NbTi superconducting wire suitable for variable magnetic field and method for manufacturing the same

Abstract

The application belongs to the technical field of superconducting wires, and relates to a NbTi superconducting wire suitable for a variable magnetic field and a preparation method thereof. The method uses Nb, Ti and Cu metal powder as raw materials, designs and establishes a gradient distribution NbTi / Cu composite ingot structure model, adopts a laser powder bed melting method to perform partition powder laying according to the designed structure model, and generates the NbTi / Cu composite ingot through layer-by-layer superposition forming through differential laser scanning. The formed NbTi / Cu composite ingot is subjected to extrusion, multi-pass stretching and aging heat treatment to obtain a NbTi / Cu composite wire. The method breaks through the limitation of traditional assembly process in structure design and arrangement precision, and makes each region synergistically act under different magnetic field strengths through the construction of a component gradient structure, so that the stability and adaptability of the wire in the application scene of the variable magnetic field or the wide magnetic field strength range are improved.

Description

Technical Field

[0001] This invention belongs to the field of superconducting wire technology, specifically relating to an NbTi superconducting wire suitable for variable magnetic fields and its preparation method. Background Technology

[0002] Niobium-titanium superconducting wires (NbTi superconducting wires) are widely used in magnetic resonance imaging systems, particle accelerators, and various scientific and engineering superconducting magnets due to their excellent processing performance, mature industrial manufacturing processes, and high critical current density under medium and low magnetic field conditions. Currently, the conventional method for preparing NbTi superconducting wires involves a combination of multiple mechanical assembly, extrusion, drawing, and heat treatment, resulting in a relatively uniform alloy composition throughout the wire cross-section. Existing NbTi superconducting wires are typically designed and manufactured for a relatively constant or narrow range of magnetic field strength. To meet the operational requirements of specific magnets, adjustments are made to the composition ratio, aging process, and dimensions to achieve a high critical current density near the target magnetic field strength. However, when used outside the designed magnetic field range, performance often deteriorates significantly. Therefore, existing NbTi superconducting wires are generally suitable for applications with relatively fixed or limited variations in the working magnetic field.

[0003] However, in practical engineering applications, the operating magnetic field of superconducting magnets is not a single constant value. As the demand for superconducting magnets in variable magnetic fields, wide magnetic field distributions, and swept-field operation continues to increase, the applicability of traditional NbTi superconducting wires in these scenarios is limited, failing to maintain stable current-carrying capacity and operational reliability within a wide or variable magnetic field range. To address this, this invention proposes an NbTi superconducting wire suitable for variable magnetic fields and its preparation method. By using a laser powder bed melting method, the composition and spatial distribution of the NbTi alloy are controllably designed to form a structure with spatially varying gradients, thereby achieving synergistic adaptation to different magnetic field ranges. Summary of the Invention

[0004] The purpose of this invention is to solve the problem that traditional NbTi superconducting wires are difficult to maintain stable operation in a wide range of magnetic fields or variable magnetic fields. To this end, this invention provides an NbTi superconducting wire suitable for variable magnetic fields and its preparation method.

[0005] On one hand, the present invention provides a method for preparing NbTi superconducting wire suitable for variable magnetic fields, comprising the following steps:

[0006] Step 1: Design a model of the NbTi / Cu composite ingot structure, the structure of which includes: The Cu matrix region is the continuous external phase of the NbTi / Cu composite ingot; A barrier layer region is disposed between the Cu matrix region and the NbTi region; The NbTi region is the core functional phase of the NbTi / Cu composite ingot. The Nb content in the NbTi region is distributed in a gradient along its own radial direction, and the gradient distribution is such that the Nb content gradually decreases from the center of the NbTi region to the side closer to the Nb barrier layer region. Step 2: Prepare Nb metal powder, Ti metal powder, and Cu metal powder, and dry them. Step 3: Mix Nb powder and Ti powder evenly according to the design to obtain multiple sets of NbTi metal powders with different compositions; Step 4: The NbTi / Cu composite ingot is formed layer by layer by using a laser powder bed melting method. The laser scanning parameters for the NbTi region are: laser power 180~300W, scanning speed 600~1200mm / s, and scanning spacing 80~120μm. The laser scanning parameters for the barrier layer region are: laser power 200~300W, scanning speed 600~1200mm / s, and scanning spacing 80~120μm; The laser scanning parameters for the Cu matrix region are: laser power 120~250W, scanning speed 700~1500mm / s, and scanning spacing 80~130μm; Step 5: The formed NbTi / Cu composite ingot is extruded, stretched in multiple passes, and subjected to aging heat treatment to obtain NbTi / Cu composite wire.

[0007] Furthermore, in the preparation method, the Nb content in the NbTi region ranges from 40 to 55 wt.%, and the gradient change step size is 0.5 to 5 wt.%.

[0008] Furthermore, in the preparation method, the NbTi region is composed of multiple concentric annular regions, and each concentric annular region corresponds to a Nb content gradient change step size.

[0009] Furthermore, in the preparation method, the Nb metal powder has a purity ≥99.9% and a particle size of 20~100μm; the Ti metal powder has a purity ≥99.9% and a particle size of 20~100μm; and the Cu metal powder has a purity ≥99.99% and a particle size of 20~200μm.

[0010] Furthermore, in the preparation method, in step two, the drying conditions are: drying at 80~120℃ under vacuum for 1~3 hours.

[0011] Furthermore, in the preparation method, in step three, the mixing method is mechanical mixing, with a mixing time of 1~12h and a rotation speed of 100~500rpm.

[0012] Furthermore, in the preparation method, in step four, the laser powder bed melting method involves first spreading powder in sections according to the design model, and then using differentiated laser scanning parameters to stack and form layers one by one, with the powder layer thickness being 20~500μm.

[0013] Furthermore, in the preparation method, in step five, the extrusion temperature is 500~800℃, the extrusion ratio is 25~60; the stretching speed of the multi-pass stretching is 5~80m / min, and the processing rate of each pass is 5%~30%.

[0014] On the other hand, it also provides NbTi superconducting wires suitable for variable magnetic fields prepared by the preparation method described in this invention.

[0015] Finally, the present invention also provides the application of the NbTi superconducting wire suitable for variable magnetic fields as a winding wire in superconducting magnets in non-uniform or changing magnetic fields.

[0016] Compared with the prior art, the technical solution provided by the present invention has at least the following beneficial effects or advantages: (1) This invention constructs a compositional structure in the NbTi superconducting region with a radial gradient distribution of Nb content, enabling different radial regions of the wire to sequentially enter the superconducting state and collaboratively carry current under corresponding magnetic field strengths. Specifically, the low Nb content region exhibits a higher critical current density under a lower magnetic field, while the high Nb content region maintains superior current-carrying performance under a higher magnetic field. The combined effect of multiple compositional regions results in a gradual decrease in the critical current density of the wire within a wide magnetic field range of 5T to 9.5T. Data from the embodiments show that the critical current density of the wire of this invention under a 9.5T magnetic field is approximately 38% higher than that of conventional uniform composition wires, and no sudden performance drop occurs in conventional NbTi superconducting wires when the magnetic field changes, achieving effective adaptation to variable magnetic fields or wide magnetic field distribution scenarios.

[0017] (2) This invention employs laser powder bed fusion additive manufacturing technology, and achieves integrated precision forming of NbTi / Cu composite ingots by switching zoned powder spreading in multiple material bins and using differentiated laser scanning parameters. Compared with the conventional "multiple assembly-extrusion-drawing" process, this invention can not only achieve radial composition gradient structures that are difficult to prepare by conventional processes, but also allows for the free design of more complex multi-region, multi-component spatial distributions according to magnetic field distribution requirements. The degree of freedom in structural design is significantly improved, opening up a new avenue for the customized preparation of superconducting wires.

[0018] (3) This invention prepares NbTi / Cu composite ingots with precise compositional distribution using a laser powder bed melting method, and then only requires one extrusion, drawing, and aging heat treatment to obtain the final wire. Compared with conventional methods that require multi-stage composite, multiple assembly, and multiple heat treatments, this invention significantly shortens the processing cycle, reduces the number of process steps, effectively reduces the introduction of interface defects and the accumulation of multi-stage processing errors, improves the uniformity of the wire's structure and consistency, and has good engineering implementation potential and cost advantages. Attached Figure Description

[0019] Figure 1 This is a flowchart of a method for preparing NbTi superconducting wires suitable for variable magnetic fields.

[0020] Figure 2 This is a schematic diagram of the cross-sectional structure of the NbTi / Cu composite ingot in Embodiment 1 of the present invention.

[0021] Figure 3 This is a schematic diagram of the compositional distribution of the NbTi region in Embodiment 1 of the present invention.

[0022] Figure 4 This is a schematic diagram of the cross-sectional structure of the NbTi / Cu composite ingot in Embodiment 2 of the present invention.

[0023] Figure 5 This is a schematic diagram of the compositional distribution of the NbTi region in Embodiment 2 of the present invention.

[0024] Figure 6 This is a schematic diagram of the cross-sectional structure of the NbTi / Cu composite ingot in Embodiment 3 of the present invention.

[0025] Figure 7 This is a schematic diagram of the compositional distribution of the NbTi region in Embodiment 3 of the present invention.

[0026] The reference numerals in the attached diagram are as follows: 1. Cu matrix region; 2. Barrier layer region; 3. NbTi region. Detailed Implementation

[0027] The technical solution of the present invention will be described below with reference to the embodiments. However, the present invention is not limited to the following embodiments.

[0028] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings, but the embodiments described are not intended to limit the present invention.

[0029] Unless otherwise specified, the experimental and detection methods described in the following embodiments are conventional methods; unless otherwise specified, the reagents and materials are commercially available.

[0030] Implementation Plan This invention provides a NbTi superconducting wire suitable for variable magnetic fields and its preparation method, the flowchart of which is shown below. Figure 1 As shown, the specific steps include: Step 1: Design and establish a structural model of NbTi / Cu composite ingot with a gradient composition distribution. The structure includes: Cu matrix region, barrier layer region, and NbTi region. The NbTi region is designed as a gradient structure with Nb content distributed radially, with Nb content ranging from 40 to 55 wt.% and gradient change step size from 0.5 to 5 wt.%.

[0031] Step 2: Prepare Nb metal powder, Ti metal powder, and Cu metal powder. The Nb metal powder should have a purity ≥99.9% and a particle size of 20–100 μm. The Ti metal powder should have a purity ≥99.9% and a particle size of 20–100 μm. The Cu metal powder should have a purity ≥99.99% and a particle size of 20–200 μm. Dry the metal powders at 80–120 °C under vacuum for 1–3 hours to remove adsorbed moisture.

[0032] Step 3: Mix Nb powder and Ti powder evenly according to the design to obtain multiple groups of NbTi metal powders with different compositions. The mixing method is mechanical mixing, the mixing time is 1~12h, and the rotation speed is 100~500rpm.

[0033] Step 4: The NbTi / Cu composite ingot is formed by layer-by-layer zonal powder spreading and shaping using the laser powder bed melting method. The Nb barrier layer and NbTi regions are scanned using a fully melted method, while the Cu matrix region is scanned using a medium energy density method. The laser powder bed melting method is to first spread the powder in zones according to the design model by switching between multiple material bins, and then use differentiated laser scanning parameters to form the ingot layer by layer. The powder layer thickness is 20~500μm, the atmosphere protection is high-purity argon gas with a flow rate of 10~50L / min, and the preheating temperature is 80~200℃. The zonal laser scanning parameters are as follows: (1) NbTi region: laser power 180~300W, scanning speed 600~1200mm / s, scanning spacing 80~120μm. (2) Barrier layer region: laser power 200~300W, scanning speed 600~1200mm / s, scanning spacing 80~120μm. (3) Cu matrix region: laser power 120~250W, scanning speed 700~1500mm / s, scanning spacing 80~130μm.

[0034] Step 5: The formed NbTi / Cu composite ingot is subjected to extrusion, multi-pass stretching, and aging heat treatment to obtain NbTi / Cu composite wire. The extrusion temperature is 500~800℃, the extrusion ratio is 25~60, the stretching speed of the multi-pass stretching is 5~80m / min, and the processing rate of each pass is 5%~30%.

[0035] Example 1 This embodiment demonstrates the preparation of NbTi superconducting wires suitable for variable magnetic fields using the method provided in the embodiments of the present invention.

[0036] Design and establish a structural model of an NbTi / Cu composite ingot with a compositional gradient distribution. The cross-sectional structure of the NbTi / Cu composite ingot is as follows: Figure 2 As shown, the NbTi region is designed with a gradient structure in which the Nb content increases radially from the outside to the inside, with the Nb content ranging from 45 to 55 wt.% and the gradient change step size being 5 wt.%. The composition distribution diagram of the NbTi region is shown below. Figure 3 As shown. The composite ingot has a diameter of Φ300mm, the NbTi mandrel has a diameter of Φ20mm, and the number of mandrels is 25. Nb and Ti metal powders with a purity ≥99.9% and a particle size of 100μm, and Cu metal powder with a purity ≥99.99% and a particle size of 200μm were prepared. The metal powders were dried at 80℃ under vacuum for 1 hour to remove adsorbed moisture. The Nb and Ti powders were uniformly mixed using a mechanical mixing method to obtain NbTi metal powder with Nb contents of 45wt.%, 50wt.%, and 55wt.%, respectively. The mixing time was 3 hours, and the rotation speed was 100 rpm. The NbTi / Cu composite ingot was layer-by-layer formed using a laser powder bed melting method, with the Nb barrier layer and NbTi region scanned using a fully melted scanning method, and the Cu matrix region scanned using a medium energy density method. The laser powder bed melting method is to first spread powder in sections by switching between multiple material bins according to the design model, and then use differentiated laser scanning parameters to stack and form the powder layer layer by layer. The powder layer thickness is 400μm, the atmosphere protection is high-purity argon gas with a flow rate of 10L / min, and the preheating temperature is 200℃. The laser scanning parameters for each section are as follows: (1) NbTi region: laser power 300W, scanning speed 600mm / s, scanning spacing 120μm. (2) Barrier layer region: laser power 200W, scanning speed 1200mm / s, scanning spacing 80μm. (3) Cu matrix region: laser power 250W, scanning speed 1500mm / s, scanning spacing 130μm. The formed NbTi / Cu composite ingot was subjected to extrusion, multi-pass stretching, and aging heat treatment to obtain NbTi / Cu composite wire. The extrusion temperature was 500℃, the extrusion ratio was 50, the stretching speed was 50m / min, and the processing rate per pass was 20%. The final specification of the NbTi / Cu composite wire (NbTi superconducting wire) was Φ2mm, and the copper ratio was 8.0.

[0037] Performance test results: The NbTi superconducting wire prepared in Example 1 and the NbTi superconducting wire prepared by conventional methods were subjected to heat treatment and performance testing. The critical current density of the NbTi superconducting wire was measured to be 2856 A / mm². 2 (4.2K, 5T), 342A / mm 2(4.2K, 9.5T). The critical current density of NbTi superconducting wire prepared by conventional methods is 3000A / mm². 2 (4.2K, 5T), 247A / mm 2 (4.2K, 9.5T). The results show that the critical current of the NbTi superconducting wire prepared in this embodiment maintains a relatively gentle decay trend in the magnetic field strength range of 5~9.5T. When the magnetic field strength is increased to 9.5T, it maintains stability and operational reliability under variable magnetic field compared with conventional NbTi superconducting wire.

[0038] Example 2 This embodiment demonstrates the preparation of NbTi superconducting wires suitable for variable magnetic fields using the method provided in the embodiments of the present invention.

[0039] Design and establish a structural model of an NbTi / Cu composite ingot with a compositional gradient distribution. The cross-sectional structure of the NbTi / Cu composite ingot is as follows: Figure 4 As shown, the NbTi region is designed with a gradient structure in which the Nb content increases radially from the outside to the inside, ranging from 49 to 54.5 wt.%, with a gradient step size of 0.5 wt.%. The compositional distribution of the NbTi region is shown in the figure. Figure 5As shown. The composite ingot has a diameter of Φ300mm, the NbTi mandrel has a diameter of Φ20mm, and the number of mandrels is 75. Nb and Ti metal powders with a purity ≥99.9% and a particle size of 20μm, and Cu metal powder with a purity ≥99.99% and a particle size of 100μm were prepared. The metal powders were dried at 120℃ under vacuum for 2 hours to remove adsorbed moisture. Nb powder and Ti powder were uniformly mixed by mechanical mixing to obtain NbTi metal powder with an Nb content of 48~55 wt.% (Nb content increasing in increments of 0.7 wt.%). The mixing time was 8 hours, and the rotation speed was 500 rpm. The NbTi / Cu composite ingot was layer-by-layered and formed using a laser powder bed melting method. The Nb barrier layer and NbTi regions were scanned using a fully melted scanning method, while the Cu matrix region was scanned using a medium energy density method. The laser powder bed melting method is to first spread powder in sections according to the design model by switching between multiple material bins, and then use differentiated laser scanning parameters to stack and form the powder layer layer by layer. The powder layer thickness is 150μm, the atmosphere protection is high-purity argon gas with a flow rate of 50L / min, and the preheating temperature is 100℃. The laser scanning parameters for each section are as follows: (1) NbTi region: laser power 180W, scanning speed 1200mm / s, scanning spacing 80μm. (2) Barrier layer region: laser power 250W, scanning speed 700mm / s, scanning spacing 90μm. (3) Cu matrix region: laser power 120W, scanning speed 700mm / s, scanning spacing 120μm. The formed NbTi / Cu composite ingot was subjected to extrusion, multi-pass stretching, and aging heat treatment to obtain NbTi / Cu composite wire. The extrusion temperature was 650℃, the extrusion ratio was 30, the stretching speed was 45m / min, and the processing rate per pass was 15~20%. The final specification of the NbTi / Cu composite wire (NbTi superconducting wire) was Φ0.620mm and the copper ratio was 2.0.

[0040] Performance test results: The NbTi superconducting wire prepared in Example 2 and the NbTi superconducting wire prepared by conventional methods were subjected to heat treatment and performance testing. The critical current density of the NbTi superconducting wire was measured to be 2902 A / mm². 2 (4.2K, 5T), 1768A / mm 2 (4.2K, 7T), 1121A / mm 2 (4.2K, 8T), 620A / mm 2 (4.2K, 9T), 357A / mm 2 (4.2K, 9.5T). The critical current density of NbTi superconducting wire prepared by conventional methods is 3000A / mm². 2 (4.2K, 5T), 1730A / mm 2 (4.2K, 7T), 1080A / mm 2 (4.2K, 8T), 500A / mm2 (4.2K, 9T), 270A / mm 2 (4.2K, 9.5T). The results show that the critical current of the NbTi superconducting wire prepared in Example 2 maintains a relatively gradual decay trend in the magnetic field strength range of 5~9.5T, without the sudden drop phenomenon of conventional NbTi superconducting wire when the magnetic field changes, and maintains stability and operational reliability under variable magnetic field.

[0041] Example 3 Design and establish a structural model of an NbTi / Cu composite ingot with a compositional gradient distribution. The cross-sectional structure of the NbTi / Cu composite ingot is as follows: Figure 6 As shown, the NbTi region is designed with a gradient structure in which the Nb content increases radially from the outside to the inside, ranging from 46 to 54 wt.%, with a gradient step size of 1.0 wt.%. The compositional distribution of the NbTi region is shown in the figure. Figure 7 As shown. The composite ingot has a diameter of Φ300mm, the NbTi mandrel has a diameter of Φ20mm, and the number of mandrels is 42. Nb and Ti metal powders with a purity ≥99.9% and a particle size of 40μm, and Cu metal powder with a purity ≥99.99% and a particle size of 120μm were prepared. The metal powders were dried at 100℃ under vacuum for 3 hours to remove adsorbed moisture. Nb powder and Ti powder were uniformly mixed by mechanical mixing to obtain NbTi metal powder with an Nb content of 47~54 wt.% (Nb content increasing in 1.0 wt.% increments). The mixing time was 5 hours, and the rotation speed was 300 rpm. The NbTi / Cu composite ingot was layer-by-layered and formed using a laser powder bed melting method. The Nb barrier layer and NbTi regions were scanned using a fully melted scanning method, while the Cu matrix region was scanned using a medium energy density method. The laser powder bed melting method is to first spread powder in sections by switching between multiple material bins according to the design model, and then use differentiated laser scanning parameters to stack and form the powder layer layer by layer. The powder layer thickness is 200μm, the atmosphere protection is high-purity argon gas with a flow rate of 30L / min, and the preheating temperature is 140℃. The laser scanning parameters for each section are as follows: (1) NbTi region: laser power 220W, scanning speed 900mm / s, scanning spacing 100μm. (2) Barrier layer region: laser power 300W, scanning speed 600mm / s, scanning spacing 90μm. (3) Cu matrix region: laser power 180W, scanning speed 1200mm / s, scanning spacing 100μm. The formed NbTi / Cu composite ingot was subjected to extrusion, multi-pass stretching, and aging heat treatment to obtain NbTi / Cu composite wire. The extrusion temperature was 620℃, the extrusion ratio was 40, the stretching speed was 50m / min, and the processing rate per pass was 15~20%. The final specification of the NbTi / Cu composite wire (NbTi superconducting wire) was Φ0.880mm, and the copper ratio was 4.0.

[0042] Performance test results: The NbTi superconducting wire prepared in Example 3 and the NbTi superconducting wire prepared by conventional methods were subjected to heat treatment and performance testing. The critical current density of the NbTi superconducting wire was measured to be 2892 A / mm². 2 (4.2K, 5T), 1760A / mm 2 (4.2K, 7T), 1118A / mm 2 (4.2K, 8T), 609A / mm 2 (4.2K, 9T), 354A / mm 2 (4.2K, 9.5T). The critical current density of NbTi superconducting wire prepared by conventional methods is 2993 A / mm². 2 (4.2K, 5T), 1720A / mm 2 (4.2K, 7T), 1073A / mm 2 (4.2K, 8T), 473A / mm 2 (4.2K, 9T), 251A / mm 2 (4.2K, 9.5T). The results show that the critical current of the NbTi superconducting wire prepared in Example 3 maintains a relatively gentle decay trend in the magnetic field strength range of 5~9.5T, without the sudden drop phenomenon of conventional NbTi superconducting wire when the magnetic field changes, and maintains stability and operational reliability under variable magnetic field.

[0043] As described above, the basic principles, main features, and advantages of the present invention have been well described. The above embodiments and specifications are merely descriptions of preferred embodiments of the present invention, and the present invention is not limited to the above embodiments. Various changes and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the spirit and scope of the present invention should fall within the protection scope defined by the present invention.

Claims

1. A method for preparing NbTi superconducting wire suitable for variable magnetic fields, characterized in that, Includes the following steps: Step 1: Design a model of the NbTi / Cu composite ingot structure, which includes: The Cu matrix region is the continuous external phase of the NbTi / Cu composite ingot; A barrier layer region is disposed between the Cu matrix region and the NbTi region; The NbTi region is the core functional phase of the NbTi / Cu composite ingot. The Nb content in the NbTi region is distributed in a gradient along its own radial direction, and the gradient distribution is such that the Nb content gradually decreases from the center of the NbTi region to the side closer to the Nb barrier layer region. Step 2: Prepare Nb metal powder, Ti metal powder, and Cu metal powder, and dry them. Step 3: Mix Nb powder and Ti powder evenly according to the design to obtain multiple sets of NbTi metal powders with different compositions; Step 4: The NbTi / Cu composite ingot is formed layer by layer by using a laser powder bed melting method. The laser scanning parameters for the NbTi region are: laser power 180~300W, scanning speed 600~1200mm / s, and scanning spacing 80~120μm. The laser scanning parameters for the barrier layer region are: laser power 200~300W, scanning speed 600~1200mm / s, and scanning spacing 80~120μm; The laser scanning parameters for the Cu matrix region are: laser power 120~250W, scanning speed 700~1500mm / s, and scanning spacing 80~130μm; Step 5: The formed NbTi / Cu composite ingot is extruded, stretched in multiple passes, and subjected to aging heat treatment to obtain NbTi / Cu composite wire.

2. The preparation method according to claim 1, characterized in that, The Nb content in the NbTi region ranges from 40 to 55 wt.%, with a gradient change step size of 0.5 to 5 wt.%.

3. The preparation method according to claim 2, characterized in that, The NbTi region is composed of multiple concentric ring-shaped regions, and each concentric ring-shaped region corresponds to a step size of Nb content gradient change.

4. The preparation method according to claim 1, characterized in that, The Nb metal powder has a purity of ≥99.9% and a particle size of 20~100μm; the Ti metal powder has a purity of ≥99.9% and a particle size of 20~100μm; the Cu metal powder has a purity of ≥99.99% and a particle size of 20~200μm.

5. The preparation method according to claim 1, characterized in that, In step two, the drying conditions are: drying at 80~120℃ under vacuum for 1~3 hours.

6. The preparation method according to claim 1, characterized in that, In step three, the mixing method is mechanical mixing, with a mixing time of 1~12 hours and a rotation speed of 100~500 rpm.

7. The preparation method according to claim 1, characterized in that, In step four, the laser powder bed melting method involves first spreading powder in sections according to the design model, and then using differentiated laser scanning parameters to stack and form layers one by one, with a powder layer thickness of 20~500μm.

8. The preparation method according to claim 1, characterized in that, In step five, the extrusion temperature is 500~800℃, and the extrusion ratio is 25~60; the stretching speed of the multi-pass stretching is 5~80m / min, and the processing rate of each pass is 5%~30%.

9. A NbTi superconducting wire suitable for variable magnetic fields, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the NbTi superconducting wire of claim 9, suitable for variable magnetic fields, as a winding wire in superconducting magnets in non-uniform or varying magnetic fields.

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