A gradient cryogenic forming method for deep cavity thin-walled curved parts

Abstract

This invention discloses a gradient cryogenic forming method for deep-cavity thin-walled curved parts, relating to the field of metal sheet processing technology. The method includes: dividing the sheet material to be formed into a flange area, a suspended area, and a molding area; establishing a stress gradient model that prevents cracking and wrinkling based on the radial and circumferential stresses of each deformation area, and accordingly determining cryogenic cooling conditions and blank holder force loading conditions; placing the sheet material on a die and pressing the flange area; injecting a cryogenic medium into a normal-pressure buffer tank through a self-pressurizing tank, allowing the liquid portion to flow into the upper surface of the sheet material in the suspended area by gravity, thus constructing a gradient temperature field radially across the flange area, suspended area, and molding area; driving the punch downwards under a set blank holder force, causing the sheet material to deform in a coordinated manner within this gradient temperature field, ultimately forming a deep-cavity thin-walled curved part. This method effectively suppresses the simultaneous defects of cracking and wrinkling caused by concentrated deformation in the suspended area, achieving coordinated deformation in each area and ensuring a stable forming effect for the curved part.

Description

Technical Field

[0001] This invention relates to the field of metal sheet processing technology, and in particular to a gradient cryogenic forming method for deep cavity thin-walled curved surface parts. Background Technology

[0002] To meet the manufacturing demands for high reliability, lightweight, low cost, and high efficiency, next-generation launch vehicles (such as rockets, aircraft, and new energy vehicles) urgently require integral structures to replace traditional multi-piece welded structures. This has given rise to a class of lightweight, high-strength alloy deep-cavity thin-walled curved surface components with integral structures. These components are characterized by large dimensions, thin walls, and semi-open, closed deep cavities (e.g., the bottom of rocket propellant tanks with diameters of 2-4 meters and wall thicknesses of less than 3 mm). The thickness-to-diameter ratio of these components is far less than the wrinkling limit, and the depth-to-diameter ratio is also less than the cracking limit. The coexistence of cracking and wrinkling defects has always been an international challenge restricting the integral forming and manufacturing of such structures.

[0003] In the forming process of deep-cavity, thin-walled curved parts, the suspended areas not compressed by the mold are the main deformation zones and also the most dangerous areas, bearing complex stress states of circumferential compression and radial tension. At room temperature, lightweight alloys have low plasticity and weak hardening ability, making them extremely prone to cracking due to excessive tension caused by concentrated deformation in the suspended areas. To prevent wrinkling in the suspended areas, the radial tensile stress needs to be increased, but this will further exacerbate the concentrated deformation in the suspended areas, creating a vicious cycle. Even with complex processes such as multi-pass forming or intermediate annealing, it is difficult to fundamentally solve this problem.

[0004] While thermoforming can improve the ductility of materials, it also leads to softening and a significant reduction in hardening stress, making deformation more difficult to transfer. Consequently, it is more likely to cause concentrated deformation and cracking in the force transmission path (i.e., the suspended area). Therefore, existing room temperature and high temperature forming technologies cannot effectively solve the core problem of concentrated deformation and cracking in the suspended area of ​​deep-cavity thin-walled curved parts. Summary of the Invention

[0005] The purpose of this invention is to provide a gradient cryogenic forming method for deep cavity thin-walled curved parts, so as to solve the problems existing in the prior art, achieve coordinated deformation in each region, and ensure the stable forming effect of the curved parts.

[0006] To achieve the above objectives, the present invention provides the following solution: This invention provides a gradient cryogenic forming method for deep-cavity thin-walled curved parts. Based on the stress constraint criterion of the deformation zone, the method controls the stress distribution at cryogenic temperatures to form deep-cavity thin-walled curved parts under gradient cryogenic conditions. The method includes the following steps: S1 divides the sheet material to be formed into three deformation zones during the forming process: the flange zone pressed by the pressure ring, the suspended zone that is not in contact with the mold and is in a free deformation state, and the mold-fitting zone that has been attached to the convex mold surface. Based on the flange area, the suspended area and the molding area, the radial stress and circumferential stress of each deformation area are mechanically analyzed. Based on the stress relationship constrained by the yield criterion, a stress gradient model that does not cause cracking and wrinkling defects is established. Based on the stress gradient model, the ultra-low temperature cooling conditions and edge clamping force loading conditions of the plate are formulated. S2, Position the plate on the die and close the pressure ring to press the flange area; S3, add the cryogenic medium from the self-pressurized storage tank into the atmospheric pressure buffer tank; S4, the liquid cryogenic medium in the atmospheric pressure buffer tank flows into the upper surface of the plate in the suspended area by gravity to construct a gradient temperature field of the plate in the flange area, the suspended area and the molding area along the radial direction; S5, under the action of the set blank holder force, the punch is driven to move downward, so that the sheet metal is deformed in a coordinated manner under the gradient temperature field, and finally formed into a deep cavity thin wall curved surface part.

[0007] Preferably, the gradient temperature field constructed in S4 satisfies: T u ≤ T p << T f ; in, T u Temperature of the suspended section of the board; T p The temperature of the substrate in the molding area; T f This refers to the plate temperature in the flange area.

[0008] Preferably, the mechanical analysis in S1 is based on the principle of force balance to analyze the radial stress of the flange area, the suspended area, and the molded area respectively, and to clarify the influencing factors and interrelationships of the radial stress in each of the deformation areas; at the same time, combined with the material flow law of sheet forming, the circumferential tensile and compressive stress boundary circle of the suspended area is defined, and the suspended area is divided into a cracking risk area with both radial and circumferential tensile stress and a wrinkling risk area with radial tension and circumferential compression.

[0009] Preferably, before implementing S4, a heating element is provided inside the flange area of ​​the plate corresponding to the pressure ring and the die, and the pressure ring and the die are heated to 100°C to recrystallization temperature to reduce the plate flow stress in the flange area and increase the stress gradient in each deformation zone of the plate.

[0010] Preferably, before S4, a step of heat insulation treatment or temperature control of the convex model surface is included to control the temperature of the plate material in the molding area.

[0011] Preferably, in S5, after the deep cavity thin-walled curved surface part is basically formed, the blank holder force is further increased to make the punch continue to move downward and generate overload displacement, so as to apply thin film stress in the component surface of the deep cavity thin-walled curved surface part.

[0012] Preferably, the cryogenic medium is one of liquid argon, liquid nitrogen, or liquid helium.

[0013] Preferably, the cryogenic medium flows into the upper surface of the plate in the suspended area, cooling it to an ultra-low temperature of -160℃ to -270℃.

[0014] Preferably, the plate material is made of aluminum alloy, stainless steel, titanium alloy, or nickel-based high-temperature alloy.

[0015] Preferably, the heat insulation treatment is to apply a cold-insulating paint to the surface of the convex mold, and the temperature control is to inject a low-temperature medium into the convex mold.

[0016] The present invention achieves the following technical effects compared to the prior art: The gradient cryogenic forming method for deep-cavity thin-walled curved parts provided by this invention divides the deformation process into three deformation zones: a flange zone, a suspended zone, and a molding zone. Based on this, mechanical analysis and a stress gradient model are established, providing directional guidance for the entire process. This ensures that the subsequent cryogenic cooling conditions and blank holder force loading conditions are based on precise theoretical drivers to prevent cracking and wrinkling defects. A liquid cryogenic medium flows into the upper surface of the sheet metal in the suspended zone by gravity, constructing a gradient temperature field in the flange zone, suspended zone, and molding zone. This keeps the suspended zone at a cryogenic temperature while the flange zone maintains a relatively high temperature, utilizing the enhanced material hardening ability and increased elongation under cryogenic conditions. The "double enhancement effect" significantly improves the flow stress and ultimate deformation capacity of the suspended area, thereby preventing cracking in the suspended area due to excessive radial tensile stress. At the same time, by controlling the gradient temperature field, the material in the flange area is easy to flow and the material in the suspended area is easy to transmit force. Furthermore, the temperature of the molding area is higher than that of the suspended area, allowing the molding area, which was originally not deforming, to continue to deform, thus achieving coordinated deformation of each deformation area. Finally, under the synergistic effect of the set blank holder force and the gradient temperature field, the punch is driven to move downward to coordinate the molding and forming of the sheet metal. While preventing defects, it fundamentally improves the deformation uniformity, reduces the wall thickness deviation of the component, and avoids the problem of machining to remove excess wall thickness after forming large components. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 Schematic diagrams of the various devices used in the gradient cryogenic forming method for deep cavity thin-walled curved parts provided by the present invention; Figure 2 This is a schematic diagram of the parameters in the gradient cryogenic forming method for deep cavity thin-walled curved surface parts provided by the present invention. Figure 3 This is a schematic diagram showing the positions of the flange area, the suspended area, and the molding area in the gradient cryogenic forming method for deep cavity thin-walled curved parts provided by the present invention. Figure 4 for Figure 3 A schematic diagram of temperature parameters corresponding to each deformation zone; Figure 5 This is a schematic diagram of the deformation process in the gradient cryogenic forming method for deep cavity thin-walled curved parts provided by the present invention. Figure 6 A schematic diagram of the structure of the deep cavity thin-walled curved surface part finally formed by the gradient cryogenic forming method for deep cavity thin-walled curved surface parts provided by the present invention; Figure 7 This is a schematic diagram showing the variation curves of flow stress (also known as hardening stress) of 2219-O state aluminum alloy under different temperatures and strains; Figure 8 This is a schematic diagram of the actual stress-strain curves of 2219-O state aluminum alloy at different temperatures.

[0019] In the diagram: 1 is the die; 2 is the sheet metal; 3 is the pressure ring; 4 is the cryogenic medium; 5 is the punch; 6 is the cryogenic valve; 7 is the atmospheric pressure buffer tank; 8 is the exhaust port; 9 is the self-pressurizing storage tank; 10 is the heating element. 2-1 is the flange area; 2-2 is the suspended area; 2-3 is the molding area. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The purpose of this invention is to provide a gradient cryogenic forming method for deep cavity thin-walled curved parts, so as to solve the problems existing in the prior art, achieve coordinated deformation in each region, and ensure the stable forming effect of the curved parts.

[0022] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] Example 1 This embodiment provides a gradient cryogenic forming method for deep-cavity thin-walled curved surface parts, such as... Figures 1-8 As shown, based on the stress constraint criterion of the deformation zone, deep cavity thin-walled curved parts are formed under gradient cryogenic conditions by controlling the stress distribution at ultra-low temperatures. The process includes the following steps: S1, the plate 2 to be formed is divided into three deformation zones during the forming process: the flange zone 2-1 which is pressed by the pressure ring 3, the suspended zone 2-2 which is not in contact with the mold and is in a free deformation state, and the mold-fitting zone 2-3 which is attached to the surface of the punch 5. Based on the flange area 2-1, the suspended area 2-2 and the molded area 2-3, the radial stress and circumferential stress of each deformation area are mechanically analyzed. Based on the stress relationship constrained by the yield criterion, a stress gradient model that does not cause cracking and wrinkling defects is established. Based on the stress gradient model, the ultra-low temperature cooling conditions and edge clamping force loading conditions of plate 2 are formulated. S2, Position the plate 2 on the die 1, and close the pressure ring 3 to press the flange area 2-1; S3, add the cryogenic medium 4 from the self-pressurized storage tank 9 into the atmospheric pressure buffer tank 7; S4, the liquid cryogenic medium 4 in the atmospheric pressure buffer tank 7 flows into the upper surface of the plate 2 in the suspended area 2-2 by gravity to construct a gradient temperature field of the plate 2 in the radial direction in the flange area 2-1, the suspended area 2-2 and the molding area 2-3. S5, under the action of the set blank holder force, drives the punch 5 to move downward, so that the sheet 2 is deformed in a coordinated manner under the gradient temperature field, and finally formed into a deep cavity thin wall curved surface part.

[0024] By dividing the deformation process into three deformation zones—flange zone 2-1, suspended zone 2-2, and molded zone 2-3—and conducting mechanical analysis and establishing a stress gradient model based on this, the entire process is guided by precise theoretical principles to ensure that subsequent cryogenic cooling and blank holder loading conditions are determined based on the prevention of cracking and wrinkling defects. A gradient temperature field is constructed in the flange zone 2-1, suspended zone 2-2, and molded zone 2-3 by gravity-fed liquid cryogenic medium 4 into the upper surface of the plate 2 in the suspended zone 2-2. This ensures that the suspended zone 2-2 is at a cryogenic temperature while the flange zone 2-1 remains at a relatively high temperature. Utilizing the "double-enhancement effect" of enhanced material hardening capacity and increased elongation at cryogenic temperatures, the material's hardening capacity is significantly improved. The flow stress and ultimate deformation capacity of the suspended zone 2-2 were increased, thus preventing cracking of the suspended zone 2-2 due to excessive radial tensile stress. At the same time, the gradient temperature field control made the material in the flange zone 2-1 easier to flow and the material in the suspended zone 2-2 easier to transmit force. Furthermore, the temperature of the molding zone 2-3 was made higher than that of the suspended zone 2-2, allowing the molding zone 2-3, which was originally no longer deformable, to continue to deform, thus achieving coordinated deformation of each deformation zone. Finally, under the synergistic effect of the set blank holder force and the gradient temperature field, the punch 5 was driven to descend to coordinate the molding and forming of the sheet metal 2. While preventing defects from occurring, the deformation uniformity was fundamentally improved, the component wall thickness deviation was reduced, and the problem of removing excess wall thickness after forming large components was avoided.

[0025] Regarding the relevant explanation of step S1: Among the optional solutions in this embodiment, the more preferred one is as follows: Figures 1-3 As shown, the mechanical analysis in S1 is based on the principle of force balance to analyze the radial stress of flange area 2-1, suspended area 2-2, and molded area 2-3 respectively, clarifying the influencing factors and interrelationships of radial stress in each deformation area; at the same time, combined with the material flow law of plate 2 forming, the circumferential tensile and compressive stress boundary circle of suspended area 2-2 is defined, dividing suspended area 2-2 into a cracking risk area with both radial and circumferential tensile stress and a wrinkling risk area with radial tension and circumferential compression.

[0026] Specifically, refer to Figure 2 Taking the forming of spherical curved surface parts as an example, the specific details are as follows: 1-Calculation of geometric parameters of the deformation zone (based on the principle of constant volume, calculate the geometric parameters of each deformation zone of the slab under different forming depths to provide geometric data support for subsequent stress mechanics analysis): Based on the principle of constant volume, and ignoring the wall thickness change during the forming process of the slab (i.e., plate 2), the area of ​​each region of the curved part with different forming heights is consistent with the initial area of ​​the slab, satisfying: (1); In the formula: R 0 The initial radius of the slab; SⅠ The area of ​​the flange zone 2-1 circular ring; S Ⅱ The area of ​​the rounded corners; S Ⅲ The area of ​​the suspended region 2-2 cone surface; S Ⅳ This refers to the area of ​​the 2-3 surface of the molding area.

[0027] The area of ​​each region can be determined according to the forming depth. H The dimensions are obtained from the already determined mold geometry. This is based on a specific forming depth. H The geometric relationship at that time can be used to derive the radius of gyration of the molded surface in the molded area 2-3. R α With cone angle α : (2); In the formula: R α The radius of the tangent point of the 2-3 punches in the molding area is 5. R p The radius of the punch is 5. H For forming depth; r d The radius of the fillet of die 1; α The cone angle is 2-3 for the molding area.

[0028] Based on this, the height of the cone surface in the suspended region 2-2 is further derived. H u : (3); In the formula: H u The height of the suspended region 2-2 cone surface; H For forming depth; r d The radius of the fillet of die 1; α The molding area has a 2-3 cone angle; R p The radius of the punch is 5. R α The radius of the tangent point of the 2-3 punches in the molding area is 5.

[0029] Therefore, the real-time areas of the spherical shell molding area 2-3, the suspended area 2-2, and the rounded corner area can be calculated: (4); In the formula: b R is the radius of the minor semi-axis of the ellipsoid.

[0030] Substituting equation (4) into equation (1), and using the annular area of ​​flange region 2-1... SⅠ Inverse calculation of the instantaneous outer diameter of the slab R : (5); 2-Analysis of radial stress in each deformation zone: During the spherical shell forming process, the slab is mainly divided into three deformation zones: the first is the flange area 2-1 where the die 1 and the blank holder 3 are pressed together; the second is the suspended area 2-2 between the flange pressing and the mold; and the third is the mold-fitting area 2-3 where it is already against the punch 5. The radial tensile stress in the flange area 2-1... Based on the radial force balance during the deformation of the slab in flange zone 2-1, the instantaneous outer diameter R of the slab can be calculated. (6); In the formula: β This indicates the difference in yield condition when considering three shear stresses versus considering only the absolute maximum shear stress; it is typically set to 1.1. This indicates the hardening stress (equivalent stress) at the edge of flange zone 2-1. r The radial radius is at any position; μ b This indicates the friction factor between the mold and the slab in flange area 2-1.

[0031] Radial stress in suspended region 2-2 The radial tensile stress at the entrance of die 1 can be determined based on the radial force balance during slab deformation in the suspended zone 2-2. (7); In the formula: This indicates the radial stress in the suspended region 2-2; This indicates the radial stress at the entrance of die 1; t Indicates the slab wall thickness.

[0032] After simplifying equation (7), the radial stress at any point in the suspended region 2-2 can be obtained: (8); To pull the plate 2 of flange area 2-1 to the die 1, the radial stress at the entrance of die 1 is subject to the radial stress of flange area 2-1. In addition, there is the blank holder force. F b Friction generated on the material surface in flange area 2-1 σ m The bending resistance generated when the material in flange area 2-1 flows through the fillet of die 1 σ w And the material in flange area 2-1 bypasses the fillet of die 1 to overcome resistance. T e μα .

[0033] (9); (10); In the formula: μ α is the friction factor between the blank and the mold in the rounded corner area of ​​die 1; α is the wrap angle at the rounded corner of die 1.

[0034] Combining equations (8), (9), and (10), the radial tensile stress at the entrance of die 1 can be obtained: (11); Substituting equation (11) into equation (7), we can obtain the radial tensile stress at any point in the suspended region 2-2: (12); Similarly, the radial tensile stress at any point in the molding area 2-3 can be calculated: (13); In the formula: This represents the radial stress at any point in the molding area 2-3; F d Indicates the drawing force; μ d This indicates the friction factor between punch 5 and the blank; β This represents the cone angle corresponding to any point.

[0035] 3. Establishment of a stress gradient model for crack prevention and control: Further stress analysis reveals that the radial stress in the suspended area 2-2 remains tensile. During the slab forming process, material flows from the flange area 2-1 into the mold, causing circumferential shrinkage and compressive stress; the punch 5 undergoes bulging deformation in the mold-attaching area 2-3, resulting in circumferential expansion and tensile stress. Therefore, the circumferential stress gradually transforms from compressive stress at the corner tangent of the die 1 to tensile stress at the tangent of the punch 5, creating a boundary where the circumferential stress is zero. This boundary is the circumferential tensile-compressive stress boundary circle (…). R σ ).

[0036] Within the stress boundary circle ( R α ≤ r≤R σ The suspended region 2-2 is in a state of double tensile stress, with radial and circumferential tensile stresses, where the radial tensile stress is always greater than the circumferential tensile stress. Under this state, the main defect form of the spherical shell is cracking caused by excessive radial tensile stress. Therefore, the maximum principal stress is the radial tensile stress, and the minimum principal stress is 0. According to the Trasca yield criterion, the radial stress in the suspended region 2-2 is equal to the corresponding hardening stress in its respective deformation zone. (14); Therefore, within the stress boundary circle (R p ≤r≤R σ Based on equations (12) and (14), we can obtain: (15); Given a fixed component geometry, the hardening stress in the suspended zone 2-2 and the flange zone 2-1 directly determines the deformation of plate 2. Therefore, to prevent cracking, the hardening stress in the suspended zone 2-2 can be increased by adjusting the temperature distribution of the slab. ), or reduce the hardening stress in flange area 2-1 ( To reduce radial tensile stress, the stress gradient parameters between the suspended region 2-2 and the flange region 2-1 are used. To evaluate the effect of slab temperature distribution on the hardening stress of plate 2: (16); Stress gradient parameters The larger the value, the higher the ultimate hardening stress in the suspended region 2-2, and the greater the process margin for forming sheet 2. The strain hardening law of sheet 2 is usually expressed by the Hollomon hardening model, in which... K i and n i For different hardening parameters: (17); Temperature is negatively correlated with the hardening ability of sheet 2, thus allowing the determination of temperatures for different zones of sheet 2. Hardening parameters for the suspended zone 2-2 and the flange zone 2-1 can be set. K i and n i Represented as temperature T i function K i = f(T i ) , n i = g(T i ) Therefore, we can obtain: (18); In the formula: Tu and T f The actual temperatures of the suspended region 2-2 and the flange region 2-1 under the ultra-low temperature gradient temperature field; LRD f This indicates the limiting drawing ratio of plate 2 at the flange zone 2-1 temperature, for 2219-O state aluminum alloy ( Figure 7and Figure 8 (related experimental curves for this material), room temperature LRD The commonly used value is 1.8, but other values ​​may vary depending on the material and temperature. LRD The value can be obtained through experiments or by consulting relevant materials.

[0037] Furthermore, since the variables contained in equation (18) are only related to... Tu and T f Relatedly, we can assume equations (19) and (20), and finally express equation (18) as equation (21): (19); (20); (twenty one); In the formula: m It is the proportionality coefficient of the hardening coefficient relative to the temperature difference, with a value of 1 to 2. l It is the proportionality coefficient of the hardening index relative to the temperature difference, with a value of 10. -3 ~2×10 -3 Stress gradient parameters Temperature difference between flange zone 2-1 and suspended zone 2-2 ( T f -T u The stress gradient is positively correlated with the deformation. Therefore, it is proposed to regulate deformation and prevent defects based on the stress gradient, and to achieve the deformation stress gradient in each zone through ultra-low temperature distribution. To prevent cracking, the hardening stress in flange zone 2-1 can be reduced by increasing the temperature of flange zone 2-1. To reduce radial tensile stress and avoid concentrated deformation in the suspended region 2-2, or to lower the temperature of the suspended region 2-2 and increase the hardening stress in the suspended region 2-2. This creates a gradient cryogenic temperature field where the suspended zone 2-2 has a low temperature and the flange zone 2-1 has a high temperature. The temperature difference between the suspended zone 2-2 and the flange zone 2-1 is ( T f -T u The larger the value, the greater the stress gradient, and the less likely the deep cavity thin-walled curved surface part is to crack due to concentrated deformation during forming.

[0038] 4-Stress gradient modeling and core criteria for temperature field in deformation uniformity: Furthermore, with λ This represents the stress gradient between the molding area 2-3 and the suspended area 2-2: (twenty two); In the formula: This indicates the hardening stress in the molding area 2-3; Tp This indicates the temperature of the molding area 2-3.

[0039] Stress gradient parameters λ Temperature difference between the molding area 2-3 and the suspended area 2-2 ( T p -T u The temperature of the plate 2, which is already in contact with the punch 5, is positively correlated with the surface temperature of the spherical shell. Due to friction, the plate 2 is generally less prone to deformation. To improve the uniformity of the spherical shell's deformation, the temperature of the contact area 2-3 can be adjusted by controlling the surface temperature of the punch 5 or the insulation conditions, and the temperature difference between the contact area 2-3 and the suspended area 2-2 of the plate 2 can be appropriately increased. T p -T u This makes the plate 2 in the molding area 2-3 "softer" and has less hardening stress than the plate 2 in the suspended area 2-2, allowing the plate 2 in the molding area 2-3, which was originally no longer deformable, to continue to deform, reducing wall thickness deviation and avoiding the problem of reprocessing after the component is formed.

[0040] It is worth noting the temperature difference between the molding area 2-3 and the suspended area 2-2 of the plate 2. T p -T u The material should not be too large to prevent concentrated deformation due to excessively soft material, which could lead to cracking.

[0041] Therefore, with the temperature of the suspended region 2-2 T u Based on the stress gradient, the temperature distribution on plate 2 can be comprehensively determined as follows: (twenty three); Therefore, the temperature relationship shown in equation (24) can be obtained: T u ≤T p ＜＜T f (twenty four); 5. Establishment of a stress model for wrinkle prevention and control: Outside the stress boundary circle ( r>R σ The suspended region 2-2 is under a state of tension and compression, with radial tensile stress being the maximum principal stress and circumferential compressive stress being the minimum principal stress. Under this state, the main defect form of the spherical shell is wrinkling caused by excessive circumferential compressive stress. According to the Trasca yield criterion, the circumferential stress in the suspended region 2-2 is: (25); Since the slab is subjected to circumferential compressive stress outside the stress boundary circle, equation (23) can be converted to: (26); When the circumferential compressive stress in the suspended region 2-2 reaches the critical wrinkling stress, that is (27); At this point, the plate 2 is subjected to excessive pressure, causing instability and wrinkling. Therefore, given a fixed component shape and dimensions, it is necessary to prevent wrinkling by increasing the radial stress and decreasing the circumferential stress. As shown in equation (12), by changing the blank holder force... F b The size is increased to improve radial tensile stress, thereby reducing circumferential compressive stress.

[0042] Regarding the relevant explanation of step S2: Step 2: Place plate 2 on the die 1 and close the pressure ring 3 to press the outer edge area (flange area 2-1) of plate 2.

[0043] In the optional solutions of this embodiment, it is more preferred that the plate 2 is made of aluminum alloy, stainless steel, titanium alloy or nickel-based high-temperature alloy.

[0044] Regarding the relevant explanation of step S3: Step 3: The cryogenic medium 4 from the self-pressurizing storage tank 9 is added to the atmospheric pressure buffer tank 7 located on the pressure ring 3. The storage tank 9 is a self-pressurizing tank, pressurized by the vaporization of the cryogenic medium 4. If it is directly added to the upper side of the plate 2, it will produce a very large gas-liquid impact and a large amount of vaporization. Therefore, by setting an atmospheric pressure buffer tank 7 on the pressure ring 3, vapor-liquid separation is achieved, allowing the liquid medium to flow directly to the upper side of the unpressed plate 2 by gravity, thus achieving direct cooling of the liquid cryogenic medium 4. This not only avoids the problem of large-scale vaporization but also achieves cooling of the liquid cryogenic medium 4 and obtains a lower ultra-low temperature. Specifically, the pressure of the self-pressurizing storage tank 9 is increased to 0.3MPa~0.6MPa, and the cryogenic medium 4 is forced into the upper inlet of the atmospheric pressure buffer tank 7 by the pressure of the cryogenic medium 4. The liquid cryogenic medium 4 falls to the lower side of the atmospheric pressure buffer tank 7 for storage under the action of gravity, and the gaseous cryogenic medium 4 is discharged through the upper exhaust port 8 of the atmospheric pressure buffer tank 7, achieving the effect of vapor-liquid separation.

[0045] In the optional schemes of this embodiment, the cryogenic medium 4 is preferably one of liquid argon, liquid nitrogen, or liquid helium. A reasonable selection is made based on the required cryogenic temperature.

[0046] Regarding the relevant explanation of step S4: Among the optional solutions in this embodiment, the more preferred one is as follows: Figures 1-4 As shown, the gradient temperature field constructed in S4 satisfies: T u ≤ T p <<T f ; in, T u Temperature of plate 2 in suspended zone 2; T p Temperature of the 2-3 sheet material in the molding area; T f The temperature of plate 2 in flange area 2-1; << means much less than.

[0047] Among the optional solutions in this embodiment, the more preferred one is as follows: Figure 1 As shown, before implementing S4, a heating element 10 is installed inside the flange area 2-1 of the plate 2 corresponding to the pressure ring 3 and the die 1 to heat the corresponding position of the pressure ring 3 and the die 1 to 100℃~recrystallization temperature, so as to reduce the flow stress of the plate 2 in the flange area 2-1 and increase the stress gradient of each deformation zone on the plate 2.

[0048] Among the optional solutions in this embodiment, the more preferred one is as follows: Figure 1 As shown, before S4, there is also a step of heat insulation treatment or temperature control of the surface of the punch 5 to control the temperature of the plate 2 in the molding area 2-3.

[0049] In the optional solutions of this embodiment, the heat insulation treatment is to apply a cold-insulating paint to the surface of the punch 5, and the temperature control is to inject a low-temperature medium 4 into the punch 5.

[0050] Regarding the relevant explanation of step S5: Step 5: After applying the set blanking force to the blanking ring 3, the punch 5 moves down at the set speed, so that the sheet 2 gradually comes into contact with the punch 5 under the gradient ultra-low temperature condition to form a deep cavity thin wall curved surface part.

[0051] In the optional schemes of this embodiment, it is more preferred that in S5, after the deep cavity thin-walled curved surface part is basically formed, the blank holder force is further increased to make the punch 5 continue to move downward and generate overload displacement, so as to apply thin film stress in the component surface of the deep cavity thin-walled curved surface part, reduce springback and improve the consistency of surface dimensions.

[0052] Regarding the relevant explanation of step S6: Step 6: Open the mold, drain the low-temperature medium 4, and take out the formed deep cavity thin-walled curved surface part.

[0053] Regarding other relevant explanations: Among the optional solutions in this embodiment, the more preferred one is as follows: Figure 1 and Figure 5 As shown, the low-temperature medium 4 flows into the upper surface of the plate 2 in the suspended area 2-2, cooling it to an ultra-low temperature of -160℃ to -270℃.

[0054] Specifically, F in the attached diagram 合模1 and F 合模2 Both refer to the blank holder force.

[0055] Specifically, supplementary explanation of the beneficial effects of this embodiment: 1) In this embodiment, a low-temperature medium 4 is introduced into the upper side of the plate 2 to directly cool the unpressed area of ​​the plate 2 to an ultra-low temperature. On the one hand, the ultra-low temperature hardening strength is used to increase the flow stress in the suspended area 2-2 (unpressed area) and avoid the radial stress from exceeding the ultimate hardening stress and cracking. On the other hand, the high elongation at ultra-low temperature is used to improve the ultimate deformation capacity of the suspended area 2-2 and avoid excessive deformation and cracking. 2) This embodiment forms a gradient ultra-low temperature field with ultra-low temperature in the suspended zone 2-2, room temperature or slightly warm temperature in the flange zone 2-1, and low temperature in the molded zone 2-3. By controlling the stress distribution in each deformation zone of the slab through temperature distribution (temperature is negatively correlated with hardening stress, the lower the temperature, the greater the hardening / flow stress), the flow stress in the suspended zone 2-2, which is prone to concentrated deformation, increases (becomes stronger) and is less likely to become concentrated in deformation again. The flow stress in the flange zone 2-1, which is difficult to deform, decreases (becomes weaker) and becomes easier to deform. This achieves the effect of easy flow of the slab in the flange zone 2-1 and easy force transmission of the slab in the suspended zone 2-2. Furthermore, the temperature of the molded zone 2-3 is higher than that of the suspended zone 2-2, so that the flow stress of the plate 2 in the molded zone 2-3 is lower than that in the suspended zone 2-2. This allows the plate 2 in the molded zone 2-3, which was originally no longer deformable, to continue to deform, achieving coordinated deformation in each area, avoiding concentrated deformation in the suspended zone 2-2, fundamentally improving the degree of uniform deformation, reducing wall thickness deviation, and avoiding the problem of removing excess wall thickness after forming large curved components.

[0056] 3) In this embodiment, gradient ultra-low temperature can prevent concentrated deformation and cracking in the suspended area 2-2. On this basis, the flow resistance of the plate 2 in the flange area 2-1 can be further increased, the radial tensile stress in the suspended area 2-2 can be increased, and the circumferential compressive stress can be reduced, thereby achieving the effect of preventing wrinkling.

[0057] 4) In this embodiment, only the cryogenic medium 4 needs to be introduced onto the upper side of the plate 2, without the need for overall mold cooling, which significantly reduces heat consumption. The larger the component size, the more obvious the advantage. Furthermore, by setting an atmospheric pressure buffer tank 7 on the pressure ring 3, vapor-liquid separation of the cryogenic medium 4 is achieved. Relying on gravity, the liquid cryogenic medium 4 flows directly to the upper side of the plate 2, which not only avoids the problem of large-scale vaporization of the cryogenic medium 4, but also allows the liquid cryogenic medium 4 to directly cool the plate 2, obtaining a gradient ultra-low temperature field with even lower temperatures.

[0058] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A gradient cryogenic forming method for deep-cavity thin-walled curved parts, characterized in that, Based on the stress constraint criterion of the deformation zone, deep cavity thin-walled curved parts are formed under gradient cryogenic conditions by controlling the stress distribution at ultra-low temperatures, including the following steps: S1 divides the sheet material to be formed into three deformation zones during the forming process: the flange zone pressed by the pressure ring, the suspended zone that is not in contact with the mold and is in a free deformation state, and the mold-fitting zone that has been attached to the convex mold surface. Based on the flange area, the suspended area and the molding area, the radial stress and circumferential stress of each deformation area are mechanically analyzed. Based on the stress relationship constrained by the yield criterion, a stress gradient model that does not cause cracking and wrinkling defects is established. Based on the stress gradient model, the ultra-low temperature cooling conditions and edge clamping force loading conditions of the plate are formulated. S2, Position the plate on the die and close the pressure ring to press the flange area; S3, add the cryogenic medium from the self-pressurized storage tank into the atmospheric pressure buffer tank; S4, the liquid cryogenic medium in the atmospheric pressure buffer tank flows into the upper surface of the plate in the suspended area by gravity to construct a gradient temperature field of the plate in the flange area, the suspended area and the molding area along the radial direction; S5, under the action of the set blank holder force, the punch is driven to move downward, so that the sheet metal is deformed in a coordinated manner under the gradient temperature field, and finally formed into a deep cavity thin wall curved surface part.

2. The gradient cryogenic forming method for deep-cavity thin-walled curved parts according to claim 1, characterized in that: The gradient temperature field constructed in S4 satisfies: T u ≤ T p << T f ; in, T u Temperature of the suspended section of the board; T p The temperature of the substrate in the molding area; T f This refers to the plate temperature in the flange area.

3. The gradient cryogenic forming method for deep-cavity thin-walled curved parts according to claim 1, characterized in that: The mechanical analysis in S1 is based on the principle of force balance to analyze the radial stress of the flange area, the suspended area, and the molded area respectively, and to clarify the influencing factors and interrelationships of the radial stress in each deformation area; at the same time, combined with the material flow law of sheet forming, the circumferential tensile and compressive stress boundary circle of the suspended area is defined, and the suspended area is divided into a cracking risk area with both radial and circumferential tensile stress and a wrinkling risk area with radial tension and circumferential compression.

4. The gradient cryogenic forming method for deep-cavity thin-walled curved parts according to claim 2, characterized in that: Before implementing S4, a heating element is installed inside the flange area of ​​the plate corresponding to the pressure ring and the die to heat the position of the pressure ring and the die to 100°C~recrystallization temperature, so as to reduce the plate flow stress in the flange area and increase the stress gradient in each deformation zone of the plate.

5. The gradient cryogenic forming method for deep-cavity thin-walled curved parts according to claim 1, characterized in that: Before S4, there is also a step of heat insulation treatment or temperature control of the convex model surface to control the temperature of the plate material in the molding area.

6. The gradient cryogenic forming method for deep-cavity thin-walled curved parts according to claim 1, characterized in that: In S5, after the deep cavity thin-walled curved surface part is basically formed, the blank holder force is increased to make the punch continue to move downward and generate overload displacement, so as to apply thin film stress in the component surface of the deep cavity thin-walled curved surface part.

7. The gradient cryogenic forming method for deep-cavity thin-walled curved parts according to claim 1, characterized in that: The cryogenic medium is one of liquid argon, liquid nitrogen, or liquid helium.

8. The gradient cryogenic forming method for deep-cavity thin-walled curved parts according to claim 1, characterized in that: The cryogenic medium flows into the upper surface of the plate in the suspended area, cooling it to an ultra-low temperature of -160℃ to -270℃.

9. The gradient cryogenic forming method for deep-cavity thin-walled curved parts according to claim 1, characterized in that: The plate material is made of aluminum alloy, stainless steel, titanium alloy, or nickel-based high-temperature alloy.

10. The gradient cryogenic forming method for deep-cavity thin-walled curved parts according to claim 5, characterized in that: The heat insulation treatment involves applying a cold-insulating paint to the surface of the convex mold, and the temperature control involves injecting a low-temperature medium into the convex mold.

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