Method for improving fatigue performance of high-strength titanium alloy

Abstract

The application discloses a method for improving fatigue performance of high-strength titanium alloy and belongs to the technical field of titanium alloy processing. The method comprises the following steps: heating a bar to Tbeta-40-Tbeta-20 DEG C; performing multi-fire forging in an alpha+beta dual-phase zone, wherein the deformation amount of each fire is 15%-20%, the bar is locally upset at both ends, then the bar is integrally upset, and then the bar is rolled to have a deformation amount of 10%-15%; when the cumulative total deformation amount reaches 40%-60%, the bar is subjected to large deformation rolling with a deformation amount of 20%-40% to obtain a rough blank; and then the rough blank is subjected to heating, pre-die forging, final forging and solid solution aging heat treatment. Through the multi-fire edge-pressing and edge-rolling process, high-density dislocations and subgrain boundaries are introduced to promote dynamic recrystallization, effectively eliminate abnormal beta grain flattening structure caused by chordwise large deformation, obtain uniform and fine equiaxed beta grains, and significantly improve the fatigue performance of the forged piece, and the method does not need to scrap the material and has low cost.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy processing technology, specifically a method for improving the fatigue performance of high-strength titanium alloys. Background Technology

[0002] TB6 titanium alloy is a near-β type titanium alloy, possessing advantages such as low density, high specific strength, good fracture toughness, low forging temperature, low flow stress, and relatively high operating temperature, enabling the manufacture of high-quality forgings with complex shapes and uniform microstructure. However, the raw material bars used for forging are typically subjected to axial large deformation rotational forging, a process involving unidirectional deformation. Uneven deformation during this process leads to elongated and flattened grains, resulting in abnormal microstructure and causing the fatigue performance of the forgings to fail to meet requirements. Therefore, it is necessary to design a reforging process that can improve grain flattening and enhance the fatigue performance of forgings, thereby refining the elongated β grains. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a method for improving the fatigue performance of high-strength titanium alloys, thereby improving the flattened grain structure of forgings and enhancing their fatigue performance.

[0004] The technical solution adopted by this invention to solve its technical problem is a method for improving the fatigue performance of high-strength titanium alloys, comprising the following steps: S1: Heat the TB6 titanium alloy bar to (Tβ-40~Tβ-20)℃ and hold it at that temperature until it is fully heated, where Tβ is the β phase transformation temperature of the titanium alloy. S2: Multi-fire forging is carried out in the α+β two-phase region, with a deformation amount ε1 of 15% to 20% per fire. During each fire deformation, the two ends of the bar are first locally upsetting to increase the diameter at both ends and form a billet shape that is large at both ends and small in the middle. Then the bar is upsetting as a whole. After the upsetting is completed, the bar is rolled, with a rolling deformation amount of 10% to 15%. When the total deformation amount of all fires reaches 40% to 60%, the billet is rolled once, with a rolling deformation amount of 20% to 40%, to obtain the raw billet. S3: Heat the billet to (Tβ-40~Tβ-20)℃ and keep it heated through; S4: Perform one pre-forging on a die forging machine, with a deformation amount ε2 of 15%~50%, and air cool after forging to obtain a pre-forging; S5: Heat the pre-forged part to (Tβ-40~Tβ-20)℃ and keep it heated through; S6: Perform a single-pass final forging on a die forging machine, with a deformation amount ε3 of 15%~50%; S7: Perform solution treatment and aging heat treatment on the forgings.

[0005] Furthermore, in steps S1, S3, and S5, the heating and holding time is (0.5~0.7)Dmin, where D is the minimum thickness of the maximum cross-section of the forging, in mm.

[0006] Furthermore, in steps S1, S3, and S5, the heating furnace is an electric furnace with a heating accuracy of ±10℃ or better.

[0007] Furthermore, in step S7, the solution treatment temperature is Tβ-40~Tβ-20℃ and the holding time is 3~8h, and the aging treatment temperature is 500~590℃ and the holding time is 5~10h.

[0008] Furthermore, in step S7, the solution treatment heating furnace adopts an electric furnace with a heating accuracy of ±10℃ or better, and the aging treatment heating furnace adopts an electric furnace with a heating accuracy of ±5℃ or better.

[0009] The beneficial effects of this invention are: by using a multi-fire side-up and side-roll forging process, utilizing the steps of first locally upsetting both ends, then overall upsetting, and finally rolling in each fire, combined with large deformation rolling after the total deformation reaches the standard, the flattened abnormal structure of TB6 titanium alloy with large tangential deformation is effectively improved, a uniform and refined forging structure is obtained, and the fatigue performance of the forging is improved. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the cumulative deformation during the deformation process of the blank in Comparative Example 1 of the present invention; Figure 2 This is a schematic diagram of the cumulative deformation during the deformation process of the blank in Embodiment 1 of the present invention; Figure 3 This is a microstructure diagram of the forging in Comparative Example 1 of the present invention; Figure 4 This is a microstructure diagram of the forging in Embodiment 1 of the present invention. Detailed Implementation

[0011] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0012] like Figure 1 As shown, the present invention provides a method for improving the fatigue properties of high-strength titanium alloys, comprising the following steps: S1: Heat the TB6 titanium alloy bar to (Tβ-40~Tβ-20)℃ and hold it at that temperature until it is fully heated, where Tβ is the β phase transformation temperature of the titanium alloy. S2: Multi-fire forging is performed in the α+β two-phase region. During each forging pass, the edges of the bar stock are locally upsetting to increase the diameter at both ends, forming a billet shape that is large at both ends and small in the middle. A narrow anvil or local pressure block is used to locally compress the edges of the bar stock at both ends, further increasing the end diameter and forming a billet shape that is large at both ends and small in the middle. The purpose of this operation is to push the end metal outward in advance to prevent folding or cracking at the ends during subsequent overall upsetting, and to create conditions for the middle part to bulge outward during overall upsetting. Then, the bar stock is upset as a whole, and a flat anvil is used to apply axial pressure to the bar stock, reducing the height and uniformly increasing the diameter. Since the ends have been pre-enlarged, the deformation during overall upsetting starts from the middle section, with the metal in the middle section bulging outward. After upsetting is completed, the bar stock is rounded. The deformation amount ε1 per forging pass is 15% to 20%. For example, a bar stock with an original height of 1000 mm becomes 800 to 850 mm in height after one forging pass. The purpose of controlling the deformation amount in each firing cycle within a small range is to avoid excessive deformation in a single firing, which would lead to violent local flow and exacerbate grain elongation, while ensuring that the deformation can gradually penetrate into the core.

[0013] The rounding deformation is 10%–15%; rounding here refers to rotating the irregularly shaped billet after upsetting and applying radial pressure to make its cross-section regularized into a circle. The 10%–15% rounding deformation refers to the percentage reduction in billet diameter during the rounding process relative to its initial diameter before the start of that pass. The purpose of this small-deformation rounding is to repair surface unevenness caused by upsetting and provide a regular billet shape for local upsetting in the next pass. Small-deformation rounding does not directly refine the grains, but it introduces high-density dislocations and subgrain boundaries, providing crucial conditions for dynamic recrystallization during subsequent large deformation passes or static recrystallization during heating and holding. Recrystallization nucleation requires reaching a certain strain energy threshold, typically corresponding to a deformation >20%–30%. Small-deformation rounding pre-stores high-density dislocations, significantly increasing the strain energy within the grains. When the next pass involves end upsetting or overall upsetting, the accumulated strain energy more easily reaches the recrystallization critical value, thus initiating recrystallization with less additional deformation. Recrystallization nucleation preferentially occurs in high-energy regions, such as primary grain boundaries, phase boundaries, and subgrain boundaries. Subgrain boundaries formed by small deformations have extremely high density, providing a vast number of potential nucleation sites for recrystallization. High-density subgrain boundaries ensure that recrystallization nuclei are uniformly distributed within the grain, resulting in small and uniformly distributed recrystallized grains.

[0014] When the cumulative total deformation from all firings reaches 40%–60%, the billet undergoes a rounding operation with a rounding deformation of 20%–40%, yielding a rough billet. When the cumulative total deformation from all firings reaches approximately 40%–60%, the billet undergoes another rounding operation, with the rounding deformation controlled at 20%–40%. This large-deformation rounding differs from the small-deformation rounding after each firing; its function is not merely shaping, but more importantly, through greater radial compression, it causes the β-grains inside the billet, which were originally elongated due to chordal deformation, to break down and reorient, promoting equiaxed grains. Simultaneously, large-deformation rounding can eliminate the fibrous structure generated by the cumulative deformation from multiple firings, resulting in a more uniform microstructure.

[0015] The original microstructure of TB6 titanium alloy billets subjected to large tangential deformation exhibits severe β-grain elongation. Direct large deformation overall upsetting would further flatten the elongated grains, making equiaxed grain formation impossible. This invention employs a multi-stage, small-deformation strategy, with each stage involving localized end upsetting, followed by overall upsetting and then small rounding. This gradually distributes the deformation evenly, preventing continuous stress on the grains in the same direction. After sufficient accumulated deformation, a large deformation rounding is performed, with the force direction perpendicular to the original tangential direction. This forces the elongated grains to bend, twist, and dynamically recrystallize, thereby refining the grains. Ultimately, a uniform and refined equiaxed β-grain microstructure is obtained, significantly improving the fatigue performance of the forging.

[0016] S3: Heat the billet to Tβ-40~Tβ-20℃ and keep it heated through; S4: A single-pass pre-forging process is performed on a die forging machine, with a deformation amount ε2 of 15%~50%. After forging, the part is air-cooled to obtain a pre-forged part. Although the billet has achieved relatively uniform equiaxed β grains after multiple passes in S2 forging, the overall deformation degree still exhibits a certain gradient. Pre-forging, with its 15%~50% deformation, further promotes uniform plastic flow across the entire cross-section, eliminating residual local flattening tendencies and providing an intermediate billet with a highly consistent microstructure for final forging. When forging TB6 titanium alloy in the α+β dual-phase region, excessive deformation can lead to significant deformation heat effects, with local temperature rises potentially exceeding the β phase transformation point, resulting in coarse β grains or β spots. Controlling the pre-forging deformation amount to within 50% can effectively control the temperature rise and prevent microstructural deterioration. If the pre-forging deformation is too large, exceeding 50%, the forging will be severely work-hardened, and the remaining deformable allowance will be insufficient, making it difficult to achieve precise surface filling during final forging. Conversely, if the pre-forging deformation is too small, below 15%, the inhomogeneity of the structure left by the blank cannot be effectively eliminated, and the structure may still retain the original flow lines after final forging.

[0017] S5: Heat the pre-forged part to Tβ-40~Tβ-20℃ and keep it heated through; S6: A single-pass final forging process is performed on a die forging machine, with a deformation amount ε3 of 15%~50%. When TB6 titanium alloy undergoes deformation of 15%~50% in the α+β dual-phase region, dynamic recrystallization occurs within the β grains. If the deformation amount is too small, the recrystallization driving force is insufficient, and the grain refinement effect is limited. A deformation amount within the range of 15%~50% can sufficiently refine the grains without causing abnormal grain growth due to excessive deformation. The microstructure after final forging consists of uniform equiaxed β grains + dispersed α phase, which is key to obtaining high fatigue performance.

[0018] S7: Perform solution treatment and aging heat treatment on the forgings. The solution treatment holding temperature is Tβ-40~Tβ-20℃, and the holding time is 3~8h. The aging treatment holding temperature is 500~590℃, and the holding time is 5~10h. The solution treatment fully dissolves the α phase in the β grains, and the aging treatment precipitates fine and dispersed α phases, improving strength while maintaining good plasticity.

[0019] Example 1

[0020] Raw material: TB6 titanium alloy Φ770mm bar stock, β phase transformation point is Tβ.

[0021] Main equipment: ±10℃ precision heating electric furnace, ±10℃ precision heat treatment furnace, ±5℃ precision heat treatment furnace, 4500MN high-speed forging machine, 800MN die forging press.

[0022] S1: Heating of the bar stock, heating to Tβ-30℃ in an electric furnace with a heating accuracy better than ±10℃, holding for thorough heating, the holding time is calculated as (0.6×D)min, where D is the diameter of the bar stock.

[0023] S2: α+β dual-phase forging, performed on a 4500MN high-speed forging mill with multiple forging passes, each pass controlling deformation to 15%–20%. The specific operation for each pass involves first locally upsetting the edges of the bar stock to increase the diameter at both ends, creating a billet shape that is larger at both ends and smaller in the middle; then, the entire bar stock is upset; after upsetting, the bar stock is rolled, with a rolling deformation of 10%–15%. This process is repeated until the cumulative total deformation reaches 50%, at which point the billet is rolled again, with a rolling deformation of 30%, to obtain the desired rough billet.

[0024] S3: Heating of the billet: The billet is heated to Tβ-30℃ in an electric furnace at ±10℃ and kept warm until thoroughly heated.

[0025] S4: Pre-forging, a single pre-forging process is performed on an 800MN forging press, with a deformation amount ε2=30%, followed by air cooling to obtain the pre-forged part.

[0026] S5: Pre-forging heating: Heat the pre-forging in an electric furnace at ±10℃ to Tβ-30℃ and keep it heated through.

[0027] S6: Final forging, a single-pass final forging is performed on an 800MN die forging press, with a deformation amount ε3=25%, to obtain the final forging, which is then air-cooled.

[0028] S7: Heat treatment. The forging is placed in an electric furnace at ±10℃ for solution treatment, with a holding temperature of Tβ-30℃ and a holding time according to process requirements. Then, it is subjected to aging treatment in an electric furnace at ±5℃, with a holding temperature of 520℃, followed by air cooling.

[0029] Comparative Example 1

[0030] Raw material: TB6 titanium alloy Φ770mm bar stock, β phase transformation point is Tβ.

[0031] Main equipment: ±10℃ precision heating electric furnace, ±10℃ precision heat treatment furnace, ±5℃ precision heat treatment furnace, 4500MN high-speed forging machine, 800MN die forging press.

[0032] S1: Heating of the bar stock, heating to Tβ-30℃ in an electric furnace with a heating accuracy better than ±10℃, holding for thorough heating, the holding time is calculated as (0.6×D)min, where D is the diameter of the bar stock.

[0033] S2: α+β dual-phase forging is carried out on a 4500MN high-speed forging machine. When the deformation reaches 50%, the billet is rolled once, and the rolling deformation is 30% to obtain the required blank.

[0034] S3: Heating of the billet: The billet is heated to Tβ-30℃ in an electric furnace at ±10℃ and kept warm until thoroughly heated.

[0035] S4: Pre-forging, a single pre-forging process is performed on an 800MN forging press, with a deformation amount ε2=30%, followed by air cooling to obtain the pre-forged part.

[0036] S5: Pre-forging heating: Heat the pre-forging in an electric furnace at ±10℃ to Tβ-30℃ and keep it heated through.

[0037] S6: Final forging, a single-pass final forging is performed on an 800MN die forging press, with a deformation amount ε3=25%, to obtain the final forging, which is then air-cooled.

[0038] S7: Heat treatment. The forging is placed in an electric furnace at ±10℃ for solution treatment, with a holding temperature of Tβ-30℃ and a holding time according to process requirements. Then, it is subjected to aging treatment in an electric furnace at ±5℃, with a holding temperature of 520℃, followed by air cooling.

[0039] Figure 1 and Figure 2The diagrams show the cumulative deformation during the blank deformation process of Comparative Example 1 and Example 1, respectively. It can be seen from the diagrams that the cumulative deformation in Example 1 is greater than that in Comparative Example 1. While the large axial deformation in Comparative Example 1 can compact some defects, the core deformation may be insufficient, leaving residual micropores that become fatigue sources. Example 1 employs multiple firing cycles of repeated deformation, "kneading" the defects in each direction for more thorough defect elimination.

[0040] Figure 3 and Figure 4 Microstructure diagrams of the forgings in Comparative Example 1 and Example 1 are shown respectively. As can be seen from the diagrams, Comparative Example 1 undergoes axial upsetting, with a single stress direction, and the flattened grains are only further flattened. Example 1, on the other hand, uses multi-directional, multi-stage cumulative large deformation, which completely breaks down and recrystallizes the flattened grains, resulting in a completely equiaxed microstructure.

[0041] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for improving the fatigue properties of high-strength titanium alloys, characterized in that, Includes the following steps: S1: Heat the TB6 titanium alloy bar to (Tβ-40~Tβ-20)℃ and hold it at that temperature until it is fully heated, where Tβ is the β phase transformation temperature of the titanium alloy. S2: Multi-fire forging is carried out in the α+β two-phase region, with a deformation amount ε1 of 15% to 20% per fire. During each fire deformation, the two ends of the bar are first locally upsetting to increase the diameter at both ends and form a billet shape that is large at both ends and small in the middle. Then the bar is upsetting as a whole. After the upsetting is completed, the bar is rolled, with a rolling deformation amount of 10% to 15%. When the total deformation amount of all fires reaches 40% to 60%, the billet is rolled once, with a rolling deformation amount of 20% to 40%, to obtain the raw billet. S3: Heat the billet to (Tβ-40~Tβ-20)℃ and keep it heated through; S4: Perform one pre-forging on a die forging machine, with a deformation amount ε2 of 15%~50%, and air cool after forging to obtain a pre-forging; S5: Heat the pre-forged part to (Tβ-40~Tβ-20)℃ and keep it heated through; S6: Perform a single-pass final forging on a die forging machine, with a deformation amount ε3 of 15%~50%; S7: Perform solution treatment and aging heat treatment on the forgings.

2. The method for improving the fatigue properties of high-strength titanium alloys according to claim 1, characterized in that, In steps S1, S3, and S5, the heating and holding time is (0.5~0.7)Dmin, where D is the minimum thickness of the maximum cross-section of the forging, in mm.

3. The method for improving the fatigue properties of high-strength titanium alloys according to claim 1, characterized in that, In steps S1, S3, and S5, the heating furnace is an electric furnace with a heating accuracy of ±10℃ or better.

4. The method for improving the fatigue properties of high-strength titanium alloys according to claim 1, characterized in that, In step S7, the solution treatment temperature is Tβ-40~Tβ-20℃ and the holding time is 3~8h, and the aging treatment temperature is 500~590℃ and the holding time is 5~10h.

5. A method for improving the fatigue properties of high-strength titanium alloys according to claim 1 or 4, characterized in that, In step S7, the solution treatment heating furnace adopts an electric furnace with a heating accuracy of ±10℃ or better, and the aging treatment heating furnace adopts an electric furnace with a heating accuracy of ±5℃ or better.

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