1 Introduction
In electric discharge machining, 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 for EDM is crucial.
EDM tool electrode materials should meet the basic requirements of high melting point, low thermal expansion coefficient, good electrical and thermal conductivity and mechanical properties, and thus have a lower loss rate and resistance to deformation during use. The microstructure of the electrode with a fine crystalline structure is also advantageous for reducing the 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 source and low cost.
As the application range of EDM continues to expand, new requirements are constantly put forward for the electrode materials (including the corresponding electrode preparation methods) that are suitable for them. With the development of materials science, people continue to explore and innovate electrode materials for electric discharge machining tools. At present, tool electrode materials that have been used in research and production include single metals such as graphite, Cu or W, and Cu or W-based alloys. Steel, cast iron, Cu-based composites, polymer composites, and diamonds.
2 Ordinary electric discharge machining tool electrode material
(1) Graphite
Graphite has good electrical and thermal conductivity and machinability, and is a widely used tool electrode material in EDM.
There are different types of graphite, which can be classified according to the size of the graphite particles, the density of the material, and the mechanical and electrical properties. Among them, fine-grained graphite has smaller particles and porosity, higher mechanical strength, and more expensive price. Generally, the electrode loss rate for EDM machining is lower, but the material removal rate is correspondingly lower. The graphite grades supplied on the market have an average particle size of less than 20 μm, depending on the working conditions of the electrode (roughing, semi-finishing, or finishing) and the electrode geometry. The surface roughness of the workpiece is directly related to the size of the graphite particles. Usually, the graphite grade with an average particle size of 1 μm or less is specifically used for finishing. KLAas uses two different grades of graphite electrodes to process deep and narrow grooves on difficult-to-machine materials, comparing their material removal rates and electrode wear rates. The research results show that the choice of graphite type mainly depends on the specific requirements of EDM on the material removal rate and electrode loss rate.
Compared with other electrode materials, the graphite electrode can use a large discharge current for EDM, so the productivity is higher; the wear rate of the electrode during roughing is smaller, but the electrode wear rate during finishing increases, and the surface roughness of the processing is higher. difference. Graphite electrodes are light weight and low in price. Due to the high brittleness of graphite, it is often difficult to make a thin and thin shape by mechanical processing. Therefore, the application in fine and complex EDM is limited, and high-speed milling can 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 molten metal (Cu or Al) and applied high pressure to the liquid metal to fill the pores of the graphite electrode with the metal 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 resistivity greatly decreases, and the surface roughness of the electrode is improved. The experimental results show that compared with the conventional graphite electrode, the new material electrode has no significant difference in electrode wear rate and material removal rate, but the surface roughness 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, commonly used electrolytic copper as a tool electrode material.
Cu has a low melting point and a large electrode loss rate. Therefore, 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 and its high melting point, low thermal expansion coefficient, and strong spark erosion resistance, making it a high-performance tool electrode material. The Cu-W electrode is mainly used for processing tool steel and WC workpieces, in which the Cu, W content ratio is generally 25:75. However, since the price of the Cu-W electrode is higher than that of an ordinary Cu or graphite electrode, it is not currently used in production.
S.Singh et al. used a Cu, Cu-W alloy, brass, and Al electrode to machine a hardened tool steel. The results show that the Cu and Al electrodes have higher processing speeds and machining accuracies, and the wear rates of Cu and Cu-W electrodes. The smallest, brass has the highest rate of electrode wear. In contrast, Cu is a good electrode material, it can obtain 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, so it can be used when the surface roughness requirement is not high. Yan Yaowei et al. [7] used Cu, W, and Cu-W alloys as electrode materials for machining hard alloys. The results show that Cu-W alloy electrodes can significantly increase the processing speed, and the electrode loss is not large at lower processing voltages. Therefore, Cu-W alloy is an ideal electrode material for machining hard alloys.
TiC is a high hardness refractory material with high melting point, good thermal shock resistance and wear resistance. L. Li et al. studied the effect of TiC on the EDM performance of the tool electrode in the sintered Cu/TiC and Cu-W/TiC electrodes. The results show that the wear rate of the Cu/TiC electrode with 5% to 45% TiC is lower than that of conventional The Cu electrode. Considering the processing performance comprehensively, 25% TiC is the ideal proportion of ingredients. Cu-W/TiC electrode materials also show good performance, and most of the EDM surface roughness is superior to the Cu-W electrode processing surface, and thus can be used for finishing. For Cu-W/TiC electrode materials, 15% TiC is added for best results.
ZrB2 and TiSi have good electrical and thermal conductivity and high melting point. HMZaw et al. studied the use of different contents of Cu and ZrB2 or TiSi to prepare EDM tool electrodes by powder metallurgy, and with electrode materials such as graphite, Cu and Cu-W. EDM performance was compared. The results show that the TiSi/Cu electrode has a large loss, a low processing speed, and a rough machining surface, and thus the material is not suitable for use as an EDM electrode. ZrB2/Cu can be used as electrode material, but the binding force between Cu matrix and ZrB2 is poor. The content of ZrB2 and the process parameters of the electrode will affect the EDM performance of this electrode.
TiB2 particles have high melting point, good electrical and thermal conductivity, low coefficient of thermal expansion, etc. TiB2/Cu composites have good electrical conductivity, high temperature resistance and mechanical properties, which are in line with the basic requirements of EDM tool electrode materials. Qiu Yan et al. used powder metallurgy TiB2/Cu composite electrode for EDM test and analyzed the EDM machining loss mechanism of composite materials. The results showed 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 electrode material has a good EDM effect.
EDM grinding usually uses Cu-based composite electrodes. KMShu et al. used the Cu/SiCp composite electrode for EDM grinding. The composite electrode containing a certain amount of SiCp has significantly improved hardness and wear resistance compared with pure Cu electrode, while the electrical properties remain almost unchanged and the thermal conductivity is good. And has a high thermal shock resistance, showing low electrode loss characteristics. The electric spark grinding effect of Cu/SiCp composite electrode containing 2% SiCp is the best.
When the electroforming method is used to prepare the electrode, the electroforming of the Cu (including the Cu-based composite material) electrodes is more researched because the electroforming Cu process is more mature. The electroformed Cu or Cu-based composite has a dense structure and can achieve a smaller grain size. Studies have shown that the pits formed on the surface of the fine grained, densely-denatured electrode due to melting of the material during spark discharge are smaller and the wear rate of the electrode can be reduced.
(3) Polymer Composites
A. Curodeau et al. used a conductive thermoplastic polymer composite as an electrode and used air or water as a working medium to perform EDM or polishing of the workpiece surface. The electrode used is made up of 60% to 65% of solid carbon material (such as fine carbon black powder, graphite powder, graphite flakes, and even carbon nanotubes) uniformly distributed in a thermoplastic matrix material (such as polystyrene) It can be repeatedly softened and molded into the required geometry. Compared with graphite electrodes, this polymer-carbon composite electrode has a lower cost and can be molded into a complex geometry. The manufacturing speed is much faster than that of milling. At the same time, the density is low and the resistivity is high, resulting in electrode loss rate. Higher, but the electrode can be trimmed by re-moulding during use.
The components of this composite are still in the research and development stage. Good plasticized electrodes should have low resistivity, high thermal conductivity, low coefficient of thermal expansion, good formability and dimensional stability in water, and heat-resistant cycling.
(4) Diamond
K.Suzuki et al. studied EDM using a conductive CVD diamond thick film (0.5mm) as an electrode material. The CVD diamond is conductive during the CVD process by doping with boron. The CVD diamond has a low electrical resistance and a high thermal conductivity, and has a strong adsorption capacity for carbon precipitated in the oil working medium during EDM. EDM tests have shown that under certain processing conditions, CVD diamond electrodes can achieve very high material removal rates, while the electrode losses are almost zero, in particular, it can be processed at high current densities that cannot be achieved with Cu or graphite electrodes. . However, conductive CVD diamonds have problems such as high cost and limited size, so K.Suzuki et al. also used polycrystalline diamond (PCD) as an electrode material for electric discharge machining. The PCD material used is made of micron-sized diamond particles sintered with Co as a binder under ultra-high pressure and temperature in the presence of a metal catalyst, and its thermal conductivity is close to that of conductive CVD diamond. Different grades of PCD materials can be obtained using diamonds of different particle sizes, and their thermal conductivity is different. Studies have shown that electrode losses are small or zero under certain EDM conditions. With the increase of thermal conductivity, the material removal rate and electrode loss of EDM materials with different grades during EDM are all reduced. Since the PCD material has a similar electric discharge machining effect as the conductive CVD diamond, the cost is relatively low, and thus it may become an ideal electrode material.
3 Electrode material for surface modification
The EDM surface modification mostly utilizes the characteristics of the electrode depletion during the EDM process, so that the electrode material is transferred to the surface of the material to be processed, thereby forming a high hardness, high wear-resistant coating, which is usually thermally decomposed in the working fluid kerosene. The carbon particles chemically react with the electrode material that is quickly lost and dropped off, and carbides are formed on the surface of the workpiece. To achieve this type of EDM surface modification, the tool electrode should be made of a material with a low thermal conductivity so that it can produce a large loss, and the electrode material should be relatively easy to form a hard carbide.
At present, the EDM surface modification mainly adopts solid electrodes of several materials, such as Si electrode, Ti electrode or W electrode, or compacted body or sintered body electrode made of various powder materials. The powder materials used include Al, Ti. , W, Ti, and Al mixed powders, WC, TiC, and ceramics and bonding agents (such as WC+Co, WC+Fe, WC+TiC+Co, TiC+Co, VC+Co), and the like. The use of such electrodes for electrical discharge machining may form one or more layers with different mechanical properties on the machined surface. When the powder material is used to prepare the electrode, the particle size of the powder has 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 (especially W) in the electrode are transferred in gradient form along with the carbon in the hydrocarbon working medium. The surface of the workpiece. HCTsai uses resin-containing Cu powder and Cr powder to form a Cu-Cr composite electrode by pressing. In the EDM process, the Cr element in the electrode migrates to the surface of the workpiece, and the processed surface obtains good corrosion resistance. As the Cr content in the electrode increases, the material removal rate during EDM decreases, but the corrosion resistance of the machined surface increases.
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, etc. used W-Powder, graphite powder and polyvinyl alcohol adhesive obtained by the body electrode of the LC4 aluminum alloy workpiece EDM surface modification. Lian Feng directly used YT15 cemented carbide material as electrode to perform EDM on 45 steel. When the positive electrode was processed, the surface of the workpiece could obtain a bright white layer with a micro-hardness that was much higher than that of the substrate.
4 micro-electrode machining electrode material
In the micro-EDM process, the use of micro-electrodes will usually increase the spark energy per unit area, resulting in greater electrode loss, and thus difficult to achieve the goal of high-precision machining. At this point, suitable EDM parameters can be selected to reduce the discharge energy per unit area, but this will extend the processing time; in addition, low-loss electrode materials can be used. Electrode materials used in the micro-EDM process mainly include Cu, W, Cu-W, and WC. Among them, electrodes used in micro-electrode drilling and milling are mainly W or WC rods or tubes.
When YYTsai et al. studied the wear resistance of the micro-EDM electrode, the electrode materials used were Ag, Al, Cu, Fe, Mo, Ni, Pt, Ti, Ta, and W. The results show that the electrode material with higher boiling point, melting point and thermal conductivity has lower loss. Among them, the loss of W electrode is minimal when machining stainless steel, pure Cu and pure Fe workpieces, and the loss of Cu and Ag electrodes is smaller than that of Fe and Ni electrodes. The greatest loss.
Ming Pingmei et al. used electroformed Cu and Cu-based composite electrodes for micro-EDM machining and studied the resistance to electrical erosion of electrode materials. The material obtained under the appropriate electroforming process conditions has a strong resistance to electrical erosion, and when more nano-germanium oxide additive is added to the electroformed Cu solution, the nano-germanium oxide is found in the electroformed Cu obtained. Phase, so that the material's conductive thermal conductivity is reduced, weakening its resistance to erosion. In the electroformed Cu solution, micro-powder graphite is added, and Cu-graphite composite material is obtained by composite electroforming. This material is introduced into the main body of the Cu powder. The micro-morphology of the material is Cu in the main body. Wrapped with Cu's sheet-like "microparticle center". Tests have shown that the composite electrode with a proper amount of graphite has much better resistance to electrical erosion than Cu. The analysis suggests that this is due to the fact that the “microparticle center†in the material combines the excellent thermal conductivity of the outer layer of Cu during the EDM process. Performance and graphite core heat storage and resistance to erosion, and graphite core exposed after a period of discharge to play a role in the skeleton, can reduce the liquid metal splash.
Since the loss of the conductive CVD diamond film as an electrode material is small, it has a good application prospect as an electrode material in the micro-electro discharge machining. Moreover, when the same diamond film is electrosparked with an electrically conductive CVD diamond film as an electrode, the shape and size of the latter can be well controlled so that it can be used as an electrode of different shapes in the micro-EDM process.
5 Conclusion
When different workpieces are processed under different process parameters, the processing effects obtained by using different electrode materials are different, so different electrode materials are suitable for different processing occasions. People have studied and applied a variety of electrode materials according to the needs of various EDM processes. The development of electrode materials has also promoted the progress of EDM processes. Some new electrode materials need further research and improvement to get practical application. When selecting electrode materials, it is necessary to comprehensively consider various factors such as EDM process methods, workpiece materials and shapes, processing requirements, and economical efficiency.
In electric discharge machining, 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 for EDM is crucial.
EDM tool electrode materials should meet the basic requirements of high melting point, low thermal expansion coefficient, good electrical and thermal conductivity and mechanical properties, and thus have a lower loss rate and resistance to deformation during use. The microstructure of the electrode with a fine crystalline structure is also advantageous for reducing the 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 source and low cost.
As the application range of EDM continues to expand, new requirements are constantly put forward for the electrode materials (including the corresponding electrode preparation methods) that are suitable for them. With the development of materials science, people continue to explore and innovate electrode materials for electric discharge machining tools. At present, tool electrode materials that have been used in research and production include single metals such as graphite, Cu or W, and Cu or W-based alloys. Steel, cast iron, Cu-based composites, polymer composites, and diamonds.
2 Ordinary electric discharge machining tool electrode material
(1) Graphite
Graphite has good electrical and thermal conductivity and machinability, and is a widely used tool electrode material in EDM.
There are different types of graphite, which can be classified according to the size of the graphite particles, the density of the material, and the mechanical and electrical properties. Among them, fine-grained graphite has smaller particles and porosity, higher mechanical strength, and more expensive price. Generally, the electrode loss rate for EDM machining is lower, but the material removal rate is correspondingly lower. The graphite grades supplied on the market have an average particle size of less than 20 μm, depending on the working conditions of the electrode (roughing, semi-finishing, or finishing) and the electrode geometry. The surface roughness of the workpiece is directly related to the size of the graphite particles. Usually, the graphite grade with an average particle size of 1 μm or less is specifically used for finishing. KLAas uses two different grades of graphite electrodes to process deep and narrow grooves on difficult-to-machine materials, comparing their material removal rates and electrode wear rates. The research results show that the choice of graphite type mainly depends on the specific requirements of EDM on the material removal rate and electrode loss rate.
Compared with other electrode materials, the graphite electrode can use a large discharge current for EDM, so the productivity is higher; the wear rate of the electrode during roughing is smaller, but the electrode wear rate during finishing increases, and the surface roughness of the processing is higher. difference. Graphite electrodes are light weight and low in price. Due to the high brittleness of graphite, it is often difficult to make a thin and thin shape by mechanical processing. Therefore, the application in fine and complex EDM is limited, and high-speed milling can 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 molten metal (Cu or Al) and applied high pressure to the liquid metal to fill the pores of the graphite electrode with the metal 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 resistivity greatly decreases, and the surface roughness of the electrode is improved. The experimental results show that compared with the conventional graphite electrode, the new material electrode has no significant difference in electrode wear rate and material removal rate, but the surface roughness 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, commonly used electrolytic copper as a tool electrode material.
Cu has a low melting point and a large electrode loss rate. Therefore, 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 and its high melting point, low thermal expansion coefficient, and strong spark erosion resistance, making it a high-performance tool electrode material. The Cu-W electrode is mainly used for processing tool steel and WC workpieces, in which the Cu, W content ratio is generally 25:75. However, since the price of the Cu-W electrode is higher than that of an ordinary Cu or graphite electrode, it is not currently used in production.
S.Singh et al. used a Cu, Cu-W alloy, brass, and Al electrode to machine a hardened tool steel. The results show that the Cu and Al electrodes have higher processing speeds and machining accuracies, and the wear rates of Cu and Cu-W electrodes. The smallest, brass has the highest rate of electrode wear. In contrast, Cu is a good electrode material, it can obtain 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, so it can be used when the surface roughness requirement is not high. Yan Yaowei et al. [7] used Cu, W, and Cu-W alloys as electrode materials for machining hard alloys. The results show that Cu-W alloy electrodes can significantly increase the processing speed, and the electrode loss is not large at lower processing voltages. Therefore, Cu-W alloy is an ideal electrode material for machining hard alloys.
TiC is a high hardness refractory material with high melting point, good thermal shock resistance and wear resistance. L. Li et al. studied the effect of TiC on the EDM performance of the tool electrode in the sintered Cu/TiC and Cu-W/TiC electrodes. The results show that the wear rate of the Cu/TiC electrode with 5% to 45% TiC is lower than that of conventional The Cu electrode. Considering the processing performance comprehensively, 25% TiC is the ideal proportion of ingredients. Cu-W/TiC electrode materials also show good performance, and most of the EDM surface roughness is superior to the Cu-W electrode processing surface, and thus can be used for finishing. For Cu-W/TiC electrode materials, 15% TiC is added for best results.
ZrB2 and TiSi have good electrical and thermal conductivity and high melting point. HMZaw et al. studied the use of different contents of Cu and ZrB2 or TiSi to prepare EDM tool electrodes by powder metallurgy, and with electrode materials such as graphite, Cu and Cu-W. EDM performance was compared. The results show that the TiSi/Cu electrode has a large loss, a low processing speed, and a rough machining surface, and thus the material is not suitable for use as an EDM electrode. ZrB2/Cu can be used as electrode material, but the binding force between Cu matrix and ZrB2 is poor. The content of ZrB2 and the process parameters of the electrode will affect the EDM performance of this electrode.
TiB2 particles have high melting point, good electrical and thermal conductivity, low coefficient of thermal expansion, etc. TiB2/Cu composites have good electrical conductivity, high temperature resistance and mechanical properties, which are in line with the basic requirements of EDM tool electrode materials. Qiu Yan et al. used powder metallurgy TiB2/Cu composite electrode for EDM test and analyzed the EDM machining loss mechanism of composite materials. The results showed 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 electrode material has a good EDM effect.
EDM grinding usually uses Cu-based composite electrodes. KMShu et al. used the Cu/SiCp composite electrode for EDM grinding. The composite electrode containing a certain amount of SiCp has significantly improved hardness and wear resistance compared with pure Cu electrode, while the electrical properties remain almost unchanged and the thermal conductivity is good. And has a high thermal shock resistance, showing low electrode loss characteristics. The electric spark grinding effect of Cu/SiCp composite electrode containing 2% SiCp is the best.
When the electroforming method is used to prepare the electrode, the electroforming of the Cu (including the Cu-based composite material) electrodes is more researched because the electroforming Cu process is more mature. The electroformed Cu or Cu-based composite has a dense structure and can achieve a smaller grain size. Studies have shown that the pits formed on the surface of the fine grained, densely-denatured electrode due to melting of the material during spark discharge are smaller and the wear rate of the electrode can be reduced.
(3) Polymer Composites
A. Curodeau et al. used a conductive thermoplastic polymer composite as an electrode and used air or water as a working medium to perform EDM or polishing of the workpiece surface. The electrode used is made up of 60% to 65% of solid carbon material (such as fine carbon black powder, graphite powder, graphite flakes, and even carbon nanotubes) uniformly distributed in a thermoplastic matrix material (such as polystyrene) It can be repeatedly softened and molded into the required geometry. Compared with graphite electrodes, this polymer-carbon composite electrode has a lower cost and can be molded into a complex geometry. The manufacturing speed is much faster than that of milling. At the same time, the density is low and the resistivity is high, resulting in electrode loss rate. Higher, but the electrode can be trimmed by re-moulding during use.
The components of this composite are still in the research and development stage. Good plasticized electrodes should have low resistivity, high thermal conductivity, low coefficient of thermal expansion, good formability and dimensional stability in water, and heat-resistant cycling.
(4) Diamond
K.Suzuki et al. studied EDM using a conductive CVD diamond thick film (0.5mm) as an electrode material. The CVD diamond is conductive during the CVD process by doping with boron. The CVD diamond has a low electrical resistance and a high thermal conductivity, and has a strong adsorption capacity for carbon precipitated in the oil working medium during EDM. EDM tests have shown that under certain processing conditions, CVD diamond electrodes can achieve very high material removal rates, while the electrode losses are almost zero, in particular, it can be processed at high current densities that cannot be achieved with Cu or graphite electrodes. . However, conductive CVD diamonds have problems such as high cost and limited size, so K.Suzuki et al. also used polycrystalline diamond (PCD) as an electrode material for electric discharge machining. The PCD material used is made of micron-sized diamond particles sintered with Co as a binder under ultra-high pressure and temperature in the presence of a metal catalyst, and its thermal conductivity is close to that of conductive CVD diamond. Different grades of PCD materials can be obtained using diamonds of different particle sizes, and their thermal conductivity is different. Studies have shown that electrode losses are small or zero under certain EDM conditions. With the increase of thermal conductivity, the material removal rate and electrode loss of EDM materials with different grades during EDM are all reduced. Since the PCD material has a similar electric discharge machining effect as the conductive CVD diamond, the cost is relatively low, and thus it may become an ideal electrode material.
3 Electrode material for surface modification
The EDM surface modification mostly utilizes the characteristics of the electrode depletion during the EDM process, so that the electrode material is transferred to the surface of the material to be processed, thereby forming a high hardness, high wear-resistant coating, which is usually thermally decomposed in the working fluid kerosene. The carbon particles chemically react with the electrode material that is quickly lost and dropped off, and carbides are formed on the surface of the workpiece. To achieve this type of EDM surface modification, the tool electrode should be made of a material with a low thermal conductivity so that it can produce a large loss, and the electrode material should be relatively easy to form a hard carbide.
At present, the EDM surface modification mainly adopts solid electrodes of several materials, such as Si electrode, Ti electrode or W electrode, or compacted body or sintered body electrode made of various powder materials. The powder materials used include Al, Ti. , W, Ti, and Al mixed powders, WC, TiC, and ceramics and bonding agents (such as WC+Co, WC+Fe, WC+TiC+Co, TiC+Co, VC+Co), and the like. The use of such electrodes for electrical discharge machining may form one or more layers with different mechanical properties on the machined surface. When the powder material is used to prepare the electrode, the particle size of the powder has 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 (especially W) in the electrode are transferred in gradient form along with the carbon in the hydrocarbon working medium. The surface of the workpiece. HCTsai uses resin-containing Cu powder and Cr powder to form a Cu-Cr composite electrode by pressing. In the EDM process, the Cr element in the electrode migrates to the surface of the workpiece, and the processed surface obtains good corrosion resistance. As the Cr content in the electrode increases, the material removal rate during EDM decreases, but the corrosion resistance of the machined surface increases.
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, etc. used W-Powder, graphite powder and polyvinyl alcohol adhesive obtained by the body electrode of the LC4 aluminum alloy workpiece EDM surface modification. Lian Feng directly used YT15 cemented carbide material as electrode to perform EDM on 45 steel. When the positive electrode was processed, the surface of the workpiece could obtain a bright white layer with a micro-hardness that was much higher than that of the substrate.
4 micro-electrode machining electrode material
In the micro-EDM process, the use of micro-electrodes will usually increase the spark energy per unit area, resulting in greater electrode loss, and thus difficult to achieve the goal of high-precision machining. At this point, suitable EDM parameters can be selected to reduce the discharge energy per unit area, but this will extend the processing time; in addition, low-loss electrode materials can be used. Electrode materials used in the micro-EDM process mainly include Cu, W, Cu-W, and WC. Among them, electrodes used in micro-electrode drilling and milling are mainly W or WC rods or tubes.
When YYTsai et al. studied the wear resistance of the micro-EDM electrode, the electrode materials used were Ag, Al, Cu, Fe, Mo, Ni, Pt, Ti, Ta, and W. The results show that the electrode material with higher boiling point, melting point and thermal conductivity has lower loss. Among them, the loss of W electrode is minimal when machining stainless steel, pure Cu and pure Fe workpieces, and the loss of Cu and Ag electrodes is smaller than that of Fe and Ni electrodes. The greatest loss.
Ming Pingmei et al. used electroformed Cu and Cu-based composite electrodes for micro-EDM machining and studied the resistance to electrical erosion of electrode materials. The material obtained under the appropriate electroforming process conditions has a strong resistance to electrical erosion, and when more nano-germanium oxide additive is added to the electroformed Cu solution, the nano-germanium oxide is found in the electroformed Cu obtained. Phase, so that the material's conductive thermal conductivity is reduced, weakening its resistance to erosion. In the electroformed Cu solution, micro-powder graphite is added, and Cu-graphite composite material is obtained by composite electroforming. This material is introduced into the main body of the Cu powder. The micro-morphology of the material is Cu in the main body. Wrapped with Cu's sheet-like "microparticle center". Tests have shown that the composite electrode with a proper amount of graphite has much better resistance to electrical erosion than Cu. The analysis suggests that this is due to the fact that the “microparticle center†in the material combines the excellent thermal conductivity of the outer layer of Cu during the EDM process. Performance and graphite core heat storage and resistance to erosion, and graphite core exposed after a period of discharge to play a role in the skeleton, can reduce the liquid metal splash.
Since the loss of the conductive CVD diamond film as an electrode material is small, it has a good application prospect as an electrode material in the micro-electro discharge machining. Moreover, when the same diamond film is electrosparked with an electrically conductive CVD diamond film as an electrode, the shape and size of the latter can be well controlled so that it can be used as an electrode of different shapes in the micro-EDM process.
5 Conclusion
When different workpieces are processed under different process parameters, the processing effects obtained by using different electrode materials are different, so different electrode materials are suitable for different processing occasions. People have studied and applied a variety of electrode materials according to the needs of various EDM processes. The development of electrode materials has also promoted the progress of EDM processes. Some new electrode materials need further research and improvement to get practical application. When selecting electrode materials, it is necessary to comprehensively consider various factors such as EDM process methods, workpiece materials and shapes, processing requirements, and economical efficiency.