J. Cent. South Univ. (2016) 23: 2550-2557

DOI: 10.1007/s11771-016-3316-5

Improving energy utilization efficiency of electrical discharge milling in titanium alloys machining

GUO Cheng-bo(郭成波)¹, WEI Dong-bo(韦东波)^{1, 2}, DI Shi-chun(狄士春)¹

1. School of Mechatronics Engineering, Harbin Institute of Technology, Harbin 150001, China;

2. State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai 200240, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: Electrical discharge milling (ED-milling) can be a good choice for titanium alloys machining and it was proven that its machining efficiency can be improved to compete with mechanical cutting. In order to improve energy utilization efficiency of ED-milling process, unstable arc discharge and stable arc discharge combined with normal discharge were implemented for material removal by adjusting servo control strategy. The influence of electrode rotating speed and dielectric flushing pressure on machining performance was investigated by experiments. It was found that the rotating of electrode could move the position of discharge plasma channel, and high pressure flushing could wash melted debris out the discharge gap effectively. Both electrode rotating motion and high pressure flushing are contributed to the improvement of machining efficiency.

Key words: electrical discharge milling; electrode rotating; dielectric flushing; energy utilization efficiency; material removal rate; tool electrode wearing rate

1 Introduction

Titanium alloys have been widely used in aerospace industry, and their application in shipbuilding, nuclear power equipment and chemical industry is also rising, due to their outstanding mechanical and chemical properties, such as exceptional corrosion resistance, high temperature strength, and high specific strength to weight [1]. In mechanical machining process, titanium element will adhere on the cutter surface, forming a build-up edge owing to its high chemical activity and poor thermal conductivity, which would decrease tool life and machining precision. Some thermal assisted machining processes such as laser assisted milling [2-3] have been proposed to improve the machinability of titanium alloys, but the problem of high tool cost and lower machining efficiency in titanium alloy machining is far from being completely solved.

Electrical discharge machining (EDM) is a popular non-traditional machining process, which is mainly used in machining materials difficult to machine by conventional method [4-6]. Electrical discharge milling (ED-milling) is an emerging electrical discharge process, which employs a simple cylindrical pipe as electrode, and its moving path is controlled by the computerized numerical control (CNC) system. Material is removed from workpiece by heat generated in discharge process, and the inter-electrode force is negligible. The tool cost saving will be significant if ED-milling can be applied to titanium alloys machining.

Studies on ED-milling have been focusing on material removal rate (MRR), tool electrode wearing and machined surface quality in recent years [7-8]. Lots of studies have been done on improving the MRR of ED-milling process. KUNIEDA et al [9] improved machining efficient by introducing high pressure oxygen as dielectric medium into ED-milling process. It has also been observed that the machining efficient can be improved by using kerosene and air mixture as dielectric [10]. HAN et al [11] proposed a novel ED-milling process charged by a DC power supply, which simplified power supply equipment. GUU and HOCHENG [12] improved materials machinability by rotating workpiece, and this method was limited to machine cylindrical workpiece. GU et al [13] improved the flushing effect by using bundled electrode. This method needs additional time to prepare electrodes. It has also been found that multi-thin electrodes can be used to improve the productivity [14]. The geometric wearing property of electrode can be reduced by using square cross-section protrusions [15].

SUBRAMANIAN et al [16] showed that current was the key element in crack and re-solidified layer formation process. It has been found that both better surface quality and high MRR could be obtained by using modified ISO current pulse generator [17]. MUTHURAMALINGAM et al [18] proposed a method that could improve the surface quality by monitoring discharge status. Electrode compensation precision can be enhanced by detecting discharge status in real time [19]. ZHOU et al [20] developed a time-varied predictive model which could be used to predict the discharge changing trend [21].

The motivation for this work follows from the observation that only normal discharge was employed for material removal in the present ED-milling process, which limited the improvement of machining efficient. For this purpose, servo control strategy of ED-milling process was adjusted based on discharge average voltage. The proposed strategy can employ unstable arc discharge and stable arc discharge combined with normal discharge for material removal. The function of rotating electrode and dielectric flushing in material removal process was also investigated by studying its effect on MRR, tool electrode wearing rate (TWR), energy utilization efficiency of workpiece (W-EUE), energy utilization efficiency of tool electrode (T-EUE) and surface morphology.

2 Experimental

2.1 Experiment setup

As illustrated in Fig. 1, ED-milling equipment contained a pulse power supply, computer numerical control (CNC) system, machine body and dielectric medium flushing system. Tool electrode and workpiece were connected to the anode and cathode of power supply respectively. The tool electrode moving path was controlled by the CNC system, and the feed speed was adjusted on the basis of detected discharge voltage. During the ED-milling process, melted debris was cooled and flushed off the discharge gap by internal dielectric flushing.

2.2 Adjustment of servo control strategy

In electrical discharge process, discharge is classified into 5 states upon ignition delay time, which are open circuit, normal discharge, unstable arc discharge, stable arc discharge and short circuit as shown in Fig. 2. In normal ED-milling process, only normal discharge which has a reasonable ignition delay time is taken for material removal, and servo control strategy will prevent discharge from unstable and stable arc states [22]. In normal discharge process, the gas bubble and plasma channel will be formed in the ignition delay time, and most of the melted materials will be ejected from the workpiece by the sudden pressure change in discharge gap that caused by the collapse of the gas bubble. Unstable and stable arc discharges are caused by the accumulation of un-ejected debris particles in the gap. The removal efficient of arc discharge process depends on the dielectric flushing effect.

In order to improve energy utilization efficiency of ED-milling, a servo control strategy was proposed, which employed normal discharge, unstable arc discharge and stable arc discharge for material removal. It was realized by utilizing a lower short circuit reference voltage, which would keep the machining process when discharge becomes unstable arc or stable arc discharge. Rotating electrode and flushing dielectric were applied to improve debris removal effect.

As shown in Fig. 3, electrical discharge status of ED-milling process was classified into open circuit, discharge machining states and short circuit based on the discharge voltage detected between electrodes. Because of the electrode rotating motion and dielectric flushing effect, the discharge voltage will not maintain at 0 when short circuit happened. Therefore, the discharge state was classified as the short circuit when detected voltage was lower than 10 V. Once short circuit happened, the path moving of tool electrode will be interrupted, and it will make a retraction motion in the z axis to terminate the short circuit. Corresponding to different discharge conditions, the detected voltage between 10 V and 55 V was divided into 9 grades, and each voltage grade had a corresponding servo control coefficient f(U_ave). The electrode feeding speed will be adjusted depending on the result of f(U_ave)·v, in which v is the preset reference feeding speed. When detected voltage was higher than 55 V, it was classified as open circuit, and electrode would feed at 200% of the preset feeding speed.

Fig. 1 Schematic diagram of ED-milling equipment

Fig. 2 Classification of discharge waveform:

Fig. 3 Servo control strategy based on discharge gap voltage

2.3 Selection of process parameters

In order to study the effect of rotating electrode and dielectric flushing on ED-milling performance, the influence of rotating speed and flushing pressure on MRR, TWR, W-EUE and T-EUE was investigated by experiments. The machined surface morphology was also studied.

The processing conditions and parameters for electrode rotating experiment are shown in Table 1. Titanium alloy TC4 was selected as difficult to machine workpiece. Cylindrical graphite pipe which has an inner diameter of 6 mm and an outer diameter of 20 mm was taken as electrode because of its good electric conductivity, high melting point and favorable thermal conductivity. A coolant water emulsion with no pressure was employed as the dielectric medium to cool discharge area. The pulse on time was set at 2000 μs which was longer than that in normal ED-milling. The pulse off time was set at 100 μs which left a short time for deionization and debris removal in the discharge gap. Those settings can help studying the role of rotating electrode in ED-milling process. Open gap voltage was set at 60 V, and TC4 workpiece was connected to the anode. Electrode rotating speed was changed from 0 to 1000 r/min in order to study its influence on MRR, TWR, W-EUE and T-EUE. In dielectric flushing experiment, all the machining conditions and process parameters were consistent with electrode rotating experiment except that electrode rotating speed was set at 600 r/min, and dielectric flushing pressure was changed between 0 and 1.4 MPa.

Figure 4(a) shows the discharge sparks of ED- milling process. Because of the high discharge energy, a large number of materials were melted during one pulse on time, and most of the melted debris was flushed off the discharge gap by dielectric flushing effect. The trajectory of high temperature particles was illustrated in the form of discharge sparks. The machined groove was presented in Fig. 4(b). It was used to calculate the MRR, TWR, W-EUE and T-EUE of ED-milling process under different machining conditions. The machined surface morphology was also detected by optical microscope and scanning electron microscope (SEM).

Table 1 Processing conditions and parameters for electrode rotating speed and dielectric flushing experiments

Fig. 4 ED-milling process:

The MRR of ED-milling process is calculated by

                (1)

where v_max is the highest feeding speed that electrode can reach; w_groove and d_groove are the width and depth of the machined groove, respectively.

The TWR of ED-milling process is calculated by

                         (2)

where ΔL_electrode is the electrode wearing in axial direction, and L_groove is the length of machined groove.

W-EUE is employed to study how much volume of material is removed by one joule discharge energy, which is given by

                       (3)

where W_discharge is the electric energy consumed in 1 min,which is calculated by measuring discharge voltage and current in machining process.

T-EUE is used to calculate the electrode length wearing by one joule discharge energy and the formula is given by

                        (4)

3 Results and discussion

3.1 Effect of proposed servo control strategy on discharge performance

The discharge voltage and current waveform that detected in ED-milling process are presented in Fig. 5(a). It can be observed that most of the discharges are stable arc discharge, and only part of the discharge are unstable arc discharge and normal discharge, which indicates that unstable and stable arc discharge is prevented from transforming to short circuit effectively.

The distribution of 1000 continuous discharge pulses is shown in Fig. 5(b). It can be seen that 55.9% of the discharge is stable arc discharge which suggests that most materials are removed by stable arc discharge process. Normal discharge which possesses 22.8% of total discharge is helpful in protecting machined surface from serious ablation, which can be demonstrated by machined surface morphology illustrated in Fig. 7.

3.2 Effect of electrode rotating speed on MRR, TWR, W-EUE and T-EUE

Figure 6 shows the influence of electrode rotating speed on MRR, TWR, W-EUE and T-EUE in ED-milling process. As can be seen from Fig. 6(a), the highest MRR reaches 300 mm³/min when electrode is not rotating. It is because discharge point moving and debris flushing effects are limited without rotating electrode. Even the discharge energy melted mass of material in one pulse on time, it cannot be flushed off from discharge gap by dielectric fluid in time. Most of the melted material will re-solidify on the original position after discharge, which results in a lower machining efficient. The MRR is increased by nearly three times when electrode rotating speed is improved from 0 to 200 r/min. It demonstrates that electrode rotating motion plays an important role in material removal process. The MRR increases with the growing of electrode rotating speed, and it reaches 1250 mm³/min when electrode rotates at 600 r/min. The MRR will decrease slightly and then becomes stable when the rotating speed continues to increase. That is because the discharge plasma channel is moved too fast when electrode rotates at a speed higher than 600 r/min, and its time to stay at each fixed point in the moving route is reduced. As a result, the moving discharge plasma channel has not enough time to melt more materials at every fixed point, which leads to the decrease of MRR. When electrode rotates at 600 r/min, discharge moving speed and material melting process will realize a good balance, and the material will be ejected from discharge gap once it is melted. The TWR is decreased with the increasing of electrode rotating speed, which suggests that the higher MRR can be realized with a lower TWR.

Fig. 5 Discharge waveform and its distribution in ED-milling process:

Fig. 6 Influence of electrode rotating speed:

From corresponding W-EUE and T-EUE in Fig. 6(b), it can be noticed that removed electrodes material by one joule discharge energy is increased when electrode rotating speed is improved. It suggests that electrode rotating motion is helpful to improve discharge energy utilization efficiency. It is observed that the changing of T-EUE is opposite to TWR. This is because the calculation of T-EUE takes the machining time into consideration, while TWR only involves the length of machined groove. The electrode rotating motion enhances the homogenization of discharge points.

3.3 Effect of rotating electrode on machined surface

As shown in Fig. 7, surface morphology machined by rotating electrode is apparently different from that machined by non-rotating electrode. It can be clearly seen from Fig. 7(a) that machined surface is covered by well distributed round pits when electrode is not rotating. When electrode is rotating, the machined surface will contain lots of tiny grooves as shown in Fig. 7(b). In discharge process, the rotating electrode can decentralize the discharge point and move the discharge plasma channel. The discharge energy will be distributed in a relatively large area and as a result, material debris will be melted in a large area with a smaller size. The decreasing of debris particle size will be beneficial to the dielectric flushing effect in discharge gap. The process of flushing off debris particles from discharge gap will become easier.

Figure 8 illustrates the discharge plasma channel moving process in ED-milling machining. As shown in Fig. 8(a), when discharge happens at the edge of electrode, the discharge plasma channel is initialed at some position that possesses a lower dielectric property, and it is moved by the rotating of electrode as the discharge continues. When discharge happens at the bottom of the electrode, the discharge plasma channel moving phenomenon is similar to that happening at the edge of the electrode as shown in Fig. 8(b).

Fig. 7 Comparison of machined surface morphology:

Fig. 8 Discharge point moving phenomenon in ED-milling process:

3.4 Effect of dielectric flushing on MRR, TWR, W- EUE and T-EUE

Dielectric flushing has a significant influence on ED-milling performance. It can be noticed from Fig. 9(a) that the MRR of ED-milling increases sharply with the rising of flushing pressure. It demonstrates that the material removal is not only depending on discharge energy but also affected by dielectric flushing effect. In the case of lacking flushing pressure, some part of melted debris cannot be flushed off discharge gap, and it will re-solidify on the workpiece after discharge, which limits the improving of machining efficient. On the other hand, with the growing of flushing pressure, the TWR is decreased gradually. It suggests that the increasing of flushing pressure can be helpful to the fast cooling of electrode. The tool electrode wearing is reduced by fast cooling process.

The influence of flushing pressure on W-EUE and T-EUE is shown in Fig. 9(b). It can be seen that removed material by one joule discharge energy increases with the growing of flushing pressure. The highest material removal of TC4 by one joule discharge energy is 0.032 mm³. Graphite electrode that can be removed by one joule discharge energy will maintain at 0.00125 μm when dielectric flushing pressure is higher than 0.5 MPa.

Fig. 9 Influence of flushing pressure:

3.5 Effect of dielectric flushing on machined surface

Figure 10 shows the influence of dielectric flushing on the morphology of machined surface. In Fig. 10(a), lots of large heaves are noticed on the machined surface when there is no dielectric flushing in ED-milling process. That is because the melted material cannot be flushed away from the discharge gap in time, and it adheres on the machined surface after discharge. Only with high pressure flushing, most of the melted material can be washed away from discharge gap sufficiently. It can be observed from Fig. 10(b) that, when high pressure flushing was employed, the machined surface became smooth, and only some tiny size of debris particles can be observed on the machined surface.

Fig. 10 Comparison of machined surface morphology:

4 Conclusions

1) By adjusting the servo control strategy of ED-milling process in titanium alloys machining, unstable and stable arc discharge combined with normal discharge are employed for material removal. The combination of different discharge pulses in material removal can improve the utilization of discharge energy.

2) Electrode rotating motion can prevent arc discharge from transferring to short circuit by moving discharge plasma channel. The melted debris particles are formed in small size, which is beneficial to the removing of debris particles from discharge gap.

3) The balance between discharge plasma channel moving pace and material melting process is realized when electrode rotates at 600 r/min. The highest MRR can reach 1250 mm³/min while 0.0047 mm³ of TC4 material is removed by one joule discharge energy. With the growing of electrode rotating speed, the TWR is decreased, meanwhile, the removed graphite electrode by one joule discharge energy is increased.

4) High pressure flushing is helpful to flush off melted debris from discharge gap in a limited time, and it can prevent melted material from re-solidifying and adhering on the machined surface after discharge. By increasing flushing pressure from 0 to 1.4 MPa, the MRR can be improved by 8 times. The T-EUE will maintain at 0.00125 μm/J when the flushing pressure is higher than 0.5 MPa which is beneficial to the wearing compensation of tool electrode.

References

[1] HASCALIK A, CAYDAS U. Electrical discharge machining of titanium alloy (Ti–6Al–4V) [J]. Applied Surface Science, 2007, 253(22): 9007-9016.

[2] KIM K S, LEE C M. Prediction of preheating conditions for inclined laser assisted machining [J]. Journal of Central South University, 2012, 19(11): 3079-3083.

[3] KIM D H, LEE C M. Development of a one-axis manipulator for laser-assisted machining [J]. Journal of Central South University, 2013, 20(2): 378-384.

[4] CUI Hai, GUO Li-bin, ZHANG Bin, LU Xi-wen. Tribological property comparison between micro WEDM and superfinishing grinding machining surfaces [J]. Journal of Central South University: Science and Technology, 2013, 44(9): 3687-3692. (in Chinese)

[5] SHEN Xiao-long, REN Cheng-gao, LI Kai-mo, ZHOU Zhi-xiong. Multi-axis movement processingmethod of spatial complicated curved surface of WEDM [J]. Journal of Central South University: Science and Technology, 2012, 43(12): 4329-4734. (in Chinese)

[6] ZHANG Cheng-guang, ZHANG Yong, ZHANG Fei-hu. Mechanism of ultrasonic-pulse electrochemical compound machining based on particles [J]. Journal of Central South University, 2014, 21(1): 151-159.

[7] RAJURKAR K P, SUNDARAM M M, MALSHE A P. Review of Electrochemical and Electrodischarge Machining [J]. Procedia CIRP, 2013, 6: 13-26.

[8] MOHD A N, SOLOMON D G, FUAD B M. A review on current research trends in electrical discharge machining (EDM) [J]. International Journal of Machine Tools and Manufacture, 2007, 47(7/8): 1214-1228.

[9] KUNIEDA M, YOSHIDA M, TANIGUCHI N. Electrical discharge machining in gas [J]. CIRP Annals - Manufacturing Technology, 1997, 46(1): 143-146.

[10] TAO Jia, SHIH A J, NI Jun. Experimental study of the dry and near-dry electrical discharge milling processes [J]. Journal of Manufacturing Science and Engineering, 2008, 130(1): 011002.

[11] HAN Fu-zhu, WANG Yong-xian, ZHOU Ming. High-speed EDM milling with moving electric arcs [J]. International Journal of Machine Tools and Manufacture, 2009, 49(1): 20-24.

[12] GUU Y H, HOCHENG H. Effects of workpiece rotation on machinability during electrical-discharge machining [J]. Materials and Manufacturing Processes, 2007, 16(1): 91-101.

[13] GU Lin, LI Lei, ZHAO Wan-sheng, RAJURKAR K P. Electrical discharge machining of Ti6Al4V with a bundled electrode [J]. International Journal of Machine Tools and Manufacture, 2012, 53(1): 100-106.

[14] SAMEH H, OKADA A, UNO Y. Improving the productivity of electrical discharge machining process by using multi-thin electrodes [J]. Machining Science and Technology, 2013, 17(1): 110-128.

[15] KUCUKTURK G. A geometrical approach for reduction of tool shape degeneration in electric discharge machining (EDM) [J]. Machining Science and Technology, 2012, 16(3): 445-458.

[16] SUBRAMANIAN R, MARIMUTHU K, SAKTHIVEL M. Study of crack formation and re-solidified layer in EDM process on T90Mn2W50Cr45 tool steel [J]. Materials and Manufacturing Processes, 2012, 30(2): 199-203.

[17] MUTHURAMALINGAM T, MOHAN B. Influence of discharge current pulse on machinability in electrical discharge machining [J]. Materials and Manufacturing Processes, 2013, 28(4): 375-380.

[18] MUTHURAMALINGAM T, MOHAN B, RAJADURAI A, SARAVANAKUMAR D. Monitoring and fuzzy control approach for efficient electrical discharge machining process [J]. Materials and Manufacturing Processes, 2014, 29(3): 281-286.

[19] BLEYS P, KRUTH J P, LAUWERS B, ZKYD A, DELPRETTI R, TRICARICO C. Real-time tool wear compensation in milling EDM [J]. CIRP Annals - Manufacturing Technology, 2002, 51(1): 157-160.

[20] ZHOU Ming, HAN Fu-zhu, SOICHIRO I. A time-varied predictive model for EDM process [J]. International Journal of Machine Tools and Manufacture, 2008, 48(15): 1668-1677.

[21] ZHOU Ming, HAN Fu-zhu, WANG Yong-xian, ISAGO S. Assessment of dynamical properties in EDM process—detecting signature of latent change to deleterious process in advance [J]. The International Journal of Advanced Manufacturing Technology, 2008, 44(3/4): 283-292.

[22] KUNIEDA M, LAUWERS B, RAJURKAR K P, SHUMACHER B M. Advancing EDM through fundamental insight into the process [J]. CIRP Annals- Manufacturing Technology, 2005, 54(2): 64-87.

(Edited by YANG Hua)

Foundation item: Project(MSV-2013-09) supported by State Key Laboratory of Mechanical System and Vibration, China

Received date: 2015-07-07; Accepted date: 2015-12-09

Corresponding author: DI Shi-chun, Professor, PhD; Tel: +86-451-86413485; E-mail: dishichun@126.com