A method for in-situ repairing of tungsten cathode defects by laser cladding tungsten-based composite material

Abstract

The present application relates to the technical field of laser cladding, and particularly relates to a method for in-situ repairing of laser cladding tungsten-based composite material of tungsten cathode defects. The method comprises the following steps: firstly, pre-treating the defect part to completely remove the corrosion layer; then, performing laser cladding in two steps: firstly, using a palladium-titanium carbide reinforced tungsten-based composite material recovery layer to recover the size and ensure the strength and toughness and compactness; and secondly, using a yttrium-zirconium doped tungsten alloy functional surface layer as a functional surface layer to improve high-temperature oxidation resistance, so that the method realizes metallurgical bonding of the repair layer and the substrate, and the repair body has high-temperature resistance and high-strength characteristics.

Description

Technical Field

[0001] This invention relates to the field of laser cladding technology, and in particular to an in-situ repair method for tungsten-based composite materials with laser cladding for defects in tungsten cathodes. Background Technology

[0002] In the process of preparing rare earth metals by molten salt electrolysis, tungsten cathodes operate in a high-temperature fluoride molten salt environment for extended periods. The area exposed near the molten salt surface experiences severe localized corrosion, known as "necking." This defect drastically reduces the effective conductive cross-sectional area of ​​the cathode, increases resistance, and can lead to overheating or even breakage, causing unplanned downtime and severely impacting production continuity and economic efficiency. Existing technologies primarily employ two solutions to address the "necking" defect in tungsten cathodes: one is complete replacement, where the failed cathode is entirely replaced with a new one. This method is costly, and undamaged material is wasted. The other is thermal spraying repair, such as spraying a tungsten or ceramic coating onto the defective area to restore dimensions. However, the thermally sprayed coating is mechanically bonded to the tungsten substrate, resulting in low bonding strength. Under high-temperature thermal shock and molten salt erosion conditions, the coating is prone to peeling off, leading to short repair life and poor reliability.

[0003] Therefore, there is an urgent need to develop an efficient method for repairing local defects in tungsten cathodes that can form a strong metallurgical bond with the substrate, possess good high-temperature performance of the repaired area, and adapt to harsh working conditions. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing an in-situ repair method for tungsten cathode defects using laser cladding of tungsten-based composite materials. This method first pre-treats the defective area to completely remove the corrosion layer. Then, laser cladding is performed in two steps: first, a palladium-titanium carbide-reinforced tungsten-based composite material recovery layer is used to restore dimensions and ensure strength, toughness, and density; then, a yttrium-zirconium-doped tungsten alloy functional surface layer is used to enhance high-temperature oxidation resistance. This method achieves a metallurgical bond between the repair layer and the substrate, resulting in a repaired body that combines high-temperature resistance and high strength.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a method for in-situ repair of tungsten cathode defects by laser cladding of tungsten-based composite materials, comprising the following steps:

[0006] Step S1: Pretreatment: The necked area of ​​the defective tungsten cathode is machined on a lathe with a single-sided cutting depth of at least 1.0 mm beyond the corrosion depth until the tungsten cathode substrate is completely exposed. The surface roughness Ra after machining is 4.2-5.4 μm. Then the workpiece is ultrasonically cleaned in acetone and anhydrous ethanol for 15 min each to obtain the pretreated defective tungsten cathode.

[0007] Step S2: Preparation of laser cladding recovery layer: Spheroidized sintered mixed powder and pure tungsten powder are mixed at a mass ratio of 1:3 as the cladding raw material for recovery layer; using a laser cladding system, under argon protection, the pretreated defective tungsten cathode is subjected to overlapping laser cladding, thereby forming a recovery layer on the surface of the pretreated defective tungsten cathode;

[0008] The laser process parameters for preparing the restoration layer are: laser power 1800W, spot diameter 2mm, scanning speed 6-7mm / s, powder feeding rate 11-12 g / min, overlap rate 40%, and cladding to restore the original cathode design size.

[0009] Step S3: Laser cladding of functional surface layer: Doped tungsten alloy powder is used as the cladding material for functional surface layer; a laser cladding system is used to laser clad the surface of the recovery layer obtained in the above steps under argon protection, thereby forming a functional surface layer on the surface of the recovery layer;

[0010] The laser process parameters for preparing the functional surface layer are as follows: laser power 1500W, spot diameter 2mm, scanning speed 8-8.2mm / s, powder feeding rate 9-10g / min, overlap rate 45%, cladding single layer, thickness approximately 0.4-0.5mm.

[0011] The spheroidized sintered mixed powder is prepared by the following steps:

[0012] Step A1: Mix 20-25% palladium powder and 75-80% titanium carbide powder by mass percentage, and transfer them to a ball mill jar. Under argon protection, at a speed of 300 rpm and a ball-to-powder ratio of 3:1, ball mill for 24 h. Using anhydrous ethanol as the system, obtain a mixed powder. Transfer the mixed powder to a graphite mold and press it for 15 min under a pressure of 4.0 MPa. Then, under argon protection, heat it to 700℃ at a heating rate of 10℃ / min and hold it for 8-10 min. Then, heat it to 1450℃ at a heating rate of 20℃ / min and hold it for 60 min. Cool it to obtain a sintered sample.

[0013] Step A2: Cut, crush and transfer the sintered sample into a ball mill. Under argon protection, at a speed of 300 rpm and a ball-to-material ratio of 12:1, ball mill for 48 hours. Use anhydrous ethanol as the system and then sieve it through a 200-250 mesh sieve to obtain sintered mixed powder.

[0014] Step A3: The sintered mixed powder is spheroidized using a radio frequency plasma spheroidizing device. The sintered mixed powder is fed into the spheroidizing chamber using a vibrating powder feeder. Argon is used as the main gas. By ionizing the argon, a high-power plasma flame capable of spheroidizing the sintered mixed powder is formed. The specific spheroidizing parameters are as follows: the central gas is argon with a flow rate of 40 L / min, the protective layer gas is argon with a flow rate of 80 L / min, the carrier gas is argon with a flow rate of 3 L / min, the powder feeding rate is 15 g / min, the pressure is 80 kPa, and the plasma power is 15 kW. The sintered mixed powder is then spheroidized to obtain spheroidized sintered mixed powder.

[0015] The doped tungsten alloy powder is prepared by the following steps:

[0016] Step B1: Yttrium hexahydrate, zirconium nitrate pentahydrate and deionized water are mixed and added to the reaction vessel. The mixture is ultrasonically dispersed for 15 min. The mixture is stirred for 15 min at a stirring rate of 300 rpm and at room temperature. Then ammonium metatungstate solution is added, the temperature is raised to 90℃ and stirred for 4 h. The mixture is cooled to room temperature and then ethanol is added. The mixture is stirred for 10-12 h, cooled, filtered, and washed to obtain the precursor powder.

[0017] Furthermore, in step B1, the ratio of yttrium nitrate, zirconium nitrate, deionized water, ammonium metatungstate solution, and ethanol is 5.7-6g: 21-23g: 180mL: 800-900mL: 40mL, wherein the molar concentration of the ammonium metatungstate solution is 0.2mol / L.

[0018] Step B2: Transfer the precursor powder to a tube furnace and heat it to 550-600℃ at a heating rate of 10℃ / min. Calcinate for 4-5 hours, cool, and transfer to a ball mill. Under argon protection, at a rotation speed of 300 rpm and a ball-to-material ratio of 10:1, ball mill for 24 hours. Using anhydrous ethanol as the system, pass the powder through a 200-250 mesh sieve to obtain the calcined precursor powder. Then, reduce the calcined precursor powder using a hydrogen reduction furnace to obtain doped tungsten alloy powder.

[0019] The reduction stage employs a two-stage reduction method. The first stage reduction temperature is 700-720℃, and the reduction time is 2 hours. The second stage reduction temperature is 900-920℃, and the reduction time is 2 hours.

[0020] Furthermore, in the preparation process of the doped tungsten alloy powder, yttrium hexahydrate and zirconium pentahydrate are used as yttrium source and zirconium source, respectively, and ammonium metatungstate is added as tungsten source. The precursor powder is obtained by azeotropic distillation, and then calcined to form calcined precursor powder of zirconium oxide, yttrium oxide and tungsten oxide. Then, the tungsten oxide is reduced by a two-stage reduction method to form doped tungsten alloy powder. The reduction reaction equation is as follows;

[0021] ;

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention discloses an in-situ repair method for tungsten-based composite materials by laser cladding of tungsten cathode defects; the method first pre-treats the defect site to completely remove the corrosion layer; then laser cladding is performed in two steps: first, a palladium-titanium carbide-reinforced tungsten-based composite material recovery layer is used to restore the dimensions and ensure strength, toughness and density; then, a yttrium-zirconium-doped tungsten alloy functional surface layer is used as the functional surface layer to improve high-temperature oxidation resistance. The method realizes the metallurgical bonding between the repair layer and the substrate, and the repaired body has both high strength and high-temperature resistance.

[0023] This invention employs a simple mechanical processing method to create a clean metallurgical bonding surface, thereby improving the problem of weak bonding between the recovery layer and the substrate. Step S1 thoroughly removes all loose and oxidized corrosion layers through turning, exposing a clean, dense, and active tungsten metal surface and forming a certain roughness. This provides an ideal interface for direct interatomic diffusion and metallurgical bonding between the molten pool and the substrate during subsequent laser cladding, which is the physicochemical basis for achieving high bonding strength.

[0024] This invention employs a palladium-titanium carbide composite reinforced tungsten-based cladding material system. Through liquid-phase palladium-assisted wetting and in-situ strengthening with titanium carbide, it solves the problems of poor density and easy cracking of the cladding layer. In the laser cladding process of step S2, the palladium powder has a lower melting point and melts first to form a liquid phase, which greatly improves the wettability and fluidity of the high-melting-point tungsten powder in the molten pool, promotes the discharge of pores and densification. At the same time, the high-hardness titanium carbide particles are dispersed in the tungsten matrix as a reinforcing phase, which plays a role in pinning grain boundaries and hindering crack propagation, significantly improving the hardness and toughness of the cladding layer, enabling it to withstand subsequent processing and working stress.

[0025] This invention employs an azeotropic distillation method combined with two-stage hydrogen reduction to prepare doped tungsten alloy powder. This powder forms a protective oxide film through the active element effect, solving the problem of insufficient high-temperature oxidation resistance of the repaired area. In step S3, the yttrium and zirconium elements in the doped tungsten alloy powder selectively diffuse to the surface and are oxidized during high-temperature service, forming a thin, dense, and strongly adherent yttrium oxide composite oxide protective film. This film effectively blocks the inward diffusion of corrosive media, significantly improving the high-temperature oxidation resistance of the repaired area above the molten salt surface, thereby delaying secondary oxidation failure of the repaired area. Attached Figure Description

[0026] Appendix Figure 1 This is a process flow diagram proposed in this invention. Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0028] Preparation Example 1: The spheroidized sintered mixed powder was prepared by the following steps:

[0029] Step A1: Mix 25% palladium powder and 75% titanium carbide powder by mass percentage, and transfer them to a ball mill jar. Under argon protection, at a speed of 300 rpm and a ball-to-powder ratio of 3:1, ball mill for 24 h. Using anhydrous ethanol as the system, obtain a mixed powder. Transfer the mixed powder to a graphite mold and press it for 15 min under a pressure of 4.0 MPa. Then, under argon protection, heat it to 700℃ at a heating rate of 10℃ / min and hold it for 8-10 min. Then, heat it to 1450℃ at a heating rate of 20℃ / min and hold it for 60 min. Cool it to obtain a sintered sample.

[0030] Step A2: Cut, crush and transfer the sintered sample into a ball mill. Under argon protection, at a speed of 300 rpm and a ball-to-material ratio of 12:1, ball mill for 48 hours. Use anhydrous ethanol as the system and then sieve it through a 200-250 mesh sieve to obtain sintered mixed powder.

[0031] Step A3: The sintered mixed powder is spheroidized using a radio frequency plasma spheroidizing device. The sintered mixed powder is fed into the spheroidizing chamber using a vibrating powder feeder. Argon is used as the main gas. By ionizing the argon, a high-power plasma flame capable of spheroidizing the sintered mixed powder is formed. The specific spheroidizing parameters are as follows: the central gas is argon with a flow rate of 40 L / min, the protective layer gas is argon with a flow rate of 80 L / min, the carrier gas is argon with a flow rate of 3 L / min, the powder feeding rate is 15 g / min, the pressure is 80 kPa, and the plasma power is 15 kW. The sintered mixed powder is then spheroidized to obtain spheroidized sintered mixed powder.

[0032] Preparation Example 2: Compared with Preparation Example 1, the ratio of palladium powder to titanium carbide powder in this preparation example is adjusted to 20:80, and the other steps are the same.

[0033] Preparation Example 3: In this preparation example, compared with Preparation Example 1, the ratio of palladium powder to titanium carbide powder was adjusted to 10:90, and the other steps were the same.

[0034] Preparation Example 4: In this preparation example, the ratio of palladium powder to titanium carbide powder was adjusted to 40:60, while the other steps were the same.

[0035] Preparation Example 5: Doped tungsten alloy powder was prepared by the following steps:

[0036] Step B1: Yttrium hexahydrate, zirconium nitrate pentahydrate and deionized water are mixed and added to the reaction vessel. The mixture is ultrasonically dispersed for 15 min. The mixture is stirred for 15 min at a stirring rate of 300 rpm and at room temperature. Then ammonium metatungstate solution is added, the temperature is raised to 90℃ and stirred for 4 h. The mixture is cooled to room temperature and then ethanol is added. The mixture is stirred for 10-12 h, cooled, filtered, and washed to obtain the precursor powder.

[0037] Furthermore, in step B1, the ratio of yttrium nitrate, zirconium nitrate, deionized water, ammonium metatungstate solution, and ethanol is 6g:21g:180mL:900mL:40mL, wherein the molar concentration of the ammonium metatungstate solution is 0.2mol / L.

[0038] Step B2: Transfer the precursor powder to a tube furnace and heat it to 550-600℃ at a heating rate of 10℃ / min. Calcinate for 4-5 hours, cool, and transfer to a ball mill. Under argon protection, at a rotation speed of 300 rpm and a ball-to-material ratio of 10:1, ball mill for 24 hours. Using anhydrous ethanol as the system, pass the powder through a 200-250 mesh sieve to obtain the calcined precursor powder. Then, reduce the calcined precursor powder using a hydrogen reduction furnace to obtain doped tungsten alloy powder.

[0039] The reduction stage employs a two-stage reduction method. The first stage reduction temperature is 700-720℃, and the reduction time is 2 hours. The second stage reduction temperature is 900-920℃, and the reduction time is 2 hours.

[0040] Preparation Example 6: Compared with Preparation Example 5, the amounts of yttrium nitrate, zirconium nitrate and ammonium metatungstate solution in this preparation example were adjusted to 5.7g:23g:800mL, while the other steps were the same.

[0041] Preparation Example 7: Compared with Preparation Example 5, the amounts of yttrium nitrate, zirconium nitrate and ammonium metatungstate solution in this preparation example are adjusted to 2g:27g:900mL, while the other steps are the same.

[0042] Preparation Example 8: Compared with Preparation Example 5, the amounts of yttrium nitrate, zirconium nitrate and ammonium metatungstate solution in this preparation example are adjusted to 6g:23g:500mL, while the other steps are the same.

[0043] Example 1: An in-situ repair method for tungsten cathode defects by laser cladding of tungsten-based composite materials, comprising the following steps:

[0044] Step S1: Pretreatment: The necked area of ​​the defective tungsten cathode is machined on a lathe with a single-sided cutting depth of at least 1.0 mm beyond the corrosion depth until the tungsten cathode substrate is completely exposed. The surface roughness Ra after machining is 4.2-5.4 μm. Then the workpiece is ultrasonically cleaned in acetone and anhydrous ethanol for 15 min each to obtain the pretreated defective tungsten cathode.

[0045] Step S2: Preparation of laser cladding recovery layer: The spheroidized sintered mixed powder of Preparation Example 1 and pure tungsten powder are mixed at a mass ratio of 1:3 as the cladding material for the recovery layer; the laser cladding system is used to perform overlapping laser cladding on the pretreated defective tungsten cathode under argon protection, thereby forming a recovery layer on the surface of the pretreated defective tungsten cathode;

[0046] The laser process parameters for preparing the restoration layer are: laser power 1800W, spot diameter 2mm, scanning speed 6mm / s, powder feeding rate 12 g / min, overlap rate 40%, and cladding to restore the original cathode design size.

[0047] Step S3: Laser cladding of functional surface layer: Tungsten alloy powder doped in Preparation Example 5 is used as the cladding material for functional surface layer; a laser cladding system is used to laser clad the surface of the recovery layer obtained in the above steps under argon protection, thereby forming a functional surface layer on the surface of the recovery layer;

[0048] The laser process parameters for preparing the functional surface layer are as follows: laser power 1500W, spot diameter 2mm, scanning speed 8mm / s, powder feeding rate 10g / min, overlap rate 45%, cladding single layer, thickness approximately 0.4-0.5mm.

[0049] Example 2: Compared with Example 1, the laser process parameters for preparing the recovery layer are adjusted to: scanning speed 7 mm / s, powder feeding rate 11 g / min, and the laser process parameters for preparing the functional surface layer are adjusted to: scanning speed 8.2 mm / s, powder feeding rate 9 g / min. Other steps are the same.

[0050] Example 3: Compared with Example 1, the spheroidized sintered mixed powder in this example was changed to the one obtained in Preparation Example 2, while the other steps were the same.

[0051] Example 4: Compared with Example 1, the spheroidized sintering mixed powder in this example was changed to the one obtained in Preparation Example 3, while the other steps were the same.

[0052] Example 5: Compared with Example 1, the spheroidized sintered mixed powder in this example was changed to the one obtained in Preparation Example 4, while the other steps were the same.

[0053] Example 6: Compared with Example 1, the tungsten alloy powder doped with tungsten was adjusted to be the same as that obtained in Example 6, while the other steps were the same.

[0054] Example 7: Compared with Example 1, the tungsten alloy powder doped with tungsten was adjusted to be the same as that obtained in Preparation Example 7, while the other steps were the same.

[0055] Example 8: Compared with Example 1, the tungsten alloy powder doped with tungsten was adjusted to be the same as that obtained in Preparation Example 8, while the other steps were the same.

[0056] Comparative Example 1: Compared with Example 3, this comparative example does not perform the S1 pretreatment step, but the other steps are the same.

[0057] Comparative Example 2: Compared with Example 3, in step S2, the cladding material is only tungsten powder, and no spheroidizing sintering mixed powder is added. The other steps are the same.

[0058] Comparative Example 3: Compared with Example 3, this comparative example does not perform the laser cladding functional surface layer preparation in step S3, but the other steps are the same.

[0059] Using the processes of Examples 1-8 and Comparative Examples 1-3, defective tungsten cathodes under similar "neck-down" defect conditions were repaired to obtain samples;

[0060] The sample was immersed in neodymium fluoride-lithium fluoride molten salt at 900℃ for 7 days. The sample was then cut along the axis, and the maximum corrosion depth at the coating / substrate interface and the maximum corrosion depth at the functional surface layer / recovery layer interface were observed and measured to evaluate the coating's resistance to molten salt corrosion. Following the static test, the sample was immersed in molten salt and rotated at 100 rpm. Argon gas was used as a simulated gas to blow along the molten salt plane at a flow rate of 0.5 L / min for 3 days. The maximum corrosion depth at the coating / substrate interface was measured using the same method. Simultaneously, according to the above test methods, the mass was measured before the molten salt corrosion experiment, and the weight change rate was calculated as: weight change rate = (initial weight - post-experiment weight) / initial weight × 100%.

[0061] The sample was kept in a muffle furnace at 950℃ for 10 minutes, and then quickly immersed in flowing deionized water at 25℃ for 10 minutes. This is one cycle. The number of cycles was recorded when obvious cracking or peeling occurred in the restored layer or surface layer after repair, observed visually or under an optical microscope.

[0062] The volume resistivity of the repaired area and its adjacent original substrate area was measured using a four-probe resistance meter, and the conductivity recovery rate was calculated. The resistance recovery rate = resistivity of the repaired area / resistivity of the adjacent original substrate area × 100%;

[0063] The test results are shown in Table 1 below:

[0064] Table 1 Test Results

[0065] Test item Static immersion maximum corrosion depth (pm) Dynamic immersion maximum corrosion depth (pm) Weight change rate (%) Cycle number (times) Resistance recovery rate (%) Example 1 25.31 40.75 1.24 >60 94.5 Example 2 28.61 45.70 1.41 >60 92.1 Example 3 26.82 42.53 1.30 57 95.2 Example 4 28.07 44.93 1.39 51 95.1 Example 5 30.47 48.18 1.54 >60 93.6 Example 6 26.82 51.46 1.27 >60 94.8 Example 7 29.42 47.85 1.43 >60 94.3 Example 8 23.17 36.51 1.14 >60 90.7 Comparative Example 1 27.16 41.04 1.28 16 84.2 Comparative Example 2 32.19 65.40 1.97 22 93.7 Comparative Example 3 92.71 >120 -6.52 47 81.4

[0066] As shown in the table, the test results indicate that Examples 1 and 2 were obtained using similar processes to Preparation Examples 1 and 2 according to the present invention. However, because the laser process parameters were changed in Example 2 compared to Example 1, the scanning rate and powder feeding rate were mismatched, resulting in a slight decrease in the corrosion performance of the repaired tungsten cathode obtained in Example 2 compared to Example 1. Nevertheless, it performed well in the thermal shock cycle test, indicating that the process itself can achieve excellent bonding between the recovery layer and the substrate. In Example 3, the spheroidized sintering mixed powder was adjusted to be the same as that obtained in Preparation Example 2. The ratio of palladium powder to titanium carbide in the spheroidized sintering mixed powder was adjusted. Since titanium carbide has better conductivity, the resistance recovery rate was improved to some extent compared to Example 1, but it lacked the necessary palladium liquid phase to assist in sintering. This affects the bonding force between the recovery layer and the substrate, leading to a certain degree of performance degradation during thermal shock cycling. Examples 4 and 5 further increased the proportion of titanium carbide and palladium powder in the spheroidized sintered mixed powder. Although the proportion of titanium carbide was increased in Example 4, its conductivity did not improve further, and its thermal shock cycling performance decreased again. Although the proportion of tungsten powder was increased in Example 5, its resistance to molten salt corrosion and conductivity decreased significantly. This indicates that the presence of palladium powder can effectively improve the bonding performance between the recovery layer and the substrate, but it will lead to a certain decrease in its conductivity. The introduction of tungsten carbide can improve the conductivity of the recovery layer to a certain extent, but the introduction of too much palladium powder or tungsten carbide will also lead to a decrease in performance. Therefore, it is necessary to select a good ratio.

[0067] Examples 1 and 6 used the tungsten-doped alloy powders prepared in Examples 5 and 6, respectively. The molecular weight ratio of zirconium to yttrium used in Examples 1 and 6 was approximately 3:1. Furthermore, the proportions of zirconium oxide and yttrium oxide in the surface layer of Example 7 were lower than those in Example 8. Therefore, due to the increased tungsten powder content in the tungsten-doped alloy powder, the molten salt corrosion barrier performance of the surface layer in Example 6 decreased to some extent, a decrease clearly observed in dynamic corrosion tests. Example 7 adjusted the tungsten-doped alloy powder to that prepared in Example 7, resulting in a significant change in the zirconium to yttrium ratio, leading to a decrease in the protective performance of its surface layer. Example 9 adjusted the tungsten-doped alloy powder to that prepared in Example 8, resulting in a significant change in the tungsten content in its surface layer. Since zirconium oxide and yttrium oxide have higher resistivity than tungsten, the resistivity of the high zirconium oxide / yttrium oxide surface layer significantly increased, which is detrimental to the use of the repaired tungsten cathode. Therefore, it is necessary to adjust the proportion of yttrium and zirconium in the tungsten-doped alloy powder to ensure good conductivity while achieving a certain level of molten salt corrosion protection.

[0068] Example 1 is compared with Comparative Example 1, Comparative Example 2 and Comparative Example 3;

[0069] Because Comparative Example 1 did not perform the S1 step for thorough mechanical pretreatment of the corrosion defect area, its laser cladding process was carried out directly on the residual tungsten oxide and other corrosion layers. This corrosion layer is characterized by being porous, loose, and weakly bonded to the substrate, and its coefficient of thermal expansion differs greatly from that of metallic tungsten. During the intense temperature fluctuations of the thermal shock test, the residual corrosion layer itself becomes a huge source of thermal resistance and stress concentration: on the one hand, it hinders effective heat conduction and coordinated deformation between the cladding layer and the tungsten substrate; on the other hand, the corrosion layer-substrate interface and the corrosion layer-cladding recovery layer interface are prone to cracking and peeling under thermal stress. This causes the repair system to peel off completely from these weak interfaces after several thermal cycles, resulting in an extremely low failure cycle count and an inability to establish a strong interface that can resist high-cycle thermal stress.

[0070] Because Comparative Example 2 only used pure tungsten powder for cladding in step S2, it lacked the dual mechanism of palladium liquid-phase assisted sintering and titanium carbide particle dispersion enhancement provided by the spheroidized sintering mixed powder in this invention. Although the pure tungsten cladding layer achieved metallurgical bonding with the substrate, its solidification structure was relatively coarse. Due to the high brittleness of tungsten itself, it was prone to micro-thermal cracks during rapid solidification. During thermal shock cycles, these micro-cracks became the starting point of stress concentration and continued to expand and connect under the drive of alternating thermal stress, eventually forming macro-cracks that led to coating failure. At the same time, the toughness of the pure tungsten layer was relatively poor, and its resistance to crack propagation was insufficient. Therefore, although its failure cycle count was higher than that of Comparative Example 1, it was significantly lower than that of Example 1. This proves that the synergistic toughening effect of palladium in improving wettability and promoting densification, and titanium carbide particles in pinning grain boundaries and hindering crack propagation, is crucial for improving the thermal fatigue life of the repair layer.

[0071] Because Comparative Example 3 did not undergo step S3 to prepare the tungsten-doped alloy functional surface layer, its outermost layer of the repair was the melt recovery layer formed in step S2. The palladium and titanium carbide in this surface material lacked stability in a high-temperature oxidizing environment. During the high-temperature holding stage of the thermal shock test, the surface layer would oxidize, which could also lead to the oxidation of titanium carbide. The formation of these oxidation products was accompanied by volume changes and had weak adhesion to the metal matrix. When quenched in cold water, the surface oxide film was prone to cracking or peeling due to its high brittleness and large thermal mismatch with the matrix. As the number of cycles increased, this oxidation-peeling process occurred repeatedly, causing the repair layer surface to be continuously consumed and become rough and porous, ultimately leading to loss of protective function and further bulk corrosion, manifested as a low failure cycle count. Furthermore, it lacked the selective oxidation of active elements in the functional surface layer to form a protective film, resulting in a lack of effective anti-oxidation barrier on the repair surface at high temperatures, thus accelerating the degradation process under thermal shock conditions.

[0072] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for in-situ repair of tungsten cathode defects by laser cladding of tungsten-based composite materials, characterized in that: Includes the following steps: Step S1: Pretreatment: The necked area of ​​the defective tungsten cathode was machined on a lathe until the tungsten cathode substrate was completely exposed. Then the workpiece was ultrasonically cleaned in acetone and anhydrous ethanol for 15 minutes each to obtain the pretreated defective tungsten cathode. Step S2: Preparation of laser cladding recovery layer: Mix spheroidized sintered mixed powder with pure tungsten powder as the cladding material for recovery layer; use a laser cladding system to perform overlapping laser cladding on the pretreated defective tungsten cathode under argon protection, thereby forming a recovery layer on the surface of the pretreated defective tungsten cathode; Step S3: Laser cladding of functional surface layer: Doped tungsten alloy powder is used as the cladding material for functional surface layer; a laser cladding system is used to laser clad the surface of the recovery layer obtained in the above steps under argon protection, thereby forming a functional surface layer on the surface of the recovery layer; Step A1: Mix 20-25% palladium powder and 75-80% titanium carbide powder by mass percentage, and transfer them to a ball mill jar. Under argon protection, at a speed of 300 rpm and a ball-to-powder ratio of 3:1, ball mill for 24 h. Using anhydrous ethanol as the system, obtain a mixed powder. Transfer the mixed powder to a graphite mold and press it for 15 min under a pressure of 4.0 MPa. Then, under argon protection, heat it to 700℃ at a heating rate of 10℃ / min and hold it for 8-10 min. Then, heat it to 1450℃ at a heating rate of 20℃ / min and hold it for 60 min. Cool it to obtain a sintered sample. Step A2: Cut, crush and transfer the sintered sample into a ball mill. Under argon protection, at a speed of 300 rpm and a ball-to-material ratio of 12:1, ball mill for 48 hours. Use anhydrous ethanol as the system and then sieve it through a 200-250 mesh sieve to obtain sintered mixed powder. Step A3: Sintered mixed powder is spheroidized using a radio frequency plasma spheroidizing device. The sintered mixed powder is fed into the spheroidizing chamber using a vibrating powder feeder. Argon is used as the main gas. By ionizing the argon, a high-power plasma flame capable of spheroidizing the sintered mixed powder is formed. The specific spheroidizing parameters are as follows: argon is used as the central gas with a flow rate of 40 L / min, argon is used as the protective layer gas with a flow rate of 80 L / min, argon is used as the carrier gas with a flow rate of 3 L / min, the powder feeding rate is 15 g / min, the pressure is 80 kPa, and the plasma power is 15 kW. The sintered mixed powder is then spheroidized to obtain spheroidized sintered mixed powder. The doped tungsten alloy powder is prepared by the following steps: Step B1: Mix yttrium hexahydrate, zirconium nitrate pentahydrate and deionized water and add them to the reaction vessel. Disperse by ultrasonication for 15-20 min. Stir at 300-400 rpm at room temperature for 10-15 min. Then add ammonium metatungstate solution, heat to 90-95℃ and stir for 3-4 h. Cool to room temperature, then add ethanol and stir for 10-12 h. Cool, filter, and wash to obtain precursor powder. Step B2: Transfer the precursor powder to a tube furnace and heat it to 550-600℃ at a heating rate of 10℃ / min. Calcinate for 4-5 hours, cool, and transfer to a ball mill. Under argon protection, at a rotation speed of 300 rpm and a ball-to-material ratio of 10:1, ball mill for 24 hours. Using anhydrous ethanol as the system, pass the powder through a 200-250 mesh sieve to obtain the calcined precursor powder. Then, reduce the calcined precursor powder using a hydrogen reduction furnace to obtain doped tungsten alloy powder.

2. The method for in-situ repair of tungsten cathode defects by laser cladding of tungsten-based composite materials according to claim 1, characterized in that: In step S1: the single-sided cutting depth exceeds the corrosion depth by at least 1.0 mm, and the surface roughness Ra after machining is 4.2-5.4 μm; In step S2, the mass ratio of spheroidized sintered mixed powder to pure tungsten powder is 1:

3.

3. The method for in-situ repair of tungsten cathode defects by laser cladding of tungsten-based composite materials according to claim 1, characterized in that: In step S2: the laser process parameters for preparing the restoration layer are: laser power 1800W, spot diameter 2mm, scanning speed 6-7mm / s, powder feeding rate 11-12 g / min, overlap rate 40%, and cladding to restore the original cathode design size; In step S3: the laser process parameters for preparing the functional surface layer are: laser power 1500W, spot diameter 2mm, scanning speed 8-8.2mm / s, powder feeding rate 9-10g / min, overlap rate 45%, cladding single layer, and thickness 0.4-0.5mm.

4. The method for in-situ repair of tungsten cathode defects by laser cladding of tungsten-based composite materials according to claim 1, characterized in that: In step B1, the ratio of yttrium nitrate, zirconium nitrate, deionized water, ammonium metatungstate solution, and ethanol is 5.7-6 g: 21-23 g: 160-180 mL: 800-900 mL: 30-40 mL, wherein the molar concentration of ammonium metatungstate solution is 0.2 mol / L.

5. The method for in-situ repair of tungsten cathode defects by laser cladding of tungsten-based composite materials according to claim 1, characterized in that: In step B2: the reduction stage adopts a two-stage reduction method. The first stage reduction temperature is 700-720℃ and the reduction time is 2h. The second stage reduction temperature is 900-920℃ and the reduction time is 2h.

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