Heat treatment method for regulating lamellar structure of TiAl alloy forge piece

Abstract

The invention discloses a heat treatment method for regulating and controlling lamellar structures of TiAl alloy forgings, and belongs to the technical field of heat treatment of alloy materials. According to the method, the obtained forged TiAl alloy is placed in a heat treatment furnace with the temperature of T alpha + / -10 DEG C, after heat preservation is conducted for 5-30 min, multi-stage step-by-step cooling is conducted, the TiAl alloy with the fine near-lamellar structure is obtained, the multi-stage step-by-step cooling mode comprises two modes, the first mode is furnace cooling + air cooling, and the second mode is furnace cooling + air cooling + oil cooling; by regulating and controlling the critical temperature point, the cooling speed and possible short-time heat preservation of each cooling stage, accurate control over lamellar regularity, lamellar cluster size and grain refinement degree is achieved, generation of Widmannstatten structures and metastable structures is effectively inhibited, and the structure uniformity and the strength and plasticity matching performance are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for alloy materials, and specifically to a heat treatment method for regulating the lamellar structure of TiAl alloy forgings. Background Technology

[0002] TiAl alloys are intermetallic compounds with intrinsic brittleness and poor room-temperature plasticity, which limits their widespread application under more complex service conditions. Studies have shown that the mechanical properties of TiAl alloys are closely related to their microstructure morphology, phase composition, and grain size. By controlling the heat treatment regime, four typical microstructures can be obtained in TiAl alloys: near-gamma microstructure (NG), dual-phase microstructure (DP), near-lamellar microstructure (NL), and fully lamellar microstructure (FL). Among these, near-lamellar and fully lamellar microstructures combine high-temperature performance with good room-temperature mechanical properties, making them the target microstructure morphologies for high-performance TiAl alloy components. In the microstructure, the γ phase is tetragonal. The structure has relatively many independent slip systems and good plasticity; while the α2 phase is hexagonal D0. 19 The structure has few slip systems and poor plasticity. Furthermore, various lamellar interfaces, such as γ / γ twinning interfaces, can be found within individual α2 / γ lamellar clusters. These interfaces hinder dislocation slip and twinning, thereby causing stress concentration at the lamellar cluster interfaces and leading to increased alloy strength. Therefore, a fine and uniformly distributed lamellar structure not only maintains the high-temperature strength of the lamellar structure but also achieves a certain balance between room-temperature strength and plasticity.

[0003] For TiAl alloys forged in near-isothermal multi-pass forging under (α+γ) two-phase conditions, the microstructure is typically bimodal or equiaxed. To obtain a near-lamellar or fully lamellar structure, subsequent heat treatment is required to re-transform the α and γ phases and form a lamellar structure. In existing technologies, lamellar structures often require prolonged holding at the α single-phase region or higher temperatures, with the transformation being induced during cooling. However, even with short holding times in the α single-phase region, rapid coarsening of α grains is easily achieved. For peritectic γ-TiAl alloys with high α transformation temperatures, heat treatment at the α transformation temperature alone not only fails to prevent abnormal growth of individual α grains but also exacerbates uneven grain growth due to the rapid dissolution of the γ phase and the loss of its pinning effect on the α grains.

[0004] Therefore, existing technologies urgently need a heat treatment method that targets the phase transformation characteristics of γ-TiAl alloys, can precisely control the heating temperature and holding process, and combine it with optimized cooling pathways to suppress abnormal grain growth while forming a regular and refined lamellar structure, so as to achieve a synergistic improvement in the room temperature strength and plasticity of forged TiAl alloys. Summary of the Invention

[0005] This invention addresses the issue of controllable microstructure in forged γ-TiAl alloys by proposing a heat treatment method that allows for precise control of microstructure morphology. This method, under near-(α+γ) two-phase heat treatment conditions, achieves a near-lamellar or fully lamellar microstructure with fine grain size, reasonable lamellar orientation, and uniform distribution through precise temperature control and staged cooling. This significantly improves the plasticity-strength matching performance of this type of alloy at room temperature. Compared with existing technologies, this invention, through a comprehensive strategy of precise phase transformation temperature measurement, narrow-temperature heating, and staged cooling, ensures uniform and controllable lamellar structure, effectively avoiding the formation of Widmanstätten structure and other metastable structures during traditional heat treatment, and significantly improving microstructure uniformity and strength-plasticity matching performance.

[0006] To achieve the above objectives, this invention proposes a heat treatment method for controlling the lamellar microstructure of TiAl alloy forgings, comprising the following steps: S1. Preparation of forged TiAl alloy: The TiAl alloy ingot is subjected to six-pass reversal forging at a temperature of 1300℃ in the (α+γ) two-phase region to obtain the forged TiAl alloy. S2, Narrow temperature zone high-temperature insulation; the forged TiAl alloy is placed at a temperature of T. α The sample was kept at ±10℃ in a heat treatment furnace for 5-30 minutes; the T... α T is the phase transformation temperature of TiAl alloy from the α phase to the (α+γ) phase. α =1340℃~1350℃; S3. Multi-stage step-by-step cooling; the TiAl alloy after high-temperature holding in the narrow temperature range is subjected to multi-stage step-by-step cooling to obtain a TiAl alloy with fine near-lamellar structure; the multi-stage step-by-step cooling method includes two methods: The first cooling method is to cool the TiAl alloy furnace after high-temperature holding in the narrow temperature zone to the first temperature and hold it for 2-10 minutes, then air cool it to room temperature. The second cooling method is to cool the TiAl alloy furnace after the narrow temperature zone high temperature holding to the first temperature and hold it for 1~2 minutes, then air cool to the second temperature, and then oil cool to room temperature.

[0007] Preferably, the chemical composition of the TiAl alloy is Ti-46.5Al-4Nb-1.8Cr-0.2Ta.

[0008] Preferably, the heat treatment furnace is a box furnace with a heating rate of 8°C / min.

[0009] Preferably, in step S3, the first temperature is 1250~1330℃, the furnace cooling rate is 5~8℃ / min, the second temperature is 800~900℃, and the air cooling time in the second cooling method is 30s.

[0010] Compared with the prior art, the present invention has the following beneficial effects: This invention introduces precise measurement of the α→α+γ phase transformation temperature in the selection of heating temperature and strictly controls the holding temperature in the narrow temperature range within T. α Short-term heat preservation is carried out within a narrow temperature range of ±10℃. Precise control of the narrow temperature range during this stage can effectively guide recrystallization and lamellar reconstruction, thereby obtaining a near-lamellar or fully lamellar structure with fine grains and regular lamellar arrangement, and significantly reducing residual stress, thereby improving the strength and plasticity of the alloy.

[0011] This invention proposes a multi-stage selectable mode, such as furnace cooling → air cooling and furnace cooling → air cooling → oil cooling. In particular, it introduces a second stage of rapid cooling (e.g., a combination mode of switching from air cooling to oil cooling). By adjusting the critical temperature point, cooling rate, and possible short-term heat preservation of each cooling stage, it achieves precise control over the lamellar regularity, lamellar cluster size, and grain refinement, effectively suppressing the formation of Widmanstätten and metastable structures, and significantly improving the uniformity of the structure and the strength-plasticity matching performance.

[0012] The method of this invention is not only applicable to Ti-46.5Al-4Nb-1.8Cr-0.2Ta alloy, but can also be extended to other TiAl alloy systems with Nb content of 3~6 at.%, demonstrating strong versatility and promotional value. Attached Figure Description

[0013] Figure 1 This is a microstructure diagram of the forged TiAl alloy in Example 1 of this invention.

[0014] Figure 2 This is a schematic diagram of the heat treatment process provided by the present invention.

[0015] Figure 3 The images show the microstructures of the alloys obtained after heat treatment in Comparative Examples 1-2 of the present invention, wherein (a) is the microstructure of the alloy in Comparative Example 1, and (b) is the microstructure of the alloy in Comparative Example 2.

[0016] Figure 4 The images shown are alloy microstructures of Examples 1-4 of the present invention, wherein (a) is the alloy microstructure of Example 1, (b) is the alloy microstructure of Example 2, (c) is the alloy microstructure of Example 3, and (d) is the alloy microstructure of Example 4.

[0017] Figure 5 The images shown are alloy microstructures of Examples 5-8 of the present invention, wherein (a) is the alloy microstructure of Example 5, (b) is the alloy microstructure of Example 6, (c) is the alloy microstructure of Example 7, and (d) is the alloy microstructure of Example 8.

[0018] Figure 6 The figures show the room temperature tensile stress-strain curves of the forged and heat-treated alloys in Examples 1-8 of this invention. Detailed Implementation

[0019] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0020] This invention proposes a heat treatment method for controlling the lamellar microstructure of TiAl alloy forgings, such as... Figure 2 The flowchart shown includes the following steps: S1. Preparation of forged TiAl alloy: The TiAl alloy ingot was subjected to six-pass reversal forging at a temperature of 1300℃ in the (α+γ) two-phase region to obtain the forged TiAl alloy. Specifically, in step S1, during the six-pass reversing forging process, intermediate heat treatment is performed between each pass to eliminate work hardening and improve plasticity, so that the microstructure of the obtained forged TiAl alloy blank is uniform. Cylindrical samples with a preferred geometric size of Φ10mm in diameter and 8mm in height are cut from the forged alloy blank using a wire EDM machine and numbered one by one in order to establish a correspondence with the subsequent heat treatment conditions and performance test results.

[0021] Specifically, the chemical composition of the TiAl alloy is Ti-46.5Al-4Nb-1.8Cr-0.2Ta.

[0022] S2, Narrow temperature zone high-temperature insulation; placing the forged TiAl alloy at a temperature preferably T α In a heat treatment furnace at ±10℃, the preferred holding time is 5~30 minutes, for example, 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes, or 30 minutes; T α T is the phase transformation temperature at which TiAl alloys transform from the α phase to the α+γ phase. α =1340℃~1350℃, therefore, T α The ±10℃ range is preferably 1330℃~1360℃, for example, it can be 1330℃, 1340℃, 1350℃, or 1360℃; Specifically, the phase transition temperature T for the α phase to transform into the α+γ phase. α The determination process is based on phase transformation characteristics as the basis for heat treatment process design. The quenching metallographic method is used to accurately determine the α→α+γ phase transformation temperature range of the target alloy, ensuring that the heating temperature is controlled within a narrow window.

[0023] Specifically, in step S2, the heat treatment furnace is preheated to T. α ±10℃, then quickly place the forged TiAl alloy into the heat treatment furnace, which is a 1700℃ box furnace with a heating rate of 8℃ / min.

[0024] S3. Multi-stage step-by-step cooling; the TiAl alloy after high-temperature holding in the narrow temperature range is subjected to multi-stage step-by-step cooling to obtain a TiAl alloy with fine near-lamellar structure; the multi-stage step-by-step cooling method includes two methods: The first cooling method is to cool the TiAl alloy furnace after high-temperature holding in a narrow temperature range to the first temperature and hold it for 2 to 10 minutes, and then quickly air-cool it to room temperature. The second cooling method is to cool the TiAl alloy furnace after high-temperature holding in a narrow temperature range to the first temperature and hold it for 1-2 minutes, then air cool it to the second temperature, and then oil cool it to room temperature. Specifically, in step S3, the first temperature is preferably 1250~1330℃, for example, it can be 1250, 1260, 1270, 1280, 1290, 1300, 1310, 1320, or 1330℃. The furnace cooling rate is preferably 5~8℃ / min, for example, it can be 5℃ / min, 6℃ / min, 7℃ / min, or 8℃ / min. The second temperature is preferably 800~900℃, for example, it can be 800, 810, 820, 830, 840, 850, 860, 870, 890, or 900℃. The air cooling time in the second cooling method is 30s.

[0025] The present invention will be further described below with reference to the embodiments.

[0026] Example 1 A heat treatment method for regulating the lamellar structure of TiAl alloy forgings to improve their room temperature strength and ductility includes the following steps: S1. An alloy ingot with the chemical composition Ti-46.5Al-4Nb-1.8Cr-0.2Ta was selected as the raw material. First, the ingot was subjected to six passes of reverse forging at a temperature of 1300℃ in the (α+γ) two-phase region. Intermediate heat treatment was performed between each pass to eliminate work hardening and improve plasticity, resulting in... Figure 1 The forged TiAl alloy blank shown; S2. The S1 forged blank was machined into cylindrical specimens with a diameter of Φ10mm and a height of 8mm using a wire EDM machine and numbered for later use. The α→α+γ phase transformation temperature of this alloy was determined to be 1340~1350℃ using the quenching metallographic method. S3. After heating the heat treatment furnace to 1350℃, quickly place the forged TiAl alloy sample into the heat treatment furnace and hold it at this temperature for 10 minutes. S4. After holding the alloy sample at the temperature for 10 minutes, use the first cooling method to cool it down to the first temperature of 1250℃ by furnace cooling, hold it at the temperature for 5 minutes, and then quickly air cool it to room temperature.

[0027] Example 2 In this embodiment, the first temperature in step S4 is 1300℃, and the remaining steps are the same as in embodiment 1.

[0028] Example 3 In this embodiment, the first temperature in step S4 is 1320℃, and the remaining steps are the same as in embodiment 1.

[0029] Example 4 In this embodiment, the first temperature in step S4 is 1330℃, and the remaining steps are the same as in embodiment 1.

[0030] Example 5 A heat treatment method for regulating the lamellar structure of TiAl alloy forgings to improve their room temperature strength and ductility includes the following steps: S1. An alloy ingot with the chemical composition Ti-46.5Al-4Nb-1.8Cr-0.2Ta was selected as the raw material. First, the ingot was subjected to six passes of reverse forging at a temperature of 1300℃ in the (α+γ) two-phase region. Intermediate heat treatment was performed between each pass to eliminate work hardening and improve plasticity, resulting in... Figure 1 The forged TiAl alloy blank shown; S2. The S1 forged blank was machined into cylindrical specimens with a diameter of Φ10mm and a height of 8mm using a wire EDM machine and numbered for later use. The α→α+γ phase transformation temperature of this alloy was determined to be 1340~1350℃ using the quenching metallographic method. S3. After heating the heat treatment furnace to 1350℃, quickly place the forged TiAl alloy sample into the heat treatment furnace and hold it at this temperature for 10 minutes. S4. After holding the alloy sample at the temperature for 10 minutes, use the second cooling method to cool it down to the first temperature of 1300℃ by furnace cooling at a rate of 8℃ / min, and hold it for 2 minutes. Then, quickly air cool it to the second temperature of 850℃ for 30 seconds. After that, immediately put it into the oil cooling medium to cool it to room temperature.

[0031] Example 6 In this embodiment, the first temperature in step S4 is 1250°C, and the remaining steps are the same as in embodiment 5.

[0032] Example 7 In this embodiment, the first temperature in step S4 is 1280℃, and the remaining steps are the same as in embodiment 5.

[0033] Example 8 In this embodiment, the first temperature in step S4 is 1280℃, and the remaining steps are the same as in embodiment 5.

[0034] Comparative Example 1 The heat preservation temperature in step S3 of this comparative example is T. α +20℃, i.e. 1370℃, and the remaining steps are the same as in Example 1.

[0035] The microstructure of the obtained TiAl alloy is shown in the figure below. Figure 3 As shown in (a), the size of the α-phase lamellar clusters inside the TiAl alloy increases significantly, resulting in more residual stress.

[0036] Comparative Example 2 The heat preservation temperature in step S3 of this comparative example is T. α -20℃, i.e. 1320℃, the remaining steps are the same as in Example 1.

[0037] The microstructure of the obtained TiAl alloy is shown in the figure below. Figure 3 As shown in (b), due to insufficient driving force for α phase nucleation inside the TiAl alloy, the lamellar reconstruction is incomplete, resulting in residual lamellae with disordered orientation in the microstructure, which leads to a large amount of residual stress.

[0038] Figure 4 Images (a)-(d) are microstructure diagrams of the TiAl alloys corresponding to Examples 1-4. Figure 5 Images (a)-(d) are microstructure diagrams of the TiAl alloys corresponding to Examples 5-8. It is clearly visible that, according to metallographic examination, the internal structure of the obtained alloys exhibits well-ordered lamellar structures, effectively suppressing the formation of Widmanstätten and metastable structures. Figure 5 The orderly effect is better than Figure 4 The orderly effect in the process.

[0039] Figure 6 The figures show the room temperature tensile stress-strain curves of the forged and heat-treated alloys in Examples 1-8. It can be seen that, compared with the forged alloy, the room temperature elongation and tensile strength of the alloy increased from 1.32% and 432.53 MPa to 1.37% and 538.76 MPa.

[0040] The above description is merely an example and illustration of the structure of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the structure of the invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

[0041] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0042] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A heat treatment method for regulating the lamellar microstructure of TiAl alloy forgings, characterized in that, Includes the following steps: S1. Preparation of forged TiAl alloy: The TiAl alloy ingot is subjected to six-pass reversal forging at a temperature of 1300℃ in the (α+γ) two-phase region to obtain the forged TiAl alloy. S2, Narrow temperature zone high-temperature insulation; the forged TiAl alloy is placed at a temperature of T. α The sample was kept at ±10℃ in a heat treatment furnace for 5-30 minutes; the T... α T is the phase transformation temperature of TiAl alloy from the α phase to the (α+γ) phase. α =1340℃~1350℃; S3. Multi-stage step-by-step cooling; the TiAl alloy after high-temperature holding in the narrow temperature range is subjected to multi-stage step-by-step cooling to obtain a TiAl alloy with fine near-lamellar structure; the multi-stage step-by-step cooling method includes two methods: The first cooling method is to cool the TiAl alloy furnace after high-temperature holding in the narrow temperature zone to the first temperature and hold it for 2-10 minutes, then air cool it to room temperature. The second cooling method is to cool the TiAl alloy furnace after the narrow temperature zone high temperature holding to the first temperature and hold it for 1~2 minutes, then air cool to the second temperature, and then oil cool to room temperature.

2. The heat treatment method for regulating the lamellar structure of TiAl alloy forgings according to claim 1, characterized in that, The chemical composition of the TiAl alloy is Ti-46.5Al-4Nb-1.8Cr-0.2Ta.

3. The heat treatment method for regulating the lamellar structure of TiAl alloy forgings according to claim 1, characterized in that, In step S2, the heat treatment furnace is a box furnace with a heating rate of 8°C / min.

4. The heat treatment method for regulating the lamellar structure of TiAl alloy forgings according to claim 1, characterized in that, In step S3, the first temperature is 1250~1330℃, the furnace cooling rate is 5~8℃ / min, the second temperature is 800~900℃, and the air cooling time in the second cooling method is 30s.

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