A surface treatment method for improving fatigue resistance of titanium alloy under high temperature and high salt environment

Abstract

The application relates to a surface treatment method for improving the fatigue resistance of titanium alloy in a high-temperature high-salt environment. The titanium alloy is processed into a rotary bending fatigue sample, high-energy deformation strengthening technology is used to pretreat the surface of the titanium alloy sample, a gradient structure is formed on the surface layer of the titanium alloy, a residual compressive stress field is introduced, and the treated sample is subjected to liquid-phase plasma arc discharge conversion film treatment. The composite treatment of the surface high-energy deformation strengthening technology and the liquid-phase plasma arc discharge conversion film technology significantly improves the fatigue resistance of the titanium alloy in a high-temperature high-salt environment. The surface forms a gradient structure with the change of microhardness and structure with depth, a residual compressive stress layer with large values and deep distribution, and good surface integrity is obtained at the same time, thereby inhibiting the initiation and propagation of the fatigue cracks of the titanium alloy. The pre-shaping deformation controls the surface layer microstructure of the titanium alloy, and improves the compactness and corrosion resistance of the liquid-phase plasma arc discharge conversion ceramic layer.

Description

Technical Field

[0001] This invention belongs to the field of surface treatment of metallic materials, and relates to a surface treatment method for improving the fatigue resistance of titanium alloys in high-temperature and high-salt environments. Background Technology

[0002] Titanium alloys, due to their excellent comprehensive mechanical properties, are widely used in the aerospace industry and have become the primary material for compressor blades and disks in aero-engines. However, for aircraft operating in marine environments, the high salt content in the atmosphere allows salt to enter the engine compressor, subjecting the compressor blades and disks to multiple factors including high temperature, high salinity, and alternating loads. Titanium alloys are prone to corrosion in high-temperature, high-salt environments, forming corrosion pits on their surface. Under cyclic stress, these pits can cause notch effects on the titanium alloy surface, leading to premature fatigue fracture of the titanium alloy compressor blades and disks, reducing engine service life, and affecting its safety. Therefore, developing surface protection technologies to improve the fatigue resistance of titanium alloys in high-temperature, high-salt environments is crucial for improving the service performance of aero-engine compressors.

[0003] High-energy surface strain strengthening technology can form a gradient structure with varying microhardness and microstructure with depth, as well as a deep and highly concentrated residual compressive stress layer on the surface of metallic materials, while simultaneously achieving good surface integrity. High-energy strain strengthening technology applies high mechanical energy to the surface of a metal specimen using a cemented carbide tool, inducing high-strain-rate plastic deformation in the surface layer. The numerous twins and dislocations introduced into the surface layer can increase element diffusion, accelerate the formation of a protective oxide film, and thus delay corrosion. Simultaneously, the good surface integrity combined with the presence of residual compressive stress can increase the resistance to fatigue crack initiation in the metallic substrate and hinder crack propagation, which is beneficial for improving the fatigue performance of metallic components. This demonstrates the great potential and further development possibilities of high-energy surface strain strengthening technology in improving the fatigue resistance of metallic materials.

[0004] Liquid-phase plasma arc discharge conversion coating treatment utilizes the instantaneous high-temperature, high-pressure plasma generated by arc discharge to enhance the electrochemical reaction on the metal surface, thereby growing a composite ceramic coating on the metal material surface, mainly composed of a base metal oxide and supplemented by electrolyte components. This technology is characterized by its simple operation and controllable film function, as well as its simple process and low environmental pollution, aligning with global environmental sustainability principles. Therefore, it has been widely used in fields such as aerospace to address the poor wear and corrosion resistance of metal substrates. However, such films have high hardness, resulting in generally poor toughness, which easily reduces the fatigue resistance of the metal substrate, thus limiting the application of this technology on engine blades. Summary of the Invention

[0005] Technical problems to be solved

[0006] To overcome the shortcomings of existing technologies, this invention proposes a surface treatment method to improve the fatigue resistance of titanium alloys in high-temperature and high-salt environments. Targeting fatigue damage of titanium alloy materials used in aero-engine blades under high-temperature and high-salt environments, this invention proposes an organic combination of surface high-energy deformation strengthening technology and liquid-phase plasma arc discharge conversion film technology. Through reasonable control of process parameters, the application goal of effectively improving the fatigue resistance of titanium alloys in high-temperature and high-salt environments is achieved.

[0007] Technical solution

[0008] A surface treatment method for improving the fatigue resistance of titanium alloys in high-temperature and high-salt environments, characterized by the following steps:

[0009] Step 1: Process the titanium alloy into a rotational bending fatigue specimen, then clean the specimen and blow it dry.

[0010] Step 2: The surface of the titanium alloy sample is pretreated using high-energy deformation strengthening technology to form a gradient structure on the surface of the titanium alloy and introduce a residual compressive stress field.

[0011] The sample is clamped on a CNC lathe. The position of the WC ball tool head is adjusted so that the center of the WC tool head is on the same horizontal plane as the axis of the titanium alloy sample. The sample is clamped and the lathe is turned on to process the sample.

[0012] During machining, the spindle speed is 56–76 rev / min, the feed rate is 0.08–0.12 mm / rev, the static load is 0.05–0.15 MPa, and the current is 0.4–0.8 A.

[0013] Remove the treated sample and clean it with anhydrous ethanol, then blow it dry.

[0014] Step 3: Treat the treated sample with a liquid phase plasma arc discharge conversion film:

[0015] After fixing the sample with titanium wire, it is suspended in the electrolyte and connected to the anode and the stainless steel tank is connected to the cathode. Then, the titanium alloy sample is treated with liquid phase plasma arc discharge conversion film technology. During the treatment, the electrolyte temperature is kept between 15 and 30°C and the solution is stirred by an electric stirrer to reduce the concentration polarization and temperature non-uniformity of the solution.

[0016] The electrolyte consists of: sodium silicate 28-32 g / L, sodium hexametaphosphate 4-8 g / L, sodium hydroxide 2-4 g / L, and sodium tetraborate 1-3 g / L.

[0017] After treatment, the sample is rinsed to remove residual electrolyte from the sample surface, and then dried with hot air. The surface treatment of the titanium alloy is now complete.

[0018] Step 1 involves processing titanium alloys according to aviation standard HB5153, namely the high-temperature rotating bending fatigue test method for metals.

[0019] The high-energy deformation strengthening technology uses a WC spherical tool head that can rotate freely. The tool head has a diameter of 10 mm, a hardness of 80 HRC, and a surface roughness Ra of 0.1 μm.

[0020] The electrical parameters used in step 3 are: current density 12-16 A / dm³. 2 The frequency is 580–620 Hz, and the duty cycle is 20–30%.

[0021] The oxidation time in step 3 is 10 to 20 minutes.

[0022] Beneficial effects

[0023] This invention proposes a surface treatment method to improve the fatigue resistance of titanium alloys in high-temperature and high-salt environments. The titanium alloy is processed into a rotational bending fatigue specimen, and the surface of the titanium alloy specimen is pretreated with high-energy deformation strengthening technology to form a gradient structure on the surface of the titanium alloy and introduce a residual compressive stress field. The treated specimen is then treated with a liquid phase plasma arc discharge conversion film.

[0024] The technical effects of this invention are as follows:

[0025] 1. This invention significantly improves the fatigue resistance of titanium alloys in high-temperature and high-salt environments through a combination of surface high-energy deformation strengthening technology and liquid-phase plasma arc discharge conversion film technology.

[0026] 2. This invention uses surface high-energy deformation strengthening technology to pre-deform the surface of titanium alloys, forming a gradient structure with varying microhardness and microstructure with depth, and a residual compressive stress layer with large values ​​and deep distribution. At the same time, it achieves good surface integrity and inhibits the initiation and propagation of fatigue cracks in titanium alloys.

[0027] 3. This invention regulates the surface microstructure of the titanium alloy substrate through pre-plastic deformation, thereby improving the density and corrosion resistance of the liquid-phase plasma arc discharge conversion ceramic layer. Attached Figure Description

[0028] Figure 1 Fatigue maximum cyclic stress (S)-life (N) curves of TC11 titanium alloy specimens with different surface treatments

[0029] The fatigue maximum cyclic stress (S)-life (N) curve test results of TC11 titanium alloy samples with different surface treatments (substrate (BM), substrate surface salt coating (BM+S), high-energy surface deformation strengthening followed by salt coating (HEDS+S), liquid phase plasma arc discharge conversion ceramic layer followed by salt coating (LPDCT+S), and high-energy surface deformation strengthening and liquid phase plasma arc discharge conversion ceramic layer composite treatment followed by salt coating (HEDS+LPDCT+S)) under high temperature conditions of 500℃. Detailed Implementation

[0030] The present invention will now be further described in conjunction with the embodiments and accompanying drawings:

[0031] The technical solution of this invention is: a surface treatment method for improving the fatigue resistance of titanium alloys in high-temperature and high-salt environments, comprising the following steps:

[0032] Step 1: According to aviation standard HB5153 (Metallic high temperature rotating bending fatigue test method), the titanium alloy is processed into a rotating bending fatigue specimen, and then the specimen is cleaned and dried.

[0033] Step 2: The surface of the titanium alloy sample is pretreated using high-energy deformation strengthening technology to form a gradient structure on the surface of the titanium alloy and introduce a residual compressive stress field.

[0034] Step 2.1: Clamp the sample cleaned in Step 1 on the CNC lathe, ensuring that the center of the WC tool head and the axis of the titanium alloy sample are on the same horizontal plane;

[0035] Step 2.2: Adjust the position of the WC ball head to ensure that the tool head presses firmly against the sample;

[0036] Step 2.3: Write a program based on the required spindle speed (56~76rev / min) and feed rate (0.08~0.12mm / rev);

[0037] Step 2.4: Adjust the static load (0.05~0.15MPa);

[0038] Step 2.5: Turn on the lubricating oil;

[0039] Step 2.6: Adjust the current (0.4~0.8A);

[0040] Step 2.7: Start the program and begin processing;

[0041] Step 2.8: Remove the treated sample and clean it with anhydrous ethanol. After cleaning, dry it for later use.

[0042] Step 3: Perform liquid phase plasma arc discharge conversion film treatment on the sample after Step 2, including the following sub-steps:

[0043] Step 3.1: Prepare the electrolyte, which consists of sodium silicate 28-32 g / L, sodium hexametaphosphate 4-8 g / L, sodium hydroxide 2-4 g / L, and sodium tetraborate 1-3 g / L;

[0044] Step 3.2: Pour the prepared electrolyte into a stainless steel tank, fix the sample with titanium wire and suspend it in the electrolyte, connecting the sample to the anode and the stainless steel tank to the cathode. Then, use liquid phase plasma arc discharge conversion film technology to treat the titanium alloy sample. During the treatment, the electrolyte temperature is maintained between 15 and 30°C, and the solution is stirred with an electric stirrer to reduce concentration polarization and temperature non-uniformity.

[0045] Step 3.3: After the test, turn off the power and remove the sample;

[0046] Step 3.4: Rinse the sample to remove residual electrolyte on the sample surface, and then dry it with hot air.

[0047] A further technical solution of the present invention is: the high-energy deformation strengthening technology in step 2 adopts a WC spherical tool head that can rotate freely, with a tool head diameter of 10mm, a hardness of 80HRC, and a surface roughness Ra of 0.1μm.

[0048] A further technical solution of the present invention is: in step 3, the electrolyte is prepared by weighing 28-32g of sodium silicate, 4-8g of sodium hexametaphosphate, 2-4g of sodium hydroxide and 1-3g of sodium tetraborate, and then diluting it with distilled water to 1L.

[0049] A further technical solution of the present invention is: the electrical parameters set in step 3.2 are: current density 12-16 A / dm³. 2 The frequency is 580–620 Hz, the duty cycle is 20–30%, and the oxidation time is 10–20 min. Specific Implementation Example 1:

[0051] The TC11 titanium alloy was processed into a rotational bending fatigue specimen according to the aviation standard (HB5153-1996 Metal High Temperature Rotational Bending Fatigue Test Method). The specimen was then ultrasonically cleaned with detergent powder, water, and anhydrous ethanol to remove the oil stains on the surface of the specimen, and then dried with a hair dryer.

[0052] High energy deformation strengthening (HEDS) technology was used to strengthen the surface of titanium alloy fatigue specimens. After strengthening, the titanium alloy specimens were ultrasonically cleaned and dried for later use. The steps of the high energy deformation strengthening treatment of titanium alloy are as follows: The fatigue specimen was clamped on a CNC lathe using a triangular clamp and a center. The axial position of the fatigue specimen and the ball head of the tool was adjusted to ensure that the center of the ball head was horizontal with the axis of the fatigue specimen. Then, the axial feed rate (0.10 mm / rev), rotation speed (76 rev / min), static load (0.15 MPa), and current (0.6 A) of the lathe were set. The lathe was then turned on to perform high energy deformation strengthening treatment on the surface of the titanium alloy.

[0053] A self-designed uniform and quantitative salt coating device was used to uniformly spray a saturated aqueous solution of chemically pure NaCl onto the surfaces of BM and HEDS samples, respectively. The salt coating amount on the sample surface was controlled to be 0.1 ± 0.05 mg / cm² by comparing the mass difference before and after salt coating. 2 .

[0054] Titanium alloy substrate (BM) and titanium alloy fatigue specimens (BM+S, HEDS+S) coated with NaCl salt film were mounted on a high-temperature rotating fatigue testing machine for high-temperature fatigue testing. The temperature was controlled at 500±5℃ to simulate the working conditions of an engine compressor.

[0055] Experimental results are as follows Figure 1 As shown, the unsalted TC11 titanium alloy substrate sample was tested at 1×10⁻⁶. 7 The fatigue limit under cyclic conditions is 540 MPa. A 0.1 mg / cm² coating is applied to the surface. 2 The fatigue limit of the TC11 titanium alloy sample coated with NaCl salt film decreased to 390 MPa, a reduction of 27.78% compared to the uncoated state, indicating that TC11 titanium alloy has high corrosion fatigue susceptibility in high-temperature and high-salt environments. The sample treated with HEDS technology, under the same salt coating amount and temperature test conditions, had a fatigue limit of 430 MPa, an improvement of 10.26% compared to the titanium alloy substrate, demonstrating that HEDS treatment can effectively improve the fatigue resistance of TC11 titanium alloy in high-temperature and high-salt environments.

[0056] Specific Implementation Case 2

[0057] The TC11 titanium alloy was processed into a rotational bending fatigue specimen according to the aviation standard (HB5153-1996 Metal High Temperature Rotational Bending Fatigue Test Method). The specimen was then ultrasonically cleaned with detergent powder, water, and anhydrous ethanol to remove the oil stains on the surface of the specimen, and then dried with a hair dryer.

[0058] TiO2 ceramic films were prepared on titanium alloy surfaces using liquid-phase plasma-arc discharge conversion technology (LPDCT). The specific experimental parameters and procedures are as follows: First, an appropriate amount of distilled water was placed in a beaker, and the weighed reagents (sodium silicate 30 g / L, sodium hexametaphosphate 6 g / L, sodium hydroxide 3 g / L, sodium tetraborate 2 g / L) were added sequentially. The mixture was stirred with a glass rod until dissolved. After all reagents were dissolved, distilled water was added to adjust the concentration to the required level. The prepared electrolyte was poured into a stainless steel tank. The sample was fixed with titanium wire and suspended in the electrolyte, with the sample connected to the anode and the stainless steel tank connected to the cathode. Then, the electrical parameters (current density 14 A / dm³) were set on the equipment panel. 2 The process was initiated with a frequency of 600 Hz, a duty cycle of 25%, and an oxidation time of 15 min. After power was switched on, the sample was treated. During treatment, the circulating cooling system was activated to maintain the electrolyte temperature within the range of 15–30 °C. The solution was stirred using an electric stirrer to reduce concentration polarization and temperature inhomogeneity. After the conversion coating treatment was completed, the power was switched off, the sample was removed, and rinsed with running tap water to remove any residual electrolyte from the sample surface. The sample was then dried with hot air and placed in a sample bag.

[0059] A self-designed uniform and quantitative salt coating device was used to uniformly spray a saturated aqueous solution of chemically pure NaCl onto the surfaces of BM and LPDCT samples. The salt coating amount on the sample surface was controlled to be 0.1 ± 0.05 mg / cm² by comparing the mass difference before and after salt coating. 2 Titanium alloy fatigue specimens (BM+S, LPDCT+S) coated with NaCl salt film were mounted on a high-temperature rotating fatigue testing machine for high-temperature fatigue testing. The temperature was controlled at 500±5℃ to simulate the working conditions of an engine compressor.

[0060] Experimental results are as follows Figure 1 As shown, the surface is coated with 0.1 mg / cm 2 The fatigue limit of the TC11 titanium alloy sample with NaCl salt film was 390 MPa, while the fatigue limit of the sample treated with LPDCT technology under the same salt coating amount and temperature test conditions was 575 MPa, which is 47.44% higher than that of the titanium alloy substrate. This shows that HEDS treatment can significantly improve the fatigue resistance of TC11 titanium alloy in high temperature and high salt environment.

[0061] Specific Implementation Case 3

[0062] The TC11 titanium alloy was processed into a rotational bending fatigue specimen according to the aviation standard (HB5153-1996 Metal High Temperature Rotational Bending Fatigue Test Method). The specimen was then ultrasonically cleaned with detergent powder, water, and anhydrous ethanol to remove the oil stains on the surface of the specimen, and then dried with a hair dryer.

[0063] High energy deformation strengthening (HEDS) technology was used to pretreat the surface of titanium alloy fatigue specimens. After strengthening, the titanium alloy specimens were ultrasonically cleaned and dried for later use. The steps of the high energy deformation strengthening treatment of titanium alloy are as follows: The fatigue specimen was clamped on a CNC lathe using a triangular clamp and a center. The axial position of the fatigue specimen and the ball head of the tool was adjusted to ensure that the center of the ball head was horizontal with the axis of the fatigue specimen. Then, the axial feed rate (0.10 mm / rev), rotation speed (76 rev / min), static load (0.15 MPa), and current (0.6 A) of the lathe were set. The lathe was then turned on to perform high energy deformation strengthening treatment on the surface of the titanium alloy.

[0064] Next, a TiO2 ceramic film was prepared on the titanium alloy surface using liquid-phase plasma-arcdischarge conversion technology (LPDCT). The specific experimental parameters and steps are as follows: First, an appropriate amount of distilled water was placed in a beaker, and the weighed reagents (sodium silicate 30 g / L, sodium hexametaphosphate 6 g / L, sodium hydroxide 3 g / L, sodium tetraborate 2 g / L) were added sequentially. The mixture was stirred with a glass rod until dissolved. After all the reagents were dissolved, distilled water was added to adjust the concentration to the required level. The prepared electrolyte was poured into a stainless steel tank. The sample was fixed with titanium wire and suspended in the electrolyte, with the sample connected to the anode and the stainless steel tank connected to the cathode. Then, the electrical parameters (current density 14 A / dm³) were set on the equipment panel. 2 The process was initiated with a frequency of 600 Hz, a duty cycle of 25%, and an oxidation time of 15 min. After power was switched on, the sample was treated. During treatment, the circulating cooling system was activated to maintain the electrolyte temperature within the range of 15–30 °C. The solution was stirred using an electric stirrer to reduce concentration polarization and temperature inhomogeneity. After the conversion coating treatment was completed, the power was switched off, the sample was removed, and rinsed with running tap water to remove any residual electrolyte from the sample surface. The sample was then dried with hot air and placed in a sample bag.

[0065] A self-designed uniform and quantitative salt coating device was used to uniformly spray a saturated aqueous solution of chemically pure NaCl onto the surfaces of BM and HEDS+LPDCT samples, respectively. The salt coating amount on the sample surface was controlled to be 0.1 ± 0.05 mg / cm² by comparing the mass difference before and after salt coating. 2Titanium alloy fatigue specimens (BM+S, HEDS+LPDCT+S) coated with NaCl salt film were mounted on a high-temperature rotating fatigue testing machine for high-temperature fatigue testing. The temperature was controlled at 500±5℃ to simulate the working conditions of an engine compressor.

[0066] Experimental results are as follows Figure 1 As shown, the surface is coated with 0.1 mg / cm 2 The fatigue limit of the TC11 titanium alloy sample with NaCl salt film was 390 MPa, while the fatigue limit of the sample treated with HEDS+LPDCT composite under the same salt coating amount and temperature test conditions was 640 MPa, which is 64.10% higher than that of the titanium alloy substrate. This shows that the composite treatment has the most significant effect on improving the fatigue resistance of TC11 titanium alloy in high temperature and high salt environment.

Claims

1. A surface treatment method for improving the fatigue resistance of titanium alloys in high-temperature and high-salt environments, characterized in that, The steps are as follows: Step 1: Process the titanium alloy into a rotational bending fatigue specimen, then clean the specimen and blow it dry. Step 2: The surface of the titanium alloy sample is pretreated using high-energy deformation strengthening technology to form a gradient structure on the surface of the titanium alloy and introduce a residual compressive stress field. The sample is clamped on a CNC lathe. The position of the WC ball tool head is adjusted so that the center of the WC tool head is on the same horizontal plane as the axis of the titanium alloy sample. The sample is clamped and the lathe is turned on to process the sample. During machining, the spindle speed is 56~76 rev / min, the feed rate is 0.08~0.12 mm / rev, the static load is 0.05~0.15 MPa, and the current is 0.4~0.8 A; Remove the treated sample and clean it with anhydrous ethanol, then blow it dry. The high-energy deformation strengthening technology uses a WC spherical tool head that can rotate freely. The tool head has a diameter of 10 mm, a hardness of 80 HRC, and a surface roughness Ra of 0.1 μm. Step 3: Treat the treated sample with a liquid phase plasma arc discharge conversion film: Prepare an electrolyte solution with the following components: sodium silicate 28-32 g / L, sodium hexametaphosphate 4-8 g / L, sodium hydroxide 2-4 g / L, and sodium tetraborate 1-3 g / L. Pour the prepared electrolyte into a stainless steel tank, fix the sample with titanium wire, suspend it in the electrolyte, and connect the sample to the anode and the stainless steel tank to the cathode. Then set the electrical parameters on the equipment panel, with a current density of 12~16. The frequency was 580~620Hz, the duty cycle was 20~30%, and the oxidation time was 10~20min. After the power was turned on, the titanium alloy sample was treated using liquid phase plasma arc discharge conversion film technology. During the treatment, the circulating cooling system was turned on to maintain the electrolyte temperature at 15~30℃. Between these, the solution is stirred by an electric stirrer to reduce concentration polarization and temperature inhomogeneity. After the conversion coating treatment is completed, turn off the power, take out the sample, rinse the sample with running tap water to remove the residual electrolyte on the sample surface, and then dry it with hot air. The surface treatment of the titanium alloy is now complete.

2. The surface treatment method for improving the fatigue resistance of titanium alloys in high-temperature and high-salt environments according to claim 1, characterized in that, Step 1 involves processing titanium alloys according to aviation standard HB5153, namely the high-temperature rotating bending fatigue test method for metals.

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