AU2020103406A4 - Method and system for predicting residual stress of large aluminum alloy part - Google Patents
Method and system for predicting residual stress of large aluminum alloy part Download PDFInfo
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Abstract
The present invention relates to a method and system for predicting residual stress of a
large aluminum alloy part, belonging to the field of heat treatment and machining. The
present invention is put forward to solve the shortcoming that an existing system cannot
accurately predict residual stress with complex changes. The method includes: arranging
thermocouples in an aluminum alloy part, heating the aluminum alloy part to a solution
temperature, holding the temperature, quenching, and recording a temperature change
of the thermocouple; generating a temperature field change result of the aluminum alloy
part according to the temperature change of the thermocouples; applying the temperature
field change result to a stress field model in the quenching process for calculation to
obtain a quenching residual stress result; introducing the quenching residual stress result
into a molding die simulation system to obtain residual stress distribution after molding;
and predicting residual stress and deformation of the aluminum alloy part. The present
invention is suitable for a residual stress control and prediction system in the whole
manufacturing process of the large aluminum alloy part.
1/7
Step 1 Arrange thermocouples in an aluminum alloy part, heat
the aluminum alloy part to a solution temperature,
uniformly keep the temperature, quench the aluminum
alloy part, and record a temperature change of the
thermocouples; generating a temperature field change
result of the aluminum alloy part according to the
temperature change of the thermocouples
Step 2
Apply the temperature field change result to a stress field
model in the quenching process for calculation to obtain
a quenching residual stress result
Step 3
Introduce the quenching residual stress result into a
molding die simulation system to obtain residual stress
distribution after molding
Step 4 IF
Predict residual stress and deformation of the aluminum
alloy part
FIG. 1
Description
1/7
Step 1 Arrange thermocouples in an aluminum alloy part, heat the aluminum alloy part to a solution temperature, uniformly keep the temperature, quench the aluminum alloy part, and record a temperature change of the thermocouples; generating a temperature field change result of the aluminum alloy part according to the temperature change of the thermocouples
Step 2 Apply the temperature field change result to a stress field model in the quenching process for calculation to obtain a quenching residual stress result
Step 3 Introduce the quenching residual stress result into a molding die simulation system to obtain residual stress distribution after molding
Step 4 IF
Predict residual stress and deformation of the aluminum alloy part
FIG. 1
[0001] The present invention relates to a method and system for predicting residual stress of a large aluminum alloy part, and belongs to the field of heat treatment and machining.
[0002] Aluminum alloy has excellent comprehensive properties and is widely used in aerospace industry. In order to obtain high strength, aluminum alloy is usually subjected to the heat treatment process of solution quenching. During quenching treatment, great residual stress is introduced, which has great impact on the dimensional stability, stress corrosion performance, fatigue strength and other performance of parts. Due to the impact of blank residual stress and machining residual stress, great machining deformation occur easily, and machining deformation is a bottleneck of the manufacturing technology of aerospace structural parts. Controlling and predicting the residual stress in aluminum alloy blank is of great significance to reduce the machining deformation and improve the dimensional stability of parts. Many processes are involved in the whole production process from aluminum alloy blank to part processing. The evolution of residual stress is a continuous and systematic process. The complex change of residual stress brings difficulties to the control and optimization of subsequent processing deformation. The existing system cannot accurately predict the residual stress, and cannot effectively solve the problems existing in the process of machining deformation control and optimization.
[0003] Therefore, it is necessary to establish a residual stress control and prediction system in the whole manufacturing process of large aluminum alloy parts, so as to describe the stress evolution of large aluminum alloy parts continuously and reliably.
[0004] In a first aspect, the present invention provides a method for predicting residual stress of a large aluminum alloy part, including: arranging thermocouples in an aluminum alloy part, heating the aluminum alloy part to a solution temperature, holding the temperature, quenching the aluminum alloy part, and recording a temperature change of the thermocouples; generating a temperature field change result of the aluminum alloy part according to the temperature change of the thermocouples; applying the temperature field change result to a stress field model in the quenching process for calculation to obtain a quenching residual stress result; introducing the quenching residual stress result into a molding die simulation system to obtain residual stress distribution after molding; and predicting residual stress and deformation of the aluminum alloy part.
[0005] In a further aspect, the present invention provides a system for predicting residual stress of a large aluminum alloy part, including: a temperature field change calculation module, configured to generate a temperature field change result of the aluminum alloy part according to a temperature change of thermocouples; a quenching residual stress calculation module, configured to apply the temperature field change result to a stress field model in the quenching process for calculation to obtain a quenching residual stress result; a post-molding residual stress calculation module, configured to introduce the quenching residual stress result into a molding die simulation system to obtain residual stress distribution after molding; and a prediction module, configured to predict residual stress and deformation of the aluminum alloy part.
[0006] The present invention has the beneficial effects that the stress evolution of the large aluminum alloy part can be described continuously and reliably, the residual stress in the whole manufacturing process of the large part can be optimized and adjusted, and the machining deformation and dimensional stability of the part can be improved. The present invention can accurately predict the quenching residual stress of the aluminum alloy part and the residual stress of the aluminum alloy part after molding. Besides, in one example, a maximum deviation between a predicted value of and an experimental test value is 13%, which shows that the present invention can better predict the deformation of the aluminum alloy part.
[0007] FIG. 1 is a flowchart of a method for predicting residual stress of a large aluminum alloy part according to an example of the present invention;
[0008] FIG. 2 is a schematic diagram of a temperature measuring device in step 1 according to an example of the present invention, where there are thermocouples A, B, C, D, E and F arranged at different positions; a hexahedron is a sample of the large aluminum alloy part;
[0009] FIG. 3 is a flowchart of step 13 according to an example of the present invention;
[0010] FIG. 4 is a comparison diagram of heat transfer coefficients at different temperatures and different surfaces according to an example of the present invention;
[0011] FIG. 5 is a diagram of a comparison between a sample temperature change obtained by heat transfer calculation and an experimental test result according to an example of the present invention;
[0012] FIG. 6 is a diagram of a comparison between a residual stress simulation result and a test result of an aluminum alloy part after quenching according to an example of the present invention, where FIG. 6(a) is a prediction result diagram of residual stress of the aluminum alloy part and FIG. 6(b) is a test result diagram of residual stress of the aluminum alloy part;
[0013] FIG. 7 is a diagram of a comparison of a residual stress simulation result and a test result of an aluminum alloy part according to an example of the present invention after molding deformation compression by 2%, where FIG. 7(a) shows the residual stress prediction result of the aluminum alloy part, FIG. 7(b) shows the residual stress test result of the aluminum alloy part;
[0014] FIG. 8 is a flowchart of deformation prediction in step 4 according to an example of the present invention;
[0015] FIG. 9 is a predicted part deformation distribution diagram according to an example of the present invention;
[0016] FIG. 10 is a diagram of a comparison between an experimental value and a predicted value of the bottom surface deformation of the part in FIG. 9; and
[0017] FIG. 11 is a flowchart of another example of the present invention.
[0018] Example 1: As shown in FIG. 1, a method for predicting residual stress of a large aluminum alloy part in this example includes the following steps.
[0019] Step 1: Arrange thermocouples in an aluminum alloy part, heat the aluminum alloy part to a solution temperature, hold the temperature, quench the aluminum alloy part, and record the temperature change of the thermocouples; and generate a temperature field change result of the aluminum alloy part according to the temperature change of the thermocouples.
[0020] The schematic diagram of arranging the thermocouples in the aluminum alloy part is shown in FIG. 2. In FIG. 2, a cube indicates an aluminum alloy part sample, the thermocouples are inserted in different positions of a temperature measuring device, and a cooling curve in quenching process can be drawn through temperature measurement results of the thermocouples.
[0021] Step 2: Apply the temperature field change result to a stress field model in the quenching process for calculation to obtain a quenching residual stress result.
[0022] Step 3: Introduce the quenching residual stress result into a molding die simulation system to obtain residual stress distribution after molding. The molding die simulation system is a software simulation program for simulating the molding process. The simulation process and result are implemented through calculation according to data by the program. The present invention does not improve the process of software simulation, so the molding die simulation system can use the existing simulation program.
[0023] Step 4: Predict residual stress and deformation of the aluminum alloy part.
[0024] Example 2: This example differs from Example 1 in that:
[0025] Stress evolution during quenching is usually caused by uneven distribution of thermal strain caused by uneven distribution of a transient temperature field in the aluminum alloy part during quenching. The prediction of quenching stress requires accurate temperature field calculation. The accurate calculation of the part temperature field depends on the calculation of a heat transfer coefficient of the quenching surface. The temperature field calculation (step 1) in the quenching process specifically includes the following sub steps.
[0026] Step 11: As shown in FIG. 2, arrange thermocouples at a plurality of preset positions in the aluminum alloy part respectively, connect the thermocouples to a data acquisition system, place the aluminum alloy part in a heating device, heat to a solution temperature, hold the temperature, put, with a transfer time less than 2 s, the aluminum alloy part into a medium for quenching, where the quenching medium is at a temperature of Tc, and record the temperature change of the thermocouples through the data acquisition system.
[0027] Step 12: Establish a three-dimensional heat conduction differential equation in a rectangular coordinate system,
PT _ (T J+ T (a'OT a(T at a.4jax) ay y )y) Oy Oz Oz
where c is specific heat capacity; p is density; A is a thermal conductivity coefficient; t is time; and T is temperature.
Initial conditions are:
T |=0= T (x,y, z)
boundary conditions are:
-A Q=e(T, -T)
where the subscript s is the boundary range of the part; k is a comprehensive heat transfer coefficient; Twis the boundary temperature of the part; and Tc is the temperature of the medium.
[0028] Step 13: based on a finite element calculation method, divide a finite element mesh of the aluminum alloy part; as shown in FIG. 3, select an initial value of the
comprehensive heat transfer coefficient k, use a finite element method to calculate a temperature field of a next time step at a current measurement position, calculate a difference between a calculated theoretical temperature and a measured temperature, then use a genetic algorithm to optimize a calculated new heat flux density, calculate a new temperature field with the optimized heat flux density, determine whether the new temperature field meets a convergence criterion, and stop the calculation if the new temperature field meets the convergence criterion; and if not, continue iteration. Herein the initial value of the comprehensive heat transfer coefficient can be selected at will, and the selected initial value has little impact on the results, and mainly affects the convergence speed. The value of the heat transfer coefficient can be set with reference to other documents. In the calculation process of this step, the temperature field solving program can be written by Matlab for calculation. The purpose of step 13 is to calculate the theoretical formula provided in step 12 through an exact solution tool. That is, step 12 is the process of establishing a theoretical model, and step 13 is the process of concretely obtaining the calculation result through program statements.
[0029] To further illustrate this implementation, the heat transfer coefficients of different surfaces can be explained with reference to FIG. 4. FIG. 4 shows the heat transfer coefficients of an upper surface, a side surface and a lower surface at different temperatures. It can be seen that the heat transfer coefficients of each surface are different under the condition of three-dimensional heat transfer. In the calculation method of one-dimensional heat transfer, it is assumed that the heat transfer coefficients of all surfaces of samples are the same, which is different from the actual heat transfer of the part.
[0030] Step 14: Input the calculated surface heat transfer coefficient into finite element analysis software as the boundary condition to calculate a temperature field change of the large aluminum alloy part. The finite element analysis software may be ABAQUS software. A drawn curve of the temperature field change is the cooling curve. FIG. 5 is a diagram of a comparison between a sample temperature change obtained by heat transfer calculation and an experimental test result. It can be seen from the figure that the calculated result is in good agreement with the experimental test result, showing that the system can accurately calculate the temperature field in the sample.
[0031] Other steps and parameters are the same as those in Example 1.
[0032] Example 3: This example differs from Example 1 or 2 in that in step 13, the convergence criterion is:
minf(x)=min||AT||=min (7;-T') 2
T' where N is the number of test points; T is a measured value of an i-th step; and is a calculated value of the i-th step.
[0033] In the calculation process, when f(x) is less than a preset value, it is considered that the solving process is convergent. For example, when f(x) is less than a minimum (1x10-6), it is considered that the solving process is convergent, and the calculation ends.
[0034] Other steps and parameters are the same as those in Example 1 or 2.
[0035] Example 4: This example differs from one of Examples 1 to 3 in that:
[0036] Stress evolution during quenching is usually caused by uneven distribution of thermal strain caused by uneven distribution of a transient temperature field in the aluminum alloy part during quenching. Thermal deformation properties and mechanical and thermal parameters of materials are the data base and boundary conditions for computer simulation, and their accuracy determines the accuracy of prediction results. The stress calculation process (step 2) of the quenching process specifically includes the following sub steps.
[0037] Step 21: establishing the following formula:
£=Ap ufexpKQ) 1
=A2 expCu) expr(N) ~RT ) (2)
=A3 [sinh(ao)]" exp(
Z = expR = A[sinh(ao)]" Z RT Q) 4 where Al, A2, A3, nI, a, P and n are material constants, Q is deformation activation energy, T is an absolute temperature (K), R is a gas constant, g is a strain rate, a is flow stress, and Z is a Zener-Hollomon parameter.
[0038] Step 22: Define a constitutive equation of a material by a UHARD subroutine in finite element analysis software ABAQUS, where the UHARD subroutine contains four variables, namely SYIELD, HARD(l), HARD(2) and HARD(3) respectively, SYIELD is the yield stress of the material under isotropy, and HARD(), HARD(2) and HARD(3) are derivatives of the yield stress to strain, deformation temperature and deformation rate respectively. The specific process of defining the constitutive equation of the material specifically includes the following steps.
[0039] Solve the four variables contained in the UHARD subroutine by the following formulas:
1Z SYIELD = u = -sinh-1 a A(5) HARD(1)=0 (6)
HARD(2)= = sinh(a) exp a T aA3 -3njsinh 2(a) (7)
HARD(3)= a0] Q-sinh(aa) T anRT 1+sinh2 (a) (8) the parameters a, A 3 , Z, n and Q is capable of being obtained by high temperature compression experiments at different rates and different temperatures. Input SYELD, HARD(1), HARD(2) and HARD(3) into the finite element analysis software ABAQUS.
[0040] Step 23: Apply the temperature field result calculated in step 1 as the boundary condition into a stress field model in the quenching process for calculation to obtain a quenching residual stress result; where the stress field model of the quenching process is the existing model in the finite element analysis software ABAQUS.
[0041] Diagrams of a comparison between a residual stress simulation result and a test result of an aluminum alloy part after quenching according to this example are shown in FIG. 6(a) and FIG. 6(b). It can be seen that the simulation and prediction result is in good agreement with the experimental test result, showing that the system can accurately predict the quenching residual stress of the aluminum alloy part.
[0042] Other steps and parameters are the same as those in one of Examples 1 to 3.
[0043] Example 5: This example is different from one of Examples 1 to 4 in that the residual stress of the large aluminum alloy part blank can be effectively reduced by molding, and the deformation in the subsequent machining process can be effectively reduced. The simulation process (step 3) of the molding process of the large aluminum alloy part blank specifically includes the following sub steps.
[0044] Step 31: Introduce a molding die into the finite element analysis software ABAQUS, set parameters and dividing the mesh by a user through a human-computer interaction interface, and assemble the large aluminum alloy part blank in the software according to the assembly information of the die and the part in the molding process; and automatically perform all operations of workpiece finite element modeling by the simulation system.
[0045] Step 32: introducing a file formed by the quenching residual stress result calculated in step 2 into the mesh model of the large aluminum alloy part blank, applying mechanical boundary conditions to the part and the die according to molding machining parameters and loading working conditions, performing stress field analysis, and obtaining residual stress distribution after molding; where the molding machining parameters include a die fixing mode, a loading rate and loading displacement.
[0046] FIG. 7 shows a comparison of a residual stress simulation result and a test result of an aluminum alloy part after molding deformation compression by 2%. The simulation and prediction result is in good agreement with the experimental test result, showing that the system can accurately predict the residual stress of the aluminum alloy part after molding.
[0047] Other steps and parameters are the same as those in one of Examples 1 to 4.
[0048] Example 6: This example differs from one of Examples 1 to 5 in that:
[0049] Most of large aviation aluminum alloy parts are complex in structure and large in size, so it is difficult to implement automatic modeling, adaptive meshing and clamping positioning of any parts in software based on the prior art. In order to predict and compensate for the machining deformation of integral structural parts, a residual stress and deformation prediction system for typical integral structural parts is developed. The deformation analysis process (step 4) of the machining process specifically includes the following sub steps.
[0050] Step 41: Select, by the user, information such as a material removal area, a machining path and numerical control machining parameters of the aluminum alloy part through the human-computer interaction interface of the finite element analysis software
ABAQUS, and perform mesh division and boundary condition definition of the aluminum alloy part blank; and perform all operations of workpiece finite element modeling by the simulation system.
[0051] Step 42: Remove materials, specifically including: according to the machining path, removing the blank by Boolean operation and a remeshing function, so as to remove the materials in the whole machining process of the part.
[0052] Step 43: Automatically transfer a functional relationship between the initial residual stress of the blank and the machining residual stress by the ABAQUS.
[0053] Step 44: Select result information to be output through the human-computer interaction interface, and extract, output and save in real time, by the simulation system, information on a stress field, a strain field and deformation of the workpiece from a result file automatically.
[0054] FIG. 8 shows the flowchart of this example. The deformation of the part in the machining process is mainly caused by the residual stress release of the blank and the surface layer residual stress introduced by machining during the machining process with the removal of materials. Therefore, the deformation of the part is calculated by calculating the residual stress release of the blank, and then introducing the machining residual stress on the part surface and coupling the stress together.
[0055] The predicted part deformation distribution in this example is shown in FIG. 9, in which the deformation of the bottom surface is tested by a three-coordinate measuring instrument.
[0056] FIG. 10 is a diagram of a comparison between an experimental value and a predicted value of the bottom surface deformation in FIG. 9. It can be seen from the figure that the predicted deformation is in good agreement with the test result, and the maximum deviation between the predicted value and the experimental test value is 13%, which indicates that the deformation of the aluminum alloy part can be better predicted by using this system.
[0057] Other steps and parameters are the same as those in one of Examples 1 to 5.
[0058] Example 7: This example is implemented based on the combination of Examples 1 to 6, and the flowchart is shown in FIG. 11. Temperature field analysis in the quenching process corresponds to step 1, stress field analysis in the quenching process corresponds to step 2, stress field analysis in the molding process corresponds to step 3, and deformation analysis in the machining process corresponds to step 4.
[0059] Example 8 provides a system for predicting residual stress of a large aluminum alloy part, including: a temperature field change calculation module, configured to generate a temperature field change result of the aluminum alloy part according to a temperature change of thermocouples; a quenching residual stress calculation module, configured to apply the temperature field change result to a stress field model in the quenching process for calculation to obtain a quenching residual stress result; a post-molding residual stress calculation module, configured to introduce the quenching residual stress result into a molding die simulation system to obtain residual stress distribution after molding; and a prediction module, configured to predict residual stress and deformation of the aluminum alloy part.
[0060] The specific functions of each module are further described below.
[0061] The temperature field change calculation module is specifically configured to establish a three-dimensional heat conduction differential equation, and divide a finite element mesh of the aluminum alloy part based on a finite element calculation method;
select an initial value of the comprehensive heat transfer coefficient k, use a finite element method to calculate a temperature field of a next time step at a current measurement position, calculate a difference between a calculated theoretical temperature and a measured temperature, then use a genetic algorithm to optimize a calculated new heat flux density, calculate a new temperature field with the optimized heat flux density, determine whether the new temperature field meets a convergence criterion, and stop the calculation if the new temperature field meets the convergence criterion; and if not, continue iteration. The temperature field change calculation module is further configured to input the surface heat transfer coefficient obtained by iteration into finite element software as the boundary condition to calculate a temperature field change of the large aluminum alloy part. Since the temperature field change calculation module can automatically perform calculation, before the module executes calculation, it is necessary to manually arrange thermocouples at a plurality of preset positions in the aluminum alloy part respectively, the thermocouples are connected to a data acquisition system, the aluminum alloy part is placed in a heating device and heated to a solution temperature, the temperature is uniformly kept, and the aluminum alloy part is put with a transfer time less than 2 s into a medium for quenching, where the quenching medium is at a temperature of Tc, and the temperature change of the thermocouples is recorded through the data acquisition system.
[0062] The quenching residual stress calculation module is configured to establish formulas (1) to (4), define a constitutive equation of a material by a UHARD subroutine in finite element analysis software ABAQUS, where the UHARD subroutine contains four variables, namely SYIELD, HARD(1), HARD(2) and HARD(3) respectively, SYIELD is the yield stress of the material under isotropy, and HARD(1), HARD(2) and HARD(3) are derivatives of the yield stress to strain, deformation temperature and deformation rate respectively. The four variables contained in the UHARD subroutine are solved through formulas (5) to (8). The quenching residual stress calculation module is also configured to apply the temperature field change result calculated by the temperature field change calculation module as the boundary condition into a stress field model in the quenching process for calculation to obtain a quenching residual stress result; where the stress field model of the quenching process is the existing model in the finite element analysis software ABAQUS.
[0063] The post-molding residual stress calculation module is configured to introduce a molding die into the finite element analysis software ABAQUS, and assemble the large aluminum alloy part blank in the software according to the assembly information of the die and the part in the molding process and in accordance with manually set parameters and a divided mesh. The post-molding residual stress calculation module is configured to introduce a file formed by the quenching residual stress result calculated by the quenching residual stress calculation module into the mesh model of the large aluminum alloy part blank, apply mechanical boundary conditions to the part and the die according to molding machining parameters and loading working conditions, perform stress field analysis, and obtain residual stress distribution after molding; where the molding machining parameters include a die fixing mode, a loading rate and loading displacement.
[0064] The prediction module is configured to perform workpiece finite element modeling according to information such as a material removal area, a machining path and numerical control machining parameters of the aluminum alloy part selected by the user on the human-computer interaction interface of the finite element analysis software ABAQUS. The prediction module is further configured to remove the blank by Boolean operation and a remeshing function according to the machining path, so as to remove the materials in the whole machining process of the part. The prediction module is further configured to automatically transfer a functional relationship between the initial residual stress of the blank and the machining residual stress by the ABAQUS. The prediction module is further configured to extract, output and save in real time information on a stress field, a strain field and deformation of the workpiece from a result file according to the result information to be output selected by the user.
[0065] The principle of this example is similar to the principles described in Examples 1 to 7, with the difference that the modules in this example are all software program modules, which can automatically perform operations and calculations without manual execution. However, some steps need to be prepared manually before being executed by the modules. For example, the process of inserting the thermocouples into the workpiece mentioned above needs to be manually executed, while the program module is an automatic operation after the hardware layout is completed.
[0066] The present invention may also have many other examples. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, and these corresponding changes and modifications should belong to the protection scope of the appended claims of the present invention.
[0067] Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to mean the inclusion of a stated feature or step, or group of features or steps, but not the exclusion of any other feature or step, or group of features or steps.
Claims (5)
1. A method for predicting residual stress of a large aluminum alloy part, including: step 1: arranging thermocouples in an aluminum alloy part, heating the aluminum alloy part to a solution temperature, holding the temperature, quenching the aluminum alloy part, and recording a temperature change of the thermocouples; generating a temperature field change result of the aluminum alloy part according to the temperature change of the thermocouples; step 2: applying the temperature field change result to a stress field model in the quenching process for calculation to obtain a quenching residual stress result; step 3: introducing the quenching residual stress result into a molding die simulation system to obtain residual stress distribution after molding; and step 4: predicting residual stress and deformation of the aluminum alloy part.
2. A method for predicting residual stress of a large aluminum alloy part according to claim 1, wherein step 1 specifically includes: step 11: arranging thermocouples at a plurality of preset positions in the aluminum alloy part respectively, connecting the thermocouples to a data acquisition system, placing the aluminum alloy part in a heating device, heating to a solution temperature, holding the temperature, putting, with a transfer time less than 2 s, the aluminum alloy part into a medium for quenching, wherein the quenching medium is at a temperature of Tc, and recording the temperature change of the thermocouples through the data acquisition system; step 12: establishing a three-dimensional heat conduction differential equation in a rectangular coordinate system PT T 0 (T OT ) ( a T at axy x) a y Oy O ay) Oz OZ wherein c is specific heat capacity; p is density; A is a thermal conductivity coefficient; t is time; T is temperature; initial conditions are:
T |0= Ton(xdyiz) boundary conditions are: aT an wherein the subscript s is the boundary range of the part; k is a comprehensive heat transfer coefficient; Twis the boundary temperature of the part; Tc is the temperature aT of the medium; To is the temperature when t=; an denotes a temperature gradient; step 13: based on a finite element calculation method, dividing a finite element mesh of the aluminum alloy part; selecting an initial value of the comprehensive heat transfer coefficient k, using a finite element method to calculate a temperature field of a next time step at a current measurement position, calculating a difference between a calculated theoretical temperature and a measured temperature, then using a genetic algorithm to optimize a calculated new heat flux density, calculating a new temperature field with the optimized heat flux density, determining whether the new temperature field meets a convergence criterion, and stopping the calculation if the new temperature field meets the convergence criterion; if not, continuing iteration; and step 14: inputting the surface heat transfer coefficient obtained by iteration into finite element software as the boundary condition to calculate a temperature field change of the large aluminum alloy part.
3. A method for predicting residual stress of a large aluminum alloy part according to claim 2, wherein in step 13, the convergence criterion is:
minf(x)=min||AT||=min X(7-T') 2
wherein N is the number of test points; T is a measured value of an i-th step;T is a calculated value of the i-th step; AT denotes a difference between a measured temperature value and a calculated value; and in the calculation process, when f(x) is less than a preset value, it is considered that the solving process is convergent.
4. A method for predicting residual stress of a large aluminum alloy part according to claim 3, wherein step 12 specifically includes: step 21: establishing the following formula:
8=A 1 " exp(Q)
- = A 2 exp(flal exp (Q2 i = A 3 [sinh(au)]"expKRT)
Z=i exp(>=A3[sinh(aa)]" RT )4
wherein A,, A 2 , A3, n, a, # and n are material constants, Q is deformation activation energy, T is an absolute temperature (K), R is a gas constant, 8 is a strain rate, a is flow stress, and Z is a Zener-Hollomon parameter; step 22: defining a constitutive equation of a material by a UHARD subroutine in finite element analysis software ABAQUS, wherein the UHARD subroutine contains four variables, namely SYIELD, HARD(1), HARD(2) and HARD(3) respectively, SYIELD is the yield stress of the material under isotropy, and HARD(1), HARD(2) and HARD(3) are derivatives of the yield stress to strain, deformation temperature and deformation rate respectively; the specific process of defining the constitutive equation of the material includes: solving the four variables contained in the UHARD subroutine by the following formulas:
1 SYIELD= a=sinILh-L a A3
HARD(1)=0 (6)
ThA ( )0a0l 2 (2)= HARD 2 = inxa) - sinh(ao7) 1*xp ira-A3 -n1+sinh (a) 2 (7)
HARD(3)= a -Q.sinh(aa) HT anRT 1+sinh2 (au) (8) wherein the parameters a, A 3, Z, n and Q can be obtained by high temperature compression experiments at different rates and different temperatures, and SYIELD, HARD(1), HARD(2) and HARD(3) are input into the finite element analysis software ABAQUS; and step 23: applying the temperature field change result calculated in step 1 as the boundary condition into a stress field model in the quenching process for calculation to obtain a quenching residual stress result; wherein the stress field model of the quenching process is the existing model in the finite element analysis software ABAQUS; wherein step 3 specifically includes: step 31: introducing a molding die into the finite element analysis software ABAQUS, setting parameters and dividing the mesh by a user through a human-computer interaction interface, and assembling the large aluminum alloy part blank in the software according to the assembly information of the die and the part in the molding process; automatically performing all operations of workpiece finite element modeling by the simulation system; and step 32: introducing a file formed by the quenching residual stress result calculated in step 2 into the mesh model of the large aluminum alloy part blank, applying mechanical boundary conditions to the part and the die according to molding machining parameters and loading working conditions, performing stress field analysis, and obtaining residual stress distribution after molding; wherein the molding machining parameters include a die fixing mode, a loading rate and loading displacement; wherein step 4 specifically includes: step 41: selecting, by the user, information such as a material removal area, a machining path and numerical control machining parameters of the aluminum alloy part through the human-computer interaction interface of the finite element analysis software ABAQUS, and performing mesh division and boundary condition definition of the aluminum alloy part blank; and performing, by the simulation system, all operations of workpiece finite element modeling according to the information for setting completion; step 42: removing materials, specifically including: according to the machining path, removing the blank by Boolean operation and a remeshing function, so as to remove the materials in the whole machining process of the part; step 43: automatically transferring a functional relationship between the initial residual stress of the blank and the machining residual stress by the ABAQUS; and step 44: selecting result information to be output through the human-computer interaction interface, and extracting, outputting and saving in real time, by the simulation system, information on a stress field, a strain field and deformation of the workpiece from a result file according to the result information to be output.
5. A system for predicting residual stress of a large aluminum alloy part, including: a temperature field change calculation module, configured to generate a temperature field change result of the aluminum alloy part according to a temperature change of thermocouples; a quenching residual stress calculation module, configured to apply the temperature field change result to a stress field model in the quenching process for calculation to obtain a quenching residual stress result; a post-molding residual stress calculation module, configured to introduce the quenching residual stress result into a molding die simulation system to obtain residual stress distribution after molding; and a prediction module, configured to predict residual stress and deformation of the aluminum alloy part.
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