High-n-value NbTi superconducting wire and preparation method thereof

Abstract

The invention belongs to the technical field of superconducting material processing, and discloses a high-n-value NbTi superconducting wire and a preparation method thereof. The method comprises the following steps: firstly, bending a special-shaped soft copper flat belt on line to form a thin-wall copper slot line, and then embedding and welding the thin-wall copper slot line to the surface of a low-copper-ratio and small-size composite round wire to obtain a large-copper-ratio NbTi superconducting composite wire with a round or round-corner square cross section. And finally, performing small-size stretching to obtain the high-n-value NbTi superconducting wire with a circular or rounded rectangle-shaped cross section. According to the preparation method, the problem that due to the fact that the hardness and work hardening difference of Cu and NbTi is large, and large-work-amount stretching and rolling are conducted, the synergistic deformation of the Cu and NbTi core wires is poor can be effectively solved, the deformation uniformity of the core wires is improved, and under the condition that the copper ratio of the prepared NbTi superconducting wire is 3-6, the n value is not lower than 43.

Description

Technical Field

[0001] This invention belongs to the field of superconducting material processing and relates to a high n-value NbTi superconducting wire and its preparation method. Background Technology

[0002] In liquid helium-free superconducting MRI magnet systems, closed-loop refrigerators directly cool the magnets, replacing traditional large-volume liquid helium immersion cooling. Cooling temperatures are typically between 4.2K and 10K, but the cooling power and temperature uniformity are weaker than liquid helium bath cooling. This places demands on the core material, NbTi superconducting wires, requiring high copper-to-superconductor ratio, large wire spacing, high critical current density (Jc), and a high n-value. A high n-value is crucial for ensuring stable magnet operation in non-uniform cryogenic environments.

[0003] The n-value describes the sharpness of the transition from the superconducting state to the normal state in a superconductor and is directly related to the uniformity of NbTi core wire deformation. A higher n-value results in more uniform core wire deformation, better consistency in the distribution of the internal spike centers, and a more uniform critical current density distribution throughout the wire. In environments without liquid helium cooling, high n-value wires can reduce the risk of localized early quenching and improve the operating margin and stability of the magnet.

[0004] Traditional processes improve the n-value by optimizing the metallurgical quality of NbTi ingots, controlling the concentricity of single core rods, and adjusting the stretching-annealing process to enhance the uniformity of core wire deformation. However, even with strict concentricity control, multi-core composite wires still exhibit phenomena such as "necking" and "twisting" during large-deformation stretching, leading to uneven stress within the core wires. This lowers the superconducting transition temperature (Tc) in stress concentration areas, resulting in a decrease in the overall n-value. The upper limit of the n-value achieved by traditional processes is only around 30, which cannot meet the requirements of liquid helium-free MRI. Therefore, developing a high-n-value NbTi superconducting wire preparation method is of great significance. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a high-n-value NbTi superconducting wire and its preparation method. This invention involves bending irregularly shaped soft copper strips online into thin-walled copper groove wires, then embedding and welding these thin-walled copper groove wires to the surface of a low-copper-ratio, small-sized composite circular wire to obtain a high-copper-ratio NbTi superconducting composite wire with a circular or rounded square cross-section. Finally, a high-n-value NbTi superconducting wire with a circular or rounded rectangular cross-section can be obtained through relatively small-scale stretching. This preparation method effectively improves the problem of poor synergistic deformation of the Cu and NbTi core wires due to the significant differences in hardness and work hardening between Cu and NbTi, as well as the large-scale stretching and rolling processes. It improves the uniformity of core wire deformation, and the prepared NbTi superconducting wire has an n-value of not less than 43 even with a copper ratio as high as 3-6, showing broad application prospects in liquid helium-free superconducting MRI magnets.

[0006] On one hand, the present invention provides a method for preparing high-n-value NbTi superconducting wires, specifically including the following steps: S1: Prepare low copper ratio composite round wire with a diameter not exceeding 1.95mm and a copper ratio not exceeding 2.3.

[0007] S2: Oxygen-free copper wire is straightened, flattened, rolled in three passes, annealed online, cooled, and passivated to obtain irregular soft copper flat strips with the short side facing up or with the flat side facing up.

[0008] Furthermore, the cross-section of the irregular soft copper strip with the short side facing upward is trapezoidal, the length of the short side of the trapezoid is 1.03 to 1.08 times the circumference of the NbTi / Cu composite circular line, the thickness of the irregular soft copper strip with the short side facing upward is 0.12 to 0.3 mm, and the length of the long side is the circumference of a circle with a radius equal to the short side length / 2π + thickness.

[0009] Furthermore, the flat, upward-facing irregularly shaped soft copper strip has a continuous wave-like shape below it, composed of concave and convex arcs. The thinnest part of the flat, upward-facing irregularly shaped soft copper strip has a thickness of 0.1~0.3mm, the length of the flat surface is 1.01~1.05 times the circumference of the NbTi / Cu composite circle, the convex arc angle is 90°, and the radius of the convex arc is calculated using the following formula: In the formula, d1 is 1.01 to 1.05 times the diameter of the NbTi / Cu composite circular wire, and h is the thickness of the thinnest part of the irregular soft copper flat strip. The concave arc parameters can be obtained by calculating the geometric constraint relationship between the convex arc angle, convex arc radius, and the thinnest part thickness through the circular arc transition.

[0010] Furthermore, the straightening process ensures that the original bending and twisting of the copper rod are completely eliminated, allowing it to enter the rolling mill straight and smoothly, with the straightening degree reaching 0~1%. During the winding process of the irregular soft copper flat strip, flatness is ensured, and no obvious unevenness is allowed in a single layer of flat strip to avoid damage or deformation of the copper flat strip, which would affect subsequent processing.

[0011] Furthermore, the annealing temperature of the online annealing is 200~400℃; the passivating agent used in the passivation treatment is RSB607 with a concentration of 5%~10%, the passivating agent solution temperature is 50±5℃, and the immersion distance is 1~2m.

[0012] S3: The irregularly shaped soft copper flat strip is passed through the bending machine and the forming concave cam group in sequence to obtain a thin-walled copper groove wire. The thin-walled copper groove wire is passed through the fluxing machine and the liquid solder bath. Then, the low copper ratio composite round wire and the thin-walled copper groove wire are passed through the pre-bonding mold and the inlay mold in sequence. Welding is performed at the inlay mold. After cooling, a circular NbTi superconducting composite wire or a rounded square NbTi superconducting composite wire is obtained.

[0013] Furthermore, flux is added to the welding machine. This invention does not specifically limit the type of flux, and those skilled in the art can use conventional flux.

[0014] Furthermore, when using the short-side-up irregular soft copper flat strip, the thin-walled copper groove obtained by the bending machine and the forming concave cam group is U-shaped. The inner width of the U-shaped thin-walled copper groove is 1.03 to 1.08 times the diameter of the low copper ratio composite round wire, and the thickness of the U-shaped thin-walled copper groove is 0.12 to 0.3 mm. When using the plane-up irregular soft copper flat strip, the thin-walled copper groove obtained by the bending machine and the forming concave cam group is a rounded pentagon with missing apex corners. The angle of the bottom two corners of the rounded pentagon with missing apex corners after bending is 120°, and the angle of the two corners on both sides of the apex corner after bending is 90°. The processing amount of the rounded pentagon with missing apex corners is 0 to 1%, and the wall thickness is obtained by calculation.

[0015] Furthermore, the pre-bonding mold is immersed in liquid solder, which is liquid SnCu solder, and the temperature of the liquid solder is 260~300℃; the pre-bonding mold bends the bottom two corners of the rounded pentagon missing the top corner to 100°; the total processing amount of the pre-bonding mold and the inlay mold is 5%~10%, and the welding speed is 20~120m / min.

[0016] S4: Stretch the NbTi superconducting composite wire with a circular cross-section to the final size of the bare wire; roll and stretch the NbTi superconducting composite wire with a rounded square cross-section to the final size of the rectangular cross-section to obtain the high n-value NbTi superconducting wire.

[0017] Furthermore, the amount of stretching and the total amount of rolling and stretching are both 5-20%.

[0018] On the other hand, the present invention claims protection for a high n-value NbTi superconducting wire, which is prepared by the above-described preparation method; the copper ratio of the NbTi superconducting wire is 3 to 6, and the n-value is not less than 43.

[0019] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects or advantages: (1) In this invention, irregularly shaped soft copper flat strips are bent into thin-walled copper groove wires online. Then, the thin-walled copper groove wires are embedded and welded to the surface of a low copper ratio, small-sized composite round wire to obtain a high copper ratio NbTi superconducting composite wire with a circular or rounded square cross-section. Finally, a high n-value NbTi superconducting wire with a circular or rounded rectangular cross-section can be obtained by rolling and stretching at a smaller scale. The preparation method of this invention can effectively improve the problem of poor synergistic deformation of Cu and NbTi core wires due to the large difference in hardness and work hardening between Cu and NbTi, and the large amount of stretching and rolling. It improves the uniformity of core wire deformation. The prepared NbTi superconducting wire has an n-value of not less than 43 under the condition of copper ratio of 3~6, ​​which is suitable for liquid helium-free superconducting MRI magnets. It obtains NbTi superconducting wires with larger core wire spacing and higher copper-to-superconducting ratio, and solves the problem that existing low copper ratio NbTi superconducting wires cannot adapt to the local thermal disturbance of the magnet, resulting in loss of superconductivity, as well as the problem of round wire eccentricity caused by excessively high copper-to-superconducting ratio. The present invention provides a NbTi superconducting composite wire with a rounded square cross-section. After rolling and stretching to a rectangular cross-section, the finished product still exhibits high core wire uniformity, solving the problem of uneven core wire size caused by the large deformation when rolling a round wire into a flat wire.

[0020] (2) This invention limits the short-side length of the irregularly shaped soft copper strip with the short side facing upwards to 1.03 to 1.08 times the circumference of the NbTi / Cu composite circular wire, and the planar length of the irregularly shaped soft copper strip with the plane facing upwards to 1.01 to 1.05 times the circumference of the NbTi / Cu composite circular wire. Beyond the limits set by this invention, the thin-walled copper groove wire cannot completely cover the NbTi / Cu composite circular wire, or it may cause the circular wire in the prepared NbTi superconducting composite wire to be eccentric, resulting in uneven thickness of the outer thin-walled copper groove wire. This invention pre-forms the composite circular wire by pressing it into the thin-walled copper groove wire, avoiding the problems of poor stability and insufficient coverage caused by direct forming. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the cross-section of an irregularly shaped soft copper strip. Figure 1 In the figure, 'a' is a schematic diagram of the cross-section of irregular soft copper strip used for circular NbTi superconducting wires; Figure 1 In the figure, b is a schematic cross-sectional view of irregularly shaped soft copper strip for square NbTi superconducting wire. The labels in the figure are as follows: 1 is the thickness at the thinnest point, 2 is the convex arc, and 3 is the concave arc.

[0023] Figure 2This is a schematic diagram of the cross-section of a thin-walled copper channel. Figure 2 In the diagram, 'a' is a cross-sectional schematic of a thin-walled copper trench wire for circular NbTi superconducting wires. Figure 2 In the diagram, b is a cross-sectional schematic of a thin-walled copper channel wire for square NbTi superconducting wire.

[0024] Figure 3 This is a schematic diagram of the cross-section of the NbTi superconducting composite wire. Figure 3 In the diagram, 'a' represents a cross-sectional schematic of a circular NbTi superconducting composite wire. Figure 3 In the diagram, b is a cross-sectional schematic of an NbTi superconducting composite wire with a rounded square cross-section. Detailed Implementation

[0025] The technical solution of the present invention will be described below with reference to embodiments. However, the present invention is not limited to the following embodiments. Unless otherwise specified, the experimental methods and detection methods described in each embodiment are conventional methods; unless otherwise specified, the reagents and materials can be purchased commercially. Unless otherwise specified, the percentages in the following embodiments refer to mass percentages.

[0026] The calculation method for the processing amount of the present invention is: (S1-S2) / S1, where S1 is the cross-sectional area of ​​the wire before processing and S2 is the cross-sectional area of ​​the wire after processing.

[0027] The preparation method of high n-value NbTi superconducting wires specifically includes the following steps: S1: Prepare low copper ratio NbTi / Cu composite round wire with a diameter not exceeding 1.5mm and a copper ratio not exceeding 2.3.

[0028] S2: Oxygen-free copper wire is straightened, flattened, rolled in three passes, annealed online, cooled, and passivated to obtain a short-side-up or flat-side-up irregular soft copper flat strip. Figure 1 ).

[0029] Furthermore, the cross-section of the irregular soft copper strip with the short side facing upward is trapezoidal, and the length of the short side of the trapezoid is 1.03 to 1.08 times the circumference of the NbTi / Cu composite circular wire. The thickness of the irregular soft copper strip with the short side facing upward is 0.12 to 0.3 mm. The length of the long side can be calculated based on the length of the short side and the thickness of the irregular soft copper strip.

[0030] Furthermore, the flat, upward-facing irregularly shaped soft copper strip has a continuous wave-like shape below it, composed of concave and convex arcs. The thinnest part of the flat, upward-facing irregularly shaped soft copper strip has a thickness of 0.1~0.3mm, the length of the flat surface is 1.01~1.05 times the circumference of the NbTi / Cu composite circle, the convex arc angle is 90°, and the radius of the convex arc is calculated using the following formula: In the formula, d1 is 1.01 to 1.05 times the diameter of the NbTi / Cu composite circular wire, and h is the thickness of the thinnest part of the irregular soft copper flat strip. The concave arc parameters can be calculated by using the geometric constraint relationship of the circular arc transition based on the convex arc angle, convex arc radius, and thinnest part thickness.

[0031] Furthermore, the straightening process ensures that the original bending and twisting of the copper rod are completely eliminated, allowing it to enter the rolling mill straight and smoothly, with a straightening degree of 0-1%. During the fabrication of the irregularly shaped soft copper flat strip, flatness is ensured; no significant unevenness should occur in a single layer of flat strip to avoid damage or deformation of the copper flat strip, which could affect subsequent processing.

[0032] Furthermore, the annealing temperature of the online annealing is 200~400℃; the passivating agent used in the passivation treatment is RSB607 with a concentration of 5%~10%, the passivating agent solution temperature is 50±5℃, and the immersion distance is 1~2m.

[0033] S3: Pass the irregularly shaped soft copper strip through the bending machine and the forming concave cam group in sequence to obtain a thin-walled copper groove ( Figure 2 The thin-walled copper channel wire is passed through a fluxing machine and a liquid solder bath. Then, the low copper ratio composite round wire and the thin-walled copper channel wire are passed through a pre-bonding mold and an inlay mold in sequence. Welding is performed at the inlay mold. After cooling, NbTi superconducting composite wire with a circular cross-section or NbTi superconducting composite wire with a rounded square cross-section is obtained. Figure 3 ).

[0034] Furthermore, flux is added to the welding machine. This invention does not specifically limit the type of flux, and those skilled in the art can use conventional flux.

[0035] Furthermore, when using the irregularly shaped soft copper flat strip with the short side facing upward, the thin-walled copper groove obtained by the bending machine and the forming concave cam group is U-shaped. The inner width of the U-shaped thin-walled copper groove is 1.03 to 1.08 times the diameter of the low copper ratio composite round wire, and the thickness of the U-shaped thin-walled copper groove is 0.12 to 0.3 mm. When using the irregularly shaped soft copper flat strip with the plane facing upward, the thin-walled copper groove obtained by the bending machine and the forming concave cam group is a rounded pentagon with missing apex corners. The processing amount of the rounded pentagon with missing apex corners is 0 to 1%, and the wall thickness is obtained by calculation. The bending angle of the bottom two corners of the rounded pentagon with missing apex corners is 120°, and the bending angle of the two corners on both sides of the apex corner is 90°.

[0036] Furthermore, the pre-bonding mold is immersed in liquid solder, which is liquid SnCu solder, and the temperature of the liquid solder is 260~300℃; the pre-bonding mold bends the bottom two corners of the rounded pentagon missing the top corner to 100°; the total processing amount of the pre-bonding mold and the inlay mold is 5%~10%, and the welding speed is 20~120m / min.

[0037] S5: Stretch the NbTi superconducting composite wire with a circular cross-section to the final size of the bare wire; roll and stretch the NbTi superconducting composite wire with a rounded square cross-section to the final size of the rectangular cross-section to obtain the high n-value NbTi superconducting wire.

[0038] Furthermore, the amount of stretching and the total amount of rolling and stretching are both 5-20%.

[0039] Example 1 This embodiment provides a method for preparing high-n-value NbTi superconducting wires, specifically including the following steps: S1: Prepare a low copper ratio NbTi / Cu composite round wire with a diameter of 0.92mm and a copper ratio of 1.5.

[0040] S2: Oxygen-free copper wire with a diameter of 0.9 mm is straightened, flattened, and rolled in 3 passes to form a flat strip with a trapezoidal cross-section. The upper base of the trapezoid is 3.1 mm long, the lower base is 3.9 mm long, and the height is 0.12 mm. It is then annealed online (annealing temperature 300℃), cooled, and passivated. The passivation uses RSB607 with a concentration of 5% and a solution temperature of 50±5℃. The immersion distance is 1 m, resulting in an irregular soft copper flat strip with the short side facing upward.

[0041] S3: The irregularly shaped soft copper flat strip is passed through a bending machine and a forming concave cam assembly in sequence to obtain a thin-walled copper groove wire. The inner width of the thin-walled copper groove wire is 0.99mm, the groove depth is 1.47mm, and the wall thickness is 0.12mm. Then, the thin-walled copper groove wire is passed through a flux machine (with flux added) and a liquid Sn0.7Cu solder bath. The solder temperature is set to 260℃ and the embedding and welding speed is 120m / min. After the solder temperature reaches the set temperature, the low copper ratio composite round wire and the thin-walled copper groove wire are passed through a 1.3mm diameter pre-bonding mold (the pre-bonding mold is immersed in liquid Sn0.7Cu solder) and a 1.14mm diameter embedding mold in sequence. The processing amount is 5.9%. Welding is performed at the embedding mold. After cooling, a round NbTi superconducting composite wire is obtained.

[0042] S4: Stretch the circular NbTi superconducting composite wire to a diameter of 1.05 mm, with a processing amount of 15%, to obtain the high n-value NbTi superconducting wire.

[0043] Using an electric field criterion of 1μV / cm, the n-value of the wire was 54.2, and the copper over-ratio of the wire was 3.

[0044] Example 2 This embodiment provides a method for preparing high-n-value NbTi superconducting wires, specifically including the following steps: S1: Prepare a low copper ratio NbTi / Cu composite round wire with a diameter of 1.50mm and a copper ratio of 2.0mm.

[0045] S2: Oxygen-free copper wire with a diameter of 1.68 mm is straightened, flattened, and rolled in 3 passes to form a flat strip with a trapezoidal cross-section. The upper base of the trapezoid is 4.87 mm long, the lower base is 6.75 mm long, and the height is 0.3 mm. It is then annealed online (annealing temperature 400℃), cooled, and passivated. The passivation uses RSB607 with a concentration of 10% and a solution temperature of 50±5℃. The immersion distance is 2 m, resulting in an irregular soft copper flat strip with the short side facing upward.

[0046] S3: The irregularly shaped soft copper flat strip is passed through a bending machine and a forming concave cam assembly in sequence to obtain a thin-walled copper groove wire. The inner width of the thin-walled copper groove wire is 1.55mm, the groove depth is 2.46mm, and the wall thickness is 0.3mm. Then, the thin-walled copper groove wire is passed through a flux machine (with flux added) and a liquid Sn0.7Cu solder bath. The solder temperature is set to 300℃ and the embedding and welding speed is 20m / min. After the solder temperature reaches the set temperature, the low copper ratio composite round wire and the thin-walled copper groove wire are passed through a 2.2mm diameter pre-bonding mold (the pre-bonding mold is immersed in liquid Sn0.7Cu solder) and a 2.05mm diameter embedding mold in sequence. The processing amount is 6.0%. Welding is performed at the embedding mold. After cooling, a round NbTi superconducting composite wire is obtained.

[0047] S4: Stretch the circular NbTi superconducting composite wire to a diameter of 1.99 mm with a processing amount of 5.8% to obtain the high n-value NbTi superconducting wire.

[0048] Using an electric field criterion of 1μV / cm, the n-value of the wire was tested to be 50.9, and the copper over-ratio of the wire was tested to be 4.95.

[0049] Example 3 This embodiment provides a method for preparing high-n-value NbTi superconducting wires, specifically including the following steps: S1: Prepare a low copper ratio NbTi / Cu composite round wire with a diameter of 1.43mm and a copper ratio of 2.3.

[0050] S2: Oxygen-free copper wire with a diameter of 1.85 mm is straightened and flattened to obtain a flat strip of 4.55 × 0.49 mm. It is rolled in 3 passes to obtain an irregular copper flat strip with a length of 4.55 mm, a minimum thickness of 0.3 mm, a concave arc radius of 0.37 mm, an angle of 67.1°, a convex arc radius of 0.51 mm, and an angle of 90°. It is then annealed online (annealing temperature 400℃), cooled, and passivated. The passivation uses RSB607 with a concentration of 10% and a solution temperature of 50±5℃, with an immersion distance of 2 m, to obtain an irregular soft copper flat strip with the plane facing upward.

[0051] S3: The irregularly shaped soft copper flat strip is passed through a bending machine and a forming concave cam assembly in sequence to obtain a thin-walled copper groove wire. The thin-walled copper groove wire has a side length of 1.87mm, a wall thickness of 0.21mm, an R angle of 0.51mm, a bending angle 1 at the bottom edge of 120°, and bending angles 2 on both sides of the notch of 90°. Then, the thin-walled copper groove wire is passed through a flux machine (with flux added) and a liquid Sn0.7Cu solder bath. The solder temperature is set to 300℃ and the embedding and welding speed is 20m / min. After the solder temperature reaches the set temperature, the low copper ratio composite round wire and the thin-walled copper groove wire are passed through a pre-bonding mold (the pre-bonding mold is immersed in liquid Sn0.7Cu solder, and the mold bending angle 1 is 100°) and an embedding mold with a size of 1.83×1.83mm in sequence. The processing amount is 10%. Welding is performed at the embedding mold, and then cooled to obtain a square NbTi superconducting composite wire.

[0052] S4: The square NbTi superconducting composite wire is rolled and stretched to a final rectangular cross-section size of 2.0×1.4mm, with a processing amount of 14.8%, to obtain the high n-value NbTi superconducting wire.

[0053] Using an electric field criterion of 1μV / cm, the n-value of the wire was 43.5, and the copper over-ratio of the wire was 6.

[0054] Example 4 This embodiment provides a method for preparing high-n-value NbTi superconducting wires, specifically including the following steps: S1: Prepare a low copper ratio NbTi / Cu composite round wire with a diameter of 0.75mm and a copper ratio of 1.8.

[0055] S2: Oxygen-free copper wire with a diameter of 0.9 mm is straightened and flattened to obtain a flat strip of 2.48 × 0.20 mm. It is rolled in 3 passes to obtain an irregular copper flat strip with a length of 2.48 mm, a minimum thickness of 0.1 mm, a concave arc radius of 0.26 mm, an angle of 73.8°, a convex arc radius of 0.22 mm, and an angle of 90°. It is then annealed online (annealing temperature 200℃), cooled, and passivated. The passivation uses RSB607 with a concentration of 5% and a solution temperature of 50±5℃, and the immersion distance is 1 m to obtain an irregular soft copper flat strip with the plane facing upward.

[0056] S3: The irregularly shaped soft copper flat strip is passed through a bending machine and a forming concave cam assembly in sequence to obtain a thin-walled copper groove wire. The thin-walled copper groove wire has a side length of 0.93mm, a wall thickness of 0.07mm, an R angle of 0.22mm, a bending angle 1 at the bottom edge of 120°, and bending angles 2 on both sides of the notch of 90°. Then, the thin-walled copper groove wire is passed through a flux machine (with flux added) and a liquid Sn0.7Cu solder bath. The solder temperature is set to 260℃ and the embedding and welding speed is 120m / min. After the solder temperature reaches the set temperature, the low copper ratio composite round wire and the thin-walled copper groove wire are passed through a pre-bonding mold (the pre-bonding mold is immersed in liquid Sn0.7Cu solder, and the mold bending angle 1 is 100°) and an embedding mold with a size of 0.89×0.89mm in sequence. The processing amount is 8.7%. Welding is performed at the embedding mold, and then cooled to obtain a square NbTi superconducting composite wire.

[0057] S4: The square NbTi superconducting composite wire is rolled and stretched to a final rectangular cross-section size of 0.99×0.75mm, with a processing amount of 19.6%, to obtain the high n-value NbTi superconducting wire.

[0058] Using an electric field criterion of 1μV / cm, the n-value of the wire was tested to be 45.5, and the copper over-ratio of the wire was tested to be 4.2.

[0059] Comparative Example 1 The preparation method of the NbTi superconducting wire in this comparative example is the same as that in Example 1, except that the thin-walled copper groove wire in S3 is replaced with the large R-angle irregular copper groove wire from the patent application number CN201711422215.2. Because the wall thickness at the R-angle of the large R-angle irregular copper groove wire is significantly greater than at other locations, uneven stress occurs during the inlay welding with the low copper ratio superconducting round wire, leading to eccentricity of the round wire. If the R-angle is further increased to eliminate the wall thickness difference, the bottom of the copper groove wire becomes close to an arc, making it impossible to ensure stable winding to the copper groove reel with the groove facing upwards. Simultaneously, since the long wire must have a relatively thick wall and sufficient strength to prevent deformation when wound to the copper groove reel, the copper ratio of the superconducting wire after inlay is too large, failing to meet the requirement of a copper ratio of 3-6 for liquid helium-free superconducting round wire.

[0060] Comparative Example 2 The preparation method of the NbTi superconducting wire in this comparative example is the same as that in Example 1, except that the trapezoidal upper base length of the S2 irregular soft copper flattening is 3.2 mm. The prepared NbTi superconducting wire exhibits localized quenching loss.

[0061] As described above, the basic principles, main features, and advantages of the present invention have been well described. The above embodiments and specifications are merely descriptions of preferred embodiments of the present invention, and the present invention is not limited to the above embodiments. Various changes and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit and scope of the present invention should fall within the protection scope defined by the present invention.

Claims

1. A method of producing a high-n-value NbTi superconducting wire, characterized by, The application relates to a high-n-value NbTi superconducting wire material and a preparation method thereof. The oxygen-free copper wire is prepared into a special-shaped soft copper flat strip, a thin-wall copper slot wire is obtained through a bending machine and a forming concave-convex wheel set; The NbTi / Cu composite round wire is placed on the thin-wall copper slot wire, and a large-copper-ratio NbTi superconducting composite wire is obtained after pre-combination and embedding and cooling; The high-n-value NbTi superconducting wire material is obtained after stretching; The special-shaped soft copper flat strip is a short-side-up special-shaped soft copper flat strip or a plane-up special-shaped soft copper flat strip; The cross section of the short-side-up special-shaped soft copper flat strip is trapezoidal, and the plane-up special-shaped soft copper flat strip is a continuous wave shape composed of a concave arc and a convex arc.

2. The method of making a high-n-value NbTi superconducting wire of claim 1, wherein, When the short-side-up special-shaped soft copper flat strip is used, a U-shaped thin-wall copper slot wire is obtained through a bending machine and a forming concave-convex wheel set; The inner width of the U-shaped thin-wall copper slot wire is 1.03-1.08 times the diameter of the low-copper-ratio composite round wire, and the thickness of the U-shaped thin-wall copper slot wire is 0.12-0.3 mm; When the plane-up special-shaped soft copper flat strip is used, the thin-wall copper slot wire obtained through a bending machine and a forming concave-convex wheel set is a round-cornered pentagon with a missing top corner; The bending angle of the two bottom corners of the round-cornered pentagon with a missing top corner is 120 DEG, the bending angle of the two corners on the two sides of the top corner is 90 DEG, and the processing amount of the round-cornered pentagon with a missing top corner is 0-1%.

3. The method of making a high-n-value NbTi superconducting wire of claim 1, wherein, The short-side length of the short-side-up special-shaped soft copper flat strip is 1.03-1.08 times the circumference of the NbTi / Cu composite round wire, and the long-side length is the circumference of a circle with a radius of the short-side length / 2pi+thickness; the thickness of the short-side-up special-shaped soft copper flat strip is 0.12-0.3 mm.

4. The method of making a high-n-value NbTi superconducting wire of claim 1, wherein, The thinnest thickness of the plane-up special-shaped soft copper flat strip is 0.1-0.3 mm, and the length of the plane is 1.01-1.05 times the circumference of the NbTi / Cu composite round wire; The convex arc angle of the plane-up special-shaped soft copper flat strip is 90 DEG; The convex arc radius calculation formula of the flat-upward special-shaped soft copper flat strip is: , wherein d1 is 1.01-1.05 times of the diameter of the NbTi / Cu composite round wire, and h is the thickness of the thinnest part of the special-shaped soft copper flat strip.

5. The method of making a high-n-value NbTi superconducting wire of claim 1, wherein, The copper ratio of the NbTi / Cu composite round wire is not higher than 2.3, and the diameter is not more than 1.5 mm.

6. The method of making a high-n-value NbTi superconducting wire of claim 1, wherein, The thin-wall copper slot wire passes through a soldering machine and a soldering slot before the pre-combination die, and liquid SnCu solder is placed in the soldering slot; The temperature of the liquid SnCu solder is 260-300 DEG C.

7. The method of making a high-n-value NbTi superconducting wire of claim 1, wherein, The pre-combination die is soaked in the liquid solder; The pre-combination die bends the two bottom corners of the round-cornered pentagon with a missing top corner to 100 DEG; The total processing amount of the pre-combination die and the embedding die is 5%-10%.

8. The method of making a high-n-value NbTi superconducting wire of claim 1, wherein, Welding is carried out at the embedding die, and the welding speed is 20-120 m / min.

9. The method of making a high-n-value NbTi superconducting wire of claim 1, wherein, When the cross section of the NbTi superconducting composite wire is a round-cornered square, rolling is further carried out before stretching; The total processing amount of the stretching or rolling and stretching is 5-20%.

10. A high n-value NbTi superconducting wire, characterized by, The high-n-value NbTi superconducting wire material is prepared by the preparation method in any one of claims 1-9; The copper super-ratio of the high-n-value NbTi superconducting wire material is 3-6, and the n value is not lower than 43.

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