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In EDM, the tool electrode is a very important factor. The performance of the electrode material will affect the EDM performance of the electrode (material removal rate, tool loss rate, workpiece surface quality, etc.). Therefore, the correct choice of electrode material is EDM is essential.
The tool electrode material for electric discharge machining should meet the basic requirements of high melting point, low thermal expansion coefficient, good electrical and thermal conductivity and mechanical properties, so that it has low loss rate and resistance to deformation during use. It is also advantageous for the electrode to have a finely crystallized structure to reduce electrode loss. It is generally believed that reducing the grain size can reduce the electrode loss rate. In addition, the tool electrode material should make the EDM process stable, high productivity, good surface quality of the workpiece, and the electrode material itself should be easy to process, rich in source and low in price.
As the application range of EDM is expanding, new requirements are constantly being applied to the electrode materials (including the corresponding electrode preparation methods). With the development of materials science, people are constantly exploring and innovating the electrode materials of EDM tools. At present, the tool electrode materials that have been used in research and production are single metal, Cu or W-based alloys such as graphite, Cu or W. Steel, cast iron, Cu-based composites, polymer composites and diamonds.
2 common EDM tool electrode material
(1) graphite
Graphite has good electrical and thermal conductivity and processability, and is a tool electrode material widely used in EDM.
Graphite has different types and can be classified according to the size of graphite particles, the density of materials, and mechanical and electrical properties. Among them, the fine graphite has small particle and porosity, high mechanical strength and high price. Generally, the electrode loss rate is lower when used in EDM, but the material removal rate is correspondingly lower. The average graphite particle size available on the market is below 20 μm, depending on the operating conditions of the electrode (roughing, semi-finishing or finishing) and the geometry of the electrode. The surface roughness of the workpiece is directly related to the size of the graphite particles. Generally, the graphite grade with an average particle size of 1 μm or less is used for finishing. KLAas uses two different grades of graphite electrodes to machine deep and narrow grooves on difficult-to-machine materials, comparing their material removal rates and electrode loss rates. The results show that the choice of graphite type depends mainly on the specific requirements of EDM for material removal rate and electrode loss rate.
Compared with other electrode materials, the graphite electrode can be electrically processed by a large discharge current, so the productivity is high; the loss rate of the electrode during roughing is small, but the electrode loss rate is increased during finishing, and the surface roughness is improved. difference. Graphite electrodes are light in weight and low in price. Due to the high brittleness of graphite, it is often difficult to form a thin and thin shape by mechanical processing, so the application in the fine-complex shape EDM is limited, and high-speed milling can better solve this problem.
In order to improve the EDM performance of the graphite electrode, O. Akira et al. immersed the graphite powder sintered electrode in a molten metal (Cu or Al) and applied a high pressure to the liquid metal to fill the pores of the graphite electrode with Cu or Al. To improve its strength and thermal conductivity. After the metal is injected, the density, thermal conductivity and bending strength of the graphite electrode increase, the electrical resistivity is greatly reduced, and the surface roughness of the electrode is improved. The experimental results show that there is no significant difference between the electrode loss rate and the material removal rate of this new material electrode compared with the conventional graphite electrode, but the surface roughness of the processing is smaller, especially the graphite electrode implanted with Cu can obtain much smaller processing. Surface roughness.
(2) Cu, Cu-based alloys and Cu-based composites
Pure Cu (electrolytic copper, commonly known as copper) is also a commonly used electrode material, especially when processing non-ferrous materials, electrolytic copper is commonly used as a tool electrode material.
Cu has a lower melting point and a higher electrode loss rate, so it is necessary to introduce another high melting point material to reduce the electrode loss rate. Cu-W alloy combines the high thermal conductivity of Cu with the high melting point of W, low coefficient of thermal expansion and strong resistance to spark erosion, making it a high-performance tool electrode material. The Cu-W electrode is mainly used for processing die steel and WC workpieces, and the Cu and W content ratio is generally 25:75. However, since the price of the Cu-W electrode is higher than that of a conventional Cu or graphite electrode, it is not currently used in production.
S.Singh et al. used Cu, Cu-W alloy, brass and Al electrode to process a hardened tool steel. The results show that the processing speed and processing precision of Cu and Al electrodes are higher, and the loss rate of Cu and Cu-W electrodes. The smallest, brass has the highest electrode loss rate. In contrast, Cu is a better electrode material, which can achieve higher processing accuracy and better surface roughness, and has high material removal rate and low electrode loss rate. The performance of Al is second only to Cu, and it can be used when the surface roughness of the machined surface is not high. Yan Yaowei et al [7] used Cu, W and Cu-W alloys as electrode materials to process hard alloys. The results show that Cu-W alloy electrodes can significantly improve the processing speed, and the electrode loss is not large at low processing voltage. Therefore, Cu-W alloy is an ideal electrode material for processing cemented carbide.
TiC is a high hardness refractory material with high melting point and good thermal shock resistance and wear resistance. L.Li et al. studied the effect of TiC on the EDM performance of tool electrodes in sintered Cu/TiC and Cu-W/TiC electrodes. The results show that the Cu/TiC electrode loss rate of 5%~45% TiC is lower than that of conventional Cu electrode. Considering the processing performance, 25% TiC is the ideal component ratio. The Cu-W/TiC electrode material also shows good performance, and most of the EDM surface roughness is superior to that of the Cu-W electrode processing surface, and thus can be used for finishing. For the Cu-W/TiC electrode material, the best effect is obtained by adding 15% TiC.
ZrB2 and TiSi have good electrical and thermal conductivity and high melting point. HMZaw et al. studied the electrodeposition of EDM tools by powder metallurgy with different contents of Cu and ZrB2 or TiSi, and with electrode materials such as graphite, Cu and Cu-W. EDM performance was compared. The results show that the TiSi/Cu electrode is seriously depleted, the processing speed is low, and the processing surface is rough, so the material is not suitable for use as an EDM electrode. ZrB2/Cu can be used as an electrode material, but the bonding strength between Cu matrix and ZrB2 is poor. The content of ZrB2 and the electrode fabrication process parameters will affect the EDM performance of this electrode.
TiB2 particles have high melting point, good electrical and thermal conductivity, low thermal expansion coefficient, etc. TiB2/Cu composites have good electrical conductivity, high temperature resistance and mechanical properties, and meet the basic requirements of electrode materials for EDM tools. Qiu Yan et al. conducted electrical discharge machining experiments using powder metallurgy TiB2/Cu composite electrodes, and analyzed the EDM loss mechanism of composites. The results show that the EDM characteristics of TiB2/Cu electrodes are similar to those of other Cu-based composite electrodes. When the volume fraction of TiB2 is 5%, the EDM effect of the electrode material is better.
Electric spark grinding usually uses Cu-based composite electrodes. KMShu and other Cu/SiCp composite electrodes for EDM grinding, this composite electrode containing a certain amount of SiCp has better hardness and wear resistance than pure Cu electrode, while the electrical properties are almost unchanged, and the thermal conductivity is good. It has high thermal shock resistance and exhibits low electrode loss rate. The Cu/SiCp composite electrode with 2% SiCp has the best EDM grinding effect.
When the electrode is prepared by electroforming, the electroforming Cu is more mature, so the electroforming Cu (including Cu-based composite) electrode is more studied. The Cu or Cu-based composite material obtained by electroforming is densely organized to achieve a small grain size. Studies have shown that the surface of the electrode with fine grain and dense structure is smaller due to the melting of the material during spark discharge, which can reduce the electrode loss rate.
(3) Polymer composites
A. Curodeau et al. use an electrically conductive thermoplastic polymer composite as an electrode to perform EDM or polishing of the surface of the workpiece using air or water as the working medium. The electrode used is made up of 60% to 65% solid carbon material (such as a mixture of fine carbon black powder, graphite powder, graphite sheet or even carbon nanotubes) uniformly distributed in a thermoplastic matrix material (such as polystyrene). Can be repeatedly softened and molded into the desired geometry. Compared with graphite electrodes, this polymer-carbon composite electrode has a lower cost and can be molded into a complex geometry, which is much faster than milling. At the same time, its density is lower, the resistivity is higher, and the electrode loss rate is higher. Higher, but the electrode can be trimmed by re-molding during use.
The composition of the composite is still in the research and development stage. The good plasticity electrode should have low electrical resistivity, high thermal conductivity, low thermal expansion coefficient, good formability and dimensional stability in water, and heat cycle resistance.
(4) Diamond
K. Suzuki et al. studied electrical discharge machining using an electrically conductive CVD diamond thick film (0.5 mm) as an electrode material. The CVD diamond has conductivity by doping with boron during the CVD process, and has low electrical resistance and high thermal conductivity, and has strong adsorption capacity for carbon precipitated in the oil working medium during electric discharge machining. EDM test shows that under certain processing conditions, CVD diamond electrode can achieve high material removal rate, and the electrode loss is almost zero, especially it can be processed at high current density without Cu or graphite electrode. . However, conductive CVD diamond has problems such as high cost and limited size, so K. Suzuki et al. used polycrystalline diamond (PCD) as an electrode material for electrical discharge machining. The PCD material used is sintered with micron-sized diamond particles under the conditions of ultra-high pressure and temperature, in the presence of a metal catalyst, with Co as a binder, and its thermal conductivity is close to that of conductive CVD diamond. Different grades of PCD materials can be obtained with different particle sizes of diamond, and their thermal conductivity is different. Studies have shown that under certain EDM conditions, the electrode loss is small or zero. As the thermal conductivity increases, the material removal rate and electrode loss of different grades of PCD material electrodes during EDM are reduced. Since the PCD material has an EDM effect similar to that of the conductive CVD diamond, but the cost is low, it may become an ideal electrode material.
3 Electrode material for surface modification of electric spark
Most of the EDM surface modification is characterized by the loss of the electrode during EDM machining, which transfers the electrode material to the surface of the material to be processed, thereby forming a coating with high hardness and high wear resistance, which is usually thermally decomposed by the working fluid kerosene. The carbon particles chemically react with the rapidly depleted electrode material to form carbides on the surface of the workpiece. To achieve this form of EDM surface modification, the tool electrode should be made of a material with low thermal conductivity, which can cause large loss, and the electrode material should be relatively easy to form hard carbide.
At present, EDM surface modification mainly uses solid electrodes of several materials, such as Si electrode, Ti electrode or W electrode, or a compacted or sintered body electrode made of various powder materials, and the powder materials used include Al and Ti. , W, Ti and Al mixed powder, WC, TiC and ceramics and bonding agents (such as WC + Co, WC + Fe, WC + TiC + Co, TiC + Co, VC + Co). Using such electrodes for EDM, one or more layers of coatings having different mechanical properties can be formed on the machined surface. When the electrode material is used to prepare the electrode, the particle size of the powder has a great influence on the manufacturing process and cost of the electrode and the roughness of the modified surface.
J.Simao et al. surface alloyed with powder metallurgy and pre-sintered WC/6% Co electrode processing tool steel. The elements in the electrode (especially W) were transferred to the carbon in the hydrocarbon working medium in a gradient form. In the surface of the workpiece. HCTsai et al. used a resin-containing Cu powder and Cr powder to form a Cu-Cr composite electrode, and the Cr element in the electrode migrated to the surface of the workpiece during EDM, so that the processed surface obtained good corrosion resistance. As the Cr content in the electrode increases, the material removal rate during EDM is reduced, but the corrosion resistance of the machined surface is enhanced.
In addition, Fang Yu et al. used a TiC+Co semi-sintered electrode to perform EDM surface modification on ordinary carbon steel workpieces. Jiang Baoqing et al. used a W-shaped electrode obtained from W powder, graphite powder and polyvinyl alcohol binder to perform EDM surface modification on LC4 aluminum alloy workpiece. Lianfeng directly used YT15 cemented carbide material as the electrode to perform EDM on 45 steel. When the positive electrode was processed, the surface hardness of the workpiece was much higher than that of the white bright layer of the substrate.
4 electrode materials for micro EDM
In the fine electric discharge machining, the use of the fine electrode usually increases the spark energy per unit area, resulting in a large electrode loss, so that it is difficult to achieve the goal of high-precision machining. At this point, the appropriate EDM parameters can be selected to reduce the discharge energy per unit area, but this will increase the processing time; in addition, low loss electrode materials can be used. The electrode materials used in micro-EDM mainly include Cu, W, Cu-W and WC, etc. Among them, the electrodes used in micro-EDM drilling and milling are mainly W or WC rods or tubes.
YYTsai et al. used the electrode materials of Ag, Al, Cu, Fe, Mo, Ni, Pt, Ti, Ta and W when studying the loss resistance of the micro EDM electrode. The results show that the electrode materials with higher boiling point, melting point and thermal conductivity have less loss. Among them, the loss of W electrode is the smallest when processing stainless steel, pure Cu and pure Fe workpiece, and the loss of Cu and Ag electrode is smaller than that of Fe, Ni electrode and Al electrode. The loss is the biggest.
Ming Pingmei and others used electroforming Cu and Cu-based composite electrodes for micro-EDM machining, and studied the electrode erosion resistance of the electrode materials. The material obtained under the appropriate electroforming process has strong anti-corrosion property, and when more nano-cerium oxide additive is added to the electroforming Cu solution, the nano-cerium oxide second appears in the electroformed Cu obtained. The phase reduces the electrical and thermal conductivity of the material and weakens its resistance to electrical corrosion. A micro-powder graphite is added into the electroforming Cu solution, and a Cu-graphite composite material is obtained by composite electroforming. The material is a fine powder of graphite introduced into the Cu main body, and its microscopic morphology is uniformly embedded in the Cu main body with flake graphite. A sheet-like "particle center" that is covered with Cu. The test shows that the corrosion resistance of the composite electrode with proper amount of graphite is much better than that of Cu. It is believed that this is because the "particle center" in the material combines the excellent thermal conductivity of the outer layer Cu in the EDM process. Performance and graphite core heat storage and anti-corrosion ability, and after a period of discharge, the exposed graphite core acts as a skeleton to reduce the splash of liquid metal.
Since the conductive CVD diamond film has little loss as an electrode material, it has a good application prospect as an electrode material in micro EDM. Further, when a conductive CVD diamond film is used as the electrode of the same diamond film for electrical discharge machining, the shape and size of the latter can be well controlled, so that it can be used as an electrode of different shapes in micro EDM.
5 Conclusion
When processing workpieces of different materials under different process parameters, the processing effects obtained by using different electrode materials are different, and thus different electrode materials are suitable for different processing occasions. According to the needs of various EDM processes, a variety of electrode materials have been researched and applied. The development of electrode materials has promoted the progress of EDM technology. Some new electrode materials require further research and improvement to get practical applications. When selecting electrode materials, it is necessary to comprehensively consider various factors such as EDM process methods, workpiece materials and shapes, processing requirements, and economics.
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Research progress on tool electrode materials for electric discharge machining
1 Introduction