Electrode wire and method for manufacturing the same

Abstract

This electrode wire (2) for electrolytic corrosion processing is - A metal core (10) having a circumferential surface, wherein this metal core is made of a single copper-zinc alloy, - A layer of zinc oxide (12) is formed directly on the circumferential surface of the metal core and covers the circumferential surface. The zinc oxide layer (12) has an average thickness of 100 nm to 461 nm.

Description

[Technical Field] 【0001】 This invention relates to an electrode wire for EDM (electrical discharge machining) processing and a method for manufacturing the electrode wire. [Background technology] 【0002】 Electrode wires are used in EDM machines to cut metal or conductive materials by electrolytic corrosion. 【0003】 The well-known electrical discharge machining (EDM) process removes material from a conductive workpiece by generating sparks in the machining area between the workpiece and a conductive electrode wire. The electrode wire extends continuously in the direction of its length near the workpiece, is held by a guide, and gradually moves laterally toward the workpiece by the lateral translation of the wire guide or the translation of the workpiece. 【0004】 A generator, connected to the electrode wire via electrical contact and separated from the workpiece, establishes a suitable potential difference between the electrode wire and the conductive workpiece being processed. The workpiece area between the electrode wire and the workpiece is immersed in a suitable dielectric fluid. The potential difference between the electrode wire and the workpiece generates sparks that gradually erode the workpiece and the electrode wire. The longitudinal movement of the electrode wire ensures that a sufficient wire diameter is always maintained to prevent wire breakage in the workpiece area. Lateral relative movement between the wire and the workpiece allows for cutting or surface treatment of the workpiece as needed. 【0005】 Particles detached from the electrode wire and workpiece by the spark are dispersed in a dielectric fluid and discharged there. 【0006】 Especially when performing small-radius angle cuts, in order to achieve machining accuracy, it is necessary to use a small-diameter wire that can withstand high mechanical cutting loads so that tension can be applied to the wire in the machining area to limit the vibration amplitude. 【0007】 Most modern EDM machines typically have a diameter of 0.25 mm and a cutting load of 700 N / mm². 2 ~1,000 N / mm 2 It is designed to use metal wire. 【0008】 When a spark is generated between the electrode wire and the workpiece, the surface of the electrode wire is rapidly heated to a very high temperature in a short time. As a result, the material of the surface layer of the electrode wire changes from solid to liquid or gas at the site of the spark, displaced on the surface of the electrode wire, and / or discharged into the dielectric fluid. The outer surface of the electrode wire affected by the spark is deformed, and it is observed that it becomes a slightly concave, crater-like shape overall, with areas where the material has melted and then solidified again. 【0009】 The efficiency of spark erosion has been shown to depend heavily on the properties and surface morphology of the electrode wire's surface layer. Therefore, significant improvements in EDM efficiency have been achieved by using electrode wires that possess the following characteristics: - A metal core made of one or more metals or alloys, ensuring good current conductivity and good mechanical strength to withstand the mechanical tensile load of the wire, - One or more other metals or alloys, and / or specific surface shapes, such as coatings on fractured areas, to reliably improve EDM efficiency, for example, to accelerate erosion. 【0010】 For example, U.S. Patent No. 8,338,735,B2 describes an electrode wire having a brass core covered with a copper-zinc alloy layer. In this application, the copper-zinc alloy layer comprises a crushed gamma phase copper-zinc alloy mixture. 【0011】 This particular coating structure is generally designed to ensure that the machining speed of EDM workpieces is significantly higher. 【0012】 Methods for manufacturing electrode wires, such as the one described in U.S. Patent No. 8,338,735,B2, generally involve depositing a zinc layer onto a metal core, typically by electroplating. This is a complex and energy-intensive method. 【0013】 Japanese Patent Publication No. 61-203223A describes an electrode wire having a brass core coated with a zinc oxide layer. The zinc oxide layer is obtained by placing the 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 atmospheres (5.07 kPa). As a result, an oxide brass wire having a 300 nm zinc oxide layer is obtained. In this manufacturing stage, the obtained oxide brass wire has a breaking load of 400 N / mm². 2 For the following reasons, it cannot be used in most EDM machines. To make this oxide brass wire usable in most EDM machines, the oxide brass wire is then drawn from a diameter of 0.4 mm to a diameter of 0.2 mm. The drawing process increases the tensile strength of the oxide brass wire, making it suitable for use in EDM machines. However, the zinc oxide layer obtained by high-temperature heating under very low pressure is very brittle. As a result, much of the zinc oxide is lost during the drawing process. Therefore, instead of obtaining a zinc oxide layer with a thickness of 150 nm after drawing, the thickness of the zinc oxide layer becomes much smaller, less than 100 nm. In addition, because the resulting zinc oxide layer is very brittle, when a workpiece is processed using this wire, the zinc oxide layer crumbles and contaminates the electrode wire guide. To improve this drawback of contaminating the guide, Japanese Patent Publication No. 61-203223A proposes coating the oxide brass wire with a varnish layer after drawing. However, simply coating the oxide brass wire with a varnish layer is insufficient. This is because the varnish layer is insoluble in water. As a result, when machining parts using brass oxide wire coated with such varnish, the varnish is not dissolved by the water present during machining and therefore remains on the electrode wire. This obstructs the flow of current between the electrode wire and the workpiece during machining. 【0014】 Prior art includes Japanese Patent Publication No. 61-103731A, German Utility Model No. 202017106956U1, and U.S. Patent Application No. 2019 / 133919A1. The teachings in Japanese Patent Publication No. 61-103731A are similar to those in Japanese Patent Publication No. 61-203223A. In particular, in Japanese Patent Publication No. 61-103731A, the oxidation of the brass core is also carried out under very low pressure, resulting in the formation of a brittle zinc oxide layer, which is largely removed during stretching. Furthermore, the temperature and time conditions for oxidizing the brass core, as described in Japanese Patent Publication No. 61-103731A, are similar to those described in Japanese Patent Publication No. 61-203223A, which again results in electrode wires with pre-stretching cutting loads insufficient for use in most EDM machines. German Utility Model No. 202017106956U1 only discloses zinc oxide layers less than 100 nm thick. US Patent Application No. 2019 / 133919A1 does not disclose zinc oxide layers formed directly on a brass core. [Overview of the project] [Problems that the invention aims to solve] 【0015】 The object of the present invention is to provide an electrode wire that is easier to manufacture, while having performance similar to that of the electrode wire described in U.S. Patent No. 8,338,735B2. [Means for solving the problem] 【0016】 The present invention is described in the appended claims. 【0017】 The present invention is presented merely as a non-limiting example and will be better understood by reading the following description made with reference to the drawings. [Brief explanation of the drawing] 【0018】 [Figure 1] This is a schematic diagram of a cross-section of an electrode wire. [Figure 2]Figure 1 is a flowchart illustrating the manufacturing method of the electrode wire. [Figure 3] This is a front view of a guide used to measure the frictional resistance of a wire. [Figure 4] Figure 3 is a longitudinal cross-sectional view of the guide shown. [Figure 5] This is a top view of the guide shown in Figure 3. [Modes for carrying out the invention] 【0019】 In these figures, the same reference numerals are used to refer to the same elements. Features and functions that are well known to those skilled in the art are not described in detail in the remainder of this specification. 【0020】 Section I provides definitions of specific terms. Chapter II describes detailed examples of embodiments with reference to figures. Chapter III then presents variations of these embodiments. Finally, Chapter IV explains the advantages of various embodiments. 【0021】 Chapter I Definitions and Terms The expression "element made of material A" or "element of material A" refers to an element in which material A constitutes at least 70% by mass, preferably at least 90% or 95% by mass of this element. 【0022】 A "copper-zinc alloy" is an alloy made entirely of copper and zinc, with unavoidable impurities removed. Copper-zinc alloys are also known as "brass." 【0023】 The term "conductivity" refers to an electrical conductivity of 10 at 20°C. 6 Exceeding S / m, preferably 10 7 This refers to materials exceeding S / m. 【0024】 The longitudinal axis of a wire is the axis along which the wire primarily extends. 【0025】 The term "cut surface" refers to a cross-section of an electrode wire perpendicular to its longitudinal axis. 【0026】 The term "longitudinal section" refers to a cross-section of an electrode wire obtained along a plane containing the longitudinal axis. 【0027】 The term "layer" refers to the annular layer of electrode wire between the inner and outer circular boundaries at each cross-section of the electrode wire. These boundaries are not actually perfect circles. However, as a first approximation, this book treats these boundaries as circles. Both the inner and outer 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 furthest from the axis of the electrode wire. Between these inner and outer circular boundaries, the chemical composition is substantially homogeneous. Conversely, the chemical composition changes abruptly at both the inner and outer circular boundaries. In particular, the change in composition when crossing these circular boundaries is much larger than the slight change in composition that can be observed within the layer. 【0028】 The term "fractured layer" refers to a layer in the longitudinal cross-section of a wire that has many fractures, where a very large number of radial fractures divide the layer into many separate regions. A very large number of radial fractures means that in the longitudinal cross-section, these radial fractures divide the layer into approximately 10 (a dozen) blocks, with about a dozen radial fractures per millimeter of electrode wire. 【0029】 The term "surface layer" refers to the outermost layer of the electrode wire. This surface layer may have a thin film of water-soluble residues such as residues of stretching lubricants on the surface of the surface layer. The outer surface of this surface layer is thus integrated with the outer surface of the electrode wire when there is no thin film, or is separated from the outer surface of the electrode wire only by this thin film. Conversely, an electrode wire layer coated with varnish, as in the case of the electrode wire described in JP-A-61-203223, is not a surface layer because the varnish applied on the electrode wire layer is not water-soluble. 【0030】 "Ambient temperature" refers to a temperature of 15°C to 35°C, typically 25°C. 【0031】 The average thickness e of the zinc oxide surface layer is determined by the following relational expression (1). e=(m i -m f ) / (ρ×π×d×L) Here, - m i is the initial mass of the wire sample provided with the zinc oxide surface layer, - m f is the mass of the same wire sample after being immersed in a bath that completely dissolves the zinc oxide surface layer, - ρ is the bulk density of zinc oxide, where ρ is 5600 kg / m 3 is taken as, - π is the ratio of a circle's circumference to its diameter, - d is the initial diameter of the wire sample before being immersed in a bath that completely dissolves the oxide layer, - L is the length of the wire sample, and - "×" is the symbol for scalar multiplication. 【0032】 The average thickness e is measured, for example, using the following procedure. 1) Using a sample of wire length L and diameter d, wind it into a coil with a diameter of approximately 5 cm. The length L is, for example, 12 m. The diameter d is often equal to 0.25 mm. 2) Rinse the sample with water, then dry it and remove dust with compressed air. 3) The initial mass m of the sample i This is measured using a precision scale. 4) Next, immerse the sample in a stirred aqueous bath of 8% to 12% sulfuric acid at a temperature of 42°C to 48°C for 20 to 30 seconds. 5) Rinse the sample with water. 6) Dry the sample using a jet of compressed air. 7) Final mass m of the sample f This is measured using a precision scale. 8) The average thickness e of the sample is calculated using the above relational equation (1). Unless otherwise specified, in the remainder of this book, the term "thickness of the zinc oxide layer" refers only to the average thickness of the zinc oxide layer. 【0033】 A "straight" wire is one that has a deflection of less than 35 mm when measured using the following method: 1) Place a wire piece 35cm to 45cm long on a horizontal sliding surface so that it can be freely bent. At this time, the wire piece placed on the horizontal surface will form approximately an arc. 2) Place a 30cm long straight ruler on a horizontal surface so that both ends of the ruler are in contact with, or nearly in contact with, the respective ends of the wire. Ensure that the wire is not deformed when bringing both ends of the ruler into contact with the wire. 3) Measure the maximum distance between the wire and the ruler. This maximum distance corresponds to the slack in the wire. The wire's deflection is generally midway between the two ends of the ruler. 【0034】 A "non-straight" wire is a wire that is not in a straight line. 【0035】 Chapter II: Examples of Implementation Forms Figure 1 shows the electrode wire 2 for EDM processing described in the introduction to this book. 【0036】 Electrode wire 2 is rated at 400 N / mm² for this purpose. 2 It exceeds 450 N / mm² in many cases. 2 , 500 N / mm 2 , or 700 N / mm2 It has a breaking load exceeding [a certain value]. The breaking load of electrode wire 2 is also generally 1100 N / mm². 2 It is less than . Here, the breaking load of electrode wire 2 is (400 N / mm²). 2 ~450N / mm 2 ), (400N / mm 2 ~500N / mm 2 ), (450N / mm 2 ~700N / mm 2 ), (500N / mm 2 ~700N / mm 2 ) within any of the ranges, or 700 N / mm 2 Wire EDM can be machined straight, slightly conical, or highly conical. When the angle α between the electrode wire and the workpiece positioning table surface is 82° to 98°, the machining is said to be "straight." This table is generally horizontal. When the angle α is 67° to 82° or 98° to 113°, the machining is said to be "slightly conical." When the angle α is 45° to 67° or 113° to 135°, the machining is said to be "highly conical." Highly conical machining generally requires a cutting load of (400 N / mm²). 2 ~450N / mm 2 ) or (400N / mm 2 ~500N / mm 2 Use electrode wires within the range of (450 N / mm²). For slightly conical machining, generally, use electrode wires with a breaking load of (450 N / mm²). 2 ~700N / mm 2 ) or (500N / mm 2 ~700N / mm 2 Use electrode wires within the range of ). Breaking load of 700 N / mm 2 Electrode wires exceeding this limit are used for straightening. Breaking load: 500 N / mm 2 Electrode wires exceeding a certain limit can be straightened by stress relief annealing, which is required when the breaking load is 450 N / mm². 2 It should also be noted that this does not apply to electrode wires less than 500 N / mm². Straight electrode wires are easier to insert and attach in an EDM machine than straight electrode wires. Therefore, wire 2 is defined as having a breaking load of 500 N / mm².2 It is a wire that exceeds [a certain limit]. 【0037】 Wire 2 extends along the longitudinal axis 4. Axis 4 is perpendicular to the plane of the paper. The length of wire 2 is greater than 1 m, typically exceeding 10 m or 50 m. 【0038】 The wire 2 has an outer surface 6 that is directly exposed to sparks when a workpiece is machined using electrical discharge machining (EDM) with this wire. The outer surface 6 is cylindrical and extends along the axis 4. The curve that is the directrix of surface 6 is essentially a circle centered on the axis 4. Therefore, the cross-section of the wire 2 is circular. The outer diameter D2 of the wire 2 is typically 50 μm to 1 mm, and quite often 70 μm to 400 μm. Here, the diameter D2 is 0.25 mm. 【0039】 In this embodiment, wire 2 comprises the following: - A central core 10 made of conductive material, and - Coating 12 applied directly to core 10. 【0040】 The function of core 10 is to secure the majority of the breaking load on wire 2 on its own. Core 10 also ensures the conductivity of wire 2. For this purpose, core 10 is made of a conductive material. Core 10 is typically made of metal or a metal alloy. 【0041】 The core 10 has a mainly cylindrical circumferential surface 14 extending along the axis 4. This circumferential surface 14 is made of a copper-zinc alloy. For this purpose, the core 10 is made entirely of a copper-zinc alloy. For example, the single copper-zinc alloy of the core 10 is an α-phase copper-zinc alloy, or a copper-zinc alloy formed of a mixture of α-phase and β-phase. The core 10 does not have a central portion of a single-phase copper-zinc alloy covered with a layer of another phase copper-zinc alloy. The zinc concentration of the core 10 is typically greater than 20 atomic percent (atomic percentage or "at%)", preferably 36 atomic percent or more, or 40 atomic percent or more. The zinc concentration of the core 10 is typically less than 42 atomic percent. 【0042】 Core 10 diameter D 10 is greater than 0.99 × D² or greater than 0.995 × D². Diameter D 10 In this case, for example, it would be 0.249 mm or more. 【0043】 Coating 12 is designed to improve machining speed, and consequently, the erosion efficiency of the electrode wire and / or the surface quality of the part obtained after EDM machining. The lower the roughness, the better the quality of the EDM-machined surface. 【0044】 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, preferably less than 0.25% of the diameter D2. 【0045】 In this embodiment, the coating 12 consists of a single layer of zinc oxide. Hereafter, the same reference numerals will be used to refer to both the coating 12 and the zinc oxide layer. 【0046】 Layer 12 is the surface layer of the electrode wire 2. 【0047】 Average thickness e of layer 12 12 The wavelength range is 160nm to 461nm, preferably 160nm to 350nm or 160nm to 300nm. 【0048】 In this embodiment, layer 12 is essentially made of zinc oxide, chemical formula ZnO. However, in some places, brass spikes may penetrate layer 12. These brass tips form projections on the circumferential surface 14 of core 10 that penetrate layer 12. These brass spikes, together with core 10, form a single block of material. 【0049】 The composition of zinc oxide may deviate slightly from the stoichiometric ratio. Compositional analysis performed using XPS (X-ray photoelectron spectroscopy) spectra showed that the zinc oxide in layer 12 has the following atomic percentage composition. - Over 90% zinc and oxygen, - More than 5% copper, and - Various manufacturing residues. 【0050】 During these compositional analyses, the presence of carbon and carbon compounds on the surface of layer 12 was not considered. This carbon actually originates from the lubricant used during the wire drawing process. According to the analysis performed, the zinc oxide matches that of zinc ore. 【0051】 Next, the method for manufacturing wire 2 will be explained with reference to Figure 2. 【0052】 To carry out this manufacturing method, typically, first the desired thickness e 12 Select the thickness e between 100nm and 461nm or 105nm and 461nm, preferably between 160nm and 461nm or 160nm and 350nm. 12 For example, here we set it to be equal to 200 nm. Next, we select a value for coefficient c1, which is described below, according to the constraints also described below. For example, we select coefficient c1 to be equal to 2 here. Finally, we select the diameter D0 as a function of the previously selected coefficient c1 and the desired final diameter D2, while adhering to the constraints stated in the following paragraphs. For example, we set the final diameter D2 to be equal to 0.25 mm. 【0053】 In step 80, a brass blank wire is first supplied. The blank wire is a brass wire with a diameter D0 of 1.3×D2 to 6×D2, preferably 1.3×D2 to 3.75×D2, or 2×D2 to 3.75×D2. In this example, the diameter D2 is equal to 0.25 mm, so the diameter D0 is 0.325 mm to 1.5 mm, preferably 0.5 mm to 1.5 mm, or 0.5 mm to 0.94 mm. Here, the diameter D0 is 0.5 mm. 【0054】 The zinc concentration of this blank wire is selected as described above for core 10. Here, the zinc concentration of the blank wire is 40 atomic percent. The higher the zinc concentration, the better the performance of wire 2. 【0055】 In this embodiment, a blank wire of a desired diameter D0 is a standard diameter D ini This is obtained by stretching a brass wire to a desired diameter D0. ini For example, it is 1.25 mm. 【0056】 Next, in step 82, the blank wire is oxidized to obtain an oxide blank wire. The oxide blank wire has a zinc oxide layer directly on its circumferential surface. This zinc oxide layer completely covers the circumferential surface of the oxide blank wire. For this purpose, in step 82, the supplied blank wire is heat-treated in the presence of oxygen. This heat treatment is carried out in an oxygen-containing gaseous medium at a pressure exceeding 50 kPa or 100 kPa. This heat treatment is carried out here simply in the Earth's atmosphere, i.e., in a medium containing more than 20 volume% oxygen, at an ambient pressure of approximately 101 kPa. This heat treatment is configured to generate a layer of zinc oxide on the circumferential surface of the blank wire with an average thickness e0 of 130 nm to 600 nm, generally 160 nm to 600 nm. In fact, if the thickness e0 is less than 130 nm, a thickness of 100 nm or more will be obtained after the stretching step 84 described below. 12It is impossible to obtain this. On the other hand, when the thickness e0 exceeds 600 nm, the zinc oxide layer does not adhere sufficiently and it was observed that it peels off at least in some places during the stretching step 84. Due to this peeling of part of the zinc oxide layer during the stretching step 84, the thickness e 12 Precise control of this becomes impossible. In fact, it is very difficult to determine in advance the amount of zinc oxide that will be peeled off during the stretching step 84, and therefore the thickness e obtained after the stretching step 84 12 It is extremely difficult to predict this in advance. Consequently, when the thickness e0 exceeds 600 nm, the reproducibility of the manufacturing method decreases. In fact, even if all manufacturing parameters are kept equal, the thickness e of the manufactured wire 2 12 The difference between them becomes large. In addition, if some of the zinc oxide peels off, the peeled zinc oxide is not used in the wire 2 that is manufactured, resulting in material waste that should be avoided or limited. 【0057】 The thickness e0 is the desired thickness of zinc oxide after stretching, within the range (130nm to 600nm). 12 The thickness e0 that produces the desired result is determined by a series of experiments testing multiple thicknesses e0 within this range. In particular, when determining the thickness e0, it must be considered that a small percentage of zinc oxide will be stripped off during the stretching step, even if the thickness e0 is still less than 600 nm. This small percentage of zinc oxide is currently estimated to be as high as 20% or 30%. It should be emphasized that this small percentage of zinc oxide lost during the stretching process is still far less than the percentage of zinc oxide lost when selecting a thickness e0 greater than 600 nm or 800 nm. In fact, when the thickness e0 is greater than 600 nm or 800 nm, the percentage of zinc oxide lost during stretching exceeds 50% or 67%. The thickness of e0 to be tested is typically (c1 × e 12 ~Min(1.3 × c1 × e 12 Preferably in the range of ,600)), (1.1 × c1 × e 12 ~Min(1.3 × c1 × e 12In the range of ,600)), and in many cases (1.2 × c1 × e 12 ~Min(1.3 × c1 × e 12 Select within the range of ,600)), and here, - c1 is equal to the diameter reduction factor of the oxide blank wire in stretching step 84, and - Min(a,b) is a function that returns the minimum value between a and b. The reduction coefficient c1 is defined as the ratio D0 / D2. Therefore, the thickness e 12 If is equal to 200 nm and coefficient c1 is equal to 2, then the desired thickness e after stretching will be 12 The thickness e0 at which it is possible to obtain this result is typically between 480 nm and 520 nm, most often equal to or very close to 500 nm. 【0058】 The heat treatment used here involves heating the supplied blank wire coils under atmospheric pressure for a duration of D four Over a certain period, at a constant temperature T four This consists of placing the blank wire in a heated furnace. Here, the furnace is not airtight, and air is circulated throughout the heat treatment process. The oxidation rate of brass blank wire is determined by temperature T four It increases as a function of temperature T. As a result, the thickness of the zinc oxide layer formed on the blank wire increases with temperature T. four The higher the value, the more rapidly it increases. Similarly, the thickness of the zinc oxide layer formed on the blank wire depends on the duration D four It increases as a function of temperature T. four and duration D four By adjusting this, it becomes possible to obtain the desired zinc oxide thickness e0. 【0059】 More specifically, thickness e0, temperature T four , and duration D four The following relationship has been established as a linear approximation: (2) below. D four =e0 2 / (k×exp(-Q / (R×T four ))) Here, - k = 2.418 × 10 -7 m 2 / sec, - Q = 152 kJ / mol, - R = 8.314 J / mol / K, and - exp(...) is an exponential function. 【0060】 Using relation (2), four The duration D required to obtain a given thickness e0. four Theoretical value D fourT It is possible to estimate this. This is shown in the table below for the specific case where the thickness e0 is equal to 600 nm. This table shows various temperatures T four Regarding this, the theoretical value D required to achieve a zinc oxide thickness of 600 nm is... fourT This is shown in units of time and in decimal places. 【0061】 [Table 1] 【0062】 The duration D over which the desired thickness e0 can be accurately obtained continues. four To obtain the exact value of the theoretical value D fourT Various durations selected in the vicinity D four Multiple trials using the value of may be necessary. The duration D selected as a result of such trials four The value of is typically (0.8 × D fourT ~1.2×D fourT ) range, (0.9 × D fourT ~1.1×D fourT ) within the range of (0.95 × D fourT ~1.05×D fourT It is within the range of ). 【0063】 Furthermore, the time required to equalize the temperature throughout the entire coil is ensured, for a duration of D. four Length D is long enough to make it significantly shorter compared to four To make it compatible, temperature Tfour It is preferably selected so as not to be too high. In fact, for the selected temperature T four if it is very high, the corresponding duration D four will be very short. However, in a very short time, there is no time for heat to spread evenly throughout the coil. Therefore, when the heat treatment consists of placing the entire coil of the blank wire inside the furnace, if the duration D four is very short, the thickness e0 of the formed oxide layer will show a large non-uniformity along the oxidized blank wire. The duration D four is advantageously selected to be longer than 4 hours or longer than 6 hours in order to avoid this problem. This constraint makes it possible to determine the maximum value of the temperature T four that must not be exceeded. On the other hand, the duration D four must not be so long as to be unsuitable for the industrial production process. The temperature T four is selected between 400 °C and 500 °C for this purpose. 【0064】 At the end of the duration D four the coil of the blank wire is taken out of the furnace. At this point, the blank wire is covered with a layer of zinc oxide with a thickness e0. The blank wire is thus called "oxidized blank wire". The coil is cooled after being taken out of the furnace. Cooling typically includes exposing the coil to the ambient air for the time required to cool it to room temperature. This completes step 82. 【0065】 In step 82, due to the oxidation of zinc, the zinc present in the brass is consumed. As a result, the zinc concentration in the brass of the blank wire near the zinc oxide layer is generally lower than the zinc concentration in the brass near axis 4. 【0066】 Next, in step 84, the oxidized and cooled blank wire is cold-drawn to produce wire 2. "Cold-drawn" refers to the fact that the drawing step 84 is performed without heating the blank wire before reducing its diameter. During step 84, a diameter reduction factor c1 is used to reduce the diameter D0 of the blank wire to the desired diameter D2 of wire 2, i.e., a diameter of 0.25 mm. 【0067】 During step 82, particularly during heat treatment, the brass recrystallizes, reducing the breaking load of the blank wire. At the end of step 82, the breaking load of the oxidized blank wire is 700 N / mm². 2 The value is considerably lower than this, and therefore, at this stage, such wires cannot be used as electrode wires. 700N / mm 2 To achieve a breaking load exceeding 800 N / mm², it has been determined that the coefficient c1 must be 1.3 or higher. More precisely, the higher the coefficient c1, the greater the breaking load. Therefore, it is preferable that the coefficient c1 be 1.6 or higher or 2.25 or higher, which results in a breaking load of 800 N / mm², respectively. 2 and 900 N / mm 2 It becomes possible to obtain a cutting load exceeding this value. The coefficient c1 must also be less than 6 in order to keep the thickness e0 below 600 nm. If the diameter D2 is equal to 0.25 mm, then in order to make the coefficient c1 equal to 1.3, the diameter D0 must be greater than 0.325 mm and less than 1.5 mm. Here, 700 N / mm 2 ~800N / mm 2 To obtain the cutting load, the coefficient c1 is selected to be equal to 2. 【0068】 Depending on the selected coefficient c1 value, the thickness c1 × e 12 If the coefficient c1 and / or thickness e are greater than 600 nm, 12 It should be emphasized that, in addition to reducing the coefficient c1, the material must have a coefficient c1 greater than 1.3 and a thickness e0 less than 600 nm. 【0069】 Here, in step 84, the oxide blank wire is stretched under the same conditions as those favorable for the unoxidized brass wire. Diameter reduction is achieved by sequentially passing the oxide blank wire through a series of dies that shorten the diameter, so that the diameter of the oxide blank wire is gradually reduced until it reaches the desired diameter D2. For example, dies with elongation rates of 15% to 22% are used. A water-soluble lubricant is used to stretch the oxide blank wire. Here, the lubricant is, for example, an aqueous solution containing a water-soluble lubricant. 【0070】 This stretching process makes it possible to create a brass tip that penetrates layer 12. 【0071】 At the end of step 84, upon reaching diameter D2, an in-line stress relief annealing process is performed before winding. This stress relief annealing minimizes residual stress in wire 2, resulting in a straight wire 2, which in turn facilitates insertion of wire 2 into the EDM machine. This stress relief annealing does not alter the composition of wire 2 and has little effect on its tensile strength. To achieve this, tension is applied to wire 2 between an upstream pulley and a downstream pulley, and the portion of wire 2 between these pulleys is heated as the wire 2 travels between these two pulleys. For example, the portion of wire 2 stretched between the two pulleys is heated using the Joule effect by passing an electric current through this portion of the wire. The temperature and duration of this stress relief annealing are much lower and shorter than those used in step 82. Typically, the stress relief annealing temperature is 300°C to 450°C, and the duration is less than 2 or 3 seconds. The electrode wire is preferably quenched immediately after stress relief annealing to cool rapidly to room temperature. For this purpose, the electrode wire is immersed in a cold bath, i.e., a bath at a temperature below room temperature. Various solutions are available for immersing the wire in this cold bath immediately after stress relief annealing. For example, the pulley downstream in the direction the wire travels is immersed in the cold bath. Alternatively, the wire is passed through the cold bath immediately after the downstream pulley. The cold bath here is an aqueous solution of polyethylene glycol (PEG). The average molar mass of PEG used is 200 to 1400 g / mol. PEG molecules may exist in the aqueous solution as ethoxylated esters of dicarboxylic acids. The concentration of PEG in this aqueous solution is typically 2 to 20 vol%, with the remainder being water. 【0072】 To demonstrate the advantages of electrode wires with a thick zinc oxide layer on the surface, the following tests were performed. A baseline EDM machining job was defined. The job involved cutting a punch from a 50 mm high steel workpiece, with the guide positioned less than 0.2 mm from the workpiece. The cutting was performed on a CUT200MS machine sold by GF Machining Solutions. The cutting was performed in three machining passes using techniques adapted for brass. The movement speed of the workpiece relative to the electrode wire during each pass was adapted to cut the punch as quickly as possible with the same final surface finish. Here, this final surface finish corresponds to a roughness of Ra 0.6 μm. More precisely, in the tests performed, only the movement speed of the workpiece relative to the electrode wire in the first and second passes was adapted depending on the wire used. The movement speed of the workpiece relative to the electrode wire in the third pass was the same in all tests performed. 【0073】 Using the manufacturing method shown in Figure 2, the thickness e 12 Various wires 2 with different properties were generated. The punching times using various wires 2 are shown in the table below. In this table, the first column shows the name of the wire. The second column shows the thickness e 12 The table shows the corresponding processing time in hours and decimal places. In this table, wires represented as L7 and L49 have a wire thickness e 12 It is identical to wire 2, except that the wavelength range is not 100nm to 461nm. The wire referred to as "Gamma" is the electrode wire taught in U.S. Patent No. 8,338,735B2. This electrode wire has a surface layer of gamma-fractured copper-zinc alloy. More specifically, this electrode wire is the electrode wire sold by Thermocompact® under the reference code Thermo SA. 【0074】 [Table 2] 【0075】 Wire 2 has a thickness e as shown in these tests. 12 From the point where the thickness exceeds 100 nm, it can be processed at almost the same speed as the wire "Gamma". Furthermore, the thickness e 12 When the thickness exceeds 160nm, it becomes possible to process it faster than with "Gamma" wire. 12 For example, it is preferable that the wavelength is 160nm to 350nm or 160nm to 300nm. 【0076】 Furthermore, it has been verified that the fabricated wire 2 has a friction resistance equivalent to or better than that of the standard wire. For this purpose, the friction resistance of wire 2 is less than 7 mg / km, preferably less than 5 mg / km. Wire 2 therefore does not contaminate the wire guide of the EDM machine as much as the 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 friction resistance of wire 2 and the friction resistance of the standard wire were measured using the following method. - Step 1) A 1km wire is run at room temperature with a tension of 12N and a speed of 80m / min over the friction surface of guide 100 (Figures 3-5), and the wire comes into contact with this friction surface along a straight path 101 parallel to direction D, and then - Step 2) Weigh the amount of dust that has detached from the wire as it travels along the path for 1 kilometer. 【0077】 The weight of this dust is a measure of the wire's frictional resistance and is expressed in mg / km units. In fact, the more brittle the zinc oxide layer is and / or the weaker its adhesion to the core, the greater the amount of zinc oxide that is peeled off when the zinc oxide layer rubs against the abrasive surfaces of the guide 100. 【0078】 Figures 3 to 5 show details of the guide 100 used in the friction resistance measurement method. In Figure 4, dimensions are shown in millimeters. 【0079】 Guide 100 is a solid of revolution. The axis of rotation of guide 100 is denoted by reference numeral 102. The cross-section of guide 100 shown in Figure 4 is cut along the cross-sectional plane AA which contains axis 102. Therefore, only the elements located on one side of axis 102 in Figure 4 are described in detail. The other elements on the opposite side can be inferred from rotational symmetry about axis 102. Axis 102 is perpendicular in Figures 3 to 5. 【0080】 The guide 100 has a friction surface 104, and the longitudinal cross-section of the friction surface 104 in cross-sectional plane AA forms a circular arc that begins at the entrance 106 and ends at the exit 108. The tangent to the arc at the exit 108 is parallel to the axis 102. The radius of the arc is 33 mm. The orthogonal projection of this circular arc onto the axis 102 forms a line with a length of 19.67 mm. The orthogonal projection of this circular arc onto a plane perpendicular to the axis 102 forms a line with a length of 6.5 mm. 【0081】 Surface 104 extends downward behind the exit 108 to a cylindrical surface 110 parallel to the axis 102. The horizontal cross-section of surface 110 is a circle centered on the axis 102 with a diameter larger than the diameter of the wire. Here, the diameter of surface 110 is 1 mm. 【0082】 As the surface 110 extends downward, it terminates with a circular opening 112 that forms an entrance to a frustoconical surface 114. 【0083】 The frustum-shaped surface 114 is centered on the axis 102. This surface 114 extends downward to the exit opening 116. 【0084】 Surface 104 is made of a material much harder than brass or zinc oxide. Surface 104 is made of ceramic here. More precisely, this ceramic is yttrium (Y)-stabilized zirconia (ZrO2). This ceramic contains about 6% yttrium, for example, in the form of Y2O3 oxide. In this embodiment, guide 100 is made entirely of yttrium (Y)-stabilized zirconia (ZrO2) in the form of Y2O3 oxide. The roughness Ra of friction surface 104 is equal to 0.03 μm. More precisely, the roughness of surface 104 was measured 30 times using the following instruments and settings. - Trademark for the equipment: MAHR - Controller part number: MarSurf M400 - Feed unit reference code: MarSurf SD26 - Stylus part number: 6852404BWF A4-4.5-2 / 90° (90° tip, 2μm radius) - Cut-off length Lc 0.08mm - Evaluation length 0.08mm (5 times) - Ls filter function The average of the 30 measurements obtained is equal to 0.0305 μm, and the standard deviation of these 30 measurements is equal to 0.0029 μm. 【0085】 Guide 100 is currently sold by GF Machining Solutions (registered trademark) as part number 326864, "Inletbush for Brake," in their online catalog, https: / / ecatalog.gfms.com / gfms / fr / USD / search / 326864. 【0086】 In step 1), the angle β between direction D and axis 102 is 30°. The wire then contacts surface 104 at point 120, which is located immediately after the entrance 106. The tangent at point 120 is parallel to direction D. Therefore, in step 1), the wire advances through the interior of the guide 100, passing through the entrance 106, then the exit 108, then the opening 112, and finally the opening 116. After the opening 116, the wire moves along a path 122 that coincides with axis 102. Under these conditions, the wire rubs only against surface 104 in step 1). 【0087】 In step 1), the axis 102 is vertical, so it is preferable that the dust generated by the wire rubbing against the surface 104 falls below the opening 116. The falling dust is collected in a container located below the opening 116 during step 1). The container is, for example, a circular adhesive pad with a diameter of 3-4 centimeters. This adhesive pad is positioned directly below the opening 116, with the adhesive surface of the adhesive pad facing the opening 116. Before the wire is fed through, an elongated hole is made in the pad, connecting the periphery of the wire to the center of the pad. This elongated hole allows the wire to be guided 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 surface. In step 2), the dust collected in this container is weighed. Here, 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. 【0088】 The frictional resistance of the standard wire measured using this method was 7 mg / km, and the frictional resistance measured for wire 2 was 2 mg / km. 【0089】 Chapter III: Transformation Forms Deformed form of electrode wire The outer surface of layer 12 can be covered with a thin film of the lubricant used in the stretching step 84. 【0090】 Variations of manufacturing methods Alternatively, if a blank wire of the desired diameter D0 is commercially available, do not stretch the blank wire in step 80 before step 82. 【0091】 Other methods are also possible for performing oxidation step 82. For example, instead of placing the entire spool of blank wire in the furnace, the blank wire can be unwound, passed through a heated tunnel, and then wound onto the spool again as it exits the tunnel. The temperature inside the tunnel is T four This is equivalent to heating the blank wire in sections one after another at temperature T four Heat until . Therefore, the problem of the time required to obtain a uniform temperature throughout the entire coil of blank wire is eliminated. In this case, a higher temperature T four and very short duration D four It is possible to use this. For example, when using a heated tunnel, the temperature value T four The temperature can be higher than 600°C or 700°C. The speed at which the blank wire passes through the tunnel is, in this case, the duration D during which a portion of the blank wire remains inside the tunnel. four However, it is adjusted to enable the realization of the desired thickness e0. 【0092】 Oxidation step 82 can also be carried out in a medium other than the Earth's atmosphere. For example, step 82 can be carried out in a medium containing more than 20% or 30% by volume of oxygen. 【0093】 In another variation of step 82, temperature T four Duration D four It changes over time. For example, temperature T four for duration D four It will be increased continuously over a period of time. 【0094】 Oxidized blank wires can also be cooled in other ways. For example, the furnace can be switched off and the coil left inside the furnace until it reaches room temperature. 【0095】 Other lubricants can be used when stretching oxide blank wires. The lubricant to be used is, for example, an aqueous PEG solution with an average molar mass of 200 to 1400 g / mol, i.e., the same solution used for cooling electrode wires after stress relief annealing. 【0096】 In a simplified embodiment, the stress relief annealing in step 84 is omitted. In this case, the electrode wire is simply immersed in a PEG cold bath. In another modified embodiment, immersion in a PEG cold bath is omitted. 【0097】 Some of the variations described above can be combined into a single embodiment. 【0098】 Chapter IV Advantages of the Described Embodiments The fact that the zinc oxide layer is formed directly on the circumferential surface of the metal core allows for the production of this zinc oxide layer simply by oxidizing the circumferential surface of the brass blank wire. In this way, there is no need to deposit the zinc layer on the circumferential surface of the blank wire, as is the case when manufacturing the wire according to the teachings presented in, for example, U.S. Patent No. 8,338,735,B2. It should be emphasized in particular that electrodeposition of the zinc layer onto the blank wire consumes far more energy than the heat treatment step. Therefore, the electrode wires described herein can be manufactured in a simpler and more economical way. 【0099】 The fact that the oxide layer thickness exceeds 100 nm improves processing speed, enabling the achievement of processing speeds close to or exceeding those of electrode wires with a fractured gamma-phase copper-zinc alloy coating, such as the electrode wire described in U.S. Patent No. 8,338,735,B2. 【0100】 The fact that the oxide layer is less than 461 nm thick enhances its adhesion to the metal core. 【0101】 thickness e 12The fact that it exceeds 160 nm makes it possible to achieve processing speeds greater than those achievable with electrode wires having a fractured gamma phase copper-zinc alloy coating, such as the electrode wire described in U.S. Patent No. 8,338,735,B2. 【0102】 The fact that the frictional resistance of the zinc oxide layer is less than 7 mg / km suppresses the amount of dust generated by the electrode wire when it is used to process a workpiece. This reduces the amount of contamination generated by the electrode wire when it is used to process a workpiece. Furthermore, the need to coat this zinc oxide layer with varnish, as described in Japanese Patent Publication No. 61-203223A, is avoided. Thus, the problems caused by the presence of this varnish on the surface of the electrode wire can be avoided. 【0103】 The fact that the zinc oxide layer is the surface layer of the electrode wire increases the processing speed. 【0104】 The fact that the zinc concentration on the outer surface exceeds 36 atomic percent further improves processing performance. 【0105】 The fact that the diameter D0 is 1.3 × D2 to 6 × D2 means that the diameter reduction factor c1 during wire drawing exceeds 1.3, and therefore the breaking load of the resulting electrode wire is 700 N / mm². 2 We guarantee it will exceed that. 【0106】 The fact that the diameter D0 is 1.3 × D2 to 6 × D2, combined with the fact that the thickness e0 is 100 × (D0 / D2) nm to 600 nm, means that the thickness e 12 This enables the manufacture of electrode wires ranging from 100 nm to 461 nm. 【0107】 The fact that the thickness e0 is less than 600 nm prevents a portion of the zinc oxide layer from peeling off during the stretching process. Since a portion of the zinc oxide layer does not peel off, the thickness e 12 It is well controlled and highly reproducible. 【0108】 The fact that the maximum temperature reached during heat treatment is 400°C to 500°C means that the heat treatment lasts for at least 6 hours. Such a long heat treatment time ensures that the temperature is uniform throughout the entire coil of blank wire being heated in the furnace, and therefore the zinc oxide layer is more uniform along the entire length of the blank wire. 【0109】 By reducing the diameter of the oxide blank wire to less than half, 770 N / mm 2 It is possible to achieve a breaking load exceeding this limit.

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