Method for preventing deformation of light alloy complex structure in welding process

Abstract

The invention discloses a method for preventing deformation of a light alloy complex structure in a welding process, which comprises the following steps: ball-milling and mixing HfO2 and VO2 nanoparticles, and then putting the mixture in a heating furnace to synthesize phase-pure HfV2O7 nanoparticles; the surfaces of the particles are subjected to nickel plating, so that the compatibility between the HfV2O7 nanoparticles and a brazing filler metal matrix is improved, and uniform dispersion of the particles in the brazing filler metal is promoted; the nano particles, polyvinylpyrrolidone and absolute ethyl alcohol are combined to be prepared into paste; determining the volume fraction of the nanoparticles on each layer of brazing filler metal foil through finite element simulation; the fins, the sealing strips and the partition plates are fully cleaned, the brazing filler metal foil is coated with the paste through a test net brush, the installed plate-fin heat exchanger is placed in a vacuum brazing furnace to be heated, and the brazing filler metal base material and the brazing filler metal foil are tightly combined; and finally, the brazing residual stress is reduced through aging treatment. According to the invention, the nanoparticles HfV2O7 are added into the brazing filler metal for welding the heat exchanger, so that the deformation of the plate-fin heat exchanger in the welding process is effectively reduced.

Description

Technical Field

[0001] The invention relates to a method for preventing deformation during welding of a light alloy complex structure, and belongs to the field of preparation of aluminum-based alloy-ceramic composite structures. Background Art

[0002] Plate-fin heat exchangers are typically constructed from multiple layers of metal plates and fins connected by brazing. These large components are prone to residual stress during the high-temperature brazing process due to mismatched thermal expansion coefficients. The different thermal expansion coefficients of aluminum and the brazing alloy lead to inconsistent expansion and contraction during heating and cooling, resulting in shear stress at the interface. This stress can cause deformation, microcracks, and even joint failure, compromising the seal and durability of the heat exchanger. During the welding process, the difference in thermal expansion coefficients between the different materials generates thermal stress, leading to structural deformation, reduced sealing performance, impeded fluid flow within the heat exchanger, and reduced heat transfer efficiency. During welding, heat is concentrated in the weld area, while the surrounding material is cooler, creating a temperature gradient. The temperature difference between the edge and center of the fin can reach 50°C to 100°C. Aluminum's high thermal conductivity exacerbates this temperature gradient. This uneven temperature distribution leads to inconsistent material contraction during cooling, further exacerbating residual stress. Stress is particularly concentrated at the joints between the fins and the bulkheads due to the difference in structural stiffness, increasing the risk of deformation and cracking. In order to improve the reliability of heat exchangers, extend their service life and reduce maintenance costs, it is crucial to prevent the deformation of heat exchangers.

[0003] Traditional solutions to this problem typically involve structural design optimization, process adjustments, and mechanical constraints. However, these solutions cannot meet the device's high precision and high performance requirements. Therefore, we decided to use HfV2O7 nanoparticles to address this issue by leveraging the material's intrinsic properties, providing superior performance.

[0004] HfV2O7 is a material with negative thermal expansion properties, and its thermal expansion coefficient is -6.7×10 -6 ℃ -1, exhibiting isotropic contraction behavior within the temperature range of 130-700°C. This property allows it to effectively offset the thermal expansion of conventional materials, thereby reducing thermal stress caused by temperature changes. Plate-fin heat exchangers are typically brazed using aluminum-based filler metals. The difference in thermal expansion coefficients between the filler metal and the plate during the brazing process can easily lead to deformation and cracking of the plate-fin heat exchanger. Adding HfV2O7 nanoparticles to the brazing filler metal leverages its isotropic contraction behavior to compensate for the positive expansion of the aluminum-based filler metal during heating, significantly reducing thermal mismatch stress. Furthermore, HfV2O7 exhibits a high elastic modulus and nanoscale effects. These properties contribute to orovan strengthening and grain refinement, improving the shear strength and creep resistance of the brazed joint. Using finite element software, the effects of varying nanoparticle volume fractions on thermal stress and deformation in the heat exchanger core were simulated before welding. This approach ensured CTE compensation while preventing uneven paste application and ensuring proper brazing filler metal flow. The synthesis temperature of HfV2O7 nanoparticles is around 600°C, which is close to the melting temperature of aluminum-based brazing. With the help of ball milling technology, the two materials can be uniformly composited, reducing costs while avoiding the complexity of traditional alloying and structural optimization.

[0005] By utilizing the negative thermal expansion characteristics of HfV2O7 nanoparticles and the nano-strengthening effect they bring to the solder, the traditional method is unable to simultaneously meet the device's requirements for high precision and high efficiency, providing a new solution to prevent deformation of large heat exchangers during the welding process. Summary of the Invention

[0006] The technical problem solved by the present invention is: during the welding process of a plate-fin heat exchanger, due to the different thermal expansion coefficients between the brazing material and the base material, interfacial shear stress is generated during the cooling process, resulting in local warping and deformation of the fins, and due to the thermal stress between the brazing material and the partition, the partition expands or contracts unevenly and generates internal stress. A method for preventing deformation of the heat exchanger by adding nanoparticles with a negative thermal expansion coefficient is provided.

[0007] The technical solutions for achieving the purpose of the present invention are:

[0008] A method for preventing deformation during welding of complex lightweight alloy structures.

[0009] Before brazing the heat exchanger core, a paste containing HfV2O7 nanoparticles is applied to the brazing foil, leveraging HfV2O7's negative thermal expansion properties to reduce thermal stress. Finite element simulations were used to determine the volume fraction of the nanoparticles between different layers, preventing overcompensation and shrinkage stress, maintaining structural stability, and reducing costs.

[0010] The device needs to be heated at three different temperatures. The first heating temperature will fully decompose the anhydrous ethanol in the paste to prevent the residual solvent from vaporizing at high temperature and causing pores; the second heating temperature will allow the adhesive pvp to completely decompose without residue; the third heating will allow the solder to fully melt; the nickel coated on the nanoparticles will increase the fluidity of the particles in the molten solder, helping the particles to be evenly dispersed in the solder.

[0011] Compared with the prior art, the present invention has the following significant advantages:

[0012] 1. Adding HfV2O7 nanoparticles to aluminum-based solder. Since HfV2O7 has stable negative thermal expansion in the range of 130℃ to 700℃, it can offset the thermal expansion of aluminum-based solder, reduce the thermal stress caused by temperature gradient during welding, and reduce the deformation risk of heat exchanger during heating;

[0013] 2. Adding HfV2O7 nanoparticles can reduce the compressive stress of the polymer matrix, relieve the residual stress of the aluminum brazing joint, and achieve stress reduction;

[0014] 3. Nanoparticles act as heterogeneous nucleation points to refine brazing seam grains, reduce local stress concentration caused by coarse grains, inhibit microcrack initiation, and achieve fine grain strengthening;

[0015] 4. HfV2O7 nanoparticles have a high elastic modulus of 78GPa, which is significantly higher than that of the aluminum matrix. Through the composite effect, they can improve the tensile strength and creep resistance of the brazing seam;

[0016] 5. HfV2O7 nanoparticles will not undergo phase change reaction below 700℃. The melting temperature of aluminum-based brazing alloy is between 500℃ and 620℃, which is suitable for aluminum brazing temperature. It has high reliability and ensures brazing quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Flowchart of the method for preventing deformation during welding of plate-fin heat exchangers

[0018] Figure 2 This is the structure diagram of the plate-fin heat exchanger

[0019] Figure 3 This is the single-layer connection diagram of the heat exchanger core

[0020] Figure 4 This is a finite element simulation diagram of the heat exchanger core heating process

[0021] Figure 5 This is the uniform distribution of nanoparticles in the solder

[0022] Figure 6 Temperature heating process diagram DETAILED DESCRIPTION

[0023] The present invention is further described in detail below with reference to the accompanying drawings.

[0024] This invention provides a method for preventing deformation during the welding process of complex lightweight alloy structures. The heat exchanger core is composed of alternating fins and plates connected by brazing filler metal. Finite element simulation is used to screen the optimal volume fraction of nanoparticles covering each layer of brazing filler metal foil. Before brazing, a paste containing HfV2O7 nanoparticles is applied to the brazing filler metal foil using a test mesh brush to varying thicknesses. This method can reduce thermal stress during brazing. The method includes the following steps:

[0025] Step 1, ball milling HfO2 and VO2 nanoparticles;

[0026] Step 2, nickel plating the surface of the synthesized particles;

[0027] Step 3, combining HfV2O7 nanoparticles with polyvinyl pyrrolidone and anhydrous ethanol to prepare a paste;

[0028] Step 4: Use Simufact Welding 2024R2 software to simulate the brazing process of the heat exchanger module and determine the volume fraction of the nanoparticles applied to each layer of brazing foil;

[0029] Step 5: Thoroughly clean the fins and partitions, apply the prepared paste on the aluminum-silicon-magnesium brazing foil, and assemble;

[0030] Step 6: Place the installed plate-fin heat exchanger into a vacuum brazing furnace for three-stage heating;

[0031] Step 7: After completion, solution aging treatment is performed.

[0032] The present invention provides a specific solution as follows: HfO2 and VO2 nanoparticles are ball-milled at a speed of 1060 times / min for 30 minutes to generate a density of about 4.87 g / cm 3 , pure phase HfV2O7 with a size of 25.4±4.4nm.

[0033] The present invention provides a specific solution as follows: HfV2O7 nanoparticles are placed in a nickel sulfate, sodium hypophosphite, and sodium citrate solution and mechanically stirred at a speed of 200 rpm to 300 rpm for 30 minutes to obtain nanoparticles with a nickel layer with a thickness of 10 nm to 20 nm coated on the surface.

[0034] The present invention provides a specific solution as follows: polyvinyl pyrrolidone accounting for 5wt% to 10wt% is used as an adhesive, anhydrous ethanol accounting for 60wt% to 80wt% is used as a solvent, and nanoparticles are mixed to prepare a paste.

[0035] The present invention provides a specific solution as follows: HfV2O7 nanoparticles, polyvinyl pyrrolidone and anhydrous ethanol are added to a container in proportion, mechanically stirred at a speed of 500 rpm for 10 minutes, and then heated at a power density of 10 W / cm 3 , ultrasonic treatment time 30 minutes.

[0036] The present invention provides a specific solution as follows: perform equivalent stress field analysis on a three-dimensional finite element model using finite element software, including

[0037] The present invention provides a specific solution as follows: based on a three-dimensional finite element model, the target heat exchanger core is divided into discrete small units, and the specific information of each structural node in the small unit is obtained. The stress vector of any node of any small unit is obtained as follows:

[0038] Wherein, σ represents the stress vector of the node, E0 represents the Young's modulus of the node material, V0 represents the Poisson's ratio of the node material, B represents the deformation matrix of the small unit where the node is located, u represents the node displacement vector, and represents the dot product.

[0039] The present invention provides a specific solution as follows: a test mesh brush is used to apply a layer of paste with a thickness of 0.084 mm to 0.166 mm on the solder foil, and the volume proportion of HfV2O7 nanoparticles in the solder foil is controlled to be 14 vol% to 16.6 vol% in the 1st to 6th layers of the heat exchanger core, 11 vol% to 13 vol% in the 7th to 11th layers, 8.5 vol% to 10 vol% in the 12th to 18th layers, 11 vol% to 13 vol% in the 19th to 23rd layers, and 14 vol% to 16.6 vol% in the 24th to 30th layers.

[0040] The present invention provides a specific solution as follows: first, the heat exchanger is placed in a furnace at 80°C to 100°C and heated for 30 minutes to remove anhydrous ethanol; then, the device is placed in a vacuum furnace and heated to 300°C to 400°C and kept at this temperature for 1 hour to completely decompose the PVP; finally, the vacuum furnace is heated to 600°C to 700°C for high-temperature brazing and kept at this temperature for 1 hour.

[0041] The present invention provides a specific solution as follows: the temperature of the post-weld aging treatment is 150° C. to 260° C., and the holding time is 2.5 hours to 4 hours.

[0042] Combine Figure 1 、 Figure 2 、 Figure 3 、 Figure 4 、 Figure 6, a method to prevent deformation during the welding process of complex structures of lightweight alloys. First, HfO2 and VO2 nanoparticles are ball-milled and mixed. This is because ball milling can achieve uniform dispersion and close contact of HfO2 and VO2 nanoparticles, reduce the temperature and duration of subsequent solid-state reactions, and ensure the generation of pure phase HfV2O7. The polished particles are sintered in a heating furnace at a temperature of 600°C for 30 minutes, because 600°C is just enough for the reaction to complete, while avoiding material decomposition or phase change side reactions that may occur due to excessive temperature. In terms of time, 30 minutes is enough for the reaction to reach equilibrium without causing excessive particle growth or structural damage. A layer of nickel is plated on the surface of the synthesized HfV2O7 nanoparticles. This measure can improve the compatibility between the HfV2O7 nanoparticles and the solder matrix and promote the uniform dispersion of the particles in the solder. Nanoparticles, polyvinyl pyrrolidone binder, and anhydrous ethanol solvent were added to a container in appropriate proportions. Mechanical stirring was performed at 500 rpm for 10 minutes to achieve initial dispersion. Ultrasonic treatment was then used to break up agglomerates and evenly distribute the nanoparticles throughout the paste. Finite element simulation software was used to simulate the brazing process of a heat exchanger after adding the nanoparticles. The volume fraction of the nanoparticles used in each layer was controlled to avoid overcompensation leading to shrinkage stress, reduce the risk of shear stress, and maintain structural stability. A layer of HfV2O7 nanoparticles with a thickness of 0.084 mm to 0.166 mm was applied to the solder foil using a test mesh brush, with the volume fraction of the HfV2O7 nanoparticles within the solder foil controlled to be between 8.4 vol% and 16.6 vol%. The cleaned device was placed in a furnace using a three-step heating regime to eliminate any residual volatile products, such as CO2 and H2O, generated by the reagents in the paste, preventing residual products from vaporizing at high temperatures and causing porosity. The final temperature of the three-stage heating process is 600°C to 700°C. At this temperature, HfV2O7 maintains its cubic phase structure and stable negative thermal expansion properties, making it suitable for high-temperature brazing environments. Simultaneously, the aluminum-based brazing filler metal fully melts, exhibiting high fluidity and filling the joint gap, resulting in a uniform brazing seam. A post-weld aging treatment further offsets the positive expansion of the substrate and brazing filler metal, reducing interfacial stress and minimized heat exchanger deformation. This also prevents structural changes in the nanoparticles caused by excessive temperatures.

[0043] The following describes in detail a method for preventing deformation during welding of a light alloy complex structure according to the present invention using an embodiment.

[0044] Example 1

[0045] Combine Figure 1Flow chart of the method for preventing deformation during the welding process of plate-fin heat exchangers, using an aluminum alloy plate with a thickness of 30 layers and a size of 90cm×50cm as an example. HfO2 and VO2 nanoparticles are ball-milled at a speed of 1060 times / min for 30 minutes to generate HfV2O7 with a size of 28.4nm, which is then placed in a nickel sulfate, sodium hypophosphite, and sodium citrate solution and mechanically stirred at a speed of 260rpm for 30 minutes to coat the surface of the particles with a layer of nickel with a thickness of 11nm. Nanoparticles, polyvinyl pyrrolidone binder, and anhydrous ethanol solvent are added to the container in a ratio of 20wt%, 6wt%, and 74wt%, respectively, and mechanically stirred at a speed of 500rpm for 10 minutes, and then subjected to a power density of 10W / cm 3 The resulting paste was then ultrasonically treated for 30 minutes and applied to the brazing foil using a 200-mesh brush. The thickness of layers 1-6 was 0.14 mm, layers 7-11 were 0.12 mm, layers 12-18 were 0.09 mm, layers 19-23 were 0.12 mm, and layers 24-30 were 0.14 mm. The corresponding volume percentage of HfV2O7 nanoparticles in the brazing foil was 14.6 vol% in layers 1-6 of the heat exchanger core, 12.5 vol% in layers 7-11, 9.3 vol% in layers 12-18, 12.5 vol% in layers 19-23, and 14.6 vol% in layers 24-30.

[0046] The device is placed in a brazing furnace and first heated to 80°C for 30 minutes, then to 350°C for 1 hour, and finally to 670°C for 1 hour. After heating, the device is aged at 200°C for 2.5 hours.

Claims

1. A method for preventing deformation during welding of complex lightweight alloy structures, characterized in that: The heat exchanger core is composed of an alternating combination of fins and plates connected by brazing filler metal. Finite element simulation is used to screen the optimal volume fraction of nanoparticles covering each layer of brazing filler metal foil. Before brazing, a paste containing HfV2O7 nanoparticles is applied to the brazing filler metal foil with a test mesh brush to reduce thermal stress during the brazing process.

2. A method for preventing deformation during welding of complex lightweight alloy structures, characterized in that: The method comprises the following steps: Step 1, ball milling HfO2 and VO2 nanoparticles; Step 2, nickel plating the surface of the synthesized particles; Step 3, combining HfV2O7 nanoparticles with polyvinyl pyrrolidone and anhydrous ethanol to prepare a paste; Step 4: Use Simufact Welding 2024R2 software to simulate the brazing process of the heat exchanger module and determine the volume fraction of the nanoparticles applied to each layer of brazing foil; Step 5: Thoroughly clean the fins and partitions, apply the prepared paste on the aluminum-silicon-magnesium brazing foil, and assemble; Step 6: Place the installed plate-fin heat exchanger into a vacuum brazing furnace for three-stage heating; Step 7: After completion, solution aging treatment is performed.

3. The method for preventing deformation during welding of a light alloy complex structure according to claim 2, characterized in that: The HfO2 and VO2 nanoparticles were ball-milled at a speed of 1060 times / min for 30 minutes to produce a density of about 4.87 g / cm 3 , pure phase HfV2O7 with a size of 25.4±4.4nm.

4. The method for preventing deformation during welding of a light alloy complex structure according to claim 2, characterized in that: The HfV2O7 nanoparticles are placed in a nickel sulfate, sodium hypophosphite, and sodium citrate solution and mechanically stirred at a speed of 200 rpm to 300 rpm for 30 minutes to obtain nanoparticles with a nickel layer with a thickness of 10 nm to 20 nm coated on the surface.

5. The method for preventing deformation during welding of a light alloy complex structure according to claim 2, characterized in that: The paste is prepared by mixing 5wt% to 10wt% of polyvinyl pyrrolidone as an adhesive, 60wt% to 80wt% of anhydrous ethanol as a solvent and nanoparticles.

6. The method for preventing deformation during welding of a light alloy complex structure according to claim 2, characterized in that: HfV2O7 nanoparticles, polyvinyl pyrrolidone and anhydrous ethanol were added to the container in proportion, and mechanical stirring was performed at a speed of 500 rpm for 10 minutes. 3 , ultrasonic treatment time 30 minutes.

7. The method for preventing deformation during welding of a light alloy complex structure according to claim 2, characterized in that: The equivalent stress field analysis of the three-dimensional finite element model is performed using finite element software, including Based on the three-dimensional finite element model, the target heat exchanger core is divided into discrete small units, and the specific information of each structural node in the small unit is obtained. The stress vector of any node of any small unit is obtained as follows: Bu; wherein, σ represents the stress vector of the node, E0 represents the Young's modulus of the node material, V0 represents the Poisson's ratio of the node material, B represents the deformation matrix of the small unit where the node is located, u represents the node displacement vector, and represents the dot product.

8. The method for preventing deformation during welding of a light alloy complex structure according to claim 2, characterized in that: A test mesh brush is used to apply a layer of paste with a thickness of 0.084 mm to 0.166 mm on the solder foil, and the volume proportion of HfV2O7 nanoparticles in the solder foil is controlled to be 14 vol% to 16.6 vol% in the 1st to 6th layers of the heat exchanger core, 11 vol% to 13 vol% in the 7th to 11th layers, 8.5 vol% to 10 vol% in the 12th to 18th layers, 11 vol% to 13 vol% in the 19th to 23rd layers, and 14 vol% to 16.6 vol% in the 24th to 30th layers.

9. The method for preventing deformation during welding of a light alloy complex structure according to claim 2, characterized in that: First, heat the heat exchanger in a furnace at 80℃~100℃ and keep it warm for 30 minutes to remove the anhydrous ethanol; then place the device in a vacuum furnace and heat it to 300℃~400℃ and keep it warm for 1 hour to completely decompose the PVP; finally, heat the vacuum furnace to 600℃~700℃ for high-temperature brazing and keep it warm for 1 hour.

10. The method for preventing deformation during welding of a light alloy complex structure according to claim 2, characterized in that: The temperature of post-weld aging treatment is between 150℃ and 260℃, and the holding time is 2.5h to 4h.

Images