Method for improving deformation uniformity of NbTi superconducting wire core wire

Abstract

The invention discloses a method for improving deformation uniformity of a core wire of an NbTi superconducting wire, and belongs to the technical field of superconducting wire processing. The method comprises the following steps: performing deep hole drilling on an oxygen-free copper ingot rod, then assembling, welding and extruding the oxygen-free copper ingot rod and a single-core rod to obtain a composite rod, and performing multi-pass drawing on the composite rod to obtain the NbTi superconducting wire with high performance, low loss and high N value. The Jc of the prepared NbTi superconducting wire is 2880-3120 A / mm < 2 >, the loss is 450-550 mJ / cm < 3 >, the N value is 70-95, compared with a superconducting wire prepared through a conventional process, the N value is remarkably increased, the loss is remarkably reduced, the critical current density is increased, and the NbTi superconducting wire has wide application prospects in MRI magnets.

Description

Technical Field

[0001] The present invention belongs to the technical field of superconducting wire processing, and relates to a method for improving the deformation uniformity of the core wire of NbTi superconducting wire. Background Art

[0002] With the development of the MRI market, MRI equipment requires high-performance MRI magnets to ensure its stability and accuracy. To prepare MRI magnets with higher performance and lower cost, researchers have developed a variety of new magnets by continuously optimizing the magnet structure and the key raw materials of the superconducting wire used in superconducting magnets, such as liquid-helium-free superconducting MRI, low-liquid-helium MRI, high-field MRI, ultra-high-field whole-body MRI, etc. NbTi / Cu superconducting wire has been widely used in MRI equipment due to its excellent superconducting properties, good mechanical properties and processing performance. Compared with conventional traditional MRI magnets, current MRI magnets of different types and different magnetic field gradients have put forward higher requirements for the performance of the NbTi / Cu superconducting wire used. It not only requires the superconducting wire to have a high critical surface current density, but also has lower losses and a high "N value". The N value reflects the voltage response characteristics of the superconductor when the current changes, and can measure the change rate of the superconductor from the superconducting state to the normal state. The higher the value, the faster the transition rate and the better the performance.

[0003] At present, the methods for improving the N value of superconducting wire mainly include: optimizing material selection, optimizing wire structure and preparation process, etc. Selecting superconducting materials with high critical temperature, high critical current density and high stability, such as NbTi, Nb3Sn, etc. can significantly improve the N value of the wire; adopting a multi-core structure design can increase the current transmission channels of the wire, improve the uniformity of current distribution, and thus improve the N value; through fine processing and heat treatment processes, control the lattice structure and grain size of the wire, and optimize the superconducting properties of the wire. At present, a hexagonal mandrel is often used as an intermediate product in the processing of NbTi / Cu superconducting wire. After being compounded with oxygen-free copper, a multi-core structure is formed. However, with the subsequent extrusion and drawing in the processing process, the shape of the NbTi core wire in the multi-core superconducting wire becomes more and more irregular, the loss of the superconducting wire is higher, and the N value is lower. Therefore, it is of great significance to provide a superconducting wire with high performance, low loss and high N value. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a method for improving the deformation uniformity of the core wire of NbTi superconducting wire. After deep-hole drilling of an oxygen-free copper ingot bar, it is assembled, welded and extruded with a single-core bar to obtain a composite bar. After multi-pass drawing of the composite bar, a high-performance, low-loss and high-N-value NbTi superconducting wire is obtained. The NbTi superconducting wire prepared by the present invention J c is 2880~3120A / mm 2, the loss is 450~550mJ / cm 3 , the N value is 70~95. Compared with the superconducting wire prepared by the conventional process, the N value of the superconducting wire of the present invention is significantly increased, the loss is significantly reduced, and the critical current density is increased. The NbTi superconducting wire of the present invention has broad application prospects in MRI magnets.

[0005] To achieve the technical purpose of the present invention, on the one hand, the present invention provides a method for improving the deformation uniformity of the NbTi superconducting wire core wire, which specifically includes the following steps: S1: Clean, assemble and weld the NbTi ingot bar, Nb tube, CuNi alloy tube or CuMnSi alloy tube, as well as the upper cover and the lower cover to obtain a NbTi / CuNi or NbTi / CuMnSi single-core ingot, and then extrude and multi-pass draw to obtain a NbTi / CuNi single-core rod or a NbTi / CuMnSi single-core rod.

[0006] Furthermore, the NbTi ingot bar is in a highly uniform state with uniform organization and composition, its grain size is greater than grade 5, the Ti content is 46~48wt%, the carbon content is less than 100ppm, the nitrogen content is less than 100ppm, the hydrogen content is less than 30ppm, and the oxygen content is less than 500ppm; the Ni content of the CuNi alloy tube is 5~30wt%; the Mn content of the CuMnSi alloy tube is 1~3wt%, and the Si content is 0.3~0.6wt%.

[0007] Furthermore, the material of the upper cover is CuNi alloy or CuMnSi alloy, the material of the upper cover is the same as that of the alloy tube, and the shape is a sub-button structure. The material of the lower cover is red copper, and the shape is a convex structure. The welding is vacuum electron beam welding, the welding current is 150~200A, and the welding speed is 150~160° / min. The extrusion is reverse hot extrusion, the extrusion temperature is 750~850°C, the extrusion speed is 15~25mm / s, the extrusion ratio is controlled at 10~15, and the reverse hot extrusion can ensure uniform deformation of the NbTi core in the NbTi / CuNi single-core rod or NbTi / CuMnSi single-core rod. After the reverse hot extrusion, the oxidized black skin on the surface of the NbTi / CuNi single-core rod or NbTi / CuMnSi single-core rod is removed by centerless turning. The processing rate of each pass in multi-pass drawing is controlled at 10~30%, the copper-to-super ratio of the single-core rod is 0.15~1.0, and the diameter of the single-core rod is 10~40mm.

[0008] S2: Machine the oxygen-free copper ingot bar, and process both ends into a convex shape. The upper cover and the lower cover of the oxygen-free copper ingot are processed into a sub-button shape, and both the upper cover and the lower cover are made of oxygen-free copper.

[0009] Furthermore, the oxygen content of the oxygen-free copper ingot bar is less than 5 ppm. After machining, the end faces of the oxygen-free copper ingot bar and the upper and lower covers need to be chamfered, and the surface roughness of the oxygen-free copper ingot bar is less than 3.2 μm.

[0010] S3: Deep-hole drilling is carried out on the oxygen-free copper ingot bar after the machining in S2. The number of drilled holes can be adjusted according to the design requirements. At the same time, in order to facilitate subsequent hoisting, hoisting holes need to be machined at both ends.

[0011] Furthermore, the holes are distributed in a hexagonal pattern. After deep-hole drilling of each hole, honing is required to ensure that the inner wall roughness of the hole is less than 1.6 μm. Burrs at both ends are polished with a polishing machine to make the surface roughness less than 3.2 μm to prevent scratching the surface of the single-core rod during subsequent assembly. Three hoisting holes are machined on the end face, and the three hoisting holes are distributed at 120°. The size of the hoisting holes is M15 - M20.

[0012] S4: The NbTi / CuNi single-core rod or NbTi / CuMnSi single-core rod is cleaned, assembled, and welded with the drilled oxygen-free copper ingot bar, the upper cover and the lower cover of the oxygen-free copper ingot to obtain the NbTi / CuNi / Cu composite ingot or NbTi / CuMnSi / Cu composite ingot, and then the NbTi / CuNi / Cu composite rod or NbTi / CuMnSi / Cu composite rod is obtained after extrusion.

[0013] Furthermore, the welding is vacuum electron beam welding, with a welding current of 180 - 230 A and a welding speed of 160 - 170 ° / min. The extrusion is reverse hot extrusion, with an extrusion temperature of 650 - 750 °C, an extrusion speed of 10 - 20 mm / s, and the extrusion ratio controlled at 8 - 12.

[0014] S5: The NbTi / CuNi / Cu composite rod or NbTi / CuMnSi / Cu composite rod is subjected to multi-pass drawing, combined with multiple aging heat treatments in the middle, and finally the high-performance, low-loss, and high-N-value NbTi / CuNi / Cu composite wire or NbTi / CuMnSi / Cu composite wire is obtained.

[0015] Furthermore, the oxidized black skin on the surface of the NbTi / CuNi / Cu composite rod or NbTi / CuMnSi / Cu composite rod is peeled off with a peeling die, and the peeling thickness is 3 - 4 mm. The multi-pass drawing die is processed with a small-angle die, and the number of aging heat treatments is 1 - 5 times. The small-angle die is a die with an angle of 4° - 10°, the aging heat treatment temperature is 300 - 500 °C, and the time is 5 h - 60 h. The diameter of the composite wire is 0.8 mm.

[0016] On the other hand, the present invention claims protection for a NbTi superconducting wire prepared by the above method.

[0017] Furthermore, the present invention claims the application of the above NbTi superconducting wire in an MRI magnet.

[0018] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects or advantages: (1) The present invention uses NbTi ingots, Nb tubes, CuNi alloy tubes or CuMnSi alloy tubes, and oxygen-free copper ingots as raw materials, and selects superconducting materials with high critical temperature, high critical current density and high stability to increase the N value of the wire. At the same time, the present invention drills deep holes in the oxygen-free copper ingot, the number of drilled holes can be flexibly adjusted, and the hole positions are distributed in a hexagonal pattern. Subsequently, the single-core rod is assembled, welded, extruded, and drawn with the oxygen-free copper ingot after deep drilling to obtain a composite wire with high performance, low loss, and high N value. After processing by the method for improving the deformation uniformity of NbTi core wires in superconducting wires provided by the present invention, the roundness of the NbTi core wires is better, the superconducting wire has low loss and high N value. The superconducting wire prepared by the present invention J c is 2880 - 3120 A / mm 2 , the loss Qh is 450 - 550 mJ / cm 3 , the N value is 70 - 95. Compared with the superconducting wires prepared by conventional processes J c which is 2750 - 3050 A / mm 2 , the loss Qh is 790 - 850 mJ / cm 3 , and the N value is 48 - 55, the N value of the superconducting wire is significantly increased, the loss is significantly reduced, and the critical current density is increased. The superconducting wire of the present invention has better performance when applied to an MRI magnet.

[0019] (2) The present invention uses a centerless lathe to remove the oxidized black skin on the surface of the single-core rod. Compared with using a conventional peeling die for removal, a large number of pits appear on the surface after peeling, and there is a risk of causing inclusions during the subsequent drawing process, which may cause the composite wire to break. The method of the present invention does not have the above defects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] 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 drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention.

[0021] Figure 1 is a schematic diagram of the assembled structure of a single-core ingot. Among them, 1 is the upper cover of the single-core ingot sheath, 2 is the barrel of the single-core ingot sheath, and 3 is the lower cover of the single-core ingot sheath.

[0022] Figure 2 is a cross-sectional view of a drilled oxygen-free copper ingot rod.

[0023] Figure 3 It is a schematic diagram of the composite ingot assembly structure. Among them, 1 is the upper cover of the oxygen-free copper ingot, 2 is the drilled oxygen-free copper ingot cylinder, and 3 is the lower cover of the oxygen-free copper ingot.

[0024] Figure 4 It is a physical cross-sectional view of the composite wire. Specific embodiments

[0025] Next, the technical solutions of the present invention will be described in conjunction with the embodiments. However, the present invention is not limited to the following embodiments. The experimental methods and detection methods described in each embodiment are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0026] The NbTi ingot bar is in a highly uniform state with uniform organization and composition. Its grain size is greater than grade 5, the Ti content is 46 - 48 wt%, the carbon content is less than 100 ppm, the nitrogen content is less than 100 ppm, the hydrogen content is less than 30 ppm, and the oxygen content is less than 500 ppm.

[0027] The Ni content of the CuNi alloy tube is 5 - 30 wt%.

[0028] The Mn content of the CuMnSi alloy tube is 1 - 3 wt%, and the Si content is 0.3 - 0.6 wt%.

[0029] The oxygen content of the oxygen-free copper ingot bar is less than 5 ppm.

[0030] A method for improving the deformation uniformity of the NbTi superconducting wire core, specifically including the following steps: S1: Clean, assemble, and weld the NbTi ingot bar, Nb tube, CuNi alloy tube or CuMnSi alloy tube, as well as the upper cover and the lower cover to obtain a NbTi / CuNi single-core ingot or a NbTi / CuMnSi single-core ingot ( Figure 1 ), and obtain a NbTi / CuNi single-core rod or a NbTi / CuMnSi single-core rod through extrusion and multiple passes of drawing.

[0031] Specifically, the material of the upper cover is CuNi alloy or CuMnSi alloy, which is the same as the material of the alloy tube, and its shape is a male buckle structure. The material of the lower cover is red copper, and its shape is a convex structure. The welding is vacuum electron beam welding, with a welding current of 150 - 200 A and a welding speed of 150 - 160 ° / min. The extrusion is reverse hot extrusion, with an extrusion temperature of 750 - 850 °C, an extrusion speed of 15 - 25 mm / s, and the extrusion ratio controlled at 10 - 15. After the reverse hot extrusion, the surface oxidation black skin of the NbTi / CuNi single core rod or NbTi / CuMnSi single core rod is removed by centerless turning, and then multi-pass drawing is carried out. The processing rate of each pass in multi-pass drawing is controlled at 10 - 30%, and after multi-pass drawing, an NbTi / CuNi single core rod or NbTi / CuMnSi single core rod with a copper super ratio of 0.15 - 1.0 and a diameter of 10 mm - 40 mm is obtained.

[0032] S2: The oxygen-free copper ingot bar is machined, and both ends are machined into a convex shape. The upper and lower covers of the oxygen-free copper ingot are machined into a male buckle shape, and both the upper and lower covers are made of oxygen-free copper.

[0033] Specifically, the end faces of the machined oxygen-free copper ingot bar and the upper and lower covers need to be chamfered, and the surface roughness is less than 3.2 μm.

[0034] S3: The oxygen-free copper ingot bar after the machining in S2 is deep drilled, and the number of drill holes can be adjusted according to the design requirements. Subsequently, hoisting holes are machined at both ends.

[0035] Specifically, after deep drilling, the holes are distributed in a hexagonal pattern. After each hole is deep drilled, honing is required to ensure that the inner wall roughness of the hole is less than 1.6 μm and the surface roughness is less than 3.2 μm. Three hoisting holes are machined on the end face, and the three hoisting holes are distributed at 120°, and the size of the hoisting holes is M15 - M20.

[0036] S4: The NbTi / CuNi single core rod or NbTi / CuMnSi single core rod, the drilled oxygen-free copper ingot bar ( Figure 2 ), the upper and lower covers of the oxygen-free copper ingot are cleaned, assembled, and welded to obtain an NbTi / CuNi / Cu composite ingot or an NbTi / CuMnSi / Cu composite ingot ( Figure 3 ), and an NbTi / CuNi / Cu composite rod or an NbTi / CuMnSi / Cu composite rod is obtained after extrusion.

[0037] Specifically, the welding is vacuum electron beam welding, with a welding current of 180 - 230 A and a welding speed of 160 - 170 ° / min. The extrusion is reverse hot extrusion, with an extrusion temperature of 650 - 750 °C, an extrusion speed of 10 - 20 mm / s, and the extrusion ratio controlled at 8 - 12.

[0038] S5: Use a peeling die to remove the surface oxide black skin of the NbTi / CuNi / Cu composite rod or NbTi / CuMnSi / Cu composite rod, and then perform multi-pass drawing, combined with multiple aging heat treatments in the middle, and finally obtain a high-performance, low-loss and high-N-value NbTi / CuNi / Cu composite wire or NbTi / CuMnSi / Cu composite wire ( Figure 4 ), the lifting hole is filled with copper after hot extrusion.

[0039] Specifically, the multi-pass drawing die uses a small-angle die, and the small-angle die is a die with an angle of 4° to 10°. The number of aging heat treatments is 1 to 5 times, the aging heat treatment temperature is 300 to 500 °C, and the time is 5 h to 60 h.

[0040] Example 1 This example provides a method for improving the deformation uniformity of the NbTi superconducting wire core, which specifically includes the following steps: S1: Clean and assemble a Nb47Ti ingot rod (Ti content is 46 - 48 wt%) with a diameter of Φ115 mm, uniform in structure and composition, a Cu15Ni alloy tube with an outer diameter of Φ150 mm and an inner diameter of Φ120 mm, a Nb tube with a diameter of Φ118 mm, an upper cover (made of Cu15Ni), and a lower cover (made of pure copper). Obtain a NbTi / Cu15Ni single-core ingot by vacuum electron beam welding under the conditions of a welding current of 150 A and a welding speed of 150 ° / min (the structure of the single-core ingot from the outside to the inside is Cu15Ni alloy tube, Nb tube, Nb47Ti ingot rod). Reverse hot extrude the single-core ingot to Φ40 mm to obtain a NbTi / Cu15Ni single-core rod, with an extrusion temperature of 750 °C, an extrusion speed of 25 mm / s, and an extrusion ratio of 15. The single-core rod is mechanically straightened, and after straightening, the surface black skin is removed by centerless turning to obtain a single-core rod with a diameter of Φ37 mm, and then it is drawn to Φ20 mm through multiple passes. The processing rate of each pass of multi-pass drawing is controlled at 10%, and the copper over ratio is 0.15.

[0041] S2: Machine a Φ220 mm oxygen-free copper ingot rod. Both ends of the oxygen-free copper ingot rod are machined into a convex shape, and the upper cover and lower cover of the oxygen-free copper ingot (both the upper cover and lower cover are made of oxygen-free copper) are machined into a male-female buckle shape. The end faces of the machined oxygen-free copper ingot rod and the upper and lower covers need to be chamfered, and the surface roughness of the copper ingot is less than 3.2 μm.

[0042] S3: Drill deep holes in the oxygen-free copper ingot rod after the machining in S2. The number of drilled holes is 55, the hole diameter is Φ20.5 mm, the distance between adjacent two holes is 3 mm, the surface roughness is less than 3.2 μm, the inner wall roughness of the hole is less than 1.6 μm, and select one end of the drilled copper ingot to machine 3 M15 lifting holes distributed at 120°.

[0043] S4: Clean and assemble the NbTi / Cu15Ni single core rod, the upper and lower covers of the 55-hole drilled copper ingot and the oxygen-free copper ingot, and perform vacuum electron beam welding under the conditions of a welding current of 180 A and a welding speed of 160° / min. After reverse hot extrusion, a NbTi / Cu15Ni / Cu composite rod with a diameter of Φ60 mm is obtained. The extrusion temperature is 650 °C, the extrusion speed is 10 mm / s, and the extrusion ratio is 8.

[0044] S5: The composite rod is mechanically straightened and peeled to Φ56 mm using a peeling die, and then drawn to Φ0.8 mm using a 4° small-angle die. During the drawing process, 5 times of aging heat treatment are carried out. The aging heat treatment temperature is 300 °C, and the aging heat treatment time is 60 h. Finally, a high-performance, low-loss, high-N value NbTi / Cu15Ni / Cu composite wire with uniform core wire deformation is obtained.

[0045] 55-core NbTi / Cu composite wire prepared by the conventional method (using oxygen-free copper as the matrix, assembly method) J c is 3050 A / mm at (4.2 K, 5 T) 2 , the loss Qh (4.2 K, ±3 T) is 850 mJ / cm 3 , and the N value is 50. The NbTi / Cu15Ni / Cu composite wire prepared by the method of this embodiment J c is 3120 A / mm at (4.2 K, 5 T) 2 , the loss Qh (4.2 K, ±3 T) is 540 mJ / cm 3 , and the N value is 70.

[0046] Example 2 This embodiment provides a method for improving the uniformity of core wire deformation of NbTi superconducting wire, which specifically includes the following steps: S1: Clean and assemble the Nb47Ti ingot rod with a diameter of Φ150 mm, uniform in structure and composition, the Cu2Mn0.5Si alloy tube with an outer diameter of Φ190 mm and an inner diameter of 155 mm, the Nb tube with a diameter of Φ153 mm, the upper cover (made of Cu2Mn0.5Si), and the lower cover (made of pure copper). Perform vacuum electron beam welding under the conditions of a welding current of 180 A and a welding speed of 155° / min to obtain a NbTi / Cu2Mn0.5Si single core ingot. Reverse hot extrude the single core ingot to Φ60 mm to obtain a NbTi / Cu2Mn0.5Si single core rod. The extrusion temperature is 800 °C, the extrusion speed is 15 mm / s, and the extrusion ratio is 10. Mechanically straighten the single core rod, and after straightening, remove the surface black skin using a centerless lathe to obtain a single core rod with a diameter of Φ57 mm. Then, draw it to Φ25 mm through multiple passes. The processing rate of each pass of the multiple passes of drawing is controlled at 20%, and the copper-to-super ratio is 0.5.

[0047] S2: The oxygen-free copper ingot bar with a diameter of Φ270 mm is machined. Both ends of the oxygen-free copper ingot bar are machined into a convex shape. The upper cover and the lower cover of the oxygen-free copper ingot (both the upper cover and the lower cover are made of oxygen-free copper) are machined into a sub-button shape. The end faces of the machined oxygen-free copper ingot bar and the upper and lower covers need to be chamfered, and the surface roughness of the copper ingot is less than 3.2 μm.

[0048] S3: Deep hole drilling is carried out on the oxygen-free copper ingot bar after the machining in S2. The number of drilled holes is 55, the diameter of the holes is Φ25.5 mm, the distance between adjacent two holes is 4 mm, the surface roughness is less than 3.2 μm, and the inner wall roughness of the holes is less than 1.6 μm. Three M15 hoisting holes distributed at 120° are machined at one end of the drilled copper ingot.

[0049] S4: The NbTi / Cu2Mn0.5Si single core bar, the 55-hole drilled copper ingot, the upper cover and the lower cover of the oxygen-free copper ingot are cleaned and assembled, and then vacuum electron beam welded under the conditions of a welding current of 200 A and a welding speed of 165° / min. After reverse hot extrusion, a NbTi / Cu2Mn0.5Si / Cu composite bar with a diameter of Φ80 mm is obtained. The extrusion temperature is 700 °C, the extrusion speed is 15 mm / s, and the extrusion ratio is 11.

[0050] S5: The composite bar is mechanically straightened and peeled to Φ76 mm by a peeling die, and then drawn to Φ0.8 mm by an 8° small-angle die. Four times of age heat treatment are carried out during the drawing process. The age heat treatment temperature is 400 °C, and the age heat treatment time is 30 h. Finally, a high-performance, low-loss, high-N-value NbTi / Cu2Mn0.5Si / Cu composite wire with uniform deformation of the core wire is obtained.

[0051] The 55-core NbTi / Cu composite wire prepared by the conventional method (using oxygen-free copper as the matrix and the assembly method) J c (4.2 K, 5 T) is 2900 A / mm 2 , and the loss Qh (4.2 K, ±3 T) is 840 mJ / cm 3 , and the N value is 55. The NbTi / Cu2Mn0.5Si / Cu composite wire prepared by the method of this embodiment J c (4.2 K, 5 T) is 2980 A / mm 2 , and the loss Qh (4.2 K, ±3 T) is 550 mJ / cm 3 , and the N value is 80.

[0052] Example 3 This embodiment provides a method for improving the uniformity of the core wire deformation of NbTi superconducting wire, which specifically includes the following steps: S1: Clean and assemble an Nb47Ti ingot rod with a diameter of Φ250mm that is uniform in structure and composition, a Cu7Ni alloy tube with an outer diameter of Φ280mm and an inner diameter of 255mm, an Nb tube with a diameter of Φ253mm, an upper cover (made of Cu7Ni), and a lower cover (made of red copper). Then, obtain an NbTi / Cu7Ni single-core ingot through vacuum electron beam welding under the conditions of a welding current of 200A and a welding speed of 160° / min. Reverse hot extrude the single-core ingot to Φ80mm to obtain an NbTi / Cu7Ni single-core rod. The extrusion temperature is 850°C, the extrusion speed is 10mm / s, and the extrusion ratio is 12. Mechanically straighten the single-core rod, and after straightening, remove the surface black skin using centerless turning to obtain a single-core rod with a diameter of Φ77mm. Then, draw it to Φ15mm through multiple passes. The processing rate for each pass of multiple-pass drawing is controlled at 30%, and the copper over-ratio is 1.0.

[0053] S2: Mechanically process an oxygen-free copper ingot rod with a diameter of Φ160mm. Both ends of the oxygen-free copper ingot rod are processed into a convex shape, and the upper and lower covers (both made of oxygen-free copper) of the oxygen-free copper ingot are processed into a male buckle shape. Chamfer the end faces of the mechanically processed oxygen-free copper ingot rod and the upper and lower covers. The surface roughness of the copper ingot is less than 3.2μm.

[0054] S3: Drill deep holes in the oxygen-free copper ingot rod after the mechanical processing in S2. The number of drilled holes is 55, the hole diameter is Φ15.5mm, the distance between adjacent two holes is 2mm, the surface roughness is less than 3.2μm, and the inner wall roughness of the holes is less than 1.6μm. Select one end of the drilled copper ingot to process 3 M15 lifting holes distributed at 120°.

[0055] S4: Clean and assemble the NbTi / Cu7Ni single-core rod, the 55-hole drilled copper ingot, and the upper and lower covers of the oxygen-free copper ingot. Then, perform vacuum electron beam welding under the conditions of a welding current of 230A and a welding speed of 170° / min. After reverse hot extrusion, obtain an NbTi / Cu7Ni / Cu composite rod with a diameter of Φ45mm. The extrusion temperature is 750°C, the extrusion speed is 20mm / s, and the extrusion ratio is 13.

[0056] S5: Mechanically straighten the composite rod and peel it to Φ42mm using a peeling die. Then, draw it to Φ0.8mm using a 10° small-angle die. During the drawing process, perform 3 times of aging heat treatment. The aging heat treatment temperature is 500°C, and the aging heat treatment time is 5h. Finally, obtain a high-performance, low-loss, high-N-value NbTi / Cu7Ni / Cu composite wire with uniform core wire deformation.

[0057] 55-core NbTi / Cu composite wire prepared by the conventional method (with oxygen-free copper as the matrix, assembly method) J c (4.2K, 5T) is 2750A / mm 2, the loss Qh (4.2K, ±3T) is 790 mJ / cm 3 , the N value is 48. The NbTi / Cu2Mn0.5Si / Cu composite wire prepared by the method of this embodiment J c (4.2K, 5T) is 2880 A / mm 2 , the loss Qh (4.2K, ±3T) is 450 mJ / cm 3 , the N value is 95.

[0058] The above-described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by relevant deductions and substitutions made by those of ordinary skill in the art under the conditions of the inventive concept of the present invention without making creative efforts fall within the scope of protection of the present invention.

Claims

1. A method for improving the deformation uniformity of the core wire of NbTi superconducting wire, characterized in that, Including: Deep drilling is performed on an oxygen-free copper ingot bar, and the holes are distributed in a hexagonal pattern to obtain a drilled oxygen-free copper ingot bar; The drilled oxygen-free copper ingot bar and the single-core rod are assembled, welded, second-extruded, and drawn to obtain a composite superconducting wire.

2. The method according to claim 1, wherein The difference between the diameter of the deep drill hole and the diameter of the single-core rod is 0.5 mm; The inner wall roughness of the deep drill hole is less than 1.6 μm; The surface roughness of the drilled oxygen-free copper ingot bar is less than 3.2 μm.

3. The method according to claim 1, characterized in that, The second extrusion is reverse hot extrusion; For the reverse hot extrusion, the extrusion temperature is 650 - 750 °C, the extrusion speed is 10 - 20 mm / s, and the extrusion ratio is 8 - 13.

4. The method according to claim 1, wherein After the second extrusion, skinning treatment is performed, and the thickness of the skinning is 3 - 4 mm; The drawing is combined with aging heat treatment, and the number of times of the aging heat treatment is 3 - 5 times; For the aging heat treatment, the temperature is 300 - 500 °C, and the time is 5 - 60 h.

5. The method according to claim 1, wherein The single-core rod is a NbTi / CuNi single-core rod or a NbTi / CuMnSi single-core rod; The raw materials of the NbTi / CuNi single-core rod are a NbTi ingot bar, a CuNi alloy tube, and a Nb cylinder; The raw materials of the NbTi / CuMnSi single-core rod are a NbTi ingot bar, a CuMnSi alloy tube, and a Nb cylinder.

6. The method according to claim 5, characterized in that The raw materials of the NbTi / CuNi single-core rod or the raw materials of the NbTi / CuMnSi single-core rod are assembled, welded, first-extruded, and drawn to obtain the NbTi / CuNi single-core rod or the NbTi / CuMnSi single-core rod.

7. The method according to claim 6, wherein The first extrusion is reverse hot extrusion; For the reverse hot extrusion, the extrusion temperature is 750 - 850 °C, the extrusion speed is 15 - 25 mm / s, and the extrusion ratio is 10 - 15.

8. The method according to claim 6, wherein After the first extrusion, skinning treatment is performed. The skinning is carried out by centerless turning, and the thickness of the skinning is 3 mm.

9. A NbTi superconducting wire, characterized in that, Obtained by the method according to any one of claims 1 - 8.

10. Application of the NbTi superconducting wire according to claim 9 in an MRI magnet.

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