Field of the invention . According to a first aspect, the present invention relates to an electrode for electrical discharge machining. According to a second aspect, the present invention relates to a method of tuning various properties of such an electrode such as mechanical tensile strength, electrical conductivity and machinability.

Background of the invention . Electro-discharge machining (EDM) uses pulsed DC waves to create sparks between a work piece and electrode material in a dielectric fluid. Very often, as in the present invention, the electrode material is a wire. Parts of the electrode and work piece material are flushed away.

An electrode wire for electrical discharge machining must meet various severe requirements. The most important requirements relate to electrical conductivity, mechanical strength and machinability. With respect to the first requirement, electrical conductivity, the electrode should be able to carry the currents without excessive heating so as to avoid breaking. The electrical current applied to the electrode may amount to 500 A and more. So the overall electrical resistance of the wire is important. With respect to the second requirement, mechanical strength, it is hereby understood that the electrode is subject to tensions during working. A minimum tensile tension or load is required in order to avoid the electrode from repelling away from the work piece or in order to avoid the electrode from vibrating. This minimum tension must be able to be maintained despite temperatures sometimes increasing up to 300 0C. With respect to the third aspect, machinability, the eroded debris m ust be removed quickly in order to prevent local shortcuts. This machinability is determined by the flushing rate of the dielectric fluid together with the nature of the electrode, more particularly the outer layer of the electrode. Various attem pts have been m ade in the prior art to meet all these requirements. As a matter of example, CH-A5- 646 083 discloses an electrode for electrical discharge machining comprising a steel core, an intermediate copper layer and various copper- zinc- nickel alloy layers. I n this way a steel core m ay be covered by following subsequent layers : a copper layer, a Cu45Ni55 layer, a Cu30Ni45Zn26 layer and a Cu30Ni20Zn50 layer. The purpose is to change gradually the properties from one layer to another layer and to avoid sudden changes.

As a matter of another example, WO- A- 98/09764 discloses an electrode for discharge machining comprising a steel core, an intermediate layer of copper or a copper containing alloy, and an outer layer containing at least 40% of zinc.

Su m m ary of the invention . It is an object of the present invention to avoid the drawbacks of the -prior art. -r It is another object of the present invention to provide an alternative electrode for discharge m achining. It is still an object of the present invention to tune in a straightforward way and independently of each other the various properties of an electrode for discharge machining.

According to a first aspect of the present invention there is provided an electrode for electrical discharge machining, where the electrode comprises a) a core out of steel wire ; b) a conductivity layer of copper or aluminum around the core ; c) a barrier layer of nickel on the conductivity layer ; d) a vaporization layer of tin, zinc, lead or cadmium or any alloy thereof on the barrier layer. The steel wire may be a carbon steel or a stainless steel wire. With respect to a carbon steel wire, the am ount of carbon in the steel composition, possibly together with the amount of micro- alloying elements such as chromium, and the degree of work hardening of the steel wire determine the ultim ate tensile strength of the electrode. Work hardening can be done by cold drawing. The thickness of the conductivity layer and the quality of the copper or aluminum determine the electrical conductivity of the electrode and the speed of cutting. The thickness of the vaporization layer and the material selected for the vaporization layer determine the speed of flushing away any eroded debris during working.

Preferably the conductivity layer is formed by cladding copper or aluminum around the steel core. Cladding provides an economical way of applying a relatively thick layer around the steel core.

I n an embodiment of the invention, the conductivity layer is made of copper and a diffusion layer- copper nickel is present between the conductivity layer and the barrier layer of nickel. This diffusion layer starts from almost pure copper. The m ore rem ote from the center, the more the content of nickel increases until the barrier layer of almost pure nickel is reached.

I n a preferable embodiment of the invention, the vaporization layer comprises pure zinc and unavoidable impurities. Such a vaporization layer gives excellent machinability to the electrode. I n this embodiment, and apart from a possible thin diffusion layer of copper nickel and making exception of unavoidable impurities, all the layers are made of a material with a single purpose : a) a core out of steel for giving the mechanical strength ; b) a conductivity layer of copper or aluminum for the appropriate electrical conductivity ; -A-

c) a barrier layer of nickel to give a thermal resistance and to prevent sparks from damaging the core ; d) a vaporization layer of zinc for a fast flushing of the eroded debris and a stable discharge. As a result, the tuning of each function corresponding with each of these layers can be done in a straightforward way.

The vaporization layer may be further provided with an oxide layer on the vaporization layer. This oxide layer may be a zinc oxide or may be a chromate. This oxide layer which is deliberately provided, has the advantage of preventing corrosion of the rest of the wire and of preventing flaking of the vaporization layer.

According to a second aspect of the present invention, there is provided a method of tuning the various functions of an electrode for electric discharge machining. The method comprises the following steps : a) providing a core out of steel wire ; b) providing a conductivity layer of copper or aluminum around the core ; c) providing a barrier layer of nickel on the conductivity layer ; d) providing a vaporization layer of tin, zinc, lead, cadmium or an alloy thereof around the barrier layer. The mechanical tensile strength may be tuned by selecting the proper amount of carbon in the wire rod composition - possibly together with the amount of micro- alloying elements such as chromium - and by applying the amount of work hardening, e.g. the am ount of cross- sectional reduction during cold drawing. The degree of electrical conductivity is tuned by choosing the quality of copper or aluminum in the conductivity layer and by determining the thickness of the conductivity layer. The degree of machinability may be tuned by the choice of the material in the vaporization layer and by determining the thickness of the vaporization layer. Brief description of the draw ings. The invention will now be described into more detail with reference to the accompanying drawings wherein FIGURE 1 is a cross- section of an electrode for electric discharge machining according to the invention.

Description of the preferred em bodi m ents of the invention . HGURE 1 is a cross-section of an electrode wire 10 for electric discharge machining according to the invention. The electrode wire 10 comprises a steel core 12. The steel core 12 is covered by a conductivity layer 14 of copper. On the conductivity layer is a diffusion layer 16 nickel - copper. Around the diffusion layer 16 is a barrier layer 18 of nickel. On the barrier layer 18 is a vaporization layer 20 of zinc.

Such an electrode wire 10 for electrical discharge machining may be made as follows. A steel core wire may have a low carbon content (lower than 0.20 %C) or a high carbon content (higher than 0.20 %C) . If the steel has a high-carbon content, a suitable steel composition may be along following lines : a carbon content ranging from 0.20 to 1.10 %, a silicon content ranging from 0.10 to 0.50 %, a manganese content ranging from 0.20 to 0.75 %, the remainder being iron, possible micro- alloying elements such as chromium, boron, vanadium and unavoidable impurities. A copper strip is cladded around a steel core wire. The cladded steel wire is drawn until an intermediate diameter. A heat treatment is carried out on the drawn cladded steel wire. The nature of the heat treatment depends upon the carbon content in the steel wire. For low carbon contents, i.e. below 0.20 %C, the cladded steel wire is subject to a glueing treatment. For higher carbon contents, i.e. higher than 0.20 %C, the cladded steel wire is patented. Before or after the heat treatment, a nickel coating is deposited on the copper layer. This is done by means of an electrolytic process. A diffusion layer copper - nickel m ay be formed. Thereafter, a zinc layer is deposited on the nickel layer by means of a hot dip layer. The nickel layer prevents a zinc alloy or zinc diffusion layer to be formed. I n other words, the top layer of zinc is - apart from unavoidable impurities - a layer of pure zinc without brittle zinc alloy layers. Alternatively, the zinc layer may be deposited by means of an electrolytic process. After the hot dip bath or the electrolytic coating with zinc, the wire is drawn until its final diameter. Performing the drawing operation after the zinc coating has the advantage of facilitating the drawing : higher speeds or less expensive drawing dies can be used than in case the wire is drawn with a nickel top coating.

The table hereunder summarizes some results obtained with invention electrodes.

Table

IACS is the I nternational Annealed Copper Standard. The elongation is the percentage elongation after fraction. CCS stands for copper cladded steel.

More generally, the steel core allows for a tunable tensile strength of the complete electrode (conductivity, barrier and evaporation layer included) in a broad range from 500 MPa to 1500 MPa, depending upon the steel composition (m ainly the carbon content) , the degree of final reduction and obviously the relative amount of mainly copper on the electrode. The 500 MPa is obtainable by means of low carbon contents (< 0.20 % G) , the higher levels require higher levels of carbon content, which m ay range up to 0.70 and 0.80 % and even up to 0.95% and more. A copper clad steel core allows for tunable conductivity in a range from 15 % IACS to 85 % IACS, e.g. from 20 % IACS to 75 % IACS. I n order to obtain a 30% IACS 25 volume per cent of copper is needed, to obtain 40% IACS 36 volume per cent of copper is required, to obtain 55% IACS 52 volume per cent of copper is required, to obtain 60% IACS 57 volume per cent of copper is required and to obtain 70 % IACS 68 volume per cent of copper is required. The thickness of the barrier layer in nickel may range from less than 1 μm to more than 10 μm, e.g. from 1 μm to 8 μm, e.g. from 2 μm to 7.5 μm. The thickness of the evaporation layer of zinc may range from 5 μm to 50 μm, e.g. from 6 μm to 40 μm, e.g. from 7 μm to 25 μm.