WO2004069460A1 - Wire for electrical discharge machining, comprising a brass core and a copper surface layer - Google Patents

Abstract

The invention relates to a wire for electrical discharge machining, comprising a brass core and a copper surface layer. According to the invention, the electrode wire for spark erosion-machining comprises a brass core (12) which is covered with at least one copper surface layer (14), the thickness (E) of said copper surface layer (14) being less than 1 micrometer. Preferably, the core-forming brass has the properties of a brass that has undergone a stress relieving treatment following wire drawing, while the surface layer-forming copper is annealed. One such wire retains all of the advantageous electrical discharge properties of a brass wire, while reducing wire peeling in the guides of the electrical discharge machine. In this way, the machine requires less frequent maintenance operations.

Description

 WIRE FOR ELECTROEROSION WITH A CORE OF BRASS AND SURFACE LAYER OF COPPER

TECHNICAL FIELD OF THE INVENTION The present invention relates to electrode wires used for machining by erosive sparking.

Erosive sparking makes it possible to machine an electrically conductive part, generating sparks between the workpiece and an electrically conductive wire. The wire electrically conductive ^'is scrolled in the vicinity of the workpiece in the longitudinal direction of the wire, and it is gradually shifted in the transverse direction relative to the workpiece or by translation of the wire or by translation of the workpiece.

The sparks gradually erode the part and the wire. The longitudinal scrolling of the wire makes it possible to permanently maintain a sufficient wire diameter to avoid breaking it in the sparking area. The relative movement of the wire and the part in the transverse direction makes it possible to cut the part or to treat its surface, if necessary. Wire EDM machines include braking and guiding means for holding and stretching a section of wire in the vicinity of the workpiece in a sparking area filled with a dielectric such as water, means for making running the wire longitudinally in the sparking area, means for generating a spark current between the wire and the workpiece, and means for producing a relative movement of the wire and the workpiece transversely to the longitudinal direction some thread .

In general, EDM machining takes place in several successive stages. During a first roughing step, a high sparking energy is generated in the machining area, in order to rapidly cut the part in order to give it substantially its final dimension. During a second finishing step, an average energy is generated in the sparking area, and a second pass is made to correct the geometry of the part and thus eliminate the geometry faults due to the high sparking energy. during the first stage. An additional medium energy finishing pass is generally necessary to further correct the geometry. During a third surfacing step, low energy is generated in the sparking area and one or more surfacing passes are carried out during which the means for relative transverse movement of the wire and the part follow the shape already cut. of the workpiece, and low energy sparking thus corrects the roughness of the workpiece.

There are currently many types of wire for EDM, which are distinguished by their structures.

Some wires have a generally homogeneous transverse structure, for example made of copper, brass, tungsten, or molybdenum. Other threads have a core of a first material, coated with one or more layers of other materials. The structure chosen must essentially meet the requirements of electrical conductivity and mechanical resistance. The electrical conductivity is necessary for the supply of energy to the spark area. Mechanical resistance is necessary to prevent breakage of the wire in the spark area. If possible, the structure is chosen so that the wire has a behavior favorable to erosion, that is to say so that the wire allows rapid erosion of the workpiece. The maximum erosion speed of a wire is the speed limit beyond which the wire breaks when increasing the spark energy to accelerate the erosion of the workpiece.

Another important parameter sought is the conformity of the part in terms of precision and surface condition of the machined part.

In general, each wire structure provides machining speed, machining precision and a surface finish.

It is difficult to find an alloy making it possible to optimize simultaneously the three parameters of machining speed, machining precision and surface finish.

Brass wires containing 35 to 37% zinc have thus been proposed, which constitute an economically acceptable compromise. One of the interests of brass wires is their mechanical resistance. Their breaking load can be greater than 900 N / mm ² . Sometimes we uses semi-hard or annealed brass wires, the breaking load of which is around 400 to 500 N / mm ² .

In other circumstances, wires covered with a layer of brass containing between 30% and 80% zinc are used, therefore relatively hard.

These brass-surface wires have good machining properties. However, it can be seen that EDM machines using such wires require more frequent cleaning of the guides, in order to eliminate the progressive congestion of the guides with a metallic powder.

According to the invention, the cause of this progressive congestion of the guides has been investigated, and it has been found that certain wires have the defect of flaking during their passage through the braking and guiding means of EDM machines. . This results in an accumulation of tiny brass shavings in the narrow areas where the wire passes. This hinders the correct passage of the thread, and can even prevent its automatic or manual threading. It is then necessary to dismantle and clean the wire guide means.

The productivity rules mean that we try to carry out cleaning operations as frequently as possible. Such brass surface layer son therefore have a drawback in this regard.

Furthermore, electrode wires have been proposed, the surface layer of which is made of zinc, for example on a brass core. It is found that such a wire also flakes excessively, unless it is covered with a sufficient and appropriate lubricant. However, the lubricant then constitutes an unfavorable element which alters the qualities of EDM, and in particular the quality of the surface finish of the part machined by the wire. Electrode wires made of copper have also been proposed. Very advantageous in terms of manufacturing cost, the copper wire does not however have sufficiently good mechanical properties, when its diameter is small, for example less than or equal to 0.25 mm. Its breaking load is indeed of the order of 450 N / mm ² . And the machining speed of a copper wire is lower than the machining speed of a brass wire. A composite wire with a steel core and a copper surface layer has also been proposed. It is thus possible to raise the mechanical properties of the wire beyond 1000 N / mm ² . But to achieve sufficient electrical conductivity, the surface layer of copper is thick, to occupy 50 to 80% of the cross section of the wire, as described in document JP 2001-030008. Despite its high mechanical strength, such a wire does not provide a satisfactory EDM speed.

In general, the state of the art therefore dissuades a person skilled in the art from using an electrode wire with a copper surface for rapid machining by electroerosion.

STATEMENT OF THE INVENTION

The present invention results from research aimed at optimizing the structure of a wire for EDM, in order to obtain both a high erosion speed and a high machining precision, while reducing the risk or the frequency of sealing of the guides in the EDM machine.

The idea which is the basis of the invention is to start from an electrode wire with a brass core, advantageously a brass having a proportion of 35% to 37% of zinc, the remainder being copper, by modifying its layer. superficial to avoid or limit flaking phenomena.

Thus, the problem proposed by the present invention is first of all to design a new electrode wire structure for machining by erosive sparking making it possible to optimize the parameters of machining speed and machining precision, while reducing the risk or frequency of blockage of the guides in the EDM machine.

The invention also aims to limit the cost of production of such an electrode wire.

The invention is based on the observation that, unexpectedly, a brass wire covered with a thin layer of copper flakes much more weakly than a bare brass wire. The chipping test can be carried out by passing a wire through a ceramic guide with conical entry and symmetry of revolution, the wire entering this guide at a certain angle, and leaving in the axis of revolution. Friction residue is collected below the guide, and the quantities of flaking chips obtained for a certain length of wire passing through the guide are measured at a certain speed and under a certain mechanical tension. However, at the same time, the invention seeks to preserve the overall mechanical and electrical properties of a brass wire, in order to preserve satisfactory machining results in terms of machining speed, machining precision and surface condition. of the workpiece. In this regard, an excessively thick layer of copper affects the properties of the brass wire, in a manner which is detrimental to the machining qualities. We find in this the teaching of the state of the art, which dissuades those skilled in the art from using copper in an electrode wire for rapid machining. A thick layer of copper also tends to favor a deposit of copper on the part being machined, which can worsen its corrosion and affect its appearance.

And a layer of copper that is too thin no longer has the property of reducing the phenomenon of flaking of the wire in the guides.

By cons, surprisingly, the invention highlights the advantageous properties of a thin layer of copper, which both does not significantly affect the machining qualities of the wire and reduces the phenomenon of flaking of the wire in the guides. Next, according to the invention, it has been sought to further improve the favorable properties of the surface layer of copper, in order to further reduce the risks of flaking of the wire in the presence of a very thin surface layer of copper. It was then found that the annealed copper has improved properties, apparently thanks to its greater ductility.

However, the annealing of copper should not affect the mechanical properties of the core of the wire, and we have sought the manufacturing conditions which make it possible to obtain both a wire with high mechanical resistance and with a surface layer of very copper ductile.

To achieve these and other objects, the present invention provides an electrode wire for flash machining. erosive comprising a brass core coated with at least one metallic layer, and in which:

- the metallic surface layer is made of copper,

- the thickness of the surface layer is less than 1 micron. Preferably, the brass of the core contains between approximately

35% and 37% zinc, the remainder being copper.

The brass of the core should preferably have a breaking load greater than 700 N / mm ² . However, wires with a lower breaking load may be suitable, depending on the applications envisaged.

When manufacturing an electrode wire, a wire drawing step is generally used to bring the diameter of the wire to a sufficiently small value, of the order of 0.25 mm. By such a drawing operation, the brass is hardened, and it thus becomes harder. But simultaneously it is the seat of internal constraints, which hinder its use. To reduce internal stresses, the wire is heat treated by heating, producing a stress relief annealing after drawing.

Under these conditions, the brass of the core has the properties of a brass having undergone a stress relief annealing after drawing. The brass of the core then presents a more regular inter-atomic distance. This can be observed by subjecting a sample of brass to X-rays and observing the X-ray diffraction; the diffraction peak widths are smaller in the cold worked and relaxed state than in the cold worked but not relaxed state.

In such an electrode wire, the thickness of the surface layer of copper can advantageously be between

0.1 and 0.5 micron. In this range of thicknesses, there is no noticeable reduction in the rate of EDM, compared to the rate of EDM of a bare brass wire.

The very thin copper surface layer can be discontinuous without affecting the quality of EDM. However, a low ratio between the apparent brass surface and the copper surface must be kept. Preferably, the surface layer has the properties of an annealed copper surface layer. In particular, the surface layer of copper may have a hardness lower than that of the same surface layer of hardened copper and not annealed. This hardness can be compared by nano-hardness methods. We can also verify that the copper of the surface layer then has a more regular inter-atomic distance, resulting from recrystallization in alpha phase, than that of the same surface layer of hardened copper and not annealed. According to a first embodiment, the brass core is covered with a single surface layer of copper.

According to another embodiment, the brass core is coated with a layer of zinc, itself coated with a surface layer of copper. To make such an electrode wire, the following steps can be used: a. a brass wire is provided, the diameter of which is of the order of 1 millimeter, advantageously between 0.9 and 1.2 millimeters, b. the brass wire is coated with a layer of copper by electrolysis in aqueous phase, according to a thickness of 0.36 to 2.4 microns, c. the wire thus coated is drawn, to bring it to the desired diameter, advantageously of the order of 0.25 millimeters.

Preferably, after drawing, the wire is subjected to heating producing a stress-relieving annealing of the brass core, simultaneously carrying out annealing of the copper of the surface layer.

SUMMARY DESCRIPTION OF THE DRAWINGS Other objects, characteristics and advantages of the present invention will emerge from the following description of particular embodiments, made in relation to the attached figures, among which:

- Figure 1 is a schematic front view of a wire EDM machine; FIG. 2 illustrates, in comparison, the properties of relative elongation of the hardened copper and the hardened brass as a function of the applied load; - Figure 3 is a similar curve illustrating the relative elongation of brass and copper, in the case of a copper wire annealed at 280 ° and a bare brass wire annealed at 450 °;

- Figure 4 illustrates the breaking load of a bare brass wire and a copper wire depending on the temperature reached during annealing;

- Figure 5 is an illustration similar to Figures 2 and 3, showing the relative elongation of a copper wire annealed at 365 ° C and a bare brass wire having undergone a stress relief annealing at 349 ° C; - Figure 6 illustrates the speed difference and the dimension difference during machining performed on the one hand using a bare brass wire, on the other hand with a copper-plated brass wire according to the present invention;

- Figure 7 illustrates the evolution of the surface condition of the part machined using a wire according to the present invention, compared to the machining performed using a bare brass wire, at during the successive stages of machining; and

- Figures 8 and 9 are schematic perspective views, on a large scale, of electrode wires according to two embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First of all, consider FIG. 1, which illustrates a machine for machining by erosive sparking by means of a wire. The EDM machine as illustrated in FIG. 1 essentially comprises a machining enclosure 1 containing a dielectric such as water, means such as pulleys 2 and 3 and wire guides 20 and 30 for holding a electrode wire 4 and tension it in a sparking area 5 inside the enclosure 1, a workpiece support 6 and means 7 for moving the workpiece support 6 with respect to the electrode wire 4 in the area d ¹ sparking 5. The workpiece 8, held by the workpiece support 6, is placed in the sparking area 5. The wire guides 20, 30 are on either side of the workpiece 8, and guide the electrode wire 4 precisely. They have for this a position close to the workpiece 8, and their diameter is little greater than that of the electrode wire 4, for example a diameter of 254 μm for an electrode wire 4 of 250 μm. The electrode wire 4 runs longitudinally as indicated by the arrow 9 in the sparking area 5 facing the workpiece 8. An ^' electric generator 10, electrically connected on the one hand to the electrode wire 4 by a line 18 and a contact 18a which touches the wire electrode 4 during its passage through the dielectric of the enclosure 1 between the pulley 2 and the wire guide 20, and on the other hand to the workpiece 8 by a line 19, generates in the sparking area 5 an energy electrical suitable for showing electric arcs between the workpiece 8 and the electrode wire 4. In general, the pulley 2 is braked, and the pulley 3 is driven, to tension the wire as it travels in the sparking area 5.

The machine comprises control means for adapting the electrical energy, the running speed of the electrode wire 4, and the movement of the workpiece 8 according to the machining steps.

As can be seen in FIG. 1, by displacement of the workpiece 8 in a transverse direction along the arrow 11, the erosive sparking gradually produces the penetration of the electrode wire 4 into the mass of the workpiece 8 which is conductive of electricity, and produces a slit. Then, by moving the workpiece 8 in a direction perpendicular to the electrode wire 4 and the arrow 11, a perpendicular cut is produced, to produce for example a workpiece with two perpendicular facets.

It is understood that a high electrical energy generated by the electric generator 10 allows rapid sparking and therefore allows faster movement of the workpiece relative to the electrode wire 4, for rapid machining. According to the invention, an electrode wire is provided having a core having good EDM properties, with a surface layer reducing the phenomena of flaking in the guides formed in particular by the wire guides 20 and 30.

FIG. 8 illustrates a first embodiment of an electrode wire 4 according to the invention, in which the core 12 is made of brass, advantageously a brass containing about 35% to 37% of zinc, and coated with a layer of copper 14 whose thickness E is less than 1 micron.

FIG. 9 illustrates a second embodiment of an electrode wire 4 according to the invention, in which the core 12 is also made of brass, advantageously containing approximately 35% to 37% of zinc, and is covered with an intermediate layer 13 made of zinc, itself covered with a surface layer 14 of copper of thickness E less than 1 micron.

A thickness E of copper layer 14 of between 0.1 and 0.5 microns is advantageous because it does not appreciably reduce the speed of EDM, but it does, however, significantly reduce the phenomena of flaking of the wire in the guides of the EDM machine. The maintenance operations of the machine can thus be spaced, without requiring the cleaning of the guides.

We will now describe the improvement in the properties of the wire by the stress relief annealing treatment after drawing.

As in all methods for manufacturing wires for EDM, the wire according to the invention is made from a wire of larger diameter, then reducing its diameter by a drawing operation to bring it to a useful diameter D on the order of 0.25 millimeters. However, such a drawing operation causes the metal constituting the wire to work hard. It is known that a hardened metal has properties of greater hardness and lower plasticity.

Thus, the Vickers hardness of the hardened copper is 110 while that of the brass containing 36% of zinc, in the same state, is 150. This information is published in the document

"the properties of copper and its alloys", published by the Copper, Brass and Alloy Information Center, Paris.

At the same time, the elongation of a hardened brass wire or a hardened copper wire is relatively low. The elongation curves as a function of the load of the hardened brass and of the hardened copper are illustrated in FIG. 2. It can be seen that a load of 400 N / mm ² deforms the copper by 1% while the brass is not deformed only 0.5%. By this figure, it is understood that, in an electrode wire according to the present invention, the surface of the wire in copper more easily follows the surface of the guides than if it were made of brass. This could explain the reduction in flaking phenomena.

It was then verified that the difference between copper and brass persists when the wire is subjected to recrystallization annealing, to be delivered with a breaking load of the order of 400 to 500 N / mm ² .

This verification is illustrated in FIG. 3, showing the tensile curves of a copper wire annealed at 280 ° C and a brass wire annealed at 450 ° C. Brass wire containing about 36% zinc. The two wires, previously hardened by wire drawing, were annealed by the Joule effect, that is to say by passing an electric current having raised their temperature for a period of approximately 5 seconds. The measurement of the voltage and the current at the end of annealing made it possible to calculate the temperature of the wire. It is noted that the difference in the mechanical properties of the wires is substantially the same when the brass has been brought to a temperature sufficient to recrystallize.

With metals in the states cited, a load of 200 N / mm ^{2 is} sufficient to deform the copper by 10%, while the brass is not substantially deformed. We then understand the advantage of having on the surface of the wire for EDM a metal such as copper in the annealed state: the copper then deforms easily and substantially, to match the surface of the guides. We then sought to obtain the same state of the annealed copper, but while retaining the mechanical properties of the work hardened wire. We tried to anneal the copper without annealing the brass core.

For this, the tests of annealing by Joule effect of wire work hardened by wire drawing were repeated, and their mechanical properties were measured.

By measuring the breaking loads as a function of the annealing temperature, on the one hand on a copper wire, on the other hand on a brass wire, the curves of FIG. 4 have been obtained. there is a temperature zone, between approximately 300 ° C and 390 ° C, allowing the copper to be annealed without significantly affecting the strength properties mechanical of brass: copper has a reduced breaking load, little more than 200 N / mm ² , while brass retains in this case a high breaking load, greater than 1000 N / mm ² . It is then possible to produce, in production, an electrode wire whose brass core has a large breaking load, for example greater than 700 N / mm ² , allowing significant traction of the wire during electroerosion, while the copper surface layer has great elongation properties to reduce flaking phenomena. In this annealing temperature range, the tensile curves of a copper wire and a brass wire are plotted, these curves being represented in FIG. 5.

In this range of annealing temperatures, copper has a low yield strength and a high elongation, while brass has a high yield strength and a low elongation. In this temperature range, the brass undergoes a stress relief annealing which removes its internal tensions.

By making a wire with a brass core and a thin layer of copper, by hardening it by wire drawing, then by subjecting it to a stress relief annealing as mentioned above, a wire is obtained having all the electrical and mechanical properties. a brass wire, with a surface comparable to that of an annealed copper wire that is not very hard and very ductile.

The phenomenon of flaking is thus further reduced, without losing the erosion properties such as the speed of EDM and the surface condition of the workpiece.

These effects are demonstrated by the following experiments, carried out with a bare brass wire and a copper-plated brass wire according to the invention, having approximately 0.5 micron of copper on a core of 0.250 millimeters in diameter in brass.

Thus, Figure 6 illustrates two curves: - the first curve, identified by diamonds, is the relative speed of machining using a copper-plated brass wire according to the invention, compared to the speed of machining using a bare brass wire, during the various machining operations carried out on a Charmilles Robofil 2020 brand machine. The machining sequence identified by the letter E3 is a roughing sequence. The machining sequences identified by the letters E17 and E8 are finishing sequences. The machining sequences identified by the letters E10, E13, E21 and E22 are facing sequences; - The second curve, illustrated by the triangles, illustrates the relative deviation of the dimensions made with the copper-plated brass wire according to the invention, compared to the dimensions made using a bare brass wire, during the same machining steps.

FIG. 7 illustrates, for the same machining steps and the same two wires of copper-plated brass according to the invention and of bare brass, the roughness parameters of the machined parts obtained. We note that there is no significant difference between the two sons.

This demonstrates that it has been possible, thanks to the wire according to the invention, to retain the machining properties of a brass wire, while significantly reducing the phenomena of flaking in the guides of the EDM machine.

The EDM wire according to the invention is much less flaking on friction, and it is possible that it corrodes more slowly. Compared with other noble or conductive metals, copper is relatively inexpensive, so that the wire according to the invention can be produced at lower cost.

Thanks to the particular properties of the electrode wire according to the invention, it is not necessary to leave a large amount of lubricant on the surface of the wire.

The EDM wire according to the invention has the appearance of a copper wire, while having the best properties of a brass wire.

When the electrode wire according to the invention has undergone a stress-relieving annealing after drawing, the brass of the core still has a breaking load greater than 700 N / mm ² . It can be seen, by appropriate measures, that the brass of the core then has a regular inter-atomic distance, in contrast to the hardened brass in which the atomic distances are irregular, as a result of the internal mechanical tensions resulting from the hardening. In the wire according to the present invention, the surface layer advantageously has the properties of an annealed copper layer. One can in particular compare the hardness of the copper surface layer of the wire according to the invention, by nanosecurity methods. It can also be checked that the copper surface layer of such a wire has a more regular interatomic distance.

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

Claims

1 - electrode wire for machining by erosive sparking, comprising a brass core (12) coated with at least one metallic layer, characterized in that: - the metallic surface layer (14) is made of copper, - The thickness (E) of the surface layer (14) is less than 1 micron. 2 - electrode wire according to claim 1, characterized in that the brass of the core (12) contains between about 35% and about 37% of zinc. 3 - electrode wire according to one of claims 1 or 2, characterized in that, the brass of the core (12) has a breaking load greater than 700 N / mm ² . 4 - electrode wire according to any one of claims 1 to 3, characterized in that the brass of the core (12) has the properties of a brass having undergone a stress relief annealing after drawing. 5 - electrode wire according to claim 4, characterized in that the brass of the core (12) has a regular inter-atomic distance. 6 - electrode wire according to any one of claims 1 to 5, characterized in that the thickness (E) of the surface layer (14) is between 0.1 and 0.5 microns. 7 - electrode wire according to any one of claims 1 to 6, characterized in that the surface layer (14) in copper is discontinuous, with a low ratio between the surface of visible brass and the surface of copper. 8 - electrode wire according to any one of claims 1 to 7, characterized in that the surface layer (14) of copper has the properties of a surface layer of annealed copper. 9 - electrode wire according to claim 8, characterized in that the surface layer (14) of copper has a lower hardness than that of the same surface layer of hardened copper and not annealed. 10 - electrode wire according to claim 9, characterized in that the copper of the surface layer (14) has a more regular inter-atomic distance than that of the same surface layer of hardened copper and not annealed. 11 - electrode wire according to any one of claims 1 to 9, characterized in that the brass core (12) is coated with a single surface layer (14) of copper. 12 - electrode wire according to any one of claims 1 to 9, characterized in that the brass core (12) is coated with a layer of zinc (13) itself coated with a surface layer (14) in copper. 13 - A method of producing an electrode wire according to any one of claims 1 to 12, characterized in that it comprises the steps: a. a brass wire is provided, the diameter of which is of the order of 1 millimeter, advantageously between 0.9 and 1.2 millimeters, b. the brass wire is coated with a layer of copper by electrolysis in aqueous phase, according to a thickness of 0.36 to 2.4 microns, c. the wire thus coated is drawn, to bring it to the desired diameter, advantageously of the order of 0.25 millimeters. 14. - Method according to claim 13, characterized in that, after drawing, the wire is subjected to heating producing a stress relief annealing of the brass core, simultaneously carrying out an annealing of the surface layer of copper.