A low yield ratio preparation method of a large-size super-high-strength beta titanium alloy rod for fasteners

Abstract

The application discloses a low yield strength ratio preparation method of a large-size super-high-strength beta titanium alloy fastener rod, and comprises the following steps: first, high-temperature forging is conducted on a super-high-strength beta titanium alloy ingot to obtain a rod blank; second, radial forging is conducted on the rod blank below a beta phase transition point in a slow-feeding and single-pass small-deformation processing mode to obtain a radial forging rod blank; and third, low-temperature slow drawing is conducted on the radial forging rod blank, and then solid solution aging treatment is conducted to obtain the large-size super-high-strength beta titanium alloy fastener rod with a low yield strength ratio. The application takes radial forging combined with low-temperature slow drawing as a core process, realizes the reduction of the yield strength ratio of the rod through precise temperature control, deformation control and speed control, solves the problems of the high yield strength ratio, insufficient plasticity reserve and easy cracking of the rod during thread rolling of the super-high-strength beta titanium alloy fastener rod, the process parameter window is clear, no special equipment is needed, the process is compatible with existing forging and drawing production lines, and is suitable for being applied to the stable batch preparation of the large-size aviation main load-bearing fastener rod.

Description

Technical Field

[0001] This invention belongs to the technical field of titanium alloy bar processing and aerospace fastener materials, specifically relating to a method for preparing low yield strength ratio of large-size ultra-high strength β titanium alloy fastener bars. Background Technology

[0002] With the rapid development of aerospace and high-end equipment towards lightweight, high reliability, and long service life, stringent requirements have been placed on the comprehensive performance of materials used in aerospace main load-bearing fasteners (such as bolts, studs, and rivets). Ultra-high strength β-titanium alloys (such as Ti-1300 or Ti-1500) with a strength of 1300MPa~1500MPa are the preferred materials for these fasteners due to their high specific strength, good hardenability, and excellent corrosion resistance. Their performance directly determines the service safety and service life of the fasteners.

[0003] β-titanium alloy bars are used as the core blanks for fasteners. Their mechanical properties and microstructure must not only meet the strength requirements of the fasteners during service, but also be compatible with subsequent processing technologies such as cold heading, hot heading, and thread rolling. Otherwise, problems such as fastener forming cracks and thread defects may occur, affecting the product qualification rate and reliability.

[0004] Large-size fasteners have deep thread profiles and large thread pitches, resulting in significant stress concentration at the thread root during thread rolling. This places higher demands on the plasticity and yield strength ratio of the bar stock: an excessively high yield strength ratio can lead to insufficient plasticity reserve, making the thread rolling prone to cracking, chipping, poor forming, and fatigue failure during service.

[0005] The higher the tensile strength of titanium alloys, especially β-titanium alloys, the greater their yield strength ratio (yield strength / tensile strength). For ultra-high strength β-titanium alloys such as Ti-1300 or Ti-1500 with tensile strengths of 1300~1500MPa, their yield strength ratio is generally higher than 0.95, and the yield strength and tensile strength tend to be close, resulting in a significant decrease in the plasticity and toughness of the bar stock. In addition, the alloy has a high work hardening rate, and the high yield strength ratio characteristic easily leads to adiabatic shearing at the threaded parts.

[0006] Currently, industry research on ultra-high strength β-titanium alloy bars largely focuses on strength enhancement, lacking specific optimization for fastener forming characteristics. An overemphasis on tensile strength leads to excessively high material hardness and insufficient plasticity reserves. This problem not only causes defects such as cracking and incomplete forming during subsequent thread rolling due to insufficient plasticity, significantly reducing the fastener forming pass rate, but also increases subsequent processing difficulty and costs, inducing various processing defects and hindering the large-scale, high-quality production of fasteners.

[0007] Therefore, in response to the actual service requirements and processing requirements of high-end aerospace fasteners, developing a method for preparing fastener bars that can effectively reduce the yield strength ratio of β-titanium alloy, optimize the microstructure, and adapt to subsequent processing techniques has become an urgent technical challenge in the field of β-titanium alloy fastener materials. Summary of the Invention

[0008] The technical problem to be solved by this invention is to address the shortcomings of the prior art by providing a method for preparing large-size ultra-high-strength β-titanium alloy fastener bars with a low yield strength ratio. This method employs a combination of slow-feed, single-pass, small-deformation machining with radial forging and low-temperature slow drawing as its core process. Through precise control of temperature, deformation, and speed, the yield strength ratio is stably controlled, significantly improving the thread rolling machinability and service reliability of large-size ultra-high-strength β-titanium alloy fastener bars, and solving the problem of excessively high yield strength ratios in existing fastener bars.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for preparing low yield strength ratio of large-size ultra-high strength β-titanium alloy fastener bars, characterized in that the method includes the following steps: Step 1: Pretreatment: The ultra-high strength β titanium alloy ingot is forged at high temperature to obtain a billet with uniform structure and no metallurgical defects. Step 2, radial forging: The billet with uniform structure and no metallurgical defects obtained in Step 1 is radially forged below the β phase transformation point using a slow feed and single-pass small deformation processing method to obtain a radially forged billet. Step 3, Low-temperature slow drawing: The radial forging billet obtained in Step 2 is subjected to low-temperature slow drawing, and then solution aging treatment to obtain large-size ultra-high strength β titanium alloy fastener bars with a low yield strength ratio of 0.86~0.90.

[0010] This invention first involves high-temperature forging of an ultra-high-strength β-titanium alloy ingot to obtain a billet with uniform microstructure and no metallurgical defects. This forms the basis for subsequently obtaining uniformly microstructured fastener bars. Then, radial forging is performed below the β-phase transformation point using slow feed and single-pass small deformation processing. This radial forging process forms… <0001> The core process of basal surface texture involves the significant preferential deflection of the edge grains in the bar under uniaxial impact compression, forming a specific filamentary texture. This texture type gives the basal slip system a larger Schmidt factor under axial force, making slip easier to initiate, thus reducing the yield strength ratio of the material from the microstructure perspective. Subsequently, low-temperature slow drawing, where the drawing deformation itself does not change the main texture type, introduces high-density dislocations and deformation substructures within the grains due to the large strain. This significantly increases the orientation gradient and deformation energy storage within the grains, ultimately forming a strong... <0001> The base surface is textured, and finally solution aging treatment is carried out to achieve the required mechanical properties of the material, resulting in large-size ultra-high strength β-titanium alloy fastener bars with low yield strength ratio.

[0011] The method for preparing a low yield strength ratio of a large-size ultra-high strength β-titanium alloy fastener bar is characterized in that the ultra-high strength β-titanium alloy in step one is Ti-1300 or Ti-1500. In this invention, Ti-1300 and Ti-1500 are 1300MPa and 1500MPa grade ultra-high strength β titanium alloys, respectively. They belong to titanium alloys with high work hardening rates. These titanium alloys achieve ultimate strength by introducing a large number of β stabilizing elements and α strengthening elements such as Al. Under the solid solution aging state, the strong solid solution strengthening effect and high β phase stability greatly improve the resistance to plastic deformation, making the work hardening rate of this type of alloy significantly higher than that of TB8 and TB9 alloys. Moreover, the yield strength ratio is usually greater than 0.95. Under such high yield strength ratio conditions, when thread rolling is performed, the deformation is extremely uneven, and it is easy to generate adiabatic shear bands, leading to crack initiation and failure. It is not possible to perform cold thread rolling after solid solution treatment in the β region like TB8 and TB9 alloys. This invention addresses the high yield strength ratio problem of ultra-high strength β titanium alloys by using radial forging and low-temperature slow drawing as the core processes to reduce the yield strength ratio of ultra-high strength β titanium alloys.

[0012] The above-mentioned method for preparing low yield strength ratio of large-size ultra-high strength β-titanium alloy fastener bars is characterized in that, in step two, the temperature below the β phase transformation point is specifically 60℃~70℃ below the β phase transformation point; the slow feed speed is 2.0m / min~2.5m / min; and the single-pass deformation amount of the single-pass small deformation processing is 12%~15%. This invention, by controlling the temperature, slow feed speed, and single-pass deformation amount, avoids the conversion of strain energy into macroscopic temperature rise, ensuring that deformation occurs within a low-temperature range.

[0013] The above-mentioned method for preparing low yield strength ratio of large-size ultra-high strength β-titanium alloy fastener bars is characterized in that the temperature of the low-temperature slow drawing in step three is 80℃~100℃ below the β phase transformation point, the drawing speed is 0.5m / min~1.0m / min, and the deformation per pass is 4%~7%. This invention controls the temperature, drawing speed, and deformation per pass, ensuring effective accumulation of deformation energy and suppressing dynamic recovery through low temperature, and ensuring uniform deformation through slow speed and deformation amount, thus avoiding localized temperature rise and adiabatic shear.

[0014] The method for preparing a low yield strength ratio rod for large-size ultra-high strength β-titanium alloy fasteners described above is characterized in that the diameter of the large-size ultra-high strength β-titanium alloy fastener rod with a low yield strength ratio in step three is not less than 18 mm. Fasteners prepared from rods with a diameter less than 18 mm have a pitch of 1.25, while fasteners prepared from rods with a diameter of 18 mm have a pitch of 1.5. Increasing the pitch from 1.25 to 1.5 means a deeper thread root, requiring a greater distance for metal flow during rolling, resulting in a significant increase in plastic deformation. This invention is applicable to larger-sized fasteners.

[0015] Compared with the prior art, the present invention has the following advantages: 1. This invention develops a core process that combines radial forging with low-temperature slow drawing, employing slow feed and single-pass small deformation machining. Through the large plastic deformation of radial forging and low-temperature slow drawing, the grains are refined while actively controlled and strong plasticity is formed. <0001> The basal texture, which positions the basal slip system of a large number of grains in a favorable orientation, increases the Schmidt factor and facilitates slip movement under axial stress, thereby significantly reducing the macroscopic yield strength. Meanwhile, the tensile strength is mainly controlled by work hardening and grain boundary strengthening mechanisms and is less affected by texture softening. Thus, the yield strength ratio of the material is actively reduced to 0.86~0.90 from the microstructure source, enabling the ultra-high strength β titanium alloy to obtain better warm working plasticity while maintaining a similar strength level.

[0016] 2. This invention combines radial forging with low-temperature slow drawing, cleverly integrating the forming characteristics of fastener thread rolling. It utilizes the subsequent deformation-induced effect to achieve secondary softening. Due to its large size, when rolling threads with a large pitch (≥1.5mm), the thread root and flank are subjected to significant radial and tangential reverse compression deformation, forcing the pre-existing grains with basal texture to rotate to a greater extent. This causes the basal slip system to shift to an orientation that is easier to move along the axial force direction, producing a significant tensile texture softening effect. This effect makes the deformation more evenly distributed in all parts of the thread, effectively alleviating local stress concentration and adiabatic temperature rise, thereby significantly reducing the risk of generating adiabatic shear structure and ensuring the thread forming quality and microstructure consistency.

[0017] 3. The process parameters of this invention are clearly defined, require no special equipment, are compatible with existing forging and drawing production lines, and are suitable for the stable batch production of large-size (diameter ≥18mm) aerospace main load-bearing fastener bars. The technical solution of the present invention will be further described in detail below through embodiments. Detailed Implementation

[0018] Example 1 This embodiment includes the following steps: Step 1, Pretreatment: The Ti-1500 ingot is forged at high temperature to obtain a billet with a diameter of 180mm, uniform structure and no metallurgical defects. Step 2, Radial Forging: The billet with uniform microstructure and no metallurgical defects obtained in Step 1 is radially forged below the β phase transformation point using slow feed and single-pass small deformation processing to obtain a radially forged billet. The billet undergoes three-pass radial forging: First pass: forging temperature is 70°C below the β phase transformation point, single-pass deformation is 15%, feed rate is 2.0 m / min, forging into a bar with a diameter of 100 mm; Second pass: forging temperature is 60°C below the β phase transformation point, single-pass deformation is 13.5%, feed rate is 2.0 m / min, forging into a bar with a diameter of 60 mm; Third pass: forging temperature is 60°C below the β phase transformation point, single-pass deformation is 12%, feed rate is 2.0 m / min, forging into a radially forged billet with a diameter of 35 mm. Step 3, Low-temperature slow drawing: The radial forging billet obtained in Step 2 is subjected to low-temperature slow drawing at a temperature 80°C below the β phase transformation point, a single-pass deformation of 4%, and a drawing speed of 0.5 m / min to obtain a bar with a diameter of 18.5 mm. Then, it is subjected to solution aging treatment, i.e., held at 770°C for 1 hour, followed by water quenching, then held at 510°C for 4 hours, and finally air-cooled to obtain a large-size ultra-high strength β titanium alloy fastener bar with a low yield strength ratio.

[0019] Example 2 This embodiment includes the following steps: Step 1, Pretreatment: The Ti-1500 ingot is forged at high temperature to obtain a billet with a diameter of 160mm, uniform structure and no metallurgical defects. Step 2, Radial Forging: The billet with uniform microstructure and no metallurgical defects obtained in Step 1 is radially forged below the β phase transformation point using slow feed and single-pass small deformation processing to obtain a radially forged billet. The billet undergoes three-pass radial forging: First pass: forging temperature is 70°C below the β phase transformation point, single-pass deformation is 14%, feed rate is 2.5m / min, forging into a bar with a diameter of 100mm; Second pass: forging temperature is 65°C below the β phase transformation point, single-pass deformation is 13%, feed rate is 2.5m / min, forging into a bar with a diameter of 60mm; Third pass: forging temperature is 65°C below the β phase transformation point, single-pass deformation is 12%, feed rate is 2.5m / min, forging into a radially forged billet with a diameter of 35mm. Step 3, Low-temperature slow drawing: The radial forging billet obtained in Step 2 is subjected to low-temperature slow drawing at a temperature 100℃ below the β phase transformation point, a single-pass deformation of 7%, and a drawing speed of 1.0m / min to obtain a bar with a diameter of 20.5mm. Then, it undergoes solution aging treatment, i.e., holding at 770℃ for 1h, followed by water quenching, then holding at 510℃ for 4h, and finally air cooling to obtain a large-size ultra-high strength β titanium alloy fastener bar with a low yield strength ratio.

[0020] Example 3 This embodiment includes the following steps: Step 1, Pretreatment: The Ti-1300 ingot is forged at high temperature to obtain a billet with a diameter of 100mm, uniform structure and no metallurgical defects. Step 2, Radial Forging: The billet with uniform microstructure and no metallurgical defects obtained in Step 1 is radially forged below the β phase transformation point using slow feed and single-pass small deformation processing to obtain a radially forged billet. The billet undergoes two-stage radial forging: First stage: The radial forging temperature is 60°C below the β phase transformation point, the single-pass deformation is 14%, the feed rate is 2.0 mm / min, and the billet is forged into a 60 mm diameter bar. Second stage: The radial forging temperature is 70°C below the β phase transformation point, the single-pass deformation is 12%, the feed rate is 2.5 m / min, and the billet is forged into a 35 mm diameter radially forged billet. Step 3, Low-temperature slow drawing: The radial forging billet obtained in Step 2 is subjected to low-temperature slow drawing at a temperature 90°C below the β phase transformation point, a single-pass deformation of 7%, and a drawing speed of 1.0 m / min to obtain a bar with a diameter of 20.5 mm. Then, solution aging treatment is performed, i.e., holding at 760°C for 1 hour, followed by air cooling, then holding at 540°C for 4 hours, and finally air cooling, to obtain a large-size ultra-high strength β titanium alloy fastener bar with a low yield strength ratio.

[0021] The mechanical properties of the large-size ultra-high strength β-titanium alloy fasteners with low yield strength ratio prepared in Examples 1 to 3 were tested, and the results are shown in Table 1.

[0022] Table 1

[0023] As can be seen from Table 1, the large-size ultra-high strength β-titanium alloy fastener bars with low yield strength ratio prepared in Examples 1 to 3 all meet the requirements for low yield strength ratio.

[0024] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing low yield strength ratio of large-size ultra-high strength β-titanium alloy fastener bars, characterized in that, The method includes the following steps: Step 1: Pretreatment: The ultra-high strength β-titanium alloy ingot is forged at high temperature to obtain a billet with uniform structure and no metallurgical defects. Step 2, radial forging: The billet with uniform structure and no metallurgical defects obtained in Step 1 is radially forged below the β phase transformation point using a slow feed and single-pass small deformation processing method to obtain a radially forged billet. Step 3, Low-temperature slow drawing: The radial forging billet obtained in Step 2 is subjected to low-temperature slow drawing, and then solution aging treatment to obtain large-size ultra-high strength β titanium alloy fastener bars with a low yield strength ratio of 0.86~0.

90.

2. The method for preparing a low yield strength ratio of a large-size ultra-high strength β-titanium alloy fastener bar according to claim 1, characterized in that, The ultra-high strength β-titanium alloy mentioned in step one is Ti-1300 or Ti-1500.

3. The method for preparing a low yield strength ratio of a large-size ultra-high strength β-titanium alloy fastener bar according to claim 1, characterized in that, In step two, the temperature below the β phase transition point is specifically 60°C to 70°C below the β phase transition point. The slow feed rate is 2.0 m / min to 2.5 m / min, and the single-pass deformation amount of the single-pass small deformation machining is 12% to 15%.

4. The method for preparing a low yield strength ratio of a large-size ultra-high strength β-titanium alloy fastener bar according to claim 1, characterized in that, The temperature of the low-temperature slow drawing in step three is 80℃~100℃ below the β phase transformation point, the drawing speed is 0.5m / min~1.0m / min, and the deformation per pass is 4%~7%.

5. The method for preparing a low yield strength ratio of a large-size ultra-high strength β-titanium alloy fastener bar according to claim 1, characterized in that, The diameter of the rods for large-size ultra-high strength β-titanium alloy fasteners with low yield strength ratio mentioned in step three shall not be less than 18 mm.

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