Study of the recast layer of a surface machined by sinking electrical discharge machining using water-

in-oil emulsion as dielectric

班級 : 碩研機械一甲學號 :MA310214

姓名 : 楊 傑

2Outline

•Abstract• Introduction• Experimental procedures• Results and discussion• Conclusion

3Abstract

• EDM caused a recast layer to form at the machined surface of the workpiece. The characteristics of the recast layer have a great relationship with the type of dielectric. The research work in this paper aims to acquire a profound knowledge of the recast layers of a surface machined by sinking EDM using water-in-oil (W/O) emulsion as dielectric.SEM, XRD, EDS and micro hardness analysis were performed. The characteristics of the recast layer formed in W/O emulsion were investigated by comparing them with those of the recast layer formed in kerosene and de-ionized water dielectric.

4Introduction

• EDM is a thermoelectric process that erodes workpiece material by a series of discrete electrical sparks between the workpiece and electrode flushed by or immersed in a dielectric fluid. After EDM, a recast layer will be formed on the machined surface regardless of the type of dielectric. It is well known that the main mode of erosion is caused by the thermal action of an electrical discharge. The non-traditional manufacturing process of sinking-EDM possesses many advantages over traditional machining during the manufacture of the hard-tocut materials. However, certain detrimental effects are also present and are due in large part to the formation of the recast layer.

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• The recast layer formed by EDM process increases surface roughness, makes the surface become hard and brittle, and decreases the fatigue strength due to the presence of microcracks and micro-voids.

• Therefore, the dielectric fluid function is to recast layer to minimize the impact of the workpiece

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• We studied the sinking-EDM performance using water-in-oil (W/O) emulsion as dielectric. The material removal rate (MRR) obtained in W/O emulsion is much higher than that obtained in kerosene, especially with rough machining parameters. In the sinking-EDM application, the traditional dielectrics are hydrogen–carbon oils. Normally water is not used as a dielectric for sinking-EDM. Although the use of plain water in sinking-EDM results in a better performance in some situations, hydrocarbon oils are superior in a wider range of machining conditions.

• In our case, W/O emulsion was used as the dielectric, the simultaneous presence of water and oil in the discharge gap may contribute to a recast layer that is different from that formed only in water or oil-based dielectric. In this paper, the composition and crystallographic and metallurgical properties of the recast layer formed in W/O emulsion were studied by comparing them with those formed in kerosene and deionized water dielectrics.

7Experimental procedures

• The W/O emulsion used in this experiment was prepared using, for the oil phase, 66 vol% of machine oil, and for the water phase, 34 vol% of de-ionized water. In order to stabilize the emulsion, 2.5 wt% of Span 80 was added to the oil phase. Emulsification was carried out with a homogenizer at 3000 rpm for 20 min. The microstructure of W/O emulsion after homogenization is shown in Fig. 1.

Fig. 1. Optical micrograph of the W/O emulsion used in this experiment.

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• Fig. 2 is schematic representation of the EDM process, viscosity W / O emulsion are larger than the kerosene and water, the emulsion consisting of a central hole (Ø4.2mm) into the gap between micro diaphragm pump and discharge, for comparison, tests are carried out kerosene and deionized water as a dielectric fluid. Processing interval of 48 seconds, range recast layer is composed of roughing to semi-finishing.

Fig. 2. Schematic representation of the EDM process (1, pulsed generator; 2,servocontrol;3, electrode; 4, specimen; 5, dielectric fluid; 6, micro pump).

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• Table 1 shows three different dielectric fluid processing parameters, the workpiece material is plain carbon steel (75mmx35mmx4mm), processing depth of 1mm

• After processing, the sample sections, respectively, with OM SEM EDS XRD Vickers hardness machine to do the analysis.

10Results and discussion

• For Fig. 3, W / O emulsion surface roughness than kerosene solution of deionized water and bigger, because the W / O emulsion is relatively high viscosity, high viscosity dielectric liquid may limit the expansion of the scope of the discharge, the impact force will be concentrated in a small area, resulting in roughness will be higher.

Fig. 3. Comparing the surface roughness obtained in different dielectrics and peakcurrents. Pulse duration = 308s.

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• For Fig. 4, The craters formed in W/O emulsion are much larger and deeper than those formed in kerosene and de-ionized water. A comparison of the surface textures reveals that the diameter and the depth of the craters are significantly affected by the type of dielectrics. The craters formed in W/O emulsion exhibit the largest depth whereas the craters formed in de-ionized water exhibit the largest diameter and the smallest depth.

• Another possible reason for the big craters is the oxidation reaction of the melted material which can intensify the discharge and contribute to larger craters.

Fig. 4. SEM photographs of the machined surface of (a) sample W9; (b) sample K9. Peak current=9A. Pulse duration = 308s.

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• Fig.5 is a recast layer thickness and pulse time comparison chart, recast layer is formed of a dielectric fluid is not washed away the molten metal in the form of processed surface cooled and solidified, it can be observed from the graph type, the dielectric liquid have a great relationship with the recast layer thickness.

• Fig. 5 also observed that the longer pulse time, recast layer thickness is higher, because the more time passes, the greater the material removal rate.

Fig. 5. Comparing the RLT of samples obtained in different dielectrics and pulse duration. Peak current=9A.

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• Fig. 6 shows the RLT obtained with various peak currents. The results have revealed that the thickness of the recast layer increases with increasing peak current regardless of the dielectric type. It should be noted that, although RLT increases as the peak current increases, the increase is not very significant.

Fig. 6. Comparing the RLT of samples obtained in different dielectrics and peak current. Pulse duration = 308s.

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• Fig. 7a and b are W / O emulsion was added deionized water, if the zoom ratio, can be seen on the surface of many oxide, which may be due to discharge energy to the discharge gap decomposition teardrop-shaped, as shown in Fig. 1 the average diameter shown, droplets of about 10 -3um, with considerable discharge gap distance.

• The area occupied by the oxides is much smaller compared with samples machined in W/O Emulsion and de-ionized water dielectric. With high magnification, the typical surface texture of samples machined in kerosene is shown in Fig. 7c.

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Fig. 7. SEM photographs show the typical surface textures of samples machined in: (a) W/O emulsion and de-ionized water; (b) W/O emulsion and de-ionized water; (c) Kerosene.

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• When the dielectric fluid is kerosene or W / O emulsion, iron carbide and α-Fe in the recast layer is no significant difference, but when the dielectric fluid when deionized water as the peak of iron oxide with pulse duration peak increase, α-Fe is opposite. This means that the discharge energy will affect the transition α → γ.

Fig. 8. X-ray diffraction pattern of samples machined in various dielectric and pulse duration. Peak current = 9A.

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Fig. 9. EDS analysis of the samples machined in various dielectric. Pulse duration = 308s; Peak current = 9A.

Table 2The element contents in the white layer of sample K9 and W9. Peak current=9A.Pulse duration = 308s.

• The carbon content of the recast layer formed in kerosene is higher than that formed in W/O emulsion and deionized water.

• Since carbon is generated due to the decomposition of the hydrocarbon oil, the lower carbon content in the recast layer formed in W/O emulsion and de-ionized water can be attributed to the presence of water in the discharge gap with respect to kerosene case.

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Fig. 10. SEM photographs show micro-cracks on the surface of samples machined in various dielectrics. Pulse duration = 308s; Peak current = 9A.

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Fig. 11. Cross-sectional micrographs of samples machined in various dielectrics. Pulse duration = 308s; Peak current = 9A.

Fig. 12. Comparing of the crack density of the recast layer formed in different dielectrics and peak currents. Pulse duration = 308s.

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• Fig. 13 compares the micro hardness of the recast layers formedin different dielectrics.

• The recast layerformed in W/O emulsion has a more inhomogeneous structure thanthat formed in kerosene and de-ionized water.

Fig. 13. Comparing of the micro hardness of the recast layer formed

in different dielectrics and peak currents. Pulse duration = 308s.

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• Fig.14 is the martensitic structure.

• In order to study the influence of cooling rate, the micro hardness of the recast layer of the samples was also measured at different distances from the interface.

Fig. 14. Comparing of the micro hardness depth profile of sample W9 and K9. Pulse duration = 308s; Peak current = 9A.

22Conclusion

• The recast layer formed in W/O emulsion dielectric exhibits a greater surface roughness and thickness than that formed in kerosene and de-ionized water dielectrics.

• Both carbide and oxide are formed in the recast layer when machining is performed in W/O emulsion dielectric, whereas only carbide is formed in the case of kerosene and only oxide is formed in the case of de-ionized water. The discharge energy has a positive effect on the formation of oxide.

• Cracks that formed in W/O emulsion and kerosene dielectrics exist in the recast layer and have much larger depth and usually terminate at the recast and heat affect layers interface. The density of crack decreases as the peak current increases regardless of the dielectric type.

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• Because of the high supersaturation of gas in the molten pool, there are many micro-voids within the recast layer when machining is performed in W/O emulsion. The micro-voids are rarely observed within the recast layer when machining is formed in kerosene and de-ionized water dielectrics.

• Due to the ingress of carbon and high cooling rate, the recast layer formed inW/Oexhibits a greater micro hardness than that formed in kerosene and de-ionized water dielectrics.

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