A brazing method for magnesium / aluminum dissimilar metals

Abstract

The application discloses a brazing method of magnesium / aluminum dissimilar metals, which comprises the following steps: pre-electroplating nickel on a magnesium alloy base material, connecting the prepared magnesium alloy with a nickel plating layer and an aluminum alloy by using an ultrasonic-assisted brazing method, and using a brazing filler metal SAC305. The method prevents the generation of Mg-Sn intermetallic compounds (IMCs) in the process of magnesium / aluminum dissimilar metal low-temperature brazing using a Sn-based brazing filler metal. After welding, the position of the original nickel plating layer is changed into a Cu-Ni solid solution, and no large block-shaped Mg2Sn IMC is found in the brazing seam. Compared with a joint without a plating layer, the shear strength of the magnesium / aluminum dissimilar metal brazing joint based on the nickel plating layer is greatly improved, and the joint strength is strengthened by the diffusion of the nickel element into the brazing seam in the welding process. In addition, the corrosion resistance of the magnesium alloy can be improved by plating nickel on the surface of the magnesium alloy, and the service life of the magnesium / aluminum component is prolonged.

Description

Technical Field

[0001] This invention belongs to the field of material interface improvement technology, specifically relating to a brazing method for magnesium / aluminum dissimilar metals. Background Technology

[0002] In recent years, with the increasing prominence of energy and environmental pollution issues, industrial transformation and upgrading, and lightweight development have become important strategic solutions. Magnesium alloys and aluminum alloys, as the two lightest metallic materials, are widely used in aerospace, high-speed trains, and the automotive industry due to their excellent properties such as high specific strength, good thermal conductivity, and good recyclability. Currently, more and more aluminum alloy parts are being replaced by lighter magnesium alloy parts, making the joining of magnesium / aluminum dissimilar metals crucial. Achieving highly reliable joining of magnesium / aluminum dissimilar metals can not only promote lightweight development but also further expand the application range of the material.

[0003] Currently, scholars have made significant progress in joining magnesium / aluminum dissimilar metals using various welding methods, including fusion welding, solid-state welding, and brazing. However, both magnesium and aluminum alloys have low melting points. Traditional fusion welding methods for magnesium / aluminum dissimilar metals easily lead to problems such as joint softening, element loss, and the formation of large amounts of intermetallic compounds that are difficult to control, affecting joint performance. This is mainly due to the high heat input during welding, causing severe heating of the base material and element loss. Furthermore, the high welding temperature intensifies the metallurgical reaction between magnesium and aluminum, promoting the formation of numerous intermetallic compounds in the joint. Therefore, controlling the welding heat input and studying low-temperature joining of magnesium / aluminum dissimilar metals is essential.

[0004] Currently, low-temperature joining of magnesium / aluminum dissimilar metals mainly utilizes Sn-based brazing filler metals. However, several problems exist in this process, primarily: First, Sn-based brazing filler metals readily form new compounds, such as Mg2Sn IMC. The presence of large Mg2Sn particles leads to low interfacial bonding strength, causing joint failure to often begin at the interface. Second, the Sn-based brazing filler metal itself has reduced strength, resulting in generally lower strength brazed joints requiring further strengthening. Furthermore, magnesium is chemically reactive, and its oxide film is not as dense as that of aluminum alloys, leading to poor corrosion resistance. Using electrodes with significantly different potentials from magnesium also increases the risk of electrochemical corrosion, affecting the usability of magnesium / aluminum joints. To achieve reliable low-temperature joining of magnesium / aluminum dissimilar metals, several prominent issues urgently need to be addressed:

[0005] 1. Suppression of IMCs: To avoid the generation of Mg2Sn IMCs during the brazing of magnesium / aluminum dissimilar metals with Sn-based brazing filler metals, which would affect the joint performance, and to achieve reliable low-temperature connection of magnesium / aluminum dissimilar metals.

[0006] 2. Joint strengthening: Improve the overall performance of the joint by adjusting the process and optimizing the alloying elements of the brazing filler metal.

[0007] 3. Improve the corrosion resistance of the joint by electroplating corrosion-resistant metals or performing surface treatments on the magnesium alloy surface to improve the corrosion resistance of the magnesium alloy and prevent corrosion cracking of the magnesium / aluminum joint. Summary of the Invention

[0008] To address the problems of easy oxidation of the base metal and difficulty in removing the oxide film during magnesium / aluminum dissimilar metal welding, as well as the difficulty in controlling Mg2Sn IMCs generated during low-temperature brazing leading to low joint strength, this invention aims to provide a method for low-temperature brazing of magnesium / aluminum dissimilar metals based on nickel plating. Specifically, nickel is pre-plated onto a magnesium alloy substrate, and then the nickel-plated magnesium alloy is welded to an aluminum alloy using ultrasonic-assisted brazing. The plating layer inhibits direct contact between Mg and Sn / Al, thus preventing the formation of IMCs in the joint. Furthermore, the dissolution of Ni in the brazing seam during the brazing process enhances the performance of the brazed joint.

[0009] The objective of this invention is achieved through the following technical solution: a brazing method for magnesium / aluminum dissimilar metals, comprising the following steps: nickel plating a magnesium alloy base material, and then ultrasonically brazing the nickel-plated magnesium alloy base material with an aluminum alloy base material after removing the oxide layer and impurities using a brazing filler metal; wherein, the thickness of the nickel plating layer in the joint after ultrasonically assisted brazing is 7.5-20μm, the magnesium alloy base material is AZ31 magnesium alloy, and the aluminum alloy base material is 6061 aluminum alloy; the brazing filler metal is Sn 96.5wt.%-Ag 3wt.%-Cu 0.5 wt.%, i.e., SAC305 brazing filler metal.

[0010] Preferably, the method for removing the oxide layer and impurities from the aluminum alloy substrate is as follows: use 600#, 800#, 1000# and 1200# sandpaper to polish the surface until it is clean, and then ultrasonically clean it in alcohol and acetone solutions for 10-15 minutes respectively.

[0011] Furthermore, the method for nickel plating on magnesium alloy substrate includes the following steps: placing pure nickel foil and magnesium alloy substrate opposite each other in an electroplating solution for electroplating; wherein, the magnesium alloy substrate needs to undergo pretreatment to remove oxide layer, impurities, and prevent oxidation.

[0012] Furthermore, the pretreatment method includes: grinding, ultrasonic cleaning, alkaline cleaning, acid pickling, activation, and zinc immersion treatment. Zinc immersion treatment prevents re-oxidation and improves the adhesion between the magnesium alloy and the nickel plating.

[0013] Preferably, the grinding method is to use 600-1200# sandpaper to grind the magnesium alloy surface to remove the oxide layer and impurities; the preferred ultrasonic cleaning method is to immerse the alloy in anhydrous ethanol solution for ultrasonic cleaning twice, 10 minutes each time.

[0014] Furthermore, the alkaline cleaning temperature is 60-65℃, the cleaning time is 10-15 min, followed by drying; the alkaline cleaning solution formula is: 40 g / L Na2CO3, 20 g / L Na2SiO3, 10 g / L Na3PO4·12H2O, 3 mL / L OP-10 emulsifier, and the remainder is deionized water.

[0015] Furthermore, the pickling time is 3-5 seconds, and after pickling, it needs to be immediately rinsed in deionized water for 3-5 minutes, followed by drying; the pickling solution formula is: 600 cm 3 / L of 85% H3PO4 and 30cm 3 69% HNO3 per liter, the remainder is deionized water.

[0016] Furthermore, the activation time is 2-3 minutes, and after activation, it needs to be immediately placed in an ethanol solution for ultrasonic cleaning for 3-5 minutes, followed by drying; the activation solution formula is: 105 g / L NH4HF2, 200 ml / L 85% H3PO4, and the remainder is deionized water.

[0017] Furthermore, the zinc immersion treatment is performed twice, with each treatment lasting 10-12 minutes. After each zinc immersion, the zinc is rinsed in deionized water and dried. The solution for the zinc immersion treatment is formulated as follows: 56 g / L ZnSO4·7H2O, 174 g / L K2PO4·3H2O, 11 g / L LiF, 5 g / L Na2CO3, with the remainder being deionized water.

[0018] Preferably, the drying method is to use cold air to dry.

[0019] Furthermore, the initial current density of the electroplating is 5 A / dm³. 2 The temperature was 50-55℃, the time was 2-3 minutes, and the constant current density was 2.5 A / dm³. 2 The electroplating solution is formulated as follows: 120 g / L NiSO4.6H2O, 40 g / L NH4.HF2, 10 g / L (NH4)3C6H5O7, 3 g / L sodium saccharin, 40 mL / L ammonia, 0.1 g / L sodium dodecyl sulfate, and the remainder is deionized water.

[0020] Preferably, the preparation method of the electroplating solution is as follows: the electroplating reagents and chemicals are added to a beaker in sequence, and the solution is continuously stirred with magnetic stirring in a constant temperature water bath at 55°C until the solid particles are completely dissolved to obtain the electroplating solution.

[0021] Furthermore, the ultrasonic-assisted brazing temperature is 240-280℃, and the holding time is 0-30min.

[0022] Furthermore, the ultrasonic power of the ultrasonic-assisted brazing is 500W, and the ultrasonic time is 5-10s.

[0023] Preferably, the ultrasonic-assisted brazing method involves placing a magnesium alloy with a nickel plating on top and an aluminum alloy on the bottom, and fixing them together using a fixture. The assembled test piece and fixture are placed in a furnace and heated to a preset temperature. The brazing filler metal is placed at the front edge of the lap joint. After the filler metal melts, ultrasonic waves are applied to fill the brazing seam. After welding is completed, the test piece is removed and allowed to cool with the fixture, resulting in a welded joint with a strengthened interface.

[0024] The innovation of this invention lies in pre-plating nickel onto a magnesium alloy base material, and then joining the nickel-plated magnesium alloy and aluminum alloy using ultrasonic-assisted brazing. The preferred nickel plating thickness is 7.5-20 μm. Because Cu and Ni are infinitely miscible, SAC305 brazing filler metal containing Cu is used, and after welding, the original nickel plating layer transforms into a Cu-Ni solid solution. This method enables magnesium / aluminum dissimilar metal joining, avoiding the formation of Mg2Sn IMCs during low-temperature brazing, resulting in a highly reliable magnesium / aluminum dissimilar metal joint without IMCs. Compared to unplated joints, the nickel-plated assisted magnesium / aluminum dissimilar metal brazed joint exhibits significantly improved shear strength, with the joint strength enhanced by the diffusion of nickel into the brazing seam during welding. Furthermore, nickel plating on the magnesium alloy surface also improves its corrosion resistance and extends the service life of magnesium / aluminum components.

[0025] The beneficial effects of this invention are:

[0026] 1. Pre-plating nickel on the surface of magnesium alloy and then using ultrasonic-assisted brazing significantly improves the strength of the joint. This method can prevent direct contact between Mg and Sn / Al during brazing, effectively avoiding the formation of IMCs and enabling reliable connection of magnesium / aluminum alloys at lower temperatures.

[0027] 2. The nickel plating process of this invention is stable, simple to operate, and easy to repeat. Furthermore, the required Ni plating thickness can be obtained by controlling the plating time, brazing time, and temperature. Previous research has also shown that pre-plating nickel on magnesium alloy surfaces can improve the corrosion resistance of the base material and the joint. This invention provides a new method for joining dissimilar metals such as magnesium and aluminum, aiming to contribute to the development of aerospace, high-speed rail, and automotive industries. Attached Figure Description

[0028] Figure 1Here is a SEM image of the connector from Example 1;

[0029] Figure 2 This is a SEM image of the connector in Example 2.

[0030] Figure 3 Here is a SEM image of the connector in Example 3;

[0031] Figure 4 Here is a SEM image of the connector in Example 4;

[0032] Figure 5 Here is a SEM image of the connector in Comparative Example 1;

[0033] Figure 6 Here is a SEM image of the connector in Comparative Example 2;

[0034] Figure 7 The image shows the SEM morphology of the connector in Comparative Example 3. Detailed Implementation

[0035] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings, but the scope of protection of the present invention is not limited to the following description.

[0036] Example 1

[0037] The difference between this embodiment and Comparative Example 1 is that the magnesium alloy in this embodiment was nickel-plated. The nickel plating time was 1 hour, and the thickness of the nickel plating layer in the welded joint was approximately 16.2 μm (Note: the thickness before welding was approximately 18.9 μm). The specific steps are as follows:

[0038] a. Sample preparation and assembly: AZ31 magnesium alloy and 6061 aluminum alloy base materials were machined into 50mm×10mm×3mm samples using wire EDM. The magnesium alloy surface was treated with a nickel plating process (grinding, ultrasonic cleaning, alkaline cleaning, acid pickling, activation, zinc immersion, and nickel plating). The aluminum alloy base material was polished with 600# sandpaper and then ultrasonically cleaned in anhydrous ethanol and acetone solutions for 10 minutes each, followed by cold air drying. The treated base materials were then assembled sequentially, with the nickel-plated magnesium alloy base material placed on top and the aluminum alloy base material placed below, and then secured using clamps.

[0039] b. Ultrasonic-assisted brazing process: The specific procedure is as follows: the sample is placed in the furnace and heated to the predetermined temperature. SAC305 brazing filler metal is placed at the edge of the lap joint, and ultrasonic waves are applied to allow the filler metal to flow into the brazing seam. Main parameters: welding temperature is 240℃, heating rate is 20℃ / min, holding time is 0min; ultrasonic power is 500W, ultrasonic time is 5s, ultrasonic pressure is 0.15MPa, and the sample is cooled to room temperature with the fixture after welding.

[0040] The joint morphology obtained in this embodiment is as follows: Figure 1 As shown, the joint morphology obtained in Comparative Example 1 is different. Figure 5 In contrast, no Mg2Sn IMC was found in the weld, the shear strength was 45.6 MPa, and the joint formation was good.

[0041] Example 2

[0042] The difference between this embodiment and Embodiment 1 is as follows: In this embodiment, the welding temperature is 260℃, the ultrasonic treatment time is 10s, the nickel plating time is 0.5H, and the thickness of the nickel plating layer in the joint after welding is approximately 7.5μm. (Note: The thickness before welding was approximately 9.8μm)

[0043] The brazed joint obtained in this embodiment was subjected to shear performance testing, and its shear strength was 48.3 MPa, which is higher than that of Comparative Example 1. No Mg2Sn IMC was observed in the weld. Figure 2 As shown, the joint is well formed.

[0044] Example 3

[0045] The difference between this embodiment and Embodiment 1 is that the welding temperature in this embodiment is 260℃, and the thickness of the nickel plating layer in the joint after welding is approximately 11.1 μm. (Note: the thickness before welding was approximately 18.9 μm)

[0046] The brazed joint obtained in this embodiment was subjected to shear performance testing, and its shear strength was 52.2 MPa, which is higher than that of Comparative Example 1. No Mg2Sn IMC was observed in the weld. Figure 3 As shown, the joint is well formed.

[0047] Example 4

[0048] The difference between this embodiment and Embodiment 1 is that the welding temperature in this embodiment is 280℃, the holding time is 10 minutes, and the thickness of the nickel plating layer in the joint after welding is approximately 8.3 μm. (Note: the thickness before welding was approximately 18.9 μm)

[0049] The brazed joint obtained in this embodiment was subjected to shear performance testing, and its shear strength was 62.3 MPa, which is higher than that of Comparative Example 1. No Mg2Sn IMC was observed in the weld. Figure 4 As shown, the joint is well formed.

[0050] Comparative Example 1

[0051] The difference between this comparative example and Example 1 is that this comparative example did not perform nickel plating on the magnesium alloy base material, but instead used the same grinding and cleaning methods as the aluminum alloy base material.

[0052] The obtained welded joint morphology is as follows Figure 5As shown, large, obvious Mg2Sn IMC is formed in the joint, and the joint shear strength is 32.2 MPa.

[0053] Comparative Example 2

[0054] The difference between this comparative example and Example 1 is that: before welding, the magnesium alloy base material of this comparative example is plated with nickel 1.5H, the thickness of the nickel plating layer before welding is about 23.6μm, and the thickness of the nickel plating layer in the joint after welding is about 20.8μm.

[0055] The obtained welded joint morphology is as follows Figure 6 As shown, although no large blocks of Mg2Sn IMC were found in the joint, there were obvious cracks, and the shear strength of the joint was 15.6 MPa.

[0056] Comparative Example 3

[0057] The difference between this comparative example and Example 4 is that the heat preservation time in this comparative example is 20 min, the ultrasonic treatment time is 10 s, and the thickness of the nickel plating layer in the welded joint is about 6.6 μm.

[0058] The brazed joint obtained in this embodiment was subjected to shear performance testing, and its shear strength was 40.5 MPa, which was higher than that of Comparative Example 1 (without Ni coating). However, compared with Example 4, the joint shear strength decreased again because Mg2Sn IMC appeared in the weld.

[0059] The experimental results of the above embodiments and comparative examples are shown in the table below:

[0060] Variations in Mg2Sn IMC, coating thickness, and shear strength in welded joints produced by different processes

[0061]

[0062]

[0063] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A brazing method of magnesium / aluminum dissimilar metals, characterized by, The method comprises the following steps: The magnesium alloy base material is plated with nickel, and then the nickel-plated magnesium alloy base material and the aluminum alloy base material with removed oxide layer and impurities are ultrasonic-assisted brazed using a brazing filler metal. The thickness of the nickel-plated layer in the joint after the ultrasonic-assisted brazing is 7.5-20 μm, the magnesium alloy base material is AZ31 magnesium alloy, the aluminum alloy base material is 6061 aluminum alloy, and the brazing filler metal is Sn96.5 wt.%-Ag3 wt.%-Cu0.5 wt.%.

2. The brazing method of a magnesium / aluminum dissimilar metal according to claim 1, characterized by, The method for plating the magnesium alloy base material with nickel comprises the following steps: placing a pure nickel foil and a magnesium alloy base material in an electroplating solution and then electroplating.

3. The brazing method of a magnesium / aluminum dissimilar metal according to claim 2, characterized by, The pretreatment method comprises grinding, ultrasonic cleaning, alkali cleaning, acid cleaning, activation, and zinc immersion treatment.

4. The brazing method of a magnesium / aluminum dissimilar metal according to claim 3, characterized by, The alkali cleaning is performed at a temperature of 60-65℃ for 10-15 min, and then drying is performed; the alkali cleaning solution is composed of 40 g / L Na2CO3, 20 g / L Na2SiO3, 10 g / L Na3PO4.12H2O, 3 mL / L OP-10 emulsifier, and the rest is deionized water.

5. The brazing method of a magnesium / aluminum dissimilar metal according to claim 3, characterized by, The pickling time is 3-5 s, and the pickling solution is 600 cm 3 / L of 85% H3PO4 and 30 cm 3 / L of 69% HNO3, with the rest being deionized water.

6. The method of brazing a magnesium / aluminum dissimilar metal according to claim 3, wherein The activation is performed for 2-3 min, and then the magnesium alloy base material is immediately placed in an ethanol solution and ultrasonic cleaned for 3-5 min, and then dried; the activation solution is composed of 105 g / L NH4HF2, 200 mL / L 85%H3PO4, and the rest is deionized water.

7. The brazing method of a magnesium / aluminum dissimilar metal according to claim 3, characterized by, The zinc immersion treatment is performed twice, each time for 10-12 min, and each time after the zinc immersion, the magnesium alloy base material is cleaned in deionized water and dried; the zinc immersion solution is composed of 56 g / L ZnSO4.7H2O, 174 g / L K2PO4.3H2O, 11 g / L LiF, 5 g / L Na2CO3, and the rest is deionized water.

8. The method of brazing a magnesium / aluminum dissimilar metal according to claim 3, wherein The initial current density of the electroplating is 5 A / dm 2 at a temperature of 50-55°C for a time of 2-3 min; the constant current density is 2.5 A / dm 2 for a time of 0.5-1 H; the electroplating solution formulation is 120 g / L NiSO4.6H2O, 40 g / L NH4.HF2, 10 g / L (NH4)3C6H5O7, 3 g / L sodium saccharin, 40 mL / L ammonia, 0.1 g / L sodium dodecyl sulfate, and the rest is deionized water.

9. The method of brazing a magnesium / aluminum dissimilar metal according to claim 1, wherein The welding temperature of the ultrasonic-assisted brazing is 240-280℃, and the holding time of the welding is 0-30 min.

10. The method of brazing a magnesium / aluminum dissimilar metal of claim 1, wherein, The ultrasonic power of the ultrasonic-assisted brazing is 500 W, and the ultrasonic time is 5-10 s.

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