A multi-curved bending super-large aluminum plate splicing welding residual stress detection system and method

Abstract

The application relates to a multi-curved bending super-large aluminum plate splicing welding residual stress detection system and method, which comprises a control follow-up platform; a cold sound coaxial flexible array probe, which comprises a flexible adhered outer film, a low-freezing-point liquid sound coupling medium layer, an ultrasonic phased array and a pulse cold spray nozzle; and a time-space coordination control system configured to: apply a transient thermal shock to a roughened rough weld surface through the pulse cold spray nozzle; drive the ultrasonic phased array to emit a detection sound wave to the weld in a temperature recovery period after the transient thermal shock, collect a scattered echo signal of the rough surface for time reversal processing and re-emission, so that a dynamic acoustic focus is formed in a target stress area; and simultaneously collect a transient sound velocity change rate in the focused area in real time, and based on a thermal-sound-elastic nonlinear coupling model, calculate and output a real welding residual stress value after eliminating the influence of the bending anisotropy. The application realizes dynamic acoustic focusing in the interior of the ungrounded weld, and improves engineering detection efficiency.

Description

Technical Field

[0001] This application relates to the technical field of special material testing, and in particular to a system and method for detecting residual stress in the welding of large aluminum plates with multiple bends. Background Technology

[0002] With the development of lightweight and streamlined designs in aerospace, rail transportation, and large building structures, the application of multi-curved bending extra-large aluminum plates is becoming increasingly widespread. Due to the limited size of extra-large aluminum plates, splicing and welding are often required during the manufacturing process. However, aluminum alloys have a large coefficient of linear expansion, which easily generates huge welding residual stress under the action of welding thermal cycles. In addition, the multi-curved bending forming process itself introduces initial forming residual stress into the plate. The coupling of these two factors can lead to structural deformation, reduced fatigue strength, and even stress corrosion cracking, seriously threatening the safety and service life of the structure.

[0003] Currently, the industry primarily uses critically refracted longitudinal waves (LCR waves) based on the acoustoelastic effect for non-destructive testing. However, in practical engineering applications targeting such specific components, existing testing methods suffer from the following serious drawbacks:

[0004] First, there is the interference of complex initial anisotropy. Before welding, aluminum plates undergo a stretch bending process, causing the internal grains to align and form a distinct grain texture. This material anisotropy caused by processing deformation also leads to significant changes in sound velocity. Traditional ultrasonic testing methods cannot physically separate the "sound velocity changes caused by grain texture" from the "sound velocity changes caused by welding residual stress." The superposition of these effects can lead to huge systematic errors in the test results, and even completely incorrect conclusions regarding the stress nature (tensile / compressive stress).

[0005] Second, there is the challenge of coupling rough surfaces with complex curved surfaces. Welded joints and their heat-affected zones often retain their original rough morphology, and the components exhibit complex, multi-directional curvature variations. Existing rigid probes are prone to generating severe scattering clutter and energy defocusing when contacting such surfaces, resulting in extremely low signal-to-noise ratios. Simultaneously, it is difficult to maintain the constant excitation angle required for critical refractive longitudinal waves under continuously varying curvature conditions, leading to extremely poor consistency and repeatability of the detection data.

[0006] Failure to accurately decouple material texture background noise and overcome acoustic beam distortion from rough surfaces will lead to the invalidation of numerous welding residual stress assessment results. The consequences of such failure are fatal: on the one hand, the high level of latent tensile stress within the component may be masked by detection errors, leading to sudden and catastrophic brittle fracture during service; on the other hand, incorrect stress assessments will guide incorrect process compensation (such as unnecessary vibration aging or heat treatment), not only causing huge energy consumption and cost waste, but also potentially causing secondary deformation due to over-treatment, resulting in the scrapping of the entire ultra-large aluminum plate. Summary of the Invention

[0007] To address the problem that existing technologies cannot achieve high-precision decoupled detection of the influence of welding residual stress and material grain texture on rough, unpolished, multi-curvature aluminum plates, this application provides a system and method for detecting welding residual stress in spliced, ultra-large multi-curvature aluminum plates.

[0008] The technical solution provided in this application for a residual stress detection system and method for splicing and welding large aluminum plates with multiple bends is as follows:

[0009] The first aspect of this application provides a residual stress detection system for splicing and welding of large multi-curved bending aluminum plates, which adopts the following technical solution:

[0010] A residual stress detection system for splicing and welding of extra-large aluminum plates with multiple bends and tensions includes:

[0011] Six-axis robot force control servo platform;

[0012] A cold-acoustic coaxial flexible array probe, installed at the end of the servo platform, includes: a flexible outer membrane with acoustic transmissivity, a low-freezing-point liquid acoustic coupling medium layer filled inside the flexible outer membrane, an ultrasonic phased array immersed in the low-freezing-point liquid acoustic coupling medium layer, and a pulsed cold-excitation nozzle penetrating the center of the ultrasonic phased array and extending to the surface of the flexible outer membrane.

[0013] The spatiotemporal collaborative control system, communicatively connected to the cold acoustic coaxial flexible array probe, is configured to: apply transient thermal shock to the unpolished rough weld surface through the pulsed cold-shock nozzle; during the temperature recovery period after the transient thermal shock, drive the ultrasonic phased array to emit probe sound waves towards the weld, collect the scattered echo signal from the rough surface, perform time reversal processing, and re-emit to form dynamic acoustic focusing in the target stress zone; simultaneously, collect the transient sound velocity change rate in the focusing zone in real time, calculate and output the true welding residual stress value after eliminating the influence of tension-bending anisotropy based on the thermo-acoustic-elastic nonlinear coupling model.

[0014] Furthermore, the end of the pulsed cooling nozzle is flush with and sealed to the inner surface of the flexible adhesive outer membrane. The pulsed cooling nozzle integrates a miniature high-frequency solenoid valve and an infrared temperature sensor, and is configured to spray liquid carbon dioxide onto the weld surface to form a cooling micro-region with a duration of 10ms to 100ms and a local temperature drop gradient of not less than 50℃ / s.

[0015] Furthermore, the low-freezing-point liquid acoustic coupling medium layer is an alcohol-water mixture or a low-viscosity silicone oil with a freezing point below -60°C, and its ultrasonic attenuation rate change within the range of -60°C to 50°C is less than 5%;

[0016] The flexible outer membrane is a polyurethane elastomer, and its acoustic impedance is matched with that of the low-freezing-point liquid acoustic coupling medium layer.

[0017] Furthermore, the spatiotemporal coordinated control system includes a hardware-level time-reversal mirror module, which is configured to perform a time-reversal operation on the received multi-channel broadband scattering clutter signal to generate a reverse excitation sequence, and drive each element of the ultrasonic phased array to perform secondary synchronous excitation with the reverse excitation sequence.

[0018] Furthermore, the ultrasonic phased array is configured to excite critically refracted longitudinal waves propagating on the metal surface, and the spatiotemporal coordinated control system controls the excitation delay time of each array element so that the equivalent incident angle of the critically refracted longitudinal wave beam re-emitted after time reversal is dynamically maintained at the first critical angle on the rough weld surface.

[0019] The second aspect of this application provides a method for detecting residual stress in the welding of multi-curved, extra-large aluminum plates, employing the following technical solution:

[0020] A method for detecting residual stress in the welding of multi-curved, ultra-large aluminum plates, based on the aforementioned system for detecting residual stress in the welding of multi-curved, ultra-large aluminum plates, includes the following steps:

[0021] S1. The follow-up platform drives the cold acoustic coaxial flexible array probe to adhere to and press against the rough weld surface of the ultra-large aluminum plate, so that the flexible outer film adapts to the surface curvature of the target area.

[0022] S2. Spatial adaptive focusing using time-reversal acoustics: emit broadband probe pulses, collect and time-reverse the scattered echoes from the rough surface, and lock the optimal focusing point phase parameters that penetrate the rough surface;

[0023] S3. Perform thermal shock-acoustic combined differential measurement: control the pulsed cooling nozzle to spray low-temperature fluid, causing transient cooling and contraction in the target area; during the relaxation period of natural temperature recovery, use the phase parameters locked in step S2 to perform high-frequency continuous ultrasonic transmission and reception, and simultaneously acquire critical refractive longitudinal wave velocity and local temperature data that change continuously with temperature.

[0024] S4. Nonlinear decoupling calculation: Extract the partial derivative characteristics of sound velocity with respect to temperature, input the preset thermo-acoustic-elastic nonlinear coupling model formula, strip away the background noise caused by the bending grain texture, and calculate and output the absolute welding residual stress value.

[0025] Furthermore, in step S2, the time-reversed spatial adaptive focusing process is independent of temperature changes. The geometric features with fixed weld surface roughness are used as a physical acoustic lens, so that the propagation path of the sound beam before and after the transient thermal shock remains strictly consistent, avoiding sound beam deflection artifacts caused by local thermal deformation.

[0026] Furthermore, in step S4, the preset formula for the thermo-acoustic-elastic nonlinear coupling model is:

[0027] ;

[0028] in, This represents the residual welding stress value. The critical refracted longitudinal wave velocity after focusing. For transient local temperature, The measured rate of change of sound speed versus temperature. The nonlinear acoustoelastic stress constant of the material, The background sound velocity temperature response constant is caused by the pre-calibrated tensile grain texture. This is the system compensation constant.

[0029] Furthermore, in step S3, the repetition frequency of the continuous emission of the ultrasonic pulse is not less than 1000Hz, so as to ensure that no less than 2000 sets of sound velocity-temperature matching data points are collected during the rapid temperature recovery period of less than 2 seconds after the transient thermal shock, which is used to perform high-precision linear fitting of the sound velocity-temperature change rate.

[0030] Furthermore, the servo platform performs step scanning measurements on the surface of the ultra-large aluminum plate according to a preset topological grid, and maps the absolute welding residual stress values ​​calculated at each grid point to a spatial curved surface coordinate system, reconstructing and outputting a three-dimensional residual stress holographic cloud map of the multi-curved ultra-large aluminum plate.

[0031] In summary, the beneficial technical effects of this application are as follows:

[0032] 1. Existing ultrasonic stress testing requires extremely smooth surfaces. However, this application combines a flexible bonding outer membrane with hardware-level time-reversal acoustic technology, treating rough surfaces as "physical acoustic lenses." By reversing time, scattered clutter is flipped and retransmitted, allowing the acoustic energy to automatically correct the phase distortion caused by the rough interface. This enables precise dynamic acoustic focusing within the unpolished weld seam, completely eliminating the cumbersome weld seam polishing process and greatly improving engineering testing efficiency.

[0033] 2. Grain deformation caused by bending and forming and welding residual stress are highly intertwined in conventional sound velocity measurements, which is a recognized problem in the industry. This application adopts "thermal shock-acoustic combined differential measurement", which applies transient cooling through a cold acoustic coaxial probe and utilizes the thermo-acoustic-elastic nonlinear coupling effect to transform the measurement of absolute sound velocity into the measurement of the partial derivative of "sound velocity-temperature change rate". This successfully physically removes the background noise of grain texture in the mathematical model, obtains the true absolute welding residual stress, and achieves high signal-to-noise ratio extraction of weak stress changes.

[0034] 3. By using a specially formulated low-freezing-point coupling medium with a constant acoustic attenuation rate at extremely low temperatures, combined with an ultrasonic emission frequency of not less than 1000Hz, it is ensured that thousands of high-resolution sound velocity-temperature matching points can be captured within a thermal relaxation period of less than 2 seconds after the cold shock. This high-frequency continuous sampling under extreme conditions not only eliminates low-frequency interference from ambient temperature drift, but also makes the nonlinear decoupling calculation have extremely high statistical accuracy and reliability.

[0035] 4. Leveraging the adaptive capabilities of a six-axis robot servo platform to multiple curved surfaces, combined with the dynamic maintenance technology of the first critical angle of the critical refraction longitudinal wave, the system can perform continuous non-destructive scanning of ultra-large aluminum plates and directly generate three-dimensional residual stress holographic cloud maps, providing an unprecedented panoramic visualization evaluation method for welding quality control in the fields of aerospace, high-speed rail and other large equipment manufacturing. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the overall structure of the detection system according to an embodiment of this application;

[0037] Figure 2 This is a cross-sectional view of the cold acoustic coaxial flexible array probe according to an embodiment of this application;

[0038] Figure 3 This is a schematic flowchart of the detection method in an embodiment of this application.

[0039] Explanation of reference numerals in the attached figures:

[0040] 1. Follow-up platform;

[0041] 2. Cold-crystal coaxial flexible array probe; 21. Flexible bonding outer membrane; 22. Low freezing point liquid acoustic coupling medium layer; 23. Pulse-cooled nozzle; 231. Miniature high-frequency solenoid valve; 232. Infrared temperature sensor; 24. Ultrasonic phased array. Detailed Implementation

[0042] The technical solutions of this application will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0043] In the splicing and welding process of large aluminum plates with multiple bends, due to the large size of the components, the complex spatial curvature, and the initial grain texture (anisotropy) introduced by the bending process, traditional residual stress detection methods often suffer from technical bottlenecks such as poor sound beam coupling, large scattered noise, and inability to distinguish between background structural noise and true stress when faced with rough weld surfaces that have not been polished.

[0044] In view of the above, this application is hereby made.

[0045] This application discloses a residual stress detection system for splicing and welding of large aluminum plates with multiple bends. (Refer to...) Figure 1 , Figure 2 and Figure 3 It includes a six-axis robot force control follow-up platform 1, a cold acoustic coaxial flexible array probe 2, and a spatiotemporal collaborative control system.

[0046] Specifically, the six-axis robot force-controlled servo platform 1, as the spatial positioning and attitude adjustment actuator of the entire inspection system, possesses translational degrees of freedom along the X, Y, and Z axes and rotational degrees of freedom around these three axes. This enables high-precision trajectory tracking on the surface of complex-curvature multi-bend aluminum plates. Furthermore, it integrates a force-controlled sensor that can monitor the contact force between the end effector and the workpiece surface in real time and provide feedback to adjust the pressing pressure, ensuring that the constant contact pressure between the probe and the rough weld surface is maintained within the range of 5N to 20N during subsequent inspections. This is a conventional technology, fully achievable by those skilled in the art, and will not be elaborated upon further.

[0047] The cold acoustic coaxial flexible array probe 2 is installed at the end of the follower platform 1 and includes: a flexible outer membrane 21 with acoustic transmissivity, a low freezing point liquid acoustic coupling medium layer 22 filled inside the flexible outer membrane 21, an ultrasonic phased array 24 immersed in the low freezing point liquid acoustic coupling medium layer 22, and a pulsed cold excitation nozzle 23 that penetrates the center of the ultrasonic phased array 24 and extends to the surface of the flexible outer membrane 21.

[0048] Traditional rigid probes or conventional water immersion probes cannot effectively adhere to rough and multi-curved weld surfaces. In this embodiment, the flexible outer membrane 21 is made of polyurethane elastomer, and its acoustic impedance matches that of the low-freezing-point liquid acoustic coupling medium layer 22. Thus, when pressed against the rough weld, the flexible outer membrane 21 can undergo elastic deformation and perfectly fill the microscopic unevenness of the rough surface, forming a continuous acoustic transmission channel, which greatly reduces the sound wave reflection and scattering loss at the interface.

[0049] Based on this, to achieve coaxial in-situ excitation of thermal shock and acoustic detection, the end of the pulsed cooling nozzle 23 is flush with and sealed to the inner surface of the flexible outer membrane 21. The pulsed cooling nozzle 23 integrates a miniature high-frequency solenoid valve 231 and an infrared temperature sensor 232, and is configured to instantaneously spray liquid carbon dioxide onto the weld surface, forming a cooling micro-region with a duration of 10ms to 100ms and a local temperature drop gradient of no less than 50℃ / s. Simultaneously, a low-freezing-point liquid acoustic coupling medium layer 22, made of an alcohol-water mixture or low-viscosity silicone oil with a freezing point below -60℃, is pressurized and filled into the cavity between the flexible outer membrane 21 and the pulsed cooling nozzle 23, and its ultrasonic attenuation rate change within the range of -60℃ to 50℃ is less than 5%. Therefore, even under transient cooling conditions of extremely low local temperatures, the coupling medium can maintain stable acoustic transmission characteristics without freezing or sudden changes in acoustic attenuation, ensuring continuous high-fidelity acquisition of acoustic signals during the subsequent extremely short temperature relaxation period.

[0050] As for the spatiotemporal collaborative control system, which is communicatively connected to its cold acoustic coaxial flexible array probe 2, it is configured to: apply transient thermal shock to the unpolished rough weld surface through the pulsed cold shock nozzle 23; during the temperature recovery period after the transient thermal shock, drive the ultrasonic phased array 24 to emit detection sound waves towards the weld, collect the scattered echo signal from the rough surface, perform time reversal processing and re-emit, so as to form dynamic acoustic focusing in the target stress area; at the same time, collect the transient sound velocity change rate in the focusing area in real time, and calculate and output the real welding residual stress value after eliminating the influence of tension and bending anisotropy based on the thermo-acoustic-elastic nonlinear coupling model.

[0051] Furthermore, to address the problem of severe sound beam distortion caused by rough surfaces, the spatiotemporal collaborative control system also includes a hardware-level time-reversal mirror module. This module is configured to flip the received multi-channel broadband scattered clutter signal along the time axis, generating a reversed excitation sequence, and driving each element of the ultrasonic phased array 24 to undergo secondary synchronous excitation using this reversed excitation sequence. The physical mechanism of time-reversal acoustics lies in its ability to treat the fixed geometric roughness of the weld surface as a natural "physical acoustic lens." No matter how rugged the surface, the sound waves, after time reversal, will propagate backward along the original path, thereby achieving spatial adaptive focusing of energy within the target detection area and completely overcoming the defocusing and artifact problems caused by surface roughness.

[0052] Meanwhile, to specifically detect welding residual stress on the surface and subsurface of the aluminum plate, the ultrasonic phased array 24 is configured to excite critically refracted longitudinal waves (LCR waves) propagating on the metal surface. However, due to the curvature of the aluminum plate and local micro-deformation caused by thermal shock, conventional fixed-angle emission often deviates from the critical angle. Therefore, in this embodiment, the spatiotemporal coordinated control system controls the excitation delay time of each array element, combined with a time-reversal algorithm, to ensure that the equivalent incident angle of the re-emitted critically refracted longitudinal wave beam on the rough weld surface is always dynamically maintained at the first critical angle, ensuring maximum coupling of LCR wave energy. Specifically, for the aluminum plate used in this embodiment, the first critical angle can be selected as 14.5° to 15.5°.

[0053] In one specific embodiment, after the flexible outer membrane 21 is bonded to the aluminum plate weld seam, the spatiotemporal collaborative control system first controls the pulsed cooling nozzle 23 to apply a transient thermal shock to the rough, unpolished weld seam surface, i.e., spraying liquid carbon dioxide to form a cooling shock. During the natural temperature recovery period after the transient thermal shock, the spatiotemporal collaborative control system then drives the ultrasonic phased array 24 to emit probe sound waves towards the weld seam. Since the weld seam surface is rough, it receives complex scattered echoes. The spatiotemporal collaborative control system collects these scattered echo signals from the rough surface and performs time-reversal processing on them by the hardware-level time-reversal mirror module (i.e., flipping the received multi-channel broadband scattered clutter signals on the time axis), generating a reversed excitation sequence. Subsequently, the spatiotemporal collaborative control system drives each element of the ultrasonic phased array 24 to perform secondary synchronous excitation with this reversed excitation sequence, thereby forming a dynamic acoustic focus within the target stress zone. This process uses the rough surface morphology as a "physical acoustic lens," allowing the sound beam to adaptively penetrate the rough layer and form a focus inside.

[0054] After achieving stable acoustic focusing, the spatiotemporal coordinated control system drives the ultrasonic phased array 24 to perform high-frequency continuous ultrasonic transmission and reception at an extremely high repetition frequency (e.g., not less than 1000Hz) during the temperature recovery period, simultaneously acquiring the transient rate of change of sound velocity within the focusing area and the local temperature data collected by the infrared temperature sensor 232. Finally, based on the built-in thermo-acoustic-elastic nonlinear coupling model, the spatiotemporal coordinated control system calculates the above measurement data and outputs the true residual welding stress value after eliminating the influence of bending anisotropy.

[0055] Thus, by combining the servo platform 1 with a flexible probe containing a built-in low-freezing-point liquid acoustic coupling medium layer 22, adaptive bonding and stable acoustic coupling between the probe and the multi-curvature, rough surface of the ultra-large aluminum plate are achieved. Moreover, by utilizing time-reversal acoustic technology, the rough weld surface is used as a "physical acoustic lens," enabling adaptive spatial focusing of the sound beam on the internal target area without grinding. This ensures stable excitation and reception of signals under complex morphologies, effectively improving the detection challenges of rough surfaces and complex curved surfaces in existing technologies.

[0056] Furthermore, traditional ultrasonic testing methods are highly susceptible to the influence of grain texture (anisotropy) generated during the bending and forming process of metallic materials, leading to deviations in stress measurement values. This application, however, utilizes a "thermal shock-acoustic joint differential measurement" mechanism. By creating localized thermal disturbances through transient cooling and combining it with high-frequency acoustic sampling, it accurately measures the sound velocity response under rapid temperature changes. Coupled with a thermo-acoustic-elastic nonlinear coupling model incorporating a built-in background noise constant, it successfully isolates the background noise caused by grain texture from the complex acoustic response, achieving high-precision, interference-resistant measurement of absolute welding residual stress values.

[0057] This application discloses a method for detecting residual stress in the welding of multi-bend ultra-large aluminum plates, based on the aforementioned system for detecting residual stress in the welding of multi-bend ultra-large aluminum plates, with reference to... Figure 1 , Figure 2 and Figure 3 It includes the following steps:

[0058] S1. The follow-up platform 1 drives the cold acoustic coaxial flexible array probe 2 to adhere to and press against the rough weld surface of the ultra-large aluminum plate, causing the flexible bonding outer film 21 to undergo elastic deformation, adapting to the surface curvature and micro-roughness of the target area, and expelling the air from the interface.

[0059] S2. Spatial Adaptive Focusing Using Time-Reversal Acoustics: An ultrasonic phased array 24 emits broadband probe pulses to acquire multipath scattered echo signals after passing through a rough surface. A hardware-level time-reversal mirror module reverses the time of the echoes, locking the optimal focusing point phase parameters for penetrating the specific rough surface. This process is independent of temperature changes, utilizing only the geometric features of the weld surface as a physical acoustic lens, ensuring that the propagation path of the sound beam remains strictly consistent before and after subsequent transient thermal shocks, avoiding beam deflection artifacts caused by local thermal deformation.

[0060] S3. Perform thermal shock-acoustic joint differential measurement: Control the pulsed cooling nozzle 23 to precisely spray liquid carbon dioxide, causing transient cooling and contraction in the targeted micro-region (temperature drop gradient ≥ 50℃ / s); during the short relaxation period of natural temperature recovery, immediately use the phase parameters locked in step S2 to drive the ultrasonic phased array 24 to perform high-frequency continuous ultrasonic transmission and reception. In particular, due to the extremely fast thermal recovery process, the continuous transmission repetition frequency of the ultrasonic pulse is not less than 1000Hz, to ensure that no less than 2000 sets of sound velocity-temperature matching data points are collected during the rapid temperature recovery period of less than 2 seconds after the transient thermal shock. This large number of high-frequency dense data points are used to perform high-precision linear fitting of the sound velocity-temperature change rate, which greatly suppresses the interference of random electrical noise.

[0061] S4. Nonlinear decoupling calculation: Extract the partial derivative features of sound velocity with respect to temperature (i.e., the rate of change of sound velocity with temperature), input the preset thermo-acoustic-elastic nonlinear coupling model formula, remove the background noise caused by the bending grain texture, calculate and output the absolute welding residual stress value.

[0062] Specifically, in step S4, the preset formula for the thermo-acoustic-elastic nonlinear coupling model is:

[0063] ;

[0064] in, This represents the residual welding stress value. The critical refracted longitudinal wave velocity after focusing. The transient local temperature is measured in real time by the infrared temperature sensor 232. To obtain a high-precision sound velocity-temperature change rate. The nonlinear acoustoelastic stress constant of the material, The background sound velocity temperature response constant is caused by the pre-calibrated tensile grain texture. This is the system compensation constant.

[0065] The decoupling principle of this formula lies in the fact that traditional absolute sound velocity measurements cannot distinguish between sound velocity changes caused by grain texture (anisotropy) and those caused by stress. This application, however, introduces transient thermal shock and utilizes the nonlinear cross-coupling effect of the stress and temperature fields in sound wave propagation (i.e., the influence of stress on temperature sensitivity differs from the influence of grain texture on temperature sensitivity). By calculating the partial derivative of sound velocity with respect to temperature using differential methods, the fixed influence term representing the initial grain texture is successfully removed, thus obtaining the pure residual stress. .

[0066] Finally, the follow-up platform 1 performs step scanning measurement on the surface of the super-large aluminum plate according to the preset topological grid, repeats the above steps in a loop, and maps the absolute welding residual stress value calculated by each grid point to a spatial curved surface coordinate system, reconstructs and outputs a three-dimensional residual stress holographic cloud map of the multi-curved super-large aluminum plate, and intuitively guides the subsequent stress relief process.

[0067] In a specific example, the 2024-T3 aluminum alloy sheet commonly used for fuselage skin was inspected. The sheet thickness was 5mm. The aluminum sheet was pre-stretched and bent and then spliced ​​using friction stir welding. The value is -0.0264 (m / s) / MPa. The value is -0.6852 (m / s) / °C. Set it to 0. After calculation using the above method, the least squares method was used to perform linear fitting on the 2400 collected data points, and the final calculated result is: A positive value indicates tensile stress.

[0068] At the same test point (after grinding), the true values ​​were compared using the recognized blind hole method (destructive testing). The weld residual stress value detected by the blind hole method was 189.2 MPa, with the error controlled within 2%; while the value detected by the traditional sonic impact method reached 312.5 MPa.

[0069] Unless otherwise defined, the technical or scientific terms used in this application shall have the ordinary meaning understood by one of ordinary skill in the art to which this application pertains. The terms "first," "second," "third," and similar terms used in this application specification and claims do not indicate any order, quantity, or importance, but are merely used to distinguish different components. The terms "a" or "an," and similar terms do not indicate a quantity limitation, but rather indicate the presence of at least one. The terms "comprising," "including," and similar terms mean that the elements or objects preceding "comprising" encompass the elements or objects listed following "comprising" or "including," and their equivalents, but do not exclude other elements or objects. "Above," "below," "left," "right," etc., are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0070] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A residual stress detection system for splicing and welding of extra-large aluminum plates with multiple bends, characterized in that, include: Six-axis robot force control servo platform; A cold-acoustic coaxial flexible array probe, installed at the end of the servo platform, includes: a flexible outer membrane with acoustic transmissivity, a low-freezing-point liquid acoustic coupling medium layer filled inside the flexible outer membrane, an ultrasonic phased array immersed in the low-freezing-point liquid acoustic coupling medium layer, and a pulsed cold-excitation nozzle penetrating the center of the ultrasonic phased array and extending to the surface of the flexible outer membrane. The spatiotemporal collaborative control system, communicatively connected to the cold acoustic coaxial flexible array probe, is configured to: apply transient thermal shock to the unpolished rough weld surface through the pulsed cold-shock nozzle; during the temperature recovery period after the transient thermal shock, drive the ultrasonic phased array to emit probe sound waves towards the weld, collect the scattered echo signal from the rough surface, perform time reversal processing, and re-emit to form dynamic acoustic focusing in the target stress zone; simultaneously, collect the transient sound velocity change rate in the focusing zone in real time, calculate and output the true welding residual stress value after eliminating the influence of tension-bending anisotropy based on the thermo-acoustic-elastic nonlinear coupling model.

2. The residual stress detection system for splicing and welding of extra-large aluminum plates with multiple bends and tensions according to claim 1, characterized in that, The end of the pulsed cooling nozzle is flush with and sealed to the inner surface of the flexible outer membrane. The pulsed cooling nozzle integrates a miniature high-frequency solenoid valve and an infrared temperature sensor, and is configured to spray liquid carbon dioxide onto the weld surface to form a cooling micro-region with a duration of 10ms to 100ms and a local temperature drop gradient of not less than 50℃ / s.

3. The residual stress detection system for splicing and welding of large aluminum plates with multiple bends and tensions according to claim 1, characterized in that, The low-freezing-point liquid acoustic coupling medium layer is an alcohol-water mixture or low-viscosity silicone oil with a freezing point below -60°C, and its ultrasonic attenuation rate change in the range of -60°C to 50°C is less than 5%. The flexible outer membrane is a polyurethane elastomer, and its acoustic impedance is matched with that of the low-freezing-point liquid acoustic coupling medium layer.

4. The residual stress detection system for splicing and welding of large aluminum plates with multiple bends and tensions according to claim 1, characterized in that, The spatiotemporal coordinated control system includes a hardware-level time-reversal mirror module. The time-reversal mirror module is configured to perform a time-reversal operation on the received multi-channel broadband scattered clutter signal to generate a reverse excitation sequence, and drive each element of the ultrasonic phased array to perform secondary synchronous excitation with the reverse excitation sequence.

5. The residual stress detection system for splicing and welding of large aluminum plates with multiple bends and tensions according to claim 4, characterized in that, The ultrasonic phased array is configured to excite critically refracted longitudinal waves propagating on the metal surface. The spatiotemporal coordinated control system controls the excitation delay time of each array element so that the equivalent incident angle of the critically refracted longitudinal wave beam re-emitted after time reversal is dynamically maintained at the first critical angle on the rough weld surface.

6. A method for detecting residual stress in the welding of multi-curved, ultra-large aluminum plates, based on the residual stress detection system for multi-curved, ultra-large aluminum plates as described in any one of claims 1-5, characterized in that... Includes the following steps: S1. The follow-up platform drives the cold acoustic coaxial flexible array probe to adhere to and press against the rough weld surface of the ultra-large aluminum plate, so that the flexible outer film adapts to the surface curvature of the target area. S2. Spatial adaptive focusing using time-reversal acoustics: emit broadband probe pulses, collect and time-reverse the scattered echoes from the rough surface, and lock the optimal focusing point phase parameters that penetrate the rough surface; S3. Perform thermal shock-acoustic combined differential measurement: control the pulsed cooling nozzle to spray low-temperature fluid, causing transient cooling and contraction in the target area; during the relaxation period of natural temperature recovery, use the phase parameters locked in step S2 to perform high-frequency continuous ultrasonic transmission and reception, and simultaneously acquire critical refractive longitudinal wave velocity and local temperature data that change continuously with temperature. S4. Nonlinear decoupling calculation: Extract the partial derivative characteristics of sound velocity with respect to temperature, input the preset thermo-acoustic-elastic nonlinear coupling model formula, strip away the background noise caused by the bending grain texture, and calculate and output the absolute welding residual stress value.

7. The method for detecting residual stress in the splicing and welding of large aluminum plates with multiple bends and tensions according to claim 6, characterized in that, In step S2, the time-reversed spatial adaptive focusing process is independent of temperature changes. The geometric features with fixed weld surface roughness are used as a physical acoustic lens, so that the propagation path of the sound beam before and after the transient thermal shock remains strictly consistent, avoiding sound beam deflection artifacts caused by local thermal deformation.

8. The method for detecting residual stress in the splicing and welding of large aluminum plates with multiple bends and tensions according to claim 6, characterized in that, In step S4, the preset formula for the thermo-acoustic-elastic nonlinear coupling model is: ； in, This represents the residual welding stress value. The critical refracted longitudinal wave velocity after focusing. For transient local temperature, The measured rate of change of sound speed versus temperature. The nonlinear acoustoelastic stress constant of the material, The background sound velocity temperature response constant is caused by the pre-calibrated tensile grain texture. This is the system compensation constant.

9. The method for detecting residual stress in the splicing and welding of large aluminum plates with multiple bends and tensions according to claim 8, characterized in that, In step S3, the continuous emission repetition frequency of the ultrasonic pulse is not less than 1000Hz to ensure that no less than 2000 sets of sound velocity-temperature matching data points are collected during the rapid temperature recovery period of less than 2 seconds after the transient thermal shock, which is used to perform high-precision linear fitting of the sound velocity-temperature change rate.

10. The method for detecting residual stress in the splicing and welding of large aluminum plates with multiple bends and tensions according to claim 6, characterized in that, The servo platform performs step scanning measurements on the surface of the super-large aluminum plate according to a preset topological grid, and maps the absolute welding residual stress values ​​calculated at each grid point to a spatial curved surface coordinate system, reconstructing and outputting a three-dimensional residual stress holographic cloud map of the multi-curved super-large aluminum plate.

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