Wire-EDM for electrode wire

Abstract

To provide an electrode wire for wire-EDM, which can increase the machining speed of wire-EDM.SOLUTION: This electrode wire for wire-EDM comprises: a core wire made of steel containing carbons of 0.60%-0.85% by mass; and a cover layer that is made of a copper-zinc alloy and that covers the outer peripheral surface of the core wire. A wire diameter is 30 μm-100 μm, a tensile strength is 1900 Mpa-2700 MPa, and an elastic limit is 98% to less than 100%. The cover layer includes an oxide film disposed to constitute the outer surface of the cover layer and having a thickness of 40 nm-80 nm.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an electrode wire for wire electric discharge machining. [Background technology]

[0002] Wire electrical discharge machining is known as one type of removal machining. In wire electrical discharge machining, an electrode wire is inserted into a pilot hole pre-formed in a workpiece and tension is applied to the electrode wire. A potential difference is generated between the workpiece and the electrode wire, causing an arc discharge between the workpiece and the electrode wire. The heat generated by the discharge melts the workpiece. Known electrode wires used in wire electrical discharge machining (wire electrical discharge machining electrode wires) include a core wire made of steel and a coating layer that coats the outer surface of the core wire (see, for example, Patent Document 1 and Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-056548 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-144582 Summary of the Invention [Problem to be solved by the invention]

[0004] In wire electrical discharge machining, an improvement in machining speed is desired. The present disclosure has been made in view of the above, and one of its objects is to provide a wire electrical discharge machining electrode wire that can improve the machining speed of wire electrical discharge machining. [Means for solving the problem]

[0005] A wire electric discharge machining electrode wire according to the present disclosure includes a core wire made of steel containing 0.60% by mass or more and 0.85% by mass or less of carbon, and a coating layer made of a copper-zinc alloy coating the outer surface of the core wire. The wire electric discharge machining electrode wire has a wire diameter of 30 μm or more and 100 μm or less. The wire electric discharge machining electrode wire has a tensile strength of 1900 MPa or more and 2700 MPa or less. The wire electric discharge machining electrode wire has an elastic limit of 98% or more and less than 100% of the tensile strength of the wire electric discharge machining electrode wire. The coating layer includes an oxide film having a thickness of 40 nm or more and 80 nm or less, arranged so as to form the outer surface of the coating layer. [Effects of the Invention]

[0006] According to the electrode wire for wire electric discharge machining according to the present disclosure, the machining speed of wire electric discharge machining can be improved. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram illustrating the structure of an electrode wire according to the present disclosure. [Figure 2] FIG. 2 is a schematic cross-sectional view of an electrode wire according to the present disclosure in a cross section perpendicular to the longitudinal direction. [Figure 3] FIG. 3 is a schematic diagram for explaining the straightness of the electrode wires in FIG. [Figure 4] FIG. 4 is a flowchart showing the steps of a method for manufacturing an electrode wire according to the present disclosure. [Figure 5] FIG. 5 is a schematic diagram for explaining the heating step for imparting straightness to the electrode wire. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Outline of the embodiment] First, embodiments of the present disclosure will be listed and described. (1) The wire includes a core wire made of steel containing 0.60% by mass or more and 0.85% by mass or less of carbon, and a coating layer made of a copper-zinc alloy that coats the outer surface of the core wire. The wire diameter of the wire electrode for wire electrical discharge machining is 30 μm or more and 100 μm or less. The tensile strength of the wire electrode for wire electrical discharge machining is 1900 MPa or more and 2700 MPa or less. The elastic limit of the wire electrode for wire electrical discharge machining is 98% or more and less than 100% of the tensile strength of the wire electrode for wire electrical discharge machining. The coating layer includes an oxide film that is arranged so as to form the outer surface of the coating layer and has a film thickness of 40 nm or more and 80 nm or less.

[0009] By ensuring that the carbon content of the steel constituting the core wire is 0.60% by mass or more, it is easy to ensure sufficient strength (tensile strength) of the core wire. This makes it possible to prevent breakage during use of the wire electrode wire. By ensuring that the carbon content of the steel constituting the core wire is 0.85% by mass or less, it is easy to process the core wire to an appropriate wire diameter. Furthermore, by ensuring that the tensile strength of the wire electrode wire is 1900 MPa or more and 2700 MPa or less, it is possible to ensure sufficient tensile strength of the wire electrode wire. As a result, it is possible to apply sufficient tension to the wire electrode wire during wire electrical discharge machining. By ensuring that the elastic limit of the wire electrode wire is 98% or more and less than 100% of the tensile strength, plastic deformation of the wire electrode wire is suppressed. This stabilizes the relationship between tension and length in the wire electrode wire, thereby suppressing vibration of the wire electrode wire. As a result, it is possible to suppress variation in the distance between the workpiece and the wire electrode wire, i.e., the discharge gap.

[0010] By making the oxide film of the coating layer 40 nm or thicker, arc discharge occurs between the workpiece and the wire EDM electrode wire only when a voltage equal to or greater than a predetermined value is applied between the workpiece and the wire EDM electrode wire. By making the oxide film of the coating layer 80 nm or thicker, the voltage required to generate arc discharge is kept to an appropriate value. This allows arc discharge to be generated with an appropriate voltage while suppressing the generation of unwanted arc discharge between the workpiece and the wire EDM electrode wire.

[0011] By suppressing variations in the discharge gap and generating arc discharges at an appropriate voltage while suppressing unwanted arc discharges between the workpiece and the wire electrode for wire electrical discharge machining, the discharge between the workpiece and the wire electrode for wire electrical discharge machining is stabilized, thereby improving the efficiency and speed of wire electrical discharge machining.

[0012] (2) In the above (1), the thickness of the coating layer may be 2% to 15% of the wire diameter of the wire electric discharge machining electrode wire, thereby improving the balance between the allowable current contributed by the coating layer and the strength contributed by the core wire.

[0013] (3) In the above (1) or (2), the copper content of the copper-zinc alloy may be 50% by mass or more and 80% by mass or less. If the copper content of the copper-zinc alloy is less than 50% by mass, the toughness decreases due to the β phase contained in the copper-zinc alloy, and the ease of wire drawing decreases. If the copper content of the copper-zinc alloy exceeds 80% by mass, arc discharge between the copper-zinc alloy and the workpiece becomes difficult to generate. As a result, it becomes difficult to increase the machining speed of wire electric discharge machining.

[0014] (4) In any of (1) to (3) above, the electrical conductivity of the wire electric discharge machining electrode wire may be 13% IACS (International Annealed Copper Standard) or more and 16% IACS or less. A conductivity of 13% IACS or more facilitates ensuring a sufficient machining speed. A conductivity of 16% IACS or less prevents deterioration of the accuracy of the machined surface of the workpiece.

[0015] (5) In any of (1) to (4) above, when the thickness of the oxide film on the wire electric discharge machining electrode wire is measured at five locations 1 m apart in the longitudinal direction of the wire electric discharge machining electrode wire, the ratio of the maximum thickness to the minimum thickness of the oxide film may be 1.4 or less. This configuration further stabilizes the discharge between the workpiece and the wire electric discharge machining electrode wire.

[0016] [Specific example of embodiment] Next, an example of a specific embodiment of a wire electric discharge machining electrode wire according to the present disclosure will be described with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated.

[0017] [Examples of electrode wire applications] FIG. 1 is a schematic diagram illustrating an application example of an electrode wire 1 (electrode wire for wire electrical discharge machining) according to the present disclosure. In FIG. 1, an arrow indicates the feeding direction of the electrode wire 1. First, an application example of the electrode wire 1 (electrode wire for wire electrical discharge machining) according to the present disclosure will be described using FIG. 1. The electrode wire 1 is used for wire electrical discharge machining (hereinafter simply referred to as "electrical discharge machining") of a workpiece W (a workpiece). The electrical discharge machining device 9 includes a support shaft 90 supporting a bobbin 9a serving as a supply source of the electrode wire 1, an upper guide 91 and a lower guide 92 arranged to face each other across the workpiece W, multiple rollers 93 for feeding the electrode wire 1 from the bobbin 9a to a collection section (not shown), and a cutting section (not shown) for cutting the electrode wire 1. The bobbin 9a is detachable from the electrical discharge machining device 9. The workpiece W is immersed in an insulating machining fluid. A pilot hole H is formed in the workpiece W in advance. Tension is applied to the electrode wire 1 while the electrode wire 1 is inserted into the pilot hole H of the workpiece W. The workpiece W and the electrode wire 1 are connected to a power supply (not shown). The power supply creates a potential difference between the workpiece W and the electrode wire 1, which causes an arc discharge between the workpiece W and the electrode wire 1. The heat generated by the discharge melts the workpiece W. Electrical discharge machining of the workpiece W progresses by moving the workpiece W relative to the electrode wire 1.

[0018] When the electric discharge machining of the workpiece W is completed, the electrode wire 1 is cut at the cut point CP by the cutting section. The electric discharge machining device 9 performs automatic wire connection by inserting the cut electrode wire 1 into the pilot hole H of a new workpiece W and sending it to the collection section.

[0019] [Electrode wire structure] FIG. 2 is a schematic cross-sectional view of a cross section CS of an electrode wire 1 according to the present disclosure, taken in a direction perpendicular to the longitudinal direction L. Next, the structure of the electrode wire 1 will be described with reference to FIG. 2. The cross section CS of the electrode wire 1 taken in a direction perpendicular to the longitudinal direction L is circular. The wire diameter D1 of the electrode wire 1 is 30 μm or more and 100 μm or less. Regarding the roundness of the electrode wire 1, the difference between the maximum diameter and the minimum diameter (diameter deviation) in the cross section CS is, for example, within 0.5 μm. The diameter deviation of the electrode wire 1 is preferably 0.3 μm or less. This further stabilizes the discharge between the workpiece and the electrode wire 1. As a result, the machining speed and machining accuracy are improved.

[0020] The tensile strength of the electrode wire 1 is 1900 MPa or more and 2700 MPa or less. This allows sufficient tension to be applied to the electrode wire 1 during electrical discharge machining. The tensile strength of the electrode wire 1 may be 2500 MPa or less. The tensile strength of the electrode wire 1 is measured in accordance with the provisions of JIS (Japanese Industrial Standards) Z 2241, Appendix C.

[0021] The elastic limit of the electrode wire 1 is 98% or more and less than 100% of the tensile strength of the electrode wire 1. This suppresses plastic deformation of the electrode wire 1. This stabilizes the relationship between tension and length in the electrode wire 1. This suppresses vibration of the electrode wire 1. This suppresses variation in the distance between the workpiece W and the electrode wire 1, i.e., the discharge gap. Furthermore, since the elastic limit of the electrode wire 1 is 98% or more and less than 100% of the tensile strength of the electrode wire 1, the electrode wire 1 can easily return to its unbent state when it is bent. For example, even if the tip of the electrode wire 1 is caught on a part of the electric discharge machining device 9 (e.g., the lower guide 92) when the electrode wire 1 is being connected in the electric discharge machining device 9 and the electrode wire 1 is bent, the electrode wire 1 can easily return to its unbent state. This suppresses buckling of the electrode wire 1. This improves the ease of automatic wire connection (automatic wire connection performance) during electric discharge machining. The elastic limit of the electrode wire 1 is the upper limit of the elastic region in the stress-strain curve obtained in measuring the tensile strength of the electrode wire 1 described above.

[0022] FIG. 3 is a schematic diagram illustrating the straightness of the electrode wire 1 in FIG. 2. For ease of explanation, the curvature of the electrode wire 1 is exaggerated in FIG. 3. Referring to FIG. 3, the straightness of the electrode wire 1 is measured using a sample 1a of the electrode wire 1, which is unwound from a reel (winding frame) such as a bobbin 9a without bending and cut to a length LS of 1000 mm. Specifically, a first end 1b of the sample 1a is fixed to a measurement table (not shown), and a second end 1c of the sample 1a is hung down from the measurement table as a free end. The bending width BW, which is the horizontal distance between the center of the first end 1b and the center of the second end 1c, corresponds to the straightness of the electrode wire 1. The bending width BW is, for example, within 360 times the wire diameter D1 of the electrode wire 1. That is, the straightness of the electrode wire 1 per 1000 mm of length is, for example, within 360 times the wire diameter D1. This makes it easier for the wire electrode 1 to be inserted into the pilot hole H of a new workpiece W during automatic wire threading. As a result, the ease of automatic wire threading (automatic wire threading performance) during electrical discharge machining can be improved. Here, the straightness of the wire electrode 1 per 1000 mm in length is the deviation from a straight line (bending width BW) per 1000 mm in length of the wire electrode 1, and can be calculated as the average value of the bending widths BW measured at five locations. The straightness can be investigated by holding a portion of the wire electrode 1 so that it hangs down vertically under its own weight and measuring the deviation from a straight line per 1000 mm in length.

[0023] 2 , the electrode wire 1 includes a core wire 2 having an outer circumferential surface 20 and a coating layer 3 that coats the outer circumferential surface 20 of the core wire 2. The core wire 2 is made of steel containing 0.60% by mass or more and 0.85% by mass or less of carbon. The core wire 2 is made of steel such as SWRS62A, SWRS62B, SWRS67A, SWRS67B, SWRS72A, SWRS72B, SWRS75A, SWRS75B, SWRS77A, SWRS77B, SWRS80A, SWRS80B, SWRS82A, or SWRS82B as specified in JIS G 3502. The steel that constitutes the core wire 2 contains, for example, 0.60% by mass or more and 0.85% by mass or less of carbon, 0.12% by mass or more and 0.32% by mass or less of silicon, and 0.30% by mass or more and 0.90% by mass or less of manganese, with the balance being iron and unavoidable impurities. The steel constituting the core wire 2 preferably contains 0.025 mass% or less of phosphorus, 0.025 mass% or less of sulfur, and 0.20 mass% or less of copper as unavoidable impurities. The steel constituting the core wire 2 is, for example, eutectoid steel. The metal structure of the steel constituting the core wire 2 has, for example, a pearlite structure throughout the entire region.

[0024] The coating layer 3 is made of a copper-zinc alloy. The proportion of copper in the copper-zinc alloy is, for example, 50% by mass or more and 80% by mass or less. This allows for easy production of the electrode wire 1 including the coating layer 3 made of the copper-zinc alloy. This also ensures appropriate discharge between the electrode wire 1 and the workpiece W. The coating layer 3 is formed, for example, by plating. The thickness T3 of the coating layer 3 is, for example, 2% to 15% of the wire diameter D1 of the electrode wire 1. This improves the balance between the allowable current contributed by the coating layer 3 and the strength contributed by the core wire 2. The thickness T3 of the coating layer 3 may be, for example, 2% to 10% of the wire diameter D1 of the electrode wire 1.

[0025] The coating layer 3 includes a surface layer 31 that includes an outer surface 30 of the coating layer 3. The hardness of the surface layer 31 of the coating layer 3 is 2.8 GPa or more. This makes it difficult for the electrode wire 1 to become curled when wound around a reel (winding frame) such as a bobbin 9a, even if the diameter of the reel is small. In other words, the straightness of the electrode wire 1 is improved. This makes it easier for the electrode wire 1 to be inserted into a pilot hole H of a new workpiece W during automatic wiring. This improves the ease of automatic wiring during electrical discharge machining (automatic wiring performance). The hardness of the surface layer 31 of the coating layer 3 is measured using a nanoindenter, with the indentation depth being the depth from the outer surface 30 of the coating layer 3 to 30% of the thickness T3 of the coating layer 3. Specifically, the average value of the hardness measured at a large number of measurement points (for example, 20 x 20 = 400 measurement points arranged two-dimensionally) arranged at intervals of 0.5 μm one-dimensionally or two-dimensionally along the outer surface 30 of the coating layer 3 corresponds to the hardness of the surface layer 31 of the coating layer 3.

[0026] The surface layer 31 includes an oxide film 32 on its outer surface. That is, the coating layer 3 includes an oxide film 32 arranged to form the outer surface 30 of the coating layer 3. The thickness Tf of the oxide film 32 is 40 nm or more and 80 nm or less. When the thickness Tf of the oxide film 32 of the coating layer 3 is 40 nm or more, an arc discharge occurs between the workpiece W and the electrode wire 1 only when a voltage equal to or greater than a predetermined value is applied between the workpiece W and the electrode wire 1. When the thickness Tf of the oxide film 32 of the coating layer 3 is 80 nm or less, the voltage required to generate an arc discharge is kept to an appropriate value. This allows an arc discharge to be generated at an appropriate voltage while suppressing the generation of unnecessary arc discharge between the workpiece W and the electrode wire 1. From the viewpoint of more stably controlling the voltage at which an arc discharge occurs, the thickness Tf of the oxide film 32 is preferably 50 nm or more. From the viewpoint of suppressing the voltage required to generate an arc discharge, the thickness Tf of the oxide film 32 is preferably 60 nm or less. The condition for the thickness Tf of the oxide film 32 only needs to be satisfied by the average value of the thickness of the oxide film 32 measured at five locations 1 m apart along the longitudinal direction of the electrode wire 1. The oxide film 32 is a region in which, when the element fraction (atomic ratio) is measured at depth points every 1 nm from the outer surface 10 of the electrode wire 1 in the depth direction, the 20-point moving average of the oxygen fraction calculated with the number of atoms at each depth point set to 100 at% is 5 at% or more. The thickness Tf of the oxide film 32 is measured by calculating the oxygen fraction in the depth direction from the outer surface 10 of the electrode wire 1 using an elemental analyzer such as a scanning electron microscope - energy dispersive X-ray spectroscopy (SEM-EDX) equipped with an energy dispersive X-ray spectroscopy.

[0027] [Method for manufacturing electrode wire] 4 is a flowchart showing the steps of a method for manufacturing the electrode wire 1 according to the present disclosure. A method for manufacturing the electrode wire 1 described above will be described with reference to FIG.

[0028] The method for manufacturing the electrode wire 1 includes the steps of preparing raw steel material constituting the core wire 2 (step ST1), patenting the prepared raw steel material (step ST2), forming a coating layer 3 on the outer peripheral surface of the raw steel wire (step ST3), drawing (drawing) the wire material 5 on which the coating layer 3 has been formed (step ST4), and imparting straightness to the wire material 5 having the coating layer 3 (step ST5). In step ST1, raw steel material (raw wire material) made of eutectoid steel containing 0.60 mass % to 0.85 mass % carbon is prepared.

[0029] In step ST2, a heat treatment is performed that includes the steps of heating the raw steel prepared in step ST1 to a first temperature range equal to or higher than the austenitizing temperature (point A1), then quenching to a second temperature range lower than point A1 and higher than point Ms, and maintaining the steel in the second temperature range. This results in the metal structure of the raw steel becoming a fine pearlite structure with small lamellar spacing (layer spacing). In the heat treatment, the step of heating the raw steel to the first temperature range equal to or higher than point A1 is performed in an inert gas atmosphere to suppress decarburization. From the same perspective, it is preferable to limit the time required for the step of heating the raw steel to the first temperature range equal to or higher than point A1 to the minimum required.

[0030] In step ST3, a plating process is performed in which a copper film and a zinc film are sequentially formed with uniform thicknesses all around the raw steel material, and a process is performed in which the copper and zinc in the film formed in the plating process are alloyed by thermal diffusion. The thermal diffusion of copper and zinc is performed, for example, at a temperature of 380°C or higher for one hour or more. This results in a wire 5 having a coating layer 3 made of a copper-zinc alloy.

[0031] In step ST4, wiredrawing is performed with a total area reduction rate of 97% or more. As a result, the elastic limit of the electrode wire 1 is adjusted to 98% or more and less than 100% of the tensile strength. When step ST4 is performed in a single wiredrawing, the total area reduction rate refers to the area reduction rate in that wiredrawing. On the other hand, when step ST4 is performed in multiple wiredrawings, the total area reduction rate refers to the area reduction rate calculated from the cross-sectional area perpendicular to the longitudinal direction of the wire 5 before the first wiredrawing and the cross-sectional area perpendicular to the longitudinal direction of the wire 5 after the last wiredrawing.

[0032] In step ST5, a heating step is performed to straighten the wire 5 (electrode wire 1) having the coating layer 3. FIG. 5 is a schematic diagram illustrating the heating step for straightening the electrode wire 1. In FIG. 5, the arrow indicates the direction in which the wire 5 is fed. The heating device S includes a first reel R1, a second reel R2, and a heater HT disposed between the first reel R1 and the second reel R2. The electrode wire 1 is wound around the first reel R1 and set in the heating device S. The electrode wire 1 is fed from the first reel R1 through the heater HT to the second reel R2. The rotation speed ω2 of the second reel R2 is greater than the rotation speed ω1 of the first reel R1. Therefore, the electrode wire 1 is pulled in a heated state. This straightens the electrode wire 1. The rotation speed ω2 of the second reel R2 is, for example, 1.01 times or more and 1.05 times or less the rotation speed ω1 of the first reel R1.

[0033] The temperature generated by the heater HT is, for example, 400°C or higher and 700°C or lower. The time that the electrode wire 1 stays in the heater HT (the residence time of the electrode wire 1) is preferably set to the minimum necessary time in order to suppress the growth (grain growth) of the crystal grains that constitute the coating layer 3. This suppresses softening of the coating layer 3 due to grain growth in the coating layer 3. When the temperature of the heater HT is 400°C or higher and 700°C or lower, the residence time of the electrode wire 1 is, for example, 0.4 seconds or higher and 1.2 seconds or lower. After the electrode wire 1 leaves the heater HT, an oxygen-containing gas, such as air, is blown onto the electrode wire 1. This suppresses grain growth in the coating layer 3 due to excessive heating, while allowing the oxide film 32 to be formed with a sufficient thickness. As a result, the oxide film 32 has a thickness Tf of 50 nm or higher and 60 nm or lower on the coating layer 3. The electrode wire 1 of this embodiment can be manufactured by the manufacturing method of the electrode wire 1 including steps ST1 to ST5 described above.

[0034] [Example: First Evaluation Test] An experiment was conducted to investigate factors affecting the machining speed of wire-cut EDM electrode wires. The experimental procedure was as follows. First, the manufacturing method for electrode wire 1 described above (including steps ST1 to ST5) was carried out to prepare samples (samples #1 to #6 and #13 to #17). For comparison, samples were also prepared that did not satisfy at least one of the conditions: the ratio of the elastic limit to the tensile strength being 98% or more but less than 100%; and the oxide film thickness being 40 nm or more and 80 nm or less (samples #7 to #12). The ratio of the elastic limit to the tensile strength was adjusted by changing the area reduction rate in step ST4. The oxide film thickness was adjusted by changing the atmosphere in step ST5.

[0035] Using the samples (electrode wires for wire EDM) prepared as described above, EDM was performed on workpieces under the same conditions, and the machining speed was investigated. The machining speed was evaluated relative to the machining speed achieved using sample #7, a conventional electrode wire for wire EDM, which was set at 100. The experimental results are shown in Table 1.

[0036] [Table 1]

[0037] In Table 1, "carbon content" refers to the carbon content of the steel that makes up each sample. "Tensile strength" refers to the tensile strength of each sample. "Ratio of elastic limit to tensile strength" refers to the ratio of elastic limit to tensile strength obtained as a result of the tensile test for each sample. "Coating layer thickness" refers to the thickness of the coating layer for each sample. "Oxide film thickness" refers to the thickness of the oxide film formed on the surface of the coating layer for each sample. "Oxide film thickness variation" refers to the ratio of the maximum to the minimum oxide film thickness measured at five locations 1 m apart along the longitudinal direction of the wire EDM electrode wire. "Machining speed" refers to a relative value, with the machining speed when sample #7 is set to 100.

[0038] Referring to Table 1, the machining speeds of all samples that did not satisfy at least one of the conditions that the ratio of the elastic limit to the tensile strength is 98% or more but less than 100% and the condition that the oxide film thickness is 40 nm or more but less than 80 nm were less than 110. In contrast, the machining speeds of all samples that satisfied these two conditions were 110 or more. This confirms that a wire electric discharge machining electrode wire that satisfies the above two conditions can improve the machining speed of wire electric discharge machining. Furthermore, among the samples that satisfied the above two conditions, sample #15, which had an "oxide film thickness variation" of more than 1.4, had higher machining speeds than the other samples with an "oxide film thickness variation" of 1.4 or less. This confirms that it is preferable to set the "oxide film thickness variation" to 1.4 or less.

[0039] [Example: Second Evaluation Test] An experiment was conducted to investigate the factors that affect the success rate of automatic wire threading for wire EDM electrodes. The experimental procedure was as follows. First, the manufacturing method for electrode wire 1 described above (including steps ST1 to ST5) was carried out to prepare samples (samples #18 to #23 and #30 to #31). For comparison, samples were also prepared that did not satisfy at least one of the conditions: the ratio of the elastic limit to the tensile strength must be 98% or more but less than 100%, and the hardness of the surface layer of the coating layer must be 2.8 GPa or more (samples #24 to #29). The ratio of the elastic limit to the tensile strength was adjusted by changing the area reduction rate in step ST4. The hardness of the surface layer of the coating layer was adjusted by changing the dwell time in the furnace in step ST5.

[0040] Using the samples (electrode wires for wire EDM) prepared as described above, automatic threading was carried out under the same conditions, and the success rate was investigated. The success rate was calculated by repeating automatic threading 100 times and counting the number of successful attempts. The experimental results are shown in Table 2.

[0041] [Table 2]

[0042] In Table 2, "carbon content" is the carbon content of the steel that makes up each sample. "Tensile strength" is the tensile strength of each sample. "Ratio of elastic limit to tensile strength" is the ratio of elastic limit to tensile strength obtained from the tensile test for each sample. "Coating layer hardness" is the hardness of the surface layer of the coating layer for each sample. "Ratio of straightness to wire diameter" is the ratio of straightness to wire diameter for each sample.

[0043] Referring to Table 2, the automatic wire threading success rate for samples that did not satisfy at least one of the conditions that the ratio of the elastic limit to the tensile strength is 98% or more but less than 100% and that the hardness of the surface layer of the coating layer is 2.8 GPa or more was less than 90%. In contrast, the automatic wire threading success rate for samples that satisfied the above two conditions was 90% or more. This confirms that by using a wire electrode wire for wire electrical discharge machining that satisfies the above two conditions, it is possible to improve the success rate of automatic wire threading in wire electrical discharge machining.

[0044] It should be understood that the embodiments and examples disclosed herein are illustrative in all respects and are not limiting in any respect. The scope of the present invention is defined not by the above description but by the claims, and it is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0045] 1 electrode wire 10 Outer surface 1a Sample 1b 1st end 1c 2nd end 2-core wire 20 Outer surface 3 Covering layer 30 Outer surface 31 Surface layer 32 Oxide film 5 wire rod 9 Electrical discharge machining equipment 9a Bobbin 90 Support shaft 91 Upper guide 92 Lower guide 93 Laura BW bending width CP Cut Point CS cross section D1 wire diameter H pilot hole HT heater L Longitudinal direction LS Length R1 1st reel R2 2nd reel S heating device T3 Coating thickness Tf oxide film thickness double work ω1 rotation speed ω2 rotation speed

Claims

1. a core wire made of steel containing 0.60% by mass or more and 0.85% by mass or less of carbon; a coating layer made of a copper-zinc alloy and coating an outer circumferential surface of the core wire; The wire diameter is 30 μm or more and 100 μm or less, The tensile strength is 1900 MPa or more and 2700 MPa or less, The elastic limit is 98% or more and less than 100% of the tensile strength, The electrode wire for wire electric discharge machining, wherein the coating layer includes an oxide film that is arranged so as to form an outer surface of the coating layer and has a film thickness of 40 nm to 80 nm.

2. 2. The wire electrode wire for electric discharge machining according to claim 1, wherein the thickness of the coating layer is 2% to 15% of the wire diameter of the wire electrode wire for electric discharge machining.

3. 2. The electrode wire for wire electric discharge machining according to claim 1, wherein the proportion of copper in the copper-zinc alloy is 50% by mass or more and 80% by mass or less.

4. 2. The wire electrode for wire electric discharge machining according to claim 1, wherein the electrical conductivity of the wire electrode for wire electric discharge machining is 13% IACS or more and 16% IACS or less.

5. 5. The wire electric discharge machining electrode wire according to claim 1, wherein when the thickness of the oxide film is measured at five locations spaced 1 m apart in the longitudinal direction of the wire electric discharge machining electrode wire, a ratio of a maximum value to a minimum value of the thickness of the oxide film is 1.4 or less.

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