A long-life electrode structure based on cell structure split arc and a preparation method thereof

Abstract

The application provides a long-life electrode structure based on cell structure arc splitting and a preparation method thereof. The electrode structure comprises a high-conductivity base and a surface cell reinforcement layer. The realization of the long-life characteristic is based on the cell structure arc splitting of the surface layer array distribution to disperse the cathode arc plasma arc root to reduce the heat input. Under the synergistic effect of the high-conductivity base and the ablation-resistant cell reinforcement layer, the electrode structure has the advantages of obvious service life gain, reusability, uniform discharge ablation and high technical transferability. The base of the long-life electrode structure completes the electric conduction and heat conduction of the discharge process, controls the overall temperature, and the surface cell reinforcement layer completes the dispersion of the arc root and improves the ablation uniformity. The application can control the cathode ablation form from concentrated ablation to dispersed and uniform ablation, improve the uniformity of the cathode service ablation, and prolong the electrode life.

Description

TECHNICAL FIELD

[0001] The application relates to a long-life electrode structure based on cell structure split arc and a preparation method thereof, and belongs to the technical field of mechanical processing. BACKGROUND

[0002] Electrical discharge machining (EDM) is a main machining method for preparing holes and cavities in the field of aviation, which has the advantages of wide adaptability and high machining efficiency. EDM is based on the high-temperature and continuous pulse discharge between the electrode and the workpiece to achieve precise material removal. With the development of aviation equipment, aircraft parts tend to be large and integrated, and the number and types of machining holes in individual parts increase accordingly. Accordingly, the EDM electrode needs to have sufficient service life. When the arc is ignited, the cathode surface provides a large number of electrons through cluster electron emission mechanisms such as thermal emission or field emission to the arc column area, while being bombarded by high-speed positive ions near the cathode surface. The large potential drop in the near-cathode region gives the positive particles arriving at the cathode surface a high average energy. In addition, the mass of the positive ions is 4-5 orders of magnitude higher than that of the electrons. Under the high-speed bombardment of a large number of positive ions, the cathode arc ablation is more serious. The cathode arc region usually contains several cathode spots, and almost all the electrons required for arc self-sustaining are provided by the cathode spots. Under the action of the cathode spots, the material at the corresponding site is burned and forms an ablation crater. Under the continuous residence and ablation of the cathode spots, the pure copper electrode surface is easy to form ablation craters deep into the interior, resulting in non-uniformity of the cathode arc ablation. When the ablation craters develop to a certain depth, even abnormal ablation failure of the cathode before reaching the theoretical service life is induced. The severe arc ablation of the cathode is the primary factor restricting the working efficiency of the EDM equipment. Therefore, it is urgent to develop a new electrode structure and a preparation method thereof.

[0003] At present, the new electrode structure mainly includes micro-alloyed cathode and surface reinforced layered cathode. The micro-alloyed cathode is prepared by adding a small amount of low work function and refractory phase into pure metal, and the main preparation methods are powder metallurgy and smelting casting. Typical low work function phases are LaB6, Cr2O3 and ZrO2. The principle of improving the ablation resistance of the micro-alloyed cathode is to improve the spot migration rate and the arc root expansion of the cathode. Studies have shown that after the micro-alloyed cathode is prepared by doping the low work function phase, the surface burn area of the cathode is significantly expanded, the ablation depth is reduced, and the overall cathode ablation tends to be moderate. The surface reinforced layered cathode is prepared by preparing a refractory conductive reinforced layer on the surface of the pure metal electrode, and the specific preparation methods are laser cladding, plasma spraying and cold spraying. The surface reinforced layer replaces the cathode matrix and withstands the arc root ablation. Correspondingly, the reinforced layer needs to have certain electric conductivity and thermal conductivity, high melting point and good plasticity and toughness, and typical reinforced layer materials are zirconium-based coating, tungsten-based coating and niobium-based coating. In addition, recent studies have shown that the surface reinforced layer can further accelerate the migration of the cathode spot and reduce the residence time to alleviate the cathode ablation by means of specific mechanical processing (such as surface rolling) to refine the grains.

[0004] However, the existing micro-alloyed cathode and surface reinforced cathode have certain limitations. The content of the low work function phase in the micro-alloyed cathode is difficult to accurately control. When the content of the low work function phase is low, the cathode spot will also be formed on the surface of the matrix, which cannot improve the arc ablation resistance of the cathode; when the content of the additional phase is high, the electric conductivity and thermal conductivity of the cathode will be reduced, which will accelerate the heat accumulation and ablation. Compared with the pure material cathode, the surface reinforced cathode can decouple the surface ablation resistance and the matrix conduction performance of the cathode, and has certain structural advantages. However, when the reinforced layer withstands the arc ablation, the arc root is often concentrated in a relatively small area, which leads to concentrated discharge ablation and shortens the service life of the cathode. Although the grain refinement and other organizational control means can accelerate the migration of the cathode spot 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 in the long-term discharge service process. SUMMARY

[0005] The purpose of the present application is to provide a long-life electrode structure based on cell structure arc splitting and a preparation method thereof. The new electrode structure specifically includes a high-conductivity matrix and a surface cell reinforced layer, which is further composed of a cell wall and a filler. The high-conductivity matrix completes the electric conduction and heat conduction of the discharge process, controls the electrode temperature, and the surface cell reinforced layer disperses the arc root and improves the ablation uniformity. The specific development process of the electrode structure is cathode material selection-precise manufacturing of lattice cell structure-filling of cell inner cavity material-post processing.

[0006] The purpose of the present application is achieved by the following technical scheme:

[0007] A long-life electrode structure based on cell structure arc splitting, decoupling the electrode surface ablation resistance requirement and the substrate conductive and thermal conductivity requirement in a layered composite structure, dispersing the arc root of the electric arc with the surface array arrangement of the cell structure, limiting the cathode spot migration to force arc splitting and reduce the input heat flux density. The specific preparation method of the new electrode structure includes electrode material selection / structure design, dot array cell structure precision manufacturing, cell cavity material filling and post-processing.

[0008] A preparation method of a long-life electrode structure based on cell structure arc splitting, including the following steps:

[0009] Step one: electrode material selection / structure design. According to the characteristics of the electrode discharge environment, service current and voltage conditions, the material types of the electrode substrate, cell wall and filler are determined. The thickness of the reinforcing layer and the structure parameters such as the cell size are designed according to the processing discharge time requirement.

[0010] Step two: dot array cell structure precision manufacturing. Using selective laser melting additive manufacturing method, the pre-designed cell structure is precisely shaped on the polished electrode substrate surface, ensuring that multiple cell structure arrays are arranged on the substrate surface.

[0011] Step three: cell cavity material filling. 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, and after the upper surface of the molten pool is leveled with the surface of the cell wall, it is condensed and formed in the furnace.

[0012] Step four: post-processing. After properly clamping the electrode blank, the upper surface of the cell wall is milled flat with high-speed milling to remove burrs, spheroidized particles and other local protrusions to obtain a flat electrode surface.

[0013] Further, the electrode material selection step has the following selection principles: 1) the substrate considers using high-strength, high-conductivity metal materials, such as copper alloy, which also needs to have certain high-temperature softening resistance; 2) the cell wall is mainly responsible for controlling the cathode spot and avoiding concentrated discharge, and preferentially selects high-melting-point, ablation-resistant, low-conductivity difficult-to-discharge metal materials, such as tungsten alloy and tantalum alloy; 3) the filler tends to use low work function, high melt viscosity metal materials, such as copper-chromium alloy and zirconium-based alloy, to induce the formation of cathode spots and continuous discharge through strong electron emission process.

[0014] Further, the electrode structure design step mainly has the single cell size and the overall reinforcing layer thickness. The single cell size is mainly determined by referring to the cathode spot diameter. To ensure that the clustered cathode spot clusters (diameter 0.5-5mm) are completely limited within a single cell during discharge, the single cell size should be no less than 6mm, and should be controlled within 10mm to avoid multiple spot clusters and concentrated ablation.

[0015] Further, the thickness parameter of the reinforcing layer is determined according to the mutual balance of electrode service performance and manufacturing cost. According to the service characteristics and required life of a specific electrode, for example, 20h continuous work under 50A current, under the premise of meeting the service life requirement, the thickness of the cell reinforcing layer is designed as thin as possible to control the manufacturing cost of a single electrode.

[0016] Further, the precise manufacturing step of the dot matrix cell structure. The manufacturing requirement is to improve the utilization rate of raw materials as much as possible on the basis of meeting the forming precision of the array distribution of multiple cell structures, and to preferentially consider the selective laser selective melting forming additive manufacturing method. At the same time, low-beam spot diameter laser powder / silk forming technology and electron beam silk melting forming technology can also be used for manufacturing.

[0017] Further, the cell structure needs to meet the fillability in the limited space range on the electrode surface, and shapes such as regular hexagon and square are preferred.

[0018] Further, the filling process of the cell cavity structure mainly uses metal high-temperature infiltration forming technology. Taking the example of filling copper-chromium alloy in tungsten cell, the array tungsten cell top copper-chromium metal block is placed in a high-temperature infiltration furnace, heated 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 inside of the cell structure below, it is cooled in the furnace.

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

[0020] Compared with the existing new cathode structure, the present application has the following advantages:

[0021] 1) The service life is obviously increased and can be reused. Based on the ablation-resistant reinforcing layer and the high-conductivity matrix, the discharge ablation process is controlled on the surface of the reinforcing layer, and the electrode matrix is less damaged. The electrode matrix surface after service can be reused after secondary preparation, which effectively prolongs the life of the cathode.

[0022] 2) The discharge ablation is uniform, and the device performance is fully played. Relying on the array structure of the cell wall and the filler, the cathode spot is fundamentally controlled in a limited area and the arc root is dispersed, avoiding concentrated discharge and burnout, thereby fully playing the discharge ablation performance of the electrode material and avoiding abnormal burnout failure.

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

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

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

[0026] Figure 3 The high-speed filter capture diagram of the zirconium-based alloy / tungsten / zirconium electrode discharge process;

[0027] Figure 4 The zirconium-based alloy / tungsten / zirconium electrode burnout surface morphology diagram;

[0028] Figure 5 The high-speed filter capture diagram of the simple layered composite zirconium-based alloy / zirconium electrode discharge process;

[0029] Figure 6 The simple layered composite zirconium-based alloy / zirconium electrode burnout surface morphology diagram.

[0030] In the figure: 1-high conductive base, 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 whole structure and arc discharge schematic diagram of the new interface cell structure arc splitting electrode, Figure 2 The manufacturing flowchart of the new electrode, Figure 3 The high-speed filter capture of the zirconium-based alloy / tungsten / zirconium electrode discharge process, Figure 4 The zirconium-based alloy / tungsten / zirconium electrode burnout surface morphology, Figure 5 The high-speed filter capture of the simple layered composite zirconium-based alloy / zirconium electrode discharge process, Figure 6 The simple layered composite zirconium-based alloy / zirconium electrode burnout surface morphology.

[0032] The present application relates to the long-life electrode structure based on cell structure arc splitting, such as Figure 1 As shown in a, specifically contains high conductive base (1) and surface cell strengthening layer (2), cell strengthening layer is further divided into cell wall (3) and filler (4). The schematic diagram of the arc discharge process of the new electrode is as shown in b. Figure 1As shown in FIG. b, the arc plasma (5) is stabilized between the top electrode (6) and the bottom cell electrode, and the cathode spots (7) are confined in the independent cells isolated from each other, separated from each other to disperse the arc root. The long-life characteristics of the cell electrode rely on the synergistic effect of each component, that is, 1) the high-conductivity matrix completes the conduction and heat conduction dissipation of the electrode discharge process, and controls the electrode temperature; 2) the surface of the cell wall of a certain size limits the cathode spots on the cathode surface, avoiding the ablation of the spots and the deepening of the ablation; 3) the filler emits dense electron beams to induce the formation of cathode spots, and the filling area separated by several cell walls carries the arc root in a dispersed manner.

[0033] The specific manufacturing process of the new electrode structure Figure 2 includes the following steps: 1, electrode material selection / structure design; 2, precise manufacturing of lattice cell structure; 3, filling of cell cavity material; 4, post-processing.

[0034] Electrode material selection / structure design. In terms of material selection, the matrix considers high-strength and high-conductivity metal materials, which also need to have certain hardness and high-temperature softening resistance; the cell wall is preferentially selected from high-melting-point, ablation-resistant, and low-conductivity difficult-to-discharge metal materials, such as tungsten alloy and nickel-based alloy; the filler uses low work function and high melt viscosity metal materials, such as copper-chromium and zirconium-based alloy. In terms of structure design, the main parameters are cell size and reinforcement layer thickness. The determination of the cell size refers to the diameter of the cathode spot, and is controlled to be more than 120% of the diameter of the cathode spot cluster, while being set to be within 10 mm to avoid the aggregation of multiple spot clusters.

[0035] Precise manufacturing of lattice cell structure. As shown in FIGS. Figure 2 a and 2b, the arrangement of the cell structure on the surface of the matrix is designed to meet the principle of close packing in a limited space, and the single cell is preferentially considered to be a regular hexagon, a square, etc. In the specific precise manufacturing of the cell, Figure 2 b, taking the regular hexagonal array cell as an example, it is required to guarantee the forming precision and improve the utilization rate of raw materials as much as possible to control the cost, so the selective laser melting method is preferentially considered.

[0036] Filling of cell cavity material. As shown in FIG. Figure 2 c, the array cell (8) obtained by precise manufacturing is placed on the pure metal block (9) at high temperature, and the temperature is raised to above the melting point of the filling metal and below the melting point of the cell material. When the molten filling material fully penetrates into the inside of the cell structure below, the furnace is cooled to form.

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

[0038] In this embodiment, the interface cell structure electrode designed and prepared is a zirconium alloy / tungsten / zirconium layered electrode. The specific electrode service condition is electrical discharge machining of through holes, and the hole type is a circular hole with a diameter of 40±0.5 mm. The electrode is discharged and ablated in an air atmosphere, and the service life of a single rod-shaped electrode is not less than 5 h. According to the above use requirements, the new electrode base material is designed to be pure zirconium, the interface cell wall material is pure tungsten, and the filling material is selected to be a zirconium-based alloy with a specific composition of Zr 65at.%, Nb 35at.%. The size of a single cell is set to 5 mm, a regular hexagonal cell configuration is adopted, and the thickness of the reinforcing layer is 4 mm.

[0039] Electrode material selection / structure design. Pure zirconium, pure tungsten and zirconium-based alloy are respectively used as the electrode base, cell wall and filling material. The cross-sectional shape of the electrode is circular with a diameter of about 40 mm. The size of a single cell is 5 mm, and the thickness of the reinforcing layer is 4 mm to complete the design of the zirconium alloy / tungsten / zirconium layered electrode.

[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 first constructed using three-dimensional modeling software, and the obtained cell model is pre-processed and sliced. Based on the optimized model, a selective laser melting forming technology is used to promote the formation of a metallurgical bond between the tungsten powder and the zirconium substrate at a relatively low laser power in the first 200 layers, and then the preset forming parameters are restored to ensure efficient and accurate forming of the tungsten cells.

[0041] Filling of zirconium-based material in the inner cavity of the tungsten cell. In this embodiment, a zirconium-based alloy is filled into the tungsten cell, and the melting point of the former is about 1500°C and the melting point of the latter is about 3422°C. To ensure that the liquid zirconium-based alloy is fully filled into the cell cavity during the infiltration forming process, the infiltration furnace temperature is set to 1700°C, and after maintaining at the corresponding temperature for 2 hours, the furnace is slowly cooled at a rate of about 100°C / h to form the electrode.

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

[0043] The new zirconium alloy / tungsten / zirconium electrode obtained based on the above series of steps is tested for arc discharge in an air atmosphere at an arc current of 40 A. A high-speed camera with a frame rate of 2000 fps is used to observe the electrode surface discharge under a 90% light filter condition, as shown in Figure 3As shown, the cell electrode is located at the bottom of the recording screen, and the top needle is the counter electrode. The filter capture screen shows that several clusters of cathode spots appear on the surface of the cell electrode, and each cluster is discrete and spaced 5-10 mm apart. Correspondingly, the arc root is forced to disperse 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. Further, the surface morphology of the cathode after discharging for 5 min is observed Figure 4 ), the burnout in the single cell of the cathode is uniform, and no ablation pit structure developing from the surface to the inside is observed, and the uniformity of the cathode arc discharge and ablation is excellent.

[0044] Comparative Example 1

[0045] In this comparative example, the service scene of the electrode is consistent with the embodiment, and both are in the environment of electric spark hole making, the hole type is a circle with a diameter of 40±0.5 mm, and the cross-sectional diameter of the hole making matching rod electrode is 40 mm. Compared with the embodiment, the electrode structure used in the comparative example is a zirconium-based alloy / zirconium alloy layered composite metal, and no cell structure is adopted.

[0046] The zirconium-based alloy / zirconium alloy electrode structure has a zirconium-based alloy strengthening layer with a thickness of 4 mm. After preparing the corresponding strengthening layer on the surface of the pure zirconium substrate by the laser additive manufacturing method, the electrode surface with similar surface flatness is also obtained by high-speed milling.

[0047] Based on the obtained zirconium alloy / zirconium layered electrode, the arc discharge process of the electrode in an air atmosphere under an arc current of 40 A is tested, and the high-speed camera with a frame rate of 2000 fps is used to observe the discharge condition of the electrode surface under the condition of filter 90%, as shown in Figure 5 As shown, continuous and dense clusters of cathode spots appear on the surface of the electrode, each cluster is closely adjacent, and is concentrated in a small range on the surface of the cathode, and the distance between each other is less than 2 mm, indicating that the cathode occurs concentrated ablation at this time, reflecting that the layered electrode obtained by simply relying on surface strengthening cannot effectively disperse the arc and improve the service performance of the cathode. The microstructure of the cathode surface after discharging for 5 min is also observed, as shown in Figure 6 As shown, a large number of circular ablation pits with a diameter of 200 μm appear on the burnout surface, which is caused by the continuous residence of the cathode spots. After the formation of the nested ablation pits, they will gradually penetrate into the inside of the cathode, leading to the increase of the electrode ablation rate and accelerating the cathode burnout. The non-uniform ablation behavior of the zirconium alloy / zirconium layered electrode confirms that only relying on surface strengthening cannot fully improve the cathode arc ablation behavior.

Claims

1. A long-lifetime electrode structure based on cell-based arc segmentation, characterized in that, The ablation resistance requirements of the electrode surface and the electrical and thermal conductivity requirements of the substrate are decoupled by a layered composite structure. The electrode structure includes an ablation-resistant reinforcement layer and a highly conductive substrate. The ablation-resistant reinforcement layer is composed of a cell structure and a filler. A cell structure is formed on the polished surface of the electrode substrate. High-temperature molten metal is filled into the cell cavity. The surface cell structure disperses the arc root, restricts the migration of the cathode spot to force arc splitting, and reduces the input heat flux density.

2. A method for fabricating a long-lifetime electrode structure based on cell-unit structure arc segmentation as described in claim 1, characterized in that, Includes the following steps: Step 1: Electrode material selection and electrode structure design; Based on the characteristics of the electrode discharge environment, service current and voltage, determine the material types of the electrode substrate, cell structure and filler; Design the reinforcement layer thickness and cell size structural parameters according to the processing discharge time requirements; Step 2: Precision manufacturing of cell structures; Using the selective laser melting additive manufacturing method, a pre-designed cell structure is precisely formed on the polished electrode substrate surface, ensuring that multiple cell structure arrays are arranged on the substrate surface. Step 3: Filling the cell cavity with material; Using high-temperature melt infiltration filling forming technology, molten metal is filled into the cavity surrounding the cell structure in the forming furnace until the upper surface of the molten metal is level with the upper surface of the cell structure, and then the furnace is cooled and formed. Step 4: Post-processing; After properly clamping the electrode blank, mill the upper surface of the cell structure using high-speed milling to remove burrs, spheroidized particles and local protrusions, and obtain a smooth electrode surface.

3. The method for fabricating a long-life electrode structure based on cell-unit structure arc segmentation as described in claim 2, characterized in that, In step one, the specific principles for selecting electrode materials are as follows: 1) The substrate uses high-strength, high-conductivity metallic materials; 2) The cell structure is responsible for controlling the cathode spot and avoiding concentrated discharge. High melting point, ablation resistant, and low conductivity difficult-to-discharge metal materials are selected. 3) The filler material induces the cathode spot to form first and continue to discharge through a strong electron emission process. Metal materials with low work function and high melt viscosity are selected.

4. The method for fabricating a long-life electrode structure based on cell-structure arc segmentation as described in claim 2 or 3, characterized in that, In step one, the electrode structure design steps are as follows: The electrode structure includes the individual cell size and the overall reinforcement layer thickness. The individual cell size is not less than 6 mm and within 10 mm.

5. A method for fabricating a long-life electrode structure based on cell-based arc segmentation as described in claim 2 or 3, characterized in that, In step two, the cell structure adopts a regular hexagon or a square.

6. A method for fabricating a long-life electrode structure based on cell-based arc segmentation as described in claim 2 or 3, characterized in that, In step three, the process of filling the cell cavity with material uses high-temperature metal infiltration forming technology. The molten metal on the upper surface of the prepared cell structure is placed in a high-temperature infiltration furnace and heated to a temperature above the melting point of the molten metal and below the melting point of the cell structure. When the molten metal has fully infiltrated into the cell structure below, it is cooled and formed in the furnace.

7. The method for fabricating a long-life electrode structure based on cell-unit structure arc segmentation as described in claim 3, characterized in that, The low work function and high melt viscosity metal material is a copper-chromium alloy or a zirconium-based alloy.

8. The method for fabricating a long-life electrode structure based on cell-unit structure arc segmentation as described in claim 3, characterized in that, The high-melting-point, ablation-resistant, and low-conductivity non-discharge metal material is a tungsten alloy or a tantalum alloy.

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