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Investigation on the influence of the dielectrics on the material removal characteristics of EDM
Journal of Materials Processing Technology 214 (2014) 1052–1061
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Investigation on the influence of the dielectrics on the material removal characteristics of EDM Yanzhen Zhang, Yonghong Liu ∗ , Yang Shen, Renjie Ji, Zhen Li, Chao Zheng College of Electromechanical Engineering, China University of Petroleum, Dongying 257061, China
a r t i c l e
i n f o
Article history: Received 24 January 2013 Received in revised form 9 December 2013 Accepted 19 December 2013 Available online 28 December 2013 Keywords: EDM Crater Dielectric Material removal Bubble
a b s t r a c t A systematical and comprehensive investigation of the material removal characteristics of the electrical discharge machining (EDM) process using various dielectrics as the working fluids was conducted in this work. Five dielectrics, including gaseous dielectrics, air and oxygen, and liquid dielectrics, de-ionized water, kerosene and water-in-oil (W/O) emulsion were used as the working fluids. The whole geometry parameters of the craters, including the recast material in the craters, were precisely determined by metallographic method. The volume of melted and removed material and removal efficiency in different dielectrics were comparatively investigated. By relating the material removal characteristics to the evolution of the discharge generated bubbles in different dielectrics which was done by computer simulation, it seems that the pressure above the discharge point is an important factor that can affect material removal characteristics. The results of this work were supposed to be helpful for further clarifying the complicated material removal mechanism of EDM. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electrical discharge machining (EDM) is a widely used manufacturing technology in the industry application. Both electric conductive and semi-conductive materials can be machined by this non-contact method regardless of their hardness; therefore many hard-to-cut and brittle materials can be economically processed by EDM. With the development of EDM, many variations of this technique, such as sinking EDM, wire EDM, EDM milling and microhole EDM drilling have been developed since it was innovated in the late 1940s by the former Soviet Union scientists. Nowadays, various types of dielectrics have been used in the area of EDM. Hydrocarbon oil based and water based working fluids are the traditional working fluids of EDM. However, with the development of this technology, gaseous dielectric, which is more environmentally friendly and more economical, attracts more and more attentions. Although the EDM technology and its fundamental mechanism have been investigated by many researchers, some additional aspects of the process still need to be thoroughly investigated for enhancing its application potential and performance in future technologies. The influence of the dielectrics and its effect on the process performance is one such aspect which needs to be thoroughly investigated. The role of dielectrics on the EDM process was not clearly stated although various types of dielectrics have been used
∗ Corresponding author. Tel.: +86 053286983303. E-mail address: [email protected] (Y. Liu). 0924-0136/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.12.012
in various variations of EDM. The physical and chemical properties of the dielectric have significant influence on the EDM performance. Investigation carried out by Zhang et al. (2011) revealed that both productivity and quality are significant affected by dielectrics; surface with different characteristics can be obtained with different dielectrics. 1.1. Type of currently used dielectrics Abbas et al. (2007) reviewed the research trends in EDM, as well as the EDM technology using water based dielectrics and gaseous dielectrics. Leão and Pashby (2004) reviewed the environmentally friendly dielectrics used in EDM. Their work revealed that the current used dielectrics can be classified basically into the following three groups. 1. Hydrocarbon oil based dielectrics. This was the original dielectric of EDM and is still used in the case of sinking EDM today. However, compared with water or gaseous based dielectrics, the oil based dielectrics were not environmentally friendly. Investigations carried out by Sivapirakasam et al. (2011) showed that aerosol and some hazardous gases will be generated when oil based dielectrics were used. 2. Water based dielectrics. They were widely used in the case of wire EDM and micro-hole EDM drilling. Compared with the oil based dielectrics, they were much more environmentally friendly. In the case of sinking EDM, the feasibility of water based dielectric had been explored by Koenig and Joerres (1987); however,
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their experiment results indicated that the performance of water based dielectrics was unfavorable and unsatisfied compared with the hydrocarbon oil based dielectrics. Recent investigation carried out by Liu et al. (2010) also showed the poor performance of water based dielectric in the case of sinking EDM. 3. Gaseous based dielectrics. Nowadays, the use of gaseous dielectrics attracts more and more attentions. Compared with the liquid dielectrics, the gaseous dielectrics were much more environmentally friendly and more economical. Kunieda and Yoshida (1997) investigated the EDM process in gas and they found that with some special conditions the machining performance can be comparable with that in liquid dielectrics. However, investigations carried out by Tao et al. (2008) showed that there are still some technical problems need to be completely resolved before the industry application of the gaseous dielectrics. Up to today, commercial EDM machine tools using gaseous dielectric are still unavailable. 1.2. Role of the dielectrics The dielectrics play a very important role in the EDM process. For a quite long period, it was thought that the liquids dielectric was indispensable during the EDM process; since it can compress the discharge generated bubble, cool the melted debris and bring the debris out of the gap. It was conventionally depicted that most of the melted material was removed by the boiling of the superheated molten material at the end of the discharge because boiling of that superheated metal is prevented by the bubble pressure during the discharge duration since the expansion of the bubble was compressed by the inertia and viscosity of the surrounding liquid dielectrics. For a quite long period, this depiction about the role of dielectrics was generally accepted by researchers. However, Kunieda et al. (2005) and Hinduja and Kunieda (2013) showed that the validity of the established theories of the role of dielectrics was challenged by some new discoveries and inventions. Hayakawa et al. (2013) investigated the material removal of single discharge with the help of high speed camera and they found that that material removal also occurs during the discharge. Yoshida and Kunieda (1998) studied the formation and distribution of the debris on the working surface formed by single discharge in liquid and gaseous dielectric respectively. They found that there are differences in the total volume of the generated debris and the distribution of the debris size between single pulse discharges in liquid and in air when the discharge duration is short; and no difference exists for discharge durations longer than 90 s. The debris volume difference is very small when the discharge duration is long. They explained this phenomenon by the bubble generated by the discharge itself. In liquids dielectric, the arc column environment becomes equivalent to that of the discharge which occurs in gas with the increase in the diameter of the bubble. Their experiment results indicate that metal removal can occur without a liquid dielectric. Fundamental research into the EDM process carried out by Imai et al. (2001), Wang et al. (2012) and Kitamura et al. (2013) showed that most of the machining area was occupied by discharge generated gas even though the gap was submerged in liquid dielectric. Experiments carried out by Yoshida and Kunieda (1998) showed that most of the melted debris reattached on the electrode surface in air because they cannot be cooled by the air; whereas in liquid dielectric, the debris can solidify into a spherical shape. Their works indicated that the dielectric liquid is important for the cooling and flushing of debris particles but not for material removal. Nowadays, numerous investigations of dry EDM have confirmed that the EDM process can be performed in gaseous dielectrics. Without the compression on discharge generated bubble by the liquid dielectrics, the material removal can also take place in the case of gaseous dielectrics. A recent investigation carried out by
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Yoshida et al. (2011) showed that over 70% of the melted material can be removed from the crater with the help of high flow rate of the airflow, whereas in liquid dielectrics, most of the material re-solidified in the crater. Yang et al. (2011) investigated the material removal mechanism in the case of micro EDM by the method of molecular dynamics simulation. In their work, the gap was assumed to be vacuum which means that the influence of dielectrics (neither liquid dielectrics nor gaseous dielectrics) was totally excluded during the material removal process. They found that the material removal mechanism in micro EDM can be explained in two ways; one by vaporization and the other by the bubble explosion of superheated metals. Note that the “bubble” is the bubble in the molten pool not the bubble between electrodes in the dielectric. From their work, it seems that the EDM process can be performed without any type of dielectrics (in vacuum). Tamura and Kobayashi (2004) measured the impulsive forces of single discharge due to the expansion and contraction of the discharge generated bubble in kerosene and gas dielectrics and examined the mechanism by which the impulsive forces affect the crater formation. Based on the experiment results, they concluded that the influence of impulsive forces on the crater formation was not significant. Zhang et al. (2013) simulated the expansion process of the discharge generated bubble and they found that the geometry shape of the crater and the material removal efficiency were significantly affected by the inter electrode distance which can determine the magnitude of the impulsive forces. Apparently, the results reported by Zhang et al. (2013) were contradicted with those reported by Tamura and Kobayashi (2004). In the actual case, besides the inter electrodes distance, the expansion of the discharge generated bubble was also affected by the inertia and viscosity of the dielectrics. Review of the published literature did not give us a clear answer to the following question: how the pressure or pressure changes in the bubble affect the material removal in the crater? The role of dielectrics which can affect the pressure in the discharge charge generated bubble was still elusive and still needs to be thoroughly investigated. 1.3. Investigation strategy of this work In our work, the material removal characteristics in different dielectrics were investigated by the method of single discharge. For a better understanding of the crater formation, it is very necessary to investigate the microstructure of the individual crater such as melting, material removal and re-solidification. Experimental determination of the geometry of the whole recast region, volume of molten material and removed material should provide useful and insightful information about the fundamental mechanism of EDM. However, most of the previously investigations only focused on the external characterization of the craters; sectional characterization of the craters was not investigated due to the difficulty in the metallographic preparation of the samples. Yoshida et al. (2011) investigate the volumes of re-solidified metal and molten metal in the crater by stacking the cross-sectional shapes of the crater; their work revealed that the metallographic preparation of the samples was feasible and could give a precise determination of the volume of the total melted material, as well as the removed material in the crater. In our work, a systemically experimental analysis of craters formed in different dielectrics was performed. The method proposed by Yoshida et al. (2011) was adopted in our research. Both the external and sectional appearance of the craters were investigated. The material removal mechanism was discussed by correlating the 3-dimentional geometry of the craters to the transient simulation results of the discharge generated bubbles in various dielectrics. Moreover, the influence of pulse duration was also investigated
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Y. Zhang et al. / Journal of Materials Processing Technology 214 (2014) 1052–1061 Table 1 Parametrical order of the single discharge experiments. Parameters, units
Description
Pulse duration, s Dielectric
52, 105, 210, 420, 840 Oxygen, air, kerosene, de-ionized water, water-in-oil (W/O) emulsion
Fig. 2. Sectional view of the crater.
Fig. 1. (a) Illustration of the single pulse discharge experimental set up and (b) equipment used for measuring the distance between each sectional layer.
since the geometry of the discharge crater would exhibits significant difference with different pulse durations even in the same dielectric. In this work, the machining polarity in deionized water and gaseous dielectrics was positive and in kerosene and W/O emulsion negative; consistent with machining polarities usually adopted in the industry application. 2. Experimental work 2.1. Experimental set-up The single pulse discharge equipment, as illustrated in Fig. 1a, was used in our experiments. The single pulse waveform was generated by an IGBT which was controlled by a singlechip. Various pulse durations can be easily generated by changing the program of the singlechip. The discharge waveform (including both the voltage and current waveforms) was recorded by an oscilloscope for further analysis. The workpiece material was mold steel 8407. Steel needle was used as the electrode. 2.2. Experimental procedure Experiments were carried out with parametrical order using five different dielectrics and five different pulse durations settings (Table 1). A big range of pulse duration was adopted to
investigate the influence of pulse duration, since Zahiruddin and Kunieda (2012) reported that the length of the pulse duration greatly affects the thermal transmission characteristics in the workpiece material. Oxygen was used as the dielectric with the aim of investigating the influence of oxidation during the material removal process by comparing the characteristic of the crater formed in air. Gaseous dielectrics were adopted with the aim of clarifying the material removal mechanism of dry EDM which attracts more and more attentions today. Zhang et al. (2012) studied the EDM performance with W/O emulsion dielectric. They found that, comparing with the traditional kerosene based dielectric, the machining efficiency can be improved and the environmental impact can be alleviated by using this new dielectric. Therefore, W/O emulsion, de-ionized water and kerosene were included in the current work to carry out a comparative investigation. The physical properties of the five dielectrics are listed in Table 2. Compared with the other two liquid dielectrics, the viscosity of the W/O emulsion is much higher. After the single pulse discharge, the workpiece was cut and embedded in epoxy (the sample shown in Fig. 1b) with the aim of getting the sectional geometry parameters of the crater. The section view of the crater was obtained by a metallurgical microscope after polished and etched with nital. After capturing one sectional image, the sample was re-grinded and re-polished to get another sectional image. For every crater, about 15–20 sectional images at different locations were obtained by the metallurgical microscope. Clear profile of the recast area can be observed after etched (Fig. 2). The distance between each layer was about 10–30 m depending on the diameters of the craters. With the help of micrometer gauge, the distance between each section can be accurately measured (as shown in Fig. 1b). With the sectional images and the distance value between each layer, the crater can be rebuilt by 3D-CAD software (Solidworks) as shown later in Fig. 3.
Table 2 Physical properties of the five dielectrics. Dielectrics
(W/m K)
ı (◦ C)
C (J/kg ◦ C)
Oxygen
0.027
−218.79
0.913
Air
0.016
–
1.005
De-ionized water Kerosene
0.62 0.14
0 –
4200 2100
W/O emulsion
0.22
–
2747
(s/m) Too small to be measured Too small to be measured 6.5 × 10−3 Too small to be measured Too small to be measured
Thermal conductivity (), melting point (ı), specific heat (C), conductivity (), viscosity (), density ().
(Pa s)
(kg/m3 )
Composition (vol%)
1.43
99.9% O2
1.29
78% N2 , 21% O2 , 1% other gases 100% H2 O 100% hydrocarbon
1 × 10−6 2.3 × 10−6
1000 860
8.7 × 10−4
890
30.8% H2 O, 68.7% hydrocarbon, 0.5% surfactant
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Fig. 3. (a and d) Display of whole re-solidified material; (b and e) display of the re-solidified material below the workpiece surface; (c and f) display of the re-solidified material above the workpiece surface.
2.3. Rebuilt of the crater with 3D-CAD software The craters were rebuilt with the help of 3D-CAD software (Solidworks). The two typical crater shapes in our experiments are shown in Fig. 3a and d respectively. The re-solidified material below the surface of the workpiece is shown in Fig. 3b and e, whereas the re-solidified material above the surface of the workpiece is shown in Fig. 3c and f, respectively. The craters shown in Fig. 3a and d were formed in W/O emulsion and de-ionized water, respectively, with pulse duration 210 s. As can be seen from Fig. 3, the removal efficiency of the crater formed in W/O emulsion is much higher than that formed in de-ionized water. In de-ionized water, most of the melted material was re-solidified in the crater. The detail forming mechanisms of the two typical appearances will be discussed in the following sections. 3. Results and discussion 3.1. Definition of the crater shape As shown in Fig. 3, there are mainly two typical geometry shapes of the craters presented in our work. The first type of crater is characterized by remarkable central depression surrounded by rim bulge caused by the flow of molten material outwards from the crater center (Fig. 3a); whereas the second type of craters is much more flat, remarkable center depression and surrounded bulge cannot be observed (Fig. 3d). The first type of the crater shape was characterized by the parameters: removal diameter drem , bulge diameter dbul , recast diameter drec , removal depth hrem , bulge height hbul , recast depth hrec , removal volume vrem , bulge volume vbul (above the surface of workpiece) and recast volume vrev (below the surface of workpiece), as shown in Fig. 4a. In the case of the
Fig. 4. Definition of discharge crater shape.
second type of craters, only the value of drec , hrec , vrem , vbul and vrec was determined since the remarkable depression and bulge cannot be observed, as shown in Fig. 4b. Apparently, the material removal efficiency of the first type of crater was much larger than that of the second type. In the following sections, emphasis was put on the value of drec and hrec because the diameter and depth of the recast material were important information which can be used to speculate and to
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Fig. 5. Comparison of the external appearances of craters formed in (a) air; (b) kerosene; (c) deionized water; (d) W/O emulsion; and (e) oxygen. Pulse duration = 105 s. The length of the scale bar was 100 m.
evaluate the heat convection characteristics of the single discharge, as described by Zahiruddin and Kunieda (2012). Besides drec and hrec , the value of hrem was also discussed since the material removal efficiency was greatly affected by removal depth.
the value of drem and dbul increases with increasing pulse duration. With longer pulse duration, the center depression and rim bulge cannot be observed in air and oxygen dielectrics. Therefore, in Fig. 7, the value of drem and dbul was not listed.
3.2. Diameter, depth and volume
3.2.2. Depth The values of recast depth hrec are shown in Fig. 8. The recast depth monotonically increases with pulse duration. As can be seen from Fig. 8, there was a great difference of recast depth in different dielectrics. The value of recast depth was biggest in de-ionized water, followed by W/O emulsion, oxygen, air and kerosene. Perhaps this can be explained by the machining polarity. As has been
The craters formed in different dielectrics are compared in Fig. 5. As shown in Fig. 5, there exists a huge difference of the external appearance of the craters. In the following section, quantitative discussions of the craters formed in different dielectrics were performed based on 3D geometry model of the craters rebuilt by Solidworks. 3.2.1. Diameter Fig. 6 shows the recast diameter of crater, drec , in different dielectrics. The value of drec increases with increasing pulse duration regardless of the dielectric types. This can be explained by the conduction of the heat in the material and expansion of the discharge plasma. The value of drec was much larger in W/O emulsion and kerosene, followed by oxygen, deionized water and air. In W/O emulsion and kerosene, the value of drec was almost one time larger than that in air. This indicates that the characteristic of the plasma, for example, the diameter and temperature distribution in it, was remarkably affected by the dielectrics. The value of drec was much bigger in oxygen than in air. Perhaps this can be explained by the oxidation reaction between melted material and the surrounding oxygen due to the high temperature caused by the discharge plasma. The values of crater removal diameter drem and bulge diameter dbul are shown in Fig. 7a and b, respectively. In the liquid dielectrics,
Fig. 6. Comparison of the diameter of recast crater in different dielectrics at various pulse durations.
Fig. 7. Comparison of the (a) crater removal diameter and (b) bulge diameter of recast crater in different dielectrics at various pulse durations.
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Fig. 8. Comparison of the recast depth in different dielectrics as various pulse durations.
Fig. 9. Comparison of the crater removal depth in different dielectrics with different pulse durations.
confirmed by Kunieda et al. (2005), the plasma diameter was much smaller at the anode than the cathode. Therefore the energy density will be higher at the anode than the cathode. The recast depth was much larger in oxygen than in air. Perhaps this can be explained by the oxidation of the melted material. In W/O emulsion, the recast depth was comparable with that in deionized water when the pulse duration was smaller than 210 s; however, when the pulse duration was longer than 210 s, the
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recast depth showed a much smoother increasing trend. Perhaps this can be explained by the energy density applied on the surface of the workpiece. At the early stage of the discharge, the pressure in the discharge generated bubble is very high due to the inertia and high viscosity of W/O emulsion; the expansion of the discharge plasma will be compressed. With the expansion of the bubble the pressure in the bubble dropped, therefore, energy density will be decreased due to the sequent expansion of the discharge plasma. The values of crater removal depth hrem are shown in Fig. 9. Not like the monotonically increasing trend of recast depth shown in Fig. 8, the relationship between pulse duration and crater removal depth was much more complicated. In kerosene and de-ionized water, the crater removal depth first decreases and then increases with pulse duration, whereas in W/O emulsion, the crater removal depth first increases and then decreases with pulse duration. The decrement of crater removal depth with increasing pulse duration is an interesting phenomenon. This indicates that the removal of the melted material is a very complex process. The appearance of the craters formed in deionized water at different pulse durations is shown in Fig. 10. The crater removal depth is 24.5, 5.6 and 43.7 m, respectively when the pulse duration is 52, 105 and 840 s. It is difficult to understand why the crater removal depth is smallest when the pulse duration is 105 s. It seems that most of the material removal occurs at the end of the discharge when the pulse duration was shorter than about 105 s, and material removal also occurs during the discharge when the pulse duration was longer than about 105 s. With short pulse duration, the pressure over the discharge point is very high and can prevent the melted material from removal. As time elapse, the pressure drops due to the expansion of the discharge generated bubble, therefore, melted material can be removed during the discharge. The crater removal depth and the difference between its maximum and minimum value were much smaller in gaseous dielectrics than those in liquid dielectrics. This indicates that in gaseous dielectrics, the pressure change over the discharge point is not as dramatic as that in liquid dielectrics. In liquid dielectrics, the expansion of the discharge generated bubble was hampered due to the inertia and viscosity of the surrounding dielectrics; whereas in gaseous dielectrics, the constriction effect was insignificant due to the extremely low density and viscosity of the gaseous dielectrics. Therefore, the pressure over the discharge point may undergoes a much dramatically “fluctuation” process due to the constriction effect of liquid dielectrics.
3.2.3. Melted and removed volume The volume of melted materials, (vrem + vrec ), was an important information that can give some insight knowledge of the energy distribution and heat conduction. The volumes of melted materials in different dielectrics are shown in Fig. 11. As shown in Fig. 11, the types of dielectric have a significant influence on the volume of melted material. The volume of melted material was largest in W/O
Fig. 10. The craters formed in deionized water with different pulse durations.
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Fig. 11. Comparison of the melted material volume (vrem + vrec ) in different dielectrics.
emulsion dielectrics. This means that in W/O emulsion, a larger part of the discharge energy was used to melt the workpiece material. In oxygen, the volume of melted material was a little larger than that in air; just as stated above, this may be due to the oxidation reaction of the material. In kerosene, the volume of melted material was much smaller than in other dielectrics. The results of our experiments seem to be very astonishing and surprising since it is a common sense that higher material removal rate can be obtained in hydrocarbon oil base dielectric rather than gaseous and water based dielectrics in the actual sinking EDM process. This indicated that there is a great difference between the volume of melted material and the volume of removed material, as well as between the case of single discharge and the case of consecutive discharge. In the following section, the removal efficiency in different dielectrics was discussed. 3.3. Removal efficiency The material removal efficiency, , is defined as the ratio of the removed volume regarding to the melted volume: (%) =
vrem − vbul × 100. vrem + vrec
The removal efficiency defined in our research was consistent with the definition of previous researchers, therefore comparative investigations and discussions can be carried out. In different dielectrics, the values of are shown in Fig. 12. As can be seen from Fig. 12, in some case, the value of was minus; this indicates that most of the melted material was re-solidified in the crater instead of removal. Another possible reason that contributes to the minus value was the metallic structure variation of the melted material which will probably decrease the density of the material. Within all the involved experiments, the maximum removal efficiency was no more than 20%. The results of our experiments were consistent with the results reported by Yang et al. (2011), who investigated the material removal mechanism of micro EDM by molecular dynamic simulation and found that the removal efficiency was no more than 10%. Investigation carried out by Yoshida et al. (2011) also showed the low removal efficiency of EDM. For a large range of pulse duration (52–420 s), the removal efficiency in W/O emulsion was higher than 10%, whereas high value of removal efficiency in kerosene and de-ionized water only occurs at short pulse duration condition (52 s). In gaseous dielectrics, the removal efficiency was no more than 5% even with short pulse
Fig. 12. Comparion of the removal efficiency in different dielectrics.
duration. With longer pulse duration (840 s), the value of removal efficiency was minus regardless of the dielectric types. In W/O emulsion, the value of was larger than 10% when the pulse duration was smaller than 420 s. However, its value become to minus when the pulse duration increased to 840 s. Similar phenomenon can be observed in the case of other dielectrics. For instance, in de-ionized water, the value of was 15.9% when the pulse duration was 52 s, whereas the value of suddenly decreased to 1.1% when the pulse duration increased to 105 s. The experiment results showed that the material removal efficiencies were much larger in liquid dielectrics than those in gaseous dielectrics. Jeanvoine et al. (2008) investigated the microstructure of single discharge crater formed with different external pressures. They found that craters formed at a low external pressure (1–4 bar) present a flat undulated molten surface without central depression; whereas at high external pressure (7–10 bar), the craters consist of one or several central depressions surrounded by a rim caused by the flow of molten material outwards to the crater center. Investigation carried out by Jeanvoine et al. (2008) confirmed that the material removal process was significantly affected by the external pressure. Perhaps the downward trend of material removal efficiency with pulse duration in liquid dielectrics can be explained by the decline of the pressure in the discharge generated bubble. In order to quantitatively study the pressure variation of the discharge generated bubble, transient simulation of the bubbles was carried out in Section 3.4. 3.4. Transient simulation of the discharge generated bubble The research in this section aims to provide a transient simulation of the bubble generated in different dielectrics. However, in gaseous dielectrics, it is not rigorous to name the discharge generated substance as “bubble” since there was no interface between gas and liquid. In the gaseous dielectrics, maybe it should be named as shock wave. In the case of gaseous dielectrics, we only focus on the variation of the pressure over the discharge point and the impulsive force act on the workpiece due to the action of the shock waves. 3.4.1. Assumption The expansion of the bubble was simulated by nonlinear transient dynamic analysis software (MSC dytran). Due to the extremely complexity of the real EDM process, the following assumptions are made to make it mathematically feasible.
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Fig. 13. Comparison of the pressure evolution over the discharge point in different dielectrics.
2. The energy of the sphere was thought to be dependent on the discharge energy and independent on the dielectric types. In our case, the initial energy of the sphere was thought to be 4% of the total discharge energy. Technically speaking, the amplitude of the impulsive force and expansion speed of the bubble in our simulation depend on this initial energy. Review of published literature showed that there were very few reports on the initial energy of the discharge generated bubble in EDM. Although the initial energy in our simulation maybe inconsistent with the real EDM process, comparative investigation can be performed in different dielectrics. In our research, we investigated the evolution of the bubble with the same energy in different dielectrics. The total energy of the bubble can be calculated by the mass and specific internal energy of the gas in the sphere, given by the following equation: Es = (4/3)r3 e; where Es is the total energy of the sphere, r is the initial radius of the sphere, is the initial density of the sphere, e is specific internal energy. 3. The expansion of the sphere was assumed to be initialed simultaneously with the discharge. In actual case, the expansion of the bubble was thought to be several microsecond latter after the discharge.
1. Before the expansion process, the bubble was replaced by a small sphere which was filled by high pressure and high temperature gas, as show in the inset figure of Fig. 14. The gas in the sphere was assumed to be ideal gas and they satisfied the ideal gas equation of state.
3.4.2. Pressure above the discharge point in different dielectrics There is a significant difference between the expansion speeds of the bubbles in different dielectrics. In WO emulsion, due to the higher viscosity of the emulsion, the bubble in it undergoes a much slower expansion process than that in kerosene and water. In the case of W/O emulsion, the pressure in the bubble can sustain at
Fig. 14. Impulsive force act on the workpiece in different dielectrics.
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a fairly high level for a longer time than in the case of kerosene and de-ionized water. The variations of the pressure above the discharge point in different dielectrics are shown in Fig. 13. In gaseous dielectric, the pressure over the discharge point suffers a much faster downward process than in liquid dielectrics; furthermore, in gaseous dielectrics, the peak value of the pressure over the discharge point was much smaller compared with that in liquid dielectrics. The pressure curves of air and oxygen almost coincident with each other due to their similar physical properties. Note that, in water and kerosene, the pressure above the discharge point dropped to a value smaller than 0.1 MPa. This indicates that the expansion of the bubble cause a subpressure in it. The second peak value of pressure above the discharge point occurred in kerosene and water dielectric at about 290 s and 350 s, respectively, due to the contraction of the bubbles. This difference can be explained by the difference of the physical prosperity (density and viscosity) of the two dielectrics. In W/O emulsion, the case of subpressure (smaller than 0.1 MPa) did not take place within the simulated time. This indicated that the bubble in W/O emulsion only undergoes the expansion process within the simulated time. The contraction process of the bubble which will lead to pressure increment did not take place due to the high viscosity of W/O emulsion. As listed in Table 2, the viscosity of emulsion was several hundreds times higher than that of water and kerosene. The simulated results were consistent with the above speculation. In W/O emulsion, the pressure over the discharge point can sustain at a high value for a much longer time than in other dielectrics. For a wide range of pulse duration (52–420 s), the removal efficiency in W/O emulsion was higher than 10%, whereas high removal efficiency only taken place at short pulse duration in other two liquid dielectrics, as described in Section 3.3. It seems that the pressure above the discharge point is an important factor that affected the material removal in the craters. 3.4.3. Impulsive force acted on the workpiece in different dielectrics The impulsive force act on the workpiece generated due to the expansion and contraction of the discharge generated bubble in liquid dielectrics is shown in Fig. 14. As can be seen in Fig. 14, the peak values of impulsive force in liquid dielectrics were much larger than those in gaseous dielectrics. The impulsive force in gaseous dielectric was very small. This can be explained by the lower density and viscosity of the gas. The simulation results in our research were consistent with the results reported by Tamura and Kobayashi (2004) who experimentally measured the impulsive force in liquid and in gaseous dielectric, and concluded that the impulsive force in gaseous dielectric was too small to be detected. This confirmed the validity of simulation results in our study. In kerosene and de-ionized water, the second impact occurred at about 290 s and 350 s, respectively, due to the contraction of the bubble. The force shown in Fig. 14 is consistent with the pressure over the discharge point shown in Fig. 13. 4. Conclusions In this paper, the material removal characteristics in different dielectrics were investigated. The whole geometry of craters, including the recast material in the crater, was precisely determined by metallographic methods. It was found that there was a huge difference of the geometry shape of the craters formed in different dielectrics even with the same experiments conditions. In kerosene, less volume of material was melted compared to other dielectrics. The removal
efficiency shows a great difference in different dielectrics and is pulse duration dependent. Generally speaking, the removal efficiency was higher in liquid dielectrics than that in gaseous dielectrics; higher at short pulse duration than that at long pulse duration. The evolution of the discharge generated bubble was investigated by computer simulation. The simulated results showed that, in gaseous dielectrics, the pressure above the discharge point undergoes a much faster downward process than that in liquid dielectrics. In W/O emulsion, high pressure can sustain for a much longer time than that in kerosene and de-ionized water due to the extremely high viscosity of the W/O emulsion. Combining the experimental and simulation results, it seems that the higher material removal efficiency in liquid dielectrics, especially in W/O emulsion, was due to the higher pressure above the discharge point. To a great extent, the simulated results were consistent with the experiments results. In conclusion, the material removal characteristics of single discharge were found to be significantly affected by dielectric types. The material removal mechanisms in actual discharge was extremely complex and it very difficult to establish a universal theory. More investigation needs to be done to clarify the material removal mechanisms of EDM. Acknowledgements The work is partially supported by Chinese National Natural Science Foundation (Grant No. 51275529) and Incubation Programme of Excellent Doctoral Dissertation of China University of Petroleum (Grant No. LW120301A). References Abbas, N.M., Solomon, D.G., Bahari Md, F., 2007. A review on current research trends in electrical discharge machining (EDM). Int. J. Mach. Tools Manuf. 47, 1214–1228. Hayakawa, S., Sasaki, Y., Itoigawa, F., Nakamura, T., 2013. Relationship between occurrence of material removal and bubble expansion in electrical discharge machining. In: Proc. of the 17th CIRP Conf. on Electro Physical and Chemical Machining (ISEM), pp. 174–179. Hinduja, S., Kunieda, M., 2013. Modelling of ECM and EDM processes. CIRP Ann. Manuf. Technol. 62 (2), 775–797. Imai, Y., Hiroi, M., Nakano, M., 2001. Investigation of EDM machining states using ultrasonic waves. In: ISEM 13, pp. 109–116. Jeanvoine, N., Holzapfel, C., Soldera, F., Mücklich, F., 2008. Microstructure characterisation of electrical discharge craters using FIB/SEM dual beam techniques. Adv. Eng. Mater. 10, 973–977. Kitamura, T., Kunieda, M., Abe, K., 2013. High-speed imaging of EDM gap phenomena using transparent electrodes. In: Proc. of the 17th CIRP Conf. on Electro Physical and Chemical Machining (ISEM), pp. 315–320. Koenig, W., Joerres, L., 1987. Aqueous solutions of organic compounds as dielectric for EDM sinking. CIRP Ann. Manuf. Technol. 36, 105–109. Kunieda, M., Lauwers, B., Rajurkar, K.P., Schumacher, B.M., 2005. Advancing EDM through fundamental insight into the process. CIRP Ann. Manuf. Technol. 54 (2), 599–622. Kunieda, M., Yoshida, M., 1997. Electrical discharge machining in gas. CIRP Ann. Manuf. Technol. 46, 143–146. Leão, F.N., Pashby, I.R., 2004. A review on the use of environmentally friendly dielectric fluids in electrical discharge machining. J. Mater. Process. Technol. 149, 341–346. Liu, Y., Ji, R., Zhang, Y., Zhang, H., 2010. Investigation of emulsion for die sinking EDM. Int. J. Adv. Manuf. Technol. 47, 403–409. Sivapirakasam, S.P., Mathew, J., Surianarayanan, M., 2011. Constituent analysis of aerosol generated from die sinking electrical discharge machining process. Process Saf. Environ. Prot. 89, 141–150. Tamura, T., Kobayashi, Y., 2004. Measurement of impulsive forces and crater formation in impulse discharge. J. Mater. Process. Technol. 149, 212–216. Tao, J., Shih, A.J., Ni, J., 2008. Experimental study of the dry and near dry electrical discharge milling processes. J. Manuf. Sci. Eng. Trans. ASME 130, 1–8. Wang, J., Han, F., Cheng, G., Zhao, F., 2012. Debris and bubble movements during electrical discharge machining. Int. J. Mach. Tools Manuf. 58, 11–18. Yang, X., Guo, J., Chen, X., Kunieda, M., 2011. Molecular dynamics simulation of the material removal mechanism in micro-EDM. Precis. Eng. 35, 51–57.
Y. Zhang et al. / Journal of Materials Processing Technology 214 (2014) 1052–1061 Yoshida, M., Hanaoka, D., Flynn, B., McGeough, J.A., 2011. Observation of craters formed by single pulse discharge by stacking cross sectional shapes: comparison of craters in liquid and air. Proc. Inst. Mech. Eng. B: J. Eng. Manuf. 225, 1311–1318. Yoshida, M., Kunieda, M., 1998. Study on the distribution of scattered debris generated by a single pulse discharge in EDM process. Int. J. Electr. Mach. 3, 39–47. Zahiruddin, M., Kunieda, M., 2012. Comparison of energy and removal efficiencies between micro and macro EDM. CIRP Ann. Manuf. Technol. 61, 187–190.
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Zhang, Y., Liu, Y., Ji, R., Cai, B., 2011. Study of the recast layer of a surface machined by sinking electrical discharge machining using water-in-oil emulsion as dielectric. Appl. Surf. Sci. 257, 5989–5997. Zhang, Y., Liu, Y., Ji, R., Cai, B., Shen, Y., 2012. Sinking EDM in water-in-oil emulsion. Int. J. Adv. Manuf. Technol., http://dx.doi.org/10.1007/s00170-012-4210-4. Zhang, Y., Liu, Y., Ji, R., Zheng, C., Shen, Y., Wang, X., 2013. Transient dynamics simulation of the electrical discharge-generated bubble in sinking EDM. Int. J. Adv. Manuf. Technol. 68, 1707–1715.
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Investigation on the influence of the dielectrics on the material removal characteristics of EDM Yanzhen Zhang, Yonghong Liu ∗ , Yang Shen, Renjie Ji, Zhen Li, Chao Zheng College of Electromechanical Engineering, China University of Petroleum, Dongying 257061, China
a r t i c l e
i n f o
Article history: Received 24 January 2013 Received in revised form 9 December 2013 Accepted 19 December 2013 Available online 28 December 2013 Keywords: EDM Crater Dielectric Material removal Bubble
a b s t r a c t A systematical and comprehensive investigation of the material removal characteristics of the electrical discharge machining (EDM) process using various dielectrics as the working fluids was conducted in this work. Five dielectrics, including gaseous dielectrics, air and oxygen, and liquid dielectrics, de-ionized water, kerosene and water-in-oil (W/O) emulsion were used as the working fluids. The whole geometry parameters of the craters, including the recast material in the craters, were precisely determined by metallographic method. The volume of melted and removed material and removal efficiency in different dielectrics were comparatively investigated. By relating the material removal characteristics to the evolution of the discharge generated bubbles in different dielectrics which was done by computer simulation, it seems that the pressure above the discharge point is an important factor that can affect material removal characteristics. The results of this work were supposed to be helpful for further clarifying the complicated material removal mechanism of EDM. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Electrical discharge machining (EDM) is a widely used manufacturing technology in the industry application. Both electric conductive and semi-conductive materials can be machined by this non-contact method regardless of their hardness; therefore many hard-to-cut and brittle materials can be economically processed by EDM. With the development of EDM, many variations of this technique, such as sinking EDM, wire EDM, EDM milling and microhole EDM drilling have been developed since it was innovated in the late 1940s by the former Soviet Union scientists. Nowadays, various types of dielectrics have been used in the area of EDM. Hydrocarbon oil based and water based working fluids are the traditional working fluids of EDM. However, with the development of this technology, gaseous dielectric, which is more environmentally friendly and more economical, attracts more and more attentions. Although the EDM technology and its fundamental mechanism have been investigated by many researchers, some additional aspects of the process still need to be thoroughly investigated for enhancing its application potential and performance in future technologies. The influence of the dielectrics and its effect on the process performance is one such aspect which needs to be thoroughly investigated. The role of dielectrics on the EDM process was not clearly stated although various types of dielectrics have been used
∗ Corresponding author. Tel.: +86 053286983303. E-mail address: [email protected] (Y. Liu). 0924-0136/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.12.012
in various variations of EDM. The physical and chemical properties of the dielectric have significant influence on the EDM performance. Investigation carried out by Zhang et al. (2011) revealed that both productivity and quality are significant affected by dielectrics; surface with different characteristics can be obtained with different dielectrics. 1.1. Type of currently used dielectrics Abbas et al. (2007) reviewed the research trends in EDM, as well as the EDM technology using water based dielectrics and gaseous dielectrics. Leão and Pashby (2004) reviewed the environmentally friendly dielectrics used in EDM. Their work revealed that the current used dielectrics can be classified basically into the following three groups. 1. Hydrocarbon oil based dielectrics. This was the original dielectric of EDM and is still used in the case of sinking EDM today. However, compared with water or gaseous based dielectrics, the oil based dielectrics were not environmentally friendly. Investigations carried out by Sivapirakasam et al. (2011) showed that aerosol and some hazardous gases will be generated when oil based dielectrics were used. 2. Water based dielectrics. They were widely used in the case of wire EDM and micro-hole EDM drilling. Compared with the oil based dielectrics, they were much more environmentally friendly. In the case of sinking EDM, the feasibility of water based dielectric had been explored by Koenig and Joerres (1987); however,
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their experiment results indicated that the performance of water based dielectrics was unfavorable and unsatisfied compared with the hydrocarbon oil based dielectrics. Recent investigation carried out by Liu et al. (2010) also showed the poor performance of water based dielectric in the case of sinking EDM. 3. Gaseous based dielectrics. Nowadays, the use of gaseous dielectrics attracts more and more attentions. Compared with the liquid dielectrics, the gaseous dielectrics were much more environmentally friendly and more economical. Kunieda and Yoshida (1997) investigated the EDM process in gas and they found that with some special conditions the machining performance can be comparable with that in liquid dielectrics. However, investigations carried out by Tao et al. (2008) showed that there are still some technical problems need to be completely resolved before the industry application of the gaseous dielectrics. Up to today, commercial EDM machine tools using gaseous dielectric are still unavailable. 1.2. Role of the dielectrics The dielectrics play a very important role in the EDM process. For a quite long period, it was thought that the liquids dielectric was indispensable during the EDM process; since it can compress the discharge generated bubble, cool the melted debris and bring the debris out of the gap. It was conventionally depicted that most of the melted material was removed by the boiling of the superheated molten material at the end of the discharge because boiling of that superheated metal is prevented by the bubble pressure during the discharge duration since the expansion of the bubble was compressed by the inertia and viscosity of the surrounding liquid dielectrics. For a quite long period, this depiction about the role of dielectrics was generally accepted by researchers. However, Kunieda et al. (2005) and Hinduja and Kunieda (2013) showed that the validity of the established theories of the role of dielectrics was challenged by some new discoveries and inventions. Hayakawa et al. (2013) investigated the material removal of single discharge with the help of high speed camera and they found that that material removal also occurs during the discharge. Yoshida and Kunieda (1998) studied the formation and distribution of the debris on the working surface formed by single discharge in liquid and gaseous dielectric respectively. They found that there are differences in the total volume of the generated debris and the distribution of the debris size between single pulse discharges in liquid and in air when the discharge duration is short; and no difference exists for discharge durations longer than 90 s. The debris volume difference is very small when the discharge duration is long. They explained this phenomenon by the bubble generated by the discharge itself. In liquids dielectric, the arc column environment becomes equivalent to that of the discharge which occurs in gas with the increase in the diameter of the bubble. Their experiment results indicate that metal removal can occur without a liquid dielectric. Fundamental research into the EDM process carried out by Imai et al. (2001), Wang et al. (2012) and Kitamura et al. (2013) showed that most of the machining area was occupied by discharge generated gas even though the gap was submerged in liquid dielectric. Experiments carried out by Yoshida and Kunieda (1998) showed that most of the melted debris reattached on the electrode surface in air because they cannot be cooled by the air; whereas in liquid dielectric, the debris can solidify into a spherical shape. Their works indicated that the dielectric liquid is important for the cooling and flushing of debris particles but not for material removal. Nowadays, numerous investigations of dry EDM have confirmed that the EDM process can be performed in gaseous dielectrics. Without the compression on discharge generated bubble by the liquid dielectrics, the material removal can also take place in the case of gaseous dielectrics. A recent investigation carried out by
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Yoshida et al. (2011) showed that over 70% of the melted material can be removed from the crater with the help of high flow rate of the airflow, whereas in liquid dielectrics, most of the material re-solidified in the crater. Yang et al. (2011) investigated the material removal mechanism in the case of micro EDM by the method of molecular dynamics simulation. In their work, the gap was assumed to be vacuum which means that the influence of dielectrics (neither liquid dielectrics nor gaseous dielectrics) was totally excluded during the material removal process. They found that the material removal mechanism in micro EDM can be explained in two ways; one by vaporization and the other by the bubble explosion of superheated metals. Note that the “bubble” is the bubble in the molten pool not the bubble between electrodes in the dielectric. From their work, it seems that the EDM process can be performed without any type of dielectrics (in vacuum). Tamura and Kobayashi (2004) measured the impulsive forces of single discharge due to the expansion and contraction of the discharge generated bubble in kerosene and gas dielectrics and examined the mechanism by which the impulsive forces affect the crater formation. Based on the experiment results, they concluded that the influence of impulsive forces on the crater formation was not significant. Zhang et al. (2013) simulated the expansion process of the discharge generated bubble and they found that the geometry shape of the crater and the material removal efficiency were significantly affected by the inter electrode distance which can determine the magnitude of the impulsive forces. Apparently, the results reported by Zhang et al. (2013) were contradicted with those reported by Tamura and Kobayashi (2004). In the actual case, besides the inter electrodes distance, the expansion of the discharge generated bubble was also affected by the inertia and viscosity of the dielectrics. Review of the published literature did not give us a clear answer to the following question: how the pressure or pressure changes in the bubble affect the material removal in the crater? The role of dielectrics which can affect the pressure in the discharge charge generated bubble was still elusive and still needs to be thoroughly investigated. 1.3. Investigation strategy of this work In our work, the material removal characteristics in different dielectrics were investigated by the method of single discharge. For a better understanding of the crater formation, it is very necessary to investigate the microstructure of the individual crater such as melting, material removal and re-solidification. Experimental determination of the geometry of the whole recast region, volume of molten material and removed material should provide useful and insightful information about the fundamental mechanism of EDM. However, most of the previously investigations only focused on the external characterization of the craters; sectional characterization of the craters was not investigated due to the difficulty in the metallographic preparation of the samples. Yoshida et al. (2011) investigate the volumes of re-solidified metal and molten metal in the crater by stacking the cross-sectional shapes of the crater; their work revealed that the metallographic preparation of the samples was feasible and could give a precise determination of the volume of the total melted material, as well as the removed material in the crater. In our work, a systemically experimental analysis of craters formed in different dielectrics was performed. The method proposed by Yoshida et al. (2011) was adopted in our research. Both the external and sectional appearance of the craters were investigated. The material removal mechanism was discussed by correlating the 3-dimentional geometry of the craters to the transient simulation results of the discharge generated bubbles in various dielectrics. Moreover, the influence of pulse duration was also investigated
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Y. Zhang et al. / Journal of Materials Processing Technology 214 (2014) 1052–1061 Table 1 Parametrical order of the single discharge experiments. Parameters, units
Description
Pulse duration, s Dielectric
52, 105, 210, 420, 840 Oxygen, air, kerosene, de-ionized water, water-in-oil (W/O) emulsion
Fig. 2. Sectional view of the crater.
Fig. 1. (a) Illustration of the single pulse discharge experimental set up and (b) equipment used for measuring the distance between each sectional layer.
since the geometry of the discharge crater would exhibits significant difference with different pulse durations even in the same dielectric. In this work, the machining polarity in deionized water and gaseous dielectrics was positive and in kerosene and W/O emulsion negative; consistent with machining polarities usually adopted in the industry application. 2. Experimental work 2.1. Experimental set-up The single pulse discharge equipment, as illustrated in Fig. 1a, was used in our experiments. The single pulse waveform was generated by an IGBT which was controlled by a singlechip. Various pulse durations can be easily generated by changing the program of the singlechip. The discharge waveform (including both the voltage and current waveforms) was recorded by an oscilloscope for further analysis. The workpiece material was mold steel 8407. Steel needle was used as the electrode. 2.2. Experimental procedure Experiments were carried out with parametrical order using five different dielectrics and five different pulse durations settings (Table 1). A big range of pulse duration was adopted to
investigate the influence of pulse duration, since Zahiruddin and Kunieda (2012) reported that the length of the pulse duration greatly affects the thermal transmission characteristics in the workpiece material. Oxygen was used as the dielectric with the aim of investigating the influence of oxidation during the material removal process by comparing the characteristic of the crater formed in air. Gaseous dielectrics were adopted with the aim of clarifying the material removal mechanism of dry EDM which attracts more and more attentions today. Zhang et al. (2012) studied the EDM performance with W/O emulsion dielectric. They found that, comparing with the traditional kerosene based dielectric, the machining efficiency can be improved and the environmental impact can be alleviated by using this new dielectric. Therefore, W/O emulsion, de-ionized water and kerosene were included in the current work to carry out a comparative investigation. The physical properties of the five dielectrics are listed in Table 2. Compared with the other two liquid dielectrics, the viscosity of the W/O emulsion is much higher. After the single pulse discharge, the workpiece was cut and embedded in epoxy (the sample shown in Fig. 1b) with the aim of getting the sectional geometry parameters of the crater. The section view of the crater was obtained by a metallurgical microscope after polished and etched with nital. After capturing one sectional image, the sample was re-grinded and re-polished to get another sectional image. For every crater, about 15–20 sectional images at different locations were obtained by the metallurgical microscope. Clear profile of the recast area can be observed after etched (Fig. 2). The distance between each layer was about 10–30 m depending on the diameters of the craters. With the help of micrometer gauge, the distance between each section can be accurately measured (as shown in Fig. 1b). With the sectional images and the distance value between each layer, the crater can be rebuilt by 3D-CAD software (Solidworks) as shown later in Fig. 3.
Table 2 Physical properties of the five dielectrics. Dielectrics
(W/m K)
ı (◦ C)
C (J/kg ◦ C)
Oxygen
0.027
−218.79
0.913
Air
0.016
–
1.005
De-ionized water Kerosene
0.62 0.14
0 –
4200 2100
W/O emulsion
0.22
–
2747
(s/m) Too small to be measured Too small to be measured 6.5 × 10−3 Too small to be measured Too small to be measured
Thermal conductivity (), melting point (ı), specific heat (C), conductivity (), viscosity (), density ().
(Pa s)
(kg/m3 )
Composition (vol%)
1.43
99.9% O2
1.29
78% N2 , 21% O2 , 1% other gases 100% H2 O 100% hydrocarbon
1 × 10−6 2.3 × 10−6
1000 860
8.7 × 10−4
890
30.8% H2 O, 68.7% hydrocarbon, 0.5% surfactant
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Fig. 3. (a and d) Display of whole re-solidified material; (b and e) display of the re-solidified material below the workpiece surface; (c and f) display of the re-solidified material above the workpiece surface.
2.3. Rebuilt of the crater with 3D-CAD software The craters were rebuilt with the help of 3D-CAD software (Solidworks). The two typical crater shapes in our experiments are shown in Fig. 3a and d respectively. The re-solidified material below the surface of the workpiece is shown in Fig. 3b and e, whereas the re-solidified material above the surface of the workpiece is shown in Fig. 3c and f, respectively. The craters shown in Fig. 3a and d were formed in W/O emulsion and de-ionized water, respectively, with pulse duration 210 s. As can be seen from Fig. 3, the removal efficiency of the crater formed in W/O emulsion is much higher than that formed in de-ionized water. In de-ionized water, most of the melted material was re-solidified in the crater. The detail forming mechanisms of the two typical appearances will be discussed in the following sections. 3. Results and discussion 3.1. Definition of the crater shape As shown in Fig. 3, there are mainly two typical geometry shapes of the craters presented in our work. The first type of crater is characterized by remarkable central depression surrounded by rim bulge caused by the flow of molten material outwards from the crater center (Fig. 3a); whereas the second type of craters is much more flat, remarkable center depression and surrounded bulge cannot be observed (Fig. 3d). The first type of the crater shape was characterized by the parameters: removal diameter drem , bulge diameter dbul , recast diameter drec , removal depth hrem , bulge height hbul , recast depth hrec , removal volume vrem , bulge volume vbul (above the surface of workpiece) and recast volume vrev (below the surface of workpiece), as shown in Fig. 4a. In the case of the
Fig. 4. Definition of discharge crater shape.
second type of craters, only the value of drec , hrec , vrem , vbul and vrec was determined since the remarkable depression and bulge cannot be observed, as shown in Fig. 4b. Apparently, the material removal efficiency of the first type of crater was much larger than that of the second type. In the following sections, emphasis was put on the value of drec and hrec because the diameter and depth of the recast material were important information which can be used to speculate and to
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Fig. 5. Comparison of the external appearances of craters formed in (a) air; (b) kerosene; (c) deionized water; (d) W/O emulsion; and (e) oxygen. Pulse duration = 105 s. The length of the scale bar was 100 m.
evaluate the heat convection characteristics of the single discharge, as described by Zahiruddin and Kunieda (2012). Besides drec and hrec , the value of hrem was also discussed since the material removal efficiency was greatly affected by removal depth.
the value of drem and dbul increases with increasing pulse duration. With longer pulse duration, the center depression and rim bulge cannot be observed in air and oxygen dielectrics. Therefore, in Fig. 7, the value of drem and dbul was not listed.
3.2. Diameter, depth and volume
3.2.2. Depth The values of recast depth hrec are shown in Fig. 8. The recast depth monotonically increases with pulse duration. As can be seen from Fig. 8, there was a great difference of recast depth in different dielectrics. The value of recast depth was biggest in de-ionized water, followed by W/O emulsion, oxygen, air and kerosene. Perhaps this can be explained by the machining polarity. As has been
The craters formed in different dielectrics are compared in Fig. 5. As shown in Fig. 5, there exists a huge difference of the external appearance of the craters. In the following section, quantitative discussions of the craters formed in different dielectrics were performed based on 3D geometry model of the craters rebuilt by Solidworks. 3.2.1. Diameter Fig. 6 shows the recast diameter of crater, drec , in different dielectrics. The value of drec increases with increasing pulse duration regardless of the dielectric types. This can be explained by the conduction of the heat in the material and expansion of the discharge plasma. The value of drec was much larger in W/O emulsion and kerosene, followed by oxygen, deionized water and air. In W/O emulsion and kerosene, the value of drec was almost one time larger than that in air. This indicates that the characteristic of the plasma, for example, the diameter and temperature distribution in it, was remarkably affected by the dielectrics. The value of drec was much bigger in oxygen than in air. Perhaps this can be explained by the oxidation reaction between melted material and the surrounding oxygen due to the high temperature caused by the discharge plasma. The values of crater removal diameter drem and bulge diameter dbul are shown in Fig. 7a and b, respectively. In the liquid dielectrics,
Fig. 6. Comparison of the diameter of recast crater in different dielectrics at various pulse durations.
Fig. 7. Comparison of the (a) crater removal diameter and (b) bulge diameter of recast crater in different dielectrics at various pulse durations.
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Fig. 8. Comparison of the recast depth in different dielectrics as various pulse durations.
Fig. 9. Comparison of the crater removal depth in different dielectrics with different pulse durations.
confirmed by Kunieda et al. (2005), the plasma diameter was much smaller at the anode than the cathode. Therefore the energy density will be higher at the anode than the cathode. The recast depth was much larger in oxygen than in air. Perhaps this can be explained by the oxidation of the melted material. In W/O emulsion, the recast depth was comparable with that in deionized water when the pulse duration was smaller than 210 s; however, when the pulse duration was longer than 210 s, the
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recast depth showed a much smoother increasing trend. Perhaps this can be explained by the energy density applied on the surface of the workpiece. At the early stage of the discharge, the pressure in the discharge generated bubble is very high due to the inertia and high viscosity of W/O emulsion; the expansion of the discharge plasma will be compressed. With the expansion of the bubble the pressure in the bubble dropped, therefore, energy density will be decreased due to the sequent expansion of the discharge plasma. The values of crater removal depth hrem are shown in Fig. 9. Not like the monotonically increasing trend of recast depth shown in Fig. 8, the relationship between pulse duration and crater removal depth was much more complicated. In kerosene and de-ionized water, the crater removal depth first decreases and then increases with pulse duration, whereas in W/O emulsion, the crater removal depth first increases and then decreases with pulse duration. The decrement of crater removal depth with increasing pulse duration is an interesting phenomenon. This indicates that the removal of the melted material is a very complex process. The appearance of the craters formed in deionized water at different pulse durations is shown in Fig. 10. The crater removal depth is 24.5, 5.6 and 43.7 m, respectively when the pulse duration is 52, 105 and 840 s. It is difficult to understand why the crater removal depth is smallest when the pulse duration is 105 s. It seems that most of the material removal occurs at the end of the discharge when the pulse duration was shorter than about 105 s, and material removal also occurs during the discharge when the pulse duration was longer than about 105 s. With short pulse duration, the pressure over the discharge point is very high and can prevent the melted material from removal. As time elapse, the pressure drops due to the expansion of the discharge generated bubble, therefore, melted material can be removed during the discharge. The crater removal depth and the difference between its maximum and minimum value were much smaller in gaseous dielectrics than those in liquid dielectrics. This indicates that in gaseous dielectrics, the pressure change over the discharge point is not as dramatic as that in liquid dielectrics. In liquid dielectrics, the expansion of the discharge generated bubble was hampered due to the inertia and viscosity of the surrounding dielectrics; whereas in gaseous dielectrics, the constriction effect was insignificant due to the extremely low density and viscosity of the gaseous dielectrics. Therefore, the pressure over the discharge point may undergoes a much dramatically “fluctuation” process due to the constriction effect of liquid dielectrics.
3.2.3. Melted and removed volume The volume of melted materials, (vrem + vrec ), was an important information that can give some insight knowledge of the energy distribution and heat conduction. The volumes of melted materials in different dielectrics are shown in Fig. 11. As shown in Fig. 11, the types of dielectric have a significant influence on the volume of melted material. The volume of melted material was largest in W/O
Fig. 10. The craters formed in deionized water with different pulse durations.
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Fig. 11. Comparison of the melted material volume (vrem + vrec ) in different dielectrics.
emulsion dielectrics. This means that in W/O emulsion, a larger part of the discharge energy was used to melt the workpiece material. In oxygen, the volume of melted material was a little larger than that in air; just as stated above, this may be due to the oxidation reaction of the material. In kerosene, the volume of melted material was much smaller than in other dielectrics. The results of our experiments seem to be very astonishing and surprising since it is a common sense that higher material removal rate can be obtained in hydrocarbon oil base dielectric rather than gaseous and water based dielectrics in the actual sinking EDM process. This indicated that there is a great difference between the volume of melted material and the volume of removed material, as well as between the case of single discharge and the case of consecutive discharge. In the following section, the removal efficiency in different dielectrics was discussed. 3.3. Removal efficiency The material removal efficiency, , is defined as the ratio of the removed volume regarding to the melted volume: (%) =
vrem − vbul × 100. vrem + vrec
The removal efficiency defined in our research was consistent with the definition of previous researchers, therefore comparative investigations and discussions can be carried out. In different dielectrics, the values of are shown in Fig. 12. As can be seen from Fig. 12, in some case, the value of was minus; this indicates that most of the melted material was re-solidified in the crater instead of removal. Another possible reason that contributes to the minus value was the metallic structure variation of the melted material which will probably decrease the density of the material. Within all the involved experiments, the maximum removal efficiency was no more than 20%. The results of our experiments were consistent with the results reported by Yang et al. (2011), who investigated the material removal mechanism of micro EDM by molecular dynamic simulation and found that the removal efficiency was no more than 10%. Investigation carried out by Yoshida et al. (2011) also showed the low removal efficiency of EDM. For a large range of pulse duration (52–420 s), the removal efficiency in W/O emulsion was higher than 10%, whereas high value of removal efficiency in kerosene and de-ionized water only occurs at short pulse duration condition (52 s). In gaseous dielectrics, the removal efficiency was no more than 5% even with short pulse
Fig. 12. Comparion of the removal efficiency in different dielectrics.
duration. With longer pulse duration (840 s), the value of removal efficiency was minus regardless of the dielectric types. In W/O emulsion, the value of was larger than 10% when the pulse duration was smaller than 420 s. However, its value become to minus when the pulse duration increased to 840 s. Similar phenomenon can be observed in the case of other dielectrics. For instance, in de-ionized water, the value of was 15.9% when the pulse duration was 52 s, whereas the value of suddenly decreased to 1.1% when the pulse duration increased to 105 s. The experiment results showed that the material removal efficiencies were much larger in liquid dielectrics than those in gaseous dielectrics. Jeanvoine et al. (2008) investigated the microstructure of single discharge crater formed with different external pressures. They found that craters formed at a low external pressure (1–4 bar) present a flat undulated molten surface without central depression; whereas at high external pressure (7–10 bar), the craters consist of one or several central depressions surrounded by a rim caused by the flow of molten material outwards to the crater center. Investigation carried out by Jeanvoine et al. (2008) confirmed that the material removal process was significantly affected by the external pressure. Perhaps the downward trend of material removal efficiency with pulse duration in liquid dielectrics can be explained by the decline of the pressure in the discharge generated bubble. In order to quantitatively study the pressure variation of the discharge generated bubble, transient simulation of the bubbles was carried out in Section 3.4. 3.4. Transient simulation of the discharge generated bubble The research in this section aims to provide a transient simulation of the bubble generated in different dielectrics. However, in gaseous dielectrics, it is not rigorous to name the discharge generated substance as “bubble” since there was no interface between gas and liquid. In the gaseous dielectrics, maybe it should be named as shock wave. In the case of gaseous dielectrics, we only focus on the variation of the pressure over the discharge point and the impulsive force act on the workpiece due to the action of the shock waves. 3.4.1. Assumption The expansion of the bubble was simulated by nonlinear transient dynamic analysis software (MSC dytran). Due to the extremely complexity of the real EDM process, the following assumptions are made to make it mathematically feasible.
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Fig. 13. Comparison of the pressure evolution over the discharge point in different dielectrics.
2. The energy of the sphere was thought to be dependent on the discharge energy and independent on the dielectric types. In our case, the initial energy of the sphere was thought to be 4% of the total discharge energy. Technically speaking, the amplitude of the impulsive force and expansion speed of the bubble in our simulation depend on this initial energy. Review of published literature showed that there were very few reports on the initial energy of the discharge generated bubble in EDM. Although the initial energy in our simulation maybe inconsistent with the real EDM process, comparative investigation can be performed in different dielectrics. In our research, we investigated the evolution of the bubble with the same energy in different dielectrics. The total energy of the bubble can be calculated by the mass and specific internal energy of the gas in the sphere, given by the following equation: Es = (4/3)r3 e; where Es is the total energy of the sphere, r is the initial radius of the sphere, is the initial density of the sphere, e is specific internal energy. 3. The expansion of the sphere was assumed to be initialed simultaneously with the discharge. In actual case, the expansion of the bubble was thought to be several microsecond latter after the discharge.
1. Before the expansion process, the bubble was replaced by a small sphere which was filled by high pressure and high temperature gas, as show in the inset figure of Fig. 14. The gas in the sphere was assumed to be ideal gas and they satisfied the ideal gas equation of state.
3.4.2. Pressure above the discharge point in different dielectrics There is a significant difference between the expansion speeds of the bubbles in different dielectrics. In WO emulsion, due to the higher viscosity of the emulsion, the bubble in it undergoes a much slower expansion process than that in kerosene and water. In the case of W/O emulsion, the pressure in the bubble can sustain at
Fig. 14. Impulsive force act on the workpiece in different dielectrics.
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a fairly high level for a longer time than in the case of kerosene and de-ionized water. The variations of the pressure above the discharge point in different dielectrics are shown in Fig. 13. In gaseous dielectric, the pressure over the discharge point suffers a much faster downward process than in liquid dielectrics; furthermore, in gaseous dielectrics, the peak value of the pressure over the discharge point was much smaller compared with that in liquid dielectrics. The pressure curves of air and oxygen almost coincident with each other due to their similar physical properties. Note that, in water and kerosene, the pressure above the discharge point dropped to a value smaller than 0.1 MPa. This indicates that the expansion of the bubble cause a subpressure in it. The second peak value of pressure above the discharge point occurred in kerosene and water dielectric at about 290 s and 350 s, respectively, due to the contraction of the bubbles. This difference can be explained by the difference of the physical prosperity (density and viscosity) of the two dielectrics. In W/O emulsion, the case of subpressure (smaller than 0.1 MPa) did not take place within the simulated time. This indicated that the bubble in W/O emulsion only undergoes the expansion process within the simulated time. The contraction process of the bubble which will lead to pressure increment did not take place due to the high viscosity of W/O emulsion. As listed in Table 2, the viscosity of emulsion was several hundreds times higher than that of water and kerosene. The simulated results were consistent with the above speculation. In W/O emulsion, the pressure over the discharge point can sustain at a high value for a much longer time than in other dielectrics. For a wide range of pulse duration (52–420 s), the removal efficiency in W/O emulsion was higher than 10%, whereas high removal efficiency only taken place at short pulse duration in other two liquid dielectrics, as described in Section 3.3. It seems that the pressure above the discharge point is an important factor that affected the material removal in the craters. 3.4.3. Impulsive force acted on the workpiece in different dielectrics The impulsive force act on the workpiece generated due to the expansion and contraction of the discharge generated bubble in liquid dielectrics is shown in Fig. 14. As can be seen in Fig. 14, the peak values of impulsive force in liquid dielectrics were much larger than those in gaseous dielectrics. The impulsive force in gaseous dielectric was very small. This can be explained by the lower density and viscosity of the gas. The simulation results in our research were consistent with the results reported by Tamura and Kobayashi (2004) who experimentally measured the impulsive force in liquid and in gaseous dielectric, and concluded that the impulsive force in gaseous dielectric was too small to be detected. This confirmed the validity of simulation results in our study. In kerosene and de-ionized water, the second impact occurred at about 290 s and 350 s, respectively, due to the contraction of the bubble. The force shown in Fig. 14 is consistent with the pressure over the discharge point shown in Fig. 13. 4. Conclusions In this paper, the material removal characteristics in different dielectrics were investigated. The whole geometry of craters, including the recast material in the crater, was precisely determined by metallographic methods. It was found that there was a huge difference of the geometry shape of the craters formed in different dielectrics even with the same experiments conditions. In kerosene, less volume of material was melted compared to other dielectrics. The removal
efficiency shows a great difference in different dielectrics and is pulse duration dependent. Generally speaking, the removal efficiency was higher in liquid dielectrics than that in gaseous dielectrics; higher at short pulse duration than that at long pulse duration. The evolution of the discharge generated bubble was investigated by computer simulation. The simulated results showed that, in gaseous dielectrics, the pressure above the discharge point undergoes a much faster downward process than that in liquid dielectrics. In W/O emulsion, high pressure can sustain for a much longer time than that in kerosene and de-ionized water due to the extremely high viscosity of the W/O emulsion. Combining the experimental and simulation results, it seems that the higher material removal efficiency in liquid dielectrics, especially in W/O emulsion, was due to the higher pressure above the discharge point. To a great extent, the simulated results were consistent with the experiments results. In conclusion, the material removal characteristics of single discharge were found to be significantly affected by dielectric types. The material removal mechanisms in actual discharge was extremely complex and it very difficult to establish a universal theory. More investigation needs to be done to clarify the material removal mechanisms of EDM. Acknowledgements The work is partially supported by Chinese National Natural Science Foundation (Grant No. 51275529) and Incubation Programme of Excellent Doctoral Dissertation of China University of Petroleum (Grant No. LW120301A). References Abbas, N.M., Solomon, D.G., Bahari Md, F., 2007. A review on current research trends in electrical discharge machining (EDM). Int. J. Mach. Tools Manuf. 47, 1214–1228. Hayakawa, S., Sasaki, Y., Itoigawa, F., Nakamura, T., 2013. Relationship between occurrence of material removal and bubble expansion in electrical discharge machining. In: Proc. of the 17th CIRP Conf. on Electro Physical and Chemical Machining (ISEM), pp. 174–179. Hinduja, S., Kunieda, M., 2013. Modelling of ECM and EDM processes. CIRP Ann. Manuf. Technol. 62 (2), 775–797. Imai, Y., Hiroi, M., Nakano, M., 2001. Investigation of EDM machining states using ultrasonic waves. In: ISEM 13, pp. 109–116. Jeanvoine, N., Holzapfel, C., Soldera, F., Mücklich, F., 2008. Microstructure characterisation of electrical discharge craters using FIB/SEM dual beam techniques. Adv. Eng. Mater. 10, 973–977. Kitamura, T., Kunieda, M., Abe, K., 2013. High-speed imaging of EDM gap phenomena using transparent electrodes. In: Proc. of the 17th CIRP Conf. on Electro Physical and Chemical Machining (ISEM), pp. 315–320. Koenig, W., Joerres, L., 1987. Aqueous solutions of organic compounds as dielectric for EDM sinking. CIRP Ann. Manuf. Technol. 36, 105–109. Kunieda, M., Lauwers, B., Rajurkar, K.P., Schumacher, B.M., 2005. Advancing EDM through fundamental insight into the process. CIRP Ann. Manuf. Technol. 54 (2), 599–622. Kunieda, M., Yoshida, M., 1997. Electrical discharge machining in gas. CIRP Ann. Manuf. Technol. 46, 143–146. Leão, F.N., Pashby, I.R., 2004. A review on the use of environmentally friendly dielectric fluids in electrical discharge machining. J. Mater. Process. Technol. 149, 341–346. Liu, Y., Ji, R., Zhang, Y., Zhang, H., 2010. Investigation of emulsion for die sinking EDM. Int. J. Adv. Manuf. Technol. 47, 403–409. Sivapirakasam, S.P., Mathew, J., Surianarayanan, M., 2011. Constituent analysis of aerosol generated from die sinking electrical discharge machining process. Process Saf. Environ. Prot. 89, 141–150. Tamura, T., Kobayashi, Y., 2004. Measurement of impulsive forces and crater formation in impulse discharge. J. Mater. Process. Technol. 149, 212–216. Tao, J., Shih, A.J., Ni, J., 2008. Experimental study of the dry and near dry electrical discharge milling processes. J. Manuf. Sci. Eng. Trans. ASME 130, 1–8. Wang, J., Han, F., Cheng, G., Zhao, F., 2012. Debris and bubble movements during electrical discharge machining. Int. J. Mach. Tools Manuf. 58, 11–18. Yang, X., Guo, J., Chen, X., Kunieda, M., 2011. Molecular dynamics simulation of the material removal mechanism in micro-EDM. Precis. Eng. 35, 51–57.
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