A quantitative method for controlling the microstructure of aerospace titanium alloys torsional fatigue properties

Abstract

This invention discloses a method for quantitatively controlling the microstructure of aerospace titanium alloys torsional fatigue properties. The method includes: designing a continuous density gradient porous structure of the titanium alloy based on a Voronoi-Tessellation mathematical model; performing additive manufacturing according to the set gradient porous structure to obtain a titanium alloy with a gradient-changing porous structure and fine grain structure; filling the fine-grained porous structure titanium alloy with electromagnetic pulse vibration to eliminate pores in the gradient porous structure, thus obtaining a porous structure after powder filling; and hot isostatic pressing sintering the porous structure after powder filling to obtain a coarse-grained structure. This invention achieves a microstructure characteristic where the fine grain content gradually decreases from the surface to the interior of the component, and the change in fine grain content can be quantitatively controlled, thereby improving torsional fatigue properties while achieving quantitative control of performance.

Description

Technical Field

[0001] This invention belongs to the field of aerospace titanium alloy materials, specifically relating to a method for quantitatively controlling the microstructure of aerospace titanium alloys torsional fatigue properties. Background Technology

[0002] Titanium alloys for aerospace applications possess properties such as lightweight, high strength, high damage tolerance, and corrosion resistance, and are gradually replacing steel in critical load-bearing components such as aircraft landing gear, engine pistons and connecting rods, drive shafts, and fasteners. Fatigue fracture is a common fracture mode in aerospace components and a significant factor affecting the safety and reliability of aircraft, consistently a hot topic in the safety design and fundamental research of mechanical components. For bearings that are constantly subjected to torque, such as drive shafts, which are often subjected to cyclic torsional loads during service, torsional fatigue damage is the main form of failure. Improving the torsional fatigue properties of aerospace titanium alloys is of great significance to promoting the development of my country's aerospace industry.

[0003] Under torsional loads, the shear stress of a component gradually decreases from the surface to the core. Cracks initiate from the surface and, once initiated, rapidly propagate inward under cyclic shear stress, leading to component failure. Therefore, to effectively improve the fatigue damage performance of components under torsional loads, it is necessary to simultaneously enhance the fatigue crack initiation resistance of the surface microstructure and the fatigue crack propagation resistance of the internal microstructure. Traditional single-microstructure types are unlikely to meet these requirements; this goal can only be achieved when the surface and core microstructures differ. Traditional fatigue resistance design methods for components with single-microstructure types are no longer applicable to the microstructure design of in-service components under torsional fatigue loads.

[0004] Therefore, obtaining a microstructure with a gradient structure to improve the torsional fatigue resistance of alloys is crucial for promoting the development of my country's aerospace industry.

[0005] However, there is currently no literature reporting a microstructure design method for controlling torsional fatigue properties, especially a quantitative microstructure control method for improving the torsional fatigue properties of aerospace titanium alloys. Summary of the Invention

[0006] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0007] In view of the problems existing in the above and / or prior art, the present invention is proposed.

[0008] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a method for quantitatively controlling the microstructure of aerospace titanium alloys torsional fatigue properties.

[0009] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for quantitatively controlling the microstructure of aerospace titanium alloys torsional fatigue properties, comprising,

[0010] Based on the Voronoi-Tessellation mathematical model, a continuous density gradient porous structure for titanium alloy was designed, wherein the porosity of the continuous density gradient porous structure is 30-90%.

[0011] Additive manufacturing is carried out according to the set gradient porous structure to obtain a fine-grained titanium alloy with a porous structure of gradient variation. The selected metal powder raw material is Ti-6.5Al-2Zr-1Mo-1V alloy.

[0012] Electromagnetic pulse vibration was used to fill powder into a porous fine-grained titanium alloy to eliminate pores in the gradient porous structure, thus obtaining a porous structure filled with powder.

[0013] The porous structure filled with powder is hot isostatically sintered to obtain a coarse-fine crystalline structure.

[0014] As a preferred embodiment of the method described in this invention, the design of the continuous density gradient porous structure of the titanium alloy includes,

[0015] A controllable irregular porous structure design method based on the probability sphere model is used to design controllable porous structures on the parametric design platform Grasshopper. Statistical methods are used to analyze the relationship between geometric parameters and design parameters, and the control method and distribution law of the porosity gradient of the gradient structure are analyzed. Porosity can be adjusted over a wide range by changing the pore size coefficient and the number of seed points.

[0016] By changing the diameter of the probability sphere, the irregularity can be controlled, enabling the design of a gradient porous titanium alloy with continuously varying porosity gradient and TPMS porous structure.

[0017] As a preferred embodiment of the method described in this invention, the step of performing additive manufacturing according to a set gradient porous structure includes,

[0018] The Ti-6.5Al-2Zr-1Mo-1V alloy powder was dried in a vacuum environment at 120℃ for 2 hours to remove the adsorbed moisture from the powder.

[0019] The dried powder was used to prepare a gradient porous titanium alloy structure using additive manufacturing. By adjusting the laser power, scanning speed and scanning spacing, the grain size of the gradient porous titanium alloy was controlled, so as to achieve a continuous density gradient porous structure with controllable design porosity in the range of 30% to 90%.

[0020] As a preferred embodiment of the method described in this invention, the Ti-6.5Al-2Zr-1Mo-1V alloy powder has the following composition: Ti:Bal, Al: 6.33wt%, Mo: 1.29wt%, Zr: 1.97wt%, V: 1.65wt%, O: 0.089wt%.

[0021] In a preferred embodiment of the method described in this invention, the powder is spherical with a particle size of 32μm±5μm, an oxygen content of ≤1000ppm, and a substrate heating temperature of 373K.

[0022] As a preferred embodiment of the method described in this invention, the additive manufacturing laser power is 150-250W, the laser scanning speed is 800-1200mm / s, the scanning mode is a 67° Z-shaped scanning mode, the laser scanning spacing is 0.12mm, and the scanning layer thickness is 0.03mm.

[0023] In a preferred embodiment of the method described in this invention, the laser power is 200W and the scanning speed is 1000mm / s.

[0024] As a preferred embodiment of the method described in this invention, the hot isostatic pressing sintering further includes weld sealing, wherein the sealing heating temperature is 500-600°C and the vacuum degree after sealing is 3-10 Pa.

[0025] In a preferred embodiment of the method described in this invention, the hot isostatic pressing (HIP) sintering is performed at a temperature of 700–900°C, a pressure of 100–150 MPa, and a holding time of 1–3 h.

[0026] As a preferred embodiment of the method described in this invention, the hot isostatic pressing temperature is 800℃, the hot isostatic pressing pressure is 120MPa, and the hot isostatic pressing time is 2h.

[0027] Beneficial effects of this invention:

[0028] This invention proposes a method for quantitatively controlling the microstructure of torsional fatigue properties of aerospace titanium alloys. The method uses modeling software to accurately model the gradient porous structure and uses additive manufacturing technology to form the designed gradient porous structure layer by layer. At the same time, due to the high-speed condensation process of additive manufacturing, a fine-grained structure will be formed in the gradient porous structure, thereby achieving precise control of the content, spatial distribution and size of the fine-grained structure.

[0029] Coarse-grained structures are prepared by hot isostatic pressing after powder filling. By controlling the hot isostatic pressing process, the size of the coarse-grained structure can be precisely controlled. Hot isostatic pressing has the advantage of closing the micro-defects that are unavoidable in the additive manufacturing process, and eliminating the texture anisotropy formed in the additive manufacturing process, so that the final material is isotropic.

[0030] Through the two methods described above, a microstructure feature in which the fine grain content gradually decreases from the surface layer to the internal structure of a component can be achieved, and the change process of the fine grain content can be quantitatively controlled, thereby improving torsional fatigue performance while achieving quantitative control of performance. Attached Figure Description

[0031] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the 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. Wherein:

[0032] Figure 1 SEM images and particle size distribution characteristics of the Ti-6.5Al-2Zr-1Mo-1V alloy powder in this invention;

[0033] Figure 2 A schematic diagram of the gradient porous structure provided by this invention;

[0034] Figure 3 Here is a physical image of the gradient porous structure prepared according to the present invention;

[0035] Figure 4 A schematic diagram of a microstructure where the content of fine grains gradually decreases from the surface to the interior, provided by this invention;

[0036] Figure 5 The microstructure from the surface to the interior prepared according to the present invention.

[0037] Figure 6 The mechanical properties of the microstructure with gradually decreasing fine grain content from the surface to the interior prepared by this invention are compared with those of fine-grained and coarse-grained microstructures. Detailed Implementation

[0038] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0039] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0040] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0041] In this invention, the Voronoi-Tessellation mathematical model is a conventional and general mathematical model. The design of the gradient porous structure is as follows: first, a unit structure model is selected, and then the gradient porosity is controlled by setting the support rods of the unit structure. The essence of the gradient porous structure is to control the diameter of each support rod and thus control the porosity of each layer.

[0042] When the unit cell is fixed, the porosity of each layer is only related to the radius, which means that the volume of each layer can be calculated, and the overall volume can also be calculated, thereby controlling the overall porosity; if the porosity difference of each layer is set to be the same, the transition will be relatively continuous.

[0043] In this embodiment of the invention, the magnetic pulse generator is manufactured by PST GmbH of Germany, model PS48-16, with a maximum discharge energy of 48kJ and a maximum discharge voltage of 16kV; specifically, the magnetic pulse generator stores electrical energy in 6 capacitors and uses electronic components to control the energy released.

[0044] Example 1

[0045] (1) Using the Voronoi-Tessellation mathematical model method, based on the probabilistic sphere model, the controllable irregular porous structure design method is carried out on the parametric design platform Grasshopper. The relationship between geometric parameters and design parameters is analyzed by statistical methods. The control method and distribution law of the porosity gradient of the gradient structure are analyzed. The porosity is adjusted over a wide range by changing the pore size coefficient and the number of seed points.

[0046] By changing the diameter of the probability sphere, the irregularity can be controlled, ultimately achieving a gradient porous titanium alloy with a porosity gradient of 90-50% continuous variation and a TPMS (Primitive) porous structure. Figure 2 As shown.

[0047] (2) Take Ti-6.5Al-2Zr-1Mo-1V alloy powder with a medium particle size of 31μm, such as... Figure 1 As shown, the composition is: Ti:Bal, Al: 6.33wt%, Mo: 1.29wt%, Zr: 1.97wt%, V: 1.65wt%, O: 0.089wt%.

[0048] The powder material was dried in a vacuum environment at 120°C for 2 hours to remove any moisture that might be adsorbed in the powder and reduce the impact of moisture absorption on the quality of the laser deposition cladding.

[0049] (3) Place the dried powder into the powder feeding cylinder of the BLT-S210 equipment, using a Ti-6Al-4V substrate, adjust the height of the substrate so that the distance between it and the high-speed steel scraper is 0.02±0.01μm, then start heating the substrate to 373K, and purge with argon gas to make the oxygen content in the equipment less than 100ppm.

[0050] (4) Using additive manufacturing process to prepare gradient porous titanium alloy structure:

[0051] Additive manufacturing was carried out according to the set gradient porous structure. The laser power was 200W, the scanning speed was 1000mm / s, the scanning spacing was 0.12mm, the scanning layer thickness was 0.03mm, and the scanning method was a Z-shaped scanning with a 67° rotation layer by layer to obtain a gradient-changing porous fine-grained titanium alloy. Figure 3 As shown.

[0052] (5) The porous titanium alloy obtained above is filled with powder by electromagnetic pulse vibration to ensure that all voids are filled with powder.

[0053] (6) The gradient porous titanium alloy filled with the powder is encased in stainless steel, and the weld sealing temperature is 600℃ and the vacuum degree is 10-3Pa.

[0054] (7) The above-mentioned cladding structure was subjected to hot isostatic pressing (HIP400) sintering at a temperature of 800℃, a pressure of 120MPa, and a time of 2 hours to obtain a titanium alloy with a microstructure in which the content of fine grains gradually decreases and the content of coarse grains gradually increases from the surface to the interior. Figure 4 As shown.

[0055] (8) EBSD analysis of the obtained microstructure showed that the fine-grained structure had a grain size of 10±3 μm, with its content gradually decreasing from 90% in the surface layer to 50% in the central part. The coarse-grained structure had a grain size of 45±5 μm, with its content gradually increasing from 10% in the surface layer to 50% in the central part. Figure 5 As shown.

[0056] (9) The obtained titanium alloy was subjected to torsional fatigue performance analysis. Torsional fatigue test conditions: sinusoidal cyclic loading was used, the loading frequency was 0.5 Hz, the equivalent amplitude was controlled, the strain ratio was R = -1, and all tests were completed at room temperature.

[0057] The results are as follows Figure 6 The results shown indicate that:

[0058] The obtained gradient-variable structure titanium alloy has a significant advantage in torsional fatigue strength and fatigue life compared with titanium alloy with uniform grain structure of the same composition.

[0059] Example 2

[0060] The difference between this embodiment and Embodiment 1 is that the gradient porosity variation of the gradient porous structure is changed:

[0061] In this embodiment, a gradient porous titanium alloy with a porosity gradient of 80-50% and a TPMS porous structure (Primitive) is selected.

[0062] The results showed that the final microstructure was as follows: fine-grained microstructure with a grain size of 10±3μm, the content of which gradually decreased from 80% in the surface layer to 50% in the central part; coarse-grained microstructure with a grain size of 45±5μm, the content of which gradually increased from 20% in the surface layer to 50% in the central part.

[0063] Compared to Example 1, the surface fine-grained structure was reduced from 90% to 80%. The subsequent process was not significantly different from that of Example 1, resulting in no significant change in the size of the fine and coarse grains. Due to the reduction of the surface fine-grained structure, the surface strength was reduced, and the resistance to torsional fatigue crack initiation was reduced compared to the titanium alloy obtained in Example 1. As a result, the torsional fatigue resistance of the alloy obtained in Example 2 was slightly reduced compared to Example 1.

[0064] Example 3

[0065] The difference between this embodiment and Embodiment 1 is that the gradient porosity variation of the gradient porous structure is changed: This embodiment selects a gradient porous titanium alloy with a porosity gradient that varies continuously from 70% to 50% and a TPMS porous structure (Primitive).

[0066] The results showed that the final microstructure was as follows: fine-grained microstructure with a grain size of 10±3μm, the content of which gradually decreased from 70% in the surface layer to 50% in the central part; coarse-grained microstructure with a grain size of 45±5μm, the content of which gradually increased from 30% in the surface layer to 50% in the central part.

[0067] Compared to Example 1, the surface fine-grained structure was reduced from 90% to 70%. The subsequent process was not significantly different from that of Example 1, resulting in no significant change in the size of the fine and coarse grains. Due to the reduction in the surface fine-grained structure, the surface strength was reduced, and the resistance to torsional fatigue crack initiation was reduced compared to the titanium alloys obtained in Examples 1 and 2. As a result, the torsional fatigue resistance of the alloy obtained in this example was slightly reduced compared to the titanium alloys obtained in Examples 1 and 2.

[0068] Example 4

[0069] The difference between this embodiment and Embodiment 1 is that the gradient porosity variation of the gradient porous structure is changed: This embodiment selects a gradient porous titanium alloy with a porosity gradient that varies continuously from 60% to 50% and a TPMS porous structure (Primitive).

[0070] The results showed that the final microstructure was as follows: fine-grained microstructure with a grain size of 10±3μm, the content of which gradually decreased from 60% in the surface layer to 50% in the central part; coarse-grained microstructure with a grain size of 45±5μm, the content of which gradually increased from 40% in the surface layer to 50% in the central part.

[0071] Compared to Example 1, the surface fine-grained microstructure was reduced from 90% to 60%. The subsequent processing steps were not significantly different from those in Example 1, resulting in no significant change in the size of the fine and coarse grains. Due to the reduction in the surface fine-grained microstructure, the surface strength was reduced, and the resistance to torsional fatigue crack initiation was lower than that of the titanium alloys obtained in Examples 1, 2, and 3. Consequently, the torsional fatigue resistance of the alloy obtained in this example was slightly lower than that of Examples 1, 2, and 3.

[0072] Example 5

[0073] The difference between this embodiment and Embodiment 1 is that the additive manufacturing process has been changed: In this embodiment, the laser power is selected as 300W, the scanning speed is 1000mm / s, the scanning spacing is 0.12mm, the scanning layer thickness is 0.03mm, and the scanning method is a Z-shaped scanning with a 67° rotation layer by layer to obtain a porous fine-grained titanium alloy with a gradient change.

[0074] The results showed that the final microstructure was as follows: fine-grained microstructure with a grain size of 15±3μm, the content of which gradually decreased from 90% in the surface layer to 50% in the central part; coarse-grained microstructure with a grain size of 45±5μm, the content of which gradually increased from 10% in the surface layer to 50% in the central part.

[0075] Compared to Example 1, the fine-grained microstructure increased from 10±3μm to 15±3μm. This is because the increased laser power in additive manufacturing led to an increase in energy density, which in turn increased the fine-grained microstructure of the gradient porous titanium alloy formed by additive manufacturing. The subsequent process was not significantly different from Example 1, and the coarse grain size did not change significantly. However, the increased size of the fine-grained microstructure on the surface led to a decrease in surface strength and a decrease in the ability to resist torsional fatigue crack initiation compared to the titanium alloy obtained in Example 1. As a result, the torsional fatigue resistance of the alloy obtained in this example was slightly lower than that of Example 1.

[0076] Example 6

[0077] The difference between this embodiment and Embodiment 1 is that the additive manufacturing process has been changed:

[0078] In this embodiment, the laser power is selected as 200W, the scanning speed is 800mm / s, the scanning interval is 0.12mm, the scanning layer thickness is 0.03mm, and the scanning method is a Z-shaped scanning with a 67° rotation layer by layer to obtain a porous fine-grained titanium alloy with a gradient change.

[0079] The results showed that the final microstructure was as follows: fine-grained microstructure with a grain size of 13±3μm, the content of which gradually decreased from 90% in the surface layer to 50% in the central part; coarse-grained microstructure with a grain size of 45±5μm, the content of which gradually increased from 10% in the surface layer to 50% in the central part.

[0080] Compared to Example 1, the fine-grained microstructure increased from 10±3μm to 13±3μm. This is because the energy density increased due to the reduced laser scanning speed in additive manufacturing, which in turn increased the fine-grained microstructure of the gradient porous titanium alloy formed by additive manufacturing. The subsequent process was not significantly different from Example 1, and the coarse grain size did not change significantly. However, the increased size of the surface fine-grained microstructure led to a decrease in surface strength and a reduction in the resistance to torsional fatigue crack initiation compared to the titanium alloy obtained in Example 1. Consequently, the torsional fatigue resistance of the alloy obtained in this example was slightly lower than that of Example 1.

[0081] Example 7

[0082] The difference between this embodiment and Embodiment 1 is that the hot isostatic pressing process has been changed:

[0083] In this embodiment, a hot isostatic pressing (HIP) temperature of 900℃, a HIP pressure of 120MPa, and a HIP time of 2h were selected to obtain a titanium alloy with a microstructure in which the content of fine grains gradually decreases from the surface to the interior, while the content of coarse grains gradually increases. The results show that the obtained fine grain structure has a grain size of 18±3μm, with the content gradually decreasing from 90% at the surface to 50% in the central region. The coarse grain size is 50±5μm, with the content gradually increasing from 10% at the surface to 50% in the central region.

[0084] Compared to Example 1, the fine-grained structure increased from 10±3μm to 18±3μm, and the coarse-grained structure increased from 45±5μm to 50±5μm. This is because the increased hot isostatic pressing temperature caused all grains, including the fine grains formed by additive manufacturing, to grow, resulting in a larger grain size in the entire component compared to Example 1. This leads to a decrease in surface strength and a lower resistance to torsional fatigue crack initiation compared to the titanium alloy obtained in Example 1. Although the increased internal grain size is beneficial in hindering crack propagation, the torsional fatigue resistance of the alloy obtained in this example is still slightly lower than that of Example 1.

[0085] Example 8

[0086] The difference between this embodiment and Embodiment 1 is that the hot isostatic pressing process has been changed:

[0087] In this embodiment, a hot isostatic pressing temperature of 800℃, a hot isostatic pressing pressure of 120MPa, and a hot isostatic pressing time of 3h were selected to obtain a titanium alloy with a microstructure in which the content of fine grains gradually decreases from the surface to the interior, while the content of coarse grains gradually increases.

[0088] The results show that the obtained fine-grained structure has a grain size of 20±3μm, and its content gradually decreases from 90% in the surface layer to 50% in the central part. The coarse-grained structure has a grain size of 53±5μm, and its content gradually increases from 10% in the surface layer to 50% in the central part.

[0089] Compared to Example 1, the fine-grained structure increased from 10±3μm to 20±3μm, and the coarse-grained structure increased from 45±5μm to 53±5μm. This is because the extended hot isostatic pressing holding time led to the growth of all grains, including the fine grains formed by additive manufacturing, resulting in a larger grain size in the entire component compared to Example 1. This resulted in a decrease in surface strength and a lower resistance to torsional fatigue crack initiation compared to the titanium alloy obtained in Example 1. Although the increased internal grain size is beneficial in hindering crack propagation, the torsional fatigue resistance of the alloy obtained in this example is still slightly lower than that of Example 1.

[0090] Comparative Example 1

[0091] The difference between Comparative Example 1 and Examples 1-8 is that:

[0092] This comparative example is a fully fine-grained structure prepared by additive manufacturing. The same 3D printing process as in Example 1 was used for shaping, and the resulting structure is a fine-grained structure with a grain size of 10±3μm. The structure is fine-grained from the surface to the interior.

[0093] Compared to Example 1, the alloy microstructure is entirely fine-grained, and the fatigue initiation resistance of the alloy surface is comparable to that of Example 1. However, the resistance to torsional fatigue crack propagation inside the alloy is reduced, ultimately leading to a decrease in the alloy's torsional fatigue resistance.

[0094] Comparative Example 2

[0095] The difference between Comparative Example 1 and Examples 1-8 is that:

[0096] This comparative example is a fully coarse-grained structure prepared by hot isostatic pressing. The same hot isostatic pressing process as in Example 1 was used for forming. The resulting structure is a coarse-grained structure with a fine grain size of 45±5μm. The structure is fine-grained from the surface to the interior.

[0097] Compared to Example 1, the alloy microstructure is entirely coarse-grained, which reduces the fatigue initiation resistance of the alloy surface. Under torsional fatigue load, cracks are more likely to initiate on the alloy surface, ultimately leading to a decrease in the alloy's torsional fatigue resistance.

[0098] This invention presents an innovative approach to preparing a controllable gradient of coarse and fine grain structures through 3D printing of a gradient porous structure followed by hot isostatic pressing (HIP) with powder filling, effectively improving torsional fatigue resistance. The controllable gradient porous structure is prepared by 3D printing, utilizing the rapid solidification of 3D printing to obtain a fine-grained microstructure. Powder is then filled into the gradient porous framework, and HIP sintering is used to prepare a coarse-grained microstructure. By designing the gradient porous structure, the content and spatial distribution of fine grains can be controlled, thus achieving a controllable distribution of coarse and fine grains. Under torque, the shear stress of a component gradually decreases from the surface to the core. Cracks generally form on the surface, but once initiated, they propagate rapidly inward under shear stress, leading to component failure. Therefore, to improve the torsional fatigue resistance of an alloy, it is necessary to simultaneously improve the resistance to fatigue crack initiation on the surface and the resistance to fatigue crack propagation in the core microstructure. In other words, torsional fatigue requires high strength and hardness on the surface of the material, while the strength and hardness in the center are low. This invention can achieve a high content of fine grains in the material to improve strength and hardness, and a high content of coarse grains in the center of the material to reduce strength and hardness. In this way, the torsional fatigue resistance of the alloy is improved, and the key point is that the distribution of coarse and fine grains can be quantitatively controlled.

[0099] High surface strength and hardness, and low core strength and hardness, can improve the torsional fatigue resistance of materials. Fine grains in titanium alloys can increase strength and hardness. This invention designs porosities of 90-50%, 80-50%, 70-50%, and 60-50% to achieve different levels of fine grains in the material's surface, thereby altering the surface strength and hardness to regulate torsional fatigue resistance. Theoretically, a porosity of 95-55% would result in a higher content of fine grains in the surface, thus increasing surface strength and hardness and improving torsional fatigue resistance. However, in practice, 3D printing a gradient porous structure with a porosity of 95-55% is difficult, and metallurgical defects are prone to occur in high-porosity regions.

[0100] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the present invention.

Claims

1. A method for quantitatively controlling the microstructure of aerospace titanium alloys torsional fatigue properties, characterized in that: include, Based on the Voronoi-Tessellation mathematical model, a continuous density gradient porous structure for titanium alloy was designed, wherein the porosity of the continuous density gradient porous structure is 30-90%. The Ti-6.5Al-2Zr-1Mo-1V alloy powder was dried in a vacuum environment at 120℃ for 2 hours to remove the adsorbed moisture from the powder. A gradient porous titanium alloy structure was prepared by additive manufacturing of dried powder. The grain size of the gradient porous titanium alloy was controlled by adjusting the laser power, scanning speed, and scanning spacing to achieve a continuous density gradient porous structure with a controllable porosity within the range of 30-90%. This resulted in a fine-grained titanium alloy with a gradient porous structure. The selected metal powder raw material was a Ti-6.5Al-2Zr-1Mo-1V alloy with the following composition: Ti:Bal, Al: 6.33wt%, Mo: 1.29wt%, Zr: 1.97wt%, V: 1.65wt%, O: 0.089wt%. The additive manufacturing laser power was 150~250W, the laser scanning speed was 800~1200mm / s, the scanning method was a 67° Z-shaped scanning method, the laser scanning spacing was 0.12mm, and the scanning layer thickness was 0.03mm. Electromagnetic pulse vibration was used to fill powder into a porous fine-grained titanium alloy to eliminate pores in the gradient porous structure, thus obtaining a porous structure after powder filling. The porous structure filled with powder is hot isostatically sintered to obtain a coarse-fine grain structure, achieving the microstructural feature of gradually decreasing fine grain content from the surface to the interior of the component.

2. The method as described in claim 1, characterized in that: The designed titanium alloy continuous density gradient porous structure includes, A controllable irregular porous structure design method based on the probability sphere model is used to design controllable porous structures on the parametric design platform Grasshopper. Statistical methods are used to analyze the relationship between geometric parameters and design parameters, and the control method and distribution law of the porosity gradient of the gradient structure are analyzed. Porosity can be adjusted over a wide range by changing the pore size coefficient and the number of seed points. By changing the diameter of the probability sphere, the irregularity can be controlled, enabling the design of a gradient porous titanium alloy with continuously varying porosity gradient and TPMS porous structure.

3. The method as described in claim 1, characterized in that: The powder is spherical with a medium particle size of 32μm±5μm, an oxygen content of ≤1000ppm, and a substrate heating temperature of 373K.

4. The method as described in claim 1, characterized in that: The laser power is 200W, and the scanning speed is 1000mm / s.

5. The method as described in claim 1, characterized in that: The hot isostatic pressing sintering also includes weld sealing, wherein the sealing heating temperature is 500~600℃ and the vacuum degree after sealing is 3~10Pa.

6. The method as described in claim 5, characterized in that: The hot isostatic pressing (HIP) sintering is performed at a temperature of 700-900℃, a pressure of 100-150MPa, and a holding time of 1-3 hours.

7. The method as described in claim 6, characterized in that: The hot isostatic pressing temperature is 800℃, the hot isostatic pressing pressure is 120MPa, and the hot isostatic pressing time is 2h.

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