Electrode wire for electrical discharge machining and method for manufacturing such a wire

Abstract

The invention relates to a wire comprising: - a metal core (10) having striations that are parallel to its longitudinal axis and which form, inside cross sections, a bumpy interface that comprises lower local extrema and upper local extrema distributed over at least 75% of the perimeter of the metal core; and - a surface layer (12) made of pure zinc. The lower and upper local extrema are spaced apart from one another by less than 12 µm. The bumpy interface is mainly comprised between a lower boundary (30) and an upper boundary (32), the difference between the lower and upper limits being between 1 µm and 5 µm. The upper limit is separated from an outer face (6) by a distance of less than 4 μm. The surface layer (12) completely fills all regions between the upper boundary (32) and the metal core.

Description

[0001] Electrode wire for machining by electroerosion and method for manufacturing such a wire

[0002] [1] The invention relates to an electrode wire for machining by electro-erosion and to a method for manufacturing this electrode wire.

[0003] [2] Electrode wires are used to cut metals or electrically conductive materials by electro-erosion in an electro-erosion machining machine.

[0004] [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.

[0005] [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.

[0006] [5] The particles detached from the electrode wire and the workpiece by the sparks disperse into the dielectric fluid, where they are removed.

[0007] [6] Achieving machining accuracy, particularly for making small-radius corner cuts, requires the use of small-diameter wires with a high tensile strength to be tensioned in the machining zone and to limit the amplitude of vibrations. [7] Most modern electrical discharge machining (EDM) machines are designed to use metal wires, typically 0.25 mm in diameter, with a tensile strength of around 700 N / mm². 2 and 1000 N / mm 2 .

[0008] [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 in 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.

[0009] [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:

[0010] - 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

[0011] - 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.

[0012]

[0010] More specifically, the invention relates to electrode wires whose metallic core is made of iron or steel and whose surface layer is made of pure zinc. These electrode wires are advantageous because they are manufactured solely from inexpensive metals that are readily available. In particular, these electrode wires are not manufactured from copper, nickel, or other much more expensive materials.

[0013]

[0011] However, it is desirable, with equal or improved performance, to further limit their manufacturing costs.

[0012] The invention aims to satisfy this desire by providing an electrode wire whose manufacture consumes less energy while offering similar or improved performance.

[0014]

[0013] The prior art relating to this invention is known from CN108857286A, US5945010A, W02019 / 066116A1, EP1295663A1 and US10589369B2.

[0015]

[0014] The invention is set forth in the attached set of claims.

[0016]

[0015] 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:

[0017] - Figure 1 is a schematic illustration of a cross-section of an electrode wire,

[0018] - Figure 2 is an enlarged image of a portion of the peripheral face of a metallic core of the electrode wire in Figure 1,

[0019] - Figures 3 and 4 are enlarged images, with different magnification factors, of portions of a cross-section of the electrode wire in Figure 1,

[0020] - Figure 5 is a schematic illustration of an angular sector of a cross-section of the electrode wire in Figure 1,

[0021] - Figure 6 is an enlarged image of the outer face of the electrode wire in Figure 1,

[0022] - Figure 7 is a flowchart of a manufacturing process for the electrode wire shown in Figure 1,

[0023] - Figure 8 is a front view of a guide used to measure the coefficient of friction of an electrode wire,

[0024] - Figure 9 is a longitudinal cross-sectional view of the guide in Figure 8,

[0025] - Figure 10 is a top view of the guide in Figure 8, and

[0026] - Figure 11 is a flowchart of another manufacturing process for the electrode wire of Figure 1.

[0027]

[0016] In this description, the terminology, conventions, and definitions of the terms used in this text are introduced in Chapter I. Detailed examples of embodiments are then described in Chapter II with reference to the figures. In Chapter III, variants of these embodiments are presented. Finally, the advantages of the different embodiments are specified in Chapter IV.

[0028]

[0017] Chapter I: Definitions, terminology and conventions:

[0029]

[0018] In the figures, the same references are used to designate the same elements.

[0030]

[0019] In the remainder of this description, the well-known characteristics and functions of a person skilled in the art are not described in detail.

[0031]

[0020] The symbol “*” denotes scalar multiplication.

[0032]

[0021] An electrically conductive material is a material whose electrical conductivity, at 20°C, is greater than 10 4 S / m or at 10 6 S / m.

[0033]

[0022] An electrically insulating material is a material whose electrical conductivity, at 20°C, is less than 10 10 S / m or at 10 14 S / m.

[0034]

[0023] The expression "an element made of a material A" or the expression "an element made of material A" means that material A represents 90% or 95% or 99% of the mass of that element.

[0035]

[0024] A “pure” component indicates that, if impurities are present in this component, then the mass of these impurities represents less than 1% and typically less than 0.1% or less than 0.05% of the total mass of the component.

[0036]

[0025] Steel is an alloy of iron and carbon. In this application, the steels used are mild or very mild steels. These mild or very mild steels are defined as steels containing less than 0.2%, by mass, of carbon and more than 0.01%, and generally more than 0.03%, by mass, of carbon. If the carbon concentration, by mass, is less than 0.01% and typically less than 0.008%, then the metal is iron and no longer steel. Here, preferably, the iron contains more than 99%, and preferably more than 99.5%, by mass, the remainder being impurities present in trace amounts (less than 0.05% by mass) and / or one or more additives.

[0037]

[0026] The longitudinal axis of a wire is the axis along which the wire mainly extends.

[0038]

[0027] The term "cross-section" refers to a section of the electrode wire perpendicular to its longitudinal axis.

[0028] The term "longitudinal section" refers to a section of the electrode wire made along a plane that contains its longitudinal axis.

[0039]

[0029] The term "layer" refers to an annular layer of the electrode wire located, in each cross-section of the electrode wire, between a substantially circular inner boundary and a substantially circular outer boundary. These substantially circular boundaries are both centered on the axis of the electrode wire. The substantially circular inner boundary is the boundary of the layer closest to the longitudinal axis of the electrode wire. Conversely, the substantially circular outer boundary is the boundary of the layer farthest from the longitudinal axis of the electrode wire. Between these substantially circular inner and outer boundaries, the chemical composition is substantially homogeneous. Conversely, at the substantially circular inner and outer boundaries, the chemical composition changes abruptly.In particular, the change in composition when these substantially circular boundaries are crossed is much greater than the slight changes in composition that can be observed within a layer.

[0040]

[0030] The term "surface layer" refers to the outermost metallic layer of the electrode wire. This surface layer may have a thin film of zinc oxide, generally less than 50 nm thick, and / or a thin film of lubricant, generally less than 0.2 g / m². 2 of matter soluble in ethanol. The outer face of this surface layer is therefore either coincident with the outer face of the electrode wire in the absence of thin films or separated from the outer face of the electrode wire only by at least one of these thin films.

[0041]

[0031] A bumpy interface is an interface between a wire core and a layer directly deposited on that core. In a cross-section of the wire, the bumpy interface forms a curve that has numerous low and high local extrema. A low local extremum is a point on this curve around which there exists a neighborhood VI such that, for any point in neighborhood VI other than the low local extremum, the distance between the longitudinal axis of the wire and this point in neighborhood VI is greater than the distance between the longitudinal axis of the wire and this low local extremum. A high local extremum is a point on the curve around which there exists a neighborhood V2 such that, for any point in neighborhood V2 other than the high local extremum, the distance between the longitudinal axis of the wire and this point in neighborhood V2 is less than the distance between the longitudinal axis and this high local extremum.

[0042]

[0032] A smooth interface between an outer face of the wire and a different external medium is an interface which, in each transverse or longitudinal section of the wire, deviates by at most 1 pm and, preferably, by at most 0.5 pm, from a segmented line. In each transverse or longitudinal section, the segmented line is constructed as follows:

[0043] - the segmented line extends continuously along the smooth interface,

[0044] - The segmented line is formed from a succession of segments,

[0045] - each segment is straight and extends from a starting point to an ending point,

[0046] - the starting point of a segment coincides with the ending point of the preceding segment in the segmented line when this segmented line is traversed clockwise,

[0047] - The starting and ending points of each segment are located on the outer face,

[0048] - the length of each segment is equal to 12 pm.

[0049]

[0033] The distance between a point on a cross-section of the wire and its longitudinal axis is the shortest distance between that point and the longitudinal axis measured in a radial direction. However, as explained later, for practical reasons this distance can be replaced by the distance between that same point on the cross-section and a 60 pm length of the electrode wire that begins before that point and ends after that point.

[0050]

[0034] The expression "ambient temperature" refers to a temperature between 15 °C and 35 °C and, typically, equal to 20 °C.

[0051]

[0035] The average thickness e of a surface layer of zinc is defined by the following relation (1): e = [mrm f ] / [p*Ti*d*L], where:

[0052] - mi is the initial mass of a sample of wire with a surface layer of zinc, - mt is the mass of the same wire sample after being immersed in a bath that completely dissolves the surface layer of zinc,

[0053] - p is the volumetric density of zinc, this density p being taken here as 7130 kg / m³ 3 ,

[0054] - n is the number pi,

[0055] - d is the initial diameter of the wire sample before it is immersed in the bath that completely dissolves the surface layer, and

[0056] - L is the length of the wire sample.

[0057]

[0036] The average thickness e is, for example, measured by successively performing the following steps:

[0058] 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.

[0059] 2) The sample is rinsed with ethanol, then dried and dusted using a jet of compressed air.

[0060] 3) The initial mass m of the sample is measured using a precision balance.

[0061] 4) The sample is then immersed in a stirred, laboratory-grade 1 mol / L aqueous hydrochloric acid bath at room temperature. The zinc is attacked by the hydrochloric acid. The reaction releases hydrogen bubbles for approximately 20 minutes. The wire is removed from the hydrochloric acid bath as soon as the gas evolution has ceased and the wire's color has changed from bright light gray to dull dark gray. If the steel wire were left to soak in hydrochloric acid after the zinc had dissolved, it would lose approximately 0.02 g / m² of zinc. 2 / min due to iron corrosion.

[0062] 5) Next, the sample is rinsed with water.

[0063] 6) The sample is dried using a jet of compressed air.

[0064] 7) The final mass m f the sample is measured using a precision balance.

[0065] 8) The average thickness e of the surface zinc layer is calculated using the previous relation (1).

[0066]

[0037] Unless otherwise specified, in the remainder of this text, the term "thickness of the surface zinc layer" alone refers to the average thickness of this zinc layer.

[0038] Chapter: Example of an embodiment

[0067]

[0039] Figure 1 represents an electrode wire 2 adapted for machining by electro-erosion as described in the introductory part of this text.

[0068]

[0040] For this purpose, the electrode wire 2 has a breaking load greater than 400 N / mm 2 and, most often, greater than 500 N / mm 2 or at 1000 N / mm 2 The breaking load of electrode wire 2 is also generally less than 2000 N / mm². 2 .

[0069]

[0041] The wire 2 extends along a longitudinal axis 4. The axis 4 is here perpendicular to the plane of the sheet. The length of the wire 2 is greater than 1 m and, typically, greater than 10 m or 50 m.

[0070]

[0042] The wire 2 has a metallic outer face 6. Face 6 is the outermost metallic face of the wire 2. This face 6 is exposed to sparks during the machining of a workpiece by electrical discharge machining (EDM) using this wire. As a first approximation, face 6 is a cylindrical face extending 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 150 µm and 1 mm and, more often, between 150 µm and 400 µm. Here, the diameter D2 is equal to 0.25 mm.

[0071]

[0043] In this embodiment, wire 2 comprises:

[0072] - a central metallic core 10 made of electrically conductive material, and

[0073] - a surface layer 12 formed directly on the core 10.

[0074]

[0044] The core 10 serves to provide, on its own, the majority of the breaking load on the wire 2. It also serves to ensure the electrical conductivity of the wire 2. To this end, it is made of an electrically conductive material. In this embodiment, the core 10 is made entirely of steel containing between 0.03% and 0.2% carbon. In this case, the core 10 consists mainly of ferrite and pearlite. On the surface of the core 10, the ferrite and pearlite are arranged in the form of respective veins, each extending mainly parallel to the axis 4. The length of these veins is typically several tens of micrometers. Ferrite is practically pure iron (Fe). Pearlite is a mixture of pure iron (Fe) and cementite (Fe3C). This arrangement of ferrite and pearlite is not necessarily found for carbon levels above 0.2%.For example, such an arrangement does not exist for carbon contents above 0.8%. Furthermore, if the carbon content is too high in the core 10, the wire 2 is very stiff and has a breaking load exceeding 2000 N / mm. 2 In this case, during the machining of a part by electrical discharge machining (EDM), wire 2 bends with difficulty in the waste wire bin. The waste wire bin therefore fills up very quickly, so machining must be stopped immediately to empty it. Thus, if the carbon content in the core 10 is too high, wire 2 becomes even more difficult to use in a conventional EDM machine.

[0075]

[0045] Preferably, the core steel 10 is unalloyed or very low alloyed. Indeed, alloy steels generally have lower electrical conductivity than unalloyed steels.

[0076]

[0046] In this embodiment, the web steel 10 corresponds to grade C4D of standard EN 16120-2 or grade 1.0300 of standard EN 10016-2. The chemical composition of the web steel 10 is as follows:

[0077] - mass percentage of Carbon (C) between 0.03% and 0.06%,

[0078] - mass percentage of silicon (Si) less than or equal to 0.3%

[0079] - mass percentage of manganese (Mn) between 0.3% and 0.6%,

[0080] - mass percentage of phosphorus (P) less than or equal to 0.035%,

[0081] - mass percentage of sulfur (S) less than or equal to 0.035%

[0082] - mass percentage of copper (Cu) less than or equal to 0.3%

[0083] - nickel (Ni) mass percentage less than or equal to 0.25%

[0084] - mass percentage of phosphorus (P) less than or equal to 0.035%,

[0085] - molybdenum (Mo) mass percentage less than or equal to 0.05%

[0086] - mass percentage of aluminium (Al) less than or equal to 0.01%, and

[0087] - mass percentage of chromium (Cr) less than or equal to 0.2%

[0088]

[0047] The electrical conductivity of the C4D grade of steel is 7.2 MS / m. Its tensile strength is approximately 1080 N / mm². 2 .

[0089]

[0048] The diameter D i0 The diameter of the core 10 is greater than 0.93*D2 or 0.96*D2. Here, the diameter D i0 is greater than or equal to 0.240 mm.

[0049] The web 10 has a generally cylindrical peripheral face 14 extending along the axis 4. This peripheral face 14 is made of the same steel as the web 10. The face 14 has grooves, each extending mainly parallel to the axis 4. These grooves on the face 14 are visible in the image in Figure 2. The image in Figure 2 is an image of a 1 mm² surface.2 of face 14 taken using an electron microscope.

[0090]

[0050] In the cross-sections of the wire 2, the striations on face 14 form bumps and hollows distributed around the entire circumference of the cross-section of face 14. These bumps and hollows form a bumpy interface between the core 10 and the surface layer 12. Typically, the bumpy interface is present over more than 75% of the wire length and, preferably, over more than 90% of the wire length. In the majority of the cross-sections of the wire 2, the bumpy interface extends over more than half or more than 75% of the circumference of face 14 and, preferably, over more than 80% or 90% or over the entire circumference of face 14.

[0091]

[0051] Portions of this bumpy interface are visible in Figures 3 and 4. Figures 3 and 4 are images of a portion of a cross-section of wire 2 taken with an electron microscope. Generally, to observe the bumpy interface in a cross-section of wire 2, the magnification factor used is greater than 2500. Under these conditions, the electron microscope only allows for the imaging of a portion of the bumpy interface. Thus, to verify the presence of the bumpy interface over more than half or more than 75% of the periphery of face 14, it is generally necessary to take images of several different portions of the cross-section of wire 2.

[0092]

[0052] In figure 4, two portions of two cross sections of wire 2 were photographed head to tail.

[0093]

[0053] The method for characterizing the bumpy interface between the core 10 and the surface layer 12 is now described with the help of the schematic illustration in Figure 5.

[0094]

[0054] Figure 5 schematically represents an angular sector Si of a cross-section of wire 2, where the index i is an angular sector identifier among the set of angular sectors that divide this cross-section. The vertex of sector Si is located on axis 4. Sector Si has two opposite sides Gi and Di. Each side Gi, Di is a half-line whose origin lies on axis 4. The angle at the vertex of sector Si is the angle co between the two sides Gi and Di. Angle CG, expressed in degrees, is generally greater than 360*50*2 / (2*TT*D2), where D2 is expressed in micrometers.

[0095]

[0055] Within each sector Si, the face 14 forms a succession of bumps and hollows that constitute the bumpy interface. These bumps and hollows have numerous local high and low extrema. To illustrate this, in Figure 5, four bumps 22 to 25 and two hollows 26 and 27 are shown. Hollow 26 is located between bumps 22 and 23. Hollow 27 is located between bumps 23 and 24. To improve the readability of Figure 5, the dimensions of these bumps and hollows have been exaggerated.

[0096]

[0056] The density of the striations on face 14 is such that the maximum distance between two consecutive lower local extrema, moving clockwise along the cross-section of the bumpy interface, is less than 12 pm. Similarly, the maximum distance between two consecutive upper local extrema, moving clockwise along the cross-section of the bumpy interface, is less than 12 pm. The depth of the striations on face 14 is such that, in each cross-section, the bumpy interface is mainly contained between a lower limit 30 and an upper limit 32, the gap between these limits 30 and 32 being between 1 pm and 5 pm and, preferably, between 2 pm and 4 pm. More precisely, along the periphery of face 14, the gap between the limits 30 and 32 varies.However, this deviation remains within the above ranges of values ​​over all or virtually all of the perimeter of the dented interface. Indeed, it may happen that, due to a defect in face 14, the deviation between limits 30 and 32 is less than 1 µm or greater than 5 µm. However, such defects are localized, so that over more than 90% or 95% of the perimeter of the dented interface in the cross-section, the deviation between limits 30 and 32 is within the above ranges of values.

[0097]

[0057] The lower limit 30 and upper limit 32 are defined as follows:

[0098] - each lower boundary 30 and upper boundary 32 extends continuously along the bumpy interface, the lower boundary 30 being closer to axis 4 than the upper boundary 32, - each lower and upper boundary is formed by a succession of segments,

[0099] - each segment is straight and extends from a starting point to an ending point,

[0100] - the starting point of a segment coincides with the ending point of the preceding segment within the same limit when this limit is traversed in a clockwise direction,

[0101] - in the case of the lower limit, the starting and ending points of each segment are located at a local low extremum,

[0102] - In the case of the upper limit, the starting and ending points of each segment are located at a local high extremum,

[0103] - in the case of the lower limit, the arrival point of each segment is located at the level of the smallest of the lower local extrema chosen from among the lower local extrema which are located at a distance from the starting point of this segment of between 6 pm and 12 pm, the smallest of the lower local extrema being the one which is closest to axis 4,

[0104] - in the case of the upper limit, the arrival point of each segment is located at the level of the largest of the upper local extrema chosen from among the upper local extrema which are located at a distance from the starting point of this segment of between 6 pm and 12 pm, the largest of the upper local extrema being the one which is furthest from axis 4.

[0105]

[0058] By way of illustration, Figure 5 shows three consecutive segments 34 to 36 of the boundary 30. The segments preceding and following these segments 34 to 36 are not shown. In Figure 5, they are symbolized by the presence of dashed lines. Segment 34 begins at point 38 and ends at point 39. Segment 35 begins at point 39 and ends at point 40. Segment 36 begins at point 40 and ends at point 41. To construct segment 34 from point 38, the smallest of the lower local extrema located, clockwise, between 6 pm and 12 pm from point 38, is selected as the endpoint 39. This construction method is then repeated step by step to construct each of the segments of the lower boundary 30. The starting point of the first segment of the boundary 30 is chosen at a lower local extremum of the bumpy interface.

[0059] In practice, an electron microscope image only shows a portion of the cross-section of wire 2. Generally, this image does not include axis 4. Under these conditions, to identify, among several lower local extrema, the one closest to axis 4, it is possible to use, instead of the distance from axis 4, a distance from a 60 pm long string. This string connects a point on the bumpy interface located before the lower local extrema among which the smallest lower local extremum is to be identified, to a point on the bumpy interface located after these lower local extrema. In this case, the measured distance is the shortest distance between a lower local extremum and this string. Using a string to identify the smallest lower local extremum makes it possible to construct a lower limit substantially identical to that which would be obtained if the distance were measured from axis 4.An example of such a chord 37 is shown in figure 4.

[0106]

[0060] In the same angular sector Si, Figure 5 represents four consecutive segments 44 to 47 of the upper limit 32. Segments 44 to 47 are constructed using the same method as that described for the limit 30 except that:

[0107] - the local high extremities are used instead of the local low extremities, and

[0108] - it is the largest local high extremum that is used as the arrival point of each segment.

[0109]

[0061] Subsequently, in each cross-section, the hollow regions located between the upper limit 32 and the bumpy interface are called "valleys". Thus, in a cross-section of wire 2, a valley is delimited, in its lower part, by the face 14 and, in its upper part, by the upper limit 32. In Figure 5, the surface of the valley located between bumps 22 and 23 has been hatched to show an example of a valley.

[0110]

[0062] The maximum depth of the valleys, relative to the upper limit 32, is typically greater than or equal to 1 pm and, preferably, greater than or equal to 2 pm. For more than half of these valleys and, typically, for more than 70% or 80% or 90% of these valleys, the maximum depth is less than 5 pm or 4 pm.

[0111]

[0063] It is noted that, since the striations on face 14 extend mainly parallel to axis 4, in a longitudinal section of wire 2, the bumpy interface is not observable. On the contrary, in a longitudinal section of wire 2, face 14 appears much smoother.

[0112]

[0064] Layer 12 is designed to increase the machining speed and therefore the erosive efficiency of the electrode wire and / or the quality of the surfaces of the part obtained after machining by electrical discharge machining (EDM). The quality of a surface cut by EDM is all the better the lower its roughness.

[0113]

[0065] The layer 12 covers more than 90% of the face 14 of the core 10. Its outer face 6 forms a smooth interface with the external environment or with a thin film of zinc oxide and / or lubricant. Thus, in each cross-section of the wire 2, the face 6 extends along a segmented line 50 (Figs. 4 and 5). Such a segmented line is defined in the preceding Chapter I. This segmented line is constructed incrementally by selecting a starting point of a segment located at the level of the face 6. Then, the ending point of this segment is constructed by selecting the point on the face 6 that is located, clockwise, 12 pm from this starting point. The starting point of the next segment coincides with the ending point of the previous segment. The ending point of the next segment is constructed using the same method as that described for the previous segment.In each cross section, the gap between face 6 and segmented line 50 is less than 1 pm and preferably less than 0.5 pm.

[0114]

[0066] To make face 6 visible in a photograph taken with an electron microscope, a layer 52 (Fig. 4) of copper is deposited directly onto face 6 using a cyanide copper electrodeposition bath. Next, strands are cut from the wire 2 coated with layer 52. One or more of these strands are then embedded in an electrically conductive resin to obtain a hard resin block in which one or more of these parallel strands are embedded. Finally, the resin block is cut to expose one face of the resin block on which a cross-section of the embedded strands is visible. This face is then polished. Finally, the polished face is observed with an electron microscope. The photograph in Figure 4 was obtained using this method.

[0115]

[0067] In this embodiment, the layer 12 is made entirely of pure zinc. The layer 12 completely fills all the valleys of the humped interface. Thus, in each cross-section, the distance separating the outer face 6 from the upper boundary 32 is greater than or equal to 0 pm. The case where this distance is equal to 0 pm corresponds to the case where the crest of a hump in the humped interface is flush with the outer face 6. However, preferably, the distance separating the upper boundary 32 from the outer face 6 is greater than 0 pm and, for example, greater than 0.2 pm or 0.5 pm. Thus, preferably, the crests of most of the humps are covered by the surface layer 12.

[0116]

[0068] To further increase machining speed and the quality of machined faces, over at least 50% and preferably at least 70%, 80%, or 90% of the perimeter of the dented interface, the distance between the upper limit 32 and the outer face 6 is less than 4 pm and typically less than 2 pm or 1 pm. For this purpose, the average thickness of the layer 12 is between E m / 2 pm and E m / 2+4 pm, where E m is the average difference between limits 30 and 32. Thus, the average thickness of layer 12 is between 0.5 pm and 7.5 pm and, most often, between 1 pm and 6 pm. Preferably, the average thickness of layer 12 is between 2 pm and 4 pm when the difference between limits 30 and 32 is between 2 pm and 4 pm.

[0117]

[0069] It is noted that the thickness of layer 12 can be of the same order of magnitude as the dimensions of the analysis volume for a method of analyzing the composition of this layer 12, such as energy-dispersive X-ray spectroscopy. In this case, such an analysis method simultaneously detects the presence of iron, originating from the core 10, within the analysis volume. However, layer 12 is indeed made of pure zinc and not of an Fe-Zn alloy.

[0118]

[0070] The presence of numerous steel bumps near the outer face 6 creates a succession of pure zinc and steel zones along the periphery of the wire 2. The pure zinc zones form areas with a low evaporation temperature. They serve to eject molten metal from the surface of the workpiece. The steel bumps, whose crests are close to the outer face 6, form areas with a high melting temperature. These high-melting-temperature zones prevent molten metal from being ejected from the wire 2 onto the workpiece and thus limit the amount of metal ejected from the wire 2 that solidifies on the workpiece.

[0071] Furthermore, the transverse dimensions of these low-evaporation-temperature and high-melting-temperature zones are much smaller than the dimensions of the electric arcs that occur during the machining process.Under these conditions, each electric arc simultaneously raises the temperature of several low-evaporation zones and several high-melting-point zones, so that the technical effects specific to each of these zones occur simultaneously. The size of an electric arc is taken here to be equal to the diameter of the crater that such an electric arc forms in a homogeneous surface layer of pure zinc. In particular, in such a homogeneous surface layer of pure zinc, there are no bumps such as bumps 22 to 25 whose crests are close to the outer face 6. It has been observed that the diameter of such a crater is often greater than or equal to 100 pm.

[0119]

[0072] A first manufacturing process for wire 2 will now be described with reference to figure 7.

[0120]

[0073] This process begins with a step 100 of supplying a steel wire whose peripheral face has the same striations as those previously described. For this purpose, during an operation 102, a steel wire made of the same steel as the core 10 is supplied. Here, this steel wire is therefore grade C4D. At this stage, the peripheral face of the steel wire does not yet have the striations previously described. The diameter D ini of this steel wire is greater than D2+20 pm or D2+50 pm. Generally, the diameter D ini is between 0.5 mm and 1 mm.

[0121]

[0074] Next, in operation 104, the peripheral face of the steel wire is grooved to obtain the same grooves as those previously described, but on the peripheral face of the steel wire. To achieve this, in operation 104, the steel wire is first degreased in an alkaline aqueous solution, preferably heated to over 40°C. Optionally, this degreasing is carried out under current, with the wire either cathodic (negatively polarized) or alternately cathodic and anodic (positively polarized). Then, the degreased steel wire is immersed in an acidic solution and, at the same time, positively polarized, to perform anodic etching. The acid used is one that dissolves the ferrite and pearlite of the steel wire at different rates. It is this difference in the dissolution rates of the ferrite and pearlite that makes it possible to obtain the desired grooves on the peripheral face of the steel wire.Indeed, as previously mentioned, ferrite and pearlite appear on the peripheral face of the steel wire in the form of veins which extend mainly parallel to the longitudinal axis of the steel wire.

[0122]

[0075] For example, the acid solution is an aqueous solution such as a 30 g / l sulfamic acid solution in water, enriched in chloride ions by the addition of 4 g / l sodium chloride. The pH of this solution is approximately 1.4 at room temperature.

[0123]

[0076] The time during which the steel wire is immersed in the acid solution and the current density for anodic etching are determined experimentally to obtain the desired striations on the peripheral face of the steel wire. For example, in the manufacturing process implemented by the depositor, the current density for anodic etching is 50 A / dm 2and the time spent in the acid solution is 1 minute for a steel wire with a diameter of 0.5 mm.

[0124]

[0077] Next, in step 110, a zinc layer is directly formed on the grooved peripheral face of the steel wire. Here, in step 110, a layer of pure zinc is deposited by electrolysis on the peripheral face of the steel wire. During electrolytic deposition, the iron from the steel wire does not diffuse into the zinc layer, so the deposited zinc layer is indeed pure zinc. The average thickness, in micrometers, of the zinc layer deposited by electrolysis is typically between (E m / 2)*(Di ni / D2) and (E m / 2 +4)*(D ini / D2) in order to obtain the desired thickness for the surface layer 12 after drawing to the final diameter D2. Preferably, the thickness of the zinc layer deposited during step 110 is between l.5*(D ini / D2) and 5.5*(D ini / D2) or between 2*(D ini / D2) and 4*(D ini / D2).

[0125]

[0078] For example, in step 110, the steel wire is first degreased in an alkaline aqueous bath while negatively polarized (cathodic degreasing). Next, it is rinsed and then pickled in an aqueous sulfuric acid bath at approximately 200 g / l. Finally, it is zinc-plated in an aqueous bath of zinc sulfates and chlorides. This bath contains approximately 100 g / l of Zn ions. 2+ Its temperature ranges from 20°C to 60°C. Its pH ranges from 2 to 3. The current density is approximately 50 A / dm³. 2 The average thickness of the deposited zinc layer is here between 3 pm and 7 pm or between 4 pm and 7 pm.

[0126]

[0079] In step 120, the zinc-coated steel wire obtained at the end of step 110 is drawn to obtain wire 2 of diameter D2. In this step 120, the diameter of the zinc-coated steel wire is reduced by at least 20 µm or 50 µm. In other words, the gap D in i-D2 is greater than 20 pm or 50 pm.

[0127]

[0080] For example, galvanized steel wire is drawn by passing it through a series of dies with elongations between 15% and 20% for each die. During drawing, the outer face of the galvanized steel wire is lubricated to limit its friction on the dies.

[0128]

[0081] During wire drawing, it was observed that the striations of the zinc-coated steel wire are preserved, which makes it possible to obtain the striations on face 14 of the core 10. It appears that this is due to the fact that, during wire drawing, the humps are constantly covered by the zinc layer. Thus, the steel humps do not come into direct contact with the dies, which prevents them from being crushed. Only the zinc layer comes into direct contact with the dies. Thus, only the zinc layer, which is of low hardness, is directly crushed by the dies, and the steel wire, which is of high hardness, is only crushed by the zinc layer, which already fills, at least partially, the valleys between the humps. It is also the crushing of the zinc layer during this wire drawing process that flattens the outer face 6 and completely fills the valleys of face 14 with zinc.

[0129]

[0082] Once the diameter D2 is reached, during step 130, an in-line stress-relieving anneal is performed before winding the electrode wire onto a spool. This stress-relieving anneal minimizes residual stresses in the wire 2 to obtain a straight wire 2 and thus facilitate its threading in an electrical discharge machining (EDM) machine. This stress-relieving anneal does not alter the composition of the wire 2 and has little effect on its breaking strength. To achieve this, the wire 2 is stretched between an upstream pulley and a downstream pulley, and the portion of the wire 2 between these two pulleys is heated as the wire 2 moves between them. For example, the portion of the wire 2 stretched between the two pulleys is heated by Joule heating by passing an electric current through this portion of the wire. Typically, the temperature for stress-relieving annealing is between 400°C and 600°C, and its duration is less than 2 or 3 seconds.

[0130]

[0083] After step 130, in step 140, the electrode wire is lubricated. For example, immediately after stress-relieving annealing, the electrode wire undergoes quenching to rapidly cool it to ambient temperature and, at the same time, to lubricate it. For this purpose, the electrode wire is quenched in a cold bath, that is, in a bath whose temperature is lower than or equal to ambient temperature. Various solutions are possible for quenching the wire in this cold bath immediately after stress-relieving annealing. For example, the pulley leading the wire, in the direction of wire movement, is quenched in this cold bath. Alternatively, just after this pulley leading the wire, the wire passes through the cold bath. Here, the cold bath is a solution that also allows the wire 2 to be lubricated so that it has a coefficient of friction less than 0.45.Furthermore, the lubricant applied to the wire acts as an antioxidant to prevent the oxidation of pure zinc, for example, in the presence of moisture. Typically, this solution is a solution of organic molecules or an oil-in-water emulsion. For example, here, the solution is an aqueous solution of polyethylene glycol (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 or a mixture of water and an antioxidant. The antioxidant is added to the solution if the PEG alone is insufficient to prevent the oxidation of the zinc.

[0131]

[0084] In this application, the coefficient of friction of an electrode wire is measured using the following method:

[0132] - Step 1): At room temperature, run 1 km of wire at a speed of 80 m / min under a tension of 12 N over a friction face of a guide 200 (Fig. 8 to 10), the wire coming into contact with this friction face following a straight trajectory 201 parallel to a direction D, and

[0133] - Step 2): In parallel with step 1), measure the average vertical force V exerted on the guide 200 by the electrode wire during step 1), then - Step 3): Calculate the coefficient p of friction of the wire using the following relation: p = [V-2*T*(sin(p / 2)) 2 ] / [2*T*sin(p / 2)*cos(p / 2)], where:

[0134] - V is the average vertical force measured,

[0135] - T is the tension of the wire during the execution of step 1),

[0136] - p is the entry angle of the wire into the guide 200.

[0137]

[0085] Figures 8 to 10 show in detail the guide 200 used in the method for measuring the coefficient p of friction. In Figure 9, the dimensions of certain parts of the guide 200 are indicated using the usual standard for indicating such dimensions in engineering drawings. Here, the dimensions are expressed in millimeters. The notation "cPx" indicates that the part of the guide 200 associated with this symbol has a diameter of x mm.

[0138]

[0086] The guide 200 is a solid of revolution. Its axis of revolution is datum 202. The cross-section of the guide 200 shown in Figure 9 is formed along a cutting plane AA (Fig. 8) which contains the axis 202. Thus, only the elements located on one side of the axis 202 in Figure 9 are described in detail. The other elements, on the opposite side, are deduced by rotational symmetry about the axis 202. In Figures 8 to 10, the axis 202 is vertical.

[0139]

[0087] The guide 200 has a friction face 204 whose longitudinal section in the cutting plane AA forms a circular arc that begins at an inlet 206 and ends at an outlet 208. The tangent of the circular arc at the outlet 208 is parallel to the axis 202. The radius R33 of this circular arc is 33 mm. The orthogonal projection of this circular arc onto the axis 202 forms a line 19.67 mm long. The orthogonal projection of this circular arc onto a plane perpendicular to the axis 202 forms a line 6.5 mm long.

[0140]

[0088] After exit 208, moving downwards, face 204 extends into a cylindrical face 210 parallel to axis 202. The horizontal cross-section of face 210 is a circle centered on axis 202 with a diameter greater than the diameter of the wire. Here, the diameter of face 210 is 1 mm.

[0141]

[0089] Moving downwards, the face 210 terminates in a circular orifice 212 which forms the entrance of a frustoconical face 214.

[0090] The frustoconical face 214 is centered on the axis 202. This face 214 flares out downwards to an outlet orifice 216.

[0142]

[0091] Face 204 is made of a very hard material. Here, face 204 is ceramic. More precisely, the ceramic is zirconia (ZrO2) stabilized with yttrium (Y). For example, this ceramic contains approximately 6% yttrium in the form of the oxide Y2O3. In this embodiment, the guide 200 is made entirely of zirconia (ZrO2) stabilized with yttrium (Y) in the form of the oxide Y2O3. The roughness Ra of the friction face 204 is 0.03 pm. More precisely, the roughness of face 204 was measured thirty times using the following equipment and settings:

[0143] - Equipment brand: MAHR

[0144] - Controller reference: MarSurf M400

[0145] - Reference of the advance unit: MarSurf SD26

[0146] - Stylus reference: 6852404 BWF A 4-4.5 - 2 / 90° (90° tip with a 2 pm radius)

[0147] - Cutting length: 0.08 mm

[0148] - Evaluation length 5 times 0.08 mm

[0149] - Ls filter active.

[0150]

[0092] 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.

[0151]

[0093] Currently, the 200 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.

[0152]

[0094] During step 1), the angle p between direction D and axis 202 is equal to 30°. Thus, the wire makes contact with the face 204 at a point 220 located just after the inlet 206. The tangent at point 220 is parallel to the direction D. Thus, during step 1), the wire advances inside the guide 200, passing successively through the inlet 206, then the outlet 208, then the orifice 212, and finally the orifice 216. After the orifice 216, the wire moves along a trajectory 222 coinciding with the axis 202. Under these conditions, during step 1), the wire only rubs against the face 204.

[0095] During step 1), the axis 202 is vertical, and the guide 200 is associated with a force sensor that measures the vertical force V that the wire exerts on the guide 200 as it passes through it.

[0153]

[0096] Using this measurement method, it is possible to ensure that once the wire 2 has been lubricated, its coefficient p of friction is well below 0.45. When the coefficient p is less than 0.45, the amount of dust produced by the wire 2 when used in an electro-erosion machining machine is low so that it does not quickly clog the guides of this machine.

[0154]

[0097] Figure 11 shows a second method for manufacturing a wire identical to wire 2 except that its metallic core 10 is made of iron or steel containing less than 0.2% carbon. This second method is identical to the first method of Figure 7 except that step 100 is replaced by a step 300 of supplying a metallic wire whose peripheral face has striations having the same characteristics as those of the striations previously described.

[0155]

[0098] Here, to striate the face of a wire, this second manufacturing process does not exploit the fact that the dissolution rates of ferrite and pearlite are different. On the contrary, the striations are produced by a mechanical process.

[0156]

[0099] More specifically, during operation 302, a starting wire made of iron or steel containing less than 0.2% carbon is supplied. For example, here, the starting wire is a wire made of iron containing 0.0075% carbon by mass. The diameter D o the diameter of this starting wire is greater than the diameter D ini For example, the diameter D o is greater than Dini+20 pm or Dini+50 pm. Generally, the diameter D o is between 0.5 mm and 1 mm.

[0157]

[0100] Then, during an operation 304, the diameter of the starting wire is brought to diameter D iniand, at the same time, its peripheral face is striated so that it has striations with the same characteristics as the striations previously described. To bring the diameter of the starting wire to diameter D iniThe starting wire is drawn by passing it through one or more dies. The elongation of each of these dies is, for example, between 5% and 20%, and most often between 10% and 20%. This is a similar process to that already described for step 120.

[0101] In particular, each die has a hole whose final diameter is smaller than the diameter of the wire entering that hole, so that the diameter of the wire exiting the hole is smaller than the diameter of the same wire entering the hole. The face of each hole, which comes into direct mechanical contact with the wire passing through it, is made of a material harder than the wire material. Moreover, it is generally advantageous for the face of the hole to be as smooth as possible to limit the coefficient of friction between this face and the wire. For this reason, it is common for the face of the hole to be formed by a single-crystal diamond coating.

[0158]

[0102] However, here, at least the last die through which the starting wire passes is arranged to produce the grooves on the peripheral face of the wire. The last die is the one that reduces the diameter of the starting wire to diameter D ini Here, the face of the hole in the last die is composed of abrasive grains embedded in a matrix made of a different material, less hard than the abrasive grains. The manufacture of such a die is carried out, for example, as follows. First, a pellet composed of these abrasive grains embedded in the matrix is ​​produced. For example, the abrasive grains and the matrix are mixed together to obtain a mixture in which the abrasive grains are uniformly distributed within the matrix. Next, this mixture is compressed and then sintered under pressure at high temperature to obtain a rigid pellet. Finally, a hole of the desired diameter is drilled into this pellet to create the die.

[0159]

[0103] The hardness of the abrasive grains is greater than the hardness of the material from which the starting wire is made. Since the hardness of iron and mild steel on the Mohs scale is less than 5, the abrasive grains are made of a material with a Mohs hardness greater than 6 and, preferably, greater than 8 or 9. Here, the abrasive grains are made of diamond or silicon carbide. The matrix serves to hold the abrasive grains mechanically bonded together. Such a matrix is ​​sometimes called a "binder" for this reason. Preferably, this matrix is ​​made of a material with a hardness equal to or greater than that of iron or mild steel. The hardness of the matrix is ​​preferably less than 7 on the Mohs scale. For example, the matrix is ​​made of a metal such as cobalt, which has a Mohs hardness of 5.5.

[0104] The abrasive grains are shaped and arranged to create the desired grooves when the peripheral face of the starting wire rubs against the face of the hole. For this purpose, typically, the average diameter of the abrasive grains is between 2 µm and 10 µm or between 2 µm and 6 µm. The average spacing between the abrasive grains is less than 12 µm or 6 µm. The diameter of an abrasive grain is taken here to be equal to the diameter of the smallest sphere that completely contains that abrasive grain. The spacing between a first and a second adjacent abrasive grain is equal to the distance between the centers of the smallest spheres that completely contain, respectively, the first and second abrasive grains.

[0160]

[0105] Since the matrix is ​​less hard than the abrasive grains, as the wire passes through the die, the matrix wears down faster than the abrasive grains. The abrasive grains therefore protrude on the inner face of the hole so that when the wire passes through this hole, the protruding abrasive grains form the desired longitudinal grooves on the peripheral face of the wire. Thus, at the exit of the last die, a wire of diameter D ini and whose peripheral face has the desired striations, is obtained.

[0161]

[0106] Next, the following steps are, for example, the same as those described with reference to Figure 7. With a carbon content of 0.0075% by mass in the iron core, the electrode wire obtained according to the process of Figure 11 typically has a breaking strength of 1000 N / mm². 2 .

[0162]

[0107] Chapter III: Variants:

[0163]

[0108] In another embodiment of operation 104, anodic etching is omitted. In this case, the wire immersed in the acid solution is not polarized.

[0164]

[0109] Step 100 or 300 can be carried out in a first plant, and the subsequent steps 110 to 140 can be carried out in a second plant different from the first. In this case, in the first plant, the steel wire may already be zinc-coated, so that the steel wire received in the second plant already has a layer of pure zinc, but its thickness is insufficient to obtain wire 2. Under these conditions, step 110, carried out in the second plant, consists of depositing an additional layer of pure zinc on top of the existing layer to obtain the desired average zinc thickness. Alternatively, in step 110, the pre-existing layer of pure zinc is completely removed, and then the layer of pure zinc of the desired thickness is deposited.

[0165]

[0110] Other embodiments of step 110, deposition of the zinc surface layer, are possible. For example, the zinc surface layer can be deposited by a process other than electrolysis. By way of example, it is also possible to deposit a zinc surface layer by hot-dip galvanizing.

[0166]

[0111] Alternatively, the stress-relieving anneal 130 is omitted.

[0167]

[0112] In another embodiment, if the coefficient of friction of the surface layer 12 obtained at the end of step 120 or 130 is already less than 0.45, then step 140 can be omitted.

[0168]

[0113] Several of the variants described above can be combined in the same embodiment.

[0169]

[0114] Chapter IV: Advantages of the described embodiments:

[0170]

[0115] It has long been considered advantageous to have an electrode wire whose surface layer has a high melting point and, at the same time, a low vaporization point. For example, this principle is taught in US patent application 5945010. To obtain such a surface layer, the proposed solutions have thus far consisted of producing the surface layer in special alloys exhibiting, as far as possible, these two properties, namely a high melting point and, at the same time, a low vaporization point. The manufacture of these alloys is very energy-intensive because it almost always involves the implementation of fairly long heat treatments at high temperatures to diffuse one element into another.For example, in US patent application 5945010, to achieve these two properties, the surface layer is made of a gamma-phase copper-zinc alloy, obtained by diffusing copper from the core into a zinc layer at 180°C for several hours. Here, these two properties are achieved simultaneously, not by creating an alloy—that is, by using a homogeneous mixture of several metals—but, on the contrary, by structuring the peripheral face 14 of the metal core so that the wire exhibits, on a scale smaller than one hundred micrometers, both areas with low evaporation temperatures and areas with high melting temperatures. The areas with low evaporation temperatures correspond to valleys filled with pure zinc, and the areas with high melting temperatures correspond to the bumps whose peaks are located near the outer face 6.It is emphasized that for a bump to form a high-melting-point zone, its apex must be close to the outer face 6, that is, less than 4 pm from the outer face 6. The structuring of the peripheral face 14 is achieved without heat treatment. Thus, the manufacture of the wires described here is less energy-intensive and therefore more economical. Furthermore, the performance of the wires described here is better than that of an identical electrode wire, except that face 14 lacks striations such as those described previously.

[0171]

[0116] Furthermore, it has already been proposed to groove the peripheral face of the metal core for various purposes. For example, application WO201966116A1 discloses a grooved peripheral face to improve the adhesion of the surface layer to the metal core. In this case, the number of grooves around the periphery of the peripheral face is small. Thus, in a cross-section of the electrode wire, the bumps formed by these grooves are very far apart, i.e., separated from each other by a distance greater than 20 pm or 50 pm. Applications EP1295663A1 and US10589369B2 teach the use of a metal core having a corrugated peripheral face to distribute the electrical discharges more evenly on the outer face of the electrode wire. In these different contexts, the peaks of the bumps are not placed less than 4 pm from the outer face because this is irrelevant to achieving the desired objective with these grooves.

[0172]

[0117] The fact that the outer face 6 is smooth limits the friction of the wire on the pulleys and guides of the electro-erosion machining machine.

[0173]

[0118] The fact that the core 10 is made of steel containing between 0.03% and 0.2% carbon allows the striations on the peripheral face 14 to be produced by dissolution in an acidic solution. Such a wire is therefore particularly simple to manufacture.

[0174]

[0119] The fact that the core 10 is made of iron improves the electrical conductivity of the electrode wire produced compared to the case where the core is made of steel. For example, it has been measured that the electrical conductivity of an iron core containing Tl

[0175] 0.0075% carbon by mass equals 9 MS / m, while the conductivity of the same core is 7.2 MS / m when made from C4D grade steel. Higher electrode wire conductivity allows for the use of a higher machining current, particularly during the roughing phase of electrical discharge machining (EDM), thus enabling faster machining during this initial stage.

[0176]

[0120] The fact that the surface layer 12 is made of pure zinc improves the performance of the wire because the evaporation temperature of pure zinc is low and lower than that of a CuZn alloy.

[0177]

[0121] The presence, on the outer face, of a lubricant containing polyethylene glycol or ethoxylated esters of dicarboxylic acids, makes it possible to obtain an electrode wire whose coefficient p of friction is less than 0.45. Such an electrode wire produces little dust when used in an electro-erosion machining machine.

[0178]

[0122] Creating the striations that form the bumps by immersing a steel wire in an acidic solution makes it possible to create these striations simply and efficiently.

[0179]

[0123] The process of producing the grooves using a die comprising abrasive grains specifically shaped and arranged to form these grooves when this die is passed through the starting wire, makes it possible to produce these grooves in an iron starting wire.

[0180]

[0124] Depositing the zinc layer by electrolysis results in a zinc layer with a much more uniform thickness than if the zinc layer were deposited by a galvanizing process. The uniform thickness of the zinc layer helps preserve the dents during wire drawing step 120.

Claims

Demands 1. Electrode wire for electrical discharge machining having an outside diameter greater than or equal to 0.15 mm, this electrode wire comprising: - a metallic core (10) made of iron or steel containing less than 0.2% carbon and extending along a longitudinal axis, this metallic core comprising: • cross-sections perpendicular to its longitudinal axis, • a peripheral face (14) having striations which each extend mainly parallel to the longitudinal axis and which form, within each of the cross-sections, a bumpy interface which has local low extremities and local high extremities distributed over at least 75% of the periphery of the peripheral face, this bumpy interface being present over at least 75% of the length of the electrode wire, and - a surface layer (12) formed directly on the peripheral face of the metallic core, this surface layer: • forming an outermost metallic outer face (6) of the electrode wire, • being made of pure zinc, characterized in that in each of the cross-sections: - the local low extremes are spaced less than 12 pm apart from each other and the local high extremes are spaced less than 12 pm apart from each other, - the bumpy interface is mainly contained between a lower limit (30) and an upper limit (32), the gap between the lower and upper limits being between 1 pm and 5 pm, the lower and upper limits being defined as follows: • Each lower and upper boundary extends continuously along the bumpy interface, • Each lower and upper boundary is formed by a succession of segments, • Each segment is straight and extends from a starting point to an ending point. • The starting point of a segment coincides with the ending point of the preceding segment within the same limit when that limit is traversed in a clockwise direction. • The starting and ending points of each segment are located at a local low extremum in the case of the lower boundary, and at a local high extremum in the case of the upper boundary. • in the case of the lower limit, the arrival point of each segment is located at the level of the smallest of the lower local extrema chosen from among the lower local extrema which are located at a distance from the starting point of this segment of between 6 pm and 12 pm, the smallest of the lower local extrema being the one which is closest to the longitudinal axis, • in the case of the upper limit, the arrival point of each segment is located at the level of the largest of the upper local extrema chosen from among the upper local extrema which are located at a distance from the starting point of this segment of between 6 pm and 12 pm, the largest of the upper local extrema being the one which is furthest from the longitudinal axis, - the upper limit is separated from the outer face by a distance that remains less than 4 pm along the perimeter of the bumpy interface, and - the surface layer completely fills all regions located between the upper limit and the peripheral face.

2. Electrode wire according to claim 1, wherein the outer face (6) forms a smooth interface which, in each of the cross-sections and in longitudinal sections of the wire, deviates by at most 1 pm from a segmented line (50) defined as follows: - the segmented line extends continuously along the smooth interface, - The segmented line is formed from a succession of segments, - each segment is straight and extends from a starting point to an ending point, - the starting point of a segment coincides with the ending point of the preceding segment in the segmented line when this segmented line is traversed clockwise, - The starting and ending points of each segment are located on the outer face, - the length of each segment is equal to 12 pm.

3. Electrode wire according to claim 2, wherein, within each of the cross sections, the segmented line is located less than 6 pm from the lower limit along the periphery of the bumpy interface.

4. Electrode wire according to any one of the preceding claims, wherein the metal core (10) is made of steel containing between 0.03% and 0.2% carbon.

5. Electrode wire according to claim 4, wherein the steel of the metal core (10) is grade C4D of standard EN16120-2.

6. Electrode wire according to any one of claims 1 to 3, wherein the metal core (10) is made of iron comprising less than 0.01%, by mass, of carbon.

7. Electrode wire according to any one of the preceding claims, wherein the outer face (6) is coated with a lubricant containing polyethylene glycol or ethoxylated esters of dicarboxylic acids.

8. Electrode wire according to any one of the preceding claims, wherein the outside diameter of the electrode wire is less than or equal to 0.4 mm.

9. Electrode wire according to any one of the preceding claims, wherein the surface layer (12) covers more than 90% of the peripheral face of the metal core.

10. A method for manufacturing an electrode wire according to any one of the preceding claims, said method comprising the following steps: - the supply (100; 300) of a metal wire made of iron or steel containing less than 0.2% carbon and extending along a longitudinal axis, this metal wire comprising: • cross-sections perpendicular to its longitudinal axis, • a peripheral face having striations which each extend mainly parallel to the longitudinal axis and which form, within each of the cross-sections, a bumpy interface which has local low extremities and local high extremities distributed over at least 75% of the periphery of the peripheral face, this bumpy interface being present over at least 75% of the length of the metal wire, and - the deposition (110) of a layer of pure zinc, directly onto the peripheral face of the metal wire, to obtain a coated wire having a surface layer directly formed on the peripheral face of a metal core, this surface layer: • forming the outermost metallic outer face of the coated wire, and • being made of pure zinc, then - the drawing (120) of the coated wire to obtain the electrode wire, characterized in that: - the supply step (100; 300) includes the supply of a metal wire in which, within each of the cross-sections: - the local low extremes are spaced less than 12 pm apart from each other and the local high extremes are spaced less than 12 pm apart from each other, - the bumpy interface is mainly contained between a lower limit and an upper limit, the gap between the lower and upper limits being between 1 pm and 5 pm, the lower and upper limits being defined as follows: • Each lower and upper boundary extends continuously along the bumpy interface, • Each lower and upper boundary is formed by a succession of segments, • Each segment is straight and extends from a starting point to an ending point. • The starting point of a segment coincides with the ending point of the preceding segment within the same limit when that limit is traversed in a clockwise direction. • The starting and ending points of each segment are located at a local low extremum in the case of the lower boundary, and at a local high extremum in the case of the upper boundary. • in the case of the lower limit, the arrival point of each segment is located at the level of the smallest of the lower local extrema chosen from among the lower local extrema which are located at a distance from the starting point of this segment of between 6 pm and 12 pm, the smallest of the lower local extrema being the one which is closest to the longitudinal axis, • in the case of the upper limit, the arrival point of each segment is located at the level of the largest of the upper local extrema chosen from among the upper local extrema which are located at a distance from the starting point of this segment of between 6 pm and 12 pm, the largest of the upper local extrema being the one which is furthest from the longitudinal axis, - The pure zinc layer deposition step involves depositing a layer of pure zinc whose average thickness, expressed in micrometers, is between (Em / 2)*(D in i / D2) and (E m / 2+ 4 pm)*Di n i / D2 such that, after drawing, in each of the cross-sections, the upper limit is separated from the outer face by a distance that remains less than 4 pm along the periphery of the dented interface and the surface layer completely fills all regions located between the upper limit and the peripheral face of the manufactured electrode wire, where E mis the average difference between the lower and upper limits, D ini is the diameter of the supplied metal wire and D2 is the diameter of the manufactured electrode wire.

11. A method according to claim 10, wherein the step (100) of supplying the wire comprises: - the supply (102) of a steel wire containing between 0.03% and 0.2% carbon and whose peripheral face is devoid of the striations forming the bumpy interface, then - the production (104) of the striations forming the bumpy interface by plunging the steel wire into an acidic solution which dissolves the ferrite and pearlite of the steel wire at different speeds.

12. A method according to claim 10, wherein the step (300) of supplying the wire comprises: - the supply (302) of a starting wire made of iron or steel containing less than 0.2% carbon, diameter D o of this starting wire being greater than the diameter D ini , Then - the creation (304) of the striations forming the bumpy interface while bringing the wire diameter to diameter D ini , by drawing the starting wire by passing it through the hole of a die, the face of this hole which comes directly to rub against the starting wire during the drawing being composed of abrasive grains embedded in a matrix made of a material less hard than the abrasive grains, the hardness of the abrasive grains being greater than the hardness of the material in which the starting wire is made and the average diameter of the abrasive grains being between 2 pm and 10 pm and the average gap between the abrasive grains being less than 12 pm.

13. Method according to claim 12, wherein the abrasive grains are made of diamond or silicon carbide.

14. A method according to any one of claims 10 to 13, wherein, during the deposition step (110), the pure zinc layer is deposited by electrolysis.

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