A magnetic field driven plasma plume opening and closing coordinated heterogeneous metal laser welding method

Abstract

This invention discloses a magnetic field-driven plasma plume opening and closing coordinated dissimilar metal laser welding method, belonging to the field of dissimilar metal laser welding technology. Addressing the problems of energy fluctuations and difficulty in suppressing brittle intermetallic compounds caused by plasma shielding effects in dissimilar metal laser welding, this invention actively regulates the expansion, contraction, opening, and closing behavior of the plasma plume through a gradient time-varying magnetic field, transforming the harmful plasma shielding effect into a controllable laser energy regulating valve. Specific steps include: workpiece pretreatment, electromagnet array arrangement, coordinated application of laser welding and gradient time-varying magnetic field, control of molten pool thermal conduction, and weld formation and microstructure optimization. This invention, by controlling the plasma's "opening and closing" behavior through a magnetic field and changing the direction of the high-energy beam velocity flow vector, achieves active regulation of the molten pool temperature and flow field, significantly suppressing the formation of intermetallic compounds (IMCs) and improving the mechanical properties of the joint. This method is applicable to various dissimilar metal combinations such as magnesium / aluminum, steel / stainless steel, titanium / stainless steel, copper / aluminum, and different grades of magnesium alloys and aluminum alloys. It has a wide process window and strong adaptability, providing a new technical approach for high-performance dissimilar metal welding.

Description

Technical Field

[0001] This invention belongs to the field of dissimilar metal laser welding technology, specifically relating to a magnetic field-driven plasma plume opening and closing coordinated dissimilar metal laser welding method, which is particularly suitable for high-quality welding between magnesium alloys and aluminum alloys, steel and stainless steel, titanium alloys and stainless steel, copper and aluminum, as well as between magnesium alloys and aluminum alloys of different grades. Background Technology

[0002] Dissimilar metal composite structures have broad application prospects in aerospace, new energy vehicles, rail transportation, and shipbuilding. By joining dissimilar metals, the performance advantages of different materials can be fully utilized, achieving lightweight structures and functional integration. However, dissimilar metal laser welding faces two major technical bottlenecks: First, the physical properties of dissimilar metals differ significantly, such as the boiling point difference between magnesium (1090℃) and aluminum (2520℃), the thermal conductivity difference between titanium (21.6 W / m·K) and stainless steel (16.2 W / m·K), and the difference in the coefficient of thermal expansion between Q345 low-alloy steel and 304 stainless steel, leading to uneven heat distribution in the molten pool and a tendency to produce welding defects. Second, brittle intermetallic compounds (IMCs) are easily formed at the dissimilar metal interface, such as Mg in the Mg-Al system. 17 Al 12 Mg2Al3, Fe2Al5 and FeAl3 in the Fe-Al system, and FeTi and Fe2Ti in the Ti-Fe system, etc., severely weaken the mechanical properties of the joint.

[0003] In the prior art, magnetic field-assisted laser welding has become an important means of improving weld quality. A search revealed the following published patents related to this application: (1) Chinese Patent No. CN209919102U, entitled "An Electromagnetic Field Assisted Laser Welding Device", was filed on April 17, 2019, and the patentee is Hunan University. This patent uses a current generator to apply current to the molten pool of the workpiece, so that the workpiece being welded is subjected to a stable electromagnetic force in a steady-state magnetic field, thereby improving the weld formation. However, this technology only uses a steady-state magnetic field and applies current directly to the workpiece. Its magnetic field action mode is different from the gradient time-varying magnetic field of this application, and it does not involve the regulation of the plasma behavior above the molten pool.

[0004] (2) Chinese Patent No. CN111112839B, entitled "A Dual-Laser Beam Double-Sided Synchronous Welding Device and Method Assisted by an External Magnetic Field," was filed on January 6, 2020, and the patentee is Nanjing University of Aeronautics and Astronautics. This patent uses a transverse magnetic field to assist dual-laser beam double-sided synchronous welding. The external magnetic field affects the distribution of plasma generated during welding, thereby improving the absorption of laser beam energy by the workpiece. Although this patent mentions the influence of the magnetic field on the plasma, it uses a transverse stable magnetic field, rather than the gradient time-varying magnetic field of this application, and does not involve the technical concept of actively controlling the "opening and closing" behavior of the plasma through the magnetic field to achieve energy regulation.

[0005] (3) Chinese Patent No. CN116197529B, entitled "A Method and System for Laser Overlap Welding of Copper-Aluminum Dissimilar Metals", was filed on December 9, 2022, and the patentee is Changsha Dake Laser Technology Co., Ltd. This patent feeds high-frequency current to the workpiece through a high-frequency current system to implement high-frequency current-assisted laser welding. This technology uses current assistance rather than magnetic field assistance, which is different from the magnetic field driving method of this application, and does not involve plasma control.

[0006] (4) Chinese Patent No. CN202410227432.X, entitled "A Narrow-Type Laser Multi-Wire Single-Power Supply Magnetically Controlled Composite Welding Device and Process", was filed on February 29, 2024, and the applicants include Zhang Hongtao et al. This patent uses magnetic field control technology to regulate the arc shape and improve welding quality. However, this technology is mainly aimed at composite applications of arc welding, and the magnetic field acts on the arc rather than the laser welding plasma, which is fundamentally different from the technical path of this application.

[0007] During laser welding, the plasma plume formed by the ionization of metal vapor above the molten pool exhibits periodic expansion and contraction behavior. Traditionally, this has been considered a harmful "shielding effect," and efforts have been made to suppress its negative impact through process parameter optimization (such as side-blown gas and pulse waveform modulation). However, after in-depth research, the inventors discovered that if the dynamic behavior of the plasma can be transformed from passive interference to active control, precise regulation of the laser energy input can be achieved. Existing technologies generally lack the understanding and means for active plasma control, and there is no known technical solution for using a gradient time-varying magnetic field to drive the plasma "opening and closing" to achieve the function of an energy regulating valve. This patent addresses this technological gap by proposing a magnetic field-driven plasma plume opening and closing method for coordinating the laser welding of heterogeneous metals. By designing a gradient magnetic field with a strong center and a weak outer edge, and a broadband time-varying magnetic field, the expansion and contraction behavior of the plasma is actively controlled, transforming harmful shielding into beneficial regulation, and achieving active regulation of the thermal conduction of the molten pool. Summary of the Invention

[0008] This invention aims to provide a magnetic field-driven plasma plume opening and closing coordinated dissimilar metal laser welding method to solve the technical problems of instability and difficulty in suppressing IMCs in the laser welding process of dissimilar metals in the prior art. By converting the plasma shielding effect into a controllable laser energy regulating valve, precise control of the thermal conduction of the molten pool is achieved.

[0009] To achieve the above objectives, the present invention adopts the following technical solution.

[0010] A method for laser welding of dissimilar metals with coordinated opening and closing of a plasma plume driven by a magnetic field includes the following steps: Step 1, Pre-treatment of workpieces to be welded: Pre-treat the surfaces of the first metal workpiece and the second metal workpiece to be welded, remove oxide film and oil stains, and dry for later use; the first metal workpiece and the second metal workpiece are a combination of dissimilar metals; Step 2, Magnetic field generation system arrangement: A two-axis, three-axis, four-axis, or six-axis electromagnet array is arranged around the laser welding head. The electromagnet array can independently control the magnitude and direction of the current on each axis to generate a spatial gradient magnetic field and a time-varying magnetic field. The gradient magnetic field strength is 0.1~1.5T / mm, generating a gradient magnetic field with different intensity distributions at the center and outer edge. The frequency of the time-varying magnetic field is 10~10KHz, and the waveform is selected from sine wave, square wave, or triangular wave. The magnetic field axis has an angle with the laser beam axis. Step 3, the synergistic application and action of laser welding and magnetic field: The laser is started for welding, and the magnetic field generation system is turned on at the same time. During the welding process, the gradient time-varying magnetic field is applied. The magnetic field acts on the plasma cloud or plume above the high-energy beam welding pool. Through Lorentz force constraint, magnetic pressure gradient drive and resonant amplification mechanism, the expansion and contraction, opening and closing behavior of the plasma is regulated, so that it forms a periodic thickening and thinning, opening and closing motion. In this way, the shielding effect of the plasma on the laser is transformed into a controllable regulating valve for the transmission behavior of laser energy, force and mass, thereby changing the spatiotemporal distribution of laser energy, force and mass reaching the surface and interior of the molten pool. Step 4, control of thermal and mass transfer in the molten pool: When welding dissimilar metals under the synergistic control of magnetic field-plasma-molten pool-defect-weld, the temperature gradient, cooling rate and flow state of the molten pool and the mass transfer behavior are actively adjusted by the expansion and contraction, opening and closing behavior of the plasma, so as to suppress the formation of brittle intermetallic compounds at the interface and obtain a high-quality weld with low content or no intermetallic compounds. Step 5, Porosity Defect Elimination: When welding dissimilar metals under the coordinated control of magnetic field-plasma-molten pool-defect-weld, a wide-frequency automatic scanning method of magnetic field is adopted to determine the resonance frequency of the external magnetic field to eliminate porosity. When the frequency of the external magnetic field matches the inherent oscillation frequency of the porosity in the molten pool, a resonance amplification effect is generated, which significantly enhances the amplitude of porosity movement, causing the porosity to overflow from the molten pool and forming a high-quality weld without porosity defects. Step 6, Multi-objective process parameter optimization: The welding process is optimized based on weld formation and microstructure properties. Through multi-objective, high-throughput machine learning methods, the magnetic field and laser welding process parameters are optimized, adjusted, and determined, including intensity gradient, direction, frequency, waveform, and intermittent parameters. This enables precise control of the spatiotemporal distribution of heat input to the molten pool, thereby optimizing the solidification structure, suppressing the formation of IMCs, and preventing defects such as porosity. A cascaded control mechanism and method for laser welding of "magnetic field-plasma-molten pool-defect-weld" is established, achieving efficient, high-quality, high-performance, and low-cost dissimilar metal laser welding technology.

[0011] Furthermore, it also includes plasma diagnostic and feedback control steps: a high-speed camera system is used to monitor the plasma morphology in real time, while a spectral diagnostic system is used to monitor the plasma electron density and temperature in real time; based on the comparison of the monitoring results with the preset threshold of machine learning, the output parameters of the magnetic field generating system are optimized and dynamically adjusted, including magnetic field frequency, intensity gradient, waveform, or intermittent, to achieve closed-loop control of the laser plasma opening and closing behavior and maintain the stability of the laser welding molten pool and weld solidification process.

[0012] Furthermore, the angle between the magnetic field axis and the laser beam axis in step 2 is adjustable in three dimensions within a wide range of 0~180°, which can adapt to the process requirements of various welding positions and material combinations, flexibly realize the control of laser welding plasma, and complete the high-quality heterogeneous metal laser welding process.

[0013] Furthermore, in step 2, the electromagnet array pre-sets the magnetic field parameters and laser parameters according to the material and thickness of the workpiece to be welded; for dissimilar metal combinations with large thickness differences, the asymmetry of the molten pool and weld caused by the difference in thermophysical properties is compensated by shifting the laser spot to the side with lower thermal conductivity by 0.1~6 mm, thereby further improving the weld formation quality.

[0014] Further, the laser welding process parameters in step 3 are as follows: laser power 0.2~8 kW, welding speed 5~40 mm / s, laser wavelength 1060~1080 nm, defocusing amount -10 mm~+5 mm, protective gas is argon or helium, flow rate 10~40 L / min, duty cycle 10~100%; the dissimilar metal combination includes combinations of magnesium alloy and aluminum alloy, low alloy steel and stainless steel, or combinations of different grades of magnesium alloy, or combinations of different grades of aluminum alloy.

[0015] Furthermore, in step 4, the magnetic field interacts with the charged particles of the laser plasma and the laser thermocurrent, driving the plasma to move under the action of the Lorentz force, electromagnetically stirring the molten metal in the pool, refining the weld grains, and improving the mechanical properties of the joint.

[0016] Furthermore, in step 4, once the magnetic field exceeds a certain threshold, the magnetic field will change the direction of the high-energy beam force vector above the weld pool, generating a high-energy beam force opposite to the direction of weight. At this time, the high-energy beam force on the weld pool is an adsorption force. The magnetic field drives the plasma to generate an adsorption force and its adsorption effect on the laser weld pool, which is completely opposite to the impact force on the conventional laser weld pool.

[0017] Furthermore, in step 4, once the magnetic field exceeds a certain threshold, the time-varying magnetic field generates a periodic high-energy beam with alternating positive and negative pressure, exerting a dual effect of impact and adsorption on the weld pool. Through the designed broadband intermittent magnetic field, intermittent periodic oscillations can be induced in the plasma and weld pool fluid, causing the high-energy beam force vector to change direction intermittently and periodically, forming an upward or downward high-energy beam force. This is beneficial for strengthening the control of plasma and weld pool stability, improving weld formation, and enhancing joint quality.

[0018] The core technical principle of this invention lies in the following: During laser welding, the plasma plume formed by the ionization of metal vapor above the molten pool has the characteristics of charged particles and is subjected to Lorentz force under the action of an external magnetic field. By designing a spatial gradient magnetic field that is strong at the center and weak at the outer edge, a magnetic pressure gradient ∇(B² / 2μ0) pointing towards the center can be formed, driving the plasma to gather towards the center or diffuse towards the outer edge. At the same time, the direction of the high-energy beam force vector acting on the molten pool can be changed to be opposite to the direction of gravity. By designing a broadband time-varying magnetic field, periodic oscillations of the plasma can be induced, and the direction of the high-energy beam force vector can be changed periodically, upwards or downwards. When the frequency of the external magnetic field matches the natural oscillation frequency of the plasma, a resonance amplification effect is generated, which significantly enhances the expansion and contraction amplitude of the plasma. Simultaneously, when the frequency of the external magnetic field matches the natural oscillation frequency of the pores in the molten pool, a resonance amplification effect is generated, which significantly enhances the movement amplitude of the pores, causing the pores to overflow from the molten pool and forming a high-quality weld without pore defects. This controlled plasma "opening and closing" motion essentially acts as a dynamic regulating valve for laser energy—when the plasma expands and thins, or even opens, laser penetration is enhanced and energy is concentrated; when the plasma contracts, concentrates, and thickens, the laser attenuates and energy is dispersed. By adjusting magnetic field parameters, including intensity gradient, direction, frequency, waveform, and interval parameters, precise control of the spatiotemporal distribution of heat input to the molten pool can be achieved, thereby optimizing the solidification structure, suppressing the formation of IMCs, and preventing defects such as porosity. The three-dimensional adjustable design of the angle between the magnetic field axis and the laser beam axis within a wide range of 0~180° can adapt to the process requirements of various welding positions and material combinations, achieving flexibility in plasma control.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) Innovative control approach: The plasma shielding effect, which is traditionally considered harmful, is transformed into a controllable laser energy regulating valve through active control of gradient time-varying magnetic field, opening up a new dimension for magnetic field-assisted welding research; Unlike existing technologies that only focus on the direct effect of magnetic field on molten pool, this invention establishes a new cascade control mechanism of "magnetic field-plasma-molten pool-defect-weld".

[0020] (2) Unique magnetic field parameter design: The gradient magnetic field with strong center and weak outer edge or weak center and strong outer edge is combined with a wide frequency time-varying and intermittent magnetic field of 10-2500 Hz, which can realize precise control of plasma opening and closing behavior. The magnetic field axis has a wide angle adjustment range of 0~180° to adapt to different welding conditions, overcoming the limitations of fixed magnetic field direction and narrow adjustment range in the existing technology.

[0021] (3) Significantly improved stability of welding process: By controlling the opening and closing behavior of plasma with magnetic field, the energy fluctuation caused by random fluctuation of plasma is eliminated, the stability of welding process is improved, spatter and porosity are reduced by more than 50-60%, the weld is uniformly formed and there is no obvious spatter.

[0022] (4) Significant suppression effect of IMCs: The magnetic field-plasma synergistic regulation optimizes the thermal conduction of the molten pool, changes the solidification path, and effectively suppresses brittle intermetallic compounds in the weld; for magnesium-aluminum dissimilar metals, Mg 17 Al 12 The volume fraction of brittle phases such as Mg2Al3 is reduced, and the tensile strength of the joint is increased by more than 30-50%. For titanium / stainless steel dissimilar metals, the thickness of Ti-Fe system IMCs is significantly reduced, and the joint strength is increased by more than 30-40%.

[0023] (5) Wide process window and strong adaptability: The method of the present invention can adapt to a variety of dissimilar metal combinations, including magnesium / aluminum dissimilar metals, steel / stainless steel dissimilar metals, titanium / stainless steel dissimilar metals, copper / aluminum dissimilar metals, between different grades of magnesium alloys, between different grades of aluminum alloys, etc. By adjusting the magnetic field parameters, the welding requirements of different material combinations can be adapted without frequent replacement of hardware equipment.

[0024] (6) High level of intelligence: It integrates plasma diagnostics and feedback control, which can realize real-time optimization and adaptive adjustment of the welding process, providing technical support for intelligent manufacturing. Attached Figure Description

[0025] Figure 1 This is a process flow diagram of the method of the present invention; Figure 2 A diagram showing the contraction and closure state of a plasma plume when the gradient time-varying magnetic field of the present invention is applied; Figure 3 A diagram showing the open state of plasma plume expansion when the gradient time-varying magnetic field of the present invention is applied; The component names corresponding to each mark in the attached diagram are as follows: 1-Attenuated laser beam; 2-Focused and contracted plasma; 3-Shallow keyhole; 4-Workpiece; 5-Enhanced laser beam; 6-Magnetic field; 7-Annular open plasma; 8-Deep keyhole; 9-Workpiece. Specific Implementation

[0026] A method for laser welding of dissimilar metals with coordinated opening and closing of a plasma plume driven by a magnetic field includes the following steps: Step 1, Pre-treatment of workpieces to be welded: Pre-treat the surfaces of the first metal workpiece and the second metal workpiece to be welded, remove oxide film and oil stains, and dry for later use; the first metal workpiece and the second metal workpiece are a combination of dissimilar metals; Step 2, Magnetic field generation system arrangement: A two-axis, three-axis, four-axis, or six-axis electromagnet array is arranged around the laser welding head. The electromagnet array can independently control the magnitude and direction of the current on each axis to generate a spatial gradient magnetic field and a time-varying magnetic field. The gradient magnetic field strength is 0.1~1.5T / mm, generating a gradient magnetic field with different intensity distributions at the center and outer edge. The frequency of the time-varying magnetic field is 10~10KHz, and the waveform is selected from sine wave, square wave, or triangular wave. The magnetic field axis has an angle with the laser beam axis. Step 3, the synergistic application and action of laser welding and magnetic field: The laser is started for welding, and the magnetic field generation system is turned on at the same time. During the welding process, the gradient time-varying magnetic field is applied. The magnetic field acts on the plasma cloud or plume above the high-energy beam welding pool. Through Lorentz force constraint, magnetic pressure gradient drive and resonant amplification mechanism, the expansion and contraction, opening and closing behavior of the plasma is regulated, so that it forms a periodic thickening and thinning, opening and closing motion. In this way, the shielding effect of the plasma on the laser is transformed into a controllable regulating valve for the transmission behavior of laser energy, force and mass, thereby changing the spatiotemporal distribution of laser energy, force and mass reaching the surface and interior of the molten pool. Step 4, control of thermal and mass transfer in the molten pool: When welding dissimilar metals under the synergistic control of magnetic field-plasma-molten pool-defect-weld, the temperature gradient, cooling rate and flow state of the molten pool and the mass transfer behavior are actively adjusted by the expansion and contraction, opening and closing behavior of the plasma, so as to suppress the formation of brittle intermetallic compounds at the interface and obtain a high-quality weld with low content or no intermetallic compounds. Step 5, Porosity Defect Elimination: When welding dissimilar metals under the coordinated control of magnetic field-plasma-molten pool-defect-weld, a wide-frequency automatic scanning method of magnetic field is adopted to determine the resonance frequency of the external magnetic field to eliminate porosity. When the frequency of the external magnetic field matches the inherent oscillation frequency of the porosity in the molten pool, a resonance amplification effect is generated, which significantly enhances the amplitude of porosity movement, causing the porosity to overflow from the molten pool and forming a high-quality weld without porosity defects. Step 6, Multi-objective process parameter optimization: The welding process is optimized based on weld formation and microstructure properties. Through multi-objective, high-throughput machine learning methods, the magnetic field and laser welding process parameters are optimized, adjusted, and determined, including intensity gradient, direction, frequency, waveform, and intermittent parameters. This enables precise control of the spatiotemporal distribution of heat input to the molten pool, thereby optimizing the solidification structure, suppressing the formation of IMCs, and preventing defects such as porosity. A cascaded control mechanism and method for laser welding of "magnetic field-plasma-molten pool-defect-weld" is established, achieving efficient, high-quality, high-performance, and low-cost dissimilar metal laser welding technology.

[0027] Furthermore, it also includes plasma diagnostic and feedback control steps: a high-speed camera system is used to monitor the plasma morphology in real time, while a spectral diagnostic system is used to monitor the plasma electron density and temperature in real time; based on the comparison of the monitoring results with the preset threshold of machine learning, the output parameters of the magnetic field generating system are optimized and dynamically adjusted, including magnetic field frequency, intensity gradient, waveform, or intermittent, to achieve closed-loop control of the laser plasma opening and closing behavior and maintain the stability of the laser welding molten pool and weld solidification process.

[0028] Furthermore, the angle between the magnetic field axis and the laser beam axis in step 2 is adjustable in three dimensions within a wide range of 0~180°, which can adapt to the process requirements of various welding positions and material combinations, flexibly realize the control of laser welding plasma, and complete the high-quality heterogeneous metal laser welding process.

[0029] Furthermore, in step 2, the electromagnet array pre-sets the magnetic field parameters and laser parameters according to the material and thickness of the workpiece to be welded; for dissimilar metal combinations with large thickness differences, the asymmetry of the molten pool and weld caused by the difference in thermophysical properties is compensated by shifting the laser spot to the side with lower thermal conductivity by 0.1~6 mm, thereby further improving the weld formation quality.

[0030] Further, the laser welding process parameters in step 3 are as follows: laser power 0.2~8 kW, welding speed 5~40 mm / s, laser wavelength 1060~1080 nm, defocusing amount -10 mm~+5 mm, protective gas is argon or helium, flow rate 10~40 L / min, duty cycle 10~100%; the dissimilar metal combination includes combinations of magnesium alloy and aluminum alloy, low alloy steel and stainless steel, or combinations of different grades of magnesium alloy, or combinations of different grades of aluminum alloy.

[0031] Furthermore, in step 4, the magnetic field interacts with the charged particles of the laser plasma and the laser thermocurrent, driving the plasma to move under the action of the Lorentz force, electromagnetically stirring the molten metal in the pool, refining the weld grains, and improving the mechanical properties of the joint.

[0032] Furthermore, in step 4, once the magnetic field exceeds a certain threshold, the magnetic field will change the direction of the high-energy beam force vector above the weld pool, generating a high-energy beam force opposite to the direction of weight. At this time, the high-energy beam force on the weld pool is an adsorption force. The magnetic field drives the plasma to generate an adsorption force and its adsorption effect on the laser weld pool, which is completely opposite to the impact force on the conventional laser weld pool.

[0033] Furthermore, in step 4, once the magnetic field exceeds a certain threshold, the time-varying magnetic field generates a periodic high-energy beam with alternating positive and negative pressure, exerting a dual effect of impact and adsorption on the weld pool. Through the designed broadband intermittent magnetic field, intermittent periodic oscillations can be induced in the plasma and weld pool fluid, causing the high-energy beam force vector to change direction intermittently and periodically, forming an upward or downward high-energy beam force. This is beneficial for strengthening the control of plasma and weld pool stability, improving weld formation, and enhancing joint quality. The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto. The materials and equipment used in each embodiment are all commercially available, and the process parameters are only specific examples of the technical solutions of the present invention and do not constitute a limitation on the scope of protection.

[0034] Example 1: Dissimilar welding of AZ31B magnesium alloy and 6061 aluminum alloy This embodiment provides a method for coordinated laser welding of magnesium-aluminum heterostructures using a magnetic field-driven plasma plume opening and closing. The specific steps are as follows: (1) Pretreatment of workpieces to be welded: Select AZ31B magnesium alloy with dimensions of 100×50×2 mm and 6061 aluminum alloy with dimensions of 100×50×2 mm. After grinding, the surfaces to be welded are ultrasonically cleaned with acetone for 10 min, rinsed with anhydrous ethanol and dried with cold air. (2) Arrangement of magnetic field generating system: Arrange a triaxial orthogonal electromagnet array around the laser welding head, set the magnetic field strength gradient to 0.5 ~ 0.6 T / mm, with the center strength to be 0.8 ~ 0.6 T and the outer edge strength to be 0.3 ~ 0.4 T, the magnetic field frequency to be 50 ~ 200 Hz, the waveform to be square wave, the duty cycle to be 40 ~ 60%, and the angle between the magnetic field axis and the laser beam axis to be 0 ~ 60°; (3) Laser welding and magnetic field application: IPG YLS-6000 fiber laser is used, with laser power set to 2.0~2.5kW, welding speed to 10~20 mm / s, defocusing amount to -2~0 mm, shielding gas to argon, flow rate to 15~25 L / min; at the same time, the magnetic field generation system is turned on to apply a gradient time-varying intermittent magnetic field during the welding process. (4) The process was optimized based on the weld formation and microstructure. The research results show that Mg 17 Al 12 The volume fraction of brittle phases such as Mg2Al3 is small and they are diffusely distributed. No continuous network IMCs layer is observed. The weld formation is good, with no defects such as porosity or cracks. The tensile strength of the joint reaches more than 70% of the strength of the magnesium alloy base material. Example 2: Dissimilar welding of Q345 carbon steel and 304 stainless steel This embodiment applies the method of the present invention to the dissimilar welding of Q345 carbon steel and 304 stainless steel. The specific steps are as follows: (1) Pretreatment of workpieces to be welded: Select Q345B low alloy steel with dimensions of 100×50×3 mm and 304 stainless steel with dimensions of 100×50×3 mm. The surfaces to be welded are ground, cleaned with acetone and then dried for later use. (2) Layout of magnetic field generating system: Set the magnetic field strength gradient to 0.4~0.6 T / mm, with the center strength to 0.5~0.6T and the outer edge strength to 0.1~0.2 T, the magnetic field frequency to 80~300 Hz, the waveform to be a triangular wave, and the angle between the magnetic field axis and the laser beam axis to 0~90°; (3) Laser welding and magnetic field application: The laser power is 2.8~3.2 kW, the welding speed is 18~26 mm / s, the defocusing amount is -1~0 mm, the shielding gas is argon, the flow rate is 18~22 L / min, and the filler is ER308L welding wire with a diameter of 1.2 mm; at the same time, the magnetic field generation system is turned on to apply a gradient time-varying magnetic field. (4) The process was optimized based on the weld formation and microstructure. The results showed that the weld formation was good, with no defects such as porosity or cracks; the microstructure showed fine lath martensite; the tensile strength was comparable to that of the 304 stainless steel base material; the weld mainly contained Cr2Ni3, Fe2Ni3, α-Fe and other phases, and no M23C6 and σ phases that were harmful to the performance were found. Example 3: Dissimilar welding of TA2 titanium alloy and 316L stainless steel This embodiment applies the method of the present invention to the challenging combination of dissimilar metals, titanium alloy and stainless steel. The specific steps are as follows: (1) Pretreatment of workpieces to be welded: Select TA2 industrial pure titanium with dimensions of 100×50×2 mm and 316L stainless steel with dimensions of 100×50×2 mm. The surfaces to be welded are ground, ultrasonically cleaned with acetone and then dried. (2) Layout of magnetic field generating system: Set the magnetic field strength gradient to 0.5~0.6 T / mm, with the center strength to 0.7~1.0T and the outer edge strength to 0.3~0.4 T, the magnetic field frequency to 80~150 Hz, the waveform to be square wave, the duty cycle to be 45~65%, and the angle between the magnetic field axis and the laser beam axis to be 0~120°; (3) Laser welding and magnetic field application: The laser power is 2.0~2.2 kW, the welding speed is 12~15 mm / s, the defocusing amount is -3~0 mm, the shielding gas is argon, the flow rate is 20~25 L / min, the laser spot is shifted to the stainless steel side by 0.1~0.5 mm to reduce the melting amount on the titanium side; at the same time, the magnetic field generation system is turned on. (4) The process was optimized based on the weld formation and microstructure. The results showed that the weld formation was good and there were no cracks; the thickness of the Ti-Fe IMCs at the interface, such as FeTi and Fe2Ti layers, was controlled within 3~5 μm and was discontinuously distributed, which was significantly improved compared with the thin and continuous network distribution without magnetic field; the tensile strength of the joint reached 60~65% of the strength of the titanium base material, which was improved compared with the non-magnetic field. Example 4: Dissimilar welding of AZ31 and AZ61 magnesium alloys This embodiment applies the method of the present invention to the welding of magnesium alloys of different grades. The specific steps are as follows: (1) Pretreatment of workpieces to be welded: AZ31B magnesium alloy and AZ61 magnesium alloy were selected, both with dimensions of 100×50×1.5mm. The surfaces to be welded were ground, chemically cleaned and then dried. (2) Layout of magnetic field generating system: Set the magnetic field strength gradient to 0.2~0.3 T / mm (center strength 0.3~0.5 T, outer edge strength 0.1~0.2 T), magnetic field frequency 20~500 Hz, waveform to be sine wave, and the angle between the magnetic field axis and the laser beam axis to be 0~30°; (3) Laser welding and magnetic field application: The laser power is 1.8~2.1 kW, the welding speed is 10~12 mm / s, the defocusing amount is -1~0 mm, the shielding gas is argon, and the flow rate is 10~15 L / min; at the same time, the magnetic field generation system is turned on; (4) The process was optimized based on the weld formation and microstructure. The results showed that the weld formation was uniform and the heat-affected zone was narrow; the hardness test showed that the hardness distribution in the weld area was uniform and there was no obvious softening zone; the tensile test showed that the tensile strength of the joint reached 70-90% of the strength of the AZ31 base material, showing good comprehensive mechanical properties. Example 5: Dissimilar welding of 2195 aluminum-lithium alloy and 6061 aluminum alloy This embodiment applies the method of the present invention to the dissimilar welding of advanced aluminum alloys for aerospace applications and commonly used aluminum alloys. The specific steps are as follows: (1) Pretreatment of workpieces to be welded: 2195 aluminum-lithium alloy with dimensions of 100×50×2 mm and 6061 aluminum alloy with dimensions of 100×50×2 mm were selected. The surfaces to be welded were ground, ultrasonically cleaned with acetone, and then dried. (2) Layout of magnetic field generating system: Set magnetic field strength gradient of 0.1~0.4 T / mm (center strength 0.4~0.7 T, outer edge strength 0.2~0.3 T), magnetic field frequency of 110~800 Hz, waveform of square wave, duty cycle of 30~70%, and angle between magnetic field axis and laser beam axis of 0~135°; (3) Laser welding and magnetic field application: The laser power is 18~2.8 kW, the welding speed is 17~22 mm / s, the defocusing amount is -2~0 mm, the shielding gas is helium, and the flow rate is 15~20 L / min; at the same time, the magnetic field generation system is turned on; (4) Plasma diagnosis and feedback control: The plasma morphology and electron density are monitored in real time using a high-speed camera and spectral diagnostic system. The magnetic field frequency is dynamically adjusted according to the monitoring results to maintain stable opening and closing of the plasma within the range of -50 to +50 Hz. (5) The process was optimized based on the weld formation and microstructure. The results showed that the weld formation was good, with no defects such as cracks or porosity; the tensile strength of the joint was 32-46% higher than that without a magnetic field. Example 6: Dissimilar welding of T2 copper and 1060 pure aluminum This embodiment applies the method of the present invention to the welding of dissimilar metals such as copper and aluminum. The specific steps are as follows: (1) Pretreatment of workpieces to be welded: Select T2 copper with dimensions of 100×50×2 mm and 1060 pure aluminum with dimensions of 100×50×2 mm. The surfaces to be welded are ground, chemically cleaned and then dried. (2) Layout of magnetic field generating system: Set the magnetic field strength gradient to 0.4~0.7 T / mm, with the center strength to 0.6~1.2T and the outer edge strength to 0.2~0.5 T, the magnetic field frequency to 10~50 Hz, the waveform to be a triangular wave, and the angle between the magnetic field axis and the laser beam axis to 0~30°; (3) Laser welding and magnetic field application: The laser power is 2.4~3.0 kW, the welding speed is 5~10 mm / s, the defocusing amount is -5~0 mm, the shielding gas is argon, the flow rate is 24~30 L / min, and the laser spot is shifted to the copper side by 0.1~0.3 mm; at the same time, the magnetic field generation system is turned on. (4) The process was optimized based on the weld formation and microstructure. The results showed that the weld formation was good and there were no obvious cracks; the thickness of Cu-Al IMCs at the interface, such as CuAl2 and Cu9Al4 layers, was controlled within 3~8 μm and was diffusely distributed; the tensile strength of the joint was improved compared with conventional welding. Example 7: Wide Frequency Range Verification This embodiment verifies the applicability of the method of the present invention under high-frequency magnetic fields. The specific steps are as follows: (1) Pretreatment of workpieces to be welded: AZ91 / 2219 dissimilar metal combination; AZ91 magnesium alloy, size 100×50×2mm, and 2219 aluminum alloy, size 100×50×2mm are selected. (2) Layout of magnetic field generating system: Set the magnetic field intensity gradient to 0.3~0.5 T / mm, the magnetic field frequency to 1800~2500Hz, the waveform to be square wave, the duty cycle to be 40~80%, and the angle between the magnetic field axis and the laser beam axis to be 0~90°; (3) Laser welding and magnetic field are applied in tandem: laser power 2.0~2.5 kW, welding speed 15~20 mm / s, defocusing amount -2~-1 mm; (4) The process was optimized based on the weld formation and microstructure. The results showed that the volume fraction of IMCs in the weld was reduced and the tensile strength of the joint was increased, which was significantly better than conventional welding, indicating that the method of the present invention can still achieve beneficial effects in the high frequency range. Example 8: Verification of Wide Defocus Range This embodiment verifies the applicability of the method of the present invention under large defocusing conditions. The specific steps are as follows: (1) Pretreatment of workpieces to be welded: Same as the ZK60 / 2195 dissimilar combination in Example 1; ZK60 magnesium alloy, size 100×50×2 mm, and 2195 aluminum alloy, size 100×50×2 mm, are selected. (2) Layout of magnetic field generating system: Set the magnetic field intensity gradient to 0.3~0.5 T / mm, the magnetic field frequency to 10~200 Hz, the waveform to be square wave, the duty cycle to be 30~50%, and the angle between the magnetic field axis and the laser beam axis to be 0~60°; (3) Laser welding and magnetic field are applied in tandem: laser power 2.0~2.8 kW, welding speed 12~15 mm / s, defocusing amount -10~-5 mm; (4) The process was optimized based on the weld formation and microstructure. The results showed that the weld formation was good, the volume fraction of IMCs was reduced, and the tensile strength of the joint was increased by 80% compared with conventional welding, indicating that the method of the present invention can still effectively control plasma behavior under the condition of large defocusing. This invention's method can be widely applied to the welding and manufacturing of dissimilar metal structural components in fields such as aerospace, new energy vehicles, rail transportation, shipbuilding, and electronic packaging. All equipment used is industrially mature; the magnetic field generation system can be modularly integrated into existing laser welding equipment. The process parameter window is wide and easy to control, demonstrating promising industrial application prospects. It is particularly suitable for high-quality welding requirements of magnesium / aluminum dissimilar light alloy structures, steel / stainless steel composite structures, titanium / stainless steel dissimilar structures, copper / aluminum electronic packaging structures, and between different grades of magnesium alloys and aluminum alloys. The method disclosed in this invention has been verified through multiple specific embodiments, covering various material combinations such as dissimilar magnesium / aluminum, dissimilar steel / stainless steel, dissimilar titanium / stainless steel, dissimilar copper / aluminum, different grades of magnesium alloys, and different grades of aluminum alloys. The process parameter ranges given in each embodiment can achieve the aforementioned beneficial effects. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the protection scope of this invention.

Claims

1. A method for laser welding dissimilar metals with coordinated opening and closing of a plasma plume driven by a magnetic field, characterized in that, Includes the following steps: Step 1, Pre-treatment of workpieces to be welded: Pre-treat the surfaces of the first and second metal workpieces to be welded, remove oxide film and oil stains, and dry them for later use. The first metal workpiece and the second metal workpiece are a combination of dissimilar metals; Step 2, Magnetic field generation system arrangement: A two-axis, three-axis, four-axis, or six-axis electromagnet array is arranged around the laser welding head. The electromagnet array can independently control the magnitude and direction of the current on each axis to generate a spatial gradient magnetic field and a time-varying magnetic field. The gradient magnetic field strength is 0.1~1.5T / mm, generating a gradient magnetic field with different intensity distributions at the center and outer edge. The frequency of the time-varying magnetic field is 10~10KHz, and the waveform is selected from sine wave, square wave, or triangular wave. The magnetic field axis has an angle with the laser beam axis. Step 3, the synergistic application and action of laser welding and magnetic field: The laser is started for welding, and the magnetic field generation system is turned on at the same time. During the welding process, the gradient time-varying magnetic field is applied. The magnetic field acts on the plasma cloud or plume above the high-energy beam welding pool. Through Lorentz force constraint, magnetic pressure gradient drive and resonant amplification mechanism, the expansion and contraction, opening and closing behavior of the plasma is regulated, so that it forms a periodic thickening and thinning, opening and closing motion. In this way, the shielding effect of the plasma on the laser is transformed into a controllable regulating valve for the transmission behavior of laser energy, force and mass, thereby changing the spatiotemporal distribution of laser energy, force and mass reaching the surface and interior of the molten pool. Step 4, control of thermal and mass transfer in the molten pool: When welding dissimilar metals under the synergistic control of magnetic field-plasma-molten pool-defect-weld, the temperature gradient, cooling rate and flow state of the molten pool and the mass transfer behavior are actively adjusted by the expansion and contraction, opening and closing behavior of the plasma, so as to suppress the formation of brittle intermetallic compounds at the interface and obtain a high-quality weld with low content or no intermetallic compounds. Step 5, Porosity Defect Elimination: When welding dissimilar metals under the coordinated control of magnetic field-plasma-molten pool-defect-weld, a wide-frequency automatic scanning method of magnetic field is adopted to determine the resonance frequency of the external magnetic field to eliminate porosity. When the frequency of the external magnetic field matches the inherent oscillation frequency of the porosity in the molten pool, a resonance amplification effect is generated, which significantly enhances the amplitude of porosity movement, causing the porosity to overflow from the molten pool and forming a high-quality weld without porosity defects. Step 6, Multi-objective process parameter optimization: The welding process is optimized based on weld formation and microstructure properties. Through multi-objective, high-throughput machine learning methods, the magnetic field and laser welding process parameters are optimized, adjusted, and determined, including intensity gradient, direction, frequency, waveform, and intermittent parameters. This enables precise control of the spatiotemporal distribution of heat input to the molten pool, thereby optimizing the solidification structure, suppressing the formation of intermetallic compounds (IMCs), and preventing defects such as porosity. A cascaded control mechanism and method for laser welding, namely "magnetic field-plasma-molten pool-defect-weld", has been established, achieving efficient, high-quality, high-performance, and low-cost dissimilar metal laser welding technology.

2. The magnetic field-driven plasma plume opening and closing coordinated heterogeneous metal laser welding method according to claim 1, characterized in that, It also includes plasma diagnostic and feedback control steps: a high-speed camera system is used to monitor the plasma morphology in real time, and a spectral diagnostic system is used to monitor the plasma electron density and temperature in real time; based on the monitoring results and the preset thresholds of machine learning, the output parameters of the magnetic field generating system are optimized and dynamically adjusted, including magnetic field frequency, intensity gradient, waveform, or intermittent, to achieve closed-loop control of the laser plasma opening and closing behavior and maintain the stability of the laser welding molten pool and weld solidification process.

3. The magnetic field-driven plasma plume opening and closing coordinated heterogeneous metal laser welding method according to claim 1, characterized in that, The angle between the magnetic field axis and the laser beam axis described in step 2 is adjustable in three dimensions within a wide range of 0 to 180°. This design can adapt to the process requirements of various welding positions and material combinations, flexibly control the laser welding plasma, and complete the high-quality laser welding process of dissimilar metals.

4. The magnetic field-driven plasma plume opening and closing coordinated heterogeneous metal laser welding method according to claim 1, characterized in that, In step 2, the electromagnet array pre-sets the magnetic field parameters and laser parameters according to the material and thickness of the workpiece to be welded. For dissimilar metal combinations with large thickness differences, the laser spot is shifted to the side with lower thermal conductivity by 0.1~6 mm to compensate for the asymmetry of the molten pool and weld caused by the difference in thermophysical properties, thereby further improving the weld formation quality.

5. The magnetic field-driven plasma plume opening and closing coordinated heterogeneous metal laser welding method according to claim 1, characterized in that, The laser welding process parameters in step 3 are as follows: laser power 0.2~8 kW, welding speed 5~40 mm / s, laser wavelength 1060~1080 nm, defocusing amount -10 mm~+5 mm, protective gas is argon or helium, flow rate 10~40 L / min, duty cycle 10~100%; the dissimilar metal combination includes combinations of magnesium alloy and aluminum alloy, low alloy steel and stainless steel, or combinations of different grades of magnesium alloy, or combinations of different grades of aluminum alloy.

6. The magnetic field-driven plasma plume opening and closing coordinated heterogeneous metal laser welding method according to claim 1, characterized in that, In step 4, the magnetic field interacts with the charged particles of the laser plasma and the laser thermocurrent. Under the action of the Lorentz force, the plasma moves and electromagnetically stirs the molten metal in the pool, refining the weld grains and improving the mechanical properties of the joint.

7. The magnetic field-driven plasma plume opening and closing coordinated heterogeneous metal laser welding method according to claim 1, characterized in that, When the magnetic field described in step 4 exceeds a certain threshold, it will change the direction of the high-energy beam force vector above the weld pool, generating a high-energy beam force opposite to the direction of weight. At this time, the high-energy beam force on the weld pool is an adsorption force. The magnetic field drives the plasma to generate an adsorption force and its adsorption effect on the laser welding weld pool, which is completely opposite to the impact force on the conventional laser welding weld pool.

8. The magnetic field-driven plasma plume opening and closing coordinated heterogeneous metal laser welding method according to claim 1, characterized in that, The magnetic field described in step 4, once it exceeds a certain threshold, generates a periodic high-energy beam with alternating positive and negative pressure, exerting a dual effect of impact and adsorption on the weld pool. Through the designed broadband intermittent magnetic field, intermittent periodic oscillations can be induced in the plasma and weld pool fluid, causing intermittent and periodic changes in the direction of the high-energy beam force vector, forming an upward or downward high-energy beam force. This is beneficial for strengthening the control of plasma and weld pool stability, improving weld formation, and enhancing joint quality.

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