A production method for processing a superconducting coil box by additive manufacturing

Abstract

This invention relates to a method for manufacturing superconducting coil boxes using additive manufacturing, belonging to the technical field of low-temperature superconducting coil boxes. The method includes: S1, establishing a three-dimensional model of the superconducting coil box; S2, slicing the three-dimensional model into layers; S3, cladding the coil box or its segments layer by layer, adapting the process parameters to the characteristics of the raw materials; S4, performing post-processing on the formed parts, including solution treatment, stress-relieving annealing, and machining. This invention achieves near-net-shape molding of complex structures, with high material utilization. Through process adaptation and post-processing, it ensures high density, high reliability, excellent mechanical properties, and high heat transfer performance of the components under extremely low temperatures and strong electromagnetic forces.

Description

Technical Field

[0001] This invention relates to a production method for superconducting coil boxes using additive manufacturing, belonging to the technical field of low-temperature superconducting coil boxes. Background Technology

[0002] Superconducting coil housings are the core structural components of superconducting magnet systems, widely used in cutting-edge technologies such as medical magnetic resonance imaging, particle accelerators, fusion devices, superconducting magnetic energy storage, electromagnetic launch, and aerospace shielding systems. Their core function is to provide high-precision, high-rigidity mechanical support and vacuum sealing protection for superconducting coils under extreme conditions of extremely low temperatures (e.g., liquid helium temperature 4.2K or liquid nitrogen temperature 77K) and strong electromagnetic forces, while maintaining extremely high dimensional stability and structural integrity. To meet specific three-dimensional magnetic field distributions (e.g., uniform regions, gradient fields, end shaping, etc.) and process requirements, many superconducting coil housings are designed with complex spatial distortions and irregular three-dimensional shapes; some also integrate internal cooling channels for efficient thermal management.

[0003] In existing technologies, the manufacturing process of superconducting coil boxes mainly adopts a combination of segmented forging, machining, and welding. First, large metal raw materials (such as low-temperature stainless steel or nickel-based alloys) are forged into blank segments that are close to the target shape. Then, these blanks are machined with high precision to form the final contour, mating surfaces, and internal cavities. Finally, the processed segments are connected into a complete and sealed box body by precision welding (such as narrow-gap laser filler wire welding, see publication number CN103252578A). This process has the following problems: 1. Low material utilization rate: a large amount of material is removed from the large blank through cutting, milling and other methods, resulting in huge material waste; 2. Long processing cycle and high cost: for complex spatial curved surfaces, multiple sets of special fixtures, complex multi-axis linkage processing paths and repeated clamping and positioning are required, resulting in a long production cycle and extremely high requirements for equipment and operators; 3. Limited manufacturing capabilities: the processing of irregular shapes, twisted structures and internally integrated complex flow channels (especially turbulent flow structures that can optimize heat exchange) is extremely difficult, or even impossible, which greatly limits the lightweight and performance optimization design of coil boxes.

[0004] Additive manufacturing technology provides a new approach for the direct forming of complex metal components.

[0005] However, directly applying additive manufacturing to superconducting coil boxes still faces the following technical bottlenecks: 1. Performance anisotropy and adaptation to extreme working conditions: Additively manufactured parts inherently exhibit anisotropy in mechanical properties. Their fatigue performance and fracture toughness differ significantly in the horizontal and vertical directions. How to ensure optimal performance in key stress directions is a problem that general additive manufacturing processes have not yet solved; 2. Internal quality and density: When using additive manufacturing to form materials such as stainless steel and nickel-based alloys, components can usually achieve high internal quality and good overall density, and their overall defect level is usually within a controllable range. While it's not necessary to rely solely on subsequent densification processes like hot isostatic pressing, for components like superconducting coil boxes that require high structural integrity and vacuum sealing, further optimization of processes and quality control are needed to reduce the impact of defects on reliability under extreme service conditions. 3. Functional design of complex internal flow channels: Although existing additive manufacturing technologies can form internal flow channels, how to flexibly incorporate high-performance internal flow channel topologies that enhance heat transfer, such as turbulence and disturbance, to meet the extreme heat transfer requirements of superconducting coil boxes, and how to integrate these with complex external surfaces, is an area not addressed by existing general additive manufacturing methods. Therefore, there is an urgent need in this field for a complete additive manufacturing solution specifically for superconducting coil boxes. Summary of the Invention

[0006] The present invention aims to solve the above-mentioned problems and thereby provides a method for manufacturing superconducting coil boxes by additive manufacturing.

[0007] The technical solution of the present invention to solve the above problems is as follows:

[0008] A method for manufacturing superconducting coil boxes using additive manufacturing includes the following steps:

[0009] S1. Establish a three-dimensional model of the superconducting coil box; when modeling, it is necessary to consider machining allowance, deformation compensation, support layout and bevels for reserved welding parts, etc.

[0010] S2. Slice the 3D model into layers and obtain the data for each layer;

[0011] S3 employs laser powder bed cladding, electron beam powder bed cladding, directional energy deposition, and additive / subtractive manufacturing technologies. It utilizes an energy beam to perform layer-by-layer cladding and deposition based on 3D model slice data. Laser powder bed fusion technology is suitable for forming small and medium-sized coil boxes with complex structures and high dimensional accuracy requirements. Directional energy deposition technology is suitable for forming large coil box segments. Additive / subtractive manufacturing technology is suitable for components that require online milling of key feature surfaces during printing to ensure real-time accuracy.

[0012] S4. Perform post-processing on the molded parts.

[0013] This invention employs additive manufacturing technology with energy beam layer-by-layer cladding as the core forming method. Its technical logic is to achieve precise material accumulation based on a three-dimensional model. The direct technical effects are: 1) It achieves near-net-shape forming of coil box components, increasing material utilization to over 90%; 2) It enables integrated manufacturing of extremely complex structures, allowing direct forming of spatially twisted irregular surfaces and complex internal flow channels inaccessible by traditional machining, thus releasing design freedom; 3) By reducing segments and welds (achieving zero welds for integral printed parts), it eliminates a major source of reliability risk at the source, improving structural integrity.

[0014] As a preferred embodiment of the above technical solution, in step S1, an internal flow channel is designed inside the wall of the superconducting coil box during modeling; the internal flow channel is integrally formed with the coil box.

[0015] An internal flow channel can be integrated into the wall of the superconducting coil box according to application requirements, and the internal flow channel is formed synchronously with the coil box. The interior of the internal flow channel can be designed with reinforcing structures to increase turbulence and improve heat exchange, including but not limited to spiral ribs, turbulence columns, fractal branch channels or surface microstructures, to increase the heat transfer area, induce secondary flow and turbulence, and improve the heat transfer efficiency from the superconducting coil to the cooling medium.

[0016] As a preferred embodiment of the above technical solution, in step S4, the post-processing includes heat treatment, pickling, machining, etc., which can be selected and implemented according to the actual requirements of the component and the service conditions.

[0017] Heat treatment methods include stress-relief annealing and solution treatment, which can be used alone or in combination. Stress-relief annealing is mainly used to reduce residual stress generated during forming and subsequent processing, reduce stress concentration, and improve the dimensional stability and service reliability of components. Solution treatment is mainly used to weaken or eliminate elemental microsegregation and non-equilibrium substructures formed during printing, promote the dissolution of precipitated phases, homogenize the microstructure, stabilize the austenite matrix, and improve the stability and reliability of components under subsequent processing and low-temperature service conditions (4.2K or 77K).

[0018] Perform necessary machining and / or pickling surface treatment on the molded parts: First, pickling is performed to remove the surface oxide layer and reduce surface roughness; then, key parts such as coil mounting surface and sealing surface are machined to ensure dimensional accuracy and surface finish.

[0019] As a preferred embodiment of the above technical solution, when the coil box is an ultra-large component, the execution content of steps S1 to S3 is to form each segment of the three-dimensional model; after step S4, the step of welding and assembling each segment into a complete component is also included; after welding and assembly, the step of performing non-destructive testing on the weld is also included.

[0020] When the coil box is an ultra-large component (exceeding the equipment's forming capacity), this solution adopts a "segmented printing - precision welding" strategy. The inlet and outlet positions of the flow channels must be planned during the modeling stage to minimize weld joints in the cross-sections of the internal flow channels between segments. Low heat input, high energy density vacuum electron beam welding is preferred for welding to reduce the heat-affected zone. After welding, a solution treatment compatible with the substrate must be performed, and all welds must undergo 100% non-destructive testing (such as ultrasonic phased array or DR X-ray inspection) to ensure weld quality meets the requirements of extreme operating conditions and guarantees the reliability of the overall component.

[0021] As a preferred embodiment of the above technical solution, after step S3, a step of pickling the molded part is also included.

[0022] As a preferred embodiment of the above technical solution, the powder raw material is low-temperature stainless steel.

[0023] The powder raw material is low-temperature stainless steel, including but not limited to grades such as 316LN or N50, preferably 316L or 316LN, and the magnetic permeability is required to meet μr≤1.05, preferably≤1.02, in order to meet the stringent requirements of superconducting magnets for low magnetic properties.

[0024] As a preferred embodiment of the above technical solution, the powder raw material is prepared by the following method: melting and processing the alloy raw material into rods or wires, and then using these rods or wires as raw materials to produce powder through an atomization process.

[0025] In the above technical solution of this invention, the powder raw material is prepared from rods or wires using atomization processes such as EIGA. When using laser powder bed melting technology, powders with a particle size of 15~53μm are typically selected; if 316LN powder is used, the nitrogen content and composition need to be controlled from the rod melting end, and can be adjusted as needed. The thickness of the printed powder layer is 30~100μm, depending on the size of the workpiece and the surface precision requirements.

[0026] As a preferred embodiment of the above technical solution, in step S2, before slicing, the three-dimensional model is first subjected to stress analysis, and the placement of the three-dimensional model is optimized based on the anisotropic characteristics of the mechanical properties of additive manufacturing.

[0027] When preparing for workpiece printing, the placement position of the workpiece in the printing chamber needs to be set in advance. First, a stress analysis is performed on the workpiece, and the placement posture of the 3D model in the printing space is optimized based on the anisotropic characteristics of additive manufacturing mechanical properties. Then, by adjusting the orientation of the "part to be printed" in the printing chamber, the stacking direction is aligned with the direction of minimum principal stress, or the scanning direction is aligned with the direction of maximum principal stress.

[0028] To address the mismatch between the anisotropic properties of additively manufactured parts and extreme operating conditions, this invention optimizes workpiece placement during printing preparation, taking into account the anisotropic characteristics of additive manufacturing mechanical properties. The technical logic involves coupling and optimizing the principal stress distribution of the part under extremely low temperatures and strong electromagnetic forces with the inherent anisotropic characteristics of the additive manufacturing process. By adjusting the orientation of the "part to be formed" in the printing chamber, aligning the deposition direction (Z-axis) with the direction of minimum principal stress (or aligning the scanning direction with the direction of maximum principal stress), the formed part is ensured to possess optimal mechanical properties (such as fatigue strength and fracture toughness) in key stress directions, thus directly adapting to extreme operating conditions.

[0029] In summary, the present invention has the following beneficial effects:

[0030] 1. This invention achieves the direct manufacturing of complex spatial curved surfaces, irregular structures and high-performance internal flow channels that are impossible or extremely costly to process using traditional processes through additive manufacturing. The material utilization rate is increased from less than 20% in traditional processing to more than 80%, which greatly liberates the design space for lightweighting, functional integration and performance optimization of coil boxes.

[0031] 2. This invention actively adapts to the anisotropy of additive manufacturing through performance-driven printing posture optimization, thereby optimizing the performance of key components; by adopting a stable printing process, high density is achieved in the printing state; and stress is eliminated and the microstructure is homogenized through heat treatment, ensuring the long-term stability of the component at extremely low temperatures. Under the condition of not using hot isostatic pressing, the low-temperature performance of the component is guaranteed, significantly reducing manufacturing costs and cycle time.

[0032] 3. This invention reduces the risk of failure at the source by reducing segments and welds; for necessary segments, precision welding and non-destructive testing are used to ensure connection reliability and significantly improve the long-term service reliability of the coil box under extremely low temperature and strong electromagnetic cyclic load.

[0033] 4. This invention eliminates the need for complex multi-axis linkage machining, special tooling fixtures, and a large number of material cutting processes, which can significantly shorten the manufacturing cycle and greatly reduce metal cutting waste, in line with the concepts of green manufacturing and sustainable development. Attached Figure Description

[0034] Figure 1 This is a diagram showing the DR X-ray detection results of Example 1;

[0035] Figure 2 This is the permeability detection diagram of Example 1. Detailed Implementation

[0036] The present invention will be further explained and described below with reference to the accompanying drawings.

[0037] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. Any changes made by those skilled in the art after reading the specification of the present invention, as long as they are within the scope of the claims, will be protected by patent law.

[0038] Example 1: A small superconducting coil box is integrally molded using laser powder bed fusion (L-PBF) technology.

[0039] This embodiment provides a small to medium-sized superconducting coil box with a complex built-in flow channel for a small fusion experimental device. It adopts an integrated molding scheme and the material is 316LN (L represents low carbon and N represents high nitrogen content).

[0040] The chemical composition of Example 316LN is shown in the table below:

[0041] ≤0.02 ≤0.75 ≤2.0 ≤0.03 ≤0.015 16.0~18.0 12.0~14.0 Mo Co N B Ta Nb Rare earth elements 2.0~3.0 ≤0.05 0.14~0.20 ≤0.0018 ≤0.010 ≤0.10 Not included

[0042] Note: Rare earth elements must not be added during the production process.

[0043] Step S101: Performance-driven modeling and printing posture optimization

[0044] 3D Digital Modeling: A 3D model of the coil box is established based on the electromagnetic and cooling requirements of the magnet system. A conformal cooling channel system is integrated into the box wall, and a periodic turbulence column array is designed inside the channel based on simulation results to enhance liquid helium heat transfer.

[0045] Stress Analysis and Orientation Optimization: Finite element analysis was performed on the model to simulate its stress distribution under 4.2K / 77K and working electromagnetic force, and the principal stress directions were identified. The model was then imported into additive manufacturing software for printing orientation optimization. The core objective was to ensure that the principal tensile stress direction of the component at extremely low temperatures was parallel to the plane of the printing substrate (i.e., the XY plane). Simultaneously, the model placement was optimized to achieve the best printing quality for key mounting surfaces and the inner walls of the flow channels, while minimizing the need for supports.

[0046] Slicing: Support generation and layer slicing are performed on the optimized model, with the layer thickness set to 30~100μm, to generate a scan path file that can be executed by the device.

[0047] Step S102: Additive manufacturing

[0048] Equipment and Materials: High-precision laser powder bed melting equipment was selected, and the oxygen content in the molding chamber was controlled below 100 ppm. The powder material was 316LN powder (particle size 15-53 μm, a small amount of satellite powder, oxygen content <200 ppm) that met the requirements of the aforementioned technical solution, prepared by the EIGA process.

[0049] Printing process: A 316L substrate is used and preheated to 80°C. Stable printing parameters are employed, such as laser power 200-300W, scanning speed 500-1300mm / s, scanning spacing 0.1mm, and a 67° rotating checkerboard scanning strategy. The entire printing process is carried out continuously under argon protection, producing a complete coil box component with complex internal flow channels in a single step.

[0050] Step S103: Post-processing and finishing

[0051] Powder removal and separation: After printing, the component is removed under a protective atmosphere and the residual powder on the inner and outer surfaces and in the flow channels is carefully cleaned by vibration, blowing and other methods. Then, the component is separated from the substrate by wire cutting.

[0052] Heat treatment: Heat treatment methods include stress-relieving annealing and solution treatment. The appropriate method can be selected based on the actual requirements and service conditions of the component. One method can be used alone or in combination.

[0053] Machining and surface treatment: After heat treatment, the components are first pickled to remove the surface oxide layer and reduce the surface roughness; then the key parts such as the coil mounting surface and sealing surface are machined to ensure dimensional accuracy and surface finish.

[0054] Comprehensive Inspection: The following inspections will be performed on the final components: 1) 100% DR radiographic inspection will be performed on all welds (3 welds, the coil box is welded and sealed after the coil is filled). The results are as follows: Figure 1 As shown in the figure, the weld is dense and defect-free; 2) Samples were taken from the component body to test the magnetic permeability, such as Figure 2 As shown, the magnetic permeability of the sample is 1.001, and the magnetic permeability of the weld meets the requirements of the ITER device (≤1.05).

[0055] Example 2 uses laser-directed energy deposition (L-DED) technology to form a large coil box in segments and then welds it.

[0056] This embodiment addresses a superconducting coil box for a stellarator device, whose dimensions exceed the molding chamber of a powder bed apparatus. It employs a segmented printing, welding, and overall post-processing approach, using 316LN as the material.

[0057] Step S201: Segmented design, printing and rough machining

[0058] Segmented design: Based on the geometric features of the coil box and the forming capabilities of the existing DED equipment, it is reasonably divided into multiple printable sections, and bevels for subsequent assembly and welding are designed. The positions of the flow channel inlet and outlet are coordinated to avoid welded joints in the cross-section of the inner flow channel between the segments.

[0059] Segmented printing: Using a large-scale laser directional energy deposition (LAD) system, 316LN metal wire or a suitable powder of the same composition is used as raw material to print each segment separately. Appropriate substrate preheating and interlayer cooling control are employed during the printing process to reduce deformation and stress, and each segment is allowed sufficient machining allowance.

[0060] Pre-welding rough machining: Rough milling is performed on the bevel surfaces and main functional surfaces of each segment to prepare for subsequent welding and finishing.

[0061] Step S202: Welding Assembly

[0062] Each segment is precisely pre-assembled and fixed on specialized tooling. Narrow-gap tungsten inert gas (TIG) welding is employed, using high-purity argon gas for protection, and high-quality welding materials with a composition matching the base metal are selected. Strict control of heat input and a multi-pass welding process ensure weld quality and minimize the heat-affected zone. After welding, the welded area is immediately subjected to stress-relief annealing.

[0063] Step S203: Overall post-processing and finishing

[0064] Overall heat treatment: The welded components are subjected to overall solution treatment (1000~1120℃, held for 0.5~2 hours and then water quenched) to homogenize the microstructure and properties of the entire component, including the weld.

[0065] Overall finishing: After heat treatment, all key feature surfaces of the component are finished on a large CNC machine tool to achieve the dimensions and tolerances required by the drawings. Pickling may be performed on the surface if necessary.

[0066] Inspection: The following inspections will be carried out on the final components: 1) 100% radiographic inspection of all welds; 2) Samples will be taken from the component body to test the magnetic permeability and confirm that the magnetic permeability of the welds meets the requirements of the ITER device (≤1.05).

Claims

1. A method for manufacturing a superconducting coil box using additive manufacturing, comprising the following steps: S1. Establish a three-dimensional model of the superconducting coil box; S2. Slice the 3D model into layers and obtain the data for each layer; S3. Using laser powder laying, electron beam powder laying, directional energy deposition, and additive / subtractive material integration technology, the energy beam is used to perform layer-by-layer cladding and deposition according to the three-dimensional model slice data. S4. Perform post-processing on the molded parts.

2. The production method according to claim 1, characterized in that: In step S2, before slicing, the three-dimensional model is first subjected to stress analysis, and the placement of the three-dimensional model is optimized based on the anisotropic characteristics of the mechanical properties of additive manufacturing.

3. The production method according to claim 1, characterized in that, In step S1, during modeling, an internal flow channel is designed within the wall of the superconducting coil box to increase turbulence and improve heat exchange.

4. The production method according to claim 1, characterized in that, In step S4, the post-processing includes, but is not limited to, annealing, solution treatment, homogenization treatment, and hot isostatic pressing.

5. The production method according to claim 1, characterized in that, When the coil box is an ultra-large component, steps S1 to S3 involve forming the segments of the three-dimensional model; after step S4, the steps include welding and assembling the segments into a complete component; after welding and assembly, the steps include performing non-destructive testing on the weld.

6. The production method according to claim 1, characterized in that, After step S4, the process also includes a pickling process for the molded part.

7. The production method according to claim 1, characterized in that, The powder raw material is low-temperature stainless steel.

8. The production method according to claim 7, characterized in that, The powder raw material is prepared by melting and processing the alloy raw material into rods or wires, and then using these rods or wires as raw materials to produce powder through an atomization process.

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