WO2024251464A1 - Electrode wire and method for the production thereof - Google Patents

Abstract

An electrode wire (2) for electrical discharge machining comprises: - a metal core (10) comprising a peripheral face, the metal core being made of a single copper-zinc alloy, and - a zinc oxide layer (12) directly formed on the peripheral face of the metal core and covering this peripheral face, wherein the average thickness of the zinc oxide layer (12) is between 100 nm and 461 nm.

Description

ELECTRODE WIRE AND METHOD FOR ITS MANUFACTURE

[1] The invention relates to an electrode wire for electroerosion machining and to a method of manufacturing this electrode wire.

[2] Electrode wires are used to cut metals or electrically conductive materials by electrical discharge machining in an electrical discharge machining machine.

[3] The well-known process 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 part to be machined and an electrically conductive electrode wire. 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 towards the part, either by transverse translation of the wire guides or by translation of the part.

[4] An electric generator, connected to the electrode wire by electrical contacts away from the machining area, establishes an appropriate potential difference between the electrode wire and the conductive part to be machined. The machining area between the electrode wire and the part is immersed in a suitable dielectric fluid. The potential difference causes sparks to appear between the electrode wire and the part to be machined, which gradually erode the part and the electrode wire. The longitudinal movement of the electrode wire makes it possible to permanently maintain a sufficient wire diameter to prevent it from breaking in the machining area. 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.

[5] Particles detached from the electrode wire and the workpiece by the sparks disperse in the dielectric fluid, where they are evacuated.

[6] Achieving machining precision, particularly the production of small radius corner cuts, requires the use of small diameter wires that can withstand a high mechanical load at break to be tensioned in the machining area and limit the amplitude of vibrations. [7] Most modern EDM machines are designed to use wire, typically 0.25 mm in diameter, and with a breaking load of between 700 N/mm ² and 1000 N/mm ² .

[8] When a spark occurs between the electrode wire and the workpiece, 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, changes from the solid state to the liquid or gaseous state, and is displaced to the surface of the electrode wire and/or evacuated into 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 been melted and re-solidified.

[9] It has been found that the efficiency of sparks in EDM depends largely on the nature and topography of the surface layer of the electrode wire. For this reason, considerable progress in EDM efficiency has been obtained by using electrode wires comprising:

- a metal core made of one or more metals or alloys ensuring good conduction of electric current and good mechanical resistance 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 the electroerosion, for example a higher erosion speed.

[10] For example, US8338735B2 discloses an electrode wire having a brass core covered with a layer of copper-zinc alloy. In this application, the layer of copper-zinc alloy comprises a mixture of fractured gamma-phase copper-zinc alloy.

[11] This particular coating structure aims to generally ensure a higher speed of machining of a part by electroerosion.

[12] The manufacturing processes for electrode wires, such as that described in patent US8338735B2, generally include a step of depositing, typically by electrodeposition, a layer of zinc on the metal core. This step is complex to carry out and consumes a lot of energy.

[13] Application JPS61203223A discloses an electrode wire having a brass core covered with a layer of zinc oxide. The zinc oxide layer is obtained by placing a brass wire in a furnace heated to 600°C for 4 h. This high-temperature heat treatment is carried out at a very low pressure of around 0.05 atm (5.07 kPa). An oxidized brass wire with a 300 nm layer of zinc oxide is obtained. At this stage of manufacture, the oxidized brass wire obtained cannot be used in most EDM machines because its breaking load is not greater than 400 N/mm ² To make this oxidized brass wire usable in most EDM machines, this wire is then drawn to go from a diameter of 0.4 mm to a diameter of 0.2 mm. The drawing operation increases the breaking load of the oxidized brass wire to make it usable in EDM machines. However, the zinc oxide layer obtained by heating at high temperature under very low pressure is very brittle. Because of this, during wire drawing, a large portion of the zinc oxide is lost. Thus, after wire drawing, instead of obtaining a 150 nm thick zinc oxide layer, the thickness of the zinc oxide layer is much smaller and less than 100 nm. In addition, since the zinc oxide layer obtained is very brittle, when using this wire to machine a part, the zinc oxide layer crumbles and fouls the electrode wire guide members. To overcome this latter drawback, application JPS61203223A proposes, after wire drawing, to coat the oxidized brass wire with a layer of varnish. However, coating the oxidized brass wire with a layer of varnish is not satisfactory. Indeed, this layer of varnish is insoluble in water. Because of this, when machining a workpiece with an oxidized brass wire coated with such a varnish, the varnish is not dissolved by the water present during machining and therefore remains on the electrode wire. This disrupts the passage of the machining current between the electrode wire and the workpiece during machining.

[14] Also known from the prior art are JPS61103731A, DE202017106956U1 and US2019/133919A1. The teaching of application JPS61103731A is similar to that of application JPS61203223A. In particular, in application JPS61103731A, the oxidation of the brass core is also carried out under very low pressure which leads to the formation of a friable zinc oxide layer which is largely removed during wire drawing. Furthermore, the temperature and time conditions described in application JPS61103731A for oxidizing the brass core are similar to those described in application JPS61203223A so that this also leads to obtaining electrode wires whose breaking load, before drawing, is insufficient to be used in most EDM machines. Application DE202017106956U1 discloses only zinc oxide layers with thicknesses less than 100 nm. Application LIS2019/133919A1 does not disclose a zinc oxide layer directly formed on a brass core.

[15] The invention aims to propose an electrode wire whose performance is similar to that of the electrode wire described in patent US8338735B2 while being simpler to manufacture.

[16] The invention is set forth in the attached set of claims.

[17] The invention will be better understood from reading the description which follows, given solely as a non-limiting example and with reference to the drawings in which:

- Figure 1 is a schematic illustration of the cross-section of an electrode wire,

- Figure 2 is a flowchart of a manufacturing process for the electrode wire of Figure 1,

- Figure 3 is a front view of a guide used to measure the frictional resistance of a wire,

- Figure 4 is a longitudinal sectional view of the guide of Figure 3, and

- Figure 5 is a top view of the guide of Figure 3.

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

[19] Subsequently, in a chapter I, the definitions of certain terms are given. In a chapter II, an example of a detailed embodiment is 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 described.

[20] Chapter I: Definitions and terminology

[21] The expression “element made of material A” or “element made of material A” means an element in which material A represents at least 70%, by mass, of this element and preferably at least 90% or 95% by mass of this element.

[22] A "copper-zinc alloy" means an alloy consisting solely of copper and zinc, with the exception of unavoidable impurities. A copper-zinc alloy is also called "brass"

". [23] The term "electrical conductor" means a material whose electrical conductivity, at 20 °C, is greater than 10 ⁶ S/m and, preferably, greater than 10 ⁷ S/m.

[24] The longitudinal axis of a wire is the axis along which the wire mainly extends.

[25] The term “cross section” means a section of the electrode wire perpendicular to its longitudinal axis.

[26] The term “longitudinal section” means a section of the electrode wire made along a plane which contains its longitudinal axis.

[27] The term “layer” refers to an annular layer of the electrode wire that is located, in each cross-section of the electrode wire, between an inner circular boundary and an outer circular boundary. In reality, these boundaries are not perfect circles. However, as a first approximation, in this text, these boundaries are considered to be circles. These circular boundaries are both centered on the axis of the electrode wire. The inner circular boundary is the boundary of the layer that is closest to the axis of the electrode wire. Conversely, the outer circular boundary is the boundary of the layer that is furthest from the axis of the electrode wire. Between these inner and outer circular boundaries, the chemical composition is substantially homogeneous. Conversely, at the inner and outer circular boundaries, the chemical composition changes abruptly. In particular, the change in composition when these circular boundaries are crossed is much greater than the slight changes in composition that can be observed within a layer.

[28] The term "fractured layer" refers to a layer that has a multitude of fractures that partition it into a multitude of zones separated from each other, in a longitudinal section of the wire, by a very large number of radial fractures. A very large number of radial fractures refers, over a length of 1 mm of the electrode wire, to more than ten radial fractures that divide the layer in question into ten or so blocks mechanically isolated from each other, in the longitudinal section, by these radial fractures.

[29] The term "surface layer" refers to the layer of the electrode wire that is located furthest to the outside of the electrode wire. This surface layer may have on its surface a thin film composed of water-soluble residues such as wire drawing lubricant residues. The outer face of this surface layer is therefore either merged with the outer face of the electrode wire in the absence of the thin film or separated from the outer face of the electrode wire only by this thin film. A conversely, a layer of the electrode wire covered with a varnish as in the case of the electrode wire described in application JPS61203223A is not a surface layer because the varnish applied is not water-soluble.

[30] The term “room temperature” means a temperature between 15°C and 35°C and, typically, equal to 25°C.

[31] The average thickness e of a surface layer of zinc oxide is defined by the following relation (1): e = [mi-m _f ]/[p*TT*d*L], where:

- mi is the initial mass of a sample of a wire with a surface layer of zinc oxide,

- m _f is the mass of the same sample of wire after being dipped in a bath which completely dissolves the surface layer of zinc oxide,

- p is the volume density of zinc oxide, this density p is here taken equal to 5600 kg/m ³ ,

- TT is the number pi,

- d is the initial diameter of the wire sample before being immersed in the bath which completely dissolves the oxide layer,

- L is the length of the wire sample, and

- “*” is the symbol that denotes scalar multiplication.

[32] The average thickness e is, for example, measured according to the following method:

1) A sample of length L and diameter d of wire is taken and then rolled into a crown of approximately 5 cm in diameter. The length L is for example equal to 12 m. The diameter d is often equal to 0.25 mm.

2) The sample is rinsed with water, then dried and dusted using a jet of compressed air.

3) The initial mass rrii of the sample is measured using a precision balance.

4) The sample is then soaked for 20 to 30 seconds in a stirred aqueous bath of sulfuric acid between 8% and 12% concentration, the temperature of which is between 42°C and 48°C.

5) The sample is rinsed with water.

6) The sample is dried using a jet of compressed air.

7) The final mass m _f of the sample is measured using a precision balance.

8) The average thickness e of the sample is calculated using the previous relation (1). Unless otherwise specified, in the remainder of this text, the term “thickness of the zinc oxide layer” alone designates the average thickness of this zinc oxide layer.

[33] A “straight” wire means a wire whose deflection is less than 35 mm when measured by the following method:

1) Place a piece of wire 35 cm to 45 cm long on a horizontal and sliding plane so that this piece of wire can freely deform. The piece of wire placed on this horizontal plane then forms approximately an arc of a circle.

2) Place a straight edge 30 cm long on the horizontal plane so that each end of the edge is in contact or almost in contact with a respective point on the wire. The ends of the edge are brought into contact with the wire with sufficient care so that it does not deform the wire.

3) Measure the maximum distance between the wire and the ruler. This maximum distance corresponds to the arrow of the wire. The arrow of the wire is generally halfway between the ends of the ruler.

[34] A “non-straight” wire is a wire that is not straight.

[35] Chapter: Example of embodiment

[36] Figure 1 shows an electrode wire 2 for EDM machining as described in the introductory part of this text.

[37] For this purpose, the electrode wire 2 has a breaking load greater than 400 N/mm ² and, most often, greater than 450 N/mm ² or 500 N/mm ² or 700 N/mm ² . The breaking load of the electrode wire 2 is also, generally, less than 1100 N/mm ² . Here, the breaking load of the electrode wire 2 is included in one of the following intervals [400 N/mm ² ; 450 N/mm ² ], [400 N/mm ² ; 500 N/mm ² ], [450 N/mm ² ; 700 N/mm ² ], [500 N/mm ² ; 700 N/mm ² ] or greater than 700 N/mm ² . In fact, wire EDM machining can be straight, slightly conical or strongly conical. Machining is said to be "straight" when the angle a between the electrode wire and the face of the table for laying the parts to be cut is between 82° and 98°. This table is generally horizontal. Machining is said to be "slightly conical" when the angle a is between 67° and 82° or between 98° and 113°. Machining is said to be "strongly conical" when the angle a is between 45° and 67° or between 113° and 135°. Electrode wires whose breaking load is in the range [400 N/mm ² ; 450 N/mm ² ] or [400 N/mm ² ; 500 N/mm ² ] are generally used for highly tapered machining. Electrode wires with a tensile strength in the range [450 N/mm ² ; 700 N/mm ² ] or [500 N/mm ² ; 700 N/mm ² ] are generally used for low-taper machining. Electrode wires with a tensile strength greater than 700 N/mm ² are used for straight machining. It is also emphasized that electrode wires with a tensile strength greater than 500 N/mm ² can be made straight by stress-relieving them, which is not the case for electrode wires with a tensile strength less than 450 N/mm ² . Straight electrode wires are easier to thread and mount in an EDM machine than non-straight electrode wires. For this reason, subsequently, wire 2 is a wire whose breaking load is greater than 500 N/mm ² .

[38] The wire 2 extends along a longitudinal axis 4. The axis 4 is here 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.

[39] The wire 2 has an outer face 6 directly exposed to sparks during the machining of a part by EDM using this wire. The outer face 6 is a cylindrical face which extends along the axis 4. The directrix 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 pm and 1 mm and, most often, between 70 pm and 400 pm. Here, the diameter D ₂ is equal to 0.25 mm.

[40] In this embodiment, the wire 2 comprises:

- a central core 10 made of electrically conductive material, and

- a coating 12 directly deposited on the core 10.

[41] The core 10 has the function of ensuring, by itself, the majority of the load at break of the wire 2. It also has the function of ensuring the electrical conductivity of the wire 2. For this purpose, it is made of electrically conductive material. Typically, it is made of metal or a metal alloy.

[42] The core 10 has a mainly cylindrical peripheral face 14 which extends along the axis 4. This peripheral face 14 is made of a copper-zinc alloy. For this purpose, the core 10 is entirely made of a single copper-zinc alloy. For example, the single copper-zinc alloy of the core 10 is an a-phase copper-zinc alloy or a copper-zinc alloy formed from a mixture of a and p phases. In particular, the core 10 does not have a central portion of a copper-zinc alloy of a given phase covered by a layer of a copper-zinc alloy in another phase. Typically the zinc concentration in the core 10 is greater than 20 atomic % and, preferably, greater than or equal to 36 atomic % or 40 atomic %. Typically, the zinc concentration of the core 10 is less than 42 atomic %.

[43] The diameter D _w of the core 10 is greater than 0.99*D ₂ or 0.995*D ₂ . For example, here, the diameter D is greater than or equal to 0.249 mm.

[44] 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 EDM machining. The quality of a face cut by EDM is all the better when its roughness is low.

[45] The average thickness of the coating 12 is very small compared to the diameter D ₂ of the wire 2, that is to say less than 0.5% of the diameter D ₂ and, preferably, less than 0.25% of the diameter D ₂ .

[46] In this embodiment, the coating 12 is formed of a single layer of zinc oxide. Thus, hereinafter, the same reference numeral is used to designate both the coating 12 and the zinc oxide layer.

[47] Layer 12 is the surface layer of electrode wire 2.

[48] The average thickness ei ₂ of the layer 12 is between 160 nm and 461 nm and, preferably, between 160 nm and 350 nm or between 160 nm and 300 nm.

[49] In this embodiment, the layer 12 is essentially made of zinc oxide of formula ZnO. However, in places, the layer 12 may be crossed by brass peaks. These brass peaks form projections on the peripheral face 14 of the core 10 which pass through the layer 12. These brass peaks form only one block of material with the core 10.

[50] The composition of zinc oxide can deviate slightly from stoichiometry. Composition analyses using XPS (X-ray Photoelectron Spectroscopy) spectra have shown that zinc oxide in layer 12 is composed, in atomic percentages:

- more than 90% zinc and oxygen,

- more than 5% copper, and

- various manufacturing residues. [51] During these composition analyses, the presence of carbon and carbon compounds on the surface of layer 12 was not taken into account. Indeed, this carbon comes from the lubricant used during the electrode wire drawing step. According to the analyses carried out, the zinc oxide corresponds to zincite.

[52] A method of manufacturing the wire 2 will now be described with reference to FIG. 2.

[53] For the implementation of this manufacturing method, typically, the desired thickness ei2 is first chosen between 100 nm and 461 nm or between 105 nm and 461 nm and, preferably, between 160 nm and 461 nm or between 160 nm and 350 nm. For example, here, the thickness ei ₂ is taken equal to 200 nm. Then, the value of a coefficient Ci described later is chosen while respecting the constraints also described later. For example, here, the coefficient Ci is chosen equal to two. Finally, a diameter D _o is chosen according to the coefficient Ci previously chosen and the desired final diameter D ₂ and while respecting the constraints stated in the following paragraph. As an illustration, the final diameter D ₂ is taken equal to 0.25 mm.

[54] In a step 80, a brass blank wire is first provided. The blank wire is a brass wire having a diameter D _o of between 1.3*D ₂ and 6*D ₂ and, preferably, between 1.3*D ₂ and 3.75*D ₂ or between 2*D ₂ and 3.75*D ₂ . In this example, the diameter D ₂ is equal to 0.25 mm so that the diameter D _o is between 0.325 mm and 1.5 mm and, preferably, between 0.5 mm and 1.5 mm or between 0.5 mm and 0.94 mm. Here, the diameter D _o is taken as 0.5 mm.

[55] The zinc concentration of this rough wire is chosen as described previously in the case of core 10. Here the zinc concentration of the rough wire is 40 atomic %. Indeed, the higher the zinc concentration, the more the performance of wire 2 is improved.

[56] In this embodiment, the blank wire having the desired diameter D _o is obtained by drawing a standard brass wire of diameter D _in j until the desired diameter D _o is obtained. For example, the diameter D _in j is equal to 1.25 mm.

[57] Then, in a step 82, the blank wire is oxidized to obtain an oxidized blank wire. The oxidized blank wire has a layer of zinc oxide directly on its peripheral face. This layer of zinc oxide completely covers the peripheral face of the oxidized blank wire. For this purpose, in step 82, the blank wire provided is subjected to a heat treatment in the presence of oxygen. This heat treatment is carried out in a gaseous medium containing oxygen and at a pressure greater than 50 kPa or 100 kPa. Here, this heat treatment is simply carried out in the Earth's atmosphere, that is to say in a medium comprising more than 20%, by volume, of dioxygen, at ambient pressure of approximately 101 kPa. This heat treatment is configured to generate a layer of zinc oxide on the peripheral face of the rough wire whose average thickness e ₀ is between 130 nm and 600 nm and, generally, between 160 nm and 600 nm. Indeed, a thickness e ₀ less than 130 nm does not allow a thickness ei ₂ greater than or equal to 100 nm to be obtained after the drawing step 84 described below. Furthermore, it has been observed that a thickness e ₀ greater than 600 nm leads to a zinc oxide layer that does not adhere well and is torn off, at least in places, during the wire drawing step 84. This tearing off of a portion of the zinc oxide layer during the wire drawing step 84 makes it impossible to precisely control the thickness ei ₂ . Indeed, it is very difficult to determine in advance the amount of zinc oxide that will be torn off during the wire drawing step 84 and therefore to predict in advance the thickness ei ₂ obtained after the wire drawing step 84. Thus, when the thickness e ₀ is greater than 600 nm, the reproducibility of the manufacturing process is degraded. Indeed, even if all the manufacturing parameters are kept equal, the differences between the thicknesses ei ₂ of the wires 2 manufactured increase. Furthermore, the removal of part of the zinc oxide constitutes a waste of material which should be avoided or limited since the removed zinc oxide is not used in the manufactured wire 2.

[58] Within the range [130 nm; 600 nm], the thickness e ₀ is determined by successive experiments by testing several thicknesses e ₀ included in this range until finding the thickness e ₀ which, after drawing, makes it possible to obtain the desired thickness ei ₂ of zinc oxide. In particular, to determine the thickness e ₀ , it must be taken into account that, even if the thickness e ₀ remains less than 600 nm, a small fraction of the zinc oxide is torn off during the drawing step. It has been estimated that, currently, this small fraction of zinc oxide can reach 20% or 30%. It is emphasized that such a small fraction of zinc oxide lost during wire drawing remains much lower than the fraction of zinc oxide lost if the thickness e ₀ was chosen to be greater than 600 nm or 800 nm. Indeed, for a thickness e ₀ greater than 600 nm or 800 nm, the fraction of zinc oxide lost during wire drawing is greater than 50% or 67%. Typically, the thicknesses e ₀ tested are generally chosen in the range [Ci*ei ₂ ; Min(1 ,3*Ci*ei ₂ ; 600)] and, preferably, in the interval [1 ,1*Ci*ei ₂ ; Min(1 ,3*Ci*ei ₂ ; 600)] and, even more often, in the interval [1 ,2*Ci*ei ₂ ; Min(1 ,3*Ci*ei ₂ ; 600)], where:

- Ci is equal to the reduction coefficient of the diameter of the oxidized rough wire during drawing step 84, and

- Min(a;b) is the function that returns the smallest of the values a and b.

The reduction coefficient Ci is defined as being equal to the ratio D ₀ /D ₂ . Thus, when the thickness ei ₂ is equal to 200 nm and the coefficient Ci is equal to two, the thickness e ₀ which makes it possible to obtain the desired thickness ei ₂ after wire drawing is typically between 480 nm and 520 nm and, most often, equal to or very close to 500 nm.

[59] Here the heat treatment used consists of placing a coil of the rough wire supplied in a furnace, in air, heated to a constant temperature T _furnace for a duration D _f0U r and at ambient pressure. Here, the furnace is not airtight and the air is stirred throughout the duration of the heat treatment. The rate of oxidation of the brass of the rough wire increases as a function of the temperature T _f0U r. Thus, the thickness of the zinc oxide layer which forms on the rough wire increases all the more rapidly as the temperature T _f0U r is high. Similarly, the thickness of the zinc oxide layer which forms on the rough wire increases as a function of the duration D _furnace . Thus, by adjusting the temperature T _f0U r and the duration D _f0U r, it is possible to obtain the desired thickness e ₀ of zinc oxide.

[60] More precisely, it has been established that the thickness e ₀ , the temperature T _four and the duration D _f0U r are related to each other, as a first approximation, by the following relation (2): D _f0U r = e ₀ ² /[k*exp(-Q/(R*T _foU r))], where:

- k = 2.418*10' ⁷ m ² /s,

- Q = 152 kJ/mol,

- R = 8.314 J/mol/K, and

- exp(... ) is the exponential function.

[62] Ensuite, plusieurs essais avec différentes valeurs de la durée D_f0Ur choisies autour de la valeur théorique D_f0UrT peuvent être nécessaires pour obtenir la valeur précise de la durée D_f0Ur qui permet d’obtenir exactement l’épaisseur e₀ souhaitée. Typiquement, la valeur de la durée D_f0Ur retenue à l’issue de ces essais est comprise dans l’intervalle [0,8*D_foUrT ; 1 ,2*D_f0UrT] ou dans l’intervalle [0,9*D_foUrT ; 1 , 1 *D_f0UrT] ou encore dans l’intervalle [0,95*D_foUrT ; 1 ,05*D_foUrT]. [61] Using the relation (2) it is possible to estimate a theoretical value D _f0U rT of the duration D _f0U r to obtain a given thickness e ₀ with a given temperature T _f0U r. This is illustrated in the following table in the particular case where the thickness e ₀ is equal to 600 nm. This table indicates the theoretical value D _f0U rT, in hours and fractions hour, required to reach a thickness of 600 nm of zinc oxide for different temperatures T _f0U r.

[62] Then, several tests with different values of the duration D _f0U r chosen around the theoretical value D _f0U rT may be necessary to obtain the precise value of the duration D _f0U r which makes it possible to obtain exactly the desired thickness e _0. Typically, the value of the duration D _f0U r retained at the end of these tests is included in the interval [0.8*D _foU rT ; 1.2*D _f0U rT] or in the interval [0.9*D _foU rT ; 1.1*D _f0U rT] or even in the interval [0.95*D _foU rT ; 1.05*D _foU rT].

[63] Furthermore, the temperature T _f0U r is preferably chosen not to be too high so as to correspond to a duration D _f0U r long enough so that the time required for the temperature to be uniform inside the entire coil is very small compared to the duration D _f0U r. Indeed, if the temperature T _f0U r chosen is very high, then the corresponding duration D _furnace is very short. However, over a very short duration, the heat does not have time to diffuse uniformly inside the entire coil. Thus, in the case where the heat treatment consists of placing an entire coil of the rough wire inside a furnace, when the duration D _f0U r is very short, the thickness e ₀ of the formed oxide layer has high inhomogeneity along the oxidized roughing wire. To avoid this problem, here, the duration D _f0U r is advantageously chosen to be greater than four hours or six hours. This constraint makes it possible to determine a maximum value for the temperature T _f0U r not to be exceeded. Conversely, the duration D _f0U r must not be too long to be suitable for an industrializable manufacturing process. For this purpose, here the temperature T _f0U r is chosen between 400°C and 500°C.

[64] At the end of the time D _f0U r, the coil of blank wire is removed from the furnace. At this stage, the blank wire is covered with a layer of zinc oxide of thickness e ₀ . It is therefore called “oxidized blank wire”. After being removed from the furnace, the coil is cooled. For this, conventionally, the coil is exposed to ambient air for the time necessary to cool to room temperature. Step 82 is then complete.

[65] In step 82, the oxidation of the zinc consumes the zinc present in the brass. Thus, the zinc concentration of the brass of the roughing wire near the zinc oxide layer is generally lower than that of the same brass located at the level of axis 4.

[66] Then, in a step 84, the rough wire, oxidized and cooled, is cold-drawn to obtain the wire 2. By “cold”, we mean the fact that the drawing step 84 is carried out without heating the rough wire prior to reducing its diameter. In step 84, the diameter reduction coefficient Ci makes it possible to bring the diameter D _o of the rough wire to the diameter D ₂ desired for the wire 2, i.e. here to a diameter of 0.25 mm.

[67] During step 82 and, in particular, during the heat treatment, the brass recrystallizes which reduces the breaking load of the blank wire. At the end of step 82, the breaking load of the oxidized blank wire is much lower than 700 N/mm ² so that such a wire cannot be used as an electrode wire at this stage. To obtain a breaking load greater than 700 N/mm ² , it has been determined that the coefficient Ci must be greater than or equal to 1.3. More precisely, the higher the coefficient Ci, the more the breaking load increases. Thus, preferably, the coefficient Ci is greater than or equal to 1.6 or 2.25 which makes it possible to obtain breaking loads greater than, respectively, 800 N/mm ² and 900 N/mm ² . The coefficient Ci must also be less than 6 so that the thickness e ₀ remains less than 600 nm. In the case where the diameter D ₂ is equal to 0.25 mm, a coefficient Ci equal to 1.3 requires that the diameter _Do be greater than 0.325 mm and less than 1.5 mm. Here, the coefficient Ci is chosen equal to two to obtain a breaking load between 700 N/mm ² and 800 N/mm ² .

[68] It is emphasized that if the chosen value of the coefficient Ci results in a thickness Ci*ei2 greater than 600 nm, then the coefficient Ci and/or the thickness ei ₂ must be reduced to have, at the same time, a coefficient Ci greater than 1.3 and a thickness e ₀ less than 600 nm.

[69] Here, in step 84, the oxidized blank wire is drawn under the same conditions as those suitable for a non-oxidized brass wire. The diameter reduction is carried out by passing the oxidized blank wire successively through several dies of decreasing diameter so as to gradually reduce the diameter of the oxidized blank wire until reaching the desired diameter D ₂ . For example, dies with elongations of between 15% and 22% are used. When drawing the oxidized blank wire, a water-soluble lubricant is used. For example, here, the lubricant is an aqueous solution containing the water-soluble lubricant.

[70] It is this drawing that can create the brass peaks that cross layer 12.

[71] At the end of step 84, once the diameter D ₂ is reached, an in-line stress-relief annealing is performed before its winding. This stress-relief annealing makes it possible to minimize the residual stresses in the wire 2 to obtain a straight wire 2 and therefore facilitate its threading into an EDM machine. This stress-relief annealing does not modify the composition of the wire 2 and has little effect on its breaking load. For this, the wire 2 is stretched between an upstream pulley and a downstream pulley and the portion of the wire 2 between these two pulleys is heated while the wire 2 runs between these pulleys. For example, the portion of the wire 2 stretched between the two pulleys is heated by the Joule effect by passing an electric current through this portion of the wire. The temperature and duration of this stress-relieving annealing are much lower than those used in step 82. Typically, the temperature for stress-relieving annealing is between 300°C and 450°C and its duration is less than 2 s or 3 s. Immediately after stress-relieving annealing, preferably, the electrode wire is dipped to cool it quickly to room temperature. For this, the electrode wire is dipped in a cold bath, i.e. in a bath whose temperature is lower than or equal to room temperature. To dip the wire in this cold bath immediately after stress-relieving annealing, different solutions are possible. For example, the pulley swallows, in the direction of travel of the wire, dipping in this cold bath. Alternatively, just after this pulley, the wire passes through the cold bath. Here the cold bath is an aqueous solution of polyethylene glycols (PEG). The average molar mass of the PEG used is between 200 and 1400 g/mol. The PEG molecules can be present in the aqueous solution in the form of ethoxylated esters of dicarboxylic acids. The concentration of PEG in this aqueous solution is typically between 2% and 20% by volume, the remainder being water.

[72] To demonstrate the benefit of an electrode wire with a thick layer of zinc oxide on its surface, the following tests were carried out. A reference EDM machining job was defined. This involves cutting a punch from a 50 mm high piece of steel with guides located less than 0.2 mm from the piece. The cutting is carried out on a CUT200MS machine marketed by the company “GF Machining Solution”. This cutting is carried out in three machining passes, with technology adapted to brass. During each pass, the speed of movement of the part relative to the electrode wire was adapted to cut the punch as quickly as possible with the same final surface condition. Here, this final surface condition corresponds to a roughness Ra of 0.6 pm. More precisely, in the tests carried out, only the speeds of movement of the part relative to the electrode wire of the first and second passes were adapted according to the wire used. The speed of movement of the part relative to the electrode wire during the third pass is the same for all the tests carried out.

[73] Using the manufacturing method of Figure 2, different wires 2 were produced with different thicknesses ei ₂ . The machining times of the punch using the different wires 2 are indicated in the table below. In this table, the first column contains the designation of the wire. The second column contains the thickness ei ₂ and the third column the time, expressed in hours and fractions of an hour, of corresponding machining. In this table, the wires designated by L7 and L49 are identical to wire 2 except that their thickness ei ₂ is not between 100 nm and 461 nm. The wire designated by “Gamma” is an electrode wire in accordance with the teaching of patent US8338735B2. It has a surface layer of copper-zinc alloy fractured in gamma phase. More precisely, it is the electrode wire marketed by the company Thermocompact® under the reference Thermo SA.

[74] As illustrated by these tests, wire 2 allows machining practically as quickly as the “Gamma” wire from the moment when the thickness ei ₂ is greater than 100 nm. In addition, for thicknesses ei ₂ greater than 160 nm, it becomes possible to machine more quickly than the “Gamma” wire. For example, preferably, the thickness ei ₂ is between 160 nm and 350 nm or between 160 nm and 300 nm.

[75] Furthermore, it was validated that the wire 2 produced has a friction resistance equal to or better than that of a standard wire. For this, the friction resistance of the wire 2 is less than or equal to 7 mg/km and, preferably, less than 5 mg/km. Thus, the wire 2 does not foul the wire guide members of an EDM machining machine any more than a standard wire. Here, the standard wire is identical to the wire 2 except that it is without a coating. The standard wire is therefore made entirely of brass. For this, the friction resistance of the wire 2 and the standard wire were measured using the following method:

- Step 1): at room temperature, run 1 km of wire at a speed of 80 m/min under a tension of 12 N on a friction face of a guide 100 (Fig. 3 to 5), the wire coming into contact with this friction face following a rectilinear trajectory 101 parallel to a direction D, then

- Step 2): weigh the amount of dust that has come off the wire when the kilometer of wire has finished running. [76] The measured weight of this quantity of dust constitutes the measurement of the friction resistance of the wire expressed in mg/km. Indeed, the more friable the zinc oxide layer is and/or the weaker its adhesion to the core, the greater the quantity of zinc oxide torn off during its friction on the friction face of the guide 100.

[77] Figures 3 to 5 show in detail the guide 100 used in the method of measuring friction resistance. In Figure 4, the dimensions indicated are expressed in millimeters.

[78] The guide 100 is a solid of revolution. Its axis of revolution bears the reference 102. The cross-section of the guide 100 shown in FIG. 4 is made along a cutting plane A-A which contains the axis 102. Thus, only the elements located on one side of the axis 102 in FIG. 4 are described in detail. The other elements, on the opposite side, are deduced by the symmetry of revolution around the axis 102. In FIGS. 3 to 5, the axis 102 is vertical.

[79] The guide 100 comprises a friction face 104 whose longitudinal section in the cutting plane A-A forms an arc of a circle which begins at an inlet 106 and ends at an outlet 108. The tangent of the arc of a circle at the outlet 108 is parallel to the axis 102. The radius of this arc of a circle is equal to 33 mm. The orthogonal projection of this arc of a circle on the axis 102 forms a line 19.67 mm long. The orthogonal projection of this arc of a circle on a plane perpendicular to the axis 102 forms a line 6.5 mm long.

[80] After the outlet 108, going downwards, the face 104 is extended by a cylindrical face 110 parallel to the axis 102. The horizontal section of the face 110 is a circle centered on the axis 102 with a diameter greater than the diameter of the wire. Here, the diameter of the face 110 is equal to 1 mm.

[81] Moving downwards, the face 110 ends with a circular orifice 112 which forms the entrance to a truncated cone-shaped face 114.

[82] The truncated face 114 is centered on the axis 102. This face 114 widens, going downwards, to an outlet orifice 116.

[83] Face 104 is made of a material much harder than brass and zinc oxide. Here, face 104 is made of ceramic. More specifically, the ceramic is zirconia (ZrO ₂ ) stabilized with Yttrium (Y). For example, this ceramic comprises about 6% Ytrrium in the form of oxide Y ₂ O ₃ . In this embodiment, the guide 100 is entirely made of zirconia (ZrO ₂ ) stabilized with Yttrium (Y) in the form of oxide Y ₂ O ₃ . The roughness Ra of the friction face 104 is equal to 0.03 pm. More precisely, the roughness of the face 104 was measured thirty times using the following equipment and settings:

- Equipment brand: MAHR

- Controller reference: MarSurf M400

- Feed unit reference: MarSurf SD26

- Stylus reference: 6852404 BWF A 4-4.5 - 2 / 90° (90° tip with a radius of 2 pm)

- Cutting length 0.08 mm

- Evaluation length 5 times 0.08 mm

- Ls filter in operation

The average of the thirty measurements obtained is equal to 0.0305 pm and the standard deviation of these thirty measurements is equal to 0.0029 pm.

[84] Currently, the guide 100 is marketed by the company GF Machining Solution® under the term “Inletbush for Brake” with the reference 326864 in their online catalog accessible at the following address: https://ecatalog.gfms.com/gfms/fr/USD/search/326864.

[85] During step 1), the angle p between the direction D and the axis 102 is equal to 30°. Thus, the wire comes into contact with the face 104 at a point 120 located just after the inlet 106. The tangent at the point 120 is parallel to the direction D. Thus, during step 1), the wire advances inside the guide 100, passing successively through the inlet 106, then the outlet 108, then the orifice 112 and finally the orifice 116. After the orifice 116, the wire moves along a trajectory 122 coincident with the axis 102. Under these conditions, during step 1), the wire rubs only on the face 104.

[86] Preferably, in step 1), the axis 102 is vertical so that the dust generated by the friction of the wire on the face 104 falls under the orifice 116. For example, in step 1), the falling dust is collected in a container located under the orifice 116. For example, the container is a circular adhesive pad three to four centimeters in diameter. This adhesive pad is placed just under the orifice 116 and its adhesive face is turned towards the orifice 116. Before the wire runs, a slot is made in this pad to connect its periphery to its center. This slot allows the wire to be inserted into the pellet until it passes through the center of the pellet. Then, in step 1), the wire passes through the pellet and the dust sticks to the adhesive side. In step 2), it is the dust collected in this container that is weighed. For this, here, the adhesive pellet is weighed before and after step 1). The difference between these two measurements of the weight of the pellet is equal to the weight of the dust collected.

[87] Using this method, the friction resistance measured for the standard wire is 7 mg/km and the friction resistance measured for wire 2 is 2 mg/km.

[88] Chapter III: Variants:

[89] Electrode wire variants:

[90] The outer face of layer 12 may be covered with a thin film of the lubricant used during wire drawing step 84.

[91] Variants of the manufacturing process:

[92] Alternatively, if the roughing wire having the desired diameter D _o is commercially available, in step 80, it is not drawn before performing step 82.

[93] Other embodiments of the oxidation step 82 are possible. For example, as a variant, instead of placing an entire coil of the blank wire in a furnace, the blank wire is unwound and then passes through a heating tunnel and is then wound again onto a coil at the exit of this tunnel. Inside the tunnel, the temperature is equal to the temperature T _furnace . Thus, in this variant, the blank wire is heated to the temperature T _f0U r portion by portion. Therefore, the problem of the time required to obtain a uniform temperature within an entire coil of the blank wire does not arise. In this case, it is possible to use a higher temperature T _furnace and a very short duration D _f0U r . For example, when a heating tunnel is used, the value of the temperature T _f0U r may be greater than 600°C or 700°C. The speed of travel of the roughing wire inside the tunnel is then adjusted so that the duration D _four , during which a portion of the roughing wire remains inside the tunnel, allows the desired thickness e ₀ to be obtained. [94] Oxidation step 82 may also be carried out in a medium other than the Earth's atmosphere. For example, step 82 may also be carried out in a medium containing more than 20% or 30%, by volume, of dioxygen.

[95] In another variant of step 82, the temperature T _f0U r varies during the duration D _f0U r. For example, the temperature T _f0U r increases continuously during the duration D _f0U r.

[96] Cooling of the oxidized roughing wire can also be done differently. For example, the furnace is turned off and the coil is left inside the furnace until it reaches room temperature.

[97] Other lubricants may be used when drawing the oxidized roughing wire. For example, the lubricant used is an aqueous solution of PEG with an average molar mass of between 200 and 1400 g/mol, i.e. the same solution as that used to cool the electrode wire after stress-relieving.

[98] In a simplified embodiment, in step 84, the stress-relieving annealing is omitted. In this case, the electrode wire is simply dipped into the cold PEG bath. In another variant, the dipping into the cold PEG bath is omitted.

[99] Several of the variants described above can be combined in a single embodiment.

[100] Chapter IV: Advantages of the embodiments described:

[101] The fact that the zinc oxide layer is directly formed on the peripheral face of the metal core makes it possible to manufacture this zinc oxide layer by simple oxidation of the brass peripheral face of a blank wire. Thus, it is not necessary to deposit a zinc layer on the peripheral face of the blank wire as, for example, in the case of the manufacture of a wire in accordance with the teaching given in patent US8338735B2. In particular, it is emphasized that the deposition, by electrodeposition, of a zinc layer on a blank wire consumes much more energy than the heat treatment step. Thus, the electrode wire described here can be manufactured by simpler and more economical processes.

[102] The fact that the thickness of the oxide layer is greater than 100 nm improves the machining speed and makes it possible, in particular, to obtain machining speeds close to or greater than that of an electrode wire comprising a fractured coating of gamma-phase copper-zinc alloy such as that described in US8338735B2.

[103] The fact that the thickness of the oxide layer is less than 461 nm makes it possible to improve the adhesion of this oxide layer to the metal core. [104] The fact that the thickness ei ₂ is greater than 160 nm makes it possible to obtain a machining speed higher than that obtained with an electrode wire comprising a fractured coating of gamma-phase copper-zinc alloy such as that described in US8338735B2.

[105] The fact that the friction resistance of the zinc oxide layer is less than 7 mg/km makes it possible to limit the amount of dust produced by the electrode wire when it is used to machine a part. This therefore limits the fouling of the guide members of EDM machining machines. This also makes it possible to avoid having to cover this zinc oxide layer with a varnish as described in application JPS61203223A. Therefore, the problems caused by the presence of this varnish on the surface of the electrode wire can be avoided.

[106] The fact that the zinc oxide layer is the surface layer of the electrode wire makes it possible to increase the machining speed.

[107] The fact that the zinc concentration of the peripheral face is greater than 36 atomic % allows for further improvement of machining performance.

[108] The fact that the diameter _Do is between 1.3*D ₂ and 6*D ₂ guarantees that the coefficient Ci of reduction of the diameter during drawing is greater than 1.3 and therefore that the breaking load of the electrode wire produced is greater than 700 N/mm ² .

[109] The fact that the diameter D _o is between 1.3*D ₂ and 6*D ₂ combined with the fact that the thickness e _o is between 100*(D _o /D ₂ ) nm and 600 nm, makes it possible to manufacture an electrode wire whose thickness ei ₂ is between 100 nm and 461 nm.

[110] The fact that the thickness e ₀ is less than 600 nm avoids the tearing off of part of the zinc oxide layer during wire drawing. Since there is no tearing off of part of the zinc oxide layer, the thickness ei ₂ is well controlled and reproducible.

[111] The fact that during the heat treatment the maximum temperature reached is between 400°C and 500°C makes it possible to have a heat treatment lasting at least six hours. Such a duration of the heat treatment makes it possible to have a uniform temperature throughout a coil of rough wire heated in the furnace and therefore to obtain a layer of zinc oxide which is more uniform over the entire length of the rough wire.

[112] Reducing the diameter of the oxidized roughing wire by a factor greater than two allows a breaking load greater than 770 N/mm ² to be obtained.

Claims

Claims 1. Electrode wire (2) for EDM machining having a breaking load greater than 400 N/mm ² , this electrode wire comprising: - a metal core (10) comprising a peripheral face, this metal core being made of a single copper-zinc alloy, and - a layer (12) of zinc oxide directly formed on the peripheral face of the metal core and which covers this peripheral face, characterized in that the average thickness of the layer (12) of zinc oxide is between 100 nm and 461 nm. 2. Electrode wire according to claim 1, in which the average thickness of the zinc oxide layer (12) is greater than or equal to 105 nm or 160 nm. 3. Electrode wire according to any one of the preceding claims, in which the friction resistance of the zinc oxide layer is less than 7 mg/km when this resistance is measured using the following method: - at room temperature, run 1 km of wire at a speed of 80 m/min under a tension of 12 N on a friction face (104) whose longitudinal section in a cutting plane is an arc of a circle with a radius of 33 mm, this arc of a circle starting at an inlet (106) and ending at an outlet (108), this friction face being made of zirconia (ZrO ₂ ) stabilized with Yttrium (Y) and the roughness Ra of this friction face being equal to 0.03 pm, the wire coming into contact with this friction face at a contact point (120) located between the inlet (106) and the outlet (108) and following a rectilinear trajectory (101 ) contained in the cutting plane and forming with the tangent at the outlet an angle of 30° and separating from this friction face at the outlet in following a trajectory (122) parallel to the tangent at this exit, then - weigh the quantity of dust that has come off the wire when the kilometer of wire has finished running, the measured weight of this quantity of dust constituting the measurement of the friction resistance expressed in mg/km. 4. Electrode wire according to claim 3, in which the zinc oxide layer is the surface layer of the electrode wire. 5. Electrode wire according to any one of the preceding claims, in which the zinc concentration of the copper-zinc alloy of the metal core (10) is greater than or equal to 36 atomic % or 40 atomic %. 6. Electrode wire according to claim 5, in which the zinc concentration of the copper-zinc alloy is less than 42 atomic %. 7. Electrode wire according to any one of the preceding claims, in which the breaking load of the electrode wire is greater than 450 N/mm ² or 500 N/mm ² . 8. Electrode wire according to claim 7, in which the breaking load of the electrode wire is greater than 700 N/mm ² . 9. Electrode wire according to any one of the preceding claims, in which the zinc oxide of the zinc oxide layer (12) is obtained by a heat treatment in the presence of oxygen so that this zinc oxide is composed, in atomic percentage: - more than 90% zinc and oxygen, - plus 5% copper, and - the remainder being formed from various residues. 10. Method of manufacturing an electrode wire according to any one of the preceding claims, this method comprising: - the supply (80) of a rough metal wire having a peripheral face, this rough wire being made of a single copper-zinc alloy whose zinc concentration is greater than 20 atomic %, the diameter D _o of this rough wire being between 1.3*D ₂ and 6*D ₂ , WHERE D ₂ is the final diameter of the electrode wire to be manufactured by this process, then - the oxidation (82) of the peripheral face of the rough wire provided to obtain an oxidized rough wire comprising a layer of zinc oxide directly on its peripheral face, this layer of zinc oxide covering this peripheral face, this oxidation of the rough wire provided being obtained by subjecting the rough wire to a heat treatment in the presence of a gas containing oxygen, this heat treatment being configured to generate a layer of zinc oxide on the peripheral face of the rough wire whose average thickness e ₀ is between 130 nm and 600 nm, then - drawing (84) the oxidized rough wire to obtain the electrode wire of diameter D ₂ and in which the zinc oxide layer forms the outer face of the electrode wire, characterized in that: - the heat treatment is configured to obtain a zinc oxide layer whose thickness e ₀ is between 1 OO*(D _o /D ₂ ) nm and 600 nm and, preferably, between 12O*(D _O /D ₂ ) nm and 600 nm, and - during heat treatment, the pressure of the gas containing oxygen is greater than 50 kPa. 11. Method according to claim 10, in which the oxidation (82) of the peripheral face of the roughing wire comprises the following operations: - heat the roughing wire to a constant temperature T _f0U r for a duration D _furnace between 0.8*e ₀ ² /[k*exp(-Q/(R*Tf _OR r))] and 1.2*e ₀ ² /[k*exp(-Q/(R*Tf _OR r))], where: - e ₀ is the desired thickness of the zinc oxide layer - k = 2.418*10' ⁷ m ² /s - Q = 152 kJ/mol, - R = 8.314 J/mol/K, and - exp(... ) is the exponential function, then - at the end of the _oven time D, cool the oxidized rough wire until its temperature drops below 35°C before carrying out the drawing. 12. Method according to claim 11, in which the oxidation (82) of the peripheral face of the roughing wire comprises: - placing a coil of the rough wire inside a furnace heated to the temperature T _f0U r, the temperature T _f0U r being between 400°C and 500°C, then - leave the coil inside the furnace for the entire duration D _f0U r then remove the coil from the furnace and cool it until the temperature of the oxidized rough wire drops below 35°C before carrying out the drawing. 13. A method according to any one of claims 10 to 12, wherein the drawing step (84) reduces the diameter D _o of the oxidized rough wire by a factor greater than two or 2.1. 14. Method according to any one of claims 10 to 13, in which, after drawing (84) the oxidized rough wire to obtain the electrode wire of diameter D ₂ , the method comprises a stress-relieving annealing which minimizes the residual stresses in the electrode wire and then dipping the electrode wire in a bath containing an aqueous solution of polyethylene glycols with an average molar mass of between 200 and 1400 g/mol, the temperature of this aqueous solution being less than 35°C.