A method for optimizing heat treatment temperature of TC4 titanium alloy bolt based on finite element simulation

Abstract

The application provides an optimization method for heat treatment temperature of TC4 titanium alloy bolts based on finite element simulation, and relates to the technical field of heat treatment numerical simulation.The optimization method firstly establishes a flow stress model of each phase zone of the TC4 titanium alloy;then establishes a phase transition equation of the heating and cooling processes of the TC4 titanium alloy;then establishes a finite element model of the heat treatment process of the TC4 titanium alloy bolt, sets initial conditions and boundary conditions of a thermal-mechanical coupling finite element analysis model of the heat treatment process of the TC4 titanium alloy bolt, and sets thermal physical parameters;numerically simulates the heat treatment process of the TC4 titanium alloy bolt, and obtains a temperature field after heat treatment and a phase transition distribution cloud picture.The application can numerically simulate the heat treatment process of the TC4 titanium alloy, obtain the temperature field after heat treatment and the phase transition distribution cloud picture, provide a technical basis for the organization distribution prediction after heat treatment of the TC4 titanium alloy, and provide a theoretical basis for process optimization.

Description

Technical Field

[0001] This invention relates to the field of heat treatment numerical simulation technology, and more specifically, to an optimization method for the heat treatment temperature of TC4 titanium alloy bolts based on finite element simulation. Background Technology

[0002] TC4 titanium alloy is an α+β dual-phase titanium alloy with excellent comprehensive mechanical properties and corrosion resistance. It also has good aerospace and aviation properties and is widely used in the aerospace field, especially in fasteners.

[0003] During the production process, it was discovered that the original heat treatment process caused a series of problems, such as unstable performance of TC4 titanium alloy bolts or failure to meet requirements. Solving these problems required repeated heat treatment, which would waste a lot of resources such as electricity, manpower, and natural gas, resulting in increased production costs.

[0004] With the development of computer numerical simulation technology, this application establishes a physical model for the heat treatment process of TC4 titanium alloy bolts and conducts numerical simulation analysis, aiming to guide the selection of temperature in the heat treatment process of TC4 titanium alloy bolts, improve production efficiency and reduce production costs. Summary of the Invention

[0005] The primary objective of this invention is to provide an optimization method for the heat treatment temperature of TC4 titanium alloy bolts based on finite element simulation, thereby addressing the issue of the influence of temperature selection on the microstructure and mechanical properties of TC4 titanium alloy bolts during the heat treatment process.

[0006] An optimization method for the heat treatment temperature of TC4 titanium alloy bolts based on finite element simulation includes:

[0007] S1. Establish phase regions and macroscopic rheological stress models for TC4 titanium alloy bolts.

[0008] S2. Establish a phase transformation model for the heating and cooling processes during the heat treatment of TC4 titanium alloy bolts.

[0009] The changes in thermal expansion and heat flux during the phase transition were collected using thermal expansion experiments and DSC experiments. The formula for the degree of phase transition, f, was used. i (T i )=1-exp(-k i (T i )τ n Formula 4 represents the phase transformation equation for the α-phase to β-phase transformation of TC4 titanium alloy during heating. Formula 5 is used to calculate the degree of phase transformation of each phase during heat treatment, simulate the microstructure transformation cloud map during heat treatment, and obtain the phase transformation content based on the microstructure transformation cloud map.

[0010] In the formula: f i (T i ) indicates the degree of phase transition, k i (T i The results are obtained through DSC experiments or by measuring the TTT curve, where τ represents the heating time, n represents the material constant, e is the natural constant, A is the material constant, and T is the material constant. s This indicates that the phase transition initiation temperature is 650℃, T e T represents the phase transition end temperature, T represents the temperature at which the phase transition occurs, and D represents the material constant.

[0011] S3. Establish a thermo-mechanical coupled finite element analysis model for the heat treatment process of TC4 titanium alloy bolts.

[0012] A TC4 titanium alloy bolt model was established using finite element software. Eight-node hexahedral elements were selected to mesh the bolt model. The thermophysical property parameters of the metal material as a function of temperature were set. The thermophysical property parameters included the values ​​of elastic modulus, thermal conductivity, coefficient of thermal expansion, specific heat, and Poisson's ratio of the metal material at different temperatures.

[0013] S4. Set boundary conditions during the heat treatment process.

[0014] Set temperature boundary conditions: Set the welding environment temperature to room temperature, set the boundary heat transfer condition parameters, specifically the heat transfer coefficient and thermal radiation condition parameters, and set all outer wall surfaces of the TC4 titanium alloy bolts to be heat exchange surfaces.

[0015] Setting structural boundary conditions: Since the TC4 titanium alloy bolt is a simple symmetrical structure, it is only necessary to take a point on the nut to constrain the translational degree of freedom in the Z direction, and the bolt can deform freely without rigid body movement;

[0016] S5. Perform numerical simulation of the heat treatment process.

[0017] Numerical simulations were performed on the heat treatment process, yielding the temperature field and phase transition contour distribution. The differential equations based on heat transfer control were then used to determine the optimal conditions. Perform heat transfer analysis,

[0018] In the formula: λ x , λ y , λ z Let ρ represent the thermal conductivity of the material in the x, y, and z directions, respectively, and let c represent the material density. p represents the specific heat capacity at constant pressure, and Q represents the intensity of the internal heat source;

[0019] S6, Instructions for heat treatment holding temperature

[0020] Based on the temperature field and phase transformation cloud diagram distribution of TC4 titanium alloy bolts during heat treatment, considering the softening and creep effects of the material during the heat treatment holding stage, the creep strain rate is:

[0021] In the formula: The creep strain rate is represented by A, the material constant is represented by σ, the creep stress is represented by n, the stress exponent is represented by Q, the nominal creep activation energy is represented by T, and the temperature is represented by T.

[0022] In some specific embodiments, the method for establishing the phase regions and macroscopic rheological stress model of TC4 titanium alloy in step S1 is as follows:

[0023] Hot compression tests were designed for the high-temperature β-phase region and the low-temperature α+β-phase region of TC4 titanium alloy. The rheological stress models for each phase region and the macroscopic rheological stress model were obtained. The formula for the rheological stress model of the α-phase of TC4 titanium alloy was also provided. Formula 1 is the formula for the rheological stress model of the β phase. Formula 2 is the macroscopic rheological stress model formula for TC4 titanium alloy, σ = X. β σ β +X α σ α Formula 3,

[0024] In the formula: This indicates the rheological rate of the α phase in TC4 titanium alloy at low temperatures. This represents the rheological rate of the β phase and σ of TC4 titanium alloy at high temperatures. α The rheological stress and σ of the α-phase of TC4 titanium alloy under hot compression are represented. β The rheological stress of TC4 titanium alloy during β-phase hot compression is represented by R, which is the air constant, and T represents the experimental temperature. α Indicating the degree of α-phase transformation and X in TC4 titanium alloy β β represents the degree of phase transformation of TC4 titanium alloy, and σ represents the macroscopic stress of TC4 titanium alloy.

[0025] In some specific implementations, the phase transition degree X in step S1 α +X β =1.

[0026] In some specific implementations, the parameter k in step S2 i (T i The values ​​of ) and n are obtained through one of the following methods: thermal expansion experiment, DSC experiment, or plotting TTT curve.

[0027] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides an optimization method for the heat treatment temperature of TC4 titanium alloy bolts based on finite element simulation. This optimization method can guide the selection of temperature for TC4 titanium alloy bolts during the heat treatment process, improve production efficiency and reduce production costs. Attached Figure Description

[0028] Figure 1 This is a schematic flowchart of the optimization method for the heat treatment temperature of TC4 titanium alloy bolts based on finite element simulation according to the present invention.

[0029] Figure 2 This is a mesh diagram of the TC4 titanium alloy bolt of the present invention;

[0030] Figure 3 This is a bar chart showing the temperature distribution during the heat treatment of the TC4 titanium alloy bolt in Embodiment 1 of the present invention.

[0031] Figure 4 This is a bar chart showing the temperature distribution during heat treatment and heat preservation of the TC4 titanium alloy bolts in Embodiment 1 of the present invention.

[0032] Figure 5 This is a bar chart showing the temperature distribution during the heat treatment and cooling of the TC4 titanium alloy bolt in Embodiment 1 of the present invention.

[0033] Figure 6 This is a bar chart showing the microstructure distribution of the TC4 titanium alloy bolt during heat treatment in Embodiment 1 of the present invention.

[0034] Figure 7 This is a bar chart showing the microstructure distribution of the TC4 titanium alloy bolt during heat treatment and heat preservation in Embodiment 1 of the present invention.

[0035] Figure 8 This is a bar chart showing the microstructure distribution of the TC4 titanium alloy bolt during heat treatment and cooling in Embodiment 1 of the present invention.

[0036] Figure 9 This is a graph showing the change of the α-phase microstructure of the TC4 titanium alloy bolt during the heat treatment process in Embodiment 1 of the present invention over time.

[0037] Figure 10 This is a bar chart showing the microstructure distribution of the TC4 titanium alloy bolt during heat treatment in Embodiment 2 of the present invention.

[0038] Figure 11 This is a bar chart showing the microstructure distribution of the TC4 titanium alloy bolt during heat treatment and heat preservation in Embodiment 2 of the present invention.

[0039] Figure 12 This is a bar chart showing the microstructure distribution of the TC4 titanium alloy bolt after heat treatment in Embodiment 2 of the present invention.

[0040] Figure 13 This is a bar chart showing the microstructure distribution of the TC4 titanium alloy bolt after heat treatment in Embodiment 3 of the present invention.

[0041] Figure 14 This is a bar chart showing the microstructure distribution of the TC4 titanium alloy bolt after heat treatment in Embodiment 3 of the present invention.

[0042] Figure 15 This is a bar chart showing the microstructure distribution of the TC4 titanium alloy bolt after heat treatment in Embodiment 3 of the present invention. Detailed Implementation

[0043] The present invention will be further described below with reference to specific embodiments. These embodiments are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention. Anything not described in detail in the present invention patent application is considered to be common knowledge in the art.

[0044] With the development of computer numerical simulation technology, people have gradually mastered the methods of establishing various physical models and are proficient in using these models to analyze the parameter curves of TC4 titanium alloys or predict the rationality of heat treatment processes. Previously, Jeoung Han Kim used artificial neural networks (ANN) and finite element (FE) simulation methods to predict the microstructure evolution of titanium alloys under isothermal and non-isothermal hot forging conditions; Dong Dashan used the finite element software DEFORM to determine the continuous cooling transformation curve of supercooled austenite, and obtained the isothermal transformation curve of bearing steel GCr15 based on considering variable physical property parameters and phase transformation; Wen Guanyun took 12CrNi3 steel helical gears as the research object, established a material property database of 12CrNi3 steel using Jmatpro software, and used Deform-3D software to simulate the carburizing and quenching heat treatment process of helical gears. The above studies all investigate the microstructure evolution of materials during processing. Based on previous research, this application conducts numerical simulation of the heat treatment of TC4 titanium alloy bolts to observe its phase transformation distribution cloud map and temperature field, guiding the selection of temperature during the heat treatment process of TC4 titanium alloy bolts, aiming to improve production efficiency and reduce production costs to a certain extent.

[0045] An optimization method for the heat treatment temperature of TC4 titanium alloy bolts based on finite element simulation includes:

[0046] S1. Establish phase regions and macroscopic rheological stress models for TC4 titanium alloy bolts.

[0047] Hot compression tests were designed for the high-temperature β-phase region and the low-temperature α+β-phase region of TC4 titanium alloy. Since the β-phase content is low at low temperatures, the stress is mainly affected by the α-phase; therefore, the rheological stress at low temperatures can be considered as the α-phase region. Rheological stress models for each phase region were calculated based on the hot compression data. A macroscopic rheological stress model was obtained based on the hybrid correction model proposed by Tamura. Specifically, the α and β-phase rheological stress models and the macroscopic rheological stress model are as follows:

[0048] Let the rheological stress model formula for the α phase of TC4 titanium alloy be given.

[0049]

[0050] Let the rheological stress model formula for the β phase of TC4 titanium alloy be given.

[0051]

[0052] Let the macroscopic rheological stress model formula for TC4 titanium alloy be σ=X β σ β +X α σ α Formula 3,

[0053] In the formula:

[0054] This indicates the rheological rate of the α phase in TC4 titanium alloy at low temperatures. σ represents the rheological rate of the β phase of TC4 at high temperatures. α The rheological stress and σ of the α-phase of TC4 titanium alloy under hot compression are represented. β This represents the rheological stress of the β-phase during hot compression of TC4 titanium alloy. σ α σ β All can be determined through a hot compression test.

[0055] R represents the air constant, with a value of 8.314 J / (mol·K).

[0056] T represents the experimental temperature.

[0057] X α Indicating the degree of α-phase transformation and X in TC4 titanium alloy β X represents the degree of phase transformation β in TC4 titanium alloy. α X β It can be determined based on thermal expansion experiments and DSC experiments.

[0058] σ represents the macroscopic stress of TC4 titanium alloy;

[0059] S2. Establish a phase transformation model for the heating and cooling processes during the heat treatment of TC4 titanium alloy bolts.

[0060] TC4 titanium alloy undergoes a diffusion phase transformation during cooling. Changes in thermal expansion and heat flux during this phase transformation were collected using thermal expansion and DSC experiments to investigate the phase transformation process.

[0061] Let f be the formula for the degree of phase transition. i (T i )=1-exp(-k i (T i )τ n Formula 4 is the correct answer.

[0062] Let the phase transformation equation for the transformation of the α phase to the β phase in TC4 titanium alloy during heating be given. Formula 5,

[0063] In the formula:

[0064] f i (T i () indicates the degree of phase transition.

[0065] k i (T i The values ​​are obtained through DSC experiments or by measuring the TTT curve.

[0066] τ represents the heating time.

[0067] n represents a material constant, obtained through thermal expansion experiments or by measuring the thermal expansion curve, and is valued at 1.275.

[0068] e is the natural constant, with a value of approximately 2.71828.

[0069] A represents the material constant, with a value of -1.86.

[0070] T s This indicates the phase transition initiation temperature, which is 600℃; T e This indicates the phase transition end temperature, which is 975℃.

[0071] T represents the temperature at which the phase transition occurs, and the unit is ℃ or K.

[0072] D represents the material constant, with a value of 4.35;

[0073] S3. Establish a thermo-mechanical coupled finite element analysis model for the heat treatment process of TC4 titanium alloy bolts.

[0074] A TC4 titanium alloy bolt model was established using finite element software. Eight-node hexahedral elements were selected to mesh the bolt model. The thermophysical property parameters of the metal material as a function of temperature were set. The thermophysical property parameters included the values ​​of elastic modulus, thermal conductivity, coefficient of thermal expansion, specific heat, and Poisson's ratio of the selected metal material at different temperatures.

[0075] S4. Set boundary conditions during the heat treatment process.

[0076] Set temperature boundary conditions: Set the welding environment temperature to room temperature, set the boundary heat transfer condition parameters, specifically the heat transfer coefficient and thermal radiation condition parameters, and set all outer wall surfaces of the TC4 titanium alloy bolts to be heat exchange surfaces.

[0077] Setting structural boundary conditions: Since the TC4 titanium alloy bolt is a simple symmetrical structure, it is only necessary to take a point on the nut to constrain the translational degree of freedom in the Z direction, and the bolt can deform freely without rigid body movement;

[0078] S5. Perform numerical simulation of the heat treatment process.

[0079] The heat source equations provided by the finite element software were invoked, and a thermo-mechanical coupled finite element analysis model of the TC4 titanium alloy bolt heat treatment process was used to numerically simulate the heat treatment process. After numerical simulation, the temperature field and phase transition contour map distribution of the heat treatment process were obtained.

[0080] Differential equations based on heat transfer control Perform heat transfer analysis,

[0081] In the formula:

[0082] λ x , λ y , λ z These represent the thermal conductivity of the material in the x, y, and z directions, respectively.

[0083] ρ represents the material density, with units of kg / m³. 3 ,

[0084] c p This represents the specific heat capacity at constant pressure, expressed in J / kg·℃.

[0085] Q represents the intensity of the internal heat source;

[0086] S6, Instructions for heat treatment holding temperature

[0087] Based on the temperature field and phase transformation cloud diagram distribution of TC4 titanium alloy bolts during heat treatment, the softening and creep effects of the material are considered during the heat treatment holding stage. The creep effect follows Norton's creep law, and the creep strain rate is...

[0088] In the formula: The creep strain rate is represented by A, the material constant is represented by σ, the creep stress is represented by n, the stress exponent is represented by Q, the nominal creep activation energy is represented by T, and the temperature is represented by T.

[0089] The heat treatment simulation of TC4 titanium alloy is a three-phase coupled field of thermo-mechanical-microstructure. The microstructure transformation of TC4 titanium alloy is affected by temperature and force. Therefore, in step S1, a model of each phase region and macroscopic rheological stress of TC4 titanium alloy is established as a prerequisite for building the TC4 titanium alloy material database, enabling the heat treatment simulation of TC4 titanium alloy to proceed smoothly and simulating the transformation of the α phase during heat treatment. When the solution temperature of TC4 alloy is lower than the β phase transformation temperature, once the solution temperature begins to rise, the α phase content decreases and the β phase content increases. The increase in β phase content causes the strength of the alloy to increase with increasing temperature, and the residual stress also increases slowly. In terms of the plasticity of TC4 titanium alloy, it shows a continuous decreasing trend with increasing temperature. At the same time, the volume fraction of the α phase has a significant impact on the yield strength. When the aging temperature is lower than the α phase transformation temperature, as the aging temperature increases, the α phase content increases, and the yield strength gradually increases. Therefore, the α-phase content affects the mechanical properties of TC4 titanium alloy. By simulating heat treatment to obtain the α-phase content at various temperatures, and based on the mechanical property requirements of actual experiments, the appropriate heat treatment temperature is selected according to the α-phase transformation law and phase content during the heat treatment simulation, thus reducing the number of heat treatment experiments. Compared with traditional experimental methods, temperature selection is more convenient and accurate.

[0090] In step S2, the phase transformations of TC4 titanium alloy during the heating and cooling processes are of different types. The phase transformation during heating is a diffusion-type phase transformation, while the phase transformation during cooling is a martensitic phase transformation, i.e., a non-diffusion-type phase transformation. In this step, a phase transformation model is established for the heating and cooling processes of TC4 titanium alloy bolts during heat treatment. This is a prerequisite for building a database of TC4 titanium alloy heat treatment materials. It is mainly used to calculate the degree of phase transformation of each phase during heat treatment, simulate the microstructure transformation cloud map during heat treatment, and obtain the phase transformation content based on the microstructure transformation cloud map. The α phase content affects the mechanical properties of TC4 titanium alloy, and the heat treatment temperature is selected based on the change in α phase content.

[0091] Example 1

[0092] The numerical simulation calculation of heat treatment at 954℃ for Φ8mm×32mm TC4 titanium alloy smooth bolts is used as an example for analysis. The chemical composition (wt.%) of TC4 titanium alloy smooth bolts is shown in Table 1.

[0093] Table 1 Chemical composition (wt.%) of TC4 titanium alloy bolts

[0094] 5.8 4.1 0.06 0.21 0.14 0.002 0.00156 89.68644

[0095] An optimization method for the heat treatment temperature of TC4 titanium alloy bolts based on finite element simulation includes:

[0096] S1. Establish phase regions and macroscopic rheological stress models for TC4 titanium alloy bolts.

[0097] Hot compression tests were designed for the high-temperature β-phase region and the low-temperature α+β-phase region of TC4 titanium alloy.

[0098] Rheological stress model of α phase

[0099] Rheological stress model of β phase

[0100] Macroscopic rheological stress model σ=X β σ β +X α σ α .

[0101] S2. Establish a phase transformation model for the heating and cooling processes during the heat treatment of TC4 titanium alloy bolts.

[0102] The changes in thermal expansion and heat flow during the phase transition were collected through thermal expansion experiments and DSC experiments. The degree of phase transition of each phase during the heat treatment process was calculated, and a microstructure transformation cloud map was simulated during the heat treatment process. The phase transition content was obtained from the microstructure transformation cloud map, and the formula for the degree of phase transition f was used. i (T i )=1-exp(-k i (T i )τ n Phase transition equation

[0103] S3. Establish a thermo-mechanical coupled finite element analysis model for the heat treatment process of TC4 titanium alloy bolts.

[0104] S3.1. Establish the three-dimensional geometry based on the actual bolt model. The result is as follows: Figure 2 As shown, the TC4 titanium alloy bolt has dimensions of Φ8mm×32mm. The model was imported into the finite element software in X_T format for mesh generation. The entire bolt model was divided into 30330 tetrahedral elements and 6905 nodes.

[0105] S3.2 Set the thermophysical property parameters of the metal material as a function of temperature, that is, the values ​​of elastic modulus, thermal conductivity, coefficient of thermal expansion, specific heat and Poisson's ratio of the selected metal at different temperatures. The thermophysical property parameters are shown in Table 2.

[0106] Table 2 Thermophysical properties of TC4 titanium alloy bolts as a function of temperature

[0107]

[0108] S4. Set boundary conditions during the heat treatment process.

[0109] In the initial step, the ambient temperature is set to 20℃; three analysis steps are established, namely the heating, holding and cooling processes in the heat treatment process. In the analysis step, the heat transfer coefficient between the heating furnace and the sample is set to 0.1, and the heat transfer coefficient ratio between the sample and water and air is set to constants of 0.599 and 0.03, respectively. The sample is cooled to room temperature by water cooling; nodes in the Z direction are selected on the nut for constraint.

[0110] S5. Perform numerical simulation of the heat treatment process.

[0111] The heat source equations provided by the finite element software were invoked, and a thermo-mechanical coupled finite element analysis model of the TC4 titanium alloy bolt heat treatment process was used to numerically simulate the heat treatment process. The temperature field and phase transition cloud map distribution of the heat treatment process were obtained after the numerical simulation. Based on the amount of α phase content in the obtained phase transition cloud map, a differential equation for heat transfer control was established. Perform heat transfer analysis, and after the calculation is completed, enter the post-processing interface to obtain the temperature field and microstructure cloud map, which are then represented by a bar chart.

[0112] like Figures 3-8 As shown, Figure 3 The corresponding bar chart shows the temperature field distribution during the heating process of the TC4 bolt. During the heating process, the edge of the nut reaches 650℃ first, followed by the center of the nut and the roll R position (the rounded corner connection between the nut and the bolt). The lowest temperature is in the bolt core, which is only 372℃, with a temperature difference of 278℃. Figure 4 The bar chart shows the temperature field distribution during the insulation process. The temperature difference is very small, but it can still be seen that the edge of the nut reaches the insulation temperature of 954℃ first, and then heat transfer occurs. The temperature of other parts of the bolt is lower, but gradually rises to 954℃ due to heat transfer. Figure 5 The bar chart shows the temperature field distribution during the cooling process. The edge of the nut cools the fastest, the bolt roll radius cools the slowest, and the core and the center of the head have the slowest temperature changes. Figure 6 The bar chart shows the distribution of the α phase during the heating process. During heating, the edge of the nut changes the most drastically, followed by the roller R and the core, while the center of the head has the lowest α phase change fraction. Figure 7 The bar chart shows the distribution of the α phase during the insulation process. The average volume fraction of the entire bolt is 0.502, and the α phase content in the core is higher than that in other parts of the bolt. Figure 8 The diagram shows the α-phase distribution during the cooling process. The phase transformation is incomplete, resulting in a smaller α-phase volume fraction at the head edge. In other locations, the slower temperature change allows for a more complete phase transformation. After the final phase transformation, the α-phase volume fraction at the edge is approximately 0.5, and at the bolt and roller R positions, it is approximately 0.58. The core has the highest α-phase volume fraction.

[0113] Figure 9The figure shows the change in the α-phase microstructure of TC4 titanium alloy bolts over time during the heat treatment process. As the temperature increases, the volume fraction of the α-phase continuously decreases until the rates of diffusion transformation and martensitic transformation remain constant, reaching a minimum of 0.5. During cooling, the volume fraction of the α-phase gradually increases, with a larger change initially. As the cooling temperature decreases, the volume fraction of the α-phase gradually levels off until it stops changing. After final cooling, the volume fraction of the α-phase is approximately 0.58.

[0114] S6, Instructions for heat treatment holding temperature

[0115] Based on the temperature field and phase transformation cloud map distribution of TC4 titanium alloy bolts during heat treatment, the temperature selection during the heat treatment process is guided by the composition content of the α phase.

[0116] Considering the softening and creep effects of the material during the heat treatment holding stage, the creep strain rate is:

[0117] Example 2

[0118] The analysis is based on numerical simulation calculation of heat treatment at 800℃ for TC4 titanium alloy smooth bolt with a diameter of 8mm×32mm. The chemical composition (wt.%) of TC4 titanium alloy smooth bolt is the same as that in Example 1.

[0119] The optimization method for the heat treatment temperature of TC4 titanium alloy bolts in this embodiment is basically the same as that in Embodiment 1, except that:

[0120] S5. Perform numerical simulation of the heat treatment process.

[0121] The heat source equations provided by the finite element software were invoked, and a thermo-mechanical coupled finite element analysis model of the TC4 titanium alloy bolt heat treatment process was used to numerically simulate the heat treatment process. The temperature field and phase transition cloud map distribution of the heat treatment process were obtained after the numerical simulation. Based on the amount of α phase content in the obtained phase transition cloud map, a differential equation for heat transfer control was established. Perform heat transfer analysis, and after the calculation is completed, enter the post-processing interface to obtain the temperature field and phase transition cloud map, which are then represented by a bar chart.

[0122] After the calculation is complete, the post-processing interface will display the phase transition contour plot, which will be converted into an α-phase distribution histogram, as shown below. Figure 10-12 As shown, Figure 10 The bar chart shows the distribution of the α phase during the heating process. The phase transition is most intense at the edge of the nut, and the degree of phase transition gradually decreases from the edge to the center of the nut, with the lowest degree of phase transition at the core. Figure 11The bar chart shows the distribution of the α phase during the heat preservation process. During heating, the edge of the nut changes the most drastically, followed by the roller and the core, while the center of the head shows the lowest α phase change fraction. Figure 12 The image shows a histogram of α-phase distribution after cooling. The average volume fraction of the entire bolt is 0.892. The α-phase content in the core is greater than that in the bolt head and the rolling radius, while the α-phase content is lowest at the edge of the bolt head.

[0123] Compared to Example 1, the lower the solution temperature, the higher the α-phase content after heat treatment. The higher the temperature, the higher the α-phase content.

[0124] Example 3

[0125] The numerical simulation calculation of TC4 titanium alloy smooth bolt with diameter of 8mm×32mm, which is subjected to solution heat treatment at 954℃ and then aging heat treatment at 450℃, 538℃ and 620℃ respectively, is used as an example for analysis. The chemical composition (wt.%) of TC4 titanium alloy smooth bolt is the same as that in Example 1.

[0126] The optimization method for the heat treatment temperature of TC4 titanium alloy bolts in this embodiment is basically the same as that in Embodiment 1, except that:

[0127] S5. Perform numerical simulation of the heat treatment process.

[0128] The heat source equations provided by the finite element software were invoked, and a thermo-mechanical coupled finite element analysis model of the TC4 titanium alloy bolt heat treatment process was used to numerically simulate the heat treatment process. The temperature field and phase transition cloud map distribution of the heat treatment process were obtained after the numerical simulation. Based on the amount of α phase content in the obtained phase transition cloud map, a differential equation for heat transfer control was established. Perform heat transfer analysis, and after the calculation is completed, enter the post-processing interface to obtain the temperature field;

[0129] After the calculation is complete, the post-processing interface will display the phase transition contour plot, which can be converted into an α-phase distribution histogram, such as... Figure 13-15 As shown, Figure 13 The image shows a bar chart of the microstructure distribution of the α phase after aging heat treatment at 450℃. The phase transformation is most intense at the edge of the nut, and the degree of phase transformation gradually decreases from the edge to the center of the nut. The degree of phase transformation is lowest in the core, with an average α phase content of 0.27%. Figure 14 The bar chart shows the distribution of the α phase after aging heat treatment at 538℃. The phase transformation is most intense at the edge of the nut, and the degree of phase transformation gradually decreases from the edge to the center of the nut. The degree of phase transformation is lowest in the core, with an average α phase content of 0.48%. Figure 15 The bar chart shows the distribution of α phase after aging heat treatment at 620°C. The average volume fraction of the entire bolt is 0.667. The α phase content in the core is greater than that in the bolt head and the rolling radius, and the α phase content is lowest at the edge of the bolt head.

[0130] Comparing Examples 1 and 2, it can be observed that during solution heat treatment, when the solution temperature is below the β phase transformation temperature, the α phase content continuously decreases while the β phase content increases with increasing solution temperature. The increased β phase content leads to an increase in alloy strength with increasing temperature, and residual stress also increases slowly. However, the TC4 titanium alloy exhibits a continuous decreasing trend in plasticity with increasing temperature. In Example 1, at a solution temperature of 954℃, the α phase content is lower and the β phase content is higher, resulting in better mechanical properties than in Example 2. Therefore, a suitable solution temperature can be selected based on the change in α phase content with solution temperature, reducing the number of heat treatment experiments. Simultaneously, Example 3 implemented three sets of aging temperatures. When the aging temperature is below the α phase transformation temperature, the higher the aging temperature, the higher the α phase content after heat treatment, and the yield strength gradually increases. Therefore, at an aging temperature of 620℃, the α phase content is higher, and the yield strength is also higher. A suitable aging temperature can be selected based on the change in α phase content with aging temperature. Therefore, the α-phase content affects the mechanical properties of TC4 titanium alloy. By simulating heat treatment to obtain the α-phase content at various temperatures, and based on the performance requirements of actual experiments, the appropriate heat treatment temperature is selected according to the α-phase transformation law and phase content during the heat treatment simulation, thus reducing the number of heat treatment experiments. Compared with traditional experimental methods, temperature selection is more convenient and accurate.

[0131] This invention enables numerical simulation of the heat treatment process of TC4 titanium alloy bolts, obtaining microstructure cloud diagrams of α-phase changes during heat treatment. This provides a technical basis for the microstructure distribution of TC4 titanium alloy bolts during heat treatment in engineering applications, allowing for the selection of heat treatment temperatures based on phase content distribution. Since the amount of α-phase content affects the mechanical properties of TC4 titanium alloy, during solution heat treatment, below the β-phase transformation temperature, the α-phase content decreases with increasing solution temperature, while the β-phase content increases. This increase in β-phase content leads to an increase in alloy strength with increasing temperature, and a slow increase in residual stress. During aging heat treatment, below the α-phase transformation temperature, the α-phase content increases with increasing aging temperature, and the yield strength gradually increases. Therefore, a suitable heat treatment temperature can be selected based on the α-phase content obtained at different heat treatment temperatures.

[0132] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for optimizing the heat treatment temperature of TC4 titanium alloy bolts based on finite element simulation, characterized in that, include: S1. Establish phase regions and macroscopic rheological stress models for TC4 titanium alloy bolts. S2. Establish a phase transformation model for the heating and cooling processes during the heat treatment of TC4 titanium alloy bolts. The changes in thermal expansion and heat flux during the phase transition were collected using thermal expansion experiments and DSC experiments. The formula for the degree of phase transition, f, was used. i (T i )=1-exp(-k i (T i )τ n Formula 4 represents the phase transformation equation for the α-phase to β-phase transformation of TC4 titanium alloy bolts during the heating process. Formula 5 is used to calculate the degree of phase transformation of each phase during heat treatment, simulate the microstructure transformation cloud map during heat treatment, and obtain the phase transformation content based on the microstructure transformation cloud map. In the formula: f i (T i ) indicates the degree of phase transition, k i (T i The results are obtained through DSC experiments or by measuring the TTT curve, where τ represents the heating time, n represents the material constant, e is the natural constant, A is the material constant, and T is the material constant. s Indicates the phase transition onset temperature, T e T represents the phase transition end temperature, T represents the temperature at which the phase transition occurs, and D represents the material constant. S3. Establish a thermo-mechanical coupled finite element analysis model for the heat treatment process of TC4 titanium alloy bolts. A TC4 titanium alloy bolt model was established using finite element software. Eight-node hexahedral elements were selected to mesh the bolt model. The thermophysical property parameters of the metal material as a function of temperature were set. The thermophysical property parameters included the values ​​of elastic modulus, thermal conductivity, coefficient of thermal expansion, specific heat, and Poisson's ratio of the metal material at different temperatures. S4. Set boundary conditions during the heat treatment process. Set temperature boundary conditions: Set the welding environment temperature to room temperature, set the boundary heat transfer condition parameters, specifically the heat transfer coefficient and thermal radiation condition parameters, and set all outer wall surfaces of the TC4 titanium alloy bolts to be heat exchange surfaces. Setting structural boundary conditions: Since the TC4 titanium alloy bolt is a simple symmetrical structure, it is only necessary to take a point on the nut to constrain the translational degree of freedom in the Z direction, and the bolt can deform freely without rigid body movement; S5. Perform numerical simulation of the heat treatment process. Numerical simulations were performed on the heat treatment process, yielding the temperature field and phase transition distribution cloud map. The differential equations based on heat transfer control were then used to obtain the final simulation results. Perform heat transfer analysis, In the formula: λ x , λ y , λ z Let ρ represent the thermal conductivity of the material in the x, y, and z directions, respectively, and let c represent the material density. p represents the specific heat capacity at constant pressure, and Q represents the intensity of the internal heat source; S6, Instructions for heat treatment holding temperature Based on the temperature field and phase transformation cloud diagram distribution of TC4 titanium alloy bolts during heat treatment, considering the softening and creep effects of the material during the heat treatment holding stage, the creep strain rate is: In the formula: The creep strain rate is represented by A, the material constant is represented by σ, the creep stress is represented by n, the stress exponent is represented by Q, the nominal creep activation energy is represented by T, and the temperature is represented by T.

2. The optimization method according to claim 1, characterized in that, In step S1, the method for establishing the phase regions and macroscopic rheological stress model of the TC4 titanium alloy bolt is as follows: Hot compression tests were designed for the high-temperature β-phase region and the low-temperature α+β-phase region of TC4 titanium alloy bolts. Rheological stress models for each phase region and the macroscopic rheological stress model were obtained. The formula for the rheological stress model of the α-phase of the TC4 titanium alloy bolt was also provided. Formula 1 is the formula for the rheological stress model of the β phase. Formula 2 is the macroscopic rheological stress model formula for TC4 titanium alloy bolts: σ = X β σ β +X α σ α Formula 3, In the formula: This indicates the rheological rate of the α phase in TC4 titanium alloy at low temperatures. This represents the rheological rate of the β phase and σ of TC4 titanium alloy at high temperatures. α The rheological stress and σ of the α-phase of TC4 titanium alloy under hot compression are represented. β The rheological stress of TC4 titanium alloy during β-phase hot compression is represented by R, which is the air constant, and T represents the experimental temperature. α Indicating the degree of α-phase transformation and X in TC4 titanium alloy β β represents the degree of phase transformation of TC4 titanium alloy, and σ represents the macroscopic stress of TC4 titanium alloy.

3. The optimization method according to claim 2, characterized in that, The phase transition degree X in step S1 α +X β =1.

4. The optimization method according to claim 1 or 3, characterized in that, The parameter k in step S2 i (T i The values ​​of ) and n are obtained through one of the following methods: thermal expansion experiment, DSC experiment, or plotting TTT curve.

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