Method for improving manufacturing precision of open-arch integral thin shell

Abstract

The present application relates to a kind of methods for improving the manufacturing precision of open arc integral shell, belong to the field of advanced manufacturing, solve the precision deficiency that the thin shell obtained by existing manufacturing process faces after forming and / or assembly, in turn lead to the problem of difficult weight control, low bearing capacity under the condition of same weight.A kind of methods for improving the manufacturing precision of open arc integral shell, comprising the following steps: S1, select suitable integral process scheme, sealing form;S2, carry out finite element simulation to thin shell after assembly subsequent process, reconstruct design, obtain thin shell reconstruction model;S3, carry out temperature finite element simulation to mould, reconstruct design and / or temperature difference design to mould;S4, carry out digitalization development to thin shell reconstruction model.The present application significantly improves the precision of manufacturing thin shell by process scheme, sealing form, thin shell reconstruction, mould reconstruction and / or temperature difference design, thin shell three-dimensional model conversion into thin shell plane development model and other key steps, and the profile precision can reach 0.3mm / 1000mm order of magnitude.

Description

Technical Field

[0001] This invention relates to the field of advanced manufacturing, and in particular to a method for improving the manufacturing precision of open, curved, integral thin-shell structures. Background Technology

[0002] Integral thin-shell structures typically refer to structures with a certain spatial shape made of thin-walled materials. They are widely used in aerospace, shipbuilding, high-speed trains and other fields. These structures often improve their load-bearing capacity and overall stiffness by adding stiffeners. Stiffeners are reinforcements set along certain directions of the thin-shell structure to improve local or overall stability. They can be raised ribs, truss structures or other types of support structures.

[0003] The manufacturing methods for integral thin-shell structures mainly include traditional machining, casting, and advanced additive manufacturing technology. However, these methods each face challenges in terms of manufacturing accuracy, especially in controlling the accuracy during forming and / or assembly. a) Machining: This is a process that forms the desired thin shell by cutting away material in three-dimensional space. For example, using a five-axis machine to mill ribs and cavities on a three-dimensional surface. However, maintaining machining accuracy in three-dimensional space is more challenging than in two-dimensional planes, especially for thin-shell structures, as they are more prone to deformation during machining. Limited by existing three-dimensional machining equipment, the forming accuracy of thin shells cannot meet high design requirements. b) Casting: The forming accuracy of thin-shell structures obtained by casting is usually low because defects such as shrinkage cavities, deformation, and cracks are easily generated during the casting process. c) Additive Manufacturing: Although additive manufacturing technology can directly manufacture integral thin shells with three-dimensional shapes from computer models, controlling the forming accuracy in 3D printing complex three-dimensional geometries remains a challenge because defects such as deformation, holes, and cracks may occur during the printing process, affecting the forming accuracy and consistency.

[0004] Furthermore, the existing method for processing integral thin shells is to directly process them according to the expected design model without fully considering and preventing dimensional deviations that may occur due to other external factors during the forming and assembly process. As a result, after the thin shell is formed and assembled, its dimensions often deviate from the original expected design model, which in turn affects the accuracy of the thin shell in actual applications.

[0005] Therefore, there is an urgent need to provide a method that can effectively improve the manufacturing precision of open arc-shaped integral thin shells for use in the manufacturing of open arc-shaped integral thin shells. This is of great significance for promoting the development of related industries where integral thin shell structures are applied. Summary of the Invention

[0006] In view of the above analysis, the present invention aims to provide a method for improving the manufacturing precision of open arc-shaped integral thin shells, in order to solve at least one of the following technical problems: the thin shells obtained by existing manufacturing processes face insufficient precision after forming and / or assembly, which leads to difficulties in weight control and low load-bearing capacity under the same weight conditions.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] This invention provides a method for improving the manufacturing precision of open, curved, integral thin-shell structures, comprising the following steps:

[0009] S1. For different types of thin-shell structures, select appropriate overall process solutions and sealing methods to improve forming accuracy during the design phase:

[0010] The thin shell has an open arc-shaped structure, including an outer skin layer and an inner reinforcing rib layer. The reinforcing rib layer is a grid structure formed by crisscrossing reinforcing ribs.

[0011] When the thin shell is not a semicircle, a process of top and bottom loading + air expansion forming is adopted; the thin shell and the mold are sealed by rigid pressure on all four sides.

[0012] When the thin shell is in the form of a semicircle, the process of loading from top to bottom and left to right and air expansion is adopted. The thin shell and the mold are sealed by applying pressure to the top, bottom, left and right, front and back inner and outer surfaces of the thin shell through three-dimensional rigid pressure.

[0013] S2. Import the theoretical model of the thin shell into the finite element analysis software, perform finite element simulation of the subsequent assembly process of the thin shell, and based on the information of deformation in different areas of the assembled thin shell, reconstruct the theoretical model of the thin shell to obtain the reconstructed model of the thin shell, so as to improve the accuracy in the design stage.

[0014] S3. Design molds for different types of thin shells, perform finite element simulation of the molds under heating, and based on the difference between the heated mold and the mold after proportional expansion in different regions, reconstruct and / or perform differential temperature design of the molds to improve forming accuracy during the design stage.

[0015] S4. Digitally unfold the thin-shell reconstruction model to obtain an accurate thin-shell planar unfolded model, which can be used to manufacture a high-precision open arc-shaped integral thin-shell.

[0016] Furthermore, S3 specifically includes the following steps:

[0017] S31: Using 3D modeling software, design a theoretical model of the mold for forming the thin shell, based on whether the thin shell is a semicircle or not.

[0018] S32: Import the theoretical model of the mold used to form the thin shell into the finite element analysis software, perform finite element simulation of the mold under heating, and based on the difference between the mold after heating and the mold after proportional expansion in different regions, perform reconstruction design and / or differential temperature design of the mold to obtain the first optimized model of the mold.

[0019] After N iterations, the dimensional difference θr between the Nth optimized model of the mold and the mold after proportional expansion is ensured to satisfy: θr≤1 / 2×αm. This reduces the dimensional deviation caused by non-proportional expansion of the mold due to heating during the design stage, thereby improving the forming accuracy. αm is the maximum allowable value for the surface accuracy of the product, and N≥1.

[0020] Furthermore, step S32 specifically includes the following steps:

[0021] S321. Geometric Model Establishment:

[0022] The theoretical model of the mold used for forming thin shells is placed in the model of the high-temperature forming furnace, and a heating platform is set on the bottom surface of the mold.

[0023] S322. Pre-processing: This includes setting boundary conditions and defining the temperature-time change curve of the heating platform.

[0024] (c) Boundary conditions: The mold and the heating platform adopt the contact heat transfer mode, the mold and other parts of the furnace adopt the radiation heat transfer mode, and the furnace interior adopts the convection heat transfer mode.

[0025] (d) Temperature-time curve: T = T0 + σ × t, until T = T C0 T0 is the initial temperature in °C; t is the time in seconds; σ is the heating coefficient in °C / s; T co The target forming temperature is initially determined based on the mold material;

[0026] S323. Perform finite element simulation of the mold during heating:

[0027] After preprocessing, the mold is subjected to finite element simulation with heating to obtain the model data of the mold after heating. The model data of the mold after heating is exported and compared with the model of the mold after proportional expansion to obtain the difference between the mold after heating and the mold after proportional expansion in different regions.

[0028] S324. Perform reconstruction design and / or differential temperature design on the mold to obtain the first optimized model of the mold; wherein, the main steps of differential temperature design include: dividing the heating platform into different heating areas, setting different target temperatures for different heating areas, and obtaining the first optimized model of the mold;

[0029] Repeat S321 to S324, and iterate N times until the difference θr between the Nth optimized model of the mold and the mold after the proportional expansion is satisfied: θr≤1 / 5×αm.

[0030] Furthermore, in S322, T0 = room temperature, σ is 0.01℃ / s~0.03℃ / s; and / or,

[0031] In S323, proportional expansion of the mold means that the mold expands in all directions in three-dimensional space with the same scaling factor, where the scaling factor λ = α × (T C0 -T0), where α is the coefficient of linear expansion.

[0032] Furthermore, when the mold material is 45# steel, in S322, T C0 =400℃~600℃; In S323, α is 14.18×10 -6 ℃ -1 .

[0033] Furthermore, in S321, the high-temperature forming furnace chamber is a hexahedral box.

[0034] Furthermore, S32 also includes, during the heating finite element simulation, using the complete adhesion of the skin layer to the mold as the evaluation criterion, obtaining the minimum gas pressure load P required for the skin layer to adhere. min Therefore, during the design phase, the process conditions that can guarantee the forming accuracy of the thin shell are determined, i.e., the following conditions are met:

[0035] The gas pressure load P ≥ P during gas expansion forming min ;

[0036] Initial tonnage of top and bottom loading: F0 ≥ P min ×S, where S is the projected area of ​​the non-semicircular thin shell in the vertical direction.

[0037] The initial tonnage of the load applied in all directions (up, down, left, right) is Fs0 ≥ P. min ×Ss,Fc0≥P min ×Sc, Fs0 is the initial tonnage of the vertical loading, Fc0 is the initial tonnage of the horizontal loading, Ss is the projected area of ​​the semicircular shell in the vertical direction, and Sc is the projected area of ​​the semicircular shell in the horizontal direction.

[0038] Furthermore, let the minimum gas pressure load P of the skin film be... min It can also be done through formula P min =H m / r×σs is calculated, where H m σs represents the thickness of the process allowance around the perimeter, r represents the minimum fillet size of the neutral layer in the final molding area of ​​the skin layer, and σs represents the yield strength of the skin layer material at the forming temperature.

[0039] Furthermore, the main steps for achieving a seal between the semi-circular thin shell and the mold through three-dimensional rigid pressure (up / down, left / right, and front / back) include: setting multiple rigid bosses along the circumferential process allowance on the inner side of the skin layer; and designing protrusions on the reinforcing ribs of each grid structure on the side of the reinforcing rib layer away from the skin layer. The width of each protrusion should ensure that each grid structure is connected. The height Ht of the protrusions designed on the reinforcing ribs satisfies: Ht = Hj - H mg Ht < 0.5mm to ensure the outer surface of the skin at the corresponding position of the reinforcing rib is properly formed; where Hj is the clearance value between the die and the mold punch, H mg It is the sum of the thickness of the skin layer and the height of the reinforcing rib layer.

[0040] The present invention also provides a method for manufacturing an integral thin shell, including the steps in the above-described method for improving the manufacturing accuracy of an open arc-shaped integral thin shell.

[0041] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0042] A. This invention provides a method for improving the manufacturing precision of open, arc-shaped integral thin-shells. Through multiple key steps (including selecting appropriate integral process schemes and sealing methods for different types of thin-shells, thin-shell reconstruction, mold reconstruction and / or differential temperature design, and converting the three-dimensional model of the thin-shell into a planar unfolded model of the thin-shell), the manufacturing precision of the thin-shell can be effectively improved, thereby achieving weight control and improving the load-bearing capacity of the thin-shell; specifically:

[0043] (1) Select the appropriate overall process scheme and sealing method: When the thin shell is not a semicircle (θ≤180°), the process scheme of top and bottom loading + air expansion forming is adopted, and the thin shell and the mold are sealed by the rigid pressure of the four sides; when the thin shell is a semicircle (θ>180°), the process scheme of top, bottom and left and right loading + air expansion forming is adopted, and the thin shell and the mold are sealed by applying pressure on the top, bottom, left and right, front and back inner and outer surfaces of the thin shell, and the three-dimensional rigid pressure of top, bottom, left and right, front and back is used to achieve the sealing between the semicircular thin shell and the mold; by selecting the best overall process scheme and sealing method for different types of thin shells, the rigid pressure of the mold and the air expansion forming based on the reliable sealing method are used to achieve rigid and flexible composite loading, thereby improving the forming accuracy, quality and efficiency;

[0044] (2) Thin shell reconstruction: Through finite element simulation, the deformation of the thin shell in the subsequent assembly process is predicted and evaluated. The thin shell is reconstructed by multiple iterations, which effectively reduces the dimensional deviation of the thin shell component caused by the assembly process in actual manufacturing. It can ensure that the formed thin shell component will not cause dimensional deviation from the expected thin shell theoretical model due to assembly, thereby improving the manufacturing accuracy of the thin shell.

[0045] (3) Mold reconstruction and / or differential temperature design: Through finite element simulation, the non-proportional expansion of the mold during the heating process of the forming process is predicted and evaluated. The mold is reconstructed and / or differential temperature designed by multiple iterations. This effectively reduces the dimensional deviation of the thin shell caused by the non-proportional expansion of the mold during heating in actual manufacturing. It can ensure that the dimensional deviation between the thin shell component and the expected thin shell reconstruction model will not occur due to the non-proportional expansion of the mold during the forming process. Furthermore, since the thin shell reconstruction design has supplemented the deformation of the thin shell caused by the subsequent assembly process, after assembling the thin shell component with other components, it can be ensured that the dimensional of the thin shell component in the product is very close to the expected thin shell theoretical model, thereby improving the manufacturing accuracy of the thin shell.

[0046] (4) Thin shell 3D model converted into thin shell planar unfolding model: Based on digital unfolding, finite element simulation is used to predict and evaluate the dimensional deviation between the thin shell planar unfolding model after forming and the expected thin shell reconstruction model. The thin shell planar unfolding model is adjusted by multiple iterations to obtain an accurate thin shell planar unfolding model. This ensures that the preform processed based on the obtained accurate planar unfolding model is close to the thin shell reconstruction model after actual forming, and close to the expected thin shell theoretical model after further assembly.

[0047] In summary, the method of the present invention effectively improves the surface accuracy of the manufactured integral thin shell in practical applications, and the surface accuracy of the product can reach the order of 0.3mm / 1000mm, which is at the leading level in the field.

[0048] Due to the improved surface accuracy, the weight deviation between the thin-shell component and the expected thin-shell theoretical model in the product obtained by the method of this invention can be controlled within 1%.

[0049] Meanwhile, due to the improved surface accuracy, the thin-shell components in the products obtained by the method of this invention have higher load-bearing capacity. In the prior art, thin-shell products obtained by traditional casting and machining have the problem of low surface accuracy. In order to avoid weak areas in mechanical properties, it is often necessary to leave a weight margin during product manufacturing. However, the thin shells manufactured by the method provided by this invention have the characteristics of high precision and have higher load-bearing capacity under the premise of the same weight.

[0050] B. This invention provides a method for improving the manufacturing accuracy of open, curved, integral thin-shell structures. Through multiple key steps (including thin-shell reconstruction, mold reconstruction and / or differential temperature design, and conversion of the thin-shell 3D model into a thin-shell planar unfolded model), it can also effectively improve thin-shell manufacturing efficiency and reduce costs; specifically:

[0051] Using the method of this invention, based on an accurate thin-shell planar unfolding model, a planar preform can be manufactured first in a two-dimensional flat state. Then, the planar preform is placed in a mold to form a thin-shell component that is very close to the expected dimensions of the thin-shell reconstruction model. Furthermore, since the thin-shell reconstruction design has already compensated for the deformation caused by subsequent assembly processes, assembling the formed thin-shell component with other components ensures that the resulting product's thin-shell component is very close to the expected dimensions of the theoretical thin-shell model. This greatly reduces rework and modifications to the thin-shell during manufacturing, thereby saving time, reducing costs, and improving the manufacturing efficiency of the thin-shell. Simultaneously, it improves product qualification rate and process stability.

[0052] Compared to existing technologies that use five-axis equipment to mill reinforcing ribs and weight-reducing cavities on a three-dimensional inner surface, or to cast or additively produce reinforcing ribs and weight-reducing cavities on a three-dimensional inner surface, this invention transforms three-dimensional into two-dimensional, greatly reducing the difficulty of manufacturing and forming, improving manufacturing efficiency, and shortening the processing cycle by more than 75%; at the same time, it reduces the requirements for processing equipment, and the manufacturing cost can be reduced by more than 80%.

[0053] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0054] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0055] Figure 1 A flowchart illustrating the method for improving the manufacturing precision of an open, arc-shaped integral thin shell according to an embodiment of the present invention;

[0056] Figure 2 In the figure, (a) is a three-dimensional schematic diagram of the thin-shell theoretical model provided in the embodiment of the present invention; (b) is a side view schematic diagram of the thin-shell theoretical model provided in the embodiment of the present invention; (c) is an enlarged schematic diagram of part A in (b);

[0057] Figure 3In the figures, (a) is a structural schematic diagram of the thin-shell theoretical model and other components (before assembly) provided in the embodiment of the present invention; (b) is a structural schematic diagram of the thin-shell theoretical model and other components (after assembly) provided in the embodiment of the present invention; (c) is a schematic diagram of the first thin-shell reconstruction model and other components (before assembly) provided in the embodiment of the present invention; and (d) is a schematic diagram of the docking and assembly parts of the thin-shell theoretical model and other components provided in the embodiment of the present invention.

[0058] Figure 4 A schematic diagram showing the pre-deformation region and magnitude verification determined after finite element simulation of subsequent assembly processes using the thin-shell theoretical model provided in this embodiment of the invention.

[0059] Figure 5 This is a schematic diagram of the structure of the first mold for forming a non-semicircular thin shell provided in an embodiment of the present invention;

[0060] Figure 6 This is a schematic diagram of the structure of the second mold for forming a semi-circular thin shell provided in an embodiment of the present invention;

[0061] Figure 7 This is a schematic diagram of the geometric model (simulated heating environment) established in the finite element simulation of mold heating provided in the embodiments of the present invention;

[0062] Figure 8 The diagrams provided in this embodiment of the invention illustrate the reconstruction design of the mold in the finite element simulation of mold heating; (a) is a schematic diagram of the mold surface before reconstruction; (b) is a schematic diagram of the mold surface after reconstruction.

[0063] Figure 9 This is a schematic diagram of differential temperature design for the mold in the finite element simulation of mold heating provided in an embodiment of the present invention.

[0064] Figure 10 The following is a schematic diagram of the process of digitally unfolding the thin shell reconstruction model provided in the embodiment of the present invention: (a) is the skin layer, (b) is the reinforcing rib layer, (c) is the combined body after the skin layer and the reinforcing rib layer are flattened, and (d) is the final combined body (with added process allowance around four sides, sealing allowance around four sides, and preform positioning allowance).

[0065] Figure 11 This is a schematic diagram of the process of adjusting the thin-shell planar unfolded model provided in the embodiment of the present invention; (a) is the top convex reconstruction compensation scheme, and (b) is a comparison diagram of the contour after the top convexity is adjusted;

[0066] Figure 12 This is a schematic diagram of the mold air passage design provided in an embodiment of the present invention;

[0067] Figure 13The embodiments of the present invention provide sealing achieved by two-dimensional rigid pressure in the upper and lower directions and sealing achieved by three-dimensional rigid pressure in the upper, lower, left, right, front and back directions, namely (a) two-dimensional rigid pressure in the upper and lower directions; (b) three-dimensional rigid pressure in the upper, lower, left, right, front and back directions.

[0068] Figure label:

[0069] 100-Thin shell theoretical model; 11-Skin layer; 12-Reinforcing rib layer; 12a-Reinforcing rib; 200-Front end frame; 300-Lower compartment; 400-Rear end frame; B-Deformation zone of thin shell; C-Reconstruction zone of thin shell; G-Welded joint; 500-First mold; 51-First upper mold; 52-Lower mold; 53-First upper platform; 54-First lower platform; 600-Second mold; 61-Second upper mold; 62-Core mold; 63-Mold base; 64-Left side mold; 65-Right side mold; 66-Second upper platform; 67-Second lower platform; 68-Side push rod of equipment; 700-High temperature forming furnace; 71-Heating platform; 81-Gas passage; 82-Ventilation hole. Detailed Implementation

[0070] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0071] This invention provides a method for improving the manufacturing precision of open, curved, integral thin-shell structures, comprising the following steps:

[0072] S1. For different types of thin-shell structures, select appropriate overall process solutions and sealing methods to improve forming accuracy during the design phase:

[0073] The thin shell has an open arc-shaped structure, including an outer skin layer and an inner reinforcing rib layer. The reinforcing rib layer is a grid structure formed by crisscrossing reinforcing ribs.

[0074] Optionally, the thin shell form can be designed as needed; the thin shell includes forms that are more than a semicircle (θ > 180°) and those that are not more than a semicircle (θ ≤ 180°); where θ is the arc angle.

[0075] Understandably, the shape of the skin is determined according to the aerodynamic shape requirements of the thin-shell product. The skin shape includes single-curvature surfaces and double-curvature surfaces; correspondingly, the thin shell includes single-curvature shells and double-curvature shells.

[0076] Preferably, the thickness of the skin layer is 1mm to 3mm, and the height-to-thickness ratio of the reinforcing ribs is H. g / T g ≤5, H g To reinforce the height, T gTo reinforce the rib thickness, by controlling the thin-shell structure parameters, based on the above-mentioned preferred skin layer thickness and the height-to-thickness ratio of the reinforcing ribs, a lightweight thin-shell that can withstand high loads can be designed.

[0077] Preferably, the reinforcing ribs adopt a cross structure with intersecting longitudinal and transverse directions; the cross structure provides support in two vertical directions, which has at least the following beneficial effects: it can improve the stability and rigidity of the overall structure, evenly distribute the load acting on the thin shell, and improve dynamic stability.

[0078] Preferably, the root of the connection between the skin and the reinforcing rib is rounded; this can have at least the following beneficial effects: rounded transitions facilitate manufacturing, reduce manufacturing defects, improve the stability and strength of the structure, and reduce stress concentration at the connection.

[0079] When the thin shell is not a semicircle, a process of top and bottom loading + air expansion forming is adopted; the thin shell and the mold are sealed by rigid pressure on all four sides.

[0080] Understandably, the main steps of “top and bottom loading” include: applying pressure to the top and bottom surfaces of the thin shell using molds (e.g., upper and lower molds), with the applied pressure being a rigid load.

[0081] The main steps of "air-expansion forming" include: applying internal pressure to a thin shell using compressed gas to make it conform to the mold surface and form it, wherein the applied pressure is a flexible load; for example, placing a preform of the thin shell in the mold cavity, filling the cavity with compressed gas, causing the preform to expand and conform to the shape of the mold.

[0082] When the thin shell is in the form of a semicircle, the process of loading from top to bottom and left to right and air expansion is adopted. The thin shell and the mold are sealed by applying pressure to the top, bottom, left and right, front and back inner and outer surfaces of the thin shell through three-dimensional rigid pressure.

[0083] It is understandable that the main steps of "achieving a seal through circumferential rigid pressure" include: applying pressure to the upper and lower surfaces of the thin shell using a mold (e.g., an upper mold and a lower mold), thereby achieving a seal between the non-semi-circular thin shell and the mold through circumferential rigid pressure; see also... Figure 12 The mold is provided with multiple ventilation holes 82, which are set to avoid the reinforcing ribs; the ventilation holes 82 and the mold cavity form a crisscrossing air channel 81; preferably, the air channel is a crisscrossing grid-shaped air channel; the air channel is mainly used to blow air into the preform during the air expansion forming process, on the one hand to make the product skin completely adhere to the mold and prevent the blank from collapsing, and on the other hand, the air blowing can also blow out the inner surface of the skin to ensure the quality of the inner surface.

[0084] It is understood that the specific steps of "applying pressure to the upper, lower, left, right, front, and back inner and outer surfaces of the thin shell" include: applying pressure to the upper, lower, left, right, front, and back inner and outer surfaces of the thin shell using molds (e.g., upper mold and core mold, side mold and core mold, local pressure-increasing blocks); the main steps of "achieving a seal between the semi-circular thin shell and the mold through three-dimensional rigid pressure" include: setting multiple rigid bosses along the circumferential process allowance on the inner side of the skin layer, and designing protrusions on the reinforcing ribs of each grid structure on the side of the reinforcing rib layer away from the skin layer. The width of each protrusion should ensure that each grid structure is connected, and the height Ht of the protrusions designed on the reinforcing ribs satisfies: Ht = Hj - H mg Ht < 0.5mm, to ensure that the outer surface of the skin at the corresponding position of the reinforcing rib is formed in place;

[0085] Where Hj is the clearance value between the punch and die of the mold, H mg It is the sum of the thickness of the skin layer and the height of the reinforcing rib layer.

[0086] The height Hy of the multi-ring rigid boss satisfies: Hy + Hm > Hj; where Hm is the thickness of the skin layer (at the surrounding process allowance), and Hj is the clearance value between the mold punch and die. Preferably, Hy + Hm = Hj + (1-3) mm.

[0087] Considering the multiple loading directions and relatively complex stress during the forming of the semi-circular thin shell, there may be weak areas at the interfaces of the loading on the top, bottom, left, and right sides. To avoid insufficient local rigid pressure affecting the forming effect, multiple rigid bosses are set on the process allowance around the inner side of the skin layer. The mold (e.g., upper mold and core mold, side mold and core mold, local pressure blocks) applies pressure to the top, bottom, left, right, front, back, inner and outer surfaces of the thin shell. The three-dimensional rigid pressure of the top, bottom, left, right, front, and back achieves the seal between the semi-circular thin shell and the mold. A protrusion of appropriate height and width is selectively designed on the reinforcing ribs of each grid structure. This ensures that the air passages between the crisscrossing reinforcing ribs are connected, and that each reinforcing rib receives sufficient rigid pressure from the punch, thereby improving the surface accuracy of the final formed product.

[0088] S2. Import the theoretical model of the thin shell into the finite element analysis software, perform finite element simulation of the subsequent assembly process of the thin shell, and based on the information of deformation in different areas of the assembled thin shell, reconstruct the theoretical model of the thin shell to obtain the reconstructed model of the thin shell, so as to improve the accuracy in the design stage.

[0089] Specifically, the thin shell theoretical model is imported into the finite element analysis software to perform finite element simulation of the subsequent assembly process of the thin shell; based on the information of deformation in different areas of the assembled thin shell, the areas that need to be reconstructed and the amount of reconstruction are determined; the thin shell theoretical model is reconstructed using 3D model design software to obtain the first thin shell reconstruction model;

[0090] After M iterations, the dimensional difference θt between the final determined thin-shell reconstruction model and the thin-shell theoretical model after finite element simulation of subsequent assembly processes is ensured to satisfy: θt≤1 / 2×αm. This reduces the deformation of the thin-shell caused by subsequent assembly processes during the design phase and improves the accuracy of the manufactured thin-shell products. αm is the maximum allowable value for the surface accuracy of the product, M≥1.

[0091] It should be noted that the subsequent assembly process mainly involves connecting the thin-shell component with other components to form a whole through welding and other methods. Since the welding process needs to be carried out under high temperature conditions, heat input and thermal stress will be generated. At the same time, the thin-shell component will also be subjected to multiple external stresses during assembly, which will inevitably cause deformation, especially for complex thin-shell components, such as large-sized complex integral thin-shell components like aircraft fuel tanks.

[0092] S2 specifically includes the following steps:

[0093] S21: Use 3D modeling software to create simulation models of the thin-shell component to be assembled and other components; any two adjacent components are assembled through welding joints.

[0094] For example, see Figure 3 (a) Establish simulation models of the thin-shell theoretical model 100 to be assembled and the other three components, namely, the front frame 200, the lower compartment 300, and the rear frame 400. Assembly is achieved between any two adjacent components via welded joints. Each welded joint is formed by overlapping the side of the thin-shell theoretical model 100 with the stepped grooves contained on the sides of the front frame 200, the lower compartment 300, and the rear frame 400, creating a flush welded joint. See [link to relevant documentation]. Figure 3 (d) The left side of the thin shell theoretical model 100 forms a welded joint with a flush surface by overlapping the stepped groove on the right side of the front frame 200.

[0095] S22: Import the simulation model into the finite element analysis software, load the welding simulation heat source at each welding joint in the simulation model, perform finite element simulation of the subsequent assembly process of the thin shell, and obtain the model data after the thin shell is assembled.

[0096] S23: Compare the assembled thin-shell model with the theoretical model of the thin-shell to determine the deformation information of different regions of the thin-shell, including the simulated deformation amount; based on the deformation information of different regions, determine the regions that need to be reconstructed and the amount of reconstruction, the reconstruction amount Dyf=δ1×Dfz, Dfz is the simulated deformation amount, δ1 is the reconstruction coefficient of the thin-shell, 0.7≤δ1≤1.3, in order to improve the efficiency of thin-shell model reconstruction;

[0097] It is understandable that there may be some error between the simulated deformation and the actual situation. By selecting appropriate reconstruction coefficients, the reconstructed model of the thin shell can be made closer to the theoretical model more quickly, thereby improving the efficiency and accuracy of model reconstruction.

[0098] For example, see Figure 3 (b) When the simulated deformation of the deformation zone B of the thin shell is a concave arc with a radius of curvature of 0.8 mm, the reconstruction coefficient δ is selected as 0.75. Accordingly, see [reference needed]. Figure 3 (c) The reverse deformation of the reconstructed region C of the thin shell is an upward convex arc with a curvature radius of 0.6 mm.

[0099] S24: Based on the area and amount of reconstruction required, use 3D model design software to reconstruct the thin shell theoretical model and obtain the first thin shell reconstruction model.

[0100] Repeat S21 to S24, and after M iterations, until the final determined thin shell reconstruction model is assembled again, the dimensional difference θt between the finite element simulation of the subsequent assembly process and the thin shell theoretical model satisfies: θt≤1 / 2×αm, where αm is the maximum allowable value of the surface accuracy of the thin shell.

[0101] Specifically, after finite element simulation of the assembly process following steps S21 to S24 of the first thin-shell reconstruction model, it is compared with the thin-shell theoretical model. If the dimensional difference θt between different regions of the two models satisfies θt≤1 / 2×αm, the iteration stops, and the first thin-shell reconstruction model is taken as the final determined thin-shell reconstruction model. If the dimensional difference θt between different regions of the two models does not satisfy θt≤1 / 2×αm, the first thin-shell reconstruction model needs to be redesigned to obtain the second thin-shell reconstruction model.

[0102] Each repetition of S21 to S24 is an "iteration". Through M iterations, until the final determined thin shell reconstruction model is assembled again, the dimensional difference θt between the finite element simulation and the thin shell theoretical model satisfies: θt≤1 / 2×αm.

[0103] Preferably, θt ≤ 1 / 5 × αm to further improve accuracy. For example, for a complex integral thin-shell structure with reinforcing ribs and skin, αm is (0.3 mm to 0.7 mm) / 1000 mm. Preferably, αm is (0.4 mm to 0.6 mm) / 1000 mm, thus achieving an optimal balance between efficiency and accuracy in thin-shell reconstruction.

[0104] It is worth noting that the thin shell reconstruction of S2 can achieve at least the following beneficial effects: (1) Improved accuracy: By finite element simulation, the deformation of the thin shell in the subsequent assembly process is predicted and evaluated. The thin shell is reconstructed by multiple iterations, which effectively reduces the dimensional deviation of the thin shell component caused by the assembly process in actual manufacturing. It can be ensured that the formed thin shell component will not cause dimensional deviation from the expected thin shell theoretical model due to assembly, thereby improving manufacturing accuracy. (2) Improved thin shell manufacturing efficiency and reduced cost: By finite element simulation, the deformation of the thin shell in the subsequent assembly process is predicted and evaluated. The thin shell is reconstructed by multiple iterations, which effectively reduces the dimensional deviation of the thin shell size caused by the assembly process in actual manufacturing. Compared with the existing method of verifying by actual manufacturing results, this invention can ensure that the thin shell will not cause dimensional deviation from the expected thin shell theoretical model due to the assembly process by reconstructing the thin shell in the design stage. This greatly reduces the rework and modification of the thin shell in the manufacturing process, thereby saving time and reducing costs, and improving the manufacturing efficiency of the thin shell; at the same time, it improves the product qualification rate and process stability.

[0105] S3. Mold design for different thin-shell forms, finite element simulation of the mold under heating, and reconstruction and / or differential temperature design of the mold based on the difference between the heated mold and the proportionally expanded mold in different regions to improve forming accuracy during the design stage; specifically including the following steps:

[0106] S31: Using 3D modeling software, design a theoretical model of the mold for forming the thin shell, based on whether the thin shell is a semicircle or not.

[0107] A. When the thin shell is not a semicircle (θ≤180°), the mold includes an upper mold and a lower mold;

[0108] For example, see Figure 5 The first mold 500 includes a first upper mold 51, a lower mold 52, a first upper platform 53, and a first lower platform 54. The first upper mold 51 is located above the lower mold 52, and the first upper mold 51 and the lower mold 52 cooperate with each other to form an arc-shaped mold cavity that is not a semicircle. The first upper platform 53 is located above the first upper mold 51, and the first lower platform 54 is located below the lower mold 52.

[0109] B. When the thin shell is a semicircle (θ>180°), the mold includes an upper mold, a core mold, a mold base, a left mold, and a right mold;

[0110] For example, see Figure 6 The second mold 600 includes a second upper mold 61, a core mold 62, a mold base 63, a left mold 64, a right mold 65, a second upper platform 66, a second lower platform 67, and a side push rod 68. The left mold 64 and the right mold 65 are respectively located on the left and right sides of the core mold 62. The second upper mold 61 is located above the core mold 62, the mold base 63 is located below the core mold 62, the second upper platform 66 is located above the second upper mold 61, and the second lower platform 67 is located below the mold base 63. When the second upper mold 61, the left mold 64, and the right mold 65 are closed relative to the core mold 62, the second upper mold 61, the left mold 64, the right mold 65, and the core mold 62 cooperate with each other to form a semi-circular arc-shaped mold cavity.

[0111] Furthermore, based on the mold material and the thin-shell material, a suitable scaling factor is selected to scale the mold.

[0112] Specifically, if the mold and the shell are made of different materials, their expansion rates will differ at the set forming temperature. If the expansion rate of the shell is greater than that of the mold, the mold needs to be scaled up during the design phase; if the expansion rate of the shell is less than that of the mold, the mold needs to be scaled down during the design phase. The scaling factor can be selected according to actual needs. For example, when the mold material is 45# steel and the shell material is 5A06 aluminum alloy, the scaling factor is +5%; when the shell material is titanium alloy, such as TA15 titanium alloy, the scaling factor is -6%.

[0113] Based on the different materials of the mold and the shell, the mold is appropriately scaled to compensate for the errors caused by the different degrees of expansion of the mold and the shell during the forming process, thereby further improving the surface accuracy of the product, increasing the product qualification rate and process stability, and reducing weight errors.

[0114] S32: Import the theoretical model of the mold used to form the thin shell into the finite element analysis software, perform finite element simulation of the mold under heating, and based on the difference between the mold after heating and the mold after proportional expansion in different regions, perform reconstruction design and / or differential temperature design of the mold to obtain the first optimized model of the mold.

[0115] After N iterations, the dimensional difference θr between the Nth optimized model of the mold and the mold after proportional expansion is ensured to satisfy: θr≤1 / 2×αm. This reduces the dimensional deviation caused by non-proportional expansion of the mold due to heating during the design stage, thereby improving the forming accuracy. αm is the maximum allowable value for the surface accuracy of the product, and N≥1.

[0116] S32 specifically includes the following steps:

[0117] S321. Geometric Model Establishment:

[0118] The theoretical model of the mold used for forming thin shells is placed in the model of the high-temperature forming furnace, and a heating platform is set on the bottom surface of the mold.

[0119] Preferably, the high-temperature forming furnace chamber is a hexahedral box; the geometry of the hexahedral box helps to simplify the preprocessing work, including mesh generation, material property setting, boundary condition definition, etc., and improves the efficiency of simulation. At the same time, the hexahedral forming furnace chamber helps to achieve the uniformity of the temperature field in the heating finite element simulation.

[0120] In one possible design, the inner surface of the bottom of the forming furnace chamber is a heating platform.

[0121] S322. Pre-processing: This includes setting boundary conditions and defining the temperature-time change curve of the heating platform.

[0122] (a) Boundary conditions: The mold and the heating platform adopt the contact heat transfer mode, the mold and other parts of the furnace adopt the radiation heat transfer mode, and the furnace interior adopts the convection heat transfer mode.

[0123] Furthermore, it also includes: using the mesh generation function and material property setting function in the finite element analysis software to perform mesh generation and material property setting for the mold and furnace respectively.

[0124] (b) Temperature-time curve: T = T0 + σ × t, until T = T C0 T0 is the initial temperature in °C; t is the time in seconds; σ is the heating coefficient in °C / s; T C0 The target forming temperature is initially determined based on the mold material;

[0125] Specifically, the heat source temperature of the bottom heating platform in contact with the furnace and the mold is set, and the heating platform temperature is set as a curve, which is a curve of temperature change over time. When the heating platform temperature T < T C0 At that time, T = T0 + σ × t, until T = T C0 ;

[0126] Preferably, T0 = room temperature, σ is 0.01℃ / s~0.03℃ / s, and starting the temperature rise from room temperature and selecting an appropriate temperature rise coefficient can more realistically simulate the actual forming process.

[0127] S323. Perform finite element simulation of the mold during heating:

[0128] After preprocessing, the mold is subjected to finite element simulation with heating to obtain the model data of the mold after heating. The model data of the mold after heating is exported and compared with the model of the mold after proportional expansion to obtain the difference between the mold after heating and the mold after proportional expansion in different regions.

[0129] It should be noted that proportional expansion of the mold means that, in three-dimensional space, the mold expands in all directions with the same scaling factor, where the scaling factor λ = 1 + α × (T) C0 -T0), where α is the coefficient of linear expansion.

[0130] In one possible design, the mold is made of 45# steel; T C0 = 400℃~600℃, T0=room temperature, from room temperature to 400℃~600℃, α is 14.18×10 -6 ℃ -1 .

[0131] Preferably, the model data after the mold is heated is exported, and the upper mold (not beyond the semicircle for thin shells), the upper mold and the left and right side molds (beyond the semicircle for thin shells) are compared with the model after the mold is expanded proportionally. For thin shell structures, compared with the lower mold, the upper mold and the side molds play an important role in determining the surface accuracy. The main comparison of the upper mold and the side molds can improve the efficiency of mold reconstruction while ensuring the surface accuracy.

[0132] Furthermore, during the finite element simulation of the heating process, the minimum gas pressure load P required for the skin layer to adhere completely to the mold was obtained, using the complete adhesion of the skin layer to the mold as the evaluation criterion. min Therefore, during the design phase, the process conditions that can guarantee the forming accuracy of the thin shell are determined, i.e., the following conditions are met:

[0133] The gas pressure load P ≥ Pmin for gas inflation forming;

[0134] The initial tonnage of the top and bottom loading is F0 ≥ Pmin × S, where S is the projected area of ​​the non-semicircular thin shell in the top and bottom directions.

[0135] The initial tonnage for loading vertically and horizontally is Fs0≥Pmin×Ss, Fc0≥Pmin×Sc, where Fs0 is the initial tonnage for vertical loading, Fc0 is the initial tonnage for horizontal loading, Ss is the projected area of ​​the semicircular thin shell in the vertical direction, and Sc is the projected area of ​​the semicircular thin shell in the horizontal direction.

[0136] In another possible design, the minimum gas pressure load P of the skin film is... min It can also be done through formula P min =H m / r×σs is calculated, where H mσs represents the thickness of the process allowance around the perimeter, r represents the minimum fillet size of the neutral layer in the final molding area of ​​the skin layer, and σs represents the yield strength of the skin layer material at the forming temperature.

[0137] S324. Reconstruct and / or perform differential temperature design on the mold to obtain the first optimized model of the mold;

[0138] The main steps of differential temperature design include: dividing the heating platform into different heating zones, setting different target temperatures for each zone, and obtaining the first optimized model of the mold; for example, designing the target forming temperature of a specific area of ​​the mold to be lower or higher than T. C0 When performing the finite element simulation for heating again, according to the differential temperature design, the different areas of the mold are heated to the target temperature set for the corresponding area.

[0139] The main steps in the mold reconstruction design include determining the area to be reconstructed and the amount of reconstruction based on the difference between the mold after heating and the mold after proportional expansion in different regions. The amount of reconstruction is Dyf = δ2 × Dfz, where Dfz is the simulated deformation amount and δ2 is the mold reconstruction coefficient, 0.7 ≤ δ2 ≤ 1.3, in order to improve the efficiency of mold reconstruction.

[0140] Repeat S321 to S324, and iterate N times until the final optimized model of the mold is determined. After the finite element simulation of the mold is heated again, the dimensional difference θr between the mold and the mold after proportional expansion satisfies: θr≤1 / 2×αm, where αm is the maximum allowable value of the surface accuracy of the thin shell.

[0141] Specifically, after performing finite element simulations of the first optimized model of the mold according to S321 to S324, the model is compared with the mold after proportional expansion. If the dimensional difference θr between different regions of the two satisfies θr≤1 / 2×αm, the iteration stops, and the first optimized model of the mold is used as the final determined model of the mold. If the dimensional difference θr between different regions of the two does not satisfy θr≤1 / 2×αm, the first optimized model of the mold needs to be redesigned and / or differential temperature designed to obtain the second optimized model of the mold.

[0142] Each repetition of S321 to S324 is an "iteration". Through N iterations, the size difference θr between the final determined mold's Nth optimized model and the mold after proportional expansion, after another finite element simulation with heating, satisfies: θr≤1 / 2×αm.

[0143] Preferably, θr ≤ 1 / 5 × αm to further improve forming accuracy. For example, for a complex integral thin-shell structure with reinforcing ribs and skin, αm is (0.3 mm to 0.7 mm) / 1000 mm. Preferably, αm is (0.4 mm to 0.6 mm) / 1000 mm, thereby achieving an optimal balance between efficiency and accuracy in mold reconstruction and / or differential temperature design.

[0144] It is worth noting that, through the mold reconstruction and / or differential temperature design of S3, at least the following beneficial effects can be achieved: (1) Improved accuracy: Through finite element simulation, the non-proportional expansion of the mold during the heating process of the forming process is predicted and evaluated. The mold is reconstructed and / or differential temperature designed by multiple iterations, which effectively reduces the dimensional deviation of the thin shell caused by the non-proportional expansion of the mold during the actual manufacturing process. It can be ensured that the thin shell component obtained by forming will not have a dimensional deviation from the expected thin shell model due to the non-proportional expansion of the mold during the forming process. Furthermore, since the thin shell reconstruction design has supplemented the deformation of the thin shell caused by the subsequent assembly process, after assembling the thin shell component obtained by forming with other components, it can be ensured that the thin shell component in the obtained product is very close to the expected thin shell theoretical model size, thereby improving the manufacturing accuracy of the thin shell. (2) Improve thin-shell manufacturing efficiency and reduce costs: Through finite element simulation, the non-proportional expansion of the mold during the heating process of the forming process is predicted and evaluated. The mold is redesigned and / or differential temperature is designed in a multi-iteration manner, which effectively reduces the dimensional deviation of the thin shell caused by the non-proportional expansion of the mold during the actual manufacturing process. Compared with the existing method of verifying by actual manufacturing results, the present invention can ensure that the thin shell will not have dimensional deviation from the expected thin shell theoretical model due to the non-proportional expansion of the mold during the forming process by redesigning the mold and / or designing the differential temperature during the design stage. This greatly reduces the rework and modification of the thin shell during the manufacturing process, thereby saving time and reducing costs, and improving the manufacturing efficiency of the thin shell; at the same time, it improves the product qualification rate and process stability.

[0145] S4. Digitally unfold the thin-shell reconstruction model to obtain an accurate thin-shell planar unfolded model, which can be used to manufacture a high-precision open arc-shaped integral thin-shell.

[0146] Specifically, using 3D modeling software or sheet metal simulation software, the final Mth thin-shell reconstruction model determined in S2 is digitally unfolded to obtain an accurate thin-shell planar unfolded model, which is then used to manufacture a high-precision open arc-shaped integral thin-shell.

[0147] S4 specifically includes the following steps:

[0148] S41: Using Boolean operations, the skin layer and stiffening rib layer included in the Mth thin-shell reconstruction model in S2 are segmented on the model to obtain two separate independent individual models; specifically including the following steps:

[0149] S411: First delete the rounded corners at the transition between the skin layer and the reinforcing rib layer;

[0150] S412: Offset the outer surface of the skin layer toward the reinforcing rib to obtain the offset sheet. The offset value is equal to the thickness of the skin layer.

[0151] S413: The thin shell is split into two parts, and the split surface is the bias sheet. At this time, the skin layer and the reinforcing rib layer are two separate independent individual models, namely, the skin layer body and the reinforcing rib layer body.

[0152] S42: Select the sheet metal function in the 3D model design software or use sheet metal simulation software to flatten two separate independent individual models using the same flattening datum, and obtain the flattened models of the skin layer and the reinforcing rib layer respectively.

[0153] (a) When the thin shell is a single-curvature shell, the main steps of S42 include:

[0154] Select the sheet metal flattening function in the sheet metal module of the three-model design software to convert the two independent individual models obtained in S41 into sheet metal respectively, and set the flattening datum of the two independent individual models and the neutral factor after conversion into sheet metal. Flatten the two independent individual models to obtain the flattened model of the skin layer and the reinforcing rib layer.

[0155] In one possible design, when the shell is a single-curvature and symmetrical shell, the flattening reference is preferably chosen on the intersection line of the skin layer and the reinforcing rib layer on the symmetry plane; this can minimize the geometric distortion that may occur during the unfolding process, which is very important for maintaining design accuracy and reducing subsequent correction work.

[0156] Preferably, the neutral factors satisfy: Xm0 < 0.5, Xj0 < 0.5, where Xm0 is the initial value of the neutral factor of the skin layer and Xj0 is the initial value of the neutral factor of the reinforcing rib layer; further, based on the results of each iteration, Xm0 and Xj0 are corrected, and in conjunction with the reconstruction design of the thin shell planar unfolding model mentioned in S44 below, the accuracy requirements of digital unfolding are met more quickly, i.e., θn ≤ 1 / 2 × αm.

[0157] (b) When the thin shell is a hyperboloid shell, the main steps of S42 include:

[0158] Choose the sheet metal forming function built into the three-model design software or use sheet metal simulation software to set the flattening reference for two independent individual models, and then perform mesh generation on the two independent individual models, setting the mesh type and mesh size;

[0159] Preferably, the mesh type includes one or a combination of solid mesh, shell mesh, and so on;

[0160] The physical network includes one or a combination of tetrahedral meshes, hexahedral meshes, and so on;

[0161] The shell mesh includes one or a combination of quadrilateral mesh, triangular mesh, and the like.

[0162] Preferably, the mesh size of the skin layer is <1 / 4×H m H m The thickness is the allowance for the process around the perimeter; the mesh size of the reinforcing rib layer is <1 / 4×T g T g The thickness of the reinforcing rib.

[0163] Preferably, the flattening reference is a point where the two curvature directions intersect;

[0164] In one possible design, when the thin shell is a hyperbolic and symmetrical shell, the flattening reference is selected as the intersection of the line of intersection of the two curvature direction symmetry planes and the separation surface of the skin layer and the stiffening layer; this can minimize the geometric distortion that may occur during the unfolding process, which is very important for maintaining design accuracy and reducing subsequent correction work.

[0165] In another possible design, when the shell is a double-curvature and asymmetrical or multi-curvature shell, the unfolding reference is selected from an easily identifiable point on the separation surface of the skin layer and the reinforcing rib layer.

[0166] S43: Using Boolean summation, merge the flattened models of the skin layer and the stiffener layer to obtain the combined body after flattening the skin layer and the stiffener layer, which is the initial model of the thin shell planar unfolding.

[0167] It should be noted that the flattened models of the skin layer and the reinforcing rib layer are assembled and merged using the same flattening datum as in S42.

[0168] S43 also includes, after merging

[0169] (a) Re-add the rounded corners where the skin layer and the reinforcing rib layer were originally deleted in S411;

[0170] (b) When Hj > H mg At that time, a protrusion is designed on the reinforcing rib of each grid structure on the side of the reinforcing rib layer away from the skin layer, and the height Ht of the protrusion on the reinforcing rib satisfies: Ht = Hj - H mgHt < 0.5mm, to ensure that the outer surface of the skin at the corresponding position of the reinforcing rib is formed in place;

[0171] Where Hj is the clearance value between the punch and die of the mold, H mg It is the sum of the thickness of the skin layer and the height of the reinforcing rib layer.

[0172] It should be noted that the crisscrossing reinforcing ribs form a mesh structure through several nodes, and the length of the reinforcing rib between any two nodes is greater than the sum of the widths of one or more protrusions (along the length direction of the reinforcing rib) between the two nodes, so as to ensure that the mesh structure formed by the crisscrossing reinforcing ribs remains connected.

[0173] Preferably, a protrusion is designed on the reinforcing rib between any two nodes.

[0174] Preferably, the width of each protrusion along the length of the reinforcing rib is 1mm to 5mm, which can ensure that the outer surface is formed in place while achieving lightweight.

[0175] S44: Using finite element simulation software, the initial thin-shell planar unfolding model is assembled onto the Nth optimized model of the mold in S3 to simulate the thin-shell forming process. The simulation output model of the initial thin-shell planar unfolding model after forming is compared with the Mth thin-shell reconstruction model in S2. Based on the dimensional deviation information of the skin and reinforcing ribs in different regions, the initial thin-shell planar unfolding model is adjusted to obtain the first adjusted model of thin-shell planar unfolding.

[0176] Specifically, (1) the initial model of the thin shell plane and the Nth optimization model of the mold in S3 are imported into the finite element simulation software, and the two are meshed. The thin shell plane is set as a plastic deformable body and the mold is set as a rigid body. Then the initial model of the thin shell plane is assembled onto the Nth optimization model of the mold in S3.

[0177] (2) Simulate the thin-shell forming process;

[0178] Molding temperature: Increase the temperature to T according to the temperature-time curve in S3. C0 Or the target temperatures for different regions determined by differential temperature design;

[0179] Forming loads include the gas pressure load of gas inflation forming and the rigid load of thermoforming; the minimum gas pressure load P for skin lamination is determined in S3. min The following conditions must be met when setting the forming load:

[0180] a) When the thin shell is not a semicircle, F = F0 + P × S, F0 ≥ P min ×S,P≥P minWhere F0 is the initial tonnage of the upper mold, F is the subsequent tonnage of the upper mold, P is the applied gas pressure load, S is the projected area of ​​the non-semicircular thin shell in the vertical direction, and P min To minimize the gas pressure load on the skin film.

[0181] b) When the thin shell is a semicircle, Fs = Fs0 + P × Ss, Fc = Fc0 + P × Sc, and Fs0 ≥ P min ×Ss,Fc0≥P min ×Sc,P≥P min Where Fs0 is the initial tonnage of the upper mold, Fs is the subsequent tonnage of the upper mold, Fc0 is the initial tonnage of the left and right side molds, Fc is the subsequent tonnage of the left and right side molds, P is the applied gas pressure load, Ss is the projected area of ​​the semicircular thin shell in the vertical direction, Sc is the projected area of ​​the semicircular thin shell in the horizontal direction, and P min To minimize the gas pressure load on the skin film.

[0182] After W iterations, the final adjusted model is compared with the simulation output model obtained in S44 and the thin shell reconstruction model in S2. The size difference θn of the skin and stiffener at different positions satisfies: θn≤1 / 2×αm, W≥1.

[0183] Specifically, the thin shell plane unfolding first adjustment model is simulated according to S44. The simulation output model after forming is compared with the thin shell reconstruction model of the Mth time in S2 to determine the size difference between the two in different regions. The thin shell plane unfolding first adjustment model is then adjusted to obtain the thin shell plane unfolding second adjustment model.

[0184] Each repetition of S44 is an "iteration". Through W iterations, until the final thin-shell planar unfolding adjustment model is obtained, the simulation output model obtained by repeating S44 is compared with the thin-shell reconstruction model in S2. The size difference θn of the skin and stiffener in different regions satisfies: θn≤1 / 2×αm. The thin-shell planar unfolding adjustment model of the Wth iteration is used as the final accurate thin-shell planar unfolding model, which is used for the subsequent manufacturing of the whole thin-shell. The preform is manufactured according to this final determined thin-shell planar unfolding model.

[0185] Preferably, θn ≤ 1 / 5 × αm to further improve forming accuracy. For example, for a complex integral thin-shell structure with reinforcing ribs and skin, αm is (0.3 mm to 0.7 mm) / 1000 mm. Preferably, αm is (0.4 mm to 0.6 mm) / 1000 mm, thereby achieving an optimal balance between efficiency and accuracy in process optimization (i.e., digital unfolding).

[0186] In one possible design, based on the thin-shell planar unfolded model, one or more combinations of the following are considered: the four-sided process allowance, the four-sided sealing allowance, and the preform positioning allowance. An additional allowance is designed at the edge of the unfolded model.

[0187] Understandably, (a) the four-sided process allowance: the extra amount of material reserved at the edge of the thin shell to facilitate subsequent processing operations. This allowance can absorb the tolerances generated during processing and reduce the risk caused by improper processing; (b) the four-sided sealing allowance: for semi-circular thin shells, to provide sufficient material to facilitate the setting of multiple rigid bosses around the perimeter inside the skin layer; (c) the preform positioning allowance: the extra dimensions reserved on the preform before the thin shell is formed to ensure the correct positioning of the preform during the forming process; for example, additional lugs can be reserved on the top and bottom of the preform, which cooperate with the positioning pins on the mold to quickly achieve accurate positioning.

[0188] It is worth noting that converting the thin-shell 3D model of S4 into a thin-shell planar unfolded model can achieve at least the following beneficial effects:

[0189] (1) Improve accuracy:

[0190] Based on digital unfolding, this invention uses finite element simulation to predict and evaluate the dimensional deviation between the unfolded thin-shell planar model and the expected reconstructed thin-shell model after forming. It employs multiple iterations to adjust the unfolded thin-shell planar model, obtaining an accurate model. This ensures that the preform processed based on the accurate unfolded model closely resembles the reconstructed thin-shell model after actual forming, and further, after assembly, closely approximates the theoretically designed thin-shell model. The method of this invention effectively improves the surface accuracy of the manufactured integral thin-shell in practical applications, achieving a product surface accuracy on the order of 0.3mm / 1000mm, reaching a leading level in the field.

[0191] Due to the improved surface accuracy, the weight deviation between the thin-shell component obtained by the method of this invention and the expected thin-shell theoretical model can be controlled within 1%. At the same time, due to the improved surface accuracy, the thin-shell component obtained by the method of this invention has a higher load-bearing capacity. In the prior art, thin-shell products obtained by traditional casting and machining have the problem of low surface accuracy. In order to avoid weak areas in mechanical properties, it is often necessary to leave a weight margin during product manufacturing. However, the thin-shell manufactured by the method provided by this invention has the characteristics of high precision and has a higher load-bearing capacity under the premise of the same weight.

[0192] (2) Improve the efficiency of thin-shell manufacturing and reduce costs:

[0193] a. First, the finalized thin-shell reconstruction model is digitally unfolded, transforming the three-dimensional model into a two-dimensional model to obtain the initial thin-shell planar unfolding model; b. Then, the forming process is simulated through finite element simulation to predict and evaluate the dimensional deviation between the initial thin-shell planar unfolding model and the three-dimensional model after forming and the thin-shell reconstruction model. The initial thin-shell planar unfolding model is adjusted through multiple iterations to obtain an accurate thin-shell planar unfolding model.

[0194] Based on the aforementioned accurate thin-shell planar unfolding model, a planar preform can be manufactured first in a two-dimensional flat state. This preform is then placed into a mold for shaping, resulting in a thin-shell component whose dimensions are very close to the expected thin-shell reconstruction model. Furthermore, since the thin-shell reconstruction design has already compensated for the deformation caused by subsequent assembly processes, assembling the shaped thin-shell component with other components ensures that the resulting product's thin-shell component dimensions are very close to the expected theoretical thin-shell model. This significantly reduces rework and modifications to the thin-shell during manufacturing, saving time and reducing costs, thus improving the manufacturing efficiency of the thin-shell. Simultaneously, it improves product qualification rate and process stability.

[0195] Compared to existing technologies that use five-axis equipment to mill reinforcing ribs and weight-reducing cavities on a three-dimensional inner surface, or to cast or additively produce reinforcing ribs and weight-reducing cavities on a three-dimensional inner surface, this invention transforms three-dimensional into two-dimensional, greatly reducing the difficulty of manufacturing and forming, improving manufacturing efficiency, and shortening the processing cycle by more than 75%; at the same time, it reduces the requirements for processing equipment, and the manufacturing cost can be reduced by more than 80%.

[0196] The present invention also provides a method for manufacturing an integral thin shell, including the steps described in the above-mentioned method for improving the manufacturing accuracy of an open arc-shaped integral thin shell.

[0197] In addition, the method for manufacturing an integral thin shell also includes the following steps:

[0198] S5. Based on S1 to S4, thin shell manufacturing is carried out in the order of preform manufacturing, thin shell forming, thin shell precision machining, and thin shell surface treatment to obtain a lightweight, high-load-bearing, high-precision thin shell.

[0199] S51: Preform manufacturing;

[0200] Based on the accurate thin-shell planar unfolding model determined in S4, select a suitable process scheme for preform manufacturing and carry out thin-shell preform manufacturing.

[0201] It should be noted that the manufacturing process of the preform can be selected from the two-dimensional processing schemes, including at least one of the following: thick plate machining scheme, skin and reinforcing rib superplastic diffusion link (SPF / DB) scheme, and additive manufacturing scheme.

[0202] S52: Thin-shell forming; specifically including the following steps:

[0203] S521: Install the mold obtained from the optimized model of the mold finally determined in S3, and heat the mold until the mold temperature reaches the preset value, i.e., T. C0 Alternatively, the target temperatures for different areas of the mold can be determined based on the differential temperature design in S3; T C0 The target forming temperature is initially determined based on the mold material;

[0204] Furthermore, before heating the mold, a temperature measuring device is installed inside the mold; for example, a temperature measuring thermocouple is installed in a temperature measuring hole in the upper mold.

[0205] Furthermore, during the heating process of the mold, preparation work for the preform manufactured by S51 is carried out simultaneously, including: cleaning the inner and outer surfaces of the preform, and spraying lubricant and / or anti-oxidant on the inner and outer surfaces of the preform. The lubricant is beneficial for demolding after forming and prevents the demolding process from affecting the surface accuracy of the thin shell. The anti-oxidant can prevent the surface of the preform from oxidizing when heated during the forming process, thereby improving the surface quality of the formed product.

[0206] S522: Once the mold temperature meets the preset value, the preform is placed in the mold cavity, kept warm, and then the mold is closed. Through a superplastic forming method combining hot pressing and air expansion forming, rigid-flexible composite synergistic loading is achieved to improve the accuracy, quality, and efficiency of thin shell forming.

[0207] (a) Insulation:

[0208] Preferably, the heat preservation time is Tb, where Tb is in minutes, and Tb = 1.5tmax to improve forming accuracy and quality; where tmax is the maximum wall thickness of the preform, and tmax is in mm.

[0209] It is understandable that a suitable heat preservation time has at least the following beneficial effects: 1) It can make the internal temperature of the preform more uniform and reduce uneven deformation caused by temperature gradient; 2) It can ensure that the preform has uniform microstructure before forming, ensure the microstructure uniformity of the thin-shell product after forming, and improve the quality of the thin-shell product; 3) It can improve production efficiency, obtain higher production efficiency while ensuring accuracy and quality, and reduce energy consumption.

[0210] (b) Superplastic forming method:

[0211] Specifically, for different types of thin shells, select the appropriate superplastic forming process.

[0212] A. When the thin shell is not a semicircle (θ≤180°), the main steps of superplastic forming include: the upper mold descends until it closes with the lower mold; the initial tonnage of the upper mold is F0; a gas pressure load P is applied; the gas pressure load P changes with time; and the subsequent tonnage F of the upper mold changes, satisfying:

[0213] F = F0 + P × S, F0 ≥ P min ×S,P≥P min Where S is the projected area of ​​the non-semicircular thin shell in the vertical direction, and P... min To minimize the gas pressure load on the skin film.

[0214] B. When the thin shell is a semicircle (θ > 180°), the main steps of superplastic forming include: the upper mold moves downward until the upper mold and the core mold close; the left and right side molds move left and right until the core mold closes; the initial tonnage of the upper mold is Fs0, and the initial tonnage of the left and right side molds is Fc0; a gas pressure load P is applied, and the gas pressure load P changes with time, causing changes in the subsequent tonnage Fs of the upper mold and the subsequent tonnage Fc of the left and right side molds, satisfying:

[0215] Fs=Fs0+P×Ss, Fc=Fc0+P×Sc, Fs0≥P min ×Ss,Fc0≥P min ×Sc,P≥

[0216] P min Where Ss is the projected area of ​​the semicircular thin shell in the vertical direction, Sc is the projected area of ​​the semicircular thin shell in the horizontal direction, and P... min To minimize the gas pressure load on the skin film.

[0217] It should be noted that the "rigid" in rigid-flexible composite synergistic loading can be understood as the rigid pressure applied by the mold to the preform in hot pressing, and the "flexible" can be understood as the flexible pressure applied by the gas to the preform in air expansion forming. Rigid-flexible composite synergistic loading can bring at least the following beneficial effects: (1) Improved accuracy: Hot pressing provides rigid pressure, while air expansion forming provides flexible pressure. This rigid-flexible composite loading method can transmit pressure more evenly, reduce local stress concentration, and thus improve forming accuracy; (2) Improved quality: The combination of hot pressing and air expansion forming helps to achieve a more uniform stress and strain distribution inside the material, reduce uneven deformation, wrinkles, cracks and other forming defects, and improve the overall quality of the formed thin shell products; (3) Improved efficiency: The combination of hot pressing and air expansion forming can shorten the forming cycle and improve production efficiency. In particular, air expansion forming can achieve the forming of complex shapes in a shorter time. At the same time, due to the improved accuracy, the need for subsequent machining and correction is reduced, saving materials and time and improving the overall manufacturing efficiency.

[0218] S53: Thin-shell precision machining;

[0219] Thin-shell finishing includes: removing the extra reserved portion at the edge of the thin shell by machining; the extra reserved portion includes one or a combination of process allowance around the perimeter, sealing allowance around the perimeter, and positioning allowance of the preform; optionally, the machining method includes, but is not limited to, one or a combination of turning, cutting, and milling.

[0220] S54: Thin-shell surface treatment;

[0221] Thin-shell surface treatment includes one or a combination of pickling, sandblasting, and laser cleaning.

[0222] The above surface treatment can bring at least the following beneficial effects: (1) Improved precision and quality: Pickling removes oxide layer and impurities, which can prevent these substances from affecting dimensional accuracy during subsequent assembly of the thin shell with other components (e.g., welding), and improves surface quality; Sandblasting can homogenize surface roughness and improve surface precision and quality; Laser cleaning provides a non-contact cleaning method that will not affect the dimensional accuracy of the thin shell, and can effectively remove tiny contaminants, avoiding affecting the quality of subsequent assembly of the thin shell with other components (e.g., welding); (2) Improved efficiency: Surface treatment can reduce rework and finishing in subsequent processes and improve production efficiency; (3) Reduced cost: By improving production efficiency and reducing rework, the cost per unit product can be reduced, and surface treatment can extend the service life of the thin shell and reduce maintenance costs.

[0223] Optionally, the 3D model design software, finite element analysis software, and sheet metal simulation software mentioned in S1 to S5 can be selected as needed; among them, 3D model design software includes but is not limited to AutoCAD, Pro / E, Autodesk Inventor, 3D Studio Max, CATIA, UG NX, SketchUp, SolidEdge, Onespace, Solidworks, and Maya; finite element analysis software includes but is not limited to ANSYS, ABAQUS, ADINA, COMSOL Multiphysics, MSCNastran, LUSAS, Algor, Femap / NX Nastran, Hypermesh, SAMCEF, SciFEA, pFEPG, SOLIDWORKSSimulation, STAR-CCM+, etc.; sheet metal simulation software includes but is not limited to AutoForm, InspireForm, Alibre Design, and FASTFORM, etc.

[0224] The technical solution of the present invention will be further described in detail below with reference to specific embodiments.

[0225] Example 1:

[0226] This embodiment provides a method for improving the manufacturing precision of open, curved, integral thin-shell structures, including the following steps:

[0227] S1. Based on the established thin-shell theoretical model 100, select a suitable overall process scheme and sealing method;

[0228] See Figure 2 The thin-shell theoretical model 100 is a semi-circle, including a skin layer 11 and a reinforcing rib layer 12 located inside the skin layer. The skin layer 11 and the reinforcing rib layer 12 are integrally connected, and the root of the connection between the skin layer 11 and each reinforcing rib 12a is a rounded transition. The material of the skin layer 11 and the reinforcing rib layer 12 is 5A06 aluminum alloy.

[0229] The outer surface of the skin layer 11 is a smooth conical curved surface, with a small end arc radius of 226 mm and an arc length of 1088 mm, a large end arc radius of 256 mm and an arc length of 1093 mm, and a shell length (i.e., the vertical distance between the small end arc and the large end arc) of 880 mm; the thickness Hm of the skin layer is 1.5 mm.

[0230] The reinforcing rib layer 12 adopts a crisscross cross structure, with 8 ribs in the horizontal direction and 7 ribs in the vertical direction, evenly distributed. The thickness of the reinforcing rib is T. g The thickness is 3mm, and the height-to-thickness ratio H of the reinforcing rib is... g / T g =5.

[0231] Overall process scheme: loading from top to bottom and left to right + air expansion forming process scheme.

[0232] Sealing method: Multiple rigid bosses are set on the inner side of the skin layer along the circumference of the mold to seal the thin shell. Pressure is applied to the upper, lower, left, right, front, and back inner and outer surfaces of the thin shell using the mold (e.g., upper mold and core mold, side mold and core mold, local pressure blocks). This three-dimensional rigid pressure achieves a seal between the semi-circular thin shell and the mold. (See also...) Figure 13 .

[0233] S2. Import the thin-shell theoretical model 100 into ANSYS finite element software and perform finite element simulation of the subsequent assembly processes of the thin-shell; specifically including the following steps:

[0234] S21: See also Figure 3(a) Using CAD design software, simulation models of the thin-shell theoretical model 100 to be assembled and the other three components, namely the front frame 200, the lower compartment 300, and the rear frame 400, are established. Assembly is achieved between any two adjacent components via welded joints. The four sides of the thin-shell theoretical model 100 overlap with the four stepped grooves on the right side of the front frame 200, the front and rear sides of the lower compartment 300, and the left side of the rear frame 400, respectively, forming flush welded joints. For example, see... Figure 3 (d) The left side of the thin shell theoretical model 100 forms a welded joint G with a flush surface by overlapping the stepped groove on the right side of the front frame 200.

[0235] S22: Will Figure 3 (a) The simulation model shown is imported into ANSYS finite element software. A welding simulation heat source is applied to each weld joint in the simulation model, and finite element simulation of subsequent assembly processes is performed to obtain the following results: Figure 3 (b) shows the model data after the thin shell is assembled.

[0236] S23: Will it be as follows Figure 3 (b) Comparison of the assembled thin-shell model with the theoretical thin-shell model 100 shown in Figure 100. Figure 4 The model reconstruction was divided into four regions: 1-1, 1-2, 1-3, and 1-4. The target reconstruction amount of the model in region 1-1 was (+0.2, +0.5) mm, in region 1-2 it was +0.6 mm, in region 1-3 it was (+0.2, +0.5) mm, and in region 1-4 it was (0, +0.5) mm. The boundaries of adjacent regions were smoothly transitioned.

[0237] S24: Based on such Figure 4 The target reconstruction amounts of the four regions shown are used to reconstruct the thin shell theoretical model 100 using CAD software to obtain the first thin shell reconstruction model. S21 to S24 are repeated, and after two iterations, the final determined second thin shell reconstruction model is assembled again. After finite element simulation of the subsequent processes, the dimensional difference θt between the model and the thin shell theoretical model 100 satisfies: θt≤1 / 5×αm, αm=0.5mm / 1000mm.

[0238] S3. Design molds for different types of thin shells, perform finite element simulation of the molds under heating, and based on the difference between the heated mold and the mold after proportional expansion in different regions, reconstruct and / or perform differential temperature design of the molds to improve forming accuracy during the design stage.

[0239] S31. Using CAD software, design a second mold 600 for forming thin shells;

[0240] See Figure 6The second mold 600 includes a second upper mold 61, a core mold 62, a mold base 63, a left mold 64, a right mold 65, a second upper platform 66, a second lower platform 67, and a side push rod 68. The left mold 64 and the right mold 65 are respectively located on the left and right sides of the core mold 62. The second upper mold 61 is located above the core mold 62, the mold base 63 is located below the core mold 62, the second upper platform 66 is located above the second upper mold 61, and the second lower platform 67 is located below the mold base 63. When the second upper mold 61, the left mold 64, and the right mold 65 are closed relative to the core mold 62, the second upper mold 61, the left mold 64, the right mold 65, and the core mold 62 cooperate with each other to form a semi-circular arc-shaped mold cavity.

[0241] S32: Import the theoretical model of the mold used to form the thin shell into the finite element analysis software and perform a heating finite element simulation of the mold.

[0242] S321. Geometric Model Establishment:

[0243] Will as Figure 6 The theoretical model of the second mold 600 for forming thin shells is shown placed in a model of a high-temperature forming furnace 700 in the shape of a hexahedral box, with a heating platform (i.e., the inner surface of the bottom of the forming furnace) provided at the bottom of the second mold 600.

[0244] S322. Pre-processing: This includes setting boundary conditions and defining the temperature-time change curve of the heating platform.

[0245] (a) Boundary conditions: The second mold 600 and the heating platform 71 adopt the contact heat transfer mode, the other parts of the second mold 600 and the furnace 700 adopt the radiation heat transfer mode, and the interior of the furnace 700 adopts the convection heat transfer mode.

[0246] (b) Temperature-time variation curve: T = 20℃ + 0.02℃ / s × t, until T = 500℃, where t is time in seconds.

[0247] S323. Perform finite element simulation of the mold during heating:

[0248] After preprocessing, a finite element simulation of the mold was performed to obtain the model data after the mold was heated. The material of the second mold 600 is 45 steel, and its coefficient of linear expansion α is 14.18×10⁻⁶ in the range of 20℃~500℃. -6 ℃ -1 Therefore, the proportional expansion coefficient λ = 1 + α × (T) C0 -T0)=1+14.18×10 -6 ℃ -1 ×(500℃-20℃)=1.0068.

[0249] Export the model data after the mold is heated, and compare the upper mold and the left and right side molds with the model of the mold that expands proportionally according to the scaling factor λ = 1.0068; obtain the difference between the heated mold and the proportionally expanded mold in different regions;

[0250] S34. Reconstruct and / or perform differential temperature design on the mold to obtain the first optimized model of the mold;

[0251] The main steps in differential temperature design include: (See below) Figure 9 The bottom heating platform is divided into nine heating zones, J1-J9, with different temperatures set for each zone. For example, J5 is set to 400℃, J2, J4, J6, and J8 to 500℃, J1 and J3 to 460℃, and J7 and J9 to 480℃, to achieve near-proportional expansion. The main steps in the mold reconstruction design include... (See...) Figure 8 Based on the difference between the mold after heating and the mold after proportional expansion in different regions, the mold surface is redesigned. The mold reconstruction coefficient δ2 = 0.8 to approximate proportional expansion.

[0252] Repeat S321 to S324, and iterate twice until the final optimized model of the mold is determined. After the second heating finite element simulation, the dimensional difference θr between the mold and the mold after proportional expansion satisfies: θr≤1 / 2×αm, αm=0.5mm / 1000mm.

[0253] S4. Digitally unfold the thin-shell reconstruction model to obtain an accurate thin-shell planar unfolded model, which can be used to manufacture a high-precision open arc-shaped integral thin-shell; specifically including the following steps:

[0254] S41: Using Boolean operations, the skin layer and stiffening rib layer included in the second thin-shell reconstruction model in S2 are segmented on the model to obtain two separate, independent individual models. See [link to relevant documentation]. Figure 10 (a) and Figure 10 (b); Specifically, it includes the following steps:

[0255] S411: First delete the rounded corners at the transition between the skin layer and the reinforcing rib layer;

[0256] S412: Offset the outer surface of the skin layer toward the reinforcing rib to obtain the offset sheet. The offset value is equal to the thickness of the skin layer, i.e., 1.5mm.

[0257] S413: Set the thin shell to be split into two bodies, with the split surface being the offset sheet body; at this time, the skin layer and the stiffener layer are two separate independent individual models, namely, the skin layer body and the stiffener layer body.

[0258] S42: Select the sheet metal flattening function in the sheet metal module of the CAD design software to convert the two separate independent individual models obtained in S41 into sheet metal, set the flattening datum of the two independent individual models and the neutral factor after conversion into sheet metal, flatten the two independent individual models to obtain the flattened model of the skin layer and the stiffener layer.

[0259] The thin-shell theoretical model 100 is a shell with single curvature and symmetry. The flattening datum is selected on the intersection line of the skin layer and the stiffening rib layer on the symmetry plane. The neutral factors satisfy: Xm0 = 0.4, Xj0 = 0.4, where Xm0 is the initial value of the neutral factor of the skin layer and Xj0 is the initial value of the neutral factor of the stiffening rib layer.

[0260] S43: Using Boolean summation, merge the flattened models of the skin layer and the stiffener layer. After merging, restore the fillet at the transition between the skin layer and the stiffener layer that was originally deleted in S411, obtaining the merged model after flattening the skin layer and the stiffener layer. See [link to documentation]. Figure 10 (c) A protrusion is designed on the reinforcing rib between any two nodes of each grid on the side of the reinforcing rib layer away from the skin layer. The height of each protrusion is Ht = 0.3 mm, and the width of each protrusion is 3 mm. Furthermore, considering the process allowance around the perimeter, the sealing allowance around the perimeter, and the positioning allowance of the preform, the following is obtained: Figure 10 The final merged body shown in (d) serves as the initial model for the thin-shell planar unfolding;

[0261] S44: Using ANSYS finite element software, the initial thin-shell planar unfolding model is assembled onto the second optimized model of the mold in S3 to simulate the thin-shell forming process.

[0262] (1) Import the following using ANSYS software: Figure 10 (d) shows the initial model of the thin-shell plane unfolding and the second optimized model of the mold in S3. The two are meshed, the thin-shell plane is set as a plastic deformable body and the mold is set as a rigid body. Then the initial model of the thin-shell plane unfolding is assembled onto the second optimized model of the mold in S3.

[0263] (2) Simulate the thin-shell forming process;

[0264] Molding temperature: Based on the temperature-time curve in S3, the temperature is increased to the value determined by the differential temperature design. Figure 9 The target temperatures for the nine different zones shown are (400℃ for J5, 500℃ for J2, J4, J6, and J8, 460℃ for J1 and J3, and 480℃ for J7 and J9).

[0265] Forming load: includes gas pressure load for air inflation forming and rigid loading load for hot pressing forming; the forming load is set under the following conditions:

[0266] Fs=Fs0+P×Ss, Fc=Fc0+P×Sc, Fs0≥P min ×Ss,Fc0≥P min ×Sc,P≥

[0267] P min Among them, based on Pmin = 0.7 MPa and Ss = 440000 mm 2 Sc = 300000mm 2 The calculations show that Fs0 ≥ 30.8 tons and Fc0 ≥ 21 tons. To better ensure proper forming, Fs0 = 40 tons and Fc0 = 30 tons. Other parameters include: displacement constraint in a 1 / 2 symmetrical direction, deformation speed of 0.5 mm / s, deformation time of 1800 s, and step parameters of 90.

[0268] (3) Compare the simulation output model of the initial thin-shell planar unfolding model after forming with the second thin-shell reconstruction model in S2. Based on the deviation dimension information of the skin and stiffeners in different regions, adjust the initial model to obtain the first adjusted model of the thin-shell planar unfolding; see Figure 11 After two iterations, the simulation output model obtained by repeating the forming process simulation in S44 after the second adjustment model is obtained is compared with the second thin shell reconstruction model in S2. The size difference θn of the skin and stiffener at different positions satisfies: θn≤1 / 2×αm, αm=0.5mm / 1000mm.

[0269] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for improving the manufacturing precision of an open, arc-shaped integral thin shell, characterized in that, Includes the following steps: S1. For different types of thin-shell structures, select appropriate overall process solutions and sealing methods to improve forming accuracy during the design phase: The thin shell is an open arc-shaped structure, including an outer skin layer and an inner reinforcing rib layer. The reinforcing rib layer is a grid structure formed by crisscrossing reinforcing ribs. When the thin shell is not a semicircle, the process of top and bottom loading + air expansion forming is adopted. A rigid pressure around the perimeter is used to achieve a seal between the thin shell and the mold. When the thin shell is in the form of a semicircle, the process of loading from top to bottom and left to right and air expansion is adopted. The thin shell and the mold are sealed by applying pressure to the top, bottom, left and right, front and back inner and outer surfaces of the thin shell through three-dimensional rigid pressure. S2. Import the theoretical model of the thin shell into the finite element analysis software, perform finite element simulation of the subsequent assembly process of the thin shell, and based on the information of deformation in different areas of the assembled thin shell, reconstruct the theoretical model of the thin shell to obtain the reconstructed model of the thin shell, so as to improve the accuracy in the design stage. S3. Design molds for different types of thin shells, perform finite element simulation of the molds under heating, and based on the difference between the heated mold and the mold after proportional expansion in different regions, reconstruct and / or perform differential temperature design of the molds to improve forming accuracy during the design stage. S3 specifically includes the following steps: S31: Using 3D modeling software, design a theoretical model of the mold for forming the thin shell, based on whether the thin shell is a semicircle or not. S32: Import the theoretical model of the mold used to form the thin shell into the finite element analysis software, perform finite element simulation of the mold under heating, and based on the difference between the mold after heating and the mold after proportional expansion in different regions, perform reconstruction design and / or differential temperature design of the mold to obtain the first optimized model of the mold. After N iterations, ensure that the dimensional difference θr between the Nth optimized model of the mold and the mold after proportional expansion, after the finite element simulation of the heating process, satisfies: θr≤1 / 2×αm; αm is the maximum allowable value for the product surface accuracy, N≥1; S4. Digitally unfold the thin-shell reconstruction model to obtain an accurate thin-shell planar unfolded model, which can be used to manufacture a high-precision open arc-shaped integral thin-shell.

2. The method for improving the manufacturing precision of open arc-shaped integral thin shells according to claim 1, characterized in that, Step S32 specifically includes the following steps: S321. Geometric Model Establishment: The theoretical model of the mold used for forming thin shells is placed in the model of the high-temperature forming furnace, and a heating platform is set on the bottom surface of the mold. S322. Pre-processing: This includes setting boundary conditions and defining the temperature-time change curve of the heating platform. (a) Boundary conditions: The mold and the heating platform adopt the contact heat transfer mode, the mold and other parts of the furnace adopt the radiation heat transfer mode, and the furnace interior adopts the convection heat transfer mode. (b) Temperature-time curve: T = T0 + σ × t, until T = T C0 T0 is the initial temperature in °C; t is the time in seconds; σ is the heating coefficient in °C / s; T C0 The target forming temperature is initially determined based on the mold material; S323. Perform finite element simulation of the mold during heating: After preprocessing, the mold is subjected to finite element simulation with heating to obtain the model data of the mold after heating. The model data of the mold after heating is exported and compared with the model of the mold after proportional expansion to obtain the difference between the mold after heating and the mold after proportional expansion in different regions. S324. Perform reconstruction design and / or differential temperature design on the mold to obtain the first optimized model of the mold; wherein, the main steps of differential temperature design include: dividing the heating platform into different heating areas, setting different target temperatures for different heating areas, and obtaining the first optimized model of the mold; Repeat S321 to S324, and iterate N times until the difference θr between the Nth optimized model of the mold and the mold after the proportional expansion is satisfied: θr≤1 / 5×αm.

3. The method for improving the manufacturing precision of an open, arc-shaped integral thin shell according to claim 2, characterized in that, In S322, T0 = room temperature, and σ is 0.01℃ / s ~ 0.03℃ / s.

4. The method for improving the manufacturing precision of open arc-shaped integral thin shells according to claim 2, characterized in that, In S323, the proportional expansion of the mold refers to the fact that, in three-dimensional space, the mold expands in all directions with the same scaling factor, where the scaling factor λ = α × (T C0 -T0), where α is the coefficient of linear expansion.

5. The method for improving the manufacturing precision of an open, arc-shaped integral thin shell according to claim 4, characterized in that, When the mold is made of 45# steel, in S322, T C0 =400℃~600℃; In S323, α is 14.18×10 -6 ℃ -1 .

6. The method for improving the manufacturing precision of open arc-shaped integral thin shells according to claim 2, characterized in that, In S321, the high-temperature forming furnace chamber is a hexahedral box.

7. The method for improving the manufacturing precision of open arc-shaped integral thin shells according to claim 1, characterized in that, S32 also includes, during the heating finite element simulation, using the complete adhesion of the skin layer to the mold as the evaluation criterion, obtaining the minimum gas pressure load P required for the skin layer to adhere. min Therefore, during the design phase, the process conditions that can guarantee the forming accuracy of the thin shell are determined, i.e., the following conditions are met: The gas pressure load P ≥ P during the gas expansion forming process min ; The initial tonnage of the upper and lower loading is F0≥P min ×S, where S is the projected area of ​​the non-semicircular thin shell in the vertical direction. The initial tonnage of the up, down, left, and right loading is Fs0≥P min ×Ss,Fc0≥P min ×Sc, Fs0 is the initial tonnage of the vertical loading, Fc0 is the initial tonnage of the horizontal loading, Ss is the projected area of ​​the semicircular shell in the vertical direction, and Sc is the projected area of ​​the semicircular shell in the horizontal direction.

8. The method for improving the manufacturing precision of an open, arc-shaped integral thin shell according to claim 7, characterized in that, The minimum gas pressure load P for attaching the skin film min Through formula P min =H m / r×σs is calculated, where H m σs represents the thickness of the process allowance around the perimeter, r represents the minimum fillet size of the neutral layer in the final molding area of ​​the skin layer, and σs represents the yield strength of the skin layer material at the forming temperature.

9. The method for improving the manufacturing precision of open arc-shaped integral thin shells according to claim 1, characterized in that, The main steps for achieving a seal between the semi-circular thin shell and the mold through three-dimensional rigid pressure (up / down, left / right, and front / back) include: setting multiple rigid bosses along the circumferential process allowance on the inner side of the skin layer; and designing protrusions on the reinforcing ribs of each grid structure on the side of the reinforcing rib layer away from the skin layer. The width of each protrusion should ensure that each grid structure is connected. The height Ht of the protrusions on the reinforcing ribs satisfies: Ht = Hj - H mg Ht < 0.5mm to ensure the outer surface of the skin at the corresponding position of the reinforcing rib is properly formed; where Hj is the clearance value between the die and the mold punch, H mg It is the sum of the thickness of the skin layer and the height of the reinforcing rib layer.

10. A method for manufacturing an integral thin-shell, characterized in that, Includes the steps described in the method for improving the manufacturing precision of open arc-shaped integral thin shells as described in any one of claims 1-9.

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