JP7603323B2 - Manufacturing method of sliding electrode - Google Patents

Description

The present invention relates to a method for manufacturing a sliding electrode of a conductive metal wire that enables a longer life.

One of the industrial applications of conductive metal wire is wire electric discharge machining. This is a machining method that cuts materials using the heat of the wire's discharge. The wire, which is insulated in a liquid environment, is brought close to the workpiece and a pulsed current flows through it, raising the temperature to several thousand degrees Celsius, melting the workpiece and cutting it into the desired shape.

Figure 1 shows a schematic diagram of the wire electric discharge machining process. Two sliding electrodes 1 are provided spaced apart in the vertical direction, and the sliding electrodes 1 are connected to a power supply unit 2. A conductive metal wire 3 used in a wire electric discharge machine is brought into contact with the sliding electrode 1, and the conductive metal wire 3 is moved continuously at a desired speed in one direction as indicated by the arrow in Figure 1.

The conductive metal wire 3 used in this wire electric discharge machining process is made of a copper alloy or molybdenum alloy wire, and passes in front of the part to be cut at a constant speed while being subjected to a constant tension. Electric current is supplied by contact with high-voltage sliding electrodes 1 installed above and below the workpiece. When the conductive metal wire 3, which is constantly under tension, passes over the sliding electrodes, a microscopic view shows that a tiny gap is constantly created, making it easier for discharge to occur.

In this environment, the wire passage area of the sliding electrode 1 gradually wears away due to sliding and discharge, forming a groove-like depression. If this area is continued to be used thereafter, the risk of the wire breaking increases, so when the depression deepens to a certain depth, it is determined that the wire has reached the end of its life, and the electrode is shifted or replaced, allowing it to pass over a new smooth surface, power supply is resumed, and machining continues.

As a material for sliding electrodes in such operating environments, cemented carbide consisting of hard particles mainly made of tungsten carbide, which has excellent wear resistance and heat resistance, and a metallic bonding phase mainly made of Co or Ni, has been used.

In recent years, as workpieces have become larger and unmanned continuous operation has become more common, the lifespan of the sliding electrodes in wire EDM machines has become an issue. The less work required to shift the electrode position or replace the electrode itself, the longer the continuous operation will be possible and the lower the cost will be, so there is a demand for methods to achieve a longer lifespan.

As an example of a measure to improve the lifespan of such sliding electrodes, as disclosed in Patent Document 1, efforts have been made to improve wear resistance and heat resistance by adjusting the particle size of the hard particles and the phase ratio and composition of the metallic bonding phase.

JP 2003-266243 A

As mentioned above, adjustments to the particle size of the hard particles in the cemented carbide and adjustments to the phase ratio and composition of the metallic bonding phase have been investigated in relation to the phenomenon in which the sliding electrode of a wire EDM machine wears out during electrical current machining, forming dents and ultimately shortening its lifespan. However, there has not been sufficient research done to understand the mechanism by which the dents form.

Regarding the wear phenomenon of the sliding electrode that constantly supplies current at high voltage, the environment in which sliding with the metal wire occurs, the pressing force is much gentler than the high-speed, high-pressure sliding that a cemented carbide die experiences during the drawing process of a metal wire. Considering this, the inventors conducted an investigation focusing on the discharge phenomenon.

The present invention was made in consideration of these circumstances, and aims to provide a method for manufacturing a sliding electrode of a conductive metal wire that suppresses the occurrence of wear during electric current processing and enables a longer life.

In order to solve the above problems, the present invention is a method for manufacturing a sliding electrode that is made of a cemented carbide containing a hard particle phase and a metallic bonding phase and is used in wire electric discharge machining using a conductive metal wire, characterized in that a lapping process is performed on the cemented carbide surface in the final finishing step of the smoothing process before use. The surface roughness of the cemented carbide surface after this lapping process is preferably 0.01 μm or less in terms of Ra.

In the present invention, it is preferable to selectively etch only the metallic bonding phase on the surface of the cemented carbide, so that the metallic bonding phase does not come into direct contact with the conductive metal wire.

This makes it possible to suppress wear during electric current machining and extend the tool's life. This reduces the need to shift the electrode position or replace the electrode itself, making it possible to operate continuously for long periods of time.

In addition, in the present invention, the depletion depth of the metallic bonding phase when only the metallic bonding phase is selectively etched can be calculated from four pieces of information: the composition of the cemented carbide base material, the composition measured by characteristic X-ray analysis from the top surface after the cemented carbide base material is processed into a sliding electrode and the metallic bonding phase is selectively etched, the composition of the metallic bonding phase remaining slightly between the hard particles in the depleted region of the selectively etched metallic bonding phase, and the analysis depth measured by the emission of characteristic X-rays in the characteristic X-ray analysis.

As a result, a small amount of metallic bonding phase remains between the hard particles in the depleted layer after selective corrosion has occurred, making it possible to efficiently calculate the amount of metallic bonding phase that changes smoothly at the interface, and accurately calculate the corrosion depth non-destructively. Note that, from the perspective of strength retention, it is assumed that the depth of selective corrosion is approximately the same as the average grain size of the hard particles.

According to the present invention, it is possible to manufacture a sliding electrode of a conductive metal wire that suppresses wear during electrical current processing and enables a longer life.

FIG. 1 is a schematic diagram showing a wire electric discharge machining process. FIG. 2 shows a smooth surface after polishing and a surface after selective etching of the metallic bonding phase. FIG. 2 is a schematic diagram showing a cross section of a surface of a cemented carbide tool after selective etching of a metallic bonding phase, and a change in the amount of _CO, which is a main component of the metallic bonding phase, during the selective etching. FIG. 2 is a diagram for defining the depletion depth of a metallic bonding phase. 1 is a flow chart of a method for measuring the depletion depth of a surface metal binder phase in a cemented carbide. This is an image of the surface of a cemented carbide alloy taken with a scanning electron microscope. FIG. 11 is a diagram showing test results of a discharge withstand voltage test. FIG. 11 is a diagram showing test results of a discharge withstand voltage test. FIG. 1 is a diagram showing the amount of wear of a sliding electrode in terms of cross-sectional area due to wear, depending on whether or not the surface of a cemented carbide alloy is subjected to selective etching treatment. FIG. 13 is a diagram showing test results regarding the wear cross-sectional area. FIG. 13 is a diagram showing test results regarding the wear cross-sectional area. FIG. 2 is a schematic diagram showing the contact state between a conductive metal wire and a sliding electrode in a test regarding a wear cross-sectional area.

Hereinafter, a method for manufacturing a sliding electrode according to the present invention will be described based on an embodiment thereof.
The method for manufacturing a sliding electrode of the present invention is a method for manufacturing a sliding electrode composed of a cemented carbide containing a hard particle phase and a metallic bonding phase, and used in wire electric discharge machining with a conductive metal wire, in which a lapping treatment is performed on the cemented carbide surface in the final finishing step of the smoothing process before use, and selectively corrodes only the metallic bonding phase on the surface of the cemented carbide, resulting in a structure in which the metallic bonding phase does not come into direct contact with the conductive metal wire.

Here, lapping is a polishing method in which a lapping agent, a mixture of abrasive grains, oil, etc., is placed between the tool and the surface of the workpiece, and the two are pressed together with an appropriate pressure while being subjected to relative motion, removing a small amount of material from the surface of the workpiece with the lapping agent, resulting in a smooth, finished surface with good dimensional accuracy. When carrying out this lapping, it is preferable that the surface roughness of the cemented carbide surface after lapping be 0.01 μm or less in Ra.

2 shows a smooth surface 4 after polishing and a surface 5 after selective etching of the metallic bonding phase 7. In the contact areas of the hard particles 6, the metallic bonding phase 8 remains.
A method for calculating the corrosion depth in the selective corrosion of the metallic bonding phase will be described below.

Fig. 3 shows a schematic diagram of a cross section of the metallic bonding phase on the surface of a cemented carbide tool after selective etching and the change in the amount of _CO, the main component of the metallic bonding phase, during the selective etching. In the depletion zone after selective etching, a small amount of metallic bonding phase remains between the hard particles, so the amount of _CO fluctuates and does not become completely zero. A gradual transition in the amount of _CO occurs from the region where the metallic bonding phase has been removed by etching to the unreacted region, and it is necessary to define the depletion depth in this region.

Therefore, the inventors have decided to define the depletion depth of the metallic bonding phase by assuming a linear change in components as shown in Fig. 4. The amount of _CO in the matrix is m1, and the small amount of _CO remaining between the hard particles is m2. It is assumed that m2 and m1 are constant values, and that the transition from m2 to m1 occurs linearly as shown in Fig. 4.

The transition position of the _CO amount where this linear change occurs is defined as the depletion depth d of the metallic bonding phase. Using these definitions, the analysis depth D measured by the X-rays irradiated in the characteristic X-ray analysis, and the amount of Co mx actually measured by the characteristic X-ray analysis, an equation is created based on the conservation of mass of the amount of Co, and the following equation (1) is obtained.

By solving equation (1) with respect to the deficiency depth d, we obtain equation (2).

Here, the concentration of the metallic bonding phase remaining between the hard particles must be estimated in advance using an analysis method attached to the scanning electron microscope. The analysis depth is determined by the X-ray intensity of the characteristic X-ray analyzer used and the composition of the cemented carbide being analyzed.

Characteristic X-ray analyzers include energy dispersive X-ray analyzers (EDS) and wavelength dispersive X-ray analyzers (WDS). Both instruments measure the chemical composition of a sample by detecting characteristic X-rays emitted from the sample surface, with EDS measuring the energy of characteristic X-rays and WDS measuring the wavelength of characteristic X-rays.

In EDS, the reaction region of characteristic X-rays is relatively shallow at a depth of a few μm, while WDS has an analysis depth of more than 10 μm. In the present invention, considering that the material to be analyzed has hard particle diameters of about a few μm, it can be said that a wavelength dispersive characteristic X-ray analyzer (WDS), which can measure the depth of multiple particles, is a more desirable analytical device.

Although this is a simplified definition of the deficiency depth, it is arithmetically uniquely determined and always intersects with the transition region of the Co content, making it a very easy definition to use when specifying it as an agreement in specifications. Furthermore, because this measurement method is non-destructive, it can be used directly for shipping inspections, and at the same time, it can also be used to analyze the cause of abnormalities.

As described above, it is possible to calculate the depletion depth d of the metallic bonding phase from four pieces of information: the composition m1 of the cemented carbide base body, the composition mx measured from the top surface by characteristic X-ray analysis after processing the same base body into a sliding electrode and selectively etching the metallic bonding phase, the composition m2 of the metallic bonding phase remaining in small amounts between the hard particles in the depleted area of the selectively etched metallic bonding phase, and the analysis depth D measured by the emission of characteristic X-rays in characteristic X-ray analysis.

Figure 5 shows a flowchart of the method for measuring the depletion depth of the surface metallic bonding phase in a cemented carbide, as explained above. By carrying out this process, when the metallic bonding phase of a cemented carbide is removed from the surface by etching, the metallic bonding phase leaves behind a mesh-like hard particle phase, and selective corrosion progresses in a tunnel-like manner. Even in this situation, the calculated corrosion depth can non-destructively and unambiguously define the depth of the corrosion front, whose boundary is actually ambiguous.

A comparison test between the calculated and measured values of the above-mentioned defect depth was carried out for three material types with different particle sizes (fine, medium, and coarse), by producing samples for each of the fine, medium, and coarse grains. The average particle size of each was 0.6 to 1.0 μm for fine grains, 2.0 to 4.0 μm for medium grains, and 5.0 μm or more for coarse grains. The actual measurements were taken at several locations using a wavelength dispersive X-ray analyzer (WDS) and the average values were calculated.

As a result of the test, the difference between the calculated and measured deficiency depths was much smaller than half the particle diameter of the hard particles for all the fine, medium, and coarse grains. The value of half the particle diameter of the hard particles can be recognized as a boundary value that determines whether or not the hard particles will fall off, and if the difference between the calculated and measured deficiency depths exceeds half the particle diameter of the hard particles, it will have a negative effect on the function of parts and tools made of cemented carbide when selective corrosion is performed. However, according to the test results, the difference between the calculated deficiency depth and the measured value was much smaller than half the particle diameter of the hard particles, so it was confirmed that the method of calculating the calculated deficiency depth according to the present invention is extremely effective.

By using this method, it is possible to selectively corrode only the metallic bonding phase on the surface of the cemented carbide that constitutes the sliding electrode, creating a structure in which the metallic bonding phase does not come into direct contact with the conductive metal wire, and it is possible to accurately calculate the corrosion depth non-destructively.

The results of experiments carried out to verify the effects of the present invention will now be described.
Specifically, discharge voltage withstand tests were conducted using the following variables as the type of cemented carbide used in the sliding electrode, the change in micro-roughness on the sliding electrode surface, and the distance from the sliding electrode surface when the metallic bonding phase, which is mainly responsible for the electrical conductivity of the cemented carbide, was selectively removed by etching treatment.

The experimental conditions are as follows.
・For the three varieties shown in Table 1, dielectric breakdown tests were conducted with two types of surface polishing (#800 and lap finish) and two types with and without selective corrosion.

・Measure Ra under two finish conditions ・Selective corrosion depth is calculated from X-ray wavelength spectroscopy ・Insulation breakdown test conditions: Room temperature (20℃±15℃) - Room humidity (45-85%)
- Voltage resistance evaluation method: A brass electrode with a 90° taper and a tip of R1 and a sliding electrode made of cemented carbide were placed with a gap of 0.05 mm between the tip of the brass electrode and the sliding surface of the sliding electrode made of cemented carbide. A voltage was applied and the voltage at the point when dielectric breakdown occurred between the electrodes was measured and evaluated as the voltage resistance.

Figure 6 shows images of the cemented carbide surface taken with a scanning electron microscope, (a) as polished and (b) after selective etching.

The test results are shown in Figures 7 and 8. Figure 7(a) shows data relating to the present invention, and Figure 7(b) shows data relating to a comparative example.
Comparisons of sample numbers 1 and 7, 3 and 9, and 5 and 11 show that lapping the cemented carbide surface increases and stabilizes the withstand voltage of all three varieties compared to #800. Comparisons of sample numbers 1 and 2, 3 and 4, and 5 and 6 show that the selective corrosion of the metallic bonding phase causes the lapping process to significantly improve the withstand voltage. On the other hand, comparisons of sample numbers 7 and 8, 9 and 10, and 11 and 12 show that the effect of selective corrosion is not apparent in samples with a rough initial surface like #800.

This shows that it is important to keep the surface roughness of the cemented carbide surface at a level of 0.01 μm or less in terms of Ra, and that by selectively corroding only the metallic bonding phase of the cemented carbide surface in this state, even more remarkable effects can be obtained.

Next, the results of tests on the amount of wear of the sliding electrode caused by selective etching of the cemented carbide surface will be described.
Fig. 9 is a schematic diagram showing the wear amount of the sliding electrode with and without selective etching treatment of the cemented carbide surface as a cross-sectional area due to wear. As shown as the sliding electrodes in Fig. 1, the sliding electrodes are arranged above and below the workpiece, and Fig. 9(a) is for pattern 1 (P1), where the wear cross-sectional area when the upper sliding electrode is selectively etched (abbreviated as "upper with") is X, and the wear cross-sectional area when the lower sliding electrode is not selectively etched (abbreviated as "lower without") is Y. The cross-sectional area ratio for this is X/Y.

Figure 9 (b) is for pattern 2 (P2), where the wear cross-sectional area when the upper sliding electrode is not subjected to selective corrosion treatment (abbreviated as "upper no") is 1.25X, and the wear cross-sectional area when the lower sliding electrode is subjected to selective corrosion treatment (abbreviated as "lower with") is 0.8Y. This assumes that the effect of selective corrosion treatment in reducing the amount of wear is reduced by 20%, so the cross-sectional area ratio in this case is 1.25X/0.8Y = 1.5625X/Y. With these settings, if the result is (upper with/lower without) < (upper without/lower with), it is considered that selective corrosion treatment is effective in reducing the amount of wear.

The results of tests on the cross-sectional wear area conducted based on the above viewpoints are shown in FIG. 10 and FIG.
10 and 11, the case where the upper sliding electrode was selectively corroded (with upper) and the lower sliding electrode was not selectively corroded (with lower) is pattern 1 (P1). The case where the upper sliding electrode was not selectively corroded (with upper) and the lower sliding electrode was selectively corroded (with lower) is pattern 2 (P2). The conductive metal wire and the sliding electrode were installed as shown in Fig. 12(a) and (b), and the usage times were 15 hours and 100 hours.

The test results showed that the relationship (top present/bottom absent) < (top absent/bottom present) was met, confirming that selective corrosion treatment was effective in reducing wear.

The contact state between the conductive metal wire and the sliding electrode in the above test is shown in Figures 12(a) and (b).

The present invention can be widely used as a manufacturing method for sliding electrodes of conductive metal wires that can suppress the occurrence of wear during electrical current processing and therefore have a long service life.

1 sliding electrode 2 power supply 3 metal conducting wire 4 smooth surface after polishing 5 surface after selective corrosion of metallic bonding phase 6 hard particles 7 metallic bonding phase 8 metallic bonding phase remaining in hard particle contact area

Claims (3)

A method for manufacturing a sliding electrode made of a cemented carbide containing a hard particle phase and a metallic bonding phase and used in wire electric discharge machining using a conductive metal wire, characterized in that in a final finishing step of a smoothing process before use, the cemented carbide surface is subjected to a lapping treatment, and only the metallic bonding phase on the surface of the cemented carbide is selectively etched, resulting in a structure in which the metallic bonding phase does not come into direct contact with the conductive metal wire . The method for manufacturing a sliding electrode according to claim 1, characterized in that the surface roughness of the cemented carbide surface after the lapping process is 0.01 μm or less in terms of Ra. 2. The method for manufacturing a sliding electrode according to claim 1, characterized in that the depletion depth of the metallic bonding phase when only the metallic bonding phase is selectively etched is calculated from four pieces of information: the composition of the cemented carbide base, the composition measured by characteristic X-ray analysis from the top surface after the cemented carbide base is processed into a sliding electrode and the metallic bonding phase is selectively etched, the composition of the metallic bonding phase remaining slightly between the hard particles in the depleted region of the selectively etched metallic bonding phase, and the analysis depth measured by emission of characteristic X-rays in the characteristic X-ray analysis.

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