Hydraulic bulging forming method for reverse double-curvature fairing skin

Abstract

The invention discloses a hydraulic bulging forming method of a reverse double-curvature fairing skin. The hydraulic bulging forming method comprises the following steps: constructing a hydraulic bulging process surface of the fairing skin; determining the size of an unfolded blank; designing a hydraulic bulging die structure of the fairing skin step by step; performing hydraulic bulging; and trimming the appearance. According to the reverse hyperbolic structure of the fairing skin, hydraulic bulging process profiles of different skins with opposite overall forming curvatures are designed, the material plastic deformation area in part hydraulic bulging is enlarged, and the problems that the material surface orange peel defect is serious, and the forming precision is low due to the fact that reverse double-curvature skin conventional drop pressing forming cannot be controlled are solved. And a duralumin material aluminum-clad layer is penetrated, and the surface quality cannot meet the requirements of customers.

Description

Technical Field

[0001] The invention relates to a sheet metal parts forming technology in the field of aircraft manufacturing, in particular to a hydraulic expansion forming method for a reverse hyperbolic fairing skin. Background Art

[0002] The function of aircraft skin is to maintain the shape of the aircraft, make it have good aerodynamic characteristics, and reduce flight resistance. Therefore, the skin material is required to have high strength, good plasticity, and a smooth appearance after forming. Complex shape skin is often seen in aircraft wing tips, fairing skins, nose covers, tail covers, etc. The fairing skin is located at the front of the fuselage. The left and right fairing skins cover both sides of the nose to protect the radar antenna device. It has a complex curvature, thin-walled multi-section structure, and reduces air resistance by adjusting the airflow flowing over the surface when the aircraft is flying, improving the aerodynamic performance of the aircraft and improving the aircraft's flight capability.

[0003] The fairing skin is made of hard aluminum alloy, and the material thickness does not exceed 1.6mm, in order to achieve the lightweight design requirements of the aircraft. The fairing skin presents a smooth transition of hyperbolic streamlines on both sides of the central axis. The curvature of the fairing skin on the left and right sides changes in the opposite direction, and the curvature of the fairing skin on the right side changes more rapidly than that on the left side, thereby protecting the front end of the aircraft from protruding radars and related discontinuities, while improving the aerodynamic performance of the aircraft. In the past, the fairing skin structure was manufactured by progressive forming using the skin drop pressing process, but the streamline characteristics of the reverse hyperbolic curvature increased the difficulty of the skin drop pressing, and the material surface had serious orange peel defects during forming, and the aluminum layer of the hard aluminum material was penetrated, and the surface quality could not meet customer requirements.

[0004] Orange peel defect is a surface roughening phenomenon of metal materials, which not only affects the appearance of the skin, but also reduces the strength, plasticity and toughness of the skin, seriously affecting the fatigue life of the aircraft. In order to avoid surface wrinkles or dead folds on the fairing skin, the amount of pressure drop is often reduced during forming, and multiple pass pressure drop forming is adopted. During the pressure drop forming process, manual trimming and repeated grinding are used to improve its surface morphology, which leads to reduced forming accuracy of parts and penetration of the aluminum cladding, affecting the appearance and assembly of the aircraft. Due to the long-term bottleneck problem of difficult to control surface and internal damage and extremely unstable product quality in pressure drop forming, rejection and scrapping occur from time to time, causing great economic losses to enterprises.

[0005] The fairing skin is about 1.6m long, 1.3m wide, and the maximum bow height of the contour is close to 300mm. The orange peel phenomenon cannot be avoided by drop forming. The open fairing skin is formed by the traditional hydraulic bulging process. Due to the large hydraulic bulging height of the skin, the deformation trends on both sides of the central axis of the shape are opposite, and the deformation amounts are unequal, which makes the skin prone to material accumulation and wrinkles. Due to the uneven plastic deformation, the skin rebounds severely after forming and cannot reach the design drawing size. It is often necessary to form the skin in sections according to the positive and negative curvatures. For the reverse hyperbolic curvature, large-size multi-section, streamlined thin-walled sheet metal fairing skin, there is an urgent need for a mechanized precision processing method to achieve no orange peel, uniform deformation, high quality, integrated and controllable forming of parts. Summary of the invention

[0006] In order to solve the problems of severe orange peel on the surface of the reverse hyperbolic thin-walled large-size fairing skin, penetration of the aluminum cladding and high scrap rate after forming, the purpose of the present invention is to provide a hydraulic expansion forming method for the reverse hyperbolic fairing skin.

[0007] The present invention provides a hydraulic expansion forming method for a reverse hyperbolic fairing skin, comprising the following steps:

[0008] Step 1, constructing a hydraulic bulging process surface of the fairing skin;

[0009] Step 2: Determine the size of the unfolded blank;

[0010] Step 3: Determine the hydraulic bulging die structure of the fairing skin according to the hydraulic bulging process surface;

[0011] Step 4, laying the skin blank that is larger than the unfolded blank size on the hydraulic bulging die structure for hydraulic bulging;

[0012] Step 5: Trim the shape.

[0013] Optionally, the step 1 is to construct a hydraulic bulging process surface of the fairing skin, and the specific process is as follows:

[0014] Step 1-1: Define the hydroforming direction

[0015] Taking the XY plane in the CAT IA environment as the design horizontal plane H, the fairing skin is rotated to ensure that the distance h between the two end points P and Q of the fairing skin and the design horizontal plane is equal, thereby reducing the fairing skin forming depth h, and the fairing skin bulging direction F is perpendicular to the design horizontal plane H;

[0016] Step 1-2: Add the fairing skin

[0017] Extract the upper surface of the fairing skin, select the vertices A and G of the elliptical notch on the upper surface of the skin, construct the spline AG with the side contour lines on both sides of the notch as tangents, fill the spline AG, the elliptical notch contour line, and the long circular notch contour line to establish the fairing skin surface;

[0018] Step 1-3: Design the transition extension surface

[0019] Select two high points A and B of the fairing skin, and establish a reference plane with reference to the ZX plane in the CAT IA environment. Set up design planes M and N by offsetting the reference plane by 200 mm in the opposite direction. Extract the contour lines of the two ends of the fairing skin surface W. Design splines on M and N according to the contours of the two ends of the fairing skin surface. Bridge planes M and N and corresponding splines. Stretch the two ends of the fairing skin by 200 mm along the curvature of the bridge surface to establish the transition flange extension surface.

[0020] Step 1-4: Design the flange extension surface

[0021] Select a midpoint C at the notch in the middle of the fairing skin, establish a design plane K with reference to the XY plane in the CATIA environment, design a spline curve in K at a distance of 300 mm from the skin contour according to the curvature of the fairing skin contour line, project the spline curve to the transition flange extension surface to obtain the flange contour line, cut the transition flange extension surface with the flange contour line, and establish the flange extension surface;

[0022] Step 1-5: Design the transition guide line of the hydraulic bulging process surface

[0023] Extract the four endpoints D1, D2, D3, and D4 of the fairing skin contour, extract the four points E1, E2, E3, and E4 on the fairing skin surface and fit the tangent lines D1 E1, D2E2, D3E3, and D4E4 with the endpoints D, E, F, and G, construct the points F1, F2, F3, and F4 at the closest distances between the four endpoints of the fairing skin contour on the inner contour line of the flange extension surface, and construct the transition guide lines D1 F1, D2F2, D3F3, and D4F4 of the hydraulic bulging process surface according to the four endpoints D1, D2, D3, and D4 of the fairing skin contour, the tangent lines D1 E1, D2E2, D3E3, and D4E4 and the construction points F1, F2, F3, and F4, and the tension of the guide lines is defined as 1;

[0024] Step 1-6: Design the transition surface

[0025] Extract the inner contour line of the flange extension surface, project points A and G onto the inner contour line of the flange extension surface to obtain points A1 and G1, cut the inner contour line of the flange extension surface at points A1 and G1, retain the inner contour line A1 G1, bridge the inner contour line A1 G1 and the spline AG, obtain the transition surface Y1 between the flange extension surface and the fairing skin surface, extract the contour lines AA1 and GG1 at both ends of the transition surface Y1, fill the contour line GG1 and the transition guide line D1 F1, the transition guide line D1 F1 and the transition guide line D2F2, the transition guide line D2F2 and the transition guide line D3F3, the transition guide line D3F3 and the transition guide line D4F4, the transition guide line D4F4 and the cross contour line AA1 to obtain transition surfaces Y2, Y3, Y4, Y5, Y6, and join Y1, Y2, Y3, Y4, Y5, Y6 to obtain the transition surface;

[0026] Step 1-7: Design the hydraulic bulging process surface

[0027] The fairing skin surface, flange extension surface and transition surface are joined, and the flange extension surface and the transition surface are transitioned with a fillet radius R to obtain a hydraulic bulging process surface.

[0028] Optionally, the step 2 is to determine the size of the unfolded blank, and the specific process is as follows:

[0029] The fairing skin surface is imported into the finite element analysis software, and the unfolded shape of the hydraulic bulging is calculated using the blank back calculation function to obtain the unfolded blank and the outer edge line of the unfolded blank;

[0030] Determine the size of the unfolded blank based on the outer edge line of the unfolded blank.

[0031] Optionally, the step 3 determines the hydraulic bulging die structure of the fairing skin according to the hydraulic bulging process surface, and the specific process is as follows:

[0032] Step 3-1: Design the die structure

[0033] The flange extension surface of the hydraulic bulging process surface is extended in all directions within the hydraulic bulging horizontal plane XY, and the expanded hydraulic bulging process surface is used as the die working surface. The outer edge line of the unfolded blank is projected to the corresponding position of the die working surface to obtain the outer contour line of the unfolded blank on the die; the die working surface is adjusted according to the unfolded blank outer contour line; a drainage hole with a diameter of 10 mm is set on the die;

[0034] Step 3-2: Design the blank holder structure

[0035] Generate the blank holder working surface according to the flange extension surface, transition surface and gap;

[0036] Gap g = (0.95-1.0) × δ;

[0037] The outer edge line of the unfolded blank is projected to the corresponding position of the working surface of the blank holder ring to obtain the outer contour line of the unfolded blank on the blank holder ring; the working surface of the blank holder ring is adjusted according to the outer contour line of the unfolded blank; a water inlet hole with a diameter of 10 mm is provided on the blank holder ring;

[0038] Where δ is the material thickness of the fairing skin.

[0039] Optionally, the die working surface is adjusted according to the outer contour of the unfolded blank, including:

[0040] Determine whether the distance between the outer edge line of the unfolded blank and the outer contour of the working surface of the die remains relatively uniform, and the distance between them is ≥80mm. If not, adjust the working surface of the die.

[0041] Optionally, in step 4, the skin blank that is larger than the unfolded blank size is laid on the hydraulic bulging die structure for hydraulic bulging, and the specific process is as follows:

[0042] Step 4-1: Install the hydraulic bulging die

[0043] Install the die and blank holder on the working table of the hydraulic bulging machine. The die is fixed to the bed of the hydraulic bulging machine. The hydraulic bulging outer surface of the die corresponds to the working surface of the blank holder. The blank holder can move up and down through the movable ejector. Lubricating oil is applied to the forming working surfaces of the die and blank holder.

[0044] Step 4-2: Expand the blank and position it

[0045] Affix a plastic film to protect the upper surface of the unfolded blank, and place it on the working surface of the die according to the outer contour of the unfolded blank on the die. Apply lubricating oil to the corresponding area of ​​the lower surface of the unfolded blank and the working surface of the die;

[0046] Step 4-3 Hydraulic bulging

[0047] The blank holder moves downward along the hydraulic bulging direction F, and after touching the die, it clamps the expanded blank with the die, and fluid is introduced from the water inlet hole of the blank holder. The fluid medium acts as a punch to apply hydraulic pressure P to the expanded blank, causing the expanded blank to undergo plastic deformation and form a part against the die cavity.

[0048] Optionally, the hydraulic pressure is adjusted by an overflow valve, and hydraulic bulging is performed according to the shape of the die, and the unit blank holding force setting range is 8MPa to 9MPa.

[0049] Optionally, the step 5 of trimming the fairing skin parts is as follows:

[0050] The cutting is completed according to the fairing skin contour line on the hydraulic bulging contour surface of the concave die, and the fairing skin parts with contour accuracy and surface quality meeting the requirements of the drawing are obtained.

[0051] The beneficial effects of the present invention are:

[0052] 1) The present invention designs the hydraulic bulging process surface of different skins with opposite overall forming curvatures based on the reverse hyperbolic structure of the fairing skin, thereby increasing the plastic deformation area of ​​the material during the hydraulic bulging of the part, and solving the problems that the conventional drop-press forming of the reverse hyperbolic skin cannot control the serious orange peel defects on the material surface, the aluminum layer of the hard aluminum material penetrates, and the surface quality cannot meet the customer's requirements;

[0053] 2) An innovative design method for hydraulic bulging process surface is established. By giving priority to establishing a flange extension surface that conforms to the part surface structure, a transition surface between the part surface and the flange extension surface is constructed by combining bridging and filling. This design method significantly reduces the difficulty of designing process surfaces in the CATIA environment, and can easily reduce the hydraulic bulging depth of large-size, multi-section, and multi-curvature parts. It has high forming accuracy and good versatility.

[0054] 3) An innovative design method for hydraulic bulging process surface has been established. This surface design method can change the two skins with opposite curvatures that can only be formed in sections due to the inability to control wrinkles in traditional drop forming to integral forming, reduce the weld at one end, and significantly improve the overall strength of the fairing and enhance the overall aerodynamic performance of the aircraft;

[0055] 4) Innovatively adopt hydraulic bulging to replace traditional drop forming. Hydraulic bulging only uses a single-sided die, and the other half is replaced by liquid medium, which reduces mold costs. Generally, mold costs can be reduced by more than 30%, making the production of complex skin parts simpler and more flexible. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is a schematic diagram of the fairing skin structure;

[0057] Figure 2 It is a schematic diagram of the hydraulic expansion direction of the fairing skin;

[0058] Figure 3 It is a schematic diagram of the fairing skin process shape design process;

[0059] Figure 4 It is a schematic diagram of the fairing skin process surface structure;

[0060] Figure 5 This is a schematic diagram of the fairing skin die structure;

[0061] Figure 6 This is a schematic diagram of the fairing skin pressure ring structure;

[0062] Figure 7 This is a schematic diagram of the unfolded blank structure of the fairing skin;

[0063] Figure 8 This is a schematic diagram of the structure of the hydraulic bulging die for the overall fairing skin;

[0064] Explanation of the accompanying drawings: 1 fairing skin, 2 fairing skin surface, 3 flange extension surface, 4 transition surface, 5 hydraulic bulging process surface, 6 trapezoidal unfolded blank, 7 unfolded blank outer edge line, 8 die, 9 pressure ring, 10 die working surface, 11 elliptical notch, 12 transition fillet surface, 13 hydraulic bulging shape surface, 14 pressure ring working surface, 15 pressure ring unfolded blank outer edge line, 16 unfolded blank, 17 die unfolded blank outer contour line, 18 die drainage hole, 19 pressure ring water inlet hole. DETAILED DESCRIPTION

[0065] The present application is further described in detail below in conjunction with the accompanying drawings of the embodiments.

[0066] Referring to the accompanying drawings, the aircraft sheet metal parts provided in the embodiment are as follows Figure 1 As shown in the figure, the fairing skin is made of hard aluminum alloy, with a length of about 1.6m, a width of about 1.3m, and a maximum bow height of nearly 300mm. The fairing skin presents a smooth transition of hyperbolic streamlines on both sides of the central axis, and the curvatures of the left and right fairing skins change in opposite directions, while the curvature of the right fairing skin changes more rapidly than that of the left fairing skin. In the past, the fairing skin structural parts were manufactured by progressive forming using the skin drop pressing process, but the streamline characteristics of the reverse hyperbolic curvature increased the difficulty of skin drop pressing, and the material surface had serious orange peel defects during forming, which caused the aluminum layer of the hard aluminum material to penetrate, and the surface quality could not meet customer requirements.

[0067] like Figure 2 to Figure 8 As shown, a hydraulic expansion forming method for a reverse hyperbolic thin-walled large-size fairing skin comprises the following steps:

[0068] Step 1: Construct the hydraulic bulging process surface of the fairing skin 5

[0069] Step 1-1: Define the hydroforming direction

[0070] Taking the XY plane in the CAT IA environment as the design horizontal plane H, the fairing skin 1 is rotated to ensure that the distances h between the two end points P and Q of the fairing skin and the design horizontal plane are equal, thereby reducing the forming depth h of the fairing skin, and the bulging direction F of the fairing skin is perpendicular to the design horizontal plane H;

[0071] Step 1-2: Add the fairing skin

[0072] Extract the upper surface of the fairing skin, select the vertices A and G of the elliptical notch 11 on the upper surface of the skin, construct the spline AG with the side contour lines on both sides of the notch as tangents, fill the spline AG, the elliptical notch contour line, and the long circular notch contour line to establish the fairing skin shape surface 2;

[0073] Step 1-3: Design the transition extension surface

[0074] Select the two endpoints P and Q of the fairing skin, and establish a reference plane with reference to the ZX plane in the CAT IA environment. Set up design planes M and N by offsetting the reference plane by 200 mm in the opposite direction. Extract the contour lines of the two ends of the fairing skin surface W. Design splines on M and N according to the contours of the two ends of the fairing skin surface. Bridge planes M and N and corresponding splines. Stretch the two ends by 200 mm along the curvature of the bridge surface to establish the transition flange extension surface.

[0075] Step 1-4: Design flange extension surface 3

[0076] Select a midpoint C at the notch in the middle of the fairing skin, establish a design plane K with reference to the XY plane in the CATIA environment, design a spline curve in K at a distance of 300 mm from the skin contour according to the curvature of the fairing skin contour line, project the spline line onto the transition extension surface to obtain the flange contour line, cut the transition extension surface with the flange contour line, and establish flange extension surface 3;

[0077] Step 1-5: Design the transition guide line of the hydraulic bulging process surface

[0078] Extract the four endpoints D1, D2, D3, and D4 of the fairing skin contour, extract the four points E1, E2, E3, and E4 on the fairing skin surface and fit the tangent lines D1 E1, D2E2, D3E3, and D4E4 with the endpoints D, E, F, and G, construct the points F1, F2, F3, and F4 at the closest distances between the four endpoints of the fairing skin contour on the inner contour line of the flange extension surface, and construct the transition guide lines D1 F1, D2F2, D3F3, and D4F4 of the hydraulic bulging process surface according to the four endpoints D1, D2, D3, and D4 of the fairing skin contour, the tangent lines D1 E1, D2E2, D3E3, and D4E4 and the construction points F1, F2, F3, and F4, and the tension of the guide lines is defined as 1;

[0079] Step 1-6: Design transition surface 4

[0080] Extract the inner contour line of the flange extension surface 3, project points A and G onto the inner contour line of the flange extension surface 3 to obtain points A1 and G1, cut the inner contour line of the flange extension surface 3 at points A1 and G1, retain the inner contour line A1 G1, bridge the inner contour line A1 G1 and the spline line AG in step 1-2, obtain the transition surface Y1 between the flange extension surface 2 and the fairing skin surface 1, extract the contour lines AA1 and GG1 at both ends of the transition surface Y1, fill the contour line GG1 and the transition guide line D1 F1, the transition guide line D1 F1 and the transition guide line D2F2, the transition guide line D2F2 and the transition guide line D3F3, the transition guide line D3F3 and the transition guide line D4F4, the transition guide line D4F4 and the transition contour line AA1 to obtain transition surfaces Y2, Y3, Y4, Y5, and Y6, and join Y1, Y2, Y3, Y4, Y5, and Y6 to obtain the transition surface 4;

[0081] Step 1-7: Design the hydraulic bulging process surface 5

[0082] The fairing skin-shaped surface 2, the flange extension surface 3, and the transition-shaped surface 4 are joined, and the flange extension surface 3 and the transition-shaped surface 4 are transitioned with a fillet radius R to obtain a hydraulic bulging process surface 5.

[0083] Step 2: Determine the size of the unfolded blank

[0084] The fairing skin surface 2 is imported into the finite element analysis software, and the unfolded shape of the hydraulic bulging is calculated using the blank back calculation function to obtain the unfolded blank 6 and the unfolded blank outer edge line 7. The unfolded blank size is determined according to the unfolded blank outer edge line 7.

[0085] Step 3: Design the hydraulic bulging die structure of the fairing skin

[0086] According to the hydraulic bulging process surface 5, the hydraulic bulging die structure of the fairing skin is determined, including the concave die 8 and the blank holder 9.

[0087] Step 3-1: Design the die 8 structure

[0088] The flange extension surface 3 of the hydraulic bulging process surface 5 is extended in all directions in the hydraulic bulging horizontal plane XY, and the enlarged hydraulic bulging process surface 5 is used as the die working surface 10, and the unfolded blank outer edge line 7 is projected to the corresponding position of the die working surface to obtain the unfolded blank outer contour line 17 on the die 9, and the die working surface 10 is adjusted according to the unfolded blank outer contour line 17, and a drainage hole 18 with a diameter of 10 mm is set on the die;

[0089] Step 3-2: Design the structure of the blank holder 9

[0090] Generate the blank holder working surface 14 according to the flange extension surface 3, the transition surface 12 and the gap;

[0091] Gap g = (0.95-1.0) × δ;

[0092] The outer edge line 7 of the unfolded blank is projected to the corresponding position of the blank holder working surface 14 to obtain the outer contour line 15 of the unfolded blank on the blank holder; the blank holder working surface 14 is adjusted according to the outer contour line 15 of the unfolded blank; a water inlet hole 19 with a diameter of 10 mm is provided on the blank holder; wherein δ is the material thickness of the fairing skin.

[0093] In addition, it is necessary to determine whether the distance between the outer edge line 15 of the unfolded blank and the outer contour of the working surface of the die 8 and the blank holder 9 remains relatively uniform, and the distance between them is ≥80mm. If not, adjust the working surfaces 10 and 14 of the die and the blank holder.

[0094] Step 4: Hydraulic bulging

[0095] Step 4-1: Install the hydraulic bulging die

[0096] Install the die 8 and the blank holder 9 on the working table of the hydraulic bulging machine. The die 8 is fixed to the bed of the hydraulic bulging machine. The hydraulic bulging outer surface 13 of the die 8 corresponds to the working surface 14 of the blank holder. The blank holder 9 can move up and down through the movable ejector. Lubricating oil is applied to the forming working surfaces of the die 8 and the blank holder 9.

[0097] Step 4-2: Expand the blank and position it

[0098] The upper surface of the unfolded blank 16 is protected by a plastic film, and is placed on the die working surface 10 according to the outer contour line 17 of the unfolded blank on the die 9, and lubricating oil is applied to the corresponding area of ​​the lower surface of the unfolded blank and the die working surface 10;

[0099] Step 4-3: Hydraulic bulging

[0100] The blank holder 9 moves downward along the hydraulic bulging direction F, and after touching the die 8, the die 8 and the blank holder 9 clamp the unfolded blank 16, and the fluid is introduced from the water inlet 19 of the blank holder. The fluid medium acts as a punch to apply hydraulic pressure P to the unfolded blank 16, causing the plate to undergo plastic deformation and form a part against the die cavity. The hydraulic pressure is adjusted by the overflow valve, and hydraulic bulging is performed according to the shape of the die 8. The unit blank holder force setting range is 8MPa to 9MPa.

[0101] Step 6: Trim the shape

[0102] Cutting is completed according to the fairing skin contour line 20 on the hydraulic bulging contour surface 10 of the concave die 8, so as to obtain a fairing skin part whose contour accuracy and surface quality meet the requirements of the drawing.

[0103] It should be noted that the present invention is suitable for aluminum skins with opposite overall forming curvatures, complex cross-sections, and an initial overall structure within the range of an open structure; after completing the hydraulic bulging die structure design and unfolding blank calculation, the hydraulic bulging effect can be predicted through hydraulic bulging finite element simulation, and the ideal variation range of the material thickness of the skin is within 8%δ~18%δ. When the material thickness variation range is greater than 18%δ, the hydraulic bulging die structure can be optimized by adjusting the liquid pressure P and adjusting the transition surface curvature; when hydraulic bulging skin parts, the unit clamping force can be adjusted according to the actual machine tool conditions.

Claims

1. A method for hydraulic expansion forming of a reverse hyperbolic fairing skin, characterized in that , including the following steps: Step 1, constructing a hydraulic bulging process surface of the fairing skin; Step 2: Determine the size of the unfolded blank; Step 3: Determine the hydraulic bulging die structure of the fairing skin according to the hydraulic bulging process surface; Step 4, laying the skin blank that is larger than the unfolded blank size on the hydraulic bulging die structure for hydraulic bulging; Step 5: Trim the shape.

2. The method for hydraulic expansion forming of a reverse hyperbolic fairing skin according to claim 1, characterized in that: The step 1 constructs the hydraulic bulging process surface of the fairing skin, and the specific process is as follows: Step 1-1: Define the hydroforming direction Take the XY plane in the CATIA environment as the design horizontal plane H, rotate the fairing skin to ensure that the distance h between the two end points P and Q of the fairing skin and the design horizontal plane is equal, thereby reducing the fairing skin forming depth h, and the fairing skin bulging direction F is perpendicular to the design horizontal plane H; Step 1-2: Add the fairing skin Extract the upper surface of the fairing skin, select the vertices A and G of the elliptical notch on the upper surface of the skin, construct the spline AG with the side contour lines on both sides of the notch as tangents, fill the spline AG, the elliptical notch contour line, and the long circular notch contour line to establish the fairing skin surface; Step 1-3: Design the transition extension surface Select two high points A and B of the fairing skin, and establish a reference plane with reference to the ZX plane in the CATIA environment. Set up design planes M and N by offsetting the reference plane by 200 mm in the opposite direction. Extract the contour lines of the two ends of the fairing skin surface W. Design splines on M and N according to the contours of the two ends of the fairing skin surface. Bridge planes M and N and corresponding splines. Stretch the two ends of the fairing skin by 200 mm along the curvature of the bridge surface to establish the transition flange extension surface. Step 1-4: Design the flange extension surface Select a midpoint C at the notch in the middle of the fairing skin, establish a design plane K with reference to the XY plane in the CATIA environment, design a spline curve in K at a distance of 300 mm from the skin contour according to the curvature of the fairing skin contour line, project the spline curve onto the transition flange extension surface to obtain the flange contour line, cut the transition flange extension surface with the flange contour line, and establish the flange extension surface; Step 1-5: Design the transition guide line of the hydraulic bulging process surface Extract the four endpoints D1, D2, D3, and D4 of the fairing skin contour, extract the four points E1, E2, E3, and E4 on the fairing skin surface and fit the tangent lines D1 E1, D2E2, D3E3, and D4E4 with the endpoints D, E, F, and G, construct the points F1, F2, F3, and F4 at the closest distances between the four endpoints of the fairing skin contour on the inner contour line of the flange extension surface, and construct the transition guide lines D1F1, D2F2, D3F3, and D4F4 of the hydraulic bulging process surface according to the four endpoints D1, D2, D3, and D4 of the fairing skin contour, the tangent lines D1 E1, D2E2, D3E3, and D4E4 and the construction points F1, F2, F3, and F4, and the tension of the guide lines is defined as 1; Step 1-6: Design the transition surface Extract the inner contour line of the flange extension surface, project points A and G onto the inner contour line of the flange extension surface to obtain points A1 and G1, cut the inner contour line of the flange extension surface at points A1 and G1, retain the inner contour line A1 G1, bridge the inner contour line A1 G1 and the spline AG, obtain the transition surface Y1 between the flange extension surface and the fairing skin surface, extract the contour lines AA1 and GG1 at both ends of the transition surface Y1, fill the contour line GG1 and the transition guide line D1 F1, the transition guide line D1 F1 and the transition guide line D2F2, the transition guide line D2F2 and the transition guide line D3F3, the transition guide line D3F3 and the transition guide line D4F4, the transition guide line D4F4 and the cross contour line AA1 to obtain transition surfaces Y2, Y3, Y4, Y5, Y6, and join Y1, Y2, Y3, Y4, Y5, Y6 to obtain the transition surface; Step 1-7: Design the hydraulic bulging process surface The fairing skin surface, flange extension surface and transition surface are joined, and the flange extension surface and the transition surface are transitioned with a fillet radius R to obtain a hydraulic bulging process surface.

3. The method for hydraulic expansion forming of a reverse hyperbolic fairing skin according to claim 1, characterized in that: The step 2 described above determines the size of the unfolded blank, and the specific process is as follows: The fairing skin surface is imported into the finite element analysis software, and the unfolded shape of the hydraulic bulging is calculated using the blank back calculation function to obtain the unfolded blank and the outer edge line of the unfolded blank; Determine the size of the unfolded blank based on the outer edge line of the unfolded blank.

4. The method for hydraulic expansion forming of a reverse hyperbolic fairing skin according to claim 3, characterized in that: The step 3 determines the hydraulic bulging die structure of the fairing skin according to the hydraulic bulging process surface, and the specific process is as follows: Step 3-1: Design the die structure The flange extension surface of the hydraulic bulging process surface is extended in all directions within the hydraulic bulging horizontal plane XY, and the expanded hydraulic bulging process surface is used as the die working surface. The outer edge line of the unfolded blank is projected to the corresponding position of the die working surface to obtain the outer contour line of the unfolded blank on the die; the die working surface is adjusted according to the unfolded blank outer contour line; a drainage hole with a diameter of 10 mm is set on the die; Step 3-2: Design the blank holder structure Generate the blank holder working surface according to the flange extension surface, transition surface and gap; Gap g = (0.95-1.0) × δ; The outer edge line of the unfolded blank is projected to the corresponding position of the working surface of the blank holder ring to obtain the outer contour line of the unfolded blank on the blank holder ring; the working surface of the blank holder ring is adjusted according to the outer contour line of the unfolded blank; a water inlet hole with a diameter of 10 mm is provided on the blank holder ring; Where δ is the material thickness of the fairing skin.

5. The method for hydraulic expansion forming of a reverse hyperbolic fairing skin according to claim 4, characterized in that: Adjust the die working surface according to the outer contour of the unfolded blank, including: Determine whether the distance between the outer edge line of the unfolded blank and the outer contour of the working surface of the die and the blank holder is relatively uniform, and the distance between them is ≥80mm. If not, adjust the working surface of the die and the blank holder.

6. The method for hydraulic expansion forming of a reverse hyperbolic fairing skin according to claim 1, characterized in that: In step 4, the skin blank larger than the unfolded blank size is laid on the hydraulic bulging die structure for hydraulic bulging. The specific process is as follows: Step 4-1: Install the hydraulic bulging die Install the die and blank holder on the working table of the hydraulic bulging machine. The die is fixed to the bed of the hydraulic bulging machine. The hydraulic bulging outer surface of the die corresponds to the working surface of the blank holder. The blank holder can move up and down through the movable ejector. Lubricating oil is applied to the forming working surfaces of the die and blank holder. Step 4-2: Expand the blank and position it Affix a plastic film to protect the upper surface of the unfolded blank, and place it on the working surface of the die according to the outer contour of the unfolded blank on the die. Apply lubricating oil to the corresponding area of ​​the lower surface of the unfolded blank and the working surface of the die; Step 4-3 Hydraulic bulging The blank holder moves downward along the hydraulic bulging direction F, and after touching the die, it clamps the expanded blank with the die, and fluid is introduced from the water inlet hole of the blank holder. The fluid medium acts as a punch to apply hydraulic pressure P to the expanded blank, causing the expanded blank to undergo plastic deformation and form a part against the die cavity.

7. The method for hydraulic expansion forming of a reverse hyperbolic fairing skin according to claim 6, characterized in that: The hydraulic pressure is adjusted by the overflow valve, and hydraulic bulging is performed according to the shape of the die. The unit blank holding force setting range is 8MPa to 9MPa.

8. The method for hydraulic expansion forming of a reverse hyperbolic fairing skin according to claim 1, characterized in that: The step 5 described is to trim the outer fairing skin parts, and the specific process is as follows: The cutting is completed according to the fairing skin contour line on the hydraulic bulging contour surface of the concave die, and the fairing skin parts with contour accuracy and surface quality meeting the requirements of the drawing are obtained.

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