Transparent restraint layer suitable for laser shock and laser shock forming method and device of metal plate

Abstract

The invention provides a transparent restraint layer suitable for laser shock and a laser shock forming method and device of a metal plate, belongs to the technical field of metal plate laser processing, and aims to solve the technical problem of failure of the restraint layer in a low-temperature environment. The forming method comprises the steps that an absorption layer and a transparent restraint layer are sequentially laid on the surface of a to-be-impacted workpiece, and the transparent restraint layer sequentially comprises a polyurea-polyurethane elastomer layer, a nanofluid layer and a hydrogel layer in the direction close to the to-be-impacted workpiece to the direction away from the to-be-impacted workpiece; a single-laser-head or multi-laser-head array is adopted to implement pulse impact, and a temperature-stress-displacement multi-physical-field closed loop is constructed through displacement sensing, temperature sensing and ultrasonic stress sensing; and the industrial controller operates a model prediction control algorithm to perform linkage optimization on the laser energy density E, the scanning speed v and the hydrogel layer volume flow q. Compared with an existing double-layer constraint and double-feedback scheme, the method has remarkable technical progress in the aspects of low-temperature stability, array consistency and forming consistency.

Description

Technical Field

[0001] This invention belongs to the technical field of laser processing and high strain rate plastic forming of metal sheets, and particularly relates to a transparent constraint layer. Background Technology

[0002] Laser shock forming utilizes a high-energy short-pulse laser to act on an absorption layer, inducing a plasma detonation wave under a transparent confinement layer. This generates transient impact pressure with amplitudes reaching several GPa, causing controllable plastic deformation of the sheet metal at a high strain rate and introducing beneficial residual compressive stress. TC4 titanium alloy, as a typical α+β dual-phase alloy, possesses advantages such as high specific strength and good corrosion resistance. Due to its combination of strength, toughness, and machinability, it is widely used in the manufacture of core components such as aero-engine blades and aircraft structural parts. However, traditional molding or hot forming suffers from problems such as complex tooling, low efficiency, and poor dimensional consistency.

[0003] Compared to traditional molding and thermoforming processes, laser shock forming, as a typical high-strain rate forming and surface strengthening integrated technology, has already seen small-batch applications in aerospace skins, engine blades, and key load-bearing components of landing gear. Existing engineering practice shows that for titanium alloys, aluminum alloys, and some high-strength steel plates with thicknesses ranging from 0.2 to 6.0 mm, when the peak single impact pressure is controlled at 2-8 GPa and the number of impacts is controlled at 1-5, beneficial residual compressive stress can be introduced within the surface layer of 150-200 μm while obtaining the target macroscopic morphology, thereby significantly improving fatigue life and stress corrosion resistance. For example, patent publication number CN104772569A discloses a laser shock microforming device and its microforming process for medical titanium alloy plates. The device, based on existing laser shock microforming devices, adds a transparent material plate as a constraint layer and a detection system; the microforming process utilizes techniques such as spraying a black paint layer onto the titanium alloy plate to be microformed as an absorption layer, making it suitable for the microforming of medical titanium alloy plates. However, existing processes are mostly optimized around a single part and a single environmental condition. Once the ambient temperature, altitude or sheet thickness changes significantly, the original empirical parameters often need to be recalibrated. Moreover, repeated molding is required many times and the cost is high, which makes it difficult to meet the comprehensive requirements of "reproducibility and high consistency" in the context of lightweighting and rapid iteration of current aerospace components. Existing publicly available technologies still have shortcomings in three aspects: (1) The constraint layer mostly uses "elastomer + water" or a single layer of hydrogel. At low temperatures, the rheology and thermal conductivity decrease, resulting in the broadening of the impact pressure time spectrum and the attenuation of the peak value. Ice formation or microcrack propagation often occurs below −10℃, causing molding failure; (2) Parallel use of multiple laser heads improves efficiency, but synchronous triggering and energy consistency are limited. It is common to have ±20ns jitter and ±5% energy deviation, resulting in differences in the morphology of sub-regions; (3) Feedback is mostly limited to displacement or temperature dual parameters, which cannot constrain the residual stress distribution in real time, and it is difficult to avoid the risk of rebound and microcracks in advance. In addition, the optical path transmission is thin at high altitudes, energy loss increases and there is a lack of systematic compensation mechanism, making it difficult to guarantee batch consistency.

[0004] Furthermore, some improvement attempts have emerged in published literature and engineering projects. These include increasing the thickness of the single-layer hydrogel, adding a small amount of plasticizer to the transparent constraint layer, or employing fixed-threshold power closed-loop control in multi-laser head systems to alleviate low-temperature icing and energy drift problems. However, these solutions often only maintain effectiveness within a narrow parameter window: when the ambient temperature fluctuates from room temperature to -20 to -25°C, the hydrogel layer still inevitably experiences localized freezing and a decrease in transmittance, resulting in a significant broadening of the impact waveform. When the number of laser heads increases to more than four, simple power closed-loop control cannot simultaneously constrain energy fluctuations and time jitter, and significant sub-region morphology errors still exist. In addition, existing feedback control is mostly based on a single sensing quantity (such as displacement or temperature), and for the key indicator of residual stress, it often adopts a "post-event offline detection" approach, lacking the ability to adjust in real time during the forming process, making it difficult to suppress springback and microcrack initiation via feedforward. These problems indicate that current technology has not yet formed a universally applicable systematic solution suitable for low-temperature, high-altitude, and laser operating conditions. Summary of the Invention

[0005] To address technical challenges such as constraint layer failure in low-temperature environments, insufficient synchronization and energy consistency in array operations, inadequate multi-physics field coupling and control, and energy loss at high altitudes, this invention proposes a laser shock forming method and apparatus for transparent constraint layers and metal sheets suitable for laser shock. The method integrates design from a system perspective encompassing materials, energy, structure, and control: at the material level, a combination of three transparent constraint layers ensures stable transmission of the impact pressure waveform under extreme conditions; at the energy and optical path level, a laser, online energy calibration, and a high-altitude pressure compensation chamber ensure high consistency of the equivalent energy incident on the workpiece surface in both spatial and temporal dimensions; at the process control level, by introducing three-dimensional feedback information on temperature, stress, and displacement, combined with model predictive control (MPC) algorithms, multi-objective collaborative optimization of key process parameters such as laser energy density, scanning speed, and outer medium flow rate is achieved, resulting in a comprehensive balance between forming accuracy, residual stress distribution, and process stability within a unified framework.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0007] A transparent constraint layer suitable for laser shock is disclosed, comprising, sequentially from the direction approaching the workpiece to the direction away from the workpiece, a polyurea-polyurethane elastomer layer, a nanofluid layer, and a hydrogel layer. The polyurea-polyurethane elastomer layer is formed by polymerizing isocyanate and amino polyether as raw materials by spraying onto the surface of the workpiece using a two-component spraying technique. The nanofluid layer is prepared by dispersing nanoparticles in water to prepare a nanoparticle solution, which forms a liquid film on the polyurea-polyurethane elastomer layer after being sprayed through a nozzle. The hydrogel layer is formed by dissolving polyvinyl alcohol in a mixed solvent of alcohol and water to form a polyvinyl alcohol gel, which is sprayed through a nozzle and forms a flow constraint layer on the surface of the nanofluid layer.

[0008] The thicknesses of each layer in the transparent constraint layer are as follows: polyurea-polyurethane elastomer layer 0.8-1.2 mm, nanofluid layer 0.6-1.0 mm, and hydrogel layer 1.0-1.5 mm.

[0009] The preparation method of the polyurea-polyurethane elastomer layer is as follows: amino polyether, amine chain extender and dynamic bond forming substance are mixed as component A, isocyanate is used as component B, and components A and B are mixed at the nozzle of the spray gun and then sprayed onto the surface of the workpiece to be impacted using a two-component spraying equipment.

[0010] The amino polyether is polyoxypropylene diamine; the amine chain extender is diethyltoluene diamine; the dynamic bond forming substance is 4-aminophenyl disulfide; the mass ratio of the amino polyether, amine chain extender, and dynamic bond forming substance is 5-8:2-4:1; the volume ratio of component A to component B is 6-7:3-4.

[0011] The nanoparticles are Al2O3 or SiO2 with a particle size of 30-80 nm; the concentration of nanoparticles in the nanoparticle solution is 3-12 wt%.

[0012] The alcohol is ethylene glycol, the concentration of polyvinyl alcohol in the polyvinyl alcohol gel is 10-15 wt%, and the volume fraction of ethylene glycol in the mixed solvent is 20-35%.

[0013] A method for laser shock forming of metal sheets includes the following steps:

[0014] (1) An absorption layer and a transparent constraint layer are sequentially laid on the surface of the workpiece to be impacted;

[0015] (2) The absorption layer is irradiated with a pulsed laser to cause plastic deformation of the workpiece to be impacted;

[0016] (3) Collect displacement, temperature and stress signals during the forming process, and construct a multi-physics field closed-loop feedback system for temperature, stress and displacement;

[0017] (4) The controller automatically adjusts the energy density, scanning speed and volume flow rate of the pulsed laser and the hydrogel layer according to the multi-physics closed-loop feedback signal to maintain the stability of the impact forming process, so that the forming contour of the workpiece to be impacted converges to the target surface. After the impact is completed, the fluid flows away and the waste liquid is recycled.

[0018] The controller is an industrial controller that runs a model predictive control algorithm. The state variable of the controller is x=[e,T,σ]T, where e is the topographic error, T is the surface temperature of the workpiece to be impacted, and σ is the residual stress surrogate quantity. The control variable is u=[E,v,q]T, where E is the laser energy density, v is the scanning speed, and q is the volumetric flow rate of the hydrogel layer. The controller adopts a discrete linearized incremental model Δx(k+1)=AΔx(k)+BΔu(k) to solve a quadratic programming problem within a control period of no more than 10ms to satisfy the constraints of T≤200℃, σ≤-150MPa, and |e|≤0.10mm.

[0019] A laser shock forming apparatus, comprising:

[0020] A laser, used to emit pulsed laser light, including a laser head array;

[0021] The synchronization control module is used to synchronize and control the laser pulse output;

[0022] Preferably, the synchronization control module is used to control the trigger jitter of each laser head to an absolute value of no more than 10 ns, and the online energy calibration module is used to control the relative error of the single pulse energy to no more than 2% within a calibration period of no more than 50 ms.

[0023] An online energy calibration module is used to calibrate the accuracy of laser energy.

[0024] The spraying module is used to lay out polyurea-polyurethane elastomer layer, nanofluid layer and hydrogel layer layer by layer;

[0025] A multi-physics detection module is used to collect displacement, temperature and stress data in real time during the forming process;

[0026] The controller is used to control the synchronization control module, the online energy calibration module, the spraying module, and the multiphysics detection module.

[0027] It also includes a workbench and a pressure compensation chamber that integrate negative pressure adsorption positioning and ultrasonic vibration auxiliary functions; the laser, synchronous control module, online energy calibration module, spraying module, multi-physics field detection module and workbench are all located in the pressure compensation chamber.

[0028] The negative pressure adsorption range of the worktable is -0.05 to -0.08 MPa, and the ultrasonic vibration frequency is 20–40 kHz with an amplitude of 3–7 μm.

[0029] The beneficial effects of this invention are:

[0030] (1) This invention employs a three-layer composite structure consisting of a polyurea-polyurethane elastomer layer, a nanofluid layer, and a hydrogel layer to construct a synergistic mechanism of "thermal conductivity and temperature control + antifreeze flow + self-healing sealing," effectively solving the problems of easy solidification of the constraint layer, sealing failure, and disordered shock wave transmission under low-temperature conditions. Specifically, the hydrogel layer, through the design of a polyvinyl alcohol and ethylene glycol-water system, maintains good transparency and fluidity even at -25℃, avoiding freezing failure; the nanofluid layer enhances thermal conductivity, quickly removing heat generated by the impact and preventing localized ablation; the polyurea-polyurethane elastomer layer, with its Shore A40-50 elasticity and 10-20 min (25℃) self-healing ability, seals the outer liquid layer, repairs microcracks generated by the impact, and ensures stable shock wave transmission. Ultimately, this achieves stable impact pressure spectrum under extreme low-temperature conditions of -25℃, eliminating constraint layer failure during the forming process and expanding the application scenarios of laser shock forming technology.

[0031] (2) Through the collaborative design of multi-laser head array synchronous control and online energy calibration, the consistency of forming is precisely controlled. The FPGA-based synchronous control module controls the trigger jitter of each laser head to an absolute value of ≤10ns, and the online energy calibration module performs real-time calibration with a period of ≤50ms, so that the relative error of single pulse energy is ≤2%. At the same time, 10-15% edge energy compensation is implemented at the junction of sub-regions to effectively suppress overshoot and undershoot errors during sub-region assembly. For the forming requirements of 1.5×1.0m skin workpieces, the sub-region morphology difference can be ≤±0.03mm, which is significantly better than the assembly accuracy of existing technologies, greatly reducing the workload of subsequent finishing work and improving the mass production efficiency of large-area complex components.

[0032] (3) An industrial controller employing the running model predictive control (MPC) algorithm is used to construct a three-dimensional multiphysics closed-loop feedback system of "displacement-temperature-stress". This system enables multi-variable linkage adjustment of laser energy density, scanning speed, and hydrogel layer volume flow rate within an extremely short control cycle. By optimizing the objective function and constraint design, active springback control of the forming process is achieved. Compared with traditional open-loop control or single-variable adjustment methods, the morphology error converges faster and is more stable. Ultimately, the morphology error of the workpiece is stably controlled within the range of ±0.07-0.09 mm, meeting the forming requirements of high-precision metal components (such as aerospace skin and precision structural parts). At the same time, it reduces the number of impacts and improves forming efficiency. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a flowchart of the present invention.

[0035] Figure 2 This is a block diagram of a multiphysics closed-loop feedback system.

[0036] Figure 3 This is a schematic diagram of the laser shock forming device of the present invention; in the figure, 1 is a laser head; 2 is a synchronous control module; 3 is an online energy calibration module; 4 is an absorption layer; 5 is a polyurea-polyurethane elastomer layer; 6 is a nanofluid layer; 7 is a hydrogel layer; 8 is a metal plate; 9 is a worktable; 10 is an ultrasonic module; 11 is a displacement sensor; 12 is an infrared temperature sensor; 13 is a stress sensor; 14 is an industrial controller; and 15 is a pressure compensation chamber.

[0037] Figure 4 This is a flowchart of the online energy calibration process. Detailed Implementation

[0038] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] A transparent constraint layer suitable for laser shock is disclosed, comprising, sequentially from the direction approaching the workpiece to the direction away from the workpiece, a polyurea-polyurethane elastomer layer, a nanofluid layer, and a hydrogel layer. The polyurea-polyurethane elastomer layer is formed by polymerizing isocyanate and amino polyether as raw materials by spraying onto the surface of the workpiece using a two-component spraying technique. The nanofluid layer is prepared by dispersing nanoparticles in water to prepare a nanoparticle solution, which forms a liquid film on the polyurea-polyurethane elastomer layer after being sprayed through a nozzle. The hydrogel layer is formed by dissolving polyvinyl alcohol in a mixed solvent of alcohol and water to form a polyvinyl alcohol gel, which is sprayed through a nozzle and forms a flow constraint layer on the surface of the nanofluid layer.

[0040] The thicknesses of each layer in the transparent constraint layer are as follows: polyurea-polyurethane elastomer layer 0.8-1.2 mm, nanofluid layer 0.6-1.0 mm, and hydrogel layer 1.0-1.5 mm.

[0041] The polyurea-polyurethane elastomer layer has a Shore A40-50 rating, an elongation at break ≥400%, and a self-healing time of 10-20 minutes (25℃). Its main functions are sealing (preventing external liquid from penetrating the absorbent layer), transmitting shock waves, and self-healing microcracks to extend its lifespan. Polyurea has excellent elasticity and impact resistance (commonly used in explosion-proof coatings). Introducing dynamic bonds (such as disulfide bonds or hydrogen bonds) to achieve self-healing is a mature technology in the polymer field. The raw materials (isocyanate, amino polyether) are commonly used chemical materials.

[0042] The preparation method of the polyurea-polyurethane elastomer layer is as follows: amino polyether, amine chain extender and dynamic bond forming substance are mixed as component A, isocyanate is used as component B, and components A and B are mixed at the nozzle of the spray gun and then sprayed onto the surface of the workpiece to be impacted using a two-component spraying equipment.

[0043] The amino polyether is polyoxypropylene diamine or polyoxypropylene triamine; the ratio of component A to component B is 6-7:3-4.

[0044] The nanoparticles are Al2O3 or SiO2 with a particle size of 30-80 nm; the concentration of nanoparticles in the nanoparticle solution is 3-12 wt%.

[0045] The nanofluid layer, as a water-based nanofluid (Al2O3 or SiO2 aqueous dispersion), primarily functions to regulate acoustic impedance (making shock wave transmission smoother) and enhance thermal conductivity (removing heat and preventing ablation). Nanoparticles can significantly improve the thermal conductivity of liquids and are inexpensive; simply purchasing ready-made nano-alumina or silica powder and dispersing it in deionized water is sufficient.

[0046] The nanofluid layer is prepared as follows: using deionized water as the base liquid, nanoparticles are added, and then dispersed using an ultrasonic disperser. The zeta potential is adjusted to ≥40mV (usually by adjusting the pH value or adding a trace amount of surfactant) to ensure the stability of the suspension and prevent sedimentation, maintaining a layer thickness of 0.6-1.0mm. This layer is a flowing liquid layer sandwiched between an inner layer (polyurea-polyurethane elastomer layer) and an outer layer (hydrogel layer), forming a liquid film in the processing area through a flow channel nozzle.

[0047] The alcohol is ethylene glycol, the concentration of polyvinyl alcohol in the polyvinyl alcohol gel is 10-15 wt%, and the volume fraction of ethylene glycol in the mixed solvent is 20-35%.

[0048] In the hydrogel layer, when the ethylene glycol content in the mixed solvent is below approximately 20%, a solidification tendency occurs around -15°C; above 35%, it significantly reduces light transmittance and alters acoustic impedance, affecting shock wave coupling efficiency. This layer provides the primary mass constraint (inertial limitation) and maintains transparency and flowability at low temperatures (freezing resistance). Ethylene glycol is a standard antifreeze agent; the PVA hydrogel has high transparency, and the combination of the two ensures that it does not freeze at -25°C while maintaining optical properties.

[0049] The hydrogel layer is prepared as follows: PVA is dissolved in a mixed solvent of water and ethylene glycol, heated and stirred until dissolved, forming a high-viscosity fluid that flows slowly, maintains its shape, and is not prone to turbulence, maintaining a thickness of 1.0-1.5 mm. This fluid is then flowed through a nozzle at a certain flow rate over the surface of the processing area, forming a flow-constrained layer with controllable thickness. It is important to note that the hydrogel layer and the nanofluid layer are not statically stacked, but rather instantaneously superimposed and covered during laser head movement using dynamic laminar flow and coaxial nozzle technology. Due to the significant viscosity difference between the two and the extremely short contact time at the moment of impact, and because they are in a laminar flow state (without vortices), they do not undergo significant chemical diffusion or physical turbulent mixing. After the impact, the fluid flows away, and the waste liquid is recycled.

[0050] Specifically, in practical engineering applications, the three-layer structure of the transparent constraint layer can be finely adjusted to accommodate different sheet materials and target surface shapes. For example, when processing high thermal conductivity aluminum alloy sheets, the mass fraction of nanoparticles can be appropriately reduced to decrease system viscosity and increase interlayer reset speed. When processing high-entropy alloys or thick plate components, the thickness of the hydrogel layer can be slightly increased while maintaining the overall volume fraction of ethylene glycol to enhance thermal buffering capacity during impact. For the polyurea-polyurethane elastomer layer, its self-healing properties can be optimized by adjusting the ratio of soft and hard segments and hydrogen bond density. Without significantly increasing the material hardness, the microcrack closure speed within the layer can be increased, thereby extending the continuous online service life of the constraint layer and reducing the frequency of downtime for replacement.

[0051] A laser shock forming method for metal sheets, such as Figure 1 As shown, it includes the following steps:

[0052] (1) An absorption layer and a transparent constraint layer are sequentially laid on the surface of the workpiece to be impacted;

[0053] (2) The absorption layer is irradiated with a pulsed laser to cause plastic deformation of the workpiece to be impacted;

[0054] (3) Collect displacement, temperature and stress signals during the forming process, and construct a multi-physics field closed-loop feedback system for temperature, stress and displacement;

[0055] (4) The controller automatically adjusts the energy density, scanning speed and volume flow rate of the pulsed laser and the hydrogel layer according to the multi-physics closed-loop feedback signal to maintain the stability of the impact forming process and make the forming contour of the workpiece to be impacted converge to the target surface.

[0056] Specifically, the absorption layer is a DLC (diamond-like carbon) modified graphite / polyimide (PI) composite material with an sp³ content ≥45%, surface energy ≤30mN / m, laser damage threshold ≥5J / cm², and a thickness of 50±5μm. It is used to improve plasma absorption efficiency and reduce the thermal impact on the workpiece surface, protecting the metal workpiece surface from ablation. Polyimide (Kapton tape) is a standard absorption layer material for laser shock stabilization (LSS), exhibiting high temperature resistance. The addition of DLC modified graphite significantly improves laser absorption (high sp³ content) and enhances wear resistance, thus enabling multiple reuses.

[0057] The method for preparing this absorbent layer is as follows: A polyimide (PI) film is selected as the substrate, and a slurry containing graphite powder is prepared. A modified coating containing a diamond-like carbon structure is deposited on the PI film surface using physical vapor deposition (PVD) or plasma-enhanced chemical vapor deposition (PECVD). In application, the absorbent layer can be prepared in advance and then applied to the surface of the workpiece to be impacted. The application method is a roll-to-roll mechanical supply, similar to the principle of old-fashioned magnetic tape or film. The device includes a feed roller and a take-up roller, which tightly adhere the absorbent layer film to the surface of the metal sheet. After each impact, if it is not damaged, it can be reused. Online thickness monitoring detects ±1μm; when the thickness wears down to 60% of its initial value, the motor automatically rolls up to replace it with a new section.

[0058] The control principle of the multiphysics closed-loop feedback system in step (3) is as follows: Figure 2 As shown, a three-dimensional feedback of "displacement e, temperature T, and residual stress σ" is constructed by using a displacement sensor (accuracy ±1μm), infrared thermography (±2℃, ≥50Hz), and an ultrasonic stress sensor (±5MPa). The controller acquires the multi-physics state quantities at a preset control cycle and automatically adjusts the laser energy density E, scanning speed v, and outer hydrogel volume flow rate q according to the deviation of morphology error, temperature, and residual stress to maintain the stability of the impact process and make the formed contour converge to the target surface.

[0059] Preferably, the controller is an industrial controller that runs a model predictive control (MPC) algorithm, and the specific forms of the state variables, control variables, objective functions, and constraints are given in the embodiments.

[0060] Specifically, the state variables and input / output definitions in the multiphysics closed-loop feedback system are as follows: The topography error e(k) is obtained from a coaxial displacement sensor (target-measured), the surface temperature T(k) of the workpiece to be impacted is obtained from infrared thermography, and the residual stress surrogate σ(k) is inverted online using an ultrasonic surface wave algorithm. The control variable u(k) = [E(k), v(k), q(k)]ᵀ, representing the laser energy density, scanning speed, and outer hydrogel volumetric flow rate, respectively.

[0061] Discrete prediction model: The linearized incremental model Δx(k+1)=AΔx(k)+BΔu(k)+w is adopted, where x=[e,T,σ]ᵀ. The parameters are identified through experiments: (i) E is negatively correlated with e and σ (increased energy → increased forming amount, deeper compressive stress); (ii) v and q affect the phase and amplitude of T and e; A and B are obtained by least squares fitting, and under nominal conditions ||A||<1 to ensure asymptotic stability.

[0062] Objective function and constraints: Minimize J = Σ(α·e^(- ... 2 +β·(T−T * ) 2 +γ·(σ−σ * ) 2 )+λ||Δu||2 2 T * ≤200℃, σ * The upper bound of the constraint is ≤−150MPa, and the ratios of α:β:γ can be 0.6:0.25:0.15. Input / state constraints are: E∈[Emin,Emax], v∈[vmin,vmax], q∈[qmin,qmax], with a Δu amplitude limit set to prevent equipment saturation. The solver uses quadratic programming (QP) with a control cycle of 10ms.

[0063] The engineering description of the linkage logic is as follows: When T ≥ 0.9·200℃ threshold, the priority rule is "reduce E by 5-10% and increase q by 10-20%"; when σ > -150MPa (insufficient compressive stress), increase the overlap rate by 5-8% or reduce v by 5-10%; when |e| approaches the upper limit, E and v are jointly fine-tuned according to the sensitivity matrix priority to ensure that the constraint is not triggered.

[0064] In implementing this invention, the establishment and updating of the MPC model adopts an "initial calibration + online fine-tuning" approach: First, under typical operating conditions, a certain number of data points on "energy density—scanning speed—hydrogel layer flow rate—morphology / temperature / stress" are obtained through orthogonal experiments or response surface methodology. Initial A and B matrices are then obtained using least squares or subspace identification methods. Subsequently, during mass production, the controller periodically performs lightweight fitting on the sampled data from the most recent period. When the model prediction error is detected to continuously deviate from a preset threshold, the model parameters are fine-tuned. This approach avoids the efficiency loss caused by frequent downtime and remodeling, while also enabling the model to gradually adapt to equipment aging and environmental changes, thereby ensuring that the multiphysics closed-loop control strategy maintains good robustness and convergence characteristics throughout its entire lifespan.

[0065] A laser shock forming apparatus, such as Figure 3 As shown, it includes:

[0066] Laser; the laser shown includes laser head 1;

[0067] Specifically, the laser head 1 is an array of 1-6 Nd:YAG pulsed laser heads (1064nm, single pulse 3-12J, pulse width 20±2ns); wherein the laser is used to pulse the absorption layer 4, the absolute value of the trigger jitter of each laser head 1 is not greater than 10ns, the relative error of online energy calibration is not greater than 2%, and 10-15% edge energy compensation is implemented at the boundary of the partition to suppress the morphological differences of the sub-region.

[0068] Synchronization control module 2 is used to synchronously control the laser pulse output;

[0069] Online energy calibration module 3 is used to calibrate the accuracy of laser energy;

[0070] Specifically, the synchronization control module 2 is an FPGA-based synchronization control module. Through the FPGA-based synchronization control module, the trigger jitter of each laser head 1 is controlled to be no more than 10ns in absolute value. The synchronization control module 2 works in conjunction with the online energy calibration module 3. The online energy calibration module performs real-time energy sampling and compensation channels, and performs online energy calibration at a period of ≤50ms, so that the relative error of the calibrated single pulse energy is no more than 2%. At the boundary of the partition, 10-15% edge energy compensation is performed based on the pre-scan data to avoid overshoot or undershoot at the boundary.

[0071] The spraying module is used to lay the polyurea-polyurethane elastomer layer 5, the nanofluid layer 6, and the hydrogel layer 7 layer by layer; specifically, the spraying module includes three sets of high-pressure spraying equipment.

[0072] A multi-physics detection module is used to acquire displacement, temperature, and stress data in real time during the forming process; the multi-physics detection module includes a displacement sensor 11, an infrared temperature sensor 12, and a stress sensor 13.

[0073] The controller is used to control the synchronous control module 2, the online energy calibration module 3, the spraying module, and the multiphysics detection module.

[0074] It also includes a worktable 9 integrating negative pressure adsorption positioning and ultrasonic vibration assisted functions, and a pressure compensation chamber 15; the laser, synchronous control module 2, online energy calibration module 3, spraying module, multiphysics field detection module, and worktable are all located inside the pressure compensation chamber 15. During operation, the metal plate is fixed on the worktable 9.

[0075] The worktable is an open-type negative pressure adsorption stage, equipped with an ultrasonic module 10. The negative pressure adsorption range of the worktable is −0.05 to −0.08 MPa, the ultrasonic vibration frequency is 20-40 kHz, and the amplitude is 3-7 μm. The synchronization error with laser triggering is ≤±50 μs to reduce the probability of local buckling of critical thickness components. When the altitude is ≥2000m, the air pressure compensation chamber 15 is activated, and the optical path area is maintained at 101.3±2 kPa.

[0076] Specifically, the thickness of the workpiece to be impacted is 0.2-6.0 mm. When the thickness is not greater than 0.5 mm, negative pressure adsorption of -0.05 to -0.08 MPa and micro-spot of 0.6-0.8 mm are used during the forming process. When the thickness is not less than 1.0 mm, multi-pass progressive impact is used and 20-40 kHz ultrasonic vibration is activated. The absolute value of the synchronization error between ultrasonic vibration and laser triggering is not greater than 50 μs.

[0077] In environments with an altitude of not less than 2000m, the air pressure compensation chamber 15 is activated to maintain an air pressure of 101.3±2kPa in the optical path area, and the main laser energy is automatically adjusted according to the pre-established altitude-transmission loss-energy compensation curve.

[0078] Example: Precision laser shock forming of TC4 titanium alloy skin (room temperature, single laser head condition)

[0079] This embodiment provides a precision forming process based on coaxial fluid nozzles and multiphysics closed-loop control.

[0080] (1) Preparation of experimental setup and materials

[0081] The experiment used, for example Figure 3 The laser shock forming apparatus shown.

[0082] Workpiece: Select TC4 titanium alloy plate with dimensions of 300mm×300mm and a thickness of 0.3mm.

[0083] Platform: Fix the workpiece on the worktable with integrated negative pressure adsorption function, and turn on the vacuum pump to maintain the negative pressure at −0.07MPa.

[0084] Absorbing layer: A 50μm thick DLC-modified graphite / polyimide composite absorbing layer is laid on the surface of the workpiece.

[0085] Sensor configuration: To ensure detection accuracy, the multi-physics detection module is configured as follows: The displacement sensor adopts a laser triangular reflector with a sampling frequency of 100Hz and an accuracy of ±1μm; the infrared temperature sensor adopts an online thermal imager with a temperature measurement range of -40~500℃; the stress sensor adopts a surface wave ultrasonic probe, which is installed on the side of the workbench.

[0086] (2) Prefabrication of solid-state substrate

[0087] The bottom layer of the transparent constraint layer—a polyurea-polyurethane elastomer layer—is pre-prepared on the surface of the absorbent layer using a spraying module: polyoxypropylene diamine, diethyltoluene diamine, and 4-aminophenyl disulfide are mixed in a mass ratio of 6:3:1 as component A, and isocyanate prepolymer is used as component B; components A and B are mixed at a volume ratio of 2:1 at the nozzle of a spray gun and sprayed onto the surface of the absorbent layer. After curing, a transparent solid elastic film with a thickness of 1.0 mm is formed.

[0088] (3) Dynamic fluid injection and laser shock

[0089] The automation program is initiated, and the laser head scans along a preset path. The coaxial fluid nozzle integrated at the end of the laser head performs the following synchronized actions with the laser pulse (synchronization delay ≤20μs):

[0090] The first annular channel is injected into the nanofluid layer (6wt% SiO2 solution) at a flow rate of 0.5 mL / s.

[0091] The second annular channel extrudes the hydrogel layer (10wt% PVA solution, 28% ethylene glycol by volume) at a flow rate of 0.3 mL / s. The thickness of this layer is controlled at 1.2 mm, which tightly compacts the inner nanofluid layer.

[0092] The absorption layer is irradiated with a pulsed laser to cause plastic deformation of the workpiece to be impacted.

[0093] (4) Multiphysics closed-loop control

[0094] The controller operates strictly according to the following logic:

[0095] Process basis: Follow Figure 1 The process is shown below.

[0096] Calibration basis: Operation Figure 4The calibration process has a calibration cycle of 50ms, ensuring that the energy error is ≤2%.

[0097] MPC control basis: operation Figure 2 The logic is as follows. Displacement, temperature, and stress signals are collected during the forming process, and a multi-physics closed-loop feedback system for temperature, stress, and displacement is constructed. The MPC constraints are T≤180℃ and σ≤−165MPa, and the hydrogel flow rate q and scanning speed v are adjusted.

[0098] Although the laser shock forming method and apparatus described in this invention primarily use TC4 titanium alloy sheet and skin components as examples in the embodiments, they are not limited to this. Those skilled in the art will understand that the process concept of this invention is applicable as long as the material to be processed still possesses a certain degree of plasticity at room temperature or slightly higher temperatures, and it is permissible to introduce residual compressive stress in a mechanical sense on the surface. For example, for 2XXX and 7XXX series high-strength aluminum alloy wing skins, nickel-based superalloy blade wrap angle transition areas, and locally thickened areas of high-entropy alloy structural components, only slight adjustments to the single-pulse energy density, number of impacts, and constraint layer thickness based on the yield strength, thermal conductivity, and acoustic impedance of different materials are needed to achieve high-precision forming and surface strengthening within a similar multiphysics closed-loop framework.

[0099] In actual production lines, this invention can be flexibly combined with processes such as sheet metal pretreatment, machining, surface shot peening or shot blasting, and coating deposition. For example, for wing skin components requiring high fatigue life, the method of this invention can first achieve precise shaping of the overall morphology and introduction of residual compressive stress, and then reinforce local high-stress concentration areas with micro-shot peening or ultrasonic rolling. For structural components with high surface roughness requirements, a slight mechanical polishing or chemical polishing can be added after laser shock forming to further reduce the Ra value, thereby balancing aerodynamic and mechanical properties. All of the above combined processes do not deviate from the basic concept of this invention.

[0100] Safety and Maintenance Considerations During Process Implementation: During the implementation of this invention, attention must be paid to laser safety, the sealing of the vacuum negative pressure system, and the replacement and recycling of the three-layer constraint materials. The laser system should be equipped with multi-level interlocks and protective shielding, and the operating area should be equipped with protective eyewear that meets wavelength requirements. The negative pressure adsorption workbench should be regularly inspected for sealing rings and vacuum pipelines to prevent air leakage caused by long-term operation from affecting the positioning accuracy of the sheet material. The mechanical and optical properties of the polyurea-polyurethane elastomer layer and hydrogel layer will gradually degrade after long-term use; a replacement threshold should be set according to the cumulative number of impacts or online monitoring indicators to prevent impact waveform distortion caused by material fatigue.

[0101] Through the above maintenance measures, the reliability and consistency of forming quality of the device of the present invention can be ensured during long-term operation. For parameter adjustment, during the equipment commissioning or introduction of new parts, it is recommended to prioritize "parameter frequency sweep" tests on small-size test plates to quickly obtain morphology, temperature, and stress response curves under different combinations of energy density, scanning speed, and hydrogel layer flow rate. Based on this, reasonable initial weights and constraint ranges can be set in the MPC controller. For production lines with large fluctuations in ambient temperature and altitude, the frequency of measurement of optical path loss and constraint layer properties can be increased during routine inspections. Once a deviation from the initial calibration value is found to exceed the preset range, the controller's model update process is triggered to ensure the forming accuracy and process stability of the method of the present invention during long-term operation.

[0102] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A transparent confinement layer suitable for laser shock, characterized in that, The transparent constraint layer comprises, in sequence from the direction close to the workpiece to be impacted to the direction away from the workpiece to be impacted, a polyurea-polyurethane elastomer layer, a nanofluid layer and a hydrogel layer; the polyurea-polyurethane elastomer layer is prepared by using isocyanate and amino polyether as raw materials and spraying on the surface of the workpiece to be impacted by a two-component spraying technology; the nanofluid layer is a nanoparticle solution prepared by dispersing nanoparticles in water, and the nanoparticle solution forms a liquid film on the polyurea-polyurethane elastomer layer after being sprayed out of a nozzle; and the hydrogel layer is a polyvinyl alcohol gel solution prepared by dissolving polyvinyl alcohol in a mixed solvent composed of alcohol and water, and the polyvinyl alcohol gel solution is shaped on the surface of the nanofluid layer and forms a flow constraint layer after being sprayed out of a nozzle.

2. The transparent confinement layer suitable for laser shocking according to claim 1, characterized in that, The thicknesses of the layers in the transparent constraint layer are respectively: the polyurea-polyurethane elastomer layer is 0.8-1.2 mm thick, the nanofluid layer is 0.6-1.0 mm thick, and the hydrogel layer is 1.0-1.5 mm thick.

3. A transparent confinement layer suitable for laser shocking according to claim 2, characterized in that, The preparation method of the polyurea-polyurethane elastomer layer is as follows: amino polyether, amine chain extender and dynamic bond forming substance are mixed as component A, isocyanate is used as component B, and components A and B are mixed after being sprayed out of a nozzle by using a two-component spraying device and sprayed on the surface of the workpiece to be impacted.

4. The transparent confinement layer suitable for laser shocking according to claim 3, characterized in that, The amino polyether is polyoxypropylene diamine; the amine chain extender is diethyl toluene diamine; the dynamic bond forming substance is 4-aminophenyl disulfide; and the mass ratio of the amino polyether, the amine chain extender and the dynamic bond forming substance is 5-8:2-4:1; and the volume ratio of component A to component B is 6-7:3-4.

5. A transparent confinement layer suitable for laser shocking according to any of claims 1-4, characterized in that, The nanoparticles are Al2O3 or SiO2, and the particle size is 30-80 nm; and the concentration of the nanoparticles in the nanoparticle solution is 3-12 wt%.

6. A transparent confinement layer suitable for laser shocking according to claim 5, characterized in that, The alcohol is ethylene glycol, the concentration of polyvinyl alcohol in the polyvinyl alcohol gel solution is 10-15 wt%, and the volume fraction of ethylene glycol in the mixed solvent is 20-35%.

7. A method of laser shock forming of a metal sheet material, characterized in that, The method comprises the following steps: (1) laying an absorbing layer and the transparent constraint layer according to any one of claims 1-6 on the surface of the workpiece to be impacted in sequence; (2) irradiating the absorbing layer by using a pulsed laser to cause plastic deformation of the workpiece to be impacted; (3) collecting displacement, temperature and stress signals in the shaping process to construct a multi-physical field closed-loop feedback system of temperature, stress and displacement; (4) the controller automatically links and adjusts the energy density of the pulsed laser, the scanning speed and the volume flow rate of the hydrogel layer according to the multi-physical field closed-loop feedback signals to maintain the stability of the impact shaping process and make the shaping profile of the workpiece to be impacted converge to a target surface.

8. The method of laser shock forming of a metal sheet according to claim 7, characterized in that, The controller is an industrial controller running a model predictive control algorithm; the state of the controller x = [e, T, σ] T where e is the topography error, T is the temperature of the workpiece to be impacted, and σ is the residual stress proxy, and the control variable u = [E, v, q] T where E is the laser energy density, v is the scanning speed, and q is the volume flow rate of the hydrogel layer; the controller uses a discrete linearized incremental model Δx(k+1) = AΔx(k) + BΔu(k) to solve a quadratic program in a control period of no more than 10 ms to satisfy the constraints T≤200℃, σ≤−150MPa, and |e|≤0.10mm.

9. A laser shock forming device for carrying out the method of claim 7 or 8, characterized in that, The method comprises: a laser; a synchronous control module for synchronously controlling the pulsed output of the laser; an online energy calibration module for calibrating the accuracy of laser energy; a spraying module for laying the polyurea-polyurethane elastomer layer, the nanofluid layer and the hydrogel layer layer by layer; a multi-physical field detection module for collecting displacement, temperature and stress data in the shaping process in real time; a controller for controlling the synchronous control module, the online energy calibration module, the spraying module and the multi-physical field detection module.

10. The laser shock forming apparatus of claim 9, wherein The workbench integrated with negative pressure adsorption positioning and ultrasonic vibration auxiliary functions and a gas pressure compensation cabin are also included; the laser, the synchronous control module, the online energy calibration module, the spraying module, the multi-physical field detection module and the workbench are located in the gas pressure compensation cabin.

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