Copper-chromium alloy for transformers and method for producing the same

Abstract

The application provides a copper-chromium alloy for transformers and a preparation method thereof. The preparation method comprises the following steps: S1, ingredients of raw materials of the copper-chromium alloy are prepared and then smelted to obtain a melt; S2, the melt is subjected to directional solidification by applying a gradient magnetic field along a solidification direction to obtain an ingot; S3, the ingot is sequentially subjected to hot rolling and cold rolling to obtain a cold-rolled alloy; and S4, the cold-rolled alloy is subjected to pulse current annealing to obtain the copper-chromium alloy for transformers. By introducing an axial gradient magnetic field in the solidification stage, the application realizes the transition of precipitated phases in the copper-chromium alloy from random distribution to directional chain arrangement for the first time. Meanwhile, by means of millisecond pulse current annealing, the application activates a high-efficiency precipitated strengthening mechanism for the first time without sacrificing the texture. Through the synergistic effect of the two, the copper-chromium alloy for transformers has high electrical conductivity, high strength and low high-frequency eddy current loss.

Description

Technical Field

[0001] This invention relates to the field of metal material preparation technology, and more specifically, to a copper-chromium alloy for transformers and its preparation method. Background Technology

[0002] Dry-type transformers, as key power conversion and isolation devices in modern power systems, have been widely used in scenarios with stringent requirements for reliability and space efficiency, such as urban smart grids, rail transit traction power supply systems, and high-density data centers, thanks to their advantages of being oil-free, fire-resistant, and maintenance-free. In these applications, the comprehensive performance of the winding materials directly determines the transformer's energy efficiency, thermal stability, and structural compactness. For a long time, high-purity electrolytic copper has been widely used due to its excellent conductivity. However, with the development of power electronics technology, especially the large-scale integration of high-frequency switching power supplies and nonlinear loads, traditional pure copper windings have revealed several inherent defects in actual operation: 1) Under continuous high-temperature conditions (above 150℃), the tensile strength of pure copper rapidly decreases, easily leading to winding geometric deformation or even insulation failure; 2) Under high-frequency harmonic excitation, the skin effect and proximity effect are significantly enhanced, resulting in non-uniform distribution of eddy currents inside the conductor, thus generating additional Joule heat loss; 3) Lower mechanical strength limits the compact design of the winding structure, making it difficult to meet the engineering requirements of the new generation of high-power-density transformers.

[0003] To overcome the aforementioned bottlenecks, researchers have turned to copper-based alloy systems. Copper-chromium alloys have attracted considerable attention due to their significant precipitation strengthening effect achieved through the aging precipitation of nanoscale chromium phases. Typical copper-chromium alloys, after smelting, hot rolling, cold rolling, and subsequent annealing, can achieve tensile strengths exceeding 300 MPa, effectively improving high-temperature mechanical stability. However, while traditional manufacturing processes enhance strength, they introduce irreconcilable contradictions at the microstructural level. Specifically, during solidification, chromium, with a partition coefficient less than 1, is prone to macroscopic and microscopic segregation, forming coarse chromium phase particles at the micrometer scale. These second phases not only disrupt the continuity of the matrix but also become strong electron scattering centers, causing the alloy's conductivity to plummet below 80% of the international annealed copper standard. Furthermore, the highly random grain orientation formed by traditional rolling and annealing processes results in a tortuous and disordered current flow path within the conductor. Under the influence of a high-frequency alternating magnetic field, eddy currents are forced to close along irregular paths, significantly exacerbating additional eddy current losses. Measured data show that the high-frequency losses are 15% to 30% higher than theoretical values. Furthermore, the traditional thermal annealing process used to restore some conductivity, while promoting dislocation annihilation and partial phase re-dissolution, inevitably induces abnormal grain growth (average size exceeding fifty micrometers), thereby weakening the strength advantage gained through work hardening and precipitation strengthening in the early stage, resulting in a dilemma where it is difficult to balance "high strength, high conductivity, and low high-frequency loss".

[0004] While existing improvement methods have achieved some breakthroughs in certain areas, they have failed to address the core of the problem. For example, some researchers have proposed introducing trace amounts of zirconium into copper-chromium alloys to refine the grains. Although this has suppressed grain coarsening to some extent, it has not effectively controlled the spatial distribution of precipitates and the orientation of grains, thus failing to improve the disorder of high-frequency eddy current paths, resulting in persistent high-frequency losses. Other studies have attempted to use strong magnetic fields to assist solidification, but the applied magnetic field is a uniform static magnetic field, mainly used to suppress melt convection and reduce compositional segregation, but it cannot provide a directional driving force to guide the precipitates to arrange themselves in an orderly manner along a specific direction. None of the above methods can truly synergistically optimize the three major performance indicators. Therefore, there is an urgent need to develop a method for preparing copper-chromium alloys that can simultaneously achieve high strength, high conductivity, and low high-frequency losses. Summary of the Invention

[0005] The main objective of this invention is to provide a copper-chromium alloy for transformers and its preparation method, so as to solve the problem that in the prior art, the copper-chromium alloy used for dry transformer windings is difficult to achieve high strength, high conductivity and low high-frequency eddy current loss due to the uncontrollable microstructure during the preparation process.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for preparing a copper-chromium alloy for transformers is provided. The method includes: step S1, mixing raw materials corresponding to the copper-chromium alloy and then melting them to obtain a melt; step S2, applying a gradient magnetic field along the solidification direction to the melt for directional solidification to obtain an ingot; step S3, sequentially hot-rolling and cold-rolling the ingot to obtain a cold-rolled alloy; and step S4, annealing the cold-rolled alloy with a pulsed current to obtain the copper-chromium alloy for transformers; wherein the magnetic field gradient of the gradient magnetic field is 0.1~0.5 T / m; and the current density of the pulsed current annealing is 500~2000 A / cm². 2 The duration of pulsed current annealing is 1~10ms.

[0007] Furthermore, in step S2 above, during the process of pouring the melt into the crystallizer, an asymmetric Helmholtz coil system is set outside the crystallizer to apply a gradient magnetic field that decreases along the solidification direction to the melt; the application of the gradient magnetic field is simultaneous with the pouring of the melt, and the asymmetric Helmholtz coil system is shut off within ≤5 seconds after the melt has completely solidified; the directional solidification time is 15~30 minutes; the asymmetric Helmholtz coil system includes a first coil and a second coil arranged coaxially; wherein, the first coil is located at the inlet end of the crystallizer, and the second coil is located at the outlet end of the crystallizer. The first and second coils have different inner diameters, and the first coil has a higher turn density than the second coil. The inner diameter of the first coil is 300-320 mm, the number of turns is 80-100, and the turn density is 267-330 turns / m. The inner diameter of the second coil is 350-380 mm, the number of turns is 60-80, and the turn density is 171-229 turns / m. A direct current in the same direction is passed through the first and second coils, and the intensity of the direct current is 100-500 A.

[0008] Furthermore, the nano-chromium precipitates in the ingot are distributed in a chain-like pattern along the solidification direction, with an average particle size of 5~20nm and a center-to-center distance of 50~150nm between adjacent nano-chromium precipitates; the ingot contains columnar crystal structure extending along the solidification direction, with a volume fraction of <111> oriented grains ≥90% in the columnar crystal structure, and a grain boundary with an orientation difference angle of less than 5° accounting for >85%.

[0009] Further, in step S4 above, the pulsed current annealing is carried out in a mixed gas, which includes nitrogen and argon, with a volume ratio of nitrogen to argon of 1~3:1; the rise time of the pulsed current in the pulsed current annealing is <0.1ms; and / or, the cold-rolled alloy is clamped by a pair of parallel copper electrodes, the surface of which contains a silver plating layer, the clamping pressure is 0.8~1.2MPa, and the contact resistance between the cold-rolled alloy and the copper electrodes is <10μΩ·cm. 2 .

[0010] Furthermore, the nano-chromium precipitates in the copper-chromium alloy for transformers are distributed in a chain-like dispersion along the solidification direction. The average grain size of the copper-chromium alloy for transformers is <10μm, the average particle size of the nano-chromium precipitates in the copper-chromium alloy for transformers is 8~15nm, the conductivity of the copper-chromium alloy for transformers is ≥85%IACS, and the tensile strength of the copper-chromium alloy for transformers is ≥300MPa.

[0011] Furthermore, in step S1 above, the copper-chromium alloy comprises 0.3-0.5% chromium by mass fraction, with the balance being copper; the raw materials include electrolytic copper and metallic chromium particles; and / or, the oxygen content during the batching process is <10ppm.

[0012] Furthermore, the smelting process includes a first smelting and a second smelting performed sequentially; the temperature of the second smelting is higher than that of the first smelting; the power of the first smelting is 15~20kW, the temperature of the first smelting is 1100~1150℃, and the holding time of the first smelting is 10~15min; the power of the second smelting is 25~30kW, the temperature of the second smelting is 1250~1300℃, and the holding time of the second smelting is 20~25min; electromagnetic stirring is performed during the second smelting process, the frequency of the electromagnetic stirring is 50~60Hz, and the electromagnetic stirring time is 5~8min.

[0013] Furthermore, hot rolling is carried out in argon gas with a dew point below -60°C, the hot rolling temperature is 700~800°C, and the total deformation of hot rolling is 70~80%.

[0014] Furthermore, the cold rolling is a multi-pass cold rolling process, with each pass having a reduction of 10-15%, and the total deformation of the cold rolling is 85-90%.

[0015] According to another aspect of the present invention, a copper-chromium alloy for transformers is provided, which is prepared by the preparation method described above.

[0016] By applying the technical solution of this invention, this application, through the introduction of an axial gradient magnetic field during the solidification stage, achieves for the first time the transformation of precipitated phases in copper-chromium alloys from random distribution to directional chain arrangement. Simultaneously, through millisecond-level pulsed current annealing, it activates a highly efficient precipitation strengthening mechanism for the first time without sacrificing texture. Through the synergistic effect of these two processes, the copper-chromium alloy material for transformers maintains high conductivity while also possessing high strength and low high-frequency eddy current losses. Specifically, in step S2, by applying a gradient magnetic field along the solidification direction to the melt and controlling the magnetic field gradient within the aforementioned range, the Cr in the melt can be... 3+Ions, carrying a positive charge, are subjected to the Lorentz force. Their axial component drives ion migration along the solidification direction (Z-axis), gradually accumulating at the growing solid-liquid interface front. When the local chromium concentration exceeds the solid solubility limit, nucleation and precipitation preferentially occur at the columnar grain boundaries, ultimately forming a directionally dispersed nanoscale chromium precipitate. This avoids the formation of coarse second phases caused by uneven chromium precipitation in traditional processes, effectively reducing electron scattering centers and further enhancing the strength of the copper-chromium alloy while maintaining high conductivity. In step S3, hot rolling effectively breaks and partially removes the oxide scale on the ingot surface, significantly refining the internal dendritic network while preserving the preferred orientation of the columnar crystals. Cold rolling introduces high-density dislocations and further strengthens the texture orientation, ensuring that the grains maintain a strong texture along the rolling direction within the rolling plane, with small grain boundary orientation differences. This provides a low-impedance, low-torsion conduction channel for high-frequency current, significantly suppressing the disordered diffusion of eddy currents under high-frequency alternating magnetic fields. In step S4, controlling the current density and duration of the pulsed current annealing within the aforementioned range allows the Joule heat generated during cold rolling of the alloy to locally raise the material to the critical temperature range for precipitate formation in a very short time, without inducing grain boundary migration or grain growth. Under this transient thermo-electric coupling field, supersaturated chromium atoms in the matrix rapidly segregate, forming a large number of Guinier-Preston (GP) regions, which further evolve into coherent or semi-coherent nano-chromium precipitates. Simultaneously, high-density dislocations undergo selective rearrangement and annihilation, significantly reducing residual stress while retaining fine precipitates, jointly providing stable precipitation strengthening and dislocation strengthening effects. This results in excellent mechanical stability of the copper-chromium alloy for transformers under high-temperature conditions. This annealing process does not introduce any external heat source or involve prolonged heat holding, thus completely avoiding the strength loss caused by abnormal grain growth in traditional hot annealing. In summary, the copper-chromium alloy material for transformers obtained by the preparation method of this application can balance high conductivity, high strength and low high-frequency eddy current loss, thus making it suitable for all dry-type transformer winding manufacturing scenarios that are sensitive to high-frequency loss and have strict requirements for thermal stability, especially suitable for high-end power equipment fields such as urban smart grids, rail transit traction power supply systems and high-density data centers. Attached Figure Description

[0017] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0018] Figure 1 SEM images of the copper-chromium alloy for transformers in Embodiment 1 of this application are shown;

[0019] Figure 2SEM images of the copper-chromium alloy for transformers in Comparative Example 1 of this application are shown. Detailed Implementation

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0021] As analyzed in the background section of this application, the prior art has the problem that the copper-chromium alloy used for dry transformer windings is difficult to balance with high strength, high conductivity and low high-frequency eddy current loss due to the uncontrollable microstructure during the preparation process. In order to solve the above problems, this application provides a copper-chromium alloy for transformers and its preparation method.

[0022] In a typical embodiment of this application, a method for preparing a copper-chromium alloy for transformers is provided. The method includes: step S1, mixing raw materials corresponding to the copper-chromium alloy and then melting them to obtain a melt; step S2, applying a gradient magnetic field along the solidification direction to the melt for directional solidification to obtain an ingot; step S3, sequentially hot-rolling and cold-rolling the ingot to obtain a cold-rolled alloy; and step S4, performing pulsed current annealing on the cold-rolled alloy to obtain the copper-chromium alloy for transformers. The gradient magnetic field has a magnetic field gradient of 0.1~0.5 T / m; the pulsed current annealing current density is 500~2000 A / cm². 2 The duration of pulsed current annealing is 1~10ms.

[0023] This application, by introducing an axial gradient magnetic field during the solidification stage, achieves for the first time the transformation of precipitated phases in copper-chromium alloys from random distribution to directional chain arrangement. Simultaneously, through millisecond-level pulsed current annealing, it activates a highly efficient precipitation strengthening mechanism for the first time without sacrificing texture. Through the synergistic effect of these two methods, the copper-chromium alloy material for transformers maintains high conductivity while also possessing high strength and low high-frequency eddy current losses. Specifically, in step S2, by applying a gradient magnetic field along the solidification direction to the melt and controlling the magnetic field gradient within the aforementioned range, the Cr in the melt can be... 3+Ions, carrying a positive charge, are subjected to the Lorentz force. Their axial component drives ion migration along the solidification direction (Z-axis), gradually accumulating at the growing solid-liquid interface front. When the local chromium concentration exceeds the solid solubility limit, nucleation and precipitation preferentially occur at the columnar grain boundaries, ultimately forming a directionally dispersed nanoscale chromium precipitate. This avoids the formation of coarse second phases caused by uneven chromium precipitation in traditional processes, effectively reducing electron scattering centers and further enhancing the strength of the copper-chromium alloy while maintaining high conductivity. In step S3, hot rolling effectively breaks and partially removes the oxide scale on the ingot surface, significantly refining the internal dendritic network while preserving the preferred orientation of the columnar crystals. Cold rolling introduces high-density dislocations and further strengthens the texture orientation, ensuring that the grains maintain a strong texture along the rolling direction within the rolling plane, with small grain boundary orientation differences. This provides a low-impedance, low-torsion conduction channel for high-frequency current, significantly suppressing the disordered diffusion of eddy currents under high-frequency alternating magnetic fields. In step S4, controlling the current density and duration of the pulsed current annealing within the aforementioned range allows the Joule heat generated during cold rolling of the alloy to locally raise the material to the critical temperature range for precipitate formation in a very short time, without inducing grain boundary migration or grain growth. Under this transient thermo-electric coupling field, supersaturated chromium atoms in the matrix rapidly segregate, forming a large number of Guinier-Preston (GP) regions, which further evolve into coherent or semi-coherent nano-chromium precipitates. Simultaneously, high-density dislocations undergo selective rearrangement and annihilation, significantly reducing residual stress while retaining fine precipitates, jointly providing stable precipitation strengthening and dislocation strengthening effects. This results in excellent mechanical stability of the copper-chromium alloy for transformers under high-temperature conditions. This annealing process does not introduce any external heat source or involve prolonged heat holding, thus completely avoiding the strength loss caused by abnormal grain growth in traditional hot annealing. In summary, the copper-chromium alloy material for transformers obtained by the preparation method of this application can balance high conductivity, high strength and low high-frequency eddy current loss, thus making it suitable for all dry-type transformer winding manufacturing scenarios that are sensitive to high-frequency loss and have strict requirements for thermal stability, especially suitable for high-end power equipment fields such as urban smart grids, rail transit traction power supply systems and high-density data centers.

[0024] Furthermore, the magnetic field gradient can be 0.1 T / m, 0.2 T / m, 0.3 T / m, 0.4 T / m, or 0.5 T / m, etc. The current density for pulsed current annealing can be 500 A / cm². 2 600A / cm 2 700A / cm 2 800A / cm 2 900A / cm 2 1000A / cm 2 1200A / cm2 1500A / cm 2 1700A / cm 2 1850A / cm 2 Or 2000A / cm 2 The duration of pulsed current annealing can be 1ms, 2ms, 3ms, 4ms, 5ms, 6ms, 7ms, 8ms, 9ms, or 10ms.

[0025] In one embodiment of this application, in step S2, during the process of pouring the melt into the crystallizer, an asymmetric Helmholtz coil system is set outside the crystallizer to apply a gradient magnetic field that decreases along the solidification direction to the melt; the gradient magnetic field is applied simultaneously with the pouring of the melt, and the asymmetric Helmholtz coil system is shut off within ≤5 seconds after the melt has completely solidified; the directional solidification time is 15~30 minutes; the asymmetric Helmholtz coil system includes a first coil and a second coil arranged coaxially; wherein, the first coil is located at the inlet end of the crystallizer, and the second coil is located at the crystallizer... The outlet end; the inner diameters of the first coil and the second coil are different, and the turns density of the first coil is greater than that of the second coil; the inner diameter of the first coil is 300~320mm, the number of turns of the first coil is 80~100 turns, and the turns density of the first coil is 267~330 turns / m; the inner diameter of the second coil is 350~380mm, the number of turns of the second coil is 60~80 turns, and the turns density of the second coil is 171~229 turns / m; a direct current in the same direction is passed through the first coil and the second coil, and the intensity of the direct current is 100~500A.

[0026] By setting the aforementioned asymmetric Helmholtz coil system outside the crystallizer, controlling the inner diameter, number of turns, and turn density of the first and second coils within the aforementioned ranges, and simultaneously applying a direct current in the same direction to both coils with the current intensity precisely controlled according to the required magnetic field gradient, it helps to form a monotonically decreasing magnetic field gradient in the solidification direction (Z-axis direction), promoting the growth of Cr in the melt. 3+ Under the influence of the Lorentz force, the axial component of the ions drives their directional migration along the Z-axis. The preferred coil includes, but is not limited to, a circular coil. Because the magnetic field gradient decreases from top to bottom, Cr... 3+ As ions migrate downwards, they gradually accumulate at the front of the growing solid-liquid interface. When the local chromium concentration exceeds the solid solubility limit, nucleation and precipitation preferentially occur at the columnar crystal boundaries, eventually forming a nanoscale chromium precipitate phase that is chain-like and diffusely distributed along the Z-axis.

[0027] The application of the gradient magnetic field is limited to the entire process from when the melt begins to contact the bottom surface of the crystallizer until it is completely solidified. The opening time of the magnetic field is triggered synchronously with the pouring action. Preferably, the closing time of the magnetic field lags behind the solidification endpoint by ≤3 seconds, which helps the entire solid-liquid interface to be under the action of the gradient magnetic field.

[0028] In one embodiment of this application, the nano-chromium precipitates in the ingot are distributed in a chain-like pattern along the solidification direction. The average particle size of the nano-chromium precipitates in the ingot is 5~20nm, and the center-to-center distance between adjacent nano-chromium precipitates is 50~150nm. The ingot contains columnar crystal structure extending along the solidification direction. The volume fraction of <111> oriented grains in the columnar crystal structure of the ingot is ≥90%, preferably 90~98%, and the proportion of grain boundaries with an orientation difference angle of less than 5° is >85%, preferably 86~95%.

[0029] The optimal distribution, average grain size, and center-to-center spacing of the nano-chromium precipitates within the aforementioned ranges contribute to a high degree of spatial periodicity and axial order, thereby further reducing electron scattering centers and ultimately enhancing the strength of the copper-chromium alloy while maintaining high conductivity. The grain size of the nano-chromium precipitates in the ingot is concentrated in the range of 8 to 18 nm, with a standard deviation of less than 3 nm. The ingot obtained through directional solidification exhibits a columnar crystal structure extending along the solidification direction, with grain orientation concentrated in… <111> The proportion of grain boundaries with orientation difference angles less than 5° reaches the above range, which helps to provide a good structural basis for the straightening of subsequent current paths.

[0030] In one embodiment of this application, in step S4, pulsed current annealing is performed in a mixed gas, which includes nitrogen and argon, with a volume ratio of nitrogen to argon of 1~3:1; the rise time of the pulsed current in the pulsed current annealing is <0.1ms; and / or, the cold-rolled alloy is clamped by a pair of parallel copper electrodes, the surface of which contains a silver plating layer, the clamping pressure is 0.8~1.2MPa, and the contact resistance between the cold-rolled alloy and the copper electrodes is <10μΩ·cm. 2 .

[0031] The preferred pulse power supply uses a square-wave pulsed current source constructed from IGBT modules. The cold-rolled alloy is placed in the chamber, pre-evacuated to 10 Pa, and then filled with the aforementioned mixed gas to atmospheric pressure before pulsed current annealing. This helps reduce surface oxidation of the cold-rolled alloy under transient high temperatures, thus maintaining the cleanliness and conductivity of the material surface. After a single pulse application, the instantaneous temperature rise peak of the copper-chromium alloy (strip) surface is approximately 510~530℃, while the depth of the heat-affected zone is limited to within 50μm of the copper-chromium alloy surface layer. The overall thermal diffusion time is <10ms, far lower than the critical holding time required for grain boundary migration (typically greater than 100ms). The rise time of the pulsed current in pulsed current annealing is the rise time from 10% amplitude to 90% amplitude. Controlling the rise time within this range helps Joule heat concentrate within the alloy in a very short time, thereby reducing grain boundary coarsening and texture destruction caused by surface overheating, and thus improving the nucleation density and uniformity of precipitated phases. By clamping the cold-rolled alloy with a pair of parallel copper electrodes and plating a silver layer on the surface of the copper electrodes, and controlling the clamping pressure within the aforementioned range, the contact resistance between the cold-rolled alloy and the copper electrodes reaches the aforementioned range. This enables efficient and uniform transmission of electrical energy to the interior of the alloy strip, reducing local overheating and energy loss. Thus, while enhancing the precipitation strengthening effect, the cold-rolled texture is fully preserved. Consequently, the final product, copper-chromium alloy for transformers, maintains high conductivity and high strength while further reducing high-frequency eddy current losses, achieving synergistic optimization of the three properties.

[0032] Under this transient thermo-electric coupling field, supersaturated chromium atoms in the copper matrix rapidly segregate at high-density dislocation lines and grain boundaries, forming a large number of Guinier-Preston regions (GP regions). The size of the GP regions is approximately 2-4 nm, and the number density is ≥1×10⁻⁶. 23 m -3 With continuous pulse energy input, some GP regions further evolved into coherent or semi-coherent nano-chromium precipitates, with a lattice mismatch of <2% with the copper matrix. Simultaneously, the high-density dislocations introduced by cold rolling underwent dynamic recovery, and the dislocation tangles decoupled into a low-energy dislocation network. The residual stress decreased from 280 MPa in the cold-rolled state to 65 MPa after annealing. The average grain size of the copper-chromium alloy for transformers decreased to <10 μm, with no significant grain boundary migration. Furthermore, the texture intensity only slightly decreased to 7.6 times the random orientation intensity after annealing, demonstrating that millisecond-level pulse annealing effectively activated precipitation while completely preserving the cold-rolled texture.

[0033] Optionally, the current control system of the asymmetric Helmholtz coil is linked with the melt temperature monitoring system. Real-time feedback of the solidification front position via thermocouples dynamically adjusts the coil current to maintain a constant magnetic field gradient. The pulsed current annealing device is equipped with a closed-loop feedback control system that automatically adjusts pulse parameters based on online monitoring results of the copper-chromium alloy's resistivity, helping to improve the consistency of annealing results. Preferably, the cold-rolled alloy is treated with an ultrasonic cleaning bath before pulsed current annealing. The cleaning medium is a mixture of deionized water and isopropanol with a volume ratio of 3~5:1. The ultrasonic frequency is 35~45kHz, and the cleaning time is 1~3 minutes. After cleaning, the alloy is rapidly dried with hot air at 110~130℃ and placed into the annealing chamber within 30 seconds to reduce contamination of the alloy surface.

[0034] In one embodiment of this application, the nano-chromium precipitates in the copper-chromium alloy for transformers are distributed in a chain-like dispersion along the solidification direction. The average grain size of the copper-chromium alloy for transformers is <10μm, preferably 5~8μm. The average particle size of the nano-chromium precipitates in the copper-chromium alloy for transformers is 8~15nm. The conductivity of the copper-chromium alloy for transformers is ≥85%IACS, preferably 86~90μm. The tensile strength of the copper-chromium alloy for transformers is ≥300MPa, preferably 310~350μm.

[0035] In copper-chromium alloys for transformers, nano-chromium precipitates are distributed in a chain-like pattern along the solidification direction. Controlling the average grain size and average particle size of the nano-chromium precipitates in the copper-chromium alloys for transformers obtained after pulsed current annealing helps achieve the desired conductivity and tensile strength while reducing high-frequency eddy current losses.

[0036] In one embodiment of this application, in step S1, the copper-chromium alloy comprises 0.3-0.5% chromium by mass fraction, with the balance being copper; the raw materials include electrolytic copper and metallic chromium particles; and / or, the oxygen content during the batching process is <10ppm.

[0037] The copper-chromium alloy raw materials are formulated according to the mass percentage of the elements, with the mass fraction of chromium controlled within the aforementioned range. This helps to balance the precipitation strengthening effect with the conductivity of the matrix. Preferably, no other alloying elements are added to the copper-chromium alloy, thereby reducing the introduction of additional electron scattering centers or complex precipitates. The raw materials are electrolytic copper with a purity ≥99.99% and metallic chromium particles with a purity ≥99.95%. Both are vacuum-dried before mixing to remove surface-adsorbed moisture. During the mixing process, the oxygen content is controlled within the aforementioned range. Weighing and mixing are carried out under the protection of high-purity argon gas. The raw materials are temporarily stored in a sealed container that undergoes three cycles of pre-vacuuming and argon purging, thereby reducing the introduction of oxide inclusions and improving the purity and microstructure uniformity of the melt.

[0038] In one embodiment of this application, the melting process includes a first melting and a second melting performed sequentially; the temperature of the second melting is higher than that of the first melting; the power of the first melting is 15~20kW, the temperature of the first melting is 1100~1150℃, and the holding time of the first melting is 10~15min; the power of the second melting is 25~30kW, the temperature of the second melting is 1250~1300℃, and the holding time of the second melting is 20~25min; electromagnetic stirring is performed during the second melting process, the frequency of the electromagnetic stirring is 50~60Hz, and the electromagnetic stirring time is 5~8min.

[0039] The raw materials are loaded into a graphite crucible and placed in a vacuum induction melting furnace for melting. Preferably, the ultimate vacuum degree of the vacuum induction melting furnace is <1×10⁻⁶. -2 Pa, after evacuating to 5 × 10 Pa -3 After Pa, high-purity argon gas is introduced to a slightly positive pressure (101~110 kPa) to reduce carbon contamination of the melt by the graphite crucible. The melting process employs a two-stage heating method. Controlling the power, temperature, and holding time of the first melting stage within the aforementioned range helps in the initial melting and degassing of the raw materials. Controlling the power, temperature, and holding time of the second melting stage within the aforementioned range, during which electromagnetic stirring is applied, helps promote homogenization of elemental composition, ultimately obtaining a homogeneous melt with a compositional deviation of less than ±0.02 wt%.

[0040] In one embodiment of this application, hot rolling is carried out in argon gas with a dew point below -60°C, the hot rolling temperature is 700~800°C, and the total deformation of hot rolling is 70~80%.

[0041] Preferably, the ingot is preheated to the aforementioned temperature in a protective atmosphere bell-type furnace before being transferred to a two-roll hot rolling mill for hot rolling. The entire hot rolling process is carried out in a high-purity argon atmosphere with a dew point within the aforementioned range, involving multiple passes. This helps to fully break down and partially remove the oxide scale on the ingot surface, thereby further refining the internal dendrite network while preserving the preferred orientation of the columnar crystals. Preferably, hot rolling is performed in an argon atmosphere with a dew point of -85°C to -60°C.

[0042] In one embodiment of this application, the cold rolling is a multi-pass cold rolling, with each pass having a reduction of 10-15% and a total deformation of 85-90%.

[0043] After hot rolling, the alloy undergoes a combined mechanical grinding and pickling treatment to remove the surface oxide layer. The pickling solution is a mixture of dilute sulfuric acid (10-15%) and hydrogen peroxide with a volume ratio of 2-4:1, and the pickling time is 2-5 minutes. It is then rinsed with deionized water and dried with hot air. Subsequently, it is cold rolled at room temperature (20-30°C) on a four-roll cold rolling mill. Preferably, the number of cold rolling passes is 6-9, with the reduction per pass controlled within the above range. Tension rolls are installed between passes, applying a constant tension of 25-35% of the material's yield strength, which helps improve the continuity and stability of the texture during rolling. At this point, the dislocation density inside the alloy increases to 5-8 × 10⁻⁶. 14 m -2 Furthermore, the grains exhibit obvious characteristics within the rolling plane. <110> The RD (roll-direction) fiber texture, with a texture strength exceeding 8.2 times that of random strength, provides a low-bending, low-scattering conduction path for high-frequency current. Gradient magnetic field-assisted directional solidification and millisecond-level pulsed current annealing are connected by an automated material transfer system. The entire transfer process is conducted under an inert atmosphere, minimizing surface contamination. All heat treatment and annealing steps are performed under an inert atmosphere. Material transfer is achieved through sealed pipelines connecting each process unit, with argon continuously supplied to maintain positive pressure, ensuring the material is completely protected from environmental influences.

[0044] In another typical embodiment of this application, a copper-chromium alloy for transformers is provided, which is prepared by the above-described preparation method.

[0045] The copper-chromium alloy material for transformers obtained by the preparation method of this application can balance high conductivity, high strength and low high-frequency eddy current loss, thus making it suitable for all dry-type transformer winding manufacturing scenarios that are sensitive to high-frequency loss and have strict requirements for thermal stability.

[0046] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0047] Example 1

[0048] The copper-chromium alloy, by mass fraction, contains 0.4% chromium and 99.6% copper. Electrolytic copper with a purity ≥99.99% and metallic chromium particles with a purity ≥99.95% are vacuum-dried and then batched under an argon atmosphere with an oxygen content <10 ppm. The batched raw materials are placed in a graphite crucible and melted in a vacuum induction melting furnace to obtain a melt with an elemental composition deviation <±0.02 wt%. Before melting, a vacuum of 5 × 10⁻⁶ ppm is applied. -3After Pa, argon gas is introduced to a slightly positive pressure (105 kPa). The melting process is as follows: first melting is carried out by heating to 1100℃ with a power of 15 kW and holding for 10 min; second melting is carried out by heating to 1250℃ with a power of 25 kW and holding for 20 min. During the second melting process, electromagnetic stirring is applied at a frequency of 50 Hz for a duration of 5 min.

[0049] The molten metal is poured into a water-cooled copper crystallizer. An asymmetric Helmholtz coil system is installed outside the crystallizer to apply a gradient magnetic field along the solidification direction to the molten metal for directional solidification, resulting in an ingot. Nano-chromium precipitates in the ingot are distributed in a chain-like pattern along the solidification direction. The ingot contains columnar crystal structures extending along the solidification direction. The asymmetric Helmholtz coil system consists of two sets of coaxially arranged circular coils: a first coil and a second coil. The first coil, located near the melt inlet, has an inner diameter of 300 mm, 80 turns, and a turn density of 267 turns / m; the second coil, located at the outlet, has an inner diameter of 350 mm, 60 turns, and a turn density of 171 turns / m. A direct current in the same direction is passed through both coils, with the intensity of the current varied within the range of 100–500 A, creating a decreasing magnetic field gradient of 0.3 T / m along the Z-axis (solidification direction). The gradient magnetic field is applied during the entire process from when the melt begins to contact the bottom surface of the crystallizer until it is completely solidified. The magnetic field is turned on at the same time as the pouring, and the magnetic field is turned off ≤3s after the solidification endpoint. The directional solidification time is 20min.

[0050] The ingot is preheated to 750°C in a protective atmosphere bell-type furnace and then transferred to a two-roll hot rolling mill for hot rolling to obtain the hot-rolled alloy. The entire hot rolling process is carried out in an argon atmosphere with the dew point controlled at -65°C. The hot rolling adopts a five-pass continuous rolling process, with a reduction of 12% in the first pass, and subsequent passes having reductions of 13%, 15%, 18%, and 20% respectively, for a total deformation of 78%.

[0051] After hot rolling, the alloy undergoes mechanical grinding and pickling. The pickling solution is a mixture of dilute sulfuric acid (10% by mass) and hydrogen peroxide in a 3:1 volume ratio, and the pickling time is 3 minutes. Following this, the alloy is rinsed with deionized water and dried with hot air. The treated alloy is then cold-rolled at room temperature (25°C) on a four-roll cold rolling mill. The cold rolling process involves eight passes, with reductions of 10%, 10%, 11%, 11%, 12%, 12%, 13%, and 13% respectively for each pass. Tension rolls are installed between passes, applying a constant tension of 30% of the material's yield strength. The alloy is cold-rolled to a thickness of 0.35 mm (strip), with a total cold-rolling deformation of 87%.

[0052] After cold rolling, the alloy is ultrasonically cleaned with a mixture of deionized water and isopropanol. Immediately after cleaning, it is dried and placed in the annealing chamber. The annealing chamber is pre-evacuated to 10 Pa and then filled with a mixture of nitrogen and argon (volume ratio 1:1) to atmospheric pressure. The cold-rolled alloy is held by a pair of parallel copper electrodes, the surfaces of which are silver-plated. The clamping pressure is 0.8 MPa, ensuring that the contact resistance between the cold-rolled alloy and the electrodes is <10 μΩ·cm. 2 The pulse power supply uses a square wave current source constructed from IGBT modules, with an output current density set to 1500 A / cm². 2 The pulse duration is 5ms and the pulse rise time is 0.08ms. The cold-rolled alloy is subjected to pulse current annealing to obtain copper-chromium alloy for transformers.

[0053] Example 2

[0054] The difference from Example 1 is that, by mass fraction, the copper-chromium alloy contains 0.3% chromium and 99.7% copper. Electrolytic copper with a purity ≥99.99% and metallic chromium particles with a purity ≥99.95% are vacuum-dried and then batched under an argon atmosphere with an oxygen content <10ppm. The batched raw materials are placed in a graphite crucible and melted in a vacuum induction melting furnace to obtain a melt with an elemental composition deviation <±0.02wt%. Before melting, a vacuum of 5×10⁻⁶ is applied. -3 After Pa, argon gas is introduced to a slightly positive pressure (105 kPa). The melting process is as follows: first melting is carried out by heating to 1120℃ with a power of 18 kW and holding for 12 min; second melting is carried out by heating to 1280℃ with a power of 28 kW and holding for 22 min. During the second melting process, electromagnetic stirring is applied at a frequency of 55 Hz for a duration of 6 min.

[0055] The molten metal is poured into a water-cooled copper crystallizer. An asymmetric Helmholtz coil system is installed outside the crystallizer to apply a gradient magnetic field along the solidification direction to the molten metal for directional solidification, resulting in an ingot. Nano-chromium precipitates in the ingot are distributed in a chain-like pattern along the solidification direction. The ingot contains columnar crystal structures extending along the solidification direction. The asymmetric Helmholtz coil system consists of two sets of coaxially arranged circular coils: a first coil and a second coil. The first coil, located near the melt inlet, has an inner diameter of 310 mm, 90 turns, and a turn density of 300 turns / m. The second coil, located at the outlet, has an inner diameter of 365 mm, 70 turns, and a turn density of 200 turns / m. A direct current in the same direction is passed through both coils, with the intensity of the current varied within the range of 100–500 A, creating a decreasing magnetic field gradient of 0.1 T / m along the Z-axis (solidification direction). The gradient magnetic field is applied during the entire process from when the melt begins to contact the bottom surface of the crystallizer until it is completely solidified. The magnetic field is turned on simultaneously with the pouring, and the magnetic field is turned off ≤4s after the solidification endpoint. The directional solidification time is 30min.

[0056] The ingot is preheated to 700°C in a protective atmosphere bell-type furnace and then transferred to a two-roll hot rolling mill for hot rolling to obtain the hot-rolled alloy. The entire hot rolling process is carried out in an argon atmosphere with the dew point controlled at -70°C. The hot rolling adopts a five-pass continuous rolling process, with a reduction of 12% in the first pass, and subsequent passes having reductions of 14%, 15%, 16%, and 18% respectively, for a total deformation of 70%.

[0057] After hot rolling, the alloy undergoes mechanical grinding and pickling. The pickling solution is a mixture of dilute sulfuric acid (12% by mass) and hydrogen peroxide in a 2:1 volume ratio, and the pickling time is 5 minutes. Following this, the alloy is rinsed with deionized water and dried with hot air. The treated alloy is then cold-rolled at room temperature (25°C) on a four-roll cold rolling mill. The cold rolling process involves eight passes, with reductions of 10%, 10%, 10%, 11%, 11%, 12%, 13%, and 14% respectively for each pass. Tension rolls are installed between passes, applying a constant tension of 30% of the material's yield strength. The alloy is cold-rolled to a thickness of 0.38 mm (strip), with a total cold-rolling deformation of 85%.

[0058] After cold rolling, the alloy is ultrasonically cleaned with a mixture of deionized water and isopropanol. Immediately after cleaning, it is dried and placed in the annealing chamber. The annealing chamber is pre-evacuated to 10 Pa and then filled with a mixture of nitrogen and argon (volume ratio 2:1) to atmospheric pressure. The cold-rolled alloy is held by a pair of parallel copper electrodes, the surfaces of which are silver-plated. The clamping pressure is 1.0 MPa, ensuring that the contact resistance between the cold-rolled alloy and the electrodes is <10 μΩ·cm. 2The pulse power supply uses a square wave current source constructed from IGBT modules, with the output current density set to 500 A / cm². 2 The pulse duration is 10ms and the pulse rise time is 0.09ms. The cold-rolled alloy is subjected to pulse current annealing to obtain copper-chromium alloy for transformers.

[0059] Example 3

[0060] The difference from Example 1 is that, by mass fraction, the copper-chromium alloy contains 0.5% chromium and 99.5% copper. Electrolytic copper with a purity ≥99.99% and metallic chromium particles with a purity ≥99.95% are vacuum-dried and then batched under an argon atmosphere with an oxygen content <10ppm. The batched raw materials are placed in a graphite crucible and melted in a vacuum induction melting furnace to obtain a melt with an elemental composition deviation <±0.02wt%. Before melting, a vacuum of 5×10⁻⁶ is applied. -3 After Pa, argon gas is introduced to a slightly positive pressure (105 kPa). The melting process is as follows: first melting is carried out by heating to 1150℃ with a power of 20 kW and holding for 15 min; second melting is carried out by heating to 1300℃ with a power of 30 kW and holding for 25 min. During the second melting process, electromagnetic stirring is applied at a frequency of 60 Hz for a duration of 8 min.

[0061] The molten metal is poured into a water-cooled copper crystallizer. An asymmetric Helmholtz coil system is installed outside the crystallizer to apply a gradient magnetic field along the solidification direction to the molten metal for directional solidification, resulting in an ingot. Nano-chromium precipitates in the ingot are distributed in a chain-like pattern along the solidification direction. The ingot contains columnar crystal structures extending along the solidification direction. The asymmetric Helmholtz coil system consists of two sets of coaxially arranged circular coils: a first coil and a second coil. The first coil, located near the molten metal inlet, has an inner diameter of 320 mm, 100 turns, and a turn density of 330 turns / m. The second coil, located at the outlet, has an inner diameter of 380 mm, 80 turns, and a turn density of 229 turns / m. A direct current in the same direction is passed through both coils, with the intensity of the current varied within the range of 100–500 A, creating a decreasing magnetic field gradient of 0.5 T / m along the Z-axis (solidification direction). The gradient magnetic field is applied during the entire process from when the melt begins to contact the bottom surface of the crystallizer until it is completely solidified. The magnetic field is turned on at the same time as the pouring, and the magnetic field is turned off ≤5s after the solidification endpoint. The directional solidification time is 15min.

[0062] The ingot is preheated to 800°C in a protective atmosphere bell-type furnace and then transferred to a two-roll hot rolling mill for hot rolling to obtain the hot-rolled alloy. The entire hot rolling process is carried out in an argon atmosphere with the dew point controlled at -75°C. The hot rolling adopts a five-pass continuous rolling process, with a reduction of 12% in the first pass, and subsequent passes having reductions of 14%, 16%, 18%, and 20% respectively, for a total deformation of 80%.

[0063] After hot rolling, the alloy undergoes mechanical grinding and pickling. The pickling solution is a mixture of dilute sulfuric acid (15% by mass) and hydrogen peroxide in a 4:1 volume ratio, and the pickling time is 2 minutes. It is then rinsed with deionized water and dried with hot air. The treated alloy is then cold-rolled at room temperature (25°C) on a four-roll cold rolling mill. The cold rolling process involves eight passes, with reductions of 10%, 10%, 10%, 11%, 11%, 12%, 13%, and 15% respectively for each pass. Tension rolls are installed between passes, applying a constant tension of 30% of the material's yield strength. The alloy is cold-rolled to a thickness of 0.32 mm (strip), with a total cold-rolling deformation of 90%.

[0064] After cold rolling, the alloy is ultrasonically cleaned with a mixture of deionized water and isopropanol. Immediately after cleaning, it is dried and placed in the annealing chamber. The annealing chamber is pre-evacuated to 10 Pa and then filled with a mixture of nitrogen and argon (volume ratio 3:1) to atmospheric pressure. The cold-rolled alloy is held by a pair of parallel copper electrodes, the surfaces of which are silver-plated. The clamping pressure is 1.2 MPa, ensuring that the contact resistance between the cold-rolled alloy and the electrodes is <10 μΩ·cm. 2 The pulse power supply uses a square wave current source constructed from IGBT modules, with an output current density set to 2000 A / cm². 2 The pulse duration is 1ms and the pulse rise time is 0.05ms. The cold-rolled alloy is subjected to pulse current annealing to obtain copper-chromium alloy for transformers.

[0065] Example 4

[0066] The difference from Example 1 lies in that the asymmetric Helmholtz coil system consists of two sets of coaxially arranged circular coils, namely a first coil and a second coil. The first coil, located near the melt inlet, has an inner diameter of 320 mm, 100 turns, and a turn density of 330 turns / m; the second coil, located at the outlet, has an inner diameter of 380 mm, 80 turns, and a turn density of 229 turns / m. A direct current in the same direction is passed through both the first and second coils, and the intensity of the direct current is varied within the range of 100~500A, creating a decreasing magnetic field gradient of 0.2 T / m along the Z-axis (i.e., the solidification direction), ultimately yielding a copper-chromium alloy for transformers.

[0067] Example 5

[0068] The difference from Example 1 lies in that the asymmetric Helmholtz coil system consists of two sets of coaxially arranged circular coils, namely a first coil and a second coil. The first coil, located near the melt inlet, has an inner diameter of 305 mm, 85 turns, and a turn density of 270 turns / m; the second coil, located at the outlet, has an inner diameter of 370 mm, 65 turns, and a turn density of 180 turns / m. A direct current in the same direction is passed through both the first and second coils, and the intensity of the direct current is varied within the range of 100~500A, creating a decreasing magnetic field gradient of 0.4 T / m along the Z-axis (i.e., the solidification direction), ultimately yielding a copper-chromium alloy for transformers.

[0069] Example 6

[0070] The difference from Example 1 is that the current density of the pulsed current annealing is 1000 A / cm. 2 The pulse current annealing lasted for 3ms, resulting in a copper-chromium alloy for transformers.

[0071] Example 7

[0072] The difference from Example 1 is that the current density of the pulsed current annealing is 1800 A / cm. 2 The pulse current annealing lasted for 8ms, resulting in a copper-chromium alloy for transformers.

[0073] Example 8

[0074] The difference from Example 1 is that the hot rolling temperature is 700°C and the total deformation of the hot rolling is 70%, ultimately yielding a copper-chromium alloy for transformers.

[0075] Example 9

[0076] The difference from Example 1 is that the hot rolling temperature is 850°C and the total deformation of the hot rolling is 85%, ultimately yielding a copper-chromium alloy for transformers.

[0077] Example 10

[0078] The difference from Example 1 is that the cold rolling process uses eight passes, and the total deformation of the cold rolling is 88%, ultimately yielding a copper-chromium alloy for transformers.

[0079] Example 11

[0080] The difference from Example 1 is that the cold rolling process uses eight passes, and the total deformation of the cold rolling is 80%, ultimately yielding a copper-chromium alloy for transformers.

[0081] Comparative Example 1

[0082] The difference from Example 1 is that no magnetic field is applied during the directional solidification process, and the annealing process involves holding at 500°C for 1 hour followed by air cooling, ultimately yielding a copper-chromium alloy for transformers.

[0083] Comparative Example 2

[0084] The difference from Example 1 is that a uniform static magnetic field is applied during the directional solidification process, ultimately resulting in a copper-chromium alloy for transformers.

[0085] Comparative Example 3

[0086] The difference from Example 1 is that the annealing process involves holding at 500°C for 1 hour followed by air cooling, ultimately yielding a copper-chromium alloy for transformers.

[0087] Comparative Example 4

[0088] The difference from Example 1 is that the magnetic field gradient of the gradient magnetic field is 0.05 T / m, and the current density of the pulsed current annealing is 450 A / cm. 2 The pulse current annealing lasted for 15ms, resulting in a copper-chromium alloy for transformers.

[0089] Comparative Example 5

[0090] The difference from Example 1 is that the magnetic field gradient of the gradient magnetic field is 0.6 T / m, and the current density of the pulsed current annealing is 2100 A / cm. 2 The pulse current annealing lasted for 0.5 ms, resulting in a copper-chromium alloy for transformers.

[0091] Test method:

[0092] Conductivity test: A digital eddy current conductivity meter was used for the test at room temperature of 25°C. Five points were measured for each sample and the average value was taken. The unit is %IACS.

[0093] Tensile strength test: The test shall be conducted in accordance with GB / T228.1-2010, wherein a universal testing machine shall be used for tensile testing, the gauge length shall be 50 mm, and the tensile speed shall be 5 mm / min.

[0094] 10kHz eddy current loss test: The test was conducted using an AC impedance analyzer at a magnetic induction intensity of 10kHz and 1T, with the unit being W / kg.

[0095] Grain size testing: The average grain size was determined by SEM and EBSD observations and the cut-off method.

[0096] <110> / / RD texture strength test: The orientation distribution function (ODF) is calculated using the EBSD test, with random texture as the baseline.

[0097] Volume fraction calculation of <111> oriented grains: The proportion of <111> oriented pixels is calculated by EBSD.

[0098] Calculation of small-angle grain boundary ratio: The proportion of grain boundary length with an orientation difference of <5° to the total grain boundary length is calculated by using EBSD.

[0099] The test results are shown in Tables 1 and 2.

[0100] Table 1

[0101]

[0102] Table 2

[0103]

[0104] As shown above, Comparative Example 1 uses traditional solidification and annealing. During solidification, chromium is prone to macroscopic and microscopic segregation, forming coarse chromium phase particles at the micrometer scale, which reduces conductivity. The grain orientation formed by annealing is highly random, resulting in a tortuous and disordered flow path of current inside the conductor. Under the action of a high-frequency alternating magnetic field, eddy currents are forced to close along irregular paths, significantly aggravating additional eddy current losses.

[0105] Comparative Example 2 uses a gradient-free magnetic field, resulting in severe chromium segregation, coarse and random precipitates with weak texture, high high-frequency loss, and low strength.

[0106] Comparative Example 3 showed that conventional hot annealing resulted in grain coarsening, texture degradation, decreased strength, and increased eddy current loss.

[0107] In Comparative Example 4, the magnetic field gradient of the gradient magnetic field is too low, the current density of the pulse current annealing is too low and the duration is too long, resulting in insufficient control of the precipitated phase and failure to meet the standards for conductivity and strength.

[0108] In Comparative Example 5, the magnetic field gradient of the gradient magnetic field is too high, the current density of the pulsed current annealing is too high and the duration is too short, resulting in excessive aggregation of the precipitated phase, decreased conductivity and uneven structure.

[0109] The embodiments of this application achieve a transformation of the precipitated phases in the copper-chromium alloy from random distribution to directional chain arrangement by introducing an axial gradient magnetic field during the solidification stage. Simultaneously, millisecond-level pulsed current annealing activates a highly efficient precipitation strengthening mechanism without sacrificing texture. Through the synergistic effect of these two processes, the copper-chromium alloy material for transformers maintains high conductivity while also possessing high strength and low high-frequency eddy current losses.

[0110] Furthermore, the copper-chromium alloy (strip) used in Example 1 was wound into a low-voltage winding of a dry-type transformer. After continuous operation in a 180°C constant temperature chamber for 1000 hours, the geometric dimensional change rate was measured. The results showed that the axial shrinkage rate was 0.18%, while the shrinkage rate of the control winding wound with pure copper strip of the same specification was 0.30%. This proves that the copper-chromium alloy used in the transformer of this application has excellent thermal dimensional stability under high temperature and long-term service conditions.

[0111] in, Figure 1 This is a SEM image of the copper-chromium alloy used in the transformer in Example 1. Figure 1 As can be seen, the nano-chromium precipitates are uniformly dispersed in a chain-like pattern along the solidification direction, with fine particle size, no coarse particles, and regular columnar crystal orientation.

[0112] Figure 2 This is a SEM image of the copper-chromium alloy used in the transformer in Comparative Example 1. Figure 2 As can be seen, the chromium precipitates are coarse, randomly agglomerated, and exhibit obvious segregation, with uneven grain size and disordered structure.

[0113] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0114] This application, by introducing an axial gradient magnetic field during the solidification stage, achieves for the first time the transformation of precipitated phases in copper-chromium alloys from random distribution to directional chain arrangement. Simultaneously, through millisecond-level pulsed current annealing, it activates a highly efficient precipitation strengthening mechanism for the first time without sacrificing texture. Through the synergistic effect of these two methods, the copper-chromium alloy material for transformers maintains high conductivity while also possessing high strength and low high-frequency eddy current losses. Specifically, in step S2, by applying a gradient magnetic field along the solidification direction to the melt and controlling the magnetic field gradient within the aforementioned range, the Cr in the melt can be... 3+Ions, carrying a positive charge, are subjected to the Lorentz force. Their axial component drives ion migration along the solidification direction (Z-axis), gradually accumulating at the growing solid-liquid interface front. When the local chromium concentration exceeds the solid solubility limit, nucleation and precipitation preferentially occur at the columnar grain boundaries, ultimately forming a directionally dispersed nanoscale chromium precipitate. This avoids the formation of coarse second phases caused by uneven chromium precipitation in traditional processes, effectively reducing electron scattering centers and further enhancing the strength of the copper-chromium alloy while maintaining high conductivity. In step S3, hot rolling effectively breaks and partially removes the oxide scale on the ingot surface, significantly refining the internal dendritic network while preserving the preferred orientation of the columnar crystals. Cold rolling introduces high-density dislocations and further strengthens the texture orientation, ensuring that the grains maintain a strong texture along the rolling direction within the rolling plane, with small grain boundary orientation differences. This provides a low-impedance, low-torsion conduction channel for high-frequency current, significantly suppressing the disordered diffusion of eddy currents under high-frequency alternating magnetic fields. In step S4, controlling the current density and duration of the pulsed current annealing within the aforementioned range allows the Joule heat generated during cold rolling of the alloy to locally raise the material to the critical temperature range for precipitate formation in a very short time, without inducing grain boundary migration or grain growth. Under this transient thermo-electric coupling field, supersaturated chromium atoms in the matrix rapidly segregate, forming a large number of Guinier-Preston (GP) regions, which further evolve into coherent or semi-coherent nano-chromium precipitates. Simultaneously, high-density dislocations undergo selective rearrangement and annihilation, significantly reducing residual stress while retaining fine precipitates, jointly providing stable precipitation strengthening and dislocation strengthening effects. This results in excellent mechanical stability of the copper-chromium alloy for transformers under high-temperature conditions. This annealing process does not introduce any external heat source or involve prolonged heat holding, thus completely avoiding the strength loss caused by abnormal grain growth in traditional hot annealing. In summary, the copper-chromium alloy material for transformers obtained by the preparation method of this application can balance high conductivity, high strength and low high-frequency eddy current loss, thus making it suitable for all dry-type transformer winding manufacturing scenarios that are sensitive to high-frequency loss and have strict requirements for thermal stability, especially suitable for high-end power equipment fields such as urban smart grids, rail transit traction power supply systems and high-density data centers.

[0115] The above are merely embodiments of the present invention and are not intended to limit the invention. Those skilled in the art will recognize that the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a copper-chromium alloy for transformers, characterized in that, The preparation method includes: Step S1: The raw materials corresponding to the copper-chromium alloy are batched and then smelted to obtain a melt; Step S2: Apply a gradient magnetic field along the solidification direction to the melt for directional solidification to obtain an ingot; Step S3: The ingot is subjected to hot rolling and cold rolling in sequence to obtain the cold-rolled alloy; Step S4: The cold-rolled alloy is subjected to pulse current annealing to obtain a copper-chromium alloy for transformers. The magnetic field gradient of the gradient magnetic field is 0.1~0.5T / m; The current density for the pulsed current annealing is 500~2000 A / cm². 2 The duration of the pulsed current annealing is 1~10ms.

2. The preparation method according to claim 1, characterized in that, In step S2, during the process of pouring the melt into the crystallizer, an asymmetric Helmholtz coil system is set outside the crystallizer to apply a gradient magnetic field that decreases along the solidification direction to the melt; the gradient magnetic field is applied simultaneously with the pouring of the melt, and the asymmetric Helmholtz coil system is turned off within ≤5 seconds after the melt has completely solidified; the directional solidification time is 15~30 minutes. The asymmetric Helmholtz coil system includes a first coil and a second coil arranged coaxially; wherein the first coil is located at the inlet end of the crystallizer, and the second coil is located at the outlet end of the crystallizer; the inner diameters of the first coil and the second coil are different, and the turns density of the first coil is greater than that of the second coil; The inner diameter of the first coil is 300~320mm, the number of turns of the first coil is 80~100 turns, and the turn density of the first coil is 267~330 turns / m; the inner diameter of the second coil is 350~380mm, the number of turns of the second coil is 60~80 turns, and the turn density of the second coil is 171~229 turns / m. A direct current in the same direction is passed through the first coil and the second coil, and the intensity of the direct current is 100~500A.

3. The preparation method according to claim 1, characterized in that, The nano-chromium precipitates in the ingot are distributed in a chain-like pattern along the solidification direction. The average particle size of the nano-chromium precipitates in the ingot is 5~20nm, and the center-to-center distance between adjacent nano-chromium precipitates is 50~150nm. The ingot contains columnar crystal structure extending along the solidification direction. The volume fraction of <111> oriented grains in the columnar crystal structure of the ingot is ≥90%, and the proportion of grain boundaries with an orientation difference angle of less than 5° is >85%.

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step S4, the pulsed current annealing is carried out in a mixed gas, which includes nitrogen and argon, with a volume ratio of nitrogen to argon of 1~3:1; the rise time of the pulsed current in the pulsed current annealing is <0.1ms; And / or, the cold-rolled alloy is held by a pair of parallel copper electrodes, the surfaces of which are silver-plated, the clamping pressure is 0.8~1.2 MPa, and the contact resistance between the cold-rolled alloy and the copper electrodes is <10 μΩ·cm. 2 .

5. The preparation method according to claim 4, characterized in that, The nano-chromium precipitates in the copper-chromium alloy for transformers are distributed in a chain-like pattern along the solidification direction. The average grain size of the copper-chromium alloy for transformers is <10μm, the average particle size of the nano-chromium precipitates in the copper-chromium alloy for transformers is 8~15nm, the conductivity of the copper-chromium alloy for transformers is ≥85%IACS, and the tensile strength of the copper-chromium alloy for transformers is ≥300MPa.

6. The preparation method according to any one of claims 1 to 3, characterized in that, In step S1, the copper-chromium alloy comprises 0.3-0.5% chromium by mass fraction, with the balance being copper; the raw materials include electrolytic copper and metallic chromium particles. And / or, the oxygen content during the preparation process is <10ppm.

7. The preparation method according to any one of claims 1 to 3, characterized in that, The smelting process includes a first smelting and a second smelting performed sequentially; the temperature of the second smelting is higher than that of the first smelting; the power of the first smelting is 15~20kW, the temperature of the first smelting is 1100~1150℃, and the holding time of the first smelting is 10~15min; the power of the second smelting is 25~30kW, the temperature of the second smelting is 1250~1300℃, and the holding time of the second smelting is 20~25min; electromagnetic stirring is performed during the second smelting process, the frequency of the electromagnetic stirring is 50~60Hz, and the duration of the electromagnetic stirring is 5~8min.

8. The preparation method according to any one of claims 1 to 3, characterized in that, The hot rolling is carried out in argon gas with a dew point below -60°C, the hot rolling temperature is 700~800°C, and the total deformation of the hot rolling is 70~80%.

9. The preparation method according to any one of claims 1 to 3, characterized in that, The cold rolling is a multi-pass cold rolling, with each pass having a reduction of 10-15%, and the total deformation of the cold rolling is 85-90%.

10. A copper-chromium alloy for transformers, characterized in that, The copper-chromium alloy for the transformer is prepared by the preparation method according to any one of claims 1 to 9.

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