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Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5 Rajesh Choudhary ⇑, Amar Kumar, Gyanendra Yadav, Rammurat Yadav, Vikas Kumar, Javed Akhtar Department of Mechanical Engineering, MIMIT Malout, Punjab 152107, India
a r t i c l e
i n f o
Article history: Received 23 November 2019 Accepted 4 January 2020 Available online xxxx Keywords: EDM Taguchi L18 OA Cryogenic tool Titanium grade 5 Tool wear rate Microstructure
a b s t r a c t The extensive use of Titanium alloy Ti-6Al-4V in defense, aerospace, automotive and biomedical sectors has made this material most demandable among industries. Due to low thermal conductivity it is difficult to machine by conventional machining processes. Electric discharge machining is a viable option to machine extremely harder and electrically conductive materials. There are a lot of possibilities in EDM to machine super alloys. Electrical discharge machining has also huge potential when it comes to biomaterials. Super alloy like Titanium grades 5 is most suitable for the biomedical field due to its desirable biocompatible properties. In this study effect of cryogenic treatment of tool, current, pulse on time, pulse off time and voltage has been investigated on tool wear rate. Optimized conditions for minimum tool wear rate have been found and microstructural analysis was also carried out to observe the surface topography of machined surface with non treated and cryogenic treated tool electrode. It has been observed that tool wear rate of cryogenic copper tool is comparatively more as compared to non treated copper tool electrode. The effect of current, on-time and type of tool electrode is found to be more pronounced as compared to other machining parameters. Thick recast layer with wide surface cracks is observed on the surface machined by the non treated tool electrode. Whereas narrow micro cracks with thin recast layer is observed on the surface machined by cryogenic treated tool electrode. Ó 2020 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.
1. Introduction One of the most investigated modern machining processes in the recent decades is electric discharge machining (EDM) process. The process involves conversion of electrical energy into heat energy that takes place in the gap between the tool electrode and the workpiece [1]. EDM is a non-conventional machining method and it is very popular due its vast applications in manufacturing of dies and moulds. The operation of EDM involves the erosion of material from the workpiece in a controlled, sensitive and systematic way. This process was firstly discovered by a British scientist Joseph Priestley in the 1770s. In the 1930s, scientists first machined the metal by electrical discharges. All conductive materials can be machined by the EDM of any hardness, a series of electrical discharges remove the material from the workpiece. When it comes to machining the complex shapes, EDM provide better solution than other machining processes. It is also suitable for ⇑ Corresponding author. E-mail address: [email protected] (R. Choudhary).
machining fragile parts that are not able to take stress of conventional machining. EDM can be economically suitable for the machining of superalloys, biomaterials and some composite materials in comparison to other non-conventional machining processes. Titanium alloys are commonly being used in various fields like aerospace industry, biomedical industry, marine building and army laboratories due to its high melting point, low weight and high corrosion resistance and high thermal conductivity. In various fields, it is being used hugely because of its toughness, strength, and durability. The human body can tolerate titanium in large doses because it has excellent biocompatibility. However, the presence of amino acids and proteins in the body fluids accelerates the corrosion process by releasing metallic ions which cause poor osseointegration and leads to cytotoxicity resulting in allergic reactions and, ultimately, to failure of the implant. Manam et al [2]. Titanium is much lighter and tougher metal than other metals that is why titanium cannot be machined in a traditional manner. After observing the suitability of Titanium grade 5 (TI 6AL 4V) in various applications of biomedical field, it has been chosen as candidate material for this study.
https://doi.org/10.1016/j.matpr.2020.01.080 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
2
R. Choudhary et al. / Materials Today: Proceedings xxx (xxxx) xxx
Ahmed et al. conducted electrical discharge machining of Ti 6Al 4V by using different tool electrodes like graphite, copper, brass and aluminum at two alternative tool polarities. Effect of tool polarity, discharge current and pulse time was investigated on tool wear rate and overcut. Positive polarity of tool was found to be more suitable for minimizing tool wear rate. Increasing discharge current and pulse time increases tool wear rate of all tool electrodes [3]. Rahul et al. found that cryogenically treatment improves electrical as well as thermal conductivities of the tool material and hence facilitates EDM performance. In this study current, voltage, pulse on time and pulse off time are chosen as the main machining parameters. As compared to normal copper tool electrode, use of cryogenically treated copper electrode has offered increased MRR, improved surface finish and reduced tool wear rate during EDM of Ti 6Al 4V [4]. Kao et al. optimized the EDM parameters with multiple quality characteristics during machining of Ti-6Al-4V using Taguchi grey relational analysis. The effect of discharge current, voltage, pulse duration and duty factor was studied on electrode wear ratio, material removal rate and surface roughness. The experimental study shows an improvement of 15% in electrode wear ratio, 12% in material removal rate and 19% in surface roughness [5]. Gupta and Akhtar performed experimental study on optimization of EDM parameters during machining of Titanium grade 5. The study reported that current and pulse-on time has major contribution on surface roughness during machining. The ANOVA analysis reveals that discharge current has maximum contribution during machining as compared to other machining parameters [6]. Prakash et al. reviewed the past research work done on surface modification of Ti based alloys by electric discharge machining process used for orthopedics applications This study highlights the advantage of EDM in surface modification of biomaterials and its use for future challenges [7]. Choudhary et al. reported the effect of cryogenic treatment of tool electrode on machining performance and surface finish during EDM of Haste alloy C4. Better surface finish and thick recast layer was obtained during machining by cryogenic tool electrode [8]. Kumar et al. concluded that cryogenically treated tool electrode ensured lower electrode wear, improved material removal ratio, and higher corner size machining accuracy [9]. Hui et al. reported that improved discharge characteristics and discharge gap could be achieved during EDM of Ti 6Al 4V using cryogenically cooled tool electrode [10]. Choudhary et al. reported experimental results for the machining of advanced materials by electric discharge surface grinding. The surface finish of advanced alloys can be improved by adopting hybrid machining processes. The study reveals that performance of non conventional machining process can be improved by combining with conventional machining operations [11–13]. In the present study titanium grade 5 alloy (Ti 6AL 4V) has been selected as work piece material. The titanium grade 5 alloy workpiece samples were cut into 50 mm 50 mm length with the help of a plasma arc cutting machine. The deep cryogenic treated copper tool electrode has been used alternatively with non treated copper tool electrode during electrical discharge machining. All the experiments were conducted as per L18 OA of Taguchi methodology. The experiments were designed by Taguchi method in MINITAB 19 software. This method reduces the number of experimental trials in comparison to other methods. The Taguchi DOE method helps in development of orthogonal array (OA) used to optimize the process parameters and there levels [14].The experiments were conducted as per Taguchi L18 orthogonal array. The machining time for each trial run is 30 min. The list of factors and their levels are shown in Table 1.
Table 1 Process parameters and their levels. Design factor Levels
Tool Electrode Current (I) Voltage (V) On time (p) Off time (s)
1
2
3
Non Treated (NT) 8 45 120 79
Cryogenic treated (CT) 10 50 150 102
—— 12 55 180 115
2. Experimental details 2.1. Experimental setup Sparkonix die sinking EDM (capacity 50 Amp.) was used to perform the experiments. The polarity of the tool electrode was set as negative. The special grade (EDM 50) dielectric fluid was used to perform the experiments. Two tool electrodes one was cryogenic treated and the other one non treated with cylindrical shape having 10 mm dia. was used during experimentation. The machining set up used for the experimentation is shown in Fig. 1. The EDM machine is equipped with all necessary requirements to conduct the experiments. The machine is having the control panel to select the machining parameters as per the design of experiments (DOE). 2.2. Selection of work piece Ti 6AL 4V is an alloy which is being used most commonly in aerospace industry, marine industry, army laboratories and it has higher corrosion resistance, low density and it is one of the lighter and high strength material to use in bio medical field as bio medical implants such as prosthesis. Table 2 shows the chemical composition of Titanium grade 5 and its physical properties are listed in Table 3. 2.3. Selection of tool electrode The material of the tool used in electrical discharge machining can be one of various metals like copper, brass, graphite, etc. depends upon specific applications and also depends on the material which is being machined. The contribution of tool cost in total operation cost is 50% approximately [15]. The shape and material of tool electrode affect the performance parameters in EDM process [16,17]. The powder metallurgy tool electrode can be used to improve the surface modification and surface finish during the process [18]. Tool material should have higher thermal and electrical conductivity and higher melting point. In this study a deep cryogenically treated and non-treated copper tool electrodes were used. Fig. 2 shows the two tool electrodes used to conduct experimentation. The physical properties of copper tool electrode are depicted in Table 4. 3. Results and discussion The experimental results recorded after conducting the trail runs are depicted in Table 5. Later on analysis of S/N ratio, means and ANOVA has been conducted through MINITAB software. 3.1. Mean ratio analyses From the mean ratio plots (Fig. 3) it is observed that nontreated tool electrode has lower electrode wear rate (EWR) as compared to cryogenic treated tool electrode. Increase in EWR was
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
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Fig. 1. Electric discharge machining setup.
Table 2 Chemical Composition of Ti 6Al 4V. Alloying Elements
O
Fe
C
H
N
Al
V
Ti
Chemical Composition (wt. %)
0.20
0.40
0.08
0.015
0.05
6.01
4.005
89.22
Table 3 Physical Properties of Ti 6Al 4V. Properties
Value
Density (g/cc) thermal conductivity at 20 °C (w/m °C) modulus of elasticity at 20 °C (N/mm2) Specific heat (j/kg °C) Hardness (HRc) Fusion temperature (°C)
4.43 6.7 114 103 560 33 1650
found with increase in gap current due to the increase in discharge energy with increase in discharge current between tool and workpiece which causes more wear of tool electrode. However decrease in EWR was observed with increase in Pulse on time. Increase in pulse on time increases the duration of spark on the workpiece which deposit carbon layer on the tool electrode that hinder the process of machining as well as wear rate of tool electrode. Similar trend has been observed for EWR with increase in voltage. There may be some chances of effect of interaction among control parameters on EWR. To verify this interaction plot between current and types of tool electrode was observed. The interaction plot in Fig. 4 clearly shows the significant interaction between cryogenic tool electrode and current at 10 A. 3.2. Analysis of 3-D TWR plot Fig. 5 shows the 3D surface plot for EWR with respect to variation in discharge current and pulse on time. EWR is found to increase with increase in current and decreases with increase in on time. The minimum value of electrode wear rate was found at gap current of 8 A and pulse on time of 180 ms i.e. 0.0009 g/min.
Fig. 2. (a) Non treated tool electrode (b) Cryogenic treated tool electrode.
3.3. ANOVA for EWR Analysis of variance was conducted to find the contribution of each input parameter toward tool wear rate. The analysis of
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
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3.4. Micro-structural analysis of machined surface
Table 4 Physical properties of copper tool electrode. Copper tool with 99% purity Properties
Value
Thermal conductivity (w/mk) Density (g/cc) Electrical resistivity (ohm-cm) Specific heat capacity (j/g °C)
391 1083 1.69 0.385
variance is reported in Table 6. In this table factor F ratio represents the significant process parameter for EWR. The lager the value of F more is its influence on the process. As per the F values tabulated in the table, it is concluded that current have significant impact on EWR followed by tool electrode and pulse on time. These three parameters are also having P value less than 0.05 which indicates that these parameters are having pronounced effect on the EWR during machining. Table 7 shows the mean values of TWR and rank of various input parameters.
EDM process involves material removal through plasma channel by utilizing the electrical energy. A series of electrical discharges take place that causes a controlled material removal from the workpiece surface. The energy dissipated to the workpiece is controlled through various input parameters. These input parameters also affect the surface morphology of the machined surface. The surface characteristic is important response variable which significantly improves the workability and life of the machining component. In this section surfaces machined through the different tools electrodes were observed through SEM and EDS to check surface integrity and to determine the chemical composition of the machined surface. Fig. 6(a-b) shows the micrograph of surface machined by a non-treaded (NT) copper tool electrode. Whereas Fig. 7(a-b) shows the micrographs of surfaces machined by cryogenic tool (CT) electrode. Thick recast layer with wide surface cracks is observed on the surface machined by the NT
Table 5 Experimental results for TWR. Tool electrode
Current (A)
On Time (ms)
Off Time (ms)
Voltage (V)
EWR (g/min)
NT NT NT NT NT NT NT NT NT CT CT CT CT CT CT CT CT CT
8 8 8 10 10 10 12 12 12 8 8 8 10 10 10 12 12 12
120 150 180 120 150 180 120 150 180 120 150 180 120 150 180 120 150 180
79 102 115 79 102 115 102 115 79 115 79 102 102 115 79 115 79 102
45 50 55 50 55 45 45 50 55 55 45 50 55 45 50 50 55 45
0.001 0.0009 0.0009 0.0018 0.0017 0.0015 0.003 0.0022 0.0015 0.0014 0.0014 0.0012 0.0018 0.0017 0.0011 0.003 0.0028 0.0031
Fig. 3. Main effect plot for means.
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
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Fig. 4. Interaction plot for means.
Table 6 ANOVA Table for means of TWR. Source
DF
Seq SS
Adj SS
Adj MS
F
P
Tool electrode Current On Time Off Time Voltage Tool electrode*Current Residual Error Total
1 2 2 2 2 2 6 17
0.000001 0.000007 0.000001 0.000000 0.000000 0.000001 0.000000 0.000009
0.000001 0.000007 0.000001 0.000000 0.000000 0.000001 0.000000
0.000001 0.000003 0.000000 0.000000 0.000000 0.000000 0.000000
10.11 68.13 6.15 3.72 2.71 5.80
0.019 0.000 0.035 0.089 0.145 0.040
Table 7 Response for Means (TWR). Level
Tool electrode
Current
On Time
Off Time
Voltage
1 2 3 Delta Rank
0.001611 0.001944
0.001133 0.001600 0.002600 0.001467 1
0.002000 0.001783 0.001550 0.000450 2
0.001600 0.001950 0.001783 0.000350 3
0.001950 0.001700 0.001683 0.000267 5
0.000333 4
tool electrode. Whereas narrow micro cracks with thin recast layer is observed on the surface machined by cryogenic treated tool electrode. This micrograph reveals that a high intensity of heat is transferred from the tool electrode to the workpiece surface during the machining process. The surface with major cracks is not suitable for the machined component as this recast layer is delicate in nature and can be removed easily by applying finishing operations. The heat distribution during machining with cryogenic tool electrode seems to be evenly distributed, as the fraction of the total heat goes to the tool itself that gives high TWR as compared to NT tool electrode. Fig. 7(a) shows that the surface has undergone melting followed by flushing out of molten metal. Some larger cavities have also been observed on the machined surface which clearly shows the effect of high intensity of spark at certain points.
3.5. Analysis of EDS spectrum of machined surface Fig. 8(a-b) shows the EDS spectrum of surface machined by non-treated and cryogenic treaded tool electrode. This spectrum indicates the presence of Ti, V, C, O, Al, Fe, Cu on the machined surface in which Ti is the key element. Whereas spectrum 8(b) shows additional elements like silicon and sulphur and higher peaks of C, Ca Al and Cu which shows the effect of cryogenic treatment of tool electrode on the machined surface during machining. No separate compound formation has observed on these machined surfaces. 4. Conclusions This study has been focused on observing the effect of various input parameters like type of tool, current, pulse on time, pulse
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
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Fig. 5. EWR vs. pulse on time, Current.
Fig. 6. SEM micrograph at (a) V-45 C-10 TON-180 TOFF-115 (b) V-50 C-12 TON-150 TOFF-115.
Fig. 7. SEM micrograph at (a) V-50 C-12 TON-150 TOFF-115 (b) V-45 C-12 TON-180 TOFF-102.
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
R. Choudhary et al. / Materials Today: Proceedings xxx (xxxx) xxx
7
Fig. 8. EDS graph of surface machined in (a) non-treated (b) cryogenic treated tool electrode.
off time and voltage on tool wear rate during machining of Titanium grade 5. The following conclusions have drawn from the experimental study. 1) Tool wear rate is found to increase with increase in current and decreases with increase in pulse on time. 2) Current and pulse on time are the main influencing parameters that effect tool wear rate during machining. 3) Tool wear rate is higher in cryogenic tool electrode as compared to non treated tool electrode. 4) Thick recast layer with wide surface cracks is observed on the surface machined by the NT tool electrode. Whereas narrow micro cracks with thin recast layer is observed on the surface machined by cryogenic treated tool electrode. 5) Cryogenic copper tool electrode is suitable to machine Titanium grade 5 at low values of gap current to achieve better surface quality. 6) The optimized condition for minimum EWR is NT, 8A, 120 ms, 79 ms, 55V. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References [1] P.S. Bains, S.S. Sidhu, H.S. Payal, Mater. Manuf. Process. 31 (2016) 1889–1894. [2] N.S. Manam, W.S.W. Harun, D.N.A. Shri, S.A.C. Ghani, T. Kurniawan, M.H. Ismail, M.H.I. Ibrahim, J. Alloys Compd. 701 (2017) 698–715. [3] N. Ahmed, K. Ishfaq, M. Rafaqat, S. Pervaiz, S. Anwar, B. Salah, Mater. Manuf. Process. (2019) 1042–6914. [4] Rahul, D.K. Mishra, S. Datta, M. Masanta, Silicon 10 (5) (2018) 2263–2277. [5] J.Y. Kao, C.C. Tsao, S.S. Wang, C.Y. Hsu, Int. J. Adv. Manuf. Technol. 47 (1–4) (2010) 395–402. [6] Anubhav Gupta, M.A. Akhtar, Int. J. Res. App. Sci. Eng. Technol. 5 (9) (2017) 981–988. [7] C. Prakash, H.K. Kansal, B.S. Pabla, S. Puri, A. Aggarwal, J. Eng. Manuf. 230 (2) (2016) 331–353. [8] R. Choudhary, H. Garg, M. Prasad, D. Kumar, Mater. Today Proc. 4 (2) (2017) 1158–1166. [9] Sanjeev Kumar, Rupinder Singh, Ajay Batish, T.P. Singh, Rajdeep Singh, J. Braz. Soc. Mech. Sci. Eng. 39 (7) (2017) 2635–2648. [10] Z. Hui, Z. Liu, Z. Cao, M. Qiu, Mater. Manuf. Process. 31 (4) (2015) 475–482. [11] H. Kumar, R. Choudhary, S. Singh, J. Mater. Engg. Perform. 23 (4) (2014) 1489– 1497. [12] R. Choudhary, H. Kumar, S. Singh, J. Mater. Eng. Perform. 22 (12) (2013) 3665– 3673. [13] R. Choudhary, H. Kumar, S. Singh, Adv. Mater. Process. Technol. (2018) 1–24. [14] A. Goswami, J. Kumar, Engg. Sci. Technol. Int. J. (1–11) (2014). [15] V. Kumar, S. Jatti, T.P. Singh, Wseas Trans. App. Theor. Mech. 11 (2016) 2224– 3429. [16] O.P. Panday, C.P. Khatter, R. Choudhary, R.K. Garg, Tribol. Mater. Surf. Interfaces 3 (2009) 74–83. [17] R. Choudhary, G. Singh, Mater. Today Proc. 5 (2) (2018) 6313–6320. [18] R. Choudhary, P. Kumar, J. Singh, Int. J. Surf. Eng. Mater. Technol. 2 (2) (2012) 53–58.
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5 Rajesh Choudhary ⇑, Amar Kumar, Gyanendra Yadav, Rammurat Yadav, Vikas Kumar, Javed Akhtar Department of Mechanical Engineering, MIMIT Malout, Punjab 152107, India
a r t i c l e
i n f o
Article history: Received 23 November 2019 Accepted 4 January 2020 Available online xxxx Keywords: EDM Taguchi L18 OA Cryogenic tool Titanium grade 5 Tool wear rate Microstructure
a b s t r a c t The extensive use of Titanium alloy Ti-6Al-4V in defense, aerospace, automotive and biomedical sectors has made this material most demandable among industries. Due to low thermal conductivity it is difficult to machine by conventional machining processes. Electric discharge machining is a viable option to machine extremely harder and electrically conductive materials. There are a lot of possibilities in EDM to machine super alloys. Electrical discharge machining has also huge potential when it comes to biomaterials. Super alloy like Titanium grades 5 is most suitable for the biomedical field due to its desirable biocompatible properties. In this study effect of cryogenic treatment of tool, current, pulse on time, pulse off time and voltage has been investigated on tool wear rate. Optimized conditions for minimum tool wear rate have been found and microstructural analysis was also carried out to observe the surface topography of machined surface with non treated and cryogenic treated tool electrode. It has been observed that tool wear rate of cryogenic copper tool is comparatively more as compared to non treated copper tool electrode. The effect of current, on-time and type of tool electrode is found to be more pronounced as compared to other machining parameters. Thick recast layer with wide surface cracks is observed on the surface machined by the non treated tool electrode. Whereas narrow micro cracks with thin recast layer is observed on the surface machined by cryogenic treated tool electrode. Ó 2020 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.
1. Introduction One of the most investigated modern machining processes in the recent decades is electric discharge machining (EDM) process. The process involves conversion of electrical energy into heat energy that takes place in the gap between the tool electrode and the workpiece [1]. EDM is a non-conventional machining method and it is very popular due its vast applications in manufacturing of dies and moulds. The operation of EDM involves the erosion of material from the workpiece in a controlled, sensitive and systematic way. This process was firstly discovered by a British scientist Joseph Priestley in the 1770s. In the 1930s, scientists first machined the metal by electrical discharges. All conductive materials can be machined by the EDM of any hardness, a series of electrical discharges remove the material from the workpiece. When it comes to machining the complex shapes, EDM provide better solution than other machining processes. It is also suitable for ⇑ Corresponding author. E-mail address: [email protected] (R. Choudhary).
machining fragile parts that are not able to take stress of conventional machining. EDM can be economically suitable for the machining of superalloys, biomaterials and some composite materials in comparison to other non-conventional machining processes. Titanium alloys are commonly being used in various fields like aerospace industry, biomedical industry, marine building and army laboratories due to its high melting point, low weight and high corrosion resistance and high thermal conductivity. In various fields, it is being used hugely because of its toughness, strength, and durability. The human body can tolerate titanium in large doses because it has excellent biocompatibility. However, the presence of amino acids and proteins in the body fluids accelerates the corrosion process by releasing metallic ions which cause poor osseointegration and leads to cytotoxicity resulting in allergic reactions and, ultimately, to failure of the implant. Manam et al [2]. Titanium is much lighter and tougher metal than other metals that is why titanium cannot be machined in a traditional manner. After observing the suitability of Titanium grade 5 (TI 6AL 4V) in various applications of biomedical field, it has been chosen as candidate material for this study.
https://doi.org/10.1016/j.matpr.2020.01.080 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and of the scientific committee of the 10th International Conference of Materials Processing and Characterization.
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
2
R. Choudhary et al. / Materials Today: Proceedings xxx (xxxx) xxx
Ahmed et al. conducted electrical discharge machining of Ti 6Al 4V by using different tool electrodes like graphite, copper, brass and aluminum at two alternative tool polarities. Effect of tool polarity, discharge current and pulse time was investigated on tool wear rate and overcut. Positive polarity of tool was found to be more suitable for minimizing tool wear rate. Increasing discharge current and pulse time increases tool wear rate of all tool electrodes [3]. Rahul et al. found that cryogenically treatment improves electrical as well as thermal conductivities of the tool material and hence facilitates EDM performance. In this study current, voltage, pulse on time and pulse off time are chosen as the main machining parameters. As compared to normal copper tool electrode, use of cryogenically treated copper electrode has offered increased MRR, improved surface finish and reduced tool wear rate during EDM of Ti 6Al 4V [4]. Kao et al. optimized the EDM parameters with multiple quality characteristics during machining of Ti-6Al-4V using Taguchi grey relational analysis. The effect of discharge current, voltage, pulse duration and duty factor was studied on electrode wear ratio, material removal rate and surface roughness. The experimental study shows an improvement of 15% in electrode wear ratio, 12% in material removal rate and 19% in surface roughness [5]. Gupta and Akhtar performed experimental study on optimization of EDM parameters during machining of Titanium grade 5. The study reported that current and pulse-on time has major contribution on surface roughness during machining. The ANOVA analysis reveals that discharge current has maximum contribution during machining as compared to other machining parameters [6]. Prakash et al. reviewed the past research work done on surface modification of Ti based alloys by electric discharge machining process used for orthopedics applications This study highlights the advantage of EDM in surface modification of biomaterials and its use for future challenges [7]. Choudhary et al. reported the effect of cryogenic treatment of tool electrode on machining performance and surface finish during EDM of Haste alloy C4. Better surface finish and thick recast layer was obtained during machining by cryogenic tool electrode [8]. Kumar et al. concluded that cryogenically treated tool electrode ensured lower electrode wear, improved material removal ratio, and higher corner size machining accuracy [9]. Hui et al. reported that improved discharge characteristics and discharge gap could be achieved during EDM of Ti 6Al 4V using cryogenically cooled tool electrode [10]. Choudhary et al. reported experimental results for the machining of advanced materials by electric discharge surface grinding. The surface finish of advanced alloys can be improved by adopting hybrid machining processes. The study reveals that performance of non conventional machining process can be improved by combining with conventional machining operations [11–13]. In the present study titanium grade 5 alloy (Ti 6AL 4V) has been selected as work piece material. The titanium grade 5 alloy workpiece samples were cut into 50 mm 50 mm length with the help of a plasma arc cutting machine. The deep cryogenic treated copper tool electrode has been used alternatively with non treated copper tool electrode during electrical discharge machining. All the experiments were conducted as per L18 OA of Taguchi methodology. The experiments were designed by Taguchi method in MINITAB 19 software. This method reduces the number of experimental trials in comparison to other methods. The Taguchi DOE method helps in development of orthogonal array (OA) used to optimize the process parameters and there levels [14].The experiments were conducted as per Taguchi L18 orthogonal array. The machining time for each trial run is 30 min. The list of factors and their levels are shown in Table 1.
Table 1 Process parameters and their levels. Design factor Levels
Tool Electrode Current (I) Voltage (V) On time (p) Off time (s)
1
2
3
Non Treated (NT) 8 45 120 79
Cryogenic treated (CT) 10 50 150 102
—— 12 55 180 115
2. Experimental details 2.1. Experimental setup Sparkonix die sinking EDM (capacity 50 Amp.) was used to perform the experiments. The polarity of the tool electrode was set as negative. The special grade (EDM 50) dielectric fluid was used to perform the experiments. Two tool electrodes one was cryogenic treated and the other one non treated with cylindrical shape having 10 mm dia. was used during experimentation. The machining set up used for the experimentation is shown in Fig. 1. The EDM machine is equipped with all necessary requirements to conduct the experiments. The machine is having the control panel to select the machining parameters as per the design of experiments (DOE). 2.2. Selection of work piece Ti 6AL 4V is an alloy which is being used most commonly in aerospace industry, marine industry, army laboratories and it has higher corrosion resistance, low density and it is one of the lighter and high strength material to use in bio medical field as bio medical implants such as prosthesis. Table 2 shows the chemical composition of Titanium grade 5 and its physical properties are listed in Table 3. 2.3. Selection of tool electrode The material of the tool used in electrical discharge machining can be one of various metals like copper, brass, graphite, etc. depends upon specific applications and also depends on the material which is being machined. The contribution of tool cost in total operation cost is 50% approximately [15]. The shape and material of tool electrode affect the performance parameters in EDM process [16,17]. The powder metallurgy tool electrode can be used to improve the surface modification and surface finish during the process [18]. Tool material should have higher thermal and electrical conductivity and higher melting point. In this study a deep cryogenically treated and non-treated copper tool electrodes were used. Fig. 2 shows the two tool electrodes used to conduct experimentation. The physical properties of copper tool electrode are depicted in Table 4. 3. Results and discussion The experimental results recorded after conducting the trail runs are depicted in Table 5. Later on analysis of S/N ratio, means and ANOVA has been conducted through MINITAB software. 3.1. Mean ratio analyses From the mean ratio plots (Fig. 3) it is observed that nontreated tool electrode has lower electrode wear rate (EWR) as compared to cryogenic treated tool electrode. Increase in EWR was
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
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Fig. 1. Electric discharge machining setup.
Table 2 Chemical Composition of Ti 6Al 4V. Alloying Elements
O
Fe
C
H
N
Al
V
Ti
Chemical Composition (wt. %)
0.20
0.40
0.08
0.015
0.05
6.01
4.005
89.22
Table 3 Physical Properties of Ti 6Al 4V. Properties
Value
Density (g/cc) thermal conductivity at 20 °C (w/m °C) modulus of elasticity at 20 °C (N/mm2) Specific heat (j/kg °C) Hardness (HRc) Fusion temperature (°C)
4.43 6.7 114 103 560 33 1650
found with increase in gap current due to the increase in discharge energy with increase in discharge current between tool and workpiece which causes more wear of tool electrode. However decrease in EWR was observed with increase in Pulse on time. Increase in pulse on time increases the duration of spark on the workpiece which deposit carbon layer on the tool electrode that hinder the process of machining as well as wear rate of tool electrode. Similar trend has been observed for EWR with increase in voltage. There may be some chances of effect of interaction among control parameters on EWR. To verify this interaction plot between current and types of tool electrode was observed. The interaction plot in Fig. 4 clearly shows the significant interaction between cryogenic tool electrode and current at 10 A. 3.2. Analysis of 3-D TWR plot Fig. 5 shows the 3D surface plot for EWR with respect to variation in discharge current and pulse on time. EWR is found to increase with increase in current and decreases with increase in on time. The minimum value of electrode wear rate was found at gap current of 8 A and pulse on time of 180 ms i.e. 0.0009 g/min.
Fig. 2. (a) Non treated tool electrode (b) Cryogenic treated tool electrode.
3.3. ANOVA for EWR Analysis of variance was conducted to find the contribution of each input parameter toward tool wear rate. The analysis of
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
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3.4. Micro-structural analysis of machined surface
Table 4 Physical properties of copper tool electrode. Copper tool with 99% purity Properties
Value
Thermal conductivity (w/mk) Density (g/cc) Electrical resistivity (ohm-cm) Specific heat capacity (j/g °C)
391 1083 1.69 0.385
variance is reported in Table 6. In this table factor F ratio represents the significant process parameter for EWR. The lager the value of F more is its influence on the process. As per the F values tabulated in the table, it is concluded that current have significant impact on EWR followed by tool electrode and pulse on time. These three parameters are also having P value less than 0.05 which indicates that these parameters are having pronounced effect on the EWR during machining. Table 7 shows the mean values of TWR and rank of various input parameters.
EDM process involves material removal through plasma channel by utilizing the electrical energy. A series of electrical discharges take place that causes a controlled material removal from the workpiece surface. The energy dissipated to the workpiece is controlled through various input parameters. These input parameters also affect the surface morphology of the machined surface. The surface characteristic is important response variable which significantly improves the workability and life of the machining component. In this section surfaces machined through the different tools electrodes were observed through SEM and EDS to check surface integrity and to determine the chemical composition of the machined surface. Fig. 6(a-b) shows the micrograph of surface machined by a non-treaded (NT) copper tool electrode. Whereas Fig. 7(a-b) shows the micrographs of surfaces machined by cryogenic tool (CT) electrode. Thick recast layer with wide surface cracks is observed on the surface machined by the NT
Table 5 Experimental results for TWR. Tool electrode
Current (A)
On Time (ms)
Off Time (ms)
Voltage (V)
EWR (g/min)
NT NT NT NT NT NT NT NT NT CT CT CT CT CT CT CT CT CT
8 8 8 10 10 10 12 12 12 8 8 8 10 10 10 12 12 12
120 150 180 120 150 180 120 150 180 120 150 180 120 150 180 120 150 180
79 102 115 79 102 115 102 115 79 115 79 102 102 115 79 115 79 102
45 50 55 50 55 45 45 50 55 55 45 50 55 45 50 50 55 45
0.001 0.0009 0.0009 0.0018 0.0017 0.0015 0.003 0.0022 0.0015 0.0014 0.0014 0.0012 0.0018 0.0017 0.0011 0.003 0.0028 0.0031
Fig. 3. Main effect plot for means.
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
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Fig. 4. Interaction plot for means.
Table 6 ANOVA Table for means of TWR. Source
DF
Seq SS
Adj SS
Adj MS
F
P
Tool electrode Current On Time Off Time Voltage Tool electrode*Current Residual Error Total
1 2 2 2 2 2 6 17
0.000001 0.000007 0.000001 0.000000 0.000000 0.000001 0.000000 0.000009
0.000001 0.000007 0.000001 0.000000 0.000000 0.000001 0.000000
0.000001 0.000003 0.000000 0.000000 0.000000 0.000000 0.000000
10.11 68.13 6.15 3.72 2.71 5.80
0.019 0.000 0.035 0.089 0.145 0.040
Table 7 Response for Means (TWR). Level
Tool electrode
Current
On Time
Off Time
Voltage
1 2 3 Delta Rank
0.001611 0.001944
0.001133 0.001600 0.002600 0.001467 1
0.002000 0.001783 0.001550 0.000450 2
0.001600 0.001950 0.001783 0.000350 3
0.001950 0.001700 0.001683 0.000267 5
0.000333 4
tool electrode. Whereas narrow micro cracks with thin recast layer is observed on the surface machined by cryogenic treated tool electrode. This micrograph reveals that a high intensity of heat is transferred from the tool electrode to the workpiece surface during the machining process. The surface with major cracks is not suitable for the machined component as this recast layer is delicate in nature and can be removed easily by applying finishing operations. The heat distribution during machining with cryogenic tool electrode seems to be evenly distributed, as the fraction of the total heat goes to the tool itself that gives high TWR as compared to NT tool electrode. Fig. 7(a) shows that the surface has undergone melting followed by flushing out of molten metal. Some larger cavities have also been observed on the machined surface which clearly shows the effect of high intensity of spark at certain points.
3.5. Analysis of EDS spectrum of machined surface Fig. 8(a-b) shows the EDS spectrum of surface machined by non-treated and cryogenic treaded tool electrode. This spectrum indicates the presence of Ti, V, C, O, Al, Fe, Cu on the machined surface in which Ti is the key element. Whereas spectrum 8(b) shows additional elements like silicon and sulphur and higher peaks of C, Ca Al and Cu which shows the effect of cryogenic treatment of tool electrode on the machined surface during machining. No separate compound formation has observed on these machined surfaces. 4. Conclusions This study has been focused on observing the effect of various input parameters like type of tool, current, pulse on time, pulse
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
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Fig. 5. EWR vs. pulse on time, Current.
Fig. 6. SEM micrograph at (a) V-45 C-10 TON-180 TOFF-115 (b) V-50 C-12 TON-150 TOFF-115.
Fig. 7. SEM micrograph at (a) V-50 C-12 TON-150 TOFF-115 (b) V-45 C-12 TON-180 TOFF-102.
Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080
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Fig. 8. EDS graph of surface machined in (a) non-treated (b) cryogenic treated tool electrode.
off time and voltage on tool wear rate during machining of Titanium grade 5. The following conclusions have drawn from the experimental study. 1) Tool wear rate is found to increase with increase in current and decreases with increase in pulse on time. 2) Current and pulse on time are the main influencing parameters that effect tool wear rate during machining. 3) Tool wear rate is higher in cryogenic tool electrode as compared to non treated tool electrode. 4) Thick recast layer with wide surface cracks is observed on the surface machined by the NT tool electrode. Whereas narrow micro cracks with thin recast layer is observed on the surface machined by cryogenic treated tool electrode. 5) Cryogenic copper tool electrode is suitable to machine Titanium grade 5 at low values of gap current to achieve better surface quality. 6) The optimized condition for minimum EWR is NT, 8A, 120 ms, 79 ms, 55V. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Please cite this article as: R. Choudhary, A. Kumar, G. Yadav et al., Analysis of cryogenic tool wear during electrical discharge machining of titanium alloy grade 5, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.080