A method for manufacturing an extension section of a large liquid rocket second stage engine nozzle

Abstract

The present application relates to the technical field of liquid rocket engine nozzle, and discloses a manufacturing method of an extension section of a large-scale liquid rocket second-stage engine nozzle, which is characterized in that the extension section is divided into n cylinder sections according to the size of the plate and the elongation rate of the plate, and each cylinder section is divided into m melon pieces to be prepared by means of tailor welding; the number n of the cylinder sections ensures that the number of the girth welds of the extension section is the least, and the welding of the girth welds is maximally avoided; the cylinder sections of the extension section are obtained by stress relaxation heat correction treatment of preformed cylinder sections, and the uncontrollable welding deformation problem is overcome. The present application solves the problem that the uncontrollable deformation caused by the springback of the sheet metal forming in the block welding leads to the difficulty in guaranteeing the profile tolerance of the inner surface of the nozzle extension section, improves the welding quality, reduces the rejection rate and production cost of the finished product, improves the production efficiency, has a wide application prospect, and any large-diameter thin-walled product can be processed according to the method of the present application.

Description

Technical Field

[0001] This invention relates to the field of liquid rocket engine nozzle technology, specifically to a method for manufacturing an extension section of a large liquid rocket second-stage engine nozzle. Background Technology

[0002] In liquid rocket propulsion systems, the engine nozzle is a key component of the rocket engine and an important energy conversion device. The extension section, in particular, primarily accelerates the high-temperature combustion gases to generate thrust, making it a crucial part of the nozzle. The nozzle extension section has a bell-shaped shape, with its inner surface being a parametric surface. The profile of this surface directly affects the engine's vacuum specific impulse performance. Second-stage engines operate in a vacuum environment. When the engine is operating in a vacuum, a larger nozzle area ratio results in higher specific impulse. Therefore, the large-end diameter of the nozzle extension section of a large liquid rocket second-stage engine is generally over 3000 mm, the height is around 3200 mm, and the wall thickness is generally less than 1 mm, classifying it as a large-size, thin-walled component.

[0003] There are two main existing methods for processing nozzle extension sections. One is to use 3D printing technology, based on digital model files, to construct the nozzle extension section by printing powdered metal layer by layer. The other is to use a modular welding method to manufacture the nozzle extension section, which is divided into several small pieces, each of which is formed by sheet metal roll bending, and then the small pieces are welded together into a whole.

[0004] The second stage of a rocket requires extremely strict quality control, and the corresponding second-stage engine also requires very strict overall quality control. Furthermore, the engine operates at high temperatures, reaching up to 3000℃. Therefore, using 3D printing technology presents several problems: First, using alloy powder as raw material results in insufficient overall strength. To ensure sufficient strength, the nozzle thickness needs to be increased, failing to meet the requirement of a wall thickness less than 1mm. Second, 3D printing requires the addition of binders, which compromises the nozzle's high-temperature resistance, necessitating additional cooling for the nozzle extension section, increasing production costs and energy consumption. In summary, this approach is suitable for manufacturing first-stage rocket engine nozzles where load requirements are relatively relaxed, but not for second-stage engine nozzles. Using a segmented welding process, the parts will spring back after sheet metal forming. The welding process after part forming involves longitudinal and circumferential seams, making welding complex and difficult to control. It is challenging to guarantee the required tolerances for the internal surface contour. Moreover, the sheet metal used to manufacture the nozzle extension section is a thin high-temperature alloy plate, making shape correction difficult. Therefore, the finished product has a high probability of being scrapped due to out-of-tolerance internal surface contour tolerances in the extension section. Summary of the Invention

[0005] In view of this, and in view of the shortcomings of the prior art, the present invention provides a method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine. This method solves the problem that the deformation caused by the springback of sheet metal forming during segmented welding is uncontrollable, which makes it difficult to guarantee the surface profile tolerance of the nozzle extension section. This method improves welding quality, reduces the scrap rate of finished products and production costs, and improves production efficiency.

[0006] To solve the above technical problems, the technical solution provided by this invention is: a method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine, which includes the following steps:

[0007] Step 1: Establish an extension section model. Divide the extension section model into n cylindrical sections (n ​​is an integer, ≥2) along the model axis. Scale down the cylindrical sections radially to obtain a pre-formed cylindrical section. The scaling ratio is cylindrical section: pre-formed cylindrical section = 1:(0.995~0.999). Divide the pre-formed cylindrical section into m segments (m is an integer, ≥2). Unfold the segments to obtain the unfolded surface. The number of cylindrical sections n and the number of segments m are determined based on the dimensions of the raw material plate. The number of cylindrical sections n ensures that the number of circumferential welds in the extension section is minimized.

[0008] Step 2: Cut the raw material plate according to the unfolded surface of the melon petals obtained in Step 1 to obtain the melon petal forming substrate. Process the melon petal forming substrate according to the size and cross-sectional contour of the melon petals described in Step 1 to obtain the melon petals described in Step 1.

[0009] Step 3: Prepare m melon segments according to Step 2, and longitudinally weld the m melon segments to obtain the preformed cylinder described in Step 1;

[0010] Step 4: The preformed cylinder prepared in Step 3 undergoes stress relaxation thermal straightening treatment to obtain the cylinder described in Step 1. The stress relaxation thermal straightening treatment involves first assembling the preformed cylinder prepared in Step 3 onto the bulging mold at room temperature and locking it. Then, the preformed cylinder and the bulging mold are placed together in a heating furnace and heated to the stress relief temperature of the raw material plate. After holding at the stress relief temperature, the preformed cylinder and the bulging mold are cooled to below 200°C and air-cooled for removal. The holding time of the preformed cylinder and the bulging mold at the stress relief temperature is determined according to the material and thickness of the raw material plate. For example, in this embodiment of the invention, the raw material plate used is a GH3128 high-temperature alloy thin plate with a wall thickness of 0.8 mm, and its holding time is 3-4 hours.

[0011] Step 5: Prepare n sections of cylinder according to the methods in steps 2 to 4, and weld the n sections of cylinder circumferentially to obtain the nozzle extension section.

[0012] Furthermore, the method for processing the melon-shaped substrate in step 2 is room temperature stretch forming and water jet cutting.

[0013] Furthermore, in step 3, the melon petal welding is performed using an automated laser welding system, and the welding process is completed by industrial robot-controlled motion.

[0014] Furthermore, in step 3, the melon petals are welded together after surface oxide removal treatment.

[0015] Furthermore, the surface oxide removal method involves first cleaning the surface of the melon petals with solvents such as acetone or alcohol to remove oil stains, then corroding with an acidic solution to remove oxides. The temperature of the acidic solution is controlled at 45–60°C, and the corrosion time is 90–150 min. After corrosion, the surface is rinsed with cold water, then neutralized with an alkaline solution at a temperature ≤80°C for 1–3 min. After neutralization, the surface is rinsed with cold water, then dried by blowing or air drying, and finally subjected to hydrogen removal treatment at a temperature of 180–250°C for 120 min.

[0016] Furthermore, in step 4, the actual size of the outer contour of the bulging mold after assembling the preformed cylinder is slightly smaller than the cylinder size described in step 1. The size difference is the difference in volume expansion between the preformed cylinder and the bulging mold at the stress relief temperature. This difference can be calculated using the volume expansion coefficients of the two materials. Therefore, the linear expansion coefficient of the raw material used to prepare the bulging mold should be greater than the linear expansion coefficient of the raw material plate used to prepare the preformed cylinder, so as to ensure that the bulging mold can be heated and expanded to the theoretical size of the inner surface of the cylinder at the stress relief temperature of the preformed cylinder, thus bulging the preformed cylinder into the cylinder described in step 1.

[0017] Furthermore, in step 5, the cylinder welding is performed using a combination of manual positioning welding and an automatic argon arc welding system.

[0018] Furthermore, the manual positioning welding method involves symmetrically welding eight positioning points, which are evenly distributed on the circumference, with the weld length at each positioning point being 10–15 mm.

[0019] Furthermore, the automatic laser welding system or automatic argon arc welding system performs welding without welding wire, using high-purity argon gas with a purity of ≥99.99% as the protective gas, and the automatic argon arc welding system uses cerium-tungsten wire as the electrode.

[0020] Furthermore, in step 5, the n sections of the cylinder are machined to accommodate the circumference of each cylinder before welding.

[0021] Compared with existing technologies, the method for manufacturing the nozzle extension section provided by this invention, taking into account the maximum size of the sheet metal available from the raw material manufacturer (standard sheet width of high-temperature alloy thin plate is 1000mm×2200mm, thickness 0.8mm) and the elongation rate of the sheet metal, divides the extension section into n cylindrical sections. Each cylindrical section is further divided into m segments, and the nozzle extension section is fabricated by welding. This solves the problem that the size of the raw material sheet metal cannot meet the requirements for manufacturing nozzle extension sections for large liquid rocket engines. Furthermore, the number of cylindrical sections n and the number of segments m are determined by the size of the raw material sheet metal. The number of cylindrical sections n ensures that the number of circumferential welds in the extension section is minimized, thus avoiding circumferential welds to the greatest extent possible. Because the circumferential dimension of the nozzle extension section is large, the circumferential weld process is more difficult than the longitudinal weld process for segments, and circumferential welds cannot avoid welding deformation, while longitudinal welds for segments can... By overcoming welding deformation through stress relaxation thermal straightening treatment, this invention minimizes the need for circumferential welding and reduces the welding deformation rate. Each section of the cylinder follows a process of "cutting sheet metal into melon-shaped substrate → stretching the melon-shaped substrate to form melon petals → welding the melon petals to form a pre-formed cylinder → undergoing stress relaxation thermal straightening treatment to form the cylinder." This eliminates the uncontrollable welding deformation problem associated with direct welding of multiple sheets. The method of stress relief in the furnace with an expansion mold effectively solves the springback problem after sheet metal forming and also addresses the difficulty in ensuring the surface profile tolerance within the nozzle extension section. The circumferential allowance machining process added after the extension section cylinder is formed effectively solves the problem of matching the circumferential weld circumferential weld circumference, making the circumferential weld controllable and better guaranteeing its quality.

[0022] The method for manufacturing the nozzle extension section provided by this invention uses existing equipment and sheet metal for processing. The processing method is conventional sheet metal processing, machining, and welding. All processing steps are controllable, which solves the problem that the deformation caused by the springback of sheet metal forming in the segmented welding process is uncontrollable, making it difficult to guarantee the surface profile tolerance of the nozzle extension section. This improves welding quality, reduces the scrap rate and production cost of finished products, and increases production efficiency. It has broad application prospects, and any large-diameter thin-walled product can be processed according to the method of this invention. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the external shape of the nozzle extension section prepared according to an embodiment of the present invention;

[0024] Figure 2 This is a schematic diagram of the segmentation of the nozzle extension section prepared according to an embodiment of the present invention;

[0025] Figure 3 This is a schematic diagram of the structure of the preformed cylinder corresponding to the front section cylinder in an embodiment of the present invention;

[0026] Figure 4 for Figure 3 Schematic diagram of the structure of the middle melon segment;

[0027] Figure 5 for Figure 4 A schematic diagram showing the unfolded shape of the melon petals.

[0028] Legend:

[0029] 1- Nozzle extension section; 11- Front section cylinder; 12- Rear section cylinder; 2- Pre-formed cylinder; 21- Petal-shaped; 3- Petal-shaped substrate. Detailed Implementation

[0030] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0031] See Figure 1 The nozzle extension section 1 of a large liquid rocket second-stage engine has a bell-shaped shape, with a large-end diameter generally exceeding 3000 mm, a height of approximately 3200 mm, and a wall thickness generally less than 1 mm, classifying it as a large-size, thin-walled component. The following embodiment uses a GH3128 high-temperature alloy sheet (compliant with GB / T14996-2010 high-temperature alloy cold-rolled sheet standard) with a wall thickness of 0.8 mm after solution treatment to prepare the nozzle extension section (large-end diameter 3100 mm, height 3200 mm, wall thickness 0.8 mm) of the large liquid rocket second-stage engine.

[0032] Example: A method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine, comprising the following steps:

[0033] Step 1: Establish the extension section model. Divide the extension section model into n cylindrical sections (n ​​is an integer, ≥2) along the model axis. Scale down each cylindrical section radially to obtain a pre-formed cylindrical section. Divide the pre-formed cylindrical section into m segments (m is an integer, ≥2). Unfold the segments to obtain the unfolded surface. The scaling ratio is cylindrical section : pre-formed cylindrical section = 1 : (0.995~0.999). The number of cylindrical sections n and the number of segments m are determined based on the size of the raw material plate. The number of cylindrical sections n ensures that the number of circumferential welds in the extension section is minimized.

[0034] In this embodiment, see Figure 2 The extension section model is divided into two cylindrical sections, including a front cylindrical section 11 and a rear cylindrical section 12. The front cylindrical section 11 is scaled down proportionally at a ratio of 1:0.995 to obtain the corresponding preformed cylindrical section, and the rear cylindrical section 12 is scaled down proportionally at a ratio of 1:0.999 to obtain the corresponding preformed cylindrical section.

[0035] Figure 3This is a structural schematic diagram of the preformed cylinder 2 corresponding to the front cylinder 11. Figure 3 As can be seen, the preformed cylinder 2 is evenly divided into ten melon-shaped segments 21. Figure 4 This is a schematic diagram of the shape of a melon segment. Figure 5 This is a schematic diagram showing the unfolding of the melon segments.

[0036] Step 2: According to the outline of the melon petal unfolding surface obtained in Step 1 (e.g., Figure 5 (As shown) Cut the raw material plate to obtain the melon petal forming substrate. Process the melon petal forming substrate according to the size and cross-sectional contour of the melon petal in step 1 to obtain the melon petal in step 1. The processing method adopts room temperature stretch forming process and water cutting to remove the stretch forming edge weight.

[0037] In this embodiment, the melon petal forming substrate is processed by room temperature stretch forming process. Specifically, the single-side dimension of the melon petal forming substrate is the single-side dimension of the unfolded surface of the melon petal plus a stretch forming blanking allowance of 30-50mm. In addition, a machining allowance of 30-50mm is added to each of the upper and lower end faces. After the melon petal forming substrate is stretched and formed at room temperature, the stretch forming blanking allowance is removed by water jet cutting to obtain the melon petal.

[0038] Step 3: Prepare m pieces of melon segments according to Step 2, and longitudinally weld the m pieces of melon segments to obtain the pre-formed cylinder described in Step 1; preferably, the melon segments need to undergo surface oxide removal treatment before welding. In this embodiment, see... Figures 3 to 5 Ten melon segments 21 are longitudinally welded together to obtain a pre-formed cylinder 2 corresponding to the front cylinder 11.

[0039] Step 4: The preformed cylinder prepared in Step 3 is subjected to stress relaxation thermal straightening treatment to obtain the cylinder described in Step 1. The stress relaxation thermal straightening treatment is as follows: First, the preformed cylinder prepared in Step 3 is assembled onto the bulging mold at room temperature and locked. Then, the preformed cylinder is placed in a heating furnace along with the bulging mold and heated to the stress relief temperature of the raw material plate (the stress relief temperature of GH3128 high temperature alloy thin plate is 850±10℃). After holding at the stress relief temperature for 3 to 4 hours, it is cooled to below 200℃ in the furnace and then air-cooled to remove the part.

[0040] In this embodiment, ductile cast iron is used to make the above-mentioned bulging mold. The linear expansion coefficient of the raw material (ductile cast iron) used to make the bulging mold is greater than that of the raw material plate (GH3128 high temperature alloy thin plate) used to make the preformed cylinder. This ensures that the bulging mold can be heated and expanded to the theoretical size of the inner surface of the cylinder at the stress relief temperature of the preformed cylinder.

[0041] Step 5: Prepare n sections of the cylinder according to the methods in steps 2 to 4, and weld the n sections of the cylinder circumferentially to obtain the nozzle extension section. In this embodiment, see... Figure 2 The front section cylinder 11 and the rear section cylinder 12 are circumferentially welded together to obtain... Figure 1 The nozzle extension shown.

[0042] The specific workflow is as follows:

[0043] 1. Purchase qualified boards that have undergone solution treatment.

[0044] 2. Using laser cutting, the sheet metal is cut into trapezoidal plates according to the process requirements of the corresponding cylinder. The trapezoidal plates are the melon-shaped substrate (the process dimensions of the trapezoidal plates are determined by theoretical calculations. The front cylinder 11 is reduced proportionally at a ratio of 1:0.995 to obtain the corresponding pre-formed cylinder, and the rear cylinder 12 is reduced proportionally at a ratio of 1:0.999 to obtain the corresponding pre-formed cylinder. The deformation of the pre-formed cylinder is required to be between 5% and 8%, and the ratio of the deformation to the elongation of the raw material is between 12% and 20%).

[0045] 3. The trapezoidal sheet is stretched into shape using a room temperature stretching process, and then the edge is cut with water jet to obtain the melon petals.

[0046] 4. Chemically clean the melon petals to remove oxides from the alloy surface. Specifically, first use solvents such as acetone or alcohol to clean and remove surface oil stains from the melon petals, then use an acidic solution to etch and remove oxides. The solution temperature should be controlled at 45-60℃ and the etching time at 90-150 minutes (depending on the plate thickness). After etching, rinse with cold water, then neutralize with an alkaline solution at a temperature ≤80℃ for 1-3 minutes. After neutralization, rinse with cold water, then blow dry or air dry, and then perform hydrogen removal treatment at a temperature of 180-250℃ for 120 minutes.

[0047] 5. An automated laser welding system is used to longitudinally weld the melon segments into pre-formed cylinders corresponding to each segment. Specifically, the melon segments are assembled onto the longitudinal seam welding fixture, with the longitudinal seam edge of the segment aligned with the center of the weld spatula. The butt weld joints are tightly fitted, allowing for local gaps of ≤0.1mm and local misalignment, but the misalignment must be ≤10% of the base material thickness. After assembly, the automated laser welding system is used for welding. The welding process is completed by controlling the movement of an industrial robot. The overall longitudinal seam welding of the pre-formed cylinders is completed according to the above requirements. This invention uses a laser power of 800W, employs a welding method without welding wire, and uses high-purity argon gas (purity ≥99.99%) as the shielding gas.

[0048] Repair the welds and remove surface defects; conduct 100% X-ray inspection on the longitudinal welds of the preformed cylinder, and ensure that the internal quality of the welds meets the requirements of Class I welds in QJ20693-2018; grind the inner surface of the preformed cylinder to ensure that the back height of the longitudinal weld of the preformed cylinder is flush with the base material, and the edge of the weld should smoothly transition to the base material, and the base material should not be damaged.

[0049] 6. Assemble the ground preformed cylinder onto the corresponding ductile iron bulging mold and lock it. Then, place the preformed cylinder and the bulging mold together in a heating furnace and heat to 850±10℃. Hold at this temperature for 3-4 hours, then cool it in the furnace to below 200℃ and air-cool it to remove the part. 7. Machin the circumferential allowance of the extension section cylinder, requiring the two extension sections to be machined to fit the outer circumference.

[0050] After the cylinder is machined, a profile inspection template and a laser tracker are used to inspect the internal profile and the roundness of the mating end face.

[0051] 8. Assemble the nozzle extension section circumferential weld tooling base and central shaft onto the automatic circumferential weld equipment, requiring the perpendicularity of the central shaft to the base to be 0.1mm;

[0052] Assemble the inner expansion ring of the circumferential seam, which is welded to the front section of the extension section, onto the central axis of the welding fixture. The inner expansion ring is required to be perpendicular to the central axis to 0.1mm, and the inner expansion ring is required to be coaxial with the base to φ0.1mm.

[0053] With the inner support expansion ring in the contracted state, assemble the rear section of the extension cylinder to the welding fixture, adjust the position of the inner support expansion ring so that the welding edge of the rear section of the extension cylinder is placed in the center of the expansion ring weld spatter plate; assemble the front section inner support frame fixture onto the central axis, requiring the frame fixture to be perpendicular to the central axis to 0.1mm, and assemble the front section of the extension cylinder onto the welding fixture.

[0054] Apply pressure to open the inner support ring. Adjust the position of the expansion ring according to the assembly of the front and rear sections of the extension section to ensure a tight fit between the rear and front sections of the extension. Local gaps are allowed at the joint, with a gap of ≤0.1mm. Local misalignment is also allowed, but the misalignment height must not exceed 10% of the base material thickness, and the total length of the local misalignment must not exceed 15% of the total weld length. After assembly, assemble the external welding pressure fixture to the butt weld of the two sections and tighten it.

[0055] 9. Perform manual tack welding on the two assembled extension sections of the cylinder. Use DC as the welding power source, locate 8 points evenly distributed around the circumference, and ensure the weld length is 10-15mm. The welding sequence should be symmetrical. The width and reinforcement of the tack weld should not exceed 75% of the corresponding weld size. Use HGH3128 high-temperature alloy welding wire (compliant with GJB 2612-1996 standard) as the tack welding wire, and use high-purity argon gas (purity ≥99.99%) as the shielding gas. Use cerium-tungsten wire as the electrode and grind the tungsten end into a flat-headed cone with a diameter of approximately 1 / 3 of the tungsten electrode diameter.

[0056] The extended section of the cylinder after the tack weld is welded around the circumference using an automatic argon arc welding machine. The circumference weld is performed without welding wire.

[0057] 10. Repair the weld and remove surface defects; conduct 100% X-ray inspection on the circumferential weld of the nozzle extension section, and ensure that the internal quality of the weld meets the requirements of QJ1842-1990 Class I weld.

[0058] 11. Grind the inner surface of the nozzle extension section to ensure that the back of the circumferential weld is flush with the base material, and the edge of the weld should smoothly transition to the base material without damaging the base material.

[0059] 12. When assembling tooling and parts, use a laser tracker to check the position of the tooling and the dimensions and position of the parts after assembly. Welding can only begin when all dimensions are qualified. After welding, check the inner and outer surface contours while keeping the external pressure tooling, inner support frame and expansion ring in a taut state.

[0060] Testing revealed that the nozzle extension section manufactured using the above method met the design requirements for both its inner and outer surface profile tolerances.

[0061] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or basic characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description. Therefore, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention, and no reference numerals in the claims should be construed as limiting the scope of the claims.

[0062] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for manufacturing an extension section of the nozzle of a large liquid rocket second-stage engine, characterized in that: Includes the following steps: Step 1: Establish an extension section model. Divide the extension section model into n cylindrical sections (n ​​is an integer, ≥2) along the model axis. Scale down the cylindrical sections radially to obtain a pre-formed cylindrical section. The scaling ratio is cylindrical section: pre-formed cylindrical section = 1:(0.995~0.999). Divide the pre-formed cylindrical section into m segments (m is an integer, ≥2). Unfold the segments to obtain the unfolded surface. The number of cylindrical sections n and the number of segments m are determined based on the dimensions of the raw material plate. The number of cylindrical sections n ensures that the number of circumferential welds in the extension section is minimized. Step 2: Cut the raw material plate according to the unfolded surface of the melon petals obtained in Step 1 to obtain the melon petal forming substrate. Process the melon petal forming substrate according to the size and cross-sectional contour of the melon petals described in Step 1 to obtain the melon petals described in Step 1. Step 3: Prepare m melon segments according to Step 2, and longitudinally weld the m melon segments to obtain the preformed cylinder described in Step 1; Step 4: The preformed cylinder prepared in Step 3 is subjected to stress relaxation thermal straightening treatment to obtain the cylinder described in Step 1. The stress relaxation thermal straightening treatment is to first assemble the preformed cylinder prepared in Step 3 onto the expansion mold at room temperature and lock it, and then place the preformed cylinder together with the expansion mold in a heating furnace and heat it to the stress relief temperature of the raw material plate. After holding it at the stress relief temperature, it is cooled in the furnace to below 200°C and then air-cooled to remove the part. Step 5: Prepare n sections of cylinder according to the methods in steps 2 to 4, and weld the n sections of cylinder circumferentially to obtain the nozzle extension section.

2. The method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine according to claim 1, characterized in that: The method for processing the melon-shaped substrate in step 2 is room temperature stretch forming and waterjet cutting.

3. The method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine according to claim 1, characterized in that: In step 3, the longitudinal welding of m melon segments is performed using an automated laser welding system.

4. The method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine according to claim 1, characterized in that: In step 3, the melon petals are welded together after surface oxide removal treatment.

5. The method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine according to claim 4, characterized in that: The surface oxide removal method involves first removing oil stains from the surface of the melon petals, then using an acidic solution to corrode and remove the oxides. The temperature of the acidic solution is controlled at 45–60°C, and the corrosion time is 90–150 min. After corrosion, the surface is rinsed with cold water, and then neutralized with an alkaline solution. The temperature of the alkaline solution is ≤80°C, and the neutralization time is 1–3 min. After neutralization, the surface is rinsed with cold water, and then dried by blowing or air drying. Finally, hydrogen removal treatment is performed at a temperature of 180–250°C for 120 min.

6. The method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine according to claim 1, characterized in that: The coefficient of linear expansion of the raw material used to prepare the bulging mold is greater than the coefficient of linear expansion of the raw material plate used to prepare the preformed cylinder.

7. The method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine according to claim 1, characterized in that: In step 5, the circumferential welding of the n-section cylinder is carried out using a combination of manual positioning welding and an automatic argon arc welding system.

8. The method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine according to claim 7, characterized in that: The manual positioning welding method involves symmetrically welding eight positioning points, which are evenly distributed on the circumference, with a weld length of 10-15 mm.

9. A method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine according to claim 3 or 7, characterized in that: The automatic laser welding system or automatic argon arc welding system performs welding without welding wire, using high-purity argon gas with a purity of ≥99.99% as the protective gas, and the automatic argon arc welding system uses cerium-tungsten wire as the electrode.

10. The method for manufacturing a nozzle extension section of a large liquid rocket second-stage engine according to claim 1, characterized in that: In step 5, the n sections of the cylinder are machined circumferentially before welding to accommodate the circumference of each cylinder.

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