Large-size grain crushing and refining unified constitutive model and construction method thereof

Abstract

The invention belongs to the field of numerical analysis of plastic forming, and particularly discloses a large-size grain crushing and refining unified constitutive model and a construction method thereof.The construction method comprises the steps that a sample is subjected to a multi-pass thermal compression simulation experiment, and a true stress-strain curve and microstructure data, namely standard values, are obtained; establishing a large-size grain crushing and refining unified constitutive model which comprises a grain crushing critical condition, a grain crushing degree evolution model, an equivalent shear strain evolution model, a dislocation density evolution model and a grain size evolution model; the critical condition of grain crushing is as follows: when the accumulated shear strain exceeds the critical shear strain, the grains are crushed, and the accumulated shear strain is determined according to the direction change of the loading shaft; and constructing a target function based on the deviation between the standard value and the simulation value, and solving through a genetic algorithm to obtain unified constitutive model parameters. The invention provides a solution for microstructure prediction, mechanical property control and processing technology formulation of the large-size grain alloy hot working blank.

Description

Technical Field

[0001] The present invention belongs to the field of numerical analysis of plastic forming, and more specifically, relates to a large-size grain crushing and refinement unified constitutive model and a construction method thereof. Background Art

[0002] In the field of hot working, in order to prepare materials with fine grains and good mechanical properties, multi-directional forging is usually used for the processing of large-sized grain alloys. During deformation, the loading direction rotates continuously, and the billet is free-forged multiple times in different directions. The columnar large-sized grains are refined into equiaxed grains, and the mechanical properties can be significantly improved. In the initial hot mechanical processing of large-sized grains of alloys, the large deformation problems of multiple passes, multiple fires, and multi-axial compression under high forging ratios lead to complex and difficult-to-control material structure evolution, and inappropriate processing technology is very likely to produce mixed crystals. As the shear stress continues to break the grains, it will further promote dynamic recrystallization. In addition, there are static recovery and static recrystallization mechanisms in the insulation process between deformation passes. Billets with uniform structure and stable mechanical properties are the basis for all subsequent processing processes. Therefore, the prediction and control of microstructure evolution during hot working of large-sized grains is crucial.

[0003] In recent years, the combination of numerical simulation and microstructure modeling has provided an effective method for predicting microstructure evolution during thermal deformation. In microstructure models, models based on internal state variables have clear physical meanings and ideal calculation speeds, and have been widely used in materials such as steel, titanium alloys, aluminum alloys, and magnesium alloys. The current progress of internal variable models is mostly focused on fine equiaxed crystal materials with good organizational morphology and performance, and lacks modeling of large-size grain crushing. Different from the continuous unidirectional compression of fine grains, the processing of large-size grains is a cumulative multi-directional compression under complex path loading conditions, and there is a heat preservation stage between passes. Predicting and controlling the grain size changes during the initial processing of large-size grains is of great significance for the formulation of forging processes. Summary of the invention

[0004] In response to the above defects or improvement needs of the prior art, the present invention provides a unified constitutive model for large-size grain crushing and refinement and a construction method thereof, the purpose of which is to establish a unified constitutive model with organizational inheritance and material universality to measure the crushing and refinement of large-size grains during processing.

[0005] To achieve the above object, according to one aspect of the present invention, a method for constructing a unified constitutive model for large-size grain crushing and refinement is proposed, comprising the following steps:

[0006] Conduct multiple hot compression simulation tests on the samples to obtain the true stress-strain curve and microstructure data of the samples, i.e., standard values;

[0007] A unified constitutive model for large-size grain crushing and refinement is established, which includes a critical condition for grain crushing, a grain crushing degree evolution model, an equivalent shear strain evolution model, a dislocation density evolution model, and a grain size evolution model; the critical condition for grain crushing is: when the cumulative shear strain E exceeds the critical shear strain E c When , the grains are broken, and the cumulative shear strain E is determined according to the change in the direction of the loading axis;

[0008] The objective function is constructed based on the deviation between the standard value and the simulated value, and solved by genetic algorithm to obtain the parameters of the unified constitutive model for large-size grain crushing and refinement.

[0009] During the solution process, the method for determining the simulation value is: based on the finite element model, simulation is performed according to the current large-size grain crushing and refinement unified constitutive model to obtain the true stress-strain curve and microstructure data, that is, the simulation value; the finite element model is pre-constructed according to the sample size and loading conditions of the hot compression simulation experiment.

[0010] As further preferred, the calculation formula of the cumulative shear strain E is:

[0011] E=γ[f1-f2cos 2 (θ L )]

[0012]

[0013] Where γ is the equivalent shear strain, f1 and f2 are proportional coefficients, θ L is the angle between two adjacent loading axes, cos(θ L ) represents the change in the direction of the loading axis, l i , l i+1 are the loading axis direction vectors for the i-th pass and the i+1-th pass respectively.

[0014] As a further preferred embodiment, the grain crushing degree evolution model is expressed as:

[0015]

[0016]

[0017] in, is the grain breakage rate, N is the grain breakage volume fraction; H N is the grain refinement efficiency, h N0 , z are coefficients, n1, n2, n3 are exponents, is the shear strain reference value, is the equivalent shear strain rate, μ is a constant, d0 is the initial grain size, and d is the average grain size.

[0018] As further preferred, the equivalent shear strain evolution model is expressed as:

[0019]

[0020]

[0021]

[0022]

[0023] Among them, A is the material constant, sinh is the hyperbolic sine function, τ is the equivalent shear stress; K, B, C are the parameters that affect the stress change, k0 is the coefficient, Q k is the activation energy, R is the gas constant, T is the temperature; b0 is the material constant, is the normalized dislocation density change rate, is the normalized dislocation density; c0 is the amplitude coefficient, n0 is the index, and S is the recrystallized volume fraction.

[0024] As further preferred, the dislocation density evolution model is expressed as:

[0025]

[0026]

[0027] in, is the recrystallization volume fraction change rate, c1, c2, c3, x, H s is the material constant, n4, n5, n6 are exponents, is the critical value of dislocation density.

[0028] As further preferred, the grain size evolution model is expressed as:

[0029]

[0030]

[0031]

[0032]

[0033] in, is the average grain size change rate, is the grain coarsening rate, is the recrystallization grain growth rate, is the rate of change of grain size due to shear crushing; G1, G2, G3, α1, α2, β1, β2, β3 are all material constants, and p is the grain crushing constant.

[0034] As further preferred, the microstructure data includes a recrystallization volume fraction S, a grain crushing volume fraction N and an average grain size d.

[0035] As a further preferred method, a thermal compression simulation experiment is performed on the sample, comprising the following steps:

[0036] Determine multiple temperature and strain rate combinations, and for each temperature and strain rate combination:

[0037] Three identical samples were used to carry out hot compression simulation experiments along the X direction, XY direction, and XYZ direction, respectively, to obtain true stress-strain curves and microstructure data, and the X, Y, and Z directions are perpendicular to each other.

[0038] As a further preferred embodiment, the objective function is: calculating the mean square error between the standard value and the simulated value of the true stress-strain curve, and the mean square error between the standard value and the simulated value of the microstructure data, and taking the minimum sum of the two mean square errors as the objective function.

[0039] According to another aspect of the present invention, a large-size grain crushing and refinement unified constitutive model is provided, which is obtained by using the above-mentioned large-size grain crushing and refinement unified constitutive model construction method.

[0040] In general, the above technical solution conceived by the present invention has the following technical advantages compared with the prior art:

[0041] 1. The present invention constructs a unified constitutive model for determining the degree of grain crushing and dynamic recrystallization during the free forging process of large-sized grain alloys, and based on the standard values ​​obtained from the hot compression simulation experiment, the model parameters are efficiently and accurately calibrated through a genetic algorithm, solving the problem of the difficulty in selecting constitutive model parameters after changing the alloy material. The unified constitutive model can not only reflect the microstructural evolution of large-sized grains during multi-pass compression, but also describe the macroscopic flow behavior of the material, and realize the accurate prediction of large-sized grain crushing and refinement.

[0042] 2. The method of the present invention can be applied to multiple compression and insulation processes under different temperature and strain rate conditions, and provides a solution for the microstructure prediction, mechanical property control and processing technology formulation of hot-processed billets of large-size grain alloys.

[0043] 3. For the critical condition of grain crushing: By defining a new geometric parameter, namely the loading axis angle of the pass, the cumulative effect of the change in loading direction on the degree of grain shear is well identified, and the cumulative shear strain in multi-directional loading is quantified.

[0044] 4. For the grain crushing degree evolution model: By establishing a quantitative correlation between grain shear strain and grain crushing volume fraction, the grain shear crushing mechanism was introduced into the grain evolution model for the first time, and the degree of shear crushing of large-size grains was quantified.

[0045] 5. For the equivalent shear strain evolution model: the hyperbolic sine function combined with the thermal activation energy parameter is used as the main control equation. It is a typical thermo-mechanical coupling viscoplastic model, which is suitable for describing the thermal deformation behavior under a wide temperature and strain rate, and is suitable for multi-pass hot processing of large-sized grains of different materials under different deformation conditions. The forming process can be optimized through numerical simulation.

[0046] 6. For the dislocation density evolution model: By accumulating shear strain and grain breakage volume fraction, the contribution of grain breakage to the total dislocation density during grain refinement is taken into account, reflecting the indirect interaction between grain breakage and dynamic recrystallization, that is, after grain refinement, the average grain size decreases and the dislocation density increases, which will feed back into the calculation of the two themselves.

[0047] 7. For the grain size evolution model: By combining dynamic recrystallization and grain shear crushing mechanisms, the full-process accurate prediction of the degree of refinement of characteristic areas under multi-directional deformation paths is achieved, and the evolution of mechanical behavior and internal variables during alternating multi-directional thermal deformation and thermal insulation is simulated.

[0048] 8. The unified constitutive model constructed by the present invention is consistent with the multi-directional forging process: due to the use of cumulative hot compression experiments in three directions, three flow stress curves are obtained under each deformation condition, which can fully reflect the thermal deformation mechanical properties of the material and clarify the influence of the thermomechanical loading path and strain rate on the grain refinement efficiency. On this basis, the unified constitutive model parameters are calibrated so that the model can truly reflect the multi-pass loading process of the multi-directional forging process of large-sized grains. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is a flow chart of a method for constructing a unified constitutive model for large-size grain crushing and refinement according to an embodiment of the present invention;

[0050] Figure 2 This is a schematic diagram of the angle of the axis direction vector of multiple-pass loading according to an embodiment of the present invention;

[0051] Figure 3 It is a process diagram of the cumulative three-way compression method according to an embodiment of the present invention;

[0052] Figure 4 A comparison diagram of the cumulative three-dimensional flow stress experimental values ​​under different deformation conditions and the finite element simulation values ​​obtained in the embodiment of the present invention;

[0053] Figure 5A comparison diagram of the experimental value of the cumulative three-dimensional grain crushing volume fraction under different deformation conditions and the finite element simulation value obtained in the embodiment of the present invention;

[0054] Figure 6 It is a comparison chart of the experimental value of the cumulative average grain size under different deformation conditions and the finite element simulation value obtained in the embodiment of the present invention. DETAILED DESCRIPTION

[0055] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0056] The embodiment of the present invention provides a method for constructing a unified constitutive model for large-size grain crushing and refinement, such as Figure 1 As shown, the following steps are included:

[0057] (1) Conduct multiple hot compression simulation tests on the sample to obtain the true stress-strain curve and microstructure data of the sample, i.e., the standard value;

[0058] (2) Construct a unified constitutive model for large-scale grain crushing and refinement, including the critical conditions for grain crushing, the grain crushing degree evolution model, the equivalent shear strain evolution model, the dislocation density evolution model, and the grain size evolution model, and program the unified constitutive model;

[0059] (3) The objective function is constructed based on the deviation between the standard value and the simulated value, and the parameters of the unified constitutive model for large-size grain crushing and refinement are calibrated by solving it through a genetic algorithm. Specifically, based on the experimental conditions in step (1), a finite element model with the same sample size and loading conditions is established, and the unified constitutive model in step (2) is called for simulation to obtain the true stress-strain curve and microstructure data, i.e., the simulated value.

[0060] Furthermore, step (1) the thermal compression simulation experiment includes the following steps:

[0061] (1-1) Rectangular samples of the alloy were cut and hot compression tests were carried out at different temperatures and strain rates to obtain true stress-strain curves.

[0062] For each temperature and strain rate combination: at least three samples are required, and the three identical samples are subjected to thermal compression simulation experiments in the X direction, the XY direction, and the XYZ direction, respectively, to obtain the true stress-strain curve and microstructure data. That is, the three samples are first subjected to the first pass of isothermal thermal compression in the X direction; after quenching, the bulge of the sample is removed, leaving one sample for testing microscopic data, and the remaining two samples are subjected to the next pass of compression in the Y direction; after quenching, the bulge of the sample is removed, leaving one sample for testing microscopic data, and the last sample is subjected to the next pass of compression in the Z direction; the three directions of X, Y, and Z are perpendicular to each other, and the deformation of each pass is controlled at 30% to 40%.

[0063] (1-2) After the experiment, metallographic microstructure and EBSD analysis tests were carried out. The large grain size of the alloy was statistically observed by metallographic microscope, and the small dynamic recrystallization grain size was tested by EBSD to obtain microstructure data.

[0064] Furthermore, the unified constitutive model for large-size grain crushing and refinement includes:

[0065] (2-1) Critical conditions for grain crushing:

[0066] In multi-pass deformation, when the accumulated shear strain E exceeds the critical shear strain E c When the grains form shear bands, the small-angle grain boundaries become large-angle grain boundaries after sufficient accumulation, and the grains are broken.

[0067] The cumulative shear strain E is shown in formula (1):

[0068] E=γ[f1-f2cos 2 (θ L )] (1)

[0069] Where γ is the equivalent shear strain, parameter f represents the influence of multi-pass loading on the cumulative shear strain, f1 and f2 are proportional coefficients used to control the amplitude change of E; cos(θ L ) represents the change in the direction of the loading axis, that is, the angle θ between the loading axis direction vectors between passes L The cosine value of is shown in formula (2):

[0070]

[0071] The subscript i represents the number of loading passes, l i , l i+1 are the loading axis direction vectors for the i-th pass and the i+1-th pass respectively.

[0072] Taking the center of mass of the sample as the reference point, connect the plane perpendicular to the loading axis through the center of mass, and the obtained three-dimensional direction vector is the loading axis direction vector l, as shown in Figure 2 If the current loading direction is at an angle θ to the previous direction, L If the current loading direction is 0° or 180° with respect to the previous pass, it indicates that the stress state of the two passes is unidirectional tension and compression, and the accumulation degree of shear crushing strain E is low. L <180°, it means that the second pass brings greater shear deformation to the blank. When the inclination angle is 90°, the accumulation degree of shear crushing strain is the highest.

[0073] (2-2) Grain crushing degree evolution model:

[0074] Define the grain crushing rate function that describes the grain shear crushing mechanism The degree of shear crushing of large grains is quantified as shown in formula (3). Compared with temperature, strain rate has a greater impact on grain shear crushing, and its efficiency is expressed as η 0.01 >η 0.1 >η 0.001 The degree of shear crushing is affected by the grain size. The shear rate is related to time and shear strain rate. The normal distribution function H N To describe the grain refinement efficiency, as shown in formula (4).

[0075]

[0076]

[0077] Where N is the volume fraction of grain breakage, and the rate-form equation controls its evolution rate, so is the grain breakage rate function; h N0 , z are coefficients, n1, n2, n3 are exponents, is the shear strain reference value, is the equivalent shear strain rate, μ is a constant that controls the inflection point of the normal distribution function; d0 is the initial grain size, and d is the average grain size.

[0078] (2-3) Equivalent shear strain evolution model:

[0079] Considering the rate and temperature dependence, the hyperbolic sine function is sufficient to describe the strain rate sensitivity in a wide range. The shear strain rate and viscoplastic constitutive equation are shown in formula (5):

[0080]

[0081] Among them, A is the material constant, K is related to temperature and thermal activation energy, Considering the influence of temperature on stress, k0 is the coefficient, Q k is the activation energy, R is the gas constant, R = 8.314 J / mol / K, T is the temperature;

[0082] Parameter B represents the effect of dislocation density evolution on stress, as shown in formula (6):

[0083]

[0084] Parameter C is the yield stress of the material, which is related to the degree of dynamic recrystallization and grain size and is expressed as formula (7):

[0085]

[0086] Where b0 is the material constant, is the normalized dislocation density change rate, is the normalized dislocation density; c0 is the amplitude coefficient, n0 is the index, and S is the recrystallized volume fraction.

[0087] (2-4) Dislocation density evolution model:

[0088] The normalized dislocation density rate equation takes into account the contribution of grain size, work hardening, dynamic recovery, static recovery, and dynamic recrystallization to the total dislocation density. As the original large-sized grains are broken, the number of fine grain boundaries increases, and the dislocations are seriously entangled, resulting in an increase in dislocation density. Considering the contribution of grain breakage to the total dislocation density, the dislocation density evolution model is expressed as formula (8):

[0089]

[0090] Among them, c1, c2, c3 are material constants, n4, n5, n6 are exponents, is the recrystallization volume fraction change rate.

[0091] When the dislocation density reaches a critical value When given enough time at high temperature, recrystallization occurs, and its evolution equation is shown in formula (9):

[0092]

[0093] in, is the normalized dislocation density, which develops from the initial state to the saturation state during the hot working process; x is the material constant, H s is a temperature-dependent parameter and is related to the activation energy, is the critical value of dislocation density.

[0094] (2-5) Grain size evolution model:

[0095] The grain boundary migration during hot working mainly includes grain coarsening under the influence of heat and grain refinement under the action of recrystallization. The grain coarsening rate at high temperature can be expressed by formula (10):

[0096]

[0097] The recrystallization grain growth rate is related to the recrystallization volume fraction change rate and the average grain size, which can be expressed as formula (11):

[0098]

[0099] The grain size is related to the grain volume fraction. The shear crushed grain size d n The rate of change is expressed as formula (12):

[0100]

[0101] Among them, G1, G2, G3, α1, α2, β1, β2, and β3 are all material constants, and p is the grain crushing constant.

[0102] The evolution equation of the average grain size can be expressed as formula (13):

[0103]

[0104] (2-6) The above formulas (1) to (13) constitute a unified constitutive model for large-scale grain crushing and refinement. Furthermore, based on the ABAQUS-VUMAT subroutine module, the explicit forward Euler algorithm is used to compile the constitutive model into a program format using the Fortran language.

[0105] Furthermore, step (3) parameter calibration includes:

[0106] (3-1) When solving by genetic algorithm, the method for determining the simulation value is as follows: a finite element model of the same size as the hot compression sample is established in ABAQUS software, including a rectangular sample and a mold punch of the same size, and the same boundary conditions are assigned. The unified constitutive model subroutine of step (2-7) is used for calculation, and the punch support reaction force and displacement data of the simulation results are converted into true stress-strain curves. The internal variables and data of the central node of the calculation model are extracted, that is, the microstructure data including the recrystallization volume fraction, the grain breakage volume fraction, and the average grain size are used to determine the simulation value.

[0107] (3-2) The constitutive model parameters are calibrated by fitting through genetic algorithms. First, a set of calculable initial model parameters is given, and the simulation values ​​are calculated and extracted. The upper and lower bounds of the material parameters are set in the genetic algorithm, and the genetic generation and population number are set. The simulation results are imported into the genetic algorithm program, and the model parameters are selected, crossed, and mutated based on the sub-objective function to minimize the sum of square errors between the experimental data and the calculated data, and the appropriate parameters are matched. After the optimized parameters are generated, the finite element calculation program is updated and recalculated, and this cycle is repeated automatically. When the sum of square errors is small enough, all the material constants of the constitutive equation can be extracted to obtain the optimal model parameters.

[0108] The established objective function is shown as follows:

[0109]

[0110] in, is the minimum value of the sum of the mean square errors of all parameters, S is the recrystallization volume fraction, N is the grain breakage volume fraction, d is the average grain size, and σ is the true stress-strain curve; the subscripts i, j, and k represent the values ​​of the internal variables under different strains, different temperature deformation conditions, and different deformation passes, respectively; the superscripts c and e represent the simulation value and the experimental value, respectively.

[0111] The following are specific embodiments:

[0112] Taking the construction of unified constitutive model of cast Ti-6.4Cr-5.44Mo-5.14V-3.72Al (Ti-6554) alloy in the process of cumulative three-pass hot compression as an example, it includes the following steps;

[0113] (1) The as-cast Ti-6554 alloy was made into a rectangular sample of 8×10×12 mm and subjected to a solution treatment at 850℃+0.5h to make the alloy enter the pure β phase region. Figure 3 As shown, the first hot compression test is carried out on the Gleeble-3500 thermal simulation test machine along the 12mm long side, i.e., the X direction. The temperature is raised to the target temperature by resistance heating at 10°C / s. After the deformation is completed, it is immediately quenched in a vacuum environment. The same operation is performed at least 3 times. Take out the sample and keep one sample as the result of the first pass. Cut off the remaining sample bulge, and further perform the next compression along the current long side, i.e., the Y direction, and keep one sample as the result of the second pass. For the last remaining sample, the next hot compression is then carried out along the Z direction as the result of the third pass. The deformation of the three passes is controlled to be 30% of the longest side of the sample.

[0114] The three representative deformation conditions selected are: temperature 1050℃ and strain rate 0.1s -1 , temperature 1050℃ and strain rate 0.01s-1 , temperature 1150℃ and strain rate 0.001s -1 After the hot compression test under a certain deformation condition, three deformed samples and three load-displacement curves can be obtained. The samples are cut along the direction parallel to the compression, and the metallographic microstructure and EBSD analysis tests are performed, and the dynamic recrystallization volume fraction, grain size and other experimental data are statistically analyzed. The load-displacement curve is processed and converted into a true stress-strain curve.

[0115] (2) After determining the loading axis angle of the direction vector between passes, the constitutive model is programmed. Based on the ABAQUS-VUMAT subroutine module, the explicit forward Euler algorithm is used to compile the constitutive model into a program format using the Fortran language. A finite element model with the same size as the hot compression sample is established in the ABAQUS software, including a rectangular sample of 8×10×12 mm and upper and lower die punches. The elastic modulus, Poisson's ratio and other material constants are calculated by inputting the element content through the JmatPro software. For other material properties, any calculable parameters are first filled in, and then solved through subsequent steps. In addition, the same displacement boundary conditions as in the experiment are given, the loading time is set, the output results are set, and the model grid is divided. The inp. file of the finite element software is exported, and after using matlab for calculation, the punch support reaction force and displacement data of the simulation results are converted into true stress-strain curves. The internal variables (recrystallization volume fraction, grain crushing volume fraction, average grain size) data of the central node of the calculation model are extracted.

[0116] (3) Calibrate the constitutive model parameters through the Matlab genetic algorithm toolkit, set the upper and lower bounds of the material parameters in the genetic algorithm, and set the genetic generation and population number. After calculating the results of the first model according to the above step (2). Import the simulation results into the genetic algorithm program, and select, crossover, and mutate the model parameters based on the sub-objective function to minimize the sum of square errors between the experimental data and the calculated data, and compare and match the appropriate parameters. After generating the optimized parameters, update and replace the finite element inp. file, and recalculate. After the calculation is completed, export the results to the genetic algorithm for comparison. This process is automatically looped in Matlab. When the sum of square errors is small enough, all the material constants of the constitutive equation are extracted. Use this parameter to return to the ABAQUS finite element software to further obtain other data such as cloud maps.

[0117] Based on the principle of minimizing the mean square error of the genetic algorithm, the material parameters are calculated as shown in Table 1. Elastic modulus E Y , Poisson's ratio v and metal material density are all constants, and the density is 4620kg / m 3 , calculated by Jmatpro software, parameters p、d0、Ec0 ,f1,f2,ρ c0 All are empirical values, and the remaining parameters are obtained through genetic algorithm.

[0118] Table 1 Ti-6554 material parameters

[0119]

[0120] Curve contrast Figure 4 As shown in Figure 2, the correlation coefficient between the predicted value of the true stress-strain curve and the experimental value reaches 0.9969, and the average relative error is 4.93%, which proves that the present invention is an accurate and reliable numerical method. Figure 5 The average grain size is shown in Figure 6 As shown, it is found that the prediction results of the internal variables are in good agreement with the experimental results as the equivalent shear strain increases, which proves that the present invention has the functions of simulating the shear crushing of large-sized grains and predicting the evolution of microstructures.

[0121] In general, the large-size grain crushing and refinement constitutive model and parameter calibration method constructed by the present invention is an accurate and reliable numerical analysis method, which can provide solutions for the microstructure prediction, mechanical property control and processing technology formulation of large-size grain hot-processed blanks.

[0122] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for constructing a unified constitutive model for large-size grain crushing and refinement, characterized in that: The steps include: Conduct multiple hot compression simulation tests on the samples to obtain the true stress-strain curve and microstructure data of the samples, i.e., standard values; A unified constitutive model for large-size grain crushing and refinement is established, which includes a critical condition for grain crushing, a grain crushing degree evolution model, an equivalent shear strain evolution model, a dislocation density evolution model, and a grain size evolution model; the critical condition for grain crushing is: when the cumulative shear strain E exceeds the critical shear strain E c When , the grains are broken, and the cumulative shear strain E is determined according to the change in the direction of the loading axis; The objective function is constructed based on the deviation between the standard value and the simulated value, and solved by genetic algorithm to obtain the parameters of the unified constitutive model for large-size grain crushing and refinement. During the solution process, the method for determining the simulation value is: based on the finite element model, simulation is performed according to the current large-size grain crushing and refinement unified constitutive model to obtain the true stress-strain curve and microstructure data, that is, the simulation value; the finite element model is pre-constructed according to the sample size and loading conditions of the hot compression simulation experiment.

2. The method for constructing a unified constitutive model for large-size grain crushing and refinement according to claim 1, characterized in that: The calculation formula of the cumulative shear strain E is: E=γ[f1-f2cos 2 (i L )] Where γ is the equivalent shear strain, f1 and f2 are proportional coefficients, θ L is the angle between two adjacent loading axes, cos(θ L ) represents the change in the direction of the loading axis, l i , l i+1 are the loading axis direction vectors for the i-th pass and the i+1-th pass respectively.

3. The method for constructing a unified constitutive model for large-size grain crushing and refinement according to claim 2, characterized in that: The grain breakage degree evolution model is expressed as: in, is the grain breakage rate, N is the grain breakage volume fraction; H N is the grain refinement efficiency, h N0 , z are coefficients, n1, n2, n3 are exponents, is the shear strain reference value, is the equivalent shear strain rate, μ is a constant, d0 is the initial grain size, and d is the average grain size.

4. The method for constructing a unified constitutive model for large-size grain crushing and refinement according to claim 3, characterized in that: The equivalent shear strain evolution model is expressed as: Among them, A is the material constant, sinh is the hyperbolic sine function, τ is the equivalent shear stress; K, B, C are the parameters that affect the stress change, k0 is the coefficient, Q k is the activation energy, R is the gas constant, T is the temperature; b0 is the material constant, is the normalized dislocation density change rate, is the normalized dislocation density; c0 is the amplitude coefficient, n0 is the index, and S is the recrystallized volume fraction.

5. The method for constructing a unified constitutive model for large-size grain crushing and refinement according to claim 4, characterized in that: The dislocation density evolution model is expressed as: in, is the recrystallization volume fraction change rate, c1, c2, c3, x, H s is the material constant, n4, n5, n6 are exponents, is the critical value of dislocation density.

6. The method for constructing a unified constitutive model for large-size grain crushing and refinement according to claim 5, characterized in that: The grain size evolution model is expressed as: in, is the average grain size change rate, is the grain coarsening rate, is the recrystallization grain growth rate, is the rate of change of grain size due to shear crushing; G1, G2, G3, α1, α2, β1, β2, β3 are all material constants, and p is the grain crushing constant.

7. The method for constructing a unified constitutive model for large-size grain crushing and refinement according to claim 1, characterized in that: The microstructure data include the recrystallization volume fraction S, the grain crushing volume fraction N and the average grain size d.

8. The method for constructing a unified constitutive model for large-size grain crushing and refinement according to claim 1, characterized in that: The thermal compression simulation experiment of the sample includes the following steps: Determine multiple temperature and strain rate combinations, and for each temperature and strain rate combination: Three identical samples were used to carry out hot compression simulation experiments along the X direction, XY direction, and XYZ direction, respectively, to obtain true stress-strain curves and microstructure data, and the X, Y, and Z directions are perpendicular to each other.

9. The method for constructing a unified constitutive model for large-size grain crushing and refinement according to any one of claims 1 to 8, characterized in that: The objective function is: calculating the mean square error between the standard value and the simulated value of the true stress-strain curve, and the mean square error between the standard value and the simulated value of the microstructure data, and taking the minimum sum of the two mean square errors as the objective function.

10. A unified constitutive model for large-size grain crushing and refinement, characterized in that: The method for constructing a unified constitutive model for large-size grain crushing and refinement as described in any one of claims 1 to 9 is used to obtain the result.

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