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Performance and Surface Integrity of Nimonic75 Alloy Machined by Electrical Discharge Machining
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2 (2015) 3481 – 3490
4th International Conference on Materials Processing and Characterization
Performance and surface integrity of Nimonic75 alloy machined by electrical discharge machining Rajesh Choudharya* , Vivek Kumar Guptab, Yogesh Batrab and Akashdeep Singhb a
Assistant Professor, Department of Mechanical Engineering, MIMIT Malout, Punjab, 152107, India b UG student, Department of Mechanical Engineering, MIMIT Malout, Punjab, 152107, India
Abstract The ongoing developments in the field of materials science have developed many high strength and temperature resistant (HSTR) metal alloys that are difficult to machine by the conventional machining. Electrical discharge machining is one of the commonly used non-conventional machining processes used to machine hardened tool steels and conductive advanced materials. In this study effect of machining parameters like tool polarity, current, pulse on-time and voltage has been investigated on surface roughness of Nimonic 75 alloy using Taguchi’s experimental design technique. From the S/N ratio plots It was observed that tool polarity, pulse on-time and current are most influential parameter that affect surface roughness in EDM. The surface machined by the negative tool polarity was found to be smoother than the positive tool polarity. The deposition rate of the negative polarity tool electrode was comparatively more than the positive polarity tool electrode. The maximum deposition of carbon was observed on the surface machined with positive polarity tool electrode. © Ltd. AllElsevier rights reserved. © 2015 2014Elsevier The Authors. Ltd. All rights reserved. Selection under responsibility of theofconference committee members of the 4th conferenceconference on Materialson Selectionand andpeer-review peer-review under responsibility the conference committee members ofInternational the 4th International Processing and Characterization. and Characterization. Materials Processing Keywords: Nimonic 75 alloy; Electrical discharge machining; Taguchi L18 OA; Tool polarity; Surface intigrity, Surface Roughness
1. INTRODUCTION The machining of newly developed hard to machine alloys of Nickel have possessed a great challenge toward the conventional manufacturing sector. * Corresponding author. Tel.:+91-8427752536;
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization. doi:10.1016/j.matpr.2015.07.324
3482
Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490
E-mail address: [email protected]
Nomenclature OA SR Ton Toff DOF ANOVA Seq. SS Adj. SS S SEM EDS
Orthogonal array Surface roughness Pulse on time Pulse off-time Degree of freedom Analysis of Variance Sum of squares Adjusted mean of squares Standard deviation Scanning Electron Microscope Energy Dispersive Spectroscopy
The tendency of advanced alloys toward galling, welding, formation of built-up-edges at low speed and presence of hard abrasive carbides in their microstructure accounts for their lower machinability [1]. Nimonic 75 is one such alloy which belongs to the family of Nickel super alloys. It is widely used in manufacturing of turbine blades, aerospace fasteners and heat treatment equipment [2]. Higher temperature is produced while conventional machining of Nimonic 75 alloy due to its low thermal diffusivity which produces poor surface integrity [3]. Surface integrity is an important machining characteristic as it improves the fatigue strength, wear and corrosion resistant properties of the workpiece. It includes work hardness, surface roughness, metallurgical transformation, microstructural changes, residual stress and surface structure [4]. EDM, a non-conventional machining process has proved to be economical in machining of these alloys with a better surface integrity. It works on the principle of electric discharge erosion. Material is removed by the controlled and repetitive electrical sparks produced by DC pulse generator when the tool and workpiece is placed in the liquid dielectric medium [5]. EDM has a large number of input parameters such as tool polarity, pulse on time, pulse of time, duty factor, voltage, peak current, flushing pressure, dielectric etc. Selection of EDM parameters for optimum output characteristics requires a large experimental work which is time consuming and costly. A lot of research has been carried out till date by the researchers to study the influence of EDM machining parameters on surface integrity for different materials. S. H. Lee et al carried out an experimentation to study the effect of pulse duration and peak current on surface integrity of tungsten carbide. They found that surface cracks increased on the surface with increasing the pulse on time and current. They also observed that there was no microstructural change in the bulk workpiece material [6]. Another investigation was carried by M. Boujelbene et al on X200Cr15 and 50CrV4 steels to study the influence of machining parameters on the surface integrity. It was found that with increasing discharge energy the surface of workpiece becomes rougher and the white layer thickness increases [7]. M.K. Das et al applied Artificial Bee colony algorithm for optimization of surface roughness and material removal rate on EDM for EN31 tool steel and found that the MRR and SR are directly proportional to the pulse on time and current [8]. K.T. Mannan et al experimented on the High speed steel to study its surface characterization after machining. They varied current and pulse on time keeping voltage and duty factor constant and found that at low current and low pulse on time craters were shallow and the density of globules and pockmarks were low [4]. K.L. Wu et al studied the effect of aluminum and surfactant added dielectric on surface finish of SKD steel. They found that the surface roughness of EDMed surface improved up to 60% as compared to that of machined surface with pure dielectric [9]. H.S. Payal et al experimented on EN31 tool steel with copper brass and graphite tool electrode and with kerosene as dielectric to study the structure of machined surface. Their micrograph study revealed that the material was removed from the surface as ligaments and sheets, surface machined with copper tool was smoother than that of graphite tool [10]. K.M. Tsai et al established semi-empirical model of surface finish on work for various materials by employing dimensional analysis based on pertinent process parameters of EDM machining. They constant parameters cannot be used for various work and tool materials [11].
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Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490
2. Experimental Details The experiments were performed on Sparkonix Die-Sinking EDM. The tool used is of pure copper and of cylindrical cross section of dimension ø10mm and 100mm. The workpiece used is Nimonic 75 alloy of dimension 75mm X 35mm X 8mm. The chemical composition of Nimonic 75 alloy is depicted in Table 1. Table 1. Chemical Composition of Nimonic 75 alloy. Alloying elements Amount
C
Si
Mn
Cr
Ni
Cu
Ti
Fe
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
.12-.15
.35-1
.65-1.0
18.5-21
77.7
.15-.50
.25-.60
2.03-5
The dielectric used was of commercial grade EDM 50 oil. Tool polarity, peak current, pulse on time and voltage were selected as input parameters. 2.1. Taguchi’s Method Dr. Genichi Taguchi developed an engineering method for quality improvement. This method uses orthogonal arrays to optimize the process parameters and their levels [12]. This method also reduces the number of experimental trials runs as compared to full factorial experimental design to reduce cost and time. Signal to noise ratio is a unique term in the Taguchi’s method that is used to analyze the robustness of the system. The experiments conducted are based on Taguchi’s L18 Orthogonal Array. The design of experiments is based on mixed OA where peak current, pulse on time and the voltage are varied to three levels and the tool polarity is varied on two levels. Table 2 describes the process parameters and their levels. Table 2. List of process parameters and their levels
Design Factors 1
Levels 2
3
-ve
+ve
--
Current (Amp.)
6
8
10
Pulse On Time(μs)
60
90
120
Voltage(Volt)
40
50
60
Tool Polarity
Two replicates of each experimental trial run were performed for 20 mins each to minimize the uncontrollable variations. After the machining of the workpiece the surface roughness of each machined cavity was measured using MITUTOYO SJ-201P portable surface roughness tester. Roughness measurements in transverse and longitudinal directions on the workpiece are performed for each machined cavity. Table 3 shows the response table for mean value of Ra.
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Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490 Table 3. Experimental Results Exp. No.
Tool Polarity
Current (A)
Pulse On-time (μs)
Voltage (V)
SR(mean) Ra μm
1
-
6
60
40
2.778
2
-
6
90
50
3.370
3
-
6
120
60
2.686
4
-
8
60
40
2.555
5
-
8
90
50
3.012
6
-
8
120
60
3.390
7
-
10
60
50
2.751
8
-
10
90
60
3.245
9
-
10
120
40
3.580
10
+
6
60
60
4.940
11
+
6
90
40
5.742
12
+
6
120
50
5.802
13
+
8
60
50
5.395
14
+
8
90
60
6.389
15
+
8
120
40
8.935
16
+
10
60
60
6.103
17
+
10
90
40
8.910
18
+
10
120
50
7.940
3. Results and Discussions The experimental results obtained by the experimental trails runs were further analyzed using Minitab 16 software to study the influence of various machining parameters on surface roughness. 3.1. Analysis of SN ratios The deviation of quality characteristics from the desired value is measured using S/N ratios. The desirable mean value of output characteristics is represented as Signal and the undesired value or standard deviation is represented as Noise [13]. Fig. 1 shows main effect plots for S/N ratios. The curve having larger inclination from the mean line will have greater impact on the performance measures. Tool polarity with the largest inclination from the mean line is the most significant parameter. This is due to the fact that higher thermal energy is imparted to the workpiece in case of positive tool polarity which leads to larger material removal thus rougher surface than negative tool polarity. Surface roughness is also found to increase with an increase in discharge current. This is because that as the current increases the discharge energy transferred to the workpiece thus removes material with larger and deeper craters making surface more rougher. Increase in pulse on time also accounts for higher surface roughness. Energy delivered in single spark increases with increase in pulse on-time causes increased size of craters on the machined surface. Increase in voltage increases the spark density and thus larger craters are formed on the workpiece making the surface more rougher. Further increase in voltage leads to expansion of plasma channel forming shallow craters [8].
Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490
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Fig 1. Main Effect plots for SN ratios.
3.2. Analysis of 3-D Surface Plot Effect of current and pulse on time on surface roughness is clearly demonstrated in Fig. 2. It shows that increase in pulse on time increases the surface roughness to a larger extent than the increase in discharge current.
Fig 2. Surface Plot of SR vs Pulse on time, Current
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Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490
3.3. Analysis of Variance Analysis of variance table was performed to obtain percentage contribution of different parameters individually on the surface roughness. Table 4 shows analysis of variance for surface roughness. F is the ratio of mean of squared deviation to the mean of squared error. Higher the value of F larger the impact of parameter on Ra. Thus the tool polarity have greatest impact on the surface roughness followed by pulse on time, current and voltage. P denotes the significant parameters affecting Ra. If the value of p < 0.05 the parameter will be the most significant. Thus the tool polarity is the most significant parameter for surface roughness. Table 4. ANOVA Table for means. Source DF
Seq. SS
Adj. SS
Adj. MS
F
P
Tool Polarity
1
59.729
59.729
59.7288
92.13
0.000
Current
2
4.396
4.396
2.1981
3.39
0.075
Pulse on time
2
5.642
5.642
2.8210
4.36
0.044
Voltage
2
2.957
2.957
1.4784
2.28
0.153
Residual Error
10
6.476
6.476
0.6476
Total
17
79.200
S = 1.0
R-Sq = 98.5%
R-Sq(adj) = 92.9%
3.4. Analysis of Response for SN ratios Table 5 shows the response for SN ratios. It is clearly observed that from this table the rank in which the parameters affect the value of Ra. Delta denotes the difference between the highest average value of each parameter and lowest average value of same parameter. Larger the value of delta larger the impact of that parameter on the value of Ra. This shows that Ra is mostly affected by tool polarity followed by pulse on time, current and voltage respectively. Table 5. Response for SN ratios Level
Tool polarity
Current
Pulse on Time
Voltage
1
-9.603
-12.056
-11.673
-13.550
2
-16.311
-13.017
-13.440
-12.805
-13.799
-13.759
-12.517
3 Delta
6.708
1.743
2.086
1.033
Rank
1
3
2
4
3.5. Analysis of Surface Topography Surface topography of the machined surface was studied using the SEM micrographs. Figure 3 shows the micrographs of the machined surface with negative and positive tool polarity at various magnifications. Fig 3(a) shows the surface machined with negative tool polarity which is smoother as compared to surface machined by positive tool polarity (Fig 3(b)). Removal of material from workpiece in case of negative tool polarity is lower which leads to uniform surface on the machined surface (fig. 3(c)). When the tool polarity is positive the removal of material is more in the form of shallow craters and the un-removed material gets resolidified as recast later. Thus higher deposition of melted metal is formed on the surface machined with positive tool polarity as shown in Fig. 3(d). Fig 3(e) shows the cracks formed on the surface due to removal of material which is shallower as compared to
Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490
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Fig 3(f). The globules in recast layer, pockmarks obtained from gases entrapped and resolidified layer is also visible from the micrographs (Fig. 3f).
Fig. 3 SEM Images (a) at –ve polarity 100x (b) at + polarity 100x (c) at –ve polarity 500x (d) at +ve polarity 500x (e) at –ve polarity 1000x (f) at +ve polarity 1000x
3.6. Analysis of Metallurgical Composition of EDMed Surface The surface machined with EDM was analyzed for its metallurgical composition using Energy Dispersive spectroscopy. Fig 4 shows the actual metallurgical constituents of pure metal surface. The transfer of foreign
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elements on the machined surface with negative and positive tool polarity is more as compared to the surface without machining (fig. 5-6). This is due to pyrolysis of dielectric fluid present between the workpiece and the tool electrode. But the Carbon content on the surface machined with positive tool polarity is more than that of surface machined with negative tool polarity. This signifies that the rate of pyrolysis of the dielectric is more in case of positive tool polarity as compared to that of negative tool polarity. The deposition of tool electrode i.e. copper is more on the surface machined with negative tool polarity than that of the surface machined with positive tool polarity. This is due to higher tool erosion in case of negative tool polarity than that of positive tool polarity.
Fig. 4 EDS graph of pure metal without machining.
Fig. 5. EDS graph of surface machined with negative tool polarity.
Fig. 6. EDS graph of surface machined with positive tool polarity.
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Increased tool wear is observed in case of negative tool polarity, this is due to bombardment of proton particles on the tool surface. As the mass of proton is higher as compared to electrons, so more energy is imparted to the tool surface which causes its erosion during machining and thus causes more deposition of tool material on the workpiece surface. 4. Conclusions In this paper the effect of tool polarity, current, pulse on time and voltage on surface roughness of Nimonic 75 alloy is concluded. Taguchi’s L18 orthogonal array is used for design of experiments. SEM and EDS analysis was carried out to study the surface topography. The following conclusions were drawn x Tool polarity is the dominating factor for surface roughness followed by pulse on time and current. x Minimum surface roughness was observed at negative tool polarity, 8 A current, 60μs pulse on-time and 40V gap voltage. x The surface machined with positive tool polarity have uneven surface with thick recast layer with globules as compared to surface machined by positive tool polarity. x Carbon content on surface machined with positive tool polarity is more than that of negative tool polarity. x Deposition of tool material on surface machined with negative tool polarity is more than that of surface machined with positive tool polarity. Acknowledgements The authors of the research work deeply acknowledge the funding given by the All India Council for Technical Education (AICTE) New Delhi for providing the EDM setup under MODROBS scheme in the Department of Mechanical Engineering at MIMIT, Malout. References [1] P.Subhash, C. Bose, C.S.P. Rao, Evaluation of Optimum Cutting Parameters In Turning of NIMONIC 75 using RSM, International Journal on Theoretical and Applied Research in Mechanical Engineering , vol. 2, no. 2, pp. 17-30, 2013. [2] S.Singh, A. pandey, Some Studies into Electrical Discharge Machining of Nimonic75 Super Alloy Using Rotary Copper Disk Electrode, Journal of Materials Engineering and Performance, vol. 22, no. 5, pp. 1290-1303, 2013. [3] E. Kuljanic, M.Sortino, G. Totis, Machinability of Difficult Machining Materials, in 14th International Research/Expert Conference Trends in the Development of Machinery and Associated Technology”, Mediterranean Cruise, 11-18 September 2010. [4] K.Tamil Mannan, A. Krishnaiah, S.P. Arikatla, Surface Characterization of Electric Discharge Machined Surface of High Speed Steel, Advanced Materials Manufacturing & Characterization, pp. 161-168, 2013. [5] Ghoreishi, S. Assarzadeh, Statistical Investigation into the Effects of Electro-Discharge Machining Parameters on WC/6%Co Composite-Part 1: Modeling through Response Surface Methodology (RSM), Advanced Materials Manufacturing & Characterization, vol. 3, no. 2, pp. 478-486, 2013. [6] S. H. Lee, X. Li Study of the surface integrity of the machined workpiece in the EDM of tungsten carbide, Journal of Materials Processing Technology, vol. 139, no. 1-3, pp. 315-321, 2003. [7] M. Boujelbene, E. Bayraktar, W. Tebni, S.B. Salem, Influence of machining parameters on the surface integrity in electrical discharge machining, Archives of Materials Science and Engineering, vol. 37, no. 2, pp. 110-116, 2009. [8] M. K. Das, K. Kumar, T.K.Burman, P. Sahoo, Application of Artificial bee Colony Algorithm for Optimization of MRR and Surface Roughness in EDM of EN31 tool steel, Procedia Materials Science , vol. 6, pp. 741-751, 2014. [9] K. L. Wu, B.H. Yan, Y. F. Huang, S.C. Chen, Improvement of surface finish on SKD steel using electro-discharge machining with aluminum and surfactant added dielectric, International Journal of Machine Tools & Manufacture, vol. 45, pp. 1195-1201, 2004. [10] H S Payal, R. Choudhary, S. Singh, Analysis of electro discharge machined surfaces of EN-31 tool steel, Journal of Scientific & Industrial Research, vol. 67, pp. 1072-1077, 2008. [11] K. M. Tsai, P.J. Wang, Semi-empirical model of surface finish on electrical discharge machining, International Journal of Machine Tools & Manufacture, vol. 41, pp. 1455-1477, 2001.
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[12]A. Goswami, J. Kumar, Optimization in wire-cut EDM of Nimonic-80A using Taguchi's approach and utility concept, Engineering Science and Technology an International Journal, pp. 1-11, 2014. [13] A. Abusada, O. Belgassim, Abdurrahman, Optimization of the EDM Parameters on the Surface Roughness of AISI D3 Tool Steel, in Proceedings of the 2012 International Conference on Industrial Engineering and Operations Management, Istanbul, Turkey, 2012.
ScienceDirect Materials Today: Proceedings 2 (2015) 3481 – 3490
4th International Conference on Materials Processing and Characterization
Performance and surface integrity of Nimonic75 alloy machined by electrical discharge machining Rajesh Choudharya* , Vivek Kumar Guptab, Yogesh Batrab and Akashdeep Singhb a
Assistant Professor, Department of Mechanical Engineering, MIMIT Malout, Punjab, 152107, India b UG student, Department of Mechanical Engineering, MIMIT Malout, Punjab, 152107, India
Abstract The ongoing developments in the field of materials science have developed many high strength and temperature resistant (HSTR) metal alloys that are difficult to machine by the conventional machining. Electrical discharge machining is one of the commonly used non-conventional machining processes used to machine hardened tool steels and conductive advanced materials. In this study effect of machining parameters like tool polarity, current, pulse on-time and voltage has been investigated on surface roughness of Nimonic 75 alloy using Taguchi’s experimental design technique. From the S/N ratio plots It was observed that tool polarity, pulse on-time and current are most influential parameter that affect surface roughness in EDM. The surface machined by the negative tool polarity was found to be smoother than the positive tool polarity. The deposition rate of the negative polarity tool electrode was comparatively more than the positive polarity tool electrode. The maximum deposition of carbon was observed on the surface machined with positive polarity tool electrode. © Ltd. AllElsevier rights reserved. © 2015 2014Elsevier The Authors. Ltd. All rights reserved. Selection under responsibility of theofconference committee members of the 4th conferenceconference on Materialson Selectionand andpeer-review peer-review under responsibility the conference committee members ofInternational the 4th International Processing and Characterization. and Characterization. Materials Processing Keywords: Nimonic 75 alloy; Electrical discharge machining; Taguchi L18 OA; Tool polarity; Surface intigrity, Surface Roughness
1. INTRODUCTION The machining of newly developed hard to machine alloys of Nickel have possessed a great challenge toward the conventional manufacturing sector. * Corresponding author. Tel.:+91-8427752536;
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the conference committee members of the 4th International conference on Materials Processing and Characterization. doi:10.1016/j.matpr.2015.07.324
3482
Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490
E-mail address: [email protected]
Nomenclature OA SR Ton Toff DOF ANOVA Seq. SS Adj. SS S SEM EDS
Orthogonal array Surface roughness Pulse on time Pulse off-time Degree of freedom Analysis of Variance Sum of squares Adjusted mean of squares Standard deviation Scanning Electron Microscope Energy Dispersive Spectroscopy
The tendency of advanced alloys toward galling, welding, formation of built-up-edges at low speed and presence of hard abrasive carbides in their microstructure accounts for their lower machinability [1]. Nimonic 75 is one such alloy which belongs to the family of Nickel super alloys. It is widely used in manufacturing of turbine blades, aerospace fasteners and heat treatment equipment [2]. Higher temperature is produced while conventional machining of Nimonic 75 alloy due to its low thermal diffusivity which produces poor surface integrity [3]. Surface integrity is an important machining characteristic as it improves the fatigue strength, wear and corrosion resistant properties of the workpiece. It includes work hardness, surface roughness, metallurgical transformation, microstructural changes, residual stress and surface structure [4]. EDM, a non-conventional machining process has proved to be economical in machining of these alloys with a better surface integrity. It works on the principle of electric discharge erosion. Material is removed by the controlled and repetitive electrical sparks produced by DC pulse generator when the tool and workpiece is placed in the liquid dielectric medium [5]. EDM has a large number of input parameters such as tool polarity, pulse on time, pulse of time, duty factor, voltage, peak current, flushing pressure, dielectric etc. Selection of EDM parameters for optimum output characteristics requires a large experimental work which is time consuming and costly. A lot of research has been carried out till date by the researchers to study the influence of EDM machining parameters on surface integrity for different materials. S. H. Lee et al carried out an experimentation to study the effect of pulse duration and peak current on surface integrity of tungsten carbide. They found that surface cracks increased on the surface with increasing the pulse on time and current. They also observed that there was no microstructural change in the bulk workpiece material [6]. Another investigation was carried by M. Boujelbene et al on X200Cr15 and 50CrV4 steels to study the influence of machining parameters on the surface integrity. It was found that with increasing discharge energy the surface of workpiece becomes rougher and the white layer thickness increases [7]. M.K. Das et al applied Artificial Bee colony algorithm for optimization of surface roughness and material removal rate on EDM for EN31 tool steel and found that the MRR and SR are directly proportional to the pulse on time and current [8]. K.T. Mannan et al experimented on the High speed steel to study its surface characterization after machining. They varied current and pulse on time keeping voltage and duty factor constant and found that at low current and low pulse on time craters were shallow and the density of globules and pockmarks were low [4]. K.L. Wu et al studied the effect of aluminum and surfactant added dielectric on surface finish of SKD steel. They found that the surface roughness of EDMed surface improved up to 60% as compared to that of machined surface with pure dielectric [9]. H.S. Payal et al experimented on EN31 tool steel with copper brass and graphite tool electrode and with kerosene as dielectric to study the structure of machined surface. Their micrograph study revealed that the material was removed from the surface as ligaments and sheets, surface machined with copper tool was smoother than that of graphite tool [10]. K.M. Tsai et al established semi-empirical model of surface finish on work for various materials by employing dimensional analysis based on pertinent process parameters of EDM machining. They constant parameters cannot be used for various work and tool materials [11].
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Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490
2. Experimental Details The experiments were performed on Sparkonix Die-Sinking EDM. The tool used is of pure copper and of cylindrical cross section of dimension ø10mm and 100mm. The workpiece used is Nimonic 75 alloy of dimension 75mm X 35mm X 8mm. The chemical composition of Nimonic 75 alloy is depicted in Table 1. Table 1. Chemical Composition of Nimonic 75 alloy. Alloying elements Amount
C
Si
Mn
Cr
Ni
Cu
Ti
Fe
(%)
(%)
(%)
(%)
(%)
(%)
(%)
(%)
.12-.15
.35-1
.65-1.0
18.5-21
77.7
.15-.50
.25-.60
2.03-5
The dielectric used was of commercial grade EDM 50 oil. Tool polarity, peak current, pulse on time and voltage were selected as input parameters. 2.1. Taguchi’s Method Dr. Genichi Taguchi developed an engineering method for quality improvement. This method uses orthogonal arrays to optimize the process parameters and their levels [12]. This method also reduces the number of experimental trials runs as compared to full factorial experimental design to reduce cost and time. Signal to noise ratio is a unique term in the Taguchi’s method that is used to analyze the robustness of the system. The experiments conducted are based on Taguchi’s L18 Orthogonal Array. The design of experiments is based on mixed OA where peak current, pulse on time and the voltage are varied to three levels and the tool polarity is varied on two levels. Table 2 describes the process parameters and their levels. Table 2. List of process parameters and their levels
Design Factors 1
Levels 2
3
-ve
+ve
--
Current (Amp.)
6
8
10
Pulse On Time(μs)
60
90
120
Voltage(Volt)
40
50
60
Tool Polarity
Two replicates of each experimental trial run were performed for 20 mins each to minimize the uncontrollable variations. After the machining of the workpiece the surface roughness of each machined cavity was measured using MITUTOYO SJ-201P portable surface roughness tester. Roughness measurements in transverse and longitudinal directions on the workpiece are performed for each machined cavity. Table 3 shows the response table for mean value of Ra.
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Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490 Table 3. Experimental Results Exp. No.
Tool Polarity
Current (A)
Pulse On-time (μs)
Voltage (V)
SR(mean) Ra μm
1
-
6
60
40
2.778
2
-
6
90
50
3.370
3
-
6
120
60
2.686
4
-
8
60
40
2.555
5
-
8
90
50
3.012
6
-
8
120
60
3.390
7
-
10
60
50
2.751
8
-
10
90
60
3.245
9
-
10
120
40
3.580
10
+
6
60
60
4.940
11
+
6
90
40
5.742
12
+
6
120
50
5.802
13
+
8
60
50
5.395
14
+
8
90
60
6.389
15
+
8
120
40
8.935
16
+
10
60
60
6.103
17
+
10
90
40
8.910
18
+
10
120
50
7.940
3. Results and Discussions The experimental results obtained by the experimental trails runs were further analyzed using Minitab 16 software to study the influence of various machining parameters on surface roughness. 3.1. Analysis of SN ratios The deviation of quality characteristics from the desired value is measured using S/N ratios. The desirable mean value of output characteristics is represented as Signal and the undesired value or standard deviation is represented as Noise [13]. Fig. 1 shows main effect plots for S/N ratios. The curve having larger inclination from the mean line will have greater impact on the performance measures. Tool polarity with the largest inclination from the mean line is the most significant parameter. This is due to the fact that higher thermal energy is imparted to the workpiece in case of positive tool polarity which leads to larger material removal thus rougher surface than negative tool polarity. Surface roughness is also found to increase with an increase in discharge current. This is because that as the current increases the discharge energy transferred to the workpiece thus removes material with larger and deeper craters making surface more rougher. Increase in pulse on time also accounts for higher surface roughness. Energy delivered in single spark increases with increase in pulse on-time causes increased size of craters on the machined surface. Increase in voltage increases the spark density and thus larger craters are formed on the workpiece making the surface more rougher. Further increase in voltage leads to expansion of plasma channel forming shallow craters [8].
Rajesh Choudhary et al. / Materials Today: Proceedings 2 (2015) 3481 – 3490
3485
Fig 1. Main Effect plots for SN ratios.
3.2. Analysis of 3-D Surface Plot Effect of current and pulse on time on surface roughness is clearly demonstrated in Fig. 2. It shows that increase in pulse on time increases the surface roughness to a larger extent than the increase in discharge current.
Fig 2. Surface Plot of SR vs Pulse on time, Current
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3.3. Analysis of Variance Analysis of variance table was performed to obtain percentage contribution of different parameters individually on the surface roughness. Table 4 shows analysis of variance for surface roughness. F is the ratio of mean of squared deviation to the mean of squared error. Higher the value of F larger the impact of parameter on Ra. Thus the tool polarity have greatest impact on the surface roughness followed by pulse on time, current and voltage. P denotes the significant parameters affecting Ra. If the value of p < 0.05 the parameter will be the most significant. Thus the tool polarity is the most significant parameter for surface roughness. Table 4. ANOVA Table for means. Source DF
Seq. SS
Adj. SS
Adj. MS
F
P
Tool Polarity
1
59.729
59.729
59.7288
92.13
0.000
Current
2
4.396
4.396
2.1981
3.39
0.075
Pulse on time
2
5.642
5.642
2.8210
4.36
0.044
Voltage
2
2.957
2.957
1.4784
2.28
0.153
Residual Error
10
6.476
6.476
0.6476
Total
17
79.200
S = 1.0
R-Sq = 98.5%
R-Sq(adj) = 92.9%
3.4. Analysis of Response for SN ratios Table 5 shows the response for SN ratios. It is clearly observed that from this table the rank in which the parameters affect the value of Ra. Delta denotes the difference between the highest average value of each parameter and lowest average value of same parameter. Larger the value of delta larger the impact of that parameter on the value of Ra. This shows that Ra is mostly affected by tool polarity followed by pulse on time, current and voltage respectively. Table 5. Response for SN ratios Level
Tool polarity
Current
Pulse on Time
Voltage
1
-9.603
-12.056
-11.673
-13.550
2
-16.311
-13.017
-13.440
-12.805
-13.799
-13.759
-12.517
3 Delta
6.708
1.743
2.086
1.033
Rank
1
3
2
4
3.5. Analysis of Surface Topography Surface topography of the machined surface was studied using the SEM micrographs. Figure 3 shows the micrographs of the machined surface with negative and positive tool polarity at various magnifications. Fig 3(a) shows the surface machined with negative tool polarity which is smoother as compared to surface machined by positive tool polarity (Fig 3(b)). Removal of material from workpiece in case of negative tool polarity is lower which leads to uniform surface on the machined surface (fig. 3(c)). When the tool polarity is positive the removal of material is more in the form of shallow craters and the un-removed material gets resolidified as recast later. Thus higher deposition of melted metal is formed on the surface machined with positive tool polarity as shown in Fig. 3(d). Fig 3(e) shows the cracks formed on the surface due to removal of material which is shallower as compared to
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Fig 3(f). The globules in recast layer, pockmarks obtained from gases entrapped and resolidified layer is also visible from the micrographs (Fig. 3f).
Fig. 3 SEM Images (a) at –ve polarity 100x (b) at + polarity 100x (c) at –ve polarity 500x (d) at +ve polarity 500x (e) at –ve polarity 1000x (f) at +ve polarity 1000x
3.6. Analysis of Metallurgical Composition of EDMed Surface The surface machined with EDM was analyzed for its metallurgical composition using Energy Dispersive spectroscopy. Fig 4 shows the actual metallurgical constituents of pure metal surface. The transfer of foreign
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elements on the machined surface with negative and positive tool polarity is more as compared to the surface without machining (fig. 5-6). This is due to pyrolysis of dielectric fluid present between the workpiece and the tool electrode. But the Carbon content on the surface machined with positive tool polarity is more than that of surface machined with negative tool polarity. This signifies that the rate of pyrolysis of the dielectric is more in case of positive tool polarity as compared to that of negative tool polarity. The deposition of tool electrode i.e. copper is more on the surface machined with negative tool polarity than that of the surface machined with positive tool polarity. This is due to higher tool erosion in case of negative tool polarity than that of positive tool polarity.
Fig. 4 EDS graph of pure metal without machining.
Fig. 5. EDS graph of surface machined with negative tool polarity.
Fig. 6. EDS graph of surface machined with positive tool polarity.
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Increased tool wear is observed in case of negative tool polarity, this is due to bombardment of proton particles on the tool surface. As the mass of proton is higher as compared to electrons, so more energy is imparted to the tool surface which causes its erosion during machining and thus causes more deposition of tool material on the workpiece surface. 4. Conclusions In this paper the effect of tool polarity, current, pulse on time and voltage on surface roughness of Nimonic 75 alloy is concluded. Taguchi’s L18 orthogonal array is used for design of experiments. SEM and EDS analysis was carried out to study the surface topography. The following conclusions were drawn x Tool polarity is the dominating factor for surface roughness followed by pulse on time and current. x Minimum surface roughness was observed at negative tool polarity, 8 A current, 60μs pulse on-time and 40V gap voltage. x The surface machined with positive tool polarity have uneven surface with thick recast layer with globules as compared to surface machined by positive tool polarity. x Carbon content on surface machined with positive tool polarity is more than that of negative tool polarity. x Deposition of tool material on surface machined with negative tool polarity is more than that of surface machined with positive tool polarity. Acknowledgements The authors of the research work deeply acknowledge the funding given by the All India Council for Technical Education (AICTE) New Delhi for providing the EDM setup under MODROBS scheme in the Department of Mechanical Engineering at MIMIT, Malout. References [1] P.Subhash, C. Bose, C.S.P. Rao, Evaluation of Optimum Cutting Parameters In Turning of NIMONIC 75 using RSM, International Journal on Theoretical and Applied Research in Mechanical Engineering , vol. 2, no. 2, pp. 17-30, 2013. [2] S.Singh, A. pandey, Some Studies into Electrical Discharge Machining of Nimonic75 Super Alloy Using Rotary Copper Disk Electrode, Journal of Materials Engineering and Performance, vol. 22, no. 5, pp. 1290-1303, 2013. [3] E. Kuljanic, M.Sortino, G. Totis, Machinability of Difficult Machining Materials, in 14th International Research/Expert Conference Trends in the Development of Machinery and Associated Technology”, Mediterranean Cruise, 11-18 September 2010. [4] K.Tamil Mannan, A. Krishnaiah, S.P. Arikatla, Surface Characterization of Electric Discharge Machined Surface of High Speed Steel, Advanced Materials Manufacturing & Characterization, pp. 161-168, 2013. [5] Ghoreishi, S. Assarzadeh, Statistical Investigation into the Effects of Electro-Discharge Machining Parameters on WC/6%Co Composite-Part 1: Modeling through Response Surface Methodology (RSM), Advanced Materials Manufacturing & Characterization, vol. 3, no. 2, pp. 478-486, 2013. [6] S. H. Lee, X. Li Study of the surface integrity of the machined workpiece in the EDM of tungsten carbide, Journal of Materials Processing Technology, vol. 139, no. 1-3, pp. 315-321, 2003. [7] M. Boujelbene, E. Bayraktar, W. Tebni, S.B. Salem, Influence of machining parameters on the surface integrity in electrical discharge machining, Archives of Materials Science and Engineering, vol. 37, no. 2, pp. 110-116, 2009. [8] M. K. Das, K. Kumar, T.K.Burman, P. Sahoo, Application of Artificial bee Colony Algorithm for Optimization of MRR and Surface Roughness in EDM of EN31 tool steel, Procedia Materials Science , vol. 6, pp. 741-751, 2014. [9] K. L. Wu, B.H. Yan, Y. F. Huang, S.C. Chen, Improvement of surface finish on SKD steel using electro-discharge machining with aluminum and surfactant added dielectric, International Journal of Machine Tools & Manufacture, vol. 45, pp. 1195-1201, 2004. [10] H S Payal, R. Choudhary, S. Singh, Analysis of electro discharge machined surfaces of EN-31 tool steel, Journal of Scientific & Industrial Research, vol. 67, pp. 1072-1077, 2008. [11] K. M. Tsai, P.J. Wang, Semi-empirical model of surface finish on electrical discharge machining, International Journal of Machine Tools & Manufacture, vol. 41, pp. 1455-1477, 2001.
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[12]A. Goswami, J. Kumar, Optimization in wire-cut EDM of Nimonic-80A using Taguchi's approach and utility concept, Engineering Science and Technology an International Journal, pp. 1-11, 2014. [13] A. Abusada, O. Belgassim, Abdurrahman, Optimization of the EDM Parameters on the Surface Roughness of AISI D3 Tool Steel, in Proceedings of the 2012 International Conference on Industrial Engineering and Operations Management, Istanbul, Turkey, 2012.