Multi-step graded laser shock finishing strengthening method

Abstract

The invention discloses a multi-step graded laser shock finishing strengthening method which comprises the following steps: performing ultrahigh-frequency electric pulse assisted laser shock strengthening treatment on the surface of a metal workpiece, forming a coarse grain layer on the surface layer of the workpiece by utilizing a high-frequency electric pulse skin effect, and constructing a heterogeneous deformation layer; and according to the thickness of the coarse grain layer, ultrasonic rolling treatment is conducted on the surface of the metal workpiece, the surface coarse grain layer is converted into a nanocrystalline layer by controlling the proportional relation between the ultrasonic rolling nanometer refining depth and the thickness of the coarse grain layer, and the heterogeneous deformation layer is converted into a gradient nanometer structure layer. According to the invention, nanometer refinement and finishing strengthening of the surface of the workpiece material can be synchronously realized.

Description

Technical Field

[0001] This invention belongs to the field of surface strengthening technology for metallic materials, and particularly relates to a multi-step graded laser shock finishing and strengthening method. Background Technology

[0002] Metal surface strengthening technology is an important way to improve the fatigue performance of key components such as aero-engine blades and spacecraft load-bearing structures. Laser shock peening technology introduces residual compressive stress into the material surface layer through high-energy laser-induced shock waves, which can effectively inhibit crack initiation. High-frequency electrical pulse-assisted laser shock peening utilizes the electroplastic effect and skin effect to further extend the depth of the strengthened layer and control the microstructure gradient distribution. Ultrasonic rolling technology achieves surface grain nanostructuring and surface finishing through the combined effect of high-frequency vibration and static load. When applied individually, these technologies can improve the surface properties of materials to a certain extent and have been widely used in the surface treatment of high-performance materials such as titanium alloys and nickel-based superalloys.

[0003] However, existing technologies still have significant limitations. The depth of the layer strengthened by single laser shock blasting is usually less than 0.5 mm, and the residual compressive stress is steeply distributed along the depth direction, which easily leads to stress concentration in the transition zone and becomes a source of fatigue cracks. Although high-frequency electrical pulses can deepen the strengthening layer, their skin effect inevitably forms a coarse-grained layer on the surface, which disrupts the continuity of the gradient nanostructure and reduces fatigue performance. Traditional laser shock blasting and rolling composite processes lack a quantitative synergistic mechanism between processes, resulting in insufficient interlayer microstructure compatibility and poor matching of process parameters.

[0004] Therefore, how to synergistically regulate surface nano-sizing, residual stress gradient distribution, and surface smoothness to achieve a balance between deep strengthening and continuous microstructure and properties has become a key bottleneck restricting the development of metal surface strengthening technology. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a multi-step graded laser shock finishing and strengthening method, comprising:

[0006] Ultra-high frequency electric pulse assisted laser shock strengthening treatment is performed on the surface of metal workpieces. The skin effect of high frequency electric pulse is used to form a coarse grain layer on the surface of the workpiece and to construct a heterogeneous deformation layer.

[0007] Based on the thickness of the coarse grain layer, the surface of the metal workpiece is subjected to ultrasonic rolling treatment. By controlling the ratio between the ultrasonic rolling nano-refinement depth and the thickness of the coarse grain layer, the surface coarse grain layer is converted into a nano-crystalline layer, and the heterogeneous deformation layer is transformed into a gradient nanostructure layer.

[0008] Preferably, the process of performing ultra-high frequency electrical pulse-assisted laser shock strengthening treatment on the surface of a metal workpiece includes:

[0009] A synchronous and synergistic strengthening process of electric pulse and laser shock is performed by applying an ultra-high frequency electric pulse with a frequency of 800-1200Hz and a current density of 50-200A / mm², and a laser with a pulse energy of 1-15J, a pulse width of 10-100ns, and a spot diameter of 1-5mm to the surface of a metal workpiece.

[0010] Preferably, the process of synchronous and synergistic enhancement of electrical pulses and laser shocks includes:

[0011] By adjusting the laser pulse frequency and electrical pulse frequency using an oscilloscope and control circuit, the synchronization timing deviation between the peak laser energy and the peak electrical pulse frequency is kept within 10μs, thereby achieving synchronous and coordinated enhancement processing of electrical pulse and laser shock.

[0012] Preferably, the formula for calculating the thickness of the coarse-grained layer is as follows:

[0013]

[0014] Where k is the material constant, μ is the material permeability, σ is the material conductivity, f is the electric pulse frequency, and d is the coarse grain layer thickness.

[0015] Preferably, the process of ultrasonic rolling treatment on the surface of the metal workpiece includes:

[0016] Nanoscale refinement of the coarse-grained surface layer is achieved by applying ultrasonic vibrations with a frequency of 15–40 kHz and an amplitude of 5–20 μm, along with a static load of 0.5–2.0 kN.

[0017] Preferably, the ratio of the ultrasonic rolling nano-refining depth to the coarse grain layer thickness is: h = 1.1~1.2d;

[0018] Where h is the ultrasonic rolling nano-refining depth and d is the coarse grain layer thickness.

[0019] Preferably, the formula for the static load is:

[0020] F = (0.3~1.8)h·HV×10 -4 =(0.5~2.0)d·HV×10 -4

[0021] Where F is the static load, h is the ultrasonic rolling nano-refining depth, d is the coarse grain layer thickness, and HV is the Vickers hardness of the material.

[0022] Preferably, the parameters of the ultrasonic rolling process satisfy:

[0023] A = (0.1~0.3)d

[0024] Where d is the thickness of the coarse-grained layer and A is the ultrasonic vibration amplitude.

[0025] Preferably, the parameters of the ultrasonic rolling process satisfy:

[0026] v=(0.2~0.5)gA

[0027] Where v is the feed speed of the vibrating head, A is the ultrasonic vibration amplitude, and g is the ultrasonic vibration frequency.

[0028] Preferably, before performing ultra-high frequency electric pulse assisted laser shock strengthening treatment, the method further includes: sandblasting or polishing the workpiece surface to make the initial roughness Ra ≤ 1.6 μm;

[0029] After ultrasonic rolling, the method further includes cleaning with acetone or anhydrous ethanol solution to remove surface contaminants and lubricating medium, and the surface roughness of the final material is Ra≤0.2μm.

[0030] Compared with the prior art, the present invention has the following advantages and technical effects:

[0031] This invention establishes a synergistic mechanism between "high-frequency electrical pulse-laser shock strengthening" and ultrasonic rolling, creating a quantitative matching relationship between the coarse grain layer thickness d and the ultrasonic rolling nano-refinement depth h. This achieves the complete elimination of the skin effect in the coarse grain layer and the transformation of the heterogeneous deformation layer into a gradient nanostructure layer. By precisely controlling process parameters, a gradient nanostructure with a depth of 1.2–1.5 mm is formed, with the residual compressive stress field transitioning smoothly and continuously along the depth direction, eliminating stress abrupt change zones. Simultaneously, surface nano-refinement and finishing are achieved, resulting in excellent surface integrity with a surface roughness Ra≤0.2 μm, thus increasing the fatigue life of metal parts by 2–3 times. Attached Figure Description

[0032] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0033] Figure 1 This is a schematic diagram of the method flow according to an embodiment of the present invention;

[0034] Figure 2 This is a schematic diagram of the surface roughness, grain size, reinforcement layer depth, and residual stress distribution curves in an embodiment of the present invention. Detailed Implementation

[0035] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0036] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0037] like Figure 1 and Figure 2 As shown, this embodiment provides a multi-step graded laser shock finishing and strengthening method, including:

[0038] Ultra-high frequency electric pulse assisted laser shock strengthening treatment is performed on the surface of metal workpieces. The skin effect of high frequency electric pulse is used to form a coarse grain layer on the surface of the workpiece and to construct a heterogeneous deformation layer.

[0039] Based on the thickness of the coarse grain layer, ultrasonic rolling is performed on the surface of the metal workpiece. By controlling the ratio between the ultrasonic rolling nano-refinement depth and the thickness of the coarse grain layer, the surface coarse grain layer is converted into a nano-crystalline layer, and the heterogeneous deformation layer is transformed into a gradient nanostructure layer.

[0040] This embodiment first performs ultra-high frequency electrical pulse-assisted laser shock strengthening treatment on the surface of the metal workpiece. During this process, high frequency electrical pulses and laser shock are applied to the workpiece simultaneously. Utilizing the skin effect of the high frequency electrical pulses and the gradient pressure distribution of the laser-induced shock wave, a coarse-grained layer with a specific thickness is formed on the surface of the workpiece. Simultaneously, a heterogeneous deformation layer exhibiting a gradient distribution of coarse-grained to fine-grained to coarse-grained characteristics is constructed from the surface inwards. After completing the above processing steps, based on the aforementioned coarse-grained layer thickness parameter as a reference input, a precise ultrasonic rolling process is performed on the surface of the workpiece after the electrical pulse-laser shock strengthening treatment. By precisely controlling the ratio between the nano-refinement depth parameter and the coarse-grained layer thickness parameter of the ultrasonic rolling process, the surface coarse-grained layer undergoes sufficient plastic deformation under the combined action of ultrasonic vibration and static pressure, thereby completely converting the coarse-grained layer into a nanocrystalline layer. This achieves the overall transformation of the heterogeneous deformation layer into a continuous gradient nanostructure layer. This gradient nanostructure layer forms a continuous transition in grain size from nanoscale to microscale in the depth direction, while obtaining excellent surface morphology.

[0041] Furthermore, the process of performing ultra-high frequency electrical pulse-assisted laser shock peening treatment on the surface of a metal workpiece includes:

[0042] A synchronous and synergistic strengthening process of electric pulse and laser shock is performed by applying an ultra-high frequency electric pulse with a frequency of 800-1200Hz and a current density of 50-200A / mm², and a laser with a pulse energy of 1-15J, a pulse width of 10-100ns, and a spot diameter of 1-5mm to the surface of a metal workpiece.

[0043] Furthermore, this embodiment focuses on the strengthening process of ultra-high frequency electric pulse assisted laser shock. During the laser shock strengthening process, an ultra-high frequency electric pulse with a frequency of f is applied to the surface of the metal workpiece. The skin effect of the ultra-high frequency electric pulse and the gradient distribution of the shock wave pressure are used to obtain a coarse grain layer with a thickness of d on the surface of the workpiece, and a heterogeneous deformation layer with a coarse-fine-coarse depth of D is formed.

[0044] Furthermore, the process of synchronously and synergistically enhancing the electrical pulse and laser shock includes:

[0045] By adjusting the laser pulse frequency and electrical pulse frequency using an oscilloscope and control circuit, the synchronization timing deviation between the peak laser energy and the peak electrical pulse frequency is kept within 10μs, thereby achieving synchronous and coordinated enhancement processing of electrical pulse and laser shock.

[0046] Specifically, in the ultra-high frequency electric pulse assisted laser shock enhancement process, a dedicated synchronous control system is used to precisely regulate and match the electric pulse parameters and laser parameters. The system uses an oscilloscope to monitor the timing status of the electric pulse and laser pulse in real time, and dynamically adjusts the phase relationship between the laser pulse frequency and the electric pulse frequency through the control circuit. This ensures that the time synchronization deviation between the peak laser energy output and the peak electric pulse frequency is strictly controlled within a precision range of no more than 10μs, thereby achieving precise coupling and superposition of the electric pulse thermal effect and the laser shock wave effect.

[0047] Specifically, the electrical pulse parameters are set to a high-frequency electrical pulse with a current density of 50–200 A / mm², a frequency of 800–1200 Hz, and a pulse width of 50–150 μs. This electrical pulse is applied through contact between the electrode and the workpiece surface. The laser parameters are set to a short-pulse high-energy laser beam with a pulse energy of 1–15 J, a single pulse width of 10–100 ns, a spot diameter of 1–5 mm, and an overlap rate of 30%–60%. The laser beam is focused by a light guiding system to ensure that it completely overlaps with the area of ​​the electrical pulse on the workpiece surface. Other process parameters, such as pulse repetition frequency and scanning speed, are adaptively set based on the material properties and workpiece geometry according to engineering experience to ensure the uniformity of energy input and process stability.

[0048] Furthermore, this embodiment achieves synchronization timing deviation control through a multi-channel synchronization triggering system. This system includes an electrical pulse generation module, a laser triggering module, a timing detection module, and a feedback control module. The electrical pulse generation module generates a frequency-adjustable high-frequency electrical pulse signal. The laser triggering module receives external trigger commands to control the laser pulse output. The timing detection module uses a high-speed photodetector and a current transformer to collect the real-time waveforms of the laser pulse and the electrical pulse, respectively. The actual time difference between the peak values ​​of the two pulses is calculated by a time interval measurement unit. The feedback control module automatically adjusts the laser trigger delay line or the electrical pulse phase based on the measured time difference, forming a closed-loop control circuit to ensure that the synchronization deviation is always maintained within 10μs. When the detected synchronization deviation exceeds the threshold, the system automatically alarms and suspends the process. It resumes operation after adjustment is satisfactory. This control mechanism effectively ensures the stability and repeatability of the electro-optic coupling effect.

[0049] Furthermore, in this embodiment, based on the skin effect in high-frequency electrical pulses, the thickness of the coarse-grained layer d satisfies:

[0050]

[0051] Where k is a material constant (for titanium alloys, take k as k). Nickel-based alloys μ is the material's magnetic permeability ( ), σ is the electrical conductivity of the material ( f is the frequency of the electrical pulse ( ), d is the thickness of the coarse-grained layer ( ).

[0052] Furthermore, in this embodiment, the thickness d of the coarse-grained layer is determined based on the electromagnetic field theory of the skin effect, and is accurately calculated using the following formula:

[0053]

[0054] Where d represents the thickness of the coarse-grained layer caused by the skin effect of the ultra-high frequency electric pulse, in μm; k is a material constant, which takes into account the intrinsic properties of the material such as crystal structure, dislocation density, and dynamic recrystallization threshold. For titanium alloy systems, k ranges from 7 to 8, and for nickel-based superalloy systems, k ranges from 4 to 6; μ is the magnetic permeability of the material, in H / m, which needs to be obtained by referring to a table or by testing based on the specific alloy composition; σ is the electrical conductivity of the material, in S / m, which needs to be determined based on the test results of the material's room temperature electrical conductivity; and f is the frequency of the applied high-frequency electric pulse, in Hz.

[0055] In practical engineering applications, the range of k values ​​is first determined based on the target material type. The accurate values ​​of μ and σ are obtained through experimental testing. Then, the electric pulse frequency f is determined according to the process requirements. Substituting this value into the formula, the thickness d of the coarse grain layer can be accurately predicted. The deviation between the predicted value and the design target value is controlled within ±5%, providing a reliable basis for the subsequent design of ultrasonic rolling process parameters.

[0056] Furthermore, the method in this embodiment only requires knowledge of the material's basic characteristic parameters, and can calculate the required electrical pulse frequency in advance by combining the skin effect formula for electrical pulses. Corresponding coarse grain layer thickness .

[0057] Meanwhile, in this embodiment, the target depth of ultrasonic rolling only needs to be determined based on the desired gradient nanostructure depth. This allows us to determine the maximum thickness of the coarse-grained layer generated by the skin effect of high-frequency electrical pulses. By combining the skin effect formula of electrical pulses, one can determine the target... Derivation of the optimal electrical pulse frequency .

[0058] Furthermore, in the formula for the skin effect of electrical pulses, the metallic material constants summarized and deduced in this embodiment... Simply choose flexibly The process parameters for similar materials can be found by adjusting the k value. The strengthening effect can be adjusted by simply adjusting the k value to adapt to different alloy systems.

[0059] Furthermore, the process of ultrasonic rolling treatment on the surface of metal workpieces includes:

[0060] Nanoscale refinement of the coarse-grained surface layer is achieved by applying ultrasonic vibrations with a frequency of 15–40 kHz and an amplitude of 5–20 μm, along with a static load of 0.5–2.0 kN. The rolling head is made of cemented carbide or diamond and has a radius of curvature of 2. 10 mm, and a lubricating medium is applied during the process.

[0061] Furthermore, in this embodiment, ultrasonic rolling with a static pressure of F, a frequency of g, and an amplitude of A is applied to the surface of the treated workpiece to achieve the transformation of the surface coarse-grained layer into a nanocrystalline layer. The plastic deformation force generated by the combined action of ultrasonic vibration and static pressure on the sample surface causes the surface coarse-grained layer to accumulate a large amount of dislocation and stacking fault energy, resulting in dislocation slip and the formation of finer grain boundaries. This achieves grain recrystallization and refinement, transforming the grains into ultrafine nanocrystals and nanosubcrystals, thereby realizing the transformation of the surface coarse-grained layer into a nanocrystalline layer, and further realizing the transformation of the heterogeneous deformation layer into a gradient nanostructure layer, while obtaining excellent surface morphology.

[0062] Specifically, the ultrasonic rolling process involved in this embodiment employs a combined loading mode of high-frequency ultrasonic vibration and static load. An ultrasonic generator produces an electrical signal with a frequency of 15–40 kHz, which is converted into mechanical vibration of the same frequency by a piezoelectric transducer. The amplitude is amplified to the range of 5–20 μm by an amplitude transformer and then applied to the workpiece surface through a spherical rolling head. Simultaneously, a static load of 0.5–2.0 kN is applied through a hydraulic or pneumatic loading system. This static load is transmitted to the workpiece surface through the rolling head, forming a constant contact stress. The rolling head is made of cemented carbide or diamond, with a spherical radius of curvature of 2–10 mm, preferably 5–8 mm, and a surface roughness Ra ≤ 0.1 μm to ensure good contact with the workpiece surface.

[0063] During the process, a lubricating medium is continuously applied between the rolling head and the workpiece. The lubricating medium is an oil-based lubricant or a water-based emulsion to reduce the coefficient of friction, prevent surface damage, and promote plastic flow. The viscosity of the lubricating medium is controlled between 20 and 100 cSt, and the supply rate is maintained at 5 to 10 ml / min to ensure the stability of the process and the integrity of the surface.

[0064] Furthermore, the formula for static load is as follows:

[0065] F = (0.3~1.8)h·HV×10 -4 =(0.5~2.0)d·HV×10 -4

[0066] Where F is the static load, h is the ultrasonic rolling nano-refining depth, d is the coarse grain layer thickness, and HV is the Vickers hardness of the material.

[0067] Furthermore, in this embodiment, the static load F is determined based on the coarse-grained layer thickness d, the Vickers hardness HV of the material, and the ultrasonic rolling plastic deformation mechanical model, and is accurately calculated using the following formula:

[0068] F = (0.5~2.0)d·HV×10 -4

[0069] Where F represents the static load in kN; d is the coarse grain layer thickness in μm; and HV is the Vickers hardness value of the material, with only the numerical part substituted when taking the value, in kgf / mm².

[0070] The derivation of this formula is based on Hertzian contact theory and the principle of volumetric work in plastic deformation. This embodiment also considers the elastic contact deformation between the rolling head and the workpiece surface, the depth of plastic zone expansion, and the material hardening characteristics. The coefficients 0.5 to 2.0 in the formula are process correction coefficients, and their specific values ​​are determined based on the workpiece geometry, the radius of curvature of the rolling head, and the material strain hardening index: for planar workpieces with a low material hardening index, 0.5 to 0.8 are used; for curved workpieces or with a high material hardening index, 1.5 to 2.0 are used; and in general, 0.8 to 1.2 are used.

[0071] In practical applications, the d value is first determined, the HV value is obtained through hardness testing, and then a correction coefficient is selected based on the workpiece characteristics. The recommended value of F is calculated by substituting the coefficients into the formula. The deviation between the calculated value and the actual optimal value is controlled within ±10%.

[0072] Furthermore, in order to achieve the transformation from the surface coarse-grained layer to the nanocrystalline layer, the ratio of the ultrasonic rolling nano-refinement depth to the thickness of the coarse-grained layer satisfies: h = 1.1~1.2d; thereby ensuring that the surface coarse-grained layer caused by the skin effect of high-frequency electrical pulses is completely refined, and ensuring that the final gradient nanostructure has dimensional consistency.

[0073] Where h is the ultrasonic rolling nano-refining depth and d is the coarse grain layer thickness.

[0074] Furthermore, the ratio of the ultrasonic rolling nanorefining depth h to the coarse-grained layer thickness d involved in this embodiment was determined through extensive process experiments and microstructure analysis, specifically satisfying the mathematical relationship h = 1.1 to 1.2d. This ratio is determined based on the following technical principles: to ensure that the surface coarse-grained layer generated by the skin effect of high-frequency electrical pulses can be completely refined into nanocrystals, the effective depth of ultrasonic rolling must be slightly greater than the thickness of the coarse-grained layer to ensure that the bottom region of the coarse-grained layer is also sufficiently subjected to plastic deformation; simultaneously, to avoid excessive processing leading to unnecessary coarsening of the deeper structure or surface damage, the value of h should not exceed 1.2 times the value of d. In actual operation, the value of d is first calculated, and then the ultrasonic rolling process parameters are designed to ensure that the value of h falls within the range of 1.1d to 1.2d. Precise control of the value of h is achieved by adjusting the matching relationship between the static load F, amplitude A, and vibration frequency g. This strict control of the ratio is a key technical guarantee for ensuring that the final gradient nanostructure layer has dimensional consistency and no abrupt structural changes.

[0075] Furthermore, the parameters of the ultrasonic rolling process satisfy:

[0076] A = (0.1~0.3)h ≈ (0.1~0.3)d

[0077] Where d is the thickness of the coarse-grained layer and A is the ultrasonic vibration amplitude.

[0078] Furthermore, in this embodiment, the determination of the ultrasonic vibration amplitude A is directly related to the coarse grain layer thickness d, satisfying the relationship A = (0.1~0.3)d, where A represents the ultrasonic vibration amplitude in μm and d is the coarse grain layer thickness in μm.

[0079] The relationship is established based on the theoretical analysis of ultrasonic vibration energy transfer and surface plastic deformation: if the amplitude is too small, it cannot provide enough dynamic energy to promote dislocation movement and grain refinement; if the amplitude is too large, it may lead to surface fatigue damage or microcrack initiation. The coefficient 0.1 to 0.3 is selected based on a comprehensive consideration of material plasticity, rolling head quality, and system stiffness: 0.25 to 0.3 for high-plasticity materials; 0.1 to 0.15 for low-plasticity materials; and 0.15 to 0.2 for general materials. The actual control of amplitude A is achieved by adjusting the output power of the ultrasonic generator and the gain coefficient of the amplitude transformer. A laser Doppler vibrometer is used to monitor the actual amplitude at the end of the rolling head in real time, with a monitoring accuracy of ±0.5μm, ensuring that the amplitude is stable within ±3% of the set value, thus guaranteeing process repeatability and microstructure uniformity.

[0080] Furthermore, the parameters of the ultrasonic rolling process satisfy:

[0081] v=(0.2~0.5)gA

[0082] Where v is the feed speed of the vibrating head, A is the ultrasonic vibration amplitude, and g is the ultrasonic vibration frequency.

[0083] Furthermore, the parameters for ultrasonic rolling treatment in this embodiment satisfy the following:

[0084]

[0085]

[0086]

[0087] Where h: rolling nano-refinement depth ( );

[0088] F: Static load ( );

[0089] HV: Vickers hardness of the material (only the numerical value is required);

[0090] g: Ultrasonic vibration frequency, taken as... ( );

[0091] A: Ultrasonic vibration amplitude ( );

[0092] v: Vibrating head feed speed ( );

[0093] Finally obtained The depth of the super-gradient nanostructure is far greater than that achieved by a single technology. (Ordinary laser shock peening is approximately...) )

[0094] Furthermore, in this embodiment, the determination of the rolling head feed speed v is based on the matching relationship between the ultrasonic vibration frequency g and the amplitude A, calculated using the formula v = (0.2~0.5)gA, where v represents the feed speed in mm / s; g represents the ultrasonic vibration frequency in kHz; and A represents the ultrasonic vibration amplitude in μm. This formula ensures a suitable overlap between adjacent rolling tracks, fully covering the plastic deformation area without omissions: excessive feed speed leads to insufficient overlap between tracks, forming untreated strips; excessive feed speed leads to local over-processing and energy waste. The coefficient 0.2~0.5 is selected based on surface roughness requirements and material response characteristics: 0.2~0.3 is used when high surface finish is required; 0.4~0.5 is used when high efficiency is required; and 0.3~0.4 is used in general cases.

[0095] In actual process planning, the g and A values ​​are first determined, and then the recommended range of v is calculated by substituting them into the formula. The feed speed is then precisely controlled by a CNC machine tool or robot system with a speed control accuracy of ±0.5mm / s to ensure the uniformity and integrity of the processing trajectory.

[0096] Furthermore, before performing ultra-high frequency electric pulse assisted laser shock strengthening treatment, the method also includes: sandblasting or polishing the workpiece surface to make the initial roughness Ra≤1.6μm;

[0097] Specifically, for workpiece surfaces with rust, corrosion, protective paint, or passivation film coatings, sandblasting is used to remove surface interference, exposing the surface to be strengthened, followed by polishing to reduce the initial roughness. And clean to remove surface contaminants.

[0098] After ultrasonic rolling, the method further includes cleaning with acetone or anhydrous ethanol solution to remove surface contaminants and lubricating medium, resulting in a final material with a surface roughness Ra ≤ 0.2 μm.

[0099] Specifically, the process before ultrasonic rolling also includes surface cleaning: removing surface ablation and oxidation residues through fine grinding or polishing.

[0100] After ultrasonic rolling, surface cleaning is performed: acetone or anhydrous ethanol solution is used to clean and remove surface contaminants and lubricating media, followed by air drying.

[0101] Furthermore, this embodiment includes a systematic pretreatment process for the workpiece surface before performing ultra-high frequency electrical pulse assisted laser shock strengthening treatment. The pretreatment process includes: for workpieces with rust, corrosion areas, protective paint layers, passivation films, or coatings on the surface, surface cleaning is first performed by sandblasting. The sandblasting medium is selected as corundum sand or glass beads with a particle size of 100-200 mesh, the sandblasting pressure is 0.3-0.5 MPa, the sandblasting angle is 45°-75°, and the treatment time is 1-3 minutes to thoroughly remove the surface coating and oxides, exposing the metal substrate. After sandblasting, polishing is performed by using diamond polishing paste or sandpaper to polish step by step. The polishing step size is P240, P600, P1200, and P2000 from coarse to fine. After each polishing step, anhydrous ethanol is used for cleaning, and the initial surface roughness Ra is ≤1.6μm. After polishing, acetone or anhydrous ethanol solution is used for ultrasonic cleaning for 15-30 minutes to remove residual polishing agent and contaminants on the surface. After cleaning, the surface is air-dried in a clean environment.

[0102] Before ultrasonic rolling, this embodiment also performs an intermediate cleaning process on the workpiece surface after electro-pulse laser shock hardening treatment. Precision grinding or chemical polishing is used to remove any micro-ablation, oxidation residues, and adsorbates that may have formed on the surface, ensuring good contact between the rolling head and the workpiece surface. After ultrasonic rolling, a final surface cleaning process is performed, using acetone or anhydrous ethanol solution for ultrasonic cleaning for 10-20 minutes to remove residual lubricating medium and metal debris from the rolling process. Subsequently, it is dried in a vacuum drying oven at 50-80°C for 30 minutes to obtain a clean and high-performance final surface.

[0103] As a preferred implementation method, this embodiment uses high-frequency electrical pulse-assisted laser shock to enhance the construction of a depth gradient structure. and a continuously decreasing residual stress field;

[0104] Using ultrasonic rolling to eliminate the skin-tightening effect of coarse crystalline layers ( Simultaneously, optimize the residual stress gradient distribution;

[0105] Establish and the depth of ultrasonic rolling action The quantitative relationship is used to determine the ultrasonic rolling process parameters and ensure the stability of the process effect.

[0106] The method in this embodiment has the advantage of gradient structure: the residual stress transitions smoothly, completely eliminates the influence of the skin effect of electric pulse, avoids early fatigue cracks caused by stress concentration, and increases the fatigue life of metal parts by 2-3 times.

[0107] This embodiment features deep strengthening, with an affected layer depth of 0.5-1.0 mm, far exceeding that of single-process methods (<0.5 mm); it also maintains surface integrity: Ra≤0.2. This reduces stress concentration caused by surface defects.

[0108] In the above embodiments, taking the titanium alloy TC4 blade as an example, the following processes are performed sequentially:

[0109] Surface pretreatment: The TC4 titanium alloy workpiece is ground with a 120# diamond grinding wheel, and the surface roughness is controlled within Ra1.6μm. The surface oil is removed by ultrasonic cleaning with acetone for 15 minutes.

[0110] Electrical pulse + laser shock peening: current density 150 A / mm², electrical pulse frequency f = 1000 Hz, electrical pulse width 100 mm. Laser pulse energy 3J, single laser pulse width 20ns, spot diameter 2mm, overlap rate 50%.

[0111] According to the formula for the skin effect of electrical pulses, the thickness d of the coarse-grained layer satisfies:

[0112]

[0113] Wherein: the electrical conductivity of TC4 metal ,

[0114] magnetic permeability ,Pick The final calculation is

[0115] Ultrasonic rolling: frequency selection The indenter is the radius of curvature of the diamond ball head. ;

[0116] static load TC4 has a Vickers hardness of approximately 340 HV, therefore we take... ;

[0117] amplitude ,Pick ;

[0118] Roller head feed rate ,Pick

[0119] Final result: Gradient nanostructure depth 1.25 mm, skin effect coarse grain layer completely eliminated;

[0120] Surface roughness is It meets the requirements;

[0121] Fatigue life increased by 2.8 times, meeting the requirements.

[0122] This embodiment employs a two-step synergistic process of "high-frequency electrical pulse-laser shock strengthening" and "ultrasonic rolling," pioneering and utilizing a synergistic mechanism of "high-frequency electrical pulse skin effect utilization-elimination." By precisely controlling the ratio between the coarse-grained layer formed by the high-frequency electrical pulse skin effect on the surface and the total gradient depth, it breaks through the depth limitations of traditional processes, achieving an extraordinary gradient structure. By constructing a continuous and gentle residual compressive stress field, it eliminates the stress abrupt change zone of traditional processes, improving the smoothness of stress transition in the depth direction by more than 40%. The method of this embodiment can also precisely control the influence depth of ultrasonic rolling to ensure the complete elimination of the coarse-grained layer on the surface caused by the high-frequency electrical pulse skin effect, simultaneously achieving nano-refinement and finishing strengthening of the workpiece material surface.

[0123] This embodiment solves the technical challenge of achieving both the skin effect of electric pulses and deep strengthening in high-frequency electric pulse-laser shock treatment, thereby increasing the fatigue life of key components such as titanium alloys and nickel-based high-temperature alloys by 2-3 times. It is particularly suitable for high-performance applications such as aero-engine blades and spacecraft load-bearing components.

[0124] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A multi-step graded laser shock finishing and strengthening method, characterized in that, include: Ultra-high frequency electric pulse assisted laser shock strengthening treatment is performed on the surface of metal workpieces. The skin effect of high frequency electric pulse is used to form a coarse grain layer on the surface of the workpiece and to construct a heterogeneous deformation layer. Based on the thickness of the coarse grain layer, the surface of the metal workpiece is subjected to ultrasonic rolling treatment. By controlling the ratio between the ultrasonic rolling nano-refinement depth and the thickness of the coarse grain layer, the surface coarse grain layer is converted into a nano-crystalline layer, and the heterogeneous deformation layer is transformed into a gradient nanostructure layer.

2. The method according to claim 1, characterized in that, The process of performing ultra-high frequency electrical pulse-assisted laser shock peening treatment on the surface of a metal workpiece includes: A synchronous and synergistic strengthening process of electric pulse and laser shock is performed by applying an ultra-high frequency electric pulse with a frequency of 800-1200Hz and a current density of 50-200A / mm², and a laser with a pulse energy of 1-15J, a pulse width of 10-100ns, and a spot diameter of 1-5mm to the surface of a metal workpiece.

3. The method according to claim 2, characterized in that, The process of synchronous and synergistic enhancement of electrical pulses and laser shocks includes: By adjusting the laser pulse frequency and electrical pulse frequency using an oscilloscope and control circuit, the synchronization timing deviation between the peak laser energy and the peak electrical pulse frequency is kept within 10μs, thereby achieving synchronous and coordinated enhancement processing of electrical pulse and laser shock.

4. The method according to claim 1, characterized in that, The formula for calculating the thickness of the coarse-grained layer is as follows: Where k is the material constant, μ is the material permeability, σ is the material conductivity, f is the electric pulse frequency, and d is the coarse grain layer thickness.

5. The method according to claim 1, characterized in that, The process of ultrasonic rolling treatment on the surface of the metal workpiece includes: Nanoscale refinement of the coarse-grained surface layer is achieved by applying ultrasonic vibrations with a frequency of 15–40 kHz and an amplitude of 5–20 μm, along with a static load of 0.5–2.0 kN.

6. The method according to claim 5, characterized in that, The ratio of the ultrasonic rolling nano-refining depth to the coarse grain layer thickness is: h = 1.1~1.2d; Where h is the ultrasonic rolling nano-refining depth and d is the coarse grain layer thickness.

7. The method according to claim 5, characterized in that, The formula for the static load is as follows: F=(0.3~1.8)h·HV×10 -4 =(0.5~2.0)d·HV×10 -4 Where F is the static load, h is the ultrasonic rolling nano-refining depth, d is the coarse grain layer thickness, and HV is the Vickers hardness of the material.

8. The method according to claim 1, characterized in that, The parameters of the ultrasonic rolling process satisfy: A = (0.1~0.3)d Where d is the thickness of the coarse-grained layer and A is the ultrasonic vibration amplitude.

9. The method according to claim 1, characterized in that, The parameters of the ultrasonic rolling process satisfy: v=(0.2~0.5)gA Where v is the feed speed of the vibrating head, A is the ultrasonic vibration amplitude, and g is the ultrasonic vibration frequency.

10. The method according to claim 1, characterized in that, Before performing ultra-high frequency electric pulse assisted laser shock strengthening treatment, the method further includes: sandblasting or polishing the workpiece surface to make the initial roughness Ra≤1.6μm; After ultrasonic rolling, the method further includes cleaning with acetone or anhydrous ethanol solution to remove surface contaminants and lubricating medium, and the surface roughness of the final material is Ra≤0.2μm.

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