DELTA-PHASE BRASS ELECTRODE WIRE FOR ELECTROEROSION MACHINING, AND METHOD FOR MANUFACTURING SAME

Abstract

An electrode wire for electroerosion machining, the electrode wire including a metal core, made of one or more layers of metal or metal alloy. On the metal core there is a coating having an alloy different from that of the metal core, and containing more than 50% by weight of zinc. The coating includes delta-phase copper-zinc alloy.

Claims

1-16. (canceled)

17. An electrode wire for machining by electrical discharge machining, said electrode wire comprising: a metallic core, made of one or more layers of metal or of metallic alloy, on the metallic core, a coating having an alloy different from that of the metallic core and containing more than 50% by weight of zinc, wherein the coating comprises delta-phase copper-zinc alloy.

18. The electrode wire as claimed in claim 17, wherein delta-phase copper-zinc alloy forms at least one layer of alloy in the coating.

19. The electrode wire as claimed in claim 18, wherein said at least one layer of delta-phase copper-zinc alloy is fractured.

20. The electrode wire as claimed in claim 18, wherein said at least one layer of delta-phase copper-zinc alloy is a surface layer of the coating.

21. The electrode wire as claimed in claim 18, wherein the coating comprises a fractured layer of gamma-phase copper-zinc alloy surmounted by said at least one layer of delta-phase copper-zinc alloy.

22. The electrode wire as claimed in claim 21, wherein said at least one layer of delta-phase copper-zinc alloy has a thickness of between 30% and 100% of the thickness of said fractured layer of gamma-phase copper-zinc alloy.

23. The electrode wire as claimed in claim 18, wherein the coating comprises a layer of beta-phase copper-zinc alloy, surmounted by a fractured layer of gamma-phase copper-zinc alloy, surmounted by said at least one layer of delta-phase copper-zinc alloy, itself surmounted by a layer of epsilon-phase copper-zinc alloy.

24. The electrode wire as claimed in claim 21, wherein the layer of gamma-phase copper-zinc alloy contains pores which are covered by said at least one layer of delta-phase copper-zinc alloy.

25. A process for manufacturing an electrode wire for machining by electrical discharge machining, said electrode wire comprising a metallic core and a coating comprising delta-phase copper-zinc alloy, which process comprises the following stages: (a) taking a blank wire made of metal, (b) producing, on this blank wire, a coating having zones, the mean composition of which corresponds to the range of existence of the delta-phase copper-zinc alloy, (c) bringing the coated blank wire to a temperature of between 559° C. and 700° C., preferably between 559° C. and 600° C., at which temperature the copper-zinc alloy delta phase is stable, (d) suddenly cooling the coated blank wire so as to keep the delta-phase copper-zinc alloy in a metastable state at ambient temperature.

26. The process as claimed in claim 25, comprising the following stages: during stage (a), choosing a blank wire having copper on the surface, producing a first coating of zinc on this blank wire, carrying out a first diffusion heat treatment so as to obtain a sub-layer of beta-phase copper and zinc alloy and an external layer of gamma-phase copper and zinc alloy, producing a second coating of zinc, carrying out a second diffusion heat treatment at a temperature of less than 170° C. so as to obtain, at the surface of the wire, an external layer made of epsilon-phase copper and zinc alloy while retaining the lower layers made of beta-phase copper-zinc alloy and gamma-phase copper-zinc alloy previously produced, bringing the wire to a temperature of between 559° C. and 700° C., preferably between 559° C. and 600° C., more preferably between 595° C. and 598° C., so as to create an intermediate layer made of delta-phase copper and zinc alloy, between the sub-layer made of gamma-phase copper and zinc alloy and the external layer of epsilon-phase copper and zinc alloy (34), suddenly cooling the wire so as to keep the delta-phase copper and zinc alloy in a metastable state at ambient temperature.

27. The process as claimed in claim 25, which process comprises the following stages: during stage (a), choosing a blank wire having copper on the surface, dipping this blank wire in a bath of molten zinc at a temperature of between 559° C. and 700° C., preferably between 559° C. and 600° C., more preferably 600° C., so as to create a coating made of delta-phase copper and zinc alloy, suddenly cooling the wire so as to keep the delta-phase copper and zinc alloy in a metastable state.

28. The process as claimed in claim 25, which process comprises the following stages: during stage (a), choosing a blank wire made of metal, depositing, at the surface of the blank wire, a layer of nickel with a thickness of approximately 5 μm, dipping this nickel-coated blank wire in a molten bath having a zinc content of between 72 and 77 atomic percent and the remainder as copper, and allowing to diffuse at a temperature of between 559° C. and 700° C., preferably between 559° C. and 600° C., more preferably 600° C., so as to create a coating made of delta-phase copper and zinc alloy, suddenly cooling the wire so as to keep the delta-phase copper and zinc alloy in a metastable state at ambient temperature.

29. The process as claimed in claim 25, which process comprises the following stages: during stage (a), choosing a blank wire made of metal, depositing, at the surface of the blank wire, a layer of nickel with a thickness of approximately 5 μm, coextruding this nickel-coated blank wire with a delta-phase copper and zinc alloy having a zinc content of between 72 and 77 atomic percent and kept at a temperature of between 559° C. and 700° C., preferably at 600° C., so as to create a coating made of delta-phase copper and zinc alloy on this nickel-coated blank wire, suddenly cooling the wire thus coated immediately after the coextrusion, so as to keep the delta-phase copper and zinc alloy in a metastable state at ambient temperature.

30. The process as claimed in claim 25, which process comprises the following stages: during stage (a), choosing a metallic blank wire, depositing, at the surface of the metallic blank wire, a layer of nickel with a thickness of approximately 5 μm, depositing, on the layer of nickel, a layer of copper and then a layer of zinc, in proportions between the copper and the zinc of between 72 and 77 atomic percent of zinc, with an excess of zinc chosen in order to compensate for the unavoidable evaporation of a part of the zinc during the subsequent diffusion stage, allowing to diffuse at a temperature of between 559° C. and 700° C., preferably between 559° C. and 600° C., more preferably 600° C., so as to create a coating having a layer made of delta-phase copper-zinc alloy, suddenly cooling the wire thus coated so as to keep the delta-phase copper-zinc alloy in a metastable state at ambient temperature.

31. The process as claimed in claim 25, in which stage (b) of producing the coating is carried out by aqueous-phase electrodeposition, using the blank wire as cathode, and using an anode made of copper and zinc alloys with a composition close to the delta phase.

32. The process as claimed in claim 25, comprising a subsequent stage of drawing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0085] Other subject matters, characteristics and advantages of the present invention will emerge from the following description of specific embodiments, which description is given in connection with the appended figures, among which:

[0086] FIG. 1 is a phase diagram of the copper-zinc system at equilibrium, with the atomic concentration (molar fraction) of zinc on the abscissa and with the temperature, expressed in degrees Kelvin, on the ordinate;

[0087] FIG. 2 is a side view in longitudinal half-section of an electrode wire of the prior art, having a core and continuous coating, before production of an electrical discharge machining spark;

[0088] FIG. 3 is a side view in longitudinal half-section of the electrode wire of FIG. 2, after production of an electrical discharge machining spark;

[0089] FIG. 4 is a top view of the electrode wire of FIG. 3, after production of an electrical discharge machining spark;

[0090] FIG. 5 is a side view in longitudinal half-section of an electrode wire of the prior art, having a core and fractured coating, before production of an electrical discharge machining spark;

[0091] FIG. 6 is a side view in longitudinal half-section of the electrode wire of FIG. 5, after production of an electrical discharge machining spark;

[0092] FIG. 7 is a side view in longitudinal half-section of an electrode wire according to one embodiment of the present invention;

[0093] FIG. 8 is a side view in longitudinal half-section of an electrode wire according to another embodiment of the present invention; and

[0094] FIG. 9 is a side view in longitudinal half-section of an electrode wire according to another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0095] First of all, in FIG. 1, the phase diagram of the copper-zinc system at equilibrium is considered.

[0096] It is noted that the delta phase is stable in a reduced range D in which the zinc concentration is between 72 and 77 atomic percent and in which the temperature is between 559° C. and 700° C.

[0097] Thus, in the zones made of delta-phase copper-zinc alloy of an electrode wire, the zinc content varies continuously while remaining between 72 and 77 atomic percent the remainder being copper, and the unavoidable impurities. The delta phase of the copper-zinc alloy has a specific crystallographic structure which can be identified by various means, for example by X-ray diffraction or by neutron diffraction. This specific crystallographic structure makes it possible to distinguish the delta phase of the copper-zinc system with respect to a mixture of fine grains made of gamma-phase brass and of fine grains made of epsilon-phase brass, which mixture would have the same overall composition. The crystallographic structure of the delta phase of the copper-zinc system, in its stable state at a temperature of 600° C., was published in 1971 by J. Lenz and K. Schubert in the Zeitschrift für Metallkunde, vol. 62, pages 810-816.

[0098] The zones of delta-phase copper and zinc alloy can only be obtained between 559° C. and 700° C., and must subsequently undergo a quenching, that is to say a sudden cooling, to bring them from 559° C. to ambient temperature in a short period of time, in order to retain their crystalline structure at ambient temperature. In practice, the quenching can be carried out by passing the electrode wire through liquid water, preferably at a temperature close to ambient temperature, before the temperature of the electrode wire drops below 559° C.

[0099] At temperatures of less than 559° C., in particular at ambient temperature, the zones of delta-phase copper and zinc alloy are in a metastable state. In such a metastable state, the transformation of the delta phase into the gamma phase is very slow, almost imperceptible after manufacture, under the conditions of storage, of transportation and of supply of the electrode wires to the machining zone of the device for electrical discharge machining.

[0100] As already indicated above, when they are subjected to an intense and short-lived machining spark, the zones of delta-phase copper-zinc alloy produce less liquid, and this favors obtaining a high speed of machining by electrical discharge machining.

[0101] It is likely that this effect, which has been observed, results from the following specific physical properties of the delta phase of the copper-zinc system.

[0102] A first physical property is linked to the metastable state of the delta phase of the copper-zinc system at ambient temperature. In fact, the molar free enthalpy of the delta phase is greater than those of the gamma phase and of the epsilon phase. Evaporation of the delta phase is therefore potentially favored by this excess of energy. The amount of heat which a spark must release to heat the delta phase from 25° C. to 561° C. is less than that required to heat a mixture of the gamma and epsilon phases of the same overall composition between the same temperatures.

[0103] A second physical property of the delta phase of the copper-zinc system is its peritectic decomposition when the temperature rises starting from 561° C. This property appears on the phase diagram of FIG. 1, by the horizontal line limiting the high end of the delta phase range. This means that the melting of the delta phase, from the high end of the delta phase range, must be accompanied by a phase change to the solid state in order to produce the gamma phase. In point of fact, the speed of the phase change to the solid state is potentially limited by the diffusion of atoms in the solid state, a diffusion which is slower than the diffusion in the liquid state. Thus, under the conditions of the machining spark, the melting of the delta phase is potentially slower than the evaporation of the metal. An interface is thus produced between the solid and its vapor, without significant presence of a liquid between the two.

[0104] In FIG. 2, there has been diagrammatically illustrated a partial longitudinal section of an electrode wire 1 having a core 2 and a continuous coating 3, before the action of a spark. The coating then exhibits a generally smooth, that is to say generally cylindrical, surface 4 along the longitudinal axis A-A of the electrode wire 1.

[0105] In FIG. 3, there has been diagrammatically illustrated the same partial longitudinal section of this same electrode wire 1 with a continuous coating 3 after a machining spark. The spark produced a crater 5, surrounded by a bead 6 of material which was melted and re-solidified.

[0106] FIG. 4 diagrammatically illustrates, in top view, the section of the electrode wire 1 with the crater 5 and its bead 6.

[0107] It is understood that the material which forms the bead 6 was moved while it was in the liquid state under the effect of the heating due to the spark. The material which forms the bead 6 results from the melting of the alloy(s) previously present in the coating, and the resulting alloy phase may be different from that of the alloy(s) previously present, and no longer has the same erosion effect during the action of the subsequent sparks in the machining zone.

[0108] The present invention, by the presence of zones made of delta-phase copper-zinc alloy, makes it possible to reduce the depth of the crater, or its diameter, or the thickness of the bead of re-solidified material. Thus, the electrode wire of the invention loses less material at each spark, and the material remaining on the surface of the electrode wire better retains its properties which were present before the spark.

[0109] In the embodiment diagrammatically illustrated in FIG. 5, showing a partial longitudinal section of an electrode wire 1 having a core 2 and coating 3, before the action of a spark, the coating 3 exhibits fractures 7.

[0110] In FIG. 6, there has been diagrammatically illustrated the same partial longitudinal section of this same electrode wire 1 having a fractured coating 3 after a machining spark. The spark produced a crater 5, surrounded by a bead 6 of material which was melted and re-solidified.

[0111] It is understood that the material which forms the bead 6 was moved while it was in the liquid state under the effect of heating due to the spark, and came to cover certain fractures or pores 7a, so that the latter can no longer be used to perform their technical effects during the subsequent sparks.

[0112] The present invention, by the presence of the zones made of delta-phase copper-zinc alloy, makes it possible to reduce the amount of liquefied material, and thus makes it possible to better preserve the presence of the fractures or pores 7, which perform their functions during the subsequent sparks.

[0113] In the embodiment illustrated in FIG. 7, the coating 3 of the electrode wire 1 comprises a lower layer 31 of beta-phase copper-zinc alloy, surmounted by a continuous first intermediate layer 32 of gamma-phase copper and zinc alloy, itself surmounted by a second intermediate layer 33 of delta-phase copper-zinc alloy, itself surmounted by a surface layer 34 of epsilon-phase copper and zinc alloy.

[0114] In the embodiment illustrated in FIG. 8, the coating 3 of the electrode wire 1 comprises a single layer 33 of delta-phase copper-zinc alloy, covering the core 2.

[0115] In the embodiment illustrated in FIG. 9, the coating 3 of the electrode wire 1 comprises a lower layer 31 of beta-phase copper-zinc alloy, surmounted by a fractured first intermediate layer 32 of gamma-phase copper-zinc alloy, itself surmounted by a second intermediate layer 33 of delta-phase copper-zinc alloy, itself surmounted by a surface layer 34 of epsilon-phase copper-zinc alloy. In this embodiment, the second intermediate layer 33 and the surface layer 34 partially cover some fractures or pores 7b of the first intermediate layer 32, forming covered pores. During a spark, the presence of zones such as the layer 33 made of delta-phase copper-zinc alloy reduces the risk of disappearance of such covered fractures or pores 7b, which covered pores 7b are favorable for obtaining a high speed of machining by electrical discharge machining with such an electrode wire 1.

[0116] The present invention is not limited to the embodiments which have been explicitly described but includes the various alternative forms and generalizations thereof contained within the scope of the claims below.