Preparation method of RRR test sample of superconducting niobium ingot with low processing strain

Abstract

The application relates to a preparation method of a low-processing-strain superconducting ni ingot RRR test sample, and belongs to the technical field of superconducting material performance detection, and solves the problem of insufficient accuracy and consistency of RRR test data caused by the lack of sampling specification, large processing strain damage and non-uniform post-processing in the prior art. The method comprises the following steps: a circular cross-section sheet is cut from a superconducting ni ingot body by using low-speed water cooling sawing, a rectangular rough blank is sawed in the center area of the sheet; the rough blank is clamped on a milling machine positioning tool, a hard alloy milling cutter is used for layered milling, and a rectangular sample is prepared; the sample is subjected to vacuum stress relief heat treatment; and after the heat treatment, organic solvent ultrasonic cleaning and mixed acid chemical pickling are sequentially carried out. The application realizes low-strain, high-precision and standardized preparation of the superconducting ni ingot RRR test sample, and effectively reduces the dispersion of RRR detection data among parallel samples.

Description

Technical Field

[0001] This invention relates to the field of superconducting material performance testing technology, and in particular to a method for preparing RRR test specimens of superconducting niobium ingots with low processing strain. Background Technology

[0002] Residual Resistivity Ratio (RR) is a core indicator for evaluating the superconducting performance and batch quality of superconducting niobium ingots. Its value directly determines the superconducting critical performance and service stability of core components such as radio frequency superconducting resonators and high-energy particle accelerators. In the industrial-scale grading and quality control and factory inspection of superconducting niobium materials, the accuracy of RRR testing directly affects the calibration reliability of downstream superconducting components.

[0003] Currently, the sample preparation method for RRR testing of superconducting niobium ingots mainly refers to the provisions of IEC 61788-23 standard, which requires the sample cross-section to be 1 mm. 2 ~20mm 2 The aspect ratio is 10 to 25, and low-stress processing methods are recommended. However, specific sampling locations are not specified, quantitative processing parameters are not provided, and supporting tooling and standardized post-processing procedures are not offered. In practice, various institutions often adopt their own internal processes: sampling is done arbitrarily from parts of the ingot, easily reaching edge defect areas or gas-absorbing layers; processing often uses wire cutting, manual filing, or free milling without clamping, lacking effective cooling and positioning tooling, the billet is prone to displacement, and the parallelism of the formed sample is difficult to meet the requirements of the four-wire resistance test; cleaning and pickling parameters vary, and there is no unified stress-relieving heat treatment specification.

[0004] The existing technology has at least the following defects: (1) There is no standard for sampling, the RRR detection value is too low, and it cannot objectively reflect the intrinsic quality; (2) The sample size accuracy and parallelism are insufficient, and the processing is prone to introduce residual strain and surface oxidation, which affects the RRR measurement value; (3) There are no standardized parameters for post-processing, and the RRR data of parallel samples of the same ingot have high dispersion, poor consistency and repeatability. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a method for preparing RRR test specimens of superconducting niobium ingots with low processing strain, so as to at least solve one of the problems of insufficient accuracy and consistency of RRR test data caused by the lack of standardized sampling, large processing strain damage and inconsistent post-processing in the prior art.

[0006] On one hand, embodiments of the present invention provide a method for preparing a superconducting niobium ingot RRR test specimen with low processing strain, comprising the following steps: S1. Thin-section sawing: Circular cross-section thin slices are cut from the superconducting niobium ingot body using a low-speed water-cooled sawing method, with continuous water cooling throughout the sawing process; S2. Rough blank cutting: A cuboid rough blank is cut in the central area of ​​the circular cross-section sheet using a water-immersion cooled band saw; S3. Precision milling: The rough blank is clamped onto the positioning fixture of the milling machine, and the rough blank is milled in layers using a carbide end mill. The milling process is continuously cooled with deionized water to obtain a cuboid sample. S4. Vacuum stress-relieving heat treatment: The rectangular sample is placed in a vacuum heat treatment furnace for low-temperature stress-relieving annealing. S5. Post-purification treatment: The heat-treated cuboid sample is subjected to ultrasonic cleaning with organic solvent and chemical acid washing with mixed acid in sequence.

[0007] Furthermore, in step S1, the thickness of the circular cross-section sheet is 8-15 mm, and the sawing speed is 0.8-1.6 mm / min.

[0008] Furthermore, in step S2, the central region is a region not less than 10mm from the edge of the circular cross-section sheet; the cross-sectional dimensions of the blank are (6-8mm) × (6-8mm), and the length is 90-120mm.

[0009] Furthermore, in step S3, the single feed rate of the layer milling is 0.4–0.8 mm / pass; the cross-sectional area of ​​the resulting cuboid sample is 2 mm². 2 ~20mm 2 The length is 60~120mm, and the parallelism is no more than 0.5mm.

[0010] Furthermore, in step S4, the specific process parameters for the vacuum stress-relief heat treatment are: furnace vacuum degree ≤ 5 × 10⁻⁶. -3 Pa, heating rate 8-12℃ / min, constant temperature annealing temperature 250-350℃, constant temperature holding time 90-120min, and after holding, cooling to room temperature with the furnace.

[0011] Furthermore, in step S5, the ultrasonic cleaning with organic solvent includes: first, ultrasonic cleaning with acetone for 10-15 minutes at an ultrasonic cleaning power of 200-300W; then, ultrasonic cleaning with anhydrous ethanol for 10-15 minutes; and finally, rinsing with deionized water for at least 3 minutes after cleaning.

[0012] Further, in step S5, the mixed acid chemical pickling is performed by using a mixed acid solution of nitric acid, hydrofluoric acid and phosphoric acid in a volume ratio of (0.8-1.2):(0.8-1.2):(1.8-2.2) at 20-25°C for 5-8 minutes, followed by rinsing with flowing deionized water.

[0013] Furthermore, in step S4, the number of cuboid samples placed in a single furnace does not exceed 8, and the interval between each cuboid sample is not less than 20 mm.

[0014] On the other hand, the present invention also discloses a milling machine positioning fixture for the above-mentioned preparation method, comprising: The tooling base is used to fix the milling machine positioning tooling as a whole onto the milling machine worktable; A limiting bracket is fixed on the tooling base. A rectangular limiting groove is provided in the middle of the limiting bracket to accommodate the rough blank. The rectangular limiting groove is used to limit the lateral displacement of the rough blank during milling. A lead screw clamping mechanism is located at the front end of the limiting seat, and includes an adjusting lead screw and a locking nut. The adjusting lead screw is used to press the blank against it to achieve axial locking. The split-type reference pressure block is located at both ends of the limiting bracket and is used to fit the two ends of the rough blank. The split-type reference pressure block is provided with multiple clamping screws to prevent the rough blank from shifting or twisting during the milling process.

[0015] Furthermore, the rectangular limiting groove has a length of 60-120mm, a width of 1.4mm, and a height of 1.4mm; the split reference pressure block is provided with 4-6 clamping screws.

[0016] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: 1) By limiting the rough blank to a central area of ​​not less than 10mm inward from the edge of the circular thin sheet and standardizing the rough blank size to 6mm×6mm×90mm~8mm×8mm×120mm, the present invention avoids the contamination of the sample by the segregation layer, gas absorption layer and impurity enrichment area at the edge of the ingot from the source, and significantly improves the consistency and repeatability of the sampling location.

[0017] 2) This invention employs a three-stage low-strain cutting mode: low-speed water-cooled thin-sheet sawing, water-cooled roughing band sawing, and layered water-cooled precision milling. The entire process is cooled by deionized water, and the single milling feed is controlled between 0.4mm and 0.8mm. Combined with the multiple constraints of the rectangular limiting groove of the special milling machine positioning fixture for lateral limiting, the lead screw clamping mechanism for axial locking, and the split reference pressure block end face clamping, the offset and torsion of the blank during the milling process are avoided. The parallelism of the finished sample is controlled within 0.5mm, and the dimensional accuracy meets the clamping and positioning requirements of the four-wire resistance test, avoiding instability and inaccuracy in the testing process.

[0018] 3) This invention adds a vacuum low-temperature stress-relieving heat treatment process after milling, with a furnace vacuum degree ≤5×10 - 3Pa, heating rate 8~12℃ / min, constant temperature annealing temperature 250~350℃, constant temperature holding 90~120min, no more than 8 samples per furnace with an interval ≥20mm, after holding, cool to room temperature with furnace, effectively eliminate residual stress introduced into the surface of the billet by layer milling, and avoid stress residue interfering with the low temperature resistance test results.

[0019] 4) This invention employs a three-stage cleaning process: ultrasonic cleaning with acetone, ultrasonic cleaning with anhydrous ethanol, and rinsing with deionized water. A ternary mixed acid solution prepared by mixing nitric acid, hydrofluoric acid, and phosphoric acid in a volume ratio of (0.8–1.2):(0.8–1.2):(1.8–2.2) is used for pickling at 20–25°C for 5–8 minutes. This process involves graded ultrasonic removal of surface oil and metal debris, mixed pickling to remove the surface strain layer and inclusions, and stress-relief heat treatment to eliminate residual milling stress. This removes both surface defects and internal stress introduced during machining, avoiding interference with low-temperature resistance testing and reducing the dispersion of RRR test data between parallel samples.

[0020] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from the description and drawings, which are particularly pointed out. Attached Figure Description

[0021] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0022] Figure 1 This is a process flow diagram of the method for preparing RRR test specimens of superconducting niobium ingots with low processing strain according to the present invention. Figure 2 This is a top view of the milling machine positioning fixture for the present invention; Figure 3 This is a three-dimensional schematic diagram of the positioning fixture for the special milling machine of the present invention; Figure 4 for Figure 3 A magnified view of a portion of the rectangular limiting groove.

[0023] Figure label: 1. Tooling base; 2. Fixing bolt; 3. Limiting bracket; 4. Rectangular limiting groove; 5. Screw clamping mechanism; 6. Adjusting screw; 7. Locking nut; 8. Split-type reference pressure block; 9. Clamping screw. Detailed Implementation

[0024] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0025] The RRR (Reverse Ratio) test results of superconducting niobium ingots are significantly affected by the sample preparation process. Processing strain, sampling position deviation, and surface contamination introduced during sample preparation can all cause the RRR test values ​​to deviate from the intrinsic properties of the material. To obtain accurate and repeatable RRR test data, a standardized sample preparation method for the entire process, from sawing the ingot body to the test specimen, needs to be established.

[0026] A specific embodiment of the present invention discloses a method for preparing a superconducting niobium ingot RRR test specimen with low processing strain, such as... Figure 1 As shown, it includes the following steps: S1. Thin-section sawing: Circular cross-section thin slices are cut from the superconducting niobium ingot body using a low-speed water-cooled sawing method, with continuous water cooling throughout the sawing process; S2. Rough blank cutting: A cuboid rough blank is cut in the central area of ​​the circular cross-section sheet using a water-immersion cooled band saw; S3. Precision milling: The rough blank is clamped onto the positioning fixture of the milling machine, and the rough blank is milled in layers using a carbide end mill. The milling process is continuously cooled with deionized water to obtain a cuboid sample. S4. Vacuum stress-relieving heat treatment: The rectangular sample is placed in a vacuum heat treatment furnace for low-temperature stress-relieving annealing. S5. Post-purification treatment: The heat-treated cuboid sample is subjected to ultrasonic cleaning with organic solvent and chemical acid washing with mixed acid in sequence.

[0027] The "low machining strain" mentioned in this invention refers to controlling the residual strain introduced into the sample by machining to a low level through multiple means such as low-speed cutting, micro-layer milling and subsequent vacuum stress relief heat treatment throughout the entire sample preparation process, so as to avoid strain interference with the accurate measurement of RRR value.

[0028] Superconducting niobium ingots refer to high-purity niobium ingots obtained through electron beam melting. Their resistance ratio (RRR) is typically required to be above 300 to meet the requirements of applications such as radio frequency superconducting cavities. RRR test specimens refer to standard cuboid specimens used for four-wire resistance measurement, whose cross-sectional dimensions and length must meet the requirements of IEC 61788-23.

[0029] Compared with existing technologies, the method for preparing RRR test specimens of superconducting niobium ingots with low processing strain provided in this embodiment solves the problems of insufficient accuracy and consistency of RRR test data caused by the lack of standardized sampling, large processing strain damage, and inconsistent post-processing in existing sample preparation methods by combining the above-mentioned steps into a complete sample preparation process. The steps work together: sampling in the central region ensures the representativeness of the specimen material from the source; graded low-strain cutting combined with specialized tooling ensures the dimensional accuracy of the specimen and controls the amount of processing strain introduced; vacuum stress-relief heat treatment eliminates the introduced residual stress; standardized purification post-treatment removes surface contamination and strain layer, enabling the final specimen to truly reflect the intrinsic RRR value of the niobium ingot.

[0030] Further, in step S1, the thickness of the circular cross-section sheet is 8-15 mm (e.g., 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm), and the sawing speed is 0.8-1.6 mm / min (e.g., 0.8 mm / min, 0.9 mm / min, 1.0 mm / min, 1.1 mm / min, 1.2 mm / min, 1.3 mm / min, 1.4 mm / min, 1.6 mm / min).

[0031] The thickness of the sheet is within this range, balancing structural strength and cutting efficiency during sawing, avoiding deformation due to excessive thinness or heat accumulation due to excessive thickness. The sawing line speed is controlled within this range, which is considered low-speed sawing, reducing frictional heat and mechanical impact, and minimizing strain hardening of the sheet surface. If the line speed is too low, the cutting efficiency is too low; if it is too high, the sawing heat increases significantly, potentially raising the sheet surface temperature to over 200°C, causing a reaction between high-purity niobium and interstitial elements, thus reducing the RRR value.

[0032] Further, in step S2, the central region is a region not less than 10mm from the edge of the circular cross-section sheet (e.g., 10mm, 12mm, 15mm, 18mm, 20mm, 25mm); the cross-sectional dimensions of the blank are (6-8mm) × (6-8mm) (e.g., 6mm×6mm, 6mm×7mm, 6mm×8mm, 7mm×7mm, 7mm×8mm, 8mm×8mm), and the length is 90-120mm (e.g., 90mm, 93mm, 96mm, 99mm, 102mm, 105mm, 108mm, 110mm, 113mm, 118mm, 120mm).

[0033] Electron beam melting of superconducting niobium ingots easily leads to the formation of segregation and absorber layers on the surface, with significantly higher interstitial element content compared to the core. If the sampling location is less than 10 mm from the edge, defective material may be mixed in, resulting in a lower RRR (Reference Rate Reduction) value. Limiting the sampling area to the central homogeneous region ensures material representativeness from the source. The rough blank cross-section dimensions are selected as (6–8 mm) × (6–8 mm) with a length of 90–120 mm, reserving a machining allowance of 1–2 mm on each side for subsequent precision milling, while ensuring the clamping rigidity of the rough blank and avoiding machining deformation.

[0034] Further, in step S3, the single feed rate of the layer milling is 0.4–0.8 mm / pass (e.g., 0.4 mm / pass, 0.5 mm / pass, 0.6 mm / pass, 0.65 mm / pass, 0.7 mm / pass, 0.8 mm / pass), and the cross-sectional area of ​​the resulting cuboid sample is 2 mm². 2 ~20mm 2 (e.g., 2mm) 2 4mm 2 8mm 2 12mm 2 16mm 2 20mm 2 The length is 60~120mm (e.g. 60mm, 70mm, 85mm, 95mm, 110mm, 120mm), and the parallelism is no greater than 0.5mm (e.g. 0.1mm, 0.2mm, 0.3mm, 0.35mm, 0.4mm, 0.5mm).

[0035] Preferably, the rectangular sample has a cross-sectional dimension of 4mm × 4mm and a length of 100mm.

[0036] When the feed rate is controlled within the above range, the cutting force is small and the cutting heat is low, which can significantly reduce the depth of the plastic deformation layer on the machined surface. If the feed rate exceeds 0.8 mm, the cutting force increases significantly, which not only forms deeper tool marks and corner collapses on the sample surface, but also introduces a lattice distortion layer in the subsurface layer. This strain layer is difficult to completely remove through subsequent acid pickling, resulting in a significant decrease in the RRR measurement value. The final sample cross-sectional area is 2 mm². 2 ~20mm 2 The length is 60~120mm, meeting the requirements of IEC 61788-23 standard regarding the cross-sectional area and aspect ratio of the specimen, and matching the fixture spacing for the four-wire resistance test. A parallelism of no more than 0.5mm ensures the accurate positioning of the specimen in the test fixture, avoiding measurement errors caused by form and position deviations.

[0037] Furthermore, the specific process parameters for the vacuum stress-relief heat treatment in step S4 are: furnace vacuum degree ≤ 5 × 10⁻⁶. -3 Pa (e.g., 1×10)-3 Pa, 2×10 -3 Pa, 3×10 -3 Pa, 3.5 × 10 -3 Pa, 4×10 -3 Pa, 5×10 -3 The heating rate is 8–12℃ / min (e.g., 8℃ / min, 9℃ / min, 10℃ / min, 11℃ / min, 11.5℃ / min, 12℃ / min), the isothermal annealing temperature is 250–350℃ (e.g., 250℃, 270℃, 290℃, 310℃, 330℃, 350℃), the isothermal holding time is 90–120 min (e.g., 90 min, 95 min, 100 min, 105 min, 110 min, 120 min), and after holding, the furnace is cooled to room temperature.

[0038] Vacuum degree ≤5×10 -3 Pa prevents oxidation of the high-purity niobium surface by residual oxygen or moisture during heating. A heating rate of 8–12 °C / min prevents uneven internal temperature and thermal stress concentration caused by rapid heating. An annealing temperature of 250–350 °C, lower than the recrystallization temperature of niobium (approximately 500 °C), eliminates dislocations and lattice distortions without altering the grain structure. Holding time of 90–120 min is sufficient to fully release residual stress. Insufficient temperature or time will result in incomplete stress relief, while excessive temperature or time may cause grain growth or redistribution of interstitial elements.

[0039] Further, the organic solvent ultrasonic cleaning in step S5 includes: first, ultrasonic cleaning with acetone for 10–15 min (e.g., 10 min, 11 min, 12 min, 13 min, 14 min, 15 min) at an ultrasonic cleaning power of 200–300 W (e.g., 200 W, 220 W, 240 W, 260 W, 280 W, 300 W); then ultrasonic cleaning with anhydrous ethanol for 10–15 min (e.g., 10 min, 11 min, 12 min, 13 min, 14 min, 15 min); and finally, rinsing with deionized water for at least 3 min (e.g., 3 min, 4 min, 5 min, 6 min, 7 min, 8 min). This two-stage organic solvent ultrasonic cleaning combined with deionized water rinsing can progressively remove residual oil, cutting fluid, and organic contaminants from the sample surface. An ultrasonic power of 200–300W and a duration of 10–15 minutes ensure that the cavitation effect fully removes the surface deposits, while avoiding ultrasonic cavitation damage to the sample surface caused by excessive power or time.

[0040] Further, the mixed acid chemical pickling in step S5 is as follows: a mixed acid solution of nitric acid, hydrofluoric acid and phosphoric acid with a volume ratio of (0.8-1.2):(0.8-1.2):(1.8-2.2) is used for pickling for 5-8 minutes (e.g. 5 minutes, 6 minutes, 6.5 minutes, 7 minutes, 7.5 minutes, 8 minutes) at 20-25°C (e.g. 20°C, 21°C, 22°C, 23°C, 24°C, 25°C), followed by rinsing with flowing deionized water.

[0041] Machining can create a strain layer on the surface of niobium samples with lattice distortion and impurity enrichment. The strain layer has a much lower RRR value than the substrate and must be removed by chemical pickling. Hydrofluoric acid in a mixed acid solution corrodes the oxide layer and strain layer, nitric acid maintains the corrosion reaction, and phosphoric acid regulates the corrosion rate and improves surface wettability. The combination of these three can achieve uniform chemical polishing at 20–25°C.

[0042] The above-mentioned mixing ratio range was determined based on the following considerations: If the proportion of hydrofluoric acid is too low, the corrosion rate will be insufficient, the strain layer cannot be completely removed within the specified time, and the residual strain layer will directly lower the RRR measurement value; if the proportion is too high, the reaction will be too vigorous, easily causing selective grain boundary erosion and increasing the risk of hydrogen absorption, which will also degrade the RRR value. If the proportion of nitric acid is too low, the oxidizing capacity will be insufficient, making it difficult for the corrosion reaction to continue; if the proportion is too high, a surface passivation film may form, hindering corrosion. Phosphoric acid, as a viscosity modifier and corrosion inhibitor, has insufficient corrosion inhibition effect and decreased surface smoothness when its proportion is below 1.8 parts, and excessive acid viscosity and reduced corrosion efficiency when its proportion is above 2.2 parts.

[0043] By limiting the volume ratio of the three components to the range of (0.8–1.2):(0.8–1.2):(1.8–2.2) (e.g., 0.8:0.8:1.8, 0.9:1.0:1.9, 1.0:1.0:2.0, 1.0:1.1:2.1, 1.1:1.2:2.1, 1.2:1.2:2.2), uniform removal of the strain layer can be achieved at 20–25°C, ensuring the corrosion effect while avoiding the introduction of new surface defects. A pickling time of 5–8 minutes ensures complete removal of the strain layer; too short a time will result in incomplete removal, while too long a time may cause over-corrosion or hydrogen absorption.

[0044] After pickling, rinse with running deionized water to thoroughly remove residual acid and dissolved products.

[0045] Furthermore, in step S4, the number of cuboid samples placed in a single furnace shall not exceed 8 (e.g., 1, 2, 3, 5, 7, or 8 samples), and the spacing between each cuboid sample shall not be less than 20 mm (e.g., 20 mm, 22 mm, 25 mm, 28 mm, 30 mm, or 35 mm). If too many samples are placed in a single furnace or the spacing is too small, thermal shielding will occur between the densely placed samples, resulting in uneven heating of each sample and inconsistent stress relief effects. Limiting the number of samples in a single furnace to no more than 8 and the spacing to no less than 20 mm ensures that there is sufficient space around each sample for radiative heat transfer and atmosphere circulation, so that parallel samples processed in the same batch achieve a consistent stress relief state.

[0046] Furthermore, the present invention also provides a milling machine positioning fixture for the above-described preparation method, such as... Figure 2 , Figure 3 As shown, it includes a tooling base 1, a limit holder 3, a screw clamping mechanism 5, and a split reference pressure block 8.

[0047] Specifically, the fixture base 1 is used to fix the milling machine positioning fixture to the milling machine worktable, and its four corners are equipped with fixing bolts 2. The limiting bracket 3 is fixed on the fixture base 1, and the center of the limiting bracket 3 has a rectangular limiting groove 4 for accommodating the rough workpiece. The rectangular limiting groove 4 is used to limit the lateral displacement of the rough workpiece during milling. The lead screw clamping mechanism 5 is located at the front end of the limiting bracket 3, including an adjusting lead screw 6 and a locking nut 7. The adjusting lead screw 6 is used to clamp the rough workpiece to achieve axial locking. The split reference pressure block 8 is located at both ends of the limiting bracket 3 to fit the two ends of the rough workpiece. The split reference pressure block 8 is equipped with multiple clamping screws 9 to prevent the rough workpiece from shifting or twisting during milling.

[0048] When using this fixture, the blank is placed into the rectangular limiting groove 4 in the middle of the limiting seat 3. The four walls of the groove restrict the lateral displacement of the blank under the milling force. Then, the adjusting screw 6 located at the front end of the limiting seat 3 is rotated so that its front end presses against the corresponding end face of the blank, achieving axial locking. The screw position is then locked by the locking nut 7 to prevent the screw from loosening due to machining vibration. Finally, the split reference pressure blocks 8 located at both ends of the limiting seat 3 are pushed towards the two end faces of the blank, so that they fit against the two end faces of the blank. The multiple clamping screws 9 on them are tightened so that the split reference pressure blocks 8 apply a stable end face clamping force to the two ends of the blank to prevent the blank from shifting or twisting during the milling process.

[0049] By using the lateral limiting of the rectangular limiting groove 4, the axial clamping of the screw clamping mechanism 5, and the clamping of both ends of the split reference pressure block 8, this fixture achieves all-round positioning of the rough blank through the synergistic effect of triple constraints, so that the slender rough blank remains stable under low strain milling conditions, ensuring that the blank does not deviate or twist during the entire milling process.

[0050] Compared to the single-point clamping of ordinary flat-jaw vises, this fixture achieves all-round active constraint on the rough workpiece, avoiding the unreliability and uneven force distribution problems caused by traditional clamping methods that rely solely on friction and single-point clamping. It is especially suitable for low-stress milling of slender rough workpieces. The fixture is precisely matched to the dimensions of the rough workpiece, eliminating the need for repeated calibration of the positioning datum. Single-sided machining can be completed in a single clamping, improving sample preparation efficiency while ensuring machining accuracy.

[0051] Furthermore, the rectangular limiting groove 4 has a length of 60-120 mm, a width of 1.4 mm, and a height of 1.4 mm; the split-type reference pressure block 8 is provided with 4 to 6 (e.g., 4, 5, or 6) clamping screws 9. The length of the rectangular limiting groove 4 is consistent with the final sample length, and the end of the groove can be directly used as the positioning reference in the length direction during milling. The 4 to 6 clamping screws 9 on the split-type reference pressure block 8 can ensure that the clamping force is applied evenly along the sample length direction, avoiding local pressure concentration that could cause bending deformation of the blank or machining vibration due to insufficient clamping points.

[0052] The present invention will be described in more detail below through specific embodiments. These embodiments are merely descriptions of the best implementation of the invention and do not limit the scope of the invention in any way.

[0053] Example 1 S1. Thin-sheet sawing: The superconducting niobium ingot is sawed at a low speed with water cooling. The sheet thickness is 8mm and the sawing line speed is 0.8mm / min. The entire process is cooled with room temperature deionized water.

[0054] S2. Rough blank cutting: Take material from the center area of ​​the circular sheet 10mm from the edge, saw it to obtain a rectangular rough blank with dimensions of 6mm×6mm×90mm, and immerse it in water for cooling throughout the process.

[0055] S3. Precision milling: The rough blank is clamped in the positioning fixture of the milling machine, and layer milling is carried out using carbide end mills with a single feed of 0.4 mm. The entire process is water-cooled, and a standard sample with a size of 4 mm × 4 mm × 80 mm is finally prepared.

[0056] S4. Vacuum low-temperature annealing: Vacuum degree controlled at 5×10 -3 Pa, heating rate 8℃ / min, hold at 250℃ for 90min; 8 samples are placed in a single furnace with a spacing of 20mm between samples, and the furnace is cooled to room temperature after the holding period.

[0057] S5. Post-purification treatment: Ultrasonic cleaning with 200W acetone for 10 min, ultrasonic cleaning with anhydrous ethanol for 10 min, and rinsing with deionized water for 3 min; then acid washing with a mixed acid solution of nitric acid, hydrofluoric acid and phosphoric acid with a volume ratio of 0.8:0.8:1.8 at 20℃ for 5 min, followed by thorough rinsing with flowing deionized water.

[0058] Example 2 The only difference between this embodiment and Embodiment 1 is that: In S1, the sheet thickness is 11 mm, and the sawing speed is 1.2 mm / min; In S2, the sampling area is 12mm from the edge, and the rough blank size is 7mm×7mm×100mm; In S3, the single feed amount is 0.6 mm, and a standard sample with a final size of 3 mm × 3 mm × 60 mm is prepared. In S4, the vacuum level is 3×10⁻⁶. -3 Pa, heating rate 10℃ / min, annealing temperature 300℃, holding time 105min; In S5, the ultrasonic cleaning power is 250W, the duration is 12min, the water washing is 4min, the acid washing ratio is 1:1:2, the temperature is 22℃, and the duration is 6min.

[0059] Example 3 The only difference between this embodiment and Embodiment 1 is that: In S1, the sheet thickness is 15mm and the sawing speed is 1.6mm / min; In S2, the sampling area is 15mm from the edge, and the rough blank size is 8mm×8mm×120mm; In S3, the single feed rate is 0.8 mm, and a standard sample with a final size of 4.4 mm × 4.4 mm × 80 mm is prepared. In S4, the vacuum level is 1×10⁻⁶. -3 Pa, heating rate 12℃ / min, annealing temperature 350℃, holding time 120min; In S5, the ultrasonic cleaning power is 300W, the duration is 15min, the water washing is 5min, the acid washing ratio is 1.2:1.2:2.2, the temperature is 25℃, and the duration is 8min.

[0060] Example 4 The only difference between this embodiment and embodiment 2 is that the special milling machine positioning fixture of the present invention is used for clamping and processing. The fixture includes a fixture base 1, a limit locator 3, a lead screw clamping mechanism 5, and a split reference pressure block 8.

[0061] Specifically, the fixture base 1 is used to fix the milling machine positioning fixture as a whole to the milling machine worktable, and its four corners are equipped with fixing bolts 2. The limiting bracket 3 is fixed on the fixture base 1, and the center of the limiting bracket 3 has a rectangular limiting groove 4 for accommodating the rough workpiece. The rectangular limiting groove 4 is used to limit the lateral displacement of the rough workpiece during milling. The lead screw clamping mechanism 5 is located at the front end of the limiting bracket 3, including an adjusting lead screw 6 and a locking nut 7. The adjusting lead screw 6 is used to clamp the rough workpiece to achieve axial locking. The split reference pressure block 8 is located at both ends of the limiting bracket 3 to fit the two end faces of the rough workpiece. The split reference pressure block 8 is equipped with multiple clamping screws 9 to prevent the rough workpiece from shifting or twisting during milling.

[0062] The remaining parameters are the same as in Example 2. For example, in S3, the single feed amount is 0.6 mm, and a standard sample with a final size of 3 mm × 3 mm × 60 mm is prepared.

[0063] Example 5 The only difference between this embodiment and embodiment 2 is that the vacuum degree in S4 is 3×10⁻⁶. -3 Pa, heating rate 10℃ / min, annealing temperature 300℃, holding time 105min, sample spacing not less than 20mm, other parameters are the same as in Example 1.

[0064] Comparative Example 1 The overall process is the same as in Example 2, except that the sampling location is a region 5mm from the edge of the sheet. The only difference between this comparative example and Example 2 is that the sampling location in S2 is a region 5mm from the edge of the sheet; all other parameters are the same as in Example 2.

[0065] Comparative Example 2 All process parameters are the same as in Example 2, except that a common flat-jaw vise is used for single-point clamping milling. The only difference between this comparative example and Example 2 is that in S3, a common flat-jaw vise is used for single-point clamping instead of the special tooling of this invention; all other parameters are the same as in Example 2.

[0066] Comparative Example 3 The only difference between this comparative example and Example 2 is that the acetone ultrasonic cleaning, anhydrous ethanol ultrasonic cleaning, and tribasic acid pickling in S5 are omitted, and only pure water is used for simple rinsing. The other parameters are the same as in Example 2.

[0067] Comparative Example 4 The only difference between this comparative example and Example 2 is that the single milling feed rate in S3 is increased to 2mm, while the other parameters are the same as in Example 2.

[0068] Comparative Example 5 The only difference between this comparative example and Example 2 is that the vacuum degree in S4 is 1×10⁻⁶. -2Pa, heating rate 15℃ / min, annealing temperature 200℃, holding time 60min, sample spacing 5mm, rapid cooling with air after holding, and other parameters are the same as in Example 2.

[0069] Characterization results and analysis The characterization results of the above embodiments and comparative examples are shown in Table 1 below.

[0070] Table 1 Comparison of test results of samples from each embodiment and comparative example

[0071] Note: The RRR value of the standard sample is 385, determined by standard method using a high-purity niobium standard sample without processing damage.

[0072] As shown in Table 1, the mean RRR values ​​of Examples 1-5 are between 383 and 385, with deviations from the standard value of 385 of only 0-2, RRR ranges of 2-4, parallelism of 0.22-0.45 mm, and no surface defects. In contrast, the RRR values ​​of Comparative Examples 1-5 decrease to 328-364, with deviations from the standard value of 385 reaching -21 to -57, RRR ranges widening to 12-16, parallelism of 0.52-0.80 mm, and defects present on the surface of all test samples. This demonstrates that the present invention solves the technical problem of poor accuracy and consistency of RRR test data in traditional sample preparation methods, and possesses good industrial applicability.

[0073] 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 changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a superconducting niobium ingot RRR test specimen with low processing strain, characterized in that, Includes the following steps: S1. Thin-section sawing: Circular cross-section thin slices are cut from the superconducting niobium ingot body using a low-speed water-cooled sawing method, with continuous water cooling throughout the sawing process; S2. Rough blank cutting: A cuboid rough blank is cut in the central area of ​​the circular cross-section sheet using a water-immersion cooled band saw; S3. Precision milling: The rough blank is clamped onto the positioning fixture of the milling machine, and the rough blank is milled in layers using a carbide end mill. The milling process is continuously cooled with deionized water to obtain a cuboid sample. S4. Vacuum stress-relieving heat treatment: The rectangular sample is placed in a vacuum heat treatment furnace for low-temperature stress-relieving annealing. S5. Post-purification treatment: The heat-treated cuboid sample is subjected to ultrasonic cleaning with organic solvent and chemical acid washing with mixed acid in sequence.

2. The preparation method according to claim 1, characterized in that, In step S1, the thickness of the circular cross-section sheet is 8-15 mm, and the sawing speed is 0.8-1.6 mm / min.

3. The preparation method according to claim 1, characterized in that, In step S2, the central region is the area at least 10 mm from the edge of the circular cross-section sheet; the cross-sectional dimensions of the blank are (6-8 mm) × (6-8 mm), and the length is 90-120 mm.

4. The preparation method according to claim 1, characterized in that, In step S3, the feed rate for the layered milling is 0.4–0.8 mm / pass; the cross-sectional area of ​​the resulting cuboid sample is 2 mm². 2 ~20mm 2 The length is 60~120mm, and the parallelism is no more than 0.5mm.

5. The preparation method according to claim 1, characterized in that, In step S4, the specific process parameters for the vacuum stress-relief heat treatment are: furnace vacuum degree ≤ 5 × 10⁻⁶. -3 Pa, heating rate 8-12℃ / min, constant temperature annealing temperature 250-350℃, constant temperature holding time 90-120min, and after holding, cooling to room temperature with the furnace.

6. The preparation method according to claim 1, characterized in that, In step S5, the ultrasonic cleaning with organic solvent includes: first, ultrasonic cleaning with acetone for 10-15 minutes at an ultrasonic cleaning power of 200-300W; then, ultrasonic cleaning with anhydrous ethanol for 10-15 minutes; and finally, rinsing with deionized water for at least 3 minutes after cleaning.

7. The preparation method according to claim 1, characterized in that, In step S5, the mixed acid chemical pickling is performed by using a mixed acid solution of nitric acid, hydrofluoric acid and phosphoric acid in a volume ratio of (0.8-1.2):(0.8-1.2):(1.8-2.2) at 20-25°C for 5-8 minutes, followed by rinsing with flowing deionized water.

8. The preparation method according to claim 5, characterized in that, In step S4, the number of cuboid samples placed in a single furnace shall not exceed 8, and the interval between each cuboid sample shall not be less than 20 mm.

9. A milling machine positioning fixture for the preparation method according to any one of claims 1 to 8, characterized in that, include: Tooling base (1) is used to fix the milling machine positioning tooling as a whole to the milling machine worktable; A limiting bracket (3) is fixed on the tooling base (1). A rectangular limiting groove (4) for accommodating the blank is provided in the middle of the limiting bracket (3). The rectangular limiting groove (4) is used to limit the lateral displacement of the blank during milling. The lead screw clamping mechanism (5) is located at the front end of the limiting seat (3) and includes an adjusting lead screw (6) and a locking nut (7). The adjusting lead screw (6) is used to press the blank to achieve axial locking. The split reference pressure block (8) is located at both ends of the limiting card seat (3) and is used to fit the two ends of the blank. The split reference pressure block (8) is provided with multiple clamping screws (9) to prevent the blank from shifting or twisting during the milling process.

10. The milling machine positioning fixture according to claim 9, characterized in that, The rectangular limiting groove (4) has a length of 60-120mm, a width of 1.4mm, and a height of 1.4mm; the split reference pressure block (8) is provided with 4-6 clamping screws (9).

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