Method for predicting damage and cracking of metal hot plastic deformation considering microstructure evolution

Abstract

The microstructure evolution considering metal hot plastic deformation damage cracking prediction method belongs to the technical field of metal hot plastic deformation and comprises the following steps: through thermal simulation test, standard sample is subjected to unidirectional compression and uniaxial tension, and dynamic recrystallization and damage related parameters are determined; on this basis, an initial expression of damage parameters containing microstructure evolution is constructed; through secondary development, the damage parameters are integrated with commercial finite element software to realize synchronous simulation of metal hot deformation-damage evolution and complete correction of the damage parameters; the secondary development process is repeated, the corrected damage parameters are used to replace the initial damage parameters and integrated with the commercial finite element software to form a final metal hot deformation-damage evolution synchronous simulation platform; and the failure criterion is inputted, so that the metal hot plastic deformation damage evolution and cracking prediction can be realized. The method has strong universality, stable damage parameter expression form, clear meaning, simple and unified failure criterion and is easy to integrate with the finite element software.

Description

Technical Field

[0001] This invention belongs to the field of metal thermoplastic deformation technology, specifically relating to a method for predicting damage and cracking in metal thermoplastic deformation that takes into account the evolution of microstructure. Background Technology

[0002] Damage during the thermoplastic deformation of metals is typically invisible and intangible on a macroscopic scale, and is also irreversible. When the damage accumulates to a certain extent, the material will crack, and any unexpected cracking can cause serious damage in terms of human, material, and financial resources. Therefore, establishing corresponding models to predict the damage evolution and cracking during the hot deformation process of metallic materials has significant engineering and social implications.

[0003] Currently, macroscopic damage models are commonly used to predict damage and cracking during thermoplastic deformation of metals. Macroscopic damage models mainly include two types: one is the existing semi-empirical model, and the other is a phenomenological model independently established by researchers based on specific materials.

[0004] Semi-empirical models are essentially a criterion for determining the energy accumulation limit, meaning that the material will fail and crack when the integral term in the model expression reaches a fixed critical damage value. Commonly used semi-empirical models include the Freudenthal model, the CL model, the Normalized C-L model, and the Brozzzo model. These models have the advantages of simple form and convenient application. However, because these models only express the influence of stress state on damage and cracking, they do not consider the influence of key factors such as microstructure, temperature, and strain rate during hot deformation. Therefore, these models cannot be directly applied to predict hot deformation damage and cracking. It is necessary to introduce the influence of hot deformation factors when calibrating the critical damage value. Moreover, the applicability and prediction accuracy of each criterion are unstable depending on the deformed material.

[0005] When independently establishing phenomenological models, most methods employ fitting experimental data to construct the correspondence between deformation parameters and damage phenomena. The advantage of this approach is its strong specificity, capable of effectively describing the damage and cracking process of a particular material. However, these models generally lack clear physical meaning, vary widely in form, and have very limited applicability. Once the material changes, the model's expressions and parameters become unusable, hindering its widespread application. Summary of the Invention

[0006] The main objective of this invention is to overcome the shortcomings of the prior art and provide a method for predicting damage and cracking of metal thermoplastic deformation that takes into account the evolution of microstructure.

[0007] This invention is achieved through the following technical solution: a method for predicting damage and cracking in metal thermoplastic deformation considering microstructure evolution, comprising the following steps:

[0008] S1. Within the range of thermal deformation parameters, the following operations are performed on the standard specimens using a thermal simulation testing device:

[0009] 1) Uniaxial compression test: Obtain the true stress-strain curve of the standard compression specimen and determine the steady-state stress, saturation stress and dynamic recrystallization volume fraction;

[0010] 2) Uniaxial tensile test: Obtain the true stress-strain curve of the standard tensile specimen, and determine the damage initiation strain (the strain corresponding to the peak point of the tensile stress-strain curve) and the fracture strain.

[0011] 3) First, prefabricate notches of different radii on several standard tensile specimens to obtain notched specimens; then conduct uniaxial tensile tests on the notched specimens to obtain the true stress-strain curves of the notched specimens, and determine the stress triaxiality and stress state influence coefficient (the ratio of the fracture strain of the specimen with the prefabricated notch radius to the fracture strain of the smooth specimen when the thermal deformation parameters are the same).

[0012] S2. Using the parameters determined in step S1, establish the metal thermoplastic deformation damage parameter d0, which includes factors influencing microstructure evolution:

[0013]

[0014] In the formula, K is a coefficient, and X dre This refers to the volume fraction of dynamic recrystallization. The effective stress is η, the stress state influence coefficient is ε. i For damage initiation strain, ε f For fracture strain, For equivalent change;

[0015] S3. First, through secondary development of commercial finite element software, the damage parameter d0 determined in step S2 is integrated with the main program of the software; then, using the experimental data obtained in step S1 as the simulation boundary conditions, the uniaxial tensile test of the standard tensile specimen in step S1 is simulated synchronously with thermal deformation and damage evolution using the secondary developed finite element software; finally, the damage parameter d0 in step S2 is calibrated based on the uniaxial tensile test results of the standard tensile specimen in step S1 and the synchronous simulation results of thermal deformation and damage evolution, and the correction coefficient γ is determined.

[0016] S4. Correct the damage parameter d0 according to the correction coefficient determined in step S3, and obtain the corrected damage parameter d1 as follows:

[0017]

[0018] The damage parameter d0 in step S2 is replaced by the modified damage parameter d1. Then, the secondary development process of step S3 is repeated to integrate d1 with the main program and establish a synchronous simulation platform for metal hot deformation-damage evolution. Then, "d1=1" is used as the judgment condition for failure and cracking (that is, when d1=1, it is considered that the corresponding material point has failed and cracks have appeared in the sample) and input into the secondary development finite element software to realize the prediction of metal thermoplastic deformation damage evolution and cracking.

[0019] Furthermore, in step S1, the hot deformation parameters include deformation temperature and deformation rate, and the specific numerical range depends on the deformed metal and the forming process.

[0020] Further, in step S1, the stress triaxiality R is calculated based on the geometry of the notched specimen. σ :

[0021]

[0022] Where R0 is the notch radius of the notched specimen, and a0 is the minimum cross-sectional radius of the notched specimen.

[0023] Furthermore, in step S2, the process of solving for the damage parameter d0 includes the following steps:

[0024] S2-1. Dynamic recrystallization and damage evolution during metal hot deformation can cause stress softening. Therefore, we first assume the existence of an ideal stress-strain behavior where neither damage nor dynamic recrystallization occurs, and under given deformation conditions, its stress is σ. W ;

[0025] S2-2、(1) When only dynamic recrystallization occurs, the resulting stress softening value is:

[0026] Δσ W-T =X dre (σ s -σ ss )=KX dre ;

[0027] In the formula, X dre The volume fraction of dynamic recrystallization; coefficient K = σ s -σ ss , σ s For saturation stress, σ ss If the stress is steady-state, then the effective stress is... (Corresponding to uniaxial compressive stress) is:

[0028]

[0029] (2) When only damage evolution occurs, the resulting stress softening value is:

[0030]

[0031] In the formula, ε i For damage initiation strain, ε f Where η is the fracture strain and η is the stress state influence coefficient;

[0032] (3) When dynamic recrystallization behavior and damage evolution behavior coexist, the resulting stress softening value is Δσ. W-T +Δσ W-D Then the damage stress (corresponding to uniaxial tensile stress) during thermoplastic deformation of the metal is:

[0033]

[0034] S2-3. According to the theory of equivalent change assumption, the following relationship exists:

[0035]

[0036] In the formula, Where σ is the effective stress, σ is the damage stress, and d is the theoretical damage parameter;

[0037] The metal thermoplastic deformation damage parameter d0, which includes factors influencing microstructure evolution, is:

[0038]

[0039] In the formula, K is a coefficient, and X dre This refers to the volume fraction of dynamic recrystallization. The effective stress is η, the stress state influence coefficient is ε. i For damage initiation strain, ε f For fracture strain, This is an equivalent change.

[0040] Further, in step S3, the method for determining the correction coefficient γ is as follows: when the standard tensile simulation specimen and the standard tensile specimen have the same length at the fracture time under given deformation conditions, the damage value at the outer radius of the minimum cross section of the standard tensile simulation specimen at that time is read, and its reciprocal is used as the corresponding correction coefficient under the set of deformation parameters.

[0041] The beneficial effects of this invention are as follows:

[0042] 1. Compared with phenomenological models based on specific materials, the damage parameter expression proposed in this invention is stable in form and can be extended to damage evolution and crack prediction during the thermoplastic deformation of different metals;

[0043] 2. Compared with semi-empirical models, this invention uses temperature, strain rate, and dynamic recrystallization degree as internal variables in the damage parameter expression, fully considering the characteristics of metal hot deformation and its influencing factors. Furthermore, this invention consistently uses d1=1 as the failure criterion, making it convenient to use and easy to integrate with finite element software.

[0044] In summary, compared with existing models, the components of the damage parameter expression in this invention have obvious physical meanings and can more comprehensively reflect the interaction between metal thermoplastic deformation, dynamic recrystallization and damage evolution. Attached Figure Description

[0045] Figure 1 This is a flowchart of the method of the present invention;

[0046] Figure 2 A schematic diagram of a standard compression specimen used in a uniaxial compression test.

[0047] Figure 3 A schematic diagram of a standard tensile specimen used in a uniaxial tensile test.

[0048] Figure 4 Photographs of the notched specimens;

[0049] Figure 5 This is a schematic diagram of the method for calculating stress triaxiality;

[0050] Figure 6 Figure 6a shows a comparison of the experimental specimen and the simulated specimen in the fracture state of the hot tensile specimen; Figure 6b shows a simulated fracture state of the experimental specimen.

[0051] Figure 7 To simulate the fracture process of the specimen and the distribution cloud map of internal damage parameters. Detailed Implementation

[0052] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0053] like Figure 1 The method for predicting damage and cracking in metal thermoplastic deformation that considers microstructure evolution includes the following steps:

[0054] S1. Taking a certain nuclear power component steel as an example, within the range of hot deformation parameters (experimental temperatures of 800℃, 900℃, 1000℃, 1100℃, and 1200℃, and strain rates of 0.001s), -1 0.01s -1 0.1s -1 1.0s -1 Within the chamber, the following operations were performed on the standard samples using a Gleeble-1500D thermal simulation testing machine:

[0055] 1) Uniaxial compression test: Obtain a standard compression specimen (e.g.) Figure 2 The actual stress-strain curve (shown) is used to determine the steady-state stress as σ. ss =92.59sinh -1 (1.071×10 16 Z 6.685 );

[0056] The saturation stress is σ s =92.59sinh -1 (1.421×10 15 Z 6.661 In the formula, Where T is the strain rate and T is the absolute temperature;

[0057] The volume fraction of dynamic recrystallization is (ε≥ε c In the formula, For equivalent strain, ε c For the critical strain of dynamic recrystallization, ε p Peak strain;

[0058] 2) Uniaxial tensile test: Obtain a standard tensile specimen (e.g. Figure 3 The actual stress-strain curve (shown) is used to determine the damage initiation strain as ε. i =25.66081-2.87532Z+0.11846Z 2 -0.00212Z 3 +1.3963×10 -5 Z 4 The fracture strain is

[0059] 3) First, pre-fabricate notches of different radii on several standard tensile specimens, with notch radii of 0.5 mm, 2 mm, and 4 mm respectively (when the notch radius is ∞, it is a smooth specimen), to obtain notched specimens (e.g. Figure 4 (As shown); then a uniaxial tensile test was performed on the notched specimen: the true stress-strain curve of the notched specimen was obtained, and the stress triaxiality R was calculated based on the geometry of the notched specimen. σ (like Figure 5 As shown below:

[0060]

[0061] Where R0 is the notch radius of the notched specimen, and a0 is the minimum cross-sectional radius of the notched specimen;

[0062] And at the same deformation temperature and strain rate, the ratio of the fracture strain of the notched specimen to the fracture strain of the smooth specimen is calculated as the stress state influence coefficient η.

[0063] After statistical analysis of the data under all deformation parameters, the stress state influence coefficient η can be obtained through data fitting:

[0064] η=[A exp(1.5R σ )+B] / 1.65A+B;

[0065] In the formula,

[0066]

[0067]

[0068] S2. Using the parameters determined in step S1, establish the metal thermoplastic deformation damage parameter d0, which includes the influencing factors of microstructure evolution. The process of solving the damage parameter d0 includes the following steps:

[0069] S2-1. Dynamic recrystallization and damage evolution during metal hot deformation can cause stress softening. Therefore, we first assume the existence of an ideal stress-strain behavior where neither damage nor dynamic recrystallization occurs, and under given deformation conditions, its stress is σ. W ;

[0070] S2-2、(1) When only dynamic recrystallization occurs, the resulting stress softening value is:

[0071] Δσ W-T =X dre (σ s -σ ss )=KX dre ;

[0072] In the formula, X dre The volume fraction of dynamic recrystallization; coefficient K = σ s -σ ss , σ s For saturation stress, σ ss If the stress is steady-state, then the effective stress is... (Corresponding to uniaxial compressive stress) is:

[0073]

[0074] (2) When only damage evolution occurs, the resulting stress softening value is:

[0075]

[0076] In the formula, ε i For damage initiation strain, ε fWhere η is the fracture strain and η is the stress state influence coefficient;

[0077] (3) When dynamic recrystallization behavior and damage evolution behavior coexist, the resulting stress softening value is Δσ. W-T +Δσ W-D Then the damage stress (corresponding to uniaxial tensile stress) during thermoplastic deformation of the metal is:

[0078]

[0079] S2-3. According to the theory of equivalent change assumption, the following relationship exists:

[0080]

[0081] In the formula, Where σ is the effective stress, σ is the damage stress, and d is the theoretical damage parameter;

[0082] The metal thermoplastic deformation damage parameter d0, which includes factors influencing microstructure evolution, is:

[0083]

[0084] In the formula, K is a coefficient, and X dre This refers to the volume fraction of dynamic recrystallization. The effective stress is η, the stress state influence coefficient is ε. i For damage initiation strain, ε f For fracture strain, For equivalent change;

[0085] S3. First, through secondary development of commercial finite element software, the damage parameter d0 determined in step S2 is integrated with the main program of the software. Taking Deform-2D finite element software as an example, the basic expression of the damage parameter d0 is written into the user subroutine file def_usr.f of Deform-2D finite element software using Fortran language. After compilation without errors, the executable file DEF_SIM.exe is generated. The newly generated DEF_SIM.exe is used to overwrite the file with the same name in the installation path of Deform-2D finite element software. Deform-2D finite element software is then run again to realize the synchronous simulation of metal thermal deformation and damage evolution.

[0086] Then, the Deform-2D finite element software, which has been developed and has embedded the d0 expression, is run. The experimental data obtained in step S1 is used as the boundary conditions to perform a synchronous simulation of the thermal deformation-damage evolution of the standard specimen uniaxial tensile test in step S1. In this way, the deformation and damage data of the specimen can be observed in the post-processing.

[0087] Finally, based on the uniaxial tensile test results of the standard specimen in step S1 and the synchronous simulation results of thermal deformation-damage evolution, the damage parameter d0 in step S2 is calibrated to determine the correction coefficient γ; the method for determining the correction coefficient γ is as follows:

[0088] When the simulated specimen and the test specimen have the same length at the fracture time under given deformation conditions, the damage value at the minimum cross-sectional outer radius of the standard simulated tensile specimen at that time is read, and its reciprocal is used as the correction coefficient corresponding to that set of deformation parameters. This process is repeated for all specimens to obtain the correction coefficient corresponding to the given deformation conditions. Through data fitting, the correction coefficient γ can be obtained as follows:

[0089]

[0090] S4. Correct the damage parameter d0 according to the correction coefficient determined in step S3 to obtain the corrected damage parameter d1.

[0091]

[0092] Replace the damage parameter d0 in step S2 with the modified damage parameter d1, and then repeat the secondary development process in step S3 to integrate d1 with the main program. Specifically, rewrite the user subroutine file def_usr.f, compile it without errors to generate the final executable file DEF_SIM.exe, and then overwrite the file with the same name in the Deform-2D finite element software installation path with the newly generated DEF_SIM.exe. This establishes a synchronous simulation platform for thermal deformation and damage evolution. Rerun the Deform-2D finite element software, inputting d1=1 as the failure criterion. This means that once an element meets this condition during the deformation simulation, it will be deleted, indicating that the corresponding material point has failed and a crack has appeared in the sample. In this way, the damage evolution and cracking prediction of the given material can be performed in subsequent thermal deformation simulations.

[0093] Figure 6 , Figure 7 The diagram shows the fracture state of the sample and the distribution cloud map of internal damage parameters during the simulated fracture process. The comparison between the simulated sample and the experimental sample proves that the method provided by this invention is real and feasible.

[0094] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for predicting damage and cracking in metal thermoplastic deformation considering microstructure evolution, characterized in that, Includes the following steps: S1. Within the range of thermal deformation parameters, the following operations are performed on the standard specimens using a thermal simulation testing device: 1) Uniaxial compression test: Obtain the true stress-strain curve of the standard compression specimen and determine the steady-state stress, saturation stress and dynamic recrystallization volume fraction; 2) Uniaxial tensile test: Obtain the true stress-strain curve of the standard tensile specimen to determine the damage initiation strain and fracture strain; 3) First, notches of different radii are pre-fabricated on several standard tensile specimens to obtain notched specimens; then, uniaxial tensile tests are performed on the notched specimens to obtain the true stress-strain curves of the notched specimens and determine the stress triaxiality and stress state influence coefficient. S2. Using the parameters determined in step S1, establish the metal thermoplastic deformation damage parameter d0, which includes factors influencing microstructure evolution: In the formula, K is a coefficient, and X dre This refers to the volume fraction of dynamic recrystallization. The effective stress is η, the stress state influence coefficient is ε. i For damage initiation strain, ε f For fracture strain, For equivalent change; S3. First, through secondary development of commercial finite element software, the damage parameter d0 determined in step S2 is integrated with the main program of the software; then, using the experimental data obtained in step S1 as the simulation boundary conditions, the uniaxial tensile test of the standard tensile specimen in step S1 is simulated synchronously with thermal deformation and damage evolution using the secondary developed finite element software; finally, the damage parameter d0 in step S2 is calibrated based on the uniaxial tensile test results of the standard tensile specimen in step S1 and the synchronous simulation results of thermal deformation and damage evolution, and the correction coefficient γ is determined. S4. Correct the damage parameter d0 according to the correction coefficient determined in step S3, and obtain the corrected damage parameter d1 as follows: The damage parameter d0 in step S2 is replaced by the modified damage parameter d1. Then, the secondary development process of step S3 is repeated to integrate d1 with the main program and establish a synchronous simulation platform for metal hot deformation-damage evolution. Then, "d1=1" is used as the judgment condition for failure and cracking and input into the secondary developed finite element software to realize the prediction of metal thermoplastic deformation damage evolution and cracking.

2. The method for predicting damage and cracking in metal thermoplastic deformation considering microstructure evolution according to claim 1, characterized in that: In step S1, the thermal deformation parameters include deformation temperature and deformation rate.

3. The method for predicting damage and cracking in metal thermoplastic deformation considering microstructure evolution according to claim 1, characterized in that: In step S1, the stress triaxiality R is calculated based on the geometric dimensions of the notched specimen. σ : Where R0 is the notch radius of the notched specimen, and a0 is the minimum cross-sectional radius of the notched specimen.

4. The method for predicting damage and cracking in metal thermoplastic deformation considering microstructure evolution according to claim 1, characterized in that: In step S2, the process of solving for the damage parameter d0 includes the following steps: S2-1. Dynamic recrystallization and damage evolution during metal hot deformation can cause stress softening. First, assume the existence of an ideal stress-strain behavior where neither damage nor dynamic recrystallization occurs, and under given deformation conditions, its stress is σ. W ; S2-2、(1) When only dynamic recrystallization occurs, the resulting stress softening value is: Board W-T =X dre (s s -s ss )=KX dre ; In the formula, X dre The volume fraction of dynamic recrystallization; coefficient K = σ s -σ ss , σ s For saturation stress, σ ss This is the steady-state stress; Then effective stress for: (2) When only damage evolution occurs, the resulting stress softening value is: In the formula, ε i For damage initiation strain, ε f Where η is the fracture strain and η is the stress state influence coefficient; (3) When dynamic recrystallization behavior and damage evolution behavior coexist, the resulting stress softening value is Δσ. W-T +Δσ W-D Then the damage stress during thermoplastic deformation of the metal is: S2-3. According to the theory of equivalent change assumption, the following relationship exists: In the formula, Where σ is the effective stress, σ is the damage stress, and d is the theoretical damage parameter; The metal thermoplastic deformation damage parameter d0, which includes factors influencing microstructure evolution, is: In the formula, K is a coefficient, and X dre This refers to the volume fraction of dynamic recrystallization. The effective stress is η, the stress state influence coefficient is ε. i For damage initiation strain, ε f For fracture strain, This is an equivalent change.

5. The method for predicting damage and cracking in metal thermoplastic deformation considering microstructure evolution according to claim 1, characterized in that: In step S3, the method for determining the correction coefficient γ is as follows: when the standard tensile simulation specimen and the standard tensile specimen have the same length at the fracture time under given deformation conditions, the damage value at the outer radius of the minimum cross section of the standard tensile simulation specimen at that time is read, and its reciprocal is used as the corresponding correction coefficient under the set of deformation parameters.

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