Low-corrosivity halogen-free soldering flux for aluminum-based part brazing and preparation method of low-corrosivity halogen-free soldering flux

Abstract

The invention discloses a low-corrosion halogen-free soldering flux for aluminum-based part brazing and a preparation method of the low-corrosion halogen-free soldering flux, and belongs to the technical field of halogen-free welding materials.The low-corrosion halogen-free soldering flux comprises, by mass, 4-6 parts of oxalic acid, 1-2 parts of triethanolamine, 0.8-1.5 parts of Tween-80, 4-6 parts of polyvinylpyrrolidone, 0.3-1 part of compound nano cerium oxide, 0.5-1.5 parts of chitosan oligomer and the balance deionized water. And the welding spot oxidation resistance and the interface self-repairing capability are further improved. Through the synergistic effect of four mechanisms of complexing, oxidation prevention, wetting and film forming, the flux achieves efficient film removal, excellent wetting, no corrosion residue and stable film forming protection, is high in shear strength, low in welding spot porosity and clean in surface after welding, and is suitable for electronic packaging, heat exchanger manufacturing and high-reliability aluminum welding occasions.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of brazing materials and environmentally friendly welding, and particularly relates to a low-corrosion halogen-free flux for brazing of aluminum-based parts and a preparation method thereof. BACKGROUND

[0002] Aluminum and its alloys are widely used in heat exchangers, electronic packaging, automobile lightweighting and other fields due to their light weight, good heat conductivity and low cost. However, the dense oxide film on the surface of aluminum seriously affects the welding wetting and brazing quality, so the use of flux to remove the oxide film and improve the joint interface becomes the key. The mainstream aluminum brazing flux on the market is mostly halogen-containing system (ZnCl2, KF), which has good film removal property, but generally has the following problems: (1) strong halogen corrosion, easy to remain, reduces the reliability of the weld; (2) a large amount of water washing or passivation treatment is required after welding, which is complicated and not environmentally friendly; (3) the flux is easy to produce carbonized residue at high temperature, which pollutes the welding surface. Therefore, it has become a research hotspot to develop a composite flux with green and environmentally friendly, halogen-free, good thermal stability, and both brazing wetting and joint protection functions. SUMMARY

[0003] To solve the problems mentioned in the background, the application relates to the technical field of brazing materials, and particularly relates to a low-corrosion halogen-free flux for brazing of aluminum-based parts and a preparation method thereof. Through the synergistic effect of complexation-oxidation prevention-wetting-film forming four mechanisms, efficient film removal, excellent wetting, no corrosion residue and stable film forming protection are realized, and the shear strength is high, the weld porosity is low, and the surface is clean after welding.

[0004] The object of the application can be achieved by the following technical solutions:

[0005] The application discloses a low-corrosion halogen-free flux for brazing of aluminum-based parts, which comprises the following components in parts by mass: oxalic acid 4-6 parts, triethanolamine 1-2 parts, Tween-80 0.8-1.5 parts, polyvinylpyrrolidone 4-6 parts, glycerol 1-3 parts, chitosan oligomer 0.2-0.8 parts, nano cerium oxide 0.3-0.7 parts, and deionized water is supplemented to 100 parts.

[0006] Optionally, the particle size of the nano cerium oxide is 10-30 nm, and the molecular weight of the chitosan oligomer is 1000-3000 Da.

[0007] Optionally, a preparation method of a low-corrosion halogen-free flux for brazing of aluminum-based parts is used to prepare the low-corrosion halogen-free flux for brazing of aluminum-based parts, and comprises the following steps:

[0008] (1) adding nano cerium oxide and part of polyvinylpyrrolidone into deionized water to form a nano stable dispersion liquid through ultrasonic dispersion;

[0009] (2) Dissolve the chitosan oligomer in the hot deionized water with pH adjusted to form a bio-protective film precursor solution;

[0010] (3) Under the condition of 45-50 DEG C, oxalic acid and triethanolamine are added into the deionized water in sequence to form a complex-acid base system, and the pH is adjusted to 6.2-6.8;

[0011] (4) Glycerol, Tween-80 and the remaining polyvinylpyrrolidone are added to form a complex liquid base after fully stirring;

[0012] (5) The nanometer cerium oxide nanodispersion solution obtained in step (1) and the chitosan solution in step (2) are added into the complex liquid in sequence, and the system is uniformly dispersed and continuously stirred for 25-30 minutes;

[0013] (6) The deionized water is supplemented to 100 parts, and is placed under vacuum condition for 15-20 minutes for defoaming;

[0014] (7) After being filtered by 180-200 mesh polytetrafluoroethylene film, it is sealed and stored in the environment of 5-25 DEG C for standby.

[0015] Optionally, the ultrasonic frequency for dispersing the nanometer cerium oxide in step (1) is 30-40 kHz, and the duration time is 15-20 minutes.

[0016] Optionally, the mass ratio of the nanometer cerium oxide, part of the polyvinylpyrrolidone and the deionized water in step (1) is 1:3.1:27.7.

[0017] Optionally, the chitosan is dissolved by adjusting the pH to 5.0-5.5 by citric acid in step (2), and is stirred at 45-50 DEG C for 25-30 minutes.

[0018] Optionally, the mass ratio of the oxalic acid, triethanolamine and deionized water in step (3) is 10:3:25.

[0019] Optionally, the mass ratio of the glycerol, Tween-80, the remaining polyvinylpyrrolidone and deionized water in step (4) is 1.2:0.9:2.4:10.

[0020] Optionally, the rate of adding the chitosan solution in step (5) is 0.8-1 mL / min.

[0021] The beneficial effects of the present application are as follows:

[0022] The low-corrosion halogen-free flux for aluminum-based piece brazing provided by the present application:

[0023] (1) The formula of the present application does not contain any halogen compounds or corrosive inorganic salts, and the use of environmentally friendly organic small molecules and bio-based polymers will not release toxic gases or form corrosive residues during welding. The residue of the flux after welding is easy to remove, and the cleaning-free or low-residue welding can be achieved, which has good environmental compatibility and operator safety.

[0024] (2) The synergistic effect of oxalic acid and triethanolamine in the flux system can effectively dissolve the aluminum surface oxide film and complex the generated aluminum ions, promote the peeling of the oxide film, significantly improve the solder wettability of aluminum and aluminum alloy and the cleanliness of the welding point interface, and provide a reliable basis for high-strength welding.

[0025] (3) The flux of the present application is compounded with Tween-80, glycerol and polyvinylpyrrolidone, which can provide good wetting and spreading performance and interface adhesion during welding, significantly reduce the surface tension, improve the solder spreading effect, and make the welding point more dense, uniform and pore-free.

[0026] (4) The present application introduces nano cerium oxide particles, which can absorb oxygen free radicals and participate in surface film reduction reaction during brazing, have good antioxidant and passivation function, can form a stable protective interface in the welding point to improve the long-term stability and reliability of the welding point. The addition of chitosan oligomer in the flux can form a continuous microfilm on the interface after welding, which has the functions of slow release, film formation and self-repairing, can inhibit the interface crack propagation and corrosion diffusion under micro stress disturbance or humid heat environment, and significantly improves the welding point life and durability. BRIEF DESCRIPTION OF DRAWINGS

[0027] The present application will be further described below in conjunction with the drawings.

[0028] Figure 1 The synergistic mechanism diagram of the environmentally friendly halogen-free flux. DETAILED DESCRIPTION

[0029] The present application will be further described below in conjunction with specific examples, but the present application is not limited to the following embodiments. Equivalent adjustments made without departing from the spirit and essence of the present application should also be considered to fall within the scope of the present application.

[0030] Example 1:

[0031] Add oxalic acid 4.0 parts, triethanolamine 1.0 part, Tween-80 0.8 part, polyvinylpyrrolidone 4.0 parts, glycerol 1.0 part, chitosan oligomer 0.2 part, and nano cerium oxide 0.3 part to deionized water, and make up to 100 parts. The preparation steps are as follows:

[0032] (1) First, add nano cerium oxide and 1g polyvinylpyrrolidone to 10g deionized water and ultrasonic dispersion for 20 minutes;

[0033] (2) In another beaker, 40 g of deionized water was heated to 50°C, oxalic acid was added and stirred for 10 minutes until completely dissolved;

[0034] (3) Triethanolamine was added dropwise to adjust the pH to 6.3, and stirring was continued for 5 minutes; glycerol, Tween-80 and the remaining polyvinylpyrrolidone were added, and stirring was continued to form a uniform system;

[0035] (4) The dispersion solution in step 1 was slowly added, and a previously dissolved chitosan solution was added; the water was supplemented to 100 g, stirred for 30 minutes, vacuum degassed for 20 minutes, and then filtered to obtain the finished product.

[0036] Example 2:

[0037] Oxalic acid 6.0 parts, triethanolamine 2.0 parts, Tween-80 1.5 parts, polyvinylpyrrolidone 6.0 parts, glycerol 3.0 parts, chitosan 0.8 parts, nano cerium oxide 0.7 parts, and the rest was deionized water, supplemented to 100 parts. The preparation process is the same as example 1, and the ultrasonic dispersion and stirring time is appropriately prolonged.

[0038] Example 3:

[0039] Oxalic acid 5.0 parts, triethanolamine 1.5 parts, Tween-80 1.2 parts, polyvinylpyrrolidone 5.5 parts, glycerol 2.0 parts, chitosan 0.5 parts, nano cerium oxide 0.5 parts, and water was supplemented to 100 parts. The preparation process is the same as example 1, and the control chitosan drop rate is 0.8 mL / min during the preparation process, and the stirring speed is kept at 400 rpm.

[0040] Comparative Example 1:

[0041] Zinc chloride 5.0 parts, potassium fluoride 3.0 parts, glycerol 1.5 parts, and rosin 4.0 parts were added to 60°C water and stirred to prepare a traditional flux. The preparation process is the same as example 1, and the residue is serious after welding, which needs to be washed several times with deionized water.

[0042] Comparative Example 2:

[0043] The formula only contains triethanolamine 1.0 parts, Tween-80 1.0 parts, glycerol 2.0 parts, and polyvinylpyrrolidone 3.0 parts, and the preparation process is the same as example 1, without any film removing component.

[0044] Performance test

[0045] Shear strength test method: Shear strength is used to evaluate the mechanical bonding ability of the solder joint. The aluminum substrate lap solder sample is prepared with a soldering area of 10 mm x 10 mm. An electronic universal material testing machine (Instron 5965) is used to load at a constant tensile speed of 1 mm / min. The maximum load is recorded and converted to MPa. Each group of samples is not less than 5 pieces, and the average value is taken as the shear strength index of the flux in the group. The higher the shear strength value, the better the strength of the solder joint.

[0046] Solder joint porosity test method: Solder joint porosity reflects the wettability, film removal completeness and solder joint density during welding. The test uses IPC-TM-650 method. The post-weld sample is cold inlaid and sectioned for observation under a metallographic microscope. The total area and void area of the solder joint are measured with the help of image analysis software. The proportion is calculated and expressed as a percentage. The lower the porosity, the more uniform the welding process and the more complete the interface.

[0047] Post-weld residue cleaning test method: To evaluate the cleanability and environmental performance of post-weld surface residues, the sample after welding is placed at 25°C and 60% relative humidity for 48 hours. Then a dust-free cotton swab is used to wipe the solder joint surface with deionized water to observe whether there are obvious residues dissolved. At the same time, an ion contamination tester is used to detect the conductivity of the wiping liquid. The less residue and the lower conductivity indicate better post-weld cleaning performance of the flux.

[0048] Metal corrosion test

[0049] Thermal stability and storage performance test method: Thermal stability reflects the physical and chemical stability of the flux in a high temperature environment. The sealed flux sample is placed in a 70°C constant temperature oven for 7 days. The viscosity change is measured daily. The sample viscosity is measured using a Brookfield rotational viscometer at a constant shear rate, and the color, layering and precipitation appearance changes are recorded. If the viscosity change rate is less than 5% within 7 days, and there is no precipitation and layering phenomenon, it indicates that it has good thermal stability and storage performance.

[0050] Solder joint oxidation resistance test method (salt spray test): used to evaluate the stability of the solder joint in a humid and corrosive environment. According to the standard GB / T 10125-2012, a 5% sodium chloride solution is used for continuous salt spray at 35°C. The test period is 48 hours. After the test, the solder joint surface is observed for corrosion, black spots and crack defects. Samples with no obvious corrosion are determined to have good oxidation resistance.

[0051] Self-repairing test method: In order to verify the self-healing ability of functional components such as chitosan on the surface of the solder joint after micro-damage, an artificial scratch method is used to make a micro-crack with a width of <50 μm on the surface of the solder joint, and then the sample is placed in a 25℃, 90% relative humidity environment for 48 hours. The crack morphology is observed by atomic force microscopy to evaluate whether it is passivated, healed or edge reconstructed. The sample with film forming or micro-repairing ability shows crack closure or slow expansion trend.

[0052] Table 1 Performance test results of low-corrosion halogen-free flux for brazing of aluminum-based parts

[0053]

[0054]

[0055] In order to verify the performance advantages of the low-corrosion halogen-free flux for brazing of aluminum-based parts of the present application, examples 1-3 and comparative examples 1 and 2 were subjected to systematic performance comparison tests. The test results show that the flux of the present application is significantly better than the traditional or incomplete formula system in terms of shear strength, hole rate of solder joint, post-weld residue, electrical conductivity, thermal stability, oxidation resistance and repairable performance. The shear strength of the solder joint of the flux of the examples is higher than 45 MPa, among which example 2 reaches 51.8 MPa, which is significantly better than comparative example 1 (41.2 MPa) and comparative example 2 (38.7 MPa), indicating that the complex film removal system constructed by the synergistic action of oxalic acid and triethanolamine can efficiently remove the oxide film on the aluminum surface to promote the real metal bonding between the solder and the base material. The hole rate test results of the solder joint show that the hole rate of the solder joint of the examples is controlled at 0.2%-0.4%, while the hole rate of the solder joint of the comparative examples is generally more than 1.5%. The synergistic wetting effect of Tween-80 and glycerol in the flux improves the spreading property of the molten solder, effectively filling the joint and reducing the generation of gas inclusions and pores.

[0056] In terms of post-weld residue cleanliness, the conductivity of the examples is all below 1 µS / cm2, reflecting very low post-weld residue and good cleanliness, suitable for no-clean applications. The conductivity of the comparative examples using traditional halide or lacking film-removing components is significantly higher, with the risk of corrosion residue. The present application uses a halogen-free formula to avoid the use of corrosive salts such as chlorine and fluorine, which is green and environmentally friendly. In the thermal stability test, the viscosity change rate of the fluxes of the examples is less than 5% after 7 days of aging at 70°C, among which the viscosity change rate of example 2 is only 2.4%, showing good storage stability. However, the samples of the comparative examples show delamination and viscosity fluctuation, and the stability is insufficient. The polyvinylpyrrolidone and glycerol components in the system of the present application synergistically enhance the physical stability of the flux system. The salt spray test further verifies the oxidation resistance of the flux. After 48 hours of continuous neutral salt spray exposure, the solder joints of the examples show no corrosion or discoloration, which is significantly better than the rust phenomenon of the samples of the comparative examples. The results show that the trace addition of nano cerium oxide in the flux forms a stable protective film on the surface of the solder joint, significantly improving the corrosion resistance of the solder joint interface. In addition, the chitosan film-forming agent introduced in some examples has a certain self-repairing ability of the solder joint. In a humid and hot environment, the fine cracks on the surface of the solder joint can be partially passivated or closed within 48 hours, improving long-term reliability. The samples of the comparative examples show obvious crack propagation and no self-healing trend.

Claims

1. A low-corrosive halogen-free flux for brazing aluminum substrates, characterized in that: It comprises the following components in parts by mass: 4-6 parts of oxalic acid, 1-2 parts of triethanolamine, 0.8-1.5 parts of Tween-80, 4-6 parts of polyvinyl pyrrolidone, 1-3 parts of glycerol, 0.2-0.8 parts of chitosan oligomers, 0.3-0.7 parts of nano-cerium oxide, and deionized water to make up to 100 parts.

2. The low-corrosion halogen-free flux for brazing aluminum substrates according to claim 1, characterized in that: The particle size of the nano-cerium oxide is 10-30 nm, and the molecular weight of the chitosan oligomer is 1000-3000 Da.

3. The method for preparing a low-corrosive halogen-free flux for brazing of aluminum-based parts according to claim 1, which is used to prepare the low-corrosive halogen-free flux for brazing of aluminum-based parts according to claims 1-2, characterized in that: The following steps are involved: (1) adding nano-cerium oxide and a portion of polyvinyl pyrrolidone into deionized water and ultrasonically dispersing the mixture to form a nano-stable dispersion; (2) dissolving chitosan oligomers in hot deionized water with adjusted pH to form a biofilm precursor solution; (3) At 45-50°C, oxalic acid and triethanolamine were sequentially added to deionized water to form a complex-acidic base system, and the pH was adjusted to 6.2-6.8; (4) adding glycerol, Tween-80 and the remaining polyvinylpyrrolidone and stirring thoroughly to form a composite liquid base; (5) adding the nano-cerium oxide nanodispersion obtained in step (1) and the chitosan solution obtained in step (2) dropwise into the composite solution, keeping the system uniformly dispersed and stirring continuously for 25 to 30 minutes; (6) Add deionized water to 100 parts and let it stand for degassing under vacuum conditions for 15 to 20 minutes; (7) After filtering through a 180-200 mesh polytetrafluoroethylene membrane, seal and store at 5-25°C until use.

4. The method for preparing a low-corrosive halogen-free flux for brazing aluminum substrates according to claim 3, characterized in that: The ultrasonic frequency of the nano-cerium oxide dispersion in step (1) is 30 to 40 kHz, and the duration is 15 to 20 minutes.

5. The method for preparing a low-corrosive halogen-free flux for brazing aluminum substrates according to claim 3, characterized in that: In the step (1), the mass ratio of nano-cerium oxide, part of polyvinyl pyrrolidone and deionized water is 1:3.1:27.

7.

6. The method for preparing a low-corrosive halogen-free flux for brazing aluminum substrates according to claim 3, characterized in that: In the step (2), the chitosan is dissolved by adjusting the pH to 5.0-5.5 with citric acid, and stirring at 45-50° C. for 25-30 minutes.

7. The method for preparing a low-corrosive halogen-free flux for brazing aluminum substrates according to claim 3, characterized in that: In the step (3), the mass ratio of oxalic acid, triethanolamine and deionized water is 10:3:

25.

8. The method for preparing a low-corrosive halogen-free flux for brazing aluminum substrates according to claim 3, characterized in that: In the step (4), the mass ratio of glycerol, Tween-80, remaining polyvinyl pyrrolidone and deionized water is 1.2:0.9:2.4:

10.

9. The method for preparing a low-corrosive halogen-free flux for brazing aluminum substrates according to claim 3, characterized in that: The chitosan solution is added dropwise at a rate of 0.8 to 1 mL / min in step (5).

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