Additive Manufacturing Forming Method for the Head of a Liquid Rocket Engine with a Special-shaped Capillary Structure

Abstract

The present invention discloses a method for additive manufacturing and forming of a special-shaped capillary structure head of a liquid rocket engine, which includes establishing an integrated head three-dimensional model; determining the growth direction of the head according to the integrated head three-dimensional model; realizing the optimization of the integrated head three-dimensional model according to the relevant parameters of the capillary determined by the expected flow resistance and expected thermal resistance of the head to obtain an optimized three-dimensional model; determining the process support and process powder cleaning ports according to the growth direction of the head and the optimized three-dimensional model to obtain the process three-dimensional model of the head; determining the forming process of laser additive manufacturing; performing laser additive manufacturing and forming by using the process three-dimensional model of the head and the forming process; removing the metal powder inside the workpiece and removing the process support by heat treatment; and chemically polishing the workpiece to obtain the head. The present invention can realize the overall precision forming of the special-shaped capillary structure head.

Description

Technical Field

[0001] The present invention belongs to the technical field of additive manufacturing, and particularly relates to a high-precision additive manufacturing method for forming the head of a liquid rocket engine with a special-shaped capillary structure. Background Art

[0002] The head is the core component in the gas generator and thrust chamber of the attitude control engine to complete the diversion and distribution of the propellant and achieve the uniform distribution of the propellant in the decomposition chamber. It contains a large number of capillary structures with a diameter of 0.6 mm to 0.8 mm, a wall thickness of 0.1 mm to 0.2 mm, and a special-shaped and twisted space. At present, the attitude control head is manufactured by the traditional process of separate manufacturing + brazing. The main problems of this method are as follows: First, poor consistency: The bending of the capillary tubes depends on manual operation, resulting in poor dimensional consistency. After brazing, the capillary tubes need to be repaired, which affects the product assembly and the stability of the liquid flow pressure drop. Second, poor reliability. The head is brazed by more than 20 parts such as capillary tubes, flange plates, and injection plates. It is difficult to control the brazing quality, the weld strength is lower than the base material, and the welds are not inspectable and measurable. If the weld strength is insufficient during the working process, the capillary tubes may explode, posing a great risk of thermal explosion and affecting the reliability of the engine. Third, long processing cycle. The manufacturing involves multiple specialties and processes such as forging, heat treatment, surface treatment, machining, testing, component assembly, and brazing. The entire production cycle takes up to 2 months, which is not conducive to the reliable research and production of the head. As one of the latest and most advanced manufacturing technologies, additive manufacturing technology has the characteristics of CAD / CAM integration and high-efficiency and agile manufacturing, providing a new solution for the design / manufacture of the attitude control engine head with a special-shaped capillary structure. However, there are technical problems such as many influencing factors in forming and the immaturity of the process leading to a decline in product quality. Summary of the Invention

[0003] The purpose of the present invention is to overcome the above defects and provide an additive manufacturing method for forming the head of a liquid rocket engine with a special-shaped capillary structure, which solves the technical problems existing in the traditional combined process of separate manufacturing + brazing of the attitude control engine head, such as poor dimensional accuracy consistency of the capillary structure, multiple welds being non-inspectable and non-measurable, resulting in insufficient reliability, high risk of thermal explosion, and long processing cycle. The present invention can realize the integral precision forming of the head with a special-shaped capillary structure.

[0004] To achieve the above invention purpose, the present invention provides the following technical solutions:

[0005] An additive manufacturing method for forming the head of a liquid rocket engine with a special-shaped capillary structure, comprising:

[0006] S1 Establish an integrated three-dimensional model of the head;

[0007] S2 Determine the growth direction of the head according to the integrated three-dimensional model of the head;

[0008] S3 determines the relevant parameters of the capillary according to the expected flow resistance and expected thermal resistance of the head, realizes the optimization of the integrated head three-dimensional model, and obtains the optimized three-dimensional model;

[0009] S4 determines the process support and process powder cleaning ports according to the growth direction of the head and the optimized three-dimensional model, and obtains the process three-dimensional model of the head;

[0010] S5 determines the forming process of laser additive manufacturing;

[0011] S6 uses the process three-dimensional model of the head obtained in step S4 and the forming process obtained in step S5 to perform laser additive manufacturing forming;

[0012] S7 removes the metal powder inside the workpiece obtained in step S6 and removes the process support through heat treatment;

[0013] S8 chemically polishes the workpiece obtained in step S7 to obtain the head.

[0014] Further, in step S1, the integrated head three-dimensional model includes a flange, a jet plate, a heat insulation frame and a capillary. The flange and the jet plate are respectively located at both ends of the heat insulation frame. Several capillaries are integrated on the inner surface of the heat insulation frame. The two ends of the capillary are respectively connected to the flange and the jet plate;

[0015] In step S2, according to the integrated head three-dimensional model, based on the principle of minimum stress, least process support and removability, the growth direction of the head is determined as: the flange is connected to the substrate and placed vertically.

[0016] Further, in step S3, the method for realizing the optimization of the integrated head three-dimensional model and obtaining the optimized three-dimensional model according to the relevant parameters of the capillary determined by the expected flow resistance and expected thermal resistance of the head includes:

[0017] S3.1 establishes a capillary shape compensation formula for the forming angle of the capillary;

[0018] S3.2 establishes a head flow resistance formula for the capillary diameter and the surface roughness of the capillary;

[0019] Based on the head flow resistance formula, the capillary diameter and the surface roughness of the capillary are determined according to the expected flow resistance of the head;

[0020] S3.3 based on the capillary shape compensation formula, determines the capillary number, capillary distribution, capillary forming angle and capillary length according to the expected thermal resistance of the head;

[0021] S3.4 uses the determined capillary diameter, capillary surface roughness, capillary number, capillary distribution, capillary forming angle and capillary length to realize the optimization of the head three-dimensional model.

[0022] Further, in step S3.1, the method for establishing the capillary shape compensation formula for the capillary forming angle is as follows:

[0023] Design a series of capillaries with the forming angle varying at equal intervals within the range of 0° to 60° and the tube diameter varying at equal intervals within the range of 0.4 to 1 mm. Use a series of head three-dimensional models containing the series of capillaries for selective laser melting forming.

[0024] Statistically analyze the collapse information of the hanging parts of the capillaries at each forming angle and each tube diameter. After fitting, obtain the capillary shape compensation formula for the capillary forming angle.

[0025] The forming angle is the angle between the inner surface of the capillary and the horizontal plane.

[0026] The capillary shape compensation formula is:

[0027]

[0028] where θ is the capillary forming angle and d0 is the tube diameter compensation value of the hanging part of the capillary.

[0029] In step S3.2, the method for establishing the head flow resistance formula for the capillary tube diameter and the capillary surface roughness is as follows:

[0030] Design a series of capillaries with the tube diameter varying at equal intervals within the range of 0.4 to 1 mm. Use a series of head three-dimensional models containing the series of capillaries for selective laser melting forming to obtain a series of heads with different capillary tube diameters.

[0031] Perform chemical polishing on the heads with the same capillary tube diameter for different times until the chemical polishing of all heads with different capillary tube diameters is completed.

[0032] Use a three-dimensional digital microscope to measure the capillary surface roughness at different chemical polishing times.

[0033] Measure the liquid flow pressure drop and the liquid flow pressure drop uniformity of each head containing different tube diameters and different surface roughnesses. After fitting, obtain the head flow resistance formula for the capillary tube diameter and the capillary surface roughness:

[0034]

[0035] In the formula, P is the pressure drop, d is the capillary tube diameter, and R is the capillary surface roughness.

[0036] In step S3.3, the number of capillaries determined according to the desired flow resistance of the head is 30 to 100. The capillaries are distributed in multiple concentric rings. The same capillary includes a vertical section and an inclined section. The forming angles of the inclined sections of the capillaries distributed in the same concentric ring are the same. The forming angles of the inclined sections of the capillaries distributed in different concentric rings increase sequentially from outside to inside. The lengths of the capillaries distributed in different concentric rings decrease sequentially from outside to inside. The diameter at the inclined section of the capillary is optimized based on the capillary shape compensation formula.

[0037] Further, in step S5, it includes determining the process parameters for in-fill scanning of the capillary with the aim of improving the dimensional accuracy of the capillary;

[0038] The process parameters include: the spot size is 38 - 42 um, the layer thickness is 0.01 - 0.03 mm, the laser power is 90 - 120 W, the scanning speed is 600 - 800 mm / s, the line spacing is 0.09 - 0.10 mm, and the phase angle is 67°.

[0039] Further, in step S5, it also includes performing a border scan at the edge of the in-fill scanning trajectory of the vertical capillary with the aim of optimizing the surface quality of the vertical capillary. The specific method is as follows:

[0040] First, perform one - pass outer - ring scanning with a higher - energy laser, and then perform one - pass outer - ring scanning with a lower - energy laser;

[0041] The process parameters for the second - outer - ring scan are: laser power 50 w, scanning speed 600 mm / s;

[0042] The process parameters for the outer - ring scan are: laser power: 80 w, scanning speed: 400 mm / s;

[0043] The diameter of the second - outer - ring scan is larger than the diameter of the outer - ring scan.

[0044] Further, in step S5, it also includes determining the scanning parameters of the upper surface and the lower surface with the aim of optimizing the upper surface and the lower surface formed during the additive manufacturing process;

[0045] The upper surface is the surface that is extra in the upper layer relative to the next layer to be formed. The lower surface is the surface that is extra in the next layer to be formed relative to the upper layer;

[0046] The scanning parameters of the upper surface are: laser power 80 - 120 w, scanning speed 350 - 450 mm / s;

[0047] The scanning parameters of the lower surface are: laser power 120 - 160 w, scanning speed 2500 - 2700 mm / s.

[0048] Further, in step S4, the process support includes a solid support structure and a hollow columnar support structure;

[0049] The lower end of the solid support structure is connected to the upper surface of the flange, the upper end of the solid support structure is connected to the lower end of the hollow columnar support structure, and the upper end of the hollow columnar support structure is connected to the lower surface of the injection plate; the solid support structure is at an angle of 45° with the vertical direction, the hollow columnar support structure is arranged along the vertical direction, and the length of the hollow columnar support structure is 10-20 mm;

[0050] The upper end of the solid support structure is provided with a diamond-shaped hole.

[0051] Further, in step S4, the process powder cleaning port is located at the lowest point of the closed cavity formed by the flange and the substrate, and the lower end of the capillary is communicated with the process powder cleaning port;

[0052] In step S7, the method for removing the metal powder inside the workpiece obtained in step S6 is: blowing high-pressure gas of 1.2-1.5 Mpa from the upper end of the capillary to blow out the metal powder from the process powder cleaning port;

[0053] In step S7, the method for removing the process support by heat treatment is: at 900-1000 °C, after heat preservation for 3-4 h, the solid support is removed by wire cutting;

[0054] The wire cutting is high-speed reciprocating wire electrical discharge machining, the pulse width is 10-15 μs, the pulse interval is 100-120 μs, and the waveform is a rectangular pulse.

[0055] Further, the above-mentioned additive manufacturing forming method for the special-shaped capillary structure head of a liquid rocket engine further includes:

[0056] S9 Conduct a liquid flow test on the head to determine whether the flow resistance of the head meets the expected flow resistance;

[0057] Conduct a micro-focus CT foreign object detection on the head to determine whether there is any residual metal powder inside the head;

[0058] The parameters of the micro-focus CT detection are: voltage 240-290 KV, current 280-330 uA, detector sensitivity 2-4, exposure time 500-1000 ms, resolution voxel size 70 um, and the ray direction is perpendicular to the vertical section of the capillary.

[0059] The present invention has at least one of the following beneficial effects compared with the prior art:

[0060] (1) Based on additive manufacturing technology, the present invention realizes an innovative change in the design of the attitude control head with a special-shaped capillary structure, integrating the traditional externally placed capillary of the head into the heat insulation frame and optimizing it to an internally placed one, solving the problems of poor consistency and weak rigidity in the manual bending of traditional externally placed capillaries with a high length-diameter ratio, being affected by vibration and thermal expansion during the operation of the engine, and having a high risk of thermal explosion;

[0061] (2) Through the optimization of dimensional accuracy based on the process of small spot and small layer thickness, the optimization of shape accuracy by compensating the cross-sectional shape of the inclined capillary, and the optimization of surface quality by controlling the scanning strategy and process parameters of the vertical surface and the upper / lower surfaces, the present invention realizes the high-precision, high-surface-quality, and high-consistency manufacturing of the head with a special-shaped capillary structure, solving the problems of poor consistency and low qualified rate of the head caused by manual bending, brazing deformation, and manual repair;

[0062] (3) By using high-precision micro-focus CT to detect foreign matters in the capillary, the present invention eliminates the risk of foreign matters in the head and ensures the reliability of the product;

[0063] (4) The present invention designs a lightweight support structure, which can be easily removed on the premise of having a stable support effect;

[0064] (5) Through the iterative optimization design technical route of the capillary diameter - surface quality - liquid flow pressure drop of the head, the present invention obtains a method for regulating the liquid flow resistance of the additive manufacturing head, that is, by adjusting the capillary diameter and surface quality, the flow resistance of the head is regulated; through the iterative optimization design technical route of the capillary number / distribution / length - thermal resistance, the present invention obtains a method for regulating the thermal resistance of the additive manufacturing head, that is, by adjusting the capillary diameter size, number, and distribution, the thermal resistance of the head is regulated;

[0065] (6) Based on the methods for regulating the flow group and thermal resistance of the additive manufacturing head, the present invention obtains the capillary diameter size, number, length, and distribution of the capillary structure of the additive manufacturing head that meet the design requirements, realizing the integrated design and manufacturing of the structure and function of the special-shaped capillary head, and integrating more than 30 parts in the traditional "machining + bending + chemical milling + brazing + fusion welding" processing technology, solving the problems of non-inspectability, non-measurability of multiple welds, insufficient reliability, long processing cycle, and many processes in the traditional processing technology;

[0066] (7) The present invention comprehensively considers various influencing factors, gives multiple relevant formulas that can realize thermal resistance regulation and flow resistance regulation, has universality, can effectively improve the product design efficiency, and optimize the thermal resistance and flow resistance performance of the product; a shape compensation method for the inclined section of the capillary is proposed, which can effectively avoid the occurrence of collapse and improve the product quality;

[0067] (8) Through the design of the circulating flow chemical polishing tooling and the control of the polishing process parameters of the present invention, the uniform and efficient polishing of the additive manufacturing capillary tube is realized. On the one hand, the requirement of the liquid flow pressure drop at the head is ensured, and on the other hand, the powder adhering to the capillary tube at the head is removed, ensuring the reliability of the product. Brief Description of the Drawings

[0068] Figure 1 It is a schematic structural diagram of the integral design of the head of the present invention;

[0069] Figure 2 It is a schematic diagram of the formation of the surface roughness of the capillary tube structure;

[0070] Figure 3 It is a schematic diagram of the scanning strategy of the outer ring and the second outer ring of the present invention;

[0071] Figure 4 It is a schematic diagram of the upper and lower surfaces of the present invention;

[0072] Figure 5 It is a schematic diagram of the optimized three-dimensional model of the capillary structure head of the present invention;

[0073] Figure 6 It is a schematic diagram of the process support structure of the present invention;

[0074] Figure 7 It is the shape of the inclined part of the capillary tube of the present invention. Detailed Description of the Invention

[0075] The present invention will be described in detail below, and the features and advantages of the present invention will become clearer and more definite with these descriptions.

[0076] The special term "exemplary" here means "serving as an example, an embodiment or an illustration". Any embodiment described as "exemplary" here does not have to be construed as superior to or better than other embodiments. Although various aspects of the embodiments are shown in the drawings, the drawings do not have to be drawn to scale unless otherwise specified.

[0077] The additive manufacturing forming method for the special-shaped capillary structure head of the liquid rocket engine of the present invention includes the following steps:

[0078] (1) Establish an integral head three-dimensional model based on additive manufacturing.

[0079] (2) According to the stress distribution and support configuration in the forming process of the integral model obtained in step (1), determine the growth direction of the head based on the principle of minimum stress, minimum support and removability.

[0080] (3) According to the growth direction in step (2), extract the typical structure in the model, that is, the characteristic capillary tube, optimize the accuracy and surface quality of the capillary structure, and obtain the process parameters and scanning paths of the capillary tube with high precision and high surface quality.

[0081] (4) Iterative optimization of process and design to obtain an optimized three-dimensional model.

[0082] (5) According to the three-dimensional model determined in step (4) and the growth direction determined in step (2), the process support and the process powder cleaning port are designed to obtain the process three-dimensional model of the head.

[0083] (6) Using the process parameters and scanning speed obtained in step (3), the process three-dimensional model obtained in step (5) is segmented, the laser scanning path parameters of each slice layer are obtained, and laser additive manufacturing is performed.

[0084] (7) removing the metal powder in the head capillary structure obtained in step (6) and removing the process support after heat treatment.

[0085] (8) The head obtained in step (7) is subjected to chemical polishing and then subjected to a liquid flow test.

[0086] (9) Perform microfocus CT redundant object detection on the capillary structure head obtained in step (8).

[0087] In the step (1), the special-shaped capillary structure characteristic component takes the head of the attitude control engine as an example, including a flange, an injection plate, an insulation frame and a capillary. In the head of the traditional capillary structure, the capillary with a high aspect ratio is exposed and independently distributed between the injection plate, the flange and the insulation frame, which has weak rigidity and is affected by vibration and thermal expansion during engine operation, and has a high risk of thermal explosion. Based on the flexibility of the additive manufacturing process design, the capillary structure is integrated into the insulation frame to increase the capillary rigidity and reliability, and realize the integrated design of additive manufacturing. Figure 1 .

[0088] In the step (2), the growth direction of the head is determined based on the stress distribution and support configuration during the forming process, with the principle of minimum stress, minimum support and removability: the large end flange is connected to the base plate and placed vertically.

[0089] In the step (3), the typical structural feature capillaries in the model are extracted to optimize the capillary structure accuracy and surface quality.

[0090] ① Capillary precision optimization

[0091] The precision optimization includes two aspects: one is the dimensional precision optimization, and the other is the shape precision optimization. For the dimensional precision optimization, the dimensional precision of the component is mainly affected by the width of the melt channel and the slice layer thickness. The width of the melt channel is determined by the size of the laser spot. Currently, the conventional spot diameter is 80 - 100um. The present invention uses a 40um small spot and a 0.02mm small layer thickness process parameter to form the capillary. By using a small spot, the width of the melt channel is reduced, and by using a small layer thickness, the slice step effect is reduced, thereby achieving high-precision forming of the capillary. The process parameters under the 40um small spot and 0.02mm small layer thickness are: laser power 90 - 120W, scanning speed 600 - 800mm / s, line spacing 0.09 - 0.10mm, and phase angle 67°.

[0092] For the shape precision optimization, it mainly includes two steps: First step, design a series of capillary structures with forming angles of 0°, 20°, 30°, 40°, 50°, tube diameters of 0.4mm - 1mm, and tube diameters increasing by 0.1mm, and perform selective laser melting forming. The forming angle refers to the angle between the inner surface of the capillary and the horizontal plane. Second step, count the collapse distance and collapse shape of the overhanging parts of the capillaries with different diameters at each forming angle, and conduct statistical analysis. Fit to obtain the shape compensation formula under different tube diameters and different forming angles:

[0093]

[0094] where θ is the capillary forming angle, and d0 is the tube diameter compensation value of the overhanging part of the capillary. The overhanging part is the upper semi-circle of the cross-section of the inclined section of the capillary.

[0095] ② Surface quality optimization

[0096] For the surface quality optimization, it includes two aspects: one is the vertical surface quality optimization, and the other is the upper and lower surface optimization. The vertical surface refers to the surface with a forming angle of 90°. To obtain a smooth surface, it is necessary to eliminate the serrated contour caused by the melt channel and reduce the powder sticking phenomenon at the same time. The present invention performs border scanning on the edge of the inner filling scanning trajectory ( Figure 3 the actual contour in) to improve the vertical surface quality. The border scanning includes the outer circle ( Figure 3 the outer circle in) and the secondary outer circle ( Figure 3 the additional outer circle in), that is, first use a laser with higher energy to perform one pass of secondary outer circle scanning to eliminate the serrated contour error, and then use a laser with lower energy to perform one pass of outer circle scanning to reduce the powder adhesion on the surface. The additional outer circle coincides with the contour of the slice to eliminate the error caused by the slice. 30% - 50% of the outer circle overlaps with the additional outer circle ( Figure 3 the overlapping area in) as Figure 2, so through the design of the sub-outer and outer paths and the setting of their energy densities, both the serrated error can be eliminated, and the phenomenon of surface powder adhesion can be reduced, improving the quality of the vertical surface of the capillary structure. The process parameters of the sub-outer ring: laser power: 50w, scanning speed: 600mm / s. The process parameters of the outer ring: laser power: 80w, scanning speed: 400mm / s. For a vertical hole with a forming angle of 90°, to eliminate Figure 2 this serrated profile caused by the melt channel, reduce the powder adhesion phenomenon, and obtain a smooth inner hole surface, border scanning can be performed on the edge of the inner filling scan trajectory.

[0097] The upper surface is the surface that the upper layer has more than the next layer to be formed, and the lower surface is the surface that the next layer to be formed has more than the upper layer. Its schematic diagram is as Figure 4 shown. For the upper surface, its growth support point is the formed entity, and there will be no molten pool collapse or slag hanging phenomenon. By setting a higher energy density, the surface is smoothed; for the lower surface, its growth support point is the loose powder, and there will be molten pool collapse and slag hanging phenomena during the forming process. By setting a lower energy density, the problems of poor surface quality caused by molten pool collapse, slag hanging and surface powder adhesion are weakened. The parameters of the upper surface are: upper surface laser power 100w, scanning speed 400mm / s, lower surface laser power 140w, scanning speed 2600mm / s.

[0098] In the step (4), based on the additive manufacturing flow resistance and heat resistance regulation method, that is, the capillary tube diameter - surface quality - liquid flow pressure drop iterative optimization design and the number / distribution / length of the head capillary tubes - heat resistance iterative optimization design heat, an integrated head structure and function design model is optimized, as Figure 5 shown.

[0099] On the basis of the overall head design, this step iteratively designs the capillary tube diameter size, quantity, distribution, etc. of the capillary structure to form a method for regulating the liquid flow resistance and heat resistance of the head. Finally, based on the flow resistance and heat resistance regulation method, an integrated head structure and function design model is optimized. The technical route for regulating the head flow resistance is the iterative optimization design of the capillary tube diameter - surface quality - liquid flow pressure drop of the head, and the technical route for regulating the head heat resistance is the iterative optimization design of the number / distribution / length of the capillary tubes - heat resistance.

[0100] (4.1) Iterative optimization of capillary tube diameter - surface quality - liquid flow pressure drop of the head

[0101] The iterative optimization method for the head capillary tube diameter - surface quality - liquid flow pressure drop mainly includes three steps: The first step is to design a series of capillary structure heads with tube diameters ranging from 0.4 mm to 1 mm and increasing by 0.1 mm; The second step is to perform chemical polishing on the heads with different tube diameters for different times (0.5 min, 1 min, 2 min, 3 min), and use a three-dimensional digital microscope to measure the surface roughness of the capillary tubes under different chemical polishing times; The third step is to measure the liquid flow pressure drop of the heads with different tube diameters and different surface roughnesses and the pressure drop uniformity in different regions, and summarize and analyze the data, and preliminarily fit to determine the corresponding relationship between the head capillary tube diameter - surface roughness - liquid flow pressure drop. The fourth step: Design a series of capillary structure heads with tube diameters ranging from 0.4 mm to 1 mm and increasing by 0.05 mm for chemical polishing with different process parameters (1 mm, 1.5 min, 2.5 min, 2 min, 3 min), and conduct liquid flow pressure drop and uniformity tests, correct and verify the corresponding relationship formula of the head capillary tube diameter - surface roughness - liquid flow pressure drop fitted in the third step, and finally determine the liquid flow pressure drop calculation formula of the head:

[0102]

[0103] In the formula, P is the pressure drop, d is the tube diameter, and R is the roughness.

[0104] Based on the above liquid flow pressure drop calculation formula of the head, according to the required design pressure drop of the head, calculate the capillary tube diameter and surface roughness, and obtain the capillary tube size and surface state, where the tube diameter is 0.4 mm and the chemical polishing time is 3 min.

[0105] For the chemical polishing of the head capillary tube, affected by the capillary structure and the solution flow resistance, the solution exchange effect in the capillary tube is weak. After polishing for a certain time, the concentration of the chemical solution is low, and there is a concentration difference in each part, resulting in incomplete or uneven chemical milling and polishing. To ensure uniform and smooth polishing of the capillary tube, design a solution circulation flow chemical milling and finishing tooling - a drainage plate-shaped tooling (a hole with a diameter matching the upper end diameter of the head is opened in the central area of the tooling, and threaded holes with a diameter of 5 mm are opened at the four corners respectively. The tooling is hermetically connected to the upper end of the head through bolts, and the pump and the head are connected through a pipe), and use an electric pump to uniformly fill the polishing solution in the capillary tube with a certain pressure and speed to ensure the uniformity of chemical solution exchange, where the pressure of the pump is 0.7 Mpa and the solution flow speed is 50 - 60 mm / s. The chemical polishing solution is a mixed solution of hydrochloric acid, nitric acid, hydrofluoric acid, etc., with a ratio of 4:3:1. The polishing removal amount for the polishing times of 0.5 min, 1 min, 2 min, and 3 min ≤ 0.05 mm, which does not affect the strength of the head.

[0106] (4.2) Iterative optimization design of the number / distribution / length of capillary tubes - thermal resistance

[0107] The head of the additive manufacturing capillary structure integrates the independently distributed capillaries in the traditional design into the heat insulation frame, resulting in a smaller thermal resistance of the head. Through the design of large flow rate, high integration, and high aspect ratio, cooling is strengthened, the heat conduction area of heat back-invasion is reduced, and the heat conduction distance is increased, thereby improving the thermal resistance of the additive manufacturing capillary head. After multiple rounds of iterative design of capillary quantity / distribution / length-thermal resistance, the quantity, distribution, and length information of the head capillaries that meet the thermal resistance requirements are obtained, that is, there are a total of 39 capillaries, which are concentrated in the heat insulation frame and divided into four categories: A, B, C, and D. The four types of special-shaped capillaries A, B, C, and D are distributed in a ring shape. The type A special-shaped capillaries are distributed in the outermost circle, with a total of 18 capillaries, an unfolded length of 160 mm, and a forming angle of 20° at the inclined part of the capillary. The capillaries of types B, C, and D are distributed towards the center in turn. There are 12 capillaries of type B, an unfolded length of 150 mm, and a forming angle of 23° at the inclined part of the capillary. There are 6 capillaries of type C, an unfolded length of 140 mm, and a forming angle of 26° at the inclined part of the capillary. There are 3 capillaries of type D, an unfolded length of 120 mm, and a forming angle of 28° at the inclined part of the capillary. According to the capillary shape compensation parabola formula in step (3), the shapes of the inclined parts of the capillaries of four diameters A, B, C, and D are calculated, as shown in Figure 7 .

[0108] Through the iterative optimization of capillary diameter-surface quality-liquid flow pressure drop and the iterative optimization design of capillary quantity / distribution / length-thermal resistance, the final optimized three-dimensional model of the capillary structure head is determined, as shown in Figure 5 .

[0109] In step (5), under the growth direction determined in step (2), the bottom surface of the upper head will be in a hanging state, and an additive manufacturing auxiliary process support needs to be designed. From the three perspectives of ensuring smooth forming, temperature field heat transfer and stress control, and easy removal, a combined support of solid + hollow columnar is designed, which not only retains the stability of the forming process and deformation control of the solid support, but also takes into account the advantage of easy removal of the hollow columnar support. For the solid + hollow columnar combined support, the solid support structure is a 45° self-supporting forming structure and forms a 45° angle with the axis, so that it falls on the plane of the injection disk and avoids falling on the arc surface with capillary characteristics. At the same time, diamond-shaped holes are designed on the support to achieve lightweight design on the basis of ensuring strength, as shown in Figure 6 . The outer diameter of the hollow columnar support is 1.23 mm, and the wall thickness is 0.2 mm. It is added to the hanging surface of the injection disk and falls on the self-supporting solid support, which is convenient for support removal.

[0110] In step (7), after the selective laser melting forming is completed, the powder material is filled in the capillary. To ensure the complete removal of the powder in the capillary, 18 circumferentially distributed process powder cleaning ports are arranged in the closed cavity formed by the bottom flange of the head and the substrate, making it at the lowest point of the closed inner cavity, and the powder cleaning ports are connected at the bottom of the flange to increase the openness of the structure, ensuring the upper and lower ports of each capillary are connected, so that the powder in the capillary can flow out smoothly under the blowing of compressed air. The shape of the process powder cleaning port is semi-circular, with a radius of 2 mm.

[0111] In step (7), high-pressure gas with a pressure of 1.2 - 1.5 Mpa is used to blow from the capillary inlet on the injection disk to completely remove the powder from the process powder cleaning port; the heat treatment system is homogenization heat treatment, with a temperature of 900 - 1000 °C, holding for 3 h, and cooling with gas injection to eliminate the thermal stress during the forming process of the head, and adjust the structure and properties to meet the design requirements; the solid support is removed by wire cutting, and the columnar support is removed by grinding. The wire cutting is high-speed reciprocating wire electrical discharge machining, with the pulse width set at 10 - 15 μs, the pulse interval at 100 - 120 μs, and the waveform being a rectangular pulse.

[0112] In step (9), micro-focus CT is used to detect foreign matters in the capillary structure head. The parameters of the micro-focus CT detection are: voltage: 240 - 290 KV, current: 280 - 330 uA, detector sensitivity: 2 - 4, exposure time: 500 - 1000 ms, resolution voxel size: 70 μm, and the ray direction is perpendicular to the capillary in the vertical section.

[0113] The present invention has been described in detail above in combination with specific embodiments and exemplary examples, but these descriptions should not be construed as limitations on the present invention. Those skilled in the art understand that without departing from the spirit and scope of the present invention, various equivalent substitutions, modifications, or improvements can be made to the technical solutions and their implementation manners of the present invention, and all of these fall within the scope of the present invention. The protection scope of the present invention is subject to the appended claims.

[0114] The content not described in detail in the specification of the present invention belongs to the well-known technology of those skilled in the art.

Claims

1. A method for additive manufacturing and forming the head of a special-shaped capillary structure of a liquid rocket engine, characterized in that, Including: S1 Establish an integrated head three-dimensional model; S2 Determine the growth direction of the head according to the integrated head three-dimensional model; S3 Optimize the integrated head three-dimensional model according to the relevant parameters of the capillary determined by the desired flow resistance and desired thermal resistance of the head to obtain an optimized three-dimensional model; S4 Determine the process support and process powder cleaning ports according to the growth direction of the head and the optimized three-dimensional model to obtain the process three-dimensional model of the head; S5 Determine the forming process of laser additive manufacturing; S6 Perform laser additive manufacturing forming using the process three-dimensional model of the head obtained in step S4 and the forming process obtained in step S5; S7 Remove the metal powder inside the workpiece obtained in step S6 and remove the process support through heat treatment; S8 Chemically polish the workpiece obtained in step S7 to obtain the head; In step S3, the method for optimizing the integrated head three-dimensional model according to the relevant parameters of the capillary determined by the desired flow resistance and desired thermal resistance of the head to obtain an optimized three-dimensional model includes: S3.1 Establish a capillary shape compensation formula for the capillary forming angle; S3.2 Establish a head flow resistance formula for the capillary diameter and capillary surface roughness; Based on the head flow resistance formula, determine the capillary diameter and capillary surface roughness according to the desired flow resistance of the head; S3.3 Based on the capillary shape compensation formula, determine the capillary number, capillary distribution, capillary forming angle and capillary length according to the desired thermal resistance of the head; S3.4 Use the determined capillary diameter, capillary surface roughness, capillary number, capillary distribution, capillary forming angle and capillary length to optimize the head three-dimensional model; In step S3.1, the method for establishing a capillary shape compensation formula for the capillary forming angle is: Design a series of capillaries with the forming angle changing at equal intervals in the range of 0° to 60° and the diameter changing at equal intervals in the range of 0.4 to 1 mm, and perform selective laser melting forming using a series of head three-dimensional models including the series of capillaries; Statistically analyze the collapse information of the hanging parts of the capillaries at each forming angle and each diameter, and obtain the capillary shape compensation formula for the capillary forming angle after fitting; The forming angle is the angle between the inner surface of the capillary and the horizontal plane; The capillary shape compensation formula is: ； Among them, θ is the forming angle of the capillary tube, is the diameter compensation value of the capillary tube.

2. The method for additive manufacturing and forming the head of a special-shaped capillary structure of a liquid rocket engine according to claim 1, characterized in that, In step S1, the integrated head three-dimensional model includes a flange, a jet plate, a heat insulation frame and capillaries. The flange and the jet plate are respectively located at both ends of the heat insulation frame. A plurality of capillaries are integrated on the inner surface of the heat insulation frame, and both ends of the capillaries are respectively connected to the flange and the jet plate; In step S2, according to the integrated head three-dimensional model, following the principle of minimum stress, least process support and removability, the growth direction of the head is determined as: the flange is connected to the substrate and placed vertically.

3. The method for additive manufacturing and forming the head of a special-shaped capillary structure of a liquid rocket engine according to claim 1, characterized in that, In step S3.2, the method for establishing a head flow resistance formula for the capillary diameter and capillary surface roughness is: Design a series of capillaries with the diameter changing at equal intervals in the range of 0.4 to 1 mm, and perform selective laser melting forming using a series of head three-dimensional models including the series of capillaries to obtain a series of heads with different capillary diameters; Chemically polish the heads with the same capillary diameter for different times until the chemical polishing of all heads with different capillary diameters is completed; Use a three-dimensional digital microscope to measure the surface roughness of the capillary at different chemical polishing times; Measure the liquid flow pressure drop and the uniformity of the liquid flow pressure drop of each head with different diameters and different surface roughnesses. After fitting, obtain the head flow resistance formula for the capillary diameter and the capillary surface roughness: In the formula, P is the pressure drop, d is the capillary tube diameter, R is the surface roughness of the capillary tube; In step S3.3, the number of capillaries determined according to the desired flow resistance of the head is 30 to 100. The capillary number distribution is multiple concentric rings. The same capillary includes a vertical section and an inclined section. The forming angles of the inclined sections of the capillaries distributed in the same concentric ring are the same. The forming angles of the inclined sections of the capillaries distributed in different concentric rings increase in sequence from outside to inside. The lengths of the capillaries distributed in different concentric rings decrease in sequence from outside to inside; the diameter at the inclined section of the capillary is optimized based on the capillary shape compensation formula.

4. The method for additive manufacturing and forming the head of a special-shaped capillary structure of a liquid rocket engine according to claim 1, characterized in that,In step S5, it includes determining the process parameters of the in-capillary filling scan for the purpose of improving the dimensional accuracy of the capillary; The process parameters include: the light spot is 38 - 42um, the layer thickness is 0.01 - 0.03mm, the laser power is 90 - 120W, the scanning speed is 600 - 800mm / s, the line spacing is 0.09 - 0.10mm, and the phase angle is 67°.

5. The additive manufacturing method for the head of a special-shaped capillary structure of a liquid rocket engine according to claim 4, wherein, In step S5, it also includes performing a border scan at the edge of the in-capillary filling scan trajectory of the vertical capillary for the purpose of optimizing the surface quality of the vertical capillary. The specific method is: First, perform one pass of outer ring scanning with a higher energy laser, and then perform one pass of outer ring scanning with a lower energy laser; The process parameters of the second outer ring scanning are: laser power 50w, scanning speed 600mm / s; The process parameters of the outer ring scanning are: laser power: 80w, scanning speed: 400mm / s; The diameter of the second outer ring scanning is larger than the diameter of the outer ring scanning.

6. The additive manufacturing method for the head of a special-shaped capillary structure of a liquid rocket engine according to claim 5, wherein, In step S5, it also includes determining the scanning parameters of the upper surface and the lower surface for the purpose of optimizing the upper surface and the lower surface formed during the additive manufacturing process; The upper surface is the surface that is extra in the upper layer relative to the next layer to be formed. The lower surface is the surface that is extra in the next layer to be formed relative to the upper layer; The scanning parameters of the upper surface are: laser power 80 - 120w, scanning speed 350 - 450mm / s; The scanning parameters of the lower surface are: laser power 120 - 160w, scanning speed 2500 - 2700mm / s.

7. The additive manufacturing method for the head of a special-shaped capillary structure of a liquid rocket engine according to claim 2, wherein, In step S4, the process support includes a solid support structure and a hollow columnar support structure; The lower end of the solid support structure is connected to the upper surface of the flange. The upper end of the solid support structure is connected to the lower end of the hollow columnar support structure. The upper end of the hollow columnar support structure is connected to the lower surface of the injection plate; the solid support structure is at an angle of 45° with the vertical direction. The hollow columnar support structure is arranged along the vertical direction, and the length of the hollow columnar support structure is 10 - 20mm; The upper end of the solid support structure is provided with a diamond-shaped hole.

8. The additive manufacturing method for the head of a special-shaped capillary structure of a liquid rocket engine according to claim 2, wherein, In step S4, the process powder cleaning port is located at the lowest point of the closed cavity formed by the flange and the substrate. The lower end of the capillary is communicated with the process powder cleaning port; In step S7, the method for removing the metal powder inside the workpiece obtained in step S6 is as follows: blow high-pressure gas of 1.2 - 1.5 Mpa into the upper end of the capillary tube to blow out the metal powder from the process powder cleaning port; In step S7, the method for removing the process support by heat treatment is as follows: at 900 - 1000 °C, after holding for 3 - 4 h, remove the solid support by wire cutting; The wire cutting is high-speed reciprocating wire electrical discharge machining, with a pulse width of 10 - 15 μs, a pulse interval of 100 - 120 μs, and a waveform of rectangular pulse.

9. The additive manufacturing method for the head of a special-shaped capillary structure of a liquid rocket engine according to claim 1, wherein, It also includes: S9 Conduct a liquid flow test on the head to determine whether the flow resistance of the head meets the expected flow resistance; Conduct a micro-focus CT foreign object detection on the head to determine whether there is residual metal powder inside the head; The parameters of the micro-focus CT detection are: voltage 240 - 290 KV, current 280 - 330 uA, detector sensitivity 2 - 4, exposure time 500 - 1000 ms, resolution voxel size 70 um, and the ray direction is perpendicular to the vertical section of the capillary tube.

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