FR2881973A1 - Composite electrode wire for use in electroerosion machining has coating of gamma-brass with fissures filled by beta-brass under-layer - Google Patents

Abstract

The electroerosion electrode wire (1), comprising a copper, copper alloy or brass core (2) and a brass coating, has the coating made from an under-layer (3) of beta-brass and an outer layer of gamma-brass (4) with fissures (5a) that are filled at least partially by beta-brass from the continuous under-layer. The combined thickness of the two brass coating layers is less than 10 per cent of the overall diameter of the electrode wire, and the outer surface can be oxidised, with either a dull or bright finish. An independent claim covers the manufacture of the electrode wire from a zinc-coated core which is drawn and heated to 200 - 400 degrees C to produce the beta- and gamma-brass phases.

Description

4656FDEP.doc

  The present invention relates to electrode wires used to cut metals or electrically conductive materials by spark erosion in a spark erosion machine.

  Most modern EDM machines are designed to use bare brass wires, typically 0.25 mm in diameter, with a breaking load of 400 to 1000 N / mm2.

  EDM wires must be electrically conductive. They act by erosive electrical discharge between the wire and a conductive workpiece in a water-based controlled dielectric medium.

  Obtaining machining precision, in particular the production of low-radius corner cut-outs, requires the use of small-diameter threads that support a large mechanical load at break in order to be stretched in the machining zone. and limit the amplitude of the vibrations. One can then be tempted to use a wire of which at least a central portion is made of steel, in order to increase the load at break.

  As EDM machining is a relatively slow process, there is simultaneously a need to maximize the machining speed. It is understood that this speed depends directly on the spark energy released in the machining zone between the wire and the workpiece, and therefore depends on the electrical energy that can lead the wire to the area of machining. But the erosive discharges in the machining area, and the Joule effect produced by the electric current passing through the wire, tend to heat up the wire.

  One of the limitations of spark erosion wires is that they break under the combined effect of heating and mechanical stress. This forces users to limit the machining power of their erosion machines, especially when the wire cooling conditions are not good, for example in conical machining, or machining of high-rise parts.

  The simplest solution to avoid breaks is to use large diameter wires, for example 0.30 mm and more. But this limits the minimum radius of the re-entrant angles that can be machined.

  It has already been proposed to use zinc-coated wires, the effect of the coating being to increase the machining speed compared to that of a bare brass wire. But the pure zinc layer wears out very quickly, and does not protect the core of the wire for a sufficient time for cutting high parts.

  It has been proposed to cover a core of wire with a brass layer in phase J3, that is to say a brass containing about 47% of zinc, avoiding the disadvantage of a too rapid wear of a metal. superficial layer of pure zinc. The cutting performance can thus be improved.

  In US Pat. No. 5,945,010, it has been proposed to anneal a galvanized brass to produce a peripheral phase brass layer y, then to draw the thus obtained blank to bring it to the final diameter and to simultaneously produce a surface layer of brass. phase and fractured. The document states that fracking of the surface layer does not adversely affect cutting speed performance.

  Finally, the document US 6,781,081 teaches to produce a wire having, on a metal core, a superposition of two continuous layers of brass, the lower layer being made of p-phase brass, the continuous outer layer being made of brass having a phase γ. The erosion rate is then greater than that of yarns having only a layer of brass in phase y or only a layer of brass in phase R. There is still a need to machine as fast as possible for a given machining intensity , and also to be able to use the highest possible machining intensity with a wire of given diameter.

  The present invention results from the surprising observation according to which, with a metal core electroerosion wire covered with an alloy layer, significantly improved electroerosion performance can be obtained by providing, on a copper core or brass, a layer of coating combining a fractured layer of brass and a brass sub-layer in phase 13. This finding runs counter to the teaching of US 5,945,010, which does not see improvement the cutting speed when using a fractured brass surface layer in phase y, and which discourages the use of phase 13 brass.

  Thus, to further improve the EDM machining speed, the present invention provides an electrode wire for EDM machining, comprising: - a core made of copper, copper alloy, or brass, - a brass coating, in which the brass coating comprises the superimposition of: - a continuous brass undercoat in phase 13, and - a fractured layer of brass structure in the fractured phase which reveals brass in phase f3 in the fractures.

  According to an advantageous embodiment, brass in phase 13 at least partially fills the fractures of the γ-phase brass surface layer.

  Best results are obtained by providing that the fractured-phase brass surface layer has a thickness of less than 5% of the wire diameter, preferably less than 2% of the wire diameter.

  Alternatively or in addition, the continuous brass undercoat layer 3 may advantageously have a thickness of between 5% and 12% of the diameter of the wire.

  An advantageous embodiment is to provide a fractured phase brass surface layer having a thickness of about 1% of the wire diameter, and the continuous phase brass underlayer has a thickness of about 7%. % to 8% of the wire diameter.

  An increase in the spark erosion rate is further obtained by providing that the outer surface of the brass in phase is sufficiently oxidized, dark in color.

  It may, however, be preferred to have an outer surface of brass wire in a less oxidized phase, which nevertheless has a glossy, light-reflecting appearance so as to be compatible with EDM machines which use this property to detect the presence of the wire. .

  A certain amount of zinc oxide on the surface of the brass in phase is even necessary to obtain the wire under advantageous conditions.

  It is preferable to choose a brass metal core with a zinc content of less than 30%, more advantageously a brass metal core with 20% zinc.

  Alternatively, we can choose a copper metal core to optimize the conductivity of the wire.

  According to another aspect, the invention proposes a method for producing such a wire electrode, comprising the steps of: a) providing a copper or brass core, b) covering the core with a layer of zinc electrolytically, for pre-roughing, c) optionally subjecting the pre-blank to a first drawing, to smooth the surface of the galvanized wire, and thus facilitate the wire feed once diffused, d) annealing the pre-blank drawn in the oven so to produce, by diffusion between the zinc of the cover layer and the copper or brass of the core, a blank having a brass sub-layer in phase 13 and a brass surface layer in phase y, itself being at less slightly oxidized at the surface, e) subjecting the blank thus diffused to a second cold drawing, to bring it to the final diameter and to fracture the brass layer in phase y.

  4656FDEP.doc During the second drawing, the outer y phase fractures first into blocks evenly distributed on the surface of the wire. Between these blocks, there are empty cracks. Then, still during drawing, these blocks tend to cluster in the longitudinal direction leaving the sub-layer in phase (3 to penetrate between them, finally to outcrop in some parts of the surface of the wire.

  Preferably, the second drawing produces a diameter reduction of between 40% and 60% approximately. This makes it possible to correctly fracture the peripheral layer of brass in phase y.

  In addition, it will be possible to choose to perform a first drawing Io realizing a diameter reduction of between 40% and 60% approximately.

  In the process, the annealing step d) can advantageously be carried out in the oven at a temperature of about 380 ° C. to 400 ° C. and for a period of about two to eight hours, with ramps of rise and fall in temperature of 0.2 C / min at about 1 C / min.

  It is not possible to simply enumerate all the conditions of time and temperatures which make it possible to obtain a certain state of diffusion. Indeed, the diffusion that is carried out to make a spark erosion wire is an outer zinc layer which is neither flat, thin nor thick enough to constitute a semi-infinite medium.

  However, for diffusion operations carried out in the air on galvanized copper wires, the thickness of the coating being approximately 18 μm, and the external diameter 0.422 mm, it has been observed that the thickness e of the intermediate layer in phase (3 grew as a function of time t according to the following law: e = (Dt) I / 2, D being the diffusion coefficient at temperature T and following a law of type D = Doe Q R'T. the molar constant of perfect gases and is 8.31 J / (mol.K) Under the conditions mentioned, Q is about 183 kJ / mol, and Do is about 9.5 m2 / s The final thickness of phase f3 is of course limited by the amount of available zinc, and according to the invention is expected to leave a little phase y surface.

  This allows the skilled person to choose his time and temperature conditions - in fact his thermal path according to the initial conditions and the desired final conditions.

  The fact that the diffusion is carried out on a wire exposed to oxygen is very important, because otherwise, in a neutral gas atmosphere, or under reduced pressure, there would be a significant evaporation of the zinc, and the thickness of the phase. remaining at the end of the broadcast would be much lower.

  4656FDEP.doc If you want to strongly oxidize the outer surface of the brass layer y, you will perform air annealing. Air must be able to diffuse to the surface of the wire faster than the oxidation of the wire requires. For this purpose, a pre-blank in the form of a sparse winding, for example in a basket, will be provided, or a ramp of very low temperature, less than 0. 5 C / min, of 0. 2 C will be used. / min for example. In the presence of a dense winding, it is mainly the outside of the coil that would oxidize while the interior would remain safe from oxygen.

  If we want to oxidize the outer surface, we will simply use the air present between the strands of son. To do this, the coil will be enclosed in a sealed or semi-sealed device, such as a thin metal foil, for example an aluminum foil, wound around it. The packaging device must allow the expansion and contraction of the gases contained in the package around and in the coil, during the heat treatment. This limits the oxidation by packaging of the pre-blank in a sealed or semi-sealed envelope.

  Diffusion in a neutral atmosphere or under reduced pressure is not recommended because the zinc will evaporate in part from the surface of the wire. This evaporation could be compensated for by a greater initial thickness. However, the evaporation of the zinc from the surface of the wire results in a relative enrichment of the copper, and therefore a large precipitation of p or a phases in the in-phase layer. This is not conducive to EDM.

  Other objects, features and advantages of the present invention will emerge from the following description of particular embodiments, with reference to the attached figures, in which: FIG. 1 is a schematic perspective view of a wire for electroerosion according to an embodiment of the present invention; FIG. 2 is a longitudinal section of a spark erosion wire according to one embodiment of the invention with a brass core; and FIG. 3 is a cross-section of a spark erosion wire according to the invention with a copper core.

  In the embodiments illustrated in the figures, an electrode wire for electro-erosion machining 1 comprises a core 2 made of copper or brass, coated with a coating consisting of a continuous underlayer 3 of brass in phase p3 and of a surface layer 4 having a brass structure in a fractured phase which reveals brass in the p3 phase in the fractures.

  In the present description and in the claims, the term "brass in phase 13" refers to an alloy of copper and zinc having substantially 0.25% zinc. At room temperature, this R phase is ordered and rather brittle, and it is usually called f3 '. If we exceed a certain temperature, the structure becomes disordered and we then speak of phase f3. The transition between the r3 and f3 'phases is inevitable, but produces little effect. Consequently, for the sake of simplicity, this brass will be designated in the present description by the single expression "brass in phase (3".

  In the description and in the claims, the term "brass in the y phase" is used to refer to a copper and zinc alloy in which zinc is present in a proportion of about 65%.

  A "phase lauton" may have a lower zinc content, for example about 35%, or even about 20%.

  With regard to the surface layer 4, there is for example a zone 5 in phase y, bordered by fractures 5a in which brass may appear in phase R. Phase brass 13 may at least partially fill the fractures 5a of the surface layer of brass in the y phase, ensuring a certain continuity of the surface of the wire.

  The advantageous effect of such a wire structure has been demonstrated by several tests carried out on wires of different structures.

  Test N 1 This first test demonstrates that a surface layer of fractured phase brass reduces the maximum intensity of electric current that the wire can withstand.

  For this purpose, different wires having the same diameter of 0.25 mm are provided.

  The wire is fixed between two electrical terminals immersed in deionized water at 20 C. The wire is not subjected to a mechanical tension. An electric power generator is connected to the terminals of the device. The electrical intensity is increased until the wire breaks, and the maximum intensity supported by the wire is noted.

  The results are shown in the table below. 10

  4656FDEP.doc Yarn Maximum strength supported Copper 130 A Brass CuZn 37 75 A Brass CuZn 37 coated with 75 A 3 μm pure zinc Brass CuZn 37 coated with 75 A 3 μm pure zinc and then diffused in the y phase, unfractured Brass CuZn 37 coated with 65 A zinc., diffused at 177 C and drawn to obtain a fractured y phase It will be noted that the last yarn tested, fractured y phase on a brass core, is consistent with the teaching of US 5,945,010 aforementioned. Test N 2 A wire A according to the invention, having a diameter of 0.25 mm, consisting of a brass core CuZn 20 covered with a layer in the apparently non-fractured phase R, and a layer of phase y visibly fractured. For this purpose, CuZn 20 brass wire having a diameter of 1.20 mm was coated with 27 μm zinc electrolytically. This wire was drawn to a diameter of 0.827 mm. The wire was annealed by passing it through an oven for two hours at 400 ° C., with ramps of rise and fall in temperature at + 1-1 ° C./min, and in an air atmosphere. Finally, the wire thus diffused was drawn to bring it back to a diameter of 0.25 mm. The coating layer obtained on the wire was about 20 μm in total thickness. It consisted of a sub-layer in phase F3 apparent on the surface of the wire in some places, and covered in other places with a fractured phase brass. It thus appears that, during the drawing step, the R-phase brass underlayer does not crack on its own when it is drawn.

  This wire, tested under the same conditions as the previous wires in test N 1, withstood a maximum intensity of 75 Å. Its breaking load was 750 N / mm 2. It has been successfully used in electroerosion under a mechanical tension of 17 N. 4656FDEP.doc This test shows a surprising effect of the brass undercoat in phase 13, which increases the capacity of the wire to withstand a current intensity electrical, and that brings this capacity to the level of that son in which the surface layer is not fractured.

  Test No. 3 A wire B was then produced in the following manner: 1.20 mm diameter CuZn brass core was coated with 27 μm zinc and then drawn to 0.25 mm diameter . An annealing of 1H15 at 380 C was carried out, to obtain a wire having an underlayer of about 16 pm in phase 13 and an outer layer of about 4 pm in unfractured phase (because not drawn).

  This wire supported a maximum intensity of 75 A. It showed a breaking load of 430 N / mm2; and it could be used successfully in spark erosion, with however a mechanical tension reduced to 10 N. Thus, the wire A according to the invention, made during the test N 2, has a better capacity of mechanical strength than thread B of the test above.

  Test No. 4 The erosion rates of wires A and B were then compared under conditions suited to the two wires, that is to say with a mechanical tension of 10 N. The test was carried out at AgieCut Evolution II SFF machine.

  The conditions were: basic technology estcca25nnn300g230050, suitable for galvanized brass wires 900 N / mm2, nozzles plated on the workpiece. The machined material was 60 mm high steel. The mechanical tension on the wire was reduced to 10 N. The spark erosion speed was: 2,515 mm / min for wire A, 2,500 mm / min for wire B. A slight increase was noted for wire A. With same machine, the same material and the technology estccw25nnn300h250050, adapted to CuZn 20 core wire and phase layer 13, in the annealed state, the force on the wire being 12 N, the following maximum speeds have been observed, gradually increasing the value of the parameter P from 1 until the wire breaks: 2.79 mm / min for the wire A (with P = 27); 1.85 mm / min for the B wire (with P = 19).

  The fact that wire A is faster than wire B is in contradiction with the data published in US 5,945,010.

  Test No. 5 The influence of the thickness of the fractured γ phase was then investigated to determine a wire having an optimum erosion rate.

  A wire according to the invention was obtained from a copper core 0.9 mm in diameter. The core was coated with zinc and then drawn to obtain an intermediate wire 0.422 mm in diameter, the outer layer of zinc is 16 to 19 microns thick. The intermediate wire has been brought to different temperatures, for varying periods of time, so as to produce external layers composed of p and y phases in different proportions. After the diffusion treatments, the lo wires were in the annealed state. A cold drawing operation made it possible to obtain 0.25 mm diameter electroerosion wires in the hardened state. The outer phase layer was fractured, while the in-phase sub-layer (3 remained continuous, the outer layer in phase y did not cover the entire surface of the wires, and the thickness was noted. this outer layer in phase y where it occurs, which gives not a mean value of the thickness, but rather a maximum value.

  The results are summarized in the table below.

  Thread Speed Thickness Conditions Maximum premature layer diffusion failure (maximum erosion thickness if E2 HSO y-layer regime) 1 400 C, 2H, (3 25pm 4.35 mm / min No +/- 0.5 C / min y 2pm in l dark appearance air 2 380 C, 3H R 18pm 4.76 mm / min No +/- 0.5 C / min y 5pm in air dark appearance 3 380 C, 3H, (3 18pm 4.61 mm / min No +/- 0. 5 C / min y 5pm wire sheltered from the air shiny appearance outside the coil 4 360 C, 2H, R 5pm 4.05 mm / min Yes +/- 0.5 C / min and 20pm in the air non-uniform color appearance 320 C, 2H, R 5pm +/- 0.5 C / min and 20pm in air non-uniform color appearance 4656F DEP.doc It can be seen that an outer layer in the fractured phase, of too great a thickness, leads to fractures premature yarn.

  From the above tests, it can be deduced that the fractured y-phase brass surface layer preferably has a thickness of less than 5% of the yarn diameter, more preferably less than 2% of the yarn diameter.

  For its part, the continuous layer of brass f3 phase can advantageously have a thickness of between 5% and 12% of the diameter of the wire, more preferably about 7 to 8%.

  A good compromise has been obtained by providing a fractured y-phase surface layer having a thickness of about 1% of the wire diameter, and a R-phase brass continuous underlayer having a thickness of about 7% to 8% of the wire diameter.

  The wire 3 of the table above shows that the erosion rate is further increased in the presence of oxidation of the outer surface of the brass in the y phase.

  Unexpected effect of the fractured-phase brass surface layer, even in very small amounts on the surface of a diffused wire, was a better electrical touch compared to a fully diffused (3) and surface-oxidized wire. The electric key consists, on an Agie Evolution II machine, in a very low-power spark, only allowing to locate the piece with precision, and not to cut it out.

  Also less fouling of the current leads has been observed with outer-phase yarns being fractured and sub-layered in phase [3, with respect to scattered wires until the phase y disappears completely. It may be thought that the fractured phase, even if present in a small quantity, makes it possible to clean the current leads. Any residues of oxide and lubricant deposited on the surface of the current leads could be carried away by the scraping effect of the surface of the wires, which surface is irregular.

  The present invention is not limited to the embodiments which have been explicitly described, but it includes the various variants and generalizations thereof within the scope of the claims below.

4656FDEP.doc

Claims (14)

  1 electrode wire (1) for electro-erosion machining comprising: - a core (2) made of copper, copper alloy, or brass, - a brass coating, characterized in that the brass coating comprises the superimposition: a continuous phase brass sub-layer (3), and a fractured phase brass structure surface layer (4) which reveals brass in phase 13 in the fractures (5a).   2 electrode wire according to claim 1, characterized in that brass in R phase at least partially fills the fractures (5a) of the surface layer (4) of brass in phase y.   3 electrode wire according to one of claims 1 or 2, characterized in that the surface layer (4) brass fractured phase has a thickness less than 5% of the diameter of the wire, preferably less than 2% of the diameter of the thread.   4 electrode wire according to any one of claims 1 to 3, characterized in that the continuous undercoat (3) of brass in R phase has a thickness of between 5% and 12% of the diameter of the wire.   5 electrode wire according to claims 3 and 4, characterized in that the surface layer (4) brass fractured phase has a thickness of about 1% of the diameter of the wire, and the continuous underlayer (3) of brass in phase 13 has a thickness of about 7% to 8% of the wire diameter.   6. An electrode wire according to any one of claims 1 to 5, characterized in that the outer surface of the brass in phase is oxidized, dark in color.   7 electrode wire according to any one of claims 1 to 5, characterized in that the outer surface brass y-phase of the wire is oxidized, but nevertheless has a glossy appearance, can reflect light.   8 electrode wire according to any one of claims 1 to 7, characterized in that the core (2) is made of brass with a zinc content of less than 30%.   9 electrode wire according to claim 8, characterized in that the core (2) is brass 20% zinc.     An electrode wire according to any one of claims 1 to 7, characterized in that the core (2) is made of copper.   11 A method of producing a wire electrode according to any one of claims 1 to 10, characterized in that it comprises the following steps: 4656FDEP.doc a) provide a core (2) of copper or brass, b) covering the core (2) with a zinc layer electrolytically, to perform a pre-roughing, c) optionally subjecting the pre-blank to a first drawing, d) annealing the pre-blank drawn in the oven so as to producing, by diffusion between the zinc of the cover layer and the copper or brass of the core (2), a blank having a brass sub-layer in phase 13 and a superficial layer of brass in the surface-oxidized phase, e) subjecting the blank thus diffused to a second cold drawing to bring it to the final diameter and fracturing the y-phase brass layer.   Process according to Claim 11, characterized in that the second drawing produces a diameter reduction of between 40% and 60% approximately.   Method according to one of claims 11 or 12, characterized in that the first drawing produces a diameter reduction of between 40% and 60% approximately.   Process according to any one of Claims 11 to 13, characterized in that the annealing step d) is carried out in the oven at a temperature of approximately 380 ° C. to 400 ° C. and for a period of approximately two to eight hours. with ramps of rise and fall in temperature of 0.2 C / min at 1 C / min approximately.     Process according to any one of Claims 11 to 14, characterized in that the annealing step d) is carried out in air, producing a surface oxidation of the γ-phase brass.   Process according to any one of Claims 11 to 14, characterized in that the annealing step d) is carried out by limiting the oxidation by packaging the pre-blank in a sealed or semi-sealed envelope.