Electrocontact discharge trepanning of metals

Journal of Materials Processing Technology, 25 (1991) 185-194

185

Elsevier

Electrocontact discharge trepanning of metals M.A. Bdjar University o[ Chile, Department o[ Mechanical Engineering, CasiUa2777, Santiago, Chile (Received December 11, 1989 )

Industrial Summary This paper presents the results of a study concerned with electrocontact discharge erosion, undertaken in order to assess the feasibility of using such a phenomenon for the trepanning of metallic materials. The process was carried out using a fixed copper cathode as the tool, a rotatory steel anode as the workpiece, a d.c. generator as the power unit, and a flow of ordinary water with soluble oil as the work fluid. The influence of: the level of the electrical current; the velocity of rotation of the anode; and the fluid pressure; on both the rate of removal of anode material and cathode wear have been determined.

1. Introduction

For about two centuries it has been known that an electrical current has an erosive effect on the surface of the electrodes [ 1,2 ]. However, the first systematic research concerned with the quantity of material removed by electro-erosion was published by E.F. Kingsbury, in 1928 [ 1 ]. Kingsbury established that the erosion increases rapidly when the electrical current is higher than a particular value, this increase being due to the arcs which appear. Latterly, other authors have reported similar results [3,14 ]. From the outset, studies of the electro-erosive effect have been focused mainly on wear, either in electrical relays or in the sliding contacts of electrical motors and generators. Only since 1943 has the erosive effect of electrical discharges been used for metal removal in manufacturing. Electro-discharge machining (EDM) is presently the best known electro-erosive manufacturing process. In such a process, the removal of material is due to the thermal action of electrical sparks (discharges), produced intermittently by means of a pulse current generator between two electrodes separated by a liquid dielectric [ 2 ]. Other electro-erosive processes and other types of electrical generators have also been used for the machining of metals [4-6 ]: it has been shown, for example, that electrocontact discharge machining with a d.c. generator can be used for grinding cutting-tool metals [ 7 ]. In electrocontact discharge machining, the electrodes (the tool and the 0924-0136/91/$03.50 © 1991--Elsevier Science Publishers B.V.

186

workpiece) are brought into contact intentionally, having either some sliding velocity with a low contact-pressure or a vibratory movement. In this way, intermittent electrical discharges can be generated without the requirement of a pulse current generator. The purpose of this paper is to study the feasibility of using electrocontact discharge machining for the trepanning of metals. 2. Description of metal removal

From the earlier studies on relays, it is clear that electro-erosion may occur when the electrodes are either approaching to complete an electrical circuit, or separating to break it. When the electrodes are approaching, the discharge may be produced by a long gap breakdown or by a short gap breakdown, depending on the interelectrode voltage. In the former case, the direction of metal transfer is, in general, from the cathode to the anode [8,9], whilst in the latter case, the erosion may be predominantly anodic (short time discharge ), predominantly cathodic (long time discharge ) or mixed [8,9 ]. When two electrodes between which a current is passing are separating, the fall of the current from its short-circuit value to zero can be divided into several stages, each one being characterized by the potential difference between the electrodes and by the mechanism of the conduction of the current. If the two electrodes are held together with a sufficiently high pressure, no destructive action on the contact points takes place. As the pressure is decreased, an increase in contact resistance and a rise in contact potential is observed [10]. Consequently, an increase in contact-point temperature occurs, which latter can be sufficiently high to produce a molten-metal bridge between the electrodes. The temperature distribution of the bridge, generally, is non-symmetrical. Typically, the hottest section does not occur at the middle of the bridge (the Thomson effect) [ 1,11 ]. In many cases, when the hottest section reaches the boiling temperature, the bridge is ruptured and metal is transferred from the electrode towards which the hottest section is displaced, to the other electrode. However, in some cases rupture occurs when the temperature is between the melting point and the boiling point [11 ], which indicates that bridge rupture is not only due to boiling, as was suggested by Ittner [12]. Indeed, it has been shown that by modifying the surface conditions - for instance by outgassing and ion bombardment [ 11 ] - the bridge rupture-temperature can be reduced. Meshcheriakov [ 13] has suggested that in electrical erosion, the separation of molten metal from its solid is governed to a large extent by interphase phenomena at the solid-molten metal boundary. Moreover, Meshcheriakov [14] has observed that bright luminous gases, positively charged, are often released in the interphase liquid-anode, driving away the molten metal from the anode. Immediately after bridge rupture, and if the voltage and the

187

current of the circuit are sufficiently high (typically greater than 10 V and 10 l° A / m 2 respectively, [15] ), a discharge will occur. For inter-electrode gaps or discharge durations smaller than a particular value, the discharge will be predominantly anodic, whereas for gaps or durations larger than such a particular value, the erosion will be predominantly cathodic [15]. In electrocontact discharge machining, the discharge duration can be controlled by means of an electrical pulse generator (as in EDM) or by the relative velocity of the electrodes. Obviously, in the case of vibratory movement, the discharge duration decreases with the increase in mechanical frequency. In the case of sliding movement, the discharge frequency increases with sliding velocity [ 14 ]. 3. Experimental procedure The disposition of the electrodes used for the electrocontact discharge trepanning tests is shown schematically in Fig. 1. A d.c. generator (Messer Griesheim GK 225 SP) with 85 V open circuit voltage - the electrical circuit diagram of which is presented elsewhere [7] - was used as the electrical source. The cathode was a fixed tube of 8-15.9 m m external diameter (De) and of 0.62.3 m m wall thickness (T). The anode (rotatory) was connected to the generator through brushes. All tests were run at about 20 V inter-electrode voltage, which was found to be an adequate value from the point of view of the stability of the process. The anode material was generally SAE-1045 steel and the cathode material was generally copper. The trepanning depth was 7-10 mm and ordinary water with soluble oil (20:1 ) was used as the work-fluid that is flushed through the centre of the cathode. The operational parameters were: the electrical current (I); the flushing pressure (P) and the velocity of rotation of the anode (N). The anode material removal rate (MRR) and cathode relative wear (RW: the loss of mass of the cathode relative to that of the anode ) were determined as functions of the operational parameters.

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4. Results

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Figure 2 shows the M R R values obtained by electrocontact discharge when trepanning a SAE-1045 steel anode with either a copper or a SAE-1045 steel cathode, as a function of the electrical current. The respective cathode relative wear values (RW, w/w) are shown in Fig. 3. At 30 N / c m 2 flushing pressure and 2000 rpm anode rotation velocity, the M R R obtained for a given electrical current (up to 40 A) when using a copper cathode (De = 12.7 mm, T= 1.5 mm ) is not much greater than the M R R obtained when using a SAE-1045 steel cathode. However, there exists a notable difference in cathode wear. With a copper cathode, the RW is lower by 20%, whereas with a SAE-1045 steel cathode, RW is greater by 45%. Consequently, it was decided to run all of the following tests using a copper cathode only. The anode material also has an influence on process performance. In Figs. 4 and 5, the values of M R R and W R for a SAE-1045 and a A I S I - H l l steel

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anode are shown. Clearly, the M R R values are lower and the WR values are greater for a A I S I - H l l steel anode, than those for a SAE-1045 steel anode. The apparent increase of the M R R with electrical current that is shown in Figs. 2, 4, 6 and 9 is a consequence of the increase of the electrical energy involved. However, for a cathode of given dimensions and for a given flushing pressure, the M R R can be increased only up to a particular maximum value. By increasing the electrical current beyond this value, an excess of anode material is deposited on the cathode and the process becomes impracticable due to short-circuits: this seems to be associated with the capacity of the workfluid for sweeping the debris from the perforation being only up to a limiting maximum quantity per unit time. Further it is probable that the gasification of the work-fluid, due mainly to the thermal action of the electrical current, contributes also to the diminishing of the removal of debris and to the localization of the electrical discharges to the exit zone of the work-fluid (the zone of greatest gas-concentration). As is shown in Figs. 4, 6 and 9, the thickness and the diameter of the cathode also have an influence on the MRR, the latter increasing with the decreasing

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of the thickness and with the increasing of the diameter. However, at low values of the electrical current, the MRR values are very similar, independently of the dimensions of the cathode. Trepanning-velocity values (TV), as a function of electrical current, are shown in Fig. 7. Cathode wear depends also on the electrical current, the cathode thickness and the cathode diameter, as is shown in Figs. 3, 5, 8 and 10, the value of the WR increasing with increasing electric current and with decreasing thickness and diameter. The influence of flushing pressure on the MRR and the WR is shown in Figs. 11 and 12. Although the MRR and the RW increase with pressure, the influence of pressure on the RW is more significant than it is on the MRR. The greater capacity of the work-fluid to remove debris as its velocity increases, as explains the MRR trend, whilst the RW trend is explained to a large extent by the increasing of mechanical erosion due to the greater velocity of the debris and of fluid turbulence. The influence of the velocity of rotation of the anode (N) on the MRR is shown in Fig. 13: up to 3500 rpm, such influence is not significant. At higher

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velocities, a large number of short-circuits will occur due to lateral accumulation of debris by centrifugal force and, consequently, the MRR values will become lower than those shown in Fig. 13. The influence of the velocity of rotation of the anode on the RW can be seen

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in Fig. 14. From this figure, the tool relative wear is seen to decrease as the velocity increases. This behaviour is hard to explain as, in general, the cathode wear decreases with increase in the duration of the discharge [16]. As the discharge frequency increases with increase in the velocity of sliding [ 14 ], the

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discharge duration must decrease with increase in velocity. It is probable that by increasing the velocity of rotation of the anode, there occurs a lower rate of mechanical erosion, due to debris being evacuated more rapidly by centrifugal force, from the frontal inter-electrode gap. Finally, it can be stated that: (i) the diametral overcut clearance and the surface-roughness values of the workpiece, obtained by electrocontact discharge trepanning, were relatively large (the minimum overcut clearance was about 1 mm whilst the minimum surface roughness, Ra, was about 5 ~tm); and (ii) electrocontact discharge has also been used for trepanning extremely hard steels. 5. Conclusions

The results of this work lead to the conclusion that electrocontact discharge trepanning, using a d.c. generator as the power unit, a copper tool and a flow of ordinary water with soluble oil as the work-fluid, can be used for the rough trepanning of metallic materials, irrespective of their hardness.

References 1 Holm, R., Electric Contacts Handbook, Springer, Berlin, 1958. 2 Pandey, P.C., and Shan, H.S., Modern Machining Processes, Tata McGraw-Hill, New Delhi, 1981. 3 Banerjee, A., Electric current effect in wear phenomena at a rotating Au-Ag interface, Wear, 86 (1983) 341-352. 4 Rasch, F.O., and Mahle, P., Electrocontact discharge machining, prospect and limitations, CIRP Ann., 23 {1974) 43. 5 Livshits, A.L., Electro-erosion Machining of Metals, {Moscow, 1957), Butterworths, London, 1960. 6 Albinsky, K., L'Usinage des M~taux par Electro-erosion, Dunod, Paris, 1960. 7 B4jar, M.A., and Orellana C., Electrocontact discharge grinding of cutting tool metals, Int. J. Mach. Tool Des. Res., 24 (2) (1984) 95-103.

194 8 9 10 11 12 13 14 15 16

Atalla, M.M., Arcing of electrical contacts in telephone switching circuits, Part V Mechanism of the short arc and erosion of contact, Bell Syst. Tech. J., 34 (1955) 1081-1102. Germer, L.H., Physical processes in contact erosion, J. Appl. Phys., 29 (7) 1958) 1067-1082. Betteridge, W, and Laird, J.A., The wear of electrical contact points, J. Inst. Elect. Eng. (London), 82 (1938) 625 632. Llewellyn Jones, F., The Physics of Electrical Contacts, Oxford University Press, Oxford, 1957. Ittner, W.B., Bridge and short arc erosion of copper, silver, and palladium contacts on break, J. Appl. Phys., 27(4) (1956) 382-388. Mescheriakov, G.N., Charugin, N.V., May, L.V., and Mescheriakov, N.G., Physico-chemical surface phenomena in EDM process and metal transfer, CIRP Ann., 29 ( 1 ) ( 1980 ), 117-122. Meshcheriakov, G.N., Electro-physical processes in electric pulse metal cutting from the point of view of efficiency and polarity of electrode wear, CIRP Ann., 18 (1970) 491-499. Holm, R., Electric Contact. Theory and Application, Springer-Verlag, Berlin, 1967. Meshcheriakov, G., Nosulenko, V., Meshcheriakov, N., and Bokov V., Physical and technological control of arc dimensional machining, CIRP Ann., 37 (1) (1988) 209-212.