A method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness
Through the waveform anti-swelling mold and the segmented strain rate forming process, the problem of difficult to control the wall thickness distribution of titanium alloy hemispherical shell in superplastic forming is solved, and the wall thickness is precisely controlled and performance maintenance is achieved, which reduces manufacturing costs and improves mechanics performance.
Patent Information
- Application Number
- CN202111028125.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2021-09-02
- Publication Date
- 2024-12-13
- Estimated Expiration
- 2041-09-02
AI Technical Summary
The prior art is difficult to accurately control the wall thickness distribution of the titanium alloy hemispherical shell during superplastic forming, especially in multiple sudden thickening structures, resulting in increased forming difficulty, high manufacturing cost and reduced performance.
The waveform anti-swelling mold design is adopted, and the reverse preforming at the top of the ball and waist is combined with the segmented strain rate forming process to achieve accurate control and performance maintenance of multiple wall thicknesses.
Accurate control of the wall thickness distribution of titanium alloy hemisphere shell, reduces processing allowance and manufacturing costs, improves the mechanical properties of the final product, and avoids grains caused by long heat exposure time in traditional forward and reverse swelling. Size increase and performance loss.
Smart Images
Figure CN113909368B_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the technical field of superplastic forming of titanium alloys, and relates to a method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness. Background Art
[0002] Superplastic forming is a forming method that utilizes the material's extremely high elongation in a temperature range close to the phase transition point and at a lower strain rate. Usually, superplastic forming uses air pressure as the forming driving force, and relies on the fit with the mold to ensure the final forming accuracy. Superplastic forming has the advantages of high precision, low forming pressure, and no residual stress, and has been increasingly widely used in various fields. At present, superplastic forming of titanium alloy pressure vessels is widely used at home and abroad. As early as the 1980s, the United States conducted experimental research on superplastic forming to manufacture spherical tanks. Japanese companies ISAS and MHI began using superplastic forming technology to manufacture tanks in 1981. The typical application of the European Space Agency using superplastic forming technology to manufacture tanks was in the 1990s, and this technology is still used today. From a domestic perspective, many universities and research institutes have used superplastic forming to manufacture titanium alloy tanks of various sizes, which have been successfully applied.
[0003] In order to meet the urgent requirements of weight reduction, titanium alloy tanks have undergone a lot of thinning designs in recent years. The thickness of the tank shell body has been reduced from 1.2mm to 0.5mm~0.8mm. While increasing the difficulty of forming, the performance requirements are also more stringent. In order to meet the strength requirements and structural interface requirements, the hemispherical shell will be thickened or locked at the top and opening, and the inner surface of the waist is designed with an interface ring rib. In order to ensure the welding strength, the interface thickness at the ring rib can reach 4mm~5mm. If traditional unidirectional superplastic forming is adopted, the wall thickness at the top and ring rib of the hemispherical ball is difficult to meet the requirements. Simply increasing the thickness of the plate not only greatly increases the subsequent processing allowance and increases the manufacturing cost, but also increases the difficulty of grain refinement of the plate, and cannot meet the requirements of superplastic forming. Therefore, special means must be taken during superplastic forming to achieve the effect of precise control of wall thickness. Wall thickness control during positive and negative forming is the most conventional method, but the anti-bulging surface of this method is usually annular, which can only guarantee the wall thickness conditions of the local area. For multiple sudden thickening structures, this method is difficult to achieve precise control of wall thickness in multiple places. At the same time, conventional forward and reverse forming greatly increases the forming time due to the addition of the reverse preforming process, which leads to an increase in the heat exposure time of the hemispherical shell at high temperatures close to the phase transition point, grain growth, and a decrease in the final strength performance. Summary of the invention
[0004] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art and propose a method for superplastic forming of a thin-walled hemispherical with sudden wall thickness and precision and performance control, which can be used to form titanium alloy hemispherical shells of various sizes with large differences in wall thickness distribution, and has better thickness distribution control and performance retention effects.
[0005] The solution of the present invention is:
[0006] A method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness comprises the following steps:
[0007] Step 1, determining the size of the thin-walled hemisphere after the thin plate blank is formed into the thin-walled hemisphere;
[0008] Step 2: Design the lower mold for forming the thin-walled hemisphere, i.e., the positive expansion mold;
[0009] Step 3: Design an upper mold for forming a thin-walled hemisphere, i.e., a reverse expansion mold;
[0010] Step 4: polishing the inner surface of the lower mold and the inner surface of the upper mold, and the roughness of the inner surface after polishing is 0.4;
[0011] Step 5: evenly apply solder stop on the inner surface of the lower mold and the inner surface of the upper mold, and perform heating treatment;
[0012] Step 6: Place the sheet blank on top of the lower mold, and cover the upper mold on the sheet blank; perform heating treatment;
[0013] Step 7, performing reverse expansion treatment and positive expansion treatment on the thin plate blank in sequence; forming a thin-walled hemisphere;
[0014] Step 8: Cool down and take out the formed thin-walled hemisphere.
[0015] In the above-mentioned superplastic forming method for controlling the precision and performance of a thin-walled hemisphere with a sudden wall thickness, in step 1, the thin plate blank is made of TC4 titanium alloy material.
[0016] In the above-mentioned method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with a sudden change in wall thickness, in step one, the wall thickness of the thin-walled hemisphere is 0.5-1 mm; the top of the thin-walled hemisphere is thickened inwardly, and the thickness δ5 of the thickened area at the top of the hemisphere is 2-3 mm; annular ribs are arranged inside the thin-walled hemisphere, and the thickness δ3 of the thickened area of the annular ribs is 4-5.5 mm; the top opening of the thin-walled hemisphere is thickened inwardly, and the thickness δ1 of the thickened area of the top opening is 4-5.5 mm; the radius R of the thin-walled hemisphere is 350-500 mm; and the height D of the annular ribs from the top of the hemisphere is 100-400 mm.
[0017] In the above-mentioned method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with a sudden change in wall thickness, in the step 2, the lower mold is a horizontally placed rectangular structure; a groove is provided on the upper surface of the lower mold, and the profile of the groove is divided into three sections, namely a spherical section adapted to the thin-walled hemisphere, a draft section and a flanging section; the spherical section is located at the bottom of the groove, the draft section is arranged at the top of the spherical section, and the flanging section is arranged at the top of the draft section; the profile size of the spherical section is consistent with the outer wall of the thin-walled hemisphere; the draft section is a straight surface, the angle between the draft section and the vertical direction is 2°-8°, and the vertical height of the draft section is 20-30mm; the flanging section is an annular groove; the flanging section and the draft section are transitioned by a fillet, and the fillet radius is 20-50mm.
[0018] In the above-mentioned superplastic forming sudden wall thickness thin-walled hemisphere precision and performance control method, in the step three, the upper mold is a horizontally placed rectangular structure; the lower mold surface of the upper mold is a wave-shaped surface, and from the center to the outside edge are the first wave peak, the wave trough, and the second wave peak; wherein the width of the first wave peak is 80 to 160 mm, and the height is 30-60 mm; the width of the second wave peak is 200-400 mm, and the height is 80-150 mm.
[0019] In the above-mentioned superplastic forming sudden wall thickness thin-walled hemisphere precision and performance control method, in the step five, the heating rate is 40-70°C / h until it reaches 750°C.
[0020] In the above-mentioned superplastic forming sudden wall thickness thin-walled hemisphere precision and performance control method, in step six, the heating rate is 40°C / h, and when the temperature is raised to 880°C, step seven is entered.
[0021] In the above-mentioned superplastic forming sudden wall thickness thin-walled hemisphere precision and performance control method, in step seven, the method of sequentially performing anti-expansion treatment on the thin plate blank is:
[0022] Blow air to the inner surface of the lower mold at the bottom of the thin plate blank to make the thin plate blank bulge upward until it fits with the inner surface of the upper mold;
[0023] The method of performing positive expansion treatment on the thin plate blank in sequence is:
[0024] Air is blown toward the inner surface of the upper mold on the upper part of the thin plate blank to make the thin plate blank bulge downward until it fits with the inner surface of the lower mold.
[0025] In the above-mentioned superplastic forming method for controlling the precision and performance of a thin-walled hemisphere with a sudden change in wall thickness, in step seven, during the reverse expansion process, the reverse expansion pressure of the blowing air is 24 bar; during the positive expansion process, the positive expansion pressure of the blowing air is 12 bar; and the forming time is 150 minutes.
[0026] In the above-mentioned method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with a sudden change in wall thickness, in step eight, the lower mold, the upper mold and the thin-walled hemisphere are cooled to room temperature.
[0027] The beneficial effects of the present invention compared with the prior art are:
[0028] (1) The present invention adopts a corrugated reverse bulging mold design, and through reverse preforming at the top of the ball and the waist and other positions, the distribution of multiple thickened areas can be accurately controlled. The existing traditional positive and negative bulging can only ensure the thickness of the top of the ball, and the waist of the hemisphere is the thinnest area, with a thinning rate of up to 60%, which cannot meet the design requirements of thickening the top of the ball and the waist at the same time. Through simulation calculation, the corrugated reverse bulging mold can adjust the unpreformed area (i.e., the trough area) to the waist thickening area after forming to ensure the wall thickness requirements. Since the top of the ball is stretched in advance to increase the area, the amount of thinning again during positive bulging is very small, which ensures the thickness requirements at this location;
[0029] (2) The present invention adopts a corrugated reverse bulging die design, which can achieve precise control of the thickness of the thickening area and achieve near-net forming. The single-side allowance in the thickening area is only 0.4-0.8mm. This further solves the problem of large thickening area and large processing allowance in traditional positive and negative bulging;
[0030] (3) The present invention adopts a corrugated reverse bulging die design. Under the premise of achieving the same ball top thickness control, the reverse bulging height is significantly reduced compared with the traditional positive and negative bulging. Therefore, the amount of high-temperature die steel required for the reverse bulging die can be greatly reduced, saving costs;
[0031] (4) The present invention adopts a corrugated anti-bulging die design, which can be accurately calculated and only one set of dies can be used to form hemispheres with different structural designs of the same diameter. This effectively reduces the mold manufacturing cost and greatly improves the research and production efficiency;
[0032] (5) The present invention adopts a process of segmented strain rate forming. This method can effectively solve the problems of long time of traditional positive and negative bulging, obvious growth of grain size, and serious loss of final product performance, thereby ensuring the mechanical properties of the final product. BRIEF DESCRIPTION OF THE DRAWINGS
[0033] Figure 1 This is a schematic diagram of a thin-walled hemisphere after molding according to the present invention;
[0034] Figure 2 It is a schematic diagram of the inner surfaces of the upper mold and the lower mold of the present invention. DETAILED DESCRIPTION
[0035] The present invention will be further described below in conjunction with the embodiments.
[0036] The present invention provides a method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness. The method adopts a corrugated reverse bulging surface design to accurately control the pre-thinning position and thinning amount in the reverse forming process, thereby achieving accurate control of the wall thickness distribution after positive bulging and meeting the processing requirements of a multi-position sudden wall thickness structure.
[0037] The method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness specifically comprises the following steps:
[0038] Step 1: Determine the size of the thin-walled hemisphere after the thin-plate blank is formed into a thin-walled hemisphere; the thin-plate blank is made of TC4 titanium alloy. The wall thickness of the thin-walled hemisphere is 0.5-1mm; the top of the thin-walled hemisphere is thickened inwards, and the thickness δ5 of the thickened area of the top of the sphere is 2-3mm; annular ribs are set inside the thin-walled hemisphere, and the thickness δ3 of the thickened area of the annular ribs is 4-5.5mm; the top opening of the thin-walled hemisphere is thickened inwards, and the thickness δ1 of the thickened area of the top opening is 4-5.5mm; the radius R of the thin-walled hemisphere is 350-500mm; the height D of the annular ribs from the top of the sphere is 100-400mm, such as Figure 1 shown.
[0039] Step 2, design a lower mold for forming a thin-walled hemisphere, that is, a positive expansion mold; the lower mold is a horizontally placed rectangular structure; the upper surface of the lower mold is provided with a groove, and the profile of the groove is divided into three sections, namely a spherical section 1 adapted to the thin-walled hemisphere, a draft section 2 and a flanging section 3; the spherical section 1 is located at the bottom of the groove, the draft section 2 is arranged at the top of the spherical section 1, and the flanging section 3 is arranged at the top of the draft section 2; the profile size of the spherical section 1 is consistent with the outer wall of the thin-walled hemisphere; the draft section 2 is a straight surface, the angle between the draft section 2 and the vertical direction is 2°-8°, and the vertical height of the draft section 2 is 20-30mm; the flanging section 3 is an annular groove; the flanging section 3 and the draft section 2 are transitioned by a fillet 4, and the radius of the fillet 4 is 20-50mm, such as Figure 2 shown.
[0040] Step 3: Design an upper mold for forming a thin-walled hemisphere, i.e., an anti-bulging mold; the upper mold is a horizontally placed rectangular parallelepiped structure; the lower mold surface of the upper mold is a wave-shaped surface, and from the center to the outside, there are the first wave crest 5, the wave trough 6, and the second wave crest 7; wherein the width of the first wave crest 5 is 80-160 mm, and the height is 30-60 mm; the width of the second wave crest 7 is 200-400 mm, and the height is 80-150 mm, such as Figure 2 shown.
[0041] Step 4: Polish the inner surfaces of the lower mold and the upper mold, and the roughness of the inner surfaces after polishing is 0.4.
[0042] Step 5: evenly apply solder stop on the inner surface of the lower mold and the inner surface of the upper mold, and perform heating treatment; the heating rate is 40-70°C / h until it reaches 750°C.
[0043] Step 6: Place the thin plate blank on top of the lower mold and cover the thin plate blank with the upper mold; perform heating treatment; the heating rate is 40°C / h, and when the temperature reaches 880°C, proceed to step 7.
[0044] Step 7: Perform reverse expansion treatment and positive expansion treatment on the thin plate blank in sequence; forming a thin-walled hemisphere; the method for performing reverse expansion treatment on the thin plate blank in sequence is:
[0045] Blow air to the inner surface of the lower mold at the bottom of the thin plate blank to make the thin plate blank bulge upward until it fits with the inner surface of the upper mold;
[0046] The method of performing positive expansion treatment on the thin plate blank in sequence is:
[0047] Air is blown toward the inner surface of the upper mold on the upper part of the thin plate blank to make the thin plate blank bulge downward until it fits with the inner surface of the lower mold.
[0048] During the reverse expansion process, the reverse expansion pressure of the blowing air is 24 bar; during the positive expansion process, the positive expansion pressure of the blowing air is 12 bar; the forming time is 150 minutes.
[0049] Step 8: Cool down and take out the formed thin-walled hemisphere. Cool the lower mold, the upper mold and the thin-walled hemisphere to room temperature.
[0050] Example
[0051] In this embodiment, TC4 titanium alloy plate conforming to GJB2505A-2008 is used, and the target hemispherical product has R=400mm, δ1=5.5mm, δ3=5.5mm, δ5=3.0mm, and D=150mm.
[0052] (1) Design of hemispherical thickness distribution after forming: The thickness of the hemispherical product after forming is designed according to the target hemispherical product diameter and wall thickness distribution. The outer surface allowance of the main body is 0.8mm, the inner surface allowance is 1.5mm, and the minimum thickness reference value is 3.0mm. The outer surface allowance of the dome thickening area δ5 is 1.5mm, the inner surface allowance is 0.5mm, and the minimum thickness reference value is 5.0mm. The outer surface allowance of the ring rib thickening area δ3 is 0.6mm, the inner surface allowance is 0.5mm, and the minimum thickness reference value is 6.1mm. The outer surface allowance of the opening δ1 is 1.5mm, the inner surface allowance is 1.0mm, and the minimum thickness reference value is 8.0mm.
[0053] (2) Design of mold positive expansion surface: Based on the hemispherical surface obtained in (1) as the lower mold surface, a draft section with a draft angle of 3° and a height of 25 mm is designed above the opening. A R30mm rounded transition is designed between the draft section and the flange. A sealing groove and a sheet metal positioning groove are designed on the flange.
[0054] (3) Design of mold anti-bulging surface: The anti-bulging mold adopts a corrugated surface design. The top area of the ball (the first wave peak) and the area between the ring rib and the hemispherical opening (the second wave peak) are the anti-bulging pre-stretching area, and the thickened area of the ring rib is the "wave valley" area. The height of the first wave peak is 40mm, and the height of the second wave peak is 80mm.
[0055] (4) Numerical simulation of the forming process: The superplastic forming process was simulated using MSC.Marc finite element numerical simulation software. The mesh was divided using three-dimensional quadrilateral shell elements, and the simulation results directly gave the predicted value of the wall thickness distribution. By comparison, the wall thickness distribution at this time meets the reference value requirements for the overall wall thickness distribution of the hemisphere, and this mold surface can be defined as an anti-bulging mold surface.
[0056] (5) Optimization of forming process parameters: The simulation process was divided into segments. The strain rate in the non-die-attached stage of the reverse expansion and positive expansion ring ribs was set to 8×10-4, at which time the strain rate sensitivity index m=0.45; the strain rate in the second half of the forming stage was 2×10-4, at which time the strain rate sensitivity index m=0.50.
[0057] (6) Mold processing: Ni7N is used as the raw material for casting and mold surface processing. The upper mold is a reverse expansion mold, and its surface is the corrugated surface determined in (4); the lower mold is a positive expansion mold, and its surface is the hemispherical surface determined in (2). After the mold processing is completed, it is polished to ensure that the roughness is 0.4.
[0058] (7) Mold heating in the furnace: After the upper and lower molds are placed in the furnace and positioned, they are installed on the upper and lower platforms respectively. The upper platform is raised, and the upper and lower mold surfaces are evenly coated with solder paste. The temperature is raised at a rate of 50℃ / h until it reaches 750℃.
[0059] (8) Plate material enters the furnace at high temperature: When the temperature of the platform furnace chamber reaches 750℃, the upper platform is raised. At this time, since the upper mold and the upper platform are fixed, they will be separated from the lower mold as the upper platform rises. Open the furnace door, put the plate material into the positioning groove, and close the furnace door.
[0060] (9) Heating to superplastic temperature: Continue heating at a rate of 40°C / h and set the temperature to 920°C.
[0061] (10) Forward and reverse superplastic forming: When the average temperature of the mold thermocouple reaches 880°C, the superplastic forward and reverse forming program is started. The program is formulated based on the curve obtained in step (5) and the equipment capacity. The reverse expansion pressure is 24 bar, the forward expansion pressure is 12 bar, and the forming time is 150 min.
[0062] (11) Cooling to the furnace exit temperature: After the superplasticization process is completed, the upper platform is lifted, and the mold and hemispherical shell are cooled to the furnace exit temperature of 800°C.
[0063] (12) Hemisphere out of the oven: After reaching the out-of-the-oven temperature, open the oven door and use the out-of-the-oven tooling to directly take out the hemisphere. The out-of-the-oven hemisphere is placed on the tooling to cool to room temperature.
[0064] (13) Fine turning: The hemispherical shell is fine turned to ensure the accuracy of all shape, position, dimensions and wall thickness.
[0065] Comparative Example
[0066] The conventional positive and negative superplastic forming reverse bulging die surface design was used to perform a forming numerical simulation analysis on a hemisphere with the same structure as the embodiment. It can be seen that the conventional reverse bulging surface is a single-region preformed surface, and after forming, the wall thickness is significantly improved only at the top of the hemisphere, but the wall thickness at the annular rib cannot meet the requirements, and adjusting the curvature and height of the reverse bulging surface still cannot effectively increase the wall thickness at this location.
[0067] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.
Claims
1. A method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness, characterized in that: The steps include: Step 1, determining the size of the thin-walled hemisphere after the thin plate blank is formed into the thin-walled hemisphere; The wall thickness of the thin-walled hemisphere is 0.5-1mm; the top of the thin-walled hemisphere is thickened inwardly, and the thickness δ5 of the thickened area of the top of the sphere is 2-3mm; an annular rib is arranged inside the thin-walled hemisphere, and the thickness δ3 of the thickened area of the annular rib is 4-5.5mm; the top opening of the thin-walled hemisphere is thickened inwardly, and the thickness δ1 of the thickened area of the top opening is 4-5.5mm; the radius R of the thin-walled hemisphere is 350-500mm; the height D of the annular rib from the top of the sphere is 100-400mm; Step 2: Design the lower mold for forming the thin-walled hemisphere, i.e., the positive expansion mold; In the step 2, the lower mold is a horizontally placed rectangular parallelepiped structure; a groove is provided on the upper surface of the lower mold, and the profile of the groove is divided into three sections, namely a spherical section (1) adapted to the thin-walled hemisphere, a draft section (2) and a flanging section (3); the spherical section (1) is located at the bottom of the groove, the draft section (2) is arranged at the top of the spherical section (1), and the flanging section (3) is arranged at the top of the draft section (2); the profile size of the spherical section (1) is consistent with the outer wall of the thin-walled hemisphere; the draft section (2) is a straight surface, the angle between the draft section (2) and the vertical direction is 2°-8°, and the vertical height of the draft section (2) is 20-30 mm; the flanging section (3) is an annular groove; the flanging section (3) and the draft section (2) are transitioned by a fillet (4), and the radius of the fillet (4) is 20-50 mm; Step 3: Design an upper mold for forming a thin-walled hemisphere, i.e., a reverse expansion mold; In the step 3, the upper mold is a horizontally placed rectangular parallelepiped structure; the lower mold surface of the upper mold is a wave-shaped surface, and from the center to the outside, there are a first wave peak (5), a wave trough (6), and a second wave peak (7); wherein the width of the first wave peak (5) is 80 to 160 mm, and the height is 30 to 60 mm; the width of the second wave peak (7) is 200 to 400 mm, and the height is 80 to 150 mm; Step 4: polishing the inner surface of the lower mold and the inner surface of the upper mold, and the roughness of the inner surface after polishing is 0.4; Step 5: evenly apply solder stop on the inner surface of the lower mold and the inner surface of the upper mold, and perform heating treatment; Step 6: Place the sheet blank on top of the lower mold and cover the upper mold on the sheet blank; perform heating treatment; Step 7, performing reverse expansion treatment and positive expansion treatment on the thin plate blank in sequence; forming a thin-walled hemisphere; In the step 7, the method of sequentially performing the anti-expansion treatment on the thin plate blank is: Blowing air to the inner surface of the lower mold at the bottom of the thin plate blank to make the thin plate blank bulge upward until it fits with the inner surface of the upper mold; The method of performing positive expansion treatment on the thin plate blank in sequence is: Blowing air onto the inner surface of the upper mold on the upper part of the thin plate blank to make the thin plate blank bulge downward until it fits with the inner surface of the lower mold; During the reverse expansion process, the reverse expansion pressure of the air is 24 bar; during the positive expansion process, the positive expansion pressure of the air is 12 bar; the forming time is 150 minutes; Step 8: Cool down and take out the formed thin-walled hemisphere.
2. The method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness according to claim 1, characterized in that: In the step 1, the thin plate blank is made of TC4 titanium alloy material.
3. The method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness according to claim 2, characterized in that: In the step 5, the heating rate is 40-70°C / h until reaching 750°C.
4. The method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness according to claim 3 is characterized in that: In the step six, the heating rate is 40° C. / h, and when the temperature reaches 880° C., step seven is entered.
5. The method for controlling the precision and performance of a superplastic forming thin-walled hemisphere with sudden wall thickness according to claim 1, characterized in that: In the step eight, the lower mold, the upper mold and the thin-walled hemisphere are cooled to room temperature.