Rigid-flexible composite pass-dividing forming method for high-strength aluminum alloy pipe fitting with large reducing ratio

Abstract

The invention provides a rigid-flexible composite pass-dividing forming method for a high-strength aluminum alloy pipe fitting with a large reducing ratio, and belongs to the technical field of metal forming and manufacturing. Comprising the steps that the initial taper pipe size is determined according to part reducing and bulging strain, a taper pipe is manufactured through plate blank roll welding, then the taper pipe is put into a rigid-flexible composite die to be subjected to two-pass reducing, the flexible die provides uniform support along with deformation, and a one-step reducing part and a preformed part are obtained in sequence; and then the preformed part is placed in a preheated hot air expansion mold for heat preservation and solid solution, a closed pipe cavity is pressurized to be attached to the mold for forming, quenching and aging treatment are conducted after mold opening, and finally machining is conducted to cut off the allowance, so that the high-strength aluminum alloy large-diameter-ratio pipe fitting is obtained. According to the method, a stress field in a deformation area is uniformly supported by a flexible mold in multi-pass rigid-flexible composite hole shrinkage to prevent wrinkling, the plasticity and the elongation of the material are improved by combining solution treatment, a strengthening phase is separated out through quenching aging treatment after bulging, and the technical effects that a formed part is not wrinkled, the wall thickness is uniform, and the strength and the fatigue performance are remarkably improved are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal forming manufacturing, and in particular to a rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter ratio change. Background Art

[0002] High-strength aluminum alloy pipe fittings with a large diameter ratio change usually have a large diameter difference and a large curvature profile, and are widely used in high-end equipment fields such as aerospace, automotive industry, energy chemical industry, and ship and ocean engineering. Such as the rear cone section of an aircraft engine, the cone of an aeroengine, etc. At the same time, the usage requirements for components with a large diameter ratio change are also constantly increasing.

[0003] Currently, the methods that can be used for the forming of high-strength aluminum alloy pipe fittings with a large diameter ratio change include split forming and welding, internal high-pressure forming, hot gas bulging forming, and superplastic forming. Specifically: Among them, split forming and welding use stamping to form the curved surface of the pipe fitting with a large diameter ratio change in blocks, and then weld the formed curved surfaces into a complete pipe fitting with a large diameter ratio change; the components obtained by this method have internal stress release, large springback, low dimensional accuracy, and uneven deformation is likely to occur in the weld and heat-affected zone, the welding difficulty is high, the microstructure at the weld is complex, and it is difficult to meet the high-quality requirements of such components; internal high-pressure forming uses a high-pressure liquid medium to drive the deformation of the component, but when forming the complex feature area of such components, it is easy to break through the forming limit of the material and cause the component to crack. At the same time, internal high-pressure forming has high requirements for equipment; hot gas bulging forming can use the high elongation rate of the material at high temperature to form. Using only the hot gas bulging method to form a pipe fitting with a large diameter ratio change, the local strain is too large, resulting in serious local thinning of the wall thickness of the component and unable to meet the service requirements of the component; superplastic forming is difficult to prepare uniformly fine-grained materials, and at the same time, the forming time is too long, resulting in a significant reduction in manufacturing efficiency.

[0004] In summary, the existing forming methods cannot meet the requirements of high-strength aluminum alloy pipe fittings with a large diameter ratio change. Therefore, aiming at the manufacturing problems of the forming accuracy and the coupling of tissue properties of high-strength aluminum alloy pipe fittings with a large diameter ratio change, there is an urgent need for an integrated forming method for high-precision shape control and property control of aluminum alloy to solve the bottleneck problems in the forming of high-strength aluminum alloy pipe fittings with a large diameter ratio change. Summary of the Invention

[0005] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter ratio change. By uniformly supporting the stress field in the deformation zone with a flexible die during multi-pass rigid-flexible composite necking to prevent wrinkling, combined with solution treatment to improve the plasticity and elongation rate of the material, and precipitation hardening phases are precipitated by quenching and aging treatment after bulging, the technical effects of no wrinkling of the formed part, uniform wall thickness, and significant improvement in strength and fatigue performance are achieved.

[0006] To achieve the above purpose, the present invention provides the following solutions: A rigid-flexible compound multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter change ratio, comprising the following steps: S1. Determine the initial cone pipe size according to the necking strain and bulging strain of the formed part, and obtain the cone pipe by rolling and welding the sheet blank; S2. Place the cone pipe into the rigid-flexible compound die, and perform die closing and necking through a press, and the flexible die deforms with the cone pipe and provides uniform support to obtain a single-step necked part; S3. Place the single-step necked part into the rigid-flexible compound die for secondary necking, and perform die closing and necking again, and the flexible die deforms with the part and provides uniform support to obtain a preformed part; S4. Place the preformed part into a hot gas expansion die for solution treatment and heat preservation, and pressurize the closed pipe cavity of the hot gas expansion die to make the preformed part conform to the die to obtain a formed part; S5. After completing S4, open the die and quickly take out the formed part, and successively perform quenching treatment, aging treatment and heat preservation, and finally machine the formed part to remove the surplus to obtain a high-strength aluminum alloy pipe fitting with a large diameter change ratio.

[0007] Preferably, in S1, the necking strain and bulging strain of the formed part do not exceed 80% - 85% of the maximum strain of the forming material; the necking strain is: ; The bulging strain is: ; ; wherein, L 1, L 2 are the characteristic dimensions before and after necking respectively, L 3, L 4 are the characteristic dimensions before and after bulging respectively; is the bulging strain, is the maximum bulging strain of the forming material; is the necking strain, is the maximum necking strain of the forming material.

[0008] Preferably, in S2 and S3, the material of the flexible support die is one or more of polyurethane, silicone rubber or TPE.

[0009] Preferably, in S2 and S3, the rigid-flexible compound die includes a flexible support die, a rigid support die, a necking die and a limit die; the flexible support die is used to provide uniform support to inhibit wrinkling on the surface of the part, the rigid support die is used to support the flexible die and jointly position the cone pipe with the limit die, and the necking die is used to apply the necking force.

[0010] Preferably, in S2 and S3, the mold clamping process is closed-loop controlled by a displacement control system, which includes a displacement sensor, a controller, a hydraulic cylinder and a feedback device, and is used to monitor and adjust the mold clamping speed and displacement in real time.

[0011] Preferably, in S4, the hot gas bulging die includes upper and lower bulging dies, a heat preservation plate, heating rods, a water cooling structure and high-pressure inflation holes; after the upper and lower bulging dies are clamped, a forming cavity matching the target part is formed, and the heating rods are controlled by a temperature control system to be heated to the solution temperature. The temperature control system consists of a temperature sensor, a controller, heating rods and a feedback adjustment mechanism, and is used to monitor and control the temperature of the bulging die in real time; the water cooling structure and the heat preservation plate are used to prevent heat from being transferred to the press.

[0012] Preferably, the closed tube cavity is formed by welding sealing plates at both ends of the preformed part.

[0013] Preferably, before the preformed part is placed in the hot gas bulging die in S4, it further includes: preheating the hot gas bulging die to the solution temperature; and determining the solution temperature and holding time according to the material state of the preformed part.

[0014] Preferably, in S5, the quenching treatment is water quenching.

[0015] Preferably, in S5, the aging treatment temperature is 160-190°C and the holding time is 6-24h.

[0016] According to the specific embodiments provided by the present invention, the following technical effects are disclosed: (1) In the multi-pass necking process of the present invention, a flexible die support member is used. Due to the low strength of the flexible die, elastic deformation occurs when it contacts the tube blank during the necking process, making the die in close contact with the tapered tube. The stress field distribution generated by this contact method is more uniform, thus effectively preventing the forming part from wrinkling.

[0017] (2) The present invention adopts the initial tapered tube coupled bulging strain and necking strain, avoiding local wrinkling of the part caused by single large-strain necking and serious wall thickness reduction caused by single bulging; this method adopts a rigid-flexible composite die for multi-pass necking, and then uses the hot gas bulging process for final forming, synchronously reducing the necking and bulging strains, making the wall thickness of the forming part more uniform.

[0018] (3) The forming method provided by the present invention promotes the complete dissolution of strengthening phases in the aluminum matrix during the solution treatment of preformed parts, forming a uniform organizational structure, significantly reducing the hardness of the material and improving its plasticity, thereby optimizing the bulging deformation process and effectively eliminating internal stresses, and further enhancing the dimensional stability and deformation resistance. Subsequently, a supersaturated solid solution is formed by rapid cooling through the quenching process, significantly increasing the strength of the material. The final aging treatment further enhances the material strength and improves the fatigue performance of the material through the formation of precipitates. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0020] Figure 1 It is a flowchart provided for the rigid-flexible composite multi-pass forming method of high-strength aluminum alloy pipe fittings with a large diameter ratio change in the present invention; Figure 2 It is a component dimension diagram obtained by the rigid-flexible composite multi-pass forming method of high-strength aluminum alloy pipe fittings with a large diameter ratio change provided in the embodiment of the present invention; Figure 3 It is a schematic diagram of multi-pass necking for the rigid-flexible composite multi-pass forming method of high-strength aluminum alloy pipe fittings with a large diameter ratio change provided in the embodiment of the present invention; among them, Figure 3 (a) in it is a forming schematic diagram of a one-step necking part, Figure 3 (b) in it is a forming schematic diagram of a preformed part; Figure 4 It is a flowchart of the heat treatment for the rigid-flexible composite multi-pass forming method of high-strength aluminum alloy pipe fittings with a large diameter ratio change provided in the embodiment of the present invention; Figure 5 It is a determination diagram of the initial blank size for the rigid-flexible composite multi-pass forming method of high-strength aluminum alloy pipe fittings with a large diameter ratio change provided in the embodiment of the present invention; Figure 6 It is a schematic diagram of the device for the rigid-flexible composite multi-pass forming method of high-strength aluminum alloy pipe fittings with a large diameter ratio change provided in the embodiment of the present invention; among them, Figure 6 (a) in it is a structural schematic diagram of a rigid-flexible composite die, Figure 6 (b) in it is a structural schematic diagram of a hot gas bulging die.

[0021] DESCRIPTION OF THE REFERENCE NUMERALS: 1. Upper table surface; 2. Reducing die; 3. Flexible support die; 4. Rigid support die; 5. Limit die; 6. Lower table surface; 7. Hot gas expansion die; 8. Upper die; 9. Lower die; 10. Preformed part; 11. High-pressure inflation hole; 12. Thermal insulation board; 13. Heating rod; 14. Water cooling structure. Specific embodiments

[0022] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0023] The present invention aims to perform local variable diameter through a rigid-flexible composite die, utilize the flexible uniform loading characteristic to achieve uniform loading in the reducing deformation zone and suppress the wrinkling defect during the reducing process, then use the non-steady hot gas expansion process for solution temperature final forming to form a large variable diameter pipe fitting, and finally perform rapid quenching and aging in a timely manner to improve the strength of the component, avoiding the disadvantage of poor reliability of the segmented welding structure.

[0024] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] Embodiment 1 As Figure 1 shown, this embodiment provides a rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter ratio, including the following steps: S1. Determine the initial cone pipe size according to the reducing strain and bulging strain of the formed part, and obtain the cone pipe by rolling and welding the plate blank; S2. Place the cone pipe into the rigid-flexible composite die, close the die through the press to reduce the diameter, and the flexible die deforms with the cone pipe and provides uniform support to obtain a one-step reduced diameter part; S3. Place the one-step reduced diameter part into the rigid-flexible composite die for secondary diameter reduction, close the die again to reduce the diameter, and the flexible die deforms with the part and provides uniform support to obtain a preformed part; S4. Place the preformed part into the hot gas expansion die for solution treatment and heat preservation, and pressurize the closed pipe cavity of the hot gas expansion die to make the preformed part conform to the die to form a formed part; S5. After completing S4, open the die and quickly take out the formed part, and successively perform quenching treatment, aging treatment and heat preservation, and finally machine the formed part to remove the surplus to obtain a high-strength aluminum alloy pipe fitting with a large diameter ratio.

[0026] Refer to Figure 2, in step S1, the upper end diameter of the tapered tube is d1, the lower end diameter is D, and the axial height is H; as Figure 5 shown, determine the maximum diameter reduction amount of the initial material according to formula (1), and determine the maximum bulging amount of the initial material according to formula (2). The diameter reduction strain is: ; (1) The bulging strain is: ; (2) Furthermore, the diameter reduction strain and the bulging strain of the formed part do not exceed 80% - 85% of the maximum strain of the forming material; the formula is as shown in formula (3) ; (3) where L 1, L 2 are the characteristic dimensions before and after diameter reduction respectively, L 3, L 4 are the characteristic dimensions before and after bulging respectively; is the bulging strain, is the maximum bulging strain of the forming material; is the diameter reduction strain, is the maximum diameter reduction strain of the forming material.

[0027] Even further, determine that the small end diameter of the initial forming material is d and the height of the tapered tube H , and their value ranges are determined by the following formulas (4) - (8): ; (4) ; (5) ; (6) ; (7) ; (8) where d is the small end diameter of the initial blank; H 1 is the height of the upper process end; H 2 is the distance from the position of the maximum bulging amount of the tapered tube to the lower bottom surface of the tapered tube; is the angle between the generatrix of the cross-section of the initial blank and the bottom edge.

[0028] Furthermore, in S2, the forming process of the single-step diameter reduction part is: refer to Figure 3 in (a) and Figure 6In (a) of [reference], first, fix the lower die of the rigid-flexible composite die on the lower table surface 6 of the press. The upper table surface 1 of the press is connected with a slider. Then, place the tapered tube on the flexible support die 3. There is a rigid support die 4 below the flexible support die 3, and limit dies 5 are arranged at both ends of the rigid support die 4. The press drives the reducing die 2 to close the die at the die closing speed v1. At the same time, the flexible support die 3 deforms with the part and plays a supporting role for the part, reducing the upper end diameter of the tapered tube from d1 to d2 to obtain a one-step reduced part. The material of the flexible support die 3 is one or more of polyurethane, silicone rubber, or TPE.

[0029] In S3, the forming process of the preformed part is as follows: Refer to Figure 3 in (b) of [reference] and Figure 6 in (a) of [reference]. First, fix the lower die of the rigid-flexible composite die for secondary reduction on the lower table surface 6 of the press. The upper table surface 1 of the press is connected with a slider. Then, place the one-step reduced part on the flexible support die 3 for secondary reduction. The press drives the reducing die 2 to close the die at the die closing speed v2, reducing the upper end diameter of the one-step reduced part from d2 to d3 to obtain the preformed part 10.

[0030] Refer to Figure 6 in (a) of [reference]. The above-mentioned rigid-flexible composite die includes a flexible support die 3, a rigid support die 4, a reducing die 2, and limit dies 5. The flexible support die 3 is used to provide uniform support to suppress wrinkling on the part surface. The rigid support die 4 is used to support the flexible die and jointly position the tapered tube with the limit dies 5. The reducing die 2 is used to apply a reducing force.

[0031] Furthermore, in S2 and S3, the die closing process is closed-loop controlled by a displacement control system. The displacement control system includes a displacement sensor, a controller, a hydraulic cylinder, and a feedback device, and is used to monitor and adjust the die closing speed and displacement in real time. Specifically, the displacement sensor monitors the position change of the press in real time and transmits the signal to the controller. The controller compares the preset parameters with the feedback signal, calculates the deviation, and then issues an instruction. At this time, the hydraulic cylinder receives the instruction and drives the upper die of the press to move. The feedback device transmits the actual displacement data back to the controller to form a closed-loop control to ensure accuracy and stability.

[0032] Further, in S4, the forming process of the preformed part 10 in the hot gas expansion die is as follows: First, heat the hot gas expansion die to the solution temperature T1. Determine the solution temperature and holding time according to the material state of the preformed part. Taking the Al2219-T6 state as an example, the solution temperature T1 is 515 - 565 °C, and the holding time t1 is 10 - 40 min. Then, place the preformed part 10 in the hot gas expansion die 7 for die closing and hold for t1. Then, at the pressurization rate Pressurize the closed cavity to p to make the part gradually conform to the mold, and hold the pressure for t2. Among them, referring to Figure 6 in (b), the hot gas bulging die 7 includes a bulging upper die 8, a lower die 9, a heat preservation plate 12, a heating rod 13, a water cooling structure 14 and a high-pressure inflation hole 11; after the bulging upper die 8 and the lower die 9 are closed, a forming cavity matching the target part is formed, the heating rod 13 is controlled by a temperature control system to be heated to the solution temperature, and the temperature control system is composed of a temperature sensor, a controller, a heating rod 13 and a feedback adjustment mechanism, and is used to monitor and control the temperature of the bulging die in real time; the water cooling structure 14 and the heat preservation plate 12 are used to prevent heat from being transferred to the press. In addition, the closed cavity is formed by sealing both ends of the preformed part 10 with welding plates.

[0033] It should be noted that the temperature control system is composed of a temperature sensor, a controller, a heating rod 13 and a feedback adjustment mechanism. The temperature sensor monitors the temperature of the bulging die in real time and transmits the signal to the controller. The controller calculates and outputs a control signal according to the difference between the set temperature and the actual temperature, and adjusts the power of the heating rod 13. If the temperature is lower than the set value, the heating rod 13 increases the heat output; if it is higher than the set value, the heating is reduced or stopped. The feedback adjustment mechanism ensures that the temperature is stable within the solution temperature range through continuous monitoring and adjustment, so as to achieve temperature control.

[0034] Further, in S5, the process of part forming is as follows: after completing step S4, open the mold and quickly take out the part, then perform quenching treatment, and immediately perform aging treatment at temperature T2 and time t3 after the end. The quenching treatment is water quenching, the temperature T2 of the aging treatment is 160-190°C, and the holding time t3 is 6-24h.

[0035] Referring to Figure 4 the shown heat treatment flow chart, the method provided in this embodiment synchronously performs solution treatment in the hot gas bulging forming stage of the preformed part. By heating the mold to the solution temperature and holding it, the strengthening phases in the high-strength aluminum alloy are fully dissolved in the aluminum matrix to form a uniform α solid solution structure, significantly improving the plasticity of the material; then, in the process of holding pressure and bulging, the high temperature and high plasticity are used to achieve conforming forming, avoiding local strain concentration; after forming, rapid quenching treatment retains the supersaturated solid solution to form a metastable structure, and then artificial aging treatment precipitates dispersed strengthening phases (such as θ' phase, GP zone). Finally, a composite strengthening mechanism of "solution strengthening + precipitation strengthening" is constructed inside the component to achieve the coordinated improvement of tissue homogenization and mechanical properties (such as strength, fatigue performance).

[0036] The above method will be further described below by taking aluminum alloy 2219 and aluminum alloy 7075 as examples respectively.

[0037] Example Two In this embodiment, a forming method of aluminum alloy 2219 is provided. The specific steps include: Step 201: Determine the initial cone tube size according to the necking strain and bulging strain of the part, and obtain the cone tube by rolling and welding the sheet blank. The upper diameter d1 of the cone tube is 100 mm, the lower diameter D is 200 mm, and the axial height H is 150 mm. Step 202: Fix the lower die of the rigid-flexible composite die on the lower table surface of the press. The upper table surface of the press is connected with a slider. Then place the cone tube on the flexible support die. There is a rigid support die below the flexible support die, and limit dies are provided at both ends of the rigid support die. The press drives the necking die to close at a closing speed of 10 mm / s. At the same time, the flexible support die deforms with the part and supports the part, reducing the upper diameter of the cone tube from 100 mm to 90 mm to obtain a one-step necked part. Step 203: Fix the lower die of the rigid-flexible composite die for secondary necking on the lower table surface of the press. The upper table surface of the press is connected with a slider. Then place the one-step necked part on the flexible support die for secondary necking. The press drives the necking die to close at a closing speed of 10 mm / s, reducing the upper diameter of the one-step necked part from 90 mm to 85 mm to obtain a preformed part. Step 204: Heat the hot gas bulging die to the solution temperature of 535 °C. Place the necked cone tube in the hot gas bulging die and close the die. Keep it warm for 40 min, and then load it to 1 MPa at a pressurization rate of 0.5 MPa / min to make the component gradually conform to the die and hold the pressure for 5 min. Step 205: Open the die and quickly take out the formed component, perform quenching treatment, and then immediately perform aging treatment at a temperature of 175 °C for 12 h. Finally, machine the formed component to cut off the excess allowance to finally obtain aluminum alloy 2219.

[0038] Embodiment Three In this embodiment, a forming method of aluminum alloy 7075 is provided. The specific steps include: Step 300: Determine the initial cone tube size according to the necking strain and bulging strain of the part, and obtain the cone tube by rolling and welding the sheet blank. The upper diameter d1 of the cone tube is 100 mm, the lower diameter D is 200 mm, and the axial height H is 150 mm. Step 301: Fix the lower die of the rigid-flexible composite die on the lower table surface of the press. The upper table surface of the press is connected with a slider. Then place the tapered tube on the flexible support die. There is a rigid support die below the flexible support die, and limit dies are provided at both ends of the rigid support die. The press drives the sizing die to close the mold at a closing speed of 10 mm / s. At the same time, the flexible support die deforms with the part and plays a supporting role for the part, reducing the upper end diameter of the tapered tube from 100 mm to 90 mm to obtain a one-step sized part. Step 302: Fix the lower die of the rigid-flexible composite die for secondary sizing on the lower table surface of the press. The upper table surface of the press is connected with a slider. Then place the one-step sized part on the flexible support die for secondary sizing. The press drives the sizing die to close the mold at a closing speed of 10 mm / s, reducing the upper end diameter of the one-step sized part from 90 mm to 85 mm to obtain a preformed part. Step 304: Heat the hot gas bulging die to the solution temperature of 475 °C. Place the sized tapered tube in the hot gas bulging die and close the mold. Keep the temperature for 40 min, and then load it to 1 MPa at a pressurizing rate of 0.5 MPa / min to make the component gradually conform to the mold and hold the pressure for 5 min. Step 305: Open the mold and quickly take out the formed component, perform quenching treatment, and then immediately perform aging treatment at a temperature of 130 °C for 20 h. Finally, machine the formed component to cut off the excess allowance to finally obtain aluminum alloy 7075.

[0039] Therefore, by adopting the above rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter ratio change, through the uniform support of the stress field in the deformation zone by the flexible die in multi-pass rigid-flexible composite sizing to prevent wrinkling, combined with solution treatment to improve the plasticity and elongation of the material, and precipitation strengthening phases after gas bulging and quenching aging treatment, the technical effects of no wrinkling of the formed part, uniform wall thickness, and significant improvement in strength and fatigue performance are achieved.

[0040] In this article, specific examples are used to elaborate on the principle and implementation mode of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation mode and application scope. To sum up, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter change ratio, characterized in that, It includes the following steps: S1. Determine the initial cone tube size according to the necking strain and bulging strain of the formed part, and obtain the cone tube by rolling and welding a sheet blank; S2. Place the cone tube into a rigid-flexible composite die, close the die by a press to neck down, and the flexible support die deforms with the cone tube and provides uniform support to obtain a one-step necked-down part; S3. Place the one-step necked-down part into the rigid-flexible composite die for secondary necking down, close the die again to neck down, and the flexible support die deforms with the part and provides uniform support to obtain a preformed part; S4. Place the preformed part into a hot gas bulging die for solution treatment and heat preservation, and pressurize the closed tube cavity of the hot gas bulging die to make the preformed part conform to the die to obtain a formed part; S5. After completing S4, open the die and quickly take out the formed part, and successively perform quenching treatment, aging treatment and heat preservation, and finally machine the formed part to cut off the surplus to obtain a high-strength aluminum alloy pipe fitting with a large diameter ratio change; 2. The rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with large diameter ratio according to claim 1, characterized in that, In S1, the necking strain and bulging strain of the formed part do not exceed 80% - 85% of the maximum strain of the forming material; the necking strain is: ； The bulging strain is: ； ； Among them, L 1, L 2 are the characteristic dimensions before and after necking down respectively, L 3, L 4 are the characteristic dimensions before and after bulging respectively; is the bulging strain, is the maximum bulging strain of the forming material; is the necking-down strain, is the maximum necking-down strain of the forming material.

3. A rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with large diameter ratio change, characterized in that, In S2 and S3, the material of the flexible support die is one or more of polyurethane, silicone rubber or TPE.

4. A rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter change ratio according to claim 3, characterized in that In S2 and S3, the rigid-flexible composite die includes a flexible support die, a rigid support die, a necking-down die and a limit die; the flexible support die is used to provide uniform support to inhibit wrinkling on the surface of the part, the rigid support die is used to support the flexible die and jointly position the cone tube with the limit die, and the necking-down die is used to apply the necking-down force.

5. A rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter ratio, characterized in that In S2 and S3, the process of closing the die is closed-loop controlled by a displacement control system, and the displacement control system includes a displacement sensor, a controller, a hydraulic cylinder and a feedback device, and is used to monitor and adjust the die closing speed and displacement in real time.

6. A rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter ratio, characterized in that, In S4, the hot gas bulging die includes an upper and lower bulging die, a heat preservation plate, heating rods, a water cooling structure and a high-pressure inflation hole; after the upper and lower bulging dies are closed, a forming cavity matching the target part is formed, the heating rods are controlled by a temperature control system to be heated to the solution temperature, and the temperature control system is composed of a temperature sensor, a controller, heating rods and a feedback adjustment mechanism, and is used to monitor and control the temperature of the bulging die in real time; the water cooling structure and the heat preservation plate are used to prevent heat from being transferred to the press.

7. A rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter ratio change, characterized in that, The closed tube cavity is formed by sealing the two ends of the preformed part with welding plates.

8. A rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with large diameter ratio change, characterized in that, In S4, before the preformed part is placed into the hot gas bulging die, it also includes: preheating the hot gas bulging die to the solution temperature; and determining the solution temperature and heat preservation time according to the material state of the preformed part.

9. A rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter ratio change, characterized in that In S5, the quenching treatment is water quenching.

10. A rigid-flexible composite multi-pass forming method for high-strength aluminum alloy pipe fittings with a large diameter ratio change, characterized in that, In S5, the temperature of the aging treatment is 160 - 190 °C, and the heat preservation time is 6 - 24 h.

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