Long-life electrode structure based on cell structure arc division and preparation method thereof

Abstract

The invention provides a long-life electrode structure based on cell structure arc division and a preparation method of the long-life electrode structure. The electrode structure comprises a high-conductivity base body and a surface cell element strengthening layer, the corresponding long-life characteristic is achieved on the basis that cell element structures distributed in a surface layer array disperse cathode arc plasma arc roots so as to reduce heat input, and under the synergistic effect of the high-conductivity base body and the ablation-resistant cell element strengthening layer, the long-life characteristic is achieved. The corresponding electrode structure has the advantages of obvious service life gain, reusability, uniform discharge ablation and high technical mobility. The substrate of the long-life electrode structure completes electric conduction and heat conduction in the discharging process, the overall temperature is controlled, and the surface cell element strengthening layer completes arc root dispersion and ablation uniformity improvement. According to the method, the cathode ablation form can be regulated and controlled from concentrated ablation to dispersed and uniform ablation, the service burning loss uniformity of the cathode is improved, and the service life of the electrode is prolonged.

Description

Technical Field

[0001] The invention relates to a long-life electrode structure based on arc division of a cell structure and a preparation method thereof, belonging to the technical field of mechanical processing. Background Art

[0002] Electrospark machining is based on high temperature and continuous pulse discharge between the electrode and the workpiece to complete the precise removal of materials. It has the advantages of wide adaptability and high processing efficiency. It is the main processing method for preparing mold holes and cavity parts in the current aviation field. With the cross-generational development of aviation equipment, aircraft parts tend to be large-scale and integrated, and the number and types of processing holes of a single part increase accordingly. Accordingly, the electrode for electrospark machining needs to have a sufficient service life. When the arc is ignited, the cathode surface continuously provides a large number of electrons to the arc column area through cluster electron emission mechanisms such as thermal emission or field emission, and at the same time withstands the high-speed bombardment of positively charged cations near the cathode surface. The large potential drop near the cathode area gives the positively charged particles reaching the cathode surface a higher average energy. In addition, the mass of cations exceeds the mass of electrons by 4 to 5 orders of magnitude. Under the high-speed bombardment of a large number of cations, the cathode arc ablation is more serious. The cathode area of ​​the arc usually contains several cathode spots. Almost all the electrons required for the arc to sustain itself are provided by the cathode spots. Under the action of the cathode spots, the corresponding site material is burned to form ablation pits. Under the continuous residence and ablation of the cathode spots, the surface of the pure copper electrode is prone to form ablation pits that penetrate deep into the interior, resulting in the arc burning of the cathode showing non-uniformity. When the ablation pits develop to a certain depth, it may even induce abnormal burning failure of the cathode before reaching its theoretical lifespan. The violent arc ablation of the cathode is the primary factor restricting the working efficiency of EDM equipment. Therefore, it is urgent to develop new electrode structures and their preparation methods.

[0003] At present, the new electrode structure mainly includes microalloy cathode and surface-strengthened layered cathode. Microalloy cathode is to add a small amount of low work function and refractory phase to pure metal, and the main preparation methods are powder metallurgy, smelting and casting. Typical low work function phases are LaB6, Cr2O3, and ZrO2. The principle of improving the ablation resistance of microalloy cathode is to increase the migration rate of cathode spots and expand the arc root. Studies have shown that the microalloy cathode made by doping with low work function phase has a significantly expanded cathode surface burnt area, reduced ablation depth, and overall cathode ablation tends to be moderate. Surface-strengthened layered cathode is to prepare a refractory conductive strengthening layer on the surface of a pure metal electrode, and the specific preparation methods are laser cladding, plasma spraying, and cold spraying. The surface strengthening layer replaces the cathode substrate and withstands arc root ablation. Accordingly, the strengthening layer needs to have certain electrical and thermal conductivity, a high melting point, and good plasticity and toughness. Typical strengthening layer materials are zirconium-based coatings, tungsten-based coatings, and niobium-based coatings. In addition, recent studies have shown that refining the grains of the surface strengthening layer by specific mechanical processing methods (such as surface rolling) can further accelerate the migration of cathode spots, reduce the residence time and alleviate cathode ablation.

[0004] However, both the existing microalloy cathodes and surface-strengthened cathodes have certain limitations. The low work function phase content of the microalloy cathode is difficult to precisely control. When the low work function phase content is low, cathode spots will also form on the substrate surface, which cannot improve the cathode's arc erosion resistance; when the added phase content is high, it tends to reduce the cathode's electrical and thermal conductivity, resulting in heat accumulation and accelerated burning. Compared with the microalloy cathode, the surface-strengthened cathode can achieve the decoupling of the cathode surface erosion resistance and the substrate's conductive properties, and shows certain structural advantages over the pure material cathode. However, when the strengthening layer is subjected to arc erosion, the arc root often converges in a relatively small area, resulting in concentrated discharge ablation, shortening the cathode's service life. Although organizational control measures such as grain refinement can accelerate the migration of cathode spots in the short term, the fine-grained structure cannot exist stably for a long time. Therefore, the existing cathode structure cannot guarantee the working stability of the cathode during long-term discharge service. Summary of the invention

[0005] The purpose of the present invention is to provide a long-life electrode structure based on arc division of a cell structure and a preparation method thereof. The novel electrode structure involved specifically comprises a highly conductive substrate and a surface cell strengthening layer, and the cell strengthening layer is further composed of a cell wall and a filler. The highly conductive substrate completes the electrical and thermal conduction of the discharge process, controls the electrode temperature, and the surface cell strengthening layer disperses the arc root and improves the ablation uniformity. The specific development process of the electrode structure is cathode material screening-precision manufacturing of lattice cell structure-cell cavity material filling-post-processing.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] A long-life electrode structure based on arc splitting of cell structure, which decouples the ablation resistance requirements of the electrode surface and the electrical and thermal conductivity requirements of the substrate with a layered composite structure, disperses the arc root with a cell structure arranged in a surface array, limits the migration of cathode spots to force arc splitting, and reduces the input heat flux density. The specific preparation method corresponding to the new electrode structure is divided into electrode material screening / structure design-lattice cell structure precision manufacturing-cell cavity material filling-post-processing.

[0008] A method for preparing a long-life electrode structure based on arc division of a cell structure comprises the following steps:

[0009] Step 1: Electrode material screening / structural design. Determine the material type of the electrode matrix, cell wall and filler according to the characteristics of the electrode discharge environment, service current and voltage, etc. Design structural parameters such as strengthening layer thickness and cell size according to the processing discharge time requirements.

[0010] Step 2: Precise manufacturing of lattice cell structure. Using the selective laser melting additive manufacturing method, the pre-designed cell structure is precisely formed on the polished surface of the electrode substrate to ensure that multiple cell structure arrays are arranged on the substrate surface.

[0011] Step 3: Filling the cell cavity with material. Using high-temperature infiltration filling forming technology, high-temperature molten metal is filled into the cavity surrounded by the cell wall in the forming furnace body until the upper surface of the molten pool is level with the surface of the cell wall, and then condensed and formed in the furnace.

[0012] Step 4: Post-processing: After properly clamping the electrode blank, use high-speed milling to flatten the upper surface of the cell wall, remove burrs, spheroidized particles and other local protrusions, and obtain a flat electrode surface.

[0013] Furthermore, the selection principles of the electrode material screening step are as follows: 1) The substrate should be made of high-strength, high-conductivity metal materials, such as copper alloys, which must also have a certain ability to resist high-temperature softening; 2) The cell wall is mainly responsible for controlling the cathode spots and avoiding concentrated discharge, and preferably uses high-melting-point, ablation-resistant, low-conductivity, difficult-to-discharge metal materials, such as tungsten alloys and tantalum alloys; 3) The filler induces the cathode spots to form first and discharge continuously through a strong electron emission process, and therefore tends to use metal materials with low work function and high melt viscosity, such as copper-chromium alloys and zirconium-based alloys.

[0014] Furthermore, the main parameters of the electrode structure design step are the size of a single cell and the thickness of the overall strengthening layer. The size of a single cell is mainly determined by referring to the cathode spot diameter. To ensure that the clustered cathode spot clusters (diameter 0.5-5mm) are completely confined to a single cell during discharge, the size of a single cell should not be less than 6mm, and should be controlled within 10mm to avoid the aggregation and concentrated ablation of multiple spot clusters.

[0015] Furthermore, the thickness parameter of the strengthening layer is determined based on the balance between the service performance and manufacturing cost of the electrode. According to the service characteristics and required life of a specific electrode, for example, continuous operation for 20 hours at a current of 50A, the thickness of the cell strengthening layer is designed to be as thin as possible while meeting the service life requirements, so as to control the manufacturing cost of a single electrode.

[0016] Furthermore, the precise manufacturing steps of the lattice cell structure. Its manufacturing requirements are to improve the utilization rate of raw materials as much as possible on the basis of satisfying the forming accuracy of multiple cell structures distributed in the array, and give priority to the selective laser selective melting forming additive preparation method. At the same time, laser powder feeding / wire forming technology with low beam spot diameter and electron beam wire feeding melting forming technology can also be used for manufacturing.

[0017] Furthermore, the outer shape of the cell structure must satisfy its fillability within the limited space of the electrode surface, with regular hexagons, squares and the like being given priority.

[0018] Furthermore, the filling process of the cell cavity structure mainly uses high-temperature metal infiltration forming technology. Taking the filling of copper-chromium alloy in tungsten cells as an example, the copper-chromium metal block on the top of the prepared array tungsten cells is placed in a high-temperature infiltration furnace, and the temperature is raised to above the melting point of the filling metal and below the melting point of the cell. When the molten copper-chromium alloy fully penetrates into the interior of the cell structure below, it is cooled and formed in the furnace.

[0019] Furthermore, the purpose of the post-processing treatment is to remove the surface undulations and local protrusions on the electrode surface generated in the previous forming step, so as to obtain a discharge surface in which the cell wall is flush with the filling metal.

[0020] Compared with the existing novel cathode structure, the present invention has the following advantages:

[0021] 1) The service life is significantly increased and can be reused. Based on the ablation-resistant strengthening layer and the highly conductive substrate, the discharge ablation process is controlled on the surface of the strengthening layer, which causes little damage to the electrode substrate. The surface of the electrode substrate after service can be reused after secondary preparation, which effectively extends the cathode life.

[0022] 2) Discharge ablation is uniform and device performance is fully utilized. Relying on the array structure of cell walls and fillers, the cathode spots are fundamentally controlled in a limited area and the arc roots are dispersed to avoid concentrated discharge and burning, thereby giving full play to the discharge ablation performance of the electrode material and avoiding abnormal burning and failure.

[0023] 3) High technology transferability and universality. The proposed new electrode structure is guided by the spot discharge characteristics and principles of the metal cathode. The corresponding structure and its preparation method have good transferability in the metal cathode arc discharge scene, and can be applied to various service environments such as electric spark discharge hole making, arc heater, arc plasma torch, arc welding, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The overall structure of the arc-splitting electrode of the new interface cell structure and the schematic diagram of arc discharge;

[0025] Figure 2 The manufacturing flow chart of the new electrode;

[0026] Figure 3 This is a high-speed filtered capture of the discharge process of the zirconium-based alloy / tungsten / zirconium electrode;

[0027] Figure 4 This is the surface morphology of the zirconium-based alloy / tungsten / zirconium electrode burnout;

[0028] Figure 5 This is a high-speed filter capture image of the discharge process of a simple layered composite zirconium-based alloy / zirconium electrode;

[0029] Figure 6 This is the surface morphology of the burned surface of a simple layered composite zirconium-based alloy / zirconium electrode.

[0030] In the figure: 1-high conductive substrate, 2-surface cell strengthening layer, 3-cell wall, 4-filler, 5-arc plasma, 6-top electrode, 7-cathode spot, 8-array cell, 9-pure metal block. DETAILED DESCRIPTION

[0031] Figure 1 The overall structure and arc discharge schematic diagram of the arc-splitting electrode of the new interface cell structure. Figure 2 The manufacturing flow chart of the new electrode is shown in Figure 2. Figure 3 High-speed optical filtering capture of the discharge process of zirconium-based alloy / tungsten / zirconium electrodes. Figure 4 The surface morphology of zirconium-based alloy / tungsten / zirconium electrode burnout. Figure 5 It is a high-speed filter capture of the discharge process of a simple layered composite zirconium-based alloy / zirconium electrode. Figure 6 It is the burnt surface morphology of a simple layered composite zirconium-based alloy / zirconium electrode.

[0032] The present invention relates to a long-life electrode structure based on arc division of a cell structure, such as Figure 1 As shown in a, it specifically includes a highly conductive substrate (1) and a surface cell strengthening layer (2), and the cell strengthening layer is further divided into a cell wall (3) and a filler (4). Figure 1As shown in Fig. b, the arc plasma (5) is stabilized between the top electrode (6) and the bottom cell electrode, and many cathode spots (7) are confined to independent cells isolated from each other and separated from each other to disperse the arc roots. The long life characteristics of the cell electrode are achieved by the synergistic effect of various components, namely 1) the highly conductive matrix completes the electrical and thermal conduction dissipation of the electrode discharge process and controls the electrode temperature; 2) the cell wall with a specific size on the surface limits the cathode spots on the cathode surface to avoid the concentration of spot clusters and the ablation penetration; 3) the filler emits a dense electron jet to induce the formation of cathode spots, and the filling area separated by several cell walls carries the arc roots in a dispersed manner.

[0033] Specific manufacturing process corresponding to the new electrode structure ( Figure 2 ) includes the following steps: 1. Electrode material screening / structure design; 2. Precision manufacturing of lattice cell structure; 3. Filling of cell cavity material; 4. Post-processing.

[0034] Electrode material screening / structural design. In terms of material screening, the matrix should consider high-strength and high-conductivity metal materials, and at the same time, it must have a certain hardness and resistance to high-temperature softening; the cell wall should preferably use high melting point, ablation resistance, low conductivity, difficult-to-discharge metal materials, such as tungsten alloys and nickel-based alloys; fillers use low work function and high melt viscosity metal materials, such as copper-chromium and zirconium-based alloys. In terms of structural design, the main parameters are cell size and strengthening layer thickness. The determination of the cell size refers to the cathode spot diameter, based on controlling it to more than 120% of the cathode spot cluster diameter, and is set within 10 mm to avoid the aggregation of multiple spot clusters.

[0035] The lattice cell structure is precisely manufactured. Figure 2 As shown in a and 2b, the arrangement design of the cell structure on the substrate surface is based on the principle of tight packing in a limited space, and the single cell is preferably in the shape of a regular hexagon, square, etc. When the cell is precisely manufactured, Figure 2 b Taking the regular hexagonal array cell as an example, it is required to ensure the forming accuracy while increasing the utilization rate of raw materials as much as possible and controlling the cost. Therefore, the selective laser selective melting forming additive preparation method is given priority.

[0036] The cell cavity is filled with material. Figure 2 As shown in c, a high-temperature metal infiltration forming technology is used to place a pure metal block (9) on top of the precisely manufactured array cell (8), and place it in a high-temperature infiltration furnace. The temperature is raised to a temperature above the melting point of the filler metal and below the melting point of the cell material. When the molten filler material fully infiltrates into the interior of the cell structure below, it is cooled in the furnace and formed.

[0037] Post-processing finishing. Figure 2 As shown in c and 2d, for the blank obtained by melt infiltration, after being properly clamped and fixed, the upper surface of the cell wall is milled flat by high-speed milling to obtain a flat electrode surface.

[0038] In this embodiment, the interface cell structure electrode designed and prepared is a zirconium alloy / tungsten / zirconium layered electrode. For the specific electrode service condition, the through hole is made by electric spark, and the hole shape is a circle with a diameter of 40±0.5mm. The cross-sectional diameter of the hole matching the rod electrode is 40mm. The arc current is 40A, and the electrode is discharged and ablated in an air atmosphere. The service life of a single rod electrode must be no less than 5h. According to the above usage requirements, the new electrode matrix material is designed to be pure zirconium, the interface cell wall material is pure tungsten, and the filling material is selected from a zirconium-based alloy, with a specific composition of Zr 65at.%, Nb 35at.%. The size of a single cell is set to 5mm, and a regular hexagonal cell configuration is adopted, and the thickness of the strengthening layer corresponds to 4mm.

[0039] Electrode material screening / structural design. Pure zirconium, pure tungsten and zirconium-based alloys are used as electrode substrate, cell wall and filling materials respectively. The electrode cross-section is circular with a diameter of ~40mm. The size of a single cell is 5mm and the thickness of the strengthening layer is 4mm. This completes the design of zirconium alloy / tungsten / zirconium layered electrodes.

[0040] Selective laser melting of pure tungsten lattice cell structure. For the manufacture of tungsten cells on the surface of pure zirconium substrate, a regular hexagonal array structure is constructed in advance using 3D modeling software, and the obtained cell model is pre-processed and sliced. Based on the optimized model, the selective laser melting forming technology is used to promote the metallurgical bonding between tungsten powder and zirconium substrate at a relatively low laser power in the initial 200 layers. After 200 layers, the preset forming parameters are restored to ensure efficient and accurate forming of tungsten cells.

[0041] The inner cavity of the tungsten cell is filled with zirconium-based materials. In this embodiment, the tungsten cell is filled with zirconium-based alloys, wherein the former has a melting point of 1500°C and the latter has a melting point of 3422°C. To ensure that the liquid zirconium-based alloy is fully filled into the inner cavity of the cell during the infiltration forming process, the infiltration furnace temperature is set to 1700°C. After being kept at the corresponding temperature for 2 hours, it is slowly cooled at a speed of 100°C / h along with the furnace to form the tungsten cell.

[0042] Post-processing and finishing: The electrode blank obtained in the previous step is properly clamped, and the upper surface of the cell wall is milled flat by high-speed milling to obtain a flat electrode surface.

[0043] Based on the novel zirconium alloy / tungsten / zirconium electrode obtained by the above series of steps, the arc discharge process in air atmosphere under 40A arc current was tested, and the discharge on the electrode surface was observed with a high-speed camera at a frame rate of 2000fps under 90% filter conditions, such as Figure 3As shown, the cell electrode is located at the bottom of the recording screen, and the needle-shaped object at the top is the counter electrode. The filtered capture screen shows that several cathode spot clusters appear on the surface of the cell electrode. Each cluster is discrete and 5 to 10 mm apart. Accordingly, the arc root is forced to be dispersed in a larger area, indicating that the new electrode based on the interface cell structure effectively achieves the design purpose of arc splitting and reducing heat input. Furthermore, the surface morphology of the cathode after 5 minutes of discharge ( Figure 4 ), the cathode single cell is burned evenly, no ablation pit structure developing from the surface to the inside is observed, and the cathode arc discharge and ablation uniformity are excellent.

[0044] Comparative Example 1

[0045] In this comparative example, the service scenario of the electrode is consistent with that of the embodiment, which is an electrospark through-hole environment. The hole shape is a circle with a diameter of 40±0.5mm, and the cross-sectional diameter of the hole-making matching rod electrode is 40mm. Compared with the embodiment, the electrode structure used in the comparative example is a zirconium-based alloy / zirconium alloy layered composite metal, and a cellular structure is not adopted.

[0046] Zirconium-based alloy / zirconium alloy electrode structure, the thickness of the zirconium-based alloy strengthening layer is 4 mm. After the corresponding strengthening layer is prepared on the surface of the pure zirconium substrate by laser additive manufacturing method, high-speed milling is also used to obtain the electrode surface with similar surface flatness.

[0047] Based on the obtained zirconium alloy / zirconium layered electrode, the arc discharge process in air atmosphere under 40A arc current was tested, and the discharge on the electrode surface was observed with a high-speed camera at a frame rate of 2000fps under 90% filter conditions, such as Figure 5 As shown in the figure, continuous and dense cathode spot clusters appear on the electrode surface. The clusters are closely adjacent and converged in a small range on the cathode surface. The distance between them is less than 2 mm, indicating that the cathode is concentratedly ablated at this time, reflecting that the layered electrode obtained by simply relying on surface strengthening cannot effectively disperse the arc and improve the cathode service performance. Similarly, the microstructure of the cathode surface after 5 minutes of discharge is observed, as shown in the figure. Figure 6 As shown in the figure, a large number of circular ablation pits with a diameter of 200 μm appeared on the burnt surface, which was caused by the continuous residence of the cathode spots. After the nested ablation pits are formed, they will gradually penetrate into the cathode, resulting in an increase in the electrode ablation rate and accelerated cathode burnout. The non-uniform ablation behavior of zirconium alloy / zirconium layered electrodes confirms that surface strengthening alone cannot fully improve the cathode arc ablation behavior.

Claims

1. A long-life electrode structure based on arc division of a cell structure, characterized in that: The layered composite structure is used to decouple the ablation resistance requirements of the electrode surface and the electrical and thermal conductivity requirements of the substrate. The cell structure arranged in a surface array disperses the arc root, restricts the migration of cathode spots to force arc division, and reduces the input heat flux density. The specific preparation methods corresponding to the new electrode structure are divided into electrode material screening / structure design-precision manufacturing of lattice cell structure-filling of cell cavity material-post-processing.

2. A method for preparing a long-life electrode structure based on arc division of a cell structure, characterized in that: The following steps are involved: Step 1: Electrode material screening / electrode structure design; determine the material types of the electrode matrix, cell wall and filler according to the characteristics of the electrode discharge environment, service current and voltage; Design the strengthening layer thickness and cell size structure parameters according to the machining discharge time requirements; Step 2: Precision manufacturing of lattice cell structure; Using the selective laser melting additive manufacturing method, the pre-designed cell structure is precisely formed on the polished surface of the electrode substrate, ensuring that multiple cell structure arrays are arranged on the substrate surface; Step 3: Filling the cell cavity with material: Using high-temperature infiltration filling forming technology, high-temperature molten metal is filled into the cavity surrounded by the cell wall in the forming furnace body until the upper surface of the molten pool is level with the surface of the cell wall, and then condensed and formed with the furnace; Step 4: Post-processing: After properly clamping the electrode blank, use high-speed milling to flatten the upper surface of the cell wall, remove burrs and local protrusions of spheroidized particles, and obtain a flat electrode surface.

3. A method for preparing a long-life electrode structure based on arc division of a cell structure as claimed in claim 2, characterized in that: In step 1, the electrode material selection principles are as follows: 1) Consider using high-strength and high-conductivity metal materials for the substrate; 2) The cell wall is mainly responsible for controlling cathode spots and avoiding concentrated discharge, and a difficult-to-discharge metal material with high melting point, ablation resistance and low conductivity is selected; 3) The filler induces the cathode spot to form first and discharge continuously through a strong electron emission process, and uses a metal material with low work function and high melt viscosity.

4. A method for preparing a long-life electrode structure based on arc division of a cell structure as claimed in claim 2 or 3, characterized in that: In step 1, the electrode structure design steps are as follows: The electrode structure includes the size of a single cell and the thickness of the overall strengthening layer. The size of a single cell should be no less than 6 mm and within 10 mm.

5. A method for preparing a long-life electrode structure based on arc division of a cell structure as claimed in claim 2 or 3, characterized in that: In the step 2, the lattice cell structure adopts a regular hexagon or a square.

6. A method for preparing a long-life electrode structure based on arc division of a cell structure as claimed in claim 2 or 3, characterized in that: In the step 2, the filling process of the cell structure uses a metal high-temperature infiltration forming technology, placing the copper-chromium metal block on the top of the prepared array tungsten cell in a high-temperature infiltration furnace, raising the temperature to above the melting point of the filling metal and below the melting point of the cell, and when the molten copper-chromium alloy fully infiltrates into the interior of the cell structure below, it is cooled and formed in the furnace.

7. The method for preparing a long-life electrode structure based on arc division of a cell structure as claimed in claim 3, characterized in that: In the step 2, the filling process of the cell structure uses a metal high-temperature infiltration forming technology, placing the copper-chromium metal block on the top of the prepared array tungsten cell in a high-temperature infiltration furnace, raising the temperature to above the melting point of the filling metal and below the melting point of the cell, and when the molten copper-chromium alloy fully infiltrates into the interior of the cell structure below, it is cooled and formed in the furnace.

8. The method for preparing a long-life electrode structure based on arc division of a cell structure as claimed in claim 2, characterized in that: The metal material with low work function and high melt viscosity is copper-chromium alloy or zirconium-based alloy.

9. The method for preparing a long-life electrode structure based on arc division of a cell structure as claimed in claim 2, characterized in that: The high melting point, ablation resistance, low conductivity and difficult-to-discharge metal material is tungsten alloy or tantalum alloy.

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