Near-isothermal forging method for improving structure uniformity and strength-plasticity-toughness matching of silicon-rich titanium alloy

Abstract

The disclosure provides a near-isothermal forging method for improving the uniformity of the microstructure of silicon-rich titanium alloy and the matching of strength-ductility-toughness, comprising: heating a silicon-rich titanium alloy ingot to 150-200 DEG C above the beta phase transition point, holding for a period of time, then two upsetting and two drawing breakdown, and strong air cooling to room temperature after forging; heating the titanium alloy ingot to 50-100 DEG C above the beta phase transition point, holding for a period of time, then one upsetting and one drawing, and strong air cooling to room temperature after forging; holding the square billet at 35-45 DEG C below the beta phase transition point, then continuously near-isothermal forging for multiple times, the preheating temperature of the forging cavity is 300-350 DEG C below the beta phase transition point, the ingot is returned to the furnace and held for a period of time between each time, and the forged product is air cooled. The microstructure of the titanium alloy forged by the method is a multi-level structure composed of 15-20% equiaxed primary alpha phase, 55-65% fine lamellar alpha phase and residual beta phase, and nanoscale silicides are uniformly precipitated at the alpha / beta phase interface, the microstructure is fine and uniform and does not require post-forging heat treatment, the matching of strength-ductility-toughness is significantly improved, and the method has good performance improvement efficiency and generalization.

Description

Near-isothermal forging method to improve the microstructure uniformity and strength-ductility-toughness matching of silicon-rich titanium alloys Technical Field

[0001] The embodiments of this disclosure relate to the field of titanium alloy hot working technology, and more specifically, to a near-isothermal forging method suitable for improving the uniformity of the microstructure and the strength-ductility-toughness matching of silicon-rich titanium alloys. Background Technology

[0002] Aero engines face harsh temperatures and quasi-static / dynamic loads during service. While structural design can optimize engine performance, the fundamental solution lies in improving the performance of the engine materials themselves. Titanium alloys possess a range of advantages, including high specific strength, specific stiffness, high damage tolerance, high creep resistance, and low fatigue sensitivity, making them ideal materials for manufacturing high thrust-to-weight ratio aero engines. For example, in international gas turbine engines, the fan system, low-pressure compressor, and approximately two-thirds of the high-pressure compressor are made of titanium alloys, accounting for about one-quarter to one-third of the engine's total weight.

[0003] Currently, most titanium alloys used in the manufacture of aero engines worldwide are near-α or α+β alloys with high aluminum equivalents, and they incorporate large amounts of β-stabilizing elements such as Mo, Zr, and Si to improve creep and fatigue performance when in service above 500°C. The combined addition of isomorphic and eutectoid β-stabilizing elements, while raising the upper limit of the alloy's mechanical properties, also significantly exacerbates the microstructural sensitivity of silicon-rich high-temperature titanium alloys, placing higher demands on their hot working processes. On the one hand, the forging temperature of silicon-rich high-temperature titanium alloys is difficult to control. Although conventional free forging in the α+β two-phase region can obtain a high volume fraction of equiaxed α phase, as the forging temperature gradually decreases, silicides undergo a eutectoid reaction with titanium. Excessive precipitation of incoherent silicides reduces the alloy's yield strength while deteriorating its plasticity and toughness. On the other hand, the forging deformation rate is difficult to control. High-strain-rate forging can refine the microstructure, but the deformation is uneven, and the material is prone to local overheating, resulting in performance fluctuations. Low-strain-rate forging produces sufficiently uniform deformation, allowing the α phase to recrystallize and grow fully, but the forging efficiency is low, energy dissipation is high, and a large amount of silicides are easily precipitated during continuous cooling. While heat treatment can control the morphology of the α phase and the precipitation behavior of silicides to some extent, for irregularly shaped forgings or large-sized forgings, the post-forging heat treatment process is complex, increasing production costs and reducing production efficiency. Summary of the Invention

[0004] The embodiments described in this paper address the problems of immature existing silicon-rich titanium alloy forging processes and unstable mechanical properties, and propose a near-isothermal forging method to improve the microstructure uniformity and strength-ductility-toughness matching of silicon-rich titanium alloys.

[0005] According to the present disclosure, a near-isothermal forging method is provided to improve the microstructure uniformity and strength-ductility-toughness matching of silicon-rich titanium alloys, comprising the following steps:

[0006] Step 1: Heat the silicon-rich titanium alloy ingot to 150℃~200℃ above the β phase transformation point, hold it at that temperature for a period of time, then upset and draw it twice to open the billet, and finally cool it to room temperature with strong air after forging.

[0007] Step 2: Heat the titanium alloy billet obtained in Step 1 to 50~100℃ above the β phase transformation point, hold it at that temperature for a period of time, then forge and draw it in one go, and finally cool it to room temperature with strong air after forging.

[0008] Step 3: Hold the titanium alloy billet obtained in Step 2 at 35~45℃ below the β phase transformation point, and then perform multiple consecutive near isothermal slow forgings. The preheating temperature of the forging die cavity is 300~350℃ below the β phase transformation point. The billet is held in the furnace for a period of time between each forging, and then air-cooled after forging.

[0009] As a further explanation of this disclosure, the titanium alloy ingot in step 1 is heated by furnace heating, and the holding time is every 1 cm. 2 The cross-sectional area is kept warm for 0.8~1 min, the upsetting deformation is 35~40%, and the upsetting and elongation strain rates are 0.06~0.09 / s.

[0010] As a further explanation of this disclosure, the titanium alloy billet in step 2 is heated by entering the furnace at a certain temperature, and the holding time is every 1 cm. 2 The cross-sectional area is kept warm for 0.4~0.6 min, the upsetting deformation is 35~40%, and the upsetting and elongation strain rates are 0.06~0.09 / s.

[0011] As a further explanation of this disclosure, the titanium alloy billet in step 3 is heated by furnace heating, and the holding time is every 1 cm. 2 The cross-sectional area is held at a constant temperature for 0.4–0.6 min. Each heat treatment includes one upsetting and one drawing. The total deformation during upsetting is 26–32%, and the strain rates during upsetting and drawing are 0.02–0.04 / s. The reheating time between heat treatments is 1 cm. 2 Insulate the cross-sectional area for 0.4~0.6 min.

[0012] As a further explanation of this disclosure, the microstructure of the silicon-rich titanium alloy forged by the method is a multi-layered structure consisting of 20% primary α phase, 60% fine lamellar α phase and 20% residual β phase, and nanoscale silicides are uniformly precipitated at the α / β phase interface.

[0013] The near-isothermal forging method provided in this application for improving the uniformity of the microstructure and the strength-ductility-toughness matching of silicon-rich titanium alloys can, through a simple forging process without subsequent heat treatment, control the formation of a multi-layered microstructure in silicon-rich titanium alloys, consisting of 15-20% equiaxed primary α phase, 55-65% fine lamellar α phase, and residual β phase. Furthermore, nanoscale silicides are uniformly precipitated at the α / β phase interface, resulting in a fine and uniform microstructure with excellent strength-ductility-toughness matching.

[0014] The above description is merely an overview of the technical solutions of the embodiments of this application. In order to better understand the technical means of the embodiments of this application and to implement them in accordance with the contents of the specification, and to make the above and other objects, features and advantages of the embodiments of this application more obvious and understandable, specific implementation methods of this application are described below. Attached Figure Description

[0015] To more clearly illustrate the technical solutions of the embodiments of this disclosure, the accompanying drawings of the embodiments will be briefly described below. It should be understood that the drawings described below only relate to some embodiments of this disclosure and are not intended to limit this disclosure, wherein:

[0016] Figure 1 is a schematic diagram of the preparation process provided in Embodiment 1 of this disclosure.

[0017] Figure 2 is a SEM image of the titanium alloy microstructure obtained in Example 1 of this disclosure.

[0018] Figure 3 is a SEM image of the titanium alloy microstructure obtained in Comparative Example 1 of this disclosure.

[0019] Figure 4 is a comparison of the room temperature tensile and impact properties of titanium alloys in the embodiments of this disclosure and the comparative examples. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the described embodiments of this disclosure without creative effort are also within the scope of protection of this disclosure.

[0021] Unless otherwise defined, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this subject matter pertains. It will be further understood that terms such as those defined in commonly used dictionaries shall be interpreted as having the meaning consistent with their meaning in the context of the specification and in the relevant art, and shall not be interpreted in an idealized or overly formal form unless otherwise explicitly defined herein. As used herein, the statement of “connecting” or “coupling” two or more parts together shall mean that these parts are directly joined together or joined through one or more intermediate components.

[0022] The term "embodiment" as used herein means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of the phrase "embodiment" in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0023] While conventional free forging in the α+β two-phase region can achieve a high volume fraction of equiaxed α phase, as the forging temperature gradually decreases, silicides undergo a eutectoid reaction with titanium. Excessive precipitation of incoherent silicides reduces the alloy's yield strength and deteriorates its plasticity and toughness. Furthermore, controlling the forging deformation rate is difficult. High-strain-rate forging can refine the microstructure significantly, but uneven deformation can lead to localized overheating and performance fluctuations. Low-strain-rate forging results in sufficiently uniform deformation, allowing for ample recrystallization and growth of the α phase, but it also results in low forging efficiency, high energy dissipation, and a tendency for significant silicide precipitation during continuous cooling. While heat treatment can control the morphology of the α phase and silicide precipitation behavior to some extent, post-forging heat treatment is complex for irregularly shaped forgings or large-sized forgings, increasing production costs and reducing efficiency.

[0024] To address the aforementioned technical problems, this disclosure proposes a near-isothermal forging method to improve the microstructure uniformity and strength-ductility-toughness matching of silicon-rich titanium alloys, comprising the following steps:

[0025] Step 1: Heat the silicon-rich titanium alloy ingot in the furnace to 150℃~200℃ above the β phase transformation point, and then heat it at a temperature of 1 cm. 2 The cross-sectional area is kept at a constant temperature for 0.8 to 1 minute, followed by two upsetting and two drawing processes. The upsetting deformation is 35 to 40%, and the upsetting and drawing strain rates are 0.06 to 0.09 / s. After forging, the material is cooled to room temperature by strong air.

[0026] Step 2: The titanium alloy billet obtained in Step 1 is heated to 50-100°C above the β-phase transformation point, with each 1cm section... 2The cross-sectional area is kept warm for 0.4~0.6 min, then it is upsetting and drawing once, with an upsetting deformation of 35~40% and an upsetting and drawing strain rate of 0.06~0.09 / s. After forging, it is cooled to room temperature by strong air.

[0027] Step 3: Heat the billet obtained in Step 2 in the furnace to 35~45℃ below the β phase transformation point and hold it there for a period of time equal to 1 cm. 2 The cross-sectional area is held at a constant temperature for 0.4~0.6 min. Then, multiple consecutive near-isothermal slow forging passes are performed. The forging die cavity preheating temperature is 300~350℃ below the β phase transformation point. Each pass includes one upsetting and one drawing pass. The total deformation during upsetting is 26~32%, and the strain rates during upsetting and drawing are 0.02~0.04 / s. The reheating time between passes is 1 cm. 2 The cross-sectional area is kept at a constant temperature for 0.4~0.6 min, and then air-cooled after forging.

[0028] This disclosure first involves two-stage forging at temperatures 150°C–200°C and 50°C–100°C above the β phase transformation point, with the holding time before forging initially longer and then shorter, followed by strong air cooling. On one hand, by rationally setting the holding time and temperature, the internal and external temperatures of the billet are kept uniform, preventing excessive growth of the original β grains. On the other hand, the aforementioned deformation amount and strain rate, while fully breaking down the original β grains, keep the alloy's deformation resistance within a controllable range, and the strong air cooling after forging limits the epitaxial growth of the original β grains during the cooling process. Through holding at a lower temperature in the β single-phase region and two-stage forging, continuous dynamic recrystallization of the original β grains is promoted, weakening the β texture and laying the microstructure foundation for forging in the two-phase region.

[0029] Then, near-isothermal forging is performed at a strain rate of 0.02~0.04 / s, within a range of 35℃~45℃ below the β phase transformation point. For most silicon-rich titanium alloys, the solid solution temperature of their silicides is approximately 40~50℃ below the β phase transformation point; below this temperature, a eutectoid reaction will occur during deformation. Forging is a highly complex thermo-mechanical coupling process, requiring simultaneous consideration of the adiabatic heating inside the billet and the heat loss outside the billet during forging. A temperature difference often exists between the inside and outside of the forged billet. If this temperature difference cannot be guaranteed to be consistent, it will lead to uneven precipitation of silicides inside the billet, significantly damaging its mechanical properties. Based on extensive experiments, this disclosure summarizes a near-isothermal forging process where the forged billet is placed in a forging chamber cavity preheated to 300~350℃ below the β phase transformation point and deformed at a strain rate of 0.02~0.04 / s. Under this process, the temperature loss around the forged billet and the temperature rise inside the forged billet reach a dynamic equilibrium, the material temperature remains stable above the silicide solid solution range, and the deformation resistance is controllable.

[0030] Furthermore, this disclosure, while avoiding uneven precipitation of silicides, also fully utilizes the advantages of deformation in the near-β temperature range of titanium alloys. For titanium alloys, deformation at 35°C to 45°C below the β phase transformation point is considered near-β deformation, in which the volume fraction of equiaxed primary α phase decreases to 15-20%. During air cooling after forging, lamellar α clusters with near-crystalline orientation precipitate between the equiaxed primary α phases, while a small amount of nanoscale silicides precipitate at the α / β phase boundary, forming a multi-layered structure composed of equiaxed primary α, lamellar α, nanoscale silicides, and the residual β matrix. During deformation, multiple slip systems are activated within the equiaxed primary α phase, ensuring continuous work hardening of the alloy and optimizing material plasticity; the lamellar α and β matrix satisfy the Borg orientation relation (BOR), facilitating dislocation passage through the phase interface, while the lamellar α phase deflects crack propagation paths, optimizing material toughness; the small amount of nanoscale silicides precipitated during air cooling pins dislocations, optimizing material strength. Through a simple forging process, without the need for subsequent heat treatment, a multi-layered microstructure consisting of 15-20% equiaxed primary α phase, 55-65% fine lamellar α phase, and residual β phase can be regulated in silicon-rich titanium alloys. Furthermore, nanoscale silicides are uniformly precipitated at the α / β phase interface, resulting in a fine and uniform microstructure with excellent strength-ductility-toughness matching.

[0031] The following is an explanation with reference to specific embodiments: Embodiment 1

[0032] The nominal composition of the silicon-rich titanium alloy used in this embodiment is Ti-5Al-7.5V-0.5Mo-0.5Zr-0.5Si-0.25Fe-0.16O, and the phase transformation point measured by metallographic method is 912℃.

[0033] The specific implementation process is as follows:

[0034] Step 1: Cut a piece with a diameter of 130mm and a cross-sectional area of ​​132cm². 2 The titanium alloy ingot was heated to 1100℃ in the furnace and held for 2 hours, followed by two upsetting and two drawing operations. The upsetting deformation was 38%, and the strain rates for upsetting and drawing were 0.09 / s, resulting in a forged cross-sectional area of ​​110cm². 2 The square billet is forged and then cooled to room temperature by strong air.

[0035] Step 2: The titanium alloy billet is placed at a warm temperature, heated to 980℃, held for 70 minutes, then upsetting and drawing are performed in one pass. The upsetting deformation is 38%, and the strain rate for both upsetting and drawing is 0.09 / s, resulting in a forged cross-sectional area of ​​100 cm². 2 The square billet is forged and then cooled to room temperature by strong air.

[0036] Step 3: Heat the billet in the furnace to 870℃ and hold for 60 minutes, then perform six consecutive near-isothermal slow forging passes. The forging die cavity preheating temperature is 600℃. Each pass includes one upsetting and one drawing. The total deformation during upsetting is 32%, and the strain rate for upsetting and drawing is 0.04 / s. The intermediate state of each pass has a cross-sectional area of ​​70 cm². 2 The square billet is held in the furnace for 30 minutes between forgings, and then air-cooled.

[0037] The process flow diagram of this embodiment is shown in Figure 1.

[0038] Figure 2 shows the SEM image of the titanium alloy microstructure obtained in this embodiment. The microstructure obtained by this forging process is uniform and fine, exhibiting multi-layered characteristics. The equiaxed primary α phase has an average size of approximately 4 μm, accounting for about 20%, surrounded by a small amount of secondary α laths with an average length of approximately 5 μm and an aspect ratio of approximately 5, accounting for about 60%. Simultaneously, uniformly distributed nanoscale silicides exist at the α phase boundaries. Mechanical property tests on the forged titanium alloy showed a tensile strength (UTS) of not less than 1080 MPa, a yield strength (YS) of not less than 1020 MPa, an elongation after fracture (E1) of not less than 16%, and an impact toughness (aKU) of not less than 49 J / cm². 2 It has an excellent balance of strength, plasticity, and toughness.

[0039] Comparative Example 1

[0040] The nominal composition of the silicon-rich titanium alloy used in this embodiment is Ti-5Al-7.5V-0.5Mo-0.5Zr-0.5Si-0.25Fe-0.16O, and the phase transformation point measured by metallographic method is 912℃.

[0041] The specific implementation process is as follows:

[0042] Step 1: Cut a piece with a diameter of 130mm and a cross-sectional area of ​​132cm². 2 The titanium alloy ingot was heated to 1100℃ in the furnace and held for 2 hours, followed by two upsetting and two drawing operations. The upsetting deformation was 38%, and the strain rates for upsetting and drawing were 0.09 / s, resulting in a forged cross-sectional area of ​​110cm². 2 The square billet is forged and then cooled to room temperature by strong air.

[0043] Step 2: The titanium alloy billet is placed at a warm temperature, heated to 980℃, held for 70 minutes, then upsetting and drawing are performed in one pass. The upsetting deformation is 38%, and the strain rate for both upsetting and drawing is 0.09 / s, resulting in a forged cross-sectional area of ​​100 cm². 2 The square billet is forged and then cooled to room temperature by strong air.

[0044] Step 3: Heat the billet in the furnace to 850℃ and hold for 60 minutes, then perform six consecutive free forging passes, each pass including one upsetting and one drawing. The total deformation during upsetting is 28%, and the strain rate during upsetting and drawing is 0.08 / s. The intermediate state of each pass has a cross-sectional area of ​​80 cm². 2 The square billet is held in the furnace for 30 minutes between forgings, and then air-cooled.

[0045] Figure 3 shows the SEM image of the titanium alloy microstructure obtained in this comparative example. The microstructure obtained by this forging process has poor uniformity, consisting only of equiaxed primary α particles with an average size of 4.5 μm, accounting for 85%, without multi-layered α characteristics. Because a near-isothermal forging method was not used to compensate for heat loss during forging, and the forging temperature was below the eutectoid temperature range of silicides, the billet temperature gradually decreased as the forging process progressed, resulting in uneven precipitation of large-sized silicides on the β-transformation matrix. Mechanical property tests were performed on the forged titanium alloy, showing a tensile strength (UTS) of 1068 MPa, a yield strength (YS) of 1017 MPa, an elongation after fracture (E1) of 13.1%, and an impact toughness (aKU) of 36 J / cm². 2 The strength, plasticity, and toughness are all reduced compared to the previous example.

[0046] Figure 4 shows a comparison of the mechanical property test curves of the examples and comparative examples. The tensile test was conducted according to GB / T228.1-2021, and the impact test was conducted according to GB / T 19748-2019.

[0047] In summary, the near-isothermal forging method proposed in this disclosure, which improves the microstructure uniformity and strength-ductility-toughness matching of silicon-rich titanium alloys, demonstrates significant benefits in microstructure optimization and performance improvement, and requires no post-forging heat treatment, making the process simple. Besides the silicon-rich titanium alloys used in the embodiments of this invention, this process can also be extended to other silicon-rich titanium alloys such as Ti55, Ti60, and TC11, showing broad application prospects.

[0048] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A near-isothermal forging method for improving the microstructure uniformity and strength-ductility-toughness matching of silicon-rich titanium alloys, characterized in that, The process includes the following steps: Step 1: Heat the silicon-rich titanium alloy ingot to 150℃~200℃ above the β phase transformation point, hold it at that temperature for a period of time, then perform two upsetting and two drawing operations to open the billet, followed by forced air cooling to room temperature; Step 2: Heat the titanium alloy billet obtained in Step 1 to 50~100℃ above the β phase transformation point, hold it at that temperature for a period of time, then perform one upsetting and one drawing operation, followed by forced air cooling to room temperature; Step 3: Hold the titanium alloy billet obtained in Step 2 at 35~45℃ below the β phase transformation point, then perform multiple consecutive near-isothermal slow forging operations: each operation includes one upsetting and one drawing operation, with a total upsetting deformation of 26~32%, and a strain rate of 0.02~0.04 / s for both upsetting and drawing. The reheating time between operations is 1 cm. 2 The cross-sectional area is kept at a constant temperature for 0.4~0.6 min, the preheating temperature of the forging die cavity is 300~350℃ below the β phase transformation point, and air cooling is performed after forging; the microstructure of the silicon-rich titanium alloy forged by the above method is a multi-layered structure consisting of 20% primary α phase, 60% fine lamellar α phase and 20% residual β phase, and nanoscale silicides are uniformly precipitated at the α / β phase interface.

2. The method according to claim 1, characterized in that, The silicon-rich titanium alloy ingot described in step 1 is heated by furnace heating, and the holding time is every 1 cm. 2 The cross-sectional area is kept warm for 0.8~1 min, the upsetting deformation is 35~40%, and the upsetting and elongation strain rates are 0.06~0.09 / s.

3. The method according to claim 1, characterized in that, The titanium alloy billet described in step 2 is heated by entering the furnace at a certain temperature, and the holding time is every 1 cm. 2 The cross-sectional area is kept warm for 0.4~0.6 min, the upsetting deformation is 35~40%, and the upsetting and elongation strain rates are 0.06~0.09 / s.

4. The method according to claim 1, characterized in that, The titanium alloy billet mentioned in step 3 is heated by furnace heating, and the holding time is every 1 cm. 2 Insulate the cross-sectional area for 0.4~0.6 min.

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