Welding method of lightweight battery pack protective cover plate for new energy electric vehicle

Abstract

The invention discloses a welding method of a lightweight battery pack protective cover plate for a new energy electric vehicle. The welding method comprises the following steps: firstly, cleaning to-be-welded surfaces of an upper-layer aluminum plate, a middle-layer steel plate and a lower-layer steel plate; nickel plating treatment is conducted on the to-be-welded faces of the upper-layer aluminum plate and the middle-layer steel plate; the to-be-welded faces of the upper-layer aluminum plate, the middle-layer steel plate and the lower-layer steel plate are coated with scaling powder; then the upper-layer aluminum plate, the middle-layer steel plate and the lower-layer steel plate are assembled, welding flux is placed between every two adjacent plate bodies to form a first combination body, and then the first combination body is integrally installed in a clamp to form a second combination body; and then the second assembly is subjected to vacuum brazing, finally, the first assembly is taken out of the clamp to be cleaned, and the battery pack protection cover plate is obtained. According to the welding method, one-time vacuum brazing of the aluminum-steel-steel sandwich structure is achieved through the welding method of nickel plating, scaling powder and welding flux, and the problems that weld joints of the battery pack protection cover plate are not fused, the surface of the battery pack protection cover plate is damaged, and welding is conducted step by step are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of automobile parts, and in particular to a welding method for a lightweight battery pack protective cover for a new energy electric vehicle. Background Art

[0002] Battery pack protective covers are common components in the new energy vehicle sector, protecting the battery cells and dissipating heat. Their performance is crucial to the safety of new energy vehicles. Currently, common battery case covers are steel and aluminum. Steel offers high strength, but increases the weight of the battery pack, contradicting the lightweighting goals of new energy vehicles. Aluminum, on the other hand, offers excellent heat dissipation but has limited strength, failing to guarantee the safety and reliability of the battery cells. Prior art steel-aluminum composite panels combine the high strength of steel with the high thermal conductivity and corrosion resistance of aluminum. These panels have been used in vehicle bodies, rear bumpers, and battery compartment floors, as exemplified by the Chinese patent application number CN202020293923.1 (authorization publication number CN212073194U), "Steel-Aluminum Composite Floor Panel for Battery Boxes."

[0003] At present, the main ways to connect steel plates and aluminum plates in stacks are resistance welding and laser powder welding. As shown in the Chinese invention "Improving the Mechanical Properties of Aluminum-Steel Welded Joints" with patent application number CN201910441287.4 (authorization announcement number CN111151854B), this method adheres an aluminum patch to the connecting surface of the steel workpiece, positions the aluminum workpiece on the aluminum patch and the steel workpiece to assemble the workpiece stack, and passes current through the entire workpiece to generate a molten aluminum welding pool. Finally, the aluminum welding pool solidifies to achieve a welding joint between the aluminum workpiece and the steel workpiece. This method will cause damage to the workpiece surface, and the welding contact surface is small and cannot achieve welding of long straight welds. In addition, as shown in the Chinese invention application "A method for flux-free laser powder welding of aluminum-steel dissimilar metal joints" with patent application number CN201110240318.3 (application publication number CN102407404A), the joint arrangement in this method is that the aluminum plate is on the top and the steel plate is on the bottom, and the brazing material is spread between the two metal plates. Through laser powder filling welding and synchronous powder feeding, the aluminum alloy and the brazing material are melted on one side and infiltrated with the steel plate to form a welded joint without melting the steel plate. However, this method requires the integration of laser equipment and powder feeding equipment, and requires precise control of the size of the powder beam and the laser spot and the relative position of the two. Remelting marks will inevitably be produced on the surface of the workpiece, and the equipment cost is high and the process is complicated, making it difficult to use in actual production.

[0004] Vacuum brazing is a surface-safe welding method suitable for long, straight welds. It offers advantages such as simple operation, excellent process reproducibility, and aesthetically pleasing welded components. Therefore, it is suitable for use in protective covers for battery packs of new energy electric vehicles. It offers significant advantages in ensuring surface integrity, ensuring joint performance through long, straight welds, and enabling process reproducibility for mass production. However, the aluminum-steel-steel sandwich structure features ultra-thin corrugated sheet metal, and the physical and chemical properties of the thinner sheets and dissimilar metals can easily lead to post-weld deformation. Furthermore, steel / steel and aluminum / steel welding require different solders, fluxes, and joining processes. If steel / steel welding is performed sequentially, followed by steel / aluminum welding, the subsequent heat treatment can lead to coarsened grains at the steel / steel interface and reduced joint performance. If steel / aluminum welding is performed first, followed by steel / steel welding, the higher temperatures during steel / steel welding can soften, deform, or even melt the aluminum base metal. Therefore, further improvements are needed in welding methods for lightweight protective covers for battery packs of new energy electric vehicles. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a welding method for a lightweight battery pack protective cover for new energy electric vehicles with simple welding operation and reliable weld strength in response to the above-mentioned existing technical status.

[0006] The technical solution adopted by the present invention to solve the above technical problems is: a welding method for a lightweight battery pack protective cover for a new energy electric vehicle, characterized by comprising the following steps:

[0007] S1. Laser cleaning the surfaces to be welded of the upper aluminum plate, the middle steel plate, and the lower steel plate, wherein the middle steel plate is a corrugated plate and the upper aluminum plate and the lower steel plate are flat plates;

[0008] S2. The surface to be welded of the upper aluminum plate and the surface to be welded of the middle steel plate cleaned in S1 are subjected to nickel plating, with a coating thickness of 15 μm to 25 μm;

[0009] S3, evenly coating the soldering flux on the surface to be welded of the upper aluminum plate and the middle steel plate that have been nickel-plated in S2, and the surface to be welded of the lower steel plate that has been cleaned in S1, and then placing the upper aluminum plate, the middle steel plate, and the lower steel plate coated with the soldering flux in an infrared heating box for baking until the moisture is completely evaporated, and then taking them out;

[0010] S4, first shear the foil solder, then place the lower steel plate obtained in S3 with the surface to be welded facing upward, and place the sheared portion of the foil solder on the surface to be welded of the lower steel plate as a first solder layer; then place the middle steel plate obtained in S3 on the surface to be welded of the lower steel plate already having the first solder layer, with the surface to be welded of the middle steel plate facing upward; then place the remaining sheared foil solder on the surface to be welded of the middle steel plate as a second solder layer; then place the upper aluminum plate obtained in S3 on the surface to be welded of the middle steel plate already having the second solder layer, with the surface to be welded of the upper aluminum plate facing downward, and align the upper aluminum plate, the middle steel plate, and the lower steel plate to obtain a first assembly; then place the entire first assembly into a fixture to form a second assembly;

[0011] S5. Place the second assembly in S4 into a vacuum brazing furnace for vacuum brazing, then take out the cooled second assembly from the vacuum brazing furnace, then take out the first assembly from the fixture of the second assembly, and finally laser clean the surface of the first assembly to obtain a battery pack protective cover.

[0012] Furthermore, the laser cleaning parameter ranges in S1 are as follows: for cleaning the upper aluminum plate, the pulse energy range is 20-24 mJ, the pulse frequency is 12-20 kHz, the pulse duration is 90-110 ns, and the pulse repetition rate is 8-12 kHz; for cleaning the middle and lower steel plates, the pulse energy range is 40-50 mJ, the pulse frequency is 18-22 Hz, the pulse duration is 90-110 ns, and the pulse repetition rate is 8-12 kHz. Laser cleaning within this parameter range can remove oil, water stains, and other contaminants from the welded surfaces of the upper aluminum plate, the middle steel plate, and the lower steel plate.

[0013] Furthermore, the soldering flux in S3 is divided into a first soldering flux and a second soldering flux. The first soldering flux is evenly applied to the surface to be welded of the upper aluminum plate and the surface to be welded of the middle steel plate that have undergone nickel plating in S2, and the second soldering flux is evenly applied to the surface to be welded of the lower steel plate that has completed cleaning in S1. Due to differences in the physical and chemical properties of dissimilar metals, different soldering fluxes are suitable for the upper aluminum plate and the middle steel plate, and for the middle steel plate and the lower steel plate. Therefore, the first soldering flux is applied between the upper aluminum plate and the middle steel plate, and the second soldering flux is applied between the middle steel plate and the lower steel plate.

[0014] Furthermore, the first flux is potassium fluoroaluminate, comprising the following components: K: 28-31%, Al: 16-20%, F: 49-53%, Fe: ≤ 0.03%, Ca: ≤ 0.2%. This composition of the first flux can break down oxide films and aid solder wetting, thereby facilitating and promoting welding of aluminum and steel plates.

[0015] Furthermore, the second soldering flux is a mixture of fluoride and borate, with the following composition: KBF4: 21.0-25.0%, B2O3: 33.0-37.0%, KF: 40.0-44.0%. This second soldering flux composition can also break down oxide films and aid solder wetting, thereby facilitating and promoting welding of steel plates.

[0016] Furthermore, the first solder layer and the second solder layer in S4 are eutectic aluminum-silicon alloys, and the first solder layer and the second solder layer are Al-12Si eutectic alloys.

[0017] Furthermore, the fixture in S4 includes a housing, a threaded hole, and a ceramic pressure plate. The housing is made of stainless steel. The threaded hole is provided on the housing and engages with a screw. The ceramic pressure plate is disposed within the housing. The screw, when tightened within the threaded hole, drives the ceramic pressure plate downward, thereby pressing the first assembly within the housing. The fixture can use the screw to drive the ceramic pressure plate to press the first assembly, thereby preventing deformation of the first assembly during welding.

[0018] Furthermore, the vacuum brazing method in S5 is as follows: the working vacuum degree of the vacuum brazing furnace is ≤6.0×10 -4 Pa, the vacuum brazing furnace is heated to 200-250°C at a rate of 5-7°C / min, kept at 200-250°C for at least 1 minute, then the vacuum brazing furnace is heated to 400-450°C at a rate of 7-9°C / min, kept at 400-450°C for at least 1 minute, then the vacuum brazing furnace is heated to 580-600°C at a rate of 8-12°C / min, kept at 580-600°C for at least 10 minutes, and finally the vacuum brazing furnace is air-cooled to 550-565°C, and then slowly cooled to room temperature with the furnace. If the temperature is raised to the target temperature at one time, the temperature of some areas of the vacuum brazing furnace may not reach the target temperature, resulting in the welding not reaching the required temperature, thereby affecting the welding effect; therefore, the vacuum brazing furnace of this method adopts a segmented heating method to ensure the temperature uniformity of the entire vacuum brazing furnace and improve the welding effect of the workpiece.

[0019] Furthermore, the parameter ranges for laser cleaning the surface of the first assembly in S5 are as follows: for cleaning the upper aluminum plate, the pulse energy range is 18-20 mJ, the pulse frequency is 12-20 kHz, the pulse duration is 70-90 ns, and the pulse repetition rate is 8-12 kHz; for cleaning the lower steel plate, the pulse energy range is 35-40 mJ, the pulse frequency is 18-22 kHz, the pulse duration is 70-90 ns, and the pulse repetition rate is 8-12 kHz. The first assembly generates oil and dust in the vacuum brazing furnace and during transportation. Laser cleaning within this parameter range can remove oil, water stains, and other contaminants from the surface of the first assembly, ensuring a clean, bright, and aesthetically pleasing appearance.

[0020] Compared with the prior art, the advantages of the present invention are:

[0021] (1) A one-time vacuum brazing of an aluminum-steel-steel sandwich structure is achieved by using a welding method using nickel plating, flux, and solder, thereby avoiding problems such as the weld seam of the battery pack protective cover not being fused, surface damage to the battery pack protective cover, and step-by-step welding. This effectively ensures the connection performance of the weld joints of the upper aluminum plate, the middle steel plate, and the lower steel plate, and also improves the appearance and strength of the welded battery pack protective cover, making the battery pack protective cover lightweight. In addition, this method can also improve the welding efficiency of the battery pack protective cover.

[0022] (2) The first assembly formed by assembling the upper aluminum plate, the middle steel plate and the lower steel plate can solve the problem of deformation of the middle steel plate during welding by means of the constraint of the clamp, thereby avoiding the problem of large deformation of the ultra-thin corrugated plate during welding;

[0023] (3) This method also has the characteristics of simple welding operation, good process reproducibility, and easy engineering implementation. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the structure of the upper aluminum plate, the middle steel plate and the lower steel plate in an embodiment of the present invention;

[0025] Figure 2 Schematic diagram of the exploded upper aluminum plate, the middle steel plate, the lower steel plate, the first solder layer and the second solder layer in an embodiment of the present invention;

[0026] Figure 3 is a cross-sectional view of a clamp according to an embodiment of the present invention;

[0027] Figure 4 1 is a curve showing the temperature change over time of the vacuum brazing furnace in Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be described in further detail below with reference to the accompanying drawings and embodiments.

[0029] like Figures 1 to 4 FIG. 1 is an embodiment 1 of the present invention.

[0030] Example 1

[0031] The welding method of the lightweight battery pack protective cover for the new energy electric vehicle of this embodiment is characterized by comprising the following steps:

[0032] S1. Laser cleaning is performed on the surfaces to be welded of the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3, wherein the middle steel plate 2 is a corrugated plate, and the upper aluminum plate 1 and the lower steel plate 3 are flat plates. The parameter range of laser cleaning is: the pulse energy range for cleaning the upper aluminum plate 1 is 22mJ, the pulse frequency is 15kHz, the pulse duration is 100ns, and the pulse repetition rate is 10kHz; the pulse energy range for cleaning the middle steel plate 2 and the lower steel plate 3 is 45mJ, the pulse frequency is 20kHz, the pulse duration is 100ns, and the pulse repetition rate is 10kHz. Laser cleaning can remove oil stains, water stains and other pollutants on the surfaces to be welded of the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3.

[0033] S2. The surfaces to be welded of the upper aluminum plate 1 and the middle steel plate 2 cleaned in S1 are subjected to nickel plating, with a coating thickness of 20 μm.

[0034] S3, evenly apply the first soldering flux 71 to the surface to be welded of the upper aluminum plate 1 and the middle steel plate 2 that have been nickel-plated in S2, then put the upper aluminum plate 1 and the middle steel plate 2 coated with the first soldering flux 71 into an infrared heating box for baking until the water is completely evaporated and then taken out; then evenly apply the second soldering flux 72 to the surface to be welded of the lower steel plate 3 that has been cleaned in S1, then put the lower steel plate 3 coated with the second soldering flux 72 into an infrared heating box for baking until the water is completely evaporated and then taken out; wherein the first soldering flux 71 is potassium fluoroaluminate (K 1-3 AlF 4-6 ), the first soldering flux 71 includes the following components: K: 30%, Al: 16%, F: 53%, Fe: ≤0.03%, Ca: ≤0.2%; the second soldering flux 72 is a flux mixed with fluoride and borate, and the second soldering flux 72 includes the following components: KBF4: 23.0%, B2O3: 35.0%, KF: 42.0%.

[0035] S4. First, shear the foil solder, then place the lower steel plate 3 obtained in S3 with the surface to be welded facing upward, and place the sheared part of the foil solder on the surface to be welded of the lower steel plate 3 as the first solder layer 4; then place the middle steel plate 2 obtained in S3 on the surface to be welded of the lower steel plate 3 having the first solder layer 4, and set the surface to be welded of the middle steel plate 2 facing upward, even if the side of the middle steel plate 2 not coated with the first soldering flux 71 is in contact with the side of the lower steel plate 3 having the first solder layer 4; then place the remaining foil solder after shearing on the surface to be welded of the middle steel plate 2 as the second solder layer 5; then place the upper aluminum plate 1 obtained in S3 on the surface to be welded on the middle steel plate 2 having the second solder layer 5, and set the surface to be welded of the upper aluminum plate 1 facing downward It is arranged that the side of the upper aluminum plate 1 coated with the first flux 71 is connected to the side of the middle steel plate 2 having the second solder layer 5; the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3 are aligned and placed to obtain a first assembly; then the first assembly is placed as a whole into the fixture 6 to form a second assembly; wherein, the first solder layer 4 and the second solder layer 5 are eutectic aluminum silicon alloys, using Al-12Si eutectic alloy, the fixture 6 includes a shell 61, a threaded hole 62 and a ceramic pressure plate 63, the shell 61 is made of stainless steel, the threaded hole 62 is arranged on the shell 61 and cooperates with the screw, the ceramic pressure plate 63 is arranged in the shell 61, and the screw can be tightened in the threaded hole 62 to drive the ceramic pressure plate 63 to press down, thereby pressing the first assembly located in the shell 61.

[0036] S5. Place the second assembly in S4 into a vacuum brazing furnace for vacuum brazing. The vacuum brazing method is as follows: the working vacuum degree of the vacuum brazing furnace is ≤6.0×10 -4 Pa, the vacuum brazing furnace is heated to 200℃ at a rate of 6℃ / min, and kept at 200℃ for 2min, then the vacuum brazing furnace is heated to 400℃ at a rate of 8℃ / min, and kept at 400℃ for 2min, then the vacuum brazing furnace is heated to 590℃ at a rate of 10℃ / min, and kept at 590℃ for 10min, finally the vacuum brazing furnace is air-cooled to 550℃, and then slowly cooled to room temperature with the furnace; then the cooled second assembly is taken out of the vacuum brazing furnace, and then the first assembly is taken out from the fixture 6 of the second assembly, and finally the surface of the first assembly is laser cleaned, and the parameter range of laser cleaning is: the pulse energy range for cleaning the upper aluminum plate 1 is 20mJ, the pulse frequency is 15kHz, the pulse duration is 80ns, and the pulse repetition rate is 10kHz; the pulse energy range for cleaning the lower steel plate 3 is 35mJ, the pulse frequency is 20kHz, the pulse duration is 80ns, and the pulse repetition rate is 10kHz. This results in a clean and shiny battery pack protective cover.

[0037] Example 2

[0038] The welding method of the lightweight battery pack protective cover for the new energy electric vehicle of this embodiment is characterized by comprising the following steps:

[0039] S1. Laser cleaning is performed on the surfaces to be welded of the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3, wherein the middle steel plate 2 is a corrugated plate, and the upper aluminum plate 1 and the lower steel plate 3 are flat plates. The parameter range of laser cleaning is as follows: the pulse energy range for cleaning the upper aluminum plate 1 is 20mJ, the pulse frequency is 13kHz, the pulse duration is 100ns, and the pulse repetition rate is 10kHz; the pulse energy range for cleaning the middle steel plate 2 and the lower steel plate 3 is 40mJ, the pulse frequency is 20kHz, the pulse duration is 100ns, and the pulse repetition rate is 10kHz. Laser cleaning can remove oil stains, water stains and other pollutants on the surfaces to be welded of the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3.

[0040] S2. The surfaces to be welded of the upper aluminum plate 1 and the middle steel plate 2 cleaned in S1 are subjected to nickel plating, with a coating thickness of 25 μm.

[0041] S3, evenly apply the first soldering flux 71 to the surface to be welded of the upper aluminum plate 1 and the middle steel plate 2 that have been nickel-plated in S2, then put the upper aluminum plate 1 and the middle steel plate 2 coated with the first soldering flux 71 into an infrared heating box for baking until the water is completely evaporated and then taken out; then evenly apply the second soldering flux 72 to the surface to be welded of the lower steel plate 3 that has been cleaned in S1, then put the lower steel plate 3 coated with the second soldering flux 72 into an infrared heating box for baking until the water is completely evaporated and then taken out; wherein the first soldering flux 71 is potassium fluoroaluminate (K 1-3 AlF 4-6 ), the first soldering flux 71 includes the following components: K: 28%, Al: 18%, F: 53%, Fe: ≤0.03%, Ca: ≤0.2%; the second soldering flux 72 is a flux mixed with fluoride and borate, and the second soldering flux 72 includes the following components: KBF4: 25.0%, B2O3: 35.0%, KF: 40.0%.

[0042] S4. First, shear the foil solder, then place the lower steel plate 3 obtained in S3 with the surface to be welded facing upward, and place the sheared part of the foil solder on the surface to be welded of the lower steel plate 3 as the first solder layer 4; then place the middle steel plate 2 obtained in S3 on the surface to be welded of the lower steel plate 3 having the first solder layer 4, and set the surface to be welded of the middle steel plate 2 facing upward, even if the side of the middle steel plate 2 not coated with the first soldering flux 71 is in contact with the side of the lower steel plate 3 having the first solder layer 4; then place the remaining foil solder after shearing on the surface to be welded of the middle steel plate 2 as the second solder layer 5; then place the upper aluminum plate 1 obtained in S3 on the surface to be welded on the middle steel plate 2 having the second solder layer 5, and set the surface to be welded of the upper aluminum plate 1 facing downward It is arranged that the side of the upper aluminum plate 1 coated with the first flux 71 is connected to the side of the middle steel plate 2 having the second solder layer 5; the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3 are aligned and placed to obtain a first assembly; then the first assembly is placed as a whole into the fixture 6 to form a second assembly; wherein, the first solder layer 4 and the second solder layer 5 are eutectic aluminum silicon alloys, using Al-12Si eutectic alloy, the fixture 6 includes a shell 61, a threaded hole 62 and a ceramic pressure plate 63, the shell 61 is made of stainless steel, the threaded hole 62 is arranged on the shell 61 and cooperates with the screw, the ceramic pressure plate 63 is arranged in the shell 61, and the screw can be tightened in the threaded hole 62 to drive the ceramic pressure plate 63 to press down, thereby pressing the first assembly located in the shell 61.

[0043] S5. Place the second assembly in S4 into a vacuum brazing furnace for vacuum brazing. The vacuum brazing method is as follows: the working vacuum degree of the vacuum brazing furnace is ≤6.0×10 -4 Pa, the vacuum brazing furnace is heated to 250℃ at a rate of 6℃ / min, and kept at 250℃ for 1min, then the vacuum brazing furnace is heated to 450℃ at a rate of 8℃ / min, and kept at 450℃ for 1min, then the vacuum brazing furnace is heated to 590℃ at a rate of 10℃ / min, and kept at 590℃ for 10min, finally the vacuum brazing furnace is air-cooled to 565℃, and then slowly cooled to room temperature with the furnace; then the cooled second assembly is taken out of the vacuum brazing furnace, and then the first assembly is taken out from the fixture 6 of the second assembly, and finally the surface of the first assembly is laser cleaned, and the parameter range of laser cleaning is: the pulse energy range for cleaning the upper aluminum plate 1 is 18mJ, the pulse frequency is 12kHz, the pulse duration is 80ns, and the pulse repetition rate is 10kHz; the pulse energy range for cleaning the lower steel plate 3 is 40mJ, the pulse frequency is 20kHz, the pulse duration is 80ns, and the pulse repetition rate is 10kHz. This results in a clean and shiny battery pack protective cover.

[0044] Example 3

[0045] The welding method of the lightweight battery pack protective cover for the new energy electric vehicle of this embodiment is characterized by comprising the following steps:

[0046] S1. Laser cleaning is performed on the surfaces to be welded of the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3, wherein the middle steel plate 2 is a corrugated plate, and the upper aluminum plate 1 and the lower steel plate 3 are flat plates. The parameter range of laser cleaning is: the pulse energy range for cleaning the upper aluminum plate 1 is 24mJ, the pulse frequency is 20kHz, the pulse duration is 100ns, and the pulse repetition rate is 10kHz; the pulse energy range for cleaning the middle steel plate 2 and the lower steel plate 3 is 50mJ, the pulse frequency is 20kHz, the pulse duration is 100ns, and the pulse repetition rate is 10kHz. Laser cleaning can remove oil stains, water stains and other pollutants on the surfaces to be welded of the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3.

[0047] S2. The surfaces to be welded of the upper aluminum plate 1 and the middle steel plate 2 cleaned in S1 are subjected to nickel plating, with a coating thickness of 15 μm.

[0048] S3, evenly apply the first soldering flux 71 to the surface to be welded of the upper aluminum plate 1 and the middle steel plate 2 that have been nickel-plated in S2, then put the upper aluminum plate 1 and the middle steel plate 2 coated with the first soldering flux 71 into an infrared heating box for baking until the water is completely evaporated and then taken out; then evenly apply the second soldering flux 72 to the surface to be welded of the lower steel plate 3 that has been cleaned in S1, then put the lower steel plate 3 coated with the second soldering flux 72 into an infrared heating box for baking until the water is completely evaporated and then taken out; wherein the first soldering flux 71 is potassium fluoroaluminate (K 1-3 AlF 4-6 ), the first soldering flux 71 includes the following components: K: 31%, Al: 19%, F: 49%, Fe: ≤0.03%, Ca: ≤0.2%; the second soldering flux 72 is a flux mixed with fluoride and borate, and the second soldering flux 72 includes the following components: KBF4: 21.0%, B2O3: 37.0%, KF: 42.0%.

[0049] S4. First, shear the foil solder, then place the lower steel plate 3 obtained in S3 with the surface to be welded facing upward, and place the sheared part of the foil solder on the surface to be welded of the lower steel plate 3 as the first solder layer 4; then place the middle steel plate 2 obtained in S3 on the surface to be welded of the lower steel plate 3 having the first solder layer 4, and set the surface to be welded of the middle steel plate 2 facing upward, even if the side of the middle steel plate 2 not coated with the first soldering flux 71 is in contact with the side of the lower steel plate 3 having the first solder layer 4; then place the remaining foil solder after shearing on the surface to be welded of the middle steel plate 2 as the second solder layer 5; then place the upper aluminum plate 1 obtained in S3 on the surface to be welded on the middle steel plate 2 having the second solder layer 5, and set the surface to be welded of the upper aluminum plate 1 facing downward It is arranged that the side of the upper aluminum plate 1 coated with the first flux 71 is connected to the side of the middle steel plate 2 having the second solder layer 5; the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3 are aligned and placed to obtain a first assembly; then the first assembly is placed as a whole into the fixture 6 to form a second assembly; wherein, the first solder layer 4 and the second solder layer 5 are eutectic aluminum silicon alloys, using Al-12Si eutectic alloy, the fixture 6 includes a shell 61, a threaded hole 62 and a ceramic pressure plate 63, the shell 61 is made of stainless steel, the threaded hole 62 is arranged on the shell 61 and cooperates with the screw, the ceramic pressure plate 63 is arranged in the shell 61, and the screw can be tightened in the threaded hole 62 to drive the ceramic pressure plate 63 to press down, thereby pressing the first assembly located in the shell 61.

[0050] S5. Place the second assembly in S4 into a vacuum brazing furnace for vacuum brazing. The vacuum brazing method is as follows: the working vacuum degree of the vacuum brazing furnace is ≤6.0×10 -4 Pa, the vacuum brazing furnace is heated to 220℃ at a rate of 6℃ / min, and kept at 220℃ for 2min, then the vacuum brazing furnace is heated to 420℃ at a rate of 8℃ / min, and kept at 420℃ for 2min, then the vacuum brazing furnace is heated to 590℃ at a rate of 10℃ / min, and kept at 590℃ for 10min, finally the vacuum brazing furnace is air-cooled to 560℃, and then slowly cooled to room temperature with the furnace; then the cooled second assembly is taken out of the vacuum brazing furnace, and then the first assembly is taken out from the fixture 6 of the second assembly, and finally the surface of the first assembly is laser cleaned, and the parameter range of laser cleaning is: the pulse energy range for cleaning the upper aluminum plate 1 is 19mJ, the pulse frequency is 20kHz, the pulse duration is 80ns, and the pulse repetition rate is 10kHz; the pulse energy range for cleaning the lower steel plate 3 is 38mJ, the pulse frequency is 20kHz, the pulse duration is 80ns, and the pulse repetition rate is 10kHz. This results in a clean and shiny battery pack protective cover.

[0051] Example 4

[0052] The welding method of the lightweight battery pack protective cover for the new energy electric vehicle of this embodiment is characterized by comprising the following steps:

[0053] S1. Laser cleaning is performed on the surfaces to be welded of the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3, wherein the middle steel plate 2 is a corrugated plate, and the upper aluminum plate 1 and the lower steel plate 3 are flat plates. The parameter range of laser cleaning is: the pulse energy range for cleaning the upper aluminum plate 1 is 20mJ, the pulse frequency is 12kHz, the pulse duration is 100ns, and the pulse repetition rate is 10kHz; the pulse energy range for cleaning the middle steel plate 2 and the lower steel plate 3 is 40mJ, the pulse frequency is 20kHz, the pulse duration is 100ns, and the pulse repetition rate is 10kHz. Laser cleaning can remove oil stains, water stains and other pollutants on the surfaces to be welded of the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3.

[0054] S2. The surfaces to be welded of the upper aluminum plate 1 and the middle steel plate 2 cleaned in S1 are subjected to nickel plating, with a coating thickness of 18 μm.

[0055] S3, evenly apply the first soldering flux 71 to the surface to be welded of the upper aluminum plate 1 and the middle steel plate 2 that have been nickel-plated in S2, then put the upper aluminum plate 1 and the middle steel plate 2 coated with the first soldering flux 71 into an infrared heating box for baking until the water is completely evaporated and then taken out; then evenly apply the second soldering flux 72 to the surface to be welded of the lower steel plate 3 that has been cleaned in S1, then put the lower steel plate 3 coated with the second soldering flux 72 into an infrared heating box for baking until the water is completely evaporated and then taken out; wherein the first soldering flux 71 is potassium fluoroaluminate (K 1-3 AlF 4-6 ), the first soldering flux 71 includes the following components: K: 29%, Al: 20%, F: 50%, Fe: ≤0.03%, Ca: ≤0.2%; the second soldering flux 72 is a flux mixed with fluoride and borate, and the second soldering flux 72 includes the following components: KBF4: 23.0%, B2O3: 33.0%, KF: 44.0%.

[0056] S4. First, shear the foil solder, then place the lower steel plate 3 obtained in S3 with the surface to be welded facing upward, and place the sheared part of the foil solder on the surface to be welded of the lower steel plate 3 as the first solder layer 4; then place the middle steel plate 2 obtained in S3 on the surface to be welded of the lower steel plate 3 having the first solder layer 4, and set the surface to be welded of the middle steel plate 2 facing upward, even if the side of the middle steel plate 2 not coated with the first soldering flux 71 is in contact with the side of the lower steel plate 3 having the first solder layer 4; then place the remaining foil solder after shearing on the surface to be welded of the middle steel plate 2 as the second solder layer 5; then place the upper aluminum plate 1 obtained in S3 on the surface to be welded on the middle steel plate 2 having the second solder layer 5, and set the surface to be welded of the upper aluminum plate 1 facing downward It is arranged that the side of the upper aluminum plate 1 coated with the first flux 71 is connected to the side of the middle steel plate 2 having the second solder layer 5; the upper aluminum plate 1, the middle steel plate 2 and the lower steel plate 3 are aligned and placed to obtain a first assembly; then the first assembly is placed as a whole into the fixture 6 to form a second assembly; wherein, the first solder layer 4 and the second solder layer 5 are eutectic aluminum silicon alloys, using Al-12Si eutectic alloy, the fixture 6 includes a shell 61, a threaded hole 62 and a ceramic pressure plate 63, the shell 61 is made of stainless steel, the threaded hole 62 is arranged on the shell 61 and cooperates with the screw, the ceramic pressure plate 63 is arranged in the shell 61, and the screw can be tightened in the threaded hole 62 to drive the ceramic pressure plate 63 to press down, thereby pressing the first assembly located in the shell 61.

[0057] S5. Place the second assembly in S4 into a vacuum brazing furnace for vacuum brazing. The vacuum brazing method is as follows: the working vacuum degree of the vacuum brazing furnace is ≤6.0×10 -4 Pa, the vacuum brazing furnace is heated to 200°C at a rate of 6°C / min, and kept at 200°C for 1 minute, then the vacuum brazing furnace is heated to 400°C at a rate of 8°C / min, and kept at 400°C for 1 minute, then the vacuum brazing furnace is heated to 590°C at a rate of 10°C / min, and kept at 590°C for 10 minutes, and finally the vacuum brazing furnace is air-cooled to 565°C, and then slowly cooled to room temperature with the furnace; then the cooled second assembly is taken out of the vacuum brazing furnace, and then the first assembly is taken out from the fixture 6 of the second assembly, and finally the surface of the first assembly is laser cleaned, and the parameter range of laser cleaning is: the pulse energy range for cleaning the upper aluminum plate 1 is 18mJ, the pulse frequency is 12kHz, the pulse duration is 80ns, and the pulse repetition rate is 10kHz; the pulse energy range for cleaning the lower steel plate 3 is 35mJ, the pulse frequency is 20kHz, the pulse duration is 80ns, and the pulse repetition rate is 10kHz. This results in a clean and shiny battery pack protective cover.

Claims

1. A welding method for a lightweight battery pack protective cover for a new energy electric vehicle, characterized in that: The following steps are involved: S1. Laser cleaning the surfaces to be welded of the upper aluminum plate (1), the middle steel plate (2), and the lower steel plate (3), wherein the middle steel plate (2) is a corrugated plate, and the upper aluminum plate (1) and the lower steel plate (3) are flat plates; S2, nickel plating the surface to be welded of the upper aluminum plate (1) and the surface to be welded of the middle steel plate (2) cleaned in S1, with the coating thickness being 15 μm to 25 μm; S3, uniformly coating the soldering flux (7) on the surface to be welded of the upper aluminum plate (1) and the surface to be welded of the middle steel plate (2) that have been nickel-plated in S2, and the surface to be welded of the lower steel plate (3) that has been cleaned in S1, and then placing the upper aluminum plate (1), the middle steel plate (2), and the lower steel plate (3) coated with the soldering flux (7) into an infrared heating box for baking until the moisture is completely evaporated, and then taking them out; S4, first shear the foil solder, then place the lower steel plate (3) obtained in S3 with the surface to be welded facing upwards, and place the sheared portion of the foil solder on the surface to be welded of the lower steel plate (3) as the first solder layer (4); then place the middle steel plate (2) obtained in S3 on the surface to be welded of the lower steel plate (3) already having the first solder layer (4) and with the surface to be welded of the middle steel plate (2) facing upwards; then place the remaining sheared foil solder Place it on the surface to be welded of the middle steel plate (2) as the second solder layer (5); then place the upper aluminum plate (1) obtained in S3 on the surface to be welded on the middle steel plate (2) already having the second solder layer (5) and the surface to be welded of the upper aluminum plate (1) facing downward, and align the upper aluminum plate (1), the middle steel plate (2) and the lower steel plate (3) to obtain a first assembly; then place the first assembly as a whole into a fixture (6) to form a second assembly; S5. Place the second assembly in S4 into a vacuum brazing furnace for vacuum brazing, then take out the cooled second assembly from the vacuum brazing furnace, then take out the first assembly from the fixture (6) of the second assembly, and finally laser clean the surface of the first assembly to obtain a battery pack protective cover.

2. The welding method according to claim 1, wherein: The parameter ranges of the laser cleaning in S1 are as follows: for cleaning the upper aluminum plate (1), the pulse energy range is 20 to 24 mJ, the pulse frequency is 12 to 20 kHz, the pulse duration is 90 to 110 ns, and the pulse repetition rate is 8 to 12 kHz; for cleaning the middle steel plate (2) and the lower steel plate (3), the pulse energy range is 40 to 50 mJ, the pulse frequency is 18 to 22 kHz, the pulse duration is 90 to 110 ns, and the pulse repetition rate is 8 to 12 kHz.

3. The welding method according to claim 1, wherein: The soldering flux (7) in S3 is divided into a first soldering flux (71) and a second soldering flux (72). The first soldering flux (71) is evenly coated on the surface to be welded of the upper aluminum plate (1) and the surface to be welded of the middle steel plate (2) that have been nickel-plated in S2, and the second soldering flux (72) is evenly coated on the surface to be welded of the lower steel plate (3) that has been cleaned in S1.

4. The welding method according to claim 3, wherein: The first soldering flux (71) is potassium fluoroaluminate, and the first soldering flux (71) comprises the following components: K: 28-31%, Al: 16-20%, F: 49-53%, Fe: ≤0.03%, Ca: ≤0.2%.

5. The welding method according to claim 3, wherein: The second soldering flux (72) is a soldering flux mixed with fluoride and borate, and the second soldering flux (72) comprises the following components: KBF4: 21.0-25.0%, B2O3: 33.0-37.0%, and KF: 40.0-44.0%.

6. The welding method according to claim 1, wherein: The first solder layer (4) and the second solder layer (5) in S4 are eutectic aluminum-silicon alloys, and the first solder layer (4) and the second solder layer (5) are Al-12Si eutectic alloys.

7. The welding method according to claim 1, wherein: The clamp (6) in S4 includes a shell (61), a threaded hole (62) and a ceramic pressure plate (63), wherein the shell (61) is made of stainless steel, the threaded hole (62) is arranged on the shell (61) and cooperates with the screw, and the ceramic pressure plate (63) is arranged in the shell (61), and the screw can drive the ceramic pressure plate (63) to press down after being tightened in the threaded hole (62), thereby pressing the first assembly located in the shell (61).

8. The welding method according to claim 1, wherein: The vacuum brazing method in S5 is as follows: the working vacuum degree of the vacuum brazing furnace is ≤6.0×10 -4 Pa, heat the vacuum brazing furnace to 200-250°C at a rate of 5-7°C / min, keep it warm at 200-250°C for at least 1 minute, then heat the vacuum brazing furnace to 400-450°C at a rate of 7-9°C / min, keep it warm at 400-450°C for at least 1 minute, then heat the vacuum brazing furnace to 580-600°C at a rate of 8-12°C / min, keep it warm at 580-600°C for at least 10 minutes, finally air-cool the vacuum brazing furnace to 550-565°C, and then slowly cool it to room temperature with the furnace.

9. The welding method according to claim 1, wherein: The parameter ranges for laser cleaning the surface of the first assembly in S5 are as follows: for cleaning the upper aluminum plate (1), the pulse energy range is 18 to 20 mJ, the pulse frequency is 12 to 20 kHz, the pulse duration is 70 to 90 ns, and the pulse repetition rate is 8 to 12 kHz; for cleaning the lower steel plate (3), the pulse energy range is 35 to 40 mJ, the pulse frequency is 18 to 22 kHz, the pulse duration is 70 to 90 ns, and the pulse repetition rate is 8 to 12 kHz.

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