A method for predicting the life of prestressed cold extrusion dies based on finite element simulation

Abstract

The present invention discloses a method for predicting the life of a prestressed cold extrusion die based on finite element simulation, step one, establishing a three-dimensionally characterized die S-N curve and strain-cycle number curve of the cold extrusion die: on the basis of the cold extrusion die strain and stress as the X and Y axes respectively, establishing a Z axis perpendicular to the stress-strain coordinate to record the number of cycles N of die strikes, and representing the number of cycles N with a bar graph, and the projection of the bar graph in the stress-cycle number coordinate plane conforms to the characteristics of the cold extrusion die life S-N curve; step two, adding a standard prestressed part in the cold extrusion die of the cold extrusion die, so that the cold extrusion die obtains the prestress relative to the die strike, and then repeats step one, and step three, performs a finite element simulation experiment. The present invention can predict the life of the die, and then obtain the optimal design.

Description

Technical Field

[0001] The invention belongs to the field of extrusion technology, and specifically, is a method for predicting the life of a prestressed cold extrusion die based on finite element simulation. Background Art

[0002] The mold is a symbol of modern industrial development. It reflects the level of industrial development of a country. The performance of the mold, especially the service life, reflects the degree of modernization of industrial equipment and directly affects the upgrading and market competitiveness of products. Due to the extremely harsh working environment of the extrusion mold, it is not only manifested in the high contact pressure and severe friction, but also the cyclic changes in stress, strain and temperature caused by cyclic loading, which causes fatigue failure of the mold. An important practical problem in extrusion production is that the mold has a short life and high consumption, which generally accounts for about 20%-30% of the extrusion cost. Therefore, it is of great significance to study the failure law of the extrusion mold and analyze its failure mechanism to improve the service life of the extrusion mold, reduce production costs, and improve the market competitiveness of products. In addition to wear failure, the main form of mold failure is fatigue fracture failure of the mold.

[0003] The nominal stress method is the earliest method to study the fatigue life of molds. This method takes the nominal stress of the critical point of the part as the basic parameter, and then considers the influence of various factors to obtain a curve of the relationship between the stress level of the material and the fatigue life of the material. This curve is called the material SN curve. In the nominal stress method, the most important parameter is the nominal stress S of the critical section of the part. The factors that affect the fatigue life of the part are not only the nominal stress, but also various other factors that will also affect the service life of the part. In this way, as long as other influencing factors are substituted into the equation for calculation, the corrected nominal stress S' can be obtained. Such a parameter is directly linked to the service life of the mold, and the fatigue life of the mold can be estimated based on S'. The nominal stress method can estimate the service life of parts with low stress levels. The nominal stress method is simple and convenient, but it also has its inherent disadvantages. This method can only be used when the load is relatively stable. When the stress level of the mold is high during operation, the nominal stress method cannot be used. The reason for this result is that the nominal stress method does not take into account the plastic deformation of the material. In general, even if the part fails, its deformation is very small, but when the part undergoes plastic deformation, the error of the part life estimated using the nominal stress method is very large. Summary of the invention

[0004] The purpose of the present invention is to solve the problems mentioned in the background technology and to provide a method for predicting the life of a prestressed cold extrusion die based on finite element simulation. In this experimental study of the life of cold dies, the traditional SN curve is used as a starting point, and the thick-walled tube theory is applied to the design of the combined die to calculate the fatigue life of the die under various stress states, and finally obtain a three-dimensionally characterized SN curve of the die. On the basis of finite element simulation, actual extrusion tests are carried out on the dies with different numbers of reinforcements to obtain a curve showing the relationship between the actual life of the die and the number of reinforcements, thereby verifying the reliability of the model. Combining finite element simulation with die life estimation can predict the fatigue life of the die more closely to reality, so that fatigue life can truly be used as an important parameter for the quality and optimization design of the die.

[0005] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0006] A method for predicting the life of a prestressed cold extrusion die based on finite element simulation comprises the following steps:

[0007] Step 1, establish the die SN curve and strain-cycle number curve of the three-dimensional characterization of the cold extrusion die: on the basis of the cold extrusion die strain and stress as the X and Y axes respectively, establish a Z axis perpendicular to the stress-strain coordinate to record the number of cycles N of the die impact, and express the number of cycles N with a bar graph. The projection of the bar graph in the stress-cycle number coordinate plane conforms to the characteristics of the cold extrusion die life SN curve, and the projection of the bar graph in the strain-cycle number coordinate plane is the strain-cycle number curve;

[0008] Step 2, adding a standard prestressed member in the cold extrusion die of the cold extrusion die, so that the cold extrusion die obtains the prestress relative to the die strike, and then repeating step 1 until the number of standard prestressed members in the cold extrusion die of the cold extrusion die reaches a preset value;

[0009] Step 3: Establish a finite element simulation model, formulate parameters in the finite element software according to the SN curve and the strain-cycle number curve, and conduct a finite element simulation experiment;

[0010] Step 4: Predict the life of prestressed cold extrusion dies using data obtained from finite element simulation experiments.

[0011] To optimize the above technical solutions, the specific measures taken also include:

[0012] In step 1, based on the thick-walled cylinder theory, a mechanical model of the prestressed cold extrusion die in the cold extrusion process is established, and then the die SN curve and strain-cycle number curve of the three-dimensional characterization of the cold extrusion die are established, specifically:

[0013] Step 1.1 Establish the mechanical model of thick-walled cylinder theory, and according to the Lamé formula, obtain the shear stress σ at the radius r of the thick-walled cylinder: t and radial stress σ r The formula is as follows:

[0014]

[0015] The cold extrusion die can be regarded as a thick-walled cylinder, where r1 and r2 are the inner and outer diameters of the thick-walled cylinder respectively; p1 and p2 are the internal and external pressures on the cylinder;

[0016] Step 1.2 Establish the mechanical model of the prestressed cold extrusion die during the cold extrusion process, and obtain the tangential prestress σ t and radial prestress σ r The calculation formula is as follows:

[0017]

[0018] Because there is an interference between the cold extrusion die and the standard prestressed part, prestressing is generated When p1=0, Substituting into equation (1), we can obtain equation (2).

[0019] In step three, a finite element simulation model is established, and parameters are set in the finite element software according to the SN curve and the strain-cycle number curve. The specific steps for the finite element simulation experiment are as follows:

[0020] Step 2.1 Establish the finite element simulation model:

[0021] The cold extrusion die, standard prestressed component and blank model required in this simulated cold extrusion forming process are constructed through modeling software, and then imported into the finite element simulation software to obtain the required finite element simulation model; the parameters of the extrusion forming process are set in the finite element software, the blank is set as a plastic body, the cold extrusion die is set as an elastic-plastic body, the elastic modulus of the cold extrusion die is determined according to the strain-cycle number curve, the standard prestressed component is set as an elastic body, and the material properties, motion conditions, loading conditions and boundary conditions are set according to the experimental content;

[0022] Step 2.2 Finite element simulation experiment results:

[0023] Finite element simulation analysis is performed using simulation software according to the set process parameters and constructed model to obtain the distribution of the stress-strain field of the cold extrusion die during the working process. By simulating standard prestressed parts with different numbers, the stress-strain distribution of the cold extrusion die under different numerical prestresses is analyzed and studied.

[0024] The specific method for predicting the life of prestressed cold extrusion die by using the data obtained from finite element simulation experiment in step 4 is:

[0025] According to the stress-strain conditions of standard prestressed parts with different numbers, since the cold extrusion die is subjected to multi-directional stress, according to the Van-Mises yield law, the multi-directional stress of the cold extrusion die is converted into unidirectional stress, i.e., equivalent stress, using formula (3);

[0026]

[0027] After calculating the equivalent stress of the multi-directional stress of the cold extrusion die, the life of the cold extrusion die is estimated using the local stress-strain method. Formula (4) is the formula of the local stress-strain method:

[0028]

[0029] Among them, N f is fatigue life; Δσ is stress range, Δσ=σ max -σ min , σ max , σ min are the maximum and minimum stresses in the stress cycle, respectively; σ' f is the fatigue strength coefficient; b is the fatigue strength index;

[0030] The corresponding stress-strain-life three-dimensional data graph was drawn, and the three-dimensional SN curve graph of the cold extrusion die was obtained, so as to predict the life of the cold extrusion die under different numerical prestress.

[0031] The blank material is AL-2017, the cold extrusion die is AISI-4120, and the standard prestressed part material is H13.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) The present invention uses finite element simulation software to conduct simulation experiments. Through the simulation experimental data, the corresponding stress-strain-life three-dimensional SN curve diagram of the mold is drawn to discuss the structural rationality of the mold, predict the life of the mold, and then obtain the optimal design.

[0034] (2) The present invention uses prestressed parts to apply different initial prestresses to the cold extrusion die, and discusses the number of times the cold extrusion die can be recycled (die life) under different prestresses. The three-dimensional SN curve can be used to scientifically plan the inspection cycle of the die, so that fatigue life can truly be used as an important parameter for the quality and optimization of the die design. At the same time, it can guide the factory to plan the die replacement cycle, greatly improving the economic benefits of the forging factory.

[0035] (3) The present invention takes into account the deformation factor of the mold during the extrusion process through an original strain-cycle number curve, which is closer to reality and more accurate than the ordinary mold life prediction model. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is the SN curve of the three-dimensional mold.

[0037] Figure 2 This is a graph showing the variation of the inner diameter of the die under different numbers of prestressed rings. DETAILED DESCRIPTION

[0038] The embodiments of the present invention are further described in detail below with reference to the accompanying drawings.

[0039] To achieve the above purpose, the present invention designs a combined die model based on thick-walled tube theory and finite element analysis theory. The DEFORM finite element software can more accurately and intuitively reflect the stress and strain distribution of the cold extrusion die during operation. The position with the largest stress and strain value is found in the simulation results, which is the dangerous part of the die. The life can be estimated based on the stress and strain at this location.

[0040] Specifically, the present invention provides a method for predicting the life of a prestressed cold extrusion die based on finite element simulation, comprising the following steps:

[0041] Step 1, establish the die SN curve and strain-cycle number curve of the three-dimensional characterization of the cold extrusion die: on the basis of the cold extrusion die strain and stress as the X and Y axes respectively, establish a Z axis perpendicular to the stress-strain coordinate to record the number of cycles N of the die impact, and express the number of cycles N with a bar graph. The projection of the bar graph in the stress-cycle number coordinate plane conforms to the characteristics of the cold extrusion die life SN curve, and the projection of the bar graph in the strain-cycle number coordinate plane is the strain-cycle number curve;

[0042] Step 2, adding a standard prestressed member in the cold extrusion die of the cold extrusion die, so that the cold extrusion die obtains the prestress relative to the die strike, and then repeating step 1 until the number of standard prestressed members in the cold extrusion die of the cold extrusion die reaches a preset value;

[0043] Step 3: Establish a finite element simulation model, formulate parameters in the finite element software according to the SN curve and the strain-cycle number curve, and conduct a finite element simulation experiment;

[0044] Step 4: Predict the life of prestressed cold extrusion dies using data obtained from finite element simulation experiments.

[0045] To optimize the above technical solutions, the specific measures taken also include:

[0046] In step 1, based on the thick-walled cylinder theory, a mechanical model of the prestressed cold extrusion die in the cold extrusion process is established, and then the die SN curve and strain-cycle number curve of the three-dimensional characterization of the cold extrusion die are established, specifically:

[0047] Step 1.1 Establish the mechanical model of thick-walled cylinder theory, and according to the Lamé formula, obtain the shear stress σ at the radius r of the thick-walled cylinder: t and radial stress σ r The formula is as follows:

[0048]

[0049] The cold extrusion die can be regarded as a thick-walled cylinder, where r1 and r2 are the inner and outer diameters of the thick-walled cylinder respectively; p1 and p2 are the internal and external pressures on the cylinder;

[0050] Step 1.2 Establish the mechanical model of the prestressed cold extrusion die during the cold extrusion process, and obtain the tangential prestress σ t and radial prestress σ r The calculation formula is as follows:

[0051]

[0052] Because there is an interference between the cold extrusion die and the standard prestressed part, prestressing is generated When p1=0, Substituting into equation (1), we can obtain equation (2).

[0053] In step three, a finite element simulation model is established, and parameters are set in the finite element software according to the SN curve and the strain-cycle number curve. The specific steps for the finite element simulation experiment are as follows:

[0054] Step 2.1 Establish the finite element simulation model:

[0055] The cold extrusion die, standard prestressed component and blank model required in this simulated cold extrusion forming process are constructed through modeling software, and then imported into the finite element simulation software to obtain the required finite element simulation model; the parameters of the extrusion forming process are set in the finite element software, the blank is set as a plastic body, the cold extrusion die is set as an elastic-plastic body, the elastic modulus of the cold extrusion die is determined according to the strain-cycle number curve, the standard prestressed component is set as an elastic body, and the material properties, motion conditions, loading conditions and boundary conditions are set according to the experimental content;

[0056] Step 2.2 Finite element simulation experiment results:

[0057] Finite element simulation analysis is performed using simulation software according to the set process parameters and constructed model to obtain the distribution of the stress-strain field of the cold extrusion die during the working process. By simulating standard prestressed parts with different numbers, the stress-strain distribution of the cold extrusion die under different numerical prestresses is analyzed and studied.

[0058] The specific method for predicting the life of prestressed cold extrusion die by using the data obtained from finite element simulation experiment in step 4 is:

[0059] According to the stress-strain conditions of standard prestressed parts with different numbers, since the cold extrusion die is subjected to multi-directional stress, according to the Van-Mises yield law, the multi-directional stress of the cold extrusion die is converted into unidirectional stress, i.e., equivalent stress, using formula (3);

[0060]

[0061] After calculating the equivalent stress of the multi-directional stress of the cold extrusion die, the life of the cold extrusion die is estimated using the local stress-strain method. Formula (4) is the formula of the local stress-strain method:

[0062]

[0063] Among them, N f is fatigue life; Δσ is stress range, Δσ=σ max -σ min , σ max , σ min are the maximum and minimum stresses in the stress cycle, respectively; σ' f is the fatigue strength coefficient; b is the fatigue strength index;

[0064] The corresponding stress-strain-life three-dimensional data graph was drawn, and the three-dimensional SN curve graph of the cold extrusion die was obtained, so as to predict the life of the cold extrusion die under different numerical prestress.

[0065] The blank material is AL-2017, the cold extrusion die is AISI-4120, and the standard prestressed part material is H13.

[0066] The cold extrusion die of this embodiment is made of a combined concave die, and the standard prestressed part is made of a prestressed ring.

[0067] The simulation test results are processed to predict the mold life. Table 1 shows the change of the inner diameter of the die at the same position with the number of prestressed rings. Figure 2 This is a diagram of changes in the Neijing.

[0068] Table 1 Inner diameter of the die for different numbers of prestressed rings

[0069]

[0070] From the above data, it can be seen that as the number of prestressing rings increases, the value of the inner diameter of the die decreases continuously, so it is correct to increase the size of the prestress by increasing the number of prestressing rings.

[0071] The maximum stress of the inner wall of the die under the action of different numbers of prestressed rings is measured by the Deform post-processing module, and the obtained data are shown in Table 2.

[0072] Table 2 Stress values ​​of the inner wall of the die under different numbers of prestressed rings

[0073]

[0074] From the above data, we can see that when the number of prestressing rings is increased, the stress value of the inner wall of the die always fluctuates around -20MPa when the combined die is not working, and the stress value of the inner wall of the die cannot be significantly changed. Therefore, adding prestressing rings will cause prestress on the inner wall of the die, but the magnitude of the prestress will not change significantly with the increase in the number of prestressing rings.

[0075] The estimated results of predicting mold life using the three-dimensional SN curve method are shown in Table 3 below.

[0076] Table 3 Estimation results of die fatigue life

[0077]

[0078]

[0079] Establish a Z axis perpendicular to the stress-strain coordinate to record the number of cycles N of the mold strikes, and express the number of cycles (number of mold strikes) in a bar graph, such as Figure 1 As shown. The projection of the bar graph in the stress-cycle number coordinate plane conforms to the characteristics of the SN curve. For molds with different structures that produce the same product, we draw the corresponding three-dimensional data graph of stress-strain-cycle number, and we get the three-dimensional SN curve of the mold. Then we can also refer to the research method of the SN curve to discuss the structural rationality of the mold and predict the life of the mold. Generally, the life of a mold without prestress is shorter, which is reflected in the figure, corresponding to point 1; point 5 is the inflection point of the mold life. The prestressed structure design above the inflection point is likely to cause the mold life to be too short and increase the manufacturing cost of a single product, which is what we need to avoid; the prestressed structure design below the inflection point is an excess design, which we need to avoid as much as possible to reduce the manufacturing cost of the mold. In summary, the prestressed structure design near the inflection point is the optimal design we need to pursue.

[0080] The above are only preferred embodiments of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should be regarded as the protection scope of the present invention.

Claims

1. A method for predicting the life of a prestressed cold extrusion die based on finite element simulation, characterized in that: The following steps are involved: Step 1. Establish a three-dimensional characterization of the cold extrusion die SN curve and strain-cycle number curve: On the basis of the cold extrusion die strain and stress as the X and Y axes respectively, establish a Z axis perpendicular to the stress-strain coordinate to record the number of cycles N of the die strike, and express the number of cycles N with a bar graph. The projection of the bar graph in the stress-cycle number coordinate plane conforms to the characteristics of the cold extrusion die life SN curve, and the projection of the bar graph in the strain-cycle number coordinate plane is the strain-cycle number curve; In step 1, firstly, based on the thick-walled cylinder theory, a mechanical model of the prestressed cold extrusion die in the cold extrusion process is established, and then the three-dimensional characterization of the cold extrusion die SN curve and strain-cycle number curve are established, specifically: Step 1.1 Establish the mechanical model of thick-walled cylinder theory, and according to the Lamé formula, obtain the shear stress σ at the radius r of the thick-walled cylinder t and radial stress σ r The formula is as follows: The cold extrusion die is regarded as a thick-walled cylinder, where r1 and r2 are the inner and outer diameters of the thick-walled cylinder respectively; p1 and p2 are the internal and external pressures of the cylinder; Step 1.2 Establish the mechanical model of the prestressed cold extrusion die during the cold extrusion process and obtain the tangential prestress σ t and radial prestress σ r The calculation formula is as follows: Due to the interference between the cold extrusion die and the standard prestressed part, the prestress p2 is generated k , when p1=0,p2=p2 k Substituting into equation (1), we get equation (2); Step 2, adding a standard prestressed member in the cold extrusion die of the cold extrusion die, so that the cold extrusion die obtains the prestress relative to the die strike, and then repeating step 1 until the number of standard prestressed members in the cold extrusion die of the cold extrusion die reaches a preset value; Step 3: Establish a finite element simulation model, formulate parameters in the finite element software according to the SN curve and the strain-cycle number curve, and conduct a finite element simulation experiment; Step 4: Predicting the life of the prestressed cold extrusion die by using the data obtained from the finite element simulation experiment; The specific method for predicting the life of the prestressed cold extrusion die by using the data obtained from the finite element simulation experiment in step 4 is: According to the stress-strain conditions of standard prestressed parts with different numbers, since the cold extrusion die is subjected to multi-directional stress, according to the Van-Mises yield law, the multi-directional stress of the cold extrusion die is converted into unidirectional stress, i.e., equivalent stress, using formula (3); After calculating the equivalent stress of the multi-directional stress of the cold extrusion die, the life of the cold extrusion die is estimated using the local stress-strain method. Formula (4) is the formula of the local stress-strain method: Among them, N f is fatigue life; Δσ is stress range, Δσ=σ max -σ min , σ max , σ min are the maximum and minimum stresses in the stress cycle, respectively; σ' f is the fatigue strength coefficient; b is the fatigue strength index; The corresponding stress-strain-life three-dimensional data graph was drawn, and the three-dimensional SN curve graph of the cold extrusion die was obtained, so as to predict the life of the cold extrusion die under different numerical prestress.

2. According to the method for predicting the life of a prestressed cold extrusion die based on finite element simulation according to claim 1, it is characterized by: In step three, a finite element simulation model is established, and parameters are set in the finite element software according to the SN curve and the strain-cycle number curve. The specific steps for the finite element simulation experiment are as follows: Step 2.1 Establish the finite element simulation model: The cold extrusion die, standard prestressed component and blank model required in this simulated cold extrusion forming process are constructed through modeling software, and then imported into the finite element simulation software to obtain the required finite element simulation model; the parameters of the extrusion forming process are set in the finite element software, the blank is set as a plastic body, the cold extrusion die is set as an elastic-plastic body, the elastic modulus of the cold extrusion die is determined according to the strain-cycle number curve, the standard prestressed component is set as an elastic body, and the material properties, motion conditions, loading conditions and boundary conditions are set according to the experimental content; Step 2.2 Finite element simulation experiment results: Finite element simulation analysis is performed using simulation software according to the set process parameters and constructed models to obtain the distribution of the stress-strain field of the cold extrusion die during the working process. By simulating standard prestressed parts with different numbers, the stress-strain distribution of the cold extrusion die under different numerical prestresses is analyzed and studied.

3. According to the method for predicting the life of a prestressed cold extrusion die based on finite element simulation according to claim 2, it is characterized in that: The blank material only considers plastic deformation, the cold extrusion die considers elastic-plastic deformation, and the standard prestressed component material only considers elastic deformation.

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