JP2007331140A - Repairing method of mold containing magnetic body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the point at issue wherein the residual stress or hair crack in a mold caused by an uncertain element at the time of mold operation or molding processing in various processing methods such as grinding, cutting, discharge processing, etc. can not be accurately estimated over a deep range from the surface of the mold in a conventional technique and the proper repairing of the mold can not be performed. <P>SOLUTION: A magnetic flux density sensor is three-dimensionally moved and scanned along the surface of a material containing a magnetic body to detect the intensity and vector of a magnetic field and the mold is repaired on the basis of the evaluation of the removal quantity of residual stress caused at the time of processing of the mold, the evaluation of the crack produced in the vicinity of the surface of the mold and the evaluation of wire discharge processing. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a mold made of a material containing a magnetic material, which is produced by various magnetoresistive methods such as grinding, cutting, electric discharge machining, electric current sintering, laser sintering, etc. The present invention relates to a method of repairing a mold that uses a magnetic flux density sensor to derive the distribution and position of residual stress and surface cracks from a magnetic field vector and magnetic field intensity generated on the surface, and removes residual stress and surface cracks.

For press dies and molding dies, from the high temperature that causes melting during processing to room temperature when making dies by various processing methods such as grinding, cutting, electric discharge machining, electric current sintering, laser sintering, etc. There is a case where it is rapidly cooled, and a large thermal stress is generated during its production or use, such as compression and cooling repeated during use as a hot mold such as injection molding. Many of these work as tensile stresses on the surface and induce the generation of fine hair cracks on the mold surface. In addition, since the mold material is a brittle material that places importance on hardness, the fracture toughness value is small, and the life of the mold is greatly reduced due to the presence of fine cracks.

For this reason, in order to reduce the residual stress existing inside the mold, efforts are being made to find a method for repairing the mold by removing the processed surface layer and an optimal machining process for creating the mold. Therefore, it is important to accurately measure the distribution of internal stress on the mold surface and the position of cracks, and to repair the mold based on this data.

On the other hand, a mold is often manufactured through a wide variety of processes. After the mold material is provided through heat treatment, quenching treatment, and tempering treatment, it is manufactured through electrical discharge machining, cutting, and grinding divided into several stages.

Among them, in electrical discharge machining, which is often used for final molding of molds that are difficult to process materials, a large thermal stress is applied to the machining surface because it is rapidly cooled from the melting temperature to the temperature of the machining fluid during cutting and molding. Often occurs.

Also in cutting and grinding, due to the necessity of highly efficient production of difficult-to-cut materials used in dies, high-speed strong machining is often performed with high cutting and grinding stress, and a large amount of residue remains on the machining surface as well. Stressed parts may occur.

However, since the mold after processing dislikes distortion and dimensional error, it cannot generally be tempered and it is difficult to avoid the influence of these residual stresses.

For this reason, surface finishing processes such as polishing and chemical treatment have been applied to remove the surface processing layer with residual stress on the mold surface, but the depth of the surface processing layer depends on the mold material and individual processing. Since it varies greatly depending on the history, an index for determining an appropriate removal amount has been demanded.

In general, in the machining of molds, combinations of machining methods that take advantage of the characteristics of each machining method, such as electric discharge machining, cutting machining, grinding machining, and final finishing machining, have been devised in consideration of the part of the mold. The evaluation method of internal residual stress including the processing history is important in terms of obtaining appropriate processing parameters in these processes.

Conventionally, the residual stress on the surface of an object such as a mold has been evaluated by measuring the change in the diffraction peak position by X-rays. For example, Non-Patent Document 1 is known. However, since X-rays penetrate only to the depth of several μm of the transition metal sample, in order to detect the residual stress inside the object existing on the order of several tens μm to several hundred μm, the measurement object inside the object It was necessary to remove the material on the object surface until the surface was exposed and to measure the residual stress on the new exposed surface. However, once the material is removed, the internal stress that was balanced inside the object before the removal is released, so stress redistribution occurs, and the residual stress state on the object surface is completely different from that before the removal. It was a thing. Therefore, in the conventional method, a method for directly measuring the residual stress distribution inside the object has not yet been realized.

In addition, since X-ray measurement uses radiation, the apparatus is large and expensive, and the handling of the apparatus has disadvantages such that it is not easy for the general public to handle due to the danger of exposure.

X-ray Stress Measurement Method Revised by Yokendo The Japan Society of Materials Science (1981)

On the other hand, various methods using the finite element method have been studied as a method of analyzing the internal stress such as the thermal stress of the mold and designing the mold according to the actual process. For example, Patent Document 1 proposes a method of obtaining a thermal stress life at a target position of a mold by obtaining a thermal stress distribution generated in the mold from an actually measured temperature distribution.

JP 2000-246394 A

However, in machining methods frequently used for molds such as electric discharge machining and cutting, the temperature distribution on the workpiece surface is not stable even under the same production conditions, and there are many uncertain factors. In addition, although the temperature at the processing tip reaches the melting temperature, there is a large temperature change locally, such as near the room temperature, and it is difficult to accurately measure the temperature, which may cause a large error in the calculation.
In addition, in order to make temperature predictions for individual molds that are frequently used in the field and to make predictions through calculations, skill and knowledge are required in addition to labor, and this is not considered an effective method. .

On the other hand, a ferromagnetic material such as iron is known to have a phenomenon in which the magnetic permeability changes due to stress. Using this property, magnetic flux is applied from the outside by a high-frequency excitation power supply and has a magnetic flux density that passes through the inspection material. A method has been proposed for measuring non-destructive deterioration of structural materials by measuring changes. For example, Patent Document 2 proposes a technique using a change in magnetic permeability when a sample is placed on a part of or the whole of a magnetization circuit and stress is applied.

On the other hand, in Patent Document 3, for example, by placing two types of magnetization circuits from different directions and passing a part of the sample as a magnetization circuit, the magnetic anisotropy caused by fatigue due to repeated stress is transmitted. A method to measure from the change of magnetic susceptibility is proposed.

Japanese Patent Laid-Open No. 2001-21538 JP 2002-350403 A

However, in these methods, magnetic flux must be excited and applied from the outside, and the magnetic flux spreads. Therefore, it is difficult to evaluate a material having high spatial resolution. Further, since the magnetic permeability behaves nonlinearly with respect to the external magnetic field, there is a problem that it is greatly influenced by the amplitude of the excitation field and quantitative measurement cannot be expected.

Further, Non-Patent Document 2 introduces a method for estimating a residual stress from a difference in strain generated by applying a DC magnetic field or an AC magnetic field from the outside.

Heizaburo Nakagawa, Machine Research, Vol.56, No.3 (2004) p.397

However, with this method, high spatial resolution cannot be expected from the spatial extent of the applied magnetic field, and local strain measurement is required to evaluate the residual stress distribution. There are problems such as complexity.

In recent years, leakage magnetic flux density that changes due to stress repeatedly applied in fatigue phenomena is measured by a highly sensitive magnetic sensor such as SQUID, and the fatigue level of austenitic stainless steel, etc. is estimated from the difference in changes over time in the stress loading process. A method to do this has been proposed (Patent Document 4).

JP 2005-055341 A

However, this method requires a special and expensive sensor such as SQUID, and requires a coolant such as liquid helium, so it cannot be a system that can be used in general factories. In addition, merely applying a repeated stress load on a trial basis and evaluating the material can only evaluate the typical characteristics of the material, and does not evaluate the characteristics of each individual mold in actual processing. In addition, fatigue alone is not sufficient for evaluating the characteristics of the mold, but rather does not provide a method for evaluating parameters affecting fracture toughness values such as hair cracks and residual stresses.

On the other hand, in the electric discharge machining of the mold, machining is performed several times at various wire positions from different directions separately on each surface of the workpiece. These machining histories are important in the sense of analyzing the machining process of the mold after creation.

Conventionally, a method for clarifying a processing history such as a wire position and a moving direction in wire electric discharge machining at the time of mold fabrication has not been proposed. Although some estimation is possible by analyzing the texture of the processed surface with an electron microscope such as SEM, it is necessary to carry out in a vacuum chamber using a large-scale apparatus, so a complex surface should be evaluated easily. Was difficult.

Furthermore, since large stresses and thermal influences are applied to the mold during production and use, cracks such as hair cracks often occur on the surface, which causes mold defects. In addition, if there is a foreign substance mixed in the manufacturing process, this also affects the strength. If it is possible to know the occurrence of cracks and the presence of foreign substances by inspection during the maintenance of the mold, repairing the mold can prevent line stoppage and manufacturing of defective products during the manufacturing process. It will be useful.

However, the cracks on the surface of the mold are generally thin and the openings on the surface are thin and show deep hair cracks in the interior, and the presence of the cracks cannot be easily known by simply observing the surface. Furthermore, if there is a crack inside, its presence cannot be detected from the outside. Also, the presence of foreign matter embedded near the surface cannot be easily detected from the surface.

Up to now, the method for inspecting hair cracks on the machined surface is generated by the penetrant flaw detection method, which is investigated from the penetration of dyed paint, the magnetic particle flaw detection method for measuring changes in the magnetic flux density on the surface to which an external magnetic field is applied, and the externally applied AC magnetic field. There have been proposed an eddy current flaw detection test method for detecting the ease of flow of eddy current from the change in impedance of an AC magnetic field coil, and an ultrasonic flaw detection test method for detecting ultrasonic echoes.

However, the penetrant testing method does not clearly distinguish between shallow and deep scratches on the surface, and it is necessary to remove the paint after the inspection. Is not suitable.

The eddy current testing method is also a large-scale device, and changes in coil impedance are also affected by distance from the surface, surface irregularities, and changes in internal tissue resistance. Does not deal with defects.

Furthermore, in the magnetic particle testing method, it is necessary to magnetize the whole once with an external magnetic field, and it is necessary to demagnetize after the testing. Further, depending on the size of the member, a large-scale device is required. Furthermore, the disturbance of the magnetic flux applied from the outside is not suitable for deep flaw detection because it is caused by surface protrusions and dents rather than hair cracks.

Finally, the ultrasonic flaw detection method is not a method suitable for the field because the sound wave spreads over the entire member, so it cannot be a method for specifically finding a flaw on the surface and the analysis is complicated. In addition, because of the limitation of the wavelength length of sound waves, small molds such as precision mold parts are not suitable for flaw detection of these members because reflection from a surface having a complicated shape occurs.

With these conventional technologies, residual stress and hair cracks inside the mold caused by uncertain factors during production in various processing methods such as grinding, cutting, and electrical discharge machining, and when used as a press mold or molding mold are eliminated. It was not possible to estimate accurately over a deep area from the surface, and therefore it was not possible to perform appropriate mold repair. In addition, since it is not possible to provide a method for simply evaluating internal stress while maintaining effective spatial resolution at the site where molds are handled, inspection of internal stress and hair cracks etc. is required for mold repairs at general factories. It was not possible to incorporate. In addition to the internal stress, no method has been proposed to give information on machining history in the wire electric discharge machining process of the mold.

Accordingly, it is an object of the present invention to provide an optimal mold repair method for accurately detecting residual stress inside a mold made of a material containing a ferromagnetic material and removing the residual stress based on the information.

The present invention evaluates residual stresses and hair cracks in the vicinity of a mold surface made of a material containing a ferromagnetic material, and repairs the mold based on this to generate the inverse magnetostriction phenomenon caused by internal stress. The magnetic field vector and magnetic field strength of the mold surface can be used as a magnetic flux density sensor for Hall elements and MI (Magneto).
Impedance) element is used, and the surface of the mold is moved and scanned three-dimensionally to detect the magnetic field strength and the magnetic field vector. In wire electrical discharge machining, in order to easily evaluate the cutting position and moving direction of the wire, the same inspection method as described above is used, and information on the machining history is obtained from the direction and change of the magnetic field sign depending on the machining history. It is characterized by obtaining.

The present invention scans a magnetic flux density sensor in a three-dimensional manner over the fracture surface of a material containing a magnetic material proposed in Patent Document 5 proposed by the present inventors. The inspection method is characterized by detecting the magnetic field strength and the magnetic field vector, and evaluating the residual stress generated during the machining of the mold, the amount of residual stress removed, the evaluation of cracks generated near the surface, and the wire It is applied to the evaluation of information at the time of machining in electric discharge machining, and the mold is repaired based on this.

Japanese Patent Application No. 2005-57442

For example, SKD steel, carbon steel, powder high-speed steel (powder high-speed steel), and high-speed cobalt-containing materials including the alpha phase and martensite phase of iron. There are iron alloys for molds, tool steels, and steel materials represented by steel (cobalt high speed steel). In addition, there are cobalt alloys and nickel alloys that are ferromagnetic materials, and cemented carbides that use iron-aluminum intermetallic compounds as a sintered matrix of metal carbides. There is also stellite, which is a tool steel mainly composed of cobalt. Further, there are nickel alloy hastelloy and superalloy, which are mold materials for high temperature. In addition, by using a two-phase separated alloy in which cobalt is mixed in several percent, the counterpart electrode material can be evaluated at the time of die-sinking electric discharge machining. Most industrial molds are materials that contain magnetic material, and are the subject of this method.

The Hall element is a sensor based on the principle that a Lorentz force acts on the current and the direction of the magnetic field perpendicularly to the magnetic field strength applied perpendicularly to the current flowing in the semiconductor to generate a voltage. By using a composite sensor in which these are arranged so as to measure the magnetic field strengths in three different directions, the three-dimensional magnetic field vector and the magnetic field strength that is the absolute value thereof can be detected simultaneously.

According to the method for inspecting a residual stress inside a mold made of a material containing a ferromagnetic material according to the present invention, an inverse magnetostriction phenomenon is caused by a residual stress inside the mold introduced when the mold is manufactured, corrected, or used. It is possible to evaluate the distribution of internal stress remaining in the vicinity of the mold surface from the change in the distribution of the generated leakage magnetic flux vector. In addition, since this inspection method is simple, inexpensive and safe, it can be easily used at the factory site, and individual inspections of molds with residual stress that are not constant even if they are manufactured using the same process. In the work to remove the residual stress inside by removing the surface processing layer of the mold by grinding, etc., the information necessary to estimate the minimum required removal amount is given, and each mold is accordingly This makes it possible to repair at the time of production and use.

In addition, this provides a guideline for optimizing the parameters of the machining process at the time of mold production, for example, in order to eliminate the residual stress inside the mold as much as possible.

In addition, by periodically measuring the distribution of the magnetic flux leakage vector during the use of the mold, for example, thermal stress due to heating / cooling in the injection mold, machining residual stress generated during grinding, and impact force in pressing It is possible to estimate the position and size of cracks and foreign matter that have entered the mold surface, etc., and greatly improve the life of the mold by replacing or repairing some defective molds. This makes it possible to prevent troubles such as a sudden line stop.

In addition, the magnetic flux vector changes in the direction of movement of the wire in wire electrical discharge machining, and the sign depends on the direction of movement, making it possible to evaluate the history of wire cuts during mold fabrication. It provides important information for understanding the machining process of the mold. In addition, information necessary for optimizing the processing procedure on different surfaces is provided so as to reduce the residual stress.

As shown in FIG. 1, a sample is placed on an XYZ stage, and a magnetic sensor capable of measuring a three-dimensional or equivalent magnetic flux density is disposed immediately above the surface of a mold to be measured. Hereinafter, the x axis and the y axis are perpendicular to each other in a plane parallel to the measurement surface of the sample, and the z axis is taken away from the normal direction of the measurement surface.
The sensor is also equipped with a soft, elastically deformable metal needle that protrudes from the sensor position by about 1 mm. By measuring the electrical conductivity between the mold sample and the metal needle, the sensor is designed to keep the distance from the mold surface constant at 1 mm or less. The relationship between the sensor and the sample is shown in FIG.

After adjusting the height of the stage in the z direction so that the distance from the surface is constant, measure the magnetic flux density in the three-dimensional direction that leaks from the mold surface, and confirm that an effective magnetic flux density is obtained. To do. The measured residual magnetic flux vector is quantified by a magnetometer, digitized, and recorded in a computer. After moving the sensor to the XY end of the mold surface using the XY stage, the z stage is controlled by a computer based on information on the electrical conductivity between the metal needle and the mold, and the height from the mold surface is kept constant. Hold to be kept. By repeating this, the stage is scanned in the XY direction below the resolution of the sensor, and a distribution of the three-dimensional magnetic flux density vector leaking onto the mold surface is obtained. This three-dimensional magnetic flux density vector is stored on a computer, and changes in the magnitude and direction of the vector are analyzed to evaluate the residual stress of the mold, the history of wire EDM process, and the distribution of hair cracks on the surface. This measuring apparatus can be the same as the apparatus in Patent Document 2, and is a probe and an elastic cantilever made of a high magnetic permeability material for efficiently collecting the magnetic flux density and keeping the distance between the sample sensors constant. Devices such as having a sensor holding bar are also effective.

The first embodiment of the method for inspecting the residual stress inside the mold made of a material containing a ferromagnetic material in the present invention will be described below. A 20 × 65 × 20 mm size plate made of several cemented carbides made of SKD steel, powdered high-speed steel, and WC-Co is prepared as a sample. This is cut in the thickness direction by wire electric discharge machining to obtain a 20 × 65 × 10 mm size plate. After that, the 20 × 65mm surface is divided into four in the longitudinal direction, and divided into four areas, one 20 × 20mm and 20 × 15mm, leaving the first area as it is, and the remaining three are 50μm, 100μm, 150μm. Precision grinding is applied to each depth. Further, in order to estimate the influence in the vertical direction due to wire electric discharge machining, electric discharge machining is performed at a boundary of each region to a depth of 1 mm. These processes are especially prepared for estimating the measuring ability of the present measuring method according to the parameters of electric discharge machining and precision grinding. An outline of a cross section of the sample processed this time is shown in FIG.

Next, the magnetic flux on the sample processed surface is measured by scanning a three-dimensional magnetic flux density sensor (scanning line interval 0.2 mm, speed 1 second / step) using the apparatus shown in FIG. At that time, the height of the stage in the z direction was adjusted so that the distance from the processing surface was constant at 0.5 mm, and the processing surface and the sensor were almost in contact with each other. The magnetic flux density in each of the x, y, and z directions was converted into an electrical signal by a magnetic flux density measuring device, digitized by an AD converter, and three-dimensional magnetic flux density vector data at each position was recorded by a PC. Thereafter, using the XY stage, the sensor was moved to the next position, and measurement was performed again. By repeating this, a distribution map of the three-dimensional leakage magnetic flux density vector existing on the XY plane of the machined surface was obtained. Since this sample has high flatness, it is not necessary to control the z stage, but depending on the sample, the z stage is controlled according to the change in the height direction, and the distance between the sensor and the sample surface is 1 mm. It is necessary to keep the following constant distance.

In the case of this sample, a leakage magnetic flux is generated on the sample surface as an inverse magnetostriction phenomenon due to thermal stress in the vicinity of the processed surface generated when wire electric discharge machining is performed. Further, a part of the surface processed layer where thermal stress is present is removed by the grinding process by the electric discharge processed surface to reduce the leakage magnetic flux density on the sample surface. From the change in the distribution of the measured leakage magnetic flux density, the distribution in the depth direction of the residual stress existing inside the sample processing surface can be clarified.

On the wire electric discharge machining surface of a die made of a material containing a ferromagnetic material, the surface machining layer melted at the time of cutting is rapidly cooled by the machining liquid, and is subjected to tensile stress. In the case of this cemented carbide, cobalt in the matrix that connects the metal carbides has the property that the magnetic field vector is oriented in the direction of the stress. Therefore, the EDM surface subjected to stronger thermal stress is finer in parallel to the machining surface direction. As a result, the magnetic flux density is changed from one pole to the other pole in the direction in which the wire discharge proceeds. The same properties are exhibited in the powdered high-speed steel and SKD steel used this time.

Therefore, a sample with a larger inclination of the change in magnetization intensity means that the surface processed layer is more strongly subjected to thermal stress, and a sample with a smaller inclination means that the thermal stress of the surface processed layer is smaller.

FIG. 4 and FIG. 5 show the distribution of leakage magnetic flux density in a sample subjected to electric discharge machining to a cemented carbide. FIG. 4 is a bird's-eye view of the three-dimensional magnetic flux vector distribution existing on the processing surface, and FIG. 5 is a distribution diagram of the three-dimensional magnetic flux vector when the processing surface is viewed from the z direction. A large change in the magnetic flux density was observed only on the surface subjected to electric discharge machining, but the gradient of the magnetic flux density could not be observed on the surface subjected to grinding to a certain depth or more. Therefore, in the case of the present cemented carbide sample subjected to the electric discharge machining this time, it was judged that the heat-affected zone of the processed layer could be sufficiently removed by grinding at a sufficient depth.

On the other hand, FIG. 6 shows the distribution of leakage magnetic flux density in a sample obtained by machining SKD tool steel. It is observed that the change in magnetic flux density decreases as grinding is performed only on the surface of EDM, but there is still a slight inclination after 150μm grinding. It can be seen that the heat-affected zone of the processed layer is not sufficiently removed. This tendency was the same also in the sample which processed the powder high speed.
In general, SKD steel and powder HSS are softer than cemented carbide and have a low Young's modulus and yield stress. Is consistent with Non-Patent Document 3, which is a conventional report. From the above results, it can be seen that the present invention is useful for semi-quantification of the residual stress of the surface processed layer, and based on this, information on the depth appropriate for repair processing by grinding of the mold surface can be given.

Satoshi Ogata et al., Journal of Japan Society for Precision Engineering, 57 (1) (1991) p.144-149

On the other hand, how much the magnetic field vectors are aligned for the same stress depends on the magnetostriction coefficient inherent to the material, and the degree of distribution in FIGS. 4 to 6 is directly converted to the value of thermal stress. is not. Since this coefficient changes with the composition even in the same iron alloy, it is necessary to clarify these physical property values for the material used as the mold. The magnetostriction coefficient that has been investigated so far is detailed in Non-Patent Document 4, for example. Fortunately, the inverse magnetostriction phenomenon of ferromagnets occurs in proportion to the elastic stress, so that polycrystalline materials can be easily estimated by mechanical tests such as bending tests and compression tests. In particular, the present invention interprets stress based on the direction of the measured magnetic field vector to determine whether the magnetic field vector is oriented in the same direction with respect to the stress or, conversely, whether the magnetic field vector is oriented perpendicularly to the stress. Is important to. Further more precise measurement requires precise measurement with a single crystal, and for practical materials, it is desired to quantify the precise magnetostriction coefficient through future research.

MagnetostrictionTheory and Applications of Magnetoelasticity, Etiennedu Tremolet de Lacheisserie, CRC Press (1993)

On the other hand, in general, the mold contributes to main processing, and the portion where stress is applied shows a more complicated shape. Therefore, the magnetic flux density vector leaking to the outside is greatly affected by these shapes. Further, in a region where a plurality of machining surfaces are in contact, the influence of magnetic flux vectors leaking from other surfaces cannot be ignored. Since all of the specific examples in FIGS. 4 to 6 were simple planes, these effects could be ignored, but shape effects were still observed in the edge region.

Strictly speaking, when this method is used as a means of evaluating the more quantitative residual stress distribution of a specific mold material, in addition to the analysis taking into account the magnetostriction coefficient depending on the material properties described above, it can be used to measure the entire sample area. It is necessary to evaluate the shape based on the shape. However, since the leakage magnetic flux density due to the main residual stress appears in the vicinity of the machined surface, it is expected to be useful enough for semi-quantitative analysis.

However, for molds that are mostly used in polycrystals, simple tests are sufficient for the purpose of semi-quantifying the degree of residual stress, and discuss the magnitude of stress between the same materials. , Simple comparisons are possible without knowing the exact coefficients.

On the other hand, in wire electric discharge machining, when the magnetic field vector is made of a material that is oriented in the direction of stress, such as cobalt in the cemented carbide, the material on the left side of the wire traveling direction is changed from the N pole to the S pole. Conversely, the material on the right changes from the S pole to the N pole.

When cutting lines are introduced in the direction perpendicular to the plane at the boundaries of the four regions shown in FIGS. 4 to 6, it is observed that opposite poles are generated on the left and right sides of the lines. This is the same phenomenon as the above result observed on a plane parallel to the cut surface.

In this way, there is always a fixed relationship between the change direction of the pole and the direction of the left and right cutting. This is considered to be caused by a rotational moment generated during cutting. When this phenomenon is applied, it becomes possible to clarify the traveling direction of the wire when creating the mold.

On the other hand, the amount of heat input by wire discharge is generally not constant, and the boundary conditions around the material change with the movement of the cutting part. Therefore, the temperature distribution is complicated, and the accompanying thermal stress distribution is not so simple. In addition, the energy input at the time of cutting may change over time due to the characteristics of the processing machine. Knowing these characteristics is essential for optimal mold design.

For example, as shown in FIG. 7, when machining a carbide sample with another electric discharge machine, periodic changes in the magnitude of the magnetic flux density may be observed from the measurement of the magnetic flux density on the electric discharge machining surface. It is suggested that there was a periodic change in machining discharge energy that gives some negative uniformity to the EDM surface that was considered uniform.

Further, among the leakage magnetic flux vectors shown in FIGS. 4 and 5 measured in the sample subjected to the electrical discharge machining to the cemented carbide, two components x and y existing in the measurement surface excluding the z component perpendicular to the measurement surface. FIG. 8 shows the distribution of the vector consisting of Although the leakage magnetic flux vector shows a characteristic flow in the machining surface, it can be seen that vortices, which are singular points of the flow, exist in several places on the machining surface. Further, all the regions where these vortices are present have a small magnetic flux vector.

When these vortex regions are observed with an optical microscope, elongated cracks are observed. Since these surface scratches are also present at positions where vortices of the xy vector of the magnetic flux are not observed, there is a possibility that the cracks near the vortex are hair cracks with deep cracks in the internal direction different from other scratches. is there.

When hair cracks or foreign substances that are cracked in the inner direction exist in the surface processing layer of the mold, the residual stress in the processing surface direction is relaxed, and the residual stress field has a singular point around the crack in the surface. Therefore, the vector distribution represented by the xy component of the leakage flux vector generated by the inverse magnetostriction phenomenon of residual stress also shows a distribution coming from a singular point such as a vortex around the crack. In materials with different magnetostriction coefficients, these singular points may be the divergence of the xy component of the vector.

From this, it is considered that vortices and divergence, which are singular points of the two-dimensional vector in the machining surface of the above-mentioned leakage magnetic flux density, indicate the presence of hair cracks and foreign matters that cause large stress relaxation. This suggests that this is a defect detection method that is not available. Since the hair rack determines the life of the mold used in press and injection molding, this method is considered to be important in industry. Based on the information on the crack position, in the present invention, the mold can be repaired by removal processing or the like.

A second embodiment of the method for inspecting the residual stress inside the mold made of a material containing a ferromagnetic material in the present invention will be described below. A 20 × 65 × 20 mm size plate made of cemented carbide made of WC-Co is prepared as a sample. This is subjected to surface grinding by high-speed grinding to form a plate of 20 × 65 × 10 mm size. After that, the 20 × 65mm surface is divided into four in the longitudinal direction, and divided into four areas, one 20 × 20mm and 20 × 15mm, leaving the first area as it is, and the remaining three are 50μm, 100μm, 150μm. Precision grinding is applied to each depth. These processes are especially prepared in order to estimate the measurement ability of this measurement method according to each parameter of high-speed grinding and precision grinding. The cross section of the sample processed this time is the same as in FIG.

Next, the magnetic flux on the sample processed surface is measured by scanning a three-dimensional magnetic flux density sensor (scanning line interval 0.2 mm, speed 1 second / step) using the apparatus shown in FIG. The measurement conditions and measurement parameters at that time are the same as those in Example 1.

In the case of this sample, a leakage magnetic flux is generated on the sample surface as an inverse magnetostriction phenomenon due to thermal stress in the vicinity of the processed surface that is generated when high-speed grinding is performed. In general, the grinding wheel has a rounded shape with a high central part and a low peripheral part, so when expanding the grinding surface by translating this grinding wheel, the surface that hits the central part of the grinding wheel A larger stress is generated than in the peripheral part.

In general, grinding is performed while running a table with a sample fixed against a fixed grindstone, but if there is a dent in the already ground line, a greater stress is applied at that position on the next line, As grinding progresses further, dents are similarly generated. Due to this phenomenon, the stress on the grinding surface changes greatly in the grinding direction of the grinding wheel on the ground surface after the surface where the unevenness appears to be too much on the surface, and the distribution of the residual magnetic flux vector corresponding to this changes.

FIG. 9 shows a residual magnetic flux vector distribution of a sample subjected to high speed grinding, and FIG. 10 shows a residual magnetic flux vector distribution of a sample subjected to normal grinding with poor sliding surface accuracy. As seen in the figure, the residual magnetic flux vector on the machined surface is distributed perpendicularly to the direction of the grinding line, and it is considered that the stress distribution due to the latter mechanism was mainly detected.

When a part of the surface processed layer is removed by grinding, the leakage magnetic flux density on the sample surface is reduced as in the first embodiment. From the change in the distribution of the measured leakage magnetic flux density, the distribution in the depth direction of the residual stress existing inside the sample processing surface can be clarified. Based on the information on the residual stress, the conditions of the final grinding process of the mold are clarified, and appropriate repairs can be made when the mold is manufactured.

As described above, by evaluating the distribution of the three-dimensional magnetic flux vector on the processing surface, it is possible to evaluate the residual stress introduced during various processing and cracks near the surface. This is the same in other processes such as cutting and sintering by laser or electric current, and it is effective for repairing such molds.

Residual leakage flux vector measurement of the mold surface made using the material containing the magnetic material of the present invention, the magnitude of residual stress on the mold surface, the presence of foreign matter and the occurrence of hair cracks that greatly affect the life of the mold Can be evaluated, and the repair of the mold based on this information creates various industrial applicability as follows.

First, most industrial molds are made of ferritic steel, mold steel, tool steel, cemented carbide containing cobalt and ferromagnetic intermetallic compounds, superalloy containing nickel α phase, etc. Therefore, most molds can be applied by the following method. Further, by using a two-phase separated alloy in which several percent of cobalt is mixed with copper, it becomes possible to perform repair by evaluating the mating electrode material at the time of die-sinking electric discharge machining.

It is possible to evaluate the distribution of residual stress on the mold surface introduced at the time of creation based on the magnitude of the residual leakage magnetic flux vector and the change in the vector direction. When optimizing the parameters during mold machining and repair It can be used as a guideline. For example, optimal electrical discharge machine process conditions can be achieved by minimizing the surface residual stress of molds made by changing values such as the amount of electric current, pulse frequency, and cutting speed of electrical discharge machining that reduce residual stress. It becomes easy to find out.

Further, by using an electrode material containing a magnetic phase, such as a copper-cobalt alloy, as a counterpart electrode material in the die-sinking electric discharge machining, it becomes possible to clarify the wear part and residual stress part of the electrode by magnetism.

The ability to optimize the machining parameters at the time of production is the same in the mold production process such as high-speed grinding, machining such as a machining center, and other powder sintering using laser, electric current discharge or current heating. Conversely, from the analysis of residual stress by this method, it is possible to clarify the characteristics of each processing method and the cause of the residual stress, so in addition to repairing the mold, improvement of the conventional mold processing process, It is also useful for creating new processing processes.

On the other hand, in the mold, the part where damage is fatal and the part where large stress is applied during use is determined, so it is possible to create and repair molds with controlled residual stress according to the part, It is also possible to improve the efficiency of mold production in a region where residual stress is not a concern.

Also, in general mold making, there is a surface processing layer that has entered the surface greatly in electric discharge machining, high-speed grinding processing, and high-speed cutting processing, and therefore includes a process of removing this by precision grinding, mechanical polishing, or electropolishing. There are many.

However, it is difficult to determine how much to remove, and if it is removed in a small amount, there is a possibility that the life of the mold will be shortened due to the presence of residual stress. Bounce back.

By using this method, as in Example 1, by measuring the magnetic flux density vector after constant removal processing, a guideline is given as to how much the residual stress of the surface processing layer can be reduced by the removal processing. Since the minimum amount of removal processing is given, appropriate repair is possible.

Further, since hair cracks and foreign matter on the surface can be estimated as will be described later, it is possible to completely remove cracks and foreign matter in the stress load portion by measuring the magnetic flux density vector after the constant removal processing. Thereby, it becomes possible to perform optimal final finishing processing and repair processing to each metal mold | die.

On the other hand, the presence of deep cracks such as hair cracks and foreign matters that occur during processing and use in the mold is a major cause of mold breakage during use, not only greatly reducing the life of expensive molds, This will cause an unexpected line stoppage and result in a significant reduction in the efficiency of the entire plant.

Compared to conventional residual stress measurement methods, this method is simpler, cheaper, safer, and faster, so it can be used in on-site lines and processes. Inspection is also possible. Therefore, it is possible to examine the presence or absence of cracks generated on the surface of the mold during repair of the mold during maintenance or during maintenance, and appropriate repair can be performed. In general, high-hardness materials such as molds require severe processing and usage environments, and the residual stress of each die surface processing layer is not uniform. The reliability of mold products is greatly improved.

This method can be applied from small precision molds to molds for automobiles and large press machines, etc. and is not particularly dependent on the size of the mold. Effective for maintenance in maintenance.

In addition, the cutting direction, grinding direction, and cutting direction in the machining process can be clarified by this method, so that it is useful for clarifying the mold process performed under unknown conditions. For example, in wire electric discharge machining, a mold is made by combining a plurality of machining processes. By knowing the cutting direction at a time from the direction of the NS magnetic field, it is possible to infer the order of cutting. Become. In addition, it is possible to estimate the rigidity of grinding and cutting devices.

As described above, according to the present invention, it is possible to provide a mold making method and a repair method for removing defects such as residual stress and hair cracks of a die made of a material containing a ferromagnetic material at a low cost, which is industrially useful. Expected to be something.

It is the figure which showed one Embodiment of this invention. It is the figure which showed the sensor part in one Embodiment of this invention. Sample sectional view of processing method of material containing magnetic material measured by the present invention It is the figure which showed the magnetic flux density vector distribution of the wire electric discharge machining surface of the cemented carbide measured by this invention. It is the figure which showed the magnetic flux density vector distribution of the wire electric discharge machining surface of the cemented carbide measured by this invention from the z direction. It is the figure which showed the magnetic flux density vector distribution of the SKD tool steel wire electric discharge machining surface measured by this invention. It is the figure which showed the magnetic flux density vector distribution of the processed surface by another wire electric discharge machine of the cemented carbide measured by this invention. It is the figure which showed distribution of the magnetic flux density vector which took out only the xy component of the wire electric discharge machining surface of the cemented carbide measured by this invention. It is the figure which showed the magnetic flux density vector distribution of the high-speed grinding processed surface of the cemented carbide measured by this invention. It is the figure which showed magnetic flux density vector distribution of the grinding surface where the sliding surface precision of the cemented carbide alloy measured by this invention was bad.

Explanation of symbols

1 3D magnetic flux density sensor element
2 3D magnetic flux density sensor control unit 3 Control computer 4 Automatic XYZ stage 5 Sample 6 Sample holding unit 7 Sensor position adjustment unit 8 Sensor holding rod 9 Probe 10 Surface 11 subjected to electric discharge machining or grinding 11 Electric discharge machining or grinding After the surface is processed, the surface is subjected to 50 μm removal processing 12 After being subjected to electric discharge machining or grinding processing, the surface is subjected to 100 μm removal processing 13 After being subjected to electric discharge processing or grinding processing, the surface 14 is subjected to 150 μm removal processing 14 Magnetization Rate (Tesla)
15 Vortex of two-dimensional magnetic flux vector

Claims (3)

In a mold made of a material containing a magnetic material, a first step of measuring a magnetic field vector and a magnetic field intensity generated on the mold surface by an inverse magnetostriction phenomenon when the mold is manufactured and used by using a magnetic flux density sensor was measured. Repair of a mold composed of a second step of deriving a residual stress distribution from the magnetic field vector and magnetic field strength distribution and a third step of mechanically or chemically removing the residual stress based on the residual stress distribution. Method. 2. The method of repairing a mold according to claim 1, wherein the position of a crack or a foreign object near the surface is derived from a singular point of the magnetic field vector obtained by the second step and a degree of decrease in the magnetic field strength. When a mold made of a material containing a magnetic material is manufactured by wire electric discharge machining, the magnetic field vector and magnetic field strength generated on the mold surface by the inverse magnetostriction phenomenon are measured using a magnetic flux density sensor, thereby Evaluation method to clarify machining history such as wire position and moving direction in EDM.

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