Aluminum alloy composite plate without flux and preparation method and application thereof

Abstract

This invention relates to the field of aluminum alloy composite sheet technology, and in particular to a flux-free aluminum alloy composite sheet, its preparation method, and its application. The flux-free aluminum alloy composite sheet includes a core layer and a brazed composite layer located on at least one surface of the core layer; the brazed composite layer includes a first brazing filler layer, a second brazing filler layer, and a third brazing filler layer; the second brazing filler layer is located between the first and third brazing filler layers, and the first brazing filler layer constitutes the outermost surface of at least one side of the flux-free aluminum alloy composite sheet; the first brazing filler layer includes at least one Al-Si brazing filler alloy layer containing a first oxide film; the second brazing filler layer includes at least one Al-Zn-Si brazing filler alloy layer containing a second oxide film; and the third brazing filler layer includes at least one Al-Si-Mg brazing filler alloy layer containing a third oxide film. This invention controls the diffusion behavior of Mg between layers by adjusting the number and thickness of the oxide film layers in the brazed composite layer, ensuring sufficient film breaking while avoiding excessively high surface Mg content.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy composite sheet technology, and in particular to a flux-free aluminum alloy composite sheet, its preparation method, and its application. Background Technology

[0002] With the rapid development of new energy vehicles, the demand for heat exchangers continues to grow. Traditional heat exchangers are typically assembled from components such as flat tubes, fins, main plates, and water chambers, and then brazed at high temperatures to form a metallurgical bond between the components. However, these heat exchangers occupy a large space, and their heat exchange efficiency is no longer sufficient to meet current demands. Therefore, stacked heat exchangers, which are smaller and more efficient, are favored by new energy vehicles. Aluminum and aluminum alloy composite plates are specifically designed for stacked heat exchangers, typically with one or more layers of aluminum-based brazing alloy laminated to one or both sides of the core material. During high-temperature brazing, the brazing alloy melts preferentially, spreads between the welding surfaces, and fills the tiny gaps between the weldments through capillary action, forming a strong weld joint after cooling. However, the high solidus temperature oxide film on the aluminum alloy surface can hinder the flow of molten brazing alloy and the metallurgical reaction between the weldments, leading to poor welding. Therefore, currently, pre-spraying flux before brazing or vacuum brazing are the main methods used to ensure welding quality.

[0003] Currently, Al-FK series fluxes are commonly used. However, the use of flux complicates the welding process, and residual flux after welding is generally considered a hazardous substance. When used in heat exchangers inside new energy vehicles, residual flux can easily clog coolant passages, and the free F... - K + Ions can increase the conductivity of coolant, posing a short-circuit risk. Furthermore, there are methods to introduce flux into the brazing alloy to eliminate the need for pre-coating. However, its cleanliness is still inferior to vacuum brazing. Vacuum brazing can achieve welding without the use of any flux, resulting in highly clean welded products and avoiding the hazards of residues. However, vacuum brazing equipment is complex, has a long welding cycle, and low output, limiting its large-scale production.

[0004] Compared to vacuum brazing, controlled atmosphere brazing (CAB brazing) also utilizes Mg to break down the oxide film in flux-free methods, but it requires stricter control over the surface Mg content, demanding that the surface be Mg-free or have it controlled at an extremely low level. For example, in Chinese patent application CN102209629A, a Mg- and Bi-containing filler material is used to coat the core alloy. Bi acts as a wetting element, and Mg diffuses to the surface through channels formed by Bi, breaking down the oxide film covering the surface through a chemical reaction to form a weld joint. However, these channels also accelerate Mg volatilization. When the oxygen content in the furnace is high, an oxide film forms on the liquid brazing filler metal, hindering its flow and severely affecting the weld quality. In Chinese patent application CN104626674A, rare earth elements are added to a Mg-containing brazing layer and a Bi-containing capping layer sequentially laminated on the surface of the core layer. These rare earth elements form high solidus temperature rare earth compounds at grain boundaries, confining Bi, Mg, and other elements within the brazing liquid crystal interface, thereby controlling their diffusion behavior. Simultaneously, the introduction of rare earth elements helps improve the microstructure and strengthen the weld joint. However, due to the addition of rare earth elements, the material requires significantly higher precision in controlling processes such as melting, cooling, and subsequent heat treatment, increasing the overall complexity and difficulty of implementation.

[0005] Therefore, developing a flux-free aluminum alloy composite plate that does not contain rare earth elements, is compatible with CAB brazing, and has good welding strength is of great significance to the development of aluminum alloy composite plates. Summary of the Invention

[0006] The purpose of this invention is to provide a flux-free aluminum alloy composite plate, its preparation method, and its application.

[0007] To achieve the objectives of this invention, the following technical solution is adopted: In a first aspect, the present invention provides a flux-free aluminum alloy composite plate, comprising a core layer and a brazed composite layer located on at least one surface of the core layer; The brazing composite layer includes a first brazing filler layer, a second brazing filler layer, and a third brazing filler layer; the second brazing filler layer is located between the first brazing filler layer and the third brazing filler layer, and the first brazing filler layer constitutes the outermost surface of at least one side of the fluxless aluminum alloy composite plate; The first solder layer includes at least one Al-Si solder alloy layer containing a first oxide film; the second solder layer includes at least one Al-Zn-Si solder alloy layer containing a second oxide film; the third solder layer includes at least one Al-Si-Mg solder alloy layer containing a third oxide film. The Zn content in the second solder layer accounts for 0.5~7.5 wt% of the total amount of the brazed composite layer; the Mg content in the third solder layer accounts for 0.03~0.24 wt% of the total amount of the brazed composite layer. The total thickness of the first oxide film layer, the second oxide film layer and the third oxide film layer is ≤100 nm.

[0008] Furthermore, the thickness of the second oxide film layer and the third oxide film layer are each independently ≥10 nm.

[0009] Furthermore, the thickness of the first oxide film layer is 5~20 nm.

[0010] Furthermore, the solidus temperature of each brazing alloy layer is ≤595℃.

[0011] Furthermore, the solidus temperature of the Al-Zn-Si solder alloy layer is lower than that of the Al-Si solder alloy layer and the Al-Si-Mg solder alloy layer.

[0012] Furthermore, the solidus temperature of the Al-Si solder alloy layer is 1~94℃ higher than that of the Al-Zn-Si solder alloy layer; preferably, it is 19~60℃ higher.

[0013] Furthermore, the solidus temperature of the Al-Si-Mg solder alloy layer is 1~61℃ higher than that of the Al-Zn-Si solder alloy layer; preferably, it is 11~57℃ higher.

[0014] Furthermore, the Al-Zn-Si solder alloy layer comprises, by mass percentage, 6%~13% Si and 5%~25% Zn.

[0015] Furthermore, the Al-Si solder alloy layer comprises 6% to 13% Si by mass percentage.

[0016] Furthermore, the Al-Si-Mg solder alloy layer comprises, by mass percentage, 6%~13% Si, ≤0.3% Mg and ≤0.2% Bi.

[0017] Furthermore, the Al-Zn-Si solder alloy layer does not contain Mg.

[0018] Furthermore, the Al-Si solder alloy layer does not contain Mg.

[0019] Furthermore, the total thickness of the first solder layer accounts for 10-80% of the total thickness of the brazing composite layer.

[0020] Furthermore, the total thickness of the second solder layer accounts for 10-30% of the total thickness of the brazing composite layer.

[0021] Furthermore, the total thickness of the third solder layer accounts for 10-80% of the total thickness of the brazing composite layer.

[0022] Furthermore, the core layer comprises at least one of 1XXX series aluminum alloys, 3XXX series aluminum alloys, and 6XXX series aluminum alloys.

[0023] In a second aspect, the present invention provides a method for preparing the flux-free aluminum alloy composite plate provided in the first aspect of the present invention, comprising the following steps: (a) Ingot preparation: Prepare core alloy ingots, Al-Si brazing alloy ingots, Al-Zn-Si brazing alloy ingots, and Al-Si-Mg brazing alloy ingots according to the target required composition and thickness. (b) Ingot oxidation process: Al-Si solder alloy ingots, Al-Zn-Si solder alloy ingots, and Al-Si-Mg solder alloy ingots are subjected to anodic coating treatment to form an oxide film layer; (c) Composite process: The core alloy, Al-Si-Mg brazing alloy, Al-Zn-Si brazing alloy and Al-Si brazing alloy are stacked in sequence and hot rolled and bonded, and then rolled to thin to the target size.

[0024] Furthermore, in the Al-Si solder alloy billet, Al-Zn-Si solder alloy billet, and Al-Si-Mg solder alloy billet after anodic coating treatment, the thickness of the oxide film layer is independently 100~1000 nm.

[0025] Furthermore, in the anode coating process, the coating solution is a sulfuric acid solution with a mass concentration of 10%~15%, and the current density is 1~2 A / dm³. 2 The coating time is 10~60 s.

[0026] Thirdly, the present invention provides a brazed composite plate that can be used as an intermediate product in the manufacture of the fluxless aluminum alloy composite plate provided in the first aspect of the present invention, the brazed composite plate comprising a first brazing filler layer, a second brazing filler layer, and a third brazing filler layer in the fluxless aluminum alloy composite plate provided in the first aspect of the present invention, wherein the second brazing filler layer is located between the first brazing filler layer and the third brazing filler layer.

[0027] Fourthly, the present invention provides the application of the fluxless aluminum alloy composite plate provided in the first aspect of the present invention or the fluxless aluminum alloy composite plate prepared by the preparation method provided in the second aspect of the present invention in fluxless CAB brazing.

[0028] Compared with the prior art, the present invention has the following beneficial effects: The fluxless aluminum alloy composite plate of this invention, through its multi-layered solder layer and oxide film layer structure, achieves control over the interlayer diffusion behavior of Mg. This ensures sufficient Mg reaches the surface to break the oxide film while preventing explosive diffusion of Mg, thus preventing excessive Mg accumulation on the surface and the formation of a stable MgO film. This avoids the risk of welding failure and ensures brazing quality. Simultaneously, the absence of Mg in the first solder layer effectively prevents darkening and yellowing; the second solder layer, containing an appropriate amount of Zn, creates localized liquid channels to promote Mg diffusion while also maintaining corrosion resistance. This results in a fluxless aluminum alloy composite plate suitable for CAB brazing and possessing good weld strength. Attached Figure Description

[0029] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the flux-free aluminum alloy composite plate provided in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the brazing composite layer provided in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the assembly structure in the brazing experiment of the test example of the present invention.

[0031] Among them, 1-core layer, 2-brazing composite layer, 21-first brazing layer, 22-second brazing layer, 23-third brazing layer, 211-Al-Si brazing alloy layer, 212-first oxide film layer, 221-Al-Zn-Si brazing alloy layer, 222-second oxide film layer, 231-Al-Si-Mg brazing alloy layer, 232-third oxide film layer. Detailed Implementation

[0032] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0033] In this invention, the term "Al-Si solder alloy layer" specifically refers to the alloy of the first solder layer, which is characterized by containing at least Si as the main alloying element. The term "Al-Si solder alloy layer" does not limit the alloy of the first solder layer from containing other alloying elements as the main alloying elements. For example, when an aluminum alloy containing 10 wt% Si and 5 wt% Zn is used as the alloy of the first solder layer, it is denoted as "Al-Si solder alloy layer", and the corresponding ingot is denoted as Al-Si solder alloy ingot.

[0034] In this invention, the term "Al-Zn-Si solder alloy layer" specifically refers to the alloy of the second solder layer, which is characterized by containing at least Si and Zn as the main alloying elements. The term "Al-Zn-Si solder alloy layer" does not limit the alloy of the second solder layer from containing other alloying elements as the main alloying elements. For example, when an aluminum alloy containing 10 wt% Si and 5 wt% Zn is used as the alloy of the second solder layer, it is denoted as "Al-Zn-Si solder alloy layer", and the corresponding ingot is denoted as Al-Zn-Si solder alloy ingot.

[0035] In this invention, the term "Al-Si-Mg solder alloy layer" specifically refers to the alloy of the third solder layer, which is characterized by containing at least Si and Mg as the main alloying elements. The term "Al-Si-Mg solder alloy layer" does not limit the alloy of the third solder layer from containing other alloying elements as the main alloying elements.

[0036] In a first aspect, the present invention provides a flux-free aluminum alloy composite plate, comprising a core layer and a brazed composite layer located on at least one surface of the core layer; The brazing composite layer includes a first brazing filler layer, a second brazing filler layer, and a third brazing filler layer; the second brazing filler layer is located between the first brazing filler layer and the third brazing filler layer, and the first brazing filler layer constitutes the outermost surface of at least one side of the fluxless aluminum alloy composite plate; The first solder layer includes at least one Al-Si solder alloy layer containing a first oxide film; the second solder layer includes at least one Al-Zn-Si solder alloy layer containing a second oxide film; the third solder layer includes at least one Al-Si-Mg solder alloy layer containing a third oxide film. The Zn content in the second solder layer accounts for 0.5~7.5 wt% of the total amount of the brazed composite layer; the Mg content in the third solder layer accounts for 0.03~0.24 wt% of the total amount of the brazed composite layer. The total thickness of the first oxide film layer, the second oxide film layer and the third oxide film layer is ≤100 nm.

[0037] In fluxless CAB soldering, magnesium (Mg) reacts with the aluminum alloy surface oxide film (Al₂O₃) at high temperatures, disrupting the dense oxide film structure and exposing and spreading the liquid solder, thus achieving fluxless soldering. However, if the Mg content diffusing to the surface is insufficient, the oxide film is not fully broken down, and the flow of the liquid solder is hindered; if the Mg content diffusing to the surface is excessive, it can easily lead to over-oxidation, which also impairs the soldering quality. Currently, it is difficult to effectively control the Mg content diffusing to the surface simply by adjusting the Mg content in the solder layer.

[0038] To address this, the present invention regulates the interlayer diffusion behavior of Mg by controlling the quantity and thickness of the oxide film layer in the brazed composite layer, thereby ensuring sufficient film breaking while avoiding excessive Mg content on the surface.

[0039] In some embodiments, the total thickness of the first oxide film layer, the second oxide film layer, and the third oxide film layer is ≤100 nm, specifically within the range of 100 nm, 95 nm, 90 nm, 85 nm, 80 nm, 75 nm, 70 nm, 65 nm, 60 nm, 55 nm, or any combination thereof. When the total thickness of the oxide film layer is too large, the Mg content reaching the surface will be insufficient, making complete film breaking difficult. Specifically, the brazing composite layer of the present invention includes multiple solder layers, wherein the Mg-containing third solder layer is disposed close to the core layer, and an oxide film is distributed between each solder layer. During the heating process of fluxless CAB brazing, the oxide film distributed between the layers extends the diffusion path of Mg through physical barriers, while simultaneously reacting with Mg through chemical capture to generate MgO. These two processes work synergistically to effectively control the diffusion rate and flux of Mg from the high-content layer to the surface. This allows Mg to reach the surface of the brazed composite layer in a stable and controllable diffusion manner to break the oxide film, avoiding the failure risk caused by excessive surface Mg enrichment and the formation of a stable MgO film due to the explosive diffusion of Mg in the early stages of brazing. Furthermore, the Mg-containing third solder layer is positioned close to the core layer, while the first solder layer does not contain Mg, thus avoiding the darkening and yellowing problems caused by Mg on the surface.

[0040] In some embodiments, the Mg content in the third solder layer accounts for 0.03~0.24 wt% of the total amount of the brazed composite layer, specifically within the range of 0.03 wt%, 0.08 wt%, 0.1 wt%, 0.15 wt%, 0.18 wt%, 0.2 wt%, 0.24 wt%, or any combination thereof. Experiments have verified that if the Mg content in the third solder layer is too low, it cannot effectively diffuse to the surface of the brazed composite layer to break the oxide film; if the Mg content is too high, a large amount of Mg will diffuse to the surface, leading to excessive oxidation and the formation of a MgO film with a higher solidus temperature, hindering brazing.

[0041] In some embodiments, the Zn content in the second solder layer accounts for 0.5 to 7.5 wt% of the total amount of the brazed composite layer, specifically within the range of 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, 5.5 wt%, 6 wt%, 6.5 wt%, 7 wt%, 7.5 wt%, or any combination thereof. In the Zn-containing second solder layer of the present invention, Zn is miscible with Al to form a low solidus temperature solution region. During brazing, the solution region preferentially melts to form local liquid channels, providing a rapid diffusion path for Mg, enabling it to quickly reach the surface in the early stages of brazing; furthermore, because the channels are limited to a local area, the overall Mg diffusion rate is suppressed, thereby effectively controlling the Mg content reaching the surface layer. As the brazing process progresses, driven by the concentration gradient, Zn continuously diffuses into the adjacent solid phase region and undergoes a dissolution reaction with the Al at the leading edge, causing the new solid Al to continuously transform into a liquid phase, propelling the liquid channel forward. When the liquid phase reaches the damaged area of ​​the oxide film, it rapidly spreads at the gap. Experiments have verified that when the Zn content in the second brazing filler layer is too low, sufficient liquid channels cannot be constructed, hindering the initial diffusion of Mg and impeding its spread after the film breaks; conversely, when the Zn content in the second brazing filler layer is too high, the corrosion resistance of the aluminum alloy composite plate deteriorates.

[0042] In some embodiments, the thickness of the second oxide film layer and the third oxide film layer are each independently ≥10 nm, specifically a range of 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 56 nm or any combination thereof.

[0043] When the thickness of the second and third oxide film layers is insufficient, the amount of Mg reaching the surface is easily out of control, making it difficult to achieve the effect of slow release and film breaking.

[0044] In some embodiments, the thickness of the first oxide film layer is 5 to 20 nm, specifically a range of 5 nm, 8 nm, 10 nm, 12 nm, 15 nm, 18 nm, 20 nm, or any combination thereof.

[0045] In CAB brazing, the outermost first oxide film layer comes into contact with oxygen and needs to be broken down by Mg to form gaps, allowing the brazing filler metal to further wet and spread rapidly. Controlling the thickness of the first oxide film layer within the aforementioned range ensures film breaking while slowing down the diffusion rate of Mg, achieving a gradual film breaking process. Unlike the inner oxide film layers (second and third oxide film layers), the control of the surface oxide film layer (first oxide film layer) is more stringent. If the thickness of the first oxide film layer is too large, it will be difficult for Mg to effectively break down the film; if the thickness of the first oxide film layer is too small, the amount of Mg reaching the surface will be uncontrolled, resulting in the formation of a MgO film with a higher solidus temperature, hindering brazing.

[0046] In some embodiments, the flux-free aluminum alloy composite plate includes a core layer and a brazed composite layer located on at least one surface of the core layer; the brazed composite layer includes a first brazing filler layer, a second brazing filler layer, and a third brazing filler layer; the second brazing filler layer is located between the first brazing filler layer and the third brazing filler layer, and the first brazing filler layer constitutes the outermost surface of at least one side of the flux-free aluminum alloy composite plate; the first brazing filler layer includes at least one Al-Si brazing filler alloy layer containing a first oxide film layer with a thickness of 5-20 nm; the second brazing filler layer includes at least one Al-Zn-Si brazing filler alloy layer containing a second oxide film layer with a thickness of ≥10 nm; the third brazing filler layer includes at least one Al-Si-Mg brazing filler alloy layer containing a third oxide film layer with a thickness of ≥10 nm; the Zn content in the second brazing filler layer accounts for 0.5-7.5 wt% of the total amount of the brazed composite layer; the Mg content in the third brazing filler layer accounts for 0.03-0.24 wt% of the total amount of the brazed composite layer. wt%; the total thickness of the first oxide film layer, the second oxide film layer and the third oxide film layer is ≤100 nm.

[0047] In some embodiments, the solidus temperature of each brazing filler alloy layer is ≤595°C, specifically within the range of 595°C, 590°C, 585°C, 580°C, 575°C, 570°C, 560°C, 550°C, 540°C, 530°C, 520°C, 515°C, or any combination thereof. Excessively high solidus temperatures of the brazing filler alloy layers make it difficult to achieve low-temperature brazing (e.g., below 600°C).

[0048] In some embodiments, the solidus temperature of the Al-Zn-Si solder alloy layer is lower than that of the Al-Si solder alloy layer and the Al-Si-Mg solder alloy layer.

[0049] In some embodiments, the solidus temperature of the Al-Si solder alloy layer is 1 to 94°C higher than that of the Al-Zn-Si solder alloy layer. Specifically, it can be a range of 1°C, 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 94°C, or any two of these ranges, preferably 19 to 60°C higher.

[0050] In some embodiments, the solidus temperature of the Al-Si-Mg solder alloy layer is 1 to 61°C higher than that of the Al-Zn-Si solder alloy layer. Specifically, it can be a range of 1°C, 10°C, 20°C, 30°C, 40°C, 50°C, 55°C, 61°C, or any two of these ranges, preferably 11 to 57°C higher.

[0051] Meeting the above conditions regarding the solidus temperature of the Al-Zn-Si solder alloy layer is beneficial for ensuring its control over Mg diffusion. If the solidus temperature of the Al-Zn-Si solder alloy layer is higher than that of the Al-Si-Mg solder alloy layer, the second solder layer cannot preferentially form liquid channels in the early stages of brazing, making it difficult for Mg to quickly reach the surface in the early stages of brazing. If the solidus temperature of the Al-Zn-Si solder alloy layer is higher than that of the Al-Si solder alloy layer, the Al-Si solder alloy layer melts before the formation of liquid channels. When the liquid channels are formed, the Mg reaching the first solder layer is not controlled by the liquid channels, which is detrimental to the control of Mg diffusion.

[0052] The Al-Si-Mg solder alloy layer may optionally contain Zn. The Zn content significantly affects the solidus temperature of the aluminum alloy. If it is desired to control the solidus temperature of the Al-Si-Mg solder alloy in the third solder layer, the Zn content of the Al-Si-Mg solder alloy layer should be appropriately controlled to adjust the solidus temperature within the target range. Obtaining the target solidus temperature through a limited number of experiments by adjusting the Zn content is existing technology and is not further limited here. For example, the Zn content may be ≤10wt%, ≤5wt%, or Zn-free.

[0053] For example, the solidus temperature of the Al-Si solder alloy in the first solder layer is 576°C, the solidus temperature of the Al-Zn-Si solder alloy in the second solder layer is 557°C, and the solidus temperature of the Al-Si-Mg solder alloy in the third solder layer is 568°C.

[0054] In some embodiments, the Al-Zn-Si solder alloy layer comprises, by mass percentage, 6%~13% Si and 5%~25% Zn. Further, the Al-Zn-Si solder alloy layer does not contain Mg.

[0055] In different embodiments, by mass percentage, the Si content in the Al-Zn-Si solder alloy layer can be 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, or any combination thereof; the Zn content can be 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, or any combination thereof.

[0056] In some embodiments, the Al-Si solder alloy layer comprises 6% to 13% Si by mass. It is known that the Si content in the aluminum alloy is related to the amount of liquid phase generated during brazing, therefore 6% to 13% Si needs to be added. Furthermore, the Al-Si solder alloy layer does not contain Mg.

[0057] In different embodiments, the Si content in the Al-Si solder alloy layer, by mass percentage, can be 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, or any combination thereof.

[0058] In some embodiments, the Al-Si-Mg solder alloy layer comprises, by mass percentage, 6%~13% Si, ≤0.3% Mg and ≤0.2% Bi.

[0059] In different embodiments, by mass percentage, the Si content in the Al-Si-Mg solder alloy layer can be 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, or any combination thereof; the Mg content can be 0.3%, 0.25%, 0.2%, 0.15%, 0.1%, or any combination thereof; and the Bi content can be 0.2%, 0.15%, 0.1%, 0.05%, 0, or any combination thereof.

[0060] Meeting the above conditions in the element content of each solder alloy layer helps to obtain a solder alloy layer with a solidus temperature that meets the requirements, and helps to ensure the control effect of the solder composite layer on the diffusion of Mg element and the spreading of liquid solder.

[0061] In some embodiments, the total thickness of the first solder layer accounts for 10-80% of the total thickness of the brazing composite layer, specifically a range of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or any two of these.

[0062] In some embodiments, the total thickness of the second solder layer accounts for 10-30% of the total thickness of the brazing composite layer, specifically 10%, 15%, 20%, 25%, 30%, or any combination thereof.

[0063] In some embodiments, the total thickness of the third solder layer accounts for 10-80% of the total thickness of the brazing composite layer, specifically a range of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or any two of these.

[0064] The total thickness of the brazed composite layer is typically designed according to the dimensions required by the customer. For example, in the field of electric vehicle heat exchangers, when flux-free aluminum alloy composite plates are used as core plates for chiillers or oil coolers, the brazed composite layer in such composite plates is typically designed to be 20% thick.

[0065] In some embodiments, the core layer comprises at least one of 1XXX series aluminum alloys, 3XXX series aluminum alloys, and 6XXX series aluminum alloys.

[0066] In a second aspect, the present invention provides a method for preparing the flux-free aluminum alloy composite plate provided in the first aspect of the present invention, comprising the following steps: (a) Ingot preparation: Prepare core alloy ingots, Al-Si brazing alloy ingots, Al-Zn-Si brazing alloy ingots, and Al-Si-Mg brazing alloy ingots according to the target required composition and thickness. (b) Ingot oxidation process: Al-Si solder alloy ingots, Al-Zn-Si solder alloy ingots, and Al-Si-Mg solder alloy ingots are subjected to anodic coating treatment to form an oxide film layer; (c) Composite process: The core alloy, Al-Si-Mg brazing alloy, Al-Zn-Si brazing alloy and Al-Si brazing alloy are stacked in sequence and hot rolled and bonded, and then rolled to thin to the target size.

[0067] In the preparation method of flux-free aluminum alloy composite plate, except for the anodic coating treatment, the other processes are all conventional techniques for preparing composite plates and are not restricted here.

[0068] In step (a), the alloy composition of each billet is proportioned according to the target range, and the processing method can be casting; the thickness of each billet can be adjusted by hot rolling if necessary. The core layer billet in step (a) can be subjected to homogenization treatment if necessary to ensure that the composition and internal structure of the billet are uniform.

[0069] The hot rolling bonding process in step (c) aims to achieve multi-layer bonding and realize the metallurgical bond between layers, which is a conventional process for preparing composite materials; for example, in step (c), the temperature of the hot rolling bonding is 470~500℃.

[0070] The purpose of rolling after hot-rolled bonding in step (c) is to thin the material and control its microstructure. This typically includes hot-rolled thinning and cold-rolled thinning. Hot-rolled thinning refers to continuing rolling at the temperature of hot-rolled bonding, which allows for rapid thinning at high temperatures. Cold-rolled thinning refers to thinning by rolling at room temperature. Compared to hot rolling, cold rolling introduces significant work hardening and internal stress. The reduction rate of cold rolling affects the final mechanical properties, grain refinement, and texture (crystal orientation) of the material. Those skilled in the art can choose the appropriate reduction rate based on the final performance of the product (such as strength, hardness, and formability) and the requirements of subsequent processes (such as annealing and brazing). For example, in step (c), the reduction rate of cold rolling is 85% to 90%.

[0071] Depending on customer requirements, the material can be annealed after step (c) to obtain the target delivery state. For example, the annealing is performed in a protective atmosphere at a temperature of 350-400°C for 1-3 hours.

[0072] In some embodiments, the thickness of the oxide film layer in the Al-Si solder alloy billet, Al-Zn-Si solder alloy billet, and Al-Si-Mg solder alloy billet after anodic coating treatment is independently 100~1000 nm, specifically within the range of 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 500 nm, 600 nm, 800 nm, 1000 nm, or any combination thereof.

[0073] In some embodiments, during the anodic coating process, the coating solution is a sulfuric acid solution with a mass concentration of 10% to 15%, and the current density is 1 to 2 A / dm³. 2 The coating time is 10~60 s.

[0074] Thirdly, the present invention provides a brazed composite plate that can be used as an intermediate product in the manufacture of the fluxless aluminum alloy composite plate provided in the first aspect of the present invention, the brazed composite plate comprising a first brazing filler layer, a second brazing filler layer, and a third brazing filler layer in the fluxless aluminum alloy composite plate provided in the first aspect of the present invention, wherein the second brazing filler layer is located between the first brazing filler layer and the third brazing filler layer.

[0075] Fourthly, the present invention provides the application of the fluxless aluminum alloy composite plate provided in the first aspect of the present invention or the fluxless aluminum alloy composite plate prepared by the preparation method provided in the second aspect of the present invention in fluxless CAB brazing.

[0076] Example (a) Ingot preparation: Prepare core alloy ingots, Al-Si brazing alloy ingots, Al-Zn-Si brazing alloy ingots, and Al-Si-Mg brazing alloy ingots according to the target alloy element composition and thickness. (b) Ingot oxidation process: Al-Si solder alloy ingots, Al-Zn-Si solder alloy ingots, and Al-Si-Mg solder alloy ingots are subjected to anodic coating treatment to form an oxide film layer; (c) Composite process: The core alloy, Al-Si-Mg brazing alloy, Al-Zn-Si brazing alloy and Al-Si brazing alloy are stacked in sequence and hot rolled and bonded, and then rolled to thin to the target size.

[0077] To better illustrate the issue, the following standard choices will be subject to uniform conditions.

[0078] In step (a), the billet is formed by casting. The core alloy billet is made of 3003 alloy, specifically: Si 0.28%, Fe 0.54%, Cu 0.11%, Mn 1.11%. The core alloy billet obtained by casting is milled and rolled thin to obtain the core alloy billet with dimensions (length * width * height) of 3800 * 1350 * 8 (unit mm).

[0079] The total composite ratio of the brazing composite layer is set to 20% (i.e., the brazing composite layer thickness accounts for 20% and the core layer thickness accounts for 80%). Based on the target set thickness ratio of each brazing layer, the corresponding billet thickness (with reserved milling surface thickness) is set, and the corresponding layer of the ingot is rolled thin to the desired size by hot rolling.

[0080] In step (b) of the ingot oxidation process, anodizing treatment is performed on Al-Si solder alloy ingots, Al-Zn-Si solder alloy ingots, and Al-Si-Mg solder alloy ingots using sulfuric acid aqueous solution to form an oxide film layer of a certain thickness on the upper surface (the oxide film layer formed on the lower surface is removed by milling). Specifically, in the anodizing treatment, the mass concentration of sulfuric acid aqueous solution is selected as 12%, and the current density is selected as 1.5 A / dm³. 2 The coating time for each billet is 10~60 s independently.

[0081] Step (c) The blanks are stacked and fixed in the order of core alloy, Al-Si-Mg brazing alloy, Al-Zn-Si brazing alloy and Al-Si brazing alloy. Then they are hot rolled and bonded at 500°C, and then further hot rolled to thin to 7 mm. Then they are cold rolled to 1 mm to obtain a composite plate with a final thickness of 1 mm.

[0082] In step (a) of Example 1, the target alloy element composition and corresponding solidus temperatures of the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are shown in Table 1; in step (b), the coating times for the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are 20 seconds, 35 seconds, and 35 seconds, respectively, and the final thicknesses of the oxide film on the upper surface are 151 nm, 300 nm, and 301 nm; in step (c), the composite plate... The specified thickness ratios (thickness ratios refer to the proportions of each brazing alloy layer to the total thickness of the brazed composite layer) for the Al-Si brazing alloy layer, Al-Zn-Si brazing alloy layer, and Al-Si-Mg brazing alloy layer are 80%, 10%, and 10%, respectively, corresponding to oxide film thicknesses of 15nm, 30nm, and 30nm, with a total oxide film thickness of 75nm. The Zn element in the Al-Zn-Si brazing alloy layer accounts for 0.5wt% of the total brazing composite layer, and the Mg element in the Al-Si-Mg brazing alloy layer accounts for 0.03wt% of the total brazing composite layer.

[0083] In step (a) of Example 2, the target alloy element composition and corresponding solidus temperatures of the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are shown in Table 1; in step (b), the coating times for the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are 30 seconds, 15 seconds, and 33 seconds, respectively, and the final oxide film thicknesses on the upper surface are 202 nm, 100 nm, and 251 nm; in step (c), the composite plate contains... The specified thickness ratios (thickness ratios refer to the proportions of each brazing alloy layer to the total thickness of the brazed composite layer) for the Al-Si brazing alloy layer, Al-Zn-Si brazing alloy layer, and Al-Si-Mg brazing alloy layer are 40%, 20%, and 40%, respectively, corresponding to oxide film thicknesses of 20nm, 10nm, and 25nm, with a total oxide film thickness of 55nm. The Zn element in the Al-Zn-Si brazing alloy layer accounts for 3.0wt% of the total brazing composite layer, and the Mg element in the Al-Si-Mg brazing alloy layer accounts for 0.08wt% of the total brazing composite layer.

[0084] In step (a) of Example 3, the target alloy element composition and corresponding solidus temperatures of the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are shown in Table 1; in step (b), the coating times for the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are 10 seconds, 60 seconds, and 15 seconds, respectively, and the final oxide film thicknesses on the upper surface are 50 nm, 556 nm, and 104 nm; in step (c), the composite plate contains... The specified thickness ratios (thickness ratios refer to the proportions of each brazing alloy layer to the total thickness of the brazed composite layer) for the Al-Si brazing alloy layer, Al-Zn-Si brazing alloy layer, and Al-Si-Mg brazing alloy layer are 10%, 10%, and 80%, respectively, corresponding to oxide film thicknesses of 5nm, 56nm, and 10nm, with a total oxide film thickness of 71nm. The Zn element in the Al-Zn-Si brazing alloy layer accounts for 1.0wt% of the total brazing composite layer, and the Mg element in the Al-Si-Mg brazing alloy layer accounts for 0.24wt% of the total brazing composite layer.

[0085] In step (a) of Example 4, the target alloy element composition and corresponding solidus temperatures of the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are shown in Table 1; in step (b), the coating times for the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are 15 seconds, 38 seconds, and 38 seconds, respectively, and the final oxide film thicknesses on the upper surface are 103 nm, 349 nm, and 350 nm; in step (c), the composite plate contains... The specified thickness ratios (thickness ratios refer to the proportions of each brazing alloy layer to the total thickness of the brazed composite layer) for the Al-Si brazing alloy layer, Al-Zn-Si brazing alloy layer, and Al-Si-Mg brazing alloy layer are 20%, 30%, and 50%, respectively, corresponding to oxide film thicknesses of 10nm, 35nm, and 35nm, with a total oxide film thickness of 80nm. The Zn element in the Al-Zn-Si brazing alloy layer accounts for 7.5wt% of the total brazing composite layer, and the Mg element in the Al-Si-Mg brazing alloy layer accounts for 0.10wt% of the total brazing composite layer.

[0086] In step (a) of Example 5, the target alloy element composition and corresponding solidus temperatures of the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are shown in Table 1; in step (b), the coating times for the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are 20 seconds, 35 seconds, and 35 seconds, respectively, and the final thicknesses of the upper surface oxide film are 151 nm, 300 nm, and 301 nm; in step (c), the composite plate... The specified thickness ratios (thickness ratios refer to the proportions of each brazing alloy layer to the total thickness of the brazed composite layer) for the Al-Si brazing alloy layer, Al-Zn-Si brazing alloy layer, and Al-Si-Mg brazing alloy layer are 80%, 10%, and 10%, respectively, corresponding to oxide film thicknesses of 15nm, 30nm, and 30nm, with a total oxide film thickness of 75nm. The Zn element in the Al-Zn-Si brazing alloy layer accounts for 0.5wt% of the total brazing composite layer, and the Mg element in the Al-Si-Mg brazing alloy layer accounts for 0.03wt% of the total brazing composite layer.

[0087] In step (a) of Example 6, the target alloy element composition and corresponding solidus temperatures of the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are shown in Table 1; in step (b), the coating times for the Al-Si, Al-Zn-Si, and Al-Si-Mg solder alloy ingots are 20 seconds, 35 seconds, and 35 seconds, respectively, and the final thicknesses of the oxide film on the upper surface are 151 nm, 300 nm, and 300 nm; in step (c), the composite plate... The thickness percentages (the percentage of thickness refers to the proportion of each solder layer to the total thickness of the brazed composite layer) of the Al-Si solder alloy layer, Al-Zn-Si solder alloy layer, and Al-Si-Mg solder alloy layer were set to 80%, 10%, and 10%, respectively, corresponding to oxide film thicknesses of 15nm, 30nm, and 30nm, with a total oxide film thickness of 75nm. The Zn element in the Al-Zn-Si solder layer accounted for 0.5wt% of the total thickness of the brazed composite layer, and the Mg element in the Al-Si-Mg solder layer accounted for 0.03wt% of the total thickness of the brazed composite layer. The solidus temperature and oxide film thickness testing methods involved in this invention are as follows: Solidus temperature was determined using differential scanning calorimetry (DSC). The DSC curve shows an endothermic peak during melting, indicating the alloy's melting process. The melting temperature of the alloy can be determined by analyzing the characteristic temperature points of this endothermic peak. Following GB / T 1425-2021 "Determination of Melting Temperature Range of Precious Metals and Their Alloys - Thermal Analysis Test Methods", a NETZSCH DSC 404 F1 differential scanning calorimeter was used for testing. An argon atmosphere was maintained throughout, with a purge gas Ar flow rate of 20.0 mL / min and a protective gas Ar flow rate of 50.0 mL / min. The temperature was increased from room temperature to 700℃ at a rate of 10.0 K / min. The DSC curve was analyzed using NETZSCH Proteus software.

[0088] Oxide film thickness: A small sample (10*5*1mm) is cut from the sample to be tested, perpendicular to the surface. The sample is then embedded in epoxy resin, ground, and polished until the cross-section has a mirror-like finish. The prepared cross-sectional sample is placed in a SEM and observed using a backscattered electron (BSE) detector. The oxide film layer will exhibit different grayscale contrasts in the BSE image, clearly showing the layered structure of the oxide film and its boundary with the substrate. The thickness of the oxide film layer can be measured vertically at 10 randomly selected locations in the image using the image analysis software provided with the SEM, and the average value is taken.

[0089] Table 1. Alloy element composition (mass percentage, wt%) of various brazing alloy billets in different embodiments

[0090] Note: "-" in the table indicates that the element was not detected. Unavoidable impurity control complies with the routine control requirements in national standards, specifically, the content of a single impurity is less than 0.05 wt%, and the total impurity content is less than 0.15 wt%.

[0091] Comparative Example 1 Comparative Example 1 follows the same preparation method as Example 1, except that the target required composition of the Al-Zn-Si solder alloy billet in step (a) is different. The rest is the same as in Example 1. The specific differences are as follows: The target required composition of the Al-Zn-Si solder alloy billet in step (a) of this comparative example, by mass percentage, is: Si 6%, Zn 1%, Mg-free; the solidus temperature of the alloy is 562℃. In the prepared composite plate, the proportion of Zn element in the Al-Zn-Si brazing alloy layer is 0.1 wt%.

[0092] Comparative Example 2 Comparative Example 2 follows the same preparation method as Example 1, except that the target composition of the Al-Si-Mg solder alloy billet in step (a) is different. The rest is the same as in Example 1. The specific differences are as follows: The target composition of the Al-Si-Mg solder alloy billet in step (a) of this comparative example, by mass percentage, is: Si 8.5%, Mg 0.1%, without Zn and Bi; the solidus temperature of the alloy is 573℃. In the prepared composite plate, the proportion of Mg element in the Al-Si-Mg solder alloy layer to the total amount of brazing composite layer is 0.01 wt%.

[0093] Comparative Example 3 Comparative Example 3 follows the same preparation method as Example 1, except that the target composition of the Al-Si-Mg solder alloy billet in step (a) is different. All other aspects are the same as in Example 1, as detailed below: The target composition of the Al-Si-Mg solder alloy billet in step (a) of this comparative example, by mass percentage, is: Si 8.5%, Mg 3.0%, free of Zn and Bi; the solidus temperature of the alloy is 559°C. In the prepared composite plate, the proportion of Mg element in the Al-Si-Mg solder alloy layer is 0.3 wt% of the total amount of brazing composite layer.

[0094] Comparative Example 4 Comparative Example 4 follows the same preparation method as Example 1, except that the coating time for the anodic coating treatment of Al-Si solder alloy billets, Al-Zn-Si solder alloy billets, and Al-Si-Mg solder alloy billets in step (b) is different. All other steps are the same as in Example 1, with the specific differences shown below: In step (b), the Al-Si solder alloy billet is subjected to anodizing treatment for 33 s to form an oxide film layer with a thickness of 250 nm on the upper surface of the Al-Si solder alloy billet; the Al-Zn-Si solder alloy billet is subjected to anodizing treatment for 50 s to form an oxide film layer with a thickness of 449 nm on the upper surface of the Al-Zn-Si solder alloy billet; and the Al-Si-Mg solder alloy billet is subjected to anodizing treatment for 38 s to form an oxide film layer with a thickness of 350 nm on the upper surface of the Al-Si-Mg solder alloy billet. In the composite board, the oxide film thicknesses on the surfaces of the Al-Si solder alloy layer, Al-Zn-Si solder alloy layer, and Al-Si-Mg solder alloy layer are 25 nm, 45 nm, and 35 nm, respectively, and the total thickness of each oxide film layer in the finished product is 105 nm.

[0095] Comparative Example 5 Comparative Example 5 follows the same preparation method as Example 1, except that the Al-Zn-Si solder alloy billet was not subjected to anodic coating treatment in step (b). All other aspects are the same as in Example 1, with the specific differences as follows: In step (b), the Al-Zn-Si solder alloy billet was not subjected to anodic coating treatment, and a 30nm thick oxide film layer was formed by natural oxidation on the surface of the Al-Zn-Si solder alloy billet. In step (c), under the same composite process, the oxide film layer thickness on the surface of the Al-Zn-Si solder alloy layer was 3nm, and the total thickness of all oxide film layers in the finished product was 48nm.

[0096] Comparative Example 6 Comparative Example 6 was prepared using the same method as Example 1, except that the Al-Si-Mg solder alloy billet was not subjected to anodic coating treatment in step (b). All other aspects were the same as in Example 1, with the specific differences as follows: In step (b), the Al-Si-Mg solder alloy billet was not subjected to anodic coating treatment, and an oxide film layer with a thickness of 31 nm was formed by natural oxidation on the surface of the Al-Si-Mg solder alloy billet. In step (c), under the same composite process, the oxide film layer on the surface of the Al-Si-Mg solder alloy layer was 3 nm thick, and the total thickness of all oxide film layers in the finished product was 48 nm.

[0097] Comparative Example 7 Comparative Example 7 follows the same preparation method as Example 1, except that the Al-Si solder alloy billet was not anodized in step (b). All other steps are identical to Example 1, with the specific differences shown below: In step (b), the Al-Si solder alloy billet was not subjected to anodic coating treatment, and an oxide film layer with a thickness of 31 nm was formed by natural oxidation on the surface of the Al-Si solder alloy billet. In step (c), under the same composite process, the oxide film layer thickness on the surface of the Al-Si solder alloy layer was 3 nm, and the total thickness of all oxide film layers in the finished product was 63 nm.

[0098] Comparative Example 8 Comparative Example 8 follows the same preparation method as Example 1, except that the coating time for the anodic coating treatment of the Al-Si solder alloy billet in step (b) is different. All other steps are the same as in Example 1, as detailed below: In step (b), the Al-Si solder alloy billet is subjected to anodic coating treatment for 33 s to form an oxide film layer with a thickness of 250 nm on the surface of the Al-Si solder alloy billet; in step (c), under the same composite process, the oxide film layer thickness on the surface of the Al-Si solder alloy layer is 25 nm, and the total thickness of each oxide film layer in the finished product is 85 nm.

[0099] Test case To illustrate the performance differences of composite plates from different embodiments and comparative examples, the composite plates were subjected to the following brazing experiment.

[0100] Brazing experiment: A T-test was used to measure the length of the welded joint. First, according to... Figure 3 The schematic diagram of the assembly structure shown illustrates the assembly of a T-shaped sample. The T-shaped sample is then placed in a CAB tunnel furnace for simulated brazing. Brazing process: O2 concentration is below 10 ppm. The brazing temperature profile is as follows: heat to 600℃ at a rate of 35℃ / min, hold at 600~603℃ for 4 min, then cool to 220℃ at a rate of 35℃ / min and allow to cool naturally in air to room temperature.

[0101] The length of the brazed T-shaped weld was measured directly along the weld trajectory using a conventional length measuring tool (such as a vernier caliper).

[0102] After brazing, the T-shaped sample was cut along a direction perpendicular to the surface of the composite plate (where the weld seam exists). The sample was then embedded in epoxy resin, followed by grinding and polishing until the cross-section achieved a mirror finish. The cross-section was observed and photographed using an optical microscope. The length of the welded portion (i.e., the weld leg size) was measured using analysis software, and the average value was taken.

[0103] Table 4 shows the test results of weld length and weld leg size in the brazing experiments of composite plates in different embodiments and comparative examples.

[0104] Table 4. Test results of brazing for different composite plates

[0105] As can be seen from the above test results, in the fluxless aluminum alloy composite plate of the present invention, the diffusion behavior of Mg element between layers is controlled by adjusting the number and thickness of the oxide film layer in the brazing composite layer, thereby ensuring sufficient film breaking while avoiding excessive Mg content on the surface.

[0106] Specifically, in Examples 1-6, ideal brazing quality can be obtained by adjusting the quantity and thickness of the oxide film layer in the brazing composite layer.

[0107] Furthermore, according to the test results of Examples 1 and 5-6, when the solidus temperature of the Al-Zn-Si solder alloy layer is higher than that of the Al-Si solder alloy layer, the Al-Si solder alloy layer melts before the formation of liquid channels. Once the liquid channels are formed, the Mg reaching the first solder layer is not controlled by the liquid channels, which is detrimental to the control of Mg element diffusion and affects the brazing effect to some extent. When the solidus temperature of the Al-Zn-Si solder alloy layer is higher than that of the Al-Si-Mg solder alloy layer, the second solder layer cannot preferentially form liquid channels in the early stages of brazing, making it difficult for Mg elements to quickly reach the surface in the early stages of brazing, thus affecting the brazing effect. Therefore, precise control of the solidus temperature of each layer to construct ideal liquid channels for Mg element diffusion will be more conducive to the control of Mg element diffusion.

[0108] According to the test results of Example 1 and Comparative Example 1, when the content of Zn element in the second solder layer is too low, sufficient liquid channels cannot be constructed, which leads to the initial diffusion of Mg element being hindered and is not conducive to the spreading after the film is broken.

[0109] According to the test results of Example 1 and Comparative Examples 2-3, when the Mg content in the third brazing filler layer is too low, it cannot effectively diffuse to the surface of the brazing composite layer to break the oxide film; if the Mg content is too high, a large amount of Mg will diffuse to the surface, causing excessive oxidation and generating a MgO film with a higher solidus temperature, which hinders brazing.

[0110] According to the test results of Example 1 and Comparative Example 4, when the total thickness of the oxide film is too large, the Mg content reaching the surface will be insufficient, making it difficult to completely break the film.

[0111] According to the test results of Example 1 and Comparative Examples 5-6, the second or third oxide film layer is an oxide film layer formed by natural oxidation without treatment. When the thickness of the second or third oxide film layer is insufficient, the amount of Mg element reaching the surface is easily out of control, making it difficult to achieve the effect of slow release and film breaking.

[0112] According to the test results of Example 1 and Comparative Examples 7-8, if the thickness of the first oxide film layer is too large, it will be difficult for Mg elements to break through the film effectively; if the thickness of the first oxide film layer is too small, the amount of Mg elements reaching the surface will be uncontrolled, and the formation of a MgO film with a higher solidus temperature will hinder brazing.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A flux-free aluminum alloy composite plate, characterized in that, It includes a core layer and a brazed composite layer located on at least one surface of the core layer; The brazing composite layer includes a first brazing filler layer, a second brazing filler layer, and a third brazing filler layer; the second brazing filler layer is located between the first brazing filler layer and the third brazing filler layer, and the first brazing filler layer constitutes the outermost surface of at least one side of the fluxless aluminum alloy composite plate; The first solder layer includes at least one Al-Si solder alloy layer containing a first oxide film; the second solder layer includes at least one Al-Zn-Si solder alloy layer containing a second oxide film; the third solder layer includes at least one Al-Si-Mg solder alloy layer containing a third oxide film. The Zn content in the second solder layer accounts for 0.5~7.5 wt% of the total amount of the brazed composite layer; the Mg content in the third solder layer accounts for 0.03~0.24 wt% of the total amount of the brazed composite layer. The total thickness of the first oxide film layer, the second oxide film layer and the third oxide film layer is ≤100 nm.

2. The flux-free aluminum alloy composite plate according to claim 1, characterized in that, The thicknesses of the second oxide film layer and the third oxide film layer are each independently ≥10 nm; Preferably, the thickness of the first oxide film layer is 5~20 nm.

3. The flux-free aluminum alloy composite plate according to claim 1, characterized in that, The solidus temperature of each brazing alloy layer is ≤595℃; Preferably, the solidus temperature of the Al-Zn-Si solder alloy layer is lower than that of the Al-Si solder alloy layer and the Al-Si-Mg solder alloy layer; Preferably, the solidus temperature of the Al-Si solder alloy layer is 1~94°C higher than that of the Al-Zn-Si solder alloy layer; more preferably, it is 19~60°C higher. Preferably, the solidus temperature of the Al-Si-Mg solder alloy layer is 1~61°C higher than that of the Al-Zn-Si solder alloy layer; more preferably, it is 11~57°C higher.

4. The flux-free aluminum alloy composite plate according to claim 1, characterized in that, The Al-Zn-Si solder alloy layer comprises, by mass percentage, 6%~13% Si and 5%~25% Zn; The Al-Si solder alloy layer contains 6% to 13% Si by mass percentage. The Al-Si-Mg solder alloy layer comprises, by mass percentage, 6%~13% Si, ≤0.3% Mg, and ≤0.2% Bi; Preferably, the Al-Zn-Si solder alloy layer does not contain Mg. Preferably, the Al-Si solder alloy layer does not contain Mg.

5. The flux-free aluminum alloy composite plate according to claim 1, characterized in that, The total thickness of the first solder layer accounts for 10-80% of the total thickness of the brazed composite layer; Preferably, the total thickness of the second solder layer accounts for 10-30% of the total thickness of the brazing composite layer; Preferably, the total thickness of the third solder layer accounts for 10-80% of the total thickness of the brazing composite layer.

6. The flux-free aluminum alloy composite plate according to claim 1, characterized in that, The core layer comprises at least one of 1XXX series aluminum alloys, 3XXX series aluminum alloys, and 6XXX series aluminum alloys.

7. The method for preparing the flux-free aluminum alloy composite plate according to any one of claims 1 to 6, characterized in that, Includes the following steps: (a) Ingot preparation: Prepare core alloy ingots, Al-Si brazing alloy ingots, Al-Zn-Si brazing alloy ingots, and Al-Si-Mg brazing alloy ingots according to the target required composition and thickness. (b) Ingot oxidation process: Al-Si solder alloy ingots, Al-Zn-Si solder alloy ingots, and Al-Si-Mg solder alloy ingots are subjected to anodic coating treatment to form an oxide film layer; (c) Composite process: The core alloy, Al-Si-Mg brazing alloy, Al-Zn-Si brazing alloy and Al-Si brazing alloy are stacked in sequence and hot rolled and bonded, and then rolled to thin to the target size.

8. The preparation method according to claim 7, characterized in that, The thickness of the oxide film in the Al-Si solder alloy billet, Al-Zn-Si solder alloy billet, and Al-Si-Mg solder alloy billet after anodic coating treatment is independently 100~1000 nm. Preferably, in the anodic coating treatment, the coating solution is a sulfuric acid solution with a mass concentration of 10%~15%, and the current density is 1~2 A / dm³. 2 The coating time is 10~60 s.

9. A brazed composite plate that can be used as an intermediate product in the manufacture of the fluxless aluminum alloy composite plate of claim 1, the brazed composite plate comprising a first brazing filler layer as described in any one of claims 1 to 5, a second brazing filler layer as described in any one of claims 1 to 5, and a third brazing filler layer as described in any one of claims 1 to 5, wherein the second brazing filler layer is located between the first brazing filler layer and the third brazing filler layer.

10. The application of the fluxless aluminum alloy composite plate according to any one of claims 1 to 6 or the fluxless aluminum alloy composite plate prepared by the preparation method according to any one of claims 7 to 8 in fluxless CAB brazing.

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