Multi-stage loading creep age forming process

Abstract

The invention discloses a multi-stage loading creep age forming process, and relates to the technical field of mechanical forming manufacturing engineering.The multi-stage loading creep age forming process comprises the steps that a wall plate is arranged in loading equipment; the wall plate is heated to the creep aging temperature and kept at the constant temperature; applying a pressure load to the wall plate and keeping the pressure load so as to enable the wall plate to generate an initial stress with a set value; after the stress of the wallboard material is relaxed to a set degree and the yield strength is improved to a set degree, continuously pressurizing on the basis of the previous pressure load to form a new pressure load so as to increase the stress of the wallboard to be the same as the initial stress; the third step and the fourth step are repeated till the wallboard is the same as the target structure and the aging duration of the process design is kept; and cooling to room temperature, and unloading to obtain the formed wallboard with the target structural shape and mechanical properties. According to the multi-stage loading creep age forming process provided by the invention, the forming limit of the wallboard can be improved, and accurate forming of the wallboard can be realized.

Description

Technical Field

[0001] This invention relates to the field of mechanical forming manufacturing engineering technology, and in particular to a multi-stage loading creep aging forming process. Background Technology

[0002] With the increasing pursuit of lightweight and high-performance structures in high-end manufacturing industries such as aerospace, the design of integral ribbed panel structures is gradually trending towards thinner skin thickness, taller rib structures, and more complex curved surface contours, which places higher demands on the forming process. However, in the traditional forming process, due to the large stress on the top of the ribs, buckling instability is very likely to occur, which has become the main reason for the failure of integral forming and seriously restricts the application and promotion of this type of structure.

[0003] Creep aging forming has become a mainstream bending forming process due to its ability to manufacture components with low residual stress, high strength, and high process repeatability. The inventors have developed a known progressive creep aging forming process for complex aluminum alloy components. By changing the tooling, the creep aging forming of aluminum alloy sheets is divided into multi-stage progressive creep aging forming. The forming process involves heating to the forming temperature, applying forming stress and holding it for 1 hour, unloading the load to near zero, holding it at that temperature for 5 minutes, and then reloading it to the forming stress. This process is repeated to achieve progressive forming.

[0004] Existing progressive creep aging forming processes unload the load to near zero after each stage of forming before proceeding to the next stage of loading and forming. This causes unnecessary springback in the panel at each stage, making it difficult to control the pressing accuracy in the next stage of loading and forming, and compromising process parameters, thus leading to difficulties in precise forming. Furthermore, the loading and unloading between each two stages introduces unnecessary residual stress and reduces the forming efficiency of the panel.

[0005] Therefore, there is an urgent need to design a technical solution that can improve the forming limit of the wall panel and achieve precise forming of the wall panel. Summary of the Invention

[0006] The purpose of this invention is to provide a multi-stage loading creep aging forming process to solve the problems existing in the prior art, improve the forming limit of the wall panel, and achieve precise forming of the wall panel.

[0007] To achieve the above objectives, the present invention provides the following solution:

[0008] This invention provides a multi-stage loading creep aging forming process, comprising the following steps:

[0009] Step 1: Install the wall panel inside the loading device;

[0010] Step 2: Heat the wall panel to the creep aging temperature and maintain a constant temperature;

[0011] Step 3: Apply a pressure load to the wall panel to generate an initial stress of a set value. During the forming process, the wall panel material undergoes stress relaxation and yield strength increase.

[0012] Step 4: After the stress of the wall panel material relaxes to the set level and the yield strength increases to the set level, continue to apply pressure on the basis of the previous pressure load to form a new pressure load, so as to increase the stress of the wall panel to the same as the initial stress.

[0013] Step 5: Repeat steps 3 and 4 until the wall panel is identical to the target structure and maintains the aging time designed in the process;

[0014] Step six: Cool down to room temperature and unload to obtain a shaped wall panel with the target structural shape and mechanical properties.

[0015] The principle of multi-stage loading creep aging forming in this invention is as follows: During the creep stage of creep aging forming, the aluminum alloy panel undergoes stress relaxation, and the forming stress in the panel slowly decreases over time. Simultaneously, the material undergoes artificial aging, increasing its yield strength (buckling strength of the component) and thus enhancing its formability. Based on the principle of real-time stress reduction and yield (buckling) strength improvement during creep aging, the forming load can be increased again after a period of material relaxation, further improving the material's forming limit.

[0016] Preferably, the loading device is an autoclave.

[0017] Preferably, the initial stress value is slightly less than the allowable yield strength of the wall panel.

[0018] Preferably, in step one, before the wall panel is placed inside the loading device, a pretreatment step is also included. The pretreatment step includes solution heat treatment of the wall panel followed by water quenching, and artificial aging heat treatment to a certain extent in combination with the material forming conditions.

[0019] Preferably, the wall panel is equipped with a stress sensor, which can monitor the stress of the wall panel in real time and control the magnitude of the pressure load applied each time.

[0020] Preferably, the pressure load in step two is close to but lower than the buckling strength of the wall panel reinforcement.

[0021] The present invention achieves the following technical effects compared to the prior art:

[0022] During the creep aging forming stage of the wall panel, stress relaxation occurs, and the forming stress in the structure slowly decreases over time. Simultaneously, the material undergoes artificial aging, increasing its yield strength (buckling strength of the component) and thus enhancing its formability. Based on the principle of real-time stress reduction and yield (buckling) strength improvement during creep aging, increasing the forming load again after a period of material relaxation can further improve the material's forming limit. This invention maintains the applied load during the creep aging stage of the wall panel, without unloading, resulting in a continuous and uninterrupted forming process. This avoids unnecessary springback and residual stress caused by continuous unloading, improving forming accuracy, forming efficiency, and forming limit. Attached Figure Description

[0023] 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.

[0024] Figure 1 This is a schematic diagram of the multi-stage loading creep aging forming process in one or more embodiments of the present invention;

[0025] Figure 2 This is a schematic diagram of the time-stress curves of the multi-stage loading creep aging forming of the present invention and the traditional creep aging forming;

[0026] Figure 3 This is a schematic diagram of the time-strain curves of the multi-stage loading creep aging forming of the present invention and the traditional creep aging forming;

[0027] Figure 4 This is a schematic diagram of the material time-true stress curves of multi-stage loading creep aging forming and conventional stress relaxation in one embodiment of the present invention;

[0028] Figure 5 This is a schematic diagram of the material time-creep strain curves of multi-stage loading creep aging forming and conventional stress relaxation in one embodiment of the present invention.

[0029] In the diagram: 1-autoclave, 2-mold, 3-wall panel. Detailed Implementation

[0030] 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.

[0031] The purpose of this invention is to provide a multi-stage loading creep aging forming process to solve the problems existing in the prior art, improve the forming limit of the wall panel, and achieve precise forming of the wall panel.

[0032] 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.

[0033] Creep refers to the slow plastic deformation of a material over time under constant temperature and sustained stress. Even if the stress is less than the material's yield strength, deformation will still occur over a long period. Aging refers to the process by which the strength and hardness of certain metallic materials (such as aluminum alloys and titanium alloys) gradually increase over time under room temperature or heating conditions (also known as age hardening). Creep aging is a process that simultaneously performs creep deformation and age hardening on a material. That is, the material is aged under constant stress, allowing it to strengthen while deforming, ultimately achieving a specific shape and properties.

[0034] Existing progressive creep aging forming processes unload the load to near zero after each stage of forming before proceeding to the next stage of loading and forming. This leads to unnecessary springback in the panel at each stage, making it difficult to control the compression accuracy in the next stage of loading and forming, and compromising process parameters, ultimately resulting in inaccurate forming. Furthermore, the loading and unloading between each two stages introduces unnecessary residual stress, reducing the forming accuracy of the panel. To address this issue, this invention provides a multi-stage loading creep aging forming process, referencing... Figure 1 , Figure 2 , Figure 3 , Figure 4 and Figure 5As shown, the process includes the following steps: Step 1, placing the wall panel 3 inside the loading device; Step 2, heating the wall panel 3 to the creep aging temperature and maintaining it at a constant temperature; Step 3, applying a pressure load to the wall panel 3 to generate an initial stress of a set value, during which the wall panel 3 material undergoes stress relaxation and yield strength increase; Step 4, after the stress of the wall panel 3 material relaxes to a set level and the yield strength increases to a set level, continuing to apply pressure on the basis of the previous pressure load to form a new pressure load, thereby increasing the stress of the wall panel 3 to the same as the initial stress; Step 5, repeating Step 3 and Step 4 until the wall panel 3 is identical to the target structure and maintains the aging time designed in the process; Step 6, cooling to room temperature and unloading to obtain a formed wall panel 3 with the target structural shape and mechanical properties. This invention does not unload after each stage of forming, maintaining pressure load during creep aging. After creep aging is complete, the next stage of forming continues, avoiding unnecessary springback of the wall panel 3 at each stage. The next stage of forming is based on the stress value of the wall panel 3, resulting in higher pressing accuracy and thus achieving precise forming. Furthermore, there is no unloading process between stages; the pressure load is simply increased from the previous stage to a new level, increasing the forming limit of the wall panel 3 while avoiding unnecessary residual stress. Figure 4 A comparison of the true stress in the multi-stage loading creep aging forming process and the traditional creep aging forming process shows that the true stress in this invention decreases after stress relaxation, rises in the next loading stage, and then slowly decreases again due to stress relaxation. This differs from the stress relaxation process in the traditional forming process, where stress gradually decreases. Creep strain refers to the amount of plastic deformation that slowly occurs over time in a material under constant stress or load. Figure 5 It can be seen that the creep strain of the present invention increases in a stepwise manner after multiple loading stages.

[0035] In one embodiment, the loading device used in this invention is an autoclave 1. The autoclave 1 is a core piece of equipment in the production of aerospace composite materials. Through the sequential control of parameters such as temperature, pressure, and vacuum, it achieves the curing and molding of resin-based composite material components. This equipment is mainly used in the manufacturing of main load-bearing structures such as aircraft doors, wings, and tail fins, as well as large components such as satellites and high-speed train bodies. Its system includes a tank, a heating / pressurizing / cooling device, and an automatic control system. A molding die 2 is installed inside the tank; details are not elaborated further.

[0036] In one embodiment, the initial stress value is slightly lower than the allowable yield strength of the wall panel 3, and in step two, a first-stage pressure load is applied to a value close to but slightly lower than the buckling strength of the stiffeners of the wall panel 3. To ensure controllability of the pressure load during the multi-stage loading process, this embodiment includes a stress sensor on the wall panel 3, which can monitor the stress of the wall panel 3 in real time and control the magnitude of the applied pressure load accordingly.

[0037] In one embodiment, the wall panel 3 is further included in a pretreatment step before being disposed inside the loading device. The pretreatment step includes solution heat treatment of the wall panel 3 followed by water quenching, and artificial aging heat treatment in combination with material forming conditions.

[0038] This invention is applicable to the integral forming of most high-ribbed wall panels 3. For wall panels 3 made of different materials and with different initial conditions, key process parameters such as pressure load, time, and temperature need to be adjusted in real time according to different forming conditions. The parameters also differ depending on the initial state of different materials, and should be flexibly selected according to the selected material. The multi-stage loading creep aging forming process of this invention is a viscoelastic forming process with a combination of creep deformation and stress relaxation for each stage of multi-stage loading. At the aging temperature, the creep deformation and stress relaxation combined viscoelastic behavior caused by multi-stage changing loads will be transformed into permanent plastic deformation. The viscoelastic deformation characteristics and deformation mechanism under multi-stage thermodynamic loading paths are different from the viscoelastic deformation process under constant stress / strain loads. Viscoelastic forming is a technique that utilizes the mechanical properties of materials that simultaneously possess elasticity and viscosity for plastic processing. Such materials exhibit both elastic deformation (reversible) and viscous deformation (irreversible, accompanied by energy dissipation) when subjected to force, and their deformation behavior is closely related to time and temperature. Common viscoelastic materials include polymers, certain metal alloys (such as magnesium alloys and titanium alloys at high temperatures), composite materials, and biological tissues.

[0039] Example 1

[0040] In this embodiment, wall panel 3 made of 7050 aluminum alloy is used as an example. The solution heat treatment of wall panel 3 is carried out on KSL-1200X heat treatment furnace, and the process forming test is carried out on RDL50 high temperature electronic creep tester.

[0041] The multi-stage loading creep aging forming process steps in this embodiment are as follows:

[0042] The 7050 aluminum alloy sample was solution heat treated (475℃, 1h) and then quenched with water; it underwent first-stage aging treatment (120℃, 5h); the sample was heated to 160℃; a load was applied at 160℃ to the initial stress of 450MPa; the sample deformation was kept constant, and stress relaxation was performed for 1 hour without unloading, at which point the material was in the creep aging stage. When the sample stress decreased and the buckling strength increased to a certain level, the forming pressure was further increased, causing the overall panel to continue to deform under a load lower than the buckling strength; without unloading, the load was increased again to the sample stress of 450MPa, and relaxation was continued for 1 hour; the above loading-relaxation process was repeated for a total of 7 times; the sample was cooled to room temperature, unloaded, and springback was performed to obtain a formed panel with a specific shape and properties. The experimental results show that after multi-stage loading creep aging tests, the permanent plastic deformation of the material was significantly increased. With artificial aging, both the material strength and the forming performance were improved. This embodiment does not involve the unloading process between the two relaxation stages, demonstrating the change in creep strain and showcasing the improvement in forming limit. For each stage of multi-level loading, which is a viscoelastic forming process combining "creep deformation + stress relaxation," the combined viscoelastic behavior of creep deformation and stress relaxation caused by multi-level varying loads at aging temperature will be transformed into permanent plastic deformation. The viscoelastic deformation characteristics and deformation mechanism under multi-level thermodynamic loading paths are different from those under constant stress / strain loads, avoiding unnecessary springback and residual stress caused by continuous unloading, and further improving the accuracy of controlling process parameters.

[0043] 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 multi-stage loading creep aging forming process, characterized in that: Includes the following steps: Step 1: Install the wall panel inside the loading device; Step 2: Heat the wall panel to the creep aging temperature and maintain a constant temperature; Step 3: Apply a pressure load to the wall panel to generate an initial stress of a set value. During the forming process, the wall panel material undergoes stress relaxation and yield strength increase. Step 4: After the stress of the wall panel material relaxes to the set level and the yield strength increases to the set level, continue to apply pressure on the basis of the previous pressure load to form a new pressure load, so as to increase the stress of the wall panel to the same as the initial stress. Step 5: Repeat steps 3 and 4 until the wall panel is identical to the target structure and maintains the aging time designed in the process; Step six: Cool down to room temperature and unload to obtain a shaped wall panel with the target structural shape and mechanical properties.

2. The multi-stage loading creep aging forming process according to claim 1, characterized in that: The loading device is an autoclave.

3. The multi-stage loading creep aging forming process according to claim 1, characterized in that: The initial stress value is less than the allowable yield strength of the wall panel.

4. The multi-stage loading creep aging forming process according to claim 1, characterized in that: In step one, before the wall panel is placed inside the loading device, a pretreatment step is also included. The pretreatment step includes solution heat treatment of the wall panel followed by water quenching, and artificial aging heat treatment in combination with material forming conditions.

5. The multi-stage loading creep aging forming process according to claim 1, characterized in that: The wall panel is equipped with a stress sensor, which can monitor the stress of the wall panel in real time and control the magnitude of the pressure load applied each time.

6. The multi-stage loading creep aging forming process according to claim 1, characterized in that: The pressure load in step two is close to but lower than the buckling strength of the wall panel reinforcement.

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