Optimization Method for the Forming Process of Solid Oxide Fuel Cell Bipolar Plates

Abstract

The present invention discloses a method for optimizing the forming process of a solid oxide fuel cell bipolar plate, belonging to the technical field of process optimization of fuel cell bipolar plates. The method includes the steps of: establishing a stress-strain constitutive model of the bipolar plate based on the scale effect generated by the changes in plate thickness and grain size; establishing a finite element model for stamping the bipolar plate, setting the structural parameter, material property parameter and grain parameter of the bipolar plate, and obtaining a prediction model for stamping forming of the bipolar plate based on the scale effect generated by the changes in plate thickness and grain size; based on the established prediction model for stamping forming of the bipolar plate, numerically simulating the stamping forming process of the bipolar plate, and analyzing the influence rules of stamping process parameters and bipolar plate die parameters on the filling depth and thinning rate, so as to obtain a parameter range that can make the filling depth meet the requirements and no tearing occurs during the forming process. The prediction model for stamping forming of the bipolar plate of the present invention can accurately simulate the real stamping process to optimize the forming process of the bipolar plate.

Description

Technical Field

[0001] The present invention relates to the technical field of solid oxide fuel cell bipolar plate forming, and in particular to a method for optimizing the forming process of a solid oxide fuel cell bipolar plate. Background Art

[0002] Solid oxide fuel cells (SOFCs) can directly convert chemical energy into electrical energy, and their energy conversion efficiency is high. The power generation efficiency can reach up to 74%, far exceeding proton membrane fuel cells (45-55%) and phosphoric acid fuel cells (37-42%). At the same time, the reaction product is water, without CO. 2 and NO x The metal bipolar plate is one of the core components of SOFC, which plays the role of connecting battery cells in series and parallel, isolating fuel gas and air, and supporting. It accounts for 60% of the volume and is the key to ensuring the functionality of the battery stack. Bipolar plate manufacturing is a high-precision forming process from ultra-thin metal plates (0.05~0.1mm) to complex multi-channel structures. Among them, stamping is more competitive in terms of cost, productivity, and bipolar plate forming effect. The factors affecting stamping forming mainly include the following two aspects: First, the complex and changeable internal stress in the stamping forming process leads to defects such as springback, wall thickness reduction, and dimensional error in the metal bipolar plate, which is an important reason for the frequent failure accidents such as corrosion, high-temperature creep and fatigue of the bipolar plate during service, and becomes a key link in the quality control of the bipolar plate; especially for the ultra-thin metal plate formed by rolling, that is, the thickness of the plate does not exceed 0.1mm, and there are often only dozens of grains in the thickness direction. The difference in grain size and orientation leads to anisotropy, which aggravates the complexity of the internal stress of the ultra-thin plate; second, the stamping process parameters and stamping die parameters directly affect the bipolar plate forming process. If the parameters do not match, it is easy to cause tearing during the stamping process. At present, before the bipolar plate is stamped, a numerical simulation method is usually used to simulate the stamping deformation process of the bipolar plate model to guide the bipolar plate structure design. Among them, the material stress-strain constitutive equation can better describe the deformation behavior in the finite element simulation, but the existing stress-strain constitutive model does not consider the influence of the scale effect caused by the thickness and grain size changes of the ultra-thin plate, which often leads to excessive stress in the numerical simulation process, affecting the accuracy of the numerical simulation of stamping.

[0003] Based on this, the present invention proposes a calculation method for the stress-strain constitutive model of ultra-thin plates that takes into account the scale effects caused by changes in plate thickness and grain size, and optimizes the solid oxide fuel cell bipolar plate forming process based on the established stress-strain constitutive model that takes into account the scale effects caused by changes in plate thickness and grain size. Summary of the invention

[0004] To solve the above technical problems, the present invention provides an optimization method for the forming process of a solid oxide fuel cell bipolar plate, which optimizes the forming process of the solid oxide fuel cell bipolar plate based on the established stress-strain constitutive model considering the scale effect caused by the changes in plate thickness and grain size.

[0005] The technical solution adopted by the present invention is as follows:

[0006] The present invention provides an optimization method for the forming process of a solid oxide fuel cell bipolar plate, including the steps of:

[0007] S1. Establish a stress-strain constitutive model of the bipolar plate based on the scale effect caused by the changes in plate thickness and grain size;

[0008] S2. Use finite element software to establish a finite element model for the stamping forming of the bipolar plate, introduce the stress-strain constitutive model of the bipolar plate established in step S1, and set the structural parameter, material property parameter, and grain parameter of the bipolar plate to obtain a stamping forming prediction model of the bipolar plate based on the scale effect caused by the changes in plate thickness and grain size;

[0009] S3. Based on the stamping forming prediction model of the bipolar plate established in step S2, perform numerical simulation on the stamping forming process of the bipolar plate, analyze the influence rules of the stamping process parameters and the bipolar plate die parameters on the filling depth and thinning rate of the bipolar plate, and obtain a parameter range that can meet the requirements of the channel depth and does not cause tearing during the forming process.

[0010] Further, the specific content of step S1 is as follows:

[0011] S11. Based on the composite model, divide the flow stress of a single grain into two parts: the internal part of the grain and the grain boundary, and obtain the flow stress of a single grain composed of the internal flow stress of the grain and the flow stress of the grain boundary ;

[0012] S12. Based on the surface layer model, divide the flow stress of the bipolar plate material into two parts: the internal part and the surface layer, and obtain the flow stress of the bipolar plate material composed of the internal flow stress of the bipolar plate material and the flow stress of the surface layer ;

[0013] S13. Considering the influence of the scale effect caused by the changes in plate thickness and grain size on the internal stress of the bipolar plate material, divide the flow stress of the bipolar plate material into the flow stress dominated by plate thickness and the flow stress dominated by grain size , and establish a stress-strain constitutive model of the bipolar plate based on the scale effect caused by the changes in plate thickness and grain size.

[0014] Furthermore, the flow stress of the crystal grains in step S11 is calculated by the formula:

[0015] ;

[0016] The flow stress of the bipolar plate material in step S12 is calculated by the formula:

[0017] ;

[0018] That is:

[0019] ;

[0020] Where:

[0021] ;

[0022] ;

[0023] ;

[0024] That is, we get:

[0025] ;

[0026] In the formula, is the flow stress of a single crystal grain, is the intragranular flow stress of a single crystal grain, is the grain boundary flow stress of a single crystal grain, is the volume fraction inside the crystal grain, is the flow stress of the bipolar plate material, is the internal flow stress of the bipolar plate material, is the surface flow stress of the bipolar plate material, is the proportion of the number of crystal grains inside the bipolar plate material, is the grain boundary layer thickness; is the average grain size, is the number of crystal grains inside the bipolar plate material, is the number of crystal grains on the surface of the bipolar plate material, and are constants for specific materials.

[0027] Furthermore, the stress-strain constitutive model of the bipolar plate established in step S13 based on the scale effect caused by the changes in plate thickness and grain size is:

[0028] ;

[0029] Wherein:

[0030] ;

[0031] ;

[0032] ;

[0033] ;

[0034] ;

[0035] ;

[0036] ;

[0037] In the formula, is the flow stress dominated by grain size, is the flow stress dominated by plate thickness, is the proportionality factor of the grain size in the scale effect influence caused by the changes in grain size and plate thickness, and respectively represent the influence factors of the scale effect caused by the changes in grain size and plate thickness, , are respectively the intragranular flow stress and grain boundary flow stress of a single grain dominated by grain size, , are respectively the intragranular flow stress and grain boundary flow stress of a single grain dominated by plate thickness, is the yield strength dominated by grain size, is the yield strength dominated by plate thickness, is the average grain size, is the bipolar plate thickness, , , , are fitting parameters, is the proportion of the number of grains inside the bipolar plate material, and are constants for specific materials.

[0038] Furthermore, the intragranular flow stress and the grain boundary flow stress of a single grain dominated by grain size are obtained by substituting the data in the true stress-strain curves Ⅰ-1 and Ⅰ-2 of two bipolar plate specimens with the same plate thickness and different average grain sizes into the calculation formula of the flow stress of the bipolar plate material and solving the equations simultaneously;

[0039] The intragranular flow stress of a single grain dominated by plate thickness and the grain boundary flow stress By substituting the data in the true stress-strain curves Ⅱ-1 and Ⅱ-2 of two bipolar plate specimens with the same average grain size but different plate thicknesses into the calculation formula of the flow stress of the bipolar plate material in step S12 and solving them simultaneously.

[0040] Furthermore, the material property parameters in step S2 include elastic modulus, yield strength, and Poisson's ratio, and the elastic modulus and yield strength are obtained through uniaxial tensile tests.

[0041] Furthermore, the grain parameters in step S2 include the average grain size, and the grain parameters are obtained by testing the bipolar plate material with an EBSD electron microscope.

[0042] Furthermore, the geometric parameters of the bipolar plate die in step S3 include draft angle, fillet radius, channel width, channel depth, and rib width.

[0043] Furthermore, the stamping process parameters in step S3 include stamping speed and stamping pressure.

[0044] The beneficial effects of the present invention are as follows:

[0045] The present invention first establishes an ultra-thin plate stress-strain constitutive model considering the scale effect caused by the changes in plate thickness and grain size, and then optimizes the forming process of the solid oxide fuel cell bipolar plate based on the established stress-strain constitutive model considering the scale effect caused by the changes in plate thickness and grain size. It can more accurately simulate the stamping forming process of the bipolar plate, obtain a more accurate range of bipolar plate die structure parameters and stamping process parameters, and provide theoretical guidance for the stamping forming of the bipolar plate. Description of the Drawings

[0046] In order to clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0047] Figure 1 is the flowchart of the present invention;

[0048] Figure 2 is the stress nephogram of the numerical simulation of the uniaxial tensile test using the traditional stress-strain constitutive model and the stress-strain constitutive model established by the present invention;

[0049] Figure 3are the stress-strain curves obtained by numerically simulating the uniaxial tensile test using the traditional stress-strain constitutive model and the stress-strain constitutive model established in the present invention, and the true stress-strain curves obtained from the true uniaxial tensile test;

[0050] Figure 4 is a two-dimensional model of a bipolar plate with three channels established in finite element software;

[0051] Figure 5 are the response surface diagrams of the thinning rate with respect to different geometric parameters of the bipolar plate die; among them, (a) is the response surface of the draft angle and fillet radius with respect to the thinning rate, (b) is the response surface of the channel depth and draft angle with respect to the thinning rate, (c) is the response surface of the channel depth and channel width with respect to the thinning rate, (d) is the response surface of the channel depth and fillet radius with respect to the thinning rate, and (e) is the response surface of the rib width and fillet radius with respect to the thinning rate;

[0052] Figure 6 are the response surface diagrams of the filling depth with respect to different geometric parameters of the bipolar plate die; among them, (a) is the response surface of the fillet radius and draft angle with respect to the filling depth; (b) is the response surface of the fillet radius and channel width with respect to the filling depth, and (c) is the response surface of the fillet radius and rib width with respect to the channel depth;

[0053] Figure 7 are the pressure load curves on the upper surface of the bipolar plate during stamping at different stamping speeds. Detailed implementation manners

[0054] The present invention provides a method for optimizing the forming process of a solid oxide fuel cell bipolar plate. To make the objectives, technical solutions and effects of the present invention clearer and more definite, the present invention will be further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0055] The present invention will be described in detail below with reference to the accompanying drawings.

[0056] Refer to Figure 1 , this embodiment provides a method for optimizing the forming process of a solid oxide fuel cell bipolar plate, including the steps of:

[0057] S1. Establish a stress-strain constitutive model of the bipolar plate based on the scale effect generated by the changes in plate thickness and grain size;

[0058] S2. Use finite element software to establish a finite element model for the stamping forming of the bipolar plate, introduce the stress-strain constitutive model of the bipolar plate established in step S1, and set the structural parameter, material property parameter and grain parameter of the bipolar plate to obtain a prediction model for the stamping forming of the bipolar plate based on the scale effect generated by the changes in plate thickness and grain size;

[0059] S3. Based on the bipolar plate stamping forming prediction model established in step S2, numerically simulate the bipolar plate stamping forming process, analyze the influence laws of stamping process parameters and bipolar plate die parameters on the filling depth and thinning rate of the bipolar plate, and obtain the parameter range that can meet the requirements of the channel depth and prevent tearing during the forming process.

[0060] Specifically, the above step S1 includes the following steps:

[0061] S11. Based on the composite model, divide the flow stress of a single grain into two parts: the inside of the grain and the grain boundary, and obtain the flow stress of the grain composed of the intragranular flow stress and the grain boundary flow stress ;

[0062] S12. Based on the surface layer model, divide the flow stress of the bipolar plate material into two parts: the inside and the surface layer, and obtain the flow stress of the bipolar plate material composed of the internal flow stress and the surface layer flow stress , and the internal flow stress of the bipolar plate material is equal to the flow stress of the grain, and the surface layer flow stress of the bipolar plate material is equal to the intragranular flow stress ;

[0063] S13. Considering the influence of the scale effect caused by the changes in plate thickness and grain size on the internal stress of the bipolar plate material, divide the flow stress of the bipolar plate material into the plate thickness-dominated flow stress and the grain size-dominated flow stress , and obtain the stress-strain constitutive model of the bipolar plate based on the scale effect caused by the changes in plate thickness and grain size.

[0064] More specifically, the calculation process of obtaining the stress-strain constitutive model of the bipolar plate based on the scale effect caused by the changes in plate thickness and grain size in the above step S1 is as follows:

[0065] First, introduce the composite model and assume that a single grain consists of a hard grain boundary region and a soft intragranular region, and express the flow stress of a single grain as:

[0066] (1);

[0067] In the formula, is the flow stress of a single grain, is the intragranular flow stress of a single grain, is the grain boundary flow stress of a single grain, is the volume fraction inside the grains;

[0068] Assume that the average grain size is , and the grain boundary layer thickness is . The relationship between the two is described by the following formula:

[0069] (2);

[0070] In the formula, and are constants of specific materials. In this embodiment, they are the material constants of the stainless steel 304 used for the bipolar plate, taking 0.133 and 0.7 respectively;

[0071] Replace the grains with equivalent spheres. The grain size is equal to the diameter of the equivalent sphere. The volume of a single grain can be obtained as:

[0072] (3);

[0073] Get as:

[0074] (4);

[0075] Then the flow stress of the grains can be expressed as:

[0076] (5);

[0077] Secondly, introduce the surface layer model. Assume that the material consists of internal grains and surface layer grains. Divide the flow stress of the material into two parts, internal and surface layer, that is:

[0078] (6);

[0079] In the formula, is the flow stress of the bipolar plate material, is the internal flow stress of the bipolar plate material, is the surface layer flow stress of the bipolar plate material, is the proportion of the number of internal grains of the bipolar plate material;

[0080] The above-mentioned internal flow stress of the bipolar plate material can be expressed by when ignoring the surface grain boundary strengthening effect; the surface layer flow stress of the bipolar plate material is equal to the internal flow stress of the grains and can be expressed by . It can be obtained that:

[0081] (7);

[0082] Since the bipolar plate material used is a plate-shaped specimen, its overall volume is:

[0083] (8);

[0084] In the formula, is the short side length, is the long side length, is the plate thickness;

[0085] Approximating the surface layer of the bipolar plate material as half of the average grain size of the material, and the grains as uniformly arranged spheres, the number of surface layer grains can be calculated as:

[0086] (9);

[0087] (10);

[0088] (11);

[0089] (12);

[0090] (13);

[0091] In the formula, 、 、 are the approximate number of grains on the three types of surfaces (i.e., the upper and lower surfaces, the left and right surfaces, and the front and back surfaces) of the bipolar plate material respectively, is the number of grains in the surface layer of the bipolar plate material;

[0092] The number of grains inside the bipolar plate material is:

[0093] (14);

[0094] (15);

[0095] In the formula, is the number of grains inside the bipolar plate material, is the volume of the bipolar plate material after removing the surface layer;

[0096] In terms of the number of grains, the proportion of the number of grains inside the bipolar plate material can be obtained as:

[0097] (16);

[0098] Substituting Equation (4) into Equation (7) gives:

[0099] (17);

[0100] Substituting Equation (2) into Equation (17) gives the flow stress of the bipolar plate material as:

[0101] (18).

[0102] To more comprehensively consider the influence of the size relationship between the grain size and the plate thickness on the internal stress of the bipolar plate, first obtain the true stress-strain curves Ⅰ-1 and Ⅰ-2 of two bipolar plate material specimens with the same plate thickness and different average grain sizes, and the true stress-strain curves Ⅱ-1 and Ⅱ-2 of two bipolar plate material specimens with the same average grain size and different plate thicknesses through uniaxial tensile tests; then substitute the data of the plastic stage and the strengthening stage of the true stress-strain curves Ⅰ-1 and Ⅰ-2 into Equation (18), and solve the equations simultaneously to obtain a set of and , that is, the intragranular flow stress and the grain boundary flow stress of a single grain dominated by the grain size. Substitute the data of the plastic stage, the strengthening stage and the fracture stage of the true stress-strain curves Ⅱ-1 and Ⅱ-2 into Equation (18), and solve the equations simultaneously to obtain a set of and , that is, the intragranular flow stress and the grain boundary flow stress of a single grain dominated by the plate thickness. That is, a total of two sets of and are obtained by solving.

[0103] It should be noted that in the above solution process, it is assumed that for two bipolar plate material specimens with basically the same average grain size and different plate thicknesses, under the same strain , the intragranular flow stress of a single grain and the grain boundary flow stress of a single grain of the two specimens are the same; for two bipolar plate material specimens with different average grain sizes and the same plate thickness, under the same strain , the intragranular flow stress of a single grain and the grain boundary flow stress of a single grain of the two specimens are the same.

[0104] Through the above calculation and solution, a total of two sets of and of the known Equation (18) are obtained. Divide these two Equations (18) into Equation (19) with a fixed plate thickness but changing grain size and Equation (20) with a fixed average grain size but changing plate thickness, that is, the calculation formula (19) of the flow stress dominated by the grain scale and the calculation formula (20) of the flow stress dominated by the plate thickness are obtained:

[0105] (19);

[0106] (20).

[0107] In actual calculations, if only formula (19) or (20) is used for calculation, the obtained data is inaccurate. Therefore, the influence of grain size and plate thickness changes on stress is further introduced, and the overall stress of this bipolar plate is obtained as follows:

[0108] (21);

[0109] The above formula (21) is the stress-strain constitutive model of the bipolar plate based on the scale effect generated by plate thickness and grain size changes;

[0110] Where:

[0111] (22);

[0112] (23);

[0113] (24);

[0114] (25);

[0115] (26);

[0116] In the formula, is the flow stress dominated by grain size, is the flow stress dominated by plate thickness, is the proportionality factor of grain size in the scale effect influence generated by grain size and plate thickness changes, and respectively represent the influence factors of the scale effect generated by grain size and plate thickness changes, is the yield strength dominated by grain size, is the yield strength dominated by plate thickness, , , , are fitting parameters.

[0117] The above formulas (25) and (26) are linear unary equations related to yield strength, plate thickness, and grain size established according to the change rules that under micro / mesoscopic conditions, the flow stress of materials increases with the increase of plate thickness and decreases with the decrease of grain size, and can be approximated as a linear relationship.

[0118] The parameters , , , Specifically, the true stress-strain curves of several bipolar plate samples with different average grain sizes and the same plate thickness are obtained through uniaxial tensile tests, and the yield strength of each sample is calculated and fitted with the grain size to obtain the parameter , ; The true stress-strain curves of several bipolar plate samples with basically the same average grain size and different plate thicknesses were obtained through uniaxial tensile tests, and the yield strength of each sample was calculated and fitted with the plate thickness to obtain the parameters , .

[0119] In order to verify the accuracy of the bipolar plate stress-strain constitutive model based on the scale effect caused by the change of plate thickness and grain size established by the present invention, and to compare it with the traditional method, the traditional method refers to the traditional stress-strain constitutive model that does not consider the scale effect caused by the change of plate thickness and grain size. In this embodiment, SUS304 stainless steel is used for uniaxial tensile test to obtain the true stress-strain curve, and the traditional stress-strain constitutive model provided by ABAQUS is used for simulation calculation. At the same time, the bipolar plate stress-strain constitutive model based on the scale effect caused by the change of plate thickness and grain size of the present invention is applied to the VUMAT subroutine of ABAQUS for simulation calculation. Specifically, the specimens used in the real uniaxial tensile test and the simulation test are 0.1mm thick SUS304 stainless steel, and the stretching rate is 1mm / min. The simulations are performed respectively by the traditional stress-strain constitutive model and the stress-strain constitutive model of the present invention, and the simulated stress cloud diagrams are as shown below. Figure 2 As shown, Figure 2 (a) is the simulation result under the stress-strain constitutive model of the present invention, and (b) is the simulation result under the traditional stress-strain constitutive model; Figure 2 It can be seen that the stress cloud maps obtained by the two simulations have obvious differences in distribution. The stress concentration area in the stress cloud map simulated by the traditional method is too large and does not conform to the actual situation. The stress concentration area in the stress cloud map simulated by the method of the present invention is mainly concentrated in the middle area, which is close to the fracture position in the actual test. In addition, the corresponding simulated real stress-strain curve is also obtained in the above simulation process, such as Figure 3 As shown in the figure, and compared with the real stress-strain curve of the real uniaxial tensile test, it can be seen that the result simulated by the method of the present invention is closer to the real stress-strain curve obtained from the real uniaxial tensile test, while the calculated stress in the result simulated by the traditional method is obviously too high. This is because the traditional method does not fully consider the influence of the scale effect, which leads to such a result.

[0120] Specifically, the above step S2 is specifically as follows:

[0121] S21. The bipolar plate is set as a three-channel structure, and the three-channel two-dimensional model of the bipolar plate is established using ABAQUS finite element software, that is, the bipolar plate stamping forming prediction model, such as Figure 4 As shown;

[0122] S22, compiling the stress-strain constitutive model of the bipolar plate established in step S1 into a VUMAT subroutine, and introducing it into ABAQUS finite element software;

[0123] S23. In the ABAQUS finite element software, the thickness of the bipolar plate structure is set to 0.1 mm, the length is 12 mm, the width is assumed to be 50 mm, the bipolar plate material is 304 stainless steel, the physical parameters of the bipolar plate material including elastic modulus, yield strength, and Poisson's ratio are set, and the grain parameters including the average grain size are set, wherein the elastic modulus and yield strength can be obtained by uniaxial tensile test, and the average grain size can be obtained by EBSD electron microscopy;

[0124] S24. Create periodic boundary conditions and mutual contact, and perform meshing. Since the stamping process is a quasi-static process, the explicit smoothing analysis step is used for simulation.

[0125] Specifically, the above step S3 is specifically as follows:

[0126] In the ABAQUS finite element software, the bipolar plate stamping forming prediction model established in step S2 is used to numerically simulate the bipolar plate stamping forming process under different bipolar plate mold geometric parameters and different stamping process parameters to obtain the corresponding filling depth and thinning rate, and analyze the influence of the bipolar plate mold geometric parameters and stamping process parameters on the filling depth and thinning rate, so as to obtain the parameter range that can make the channel depth meet the requirements and no tearing occurs during the forming process.

[0127] It should be noted that the filling depth mainly affects the depth of the channel formed, and the thinning rate is mainly used to determine whether tearing occurs in the bipolar plate during the forming process. Usually, tearing occurs when the thinning rate exceeds 33.4%.

[0128] First, in step S3, the influence rules of the geometric parameters of the bipolar plate mold on the filling depth and thinning rate are studied first. The geometric parameters of the bipolar plate mold mainly include the draft angle α, the fillet radius R, the channel width W, the channel depth h, and the rib width S. The value range of the draft angle α is set to 5 - 25°, the value range of the fillet radius R is 0.1 - 0.3 mm, the value range of the channel width W is 1.2 - 1.5 mm, the value range of the channel depth h is 0.45 - 0.65 mm, and the rib width S is 0.7 - 1.2 mm. The above 5 mold parameters are designed into 45 groups of data with different value combinations for numerical simulation, and the filling depth and thinning rate corresponding to each numerical simulation are obtained.

[0129] Analysis of variance was carried out on the influence of the five mold parameters of the draft angle α, the fillet radius R, the channel width W, the channel depth h, and the rib width S and their combinations on the thinning rate, as shown in Table 1 below.

[0130] Table 1 Analysis of variance results of mold parameters on thinning rate

[0131]

[0132] It can be seen from Table 1 above that for the thinning rate, the contribution rates of the fillet radius R and the channel depth h are large, and the contribution rate of the fillet radius R is the most prominent.

[0133] Analysis of variance was carried out on the influence of the four mold parameters of the draft angle α, the fillet radius R, the channel width W, and the rib width S on the filling depth, as shown in Table 2 below.

[0134] Table 2 Analysis of variance results of mold parameters on filling depth

[0135]

[0136] It can be seen from Table 2 above that for the filling depth, the contribution rate of the fillet radius R is the largest, and the influence of other parameters can be ignored.

[0137] In addition, according to the above simulation results, in this embodiment, the response surface method is used to analyze the influence of different geometric parameters of the bipolar plate mold on the thinning rate, as Figure 5 shown. Among them, Figure 5 in (a) is the response surface of the draft angle and the fillet radius on the thinning rate, (b) is the response surface of the channel depth and the draft angle on the thinning rate, (c) is the response surface of the channel depth and the channel width on the thinning rate, (d) is the response surface of the channel depth and the fillet radius on the thinning rate, and (e) is the response surface of the rib width and the fillet radius on the thinning rate. Through Figure 5It can be known that increasing the draft angle, fillet radius, and channel width will result in a decrease in the thinning rate; while increasing the channel depth and rib width will increase the thinning rate. Therefore, if a deeper channel depth needs to be designed, it is necessary to prevent tearing by increasing the draft angle, fillet radius, and channel width and reducing the rib width.

[0138] In this embodiment, based on the above simulation results, the response surface method is also used to analyze the influence of different bipolar plate die geometric parameters on the filling depth, as Figure 6 shown. Among them, Figure 6 in (a) is the response surface of the fillet radius and draft angle to the filling depth; (b) is the response surface of the fillet radius and channel width to the filling depth, and (c) is the response surface of the fillet radius and rib width to the channel depth. Through Figure 6 it can be known that the fillet radius has a great influence on the filling depth. When the fillet radius increases, it helps to improve the filling depth; while the draft angle, channel width, and rib width have basically no influence on the filling depth.

[0139] Within the range of the bipolar plate die geometric parameters set in this embodiment, through numerical simulation and result analysis, the preferred range of each parameter is determined as follows: when a channel depth of more than 0.5 mm needs to be achieved, the draft angle α≥15°, the fillet radius R≥0.2 mm, the channel width W≥1.4 mm, and the rib width S≤1 mm.

[0140] Secondly, this step S3 also studies the influence law of stamping process parameters on the filling depth and thinning rate. The stamping process parameters mainly include the stamping speed and stamping pressure. Under the preferred range of the bipolar plate die geometric parameters determined above, the specific values of each parameter are determined as follows: the draft angle α is taken as 15°, the fillet radius R is taken as 0.3 mm, the channel depth is taken as 0.65 mm, the channel width W is taken as 1.5 mm, and the rib width S is taken as 1 mm. Under these bipolar plate die geometric parameters, the influence law of stamping process parameters on the filling depth and thinning rate is studied.

[0141] For the stamping speed, the stamping speed is controlled by the analysis step time during the punch pressing process. The analysis step time is set to 8 - 12 s for numerical simulation to obtain the pressure load on the upper surface of the bipolar plate during stamping, the corresponding filling depth, and the thinning rate at different stamping speeds. Among them, the pressure load curves on the upper surface of the bipolar plate during stamping at different stamping speeds are as Figure 7 shown, and the filling depth and thinning rate are shown in Table 3 below.

[0142] Table 3 Filling depth and thinning rate at different stamping speeds

[0143]

[0144] Through Figure 7It can be obtained from Table 3 that the fluctuation of the stamping speed has little effect on the magnitude of the pressure load on the bipolar plate surface, and the changes in the filling depth and thinning rate are also small.

[0145] For the stamping pressure, when the analysis step time of the punch pressing process is set to 10 s, the punch pressure load is adjusted to 400 - 750 MPa for numerical simulation, and the filling depth and thinning rate under different stamping pressures are obtained as shown in Table 4 below.

[0146] Table 4 Filling depth and thinning rate under different stamping pressures

[0147]

[0148] It can be obtained from Table 4 that when the stamping pressure is 400 - 500 Mpa, the filling depth is relatively small and the thinning rate is also small; when the stamping pressure is 600 - 700 Mpa, the filling depth is relatively large and the thinning rate is still small; when the stamping pressure reaches 750 Mpa, although the filling depth is large, the thinning rate is too large at this time and there is a great risk of fracture. Therefore, if a channel depth of 0.65 mm needs to be achieved, a stamping pressure of 700 Mpa is most suitable.

[0149] In this embodiment, an ultra-thin plate stress-strain constitutive model considering the scale effect generated by the changes in plate thickness and grain size is first established, and then the forming process of the solid oxide fuel cell bipolar plate is optimized based on the established stress-strain constitutive model considering the scale effect generated by the changes in plate thickness and grain size, which can more accurately simulate the stamping forming process of the bipolar plate, obtain a more accurate range of the best structural parameters and stamping process parameters of the bipolar plate, and provide a theoretical guidance for the stamping forming of the bipolar plate.

[0150] It should be noted that the parts not described in this invention can be implemented by adopting or referring to the existing technologies.

[0151] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. The changes, modifications, additions or substitutions made by those skilled in the art within the essence of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for optimizing the forming process of a solid oxide fuel cell bipolar plate, characterized in that: Includes steps: S1. Establish a stress-strain constitutive model of bipolar plates based on the scale effect caused by changes in plate thickness and grain size; S2. Using finite element software to establish a bipolar plate stamping finite element model, introducing the bipolar plate stress-strain constitutive model established in step S1, and setting the structural parameters, material property parameters and grain parameters of the bipolar plate, to obtain a bipolar plate stamping prediction model based on the scale effect caused by changes in plate thickness and grain size; S3. Based on the bipolar plate stamping forming prediction model established in step S2, the bipolar plate stamping forming process is numerically simulated, and the influence of the stamping process parameters and the bipolar plate mold parameters on the filling depth and thinning rate of the bipolar plate is analyzed to obtain the parameter range that can make the filling depth meet the requirements and no tearing occurs during the forming process.

2. A solid oxide fuel cell bipolar plate forming process optimization method according to claim 1, characterized in that: The step S1 is specifically as follows: S11. Based on the composite model, the flow stress of a single grain is divided into two parts: the grain interior and the grain boundary. and grain boundary flow stress The flow stress of a single grain ; S12. Based on the surface layer model, the flow stress of the bipolar plate material is divided into two parts: the internal part and the surface part. and surface flow stress Flow stress of bipolar plate materials ; S13. Considering the influence of the scale effect caused by the change of plate thickness and grain size on the internal stress of the bipolar plate material, the flow stress of the bipolar plate material is Grain size dominated flow stress and thickness-dominated flow stress , a stress-strain constitutive model of bipolar plates is established based on the scale effect caused by changes in plate thickness and grain size.

3. A solid oxide fuel cell bipolar plate forming process optimization method according to claim 2, characterized in that: The flow stress of the grains in step S11 The calculation formula is: （1）； The flow stress of the bipolar plate material in step S12 The calculation formula is: （6）； Right now: （7）； in: （4）； （16）； （2）； That is, we get: （18）； In the formula, is the flow stress of a single grain, is the intragranular flow stress of a single grain, is the grain boundary flow stress of a single grain, is the volume fraction inside the grain, is the flow stress of the bipolar plate material, is the internal flow stress of the bipolar plate material, is the surface flow stress of the bipolar plate material, is the ratio of the number of grains inside the bipolar plate material, is the grain boundary layer thickness; is the average grain size, is the number of grains inside the bipolar plate material, is the number of grains on the surface of the bipolar plate material, and is a constant of the bipolar plate material.

4. A method for optimizing a solid oxide fuel cell bipolar plate forming process according to claim 3, characterized in that: The stress-strain constitutive model of the bipolar plate based on the scale effect caused by the change of plate thickness and grain size established in step S13 is: （21）； in: （22）； （23）； （24）； （19）； （20）； （25）； （26）； In the formula, is the flow stress dominated by the grain size, is the flow stress dominated by the plate thickness, is the proportional factor of grain size in the scale effect caused by the change of grain size and plate thickness, and They represent the influencing factors of the scale effect caused by the change of grain size and plate thickness, , are the intragranular flow stress and grain boundary flow stress of a single grain dominated by grain size, , are the intragranular flow stress and grain boundary flow stress of a single grain dominated by plate thickness, is the yield strength dominated by the grain size, is the yield strength dominated by the plate thickness, is the average grain size, is the bipolar plate thickness, , , , is the fitting parameter, is the ratio of the number of grains inside the bipolar plate material, and is a constant of the bipolar plate material.

5. A method for optimizing a solid oxide fuel cell bipolar plate forming process according to claim 4, characterized in that: Grain size dominates the intragranular flow stress of a single grain and grain boundary flow stress By substituting the data of the plastic stage and the strengthening stage in the true stress-strain curves Ⅰ-1 and Ⅰ-2 of two bipolar plate samples with the same plate thickness and different average grain sizes into the flow stress of the bipolar plate material in step S12, The calculation formula (18) is solved together to obtain: Intragranular flow stress of a single grain dominated by plate thickness and grain boundary flow stress By substituting the data of the plastic stage, strengthening stage and fracture stage in the true stress-strain curves II-1 and II-2 of two bipolar plate samples with the same average grain size and different plate thickness into the flow stress of the bipolar plate material in step S12, The calculation formula (18) is solved together to obtain: In the solution process, it is assumed that two bipolar plate samples with different average grain sizes and the same plate thickness experience the same strain Under the same strain, the intragranular flow stress of a single grain and the grain boundary flow stress of a single grain of the two samples are the same; two bipolar plate samples with the same average grain size and different plate thicknesses undergo the same strain. Under the condition of , the intragranular flow stress of a single grain and the grain boundary flow stress of a single grain of the two specimens are the same.

6. The method for optimizing the forming process of a solid oxide fuel cell bipolar plate according to claim 1, characterized in that: The material property parameters in step S2 include elastic modulus, yield strength, and Poisson's ratio, and the elastic modulus and yield strength are obtained through a uniaxial tensile test.

7. The method for optimizing the forming process of a solid oxide fuel cell bipolar plate according to claim 1, characterized in that: The grain parameters in step S2 include an average grain size, and the grain parameters are obtained by testing the bipolar plate material using an EBSD electron microscope.

8. The method for optimizing the forming process of a solid oxide fuel cell bipolar plate according to claim 1, characterized in that: The bipolar plate mold parameters in step S3 include draft angle, fillet radius, channel width, channel depth, and rib width.

9. The method for optimizing the forming process of a solid oxide fuel cell bipolar plate according to claim 1, characterized in that: The stamping process parameters in step S3 include stamping speed and stamping pressure.

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