ELECTRODE WIRE

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 of International Patent Application PCT/EP2022/077239, filed Sep. 29, 2022, designating the United States of America and published as International Patent Publication WO 2023/088601 A1 on May 25, 2023, which claims benefit, under Article 8 of the Patent Cooperation Treaty, of French Patent Application Serial No. FR2112098, filed Nov. 16, 2021.

TECHNICAL FIELD

This disclosure relates to an electrode wire capable of being used as an electrode wire for electro-erosion machining. The disclosure also relates to a method for manufacturing this electrode wire.

BACKGROUND

Electrode wires may be used to cut metals or electrically conductive materials, by electro-erosion in an electro-erosion machining machine.

The well-known method of electro-erosion machining, or spark erosion, allows material to be removed from an electrically conductive part by generating sparks in a machining zone between the workpiece and an electrically conductive electrode wire. The electrode wire continuously unwinds in the vicinity of the part along the length of the wire, which is held by guides, and it is gradually moved toward the part in the transverse direction, either by a transverse translation movement of the wire guides or by translationally moving the part.

An electrical generator, connected to the electrode wire by electrical contacts at a distance from the machining zone, establishes an appropriate potential difference between the electrode wire and the conductive part to be machined. The machining zone between the electrode wire and the part is immersed in a suitable dielectric fluid. The potential difference causes, between the electrode wire and the workpiece, sparks to appear that gradually erode the part and the electrode wire. The longitudinal unwinding of the electrode wire allows a sufficient wire diameter to be constantly maintained in order to prevent it from breaking in the machining zone. The relative movement of the wire and of the part in the transverse direction allows the part to be cut or its surface to be treated, if applicable.

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

Obtaining precision machining, notably producing small-radius corner cuts, requires the use of wires with a small diameter and that are able to withstand a significant mechanical tensile strength in order to be tensioned in the machining zone and limit the amplitude of any vibrations.

Most modern electro-erosion machining machines are designed to use metal wires, generally with a diameter of 0.25 mm, with a tensile strength ranging between 400 N/mm² and 1,000 N/mm².

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 duration. As a result, the material of the surface layer of the electrode wire, at the site of the spark, transitions from the solid state to the liquid or gaseous state, and is moved to the surface of the electrode wire and/or is discharged into the dielectric fluid. It can be seen that the outer face of the electrode wire reached by the spark has been deformed, generally assuming a slightly concave, crater shape, with zones where the material has been melted and solidified again.

It has been observed that the efficiency of the sparks with respect to electro-erosion mainly depends on the nature and on the topography of the surface layer of the electrode wire. To this end, considerable progress in electro-erosion efficiency has been achieved by using electrode wires comprising:

• • a core made of one or more metals or alloys ensuring good conduction of the electrical current and good mechanical strength in order to hold the mechanical tensile strength of the wire; and
  • a coating made of one or more other metals or alloys and/or a particular topography, for example fractures, ensuring better efficiency of the electro-erosion, for example a higher erosion speed.

For example, U.S. Pat. No. 5,945,010 A describes an electrode wire having a brass core covered with a layer of copper-zinc gamma phase alloy optionally overlaid by a layer of copper-zinc epsilon phase alloy. This disclosure teaches that such a layer of copper-zinc gamma phase alloy allows the performance capabilities of the electrode wire to be improved. In particular, this disclosure describes an electrode wire specimen No. 3 comprising a layer of copper-zinc gamma phase alloy overlaid by a surface layer of copper-zinc epsilon phase alloy. The surface layer is 3 μm thick. The zinc concentration of the layer of copper-zinc gamma phase alloy is 68 atomic %. This disclosure also describes an electrode wire specimen No. 4 comprising a surface layer of copper-zinc gamma phase alloy. In this case, the zinc concentration of the surface layer of copper-zinc gamma phase alloy is 65 atomic %.

The prior art is also known from documents US 2017/259361 A, US 2008/179296 A1 and US 2009/025959 A1.

The electrode wires of U.S. Pat. No. 5,945,010 A have high performance capabilities. However, it is still desirable for electrode wires with even better performance capabilities to be obtained and, notably, electrode wires exhibiting improved erosive yield and/or a higher machining speed.

BRIEF SUMMARY

This disclosure aims to meet the above-discussed requirement by proposing an electrode wire as claimed in independent claim(s) herein.

A further aim of the disclosure is a method for manufacturing the claimed electrode wire.

BRIEF DESCRIPTION OF THE DRAWINGS

This disclosure will be better understood upon reading the following description, which is provided solely by way of a non-limiting example and with reference to the drawings, in which:

FIG. 1 is a schematic illustration of the cross-section of an electrode wire;

FIG. 2 is a flowchart of a method for manufacturing the electrode wire of FIG. 1;

FIG. 3 is a black and white illustration of a portion of a cross-section of the wire manufactured according to the method of FIG. 2 before the drawing step; and

FIG. 4 is an image of a portion of a cross-section of the electrode wire manufactured according to the method of FIG. 2 after the drawing step.

DETAILED DESCRIPTION

Throughout the figures, the same reference signs are used to denote the same elements. Throughout the remainder of this description, the features and functions that are well known to a person skilled in the art are not described in detail.

The definitions of certain terms are provided in Chapter I hereafter. In Chapter II, detailed examples of embodiments are described with reference to the figures. Then, in Chapter III, alternative embodiments of these embodiments are introduced. Finally, in Chapter IV, the advantages of the various embodiments are introduced.

Chapter I: Definitions and Terminology

The expression “element produced from material A” or “element made of material A” denotes an element in which the material A represents at least 90% by weight of this element, and, preferably, at least 95% or 98% by weight of this element.

A “copper-zinc alloy” denotes an alloy solely formed by copper and zinc to the nearest unavoidable impurities.

A “phase” of the copper-zinc alloy denotes a solid phase of the copper-zinc alloy that exhibits a particular crystallographic structure. More specifically, the phases of the copper-zinc system are distinguished from one another in terms of their composition and their particular crystallographic structure. This particular crystallographic structure allows a phase of the copper-zinc alloy to be distinguished 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 the alpha phase, the beta phase, the gamma phase, the delta phase, the epsilon phase and the eta phase. The particular crystallographic structure of a phase can be identified using various means. For example, optical or metallography microphotographs of polished samples show shades of different colors for each phase, as long as the sample has been appropriately attacked. Thus, in order 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 grey when it is zinc-poor and greyed-out with brown shades when it is zinc-rich. The epsilon phase appears as a 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 this case, the wire sample is placed under an incident beam of X-rays with a precise wavelength. For example, the Kα ray of the copper, with an average wavelength of 0.1541 nm, is used. The intensity of the diffracted rays is assessed for each diffraction angle. The gamma phase has a known X-ray diffraction spectrum, and it is different from that of the other phases of the copper-zinc system, and from zinc oxide ZnO, which is often located on the surface of the wires. If the copper-zinc alloy is not crystallized in the form of at least one of the alpha, beta, gamma, delta, epsilon, or eta phases, it is amorphous, and the X-ray diffraction spectrum then exhibits flattened bumps rather than protruding peaks.

At a given temperature, the various phases of the copper-zinc alloy each correspond to a specific range of zinc concentration. The extent of each of these specific ranges of zinc concentration varies as a function of the temperature. The zinc concentration of a phase of a sample can be obtained by compositional microanalysis. A compositional microanalysis is carried out using a scanning electron microscope equipped with a spectrometry probe. An electron beam, accelerated, for example, in a 20 kV electric field, impacts the surface of the sample and causes X-rays to be emitted. These X-rays have an energy spectrum that is characteristic of the composition of the surface of the sample that has been impacted by the electron beam. The spectrum of the X-rays emitted by the surface of the sample is measured with an Energy-Dispersive Spectroscopy (EDS) or a Wavelength-Dispersive Spectroscopy (WDS) analysis probe. Algorithms allow the analyzed elements to be selected (and therefore the effect of any impurities to be eliminated), and the composition of the sample impacted by the electron beam to be computed based on the measured spectra. It should be noted that, due to the interactions between the X-rays and the material, the volume analyzed by EDS (or WDS) is generally approximately one cubic micrometer. At the boundary between two phases, an average concentration, which does not actually exist in any of the two phases, can be measured. The concentrations indicated in this case relate to pure phases in their analysis volume. The zones in which a concentration is measured are greater than micrometer-sized cubes.

The expression “the zinc concentration inside a zone of copper-zinc alloy is greater than X atomic %” means that the average zinc concentration inside this zone is greater than X atomic %. An average concentration is obtained, for example, by measuring the zinc concentration at different sites inside this zone and then by averaging these concentration measurements. The sites where the measurements are carried out are equally located at the sites where the concentration is likely to be the lowest, at the sites where the concentration is likely to be close to the average, and at the sites where the concentration is likely to be maximal. To this end, typically, the sites where the measurements are carried out are distributed along an axis passing through the axis of the electrode wire.

The expression “electrically conductive” denotes a material with electrical conductivity, at 20° C., that is greater than 106 S/m and, preferably, greater than 107 S/m.

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

The expression “cross-section” denotes a section of the electrode wire perpendicular to its longitudinal axis.

The expression “layer of the electrode wire” denotes an annular layer of the electrode wire that 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 an initial approximation, in this document, these limits are likened to circles. These circular limits are both centered on the longitudinal axis of the electrode wire. The inner circular limit is the limit of the layer that is closest to the longitudinal axis of the electrode wire. Conversely, the outer circular limit is the limit of the layer that is furthest 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 by an uneven enmeshment of different phases of the copper-zinc alloy. Conversely, at the inner and outer circular limits, the chemical composition and/or the crystallographic form change suddenly.

A “homogeneous” layer is a layer formed by a single phase of the copper-zinc alloy.

A “uniform” layer denotes a layer formed by a material that, in a cross-section of the wire, extends continuously or practically continuously around the axis of the wire and inside this layer. Thus, a uniform layer does not comprise a multitude of fractures that divide it into a multitude of zones separated from one another, in a cross-section of the wire, by these numerous radial fractures. “Numerous radial fractures” denotes more than approximately ten radial fractures that divide the layer in question into approximately ten zones that are mechanically isolated from one another, in the cross-section, by these radial fractures.

Conversely, the term “fractured layer” denotes a layer that comprises a multitude of fractures that divide it into a multitude of zones separated from each other, in a cross-section of the wire, by numerous radial fractures.

The expression “metal surface layer,” or simply “surface layer,” denotes the copper-zinc alloy or zinc layer of the electrode wire that is the outermost layer of the electrode wire. This metal surface layer can comprise a thin oxide film on its surface. Typically, this oxide film is mainly made up of zinc oxide, zinc hydroxides, zinc carbonate, as well as possible residues such as drawing lubricant residues. The outer face of this metal 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 mainly extends, within a cross-section of the electrode wire, in a radial direction.

The expression “ambient temperature” denotes a temperature ranging between 15° C. and 30° C. and, typically, that is equal to 25° C.

The “erosive yield” of an electrode wire is equal to the ratio of the surface area cut in one minute to the average intensity of the current passing through the electrode wire when cutting this surface area. For example, if the feed rate of the wire in the cut material is 2 mm/min in a 50 mm-high steel part, the machining speed is then 100 mm²/min. If the average machining current is 10 A, the erosive yield of the wire under these conditions is 10 mm²/min/A.

Chapter II: Examples of Embodiments

FIG. 1 shows an electrode wire 2 (also referred to herein as a “wire” 2) for electro-erosion machining as described in the introductory part of this document.

To this end, the electrode wire 2 has a tensile strength ranging between 400 N/mm² and 1,000 N/mm². The wire 2 extends along a longitudinal axis 4. The axis 4 in this case is perpendicular to the plane of the sheet. The length of the 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 when machining a part by electro-erosion using this wire. The outer face 6 is a cylindrical face that extends along the axis 4. The directrix of the outer face 6 is mainly a circle centered on the axis 4. Thus, the cross-section of the wire 2 is circular. The outer diameter D₂ of the wire 2 typically ranges between 50 μm and 1 mm and, most often, ranges between 70 μm and 400 μm. In this case, the diameter of the wire 2 is equal to 250 μm.

In this embodiment, the wire 2 comprises:

• • a central core 10 (also referred to herein as a “core” 10) made of electrically conductive material; and
  • a coating 12 directly deposited onto the core 10.

The function of the core 10 is to itself provide most of the tensile strength of the wire 2. A further function of the core is to provide the electrical conductivity of the wire 2. To this end, it is made of an electrically conductive material. Typically, it is made of metal or of metal alloy. For example, in this embodiment, the core 10 is made of copper.

The diameter D₁₀ of the core 10 ranges between 0.75 D₂ and 0.98 D₂ and, typically, between 0.85 D₂ and 0.95 D₂, where D₂ is the outer diameter of the electrode wire 2. For example, in this case, the diameter D₁₀ is equal to 230 μm.

The coating 12 is designed to increase the machining speed and therefore the erosive yield of the electrode wire and/or the quality of the faces of the part obtained after electro-erosion machining. The quality of a face cut by electro-erosion is even better when it exhibits low roughness.

The thickness of the coating 12 is low compared to the diameter D₂ of the wire 2, i.e., less than 10% of the diameter D₂ and, preferably, less than 8% of the diameter D₂. The thickness of the coating 12 corresponds to the shortest distance, in a cross-section, between the circular limit that separates the core 10 from the coating 12 and the outer face 6.

In this embodiment, the coating 12 is formed by:

• • two layers 14 and 16 successively and directly stacked on top of each other by proceeding from the core 10 toward the outer face 6; and
  • a possible surface layer 18 directly deposited onto the layer 16.

Hereafter, the structure of the electrode wire 2 will be described in the particular case whereby the layer 18 exists. The entire description provided for this particular case also applies to the case whereby the layer 18 does not exist. In this case, the surface layer of the electrode wire 2 is directly the layer 16.

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

The layer 16 is a homogeneous layer made of a copper-zinc gamma phase alloy. The zinc concentration of the layer 16 is high, i.e., in this case greater than or equal to a threshold S₁₆. This threshold S₁₆ is greater than or equal to 65.4 atomic % and, preferably, greater than 66.4 atomic % or 68.4 atomic % or even more than 70 atomic %, with the remainder being copper and unavoidable impurities. The zinc concentration of the layer 16 is generally less than 84 atomic % or 75 atomic %.

According to the recently updated equilibrium phase diagram of the copper-zinc system, in a stable state, the copper-zinc gamma phase alloy has a zinc concentration that ranges between 60 atomic % and 62 atomic % at ambient temperature, with the remainder being copper. A recently updated equilibrium phase diagram of the copper-zinc system has been published, for example, 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, pages 224 to 232.

Thus, with a zinc concentration that is greater than or equal to the threshold S₁₆, the copper-zinc gamma phase alloy of the layer 16 is not in a stable state at ambient temperature. In this case, it is in a metastable state. In a metastable state, the conversion of the copper-zinc gamma phase alloy to its stable state, and therefore the reduction of its zinc concentration, is very slow at ambient temperature. In other words, this transformation of the gamma phase to its stable state at ambient temperature is practically imperceptible by a human being. Thus, the composition of this gamma phase in its metastable state hardly varies from when it is manufactured until it reaches a machining zone of an electro-erosion machine when this wire 2 is stored and transported under normal conditions and therefore maintained at ambient temperature. Methods for manufacturing such a metastable layer of copper-zinc alloy are described hereafter.

The thickness of the layer 16 is greater than the thickness of the layer 18. In FIG. 1, by way of an illustration, the thickness of the layer 16 is also greater than the thickness of the layer 14. Advantageously, the thickness of the layer 16 is greater than 10% or 20% or 30% of the total thickness of the coating 12. To this end, the thickness of the layer 16 is typically greater than 1% or 2% of the diameter D₂. For example, the thickness of the layer 16 is greater than 2.5 μm or 5 μm or 10 μm. The thickness of the layer 16 is also quite low so that it can be fractured when drawing the electrode wire. To this end, for example, the layer 16 is less than 25 μm or 20 μm thick.

The layer 16 is located less than 1 μm and, preferably, less than 0.5 μm, from the outer face 6. In this case, it is separated from the outer face 6 solely by the layer 18. Thus, to this end, when it exists, the thickness of the layer 18 is less than 1 μm and, preferably, less than 0.5 μm.

The layer 18 is a metal surface layer that is even more zinc-rich than the layer 16. For example, the layer 18 is made of a copper-zinc alloy, the zinc concentration of which is greater than that of the layer 16. Typically, the difference between the zinc concentrations of the layers 16 and 18 is greater than 2 atomic % or 5 atomic % or 10 atomic %. Typically, the layer 18 is a layer of copper-zinc epsilon phase or delta phase or eta phase alloy or is made of zinc.

In this embodiment, the layers 16 and 18 are fractured. Thus, the layers 16 and 18 comprise fractures that divide each of these layers into several zones that are mechanically separated from one another, in a cross-section, by radial fractures. As described hereafter, 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 continuously extends entirely around the axis 4 but is divided into several zones of material, which, in a cross-section, are mechanically separated from one another by radial fractures. These fractures mainly extend radially and fully pass through the layers 16 and 18.

For example, a fracture starts at the circular limit between the layers 14 and 16 and emerges on the outer face 6.

FIG. 1 schematically shows three fractures 22 to 24. These three fractures 22 to 24 divide the layer 18 into three distinct zones 26 to 28 and divide the layer 16 into three distinct zones 30 to 32.

These fractures correspond to empty recesses or hollows of solid or liquid material. The width of a fracture, in a direction perpendicular to the radial direction along which it extends, is generally less than 2 μm. It should be highlighted in this case that, given the very low thickness of the layer 18, the copper-zinc alloy of the layer 18 does not penetrate into the cracks and also does not cover these cracks.

Each zone of the layer 16 is typically longer than the thickness of the layer 16. In this case, each zone of the layer 16 is longer than 5 μm or 10 μm. In this document, the length and the width of a zone, in a cross-section, are defined as being equal, respectively, to the length and the width of the rectangle with the smallest surface area that fully incorporates this zone.

Chapter II.1: Manufacturing Examples Using Rapid Copper Diffusion

Example 1: Rapid Diffusion in a Tunnel Furnace

This first example of a method for manufacturing the wire 2 is described with reference to the flowchart of FIG. 2. In this first example, the threshold S₁₆ is selected so as to be equal to 66.4 atomic % and the layer 18 is omitted.

During a step 80, a metal rough wire is initially provided. In this example, the rough wire is a copper wire with a diameter of 1.25 mm.

Then, during a step 82, a coating is produced on the rough wire. This coating continuously covers the entire outer face of the rough wire. This coating is made of a material or of several materials with the ability to form the layer 16 overlaid by a layer that is even more zinc-rich when its temperature ranges between 500° C. and 700° C. In this example, the coating is only formed, at this stage, by a zinc layer directly deposited onto the outer face of the rough wire. To this end, the zinc layer is deposited onto the rough wire by an electrolytic galvanization method for obtaining an electrogalvanized wire with a diameter of more than 1.25 mm.

In this case, 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 zinc coating is 25 μm thick 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 ranges between 500° C. and 700° C. In this first manufacturing example, the temperature T_c ranges between 500° C. and 600° C. and even more advantageously ranges between 559° C. and 600° C. Selecting a temperature T_c that is less than or equal to 600° C. limits the formation of molten zinc droplets during heating. In this case, 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, the internal temperature of which is equal to T_c. Moreover, preferably, this heat treatment is carried out in air under atmospheric pressure in order to oxidize the outer face of the electrode wire.

In this case, the electrogalvanized wire is considered to be formed by a series of contiguous portions of electrogalvanized wire placed one after the other along the axis 4, with each of these portions being very short. For example, for the explanation provided herein, a short portion is a portion that is 0.1 mm long. The successive portions of the electrogalvanized wire enter the heating zone one after the other so that these successive portions are heated to the temperature T_c one after the other. More specifically, the heating zone comprises an inlet and an outlet, between which the temperature is equal to the temperature T_c. Before the inlet and after the outlet, the temperature is two or three times lower than the temperature T_c. This heating zone in this case is a tunnel of a tunnel furnace.

Each portion of the electrogalvanized wire enters the heating zone through the inlet, then moves inside the heating zone at a constant speed. Finally, this portion emerges from the heating zone through the outlet after having remained inside the heating zone for a duration d₀. The duration d₀ is equal to the time interval that separates:

• • an instant t_ini at which the portion of the electrogalvanized wire enters the heating zone; and
  • the instant to at which this same portion of the electrogalvanized wire exits the heating zone.

The duration d₀ is adjusted by setting the speed of movement of the electrogalvanized wire inside the heating zone. The entire electrogalvanized wire passes through the heating zone so that, on completion of step 84, the entire zinc coating has been heated to the temperature T_c.

As taught in U.S. Pat. No. 5,762,726 A, at the temperature T_c, the copper gradually diffuses inside the zinc coating. Thus, at a given site of the coating that is initially made of zinc, the copper concentration gradually increases over time.

In addition, since the copper diffuses inside the coating by proceeding from the rough copper wire toward the outside of the wire, a copper concentration gradient exists in the thickness of the coating. The copper concentration within the coating gradually decreases proceeding from the rough wire outward. Conversely, the zinc concentration increases when approaching the outer face of the wire. Due to this copper concentration gradient, during step 84, an overlay appears of several layers of copper-zinc alloy in different phases. In this overlay of layers of copper-zinc alloy, the layers are ordered by increasing zinc concentration when approaching the outer face. The surface layer of copper-zinc alloy is therefore always that with the highest zinc concentration.

In this case, the aim of step 84 is to form the layer 16 of copper-zinc gamma phase alloy that is located less than 1 μm from the outer face 6 of the electrode wire and that also exhibits a high zinc concentration.

It has been noted that when the method described in U.S. Pat. No. 5,762,726 A is implemented, the zinc concentration of the layer of copper-zinc gamma phase alloy is close to its maximum value near an instant t_opt where the layer of copper-zinc epsilon phase alloy that overlays it disappears. This is actually explained by the fact that the zinc concentration of the gamma phase tends toward the zinc concentration of the epsilon phase as long as this epsilon phase exists. However, once the epsilon phase has disappeared, the zinc concentration of the gamma phase tends to balance with the zinc concentration of the layer of copper-zinc beta phase alloy that appeared under the layer of copper-zinc gamma phase alloy. Thus, once the layer of copper-zinc epsilon phase alloy has disappeared, the zinc concentration of the layer of copper-zinc gamma phase alloy decreases rapidly. The zinc concentration of the layer of copper-zinc gamma phase alloy is therefore optimal near the instant t_opt where the layer of copper-zinc epsilon phase alloy disappears. It should be noted that this teaching is absent from U.S. Pat. No. 5,762,726 A. Indeed, this document does not teach that it is advantageous to interrupt the heating of the electrogalvanized wire at the instant t_opt or at an instant very close to the instant t_opt. On the contrary, U.S. Pat. No. 5,762,726 A promotes interrupting the heating well before the instant t_opt in order to obtain a surface layer of copper-zinc epsilon phase alloy or, on the contrary, well after the instant t_opt in order to obtain a surface layer of copper-zinc gamma phase alloy.

Consequently, in order to take advantage of this observation, in this case, for each portion of the electrogalvanized wire, step 84 is interrupted at an instant to ranging between the instants t_0min and t_0max. The instant t_0min is the instant at which the thickness of the surface layer of copper-zinc epsilon phase alloy that overlays the layer of copper-zinc gamma phase alloy is equal to 1 μm. Indeed, when the thickness of the layer of copper-zinc epsilon phase alloy becomes less than 1 μm, the zinc concentration in the layer of copper-zinc gamma phase alloy is maximal or very close to the maximum. Moreover, it is important that the thickness of the layer of copper-zinc epsilon phase alloy is low in order to limit the deformation of the topography of the outer face during the electro-erosion. Moreover, it has been noted that the erosive yield of the copper-zinc epsilon phase alloy is lower than the erosive yield of the copper-zinc gamma phase alloy.

The instant t_0max occurs after the instant t_opt at which the surface layer of copper-zinc epsilon phase alloy has disappeared. After the instant t_opt, the layer of copper-zinc gamma phase alloy forms the surface layer of the electrode wire at this manufacturing stage. After the instant t_opt, the zinc concentration of the layer of copper-zinc gamma phase alloy decreases rapidly and drops below the threshold S₁₆ at the instant 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 selecting the instant to lying within the interval [t_0min; t_0max], for each portion of the electrogalvanized wire, step 84 is interrupted at an instant when the zinc concentration of the layer of copper-zinc gamma phase alloy is greater than S₁₆, while forming the surface layer of the electrode wire or by only being covered with a very thin surface layer that is even more zinc-rich, i.e., in this case, with a very thin layer of copper-zinc epsilon phase alloy.

Typically, the duration d₀, and therefore the instant to, is determined by successive experiments. Indeed, as indicated in Chapter I, methods exist that allow:

• • 1) the thicknesses of the layers of copper-zinc gamma phase and epsilon phase alloy to be measured in a cross-section of an electrode wire; and
  • 2) the zinc concentration of the layer of copper-zinc gamma phase alloy to be determined.

Thus, the duration d₀ can be determined by successively trying several possible values for this duration d₀.

Subsequently, the instant to lies within the interval [t_opt; t_0max] so that the layer 18 is omitted.

By way of an illustration, it has been noted that, under these conditions, a duration d₀ equal to 10 seconds allows a surface layer of copper-zinc gamma phase alloy to be obtained with a zinc concentration of 67 atomic %.

At the end of the duration d₀, the coating deposited onto the rough copper wire is made up of the layer 14 of copper-zinc beta phase alloy overlaid by the surface layer 16 of copper-zinc gamma phase alloy.

At this stage, it should be noted that such rapid heating of the electrogalvanized wire cannot be obtained, for example, by placing a coil, with several thousand turns, of the electrogalvanized wire inside a conventional static furnace heated to the temperature T_c. A static furnace is a furnace containing the electrogalvanized wire that is immobile throughout the heating period. Indeed, in such a case, the heating speed of the turns of the electrode wire that are mechanically isolated from the air heated to the temperature T_c by other turns overlaid thereon is much slower than for the turns directly in contact with the heated air. Thus, by placing a coil in the air heated to the temperature T_c for 10 seconds, only a small part of the electrogalvanized wire is rapidly heated, 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 gamma phase alloy exhibiting the features of the layer 16, while a significant part of the electrogalvanized wire, which has undergone slower heating, does not comprise such a layer of a copper-zinc gamma phase alloy with a high zinc concentration. In other words, with a conventional static furnace, it is not possible to precisely control the duration d₀ for each portion of the electrogalvanized wire.

For each portion of the electrogalvanized wire, as soon as the instant to is reached, i.e., in this case, from the end of the duration d₀, 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 the instant to and therefore to bring it into a metastable state at ambient temperature. To this end, immediately after the instant to, during step 90, each portion of the wire is subjected to rapid cooling for a duration d₁, which abruptly drops the temperature of the layer 16 of this portion to 30° C. in less than ten seconds.

This cooling is referred to as rapid cooling since the duration d₁ is less than 10 seconds. Preferably, the duration d₁ is less than 1 second or 0.5 second. In order to obtain such a short duration d₁, the cooling speed during step 90 is high. In this first example, the duration d₁ is less than or equal to 1 second. The average cooling speed for the duration d₁ is therefore greater than or equal to (T_c−30)°/s. Therefore, in this first manufacturing example, the average cooling speed is greater than 570° C./s.

In this case, step 90 is successively applied to each of the wire portions that exit the heating zone so that each portion of the wire undergoes this rapid cooling. This therefore allows the zinc concentration of the layer 16 to be frozen over the entire length of the electrode wire 2.

To this end, as soon as it exits the heating zone, the portion of the wire is immersed in a fluid at ambient temperature. For example, in this case, a liquid bath at 25° C. is installed at the outlet of the heating zone. For example, the liquid is water. In this case, each portion of the wire passes through, between the outlet of the heating zone and the inlet in the bath, an initial section of the trajectory during which this portion is firstly immersed in air at ambient temperature. Then, this portion of the wire enters the bath at ambient temperature and passes through a second section in this bath during which it is directly in contact with the liquid at ambient temperature. At the end of the second section, the portion of wire emerges from the bath. The lengths of the first and second sections are adapted so that the temperature of each portion of the wire drops below 30° C. less than 10 seconds after the instant to. In this case, each portion of wire passes through the first section in less than one second. The average cooling speed of a portion of the wire in water at ambient temperature is approximately 20,000° C./s. Under these conditions, in this first manufacturing example, the duration d₁ is less than or equal to 1 second.

At this stage, it should be noted that such rapid cooling cannot be obtained, for example, by immersing a coil, with several thousand turns, of the electrode wire heated to the temperature T_c in a bath, even a liquid bath, at ambient temperature. Indeed, in such a case, for reasons similar to those disclosed in the case of rapid heating, the cooling speed of the turns of the electrode wire, which are mechanically isolated from the liquid by other turns overlaid thereon, is much slower than for the 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 d₁ for cooling each portion of the electrode wire.

At the end of step 90, the layer 16 is in a metastable state and its zinc concentration, at ambient temperature, is greater than the threshold S₁₆, as long as the wire is maintained at ambient temperature.

Next, during a step 94, the wire obtained on completion of step 90 is drawn in order to obtain the electrode wire 2. This drawing step 94 allows the diameter of the electrode wire to reach the desired diameter, i.e., in this case a diameter of 250 μm. Step 94 fractures the layers 16 and 18. Thus, it is during this step 94 that most of the fractures located in the layers 16 and 18 are created.

FIG. 3 is an image of a portion of the cross-section of the electrode wire 2 obtained at the end of step 90 and before the drawing step 94. This image was obtained using an optical microscope. The composition of the layer 16 was measured with an Energy-Dispersive Spectroscopy (EDS) spectrometric analysis probe. The zinc concentration in the layer 16 is 67 atomic %.

FIG. 4 is an image of a portion of the electrode wire 2 obtained on completion of the drawing step 94. This image was also obtained using an optical microscope. The layer 16 is fractured. It is therefore divided into a multitude of zones separated from one another by fractures. This image shows that some of the zones of copper-zinc gamma phase alloy can be separated from the outer face 6 by another zone of copper-zinc gamma phase alloy. In this case, this zone is located more than 1 μm from the outer face 6. However, the majority, and, typically, more than 70% or 80%, of the zones of copper-zinc gamma phase alloy are less than 1 μm and, in this case, less than 0.5 μm from the outer face 6. In this document, the “majority” of the zones of copper-zinc gamma phase alloy corresponds to more than 50% of the zones of copper-zinc gamma phase alloy present in the electrode wire.

This image also shows that some of these zones can be covered with the remainder 100 of the layer 18.

Example 2: Joule Effect Heating

The second manufacturing example is identical to the first example except that:

• • the zinc is 18 μm thick;
  • the heating is carried out by the Joule effect; and
  • the duration d₀ is selected so that the instant to ranges between t_0min and t_opt, so that the layer 18 is present in the manufactured electrode wire.

In the case of Joule effect heating, the heating zone is a segment of the electrogalvanized wire located between a first and a second conductive pulley polarized by a direct current electrical generator. The potential difference between these two pulleys generates a direct current that flows in the segment of the electrogalvanized wire located between these two pulleys. The segment of the electrogalvanized wire located between the two pulleys is then heated by the Joule effect. Typically, the intensity of the direct current is greater than 10 A.

The various parameters of the Joule effect heating are adjusted so that the instant to when each portion of the electrogalvanized wire exits this heating zone lies within the previously defined interval [t_0min; t_0max]. The adjustable parameters of Joule effect heating are notably the intensity of the direct current, the potential difference between the two pulleys, the unwinding speed of the electrogalvanized wire and the length of the heating zone of the electrogalvanized wire lying between the two pulleys. For example, in this case, the length of the heating zone before the electrogalvanized wire directly enters a water bath at 25° C. is considered to be equal to 1,530 mm. The unwinding 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 d₀ 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 air before soaking in the bath.

The wire obtained after step 90 was analyzed. At certain sites around the wire, the layer 16 is covered with a layer of copper-zinc alloy that is even more zinc-rich, while at other sites, on the contrary, the layer 16 is the only metal alloy present on the surface of the wire. In both cases, zinc oxide is also present on the surface of the wire.

In the zones covered with the layer that are even more zinc-rich, the layer 16 has a zinc concentration that basically ranges between 65.4 and 69.4 atomic %. In the zones where the layer 16 is not covered by the layer that is even more zinc-rich, the zinc concentration of the layer 16 basically ranges between 65.4 and 66.3 atomic %.

It would appear that the layer that is even more zinc-rich, at the sites where it exists, is made of a copper-zinc delta phase alloy.

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 manufacture a layer of copper-zinc alloy with a high zinc concentration by implementing only the teaching of U.S. Pat. No. 5,762,726 A.

A 420 μm diameter and 100 mm long copper wire, coated with 18 μm of zinc, was placed in a static furnace at a temperature of 700° C., for a certain duration, then it was rapidly, in less than one second, extracted from the furnace, and soaked in water at ambient temperature.

A residence time do of 6 seconds in the static furnace allows a layer of copper-zinc gamma phase alloy to be obtained, the zinc concentration of which is 68 atomic %, overlaid by a layer 18 of copper-zinc epsilon phase alloy, which is less than 0.5 μm thick. Under identical conditions but for a residence time of 7 seconds in the static furnace, the obtained electrode wire does not comprise the surface layer 18 of copper-zinc epsilon phase alloy and the zinc concentration of the surface layer of copper-zinc gamma phase alloy is 63 atomic % only.

This observation was also confirmed by implementing the method of Example 1, but in the case whereby:

• • the rough 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 d₀ is equal to 9 seconds.

Under these conditions, the zinc concentration of the obtained surface layer 16 is equal to 63 atomic % and therefore is 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 epsilon phase alloy disappears occurs earlier. Consequently, by interrupting step 84 after 9 seconds, the rapid cooling step 90 is triggered too late and after the instant t_0max.

Chapter II.2: Manufacturing Example by Slow Copper Diffusion

Example 4

The manufacturing method is identical to the manufacturing method of Example 1 except that during step 84:

• • the zinc is 18 μm thick;
  • a static furnace is used;
  • the temperature T_c is equal to 250° C.;
  • the duration d₀ 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, i.e., typically less than 300° C., the duration d₀ is long enough for the heat treatment applied to the electrogalvanized wire to be practically the same in all the portions of this electrogalvanized wire, even if the heated electrogalvanized wire is present in the form of a coil comprising thousands of turns.

During the cooling step 90, at the outlet of the heating zone, the wire is only immersed in ambient air at 25° C., without having to carry out rapid cooling. Indeed, tests have shown that such rapid cooling of the wire after it exits the static furnace is not necessary when the temperature T_c is low. In other words, the zinc concentration of the layer 16 is the same in the event of rapid cooling and in the absence of such rapid cooling.

On completion of step 94, the obtained wire comprises a layer 18 that is less than 0.5 μm thick. This layer 18 is a copper-zinc epsilon phase alloy. The layer 16 has a zinc concentration ranging between 65.3 and 68.3 atomic %. The copper-zinc gamma phase alloy of this layer 16 appears to be less ductile than a layer that has a similar composition but that is obtained at 600° C.

Chapter II.3: Manufacturing Using Electroplating

Example 5

Manufacturing using electroplating involves directly depositing, by aqueous phase electroplating, the layer 16 without proceeding with a step of copper diffusion.

To this end, the rough wire forms the cathode, and an anode is used comprising a mixture of copper and zinc, in which anode the zinc concentration is greater than 65.4 or 77 atomic %. For example, for the tests described herein, the anode is formed by a mixture of copper balls and zinc plates. The electrolysis bath is adapted in order to deposit a layer of copper-zinc gamma phase alloy onto the rough wire with a high zinc concentration.

In this example, the bath is a 200 liter volume “Oplinger” type bath containing:

• • water as a solvent;
  • 12 kg of sodium hydroxide (beads) NaOH, i.e., 60 g/l;
  • 12 kg of sodium cyanide NaCN, i.e., 60 g/l;
  • 3.4 kg of copper cyanide CuCN, i.e., 17 g/l;
  • 12 kg of zinc cyanide Zn(CN)₂, i.e., 60 g/l; and
  • 80 g of sodium sulfite Na₂SO₃, i.e., 0.4 g/l.

The temperature of the bath is 45° C. The current density is 20 amperes per square decimeter (20 A/dm²). The faradaic yield is approximately 56%. A brass rough wire with a diameter of 0.51 mm was thus coated with a 7 μm thick layer 16.

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

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

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

Example 6

The manufacturing method of Example 6 is identical to that of Example 5, except that the bath is modified in order to increase the zinc concentration of the layer 16. To this end, the bath with the following features is used instead of the bath of 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 sulfite (Na₂SO₃) is 0.6 g/l.

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

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

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

Chapter II.4: Performance

The erosive yields of various 0.25 mm diameter wires are compared. Each test was carried out under the following conditions:

• • the electro-erosion machine used is the machine manufactured by GFMS (GF Machining Solutions), reference “CUT 200 MS”;
  • the cut part is a 50 mm-high steel block;
  • the injection nozzles are separated from the part by approximately 5 mm at the top and at the bottom;
  • the machining parameters are those of a standard bare brass wire with a diameter of 0.25 mm and a tensile strength of 900 N/mm², under conditions for plated injection nozzles.

In order to avoid breaking the wires (the nozzles are effectively separated), and to clearly demonstrate the effect of the layer 16, in order to measure the erosive yields, the sparking frequency was 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, in order to measure the maximum cutting speed of the tested wires, before they break.

The compared electrode wires are as follows:

• • Wire A: this is the wire sold under the trademark “Thermo SA” by THERMOCOMPACT® and described in patent application EP 1949995: it is a wire with a brass core (64% copper and 36% zinc by mass) and which comprises a surface layer of copper-zinc gamma phase alloy divided into blocks. The zinc concentration of these blocks in the gamma phase is 61.3 atomic %;
  • Wire B: this is the wire manufactured according to Example 5 described above, with the zinc concentration of the surface layer 16 being 66.4 atomic %; and
  • Wire C: this is the wire manufactured according to Example 6 described above, with the zinc concentration of the layer 16 being 82.3 atomic %.

		Erosive yield	Maximum erosion speed
	Wire	(mm2/min/A)	(mm2/min)

	Wire A	8.0	99.2
	Wire B	8.2	99.2
	Wire C	7.6	104.3

Wire B has a better erosive yield than wire A. Wire C withstands greater machining intensities before breaking. It therefore allows a higher maximum machining speed to be reached than the other wires.

It should be noted that wire A has an oxide layer of the order of 50 nm thick, which is favorable for its erosive yield. Wires B and C have a much thinner oxide layer. It is estimated that if the manufacturing conditions for wire B are modified to obtain an oxide thickness of the order of 50 nm thick, then its erosive yield would be even higher.

Chapter III: Alternative Embodiments

Alternative Embodiments of the Electrode Wire

The core of the electrode wire is not necessarily made of copper or of an alloy comprising copper such as, for example, brass. For example, the core also can be made of steel or of another electrically conductive metal. In the case whereby the core does not comprise copper, the layer 16 is obtained by electroplating.

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

The layer 14 can be omitted. This is notably the case if the layer 16 is produced by electroplating.

As an alternative embodiment, the layer 16 is uniform and is not fractured. The layer 16 is therefore formed by a single zone that continuously extends over the entire periphery of the core 10. For example, in order to manufacture this alternative embodiment, during step 82, the electrogalvanized wire is drawn in order to directly obtain the desired final diameter and the drawing step 94 is omitted. The other steps of the method of FIG. 2 remain unchanged, for example.

As an alternative embodiment, the layer 18 is a copper-zinc delta phase or eta phase alloy or is made of zinc.

As an alternative embodiment, the layer 18 is absent and the layer 16 then forms the surface layer of electrode wire.

Alternative Embodiments of the Manufacturing Method

Numerous other methods for manufacturing the wire 2 are possible. For example, manufacturing methods involving rapid or slow copper diffusion, described in Chapter II, can be implemented with a rough wire that is not necessarily entirely made of copper. For example, as an alternative embodiment, the rough wire comprises only a surface layer, the copper concentration of which is greater than 50 or 60 atomic % and less than 95 or 90 atomic %. Similarly, it also can be implemented with a coating with a zinc concentration that is less than 100 atomic %. However, preferably, the zinc concentration of the coating is high, i.e., greater than 95 atomic % or 98 atomic %.

The temperature T_c for implementing the method for manufacturing by slow copper diffusion can be selected between 150° C. and 500° C. The duration d₀ then must be adapted as a function of the selected temperature T_c. However, preferably, the temperature T_c is selected so as to be less than 300° C. or 250° C. or 200° C. so as to be able to use a static furnace and avoid having to execute rapid cooling. If the temperature T_c exceeds 300° C. or 400° C., a tunnel furnace is used instead of the static furnace in order to better control the durations d₀ and d₁. In this case, the method for manufacturing by slow copper diffusion is identical to the method of Example 1, except that the temperature T_c ranges between 150° C. and 500° C. and not between 500° C. and 700° C.

The drawing step 94 can be omitted.

Chapter IV: Advantages of the Described Embodiments

An electrode wire in which the zones of copper-zinc gamma phase alloy are less than 1 μm from the outer face of the electrode wire, while having a high zinc concentration, has at least one of the following advantages compared to the electrode wire of U.S. Pat. No. 5,945,010 A:

• • the erosive yield is improved; and/or
  • the maximum erosion speed is improved.

Presently, the fact that the electrode wire described in this document has improved performance capabilities is explained as follows. Since the layer 16, which is in the gamma phase, has a high zinc concentration, its melting point is high and its sublimation temperature is low. These are two features that are recognized as improving the performance capabilities of an electrode wire. In addition, the fact that the thickness of the surface layer that is even more zinc-rich is zero or less than 1 μm also improves the performance capabilities. Indeed, the melting point of the layer 18 is lower than the melting point of the layer 16. Thus, during electro-erosion, this layer 18 melts before the copper-zinc gamma phase alloy. By reducing the thickness or by eliminating this surface layer 18, the amount of liquid that appears on the surface of the electrode wire during the electro-erosion method is significantly reduced. Consequently, for example, the craters resulting from electro-erosion sparks have fewer re-solidified zones. The electrode wire also loses less material during the spark. Furthermore, there are fewer fractures or pores that are hidden 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 unwinding speed of the electrode wire, and therefore the consumption of electrode wire, while maintaining a good machining speed; or
  • increase the machining speed while keeping the unwinding speed unchanged.

It should be noted herein that, in U.S. Pat. No. 5,945,010 A, the layer of copper-zinc gamma phase alloy of specimen No. 4 has a lower zinc concentration than that of specimen No. 3. However, U.S. Pat. No. 5,945,010 A teaches that specimen No. 4 has the best performance capabilities. Furthermore, U.S. Pat. No. 5,945,010 A does not teach how to further increase the zinc concentration of the surface layer of specimen No. 4. In particular, this disclosure does not teach that heating of the electrode wire should be rapidly interrupted close to the instant t_opt.

The fact that each zone of copper-zinc gamma phase alloy is directly flush with the outer face further reduces the amount of liquid that appears during the electro-erosion. This further improves the performance capabilities of the electrode wire.

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

The fact that the layer of copper-zinc alloy is fractured allows the erosive yield of the electrode wire to be increased.

The diffusion heat treatment at more than 500° C. allows an electrode wire to be obtained that further comprises the layer 14 under the layer 16, which is advantageous.

Conversely, manufacturing the electrode wire by slow copper diffusion does not allow as thick a layer 14 to be obtained under the layer 16. However, slow diffusion allows a layer 16 to be obtained with a more even thickness.

Rapidly cooling the layer 16 limits or prevents the reduction of its zinc concentration during step 90.

Depositing the layer 16 by electroplating allows a layer 16 to be obtained with a zinc concentration that is greater than 66 or 70 atomic %. In addition, the thickness of the layer 16 is more even.