Large-sized TC4 titanium alloy bar for large aero-engine fan blisk and method for preparing same

Abstract

The present disclosure discloses a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk and a method for preparing the same, comprising blooming a large-sized TC4 titanium alloy ingot having a diameter of Φ860∼1000 mm by two or three cycles of heating, upsetting and drawing with the a amount in each cycle of 40-70% to achieve sufficient deformation, and then subjecting to three to seven cycles of heating, upsetting and drawing with a deformation amount in each cycle of 40-95%. The large-sized TC4 titanium alloy bar has a microstructure that is a uniformly distributed equiaxed structure, a grain size of ≤20 µm, an anisometry KD of ≤1.5, a primary α phase content of 70-90% by area, and a primary α phase grain size rating of Level 10-12. The bar has a diameter that reaches Φ400~800 mm, and can be used for direct production of blisks.

Description

Technical Field

[0001] The present disclosure pertains to the technical field of hot working of titanium alloy, and in particular relates to a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk and a method for preparing the same.Background Art

[0002] Titanium alloy is characterized by high specific strength, excellent high / low temperature endurance, among others. It is the material of first choice for important components such as aero-engine fans, compressor discs and blades, etc. The linear friction welding technology is mostly employed to process aero-engine blisks. However, this technology is prone to cause defects at the welding points, especially lack of fusion frequently observed at the edges and corners of the welds, which may cause joint breakage and in turn lead to flight accidents. In order to improve the welding quality, the isothermal upsetting-and-drawing process may be used to achieve uniform and controllable change in the material microstructure, thereby providing a denser and more uniform blisk with stable performances.

[0003] Along with the rapid development of the aviation industry, aircrafts become larger and larger. Accordingly, higher requirements are imposed on engine size and blank materials to be processed in terms of size, structural uniformity, etc. Titanium alloy has lower thermal conductivity and poorer temperature sensitivity than those materials like steel, such that it has a narrow hot working window. As such, it's more difficult to process a large-sized TC4 titanium alloy bar. Especially, when the size increases, it's difficult to guarantee the deformation effect of the bar and the uniformity of the internal temperature distribution, which has a direct influence on its structural transformation and mechanical properties.

[0004] Chinese Patent Publication No. CN116765121A discloses "a preparation method, use and a production system for a TC4 bar". The preparation method includes heating, rough rolling, intermediate rolling and finish rolling in sequence. A primary temperature compensation is provided before the rough rolling to make up for the temperature difference during the rough rolling. A secondary temperature compensation is provided between the finish rolling and the final rolling to make up for the temperature difference caused during the rough rolling and the finish rolling. This improves the uniformity of the structure and appearance of the resulting TC4 bar.

[0005] Chinese Patent Publication No. CN116673422A discloses "an upsetting-and-drawing method for a large-sized TC4 titanium alloy forging bar". The bar is upset and drawn 2 to 3 times in each cycle of heating, upsetting and drawing. In each cycle of heating, upsetting and drawing, the bar is first upset to a predetermined amount, and then drawn to a predetermined aspect ratio, while the deformation amount of the upsetting is different from that of the drawing. This provides a way to effectively combine the gradient distribution of the deformation amount and the drawing crossing cycles, thereby reducing the degree of cracking of the billet during the thermal deformation process in each cycle, improving the uniformity of the bar structure, and increasing the yield ratio of the TC4 forging bar. The bar obtained has a diameter of Φ200~300 mm and a length of 1800~2000 mm.

[0006] Chinese Patent Publication No. CN118600279A discloses a method for preparing a large-sized TC11 high-temperature titanium alloy bar. It mainly aims to address the instability of mechanical properties of titanium alloy bars finally obtained by traditional methods. Although it mentions that a large-sized TC11 high-temperature titanium alloy bar can be prepared, the specific size that is available is not disclosed, and the patent involves TC11 titanium alloy, not TC4 titanium alloy.

[0007] Chinese Patent Publication No. CN116727583A discloses a method for preparing a large-sized titanium alloy bar. Water cooling is utilized to increase the nucleation rate of the billet, increase the number and uniform distribution of nuclei of the material, and improve the overall uniformity of the material's structure and performances. However, the maximum size can only be Φ450mm in diameter.

[0008] Chinese Patent Publication No. CN114888219A discloses a method for preparing a large-sized Ti6Al4V titanium alloy bar. By designing the corresponding deformation amounts after heating for different phase zones, in the blooming stage, blooming, upsetting and drawing are performed at a large deformation amount in the β single-phase zone to fully break up the cast grains and refine the grain size; and finally, upsetting and drawing are performed at a small deformation amount in the α+β two-phase zone in the forming stage. By means of multiple small deformations, the stability of the process and the further refinement of the grains are ensured. This method optimizes the hot working process and flow, reduces the number of heaing, upsetting and drawing, improves the uniformity of the bar structure and the consistency of performances, and provides a bar diameter of Φ200~450 mm. The bar diameter obtained by this patent is small.

[0009] Chinese Patent Publication No. CN111889597A discloses a method for upsetting and drawing a large-sized TC4 titanium alloy bar. By strictly and effectively controlling the upsetting-and-drawing temperature, upsetting and drawing frequency, upsetting and drawing speed, and deformation amount of each pass, the bar temperature is advantageously controlled, so that the internal and external temperature uniformity of the upset and drawn bar is higher, and the bar temperature is prevented from exceeding the transformation temperature of the β phase of the TC4 titanium alloy during the upsetting-and-drawing process, thereby ensuring the uniformity of the bar structure. The large-sized TC4 titanium alloy bar has no complete original β grain boundary; instead, the grain boundary is composed of equiaxed α + β grain boundary. The initial α content is about 50%. The bar obtained has a diameter of ®< 200~450 mm. The bar diameter obtained by this patent is small.

[0010] Chinese Patent Publication No. CN111286686A discloses a short-process method for preparing a large-sized TC4 titanium alloy bar with a fine equiaxed structure. According to this patent, the primary strip-shaped α phase in the TC4 titanium alloy ingot is broken up effectively by near-β reforging and rapid post-forging cooling. It's converted into fine equiaxed α spheres, and a fine-grained structure is formed. This solves the problem of excessive growth of primary α phase caused by the excessively slow air cooling rate after the large-sized billet is forged. In addition, a large number of vacancies are introduced to provide energy for static recrystallization in the heating and holding process in a subsequent cycle of heating, thereby significantly improving the grain refinement efficiency of a single cycle of heating. An intermediate forging billet composed of a fine equiaxed structure, an acicular secondary α phase and a β matrix is obtained. The TC4 titanium alloy bar obtained has a cross-sectional diameter of 200~300 mm, and a length of 5000mm~8000mm. The primary equiaxed α phase grain size rating is not lower than the requirement of Level 8 according to GB / T 6394-2017 Determination of Average Grain Size of Metal. The bar obtained by this patent also has a low grain size and a small diameter.

[0011] Chinese Patent Publication No. CN108097852A discloses a method for upsetting and drawing a large-sized TC4 titanium alloy bar. According to this patent, the process consisting of one upsetting operation and one drawing operation is used to replace the original process consisting of three upsetting operations and three drawing operations, thereby simplifying the process, reducing the production cost and material loss, increasing the degree of material deformation, and enabling fuller crushing of the core structure to achieve equiaxedization. Moreover, as the operations proceed, the temperature of the forging billet decreases gradually, which is conducive to the crushing of the grains and the transformation of the structural morphology, thereby improving the structural uniformity of the large-sized TC4 alloy bar. The diameter of the bar obtained is Φ120mm.

[0012] Chinese Patent Publication No. CN114457259A discloses a fine-grained TC4 titanium alloy bar and a method for preparation the same. Trace amounts of P and S elements are added to an existing TC4 titanium alloy, and the mass ratio of the two elements is controlled to be (1~4): (1~10). The P and S elements are used to weaken the grain boundary bonding force of the titanium alloy, which facilitates the subsequent upsetting-and-drawing process. The main purpose is to fully deform and refine the β grains. It is suitable for industrial production of fine-grained TC4 titanium alloy ingots having a size of ®< 280~920 mm. However, the diameter of the bar obtained is only Φ300mm.

[0013] In summary, the sizes of the currently available TC4 titanium alloy bars are relatively small, and cannot meet the requirement for the manufacturing of large aero-engines. Therefore, it is necessary to prepare a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk to cope with the challenge of manufacturing large-sized aero-engines in the future.Summary

[0014] An object of the present disclosure is to provide a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk and a method for preparing the same. The diameter of the TC4 titanium alloy bar obtained is Φ400~800 mm. In the microstructure of the TC4 titanium alloy bar, the primary α phase content is as high as 70%~90%, and the structure is equiaxed and evenly distributed, wherein the grain size is ≤20 µm, and the anisometry K D is ≤1.5. The bar has a tensile strength of ≥950 MPa, a yield strength of ≥900 MPa, an elongation of ≥13%, a cross-sectional shrinkage of ≥38%, a Brinell hardness HB (indentation diameter) of ≥3.36 mm, and a fracture toughness of ≥65 MPa·m 1 / 2< . At a high temperature of 400°C, the TC4 titanium alloy bar has a tensile strength of ≥630 MPa, an elongation of ≥18%, and a cross-sectional shrinkage of ≥45%, meeting the requirements for preparing a large aero-engine fan blisk.

[0015] To achieve the above object, the technical solution of the present disclosure is as follows: A method for preparing a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk, comprising the following steps: S1: Preparation of a titanium alloy ingot wherein the TC4 titanium alloy ingot has a diameter of Φ860~1000 mm; S2: Preparation of a blank wherein the TC4 titanium alloy ingot obtained in Step S1 is subjected to two or three cycles of heating, upsetting and drawing to obtain a blank, wherein a deformation amount in each cycle is 40-70%, and an upsetting-and-drawing temperature is 1020~1200 °C; S3: Preparation of a bar wherein the blank is subjected to three to seven cycles of heating, upsetting and drawing to obtain a bar having a diameter of Φ400~800 mm, wherein a deformation amount in each cycle is 40~95%, wherein a heating-and-holding temperature of the blank in the upsetting-and-drawing process alternates in a β phase zone and a two-phase zone, wherein the temperature in the β phase zone is 1020~1080 °C, and the temperature in the two-phase zone is 950~980 °C, wherein after upsetting and drawing in the β phase zone, water cooling is carried out, and after upsetting and drawing in the two-phase zone, water cooling or air cooling is carried out.

[0016] Preferably, in Step S1, the titanium alloy ingot is obtained by at least three times of vacuum arc remelting.

[0017] Preferably, in Step S2, the TC4 titanium alloy ingot is subjected to two cycles of heating, upsetting and drawing to obtain the blank, and the specific operations are as follows: the TC4 titanium alloy ingot is held at 700~900 °C for 1~5 h, then heated to 1100~1180 °C and held for 5~8 h, and then upset and drawn to Φ700~900 mm to obtain an octagonal billet; thereafter, the octagonal billet is held at 1020~1080 °C for 1~3 h, and then upset and drawn to Φ400~800 mm to obtain the blank which is then water cooled.

[0018] Preferably, in Step S3, the blank is heated and held in the β phase zone or two-phase zone before upsetting and drawing in each cycle; wherein if the heating-and-holding temperature before upsetting and drawing in the first cycle is in the β phase zone, then the heating-and-holding temperature before upsetting and drawing in the second cycle is in the two-phase zone, and the heating-and-holding temperature after upsetting and drawing in the second cycle is in the β phase zone; wherein if the heating-and-holding temperature before upsetting and drawing in the first cycle is in the two-phase zone, then the heating-and-holding temperature before upsetting and drawing in the second cycle is in the β phase zone, the heating-and-holding temperature before upsetting and drawing in the third cycle is in the two-phase zone, and the heating-and-holding temperature after upsetting and drawing in the third cycle is in the β phase zone.

[0019] Preferably, in Step S3, the blank is subjected to five cycles of heating, upsetting and drawing to obtain a bar having a diameter of Φ400-800 mm, and the specific operations are as follows: S31: The blank is put into a furnace at a furnace temperature of 700-900 °C , held for 3-6 h when the temperature reaches 1020-1080 °C after a heating time of ≥2.5 h, and then upset and drawn to Φ 400-800 mm, followed by water cooling to obtain a first bar blank; S32: The first bar blank is held at 950-980 °C for 2-8 h; after the holding, the first bar blank is upset and drawn to Φ400-800 mm; it is upset and drawn to Φ400-800 mm again; it is then held at 1020-1080 °C for 5-7 h, followed by water cooling to obtain a second bar blank; S33: The second bar blank is held at 950-980 °C for 4-6 h; after the holding, the second bar blank is upset and drawn to Φ400-800 mm, followed by water cooling or air cooling to obtain a third bar blank; S34: The third bar blank is held at 1020-1080 °C for 5-7 h; after the holding, the third bar blank is upset and drawn to Φ400-800 mm, followed by water cooling to obtain a fourth bar blank; S35: The fourth bar blank is held at 950-980 °C for 1-3 h; after the holding, the fourth bar blank is upset and drawn to Φ400-800 mm, followed by water cooling or air cooling to finally obtain a large-sized bar having a diameter of Φ400-800 mm.

[0020] Preferably, the water cooling or air cooling is carried out for a cooling time of ≥ 1 h.

[0021] A large-sized TC4 titanium alloy bar for a large aero-engine fan blisk prepared by the method according to the present disclosure, wherein the TC4 titanium alloy bar comprises, by weight percentage, the following components: Al: 5.50~6.75%, V: 3.5~4.5%, Fe≤0.3%, Si≤0.15%, C≤0.05%, B≤0.0014%, Y≤0.005%, N≤0.05%, H≤0.0125%, 0≤0.20%, and a balance of Ti and unavoidable impurities, wherein a single impurity is ≤0.1%, and a total amount of the impurities is ≤0.2%.

[0022] The microstructure of the TC4 titanium alloy bar is a uniformly distributed equiaxed structure, having a grain size of ≤20 µm, an anisometry K D of ≤1.5, a primary α phase content of 70~90% by area, and a primary α phase grain size rating of Level 10~12 according to GB / T 6394-2017 Determination of Average Grain Size of Metal.

[0023] The diameter of the TC4 titanium alloy bar is Φ400-800 mm.

[0024] In some embodiments, the diameter of the TC4 titanium alloy bar is Φ500-800 mm.

[0025] In some embodiments, the diameter of the TC4 titanium alloy bar is Φ600-800 mm.

[0026] In some embodiments, the primary α phase content of the TC4 titanium alloy bar is 80~90%.

[0027] In some embodiments, in the TC4 titanium alloy bar, the equiaxed primary α phase is distributed on the β transformed structure matrix, all the original β grain boundaries are fully broken, and there is no continuous reticular α phase on the original β grain boundaries.

[0028] The TC4 titanium alloy bar according to the present disclosure has a tensile strength of ≥950 MPa, a yield strength of ≥900 MPa, an elongation of ≥13%, a cross-sectional shrinkage of ≥38%, a Brinell hardness HB (indentation diameter) of ≥3.36 mm, and a fracture toughness of ≥65 MPa·m 1\2< ; at a high temperature of 400 °C, the TC4 titanium alloy bar has a tensile strength of ≥630 MPa, an elongation of ≥18%, and a cross-sectional shrinkage of ≥45%.

[0029] Disclosed is an aero-engine fan blisk with its material being the TC4 titanium alloy bar as described in any embodiments of the present disclosure or formed by the TC4 titanium alloy bar.

[0030] In the manufacturing method of the present disclosure: The TC4 titanium alloy ingot having a diameter of Φ860~1000mm used in the present disclosure is obtained by at least three times of vacuum arc remelting. The large ingot is selected to meet the requirement for preparing a large-sized bar by upsetting and drawing. For preparing a conventional small-sized bar, the ingot selected is also smaller. For the smaller ingot, the ingot material exhibits better forging penetration efficiency in the upsetting-and-drawing process, and thus it's easier to forge. In contrast, a large ingot is used in the present disclosure for upsetting and drawing. The difficulty in upsetting and drawing increases accordingly if a better upsetting-and-drawing effect is desired.

[0031] According to the present disclosure, the blank is prepared by two or three cycles of heating, upsetting and drawing, wherein the upsetting-and-drawing temperature is controlled at 1020~1200 °C in the β phase zone, and the deformation amount in each cycle is controlled at 40~70%, so as to break up the cast structure of the ingot, refine the grains, and modify the structural morphology, thereby providing a ready-to-break structure (a basketweave structure, a martensite structure, etc.) for subsequent upsetting and drawing. At the same time, the upsetting and drawing operations at the β phase temperature reduce surface cracking during the upsetting-and-drawing process, thereby increasing the yield ratio. After cooling, the blank is split by sawing, providing a suitable aspect ratio for subsequent upsetting and drawing, thereby improving the structure-breaking effect in subsequent upsetting and drawing.

[0032] Preferably, in the preparation of the blank by upsetting and drawing, the TC4 titanium alloy ingot is held at 700-900 °C for 1-5 h, then heated to 1100-1180 °C and held for 5-8 h, and then upset and drawn to Φ700-900 mm to obtain an octagonal blank. (1) Gradual heating to the blooming temperature: Since the thermal conductivity of titanium alloy is relatively small, the large ingot having a relatively large diameter (thickness) used in the present disclosure needs to be preheated at a low temperature to prevent cracking inside the ingot material, and then the temperature is increased gradually, which helps to control the structural changes during the heating process and avoid excessive heating that will degrade the material's performances. Gradual heating can reduce the thermal stress caused by the temperature gradient, thereby reducing the risk of cracking in the material during the upsetting-and-drawing process. (2) Structural changes: The large ingot obtained after smelting has a classic Widmanstatten structure. Specifically, the coarse original β grains are clear and complete in the microstructure; there is a continuousα phase grain boundary; large "colonies" are formed in the original β grains; in the same "colony", there are a number of α phases parallel to each other in the same orientation; and there are coarse lamellar α phase and interlamellar β phase in the grains. This structure usually leads to reduced plasticity and toughness of the material, and increased brittleness. After upsetting and drawing at 1100-1180 °C in the β phase zone according to the present disclosure, the β grains are elongated and flattened in the direction of metal flow. Then, nucleation and recrystallization are first developed on the original β grain boundary, so that the β grains are refined. When the new β grains recrystallized on the original β grain boundary grow and contact each other, the finest grain size of the β grains is obtained. (3) Heating to 1100-1180 °C and holding for 5-8 h: This temperature is the blooming temperature of the ingot. The holding ensures uniformity of the temperature across the ingot, avoiding uneven processing and internal stress caused by non-uniformity of the temperature. Owing to the holding, the thermal stress inside the material generated by rapid heating can be reduced, thereby reducing the risk of cracking. (4) The octagonal billet obtained as an intermediate blank: Compared with a round or square cross-section, the corners of the octagonal cross-section can better guide metal flow, making the metal more uniformly distributed during the upsetting-and-drawing process, and reducing the stress concentration at the corners and edges. By upsetting and drawing the octagonal billet, the deformation amount of the material in the β phase zone can be increased, thereby improving the structural uniformity and mechanical performances of the material. Upsetting and drawing the octagonal billet can reduce defects such as cracks and folds during the upsetting-and-drawing process, because octagon can distribute the upsetting and drawing forces more uniformly, and reduce local stress concentration. The octagonal cross-section has more sides and corners than a round or square cross-section, which can improve heat conduction and make the temperature of the material more uniform during heating.

[0033] The octagonal billet obtained is held at 1020-1080 °C for 1-3 h, and then upset and drawn again to Φ400-800 mm to obtain the blank which is then water cooled or air cooled. (1) Structural changes during upsetting and drawing: The Widmanstätten structure has strong heredity, but this heredity can be weakened by the thermoplastic deformation during the first upsetting-and-drawing process, such that the impurities in the ingot are broken up, and distribute more dispersively. The retained β structure is further broken up, which leads to broken β grains. After the deformation of the alloy above the β transformation temperature is ended, during the air cooling process, when the temperature drops to the β phase transition temperature, the β→α transformation occurs. The lamellar α phase first precipitates along the boundary of the original β grains, and then the lamellar α phase precipitates within the grains. After this step, finer β grains, a thinner grain boundary α structure and an intragranular α lamellar structure are obtained. (2) The reason for upsetting before drawing: During upsetting and drawing, upsetting first can provide a larger deformation amount, remove defects, refine impurities, and store energy at the same time to provide energy for recrystallization during drawing. When a large deformation amount is required in the high temperature zone, upsetting first can improve forging penetration efficiency, make the metal flow more uniformly, and reduce the need for frequent trimming. Improving structural uniformity: Upsetting first can increase the deformation amount of the β phase zone, which helps to improve the structural uniformity across the bar, especially between different parts of the bar (such as the edge, 1 / 2R and the core). This is critical to ensure consistent performances of the bar. Upsetting before drawing helps to optimize metal flow, such that the material flows more uniformly during the drawing process, and defects caused by non-uniform local deformation are reduced. (3) The deformation amount in each upsetting-and-drawing process is controlled at 40~95% in order to refine the grains, improve the microstructure, reduce the defects caused by upsetting and drawing, and increase the forging penetration efficiency. A larger deformation amount can increase energy storage, and help to promote recrystallization of the material, such as α→β transformation, thereby influencing the microstructure and macroscopic properties of the material, reducing the anisotropy of the material, and ensuring the performances of the core part, so that the overall performances of the titanium alloy bar and the production efficiency can be improved.

[0034] The blank obtained is subjected to three to seven cycles of heating, upsetting and drawing to obtain a large-sized TC4 titanium alloy bar having a diameter of Φ400~800 mm. During the upsetting-and-drawing process, the upsetting and drawing operations are controlled in two upsetting-and-drawing temperature zones, i.e., the β phase zone and the two-phase zone. In addition, the deformation amount in each cycle is controlled at 40~95%. In coordination with sufficient upsetting and drawing as well as variation in cooling mode (water cooling is employed after upsetting and drawing in the β phase zone, because accelerating the cooling rate will be conducive to recrystallization to obtain finer β grains, a thinner grain boundary α structure and an intragranular α lamellar structure, which is beneficial to further refine the structure, so water cooling is employed. Water cooling or air cooling is employed after upsetting and drawing in the two-phase zone. On the one hand, it reduces residual stress to prevent deformation; on the other hand, it is conducive to further refinement of the lamellar or equiaxed structure with high internal energy, that is, sub-dynamic recrystallization and subsequent static recrystallization occur), recrystallization is carried out for a plurality of times to further refine the grains, and at the same time, the structural morphology is continuously adjusted to gradually transform from a basketweave structure and a martensite structure to a basketweave structure and a partially equiaxed structure, and then further to an equiaxed structure and a partial basketweave structure. Finally, a full equiaxed structure distributed uniformly is obtained, which has a grain size of ≤20 µm and an anisometry K D of ≤1.5. At the same time, the primary α phase content is increased, with the primary α phase content reaching 70~90% by area. The grain size rating reaches Level 10~12 according to GB / T6394-2017 Determination of Average Grain Size of Metal. Hence, the resulting TC4 titanium alloy bar having a larger diameter has better strength and plasticity.

[0035] It should be understood that, as used herein, each cycle of heating, upsetting and drawing includes a heat treatment, upsetting and drawing. In some embodiments, two or more upsettings and drawings can be performed after heat treatment.

[0036] Preferably, the blank is subjected to five cycles of heating, upsetting and drawing to obtain a bar having a diameter of Φ400-800 mm.

[0037] S31: The bar blank is put into a furnace at a furnace temperature of 700~900 °C, held for 3-6 h when the temperature reaches 1020-1080 °C after a heating time of ≥2.5 h, and then upset and drawn to Φ400-800 mm to obtain a first bar blank.

[0038] Structural changes: β grains are further broken up, and the structure is fully broken up by upsetting and drawing, such that the grains are small and uniform. In the β phase zone, the grains are recrystallized to refine the grains, and the structural morphology is adjusted to a basketweave structure and a partially equiaxed structure.

[0039] S32: The first bar blank is held at 950-980 °C for 2-8 h; after the holding, the first bar blank is upset and drawn to Φ400-800 mm; it is upset and drawn to Φ400-800 mm again; it is then held at 1020-1080 °C for 5-7 h, followed by water cooling to obtain a second bar blank.

[0040] The structure is fully broken up by upsetting and drawing, so that the grains are fine and uniform. The heating in the two-phase zone allows for recrystallization of the grains to refine the grains, and the structural morphology is adjusted to a basketweave structure and a partially equiaxed structure. The grain boundary α phase and the lamellar α phase are flattened at the same time, elongated in the direction of metal flow, and broken up. The difference between the grain boundary α phase and the intragranular α phase disappears gradually. After 40~95% deformation, there is no sign of a lamellar structure. During the deformation in the two-phase zone, the alloy undergoes full recrystallization. Moreover, the recrystallization of the α phase is faster than that of the β phase, which promotes formation of the primary α phase. The grains after recrystallization are spherical, named as primary equiaxed α grains. The structure at this time comprises an equiaxed primary α phase and structure of β transformation.

[0041] Cooling after plastic deformation in the β zone is very important, because accelerating the cooling rate will be conducive to recrystallization to obtain finer β grains, a thinner grain boundary α structure and an intragranular α lamellar structure, which is beneficial to further refine the structure. Therefore, water cooling is employed after holding.

[0042] S33: The second bar blank is held at 950-980 °C for 4-6 h; after the holding, the second bar blank is upset and drawn to Φ400-800 mm, followed by water cooling or air cooling to obtain a third bar blank.

[0043] The structure is fully broken up by upsetting and drawing, so that the grains are fine and uniform. Heating in the two-phase zone allows for recrystallization of the grains to refine the grains, and the structural morphology is adjusted to a basketweave structure and a partially equiaxed structure.

[0044] S34: The third bar blank is held at 1020-1080 °C for 5-7 h; after the holding, the third bar blank is upset and drawn to Φ400-800 mm, followed by water cooling to obtain a fourth bar blank.

[0045] The β grains are further broken up, and the structure is fully broken up by upsetting and drawing, such that the grains are small and uniform. In the β phase zone, the grains are recrystallized to refine the grains, and the structural morphology is adjusted to a basketweave structure and a partially equiaxed structure.

[0046] S35: The fourth bar blank is held at 950-980 °C for 1-3 h; after the holding, the fourth bar blank is upset and drawn to Φ400-800 mm, followed by water cooling or air cooling to finally obtain a large-sized bar having a diameter of Φ400-800 mm.

[0047] The structure is fully broken up by upsetting and drawing, so that the grains are small and uniform. The grain boundary is fully broken by heating in the two phase zones respectively, and the final structural morphology is a full equiaxed structure.

[0048] Compared with the prior art, the present disclosure has the following beneficial effects: 1. The present disclosure is directed to a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk.

[0049] In the past, the TC4 titanium alloy bar selected for some aero-engine fan blisks has a diameter of approximately Φ400 mm or less (mostly Φ300 mm or less). The weight of the bar used for a single blisk is also relatively light, and the ingot selected for the bar also has a diameter of Φ860 mm or less. As the bar size becomes smaller, the ingot size also becomes smaller, and the forging penetration efficiency of the ingot material in the upsetting-and-drawing process is also better. The following operations are used in the upsetting-and-drawing process: blooming by two to four cycles of heating, upsetting and drawing in the β phase zone, followed by four to five cycles of heating, upsetting and drawing in the (α+β) two phase zone, forming by one or two cycles of heating, and four to five cycles of heating, upsetting and drawing to obtain a bar, the performances of which meet the requirements.

[0050] In contrast, the blisk made of the bar according to the present disclosure is very large. In the past, a TC17 titanium alloy material is used for this type of blisk. However, due to the low plasticity of the TC17 titanium alloy, it is not suitable in certain usage environment. For this reason, TC4 titanium alloy is selected. The size of the bar is also required to be larger in light of the size of the blisk. In order to make the large-sized bar meet users' technical requirements and that each batch has a certain weight, it is necessary to use a large ingot to meet the requirements of the large-sized bar in terms of the deformation amount in upsetting and drawing and the batch weight, and optimize the upsetting-and-drawing process for the large-sized bar.

[0051] The present disclosure uses a large-sized TC4 titanium alloy ingot having a diameter of Φ860-1000 mm, obtained by at least three times of vacuum arc remelting. By blooming, two or three cycles of heating, upsetting and drawing in coordination with sufficient deformation, a blank is prepared at first to break the cast structure of the ingot to refine the grains, and adjust the structural morphology to a basketweave structure, a martensite structure, etc., so as to provide a ready-to-break blank structure for subsequent upsetting and drawing. Then, three to seven cycles of heating, upsetting and drawing operations are performed, with the deformation amount being controlled at 40~95% in each cycle, and the upsetting-and-drawing temperature being controlled in the β phase zone and the two-phase zone for heating. In coordination with sufficient upsetting and drawing as well as variation in cooling mode, recrystallization is carried out for a plurality of times to further refine the grains, and at the same time, the structural morphology is continuously adjusted to finally transform the structural morphology to an equiaxed structure. At the same time, the primary α phase content is increased, with the primary α phase content reaching 70~90% by area, so that the large-sized TC4 titanium alloy bar obtained has good strength and plasticity, and the bar has a tensile strength of ≥950 MPa, a yield strength of ≥900 MPa, an elongation of ≥13%, a cross-sectional shrinkage of ≥38%, a Brinell hardness HB (indentation diameter d) of ≥3.36 mm, and a fracture toughness of ≥65 MPa·m 1\2< . At a high temperature of 400 °C, the TC4 titanium alloy bar has a tensile strength of ≥630 MPa, an elongation of ≥18%, and a cross-sectional shrinkage of ≥45%.

[0052] 2. The large-sized TC4 titanium alloy bar prepared according to the present disclosure has a diameter that reaches Φ400~800 mm, and has a uniformly distributed equiaxed structure. The grain size is ≤20 µm, and the anisometry K D is ≤1.5. The primary α phase content in the structure is increased. The primary α phase content is as high as 70~90% by area. This percentage content far exceeds the standard for a bar used to prepare an aero-engine fan blisk (>40%). The grain size rating reaches Level 10~12 according to GB / T 6394-2017 Determination of Average Grain Size of Metal. As such, there is obtained a TC4 titanium alloy bar which has a large diameter and whose strength and plasticity can both meet the requirements. The TC4 titanium alloy bar can be used for direct production of blisks. Therefore, the potential for processing and application of titanium alloy is further expanded, and the application fields of titanium alloy are enriched. On the contrary, most of the existing TC4 titanium alloy bars have a diameter of Φ400 mm or less, failing to meet the requirements for producing blisks.

[0053] 3. According to the present disclosure, the large-sized TC4 titanium alloy bar obtained by the method described herein is used to prepare a blisk, whereas the blisks in the prior art are mostly made of TC17 titanium alloy materials. However, due to the low plasticity of TC17 titanium alloy, it cannot be used in particular environments. The use of the large-sized TC4 titanium alloy bar according to the present disclosure to prepare a blisk allows the blisk to possess both high strength and high plasticity, thereby meeting the requirements for use in more environments.

[0054] 4. The large-sized TC4 titanium alloy bar obtained according to the present disclosure can be used directly for isothermal upsetting and drawing to make an aero-engine fan blisk. Therefore, the potential for processing and application of titanium alloy is further expanded, and the application fields of titanium alloy are enriched.Description of the Drawings

[0055] FIG. 1 is a low-magnification photograph of the structure of the TC4 titanium alloy bar having a diameter of Φ420 mm in Example 1 according to the present disclosure; FIG. 2 is a high-magnification photograph of the structure of the TC4 titanium alloy bar having a diameter of Φ420 mm in Example 1 according to the present disclosure; FIG. 3 is a low-magnification photograph of the structure of the TC4 titanium alloy bar having a diameter of Φ800 mm in Example 2 according to the present disclosure; FIG. 4 is a high-magnification photograph of the structure of the TC4 titanium alloy bar having a diameter of Φ800 mm in Example 2 according to the present disclosure; FIG. 5 is a low-magnification photograph of the structure of the TC4 titanium alloy bar having a diameter of Φ700 mm in Example 3 according to the present disclosure; FIG. 6 is a high-magnification photograph of the structure of the TC4 titanium alloy bar having a diameter of Φ700 mm in Example 3 according to the present disclosure; FIG. 7 is a low-magnification photograph of the structure of the TC4 titanium alloy bar in the Comparative Example; FIG. 8 is a high-magnification photograph of the structure of the TC4 titanium alloy bar in the Comparative Example. Detailed Description

[0056] The present disclosure will be further illustrated with reference to the examples and drawings.

[0057] In the following examples, the structure is tested according to GB / T 5168. The tensile strength, yield strength, elongation after fracture and cross-sectional shrinkage are tested according to GB / T 228.1. The fracture toughness is tested according to GB / T 4161. The hardness is tested according to GB / T 231.1.Example 1

[0058] The chemical formulation of the TC4 titanium alloy bar having a diameter of Φ420 mm is shown in Table 1. The method for preparing it comprises the following steps: S1 Preparation of a titanium alloy ingot A Φ860 mm titanium alloy ingot was obtained by three times of vacuum arc remelting. S2 Preparation of a blank S21 The alloy ingot obtained in Step S1 was held at 700 °C for 1 h, then heated to 1130 °C in 2.5 h and held for 5 h, and then upset and drawn from Φ860 to Φ700 mm with a deformation amount being 43% to obtain an octagonal billet; S22 The octagonal billet obtained was held at 1050 °C for 1 h, and then upset and drawn to Φ500 mm with a deformation amount of 50% to obtain a blank which was then water cooled for 10h; S3 Preparation of a bar S31 The blank obtained in Step S22 was put into a furnace at a furnace temperature of 850 °C, heated to 1020 °C in 2.5 h and held for 3 h, and then upset and drawn to Φ500 mm with a deformation amount of 50% to obtain a first bar blank; S32 The first bar blank was held at 950 °C for 2 h, upset and drawn to Φ500 mm, then upset and drawn to Φ500 mm again, with a deformation amount of 50%, and then held at 1080 °C for 5 h, followed by water cooling for 10 h to obtain a second bar blank; S33 The second bar blank was held at 950 °C for 4 h, upset and drawn to Φ500 mm with a deformation amount of 64%, followed by water cooling to obtain a third bar blank; S34 The third bar blank was held at 1020 °C for 5 h, and then upset and drawn to Φ420 mm with a deformation amount of 63%, followed by water cooling to obtain a fourth bar blank; S35 The fourth bar blank obtained was held at 950 °C for 1 h, upset and drawn to Φ420 mm with a deformation amount of 65%, and water cooled for 10 h after the forging to obtain a large-sized TC4 titanium alloy bar having a diameter of Φ420 mm.

[0059] The bar obtained was tested for its structure and mechanical properties. The properties are shown in Table 2.

[0060] FIG. 1 is a low-magnification photograph of the structure of the Φ420 mm TC4 titanium alloy bar obtained. As it can be seen from the photograph, in the structure, there are no visible cracks, shrinkage cavities, pores, folds, inclusions, segregation or other defects that affect its use, and there are no obvious, visually visible clear grains in the low-magnification structure.

[0061] FIG. 2 is a high-magnification photograph of the structure of the Φ420 mm TC4 titanium alloy bar obtained. As it can be seen from the photograph, the structure is an equiaxed structure, and the primary α phase content in the structure reaches 85% or higher. The primary α phase grain size rating reaches Level 10~12 according to GB / T6394-2017 Determination of Average Grain Size of Metal. The equiaxed primary α phase is distributed on the β transformed structure matrix. All the original β grain boundaries are fully broken, and there is no continuous reticular α phase on the original β grain boundaries.

[0062] The Φ420mm large-sized TC4 titanium alloy bar obtained has a grain size of ≤ 10 µm and an anisometry K D of ≤ 1.5. It can be used directly to prepare an aero-engine fan blisk by an isothermal upsetting-and-drawing process.Example 2

[0063] The chemical formulation of the TC4 titanium alloy bar having a diameter of Φ800 mm is shown in Table 1. The method for preparing it comprises the following steps: S1 Preparation of a titanium alloy ingot A Φ1000 mm titanium alloy ingot was obtained by three times of vacuum arc remelting. S2 Preparation of a blank S21 The alloy ingot obtained in Step S1 was held at 850 °C for 4 h, then heated to 1170 °C in 4 h and held for 8 h, and then upset and drawn from Φ1000 mm to Φ700 mm to obtain an octagonal billet, with a deformation amount of 70% in this cycle ; S22 The octagonal billet obtained was held at 1080 °C for 2 h, and then upset and drawn to Φ600 mm with a deformation amount of 65% in this cycle to obtain a blank which was then water cooled for 15 h; S3 Preparation of a bar S31 The blank obtained in Step S22 was put into a furnace at a furnace temperature of 750 °C, heated to 1070 °C in 3.5 h and held for 4 h, and then upset and drawn to Φ600mm with a deformation amount of 92% in this cycle, followed by water cooling to obtain a first bar blank; S32 The first bar blank was held at 980 °C for 3 h, upset and drawn to Φ600 mm with a deformation amount of 85% in this cycle, then upset and drawn to Φ600 mm again, and then held at 1050 °C for 7 h, followed by water cooling for 15 h to obtain a second bar blank; S33 The second bar blank was held at 980 °C for 5 h, upset and drawn to Φ600 mm with a deformation amount of 83% in this cycle to obtain a third bar blank; S34 The third bar blank was held at 1070 °C for 6 h, and then upset and drawn to Φ800 mm with a deformation amount of 83% in this cycle, followed by water cooling to obtain a fourth bar blank; S35 The fourth bar blank was held at 980 °C for 2 h, upset and drawn to Φ800 mm with a deformation amount of 86% in this cycle, and water cooled for 15 h after the forging to obtain a large-sized TC4 titanium alloy bar having a diameter of Φ800 mm.

[0064] Low-magnification structure and mechanical properties were obtained by testing. The properties are shown in Table 2.

[0065] FIG. 3 is a low-magnification photograph of the structure of the Φ800 mm large-sized TC4 titanium alloy bar. As it can be seen from the photograph, there are no visible cracks, shrinkage cavities, pores, folds, inclusions, segregation or other defects that affect its use, and there are no obvious, visually visible clear grains in the low-magnification structure.

[0066] FIG. 4 is a high-magnification photograph of the structure of the Φ800 mm large-sized TC4 titanium alloy bar. As it can be seen from the photograph, the structure is an equiaxed structure, and the primary α phase content in the structure reaches 80% or higher. The primary α phase grain size rating reaches Level 10~12 according to GB / T6394-2017 Determination of Average Grain Size of Metal. The equiaxed primary α phase is distributed on the β transformed structure matrix. All the original β grain boundaries are fully broken, and there is no continuous reticular α phase on the original β grain boundaries.

[0067] The Φ800 mm large-sized TC4 titanium alloy bar obtained has a grain size of ≤15 µm and an anisometry K D of ≤1.5. It can be used directly to prepare an aero-engine fan blisk by an isothermal upsetting-and-drawing process.Example 3

[0068] The chemical formulation of the TC4 titanium alloy bar having a diameter of Φ700 mm is shown in Table 1. The method for preparing it comprises the following steps: S1 Preparation of a titanium alloy ingot The Φ900 mm titanium alloy ingot was obtained by three times of vacuum arc remelting. S2 Preparation of a blank S21: The alloy ingot obtained in Step S1 was held at 880 °C for 3 h, then heated to 1180 °C in 3 h and held for 6 h, and then upset and drawn from Φ900 mm to Φ700 mm to obtain an octagonal billet with a deformation amount of 59% in this cycle, followed by water cooling after the forging; S22: The octagonal billet obtained was held at 1020 °C for 2 h, and then upset and drawn to Φ600 mm with a deformation amount of 62% in this cycle to obtain a blank which was then water cooled for 15 h; S3 Preparation of a bar S31 The blank obtained in Step S22 was put into a furnace at a furnace temperature of 890 °C, heated to 1040 °C in 3.5 h and held for 4 h, and then upset and drawn to Φ600 mm with a deformation amount of 85% in this cycle to obtain a first bar blank; S32 The first bar blank was held at 975 °C for 8 h, upset and drawn to Φ600 mm with a deformation amount of 82% in this cycle, then upset and drawn to Φ600 mm again, and then held at 1040 °C for 6 h, followed by water cooling for 15 h to obtain a second bar blank; S33 The second bar blank was held at 975 °C for 5 h, upset and drawn to Φ600 mm with a deformation amount of 84% in this cycle, followed by air cooling to obtain a third bar blank; S34 The third bar blank was held at 1040 °C for 6 h, and then upset and drawn to Φ700 mm with a deformation amount of 86% in this cycle, followed by water cooling to obtain a fourth bar blank; S35 The fourth bar blank was held at 975 °C for 2 h, upset and drawn to Φ700 mm with a deformation amount of 80% in this cycle, and air cooled for 15 h after the forging to obtain a fifth bar blank; S36 The fifth bar blank was held at 1040 °C for 7 h, and then upset and drawn to Φ700 mm with a deformation amount of 78% in this cycle, followed by water cooling to obtain a sixth bar blank; S37 The sixth bar blank was held at 975 °C for 3 h, upset and drawn to Φ700 mm with a deformation amount of 75% in this cycle, and air cooled for 13 h after the forging to obtain a large-sized TC4 titanium alloy bar having a diameter of Φ700 mm.

[0069] Low-magnification structure and mechanical properties were obtained by testing. The properties are shown in Table 2.

[0070] FIG. 5 is a low-magnification photograph of the structure of the Φ700 mm large-sized TC4 titanium alloy bar. As it can be seen from the photograph, there are no visible cracks, shrinkage cavities, pores, folds, inclusions, segregation or other defects that affect its use, and there are no obvious, visually visible clear grains in the low-magnification structure.

[0071] FIG. 6 is a high-magnification photograph of the structure of the Φ700 mm large-sized TC4 titanium alloy bar. As it can be seen from the photograph, the structure is an equiaxed structure, and the primary α phase content in the structure reaches 85% or higher. The primary α phase grain size rating reaches Level 10~12 according to GB / T6394-2017 Determination of Average Grain Size of Metal. The equiaxed primary α phase is distributed on the β transformed structure matrix. All the original β grain boundaries are fully broken, and there is no continuous reticular α phase on the original β grain boundaries.

[0072] The Φ700 mm large-sized TC4 titanium alloy bar obtained has a grain size of ≤13 µm and an anisometry K D of ≤1.5. It can be used directly to prepare an aero-engine fan blisk by an isothermal upsetting-and-drawing process.Comparative Example

[0073] The chemical formulation of the TC4 titanium alloy bar having a diameter of Φ400 mm is shown in Table 1. The method for preparing it comprises the following steps: S1 Preparation of a titanium alloy ingot The Φ860 mm titanium alloy ingot was obtained by three times of vacuum arc remelting. S2 Preparation of a blank S21 The alloy ingot obtained in Step S1 was held at 850 °C for 1 h, then heated to 1150 °C in 2.5 h and held for 5 h, and then upset and drawn from Φ860 to Φ700 mm to obtain an octagonal billet with a deformation amount of 50% in this cycle, followed by water cooling after the forging; S22 The octagonal billet obtained was held at 1050 °C for 1h, and then upset and drawn to Φ500 mm with a deformation amount of 52% in this cycle to obtain a blank which was then water cooled for 10 h; S3 Preparation of a bar S31 The blank obtained in Step S22 was put into a furnace at a furnace temperature of 850 °C, heated to 1050 °C in 2.5 h and held for 3 h, and then upset and drawn to Φ500 mm with a deformation amount of 60% in this cycle to obtain a first bar blank; S32 The first bar blank obtained was held at 960 °C for 1 h, upset and drawn to Φ400 mm with a deformation amount of 62% in this cycle, and air cooled for 10 h after the forging to obtain a large-sized TC4 titanium alloy bar having a diameter of Φ400 mm.

[0074] The bar obtained was tested for its structure and mechanical properties. The properties are shown in Table 2.

[0075] As it can be seen from Table 2, the bars prepared in Examples 1~3 according to the present disclosure have a diameter that can reach 400~800 mm, a tensile strength of ≥950 MPa, a yield strength of ≥900 MPa, an elongation of ≥13%, a cross-sectional shrinkage of ≥38%, a Brinell hardness HB of ≥3.36mm, and a fracture toughness of ≥65 MPa·m 1\2< ; and at a high temperature of 400°C, the TC4 titanium alloy bars have a tensile strength of ≥630 MPa, an elongation of ≥18%, and a cross-sectional shrinkage of ≥45%, indicating that the TC4 titanium alloy bar prepared by the method according to the present disclosure has high strength and high plasticity while having a large size, and can meet the requirements for preparing a blisk.

[0076] In contrast, in the Comparative Example, the heating, upsetting and drawing operations were performed only in two cycle during the preparation of the blank, and the cooling mode after upsetting and drawing was not clarified. The bar obtained has a yield strength of 820 MPa and a high-temperature tensile strength of 600 MPa. It fails to meet the technical requirements of large aero-engine fan blisks, and thus cannot be used to prepare large aero-engine fan blisks.

[0077] FIG. 7 is a low-magnification photograph of the structure of the Φ400 mm TC4 titanium alloy bar obtained in the Comparative Example. It can be seen from the photograph that the low-magnification structure is not uniform, and the low-magnification structure does not meet the requirements on Φ220 mm bars according to GJB1538.

[0078] FIG. 8 is a high-magnification photograph of the structure of the Φ400 mm TC4 titanium alloy bar obtained in the Comparative Example. As it can be seen from the photograph, the primary α phase content in the structure is about 40%, and there is little primary equiaxed α phase. Elongated α phase is distributed on the β transformed structure matrix. The original β grain boundary is not fully broken, and there is continuous fine reticular α phase on the original β grain boundary. The grain size is ≥20 µm, and the anisometry K D is ≥10. The high-magnification structure rating in the T direction does not meet the requirement on Φ220 mm bars according to GJB1538.

[0079] The Examples described above are only intended to illustrate the preferred embodiments of the present disclosure, not to limit the scope of the present disclosure. Without departing from the design spirit of the present disclosure, various changes and modifications made to the technical solutions of the present disclosure by those skilled in the art should all fall within the protection scope determined by the claims of the present disclosure. Table 1 (Unit: mass percentage)AlVFeSiCBYNHOOther elementsTiSingleTotalStandard value5.50-6.753.50-4.50≤0.30≤0.15≤0.05≤0.0014≤0.005≤0.05≤0.0125≤0.20≤0.1≤0.2BalanceEx. 16.124.000.290.130.040.00130.0040.030.01050.15≤0.02≤0.15BalanceEx. 26.434.200.240.110.020.00110.0030.020.01080.15≤0.02≤0.15BalanceEx. 36.494.290.220.120.010.00100.0030.030.0090.16≤0.02≤0.15BalanceComp. Ex.6.494.290.220.120.010.00100.0030.030.0090.16≤0.02≤0.15Balance Table 2 Tensile performancesTensile performances at a high temperature of 400 °CHardnessFracture toughness (MPa·m 1\2< )Tensile strength Rm (MPa)Yield strength Rp 0.2 (MPa)Elongation after fracture A (%)cross-sectional shrinkage Z (%)Tensile strength Rm (MPa)Elongation after fracture A (%)cross-sectional shrinkage Z (%)Brinell hardness HB(d)Standard value≥895≥825≥10≥25≥615≥12≥40≥3.35mm≥55Ex. 1102694813.53868719.5583.41mm67.9Ex. 2101496813.038.569020.14603.54mm66.73Ex. 31105102116.54471021.5563.39mm68.29Comp. Ex.950820122860018.5423.65mm56

Claims

1. A method for preparing a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk, comprising the following steps: S1: Preparation of a titanium alloy ingot wherein a TC4 titanium alloy ingot has a diameter of Φ860~1000 mm; S2: Preparation of a blank wherein the TC4 titanium alloy ingot obtained in Step S1 is subjected to two or three cycles of heating, upsetting and drawing to obtain a blank, wherein a deformation amount in each cycle is 40~70%, and an upsetting-and-drawing temperature is 1020~1200 °C; S3: Preparation of a bar wherein the blank is subjected to three to seven cycles of heating, upsetting and drawing to obtain a bar having a diameter of Φ400~800 mm, wherein a deformation amount in each cycle is 40~95%, wherein a heating-and-holding temperature of the blank in an upsetting-and-drawing process alternates in a β phase zone and a two-phase zone, wherein the temperature in the β phase zone is 1020~1080°C, and the temperature in the two-phase zone is 950~980 °C, wherein after upsetting and drawing in the β phase zone, water cooling is carried out, and after upsetting and drawing in the two-phase zone, water cooling or air cooling is carried out; wherein the TC4 titanium alloy bar has a microstructure that is a uniformly distributed equiaxed structure, a grain size of ≤20 µm, an anisometry KD of ≤1.5, a primary α phase content of 70~90%, and a primary α phase grain size rating of Level 10~12 according to GB / T 6394-2017 Determination of Average Grain Size of Metal; the TC4 titanium alloy bar has a diameter of Φ400-800 mm.

2. The method for preparing a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk according to claim 1, wherein in Step S1, the TC4 titanium alloy ingot is obtained by at least three times of vacuum arc remelting.

3. The method for preparing a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk according to claim 1, wherein in Step S2, the TC4 titanium alloy ingot is subjected to two cycles of heating, upsetting and drawing to obtain the blank, and specific operations are as follows: the TC4 titanium alloy ingot is held at 700~900 °C for 1~5 h, then heated to 1100~1180 °C and held for 5~8 h, and then upset and drawn to ®700~900 mm to obtain an octagonal billet; thereafter, the octagonal billet is held at 1020~1080 °C for 1~3 h, and then upset and drawn to Φ400~800 mm to obtain the blank which is then water cooled.

4. The method for preparing a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk according to claim 1, wherein in Step S3, the blank is heated and held in the β phase zone or two-phase zone before upsetting and drawing in each cycle; if the heating-and-holding temperature before upsetting and drawing in the first cycle is in the β phase zone, and upsetting and drawing are subsequently carried out in the first cycle, then the heating-and-holding temperature before upsetting and drawing in the second cycle is in the two-phase zone, and upsetting and drawing are subsequently carried out in the second cycle, followed by holding in the β phase zone after upsetting and drawing in the second cycle; if the heating-and-holding temperature before upsetting and drawing in the first cycle is in the two-phase zone, and upsetting and drawing are subsequently carried out in the first cycle, then the heating-and-holding temperature before upsetting and drawing in the second cycle is in the β phase zone, and upsetting and drawing are subsequently carried out in the second cycle; and the heating-and-holding temperature before upsetting and drawing in the third cycle is in the two-phase zone, and upsetting and drawing are subsequently carried out in the third cycle, followed by holding in the β phase zone after upsetting and drawing in the third cycle.

5. The method for preparing a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk according to claim 1 or 4, wherein in Step S3, the blank is subjected to upsetting and drawing in five cycles to obtain a bar having a diameter of Φ400-800 mm, and specific operations are as follows: S31: The blank is put into a furnace at a furnace temperature of 700~900 °C, held for 3-6 h when the temperature reaches 1020-1080°C after a heating time of ≥2.5h, and then upset and drawn to Φ400-800 mm, followed by water cooling to obtain a first bar blank; S32: The first bar blank is held at 950-980 °C for 2-8 h; after the holding, the first bar blank is upset and drawn to Φ400-800 mm; it is upset and drawn to Φ400-800 mm again; it is then held at 1020-1080 °C for 5-7 h, followed by water cooling to obtain a second bar blank; S33: The second bar blank is held at 950~980°C for 4~6 h; after the holding, the second bar blank is upset and drawn to Φ400-800 mm, followed by water cooling or air cooling to obtain a third bar blank; S34: The third bar blank is held at 1020-1080 °C for 5~7 h; after the holding, the third bar blank is upset and drawn to Φ400-800 mm, followed by water cooling to obtain a fourth bar blank; S35: The fourth bar blank is held at 950~980 °C for 1~3 h; after the holding, the fourth bar blank is upset and drawn to Φ400~800 mm, followed by water cooling or air cooling to finally obtain a large-sized bar having a diameter of Φ400~800 mm.

6. The method for preparing a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk according to claim 1, wherein the water cooling or air cooling is carried out for a cooling time of ≥ 1 h.

7. The method for preparing a large-sized TC4 titanium alloy bar for a large aero-engine fan blisk according to claim 5, wherein the water cooling or air cooling is carried out for a cooling time of ≥ 1 h.

8. A large-sized TC4 titanium alloy bar for a large aero-engine fan blisk prepared by the method according to any one of claims 1~7, wherein the TC4 titanium alloy bar comprises, by weight percentage, the following components: Al: 5.50~6.75%, V: 3.5~4.5%, Fe≤0.3%, Si≤0.15%, C≤0.05%, B≤0.0014%, Y≤0.005%, N≤0.05%, H≤0.0125%, O≤0.20%, and a balance of Ti and unavoidable impurities, wherein a single impurity is ≤0.1%, and a total amount of the impurities is ≤0.2%; The TC4 titanium alloy bar has a microstructure that is a uniformly distributed equiaxed structure, a grain size of ≤20 µm, an anisometry KD of ≤1.5, a primary α phase content of 70~90% by area, and a primary α phase grain size rating of Level 10~12 according to GB / T 6394-2017 Determination of Average Grain Size of Metal; The TC4 titanium alloy bar has a diameter of Φ400-800 mm.

9. The large-sized TC4 titanium alloy bar for a large aero-engine fan blisk according to claim 8, wherein the TC4 titanium alloy bar has a tensile strength of ≥950 MPa, a yield strength of ≥900 MPa, an elongation of ≥13%, a cross-sectional shrinkage of ≥38%, a Brinell hardness HB indentation diameter of ≥3.36mm, and a fracture toughness of ≥65MPa·m1\2; at a high temperature of 400 °C, the TC4 titanium alloy bar has a tensile strength of ≥630 MPa, an elongation of ≥18%, and a cross-sectional shrinkage of ≥45%.

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