Thick plate laser hole-forming construction method based on complex curved surface

Abstract

The invention provides a laser hole forming construction method based on a thick plate with a complex curved surface, and belongs to the field of bolt trepanning, and the method comprises the following steps: step 1, pretreatment and positioning of the thick plate with the complex curved surface of a steel box arch bridge, step 2, optimization of dynamic technological parameters of laser hole forming construction, step 3, self-adaptive path planning and real-time focus following, and step 4, real-time follow-up of the laser hole forming construction. And 4, real-time molten pool state monitoring and plasma characteristic analysis are carried out, and then closed-loop laser control is carried out. Based on a material, structure and process database and a machine learning model, parameters such as laser energy and scanning speed are dynamically optimized, the problem of uneven energy distribution caused by curvature change is solved, visual, infrared and spectrum multi-sensor data are fused, the molten pool state and plasma characteristics are sensed in real time, and active suppression of machining defects is achieved.

Description

Technical Field

[0001] The invention relates to the field of bolt hole drilling, and in particular to a laser hole forming construction method based on a thick plate with a complex curved surface. Background Art

[0002] Existing bolt holes for complex curved thick plates in steel box arch bridges are drilled using flat-surface drilling machines or small magnetic drilling rigs, resulting in low precision, inefficiency, and poor quality. Traditional mechanical drilling suffers from rapid drill bit wear, rough hole walls, and a large heat-affected zone. Conventional laser drilling, on complex curved thick plates, also suffers from melt pool instability, hole shape distortion, and a limited aspect ratio.

[0003] In traditional mechanical drilling, the drill bit rubs violently against thick plates, easily leading to drill bit breakage, difficult chip removal, hole wall scratches, surface roughness Ra ≥ 12.5μm, thermal stress concentration, and edge cracking. Conventional laser drilling, using fixed parameters, can cause uneven laser energy distribution due to the varying curvature of complex surfaces. Convex surfaces experience excessive energy, leading to ablation, while concave surfaces experience insufficient energy, making penetration difficult. Thick plates with a depth-to-diameter ratio ≥ 10:1 can lead to slow melt pool cooling, severe remelting, slag on the hole wall, plasma shielding, and reduced energy utilization. Therefore, a laser drilling method for thick plates with complex curved surfaces is needed. Summary of the Invention

[0004] The purpose of the present invention is to provide a construction method based on laser drilling of complex curved thick plates to solve the technical problems of low efficiency of bolt drilling of complex curved thick plates in existing steel box arch bridges, hole shape distortion and limited depth-to-diameter ratio.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A method for laser drilling in thick plates with complex curved surfaces, comprising the following steps:

[0007] Step 1: Pretreatment and positioning of complex curved thick plates of steel box arch bridges;

[0008] Step 2: Optimization of dynamic process parameters of laser drilling construction;

[0009] Step 3: Perform adaptive path planning and real-time focus tracking;

[0010] Step 4: Real-time molten pool status monitoring and plasma feature analysis, followed by closed-loop control of the laser.

[0011] Furthermore, the specific process of step 1 is as follows: use pulsed laser pre-cleaning to remove surface oil and oxide layer to ensure effective coupling of laser energy, scan the surface of the thick plate through the laser triangulation sensor, build a point cloud model, combine the CAD model for surface alignment, determine the hole position and normal vector, and use a deformable vacuum adsorption fixture for the local curvature mutation area to avoid positioning deviation caused by processing vibration.

[0012] Furthermore, the specific process in step 2 is: based on the thermophysical properties and structural characteristics of complex curved thick plate materials, the optimal process parameter combination is calculated in real time through a machine learning model. The process parameters include laser energy density, pulse frequency, scanning speed, auxiliary gas parameters and focal position.

[0013] Furthermore, the thermophysical properties include thermal conductivity λ, melting point Tm and vaporization heat Lv, the structural characteristics include the processing point curvature radius R, plate thickness h, depth z, laser energy density E = ηP / (πr 2 ), where η is the coupling efficiency, P is the average power, and r is the spot radius. When the aspect ratio is greater than 10, the laser energy density E is increased by 20%-30% to avoid energy attenuation. In the pulse frequency f, when the convex surface R is greater than 50mm, the frequency f = 50-100kHz is used to reduce heat accumulation. When the concave surface R is less than 10mm, the frequency f = 1-10kHz is used to enhance the penetration of the molten pool. The scanning speed v = k·h / R, where k is the correction coefficient, h is the plate thickness, and R is the curvature radius. This is to avoid the molten pool not being filled due to the speed being greater than the set value when scanning the concave surface. When the thick plate is convex, the laser focus is dynamically adjusted to the normal distance Δz = R / 2 to the processing surface. When the thick plate is concave, the laser focus is dynamically adjusted to the normal distance Δz = -R / 3 to the processing surface to compensate for the energy discreteness caused by the curved surface.

[0014] Furthermore, the specific process of calculating the optimal process parameter combination is as follows: first input the thick plate material characteristic data, scan the thick plate structural characteristics, obtain the structural characteristics by laser scanning, the structural characteristics include the current curvature radius, plate thickness and processed depth, calculate the key derivative characteristics of heat impact, the derivative characteristics include thermal diffusivity, curvature Biot number and safety temperature threshold, solve the thermomechanical coupling equation, use the curvature to encrypt the grid, scan the parameter range, generate a training set, use the XGBoost model to calculate, input the construction vector, basic parameters and derivative characteristics, and output the optimal parameters of the optimal speed, feed rate and cooling intensity.

[0015] Furthermore, the specific process of step 3 is as follows: according to the normal vector field and depth gradient of the surface, a heuristic search path planning algorithm is used to generate a spiral and straight line composite path, where the spiral angle α is 30°-60°, the pitch p=h / 10, and h is the height, to ensure that the laser beam always vertically impinges on the processing surface, and the incident angle is ≤5°. The six-axis robot is equipped with a laser head at the end, and the height change of the processing surface is fed back in real time through the laser ranging sensor. The robot joint angle and dynamic focusing module parameters are adjusted in combination with the PID controller to achieve a focus position error of ≤0.02mm. The main pulse and auxiliary pulse mode is used in the thick plate penetration stage, the main pulse energy accounts for 70%, and the auxiliary pulse interval τ=10-50μs. The auxiliary pulse is used to preheat the lower layer material to reduce the hole wall cracks caused by thermal stress.

[0016] Furthermore, the specific process of step 4 is as follows: using a microscope to collect the hole wall image in real time, identifying the molten pool width W and depth D through the edge detection algorithm, combining with the infrared thermal imager to obtain the molten pool temperature T, calculating the heat input Q = α·k·(T-T0), where α is the thermal diffusivity, k is the thermal conductivity, T0 is the temperature at a certain moment, and using a spectrometer to collect the plasma emission spectrum, inverting the electron temperature Te and electron density Ne through the Boltzmann equation to judge the plasma shielding effect. If the molten pool width W deviates from the target value by ±10% or the temperature T exceeds the material boiling point Tb+200K, the laser power P or scanning speed v is automatically adjusted. If microcracks appear on the hole wall, the processing is suspended and a low-energy repair pulse is started, with an energy density of 30% of normal processing.

[0017] Furthermore, in the plasma shielding effect, when Ne>10 1 At 6cm-3, the trigger power is reduced by 10%-15%.

[0018] The present invention has the following beneficial effects due to the adoption of the above technical solution:

[0019] Based on the material, structure and process database and machine learning model, the present invention dynamically optimizes parameters such as laser energy and scanning speed to solve the problem of uneven energy distribution caused by curvature changes. It integrates visual, infrared and spectral multi-sensor data to perceive the molten pool state and plasma characteristics in real time, and realizes active suppression of processing defects. It adopts the main pulse and auxiliary pulse superposition strategy and dynamic focus following to increase the depth-to-diameter ratio of thick plate laser drilling from 8:1 of the traditional method to more than 15:1. Through path planning optimization (spiral and linear composite) and gas injection angle control, the hole wall crack rate is reduced from 0.5% of conventional laser drilling to below 0.1%. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a flow chart of the method of the present invention. DETAILED DESCRIPTION

[0021] For the purposes of the present invention, the technical solutions and advantages are more clearly and clearly understood, the following refers to the drawings and preferred embodiments are given, the present invention is further described in detail. However, it should be noted that many details listed in the specification are only to enable the reader to have a thorough understanding of one or more aspects of the present invention, even without these specific details, the aspects of the present invention can be realized.

[0022] As Figure 1 shown, a complex curved thick plate laser hole forming construction method, the method comprises the following steps:

[0023] Step 1: Pretreatment and positioning of complex curved thick plate of steel box arch bridge. Pulse laser pre-cleaning is used to remove surface dirt and oxide layer, to ensure effective coupling of laser energy, to scan the thick plate surface by laser triangulation sensor, to construct point cloud model, to combine with CAD model for curved surface registration, to determine the hole forming position and normal vector, and to use deformable vacuum suction clamp for local curvature mutation area to avoid positioning deviation caused by processing vibration.

[0024] Laser adopts nanosecond pulse laser or femtosecond ultrashort pulse laser, nanosecond pulse laser wavelength 1064nm, single pulse energy 5-50mJ, repetition frequency 1-100kHz. The wavelength of femtosecond ultrashort pulse laser is 532nm, the pulse width is 300fs, the average power is 1-5kW, the focal length of dynamic focusing module is adjustable in the range of 50-500mm, and the response time is ≤10ms. The six-axis linkage industrial robot has a repeat positioning accuracy of ±0.02mm, or the five-axis numerical control machine tool has a feeding speed of 0.1-10m / min. The accuracy of laser triangulation sensor is ±0.01mm, which is used for real-time scanning of curved surface profile, the resolution of high-speed confocal microscope is 500nm, and the frame rate is 1000fps, which is used for monitoring hole wall morphology. The wave band of infrared thermal imager is 8-14μm, and the temperature resolution is 0.1K, which is used for monitoring the temperature field of molten pool. The wavelength range of spectrometer is 400-1000nm, which is used for plasma feature analysis.

[0025] Step 2: Dynamic process parameter optimization of laser hole forming construction. Based on the thermal physical properties and structural characteristics of complex curved thick plate material, the optimal process parameter combination is calculated in real time by machine learning model, including laser energy density, pulse frequency, scanning speed, auxiliary gas parameters and focal point position.

[0026] Thermal physical properties include thermal conductivity λ, melting point Tm and vaporization heat Lv, structural characteristics include processing point curvature radius R, plate thickness h, depth z, laser energy density E = ηP / (πr 2), where η is the coupling efficiency, P is the average power, and r is the spot radius. When the aspect ratio is greater than 10, the laser energy density E is increased by 20%-30% to avoid energy attenuation. In the pulse frequency f, when the convex surface R is greater than 50mm, the frequency f = 50-100kHz is used to reduce heat accumulation. When the concave surface R is less than 10mm, the frequency f = 1-10kHz is used to enhance the penetration of the molten pool. The scanning speed v = k·h / R, where k is the correction coefficient, h is the plate thickness, and R is the curvature radius. This is to avoid the molten pool not being filled due to the speed being greater than the set value when scanning the concave surface. When the thick plate is convex, the laser focus is dynamically adjusted to the normal distance Δz = R / 2 to the processing surface. When the thick plate is concave, the laser focus is dynamically adjusted to the normal distance Δz = -R / 3 to the processing surface to compensate for the energy discreteness caused by the curved surface.

[0027] The specific process of calculating the optimal process parameter combination is as follows: first input the thick plate material characteristic data, scan the thick plate structural characteristics, obtain the structural characteristics by laser scanning, the structural characteristics include the current curvature radius, plate thickness and processed depth, calculate the key derivative characteristics of heat impact, the derivative characteristics include thermal diffusivity, curvature Biot number and safety temperature threshold, solve the thermomechanical coupling equation, use the curvature to encrypt the grid, scan the parameter range, generate a training set, use the XGBoost model to calculate, input the construction vector, basic parameters and derivative characteristics, and output the optimal parameters of the optimal speed, feed rate and cooling intensity.

[0028] Step 3: Perform adaptive path planning and real-time focus tracking. Based on the surface normal vector field and depth gradient, a heuristic search path planning algorithm is used to generate a spiral and straight line composite path, where the spiral angle α is 30°-60°, the pitch p = h / 10, and h is the height. This ensures that the laser beam always perpendicularly impacts the processing surface, with an incident angle of ≤5°. The six-axis robot is equipped with a laser head at the end, and the laser ranging sensor provides real-time feedback on changes in the processing surface height. The robot joint angle and dynamic focusing module parameters are adjusted in conjunction with a PID controller to achieve a focus position error of ≤0.02mm. The main pulse and auxiliary pulse mode is used in the thick plate penetration stage, with the main pulse energy accounting for 70% and the auxiliary pulse interval τ = 10-50μs. The auxiliary pulse is used to preheat the underlying material to reduce hole wall cracks caused by thermal stress.

[0029] Step 4: Real-time monitoring of the molten pool state and analysis of plasma characteristics, followed by closed-loop control of the laser. Use a microscope to collect images of the hole wall in real time, identify the molten pool width W and depth D through an edge detection algorithm, and use an infrared thermal imager to obtain the molten pool temperature T. Calculate the heat input Q = α·k·(T-T0), where α is the thermal diffusivity, k is the thermal conductivity, and T0 is the temperature at a certain moment. Use a spectrometer to collect the plasma emission spectrum, and invert the electron temperature Te and electron density Ne through the Boltzmann equation to determine the plasma shielding effect. If the molten pool width W deviates from the target value by ±10% or the temperature T exceeds the material boiling point Tb+200K, automatically adjust the laser power P or scanning speed v. If microcracks appear on the hole wall, pause the processing and start a low-energy repair pulse with an energy density of 30% of normal processing. In the plasma shielding effect, when Ne>10 1 At 6cm-3, the trigger power is reduced by 10%-15%.

[0030] After machining, confocal microscopy is used to measure the hole diameter tolerance to ±0.02mm, the hole wall roughness Ra ≤3.2μm, and the inlet / outlet edge chipping ≤0.05mm. X-ray inspection is used to detect internal defects, including pores and slag inclusions, and ultrasonic thickness measurement verifies that the depth-to-diameter ratio error is ≤2%.

[0031] Matters not covered by the present invention are known technologies.

[0032] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for laser drilling in thick plates with complex curved surfaces, characterized by: The method comprises the following steps: Step 1: Pretreatment and positioning of complex curved thick plates of steel box arch bridges; Step 2: Optimization of dynamic process parameters of laser drilling construction; Step 3: Perform adaptive path planning and real-time focus tracking; Step 4: Real-time molten pool status monitoring and plasma feature analysis, followed by closed-loop control of the laser.

2. The method for laser drilling a thick plate with a complex curved surface according to claim 1, characterized in that: The specific process of step 1 is as follows: use pulsed laser pre-cleaning to remove surface oil and oxide layer to ensure effective coupling of laser energy, scan the surface of the thick plate with a laser triangulation sensor, build a point cloud model, combine it with the CAD model for surface alignment, determine the hole position and normal vector, and use a deformable vacuum adsorption fixture for local curvature mutation areas to avoid positioning deviation caused by processing vibration.

3. The method for laser drilling a thick plate with a complex curved surface according to claim 1, characterized in that: The specific process in step 2 is: based on the thermophysical properties and structural characteristics of complex curved thick plate materials, the optimal process parameter combination is calculated in real time through a machine learning model. The process parameters include laser energy density, pulse frequency, scanning speed, auxiliary gas parameters and focal position.

4. The method for laser drilling a thick plate with a complex curved surface according to claim 3, characterized in that: Thermophysical properties include thermal conductivity λ, melting point Tm and vaporization heat Lv. Structural characteristics include processing point curvature radius R, plate thickness h, depth z, laser energy density E=ηP / (πr 2 ), where η is the coupling efficiency, P is the average power, and r is the spot radius. When the aspect ratio is greater than 10, the laser energy density E is increased by 20%-30% to avoid energy attenuation. In the pulse frequency f, when the convex surface R is greater than 50mm, the frequency f = 50-100kHz is used to reduce heat accumulation. When the concave surface R is less than 10mm, the frequency f = 1-10kHz is used to enhance the penetration of the molten pool. The scanning speed v = k·h / R, where k is the correction coefficient, h is the plate thickness, and R is the curvature radius. This is to avoid the molten pool not being filled due to the speed being greater than the set value when scanning the concave surface. When the thick plate is convex, the laser focus is dynamically adjusted to the normal distance Δz = R / 2 to the processing surface. When the thick plate is concave, the laser focus is dynamically adjusted to the normal distance Δz = -R / 3 to the processing surface to compensate for the energy discreteness caused by the curved surface.

5. The method for laser drilling a thick plate with a complex curved surface according to claim 3, characterized in that: The specific process of calculating the optimal process parameter combination is as follows: first input the thick plate material characteristic data, scan the thick plate structural characteristics, obtain the structural characteristics by laser scanning, the structural characteristics include the current curvature radius, plate thickness and processed depth, calculate the key derivative characteristics of heat impact, the derivative characteristics include thermal diffusivity, curvature Biot number and safety temperature threshold, solve the thermomechanical coupling equation, use the curvature to encrypt the grid, scan the parameter range, generate a training set, use the XGBoost model to calculate, input the construction vector, basic parameters and derivative characteristics, and output the optimal parameters of the optimal speed, feed rate and cooling intensity.

6. The method for laser drilling a thick plate with a complex curved surface according to claim 1, characterized in that: The specific process of step 3 is as follows: based on the normal vector field and depth gradient of the surface, a heuristic search path planning algorithm is used to generate a spiral and straight line composite path, where the spiral angle α is 30°-60°, the pitch p = h / 10, and h is the height, to ensure that the laser beam always enters the processing surface perpendicularly, and the incident angle is ≤5°. The laser head is equipped at the end of the six-axis robot, and the height change of the processing surface is fed back in real time through the laser ranging sensor. The robot joint angle and dynamic focusing module parameters are adjusted in combination with the PID controller to achieve a focus position error of ≤0.02mm. The main pulse and auxiliary pulse mode is used in the thick plate penetration stage, with the main pulse energy accounting for 70% and the auxiliary pulse interval τ = 10-50μs. The auxiliary pulse is used to preheat the lower layer material to reduce the hole wall cracks caused by thermal stress.

7. The method for laser drilling in thick plates with complex curved surfaces according to claim 1, characterized in that: The specific process of step 4 is as follows: use a microscope to collect the hole wall image in real time, identify the molten pool width W and depth D through the edge detection algorithm, use the infrared thermal imager to obtain the molten pool temperature T, calculate the heat input Q = α·k·(T-T0), where α is the thermal diffusivity, k is the thermal conductivity, and T0 is the temperature at a certain moment, and use a spectrometer to collect the plasma emission spectrum. The electron temperature Te and electron density Ne are inverted by the Boltzmann equation to judge the plasma shielding effect. If the molten pool width W deviates from the target value by ±10% or the temperature T exceeds the material boiling point Tb+200K, the laser power P or scanning speed v is automatically adjusted. If microcracks appear on the hole wall, the processing is suspended and a low-energy repair pulse is started with an energy density of 30% of normal processing.

8. The method for laser drilling a thick plate with a complex curved surface according to claim 7, characterized in that: In the plasma shielding effect, when Ne>10 1 At 6cm-3, the trigger power is reduced by 10%-15%.

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