Study on mechanical pulse electric discharge machining

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Study on mechanical pulse electric discharge machining Jia Zhixin, Ai Xing, and Zhang Jianhua Mechanical Engineering Department, Shandong University of Technology, Jinan 250014, People's Republic of China In this paper, a mechanical pulse electric discharge machining (MPEDM) technique has been developed to produce holes in the conductive hard and brittle materials, and the mechanism of the MPEDM is described. The sparks of electric discharge in the MPEDM are caused by the ultrasonic vibration of the tool, which is used in place of the conventional special pulse generator. Ultrasonic vibration of the tool also acts as a gap-flushing method. Tap water is used as the working fluid in the MPEDM technique. It has been confirmed through the experiments that the MPEDM is effective to attain a higher material removal rate (MRR) for conductive difficult-to-machine materials.

Keywords: electric discharge machining; ultrasonic vibration; material removal rate; hard and brittle materials Introduction Material scientists are continually looking for materials that can operate at elevated temperatures and under harsh environmental conditions. Such materials cause considerable difficulty during machining and fabrication. On the other hand, design engineers who choose the new materials are also specifying high tolerances and better quality finishes. Therefore, there is an increased requirement for machining researchers to keep up with the new material development. The need for efficient and highly accurate machining processes of new materials is as important as the development of the material itself if a practical application is ever to be found. The demand for hole drilling in hard and brittle materials is steadily increasing in many applications. To date, several techniques for machining holes have been available, including mechanical drilling, electric discharge machining (EDM), ultrasonic machining (USM), laser beam machining (LBM), electron beam machining (EBM), and other methods. Mechanical drilling of holes in hard and brittle materials presents several problems related to surface cracks and tool life. Laser beam machining and EBM usually result in holes with funnel or pear-like shapes; holes with straight profiles are difficult to obtain. Ultrasonic machining produces holes with better surface quality. However, the material removal rate (MRR) is extremely low.

Address reprint requests to ,7. Zhixin, Mechanical Engineering Department, Shandong University of Technology, Jinan 250014, People's Republic of China. Precision Engineering 17:89-93, 1995 © Elsevier Science Inc., 1995 655 Avenue of the Americas, New York, NY 10010

Electric discharge machining has gained importance in the manufacturing world since its discovery 50 years ago by B. R. Lazarenko and N. I. Lazarenko. In recent years, research has successfully shown that EDM can be applied to conductive hard and brittle materials if the electrical resistivety is below 100,q,cm. 1-3 One underable characteristic of the EDM is very low efficiency of sparking in the forms of frequent open circuit, arcing pulse, and short circuit. `= Combining two or more machining processes has an advantage of combining their virtues. This is a worthwhile approach, especially for the machining of difficult-to-machine materials. Recently, combined machining of different machining process has attracted special interest in the field of new materials and difficult-to-machine materials. 5 In this paper, mechanical pulse electric discharge machining is proposed. The aim of this study has been to decrease the equipment cost and to increase the discharge efficiency and give a higher MRR. This MPEDM can be applied to drilling of all conductive hard and brittle materials.

Equipment and principle of mechanical pulse electric discharge machining An ultrasonic machine looks similar to an electric discharge machine, where: 1) the head is replaced by an acoustic system (vibrator); 2) the approach movement is simultaneous with material removal; 3) as with an electric discharge machine, an ultrasonic machine should be fitted with a system allowing injection to bring working fluid to the machining zone and allowing suction to remove machining debris from it, 0141-6359/95/$10.00 SSDI 0141-6359(94)00007-M

Zhixin et aL: Study on mechanical pulse electric discharge machining The machine-tool used in the present MPEDM technique is an ultrasonic drilling machine. The schematic diagram of the apparatus is shown in Figure 1. The ultrasonic generator produces a highfrequency electrical signal with a power of 250 W and a frequency range of 17-25kHz. The electrical signal is transformed into a mechanical vibration signal with the same frequency by a magnetostrictive nickle-stack transducer. A horn amplifies the amplitude and converts it to the tool. The workpiece and the tool are connected to the positive pole and negative pole of a DC source, respectively. Tap water flushes the gap between the workpiece and the tool. The tap water is supplied under pressure by a single jet. In traditional EDM, cold emission of electrons occurs when a voltage is applied to both electrodes, and this in turn, produces a state of ionization in a particular space. At given voltages, the ionization ends at a certain distance from the cathod because dielectrics display a considerable capacity for attenuating the ionization process; that is, they bring about deionization. An increase in voltage extends the ionization zone and intensifies the ionization. At a certain moment, the state of ionization becomes sufficient for a flow of charge from cathod to anode. When the voltage remains constant, a similar phenomenon can be attained if both electrodes are drawn closer together. This phenomenon is applied to the MPEDM. Figure 2 shows the principle of electric discharge in this MPEDM technique. Applying a certain voltage DC across the spark gap generates an electric field between the workpiece electrode and the tool electrode. At first, the two electrodes are insulated by the working fluid (tap water), so no current flows. With the ultrasonic vibration of the tool, the tool front surface moves down toward the workpiece surface, and the electric field intensity increases. The resulting electric field, however, causes the ultrafine impurities in the working fluid

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Discharge process: a) build-up of an electrical field; b) formation of bridge by conductive particles; c) beginning of discharge caused by emission of negative particles; d) flow of current by means of negative and particles; e) development of discharge channel resulting from a rise in temperature and pressure, formation of vapor bubble; f) reduced heat input after drop in current, explosionlike removal of material; g) collapse of vapor bubbles; h) residues of material particles and gas

to be suspended and forms a bridge across the gap (Fig. 2b). When the gap reaches a certain very small size, it results in the deionization or breakdown of the working fluid (Fig. 2c). The voltage falls to a constant value, and the current rises to a certain value set by the operator. A plasma channel forms (Fig. 2d). A vapor bubble forms around the channel (Fig. 2e). When the tool front surface moves up far from the workpiece surface, the discharge voltage begins to rise and the current begins to drop. The plasma channel collapses very rapidly when the gap reaches a certain large size (Fig. 2g). The process begins again when the tool moves down toward the workpiece again. During the time of discharge, current is converted into heat. The workpiece surface is very strongly heated in the areas of the plasma channel. A violent collapse of the plasma channel causes a superheated vapor bubble. The high temperature causes melting and vaporization of the electrode materials. 6 Molten liquid on the electrode surfaces is exploded into the gap, resulting in small craters on both electrodes. Thermal spalling may play an important role in the MPEDM of ceramics (see Refs. 3 and 7).

Theoretical aspect of effect of ultrasonic vibration on mechanical pulse electric discharge machining In EDM, the dielectric fluid acts as an insulator between the tool and the workpiece, convects away the small amount of heat generated by the disAPRIL 1995 VOL 17 NO 2

Zhixin et al.: Study on mechanical pulse electric discharge machining charges, and flushes off the discharge byproducts from the working gap. As machining proceeds, the concentration of the particles in the gap increases rapidly. It is imperative to remove the wear debris from the gap so that fresh dielectric fluid enters for spark discharges. Thus, flushing is decisive for process efficiency. Insufficient flushing results in the stagnation of the dielectric, build-up of machining residue in the gap, short circuit, arcs, and low MRR. In MPEDM, ultrasonic vibration of the tool acts as a gap-flushing method. The results of ultrasonic vibration of the tool on fluid particle displacement, velocity, acceleration, and pressure are cyclic. The ultrasonic waves propogate as elastic waves, exciting oscillations at a point in the fluid medium with alternate compression and rarefaction. Apart from this, ultrasonic irradiation of a fluid medium gives rise to the following pronounced induced effects. 1. Cavitation: associated with nucleation, growth and burst of a swarm of gaseous bubbles. 2. Ultrasonic field force: any solid particle present in the ultrasonic field undergoes the following forces 8'9 a. Radiation force caused by the intensity of the ultrasonic field b. Stokes's force or viscous drag force caused by the different viscosity of the fluid in compression and rarefaction during the vibration c. Bernoulli's attraction resulting from the presence of different size particles in a flow regime 3. Acoustic streaming: the propagation of ultrasonic waves in a rarefactive fluid medium gives rise to mass flow, witnessed as a pattern of steady vortices termed as microstreaming. 4 Jackson 1° reported three distinct pairs of vortices, and developed a particular solution for the tangential component of the streaming velocity. In MPEDM, cavitating bubbles and the ultrasonic field force prevent the sedimentation of the debris particles in the working gap and result in their animated suspension in working fluid. The high-frequency pumping action or the flushing action of acoustic streaming improves the working fluid circulation by pushing the debris away and sucking fresh working fluid into the spark gap. These stoutly increase discharge efficiency and give higher erosion rates.

Experimental procedures The experimental setup is shown schematically in Figure 1. The experiments were carried out with a J93025 ultrasonic drilling machine (made in China). The generator has a power output of 250W and a frequency range of 17-25kHz. The transducer is a PRECISION ENGINEERING

magnetostrictive nickle-stack. Water cooled the transducer and air cooled power tubes. The horn yields a wide range of amplitudes up to 40 i~m (peak-to-peak). In these experiments, the amplitudes of the tool are 8-301~m. The experimental conditions are as shown in Table 1. Ceramics are the typical examples of difficult-to-machine hard and brittle materials. Silicon carbide (SIC) and SG-4 ceramics were used as the workpiece materials. Some properties of SiC and SG-4 ceramics are shown in Table 2.

Table 1

Experimental conditions J93025 Ultrasonic drilling machine

Machine Frequency Output power Amplitude Working fluid Applied voltage Tool material Diameter of the tool Workpiece materials

Table 2

20000 Hz 150-250 W 8-30 I~m Tap water 20-100 V Mild steel, HV 160 kg/mm 2 5.2 mm Silicon carbide SG-4 ceramic, (aluminabased composite ceramic

Properties of Workpiece Materials

Property

SiC

SG-4

Density, kg/m 3 Hardness, Hv Flexural strength, MPa Fracture toughness, MPa m 1/2 Young's modulus, GPa Possion's ratio Termal expansion coefficient, 1/K Thermal conductivity, W/m K Electrical resistivity, ~,cm

3,200 2,000

6,650 2,760

460

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4.9 435 0.25

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160 0.025

A frequency meter is used for measuring the vibration frequency of the tool. A dual-trace storage oscilliscope is used for recording the voltage waveforms and current waveforms between the workpiece and the tool. A LFDZ-2 discharge condition detector (made by Haerbin Institute of Technology, China) is used for estimating the percentage of efficient discharge. 91

Zhixin et aL: Study on mechanical pulse electric discharge machining Results and discussion Effect o f ultrasonic vibration on discharge efficiency

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Test results showed that the percentage of efficient discharge pulses of the MPEDM is always over 80%. The effect of the amplitude of the tool vibration on MRRs is shown in Figure 3. As we expected, it is seen that the MRRs rise slightly with the amplitude of the tool vibration. The higher amplitude of the tool results in a higher flushing action in the spark gap with a corresponding effect on the discharge pulses. It should be pointed out that if the amplitude of the tool vibration is below 8 i~m (peakto-peak), the stable spark of electric discharge is difficult to maintain. Table 3 gives comparations of machining rates and surface roughness of USM, EDM, and MPEDM. The results showed that the MRR of the MPEDM is about three times greater than that of USM, and about two times greater than that of conventional EDM.

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First, the mechanical pulse electric discharge machining can be performed by the utrasonic vibration of the tool surface with a DC source. It is simple in principle, with good operational performance and low equipment cost.

Table 3

C o m p a r a t i o n of different machining

methods MRR, mm3/min

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Method USM EDM MPEDM

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Effect o f applied voltage on MRR The effect of the applied voltages between the workpiece and the tool on MRR is shown in Figure 4. As the applied voltage is increased and becomes higher than 15-20V, the sparks of electric discharge begin to be observed, and the MRR is increased. When the applied voltage is above 80V in the case of 15 i~m amplitude (100V in the case of 25 i~m amplitude), electric arcs form between the tool and the workpiece. This will cause the decrease of MRR and initiation of cracks on the workpiece surface. Such cases should be avoided. This result also indicates that the ultrasonic vibration allows more delicate work: for example a hole with large diameter can be machined without arcing.

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Two, test results show that the MRR of the present technique is twice that of conventional EDM. Third, tap water is used as the working fluid. In this case, a potential fire hazard is avoided and no carbonaceous material is produced during electric discharge. Therefore, we would expect deep hole drilling to be possible by stable electric discharge and also that the machining time would be shorter when water is used as the working fluid. The aim of this study is to seek new machining method of difficult-to-machine materials. It is hoped that this paper will stimulate others to pursue in-depth studies of machining of such materials.

References 1 Petrofes, N. F. and Gadalla, A. M. "Processing aspects of shaping advanced materials by electrical discharge machining," Adv ManufgProcs 1988, 3, 127-153 2 Petrofes,N. F. and Gadalla, A. M. "Electrical discharge machining of advanced ceramics," CeramicBull 1988, 67, 1,048-1,052 3 Nakamura,M., Shigematsu, I., Kanayama, K., and Hirai, Y. "Surface damage in ZrBz-based composite ceranics induced by electrodischarge machining," J Mats Sci, 1991, 26, 6,078-6,082 APRIL 1995 VOL 17 NO 2

Z h i x i n et aL: S t u d y on m e c h a n i c a l p u l s e electric discharge m a c h i n i n g 4 Murthy, V. S. R. and Philip, P. K. "Pulse train analysis in uItrasonic assisted EDM," Int J Mach Tools Manufact, 1987, 27, 469-477 5 Aoyama, T. and Inasaki, I. "Hybrid machining--Combination of electrical discharge machining and grinding," Proceedings of the 14th North American Manufacturing Research Conference, SME, 1986, 654-661 6 Zhang, Jianhua and Ai, Xingo "Study on the combined technology of ultrasonic and EDM," Proceedings of the 5th IMCC, (Guangzhou, People's Republic of China), 1991

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7 Zhang, Jianhua and Ai, Xing. "Study on the machining of ceramics," Proceedings of the 11th International Conference on Production Research (Hefei, People's Republic of China), 1991 (suppl), 1,935-1,938 8 Nyborg, W. L. "Acoustic streaming," in PhysicalAcoustics, Vol. II(B), W.P. Mason, ed. New York: Academic Press, 1965, 265-331 9 Vigoureux, P. Ultrasonics, Wiley, New York, 1952 10 Jackson, F.J. "Sonically induced microstreaming near a plane boundary--II," JAcoust SocAm 1960, 32, 1387-1395

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