FR3129098A1 - Electrode wire - Google Patents

Abstract

Electrode wire This electrode wire comprises: - a metal core (10) which extends along a longitudinal axis, and - on the metal core, a coating comprising one or more areas (30-32) of copper alloy -zinc in gamma phase, each of these zones being formed solely of copper-zinc alloy in gamma phase and the zinc concentration, at an ambient temperature of 25°C, inside each of these zones of copper alloys -zinc in gamma phase, being greater than 65.4 atomic %. The majority of the zones (30-32) of copper-zinc alloy in the gamma phase are located less than 1 μm from the outer face of the electrode wire. Fig. 1

Description

Wire electrode

The invention relates to an electrode wire suitable for use as an electrode wire for electroerosion machining. The invention also relates to a method of manufacturing this electrode wire.

Wire electrodes are used to cut metals or electrically conductive materials by spark erosion in an spark erosion machine.

The well-known method of machining by electroerosion, or erosive sparking, makes it possible to remove material from an electrically conductive part, by generating sparks in a machining zone between the part to be machined and a conductive electrode wire electricity. The electrode wire runs continuously in the vicinity of the part in the direction of the length of the wire, held by guides, and it is gradually moved in the transverse direction in the direction of the part, either by transverse translation of the guides of the wire, or by translation of the part.

An electric generator, connected to the wire electrode by electrical contacts away from the machining area, establishes an appropriate potential difference between the wire electrode and the conductive workpiece. The machining area between the wire electrode and the part is immersed in an appropriate dielectric fluid. The potential difference causes, between the electrode wire and the part to be machined, the appearance of sparks which progressively erode the part and the electrode wire. The longitudinal scrolling of the electrode wire makes it possible to permanently maintain a sufficient wire diameter to prevent it from breaking in the machining zone. The relative movement of the wire and the part in the transverse direction makes it possible to cut the part or to treat its surface, if necessary.

The particles detached from the electrode wire and the part by the sparks disperse in the dielectric fluid, where they are evacuated.

Obtaining machining precision, in particular the production of small radius corner cutouts, requires the use of wires of small diameter and supporting a large mechanical load at breakage to be stretched in the machining area. and limit the amplitude of the vibrations.

Most modern EDM machines are designed to use metal wires, typically 0.25 mm in diameter, and with a breaking load of between 400 N/mm ² and 1000 N/mm ² .

When a spark occurs between the electrode wire and the part, the surface of the electrode wire is suddenly heated to a very high temperature for a short time. As a result, the material of the surface layer of the electrode wire, at the location of the spark, passes from the solid state to the liquid or gaseous state, and is displaced to the surface of the electrode wire and/or evacuated in the dielectric fluid. It can be seen that the outer face of the electrode wire reached by the spark has been deformed, generally taking on a slightly concave crater shape, with areas where the material has melted and solidified again.

It has been observed that the effectiveness of the sparks with regard to electroerosion depends largely on the nature and the topography of the surface layer of the electrode wire. For this, considerable progress in spark erosion efficiency has been obtained by using electrode wires comprising:
- a core in one or more metals or alloys ensuring good conduction of electric current and good mechanical strength to withstand the mechanical tension load of the wire, and
- A coating of one or more other metals or alloys and/or a particular topography, for example fractures, ensuring better efficiency of electroerosion, for example a higher erosion rate.

For example, application US5945010A describes an electrode wire having a brass core covered with a layer of copper-zinc alloy in the gamma phase, optionally surmounted by a layer of copper-zinc alloy in the epsilon phase. This application teaches that such a layer of copper-zinc alloy in the gamma phase makes it possible to improve the performance of the electrode wire. In particular, this application describes a specimen No. 3 of electrode wire comprising a layer of copper-zinc alloy in the gamma phase surmounted by a surface layer of copper-zinc alloy in the epsilon phase. The thickness of the surface layer is 3 µm. The zinc concentration of the copper-zinc alloy layer in the gamma phase is 68 atomic %. This application also describes a specimen No. 4 of electrode wire comprising a surface layer of copper-zinc alloy in gamma phase. In this case, the zinc concentration of the superficial layer of copper-zinc alloy in the gamma phase is 65 atomic %.

The electrode wires of application US5945010A are efficient. However, it is still desirable to obtain even more efficient electrode wires and, in particular, having an improved erosive efficiency and/or an increased machining speed.

The invention aims to satisfy this need by proposing an electrode wire capable of being used as an electrode wire for electroerosion machining, this electrode wire comprising:
- a metal core which extends along a longitudinal axis, and
- on the metal core, a coating comprising one or more zones of copper-zinc alloy in gamma phase, each of these zones being formed solely of copper-zinc alloy in gamma phase and the zinc concentration, at an ambient temperature of 25° C., inside each of these zones of copper-zinc alloys in gamma phase, being greater than 65.4 atomic %,
wherein the majority of the gamma-phase copper-zinc alloy areas are located within 1 µm of the outer face of the electrode wire.

Embodiments of this wire electrode may include one or more of the following features:
1) At room temperature, the majority of the copper-zinc alloy zones in the gamma phase are directly flush with the outer face of the electrode wire.
2) At ambient temperature, inside each zone of copper-zinc alloy in gamma phase, the zinc concentration is greater than or equal to 68.4 atomic %.
3) At room temperature, the coating successively comprises, going from the outside towards the metal core of the electrode wire:
- a first superficial layer whose zinc concentration is greater than 72 atomic %, the thickness of this superficial layer being less than 1 μm or 0.5 μm, and immediately under this superficial layer
- a second layer which each contains zones of copper-zinc alloy in gamma phase.
4) The surface layer is made of copper-zinc alloy in the epsilon phase.
5) The coating comprises, immediately below the second layer, a third homogeneous layer of copper-zinc alloy formed solely of beta-phase copper-zinc alloy.
6) The electrode wire comprises fractures which, in a cross section of the electrode wire, mechanically separate the different zones of copper-zinc alloy in the gamma phase.
7) The majority of gamma-phase copper-zinc alloy zones have a cross-section, perpendicular to the longitudinal axis, whose length is greater than 5 μm and whose width is greater than 4 μm, the length and width of a cross-section of a zone of gamma-phase copper-zinc alloy being equal, respectively, to the width and length of the rectangle of smallest area which entirely contains this cross-section.

The invention also relates to a process for manufacturing the above electrode wire, in which this process comprises the production, on a metal core, of a layer of copper-zinc alloy in gamma phase which, at an ambient temperature from 25°C:
- is located less than 1 µm from the outer face of the electrode wire, and
- in which the zinc concentration is greater than a threshold S ₁₆ , this threshold S ₁₆ being greater than or equal to 65.4 atomic %.

Embodiments of this method may include one or more of the following features:
1) The production of the copper-zinc alloy layer in gamma phase comprises the following steps:
a) the production, on a metal blank wire comprising a surface layer whose copper concentration is greater than 50% or 60% at atomic, of a coating having the capacity to form a layer of copper-zinc alloy in gamma phase by diffusion of copper from the surface layer of the blank wire into this coating when heated above 150°C, then
b) heating, one after the other, each successive portion of the blank yarn on which the coating is made, to a temperature T _c greater than 150° C. so that part of the coating, in the heated portion, is transformed in a layer of copper-zinc alloy in gamma phase surmounted by a surface layer even richer in zinc, for this each portion of the blank wire entering a heating zone at a time t _ini and leaving this heating zone at a time t ₀ , the running speed of the rough wire inside the heating zone being constant and determined so that time t ₀ corresponds to a time when:
- the thickness of the superficial layer richer in zinc on this portion of the blank wire which comes out of the heating zone is less than 1 μm or has disappeared, and
- the zinc concentration of the layer of copper-zinc alloy in gamma phase of this portion of the blank wire which comes out of the heating zone is still greater than 65.4 atomic%, then
c) from time t ₀ , cooling the temperature of the layer of copper-zinc alloy in gamma phase of the portion of the blank wire which leaves the heating zone at this time t ₀ , to drop its temperature to 30° C. in less than ten seconds, this cooling step being applied to each of the portions of the preform yarn which comes out of the heating zone.
2) During step b), each portion of the coating is brought to a temperature T _c of between 500°C and 700°C.
3) The production of the coating includes the production, directly on a surface layer of the blank wire in which the copper concentration is greater than 50% or 60 atomic%, of a layer whose zinc concentration is greater than 98% atomic.
4) The production of the copper-zinc alloy layer in gamma phase comprises the following steps:
a) the production, on a metal blank wire comprising a surface layer whose copper concentration is greater than 50% or 60% at atomic, of a coating having the capacity to form a layer of copper-zinc alloy in gamma phase by diffusion of copper from the surface layer of the blank wire into this coating when heated above 150°C, then
b) heating, in a static furnace, the blank wire on which the coating is made, to a temperature T _c of between 150°C and 300°C so that part of the coating is transformed into a layer of copper alloy -zinc in gamma phase surmounted by a superficial layer even richer in zinc, the thickness of this superficial layer even richer in zinc then gradually decreasing as the heating continues, then
c) interrupting step b) at a time t ₀ and allowing the temperature of the electrode wire to drop below 30° C., time t ₀ corresponding to a time when:
- the thickness of the surface layer richer in zinc on the roughing wire is less than 1 µm or has disappeared, and
- the zinc concentration of the copper-zinc alloy layer in the gamma phase on the blank wire is still greater than 65.4 atomic %.
5) The production of the copper-zinc alloy layer comprises the deposition by electrodeposition, on the metal core, of a layer of copper-zinc alloy in the gamma phase in which the zinc concentration is greater than or equal to 65 .4 atomic %.

The invention will be better understood on reading the following description, given solely by way of non-limiting example and made with reference to the drawings in which:
- there is a schematic illustration of the cross section of an electrode wire,
- there is a flowchart of a process for manufacturing the wire electrode of the , And
- there is a photograph, in black and white, of a portion of a cross-section of yarn made by the process of before the drawing step, and
- there is an image of a portion of a cross section of the wire electrode made by the method of after the drawing step.

In these figures, the same references are used to designate the same elements. In the rest of this description, the characteristics and functions well known to those skilled in the art are not described in detail.

Thereafter, in chapter I, the definitions of certain terms are given. In Chapter II, examples of detailed embodiments are described with reference to the figures. Then, in a chapter III, variants of these embodiments are presented. Finally, in a chapter IV, the advantages of the different embodiments are presented.

CHAPTER I: Definitions and terminology

The expression "element made of material A" or "element made of material A" designates an element in which material A represents at least 90%, by mass, of this element and preferably at least 95% or 98% by mass. of this element.

A "copper-zinc alloy" designates an alloy formed solely of copper and zinc apart from the inevitable impurities.

A "phase" of the copper-zinc alloy refers to a solid phase of the copper-zinc alloy that has a particular crystallographic structure. More precisely, the phases of the copper-zinc system are distinguished from each other by their composition and by their particular crystallographic structure. This particular crystallographic structure makes it possible to distinguish a phase of the copper-zinc alloy from a simple mixture of fine grains of copper and zinc, which mixture would have the same overall composition. Typically known phases of the copper-zinc alloy are alpha phase, beta phase, gamma phase, delta phase, epsilon phase and eta phase. The particular crystallographic structure of a phase is identifiable by various means. For example, optical micrographs or metallography of polished samples show different shades of color for each phase, provided the sample has been suitably etched. Thus, to distinguish the gamma phase from the epsilon phase, an attack with “Nital”, which is a solution of 3% nitric acid diluted in ethanol, is carried out. The gamma phase then appears in gray when it is low in zinc and in gray with shades of brown when it is rich in zinc. The epsilon phase appears darker brown. It is also possible to distinguish the gamma phase from the epsilon phase, by observing the sample under a scanning electron microscope, using a backscattered electron detector. It is also possible to identify the phase of a sample by X-ray diffraction. In the latter case, the wire sample is placed under an incident beam of X-rays of precise wavelength. For example, the Kα line of copper, with an average wavelength of 0.1541 nm, is used. The intensity of the diffracted rays is evaluated for each angle of diffraction. The gamma phase has a known X-ray diffraction spectrum, and different from that of the other phases of the copper-zinc system, and from the zinc oxide ZnO which is often found on the surface of the wires. If the copper-zinc alloy is not crystallized as at least one of the alpha, beta, gamma, delta, epsilon, or eta phases, it is amorphous, and the X-ray diffraction pattern then shows flattened bumps rather than sharp peaks.

At a given temperature, the different phases of the copper-zinc alloy each correspond to a specific range of zinc concentration. The extent of each of these specific zinc concentration ranges varies with temperature. The zinc concentration of a phase of a sample can be obtained by microanalysis of composition. A microanalysis of composition is carried out, with a scanning electron microscope equipped with a spectrometry probe. A beam of electrons, accelerated for example in an electric field of 20 kV, impacts the surface of the sample and causes an emission of X-rays. These X-rays have an energy spectrum characteristic of the composition of the surface of the sample. sample that was impacted by the electron beam. With an energy dispersion spectrometric (EDS) or wavelength selection (WDS) analysis probe, the spectrum of X-rays emitted by the surface of the sample is measured. Algorithms make it possible to select the elements analyzed (therefore to eliminate the effect of impurities), and to calculate the composition of the sample impacted by the electron beam, from the measured spectra. It should be noted that due to the interactions between X-rays and matter, the volume analyzed by EDS (or WDS) is generally around one cubic micrometer. At the boundary between two phases, an average concentration, which does not actually exist in either of the two phases, can be measured. The concentrations indicated here relate to pure phases in their analysis volume. The areas in which a concentration is measured are larger than cubes with a side of one micrometer.

The expression "the zinc concentration inside a copper-zinc alloy zone is greater than X atomic %" means that the average zinc concentration inside this zone is greater than X atomic %. An average concentration is for example obtained by measuring the zinc concentration at different places inside this zone and then by averaging these concentration measurements. The places where the measurements are carried out are located both at the places where the concentration is likely to be the lowest, at the places where the concentration is likely to be close to the average, and at the places where the concentration has chances of being maximum. For this, typically, the places where the measurements are carried out are distributed along an axis passing through the axis of the electrode wire.

The expression “electrical conductor” denotes a material whose electrical conductivity, at 20° C., is greater than 10 ⁶ S/m and, preferably, greater than 10 ⁷ S/m.

The longitudinal axis of a wire is the axis along which that wire mainly extends.

The expression "cross-section" designates a section of the electrode wire perpendicular to its longitudinal axis.

The expression "layer of the electrode wire" means an annular layer of the electrode wire which is located, in each cross-section of the electrode wire, between an inner circular limit and an outer circular limit. In reality, these limits are not necessarily perfect circles. However, as a first approximation, in this text, these limits are likened to circles. These circular boundaries are both centered on the longitudinal axis of the electrode wire. The inner circular boundary is the boundary of the layer that is closest to the longitudinal axis of the wire electrode. Conversely, the outer circular boundary is the boundary of the layer that is farthest from the longitudinal axis of the electrode wire. Between these inner and outer circular limits, the phase of the copper-zinc alloy is homogeneous or formed from an irregular entanglement of different phases of the copper-zinc alloy. Conversely, at the inner and outer circular boundaries, the chemical composition and/or crystallographic form changes abruptly.

A "homogeneous" layer is a layer formed from a single phase of the copper-zinc alloy.

A "uniform" layer means a layer formed of a material which, in a cross-section of the wire, extends, around the axis of the wire and inside this layer, continuously or practically continuously. Thus, a uniform layer does not include a multitude of fractures which partitions it into a multitude of zones separated from each other, in a cross-section of the wire, by these very numerous radial fractures. Very many radial fractures means more than ten radial fractures which divide the layer in question into ten zones mechanically isolated from each other, in the cross section, by these radial fractures.

Conversely, the term "fractured layer" designates a layer which comprises a multitude of fractures which partition it into a multitude of zones separated from each other, in a cross-section of the wire, by very many radial fractures.

The expression "metallic surface layer" or simply "surface layer" refers to the copper-zinc alloy or zinc layer of the electrode wire which is the outermost of the electrode wire. This metallic surface layer may have a thin oxide film on its surface. Typically, this oxide film is mainly composed of zinc oxide, zinc hydroxides, zinc carbonate as well as possible residues such as wire drawing lubricant residues. The outer face of this metallic surface layer is therefore either coincident with the outer face of the electrode wire in the absence of the thin oxide film or separated from the outer face of the electrode wire solely by this thin oxide film.

A "radial fracture" is a fracture that extends primarily, within a cross section of the wire electrode, in a radial direction.

The expression "ambient temperature" means a temperature between 15°C and 30°C and, typically, equal to 25°C.

The "erosive yield" of an electrode wire is equal to the ratio of the surface cut in one minute by the average intensity of the current crossing the electrode wire during the cutting of this surface. For example, if the feed rate of the wire in the cut material is 2 mm/min in a piece of steel 50 mm high, the machining speed is then 100 mm²/min. If the average machining current is 10 A, the erosive efficiency of the wire under these conditions is 10 mm²/min/A.

CHAPTER II: Examples of embodiments

There represents an electrode wire 2 for electroerosion machining as described in the introductory part of this text.

For this purpose, the electrode wire 2 has a breaking load of between 400 N/mm ² and 1000 N/mm ² . The wire 2 extends along a longitudinal axis 4. Axis 4 is here perpendicular to the plane of the sheet. The length of wire 2 is greater than 1 m and, typically, greater than 10 m or 50 m.

The wire 2 has an outer face 6 directly exposed to sparks during the machining of a part by spark erosion using this wire. The outer face 6 is a cylindrical face which extends along the axis 4. The guiding curve of the face 6 is mainly a circle centered on the axis 4. Thus, the cross section of the wire 2 is circular. The outer diameter D ₂ of the wire 2 is typically between 50 μm and 1 mm and, most often, between 70 μm and 400 μm. Here, the diameter of wire 2 is equal to 250 μm.

In this embodiment, wire 2 comprises:
- a central core 10 made of electrically conductive material, and
- a coating 12 directly deposited on the core 10.

The core 10 has the function of ensuring, on its own, the bulk of the breaking load of the wire 2. It also has the function of ensuring the electrical conductivity of the wire 2. For this purpose, it is made made of electrically conductive material. Typically, it is made of metal or metal alloy. For example, in this embodiment, the core 10 is made of copper.

The diameter D ₁₀ of the core 10 is between 0.75D ₂ and 0.98D ₂ and, typically, between 0.85D ₂ and 0.95D ₂ , where D ₂ is the outside diameter of the electrode wire 2. For example , here, the diameter D ₁₀ is equal to 230 μm.

The coating 12 is designed to increase the machining speed and therefore the erosive efficiency of the electrode wire and/or the quality of the faces of the part obtained after electroerosion machining. The quality of a face cut by electroerosion is all the better as its roughness is low.

The thickness of the coating 12 is small compared to the diameter D ₂ of the wire 2, that is to say less than 10% of the diameter D ₂ and, preferably, less than 8% of the diameter D ₂ . The thickness of the coating 12 corresponds to the shortest distance, in a cross section, between the circular limit which separates the core 10 of the coating 12 and the outer face 6.

In this embodiment, the coating 12 is formed:
- two layers 14 and 16 successively and directly stacked one on the other going from the core 10 towards the outer face 6, and
- a possible surface layer 18 directly deposited on layer 16.

Subsequently, the structure of the electrode wire 2 is described in the particular case where the layer 18 exists. Everything that is described in this particular case also applies to the case where layer 18 does not exist. In the latter case, the surface layer of electrode wire 2 is directly layer 16.

Layer 14 is a homogeneous and uniform layer made of beta-phase copper-zinc alloy. The zinc concentration of layer 14 is therefore typically between 45 atomic % and 50 atomic %, the remainder being copper and the inevitable impurities. The thickness of layer 14 is, for example, less than 5 μm.

Layer 16 is a homogeneous layer made of copper-zinc alloy in gamma phase. The zinc concentration of layer 16 is high, that is to say here greater than or equal to a threshold S ₁₆ . This threshold S ₁₆ is greater than or equal to 65.4 atomic % and preferably greater than 66.4 atomic % or 68.4 atomic % or even greater than 70 atomic %, the remainder being copper and the inevitable impurities. The zinc concentration of layer 16 is generally less than 84 atomic % or 75 atomic %.

It follows from the phase equilibrium diagram of the copper-zinc system as recently updated that, in a stable state, the copper-zinc alloy in the gamma phase has a zinc concentration which is between 60 atomic % and 62 Atomic % at room temperature, remainder copper. A recently updated phase equilibrium diagram of the copper-zinc system has, for example, been published in the following article: Liang et al. : “Thermodynamic assessment of the Al-Cu-Zn system, part I: Cu-Zn binary system”, CALPHAD, volume 51, 2015, page 224 to 232.

Thus, with a zinc concentration greater than or equal to the threshold S ₁₆ , the copper-zinc alloy in the gamma phase of the layer 16 is not in a stable state at ambient temperature. Here it is in a metastable state. In a metastable state, the transformation of the copper-zinc alloy in the gamma phase towards its stable state, and therefore the decrease in its zinc concentration, is very slow at room temperature. In other words, this transformation of the gamma phase towards its stable state at room temperature is practically imperceptible by a human being. Thus, the composition of this gamma phase in its metastable state practically does not vary from its manufacture until it is brought into a machining zone of an electroerosion machine when this wire 2 is stored and transported under normal conditions and therefore kept at room temperature. Methods of making such a metastable copper-zinc alloy layer are described below.

The thickness of layer 16 is greater than the thickness of layer 18. On the , by way of illustration, the thickness of layer 16 is also greater than the thickness of layer 14. Advantageously, the thickness of layer 16 is greater than 10% or 20% or 30% of the thickness total of the coating 12. For this purpose, the thickness of the layer 16 is typically greater than 1% or 2% of the diameter D ₂ . For example, the thickness of layer 16 is greater than 5 μm or 10 μm. The thickness of layer 16 is also low enough for it to be fractured during wire drawing of the electrode wire. For this purpose, for example, the thickness of layer 16 is less than 25 μm or 20 μm.

Layer 16 is located less than 1 μm and preferably less than 0.5 μm from outer face 6. Here, it is separated from outer face 6 only by layer 18. Thus, for this purpose, when it exists, the thickness of the layer 18 is less than 1 μm and, preferably, less than 0.5 μm.

Layer 18 is a metallic surface layer even richer in zinc than layer 16. For example, layer 18 is made of copper-zinc alloy whose zinc concentration is higher than that of layer 16. Typically, the difference between the zinc concentrations of layers 16 and 18 is greater than 2 atomic % or 5 atomic % or 10 atomic %. Typically, layer 18 is an epsilon-phase or delta-phase or eta-phase or zinc copper-zinc alloy layer.

In this embodiment, layers 16 and 18 are fractured. Thus, the layers 16 and 18 comprise fractures which divide each of these layers into several zones mechanically separated from each other, in a cross section, by radial fractures. As described later, these fractures are obtained by drawing a wire in which the layers 16 and 18 are uniform or practically uniform. After drawing, the same material no longer extends continuously all around axis 4 but is divided into several areas of material which, in a cross section, are mechanically separated from each other by radial fractures. These fractures extend mainly radially and cross layers 16 and 18 right through.

For example, a fracture begins at the level of the circular boundary between layers 14 and 16 and leads to the outer face 6.

On the , three fractures 22 to 24 are schematically represented. These three fractures 22 to 24 divide layer 18 into three distinct zones 26 to 28 and divide layer 16 into three distinct zones 30 to 32.

These fractures correspond to recesses or voids of solid or liquid matter. The width of a fracture, in a direction perpendicular to the radial direction along which it extends, is generally less than 2 µm. It is emphasized here that, given the very small thickness of the layer 18, the copper-zinc alloy of the layer 18 does not penetrate inside the cracks and does not cover these cracks either.

Each zone of layer 16 typically has a length greater than the thickness of layer 16. Here, each zone of layer 16 has a length greater than 5 μm or 10 μm. In this text, the length and the width of an area, in a cross-section, are defined as being equal, respectively, to the length and the width of the rectangle of smallest area which entirely contains this area.

Chapter II.1: Examples of manufacture by rapid diffusion of copper:

Example 1: Rapid diffusion in a tunnel kiln

This first example of the process for manufacturing yarn 2 is described with reference to the flowchart of the . In this first example, the threshold S ₁₆ is chosen equal to 66.4 atomic % and the layer 18 is omitted.

In a step 80, a metal blank wire is first provided. In this example, the draft wire is a 1.25mm diameter copper wire.

Then, during a step 82, a coating is made on the roughing wire. This coating continuously covers the entire outer face of the roughing wire. This coating is made in a material or in several materials having the capacity to form the layer 16 surmounted by a layer even richer in zinc when its temperature is between 500°C and 700°C. In this example, the coating is only formed, at this stage, by a zinc layer directly deposited on the outer face of the blank wire. For this, the zinc layer is deposited on the rough wire by an electrolytic zinc plating process to obtain an electrogalvanized wire with a diameter greater than 1.25 mm.

Here, at the end of step 82, this electrogalvanized wire is drawn until its diameter is equal to 420 µm. At this stage, in this first embodiment, the thickness of the zinc coating is equal to 25 μm in order to obtain a thick layer 16 in which it is easier to measure the zinc concentration.

During a step 84, the electrogalvanized and drawn wire is heated to a temperature equal to T _c . The temperature T _c is between 500°C and 700°C. In this first manufacturing example, the temperature T _c is between 500°C and 600°C and even more advantageously between 559°C and 600°C. Choosing a temperature T _c of less than or equal to 600° C. makes it possible to limit the formation of drops of molten zinc during heating. Here, the temperature T _c is equal to 600°C.

For example, during step 84, the electrogalvanized and drawn wire is introduced into a heating zone whose internal temperature is equal to T _c . Furthermore, this heat treatment is preferably carried out in air under atmospheric pressure to oxidize the outer face of the electrode wire.

Here, the electrogalvanized wire is considered to be formed from a succession of contiguous portions of electrogalvanized wire placed one after the other along the axis 4, each of these portions being very short. For example, for the explanation given here, a short portion is a portion whose length is equal to 0.1 mm. The successive portions of the electrogalvanized wire return one after the other to the heating zone so that these successive portions are heated to the temperature T _c one after the other. More precisely, the heating zone comprises an inlet and an outlet between which the temperature is equal to the temperature T _c . Before entry and after exit, the temperature is two or three times lower than the temperature T _c . This heating zone is here a tunnel of a tunnel oven.

Each portion of the electrogalvanized wire enters the interior of the heating zone through the entrance, then moves inside the heating zone at a constant speed. Finally, this portion comes out of the heating zone via the outlet after having stayed inside the heating zone for a duration d ₀ . The duration d ₀ is equal to the time interval between:
- a time t _ini at which the portion of the electrogalvanized wire enters the interior of the heating zone, and
- The instant t ₀ at which this same portion of the electrogalvanized wire leaves the heating zone.

The duration d ₀ is adjusted by adjusting the speed of movement of the electrogalvanized wire inside the heating zone. All of the electrogalvanized wire passes through the heating zone so that at the end of step 84 the entire zinc coating has been heated to the temperature T _c .

As taught in application US5762726A, at temperature T _c , the copper gradually diffuses inside the zinc coating. Thus, at a given location of the initially zinc coating, the copper concentration gradually increases over time.

In addition, since the copper diffuses inside the coating from the copper blank wire to the outside of the wire, there is a copper concentration gradient in the thickness of the coating. The copper concentration inside the coating gradually decreases going from the roughing wire outwards. Conversely, the zinc concentration increases as one approaches the outer face of the wire. Because of this copper concentration gradient, during step 84, a superposition of several layers of copper-zinc alloy in different phases appears. In this superposition of copper-zinc alloy layers, the layers are ordered by increasing concentration of zinc as one approaches the outer face. The surface layer of copper-zinc alloy is therefore always the one with the highest zinc concentration.

Here, the objective of step 84 is to form the layer 16 of copper-zinc alloy in gamma phase which is less than 1 μm from the outer face 6 of the electrode wire and which also has a high zinc concentration. .

It has been observed that when the process described in application US5762726A is implemented, the zinc concentration of the copper-zinc alloy layer in gamma phase is close to its maximum value near a time t _opt where the layer copper-zinc alloy in the epsilon phase which overcomes it disappears. This is currently explained by the fact that the zinc concentration of the gamma phase tends towards the zinc concentration of the epsilon phase as long as this epsilon phase exists. On the other hand, once the epsilon phase has disappeared, the zinc concentration of the gamma phase tends to equilibrate with the zinc concentration of the beta-phase copper-zinc alloy layer which has appeared under the copper alloy layer. -zinc in gamma phase. Thus, once the copper-zinc alloy layer in the epsilon phase has disappeared, the zinc concentration of the copper-zinc alloy layer in the gamma phase decreases rapidly. The zinc concentration of the copper-zinc alloy layer in gamma phase is therefore optimal close to the instant t _opt where the copper-zinc alloy layer in epsilon phase disappears. It is emphasized that this teaching is absent from application US5762726A. Indeed, this document does not teach that it is advantageous to interrupt the heating of the electrogalvanized wire at time t _opt or at a time very close to time t _opt . On the contrary, application US5762726A encourages interrupting the heating well before time t _opt to obtain a surface layer of copper-zinc alloy in the epsilon phase or, on the contrary, well after time t _opt to obtain a surface layer of alloy copper-zinc in gamma phase.

Therefore, to exploit this finding, here, for each portion of the electrogalvanized wire, step 84 is interrupted at a time t ₀ between times t _0min and t _0max . The instant t _0min is the instant at which the thickness of the superficial layer of copper-zinc alloy in the epsilon phase which surmounts the layer of copper-zinc alloy in the gamma phase is equal to 1 μm. Indeed, when the thickness of the copper-zinc alloy layer in the epsilon phase becomes less than 1 μm, the zinc concentration in the copper-zinc alloy layer in the gamma phase is maximum or very close to the maximum. In addition, it is important that the thickness of the copper-zinc alloy layer in the epsilon phase be small in order to limit the deformation of the topography of the external face during electroerosion. Moreover, it was observed that the erosion efficiency of the copper-zinc alloy in the epsilon phase is lower than the erosion efficiency of the copper-zinc alloy in the gamma phase.

Time t _0max is located after time t _opt at which the surface layer of copper-zinc alloy in epsilon phase has disappeared. After time t _opt , it is the layer of copper-zinc alloy in gamma phase which then forms the surface layer of the electrode wire at this stage of manufacture. After time t _opt , the zinc concentration of the copper-zinc alloy layer in gamma phase decreases rapidly and falls below threshold S ₁₆ at time t _0max . Typically, the instant t _0max is such that the duration d _0max of the interval [t _ini ; t _0max ] is less than 1.2*d _opt or 1.1*d _opt , where the duration d _opt is equal to the duration of the interval [t _ini ; t _opt ].

Thus, by choosing the instant t ₀ included in the interval [t _0min ; t _0max ], for each portion of the electrogalvanized wire, step 84 is interrupted at a time when the zinc concentration of the copper-zinc alloy layer in gamma phase is greater than S ₁₆ while forming the surface layer of the electrode wire or by being only covered with a very thin surface layer even richer in zinc, that is to say here with a very thin layer of copper-zinc alloy in the epsilon phase.

Typically, the duration d ₀ , and therefore the instant t ₀ , is determined by successive experiments. Indeed, as indicated in chapter I, there are methods that allow:
1) to measure in a cross section of an electrode wire, the thicknesses of the layers of copper-zinc alloy in the gamma phase and in the epsilon phase, and
2) to determine the zinc concentration of the copper-zinc alloy layer in gamma phase.
Thus, the duration d ₀ can be determined by successively trying several possible values for this duration d ₀ .

Subsequently, time t ₀ is included in the interval [t _opt ; t _0max ] so layer 18 is omitted.

By way of illustration, it has been observed that, under these conditions, a duration d ₀ equal to 10 seconds makes it possible to obtain a surface layer of copper-zinc alloy in gamma phase whose zinc concentration is 67 atomic %. .

At the end of the duration d ₀ , the coating deposited on the copper blank wire consists of the layer 14 of copper-zinc alloy in the beta phase surmounted by the surface layer 16 of copper-zinc alloy in the gamma phase.

At this point, it is pointed out that such rapid heating of the electrogalvanized wire cannot be obtained, for example, by placing a coil, of several thousand turns, of the electrogalvanized wire inside a heated conventional static furnace at the temperature T _c . A static oven is an oven inside which the electrogalvanized wire is immobile throughout the duration of the heating. Indeed, in such a case, the heating rate of the turns of the electrode wire which are mechanically isolated from the air heated to the temperature T _c by other turns superimposed on them, is much slower than for the turns directly in contact with the heated air. Thus, by placing a coil in air heated to the temperature T _c for 10 seconds, only a small part of the electrogalvanized wire is heated rapidly while another part of the electrogalvanized wire is heated much more slowly. Consequently, only part of the electrogalvanized wire comprises a layer of copper-zinc alloy in gamma phase having the characteristics of layer 16, whereas a large part of the electrogalvanized wire, which has undergone slower heating, does not comprise such a layer of copper-zinc alloy in gamma phase with a high concentration of zinc. In other words, with a conventional static furnace, it is not possible to precisely control the duration d ₀ for each portion of the electrogalvanized wire.

For each portion of the electrogalvanized wire, as soon as the instant t ₀ is reached, that is to say here from the end of the duration d ₀ , a rapid cooling step 90 is executed. The purpose of the cooling step 90 is to freeze the composition of the layer 16 obtained at time t ₀ and therefore to bring it into a metastable state at room temperature. For this, immediately after time t ₀ , during step 90, each portion of the wire is subjected to rapid cooling for a period d ₁ which suddenly causes the temperature of layer 16 of this portion to drop to 30° C. in less than ten seconds.

This cooling is qualified as rapid because the duration d ₁ is less than 10 seconds. Preferably, the duration d ₁ is less than 1 second or 0.5 second. To obtain such a short duration d ₁ , the cooling rate during step 90 is high. In this first example, the duration d ₁ is less than or equal to 1 second. The average cooling rate during the duration d ₁ is therefore greater than or equal to (T _c -30)°/s. Therefore, in this first manufacturing example, the average cooling rate is greater than 570° C./s.

Here, step 90 is successively applied to each of the portions of wire which come out of the heating zone so that each portion of the wire undergoes this rapid cooling. This therefore makes it possible to freeze the zinc concentration of layer 16 over the entire length of electrode wire 2.

For this, as soon as it leaves the heating zone, the portion of the wire is immersed in a fluid at ambient temperature. For example, here, a 25°C liquid bath is installed at the outlet of the heating zone. For example, the liquid is water. In this case, each portion of the wire traverses, between the exit from the heating zone and the entry into the bath, a first section of trajectory during which this portion is first immersed in air at ambient temperature. Then, this portion of the wire enters the bath at room temperature and travels through a second section in this bath during which it is in direct contact with the liquid at room temperature. At the end of the second section, the wire portion emerges from the bath. The lengths of the first and second sections are adapted so that the temperature of each portion of the wire falls below 30° C. less than 10 seconds after time t ₀ . Here, each portion of wire travels the first section in less than one second. The average cooling rate in water at ambient temperature of a portion of the wire is approximately 20,000° C./s. Under these conditions, in this first manufacturing example, the duration d ₁ is less than or equal to 1 second.

At this stage, it is emphasized that such rapid cooling cannot be obtained, for example, by immersing a coil, of several thousand turns, of electrode wire heated to the temperature T _c in a bath, even liquid, at ambient temperature. Indeed, in such a case, for reasons similar to those explained in the case of rapid heating, the cooling rate of the turns of the electrode wire which are mechanically isolated from the liquid by other turns superimposed on them, is much slower than for turns directly in contact with the liquid. In other words, by immersing a coil of turns in a liquid bath, it is not possible to precisely control the duration d ₁ of cooling of each portion of the electrode wire.

At the end of step 90, layer 16 is in a metastable state and its zinc concentration, at ambient temperature, is greater than threshold S ₁₆ as long as the wire is stored at ambient temperature.

Then, during a step 94, the wire obtained at the end of step 90 is drawn to obtain the electrode wire 2. This drawing step 94 makes it possible to bring the diameter of the electrode wire to the desired diameter, it that is to say here at a diameter of 250 μm. Stage 94 fractures layers 16 and 18. Thus, it is during this stage 94 that most of the fractures located in layers 16 and 18 are created.

There is an image of a portion of the cross-section of wire electrode 2 obtained at the end of step 90 and before step 94 of drawing. This image was obtained using an optical microscope. The composition of layer 16 was measured with an energy dispersion spectrometric (EDS) analysis probe. The zinc concentration in layer 16 is 67 atomic %.

There is an image of a portion of the electrode wire 2 obtained at the end of step 94 of drawing. This image was also obtained using an optical microscope. Layer 16 is fractured. It is therefore divided into a multitude of zones separated from each other by fractures. This image shows that some of the copper-zinc alloy zones in the gamma phase can end up separated from the outer face 6 by another copper-zinc alloy zone in the gamma phase. In this case, this zone is more than 1 μm from the outer face 6. However, the majority and, typically, more than 70% or 80% of the copper-zinc alloy zones in the phase range are less than 1 µm and, here, less than 0.5 µm from the outer face 6.

This image also shows that some of these areas may be covered with remnant 100 of layer 18.

Example 2: Heating by Joule effect

The second manufacturing example is identical to the first example except that:
- The zinc thickness is 18 µm.
- the heating is carried out by Joule effect, and
- the duration d ₀ is chosen so that the instant t ₀ is between t _0min and t _opt so that the layer 18 is present in the manufactured electrode wire.

In the case of heating by Joule effect, the heating zone is a segment of the electrogalvanized wire located between a first and a second conductive pulleys biased by a direct current electric generator. The difference in potential between these two pulleys generates a direct current which circulates in the segment of electrogalvanized wire located between these two pulleys. The segment of the electro-galvanized wire located between the two pulleys is then heated by the Joule effect. Typically, the intensity of direct current is greater than 10 A.

The various parameters of the heating by Joule effect are adjusted so that the instant t ₀ , when each portion of the electrogalvanized wire leaves this heating zone, is included in the interval [t _0min ; t _0max ] previously defined. The adjustable parameters of a heating by Joule effect are in particular the intensity of the direct current, the potential difference between the two pulleys, the running speed of the electrogalvanized wire and the length of the heating zone of the electrogalvanized wire included between the two pulleys. For example, here, the length of the heating zone before the electrogalvanized wire enters directly into a water bath at 25° C. is taken as 1530 mm. The running speed of the electrogalvanized wire in the heating zone is 4.59 m/min. The intensity of the current flowing in the electrogalvanized wire between the two conductive pulleys is 17.9 A. Under these conditions, the duration d ₀ is equal to 20 seconds. In this embodiment, each portion of the electrogalvanized wire can enter the bath before reaching the second pulley so that there is no soaking in the air before soaking in the bath.

The yarn obtained after step 90 was analyzed. In some places around the wire, layer 16 is covered with a layer of copper-zinc alloy even richer in zinc, while in other places, on the contrary, layer 16 is the only metal alloy present at the wire surface. In both cases, there is also zinc oxide on the surface of the wire.

In the zones covered with the layer even richer in zinc, the layer 16 has a zinc concentration essentially comprised between 65.4% and 69.4% at atomic. In areas where layer 16 is not overlain by the even richer layer of zinc, the concentration of layer 16 in zinc is essentially between 65.4% and 66.3at.

It appears that the layer even richer in zinc, where it exists, is a copper-zinc alloy in the delta phase.

Example 3: Diffusion temperature equal to 700°C

The following experiments were carried out to show that the interval [t _0min ; t _0max ] is very small when the temperature T _c is greater than or equal to 400°C. This shows that it is not possible to succeed in manufacturing a copper-zinc alloy layer having a high zinc concentration by implementing only the teaching of application US5762726A.

A copper wire 420 µm in diameter and 100 mm in length, coated with 18 µm of zinc was placed in a static oven at a temperature of 700°C, for a certain period of time, then it was quickly, in less of a second, extracted from the oven, and soaked in water at room temperature.

A residence time d ₀ in the static furnace equal to 6 seconds makes it possible to obtain a layer of copper-zinc alloy in gamma phase whose zinc concentration is 68 atomic % surmounted by a layer 18 of copper-zinc alloy. zinc in the epsilon phase whose thickness is less than 0.5 μm. Under identical conditions but for a residence time in the static furnace equal to 7 seconds, the electrode wire obtained does not include the surface layer 18 of copper-zinc alloy in the epsilon phase and the zinc concentration of the surface layer of copper alloy -zinc in the gamma phase is only 63 atomic%.

This observation was also confirmed by implementing the method of example 1 but in the case where:
- the draft wire is a brass wire in which the zinc concentration is 37 atomic%,
- during step 82, the wire is drawn to a diameter of 460 μm and, after drawing, the thickness of the zinc coating is only 5 μm, and
- the duration d ₀ equal to 9 seconds.
Under these conditions, the zinc concentration of the surface layer 16 obtained is equal to 63 atomic % and therefore much less than 65.4 atomic %. Indeed, since the thickness of the zinc coating is reduced, the instant t _opt at which the layer of copper-zinc alloy in the epsilon phase disappears occurs earlier. Consequently, by interrupting step 84 after 9 seconds, step 90 of rapid cooling is triggered too late and after time t _0max .

Chapter II.2: Example of manufacture by slow diffusion of copper:

Example 4:

The manufacturing process is identical to the manufacturing process of Example 1 except that during step 84:
-Zinc thickness is 18 µm
- a static oven is used,
- the temperature T _c is equal to 250°C,
- the duration d ₀ is equal to 65 minutes, and
- during step 90 the cooling is not rapid.

In this embodiment, given that the temperature T _c is low, that is to say typically less than 300° C., the duration d ₀ is long enough for the heat treatment applied to the electrogalvanized wire to be practically the same in all portions of this electrogalvanized wire, even if the heated electrogalvanized wire is in the form of a coil comprising thousands of turns.

During the cooling step 90, at the exit from the heating zone, the wire is only immersed in ambient air at 25° C. without it being necessary to carry out rapid cooling. Indeed, tests have shown that such rapid cooling of the wire after it leaves the static oven is not necessary when the temperature T _c is low. In other words, the zinc concentration of layer 16 is the same in the case of rapid cooling and in the absence of such rapid cooling.

At the end of step 94, the yarn obtained comprises a layer 18 less than 0.5 μm thick. This layer 18 is made of copper-zinc alloy in the epsilon phase. Layer 16 has a zinc concentration of between 65.3% and 68.3 atomic%. The copper-zinc alloy in gamma phase of this layer 16 seems less ductile than a layer of similar composition, but obtained at 600°C.

CHAPTER II.3: Manufacture by electrodeposition

Example 5:

Manufacturing by electrodeposition consists in depositing, by electrodeposition in the aqueous phase, the layer 16 directly without carrying out a copper diffusion step.

For this, the blank wire constitutes the cathode, and an anode is used comprising a mixture of copper and zinc in which the zinc concentration is greater than 65.4% or 77% at atomic. For example, for the tests described here, the anode is formed from a mixture of copper balls and zinc plates. The electrolysis bath is adapted to deposit a layer of gamma-phase copper-zinc alloy with a high zinc concentration on the blank wire.

In this example, the bath is an "Oplinger" type bath with a volume of 200 liters containing:
- water as a solvent,
- 12 kg of sodium hydroxide (pearls) NaOH, i.e. 60 g/l,
- 12 kg of sodium cyanide NaCN, i.e. 60 g/l,
- 3.4 kg of CuCN copper cyanide, i.e. 17 g/l,
- 12 kg of zinc cyanide Zn(CN) ₂ , i.e. 60 g/l, and
- 80 g of sodium sulphite Na ₂ SO ₃ i.e. 0.4 g/l.

The bath temperature is 45°C. The current density is 20 Amps per square decimeter (20 A/dm²). Faradaic efficiency is about 56%. A brass blank wire 0.51 mm in diameter was thus coated with a layer 16 7 μm thick.

By EDS analysis of this layer 16, the measured zinc concentration is 66.4 atomic %.

The wire coated with this electrodeposited gamma phase is then drawn to obtain a diameter of 0.25 mm.

The advantage of electroplating a copper-zinc alloy is that its composition is constant through the thickness of the coating, unlike diffusion of zinc onto a copper or brass substrate, which exhibits a composition gradient. Thus, the electrode wire manufactured according to this example 5 comprises neither layer 14 nor layer 18 and only layer 16.

Example 6:

The manufacturing process of Example 6 is identical to that of Example 5 except that the bath is modified in order to increase the zinc concentration of layer 16. For this, the bath having the following characteristics is used at the instead of the bath in example 5:
- water as a solvent,
- the concentration of sodium hydroxide (NaOH) is 90 g/l,
- the concentration of sodium cyanide (NaCN) is 60 g/l,
- the concentration of copper cyanide (CuCn) is 17 g/l,
- the concentration of zinc cyanide (Zn(CN) ₂ ) is 90 g/l, and
- the concentration of sodium sulphite (Na ₂ SO ₃ ) is 0.6 g/l.

The zinc concentration, measured by EDS analysis, in layer 16 is 82.3 atomic % under the same electrodeposition conditions.

By drawing this wire to a diameter of 0.25 mm, the fracturing of the coating was observed, which is characteristic of a copper-zinc alloy in the gamma phase and not of a copper-zinc alloy in the epsilon phase.

If, however, the coating obtained using the above bath should still have a residue of copper-zinc alloy in the epsilon phase, this residue can be reduced or eliminated by reducing the NaOH and Zn(CN) ₂ concentration. For example, the NaOH and Zn(CN) ₂ concentrations are then between 60 g/l and 90 g/l.

CHAPTER II.4: Performance

The erosive efficiencies of different 0.25 mm diameter wires are compared. Each test was carried out under the following conditions:
- the spark erosion machine used is the machine manufactured by the company GFMS (GF Machining Solutions) whose reference is "CUT 200MS",
- the cut piece is a block of steel 50 mm high,
- the injection nozzles are detached from the part by approximately 5 mm at the top and bottom,
- the machining parameters are those of a standard bare brass wire with a diameter of 0.25 mm and a breaking strength of 900 N/mm², under conditions of plated injection nozzles.

In order not to break the wires (the nozzles are effectively unstuck) and to clearly highlight the effect of layer 16, to measure the erosive yields, the sparking frequency has been lowered to an interval of 25 µs between two sparks, which corresponds to an average intensity of approximately 6.5 A. The frequency was then increased, by reducing the time interval between two sparks, to measure the maximum cutting speed of the tested wires, before they broke.

The electrode wires compared are as follows:
- Wire A: this is the wire marketed under the "Thermo SA" brand by the company Thermocompact® and described in application EP1949995: it is a wire whose core is made of brass (64% copper and 36% zinc by mass) and which comprises a superficial layer of copper-zinc alloy in gamma phase fractured into blocks. The zinc concentration of these blocks in gamma phase is 61.3 atomic %.
- Wire B: this is the wire manufactured according to example 5 previously described, the zinc concentration of the surface layer 16 of which is 66.4 atomic %.
- Wire C: this is the wire manufactured according to Example 6 previously described, the zinc concentration of layer 16 of which is 82.3 atomic %. Thread Erosion yield (mm²/min/A) Maximum erosion rate (mm²/min) Wire A 8.0 99.2 Wire B 8.2 99.2 C-wire 7.6 104.3

Wire B has a better erosive performance than wire A. Wire C withstands higher machining intensities before breaking. It therefore makes it possible to reach a higher maximum machining speed than other wires.

It is emphasized that wire A has an oxide layer about 50 nm thick, which is favorable to its erosive performance. Wires B and C have a much thinner oxide layer. It is estimated that if the manufacturing conditions of wire B are modified to obtain an oxide thickness of the order of 50 nm in thickness, then its erosive efficiency will be even higher.

CHAPTER III: Variants

Variants of the electrode wire:

The core of the electrode wire is not necessarily made of copper or an alloy containing copper such as, for example, brass. For example, the core can also be made of steel or another electrically conductive metal. If the core does not contain copper, layer 16 is obtained by electrodeposition.

The core is not necessarily made of a single metal or a single metal alloy. Alternatively, the core comprises several layers each made of a respective metal or metal alloy. For example, the core has a central body of copper or steel coated with a layer of brass. This brass layer may be a beta-phase copper-zinc alloy layer.

Layer 14 can be omitted. This is particularly the case if the layer 16 is produced by electrodeposition.

Alternatively, layer 16 is uniform and unfractured. Layer 16 is therefore formed from a single zone which extends continuously over the entire circumference of core 10. For example, to manufacture this variant, during step 82, the electrogalvanized wire is drawn to directly obtain the desired final diameter and step 94 of drawing is omitted. The other steps in the process of remain, for example, unchanged.

Alternatively, layer 18 is a delta-phase or eta-phase copper-zinc alloy or zinc.

Alternatively, layer 18 is absent and layer 16 then forms the surface layer of electrode wire.

Variants of the manufacturing process:

Many other methods of manufacturing the yarn 2 are possible. For example, the manufacturing processes by fast or slow diffusion of copper, described in chapter II, can be implemented with a blank wire which is not necessarily made entirely of copper. For example, as a variant, the blank wire comprises only a surface layer whose copper concentration is greater than 50% or 60 atomic% and less than 95% or 90 atomic%. Similarly, it can also be implemented with a coating whose zinc concentration is less than 100 atomic %. However, preferably, the zinc concentration of the coating is high, that is to say greater than 95 atomic % or 98 atomic %.

The temperature T _c for implementing the manufacturing process by slow copper diffusion can be chosen between 150°C and 500°C. The duration d ₀ must then be adapted according to the temperature T _c chosen. However, the temperature T _c is preferably chosen below 300° C. or 250° C. or 200° C. in order to be able to use a static oven and avoid having to carry out rapid cooling. If the temperature T _c exceeds 300° C. or 400° C., a tunnel oven is used instead of the static oven to better control the durations d ₀ and d ₁ . In the latter case, the manufacturing process by slow copper diffusion is identical to the process of Example 1 except that the temperature T _c is between 150° C. and 500° C. and not between 500° C. and 700° C. .

Step 94 of drawing can be omitted.

CHAPTER IV: Advantages of the embodiments described:

A wire electrode in which the gamma-phase copper-zinc alloy areas are within 1 µm of the outer face of the wire electrode while having a high zinc concentration has at least one of the following advantages over the wire electrode of application US5945010A:
- the erosive yield is improved, and/or
- the maximum erosion speed is improved.

Currently, the fact that the electrode wire described in this text exhibits improved performance is explained as follows. Since layer 16, which is in gamma phase, has a high zinc concentration, its melting temperature is high and its sublimation temperature is low. These are two characteristics known to improve the performance of an electrode wire. In addition, the fact that the thickness of the surface layer even richer in zinc is zero or less than 1 μm also improves performance. Indeed, the melting temperature of layer 18 is lower than the melting temperature of layer 16. Thus, during spark erosion, this layer 18 melts before the copper-zinc alloy in gamma phase. By reducing the thickness or by eliminating this surface layer 18, the quantity of liquid which appears on the surface of the electrode wire during the electroerosion process is greatly reduced. Therefore, for example, craters resulting from EDM sparks have fewer re-solidified areas. The electrode wire also loses less material during the spark. Furthermore, there are fewer fractures or pores which are obscured by the flow of molten metal. Thus, the topography of the surface of the electrode wire is better preserved. It is therefore possible:
- to reduce the running speed of the electrode wire, and therefore the consumption of electrode wire, while maintaining a good machining speed, or
- to increase the machining speed while keeping the scrolling speed unchanged.

It is emphasized here that, in application US5945010A, the copper-zinc alloy layer in gamma phase of specimen No. 4 has a zinc concentration lower than that of specimen No. 3. However, application US5945010A teaches that it is specimen No. 4 which exhibits the best performance. Moreover, application US5945010A does not teach how to further increase the zinc concentration of the surface layer of specimen No. 4. In particular, this application does not teach that it is necessary to quickly interrupt the heating of the electrode wire close to the instant t _opt .

Having each gamma-phase copper-zinc alloy area directly flush with the exterior face further reduces the amount of liquid that appears during EDM. This further improves the performance of the electrode wire.

Further increasing the zinc concentration in each gamma-phase copper-zinc alloy zone and, in particular, exceeding a zinc concentration of 68.4 atomic %, further improves the performance of the wire. electrode.

The fact that the copper-zinc alloy layer is fractured makes it possible to increase the erosive efficiency of the electrode wire.

The diffusion heat treatment at more than 500° C. makes it possible to obtain an electrode wire additionally comprising layer 14 under layer 16, which is advantageous.

Conversely, the fact of manufacturing the electrode wire by slow diffusion of copper does not make it possible to obtain such a thick layer 14 under layer 16. On the other hand, slow diffusion makes it possible to obtain a layer 16 whose thickness is more regular.

The fact of rapidly cooling layer 16 makes it possible to limit or prevent its zinc concentration from decreasing during step 90.

The deposition of the layer 16 by electrodeposition makes it possible to obtain a layer 16 whose zinc concentration is greater than 66% or 70% at atomic. In addition, the thickness of layer 16 is more regular.

Claims (14)

Electrode wire suitable for use as an electrode wire for electroerosion machining, this electrode wire comprising:
- a metal core (10) which extends along a longitudinal axis, and
- on the metal core, a coating comprising one or more zones (30-32) of copper-zinc alloy in gamma phase, each of these zones being formed solely of copper-zinc alloy in gamma phase and the zinc concentration , at an ambient temperature of 25° C., inside each of these zones of copper-zinc alloys in gamma phase, being greater than 65.4 atomic %,
characterized in that the majority of the zones (30-32) of copper-zinc alloy in the gamma phase are located less than 1 µm from the outer face of the electrode wire. A wire electrode according to claim 1, in which, at room temperature, the majority of the zones (30-32) of gamma-phase copper-zinc alloy are directly flush with the exterior surface of the electrode wire. A wire electrode according to any preceding claim, wherein at room temperature within each zone (30-32) of gamma-phase copper-zinc alloy the zinc concentration is greater than or equal to 68.4 atomic %. Wire electrode according to any one of Claims 1 and 3, in which, at ambient temperature, the coating comprises successively, going from the outside towards the metal core of the wire electrode:
- a first surface layer (18) whose zinc concentration is greater than 72 atomic %, the thickness of this surface layer being less than 1 μm or 0.5 μm, and immediately under this surface layer
- a second layer (16) which each contains areas (30-32) of copper-zinc alloy in gamma phase. A wire electrode according to claim 4, wherein the surface layer (18) is an epsilon phase copper-zinc alloy. A wire electrode according to claim 4 or 5, wherein the coating comprises, immediately below the second layer (16), a homogeneous third layer (14) of copper-zinc alloy formed solely from beta-phase copper-zinc alloy. Wire electrode according to any one of the preceding claims, in which the wire electrode has fractures (22-24) which, in a cross-section of the wire electrode, mechanically separate the different zones of copper-zinc alloy in the gamma phase. Wire electrode according to any of the preceding claims, in which the majority of the zones (30-32) of gamma-phase copper-zinc alloy comprise a cross-section, perpendicular to the longitudinal axis, the length of which is greater than 5 µm and whose width is greater than 4 µm, the length and the width of a cross-section of a gamma-phase copper-zinc alloy zone being equal, respectively, to the width and the length of the rectangle of smaller surface that entirely contains this cross-section. Process for the manufacture of an electrode wire in accordance with any one of the preceding claims, characterized in that this process comprises the production, on a metal core, of a layer of copper-zinc alloy in gamma phase which, at a ambient temperature of 25°C:
- is located less than 1 µm from the outer face of the electrode wire, and
- in which the zinc concentration is greater than a threshold S ₁₆ , this threshold S ₁₆ being greater than or equal to 65.4 atomic %. Process according to Claim 9, in which the production of the layer of copper-zinc alloy in the gamma phase comprises the following steps:
a) the production (82), on a metal blank wire comprising a surface layer whose copper concentration is greater than 50% or 60% at atomic, of a coating having the capacity to form a layer of copper alloy -zinc in gamma phase by diffusion of copper from the surface layer of the blank wire in this coating when it is heated to more than 150°C, then
b) heating (84), one after the other, each successive portion of the blank yarn on which the coating is made, to a temperature T _c greater than 150° C. so that part of the coating, in the heated portion , is transformed into a layer of copper-zinc alloy in gamma phase surmounted by a surface layer even richer in zinc, for this each portion of the blank wire entering a heating zone at a time t _ini and leaving this heating zone at a time t ₀ , the running speed of the rough wire inside the heating zone being constant and determined so that time t ₀ corresponds to a time when:
- the thickness of the superficial layer richer in zinc on this portion of the blank wire which comes out of the heating zone is less than 1 μm or has disappeared, and
- the zinc concentration of the layer of copper-zinc alloy in gamma phase of this portion of the blank wire which comes out of the heating zone is still greater than 65.4 atomic%, then
c) from time t ₀ , cooling (90) the temperature of the copper-zinc alloy layer in gamma phase of the portion of the blank wire which leaves the heating zone at this time t ₀ , to cause its temperature to drop to 30° C. in less than ten seconds, this cooling step being applied to each of the portions of the preform yarn which comes out of the heating zone. Process according to Claim 10, in which, during stage b), each portion of the coating is brought to a temperature T _c of between 500°C and 700°C. Process according to any one of Claims 9 to 11, in which the production of the coating comprises the production, directly on a surface layer of the blank wire in which the concentration of copper is greater than 50% or 60 atomic %, of a layer whose zinc concentration is greater than 98 atomic %. Process according to Claim 9, in which the production of the layer of copper-zinc alloy in the gamma phase comprises the following steps:
a) the production (82), on a metal blank wire comprising a surface layer whose copper concentration is greater than 50% or 60% at atomic, of a coating having the capacity to form a layer of copper alloy -zinc in gamma phase by diffusion of copper from the surface layer of the blank wire in this coating when it is heated to more than 150°C, then
b) heating, in a static furnace, the blank wire on which the coating is made, to a temperature T _c of between 150°C and 300°C so that part of the coating is transformed into a layer of copper alloy -zinc in gamma phase surmounted by a superficial layer even richer in zinc, the thickness of this superficial layer even richer in zinc then gradually decreasing as the heating continues, then
c) interrupting step b) at a time t ₀ and allowing the temperature of the electrode wire to drop below 30° C., time t ₀ corresponding to a time when:
- the thickness of the surface layer richer in zinc on the roughing wire is less than 1 µm or has disappeared, and
- the zinc concentration of the layer of copper-zinc alloy in gamma phase on the blank wire is still greater than 65.4 atomic %. Process according to Claim 9, in which the production of the layer of copper-zinc alloy comprises the deposition by electrodeposition, on the metal core, of a layer of copper-zinc alloy in gamma phase in which the concentration of zinc is greater than or equal to 65.4 atomic %.