Method for improving mechanical properties of a welded joint in laser welding and laser welded sheet

Abstract

This invention discloses a method for improving the mechanical properties of welded joints in laser welding and laser-welded plates, relating to the field of welding technology. The method for improving the mechanical properties of welded joints in laser welding utilizes a 3D-printed eutectic high-entropy alloy as an intermediate layer material. By precisely adjusting the line energy and laser power density and controlling the temperature field distribution, the laser beam temperature reaches above the liquidus temperature of both the eutectic high-entropy alloy and the steel plate, forming a molten weld on the steel plate side. On the aluminum alloy plate side, a brazing weld is formed due to the lower melting point of aluminum. A second laser process is applied to the weld to achieve a heat treatment effect. The high-entropy alloy intermediate layer increases the entropy of the welded joint, suppressing the formation of brittle Fe-Al intermetallic compounds and reducing the brittleness of the joint. The second laser processing eliminates residual stress, improving the mechanical properties of the joint, thereby achieving an effective connection between the aluminum alloy and the steel.

Description

Technical Field

[0001] This invention relates to the field of welding technology, and more specifically, to a method for improving the mechanical properties of welded joints in laser welding and laser-welded plates. Background Technology

[0002] Steel possesses excellent mechanical properties and weldability, occupying a crucial position in manufacturing. Aluminum and aluminum alloys have high specific strength, low density, and abundant resources, making them the most widely used non-ferrous metals. Dissimilar welding between steel and aluminum can fully combine the characteristics of both materials. However, due to the significant differences in the physical properties of aluminum and steel, and their low solid solubility, large residual stress distributions are easily generated in the weld joint, leading to cracking and the formation of many hard and brittle intermetallic compounds. This reduces the plasticity and toughness of the joint, thereby degrading its performance. Reducing the formation of intermetallic compounds is key to improving the quality of aluminum / steel welded joints.

[0003] The traditional method for welding aluminum and steel is brazing, which involves controlling the composition of the filler metal and optimizing the local chemical composition at the interface between the high-melting-point workpiece and the filler metal. With further research, more welding technologies can be applied to aluminum / steel welding. Laser welding is one of the important applications of laser processing technology. Laser welding uses a high-energy, high-density laser beam as a heat source to weld workpieces. Compared with traditional welding methods, laser welding has many advantages, including higher welding quality and faster efficiency. Laser brazing is a novel welding method for achieving steel / aluminum welding.

[0004] However, laser brazing is prone to cracking or fracture in the weld joint. The main cause of cracking or fracture is residual stress, which can cause the weld to partially or completely break. Residual stress during machining can also cause workpiece deformation. Furthermore, welding residual stress can lead to brittle fracture of the structure, reducing fatigue strength and corrosion resistance. Therefore, welding residual stress has always been a key concern in the welding industry.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a method for improving the mechanical properties of welded joints in laser welding and a laser-welded plate.

[0007] This invention is implemented as follows:

[0008] In a first aspect, the present invention provides a method for improving the mechanical properties of welded joints in laser welding, comprising:

[0009] A 3D-printed eutectic high-entropy alloy is used as the intermediate layer between the first workpiece to be welded and the second workpiece to be welded, and the first workpiece to be welded and the second workpiece to be welded are joined together.

[0010] Before welding, the linear energy and laser power density of the first laser processing are adjusted to improve the energy distribution at the weld, so that the heat input simultaneously reaches above the liquidus temperature of both the 3D-printed eutectic high-entropy alloy and the first workpiece to be welded, causing both the 3D-printed eutectic high-entropy alloy and the first workpiece to be welded to melt and form a molten weld. At the same time, the heat input reaches below the liquidus temperature of the second workpiece to be welded, while the second workpiece to be welded remains in a solid state. The molten 3D-printed eutectic high-entropy alloy wets and spreads onto the second workpiece to be welded, forming a brazed weld. 1-2 seconds after the start of the first laser processing, the weld is heat-treated by a second laser processing.

[0011] In an optional embodiment, the 3D-printed eutectic high-entropy alloy is a sheet or block prepared by 3D printing using AlCoCrFeNi2.1 powder;

[0012] Preferably, the overall thickness of the 3D printed eutectic high-entropy alloy is 0.2mm to 0.8mm.

[0013] In an optional embodiment, the method for 3D printing AlCoCrFeNi2.1 powder is SLM 3D printing;

[0014] Preferably, the parameters of the SLM 3D printing include a laser power of 700-900w, a scanning speed of 900-1100mm / min, a laser spot size of 1.3-1.7mm, and a powder feed rate of 8-12g / min.

[0015] In an optional embodiment, the first workpiece to be welded and the second workpiece to be welded are each independently selected from the same or different types of steel plate, aluminum alloy plate, titanium alloy plate, nickel alloy plate and copper alloy plate.

[0016] Preferably, the first workpiece to be welded is a steel plate, and the second workpiece to be welded is an aluminum alloy plate.

[0017] In an optional embodiment, the thickness of the first workpiece to be welded and the second workpiece to be welded is 1 mm to 4 mm.

[0018] In an optional embodiment, the first laser processing includes welding the weld seam on the surface of the steel plate using a laser beam.

[0019] In an optional embodiment, the line energy and laser power density are adjusted by changing the laser power, laser welding speed and spot radius. The laser power density LPD is represented by the formula: LPD = P / Aspot, and the line energy Q is represented by the formula: Q = P / V, where the laser power is P, the laser welding speed is V, and the laser spot area is Aspot.

[0020] In an optional embodiment, the laser power of the first laser processing is 1900w to 2300w, the laser welding speed is 0.02m / s to 0.04m / s, the defocusing amount is -5mm to 5mm, and the spot radius R1 is 0.5mm to 1.5mm.

[0021] The laser power of the second laser processing is 1200w to 1400w, the laser welding speed is 0.02m / s to 0.04m / s, the defocusing amount is increased by 5mm compared with the first laser processing, and the spot radius R2 is 1mm to 2mm.

[0022] Preferably, during the first and second laser processing, argon gas with a purity of 99.9% is used as a protective gas on the upper surface of the weld, with a gas flow rate of 5L / min to 15L / min.

[0023] In an optional embodiment, the step of using a 3D-printed eutectic high-entropy alloy as an intermediate layer between the first workpiece to be welded and the second workpiece to be welded includes: placing the 3D-printed eutectic high-entropy alloy on the part to be welded of the second workpiece to be welded, overlapping the first workpiece to be welded onto the 3D-printed eutectic high-entropy alloy, fixing the first workpiece to be welded, the 3D-printed eutectic high-entropy alloy and the second workpiece to be welded on a fixture using a pressure plate, and applying a preload force of 10-20N.

[0024] Secondly, the present invention provides a laser-welded plate, which is welded by a method for improving the mechanical properties of the weld joint in laser welding as described in any of the foregoing embodiments.

[0025] The present invention has the following beneficial effects:

[0026] The method for improving the mechanical properties of welded joints in laser welding provided by this invention uses a 3D-printed eutectic high-entropy alloy as an intermediate layer material. By precisely adjusting the line energy and laser power density and controlling the temperature field distribution, the laser beam temperature reaches above the liquidus temperature of both the eutectic high-entropy alloy and the steel plate, forming a molten weld on the steel plate side. On the aluminum alloy side, a brazing weld is formed due to the lower melting point of aluminum. A second laser process is applied to the weld to achieve a heat treatment effect. The high-entropy alloy intermediate layer increases the entropy of the welded joint, suppressing the formation of brittle Fe-Al intermetallic compounds and reducing the brittleness of the joint. The second laser processing eliminates residual stress and improves the mechanical properties of the joint, thereby achieving an effective connection between the aluminum alloy and the steel. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 A schematic diagram of the welding process for the method of improving the mechanical properties of welded joints in laser welding provided by the present invention;

[0029] Figure 2 A schematic diagram of the laser spot radius during the welding process for the method of improving the mechanical properties of welded joints in laser welding provided by the present invention;

[0030] Figure 3 This is a schematic diagram of the weld morphology after welding, showing the method for improving the mechanical properties of welded joints in laser welding provided in Embodiment 1 of the present invention.

[0031] Figure 4 Force-displacement diagram after welding for the method of improving the mechanical properties of welded joints in laser welding provided in Embodiment 1 of the present invention.

[0032] Icons: 1-First workpiece to be welded; 2-Second workpiece to be welded; 3-3D printed eutectic high-entropy alloy; 4-First laser; 5-Second laser; 6-Welding area. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0034] Please see Figure 1 and Figure 2 This invention provides a method for improving the mechanical properties of welded joints in laser welding, comprising: using a 3D-printed eutectic high-entropy alloy 3 as an intermediate layer between a first workpiece 1 and a second workpiece 2 to be welded, wherein the first workpiece 1 and the second workpiece 2 are overlapped; forming a composite joint of a molten weld on the side of the first workpiece 1 and a brazed weld on the side of the second workpiece 2 by a first laser processing 4; and performing heat treatment on the weld by a second laser processing 5 1-2 seconds after the start of the first laser processing 4.

[0035] Specifically, it includes the following steps:

[0036] (1) Preprocessing.

[0037] In this invention, the first workpiece to be welded 1 and the second workpiece to be welded 2 can be made of the same material or different materials. The first workpiece to be welded 1 and the second workpiece to be welded 2 are each independently selected from the same or different materials among steel plate, aluminum alloy plate, titanium alloy plate, nickel alloy plate, and copper alloy plate. Preferably, the first workpiece to be welded 1 is a steel plate, and the second workpiece to be welded 2 is an aluminum alloy plate. The thickness of the first workpiece to be welded 1 and the second workpiece to be welded 2 is 1mm to 4mm.

[0038] Before welding, use conventional pretreatment processes to remove oil, coatings and oxide films from the surfaces of the first workpiece 1 and the second workpiece 2 to be welded: wipe the surface of the first workpiece 1 to be welded with alcohol, and use 2000# sandpaper to polish the welding area 6 of the second workpiece 2 to be welded.

[0039] (2) Overlap.

[0040] A 3D-printed eutectic high-entropy alloy 3 is placed on the part of the second workpiece 2 to be welded. The first workpiece 1 to be welded is overlapped on the 3D-printed eutectic high-entropy alloy 3. The first workpiece 1 to be welded, the 3D-printed eutectic high-entropy alloy 3 and the second workpiece 2 to be welded are fixed on the fixture with a pressure plate and a preload force of 10-20N is applied.

[0041] In this invention, taking the first workpiece to be welded, 1, as a steel plate and the second workpiece to be welded, 2, as an aluminum alloy plate, as an example, the invention adopts a steel-on-aluminum-on-aluminum overlapping method. A pressure plate is used to place the aluminum alloy plate and a backing plate of equal thickness together at the bottom. A certain thickness of AlCoCrFeNi2.1 eutectic high-entropy alloy is filled into the part of the aluminum alloy plate to be welded. The steel plate is then overlapped onto the aluminum alloy plate, and the backing plate of equal thickness is placed together with the steel plate. The pressure plate is used to fix it to the fixture. The fixing knob of the fixture platform is adjusted, and the applied preload force is 10-20N to ensure uniform pressure on the steel plate, high-entropy alloy, and aluminum alloy plate.

[0042] The development of high-entropy alloys (HEAs) offers a new research approach to addressing the challenges of aluminum / steel welding. Compared to traditional metal alloys, the design philosophy of HEAs is entirely different. HEAs possess excellent high-temperature structural stability, corrosion resistance, and superior mechanical properties, such as tensile strength, fracture toughness, and creep resistance, making them suitable for use in many extreme environments.

[0043] The eutectic high-entropy alloy in this invention is prepared using 3D printing technology. Conventional eutectic high-entropy alloys are typically produced through traditional induction melting or vacuum arc melting, followed by casting, which requires repeated remelting to achieve chemical homogeneity. However, traditional casting often leads to significant phase separation during the manufacturing process, necessitating post-processing to further adjust the microstructure and obtain the desired properties. Furthermore, controlling the inherent complexity required to produce homogeneous bulk alloys using traditional manufacturing processes remains challenging. The 3D printing method in this invention, however, allows for the creation of complex part geometries and high-degree-of-freedom designs, offering significant potential for producing high-entropy alloys used in high-performance engineering materials.

[0044] Specifically, the 3D-printed eutectic high-entropy alloy 3 is a sheet or block formed by 3D printing using AlCoCrFeNi2.1 powder; the method used for AlCoCrFeNi2.1 powder 3D printing is SLM 3D printing. SLM is a commonly used 3D printing technology for the research and development of high-entropy alloys. SLM (selected laser melting) selectively heats metal powder by irradiating it with a laser beam, causing it to melt and form a shape. The eutectic high-entropy alloy printed by SLM exhibits a higher yield strength than that produced by other 3D printing processes. The printed high-entropy alloy has finer grains, a more uniform composition distribution, and no intermetallic compound phase precipitation. Parameters such as laser power and laser scanning speed can be changed to create high-entropy alloys with optimized microstructure and properties. Specifically, the parameters for SLM 3D printing include a laser power of 700-900W, a scanning speed of 900-1100mm / min, a laser spot size of 1.3-1.7mm, and a powder feed rate of 8-12g / min. This application selects SLM 3D printed high-entropy alloy as the intermediate layer for welding. Compared with the traditional method of casting, the high-entropy alloy intermediate layer obtained by this invention has superior performance. The overall thickness of the 3D printed eutectic high-entropy alloy 3 is 0.2mm to 0.8mm.

[0045] Furthermore, the use of 3D printing in this invention improves the strength and toughness of the high-entropy alloy, resulting in better performance compared to conventional nickel-based high-entropy alloy foils. In addition, the high-entropy alloy composition of this application contains Al and Fe elements as base materials. This high-entropy alloy has FCC and BCC phases, exhibiting excellent mechanical properties. The Fe and Al elements can enhance the wettability and bonding to the base material.

[0046] In this invention, by precisely controlling the thickness of the eutectic high-entropy alloy, the connection area at the interface between the aluminum alloy plate and the steel plate can be appropriately controlled. The precise thickness of the AlCoCrFeNi2.1 eutectic high-entropy alloy ensures that the molten aluminum fully wets and spreads at the fusion zone interface, which helps improve the mechanical properties of the lap joint while reducing defects such as cracks and porosity. Furthermore, during welding, using the AlCoCrFeNi2.1 eutectic high-entropy alloy as an intermediate layer effectively prevents the diffusion of elements from the base materials of the first and second workpieces 1 and 2 from the weld, inhibiting the formation of Fe-Al intermetallic compounds. This achieves high entropy in the weld, reduces the formation of intermetallic compounds, and ensures that the molten aluminum fully wets and spreads at the fusion zone interface, contributing to improved mechanical properties of the lap joint while reducing defects such as cracks and porosity.

[0047] (3) Welding.

[0048] The first laser processing (4) forms a composite joint with a fusion weld on the steel plate side and a brazed weld on the aluminum alloy plate side; 1-2 seconds after the start of the first laser processing (4), the weld is heat-treated by a second laser processing (5).

[0049] The first laser processing step 4 involves using a laser beam to weld the weld seam on the surface of the steel plate. Before welding, the linear energy and laser power density of the first laser processing step 4 are adjusted to improve the energy distribution at the weld seam, so that the heat input reaches above the liquidus temperature of both the 3D printed eutectic high-entropy alloy 3 and the first workpiece 1 to be welded. Both the 3D printed eutectic high-entropy alloy 3 and the first workpiece 1 to be welded melt to form a molten weld seam. At the same time, the heat input reaches below the liquidus temperature of the second workpiece 2 to be welded. The second workpiece 2 to be welded is still in a solid state. The molten 3D printed eutectic high-entropy alloy 3 wets and spreads on the second workpiece 2 to form a brazing weld seam.

[0050] In this invention, the purpose of adding the second laser 5 is not to melt the weld, but to heat treat the weld and reduce residual stress. In addition, the second laser 5 processing and the first laser 4 processing have a fixed time interval and are not performed simultaneously. The purpose of this setting is to ensure that heat treatment is performed within a certain time after the weld melts, thereby improving the weld effect.

[0051] During laser treatment, power density typically determines whether a threshold is reached for damaging, ablating, or otherwise affecting the material. By controlling the laser power, laser welding speed, and laser spot area, the linear energy and laser power density can be effectively managed, temperature distribution can be reduced, stress distribution at the lap joint can be decreased, and the mechanical properties of the joint can be improved. Effective connection between aluminum alloy plates and steel plates can be achieved through precise control of the temperature field distribution. The laser power density (LPD) is represented by the formula: LPD = P / Aspot, and the linear energy (Q) is represented by the formula: Q = P / V, where P is the laser power, V is the laser welding speed, and Aspot is the laser spot area.

[0052] In this invention, both the first laser processing 4 and the second laser processing 5 use high-power lasers, specifically continuous fiber lasers. The power of the high-power laser can be adjusted from 100W to 10000W, and the spot radius R can be adjusted from 0mm to 4mm.

[0053] In this invention, the laser power of the first laser processing (4) is adjusted to 1900W to 2300W, the laser welding speed is 0.02m / s to 0.04m / s, the defocusing amount is -5mm to 5mm, and the spot radius R1 is 0.5mm to 1.5mm; the laser power of the second laser processing (5) is 1200W to 1400W, the laser welding speed is 0.02m / s to 0.04m / s, the defocusing amount is increased by 5mm compared to the first laser processing (4), and the spot radius R2 is 1mm to 2mm; furthermore, during the first laser processing (4) and the second laser processing (5), argon gas with a purity of 99.9% is used as a protective gas on the upper surface of the weld, and the gas flow rate is 5L / min to 15L / min.

[0054] This invention employs laser welding, which modulates the penetration depth of the weld joint by changing the linear energy and laser power density, thereby reducing the thickness of intermetallic compounds, allowing the weld to undergo heat treatment, thus reducing the residual stress of the weld joint and improving the performance of the aluminum-steel weld joint.

[0055] Laser-welded plates produced by using the above-mentioned methods to improve the mechanical properties of welded joints in laser welding exhibit excellent mechanical properties.

[0056] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0057] Example 1

[0058] This embodiment provides a method for improving the mechanical properties of welded joints in laser welding, which includes the following steps:

[0059] (1) Select a 6061 aluminum alloy plate with dimensions of 50×100×2 mm and a 304 stainless steel plate with dimensions of 50×100×2 mm for laser welding. Before welding, wipe the surface of the steel plate with alcohol, and use 600#, 800#, 1000#, 1500# and 2000# sandpaper to grind the area to be welded of the aluminum alloy plate in sequence; remove the oil and oxide layer on the metal surface, and then dry it in a vacuum drying oven.

[0060] (2) The aluminum alloy plate and steel plate are joined in an overlapping manner with the steel plate on top and the aluminum plate on the bottom. A pressure plate is used to place the aluminum alloy plate and the equal-thickness pad at the bottom, and the steel plate is overlapped on the aluminum alloy plate. The equal-thickness pad is then placed on the steel plate. A 3D-printed AlCoCrFeNi2.1 eutectic high-entropy alloy is placed between the two and fixed to the fixture with a pressure plate. The fixing knob of the fixture platform is adjusted to ensure that the pressure on the steel plate, high-entropy alloy and aluminum alloy plate is uniform.

[0061] (3) Welding is performed using a continuous fiber laser. The linear energy and laser power density of the first laser processing (4) are adjusted to improve the energy distribution at the weld. The laser power of the first laser processing (4) is 2100W, the laser welding speed is 0.03m / s, and the spot radius R1 is 1mm. The heat input is simultaneously made to reach above the liquidus temperature of both the high-entropy alloy metal sheet and the steel plate, causing both the AlCoCrFeNi2.1 eutectic high-entropy alloy and the steel plate to melt and form a molten weld. At the same time, the heat input reaches below the liquidus temperature of the aluminum alloy plate, which remains in a solid state. The molten AlCoCrFeNi2.1 eutectic high-entropy alloy wets and spreads on the aluminum alloy plate to form a brazed weld. The defocusing amount of the laser beam is adjusted to 0mm to avoid the phenomenon of the AlCoCrFeNi2.1 eutectic high-entropy alloy not melting or the aluminum alloy plate being completely melted.

[0062] One to two seconds after the first laser processing (laser 4), the weld was subjected to a second laser processing (laser 5) for heat treatment. The laser power of the second laser processing (laser 5) was 1300W, the laser welding speed was 0.03m / s, the defocusing amount was increased by 5mm compared to the first laser processing (laser 4), and the spot radius R2 was 1.5mm; this heat treatment of the weld reduced residual stress. High-purity argon gas was used during welding, with 99.99% pure argon gas used to protect the upper surface of the weld. The gas flow rate for the upper surface protection gas was adjusted to 10L / min.

[0063] from Figure 3 The resulting weld was free of defects such as porosity and spatter, and macroscopically, it appeared as a bright, continuous weld. Figure 4As can be seen, through optimization of the welding process, when the laser power is 2100W, a fusion weld is formed on the steel side and a brazing weld is formed on the aluminum side. Compared with 1900W, the maximum load of the joint increased by 82.3%, successfully improving the mechanical properties of the welded joint.

[0064] Example 2

[0065] This embodiment provides a method for improving the mechanical properties of welded joints in laser welding, which includes the following steps:

[0066] (1) Select a 6061 aluminum alloy plate with dimensions of 50×100×2 mm and a 304 stainless steel plate with dimensions of 50×100×2 mm for laser welding. Before welding, wipe the surface of the steel plate with alcohol, and use 600#, 800#, 1000#, 1500# and 2000# sandpaper to grind the area to be welded of the aluminum alloy plate in sequence; remove the oil and oxide layer on the metal surface, and then dry it in a vacuum drying oven.

[0067] (2) The aluminum alloy plate and steel plate are joined in an overlapping manner with the steel plate on top and the aluminum plate on the bottom. A pressure plate is used to place the aluminum alloy plate and the equal-thickness pad at the bottom, and the steel plate is overlapped on the aluminum alloy plate. The equal-thickness pad is then placed on the steel plate. A 3D-printed AlCoCrFeNi2.1 eutectic high-entropy alloy is placed between the two and fixed to the fixture with a pressure plate. The fixing knob of the fixture platform is adjusted to ensure that the pressure on the steel plate, high-entropy alloy and aluminum alloy plate is uniform.

[0068] (3) Welding is performed using a continuous fiber laser. The linear energy and laser power density of the first laser processing (4) are adjusted to improve the energy distribution at the weld. The laser power of the first laser processing (4) is 1900W, the laser welding speed is 0.02m / s, and the spot radius R1 is 0.5mm. The heat input is simultaneously made to reach above the liquidus temperature of both the high-entropy alloy metal sheet and the steel plate, causing both the AlCoCrFeNi2.1 eutectic high-entropy alloy and the steel plate to melt and form a molten weld. At the same time, the heat input reaches below the liquidus temperature of the aluminum alloy plate, which remains in a solid state. The molten AlCoCrFeNi2.1 eutectic high-entropy alloy wets and spreads on the aluminum alloy plate to form a brazed weld. The defocusing amount of the laser beam is adjusted to -5mm to avoid the phenomenon of the AlCoCrFeNi2.1 eutectic high-entropy alloy not melting or the aluminum alloy plate being completely melted.

[0069] One to two seconds after the first laser processing (laser 4), the weld was subjected to a second laser processing (laser 5) for heat treatment. The laser power of the second laser processing (laser 5) was 1200W, the laser welding speed was 0.02m / s, the defocusing amount was increased by 5mm compared to the first laser processing (laser 4), and the spot radius R2 was 1mm; this heat treatment of the weld reduced residual stress. High-purity argon gas was used during welding, with 99.99% pure argon gas passing through the upper surface of the weld for protection. The gas flow rate for the upper surface protection gas was adjusted to 5L / min.

[0070] Example 3

[0071] This embodiment provides a method for improving the mechanical properties of welded joints in laser welding, which includes the following steps:

[0072] (1) Select a 6061 aluminum alloy plate with dimensions of 50×100×2 mm and a 304 stainless steel plate with dimensions of 50×100×2 mm for laser welding. Before welding, wipe the surface of the steel plate with alcohol, and use 600#, 800#, 1000#, 1500# and 2000# sandpaper to grind the area to be welded of the aluminum alloy plate in sequence; remove the oil and oxide layer on the metal surface, and then dry it in a vacuum drying oven.

[0073] (2) The aluminum alloy plate and steel plate are joined in an overlapping manner with the steel plate on top and the aluminum plate on the bottom. A pressure plate is used to place the aluminum alloy plate and the equal-thickness pad at the bottom, and the steel plate is overlapped on the aluminum alloy plate. The equal-thickness pad is then placed on the steel plate. A 3D-printed AlCoCrFeNi2.1 eutectic high-entropy alloy is placed between the two and fixed to the fixture with a pressure plate. The fixing knob of the fixture platform is adjusted to ensure that the pressure on the steel plate, high-entropy alloy and aluminum alloy plate is uniform.

[0074] (3) Welding is performed using a continuous fiber laser. The linear energy and laser power density of the first laser processing (4) are adjusted to improve the energy distribution at the weld. The laser power of the first laser processing (4) is 2300W, the laser welding speed is 0.04m / s, and the spot radius R1 is 1.5mm. The heat input is simultaneously brought above the liquidus temperature of both the high-entropy alloy metal sheet and the steel plate, causing both the AlCoCrFeNi2.1 eutectic high-entropy alloy and the steel plate to melt and form a molten weld. At the same time, the heat input is brought below the liquidus temperature of the aluminum alloy plate, which remains in a solid state. The molten AlCoCrFeNi2.1 eutectic high-entropy alloy wets and spreads on the aluminum alloy plate to form a brazed weld. The defocusing amount of the laser beam is adjusted to 5mm to avoid the phenomenon of the AlCoCrFeNi2.1 eutectic high-entropy alloy not melting or the aluminum alloy plate being completely melted.

[0075] One to two seconds after the first laser processing (laser 4), the weld was subjected to a second laser processing (laser 5) for heat treatment. The laser power of the second laser processing (laser 5) was 1400W, the laser welding speed was 0.04m / s, the defocusing amount was increased by 5mm compared to the first laser processing (laser 4), and the spot radius R2 was 2mm; this heat treatment of the weld reduced residual stress. High-purity argon gas was used during welding, with 99.99% pure argon gas passing through the upper surface of the weld for protection. The gas flow rate for the upper surface protection gas was adjusted to 15L / min.

[0076] Comparative Example 1

[0077] This comparative example is basically the same as Example 1, except that the high-entropy alloy is replaced with CoTiCuNiSi in this comparative example. 0.4 .

[0078] Because Comparative Example 1 uses CoTiCuNiSi 0.4 As a high-entropy alloy, the AlCrCoFeNi2.1 high-entropy alloy used in Example 1 contains Fe and Al elements from the base material, which can increase the wettability between the base material and the intermediate layer. The SLM-printed eutectic high-entropy alloy used in Example 1 exhibits a higher yield strength than other traditional casting processes. The printed high-entropy alloy has fine grains, uniform composition distribution, and no intermetallic compound phase precipitation.

[0079] Comparative Example 2

[0080] This comparative example is basically the same as Example 1, except that the first laser and the second laser are performed simultaneously in this comparative example.

[0081] When the first and second lasers are applied simultaneously, their sole purpose is to alter the heat input distribution and melt the weld. However, the second laser used in Example 1 effectively reduces residual stress, whereas Comparative Example 2 does not achieve this effect.

[0082] Comparative Example 3

[0083] This comparative example is basically the same as Example 1, except that the high-entropy alloy in this comparative example is directly prepared by the traditional casting process.

[0084] The SLM-printed eutectic high-entropy alloy used in Example 1 exhibits a higher yield strength than the traditional casting process in Comparative Example 3. The printed high-entropy alloy has finer grains, more uniform composition distribution, and no intermetallic compound phase precipitation.

[0085] Comparative Example 4

[0086] This comparative example is basically the same as Example 1, except that in this comparative example, the laser power of the second laser is 1000w, and the defocusing amount is the same as that of the first laser, which is 0mm.

[0087] The parameters of the second laser in this comparative example are not within the scope of this application. When the power of the second laser is 1000W, the power is too low and the heat input is too low to obtain good weld formation.

[0088] The welds obtained in the above embodiments were compared, and the detection methods included optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, and mechanical property testing. The detection results are shown in the table below.

[0089]

[0090] As can be seen from the table above, effective connections can be obtained within a certain process range. However, the experimental results show that deeper penetration is not always better; there is a relatively optimal value. In this embodiment, the addition of a high-entropy alloy successfully achieved the slow diffusion effect of the high-entropy alloy, preventing the main Fe and Al elements in the base material from combining to form intermetallic compounds. The heat input is just enough to melt the steel side, forming a fusion weld, while the aluminum side does not melt, forming a brazing weld. When the second laser beam performs heat treatment and reduces residual stress, the performance of Example 1 reaches its optimal level.

[0091] In summary, the method for improving the mechanical properties of welded joints in laser welding provided by this invention uses a 3D-printed eutectic high-entropy alloy 3 as an intermediate layer material. By precisely adjusting the line energy and laser power density and controlling the temperature field distribution, the laser beam temperature reaches above the liquidus temperature of both the eutectic high-entropy alloy and the steel plate, forming a molten weld on the steel plate side. On the aluminum alloy plate side, a brazing weld is formed due to the lower melting point of aluminum. A second laser 5 further processes the weld to achieve a heat treatment effect. The intermediate high-entropy alloy layer increases the entropy of the welded joint, suppressing the formation of brittle Fe-Al intermetallic compounds and reducing the brittleness of the joint. The second laser 5 process eliminates residual stress, improving the mechanical properties of the joint, thereby achieving an effective connection between the aluminum alloy and the steel.

[0092] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of improving the mechanical properties of a welded joint in laser welding, characterized in that, It includes: A 3D-printed eutectic high-entropy alloy is used as an intermediate layer between the first and second workpieces to be welded. The first and second workpieces to be welded are joined together. The 3D-printed eutectic high-entropy alloy is a sheet or block made by 3D printing AlCoCrFeNi2.1 powder. The AlCoCrFeNi2.1 powder 3D printing method is SLM 3D printing. Before welding, the linear energy and laser power density of the first laser processing are adjusted to improve the energy distribution at the weld, so that the heat input simultaneously reaches above the liquidus temperature of both the 3D-printed eutectic high-entropy alloy and the first workpiece to be welded, causing both the 3D-printed eutectic high-entropy alloy and the first workpiece to be welded to melt and form a molten weld. At the same time, the heat input reaches below the liquidus temperature of the second workpiece to be welded, while the second workpiece to be welded remains in a solid state. The molten 3D-printed eutectic high-entropy alloy wets and spreads onto the second workpiece to be welded, forming a brazed weld. 1-2 seconds after the start of the first laser processing, the weld is heat-treated by a second laser processing. The line energy and laser power density are adjusted by changing the laser power, laser welding speed and spot radius. The laser power density LPD is represented by the formula: LPD=P / Aspot, and the line energy Q is represented by the formula: Q=P / V, where laser power P, laser welding speed V, and laser spot area Aspot are the laser spot areas. The laser power for the first laser processing is 1900W~2300W, the laser welding speed is 0.02m / s~0.04m / s, the defocusing amount is -5mm~5mm, and the spot radius R1 is 0.5mm~1.5mm; the laser power for the second laser processing is 1200W~1400W, the laser welding speed is 0.02m / s~0.04m / s, the defocusing amount is increased by 5mm compared to the first laser processing, and the spot radius R2 is 1mm~2mm.

2. The method of improving mechanical properties of a welded joint in laser welding according to claim 1, characterized in that, The overall thickness of the 3D-printed eutectic high-entropy alloy is 0.2mm~0.8mm.

3. The method of improving mechanical properties of a welded joint in laser welding according to claim 1, characterized in that, The parameters for SLM 3D printing include a laser power of 700-900w, a scanning speed of 900-1100mm / min, a laser spot size of 1.3-1.7mm, and a powder feed rate of 8-12g / min.

4. The method of improving mechanical properties of a welded joint in laser welding according to claim 1, characterized in that, The first workpiece to be welded and the second workpiece to be welded are each independently selected from the same or different types of steel plate, aluminum alloy plate, titanium alloy plate, nickel alloy plate and copper alloy plate.

5. The method of improving mechanical properties of a welded joint in laser welding according to claim 4, characterized in that, The first workpiece to be welded is a steel plate, and the second workpiece to be welded is an aluminum alloy plate.

6. The method of improving mechanical properties of a welded joint in laser welding according to claim 4, characterized in that, The thickness of the first workpiece to be welded and the second workpiece to be welded is 1mm to 4mm.

7. The method of improving mechanical properties of a welded joint in laser welding according to claim 4, characterized in that, The first laser processing involves using a laser beam to weld the weld seam on the surface of the steel plate.

8. The method of improving mechanical properties of a weld joint in laser welding according to claim 1, wherein During the first and second laser processing, argon gas with a purity of 99.9% is used as a protective gas on the upper surface of the weld, with a gas flow rate of 5L / min to 15L / min.

9. The method for improving the mechanical properties of welded joints in laser welding according to any one of claims 1-8, characterized in that, The steps of using a 3D-printed eutectic high-entropy alloy as an intermediate layer between the first workpiece to be welded and the second workpiece to be welded include: placing the 3D-printed eutectic high-entropy alloy on the part to be welded of the second workpiece to be welded; overlapping the first workpiece to be welded onto the 3D-printed eutectic high-entropy alloy; fixing the first workpiece to be welded, the 3D-printed eutectic high-entropy alloy, and the second workpiece to be welded onto a fixture using a pressure plate; and applying a preload force of 10~20N.

10. A laser welded sheet material, characterized by It is welded using the method described in any one of claims 1-9 for improving the mechanical properties of the welded joint in laser welding.

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