Aluminum-copper dissimilar metal laser welding process

Abstract

This invention belongs to the field of dissimilar metal welding and relates to a laser welding process for aluminum-copper dissimilar metals. The welding process includes: preparing a first microstructure on the upper surface of a copper plate in the area to be welded; preparing a second microstructure on the lower surface of the copper plate in the overlapping area; then stacking the copper plate onto an aluminum plate, with the second microstructure of the copper plate overlapping the aluminum plate; and performing laser brazing under a protective atmosphere, with the laser beam focused on the first microstructure of the copper plate and welded along a predetermined path. This process, through the pretreatment of differentiated microstructures on the upper and lower surfaces of the copper plate, simultaneously achieves high laser absorption, low heat input, controllable interface, and mechanical anchoring, overcoming the core challenges of high reflectivity, thick brittle phases, and low joint strength in copper-aluminum laser welding, resulting in a high-strength, highly stable, and low-defect copper-aluminum dissimilar metal joint.

Description

Technical Field

[0001] This invention belongs to the field of dissimilar metal welding, and relates to an aluminum-copper dissimilar metal welding process, specifically an aluminum-copper dissimilar metal laser welding process. Background Technology

[0002] Copper-aluminum dissimilar metals combine the high electrical and thermal conductivity of copper with the lightweight and low-cost advantages of aluminum, leading to a continuous increase in demand for their applications in fields such as power transmission, electronic packaging, new energy power batteries, and refrigeration and heating. Currently, a diversified technology system dominated by solid-state welding, brazing, and laser welding has been formed.

[0003] In existing technologies, laser welding has become the mainstream direction for high-end precision connections due to its high energy density, high precision, and strong automation adaptability. The mainstream process adopts the "aluminum on top, copper on the bottom" overlapping mode, while optimization schemes such as filler metal, dual beam, oscillating spot and short wavelength blue laser have been derived to improve the stability of the molten pool and the degree of interface reaction.

[0004] Although existing technologies have achieved preliminary copper-aluminum bonding, they still struggle to completely overcome the core challenges posed by the differences in the physicochemical properties of copper and aluminum. Overall, there are issues with performance stability, process applicability, and quality control. In the laser welding "aluminum on top, copper on bottom" process, the poor wettability of molten aluminum on the copper surface and the intense interfacial metallurgical reaction lead to uncontrolled IMC layer thickness, resulting in joint embrittlement and a sharp decline in mechanical and electrical properties. However, if the process is reversed to "copper on top, aluminum on bottom," the heat source acts on the high-melting-point copper side. The heat is conducted downwards through the copper layer, causing the aluminum plate at the interface to melt precisely while the copper plate remains solid. Theoretically, this allows for more precise control over the amount of aluminum melted and the degree of interfacial reaction, thereby suppressing excessive IMC growth. However, copper has a reflectivity of over 95% for near-infrared fiber lasers. When the "copper on top, aluminum on the bottom" process is used directly, most of the laser energy is reflected, making it impossible to form an effective molten pool. This "high reflectivity" bottleneck makes it difficult to realize the advantages of the "copper on top, aluminum on the bottom" process.

[0005] Therefore, how to overcome the high reflectivity barrier of copper and unleash the potential of the "copper on top, aluminum on the bottom" process in controlling interface reaction and improving joint strength is a technical problem that urgently needs to be solved in the field of laser welding of dissimilar copper and aluminum. Summary of the Invention

[0006] In view of the defects and shortcomings of the existing technology, the present invention provides a laser welding process for aluminum and copper dissimilar metals.

[0007] This invention fabricates a first microstructure on the surface of a copper plate using nanosecond laser direct writing, increasing the copper's absorption rate of fiber lasers from less than 5% to greater than 85%, and for the first time, achieving stable implementation of the "copper on top, aluminum on the bottom" process. Compared to the traditional "aluminum on top, copper on the bottom" process, this invention utilizes copper as a heat-conducting intermediate layer, allowing the aluminum plate to be indirectly heated and melted, avoiding overheating and excessive flow of the molten aluminum. Combined with the mechanical anchoring effect of the microstructure on the lower surface, the thickness of the Al-Cu compound layer at the interface is controlled below 5μm, increasing the shear strength of the joint by more than 60%.

[0008] A laser welding process for dissimilar metals, aluminum and copper, includes the following steps: Step 1: Prepare the first microstructure on the solderable area on the upper surface of the copper plate; prepare the second microstructure on the overlapping area on the lower surface of the copper plate. Step 2: The copper plate is stacked on the aluminum plate, and the second microstructure of the copper plate overlaps with the aluminum plate; Step 3: Perform laser brazing under a protective atmosphere. The laser beam is focused on the first microstructure of the copper plate and welded along a preset path.

[0009] Preferably, in step 1, the first microstructure includes multiple parallel grooves, the width of which is 20~50μm, the depth of which is 10~30μm, and the spacing between two adjacent grooves is 30~70μm.

[0010] Preferably, in step 1, the second microstructure is any one or more of pits, grooves, and grids; the grooves are multiple and adjacent grooves are parallel to each other; the pits include multiple pits that are distributed in an array; the grids include multiple grids that are distributed in an array.

[0011] Further preferred, in step 1, when the second microstructure is an array of pits, the length × width × depth of the pit is (200~500μm) × (200~500μm) × (100~400μm).

[0012] Further preferred, the pits in two adjacent columns are staggered or the pits in two adjacent rows are staggered; the spacing between pit units in two adjacent columns is 150~550μm, and the spacing between two adjacent pits in the same column is 150~550μm.

[0013] Further preferred, in step 1, when the second microstructure is a parallel distribution of grooves, the width of the groove is 200~500μm, the depth is 100~400μm, and the spacing between two adjacent grooves is 150~550μm.

[0014] Further optimization: In step 1, when the second microstructure is a mesh, the size of the mesh unit is (200~500)μm×(200~500)μm, the width of the mesh groove is 50~150μm, and the depth is 100~400μm.

[0015] Preferably, in step 1, the first microstructure and the second microstructure are prepared using nanosecond laser technology.

[0016] Preferably, a pretreatment step is included after step 1 and before step 2: the copper plate and aluminum plate are immersed in acetone solution for degreasing and cleaning, and then the surfaces of the copper plate and aluminum plate are wiped with acetone reagent.

[0017] Preferably, in step 3, during the welding process, the deflection angle between the laser beam and the copper plate is 5~15°, the laser power is 3.5~4.5kW, the welding speed is 1.0~3.0m / min, the defocusing amount is +10~+30mm, and the gas flow rate of the shielding gas is 15-25L / min.

[0018] Compared with the prior art, one or more technical solutions provided by the present invention have at least one of the following beneficial effects: (1) This process achieves high laser absorption, low heat input, controllable interface, and mechanical anchoring simultaneously through differential microstructure pretreatment of the upper and lower surfaces of the copper plate. It overcomes the core problems of high reflection, thick brittle phase, and low joint strength in copper-aluminum laser welding, and obtains high-strength, high-stability, and low-defect copper-aluminum dissimilar metal joints.

[0019] (2) The absorption rate of the copper surface after nanosecond laser direct writing treatment is greatly increased from less than 5% to more than 85%. The laser energy is efficiently absorbed by the copper surface and conducted downward, which raises the temperature of the copper-aluminum interface. Since the melting point of aluminum (about 660℃) is much lower than that of copper (about 1083℃), the aluminum plate melts while the copper plate only melts slightly on the surface or remains solid. The molten aluminum fills the microstructure grooves on the lower surface of the copper plate under wetting and capillary action. After cooling, the joint forms two bonding forms at the same time: First, the liquid aluminum undergoes atomic diffusion and interfacial reaction with the solid or slightly molten copper to form a continuous and uniform Al-Cu intermetallic compound layer with a thickness of less than 5μm, achieving a dense and reliable metallurgical bond and providing basic strength for the joint; Second, the solidified aluminum filled in the microstructure forms a mechanical anchoring effect, similar to a pin and anchoring structure, which can effectively resist shear loads and work synergistically with the metallurgical bond to significantly improve the overall strength and reliability of the joint.

[0020] (3) By optimizing the microstructure parameters, it can adapt to different grades of copper and aluminum alloys, as well as different plate thicknesses, and has good process adaptability and industrial application prospects. Attached Figure Description Figure 1 This is a SEM image of the first microstructure obtained by nanosecond laser etching in step 1 of Example 1; Figure 2 This is a SEM image of the second microstructure obtained by nanosecond laser etching in step 1 of Example 1; Figure 3This is a metallographic cross-sectional view of the copper-aluminum dissimilar metals after welding in Example 1. Figure 4 The image shows the SEM image of the fracture surface of the copper-aluminum dissimilar metal joint obtained by welding in Example 1 after a tensile shear test. Detailed Implementation

[0021] The present invention provides the following specific technical solutions.

[0022] A laser welding process for dissimilar metals, aluminum and copper, includes the following steps: Step 1: Prepare the first microstructure on the solderable area on the upper surface of the copper plate; prepare the second microstructure on the overlapping area on the lower surface of the copper plate. Step 2: The copper plate is stacked on the aluminum plate, and the second microstructure of the copper plate overlaps with the aluminum plate; Step 3: Perform laser brazing under a protective atmosphere. The laser beam is focused on the first microstructure of the copper plate and welded along a preset path.

[0023] Research has revealed that a first microstructure on the upper surface of the copper plate significantly enhances the laser absorption rate from <5% to >85% through multiple reflections and localized surface plasmon resonance. Laser energy is efficiently absorbed by the copper surface, and heat is conducted downwards, raising the temperature at the interface between the copper and aluminum plates. Since the melting point of aluminum (approximately 660℃) is much lower than that of copper (1083℃), the aluminum plate melts, while the copper plate only partially melts or remains solid. The molten aluminum, through wetting and capillary action, fills the microstructured grooves on the copper plate surface. Upon cooling, the joint forms both metallurgical and mechanical bonds. Metallurgical bonding: At the interface, liquid aluminum undergoes atomic diffusion and reaction with solid (or partially molten) copper, forming an extremely thin (typically <5μm) and continuously distributed Al-Cu intermetallic compound layer, ensuring joint density and basic strength. Mechanical bonding: The solidified aluminum filling the microstructure pits acts like a "pin" or "anchor" when the joint is under stress, effectively resisting shear force and significantly improving the overall strength of the joint.

[0024] The area to be welded on the upper surface of the copper plate refers to the area directly heated and melted by laser, which is responsible for weld formation; the overlapping area on the lower surface of the copper plate is the aluminum-copper bonding interface area, which is responsible for connecting dissimilar metals.

[0025] In practical applications, the absorption rate of the copper plate surface to the laser was tested using calorimetry. The absorption rate of the untreated copper plate to the laser was found to be <5%. After surface treatment of the copper plate and the setting of the first microstructure, the absorption rate of the copper plate surface to the laser was found to be >85%.

[0026] Preferably, in step 1, the thickness of the copper plate is 2-3 mm; the thickness of the aluminum plate is 2-3 mm.

[0027] In a specific embodiment of the present invention, the aluminum plate material is 6061 aluminum alloy, because 6061 aluminum alloy is one of the most commonly used aluminum materials in industry, and the experimental results are of greater engineering reference value. Compared with pure aluminum, 6061 has a slightly lower melting point and slightly lower thermal conductivity. These two aspects are actually beneficial to the "copper on top, aluminum on the bottom" process. The slightly lower melting point reduces the heat input requirement, and the slightly lower thermal conductivity reduces heat loss, allowing the molten aluminum to fill the microstructure more stably. The trace amounts of Mg and Si elements in 6061 can improve the wettability of the aluminum-copper interface and inhibit the growth of brittle phases, which is beneficial to improving the joint quality.

[0028] Preferably, in step 1, the first microstructure includes multiple parallel grooves, the width of which is 20~50μm, the depth of which is 10~30μm, and the spacing between two adjacent grooves is 30~70μm.

[0029] Research has shown that by optimizing the trench parameters, the reflection of laser light on the copper surface can be significantly reduced, the energy coupling efficiency can be improved, and the best results of high absorption, low reflection, uniform thermal field, and stable process can be achieved.

[0030] Preferably, in step 1, the second microstructure is any one or more of pits, grooves, and grids; the grooves are multiple and adjacent grooves are parallel to each other; the pits include multiple pits that are distributed in an array; the grids include multiple grids that are distributed in an array.

[0031] Research has shown that pits, grooves, and grid structures can all form open and easily filled cavities at the copper-aluminum interface. Molten aluminum can quickly and fully enter the structure through capillary action and wetting, forming a stable and reliable mechanical anchoring effect after cooling, thereby improving the shear resistance of the joint. At the same time, pits, grooves, and grids can increase the copper-aluminum contact area and uniformly distribute the interface temperature, which is beneficial to inhibiting the excessive growth of brittle intermetallic compound layers and improving the interface bonding quality.

[0032] Further preferred, in step 1, when the second microstructure is an array of pits, the length × width × depth of the pit is (200~500μm) × (200~500μm) × (100~400μm).

[0033] Further preferred, in step 1, when the second microstructure is a parallel distribution of grooves, the width of the groove is 200~500μm, the depth is 100~400μm, the spacing between two adjacent grooves is 150~550μm, the spacing between two adjacent columns of pit units is 150~550μm, and the spacing between two adjacent pits in the same column is 150~550μm.

[0034] In practical applications, when the second microstructure is an array of pits, adjacent rows (upper and lower rows or left and right rows) of pits can be aligned or staggered. The staggered distribution is more effective, as it can break the continuous straight channel of the interface, disperse the stress concentration at the interface and effectively prevent crack propagation. At the same time, it can make the molten aluminum spread more evenly during the welding process, reduce porosity and unwelded defects, and the resulting staggered interlocking structure can significantly enhance the mechanical locking effect, improve the overall shear strength and bonding reliability of the joint.

[0035] Further preferred, in step 1, when the second microstructure is a parallel distribution of grooves, the width of the groove is 200~500μm, the depth is 100~400μm, and the spacing between two adjacent grooves is 150~550μm.

[0036] Further optimization: In step 1, when the second microstructure is a mesh, the size of the mesh unit is (200~500)μm×(200~500)μm, the width of the mesh groove is 50~150μm, and the depth is 100~400μm.

[0037] Research has shown that the optimized pit and grid sizes provide sufficient space and anchoring capacity for the second microstructure. This ensures that the molten aluminum is fully filled by capillary action without significant porosity or defects, and that a stable mechanical anchoring effect is formed after cooling, significantly improving the shear strength of the joint. Simultaneously, this size effectively increases the contact area between the copper and aluminum interfaces, promotes uniform interfacial heat distribution, inhibits excessive growth of the brittle Al-Cu intermetallic compound layer, and guarantees the quality of the interfacial metallurgical bond. Furthermore, this size range is compatible with nanosecond laser direct writing technology, ensuring good processing consistency without weakening the strength of the copper substrate, thus balancing anchoring effect and structural stability.

[0038] Preferably, in step 1, the first microstructure and the second microstructure are prepared using nanosecond laser technology.

[0039] In practical applications, nanosecond laser technology enables high-precision, high-efficiency, and low-damage processing of microstructures on both upper and lower surfaces. It can form a stable and efficient light-absorbing and anti-reflection structure on the upper surface, significantly improving the absorption rate of copper to the laser, while precisely forming uniformly sized anchoring microstructures on the lower surface. This ensures the molten aluminum is fully filled and forms a reliable mechanical interlock. Furthermore, nanosecond laser technology results in a small heat-affected zone, no burrs, and no contaminant residue. The process offers good controllability and repeatability, effectively guaranteeing welding stability and joint quality, making it more suitable for industrial-scale mass production. Other micron-level precision processes can also be used to prepare the first and second microstructures.

[0040] In practical applications, the first microstructure is prepared by continuous scanning and direct writing on the upper surface of a copper plate using a nanosecond laser beam. The laser parameters are: scanning speed 50~150mm / s, direct writing laser power 10~20W, number of scans 3~5, and scanning line spacing 30~70μm.

[0041] The second microstructure is prepared by continuous scanning and direct writing on the lower surface of a copper plate using a nanosecond laser beam. When the second microstructure is a pit, the laser parameters are: laser power 15~20W, scanning speed 1~10mm / s (or fixed-point drilling mode), number of scans 5~15, and pulse frequency 20~50kHz. When the second microstructure is a groove, the laser parameters are: laser power 15~20W, scanning speed 10~50mm / s, number of scans 3~10 times, pulse frequency 20~100kHz. When the second microstructure is a grid, the laser parameters are: laser power 15~20W, scanning speed 10~50mm / s (X / Y bidirectional), number of scans 3~10 times (per direction), and pulse frequency 20~100kHz.

[0042] Preferably, in step 2, the width of the overlapping area is 8~12mm.

[0043] In this invention, the width of the overlapping area is preferably 8~12mm. The width of the overlapping area is affected by both the actual device and the laser power. In the actual production process, the overlapping area can be adjusted according to actual needs. It is only necessary to ensure that the overlapping area has sufficient width to avoid the weld width being greater than the width of the overlapping area.

[0044] Preferably, in step 3, during the welding process, the deflection angle between the laser beam and the copper plate is 5~15°, the laser power is 3.5~4.5kW, the welding speed is 1.0~3.0m / min, the defocusing amount is +10~+30mm, and the gas flow rate of the shielding gas is 15-25L / min.

[0045] Research has shown that step 3, using parameters such as a laser deflection angle of 5~15°, laser power of 3.5~4.5kW, welding speed of 1.0~3.0m / min, and positive defocusing amount of +10~+30mm, combined with the first microstructure, can expand the laser spot and reduce the peak energy density, avoiding local overheating and vaporization of the copper plate and spattering. This results in the copper plate only slightly melting or not melting, allowing the molten aluminum to be smoothly embedded in the second microstructure on the lower surface of the copper plate and effectively alloy with the copper interface. At the same time, the microgrooves can regulate the flow of the molten pool to be more stable, improving the wettability of the aluminum liquid at the copper interface. The moderate welding speed shortens the high-temperature residence time at the interface, inhibiting the formation of brittle intermetallic compounds between aluminum and copper. Combined with an appropriate protective gas flow rate, a stable gas curtain is formed, reducing porosity and oxidation defects, and improving the joint forming quality and bonding strength.

[0046] The deflection angle is defined as the angle between the laser beam axis and the normal to the surface of the copper plate being welded.

[0047] Preferably, in step 3, the relationship between the moving direction of the laser beam and the length direction of the first microstructure is either parallel or perpendicular.

[0048] To make the technical problems, technical solutions and technical advantages of the present invention clearer, a detailed description will be given below with reference to specific examples. However, the scope of protection of the present invention is not limited to the following specific embodiments.

[0049] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0050] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0051] To facilitate the differentiation of the arrangement of the first array, the copper plate used in the specific embodiment of the present invention is a rectangular sheet copper plate, with the longer side of the rectangular sheet copper plate defined as the length direction and the shorter side defined as the width direction.

[0052] Example 1: A laser welding process for dissimilar metals, aluminum and copper, includes the following steps: Step 1: Prepare a copper plate with a thickness of 2mm and an aluminum plate with a thickness of 2mm. Use a nanosecond laser etching machine to etch the first microstructure on the soldering area on the upper surface of the copper plate and etch the second microstructure on the overlapping area on the lower surface of the copper plate.

[0053] The first microstructure unit is a trench. Multiple trenches are evenly spaced along the length of the copper plate. The length of the trench is parallel to the width of the copper plate. The trench width is 20μm, the depth is 20μm, and the spacing between two adjacent trenches is 30μm.

[0054] The second microstructure includes an array of pits, each pit measuring 300 μm × 300 μm × 250 μm (length × width × depth). The spacing between two adjacent rows of pits is 300 μm, and the spacing between two adjacent pits in the same row is 300 μm. The pits in adjacent rows are arranged in an alternating pattern.

[0055] Step 2, pretreatment process: Place the copper plate and aluminum plate from Step 1 into acetone for ultrasonic cleaning for 10 minutes, then wipe them clean with acetone and dry them with cold air.

[0056] Step 3: Place the aluminum plate on the workbench and stack the copper plate on top of the aluminum plate. The width of the stacking area is 10mm.

[0057] Step 4: Perform laser brazing under a protective atmosphere. The laser beam is focused on the first microstructure of the copper plate, and the direction of laser beam movement is perpendicular to the length of the groove. The laser beam welds along a preset path. During the welding process, the deflection angle between the laser beam and the copper plate is controlled at 10°, the laser power is 4kW, the welding speed is 2.0m / min, the defocusing amount is +20mm, and the shielding gas flow rate is 20L / min.

[0058] Figure 1 The image shown is an SEM image of the first microstructure obtained by nanosecond laser etching in step 1 of Example 1. The first microstructure obtained by nanosecond laser etching exhibits a uniform and continuous trench morphology with a trench width of approximately 20 μm. A dense porous rough structure is formed on the surface of the trench. This morphology can significantly increase the specific surface area of ​​the copper surface, effectively reduce the reflectivity to infrared lasers, improve energy coupling efficiency, and provide favorable conditions for subsequent stable welding.

[0059] Figure 2 The image shows the SEM image of the second microstructure obtained by nanosecond laser etching in step 1 of Example 1. The second microstructure exhibits regularly distributed square pits, with each pit measuring approximately 300 μm x 300 μm in length and width. The pits in adjacent columns are staggered, and the overall distribution is uniform and consistent. This structure can provide filling space for molten aluminum to form a mechanical lock, and can also disperse stress and prevent crack propagation at the interface, which is beneficial to improving the shear strength and bonding reliability of the joint.

[0060] Figure 3 The figure shows a metallographic cross-section of the copper and aluminum dissimilar metals after welding in Example 1. As can be seen from the figure, laser brazing achieved an effective connection between the copper plate and the aluminum plate. The weld formation was good, with no obvious cracks, pores or other defects. The copper side remained basically solid, while the aluminum side formed a typical brazing pool morphology. The fusion zone had a clear outline and a smooth transition, indicating that the microstructure-assisted process of the present invention can stably control the heat input and achieve a good metallurgical bond at the copper-aluminum interface, providing a foundation for obtaining a high-strength and high-reliability joint.

[0061] Example 2: A laser welding process for dissimilar metals, aluminum and copper, includes the following steps: Step 1: Prepare a copper plate with a thickness of 2mm and an aluminum plate with a thickness of 2mm. Use a nanosecond laser etching machine to etch the first microstructure on the soldering area on the upper surface of the copper plate and etch the second microstructure on the overlapping area on the lower surface of the copper plate.

[0062] The first microstructure unit is a trench with a width of 25 μm and a depth of 15 μm. The spacing between two adjacent trenches is 40 μm. Multiple trenches are evenly spaced along the length of the copper plate, and the length of the trenches is parallel to the width of the copper plate.

[0063] The second microstructure includes pits arranged in an array, each pit being 300 μm wide and 200 μm deep, with both the lateral and longitudinal spacing between the pits being 300 μm, and adjacent rows of pits being aligned.

[0064] Step 2, pretreatment process, the pretreatment process is the same as in Example 1.

[0065] Step 3: Place the aluminum plate on the workbench and stack the copper plate on top of the aluminum plate. The width of the stacking area is 8mm.

[0066] Step 4: Perform laser brazing under a protective atmosphere. The laser beam is focused on the first microstructure of the copper plate, and the direction of laser beam movement is perpendicular to the length of the groove. The laser beam welds along a preset path. During the welding process, the deflection angle between the laser beam and the copper plate is controlled at 5°, the laser power is 3.5kW, the welding speed is 1.0m / min, the defocusing amount is +10mm, and the shielding gas flow rate is 15L / min.

[0067] Example 3: A laser welding process for dissimilar metals, aluminum and copper, includes the following steps: Step 1: Use a nanosecond laser etching machine to etch the first microstructure on the soldering area on the upper surface of the copper plate, and etch the second microstructure on the overlapping area on the lower surface of the copper plate.

[0068] The first microstructure unit is a trench. Multiple trenches are evenly spaced along the length of the copper plate. The length of the trench is parallel to the width of the copper plate. The width of the trench is 40 μm and the depth is 25 μm. The spacing between two adjacent trenches is 60 μm.

[0069] A second microstructure is etched on the lower surface of the copper plate. The second microarray structure is a grid structure with a grid unit size of 400μm×400μm and a grid unit groove width of 100μm and a depth of 300μm.

[0070] Step 2, pretreatment process, the pretreatment process is the same as in Example 1.

[0071] Step 3: Place the aluminum plate on the workbench and stack the copper plate on top of the aluminum plate. The width of the stacking area is 12mm.

[0072] Step 4: Perform laser brazing under a protective atmosphere. The laser beam is focused on the first microstructure of the copper plate, and the direction of laser beam movement is perpendicular to the length of the groove. The laser beam welds along a preset path. During the welding process, the deflection angle between the laser beam and the copper plate is controlled at 15°, the laser power is 4.5kW, the welding speed is 3.0m / min, the defocusing amount is +30mm, and the shielding gas flow rate is 25L / min.

[0073] Example 4: A laser welding process for dissimilar metals, aluminum and copper, includes the following steps: Step 1: Prepare a copper plate with a thickness of 2mm and an aluminum plate with a thickness of 2mm. Use a nanosecond laser etching machine to etch the first microstructure on the soldering area on the upper surface of the copper plate and etch the second microstructure on the overlapping area on the lower surface of the copper plate.

[0074] The first microstructure unit is a trench. Multiple trenches are evenly spaced along the width direction of the copper plate. The length direction of the trench is parallel to the length direction of the copper plate. The trench width is 20μm, the depth is 20μm, and the spacing between two adjacent trenches is 30μm.

[0075] The second microstructure includes pits arranged in an array, each pit being 300 μm wide and 200 μm deep, with both the lateral and longitudinal spacing between the pits being 300 μm, and adjacent rows of pits being aligned.

[0076] Steps 2-3 are the same as in Example 1.

[0077] Step 4: Perform laser brazing under a protective atmosphere. The laser beam is focused on the first microstructure of the copper plate, and the direction of laser beam movement is parallel to the length of the groove. The laser beam welds along a preset path. During the welding process, the deflection angle between the laser beam and the copper plate is controlled at 10°, the laser power is 4kW, the welding speed is 2.0m / min, the defocusing amount is +20mm, and the shielding gas flow rate is 20L / min.

[0078] Comparative Example 1: A laser welding process for dissimilar metals, aluminum and copper, includes the following steps: Step 1, pretreatment process: Prepare a copper plate with a thickness of 2mm and an aluminum plate with a thickness of 2mm. Place the copper plate and aluminum plate in acetone for ultrasonic cleaning for 10 minutes. Then, use 800# sandpaper to gently polish the surface of the aluminum plate to be welded and the lower surface of the copper plate (to remove the natural oxide film). Wipe them clean with acetone and dry them with cold air.

[0079] Step 2: Place the aluminum plate on the workbench and stack the copper plate on top of the aluminum plate. The width of the stacking area is 10mm.

[0080] Step 3: Perform laser brazing under a protective atmosphere. The laser beam is focused on the copper plate and welded along a preset path. During the welding process, the deflection angle between the laser beam and the copper plate is controlled at 10°, the laser power is 4kW, the welding speed is 2.0m / min, the defocusing amount is +20mm, and the shielding gas flow rate is 20L / min.

[0081] Comparative Example 2: Step 1: Prepare a 2mm thick copper plate and a 2mm thick aluminum plate. Use a nanosecond laser etching machine to etch a second microstructure on the overlapping area of ​​the lower surface of the copper plate. The second microstructure includes an array of pits, each pit measuring 300μm × 300μm × 250μm (length × width × depth). The spacing between two adjacent rows of pits is 300μm, and the spacing between two adjacent pits in the same row is also 300μm. The pits in adjacent rows are arranged in an alternating pattern.

[0082] Steps 2-4 are the same as steps 2-4 in Example 1. In step 4, the laser beam is focused on the copper plate and welded along a preset route.

[0083] Comparative Example 3: Step 1: Prepare a 2mm thick copper plate and a 2mm thick aluminum plate. Use a nanosecond laser etching machine to etch the first microstructure on the solderable area of ​​the upper surface of the copper plate. The unit of the first microstructure is a trench. Multiple trenches are evenly spaced along the length of the copper plate, and the length of the trenches is parallel to the width of the copper plate. The trench width is 20μm, the depth is 20μm, and the spacing between two adjacent trenches is 30μm.

[0084] Steps 2-4 are the same as steps 2-4 in Example 1. In step 4, the laser beam is focused on the copper plate and welded along a preset route.

[0085] Comparative Example 4: Step 1, pretreatment process: Prepare a copper plate with a thickness of 2mm and an aluminum plate with a thickness of 2mm. Place the copper plate and aluminum plate in acetone for ultrasonic cleaning for 10 minutes. Then, use 800# sandpaper to gently polish the surface of the aluminum plate to be welded and the lower surface of the copper plate (to remove the natural oxide film). Wipe them clean with acetone and dry them with cold air.

[0086] Step 3: Place the copper plate on the workbench, and then stack the aluminum plate on top of the copper plate. The width of the stacking area is 10mm.

[0087] Step 4: Perform laser brazing under a protective atmosphere. The laser beam is focused on the area to be welded on the aluminum plate and welds along a preset path. The deflection angle between the laser beam and the aluminum plate is controlled at 10°, the laser power is 2.5kW, the welding speed is 1.5m / min, the defocusing amount is 0mm, and the shielding gas flow rate is 20L / min.

[0088] The copper-aluminum metal welded joints obtained from Examples 1-4 and Comparative Examples 1-4 were subjected to the following treatment: (1) Thickness of Al-Cu compound layer at interface: After cutting, embedding, grinding and polishing the copper-aluminum metal welded joint along the cross section, it was etched with a 5% nitric acid alcohol solution by volume. The average thickness of the continuous compound layer at the interface was observed and measured under a scanning electron microscope (SEM) (5 locations were randomly selected and the average value was taken).

[0089] (2) Joint shear strength: According to GB / T 11363-2017 "Test method for strength of brazed joint", lap shear test specimens were used and tensile shear test was carried out on a universal testing machine with a loading rate of 1 mm / min. The maximum breaking load was recorded.

[0090] (3) Determining the fracture location: After the tensile test, observe the fracture morphology and combine it with energy dispersive spectroscopy (EDS) to determine whether the fracture occurred in the IMC layer (intermetallic compound layer), the aluminum substrate, or the interface microstructure. At the same time, use a metallographic microscope to observe the longitudinal section of the fracture to confirm the crack propagation path.

[0091] Figure 4 The image shows the SEM image of the fracture surface of the copper-aluminum dissimilar metal joint obtained by welding in Example 1 after a tensile shear test. The fracture surface exhibits typical ductile fracture characteristics of aluminum base material, with a large number of tear ridges and plastic deformation traces distributed on the fracture surface. No brittle fracture morphology with straight extension along the interface is observed, indicating that the interfacial bonding strength of the joint prepared by the process of the present invention exceeds the strength of the aluminum base material itself. The fracture mainly occurs in the aluminum base material region, rather than the brittle IMC layer, and the load-bearing capacity of the joint is significantly improved.

[0092] Table 1 shows the thickness of the Al-Cu compound layer at the interface of the copper-aluminum metal welded joints obtained in Examples 1-4 and Comparative Examples 1-4, the shear strength of the joints, and the location of fracture.

[0093] Table 1 Performance parameters of copper-aluminum welded joints obtained in Examples 1-4 and Comparative Examples 1-4 As shown in Table 1, the shear strength of the joints in Examples 1 to 4 of the present invention is significantly higher than that of the comparative examples. Among them, Example 1 reaches 2030N, which is 62.4% higher than the traditional "aluminum on top, copper on the bottom" process (comparative example 4) and 43.0% higher than the microstructure on the upper surface only (comparative example 3).

[0094] The IMC layer thickness in Example 1 was only 2.8 μm, much smaller than that in Comparative Example 4 (>10 μm), indicating that the double-sided microstructure effectively suppressed the excessive growth of brittle compounds. Comparative Examples 1 and 2, lacking an upper surface anti-reflection structure, suffered severe laser reflection and were unable to form effective joints.

[0095] The comparison of fracture locations shows that the fracture in Example 1 occurred at the root of the aluminum base material and the interface microstructure, indicating that the overall strength of the joint is close to or even partially exceeds that of the aluminum base material, and the interface bonding strength is much higher than that of the brittle IMC layer. In contrast, Comparative Example 3 fractured only at the IMC layer, and Comparative Example 4 fractured completely along the IMC layer, indicating that the weak point of the joint in the traditional process and the single-sided microstructure process is the brittle intermetallic compound layer, which has limited strength and is prone to brittle fracture. This fully demonstrates that the double-sided microstructure of the present invention can significantly optimize the interface bonding state and improve the joint's load-bearing capacity and toughness.

[0096] The above-described embodiments are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope of the present invention, based on the technical solution and concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A laser welding process for dissimilar metals, aluminum and copper, characterized in that, Includes the following steps: Step 1: Prepare the first microstructure on the solderable area on the upper surface of the copper plate; prepare the second microstructure on the overlapping area on the lower surface of the copper plate. Step 2: The copper plate is stacked on the aluminum plate, and the second microstructure of the copper plate overlaps with the aluminum plate; Step 3: Perform laser brazing under a protective atmosphere. The laser beam is focused on the first microstructure of the copper plate and welded along a preset path.

2. The aluminum-copper dissimilar metal laser welding process as described in claim 1, characterized in that, In step 1, the first microstructure includes multiple parallel grooves, the width of which is 20~50μm, the depth of which is 10~30μm, and the spacing between two adjacent grooves is 30~70μm.

3. The aluminum-copper dissimilar metal laser welding process as described in claim 1, characterized in that, In step 1, the second microstructure is any one or more of pits, grooves, and meshes; The trenches are designed as multiple trenches, with adjacent trenches being parallel to each other; The pits consist of multiple pits, which are distributed in an array. The grid consists of multiple grids, which are distributed in an array.

4. The aluminum-copper dissimilar metal laser welding process as described in claim 3, characterized in that, In step 1, when the second microstructure is an array of pits, the length × width × depth of the pit is (200~500μm) × (200~500μm) × (100~400μm).

5. The aluminum-copper dissimilar metal laser welding process as described in claim 4, characterized in that, In step 1, when the second microstructure is an array of pits, the pits in adjacent columns are staggered or the pits in adjacent rows are staggered. The spacing between pit units in adjacent columns is 150~550μm, and the spacing between two adjacent pits in the same column is 150~550μm.

6. The aluminum-copper dissimilar metal laser welding process as described in claim 3, characterized in that, In step 1, when the second microstructure is a parallel distribution of grooves, the width of the grooves is 200~500μm, the depth is 100~400μm, and the spacing between two adjacent grooves is 150~550μm.

7. The aluminum-copper dissimilar metal laser welding process as described in claim 3, characterized in that, In step 1, when the second microstructure is a mesh, the size of the mesh unit is (200~500)μm×(200~500)μm, the width of the mesh groove is 50~150μm, and the depth is 100~400μm.

8. The aluminum-copper dissimilar metal laser welding process as described in claim 1, characterized in that, In step 3, during the welding process, the deflection angle between the laser beam and the copper plate is 5~15°, the laser power is 3.5~4.5kW, the welding speed is 1.0~3.0m / min, the defocusing amount is +10~+30mm, and the gas flow rate of the shielding gas is 15~25L / min.

9. The aluminum-copper dissimilar metal laser welding process as described in claim 1, characterized in that, In step 1, the first and second microstructures are prepared using nanosecond laser technology.

10. The aluminum-copper dissimilar metal laser welding process as described in claim 1, characterized in that, The process includes a pretreatment step after step 1 and before step 2: immersing the copper and aluminum plates in an acetone solution for degreasing and cleaning, and then wiping the surfaces of the copper and aluminum plates with an acetone reagent.

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