Wire-cut WEDM molds have been widely used in mold making. However, the wire cutting process will also cause some negative effects on the mold surface. Under the instantaneous high temperature of the wire cutting and rapid cooling of the working fluid, the surface of the mold will form a metamorphic layer after wire cutting, which will reduce the surface hardness and generate microscopic Cracks and other ills seriously affect the quality and service life of molds and should be given enough attention. In addition, on-line cutting centers, in order to prevent cracks and deformation, processing conditions should also be carefully selected, especially for those large, thick-walled, complex shapes, uneven thickness mold parts, should adopt multiple cutting method, which is reduced and A very effective method to remove surface defects. It is also important to choose a flat, easy-to-finish tool or set the starting point of the wire-cutting for areas that have little effect on the performance of the workpiece. For some molds with high requirements, a variety of effective measures can be taken. After the on-line cutting, the surface metamorphic layer is thrown away and ground away to improve the surface quality of the parts. BOPP FILM Bopp Film,Bopp Material,Bopp Lamination Film,Bopp Thermal Lamination Film SHAOXING TIANPU PLASTIC PRODUCTS CO.,LTD , https://www.fulingholdinggroup.com
Formation of a metamorphic layer
Spark wire cutting is the processing method that utilizes the thermal effect of instantaneous discharge energy to melt and evaporate the workpiece material to the required size. Since the wire-cut working fluid uses a liquid having a dielectric effect, a certain amount of electrolysis is also accompanied during the processing. The thermal effect and electrolytic action during cutting usually produce a degeneration layer with a certain thickness on the processing surface. For example, the hardness of the surface layer is reduced, microcracks occur, etc., and the wire-cutting die is prone to early wear, directly affecting the maintenance of the die blanking gap and The cutting edge of the die is easy to chip, shortening the service life of the die.
For carbon steel, the melted layer on the surface of the workpiece (composed of a melted solidified layer and a heat-affected layer) appears white on the metallographic photograph and has a white layer. It is completely different from the matrix metal and is a dendritic quenched casting structure. The combination with the inner layer is not very firm. It is mainly composed of martensite, a large number of fine-grained retained austenite and some carbides.
The influencing factors of the second metamorphic layer
1. Metallurgical structure and elemental composition of workpiece materials Due to the spark discharge effect, the metallurgical structure of the surface layer of the workpiece material has undergone significant changes, resulting in discontinuities. Non-uniform thickness of the degenerating layer. It is related to the parameters of the workpiece material, electrode wire material, pulse power supply, and working fluid. According to metallographic analysis, large austenite remains in the metamorphic layer. When molybdenum wire electrode wire and carbon-containing working fluids are used, spectroscopic analysis and electron probe detection show that the content of molybdenum and carbon in the metamorphic layer is greatly increased, while the working fluid using copper wire electrode wire and deionized water is used. At the time, it was found that the copper content in the metamorphic layer increased without carburization.
2. The thickness of the metamorphic layer Generally, the thickness of the metamorphic layer becomes thicker as the pulse energy increases. Due to the randomness of the EDM process, the thickness of the metamorphic layer is often not uniform under the same processing conditions. The data from the measured thickness of the metamorphic layer on the test piece indicates that the wire cutting criterion is correct for the thickness of the metamorphic layer. There are obvious effects. For example, the electrode wire is a brass wire, and the processing voltage is 60 V at a low speed (0.6 m/s), the current is 5.5 A, and the maximum thickness of the modified layer is 20.0 μm, with an average increase of 13.8 μm.
3, micro-hardness decreased significantly, and microcracks appeared due to changes in the microstructure of the metamorphic layer and the content of elements, so that the micro-hardness of the workpiece surface decreased significantly. For example, after WEDM in deionized water, the surface hardness value of the workpiece dropped from 970 HV before the wire cutting to 670 HV after the wire cutting process, and the soft wire layer of the wire cutting usually appeared within a depth of more than ten micrometers from the surface. At the same time, the surface metamorphic layer generally has tensile stress and microcracks occur. In particular, when cutting cemented carbides, cracks are more likely to occur under conventional electrical gauge parameters and voids are present. Great harm.
Formation Mechanism and Preventive Measures of Three Microscopic Cracks
Wire cutting is the use of discharge heat effect processing, mold material surface due to the discharge of high temperature and melting, and then quenched to produce metamorphic layer, metamorphic layer often appear more micro-cracks, this micro-cracks is mostly due to the metal from the molten state Sudden cold solidification, shrinkage of the material caused by tensile thermal stress.
Different workpiece materials also have different third crack properties, and hard brittle materials tend to crack. Pre-heat treatment of the workpiece has a great influence on the cracks. Processing the quenched material is easier than cracking or quenching of the annealed material, because the quenched material is hard and brittle, and the original stress is also large.
In order to prevent micro cracks on the surface of the mold, all aspects of the steel hot working (casting, forging) and heat treatment until the mold is made should be fully noticed and valued, and corresponding measures taken. 1 The heat treatment before the on-line cutting process should not avoid overheating, carburizing, and decarburization of the material; the two-wire cutting should optimize the power regulation: 1) The high peak and narrow pulse electrical parameters are used to make the workpiece material to be ejected in the gas phase. The heat is much higher than the heat of melting to take away most of the heat and avoid overheating of the workpiece surface; 2) Effectively perform pulse-by-pulse detection, control the length of the concentrated discharge pulse train, and also solve the problem of local overheating, eliminating microcracking Produce; 3) The impact of pulse energy on microcrack is extremely obvious, the greater the energy, the wider and deeper the microcrack; when the pulse energy is very small, for example using precision machining, the surface roughness value is less than Ra1.25μm, Generally not easy to appear micro-cracks; 3 electrolytic corrosion products in the working fluid (such as liquid slag, etc.) often lead to concentrated discharge, the formation of micro-cracks.
Four conclusions
Optimize the technological process of wire cutting, select reasonable process parameters to prevent overheating of the mold surface, reduce and avoid the formation of metamorphic layers, eliminate surface micro-cracks, and it is very important for improving mold manufacturing quality and prolonging the service life of molds. Effective
June 04, 2021