Abstract. EDM is a nontraditional method of removing material by a series of rapidly recurring electric discharges between an electrode (the cutting tool) and the workpiece, in a medium of a dielectric fluid. EDM is a precision machining technique and is used in making dies and molds of extremely hard materials that cannot be machined by conventional techniques. The present work was conducted in order to investigate the surface finish, material removal rate and the surface damage during EDM. Copper and carbide were taken as the electrode and the work materials for the present study. The influence of current and pulse-on time on the responses were studied. Design of Experiment (DOE) was used to conduct the investigation. It was found that MRR and surface roughness increases with both current and pulse on time. Tool wear, work surface damage and materials migration between the electrode and the workpiece was found to be increased with current.
Introduction
Electrical discharge machining (EDM) is one of the most common non-traditional machining processes employed to machine difficult-to-machine materials. High material removal rate (MRR) and low electrode wear are desirable with minimum damage to the work surface. However, heat treatments after EDM improve the quality in terms of microstructures and surface roughness [1]. Development of a recast layer on the machined surface, surface roughness, micro cracks and spalling are the main types of surface damage. It was reported by T.R. Newton et al. [2] that a recast layer was observed to be between 5 and 9 µm in average thickness during EDM of Inconel 718. It was found that average recast layer thickness increased primarily with energy per spark, peak discharge current, and current pulse duration. Sometimes molds and dies desire to remove the recast layer even though it is hard and has good matrix adherence [3]. A smooth surface is the primary requirement for precision dies and moulds. It was established that lower current, lower pulse time and relatively higher pulse pause time produce a better surface finish [4]. Micro cracks in the surface and loose grains in the subsurface were found during EDM of Ti3SiC2 ceramic [5] resulted from thermal shock during the process that led to a degradation of both strength and reliability of the work material. Carbides are increasingly being used in making tools and precision parts due to their unique properties of high hardness and wear resistance. Due to its high hardness it is difficult to machine it by conventional techniques. Therefore, EDM is the only potential technique for machining carbides. In the present study investigations have been carried out on surface damage of carbide grade GC-US16 during EDM.
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Work and the electrode Material. In the present study the work material was carbide of grade GC-US16. Its principal chemical composition and mechanical properties are: 84% WC, 25% Co, the grain size of the constituents 0.8 microns, hardness 90.8 HRA and density 13.8 g/cc. The material of the electrode was copper. The major properties of the electrode material are shown in Table 1.
Table 1: Major properties of copper electrode Electrical resistivity 1.71 x 10
-6 Ω/cm
Coefficient of thermal expansion 17 µm/m-oC
Thermal conductivity 391 Wm-K
Melting point 1083 oC
Heat capacity 0.385 J/g-oC
Experimental Procedure. In the present study the experimental plan was designed using Design of Experiments (DOE). The software used is Design Expert. Four machining parameters like current, on-time, off time and voltage were taken as the input parameters. For analysis, the influence of current and on-time on the responses were considered as the two most significant factors according to literature survey. The responses were MRR and work surface roughness (Ra). The machining parameters and their selected levels are shown in Table 2. The machining was performed in a die sinking EDM machine Mitsubishi EX 22 model C11E FP60E. Kerosene was used as the dielectric fluid.
Table 2: Machining parameters and their selected levels Factors Levels
-1 +1
A Current (I) 4.5 6.5
B On-time (µs) 3.5 5.5
C Off time (µs) 5 7
D Voltage (V) 12 14
The weight of the workpiece was measured before and after the machining using a precision electronic balance. MRR was calculated as the ratio of the difference of the weights of the workpiece before and after machining to the machining time. The work surface roughness was measured using a surface roughness tester Mitutoyo Surftest SV-514. The tester uses the software Surfpak V4.10 (2) with a resolution of 0.01 µm and the stylus speed was 0.1 mm/s. The work surface was analyzed using JOEL scanning electron microscope model JSM 56000.
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Std Run Block A:Current B:ON time C:OFF time D:Voltage Surface
Roughness MRR
Ampere µs µs Volt µm µm3/s
15 1 Block 1 4.5 5.5 7 14 4.86 16.7
3 2 Block 1 4.5 5.5 5 12 4.76 41.84
7 3 Block 1 4.5 5.5 7 12 5 18.33
1 4 Block 1 4.5 3.5 5 12 2.98 23.93
6 5 Block 1 6.5 3.5 7 12 3.31 7.17
14 6 Block 1 6.5 3.5 7 14 3.31 6.53
17 7 Block 1 5.5 4.5 6 13 2.2 30.68
5 8 Block 1 4.5 3.5 7 12 1.77 6.77
9 9 Block 1 4.5 3.5 5 14 1.79 20.52
11 10 Block 1 4.5 5.5 5 14 4.04 35.77
8 11 Block 1 6.5 5.5 7 12 4.01 53.73
16 12 Block 1 6.5 5.5 7 14 5.25 48.03
2 13 Block 1 6.5 3.5 5 12 2.07 56.91
10 14 Block 1 6.5 3.5 5 14 2.3 43.41
4 15 Block 1 6.5 5.5 5 12 3.39 124.21
13 16 Block 1 4.5 3.5 7 14 0.79 6.11
12 17 Block 1 6.5 5.5 5 14 2.58 130.73
24 18 Block 2 5.5 4.5 6 11 1.12 38.29
21 19 Block 2 5.5 6.5 6 13 5.17 41.19
26 20 Block 2 5.5 4.5 6 13 1.53 30.57
22 21 Block 2 5.5 4.5 4 13 4.11 73.64
25 22 Block 2 5.5 4.5 6 15 3.5 26.39
20 23 Block 2 5.5 2.5 6 13 2.3 5.26
23 24 Block 2 5.5 4.5 8 13 4.26 8.02
19 25 Block 2 7.5 4.5 6 13 3.89 73.01
18 26 Block 2 3.5 4.5 6 13 2.42 6.34
Work surface roughness. Fig. 1 shows the influence of current and on-time on work surface roughness. It is observed that Ra increases with increase in current. This can be explained by the fact that at a given pulse duration, if the pulse current is increased, the thermal energy which is being induced in the workpiece through the spark is also increased. If the sparks are energetic, the surface finish will be rough. It can be observed that Ra also increases as on-time is increased. A larger on-time removes more material and results higher Ra. Smoother surface can be obtained when both current and on-time are smaller (Fig. 1a).
Fig.1: Relationship of Ra with current and on-time Material removal Rate. Fig.2 illustrates the relationship of MRR with current and on-time. From the Fig. it is obvious that MRR maintains almost linear relationships with current and on-time. As the current is increased, a stronger spark is produced generating more heat that results more MRR. This result is supported by the work done by Puertas et.al [6]. Similarly, with increase in on-time, more time is available for the carbide to melt and vaporize. Fig. 2a clearly shows that a higher MRR can be achieved with a higher current and a higher on-time.
(a) Contour plot (b) 3D plot for MRR
Fig.2: Relation of MRR with current and on-time
Micro cracks. It strongly influences the fatigue properties of a part. Micro cracks were observed by SEM with a magnification of 600. Fig. 3 shows that the surface machined with a higher currant of 7.5 A exhibits pronounced defects with more micro cracks compared to the one machined with a lower current of 3.5 A. Moreover, spalling effect most often is related to the generation of larger micro cracks during EDM. These larger micro cracks make the separation of a volume during successive discharges much easier
Micro cracks at 7.5 A Micro cracks at 3.5 A
Fig.3: Micro cracks at different currents
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Recast layer. There is a recast white layer formed on the EDMed machined surface. J.P. Kruth et al. [7] reported that a white layer has been observed to form under all machining conditions even when using water as dielectric, though this white layer differs from the one found on samples machined in an oil dielectric. Fig.4 shows the effect of the pulse current on the thickness of recast layer. It can be seen from Fig.4 that the surface EDMed with 7.5A pulse current produces a thicker recast layer compared to that EDMed with 3.5A pulse current.
Recast layer at 7.5 A Recast layer at 3.5 A
Fig.4: Recast layer at different currents
Salling at 7.5 A Spalling at 3.5 A
Fig.5: Spalling effect at different currents Spalling. Due to the melting of materials from the grain boundaries, some material is separated from the main mass. Since a higher current melts more material and more weakens the grain boundaries, more spalling effect is observed at a higher current as demonstrated in Fig.5.
Conclusions
1. Pulse current strongly influences the surface quality. A larger on-time removes more material and results higher Ra. Smoother surface can be obtained when both current and on-time are smaller.
2. Material removal rate strongly depends on current and on-time. A higher current produces a stronger spark removing more material and when on-time is higher, more time is available to melt carbide.
3. More micro cracks on the EDMed surface have been observed when the surface is machined by a higher current. This is due to heating the surface to a very high temperature at a higher current followed by rapid cooling.
4. A thicker layer of recast layer on the surface is observed when it is machined with a higher current. This is due to more melting and depositing of material at a higher current. A higher current melts more material from the grain boundaries and causes more spalling of material.
5. In order to reduce micro cracks, spalling and thickness of the recast layer a low current should be used.
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[7.] J.P. Kruth, L. Stevens, L. Froyen and B. Lauwers, Study of the White Layer of a Surface Machined by Die-Sinking Electro-Discharge Machining, CIRP Annals - Manufacturing Technology, Vol. 44(1), (1995), 169-172.
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Advances in Materials and Processing Technologies II doi:10.4028/www.scientific.net/AMR.264-265 Investigation on the Surface Finish, MRR and the Surface Damage during the EDM ofTungsten Carbide doi:10.4028/www.scientific.net/AMR.264-265.1073
