A device and method for preparing secondary micro-pits by near-wall laser-induced double cavitation

Abstract

The application provides a device and method for preparing secondary micro-dimple by near-wall laser-induced double cavitation bubbles, and relates to the technical field of laser strengthening, and comprises a laser system, a beam expanding and focusing system, a climbing system, a light splitting system, a glass tank, a liquid medium, a material to be processed, an electric three-dimensional moving platform, and the beam expanding and focusing system comprises a beam expander, a left focusing lens and a right focusing lens; the laser emitted by the laser system is evenly divided into two symmetrical paths through the beam expander, the climbing system and the light splitting system, and the two paths of laser are focused by the left focusing lens and the right focusing lens respectively, and then simultaneously break through the liquid medium from top to bottom, the focal points act on the surface of the material to be processed, and cavitation bubble one and cavitation bubble two are simultaneously formed on the surface of the material to be processed, the double cavitation bubbles generate superposition to form a composite interference zone in the pulsation evolution process, and the secondary micro-dimple is prepared on the surface of the material to be processed in the composite interference zone. The secondary micro-dimple is prepared by the double cavitation bubble interference zone, and the strengthening effect on the surface of the material is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of laser enhancement technology, and in particular to an apparatus and method for preparing secondary micro-pits by near-wall laser-induced double cavitation. Background Technology

[0002] Near-wall laser-induced cavitation technology utilizes laser penetration of the liquid threshold to form cavitation bubbles near the material wall. During the expansion-collapse process of these cavitation bubbles, the resulting burst waves, oscillating waves, collapse waves, and microjets influence the material. Compared to traditional laser processing techniques, it offers advantages such as lower material oxidation and less thermal impact; compared to other cavitation-induced technologies, it features better spherical symmetry and easier control.

[0003] Research on laser-induced cavitation technology mainly includes mechanistic studies and strengthening techniques. Specifically, regarding laser-induced cavitation strengthening technology, the invention patent CN106086319B, after its authorization announcement, discloses a device and method for strengthening a pump valve core using laser-induced cavitation. The cavitation collapse impacts the workpiece surface, creating residual compressive stress and improving the surface strength and cavitation resistance of the pump valve core. Currently, research on laser-induced cavitation technology primarily focuses on the formation of multiple cavitation bubbles and the strengthening of single cavitation bubbles, with little research on double-cavitation strengthening techniques.

[0004] Although the invention patent CN106271063B, after its authorization announcement, discloses a method and apparatus for laser-induced double cavitation, which superimposes strong shock waves induced by cavitation in a liquid medium and strong shock waves induced by cavitation on the surface of the material to be processed, repeatedly or simultaneously acting on the surface of the material to be processed to improve its surface strength and other properties, and has advantages such as high impact pressure and low material oxidation, the aforementioned double cavitations are not on the same plane of the material to be processed and the types of double cavitations are different. Therefore, it is impossible to simultaneously prepare secondary micro-cavities when preparing single-cavity pits. If single cavitations are repeatedly superimposed to prepare secondary micro-cavities, there will be a large thermal effect, poor morphological quality of the secondary micro-cavities, and uneven surface strengthening effect on the material. Summary of the Invention

[0005] The purpose of this invention is to provide an apparatus and method for preparing secondary micro-pits by near-wall laser-induced double cavitation. This method can induce the generation of double cavitation located on the same plane, and by adjusting the distance between the centers of the double cavitation, an interference region is created between the double cavitation. Secondary micro-pits are then prepared through the interference region of the double cavitation. At the same time, the secondary micro-pits have a small thermal effect and good morphological quality, effectively improving the strengthening effect on the material surface.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0007] A device for preparing secondary micro-pits by near-wall laser-induced double cavitation includes a laser system, a beam expanding and focusing system, a climbing system, a beam splitting system, a glass water tank, and an electric three-dimensional moving platform. The glass water tank is placed inside the electric three-dimensional moving platform and is filled with a liquid medium. The material to be processed is fixed inside the glass water tank and located below the liquid medium surface.

[0008] The laser system includes a laser control terminal and a laser body connected by a control connection. The laser control terminal controls the laser parameters and laser output of the laser body.

[0009] The beam expanding and focusing system includes a vertically arranged beam expander, an inclined left focusing lens, and a right focusing lens. The climbing system includes a vertically arranged climbing frame and an upper reflecting mirror and a lower reflecting mirror respectively arranged at the upper and lower ends of the climbing frame. The beam expander is located between the laser body and the lower reflecting mirror, the climbing frame is located between the laser body and the motorized three-dimensional moving platform, and the left and right focusing lenses are located above the glass water tank and are symmetrical about the beam splitting system.

[0010] The lower reflector reflects the laser beam emitted horizontally from the laser body through the beam expander vertically upwards to the upper reflector. The upper reflector reflects the laser beam to the beam splitting system, which splits the laser beam into two symmetrical paths. The two laser beams are focused by the left and right focusing lenses respectively and injected into the liquid medium from top to bottom, with their focal points on the upper surface of the material to be processed, forming symmetrical spherical cavitation bubbles one and two on the upper surface of the material to be processed. The radii of cavitation bubble one and cavitation bubble two are R1 and R2 respectively, and the distance between the centers of cavitation bubble one and cavitation bubble two is L, where L satisfies {R1, R2}min < L < R1 + R2, so that cavitation bubble one and cavitation bubble two form an overlapping interference region.

[0011] Furthermore, the beam splitting system includes a beam splitter located on the plane of symmetry between the left and right focusing lenses. A left and right reflecting mirror are vertically arranged and symmetrical about the beam splitter on both sides below it. The beam splitter evenly splits the laser reflected from the upper reflecting mirror into two symmetrical paths. The two laser paths are reflected by the left and right reflecting mirrors to the corresponding left and right focusing lenses for focusing, respectively.

[0012] Furthermore, it also includes a high-speed camera and a fiber optic hydrophone system 19, wherein the high-speed camera is positioned on one side of the glass tank with its lens facing the material to be processed, and the fiber optic hydrophone system 19 is positioned on one side of the glass tank with its front fiber optic probe placed in the liquid medium.

[0013] Furthermore, it also includes a synchronization controller and a computer. The laser control terminal, high-speed camera, and fiber optic hydrophone system 19 are all connected to the synchronization controller, which is connected to the computer.

[0014] Furthermore, the lower reflecting mirror has an angle of 45°, the upper reflecting mirror has an angle of 22.5°, the left and right focusing lenses have angles of 45° that are symmetrical, and the beam splitter has an angle of 90°.

[0015] Furthermore, the focal length of the left focusing lens is greater than 10 times the radius of cavitation 1 R1, and the focal length of the right focusing lens is greater than 10 times the radius of cavitation 2 R2.

[0016] A method for preparing secondary micro-pits using near-wall laser-induced double cavitation involves employing the aforementioned apparatus. The laser emits a laser beam, which is then split into two symmetrical paths by a beam expander, a climbing system, and a beam splitter. These two laser paths are focused by the left and right focusing lenses, respectively, and simultaneously penetrate the liquid medium from top to bottom. The focal point acts on the surface of the material to be processed, simultaneously forming cavitation bubble one and cavitation bubble two on the surface. The radii of cavitation bubble one and cavitation bubble two are R1 and R2, respectively. The distance between the centers of cavitation bubble one and cavitation bubble two is L, where L satisfies {R1, R2}min < L < R1 + R2. During the pulsating evolution process, cavitation bubble one and cavitation bubble two superimpose to form a composite interference region. Simultaneously, cavitation bubble one and cavitation bubble two form a single pit on the surface of the material to be processed, and the shock wave from the composite interference region acts on the surface of the material to be processed, forming a secondary micro-pit.

[0017] Furthermore, the laser energy E emitted by the laser body satisfies E>max(J0, J1)+Es, where J0 is the breakdown threshold of the liquid medium, J1 is the laser breakdown threshold of the material to be processed, and Es is the laser loss value in the liquid medium.

[0018] Furthermore, the liquid medium is pure water, the material to be processed is aluminum alloy, and Es = 1.6-1.8 mJ.

[0019] Furthermore, the distance L between the first bubble center and the second bubble center is adjusted by a moving electric three-dimensional moving platform. The adjustable range of the electric three-dimensional moving platform in the three dimensions of X-axis, Y-axis and Z-axis is 0-100mm, and the displacement accuracy of X-axis and Y-axis is 0.1mm, and the displacement accuracy of Z-axis is 0.05mm.

[0020] In summary, the present invention has the following beneficial effects:

[0021] 1. This invention obtains two symmetrical laser beams through a beam splitting system. The two symmetrical laser beams simultaneously induce two cavitation bubbles on the surface of the material to be processed, and there is a composite interference region between the two cavitation bubbles. In this way, the near-wall laser-induced double cavitation technology can simultaneously form double pits and secondary micro-pits on the material surface. The secondary micro-pits are affected by the superposition of the shock waves from the two cavitation bubbles at the same time. Therefore, the formed secondary micro-pits have good morphology, small thermal effect, and excellent processing effect. This avoids the adverse effects of secondary processing on the surface of the primary micro-pits in the prior art, thereby effectively improving the surface strengthening quality of the material to be processed.

[0022] 2. In this invention, the two cavitation bubbles induced by the near-wall laser are of the same type, both being cavitation bubbles generated on the surface of the material to be processed, and the two cavitation bubbles are on the same plane. The same type of two cavitation bubbles in the same plane form a composite interference region during the "expansion-collapse" process, which can not only ensure the uniformity of the quality of the two pits, but also the secondary micro-pits in the composite interference region are secondary processing of the primary micro-pits in the same time dimension, avoiding the adverse effects of secondary processing at different times on the surface of the micro-pit structure in the prior art;

[0023] 3. In this invention, the distance L between the bubble centers of bubble one and bubble two can be adjusted by an electric three-dimensional moving platform, thereby adjusting the composite interference region. The larger L is, the smaller the composite interference region and its impact force, and the smaller and shallower the secondary micro-pits. Conversely, the smaller L is, the larger and deeper the secondary micro-pits. Therefore, secondary micro-pits of different sizes and depths can be prepared according to actual needs. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a device for preparing secondary micro-pits using near-wall laser-induced double cavitation;

[0025] Figure 2 This is a pulsation evolution diagram of laser-induced double cavitation in Example 1;

[0026] Figure 3 This is an underwater acoustic signal diagram of laser-induced double cavitation in Example 1;

[0027] Figure 4 This is a three-dimensional morphology diagram of the laser-induced double cavitation in Example 1;

[0028] Figure 5 This is a diagram showing the pulsation evolution of laser-induced double cavitation in Comparative Example 1;

[0029] Figure 6 This is a diagram of the underwater acoustic signal of a laser-induced double cavitation in Comparative Example 1.

[0030] Figure 7 This is a three-dimensional morphology diagram of the laser-induced double cavitation in Comparative Example 2;

[0031] Figure 8This is a diagram showing the pulsation evolution of laser-induced double cavitation in Comparative Example 2;

[0032] Figure 9 This is a diagram of the underwater acoustic signal of a laser-induced double cavitation in Comparative Example 2.

[0033] Figure 10 This is a three-dimensional morphology diagram of the laser-induced double cavitation in Comparative Example 2;

[0034] In the diagram: 1. Computer; 2. Synchronization controller; 3. Laser control terminal; 4. Laser body; 5. Beam expander; 6. Lower reflector; 7. Climbing frame; 8. Upper reflector; 9. Beam splitter; 10. Left reflector; 11. Right reflector; 12. Left focusing lens; 13. Right focusing lens; 14. Glass water tank; 15. Liquid medium; 16. Material to be processed; 17. Electric three-dimensional moving platform; 18. High-speed camera; 19. Fiber optic hydrophone system; 20. Cavitation 1; 21. Cavitation 2; 22. Composite interference region. Detailed Implementation

[0035] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0036] A device for preparing secondary micro-pits using near-wall laser-induced double cavitation, such as... Figure 1 As shown, the system includes a computer 1, a synchronous controller 2, a laser system, a beam expander and focusing system, a climbing system, a beam splitter system, a glass water tank 14, an electric three-dimensional moving platform 17, a high-speed camera 18, and a fiber optic hydrophone system 19. The glass water tank 14 is placed inside the electric three-dimensional moving platform 17 and is filled with a liquid medium 15. The material to be processed 16 is fixed inside the glass water tank 14 and located below the surface of the liquid medium 15. The electric three-dimensional moving platform 17 realizes the movement and adjustment of the three-dimensional position of the glass water tank 14, i.e. the material to be processed 16.

[0037] like Figure 1 As shown, the laser system includes a laser control terminal 3 and a laser body 4 connected to the control system. The laser body 4 is located on the far left of the entire device. The laser control terminal 3 controls the laser parameters and laser output of the laser body 4. The laser output from the laser body 4 is input into the liquid medium 15 in the glass water tank 14 through the beam expansion and focusing system, the climbing system and the beam splitting system, and the focal point acts on the upper surface of the material to be processed 16.

[0038] like Figure 1As shown, the beam expanding and focusing system includes a vertically arranged beam expander 5, an inclined left focusing lens 12, and a right focusing lens 13. The climbing system includes a vertically arranged climbing frame 7 and an upper reflecting mirror 8 and a lower reflecting mirror 6 respectively located at the upper and lower ends of the climbing frame 7. The beam expander 5 is located between the laser body 4 and the lower reflecting mirror 6, and is used to amplify the diameter of the laser beam emitted by the laser body 4, which is beneficial for better forming of spherical cavitation. The climbing frame 7 is located between the laser body 4 and the motorized three-dimensional moving platform 17. The left focusing lens 12 and the right focusing lens 13 are located above the glass water tank 14 and are symmetrical about the beam splitting system.

[0039] like Figure 1 As shown, the lower reflector 6 reflects the laser beam emitted horizontally from the laser body 4 through the beam expander 5 vertically upwards to the upper reflector 8, which then reflects the laser beam to the beam splitting system. The elevation system raises the laser beam path, facilitating its entry into the liquid medium 15 from top to bottom and its application to the surface of the material to be processed 16, while simultaneously preventing instability caused by raising the laser body 4. Specifically, the lower reflector 6 has an angle of 45°, and the upper reflector 8 has an angle of 22.5°, ensuring that the lower reflector 6 reflects the laser beam vertically upwards, while the upper reflector 8 tilts downwards and to the right to reflect the laser beam to the beam splitting system, thus generating two symmetrical beam paths.

[0040] like Figure 1 As shown, the beam splitting system includes a beam splitter 9 located on the symmetrical plane of the left focusing lens 12 and the right focusing lens 13. A left reflecting mirror 10 and a right reflecting mirror 11, vertically arranged and symmetrical about the beam splitter 9, are positioned on either side below it. The left focusing lens 12 and the right focusing lens 13 are located slightly below the side of the left and right reflecting mirrors 10 and 11 that are close to each other, also symmetrical about the beam splitter 9. The beam splitter 9 evenly splits the laser light reflected from the upper reflecting mirror 8 into two symmetrical paths. The two laser paths are reflected by the left reflecting mirror 10 and the right reflecting mirror 11, respectively, and then focused by the corresponding left focusing lens 12 and right focusing lens 13. Specifically, the angles of the beam splitter 9, the left reflecting mirror 10, and the right reflecting mirror 11 are all 90°, and the angles of the left focusing lens 12 and the right focusing lens 13 are symmetrically 45°.

[0041] like Figure 1 As shown, the beam splitting system divides the laser into two symmetrical paths. These two lasers are focused from top to bottom into the liquid medium 15 by the left focusing lens 12 and the right focusing lens 13, respectively, with their focal points on the upper surface of the material to be processed 16. This forms symmetrical spherical cavitation bubbles 20 and 21 on the upper surface of the material 16. The radii of bubble 20 and 21 are R1 and R2, respectively. The distance between the centers of bubble 20 and 21 is L, and L satisfies {R1, R2}min < L < R1 + R2, resulting in an overlapping interference region between bubble 20 and 21.

[0042] In order to avoid the cavitation generated by the laser-induced liquid medium 15 from affecting the cavitation bubble 20 and cavitation bubble 21 on the material surface, the focal length of the left focusing lens 12 is greater than 10 times the radius R1 of cavitation bubble 20, and the focal length of the right focusing lens 13 is greater than 10 times the radius R2 of cavitation bubble 21.

[0043] like Figure 1 As shown, a high-speed camera 18 is positioned on one side of the glass tank 14 with its lens facing the material 16 to be processed, for acquiring images of the pulsating evolution of the double cavitation. A fiber optic hydrophone system 19 is positioned on one side of the glass tank 14, with its front-end fiber optic probe placed in the liquid medium 15, for acquiring underwater acoustic signals from the double cavitation.

[0044] like Figure 1 As shown, the laser control terminal 3, the high-speed camera 18, and the fiber optic hydrophone system 19 are all connected to the synchronous controller 2. The synchronous controller 2 realizes the synchronous control of the entire device. The synchronous controller 2 is connected to the computer 1, and the computer 1 directly controls the start and stop of the entire device.

[0045] A method for preparing secondary micro-pits using near-wall laser-induced double cavitation, employing the aforementioned device, combined with... Figure 1 The laser body 4 emits a laser beam, which is magnified by the beam expander 5, reflected upwards by the lower reflector 6, and reflected by the upper reflector 8 to the beam splitting system. The beam splitting system divides the laser beam into two symmetrical paths. The two laser beams are focused by the left focusing lens 12 and the right focusing lens 13 respectively, and then simultaneously penetrate the liquid medium 15 from top to bottom. The focal points of the two laser beams act on the surface of the material to be processed 16, and simultaneously induce the formation of cavitation bubble 1 20 and cavitation bubble 21 on the surface of the material to be processed 16. The radii of cavitation bubble 1 20 and cavitation bubble 21 are R1 and R2 respectively, and the distance between the center of cavitation bubble 1 20 and the center of cavitation bubble 21 is L, and L satisfies {R1, R2}min<L<R1+R2. During the pulsation evolution process, cavitation bubble 1 20 and cavitation bubble 21 superimpose to form a composite interference region 22. Cavitation bubble 1 20 and cavitation bubble 21 simultaneously form a single pit on the surface of the material to be processed 16, and the shock wave of the composite interference region 22 acts on the surface of the material to be processed 16 to prepare a secondary micro-pit.

[0046] The high-speed camera 18 and fiber optic hydrophone system 19 capture the graphic and acoustic signals of the laser-induced double cavitation. The bubble spacing L between cavitation 1 20 and cavitation 2 21 can be adjusted by moving the electric three-dimensional moving platform 17 in three-dimensional space, controlling the size and depth of the secondary micro-pits. The laser energy E emitted by the laser body 4 is controlled by the laser control terminal 3, controlling the radii R1 and R2 of cavitation 1 20 and cavitation 2 21. The laser energy E emitted by the laser body 4 satisfies E > max(J0, J1) + Es, where J0 is the breakdown threshold of the liquid medium 15, J1 is the laser breakdown threshold of the material to be processed 16, and Es is the laser loss value in the liquid medium 15.

[0047] In this invention, two symmetrical laser beams simultaneously penetrate the liquid medium 15 to generate two symmetrical cavitation bubbles 20 and 21 on the surface of the material to be processed 16. Cavitation bubbles 20 and 21 are of the same type, both being cavitation bubbles on the surface of the material to be processed 16, and are located on the same plane. During the pulsating evolution of the two cavitation bubbles, a composite interference region 22 is generated by superimposing them. The collapse shock waves of cavitation bubbles 20 and 21 simultaneously form double pits on the surface of the material to be processed 16, and the shock waves of the composite interference region 22 superimpose on the upper surface of the material to be processed 16, forming secondary micro-pits. The simultaneous formation of double pits and secondary micro-pits, and the large amplitude of the shock waves in the composite interference region 22, play a positive role in modifying the material surface. At the same time, the secondary micro-pits are subjected to secondary processing on the surface of the material to be processed 16 in the same time dimension, avoiding the adverse effects of secondary processing at different times on the surface of the micro-pit structure in the prior art.

[0048] The specific method for using a device for preparing secondary micro-pits by near-wall laser-induced double cavitation is as follows:

[0049] S1. Install equipment; according to... Figure 1 As shown, the glass water tank 14 is placed and fixed on the electric three-dimensional moving platform 17.

[0050] S2. Place the material to be processed 16; place the material to be processed 16 in the center of the glass water tank 14 and fix it.

[0051] S3. Optical path setting and adjustment: Based on the position of the material to be processed 16, the optical path is set. The beam splitting system ensures that the laser can be evenly divided into two symmetrical paths according to the principle of equal division and the basic theorem of isosceles right triangle.

[0052] S4. Set laser parameters; Set the laser energy parameter E at the laser control terminal 3 so that E satisfies E>max(J0, J1)+Es, where J0 is the breakdown threshold of the liquid medium 15, J1 is the laser breakdown threshold of the material to be processed 16, and Es is the laser loss value in the liquid medium 15.

[0053] S5. Adjust the distance L between the two cavitation bubbles; the laser energy parameter E determines the radii R1 and R2 of cavitation bubble 1 20 and cavitation bubble 2 21. By adjusting the three-dimensional spatial position of the electric three-dimensional moving platform 17, the distance L between the centers of cavitation bubble 1 20 and cavitation bubble 2 21 is adjusted, where L satisfies {R1, R2}min < L < R1 + R2. The electric three-dimensional moving platform 17 has an adjustable range of 0-100mm in the X, Y, and Z axes, with a displacement accuracy of 0.1mm for the X and Y axes and 0.05mm for the Z axis.

[0054] S6. Pour in liquid medium 15; Slowly pour liquid medium 15 into glass water tank 14 until the liquid medium 15 fills the entire container, and control the pouring speed of liquid medium 15 to prevent the generation of excess small air bubbles.

[0055] S7. Set up a high-speed camera 18; set up the high-speed camera 18 so that its lens is facing the material to be processed 16, and set the parameters of the high-speed camera 18 so that it can clearly capture the pulsating evolution image of the laser-induced double cavitation near the wall.

[0056] S8. Set up the fiber optic hydrophone system 19; set up the fiber optic hydrophone system 19, place its front probe in the liquid medium 15, and the vertical distance between the probe and the composite interference region 22 of cavitation bubble 1 20 and cavitation bubble 21 is about 10mm.

[0057] S9. Set up a synchronization controller 2; the synchronization controller 2 is connected to the computer 1, the laser control terminal 3, the high-speed camera 18, and the fiber optic hydrophone system 19, and controls the laser control terminal 3, the high-speed camera 18, and the fiber optic hydrophone system 19 to work synchronously. The high-speed camera 18 and the fiber optic hydrophone system 19 start 50ns after the laser control terminal 3 starts. When the laser control terminal 3 starts, the synchronization controller 2 records all signals and synchronously inputs them to the computer 1.

[0058] S10, Inducing the generation of double cavitation bubbles; Computer 1 controls the start-up of the entire device. The laser emitted by the main body of the laser 4 is divided into two symmetrical paths by the beam expander 5, the climbing system, and the beam splitting system. The two lasers are focused and simultaneously penetrate the liquid medium 15 from top to bottom, and the focal points act on the surface of the material to be processed 16. Cavitation bubble 1 20 and cavitation bubble 21 are simultaneously induced on the surface of the material to be processed 16. The double cavitation bubbles simultaneously form double pits on the surface of the material to be processed 16, and the composite interference region 22 forms a secondary micro-pit.

[0059] Example 1:

[0060] The laser body 4 has the following parameters: wavelength 1064nm, pulse width 9ns, and laser repetition frequency 5Hz. The liquid medium 15 is pure water with J0 = 37mJ. The material to be processed 16 is aluminum alloy with J1 = 48mJ. The 1064nm laser attenuation coefficient is 0.8-0.9mJ / m. The loss value in the pure water liquid medium 15 is Es = 1.6-1.8mJ. According to E > max(J0, J1) + Es, the laser energy parameter E is selected as 50mJ in this embodiment. Under this laser energy parameter, the maximum radius of cavitation bubble 1 20 and cavitation bubble 2 21 is generally 2mm. Theoretically, the radii R1 and R2 of cavitation bubble 1 20 and cavitation bubble 2 21 are equal. According to {R1, R2}min < L < R1 + R2, L = 3mm is selected in this embodiment.

[0061] A high-speed camera (18Phantom v2012) and a fiber optic hydrophone system (19FOHS v2) were used to capture the pulsation evolution of laser-induced double cavitation and underwater acoustic signals. For example... Figure 2 The figure shows the pulsation evolution of a laser-induced double cavitation when L = 3 mm and E = 50 mJ. Figure 3 The image shows the underwater acoustic signal spectrum of a laser-induced double cavitation bubble at L = 3 mm and E = 50 mJ. Due to simultaneous surface breakdown of the material 16 to be processed, white-light-emitting plasma is generated in the breakdown area. Simultaneously, the plasma expands rapidly outward, generating shock waves and double cavitation bubbles. Initially, the internal pressure of the cavitation bubble is higher than that of the surrounding liquid medium 15. Under the influence of this pressure difference, the cavitation bubble expands outward. When the cavitation bubble radius reaches its maximum, it begins to be compressed into a single cavitation bubble under the constraint of external pressure and surface tension. When compressed to its minimum, the internal pressure of the bubble exceeds the static pressure of the surrounding liquid medium 15, and the double cavitation bubbles merge into a single cavitation bubble and begin to rebound, repeating this process. The more times this process is repeated, the greater the loss, until the cavitation bubble collapses. Figure 4 The image shows the three-dimensional morphology of the laser-induced double cavitation when L=3mm and E=50mJ. Obvious secondary micro-pits appeared on the surface of the material to be processed 16.

[0062] Comparative Example 1:

[0063] The value of L is selected to be slightly larger than the sum of the radii of cavitation bubble 1 (20) and cavitation bubble 2 (21), and all other conditions are the same as in Example 1, with L = 5 mm. Figure 5 The figure shows the pulsation evolution of a laser-induced double cavitation when L = 5 mm and E = 50 mJ. Figure 6 The image shows the underwater acoustic signal spectrum of laser-induced double cavitation when L = 5 mm and E = 50 mJ. At this time, the cavitation bubbles 20 and 21 induced by the two symmetrical lasers on the surface of the material 16 do not have a composite interference region 22; they are two non-interfering bubbles, each accompanied by an "expansion-collapse" state. Figure 7The image shows the three-dimensional morphology of the laser-induced double cavitation when L=5mm and E=50mJ. Two non-interfering micro-pits appeared on the surface of the material to be processed 16.

[0064] Comparative Example 2:

[0065] The radius of L is selected to be slightly smaller than that of cavitation bubble 20 or cavitation bubble 21, and all other conditions are the same as in Example 1, with L = 1 mm. Figure 8 The figure shows the pulsation evolution of a laser-induced double cavitation when L = 1 mm and E = 50 mJ. Figure 9 The image shows the underwater acoustic signal spectrum of laser-induced double cavitation when L = 1 mm and E = 50 mJ. At this point, the distance between cavitation bubble 1 (20) and cavitation bubble 2 (21) induced by the two symmetrical lasers on the surface of the material 16 is very close, and can be considered as forming only a single cavitation bubble, accompanied by an "expansion-collapse" state. Figure 10 The image shows the three-dimensional morphology of the laser-induced double cavitation when L=1mm and E=50mJ. A micro-pit slightly larger than the single micro-pit in Comparative Example 1 appears on the surface of the material to be processed 16.

[0066] The foregoing description illustrates and describes preferred embodiments of the present invention. As previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A device for preparing secondary micro-pits using near-wall laser-induced double cavitation, characterized in that: It includes a laser system, a beam expanding and focusing system, a climbing system, a beam splitting system, a glass water tank (14), and an electric three-dimensional moving platform (17). The glass water tank (14) is placed inside the electric three-dimensional moving platform (17) and is filled with a liquid medium (15). The glass water tank (14) contains a material to be processed (16) located below the liquid surface of the liquid medium (15). The laser system includes a laser control terminal (3) and a laser body (4) connected by a control connection. The laser control terminal (3) controls the laser parameters and laser output of the laser body (4). The beam expanding and focusing system includes a vertically arranged beam expander (5), an inclined left focusing lens (12) and a right focusing lens (13). The climbing system includes a vertically arranged climbing frame (7) and an upper reflecting mirror (8) and a lower reflecting mirror (6) respectively arranged at the upper and lower ends of the climbing frame (7). The beam expander (5) is located between the laser body (4) and the lower reflecting mirror (6). The climbing frame (7) is located between the laser body (4) and the electric three-dimensional moving platform (17). The left focusing lens (12) and the right focusing lens (13) are located above the glass water tank (14) and are symmetrical about the beam splitting system. The lower reflector (6) reflects the laser beam emitted horizontally from the laser body (4) through the beam expander (5) vertically upward to the upper reflector (8). The upper reflector (8) reflects the laser beam to the beam splitting system, which splits the laser beam into two symmetrical paths. The two laser beams are focused by the left focusing lens (12) and the right focusing lens (13) respectively and shot into the liquid medium (15) from top to bottom. Their focal points are on the upper surface of the material to be processed (16), forming symmetrical spherical cavitation bubbles one (20) and two (21) on the upper surface of the material to be processed (16). The radii of the first cavitation bubble (20) and the second cavitation bubble (21) are R1 and R2 respectively. The distance between the center of the first cavitation bubble (20) and the center of the second cavitation bubble (21) is L, and L satisfies {R1, R2}min < L < R1 + R2, so that the first cavitation bubble (20) and the second cavitation bubble (21) form an overlapping interference region.

2. The apparatus for preparing secondary micro-pits using near-wall laser-induced double cavitation according to claim 1, characterized in that: The beam splitting system includes a beam splitter (9) located on the symmetrical plane of the left focusing lens (12) and the right focusing lens (13). The beam splitter (9) has a vertically arranged left reflecting mirror (10) and a right reflecting mirror (11) symmetrical about it on both sides below it. The beam splitter (9) evenly splits the laser reflected from the upper reflecting mirror (8) into two symmetrical paths. The two laser paths are reflected by the left reflecting mirror (10) and the right reflecting mirror (11) respectively and focused by the corresponding left focusing lens (12) and right focusing lens (13).

3. The apparatus for preparing secondary micro-pits using near-wall laser-induced double cavitation according to claim 2, characterized in that: It also includes a high-speed camera (18) and a fiber optic hydrophone system (19), wherein the high-speed camera (18) is positioned on one side of the glass tank (14) with its lens facing the material to be processed (16), and the fiber optic hydrophone system (19) is positioned on one side of the glass tank (14) with its front fiber optic probe placed in the liquid medium (15).

4. The apparatus for preparing secondary micro-pits using near-wall laser-induced double cavitation according to claim 3, characterized in that: It also includes a synchronization controller (2) and a computer (1). The laser control terminal (3), the high-speed camera (18), and the fiber optic hydrophone system (19) are all connected to the synchronization controller (2), which is connected to the computer (1).

5. The apparatus for preparing secondary micro-pits using near-wall laser-induced double cavitation according to claim 4, characterized in that: The lower reflector (6) has an angle of 45°, the upper reflector (8) has an angle of 22.5°, the left focusing lens (12) and the right focusing lens (13) have angles of 45° that are symmetrical from left to right, and the beam splitter (9) has an angle of 90°.

6. The apparatus for preparing secondary micro-pits using near-wall laser-induced double cavitation according to claim 5, characterized in that: The focal length of the left focusing lens (12) is greater than 10 times the radius R1 of cavitation 1 (20), and the focal length of the right focusing lens (13) is greater than 10 times the radius R2 of cavitation 2 (21).

7. A method for preparing secondary micro-pits using near-wall laser-induced double cavitation, characterized in that: Using the apparatus described in any one of claims 4-6, the laser body (4) emits a laser beam, which is then divided into two symmetrical paths by a beam expander (5), a climbing system, and a beam splitting system. The two laser beams are focused by the left focusing lens (12) and the right focusing lens (13) respectively, simultaneously penetrating the liquid medium (15) from top to bottom. The focal point acts on the surface of the material to be processed (16), simultaneously forming cavitation bubble one (20) and cavitation bubble two (21) on the surface of the material to be processed (16). The cavitation bubble one (20) and cavitation bubble two (21)... The radii are R1 and R2 respectively. The distance between the bubble center of bubble one (20) and the bubble center of bubble two (21) is L, and L satisfies {R1, R2}min<L<R1+R2. Bubble one (20) and bubble two (21) superimpose to form a composite interference region (22) during the pulsation evolution process. Bubble one (20) and bubble two (21) simultaneously form a single pit on the surface of the material to be processed (16), and the shock wave of the composite interference region (22) acts on the surface of the material to be processed (16) to form a secondary micro pit.

8. The method for preparing secondary micro-pits using near-wall laser-induced double cavitation according to claim 7, characterized in that: The laser energy E emitted by the laser body (4) satisfies E>max(J0,J1)+Es, where J0 is the breakdown threshold of the liquid medium (15), J1 is the laser breakdown threshold of the material to be processed (16), and Es is the laser loss value in the liquid medium (15).

9. The method for preparing secondary micro-pits using near-wall laser-induced double cavitation according to claim 8, characterized in that: The liquid medium (15) is pure water, and the material to be processed (16) is aluminum alloy with Es = 1.6-1.8 mJ.

10. The method for preparing secondary micro-pits using near-wall laser-induced double cavitation according to claim 7, characterized in that: The distance L between the bubble center of bubble one (20) and the bubble center of bubble two (21) is adjusted by a moving electric three-dimensional moving platform (17). The adjustable range of the electric three-dimensional moving platform (17) in the three dimensions of X-axis, Y-axis and Z-axis is 0-100mm, and the displacement accuracy of X-axis and Y-axis is 0.1mm, and the displacement accuracy of Z-axis is 0.05mm.

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