A method for manufacturing a TiAl alloy low-pressure turbine blade

Abstract

The present application belongs to the field of TiAl intermetallic compound, and relates to a TiAl alloy composition and a preparation method of high-performance TiAl alloy low-pressure turbine blade. Based on a TiAl alloy composition, the alloy is first made to have fine grain structure through reasonable extrusion + recrystallization annealing process, then the special molybdenum alloy die and the process method of vacuum isothermal die forging are adopted, so that the brittle material realizes the forming of blade structure at high temperature and low speed, and the technical bottleneck problems such as large blade machining allowance, serious material waste, uneven blade structure, unstable mechanical properties of parts and the like caused by the need of machining preform of TiAl alloy at high temperature and preheating at low temperature during the near isothermal forming in the conventional preparation method are solved, and the blade quality is significantly improved and the manufacturing cost is saved. The machining allowance of the TiAl blade die forging part of the present application is only 2mm, and the raw material consumption can be saved by about 30%.

Description

Technical Field

[0001] This invention belongs to the field of TiAl intermetallic compounds and relates to a method for preparing TiAl alloy low-pressure turbine blades. Background Technology

[0002] TiAl alloys possess characteristics such as low density, high specific strength, high specific stiffness, flame retardancy, and excellent high-temperature oxidation and creep resistance, making them a highly promising lightweight high-temperature structural material with broad application prospects in aerospace, automotive manufacturing, and other industrial fields. Given these properties, TiAl alloys are suitable for manufacturing aero-engine blades. On one hand, replacing existing materials can reduce component weight, achieving weight reduction. On the other hand, it can reduce the centrifugal force generated during rotation, lowering the load on disks and shafts, thus achieving structural weight reduction for the system. It is particularly suitable for large-sized low-pressure turbine blades.

[0003] The unique crystal structure of TiAl alloys limits their hot deformation capabilities, making hot forging of TiAl alloy blades extremely difficult. During hot forging, TiAl alloy billets are prone to cracking, incomplete filling, uneven blade microstructure, and unstable blade performance. Furthermore, the fabrication of large-sized TiAl alloy low-pressure turbine blades through forging has always been a global challenge.

[0004] In recent years, research on TiAl alloy blade forging processes has utilized nickel-based superalloy dies. The maximum operating temperature of these dies is 1050℃, thus limiting the forging temperature of TiAl alloy blades to no higher than 1050℃, failing to reach the optimal forging temperature for TiAl alloy blades. For this reason, researchers have adopted a near-isothermal forging process to prepare TiAl alloy blades. This involves preheating the billet at a high temperature (approximately 1200℃) and then forging it at a lower temperature (approximately 950℃~1050℃). This method can produce small-sized TiAl alloy compressor blade forgings. However, because the hot deformation characteristics of TiAl alloys are extremely sensitive to microstructure and temperature, the difference between the billet preheating temperature and the die temperature during near-isothermal forging causes continuous temperature fluctuations within the billet, resulting in uneven microstructure in the blade forgings and affecting blade performance. Furthermore, the forging temperature of TiAl alloy blades in near-isothermal forging is relatively low, making it difficult to achieve large deformation amounts during forging. Therefore, near-isothermal forging employs a two-stage forging process involving pre-forging, surface treatment, and final forging. This method is time-consuming, costly, and has low material utilization, which is particularly significant for large-sized low-pressure turbine blades. For example, invention CN112122523A proposes a method for preparing TiAl alloy compressor blades using near-isothermal forging, involving multiple processes such as pre-forging, surface treatment, and final forging.

[0005] Studies have shown that fine-grained TiAl alloys exhibit superplastic deformation characteristics between 1140℃ and 1200℃. At this temperature, TiAl alloys can be subjected to large-variable die forging, which is beneficial for one-fire forging of blades. Furthermore, during the superplastic deformation process of TiAl alloys, significant dynamic recrystallization does not occur within the TiAl alloy billet, a characteristic that contributes to the uniformity of the microstructure in the blade forging. Therefore, superplastic isothermal die forging of TiAl alloys at 1140℃ to 1200℃ is an effective method for preparing high-quality TiAl alloy blade forgings. Summary of the Invention

[0006] The purpose of this invention is to solve the problems of difficult forming, large machining allowance, uneven microstructure, unstable performance, and low material utilization in the traditional near-isothermal forging of TiAl alloy blades, and to provide a vacuum isothermal forging method for TiAl alloy low-pressure turbine blades, so as to obtain TiAl alloy low-pressure turbine blades with small allowance, uniform microstructure, and stable performance.

[0007] To solve this technical problem, the technical solution of the present invention is as follows:

[0008] A method for manufacturing TiAl alloy low-pressure turbine blades specifically includes the following steps:

[0009] S1: Prepare TiAl alloy ingots and use hot extrusion + recrystallization annealing process to obtain fine-grained rods with a grain size of not less than 8.

[0010] S2: Based on the structural dimensions and volume of the blade, the fine-grained bar is machined to obtain a simple cylindrical blank for die forging;

[0011] S3: The blank is formed into blades using a vacuum isothermal forging method and a special Mo alloy mold. The mold is placed in a vacuum heating chamber, and then the vacuum heating chamber is evacuated. When the vacuum degree is ≤1Pa, the mold is heated. The temperature is held at 1140-1200℃ for 60-120 minutes.

[0012] S4: Place the billet in the transfer chamber, then close the transfer chamber and draw a vacuum. When the vacuum degree is ≤1Pa, open the vacuum heating chamber door, use a robotic arm to grab the billet and place it in the mold. After holding it at the temperature for 5min to 10min, start forging.

[0013] S5: The vacuum isothermal forging rate of low-pressure turbine blades is 0.001s. -1 -1s -1 The forging deformation is 60%-80%, resulting in TiAl alloy low-pressure turbine blade forgings.

[0014] S6: The blade forgings are subjected to solution treatment and aging treatment: The solution treatment process involves heating the furnace to 1250℃-1300℃, holding for 2-4 hours, removing from the furnace, and air cooling to room temperature; the aging process involves holding at 850-950℃ for 8-12 hours, cooling in the furnace to below 200℃-300℃, removing from the furnace, and air cooling to room temperature to obtain high-performance TiAl alloy low-pressure turbine blades.

[0015] S7: Perform precision machining according to the external dimensions of the TiAl alloy low-pressure turbine blades, i.e., manufacture the required TiAl alloy low-pressure turbine blades.

[0016] The recrystallization annealing process mentioned in step S1 is as follows: the annealing temperature is T. α - (15~35℃), keep warm for 2~5 hours, then slowly cool. α This is the critical temperature at which the (α+γ) two-phase structure of the TiAl alloy transforms into a single α phase.

[0017] The chemical composition of the TiAl alloy, in atomic percentage, is: Ti-(41-47)Al-(3-5)Nb-(1-2)Cr-(0.1~0.2)Ta-(0.05-0.2)B-(0.~0.5)Mn.

[0018] Preferably, the chemical composition of the TiAl alloy, in terms of atomic percentage, is: Ti-(41-45)Al-(3-5)Nb-(1-2)Cr-(0.05~0.2)Ta-(0.05-0.2)B-(0.05~0.5)Mn.

[0019] This invention innovatively proposes a method for preparing TiAl alloy low-pressure turbine blade forgings via vacuum isothermal forging. The method involves a thermomechanical treatment process for fine-grained TiAl alloy bars to provide fine-grained bars as blanks; using a molybdenum alloy as a die, with a maximum temperature reaching 1250℃, achieving the same preheating and forging temperatures for the TiAl billet; and employing a vacuum isothermal forging method, under scientifically reasonable process parameters, enabling brittle TiAl alloy materials to achieve large-scale plastic deformation under near-superplastic conditions. This eliminates the need for processing complex blanks containing blades and tenons; using regular cylindrical blanks, a TiAl alloy low-pressure turbine blade forging with a machining allowance of only 2mm can be prepared through a single forging process.

[0020] The TiAl alloy blade forgings prepared by the above method have a room temperature tensile strength of 720–760 MPa, a specified non-proportional elongation strength of 500–550 MPa, and an elongation after fracture (A) of 3.0%–3.5% after heat treatment. At a high temperature of 750℃, the tensile strength is 680–700 MPa, the residual yield strength is 430–470 MPa, the elongation after fracture (A) is 6%–10%, and in the creep rupture test at 750℃, the applied stress σ is 250 MPa, and the creep rupture time τ is 100 h without fracture. The vibration fatigue strength of the blade parts is above 450 MPa, which is 10% higher than that of TiAl alloy blades prepared by the traditional near-isothermal forging method.

[0021] The beneficial effects of this invention are:

[0022] (1) Since this method does not require machining of the TiAl alloy rod for blade body and tenon blanking, it can not only save about 30% of raw material consumption and reduce the cost of blade production, but also shorten the production cycle of blade blanking and reduce the difficulty of machining. The above advantages are more obvious for large-sized low-pressure turbine blades.

[0023] (2) Utilizing the superplasticity of TiAl alloy at high temperatures for blade forging can improve the deformation capacity of TiAl alloy, suppress the generation of micro-defects in the blade, reduce the cracking tendency of TiAl alloy billet during forging, and help improve the yield rate of TiAl alloy blade forgings. Furthermore, it can realize one-time forging of TiAl alloy low-pressure turbine blades, significantly shortening the production cycle of TiAl alloy low-pressure turbine blades.

[0024] (3) The low-pressure turbine blade preparation method provided by this invention does not cause dynamic recrystallization of TiAl alloy during the blade forming process. This phenomenon is beneficial for obtaining a uniform and fine microstructure of TiAl alloy blades, ultimately significantly improving the microstructure and properties of the forgings. Using the method of this invention, the material consumption of a single TiAl alloy blade can be significantly reduced, and the uniformity of the TiAl alloy microstructure can also be improved, thereby enhancing the performance of TiAl alloy blades.

[0025] (4) The method adopts isothermal die forging process, that is, the preheating temperature of TiAl alloy billet is the same as the forging temperature. Therefore, the internal temperature of TiAl alloy billet will not change significantly during the forging process. This is beneficial to the uniformity of rheology of TiAl alloy material, and to the uniformity of internal structure of blade and the stability of blade mechanical properties. Attached Figure Description

[0026] Figure 1 The microstructure of the TiAl alloy low-pressure turbine blade forging proposed in this invention is shown. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The features of various aspects of the embodiments of the present invention will now be described in detail. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can also be practiced without these specific details. The following description of the embodiments is merely intended to provide a better understanding of the invention by illustrating examples. The invention is not limited to any specific setups and methods provided below, but covers all improvements, substitutions, etc., to product structures and methods without departing from the spirit of the invention.

[0029] In the following description, well-known structures and techniques are not shown in order to avoid unnecessarily obscuring the invention.

[0030] To illustrate the technical advantages of the vacuum isothermal forging method for preparing TiAl alloy low-pressure turbine blades proposed in this invention compared with the traditional near-isothermal forging method, a comparative experiment was conducted, as shown in Comparative Example 1.

[0031] Comparative Example 1

[0032] Prepare TiAl alloy bars with a nominal composition of Ti-46Al-4Nb-1.8Cr-0.2Ta-0.1B and a grain size of grade 8, with an extrusion ratio of not less than 4. Using a near-isothermal forging method, design and machine blade preforms according to the dimensions of TiAl alloy low-pressure turbine blades. Heat the forging die to 1000℃, then heat the preforms to 1200℃ in an air furnace. After holding the preforms at this temperature for 60 minutes, transfer them to the die for forging. Use a forging strain rate of 0.001 s⁻¹. -1 -0.01s -1 The forging deformation amount is 50%, resulting in TiAl alloy blade forgings.

[0033] Example 1:

[0034] TiAl alloy bars with a nominal composition of Ti-46Al-4Nb-1.8Cr-0.2Ta-0.1B and an extrusion ratio of not less than 4 were prepared. After holding at 1290℃ for 3 hours, they were slowly cooled to 300℃ at a rate of 0.5℃ / min to obtain TiAl alloy bars with a grain size of grade 8. Based on the volume of the TiAl alloy blades, the fine-grained bars were machined to obtain cylindrical billets for die forging. A molybdenum alloy mold was used, placed in a vacuum heating chamber, and then the chamber was evacuated. When the vacuum degree was ≤1Pa, the molybdenum alloy mold was heated to 1200℃ and held for 120 minutes. The billet was placed in a transfer chamber, which was then sealed and evacuated. When the vacuum degree was ≤1Pa, the vacuum heating chamber door was opened, and a robotic arm was used to grab the billet and place it in a special mold. The holding time was 10 minutes. A forging strain rate of 0.001s was used. -1 -0.01s -1 The forging deformation is 70%, resulting in TiAl alloy blade forgings. The machining allowance for TiAl blade forgings is only 1.5mm, saving approximately 30% of raw material consumption.

[0035] Example 2

[0036] Prepare TiAl alloy bars (nominal composition: Ti-45Al-4Nb-1Cr-0.1Ta-0.05B-0.1Mn) with an extrusion ratio of not less than 4. Hold at 1280℃ for 3 hours, then slowly cool to 300℃ at a rate of 0.5℃ / min to obtain TiAl alloy bars with a grain size of grade 8. Based on the TiAl alloy blade volume, machine the fine-grained bars to obtain cylindrical billets for die forging. Use a molybdenum alloy mold, place it in a vacuum heating chamber, and then evacuate the chamber. When the vacuum degree is ≤1Pa, start heating the molybdenum alloy mold to 1150℃ and hold for 120 minutes. Place the billet in a transfer chamber, then close the chamber and evacuate it. When the vacuum degree is ≤1Pa, open the vacuum heating chamber door, use a robotic arm to grab the billet and place it in a special mold, holding for 5 minutes. The forging strain rate is 0.001s. -1 -1s -1 The forging deformation is 70%, resulting in TiAl alloy blade forgings. The machining allowance for TiAl blade forgings is only 2mm, saving approximately 30% of raw material consumption.

[0037] Example 3

[0038] Prepare TiAl alloy bars (nominal composition: Ti-44Al-4Nb-1Cr-0.2Ta-0.05B-0.05Mn) with an extrusion ratio of not less than 4. Hold at 1240℃ for 3 hours, then slowly cool to 300℃ at a rate of 0.5℃ / min to obtain TiAl alloy bars with a grain size of grade 8. Based on the TiAl alloy blade volume, machine the fine-grained bars to obtain cylindrical billets for die forging. Use a molybdenum alloy mold, place it in a vacuum heating chamber, and then evacuate the chamber. When the vacuum degree is ≤1Pa, start heating the molybdenum alloy mold to 1180℃ and hold for 120 minutes. Place the billet in a transfer chamber, then close the chamber and evacuate it. When the vacuum degree is ≤1Pa, open the vacuum heating chamber door, use a robotic arm to grab the billet and place it in a special mold, holding for 8 minutes. The forging strain rate is 0.001s. -1 -1s -1 The forging deformation is 70%, resulting in TiAl alloy blade forgings. The machining allowance for the TiAl blade forgings is only 1.8 mm, saving approximately 22% of raw material consumption.

[0039] Example 4

[0040] TiAl alloy bars (nominal composition: Ti-43.5Al-4Nb-1.8Cr-0.2Ta-0.05B-0.05Mn) with an extrusion ratio of not less than 4 were prepared. After holding at 1235℃ for 3 hours, they were slowly cooled to 300℃ at a rate of 0.5℃ / min to obtain TiAl alloy bars with a grain size of grade 8. Based on the TiAl alloy blade volume, the fine-grained bars were machined to obtain cylindrical billets for die forging. A molybdenum alloy mold was used, placed in a vacuum heating chamber, and then the chamber was evacuated. When the vacuum degree was ≤1Pa, the molybdenum alloy mold was heated to 1200℃ and held for 120 minutes. The billet was placed in a transfer chamber, which was then sealed and evacuated. When the vacuum degree was ≤1Pa, the vacuum heating chamber door was opened, and a robotic arm was used to grab the billet and place it in a special mold, holding it for 10 minutes. The forging strain rate is 0.001s. -1 -1s -1 The forging deformation is 70%, resulting in TiAl alloy blade forgings. The machining allowance for TiAl blade forgings is only 1.5mm, saving approximately 25% of raw material consumption.

[0041] The TiAl alloy blade preparation method proposed in this invention is suitable for mass production of blades, and the blade quality is stable. A batch of TiAl alloy blade forgings prepared using the method in Example 1, after heat treatment, has a room temperature tensile strength of 720-760 MPa, a specified non-proportional elongation strength of 500-550 MPa, and an elongation after fracture A of 3.0%-3.5%. At a high temperature of 750℃, the tensile strength is 680-700 MPa, the residual yield strength is 430-470 MPa, the elongation after fracture A is 6%-10%, and in the creep rupture test at 750℃, the applied stress σ is 250 MPa, and the creep rupture time τ is 100 h without fracture.

[0042] Vibration fatigue performance of TiAl alloy blades is an important indicator for evaluating the application performance of TiAl alloys. It is the result obtained after vibrating dozens of blades. We tested the TiAl alloy low-pressure turbine blades prepared in Comparative Example 1 and Example 1. As shown in Table 1, the vibration fatigue strength of the blade part in Example 1 is above 450 MPa, which is 10% higher than that of TiAl alloy blades prepared by the traditional near-isothermal forging method. In the field, it is already very difficult to improve the vibration fatigue strength of low-pressure turbine blades by 5%.

[0043] Table 1

[0044] temperature Blade vibration fatigue strength Remark Comparative Example 1 25℃ 409MPa - Example 1 25℃ 451MPa Increase by about 10%

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should be covered within the protection scope of the present invention.

Claims

1. A method for manufacturing TiAl alloy low-pressure turbine blades, characterized in that, Includes the following steps: S1: Prepare TiAl alloy ingots and use hot extrusion + recrystallization annealing process to obtain fine-grained rods with an extrusion ratio of not less than 4 and a grain size of not less than grade 8; the chemical composition of TiAl alloy, in atomic percentage, is: Ti-44Al-4Nb-1Cr-0.2Ta-0.05B-0.05Mn or Ti-43.5Al-4Nb-1.8Cr-0.2Ta-0.05B-0.05Mn; S2: Based on the structural dimensions and volume of the blade, the fine-grained bar is processed to obtain a cylindrical blank for die forging; S3: The blank is formed into blades using a vacuum isothermal forging method and a Mo alloy mold. The mold is placed in a vacuum heating chamber, and then the vacuum heating chamber is evacuated. When the vacuum degree is ≤1Pa, the mold is heated. The mold is heated to 1180-1200°C and then held for 60min-120min. Place the billet in the transfer chamber, then seal the transfer chamber and draw a vacuum. When the vacuum degree is ≤1Pa, open the vacuum heating chamber door, grab the billet and place it in the mold. After holding it at the temperature for 5 to 10 minutes, start forging. The vacuum isothermal forging rate of low-pressure turbine blades is 0.001 s. -1 -1s -1 The forging deformation is 70%-80%, resulting in TiAl alloy low-pressure turbine blade forgings. S4: The blade forgings are subjected to solution treatment and aging treatment: The solution treatment process involves heating the furnace to 1250°C-1300°C, holding for 2-4 hours, removing from the furnace, and air cooling to room temperature; the aging process involves holding at 850-950°C for 8-12 hours, cooling in the furnace to below 200°C-300°C, removing from the furnace, and air cooling to room temperature to obtain high-performance TiAl alloy low-pressure turbine blades.

2. The preparation method according to claim 1, characterized in that, Step S1, recrystallization annealing, involves annealing at a temperature of T. α - (15~35℃), hold for 2~5 hours, then reheat and cool; T α This is the critical temperature at which the (α+γ) two-phase structure of the TiAl alloy transforms into a single α phase.

3. The preparation method according to claim 1, characterized in that, S2 uses machining methods to process the fine-grained rod.

4. The preparation method according to claim 1, characterized in that, The S3 uses a robotic arm to grip the billet.

5. The preparation method according to claim 1, characterized in that, It also includes step S5: precision machining according to the external dimensions of the TiAl alloy low-pressure turbine blade to manufacture the required TiAl alloy low-pressure turbine blade.

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