Electrode wire and method for the production thereof

Abstract

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

Description

Electrode wire and method of its manufacture [1] The invention relates to an electrode wire for machining by electro-erosion and to a method for manufacturing this electrode wire. [2] Electrode wires are used to cut metals or electrically conductive materials by electro-erosion in an electro-erosion machining machine. [3] 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 along the length of the wire, held by guides, in the vicinity of the workpiece, and is progressively moved transversely towards the workpiece, either by transverse translation of the wire guides or by translation of the workpiece. [4] 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 gradually erode both the workpiece and the electrode wire. The longitudinal movement of the electrode wire ensures that a sufficient wire diameter is maintained at all times 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 treated, as appropriate. [5] The particles detached from the electrode wire and the workpiece by the sparks disperse into the dielectric fluid, where they are removed. [6] 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. [7] Most modern electrical discharge machining (EDM) machines are designed to use metal wires, usually 0.25 mm in diameter, with a breaking load of around 700 N / mm 2 and 1000 N / mm 2 . [8] When a spark occurs between the electrode wire and the workpiece, the surface of the electrode wire is suddenly heated to a very high temperature for a short time. As a result, the material 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. The outer face of the electrode wire affected by the spark is observed to be deformed, generally taking on a slightly concave, cratered shape, with areas where the material has been melted and then solidified again. [9] It has been observed that the effectiveness of sparks in electrical discharge machining (EDM) depends largely on the nature and topography of the electrode wire's surface layer. For this reason, considerable improvements in EDM efficiency have been achieved by using electrode wires comprising: - a metallic core made of one or more metals or alloys ensuring good electrical conductivity and good mechanical strength to withstand the mechanical tension load of the wire, and - a coating in one or more other metals or alloys and / or a particular topography, for example fractures, ensuring better efficiency of electro-erosion, for example a higher erosion rate.

[0010] 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.

[0011] This particular coating structure is generally intended to ensure a higher machining speed of a part by electro-erosion.

[0012] The manufacturing processes for electrode wires, such as the one described in US patent 8338735B2, generally involve a step of depositing, typically by electrodeposition, a layer of zinc onto the metal core. This step is complex to perform and consumes a great deal of energy.

[0013] Application JPS61203223A describes an electrode wire having a brass core coated with a layer of zinc oxide. The zinc oxide layer is obtained by A brass wire is placed 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 layer of zinc oxide is obtained. At this stage of production, the oxidized brass wire is not usable in most electrical discharge machining (EDM) machines because its tensile strength is no greater than 400 N / mm². 2To make this oxidized brass wire usable in most electrical discharge machining (EDM) machines, it is then drawn from a diameter of 0.4 mm to 0.2 mm. The drawing process increases the breaking strength of the oxidized brass wire, making it suitable for EDM. However, 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 drawing, instead of obtaining a 150 nm thick zinc oxide layer, the thickness is much smaller, less than 100 nm. Furthermore, since the zinc oxide layer obtained is very brittle, when using this wire to machine a part, the zinc oxide layer crumbles and fouls the guiding elements of the electrode wire.To address this drawback, application JPS61203223A proposes coating the oxidized brass wire with a layer of varnish after drawing. However, coating the oxidized brass wire with varnish is not a satisfactory solution. This varnish layer is insoluble in water. Consequently, when machining a workpiece with oxidized brass wire coated with such 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.

[0014] The prior art is also known from JPS61103731A, DE202017106956U1 and US2019 / 133919A1. The teaching of application JPS61103731A is similar to that of application JPS61203223A. In particular, in application JPS61103731A, the oxidation of the brass core is also carried out under very low pressure, which leads to the formation of a friable zinc oxide layer that is largely removed during wire drawing. Furthermore, the temperature and time conditions described in application JPS61103731A for oxidizing the brass core are similar to those described in application JPS61203223A, such that this also leads to the production of electrode wires whose breaking strength, before drawing, is insufficient for use in most electrical discharge machining (EDM) machines. Application DE202017106956U1 discloses only zinc oxide layers with thicknesses less than 100 nm. Application LIS2019 / 133919A1 does not disclose a zinc oxide layer directly formed on a brass core.

[0015] The invention aims to provide an electrode wire whose performance is similar to that of the electrode wire described in patent US8338735B2 while being simpler to manufacture.

[0016] The invention is described in the attached set of claims.

[0017] 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: - Figure 1 is a schematic illustration of the cross-section of an electrode wire, - Figure 2 is a flowchart of a manufacturing process for the electrode wire shown in Figure 1, - Figure 3 is a front view of a guide used to measure the frictional resistance of a wire, - Figure 4 is a longitudinal cross-sectional view of the guide in Figure 3, and - Figure 5 is a top view of the guide in Figure 3.

[0018] 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 a person skilled in the art are not described in detail.

[0019] 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, variations of these embodiments are presented. Finally, in Chapter IV, the advantages of the different embodiments are described.

[0020] Chapter I: Definitions and Terminology

[0021] The expression "element made of material A" or "element made of material A" means an element in which material A represents at least 70%, by mass, of that element and preferably at least 90% or 95% by mass of that element.

[0022] A "copper-zinc alloy" refers to an alloy composed solely of copper and zinc, with the inevitable impurities present. A copper-zinc alloy is also called "brass." "

[0023] The term "electrical conductor" refers to a material whose electrical conductivity, at 20 °C, is greater than 10 6 S / m and, preferably, greater than 10 7 S / m.

[0024] The longitudinal axis of a wire is the axis along which the wire mainly extends.

[0025] The term "cross section" refers to a section of the electrode wire perpendicular to its longitudinal axis.

[0026] The term "longitudinal section" refers to a section of the electrode wire made along a plane that contains its longitudinal axis.

[0027] 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.

[0028] The term "fractured layer" refers to a layer containing numerous fractures that partition it into many separate zones in a longitudinal section of the wire, separated by numerous radial fractures. Numerous radial fractures, over a 1 mm length of the electrode wire, refer to more than ten radial fractures that divide the layer into approximately ten blocks mechanically isolated from each other in the longitudinal section by these radial fractures.

[0029] 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 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.

[0030] The term "room temperature" refers to a temperature between 15°C and 35°C and, typically, equal to 25°C.

[0031] The average thickness e of a surface layer of zinc oxide is defined by the following relation (1): e = [mi-m f ] / [p*TT*d*L], where: - mi is the initial mass of a sample of a wire having a surface layer of zinc oxide, - m f is the mass of the same wire sample after being immersed in a bath that completely dissolves the surface layer of zinc oxide, - p is the volumetric density of zinc oxide; this density p is taken here to be 5600 kg / m³ 3 , - TT is the number pi, - d is the initial diameter of the wire sample before being immersed in the bath that completely dissolves the oxide layer, - L is the length of the wire sample, and - “*” is the symbol that denotes scalar multiplication.

[0032] The average thickness e is, for example, measured according to the following method: 1) A sample of wire of length L and diameter d is taken and then wound into a coil approximately 5 cm in diameter. The length L is, for example, equal to 12 m. The diameter d is often equal to 0.25 mm. 2) The sample is rinsed with water, then dried and dusted using a jet of compressed air. 3) The initial mass rrii of the sample is measured using a precision balance. 4) The sample is then soaked for 20 to 30 seconds in an agitated aqueous bath of sulfuric acid between 8% and 12% concentration, with a temperature between 42°C and 48°C. 5) The sample is rinsed with water. 6) The sample is dried using a jet of compressed air. 7) The final mass m f the sample is measured using a precision balance. 8) The average thickness e of the sample is calculated using the previous relation (1). Unless otherwise specified, in the remainder of this text, the term "zinc oxide layer thickness" alone refers to the average thickness of this zinc oxide layer.

[0033] A "straight" wire is defined as a wire whose sag is less than 35 mm when measured using the following method: 1) Place a piece of wire 35 cm to 45 cm long on a horizontal, sliding surface so that the wire can freely deform. The piece of wire placed on this horizontal surface will then form approximately an arc of a circle. 2) Place a 30 cm long straight ruler on the horizontal plane so that each end of the ruler is in contact, or almost in contact, with a respective point on the thread. The contact between the ends of the ruler and the thread should be made carefully enough so as not to deform the thread. 3) Measure the maximum distance between the string and the ruler. This maximum distance corresponds to the string's deflection. The string's deflection is usually located halfway between the ends of the ruler.

[0034] A "non-straight" wire is a wire that is not straight.

[0035] Chapter: Example of a method of implementation

[0036] Figure 1 represents an electrode wire 2 for electro-erosion machining as described in the introductory part of this text.

[0037] For this purpose, electrode wire 2 has a breaking load greater than 400 N / mm 2 and, most often, greater than 450 N / mm 2 or at 500 N / mm 2 or at 700 N / mm 2 The breaking load of electrode wire 2 is also generally less than 1100 N / mm². 2 Here, the breaking load of electrode wire 2 is within one of the following ranges [400 N / mm²]. 2 450 N / mm² 2], [400 N / mm 2 500 N / mm 2 ], [450 N / mm 2 700 N / mm 2 ], [500 N / mm 2 700 N / mm 2 or greater than 700 N / mm 2 Indeed, wire electrical discharge machining (EDM) can be straight, slightly tapered, or highly tapered. Machining is considered "straight" when the angle α between the electrode wire and the face of the workpiece setup table is between 82° and 98°. This setup table is generally horizontal. Machining is considered "slightly tapered" when the angle α is between 67° and 82° or between 98° and 113°. Machining is considered "highly tapered" when the angle α is between 45° and 67° or between 113° and 135°. The electrode wires have a tensile strength within the range of 400 N / mm². 2 450 N / mm 2 ] or [400 N / mm 2 500 N / mm 2] are generally used for highly tapered machining. Electrode wires with a breaking strength in the range of [450 N / mm²] 2 700 N / mm 2 ] or [500 N / mm 2 700 N / mm 2 These are generally used for machining with slight tapers. Electrode wires with a breaking strength greater than 700 N / mm² are used. 2 are used for straight machining. It is also emphasized that electrode wires with a breaking strength greater than 500 N / mm² are used. 2 They can be straightened by applying a stress-relieving anneal, which is not the case for electrode wires whose breaking strength is less than 450 N / mm². 2 Straight electrode wires are easier to thread and mount in an electrical discharge machining (EDM) machine than non-straight electrode wires. For this reason, wire 2 is subsequently described as having a breaking strength greater than 500 N / mm². 2 .

[0038] Thread 2 extends along a longitudinal axis 4. Axis 4 is perpendicular to the plane of the sheet. The length of thread 2 is greater than 1 m and, typically, greater than 10 m or 50 m.

[0039] 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 cylindrical and extends along axis 4. The direction curve of face 6 is essentially a circle centered on axis 4. Thus, the cross-section of wire 2 is circular. The outer diameter D2 of wire 2 is typically between 50 µm and 1 mm, and most often between 70 µm and 400 µm. Here, the diameter D2 is 0.25 mm.

[0040] In this embodiment, wire 2 comprises: - a central core 10 made of electrically conductive material, and - a coating 12 directly deposited on the core 10.

[0041] The core 10 is responsible for bearing most of the load at the break of wire 2. It also ensures the electrical conductivity of wire 2. For this purpose, it is made of an electrically conductive material. Typically, it is made of metal or a metal alloy.

[0042] 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 the a phase or a copper-zinc alloy formed from a mixture of a and p phases. In particular, the core 10 does not include a central portion of copper-zinc alloy in one phase covered by 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 in the core 10 is less than 42 atomic percent.

[0043] Diameter D w of the core 10 is greater than 0.99*D2 or 0.995*D2. For example, here, the diameter D is greater than or equal to 0.249 mm.

[0044] Coating 12 is designed to increase machining speed and therefore the erosive efficiency of the electrode wire and / or the quality of the surfaces of the part obtained after electrical discharge machining (EDM). The quality of a surface cut by EDM is directly proportional to its roughness.

[0045] The average thickness of the coating 12 is very small compared to the diameter D2 of the wire 2, that is less than 0.5% of the diameter D2 and preferably less than 0.25% of the diameter D2.

[0046] In this embodiment, the coating 12 consists of a single layer of zinc oxide. Therefore, the same numerical reference is subsequently used to designate both the coating 12 and the zinc oxide layer.

[0047] Layer 12 is the surface layer of electrode wire 2.

[0048] The average thickness ei2 of layer 12 is between 160 nm and 461 nm and preferably between 160 nm and 350 nm or between 160 nm and 300 nm.

[0049] In this embodiment, layer 12 is essentially made of zinc oxide with the formula ZnO. However, in certain areas, layer 12 may be traversed by brass peaks. These brass peaks form projections on the peripheral face 14 of the core 10 that penetrate layer 12. These brass peaks form a single block of material with the core 10.

[0050] The composition of zinc oxide can deviate slightly from stoichiometry. Compositional analyses performed using XPS (X-ray Photoectron Spectroscopy) spectra have shown that zinc oxide in layer 12 is composed, in atomic percentages: - containing over 90% zinc and oxygen, - more than 5% copper, and - various manufacturing residues.

[0051] 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 stage of the electrode. According to the analyses performed, the zinc oxide corresponds to zincite.

[0052] A manufacturing process for wire 2 will now be described with reference to Figure 2.

[0053] For the implementation of this manufacturing process, typically, the desired thickness ei2 is first chosen between 100 nm and 461 nm or between 105 nm and 461 nm, and preferably between 160 nm and 461 nm or between 160 nm and 350 nm. For example, here, the thickness ei2 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 D ois chosen based on the previously selected coefficient Ci and the desired final diameter D2, while respecting the constraints stated in the following paragraph. For illustration purposes, the final diameter D2 is taken to be 0.25 mm.

[0054] In step 80, a brass blank wire is first supplied. The blank wire is a brass wire with a diameter D o 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 the diameter D o is between 0.325 mm and 1.5 mm and, preferably, between 0.5 mm and 1.5 mm or between 0.5 mm and 0.94 mm. Here, the diameter D o is taken to be equal to 0.5 mm.

[0055] The zinc concentration of this blank wire is chosen as described previously for 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 wire 2.

[0056] In this embodiment, the roughing wire has diameter D o The desired diameter is obtained by drawing a brass wire of diameter D in j standard until diameter D is obtained o desired diameter D. For example, the diameter D in j is equal to 1.25 mm.

[0057] 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 periphery. This zinc oxide layer completely covers the periphery of the oxidized blank wire. To achieve this, 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, 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 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 for obtaining a thickness ei2 greater than or equal to 100 nm after the wire drawing step 84 described later. Furthermore, it has been observed that a thickness greater than 600 nm leads to a layer of zinc oxide which does not adhere well and which is torn off, at least in places, during step 84 of wire drawing.This removal of part of the zinc oxide layer during drawing step 84 makes precise control of the thickness ei2 impossible. Indeed, it is very difficult to determine in advance the quantity of zinc oxide that will be removed during drawing step 84 and therefore to predict in advance the thickness ei2 obtained after drawing step 84. Thus, when the thickness e0 is greater than 600 nm, the reproducibility of the manufacturing process is degraded. In fact, even if all manufacturing parameters are kept constant, the differences between the thicknesses ei2 of the manufactured wires 2 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.

[0058] Within the range [130 nm; 600 nm], the thickness e0 is determined through successive experiments, testing several thicknesses e0 within this range until the thickness e0 is found which, after wire drawing, yields the desired zinc oxide thickness ei2. 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 lost 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 much smaller than the fraction of zinc oxide lost if the thickness e0 were chosen to be greater than 600 nm or 800 nm. Indeed, for a thickness 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 e0 thicknesses are usually chosen within the range. [Ci*ei2; Min(1 ,3*Ci*ei2; 600)] and, preferably, in the interval [1 ,1*Ci*ei2; Min(1 ,3*Ci*ei2; 600)] and, even more often, in the interval [1 ,2*Ci*ei2; Min(1 ,3*Ci*ei2; 600)], where: - Ci is equal to the reduction coefficient of the diameter of the oxidized rough wire during drawing step 84, and - Min(a;b) is the function that returns the smallest of the values ​​a and b. The reduction coefficient Ci is defined as being equal to the ratio D0 / D2. Thus, when the thickness ei2 is equal to 200 nm and the coefficient Ci is equal to two, the thickness e0 which allows obtaining the desired thickness ei2 after wire drawing is typically between 480 nm and 520 nm and, most often, equal to or very close to 500 nm.

[0059] The heat treatment used here consists of placing a coil of the supplied blank wire in an oven, under air, heated to a temperature T four constant for a duration D f0U r and at ambient pressure. Here, the furnace is not airtight, and the air is circulated throughout the heat treatment. The oxidation rate of the brass blank wire increases with temperature T f0U r. Thus, the thickness of the zinc oxide layer that forms on the blank wire increases all the more rapidly as the temperature T f0U r is high. Similarly, the thickness of the zinc oxide layer that forms on the blank wire increases with time D four Thus, by adjusting the temperature T f0U r and the duration D f0U r, it is possible to obtain the desired thickness e0 of zinc oxide.

[0060] More specifically, it has been established that the thickness e0, the temperature T four and the duration Df0U r are related to each other, to a first approximation, by the following relation (2): D f0U r = e0 2 / [k*exp(-Q / (R*T foU r))], where: - k = 2.418*10' 7 m 2 / s, - Q = 152 kJ / mol, - R = 8.314 J / mol / K, and - exp(... ) is the exponential function.

[0061] Using relation (2) it is possible to estimate a theoretical value D f0U rT of the duration D f0U r to obtain a given thickness e0 at a temperature T f0U given r. 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 D f0U rT, in hours and fractions of an hour, necessary to achieve a thickness of 600 nm of zinc oxide for different temperatures T f0U r.

[0062] Next, several trials with different values ​​of the duration Df0U r chosen around the theoretical value D f0U rT may be necessary to obtain the precise value of the duration D f0U r which allows us to obtain exactly the desired thickness e0. Typically, the value of the duration D f0U The value retained at the end of these tests is within the interval [0.8*D foU rT; 1, 2*D f0U rT] or in the interval [0.9*D foU rT; 1, 1 *D f0U rT] or in the interval [0.95*D foU rT; 1.05*D foU rT].

[0063] Furthermore, the temperature T f0U r is preferably chosen so as not to be too high in order to correspond to a duration D f0U r long enough so that the time required for the temperature to become uniform throughout the coil is very small compared to the duration D f0U r. Indeed, if the temperature T f0U The chosen r is very high, so the duration D fourThe corresponding time 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 time D f0U r is very short, The thickness e0 of the oxide layer formed exhibits strong inhomogeneity along the oxidized blank wire. To avoid this problem, here, the duration D f0U r is advantageously chosen to be greater than four or six hours. This constraint allows us to determine a maximum value for the temperature T f0U r not to be exceeded. Conversely, the duration D f0U r must not be too long to be suitable for an industrializable manufacturing process. For this purpose, here the temperature T f0U r is chosen between 400°C and 500°C.

[0064] At the end of period D f0UThe 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. Typically, 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.

[0065] 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.

[0066] Next, 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 D to o of the roughing wire to the desired diameter D2 for wire 2, i.e. here to a diameter of 0.25 mm.

[0067] During step 82, and particularly during 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 significantly less than 700 N / mm². 2 Therefore, such a wire is not usable as an electrode wire at this stage. To obtain a breaking strength greater than 700 N / mm² 2It 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 breaking load. Thus, preferably, the coefficient Ci is greater than or equal to 1.6 or 2.25, which allows for breaking loads exceeding 800 N / mm², respectively. 2 and 900 N / mm 2 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 D o either 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 / mm 2 and 800 N / mm 2 .

[0068] 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.

[0069] Here, in step 84, the oxidized blank wire is drawn under the same conditions as those suitable for unoxidized brass wire. The diameter reduction is achieved by passing the oxidized blank wire successively through several dies of decreasing diameter, progressively reducing 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. A water-soluble lubricant is used during the drawing of the oxidized blank wire. For example, here, the lubricant is an aqueous solution containing the water-soluble lubricant.

[0070] It is this wire drawing process that can create the brass spikes that pass through layer 12.

[0071] At the end of step 84, once the diameter D2 is reached, an in-line stress-relieving anneal is performed before winding. This stress-relieving anneal minimizes residual stresses in wire 2 to obtain a straight wire 2, thus facilitating its threading in an electrical discharge machining (EDM) machine. This stress-relieving anneal does not alter the composition of wire 2 and has little effect on its breaking strength. To achieve this, wire 2 is stretched between an upstream pulley and a downstream pulley, and the portion of wire 2 between these two pulleys is heated as it passes between them. For example, the portion of wire 2 stretched between the two pulleys is heated by Joule heating by passing an electric current through this section of wire. The temperature and duration of this stress-relieving anneal are much lower than those used in step 82. Typically, the temperature for a stress-relieving anneal is between 300°C and 450°C and its duration is less than 2 or 3 seconds.Immediately after stress-relieving annealing, the electrode wire is preferably quenched to cool it rapidly to room temperature. For this, the electrode wire is immersed in a cold bath, that is, a bath whose temperature is at or below room temperature. Several methods are possible for immersing the wire in this cold bath immediately after stress-relieving annealing. For example, the pulley, in the direction of wire feed, is immersed in this cold bath. Alternatively, immediately after this downstream pulley, the wire passes through the cold bath. Here, the cold bath is an aqueous solution of polyethylene glycols (PEG). The average molar mass of the PEG used is between 200 and 1400 g / mol. The PEG molecules may be present in the aqueous solution as ethoxylated esters of dicarboxylic acids. The concentration of PEG in this aqueous solution is typically between 2% and 20% by volume, the remainder being water.

[0072] To demonstrate the advantages of an electrode wire with a thick zinc oxide coating on its surface, the following tests were conducted. 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 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 performed, 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 was the same for all tests.

[0073] Using the manufacturing process shown in Figure 2, different wires 2 were produced with varying thicknesses ei2. The machining times for 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 ei2, 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 ei2 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.

[0074] As illustrated by these tests, wire 2 allows machining almost as fast as wire "Gamma" as long as the thickness ei2 is greater than 100 nm. Furthermore, for thicknesses ei2 greater than 160 nm, it becomes possible to machine faster than wire "Gamma". For example, preferably, the thickness ei2 is between 160 nm and 350 nm or between 160 nm and 300 nm.

[0075] 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 components of an electrical discharge machining (EDM) machine any more than a standard wire. The standard wire is identical to wire 2 except that it is uncoated. The standard wire is therefore made entirely of brass. To this end, the frictional resistance of wire 2 and the standard wire was measured using the following method: - Step 1): At room temperature, wind 1 km of wire at a speed of 80 m / min under a tension of 12 N over a friction face of a guide 100 (Figs. 3 to 5), the wire coming into contact with this friction face following a straight path 101 parallel to a direction D, then - Step 2): Weigh the amount of dust that has detached from the thread when the kilometer of thread has been wound.

[0076] The measured weight of this quantity of dust constitutes a 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 amount of zinc oxide torn off during friction on the friction face of guide 100.

[0077] Figures 3 to 5 show in detail the guide 100 used in the method for measuring friction resistance. In Figure 4, the dimensions indicated are expressed in millimeters.

[0078] Guide 100 is a solid of revolution. Its axis of revolution is referenced as 102. The cross-section of guide 100 shown in Figure 4 is formed along a cutting plane AA that contains the axis 102. Thus, only the elements located on one side of the axis 102 in Figure 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.

[0079] 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.

[0080] 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 wire diameter. Here, the diameter of face 110 is 1 mm.

[0081] Moving downwards, face 110 ends with a circular orifice 112 which forms the entrance to a truncated conical face 114.

[0082] The truncated conical face 114 is centered on the axis 102. This face 114 flares out, going downwards, to an outlet orifice 116.

[0083] Face 104 is made of a material much harder than brass and zinc oxide. Here, 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, 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 0.03 pm. More precisely, the roughness of face 104 was measured thirty times using the following equipment and settings: - Equipment brand: MAHR - Controller reference: MarSurf M400 - Reference of the advance unit: MarSurf SD26 - Stylus reference: 6852404 BWF A 4-4.5 - 2 / 90° (90° tip with a 2 pm radius) - Cutting length: 0.08 mm - Evaluation length 5 times 0.08 mm - Ls filter in operation The average of the thirty measurements obtained is equal to 0.0305 pm and the standard deviation of these thirty measurements is equal to 0.0029 pm.

[0084] 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.

[0085] During step 1), the angle p 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 the inlet 106. The tangent at point 120 is parallel to direction D. Thus, during step 1), the wire advances inside the 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.

[0086] 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 opening 116. For example, in step 1), the falling dust is collected in a container located below the opening 116. For example, the container is a circular adhesive pad three to four centimeters in diameter. This adhesive pad is placed just below the opening 116 with its adhesive side facing the opening 116. Before the wire is wound, a slot is made in this pad to connect its periphery to its center. This slot This allows the wire to be inserted into the pad until it passes through the center. Then, in step 1), the wire passes through the pad and the dust adheres to the adhesive side. In step 2), the dust collected in this container is weighed. For this, the adhesive pad is weighed before and after step 1). The difference between these two measurements of the pad's weight is equal to the weight of the collected dust.

[0087] 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.

[0088] Chapter III: Variants:

[0089] Electrode wire variations:

[0090] The outer face of layer 12 can be covered with a thin film of the lubricant used during wire drawing step 84.

[0091] Variations in the manufacturing process:

[0092] Alternatively, if the roughing wire has diameter D o If the desired material is commercially available, during step 80, it is not drawn before executing step 82.

[0093] Other embodiments of oxidation step 82 are possible. For example, as an alternative, instead of placing an entire spool of blank wire in a furnace, the blank wire is unwound, then passes through a heated tunnel, and is rewound onto a spool at the tunnel's exit. Inside the tunnel, the temperature is equal to temperature T four Thus, in this variant, the blank wire is heated to temperature T f0U portion by portion. Therefore, the problem of the time required to obtain a uniform temperature within an entire coil of blank wire does not arise. In this case, it is possible to use a temperature T four higher and a duration D f0Ur very short. For example, when a heating tunnel is used, the temperature value T f0U r can be greater than 600°C or 700°C. The feed speed of the blank wire inside the tunnel is then adjusted so that the duration D four , during which a portion of the roughing wire remains inside the tunnel, allows the desired thickness e0 to be obtained.

[0094] 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.

[0095] In another variant of step 82, the temperature T f0U r varies during the duration D f0U r. For example, the temperature T f0U r increases continuously during the duration D f0U r.

[0096] The cooling of the oxidized blank wire can also be done differently. For example, the oven is turned off and the coil is left inside the oven until it reaches room temperature.

[0097] Other lubricants can be used during the drawing of the oxidized blank wire. For example, the lubricant used is an aqueous PEG solution with an average molar mass between 200 and 1400 g / mol, i.e., the same solution used to cool the electrode wire after stress-relieving annealing.

[0098] In a simplified embodiment, stress-relieving annealing is omitted in step 84. In this case, the electrode wire is simply immersed in the cold PEG bath. In another variant, immersion in the cold PEG bath is omitted.

[0099] Several of the variants described above can be combined in the same embodiment.

[0100] Chapter IV: Advantages of the described embodiments:

[0101] The fact that the zinc oxide layer is formed directly on the outer face of the metal core makes it possible to manufacture this zinc oxide layer by simply oxidizing the outer brass face of a blank wire. Thus, it is not necessary to deposit a zinc layer on the outer 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. Therefore, the electrode wire described here can be manufactured by simpler and more economical processes.

[0102] 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 that of an electrode wire having a fractured copper-zinc alloy coating in gamma phase such as that described in US8338735B2.

[0103] The fact that the thickness of the oxide layer is less than 461 nm improves the adhesion of this oxide layer to the metal core.

[0104] The fact that the thickness ei2 is greater than 160 nm allows for a higher machining speed than that obtained with an electrode wire having a fractured copper-zinc alloy coating in gamma phase such as that described in US8338735B2.

[0105] 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, in turn, reduces the fouling of the guide components 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 electrode wire surface can be avoided.

[0106] The fact that the zinc oxide layer is the surface layer of the electrode wire allows for increased machining speed.

[0107] The fact that the zinc concentration of the peripheral face is greater than 36% atomic allows for even better machining performance.

[0108] The fact that the diameter D oThe value being between 1.3*D2 and 6*D2 guarantees that the diameter reduction coefficient Ci during wire drawing is greater than 1.3, and therefore that the breaking strength of the manufactured electrode wire is greater than 700 N / mm². 2 .

[0109] The fact that the diameter D o be between 1.3*D2 and 6*D2 combined with the fact that the thickness e o is between 100*(D o / D2) nm and 600 nm, allows the manufacture of an electrode wire whose thickness ei2 is between 100 nm and 461 nm.

[0110] 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 ei2 is well controlled and reproducible.

[0111] 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 ensures a uniform temperature throughout the entire coil of blank wire heated in the furnace, resulting in a more uniform zinc oxide layer along the entire length of the blank wire.

[0112] Reducing the diameter of the oxidized blank wire by a factor greater than two results in a breaking strength exceeding 770 N / mm². 2 .

Claims

Claims 1. Electrode wire (2) for EDM machining with a breaking load greater than 400 N / mm 2 , this electrode wire comprising: - a metal core (10) comprising a peripheral face, this metal core being made from a single copper-zinc alloy, and - a layer (12) of zinc oxide directly formed on the peripheral face of the metal core and which covers this peripheral face, characterized in that the average thickness of the layer (12) of zinc oxide is between 100 nm and 461 nm.

2. Electrode wire according to claim 1, in which the average thickness of the zinc oxide layer (12) is greater than or equal to 105 nm or 160 nm.

3. Electrode wire according to any one of the preceding claims, in which the friction resistance of the zinc oxide layer is less than 7 mg / km when this resistance is measured using the following method: - at room temperature, run 1 km of wire at a speed of 80 m / min under a tension of 12 N on a friction face (104) whose longitudinal section in a cutting plane is an arc of a circle with a radius of 33 mm, this arc of a circle starting at an inlet (106) and ending at an outlet (108), this friction face being made of zirconia (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 exit, then - weigh the quantity of dust that has detached from the wire when the kilometer of wire has been run, the measured weight of this quantity of dust constituting the measurement of the friction resistance expressed in mg / km.

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

5. Electrode wire according to any one of the preceding claims, in which the zinc concentration of the copper-zinc alloy of the metal core (10) is greater than or equal to 36 atomic % or 40 atomic %.

6. Electrode wire according to claim 5, 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, wherein the breaking load of the electrode wire is greater than 450 N / mm 2 or at 500 N / mm 2 .

8. Electrode wire according to claim 7, wherein the breaking load of the electrode wire is greater than 700 N / mm 2 .

9. Electrode wire according to any one of the preceding claims, in which the zinc oxide of the zinc oxide layer (12) is obtained by a heat treatment in the presence of oxygen so that this zinc oxide is composed, in atomic percentage: - more than 90% zinc and oxygen, - plus 5% copper, and - the remainder being formed from various residues.

10. Method for manufacturing an electrode wire according to any one of the preceding claims, this method comprising: - the supply (80) of a rough metal wire comprising a peripheral face, this rough wire being made of a single copper-zinc alloy whose zinc concentration is greater than 20 atomic %, the diameter D oof 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 - the oxidation (82) of the peripheral face of the rough wire provided to obtain an oxidized rough wire comprising a layer of zinc oxide directly on its peripheral face, this layer of zinc oxide covering this peripheral face, this oxidation of the rough wire provided being obtained by subjecting the rough wire to a heat treatment in the presence of a gas containing oxygen, this heat treatment being configured to generate a layer of zinc oxide on the peripheral face of the rough wire whose average thickness e0 is between 130 nm and 600 nm, then - drawing (84) the oxidized rough 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 layer of zinc oxide whose thickness e0 is between 1 OO*(D o / D2) nm and 600 nm and, preferably, between 12O*(D O / D2) nm and 600 nm, and - during heat treatment, the pressure of the gas containing oxygen is greater than 50 kPa.

11. Method according to claim 10, in which the oxidation (82) of the peripheral face of the roughing wire comprises the following operations: - heat the roughing wire to a temperature T f0U r constant for a duration D four between 0.8*e0 2 / [k*exp(-Q / (R*Tf OU r))] and 1,2*e0 2 / [k*exp(-Q / (R*Tf OU r))], where: - e0 is the desired thickness of the zinc oxide layer - k = 2, 418*10' 7 m 2 / s - Q = 152 kJ / mol, - R = 8.314 J / mol / K, and - exp(... ) is the exponential function, then - at the end of duration D four , cool the oxidized rough wire until its temperature drops below 35°C before drawing.

12. Method according to claim 11, in which the oxidation (82) of the peripheral face of the roughing wire comprises: - place a coil of the rough wire inside a furnace heated to temperature T f0U r, the temperature T f0U r being between 400°C and 500°C, then - leave the coil inside the oven for the entire duration D f0U r then remove the coil from the furnace and cool it until the temperature of the oxidized rough wire drops below 35°C before carrying out the drawing.

13. A method according to any one of claims 10 to 12, wherein the drawing step (84) reduces the diameter D o roughing wire oxidized by a factor greater than two or 2.

1.

14. Method according to any one of claims 10 to 13, in which, after drawing (84) the oxidized rough wire to obtain the electrode wire of diameter D2, the method comprises a stress relief annealing which minimizes the residual stresses in the electrode wire then a soaking of the electrode wire in a bath containing an aqueous solution of polyethylene glycols with an average molar mass of between 200 and 1400 g / mol, the temperature of this aqueous solution being less than 35°C.

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