A forward optimization design method for a hollow cast subframe

Abstract

The present application belongs to the technical field of CAE simulation, and particularly relates to a forward optimization design method for a hollow cast auxiliary frame. According to the shape features of the hollow cast part, i.e. irregular shape and complex cavity, a series of different optimization algorithms are used for accurate shape description in the design process. The stiffness performance and the strength performance are decoupled into two stages in the whole design process. Since the weighted flexibility minimization and the mass minimization are used in the whole design process, and the fixed feature thickness t is used in the early conceptual design, the height and the width are solved under the feature, and the material is efficiently utilized, so that the whole design process has the effect of lightweight design.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of CAE simulation technology, in particular to a forward optimization design method for a hollow cast subframe. BACKGROUND

[0002] Reference Figure 1 , Figure 1 is a hollow cast aluminum subframe geometry example, the use of aluminum material and thin-walled cavity structure is beneficial to the lightweight of the part. In the development process of the subframe, the earlier the optimization calculation is involved, the earlier a reasonable general frame trend and contour can be determined, and the congenital deficiency of the structure can be avoided.

[0003] The common optimization design method for the current hollow cast aluminum subframe is to first determine the design space, then calculate the force transmission path through topology optimization, then draw a three-dimensional geometry according to the topology optimization result, and finally analyze the stiffness and durability of the part through finite element calculation. According to the calculation results and experience, modify and iterate until the requirements are met and the design is completed. The main shortcomings of this method are as follows: first, due to the characteristics of the algorithm itself, the results calculated by the current topology optimization program cannot well describe the shape characteristics of the hollow cast process part (complex cavity body), so there is a large difference between the calculated geometry and the actually drawn geometry, the precision is low, and a lot of work is needed after topology optimization to achieve the strength and durability performance; second, the topology optimization algorithm itself requires a large number of three-dimensional finite element meshes, and the topology optimization of the subframe usually requires hundreds of thousands of meshes, so the data volume and workload of modeling and calculation are relatively large. SUMMARY

[0004] The present application provides a forward optimization design method for a hollow cast subframe, which describes the shape more accurately in the design process with a series of different optimization algorithms according to the shape characteristics of the "irregular shape complex cavity body" of the hollow cast part. The stiffness performance and strength performance are decoupled into two stages in the entire design process, and the weighted flexibility minimization and mass minimization are used in the entire design process, and a fixed feature thickness t is used in the early conceptual design, so that the height and width are solved with the effect of efficient material utilization, so that the entire design process has the effect of lightweight design, and solves the above problems existing in the optimization design method of the existing hollow cast aluminum subframe.

[0005] The technical scheme of the present application is described in combination with the drawings as follows:

[0006] In a first aspect, the present application provides a forward optimization design method for a hollow cast subframe, comprising the following steps:

[0007] Step one, according to the suspension hard point arrangement and interface definition, design the force transmission path and structural frame of the subframe;

[0008] Step two, according to the force transmission path and structure frame of the subframe, the entire frame structure is discretized, and a simplified frame finite element model of the subframe is built by using beam elements with hollow rectangular cross sections;

[0009] Step three, determine the subframe conceptual frame design line;

[0010] Step four, determine the three-dimensional conceptual profile of the subframe;

[0011] Step five, draw the initial version of the conceptual three-dimensional geometry;

[0012] Step six, verify and iterate the stiffness and modal performance of the initial version of the conceptual three-dimensional geometry;

[0013] Step seven, determine the location and priority order of the process hole and weight reduction hole;

[0014] Step eight, draw the initial version of the detailed three-dimensional geometry;

[0015] Step nine, perform finite element strength and durability performance analysis and process feasibility analysis on the initial version of the detailed three-dimensional geometry data;

[0016] Step ten, design is completed.

[0017] Further, the specific method of step one is as follows:

[0018] The hard point arrangement includes the subframe front mounting point, rear mounting point, control arm connecting point, suspension connecting point and steering machine connecting point; the force transmission path requirements of all loads of the subframe are determined, and the components of the subframe are determined, the force transmission path of the subframe is designed according to the force transmission path requirements; the structure frame is designed according to the components of the subframe.

[0019] Further, the specific method of step two is as follows:

[0020] Build a simplified frame finite element model of the subframe by using beam elements with hollow rectangular cross sections: divide each part of the subframe into small sections, and replace each small section with a beam element; after the segmentation and discretization is completed, fix the thickness of all elements to a uniform characteristic thickness t, t is the minimum thickness that can be stably achieved in the entire field of casting process, the cross-sectional width W and height H of all beam elements are set to consistent statistical data, and the statistical data W and H are derived from the average value calculated from the cross-sectional dimensions of the existing vehicle subframe.

[0021] Further, the specific method of step three is as follows:

[0022] Take one side of the symmetric model, take the x, y, z coordinates of all nodes as design variables, take the size limit of the design space as constraint, take the weighted flexibility of all stiffness cases as target, calculate the position change of each node in the design space when the weighted flexibility is minimized, calculate the new position coordinates of each node from the original coordinates and coordinate change, and the line connected by the new positions of all nodes is the design line of the subframe conceptual framework.

[0023] Further, the specific method of step four is as follows:

[0024] Symmetrize the model determined in step three along the symmetry axis, fill in the units on the other side of the symmetry axis, and use the same size variable for each pair of left and right symmetric units; still fix the thickness of the beam unit as the characteristic thickness t, take the width Wi and height Hi of each beam unit as variables, take the stiffness target value and modal frequency target value of all stiffness cases as constraints, and perform mass minimization parameter optimization to calculate the new size of all beam units. The stepped discontinuous surface composed of all beam units of different widths and heights is the three-dimensional conceptual contour surface of the subframe.

[0025] Further, the specific method of step five is as follows:

[0026] For each component part of the subframe body, a continuous smooth curve, surface and three-dimensional entity are generated in sequence from the discrete three-dimensional conceptual contour surface; then two smooth surfaces are generated from two adjacent lines in the continuous smooth curve, and then a smooth surface is connected by a round corner to obtain a continuous cavity surface, which is taken as the outer surface of any component part of the subframe; the characteristic thickness t is pushed from the outer surface to the inner side of the cavity as the inner surface of any component part of the subframe; the hollow casting three-dimensional entity is obtained from the inner and outer surfaces of any component part of the subframe; each part of the subframe is sequentially smoothed, and then all parts are joined together to obtain the initial version of the conceptual three-dimensional geometry of the entire subframe.

[0027] Further, the specific method of step six is as follows:

[0028] Perform finite element stiffness and modal analysis on the initial version of the conceptual geometry to verify the deviation of the stiffness and modal performance of the conceptual geometry. If the deviation is within an acceptable range, no iteration is needed and the next step is directly entered. If the stiffness and modal deviation needs to be adjusted, the new target values of each case are reset according to the proportional relationship between the actual results and the original design target, and optimization iteration calculation is performed to obtain new discrete geometry, then new continuous geometry is drawn and verified by finite element, until the deviation meets the requirements, and the conceptual three-dimensional geometry that meets the requirements of stiffness and modal performance is obtained.

[0029] Further, the specific method of step seven is as follows:

[0030] The position that cannot be dug is set as a part not participating in the topology optimization, and the rest is set as a part participating in the topology optimization, and the feasible position of the hole is obtained by topology optimization of the conceptual three-dimensional geometry; different levels of mass percentages are set in sequence, and the priority order of the hole is obtained through the order and size change of the hole, so that the positions of the process hole and the weight-reducing hole are determined.

[0031] Further, the specific method of step eight is as follows:

[0032] According to the conceptual three-dimensional geometry, the positions of the process hole and the weight-reducing hole, and the size requirements of all interfaces on the subframe, the initial version of the detailed three-dimensional geometry data is drawn.

[0033] Further, the specific method of step nine is as follows:

[0034] The initial version of the detailed three-dimensional geometry data is subjected to finite element strength and durability performance analysis and process feasibility analysis, if the requirements are not met, fine adjustment and modification are carried out, for the positions not meeting the strength and durability performance, local fine adjustment is carried out through local thickening, local stiffening and internal surface through rib, including external surface local thickening, internal surface local thickening, external surface local simple stiffening, internal surface local simple stiffening, internal surface with upper and lower communication through rib; for the positions not meeting the process requirements, local adjustment is carried out by adjusting the pouring position and increasing the pouring channel, until the design structure meeting the requirements of strength and durability performance and process is obtained.

[0035] The beneficial effects of the present application are:

[0036] 1) The present application is a set of forward optimization method specially designed for complex cavity structure, which solves the problem that the results obtained by common optimization design method cannot well describe the shape characteristics of the hollow casting part;

[0037] 2) In the early stage of design, the present application does not need to draw complex geometric entities and divide tens of thousands of finite element grids, only uses a simple model, quickly finds out a reasonable trend and a complex cavity shape contour meeting the stiffness requirements through forward design optimization, and the calculation model only needs several hundred units, the calculation data is very small, and a large amount of calculation resources and design and simulation time can be saved;

[0038] 3) The present application can be applied in the initial stage of design, can predict whether the frame layout and shape trend are reasonable, and avoids the congenital deficiency of the structure layout;

[0039] 4) The present application can obtain a shape result meeting the requirements under different spaces by changing the position and size of the design space, weigh and compare the weights of the shape results under different design spaces, predict the influence of different space arrangements on the weight of the part in the initial stage of design, and help to provide reference data for determining a reasonable space arrangement;

[0040] 5) The application adopts a specified feature thickness t according to the mechanical characteristics of the different degrees of influence of the height, width and thickness of the cavity on the stiffness strength performance, and the height and width that meet the stiffness target are solved at this thickness to efficiently utilize the material and achieve the lightweight design effect;

[0041] 6) The stiffness design and strength durability design are carried out in stages in a similar "decoupling" manner, the stiffness target is achieved through the cavity cross-section size design in the early stage, and the strength durability target is achieved through local thickness fine-tuning in the later stage, which can improve the design efficiency, reduce the design difficulty, and reduce the design and simulation workload in the whole process. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments, and it should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope, and for those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0043] Figure 1 A hollow cast aluminum subframe geometric diagram;

[0044] Figure 2 A flowchart of the present application;

[0045] Figure 3 A subframe conceptual framework design line diagram;

[0046] Figure 4 A subframe three-dimensional conceptual profile surface generation process diagram;

[0047] Figure 5 A smooth geometry generation rule diagram. DETAILED DESCRIPTION

[0048] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application, and obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0049] It should be noted that similar reference numerals and letters refer to like items in the accompanying drawings, and once an item is defined in one drawing, it need not be further defined and explained in subsequent drawings. Meanwhile, in the description of the present application, the terms "first", "second", and the like are used only to distinguish descriptions, and cannot be understood as indicating or implying relative importance.

[0050] Figure 2 A flow chart of a forward optimization design method of a hollow cast subframe is provided for an embodiment of the present application. The embodiment can be applicable to the case of forward optimization design of a hollow cast subframe. The method can be executed by a forward optimization design device of a hollow cast subframe in the embodiments of the present application. The device can be realized in the form of software and / or hardware.

[0051] A forward optimization design method of a hollow cast subframe includes the following steps:

[0052] Step one, according to the suspension hard point arrangement and interface definition, design the force transmission path and structure frame of the subframe;

[0053] The specific method is as follows:

[0054] The hard point arrangement includes the front mounting point, rear mounting point, control arm connecting point, suspension connecting point, and steering machine connecting point of the subframe. The force transmission path requirements of all loads of the subframe such as vertical, longitudinal, lateral, suspension, and steering machine are clarified, and the components of the subframe are determined, such as whether to have front arm, rear arm, front cross beam, rear cross beam, left and right longitudinal beams, etc. Then, the force transmission path of the subframe is designed according to the force transmission path requirements. The structure frame is designed according to the components of the subframe.

[0055] Step two, according to the force transmission path and structure frame of the subframe, the entire frame structure is discretized, and a simplified frame finite element model of the subframe is built by using beam elements with hollow rectangular cross sections.

[0056] The specific method is as follows:

[0057] Beam element with hollow rectangular section is used to build the simplified frame finite element model of the subframe: each part of the subframe is divided into 2-4 small segments, each small segment is replaced by a beam element, for the part with long length or the part with sharp transition of shape, each small segment is divided into 2-4 sub-segments, each sub-segment is replaced by a beam element. For example: the front arm and the rear arm are respectively divided into three small segments of front, middle and rear, each small segment is divided into two sub-segments, and three* two beam elements are used to replace; the front and rear cross beams are divided into four small segments on one side of the symmetry plane, and four beam elements are used to replace on one side; the longitudinal beam is divided into three small segments of front, middle and rear, and then each of the three small segments is divided into three sub-segments, and three* three beam elements are used to replace; the ear piece connected with the control arm is divided into three small segments, and three beam elements are used to replace, etc. After the segmentation and discretization is completed, the thickness of all elements is fixed to a uniform feature thickness t, t is the minimum thickness that can be stably reached in the whole field of casting process, the cross-sectional width W and the height H of all beam elements are set to be consistent statistical data in the whole field, and the statistical data W and H are obtained by calculating the average value of the cross-sectional dimensions of the subframe of the existing vehicle model.

[0058] In some specific cases, other shapes of beam elements can be used, such as hollow circular, solid circular, solid rectangular, etc.

[0059] Step three, determine the subframe conceptual frame design line;

[0060] The specific method is as follows:

[0061] Take one side of the symmetric model, take the x, y, z coordinates of all nodes as design variables, take the size limit of the design space as constraint, take the weighted flexibility of all stiffness conditions as target, calculate the position change of each node in the design space when the weighted flexibility is minimized, calculate the new position coordinates of each node from the original coordinates and the coordinate change, and the line connected by the new positions of all nodes is the subframe conceptual frame design line, as shown in Figure 3 It can be seen from the figure that in X, Y and Z directions, L1 has different degrees of offset compared with L0. The advantage of using the conceptual frame design line is to determine a reasonable center line trend before three-dimensional modeling, which can balance and match all stiffness conditions, and can avoid excessive stiffness or small stiffness of a certain point in subsequent design.

[0062] Step four, determine the three-dimensional conceptual profile of the subframe;

[0063] The specific method is as follows:

[0064] The model determined in step three is symmetrized along the symmetry axis, and the units on the other side of the symmetry axis are completed, and the same size variable is used for each pair of left and right symmetrical units to ensure that the final shape is left and right symmetrical. The thickness of the beam unit is still fixed as the characteristic thickness t, and the width Wi and the height Hi of each beam unit are used as variables, and the stiffness target value and the modal frequency target value of all stiffness working conditions are used as constraints to perform mass minimization parameter optimization, and the new size of all beam units is calculated. The stepped discontinuous surface composed of beam units of all different widths and heights is the three-dimensional conceptual contour surface of the subframe, as shown in Figure 4 The three-dimensional conceptual contour surface uses very few units, has very small calculation amount, and discretely describes the part shape size that meets the stiffness requirement, and the approximate shape of the final part can be seen. Since the thickness, height and width of the cavity structure have different degrees of influence on the stiffness and strength performance of the cavity, within a certain range, the "thinning and heightening" of the cavity can improve the "use efficiency" of the material to achieve the same stiffness and strength performance with lighter mass. Therefore, fixing the thickness as the stable minimum value t that can be achieved by the process can ensure that the material is almost "used most efficiently", and the benefits are to find a cavity structure within the design space that does not interfere with surrounding parts, is easy to stabilize and realize by the process, meets the stiffness target, and has a good lightweight effect.

[0065] At this time, if the design space arrangement needs to be analyzed, the position and size limit of the design space can be changed, and optimization calculation under different design spaces can be performed to obtain different three-dimensional conceptual contour surfaces. By comparing the weights and shapes of different conceptual contour surfaces, the influence degree of different design space arrangements on the subframe can be obtained. The benefits of this are that the influence analysis and prediction of space arrangement can be performed with very small calculation amount before drawing the part geometric data, which can provide data reference for determining a reasonable space arrangement scheme.

[0066] Step five, draw the initial version of the three-dimensional conceptual geometry;

[0067] The specific method is as follows:

[0068] For each component part of the subframe main body, a continuous smooth curve, surface and three-dimensional entity are sequentially generated from the discrete three-dimensional conceptual contour surface according to the given method. Figure 5is an example of a subframe rear arm, each beam unit outer layer corner point is marked as N11, N12, N13, N14, N21, N22, N23, N24, N31, N32, N33, N34, N41, N42, N43, N44, respectively, and the series of discrete corner points N11-N12-N13-N14 are connected with a smooth curve to obtain a smooth curve L1, and the same method is used in turn to obtain smooth curves L2, L3, L4. Then, two adjacent lines (L1 and L2, L2 and L3, L3 and L4, L4 and L1) from the four curves are used to generate four smooth surfaces, respectively, and then a fillet of appropriate size is used to connect the four smooth surfaces to obtain a continuous cavity surface, which is used as the outer surface of the subframe rear arm. The inner surface of the subframe rear arm is obtained by pushing the characteristic thickness t from the outer surface to the inside of the cavity. The hollow casting three-dimensional entity is obtained from the inner surface and the outer surface of the subframe rear arm. The same smooth processing is performed on each part of the subframe in turn, and all the parts are combined together according to the design experience to obtain the initial version of the concept three-dimensional geometry of the entire subframe.

[0069] There are many geometric generation methods for drawing continuous three-dimensional geometric data from discrete concept contour surfaces, such as connecting the middle points of the beam units instead of the corner points to generate smooth curves, and the same effect can be achieved after a small number of iteration steps.

[0070] In the smooth geometry generation process, the entity can be generated first, and then the outer surface / inner surface / intermediate surface of the entire subframe is generated, and then the characteristic thickness t is set in the finite element analysis process to achieve the same effect.

[0071] In addition, this method can also be used for parts of other processes or materials, such as subframes containing steel pipe beams, welded steel subframes, hollow stabilizer bars, etc., as long as there is a certain length of thin-walled cavity body.

[0072] Step six, verify and iterate the stiffness and modal performance of the initial version of the concept three-dimensional geometry;

[0073] The specific method is as follows:

[0074] The initial version of the concept geometry is subjected to finite element stiffness and modal analysis to verify the stiffness and modal performance deviation of the concept geometry. If the deviation is within an acceptable range, no iteration is needed and the next step is directly entered. If the stiffness and modal deviation needs to be adjusted, the new target values of each working condition are set according to the proportional relationship between the actual results and the original design targets, and the optimization iteration calculation is performed to obtain a new discrete geometry, which is then drawn into a new continuous geometry and subjected to finite element verification until the deviation meets the requirements, and a concept three-dimensional geometry that meets the stiffness and modal performance requirements is obtained.

[0075] The first design stage is completed. In this stage, the amount of data and workload of geometry drawing and finite element analysis is greatly reduced compared with traditional methods, and the occupation of computing resources is also reduced.

[0076] Step seven, determine the position and priority of process holes and weight reduction holes;

[0077] The specific method is as follows:

[0078] The positions that cannot be dug are set as parts not participating in topology optimization, and the remaining parts are set as parts participating in topology optimization. Topology optimization is performed on the concept three-dimensional geometry to obtain feasible positions for digging holes. Different levels of mass percentages are set in turn, such as 95% mass-90% mass-85% mass-80 mass-75% mass. Through the order and size changes of the holes in this process, the priority of the holes is obtained. Finally, the positions of the process holes and the weight reduction holes are determined in combination with engineering experience.

[0079] Step eight, draw the initial version of detailed three-dimensional geometry;

[0080] The specific method is as follows:

[0081] According to the concept three-dimensional geometry, the positions of the process holes and the weight reduction holes, and the size requirements of all interfaces on the subframe, the initial version of detailed three-dimensional geometry data is drawn.

[0082] Step nine, perform finite element strength and durability performance analysis and process feasibility analysis on the initial version of detailed three-dimensional geometry data;

[0083] Perform finite element strength and durability performance analysis and process feasibility analysis on the initial version of detailed three-dimensional geometry data. If the requirements are not met, fine-tune and modify in combination with engineering experience-analysis iteration. For positions that do not meet the strength and durability performance, mainly perform local fine-tuning through local thickening and stiffening, including local thickening of the outer surface, local thickening of the inner surface, local simple stiffening of the outer surface, local simple stiffening of the inner surface, and through stiffening with upper and lower communication. For positions that do not meet the process requirements, mainly perform local fine-tuning by adjusting the pouring position and adding pouring channels until a design structure M that meets the requirements of strength and durability performance and process is obtained. The advantage of local fine-tuning is to greatly improve the strength and durability performance without significantly changing the stiffness performance, to separate the strength design stage and the stiffness design stage, and to reduce the complexity and difficulty of the design process

[0084] Step ten, the design is completed.

[0085] The preferred embodiments of the present application are described in detail above with reference to the drawings, but the scope of protection of the present application is not limited to the specific details of the above-described embodiments. Any person skilled in the art, within the scope of the technical concept of the present application, can make equivalent replacements or changes to the technical scheme of the present application and the inventive concept according to the technical scheme of the present application, and these simple modifications all belong to the scope of protection of the present application.

[0086] In addition, it should be noted that each specific technical feature described in the above specific embodiments can be combined in any appropriate manner without contradiction, and in order to avoid unnecessary repetition, the present application will not further describe various possible combinations.

[0087] In addition, various different embodiments of the present application can be combined in any manner, as long as they do not deviate from the idea of the present application, and they should also be considered as disclosed by the present application.

Claims

1. A method for forward optimization design of a hollow cast subframe, characterized in that, It comprises the following steps: Step one, according to the suspension hard point arrangement and interface definition, design the subframe force transmission path and structure frame; Step two, according to the subframe force transmission path and structure frame, discretize the whole frame structure, and build a subframe simplified frame finite element model with a hollow rectangular section beam element; Step three, determine the subframe concept frame design line; Step four, determine the subframe three-dimensional concept profile; Step five, draw the initial version of the concept three-dimensional geometry; Step six, verify and iterate the stiffness and modal performance of the initial version of the concept three-dimensional geometry; Step seven, determine the position and priority of the process hole and weight reduction hole; Step eight, draw the initial version of the detailed three-dimensional geometry; Step nine, perform finite element strength durability performance analysis and process feasibility analysis on the initial version of the detailed three-dimensional geometry data; Step ten, design is completed; The specific method of step seven is as follows: The positions that cannot be dug are set as parts not participating in topology optimization, and the rest are set as parts participating in topology optimization, and the topology optimization of the concept three-dimensional geometry is performed to obtain the feasible positions of the holes; Different levels of mass percentage are set in turn, and the priority of the holes is obtained through the order and size change of the holes, and the positions of the process holes and weight reduction holes are determined; The specific method of step nine is as follows: Perform finite element strength durability performance analysis and process feasibility analysis on the initial version of the detailed three-dimensional geometry data, if not meet the requirements, then make fine adjustment and modification, for the positions that do not meet the strength durability performance, make local fine adjustment through local thickening, local stiffening and internal surface through-hole rib, including external surface local thickening, internal surface local thickening, external surface local simple stiffening, internal surface local simple stiffening, internal surface with upper and lower connected through-hole rib; for the positions that do not meet the process requirements, make local adjustment by adjusting the pouring position and increasing the pouring channel, until the design structure meets the requirements of strength durability performance and process.

2. The method of claim 1, wherein, The specific method of step one is as follows: The hard point arrangement includes the front mounting point, rear mounting point, control arm connecting point, suspension connecting point and steering machine connecting point of the subframe; the force transmission path requirements of all loads of the subframe are determined, and the components of the subframe are determined, and the force transmission path of the subframe is designed according to the force transmission path requirements; the structure frame is designed according to the components of the subframe.

3. The method of claim 1, wherein, The specific method of step two is as follows: Build a subframe simplified frame finite element model with a hollow rectangular section beam element: divide each part of the subframe into small sections, and replace each small section with a beam element; after the segmentation and discretization is completed, fix the thickness of all elements to a uniform feature thickness t, t is the minimum thickness that can be stably reached in the whole field of casting process, and the cross-sectional width W and height H of all beam elements are set to consistent statistical data, and the statistical data W and H are obtained by calculating the average value from the cross-sectional dimensions of the existing vehicle subframe.

4. The method of claim 1, wherein, The specific method of step three is as follows: Take one side of the symmetric model, take the x, y, z coordinates of all nodes as design variables, take the size limit of the design space as constraint, take the weighted flexibility of all stiffness cases as target, calculate the position change of each node in the design space when the weighted flexibility is minimized, calculate the new position coordinates of each node from the original coordinates and coordinate change, and the line connected by the new positions of all nodes is the design line of the subframe conceptual framework.

5. The method of claim 1, wherein, The specific method of step four is as follows: Symmetrize the model determined in step three along the symmetry axis, fill in the units on the other side of the symmetry axis, use the same size variable for each pair of left and right symmetric units, still fix the thickness of the beam unit as the characteristic thickness t, take the width Wi and height Hi of each beam unit as variables, take the stiffness target value and modal frequency target value of all stiffness cases as constraints, and perform mass minimization parameter optimization to calculate the new size of all beam units. The stepped discontinuous surface composed of all beam units with different widths and heights is the three-dimensional conceptual contour surface of the subframe.

6. The method of forward optimization design of a hollow cast subframe according to claim 1, wherein, The specific method of step five is as follows: For each component part of the subframe main body, generate a continuous and smooth curve, surface and three-dimensional entity from the discrete three-dimensional conceptual contour surface in sequence; then generate a smooth surface from two adjacent lines in the continuous and smooth curve, and then connect the smooth surfaces with rounded corners to obtain a continuous cavity surface, which is taken as the outer surface of any component part of the subframe; push the characteristic thickness t from the outer surface to the inside of the cavity as the inner surface of any component part of the subframe; obtain the three-dimensional entity of hollow casting from the inner and outer surfaces of any component part of the subframe; sequentially smooth each part of the subframe, and then join all the parts together to obtain the preliminary conceptual three-dimensional geometry of the entire subframe.

7. The method of forward optimization design of a hollow cast subframe according to claim 1, wherein, The specific method of step six is as follows: Perform finite element stiffness and modal analysis on the preliminary conceptual geometry to verify the deviation of the stiffness and modal performance of the conceptual geometry. If the deviation is within an acceptable range, no iteration is needed and the next step is directly entered. If the stiffness and modal deviation needs to be adjusted, set new target values for each case according to the proportional relationship between the actual results and the original design targets, perform optimization iteration calculation to obtain new discrete geometry, draw new continuous geometry and perform finite element verification until the deviation meets the requirements, and obtain the conceptual three-dimensional geometry that meets the stiffness and modal performance requirements.

8. The method of forward optimization design of a hollow cast subframe according to claim 1, wherein, The specific method of step eight is as follows: According to the conceptual three-dimensional geometry, the location and size requirements of process holes, weight reduction holes and all interfaces on the subframe, draw the preliminary detailed three-dimensional geometry data.

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