A preparation method of NbTi superconducting wire for a cryogen-free superconducting magnet

Abstract

The present invention relates to a method for preparing NbTi superconducting wire for a liquid-helium-free superconducting magnet, comprising the following steps: processing an oxygen-free copper single-core ingot sheath and an oxygen-free copper composite sheath; uniformly wrapping an Nb barrier layer around an NbTi ingot, placing it into the oxygen-free copper single-core ingot sheath, and performing vacuum electron beam welding to obtain an NbTi single-core ingot, and subjecting the NbTi single-core ingot to superconducting induction heating to obtain an NbTi single-core rod; uniformly arranging multiple NbTi single-core rods and oxygen-free copper rods of the same specification in the oxygen-free copper composite sheath, performing vacuum electron beam welding to obtain an NbTi composite ingot, subjecting the NbTi composite ingot to superconducting induction heating, and performing backward extrusion to obtain an NbTi composite rod; adopting a combination of horizontal stretching and vertical stretching for the NbTi composite rod to obtain an NbTi composite bare wire; coating the NbTi composite bare wire with paint for insulation to obtain the NbTi superconducting wire for a liquid-helium-free magnet. This method optimizes from aspects of raw materials, design, processing technology, and enameled insulation, greatly improving the thermal conductivity of the superconducting wire itself.

Description

Technical Field

[0001] The present invention belongs to the technical field of superconducting materials and relates to a method for preparing NbTi superconducting wire for a cryogen-free superconducting magnet. Background Art

[0002] With the continuous improvement of the global medical level, the installation and popularization of nuclear magnetic resonance imaging (MRI) machines have expanded from developed countries to developing countries. In China, it has also started to develop from provincial capitals and prefecture-level cities to county and township levels. For a conventional 1.5T (tesla, magnetic field unit) MRI device, its superconducting magnet must operate immersed in 2000 liters of liquid helium. As liquid helium is a scarce strategic resource, it basically relies on imports. On the other hand, the price of each liter of liquid helium is over a hundred yuan, and regularly maintaining and replenishing liquid helium poses a relatively large cost pressure on hospitals. At the same time, the maintenance process will also affect the normal detection and treatment in hospitals. Therefore, major MRI R & D and manufacturing enterprises, considering various factors such as customer usage requirements and cost reduction, have started the research and development work on cryogen-free superconducting magnets.

[0003] For cryogen-free superconducting magnets, one of the greatest difficulties in their development is the heat conduction problem of superconducting coils, which also poses more stringent requirements for the preparation of superconducting wires. For conventional Wire in channel (WIC) NbTi superconducting wires, considering the following factors: 1. The copper ratio of the wires is generally large. When achieving the same current-carrying capacity, the corresponding wire volume is large. Therefore, the amount of liquid helium required for cooling and heat dissipation is also large. 2. Polyester filaments are used for braided insulation, with a relatively thick thickness and general compactness, which is not conducive to heat conduction between turns and layers of the wire. Therefore, the deficiency of conventional WIC superconducting wires in heat conduction limits their batch application in the field of cryogen-free magnets. For conventional NbTi painted wires, considering the following points: 1. The NbTi core filaments of the wires are generally evenly distributed from the inside to the outside. Especially for the core filaments in the central region, heat dissipation and heat transfer are relatively blocked. 2. The wires generally use NbTi single-core rods with a copper ratio of 0.5, resulting in a relatively thin copper layer around each NbTi core filament and a relatively narrow heat transfer channel, which is not conducive to rapid heat transfer. 3. The painted insulation of superconducting wires generally uses traditional acetal paint, and its excellent insulation at low temperatures also means relatively poor heat conductivity. For the above reasons, it is of great significance to develop a NbTi superconducting wire for cryogen-free superconducting magnets. Summary of the Invention

[0004] The object of the present invention is to overcome the above-mentioned disadvantages of the prior art and propose a method for preparing NbTi superconducting wire for a liquid-helium-free superconducting magnet. This method optimizes aspects such as raw materials, design, processing technology, and enamel insulation, greatly improving the thermal conductivity of the superconducting wire itself. The NbTi superconducting wire processed by this technology not only has excellent thermal conductivity at low temperatures, but also has high production efficiency. It can also be customized according to the batch requirements of customers, meeting the special needs of different customers for superconducting wire for liquid-helium-free magnets.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for preparing NbTi superconducting wire for a liquid-helium-free superconducting magnet, characterized by comprising the following steps:

[0007] S1. Process an oxygen-free copper single-core ingot sheath and the upper and lower covers supporting both ends thereof, and an oxygen-free copper composite sheath and the upper and lower covers supporting both ends thereof;

[0008] S2. Uniformly wrap the Nb barrier layer around the NbTi ingot, place it in the oxygen-free copper single-core ingot sheath, match the upper and lower covers at both ends of the oxygen-free copper single-core ingot sheath, and perform vacuum electron beam welding to obtain an NbTi single-core ingot. Superconducting induction heating is carried out on the NbTi single-core ingot, and after the heat preservation is completed, reverse extrusion is carried out on the NbTi single-core ingot to obtain an NbTi single-core rod;

[0009] S3. Uniformly arrange multiple NbTi single-core rods and oxygen-free copper rods of the same specification in the oxygen-free copper composite sheath. Among them, multiple NbTi single-core rods are arranged from the outermost layer close to the inner wall of the sheath inward. Match the upper and lower covers at both ends of the oxygen-free copper composite sheath, and perform vacuum electron beam welding to obtain an NbTi composite ingot. Superconducting induction heating is carried out on the NbTi composite ingot, and after the heat preservation is completed, reverse extrusion is carried out on the NbTi composite ingot to obtain an NbTi composite rod;

[0010] S4. Adopt a combination of horizontal stretching and vertical stretching for the NbTi composite rod to obtain an NbTi composite bare wire;

[0011] S5. Adopt the "thin-layer multi-coating" enamel insulation process for the NbTi composite bare wire to finally obtain the NbTi superconducting wire for a liquid-helium-free magnet.

[0012] Further, in the S1, when processing the oxygen-free copper single-core ingot sheath and the upper and lower covers supporting both ends thereof, and the oxygen-free copper composite sheath and the upper and lower covers supporting both ends thereof, 6N-level high-purity oxygen-free copper raw materials are used, and the purity requirement is >99.9999%.

[0013] Furthermore, the thickness of the Nb barrier layer in S2 is 0.3 mm, the diameter of the NbTi ingot is Φ143.2 mm, the length is ≤1200 mm, the outer diameter of the oxygen-free copper single-core ingot sheath is Φ250.0 mm, the inner diameter is Φ144.5 mm, and the copper ratio of the NbTi single-core ingot is 0.9-1.1.

[0014] Furthermore, the NbTi single core rod in S3 is arranged close to the inner wall of the oxygen-free copper composite sheath, ensuring that when the NbTi composite wire is stretched to a specification of Φ1.000mm, the shortest distance between the NbTi core wire and the oxygen-free copper shell outside the wire is less than 25μm.

[0015] Furthermore, the magnetic induction intensity in the space in which the NbTi single-core ingot is superconductingly heated in S2 and the NbTi composite ingot is superconductingly heated in S3 is 0.4-0.5 T, the induction heating insulation temperature is controlled at 740°C-760°C, and the rotation speed of the NbTi single-core ingot during the heating process is 300 r / min; the NbTi single-core ingot and the NbTi composite ingot are reversely extruded, and the extrusion process rate is controlled at 40-60 mm / s.

[0016] Furthermore, the number of NbTi single core rods in S3 is 36 to 180, and the corresponding composite ingot copper ratio ranges from 5 to 7.

[0017] Furthermore, in S4, when the diameter of the NbTi composite rod is greater than Φ10 mm, a horizontal stretching method using a die is adopted; when the diameter of the NbTi composite rod is less than or equal to Φ10 mm, a vertical stretching method is adopted, and a pressure drawing method is adopted in the vertical stretching to improve the uniformity of wire deformation.

[0018] Furthermore, the S5 paint insulation process goes through 16 to 20 die painting processes, the total thickness of the paint layer is 0.05 to 0.10 mm, and the overall dimensional accuracy of the wire after painting is controlled within ±0.003 mm.

[0019] Furthermore, the painting process in S5 adopts a new filler-modified acetal paint.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The present invention requires the purity of high-purity oxygen-free copper raw materials in NbTi wires, avoiding the influence of impurities on the thermal conductivity of wires from the source of raw materials. The product requires that the purity of oxygen-free copper must reach 6N level. Compared with the conventional high-purity oxygen-free copper materials, 6N-level copper materials have lower resistance at low temperatures, are more conducive to heat transfer, and effectively ensure the stable operation of liquid helium-free magnets;

[0022] 2. In the present invention, a thick-walled copper tube is used for the assembly of the NbTi single-core ingot, ensuring that there is sufficient copper layer wrapping around the NbTi core wire in the finally formed NbTi single-core rod, providing an effective channel for heat transfer, thus avoiding heat accumulation and being beneficial to improving the thermal conductivity of the superconducting wire;

[0023] 3. In the extrusion process of the present invention, the method of "superconducting induction heating + backward extrusion" is adopted. The superconducting induction heating frequency is extremely low (the lowest heating frequency can reach 5 Hz), and the corresponding penetration depth of the induced current is deep, which can directly heat the core of the ingot billet and effectively control the temperature difference between the core and the surface (controlled within 10 °C). On the premise of ensuring uniform heating of the material, the heating efficiency of the superconducting ingot billet is greatly improved. In the backward extrusion method, there is no relative movement between the ingot billet and the extrusion cylinder, so there is no external friction. After extrusion, the deformation of the core wire of the product is overall uniform and the yield is high. Compared with the conventional extrusion process, the method of "superconducting induction heating + backward extrusion" has low process cost and high production efficiency. The most crucial thing is that the deformation of the core wire of the wire is uniform, effectively improving the stability of the superconducting performance of the wire;

[0024] 4. In the present invention, a thin-walled jacket is used for the assembly of the composite ingot billet, and the NbTi single-core rods are evenly distributed near the inner wall of the jacket, making the distribution of the NbTi core wires in the finally prepared wire relatively dispersed and close to the edge of the wire, which is beneficial to the heat exchange between the NbTi core wires and the base high-purity oxygen-free copper in the superconducting wire, effectively improving the thermal stability of the superconducting wire. In this process, when the wire is stretched to a specification of Φ1.000 mm, the minimum distance between the outermost core wire and the copper layer of the jacket ≤ 25 μm; in the conventional process, the NbTi single-core rods are generally evenly distributed from the inner layer to the outer layer and are arranged tightly. Especially for the NbTi single-core rods in the core part, there is a situation where the heat exchange of the NbTi core wires is limited and blocked, and there is a large difference in the thermal stability between the core wires;

[0025] 5. In the present invention, when the composite wire specification ≤ Φ10 mm, the method of combining "vertical stretching + pressure drawing" is adopted. A pressurizing device is used to pressurize the inside of the drawing die device. Under the action of high pressure, the lubricating liquid will strongly adhere to the surface of the wire, greatly reducing the friction between the wire and the die, improving the surface state of the drawn wire. At the same time, with the significant reduction of friction, the processing rate of each pass of the wire is also adjusted and increased, from 15% - 20% to 25 - 30%. After adjustment, the stretching process of the wire is normal, greatly improving the production stretching efficiency; when stretching vertically, the wire and the die are more likely to maintain a coaxial state, making the drawing smoother, and at the same time ensuring the roundness and dimensional accuracy of the stretched wire;

[0026] 6. In the present invention, the painting insulation method of "thin layer and multiple coatings" is adopted. The number of painting passes is generally controlled within 16 - 20 passes, and the total thickness of the paint layer is generally controlled within 0.05 - 0.10 mm. According to the product specifications and the required film thickness, the number of painting passes and the paint amount per pass can be appropriately adjusted to effectively ensure the dimensional accuracy of the wire, reduce painting defects, improve the RRR value of the superconducting wire, enhance the quench protection of the superconducting wire in a liquid-helium-free environment, and the dimensional fluctuation of the superconducting painted product after insulation can be controlled within ±0.005 mm. Additionally, a new type of filler-modified acetal paint with both heat conductivity and low-temperature resistance is used in the painting process. By adding a certain amount of modified nano-silica powder filler to the conventional acetal paint, while ensuring the low-temperature insulation of the paint film, the thermal conductivity of the acetal paint can be increased from ~0.2 W / (m·K) to ~0.4 W / (m·K), significantly improving the thermal conductivity of the wire.

[0027] 7. The present invention can be customized according to the mass production requirements of customers to meet the special needs of different customers for superconducting wires used in liquid-helium-free magnets.

[0028] In the present invention, through the following detailed description of the exemplary embodiments of the present invention with reference to the accompanying drawings, other features and advantages of the present invention will become clear. Description of the Drawings

[0029] The accompanying drawings herein are incorporated into the specification and form a part of this specification, and are used together with the specification to explain the principles of the present invention.

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0031] Figure 1 It is a cross-sectional view of a 36 NbTi composite ingot with a copper ratio of 7 and a core number of 36 in Embodiment 1 of the present invention;

[0032] Figure 2 It is a cross-sectional view of a 180 NbTi composite ingot with a copper ratio of 5 and a core number of 180 in Embodiment 2 of the present invention. Detailed Embodiments

[0033] Here, the exemplary embodiments will be described in detail, and the examples are shown in the accompanying drawings. When the following description refers to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. On the contrary, they are only examples of devices consistent with some aspects of the present invention detailed in the appended claims.

[0034] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0035] A method for preparing NbTi superconducting wire for a cryogen-free superconducting magnet, characterized by comprising the following steps:

[0036] S1. Process an oxygen-free copper single-core ingot sheath and the upper and lower covers supporting both ends thereof, and an oxygen-free copper composite sheath and the upper and lower covers supporting both ends thereof; the oxygen-free copper single-core ingot sheath and the upper and lower covers supporting both ends thereof, and the oxygen-free copper composite sheath and the upper and lower covers supporting both ends thereof are made of 6N-grade high-purity oxygen-free copper raw materials, and the purity requirement of the high-purity oxygen-free copper raw materials is >99.9999%. The present invention requires the purity of the high-purity oxygen-free copper raw materials in the NbTi wire, avoiding the influence of impurity introduction on the thermal conductivity of the wire from the source of the raw materials. The product requires that the oxygen-free copper purity must reach the 6N level. Compared with the commonly used high-purity oxygen-free copper materials, the 6N-grade copper material has a smaller resistance at low temperature and is more conducive to heat transfer, effectively ensuring the stable operation of the cryogen-free magnet. Among them, the outer diameter of the oxygen-free copper single-core ingot sheath is Φ250.0 mm, the inner diameter is Φ144.5 mm, the wall thickness of the oxygen-free copper composite ingot sheath is 12-18 mm, and the NbTi single-core ingot is assembled with a thick-walled oxygen-free copper single-core ingot sheath, ensuring that there is sufficient copper layer wrapping around the NbTi core wire in the finally formed NbTi single-core rod, providing an effective channel for heat transfer, thus avoiding heat accumulation and being conducive to improving the thermal conductivity of the superconducting wire. The NbTi composite ingot uses a thin-walled oxygen-free copper composite sheath, and the NbTi single-core rods are evenly distributed near the inner wall of the sheath, so that the NbTi core wires in the finally prepared wire are relatively dispersed and close to the edge of the wire, which is conducive to the heat exchange between the NbTi core wires and the base high-purity oxygen-free copper in the superconducting wire, effectively improving the thermal stability of the superconducting wire. Under this process, when the wire is stretched to the Φ1.000 mm specification, the minimum distance from the outermost core wire to the copper layer of the sheath is ≤25 μm. Under the conventional process, the NbTi single-core rods are generally evenly distributed from the inner layer to the outer layer and are arranged closely. Especially for the NbTi single-core rods in the core part, there is a situation where the heat exchange of the NbTi core wires is limited and blocked, and there is a large difference in the thermal stability between the core wires.

[0037] S2. Uniformly wrap the Nb barrier layer around the NbTi ingot, place it in the oxygen-free copper single-core ingot sheath, match the upper and lower covers at both ends of the oxygen-free copper single-core ingot sheath, and perform vacuum electron beam welding to obtain a NbTi single-core ingot. The NbTi single-core ingot is subjected to superconducting induction heating, and after the heat preservation is completed, the NbTi single-core ingot is subjected to reverse extrusion to obtain a hexagonal NbTi single-core rod; the thickness of the Nb barrier layer is 0.3 mm, the diameter of the NbTi ingot is Φ143.2 mm, and the length is 500 mm - 1200 mm. The copper ratio range of the NbTi single-core ingot is 0.9 - 1.1;

[0038] Since the copper layer of the hexagonal NbTi single core rod is relatively thick, it is beneficial for the wrapped NbTi core wire to have sufficient heat transfer area. And when multiple NbTi single core rods are assembled by secondary composite, the heat transfer channel between the core wires is relatively wide, which is beneficial for the heat transfer inside the core wires.

[0039] In the induction heating space for superconducting induction heating of the NbTi single core ingot, the magnetic induction intensity is 0.4 - 0.5 T, the induction heating holding temperature is controlled at 740 °C - 760 °C, the rotation speed of the NbTi single core ingot during the heating process is 300 r / min, and the NbTi single core ingot is subjected to backward extrusion, and the extrusion process rate is controlled at 40 - 60 mm / s; the friction force during backward extrusion is small, which is beneficial for the deformation of the NbTi single core ingot.

[0040] The superconducting induction heating frequency is extremely low, and the lowest heating frequency can reach 5 Hz. The corresponding penetration depth of the induced current is deep, which can directly heat to the core of the ingot billet, and effectively control the temperature difference between the core and the surface within 10 °C. On the premise of ensuring uniform heating of the material, the heating efficiency of the superconducting ingot billet is greatly improved. In the backward extrusion mode, there is no relative movement between the ingot billet and the extrusion cylinder, so there is no external friction force, and the deformation of the core wire of the product after extrusion is overall uniform, and the yield is high.

[0041] Compared with the conventional extrusion process, the combination of "superconducting induction heating" and "backward extrusion" has low process cost and high production efficiency. The most crucial thing is that the deformation of the core wire of the wire is uniform, effectively improving the stability of the superconducting performance of the wire.

[0042] S3. Arrange multiple NbTi single core rods and oxygen-free copper rods of the same specification evenly in the oxygen-free copper composite sheath, match the upper and lower covers at both ends of the oxygen-free copper composite sheath, and carry out vacuum electron beam welding to obtain the NbTi composite ingot. Then, perform superconducting induction heating on the NbTi composite ingot, and perform backward extrusion on the NbTi composite ingot after the heat preservation is completed to obtain the NbTi composite rod; the NbTi single core rods are arranged in the outermost region of the oxygen-free copper composite sheath, ensuring that when the NbTi composite wire is stretched to the required product specification, the distance between the NbTi core wire and the oxygen-free copper on the outer side of the wire is < 25 μm.

[0043] Since the wall of the oxygen-free copper composite ingot sheath is thin, once heat is generated in the core wire, it can be quickly conducted to the copper matrix and then taken away by the circulating refrigeration equipment, improving the stability of the wire.

[0044] In the space for superconducting induction heating of the NbTi composite ingot, the magnetic induction intensity is 0.4 - 0.5 T, and the induction heating holding temperature is controlled at 740 °C - 760 °C. The number of NbTi single core rods is 36 - 180, and the corresponding copper ratio range of the composite ingot is 0.6 - 10. The induction heating method makes the heating of the material more uniform and has a higher heating efficiency.

[0045] S4. The NbTi composite bar is subjected to a combination of horizontal stretching and vertical stretching to obtain NbTi composite bare wire. When the diameter of the NbTi composite bar > Φ10 mm, the die horizontal stretching method is adopted; when the diameter of the NbTi composite bar ≤ Φ10 mm, the vertical stretching method is adopted. A pressurizing device is used to pressurize the drawing die device. Under the action of high pressure, the lubricating fluid will strongly adhere to the surface of the wire, greatly reducing the friction between the wire and the die, improving the surface state of the drawn wire. At the same time, with the significant reduction of friction, the processing rate of each pass of the wire is also adjusted and increased, from 15% - 20% to 25 - 30%. After adjustment, the wire drawing process is normal, greatly improving the production drawing efficiency. The pressure drawing method is adopted in vertical stretching to improve the uniformity of the core wire deformation. It is easier for the wire and the die to maintain a coaxial state, making the drawing smoother. At the same time, the roundness and dimensional accuracy of the drawn wire are also ensured.

[0046] S5. The NbTi composite bare wire is subjected to a paint insulation process with multiple thin coatings to finally obtain NbTi superconducting wire for a cryogen-free magnet. The paint insulation process passes through 16 - 20 die painting processes. Multiple passes of painting ensure the uniformity of the wire. The total thickness of the paint layer is 0.05 - 0.10 mm. According to the product specifications and the required paint film thickness, the number of painting passes and the paint volume per pass can be appropriately adjusted to effectively ensure the dimensional accuracy of the wire, reduce painting defects and improve the RRR value of the superconducting wire, enhancing the quench protection of the superconducting wire in a cryogen-free environment. The dimensional fluctuation of the superconducting painted product after insulation can be controlled within ±0.005 mm. In addition, a new type of filler-modified acetal paint with both heat conductivity and low-temperature resistance is used in the painting process. By adding a certain amount of modified nano-silica powder filler to the conventional acetal paint, while ensuring the low-temperature insulation of the paint film, the thermal conductivity of the acetal paint can be increased from ~0.2 W / (m·K) to ~0.4 W / (m·K), greatly improving the thermal conductivity of the wire.

[0047] The following is an illustration in combination with the specific process treatment:

[0048] Example 1:

[0049] As Figure 1 shown, the present invention provides a method for preparing NbTi superconducting wire for a cryogen-free superconducting magnet, and the specific steps are as follows:

[0050] S1. Process the oxygen-free copper single-core ingot sheath and the upper and lower covers supporting both ends thereof, and the oxygen-free copper composite sheath and the upper and lower covers supporting both ends thereof; process the oxygen-free copper single-core ingot sheath and the upper and lower covers supporting both ends thereof, and the oxygen-free copper composite sheath and the upper and lower covers supporting both ends thereof adopt 6N-level high-purity oxygen-free copper raw materials, and the purity requirement of the high-purity oxygen-free copper raw materials > 99.9999%.

[0051] S2. Uniformly wrap the Nb barrier layer around the NbTi ingot, place it in the single-core ingot sheath of oxygen-free copper, match the upper and lower covers at both ends of the single-core ingot sheath of oxygen-free copper, and perform vacuum electron beam welding to obtain a single-core NbTi ingot. Conduct superconducting induction heating on the single-core NbTi ingot. After the heat preservation is completed, perform reverse extrusion on the single-core NbTi ingot, and then after multiple passes of cold drawing, cut it to a specified length to obtain a large copper ratio hexagonal single-core NbTi rod with a size of H21.5*800mm; the thickness of the Nb barrier layer is 0.3mm, the diameter of the NbTi ingot is Φ143.2mm, the length is 500mm, the outer diameter of the single-core ingot sheath of oxygen-free copper is Φ250.0mm, and the inner diameter is Φ144.5mm. The copper ratio range of the single-core NbTi ingot is 0.9 - 1.1;

[0052] In the induction heating space for superconducting induction heating of the single-core NbTi ingot, the magnetic induction intensity is 0.4 - 0.5T, the induction heating and heat preservation temperature is controlled at 740°C - 760°C, the rotation speed of the single-core NbTi ingot during the heating process is 300r / min, perform reverse extrusion on the single-core NbTi ingot, and the extrusion process rate is controlled at 40 - 60mm / s;

[0053] S3. Uniformly arrange 36 single-core NbTi rods and multiple oxygen-free copper rods of the same specification in the oxygen-free copper composite sheath, match the upper and lower covers at both ends of the oxygen-free copper composite sheath, and perform vacuum electron beam welding to obtain a NbTi composite ingot. Conduct superconducting induction heating on the NbTi composite ingot. After the heat preservation is completed, perform reverse extrusion on the NbTi composite ingot to obtain a NbTi composite rod;

[0054] The single-core NbTi rods are arranged in the area closest to the inner wall of the oxygen-free copper composite sheath, and multiple oxygen-free copper rods are located inside the 36 single-core NbTi rods, ensuring that when the NbTi composite wire is drawn to the required product specifications, the distance between the NbTi core wire and the oxygen-free copper on the outer side of the wire is <25μm.

[0055] The wall thickness of the oxygen-free copper composite ingot sheath is 12mm. In the induction heating space for superconducting induction heating of the NbTi composite ingot, the magnetic induction intensity is 0.4 - 0.5T, and the induction heating and heat preservation temperature is controlled at 740°C - 760°C. The number of single-core NbTi rods is 36, and the corresponding copper ratio of the composite ingot is 7.

[0056] S4. The NbTi composite bar is subjected to a combination of horizontal and vertical stretching to obtain NbTi composite bare wire. When the diameter of the NbTi composite bar > Φ10 mm, the die horizontal stretching method is used. During the process, to precisely control the copper ratio and avoid the influence of surface impurities after extrusion on subsequent stretching, the surface of the material is skinned. The outer diameter is skinned from Φ61.50 mm to Φ60.50 mm, and from Φ52.00 mm to Φ51.10 mm. The final designed copper ratio is 6.968. During the stretching process, according to the product performance requirements, multi-pass aging heat treatment processes are carried out to improve the superconducting performance of the NbTi wire. When the diameter of the NbTi composite bar ≤ Φ10 mm, the vertical stretching method is used. In the vertical stretching, the pressure drawing method is used to improve the uniformity of the core wire deformation. The average processing amount per pass is controlled at ~28%. Finally, through twisting and final stretching, a round wire bare wire with a specification of Φ1.500 mm is prepared.

[0057] S5. The NbTi composite bare wire is subjected to a paint insulation process with thin-layer multi-coating to finally obtain NbTi superconducting wire for a cryogen-free magnet. After repeatedly painting and baking 16 passes, a finished wire with a specification of Φ1.560 mm is finally obtained. The multi-pass painting ensures the uniformity of the wire. The total thickness of the paint layer is 0.05 - 0.10 mm. The overall dimensional accuracy of the wire after painting is controlled within ±0.003 mm. A new type of filler-modified acetal paint is used in the painting process, which has stronger thermal conductivity.

[0058] At this time, the obtained final product is sampled and tested. The measured copper ratio at the ingot head is 7.102, and the critical current Ic at 4 T and 4.2 K is 825 A. The measured copper ratio at the ingot tail is 6.913, and the critical current Ic at 4 T and 4.2 K is 841 A. The closest distance between the outermost core wire of the wire and the inner wall of the oxygen-free copper tube is ~21 μm.

[0059] Example 2

[0060] As Figure 2 shown, the present invention provides a method for preparing NbTi superconducting wire for a cryogen-free superconducting magnet, and the specific steps are as follows:

[0061] S1. Process the oxygen-free copper single-core ingot sheath and its upper and lower covers supporting both ends, and the oxygen-free copper composite sheath and its upper and lower covers supporting both ends; process the oxygen-free copper single-core ingot sheath and its upper and lower covers supporting both ends, and the oxygen-free copper composite sheath and its upper and lower covers supporting both ends use 6N-level high-purity oxygen-free copper raw materials, and the purity requirement of the high-purity oxygen-free copper raw materials > 99.9999%.

[0062] S2. Uniformly wrap the Nb barrier layer around the NbTi ingot, place it in the single-core ingot sleeve of oxygen-free copper, match the upper and lower covers at both ends of the single-core ingot sleeve of oxygen-free copper, and perform vacuum electron beam welding to obtain the NbTi single-core ingot. Conduct superconducting induction heating on the NbTi single-core ingot. After the heat preservation is completed, perform reverse extrusion on the NbTi single-core ingot, and then after multiple passes of cold drawing, cut it to a specified length to obtain a large copper ratio hexagonal NbTi single-core rod with a size of H21.5*800mm; the thickness of the Nb barrier layer is 0.3mm, the diameter of the NbTi ingot is Φ143.2mm, the length is 1200mm, the outer diameter of the single-core ingot sleeve of oxygen-free copper is Φ250.0mm, the inner diameter is Φ144.5mm, and the copper ratio range of the NbTi single-core ingot is 0.9 - 1.1;

[0063] In the induction heating space for superconducting induction heating of the NbTi single-core ingot, the magnetic induction intensity is 0.4 - 0.5T, the induction heating and heat preservation temperature is controlled at 740°C - 760°C, the rotation speed of the NbTi single-core ingot during the heating process is 300r / min, perform reverse extrusion on the NbTi single-core ingot, and the extrusion process rate is controlled at 40 - 60mm / s;

[0064] S3. Uniformly arrange 180 NbTi single-core rods and multiple oxygen-free copper rods of the same specification in the oxygen-free copper composite sleeve, match the upper and lower covers at both ends of the oxygen-free copper composite sleeve, and perform vacuum electron beam welding to obtain the NbTi composite ingot. Conduct superconducting induction heating on the NbTi composite ingot. After the heat preservation is completed, perform reverse extrusion on the NbTi composite ingot to obtain the NbTi composite rod;

[0065] The NbTi single-core rods are arranged in the area closest to the inner wall of the oxygen-free copper composite sleeve to ensure that when the NbTi composite wire is stretched to the required product specification, the distance between the NbTi core wire and the oxygen-free copper on the outer side of the wire is <25μm.

[0066] The wall thickness of the oxygen-free copper composite ingot sleeve is 18mm. In the induction heating space for superconducting induction heating of the NbTi composite ingot, the magnetic induction intensity is 0.4 - 0.5T, and the induction heating and heat preservation temperature is controlled at 740°C - 760°C. The number of NbTi single-core rods is 180, and the corresponding copper ratio of the composite ingot is 5.

[0067] S4. The NbTi composite bar is subjected to a combination of horizontal stretching and vertical stretching to obtain the NbTi composite bare wire. When the diameter of the NbTi composite bar is > Φ10 mm, the die horizontal stretching method is used. During the process, to precisely control the copper ratio and avoid the influence of surface impurities after extrusion on subsequent stretching, the surface of the material is peeled. The outer diameter is peeled from Φ61.50 mm to Φ60.50 mm, and from Φ52.00 mm to Φ51.10 mm. The final designed copper ratio is 6.968. During the stretching process, according to the product performance requirements, multi-pass aging heat treatment processes are carried out to improve the superconducting performance of the NbTi wire. When the diameter of the NbTi composite bar is ≤ Φ10 mm, the vertical stretching method is used. In vertical stretching, the pressure drawing method is used to improve the uniformity of the core wire deformation. The average processing amount per pass is controlled at ~28%. Finally, through twisting and final stretching, a round wire bare wire with a specification of Φ1.500 mm is prepared.

[0068] S5. The NbTi composite bare wire processed in S4 is rolled flat. Through the precise control of a four-roll mill, the wire is processed to 1.23 * 0.78 mm, and the 4 R corners are controlled at 0.50 ± 0.10 mm. The rolled flat wire is then painted. The painting process uses a new type of filler-modified acetal paint with both heat conductivity and low-temperature resistance. After repeated painting and baking 20 times, the total thickness of the paint layer is 0.05 - 0.10 mm, and the overall dimensional accuracy of the painted wire is controlled within ±0.003 mm. Finally, a finished wire with a specification of Φ1.31 * 0.86 mm is obtained.

[0069] The above are only specific embodiments of the present invention, enabling those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention.

[0070] It should be understood that the present invention is not limited to the above-described content and can be modified and changed without departing from its scope. The scope of the present invention is only limited by the appended claims.

Claims

1. A method for preparing NbTi superconducting wire for a cryogen-free superconducting magnet, characterized in that, It includes the following steps: S1. Process the single-core ingot sheath of oxygen-free copper and the upper and lower covers matching both ends thereof, and the composite sheath of oxygen-free copper and the upper and lower covers matching both ends thereof; S2. Uniformly wrap the Nb barrier layer around the NbTi ingot, place it in the single-core ingot sheath of oxygen-free copper, match the upper and lower covers at both ends of the single-core ingot sheath of oxygen-free copper, and perform vacuum electron beam welding to obtain a NbTi single-core ingot. Perform superconducting induction heating on the NbTi single-core ingot. After the heat preservation ends, perform backward extrusion on the NbTi single-core ingot to obtain a NbTi single-core rod; S3. Arrange multiple NbTi single-core rods and oxygen-free copper rods of the same specification in the composite sheath of oxygen-free copper. Multiple NbTi single-core rods are distributed near the inner wall of the composite sheath of oxygen-free copper, and the oxygen-free copper rods are distributed inside the NbTi single-core rods. Match the upper and lower covers at both ends of the composite sheath of oxygen-free copper, and perform vacuum electron beam welding to obtain a NbTi composite ingot. Perform superconducting induction heating on the NbTi composite ingot. After the heat preservation ends, perform backward extrusion on the NbTi composite ingot to obtain a NbTi composite rod; S4. Adopt a combination of horizontal stretching and vertical stretching for the NbTi composite rod to obtain a NbTi composite bare wire; S5. Adopt a "thin layer and multiple coatings" paint insulation process for the NbTi composite bare wire to finally obtain NbTi superconducting wire for a liquid-helium-free magnet.

2. The preparation method of NbTi superconducting wire for a liquid-helium-free superconducting magnet according to claim 1, characterized in that, In the S1, when processing the single-core ingot sheath of oxygen-free copper and the upper and lower covers matching both ends thereof, and the composite sheath of oxygen-free copper and the upper and lower covers matching both ends thereof, 6N-level high-purity oxygen-free copper raw materials are used, and the purity requirement is >99.9999%; 3. A method for preparing NbTi superconducting wire for a cryogen-free superconducting magnet according to claim 1, characterized in that, In the S2, the thickness of the Nb barrier layer is 0.3 mm, the diameter of the NbTi ingot is Φ143.2 mm, the length ≤1200 mm, the outer diameter of the single-core ingot sheath of oxygen-free copper is Φ250.0 mm, and the inner diameter is Φ144.5 mm. The copper ratio of the NbTi single-core ingot is 0.9 - 1.

1.

4. A method for preparing NbTi superconducting wire for a liquid-helium-free superconducting magnet according to claim 1, characterized in that, In the S3, the arrangement of the NbTi single-core rods is close to the inner wall of the composite sheath of oxygen-free copper, ensuring that when the NbTi composite wire is stretched to a Φ1.000 mm specification, the shortest distance between the NbTi core wire and the oxygen-free copper shell on the outer side of the wire is <25 μm.

5. A method for preparing NbTi superconducting wire for a cryogen-free superconducting magnet according to claim 1, characterized in that, In the S2, the magnetic induction intensity in the space for superconducting induction heating of the NbTi single-core ingot and in the S3 for the NbTi composite ingot is 0.4 - 0.5 T, the induction heating and heat preservation temperature is controlled at 740°C - 760°C, and the rotation speed of the NbTi single-core ingot during the heating process is 300 r / min; for the NbTi single-core ingot and the NbTi composite ingot, backward extrusion is performed, and the extrusion process rate is controlled at 40 - 60 mm / s.

6. A method for preparing NbTi superconducting wire for a non-liquid-helium superconducting magnet according to claim 1, characterized in that, In the S3, the number of NbTi single-core rods is 36 - 180, and the corresponding copper ratio range of the composite ingot is 5 - 7.

7. A method for preparing NbTi superconducting wire for a liquid-helium-free superconducting magnet according to claim 1, characterized in that, In the S4, when the diameter of the NbTi composite rod > Φ10 mm, the die horizontal stretching method is adopted; when the diameter of the NbTi composite rod ≤ Φ10 mm, the vertical stretching method is adopted, and in the vertical stretching, the pressure drawing method is adopted to improve the uniformity of wire deformation.

8. A method for preparing NbTi superconducting wire for a cryogen-free superconducting magnet according to claim 1, characterized in that, The painting and insulation process of the S5 undergoes 16 to 20 die painting processes, with the total thickness of the paint layer being 0.05 to 0.10 mm, and the overall dimensional accuracy of the wire after painting being controlled within ±0.003 mm.

9. A method for preparing NbTi superconducting wire for a cryogen-free superconducting magnet according to claim 1, characterized in that, In the S5, a modified polyvinyl acetal paint is used in the painting process.

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