A method of controllably variable rate hot gas pressure forming of titanium alloy thin walled structures

Abstract

The present application belongs to the field of titanium alloy part forming processing method, and proposes a controllable variable rate hot gas pressure forming method for titanium alloy thin-walled components, which comprises the following steps: selecting a blank to determine the rolled state or annealed state of the blank, placing the blank in a hot gas pressure forming die, and heating; according to the initial microstructure state of the blank, selecting a variable rate loading path corresponding to the initial microstructure state of the blank; according to the shape regularity degree of the target component, designing a gas pressure-time loading path, adjusting the gas pressure in real time, and driving the deformation of the blank; after forming is completed, stop heating, remove the gas pressure, open the die, and take out the formed component. According to the initial microstructure state of the titanium alloy blank, the opposite variable rate loading path is selected, the forming mechanism of the blank at the microstructure level under different loading rates is reasonably utilized, the performance of the formed component in the microstructure aspect is improved, and the mechanical properties are significantly improved, while the forming efficiency and forming performance are considered.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy forming and processing methods, and particularly relates to a controllable variable rate hot gas pressure forming method for thin-walled titanium alloy components. Background Technology

[0002] Titanium alloys are widely used in the aerospace field for thin-walled components, such as plate and shell components and irregularly shaped tubing, due to their high specific strength and good high-temperature resistance. Hot gas compression forming is an important process for manufacturing such components. It typically involves heating the sheet metal to a two-phase temperature and then using high-pressure gas to mold the sheet metal or tubing into shape.

[0003] Traditional hot gas compression forming processes often employ constant strain rate or constant pressure loading. For example, typical process parameters are: forming temperature 900℃, strain rate 0.001 s⁻¹. -1 The following are some significant drawbacks of this process: 1) Low forming efficiency: Low strain rate results in forming times of tens of minutes to several hours for a single part; 2) Grain coarsening and decreased mechanical properties: Long-term high-temperature deformation promotes full dynamic recrystallization, grain growth, and a decrease in dislocation density, leading to a significant decrease in yield strength and tensile strength of the component at its service temperature; 3) High-rate forming easily produces micropores: If the strain rate is increased (e.g., 0.01 s) to improve efficiency... -1 If the deformation coordination of each phase / grain is insufficient, microscopic pores are easily generated at the grain boundaries, leading to premature necking or fracture of the component. Summary of the Invention

[0004] This invention addresses the technical problems existing in the above-mentioned titanium alloy forming methods by proposing a controllable variable-rate hot gas pressure forming method for thin-walled titanium alloy components. Based on the initial microstructure of the blank, an appropriate variable-rate loading path is selected to solve the technical problem in the prior art that it is difficult to simultaneously achieve high forming efficiency, fine grain strengthening, dislocation strengthening, and suppression of micropores.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The present invention provides a controllable variable-rate hot gas pressure forming method for thin-walled titanium alloy components, comprising the following specific steps:

[0006] S1. Prepare billet and mold: Select billet of titanium alloy plate or tube, determine the initial microstructure of billet, place billet in hot air pressure forming mold, the heating end of mold is equipped with heating device;

[0007] S2. Heating: Heat the mold and blank to the preset forming temperature, which is the two-phase temperature of the titanium alloy.

[0008] S3. Select the variable rate loading path: Based on the initial microstructure of the billet determined in step S1, select the variable rate loading path corresponding to the initial microstructure of the billet.

[0009] S4. Variable rate pneumatic loading: Based on the variable rate loading path selected in step S3 and the regularity of the target component's shape, design a pneumatic-time loading path. Through the pressure control system, high-pressure gas is introduced into the mold according to the pneumatic-time loading path and the air pressure is adjusted in real time to drive the blank deformation.

[0010] S5. Depressurization and Part Removal: After molding is completed, stop heating, release the gas pressure, open the mold, and remove the molded part.

[0011] Preferably, the billet selected in step S1 is a rolled billet, and the variable-rate loading path selected in step S3 for the rolled billet consists of two stages: a low rate followed by a high rate. Specifically, the first stage includes a low strain rate of 0.0005 s⁻¹. -1 ~0.005 s -1 The rolled billet is deformed to the first preset strain, with a true strain of 0.2~0.4; the second stage involves a high strain rate of 0.005s. -1 ~0.1s -1 The rolled billet is further deformed to the target strain.

[0012] Preferably, the billet selected in step S1 is an annealed billet, and the annealed billet corresponds to the variable rate loading path selected in step S3, which consists of two stages: a high rate followed by a low rate. Specifically, it includes a first stage: a high strain rate of 0.005 s. -1 ~0.1s -1 The annealed billet is deformed to the first preset strain, with a true strain of 0.2~0.4; the second stage: at a low strain rate of 0.0005 s. -1 ~0.005 s -1 The annealed billet is further deformed to the target strain.

[0013] Preferably, the target component in step S4 is a regular-shaped component, and the air pressure-time loading path is calculated using the free expansion theory formula of sheet metal to ensure that the strain rate at the location of the maximum deformation of the component is a preset value.

[0014] Preferably, the target component in step S4 is an irregularly shaped and complex component. The pressure-time loading path is determined by finite element simulation, specifically including: establishing a finite element model of the target component, taking the equivalent strain rate at the critical dangerous section on the billet as the control target, and calculating the pressure-time curve that can make the critical dangerous section on the billet deform according to the preset strain rate law through iterative calculation.

[0015] Preferably, the forming temperature in step S2 is 700℃~850℃.

[0016] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0017] 1. The present invention provides a controllable variable rate hot gas pressure forming method for thin-walled titanium alloy components. The method selects opposite variable rate loading paths according to the initial titanium alloy billet in the rolled or annealed state, and makes reasonable use of the forming mechanism of the billet micro-layer under different loading rates. This improves the performance of the formed component in terms of microstructure and significantly enhances its mechanical properties, while taking into account both forming efficiency and forming performance. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A technical roadmap for a controllable variable-rate hot gas pressure forming method for thin-walled titanium alloy components provided by the present invention;

[0020] Figure 2 This is a microstructure diagram of the billet in the initial rolled state in Example 1;

[0021] Figure 3 This is a microstructure diagram of the billet in the annealed state in Example 2;

[0022] Figure 4 This invention provides a schematic diagram of the variable-rate loading path for a controllable variable-rate hot gas pressure forming method for thin-walled titanium alloy components; wherein, curve 1 is a variable-rate loading path that starts high and then decreases, and curve 2 is a variable-rate loading path that starts low and then increases.

[0023] Figure 5 The pneumatic-time loading path diagram of a regular-shaped component (hemispherical part) in a controllable variable rate hot pneumatic forming method for a titanium alloy thin-walled component provided in the embodiment is shown. Curve 1 corresponds to the variable rate loading path of first low and then high, and curve 2 corresponds to the variable rate loading path of first high and then low.

[0024] Figure 6 This is a schematic diagram of a hot air pressure forming mold structure; where 1 is the upper mold; 2 is the lower mold; 3 is the embedded mold; 4 is the high-pressure air pipe; 5 is the heating furnace; and 6 is the thermocouple.

[0025] Figure 7 This is a physical image of the molded component from Example 1;

[0026] Figure 8 This is a physical image of the molded component from Example 2;

[0027] Figure 9 This is a schematic diagram showing how different microscopic parameters of titanium alloy billets vary with forming conditions.

[0028] Figure 10 This is a schematic diagram showing how the mechanical properties, room temperature hardness, and yield strength of the formed component change with forming conditions. Detailed Implementation

[0029] To better understand the above-mentioned objectives, features and advantages of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0030] Numerous specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways than those described herein, and therefore the invention is not limited to the specific embodiments disclosed in the following specification.

[0031] Example 1

[0032] A controllable variable-rate hot gas pressure forming method for thin-walled titanium alloy components, more specifically, a variable-rate hot gas pressure forming method for rolled TA15 titanium alloy sheets, includes the following specific steps:

[0033] S1. Prepare billet and mold: Select billet of titanium alloy plate or tube, determine the initial microstructure of billet, place billet in hot air pressure forming mold, the heating end of mold is equipped with heating device.

[0034] This embodiment uses TA15 titanium alloy rolled sheet, whose microstructure is as follows: Figure 2 As shown, the interior contains numerous dislocations and subgrain boundaries. EBSD characterization revealed an average grain size of 3.2 μm, an initial recrystallization fraction of 15.4%, and a dislocation density of 13.8 × 10⁻⁶. 14 m -2 The subgrain boundary fraction is 54.6%. This state belongs to the high dislocation density and high deformation energy storage type.

[0035] like Figure 6As shown, this embodiment uses a hot air pressure forming mold. The target component is a hemispherical part. The upper and lower molds are used for mold closing and sealing. An insert mold for forming the target component is provided on the opposite surfaces of the upper and lower molds. The insert mold has a cavity matching the hemispherical part (diameter 60 mm, bulging height 17.75 mm, equivalent strain 0.6 corresponding to the apex of the hemispherical part). The slab is placed in the insert mold, and a copper gasket serves as a sealing element between the lower mold and the insert mold to achieve airtightness. The lower mold is connected to a high-pressure air source via a high-pressure air pipe. The heating device is a resistance wire heating furnace, with real-time temperature measurement via thermocouples.

[0036] S2. Heating: Heat the mold and blank to the preset forming temperature of 750℃. The forming temperature is the two-phase temperature of the titanium alloy.

[0037] S3. Selecting a variable-rate loading path: Based on the initial microstructure of the TA15 titanium alloy in the rolled state as described in step S1, select a variable-rate loading path corresponding to the initial microstructure of the billet; based on... Figure 1 The decision-making process adopts a low-to-high rate approach; for example... Figure 4 As shown in curve 2: the first stage (ε=0~0.3) is characterized by a low strain rate of 0.001 s. -1 The second stage (ε=0.3~0.6) is characterized by a high strain rate of 0.01 s⁻¹. -1 The true strain at the switching point is ε=0.3.

[0038] S4. Variable-rate pneumatic loading: Based on the variable-rate loading path of low to high rate and the regularity of the shape of the target component, i.e., the hemispherical part, a pneumatic-time loading path is designed; high-pressure gas is introduced into the mold according to the pneumatic-time loading path through the pressure control system and the pneumatic pressure is adjusted in real time to drive the deformation of the billet. The actual strain rate at the position of maximum deformation of the billet is consistent with the preset variable-rate curve.

[0039] Pneumatic loading path design: The billet is TA15 titanium alloy rolled sheet, based on the free bulging theory formula of sheet metal, with... Figure 4 Curve 2 represents the target strain rate, calculated as follows: Figure 5 The pressure-time loading path is shown in curve 2. The total forming time is 397 seconds. The first 350 seconds constitute the first stage, during which the pressure slowly rises to approximately 10.6 MPa. The next 47 seconds constitute the second stage, during which the pressure first rapidly rises to approximately 18.8 MPa, and then slowly decreases to around 8 MPa. The theoretical formula for free bulging of the sheet metal is: , In the formula The length of the major axis of the sheet metal during free bulging; The length of the short axis of the free bulging of the sheet metal; This refers to the initial thickness of the sheet metal. t is the equivalent strain rate; t is the bulging time. For flow stress; is a dimensionless constant, and P is the loading pressure.

[0040] S5. Depressurization and Part Removal: After molding is completed, stop heating, release the gas pressure, open the mold, and remove the molded part.

[0041] The final formed component of this embodiment 1 exhibits the following characteristics in several aspects: Figure 7 The formed hemispherical part shown has a height of 17.75 mm. The component exhibits a clean appearance, with no necking, cracks, or fractures. Regarding the microstructure, such as... Figure 9 As shown, EBSD analysis revealed an average grain size of 2.24 μm and a dislocation density of 10.6 × 10⁻⁶. 14 m -2 The micropore fraction is only 0.75%. Compared to Figure 2 The original rolled sheet shown exhibits approximately 30% grain refinement, a slightly reduced but still high dislocation density, and no obvious micropores. In terms of mechanical properties, under these deformation conditions, the sheet achieves a yield strength of 588.2 MPa and an elongation of 10.02% at its service temperature (500℃). Room temperature hardness test results are as follows... Figure 10 As shown, the hardness of the formed component in this embodiment is 343.78 HV, which is 6.3% higher than that of the original rolled sheet (323.47 HV) and 15.0% higher than that of the traditional 900℃ constant low speed formed component (299.06 HV).

[0042] Example 2

[0043] A controllable variable-rate hot gas pressure forming method for thin-walled titanium alloy components, more specifically, a variable-rate hot gas pressure forming method for annealed TA15 titanium alloy sheets, includes the following specific steps:

[0044] S1. Preparing the billet and mold: In this embodiment, the billet is an annealed TA15 titanium alloy sheet, and its microstructure is as follows: Figure 4 As shown: average grain size 3.5 μm, initial recrystallization fraction 62.5%, dislocation density approximately 6 × 10⁻⁶. 14 m -2 This state belongs to the low dislocation density, low deformation energy storage type, and the grains are in a relatively stable equiaxed state. Therefore, a high-to-low rate of change path is chosen. The forming apparatus is the same as in Example 1, employing... Figure 6 The mold shown.

[0045] S2. Heating: Heat the mold and blank to the preset forming temperature of 750℃. The forming temperature is the two-phase temperature of the titanium alloy.

[0046] S3. Select the variable rate loading path: such as... Figure 4As shown in curve 1: the strain rate in the first stage (ε=0~0.3) is 0.01s. -1 The strain rate in the second stage (ε=0.3~0.6) was 0.001 s⁻¹. -1 The true strain at the switching point is ε=0.3.

[0047] S4. Air pressure loading path design: The target component is a hemispherical part of the same size (true strain 0.6). Figure 4 Using curve 1 as the target, the calculation yielded... Figure 5 The pressure-time loading path is shown in curve 1. The total forming time is 505 seconds, of which the first 35 seconds are the first stage, during which the pressure rises rapidly to about 18.8 MPa; the next 470 seconds are the second stage, during which the pressure first drops rapidly to 10.6 MPa, and then slowly drops to about 6 MPa.

[0048] S5. Depressurize and remove parts.

[0049] The final formed component of this embodiment 2 exhibits the following characteristics in several aspects: Figure 8 The hemispherical part shown has a height of 17.75 mm after forming, and its appearance is intact with no macroscopic defects. Regarding the microstructure, the microstructure at the highest point of the formed component is as follows: Figure 9 As shown, analysis revealed an average grain size of 2.45 μm and a dislocation density of 10.4 × 10⁻⁶. 14 m -2 The micropore fraction is only 0.55%. Compared to the original annealed state, the grain size is refined by about 30%, the dislocation density is significantly increased, and micropores are extremely rare. In terms of mechanical properties, under this deformation condition, the yield strength at the service temperature (500℃) reaches 588.4MPa, and the elongation is 10.09%; Figure 10 As shown, the hardness of the formed component in this embodiment is 353.74 HV, which is 9.4% higher than that of the original rolled sheet (323.47 HV) and 18.3% higher than that of the traditional 900℃ constant low speed formed component (299.06 HV), and is the highest among all embodiments and comparative examples.

[0050] Comparative Example 1

[0051] Rolled sheet metal was formed at a constant height and speed, with forming parameters of 750℃ and a constant strain rate of 0.01 s. -1 True strain 0.6. Forming result: The component exhibits numerous micropores, with a pore volume fraction of 2.05%. Room temperature hardness is 330.25 HV, and service temperature elongation is only 7.41%. This comparative example illustrates that simply forming at high rates can lead to the formation of micropores and a decrease in mechanical properties.

[0052] Comparative Example 2

[0053] Rolled sheet metal was formed at a constant low rate with the following forming parameters: 750℃ and a constant strain rate of 0.001 s⁻¹. -1 True strain 0.6. Forming results: The component showed no necking, the grains were first refined and then grown to 3.15 μm, and the dislocation density decreased to 9.88 × 10⁻⁶. 14 m -2 The room temperature hardness is 334.02 HV. This comparative example illustrates that simply forming at a low rate leads to grain growth, reduced dislocation density, and insufficient strength.

[0054] Comparative Example 3

[0055] Traditional high-temperature, low-speed forming, forming parameters: 900℃, constant strain rate 0.001 s. -1 True strain 0.6. Forming results: The component was completely recrystallized, the grain size coarsened to 6.22 μm, and the dislocation density decreased to 6.6 × 10⁻⁶. 14 m -2 The room temperature hardness is only 299.06 HV. This comparative example illustrates that traditional processes are insufficient to obtain high-performance components.

[0056] Example 3

[0057] S1~S3 and S5 are the same as in Example 1 or Example 2.

[0058] S4. The target component is an irregularly shaped component that cannot be described by analytical formulas. Finite element simulation is used to determine the pressure-time loading path, such as... Figure 1 As shown, the specific steps are as follows:

[0059] S41. Establish the finite element model of the component, define the high-temperature constitutive model of TA15 titanium alloy, including subroutines for dynamic recrystallization and void evolution;

[0060] S42. Identify the most critical sections of a component, such as small fillet transition areas and other locations with large strain;

[0061] S43. Based on the initial state of the billet, select a variable rate path of low-to-high or high-to-low, and set the target strain rate curve for the critical point.

[0062] S44. Through iterative calculations, adjust the air pressure loading curve until the simulated strain rate at the critical point matches the target curve;

[0063] S45. Actual forming is carried out using the simulated air pressure path.

[0064] In Examples 1 and 3, rolled titanium alloy billets were used. The rolled, unannealed billets exhibited high dislocation density and a high subgrain boundary fraction, with a relatively low initial recrystallization fraction (approximately 15%). For these billets, a variable-rate loading path was employed, starting with a low rate and then increasing to a high rate. The forming mechanism at the low strain rate stage was as follows: the high forming defects in the rolled billet promoted the recrystallization process, while the low-rate deformation provided sufficient recrystallization time, promoting both continuous and discontinuous dynamic recrystallization. This transformed the initial high dislocation density and high subgrain boundary fraction deformed microstructure into fine recrystallized grains, resulting in grain homogenization and refinement. The forming mechanism at the high strain rate stage was as follows: high-rate deformation reintroduced dislocations into the already refined recrystallized grains, increasing the dislocation density. Simultaneously, due to the fine grain size, deformation coordination was better, making it less prone to microscopic voids; ultimately, an ideal microstructure of fine grains and high dislocation density was obtained.

[0065] In Examples 1 and 3, annealed titanium alloy billets were used. Annealed billets have low dislocation density and low subgrain boundary fraction, with grains in a relatively stable equiaxed state. For these billets, a variable-rate loading path was adopted, starting with a high strain rate and then decreasing it. The forming mechanism at the high strain rate stage is that high-rate deformation rapidly accumulates deformation defects such as dislocations and subgrain boundaries, providing nucleation driving force and nucleation sites for subsequent recrystallization. The forming mechanism at the low strain rate stage is that low-rate deformation promotes the formation of fine recrystallized grains from the accumulated dislocations and other deformation structures through dynamic recrystallization, while avoiding the initiation and expansion of micropores. Ultimately, an ideal microstructure with fine grains and a relatively high dislocation density is obtained.

[0066] This invention provides a controllable variable-rate hot gas pressure forming method for thin-walled titanium alloy components. The method selects opposite variable-rate loading paths based on the initial titanium alloy billet in the rolled or annealed state, which reflects a profound understanding and precise control of the microstructure evolution law. It rationally utilizes the forming mechanism of the billet microstructure under different loading rates, thereby improving the performance of the formed components.

[0067] In terms of microstructure performance, the average grain size of the formed component can be controlled at 2.4~2.9 μm, which is 10%~25% finer than the original sheet material; the geometrically necessary dislocation density is maintained at 10×10⁻⁶. 14 m -2 The volume fraction of micropores is less than 1%.

[0068] Mechanical properties have been improved: taking the TA15 titanium alloy hemispherical part as an example, the room temperature hardness of the variable rate hot gas pressure forming component can reach 353.74 HV, which is 9.4% higher than the original rolled plate (323.47 HV) and 18.3% higher than the traditional 900℃ constant low speed forming component (299.06 HV).

[0069] In terms of forming efficiency, it balances efficiency and performance: compared to traditional constant low-rate processes (such as 0.001s) -1 The variable-rate process can shorten the forming time by 30% to 50% by introducing a high-rate stage; compared with the constant high-rate process, the introduction of a low-rate stage avoids the generation of micro-pores and ensures the integrity and mechanical properties of the component.

[0070] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for controllable variable-rate hot gas pressure forming of thin-walled titanium alloy components, characterized in that, The specific steps include the following: S1. Prepare billet and mold: Select billet of titanium alloy plate or tube, determine the initial microstructure of billet, place billet in hot air pressure forming mold, the heating end of mold is equipped with heating device; S2. Heating: Heat the mold and blank to the preset forming temperature, which is the two-phase temperature of the titanium alloy. S3. Select the variable rate loading path: Based on the initial microstructure of the billet determined in step S1, select the variable rate loading path corresponding to the initial microstructure of the billet. S4. Variable rate pneumatic loading: Based on the variable rate loading path selected in step S3 and the regularity of the target component's shape, design a pneumatic-time loading path. Through the pressure control system, high-pressure gas is introduced into the mold according to the pneumatic-time loading path and the air pressure is adjusted in real time to drive the blank deformation. S5. Depressurization and Part Removal: After molding is completed, stop heating, release the gas pressure, open the mold, and remove the molded part.

2. The controllable variable-rate hot gas pressure forming method for thin-walled titanium alloy components according to claim 1, characterized in that, The billet selected in step S1 is a rolled billet. The variable-rate loading path selected in step S3 for the rolled billet consists of two stages: a low rate followed by a high rate. Specifically, it includes a first stage: a low strain rate of 0.0005 s. -1 ~0.005s -1 The rolled billet is deformed to the first preset strain, with a true strain of 0.2~0.4; the second stage involves a high strain rate of 0.005s. -1 ~0.1s -1 The rolled billet is further deformed to the target strain.

3. The controllable variable-rate hot gas pressure forming method for a titanium alloy thin-walled component according to claim 1, characterized in that, The billet selected in step S1 is an annealed billet. The annealed billet corresponds to the variable rate loading path selected in step S3, which consists of two stages: a high rate followed by a low rate. Specifically, it includes the first stage: a high strain rate of 0.005 s. -1 ~0.1s -1 The annealed billet is deformed to the first preset strain, with a true strain of 0.2~0.

4. Second stage: with a low strain rate of 0.0005 s -1 ~0.005 s -1 The annealed billet is further deformed to the target strain.

4. A controllable variable-rate hot gas pressure forming method for a thin-walled titanium alloy component according to claim 2 or 3, characterized in that, The target component in step S4 is a regular-shaped component. The air pressure-time loading path is calculated using the free expansion theory formula of sheet metal to ensure that the strain rate at the location of the maximum deformation of the component is a preset value.

5. A controllable variable-rate hot gas pressure forming method for a thin-walled titanium alloy component according to claim 2 or 3, characterized in that, The target component mentioned in step S4 is an irregular and complex component. The pressure-time loading path is determined by finite element simulation. Specifically, it includes: establishing a finite element model of the target component, taking the equivalent strain rate at the critical dangerous section on the billet as the control target, and calculating the pressure-time curve that can make the critical dangerous section on the billet deform according to the preset strain rate law through iterative calculation.

6. The controllable variable-rate hot gas pressure forming method for a titanium alloy thin-walled component according to claim 3, characterized in that, The forming temperature in step S2 is 700℃~850℃.

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