A gradient variable parameter foil rolling and annealing process for high specific capacity cathode foils

Abstract

The application discloses a gradient variable parameter foil rolling and annealing process for high specific capacity cathode foil, comprising the following steps: S1, blank pretreatment: selecting high-purity aluminum cathode foil blank, carrying out surface oil removal, water washing and drying treatment on the blank, removing oil stains and oxidized impurities on the surface of the blank, ensuring the cleanliness of the surface of the blank, and reserving; S2, gradient variable parameter foil rolling forming: carrying out final deformation foil rolling processing on the pretreated blank, and adopting a gradient change coupling control mode of rolling force and rolling speed in the thickness direction of the foil rolling. The application adopts the rolling force, rolling speed and step-down amount which change along the thickness gradient to carry out foil rolling, constructs a gradient strain field, and forms a surface fine core coarse gradient texture in cooperation with three-stage gradient annealing; the organization is suitable for electrochemical corrosion characteristics, uniform growth of etching holes is achieved, the problems of over-etching of the surface layer and few holes in the core are avoided, the effective specific surface area of the foil material is greatly improved, and the specific capacity of the finished cathode foil is effectively improved compared with a traditional homogeneous process.

Description

Technical Field

[0001] This invention relates to the field of cathode foil production technology, and specifically to a gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil. Background Technology

[0002] Aluminum electrolytic capacitors are the most widely used passive energy storage components in consumer electronics, new energy vehicles, 5G communications, and industrial power supplies. The cathode foil, as the core conductive carrier of the capacitor, directly determines the capacitor's specific capacitance, equivalent series resistance, and lifespan through its microstructure and corrosion diffusion capabilities. Currently, electronic devices are continuously iterating towards miniaturization, high frequency, and large capacity, placing stringent demands on the cathode foil for higher specific capacitance, lower impedance, and higher corrosion uniformity. Traditional cathode foil rolling and annealing processes are no longer sufficient to meet the needs of high-end products.

[0003] Current mass production processes for cathode foil mostly employ a uniform foil rolling mode with constant rolling force and constant rolling speed. The billet undergoes a consistent amount of deformation in the thickness direction, resulting in no difference in the degree of plastic deformation between the foil surface and core. After rolling, the strain field in the thickness direction is uniform, and the internal grain deformation and dislocation distribution tend to be consistent. The accompanying annealing process typically uses a single constant-temperature holding and uniform heating / cooling rates, which can only obtain an overall uniform equiaxed grain structure and cannot create differentiated grain and texture gradients in the foil thickness direction. This type of conventional process still has some shortcomings: First, poor corrosion uniformity limits the improvement of specific capacitance. Under uniform grain structure, the corrosion activity of aluminum crystals on the surface and in the core tends to be the same. The electrochemical corrosion process is prone to excessive dissolution of the surface layer and insufficient development of core pits. The surface pores are large and collapsed, while the core micropores are sparse. The effective specific surface area is difficult to increase significantly, and the specific capacitance of the finished cathode foil remains at a low level for a long time, which cannot meet the high-capacity requirements of micro-chip capacitors and automotive high-voltage capacitors.

[0004] Second, the residual stress distribution in the thickness direction is unbalanced. Constant parameter rolling will generate a uniform high-density dislocation throughout the foil, and ordinary annealing is not enough to release the stress in the surface and core layers in layers; rapid cooling regimes are prone to inducing stress rebound in the structure, and warping and strip breakage are likely to occur during subsequent slitting and winding of the foil, resulting in a low yield rate.

[0005] Third, the uniform grain structure makes it difficult to achieve both mechanical and electrochemical properties. Under uniform deformation and annealing conditions, it is impossible to achieve a gradient match between fine surface grains and coarse core grains: if the overall grains are refined, the foil will lack toughness and the hole walls will easily crack after corrosion; if the overall grains are coarse, the number of corrosion nucleation sites will decrease, and the surface area expansion efficiency will drop significantly, resulting in a performance contradiction of "high toughness and low specific volume, high specific volume and low strength".

[0006] At present, a few studies have improved the performance of cathode foil by adjusting the total reduction or a single annealing temperature, but this can only achieve overall microstructure optimization and cannot actively construct gradient strain and gradient texture in the thickness direction. Some gradient corrosion processes only perform differentiated channel control on the foil surface and do not control the microstructure of the substrate from the rolling and annealing source. The improvement in corrosion adaptability is limited, and the early rolling and heat treatment processes still use traditional uniform parameters, so the inherent microstructure defects of the substrate cannot be eliminated.

[0007] Therefore, it is necessary to invent a gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil to solve the above problems. Summary of the Invention

[0008] The purpose of this invention is to provide a gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil, so as to solve the problems in the above-mentioned technology.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil, comprising the following steps: S1. Pre-treatment of billet: Select high-purity aluminum cathode foil billet, and perform surface degreasing, water washing and drying treatment on the billet to remove oil stains and oxide impurities on the billet surface and ensure the cleanliness of the billet surface for later use. S2, Gradient variable parameter foil rolling: The pretreated billet is subjected to final deformation foil rolling. The rolling force and rolling speed are coupled and controlled in the foil thickness direction to generate differentiated plastic deformation from the surface to the core of the foil, and a gradient strain field is constructed in the foil thickness direction. S3, graded precision annealing: a segmented gradient annealing process is adopted for the foil after gradient rolling. By controlling the annealing temperature, holding time, heating rate and cooling rate, a gradient texture structure is induced in the thickness direction of the foil to obtain a texture gradient structure with different grain size and crystal orientation distribution between the surface and the core. S4. Post-processing and shaping: The annealed foil is flattened, cut, and cleaned to remove the annealed oxide layer on the surface of the foil, and finally a high specific capacity cathode foil is obtained.

[0010] Preferably, in step S1, the purity of the high-purity aluminum cathode foil blank is ≥99.99%, the original thickness of the blank is 0.8mm~1.2mm, and the surface roughness Ra≤0.8μm.

[0011] Preferably, in step S2, the gradient-changing rolling parameters are as follows: the rolling force changes linearly from the surface to the core along the thickness direction of the foil, and the rolling speed changes linearly from the surface to the core.

[0012] Preferably, the rolling force gradient control range is 800kN to 1500kN, the foil surface rolling force is 800kN to 1000kN, and the foil core rolling force is 1200kN to 1500kN; the rolling speed gradient control range is 60m / min to 120m / min, the foil surface rolling speed is 100m / min to 120m / min, and the foil core rolling speed is 60m / min to 90m / min.

[0013] Preferably, in step S2, the total foil reduction is controlled at 85% to 95%, and the reduction along the foil thickness direction gradually changes synchronously with the rolling parameter gradient, with the surface reduction at 85% to 90% and the core reduction at 90% to 95%.

[0014] Preferably, in step S3, the segmented gradient annealing process is divided into three stages: a low-temperature preheating stage, a high-temperature heat preservation stage, and a gradient slow cooling stage.

[0015] Preferably, the low-temperature preheating section has a heating rate of 3℃ / min to 5℃ / min, a preheating temperature of 180℃ to 220℃, and a preheating holding time of 10min to 15min, which is used to eliminate residual stress from foil rolling; the high-temperature holding section heats up to 320℃ to 380℃ and holds for 25min to 40min, which promotes the gradient weaving of the foil in the thickness direction.

[0016] Preferably, the gradient slow cooling section adopts a segmented cooling mode. The initial cooling rate is 4℃ / min to 6℃ / min. After cooling to 250℃, the subsequent cooling rate is reduced to 1℃ / min to 2℃ / min, and the furnace is cooled to room temperature to avoid texture disorder and residual stress recovery caused by rapid cooling.

[0017] Preferably, in step S3, the texture gradient in the thickness direction of the foil is a uniform corrosion-adaptive texture gradient, which enables the cathode foil to achieve uniform overall etch hole formation in the corrosion process, thereby improving the effective specific surface area and specific volume performance of the foil.

[0018] Preferably, in step S4, the leveling process adopts a low elongation leveling, with the elongation controlled at 0.3% to 1.2%; the cleaning adopts a multi-stage spray cleaning process, which sequentially passes through weak alkali cleaning, deionized water rinsing, and hot air drying. The pH value of the weak alkali cleaning solution is 8.5 to 11.0, the cleaning temperature is 40℃ to 65℃, and the linear speed is 80m / min to 120m / min, which fully removes the uneven oxide layer on the surface generated by annealing and avoids damage to the thickness gradient texture of the foil.

[0019] The technical effects and advantages provided by the present invention in the above technical solution are as follows: 1. Foil rolling is performed using rolling force, rolling speed, and graded reduction that are coupled along the thickness gradient. This creates a continuous gradient strain field within the foil. Combined with a three-stage gradient annealing process, this precisely induces a gradual texture gradient in the foil, with fine grains on the surface and coarse grains in the core. This gradient structure is adapted to the electrochemical corrosion characteristics. During corrosion, the pores grow uniformly from the surface to the core, avoiding the problems of surface over-corrosion collapse and sparse micropores in the core. This significantly increases the effective specific surface area of ​​the foil, and the specific volume of the finished cathode foil is effectively improved compared to traditional homogeneous processes. 2. By setting up a low-temperature preheating section to release residual rolling stress in layers, combined with a segmented gradient slow cooling system, the texture disorder and stress rebound caused by rapid cooling are effectively avoided. The stress distribution in the thickness direction of the foil is uniform, and it is not easy to warp or break during subsequent flattening, shearing and winding processes, which significantly improves the production yield. At the same time, the post-treatment adopts flattening with a small elongation rate and gentle multi-stage spray cleaning, which can remove the annealing oxide layer without destroying the gradient grain texture inside the foil, thus taking into account the surface quality and electrochemical performance stability of the finished product. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the overall process flow of the present invention. Detailed Implementation

[0021] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0022] This invention provides, for example Figure 1 The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil, as shown, includes the following steps: S1. Pre-treatment of billet: Select high-purity aluminum cathode foil billet, and perform surface degreasing, water washing and drying treatment on the billet to remove oil stains and oxide impurities on the billet surface and ensure the cleanliness of the billet surface for later use. S2, Gradient variable parameter foil rolling: The pretreated billet is subjected to final deformation foil rolling. The rolling force and rolling speed are coupled and controlled in the foil thickness direction to generate differentiated plastic deformation from the surface to the core of the foil, and a gradient strain field is constructed in the foil thickness direction. S3, graded precision annealing: a segmented gradient annealing process is adopted for the foil after gradient rolling. By controlling the annealing temperature, holding time, heating rate and cooling rate, a gradient texture structure is induced in the thickness direction of the foil to obtain a texture gradient structure with different grain size and crystal orientation distribution between the surface and the core. S4. Post-processing and shaping: The annealed foil is flattened, cut, and cleaned to remove the annealed oxide layer on the surface of the foil, and finally a high specific capacity cathode foil is obtained.

[0023] In one aspect of this embodiment, the high-purity aluminum billet is selected from aluminum coils with a purity of 99.993%, the original billet thickness is 1.0 mm, and the surface of the billet is precisely ground to control Ra≤0.6 μm; the foil rolling process uses a surface rolling force of 900 kN and a core rolling force of 1350 kN, a surface rolling speed of 110 m / min, a core rolling speed of 75 m / min, a surface reduction of 88%, and a core reduction of 92%; the annealing low-temperature preheating heating rate is 4 °C / min, preheating to 200 °C and holding for 12 min, and a high-temperature holding temperature of 350 °C and holding for 32 min; the gradient slow cooling first reduces the temperature to 250 °C at 5 °C / min, and then cools it to room temperature in the furnace at 1.5 °C / min; the post-treatment flattening elongation rate is 0.6%, the weak alkaline cleaning solution pH=9.5, the cleaning temperature is 50 °C, and the line speed is 100 m / min. The gradient textured cathode foil is prepared by this complete process.

[0024] Working principle of this invention: Refer to the instruction manual appendix Figure 1 When using this invention, first select high-purity aluminum billet to complete degreasing, water washing and drying pretreatment to eliminate surface impurities and ensure that the subsequent rolling deformation and heat treatment structure are uniform and controllable. Then, the gradient variable parameter foil rolling process is entered. The rolling force is linearly increased and the rolling speed is reduced from the surface to the core along the thickness direction of the foil. The gradient reduction is matched simultaneously, so that the plastic deformation degree of the foil surface is lower and the deformation degree of the core is higher. A continuous gradient strain field is formed in the thickness direction. The surface layer accumulates low dislocation density and the core is enriched with high density deformed dislocations, which provides the microstructure basis for subsequent gradient grain and texture formation. The material is then fed into a segmented gradient annealing furnace. First, it is preheated at a low temperature to slowly release the residual stress on the surface of the rolled material and avoid the formation of micro-cracks on the surface. Then, it is heated to a high temperature range and held for a long time. Differential recrystallization is driven by the difference in dislocation density between the surface and the core: the surface has fewer dislocations, and after recrystallization, it forms fine grains with abundant corrosion nucleation sites; the core has a high dislocation density, and the driving force for grain growth is stronger, forming larger equiaxed grains and simultaneously generating corrosion-adaptive textures with varying thickness. After annealing, a two-stage slow cooling is used to suppress texture disorder and prevent internal stress rebound. Finally, the foil warping and deformation are eliminated by flattening with a small elongation rate, and the surface oxide film is removed by multi-stage gentle spray cleaning without damaging the internal gradient grain structure. The final product foil has fine grains on the surface and coarse grains in the core, with a matching corrosion activity gradient in the thickness direction. During electrochemical corrosion, the pores develop uniformly from the surface to the core, which greatly increases the effective specific surface area and achieves the high specific capacity characteristics of the cathode foil.

[0025] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil, characterized in that, Includes the following steps: S1. Pre-treatment of billet: Select high-purity aluminum cathode foil billet, and perform surface degreasing, water washing and drying treatment on the billet to remove oil stains and oxide impurities on the billet surface and ensure the cleanliness of the billet surface for later use. S2, Gradient variable parameter foil rolling: The pretreated billet is subjected to final deformation foil rolling. The rolling force and rolling speed are coupled and controlled in the foil thickness direction to generate differentiated plastic deformation from the surface to the core of the foil, and a gradient strain field is constructed in the foil thickness direction. S3, graded precision annealing: a segmented gradient annealing process is adopted for the foil after gradient rolling. By controlling the annealing temperature, holding time, heating rate and cooling rate, a gradient texture structure is induced in the thickness direction of the foil to obtain a texture gradient structure with different grain size and crystal orientation distribution between the surface and the core. S4. Post-processing and shaping: The annealed foil is flattened, cut, and cleaned to remove the annealed oxide layer on the surface of the foil, and finally a high specific capacity cathode foil is obtained.

2. The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil according to claim 1, characterized in that: In step S1, the purity of the high-purity aluminum cathode foil blank is ≥99.99%, the original thickness of the blank is 0.8mm~1.2mm, and the surface roughness Ra≤0.8μm.

3. The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil according to claim 1, characterized in that: In step S2, the gradient-changing rolling parameters are as follows: along the thickness direction of the foil from the surface to the core, the rolling force changes in a linearly increasing gradient, and the rolling speed changes in a linearly decreasing gradient.

4. The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil according to claim 3, characterized in that: The rolling force gradient control range is 800kN to 1500kN, the rolling force of the foil surface layer is 800kN to 1000kN, and the rolling force of the foil core is 1200kN to 1500kN; the rolling speed gradient control range is 60m / min to 120m / min, the rolling speed of the foil surface layer is 100m / min to 120m / min, and the rolling speed of the foil core is 60m / min to 90m / min.

5. The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil according to claim 1, characterized in that: In step S2, the total foil reduction is controlled at 85% to 95%, and the reduction along the foil thickness direction gradually changes synchronously with the rolling parameter gradient, with the surface reduction at 85% to 90% and the core reduction at 90% to 95%.

6. The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil according to claim 1, characterized in that: In step S3, the segmented gradient annealing process is divided into three stages: a low-temperature preheating stage, a high-temperature heat preservation stage, and a gradient slow cooling stage.

7. The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil according to claim 6, characterized in that: The low-temperature preheating section has a heating rate of 3℃ / min to 5℃ / min, a preheating temperature of 180℃ to 220℃, and a preheating holding time of 10min to 15min, which is used to eliminate residual stress from foil rolling; the high-temperature holding section heats up to 320℃ to 380℃ and holds for 25min to 40min, which promotes the gradient weaving of the foil in the thickness direction.

8. The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil according to claim 6, characterized in that: The gradient slow cooling section adopts a segmented cooling mode. The initial cooling rate is 4℃ / min to 6℃ / min. After cooling to 250℃, the subsequent cooling rate is reduced to 1℃ / min to 2℃ / min, and the furnace is cooled to room temperature to avoid texture disorder and residual stress recovery caused by rapid cooling.

9. The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil according to claim 1, characterized in that: In step S3, the texture gradient in the thickness direction of the foil is a uniform corrosion-adaptive texture gradient, which enables the cathode foil to achieve uniform overall etch hole formation in the corrosion process, thereby improving the effective specific surface area and specific volume performance of the foil.

10. The gradient variable parameter foil rolling and annealing process for high specific capacitance cathode foil according to claim 1, characterized in that: In step S4, the leveling process adopts a low elongation leveling, with the elongation controlled between 0.3% and 1.2%. The cleaning adopts a multi-stage spray cleaning process, which sequentially passes through weak alkali cleaning, deionized water rinsing, and hot air drying. The pH value of the weak alkali cleaning solution is 8.5 to 11.0, the cleaning temperature is 40℃ to 65℃, and the linear speed is 80m / min to 120m / min. This process thoroughly removes the uneven oxide layer generated during annealing and avoids damage to the thickness gradient texture of the foil.

Images