A method for laser shock peening of bcc metal sheet and applications thereof

Abstract

This invention relates to the field of laser processing, and more particularly to a laser shock peening method and its application for BCC metal. The method involves applying a constraint layer of appropriate thickness to the front and back sides of the BCC metal specimen to constrain the laser shock wave and increase its impact force; then, shock treatment is performed using a laser with a pulse width of 1–10 ns, a wavelength of 200–1000 nm, a laser energy of 10–50 J, a repetition frequency of 0.1–10 Hz, and a beam diameter of 0.5–5 mm. The BCC metal specimen treated by this method forms a large number of dislocation groups, dislocation entanglements, dislocation pile-ups, and recrystallized nanocrystals within its interior, increasing the surface microhardness by 20%–40%. The resulting surface strengthening effect can significantly inhibit the initiation and propagation of fatigue cracks, and the process is simple, efficient, low-cost, and pollution-free.

Description

Technical Field

[0001] This invention relates to the field of laser processing technology, specifically to a laser shock peening method and application for BCC metal sheets. The strengthened BCC metal is suitable for high-temperature metal structural materials, engine nozzles, missiles, rocket propulsion units, or aero-engine components used in the aerospace industry. Background Technology

[0002] Tungsten (W), molybdenum (Mo), tantalum (Ta), and other body-centered cubic (BCC) metals possess high strength and hardness, excellent wear resistance, and impact resistance. They are primarily used in structural materials, mechanical parts, and tool manufacturing, playing a vital role in industries such as construction, aerospace, automotive, and energy. The high strength and hardness of BCC metals enable them to withstand significant stress and loads, providing excellent support and durability, making them ideal structural materials for buildings, bridges, and aircraft. In the machinery field, BCC metals are widely used in manufacturing machine parts such as shafts, gears, and bolts. Their high strength and wear resistance allow these parts to withstand high pressure and frequent movement, extending equipment life and reliability. Furthermore, BCC metals play a crucial role in tool manufacturing. Due to their excellent wear resistance, cutting tools and cutters made from BCC metals can perform cutting operations efficiently, improving production efficiency and product quality. However, with rapid technological advancements, the service life and reliability of traditional BCC metals often fail to meet the demands of various complex service environments. Severe surface plastic deformation (S) is a significant issue. 2 PD (Polymerization Processing) is a surface modification technology that induces elastoplastic deformation by impacting or applying pressure to the surface of metallic materials, resulting in beneficial effects such as residual compressive stress, work hardening, and substructure refinement. This, in turn, enhances the strength, hardness, and ductility of the material, thereby increasing its service life and reliability.

[0003] Laser shock peening (LSP), a typical technique for severe plastic deformation of high-strain-rate surfaces, can achieve the fabrication of gradient nanostructures (GNS) and the formation of deeper levels of compressive residual stress (CRS). In LSP, metallic materials are irradiated with short-pulse (nanosecond and even picosecond and femtosecond levels) and high-power (GW / cm) lasers. Energy is absorbed in the surface absorption layer, causing explosive vaporization and forming high-temperature (>107 K) and high-pressure (>GPa) plasma. The plasma continues to absorb laser energy, undergoing violent outward expansion to form a laser shock wave (LSW). When the plasma pressure exceeds the elastic limit of the metallic material, dynamic plastic deformation occurs. Simultaneously, LSP can refine the grain size of metallic materials to submicron or nanoscale, and use a "top-down" grain refinement method to form gradient nanostructures and generate deeper levels of compressive residual stress. Through gradient nanostructures and residual compressive stress, numerous dislocation groups, dislocation tangles, dislocation pile-ups, and nanocrystalline recrystallized grains are formed within the sample. Furthermore, the recrystallized grains also contain a large number of dislocation tangles, forming a complex dislocation network. These two characteristics induced by laser shock peening are used to improve the strength, hardness, fatigue resistance, and corrosion resistance of metallic materials. Some research has been conducted in the prior art; for example, Weiwei Deng's paper "Progressive developments, challenges and future trends in laser shock peening of metallic materials and alloys: A comprehensive review" introduces the application of laser shock peening methods, but it does not address the relationship between the optically transparent glass constraint layer and the thickness of the thin plate to be treated. Summary of the Invention

[0004] To address the problem that the strength and hardness of traditional BCC metals used in existing engineering applications often fail to meet the requirements of various complex service environments, the first objective of this invention is to provide a laser shock peening method for BCC metal sheets. This method can enhance the strength and hardness of BCC metal sheets, and is simple to operate, low in cost, environmentally friendly, and highly effective.

[0005] Unlike ordinary BCC metal thick plates or blocks, thin BCC metal sheets are prone to problems such as surface quality damage, internal stress concentration, severe uneven deformation, and low strengthening effect when subjected to laser shock strengthening.

[0006] The second objective of this invention is to provide a laser-shock-strengthened BCC metal sheet prepared by the above-described strengthening method.

[0007] The third objective of this invention is to provide an application for high-quality BCC metal sheets prepared by the above-mentioned strengthening method using laser shock peening.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A laser shock strengthening method for BCC metal sheets, comprising the following steps: Laser shock strengthening technology is used to strengthen BCC metal materials.

[0010] (1) Surface pretreatment of BCC metal material thin plate: Before laser shock strengthening treatment, the surface of BCC metal material sample with a thickness of 100-1000μm is pretreated to ensure that there are no residual organic matter and other impurities on the sample.

[0011] In a preferred embodiment, the sample is placed in anhydrous ethanol and then placed in an ultrasonic cleaner. The ultrasonic frequency is set to 40–100 kHz, the cleaning temperature to 20–35 °C, and the ultrasonic oscillation cleaning is performed for 10–30 minutes.

[0012] The inventors discovered that anhydrous ethanol possesses excellent solvent properties, effectively removing surface grease, organic impurities, and dirt. Ultrasonic waves induce high-frequency vibrations in liquids, generating tiny bubbles. The collapse of these bubbles produces strong localized impact forces, effectively removing dirt from the surface. However, excessively high ultrasonic frequencies and temperatures may cause the peeling of microparticles and oxides from the material surface, leading to surface damage. Furthermore, the ultrasonic field can cause stress and dislocation effects on the grains within the material, exceeding the material's original lattice capabilities, potentially leading to grain growth over prolonged exposure. Conversely, excessively low ultrasonic temperatures and frequencies fail to completely remove surface organic matter and other impurities using anhydrous ethanol.

[0013] Further preferably, in the anhydrous ethanol ultrasonic oscillation cleaning, the ultrasonic frequency is 80KHz, the ultrasonic cleaning temperature is 25℃, and the ultrasonic cleaning time is 20min.

[0014] The preferred method involves removing the sample after ultrasonic cleaning with anhydrous ethanol, placing it in deionized water, and then placing it in an ultrasonic cleaner. The ultrasonic frequency is set to 30–100 kHz, the cleaning temperature to 20–25°C, and the ultrasonic cleaning is performed for 10–20 minutes. The cleaned sample is then removed, and surface moisture is dried using a 0.5–1 MPa nitrogen gas gun. Finally, it is placed in a vacuum drying oven at 100–150°C for 10–20 minutes to ensure complete removal of surface moisture.

[0015] The inventors discovered that ultrasonic cleaning of BCC metal surfaces with anhydrous ethanol may leave trace amounts of solvent residue. To ensure surface purity, deionized water can be used for a second cleaning to remove residual anhydrous ethanol and other potential impurities. However, it's important to note that after ultrasonic cleaning with deionized water removes residual anhydrous ethanol, moisture may remain on the BCC metal surface, potentially leading to oxidation and corrosion. To ensure surface purity and dryness and prevent oxidation and corrosion, drying with a nitrogen gas gun and a vacuum drying oven is a common method. Nitrogen is a dry, colorless, and odorless gas, free of moisture and other impurities, effectively drying the metal surface and preventing corrosion caused by residual moisture. Similarly, when ultrasonically cleaning with deionized water, appropriate ultrasonic frequency, temperature, and time must be selected; when drying surface moisture with a nitrogen gas gun, appropriate pressure must be selected to ensure effective moisture removal without damaging the metal surface. Too low a pressure may result in incomplete drying, while too high a pressure may scratch the metal surface; when thoroughly drying in a vacuum drying oven, appropriate temperature and time must also be selected to avoid damage caused by excessively high temperatures or prolonged drying times.

[0016] Further preferably, in the ultrasonic oscillation cleaning of deionized water, the ultrasonic frequency is 50KHz, the ultrasonic cleaning temperature is 25℃, and the ultrasonic cleaning time is 15min; the nitrogen gas gun pressure is 0.5MPa; and the vacuum drying oven drying temperature is 120℃ and the drying time is 15min.

[0017] (2) Applying a constraint layer: The pretreated BCC metal material sample is mounted on the worktable, and an optical transparent glass constraint layer with a thickness of 50 to 500 μm is applied to the front and back sides respectively, and the thickness of a single optical transparent glass constraint layer is 1 / 3 to 3 / 5 of the thickness of the BCC metal.

[0018] The inventors discovered that laser shock peening of BCC metals, with the addition of a constraint layer of appropriate thickness, can increase the strength and durability of BCC metals and prevent the material from cracking or deforming during impact. The constraint layer of appropriate thickness can limit the free deformation of the material, distribute the stress generated during impact, reduce fatigue damage and deformation, and control the material's shape changes. This improves material strength, increases tensile strength and toughness, reduces stress concentration and crack formation, thereby improving material durability and processing accuracy, making it more suitable for industrial and engineering applications. The constraint layer on the front of the metal primarily serves a protective and isolating function. It can prevent excessive deformation and damage to the metal surface caused by the laser shock wave, reducing surface crack formation. It can also control the propagation range of the laser shock wave, ensuring that it only acts on the metal surface, thus subjecting the metal surface to a uniform impact force. The constraint layer on the back of the metal primarily serves a supporting and reflective function. When the laser shock wave acts on the metal surface, the constraint layer forms a certain reaction force on the back of the metal, slowing down or hindering the further propagation of the shock wave by supporting the metal material. This helps reduce internal stress and deformation of the metal and improves the fatigue life and crack propagation resistance of the metal material. Furthermore, the thickness of the confinement layer plays a crucial role in laser shock annealing (LSU) of BCC metals. An excessively thick confinement layer can lead to stress concentration, reduced impact effectiveness, and processing difficulties; conversely, an excessively thin confinement layer may result in insignificant confinement, limited strengthening, and adverse stress effects on the material. Therefore, appropriately selecting the confinement layer thickness provides a more stable processing procedure and dimensional control, and is a key factor in ensuring optimal performance and processing quality of LSU-annealed BCC metals.

[0019] Further preferably, the thickness of the optically transparent glass constraint layer is 1 / 3 to 1 / 2 of the thickness of the BCC metal.

[0020] (3) Laser shock treatment: The working end face of the BCC metal material sample with the applied constraint layer is facing the laser beam, and the laser is used for shock treatment with a pulse width of 1-10 ns, wavelength of 200-1000 nm, laser energy of 10-50 J, repetition frequency of 0.1-10 Hz, and beam diameter of 0.5-5 mm.

[0021] The inventors discovered that selecting appropriate pulse width, wavelength, laser energy, repetition frequency, and beam diameter are key factors in laser shock peening of BCC metals. A moderately shortened pulse width can improve surface quality; wavelength selection must consider the material's absorption capacity; laser energy and repetition frequency depend on the balance between impact effect and processing speed; and an appropriate beam diameter controls the depth of impact to ensure uniformity of the impact area.

[0022] Furthermore, excessively large pulse widths can lead to increased heat conduction and expansion of the heat-affected zone. Prolonged heating times can cause excessive heat transfer to surrounding areas, expanding unheated regions and reducing the impact strengthening effect. While excessively small pulse widths can generate high temperatures and rapid cooling, if the pulse is too short, the material cannot fully absorb the energy, and the heating process cannot proceed effectively. Excessively large wavelengths result in low photon energy and weak absorption by metals, potentially leading to insufficient laser energy absorption and heat conduction to surrounding areas, affecting the strengthening effect. Conversely, excessively small wavelengths, while providing higher heating temperatures, can cause excessive thermal damage and affect thermal expansion and cooling capacity. Excessively large laser energy can exceed the material's melting point or vaporization temperature, causing excessive thermal damage and excessive heat conduction, which also expands the heat-affected zone, impacting the impact strengthening effect. Insufficient laser energy cannot provide enough heating and cooling energy to reach the necessary temperature and thermal expansion state, resulting in unsatisfactory processing results. Excessively high repetition frequencies can prevent sufficient cooling of the metal material. Continuous pulse heating leads to temperature accumulation and heat conduction, preventing the metal material from reaching sufficient cooling time, potentially causing overheating, deformation, or even material damage. If the repetition frequency is too low, the heating and cooling cycle time of the metal material will be longer. This may cause the heating process to be discontinuous, affecting the impact strengthening effect. If the beam diameter is too large, the total energy density will be reduced, and too much energy will be lost, failing to provide sufficient energy density to achieve effective heating and impact strengthening. It will also result in a larger heat-affected zone and increased heat conduction, reducing accuracy and processing efficiency. If the beam diameter is too small, the metal may be overheated, melted, or vaporized, increasing the risk of overheating and thermal damage due to heat conduction spreading to the surrounding area, thus reducing the impact strengthening effect.

[0023] This invention provides BCC metals with high strength, high hardness, excellent corrosion resistance and fatigue resistance, including W, Mo, Ta, etc., strengthened by the above-mentioned strengthening method.

[0024] When processing BCC metal material of size W, 5mm*5mm*0.3mm, the preferred scheme is: constraint layer thickness 100μm, pulse width 5ns, wavelength 527nm, laser energy 30J, repetition frequency 5Hz, and beam diameter 2mm.

[0025] When processing BCC metal material of size W, 5mm*5mm*0.1mm, the preferred scheme is: constraint layer thickness 50μm, pulse width 5ns, wavelength 527nm, laser energy 15J, repetition frequency 3Hz, and beam diameter 2mm.

[0026] When processing BCC metal material of size W, 5mm*5mm*0.5mm, the preferred scheme is: constraint layer thickness 200μm, pulse width 3ns, wavelength 527nm, laser energy 35J, repetition frequency 5Hz, and beam diameter 2mm.

[0027] When processing BCC metal material of size W, 3mm*3mm*0.1mm, the preferred scheme is: constraint layer thickness 50μm, pulse width 6ns, wavelength 527nm, laser energy 15J, repetition frequency 2Hz, and beam diameter 1mm.

[0028] When processing BCC metal material of size W, with dimensions of 3mm*3mm*0.5mm, the preferred scheme is: constraint layer thickness 200μm, pulse width 4ns, wavelength 527nm, laser energy 35J, repetition frequency 4Hz, and beam diameter 1mm.

[0029] When processing BCC metal material of Mo with dimensions of 5mm*5mm*0.5mm, the preferred scheme is: constraint layer thickness of 250μm, pulse width of 6ns, wavelength of 527nm, laser energy of 30J, repetition frequency of 3Hz, and beam diameter of 2mm.

[0030] When processing BCC metal material Mo with dimensions of 5mm*5mm*0.3mm, the preferred scheme is: constraint layer thickness 150μm, pulse width 8ns, wavelength 527nm, laser energy 20J, repetition frequency 3Hz, and beam diameter 2mm.

[0031] When processing BCC metal material of Ta with dimensions of 5mm*5mm*0.5mm, the preferred scheme is: constraint layer thickness of 300μm, pulse width of 7ns, wavelength of 527nm, laser energy of 20J, repetition frequency of 3Hz, and beam diameter of 2mm.

[0032] When processing BCC metal material of the type Ta with dimensions of 5mm*5mm*0.3mm, the preferred scheme is: constraint layer thickness of 180μm, pulse width of 9ns, wavelength of 527nm, laser energy of 15J, repetition frequency of 2Hz, and beam diameter of 2mm.

[0033] (4) Post-laser shock treatment: After laser shock treatment, the BCC metal sample was placed in anhydrous ethanol, and then placed in an ultrasonic cleaner. The ultrasonic frequency was set to 50-100 kHz, the cleaning temperature to 20-25℃, and the ultrasonic oscillation cleaning was performed for 10-20 min. Next, the cleaned sample was removed and placed in deionized water, and again placed in an ultrasonic cleaner. The ultrasonic frequency was set to 50-100 kHz, the cleaning temperature to 20-25℃, and the ultrasonic oscillation cleaning was performed for 10-20 min. Finally, the cleaned sample was removed and the surface was dried using a nitrogen gas gun at 0.5-1 MPa.

[0034] The inventors discovered that laser shock annealing of BCC metals produces some undesirable effects, such as surface residues and a molten layer, necessitating cleaning. Anhydrous ethanol was chosen for ultrasonic cleaning due to its excellent dissolving properties, capable of dissolving surface contaminants and residues without introducing moisture, thus preventing corrosion and oxidation. Simultaneously, the use of ultrasound helps remove contaminants from the metal surface, ensuring the material's performance and quality. Similar to the pretreatment before laser shock annealing of BCC metals, the parameters for anhydrous ethanol ultrasonic oscillation cleaning need to be appropriately selected; similarly, the parameters for subsequent deionized water ultrasonic cleaning and nitrogen gas gun drying also require careful selection.

[0035] Further preferably, in the anhydrous ethanol ultrasonic oscillation cleaning, the ultrasonic frequency is 80KHz, the ultrasonic cleaning temperature is 25℃, and the ultrasonic cleaning time is 10min; in the deionized water ultrasonic oscillation cleaning, the ultrasonic frequency is 50KHz, the ultrasonic cleaning temperature is 25℃, and the ultrasonic cleaning time is 10min; the nitrogen gas gun pressure is 0.5MPa.

[0036] This invention also provides applications of BCC metals such as W, Mo, and Ta strengthened by the above-mentioned strengthening methods, applying the BCC metals to at least one of high-temperature structural materials, high-speed cutting tools, electrodes, filaments, electronic components and resistors, superconducting magnets, and power transmission lines.

[0037] Beneficial effects

[0038] This invention, by designing reasonable laser shock pretreatment and posttreatment schemes for the sample, as well as a constraint layer of appropriate thickness and the ratio of the constraint layer to the thickness of the thin plate to be treated, can effectively prevent the initiation of microcracks on the sample surface, ensure the effective utilization of the laser shock wave, and reduce the damage rate.

[0039] This invention improves the microhardness of BCC metal materials by 20-40% by designing appropriate laser shock parameters, including pulse width, wavelength, laser energy, repetition frequency, and beam diameter.

[0040] The laser shock peening method designed in this invention induces elastoplastic deformation in BCC metal samples, generating residual compressive stress. This causes a large number of dislocation groups, dislocation tangles, dislocation pile-ups, and nanocrystalline recrystallized grains to form inside the sample. Furthermore, a large number of dislocation tangles also exist inside the recrystallized grains, forming a complex dislocation network, thereby improving the strength and hardness of the material.

[0041] The laser-strengthened BCC metal provided by this invention can be applied to high-temperature structural materials, high-speed cutting tools, electrodes, filaments, electronic components and resistors, superconducting magnets and power transmission lines, etc., greatly expanding the application of BCC metal materials. Attached Figure Description

[0042] Figure 1 The present invention provides a schematic diagram of the apparatus and a process flow diagram for the laser shock strengthening method; wherein, (a) is a schematic diagram of the apparatus for the laser shock strengthening method; and (b) is a process flow diagram for the laser shock strengthening method.

[0043] Figure 2 The images show the scanning electron microscope (SEM) images of the laser-shock-enhanced sample obtained in Example 1 and the SEM images of the sample obtained in Comparative Example 1; where (a) is the tissue morphology image of Example 1 and (b) is the tissue morphology image of Comparative Example 1.

[0044] Figure 3 The transmission electron microscope (TEM) analysis of the laser shock-strengthened sample obtained in Example 1 is shown below; (a) is a bright-field TEM image; (b) is a high-resolution TEM image; and (c) is a selected-area electron diffraction pattern.

[0045] Figure 4 The images show the transmission electron microscopy (TEM) analysis of the recrystallized grains of the laser-shock-strengthened sample obtained in Example 1; where (a) is a bright-field TEM image of the recrystallized grains; and (b) is a high-resolution TEM image.

[0046] Figure 5 The hardness data are those of the laser shock-strengthened specimens obtained in Examples 1-3 and the specimens of Comparative Examples 1-3; where (a) is the hardness data of Example 1 and Comparative Example 1; (b) is the hardness data of Example 2 and Comparative Example 2; and (c) is the hardness data of Example 3 and Comparative Example 3.

[0047] from Figure 2 It can be observed that the BCC metal W microstructure in Example 1 is an irregular polygon with a grain size of 20-80 μm. After laser shock strengthening, a small number of grains showed grain boundary separation, but no obvious damage was caused to the sample surface.

[0048] from Figure 3 As can be observed, after W laser shock strengthening of BCC metal in Example 1, a distinct white band appears in the bright-field image of the transmission electron microscope. Numerous dislocation groups, dislocation tangles, and dislocation pile-ups exist on both sides of the band. High-resolution transmission electron microscopy reveals that the band belongs to a region of severe stress distortion, containing numerous edge dislocations and exhibiting significant lattice distortion.

[0049] from Figure 4 As can be seen in Example 1, after W laser shock strengthening of BCC metal, recrystallized grains were formed. Bright-field transmission electron microscopy revealed numerous dislocation tangles within the recrystallized grains, forming a complex dislocation network. High-resolution transmission electron microscopy revealed significant screw dislocation pile-ups at the recrystallized grain boundaries.

[0050] from Figure 5 It can be observed that the hardness of BCC metal strengthened by Examples 1-3 is significantly improved. Detailed Implementation

[0051] Example 1

[0052] A laser shock peening method for strengthening BCC metal W includes the following steps:

[0053] Step 1: Surface pretreatment of metal W

[0054] Take a BCC metal W with dimensions of 5mm*5mm*0.3mm, place the sample in anhydrous ethanol, put it in an ultrasonic cleaner, set the ultrasonic frequency to 80KHz, the cleaning temperature to 25℃, and ultrasonically vibrate for 20min.

[0055] After ultrasonic cleaning with anhydrous ethanol, the sample was placed in deionized water and then in an ultrasonic cleaner. The ultrasonic frequency was set to 50 kHz, the cleaning temperature to 25°C, and the ultrasonic oscillation cleaning was performed for 15 minutes. Then, the cleaned sample was removed and the surface moisture was dried using a 0.5 MPa nitrogen gas gun. Finally, it was placed in a vacuum drying oven at 120°C for 15 minutes to ensure complete removal of surface moisture.

[0056] Step 2: Apply constraint layers

[0057] The pretreated BCC metal W is mounted on the worktable, and a 100μm thick optical transparent glass constraint layer is applied to the front and back sides respectively.

[0058] Step 3: Metal W laser shock treatment

[0059] The working end face of the BCC metal W with the applied constraint layer is facing the laser beam, and the laser is used for impact treatment with a pulse width of 3ns, a wavelength of 527nm, a laser energy of 30J, a repetition frequency of 5Hz, and a beam diameter of 2mm.

[0060] Step 4: Post-processing of metal W laser shock

[0061] After laser shock treatment, the BCC metal W was placed in anhydrous ethanol and then in an ultrasonic cleaner. The ultrasonic frequency was set to 80 kHz, the cleaning temperature to 25°C, and the ultrasonic cleaning was performed for 10 minutes. After cleaning, the sample was placed in deionized water and then in an ultrasonic cleaner. The ultrasonic frequency was set to 50 kHz, the cleaning temperature to 25°C, and the ultrasonic cleaning was performed for 10 minutes. Finally, the cleaned sample was removed and the surface was dried using a 0.5 MPa nitrogen gas gun.

[0062] After laser shock strengthening, the average grain size and average density of metal W did not change significantly, but the average hardness increased from 453.72 HV to 606.96 HV. A large number of edge dislocation entanglements and recrystallized grains with a size of 50-300 nm appeared, and a large number of screw dislocations were accumulated in the recrystallized grains.

[0063] The inventors noted that changing the thickness of the metal W significantly impacts the sample's performance when other laser shock peening process parameters remain constant. When the metal W's dimensions are 3mm*3mm*0.1mm, laser shock peening increases the hardness to 642.58 HV, but significant grain boundary separation occurs, causing noticeable damage to the sample surface. When the metal W's dimensions are 3mm*3mm*0.8mm, laser shock peening increases the surface hardness to 527.81 HV, while the back surface hardness is only 481.77 HV, showing a clear hardness gradient. These results demonstrate that different process parameters must be selected for laser shock peening of metal W with varying thicknesses, and the optimal process must be chosen for each thickness.

[0064] Further exploration revealed:

[0065] When processing BCC metal material of size W, 5mm*5mm*0.1mm, the preferred scheme is: constraint layer thickness 50μm, pulse width 5ns, wavelength 527nm, laser energy 15J, repetition frequency 3Hz, and beam diameter 2mm; the resulting product has a hardness of 682.19HV, with a large number of edge dislocation entanglements and recrystallized grains of size 50-200nm, and a large number of screw dislocations are accumulated in the recrystallized grains.

[0066] When processing BCC metal material of size W (5mm*5mm*0.5mm), the preferred scheme is: constraint layer thickness 200μm, pulse width 3ns, wavelength 527nm, laser energy 35J, repetition frequency 5Hz, and beam diameter 2mm. The resulting product has a hardness of 615.73HV, exhibits numerous edge dislocation tangles and recrystallized grains with sizes ranging from 50 to 300nm, with a large number of screw dislocations stacked within the recrystallized grains.

[0067] When processing BCC metal material of size W (3mm*3mm*0.1mm), the preferred configuration is: constraint layer thickness 50μm, pulse width 6ns, wavelength 527nm, laser energy 15J, repetition frequency 2Hz, and beam diameter 1mm. The resulting product has a hardness of 667.54HV, exhibits numerous edge dislocation tangles and recrystallized grains with sizes ranging from 50 to 200nm, with a large number of screw dislocations stacked within the recrystallized grains.

[0068] When processing BCC metal material of size W (3mm*3mm*0.5mm), the preferred scheme is: confinement layer thickness 200μm, pulse width 4ns, wavelength 527nm, laser energy 35J, repetition frequency 4Hz, and beam diameter 1mm. The resulting product has a hardness of 624.38HV, exhibits numerous edge dislocation tangles and recrystallized grains with sizes ranging from 50 to 300nm, with a large number of screw dislocations stacked within the recrystallized grains.

[0069] Example 2

[0070] A laser shock peening method for strengthening BCC metallic Mo includes the following steps:

[0071] Step 1: Surface pretreatment of Mo metal

[0072] Take a 5mm*5mm*0.3mm BCC metal Mo, place it in anhydrous ethanol, put it in an ultrasonic cleaner, set the ultrasonic frequency to 80KHz, the cleaning temperature to 25℃, and ultrasonically vibrate for 20 minutes.

[0073] After ultrasonic cleaning with anhydrous ethanol, the sample was placed in deionized water and then in an ultrasonic cleaner. The ultrasonic frequency was set to 50 kHz, the cleaning temperature to 25°C, and the ultrasonic oscillation cleaning was performed for 15 minutes. Then, the cleaned sample was removed and the surface moisture was dried using a 0.5 MPa nitrogen gas gun. Finally, it was placed in a vacuum drying oven at 120°C for 15 minutes to ensure complete removal of surface moisture.

[0074] Step 2: Apply constraint layers

[0075] The pretreated BCC metal Mo was mounted on the worktable, and a 150μm thick optical transparent glass constraint layer was applied to the front and back sides respectively.

[0076] Step 3: Mo laser shock treatment

[0077] The working end face of the BCC metal Mo with the applied constraint layer is facing the laser beam, and the laser is used for impact treatment with a pulse width of 8ns, a wavelength of 527nm, a laser energy of 20J, a repetition frequency of 3Hz, and a beam diameter of 2mm.

[0078] Step 4: Post-treatment of Mo metal by laser shock

[0079] After laser shock treatment, the BCC metallic Mo was placed in anhydrous ethanol and then in an ultrasonic cleaner. The ultrasonic frequency was set to 80 kHz, the cleaning temperature to 25°C, and the ultrasonic cleaning was performed for 10 minutes. Next, the cleaned sample was removed and placed in deionized water, then in an ultrasonic cleaner. The ultrasonic frequency was set to 50 kHz, the cleaning temperature to 25°C, and the ultrasonic cleaning was performed for 10 minutes. Finally, the cleaned sample was removed and the surface was dried using a 0.5 MPa nitrogen gas gun.

[0080] After laser shock strengthening, the average grain size and average density of metallic Mo did not change significantly, but the average hardness increased from 218.08 HV to 347.44 HV. A large number of edge dislocation tangles and recrystallized grains with sizes of 100-400 nm appeared, and a large number of screw dislocations were accumulated in the recrystallized grains.

[0081] Example 2-1

[0082] All other conditions are the same as in Example 2, except that:

[0083] When processing Mo, a BCC metal material with dimensions of 5mm*5mm*0.5mm, the constraint layer thickness was 250μm, the pulse width was 6ns, the wavelength was 527nm, the laser energy was 30J, the repetition frequency was 3Hz, and the beam diameter was 2mm. The resulting product had a hardness of 339.21HV, exhibited numerous edge dislocation tangles and recrystallized grains with sizes ranging from 100 to 300nm, and contained a large number of screw dislocations.

[0084] Example 3

[0085] A laser shock peening method for strengthening BCC metal Ta includes the following steps:

[0086] Step 1: Pretreatment of Ta metal surface

[0087] Take a BCC metal Ta sample with dimensions of 3mm*3mm*0.3mm, place it in anhydrous ethanol, put it in an ultrasonic cleaner, set the ultrasonic frequency to 80KHz, the cleaning temperature to 25℃, and ultrasonically vibrate for 20 minutes.

[0088] After ultrasonic cleaning with anhydrous ethanol, the sample was placed in deionized water and then in an ultrasonic cleaner. The ultrasonic frequency was set to 50 kHz, the cleaning temperature to 25°C, and the ultrasonic oscillation cleaning was performed for 15 minutes. Then, the cleaned sample was removed and the surface moisture was dried using a 0.5 MPa nitrogen gas gun. Finally, it was placed in a vacuum drying oven at 120°C for 15 minutes to ensure complete removal of surface moisture.

[0089] Step 2: Apply constraint layers

[0090] The pretreated BCC metal Ta is mounted on the worktable, and an optically transparent glass constraint layer with a thickness of 180μm is applied to the front and back sides respectively.

[0091] Step 3: Metal Ta laser shock treatment

[0092] The working end face of the BCC metal Ta with the applied constraint layer is facing the laser beam, and the laser is used for impact treatment with a pulse width of 9ns, a wavelength of 527nm, a laser energy of 15J, a repetition frequency of 2Hz, and a beam diameter of 2mm.

[0093] Step 4: Post-treatment of metal Ta laser shock

[0094] After laser shock treatment, the BCC metal Ta was placed in anhydrous ethanol and then placed in an ultrasonic cleaner. The ultrasonic frequency was set to 80 kHz, the cleaning temperature to 25 °C, and the ultrasonic oscillation cleaning was performed for 10 minutes. Next, the cleaned sample was removed and placed in deionized water, then placed in an ultrasonic cleaner. The ultrasonic frequency was set to 50 kHz, the cleaning temperature to 25 °C, and the ultrasonic oscillation cleaning was performed for 10 minutes. Finally, the cleaned sample was removed and the surface was dried using a 0.5 MPa nitrogen gas gun.

[0095] After laser shock strengthening, the average grain size and average density of metallic Ta did not change significantly, but the average hardness increased from 164.92 HV to 237.38 HV. A large number of edge dislocation tangles and recrystallized grains with sizes of 100-500 nm appeared, and a large number of screw dislocations were accumulated in the recrystallized grains.

[0096] Example 3-1

[0097] When processing BCC metal material of type Ta with dimensions of 5mm*5mm*0.5mm, the constraint layer thickness was 300μm, the pulse width was 7ns, the wavelength was 527nm, the laser energy was 20J, the repetition frequency was 3Hz, and the beam diameter was 2mm. The resulting product had a hardness of 235.84HV, exhibited numerous edge dislocation tangles and recrystallized grains with sizes ranging from 80 to 400nm, and contained a large number of screw dislocations.

[0098] Comparative Example 1

[0099] All other conditions were the same as in Example 1, except that no constraint layer was applied when laser shock strengthening of BCC metal W;

[0100] Using Comparative Example 1, the hardness of BCC metal W was increased by about 25%, but obvious laser ablation surface and microcrack initiation appeared on the front of the sample, and delamination tendency appeared on the back of the sample.

[0101] The inventors noted that, due to the lack of a restraining layer, the laser shock wave could directly act on the metal surface, causing damage due to its high energy. Without a front restraining layer to limit the shock wave's propagation range, the laser impact force would act unevenly on the metal surface, leading to uneven local stress distribution. Without the reaction force support of a back restraining layer, the laser shock wave propagated within the metal, resulting in significant stress accumulation.

[0102] Comparative Example 2

[0103] All other conditions are the same as in Example 1, except that the laser energy for laser shock enhancement is different: 200J laser energy is selected.

[0104] Using Comparative Example 2, the BCC metal W obtained by strengthening showed a very obvious laser ablation surface on the front side, as well as a large number of microcracks and even peeling. On the back side of the sample, delamination occurred and the sample was directly penetrated.

[0105] The inventors noted that excessively high laser energy during laser shock peening causes excessive damage to the metal surface, such as cracking, peeling, or melting; internal deformation and stress concentration lead to crack formation and metal material breakage; high-energy lasers also cause changes in the metal crystal structure and excessive deformation or damage in localized areas. Therefore, to obtain effective and controllable laser shock peening results, it is necessary to carefully control the laser energy and avoid using excessively high energy.

[0106] Comparative Example 3

[0107] All other conditions are the same as in Example 1, except that the pulse width set during the laser shock process is different: a pulse width of 8 ns is selected.

[0108] The average grain size and average density of BCC metal W obtained by strengthening with Comparative Example 3 did not change significantly, but the average hardness increased from 438.26 HV to 521.83 HV. Dislocation clusters appeared, mainly a large number of edge dislocation entanglements, and no nanoscale or submicron-scale recrystallized grains appeared.

[0109] The inventors noted that excessively large pulse widths cause laser energy to be dispersed over a longer period, resulting in uneven heating and blurred impact areas, while also increasing energy loss. This, in turn, prevents the achievement of precise impact strengthening effects and reduces impact strengthening efficiency.

[0110] Comparative Example 4

[0111] All other conditions were the same as in Example 1, except that the wavelength set during the laser shock process was different: a wavelength of 1064 nm was selected.

[0112] The average grain size and average density of BCC metal W obtained by strengthening using Comparative Example 4 did not change significantly, but the average hardness increased from 468.59 HV to 553.17 HV. Dislocation clusters appeared, mainly a large number of edge dislocation entanglements, and a small number of recrystallized grains and screw dislocations inside the recrystallized grains.

[0113] The inventors noted that the absorption of laser light by the W-plate is closely related to the laser wavelength. Excessively long wavelengths reduce the absorption of laser light by the W-plate, making it difficult to achieve a sufficient heating effect. Furthermore, longer wavelengths result in weaker absorption on the surface of the W-plate, making it difficult for laser energy to be uniformly transmitted to the deeper layers of the metal, thus affecting the strengthening effect.

[0114] Comparative Example 5

[0115] All other conditions are the same as in Example 1, except that the repetition frequency set during the laser shock process is different: a repetition frequency of 0.5 Hz is selected.

[0116] The average grain size and average density of BCC metal W obtained by strengthening with Comparative Example 5 did not change significantly. The average hardness increased from 445.29 HV to 487.61 HV. No dislocation groups were found, only a small number of edge dislocation entanglements were found. Similarly, no recrystallized grains or screw dislocation accumulation inside recrystallized grains were found.

[0117] The inventors noted that during laser shock peening, a low laser repetition frequency leads to reduced heating efficiency, slower processing speed, and uneven heating. A low repetition frequency means that the metal surface has more time to dissipate heat between pulses, reducing heating efficiency and preventing the metal surface from reaching a sufficient temperature, thus failing to achieve the desired shock peening effect.

Claims

1. A method of laser shock peening of a BCC metal sheet, characterized in that, It comprises the following steps: (1) BCC metal material sheet surface pretreatment: before laser shock peening treatment, the surface of BCC metal material sample with a thickness of 100-1000 μm is pretreated; (2) Apply a constraint layer: the pretreated BCC metal material sample is installed on a workbench, and a layer of optical transparent glass constraint layer with a thickness of 50-500 μm is applied on the front and back surfaces, and the thickness of the single-layer optical transparent glass constraint layer is 1 / 3-1 / 2 of the thickness of the BCC metal; (3) Laser shock treatment: the working end surface of the BCC metal material sample with the constraint layer is directed towards the laser beam, and the laser with a pulse width of 1-10 ns, a wavelength of 200-1000 nm, a laser energy of 10-50 J, a repetition frequency of 0.1-10 Hz, and a beam diameter of 0.5-5 mm is used for impact treatment; The material of the BCC metal sheet is selected from one of W, Mo and Ta; When the BCC metal material is W with a size of 5 mm×5 mm×0.3 mm, the constraint layer thickness is 100 μm, the pulse width is 3 ns, the wavelength is 527 nm, the laser energy is 30 J, the repetition frequency is 5 Hz, and the beam diameter is 2 mm; When the BCC metal material is W with a size of 5 mm×5 mm×0.1 mm, the constraint layer thickness is 50 μm, the pulse width is 5 ns, the wavelength is 527 nm, the laser energy is 15 J, the repetition frequency is 3 Hz, and the beam diameter is 2 mm; When the BCC metal material is W with a size of 5 mm×5 mm×0.5 mm, the constraint layer thickness is 200 μm, the pulse width is 3 ns, the wavelength is 527 nm, the laser energy is 35 J, the repetition frequency is 5 Hz, and the beam diameter is 2 mm; When the BCC metal material is W with a size of 3 mm×3 mm×0.1 mm, the constraint layer thickness is 50 μm, the pulse width is 6 ns, the wavelength is 527 nm, the laser energy is 15 J, the repetition frequency is 2 Hz, and the beam diameter is 1 mm; When the BCC metal material is W with a size of 3 mm×3 mm×0.5 mm, the constraint layer thickness is 200 μm, the pulse width is 4 ns, the wavelength is 527 nm, the laser energy is 35 J, the repetition frequency is 4 Hz, and the beam diameter is 1 mm; When the BCC metal material is Mo with a size of 5 mm×5 mm×0.5 mm, the constraint layer thickness is 250 μm, the pulse width is 6 ns, the wavelength is 527 nm, the laser energy is 30 J, the repetition frequency is 3 Hz, and the beam diameter is 2 mm; When the BCC metal material is Mo with a size of 5 mm×5 mm×0.3 mm, the constraint layer thickness is 150 μm, the pulse width is 8 ns, the wavelength is 527 nm, the laser energy is 20 J, the repetition frequency is 3 Hz, and the beam diameter is 2 mm; When the BCC metal material is Ta with a size of 5 mm×5 mm×0.5 mm, the constraint layer thickness is 300 μm, the pulse width is 7 ns, the wavelength is 527 nm, the laser energy is 20 J, the repetition frequency is 3 Hz, and the beam diameter is 2 mm; When the BCC metal material is Ta, the size is 3mm*3mm*0.3mm, the constraint layer thickness is 180μm, the pulse width is 9ns, the wavelength is 527nm, the laser energy is 15J, the repetition frequency is 2Hz, and the beam diameter is 2mm; (4) Laser shock post-processing: after laser shock processing, clean it, dry it with nitrogen after cleaning, and obtain the product.

2. The method of laser shock peening of a BCC metal sheet according to claim 1, wherein: In step (1), the sample is placed in anhydrous ethanol, and then it is placed in an ultrasonic cleaner, the ultrasonic frequency is set to 40-100KHz, the cleaning temperature is set to 20-35℃, and the ultrasonic oscillation cleaning time is set to 10-30min.

3. The method of laser shock peening of a BCC metal sheet according to claim 2, wherein: In step (1), the ultrasonic frequency in the anhydrous ethanol ultrasonic oscillation cleaning is 80KHz, the ultrasonic cleaning temperature is 25℃, and the ultrasonic cleaning time is 20min.

4. The method of laser shock peening of a BCC metal sheet according to claim 3, wherein: In step (1), after the sample is taken out from the anhydrous ethanol ultrasonic cleaning, it is placed in deionized water, and it is also placed in an ultrasonic cleaner, the ultrasonic frequency is set to 30-100KHz, the cleaning temperature is set to 20-25℃, and the ultrasonic oscillation cleaning time is set to 10-20min; then the cleaned sample is taken out, the surface moisture is dried using a nitrogen gas gun with a pressure of 0.5-1MPa, and finally it is placed in a vacuum drying oven, the temperature is set to 100-150℃, and the drying time is set to 10-20min to ensure that the surface moisture is completely removed.

5. The method of laser shock peening of a BCC metal sheet according to claim 4, wherein: In step (1), the ultrasonic frequency in the deionized water ultrasonic oscillation cleaning is 50KHz, the ultrasonic cleaning temperature is 25℃, the ultrasonic cleaning time is 15min, the pressure of the nitrogen gas gun is 0.5MPa, the drying temperature of the vacuum drying oven is 120℃, and the drying time is 15min.

6. Use of a method of laser shock peening of a BCC metal sheet according to any one of claims 1 to 5, characterized in that: The product is applied to at least one of high-temperature structural materials, high-speed cutting tools, electrodes, filaments, electronic components and resistors, superconducting magnets, and power transmission lines.

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