A TC4-DT titanium alloy bar for aerospace deep cryogenic fastener and a preparation method thereof

Abstract

This invention belongs to the field of non-ferrous metal processing technology, and relates to a TC4-DT titanium alloy bar for aerospace cryogenic fasteners and its preparation method. The method includes the following steps: Step 1, selecting TC4-DT titanium alloy R-state round bar billets that have passed ultrasonic testing and have uniform microstructure under low magnification; Step 2, heating and holding the selected billets in a heating furnace, followed by reversible rolling in two stages with diamond-shaped and elliptical-round hole patterns, and then air cooling; Step 3, performing a centerless turning process to remove the outer layer of the rolled billet; Step 4, subjecting the stripped billet to atmospheric heat treatment; Step 5, straightening the heat-treated billet; Step 6, grinding and polishing the straightened billet to obtain TC4-DT titanium alloy bars for aerospace cryogenic fasteners, meeting the requirements of tensile strength ≥1800MPa and A ≥4% under cryogenic conditions of -253℃, thus satisfying the needs of aerospace cryogenic applications.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metal processing technology, and relates to a TC4-DT titanium alloy bar for aerospace cryogenic fasteners and its preparation method. Background Technology

[0002] In the field of cryogenic applications of titanium alloys, international research on titanium alloys used at -253℃ mainly focuses on the 700MPa grade near-α type titanium alloy TA7 ELI and the 1100MPa grade α+β type titanium alloy Ti6Al4V ELI. Both titanium alloys have shown significant improvements in strength and toughness at extremely low temperatures. Ti6Al4V ELI has been widely used as a primary material for liquid hydrogen containers and liquid hydrogen conduits, achieving good results.

[0003] Based on Ti-6Al-4V titanium alloy, TC4-DT alloy achieves higher fracture toughness and lower crack propagation rate by optimizing the range of alloying elements such as Al, V, and Fe, and reducing the contents of Fe, C, N, and O. Its impurity and interstitial element contents are also lower than those of Ti6Al4V ELI, allowing it to be used in liquid nitrogen environments, where temperatures are lower than liquid hydrogen. Due to its excellent corrosion resistance, low-temperature performance, and weldability, making it suitable for deep-sea environments, it has been widely used in deep-sea equipment such as manned submersibles, deep-sea oil and gas stations, and deep-sea submarines. The manned cabin of China's 4500-meter deep-sea submersible also uses TC4-DT titanium alloy as its pressure hull. Under the premise of comparable strength and plasticity, TC4-DT titanium alloy has better damage tolerance and superior overall performance compared to other titanium alloys, making it a promising candidate for application in cryogenic environments down to -253℃.

[0004] Currently, TC4-DT is generally used for bars with a diameter greater than 100mm. The transverse microstructure is generally controlled at grade A3 to A6 in ETTC2, while the longitudinal microstructure is not graded and is generally coarser than grade AA5. However, the optimal microstructure for the TC4-DT used in this project is grade A1 in the transverse and AA1 in the longitudinal direction to stably meet the fastener connection design requirements under ultra-low temperature conditions of -253℃.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a TC4-DT titanium alloy rod for aerospace cryogenic fasteners and its preparation method.

[0007] To achieve the above objectives, the present invention provides the following technical solution: On one hand, the present invention provides a method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners, specifically including the following steps: Step 1: Select TC4-DT titanium alloy R-state round bar billets that have passed ultrasonic testing and have uniform microstructure at low magnification. The billet billets, by mass percentage, include the following chemical composition: Al 5.5%–6.5%, V 3.5%–4.5%, O ≤0.13%, Fe ≤0.10%, C ≤0.08%, N ≤0.03%, H ≤0.0012%, and the total of other impurity elements ≤0.30%. The billet billet specifications are Φ100mm–Φ180mm. Step 2: The selected billet is heated and held in a heating furnace, and then subjected to reversible rolling in two separate heats with diamond-shaped and elliptical-rounded holes, followed by air cooling; wherein, the heating temperature is 50℃~100℃ below the phase transformation point, and the temperature is held for 1h~4h after reaching the temperature. Step 3: The rolled billet is peeled off using a centerless lathe; Step 4: Perform atmospheric heat treatment on the stripped billet; Step 5: Straighten the heat-treated billet. Step 6: Grind and polish the straightened billet to obtain the target TC4-DT titanium alloy billet.

[0008] Specifically, in step 2, the reversible rolling is as follows: first, a reversible rolling mill with a roll diameter of 650mm is used to roll the material 1-2 times to Φ45mm-Φ60mm, with 7-24 rolling passes; then, a transverse reversible rolling mill is used to roll the material 1 time, with 7-11 rolling passes; the deformation per pass is determined according to the different distributions of diamond-shaped holes and elliptical-circular holes, and the deformation per pass is 10%-30%, with a total rolling deformation of 89%-99%.

[0009] Specifically, the rolled bar blank has a specification of Φ11.5mm~Φ32mm and a diameter tolerance of ±0.5mm.

[0010] Specifically, in step 4, the atmospheric heat treatment process is as follows: the heat treatment temperature is 700℃~800℃, the holding time is 0.5h~2h, AC.

[0011] Specifically, in step 5, thermal tension straightening is used for straightening, and the straightening temperature is 650℃~800℃.

[0012] Specifically, in step 6, the total grinding amount is ≥0.4mm, and the grinding amount per pass is ≤0.3mm.

[0013] Furthermore, in step 6, the specifications of the target TC4-DT titanium alloy rod are Φ10mm~Φ30mm.

[0014] On the other hand, the TC4-DT titanium alloy rods prepared by the above-described methods, under ultra-low temperature conditions of -253℃, have a tensile strength ≥1800MPa and an elongation at section A ≥4%.

[0015] In addition, TC4-DT titanium alloy bars prepared by some or all of the preparation methods described above are used in aerospace cryogenic fasteners.

[0016] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects: The preparation method provided by this invention can meet the requirements of tensile strength ≥1800MPa and A ≥4% under deep cryogenic conditions of -253℃, satisfying the needs of aerospace deep cryogenic applications. This preparation method achieves precise control of deep cryogenic performance through the following two aspects: On the one hand, by purifying the ingot, the content of impurities and interstitial elements in the ingot is reduced, so that the finished bar wire has higher fracture toughness; the TC4-DT selected in this invention is purer than Ti6Al4V ELI, so it has higher deep low temperature plasticity and fracture toughness while maintaining high strength. On the other hand, by designing the overall hole shape (rhomboid hole and elliptical-circular hole), and by adjusting the heating temperature and deformation amount, the microstructure can be controlled, resulting in obvious twinning deformation characteristics (two types of twins, 85°<11-20> and 65°<-1-12-1>), which helps to improve plasticity. Attached Figure Description

[0017] The accompanying drawings are incorporated in and form part of this specification, and together with the description serve to explain the principles of the invention.

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A flowchart illustrating the preparation method of TC4-DT titanium alloy rods for aerospace cryogenic fasteners provided by this invention; Figure 2 This is a microstructure image of the 10mm TC4-DT titanium alloy bar for aerospace cryogenic fasteners prepared in Example 1 of the present invention. Figure 3 The image shows the microstructure of the 22mm TC4-DT titanium alloy bar for aerospace cryogenic fasteners prepared in Example 2 of this invention. Figure 4 This is a microstructure image of the 30mm TC4-DT titanium alloy bar for aerospace cryogenic fasteners prepared in Example 3 of the present invention. Detailed Implementation

[0020] Exemplary embodiments will now be described in detail. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples consistent with some aspects of the invention as detailed in the appended claims.

[0021] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0022] See Figure 1 As shown, this invention provides a method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners, specifically including the following steps: Step 1: Select TC4-DT titanium alloy R-state round bar billets that have passed ultrasonic testing and have uniform microstructure at low magnification. The billets, by mass percentage, include the following chemical composition: Al 5.5%–6.5%, V 3.5%–4.5%, O ≤0.13%, Fe ≤0.10%, C ≤0.08%, N ≤0.03%, H ≤0.0012%, and the total of other impurity elements ≤0.30%. Among them, the specifications of the round bar billet are Φ100mm~Φ180mm, the microstructure is a forging microstructure above the phase transformation point, and the low magnification microstructure is uniform. The surface of the billet is treated by turning, spot grinding or 100% light grinding. The billet length is 1200mm~1800mm. The two ends are treated by turning or grinding to form a bite angle. The treated bite angle is a frustum shape that meets the rolling bite conditions. Step 2: The selected billet is heated and held in a heating furnace, and then subjected to reversible rolling in two separate heats with diamond-shaped and elliptical-rounded holes, followed by air cooling; wherein, the heating temperature is 50℃~100℃ below the phase transformation point, and the temperature is held for 1h~4h after reaching the temperature. The reversible rolling process is as follows: First, a reversible rolling mill with a roll diameter of 650mm is used for 1-2 passes to roll to Φ45mm-Φ60mm, with 7-24 rolling passes; then, a transverse reversible rolling mill is used for 1 pass, with 7-11 rolling passes; the deformation per pass is determined according to the different distributions of diamond-shaped and elliptical-circular holes, with a single pass deformation of 10%-30%, and the total rolling deformation is 89%-99%; Step 3: The rolled billet is peeled off using a centerless lathe; Step 4: Perform atmospheric heat treatment on the stripped billet; Step 5: Straighten the heat-treated billet. Step 6: Grind and polish the straightened billet to obtain the target TC4-DT titanium alloy billet.

[0023] Example 1 This embodiment provides a method for preparing 10mm TC4-DT titanium alloy bars for aerospace cryogenic fasteners, specifically including the following steps: Step 1: Select TC4-DT titanium alloy R-state round bar billets that have passed ultrasonic testing and have uniform microstructure at low magnification. The billets, by mass percentage, include the following chemical composition: Al 5.8%–6.5%, V 3.8%–4.5%, O ≤0.13%, Fe ≤0.05%, C ≤0.05%, N ≤0.03%, H ≤0.0012%, and the total of other impurity elements ≤0.30%. The phase transformation point is 975℃–980℃. Among them, the specifications of the round bar billet are Φ100mm, the microstructure is a forging microstructure above the phase transformation point, and the low magnification microstructure is uniform. The surface of the billet is treated by turning, spot grinding or 100% light grinding. The length of the billet is 1200mm~1800mm. The two ends are treated by turning or grinding to form a bite angle. The treated bite angle is a frustum shape that meets the rolling bite conditions. Step 2: The selected billet is heated and held in a heating furnace, and then subjected to reversible rolling in two separate heats with diamond-shaped and elliptical-rounded holes. The rolled surface is required to be smooth and free of defects such as folds and cracks. After rolling, it is air-cooled. The heating temperature is 930℃, and the temperature is held for 1 hour after reaching the temperature. The rolling process is as follows: First, a reversible rolling mill with a roll diameter of 650mm is used for one pass, rolling to Φ45mm using a diamond-shaped pass, for 10 passes; then, a transverse reversible rolling mill is used for one pass using an elliptical-round pass, for 11 passes; the finished bar billet has a diameter of Φ11.5mm, a diameter tolerance of ±0.5mm, and a total rolling deformation of 98.7%. Step 3: The rolled billet is peeled off using a centerless lathe; Step 4: Perform atmospheric heat treatment on the stripped billet: heat treatment temperature is 700℃, holding time is 1h, AC; Step 5: Straighten the heat-treated billet: The straightening method is thermal tension straightening, and the straightening temperature is 650℃. Step 6: Grind and polish the straightened billet. The total grinding amount is ≥0.4mm and the grinding amount per pass is ≤0.3mm to obtain TC4-DT titanium alloy Φ10mm rod for aerospace cryogenic fasteners.

[0024] The titanium alloy rods prepared in this embodiment were tested, and their product performance is shown in Table 1. The microstructure metallographic images are shown in [Table 1]. Figure 2 , Figure 2 The left image shows the metallographic microstructure of the transverse section, and the right image shows the metallographic microstructure of the longitudinal section. The resulting bar stock meets the processing requirements for fasteners.

[0025] Table 1 Performance results of TC4-DT bar stock Φ10mm Example 2 This embodiment provides a method for preparing 22mm TC4-DT titanium alloy bars for aerospace cryogenic fasteners, specifically including the following steps: Step 1: Select TC4-DT titanium alloy R-state round bar billets that have passed ultrasonic testing and have uniform microstructure at low magnification. The billets, by mass percentage, include the following chemical composition: Al 5.8%–6.5%, V 3.8%–4.5%, O ≤0.13%, Fe ≤0.05%, C ≤0.05%, N ≤0.03%, H ≤0.0012%, and the total of other impurity elements ≤0.30%. The phase transformation point is 975℃–980℃. The round bar billet has a specification of Φ180mm, a forging structure above the phase transformation point, a uniform low-magnification structure, and the surface is treated by turning, spot grinding, or 100% light grinding. The billet length is 1200mm to 1800mm, and the two ends are treated by turning or grinding to form a frustum shape that meets the rolling bite conditions. Step 2: The selected billet is heated and held in a heating furnace, and then subjected to reversible rolling in two separate heats with diamond-shaped and elliptical-rounded holes. The rolled surface is required to be smooth and free of defects such as folds and cracks. After rolling, it is air-cooled. The heating temperature is 930℃, and the temperature is held for 1 hour after reaching the temperature. The rolling process is as follows: First, a reversible rolling mill with a roll diameter of 650mm is used for two passes, rolling to Φ100mm using a diamond-shaped pass, then rolling to Φ55mm, for a total of 24 passes; then, a transverse reversible rolling mill is used for one pass using an elliptical-round pass, for a total of 9 passes; the finished bar billet has a diameter of Φ11.5mm, a diameter tolerance of ±0.5mm, and a total rolling deformation of 98.3%. Step 3: The rolled billet is peeled off using a centerless lathe; Step 4: Perform atmospheric heat treatment on the stripped billet: heat treatment temperature is 750℃, holding time is 2h, AC; Step 5: Straighten the heat-treated billet: The straightening method is thermal tension straightening, and the straightening temperature is 700℃. Step 6: Grind and polish the straightened billet. The total grinding amount is ≥0.4mm and the grinding amount per pass is ≤0.3mm to obtain 22mm TC4-DT titanium alloy rod for aerospace cryogenic fasteners.

[0026] The titanium alloy rods prepared in this embodiment were tested, and their product performance is shown in Table 2. The microstructure metallographic images are shown below. Figure 3 , Figure 3 The left image shows the metallographic microstructure of the transverse section, and the right image shows the metallographic microstructure of the longitudinal section. The resulting bar stock meets the processing requirements for fasteners.

[0027] Table 2 Performance results of TC4-DT bar stock Φ22mm Example 3 This embodiment provides a method for preparing 30mm TC4-DT titanium alloy bars for aerospace cryogenic fasteners, specifically including the following steps: Step 1: Select TC4-DT titanium alloy R-state round bar billets that have passed ultrasonic testing and have uniform microstructure at low magnification. The billets, by mass percentage, include the following chemical composition: Al 5.8%–6.5%, V 3.8%–4.5%, O ≤0.13%, Fe ≤0.05%, C ≤0.05%, N ≤0.03%, H ≤0.0012%, and the total of other impurity elements ≤0.30%. The phase transformation point is 975℃–980℃. Among them, the specifications of the round bar billet are Φ100mm, the microstructure is a forging microstructure above the phase transformation point, and the low magnification microstructure is uniform. The surface of the billet is treated by turning, spot grinding or 100% light grinding. The length of the billet is 1200mm~1800mm. The two ends are treated by turning or grinding to form a bite angle. The treated bite angle is a frustum shape that meets the rolling bite conditions. Step 2: The selected billet is heated and held in a heating furnace, and then subjected to reversible rolling in two separate heats with diamond-shaped and elliptical-round-shaped holes. The rolled surface is required to be smooth and free of defects such as folds and cracks. After rolling, it is air-cooled. The heating temperature is 880℃, and the temperature is held for 2 hours after reaching the temperature. The rolling process is as follows: First, a reversible rolling mill with a roll diameter of 650mm is used for one pass, rolling to Φ60mm using a diamond-shaped pass, for 8 passes; then, a transverse reversible rolling mill is used for one pass using an elliptical-round pass, for 7 passes; the finished bar billet has a diameter of Φ32mm, a diameter tolerance of ±0.5mm, and a total rolling deformation of 90%. Step 3: The rolled billet is peeled off using a centerless lathe; Step 4: Perform atmospheric heat treatment on the stripped billet: heat treatment temperature is 800℃, holding time is 0.5h, AC; Step 5: Straighten the heat-treated billet: The straightening method is thermal tension straightening, and the straightening temperature is 800℃. Step 6: Grind and polish the straightened billet. The total grinding amount is ≥0.4mm and the grinding amount per pass is ≤0.3mm to obtain TC4-DT titanium alloy Φ30mm rod for aerospace cryogenic fasteners.

[0028] The titanium alloy rods prepared in this embodiment were tested, and their product performance is shown in Table 3. The microstructure metallographic images are shown in [Table 3]. Figure 4 , Figure 4 The left image shows the metallographic microstructure of the transverse section, and the right image shows the metallographic microstructure of the longitudinal section. The 30mm TC4-DT titanium alloy bar material used for the aerospace cryogenic fastener meets the processing requirements of the fastener.

[0029] Table 3 Performance results of TC4-DT bar stock Φ30mm In summary, the TC4-DT titanium alloy rod prepared by this invention can meet the requirements of tensile strength ≥1800MPa and A ≥4% under deep cryogenic conditions of -253℃, and is suitable for aerospace deep cryogenic application scenarios.

[0030] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention.

[0031] It should be understood that the present invention is not limited to the content already described above, and various modifications and changes can be made without departing from its scope. The scope of the present invention is limited only by the appended claims.

Claims

1. A method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners, characterized in that, Specifically, the following steps are included: Step 1: Select TC4-DT titanium alloy R-state round bar billets that have passed ultrasonic testing and have uniform microstructure at low magnification. The billets, by mass percentage, include the following chemical composition: Al 5.5%–6.5%, V 3.5%–4.5%, O ≤0.13%, Fe ≤0.10%, C ≤0.08%, N ≤0.03%, H ≤0.0012%, and the total of other impurity elements ≤0.30%. Step 2: The selected billet is heated and held in a heating furnace, and then subjected to reversible rolling in two separate heats with diamond-shaped and elliptical-rounded holes, followed by air cooling; wherein, the heating temperature is 50℃~100℃ below the phase transformation point, and the temperature is held for 1h~4h after reaching the temperature. Step 3: The rolled billet is peeled off using a centerless lathe; Step 4: Perform atmospheric heat treatment on the stripped billet; Step 5: Straighten the heat-treated billet. Step 6: Grind and polish the straightened billet to obtain the target TC4-DT titanium alloy billet.

2. The method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners according to claim 1, characterized in that, In step 1, the specifications of the bar blank are Φ100mm~Φ180mm.

3. The method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners according to claim 2, characterized in that, In step 2, the reversible rolling process is specifically as follows: First, a reversible rolling mill with a roll diameter of 650mm is used to roll the material 1-2 times to a diameter of Φ45mm-Φ60mm, with 7-24 rolling passes. Then, a transverse reversible rolling mill is used to roll the material 1 time, with 7-11 rolling passes. The deformation per pass is 10%-30%, and the total deformation is 89%-99%.

4. The method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners according to claim 3, characterized in that, In step 3, the rolled bar blank has a specification of Φ11.5mm~Φ32mm and a diameter tolerance of ±0.5mm.

5. The method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners according to claim 1, characterized in that, In step 4, the atmospheric heat treatment process is as follows: the heat treatment temperature is 700℃~800℃, the holding time is 0.5h~2h, AC.

6. The method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners according to claim 1, characterized in that, In step 5, thermal tension straightening is used for straightening, and the straightening temperature is 650℃~800℃.

7. The method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners according to claim 1, characterized in that, In step 6, the total grinding amount is ≥0.4mm and the grinding amount per pass is ≤0.3mm.

8. The method for preparing TC4-DT titanium alloy rods for aerospace cryogenic fasteners according to claim 1, characterized in that, In step 6, the target TC4-DT titanium alloy bar has a specification of Φ10mm~Φ30mm.

9. A TC4-DT titanium alloy rod prepared by the preparation method according to any one of claims 1 to 8, characterized in that, Under ultra-low temperature conditions of -253℃, the tensile strength is ≥1800MPa and the elongation at section A is ≥4%.

10. The application of TC4-DT titanium alloy bars prepared by the preparation method according to any one of claims 1 to 8 in aerospace cryogenic fasteners.

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