Preparation method of reverse gradient nanocrystalline high-toughness high-conductivity copper material

Abstract

The invention discloses a preparation method of a reverse gradient nanocrystalline high-toughness high-conductivity copper material, and belongs to the technical field of metal material processing. According to the method, through cross section shape design and strain distribution control, the annealed and polished copper material is subjected to multi-pass rolling treatment, and a multi-scale microstructure with gradient nanocrystalline and dislocation density is constructed in the copper material. And a reverse gradient nanostructure from surface coarse crystals to core nanocrystalline / subcrystals is constructed by regulating and controlling the pass rolling reduction and the rolling temperature. Compared with an annealed copper material, the prepared reverse gradient nano-structure copper material has excellent mechanical properties and high conductivity, the yield strength is improved by 50-150%, and the elongation is gt; 20%; the conductivity is gt; 95%. The technology is compatible with an existing rolling production line, has the advantages of being low in cost, high in efficiency, capable of achieving large-scale production and the like, and becomes an irreplaceable key material in occasions with high requirements for current transmission, efficient heat dissipation and strength and plasticity.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, specifically relating to a method for preparing a high-strength, high-toughness, and high-conductivity copper material with reverse gradient nanocrystals. Background Technology

[0002] As an indispensable basic conductive material in modern industry, the synergistic improvement of copper's comprehensive mechanical properties and electrical conductivity has always been a research challenge in related fields. A strength-ductility trade-off effect is common in metallic materials, and this is particularly pronounced in copper. Traditional strengthening methods such as cold deformation, alloying, or grain refinement, while increasing strength, often lead to a significant decrease in electrical conductivity and ductility. This phenomenon stems from the mutual constraint between dislocation movement and the hindering mechanism during material deformation. Specifically, when the strength of a material is improved through methods such as grain refinement, solid solution strengthening, or precipitation, the hindered dislocation movement leads to a significant decrease in plastic deformation capacity; conversely, improving ductility often comes at the cost of sacrificing strength. Especially in the large-scale production of copper wire, in order to achieve the required wire diameter and surface quality, it is often necessary to undergo multiple drawing or rolling processes, resulting in a large cumulative deformation (usually exceeding 90%). Although this can improve strength, it also causes a sharp increase in dislocation density, internal stress concentration and deterioration of toughness. At the same time, the electrical conductivity is significantly reduced (for example, when the cold deformation increases to 40%, the electrical conductivity of industrial pure copper can drop rapidly from 100% IACS (International Standard for Annealed Copper) in the annealed state to 85% IACS or even below), making it difficult to meet the stringent requirements of high-end applications for the comprehensive performance of materials.

[0003] Currently, preparing copper materials with gradient nanostructures is one of the effective methods to simultaneously improve comprehensive mechanical properties. Existing methods for preparing gradient nanocopper mainly include surface mechanical grinding, rolling and extrusion, deposition and coating technologies, 3D printing and additive manufacturing technologies, etc. However, these methods have obvious technical bottlenecks: the effective strengthening layer thickness is usually less than 500 μm, which has limited contribution to the strengthening of thick cross-section components; the processing efficiency is extremely low (0.2~0.5 m² / h), requiring subsequent processing; and the equipment investment is high. This results in existing gradient copper materials mostly exhibiting a single pattern of monotonically increasing grain size from the surface to the interior, leading to strain mismatch at the soft / hard phase interface, which easily induces early cracks; and the grain size gradient in the transition region is not gentle (only 3~5 grain spans), making it difficult to achieve synergistic optimization of stress redistribution and dislocation storage capacity.

[0004] Copper wire is a type of round or irregularly shaped metal wire produced through plastic processing such as rolling and drawing. Due to its excellent conductivity, toughness, and corrosion resistance, it is widely used in various sectors of the national economy, including energy and power, electronics and information, transportation, high-end manufacturing, and daily necessities. Applications include, but are not limited to, power cables and winding wires, motor and electrical coils, communication cables, braided shielding meshes, heat exchanger channels, and precision instrument springs. With economic and social development, fields such as intelligent manufacturing, new energy, and new energy vehicles have all placed demands on high-quality development. This has led to a strong demand for high-efficiency motors in wind power generation, new energy vehicles, and intelligent manufacturing, which places higher demands on the performance of copper wire. For example, the flat-wire motors widely used in new energy vehicle drive motors and wind turbines require the copper flat wires used for winding to possess high conductivity, high strength, and excellent bending toughness within a limited slot fill factor, in order to withstand the harsh processing and winding process and long-term vibration fatigue conditions. Traditional high-strength, high-conductivity copper wires rely on cold work hardening or complex heat treatment with a large amount of deformation. Their strength improvement often comes at the cost of sacrificing plasticity, toughness and electrical conductivity, making it difficult to meet the integrated requirements of "high strength, high conductivity and high toughness" for winding materials in the new generation of high-efficiency motors.

[0005] Therefore, there is an urgent need to develop a processing method for copper materials that can effectively improve strength while enhancing ductility without affecting the conductivity of copper. Summary of the Invention

[0006] To address the aforementioned technical problems, the present invention aims to provide a method for preparing high-strength, high-toughness, and high-conductivity copper materials with reverse gradient nanocrystalline structure based on low-temperature (-100~120 ℃) ​​multi-pass rolling. By controlling the rolling process parameters and cross-sectional shape design, a reverse gradient structure from surface coarse grains to core nanocrystalline / subcrystalline structures is constructed in the metal material, breaking through the technical bottleneck of traditional metal materials where strength and plasticity are difficult to achieve simultaneously.

[0007] To achieve the above objectives, the present invention first provides a method for preparing a reverse gradient nanocrystalline high-strength, high-toughness, and high-conductivity copper material, comprising the following steps: annealing and polishing the copper material, rolling it using a multi-pass rolling process, controlling the total deformation to be 10-50%, and preparing a reverse gradient nanostructure from central nanocrystals / subcrystals to surface coarse grains.

[0008] In one embodiment of the present invention, the total deformation is preferably 10-40%, more preferably 12-38%, and can be specifically selected from 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, etc.

[0009] In one embodiment of the present invention, the reduction amount per pass is controlled between 0.5% and 20%.

[0010] In one embodiment of the present invention, the copper material is selected from pure copper, copper alloys, and other FCC system copper materials that do not exhibit strain-induced twinning, precipitation, or phase transformation. Copper wire is preferred, with a diameter Φ of 1~30mm, an initial microstructure of equiaxed crystals, and a grain size of 1~100μm.

[0011] In one embodiment of the present invention, the annealing process involves raising the temperature of the annealing environment to 350-650°C, holding it at this temperature for 5-200 minutes, and then rapidly transferring it to water for quenching after annealing, with a dwell time of <3 seconds. The purpose of annealing is to eliminate internal stress and defects generated during the previous processing of the copper material and to improve the uniformity of the metal's microstructure.

[0012] In one embodiment of the present invention, the polishing is carried out by using common polishing methods such as mechanical polishing, chemical polishing, and electrochemical polishing to reduce the surface roughness of the annealed copper material to Ra<0.8μm, so as to reduce rolling defects.

[0013] In one embodiment of the present invention, the multi-pass rolling process is carried out under low temperature conditions, wherein the low temperature refers to -100~120 ℃.

[0014] In one embodiment of the present invention, the specific steps of the multi-pass rolling process are as follows: (1) Adjust the initial rolling gap of the twin-roll mill to the original diameter of the copper material to be processed; (2) Control the amount of reduction in each pass, and continuously roll the copper material to be processed in each pass. Repeat this step until the total deformation reaches the required amount.

[0015] In one embodiment of the present invention, the rolling rate is 0.1~100 m / min.

[0016] In one embodiment of the present invention, the cumulative reduction of the previous pass accounts for more than half of the total reduction, preferably 50-90% of the total reduction.

[0017] In one embodiment of the present invention, the rolling process employs a circular, elliptical, or other cross-sectional design with a circular arc structure, and the microstructure gradient distribution is achieved by controlling the strain gradient distribution.

[0018] The present invention also provides a copper material with a reverse gradient nanostructure prepared according to the above preparation method.

[0019] In one embodiment of the present invention, the grain size of the central region is 30~100nm, which belongs to nanocrystals / subcrystals. This region has a high dislocation density. The transition region consists of ultrafine and micron-sized grains. The size of the micron-sized grains is 0.1~10μm, and the dislocation density within the grains is relatively high. The edge region consists of coarse grains with a grain size of 1~100μm.

[0020] In one embodiment of the present invention, the size of the nanocrystals / subcrystals decreases continuously from the edge to the center.

[0021] In one embodiment of the present invention, when the material is pure copper, its mechanical properties meet the following requirements: tensile strength ≥ 250 MPa; elongation ≥ 20%.

[0022] In one embodiment of the present invention, when the material is a C18150 copper alloy, its properties meet the following requirements: yield strength ≥ 250 MPa; tensile strength ≥ 300 MPa; elongation ≥ 10%.

[0023] The present invention also provides an application of the above-mentioned reverse gradient nanostructured copper material in superconducting magnet windings, high-power electronic packaging, cable conductors, or other structural components with stringent requirements for strength and toughness.

[0024] This invention also provides a method for optimizing the performance of gradient nanostructured copper materials, characterized in that the method includes the following steps: deforming the copper material by 10-50% and reducing it by 0.5-20% per pass, thereby improving plasticity at low deformation rate and increasing strength at high deformation rate by controlling the deformation rate and deformation amount. The low deformation rate refers to a reduction of 0.5-5% per pass, and the high deformation rate refers to a reduction of 5-20% per pass.

[0025] Beneficial effects: (1) This invention utilizes an innovative low-temperature (-100~120 ℃) ​​multi-pass rolling process to construct, for the first time, a reverse gradient structure in pure copper, ranging from coarse surface grains (1~100 μm) to nanocrystalline / subcrystalline core grains (30~100 nm). This unique structural design breaks through the single mode of "surface nanostructuring" in traditional gradient materials. By coordinating plastic deformation through the coarse surface and providing reinforcement through the nanocrystalline core, the synergistic optimization of stress distribution and strain hardening is achieved. The grain size in the gradient transition region changes continuously and gently, effectively avoiding the interfacial stress concentration problem of traditional gradient materials.

[0026] (2) Simple and efficient preparation process: The present invention is based entirely on low temperature (-100~120 ℃) ​​multi-pass rolling process, without the need for extreme conditions such as deep cryogenic treatment or high temperature annealing. It can be achieved using a conventional rolling mill. Through multi-pass strain distribution design (high strain accumulation in the front section + precise control in the back section), it has significant advantages for industrial application.

[0027] (3) Breakthrough performance improvement: The reverse gradient structure pure copper obtained by this invention exhibits an excellent combination of strength and plasticity: the yield strength is increased by 50~150% compared with the annealed state, while the elongation is ≥20%, which is significantly better than traditional uniform nanocrystalline copper (the elongation is usually <10%). This performance breakthrough is due to: 1) the gradient dislocation density distribution enables the regulation of strain hardening ability; 2) the synergistic deformation mechanism of nanocrystalline region and coarse-grained region; 3) the multi-scale interface promotes dislocation storage and rearrangement.

[0028] (4) Broad application prospects: This method can be extended to various metal systems, and the gradient characteristics can be precisely controlled by adjusting the process parameters. The prepared materials are particularly suitable for applications that require both high strength and high conductivity, such as superconducting magnet windings, long-distance high-voltage power transmission, and cable conductors, providing a brand-new solution for the development of high-performance copper-based materials.

[0029] (5) The present invention relates to a process for preparing high-strength and high-toughness copper flat wire with reverse gradient nanocrystalline structure. The present invention directionally introduces and precisely controls a grain / subgrain structure with macroscopic gradient distribution in copper material, thereby obtaining excellent strength-plasticity matching and maintaining high electrical conductivity. Studies have shown that when the total compression deformation is controlled within a specific range, a reverse gradient distribution from surface coarse grains (1~100 μm) to core nanocrystalline / subgrain structure (30~100 nm) can be effectively achieved, and its performance synergy effect is the most significant. Attached Figure Description

[0030] Figure 1 The image shows the morphology of the pure copper solid solution grains after annealing in Example 1. Figure 2 The hardness gradient of pure copper rolled at low strain rates in Examples 1 and 2; Figure 3 The mechanical property test results are for pure copper rolled at low strain rates in Examples 1 and 2. Figure 4 This is a representative microstructure of the central region of pure copper rolled at a low strain rate in Example 1. Figure 5 This is a representative microstructure of the transition zone of pure copper rolled at a low strain rate in Example 1. Figure 6 This is a representative microstructure of the edge region of pure copper rolled at a low strain rate in Example 1. Figure 7 The TEM results show the nanocrystals in the central region of the gradient nanostructure of pure copper rolled at a low strain rate in Example 1. Figure 8 Tensile curves of uniformly deformed pure copper plates prepared in Comparative Example 1 and Comparative Example 2. Figure 9The diagram shows the inverted relationship between yield strength and uniform elongation of pure copper prepared in Example 1 and Comparative Example 1. Figure 10 The hardness gradient characterization results of the high strain rate gradient deformation pure copper prepared in Examples 3 and 4; Figure 11 Tensile curves of high strain rate gradient deformation pure copper prepared for Examples 3 and 4; Figure 12 Tensile curves of high strain rate uniformly deformed pure copper prepared for comparative examples 3 and 4. Figure 13 The hardness gradient characterization results of the low strain rate gradient deformation C18150 copper alloys prepared in Examples 5 and 6; Figure 14 Tensile curves of low strain rate gradient deformation C18150 copper alloys prepared for Examples 5 and 6; Figure 15 Tensile curves of C18150 copper alloys prepared at low strain rates and with uniform deformation for comparative examples 5 and 6. Figure 16 Characterization results of hardness gradient of C18150 prepared under high strain rate gradient deformation in Examples 7 and 8; Figure 17 Tensile curves of high strain rate gradient deformation C18150 copper alloys prepared for Examples 7 and 8; Figure 18 Tensile curves of C18150 copper alloys prepared at high strain rates with uniform deformation for comparative examples 7 and 8. Detailed Implementation

[0031] To better understand the above-mentioned objectives, features, and advantages of the embodiments of this application, the application will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the features in the embodiments of this application can be combined with each other.

[0032] The following description sets forth numerous specific details to provide a thorough understanding of the embodiments of this application. The described implementations are only a portion, not all, of the embodiments described herein. All other implementations obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of the embodiments of this application.

[0033] Terminology Explanation: The "multi-pass rolling" involved in this invention refers to a cold rolling process in which the plastic deformation of metal materials is completed in multiple consecutive passes through rolls because the deformation amount of a single rolling pass is limited by factors such as material plasticity and equipment load, and the target thickness and performance cannot be achieved in one pass.

[0034] The "reduction per pass" involved in this invention refers to the relative change in thickness of a metal workpiece after being rolled by the rolls in a single rolling pass, and its calculation formula is ε. i =Δh i / h i-1 It represents the degree of deformation in a single pass.

[0035] The "total deformation" involved in this invention refers to the relative change in the cumulative thickness of a metal rolled piece from the initial cold-rolled billet to the final finished product, and its calculation formula is ε. 总 =ΔH / H0.

[0036] The "rolling rate" involved in this invention refers to the linear velocity of the workpiece (the metal material being rolled) as it passes through the rolls. It is one of the core parameters of the cold rolling process and directly affects the rolling efficiency, product accuracy, and rolling force.

[0037] The "strain rate" involved in this invention refers to the rate at which the rolled piece undergoes plastic strain per unit time during each deformation process by precisely controlling the amount of reduction in each pass.

[0038] The "preliminary passes" involved in this invention refer to the rolling passes in the preceding stage of a complete multi-pass rolling sequence, accounting for 1 / 3 to 2 / 3 of the total number of passes. The process characteristics of these passes are the use of a large reduction rate, which lays the organizational foundation for the finishing rolling process of the middle and later passes.

[0039] The "gradient nanostructure" involved in this invention refers to a nanoscale structure in metallic or alloy materials where microscopic features such as grain size, dislocation density, phase composition, or twin structure exhibit a continuous gradient change in space. Unlike traditional uniform nanomaterials (with uniform grain size), gradient nanostructures achieve optimized combinations of material properties, such as high strength, high toughness, fatigue resistance, and wear resistance, through a controllable gradient distribution of microstructure. "Reverse gradient nanostructure" refers to a novel nanocomposite structure where microscopic features such as grain size, dislocation density, phase composition, or twin structure exhibit a reverse gradient distribution along the thickness / depth direction of the material. Its core characteristic is the opposite of traditional forward gradient nanostructures: the core of the material is composed of nanocrystals / ultrafine crystals (grain size 1~100 nm), while the surface layer is composed of coarse crystals / micron crystals (grain size > 1 μm), and the grain size exhibits a continuous or stepwise gradient evolution from the core to the surface.

[0040] The "central region" in this invention refers to the area in the cross-section of a copper rolled piece where the vertical distance from the upper and lower surfaces is not less than 1 / 3 of the total cross-sectional thickness. This region is the concentrated area of ​​cold-rolled plastic deformation. The "edge region" refers to the outermost region of the copper rolled piece's cross-section, the area in contact with the rolling tool or environmental medium. Its location can be quantitatively defined as: the area where the vertical distance from the outer surface of the copper cross-section does not exceed 30% of the total cross-sectional thickness. The core characteristic of this region is dominated by interfacial friction or continuum constraint stress during processing, specifically manifested as: significant orientation deformation of grains under shear stress, forming a fibrous structure arranged along the rolling direction. The "transition region" refers to the intermediate region in the cross-section of a copper rolled piece between the central region and the edge region, serving as a structural-performance buffer region connecting the core high-strength region and the surface tough-plastic region. Its location can be quantitatively defined as: the area where the vertical distance from the outer surface of the copper cross-section is 20-40% of the total cross-sectional thickness.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of this application belong. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the embodiments of this application.

[0042] Example 1: Low-rate gradient deformation multi-pass controlled rolling of pure copper A method for preparing a high-strength, high-toughness, and high-conductivity copper material with reverse gradient nanocrystalline structure includes the following steps: (1) Raw material selection: Pure copper wire with a diameter of Φ is selected; (2) Pretreatment: The pure copper wire is annealed and then polished. The annealing process involves placing the copper wire at a temperature of 350~500℃ for 90 minutes and then quickly transferring it to water for quenching. The surface oxide scale and impurities are removed by using sulfuric acid or nitric acid solution for polishing to obtain a clean surface, which facilitates lubrication during rolling and prevents surface defects. The surface roughness of the polished copper wire is Ra<0.8μm.

[0043] (3) Multi-pass rolling: Adjust the roller spacing to 3 mm, control the reduction per pass to 3%, and the rolling speed to 5 m / min. Keep the sample orientation unchanged and feed it into the mill from one end for axial cold rolling. Monitor the evolution of the cross-sectional shape during rolling and detect the sample cross-sectional dimensions (major axis, minor axis) online. Repeat this process until the total deformation is 15%, forming a reverse gradient nanostructure from central nanocrystals / subcrystals to surface coarse grains. Gradually reduce the strain rate in subsequent passes until the total deformation reaches 20%. Name it Cu-GCR20-low strain rate. In subsequent passes, the purpose of reducing the strain rate is to ensure precise control of the interface dimensions. The strain rate can be reduced by successively decreasing the reduction per pass to 2%, 1.5%, 1%, etc., until the total deformation reaches 20%.

[0044] Example 2 The difference between Example 2 and Example 1 is that the total deformation in step (3) is 40%. During rolling, the rolling is first carried out with a reduction of 3% until the total deformation is 30%, and then the reduction of each pass is gradually reduced until the total deformation is 40%, which is named Cu-GCR40-low strain rate.

[0045] The solid solution grain morphology of the copper wire after annealing in Example 1 is as follows: Figure 1 As shown, Figure 1 The grain size of the copper wire after annealing is 10~50μm.

[0046] Hardness gradient characterization: Pure copper wire processed according to Example 1 was selected, and a microhardness tester (load 100g, holding pressure 15s) was used for hardness testing. The test direction was the horizontal and vertical directions of the sample cross-section. The test path was to take points every 150μm from the surface to the core (≥12 data points per group).

[0047] Mechanical property testing: Figure 2 The results show the hardness distribution at 20% and 40%, where Cu-ST represents annealed (solution treated) copper. Both exhibit a clear reverse gradient distribution, meaning the hardness increases from the edge to the center of the sample cross-section. The horizontal gradient slope is significantly greater than the vertical gradient; for example, in GCR20, the horizontal hardness difference can reach over 25 Hv, while the vertical difference is approximately 10 Hv. With increasing indentation (20%→40%), both the horizontal and vertical gradients significantly increase, indicating a noticeable improvement in hardness.

[0048] By comparing the tensile stress-strain curves of samples with 20% deformation in Example 1 and 40% deformation in Example 2, the influence of different deformation amounts on the mechanical properties of the materials is analyzed. Figure 3It exhibits a 20% yield strength of approximately 250 MPa, displaying typical ductile fracture characteristics, with the strain hardening stage lasting until approximately 25% elongation and a peak stress of 275 MPa. 40% exhibits an ultra-high yield strength of approximately 350 MPa, but fractures immediately after reaching an ultimate tensile strength of 365 MPa, with an elongation of only 2-3%.

[0049] These two examples demonstrate that the rolling amount has a decisive influence on the tensile behavior of pure copper wire. 20% is suitable for forming processes requiring high plasticity and exhibits the best performance in terms of strength-plasticity matching; 40% can meet the requirements of ultra-high strength applications. These results verify the effect of different strain distributions of rolling deformation on hardness regulation, providing a quantitative basis for gradient structure design.

[0050] Characterization of reverse gradient nanostructures: The microstructure of the sample with 20% deformation in Example 1 was characterized using scanning electron microscopy. The sample was taken from the longitudinal section of the rolled sheet, and the test area covered a complete gradient structure from the edge to the center.

[0051] Figures 4-6 The microstructure characteristics of the reverse gradient nanostructured copper material in Example 1 are given. Figure 4 The central region (strain ε>1.5) was characterized by nanocrystals / subcrystals (50~100 nm) and high-density dislocations. The nanostructural unit was determined to be nanocrystals, with the grain size gradually decreasing from the edge to the center, eventually reaching the nanoscale.

[0052] Figure 5 The transition region (ε=0.3~1.5) shows the formation of a multi-scale grain and dislocation system, with ultrafine grains (100~500nm) accounting for about 40%; micron-sized grains (1~2 μm) accounting for about 30%; and a dislocation gradient distribution that changes continuously from the edge to the center.

[0053] Figure 6 The matrix structure in the edge region (ε<0.3) is coarse-grained (10~50μm), retaining the original annealed structure.

[0054] This invention achieves the construction of three-dimensional reverse gradient nanostructures through an innovative rolling process, with the characteristic size continuously transitioning from micron-level grains (edges) to nano-level grains (center); multi-scale microstructure regulation forms a nanocrystal / dislocation composite reinforcement system; and structure-performance correlation design provides a new approach for developing next-generation gradient functional materials.

[0055] TEM characterization of nanocrystalline / subcrystalline structures: The central region of the sample with 20% deformation in Example 1 was characterized by fine tissue analysis using transmission electron microscopy (TEM). Figure 7TEM characterization results show that bright-field imaging reveals nanocrystals / subcrystals (30–100 nm in size) distributed in the central region of the sample. Selected area diffraction confirmed the presence of numerous nanocrystals / subcrystals in addition to a significant high density of dislocations. TEM results further confirm that the nanostructural units in this patent are nanoscale grains / subcrystals.

[0056] Comparative Example 1: A Comparative Study of Low-Rate Uniform Deformation and Gradient Deformation of Pure Copper The difference between Comparative Example 1 and Example 1 is that the copper material used in Example 1 is a wire with a circular cross-section, while the comparative example uses a plate with a rectangular cross-section (thickness of 3 mm). The rest is the same as Example 1, and it is named Cu-CR20-low strain rate.

[0057] Comparative Example 2 The difference between Comparative Example 2 and Comparative Example 1 is that the total deformation is 40%, and it is named Cu-CR40-low strain rate.

[0058] Comparative Examples 1-3 involve rolling rectangular plates. The overall shape of the plates did not change significantly before and after rolling deformation, and the microstructure distribution indicates uniform deformation. The microstructure and properties of uniformly deformed plates are compared with those of gradient-deformed plates. The tensile curves for low-strain-rate uniform deformation are shown below. Figure 8 As shown.

[0059] Table 1. Tensile properties of pure copper nanostructures prepared in Example 1 and Comparative Example 1

[0060] Compared to the annealed state, the conductivity of samples with the same amount of uniform deformation and gradient deformation is as follows: the conductivity of the 20% gradient deformation sample decreases to 96% IACS, while that of the 20% uniform deformation sample decreases to 85% IACS; the conductivity of the gradient sample is 11% higher than that of the uniform sample.

[0061] Figure 9 The diagram shows the inverse relationship between the yield strength and uniform elongation of pure copper in Example 1 and Comparative Example 1 after uniform deformation at low strain rates, and the mechanical properties of deformed pure copper in the literature (Yan HT, et. science 331 (2011) 1587; Cheng Z, et. Acta Materialia 256 (2023) 119138). The results indicate that the mechanical properties of pure copper subjected to gradient deformation treatment in this invention are significantly improved, demonstrating a clear advantage.

[0062] This embodiment confirms that the gradient rolling process of this patent (Example 1) increases the elongation by more than 200% compared with the traditional uniform deformation process, while maintaining a comparable strength level. The reverse gradient structure achieves a synergistic improvement in strength and plasticity through strain distribution optimization, and its electrical conductivity is superior to that of the uniform sample. Although the uniform deformation process has a uniform hardness distribution, it sacrifices the overall plasticity of the material.

[0063] Example 3: High Strain Rate Gradient Deformation Multi-Pass Control Rolling of Pure Copper The difference between Example 3 and Example 1 is that the reduction amount per pass is 10%, and it is named Cu-GCR20-high strain rate.

[0064] Example 4 The difference between Example 4 and Example 2 is that the reduction amount per pass is 10%, and it is named Cu-GCR40-high strain rate.

[0065] In Examples 3 and 4, the reduction per pass was 10%, which falls within the high strain rate range. Other rolling parameters were the same as in Example 1.

[0066] Hardness gradient characterization and mechanical property testing: Pure copper wires from Examples 3 and 4, processed using a high strain rate rolling process, were selected and their hardness was tested using a microhardness tester (load 100g, holding pressure 15s). The testing directions were the horizontal and vertical directions of the sample cross-section, and the testing path involved sampling points every 150μm from the surface to the core (≥12 data points per group). Tensile property tests were also performed simultaneously to obtain engineering stress-strain curves.

[0067] Figure 10 Hardness gradient characterization results show that all samples exhibit a clear reverse gradient distribution, meaning that hardness increases from the surface to the core. The gradient characteristic is positively correlated with the strain rate, with samples at higher strain rates showing a greater gradient.

[0068] By comparing the tensile stress-strain curves under rolling deformation of 20% in Example 3 and 40% in Example 4, the influence of different deformation amounts on the mechanical properties of materials under high strain rates is analyzed. Figure 11 It exhibits a 20% yield strength of approximately 260 MPa, displaying typical ductile fracture characteristics, with the strain hardening stage lasting until approximately 24% elongation and a peak stress of 280 MPa. 40% exhibits an ultra-high yield strength of approximately 330 MPa, but fractures immediately after reaching an ultimate tensile strength of 350 MPa, with an elongation of only 3%.

[0069] Comparative Example 3 The difference between Comparative Example 3 and Comparative Example 1 is that the reduction amount per pass is 10%, and it is named Cu-CR20-High Strain Rate.

[0070] Comparative Example 4 The difference between Comparative Example 4 and Comparative Example 2 is that the reduction amount per pass is 10%, and it is named Cu-CR40-High Strain Rate.

[0071] The microstructure and properties of uniform deformation and gradient deformation are compared. The tensile curve of uniform deformation at high strain rate is shown in the figure. Figure 12 As shown.

[0072] Table 2 Tensile properties of samples from Example 3 and Comparative Example 3

[0073] from Figure 12 As can be seen from Table 2, the gradient rolling process of this patent increases the elongation by 100-300% compared with the traditional uniform deformation process, while maintaining a comparable strength level. The reverse gradient structure achieves a synergistic improvement in strength and plasticity through strain distribution optimization. Although the uniform deformation process has a uniform hardness distribution, it sacrifices the overall plasticity of the material.

[0074] Example 5 The difference between Example 5 and Example 1 is that the copper raw material used is C18150 copper alloy, the copper alloy wire diameter is Φ3mm, and it is named C18150-GCR20-low strain rate.

[0075] Example 6 The difference between Example 6 and Example 2 is that the copper raw material used is C18150 copper alloy, the copper alloy wire diameter is Φ 3mm, and it is named C18150-GCR40-low strain rate.

[0076] Hardness characterization analysis and mechanical property testing: Gradient and uniformly deformed C18150 copper alloys processed by Examples 5 and 6 were selected, and hardness tests were performed using a microhardness tester (load 100g, holding pressure 15s). The test directions were the horizontal and vertical directions of the sample cross-section. The test path was to take points every 150 μm from the surface to the core (≥15 data points per group).

[0077] Figure 13 The hardness distribution results at 20% and 40% both exhibit a clear reverse gradient distribution characteristic, meaning the hardness increases from the edge to the center of the sample cross-section. The slope of the horizontal gradient is significantly greater than that in the vertical direction; for example, in GCR20, the horizontal hardness difference can reach over 40 Hv, while the vertical difference is approximately 25 Hv. With increasing reduction (20%→40%), the horizontal gradient characteristic weakens, but a significant increase in hardness is still visible at the edges. These results verify the effect of different strain distributions of rolling deformation on hardness regulation, providing a quantitative basis for gradient structure design.

[0078] Comparative Example 5 The difference between Comparative Example 5 and Comparative Example 1 is that the copper raw material used is C18150 copper alloy, the copper alloy wire diameter is Φ 3mm, and it is named C18150-CR20-low strain rate.

[0079] Comparative Example 6 The difference between Comparative Example 6 and Comparative Example 2 is that the copper raw material used is C18150 copper alloy, the copper alloy wire diameter is Φ 3mm, and it is named C18150-CR40-low strain rate.

[0080] By comparing the tensile stress-strain curves at 20% and 40% in Examples 5 and 6, the influence of different deformation modes on the mechanical properties of the material is analyzed, such as... Figure 14 As shown. In Figure 14 In low-rate gradient deformation, the 20% yield strength is approximately 270 MPa, exhibiting typical ductile fracture characteristics. The strain hardening stage lasts until the elongation reaches approximately 18%, with a peak stress of 310 MPa. The 40% yield strength exhibits a high yield strength of approximately 350 MPa, but fractures immediately after reaching the ultimate tensile strength of 380 MPa, with an elongation of only 2-3%. Figure 15 The figures in the middle show the tensile stress-strain curves for Comparative Examples 5 and 6 after low-rate uniform deformation of 20% and 40%. The 20% deformation results in a yield strength of approximately 300 MPa, a uniform elongation of only 4%, and a peak stress of 340 MPa. The 40% deformation exhibits a high yield strength of approximately 370 MPa, but fractures immediately after reaching its ultimate tensile strength of 390 MPa, with an elongation of only 2%.

[0081] The above results demonstrate that the deformation mode has a decisive influence on the tensile behavior of C18150 copper alloy. For gradient deformation samples, 20% are suitable for forming and processing applications requiring high plasticity, while 40% can meet the requirements of ultra-high strength applications.

[0082] Example 7 The difference between Example 7 and Example 3 is that the copper raw material used is C18150 copper alloy, the copper alloy wire diameter is Φ 3mm, and it is named C18150-GCR20-high strain rate.

[0083] Example 8 The difference between Example 8 and Example 4 is that the copper raw material used is C18150 copper alloy, the copper alloy wire diameter is Φ 3mm, and it is named C18150-GCR40-high strain rate.

[0084] Comparative Example 7 The difference between Comparative Example 7 and Comparative Example 3 is that the copper raw material used is C18150 copper alloy, the copper alloy wire diameter is Φ 3mm, and it is named C18150-GCR20-high strain rate.

[0085] Comparative Example 8 The difference between Comparative Example 8 and Comparative Example 4 is that the copper raw material used is C18150 copper alloy, the copper alloy wire diameter is Φ 3mm, and it is named C18150-GCR40-high strain rate.

[0086] Hardness characterization analysis and mechanical property testing: Gradient and uniformly deformed C18150 copper alloys processed by Examples 7 and 8 were selected, and hardness tests were performed using a microhardness tester (load 100g, holding pressure 15s). The test directions were the horizontal and vertical directions of the sample cross-section. The test path was to take points every 150 μm from the surface to the core (≥15 data points per group).

[0087] Figure 16 The hardness distribution results for copper materials with gradient deformation of 20% and 40% in Examples 7 and 8 are presented. Both exhibit a clear reverse gradient distribution characteristic, that is, the hardness increases from the edge to the center of the sample cross-section. This result verifies the effect of different strain distributions of rolling deformation on hardness regulation and provides a quantitative basis for gradient structure design.

[0088] By comparing the tensile stress-strain curves of C18150 copper alloy under high-rate gradient deformation in Examples 7 and 8 with those under uniform deformation at 20% and 40%, the influence of gradient deformation and uniform deformation on the mechanical properties of the material under high strain rates is analyzed. Figure 17 In medium-to-high rate gradient deformation, 20% exhibits a yield strength of approximately 250 MPa, displaying typical ductile fracture characteristics. The strain hardening stage lasts until the elongation reaches approximately 26%, with a peak stress of 280 MPa. 40% exhibits an ultra-high yield strength of approximately 340 MPa, but fractures immediately after reaching the ultimate tensile strength of 360 MPa, with an elongation of only 2%. Figure 18 During medium-to-high rate uniform deformation, 20% exhibits a yield strength of approximately 260 MPa, a tensile strength of approximately 320 MPa, and an elongation of up to 3%. 40% exhibits a high yield strength of approximately 340 MPa, but fractures immediately after reaching an ultimate tensile strength of 355 MPa, with an elongation of 2%.

[0089] This embodiment demonstrates that the deformation mode has a decisive influence on the tensile behavior of C18150 copper alloy. For the gradient deformation sample, 20% is suitable for forming and processing applications that require high plasticity and performs best in terms of strength-plasticity matching, while 40% can meet the requirements of ultra-high strength applications.

[0090] Comparative Example 9 The difference between Comparative Example 9 and Example 1 is that the pretreatment step is omitted, that is, the annealing and polishing processes are omitted.

[0091] The method used in Comparative Example 9 for rolling copper material yielded poor results because the high level of crystallographic defects in the unannealed state prevented the formation of a sufficient microstructural gradient after gradient rolling. The poor results were also attributed to the presence of numerous defects in the edge region of the flat wire, negating its role as a buffer for toughness and ductility. Annealing significantly eliminates the original wire defects, and subsequent moderate gradient rolling (10–50%) can produce high-strength and high-toughness metallic materials with excellent reverse gradient nanocrystalline (or subcrystalline) properties.

[0092] Compared with the traditional uniform deformation process, the elongation of the gradient rolling process of this patent is increased by 200-300% while maintaining a comparable strength level. The reverse gradient structure achieves a synergistic improvement in strength and plasticity through strain distribution optimization. Although the uniform deformation process has a uniform hardness distribution, it sacrifices the overall plasticity of the material.

[0093] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A method for preparing a high-strength, high-toughness, and high-conductivity copper material with reverse gradient nanocrystalline structure, characterized in that, Includes the following steps: After annealing and polishing, copper material is rolled using a multi-pass rolling process, with the total deformation controlled at 10-40% and the reduction per pass controlled at 0.5-20%, to prepare a reverse gradient nanostructure from central nanocrystals / subcrystals to surface coarse grains.

2. The preparation method according to claim 1, characterized in that, The total deformation should be controlled at 12-38%, and the reduction in each pass should be controlled at 1-10%.

3. The preparation method according to claim 1, characterized in that, The copper material is selected from FCC system copper materials without strain-induced twinning, precipitation or phase transformation, including pure copper or copper alloy. The copper material is selected as copper wire with a diameter Φ of 1~30mm, the initial structure is equiaxed crystal, and the grain size is 1~100μm.

4. The preparation method according to claim 1, characterized in that, The annealing process involves raising the temperature of the annealing environment to 350-650℃ and holding it at this temperature for 5-200 minutes. After annealing, the material is quickly transferred to water for quenching, with a dwell time of <3 seconds. The polishing process involves reducing the surface roughness of the annealed copper material to Ra<0.8μm through mechanical polishing, chemical polishing, or electrochemical polishing.

5. The preparation method according to claim 1, characterized in that, The multi-pass rolling process is carried out at -100~120℃, and the specific steps of the multi-pass rolling process are as follows: (1) Adjust the initial rolling gap of the twin-roll mill to the original diameter of the copper material to be processed; (2) Control the amount of reduction in each pass, and continuously roll the copper material to be processed in each pass. Repeat this step until the total deformation reaches the required amount.

6. The preparation method according to claim 1, characterized in that, The rolling speed is 0.1~100 m / min, and the cumulative reduction in the previous pass accounts for 50~90% of the total reduction.

7. A copper material with a reverse gradient nanostructure prepared by the preparation method according to any one of claims 1 to 6.

8. The copper material with a reverse gradient nanostructure according to claim 7, characterized in that, The copper material with reverse gradient nanostructure has nanocrystals / subcrystals in the central region with a grain size of 30~100nm, ultrafine crystals and microcrystals in the transition region with a microcrystal size of 0.1~10μm, and coarse crystals in the edge region with a grain size of 1~100μm. The size of the nanocrystals / subcrystals decreases continuously from the edge to the center.

9. The copper material with a reverse gradient nanostructure according to claim 7, characterized in that, When the material is pure copper, its mechanical properties meet the following requirements: tensile strength ≥ 250 MPa; elongation ≥ 20%. When the material is C18150 copper alloy, its properties meet the following requirements: yield strength ≥ 250 MPa; tensile strength ≥ 300 MPa; elongation ≥ 10%.

10. The application of the copper material with reverse gradient nanostructure as described in any one of claims 7 to 9 in superconducting magnet windings, high-power electronic packaging, and cable conductors.

11. A method for performance optimization of gradient nanostructured copper materials, characterized in that: The method includes the following steps: the deformation amount of copper material is 10~40%, and the reduction amount per pass is 0.5~20%. By controlling the deformation rate and deformation amount, the plasticity is improved at a low deformation rate and the strength is improved at a high deformation rate. The low deformation rate refers to a reduction amount of 0.5~5% per pass, and the high deformation rate refers to a reduction amount of 5~20% per pass.

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