experimental research on machining characteristics of … · ... and the material removal mechanism...

Journal of Mechanical Science and Technology 25 (6) (2011) 1535~1542

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-011-0327-x

Experimental research on machining characteristics of SiC ceramic with end

electric discharge milling† Renjie Ji, Yonghong Liu*, Yanzhen Zhang, Xin Dong, Zhili Chen and Baoping Cai

College of Electromechanical Engineering, China University of Petroleum, Shandong, 257061, China

(Manuscript Received September 20, 2010 ; Revised February 19, 2011 ; Accepted March 1, 2011)

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Abstract Silicon carbide (SiC) ceramic has been widely used in modern industry. However, the beneficial properties of SiC ceramic make ma-

chining difficult and costly by conventional machining methods. This paper proposes a new process of machining SiC ceramic using end electric discharge (ED) milling. The process is able to effectively machine a large surface area on SiC ceramic at low cost and no envi-ronmental pollution. The effects of emulsion concentration, emulsion flux, milling depth, copper electrode number, and copper electrode diameter on the process performance such as the material removal rate, electrode wear ratio, and surface roughness have been investi-gated. In addition, the microstructure of the machined surface is examined with a scanning electron microscope, and the material removal mechanism of SiC ceramic during end ED milling is obtained.

Keywords: Electrical discharge machining; End electric discharge milling; Machining characteristics; Silicon carbide ceramic; Technological parameter ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Silicon carbide (SiC) is regarded as one of the most promis-ing engineering ceramics, and it has excellent physical and mechanical properties such as low density, high intensity, high rigidity, low thermal expansion coefficient and corrosion re-sistance even at elevated temperatures [1-3]. Due to its supe-rior properties, SiC ceramic is increasingly employed in opti-cal mirrors, accelerometers, refractories, electronic compo-nents, and in the biomedical, aerospace, and defense industries [4-6]. However, the beneficial properties that make SiC ce-ramic appealing to use also create a major challenge in ma-chining.

Diamond grinding and diamond turning are the major ma-chining methods for SiC ceramic. Agarwal and Rao [7] ground SiC ceramic with a diamond wheel and investigated the grinding characteristics, surface integrity and material removal mechanisms. Yin et al. [8] studied the plastic defor-mation, fracture damage and material removal mechanisms during spherical grinding of polycrystalline silicon carbide with a diamond tool. Gopal and Rao [9] developed a new chip-thickness model for the performance assessment of sili-con carbide grinding by incorporating elastic properties of the diamond grinding wheel and the workpiece. Zhang et al. [10]

studied the precision machinability of reaction-bonded silicon carbide by single-point diamond turning and investigated the influence of the depth of the cut and tool feed rate on surface roughness and cutting force. The above-mentioned studies have many advantages; however, diamond grinding and dia-mond turning give rise to difficulties associated with the high cost of diamond tools, large consumption of diamonds and laborious processing due to the high hardness and brittleness of the silicon carbide ceramic.

Electrical discharge machining (EDM) is one of the most widely applied non-conventional processes for machining silicon carbide. In this electro-thermal process, material is mainly removed by a succession of electrical discharges oc-curring between an electrode and a workpiece which is im-mersed in a dielectric liquid medium. The most important advantage of EDM is that its effectiveness is independent of the mechanical properties of the machined materials. Hence, silicon carbide ceramic, which is a difficult-to-machine mate-rial, can be machined effectively by EDM. Clijsters et al. [11] examined the influences of discharge current, open gap volt-age, discharge duration and pulse interval on the machining performance, including the material removal rate, tool wear ratio and surface roughness, and developed a die-sinking EDM strategy for machining silicon infiltrated silicon carbide. Luis et al. [12] investigated die-sinking electrical discharge machining of siliconized silicon carbide with a 25-1 fractional factorial design and multiple linear regression statistical analy-sis. However, die-sinking EDM shows low efficiency when

† This paper was recommended for publication in revised form by Editor Dae-Eun Kim

*Corresponding author. Tel.: +86 546 8392303, Fax.: +86 546 8393620 E-mail address: [email protected]; [email protected]

© KSME & Springer 2011

1536 R. Ji et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1535~1542

machining a large surface area on SiC ceramic. Milling EDM has been developed and demonstrated to be beneficial for its simplicity of electrode geometry. Liu et al. [13] developed a process of electric discharge milling SiC ceramic with a steel-toothed wheel as the tool electrode. Lauwers et al. [14] inves-tigated the manufacturability of SiC ceramic by milling EDM. The results indicated that the performance of milling EDM in terms of material removal rate and surface quality was much better than conventional EDM, but it still could not meet the requirements for industrial applications.

A new technique of machining SiC ceramic using end elec-tric discharge (ED) milling is proposed in this paper. The process employs a turntable with several cylindrical copper rods as the tool electrode and uses a water-based emulsion as the machining fluid. It is able to effectively machine a large surface area on SiC ceramic with low cost and no environ-mental pollution. The material removal rate can reach 100.5 mm3/min. The effects of emulsion concentration, emulsion flux, milling depth, copper electrode number, and copper elec-trode diameter on the process performance such as the mate-rial removal rate (MRR), electrode wear ratio (EWR), and surface roughness (SR) have been investigated. In addition, the microstructure of the machined surface is examined with a scanning electron microscope (SEM).

2. Principle for end ED milling of SiC ceramic

Fig. 1 shows the schematic representation for end ED mill-ing of SiC ceramic. The tool and the workpiece are connected to the negative and positive poles of the pulse generator, re-spectively. The tool is a turntable with several small cylindri-cal rods rotating rapidly around its axis. The tool is mounted onto a rotary spindle, driven by an AC motor. The workpiece is SiC ceramic blank and is mounted onto a numerically con-trolled (NC) table. The machining fluid is a water-based emul-sion.

During machining, the tool rotates at a high speed and the SiC ceramic workpiece is fed towards the tool driven by a DC motor. The machining fluid is flushed to the discharge gap with the nozzles. As the workpiece approaches the tool and the distance between the workpiece and the electrode reaches the discharge gap, electrical discharges occur from the edge and the bottom of the cylindrical electrodes. A plasma channel grows during the pulse on-time. A vapor bubble forms around this channel. The surrounding water-based emulsion restricts plasma growth and makes the plasma energy densities rise to very high levels. Petrofes and Gadalla [15] showed that the plasma temperature could reach nearly 40000 K and the plasma pressure could rise to 300 MPa in EDM of conducting advanced ceramics. The high instantaneous temperature and plasma pressure cause melting, vapor, and thermal spalling in SiC ceramic. A large diameter turntable with several cylindri-cal copper rods is used as the tool, so the process is able to effectively machine a large surface area on SiC ceramic at low cost, and the material removal rate can reach 100.5 mm3/min.

The cylindrical electrodes are fixed on the turntable separately, the machining fluid is flushed into the gap, and the chips are flushed away easily, which makes the processing stable. The tool rotates to ensure that the cylindrical electrodes are worn homogeneously. In addition, a water-based emulsion is used as the machining fluid, so harmful gas is not generated during end ED milling, and the equipment is not corroded.

3. Experimental procedures

In the following experiments, the workpiece material was SiC ceramic, and the tool was a turntable with several uni-formly-distributed cylindrical copper electrodes in the circum-ference. The diameter of the turntable was 90 mm and the rotational speed of the spindle was 3000 rpm. The pulse on-time was 400 µs, the pulse off-time was 300 µs, the peak volt-age was 150 V, the peak current was 75 A, and the tool polar-ity was negative. The machining fluid was a water-based emulsion composed of emulsified oil and distilled water, which were mixed with a constant speed power-driver mixer.

The material removal rate (MRR) and electrode wear ratio (EWR) were obtained through measuring the dimensions of the workpiece and the electrode before and after machining with a dial indicator and a Vernier caliper. The surface rough-ness (SR) was measured by a surface roughness tester. The microstructure of the workpiece surface was examined with a scanning electron microscope (SEM). All the observed speci-mens had been cleaned ultrasonically and dried with a hot-air blower before the examination.

4. Results and discussion

4.1 Effect of emulsion concentration on the process per-formance

The effect of emulsion concentration on MRR, EWR and SR is illustrated in Fig. 2 for an emulsion flux of 200 mL/s, milling depth of 0.1 mm, electrode diameter of 10 mm, and 8 copper electrodes. As shown in Fig. 2(a), the MRR initially increases with an increase in emulsion concentration and then decreases with continued increase in emulsion concentration. There are many

Nozzle generator +

_

Pulse

NC table Workpiece

Turntable

Spindle

ElectrodeMachining fluid

Servo

controller

D. C. motor

Fig. 1. Schematic representation for end ED milling of SiC ceramic.

R. Ji et al. / Journal of Mechanical Science and Technology 25 (6) (2011) 1535~1542 1537

reasons causing this phenomenon. The dielectric strength, washing capability, density and viscosity of the machining fluid increase with an increase in emulsion concentration, pinch effect and energy density of the discharge channel are enhanced and ejection effect of the eroded material increases; therefore, MRR rises. However, with a very high viscosity of the machining fluid, the eroded materials are difficult to be flushed away and the stability of electrical discharges becomes unsatisfactory, so MRR falls.

Fig. 2(b) shows the influence of emulsion concentration on EWR. It can be seen from Fig. 2(b) that the EWR initially decreases with an increase in emulsion concentration and then increases with continued increase in emulsion concentration. This phenomenon can be explained as follows. There is hy-

drocarbon in the emulsion, and a deposition layer forms on the electrode surface due to the decomposition of the hydrocarbon during electrical discharge, which can prevent electrode wear. As the emulsion concentration increases, the hydrocarbon con-tent in the dielectric increases and the decomposition of the hydrocarbon and the deposition on the electrode surface are enhanced; therefore, the EWR decreases. However, as the emulsion concentration is higher than 12 mass%, the viscosity of the machining fluid increases largely. The eroded materials are difficult to be flushed away, and they are gathered in the machining zone. The electrical discharge energy supplied to the machining zone repeatedly strikes the un-expelled eroded mate-rials that become concentrated on the machined surface, causing unnecessary electrode wear; therefore, the EWR is high.

The effect of emulsion concentration on SR is shown in Fig. 2(c). It can be seen from this figure that the SR increases with an increase in emulsion concentration. This is because energy density of the discharge channel increases with the increase of emulsion concentration; the crater size generated by a single pulse becomes larger and deeper; therefore, the SR rises with an increase in emulsion concentration.

4.2 Effect of emulsion flux on the process performance

The effect of emulsion flux on MRR, EWR and SR is illus-trated in Figs. 3(a)-(c), respectively, for an emulsion concen-tration of 8 mass%, milling depth of 0.1 mm, electrode diame-ter of 10 mm, and 8 copper electrodes.

Fig. 3(a) shows the influence of emulsion flux on MRR. It can be seen from Fig. 3(a) that the MRR increases with an increase in emulsion flux. This phenomenon can be explained as follows. The high emulsion flux can enhance the cooling effect of the electrode and the ejecting effect of the eroded materials, the electrical discharges become strong and stable; therefore, the MRR increases.

As shown in Fig. 3(b), the EWR increases with an increase in emulsion flux. This is because the cooling effect of the elec-trode and the ejecting effect of the eroded materials are en-hanced with an increase in emulsion flux. The deposition ef-fect on the electrode surface decreases and the tool electrode wear increases; therefore, the EWR increases.

The effect of emulsion flux on SR is shown in Fig. 3(c). It can be seen from this figure that the SR decreases with the increase of emulsion flux. This phenomenon can be explained as follows. The high emulsion flux can enhance the cooling effect of the electrode and the ejecting effect of the eroded materials, the eroded materials are flushed away easily, and the discharge points on the workpiece become dispersive and uniform. The craters generated by electrical discharges are shallow and uniformly distributed on the workpiece surface, so the SR decreases with an increase in emulsion flux.

4.3 Effect of milling depth on the process performance

The effect of milling depth on MRR, EWR and SR is illus-

(a)

(b)

(c)

Fig. 2. Effect of emulsion concentration on the process performance (a) Effect of emulsion concentration on MRR; (b) Effect of emulsionconcentration on EWR; (c) Effect of emulsion concentration on SR.


trated in Figs. 4(a)-(c), respectively, for an emulsion concen-tration of 8 mass%, emulsion flux of 200 mL/s, electrode di-ameter of 10 mm, and 8 copper electrodes. As shown in Fig. 4(a), the MRR initially increases with an increase in milling depth and then decreases with continued increase in milling depth. There are many reasons causing this phenomenon. Shallower the milling depths with higher dis-charge current density result in very high thermal energy den-sity of the discharge channel. At this time, the material is re-moved by vaporization. As the vaporization heat consumption is high, the MRR is low. However, with a very deep milling depth, the discharge current density is low, the SiC ceramic is difficult to be removed by the low current density of electrical discharge, and the electrical discharge becomes unstable.

Therefore, as the milling depth is greater than 0.1 mm, the MRR decreases with an increase in milling depth.

Fig. 4(b) shows the effect of milling depth on EWR. The EWR initially decreases with an increase in milling depth and then increases with an increase in milling depth. The reason for this is that the shallower the milling depth with higher discharge current density, the thermal energy density and the discharge explosion forces are very high. At this time, the released carbon decomposed from the hydrocarbon in the emulsion is not easily attached to the electrode surface, and the deposition effect on the electrode surface is weak; there-fore, the EWR is high. However, with a very deep milling depth, the discharge current density is low, the electrical dis-charge becomes unstable and arcs are easily generated, caus-

(a)

(b)

(c)

Fig. 3. Effect of emulsion flux on the process performance (a) Effect of emulsion flux on MRR; (b) Effect of emulsion flux on EWR; (c) Effect of emulsion flux on SR.

(a)

(b)

(c)

Fig. 4. Effect of milling depth on the process performance (a) Effect of milling depth on MRR; (b) Effect of milling depth on EWR; (c) Effect of milling depth on SR.


ing unnecessary electrode wear; therefore, the EWR is high. Fig. 4(c) shows the influence of milling depth on SR. It can

be seen from Fig. 4(c) that the SR increases gradually with an increase in milling depth. The reason for this is that the dis-charge current density decreases with an increase in milling depth. The electrical discharge becomes unstable and arcs are easily generated; therefore, the SR increases with an increase in milling depth.

4.4 Effect of electrode number on the process performance

The effect of electrode number on MRR, EWR and SR is illustrated in Figs. 5(a)-(c), respectively, for an emulsion con-centration of 8 mass%, emulsion flux of 200 mL/s, milling

depth of 0.1 mm, and electrode diameter of 10 mm. Fig. 5(a) shows the influence of electrode number on MRR.

It can be seen from Fig. 5(a) that the MRR initially increases with an increase in electrode number and then decreases with continued increase in electrode number. There are many rea-sons causing this phenomenon. The electrode diameter and the pitch circle diameter in the turntable are constant, as the elec-trode number is low, the circular pitch between two electrodes is large. The electrical discharge times at a unite time is few, the electrical discharge times at a unite time increases with an increase in electrode number; therefore, the MRR increases with an increase in electrode number. However, when the electrode number is too great, the circular pitch between the two electrodes is very small. The effect of mechanical deioni-zation becomes weak, the electrical discharges become unsta-ble, and arcs are easily produced; therefore, when the elec-trode number is higher than 6, the MRR decreases as electrode number increases.

The effect of electrode number on EWR is shown in Fig. 5(b). It can be seen from this figure that the EWR decreases with an increase in electrode number. This phenomenon can be explained as follows. The electrical discharge times at a unite time increases with an increase in electrode number, the discharge energy delivered to the machining gap increases, the dielectric and workpiece are heated for more time, the released carbon decomposed from the hydrocarbon in the emulsion is easily attached to the electrode surface, the deposition effect is enhanced, and the tool electrode wear is compensated, there-fore decreasing the EWR.

As shown in Fig. 5(c), the SR initially decreases with an in-crease in electrode number and then increases with an increase in electrode number. The reason for this is that when the elec-trode number is little, the circular pitch between two electrodes is large; the cooling effect of the electrode, the effect of me-chanical deionization and the ejecting effect of the eroded ma-terials become favorable; electrical discharge explosion force becomes strong; the crater size generated by electric discharge becomes large and deep; therefore the SR is high. However, with a much greater electrode number, the circular pitch be-tween two electrodes is small. The effect of mechanical deioni-zation becomes weak, the electrical discharges become unsta-ble, and arcs are easily produced; therefore, the SR is high.

4.5 Effect of electrode diameter on the process performance

The effect of electrode diameter on MRR, EWR and SR is illustrated in Figs. 6(a)-(c), respectively, for an emulsion con-centration of 8 mass%, emulsion flux of 200 mL/s, milling depth of 0.1 mm, and 8 copper electrodes.

Fig. 6(a) shows the effect of electrode diameter on MRR. The MRR initially increases with an increase in electrode diameter and then decreases with an increase in electrode di-ameter. This is because a smaller electrode diameter with high current density results in very high thermal energy density. At this time, the material is removed by vaporization. As the va-

(a)

(b)

(c)

Fig. 5. Effect of electrode number on the process performance (a) Effect of electrode number on MRR; (b) Effect of electrode number on EWR; (c) Effect of electrode number on SR.


porization heat consumption is high, the MRR is low. How-ever, with a large electrode diameter, the current density and the thermal energy density are low, and electrical discharges become weak; therefore, the MRR is low. As shown in Fig. 6(b), the EWR initially decreases with the increase of electrode diameter and then increases with the continued increase of electrode diameter. The reason for this is that smaller electrode diameter with high discharge current density result in high thermal energy density and discharge explosion force. At this time, the released carbon decomposed from the hydrocarbon in the emulsion does not easily attach to the electrode surface, and the deposition effect on the elec-trode surface is weak; therefore, the EWR is high. However,

with a large electrode diameter the discharge current density is low, the electrical discharge becomes unstable, and arcs are easily generated, causing unnecessary electrode wear; there-fore, the EWR is high.

Fig. 6(c) shows the influence of electrode diameter on SR. It can be seen from Fig. 6(c) that the SR decreases with an in-crease in electrode diameter. This phenomenon can be ex-plained as follows. The current density and the thermal energy density decreases with an increase in electrode diameter, elec-trical discharges become weak, and the craters generated by electric discharges are little and shallow; therefore, the SR decreases with an increase in electrode diameter.

4.6 Microstructure character of SiC ceramic surface ma-

chined by end ED milling

Surface morphology plays an important part in understand-ing the characteristics of machined surfaces. Fig. 7 shows the SEM micrograph of the SiC ceramic surface machined by end ED milling. It can be seen from this micrograph that the sur-face is covered with craters, droplets in clusters and micropores. The possible mechanism for the formation of the craters on machined surface is that sparks are formed on the SiC ceramic at the electrical conductive phase. The discharged energy pro-duces very high temperatures at the point of the spark, causing a minute part of the workpiece to melt and vaporize. Most mol-ten material is flushed away from the gap by the dielectric, whereas a small amount of melt resolidifies to form droplets on the surface. The formation of micropores is ascribed to the ejection of gases that escape from the solidified material.

Higher magnification SEM view of microstructure ma-chined by end ED milling is illustrated in Fig. 8. It can be seen from this figure that there are some microcracks on the ma-chined surface. The microcracks’ formation is associated with the development of high thermal stresses exceeding the frac-ture strength of the SiC ceramic material. The microcracks on the surface may be responsible for the separation of small volumes of material in the form of flake detachment from the base material. Therefore, the material removal mechanism may be called thermal spalling, which is another material re-moval mechanism for the SiC ceramic during machining. The

(a)

(b)

(c)

Fig. 6. Effect of electrode diameter on the process performance (a) Effect of electrode diameter on MRR; (b) Effect of electrode diameteron EWR; (c) Effect of electrode diameter on SR.

Fig. 7. SEM micrograph of SiC ceramic surface machined by end ED milling.


material removal mechanisms of SiC ceramic in end ED mill-ing include removal of most of the materials by the effects of melting and evaporation, while a few materials are removed by thermal spalling.

The current paper mainly discusses the effects of emulsion concentration, emulsion flux, milling depth, copper electrode number, and copper electrode diameter on the process per-formance such as the material removal rate, electrode wear ratio, and surface roughness with rough machining parameters. The finish machining experiments have also been carried out. The results show that the surface quality of machined SiC ceramic is improved greatly, there are no evident micro-cracks in the machined surface, and the surface roughness (SR Ra) can reach 0.56µm with the appropriate machining parameters.

5. Conclusions

(1) A novel high speed end electric discharge (ED) milling technique using a turntable with several small cylindrical rods as the tool electrode is proposed in this paper. The process employs a water-based emulsion as the machining fluid, and it shows high MRR and good working environmental practice.

(2) With a suitable emulsion concentration, higher MRR and lower EWR can be obtained. However, the SR increases with an increase in emulsion concentration. The MRR and the EWR increase with an increase in emulsion flux, whereas the SR decreases with an increase in emulsion flux. During end ED milling of SiC ceramic, higher MRR results in lower EWR, and lower SR can be acquired with the appropriate milling depth, electrode diameter and electrode number.

(3) The machined surface is characterized by craters, drop-lets in clusters, micropores and microcracks. The mechanisms by which material is removed are melting, evaporation and thermal spalling in end ED milling of the SiC ceramic.

Acknowledgments

The work was supported by the National Natural Science Foundation of China (Grant No. 50675225) and the Ministry of Science and Technology of the People's Republic of China (Grant No. 2009GJC60047).

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Fig. 8. SEM micrograph showing microcracks on the machined surfaceby end ED milling.


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Renjie Ji received his B. S. and M. S. degree in Electromechanics Engineering from China University of Petroleum in 2005 and 2007, respectively. Currently, he is a Ph.D. candidate in College of Electromechanical Engineering in China University of Petroleum, China. His recent research interest is electrical dis-

charge machining of engineering ceramics.

Yonghong Liu received his Ph.D. de-gree in Mechanical Manufacture from Harbin Institute of Technology, Harbin, China, in 1996. Currently, he is a pro-fessor and doctoral supervisor in College of Electromechanical Engineering, China University of Petroleum, China. He has published over 120 papers in

some international or national journals and conferences. His research fields include electrical discharge machining of engi-neering ceramics, expansion sand screen for sand control and control system of subsea drilling equipments.

experimental research on machining characteristics of … · ... and the material removal mechanism...

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