Ultrasonic vibration pulse electro-discharge machining of holes in engineering ceramics.pdf
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Journal of Materials Processing Technology ELSEVIER Journal of Materials Processing Technology 53 (1995) 811-816 Ultrasonic vibration pulse electro-discharge machining of holes in engineering ceramics Jia Zhixin*, Zhang Jianhua, Ai Xing Department of Mechanical Engineering, Shandong University of Technology, Jinan 250014, Peoples Republic of China Received 27 April, 1994 Industrial Summary In this paper, an ultrasonic vibration pulse electro-discharge machining (UVPEDM) technique has been developed to produce holes in engineering ceramics. The principle of UVPEDM is described. Pulse discharge in the UVPEDM technique is caused by ultrasonic vibration of the tool electrode, instead of the special pulse generator in traditional electro- discharge machining. Ultrasonic vibration of the tool electrode also acts as a gap-flushing method. It has been confirmed by experiment that this new technique is effective in the obtaining of a high material removal rate. I. Introduction With the rapid development of the machinery industry toward high accuracy, high efficiency and automation, machines are usually needed to operate at elevated temper- ature and under harsh environmental conditions. Obviously, metallic materials cannot meet these new requirements, therefore various new types of non-metallic materials that have superior properties, such as engineering ceramics, have been used increasingly in electro-mechanical products over the last two decades. The demand for hole drilling in ceramics is increasing steadily in many applications. At present, several techniques for drilling holes are available, including mechanical drilling, ultrasonic machining (USM), electro-discharge machining (EDM), laser- beam machining (LBM), electron-beam machining (EBM) and other methods. The mechanical drilling of holes in ceramics presents several problems related to surface cracking and tool life. LBM and EBM usually result in holes with a funnel or pear-like shape: holes with straight profiles are difficult to obtain. USM produces holes with *Corresponding author. 0924-0136/95/$09.50 1995 Elsevier Science S.A. All rights reserved. SSDI 0924-0136(94)01749-Q 812 J. Zhixin et al. / Journal of Materials Processing Technolo, 53 (1995) 811 816 better surface quality, however the material removal rate (MRR) is very low. EDM has gained importance in the manufacturing world since its discovery 50 years ago by Lazarenko and Lazarenko. In recent years, it has been demonstrated successfully that EDM can be applied to ceramics if their electrical resistivity is below 100 cm 1 3. One undesirable characteristic of EDM is the very low efficiency of sparking in the forms of frequency of open circuit, short circuit and arcing pulse 4. The situation is aggravated further in the drilling of deep holes. This paper presents an ultrasonic vibration pulse electro-discharge machining technique to produce holes in ceramics, the aim of the study being to decrease the equipment cost, to increase the discharge efficiency and to give again a higher MRR. 2. Principle of ultrasonic vibration pulse electro-discharge machining In traditional EDM, cold emission of electrons occurs when a voltage is applied to both electrodes, this in turn producing a state of ionization in a particular space. At a given voltage, the ionization ends at a particular distance from the cathode, since dielectrics display a considerable capacity for attenuating the ionization process, i.e. they bring about de-ionization. An increase in voltage extends the ionization zone and intensifies the ionization. At a particular moment, the state of ionization becomes sufficient for a flow of charge from the cathode to the anode. With the voltage remaining constant, a similar phenomenon can be attained if both electrodes are drawn closer together, this phenomenon being applied to UVPEDM. The machine tool used in this UVPEDM technique is an ultrasonic drilling machine, a schematic diagram of the apparatus is shown in Fig. 1. The ultrasonic I Ultrasnicl, J I generator J transducer horn tool t D.C. l working fluid Source workpiece Fig. 1. Schematic diagram of the UVPEDM apparatus. J. Zhixin et aL / Journal of Materials Processing Technology 53 (1995) 811-816 813 tool workpmce tool workpiece I tool 1 ii (a) (b) (c) tool ece (d) tool tool 1 tool I tool o, , 11_ i1),.1 o 4 . : . workpiece workpiece workpiece (e) (f) (g) (h) Fig. 2. Discharge process: (a) build-up of an electrical field; (b) formation of a bridge by conductive particles; (c) beginning of the discharge due to the emission of negative particles; (d) flow of current by means of negative and positive particles; (e) development of the discharge channel resulting from a rise in temperature and pressure, with the formation of a vapour bubble; (f) reduced heat input after a drop in the current, with an explosion-like removal of material; (g) collapse of the vapour bubble; (h) residues of material particles and gas. generator produces a high-frequency electrical signal (17-25 KHz), this signal is transformed into a mechanical vibration signal of the same frequency by a transducer. A horn amplifies the amplitude and transfers it to the tool. The workpiece and the tool are connected to the positive pole and the negative pole of a DC source, respectively. Fig. 2 shows the discharge process in UVPEDM 5. Applying a particular voltage across the working gap generates an electric field between the workpiece and the tool. Initially, the two electrodes are insulated by dielectric fluid (tap water), so no current flows. With the ultrasonic vibration of the tool, the front surface of the tool moves down towards the workpiece surface and the electric field intensity increase, the resulting electric field causing ultra-fine solid impurities to be suspended and form a bridge across the gap (Fig. 2(b). When the gap reaches a particular, very small size, it results in the breakdown or de-ionization of the dielectric fluid (Fig. 2(e). The voltage falls to a constant value, and the current rises to a value set by the operator. Plasma is created and a vapour bubble forms around the channel (Fig. 2(e). As the front surface of the tool moves away from the workpiece surface, the voltage begins to rise and the current begins to drop, the discharge channel collapsing very rapidly when the gap reaches a particular, large size (Fig. 2(g). The process begins again when the tool moves down towards the workpiece again. Briefly, the discharge in UVPEDM is formed when a voltage is applied between the tool and the workpiece, which approach and retract periodically by the mechanical vibration of one of them. 814 J. Zhixin et al. / Journal of Materials Processing Technology. 53 (1995) 811 816 Table 1 Machining conditions Machine J93025 ultrasonic drilling machine Frequency 20 kHz Power output 250 W Tool material Mild steel Workpiece material SG-4 ceramic Tool size 10 mm outer diameter 8 mm inner diameter Applied voltage 25 90 V Table 2 Some properties of alumina ceramics Density (kg/m 3) 6650 Flexural strength (MPa) 980 Vickers hardness (Gpa) 2690 Fracture toughness (MN m - 3/2) 4.9 Youngs modulus (GPa) 430 Poissons ratio 0.23 Melting point (K) 2330 Thermal expansion coefficient (k) 7.5 x 10 6 Electrical resistivity (f cm) 3.5 x l0 2 3. Experimental procedure The experimental set-up is shown schematically in Fig. 1. The experiments were carried out using a J93025 ultrasonic machine. The generator has a power output of 250 W and a frequency range of 17-25 kHz. The transducer is a magnetostrictive nickel-stack and is Water cooled, whilst the power tubes are air cooled. The horn yields a wide range of amplitudes up to 40 tm (peak-to-peak). In these experiments, the amplitudes of the tool (peak-to-peak) are 8 25 lam. The experimental conditions are shown in Table 1. Alumina ceramics were used as the workpieces, some properties of alumina ceramics being shown in Table 2. 4. Results and discussion 4.1. Effect of the applied voltage on the MRR The effect of the applied voltages on the MRR is shown in Fig. 3. As the applied voltage is increased to beyond 15 20 V, the sparks of the discharge start to be observed and the material removal rate increases. When the applied voltage is above J. Zhixin et al. / Journal of Materials Processing Technology 53 (1995) 811-816 815 A - E m E 5 E 4 3 0 E 2 ,= I 0 o I I 10 20 m gto o oO ; o o O O o I I I I I I I 30 40 50 60 70 80 90 Applied voltage ( V ) Fig. 3. The effect of the applied voltage on the MRR (hollow circle: 15 lain amplitude; full circles: 25 gm amplitude). r- E E O E I I I I I I I I i - 8 10 12 14 16 18 20 22 24 Amplitude ( gm ) Fig. 4. The effect of the amplitude of tool vibration on the MRR, 80 V in the case of 15 gm amplitude (100 V in the case of 25 gm amplitude), electric arcs form between the tool and the workpiece: such case should be avoided. 4.2. Effect of the amplitude of tool vibration on the MRR The effect of the amplitude of tool vibration on the MRR is shown in Fig. 4, where it is seen that the material removal rates rise slightly with increase in the amplitude of tool vibration. When the amplitude is under 8 p.m (peak-to-peak), the electric dis- charge is difficult to maintain. During the time of discharge, current is converted into heat, the surface of the workpiece being heated very strongly in the area of the plasma channel, this high temperature causing the melting and vaporization of the electrode 816 J. Zhixin el al. / Journal oMaterials Processing Technolog), 53 (1995) 811 816 materials 6. Melting and evaporation of the electrode is the most-common method for erosion and is that traditionally explained and modeled 7 93. As the current ceases, a violent collapse of the plasma channel and vapour bubble causes super- heated, molten liquid on the surfaces of both of the electrodes to explode into the gap. However, not all of the molten materials can be removed, because of surface tension, tensile strength, and bonding force between the liquid and the solid. In UVPEDM, with the ultrasonic vibration of the tool, the tool makes the gap vary very rapidly, and a high frequency alternate pressure variation is generated. During the decreasing phase of the gap, the pressure is increased and the growth in the diameter of the plasma channel is slowed, whilst during the increasing phase of the gap, a large pressure drop occurs and an increase in material evaporation should arise owing to a decrease in the evaporation temperature. This, of course, gives a better ejection of the molten materials and less molten material is re-cast onto the surface of the electrode. In addition, the ultrasonic vibration of the tool prevents the sedimentation of the debris particles in the working ga
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