Electrode wire

Abstract

Electrode wire. This electrode wire (2) for machining by electrical discharge machining comprises: - a metal core (10) having 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 covering this peripheral face, in which the average thickness of the zinc oxide layer (12) is between 100 nm and 461 nm. Fig. 1

Description

Title of the invention: Electrode wire

[0001] The invention relates to an electrode wire for electroerosion machining and to a method for manufacturing this electrode wire.

[0002] Electrode wires are used to cut metals or electrically conductive materials by electroerosion in an electroerosion machining machine.

[0003] The well-known method of electroerosion machining, or spark erosion, makes it possible to remove material from an electrically conductive part, by generating sparks in a machining zone between the part to be machined and 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.

[0004] An electrical 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.

[0005] The particles detached from the electrode wire and the part by the sparks disperse in the dielectric fluid, where they are evacuated.

[0006] Achieving machining precision, particularly the production of small radius corner cuts, requires the use of small diameter wires which can withstand a high mechanical load at break to be tensioned in the machining area and limit the amplitude of vibrations.

[0007] Most modern EDM machines are designed to use wires, generally 0.25 mm in diameter, and with a breaking load of between 700 N / mm2 and 1000 N / mm2.

[0008] 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 is found that the outer face of the electrode wire reached by the spark has been deformed, usually taking a slightly concave, cratered shape, with areas where the material has melted and re-solidified.

[0009] It has been observed that the efficiency of sparks with regard to electroerosion depends largely on the nature and topography of the surface layer of the electrode wire. For this, considerable progress in electroerosion efficiency has been obtained by using electrode wires comprising:

[0010] - a metallic core made of one or more metals or alloys ensuring good conduction of electric current and good mechanical resistance to withstand the mechanical load of wire tension, and

[0011] - a coating of one or more other metals or alloys and / or a topography particular, for example fractures, ensuring better efficiency of electroerosion, for example a higher erosion speed.

[0012] For example, patent US8338735B2 describes 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.

[0013] This particular coating structure generally aims to ensure a higher machining speed of a part by electroerosion.

[0014] The methods for manufacturing 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.

[0015] Application JPS61203223A describes 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 hours. This high-temperature heat treatment is carried out at a very low pressure of the order of 0.05 atm (5.07 kPa). An oxidized brass wire having a 300 nm layer of zinc oxide is obtained. Then, the oxidized brass wire is drawn to go from a diameter of 0.4 mm to a diameter of 0.2 mm. 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 part of the zinc oxide is lost. So, after wire drawing, instead of getting a 150 nm thick zinc oxide layer, the thickness of the zinc oxide layer is much smaller and less than 100 nm.Furthermore, since the resulting zinc oxide layer is very brittle, when using this wire to machine a part, the zinc oxide layer crumbles and clogs the electrode wire guide members. To overcome this last 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 is because when machining a part 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 part during machining.

[0016] 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.

[0017] The invention therefore relates to an electrode wire for electroerosion machining, this electrode wire comprising:

[0018] - a metal core comprising a peripheral face, this metal core being made of a single copper-zinc alloy, and

[0019] - a layer of zinc oxide directly formed on the peripheral face of the core metallic and which covers this peripheral face,

[0020] wherein the average thickness of the zinc oxide layer is between 100 nm and 461 nm.

[0021] The embodiments of this electrode wire may comprise one or more of the following characteristics:

[0022] 1) The average thickness of the zinc oxide layer is greater than or equal to 160 nm.

[0023] 2) The friction resistance of the zinc oxide layer is less than 7 mg / km when this resistance is measured using the following method:

[0024] - 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 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 and ending at an outlet, this friction face being made of zirconia (ZrO2) 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 point of contact located between the inlet and the outlet and following a rectilinear trajectory 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 following a trajectory parallel to the tangent at this outlet, then

[0025] - weigh the quantity of dust that has detached from the wire when the scrolling of the kilometer of wire is completed, the measured weight of this quantity of dust constituting the measure of frictional resistance expressed in mg / km.

[0026] 3) The zinc oxide layer is the surface layer of the electrode wire.

[0027] 4) The zinc concentration of the copper-zinc alloy of the metal core is higher or equal to 36 atomic % or 40 atomic %.

[0028] 5) The zinc concentration of the copper-zinc alloy is less than 42 atomic %.

[0029] 6) The zinc oxide of the zinc oxide layer is obtained by a treatment thermal in the presence of oxygen so that this zinc oxide is composed, in atomic percentage:

[0030] - more than 90% zinc and oxygen,

[0031] - plus 5% copper, and

[0032] - the remainder being formed from various residues.

[0033] The invention also relates to a method for manufacturing the above electrode wire, this method comprising:

[0034] - the provision of a rough metal wire comprising a peripheral face, this wire rough wire being made from a single copper-zinc alloy with a zinc concentration greater than 20 atomic %, the diameter Do of this rough wire being between 1.3*D2 and 6*D2, where D2 is the final diameter of the electrode wire to be manufactured by this process, then

[0035] - oxidation of the peripheral face of the rough wire supplied to obtain a wire 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 supplied 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 e0 is between 130 nm and 600 nm, then

[0036] - drawing the oxidized rough wire to obtain the electrode wire of diameter D2 and wherein the zinc oxide layer forms the outer face of the electrode wire,

[0037] wherein:

[0038] - the heat treatment is configured to obtain a zinc oxide layer of which the thickness e0 is between 100*(Do / D2) nm and 600 nm and, preferably, between 120*(Do / D2) nm and 600 nm, and

[0039] - during the heat treatment, the pressure of the oxygen-containing gas is su above 50 kPa.

[0040] Embodiments of this method may include one or more of the following features:

[0041] 1) The oxidation of the peripheral face of the roughing wire comprises the operations following:

[0042] - heating the roughing wire to a constant temperature Tfour for a duration Dfour between 0.8*e02 / [k*exp(-Q / (R*Tfour))] and 1.2*e02 / [k*exp(-Q / (R*Tfour))], where:

[0043] - e0 is the desired thickness of the zinc oxide layer

[0044] - k = 2.418*107 m2 / s

[0045] - Q = 152 kJ / mol,

[0046] - R = 8.314 J / mol / K, and

[0047] - exp(...) is the exponential function, then

[0048] - at the end of the duration Dfour, cool the oxidized rough wire until its temperature temperature drops below 35°C before drawing.

[0049] 2) The oxidation of the peripheral face of the roughing wire comprises:

[0050] - placing a coil of the rough wire inside a furnace heated to temperature T oven, the temperature Toven being between 400°C and 500°C, then

[0051] - leave the coil inside the oven for the entire duration of the oven then remove it coil from the furnace and cool it until the temperature of the oxidized rough wire drops below 35°C before drawing.

[0052] 3) The drawing step reduces the diameter Do of the oxidized rough wire by a factor greater than two or 2.1.

[0053] The invention will be better understood on reading the description which follows, given solely by way of non-limiting example and made with reference to the drawings in which:

[0054] - [Fig.l] is a schematic illustration of the cross-section of a wire electrode,

[0055] - [Fig.2] is a flowchart of a manufacturing process for the electrode wire of the [Fig.l],

[0056] - [Fig.3] is a front view of a guide used to measure the resistance to friction of a thread,

[0057] - [Fig.4] is a longitudinal sectional view of the guide of [Fig.3], and

[0058] - [Fig.5] is a top view of the guide of [Fig.3].

[0059] 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.

[0060] 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.

[0061] Chapter I: Definitions and terminology

[0062] The expression “element made of material A” or “element of material A” designates 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.

[0063] A "copper-zinc alloy" means an alloy formed solely of copper and zinc, excluding unavoidable impurities. A copper-zinc alloy is also called "brass".

[0064] The expression “electrical conductor” designates a material whose electrical conductivity, at 20°C, is greater than 106 S / m and, preferably, greater than 107 S / m.

[0065] The longitudinal axis of a wire is the axis along which the wire primarily extends.

[0066] The expression "cross section" designates a section of the electrode wire perpen dicular to its longitudinal axis.

[0067] The expression “longitudinal section” designates a section of the electrode wire made along a plane which contains its longitudinal axis.

[0068] The expression "layer" designates an annular layer of the electrode wire which 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 assimilated to circles. These circular boundaries are both centered on the axis of the electrode wire. The inner circular boundary is the boundary of the layer which is closest to the axis of the electrode wire. Conversely, the outer circular boundary is the boundary of the layer which 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 compositional change when these circular boundaries are crossed is much larger than the slight compositional changes that can be observed within a layer.

[0069] The term "fractured layer" designates a layer which comprises a multitude of fractures which partition it into a multitude of zones separated from each other, in a longitudinal section of the wire, by a very large number of radial fractures. A very large number of radial fractures designates, over a length of 1 mm of the electrode wire, more than ten radial fractures which divide the layer in question into ten blocks mechanically isolated from each other, in the longitudinal section, by these radial fractures.

[0070] The expression "surface layer" designates the layer of the electrode wire which is located furthest to the outside of the electrode wire. This surface layer may comprise 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. 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.

[0071] The expression “room temperature” designates a temperature between 15°C and 35°C and, typically, equal to 25°C.

[0072] The average thickness e of a surface layer of zinc oxide is defined by the following relation (1): e = [mi-mf] / [p*jr*d*L], where:

[0073] - m; is the initial mass of a sample of a wire comprising a super layer zinc oxide powder,

[0074] - mf is the mass of the same sample of wire after being immersed in a bath which completely dissolves the surface layer of zinc oxide,

[0075] - p is the volume density of zinc oxide, this density p is here taken equal to 5600 kg / m3,

[0076] - ir is the number pi,

[0077] - d is the initial diameter of the wire sample before being immersed in the bath which completely dissolves the oxide layer,

[0078] - L is the length of the wire sample, and

[0079] - “*” is the symbol that denotes scalar multiplication.

[0080] The average thickness e is, for example, measured according to the following method:

[0081] 1) A sample of length L and diameter d of wire is taken then wound under the shape of a crown of about 5 cm in diameter. The length L is for example equal to 12 m. The diameter d is often equal to 0.25 mm.

[0082] 2) The sample is rinsed with water, then dried and dusted using an air jet compressed.

[0083] 3) The initial mass m; of the sample is measured using a balance of precision.

[0084] 4) The sample is then soaked for 20 to 30 seconds in an aqueous bath stirred with sulfuric acid between 8% and 12% concentration, the temperature of which is between 42°C and 48°C.

[0085] 5) The sample is rinsed with water.

[0086] 6) The sample is dried using a jet of compressed air.

[0087] 7) The final mass mf of the sample is measured using a balance of precision.

[0088] 8) The average thickness e of the sample is calculated using the relation (1) previous.

[0089] 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.

[0090] Chapter II: Example of embodiment

[0091] [Fig. 1] represents an electrode wire 2 for electroerosion machining as described in the introductory part of this text.

[0092] For this purpose, the electrode wire 2 has a breaking load greater than 400 N / mm2 or 700 N / mm2 and, generally, less than 1100 N / mm2. The wire 2 extends along of a longitudinal axis 4. Axis 4 is here perpendicular to the plane of the sheet. The length of wire 2 is greater than 1 m and, typically, greater than 10 m or 50 m.

[0093] The wire 2 has an outer face 6 directly exposed to sparks during the machining of a part by electroerosion using this wire. The outer face 6 is a cylindrical face which extends along the axis 4. The direction 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 D2 of the wire 2 is typically between 50 μm and 1 mm and, most often, between 70 μm and 400 μm. Here, the diameter D2 is equal to 0.25 mm.

[0094] In this embodiment, the wire 2 comprises:

[0095] - a central core 10 made of electrically conductive material, and

[0096] - a coating 12 directly deposited on the core 10.

[0097] 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 metal alloy.

[0098] 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 α-phase copper-zinc alloy or a copper-zinc alloy formed from a mixture of α and β phases. In particular, the core 10 does not have a central portion made of a copper-zinc alloy of a given phase covered with 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 %.

[0099] The diameter Dio of the core 10 is greater than 0.99*D2 or 0.995*D2. For example, here, the diameter D10 is greater than or equal to 0.249 mm.

[0100] 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.

[0101] The average thickness of the coating 12 is very small compared to the diameter D2 of the wire 2, that is to say less than 0.5% of the diameter D2 and, preferably, less than 0.25% of the diameter D2.

[0102] In this embodiment, the coating 12 is formed from 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.

[0103] Layer 12 is the surface layer of the electrode wire 2.

[0104] The average thickness e^ 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.

[0105] 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 a single block of material with the core 10.

[0106] The composition of zinc oxide may deviate slightly from stoichiometry. Composition analyses carried out using XPS (X-ray Photoelectron Spectroscopy) spectra have shown that the zinc oxide of layer 12 is composed, in atomic percentages:

[0107] - more than 90% zinc and oxygen,

[0108] - more than 5% copper, and

[0109] - various manufacturing residues.

[0110] 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.

[0111] A method of manufacturing the wire 2 will now be described with reference to [Fig.2].

[0112] For the implementation of this manufacturing method, typically, the desired thickness e^ is first chosen between 100 nm and 461 nm and, preferably, between 160 nm and 461 nm or between 160 nm and 350 nm. For example, here, the thickness el2 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 Do is chosen according to the coefficient Ci previously chosen and the desired final diameter D2 and while respecting the constraints stated in the following paragraph. By way of illustration, the final diameter D2 is taken equal to 0.25 mm.

[0113] In a step 80, a brass blank wire is first provided. The blank wire is a brass wire having a diameter Do between 1.3*D2 and 6*D2 and, preferably, between 1.3*D2 and 3.75*D2 or between 2*D2 and 3.75*D2. In this example, the diameter D2 is equal to 0.25 mm so that the diameter Do 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 Do is taken equal to 0.5 mm.

[0114] The zinc concentration of this rough wire is chosen as described previously in the case of the core 10. Here the zinc concentration of the rough wire is 40 atomic %. Indeed, the higher the zinc concentration, the better the performance of the thread 2 are improved.

[0115] In this embodiment, the blank wire having the desired diameter Do is obtained by drawing a brass wire of standard diameter Dini until the desired diameter Do is obtained. For example, the diameter Dini is equal to 1.25 mm.

[0116] Then, during 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, during 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 oxygen, at the 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 e0 is between 130 nm and 600 nm and, generally, between 160 nm and 600 nm.Indeed, a thickness e0 less than 130 nm does not allow obtaining a thickness ei2 greater than or equal to 100 nm after the drawing step 84 described later. Furthermore, it has been observed that a thickness e0 greater than 600 nm leads to a zinc oxide layer which does not adhere well and which is torn off, at least in places, during the drawing step 84. This tearing off of a part of the zinc oxide layer during the drawing step 84 makes it impossible to precisely control the thickness ei2. Indeed, it is very difficult to determine in advance the quantity of zinc oxide which will be torn off during the drawing step 84 and therefore to predict in advance the thickness e^ obtained after the drawing step 84. Thus, when the thickness eo 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 e^ of the manufactured wires 2 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.

[0117] Within the range [130 nm; 600 nm], the thickness e0 is determined by successive experiments by testing several thicknesses e0 included in this range until finding the thickness e0 which, after wire drawing, makes it possible to obtain the desired thickness e^ of zinc oxide. In particular, to determine the thickness e0, it must be taken into account that, even if the thickness e0 remains less than 600 nm, a small fraction of the zinc oxide is torn off during the wire drawing step. It has been evaluated 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 very in lower than the fraction of zinc oxide lost if the thickness e0 was chosen to be greater than 600 nm or 800 nm. Indeed, for a thickness e0 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 e0 tested are generally chosen in the interval [ci*ei2; Min(l,3*Ci*ei2; 600)] and, preferably, in the interval [l,l*Ci*ei2; Min(l,3*Ci*ei2; 600)] and, even more often, in the interval [l,2*Ci*ei2; Min(l,3*Ci*ei2; 600)], where:

[0118] - Ci is equal to the reduction coefficient of the diameter of the oxidized roughing wire during step 84 of wire drawing, and

[0119] - Min(a;b) is the function that returns the smallest of the values ​​a and b.

[0120] The reduction coefficient Ci is defined as being equal to the ratio Do / D2. Thus, when the thickness e12 is equal to 200 nm and the coefficient Ci is equal to two, the thickness e0 which makes it possible to obtain the desired thickness e12 after wire drawing is typically between 480 nm and 520 nm and, most often, equal to or very close to 500 nm.

[0121] Here the heat treatment used consists of placing a coil of the supplied rough wire in a furnace, in air, heated to a constant temperature Tfurnace for a duration Dfurnace and at ambient pressure. Here, the furnace is not airtight and the air is stirred throughout the duration of the heat treatment. The oxidation rate of the brass of the rough wire increases as a function of the temperature Tfurnace. Thus, the thickness of the zinc oxide layer that forms on the rough wire increases all the more rapidly as the temperature Tfurnace is high. Similarly, the thickness of the zinc oxide layer that forms on the rough wire increases as a function of the duration Dfurnace. Thus, by adjusting the temperature Tfurnace and the duration Dfurnace, it is possible to obtain the desired thickness e0 of zinc oxide.

[0122] More precisely, it has been established that the thickness e0, the temperature Tfour and the duration Dfour are related to each other, as a first approximation, by the following relation (2): Dfour = e02 / [k*exp(-Q / (R*Tfour))], where:

[0123] - k = 2.418*107 m2 / s,

[0124] - Q = 152 kJ / mol,

[0125] - R = 8.314 J / mol / K, and

[0126] - exp(... ) is the exponential function.

[0127] Using relation (2) it is possible to estimate a theoretical value DfourT of the duration Dfour to obtain a given thickness e0 with a given temperature Tfour. This is illustrated in the following table in the particular case where the thickness e0 is equal to 600 nm. This table indicates the theoretical value DfourT, in hours and fractions of an hour, necessary to achieve a thickness of 600 nm of zinc oxide for different temperatures Tfour. [Tables 1] T4°C) DfourT (h) 200 25 287 835 250 628 198 300 29 741 350 2 297 400 260 450 39.7 500 7.73 550 1.84 600 0.51 650 0.17 700 0.06 750 0.02

[0128] Then, several tests with different values ​​of the duration Dfour chosen around the theoretical value DfourT may be necessary to obtain the precise value of the duration Dfour which makes it possible to obtain exactly the desired thickness e0. Typically, the value of the duration Dfour retained at the end of these tests is included in the interval [0.8*DfourT; 1.2*DfourT] or in the interval [0.9*DfourT; 1.1*DfourT] or even in the interval [0.95*DfourT; 1.05*DfourT].

[0129] Furthermore, the temperature Tfour is preferably chosen not to be too high so as to correspond to a duration Dfour that is long enough so that the time required for the temperature to be uniform inside the entire coil is very small compared to the duration Dfour. Indeed, if the temperature Tfour chosen is very high, then the corresponding duration Dfour 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 Dfour is very short, the thickness e0 of the oxide layer formed has significant inhomogeneity along the oxidized rough wire. To avoid this problem, here, the duration Dfour 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 Tfour that must not be exceeded. Conversely, the duration Dfour must not be too long to be suitable for an industrializable manufacturing process. For this purpose, here the temperature Tfour is chosen between 400°C and 500°C.

[0130] At the end of the duration Dfour, the coil of rough wire is removed from the furnace. At this stage, the rough wire is covered with a layer of zinc oxide of thickness e0. It is therefore called “oxidized rough 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 ambient temperature. Step 82 is then completed.

[0131] During 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 this same brass located at the axis 4.

[0132] Then, during 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. During step 84, the diameter reduction coefficient Ci makes it possible to bring the diameter Do of the rough wire to the diameter D2 desired for the wire 2, that is to say here to a diameter of 0.25 mm.

[0133] 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 / mm2 so that such a wire cannot be used as an electrode wire at this stage. To obtain a breaking load greater than 700 N / mm2, 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 / mm2 and 900 N / mm2. The coefficient Ci must also be less than 6 so that the thickness e0 remains less than 600 nm.In the case where the diameter D2 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 / mm2 and 800 N / mm2.

[0134] It is emphasized that if the value of the coefficient Ci chosen results in a thickness Ci*ei2 greater than 600 nm, then the coefficient Ci and / or the thickness ei2 must be reduced to have, at the same time, a coefficient Ci greater than 1.3 and a thickness e0 less than 600 nm.

[0135] 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 achieved by passing the oxidized blank wire successively through several dies of decreasing diameter so as to progressively reduce the diameter of the oxidized blank wire until the desired diameter D2 is reached. For example, dies with elongations of between 15% and 22% are used. When drawing the oxidized roughing wire, a water-soluble lubricant is used. For example, here, the lubricant is an aqueous solution containing the water-soluble lubricant.

[0136] It is this drawing that can create the brass peaks that pass through layer 12.

[0137] At the end of step 84, once the diameter D2 is reached, an in-line stress relief annealing is carried out before its winding. This stress relief annealing makes it possible to minimize the residual stresses in the wire 2 to obtain a wire 2 whose axis 4 is very straight and therefore facilitates 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 two pulleys and the portion of the wire 2 between the two pulleys is heated while the wire 2 runs between these pulleys. The temperature and duration of this stress relief annealing are much lower than those used during step 82. Typically, the temperature for a stress relief annealing is between 300°C and 450°C and its duration is less than 2 s or 3 s.

[0138] 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 steel part with guides located less than 0.2 mm from the part. 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 specifically, 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.

[0139] Using the manufacturing method of [Fig. 2], different wires 2 were produced with different thicknesses e^. 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 el2 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 ei2 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. [Tables 2] Wire in (nm) Time (h) L7 7 0.88 L49 49 0.87 L106 106 0.84 L146 146 0.82 L218 218 0.80 L252 252 0.79 L280 280 0.80 L380 380 0.83 L436 436 0.79 Gamma 44 0.82

[0140] As illustrated by these tests, wire 2 makes it possible to machine practically as quickly as the “Gamma” wire from the moment when the thickness e^ is greater than 100 nm. In addition, for thicknesses el2 greater than 160 nm, it becomes possible to machine more quickly than the “Gamma” wire. For example, preferably, the thickness eu is between 160 nm and 350 nm or between 160 nm and 300 nm.

[0141] Furthermore, it has been 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 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:

[0142] - 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

[0143] - Step 2): weigh the quantity of dust that has detached from the wire when the die The kilometer of thread is finished.

[0144] 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.

[0145] Figures 3 to 5 show in detail the guide 100 used in the method of measuring friction resistance. In [Fig.4], the dimensions indicated are expressed in millimeters.

[0146] 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 section plane AA 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 Figures 3 to 5, the axis 102 is vertical.

[0147] The guide 100 comprises a friction face 104 whose longitudinal section in the cutting plane AA 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 onto the axis 102 forms a line 19.67 mm long. The orthogonal projection of this arc of a circle onto a plane perpendicular to the axis 102 forms a line 6.5 mm long.

[0148] 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.

[0149] Going downwards, the face 110 ends with a circular orifice 112 which forms the entrance to a frustoconical face 114.

[0150] The truncated face 114 is centered on the axis 102. This face 114 widens, going downwards, to an outlet orifice 116.

[0151] The face 104 is made of a material much harder than brass and zinc oxide. Here, the face 104 is made of ceramic. More precisely, the ceramic is zirconia (ZrO2) stabilized with Yttrium (Y). For example, this ceramic comprises approximately 6% of Ytrrium in the form of Y2O3 oxide. In this embodiment, the guide 100 is entirely made of zirconia (ZrO2) stabilized with Yttrium (Y) in the form of Y2O3 oxide. 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:

[0152] - Equipment brand: MAHR

[0153] - Controller reference: MarSurf M400

[0154] - Feed unit reference: MarSurf SD26

[0155] - Stylus reference: 6852404 BWF A 4-4.5 - 2 / 90° (90° tip with a radius from 2 pm)

[0156] - Cutting length 0.08 mm

[0157] - Evaluation length 5 times 0.08 mm

[0158] - Ls filter in operation

[0159] 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.

[0160] 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.

[0161] During step 1), the angle [3 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.

[0162] Preferably, during 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, during 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 introduced into the pad until it passes through the center of the pad. Then, during step 1), the wire passes through the pad and the dust sticks to the adhesive face. During step 2), it is the dust collected in this container that is weighed. For this, here, the adhesive pad is weighed before and after step 1).The difference between these two pellet weight measurements is equal to the weight of the collected dust.

[0163] Using this method, the friction resistance measured for the standard yarn is 7 mg / km and the friction resistance measured for yarn 2 is 2 mg / km.

[0164] Chapter III: Variants:

[0165] Electrode wire variants:

[0166] The outer face of layer 12 may be covered with a thin film of the lubricant used during wire drawing step 84.

[0167] Variants of the manufacturing process:

[0168] Alternatively, if the roughing wire having the desired diameter Do is commercially available, during step 80, it is not drawn before performing step 82.

[0169] 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 onto a coil at the exit of this tunnel. Inside the tunnel, the temperature is equal to the temperature Tfurnace. Thus, in this variant, the blank wire is heated to the temperature Tfurnace 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 Tfurnace and a very short duration Dfurnace. For example, when a heating tunnel is used, the value of the temperature Tfurnace 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 Dfour, during which a portion of the roughing wire remains inside the tunnel, allows the desired thickness e0 to be obtained.

[0170] 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 oxygen.

[0171] In another variant of step 82, the temperature Tfour varies during the duration Dfour. For example, the temperature Tfour increases continuously during the duration Dfour.

[0172] Cooling of the oxidized blank wire can also be carried out differently. For example, the furnace is turned off and the coil is left inside the furnace until it reaches room temperature.

[0173] In a simplified embodiment, during step 84, the stress relief annealing is omitted.

[0174] Several of the variants described above can be combined in the same embodiment.

[0175] Chapter IV: Advantages of the described embodiments:

[0176] 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.

[0177] 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 alloy coating. gamma-phase copper-zinc as described in US8338735B2.

[0178] 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.

[0179] The fact that the thickness e^ 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 copper-zinc alloy in gamma phase such as that described in US8338735B2.

[0180] The fact that the friction resistance of the zinc oxide layer is less than 7 mg / km makes it possible to limit the quantity 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.

[0181] The fact that the zinc oxide layer is the surface layer of the electrode wire makes it possible to increase the machining speed.

[0182] The fact that the zinc concentration of the peripheral face is greater than 36 atomic % makes it possible to further improve the machining performance.

[0183] The fact that the diameter Do is between 1.3*D2 and 6*D2 guarantees that the coefficient Ci of reduction of the diameter during wire drawing is greater than 1.3 and therefore that the breaking load of the manufactured electrode wire is greater than 700 N / mm2.

[0184] The fact that the diameter Do is between 1.3*D2 and 6*D2 combined with the fact that the thickness eo is between 100*(D0 / D2) nm and 600 nm, makes it possible to manufacture an electrode wire whose thickness e12 is between 100 nm and 461 nm.

[0185] The fact that the thickness e0 is less than 600 nm prevents 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 e12 is well controlled and reproducible.

[0186] 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 which lasts 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.

[0187] Reducing the diameter of the oxidized roughing wire by a factor greater than two makes it possible to obtain a breaking load greater than 770 N / mm2.

Claims

Claims

1. Electrode wire (2) for electroerosion machining, 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. An electrode wire according to claim 1, wherein the average thickness of the zinc oxide layer (12) is greater than or equal to 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 (ZrO2) 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 point (120) of contact 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 following a trajectory (122) parallel to the tangent at this outlet, then - weigh the quantity of dust which has detached from the wire when the running of the kilometer of wire is finished, the measured weight of this quantity of dust constituting the measurement of the friction resistance expressed in mg / km.,

4. An electrode wire according to claim 3, wherein the zinc oxide layer is the surface layer of the electrode wire.

5. An electrode wire according to any preceding claim, wherein the zinc concentration of the copper-zinc alloy of the metallic core (10) is greater than or equal to 36 atomic % or 40 atomic %.

6. An electrode wire according to claim 5, wherein 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 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: - of more than 90% zinc and oxygen, - of more than 5% copper, and - the remainder being formed from various residues.

8. A method of manufacturing an electrode wire according to any one of the preceding claims, this method comprising: - providing (80) a blank wire made of metal having a peripheral face, this blank wire being made of a single copper-zinc alloy whose zinc concentration is greater than 20 atomic %, the diameter Do of this blank wire being between 1.3*D2 and 6*D2, where D2 is the final diameter of the electrode wire to be manufactured by this method, then - oxidizing (82) the peripheral face of the blank wire provided to obtain an oxidized blank wire having a layer of zinc oxide directly on its peripheral face, this layer of zinc oxide covering this peripheral face, this oxidation of the blank wire provided being obtained by subjecting the blank 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 e0 is between 130 nm and 600 nm, then - the drawing (84) of the oxidized rough wire to obtain the electrode wire of diameter D2 and in which the layer of zinc oxide forms the outer face of the electrode wire, characterized in that: - the heat treatment is configured to obtain a layer of zinc oxide whose thickness e0 is between 100*(Do / D2) nm and 600 nm and, preferably, between 120*(Do / D2) nm and 600 nm, and - during the heat treatment, the pressure of the gas containing oxygen is greater than 50 kPa.,

9. A method according to claim 8, wherein the oxidation (82) of the peripheral face of the roughing wire comprises the following operations: - heating the roughing wire to a constant temperature Tfour for a duration Dfour of between 0.8*eo2 / [k*exp(-Q / (R*Tfour))] and 1.2*e02 / [k*exp(-Q / (R*Tfour))], where: - e0 is the desired thickness of the zinc oxide layer - k = 2.418*107 m2 / s - Q = 152 kJ / mol, - R = 8.314 J / mol / K, and - exp(...) is the exponential function, then - at the end of the Dfour time, cool the oxidized rough wire until its temperature drops below 35°C before drawing.

10. A method according to claim 9, wherein the oxidation (82) of the peripheral face of the blank wire comprises: - placing a coil of the rough wire inside a furnace heated to temperature Tfurnace, the temperature Tfurnace being between 400°C and 500°C, then - leave the coil inside the furnace for the entire duration of the Dfour then remove the coil from the furnace and cool it until the temperature of the oxidized rough wire drops below 35°C before drawing.

11. A method according to any one of claims 8 to 10, wherein the drawing step (84) reduces the diameter Do of the oxidized blank wire by a factor greater than two or 2.1.

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