Metal part defect detection system and detection method based on photoacoustic and laser scanning

Abstract

The invention discloses a metal part defect detection system and detection method based on photoacoustic and laser scanning, which are applied to detection of a metal material to be detected, and comprise a signal detection and synchronous triggering system, a data acquisition device, a processing unit, an ultrasonic probe and a camera, the data acquisition device is respectively connected with the signal detection and synchronous triggering system, the ultrasonic probe and the processing unit, and the processing unit is connected with the camera; the system further comprises a blue light linear polarization laser and a green light pulse laser. The blue light linear polarization laser is provided with a first beam expanding system and an orthogonal polarization beam generation module along the transmission direction; the green light pulse laser is provided with a second beam expanding system along the transmission direction; a transflective mirror, a one-dimensional laser galvanometer and a rotary table are sequentially arranged in the transmission direction of the orthogonal polarized light beam generation module and the second beam expanding system. According to the invention, the internal structure and surface defects of the metal material can be comprehensively and accurately detected, and the sensitivity and adaptability of defect detection are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material defect detection, and in particular to a metal part defect detection system and a detection method based on photoacoustic and laser scanning. Background Art

[0002] Bearings play an important role in supporting and rotating mechanical equipment. Defects on the roller surface and inside the bearings can seriously affect the stability and life of the equipment. Currently, bearing defect detection technologies include conventional acoustic detection, vibration analysis, and visual detection. However, these technologies have limitations in practical applications, such as insufficient sensitivity, inability to achieve real-time online monitoring, and susceptibility to interference from environmental noise. In some technical solutions, a combination of half-wave plates and quarter-wave plates is used to achieve high-precision signal detection through beam separation and synthesis. However, these existing technologies also have certain shortcomings, such as sensitivity to external interference, high system complexity, and reliance on a large number of optical component configurations.

[0003] The technical solution closest to the present invention in the existing technology is a multimodal nondestructive testing system technology, whose design goal is to achieve efficient detection of defects in metal materials. In order to achieve this goal, the system integrates three signals: optical detection, photoacoustic detection, and ultrasonic detection, and can effectively diagnose different types of defects (such as cracks, holes, peeling, etc.). The system uses a multimodal detection unit composed of an area array camera, an Nd-YAG laser, an ultrasonic probe, etc. The optical image is transmitted to the PC for processing through the USB interface, the photoacoustic signal and the ultrasonic signal are synchronously collected through the data acquisition card, and then the laser scanning is realized by controlling the XY motion platform to finally complete the detection and identification of defects. However, this technology has the following problems:

[0004] 1. Insufficient sensitivity: Laser interferometry relies on phase changes in the signal to identify defects, but tiny surface defects may not cause significant phase changes. Therefore, local changes in light intensity may be masked by noise, resulting in ineffective detection of subtle defects.

[0005] 2. Poor real-time processing capabilities: Traditional signal processing methods (such as fast Fourier transforms) take a long time to process data and lack real-time performance. As a result, the system cannot achieve real-time monitoring in high-speed production environments, increasing the risk of potential defects going undetected.

[0006] 3. Significant environmental interference: External environmental factors (such as temperature changes, vibrations, and electromagnetic interference) can affect the propagation and interference effects of laser signals. As a result, the stability and detection accuracy of the system are reduced, which may lead to erroneous judgments and missed detections. Summary of the Invention

[0007] The purpose of the present invention is to provide a metal part defect detection system and method based on photoacoustic and laser scanning. The present invention can comprehensively and accurately detect the internal structure and surface defects of metal materials, thereby improving the sensitivity and adaptability of defect detection.

[0008] The technical solution of the present invention is: a metal part defect detection system based on photoacoustic and laser scanning, which is used to detect metal materials to be tested, including a signal detection and synchronous triggering system, a data acquisition device, a processing unit, an ultrasonic probe and a camera; the data acquisition device is respectively connected to the signal detection and synchronous triggering system, the ultrasonic probe and the processing unit, and the processing unit is connected to the camera; it also includes a blue light polarized laser and a green light pulse laser; the blue light polarized laser is provided with a first beam expansion system and an orthogonal polarized light beam generating module along the transmission direction; the green light pulse laser is provided with a second beam expansion system along the transmission direction; the orthogonal polarized light beam generating module and the second beam expansion system are sequentially arranged in the transmission direction with a reflective mirror, a one-dimensional laser galvanometer and a turntable, and the turntable is used to place the metal material to be tested; the ultrasonic probe and the camera are facing the turntable for receiving signals.

[0009] In the aforementioned metal part defect detection system based on photoacoustic and laser scanning, a laser speckle reducer and a half-wave plate are sequentially provided between the blue light polarized laser and the first beam expansion system along the transmission direction.

[0010] The aforementioned metal part defect detection system based on photoacoustic and laser scanning, the orthogonal polarized beam generating module includes a polarization beam splitter, a first reflector, a second reflector and a quarter wave plate; the laser light emitted by the blue light polarized laser is divided into a reflected beam and a polarized beam by the polarization beam splitter, the polarized beam is polarized by the quarter wave plate and then reflected by the second reflector, returns to the polarization beam splitter, and then synthesizes an orthogonal polarized beam with the reflected beam.

[0011] In the aforementioned metal part defect detection system based on photoacoustic and laser scanning, a green light cutoff filter is embedded in the camera to block green light.

[0012] In the aforementioned metal part defect detection system based on photoacoustic and laser scanning, a blue light polarized laser emits laser light to an orthogonal polarized beam generating module. The orthogonal polarized beam generating module synthesizes the laser light into orthogonal polarized beams after polarization beam splitting. The orthogonal polarized beams are reflected by a reflective mirror and then irradiated by a one-dimensional laser galvanometer onto the metal material to be tested on the turntable. The optical signal reflected by the metal material to be tested is received by a camera and transmitted to a processing unit. At the same time, the camera captures an image of the metal material to be tested and transmits it to the processing unit.

[0013] The laser emitted by the green pulse laser is expanded by the second beam expansion system, and then transmitted through the reflective mirror to hit the surface of the metal material to be tested to generate a photoacoustic signal. The photoacoustic signal is received by the ultrasonic probe, and then collected by the data acquisition device and transmitted to the processing unit;

[0014] The ultrasonic probe transmits ultrasonic waves to the metal material to be tested. The ultrasonic signal reflected by the metal material to be tested is received by the ultrasonic probe, collected by the data acquisition device, and then transmitted to the processing unit;

[0015] The processing unit obtains the position and size of the surface defects of the metal material to be tested based on the optical signal, obtains the position, size and depth of the shallow surface defects of the metal material to be tested based on the photoacoustic signal, and obtains the position, size and depth of the internal defects of the metal material to be tested based on the ultrasonic signal, and then combines the image reconstruction of the metal material to be tested to obtain a two-dimensional image of the surface and interior of the metal to be tested and a three-dimensional image of the surface of the metal to be tested.

[0016] In the aforementioned detection method of the metal part defect detection system based on photoacoustic and laser scanning, the specific process by which the processing unit obtains the position and size of the surface defect of the metal material to be tested based on the optical signal is to smooth the image through Gaussian filtering, then process the image through an edge extraction algorithm to locate the contour edge, and then calculate the image gradient to locate the defect edge strength and direction;

[0017] The formula for smoothing the image by Gaussian filtering is:

[0018] I(x,y)=G(x,y)*f(x,y);

[0019] Where I(x,y) represents the obtained smoothed image, G(x,y) is the Gaussian kernel function, and f(x,y) is the optical signal reflected by the metal material to be tested and received by the camera;

[0020] The gradient calculation formula is:

[0021]

[0022] Among them, M(i,j) represents the image gradient and θ(i,j) represents the gradient direction.

[0023] In the aforementioned detection method of the metal part defect detection system based on photoacoustic and laser scanning, the photoacoustic signal generation process is that the laser is irradiated on the surface of the metal material to be tested, so that part of the laser energy is absorbed, resulting in a local temperature increase of the metal material to be tested, thereby causing thermal expansion of the metal material to be tested, generating an elastic stress field, and then stimulating the photoacoustic signal;

[0024] The laser has a certain power density I0, and the laser energy density A absorbed by the metal material to be measured is expressed as:

[0025] A(x,y,z,t)=I0αf(x,y,z)g(t);

[0026] Where I0 is the central intensity of the laser; α is the laser absorption coefficient of the metal material to be measured; f(x, y, z) is the spatial distribution function of the laser energy; and g(t) is the temporal distribution function of the laser pulse.

[0027] The aforementioned detection method of the metal part defect detection system based on photoacoustic and laser scanning, the photoacoustic signal includes body wave and surface wave, the velocity of body wave is divided into longitudinal wave velocity L c and shear wave velocity T c , the calculation formula is:

[0028]

[0029] Where E is the elastic modulus of the metal material to be tested, ρ is the density of the metal material to be tested, and σ is the Poisson's ratio;

[0030] The velocity of the surface wave R c The calculation formula is:

[0031]

[0032] In the aforementioned detection method of the metal part defect detection system based on photoacoustic and laser scanning, the solution model of the photoacoustic signal is as follows:

[0033]

[0034] in, represents the photoacoustic signal, ρ is the material density, C is the specific heat capacity, k is the thermal conductivity, T is the temperature, Q is the heat source term, u is the displacement, ξ is the stress tensor, and f is the body force.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] This invention utilizes a blue polarized laser and a green pulsed laser, utilizing lasers with specific polarization characteristics to analyze signals through optical interferometry, offering the advantages of high sensitivity and high resolution. Furthermore, by combining optical, photoacoustic, and ultrasonic detection methods, the invention can comprehensively inspect metal materials from different angles and layers, providing more accurate and reliable detection information and enhancing the accuracy and comprehensiveness of detection. Furthermore, by utilizing nondestructive testing technology, the system can detect internal and surface defects without destroying the material, ensuring its integrity and safety. The invention also records ultrasonic signals and camera image data in real time, ensuring data synchronization and accuracy, enabling efficient and accurate material testing. This makes it particularly suitable for nondestructive testing applications in complex and harsh environments. Furthermore, the camera's high-resolution imaging capability effectively captures minute defects, while the ultrasonic probe can detect deep defects such as cracks and pores within the material that are invisible to optical imaging. The orthogonal polarization beam module generates orthogonal polarization beams through a combination of a quarter-wave plate and a polarization beam splitter, further enhancing the detection capabilities of metal surface defects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the device of the present invention. DETAILED DESCRIPTION

[0038] The present invention will be further described below with reference to the accompanying drawings and examples, but they are not intended to limit the present invention.

[0039] Example 1: A metal part defect detection system based on photoacoustic and laser scanning, such as Figure 1 As shown, in this embodiment, the defects of the bearing rollers are detected, including a signal detection and synchronous triggering system, a data acquisition device, a processing unit, an ultrasonic probe and a camera; the data acquisition device is respectively connected to the signal detection and synchronous triggering system, the ultrasonic probe and the processing unit, and the processing unit is connected to the camera. The data acquisition device receives the trigger signal from the signal detection and synchronous triggering system and the ultrasonic signal from the ultrasonic probe, and transmits the signal to the processing unit (computer); after the processing unit synchronously processes the signal, it sends the control instruction to the camera to realize image acquisition and data storage. The system also includes a blue light polarized laser and a green light pulse laser; the blue light polarized laser is provided with a first beam expansion system and an orthogonal polarized light beam generating module along the transmission direction; the green light pulse laser is provided with a second beam expansion system along the transmission direction; the orthogonal polarized light beam generating module and the second beam expansion system are sequentially provided with a reflective mirror, a one-dimensional laser galvanometer and a turntable in the transmission direction, and the turntable is used to place the metal material to be tested; the ultrasonic probe and the camera are directed towards the turntable to receive signals.

[0040] In this embodiment, the blue light polarization laser uses a Thorlabs SLPB-405-100-SH blue light polarization laser, which is used to emit blue light with a wavelength of 405nm to the orthogonal polarization beam generation module. The 405nm blue light has high sensitivity to metal surface defects (such as scratches and oxide layers). The orthogonal polarization beam generation module is used to synthesize orthogonal polarization beams after polarization beam splitting of the laser. The orthogonal polarization beams are reflected by the reflective mirror and vertically incident on the surface of the metal material to be tested, and surface defects are detected by polarization characteristic differences. The green light pulse laser uses a green light fiber laser with a model number of NLR-532-1000-SMA, which is used to emit 532nm green light. The 532nm wavelength has a high absorption rate for metal materials. After being transmitted by the reflective mirror, the green light is spatially separated from the orthogonal polarization beam and hits the surface of the bearing roller to independently stimulate the photoacoustic effect. Compared with 405nm blue light, green light penetrates deeper. Green light can efficiently excite photoacoustic signals (especially shallow surface defects) and can be combined with ultrasonic signals to detect shallow surface defects. The first beam expansion system and the second beam expansion system use a Thorlabs BE05M-B beam expander with a wavelength range of 350-1100nm, which is compatible with blue light 405nm and green light 532nm. At the same time, it enhances the intensity of the laser beam to ensure that it produces a strong photoacoustic effect on the surface of the bearing roller. The one-dimensional laser galvanometer is of the Cambridge Technology CT6210H model, which is used for the rapid movement and positioning of green light and orthogonal polarized light beams. A green light cutoff filter is embedded in the camera to block green light. The ultrasonic probe is of the Olympus V156-RM model, which matches the frequency range of the photoacoustic signal excited by the green light pulse laser (532nm). It has the characteristics of high sensitivity and deep penetration, and is suitable for detecting internal defects of bearing rollers (such as inclusions and looseness). The ultrasonic probe detects superficial defects on the bearing roller surface by receiving photoacoustic signals generated by the photoacoustic effect, while the ultrasonic probe detects internal defects by emitting ultrasonic waves. The camera uses a FLIR Chameleon 3 CMOS 5MP high-speed industrial camera, which supports the installation of an external green filter and is compatible with 405nm blue light detection. It provides high-resolution imaging capabilities in both three-dimensional spatial information and time dimensions. The camera simultaneously captures the optical signal and image reflected from the bearing roller surface, while the data acquisition device records the ultrasonic and photoacoustic signals. A signal detection and synchronous triggering system ensures data synchronization and accuracy.

[0041] Specifically, a laser speckle reducer and a half-wave plate are sequentially provided between the blue light polarization laser and the first beam expansion system along the transmission direction. The laser speckle reducer adopts the model of Thorlabs SPF10-405-SH, which reduces spatial noise and improves the spatial uniformity of the laser beam through dynamic speckle suppression technology. The half-wave plate is used to adjust the polarization direction of the laser to ensure the polarization matching of the subsequent optical path. The reflector adopts the model of Edmund Optics-Geo-AlignPolarizing Beam Splitter 87-545-RG, which has a spectral width of 400-700nm, can transmit 532nm green light and reflect the orthogonal polarized beam after the 405nm blue light polarization splitting synthesis.

[0042] Specifically, the orthogonal polarized beam generation module includes a polarization beam splitter (PBS), a first reflector, a second reflector, and a quarter-wave plate. The quarter-wave plate is used to convert the polarized beam into circularly polarized light to achieve orthogonal polarization synthesis. The laser light emitted by the blue light polarization laser is split into a reflected beam and a polarized beam by the polarization beam splitter. The polarized beam is adjusted by the quarter-wave plate, then reflected by the second reflector and synthesized with the reflected beam to form an orthogonal polarized beam.

[0043] Example 2: Based on the detection method of the metal part defect detection system based on photoacoustic and laser scanning in Example 1, a blue light polarized laser emits laser to an orthogonal polarized beam generating module, and the orthogonal polarized beam generating module synthesizes the laser into an orthogonal polarized beam after polarization splitting. The orthogonal polarized beam is reflected by a reflective mirror and then irradiated to the metal material to be tested on the turntable through a one-dimensional laser galvanometer. The optical signal reflected by the metal material to be tested is received by the camera and transmitted to the processing unit. At the same time, the camera takes an image of the metal material to be tested and transmits it to the processing unit. The optical signal is the reflected light signal of the blue light polarized laser after being modulated by the orthogonal polarized beam generating module.

[0044] The laser emitted by the green pulse laser is expanded by the second beam expansion system, then transmitted through the reflective mirror to hit the surface of the metal material to be tested to generate a photoacoustic signal. The photoacoustic signal is received by the ultrasonic probe, and then collected by the data acquisition device and transmitted to the processing unit.

[0045] The ultrasonic probe transmits ultrasonic waves to the metal material to be tested. The ultrasonic signal reflected by the metal material to be tested is received by the ultrasonic probe, then collected by the data acquisition device and transmitted to the processing unit. The ultrasonic signal is a mechanical wave signal actively emitted and received by the ultrasonic probe.

[0046] The processing unit obtains the position and size of the surface defects of the metal material to be tested based on the optical signal, obtains the position, size and depth of the shallow surface defects of the metal material to be tested based on the photoacoustic signal, and obtains the position, size and depth of the internal defects of the metal material to be tested based on the ultrasonic signal, and then combines the image reconstruction of the metal material to be tested to obtain a two-dimensional image of the surface and interior of the metal to be tested and a three-dimensional image of the surface of the metal to be tested.

[0047] The specific process of the processing unit obtaining the position and size of the surface defect of the metal material to be tested according to the optical signal is to smooth the image through Gaussian filtering, then process the image through edge extraction algorithm to locate the contour edge, and then calculate the image gradient to locate the edge strength and direction of the defect;

[0048] The formula for smoothing the image by Gaussian filtering is:

[0049] I(x,y)=G(x,y)*f(x,y);

[0050] Where I(x,y) represents the obtained smoothed image, G(x,y) is the Gaussian kernel function, and f(x,y) is the optical signal reflected by the metal material to be tested and received by the camera;

[0051] The gradient calculation formula is:

[0052]

[0053] Among them, M(i,j) represents the image gradient and θ(i,j) represents the gradient direction.

[0054] The photoacoustic signal is generated due to the photoacoustic effect: when laser light is irradiated on the surface of the metal material being tested, some of the laser energy is absorbed, causing the local temperature of the metal material to rise. This in turn causes thermal expansion of the metal material, generating an elastic stress field, which in turn stimulates the photoacoustic signal. The laser power density is I0, and the laser energy density A absorbed by the metal material is expressed as:

[0055] A(x,y,z,t)=I0αf(x,y,z)g(t);

[0056] Where I0 is the central intensity of the laser; α is the laser absorption coefficient of the metal material to be measured; f(x, y, z) is the spatial distribution function of the laser energy; and g(t) is the temporal distribution function of the laser pulse.

[0057] Photoacoustic signals are acoustic wave signals generated by the laser-induced thermoelastic effect. There are two types of photoacoustic signals:

[0058] (1) Body wave: longitudinal wave (speed L c ) and shear wave (speed T c ), satisfying the elastic mechanics equation; the longitudinal wave velocity Lc and shear wave velocity T c The calculation formula is:

[0059]

[0060] Where E is the elastic modulus of the material, ρ is the material density, and σ is Poisson's ratio.

[0061] (2) Surface wave: Rayleigh wave (speed R c ), used to characterize near-surface defects, whose velocity R c The calculation formula is:

[0062]

[0063] Thermoelastic mechanism: involves solving the temperature field equation and the stress field equation simultaneously to obtain the solution of the photoacoustic signal generated by the photoacoustic effect.

[0064] Temperature field equation:

[0065] Stress field equation:

[0066] Where ρ is the material density, C is the specific heat capacity, k is the thermal conductivity, T is the temperature, Q is the heat source term, u is the displacement, ξ is the stress tensor, and f is the body force.

[0067] This invention utilizes a blue polarized laser and a green pulsed laser, utilizing lasers with specific polarization characteristics to analyze signals through optical interferometry, offering the advantages of high sensitivity and high resolution. Furthermore, by combining optical, photoacoustic, and ultrasonic detection methods, the invention can comprehensively inspect metal materials from different angles and layers, providing more accurate and reliable detection information and enhancing the accuracy and comprehensiveness of detection. Furthermore, by utilizing nondestructive testing technology, the system can detect internal and surface defects without destroying the material, ensuring its integrity and safety. The invention also records ultrasonic signals and camera image data in real time, ensuring data synchronization and accuracy, enabling efficient and accurate material testing. This makes it particularly suitable for nondestructive testing applications in complex and harsh environments. Furthermore, the camera's high-resolution imaging capability effectively captures minute defects, while the ultrasonic probe can detect deep defects such as cracks and pores within the material that are invisible to optical imaging. The orthogonal polarization beam module generates orthogonal polarization beams through a combination of a quarter-wave plate and a polarization beam splitter, further enhancing the detection capabilities of metal surface defects.

[0068] In summary, the present invention can perform comprehensive and accurate detection of the internal structure and surface defects of metal materials, thereby improving the sensitivity and adaptability of defect detection.

Claims

1. A metal defect detection system based on photoacoustic and laser scanning, used for detecting metal materials to be tested, comprising a signal detection and synchronous triggering system, a data acquisition device, a processing unit, an ultrasonic probe, and a camera; the data acquisition device is respectively connected to the signal detection and synchronous triggering system, the ultrasonic probe, and the processing unit, which is connected to the camera; characterized in that: It also includes a blue light polarized laser and a green light pulse laser; the blue light polarized laser is provided with a first beam expansion system and an orthogonal polarized light beam generating module along the transmission direction; the green light pulse laser is provided with a second beam expansion system along the transmission direction; the orthogonal polarized light beam generating module and the second beam expansion system are sequentially provided with a reflective mirror, a one-dimensional laser galvanometer and a turntable in the transmission direction, the turntable is used to place the metal material to be tested; the ultrasonic probe and the camera are directed towards the turntable to receive signals.

2. The metal part defect detection system based on photoacoustic and laser scanning according to claim 1, characterized in that: A laser speckle attenuator and a half-wave plate are sequentially arranged between the blue light polarization laser and the first beam expansion system along the transmission direction.

3. The metal part defect detection system based on photoacoustic and laser scanning according to claim 1, characterized in that: The orthogonal polarized beam generation module includes a polarization beam splitter, a first reflector, a second reflector and a quarter-wave plate; the laser light emitted by the blue light polarization laser is divided into a reflected beam and a polarized beam by the polarization beam splitter, the polarized beam is polarized by the quarter-wave plate, and then reflected by the second reflector, returns to the polarization beam splitter, and then synthesizes an orthogonal polarized beam with the reflected beam.

4. The metal part defect detection system based on photoacoustic and laser scanning according to claim 1, characterized in that: A green light cutoff filter is embedded in the camera to block green light.

5. The detection method of the metal part defect detection system based on photoacoustic and laser scanning according to any one of claims 1 to 4, characterized in that: A blue polarized laser emits laser light to an orthogonal polarized beam generating module. The orthogonal polarized beam generating module synthesizes orthogonal polarized beams after polarization beam splitting. The orthogonal polarized beams are reflected by a reflective mirror and then irradiated by a one-dimensional laser galvanometer onto the metal material to be tested on the turntable. The optical signal reflected by the metal material to be tested is received by a camera and transmitted to a processing unit. At the same time, the camera captures an image of the metal material to be tested and transmits it to the processing unit. The laser emitted by the green pulse laser is expanded by the second beam expansion system, and then transmitted through the reflective mirror to hit the surface of the metal material to be tested to generate a photoacoustic signal. The photoacoustic signal is received by the ultrasonic probe, and then collected by the data acquisition device and transmitted to the processing unit; The ultrasonic probe transmits ultrasonic waves to the metal material to be tested. The ultrasonic signal reflected by the metal material to be tested is received by the ultrasonic probe, collected by the data acquisition device, and then transmitted to the processing unit; The processing unit obtains the position and size of the surface defects of the metal material to be tested based on the optical signal, obtains the position, size and depth of the shallow surface defects of the metal material to be tested based on the photoacoustic signal, and obtains the position, size and depth of the internal defects of the metal material to be tested based on the ultrasonic signal, and then combines the image reconstruction of the metal material to be tested to obtain a two-dimensional image of the surface and interior of the metal to be tested and a three-dimensional image of the surface of the metal to be tested.

6. The detection method of the metal part defect detection system based on photoacoustic and laser scanning according to claim 5, characterized in that: The specific process of the processing unit obtaining the position and size of the surface defect of the metal material to be tested according to the optical signal is to smooth the image through Gaussian filtering, then process the image through edge extraction algorithm to locate the contour edge, and then calculate the image gradient to locate the edge strength and direction of the defect; The formula for smoothing the image by Gaussian filtering is: I(x,y)=G(x,y)*f(x,y); Where I(x,y) represents the obtained smoothed image, G(x,y) is the Gaussian kernel function, and f(x,y) is the optical signal reflected by the metal material to be tested and received by the camera; The gradient calculation formula is: Among them, M(i,j) represents the image gradient and θ(i,j) represents the gradient direction.

7. The detection method of the metal part defect detection system based on photoacoustic and laser scanning according to claim 5, characterized in that: The photoacoustic signal is generated by irradiating the surface of the metal material to be tested with laser light, so that part of the laser energy is absorbed, causing the local temperature of the metal material to be tested to rise, thereby causing thermal expansion of the metal material to be tested, generating an elastic stress field, and then stimulating the photoacoustic signal; The power density of the laser is I0, and the laser energy density A absorbed by the metal material to be tested is expressed as: A(x,y,z,t)=I0αf(x,y,z)g(t); Where I0 is the central intensity of the laser; α is the laser absorption coefficient of the metal material to be measured; f(x, y, z) is the spatial distribution function of the laser energy; and g(t) is the temporal distribution function of the laser pulse.

8. The detection method of the metal part defect detection system based on photoacoustic and laser scanning according to claim 7, characterized in that: The photoacoustic signal includes body waves and surface waves. The velocity of the body wave is divided into the longitudinal wave velocity L c and shear wave velocity T c , the calculation formula is: Where E is the elastic modulus of the metal material to be tested, ρ is the density of the metal material to be tested, and σ is the Poisson's ratio; The velocity of the surface wave R c The calculation formula is:

9. The detection method of the metal part defect detection system based on photoacoustic and laser scanning according to claim 7, characterized in that: The solution model of the photoacoustic signal is as follows: where ▽ represents the photoacoustic signal, ρ is the material density, C is the specific heat capacity, k is the thermal conductivity, T is the temperature, Q is the heat source term, u is the displacement, ξ is the stress tensor, and f is the body force.

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