Preparation process for increasing RRR value of welding seam of high-purity niobium plate

Abstract

The invention discloses a preparation process for increasing the RRR value of a welding seam of a high-purity niobium plate. The preparation process comprises the following steps: 1, a multi-cleaning process: carrying out ultrasonic oil removal, chemical slow-release polishing, ultrapure water cleaning and ultrapure water ultrasonic treatment on a high-purity niobium plate base material; 2, performing vacuum electron beam welding to obtain a welded niobium plate; 3, a surface treatment process is conducted, specifically, weld joint mechanical grinding, chemical slow-release polishing and ultrapure water washing are conducted on the welded niobium plate; and 4, detecting the RRR value and the surface roughness of the welding seam. Through the synergistic effect of the multiple cleaning processes, the vacuum electron beam welding and the post-welding surface treatment process, the impurity introduction risk before welding is effectively reduced, grain growth and impurity diffusion in a heat affected zone are remarkably inhibited in the welding process, the oxidation risk is avoided, microcracks and a residual oxidation layer are removed, the surface flatness is improved, and the welding quality is improved. The high-purity niobium welded niobium plate which is smooth in weld joint, good in surface quality and suitable for the field of superconducting materials, and the RRR value of the high-purity niobium welded niobium plate meets the requirement of a radio frequency superconducting cavity.

Description

Technical Field

[0001] This invention belongs to the field of superconducting material processing technology, specifically relating to a preparation process for improving the RRR value of weld seams in high-purity niobium plates. Background Technology

[0002] High-purity niobium plates (RRR>300) are key materials in the fabrication of superconducting cavities, and structural connections are often achieved through welding during manufacturing. However, traditional welding processes (such as argon arc welding) cause a significant decrease in the RRR value of the weld area under high temperatures, typically dropping to 40%–60% of the base material. This performance degradation mainly stems from grain coarsening in the heat-affected zone, segregation of impurities such as oxygen / nitrogen, and the accumulation of residual welding stress.

[0003] Due to the high melting point of niobium (2468℃), it readily adsorbs gas molecules in the air, making it difficult to obtain high-quality joints using conventional welding methods. Therefore, vacuum electron beam welding is widely used in the manufacturing process of superconducting niobium cavities. While electron beam welding can reduce some defects by lowering heat input, improper control of process parameters can still lead to grain boundary defects and microstructural inhomogeneities. Furthermore, although conventional electron beam welding requires a vacuum environment, it does not address the issues of pre-weld substrate contamination and post-weld surface defects, resulting in limited improvement in the RRR (Rapid Reduction Rate) value. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide a preparation process for improving the RRR value of high-purity niobium plate welds, addressing the shortcomings of the prior art. This method sequentially performs multi-stage cleaning, vacuum electron beam welding, and post-weld surface treatment on the high-purity niobium plate substrate. This achieves deep purification of the substrate, reduces the risk of impurity introduction before welding, significantly inhibits grain growth and impurity diffusion in the heat-affected zone during welding, avoids oxidation risks, and removes microcracks and residual oxide layers from the welded niobium plate. This optimizes the purity and superconducting properties of the welded niobium plate, achieving an RRR value of over 300 for the high-purity niobium weld with a surface roughness Ra ≤ 1.2 μm, meeting the requirements of high-precision welds for superconducting cavities. This solves the problem in the prior art where pre-weld substrate contamination and post-weld surface defects limit the improvement of the RRR value of welded niobium plates from high-purity niobium plates.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a preparation process for improving the RRR value of weld seams in high-purity niobium plates, characterized in that the method includes the following steps: Step 1: Multiple Cleaning Processes Step 101, Ultrasonic Degreasing: Immerse the high-purity niobium plate substrate in an alkaline solution containing sodium hydroxide for ultrasonic cleaning to remove surface grease and organic contaminants; Step 102, Chemical slow-release polishing: The high-purity niobium plate after ultrasonic degreasing in step 101 is subjected to heavy etching using a chemical slow-release polishing solution prepared from nitric acid, phosphoric acid and hydrofluoric acid to dissolve the surface niobium oxide layer and microscopic impurities. Step 103, Ultrapure water cleaning: Use ultrapure water to rinse the surface of the high-purity niobium plate after heavy etching in step 102 to remove residual acid. Step 104, Ultrapure Water Ultrasonic Cleaning: Place the high-purity niobium plate rinsed in Step 103 into ultrapure water for ultrasonic cleaning. Step 2, Vacuum Electron Beam Welding: The high-purity niobium plate, after ultrasonic cleaning in step 104, is welded under a vacuum of less than 10⁻⁻⁶. 3 Electron beam welding was performed under a high vacuum environment of Pa to obtain welded niobium plates; Step 3: Surface Treatment Process Step 301, Mechanical grinding of weld seam: Use a small flap wheel to grind the surface of the weld seam of the niobium plate welded in step two in one direction to remove oxide layer, weld beads and micro protrusions; Step 302, Chemical slow-release polishing: The welded niobium plate after unidirectional grinding in step 301 is placed in a chemical slow-release polishing solution prepared with nitric acid, phosphoric acid and hydrofluoric acid for heavy etching, and the surface is further smoothed by chemical dissolution. Step 303, Ultrapure water rinsing: Immediately place the heavily etched niobium plate from step 302 into ultrapure water for high-pressure rinsing to thoroughly remove residual acid. Step 4, Testing: After high-pressure rinsing in step 303, a sample was taken from the weld of the welded niobium plate. After milling, the RRR value of the weld was measured by the four-probe method. The results showed that the RRR test value was more than 75% of that of the high-purity niobium plate substrate, and the surface roughness Ra of the weld was ≤1.2μm.

[0006] Typically, in step 101, the sodium hydroxide content in the alkaline solution is 5% by mass, the ultrasonic cleaning frequency is 40 kHz, and the time is 2 min; in step 102, the heavy etching time is 3 min to 6 min; in step 103, the ultrapure water rinsing flow rate is 0.5 L / min, and the duration is 3 min; in step 104, the ultrasonic cleaning frequency is 30 kHz, and the time is 40 min to 60 min; in step four, the length × width of the sample is 100 mm × 5 mm, and the height is the weld height.

[0007] The above-mentioned preparation process for improving the RRR value of weld seams in high-purity niobium plates is characterized in that, in step 101, the high-purity niobium plate substrate is a high-purity niobium plate with a thickness of δ 2.8mm~3.3mm and a surface roughness Ra ≤ 3.2μm, composed of the following components by mass percentage: Ta ≤ 0.015%, Ni ≤ 0.001%, Zr ≤ 0.001%, Si ≤ 0.001%, Fe ≤ 0.001%, W ≤ 0.002%, Mo ≤ 0.002%, Ti ≤ 0.001%, Hf ≤ 0.001%, Cr ≤ 0.001%, C ≤ 0.001%, N ≤ 0.001%, O ≤ 0.001%, H ≤ 0.002%, with the balance being Nb. The high-purity niobium welding substrate is in an annealed state, with an RRR value of 350 or higher, and uniform thickness and good surface quality.

[0008] The above-mentioned preparation process for improving the RRR value of weld seams in high-purity niobium plates is characterized in that the chemical slow-release polishing solution in step 102 is prepared by mixing 40% nitric acid, 60% phosphoric acid, and 60% hydrofluoric acid in a volume ratio of 1:2.5:1. The above-mentioned preparation process for improving the RRR value of weld seams in high-purity niobium plates is characterized in that the thickness of the heavy etching in step 102 is 10 μm, and the temperature of the chemical slow-release polishing solution is controlled to be less than 20°C, and the flow rate is 1.0 L / min.

[0009] The above-mentioned preparation process for improving the RRR value of weld seams in high-purity niobium plates is characterized in that, in step two, the vacuum electron beam welding employs a spot welding beam current of 15mA~20mA, a welding beam current of 35mA~40mA, a welding voltage of 85kV, and a scanning speed of 400mm / min~600mm / min. Preferably, the scanning speed of vacuum electron beam welding in this invention is 500mm / min~600mm / min.

[0010] The above-mentioned preparation process for improving the RRR value of weld seams in high-purity niobium plates is characterized in that, during vacuum electron beam welding in step two, the welding temperature is monitored by an infrared thermal imager, and the heat input is controlled at 200℃~300℃ to ensure that the heat-affected zone is minimized.

[0011] The above-mentioned preparation process for improving the RRR value of high-purity niobium plate welds is characterized in that, in step 301, 200... 400 Use a 600-grit grinding wheel for unidirectional grinding.

[0012] The above-mentioned preparation process for improving the RRR value of weld seams in high-purity niobium plates is characterized in that the chemical slow-release polishing solution in step 302 is prepared by mixing 40% nitric acid, 60% phosphoric acid and 60% hydrofluoric acid in a volume ratio of 1:2:1, and the temperature of the chemical slow-release polishing solution is controlled to be less than 20°C, the flow rate is 1.0 L / min, and the time for heavy etching is 2 min to 5 min.

[0013] The above-mentioned preparation process for improving the RRR value of weld seams in high-purity niobium plates is characterized in that the ultrapure water flow rate during high-pressure rinsing in step 303 is 5 L / min, and the time is 20 min to 40 min.

[0014] The above-mentioned preparation process for improving the RRR value of weld seams in high-purity niobium plates is characterized in that, in step four, the test value of the weld seam RRR value is increased from 150 in the traditional process to over 300, and the weld seam width is greater than 3mm, with a surface roughness Ra≤1.2μm, which meets the high precision and superconducting performance requirements for welding radio frequency superconducting cavities.

[0015] The purity of high-purity niobium and the cleanliness of electron beam welding significantly affect the resistivity-resistivity (RRR) value and superconducting properties of the material. High-purity niobium (e.g., RRR value > 300) effectively reduces electron scattering caused by impurities, improving the quality factor (Q value) and acceleration gradient of the superconducting cavity. Introducing contaminants such as oxides and greases during welding can create interface defects, increase resistivity, lead to a decrease in RRR value, and potentially cause local quenching failure, disrupting the superconducting state. Therefore, strict cleanliness must be maintained throughout the welding process, including the welding substrate, or the environmental cleanliness must be controlled, ensuring that the RRR value at the weld is greater than 250 after welding to guarantee material performance. High-RRR niobium materials are suitable for large-scale scientific projects such as particle accelerators, but welding defects can pose long-term risks, necessitating process optimization and design improvements to ensure the reliability and performance of superconducting devices.

[0016] Compared with the prior art, the present invention has the following advantages: 1. This invention employs a multi-stage cleaning process. First, ultrasonic degreasing thoroughly removes grease from the surface of the high-purity niobium plate substrate (peeling rate of over 95%). Then, chemical slow-release polishing dissolves the niobium oxide layer on the surface of the high-purity niobium plate substrate. Finally, ultrapure water rinsing and ultrasonic cleaning with ultrapure water remove acid from the surface of the high-purity niobium plate substrate (residual amount <0.1ppm). This process can control the RRR value fluctuation of the high-purity niobium plate substrate within ±5%, achieving deep purification of the substrate surface, significantly reducing the risk of impurity introduction before welding, and laying a good clean foundation for weld quality.

[0017] 2. This invention employs vacuum electron beam welding in a high vacuum environment (as low as 10⁻⁻⁴). 3(Pa) First, a smaller beam current is used for spot welding. By precisely adjusting parameters such as welding current and resistance, the welding heat can be accurately controlled, avoiding thermal deformation and stress concentration in the thin niobium plate substrate caused by continuous welding. Furthermore, the heat-affected zone of spot welding is small, and metal deformation is less, significantly inhibiting grain growth and impurity diffusion in the heat-affected zone and avoiding oxidation risks. This method is suitable for welding superconducting cavities, which have high requirements for dimensional accuracy. Then, the beam current intensity is increased, which increases the arc heat input, moves the heat source position downward, forms a stable molten pool, increases the penetration depth, and improves the weld strength.

[0018] 3. This invention, by combining infrared thermal imager monitoring during vacuum electron beam welding, stabilizes the welding temperature within the range of 200℃ to 300℃, further limiting heat input and reducing the thermal impact on the surrounding materials of the niobium plate substrate. This effectively controls welding deformation and is particularly suitable for welding precision components with high accuracy requirements. Simultaneously, the 200℃ to 300℃ temperature range helps prevent harmful gases from contaminating the high-purity niobium weld and facilitates degassing and purification of the weld metal, resulting in a purer, higher-quality weld. This provides excellent controllability for the welding process, making welding parameters easy to adjust and offering strong process adaptability.

[0019] 4. The present invention employs a surface treatment process after welding, which uses mechanical grinding and chemical slow-release polishing to remove microcracks and residual oxide layers and improve surface smoothness. Ultrapure water rinsing thoroughly removes polishing fluid residue, thereby significantly eliminating micro-defects, reducing surface defect density, and further optimizing the purity and superconducting performance of the welded niobium plate.

[0020] 5. This invention, through step-by-step synergistic action, obtains high-purity niobium welded niobium plates with smooth welds, good surface quality, and RRR values ​​exceeding 300, and a surface roughness Ra ≤ 1.2 μm, meeting the requirements of superconducting cavities for high-precision and high RRR welds. This process provides a reliable solution for superconducting material joining technology, meets the requirements for radio frequency superconducting cavity fabrication technology, and provides a more reliable processing method for the application of high-purity niobium materials in large scientific devices such as superconducting materials, thereby indirectly promoting the development of superconducting materials.

[0021] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0022] Figure 1 These are photographs of the weld seam of the welded niobium plate and the metallographic structure of the weld center / fusion region, which were detected in step four of Embodiment 1 of the present invention.

[0023] Figure 2 These are photographs of the weld seam of the welded niobium plate and the metallographic structure of the weld center / fusion region, which were detected in step four of embodiment 4 of the present invention. Detailed Implementation

[0024] Example 1 This embodiment includes the following steps: Step 1: Multiple Cleaning Processes Step 101, Ultrasonic Degreasing: A high-purity niobium plate is selected as the substrate, composed of the following components by mass percentage: Ta 0.0074%, Ni ≤ 0.001%, Zr ≤ 0.001%, Si 0.001%, Fe < 0.001%, W 0.0009%, Mo 0.0004%, Ti < 0.001%, Hf < 0.001%, Cr < 0.001%, C < 0.0005%, N < 0.0005%, O < 0.0005%, H < 0.0001%, with the balance being Nb. This high-purity niobium plate substrate is prepared by forging, hot rolling, and cold rolling from an ingot. The high-purity niobium plate substrate is in the annealed state, with a thickness of δ 2.8 mm, a surface roughness Ra ≤ 3.2 μm, an RRR value of 417, and uniform thickness and good surface quality. The high-purity niobium plate substrate was immersed in an alkaline solution containing 5% sodium hydroxide and ultrasonically cleaned at a frequency of 40kHz for 2 minutes to remove surface grease and organic contaminants. Step 102, Chemical Slow-Release Polishing: Prepare a chemical slow-release polishing solution by mixing 40% nitric acid, 60% phosphoric acid, and 60% hydrofluoric acid at a volume ratio of 1:2.5:1. Then, place the high-purity niobium plate after ultrasonic degreasing in Step 101 into the solution for heavy etching for 3 minutes to dissolve the niobium oxide layer and microscopic impurities on the surface. The thickness of the heavy etching is about 10 μm. Strictly control the temperature of the chemical slow-release polishing solution to be less than 20°C and the flow rate to be 1.0 L / min.

[0025] Step 103, Ultrapure water cleaning: Use ultrapure water to rinse the surface of the high-purity niobium plate after heavy etching in step 102, with a flow rate of 0.5 L / min and a continuous flow rate of 3 min, to remove residual acid. Step 104, Ultrapure Water Ultrasonic Cleaning: Place the high-purity niobium plate rinsed in Step 103 into ultrapure water and ultrasonically clean it at a frequency of 30kHz for 40 minutes. Step 2, Vacuum Electron Beam Welding: The high-purity niobium plate, after ultrasonic cleaning in step 104, is welded under a vacuum of less than 10⁻⁻⁶. 3 Electron beam welding was performed under a high vacuum environment of Pa using welding parameters of 15mA spot welding current, 35mA welding current, 85kV welding voltage, and 400mm / min scanning speed. At the same time, the welding temperature was monitored by an infrared thermal imager, and the heat input was controlled at 200℃~300℃ to ensure that the heat-affected zone was minimized, resulting in a welded niobium plate with a weld width greater than 3mm. Step 3: Surface Treatment Process Step 301, Mechanical grinding of weld seams: Using a small flap wheel, grind 200mm diameter plates in sequence. 400 A 600-grit grinding wheel is used to grind the weld surface of the niobium plate in step two in one direction to remove the oxide layer, weld beads and micro protrusions. Step 302, Chemical Slow-Release Polishing: The welded niobium plate after unidirectional grinding in Step 301 is placed in a chemical slow-release polishing solution prepared by mixing 40% nitric acid, 60% phosphoric acid and 60% hydrofluoric acid in a volume ratio of 1:2:1 for heavy etching. The surface is further smoothed by chemical dissolution. The temperature of the chemical slow-release polishing solution is strictly controlled to be less than 20°C, the flow rate is 1.0 L / min, and the heavy etching time is 5 min. Step 303, Ultrapure water rinsing: Immediately place the heavily etched welded niobium plate from step 302 into ultrapure water for high-pressure rinsing. The ultrapure water flow rate is 5L / min and the time is 20min to completely remove residual acid. Step 4, Testing: After high-pressure rinsing in step 303, take a sample with a length × width of 100mm × 5mm and a height equal to the weld height from the weld seam of the welded niobium plate. After milling the sample, measure the weld RRR value using the four-probe method. The test value is shown in Table 1 and is 75% of the high-purity niobium plate substrate.

[0026] Figure 1 These are photographs of the weld seam of the niobium plate and the metallographic structure of the weld center / fusion region, taken in step four of this embodiment. Figure 1 It can be seen that the surface of the weld is flat and uniform, without defects such as cracks, lack of fusion, porosity, slag inclusions, or weld beads, and the grain structure of the weld area is uniform.

[0027] Example 2 This embodiment includes the following steps: Step 1: Multiple Cleaning Processes Step 101, Ultrasonic Degreasing: A high-purity niobium plate is selected as the substrate, composed of the following components by mass percentage: Ta 0.0074%, Ni ≤ 0.001%, Zr ≤ 0.001%, Si 0.001%, Fe < 0.001%, W 0.0009%, Mo 0.0004%, Ti < 0.001%, Hf < 0.001%, Cr < 0.001%, C < 0.0005%, N < 0.0005%, O < 0.0005%, H < 0.0001%, with the balance being Nb. This high-purity niobium plate substrate is prepared by forging, hot rolling, and cold rolling from an ingot. The high-purity niobium plate substrate is in the annealed state, with a thickness of δ 2.8 mm, a surface roughness Ra ≤ 3.2 μm, an RRR value of 417, and uniform thickness and good surface quality. The high-purity niobium plate substrate was immersed in an alkaline solution containing 5% sodium hydroxide and ultrasonically cleaned at a frequency of 40kHz for 2 minutes to remove surface grease and organic contaminants. Step 102, Chemical Slow-Release Polishing: Prepare a chemical slow-release polishing solution by mixing 40% nitric acid, 60% phosphoric acid, and 60% hydrofluoric acid at a volume ratio of 1:2.5:1. Then, place the high-purity niobium plate after ultrasonic degreasing in Step 101 into the solution for heavy etching for 6 minutes to dissolve the niobium oxide layer and microscopic impurities on the surface. The thickness of the heavy etching is about 10 μm. Strictly control the temperature of the chemical slow-release polishing solution to be less than 20°C and the flow rate to be 1.0 L / min.

[0028] Step 103, Ultrapure water cleaning: Use ultrapure water to rinse the surface of the high-purity niobium plate after heavy etching in step 102, with a flow rate of 0.5 L / min and a continuous flow rate of 3 min, to remove residual acid. Step 104, Ultrapure Water Ultrasonic Cleaning: Place the high-purity niobium plate rinsed in Step 103 into ultrapure water and ultrasonically clean it at a frequency of 30kHz for 50 minutes. Step 2, Vacuum Electron Beam Welding: The high-purity niobium plate, after ultrasonic cleaning in step 104, is welded under a vacuum of less than 10⁻⁻⁶. 3 Electron beam welding was performed under a high vacuum environment of Pa using welding parameters of 20mA spot welding current, 35mA welding current, 85kV welding voltage, and 500mm / min scanning speed. At the same time, the welding temperature was monitored by an infrared thermal imager, and the heat input was controlled at 200℃~300℃ to ensure that the heat-affected zone was minimized, resulting in a welded niobium plate with a weld width greater than 3mm. Step 3: Surface Treatment Process Step 301, Mechanical grinding of weld seams: Using a small flap wheel, grind 200mm diameter plates in sequence. 400 A 600-grit grinding wheel is used to grind the weld surface of the niobium plate in step two in one direction to remove the oxide layer, weld beads and micro protrusions. Step 302, Chemical Slow-Release Polishing: The welded niobium plate after unidirectional grinding in Step 301 is placed in a chemical slow-release polishing solution prepared by mixing 40% nitric acid, 60% phosphoric acid and 60% hydrofluoric acid in a volume ratio of 1:2:1 for heavy etching. The surface is further smoothed by chemical dissolution. The temperature of the chemical slow-release polishing solution is strictly controlled to be less than 20°C, the flow rate is 1.0 L / min, and the heavy etching time is 2 min. Step 303, Ultrapure water rinsing: Immediately place the heavily etched niobium plate from step 302 into ultrapure water for high-pressure rinsing. The ultrapure water flow rate is 5L / min and the time is 30min to completely remove residual acid. Step 4, Testing: After high-pressure rinsing in step 303, take a sample with a length × width of 100mm × 5mm and a height equal to the weld height from the weld seam of the welded niobium plate. After milling the sample, measure the weld RRR value using the four-probe method. The test value is shown in Table 1, which is 76% of the high-purity niobium plate substrate.

[0029] Example 3 This embodiment includes the following steps: Step 1: Multiple Cleaning Processes Step 101, Ultrasonic Degreasing: A high-purity niobium plate is selected as the substrate, composed of the following components by mass percentage: Ta 0.0070%, Ni ≤ 0.001%, Zr ≤ 0.001%, Si 0.001%, Fe < 0.001%, W 0.0008%, Mo 0.0004%, Ti < 0.001%, Hf < 0.001%, Cr < 0.001%, C < 0.0005%, N < 0.0005%, O < 0.0005%, H < 0.0001%, with the balance being Nb. This high-purity niobium plate substrate is prepared by forging, hot rolling, and cold rolling from an ingot. The high-purity niobium plate substrate is in the annealed state, with a thickness of δ 3.0 mm, a surface roughness Ra ≤ 3.2 μm, an RRR value of 396, and uniform thickness and good surface quality. The high-purity niobium plate substrate was immersed in an alkaline solution containing 5% sodium hydroxide and ultrasonically cleaned at a frequency of 40kHz for 2 minutes to remove surface grease and organic contaminants. Step 102, Chemical Slow-Release Polishing: Prepare a chemical slow-release polishing solution by mixing 40% nitric acid, 60% phosphoric acid, and 60% hydrofluoric acid at a volume ratio of 1:2.5:1. Then, place the high-purity niobium plate after ultrasonic degreasing in Step 101 into the solution for heavy etching for 6 minutes to dissolve the niobium oxide layer and microscopic impurities on the surface. The thickness of the heavy etching is about 10 μm. Strictly control the temperature of the chemical slow-release polishing solution to be less than 20°C and the flow rate to be 1.0 L / min.

[0030] Step 103, Ultrapure water cleaning: Use ultrapure water to rinse the surface of the high-purity niobium plate after heavy etching in step 102, with a flow rate of 0.5 L / min and a continuous flow rate of 3 min, to remove residual acid. Step 104, Ultrapure Water Ultrasonic Cleaning: Place the high-purity niobium plate rinsed in Step 103 into ultrapure water and ultrasonically clean it at a frequency of 30kHz for 60 minutes. Step 2, Vacuum Electron Beam Welding: The high-purity niobium plate, after ultrasonic cleaning in step 104, is welded under a vacuum of less than 10⁻⁻⁶. 3Electron beam welding was performed under a high vacuum environment of Pa using welding parameters of 20mA spot welding current, 40mA welding current, 85kV welding voltage, and 600mm / min scanning speed. At the same time, the welding temperature was monitored by an infrared thermal imager and the heat input was controlled at 200℃~300℃ to ensure that the heat-affected zone was minimized, resulting in a welded niobium plate with a weld width greater than 3mm. Step 3: Surface Treatment Process Step 301, Mechanical grinding of weld seams: Using a small flap wheel, grind 200mm diameter plates in sequence. 400 A 600-grit grinding wheel is used to grind the weld surface of the niobium plate in step two in one direction to remove the oxide layer, weld beads and micro protrusions. Step 302, Chemical Slow-Release Polishing: The welded niobium plate after unidirectional grinding in Step 301 is placed in a chemical slow-release polishing solution prepared by mixing 40% nitric acid, 60% phosphoric acid and 60% hydrofluoric acid in a volume ratio of 1:2:1 for heavy etching. The surface is further smoothed by chemical dissolution. The temperature of the chemical slow-release polishing solution is strictly controlled to be less than 20°C, the flow rate is 1.0 L / min, and the heavy etching time is 5 min. Step 303, Ultrapure water rinsing: Immediately place the heavily etched niobium plate from step 302 into ultrapure water for high-pressure rinsing. The ultrapure water flow rate is 5L / min and the time is 40min to completely remove residual acid. Step 4, Testing: After high-pressure rinsing in step 303, take a sample with a length × width of 100mm × 5mm and a height equal to the weld height from the weld seam of the welded niobium plate. After milling the sample, measure the weld RRR value using the four-probe method. The test values ​​are shown in Table 1, which is 80% of the high-purity niobium plate substrate.

[0031] Example 4 This embodiment includes the following steps: Step 1: Multiple Cleaning Processes Step 101, Ultrasonic Degreasing: A high-purity niobium plate is selected as the substrate, composed of the following components by mass percentage: Ta 0.0070%, Ni ≤ 0.001%, Zr ≤ 0.001%, Si 0.001%, Fe < 0.001%, W 0.0008%, Mo 0.0004%, Ti < 0.001%, Hf < 0.001%, Cr < 0.001%, C < 0.0005%, N < 0.0005%, O < 0.0005%, H < 0.0001%, with the balance being Nb. This high-purity niobium plate substrate is prepared by forging, hot rolling, and cold rolling from an ingot. The high-purity niobium plate substrate is in the annealed state, with a thickness of δ 3.3 mm, a surface roughness Ra ≤ 3.2 μm, an RRR value of 395, and uniform thickness and good surface quality. The high-purity niobium plate substrate was immersed in an alkaline solution containing 5% sodium hydroxide and ultrasonically cleaned at a frequency of 40kHz for 2 minutes to remove surface grease and organic contaminants. Step 102, Chemical Slow-Release Polishing: Prepare a chemical slow-release polishing solution by mixing 40% nitric acid, 60% phosphoric acid, and 60% hydrofluoric acid at a volume ratio of 1:2.5:1. Then, place the high-purity niobium plate after ultrasonic degreasing in Step 101 into the solution for heavy etching for 4 minutes to dissolve the niobium oxide layer and microscopic impurities on the surface. The thickness of the heavy etching is about 10 μm. Strictly control the temperature of the chemical slow-release polishing solution to be less than 20°C and the flow rate to be 1.0 L / min.

[0032] Step 103, Ultrapure water cleaning: Use ultrapure water to rinse the surface of the high-purity niobium plate after heavy etching in step 102, with a flow rate of 0.5 L / min and a continuous flow rate of 3 min, to remove residual acid. Step 104, Ultrapure Water Ultrasonic Cleaning: Place the high-purity niobium plate rinsed in Step 103 into ultrapure water and ultrasonically clean it at a frequency of 30kHz for 50 minutes. Step 2, Vacuum Electron Beam Welding: The high-purity niobium plate, after ultrasonic cleaning in step 104, is welded under a vacuum of less than 10⁻⁻⁶. 3 Electron beam welding was performed under a high vacuum environment of Pa using welding parameters of 18mA spot welding current, 40mA welding current, 85kV welding voltage, and 600mm / min scanning speed. At the same time, the welding temperature was monitored by an infrared thermal imager and the heat input was controlled at 200℃~300℃ to ensure that the heat-affected zone was minimized, resulting in a welded niobium plate with a weld width greater than 3mm. Step 3: Surface Treatment Process Step 301, Mechanical grinding of weld seams: Using a small flap wheel, grind 200mm diameter plates in sequence. 400 A 600-grit grinding wheel is used to grind the weld surface of the niobium plate in step two in one direction to remove the oxide layer, weld beads and micro protrusions. Step 302, Chemical Slow-Release Polishing: The welded niobium plate after unidirectional grinding in Step 301 is placed in a chemical slow-release polishing solution prepared by mixing 40% nitric acid, 60% phosphoric acid and 60% hydrofluoric acid in a volume ratio of 1:2:1 for heavy etching. The surface is further smoothed by chemical dissolution. The temperature of the chemical slow-release polishing solution is strictly controlled to be less than 20°C, the flow rate is 1.0 L / min, and the heavy etching time is 3 min. Step 303, Ultrapure water rinsing: Immediately place the heavily etched niobium plate from step 302 into ultrapure water for high-pressure rinsing. The ultrapure water flow rate is 5L / min and the time is 40min to completely remove residual acid. Step 4, Testing: After high-pressure rinsing in step 303, take a sample with a length × width of 100mm × 5mm and a height equal to the weld height from the weld seam of the welded niobium plate. After milling the sample, measure the weld RRR value using the four-probe method. The test value is shown in Table 1, which is 81% of the high-purity niobium plate substrate.

[0033] Figure 2 These are photographs of the weld seam of the niobium plate and the metallographic structure of the weld center / fusion region, taken in step four of this embodiment. Figure 2 It can be seen that the surface of the weld is flat and uniform, without defects such as cracks, lack of fusion, porosity, slag inclusions, or weld beads, and the grain structure of the weld area is uniform.

[0034] Comparative Example 1 This comparative example includes the following steps: Step 1: Multiple Cleaning Processes Step 101: Select a high-purity niobium plate as the substrate, which is composed of the following components by mass percentage: Ta 0.0074%, Ni≤0.001%, Zr≤0.001%, Si 0.001%, Fe<0.001%, W 0.0009%, Mo 0.0004%, Ti<0.001%, Hf<0.001%, Cr<0.001%, C<0.0005%, N<0.0005%, O<0.0005%, H<0.0001%, with the balance being Nb. This high-purity niobium plate substrate is prepared by forging, hot rolling, and cold rolling from an ingot. The high-purity niobium plate substrate is in the annealed state, with a thickness of δ2.8mm, a surface roughness Ra≤3.2μm, an RRR value of 417, and uniform thickness and good surface quality. The high-purity niobium plate substrate was immersed in an alkaline solution containing 5% sodium hydroxide and ultrasonically cleaned at a frequency of 40kHz for 2 minutes to remove surface grease and organic contaminants. Step 102, Chemical Slow-Release Polishing: Prepare a chemical slow-release polishing solution by mixing 40% nitric acid, 60% phosphoric acid, and 60% hydrofluoric acid at a volume ratio of 1:2.5:1. Then, place the high-purity niobium plate after ultrasonic degreasing in Step 101 into the solution for heavy etching for 3 minutes to dissolve the niobium oxide layer and microscopic impurities on the surface. The etching thickness is about 10 μm. Strictly control the temperature of the chemical slow-release polishing solution to be less than 20°C and the flow rate to be 1.0 L / min. Step 2, Vacuum Electron Beam Welding: The high-purity niobium plate, after heavy etching in step 102, is then welded under a vacuum of less than 10⁻⁻⁶. 3 Electron beam welding was performed under a high vacuum environment of Pa using welding parameters of 20mA welding beam current, 85kV welding voltage, and 400mm / min scanning speed to obtain welded niobium plates. Step 3, Testing: Take a sample with a length × width of 100mm × 5mm and a height equal to the weld height at the weld seam of the niobium plate in Step 2. After milling the sample, measure the RRR value of the weld seam using the four-probe method. The test value is shown in Table 1, which is 34% of the high-purity niobium plate substrate.

[0035] Comparative Example 2 The difference between this comparative example and Example 1 is that the high-purity niobium plate after ultrasonic cleaning in step two is subjected to a vacuum of 5×10⁻⁶. -2 Welding was carried out in a low vacuum environment of Pa; the test values ​​of the weld RRR of the obtained niobium plate are shown in Table 1, which is 54% of that of the high-purity niobium plate substrate.

[0036] The RRR value, weld width, and surface roughness Ra of the welded niobium plates detected in step four of Examples 1-4 and Comparative Examples 1-2 of this invention are shown in Table 1 below.

[0037] Table 1

[0038] As shown in Table 1, the RRR values ​​of the welded niobium plates prepared in Examples 1-4 of this invention have increased from less than 150 in the conventional process (comparative Example 1, which uses conventional chemical degreasing + pickling + vacuum electron beam welding) to over 300. These values ​​are all above 75% of the RRR values ​​of the high-purity niobium plate substrate, and are much higher than the RRR values ​​of the welded niobium plates prepared in Comparative Example 2 using electron beam welding under low vacuum. Furthermore, the weld width is greater than 3 mm, and the surface roughness Ra ≤ 1.2 μm, meeting the high precision and superconducting performance requirements for welding radio frequency superconducting cavities. This indicates that thorough cleaning before and after welding and the high vacuum of the welding machine are necessary conditions to ensure that the RRR values ​​meet the technical requirements.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A preparation process for improving the RRR value of weld seams in high-purity niobium plates, characterized in that, The method includes the following steps: Step 1: Multiple Cleaning Processes Step 101, Ultrasonic Degreasing: Immerse the high-purity niobium plate substrate in an alkaline solution containing sodium hydroxide for ultrasonic cleaning to remove surface grease and organic contaminants; Step 102, Chemical slow-release polishing: The high-purity niobium plate after ultrasonic degreasing in step 101 is subjected to heavy etching using a chemical slow-release polishing solution prepared from nitric acid, phosphoric acid and hydrofluoric acid to dissolve the surface niobium oxide layer and microscopic impurities. Step 103, Ultrapure water cleaning: Use ultrapure water to rinse the surface of the high-purity niobium plate after heavy etching in step 102 to remove residual acid. Step 104, Ultrapure Water Ultrasonic Cleaning: Place the high-purity niobium plate rinsed in Step 103 into ultrapure water for ultrasonic cleaning. Step 2, Vacuum Electron Beam Welding: The high-purity niobium plate, after ultrasonic cleaning in step 104, is welded under a vacuum of less than 10⁻⁻⁶. 3 Electron beam welding was performed under a high vacuum environment of Pa to obtain welded niobium plates; Step 3: Surface Treatment Process Step 301, Mechanical grinding of weld seam: Use a small flap wheel to grind the surface of the weld seam of the niobium plate welded in step two in one direction to remove oxide layer, weld beads and micro protrusions; Step 302, Chemical slow-release polishing: The welded niobium plate after unidirectional grinding in step 301 is placed in a chemical slow-release polishing solution prepared with nitric acid, phosphoric acid and hydrofluoric acid for heavy etching, and the surface is further smoothed by chemical dissolution. Step 303, Ultrapure water rinsing: Immediately place the heavily etched niobium plate from step 302 into ultrapure water for high-pressure rinsing to thoroughly remove residual acid. Step 4, Testing: After high-pressure rinsing in step 303, a sample was taken from the weld of the welded niobium plate. After milling, the RRR value of the weld was measured by the four-probe method. The results showed that the RRR test value was more than 75% of that of the high-purity niobium plate substrate, and the surface roughness Ra of the weld was ≤1.2μm.

2. The preparation process for improving the RRR value of high-purity niobium plate welds according to claim 1, characterized in that, The high-purity niobium plate substrate mentioned in step 101 is a high-purity niobium plate with a thickness of δ 2.8mm~3.3mm and a surface roughness Ra≤3.2μm, composed of the following components by mass percentage: Ta≤0.015%, Ni≤0.001%, Zr≤0.001%, Si≤0.001%, Fe≤0.001%, W≤0.002%, Mo≤0.002%, Ti≤0.001%, Hf≤0.001%, Cr≤0.001%, C≤0.001%, N≤0.001%, O≤0.001%, H≤0.002%, with the balance being Nb. The high-purity niobium welding substrate is in the annealed state, with an RRR value of ≥350, and has uniform thickness and good surface quality.

3. The preparation process for improving the RRR value of weld seams in high-purity niobium plates according to claim 1, characterized in that, The chemical slow-release polishing solution described in step 102 is prepared by mixing 40% nitric acid, 60% phosphoric acid, and 60% hydrofluoric acid in a volume ratio of 1:2.5:

1.

4. The preparation process for improving the RRR value of weld seams in high-purity niobium plates according to claim 1, characterized in that, In step 102, the thickness of the heavy etching is 10 μm, and the temperature of the chemical slow-release polishing solution is controlled to be less than 20°C, and the flow rate is 1.0 L / min.

5. The preparation process for improving the RRR value of weld seams in high-purity niobium plates according to claim 1, characterized in that, In step two, the vacuum electron beam welding uses a spot welding current of 15mA~20mA, a welding beam current of 35mA~40mA, a welding voltage of 85kV, and a scanning speed of 400mm / min~600mm / min.

6. The preparation process for improving the RRR value of weld seams in high-purity niobium plates according to claim 1, characterized in that, During the vacuum electron beam welding described in step two, the welding temperature is monitored by an infrared thermal imager, and the heat input is controlled at 200℃~300℃ to ensure that the heat-affected zone is minimized.

7. The preparation process for improving the RRR value of weld seams in high-purity niobium plates according to claim 1, characterized in that, In step 301, 200 are used sequentially. 400 Use a 600-grit grinding wheel for unidirectional grinding.

8. The preparation process for improving the RRR value of high-purity niobium plate welds according to claim 1, characterized in that, The chemical slow-release polishing solution described in step 302 is prepared by mixing 40% nitric acid, 60% phosphoric acid and 60% hydrofluoric acid in a volume ratio of 1:2:

1. The temperature of the chemical slow-release polishing solution is controlled to be less than 20°C, the flow rate is 1.0 L / min, and the time for heavy etching is 2 min to 5 min.

9. The preparation process for improving the RRR value of weld seams in high-purity niobium plates according to claim 1, characterized in that, In step 303, the ultrapure water flow rate during high-pressure rinsing is 5 L / min, and the time is 20 min to 40 min.

10. The preparation process for improving the RRR value of high-purity niobium plate welds according to claim 1, characterized in that, In step four, the test value of the weld RRR value is increased from 150 in the traditional process to over 300, and the weld width is greater than 3mm, with a surface roughness Ra≤1.2μm, which meets the high precision and superconducting performance requirements for welding radio frequency superconducting cavities.

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