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 machining by electro-erosion and to a method for manufacturing this electrode wire.

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

[0003] The well-known process of electrical discharge machining, or erosive sparking, removes material from an electrically conductive workpiece by generating sparks in a machining zone between the workpiece and an electrically conductive electrode wire. The electrode wire moves continuously in the vicinity of the workpiece along its length, held by guides, and is progressively moved transversely towards the workpiece, either by transverse translation of the wire guides or by translation of the workpiece.

[0004] An electric generator, connected to the electrode wire by electrical contacts away from the machining area, establishes a suitable potential difference between the electrode wire and the conductive workpiece. The machining area between the electrode wire and the workpiece is immersed in a suitable dielectric fluid. The potential difference causes sparks to appear between the electrode wire and the workpiece, which progressively erode both the workpiece and the electrode wire. The longitudinal movement of the electrode wire ensures that a sufficient wire diameter is maintained to prevent breakage in the machining area. The relative movement of the wire and the workpiece in the transverse direction allows the workpiece to be cut or its surface to be treated, as appropriate.

[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 accuracy, particularly for making small radius angle 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.

[0007] Most modern electro-erosion machining machines are designed to use metal wires, generally 0.25 mm in diameter, and with a breaking load 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 in the surface layer of the electrode wire, at the point of the spark, changes from a solid to a liquid or gaseous state and is displaced across the surface of the electrode wire and / or carried away into the dielectric fluid. It is observed that the outer face of the electrode wire affected by the spark has been deformed. generally taking a slightly concave, crater-like shape, with areas where the material has been melted and solidified again.

[0009] It has been observed that the effectiveness of sparks in electrical discharge machining (EDM) depends largely on the nature and topography of the surface layer of the electrode wire. Therefore, considerable improvements in EDM efficiency have been achieved 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 strength to withstand the mechanical tension load of the wire, and

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

[0012] For example, US patent 8338735B2 describes an electrode wire having a brass core coated with a layer of copper-zinc alloy. In this application, the copper-zinc alloy layer comprises a mixture of fractured gamma-phase copper-zinc alloy.

[0013] This particular coating structure is generally intended to ensure a higher machining speed of a part by electro-erosion.

[0014] The manufacturing processes for electrode wires, such as that described in US patent 8338735B2, generally include a step of depositing, typically by electrodeposition, a layer of zinc onto 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 coated 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 approximately 0.05 atm (5.07 kPa). An oxidized brass wire with a 300 nm zinc oxide layer is obtained. The oxidized brass wire is then drawn 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, a large portion of the zinc oxide is lost during the drawing process. Thus, after wire drawing, instead of obtaining a zinc oxide layer 150 nm thick, 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 this wire is used to machine a workpiece, the zinc oxide layer crumbles and fouls the electrode wire's guide components. To remedy this drawback, application JPS61203223A proposes coating the oxidized brass wire with a layer of varnish after drawing. However, coating the oxidized brass wire with a layer of varnish is not satisfactory. Indeed, this varnish layer is insoluble in water. Because of... This occurs when machining a workpiece with 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 flow of machining current between the electrode wire and the workpiece during machining.

[0016] The invention aims to provide an electrode wire whose performance is similar to that of the electrode wire described in US patent 8338735B2 while being simpler to manufacture.

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

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

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

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

[0021] Embodiments of this electrode wire may include 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 frictional 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 A tension of 12 N is applied to a friction face whose longitudinal cross-section in a cutting plane is a circular arc of radius 33 mm, this circular arc starting at an entry point and ending at an exit point. This friction face is made of zirconia (ZrO2) stabilized with yttrium (Y), and its roughness Ra is 0.03 pm. The wire enters contact with this friction face at a point of contact located between the entry and exit points, following a straight path contained within the cutting plane and forming a 30° angle with the tangent at the exit point. It then separates from this friction face at the exit point along a path parallel to the tangent at this exit point.

[0025] - weigh the amount of dust that came off the wire when the wire was being wound The kilometer of thread is finished, the measured weight of this quantity of dust constituting the measure of friction 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 greater 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 reaction in the presence of oxygen such that this zinc oxide is composed, in atomic percentage:

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

[0031] - plus 5% copper, and

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

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

[0034] - the supply of a metal blank wire having a peripheral face, this wire the roughing wire being made of a single copper-zinc alloy with a zinc concentration greater than 20 atomic percent, the diameter Do of this roughing 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] - the oxidation of the peripheral face of the blank wire supplied to obtain a wire oxidized blank wire having a zinc oxide layer directly on its peripheral face, this zinc oxide layer covering this peripheral face, this oxidation of the supplied blank wire being obtained by subjecting the blank wire to a heat treatment in the presence of an oxygen-containing gas, this heat treatment being configured to generate a zinc oxide layer on the peripheral face of the blank wire whose average thickness e0 is between 130 nm and 600 nm, then

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

[0037] in which:

[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 heat treatment, the pressure of the oxygen-containing gas is su greater than 50 kPa.

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

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

[0042] - heat the blank wire to a constant temperature Tfour for a duration Dfour between 0.8*e02 / [k*exp(-Q / (R*Tfour))] and l.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 Dfour time, cool the oxidized blank wire until its temperature the temperature drops below 35°C before the wire drawing process is carried out.

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

[0050] - place a coil of the blank wire inside an oven heated to temperature T oven, the temperature Tfour being between 400°C and 500°C, then

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

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

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

[0054] - [Fig. 1] is a schematic illustration of the cross-section of a wire electrode,

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

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

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

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

[0059] In these figures, the same reference numerals 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 Chapter I, definitions of certain terms are given. In Chapter II, a detailed example of an embodiment is described with reference to the figures. Then, in Chapter III, variants of these embodiments are presented. Finally, in 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 made of material A" designates an element in which material A represents at least 70%, by mass, of that element and preferably at least 90% or 95% by mass of that element.

[0063] A "copper-zinc alloy" refers to an alloy formed solely of copper and zinc, with the exception of unavoidable impurities. A copper-zinc alloy is also called "brass".

[0064] The term "electrical conductor" refers to 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 mainly extends.

[0066] The expression "cross-section" refers to a section of the electrode wire perpendicular to dicular to its longitudinal axis.

[0067] The expression "longitudinal section" refers to a section of the electrode wire made along a plane which contains its longitudinal axis.

[0068] The term "layer" refers to an annular layer of the electrode wire 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. Both of these circular boundaries are centered on the axis of the electrode wire. The inner circular boundary is the boundary of the layer closest to the axis of the electrode wire. Conversely, the outer circular boundary is the boundary of the layer farthest from the axis of the electrode wire. Between these inner and outer circular boundaries, the chemical composition is essentially 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.

[0069] The term "fractured layer" refers to a layer containing numerous fractures that partition it into a multitude of zones separated from each other, in a longitudinal section of the wire, by numerous radial fractures. Numerous 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 approximately ten blocks mechanically isolated from each other, in the longitudinal section, by these radial fractures.

[0070] The term "surface layer" refers to the outermost layer of the electrode wire. This surface layer may have a thin film on its surface composed of water-soluble residues, such as wire-drawing lubricant residues. The outer face of this surface layer is therefore either indistinguishable from 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 coated with a varnish, as in the case of the electrode wire described in application JPS61203223A, is not a surface layer because the applied varnish is not water-soluble.

[0071] The expression "room temperature" refers to 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 filament,

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

[0075] - p is the volume density of zinc oxide; this density p is here taken to be 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 and then wound under the shape of a crown 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.

[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 agitated 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 relation (1) previous.

[0089] Unless otherwise specified, in the remainder of this text, the term "zinc oxide layer thickness" alone refers to the average thickness of this zinc oxide layer.

[0090] Chapter II: Example of an embodiment

[0091] Fig. 1 represents an electrode wire 2 for electro-erosion 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 workpiece by electrical discharge machining (EDM) using this wire. The outer face 6 is a cylindrical face extending along the axis 4. The direction curve of the face 6 is essentially 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, more often, between 70 µm and 400 µm. Here, the diameter D2 is equal to 0.25 mm.

[0094] In this embodiment, 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 serves to provide, on its own, the majority of the load at the break of the wire 2. It also serves to ensure the electrical conductivity of the wire 2. For this purpose, it is made of an electrically conductive material. Typically, it is made of metal or a metal alloy.

[0098] The core 10 has a predominantly cylindrical peripheral face 14 extending 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 a copper-zinc alloy in phase α or a copper-zinc alloy formed from a mixture of phases α and β. In particular, the core 10 does not have a central portion of a copper-zinc alloy in a given phase covered with a layer of copper-zinc alloy in another phase. Typically, the zinc concentration in the core 10 is greater than 20 atomic percent and, preferably, greater than or equal to 36 atomic percent or 40 atomic percent. Typically, the zinc concentration of the core 10 is less than 42 atomic percent.

[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 surfaces of the part obtained after machining by electrical discharge machining (EDM). The quality of a surface cut by EDM is all the better the lower its roughness.

[0101] The average thickness of the coating 12 is very small compared to the diameter D2 of the wire 2, i.e. 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 of a single layer of zinc oxide. Thus, hereafter, the same numerical reference 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, layer 12 is essentially made of zinc oxide with the formula ZnO. However, in certain places, layer 12 may be traversed by brass peaks. These brass peaks form projections on the peripheral face 14 of the core 10 that pass through layer 12. These brass peaks form a single block of material with the core 10.

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

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

[0108] - of more than 5% copper, and

[0109] - of various manufacturing residues.

[0110] During these compositional analyses, the presence of carbon and carbon compounds on the surface of layer 12 was not taken into account. This carbon originates from the lubricant used during the wire drawing step of the electrode wire. According to the analyses performed, the zinc oxide corresponds to zincite.

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

[0112] For the implementation of this manufacturing process, 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 to be 200 nm. Next, 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 to be two. Finally, a diameter Do is chosen based on the previously chosen coefficient Ci 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 to be 0.25 mm.

[0113] In 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 0.25 mm, so 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 to be 0.5 mm.

[0114] The zinc concentration of this blank wire is chosen as described previously in the case of the core 10. Here, the zinc concentration of the blank wire is 40 atomic percent. 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] Next, in 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 zinc oxide layer completely covers the peripheral face of the oxidized blank wire. To this end, in step 82, the supplied blank wire is subjected to 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 containing more than 20%, by volume, dioxygen, at an ambient pressure of approximately 101 kPa. This heat treatment is configured to generate a zinc oxide layer on the peripheral face of the blank wire with an average thickness e0 of between 130 nm and 600 nm and, generally, between 160 nm and 600 nm.Indeed, a thickness e0 of less than 130 nm does not allow for a thickness ei2 greater than or equal to 100 nm after the wire drawing step 84 described below. Furthermore, it has been observed that a thickness e0 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 of part of the zinc oxide layer during the wire drawing step 84 makes precise control of the thickness ei2 impossible. In fact, it is very difficult to determine in advance the quantity of zinc oxide that will be torn off during the wire drawing step 84 and therefore to predict in advance the thickness e^ obtained after the wire 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 manufacturing parameters are kept equal, the differences between the thicknesses e^ of the manufactured wires increase.Furthermore, the removal of some of the zinc oxide constitutes a waste of material that 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, testing several thicknesses e0 within this range until the thickness e0 is found 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 below 600 nm, a small fraction of the zinc oxide is removed during the wire 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 very important. less than the fraction of zinc oxide lost if the thickness e0 were chosen 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 tested thicknesses e0 are generally chosen in the range [ci*ei2 ; Min(l,3*Ci*ei2; 600)] and, preferably, in the range [l,l*Ci*ei2; Min(l,3*Ci*ei2 ; 600)] and, even more often, in the range [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 blank 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 allows obtaining 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 blank wire in an air-heated furnace, heated to a constant temperature Tfour for a duration Dfour and at ambient pressure. The furnace is not airtight, and the air is circulated throughout the heat treatment. The oxidation rate of the brass in the blank wire increases with the temperature Tfour. Thus, the thickness of the zinc oxide layer that forms on the blank wire increases more rapidly as the temperature Tfour increases. Similarly, the thickness of the zinc oxide layer that forms on the blank wire increases with the duration Dfour. Therefore, by adjusting the temperature Tfour and the duration Dfour, 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, to 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 for the time Dfour required to obtain a given thickness e0 at a given temperature Tfour. This is illustrated in the following table in the specific case where the thickness e0 is equal to 600 nm. This table indicates the theoretical value DfourT, in hours and fractions of an hour, required 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] Next, several trials with different values ​​of the time Dfour chosen around the theoretical value DfourT may be necessary to obtain the precise value of the time Dfour which makes it possible to obtain exactly the desired thickness e0. Typically, the value of the time Dfour retained at the end of these trials is within the interval [0.8*DfourT ; l.2*DfourT] or within the interval [0.9*DfourT ; l.l*DfourT] or even within the interval [0.95*DfourT ; l.05*DfourT].

[0129] Furthermore, the temperature Tfour is preferably chosen not to be too high so as to correspond to a sufficiently long duration Dfour, ensuring that the time required for the temperature to become uniform throughout the entire coil is very small compared to the duration Dfour. Indeed, if the chosen temperature Tfour is very high, then the corresponding duration Dfour is very short. However, over a very short period, the heat does not have time to diffuse uniformly throughout the entire coil. Thus, in the case where the heat treatment consists of placing an entire coil of blank wire inside a furnace, when the duration Dfour is very short, the thickness e0 of the oxide layer formed exhibits significant inhomogeneity along the oxidized blank wire. To avoid this problem, the duration Dfour is advantageously chosen to be greater than four or six hours.This constraint allows us to determine a maximum value for the Tfour temperature that must not be exceeded. Conversely, the Dfour duration must not be too long to be suitable for an industrializable manufacturing process. For this reason, the Tfour temperature is chosen here between 400°C and 500°C.

[0130] At the end of the time Dfour, the spool of blank wire is removed from the furnace. At this stage, the blank wire is coated with a layer of zinc oxide of thickness e0. It is then called "oxidized blank wire". After being removed from the furnace, the spool is cooled. Conventionally, this is done by exposing the spool to ambient air for the time necessary to cool down to room temperature. Step 82 is then complete.

[0131] During step 82, the oxidation of zinc consumes the zinc present in the brass. Thus, the zinc concentration of the brass in the blank wire near the zinc oxide layer is generally lower than that of the same brass located at axis 4.

[0132] Then, in step 84, the oxidized and cooled blank wire is cold-drawn to obtain wire 2. "Cold-drawing" means that the drawing step 84 is carried out without heating the blank wire prior to reducing its diameter. In step 84, the diameter reduction coefficient Ci brings the diameter Do of the blank wire to the desired diameter D2 for wire 2, i.e., in this case, a diameter of 0.25 mm.

[0133] During step 82, and in particular during heat treatment, the brass recrystallizes, which reduces the tensile strength of the blank wire. At the end of step 82, the tensile strength of the oxidized blank wire is much lower than 700 N / mm², so such a wire is not usable as an electrode wire at this stage. To obtain a tensile strength 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 greater the tensile strength. Thus, preferably, the coefficient Ci is greater than or equal to 1.6 or 2.25, which makes it possible to obtain tensile strengths greater than 800 N / mm² and 900 N / mm², respectively. 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 to be 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 an unoxidized 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 between 15% and 22% are used. During the drawing of the For oxidized blank 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 wire drawing that can create the brass peaks that pass through layer 12.

[0137] At the end of step 84, once diameter D2 is reached, an in-line stress-relieving anneal is This stress-relieving annealing is performed before winding. It minimizes residual stresses in wire 2, resulting in a wire with a perfectly straight axis 4, thus facilitating its threading in an electrical discharge machining (EDM) machine. This stress-relieving annealing does not alter the composition of wire 2 and has little effect on its breaking strength. To achieve this, wire 2 is stretched between two pulleys, and the portion of wire 2 between the pulleys is heated as the wire passes between them. The temperature and duration of this stress-relieving annealing are significantly 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 or 3 seconds.

[0138] To demonstrate the advantages of an electrode wire with a thick layer of zinc oxide on its surface, the following tests were carried out. A reference electrical discharge machining (EDM) job was defined. This involved cutting a punch from a 50 mm high steel part with guides located less than 0.2 mm from the workpiece. The cutting was performed on a CUT200MS machine marketed by the company "GF Machining Solution". This cutting was carried out in three machining passes, using a technology adapted for brass. During each pass, the workpiece travel speed relative to the electrode wire was adjusted to cut the punch as quickly as possible while maintaining the same final surface finish. In this case, the final surface finish corresponds to a roughness Ra of 0.6 µm.More specifically, in the tests carried out, only the workpiece travel speeds relative to the electrode wire during the first and second passes were adjusted according to the wire used. The workpiece travel speed relative to the electrode wire during the third pass is the same for all tests carried out.

[0139] Using the manufacturing process of [Fig. 2], different wires 2 were produced with different thicknesses e^. The machining times of the punch using the different wires 2 are shown in the table below. In this table, the first column contains the wire designation. The second column contains the thickness e12, and the third column contains the corresponding machining time, expressed in hours and fractions of an hour. In this table, the wires designated L7 and L49 are identical to wire 2 except that their thickness e12 is not between 100 nm and 461 nm. The wire designated "Gamma" is an electrode wire conforming to the teachings of US patent 8338735B2. It has a surface layer of gamma-fractured copper-zinc alloy. More specifically, it is the electrode wire marketed by 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 allows machining to be done almost as fast as wire "Gamma" as long as the thickness e^ is greater than 100 nm. Moreover, for thicknesses el2 greater than 160 nm, it becomes possible to machine faster than wire "Gamma". 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 verified that the produced wire 2 exhibits a frictional resistance equal to or better than that of a standard wire. To this end, the frictional resistance of wire 2 is less than or equal to 7 mg / km and, preferably, less than 5 mg / km. Thus, wire 2 does not foul the wire guide elements of an electrical discharge machining (EDM) machine any more than a standard wire. Here, the standard wire is identical to wire 2 except that it is uncoated. The standard wire is therefore made entirely of brass. For this purpose, the frictional resistance of wire 2 and the standard wire were measured using the following method:

[0142] - Step 1): at room temperature, wind 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 straight trajectory 101 parallel to a direction D, then

[0143] - Step 2): Weigh the amount of dust that detached from the wire when the The spinning of the kilometer of thread is finished.

[0144] The measured weight of this quantity of dust constitutes the measure of the wire's friction resistance expressed in mg / km. Indeed, the more friable the zinc oxide layer 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 for 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 is datum 102. The cross-section of the guide 100 shown in [Fig. 4] is formed along a cutting 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 rotational symmetry about the axis 102. In Figures 3 to 5, the axis 102 is vertical.

[0147] The guide 100 has a friction face 104 whose longitudinal section in the cutting plane AA forms a circular arc that begins at an inlet 106 and ends at an outlet 108. The tangent of the circular arc at the outlet 108 is parallel to the axis 102. The radius of this circular arc is 33 mm. The orthogonal projection of this circular arc onto the axis 102 forms a line 19.67 mm long. The orthogonal projection of this circular arc onto a plane perpendicular to the axis 102 forms a line 6.5 mm long.

[0148] After exit 108, moving downwards, face 104 extends into a cylindrical face 110 parallel to axis 102. The horizontal cross-section of face 110 is a circle centered on axis 102 with a diameter greater than the diameter of the wire. Here, the diameter of face 110 is 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 frustoconical face 114 is centered on the axis 102. This face 114 flares out, 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 contains approximately 6% yttrium in the form of the oxide Y2O3. In this embodiment, the guide 100 is made entirely of zirconia (ZrO2) stabilized with yttrium (Y) in the form of the oxide Y2O3. 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] - Advance unit reference: MarSurf SD26

[0155] - Stylus reference: 6852404 BWF A 4-4.5 - 2 / 90° (90° point with a radius (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 100 guide is marketed by GF Machining Solution® under the term "Inletbush for Brake" with reference 326864 in their online catalogue accessible at the following address: https: / / ecatalog.gfms.com / gfms / fr / USD / search / 326864.

[0161] During step 1), the angle [3] between direction D and axis 102 is equal to 30°. Thus, the wire comes into contact with face 104 at a point 120 located just after inlet 106. The tangent at point 120 is parallel to direction D. Thus, during step 1), the wire advances inside guide 100, passing successively through inlet 106, then outlet 108, then orifice 112, and finally orifice 116. After orifice 116, the wire moves along a path 122 coinciding with axis 102. Under these conditions, during step 1), the wire only rubs against face 104.

[0162] 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 below the orifice 116. For example, in step 1), the falling dust is collected in a container located below the orifice 116. For example, the container is a circular adhesive pad three to four centimeters in diameter. This adhesive pad is placed just below the orifice 116, with its adhesive face facing the orifice 116. Before the wire is advanced, a slot is made in this pad to connect its periphery to its center. This slot allows the wire to be inserted into the pad until it passes through the center of the pad. Then, in step 1), the wire passes through the pad, and the dust adheres to the adhesive face. In step 2), the dust collected in this container is weighed. For this purpose, here, the adhesive pad is weighed before and after step 1).The difference between these two measurements of the pellet's weight is equal to the weight of the collected dust.

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

[0164] Chapter III: Variants:

[0165] Electrode wire variants:

[0166] The outer face of layer 12 can 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, in step 80, it is not drawn before carrying out step 82.

[0169] Other embodiments of the oxidation step 82 are possible. For example, alternatively, instead of placing an entire spool of the blank wire in a furnace, the blank wire is unwound, then passes through a heating tunnel, and is rewound onto a spool at the tunnel's exit. Inside the tunnel, the temperature is equal to the furnace temperature (Tfour). Thus, in this embodiment, the blank wire is heated to the furnace temperature (Tfour) section by section. Consequently, the problem of the time required to obtain a uniform temperature within an entire spool of blank wire does not arise. In this case, it is possible to use a higher furnace temperature (Tfour) and a very short heating time (Dfour). For example, when a heating tunnel is used, the furnace temperature (Tfour) can be greater than 600°C or 700°C.The feed rate of the blank wire inside the tunnel is then adjusted so that the duration Dfour, during which a portion of the blank wire remains inside the tunnel, allows the desired thickness e0 to be obtained.

[0170] Oxidation step 82 can also be carried out in a medium other than the Earth's atmosphere. For example, step 82 can also be carried out in a medium containing more than 20% or 30%, by volume, of dioxygen.

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

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

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

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

[0176] The fact that the zinc oxide layer is formed directly on the peripheral face of the metal core makes it possible to manufacture this zinc oxide layer by simply oxidizing 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 manufacturing a wire conforming to the teaching given in US patent 8338735B2. In particular, it is emphasized that the 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 oxide layer thickness is greater than 100 nm improves the machining speed and makes it possible, in particular, to obtain machining speeds close to or greater than those of an electrode wire having 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 allows a machining speed greater than that obtained with an electrode wire having a fractured coating of copper-zinc alloy in gamma phase such as that described in US8338735B2.

[0180] The fact that the frictional strength of the zinc oxide layer is less than 7 mg / km limits the amount of dust produced by the electrode wire when used to machine a workpiece. This therefore limits the fouling of the guide elements of electrical discharge machining (EDM) machines. It also eliminates the need to coat this zinc oxide layer with a varnish as described in application JPS61203223A. Consequently, 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 allows for further improvement of machining performance.

[0183] The fact that the diameter Do is between 1.3*D2 and 6*D2 guarantees that the coefficient Ci of diameter reduction 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 removal of part of the zinc oxide layer during wire drawing. Since there is no removal of part of the zinc oxide layer, the thickness e12 is well controlled and reproducible.

[0186] The fact that the maximum temperature reached during heat treatment is between 400°C and 500°C allows for a heat treatment lasting at least six hours. This duration of heat treatment ensures a uniform temperature throughout a coil of blank wire heated in the furnace, thus resulting in a more uniform zinc oxide layer along the entire length of the blank wire.

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

Claims

Demands

1. Electrode wire (2) for machining by electro-erosion having a breaking load greater than 400 N / mm2, this electrode wire comprising: - 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 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 160 nm.

3. Electrode wire according to any one of the preceding claims, wherein the frictional 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 over a friction face (104) whose longitudinal section in a cutting plane is an arc of a circle of radius 33 mm, this arc of a circle starting at an entry (106) and ending at an exit (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 straight 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 by following a trajectory (122) parallel to the tangent at this outlet, then - weigh the quantity of dust that has detached from the wire when the unwinding of the kilometer of wire is complete, the measured weight of this quantity of dust constituting the measure of the friction resistance expressed in mg / km.

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

5. Electrode wire according to any one of the preceding claims, wherein the zinc concentration of the copper-zinc alloy the metallic 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, wherein the zinc oxide of the zinc oxide layer (12) is obtained by heat treatment in the presence of oxygen such 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 of various residues.

8. A method for manufacturing an electrode wire according to any one of the preceding claims, this method comprising: - supplying (80) a blank wire of metal having a peripheral face, this blank wire being made of a single copper-zinc alloy having a zinc concentration 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 supplied blank wire 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 supplied blank wire 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 zinc oxide layer on the peripheral face of the blank wire, the average thickness e0 of which is between 130 nm and 600 nm, then - the drawing (84) of the oxidized blank wire to obtain the electrode wire of diameter D2 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 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 oxygen-containing gas is greater than 50 kPa.

9. A method according to claim 8, wherein the oxidation (82) of the peripheral face of the blank wire comprises the following operations: - heat the blank wire to a constant temperature Tfour for a duration Dfour 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 falls below 35°C before carrying out wire drawing.

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

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

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