Surface treatment method for laser metal deposition TC4 alloy based on laser shock peening

Abstract

The invention belongs to the technical field of material surface treatment, and provides a TC4 alloy surface treatment method based on laser shock peening and laser metal deposition, and the method comprises two parts of preparation of a TC4 alloy material test piece and laser shock peening of the formed test piece. According to the surface treatment method for preparing the TC4 alloy material based on selective laser deposition, various properties of the surface of the TC4 alloy are improved in the mode of regulating and controlling the laser power density, the problem that residual stress is generated in the material in the process of preparing the TC4 alloy through laser metal deposition is solved in the mode of laser shock peening, and the quality of the TC4 alloy material is improved. Compared with the prior art, the method has the advantages that various properties of the material surface can be improved, residual tensile stress on the surface of a part can be reduced through plastic deformation generated in the impact process, the deformation resistance of a test piece is improved, and initiation, expansion and cracking of cracks of the part are inhibited.

Description

Technical Field

[0001] This invention belongs to the field of material surface treatment technology, and in particular relates to a surface treatment method for TC4 alloy based on laser shock strengthening laser metal deposition. Background Technology

[0002] TC4 alloy, a typical α+β dual-phase titanium alloy, possesses advantages such as low density, high specific strength, excellent comprehensive mechanical and chemical properties, and strong corrosion resistance, making it widely used in key fields such as aerospace, chemical engineering, biomedicine, energy and power, and marine engineering. However, its high melting point necessitates specialized and expensive equipment and complex operations for traditional smelting and casting. Furthermore, its poor thermal conductivity further increases the difficulty and cost of manufacturing complex structural components using traditional processes, limiting its large-scale application in high-end equipment. Laser additive manufacturing technology, a combination of laser cladding and rapid prototyping technologies, offers an effective solution for manufacturing complex TC4 alloy components due to its short manufacturing cycle, high material utilization, and high forming flexibility. Laser metal deposition technology, in particular, achieves layer-by-layer material deposition by simultaneously melting metal powder with a high-energy laser, enabling the fabrication of high-density, complex-shaped TC4 alloy components. This effectively overcomes the limitations of traditional processes and has become one of the core technologies for advanced manufacturing of TC4 alloy. However, the rapid melting and solidification characteristics of laser metal deposition process can easily lead to internal defects such as pores, cracks, and lack of fusion in the formed TC4 alloy components. In addition, the high residual tensile stress results in generally low ductility of the components (about 5%–9%), which significantly weakens the overall mechanical properties and service reliability, restricting its engineering application in high-end fields. There is an urgent need for suitable post-processing technology to improve the microstructure and properties. Summary of the Invention

[0003] The purpose of this invention is to provide a surface treatment method for TC4 alloy based on laser shock strengthening laser metal deposition, which improves the various properties of TC4 alloy and solves the problem of residual stress generated inside the alloy during the process of preparing TC4 alloy composite specimens by laser metal deposition (LMD).

[0004] The objective of this invention is achieved as follows: A surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition includes the following steps: Step 1: Weigh TC4 alloy powder and pretreat it; Step 2: Pre-treat the substrate surface, then purge the printing chamber with inert gas until the oxygen content in the printing chamber is below 500 ppm; Step 3: The laser of the laser metal deposition equipment prints the alloy powder layer by layer according to the set process parameters. The laser of the printing equipment adopts a Z-shaped scanning method to obtain TC4 alloy specimens. Step 4: Perform surface treatment on the deposited layer of the TC4 alloy specimen to obtain the surface to be strengthened; Step 5: Apply an 80 μm thick black tape as an absorption layer to the surface of the TC4 alloy specimen to be strengthened, and use a 1 mm thick water film as a constraint layer. Step 6: Perform laser shock hardening treatment on the TC4 alloy specimen from step (5); Step 7: Clean the surface of the laser-shock-strengthened TC4 alloy specimen; Step 8: Perform performance tests on TC4 alloy specimens before and after laser shock strengthening treatment.

[0005] In the above technical solution, in step 1, the pretreatment is a drying treatment of the powder, and the oven temperature in the drying treatment is 100-120℃, and the time is 4-8h.

[0006] In the above technical solution, in step 2, the substrate pretreatment involves first removing the surface oxide film with sandpaper, then spraying acetone and alcohol onto the surface to remove oil stains, and finally using argon as the inert gas.

[0007] In the above technical solution, in step 3, the laser metal deposition process parameters are: laser power of 1700 W, scanning speed of 350 mm / min, spot diameter of 6 mm, powder feed rate of 6 g / min, defocusing amount of 36 mm, and coating overlap rate of 50%; The laser spot of the printing equipment scans the printing substrate in a zigzag pattern, sintering and melting the alloy powder, which is then rapidly cooled and solidified onto the pre-laid substrate, completing the first printing. The laser is then raised, and the previous scanning path is repeated, depositing the alloy powder onto the substrate in a zigzag pattern within the layer, completing the second printing. These steps are repeated until a TC4 alloy material specimen is obtained.

[0008] In the above technical solution, in step 4, the surface is wet-polished with 240#, 600#, 1000# and 2000# sandpaper in sequence, then polished with W5 alumina polishing liquid and W1.5 polishing paste, and then ultrasonically cleaned with anhydrous ethanol to obtain the surface to be strengthened.

[0009] In the above technical solution, the black tape mentioned in step 5 is a polyester film tape with a laser absorption rate of ≥90%. When applied, it completely adheres to the surface of the TC4 alloy workpiece to be strengthened, without bubbles or wrinkles, and the edge of the black tape extends 5-10 mm beyond the boundary of the area to be strengthened. The water film is prepared using deionized water, with a thickness of 1 mm ± 0.1 mm. It is evenly distributed using a constant pressure water spraying device mounted on a Guangzhou CNC robotic arm, and the water film completely covers the surface of the black tape without any exposed or broken areas. Furthermore, the absorption layer (black tape) and the constraint layer (water film) are sequentially superimposed on the surface to be strengthened, avoiding secondary contamination or damage to the surface during the process.

[0010] In the above technical solution, in step 6, the laser shock strengthening process uses a laser beam with a wavelength of 1064 nm, the laser spot is designed as a 4 mm × 4 mm square spot structure, the laser output frequency is set to 1 Hz, the scanning path adopts a serpentine scanning method, and the overlap rate of adjacent shock areas is controlled to be 50%. To meet the microstructure and performance requirements of laser metal deposition forming of TC4 alloy, four sets of gradient laser power density parameters are set: 1.974 GW / cm², 2.5 GW / cm², 2.634 GW / cm², and 3.333 GW / cm².

[0011] In the above technical solution, in step 7, the surface cleaning method is to tear off the black tape residue left on the impacted surface and use ultrasonic cleaning to ensure that there are no black tape residue fragments left on the surface.

[0012] In the above technical solution, in step 8, the TC4 alloy material specimen after laser shock strengthening treatment is polished, ground and etched, and its microscopic properties are observed using a metallographic microscope, scanning electron microscope and electron backscatter diffraction (EBSD) technology, and its mechanical properties are tested using a Vickers hardness tester, X-ray diffractometer and friction and wear tester.

[0013] Compared with the prior art, the present invention has the following advantages: This invention uses laser metal deposition technology to form TC4 alloy specimens, followed by laser shock peening to treat their surface. Combined with targeted process parameter control (laser power density), this effectively eliminates internal defects such as porosity, cracks, and incomplete fusion that are prone to occur during the laser metal deposition process of TC4 alloy, significantly reduces residual tensile stress within the component, and retains the original core advantages of TC4 alloy, such as low density and high wear resistance. Furthermore, this surface treatment method improves the microstructure of TC4 alloy. The intense plastic deformation induced by laser shock promotes the formation of refined grains and high-density dislocations on the component surface, resulting in a more uniform microstructure distribution, reducing anisotropy caused by rapid solidification, and simultaneously improving the surface microhardness and toughness of the material, ensuring that the component meets the yield strength required for engineering applications. Therefore, TC4 alloy specimens prepared by laser metal deposition and supplemented by laser shock peening treatment have better stability and comprehensive mechanical properties. They can not only meet the stringent application requirements of complex and precision titanium alloy components in aerospace, biomedicine and other fields, but also significantly improve the overall quality and service reliability of TC4 alloy components, effectively expanding the application scope of laser metal deposition TC4 alloy in high-end equipment. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 The images show the EBSD analysis results of the laser metal deposition specimens at different depths in the embodiments of the present invention, where (a)–(d) are the image quality (BC) superimposed inverse pole figure (IPF) at a distance of 100–400 μm from the surface; (e)–(h) are the pole figures (PF) at a distance of 100–400 μm from the surface; and (i)–(l) are the mean orientation difference (KAM) plots at a distance of 100–400 μm from the surface. Figure 2 The power density in this embodiment of the invention is 2.5 GW / cm². 2 The EBSD analysis results of the laser shock strengthened specimens at different depths are shown below, where (a)–(d) are the image quality (BC) superimposed inverse pole figure (IPF) at a distance of 100–400 μm from the surface; (e)–(h) are the pole figures (PF) at a distance of 100–400 μm from the surface; and (i)–(l) are the mean orientation difference (KAM) plots at a distance of 100–400 μm from the surface. Figure 3 The power density in this embodiment of the invention is 3.333 GW / cm².2 The following are the EBSD analysis results of laser shock-strengthened specimens at different depths, where (a)–(d) are the image quality (BC) superimposed inverse pole figure (IPF) at a distance of 100–400 μm from the surface; (e)–(h) are the pole figures (PF) at a distance of 100–400 μm from the surface; and (i)–(l) are the mean orientation difference (KAM) plots at a distance of 100–400 μm from the surface. Figure 4 Sedimentary state, 2.5 GW / cm 2 and 3.333GW / cm 2 Grain size distribution diagrams at depths of 100µm, 200µm, 300µm, and 400µm, where (a)-(d) are specimens in the deposited state; (e)-(h) are specimens at 2.5GW / cm². 2 Specimens (i)-(l) have a strength of 3.333 GW / cm³. 2 Specimen; Figure 5 XRD patterns of specimens in the sedimentary state and shot-peened state at different power densities; Figure 6 The gradient hardness of the laser metal deposition specimen and the shot-peened specimen with different power densities in the embodiments of the present invention; Figure 7 The images show the surface wear morphology of laser-deposited metal specimens and shot-peened specimens with different power densities in this invention (a, b, c, and d represent the deposited state, 1.974 GW / cm², and 1.974 GW / cm², respectively). 2 ; 2.5 GW / cm 2 2.632 GW / cm 2 3.333 GW / cm 2 ); Figure 8 The wear amount and wear rate of laser metal deposition specimens and shot-peened specimens with different power densities in the embodiments of the present invention are shown. Figure 9 The graph shows the change of friction coefficient over time for laser metal deposition specimens and shot-peened specimens with different power densities in the embodiments of the present invention. Figure 10 The graph shows the change of friction coefficient over time for laser metal deposition specimens and shot-peened specimens with different power densities in the embodiments of the present invention. Figure 11 This is a schematic diagram of the surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to the present invention. Detailed Implementation

[0016] To enable those skilled in the art to better understand the present invention, the technical solution of the present invention will be further described in detail below with reference to the accompanying drawings in the embodiments of the present invention.

[0017] Laser shock peening is a surface modification technique that uses high-power-density, nanosecond-level short-pulse lasers to act on the absorption layer of a metal surface (such as black tape or aluminum foil), inducing a high-temperature, high-pressure plasma explosion to form a shock wave. This causes severe plastic deformation of the metal surface, resulting in microstructure refinement, the introduction of residual compressive stress, and effective improvement of the material's microhardness and fatigue resistance. It is superior to traditional shot peening and rolling processes and has been successfully applied in the strengthening of various metal materials, showing a significant strengthening effect on TC4 alloy.

[0018] This invention provides a surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition, such as... Figure 11 As shown, Figure 11 This is a flowchart illustrating the surface treatment process of TC4 alloy based on laser shock peening and laser metal deposition in an embodiment of the present invention, specifically including the following steps: Step 1: Weigh TC4 alloy powder and pre-treat it. The pre-treatment is the drying treatment of the powder. The oven temperature is 100-120℃ and the time is 4-8h. Step 2: Pre-treat the substrate surface, then clean the printing chamber with inert gas until the oxygen content in the chamber is below 500ppm. The substrate pre-treatment involves first removing the surface oxide film with sandpaper, then spraying acetone and alcohol on the surface to remove oil stains, and using argon as the inert gas. Step 3: The laser of the laser metal deposition equipment prints the alloy powder layer by layer according to the set process file. In order to ensure the density and uniformity of the formed parts and reduce internal defects, this step adopts the Z-shaped scanning method. The symmetrical and orderly trajectory design balances the temperature field and reduces internal stress, thereby effectively reducing the risk of deformation and cracking of the formed parts.

[0019] The laser metal deposition process parameters are as follows: laser power of 1700 W, scanning speed of 350 mm / min, spot diameter of 6 mm, powder feed rate of 6 g / min, defocusing amount of 36 mm, and coating overlap rate of 50%. The laser spot of the printing equipment scans the printing substrate in a zigzag pattern, sintering and melting the alloy powder, which is then rapidly cooled and solidified onto the pre-laid substrate, completing the first printing. The laser is then raised, and the previous scanning path is repeated, depositing the alloy powder onto the substrate in a zigzag pattern within the layer, completing the second printing. These steps are repeated until a TC4 alloy material specimen is obtained.

[0020] Step 4: Wet sanding with sandpaper, then surface polishing with alumina polishing liquid and polishing paste, followed by ultrasonic cleaning with anhydrous ethanol to obtain the surface to be strengthened; The process involves wet sanding with 240#, 600#, 1000# and 2000# sandpaper in sequence, followed by surface polishing with W5 alumina polishing liquid and W1.5 polishing paste, and then ultrasonic cleaning with anhydrous ethanol to obtain the surface to be strengthened.

[0021] Step 5: Apply an 80 μm thick black tape as an absorption layer to the surface of the specimen, and use a 1 mm thick deionized water film as a constraint layer. The two are then stacked on the surface to be strengthened. The black adhesive tape is a polyester film tape with a laser absorption rate of ≥90%. During application, it adheres completely to the surface of the TC4 alloy workpiece to be strengthened, without bubbles or wrinkles, and the edge of the black tape extends 5-10 mm beyond the boundary of the area to be strengthened. The water film is prepared using deionized water, with a thickness of 1 mm ± 0.1 mm. It is evenly distributed using a constant-pressure water spray device mounted on a Guangzhou CNC robotic arm, and the water film completely covers the surface of the black adhesive tape without any exposed or broken areas. The absorption layer (black adhesive tape) and the constraint layer (water film) are sequentially superimposed on the surface to be strengthened, avoiding secondary contamination or damage to the surface during the process.

[0022] Step 6: Perform laser shock strengthening treatment on the sample using energy beams with power energy densities ranging from 1.974 GW / cm² to 2.5 GW / cm². The laser shock peening process uses a 1064 nm laser beam with a 4 mm × 4 mm square spot structure. The laser output frequency is set to 1 Hz, and a serpentine scanning path is used. The overlap rate between adjacent shock regions is controlled to be 50%. To meet the microstructure and performance requirements of laser metal deposition forming of TC4 alloy, four sets of gradient laser power density parameters are set: 1.974 GW / cm², 2.5 GW / cm², 2.634 GW / cm², and 3.333 GW / cm².

[0023] Step 7: Clean the surface of the laser-strengthened specimen by removing the black tape remaining on the impacted surface and using ultrasonic cleaning to ensure that there are no black tape fragments remaining on the surface. Step 8: Perform performance tests on TC4 alloy specimens before and after laser shock strengthening treatment. After laser shock strengthening treatment, the TC4 alloy material specimens are polished, ground and etched. The microscopic observation is carried out using metallographic microscope, scanning electron microscope and electron backscatter diffraction (EBSD) technology. The mechanical properties are tested using Vickers hardness tester, X-ray diffractometer and friction and wear tester.

[0024] Tests of Examples 1-4 and Comparative Example 1 Examples 1-4 were laser shock peening (LSP) treatments on the deposited samples using different power densities. Examples 1-4 were optimized process groups with power densities of 1.974 GW / cm², 2.5 GW / cm², 2.634 GW / cm², and 3.333 GW / cm², respectively. Comparative Example 1 was a sample without LSP treatment. All other laser parameters followed the same parameters: wavelength 1064 nm, 4 mm × 4 mm square spot, frequency 1 Hz, serpentine scanning, and overlap rate 50%.

[0025] Phase composition, microstructure, and residual stress: The prepared samples were cut into specimens of the required size using wire cutting technology. These cut specimens were then mounted using a mounting machine, ground and polished using a grinding machine, and etched with a hydrochloric acid:nitric acid = 3:1 etching solution for 60 s, followed immediately by cleaning with ethanol. The microcrystalline structure of the specimen surface was characterized using electron backscatter diffraction (EBSD). TC4 alloy specimens were cut into 10 mm × 10 mm × 10 mm pieces using wire cutting. The specimen surfaces were then finely ground and polished. The phase composition of the coating was analyzed using a Rigaku Ultima IV X-ray diffractometer (XRD), and its residual stress was measured.

[0026] Microhardness: The hardness of the coating was tested using an HVS-1000 Vickers microhardness tester along the vertical direction of the specimen. A load of 0.4 kg was applied, and the holding time was 15 s. Five different measurement points were selected at the same height position, 200 μm apart along the vertical direction of the coating, and the hardness of each point was tested. The average of these five hardness values ​​was then calculated as the representative hardness value of that point.

[0027] Friction and wear: Friction and wear tests were conducted using a UMT TriboLab friction and wear testing machine. The specimens were first polished with sandpaper, then cleaned with alcohol and deionized water and dried. The average weight of the specimens was measured. The experimental conditions were: 25 N load, 30 mm / s sliding speed, 5 mm amplitude, 1 Hz frequency, and duration for 30 minutes. After the test, the specimens were cleaned, their weight was measured, the wear rate was calculated, and the surface morphology was observed using a CIQTEK SEM3100 scanning electron microscope (SEM) and an Oxford Xplore15 energy dispersive spectroscopy (EDS) instrument.

[0028] Figure 1 The results are EBSD values ​​for the deposited specimen. Figure 1Images (a)-(d) show the superimposed results of image quality (BC) and inverse pole figure (IPF) at depths of 100µm, 200µm, 300µm, and 400µm from the specimen surface. Red and blue lines were added to the superimposed images; red lines represent low-angle grain boundaries (LAGBs, 0°–15°), and blue lines represent high-angle grain boundaries (HAGBs, 15°–180°). At depths of 100µm, 200µm, 300µm, and 400µm, the proportions of LABs were 8.9%, 8.5%, 7.3%, and 6.3%, respectively. It can be observed that LABs are slightly higher at depths of 100µm and 200µm than at depths of 300µm and 400µm. The IPF distribution images show a certain degree of concentration in grain orientation at different depths, exhibiting distinct texture characteristics. Figure 1 (e)-(h) show the distribution of {001} polar plots (along the deposition direction) of the TC4 alloy during laser deposition. It can be observed that the polar plots at different depths exhibit a concentrated zonal distribution. Figure 1 (i)-(l) show the distribution of nucleus average orientation difference (KAM). It can be observed that the green area gradually decreases and the blue area gradually increases with increasing depth from the alloy surface. This indicates that the dislocation density of the material gradually decreases with increasing depth from the alloy surface.

[0029] Figure 2 2.5GW / cm 2 EBSD results of shot-peened specimens. Figure 2 Images (a)-(d) show the image quality (BC) and inverse pole figure (IPF) overlay results at depths of 100µm, 200µm, 300µm, and 400µm from the impact surface. The proportions of LABs (Laminated Graphite Bars) on the LSP specimen at depths of 100µm, 200µm, 300µm, and 400µm are 11.5%, 11.0%, 8.5%, and 6.68%, respectively. Compared to the deposited specimen, the proportion of LABs on the surface of the shot-peened specimen is significantly improved. Furthermore, this improvement gradually decreases with increasing depth from the impact surface. Secondly, the IPF images show that the LSP specimen also exhibits obvious texture characteristics along the depth gradient direction, indicating that LSP did not alter the original grain orientation. Figure 2 (e)-(h) show the distribution of the {001} crystal pole figures (along the deposition direction) of the TC4 alloy after laser shock peening. From the PF figures, it can be observed that, compared to the deposited specimen, the shot-peened specimen shows a significant decrease in the overall texture intensity of the poles in the 100µm and 200µm regions. Figure 2(i)-(l) show the distribution of the nucleus average orientation difference (LAGB). It can be observed that the green area in the shot-peened specimen is significantly higher than that in the deposited specimen, with the most significant difference at depths of 100 µm and 200 µm. This indicates that LSP treatment induces a higher density of dislocations in the subsurface region of the material, and further enhances the orientation inconsistency within the grains. Simultaneously, it can be observed that the KAM at depths of 100 µm and 200 µm is uniformly distributed and not concentrated near grain boundaries. Based on the above analysis, the increased LABs in this region are mainly subgrains. Furthermore, combining the BC and KAM images of the LSP specimen, it can be observed that the plastic deformation and dislocation density gradually decrease with increasing depth, indicating that the effect of LSP decreases along the impact direction with increasing depth.

[0030] Figure 3 It is 3.333 GW / cm 2 EBSD analysis results of shot-peened specimens at different depths (100 μm, 200 μm, 300 μm, and 400 μm), from... Figure 3 From the BC+IPF plots (a)–(d), it can be observed that the proportions of LAGBs at different depths are 13.7%, 11.7%, 9.95%, and 9.25%, respectively. It can be seen that the shallow region (100–200 μm) near the impact surface has a higher proportion of LAGBs, indicating that this region induces the formation of a large number of subgrain structures. Compared to 2.5 GW / cm², 2 Specimen (see) Figure 2 The higher overall proportion of LAGBs indicates that more intense grain fragmentation and dislocation rearrangement occurred on the material surface under higher laser power density. The IPF plot shows that the increase in laser power density did not change the original grain orientation. Figure 3 (e)–(h) are the {001} crystal plane pole figures at the corresponding depths. (and 2.5GW / cm) 2 Compared to the specimen, 3.333GW / cm 2 The maximum polar density values ​​of the specimen pole plot are generally lower, indicating a more significant reduction in texture strength. Figure 3 (i)–(l) demonstrates 3.333 GW / cm². 2 The distribution of the nucleus average orientation difference (KAM) at different depths of the specimen was studied to reveal the spatial distribution characteristics of dislocation density under different laser power densities. Figure 2 The figure of 2.5 GW / cm shown 2 Compared to the specimens, it is clearly observed that at the same depth (especially 100 μm and 200 μm), 3.333 GW / cm 2The specimen has a higher proportion of green areas and a larger overall KAM value, indicating that the material has generated a higher density of dislocation accumulation under the action of high power density laser, and the orientation difference inside the grains is more randomly dispersed, which characterizes a stronger plastic deformation behavior.

[0031] Figure 4 Sedimentary state, 2.5 GW / cm 2 and 3.333GW / cm 2 Grain size distribution at different depths (100 μm, 200 μm, 300 μm, and 400 μm) of the specimens. In the deposited specimens, the average grain size ranged from 5.80 to 6.28 μm. In comparison, 2.5 GW / cm²... 2 The grain size at the four depth locations of the specimen decreased to 4.24 μm, 4.39 μm, 4.51 μm, and 5.45 μm, respectively, with an average refinement degree between 0.25 and 1.07 μm. After further increasing the laser power, the result was 3.333 GW / cm². 2 The grain size of the specimens decreased to 3.69 μm, 3.78 μm, 3.91 μm, and 5.20 μm, with a more significant grain refinement effect, and the average refinement degree was between 1.08 and 2.11 μm. Comparative analysis showed that the grain refinement degree was significantly enhanced with the increase of LSP laser power density, especially in the 100–300 μm region.

[0032] Figure 5 The X-ray diffraction (XRD) patterns of the deposited and shot-peened TC4 alloy specimens are shown. It can be seen that LSP treatment did not introduce new diffraction peaks; the diffraction patterns of the specimens still mainly consist of α-Ti (hexagonal close-packed, HCP) and β-Ti (body-centered cubic, BCC) phases, indicating that no new phase transformation occurred during the LSP process. The diffraction peaks of the shot-peened specimens all exhibit significant broadening, accompanied by a shift towards higher angles. This indicates that the intense plastic deformation caused by the laser shock process leads to a large number of dislocations and lattice distortions within the material, forming a non-uniform micro-stress field.

[0033] Figure 6 The Vickers hardness distribution along the depth direction is shown for sedimentary and shot-peened specimens. It can be observed that the hardness value of the shot-peened specimen decreases with increasing depth, while the hardness value of the sedimentary specimen shows only slight fluctuations with depth. The average hardness of the sedimentary specimen is 321.6 HV1. (The last sentence appears to be incomplete and possibly refers to a different specimen.) 2 The power density was used to strengthen the surface of the alloy, resulting in a subsurface hardness of 342.5 HV1 and an average microhardness of 333.9 HV1. 2.5 GW / cm² 2 The microhardness value of the shot-peened specimen was 1.974 GW / cm. 2Similarly, the subsurface hardness is 345.8 HV1, and the average microhardness reaches 335.8 HV1. When the power density increases to 2.632 GW / cm²... 2 At this point, the subsurface hardness increased to 354.9 HV1, and the average microhardness was 344.1 HV1. The power density further increased to 3.333 GW / cm². 2 At that time, the subsurface hardness increased to 367.7 HV1, and the average microhardness was 352.7 HV1. Compared with the sedimentary specimen, the hardness was 1.974 GW / cm. 2 2.5GW / cm 2 2.632GW / cm 2 and 3.333GW / cm 2 The subsurface hardness of the specimens increased by 6.4%, 7.5%, 10.3% and 14.3%, respectively, and the average microhardness increased by 3.8%, 4.4%, 6.9% and 9.6%, respectively.

[0034] Figure 7 The residual stress diagrams on the surfaces of each specimen show that the residual stress on the surface of the deposited specimen is 331 MPa, which is a typical tensile stress state, at 1.974 GW / cm². 2 2.5GW / cm 2 2.632GW / cm 2 and 3.333GW / cm 2 At the specified power densities, the residual surface stresses of the shot-peened specimens were 54.9 MPa, −64 MPa, −120.7 MPa, and −177.5 MPa, respectively. This demonstrates that LSP can significantly modulate the stress state of the TC4 alloy surface. When the laser power density exceeds 1.974 GW / cm², the stress remains high. 2 Subsequently, the residual stress changed from tensile stress to compressive stress, and the compressive stress level increased significantly with the increase of power density. This indicates that LSP treatment has good controllability in inducing plastic deformation and introducing residual compressive stress.

[0035] Figure 8 The figures show the friction coefficient variation curves of the deposited and shot-peened specimens during the tribological test. The deposited state has a friction coefficient of 1.974 GW / cm. 2 2.5GW / cm 2 2.632GW / cm 2 and 3.333GW / cm 2 The friction coefficients of the specimens fluctuated around 0.46, 0.42, 0.38, 0.35, and 0.31, respectively. The deposited state and the specimen with a friction coefficient of 1.974 GW / cm² exhibited lower hardness, poorer wear resistance, and a wider range of friction coefficient fluctuations. Therefore, the wear resistance ranking from best to worst is: 3.333 GW / cm². 2 >2.632GW / cm2 >2.5GW / cm 2 >1.974GW / cm 2 >Sedimentary state.

[0036] Figure 9 The figures show the wear loss of the deposited and shot-peened specimens. As shown, the deposited specimen exhibits the highest wear amount, 6.7 mg, with a wear rate of 2.977 mm. 3· N −1 ·m −1 1.974GW / cm 2 2.5GW / cm 2 2.632GW / cm 2 and 3.333GW / cm 2 The wear rate of the specimens was 2.755 mm. 3 ·N −1 ·m −1 2.266 mm 3 ·N −1 ·m −1 2.133mm 3 ·N −1 ·m −1 and 1.56 mm 3 ·N −1 ·m −1 Compared to the deposited specimens, the wear resistance was reduced by 7.4%, 23.8%, 28.3%, and 47.5%, respectively. The wear loss data clearly show that the wear resistance of the shot-peened specimens is significantly improved compared to the deposited specimens. This also confirms the conclusion that LSP treatment can improve the wear resistance of LMD-formed TC4 alloys.

[0037] Figure 10 The surface wear morphology of the deposited and shot-peened specimens is shown. Figure 10 (a) shows the wear morphology of the deposited specimen. The wear mechanism is mainly dominated by abrasive wear and adhesive wear, accompanied by obvious oxidative wear. Figure 10 (b), 10 (c), 10 (d), and 10 (e) respectively show 1.974 GW / cm². 2 2.5 GW / cm 2 2.632GW / cm 2 and 3.333GW / cm 2 Wear morphology of the specimen. Among them, 1.974GW / cm 2 and 2.5GW / cm 2 The wear morphologies of the specimens were quite similar, and the wear mechanisms were also dominated by adhesive wear and abrasive wear, accompanied by oxidative wear. When the power density increased to 2.632 GW / cm²... 2Subsequently, the wear mechanism at this stage is mainly abrasive wear, along with slight adhesive wear and oxidative wear. When the power density further increases to 3.333 GW / cm²... 2 Subsequently, the wear morphology of the specimens showed that, compared with the wear morphology of other specimens, the oxygen content was the lowest, the compacted layer was the densest, the surface furrows were shallow and fine with only a small amount of layered adhesive, the surface was smoother, and the abrasive grains were finer, exhibiting slight abrasive wear, adhesive wear, and oxidative wear.

[0038] In summary, the present invention provides a method and process for controlling the surface treatment of TC4 alloy based on laser shock peening laser metal deposition. The resulting TC4 alloy composite coating has improved comprehensive mechanical properties, effectively expanding the application of TC4 alloy in the field of mechanical manufacturing.

Claims

1. A surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition, characterized in that, Includes the following steps: Step 1: Weigh TC4 alloy powder and pretreat it; Step 2: Pre-treat the substrate surface, then purge the printing chamber with inert gas until the oxygen content in the printing chamber is below 500 ppm; Step 3: The laser of the laser metal deposition equipment prints the alloy powder layer by layer according to the set process parameters. The laser of the printing equipment adopts a Z-shaped scanning method to obtain TC4 alloy specimens. Step 4: Perform surface treatment on the deposited layer of the TC4 alloy specimen to obtain the surface to be strengthened; Step 5: Apply an 80 μm thick black tape as an absorption layer to the surface of the TC4 alloy specimen to be strengthened, and use a 1 mm thick water film as a constraint layer. Step 6: Perform laser shock hardening treatment on the TC4 alloy specimen from step (5); Step 7: Clean the surface of the laser-shock-strengthened TC4 alloy specimen; Step 8: Perform performance tests on TC4 alloy specimens before and after laser shock strengthening treatment.

2. The surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to claim 1, characterized in that, In step 1, the pretreatment is the drying of the powder. The drying oven temperature is 100-120℃ and the time is 4-8 hours.

3. The surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to claim 1, characterized in that, In step 2, the substrate pretreatment involves first sanding off the surface oxide film with sandpaper, and then spraying acetone and alcohol onto the surface to remove oil stains.

4. The surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to claim 1, characterized in that, In step 3, the process parameters for laser metal deposition are as follows: laser power of 1700 W, scanning speed of 350 mm / min, spot diameter of 6 mm, powder feed of 6 g / min, defocusing amount of 36 mm, and coating overlap rate of 50%.

5. The surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to claim 1, characterized in that, In step 3, the printing process layer by layer is as follows: The laser spot of the printing equipment laser scans the printing substrate in a zigzag scanning manner, sintering and melting the alloy powder and rapidly cooling and solidifying it on the pre-laid substrate to complete the first printing. Then, the laser height is increased and the scanning path is repeated. The alloy powder is deposited on the substrate in a zigzag scanning manner within the layer to complete the second printing. The first and second printings are repeated alternately to obtain TC4 alloy specimens.

6. The surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to claim 1, characterized in that, The surface treatment in step 4 involves wet grinding the TC4 alloy specimen with 240-grit sandpaper, 600-grit sandpaper, 1000-grit sandpaper, and 2000-grit sandpaper in sequence, followed by surface polishing with W5 alumina polishing liquid and W1.5 polishing paste, and then ultrasonic cleaning with anhydrous ethanol to obtain the surface to be strengthened.

7. The surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to claim 1, characterized in that, In step 5, the black tape is a polyester film tape with a laser absorption rate of ≥90%. When applying the tape, it is completely adhered to the surface of the TC4 alloy specimen to be strengthened without bubbles or wrinkles, and the edge of the black tape extends 5~10mm beyond the boundary of the area to be strengthened. The water film is prepared with deionized water and has a thickness of 1 mm ± 0.1 mm. It is evenly distributed and completely covers the surface of the black tape without any exposed or broken parts. The black tape and the water film are stacked sequentially on the surface to be strengthened.

8. The surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to claim 1, characterized in that, In step 6, the power density of the laser shock peening treatment is 1.974 GW / cm². 2 2.5GW / cm 2 2.634GW / cm 2 and 3.333GW / cm 2 The laser shock peening process has a wavelength of 1064 nm, a spot shape of 4 mm × 4 mm, a frequency of 1 Hz, a serpentine scanning path, and an overlap rate of 50%.

9. The surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to claim 1, characterized in that, In step 7, remove the black tape residue from the impacted surface and use ultrasonic cleaning to ensure that there are no black tape residue fragments left on the surface.

10. The surface treatment method for TC4 alloy based on laser shock peening and laser metal deposition according to claim 1, characterized in that, In step 8, the TC4 alloy specimens after laser shock strengthening were polished, ground, and etched. The microscopic observation was performed using a metallographic microscope, scanning electron microscope, and electron backscatter diffraction method. The mechanical properties were tested using a Vickers hardness tester, X-ray diffractometer, and friction and wear tester.

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