Metal stress regulation and control method based on expanded graphite

Abstract

The invention discloses a metal stress regulation and control method based on expanded graphite. The method comprises the following steps that 1, a scanning electron microscope is used for obtaining data such as workpiece morphology; 2, establishing a cavity metal thin-wall part crystal plasticity-finite element model; 3, establishing a macroscopic geometric model and a finite element model of the cavity thin-wall part; step 4, determining a bulging process parameter range; 5, determining the proportion of the expanded graphite in the mixed solid filling structure; 6, purified water and wheat flour are added to form a mixed solid filling structure; 7, the cavity structure of the target regulation and control metal is filled, and heating bulging is carried out to eliminate residual stress; and step 8, performing a sampling experiment, and verifying the residual stress eliminating effect. According to the method, the thermally induced volume deformation characteristic of the expanded graphite-based mixed solid filling structure is utilized, and the cavity metal thin-wall part is induced to complete stress release and relaxation through the controllable bulging force, so that the residual stress is efficiently eliminated. According to the method, key conditions and internal rules of stress release and plastic strengthening of the thin-wall part under the working conditions of heating, filling and bulging can be defined, and the method has important practical significance on residual stress control of the cavity metal thin-wall weak-rigidity structure.

Description

Technical Field

[0001] This invention relates to a method for controlling the residual stress of a cavity metal thin-walled structure by utilizing the volume change of a hybrid solid-filled structure based on expanded graphite. Background Technology

[0002] Hollow metal thin-walled parts are characterized by large material removal volumes, complex structural features, weak overall rigidity, and stringent machining tolerance requirements. During their forming and machining processes, residual stress is generated; in the semi-finishing and finishing stages, this residual stress is gradually released and redistributed, leading to workpiece warping or torsional deformation and a decrease in machining accuracy. Furthermore, due to their inherent weak rigidity, coupled with the dual driving forces of cutting loads and residual stress loads, hollow metal thin-walled parts are highly susceptible to dimensional deviations and surface defects during machining, which in severe cases can lead to direct scrapping of the parts and consequently, product delivery delays.

[0003] Residual stress in metals can be reduced or homogenized through pre-stretching, which is one of the existing methods for controlling residual stress in metals. Meanwhile, expanded graphite undergoes volume expansion under heating conditions, with the expansion amount reaching up to 100 times its original size. The pre-stretching deformation in metal pre-stretching processes is generally 1.5%-3%, and the deformation amount of the pre-stretching process can be satisfied by adjusting the volume expansion caused by the expansion.

[0004] To address this, this invention proposes a method for controlling metal stress based on expanded graphite. The method first utilizes EBSD technology to collect relevant data, constructing a plastic constitutive-finite element coupled model, a macroscopic geometric model, and their corresponding finite element analysis models for controlling the metal. Simulation calculations clarify the precise deformation parameters required for the hollow thin-walled structure. Based on this, and combined with the test results of temperature-varying experiments, the composition ratio of expanded graphite is optimized. Water, wheat flour, and expanded graphite powder are uniformly mixed in a specific ratio to prepare a high-expansion-coefficient hybrid solid-filled structure with temperature-sensitive properties. By adjusting the doping ratio of graphite powder, the structure and composition of the composite material can be flexibly controlled. After filling this composite-filled structure into the hollow thin-walled component, the ambient temperature is adjusted to drive the composite structure to produce quantitative and directional expansion deformation, thereby achieving precise control and efficient elimination of residual stress. Finally, the treated component is sampled and tested to verify the residual stress elimination effect. This method has significant engineering application value for improving the stress control capability of hollow thin-walled weakly rigid structures. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] To address the shortcomings of existing technologies, this invention provides a metal stress control method based on expanded graphite, which solves the problem of residual stress release inside hollow metal thin-walled weak rigidity structures after processing. This method is of great significance for hollow metal thin-walled weak rigidity structures and for reducing structural stress.

[0007] (II) Technical Solution

[0008] To achieve the above objectives, the present invention provides the following technical solution: a method for regulating metal stress based on expanded graphite. The steps are as follows:

[0009] Step 1: Using scanning electron microscopy (SEM) electron backscattered diffraction (EBSD) technique, key information such as the fine grain morphology, grain orientation, and texture characteristics of the metal workpiece sample is acquired, and the feature files in *.ctf or *.ang format are exported. Based on this, a Spectral solver based on fast Fourier transform is used to construct the geometric model of the microstructure of the hollow thin-walled metal part.

[0010] Step 2: Establish a Crystal Plasticity Finite Element Method (CP-FEM) model for the hollow metal thin-walled component, fully incorporating various state variables during the microstructure evolution process. Solve the nonlinear state dynamics equations using a macro program. These equations describe the evolution of state parameters under preset boundary and initial conditions, thereby obtaining the change curves of state variables throughout the phase transition process.

[0011] Step 3: Using general-purpose finite element analysis software, build the macroscopic geometric model and finite element analysis model of the hollow thin-walled part, and map the state variables calculated by the crystal plasticity model to the macroscopic finite element model; at the same time, define the macroscopic mechanical property parameters of related materials such as expanded graphite and metal thin-walled parts, carry out thermo-mechanical coupled finite element simulation analysis, solve the physical field data such as stress, strain, deformation and temperature distribution during the variable temperature filling process, and finally realize the cross-scale simulation of the variable temperature filling process.

[0012] Step 4: Based on the established macro-micro cross-scale simulation model, analyze the expansion volume and bulging force threshold required for residual stress control in typical hollow metal structures. Compare the evolution of tensile and compressive residual stresses during the bulging process, and establish a quantitative correlation between filling process parameters and bulging volume and bulging load. Through quantitative iterative calculations, determine the range of filling process parameters that can achieve the desired residual stress control effect as required by engineering.

[0013] Step 5: Analyze the thermally induced volume change characteristics and expansion force response of expanded graphite-based mixed solid filler structures with different ratios. Conduct thermal expansion characteristic tests on multi-component mixed fillers, explore the expansion force distribution law of expanded graphite under different cavity configurations, and determine the optimal dosing ratio of expanded graphite in the mixed filler based on the stress control target requirements of thin-walled metal structures.

[0014] Step 6: Prepare a solid filling medium with expanded graphite as the core. To solve the problem that expanded graphite is difficult to form into a stable filling structure, and to adapt to the expansion force requirements of different cavity structures, pure water and wheat flour are introduced as molding aids and compounded with expanded graphite to prepare a plastically formable mixed solid filling structure. By adjusting the proportion of wheat flour, the volume of the mixed filler and the direction of thermo-expansion can be precisely controlled.

[0015] Step 7: Place the hollow thin-walled component filled with the mixed solid medium in a thermal aging device and heat the entire component in a gradient manner; after the temperature rises to the volume deformation threshold of the expanded graphite, maintain the temperature constant for a period of time until the expanded graphite completes directional expansion.

[0016] Step 8: Samples were taken from specimens with different process parameters, and tensile property tests, EBSD microscopic characterization, and transmission electron microscopy (TEM) analysis were conducted to explore the influence of different process parameters on the residual stress relief effect, mechanical properties, and microstructure characteristics of the specimens. The effect of graphite doping control-heat bulging process on residual stress relief was verified, and the systematic errors of the simulation model were corrected based on the experimental results to improve the model's calculation accuracy and engineering applicability.

[0017] Preferably, in step 2, the state variables are set as microstructure parameters, such as dislocation density, dislocation rate, stacking fault energy, twin integral number, volume fraction of phase transition, and related variables, and an evolution equation for each state variable is established.

[0018] Preferably, in step 5, the investigation of the expansion force distribution law of expanded graphite with different proportions in different cavity shapes can be carried out by analyzing the influence of different parameters such as graphite mass fraction, deformation expansion rate, and heat preservation time on expansion deformation and expansion load.

[0019] Preferably, in step 7, in order to make the mixed solid filling structure firmly and tightly fill the cavity structure, after the mixture is placed into the cavity, it is necessary to use a flat mold of the same size as the cavity opening for extrusion, and refill the mixed solid filling structure with the empty space generated by extrusion until the cavity is tightly filled.

[0020] Preferably, in step 7, to prevent the graphite from escaping from the opening of the filled cavity thin-walled structure due to heat, thus causing the expansion amount to fail to meet expectations, the cavity opening position can be adjusted using a method...

[0021] The flange and stud are used to seal and limit the opening, ensuring the filling of the mixed solid containing expanded graphite.

[0022] The filled structure does not escape from the opening of the thin-walled cavity after being heated. Attached Figure Description

[0023] Figure 1 This is a flowchart of the analysis process of the present invention.

[0024] Figure 2 This is a schematic diagram of the cavity thin-walled structure after filling according to the present invention.

[0025] Figure 3 This is a schematic diagram of the flange and stud limiting mechanism of the present invention.

[0026] Figure 4 This is a schematic diagram of the planar mold of the present invention.

[0027] Figure 5 This is a schematic diagram of the compaction of the solid-filled structure of the present invention.

[0028] In the diagram: 1-Thin-walled hollow structure, 2-Hybrid solid filling structure containing expanded graphite, 3-Stud, 4-Flange, 5-Flat mold Detailed Implementation

[0029] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0030] like Figure 1 As shown, a method for controlling metal stress based on expanded graphite includes the following steps:

[0031] First, the fine grain morphology, orientation, and texture of the workpiece sample are obtained by scanning electron microscopy (SEM) electron backscatter diffraction, and exported as *.ctf or *.ang files. Then, a Spectral solver based on fast Fourier transform is applied to construct a geometric model of the microstructure of the thin-walled metal material.

[0032] Secondly, a Crystal Plasty Finite Element Method (CP-FEM) model for controlling the crystal plasticity of thin-walled metal parts is established. In this model, the state variables are actual microstructural parameters, such as dislocation density, dislocation rate, stacking fault energy, twin integral number, volume fraction of phase transition, and related variables. An evolution equation for each state variable is established. By writing a macro program, the nonlinear state dynamics equations describing the evolution of state parameters under application boundary or initial conditions are solved, thereby obtaining the change curves of state variables during the phase transition process.

[0033] Next, using general-purpose finite element software, we established a macroscopic geometric model and a finite element model of the thin-walled part, and solved the physical field results such as stress, strain, deformation, and temperature during the variable-temperature filling process, thus realizing cross-scale simulation of the filling control process.

[0034] Furthermore, based on the constructed cross-scale macro-micro simulation model, the evolution of tensile and compressive residual stress states during the bulging process was compared, and a quantitative relationship was established between the expanded graphite ratio parameter and the bulging volume and bulging load. Through quantitative iterative calculations, the range of filling process parameters that achieve the desired residual stress control effect was obtained.

[0035] Furthermore, by analyzing the volume change and expansion force characteristics of mixed solid filler structures composed of different proportions of expanded graphite as a function of temperature, the effects of pressure and filling amount generated by different proportions of expanded graphite on expansion volume, expansion force and residual stress can be calculated. By comparing the thermal expansion characteristics of components with different graphite proportions and combining them with the matching with the target thin-walled structure, the proportion of graphite in the mixed filler can be determined.

[0036] Furthermore, a mixed solid filling structure was constructed using wheat flour, purified water, and the required amount of expanded graphite. The volume and thermal expansion direction of the mixed filling were controlled by varying the proportion of wheat flour. The distribution of expansion force of expanded graphite with different proportions in different cavity shapes was analyzed, including the influence of parameters such as graphite mass fraction, deformation expansion rate, and holding time on the expansion deformation and expansion load.

[0037] Furthermore, such as Figure 2 As shown, a mixed solid-filled structure containing expanded graphite (2) is placed inside a thin-walled cavity structure (1). The thin-walled structure, now filled with the mixed solid-filled structure, is placed in a thermal aging apparatus. The entire container is heated until the temperature reaches the level required for the expanded graphite to undergo volume change. The temperature is then maintained for a period until the expanded graphite has fully expanded. Figure 3 As shown, before heating and expanding, to prevent the mixed solid filling structure from escaping from the cavity opening after heating, a 3-screw and a 4-flange can be used for sealing and limiting. For example... Figure 4As shown, before heating and expanding, the mixed solid filling structure can be compacted using a 5-plane mold. Figure 5 As shown, before heating and expanding, in order to make the 2-mixed solid filling structure containing expanded graphite firmly and tightly fill the 1-thin-walled cavity structure, after the mixture is placed into the cavity, a 5-flat die of the same size as the cavity opening can be used for extrusion, and the empty space generated by extrusion is refilled into the 2-mixed solid filling structure containing expanded graphite until the cavity is tightly filled.

[0038] Furthermore, samples were taken from different specimens, and tensile tests, EBSD analysis, and transmission electron microscopy were conducted to investigate the effects of different parameters on the residual stress relief, mechanical properties, and microstructure of the specimens. The experimental results were compared with CP-FEM simulation results to verify the residual stress removal effect and revise the simulation model.

[0039] The workflow of this invention is as follows: First, an electron backscatter diffraction (ESD) model of the microstructure of a thin-walled metal part is constructed using electron microscopy. Second, a crystal plasticity-finite element model of the thin-walled metal part is established. Next, a cross-scale simulation of the heating and bulging process is performed using general-purpose finite element software. Next, the parameter range of the filling process that meets the requirements is determined. Next, the proportion of expanded graphite in the mixed filler is determined. Next, wheat flour and pure water are added to the calculated mass of graphite to form a mixed solid filling structure. Next, the controlled cavity thin-walled structure is filled and placed in a thermal aging furnace to generate bulging through heating, thus eliminating residual stress. Finally, the simulation model is modified according to the actual stress condition of the specimen to verify the residual stress elimination effect.

[0040] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0041] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for controlling metal stress based on expanded graphite, comprising the following steps: Step 1: Using scanning electron microscopy (SEM) electron backscattered diffraction (EBSD) technique, key information such as the fine grain morphology, grain orientation, and texture characteristics of the metal workpiece sample is acquired, and the feature files in *.ctf or *.ang format are exported. Based on this, a Spectral solver based on fast Fourier transform is used to construct the geometric model of the microstructure of the hollow thin-walled metal part. Step 2: Establish a Crystal Plasticity-Finite Element Method (CP-FEM) model for the hollow thin-walled metal part, fully incorporating various state variables during the microstructure evolution process. Solve the nonlinear state dynamics equations using a macro program. These equations describe the evolution of state parameters under preset boundary and initial conditions, thereby obtaining the change curves of state variables throughout the entire phase transition process. Step 3: Using general-purpose finite element analysis software, build the macroscopic geometric model and finite element analysis model of the hollow thin-walled part, and map the state variables calculated by the crystal plasticity model to the macroscopic finite element model; at the same time, define the macroscopic mechanical property parameters of related materials such as expanded graphite and metal thin-walled parts, carry out thermo-mechanical coupled finite element simulation analysis, solve the physical field data such as stress, strain, deformation and temperature distribution during the variable temperature filling process, and finally realize the cross-scale simulation of the variable temperature filling process. Step 4: Based on the established macro-micro cross-scale simulation model, analyze the expansion volume and bulging force threshold required for residual stress control in typical hollow metal structures. Compare the evolution of tensile and compressive residual stresses during the bulging process, and establish a quantitative correlation between filling process parameters and bulging volume and bulging load. Through quantitative iterative calculations, determine the range of filling process parameters that can achieve the desired residual stress control effect as required by engineering. Step 5: Analyze the thermally induced volume change characteristics and expansion force response of expanded graphite-based mixed solid filler structures with different ratios. Conduct thermal expansion characteristic tests on multi-component mixed fillers, explore the expansion force distribution law of expanded graphite under different cavity configurations, and determine the optimal dosing ratio of expanded graphite in the mixed filler based on the stress control target requirements of thin-walled structures. Step 6: Prepare a solid filling medium with expanded graphite as the core. To solve the problem that expanded graphite is difficult to form into a stable filling structure, and to adapt to the expansion force requirements of different cavity structures, pure water and wheat flour are introduced as molding aids and compounded with expanded graphite to prepare a plastically formable mixed solid filling structure. By adjusting the proportion of wheat flour, the volume of the mixed filler and the direction of thermo-expansion can be precisely controlled. Step 7: Place the hollow metal thin-walled component filled with the mixed solid medium into a thermal aging device and heat the entire component in a gradient manner; after the temperature rises to the volume deformation threshold of the expanded graphite, maintain the temperature constant for a period of time until the expanded graphite completes directional expansion. Step 8: Samples were taken from specimens with different process parameters, and tensile property tests, EBSD microscopic characterization, and transmission electron microscopy (TEM) analysis were conducted to explore the influence of different process parameters on the residual stress relief effect, mechanical properties, and microstructure characteristics of the specimens. The effect of graphite doping control-heat bulging process on residual stress relief was verified, and the systematic errors of the simulation model were corrected based on the experimental results to improve the model's calculation accuracy and engineering applicability.

2. The method for regulating metal stress based on expanded graphite according to claim 1, characterized in that: This invention utilizes a mixed solid filling structure prepared by mixing expanded graphite with pure water and wheat flour. Upon heating, the volume change generates an expansion force on the hollow metal thin-walled structure, causing the metal to undergo tensile deformation, thereby achieving the control of stress reduction or homogenization in the hollow thin-walled metal structure.

3. The method for controlling metal stress based on expanded graphite according to claim 1, characterized in that: In step 5, the distribution law of expansion force of expanded graphite with different proportions in the mixed solid filling structure in different cavity shapes is investigated. This can help analyze the influence of different parameters such as graphite mass fraction, deformation expansion rate, and heat preservation time on the expansion deformation and expansion load of the expanded graphite-based mixed solid filling structure.

4. The method for controlling metal stress based on expanded graphite according to claim 1, characterized in that: In step 7, to ensure the mixed solid filling structure firmly and tightly fills the cavity structure, after the mixture is placed into the cavity, an extrusion step is required using a flat mold of the same size as the cavity opening. The excess space created by the extrusion is then refilled with the mixed solid filling structure until the cavity is completely and tightly filled. This step ensures that the mixed solid filling structure completely and tightly fills the cavity structure, guaranteeing the subsequent expansion effect.

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