A device and method for bipolar laser composite welding and gradient coating preparation

Abstract

This invention discloses an apparatus and method for bipolar plate laser composite welding and gradient coating preparation. The apparatus includes: a support worktable with a through-flow guide groove; a three-axis gantry motion mechanism disposed above the support worktable; a composite processing head assembly suspended from the execution end of the three-axis gantry motion mechanism and including coaxial double-ring powder feeding nozzles that emit a leading main laser beam and a rear broadband defocused laser beam downwards; and a back-side cooling and clamping assembly disposed at the bottom of the support worktable and including an abutting slider and a pneumatic lifting mechanism. The pneumatic lifting mechanism drives the abutting slider to pass through the through-flow guide groove and abut against the lower surface of the substrate, and a low-temperature airflow is introduced into the internal cavity of the abutting slider and follows its movement. This application can suppress thermal deformation during thin-walled plate welding and prepare a high-performance anti-corrosion coating in situ.

Description

Technical Field

[0001] This invention relates to the fields of new energy equipment manufacturing and laser processing technology, specifically to a device and method for bipolar plate laser composite welding and gradient coating preparation in the scenario of splicing large-size thin-walled bipolar plates for alkaline water electrolysis hydrogen production. Background Technology

[0002] Alkaline water electrolysis hydrogen production systems are currently the mainstream equipment for large-scale green hydrogen production. Inside the electrolyzer of this system, large-sized bipolar plates are the core framework supporting the electrodes, separating gases, and conducting electrons. Typically, the diameter of these plates often exceeds 1.5 meters, while the thickness is usually between 1.0 and 2.0 millimeters. In actual working environments, the bipolar plates need to be immersed in a potassium hydroxide solution with a mass percentage concentration of approximately 30% for extended periods, accompanied by vigorous hydrogen and oxygen evolution reactions. To balance manufacturing costs and mechanical strength, the industry widely uses spliced ​​low-carbon steel or ordinary stainless steel sheets as the basic framework, followed by the preparation of a highly corrosion-resistant nickel-based anti-corrosion coating on its surface.

[0003] There are two main technical approaches for manufacturing large-size thin-walled bipolar plates, but both have significant industrial drawbacks. The first approach involves first welding the plates together, then immersing the entire plate in an electroplating bath to prepare the anti-corrosion layer. Due to the extremely large size of the plates, this method necessitates the use of ultra-large temperature-controlled electroplating baths in factories. This not only increases fixed asset investment and environmental treatment costs, but also results in extremely uneven plating thickness between the central and edge areas of the plate due to the edge effect of the large-area electric field distribution, reducing the overall service life of the plate. The second approach involves first preparing the plating layer on small plates, then performing laser welding. However, the extremely high localized heat input from laser welding causes the original anti-corrosion plating layer in the weld area to instantly vaporize and burn away, creating fatal weak points at the joint. During subsequent operation, these tiny exposed weld areas become anodes for electrochemical reactions, causing corrosion to rapidly extend into the carbon steel substrate, leading to perforation of the substrate.

[0004] Furthermore, because the substrate is a large-sized, thin-walled component, when using conventional laser self-fusion welding technology for splicing, the locally concentrated high-energy heat input can easily cause severe warping and wavy deformation of the thin plate during the cooling and shrinkage phase. This uncontrolled residual thermal stress not only reduces the adhesion of subsequent surface coatings but also makes it easier for uneven stress to occur on the sealing gaskets during the assembly and stacking of electrolytic cells, leading to production accidents such as hydrogen-oxygen mixture leakage or leakage of high-concentration alkali solutions. Therefore, there is an urgent need for a processing system that can achieve continuous, high-quality in-situ preparation of anti-corrosion coatings in the weld area while controlling welding thermal deformation. Summary of the Invention

[0005] To address the technical problems of large welding thermal deformation, inability to continuously prepare corrosion-resistant coatings in the weld zone, and decreased coating adhesion caused by residual stress after welding during the splicing of large-size thin-walled carbon steel bipolar plates, this application provides a device and method for bipolar plate laser composite welding and gradient coating preparation.

[0006] This application provides a bipolar laser composite welding and gradient coating preparation apparatus, comprising: a support worktable having a through-flow follower guide groove; a three-axis gantry motion mechanism disposed in the space above the support worktable; a composite processing head assembly suspended on the execution end of the three-axis gantry motion mechanism, the composite processing head assembly including a coaxial double-ring powder feeding nozzle, the composite processing head assembly being configured to emit a leading main laser beam and a rear broadband defocused laser beam downwards; and a back-side chilling and clamping assembly disposed at the bottom of the support worktable, the back-side chilling and clamping assembly including an abutment slider and a pneumatic lifting mechanism, the pneumatic lifting mechanism being connected to the abutment slider to drive the abutment slider through the through-flow follower guide groove and abut against the lower surface of the substrate; the abutment slider being configured to move synchronously along the through-flow follower guide groove with the composite processing head assembly, and the internal cavity being configured to allow the introduction of a low-temperature airflow.

[0007] The coaxial dual-ring powder feeder includes an inner ring flow channel located inside and an outer ring flow channel sleeved on the outside; the optical path system of the composite processing head assembly is configured to emit a leading main laser beam with a circular spot vertically downward from the center position, and to emit a rear broadband defocused laser beam with a rectangular spot obliquely inward from the outer edge; the leading main laser beam penetrates and heats the splicing gap of the substrate, and the rear broadband defocused laser beam melts the particulate material ejected from the coaxial dual-ring powder feeder.

[0008] Specifically, the focal point of the leading main laser beam is positioned within the thickness dimension of the substrate; the projection irradiation area of ​​the rear broadband defocused laser beam on the horizontal plane completely covers the area behind the heating region acted upon by the leading main laser beam; the inner annular flow channel is configured to connect to a first powder supply source to output a first type of metal powder, and the outer annular flow channel is configured to connect to a second powder supply source to output a second type of metal powder; the rear broadband defocused laser beam is configured to simultaneously melt and converge the first type of metal powder and the second type of metal powder to the irradiation area, wherein the melting point of the first type of metal powder is higher than the melting point of the second type of metal powder.

[0009] The abutting slider has a regularly arranged array of micropores on its top surface facing the substrate; the back-side cooling and pressing assembly also includes a liquid nitrogen vaporization mixer disposed below the supporting worktable; the output end of the liquid nitrogen vaporization mixer is connected to the internal cavity of the abutting slider through a flexible pipeline, supplying the abutting slider with an extremely low temperature gas-liquid mixture; the extremely low temperature gas-liquid mixture is sprayed upward through the micropore array and directly contacts the substrate.

[0010] Specifically, the main structure of the contact slider is made of polytetrafluoroethylene; the cryogenic gas-liquid mixture is a liquid nitrogen vaporization flow with a temperature between a first preset temperature and a second preset temperature; the thrust shaft of the pneumatic lifting mechanism is configured to provide a constant upward air pressure thrust to keep the contact slider tightly pressed against the back of the substrate during movement; the extension axis of each micropore in the micropore array is inclined at a set angle relative to the vertical direction in the opposite direction of the travel trajectory, so that the ejected liquid nitrogen vaporization flow directly covers the solid-phase transformation region directly behind the travel path of the composite processing head assembly.

[0011] The bottom end of the composite processing head assembly is also equipped with a powder gathering cone nozzle, and the coaxial double-ring powder feeding nozzle is coaxially nested inside the powder gathering cone nozzle.

[0012] This application embodiment also provides a method for bipolar laser composite welding and gradient coating preparation, including: obtaining a substrate and placing the substrate on a support worktable; controlling the composite processing head assembly to output a leading main laser beam and a rear broadband defocused laser beam to simultaneously perform welding and cladding operations on the seam position on the substrate; during the cladding process, transition alloy powder and anti-corrosion alloy powder are continuously transported through the inner and outer ring channels of the coaxial dual-ring powder feeding nozzle, respectively, and the air supply flow parameters of the carrier gas are dynamically adjusted according to the travel speed variable to form a material concentration gradient in the vertical direction; when the surface temperature of the weld metal cools to a preset temperature range, the back cooling and pressing assembly is controlled to move synchronously with the composite processing head assembly, and a mixed airflow is sprayed onto the back of the substrate through the abutment slider.

[0013] In the process of forming a material concentration gradient, the transition alloy powder preferentially deposits and adheres to the surface of the substrate, and the anti-corrosion alloy powder is stacked and covers the top surface of the transition alloy powder. The travel position data and the current material deposition thickness value are acquired, and the feedback signal is calculated in real time according to the built-in mass concentration integral function model. The rotation speed of the powder feeders input to the inner ring flow channel and the outer ring flow channel are adjusted respectively to complete the nonlinear material transition from pure transition alloy powder to pure anti-corrosion alloy powder.

[0014] Specifically, the transition alloy powder comprises nickel, iron, and chromium, and the anti-corrosion alloy powder comprises nickel, molybdenum, and titanium. The control process for the nonlinear material transition includes: as the material deposition thickness increases, a first control command is output to gradually reduce the spray velocity of the transition alloy powder according to a preset slope, while a second control command is output to increase the spray velocity of the anti-corrosion alloy powder in a nonlinear increment; when the cladding process enters the set final stage, the powder supply source of the inner ring channel is cut off, so that the anti-corrosion alloy powder forms a continuous anti-corrosion sealing structure on the outermost side of the substrate.

[0015] Specifically, the step of blowing mixed airflow onto the back of the substrate via the abutting slider includes: when the temperature of the weld metal drops to a preset first crystallization intervention temperature value, triggering a control command to cause the abutting slider of the back cooling pressing assembly to mechanically press against the back of the substrate in the area directly opposite it; in the pressed state, supplying liquid gas into the interior of the abutting slider and mixing and vaporizing it, and continuously spraying the mixed airflow within the target temperature range from the surface of the abutting slider outward; the target temperature range is configured and maintained between -150 degrees Celsius and -120 degrees Celsius.

[0016] The beneficial effects of this application's embodiments are as follows: By designing a structural combination of a dual-beam laser head and a coaxial dual-ring powder feeding nozzle, the deep penetration of carbon steel and the metallurgical coating of surface alloy materials can be completed simultaneously in the electrode splice area in one go, eliminating the limitations of secondary transfer or large-tank electroplating, and avoiding the risk of coating burn-out caused by plating before welding. By setting a follow-up back-side cooling and clamping component on the back of the workpiece, not only is a rigid anti-warping constraint applied to the large-size thin plate in terms of physical structure, but the instantaneously ejected ultra-low temperature gas flow also forces the anti-corrosion molten pool to undergo non-equilibrium rapid solidification, forming nanoscale microcrystals or even amorphous structures. This forced cooling combined with dual-ring dynamic powder feeding achieves a non-linear gradient transition of material composition, eliminating residual stress and microcrack sources at the interface of dissimilar metals, enabling the bipolar plate to achieve a leap in resistance to strong alkali corrosion and stress peeling resistance while maintaining the extremely high flatness required for assembly. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the overall structure provided in an embodiment of the present invention.

[0018] Figure 2 This is a partial sectional view provided in an embodiment of the present invention.

[0019] Figure 3 This is a flowchart of the bipolar plate laser composite welding and gradient coating preparation method provided in the embodiments of the present invention.

[0020] Explanation of reference numerals in the attached figures: In the diagram: 101-Supporting worktable, 102-Through-through follower guide groove, 103-Three-axis gantry motion mechanism, 104-Composite processing head assembly, 105-Coaxial double-ring powder feeding nozzle, 106-Backside rapid cooling and clamping assembly, 107-Abutting slider, 108-Pneumatic lifting mechanism, 111-Control system, 114-Leading main laser beam, 115-Rear broadband defocused laser beam, 116-Liquid nitrogen vaporization mixer, 117-Inner annular flow channel, 118-Outer annular flow channel, 119-Substrate. V 101 -Powder gathering cone nozzle V 102 - Micropore array. Detailed Implementation

[0021] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0022] Example 1

[0023] like Figure 1 and Figure 2 As shown, this embodiment provides a bipolar laser composite welding and gradient coating preparation device. The device's spatial layout uses a large mechanical motion platform as its framework, deeply integrating a high-energy optical path system, a multi-stage powder conveying system, and a cryogenic pneumatic pressing system. Specifically, the basic support structure of the device is a support worktable 101. Considering that the processing object is a large-sized bipolar substrate 119 with a diameter greater than 1.5 meters and a thickness between 1.0 mm and 2.0 mm, the support worktable 101 is designed as a large cast iron vibration isolation platform with high rigidity. In the central axis region of the support worktable 101, a through-type guide groove 102 is longitudinally formed, penetrating its upper and lower surfaces. The width of this through-type guide groove 102 allows the mechanical components below to pass through without interference, and flexibly adjustable vacuum suction cups or mechanical pressure plates are arranged on both sides for initial peripheral boundary fixation of the large-sized substrate 119.

[0024] A three-axis gantry motion mechanism 103 is disposed in the space above the support worktable 101. The three-axis gantry motion mechanism 103 includes support columns on both sides of the worktable, a transverse Y-axis beam, and suspended X-axis and Z-axis feed modules. Through high-precision servo motors paired with ball screws and linear guides, this mechanism can perform complex three-dimensional spatial interpolation trajectory movements. To achieve the control closed loop of this application, the device is equipped with a conventional CNC machine tool position and machining path control system 111. This control system 111 interacts with the three-axis gantry motion mechanism 103 via electrical signals to parse G-code and precisely control the translation coordinates and feed speed of the actuator. Through the aforementioned high-precision rigid gantry structure combined with CNC commands, it is possible to ensure that the trajectory does not jitter or drift during long-distance welding of large parts.

[0025] Below the Z-axis actuator of the three-axis gantry motion mechanism 103, a composite machining head assembly 104 is suspended. This composite machining head assembly 104 integrates a dual-path fiber laser optical path and a powder feeding optical path, serving as the core energy emission terminal for realizing machining operations. The composite machining head assembly 104 further includes a coaxial dual-ring powder feeding nozzle 105 mounted at its bottom. Through the arrangement of internal reflectors, collimating lenses, and composite focusing lenses, the optical system of the composite machining head assembly 104 is configured to simultaneously emit two spatially independent but logically coordinated laser beams downwards: a leading main laser beam 114 and a rear broadband defocused laser beam 115.

[0026] To eliminate the instability and warping of large-area thin-walled plates during heating and to intervene in the crystallization morphology of the alloy cladding layer at the microscopic level, a back-side chilling and clamping assembly 106 is independently configured in the bottom space of the supporting worktable 101. The back-side chilling and clamping assembly 106 mainly includes an abutment slider 107 and pneumatic lifting mechanisms 108 arranged on both sides thereof. The pneumatic lifting mechanism 108 includes a high-frequency response proportional valve and a linear cylinder structure, with its piston thrust shaft connected to the bottom of the abutment slider 107. In the processing state, the pneumatic lifting mechanism 108 drives the abutment slider 107 to extend upwards and pass through the through-feeder guide groove 102, so that the top end face of the abutment slider 107 directly and physically adheres to and abuts against the lower surface of the substrate 119. The back-side chilling and clamping assembly 106 is mounted on an independent bottom guide rail, driven by a separate linear motor and kept in coordinate synchronization with the three-axis gantry motion mechanism 103. This means that the abutting slider 107 can move synchronously and in the same direction as the composite processing head assembly 104 along the through-path follower guide groove 102. The interior of the abutting slider 107 is hollowed out to form an internal cavity, which is configured to continuously supply an airflow of extremely low temperature. Through this structural design, the slider provides an upward rigid support point from a physical and mechanical perspective to suppress thermal deformation of the thin plate, and also acts as a follower cold source for forced convection heat exchange.

[0027] Further reference Figure 2 The optical path and powder fluid dynamics design shown features a coaxial dual-ring powder feed nozzle 105 internally divided by special metal partitions, forming an inner annular flow channel 117 located inside and an outer annular flow channel 118 sleeved on the outside. To ensure precise material aggregation, a high-temperature resistant powder aggregation cone nozzle is also installed at the bottom of the composite processing head assembly 104. V 101 The coaxial double-ring powder feeding nozzle 105 is coaxially nested within the powder gathering cone nozzle. V 101 Inside the substrate 119, based on a dual-path optical shaping mechanism, the composite processing head assembly 104 emits a leading main laser beam 114 with a circular spot vertically downward from its center. In terms of specific parameter configuration, the power of the leading main laser beam 114 is set to 1800 watts to 2200 watts, and its circular spot diameter is compressed to only 0.2 mm to achieve extremely high energy density. More importantly, the optical path system positions the focal point of the leading main laser beam 114 within the thickness dimension of the substrate 119, specifically 0.5 mm below the centerline of the seam of the substrate 119. This deep focal positioning can penetrate and instantly melt the seam of the substrate 119, forming a stable keyhole effect to ensure full-thickness melting.

[0028] Simultaneously, the optical path system emits a rear broadband defocused laser beam 115 with a rectangular spot from the outer edge of the coaxial dual-ring powder feed nozzle 105 inwards at an angle. The long side of the rectangular spot is configured perpendicular to the processing direction of the device. The power of the rear broadband defocused laser beam 115 is set to 2500 watts to 3000 watts, the spot size is expanded to 5 mm × 2 mm, and its focal plane is intentionally raised to 5 mm above the upper surface of the substrate 119, thereby forming a defocused spot with low energy density but wide coverage on the workpiece surface. In terms of spatial arrangement, the physical distance between the leading main laser beam 114 and the rear broadband defocused laser beam 115 in the processing direction is strictly set between 18 mm and 25 mm. This specific distance setting ensures that the projected irradiation area of ​​the rear broadband defocused laser beam 115 on the horizontal plane completely covers the area directly behind the heating area acted upon by the leading main laser beam 114. The underlying physical principle is that the high-temperature residual heat generated by the leading deep penetration welding increases the optical absorption rate of the carbon steel substrate to the subsequent broadband defocused laser beam 115 and the falling alloy powder, enabling the defocused laser to completely melt the alloy powder particles at a relatively low power threshold. This mechanism avoids the risk of high-energy lasers directly burning away trace elements in the powder and improves the stability of the cladding process.

[0029] Based on this, since the system needs to establish a transition gradient of different material properties in the vertical direction, the inner annular flow channel 117 is configured to connect to an external first powder supply source to continuously output the first type of metal powder, and the outer annular flow channel 118 is configured to connect to a second powder supply source to output the second type of metal powder. The first type of metal powder is a bridging transition alloy powder (Ni-Fe alloy), while the second type of metal powder is a protective catalytic corrosion-resistant alloy powder (Ni-Mo alloy). The post-wideband defocused laser beam 115 is configured to simultaneously melt and converge the first and second types of metal powders to its defocused irradiation area. To ensure the order of coherent crystallization of the metal phase in the molten pool, the melting point of the first type of metal powder is higher than that of the second type of metal powder. The melting point difference, combined with the inherent difference in fluid cone angle during the spraying of the inner and outer annular flow channels, causes the high-melting-point first type of powder to preferentially contact the bottom layer of red-hot molten steel to form metallurgical roots, while the low-melting-point second type of powder subsequently spreads gently on the top layer. Throughout the powder transfer process, the device is also equipped with a high-purity argon gas supply system for protection, to prevent the high-temperature metal powder from oxidizing in the air.

[0030] For the follow-up cooling system on the back of the electrode plate, to prevent wear or thermal failure of the mechanical slider under the harsh conditions between the high-temperature steel plate and the ultra-low temperature gas, the main structure of the abutting slider 107 is integrally manufactured from wear-resistant polytetrafluoroethylene material. A regularly arranged array of micropores is formed on the top surface of the abutting slider 107 facing the substrate 119. V 102 To provide sufficient cold source, the back-side cooling and pressing assembly 106 also includes a liquid nitrogen vaporization mixer 116 disposed below the support worktable 101. The output end of the liquid nitrogen vaporization mixer 116 is connected to the internal cavity of the abutment slider 107 through a heat-insulating flexible corrugated pipe wrapped with aerogel material, thereby enabling the continuous pumping of an extremely low-temperature gas-liquid mixture into the abutment slider 107 without leakage. Specifically, the extremely low-temperature gas-liquid mixture is a liquid nitrogen vaporization flow with a temperature between a first preset temperature (-150°C) and a second preset temperature (-120°C), and its gas source pressure is maintained at 0.5 MPa. During this stage, the thrust shaft of the pneumatic lifting mechanism 108 is configured to provide a constant upward air pressure thrust based on the air source feedback, so as to keep the abutment slider 107 tightly pressed against the back of the substrate 119 during movement.

[0031] Furthermore, in order to enable the cooling jet to produce the best metallurgical intervention effect, a micropore array... V 102The extension axes of each micropore are tilted at an angle relative to the vertical direction in the opposite direction of the travel trajectory. This hydrodynamic deflection design allows the ultra-low temperature liquid nitrogen vaporized flow ejected at 0.5 MPa pressure to accurately cover the solid-phase transformation region directly behind the travel path of the composite processing head assembly 104. Specifically, the cold airflow passes through the micropore array. V 102 When the gas is sprayed upwards and directly contacts the surface of the substrate 119, it is positioned at the middle rear section where the leading deep melt pool and the rear cladding pool overlap. This instantaneous purging action utilizes the physical property that liquid gas absorbs a large amount of latent heat during vaporization to create a strong local temperature gradient on the back of the electrode, forcing the newly solidified anti-corrosion coating lattice on the front to undergo forced destabilization and crystallization.

[0032] To ensure the feasibility of the entire device's energy and signal closed-loop operation, the device is also equipped with a laser generator, a high-power fiber optic cooling water circulation system, and a high-speed temperature sensor integrated behind the processing head. The high-speed temperature sensor uses an infrared colorimetric array to scan the temperature field of the molten pool wake in real time and feeds back the electrical signal to the main control unit to determine the precise timing for activating the cooling flow in the liquid nitrogen vaporization mixer 116. Through the interconnection of all the aforementioned electromechanical components, the device of this application forms a super-composite processing platform integrating mechanical constraints, optical deep melting, and extreme thermal exchange.

[0033] Example 2

[0034] Based on the electromechanical device architecture described in Embodiment 1, this embodiment further proposes a method for bipolar plate laser composite welding and gradient coating preparation. Please refer to... Figure 3 As shown, the macroscopic process steps of this preparation method include: Step S301: Obtain the large-size thin-walled substrate 119 to be spliced, and place the substrate 119 on the support worktable 101 to complete the physical positioning of the splicing seam. In the specific test, the substrate 119 selected is a Q235 low carbon steel sheet with a thickness of 1.2 mm, and its splicing seam length is set to 1200 mm in a single test.

[0035] Step S302: Control the composite processing head assembly 104 to move along a preset trajectory at a welding speed of 1.5 meters per minute, and simultaneously output a high-energy-density leading main laser beam 114 and a broadband-covered rear broadband defocused laser beam 115 to simultaneously perform bottom layer welding and surface cladding operations on the seam position on the substrate 119. A bonding foundation is formed through forward deep penetration, and the coating is applied using residual heat through backward defocusing.

[0036] Step S303: During the synchronous cladding process, a dual powder transfer command is initiated. Transition alloy powder, serving as the first type of metal powder, is continuously transported through the inner annular flow channel 117 of the coaxial dual-ring powder feeder 105, while anti-corrosion alloy powder, serving as the second type of metal powder, is continuously transported through the outer annular flow channel 118. The system dynamically adjusts the supply flow parameters of the two carrier gases (argon) based on the travel speed variable, maintaining the total powder delivery rate between 12 grams per minute and 18 grams per minute, thereby creating a nonlinear material concentration gradient in situ along the vertical thickness direction of the weld.

[0037] In this gradient construction step, the transition alloy powder used comprises nickel, iron, and chromium, with a preferred mass percentage composition of Ni-40Fe-5Cr and a particle size of 45 to 105 micrometers. The anti-corrosion alloy powder used comprises nickel, molybdenum, and titanium, with a mass percentage composition of Ni-15Mo-3Ti and a finer particle size of 20 to 60 micrometers. During the dynamic physical process of forming the material concentration gradient, due to the smaller emission cone angle and higher particle potential energy of the inner annular channel 117, the Ni-Fe system transition alloy powder preferentially penetrates the outer protective gas and deposits onto the liquid surface of the substrate 119. Meanwhile, the outer annular anti-corrosion alloy powder with a larger cone angle is uniformly stacked and covers the top surface of the bottom transition alloy powder.

[0038] To achieve the optimal micro-stress distribution for peel resistance, the concentration gradient is not a simple linear superposition, but rather nonlinearly controlled through a rigorous mathematical closed-loop model. The controller acquires real-time travel position data from the encoder and the current material deposition thickness value transmitted from the ultrasonic rangefinder, and calculates the feedback signal in real time based on the system's built-in mass concentration integral function model. Specifically, the system executes the calculation of the mass concentration integral function model, the specific mathematical expression of which is as follows:

[0039] in, C Ni (h) This indicates the height from the bottom of the cladding layer is h Local mass fraction of nickel at the cross section; C 0 This represents the constant of the background trace nickel mass fraction that diffuses and penetrates from the underlying carbon steel substrate to the bottom of the molten pool during the deep melting stage. V ext (t) and V int (t) These represent the pneumatic conveying system based on the time variable. tReal-time dynamic output of instantaneous powder delivery volumetric flow rate to outer ring channel 118 and inner ring channel 117; h This represents the vertical height coordinate variable that is integraled upwards from the bottom interface as the zero point. t Indicates processing timing parameters; and These represent the steady-state loose density constants of the external anti-corrosion powder and the internal transition powder, respectively. By solving this definite integral model in real time, the system continuously feeds back and adjusts the speed of the stepper powder feeding motors input to the inner annular flow channel 117 and the outer annular flow channel 118, respectively, to complete the nonlinear material transformation from pure transition alloy powder to the bottom substrate, and then smoothly transitioning upward to pure anti-corrosion alloy powder.

[0040] The specific steps in the execution of nonlinear material transition include: [following the obtained material deposition thickness value] h As the flow rate increases, the main controller outputs a first control command, causing the gas valve actuator to gradually reduce the inner ring spray velocity corresponding to the transition alloy powder according to the system's preset negative slope parameter. Simultaneously, a second control command is output in parallel, driving the external flow valve to increase the outer ring spray velocity corresponding to the anti-corrosion alloy powder in a non-linear, incremental manner. Through the coordination of these parameters, the system causes the overall mass concentration of nickel in the vertical cladding layer to non-linearly and smoothly increase from an initial 40% to a final concentration of over 85% from bottom to top. At the final stage of the cladding process, when the geometric boundary is set, the system directly sends a cutoff command to completely cut off the transition alloy powder supply source of the inner ring flow channel 117, allowing the highly corrosion-resistant anti-corrosion alloy powder to solidify separately at the outermost environmental contact surface of the substrate 119, thereby forming a dense and continuous ultimate anti-corrosion sealing structure.

[0041] Step S304: Synchronously activate the rapid cooling effect. When the infrared scanning probe detects that the surface temperature of the weld metal has cooled from the high heat peak to the preset temperature range, the back-side rapid cooling clamping component 106 is controlled to start tracking. Specifically, the preset judgment condition is: when the cooling curve of the weld metal is detected to drop to the preset first crystallization intervention temperature value (strictly set to the critical point of 800 degrees Celsius in this embodiment), the programmable logic controller instantly triggers a control command. This command first opens the pneumatic lifting mechanism 108, causing the abutment slider 107 to follow and adhere to the physical area behind the 800-degree Celsius molten pool on the back of the substrate 119 for mechanical clamping, eliminating the thermal shrinkage torque that will be generated. While maintaining constant clamping, a large flow of liquid gas is immediately supplied into the abutment slider 107 and mixed and vaporized, exiting from the inclined micro-pore array on the surface of the abutment slider 107. V 102 A mixed airflow within the target temperature range is continuously ejected upwards and outwards. This target temperature range is precisely configured and maintained between -150°C and -120°C through control via insulated piping.

[0042] Ultra-low temperature nitrogen gas at -150°C to -120°C is ejected from the back-side cooling slider, instantly passing over the 800°C back side of the thin plate. This extreme physical temperature gradient generates an extremely powerful instantaneous heat extraction effect. This quenching action forcibly severs the long-range regular diffusion channels of atoms in the front Ni-Mo alloy cladding layer, forcing the high-temperature metal to undergo violent non-equilibrium solidification. The microscopic material mechanism is that traditional slow air cooling easily precipitates coarse and directional dendrites in thick coatings, and these free dendrite grain boundaries will become weak channels for electrochemical corrosion by strong alkali penetration of potassium hydroxide during future service. The method of this application utilizes deep cryogenic instantaneous intervention to directly form a large number of dense nanoscale microcrystals during the surface solidification stage, and even induces amorphous glass structures without grain boundary defects in local surface layers.

[0043] To verify the corrosion resistance and stress control effects of the method and apparatus of this application in practical engineering applications, samples prepared using the method of this embodiment and samples prepared using a conventional comparative process (i.e., first using conventional gas metal arc welding of carbon steel plates, then immersing the entire plate in a large electrolyte bath for overall electrochemical pure nickel plating) were placed in a simulated harsh service environment for comparative analysis and testing. All samples were tested under the reference conditions of 1.2 mm thick plates and 1200 mm joint length. The summarized test data are shown in the table below:

[0044] In summary, the laser composite device and gradient fabrication method provided in this application integrate coaxial dual-beam energy distribution, dual-powder integral model concentration control, and back-side sub-zero cooling constraint into a unified physical process. This technical solution not only protects large-size thin-walled components from thermal deformation waves during welding, but also completely blocks intergranular corrosion channels by inducing the formation of a dense coating of non-equilibrium microcrystals, achieving globally optimal synergy between continuous large-plate operation, extremely high flatness assurance, and strong alkali corrosion and peeling resistance.

[0045] It should be noted that the entities or parameters mentioned in the above embodiments, such as the Q235 carbon steel substrate, infrared colorimetric sensor, and control system, are merely preferred examples to facilitate the demonstration of engineering implementation details. Those skilled in the art should understand that, without departing from the design concept of this application, using stainless steel sheets instead of low-carbon steel, or introducing a higher-precision thermocouple feedback control loop, can also achieve the same technical effect and realize the core purpose of this invention. Such equivalent replacement structures also fall within the technical protection scope of this invention.

Claims

1. A device for bipolar laser composite welding and gradient coating preparation, characterized in that, include: The supporting worktable has a through-type follower guide groove; A three-axis gantry motion mechanism is installed in the space above the bearing worktable; A composite processing head assembly is suspended and mounted on the execution end of the three-axis gantry motion mechanism. The composite processing head assembly includes a coaxial double-ring powder feeding nozzle and is configured to emit a leading main laser beam and a rear broadband defocused laser beam downwards. A back-side chilling and clamping assembly is disposed at the bottom of the supporting worktable. The back-side chilling and clamping assembly includes an abutting slider and a pneumatic lifting mechanism. The pneumatic lifting mechanism is connected to the abutting slider to drive the abutting slider through the through-feed guide groove and abut against the lower surface of the substrate. The abutting slider is configured to move synchronously with the composite processing head assembly along the through-feed guide groove, and the internal cavity is configured to allow the introduction of a low-temperature airflow.

2. The bipolar laser composite welding and gradient coating preparation apparatus as described in claim 1, characterized in that, The coaxial dual-ring powder feeder includes an inner ring flow channel located inside and an outer ring flow channel sleeved on the outside; the optical path system of the composite processing head assembly is configured to emit a leading main laser beam with a circular spot vertically downward from the center position, and to emit a rear broadband defocused laser beam with a rectangular spot obliquely inward from the outer edge; the leading main laser beam penetrates and heats the splicing gap of the substrate, and the rear broadband defocused laser beam melts the particulate material ejected from the coaxial dual-ring powder feeder.

3. The bipolar laser composite welding and gradient coating preparation apparatus as described in claim 2, characterized in that, The focal point of the leading main laser beam is positioned within the thickness dimension of the substrate; the projection irradiation area of ​​the rear broadband defocused laser beam on the horizontal plane completely covers the area behind the heating region acted upon by the leading main laser beam; the inner annular flow channel is configured to connect to a first powder supply source to output a first type of metal powder, and the outer annular flow channel is configured to connect to a second powder supply source to output a second type of metal powder; the rear broadband defocused laser beam is configured to simultaneously melt and converge the first type of metal powder and the second type of metal powder to the irradiation area, wherein the melting point of the first type of metal powder is higher than the melting point of the second type of metal powder.

4. The bipolar laser composite welding and gradient coating preparation apparatus as described in claim 1, characterized in that, The top surface of the contact slider facing the substrate has a regularly arranged array of micropores; the back-side cooling and pressing assembly also includes a liquid nitrogen vaporization mixer disposed below the support worktable; the output end of the liquid nitrogen vaporization mixer is connected to the internal cavity of the contact slider through a flexible pipeline, supplying the contact slider with an extremely low temperature gas-liquid mixture; the extremely low temperature gas-liquid mixture is sprayed upward through the micropore array and directly contacts the substrate.

5. The bipolar laser composite welding and gradient coating preparation apparatus as described in claim 4, characterized in that, The main structure of the contact slider is made of polytetrafluoroethylene; the ultra-low temperature gas-liquid mixture is a liquid nitrogen vaporization flow with a temperature between a first preset temperature and a second preset temperature; the thrust shaft of the pneumatic lifting mechanism is configured to provide a constant upward air pressure thrust, so that the contact slider is tightly pressed against the back of the substrate during movement; the extension axis of each micropore in the micropore array is tilted at a set angle relative to the vertical direction in the opposite direction of the travel trajectory, so that the ejected liquid nitrogen vaporization flow covers the solid-phase transformation region directly behind the travel path of the composite processing head assembly.

6. The bipolar laser composite welding and gradient coating preparation apparatus as described in claim 1, characterized in that, The bottom end of the composite processing head assembly is also equipped with a powder gathering cone nozzle, and the coaxial double-ring powder feeding nozzle is coaxially nested inside the powder gathering cone nozzle.

7. A method for preparing bipolar laser composite welding and gradient coating, characterized in that, include: Obtain the substrate and place it on the support worktable; The composite processing head assembly outputs a leading main laser beam and a rear broadband defocused laser beam to simultaneously perform welding and cladding operations on the seam positions on the substrate. During the cladding process, the transition alloy powder and the anti-corrosion alloy powder are continuously transported through the inner and outer ring channels of the coaxial double-ring powder feeding nozzle, respectively. The air supply flow parameters of the carrier gas are dynamically adjusted according to the travel speed variable to form a material concentration gradient in the vertical direction. When the surface temperature of the weld metal cools to a preset temperature range, the back-side cooling and pressing assembly moves synchronously with the composite processing head assembly and sprays a mixed airflow onto the back of the substrate through the abutting slider.

8. The method for preparing bipolar laser composite welding and gradient coating as described in claim 7, characterized in that, During the formation of the material concentration gradient, the transition alloy powder preferentially deposits and adheres to the surface of the substrate, and the anti-corrosion alloy powder is stacked and covers the top surface of the transition alloy powder. The travel position data and the current material deposition thickness value are acquired, and the feedback signal is calculated in real time according to the built-in mass concentration integral function model. The rotation speed of the powder feeders input to the inner ring flow channel and the outer ring flow channel are adjusted respectively to complete the nonlinear material transition from pure transition alloy powder to pure anti-corrosion alloy powder.

9. The method for preparing bipolar laser composite welding and gradient coating as described in claim 8, characterized in that, The transition alloy powder comprises nickel, iron, and chromium, and the anti-corrosion alloy powder comprises nickel, molybdenum, and titanium. The control process for the nonlinear material transition specifically includes: as the material deposition thickness increases, a first control command is output to gradually decrease the spray velocity of the transition alloy powder according to a preset slope, while a second control command is output to increase the spray velocity of the anti-corrosion alloy powder in a nonlinear function increment; when the cladding process enters the set final stage, the powder supply source of the inner ring channel is cut off, so that the anti-corrosion alloy powder forms a continuous anti-corrosion sealing structure on the outermost side of the substrate.

10. The method for preparing bipolar laser composite welding and gradient coating as described in claim 7, characterized in that, The step of blowing mixed airflow onto the back of the substrate via the abutting slider specifically includes: when the temperature of the weld metal drops to a preset first crystallization intervention temperature value, triggering a control command to cause the abutting slider of the back cooling pressing assembly to mechanically press against the back of the substrate in the area directly opposite it; in the pressed state, supplying liquid gas into the interior of the abutting slider and mixing and vaporizing it, and continuously spraying the mixed airflow within the target temperature range from the surface of the abutting slider outward; the target temperature range is configured and maintained between -150 degrees Celsius and -120 degrees Celsius.

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