A residual stress reduction method of high-frequency alternating current composite nanosecond pulse laser cleaning

Abstract

This invention relates to a method for residual stress reduction in high-frequency alternating current combined with nanosecond pulsed laser cleaning. The method includes the following steps: preparing a workpiece to be cleaned and connecting a high-frequency AC power supply to the workpiece; preheating the workpiece to 150-300°C based on the Joule heating and skin effect of the pulsed current; performing nanosecond laser scanning cleaning on the focal plane of the workpiece based on a predetermined scanning path; increasing the peak current density of the power supply to achieve dislocation slip and residual stress release based on the electroplastic effect, and reducing the surface temperature of the workpiece to below 200°C; and turning off the power supply and air-cooling the workpiece to room temperature. This invention not only effectively removes contaminants from metal surfaces and significantly inhibits secondary oxidation, but also precisely controls the thermal effects generated in the cleaning area. By effectively controlling the distribution of residual stress on the surface through high-frequency alternating current, it effectively improves the fatigue performance of metal materials after laser cleaning, thereby achieving a highly efficient and non-destructive laser cleaning technology.

Description

Technical Field

[0001] This invention relates to the field of laser processing technology, and in particular to a method for reducing residual stress in high-frequency alternating current composite nanosecond pulse laser cleaning. Background Technology

[0002] Titanium alloys, high-temperature alloys, and other metallic materials, due to their excellent strength, corrosion resistance, and heat resistance, have become the main materials used in key components of aero-engines. However, during hot forming and long-term storage, these materials are prone to forming an oxide layer on their surface, which can adversely affect subsequent processing. Therefore, it is necessary to remove the surface oxide film before processing. Laser cleaning, as an industrial cleaning method, offers advantages over traditional chemical or mechanical cleaning methods. It is non-abrasive, non-contact, has no thermal effect, and is applicable to various materials, making it considered the most reliable and effective solution. Furthermore, laser cleaning can solve problems that traditional cleaning methods struggle with. For example, when submicron-sized contaminant particles adhere to the workpiece surface, these particles are usually tightly bonded and difficult to remove using conventional cleaning methods. However, cleaning the workpiece surface using nano-laser irradiation is highly effective. In addition, because laser cleaning is non-contact, it is very safe for cleaning precision workpieces or their delicate parts, ensuring accuracy. Therefore, laser cleaning has unique advantages in the cleaning industry.

[0003] Chinese patent CN109226096A discloses a laser cleaning process for the surface of Al-Mg-Zn alloy workpieces, comprising: cleaning the Al-Mg-Zn alloy surface using nanosecond pulsed fiber laser technology; wherein the cleaning parameters are: pulse energy: 20~24mJ; pulse frequency: 10~20KHz; working distance from the workpiece surface: 100mm; wavelength: 1064nm; power: 500W. Nanosecond pulsed lasers have become a mainstream cleaning technology due to their high energy density and controllable heat input. However, the local high-temperature gradient and rapid cooling induced by its thermal ablation mechanism can form tensile stress peaks on the surface, significantly reducing the fatigue strength and stress corrosion resistance of the material. To eliminate such residual stress, traditional post-treatment processes such as heat treatment and shot peening are often used. However, these traditional post-treatment processes require offline operation, resulting in complex procedures, high energy consumption, and a high risk of deformation.

[0004] Chinese Patent CN109926403B discloses a method for reducing residual stress on metal surfaces after high-power pulsed laser cleaning. The method involves cleaning contaminants from the metal surface with a high-power short-pulse laser, then spraying a water-based mixture onto the cleaned metal surface. Low-power laser cleaning parameters are selected, and laser scanning is performed along a specific scanning path. This method reduces residual stress on the metal surface after high-power pulsed laser cleaning by converting residual tensile stress into residual compressive stress. Specifically, the average laser power in the high-power short-pulse laser cleaning is 500W; the average laser power in the low-power laser cleaning is 50-150W; and the water-based mixture is a mixture containing water, sodium orthosilicate, and γ-glycidoxypropyltrimethoxysilane. The combination of low-power laser scanning and water-based mixed liquid spraying used in the above method has the following problems in practical applications: the process steps are relatively complicated, the requirements for the composition and ratio of the water-based mixed liquid are strict, and the cost is high. At the same time, the effect of low-power laser is limited, and its ability to control deeper or more complex stress states is insufficient. It is difficult to achieve efficient, accurate and economical reduction of residual stress on the surface of metal workpieces, and it also fails to fully consider the synergistic effect of laser and other energy forms to optimize the stress reduction effect.

[0005] Currently, using pulsed current to regulate the residual stress distribution of components is an emerging technology for extending their service life. While this technology has advantages such as excellent regulation effect, short processing time, and minimal damage to the substrate, the high-energy pulsed current used in current mainstream solutions suffers from excessive energy consumption. Therefore, this invention proposes a residual stress reduction method using a composite cleaning process of high-frequency alternating current and nanosecond pulsed laser. Utilizing a time-series coupling mechanism of "laser cleaning - current stress relief," it achieves in-situ dynamic regulation of residual stress during the cleaning process, overcoming the technical bottlenecks of in-situ residual stress control and energy consumption optimization. Summary of the Invention

[0006] The problem this invention aims to solve is to provide a residual stress reduction method for high-frequency alternating current combined with nanosecond pulsed laser cleaning, which addresses the aforementioned shortcomings of existing technologies. This method not only effectively removes contaminants from metal surfaces and significantly inhibits secondary oxidation, achieving oxygen content close to that of an inert gas environment, but also precisely controls the thermal effects generated within the cleaning area. By effectively controlling the distribution of residual stress on the surface through high-frequency alternating current, it effectively improves the fatigue performance of metal materials after laser cleaning, thereby realizing a highly efficient and non-destructive laser cleaning technology.

[0007] The above-mentioned objective of this invention is achieved through the following technical solutions:

[0008] A method for reducing residual stress in high-frequency alternating current combined with nanosecond pulsed laser cleaning includes the following steps:

[0009] S1 Prepare a workpiece to be cleaned and connect a high-frequency AC power supply to the workpiece;

[0010] S2 preheats the workpiece to 150~300°C based on the Joule heating effect and skin effect of the pulse current of the power supply.

[0011] S3 completes the scanning and cleaning of the workpiece using a nanosecond laser on the focal plane based on a predetermined scanning path;

[0012] S4 increases the peak current density of the power supply, completes the dislocation slip and residual stress release of the workpiece based on the electroplastic effect, and reduces the surface temperature of the workpiece to below 200°C.

[0013] S5 turns off the power supply and air-cools the workpiece to room temperature.

[0014] Furthermore, in S1, the workpiece is made of titanium alloy, high-temperature alloy, stainless steel or aluminum alloy, and has a thickness of 0.5~50mm.

[0015] Furthermore, in S1, the surface contaminants of the workpiece include oxide layers, oil stains, paint layers, or coatings, and the thickness of the surface contaminants does not exceed 100 μm.

[0016] Furthermore, in S2 and S3, the peak current density of the control power supply is 50~800A / mm². 2 The current frequency is 100kHz~5MHz, and the pulse current waveform is a sine wave.

[0017] Furthermore, in step S3, the scanning cleaning is performed in an air environment. High-frequency, short-pulse nanosecond laser process parameters induce the laser pulse to generate high-temperature, high-pressure plasma microcapsules, which continuously cover the area of ​​the workpiece to be cleaned. These plasma microcapsules effectively isolate oxygen, inhibiting secondary oxidation of the material surface. Simultaneously, the accompanying shock wave pressure assists in the removal of surface contaminants and applies plasma impact to the substrate, introducing compressive stress on the surface layer, thereby partially offsetting the residual tensile stress caused by the laser thermal effect.

[0018] Furthermore, in S3, the wavelength of the nanosecond laser is controlled to be 1000~1100nm, the pulse width to be 25~35ns, the pulse repetition frequency to be 250~300kHz, and the pulse energy density to be 1.2~5.0J / cm². 2 .

[0019] Furthermore, in S3, the scanning path is one or a combination of several of the following: line-by-line scanning path, bow-shaped scanning path, grid-interwoven scanning path, concentric circle scanning path, and spiral scanning path.

[0020] Furthermore, in S3, the scanning line spacing is controlled to be 40~60μm, and the scanning point overlap rate is 40~70%.

[0021] Furthermore, in S4, the peak current density of the control power supply is 400~1000A / mm. 2 The current frequency is 100kHz~5MHz, and the pulse current waveform is a sine wave.

[0022] Furthermore, in S4, the pulse current of the power supply is applied for a duration of 5 to 60 seconds.

[0023] In summary, the beneficial technical effects of the present invention are as follows:

[0024] 1. This invention uses high-frequency alternating current to preheat the workpiece before laser cleaning. First, the skin effect of pulsed current is used to concentrate the current density on the metal surface, thereby achieving a high current density at a low current intensity. The Joule heating effect of the current softens the contaminants on the workpiece surface and reduces the cleaning threshold. At the same time, it increases the thermal conductivity of the workpiece substrate, accelerates the diffusion of heat into the interior, reduces the temperature gradient, weakens the local accumulation of laser energy, and reduces the thermal stress amplitude.

[0025] 2. In the laser cleaning process, the present invention uses a galvanometer to achieve a predetermined scanning path, accurately covering the surface contaminant area. Through the high-frequency, short-pulse nanosecond laser process, the laser action time is shortened to suppress the thermal effect. The high repetition rate laser makes the interval between adjacent pulses smaller than the plasma dissipation time, forming a continuously covering plasma microcapsule, effectively isolating oxygen and avoiding secondary oxidation in the air environment. At the same time, the accompanying shock wave pressure can assist in the peeling of surface contaminants and apply plasma impact to the substrate, introducing compressive stress on the surface layer, thereby partially offsetting the residual tensile stress caused by the laser thermal effect.

[0026] 3. This invention utilizes the electroplastic effect and skin effect of electric current throughout the process. When the pulsed current flows through the metal surface, the momentum exchange between free electrons and dislocations generates an electron wind, which helps overcome lattice resistance, induces dislocation slip and recombination, and releases the tensile stress energy of the surface generated by the laser thermal effect. At the same time, the plastic deformation of the surface induces compressive stress. In addition, the instantaneous Joule heat generated by the current causes the material surface to soften instantaneously and the yield strength to decrease, further promoting plastic deformation and maintaining the surface temperature at 200~500℃, slowing down the cooling rate and producing an aging-like effect.

[0027] 4. The mechanism of high-frequency current stress reduction of the present invention includes: (a) Joule heating effect: maintaining temperature field uniformity and suppressing thermal gradient stress; (b) electroplastic effect: high-density electron flow reduces the critical shear stress of dislocation slip; (c) electron wind: directional electron impact promotes dislocation unpinning motion; (d) plasma shock wave: laser-induced plasma causes impact force on the cleaning surface.

[0028] 5. This high-frequency alternating current combined with nanosecond pulsed laser cleaning method can effectively remove surface contaminants in an air environment, avoid secondary oxidation, significantly reduce thermal stress and regulate residual stress, and ultimately achieve efficient, non-destructive precision cleaning of typical materials for aero-engines. Attached Figure Description

[0029] Figure 1 This is a flowchart of the method provided in Embodiment 1 of the present invention.

[0030] Figure 2 This is a schematic diagram of the connection relationship between the high-frequency AC power supply and the workpiece to be cleaned in Embodiment 1 of the present invention.

[0031] Figure 3 These are surface morphology images of the workpieces obtained by processing in Examples 2-6 and Comparative Examples 1-5 of the present invention.

[0032] Figure 4 These are oxygen content diagrams of the workpieces obtained from the treatment of Examples 2-6 and Comparative Examples 1-5 of the present invention.

[0033] Figure 5 These are residual stress diagrams of the workpieces obtained by processing in Examples 2-6 and Comparative Examples 1-5 of the present invention. Detailed Implementation

[0034] To make the technical means, creative features, objectives and effects of this invention clearer and easier to understand, the invention will be further described below in conjunction with the accompanying drawings and specific embodiments. Example

[0035] Example 1: Refer to Figure 1 and Figure 2 This invention discloses a method for residual stress reduction in high-frequency alternating current combined with nanosecond pulsed laser cleaning, comprising the following steps:

[0036] S1 prepares a workpiece to be cleaned and connects a high-frequency AC power supply to the workpiece.

[0037] S2 preheats the workpiece to 150~300℃ based on the Joule heating effect and skin effect of the pulse current of the power supply.

[0038] S3 completes the scanning and cleaning of the workpiece by nanosecond laser on the focal plane based on a predetermined scanning path;

[0039] S4 increases the peak current density of the power supply, completes the dislocation slip and residual stress release of the workpiece based on the electroplastic effect, and reduces the surface temperature of the workpiece to below 200℃.

[0040] S5 turns off the power and allows the workpiece to air-cool to room temperature.

[0041] Example 2: This invention discloses a method for residual stress reduction in high-frequency alternating current combined with nanosecond pulsed laser cleaning. The difference from Example 1 is that it includes the following steps:

[0042] S1 Prepare a workpiece to be cleaned. The workpiece is made of titanium alloy TC4 with a thickness of 20mm. The surface contaminants of the workpiece include oxide layer, oil stains, paint layer or coating. The thickness of the surface contaminants does not exceed 100μm. The positive and negative terminals of the high-frequency AC power supply are electrically connected to the electrodes on both sides of the workpiece through a sliding resistor.

[0043] The peak current density of the S2 control power supply is 500A / mm². 2 The current frequency is 230kHz, the pulse current waveform is a sine wave, and the workpiece is preheated to 200℃ based on the Joule heating effect and skin effect of the pulse current of the power supply.

[0044] S3 maintains the applied pulsed current, applying a nanosecond laser to the focal plane of the workpiece based on a predetermined bow-shaped scanning path. The scanning line spacing is controlled at 46 μm, and the scanning point overlap rate is 60%. The scanning cleaning is performed in an air environment, utilizing high-frequency, short-pulse nanosecond laser process parameters: laser wavelength of 1060 nm, pulse width of 30 ns, pulse repetition frequency of 270 kHz, and pulse energy density of 2.14 J / cm². 2 High-frequency laser pulse-induced high-temperature and high-pressure plasma microcapsules continuously cover the area of ​​the workpiece to be cleaned, isolating oxygen and inhibiting secondary oxidation. At the same time, the accompanying shock wave pressure can help remove surface contaminants and apply plasma impact to the substrate, introducing compressive stress on the surface layer, thereby partially offsetting the residual tensile stress caused by the laser thermal effect, and completing the nanosecond laser scanning cleaning on the focal plane of the workpiece.

[0045] S4 increases the peak current density of the power supply to 800A / mm². 2 The current frequency is 230kHz, the pulse current waveform is a sine wave, and the pulse current action time is 5~60s (adjusted according to the residual stress level). Based on the electroplastic effect, the dislocation slip and residual stress release of the workpiece are completed, and the surface temperature of the workpiece is reduced to below 200℃.

[0046] S5 turns off the power and allows the workpiece to air-cool to room temperature.

[0047] Example 3: This invention discloses a method for reducing residual stress in high-frequency alternating current combined with nanosecond pulsed laser cleaning. The difference from Example 1 is that it includes the following steps:

[0048] S1 Prepare a workpiece to be cleaned. The workpiece is made of titanium alloy TA15 with a thickness of 20mm. The surface contaminants of the workpiece include oxide layer, oil stains, paint layer or coating. The thickness of the surface contaminants does not exceed 100μm. The positive and negative terminals of the high-frequency AC power supply are electrically connected to the two electrodes of the workpiece through a sliding resistor.

[0049] The peak current density of the S2 control power supply is 375A / mm². 2 The current frequency is 180kHz, the pulse current waveform is a sine wave, and the workpiece is preheated to 200℃ based on the Joule heating effect and skin effect of the pulse current of the power supply.

[0050] S3 maintains the applied pulsed current, applying a nanosecond laser to the focal plane of the workpiece based on a predetermined bow-shaped scanning path. The scanning line spacing is controlled at 58 μm, and the scanning point overlap rate is 50%. The scanning cleaning is performed in an air environment, utilizing high-frequency, short-pulse nanosecond laser process parameters: laser wavelength of 1060 nm, pulse width of 30 ns, pulse repetition frequency of 270 kHz, and pulse energy density of 1.57 J / cm². 2 High-temperature, high-pressure plasma microcapsules induced by high-frequency laser pulses continuously cover the area of ​​the workpiece to be cleaned. These plasma microcapsules can effectively isolate oxygen and inhibit secondary oxidation of the material surface. At the same time, the accompanying shock wave pressure can assist in the removal of surface contaminants and apply plasma impact to the substrate, introducing compressive stress on the surface layer, thereby partially offsetting the residual tensile stress caused by the laser thermal effect, and completing the scanning cleaning of the workpiece by nanosecond laser on the focal plane.

[0051] S4 increases the peak current density of the power supply to 600A / mm². 2 The current frequency is 230kHz, the pulse current waveform is a sine wave, and the pulse current action time is 5~60s (adjusted according to the residual stress level). Based on the electroplastic effect, the dislocation slip and residual stress release of the workpiece are completed, and the surface temperature of the workpiece is reduced to below 200℃.

[0052] S5 turns off the power and allows the workpiece to air-cool to room temperature.

[0053] Example 4: This invention discloses a method for reducing residual stress in high-frequency alternating current combined with nanosecond pulsed laser cleaning. The difference from Example 1 is that it includes the following steps:

[0054] S1 Prepare a workpiece to be cleaned. The workpiece is made of stainless steel 0Cr18 with a thickness of 20mm. The surface contaminants of the workpiece include oxide layer, oil stains, paint layer or coating. The thickness of the surface contaminants does not exceed 100μm. Connect the positive and negative terminals of the high-frequency AC power supply to the electrodes on both sides of the workpiece through a sliding resistor.

[0055] The peak current density of the S2 control power supply is 800A / mm². 2 The current frequency is 360kHz, the pulse current waveform is a sine wave, and the workpiece is preheated to 300℃ based on the Joule heating effect and skin effect of the pulse current of the power supply.

[0056] S3 maintains the applied pulsed current, applying a nanosecond laser to the focal plane of the workpiece based on a predetermined bow-shaped scanning path. The scanning line spacing is controlled at 46 μm, and the scanning point overlap rate is 60%. The scanning cleaning is performed in an air environment, utilizing high-frequency, short-pulse nanosecond laser process parameters: laser wavelength of 1060 nm, pulse width of 30 ns, pulse repetition frequency of 270 kHz, and pulse energy density of 1.43 J / cm². 2 High-temperature, high-pressure plasma microcapsules induced by high-frequency laser pulses continuously cover the area of ​​the workpiece to be cleaned. These plasma microcapsules can effectively isolate oxygen and inhibit secondary oxidation of the material surface. At the same time, the accompanying shock wave pressure can assist in the removal of surface contaminants and apply plasma impact to the substrate, introducing compressive stress on the surface layer, thereby partially offsetting the residual tensile stress caused by the laser thermal effect, and completing the scanning cleaning of the workpiece by nanosecond laser on the focal plane.

[0057] S4 increases the peak current density of the power supply to 1000A / mm². 2 The current frequency is 360kHz, the pulse current waveform is a sine wave, and the pulse current action time is 5~60s (adjusted according to the residual stress level). Based on the electroplastic effect, the dislocation slip and residual stress release of the workpiece are completed, and the surface temperature of the workpiece is reduced to below 200℃.

[0058] S5 turns off the power and allows the workpiece to air-cool to room temperature.

[0059] Example 5: This invention discloses a method for residual stress reduction in high-frequency alternating current combined with nanosecond pulsed laser cleaning. The difference from Example 1 is that it includes the following steps:

[0060] S1 Prepare a workpiece to be cleaned. The workpiece is made of high-temperature alloy GH188 with a thickness of 20mm. The surface contaminants of the workpiece include oxide layer, oil stains, paint layer or coating. The thickness of the surface contaminants does not exceed 100μm. The positive and negative terminals of the high-frequency AC power supply are electrically connected to the two electrodes of the workpiece through a sliding resistor.

[0061] The peak current density of the S2 control power supply is 200A / mm². 2 The current frequency is 150kHz, the pulse current waveform is a sine wave, and the workpiece is preheated to 200℃ based on the Joule heating effect and skin effect of the pulse current of the power supply.

[0062] S3 maintains the applied pulsed current, applying a nanosecond laser to the focal plane of the workpiece based on a predetermined bow-shaped scanning path. The scanning line spacing is controlled at 46 μm, and the scanning point overlap rate is 60%. The scanning cleaning is performed in an air environment, utilizing high-frequency, short-pulse nanosecond laser process parameters: laser wavelength of 1060 nm, pulse width of 30 ns, pulse repetition frequency of 270 kHz, and pulse energy density of 1.43 J / cm². 2 High-temperature, high-pressure plasma microcapsules induced by high-frequency laser pulses continuously cover the area of ​​the workpiece to be cleaned. These plasma microcapsules can effectively isolate oxygen and inhibit secondary oxidation of the material surface. At the same time, the accompanying shock wave pressure can assist in the removal of surface contaminants and apply plasma impact to the substrate, introducing compressive stress on the surface layer, thereby partially offsetting the residual tensile stress caused by the laser thermal effect, and completing the scanning cleaning of the workpiece by nanosecond laser on the focal plane.

[0063] S4 increases the peak current density of the power supply to 400A / mm². 2 The current frequency is 150kHz, the pulse current waveform is a sine wave, and the pulse current action time is 5~60s (adjusted according to the residual stress level). Based on the electroplastic effect, the dislocation slip and residual stress release of the workpiece are completed, and the surface temperature of the workpiece is reduced to below 200℃.

[0064] S5 turns off the power and allows the workpiece to air-cool to room temperature.

[0065] Example 6: This invention discloses a method for residual stress reduction in high-frequency alternating current combined with nanosecond pulsed laser cleaning. The difference from Example 1 is that it includes the following steps:

[0066] S1 Prepare a workpiece to be cleaned. The workpiece is made of high-temperature alloy GH625 with a thickness of 20mm. The surface contaminants of the workpiece include oxide layer, oil stains, paint layer or coating. The thickness of the surface contaminants does not exceed 100μm. The positive and negative terminals of the high-frequency AC power supply are electrically connected to the electrodes on both sides of the workpiece through a sliding resistor.

[0067] The peak current density of the S2 control power supply is 200A / mm². 2 The current frequency is 150kHz, the pulse current waveform is a sine wave, and the workpiece is preheated to 200℃ based on the Joule heating effect and skin effect of the pulse current of the power supply.

[0068] S3 maintains the applied pulsed current, applying a nanosecond laser to the focal plane of the workpiece based on a predetermined bow-shaped scanning path. The scanning line spacing is controlled at 46 μm, and the scanning point overlap rate is 60%. The scanning cleaning is performed in an air environment, utilizing high-frequency, short-pulse nanosecond laser process parameters: laser wavelength of 1060 nm, pulse width of 30 ns, pulse repetition frequency of 270 kHz, and pulse energy density of 1.43 J / cm². 2 High-temperature, high-pressure plasma microcapsules induced by high-frequency laser pulses continuously cover the area of ​​the workpiece to be cleaned. These plasma microcapsules can effectively isolate oxygen and inhibit secondary oxidation of the material surface. At the same time, the accompanying shock wave pressure can assist in the removal of surface contaminants and apply plasma impact to the substrate, introducing compressive stress on the surface layer, thereby partially offsetting the residual tensile stress caused by the laser thermal effect, and completing the scanning cleaning of the workpiece by nanosecond laser on the focal plane.

[0069] S4 increases the peak current density of the power supply to 400A / mm². 2 The current frequency is 150kHz, the pulse current waveform is a sine wave, and the pulse current action time is 5~60s (adjusted according to the residual stress level). Based on the electroplastic effect, the dislocation slip and residual stress release of the workpiece are completed, and the surface temperature of the workpiece is reduced to below 200℃.

[0070] S5 turns off the power and allows the workpiece to air-cool to room temperature.

[0071] Example 7: This invention discloses a method for residual stress reduction in high-frequency alternating current combined with nanosecond pulsed laser cleaning. The difference from Example 1 is that it includes the following steps:

[0072] S1 Prepare a workpiece to be cleaned. The workpiece is made of titanium alloy TC4 with a thickness of 0.5mm. The surface contaminants of the workpiece include oxide layer, oil stains, paint layer or coating. The thickness of the surface contaminants does not exceed 100μm. The positive and negative terminals of the high-frequency AC power supply are electrically connected to the two electrodes of the workpiece through a sliding resistor.

[0073] The peak current density of the S2 control power supply is 50A / mm². 2 The current frequency is 100kHz, the pulse current waveform is a sine wave, and the workpiece is preheated to 150℃ based on the Joule heating effect and skin effect of the pulse current of the power supply.

[0074] S3 maintains the applied pulsed current, applying a nanosecond laser to the focal plane of the workpiece based on a predetermined bow-shaped scanning path. The scanning line spacing is controlled at 40 μm, and the scanning point overlap rate is 40%. The scanning cleaning is performed in an air environment, utilizing high-frequency, short-pulse nanosecond laser process parameters: laser wavelength of 1060 nm, pulse width of 30 ns, pulse repetition frequency of 270 kHz, and pulse energy density of 1.2 J / cm².2 High-temperature, high-pressure plasma microcapsules induced by high-frequency laser pulses continuously cover the area of ​​the workpiece to be cleaned. These plasma microcapsules can effectively isolate oxygen and inhibit secondary oxidation of the material surface. At the same time, the accompanying shock wave pressure can assist in the removal of surface contaminants and apply plasma impact to the substrate, introducing compressive stress on the surface layer, thereby partially offsetting the residual tensile stress caused by the laser thermal effect, and completing the scanning cleaning of the workpiece by nanosecond laser on the focal plane.

[0075] S4 increases the peak current density of the power supply to 400A / mm². 2 The current frequency is 100kHz, the pulse current waveform is a sine wave, and the pulse current action time is 5~60s (adjusted according to the residual stress level). Based on the electroplastic effect, the dislocation slip and residual stress release of the workpiece are completed, and the surface temperature of the workpiece is reduced to below 200℃.

[0076] S5 turns off the power and allows the workpiece to air-cool to room temperature.

[0077] Example 8: This invention discloses a method for residual stress reduction in high-frequency alternating current combined with nanosecond pulsed laser cleaning. The difference from Example 1 is that it includes the following steps:

[0078] S1 Prepare a workpiece to be cleaned. The workpiece is made of titanium alloy TC4 with a thickness of 50mm. The surface contaminants of the workpiece include oxide layer, oil stains, paint layer or coating. The thickness of the surface contaminants does not exceed 100μm. The positive and negative terminals of the high-frequency AC power supply are electrically connected to the electrodes on both sides of the workpiece through a sliding resistor.

[0079] The peak current density of the S2 control power supply is 800A / mm². 2 The current frequency is 5MHz, the pulse current waveform is a sine wave, and the workpiece is preheated to 300℃ based on the Joule heating effect and skin effect of the pulse current of the power supply.

[0080] S3 maintains the applied pulsed current, applying a nanosecond laser to the focal plane of the workpiece based on a predetermined bow-shaped scanning path. The scanning line spacing is controlled at 40~60μm, and the scanning point overlap rate is 70%. Scanning and cleaning are performed in an air environment, utilizing high-frequency, short-pulse nanosecond laser process parameters: laser wavelength 1060nm, pulse width 30ns, pulse repetition frequency 270kHz, and pulse energy density 5.0J / cm². 2High-temperature, high-pressure plasma microcapsules induced by high-frequency laser pulses continuously cover the area of ​​the workpiece to be cleaned. These plasma microcapsules can effectively isolate oxygen and inhibit secondary oxidation of the material surface. At the same time, the accompanying shock wave pressure can assist in the removal of surface contaminants and apply plasma impact to the substrate, introducing compressive stress on the surface layer, thereby partially offsetting the residual tensile stress caused by the laser thermal effect, and completing the scanning cleaning of the workpiece by nanosecond laser on the focal plane.

[0081] S4 increases the peak current density of the power supply to 1000A / mm². 2 The current frequency is 5MHz, the pulse current waveform is a sine wave, and the pulse current action time is 5~60s (adjusted according to the residual stress level). Based on the electroplastic effect, the dislocation slip and residual stress release of the workpiece are completed, and the surface temperature of the workpiece is reduced to below 200℃.

[0082] S5 turns off the power and allows the workpiece to air-cool to room temperature. Comparative Example

[0083] Comparative Example 1: This is a residual stress reduction method for high-frequency alternating current composite nanosecond pulse laser cleaning disclosed in this invention. The difference from Example 2 is that it does not include S3.

[0084] Comparative Example 2: This is a residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning disclosed in this invention. The difference from Example 3 is that it does not include S3.

[0085] Comparative Example 3: This is a residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning disclosed in this invention. The difference from Example 4 is that it does not include S3.

[0086] Comparative Example 4: This is a residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning disclosed in this invention. The difference from Example 5 is that it does not include S3.

[0087] Comparative Example 5: This is a residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning disclosed in this invention. The difference from Example 6 is that it does not include S3.

[0088] Performance testing

[0089] Experimental Example 1: The workpieces to be cleaned in Examples 2-6 were used as the original samples, the workpieces treated in Examples 2-6 were used as the cleaned-on samples, and the workpieces treated in Comparative Examples 1-5 were used as the cleaned-untreated samples. The surface morphology, oxygen content before and after cleaning, and residual stress of these samples were tested. The test results are as follows: Figures 2-5 As shown.

[0090] from Figures 2-5It can be seen that the surface morphology of the untreated sample after cleaning and the sample after cleaning and energizing are extremely similar, indicating that the thermal effect introduced by the high-frequency current did not have a negative impact on the cleaning effect. For aerospace metal materials such as TC4, TA15, 0Cr18, GH188, and GH625, the residual stress decreased significantly after high-frequency current combined with nanosecond pulsed laser cleaning, with reductions of 73.98%, 59.30%, 64.66%, 57.04%, and 63.63%, respectively.

[0091] The stress reduction mechanism of this technology mainly stems from the synergistic effect of the following aspects: First, the Joule heating generated by the high-frequency current has a preheating effect, which can reduce the temperature gradient during laser irradiation, thereby reducing the residual tensile stress caused by thermal stress. Second, this technology utilizes the skin effect to precisely confine electrical energy to the micron-level surface layer of the laser heat-affected zone, and reduces the critical shear stress for dislocation slip by means of the electroplastic effect, promoting dislocation movement. At the same time, the thermal effect of the laser and the electron wind work together to enable the directional electron flow to more effectively drive dislocations and alleviate pinning. In addition, by using high-frequency, short-pulse laser parameters, plasma can be rapidly excited on the surface, and the compressive stress introduced by its impact force can partially offset the residual tensile stress.

[0092] In summary, under the combined effects of electroplasticity, electron wind, and plasma impact, this technology can effectively suppress and reduce the residual tensile stress on the surface generated during laser cleaning, achieving a significant improvement in the integrated "cleaning-modification" of the surface integrity of aerospace metal materials.

[0093] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for reducing residual stress in high-frequency alternating current combined with nanosecond pulsed laser cleaning, characterized in that: Includes the following steps, S1 Prepare a workpiece to be cleaned and connect a high-frequency AC power supply to the workpiece; S2 preheats the workpiece to 150~300°C based on the Joule heating effect and skin effect of the pulse current of the power supply. S3 completes the scanning and cleaning of the workpiece using a nanosecond laser on the focal plane based on a predetermined scanning path; S4 increases the peak current density of the power supply, completes the dislocation slip and residual stress release of the workpiece based on the electroplastic effect, and reduces the surface temperature of the workpiece to below 200°C. S5. Turn off the power supply and air-cool the workpiece to room temperature; In S2 and S3, the peak current density of the control power supply is 50~800A / mm. 2 The current frequency is 100kHz~5MHz, and the pulse current waveform is a sine wave; In step S3, the wavelength of the nanosecond laser is controlled to be 1000~1100nm, the pulse width to be 25~35ns, the pulse repetition frequency to be 250~300kHz, and the pulse energy density to be 1.2~5.0J / cm². 2 .

2. The residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning according to claim 1, characterized in that: In S1, the workpiece is made of titanium alloy, high-temperature alloy, stainless steel or aluminum alloy, and has a thickness of 0.5~50mm.

3. The residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning according to claim 1, characterized in that: In S1, the surface contaminants of the workpiece include oxide layers, oil stains, paint layers, or coatings, and the thickness of the surface contaminants does not exceed 100 μm.

4. The residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning according to claim 1, characterized in that: In S3, the scanning cleaning is performed in an air environment. High-frequency, short-pulse nanosecond laser process parameters are used to induce the generation of high-temperature and high-pressure plasma microcapsules, which continuously cover the area of ​​the workpiece to be cleaned.

5. The residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning according to claim 4, characterized in that: In S3, the scanning path is one or a combination of several of the following: line-by-line scanning path, bow-shaped scanning path, grid interlacing scanning path, concentric circle scanning path, and spiral line scanning path.

6. The residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning according to claim 5, characterized in that: In S3, the scanning line spacing is controlled to be 40~60μm, and the scanning point overlap rate is 40~70%.

7. The residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning according to claim 1, characterized in that: In step S4, the peak current density of the control power supply is 400~1000 A / mm². 2 The current frequency is 100kHz~5MHz, and the pulse current waveform is a sine wave.

8. The residual stress reduction method for high-frequency alternating current combined with nanosecond pulse laser cleaning according to claim 1, characterized in that: In S4, the pulse current of the power supply is applied for 5 to 60 seconds.

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