Method for matching strength and plasticity to enhance anti-fretting damage performance of high-strength titanium alloy fastener

Abstract

The application discloses a method for enhancing the anti-fretting damage performance of high-strength titanium alloy fasteners, which comprises the following steps: preparing samples with different microstructures by adopting a solid solution and aging process, so that the samples have different combinations of primary alpha phase and secondary alpha phase volume fractions, thereby producing different strength-plasticity matching; determining the microstructure and testing the macro hardness and tensile properties of the different microstructure samples; using a three-dimensional profilometer to characterize the fretting damage behavior of the samples after the fretting friction test; and combining the strength-plasticity matching and the fretting damage analysis to enhance the strength-plasticity matching strategy of the anti-fretting damage performance of the high-strength titanium alloy fasteners. When the best matching strategy of improving the plasticity on the basis of the ultra-high strength is adopted, the fretting wear damage is reduced by 5 times compared with the strategy with lower strength and plasticity level, and the anti-fretting damage performance of the fastener is significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy engineering application technology, specifically relating to a method for matching strength and plasticity to enhance the resistance of high-strength titanium alloy fasteners to fretting damage. Background Technology

[0002] The intrinsic factors of materials—microstructure characteristics—are the root cause of differences in fretting damage in alloys. Differences in the size, volume fraction, distribution location, and morphology of α-precipitates in high-strength titanium alloys lead to different microstructure characteristics, which in turn affect the alloy's strength, plasticity, hardness, and elasticity, thus influencing its fretting damage sensitivity. Since fretting damage under actual service conditions is accompanied by significant plastic deformation, and strength is generally positively correlated with the hardness of metallic materials, it is traditionally believed that strength or plasticity alone has a certain effect on improving fretting damage resistance, and is one of the two most important mechanical indicators affecting fretting damage. Therefore, providing a suitable strength-plasticity matching strategy achieved through microstructure control, tailored to the actual service conditions of fasteners, is of great significance for significantly improving the fretting damage resistance and service reliability of high-strength titanium alloy fasteners during service, and this strategy can be extended to other alloys. Summary of the Invention

[0003] This invention addresses the problem of fretting damage in high-strength titanium alloy fasteners that severely restricts their service reliability. By controlling the microstructure of the alloy, different strength-plasticity matchings are achieved. By analyzing the fretting damage behavior of fasteners under different strength-plasticity matchings, and combining the external stress level applied under actual service conditions, a suitable strength-plasticity matching strategy is proposed to significantly reduce fretting damage in fasteners.

[0004] This invention is specifically achieved through the following technical solution: This invention provides a method for enhancing the resistance of high-strength titanium alloy fasteners to fretting damage through a strong-plasticity matching process, the method comprising the following steps: S1. High-strength titanium alloy samples with different microstructures are prepared by solution treatment and aging heat treatment processes. The microstructures of the samples contain primary α phase and secondary α phase with different volume fractions, thereby producing different strength-plasticity matching. S2. Determine the microstructure of the sample and test the macrohardness and tensile properties of samples with different microstructures; S3. Characterize the micro-motion damage of the sample after micro-motion friction test using a three-dimensional optical profilometer; S4. Combining the strength-plasticity matching and the fretting damage, analyze the strength-plasticity matching strategy to improve the fretting damage resistance of the high-strength titanium alloy fastener.

[0005] As a further explanation of the present invention, the heat treatment process temperatures in S1 are all lower than the β phase transformation temperature of the high-strength titanium alloy.

[0006] As a further illustration of the present invention, the sample was solution treated at 973 K, 1023 K and 1073 K for 1 h to obtain microstructures containing primary α phases with different volume fractions.

[0007] As a further illustration of the present invention, the sample was solution treated at 973 K for 1 h, and then aged at 773 K for 55 min, 90 min and 6 h to obtain microstructures containing secondary α phases with different volume fractions.

[0008] As a further explanation of the present invention, the volume fraction of the α phase in the microstructure of the sample is uniformly distributed within the range of α phases that can precipitate in the alloy, making the microstructure and strength-plasticity matching representative.

[0009] As a further explanation of the present invention, in S3, when performing the fretting friction test, the normal load is set to 35 N, the stroke amplitude is set to 80 μm, the frequency is set to 30 Hz, and the test duration is set to 2 × 10 5 The cycle is repeated to simulate the actual service conditions of the high-strength titanium alloy fastener.

[0010] As a further explanation of the present invention, S3 specifically includes: deriving the friction coefficient variation curve of the sample after fretting friction test from the fretting friction tester as a function of friction time, and then using the three-dimensional optical profilometer to measure the surface morphology of the wear scar and the wear volume of the sample after fretting friction test.

[0011] As a further explanation of the present invention, the strategy is derived by analyzing the strength-plasticity matching, the friction coefficient variation curve with friction time, the wear scar surface morphology, and the wear volume.

[0012] As a further explanation of the present invention, the nominal chemical composition of the high-strength titanium alloy is Ti-15Mo-2.7Nb-3Al-0.25Si (wt. %).

[0013] Compared with the prior art, the present invention has the following advantages: Considering the influence of the volume fraction, size, and shape of the primary α phase (Pα) and secondary α precipitates (Sα) on strength and plasticity, six microstructures with different volume fractions of primary and secondary α phases were selected as typical representatives (1%Pα, 10%Pα, 20%Pα, 20%Pα+7%Sα, 20%Pα+14%Sα, 20%Pα+24%Sα). The total volume fraction of the α phase is almost uniformly distributed within the volume fraction range of precipitable α phases in this high-strength titanium alloy (0%~50%). Therefore, the achieved strength-plasticity matching is representative and effectively reflects the relationship between different strength-plasticity matching and fretting damage. The strength-plasticity matching strategy for improving the fretting damage resistance of alloys proposed in this invention is based on controlling the microstructure characteristics of the alloy. Compared with controlling external working conditions such as stress state, displacement amplitude, temperature, and environmental medium, it fundamentally improves the fretting damage resistance of the alloy and can be extended to other alloys. Furthermore, compared with surface treatment and surface protective coating technologies, the alloy preparation method involved in this strategy is simple, easy to operate, low in cost, highly efficient, and highly repeatable. When an optimal matching strategy is adopted to improve plasticity on the basis of ultra-high strength (the alloy tensile strength is much greater than the applied stress under actual service conditions), the alloy has both strong deformation resistance and can coordinate the externally applied deformation through a large amount of uniform subsurface plastic deformation. Compared with a strategy with lower strength and plasticity levels (the alloy tensile strength is slightly greater than the applied stress under actual service conditions), fretting wear damage is reduced by 5 times. Compared with a strategy with ultra-high strength and a small amount of plasticity, wear damage is reduced by nearly 2.5 times, significantly improving the fastener's resistance to fretting damage. 4. Combining the matching relationship between actual service conditions and the properties of selected materials, this paper comprehensively explains the dominant and secondary alloy properties corresponding to improving damage resistance under different conditions. When the tensile strength of the alloy is much lower than the applied stress under actual service conditions, strength (improving the alloy's deformation resistance) is the primary factor, while the influence of plasticity is weak. When the tensile strength of the alloy is slightly higher than the applied stress under actual service conditions, strength is still the primary factor, but the influence of plasticity (coordinating deformation to consume the applied deformation property) increases, which can reduce damage to some extent. When the tensile strength of the alloy is much higher than the applied stress under actual service conditions, plasticity is the primary factor, and the influence of strength is not significant. This makes this strategy applicable to other alloys and has engineering application value. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the heat treatment process and fretting friction test provided by the present invention; Figure 2 These are SEM-BSE images of the microstructure of the heat-treated samples provided by this invention; where (a)-(f) represent the microstructure of the six heat-treated samples, respectively. Figure 3 The following are (a) tensile engineering stress-strain curves and (b) macroscopic Vickers hardness diagrams of six high-strength titanium alloy samples provided by this invention. Figure 4 The following figures show the results of room temperature fretting friction tests on six high-strength titanium alloy samples provided by this invention: (a) friction coefficient changes with friction time; (b)-(g) wear scar surface morphology. Figure 5 The wear volume of six high-strength titanium alloy samples after room temperature fretting friction test provided by this invention is shown. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] A method for enhancing the resistance of high-strength titanium alloy fasteners to fretting damage through strength-ductility matching, the method comprising the following steps: S1. Heat treatment to prepare different microstructures: Several cylindrical samples (Φ25 mm × 8 mm) were cut from a hot-rolled bar with a nominal chemical composition of Ti-15Mo-2.7Nb-3Al-0.25Si (wt. %) and subjected to six different heat treatment regimes. Figure 1 -(a) is a schematic diagram of the heat treatment process for standard-sized alloy specimens after cutting, as provided by the present invention. Figure 1 This invention achieves the control of primary and secondary α phases with different volume fractions, shapes, and sizes by combining different solution treatment and aging processes. The phase transformation point of the high-strength titanium alloy, determined by metallography, is 1103 K. Therefore, three samples (labeled Pα-A, Pα-B, and Pα-C) that were solution treated for 1 h at 973 K, 1023 K, and 1073 K respectively and then water-quenched will exhibit varying degrees of primary α phase residue. Three samples (labeled Sα-A, Sα-B, and Sα-C) that were first solution treated for 1 h at 973 K and then water-quenched, followed by aging treatment at 773 K for 55 min, 90 min, and 6 h respectively and then water-quenched, will continue to precipitate secondary α phases with different volume fractions from the β phase on top of a large amount of residual primary α phase. Therefore, the final volume fraction of the α phase in each sample is provided by both the primary and secondary α phases.

[0017] S2. Fretting Friction Test: Before the fretting friction test, the surface of the heat-treated sample was polished with sandpaper of 80, 400, 800, 1000 and 2000 grit respectively to remove the oxide scale. Then the sample surface was mechanically polished so that the surface roughness was Rp < 0.5 μm. Finally, there were no obvious scratches on the sample surface. Figure 1 -(b) is a schematic diagram of the fretting friction test after heat treatment provided by the present invention. The friction test was conducted at room temperature using a ball-disc contact mode, wherein the grinding pair used 316L stainless steel balls with a diameter of 10 mm. Friction tests with the same parameters were performed on all six samples. Each test involved reciprocating sliding along the horizontal direction for 2 × 10 Hz at a frequency of 30 Hz under a normal load of 35 N and a stroke amplitude of 80 μm. 5 To ensure the repeatability of the friction test, the experiment for each parameter was performed at least three times at different locations widely distributed on the sample surface.

[0018] S3. Microstructure Characterization: The sample surface was mechanically and electrolytically polished, and then the microstructure was characterized using a scanning electron microscope in backscatter mode. Scanning observation was performed at an accelerating voltage of 20 kV. The results are as follows: Figure 2 As shown, Pα-A, Pα-B, and Pα-C contain approximately 20%, 10%, and 1% primary α phase by volume, respectively. These phases are mainly distributed as coarse granules or elongated strips along the grain boundaries and within the β grains. With increasing solution temperature, the grain size of the β grains increases, and the content of the primary α phase decreases; only a very small amount remains within the β grains of Pα-C. In addition to containing approximately 20% primary α phase by volume, Sα-A, Sα-B, and Sα-C also contain approximately 7%, 14%, and 24% secondary α phase by volume, respectively. These secondary α phases are mainly distributed as extremely fine needle-like interwoven patterns within the β grains. Therefore, the six samples contain 1%, 10%, 20%, 27%, 34%, and 44% volume fraction of α phase, respectively, which are almost uniformly distributed within the range of precipitable α phase (0-50%) of this type of high-strength titanium alloy with β-stabilizing element content. This can effectively and representatively reflect different microstructures and corresponding strength-plasticity matching.

[0019] S4. Hardness and Plasticity Testing: Multiple bone-shaped tensile specimens, 8 mm in length and 1.5 mm in width, were cut from each heat-treated sample using an electrical discharge wire cutter. The surfaces were then sanded to remove the oxide layer. Tensile tests were performed on the bone specimens using a tensile testing machine, with all results exceeding 10... -3 s -1 The tests were conducted at strain rates, and each tissue sample was tested at least three times. The results are as follows: Figure 3As shown in (a), it can be seen that since the β phase has a bcc structure and the α phase has an hcp structure, and the hcp structure has fewer slip systems, the sample with less α phase content has better plasticity. Furthermore, compared to the coarse primary α phase, the extremely fine secondary α phase has limited variability and poorer transformability. Therefore, Sα-A, Sα-B, and Sα-C, due to their greater secondary α phase content, exhibit significantly worse plasticity than Pα-A, Pα-B, and Pα-C, which only contain the primary α phase. The tensile strengths of the six samples are indicated in [the table / reference]. Figure 3 In (a), it can be observed that samples with poorer plasticity tend to exhibit better tensile strength. Ten locations (approximately 100 μm apart) were randomly selected on the surface of six samples after the surface oxide scale was removed, and their microVickers hardness was tested using a Vickers hardness tester. The average value was calculated as the final hardness. During the measurement, the loading time was fixed at 200 gf, and the dwell time was 10 s. The results are as follows... Figure 3 As shown in (b), the better the plasticity of the sample, the lower the hardness.

[0020] S5. Characterization of fretting friction behavior: The change in friction coefficient as a function of friction time after room temperature fretting friction testing was derived from the SRV5 friction and wear testing machine. The results are as follows: Figure 4 As shown in (a), it can be seen that the friction coefficients of Sα-A, Sα-B, and Sα-C are lower than those of Pα-A, Pα-B, and Pα-C in the steady-state phase, and their surfaces are easier to slide. The sample surface after the fretting wear test was gently wiped with ethanol to remove loose wear debris adhering to the wear surface. Then, a three-dimensional optical profilometer was used to measure the surface morphology and wear volume of the wear scar, as shown in (a). Figure 4 As shown in (bg), it can be observed that the wear scar depth of samples with secondary α phase is generally shallower, while the wear scar depth of Pα-A, Pα-B, and Pα-C, which have lower α phase content, is deeper, accompanied by more severe wear.

[0021] S6. Strength-Plasticity Matching Strategy Analysis: Figure 5 The wear volume of six high-strength titanium alloy samples provided in this invention after room temperature fretting friction testing is shown. It can be seen that Sα-A, Sα-B, and Sα-C exhibit significantly less fretting damage than Pα-A, Pα-B, and Pα-C. For Pα-A, Pα-B, and Pα-C samples, which also possess only the primary α phase, samples with higher hardness generally show less fretting damage. However, for Sα-A, Sα-B, and Sα-C samples, which also possess the secondary α phase, samples with higher hardness show more severe fretting damage; conversely, fretting damage decreases with increasing plasticity. According to Hertzian contact theory, the maximum surface contact stress in the fretting friction experiment is approximately 742 MPa, which is the externally applied stress under the simulated actual service conditions in this experiment. Combined with... Figure 3Analysis of the strength and ductility test results reveals that if the tensile strength of the material is much lower than the externally applied stress under actual service conditions, the alloy will experience significant deformation stress and severe plastic deformation. In this case, increasing the strength, which reflects resistance to cracking and wear, is crucial, while increasing the ductility, which reflects cooperative deformation capability, is less significant. Therefore, materials with better strength should be selected whenever possible. If the tensile strength of the material is approximately equal to or slightly greater than the externally applied stress under actual service conditions, the alloy possesses a certain resistance to deformation. The ductility-reflected cooperative deformation capability has an increased impact on reducing fretting damage. For example, Pα-C, due to its excellent ductility, actually has lower damage than Pα-B, but resistance to deformation remains the primary factor. If the tensile strength of the material is much greater than the externally applied stress under actual service conditions, the alloy itself has strong resistance to deformation and will not experience severe plastic deformation under reciprocating tangential forces. In this case, if the alloy can withstand more external deformation and uniformly transmit deformation over a large area, friction damage will be significantly reduced. Therefore, materials with better ductility should be selected whenever possible.

[0022] When an optimal strength-plasticity matching strategy is adopted, which enhances plasticity on the basis of ultra-high strength, the alloy possesses both strong deformation resistance and can coordinate externally applied deformation through a large amount of uniform subsurface plastic deformation. Compared with strategies with lower strength and plasticity levels, fretting wear damage is reduced by 5 times; compared with strategies with ultra-high strength and low plasticity, wear damage is reduced by nearly 2.5 times, significantly improving the fastener's resistance to fretting damage. Therefore, appropriate strength-plasticity matching strategies should be selected under different service conditions.

[0023] It should be noted that, in this document, terms such as “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0024] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for achieving a high-strength titanium alloy fastener with a high-ductility matching property to enhance its resistance to fretting damage, characterized in that, The method includes the following steps: S1. High-strength titanium alloy samples with different microstructures are prepared by solution treatment and aging heat treatment processes. The microstructures of the samples contain primary α phase and secondary α phase with different volume fractions, thereby producing different strength-plasticity matching. S2. Determine the microstructure of the sample and test the macrohardness and tensile properties of samples with different microstructures; S3. Characterize the fretting damage of the sample after fretting friction test using a three-dimensional optical profilometer: Derive the friction coefficient variation curve of the sample after fretting friction test with friction time from the fretting friction testing machine, and then use the three-dimensional optical profilometer to measure the surface morphology of the wear scar and the wear volume of the sample after fretting friction test; the relationship between the fretting damage of the sample and the α phase of the sample is as follows: the sample containing the secondary α phase has a lower friction coefficient and less wear in the stable stage than the sample without the secondary α phase; S4. Combining the strength-plasticity matching and the fretting damage, analyze the strength-plasticity matching strategy to improve the fretting damage resistance of the high-strength titanium alloy fastener; the strategy is derived by comprehensively analyzing the strength-plasticity matching, the friction coefficient variation curve with friction time, the wear scar surface morphology, and the wear volume: if the tensile strength of the material used is much less than the external stress applied in actual service conditions, selecting a material with better strength can reduce fretting damage; if the tensile strength of the material used is much greater than the external stress applied in actual service conditions, selecting a material with better plasticity can reduce fretting damage.

2. The method for enhancing the resistance to fretting damage of high-strength titanium alloy fasteners according to claim 1, characterized in that, The heat treatment process temperatures in S1 are all lower than the β phase transformation temperature of the high-strength titanium alloy.

3. The method for enhancing the resistance to fretting damage of high-strength titanium alloy fasteners according to claim 2, characterized in that, The samples were solution treated at 973 K, 1023 K and 1073 K for 1 h to obtain microstructures containing primary α phases with different volume fractions.

4. The method for enhancing the resistance to fretting damage of high-strength titanium alloy fasteners according to claim 2, characterized in that, The sample was solution treated at 973 K for 1 h, and then aged at 773 K for 55 min, 90 min and 6 h to obtain microstructures containing secondary α phases with different volume fractions.

5. The method for enhancing the resistance to fretting damage of high-strength titanium alloy fasteners according to claim 1, characterized in that, The volume fraction of the α phase in the microstructure of the sample is uniformly distributed within the range of precipitable α phases in the alloy, making the microstructure and strength-plasticity matching representative.

6. The method for enhancing the resistance to fretting damage of high-strength titanium alloy fasteners according to claim 1, characterized in that, In S3, during the fretting friction test, the normal load was set to 35 N, the stroke amplitude to 80 μm, the frequency to 30 Hz, and the test duration to 2 × 10⁻⁶. 5 The cycle is repeated to simulate the actual service conditions of the high-strength titanium alloy fastener.

7. The method for enhancing the resistance to fretting damage of high-strength titanium alloy fasteners according to claim 1, characterized in that, The nominal chemical composition of the high-strength titanium alloy is Ti-15Mo-2.7Nb-3Al-0.25Si.

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