cu-W) electrode and without cryogenically treated copper-tungsten electrode. The parameters selected for the study were cryogenic treatment of electrode material, peak current, pulse-on-time, pulse-off- time and flushing pressure. Machining efficiency is evaluated in terms of reduction in TWR.

2. Materials and methods The experiments were conducted on commercial CNC Electric Discharge Machine (Model OSCARMAX S 645 CMAX manufactured

Fig. 1. (a) Pictorial view of CNC EDM machine, (b) Actual machining operation, and (c) Pictorial view of copper-tungsten electrode.

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Table 1 Chemical analysis (%) of Ti-5Al-2.5Sn alloy.

225

Table 3 Experimental levels of the machining parameters.

Element

Ti

Al

Sn

Fe

C

O

N

H

Symbol Control parameters

Level-1

Level-2

Level-3

Wt (%)

91.806

5.32

2.76

0.025

0.013

0.069

0.0032

0.0035

A

WCT*

DCT*

–

06 amp 90 ms 30 ms 0.5 kg/cm2

10 amp 120 ms 45 ms 0.6 kg/cm2

14 amp 150 ms 60 ms 0.7 kg/cm2

B C D E

Table 2 Chemical analysis (%) of copper-tungsten electrode. Element

Cu

Zn

W

Pb

Sn

Fe

Ni

Wt (%)

29.27

0.68

68.10

0.023

0.012

0.46

0.56

by OSCAR EDM Co. Ltd., Taichung, Taiwan) as shown in Fig. 1(a). The workpiece material Ti-5Al-2.5Sn alloy was ground on surface grinding machine to ensure parallelism before running the experiments. The chemical composition of the alloy is shown in Table 1. A 18 mm diameter and 40 mm length copper-tungsten electrode was used during machining. The face of each electrode was ground using emery paper to ensure flatness as shown in Fig. 1(c). Face of each electrode and workpiece was properly polished/ground to ensure alignment of two faces. Actual machining operation performed is shown in Fig. 1(b). The chemical composition of the copper-tungsten electrode material is given in Table 2. In addition, FERROLAC 3M EDM Oil was used as dielectric medium during this study. A suitable fixture was developed to hold the Ti5Al-2.5Sn alloy because of its non-magnetic property. Deep cryogenic treatment (DCT) of Cu-W electrode was carried out in cryogenic processor (make; Primero EnServe, CP 220LH) with temperature range
184 °C to 150 °C having a size of 450 mm  1200 mm  450 mm to investigate their effect on TWR. During the DCT, the Cu-W electrodes were cooled down from ambient temperature to ultra-sub low temperature i.e. temperature of liquid nitrogen. During the DCT process, the temperature of the tool electrodes was brought to
184 °C at a cooling rate of 1 °C/min. The electrodes were then kept in the insulated chamber for 24 h at constant temperature. The temperature was thereafter raised to room temperature slowly at a heating rate of 1 °C/min. The deep cryogenic treatment cycle for copper-tungsten electrode is shown in Fig. 2. However, there are a number of machining parameters to be considered during machining of Ti-5Al-2.5Sn alloy. The input process and operating parameters used were cryogenic treatment of electrode material, peak current, pulse-on-time, pulse-off- off time and flushing pressure. These parameters and their respective levels are presented in Table 3 (please refer table foot note for abbreviations, e.g. WCT - without cryogenic treatment, DCT - deep

* WCT- Without cryogenic treatment, DCT- Deep cryogenic treatment.

cryogenic treatment). These experiments used a positive polarity to electrode and negative polarity to workpiece material. In this study, the electrode material was varied at two levels while the remaining parameters were varied at three levels. Since we had a mixed two level and three level design a mixed level L18 (21  37) orthogonal array with five columns and eighteen rows was used for designing the experiments. MINITAB statistical software, version 17 was used for assigning the factors to the array. Another important thing is that to reduce the influence of noise factors, all the 18 experiments were repeated with the same set of input parameters three times in random order. Two level factor, cryogenic treatment of electrode materials (A) was assigned in the first column of the array and the other factors (all three levels) namely peak current (Ip) (B), pulse-on-time (Ton) (C), pulse-off-time (Toff) (D), flushing pressure (E) in the remaining columns. The assignment of actual parameter values with trial run conditions is shown in Table 4. The microscopic study of selected electrode was analyzed using scanning electron microscope (SEM), energy dispersive spectrograph (EDS) and X-ray diffraction (XRD). Micro-structural analysis was carried out on a Scanning Electron Microscopy (SEM) analyzer (JEOL, JSM-6610LV, JAPAN) to examine the surface characteristics. The weight percentages of various chemical elements were measured using Energy Dispersive Spectroscopy (EDS)/Energy Dispersive X-ray analysis (EDX) analyzer. To investigate the possibility of formation of various compounds on the machined workpiece surface, X-Ray Diffraction (XRD) analysis was performed using PANALYTICAL X- ray diffractometers, Netherlands along with X’Pert High Score Plus and Origin Pro 8 software. Before conducting the SEM, EDS and XRD analysis, tool sample was cleaned properly with acetone solution and then etched with a solution Table 4 L18 (21  37) orthogonal array, control parameters and observed mean values of TWR. Exp No. Control parameters

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. Fig. 2. Deep cryogenic treatment cycle for copper-tungsten tool material.

Cryogenic treatment of electrode Peak current (Ip) Pulse-on-time (Ton) Pulse-off-time (Toff) Flushing pressure

A

B

C

D

E

Mean of TWR (mm3/min)

WCT WCT WCT WCT WCT WCT WCT WCT WCT DCT DCT DCT DCT DCT DCT DCT DCT DCT

6 6 6 10 10 10 14 14 14 6 6 6 10 10 10 14 14 14

90 120 150 90 120 150 90 120 150 90 120 150 90 120 150 90 120 150

30 45 60 30 45 60 45 60 30 60 30 45 45 60 30 60 30 45

0.5 0.6 0.7 0.6 0.7 0.5 0.5 0.6 0.7 0.7 0.5 0.6 0.7 0.5 0.6 0.6 0.7 0.5

0.145 0.157 0.135 0.165 0.195 0.245 0.458 0.585 0.535 0.115 0.145 0.135 0.115 0.205 0.145 0.465 0.400 0.475

Std. Deviation S/N Ratio (dB)

0.0087 0.0044 0.0087 0.0132 0.0132 0.0087 0.0122 0.0087 0.0132 0.0050 0.0050 0.0132 0.0087 0.0087 0.0050 0.0087 0.0304 0.0087

16.7726 16.0820 17.3933 15.6503 14.1993 12.2167 6.7827 4.6569 5.4329 18.7860 16.7726 17.3933 18.7860 13.7649 16.7726 6.6509 7.9588 6.4661

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S. Kumar et al. / Wear 386-387 (2017) 223–229

having compositions of 20 ml of Sodium-Hydroxide (NH4OH), 20 ml of Water (H2O) and 20 ml of Hydrogen-Peroxide (3% H2O2). Secondary electron image (SEI) detector working distance 15 mm and acceleration voltage 15 kV is used for SEM analysis.

3. Results and discussions

Table 6 Response table for signal to noise ratios (smaller is better). Level

A

B

C

D

E

1 2 3 Delta Rank

12.132 13.706 – 1.574 4

17.200 15.232 6.325 10.875 1

13.905 12.239 12.613 1.666 2

13.227 13.285 12.245 1.040 5

12.129 12.868 13.759 1.630 3

TWR was evaluated after each experimental run. The volumetric loss in weight of electrode per unit machining time is known as tool wear rate (TWR). The weight of tool before and after each machining operation was taken carefully three times on electronic weighing machine. Finally, average value of loss in electrode weight was taken as a measure of TWR. The average values of TWR after each run is presented in Table 4. Since TWR is a ‘lower the better’ characteristic, Eq. (1) was used to measure the signal – to-noise ratio (S/N ratio). n

⎤

i=1

⎥⎦

∑ yi2⎥

(1)

The observed values of TWR and calculated values of S/N ratios are shown in Table 4. The significant parameters which influence the TWR were determined by using analysis of variance (ANOVA) approach by comparing the statistically calculated F-value with standard tabulated Ftab-values at 95% confidence level. For significance of different parameters, F-values should be greater than Ftab- values (refer Table 5). By observing Table 5, it can be concluded that peak current (factor-B) is the most significant factor that significantly affected the TWR. Cryogenic treatment of electrode materials (A), pulse-on-time (C) and flushing pressure (E) also significantly influences the TWR, whereas, pulse-off-time (D) shows insignificant effect on TWR as compared to other control parameters as shown in Table 5. The delta value and ranking of individual factors is provided in Table 6, which shows peak current has the top rank, pulse-on-time second, flushing pressure third, cryogenic treatment of electrode material fourth and pulse-off-time fifth rank. Fig. 3(a) shows the graphical representation of TWR for each experiment. In this fig. experiment number 01–09 shows TWR of without cryogenic treatment (WCT) Cu-W electrode, whereas, experiment number 10–18 shows the TWR of DCT Cu-W electrode. It can be seen from this fig. that TWR decreases in case of DCT electrode (exp. 10–18). Fig. 3(b) shows the graphical representation of standard deviation. A graph is plotted for TWR of WCT and DCT electrode by taking the average value of TWR (experiment 1–9) and (experiment 10–18) as shown in Fig. 4. Improvement in TWR with DCT electrode was observed by 15.86%. The main effect plot of TWR for S/N ratio is shown in Fig. 5. This main effect plot suggests that DCT copper-tungsten electrode, 6 amp peak current, 90 ms pulse-on-time, 30 ms pulse-off-time and 0.7 kg/cm2 flushing pressure gives the low TWR. Therefore, the optimum condition of input process parameters produce Table 5 Analysis of variance for SN ratios. Source of variation

DF Seq SS

Adj MS

F

P

Remarks

CT of electrode Peak current (Ip) Pulse-on-time (Ton) Pulse-off-time (Toff) Flushing pressure Residual error Total

1 2 2 2 2 8 17

11.147 201.479 4.584 2.049 3.998 0.734

15.19 274.54 6.25 2.79 5.45

0.005 0.000 0.023 0.120 0.032

Significant Most Significant Significant Not Significant Significant

11.147 402.958 9.168 4.099 7.995 5.871 441.238

S¼ 0.8567, R-Sq¼ 98.7%, R-Sq(adj)¼ 97.2%, Ftab ¼ F (1,8) ¼5.3177, Ftab ¼ (2,8) ¼ 4.4590.

(a 0.0350 Standard Deviation

⎡1 LB: S/Nratio = − 10 log10 ⎢ ⎢⎣ n

0.0300 0.0250 0.0200 0.0150 0.0100 0.0050 0.0000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Experiment Number

Fig. 3. (a): Graphical representation of TWR. (b): Graphical representation of standard deviation.

Fig. 4. Difference between TWR for WCT electrode and DCT electrode.

minimum TWR can be written as A2B1C1D1E3. Where A, B, C…. represent the control factors and 1, 2 or 3 depict the level at which factors were varied. By referring to Fig. 5, it is observed that DCT copper-tungsten electrode contributes in low TWR. Higher S/N ratio is shown for DCT electrode which indicates the reduction in TWR as compared to without cryogenic treatment (WCT). It is well known that, cryogenic treatment has a history to improve mechanical, electrical and thermal properties of materials [23]. More refinement of grain particles observed in case of DCT electrode as compared to untreated electrode. During cryogenic treatment as temperature

S. Kumar et al. / Wear 386-387 (2017) 223–229

227

Fig. 5. Main Effects Plot of TWR for S/N Ratios (Smaller-the-better).

goes down, thermal vibration of atoms become weaker resulting in easy movement of electrons in a metal. Due to this phenomenon, electrical conductivity of electrode increases which reduces the bulk electrical heating of metal. As per Wiedemann–Franz– Lorenz Law, thermal conductivity of metal increases with the increase in electrical conductivity. This issue leads to quick removal of heat from the surface of electrode, which reduces the excessive melting and vaporization of tool material, hence, reduces the TWR [22,23]. Peak current is one of the most critical machine input parameter which adversely influences TWR followed by pulse-ontime. The electric discharge energy is directly proportional to current and inversely proportional to pulse-on-time. As the value of current and pulse-on-time is increased, it results in more discharge energy to the machining zone. Pulse-on-time is the period for which energy is supplied to the machining zone. Higher energy level directly raises the temperature in spark zone. This issue leads to more melting and evaporation from bottom surface of tool material or in other words TWR would be increased [2,30]. Thus, for low TWR, low value of current and pulse-on-time should be used. By observing Fig. 5, TWR increases sharply from 6 A to 10 A current because pulse energy during electric discharge is increased, thus, metal removal from the Cu-W electrode increased, consequently TWR increased. But further increase in current from 10 A to 14 A slope of TWR curve declines. At higher current 14 A arcing occurs which results in unstable machining, thus, increasing TWR. Moreover, at higher current due to high heat energy, decomposition of hydrocarbon dielectric takes place. Large amount of carbon decomposed from dielectric. This cracked carbon interacts with the elements of copper-tungsten electrode and Ti-5Al2.5Sn alloy and produces various compounds, specially, titanium carbide (TiC). This, TiC compound is stick on the bottom surface of electrode in layer form, which results in arcing and results in higher TWR and low MRR [2]. It is clear in Fig. 5, that increasing flushing pressure (factor E) reducing the TWR. At low flushing pressure, the concentration of the debris particles in the gap are increases rapidly as the machining progress, thus, creating the problem during the machining operation. For effective machining these particles must be removed from the gap quickly so that fresh dielectric enters the gap for spark discharges. Thus, higher flushing pressure quickly removes the eroded particles from the spark gap and cleans the surface of both workpiece and electrode, resulting in higher MRR and low TWR.

4. Microscopic analysis In EDM process, due to generation of high temperature, surface and metallurgical properties of tool surface is also affected. From the micrographs, it can be seen that the process produces irregular surface texture and also defects such as globules of debris, pinholes, and spherical particles with craters of varying sizes, microcracks, pock marks, spherical nodules, recast layer, pull out materials on tool surface. The peak current was observed to be the most significant factor, which highly affected the surface properties of electrode surface. The SEM images are in agreement with the results reported by Hascalik and Caydas [2]. Fig. 6(a) represents micrograph of WCT Cu-W tool for experiments number 3 conducted at 6 A current, Ton 150 μs, Toff 60 μs, flushing pressure 0.7 kg/cm2, whereas, Fig. 6(b) shows the micrograph of DCT Cu-W electrode for experiment number 12 conducted at 6 A current, Ton 150 μs, Toff 45 μs, flushing pressure 0.6 kg/cm2 at a magnification of 1000 X. Various surface defects can be clearly seen in the micrographs. In both the figures surface crack, spherical nodules, craters, holes, compound formation can be clearly seen on the tool surface. Formation of recast layer on tool surface is also seen. Due to high thermal energy generated in the spark zone, the materials was also removed from the surface of the electrode, but significantly lower than what is eroded from the surface of workpiece surface. Since the dielectric used is hydrocarbon oil. During the process, due to high temperature this dielectric oil is decomposed to its chemical components especially carbon. Carbon is precipitated on workpiece surface as well as electrode surface in free form or compound form. This cracked carbon interacts with the base element of workpiece surface and electrode surface in different carbide forms. Titanium is the major element which interacts with cracked carbon and produces the titanium carbide (TiC). There is a sufficient time of diffusion and the formation of the TiC compounds on both surfaces. The formation of TiC layer on tool surface reduces the wear of electrode. Various elements such as: copper, titanium, aluminum, oxygen, aluminum, and tungsten were also observed on the tool surface. Carbon has the maximum weight percentage of 60.79% followed by oxygen 19.79%, aluminum 3.99%, titanium 2.85%, copper 12.21% and tungsten 0.37% respectively. Fig. 7 shows the XRD pattern of WCT Cu-Cr electrode obtained after machining of Ti-5Al-2.5Sn titanium alloy for experiment

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to a very low value, these compounds are not shown on the XRD pattern.

5. Conclusions In the present work, Ti-5Al-2.5Sn alloy was machined by EDM process with different machining conditions to investigate their effects on TWR. Moreover, WCT and DCT copper-tungsten electrode were used to study their effects on TWR. Peak current was observed to be most significant process parameter that badly affected the TWR. Cryogenic treatment of electrode material, pulseon-time and flushing pressure also observed the significant factor. Insignificant effect of pulse-off-time was noticed on the TWR. By comparing the TWR of WCT electrode and DCT electrode, a total improvement of 15.86% was observed in case of DCT electrode than WCT electrode. This improvement is because of the refinement of grain particles in case of DCT electrode as compared to untreated electrode. As the temperature is reduced during cryogenic treatment, thermal vibration of atoms become weaker resulting in easy movement of electrons in a metal which increases the electrical conductivity of electrode. This reduces the bulk electrical heating of metal. This leads to quick removal of heat from the surface of electrode thereby reducing the TWR. The micrograph of selected electrode sample showed various surface defects such as micro-cracks, craters of bigger and smaller sizes, pin holes, pock-marks, spherical nodules etc. which is increased due to higher peak currents. Migration of various elements from workpiece and dielectric can be seen in EDX graph. Carbon is the major element transferred from the dielectric after its decomposition at higher temperature. This cracked carbon produces different carbides which can be seen on surface of tool. Overall conclusions of the present study is that peak current highly influences TWR and after that deep cryogenic treatment of electrode material. Fig. 6. (a) SEM Micrograph of WCT Cu-W Electrode for Exp. No.3 (Ip 6 A, Ton 150 μs, Toff 60 μs, flushing pressure 0.7 kg/cm2). (b) SEM Micrograph of DCT Cu-W Electrode for Exp. No. 12 (Ip 6 A, Ton 150 μs, Toff 45 μs, flushing pressure 0.6 kg/cm2).

Fig. 7. XRD pattern of Cu-W electrode (WCT) after the machining of Ti-5Al-2.5Sn alloy.

number 3. Titanium-Carbide (TiC), Aluminum-Titanium-Carbide (Al2Ti4C2), Tin-Titanium-Carbide (SnTi2C), Nickel-Aluminum-Titanium-Carbide (Al0.5Cni3Ti0.5), Titanium-Tungsten-Carbide (WTiC) were observed at different 2-theta positions from 34.70 to 114.94 as shown in Fig. 7. Moreover, the formation of some other low score compounds were also observed such as Titanium-ZincCarbide (Zn2Ti4C), Zinc-Iron-Titanium-Oxide (ZnFeTiO4), but due

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