A flux for aluminum alloy brazing and its preparation method

Abstract

This invention relates to the field of brazing technology, specifically to a flux for brazing aluminum alloys and its preparation method, comprising the following raw materials in parts by weight: 40-60 parts potassium fluoride, 40-60 parts aluminum fluoride, 3-6 parts composite powder, and 0.4-0.6 parts potassium fluorosilicate. In this invention, the composite powder is formed by ball milling and heat treatment of cesium fluoride, potassium fluoride, and aluminum fluoride under argon protection to pre-form a composite fluoride structure with better thermal stability. The additives further form a complex salt phase by rapidly cooling modified nano-silica, cesium carbonate, potassium carbonate, and potassium fluorotitanate after high-temperature melting. This composite powder, in combination with the base fluoride, significantly promotes the wetting and spreading of the brazing filler metal on the base material, ensuring a strong and tough metallurgical bond at the brazing interface, and ultimately improving the strength and reliability of the brazed joint, enabling it to handle brazing operations of large structural components with ease.

Description

Technical Field

[0001] This invention relates to the field of brazing technology, specifically to a flux for brazing aluminum alloys and its preparation method. Background Technology

[0002] Aluminum alloy brazing is an important process that connects parts by melting a filler metal with a melting point lower than that of the base metal and filling the joint gap. In this process, flux plays a crucial role. It is mainly used to remove the oxide film on the surface of the base metal and the filler metal, reduce the surface tension of the liquid filler metal, improve wetting and spreading performance, and protect the brazing process from secondary oxidation. Non-corrosive and highly active fluoroaluminate fluxes are key materials widely used in aluminum alloy brazing, especially vacuum brazing.

[0003] In existing technologies, common potassium fluoroaluminate-based fluxes, when used for brazing large structural components, suffer from insufficient thermal stability of their active components at high temperatures due to the relatively long brazing thermal cycle time. This leads to premature decomposition and performance degradation, resulting in decreased flux activity and weakened film removal ability in the later stages of brazing. Consequently, the interfacial reaction between the liquid filler metal and the aluminum alloy base material is incomplete, negatively impacting the mechanical strength of the final brazed joint. Therefore, this invention provides a flux for brazing aluminum alloys and its preparation method. Summary of the Invention

[0004] The purpose of this invention is to provide a flux for aluminum alloy brazing and its preparation method. The flux for aluminum alloy brazing prepared by this invention not only has good brazed joint strength and activity retention ability under long-term operation, but also significantly reduces the residue after brazing and improves the surface cleanliness of the workpiece.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a flux for brazing aluminum alloys, comprising the following raw materials in parts by weight: 40-60 parts potassium fluoride, 40-60 parts aluminum fluoride, 3-6 parts composite powder, and 0.4-0.6 parts potassium fluorosilicate; The composite powder is prepared by the following method: S1: Preparation of mixed powder, wherein the raw materials of the mixed powder include cesium fluoride, potassium fluoride, aluminum fluoride and a mixture; S2: Preparation of additives, wherein the raw materials for the additives include modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate, and the mass of the additives is 25-35% of the mass of the mixed powder; S3: Mixing process, after mixing the powder and additives, a composite powder is obtained.

[0006] Preferably, the method for preparing the mixed powder is as follows: weigh cesium fluoride, potassium fluoride, and aluminum fluoride and add them to a ball mill jar. Fill the ball mill jar with argon gas and treat it at room temperature at 300-400 rpm for 2-4 hours. Then, heat the ball mill jar to 500-540℃ at a heating rate of 3-5℃ / min and hold it at that temperature for 1 hour. During the heating and holding process, the ball mill jar is set to rotate at 50-100 rpm to obtain a premix. Add the premix and the mixed material to a mixer and stir at 100-200 rpm for 30-50 minutes to obtain the mixed powder.

[0007] Preferably, the mass ratio of cesium fluoride, potassium fluoride, and aluminum fluoride is 40-42:22-25:33-35, and the mass ratio of premix to mixture is 1:0.3-0.5.

[0008] Preferably, the mixture is prepared by the following method: lithium fluoride and potassium fluoroaluminate are mixed and dried at 120-150°C for 4-5 hours. The resulting product is added to a ball mill jar, which is filled with argon gas and treated at room temperature at 300-400 rpm for 2-4 hours. Then, the ball mill jar is heated to 520-550°C at a heating rate of 3-5°C / min and held at that temperature for 1 hour. During the heating and holding process, the ball mill jar is set to rotate at 50-100 rpm to obtain the mixture.

[0009] Preferably, the mass ratio of lithium fluoride to potassium fluoroaluminate is 80-85:15-20.

[0010] Preferably, the method for preparing the additive is as follows: weigh modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate and mix them. Under argon protection, melt them at 580-620℃ for 1-2 hours. The resulting melt is quickly poured into deionized water and cooled rapidly. The resulting product is dried and ground until the particle size is less than 40μm to obtain the additive.

[0011] Preferably, the mass ratio of modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate is 5-8:3-5:7-10:2-4.

[0012] Preferably, the modified nano-silica is prepared by the following method: fumed nano-silica is dispersed in anhydrous ethanol to obtain a suspension. Under stirring and a water bath at 60-80°C, a mixed solution is added dropwise to the suspension. After the addition is complete, the reaction is continued at the temperature for 4-6 hours. The obtained product is centrifuged and separated. The solid product is washed three times with ethanol and dried under vacuum at 120°C for 6 hours to obtain the modified nano-silica.

[0013] Preferably, the mass ratio of fumed silica nanoparticles to anhydrous ethanol is 1:25-50, the mass of the mixture is 10-15% of the mass of the suspension, and the mixture is prepared by mixing KH550, potassium fluorozirconate and anhydrous ethanol in a mass ratio of 1:0.3-0.5:8-10.

[0014] A preferred method for preparing flux for aluminum alloy brazing includes the following steps: weighing potassium fluoride and aluminum fluoride, mixing them, placing them in a graphite crucible, melting them at 580-600℃ for 1 hour under argon protection, cooling the melt, crushing it with a pulverizer, and sieving it through a 200-mesh sieve to obtain a base powder, adding the base powder, composite powder, and potassium fluorosilicate to a mixer, and stirring at 40-60 rpm for 2-4 hours to obtain the flux for aluminum alloy brazing.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. In this invention, the mixed powder in the composite powder is formed by ball milling and heat treatment of cesium fluoride, potassium fluoride, and aluminum fluoride under argon protection, resulting in a composite fluoride structure with better thermal stability. The additives further form a complex salt phase by rapidly cooling modified nano-silica, cesium carbonate, potassium carbonate, and potassium fluorotitanate after high-temperature melting. This composite powder, in combination with the base fluoride, can release active fluoride ions more controllably and for a longer period of time at the brazing temperature, effectively maintaining the film removal ability throughout the entire brazing cycle. This significantly promotes the wetting and spreading of the brazing filler metal on the base material, ensuring a strong and tough metallurgical bond at the brazing interface, and ultimately improving the strength and reliability of the brazed joint. It can easily handle the brazing of large structural components.

[0016] 2. In this invention, by introducing nano-silica that has been synergistically modified with KH550 and potassium fluorozirconate and dispersing it uniformly in the flux system, the physical morphology and removability of the brazing residue are effectively improved. During the brazing process, these surface-modified nanoparticles can act as nucleation cores, refine the brazing seam structure, and play a supporting role in the skeleton after the flux solidifies. Together with the stable structure in the composite powder, they make the residual flux layer structure more porous and brittle, and weaken the bonding force with the workpiece surface. This makes the flux residue easier to remove completely in the subsequent cleaning process, greatly reducing the amount of residue on the workpiece surface and improving the cleanliness and quality of the product. Attached Figure Description

[0017] Figure 1 This is a flowchart of a flux preparation method for aluminum alloy brazing proposed in this invention. Detailed Implementation

[0018] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] It should be noted that the raw materials used in the following embodiments are all commercially available.

[0020] Example 1: Prepare the following raw materials by weight: 40 parts potassium fluoride, 40 parts aluminum fluoride, 3 parts composite powder, and 0.4 parts potassium fluorosilicate; The composite powder is prepared by the following method: S1: Preparation of mixed powder, the raw materials of which include cesium fluoride, potassium fluoride, aluminum fluoride and mixture; S2: Additive preparation, the raw materials for the additive include modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate, and the mass of the additive is 25% of the mass of the mixed powder; S3: Mixing process, after mixing the powder and additives, a composite powder is obtained.

[0021] The method for preparing the mixed powder is as follows: Cesium fluoride, potassium fluoride, and aluminum fluoride are weighed and added to a ball mill jar at a mass ratio of 40:22:33. Argon gas is introduced into the ball mill jar, and the mixture is treated at 300 rpm for 2 hours at room temperature. Then, the ball mill jar is heated to 500℃ at a heating rate of 3℃ / min and held at that temperature for 1 hour. During the heating and holding process, the ball mill jar is set to rotate at 50 rpm to obtain a premix. The premix and the mixed powder are added to a mixer at a mass ratio of 1:0.3 and stirred at 100 rpm for 30 minutes to obtain the mixed powder.

[0022] The mixture was prepared by the following method: lithium fluoride and potassium fluoroaluminate were mixed at a mass ratio of 80:15 and dried at 120°C for 4 hours. The resulting product was added to a ball mill jar, which was filled with argon gas and treated at room temperature at 300 rpm for 2 hours. Then, the ball mill jar was heated to 520°C at a heating rate of 3°C / min and held at that temperature for 1 hour. During the heating and holding process, the ball mill jar was set to rotate at 50 rpm to obtain the mixture.

[0023] The additive is prepared by weighing modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate in a mass ratio of 5:3:7:2 and mixing them. Under argon protection, the mixture is melted at 580℃ for 1 hour. The resulting melt is then rapidly poured into deionized water and cooled. The resulting product is dried and ground until the particle size is less than 40μm to obtain the additive.

[0024] Modified nano-silica was prepared by the following method: fumed nano-silica was dispersed in anhydrous ethanol to obtain a suspension. Under stirring and a 60°C water bath, a mixed solution was added dropwise to the suspension. After the addition was complete, the reaction was continued at this temperature for 4 hours. The resulting product was centrifuged, and the solid product was washed three times with ethanol and then vacuum dried at 120°C for 6 hours to obtain modified nano-silica. The mass ratio of fumed nano-silica to anhydrous ethanol was 1:25, and the mass of the mixed solution was 10% of the mass of the suspension. The mixed solution was prepared by mixing KH550, potassium fluorozirconate, and anhydrous ethanol in a mass ratio of 1:0.3:8.

[0025] A method for preparing flux for aluminum alloy brazing includes the following steps: weighing potassium fluoride and aluminum fluoride, mixing them and placing them in a graphite crucible, melting them at 580℃ for 1 hour under argon protection, cooling the melt and crushing it with a pulverizer, sieving it through a 200-mesh sieve to obtain a base powder, adding the base powder, composite powder and potassium fluorosilicate into a mixer, stirring at 40 rpm for 2 hours to obtain the flux for aluminum alloy brazing.

[0026] Example 2: Prepare the following raw materials by weight: 50 parts potassium fluoride, 50 parts aluminum fluoride, 4 parts composite powder, and 0.5 parts potassium fluorosilicate; The composite powder is prepared by the following method: S1: Preparation of mixed powder, the raw materials of which include cesium fluoride, potassium fluoride, aluminum fluoride and mixture; S2: Additive preparation, the raw materials for the additive include modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate, and the mass of the additive is 30% of the mass of the mixed powder; S3: Mixing process, after mixing the powder and additives, a composite powder is obtained.

[0027] The method for preparing the mixed powder is as follows: Cesium fluoride, potassium fluoride, and aluminum fluoride are weighed and added to a ball mill jar at a mass ratio of 41:23:34. Argon gas is introduced into the ball mill jar, and the mixture is treated at 350 rpm for 3 hours at room temperature. Then, the ball mill jar is heated to 520℃ at a heating rate of 4℃ / min and held at that temperature for 1 hour. During the heating and holding process, the ball mill jar is set to rotate at 80 rpm to obtain a premix. The premix and the mixed powder are added to a mixer at a mass ratio of 1:0.4 and stirred at 150 rpm for 40 minutes to obtain the mixed powder.

[0028] The mixture was prepared by the following method: lithium fluoride and potassium fluoroaluminate were mixed at a mass ratio of 83:18 and dried at 135°C for 4.5 hours. The resulting product was added to a ball mill jar, which was filled with argon gas and treated at room temperature at 350 rpm for 3 hours. Then, the ball mill jar was heated to 535°C at a heating rate of 4°C / min and held at that temperature for 1 hour. During the heating and holding process, the ball mill jar was set to rotate at 80 rpm to obtain the mixture.

[0029] The additive is prepared by weighing modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate in a mass ratio of 6:4:8:3 and mixing them. Under argon protection, the mixture is melted at 600℃ for 1.5h. The resulting melt is then rapidly poured into deionized water and quenched. The resulting product is dried and ground until the particle size is less than 40μm to obtain the additive.

[0030] Modified nano-silica was prepared by the following method: fumed nano-silica was dispersed in anhydrous ethanol to obtain a suspension. Under stirring and a 70°C water bath, a mixed solution was added dropwise to the suspension. After the addition was complete, the reaction was continued at this temperature for 5 hours. The resulting product was centrifuged, and the solid product was washed three times with ethanol and then vacuum dried at 120°C for 6 hours to obtain modified nano-silica. The mass ratio of fumed nano-silica to anhydrous ethanol was 1:40, and the mass of the mixed solution was 13% of the mass of the suspension. The mixed solution was prepared by mixing KH550, potassium fluorozirconate, and anhydrous ethanol in a mass ratio of 1:0.4:9.

[0031] A method for preparing flux for aluminum alloy brazing includes the following steps: weighing potassium fluoride and aluminum fluoride, mixing them and placing them in a graphite crucible, melting them at 600℃ for 1 hour under argon protection, cooling the melt and crushing it with a pulverizer, sieving it through a 200-mesh sieve to obtain a base powder, adding the base powder, composite powder and potassium fluorosilicate into a mixer, stirring at 50 rpm for 3 hours to obtain the flux for aluminum alloy brazing.

[0032] Example 3: Prepare the following raw materials by weight: 60 parts potassium fluoride, 60 parts aluminum fluoride, 6 parts composite powder, and 0.6 parts potassium fluorosilicate; The composite powder is prepared by the following method: S1: Preparation of mixed powder, the raw materials of which include cesium fluoride, potassium fluoride, aluminum fluoride and mixture; S2: Additive preparation, the raw materials for the additive include modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate, and the mass of the additive is 35% of the mass of the mixed powder; S3: Mixing process, after mixing the powder and additives, a composite powder is obtained.

[0033] The method for preparing the mixed powder is as follows: Cesium fluoride, potassium fluoride, and aluminum fluoride are weighed in a mass ratio of 42:25:35 and added to a ball mill jar. Argon gas is introduced into the ball mill jar, and the mixture is treated at room temperature at 400 rpm for 4 hours. Then, the ball mill jar is heated to 540℃ at a heating rate of 5℃ / min and held at that temperature for 1 hour. During the heating and holding process, the ball mill jar is set to rotate at 100 rpm to obtain a premix. The premix and the mixed powder are added to a mixer in a mass ratio of 1:0.5 and stirred at 200 rpm for 50 minutes to obtain the mixed powder.

[0034] The mixture was prepared by the following method: lithium fluoride and potassium fluoroaluminate were mixed at a mass ratio of 85:20 and dried at 150°C for 5 hours. The resulting product was added to a ball mill jar, which was filled with argon gas and treated at room temperature at 400 rpm for 4 hours. Then, the ball mill jar was heated to 550°C at a heating rate of 5°C / min and held at that temperature for 1 hour. During the heating and holding process, the ball mill jar was set to rotate at 100 rpm to obtain the mixture.

[0035] The additive is prepared by weighing modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate in a mass ratio of 8:5:10:4 and mixing them. Under argon protection, the mixture is melted at 620℃ for 2 hours. The resulting melt is then rapidly poured into deionized water and cooled. The resulting product is dried and ground until the particle size is less than 40μm to obtain the additive.

[0036] Modified nano-silica was prepared by the following method: fumed nano-silica was dispersed in anhydrous ethanol to obtain a suspension. Under stirring and an 80°C water bath, a mixed solution was added dropwise to the suspension. After the addition was complete, the reaction was continued at this temperature for 6 hours. The resulting product was centrifuged, and the solid product was washed three times with ethanol and then vacuum dried at 120°C for 6 hours to obtain modified nano-silica. The mass ratio of fumed nano-silica to anhydrous ethanol was 1:50, and the mass of the mixed solution was 15% of the mass of the suspension. The mixed solution was prepared by mixing KH550, potassium fluorozirconate, and anhydrous ethanol in a mass ratio of 1:0.5:10.

[0037] A method for preparing flux for aluminum alloy brazing includes the following steps: weighing potassium fluoride and aluminum fluoride, mixing them and placing them in a graphite crucible, melting them at 600℃ for 1 hour under argon protection, cooling the melt and crushing it with a pulverizer, sieving it through a 200-mesh sieve to obtain a base powder, adding the base powder, composite powder and potassium fluorosilicate into a mixer, stirring at 60 rpm for 4 hours to obtain the flux for aluminum alloy brazing.

[0038] Comparative Example 1: The difference between this comparative example and Example 1 is that no composite powder is added in this comparative example, that is, the flux is made only of potassium fluoride, aluminum fluoride and potassium fluorosilicate.

[0039] Comparative Example 2 differs from Example 1 in that: no modified nano-silica is added during the preparation of the additive in this comparative example, that is, the additive is only prepared by melting and rapidly cooling cesium carbonate, potassium carbonate and potassium fluorotitanate.

[0040] Comparative Example 3 differs from Example 1 in that: no mixing liquid is added during the preparation of modified nano-silica in this comparative example, that is, an equal amount of gas-phase nano-silica is directly used to replace the modified nano-silica.

[0041] Performance testing: The fluxes prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance testing.

[0042] Shear strength test of brazed joints: Referring to GB / T11363-2008 standard, 3003 aluminum alloy was used as the base material to prepare standard 100mm×20mm×2mm lap joint samples. BAISI-4 aluminum silicon brazing filler metal was used. Brazing was performed using the fluxes of Examples 1-3 and Comparative Examples 1-3 respectively, and shear tests were performed on the brazed joints until joint failure. The shear strength (MPa) of the brazed joints was recorded in Table 1. This index directly reflects the metallurgical bonding quality of the brazed seam and the reliability of the joint. Solder spread area test: Referring to GB / T11364-1989 standard, 0.2g of solder was placed in the center of a 50mm×50mm×2mm 3003 aluminum alloy plate, and the flux to be tested was covered on the solder. The test was conducted at a brazing temperature of 600℃ and a holding time of 5min. After cooling, the flux residue was cleaned, and the spread area (mm²) of the solder on the base material was measured and recorded in Table 1. The larger the spread area, the stronger the flux's film removal and wetting ability, which indirectly reflects its activity and thermal stability. High-temperature long-term heat preservation and spreading area retention rate test: Based on the brazing filler metal spreading area test, the room temperature heat preservation time was extended from 5 min to 30 min. The spreading area (mm²) after long-term heat preservation was tested and the spreading area retention rate (%) was calculated and recorded in Table 1. The calculation formula is: Spreading area retention rate (%) = (spreading area after long-term heat preservation / spreading area after conventional heat preservation) × 100%; Flux Residue Test: 0.5g of the flux to be tested was evenly coated on the surface of a clean 304 stainless steel plate and weighed (M1). The plate was then placed in a muffle furnace and heated to 600℃ in an air atmosphere and held for 5 minutes. After cooling, the plate was immersed in 80℃ deionized water for ultrasonic cleaning for 10 minutes. After drying with hot air, the plate was weighed (M2). The flux residue rate (%) was calculated and recorded in Table 1. The calculation formula is: Residue rate (%) = [(M2-M1) / initial flux mass] × 100%. The lower the residue rate, the easier it is to remove flux residue and the higher the cleanliness after brazing.

[0043] Table 1 Analysis and comparison of the data in Table 1 show that the fluxes prepared by the methods of Examples 1-3 have better performance than those of Comparative Examples 1-3. Among these, the shear strength and brazing filler metal spread area of ​​the brazed joint directly reflect the flux's film removal and wetting ability, as well as the final joint quality. Examples 1-3 exhibit excellent and stable high strength (108.5-110.2 MPa) and large spread area (365-372 mm²), which is attributed to the core role of the composite powder. The composite fluoride structure pre-formed by ball milling and heat treatment of the mixed powder, and the complex salt phase formed by the additives through melt quenching, together constitute a more thermally stable active fluoride source. During the brazing process, this structure can release fluoride ions more controllably and persistently, effectively maintaining the film removal ability at long-term high temperatures, ensuring sufficient wetting of the base material and interfacial metallurgical bonding of the filler metal. In contrast, Comparative Example 1 lacks the complex... The sharp decrease in shear strength and spread area after mixing the powder indicates that the absence of this core component leads to insufficient flux activity, poor thermal stability, and ineffective removal of the oxide film, severely weakening the joint bonding force. Although the performance of Comparative Example 2 (lacking modified nano-silica) and Comparative Example 3 (without modified nano-silica) is better than that of Comparative Example 1, it is still significantly lower than that of the example. This is because modified nano-silica not only affects the morphology of the residue, but also plays a role in refining the brazing seam structure as a nucleation core, which also contributes positively to improving the joint strength. The performance of Comparative Example 3 is better than that of Comparative Example 2. This shows that even without modification, the physical addition of nanoparticles still has a certain effect, but it is far less than the good compatibility and dispersibility with the flux system after synergistic modification with KH550 and potassium fluorozirconate.

[0044] The retention rate of the spread area after long-term high-temperature heat preservation mainly reflects whether the flux can overcome the adverse effects of activity decay under long-term working conditions. The retention rate of Examples 1-3 is as high as 91% or more, indicating that its active components have excellent thermal stability. After 30 minutes of long-term heat exposure, the film removal ability decays very little, which can meet the requirements of long-term brazing of large structural parts. The retention rate of Comparative Example 1 is the lowest (58.4%), which verifies that the lack of composite powder is the fundamental reason for the rapid decline of the activity of conventional flux under long-term high temperature. Although the retention rates of Comparative Examples 2 and 3 are higher than those of Comparative Example 1, they are still significantly lower than those of Examples. This shows that the introduction of modified nano silica, through synergistic effect with composite powder, further stabilizes the structure of the flux system and delays the performance degradation at high temperature.

[0045] Among them, the flux residue rate mainly reflects the cleanliness of the workpiece after brazing. The residue rates of Examples 1-3 are extremely low (1.5%-2.0%), which fully demonstrates the key role of modified nano-silica. After synergistic modification with KH550 and potassium fluorozirconate, the nano-silica is evenly dispersed in the flux and plays a skeletal support role during solidification, making the residue structure looser and more brittle, and weakening its bonding force with the matrix, thus making it easier to be completely removed in subsequent cleaning. The comparative example 1 without the addition of composite powder has the highest residue rate, which is because of the lack of... After the entire composite powder is lost, the behavior of the flux system becomes uncontrollable at high temperatures, resulting in a denser residue structure that is firmly bonded to the matrix, making it difficult to remove. In contrast, the residue rate of unmodified nano-silica in Comparative Example 2 is the second lowest. This is because the absence of this key component leads to denser residue and increased difficulty in removal. Although the residue rate of unmodified nano-silica in Comparative Example 3 is lower than that of Comparative Example 2, it is still much higher than that of the Example. This is mainly because untreated nano-silica is prone to agglomeration and has poor dispersibility, thus failing to effectively improve the morphology of the residue.

[0046] By comparing and analyzing the relevant data in the table, it can be seen that the flux for aluminum alloy brazing prepared by this invention not only has good brazed joint strength and activity retention under long-term operation, but also significantly reduces residue after brazing and improves the surface cleanliness of the workpiece. This indicates that the flux for aluminum alloy brazing provided by this invention has a broader market prospect and is more suitable for widespread application.

[0047] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0048] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A flux for brazing aluminum alloys, characterized in that: It includes the following raw materials in parts by weight: 40-60 parts potassium fluoride, 40-60 parts aluminum fluoride, 3-6 parts composite powder, and 0.4-0.6 parts potassium fluorosilicate; The composite powder is prepared by the following method: S1: Preparation of mixed powder, wherein the raw materials of the mixed powder include cesium fluoride, potassium fluoride, aluminum fluoride and a mixture; S2: Preparation of additives, wherein the raw materials for the additives include modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate, and the mass of the additives is 25-35% of the mass of the mixed powder; S3: Mixing process, after mixing the powder and additives, a composite powder is obtained.

2. The flux for brazing aluminum alloys according to claim 1, characterized in that, The method for preparing the mixed powder is as follows: weigh cesium fluoride, potassium fluoride, and aluminum fluoride and add them to a ball mill jar. Fill the ball mill jar with argon gas and treat it at room temperature at 300-400 rpm for 2-4 hours. Then, heat the ball mill jar to 500-540℃ at a heating rate of 3-5℃ / min and hold it at that temperature for 1 hour. During the heating and holding process, the ball mill jar is set to rotate at 50-100 rpm to obtain a premix. Add the premix and the mixed material to a mixer and stir at 100-200 rpm for 30-50 minutes to obtain the mixed powder.

3. The flux for brazing aluminum alloys according to claim 2, characterized in that, The mass ratio of cesium fluoride, potassium fluoride, and aluminum fluoride is 40-42:22-25:33-35, and the mass ratio of premix to mixture is 1:0.3-0.

5.

4. The flux for brazing aluminum alloys according to claim 2, characterized in that, The mixture is prepared by the following method: lithium fluoride and potassium fluoroaluminate are mixed and dried at 120-150℃ for 4-5 hours. The resulting product is added to a ball mill jar, which is filled with argon gas and treated at room temperature at 300-400 rpm for 2-4 hours. Then, the ball mill jar is heated to 520-550℃ at a heating rate of 3-5℃ / min and held at that temperature for 1 hour. During the heating and holding process, the ball mill jar is set to rotate at 50-100 rpm to obtain the mixture.

5. The flux for brazing aluminum alloys according to claim 4, characterized in that, The mass ratio of lithium fluoride to potassium fluoroaluminate is 80-85:15-20.

6. The flux for brazing aluminum alloys according to claim 1, characterized in that, The additive is prepared by weighing and mixing modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate, melting and treating them at 580-620℃ for 1-2 hours under argon protection, rapidly pouring the resulting melt into deionized water for rapid cooling, and drying and grinding the resulting product to a particle size of less than 40μm to obtain the additive.

7. The flux for brazing aluminum alloys according to claim 6, characterized in that, The mass ratio of modified nano-silica, cesium carbonate, potassium carbonate and potassium fluorotitanate is 5-8:3-5:7-10:2-4.

8. The flux for brazing aluminum alloys according to claim 6, characterized in that, The modified nano-silica was prepared by the following method: fumed nano-silica was dispersed in anhydrous ethanol to obtain a suspension. Under stirring and a water bath at 60-80℃, a mixed solution was added dropwise to the suspension. After the addition was complete, the reaction was continued at the temperature for 4-6 hours. The obtained product was centrifuged and separated. The solid product was washed three times with ethanol and dried under vacuum at 120℃ for 6 hours to obtain the modified nano-silica.

9. The flux for brazing aluminum alloys according to claim 8, characterized in that, The mass ratio of fumed silica nanoparticles to anhydrous ethanol is 1:25-50, and the mass of the mixture is 10-15% of the mass of the suspension. The mixture is prepared by mixing KH550, potassium fluorozirconate and anhydrous ethanol in a mass ratio of 1:0.3-0.5:8-10.

10. The method for preparing flux for aluminum alloy brazing according to claims 1-9, characterized in that, The process includes the following steps: weigh potassium fluoride and aluminum fluoride, mix them, and place them in a graphite crucible. Melt the mixture at 580-600℃ for 1 hour under argon protection. After the melt cools, crush it using a pulverizer and sieve it through a 200-mesh sieve to obtain a base powder. Add the base powder, composite powder, and potassium fluorosilicate to a mixer and stir at 40-60 rpm for 2-4 hours to obtain a flux for brazing aluminum alloys.

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