A high-performance, low-loss NbTi superconducting wire and its preparation method

Abstract

This invention belongs to the field of superconducting wire technology and discloses a high-performance, low-loss NbTi superconducting wire and its preparation method. First, an NbTi rod, an Nb cylinder, and a copper tube are assembled to obtain an NbTi / Nb / Cu composite rod. After removing the surface copper, an NbTi / Nb composite rod is obtained. This NbTi / Nb composite rod is then inserted into a drilled Cu alloy ingot and fitted with a first oxygen-free copper tube to obtain a hexagonal composite rod. Finally, this hexagonal composite rod is arranged in a second oxygen-free copper tube, the gaps are filled, and then subjected to multiple drawing passes, multiple aging heat treatments, and twisting to obtain the NbTi superconducting wire. This method effectively improves the problem of poor core wire deformation in conventional NbTi / Cu superconducting wires, increases the critical current density of the wire, and reduces AC losses, meeting the requirements for superconducting magnets in nuclear fusion devices and other applications.

Description

Technical Field

[0001] This invention belongs to the field of superconducting wire technology, and relates to a high-performance, low-loss NbTi superconducting wire and its preparation method. Background Technology

[0002] As superconducting magnet technology continues to evolve, it is gradually developing towards larger current, higher magnetic field, and larger aperture. Especially in high-end fields such as nuclear fusion, superconducting magnets, represented by the Burning Plasma Experimental Superconducting Tokamak (BEST), have placed stringent requirements on NbTi superconducting wires for high critical current density and low AC loss.

[0003] Currently, conventional NbTi / Cu superconducting wires generally use high-purity oxygen-free copper as the matrix and are prepared through a one-step composite process. However, this preparation method has significant technical limitations: on the one hand, the overall deformation effect of the core wire is poor, resulting in low overall wire performance; on the other hand, the characteristics of the oxygen-free copper matrix make the NbTi core wire prone to coupling under high current conditions, leading to a large amount of AC loss, causing severe heating of the magnet, and ultimately resulting in frequent quench loss of the magnet.

[0004] In view of this, existing technologies can no longer meet the requirements of high-end equipment such as next-generation nuclear fusion devices for superconducting magnets. Developing high-performance, low-loss NbTi superconducting wires to completely solve the problems of insufficient performance and excessive AC loss of conventional wires has become the key to promoting the further development of superconducting magnet technology and has important technical value and practical application needs. Summary of the Invention

[0005] To address the core defects of existing conventional NbTi / Cu superconducting wires prepared by one-time composite fabrication with a high-purity oxygen-free copper matrix: poor uniformity of core wire deformation, resulting in low core performance such as critical current density; and the tendency of NbTi core wires to undergo electromagnetic coupling under high current conditions, generating significant AC losses, causing severe magnet heating and frequent quench loss, which cannot meet the stringent requirements of high critical current density and low AC loss for superconducting wires in next-generation nuclear fusion devices (such as the BEST project), this invention aims to provide a high-performance, low-loss NbTi superconducting wire and its preparation method, thoroughly solving the above-mentioned technical problems from the perspectives of process design and structural optimization.

[0006] In a first aspect, the present invention provides a method for preparing high-performance, low-loss NbTi superconducting wire, comprising the following steps:

[0007] Step 1, Preparation of single-core composite rod: After cleaning, assembling, welding, extruding, drawing, cutting to length and straightening, NbTi rod, Nb cylinder and copper tube are obtained by removing the surface copper layer. Step 2, preparation of secondary composite rod: Drill holes in Cu alloy ingot to obtain porous ingot, insert the NbTi / Nb single core composite rod obtained in step 1 into the porous ingot, and then coat it with a first oxygen-free copper tube. After cleaning, degassing, welding and extrusion, the secondary composite rod is obtained. Step 3, hexagonal forming process: The secondary composite rod is drawn, surface peeled, hexagonal formed, cut to length and straightened to obtain a hexagonal composite rod; Step 4, Preparation of finished wire: Multiple hexagonal composite rods are neatly arranged inside the second oxygen-free copper tube, and the gaps are filled with oxygen-free copper inserts. After integral rotary forging, cold drawing, twisting and final stretching, the finished NbTi superconducting wire is obtained.

[0008] Furthermore, in step one of the above preparation method, the thickness of the Nb tube is at least four-thousandths of the diameter of the NbTi rod; the copper tube is T2 copper or T3 copper.

[0009] Furthermore, in step one of the above preparation method, the drawing pass processing rate is 5~30%; the straightness after straightening is ≤3mm / 1000mm; the surface copper layer is removed by nitric acid, and the straightness after copper removal by nitric acid is ≤2mm / 1000mm.

[0010] Furthermore, in step two of the above preparation method, the Cu alloy ingot is a CuNi ingot or a CuMn ingot, the Ni content of the CuNi ingot is 1~60%, and the Mn content of the CuMn ingot is 1~40%; when the Ni mass fraction is ≥40% or the Mn mass fraction is ≥30%, the secondary composite rod needs to be subjected to high-temperature vacuum annealing; the pores of the porous ingot are hexagonally and uniformly arranged.

[0011] Furthermore, in step three of the above preparation method, the secondary composite rod is formed in two passes. The first processing rate is 15-25%, and the mold radius is 0.7-0.9 mm; the second processing rate is 8-15%, and the mold radius is 0.1-0.3 mm.

[0012] Furthermore, in the above preparation method, before step four, eddy current testing is required, and the filling rate detection threshold for eddy current testing is not less than 60%.

[0013] Furthermore, in step four of the above preparation method, multiple aging heat treatments are performed during the cold drawing process, with 3 to 8 aging heat treatments at a temperature of 200 to 600°C and an aging time of 5 to 200 hours for each treatment.

[0014] Furthermore, in step four of the above preparation method, the twisting pitch is 5~20mm, and the twisting direction is right-handed.

[0015] Secondly, the present invention provides a high-performance, low-loss NbTi superconducting wire prepared by the above-mentioned preparation method. The wire is composed of multiple composite units, oxygen-free copper inserts and an outer layer of second oxygen-free copper tubes. The composite units are arranged in a hexagonal uniform arrangement, and the gaps between the composite units and the second oxygen-free copper tubes are filled with oxygen-free copper inserts. The composite unit is formed by sequentially covering an NbTi core wire, an Nb barrier layer covering the NbTi core wire, a CuNi or CuMn alloy layer covering the Nb barrier layer, and a first oxygen-free copper tube layer.

[0016] Furthermore, the diameter of the NbTi superconducting wire is 0.7~0.9mm, and the total number of core wires is 2000~6500.

[0017] Compared with existing one-time composite preparation technologies, the present invention has the following significant advantages: This invention addresses the stringent requirements of the BEST fusion device's superconducting magnets for large core counts, high critical current densities, and low AC losses in NbTi superconducting wires. It overcomes the technical shortcomings of conventional multi-core NbTi / Cu wires, such as uneven core wire deformation, high core wire coupling losses, and the difficulty in balancing performance and cost. The invention forms an integrated technical solution encompassing structural isolation, step-by-step molding, and precise parameter control. The core technical improvements are as follows: 1. Construct a four-layer functional partitioned composite core wire structure to suppress core wire coupling and AC loss at the source. Breaking away from the traditional binary NbTi / Cu single structure, a four-layer gradient barrier structure of NbTi / Nb / CuNi(CuMn) / Cu is constructed, eliminating the root cause of AC loss at the material level through functional stratification. Precisely Thick Nb Barrier Layer: An Nb barrier layer with a thickness ≥ 0.4% of the NbTi rod diameter is set outside the NbTi core wire to completely isolate the electrical connection between the NbTi core wire and the external conductive substrate, blocking the current coupling between the core wires from the source. At the same time, the processing technology is strictly controlled to ensure that the Nb layer is free from damage and wrinkles, ensuring a stable barrier effect.

[0018] High-resistivity alloy intermediate layer: CuNi (Ni content 1~60%) or CuMn (Mn content 1~40%) alloys are used to replace the traditional pure copper matrix. The high resistivity of the alloy significantly increases the resistance between the core wires, effectively suppresses the generation of eddy currents, and further reduces AC loss. At the same time, the alloy layer can optimize the uniformity of core wire deformation and alleviate the stress unevenness during the processing.

[0019] High-purity oxygen-free copper outer layer: It balances current carrying and shunting with thermal conductivity and safety. When the wire is overloaded, it can quickly shunt the current to avoid local overheating, while efficiently dissipating working heat to prevent magnet overrun and solve the safety hazards caused by the barrier structure.

[0020] 2. A unique "three-step composite" preparation process solves the problem of uneven core wire deformation in multi-core NbTi superconducting wires. Abandoning the conventional method of preparing multi-core NbTi superconducting wires through a single composite process, this invention pioneers a three-step forming process: single-core composite → porous ingot composite → integral cable assembly. Combined with a refined arrangement design, this achieves synchronous and uniform deformation of the multi-core wires. Step-by-step composite with precise shape control: First, high-precision NbTi / Nb single core rods are prepared. Then, the single core rods are inserted into hexagonal porous alloy ingots with equal spacing to form multi-core composite rods. Finally, the multi-core rods are arranged into a cable. The deformation accuracy is controlled independently at each step to avoid the problems of uneven stress and large deformation differences in the core wires caused by single composite.

[0021] Progressive hexagonal molding process: For the hexagonal molding of multi-core rods, a two-stage progressive molding process is adopted to replace the traditional one-stage molding: the first molding has a processing rate of ≤25% and a mold radius of 0.8mm, and the second molding has a processing rate of ≤15% and a mold radius of 0.2mm. This effectively avoids core wire extrusion damage and structural deformation caused by one-stage molding, and ensures the accuracy and core wire integrity of hexagonal molding.

[0022] Hexagonal equidistant arrangement design: The drilling of the multi-hole ingot adopts a hexagonal equidistant arrangement to ensure that the spacing between adjacent NbTi core wires is completely consistent, so that the stress distribution of each core wire is uniform during processing, and the deformation consistency of the multi-core wire is guaranteed from the assembly level.

[0023] 3. A refined parameter control system throughout the entire process achieves both high performance and stability while improving efficiency. To address the challenges in processing multi-core NbTi superconducting wires, a targeted and refined parameter control system was established, overcoming the performance degradation bottleneck after processing high-core-count wires. Phased drawing parameter control: The processing rate is controlled in stages during the drawing process. The processing rate of the first few passes is ≤20% to eliminate assembly gaps and achieve tight bonding of components. The processing rate of subsequent passes is ≤30% to ensure uniform deformation of the core wire. Small-angle dies are used throughout the process. At the same time, the black skin is not removed during the single-core preparation stage and the core wire is drawn directly, which ensures the roundness of the core wire and reduces processing loss.

[0024] Stepped aging heat treatment: During the drawing process, 3 to 8 stepped aging heat treatments (temperature 200 to 600℃, single aging 5 to 200h) are inserted. Through multiple aging processes, the microstructure of the NbTi core wire is optimized, the uniform distribution of α-Ti precipitates is promoted, the critical current density (Jc) of the wire is significantly increased, and the problem of performance degradation after processing of multi-core NbTi superconducting wire is solved.

[0025] Low-loss twisting process: Right-hand twisting combined with a small pitch design of 5~20mm is adopted to shorten the electromagnetic coupling path of the core wire and further suppress eddy current loss; at the same time, the final stretching rate after twisting is strictly controlled at 5~30% to avoid the performance degradation of the core wire caused by twisting. Finally, the wire specification is precisely controlled to Φ0.8mm to meet the assembly requirements of BEST magnet.

[0026] 4. Optimize low cost and reliability in a synergistic way, taking into account both technological advancement and industrialization feasibility. Low-cost raw material design: In the single-core preparation stage, inexpensive T2 / T3 copper tubes are used to replace high-purity oxygen-free copper, and the wall thickness of the copper tubes is reduced as much as possible. Without affecting the performance of the final wire, the cost of raw materials is greatly reduced, which solves the industrialization problem of high cost of high-performance superconducting materials.

[0027] Defect control: After hexagonal molding, a targeted eddy current testing process is added. According to the requirement of ≥60% fill rate, a special probe is matched to detect defects such as core wire gaps and structural damage in advance, avoid finished product failure, and ensure the reliability of mass production of multi-core wires.

[0028] Through the above innovations, this invention overcomes the technical bottleneck of balancing "large core count" and "high performance and low loss" in multi-core NbTi superconducting wires: The critical current density Jc (4.2K, 5T) is 2850A / mm² for conventional wire. 2 Increased to 2988~3210A / mm 2 ; AC losses have been reduced significantly from 15W / m in conventional cables to 4-8W / m, a reduction of over 45%. It can stably produce multi-core wires of different specifications such as 2035 cores, 4675 cores, and 6270 cores, which are fully compatible with the core requirements of superconducting magnets for the BEST fusion device, providing key material support for the localization of superconducting magnets for compact fusion devices. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the assembly structure of the NbTi / Nb / CuNi / Cu secondary composite rod in Embodiment 1 of the present invention.

[0030] Figure 2 This is a schematic diagram of the finished wire assembly structure of Embodiment 1 of the present invention.

[0031] Figure 3 This is a schematic diagram of the assembly structure of the NbTi / Nb / CuNi / Cu secondary composite rod in Embodiment 2 of the present invention.

[0032] Figure 4 This is a schematic diagram of the finished wire assembly structure of Embodiment 2 of the present invention.

[0033] Figure 5 This is a schematic diagram of the assembly structure of the NbTi / Nb / CuMn / Cu secondary composite rod in Embodiment 3 of the present invention.

[0034] Figure 6 This is a schematic diagram of the finished wire assembly structure of Embodiment 3 of the present invention.

[0035] Figure 7 This is a diagram showing the morphology of the core wire inside the finished wire of the present invention.

[0036] Figure 8 This is a metallographic cross-sectional view of the interior of the finished wire of the present invention.

[0037] Explanation of reference numerals in the attached drawings: 1. NbTi / Nb single-core composite rod; 2. Cu alloy ingot; 3. First oxygen-free copper tube; 4. Secondary composite rod; 5. Oxygen-free copper insert; 6. Second oxygen-free copper tube. Detailed Implementation

[0038] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0039] This invention provides a method for preparing the high-performance, low-loss NbTi superconducting wire, specifically including the following steps: Step 1, Preparation of single-core composite rod: After cleaning, assembling, welding, extruding, drawing, cutting to length and straightening, NbTi rod, Nb cylinder and copper tube are obtained by removing the surface copper layer. Step 2, preparation of secondary composite rod: Drill holes in Cu alloy ingot to obtain porous ingot, insert the NbTi / Nb single core composite rod obtained in step 1 into the porous ingot, and then coat it with a first oxygen-free copper tube. After cleaning, degassing, welding and extrusion, the secondary composite rod is obtained. Step 3, hexagonal forming process: The secondary composite rod is drawn, surface peeled, hexagonal formed, cut to length and straightened to obtain a hexagonal composite rod; Step 4, Preparation of finished wire: Multiple hexagonal composite rods are neatly arranged inside the second oxygen-free copper tube, and the gaps are filled with oxygen-free copper inserts. After integral rotary forging, cold drawing, twisting and final stretching, the finished NbTi superconducting wire is obtained.

[0040] For example, in step one of the above preparation method, the thickness of the Nb tube is at least four-thousandths of the diameter of the NbTi rod; the copper tube is T2 copper or T3 copper. Preferably, the copper tube wall thickness is as thin as possible to reduce production costs; the extrusion temperature is as low as possible to ensure the roundness of NbTi / Nb (reasonable control of the extrusion temperature can improve the plasticity matching of NbTi, Nb, and Cu, and effectively improve the roundness of the NbTi / Nb single-core composite rod).

[0041] For example, in step one of the above preparation method, the drawing pass rate is 5-30%; the straightness deviation after straightening is ≤3mm / 1000mm; the surface copper layer is removed with nitric acid, and the straightness deviation after copper removal with nitric acid is ≤2mm / 1000mm. Preferably, a small-angle die is used for drawing to ensure the roundness of the NbTi / Nb single-core composite rod during the drawing process, and the entire processing does not require the removal of the surface black skin before drawing; the processed Nb barrier layer is free from defects such as breakage, wrinkles, and inclusions.

[0042] For example, in step two of the above preparation method, the Cu alloy ingot is a CuNi ingot or a CuMn ingot, wherein the Ni content of the CuNi ingot is 1-60%, and the Mn content of the CuMn ingot is 1-40%. When the Ni or Mn content is high (Ni mass fraction ≥ 40% or Mn mass fraction ≥ 30%), the secondary composite rod undergoes severe work hardening during subsequent processing, requiring high-temperature vacuum annealing. The pores of the porous ingot are hexagonally and uniformly arranged to ensure equal spacing between adjacent NbTi core wires and uniform stress distribution during processing. Preferably, the high-temperature vacuum annealing temperature is 300-900℃, the annealing time is 2-10h, and the vacuum degree is less than 10. -3 Pa. The purpose of the first oxygen-free copper tube is to facilitate good cold bonding at the Cu interface during subsequent cold-piercing processing, reduce the risk of wire breakage, and at the same time, to divert and conduct heat in a timely manner when the wire carries too much current.

[0043] For example, in step three of the above preparation method, the secondary composite rod is formed in two passes. The first pass has a processing rate of 15-25% and a die radius of 0.7-0.9 mm; the second pass has a processing rate of 8-15% and a die radius of 0.1-0.3 mm. The purpose of using two passes is to ensure a good hexagonal forming effect.

[0044] For example, before proceeding to step four, eddy current testing is required, and the fill rate detection threshold for eddy current testing is not less than 60%.

[0045] For example, in step four of the above preparation method, multiple aging heat treatments are performed during the cold drawing process. The number of aging heat treatments is 3 to 8, the temperature is 200 to 600°C, and the aging time is 5 to 200 hours each time. Preferably, the wall thickness of the second oxygen-free copper tube in step four should not be too thin, as this is not conducive to the transfer of drawing stress. The processing rate of the first few drawing passes is controlled within 20% to eliminate assembly gaps and ensure that the components are tightly bonded. The processing rate of subsequent multiple drawing passes is controlled within 30%, and a small-angle die is used to ensure that the composite rod is deformed uniformly.

[0046] For example, in step four of the above preparation method, in order to reduce losses, the twisting pitch is kept as small as possible. Specifically, the twisting pitch is 5~20mm, and the twisting direction is right-handed. Preferably, the final stretching rate is controlled at 5~30%, and the final specification of the finished NbTi superconducting wire is Φ0.7~0.9mm.

[0047] Secondly, embodiments of the present invention provide a high-performance, low-loss NbTi superconducting wire prepared by the above method. The wire is composed of multiple composite units, oxygen-free copper inserts, and an outer layer of second oxygen-free copper tubes. The composite units are arranged in a hexagonal uniform arrangement, and the gaps between the composite units and the second oxygen-free copper tubes are filled with oxygen-free copper inserts. The composite unit is formed by sequentially covering an NbTi core wire, an Nb barrier layer covering the NbTi core wire, a CuNi or CuMn alloy layer covering the Nb barrier layer, and a first oxygen-free copper tube layer.

[0048] For example, the wire diameter is 0.7~0.9mm and the total number of core wires is 2000~6500.

[0049] Example 1 This embodiment describes the fabrication of the high-performance, low-loss NbTi superconducting wire. The specific fabrication steps are as follows: Step 1: Prepare NbTi / Nb single-core composite rods Select an 80mm NbTi rod, a 0.4mm thick Nb cylinder, and a 6mm thick T2 copper tube (using commercially available low-cost T2 copper to control production costs; the copper tube wall thickness is as thin as possible according to the design). Also prepare matching upper and lower covers. First, thoroughly clean all components to remove surface oil, impurities, and oxide layers. Then, assemble them according to the pre-defined assembly relationship. Place the NbTi rod inside the Nb cylinder, and then insert them into the copper tube. Seal the upper and lower covers using a welding process to form a sealed composite blank.

[0050] The composite billet was subjected to low-temperature extrusion (extrusion temperature controlled at 600℃ to ensure the roundness of NbTi / Nb). After extrusion, it underwent multiple drawing passes, with each pass's processing rate controlled at 5-30%. A small-angle die was used for drawing to ensure that the roundness of NbTi / Nb was not affected during the drawing process. The entire process did not require the removal of the surface black skin; continuous drawing was performed until the composite billet was drawn to Φ12mm. The drawn composite rod was then cut to a predetermined length and straightened. The straightness deviation of the straightened NbTi / Nb / Cu composite rod was controlled within 3mm / 1000mm. Finally, the straightened composite rod was etched with nitric acid to completely remove the Cu layer on its surface, obtaining a nearly circular NbTi / Nb single-core composite rod with a Φ10.5mm diameter. The straightness deviation of this single-core composite rod was controlled within 2mm / 1000mm, and the surface Nb barrier layer was free of defects such as breakage, wrinkles, and inclusions.

[0051] Step 2: Prepare NbTi / Nb / CuNi / Cu secondary composite rods A CuNi alloy ingot with a diameter of 120mm (Ni content of 10%) was selected and deep-hole drilling was performed on it, resulting in 55 holes with a diameter of 11mm. All holes were arranged in a hexagonal pattern with a spacing of 3mm between adjacent holes to ensure that the spacing between adjacent NbTi core wires was equal and the stress distribution was uniform during the processing. This process yielded a drilled porous ingot.

[0052] After cleaning the NbTi / Nb single-core composite rod prepared in step 1, it was sequentially inserted into each hole of the drilled CuNi alloy ingot. Then, a high-purity oxygen-free copper tube with an outer diameter of Φ150mm and a wall thickness of 15mm was fitted over the outside of the drilled CuNi alloy ingot. For example... Figure 1 As shown, in this assembly structure, the first oxygen-free copper tube facilitates good cold bonding at the Cu interface during subsequent cold-piercing processing, reducing the risk of wire breakage. It also provides timely heat diversion when the wire current is too high. The assembled components are then cleaned and degassed again to remove internal gases and surface impurities before welding and sealing. Following this, extrusion processing is performed to obtain a secondary NbTi / Nb / CuNi / Cu composite rod.

[0053] Step 3, hexagonal molding process The NbTi / Nb / CuNi / Cu secondary composite rod obtained in step 2 was subjected to multiple cold drawing processes. During the drawing process, a peeling die was used to remove the black oxide layer and surface defects generated during the drawing process. To ensure a good hexagonal forming effect, a two-pass forming process was adopted: the processing rate of the first forming was controlled within 25%, and a die with an R-angle of 0.8 mm was selected; the processing rate of the second forming was controlled within 15%, and a die with an R-angle of 0.2 mm was selected. Finally, the composite rod was formed into a hexagonal structure with an H5.0 mm diameter. After forming, it was cut to length, and hexagonal composite rods with a length of 4000 mm were cut. After straightening, eddy current testing was performed using a suitable eddy current probe (the probe was selected based on the requirement that the filling rate should not be less than 60%) to ensure that there were no obvious defects inside the composite rod and that the filling was dense.

[0054] Step 4, Prepare finished wire A high-purity oxygen-free copper tube with an outer diameter of Φ42mm and an inner diameter of Φ36mm was selected as the outer coating tube. Thirty-seven hexagonal NbTi / Nb / CuNi / Cu secondary composite rods, each 4000mm long and H5.0mm in diameter, prepared in step 3, were neatly arranged inside the second oxygen-free copper tube. To fill the gap between the second oxygen-free copper tube and the composite rods, six Φ3.0mm oxygen-free copper inserts and twelve Φ2.0mm oxygen-free copper inserts were used to ensure a tight fit between the components without significant gaps. Figure 2 As shown, this filling structure further ensures the uniform transmission of stress during subsequent drawing processes.

[0055] The filled monolithic billet is subjected to integral rotary forging, followed by multiple cold drawing passes: the first few drawing passes have a machining rate controlled within 20% to eliminate assembly gaps and ensure tight bonding of the components; subsequent drawing passes have a machining rate controlled within 30%, using small-angle dies to ensure uniform deformation of the composite bar. Five aging heat treatments are performed during the drawing process, each at a temperature controlled at 400℃ for 30 hours. After drawing to near the final specifications, the composite wire is right-hand twisted with a twist pitch controlled at 6mm (to reduce AC loss), and finally, a final stretching process (machining rate controlled between 5% and 30%) yields a finished wire with a diameter of 0.8mm.

[0056] The finished product prepared in this embodiment is a 2035-core NbTi / Nb / CuNi / Cu composite superconducting wire. Through a multi-layered composite structure and precise process control, the uniformity of core wire deformation and electromagnetic properties are effectively optimized. Testing showed that the wire achieves a critical current density Jc of 3210 A / mm² under conditions of 4.2 K and 5 T. 2 The AC loss is 8W / m, compared to the conventional assembly method + hot extrusion prepared NbTi / Cu multi-core composite wire (Jc is 2850A / mm). 2With an AC loss of 15W / m, its performance has been significantly improved, meeting the requirements for superconducting magnets in next-generation nuclear fusion devices.

[0057] Example 2 This embodiment describes the fabrication of the high-performance, low-loss NbTi superconducting wire. The specific fabrication steps are as follows: Step 1: Prepare NbTi / Nb single-core composite rods Select NbTi rods with a diameter of 120mm, Nb cylinders with a thickness of 0.5mm, and T3 copper tubes with a wall thickness of 6mm (use commercially available low-cost T3 copper to control production costs, and make the copper tubes as thin as possible according to the design). Also prepare matching upper and lower covers. First, thoroughly clean all the components to remove surface oil, impurities, and oxide layers. Then, assemble them according to the pre-set assembly relationship, placing the NbTi rods inside the Nb cylinders, and then inserting them into the copper tubes. Seal the upper and lower covers using a welding process to form a sealed composite blank.

[0058] The composite billet was subjected to low-temperature extrusion (extrusion temperature controlled at 600℃ to ensure the roundness of NbTi / Nb). After extrusion, it underwent multiple drawing passes, with each pass's processing rate controlled at 5-30%. A small-angle die was used for drawing to ensure that the roundness of NbTi / Nb was not affected during the drawing process. The entire process did not require the removal of the surface black skin; continuous drawing was performed until the composite billet was drawn to Φ6mm. The drawn composite rod was then cut to a predetermined length and straightened. The straightness deviation of the straightened NbTi / Nb / Cu composite rod was controlled within 3mm / 1000mm. Finally, the straightened composite rod was etched with nitric acid to completely remove the Cu layer on its surface, obtaining a nearly circular NbTi / Nb single-core composite rod with a diameter of Φ5.5mm. The straightness deviation of this single-core composite rod was controlled within 2mm / 1000mm, and the surface Nb barrier layer was free of defects such as breakage, wrinkles, and inclusions.

[0059] Step 2: Prepare NbTi / Nb / CuNi / Cu secondary composite rods A CuNi alloy ingot with a diameter of 75mm (40% Ni content, which is relatively high and prone to work hardening) was selected and deep-hole drilled. A total of 85 holes with a diameter of 6mm were drilled. All holes were arranged in a hexagonal pattern with a spacing of 2mm between adjacent holes to ensure that the spacing between adjacent NbTi core wires was equal and the stress distribution was uniform during the processing. This resulted in a drilled porous ingot.

[0060] After cleaning the NbTi / Nb single-core composite rod prepared in step 1, it was sequentially inserted into each hole of the drilled CuNi alloy ingot. Then, a high-purity oxygen-free copper tube with an outer diameter of Φ125mm and a wall thickness of 25mm was fitted over the drilled CuNi alloy ingot. For example... Figure 3 As shown, in this assembly structure, the first oxygen-free copper tube facilitates good cold bonding at the Cu interface during subsequent cold-piercing processing, reducing the risk of wire breakage. It also provides timely heat diversion when the wire current is too high. The assembled components are then cleaned and degassed again to remove internal gases and surface impurities before welding and sealing. Following this, extrusion processing is performed to obtain a secondary NbTi / Nb / CuNi / Cu composite rod. Due to the high Ni content in this composite rod, severe work hardening occurs during subsequent processing, necessitating high-temperature vacuum annealing at 850℃ for 6 hours with a vacuum degree less than 10. -3 Pa, to improve its processing performance.

[0061] Step 3, hexagonal molding process The NbTi / Nb / CuNi / Cu secondary composite rod, after annealing in step 2, underwent multiple cold drawing processes. During the drawing process, a peeling die was used to remove the black oxide layer and surface defects generated during the drawing process. To ensure a good hexagonal forming effect, a two-pass forming process was adopted: the processing rate of the first forming was controlled within 25%, and a die with an R-angle of 0.8mm was selected; the processing rate of the second forming was controlled within 15%, and a die with an R-angle of 0.2mm was selected, ultimately forming the composite rod into a hexagonal structure with an H6.5mm diameter. After forming, it was cut to length, and hexagonal composite rods with a length of 6000mm were cut. After straightening, eddy current testing was performed using a suitable eddy current probe (the probe was selected based on the requirement that the filling rate should not be less than 60%) to ensure that there were no obvious defects inside the composite rod and that the filling was dense.

[0062] Step 4, Prepare finished wire A high-purity oxygen-free copper tube with an outer diameter of Φ64mm and an inner diameter of Φ56mm was selected as the outer coating tube. Fifty-five hexagonal NbTi / Nb / CuNi / Cu secondary composite rods, each 6000mm long and H6.5mm in diameter, prepared in step 3, were neatly arranged inside the second oxygen-free copper tube. To fill the gap between the second oxygen-free copper tube and the composite rods, six Φ4.0mm oxygen-free copper inserts were used to ensure a tight fit between the components without significant gaps. Figure 4 As shown, this filling structure further ensures the uniform transmission of stress during subsequent drawing processes.

[0063] The filled monolithic billet is subjected to integral rotary forging, followed by multiple cold drawing passes: the first few drawing passes have a machining rate controlled within 20% to eliminate assembly gaps and ensure tight bonding of the components; subsequent drawing passes have a machining rate controlled within 30%, and small-angle dies are used to ensure uniform deformation of the composite bar. Seven aging heat treatments are performed during the drawing process, with each aging temperature controlled at 500℃ and each aging time at 15 hours. After drawing to near the final specifications, the composite wire is subjected to right-hand twisting with a twist pitch controlled at 10mm (to reduce AC loss). Finally, a final stretching process (machining rate controlled at 5-30%) yields a finished wire with a diameter of Φ0.8mm.

[0064] The finished product prepared in this embodiment is a 4675-core NbTi / Nb / CuNi / Cu composite superconducting wire. Through process optimization adapted to high Ni content scenarios and a multi-layer composite structure design, the uniformity of core wire deformation and processing stability are effectively improved. Testing showed that the critical current density Jc of this wire reaches 3015 A / mm² under conditions of 4.2K and 5T. 2 The AC loss is 6W / m, compared to the conventional assembly method + hot extrusion prepared NbTi / Cu multi-core composite wire (Jc is 2850A / mm). 2 With an AC loss of 15W / m, its performance has been significantly improved, meeting the stringent requirements of superconducting magnets in next-generation nuclear fusion devices.

[0065] Example 3 This embodiment describes the fabrication of the high-performance, low-loss NbTi superconducting wire. The specific fabrication steps are as follows: Step 1: Prepare NbTi / Nb single-core composite rods Select Φ50mm NbTi rods, 0.25mm thick Nb cylinders, and 4mm thick T2 copper tubes (using commercially available low-cost T2 copper to control production costs, and ensuring the copper tube wall thickness is as thin as possible according to the design). Also prepare matching upper and lower covers. First, thoroughly clean all components to remove surface oil, impurities, and oxide layers. Then, assemble them according to the pre-defined assembly relationship, placing the NbTi rods inside the Nb cylinders, and then fitting them into the copper tubes. Seal the upper and lower covers using a welding process to form a sealed composite blank.

[0066] The composite billet was subjected to low-temperature extrusion (extrusion temperature controlled at 600℃ to ensure the roundness of NbTi / Nb). After extrusion, it underwent multiple drawing passes, with each pass's processing rate controlled at 5-30%. Small-angle dies were used for drawing to ensure that the roundness of NbTi / Nb was not affected during the drawing process. The entire process did not require the removal of the surface black skin; continuous drawing was performed until the composite billet was drawn to Φ9.3mm. The drawn composite rod was then cut to a predetermined length and straightened. The straightness deviation of the straightened NbTi / Nb / Cu composite rod was controlled within 3mm / 1000mm. Finally, the straightened composite rod was etched with nitric acid to completely remove the Cu layer on its surface, obtaining a nearly circular NbTi / Nb single-core composite rod with a diameter of Φ8.0mm. The straightness deviation of this single-core composite rod was controlled within 2mm / 1000mm, and the surface Nb barrier layer was free of defects such as breakage, wrinkles, and inclusions.

[0067] Step 2: Prepare NbTi / Nb / CuMn / Cu secondary composite rods A CuMn alloy ingot with a diameter of 140mm (Mn content of 7%) was selected and deep-hole drilling was performed on it, resulting in 114 holes with a diameter of 8.5mm. All holes were arranged in a hexagonal pattern with a spacing of 2.5mm between adjacent holes to ensure that the spacing between adjacent NbTi core wires was equal and the stress distribution was uniform during the processing. This process yielded a drilled porous ingot.

[0068] After cleaning the NbTi / Nb single-core composite rod prepared in step 1, it was sequentially inserted into each hole of the drilled CuMn alloy ingot. Then, a high-purity oxygen-free copper tube with an outer diameter of Φ180mm and a wall thickness of 20mm was fitted over the outside of the drilled CuMn alloy ingot. Figure 5 As shown, in this assembly structure, the first oxygen-free copper tube facilitates good cold bonding at the Cu interface during subsequent cold-piercing processing, reducing the risk of wire breakage. It also provides timely heat diversion when the wire current is too high. The assembled components are then cleaned and degassed again to remove internal gases and surface impurities before welding and sealing. Following this, extrusion processing is performed to obtain a secondary NbTi / Nb / CuMn / Cu composite rod.

[0069] Step 3, hexagonal molding process The NbTi / Nb / CuMn / Cu secondary composite rod obtained in step 2 was subjected to multiple cold drawing processes. During the drawing process, a peeling die was used to remove the black oxide layer and surface defects generated during the drawing process. To ensure a good hexagonal forming effect, a two-pass forming process was adopted: the processing rate of the first forming was controlled within 25%, and a die with an R-angle of 0.8 mm was selected; the processing rate of the second forming was controlled within 15%, and a die with an R-angle of 0.2 mm was selected. Finally, the composite rod was formed into a hexagonal structure with an H of 5.3 mm. After forming, it was cut to length, and hexagonal composite rods with a length of 8000 mm were cut. After straightening, eddy current testing was performed using a suitable eddy current probe (the probe was selected according to the requirement that the filling rate should not be less than 60%) to ensure that there were no obvious defects inside the composite rod and that the filling was dense.

[0070] Step 4, Prepare finished wire A high-purity oxygen-free copper tube with an outer diameter of Φ56mm and an inner diameter of Φ46mm was selected as the outer coating tube. Fifty-five hexagonal NbTi / Nb / CuMn / Cu secondary composite rods, each 6000mm long and with an H5.3mm diameter prepared in step 3, were neatly arranged inside the second oxygen-free copper tube. To fill the gap between the second oxygen-free copper tube and the composite rods, six Φ3.3mm oxygen-free copper inserts were used to ensure a tight fit between the components without significant gaps. Figure 6 As shown, this filling structure further ensures the uniform transmission of stress during subsequent drawing processes.

[0071] The filled monolithic billet is subjected to integral rotary forging, followed by multiple cold drawing passes: the first few drawing passes have a machining rate controlled within 20% to eliminate assembly gaps and ensure tight bonding of the components; subsequent drawing passes have a machining rate controlled within 30%, using small-angle dies to ensure uniform deformation of the composite bar. Three aging heat treatments are performed during the drawing process, each at a temperature controlled at 300℃ for 40 hours. After drawing to near the final specifications, the composite wire is right-hand twisted with a twist pitch controlled at 8mm (to reduce AC loss), and finally, a final stretching process (machining rate controlled between 5% and 30%) yields a finished wire with a diameter of 0.8mm.

[0072] The finished product prepared in this embodiment is a 6270-core NbTi / Nb / CuMn / Cu composite superconducting wire. Through the synergistic design of the CuMn alloy matrix and the multilayer composite structure, the uniformity of the core wire distribution and electromagnetic properties are effectively optimized. Testing showed that the critical current density Jc of this wire reaches 2988 A / mm² under conditions of 4.2 K and 5 T. 2 The AC loss is 4W / m, compared to the conventional assembly method + hot extrusion prepared NbTi / Cu multi-core composite wire (Jc is 2850A / mm). 2With an AC loss of 15W / m, its performance has been significantly improved, meeting the stringent requirements of superconducting magnets in next-generation nuclear fusion devices.

[0073] Example 4 This embodiment describes the performance of the high-performance, low-loss NbTi superconducting wires prepared in Examples 1-3.

[0074] 1. Test Objective For the NbTi superconducting wires prepared in Examples 1-3, we verified whether their structural characteristics and core performance met the design standards, and clarified the wires' performance in terms of critical current density, AC loss and microstructure integrity.

[0075] 2. Test Sample The finished wires prepared in Examples 1-3 were selected as test samples. Meanwhile, commercially available conventional NbTi / Cu superconducting wires (Jc=2850A / mm under 4.2K, 5T conditions) were also selected. 2 As a comparison sample (AC loss = 15W / m), the basic information of the sample is shown in Table 1 below: Table 1. Basic Information of Test Samples

[0076] 3. Testing Methods 1) Microstructure characterization Each test sample was cut into 10mm segments and etched with a 30% nitric acid aqueous solution to remove the surface CuNi / CuMn alloy layer and the first oxygen-free copper tube layer, while retaining the NbTi core wire and the Nb barrier layer. The morphology of the core wire and the metallographic cross-sectional structure were observed using a scanning electron microscope (SEM, model: ZEISS Sigma 300) with an accelerating voltage of 15kV.

[0077] 2) Critical current density (Jc) test The test was conducted according to GB / T 21546 standard. Test conditions: ambient temperature 4.2K (liquid helium temperature range), applied magnetic field 5T, current rise rate 0.1A / s, and the critical current (Ic) was determined using a voltage criterion of 1μV / cm. Calculation method: Critical current density Jc = Ic / S, where S is the total cross-sectional area of ​​all NbTi core wires in the wire.

[0078] 3) AC loss test The test platform was built independently (equipped with AC power supply, nanovoltmeter, lock-in amplifier and four-probe fixture). The test conditions were: ambient temperature 4.2K, external alternating magnetic field ±3T, test frequency 50Hz (simulating the actual working frequency of nuclear fusion device), and the AC power loss of the wire was measured by the four-probe method.

[0079] 4. Test Results and Analysis 1) Microstructure characterization results SEM observation showed that the NbTi core wires of samples S1, S2, and S3 had smooth and flat surfaces, uniform thickness, and the Nb barrier layer continuously and completely covered the surface of the NbTi core wires without any damage, wrinkles, or inclusions. Figure 7 Metallographic cross-section shows that the white layer is an Nb barrier layer, uniformly coating the NbTi surface, continuous, intact, and without damage. Figure 8 ).

[0080] 2) Critical current density (Jc) Test results show that the critical current density of samples S1, S2, and S3 is not lower than 2988 A / mm². 2 Compared with the control sample CK (2850A / mm) 2 Significant improvement, with specific data shown in Table 2 below: Table 2. Critical current density test results (4.2K, 5T)

[0081] As shown in Table 2, this invention solves the problem of uneven core wire deformation in conventional one-time composite processes by using a multi-stage composite molding process combined with precise control such as small-angle die drawing and multi-stage hexagonal molding. At the same time, multiple aging heat treatments optimize the microstructure of the NbTi core wire, improve the superconducting performance, and ultimately achieve a significant increase in Jc.

[0082] 3) AC loss Test results show that the AC losses of samples S1, S2, and S3 are all no higher than 8 W / m, which is much lower than that of the control sample CK (15 W / m). The specific data are shown in Table 3 below: Table 3. AC loss test results (4.2K, ±3T, 50Hz)

[0083] The Nb barrier layer effectively blocks the electromagnetic coupling path between NbTi core wires, avoiding current interference between core wires; the conductivity of the CuNi / CuMn alloy matrix is ​​lower than that of high-purity oxygen-free copper, suppressing current conduction interference and reducing coupling loss; the small-pitch twisted design (5~20mm) further disrupts the electromagnetic coupling conditions between core wires, reducing eddy current losses under alternating magnetic fields; the synergistic effect of the above structure and process fundamentally reduces the AC loss of the wire, avoiding magnet heating and frequent quenching.

[0084] The high-performance, low-loss NbTi superconducting wires prepared in Examples 1-3 exhibit good core wire uniformity and a continuous and intact Nb barrier layer; the critical current density at 4.2K and 5T is not less than 2988A / mm². 2The AC loss is ≤8W / m, significantly better than conventional NbTi / Cu superconducting wires. The results of this embodiment fully verify the effectiveness of the multi-stage composite process and structural design of the present invention, and the wire performance can meet the stringent requirements of high-end equipment such as next-generation nuclear fusion devices for superconducting magnets.

[0085] The embodiments described above are only some, not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art through related deductions and substitutions based on the inventive concept, without inventive effort, are within the scope of protection of the present invention.

Claims

1. A method for producing a high performance, low loss NbTi superconducting wire, characterized by, The method comprises the following steps: Step one, preparing a single-core composite rod: after cleaning, assembling, welding, extruding, drawing, cutting to size, and straightening, the NbTi rod, Nb cylinder, and red copper pipe are removed from the surface to obtain a NbTi / Nb single-core composite rod; Step two, preparing a secondary composite rod: after drilling a Cu alloy ingot to obtain a porous ingot, inserting the NbTi / Nb single-core composite rod obtained in step one into the porous ingot, and then sleeving a first oxygen-free copper pipe, the secondary composite rod is obtained through cleaning, degassing, welding, and extruding; Step three, hexagonal forming treatment: the secondary composite rod is subjected to drawing, surface peeling treatment, hexagonal forming, cutting to size, and straightening to obtain a hexagonal composite rod; Step four, preparing a finished wire rod: a plurality of hexagonal composite rods are arranged in a second oxygen-free copper pipe, the gap is filled with an oxygen-free copper plug, and the finished NbTi superconducting wire rod is obtained through overall rotary swaging, cold drawing, twisting, and final stretching.

2. The production method according to claim 1, characterized by, In step one: the thickness of the Nb cylinder is at least 0.004 times the diameter of the NbTi rod; and the red copper pipe is T2 copper or T3 copper.

3. The preparation method according to claim 1, characterized in that, In step one, the drawing pass processing rate is 5-30%, and the straightness after straightening is ≤3 mm / 1000 mm; nitric acid is used to remove the surface copper layer, and the straightness after nitric acid copper removal is ≤2 mm / 1000 mm.

4. The method of claim 1, wherein, In step two, the Cu alloy ingot is a CuNi ingot or a CuMn ingot, the Ni content of the CuNi ingot is 1-60%, and the Mn content of the CuMn ingot is 1-40%; when the mass fraction of Ni is ≥40% or the mass fraction of Mn is ≥30%, the secondary composite rod needs to be subjected to high-temperature vacuum annealing; and the holes of the porous ingot are uniformly arranged in a hexagonal shape.

5. The preparation method according to claim 1, characterized in that, In step three, the secondary composite rod is formed in two passes, the first pass processing rate is 15-25%, and the die R angle is 0.7-0.9 mm; the second pass processing rate is 8-15%, and the die R angle is 0.1-0.3 mm.

6. The method of claim 1, wherein, Before step four, eddy current testing is also required, and the filling rate detection threshold of the eddy current testing is not less than 60%.

7. The preparation method according to claim 1, characterized in that, In step four, multiple aging heat treatments are performed during cold drawing, the number of aging heat treatments is 3-8 times, the temperature is 200-600°C, and the aging time of each time is 5-200 h.

8. The method of claim 1, wherein, In step four, the twisting pitch is 5-20 mm, and the twisting direction is right-handed.

9. A high performance low loss NbTi superconducting wire produced by the method of claim 1, wherein, The wire rod is composed of a plurality of composite units, oxygen-free copper plugs, and a second oxygen-free copper pipe; the composite units are uniformly arranged in a hexagonal shape, and the gap between the composite units and the second oxygen-free copper pipe is filled with oxygen-free copper plugs; The composite unit is formed by sequentially coating a NbTi core wire, a Nb barrier layer coated on the NbTi core wire, a CuNi or CuMn alloy layer coated on the Nb barrier layer, and a first oxygen-free copper pipe layer.

10. The high performance low loss NbTi superconducting wire of claim 9, wherein, The diameter of the wire rod is 0.7-0.9 mm, and the total number of core wires is 2000-6500.

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