Variable thickness titanium alloy spinner precise forming method

Abstract

The application discloses a precision forming method for a variable-thickness titanium alloy spinner, and comprises the following steps: according to a three-dimensional developed flat numerical control milling cutting processing, a developed blank is obtained, double-side milling cutting is performed to form an outer contour of a part and an initial gradient variable-thickness feature, so that a part developed flat is obtained; a local surface forming die is designed to be used for preforming a key area in the titanium alloy spinner; and a full-surface correcting die is designed to be used for full-surface correcting the preformed part. Through the method, the combined step-by-step hot forming process of the local surface pre-clamping forming and the full-surface correcting is used to form the developed gradient-thickness blank in a high-temperature state, and the formed part has no residual stress and will not produce delayed deformation. Meanwhile, the positioning ear piece is used to realize the positioning reference transmission in the clamping state and the positioning pin cooperation, so that the final manufacturing precision is ensured.

Description

Technical Field

[0001] This invention belongs to the field of metal material plastic processing technology, specifically relating to a method for precise forming of a variable thickness titanium alloy fairing. Background Technology

[0002] Fairing-type components are generally located on the outer surface of an aircraft and are directly exposed to the air during flight, forming the aerodynamic shape of the aircraft. The assembly structure of this type of component is mainly used to cover the internal frame structure, forming a continuous, integral, thin-walled hollow shell structure along the overall aerodynamic profile of the aircraft.

[0003] Titanium alloys are widely used in the manufacture of aerospace and other aircraft structures due to their excellent corrosion resistance and high-temperature resistance, especially in the vicinity of engine nozzles where they need to operate in high-temperature environments for extended periods. Furthermore, to further reduce the overall weight of the aircraft structure, the component design incorporates numerous integrated features, significantly reducing the number of parts and connectors through specialized component configurations.

[0004] The titanium alloy fairing has a polygonal shape, and its surface is designed with varying thickness, which can easily cause problems such as material sway and stress deformation during processing, making it difficult to achieve high-precision machining and manufacturing of fairing parts. Summary of the Invention

[0005] To address the problems existing in the prior art, the purpose of this invention is to provide a method for precise forming of a variable thickness titanium alloy fairing. This method utilizes CNC machining to pre-obtain variable thickness feature blanks, and then combines precise positioning and transfer methods to hot-form and pre-bend the part's profile. Finally, hot-forming ensures high precision of the overall features of the part.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A method for precisely forming a variable-thickness titanium alloy fairing, wherein the spatial outline of the titanium alloy fairing is a zigzag structure with a central protrusion, and the two ends of the protrusion are rounded. The titanium alloy fairing includes a main body, left edge strip structures and right edge strip structures on both sides of the main body. The left edge strip structures and right edge strip structures overlap with the sides of the main body, forming a gradient variable thickness feature. The method includes: Based on the three-dimensional solid digital model of the titanium alloy fairing, design the three-dimensional unfolded flat plate digital model of the part; The unfolded blank is processed by CNC milling of a three-dimensional unfolded flat plate, and double-sided milling is used to form the outer contour of the part and the initial gradient thickness characteristics, thereby obtaining the unfolded flat plate of the part; A partial surface forming mold is designed for pre-forming key areas in a titanium alloy fairing. The key areas include protrusions and rounded corners at both ends of the protrusions. The partial surface forming mold is provided with a first punch and a first die that cooperate with each other, and an ejector that can move up and down. The upper surface of the ejector is a bearing surface that matches the outer surface of the protrusions of the titanium alloy fairing. First positioning pins that cooperate with positioning lugs on the unfolded plate of the part are provided on both sides of the bearing surface. The partial surface forming mold is installed in the thermoforming machine and heated. When the temperature reaches the forming temperature of the titanium alloy part, the unfolded flat part is placed in the partial surface forming mold and fixed with the first pin. After preheating, the mold is closed and pressure is applied, causing the two sides of the unfolded flat part to flip up and partially suspend outside the partial surface forming mold. At this time, pressure is held. After the pressure is held, the preformed part is taken out and cooled. Design a full-surface correction mold for full-surface correction of preformed parts; the full-surface correction mold is equipped with a second punch and a second die that cooperate with each other. After the mold is closed, the second die and the second punch can completely cover the preformed parts; the second punch has second locating pins on both sides of the middle part. The full-surface shaping mold is installed in the thermoforming machine and the temperature is raised. After the forming temperature is reached, the pre-formed part is installed and fixed with the second pin. The pre-formed part is then preheated. After preheating, the mold is closed and pressure is applied and held. After holding the pressure, the mold is opened and the formed part is taken out and transferred to other areas for cooling and post-processing to obtain the processed titanium alloy fairing.

[0007] Furthermore, the outer surface a of the main body of the titanium alloy fairing has a uniform thickness C with the inner surface d of the main body; the two sides of the inner surface d of the main body extend outward to form a left edge strip structure and a right edge strip structure, respectively. The outer surface b and inner surface e of the left edge strip structure have a uniform thickness A, and the outer surface c and inner surface f of the right edge strip structure have a uniform thickness E; the left edge strip structure and the right edge strip structure form a left overlapping area with a thickness B and a right overlapping area with a thickness D at the junction with the left side and the right side of the main body, thereby making the titanium alloy fairing exhibit a gradient thickness characteristic; the step difference between the outer surface b of the left edge strip structure, the outer surface c of the right edge strip structure and the outer surface a of the main body is the thickness of the front and rear end mating skin when the titanium alloy fairing is assembled.

[0008] Furthermore, positioning lugs g and h are designed on both sides of the center of the three-dimensional unfolded flat plate digital model; the width of positioning lugs g and h is consistent with the protrusion of the titanium alloy fairing, and pin holes are evenly provided on positioning lugs g and h respectively.

[0009] Furthermore, based on the three-dimensional solid model of the titanium alloy fairing, a three-dimensional unfolded flat plate model of the part is designed, including: In CATIA software, a 3D solid model of the titanium alloy fairing is obtained. Then, the outer surface 'a' of the main body is used as the reference plane. The reference plane is extended around to cover the outer contour of the titanium alloy fairing. The 3D solid model is then simplified and modified by removing all chamfers and rounded edges. The outer contour edges of the left and right edge strip structures, the gradient step edges, the positioning lug contours, and the pin holes are all projected onto the reference plane. All curved surfaces and edges are then simultaneously unfolded into a plane. Subsequently, the unfolded model is modeled in reverse according to the maximum outer contour and maximum thickness. Finally, based on the gradient configuration formed by the left and right edge strip structures and the two sides of the main body, the steps are removed on the flat plate solid. The positioning lugs, chamfers, and rounded edges are reconstructed on the unfolded model with reference to the 3D solid model, thus obtaining the 3D unfolded flat plate model of the part.

[0010] Furthermore, the partial surface forming mold includes a first upper mold and a first lower mold; the first upper mold can move up and down relative to the first lower mold, and an upper boss is provided at the lower part of the first upper mold, and a lower cavity is provided in the surface of the first lower mold; wherein, a first punch adapted to the inner surface of the titanium alloy fairing is provided on the upper boss, and a first concave mold adapted to the outer surface of the titanium alloy fairing is provided on the concave platform; a lifting groove is provided downward in the lower cavity, and an ejector that can move up and down is provided in the lifting groove; the upper surface of the ejector is a bearing surface adapted to the outer surface of the protrusion of the titanium alloy fairing, and first positioning pins that cooperate with the pin holes on the positioning lugs are provided on both sides of the bearing surface; wherein, when the ejector moves downward to fully enter the lifting groove, the bearing surface of the ejector will serve as the bottom surface of the first concave mold; first guide posts that cooperate with the upper boss are provided at both ends of the lower cavity.

[0011] Furthermore, once the temperature reaches the forming temperature of the titanium alloy part, the first upper mold is opened, and then the ejector is driven upward by the hydraulic ejector rod until the bearing surface of the ejector is flush with the surface of the first lower mold. The middle part of the unfolded flat plate of the part is placed on the bearing surface; and the first pin is fastened by passing through the pin hole on the positioning lug. The part unfolding plate is preheated for a first preset time, and the mold is closed and pressure is applied. The first upper mold is driven to move downward so that it contacts the upper surface of the part unfolding plate, and then continues to move downward until the mold is completely closed. At this time, the two sides of the part unfolding plate are flipped up and partially suspended outside the local surface forming mold. At this time, the pressure is held for a second preset time to obtain the pre-formed part.

[0012] Furthermore, in the design of the partial surface forming mold, measures are taken to reduce the depth of the first cavity; in the design of the first cavity, its depth only needs to cover the bending radius at both ends of the protrusion of the titanium alloy fairing.

[0013] Furthermore, the full-surface shaping mold includes a second upper mold, a second lower mold, a second guide post, and a second positioning pin; wherein, the lower part of the second upper mold has an upper cavity, and the interior of the upper cavity forms a second concave mold that matches the outer surface of the titanium alloy fairing; the upper part of the second lower mold has a lower boss, and the surface of the lower boss forms a second punch that matches the inner surface of the titanium alloy fairing; wherein the outline length of the second concave mold and the second punch is greater than the outer surface and the inner surface, so that the second concave mold and the second punch can completely cover the preformed part after the mold is closed; the second upper mold can move up and down relative to the second lower mold; the second guide post is provided on both sides of the lower boss, and the second guide post cooperates with the positioning hole on the upper cavity; the second positioning pin is provided on both sides of the middle part of the second punch, and cooperates with the positioning lug on the preformed part.

[0014] Furthermore, the scaling factors for the first and second dies relative to the outer surfaces of the titanium alloy fairing, and for the first and second punches relative to the inner surfaces of the titanium alloy fairing, are calculated using the following formulas: Ks=[(αj-αm)·ΔT / (1+αm·ΔT)] Where Ks is the scaling factor; αj is the thermal expansion coefficient of the titanium alloy fairing material at the forming temperature; αm is the thermal expansion coefficient of the local surface forming mold and the full surface straightening mold material at the forming temperature; ΔT is the temperature difference between the forming temperature and room temperature; and T is the forming temperature.

[0015] Furthermore, equidistant grooves of 2-3 mm are designed in the first die, the first punch, the second die, and the second punch to compensate for thermal expansion and positioning deviations, taking into account the gradient thickness characteristics.

[0016] Compared with the prior art, the present invention has the following technical features: By employing the method of this invention, a step-by-step thermoforming process combining local surface pre-clamping and full surface correction is used to form gradient thickness blanks at high temperatures. The formed parts have no residual stress and do not experience delayed deformation. Simultaneously, the positioning lugs, in the clamping state, cooperate with the positioning pins to achieve positioning reference transfer, ensuring final manufacturing accuracy. CNC machining is used to unfold and mill the flat blanks, achieving high precision in gradient features and edge contour coordination, using minimal raw material, and maximizing production efficiency. This method is highly suitable for the precise forming of non-uniform thickness blanks, with excellent results. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the titanium alloy fairing structure; Figure 2 This is a schematic diagram of the gradient thickness section of the titanium alloy fairing; Figure 3 A schematic diagram of the three-dimensional unfolded flat plate of the titanium alloy fairing; Figure 4 This is a schematic diagram of a partial surface forming mold; Figure 5 This is a sectional view of a partial surface forming mold; Figure 6 This is a schematic diagram of the full-surface correction mold structure; Figure 7 This is a sectional view of the full-surface correction mold. Detailed Implementation

[0018] like Figure 1 , Figure 2 As shown, the spatial outline of the titanium alloy fairing of this invention is a zigzag structure with a protrusion in the middle, and the two ends of the protrusion are rounded. The outer surface a (aerodynamic outer surface) of the main body of the titanium alloy fairing has a uniform thickness C with the inner surface d of the main body. In the integrated design of the titanium alloy fairing, in order to meet the assembly and connection requirements with the installation parts on the aircraft, a left edge strip structure and a right edge strip structure are formed on both sides of the inner surface d, respectively. The outer surface b and inner surface e of the left edge strip structure have a uniform thickness A, and the outer surface c and inner surface f of the right edge strip structure have a uniform thickness E. At the junction of the left edge strip structure and the left side of the main body, a left overlapping area with a thickness of B is formed while ensuring the misalignment. At the junction of the right edge strip structure and the right side of the main body, a right overlapping area with a thickness of D is formed while ensuring the misalignment, thus making the titanium alloy fairing have a gradient thickness characteristic. Among them, the step difference between the outer surface b of the left edge strip structure and the outer surface a of the main body, and the step difference between the outer surface c of the right edge strip structure and the outer surface a of the main body are the thicknesses of the front and rear end mating skins when assembling the titanium alloy fairing. Therefore, the left edge strip structure and the right edge strip structure are equivalent to replacing the original independent seam lining and other wide strips.

[0019] To ensure a seamless transition between the front and rear ends of the main body's outer surface a after assembly, high precision is required for the surface profile accuracy and varying thickness contour of the main body's outer surface a, as well as the step depth of the outer surface b of the left edge strip structure. Using a machining milling and bending forming method, the titanium alloy fairing exhibits two distinct bending features. During forming, an approximate three-point bend is created between the punch and die, leading to material bulging in the middle flat area. Due to the inconsistent contact width between the blank and the die, different frictional forces during die closing cause blank sway. Conversely, using a thick plate thermoforming bending followed by CNC milling results in stress deformation due to the thin remaining wall thickness. Therefore, this invention designs a manufacturing method for the gradient thickness and surface bending configuration of the titanium alloy fairing, ensuring high-precision manufacturing of the fairing parts.

[0020] The overall concept of this invention is as follows: A CNC milling and unfolding technique is employed for thin, flat materials, followed by precise positioning and forming. First, a three-dimensional unfolded flat plate model is obtained using reverse unfolding technology, ensuring the continuous consistency of the gradient thickness feature step-by-step. Second, CNC machining is used to process the part outline, gradient thickness feature, and positioning lugs on the flat material in a single operation, fully utilizing the precision of CNC milling. The resulting part outline and the gradient thickness feature within the molded surface exhibit harmonious consistency. Finally, precise positioning and plastic deformation processing are used to obtain the overall molded surface and solidify the gradient thickness feature, eliminating the need for secondary cutting of the part outline. This method offers high overall efficiency, guaranteed precision, and the highest utilization rate of raw materials. The specific steps of this invention are as follows: Step 1: Design the 3D unfolded flat plate digital model of the part; such as... Figure 1 As shown, in CATIA software, a 3D solid model of the titanium alloy fairing is first obtained. Then, the outer surface 'a' of the main body is used as the reference plane. The reference plane is extended around to cover the outer contour of the titanium alloy fairing. The 3D solid model is then simplified and modified, removing all chamfers, rounded edges, and other edge features. Next, the outer contour edges of the left and right edge structures, the gradient step edges, the positioning lug contours, and the pin holes are all projected onto the reference plane. All surfaces and edges are then simultaneously unfolded into a plane. Subsequently, the unfolded model is modeled in reverse according to the maximum outer contour and maximum thickness. Finally, based on the gradient configuration formed by the left and right edge structures and the two sides of the main body, steps are removed on the flat plate. Referring to the 3D solid model, the positioning lugs, chamfers, and rounded edges are reconstructed on the unfolded model, thus obtaining the 3D unfolded flat plate model of the part. Figure 3 As shown.

[0021] In this step, such as Figure 3 As shown, to improve the stability of the blank positioning during forming, positioning lugs g and h are designed on both sides of the center (the part with gentle curvature) of the three-dimensional unfolded flat plate digital model of the part; the width of the positioning lugs g and h is consistent with the protrusion of the titanium alloy fairing, ensuring a certain resistance to deformation at high temperatures; to prevent the blank from swaying after positioning, two pin holes are evenly provided on the positioning lugs g and h respectively; there is a certain amount of compensation during installation to avoid the positioning holes being over-positioned and the pins not being able to cooperate at the same time.

[0022] Step 2: The unfolded blank is CNC milled using a 3D unfolded flat plate. Double-sided milling forms the outer contour of the part and the initial gradient thickness characteristics, thus obtaining the unfolded flat plate of the part, as shown below. Figure 2 As shown.

[0023] Since the common heat treatment temperature range for relieving internal stress after room temperature forming of titanium alloys is 560-710℃, the specific temperature depends on the material grade. However, titanium alloys have a large springback at room temperature and are not easy to form. Titanium alloys have the best plastic deformation characteristics at high temperatures of 600-750℃. Therefore, the overlap between the heat treatment temperature and forming temperature range of titanium alloys can be utilized to perform high-temperature forming within the same temperature range, while simultaneously completing surface machining and relieving internal stress.

[0024] Step 3: Since there is a protrusion in the middle of the titanium alloy fairing and its two ends are bent rounded corners, the process of forming the unfolded blank with a punch and die will form an approximate three-point bend, which will cause the flat area of ​​the middle protrusion to deform and cause positioning failure. Therefore, it is not possible to use the punch and die to close the mold in one step and process it. It is necessary to take a forced fixing method for the unfolded blank.

[0025] like Figure 4 As shown, this solution designs a partial surface forming mold for pre-forming key areas of the titanium alloy fairing; the key areas include protrusions and the rounded corners at both ends of the protrusions; the partial surface forming mold includes a first upper mold 1 and a first lower mold 3; the first upper mold 1 can move up and down relative to the first lower mold 3, and an upper boss is provided at the lower part of the first upper mold 1, and a lower cavity is provided in the surface of the first lower mold 3; wherein, a first punch adapted to the inner surface of the titanium alloy fairing is provided on the upper boss, and a first concave mold adapted to the outer surface of the titanium alloy fairing is provided on the concave platform; the lower cavity has a downward opening. The lifting groove contains an ejector 4 that can move up and down (e.g., hydraulically driven). The upper surface of the ejector 4 is a bearing surface that matches the outer surface of the protrusion of the titanium alloy fairing. First positioning pins 6 are provided on both sides of the bearing surface to cooperate with the pin holes on the positioning lugs g and h. When the ejector 4 moves down to fully enter the lifting groove, the bearing surface of the ejector 4 will serve as the bottom surface of the first cavity. First guide posts 5 are provided at both ends of the lower cavity to cooperate with the upper boss (which has guide holes that cooperate with it). These guide posts are used to guide the first upper mold 1 when it is pressed down, ensuring accurate mold closing.

[0026] Step 4: First, install the partial surface forming mold in the thermoforming machine for heating, such as... Figure 4 As shown; after the temperature reaches the forming temperature of the titanium alloy part, the first upper mold 1 is opened, and then the ejector 4 is driven to move upward by the hydraulic ejector rod until the bearing surface of the ejector 4 is flush with the surface of the first lower mold 3, and the middle part of the unfolded plate is placed on the bearing surface; the first pin 6 is fastened by passing through the pin holes on the positioning lugs g and h, so that the part is stably fixed in the width direction, and no positioning deformation will occur even if the unfolded plate of the part is subjected to force in the subsequent forming process.

[0027] Step 5: Preheat the unfolded part platen for 5-10 minutes, then begin mold closing and pressurization; drive the first upper mold 1 downwards, so that the first upper mold 1 contacts the upper surface of the unfolded part platen, clamping the unfolded part platen, and then continue to move downwards until the mold is fully closed. At this time, the two sides of the unfolded part platen are flipped upwards, and part of it is suspended outside the local surface forming mold, such as... Figure 5 As shown; at this point, hold the pressure for 15-25 minutes to obtain the pre-formed part.

[0028] Considering the significant drop in height of the rear chord during the formation of the preformed part, even if it does not exceed the ejector rod stroke of the ejector 4, it can easily lead to a decrease in the stability of the ejector 4 during use. Therefore, a measure is adopted to reduce the depth of the first die cavity. That is, the depth of the first die is designed to cover the bending radius at both ends of the protrusion of the titanium alloy fairing (without completely covering the formed titanium alloy fairing). This ensures the effective bending and forming of the main profile of the titanium alloy fairing, while avoiding excessively long forming strokes and reducing the opening height of the partial profile forming mold. Therefore, the partial profile forming mold is not a full profile mold, that is, after the mold is closed, the two ends of the part unfolding plate are still outside the partial profile forming mold.

[0029] Step 6: After the pressure holding is completed, open the first upper mold 1, take out the pre-formed part and cool it; for batch processing of parts, steps 4 and 5 can be repeated to complete the first forming process of a batch of parts.

[0030] Step 7: Due to the use of a non-full-surface thermoforming mold design, although the forming difficulty is reduced, there are still deviations in the surface of the suspended parts. Therefore, a step-by-step solution is adopted for precise forming, and a full-surface correction mold is designed, such as... Figure 6 As shown.

[0031] The full-surface shaping mold includes a second upper mold 7, a second lower mold 8, a second guide post 9, and a second positioning pin 10. The second upper mold 7 has an upper cavity at its lower part, inside which a second concave mold is formed that matches the outer surface of the titanium alloy fairing. The second lower mold 8 has a lower boss at its upper part, and a second punch is formed on the surface of the lower boss that matches the inner surface of the titanium alloy fairing. The outline length of the second concave mold and the second punch is greater than the outer and inner surfaces, allowing the second concave mold and the second punch to completely cover the preformed part after mold closing. The second upper mold 7 can move up and down relative to the second lower mold 8. The second guide post 9 is provided on both sides of the lower boss, and the second guide post 9 cooperates with the positioning holes on the upper cavity to ensure accuracy during mold closing. The second positioning pin 10 is provided on both sides of the middle part of the second punch, cooperating with the positioning lugs g and h on the preformed part to achieve precise positioning of the preformed part.

[0032] The full-surface shaping mold is installed in the thermoforming machine, and the temperature is raised.

[0033] Step 8: After the full-surface shaping mold reaches the forming temperature, open the second upper mold 7, place the pre-formed part on the second punch, and use the second pin 10 to assemble and fix it.

[0034] Step 9: Preheat the pre-formed part for 5-10 minutes, then start driving the second upper mold 7 to move up and down to close the mold and apply pressure. Figure 7 As shown; the part's surface is formed completely according to the full-surface shaping mold, and pressure is maintained for 15-25 minutes to obtain the processed titanium alloy fairing part; after the part is formed, the mold is opened, the formed part is taken out, and transferred to other areas for cooling, shaping and other post-processing (cleaning, dimensional correction, quality inspection, performance enhancement, etc.) to obtain the processed titanium alloy fairing.

[0035] In the design of the partial surface forming mold and the full surface straightening mold, the difference in the coefficients of thermal expansion between the titanium alloy parts and the cast steel mold materials at the forming temperature may cause the formed titanium alloy fairing to be larger than the theoretical digital model size, or cause edge damage due to the varying thickness gradient within the surface. To ensure the overall forming effect of the gradient varying thickness feature, the partial surface forming mold and the full surface straightening mold adopt a scaled-down design. The first concave mold and the first convex mold on the partial surface forming mold, as well as the second concave mold and the second convex mold on the full surface straightening mold, are all scaled relative to the inner and outer surfaces of the titanium alloy fairing to ensure dimensional consistency under high temperature conditions. That is, the scaling factor calculation formulas for the first and second dies relative to the outer surfaces of the titanium alloy fairing, and for the first and second punches relative to the inner surfaces of the titanium alloy fairing, are as follows: Ks=[(αj-αm)·ΔT / (1+αm·ΔT)]≈(αj-αm)·ΔT≈(αj -αm)·T Where Ks is the scaling factor, positive for scaling and negative for scaling; αj is the coefficient of thermal expansion of the titanium alloy fairing material at the forming temperature (1 / ℃); αm is the coefficient of thermal expansion of the local surface forming mold and the full surface straightening mold material at the forming temperature (1 / ℃); ΔT is the temperature difference between the forming temperature and room temperature (℃); and T is the forming temperature (℃).

[0036] Meanwhile, equidistant grooves of 2-3 mm are designed in the first die, the first punch, the second die, and the second punch to compensate for thermal expansion and positioning deviation, and can also effectively avoid damage.

[0037] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A method for precise forming of a variable thickness titanium alloy fairing, characterized in that, The titanium alloy fairing has a spatial outline that is a zigzag structure with a central protrusion and rounded corners at both ends. The fairing includes a main body, left and right edge structures on either side of the main body, and overlapping areas between the left and right edge structures and the sides of the main body, creating a gradient thickness characteristic. The method includes: Based on the three-dimensional solid digital model of the titanium alloy fairing, design the three-dimensional unfolded flat plate digital model of the part; The unfolded blank is processed by CNC milling of a three-dimensional unfolded flat plate, and double-sided milling is used to form the outer contour of the part and the initial gradient thickness characteristics, thereby obtaining the unfolded flat plate of the part; A partial surface forming mold is designed for pre-forming key areas in a titanium alloy fairing. The key areas include protrusions and rounded corners at both ends of the protrusions. The partial surface forming mold is provided with a first punch and a first die that cooperate with each other, and an ejector that can move up and down. The upper surface of the ejector is a bearing surface that matches the outer surface of the protrusions of the titanium alloy fairing. First positioning pins that cooperate with positioning lugs on the unfolded plate of the part are provided on both sides of the bearing surface. The partial surface forming mold is installed in the thermoforming machine and heated. When the temperature reaches the forming temperature of the titanium alloy part, the unfolded flat part is placed in the partial surface forming mold and fixed with the first pin. After preheating, the mold is closed and pressure is applied, causing the two sides of the unfolded flat part to flip up and partially suspend outside the partial surface forming mold. At this time, pressure is held. After the pressure is held, the preformed part is taken out and cooled. Design a full-surface correction mold for full-surface correction of preformed parts; the full-surface correction mold is equipped with a second punch and a second die that cooperate with each other. After the mold is closed, the second die and the second punch can completely cover the preformed parts; the second punch has second locating pins on both sides of the middle part. The full-surface shaping mold is installed in the thermoforming machine and the temperature is raised. After the forming temperature is reached, the pre-formed part is installed and fixed with the second pin. The pre-formed part is then preheated. After preheating, the mold is closed and pressure is applied and held. After holding the pressure, the mold is opened and the formed part is taken out and transferred to other areas for cooling and post-processing to obtain the finished titanium alloy fairing.

2. The method for precise forming of variable thickness titanium alloy fairing according to claim 1, characterized in that, The outer surface a of the main body of the titanium alloy fairing has a uniform thickness C with the inner surface d of the main body. The two sides of the inner surface d of the main body extend outward to form a left edge strip structure and a right edge strip structure, respectively. The outer surface b and inner surface e of the left edge strip structure have a uniform thickness A, and the outer surface c and inner surface f of the right edge strip structure have a uniform thickness E. The left edge strip structure and the right edge strip structure form a left overlapping area with a thickness B and a right overlapping area with a thickness D at the junction with the left side and the right side of the main body, respectively, so that the titanium alloy fairing has a gradient thickness characteristic. The step difference between the outer surface b of the left edge strip structure, the outer surface c of the right edge strip structure and the outer surface a of the main body is the thickness of the front and rear end mating skin when the titanium alloy fairing is assembled.

3. The method for precise forming of variable thickness titanium alloy fairing according to claim 1, characterized in that, Positioning lugs are designed on both sides of the center of the three-dimensional unfolded flat plate digital model; the width of the positioning lugs is consistent with the protrusion of the titanium alloy fairing, and pin holes are evenly set on the positioning lugs.

4. The method for precise forming of variable thickness titanium alloy fairing according to claim 1, characterized in that, Based on the three-dimensional solid model of the titanium alloy fairing, a three-dimensional unfolded flat plate model of the part is designed, including: In CATIA software, a 3D solid model of the titanium alloy fairing is obtained. Then, the outer surface 'a' of the main body is used as the reference plane. The reference plane is extended around to cover the outer contour of the titanium alloy fairing. The 3D solid model is then simplified and modified by removing all chamfers and rounded edges. The outer contour edges of the left and right edge strip structures, the gradient step edges, the positioning lug contours, and the pin holes are all projected onto the reference plane. All curved surfaces and edges are then simultaneously unfolded into a plane. Subsequently, the unfolded model is modeled in reverse according to the maximum outer contour and maximum thickness. Finally, based on the gradient configuration formed by the left and right edge strip structures and the two sides of the main body, the steps are removed on the flat plate solid. The positioning lugs, chamfers, and rounded edges are reconstructed on the unfolded model with reference to the 3D solid model, thus obtaining the 3D unfolded flat plate model of the part.

5. The method for precise forming of variable thickness titanium alloy fairing according to claim 1, characterized in that, The partial surface forming mold includes a first upper mold and a first lower mold. The first upper mold can move up and down relative to the first lower mold, and an upper boss is provided at the lower part of the first upper mold. A lower cavity is provided in the surface of the first lower mold. The upper boss is provided with a first punch that matches the inner surface of the titanium alloy fairing, and the lower cavity is provided with a first concave mold that matches the outer surface of the titanium alloy fairing. A lifting groove is provided downward in the lower cavity, and an ejector that can move up and down is provided in the lifting groove. The upper surface of the ejector is a bearing surface that matches the outer surface of the protrusion of the titanium alloy fairing. First positioning pins that mate with the pin holes on the positioning lugs are provided on both sides of the bearing surface. When the ejector moves downward to fully enter the lifting groove, the bearing surface of the ejector will serve as the bottom surface of the first concave mold. First guide posts that mate with the upper boss are provided at both ends of the lower cavity.

6. The method for precise forming of variable thickness titanium alloy fairing according to claim 1, characterized in that, Once the temperature reaches the forming temperature of the titanium alloy part, the first upper mold is opened, and then the ejector is driven upward by the hydraulic ejector rod until the bearing surface of the ejector is flush with the surface of the first lower mold. The middle part of the unfolded flat plate of the part is placed on the bearing surface; the first pin is fastened by passing through the pin hole on the positioning lug. After preheating the flat plate of the part for the first preset time, start mold closing and pressurization; The first upper mold is driven to move downwards, so that it contacts the upper surface of the part unfolding platen. It continues to move downwards until the mold is fully closed. At this time, the two sides of the part unfolding platen are flipped up and partially suspended outside the local surface forming mold. At this time, the pressure is held for a second preset time to obtain the pre-formed part.

7. The method for precise forming of variable thickness titanium alloy fairing according to claim 1, characterized in that, When designing a partial surface forming mold, measures are taken to reduce the depth of the first die cavity; when designing the first die cavity, its depth only needs to cover the bending radius at both ends of the protrusion of the titanium alloy fairing.

8. The method for precise forming of variable thickness titanium alloy fairing according to claim 1, characterized in that, The full-surface shaping mold includes a second upper mold, a second lower mold, a second guide post, and a second positioning pin. The second upper mold has an upper cavity at its lower part, inside which a second concave mold is formed that matches the outer surface of the titanium alloy fairing. The second lower mold has a lower boss at its upper part, and a second punch is formed on the surface of the lower boss that matches the inner surface of the titanium alloy fairing. The outline lengths of the second concave mold and the second punch are greater than the outer and inner surfaces, allowing the second concave mold and the second punch to completely cover the preformed part after mold closing. The second upper mold can move up and down relative to the second lower mold. The second guide post is provided on both sides of the lower boss, and the second guide post engages with positioning holes on the upper cavity. The second positioning pin is provided on both sides of the middle part of the second punch, engaging with positioning lugs on the preformed part.

9. The method for precise forming of variable thickness titanium alloy fairing according to claim 1, characterized in that, The scaling factors for the first and second dies relative to the outer surfaces of the titanium alloy fairing, and the first and second punches relative to the inner surfaces of the titanium alloy fairing, are calculated using the following formulas: Ks=[(αj-αm)·ΔT / (1+αm·ΔT)] Where Ks is the scaling factor; αj is the thermal expansion coefficient of the titanium alloy fairing material at the forming temperature; αm is the thermal expansion coefficient of the local surface forming mold and the full surface straightening mold material at the forming temperature; ΔT is the temperature difference between the forming temperature and room temperature; and T is the forming temperature.

10. The method for precise forming of a variable thickness titanium alloy fairing according to claim 1, characterized in that, In the first die cavity, the first punch, the second die cavity, and the second punch, equidistant grooves of 2-3 mm are designed to compensate for thermal expansion and positioning deviations due to the gradient thickness characteristics.

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