A method for resistance brazing of copper-aluminum dissimilar metals for wind turbine generators

Abstract

The application provides a copper-aluminum dissimilar metal resistance brazing method for a wind power generator, comprising the following steps: S1, temperature field regulation of partition resistance heating: a partition resistance heating mode of double-electrode independent power supply is adopted, current is respectively input to the copper side and the aluminum side for independent control, and the brazing interface temperature is controlled in the range of 580-620 DEG C; S2, interface diffusion control of nano nickel barrier layer: a nano-crystalline nickel barrier layer is prepared in advance on the surface of the copper matrix to be connected; S3, wetting and filling of zinc-based composite filler material: zinc-based composite filler material is used for brazing; S4, pressure-assisted brazing seam densification: axial pressure in the range of 0.5-2 MPa is applied during brazing; S5, atmosphere protection and surface activation: the brazing process is carried out under the condition of inert protective atmosphere. Through the cooperation of partition heating and nano nickel barrier layer, the strength of the copper-aluminum brazed joint is improved to more than 80% of the base material, and the contact resistance is reduced to 3 mu omega cm 2 The energy consumption of the single piece is reduced by 80%, and the qualified rate reaches more than 95%.

Description

Technical Field

[0001] This invention relates to the field of copper-aluminum dissimilar metal joining technology, and specifically to a method for resistance brazing of copper-aluminum dissimilar metals for wind turbine generators. Background Technology

[0002] With the development of the wind power industry, installed capacity continues to increase. Copper-aluminum dissimilar metal connections are widely used inside wind turbines, such as connecting copper busbars to aluminum busbars, and connecting stator lead terminals. Copper has good electrical conductivity and mechanical properties, but it is more expensive and has a higher density. Aluminum is lightweight and inexpensive, but its electrical conductivity and strength are relatively lower. Therefore, in practical engineering, a copper-aluminum combination is often used to balance performance and cost requirements.

[0003] In copper-aluminum brazing, several key issues arise due to the significant differences in the physical and chemical properties of the two materials: First, at high temperatures, the copper-aluminum interface is prone to strong diffusion reactions, forming brittle intermetallic compounds such as CuAl2. When this compound layer exceeds a certain thickness, it significantly reduces the joint toughness, making it susceptible to fracture under stress or thermal cycling. Second, the interface oxide film and porous structure increase contact resistance. When the resistance is high, a large current passing through will generate significant Joule heating, leading to energy loss and accelerated aging. Third, different heating methods have their limitations. For example, flame brazing results in uneven temperature distribution, easily causing overheating on the aluminum side; furnace brazing has lower efficiency and a larger heat-affected zone; induction heating is more sensitive to workpiece shape and has higher equipment costs. Furthermore, the oxide film on the copper and aluminum surfaces is difficult to remove, especially the Al2O3 on the aluminum surface, which has an extremely high melting point and severely affects wetting. Conventional fluxes also present problems of residual corrosion and cleaning difficulties. Finally, due to the significant difference in diffusion coefficients between copper and aluminum, Kirkendall voids are easily formed during brazing, thereby reducing electrical and mechanical properties. Meanwhile, traditional processes rely heavily on human experience, resulting in large parameter fluctuations and poor batch consistency.

[0004] The shortcomings of existing technologies are specifically manifested as follows:

[0005] Excessive thickness of brittle intermetallic compounds: Copper and aluminum undergo solid-state diffusion reactions at high temperatures to form compounds such as CuAl2 and Cu9Al4. These structures are hard but brittle; when the thickness exceeds 5 μm, interfacial fracture easily occurs under mechanical loads or thermal cycling. Traditional processes, due to long heating times and insufficient temperature control precision, often result in this layer thickness reaching 15-30 μm, significantly reducing joint reliability.

[0006] High contact resistance: Interfacial oxide film, porosity, and brittleness all contribute to increased contact resistance. When the resistance exceeds 5 μΩ·cm... 2 At high current conditions, significant heat generation will occur, affecting long-term stability.

[0007] Limitations of heating methods: Flame heating is difficult to control temperature evenly; furnace heating is inefficient and unsuitable for large on-site workpieces; induction heating is structurally sensitive and requires high equipment investment, all of which are difficult to meet the engineering needs of the wind power industry.

[0008] The oxide film is difficult to remove: Al2O3 has a stable structure and an extremely high melting point, making it difficult to destroy using conventional methods. While flux plays a role, it poses risks of residue and corrosion.

[0009] Kirkendall void problem: The difference in diffusion rates between copper and aluminum leads to an imbalance in vacancy migration, resulting in void structures at the interface, which reduces the effective conductivity and load-bearing area.

[0010] Poor batch consistency: The process relies on manual experience and the parameters fluctuate greatly, resulting in poor product consistency and difficulty in meeting the high reliability requirements of wind power equipment. Summary of the Invention

[0011] The purpose of this invention is to provide a method for resistance brazing copper and aluminum dissimilar metals in wind turbines. This method achieves precise temperature control at the interface through zoned resistance heating, combines a nano-nickel barrier layer technology to inhibit the growth of brittle intermetallic compounds, uses a zinc-based composite brazing filler metal to improve wetting properties, and enhances the quality of the brazed joint through pressure assistance and atmosphere protection. This results in a high-strength, low-resistance, and highly reliable copper-aluminum joint. The entire process forms a complete technical chain: precise heating to establish a temperature field, barrier layer to control diffusion, brazing filler metal wetting and filling, pressure densification, atmosphere protection, and intelligent monitoring. Each step supports and synergizes with the others.

[0012] To achieve the above objectives, the present invention adopts the following technical solution:

[0013] A resistance brazing method for copper-aluminum dissimilar metals in wind turbine generators, characterized by comprising the following steps:

[0014] S1. Temperature Field Control of Partitioned Resistance Heating: Partitioned resistance heating with independent power supply for dual electrodes is adopted. The current is applied to the copper side and the aluminum side separately and controlled independently, so that the two sides form differentiated energy input during the heating process, thereby achieving uniform temperature distribution at the interface. Through comprehensive analysis of the differences in resistivity and thermal conductivity between copper and aluminum materials, a specific proportional relationship is determined between the current on the copper side and the current on the aluminum side, so that the interface region reaches the target brazing temperature range in a short time. The brazing interface temperature is controlled within the range of 580-620℃ and a stable temperature field is maintained for a relatively short time, thereby providing the necessary conditions for the melting of the brazing filler metal and the metallurgical reaction at the interface, while significantly shortening the high-temperature dwell time and inhibiting the excessive growth of intermetallic compounds.

[0015] S2. Interfacial diffusion control of nano-nickel barrier layer: A nanocrystalline nickel barrier layer is pre-prepared on the copper substrate surface to be joined. This barrier layer is formed by chemical plating or physical deposition, with its thickness controlled in the nanometer to micrometer range and possessing nanocrystalline structure characteristics, thereby forming a high-density grain boundary network structure at the interface. This structure can significantly reduce the diffusion rate of copper atoms to the aluminum side, inhibiting the formation and growth of intermetallic compounds in the Cu-Al system from the perspective of diffusion kinetics. At the same time, since nickel can form a good metallurgical bonding interface with both copper and aluminum, this barrier layer will not significantly increase the interfacial contact resistance and can participate in the final interface structure formation as a stable transition layer.

[0016] S3. Wetting and filling of zinc-based composite brazing filler metal: Brazing is performed using zinc-based composite brazing filler metal;

[0017] S4. Pressure-assisted brazing seam densification: Applying axial pressure in the range of 0.5-2MPa during brazing;

[0018] S5. Atmosphere protection and surface activation: The brazing process is carried out under an inert protective atmosphere.

[0019] Furthermore, in the partitioned resistance heating, the current on the copper side and the current on the aluminum side meet a preset ratio, wherein the current on the aluminum side is greater than the current on the copper side, and the ratio ranges from 1.8 to 2.2 times, to compensate for the difference in resistivity and thermal conductivity between copper and aluminum materials, and to achieve uniform control of the interface temperature. Specifically, considering the differences in physical properties between copper and aluminum, this invention designs a dual-electrode system with independent power supply for each partition. The resistivity of copper is approximately 1.7 μΩ·cm, and the resistivity of aluminum is approximately 2.8 μΩ·cm, with aluminum's resistivity being 1.65 times that of copper. Simultaneously, the thermal conductivity of copper is approximately 400 W / (m·K), and the thermal conductivity of aluminum is approximately 237 W / (m·K), with copper's thermal conductivity being 1.69 times that of aluminum. This means that under the same current density, aluminum generates more heat but dissipates it more slowly, easily leading to overheating on the aluminum side and insufficient temperature on the copper side.

[0020] To resolve this contradiction, this invention employs a zoned current control strategy. Let the current flowing through the copper-side electrode be... The current flowing through the aluminum side electrode is The heat generation capacities on both sides are as follows:

[0021]

[0022]

[0023] in The current on the copper side is expressed in A. Current on the aluminum side (unit: A). and The equivalent resistances (in Ω) of the copper and aluminum sides of the workpiece are respectively. and The heat generation power (in W) is represented by the copper and aluminum sides, respectively. Considering the heat dissipation characteristics of the materials, thermal equilibrium conditions need to be met to ensure uniform temperature at the brazing interface. Through theoretical analysis and experimental verification, this invention determines the optimal current ratio:

[0024]

[0025] in and The resistivity of copper and aluminum are shown below (unit: μΩ·cm). and The thermal conductivity of copper and aluminum are respectively (unit: W / (m·K)). The correction factor (dimensionless) ranges from 1.2 to 1.4, with the specific value determined based on the workpiece's geometry and contact condition. This formula comprehensively considers the material's heat generation capacity (determined by resistivity) and heat dissipation capacity (determined by thermal conductivity), adjusting the current ratio to ensure both interfaces reach the same temperature.

[0026] By using this zoned current control, the brazing interface temperature can be kept within the optimal range of 580-620℃. This temperature is slightly higher than the liquidus temperature of zinc-based brazing filler metal, which is sufficient to ensure complete melting and wetting of the filler metal, while avoiding softening of the base metal and excessive growth of brittle phases caused by excessively high temperatures. The uniformity of the interface temperature is controlled within ±8℃, and the temperature at a distance of 5mm from the interface has dropped below 400℃. The width of the heat-affected zone is reduced by more than 50% compared to traditional flame brazing.

[0027] The heating process utilizes a pulsed DC power supply, achieving a heating rate of 50-80℃ / s with a total heating time of 15-25 seconds. Compared to the several minutes or even tens of minutes required for traditional furnace brazing, this represents an efficiency improvement of over 10 times. Rapid heating not only enhances production efficiency but, more importantly, significantly shortens the contact time between copper and aluminum at high temperatures. This kinetically inhibits the growth of intermetallic compounds, providing sufficient time for the nano-nickel barrier layer to exert its diffusion control function.

[0028] Furthermore, the thickness of the nanocrystalline nickel barrier layer is 500nm-2μm, and the grain size is 20-50nm. Specifically, nickel has multiple advantages as a barrier layer material: First, nickel can form good metallurgical bonds with both copper and aluminum, resulting in high interfacial bonding strength; second, nickel has a melting point of 1455℃, which is much higher than the aforementioned brazing temperature of 580-620℃, allowing it to remain solid throughout the brazing process and act as a stable diffusion barrier; third, the Cu-Ni solid solution formed by nickel and copper, and the Ni-Al solid solution formed by nickel and aluminum, both have good plasticity and conductivity, unlike CuAl2 which can become a brittle weak point, thus ensuring the mechanical and electrical properties of the joint.

[0029] The inhibitory effect of the nickel barrier layer on copper atom diffusion is characterized by the effective diffusion coefficient:

[0030]

[0031] in The intrinsic diffusion coefficient of copper in aluminum without a barrier layer (unit: m). 2 / s), Effective diffusion coefficient with a barrier layer (unit: m) 2 / s), The thickness of the nickel layer (unit: μm). The value represents the grain size of the nickel layer (unit: nm). The value is a dimensionless coefficient related to grain boundary diffusion characteristics, typically ranging from 2 to 4. This formula indicates that the greater the nickel layer thickness and the smaller the grain size, the more significant the decrease in the effective diffusion coefficient. When the nickel layer thickness is 1 μm and the grain size is 30 nm, the effective diffusion coefficient can be reduced to 1 / 100–1 / 200 of its original value. Since the growth rate of the intermetallic compound is proportional to the square root of the diffusion coefficient, the growth rate is reduced to 1 / 10–1 / 14 of its original value.

[0032] Experimental results show that, with the use of a nano-nickel barrier layer, under the same brazing process conditions, the thickness of the intermetallic compound layer decreases from 15-25 μm without the barrier layer to 3-5 μm, the tensile strength of the joint increases from 55-60% of that of the aluminum base material to 80-85%, and the contact resistance decreases from 8-12 μΩ·cm. 2 Reduced to 2-3 μΩ·cm 2 This thin and dense transition layer provides a good interfacial foundation for the subsequent wetting and filling of the solder.

[0033] Furthermore, the zinc-based composite brazing filler metal is a Zn-Al-Cu-Mg quaternary alloy system, wherein the mass percentages of each element are: Zn 85-90%, Al 5-8%, Cu 2-4%, and Mg 0.5-1.5%. Specifically, zinc, as the matrix element, provides low melting point characteristics; aluminum is used to adjust the melting temperature and improve the wetting performance on the aluminum base material; copper is used to improve the wetting ability on the copper side and nickel layer and refine the microstructure; magnesium, as an active element, is used to break down the interfacial oxide film and promote the wetting and spreading process. This brazing filler metal system has a low liquidus temperature range and can achieve sufficient melting and rapid spreading within the target brazing temperature range, thereby improving the interfacial filling ability and the density of the brazed joint. The wettability of the brazing filler metal to the base material can be characterized by the wetting angle; the smaller the wetting angle, the better the wettability. The Zn-Al-Cu-Mg solder of this invention exhibits a wetting angle of less than 15° on copper surfaces and less than 20° on aluminum surfaces, a significant improvement compared to the 25-35° of traditional Zn-Al solders. This excellent wettability ensures the solder can fully spread and fill interfacial gaps, reducing porosity defects and creating conditions for subsequent pressure-assisted densification. The increase in the solder's spreading area over time is described by a kinetic equation:

[0034]

[0035] in The spread area at time t (unit: mm) 2 ), To achieve the final balanced spread area (unit: mm) 2 ), t s The spreading time constant (in seconds) reflects the spreading rate. This formula is a first-order exponential growth model, describing the spreading kinetics of liquid solder on the base material surface. The t of the solder of this invention... s The value is about 2-3 seconds, which is 60-70% shorter than the 6-9 seconds of traditional brazing filler metal. This means that more thorough wetting and filling can be achieved within the aforementioned 15-25 seconds of rapid heating time, and the brazing filler density can reach more than 98%, with a porosity of less than 1%.

[0036] Furthermore, the axial pressure control includes staged control: in the pre-compression stage, the interfacial oxide film is broken up and the actual contact area is increased by applying pressure; in the holding pressure stage, the liquid brazing filler metal is encouraged to fully fill the interfacial gaps and Kirkendall voids are suppressed; in the solidification stage, pressure is maintained to reduce solidification shrinkage defects and improve the tightness of the brazing seam. This pressure control method can significantly reduce interfacial porosity and increase the effective area of ​​the conductive cross-section.

[0037] Furthermore, the inert protective atmosphere is nitrogen or argon, with an oxygen content controlled at <50ppm. 0.5-2% formic acid vapor or triethanolamine vapor is added to the protective atmosphere as a vapor-phase flux for in-situ reduction of the oxide film and activation of the interface. This method enables interface cleaning without introducing solid residues, improves solder wettability and spreading performance, and reduces the risk of subsequent corrosion.

[0038] Furthermore, an online monitoring system is set up during the brazing process to monitor interface temperature, current, voltage, and pressure parameters in real time. The current output is dynamically adjusted through closed-loop control to maintain the interface temperature within the target range. At the same time, the parameters of different workpieces are adaptively matched by combining the process database to realize intelligent adjustment of process parameters, thereby ensuring the consistency and stability of the mass production process.

[0039] Furthermore, the method is applied to dissimilar metal connections of copper and aluminum in wind turbine stator leads, busbars, and terminals, as well as dissimilar metal conductive connection structures in transformer bus systems, new energy vehicle battery connection structures, photovoltaic inverter conductive components, ship power systems, and rail transit power supply systems.

[0040] The beneficial effects of this invention are as follows: Precise control of the welding interface temperature is achieved through zoned resistance heating. Simultaneously, a nano-nickel barrier layer is introduced into the interface region to slow the formation and propagation of brittle intermetallic compounds. Regarding the brazing filler metal, a zinc-based composite filler metal is used to improve wetting and spreading capabilities, and external pressure is combined to enhance the density of the brazed joint. Through the above combined process, a copper-aluminum joint with low resistance, high strength, and high reliability can be obtained, thus solving problems such as low heating efficiency, high contact resistance, non-dense brazed joints, and poor batch consistency in traditional methods. This provides a low-cost and high-performance process solution for copper-aluminum connections in wind turbines. Specific advantages are as follows:

[0041] 1. Significantly improved joint strength and conductivity: Through the synergistic effect of the nano-nickel barrier layer and rapid heating, the thickness of the intermetallic compound layer is reduced from 15-25 μm in traditional methods to 3-5 μm. The tensile strength of the joint is increased from 55-60% of that of the aluminum base material to 80-85%, and the contact resistance is reduced from 8-12 μΩ·cm. 2 Reduced to 2-3 μΩ·cm 2 It meets the stringent requirements for high-current and high-reliability connections in wind turbine generators.

[0042] 2. Significantly Improved Production Efficiency: Due to the use of resistance heating for rapid temperature rise, the effective heating time in the brazing process is significantly shortened. The brazing time per piece is reduced from 30-40 minutes in traditional furnace brazing to approximately 3 minutes, significantly accelerating the overall production cycle and increasing production efficiency by more than 10 times. This is suitable for mass production and on-site assembly of large equipment such as wind turbines, thereby reducing production cycles and lowering delivery time costs.

[0043] 3. Significantly reduced energy consumption: The energy from resistance heating is applied directly to the workpiece, achieving a thermal efficiency of over 85%, which is significantly higher than the 30-40% of flame brazing and the 50-60% of furnace brazing. The energy consumption per piece is reduced from 0.15 kWh using traditional methods to 0.03 kWh, a reduction of 80%, aligning with the development direction of energy conservation and emission reduction.

[0044] 4. Small heat-affected zone and good preservation of base material properties: Due to the use of localized rapid heating, the heat input time is significantly shortened, reducing the width of the heat-affected zone from 15-20mm in traditional processes to 6-7mm. The softening degree of the aluminum side material is reduced, and the oxidation degree of the copper side is effectively controlled, allowing the original mechanical and electrical properties of the base material to be well preserved. From the perspective of overall structure, this improves the performance consistency and structural stability between the joint area and the base material area.

[0045] 5. Excellent process stability and consistency: The integrated online monitoring and intelligent control system enables precise control and adaptive adjustment of process parameters, eliminating the influence of human factors. The batch production qualification rate has increased from 75-85% using traditional methods to over 95%, with low performance dispersion (coefficient of variation <5%). This ensures the stability of product quality.

[0046] 6. Environmentally friendly with no harmful residues: The use of a vapor-phase flux system replaces traditional chloride or fluoride solid fluxes, avoiding solid flux residues in the joint area and reducing the risk of long-term corrosion. It also reduces post-soldering cleaning processes and wastewater discharge. The overall process better meets environmental protection and clean manufacturing requirements.

[0047] 7. Strong adaptability and wide range of applications: In addition to being applicable to copper-aluminum connections in wind turbines, the method of this invention can also be used in transformer bus systems, new energy vehicle battery connection structures, photovoltaic inverter conductive components, ship power systems, and rail transit power supply systems, among other application scenarios. It has good engineering applicability and promotional application value.

[0048] 8. Reasonable equipment investment and easy to promote: Resistance brazing equipment mainly consists of a power supply system, electrode clamping mechanism, and control system. Its structure is relatively simple, resulting in low equipment investment costs. Furthermore, the equipment is easy to maintain and operate, possessing good engineering feasibility, which is conducive to its promotion and application in small and medium-sized manufacturing enterprises.

[0049] 9. Excellent long-term reliability: Thermal cycling tests show that after 500 cycles from -40℃ to 80℃, the joint performance changes by less than 15%, while joints made by traditional methods show obvious cracks after 200 cycles, ensuring the design life requirement of more than 20 years for wind turbines in harsh environments.

[0050] 10. Outstanding Technological Innovation: This invention is the first to systematically integrate multiple technologies, including zoned resistance heating, nano-nickel barrier layer, zinc-based composite solder, pressure assistance, vapor phase flux, and intelligent control, forming a complete and synergistic technological chain. This overcomes the technical bottleneck of traditional methods characterized by "thick brittle layers and poor performance." It achieves a technological leap from "low performance and high cost" to "high performance and low cost," demonstrating significant innovation and advancement. Attached Figure Description

[0051] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0052] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0053] This embodiment details the specific implementation process of a copper-aluminum dissimilar metal resistance brazing welding method for wind turbine generators. Taking the connection of copper leads and aluminum terminals of a 3MW wind turbine generator stator as an application scenario, it demonstrates how the various technologies in the aforementioned invention work synergistically in actual production.

[0054] 1. Workpiece preparation and barrier layer fabrication

[0055] The workpieces to be soldered consist of copper leads (99.9% purity, 30mm × 5mm cross-section) and aluminum terminals (6061 aluminum alloy, 30mm × 8mm cross-section). First, the workpiece surfaces underwent pretreatment: acetone ultrasonic cleaning was used to remove oil stains, followed by immersion in a 5% dilute nitric acid solution for 30 seconds to remove the oxide layer on the copper parts, and immersion in a 10% sodium hydroxide solution for 20 seconds to remove the oxide film on the aluminum parts. Subsequently, the surfaces were thoroughly rinsed with deionized water and dried with nitrogen. This surface cleaning treatment created favorable conditions for the subsequent deposition of the nano-nickel barrier layer and the wetting of the solder.

[0056] Depositing a nano-nickel barrier layer on the surface of the copper leads to be soldered is a key step in achieving diffusion control. This embodiment employs a chemical nickel plating process: the cleaned copper components are immersed in a chemical nickel plating solution with the following composition: nickel sulfate 30 g / L, sodium hypophosphite 25 g / L, sodium citrate 20 g / L, and sodium acetate 15 g / L. The solution temperature is 85°C, and the pH is 4.8. During the chemical plating process, sodium hypophosphite decomposes under catalysis, releasing active hydrogen that reduces nickel ions to metallic nickel, which is then deposited on the copper surface. The plating time is controlled to be 15 minutes, resulting in a nickel layer with a thickness of approximately 1.2 μm. = 1.2 μm will play an important role in subsequent diffusion control.

[0057] After the plating was completed, the nickel layer was characterized: the surface morphology was observed using scanning electron microscopy, and the nickel layer surface was smooth and dense, without obvious pores and cracks; the crystal structure was analyzed by X-ray diffraction, and the diffraction peaks were significantly broadened. According to the Scherrer formula, the grain size was calculated to be approximately 35 nm. = 35nm, consistent with nanocrystalline characteristics; hardness was tested using a microhardness tester, the nickel layer hardness was HV520, higher than the conventional nickel plating layer HV400, indicating the strengthening effect of the nanocrystalline structure. These parameters ensure that the barrier layer can effectively suppress copper-aluminum diffusion.

[0058] 2. Brazing filler metal preparation and assembly

[0059] The brazing filler metal used in this embodiment has a composition of Zn-6.5Al-3Cu-1Mg (mass percentage), which is within the optimal range of Zn (85-90%)-Al (5-8%)-Cu (2-4%)-Mg (0.5-1.5%) described in the invention. The brazing filler metal is prepared using vacuum induction melting: high-purity zinc (99.99%), aluminum (99.9%), copper (99.9%), and magnesium (99.8%) are loaded into a graphite crucible according to the specified ratio, and the mixture is melted under a vacuum of 10... -5 The alloy is heated to 650°C in an induction furnace and held for 10 minutes to ensure thorough and uniform mixing of the alloying elements. Then, it is cast into an ingot. After homogenization annealing (holding at 350°C for 4 hours), the ingot is rolled into a 0.2 mm thick brazing foil.

[0060] The melting characteristics of the solder were determined using differential scanning calorimetry (DSC): solidus temperature 382℃, liquidus temperature 415℃, and melting temperature range 33℃. This temperature range is consistent with the liquidus temperature of 380-420℃ described in the invention, ensuring that the solder can fully melt at the subsequent interface temperature of 580-620℃. The wetting properties of the solder were tested using the droplet method: on a copper substrate, the wetting angle of the solder at 450℃ was 12°, and on an aluminum substrate, it was 18°, both less than 20°, indicating good wettability and laying the foundation for subsequent rapid spread.

[0061] Cut the brazing foil into 30mm x 30mm squares and clamp them between the copper leads and the aluminum terminals. To ensure accurate brazing placement, pre-weld positioning points onto the surface of the aluminum terminals. After assembly, place the workpiece into the fixture of the resistance brazing equipment, adjust the electrode positions to press the copper and aluminum parts together, forming a dual-electrode system with zoned power supply, preparing for subsequent precise heating.

[0062] 3. Resistance heating brazing process

[0063] The resistance brazing equipment is started, and heating is performed according to the preset process curve. This is the starting point of the entire process chain. The process parameters used in this embodiment are as follows:

[0064] Current parameter setting: Set the copper-side electrode current according to the current ratio formula in the invention. =2800A, aluminum side electrode current = 5600A. Verify current ratio:

[0065]

[0066] According to the formula in the invention description:

[0067]

[0068] Substitution = 2.0, so the correction factor can be obtained. = 2.0 / 1.67 = 1.2, which is within the reasonable range of 1.2-1.4. This value was determined through finite element simulation and experimental verification based on the workpiece geometry (5mm thickness for copper parts and 8mm thickness for aluminum parts). This current ratio ensures the uniformity of the interface temperature, creating suitable conditions for the barrier layer and solder to function effectively.

[0069] Heating curve execution: A three-stage heating strategy is adopted, consistent with the rapid heating concept in the invention: In the first stage (0-8 seconds), the current linearly increases from 0 to the set value, with a heating rate of approximately 60℃ / s, and the interface temperature rises from room temperature to 480℃; in the second stage (8-18 seconds), a constant current is maintained, and the interface temperature stabilizes at 600±5℃. At this point, the solder fully melts and spreads, and the nano-nickel barrier layer acts as a diffusion barrier; in the third stage (18-22 seconds), the current linearly decreases to 0, the interface temperature drops below the solidus line of the solder, and the solder solidifies. The total heating time is 22 seconds, meeting the rapid heating requirement of 15-25 seconds described in the invention.

[0070] Temperature monitoring and control: An infrared thermometer monitors the interface temperature in real time. Sampling data shows that during the second-stage heat preservation process, the interface temperature fluctuates between 595-605℃, with a fluctuation range of ±5℃, meeting the ±8℃ uniformity requirement of the invention. The temperature on the copper side at 3mm from the interface is 420℃, and the temperature on the aluminum side is 450℃. The temperatures at 5mm from the interface drop to 380℃ and 410℃ respectively. The width of the heat-affected zone is approximately 6-7mm, significantly smaller than the 15-20mm of traditional flame brazing, verifying the accuracy of zoned heating.

[0071] Pressure control and coordination: Pressure is applied at the start of heating and coordinated with the heating process. The pressure curve is as follows: 0-5 seconds, the pressure rises from 0 to 1.2 MPa (pre-compression stage). At this time, the temperature has not yet reached the melting point of the solder, and the pressure promotes oxide film breakage and surface plastic deformation; 5-20 seconds, a constant pressure of 1.2 MPa is maintained (holding pressure stage). At this time, the solder has melted, and the pressure drives the liquid solder to fully fill the interface, while inhibiting the formation of Kirkendall voids, which complements the effect of the nano-nickel barrier layer in reducing the diffusion rate; 20-25 seconds, the pressure linearly drops to 0 (decompression stage), and the solder solidification is complete. Displacement sensor monitoring data shows that during the pre-compression stage, the workpiece gap is compressed from the initial 0.25 mm to 0.18 mm, indicating that plastic deformation has occurred in the surface micro-protrusions; during the holding pressure stage, the gap is further reduced to 0.15 mm, and the liquid solder fully fills the interface.

[0072] Atmosphere protection implementation: The brazing chamber is evacuated to 100 Pa before heating, and then filled with high-purity nitrogen to atmospheric pressure. The oxygen content monitoring shows 35 ppm, which meets the requirement of <50 ppm in the invention. 1.2% formic acid vapor is added to the nitrogen as a vapor-phase flux. Formic acid decomposes at high temperature to produce CO and H2, which reduces the oxide film and activates the surface. Combined with the pre-pressure to break up the oxide film, the interface is further cleaned to ensure good wetting of the brazing filler metal.

[0073] 4. Microstructure and properties of brazed joints

[0074] After brazing is completed, the joint is subjected to systematic microstructure analysis and performance testing to verify the synergistic effect of the aforementioned technologies.

[0075] Metallographic analysis: The joint was cut perpendicular to the interface, then inlaid, ground, and polished. It was then etched with Keller's reagent, and the interface microstructure was observed under an optical microscope and a scanning electron microscope. Results showed:

[0076] A nickel barrier layer with a thickness of approximately 1.1 μm exists at the copper side interface (slightly thinner than the 1.2 μm thickness during plating, due to some nickel diffusion into the copper substrate and brazing seam at high temperatures). The nickel layer is tightly bonded to the copper substrate without peeling or cracking, verifying the stability of the barrier layer.

[0077] A transition layer approximately 2.5 μm thick was formed between the nickel layer and the brazing seam. Energy dispersive spectroscopy (EDS) analysis showed that this layer mainly consisted of Ni-Al solid solution and a small amount of Ni2Al3 intermetallic compound. Although Ni2Al3 is also an intermetallic compound, its brittleness is much lower than that of CuAl2, and its thickness is very thin, thus having little impact on the joint performance. Based on the diffusion coefficient formula in the invention description, substituting... = 1.1μm, = 35nm, γ = 3 (experimental fitted value), the effective diffusion coefficient can be estimated:

[0078]

[0079] That is, the effective diffusion coefficient is reduced to about 1 / 95 of the original, which is consistent with the order of magnitude of 1 / 100-1 / 200 described in the invention, verifying the diffusion control effect of the nano-nickel barrier layer.

[0080] The brazed seam region is approximately 150 μm thick, with a dense and uniform microstructure, mainly composed of α-Al solid solution and Zn-Al eutectic structure. The grains are fine (average grain size 15 μm), and no obvious pores or inclusions were found. The brazed seam density reaches 98.5%, verifying the good wettability of the brazing filler metal and the pressure-assisted densification effect. A small amount of fine Cu-Zn intermetallic compound particles (1-3 μm in size) are distributed within the brazed seam, playing a role in dispersion strengthening. The aluminum-side interface transition layer is approximately 1.8 μm thick, mainly composed of Al-Zn solid solution, exhibiting good metallurgical bonding with the aluminum matrix. The total thickness of the intermetallic compound layer at the entire interface (2.5 μm nickel transition layer + 1.8 μm aluminum-side transition layer) is approximately 4.3 μm, which is less than the 5 μm control target described in the invention. Comparative experiment: In the control sample without a nickel barrier layer, under the same brazing process, a CuAl2 brittle layer with a thickness of up to 18 μm was formed at the copper-aluminum interface, and a large number of microcracks existed within this layer. This fully demonstrates the significant effect of the nano-nickel barrier layer on inhibiting the growth of brittle phases and verifies the effectiveness of the "time-space" dual inhibition mechanism in the invention.

[0081] Mechanical property testing: Tensile specimens were prepared according to GB / T228 standard and subjected to room temperature tensile tests on a universal testing machine. Test results: The tensile strength of the joint was 185 MPa, while the tensile strength of the 6061 aluminum alloy base material was 220 MPa. The joint strength reached 84% of the base material strength, exceeding the 80% target stated in the invention. The fracture location was in the heat-affected zone on the aluminum side, and the fracture morphology was dimple-like, indicating ductile fracture, demonstrating good interfacial bonding and effective control of the brittle phase.

[0082] The tensile strength of the control sample (without nickel barrier layer) was only 128 MPa, which is 58% of that of the base material. The fracture location was in the CuAl2 brittle layer, and the fracture surface showed brittle cleavage characteristics, which verified the necessity of barrier layer technology.

[0083] Electrical performance testing: The contact resistance of the connector was tested using the four-probe method. A 100A DC current was applied across the connector, and the voltage drop across the connector area (10mm in length) was measured using a microvoltmeter. The resistance was calculated using Ohm's law and then converted to contact resistance per unit area. Test result: Contact resistance was 2.6μΩ·cm. 2 It meets the requirement of <3μΩ·cm as described in the invention. 2 It also meets the requirements for high-current connection of wind turbines (typically <5μΩ·cm). 2 ).

[0084] The contact resistance of the control sample was 9.8 μΩ·cm. 2 The resistance is 3.8 times that of the method of the present invention. The main reason is that the thick CuAl2 brittle layer and interface pores lead to a reduction in the conductive cross section and an increase in resistivity, which verifies the significant advantage of the method of the present invention in reducing contact resistance.

[0085] Reliability Testing: Thermal cycling tests were conducted to simulate the actual operating environment of a wind turbine. Test conditions: -40℃ for 30 minutes, then heated to 80℃ and held for 30 minutes, constituting one cycle. After 500 thermal cycles, the tensile strength of the joint was 178 MPa, a decrease of 3.8% compared to the initial value; the contact resistance was 2.9 μΩ·cm. 2 The value increased by 11.5% compared to the initial value. The small change in performance indicates that the joint has good thermal cycling stability, verifying the reliability of the entire process chain.

[0086] The control sample developed obvious cracks after 200 thermal cycles, with its tensile strength decreasing to 85 MPa and its contact resistance increasing to 18 μΩ·cm. 2 This method no longer meets the usage requirements, further demonstrating the superiority of the method of the present invention.

[0087] 5. Process parameter optimization and batch production verification

[0088] To verify the stability and repeatability of the process, as well as the effectiveness of the intelligent control system, a batch production test was conducted. Under the same process parameters, 100 copper-aluminum brazed joints were continuously produced. Each product underwent visual inspection, ultrasonic testing, and sampling mechanical testing. Visual quality: The brazed seam surfaces of all 100 products were smooth and flat, with no obvious porosity, cracks, or incomplete penetration. The visual pass rate was 100%, verifying the effectiveness of the atmosphere protection and pressure-assisted brazing.

[0089] Ultrasonic testing: C-scan mode was used to perform non-destructive testing on the joints, with the testing standard being interface defect area <5%. The results showed that 98 products were fully qualified, and 2 products had small-area (2-3%) incomplete penetration defects, with a pass rate of 98%, exceeding the 95% target stated in the invention, thus verifying the role of the intelligent control system in ensuring process consistency.

[0090] Mechanical properties: Twenty randomly selected products were subjected to tensile testing. The tensile strength ranged from 180 to 192 MPa, with an average value of 186 MPa, a standard deviation of 3.2 MPa, and a coefficient of variation of 1.7%. The contact resistance ranged from 2.4 to 2.9 μΩ·cm. 2 The average value is 2.65 μΩ·cm. 2 Standard deviation 0.15 μΩ·cm 2 The coefficient of variation was 5.7%. The low performance dispersion indicates good process stability and verifies the effectiveness of the online monitoring and intelligent control system.

[0091] Production efficiency: The brazing time per piece (including clamping, heating, cooling, and unloading) is approximately 3 minutes, which is more than 10 times more efficient than the 30-40 minutes of traditional furnace brazing, consistent with the expectations of the invention. The equipment power is 50kW, and the energy consumption per piece is approximately 0.03kWh, which is 80% lower than the 0.15kWh of furnace brazing, exceeding the 60% target stated in the invention.

[0092] This detailed description of the embodiment verifies the effectiveness of the complete process chain proposed in the invention, namely "precise heating to establish a temperature field → barrier layer to control diffusion → brazing filler metal wetting and filling → pressure densification → atmosphere protection → intelligent monitoring," and the synergistic effect of each link. Those skilled in the art can clearly understand the technical solution of this invention and can implement it in actual production according to the described methods and process parameters to obtain high-quality copper-aluminum brazed joints.

[0093] The technical means disclosed in this invention are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention.

Claims

1. A method for resistance brazing of dissimilar metals (copper and aluminum) in wind turbine generators, characterized in that, Includes the following steps: S1. Temperature field control of zoned resistance heating: The zoned resistance heating method with independent power supply of dual electrodes is adopted. The current is applied to the copper side and the aluminum side separately and controlled independently to keep the brazing interface temperature within the range of 580-620℃. S2. Interface diffusion control of nano-nickel barrier layer: A nanocrystalline nickel barrier layer is pre-prepared on the copper substrate surface to be joined. S3. Wetting and filling of zinc-based composite brazing filler metal: Brazing is performed using zinc-based composite brazing filler metal; S4. Pressure-assisted brazing densification: Applying axial pressure in the range of 0.5-2MPa during brazing; S5. Atmosphere protection and surface activation: The brazing process is carried out under an inert protective atmosphere.

2. The method according to claim 1, characterized in that, In the partitioned resistance heating, the copper-side current and the aluminum-side current satisfy a preset ratio, wherein the aluminum-side current is greater than the copper-side current, and the ratio ranges from 1.8 to 2.2 times, to compensate for the difference in resistivity and thermal conductivity between copper and aluminum materials, thereby achieving uniform control of the interface temperature; specifically, Assume the current flowing through the copper side electrode is The current flowing through the aluminum side electrode is The heat generation capacities on both sides are as follows: in For copper-side current, For aluminum side current, and These are the equivalent resistances of the copper-side and aluminum-side workpieces, respectively. and The heat generation power on the copper side and the aluminum side are respectively; determine the optimal current ratio: in and The resistivity of copper and aluminum are respectively. and The thermal conductivity of copper and aluminum are respectively. This is a correction factor, with a value range of 1.2-1.4; The heating process uses a pulsed DC power supply, with a heating rate of 50-80℃ / s and a total heating time of 15-25 seconds.

3. The method according to claim 1, characterized in that, The nanocrystalline nickel barrier layer has a thickness of 500 nm-2 μm and a grain size of 20-50 nm; specifically, the inhibitory effect of the nickel barrier layer on copper atom diffusion is characterized by the effective diffusion coefficient. in The intrinsic diffusion coefficient of copper in aluminum without a barrier layer. The effective diffusion coefficient when a barrier layer is present. For the thickness of the nickel layer, For the nickel layer grain size, This is a dimensionless coefficient related to grain boundary diffusion characteristics, with typical values ​​of 2-4.

4. The method according to claim 1, characterized in that, The zinc-based composite brazing filler metal is a Zn-Al-Cu-Mg quaternary alloy system, wherein the mass percentage of each element is: Zn 85-90%, Al 5-8%, Cu 2-4%, and Mg 0.5-1.5%. Specifically, the growth of the brazing filler metal's spreading area over time is described by a kinetic equation: in Let be the area spread out at time t. To achieve the final balanced spread area, t s This is the spreading time constant, reflecting the spreading rate.

5. The method according to claim 1, characterized in that, The control of axial pressure includes staged control: in the pre-pressure stage, the interface oxide film is broken by pressure and the actual contact area is increased; in the holding pressure stage, the liquid brazing filler metal is promoted to fully fill the interface gap and suppress the formation of Kirkendall voids; in the solidification stage, the pressure is maintained to reduce solidification shrinkage defects and improve the tightness of the brazing seam.

6. The method according to claim 1, characterized in that, The inert protective atmosphere is nitrogen or argon, with an oxygen content controlled at <50ppm. 0.5-2% formic acid vapor or triethanolamine vapor is added to the protective atmosphere as a vapor-phase flux for in-situ reduction of the oxide film and activation of the interface.

7. The method according to claim 1, characterized in that, An online monitoring system is set up during the brazing process to monitor the interface temperature, current, voltage and pressure parameters in real time, and dynamically adjust the current output through closed-loop control to maintain the interface temperature stable within the target range; at the same time, it is combined with the process database to adaptively match the parameters of different workpieces.

8. The method according to any one of claims 1-7, characterized in that, The method is applied to dissimilar metal connections of copper and aluminum in wind turbine stator leads, busbars, and terminals, as well as dissimilar metal conductive connection structures in transformer bus systems, new energy vehicle battery connection structures, photovoltaic inverter conductive components, ship power systems, and rail transit power supply systems.

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