A method for controlling the instability of deep drawing of skin with small rotation angle, narrow taper and variable curvature

Abstract

The present invention provides a method for controlling instability during deep drawing of a small-angle, narrow-tapered variable-curvature skin. The method primarily comprises the following steps: constructing a variable-curvature skin drawing process profile; determining the dimensions of the unfolded blank; designing the drawing die structure for the variable-curvature skin; and performing deep drawing. Based on the variable-curvature skin, the present invention designs a hemispherical closed structure at the end, further constructs the variable-curvature skin drawing process profile, and thereby determines the drawing die structure. This method effectively controls the material flow rate and volume, thereby avoiding defects such as wrinkling, cracking, and springback, achieving uniform part deformation, and significantly improving the dimensional accuracy and surface quality of the formed variable-curvature skin.

Description

Technical Field

[0001] The invention relates to sheet metal parts forming technology in the field of aircraft manufacturing, in particular to a method for controlling deep drawing instability of a small-angle, narrow-cone, variable-curvature skin. Background Art

[0002] Variable curvature skin is an important component of the aircraft. In the aircraft structure, it not only bears the role of maintaining the aerodynamic shape and reducing air resistance, but also needs to withstand various loads and stresses during flight. Therefore, its manufacturing precision and quality requirements are extremely high. The shape of the variable curvature skin is usually more complex and has certain special designs. One type of small-angle, narrow-cone variable curvature skin has an overall shape extending from the root to the tip, forming a gradually narrowing cone with a sharp tip. The transition angle is relatively small, resulting in uneven and unpredictable flow of the material during the forming process. For a long time, the traditional drop forming process with a high degree of manual intervention has been used. During the forming process, defects such as cracking, wrinkling, thinning, and rebound are prone to occur. In addition, the production efficiency is low, the risk of internal damage is large, and the scrap rate is high, which seriously affects the forming precision and surface quality of this type of skin and cannot meet the high quality and high stability requirements of modern aviation manufacturing.

[0003] In order to solve the problems of uneven material flow, poor surface quality and low tire attachment accuracy in the forming of small-angle, narrow-cone variable curvature skins, it is urgent to improve the forming process methods and mold structures to improve the manufacturing accuracy and surface quality of variable curvature skins and meet the high quality and high stability requirements of modern aircraft parts production. Summary of the Invention

[0004] In order to solve the problems of unstable quality and uncontrollable deformation of long-term drop forming of small-angle, narrow-cone variable curvature skins, easy instability and wrinkling, fatigue cracking during the forming process, low precision of variable curvature skin tire attachment, and difficulty in meeting assembly requirements, the purpose of the present invention is to provide a method for controlling deep drawing instability of small-angle, narrow-cone variable curvature skins.

[0005] The present invention provides a method for controlling the instability of deep drawing of a skin with a small rotation angle and narrow tapered curvature, comprising the following steps:

[0006] Step 1: constructing a variable curvature skin drawing process profile;

[0007] Step 2: Determine the size of the unfolded blank;

[0008] Step 3: Design the drawing die structure of the variable curvature skin according to the variable curvature skin drawing process profile;

[0009] Step 4: Deep drawing;

[0010] Step 5: Cut the shape.

[0011] Optionally, the step 1 is to construct a variable curvature skin drawing process profile, and the specific process is as follows:

[0012] In the CATIA environment, the drawing process surface is designed based on the variable curvature skin surface;

[0013] Step 1-1: Construct auxiliary horizontal plane α

[0014] An auxiliary horizontal plane α is established with the endpoints A, B, C, and D of the variable curvature skin; the drawing direction F of the variable curvature skin is perpendicular to the auxiliary horizontal plane α;

[0015] Step 1-2: Create the initial work surface

[0016] The gradual concave cavity and outer flange of the variable curvature skin are extracted separately, excluding the transition corner area; the gradual concave cavity is extended outward along the curvature to obtain the transition extension surface, and the outer flange is extended inward and outward to obtain the transition flange surface; the transition extension surface and the transition flange surface intersect at the conical curve L1, and the conical curve L1 is used to cut the transition extension surface and the transition flange surface to obtain the initial working surface;

[0017] Step 1-3: Design the hemispherical closure at the end

[0018] Extract the contour line of the opening of the variable curvature skin end, and extend it 15mm to 20mm outward along the curvature of the variable curvature skin to obtain the transition margin area; create a spline L2 with points E and N as control points and AN and BE as tangent directions; take the midpoint F of the spline L2 and the contour arc of the variable curvature skin end Midpoint G, establish guide line L3 with midpoints F and G as control points; through The spline L2 and guide line L3 create a multi-section surface, and finally obtain a hemispherical closed body;

[0019] Step 1-4: Design the drawing process surface

[0020] The filling function is used to fill the gap between the outer edge flange and the hemispherical closed body to obtain the annular flange surface, the hemispherical closed body and the annular flange surface are spliced ​​into a whole, and a variable fillet transition is performed to obtain a closed extension body; the fillet radius of the initial working surface is transitioned according to the variable curvature skin corner radius R = 3.8mm and then combined with the closed extension body to obtain the initial process surface; the flange outer contour line L4 of the initial process surface is extracted and extended outward by 90mm along the flange curvature direction to obtain the initial flange surface; the initial process surface and the initial flange surface are combined to form a variable curvature skin drawing process surface.

[0021] Optionally, the step 2 of determining the size of the unfolded blank is as follows:

[0022] Step 2-1: Import the variable curvature skin surface into the finite element analysis software, and use the blank inverse calculation function to calculate the unfolded blank size and the unfolded outline of the variable curvature skin.

[0023] Optionally, the step 3 is to design a drawing die structure for the variable curvature skin, and the specific process is as follows:

[0024] Step 3-1: Design the die structure

[0025] The initial flange surface of the drawing process profile is extended outward along the curvature, and the extended drawing process profile is used as the concave model surface; the shape of the outer contour surface of the die is adapted to the concave model surface, the outer contour surface of the die can entirely contain the drawing process profile, the outer contour surface of the die is relatively uniformly spaced from the outer contour line of the initial flange surface, and the distance between the two is ≥50mm, and a variable curvature skin outline is drawn on the outer contour surface of the die;

[0026] Step 3-2: Design the blank holder structure

[0027] The outer contour surface of the die is extracted as the working surface of the blank holder. Three rolling ribs are set on the working surface of the blank holder: one V-shaped rolling rib and two arc-shaped rolling ribs to control the material flow and ensure uniform deformation of the raw material. The center line of the rolling rib matches the shape of the outer contour line of the flange of the initial process profile. The center line of the arc-shaped rolling rib is offset 15mm to 18mm outward from the outer contour line of the annular flange surface, and then evenly divided into three equal parts. The middle part is removed and the arcs at both ends are retained.

[0028] A splitting horizontal plane β is established through the midpoint G, and the splitting horizontal plane β is perpendicular to the auxiliary horizontal plane α. The center line of the V-shaped rolled bead is offset outward by 15mm to 18mm from the outer edge flange contour line of the variable curvature skin, and the redundant line segments at both ends are removed using the splitting horizontal plane β. The cross-sectional dimensions of the V-shaped rolled bead and the arc-shaped rolled bead are both semicircular, but the dimensions are different. The V-shaped rolled bead is distributed around the gradually changing deep cavity of the variable curvature skin, where the external curvature changes significantly. The fillet radius R1 of the V-shaped rolled bead is 3.5mm, and the height H1 is 6mm to 7mm. Two arc-shaped rolled bead are distributed around the closed extension body, with a fillet radius R2 = 4.5mm and a height H2 = 5mm.

[0029] Step 3-3: Design the calendering ribs of the die

[0030] The die's calendering ribs are distributed on the initial flange surface of the die's outer contour. The die's calendering rib structure matches the blank holder's calendering ribs, and their shapes and sizes correspond to each other.

[0031] Step 3-4: Design the punch structure

[0032] The punch structure is designed based on the outer contour surface of the die, and the shape of the punch working surface is adapted to the inner shape of the drawing process surface.

[0033] Optionally, the gap between the punch working surface and the outer contour surface of the die is i=(1.0-1.1)δ;

[0034] δ is the material thickness of the variable curvature skin.

[0035] Optionally, the step 4 of deep drawing is performed as follows:

[0036] Step 4-1: Install the drawing die

[0037] Install the die on the work surface of the drawing machine, and install the punch and blank holder on the lower work surface of the drawing machine, ensuring the matching accuracy between the blank holder and the die; ensure the parallelism and perpendicularity of the die and the work surface, and install them securely; use the adjustment device of the press to adjust the closing height of the die; apply an appropriate amount of lubricant to the contact surface between the die and the blank;

[0038] Step 4-2: Expand the blank and position it

[0039] Use a clean cotton cloth to clean the unrolled blank to be formed, and lay a plastic film evenly on the surface of the unrolled blank; clean the effective molding surface and the effective pressing surface of the mold again, and apply an appropriate amount of lubricant; position the unrolled blank on the blank holder;

[0040] Step 4-3: Deep drawing

[0041] The die descends along the drawing direction F until it closes with the blank holder, then continues downwards while holding the expanded blank with the blank holder. The expanded blank is subjected to a certain blank holder force. Under the action of the punch, the expanded blank gradually deforms and enters the cavity of the die. During the drawing process, the presence of V-shaped and arc-shaped ribs increases the resistance to material flow, preventing wrinkles and cracks during forming. When the expanded blank is completely inserted into the gap between the die and punch, the drawing is completed, and the initial process part is obtained after removing the plastic film.

[0042] Step 4-4: Heat treatment

[0043] Solution heat treatment is performed on the initial process parts according to the requirements of the digital model to improve their strength and hardness;

[0044] Step 4-5: Reshaping after heat treatment

[0045] During the heat treatment process, the deformed initial process parts are corrected by drawing equipment or manual shaping to make them fit perfectly with the die.

[0046] Optionally, the specific process of cutting the shape in step 5 is as follows:

[0047] The initial process parts are cut and sanded according to the variable curvature skin outline on the outer contour surface of the die, and finally the variable curvature skin parts that meet the requirements of the design digital model are obtained.

[0048] Optionally, the variable fillet radius Rf gradually increases from both ends to the middle, and the gradient value range is Rf = 3.8 mm to 25 mm.

[0049] The beneficial effects of the present invention are as follows: the present invention provides a method for controlling the instability of deep drawing of a skin with a small turning angle and a narrow taper and variable curvature. A hemispherical closed structure of the end is designed based on the variable curvature skin, which increases the plastic deformation area of ​​the variable curvature skin during the deep drawing process, balances the material flow, and avoids transition accumulation or insufficiency in local areas. V-shaped calendering ribs and circular arc calendering ribs are designed according to the principle of being perpendicular to the flow stress of the raw material, which effectively controls the flow speed and flow amount of the material, thereby avoiding the occurrence of defects such as wrinkling, cracking and rebound, and achieving uniform deformation of parts. The present invention provides a method for deep drawing of a skin with a narrow taper and variable curvature with a relatively small transition angle, which replaces the traditional drop forming method, can make the material better fit the shape of the mold, and greatly improves the dimensional accuracy and surface quality of the variable curvature skin after forming. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a schematic diagram of the variable curvature skin structure;

[0051] Figure 2 This is a schematic diagram of the initial working surface of the variable curvature skin;

[0052] Figure 3 This is a schematic diagram of the initial process profile of the variable curvature skin;

[0053] Figure 4 This is a schematic diagram of the variable curvature skin drawing process profile;

[0054] Figure 5 This is a schematic diagram of the variable curvature skin die structure;

[0055] Figure 6 This is a schematic diagram of the variable curvature skin pressure ring structure;

[0056] Figure 7 This is a schematic diagram of the variable curvature skin drawing die structure;

[0057] Description of reference numerals:

[0058] 1 variable curvature skin, 2 variable curvature skin outer surface, 3 gradual concave cavity, 4 outer edge flange, 5 transition extension surface, 6 transition flange surface, 7 initial working surface, 8 transition allowance area, 9 end hemispherical closed body, 10 annular flange surface, 11 closed extension body, 12 initial process surface, 13 initial flange surface, 14 drawing process surface, 15 unfolded blank, 16 concave model surface, 17 die outer contour, 18 die, 19 V-shaped rolling rib, 20 arc-shaped rolling rib, 21 pressure ring, 22 rolling concave rib, 23 punch, 24 variable curvature skin outer line, L1 conical curve, L2 spline, L3 guide line, L4 initial process surface flange outer contour line, L5 rough material unfolded outer contour line, L6 initial flange surface outer contour line. DETAILED DESCRIPTION

[0059] The present application is described in further detail below with reference to the accompanying drawings of the embodiments.

[0060] The present invention provides a method for controlling instability in deep drawing of a skin with a small turning angle and narrow taper and variable curvature. The drawing die consists of a concave die, a punch, and a blank holder. Using CATIA 3D design software, a structural model of the drawing die is established based on a digital model of the variable curvature skin product. The forming method includes the following steps:

[0061] Step 1: Construct variable curvature skin drawing process surface

[0062] In the CATIA environment, the drawing process surface is designed based on the variable curvature skin surface;

[0063] Step 1-1: Construct auxiliary horizontal plane α

[0064] An auxiliary horizontal plane α is established with the endpoints A, B, C, and D of the variable curvature skin; the drawing direction F of the variable curvature skin is perpendicular to the auxiliary horizontal plane α;

[0065] Step 1-2: Create the initial work surface

[0066] Extract the gradual concave cavity and outer flange of the variable curvature skin separately, excluding the transition corner area. Extend the gradual concave cavity outward along the curvature to obtain the transition extension surface, and extend the outer flange inward and outward to obtain the transition flange surface. The transition extension surface and the transition flange surface intersect at the conical curve L1. Use the conical curve L1 to cut the transition extension surface and the transition flange surface to obtain the initial working surface.

[0067] Step 1-3: Design the hemispherical closure at the end

[0068] Extract the contour line of the opening of the variable curvature skin end, and extend it 15mm to 20mm outward along the curvature of the variable curvature skin to obtain the transition margin area. Create a spline L2 with points E and N as control points and AN and BE as tangent directions; take the midpoint F of the spline L2 and the contour arc of the variable curvature skin end Midpoint G, establish guide line L3 with midpoints F and G as control points; through The spline L2 and the guide line L3 create a multi-section surface, and finally a hemispherical closed body is obtained.

[0069] Step 1-4: Design the drawing process surface

[0070] Use the filling function to fill the gap between the outer flange and the hemispherical closed body to obtain the annular flange surface, splice the hemispherical closed body and the annular flange surface into a whole, and perform variable fillet transition to obtain a closed extension body. Variable fillet radius R f The gradient value range is R f =3.8mm~25mm. The fillet radius of the initial working surface is transitioned to the variable curvature skin corner radius R = 3.8mm and then combined with the closed extension to form the initial process profile. The flange outer contour line L4 of the initial process profile is extracted and extended outward 90mm along the flange curvature direction to form the initial flange surface. The initial process profile and the initial flange surface are combined to form the variable curvature skin deep drawing process profile.

[0071] Step 2: Determine the size of the unfolded blank

[0072] Step 2-1: Import the variable curvature skin surface into the finite element analysis software, and use the blank inverse calculation function to calculate the unfolded blank size and the unfolded outline of the variable curvature skin.

[0073] Step 3: Design the drawing die structure of the variable curvature skin

[0074] The drawing die structure is designed based on the drawing process profile of the variable curvature skin and the unfolded blank, including the die, punch and blank holder, a total of three parts.

[0075] Step 3-1: Design the die structure

[0076] The initial flange surface of the drawing process profile is extended outward along the curvature, and this extended drawing process profile serves as the concave mold surface. The shape of the die outer contour surface is adapted to the concave mold surface, and its dimensions are designed with reference to the drawing process profile surface. The die outer contour surface should fully encompass the drawing process profile, and the distance between the die outer contour surface and the initial flange surface outer contour line should be relatively uniform and ≥50mm, ensuring the mold has sufficient strength and rigidity to withstand the various loads during the forming process. The variable curvature skin outline is drawn on the die outer contour surface.

[0077] Step 3-2: Design the blank holder structure

[0078] The outer contour surface of the die is extracted as the working surface of the blank holder. There are three ribs on the working surface of the blank holder: one V-shaped rib and two arc-shaped ribs, which control the material flow and ensure uniform deformation of the raw material. The center line of the rib matches the shape of the outer contour line of the flange of the initial process profile. The center line of the arc-shaped rib is offset outward by 15mm to 18mm from the outer contour line of the annular flange surface, and then evenly divided into three equal parts. The middle part is removed and the arcs at both ends are retained. A splitting horizontal plane β is established through the endpoint G, and the splitting horizontal plane β is perpendicular to the auxiliary horizontal plane α. The center line of the V-shaped rib is offset outward by 15mm to 18mm from the outer edge flange contour line of the variable curvature skin, and the redundant line segments at both ends are removed using the splitting horizontal plane β. The cross-sectional dimensions of the V-shaped rib and the arc-shaped rib are both semicircular, but the dimensions are different. V-shaped ribs are distributed around the gradually deepening cavity of the variable curvature skin, where the curvature of the outer shape changes significantly. The V-shaped ribs have a fillet radius R1 of 3.5mm and a height H1 of 6mm to 7mm. Two arc-shaped ribs are distributed around the closed extension body, with a fillet radius R2 of 4.5mm and a height H2 of 5mm.

[0079] Step 3-3: Design the calendering ribs of the die

[0080] The die's calendering ribs are distributed on the initial flange surface of the die's outer contour. The die's calendering rib structure matches the blank holder's calendering ribs. Their shapes and sizes correspond to each other, and an appropriate gap is maintained to achieve good material flow.

[0081] Step 3-4: Design the punch structure

[0082] The punch structure is designed based on the outer contour of the die. The shape of the die's working surface is adapted to the inner shape of the drawing process surface. The clearance between the punch's working surface and the outer contour of the die is denoted as i. The variable curvature skin is made of aluminum alloy and requires high manufacturing precision. Using a drawing gap i = (1.0-1.1)δ ensures uniform wall thickness, thereby improving surface quality and dimensional accuracy.

[0083] Step 4: Deep drawing;

[0084] Step 4-1: Install the drawing die

[0085] Install the die on the work surface of the drawing machine, and the punch and blank holder on the lower work surface of the drawing machine, ensuring the precise fit between the blank holder and the die. Ensure the parallelism and perpendicularity of the die and punch to the work surface, and ensure they are securely mounted. Use the press's adjustment device to adjust the die's closing height. Apply an appropriate amount of lubricant to the contact surface between the die and the workpiece to reduce friction and improve production efficiency.

[0086] Step 4-2: Expand the blank and position it

[0087] Clean the unrolled blank with a clean cotton cloth and evenly lay plastic film on the surface of the unrolled blank. Clean the effective molding surface and the effective pressing surface of the mold again and apply an appropriate amount of lubricant. Position the unrolled blank on the blank holder.

[0088] Step 4-3: Deep drawing

[0089] The die descends in the drawing direction F until it closes with the blank holder. The die then continues downward, clamping the expanded blank with the blank holder. Under the action of the punch, the expanded blank gradually deforms and enters the cavity of the die. During the drawing process, the presence of V-shaped and arc-shaped ribs increases resistance to material flow, preventing wrinkling and cracking during forming. Drawing concludes when the expanded blank fully enters the gap between the punch and the die, and the plastic film is removed to obtain the initial part.

[0090] Step 4-4: Heat treatment

[0091] According to the requirements of the digital model, the initial process parts are subjected to solution heat treatment to improve their strength and hardness. Taking advantage of the good plasticity of the material during the aging period, subsequent shape trimming and cutting are carried out.

[0092] Step 4-5: Reshaping after heat treatment

[0093] During the heat treatment process, the initial workpiece may be deformed due to thermal stress and structural changes. The deformed initial workpiece is corrected by deep drawing equipment or manual shaping to make it fit perfectly with the die.

[0094] Step 5: Cutting the Outline

[0095] The initial process parts are cut and sanded according to the variable curvature skin outline on the outer contour surface of the die, and finally the variable curvature skin parts that meet the requirements of the design digital model are obtained.

[0096] 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 prominent features of the small-angle, narrow-cone variable-curvature skin are: the overall shape is saddle-shaped, with a smooth and continuous surface; when viewed from the side, the distance between the highest point and the bottom of the skin is about 130mm, presenting an asymmetric shape; the transition curve from the root to the tip is relatively gentle, the corner radius is relatively small, about 3.8mm; and the end opening size is close to 270mm. Due to the complex structural characteristics of this type of variable-curvature skin, there are significant differences in the flow speed and flow volume of the material in different areas. In the narrow conical area, the material flow speed gradually increases and the flow volume gradually decreases from the wide end to the narrow end; the presence of a small angle makes the material flow relatively slow, which is prone to stress concentration.

[0097] The variable curvature skin 1 has an overall saddle-shaped shape with a relatively small transition angle, resulting in uneven and unpredictable material flow during the forming process. Variable curvature skin 1 has long been processed using a step-down press forming process, which is prone to defects such as cracking, wrinkling, thinning, and springback. This process also results in low production efficiency, a high risk of internal damage, and a high scrap rate, severely impacting the forming accuracy and surface quality of this type of skin.

[0098] like Figures 1 to 7 As shown, a method for controlling the instability of deep drawing of a skin with a small rotation angle and narrow tapered curvature includes the following steps:

[0099] Step 1: Constructing the variable curvature skin drawing process profile 14

[0100] In the CATIA environment, the drawing process surface is designed based on the variable curvature skin outer surface 2;

[0101] Step 1-1: Construct auxiliary horizontal plane α

[0102] An auxiliary horizontal plane α is established with the endpoints A, B, C, and D of the variable curvature skin 1; the drawing direction F of the variable curvature skin 1 is perpendicular to the auxiliary horizontal plane α;

[0103] Step 1-2: Create the initial work surface 7

[0104] Extract the gradual concave cavity 3 and outer flange 4 of the variable curvature skin 1, excluding the transition corner area. Extend the gradual concave cavity 3 outward along the curvature to obtain a transition extension surface 5. Extend the outer flange 4 inward and outward to obtain a transition flange surface 6. Transition extension surface 5 and transition flange surface 6 intersect at conical curve L1. Use conical curve L1 to cut transition extension surface 5 and transition flange surface 6 to obtain initial working surface 7.

[0105] Step 1-3: Design the end hemispherical closure 9

[0106] Extract the contour line of the opening of the variable curvature skin 1 and extend it 15mm to 20mm outward along the curvature of the variable curvature skin to obtain the transition margin area 8. Create a spline L2 with points E and N as control points and AN and BE as tangent directions; take the midpoint F of the spline L2 and the contour arc of the variable curvature skin 1 end The midpoint G, with the midpoints F and G as control points, establish the guide line L3. The spline L2 and the guide line L3 create a multi-section surface, and finally a hemispherical closed body 9 is obtained.

[0107] Step 1-4: Design the drawing process surface 14

[0108] The gap between the outer flange 4 and the hemispherical closed body 9 is filled with a filling function to obtain an annular flange surface 10. The hemispherical closed body 9 and the annular flange surface 10 are spliced ​​into a whole, and a variable fillet transition is performed to obtain a closed extension body 11. Variable fillet radius R f The gradient value range is R f =3.8mm-25mm. The fillet radius of the initial working surface 7 is transitioned to the corner radius R = 3.8mm of the variable curvature skin 1 and then combined with the closed extension 11 to form the initial process profile 12. The flange outer contour line L4 of the initial process profile 12 is extracted and extended outward 90mm along the flange curvature direction to form the initial flange surface 13. The initial process profile 12 and the initial flange surface 13 are combined to form the variable curvature skin 1 deep drawing process profile 14.

[0109] Step 2: Determine the size of the unfolded blank 15

[0110] Step 2-1: Import the variable curvature skin 1 profile into the finite element analysis software, and use the blank inverse calculation function to calculate the size of the unfolded blank 15 and the unfolded outline L5 of the variable curvature skin 1;

[0111] Step 3: Design the drawing die structure of the variable curvature skin 1

[0112] The drawing die structure is designed based on the drawing process profile 14 of the variable curvature skin 1 and the unfolded blank 15, including the die 18, the punch 23 and the blank holder 21, a total of three parts.

[0113] Step 3-1: Design the structure of the die 18

[0114] The initial flange surface 13 of the drawing process profile 14 is extended outward along the curvature, and this extended drawing process profile 14 serves as the concave mold surface 16. The shape of the die outer contour surface 17 matches the concave mold surface 16, and the die outer contour surface 17 completely encompasses the drawing process profile 14. The distance between the die outer contour surface 17 and the outer contour line L6 of the initial flange surface 13 is relatively uniform, and the distance between the two is ≥ 50 mm, ensuring that the mold has sufficient strength and rigidity to withstand the various loads during the forming process. The variable curvature skin outline 24 is drawn on the die outer contour surface 17.

[0115] Step 3-2: Design the structure of the blank holder 21

[0116] The outer contour surface 17 of the die is extracted as the working surface of the blank holder 21. Three ribs are provided on the working surface of the blank holder 21: one V-shaped rib 19 and two arc-shaped ribs 20. These ribs control material flow and ensure uniform deformation of the blank. The centerline of the rib matches the shape of the flange outer contour line L4 of the initial process profile 12. The centerline of the arc-shaped rib 20 is offset 15mm to 18mm outward from the outer contour line of the annular flange surface 10, then evenly divided into three equal parts, removing the middle portion and retaining the arcs at both ends. A splitting horizontal plane β is established through the endpoint G, and the splitting horizontal plane β is perpendicular to the auxiliary horizontal plane α. The centerline of the V-shaped rib 19 is offset 15mm to 18mm outward from the outer edge flange 4 contour line of the variable curvature skin 1, and then the redundant line segments at both ends are removed using the cutting horizontal plane β. The cross-sectional dimensions of the V-shaped rib 19 and the arc-shaped rib 20 are both semicircular, but the dimensions are different. V-shaped rolled ribs 19 are distributed around the gradually deepening cavity 3 of the variable-curvature skin 1, where the curvature of the outer shape changes significantly. The V-shaped rolled ribs 19 have a fillet radius R1 of 3.5 mm and a height H1 of 6 to 7 mm. Two arc-shaped rolled ribs 20 are distributed around the closed extension 11, with a fillet radius R2 of 4.5 mm and a height H2 of 5 mm.

[0117] Step 3-3: Design the calendering rib 22 of the die 18

[0118] The rolled ribs 22 of the die 18 are distributed on the initial flange surface 13 of the die outer contour surface 17. The rolled ribs 22 of the die 18 match the rolled ribs of the blank holder 21. The shapes and sizes of the two correspond to each other, and an appropriate gap is maintained to achieve good material flow.

[0119] Step 3-4: Design the structure of the punch 23

[0120] The punch 23 is designed based on the die's outer contour 17. The working surface of the die 18 is shaped to match the inner shape of the drawing process surface 14. The clearance between the punch 23's working surface and the die's outer contour 17 is denoted as i. The variable curvature skin 1 is made of aluminum alloy and requires high manufacturing precision. Using a drawing clearance i = (1.0-1.1)δ ensures a uniform wall thickness for the variable curvature skin 1, thereby improving its surface quality and dimensional accuracy.

[0121] Step 4: Deep drawing;

[0122] Step 4-1: Install the drawing die

[0123] Install the die 18 on the work surface of the drawing machine, and the punch 23 and blank holder 21 on the lower work surface of the drawing machine, ensuring the precision of the fit between the blank holder 21 and the die 23. Ensure that the die 18 and the punch 23 are parallel and perpendicular to the work surface, and that they are securely installed. Use the adjustment device of the press to adjust the closing height of the die. Apply an appropriate amount of lubricant to the contact surface between the die and the unfolded blank to reduce friction and improve production efficiency.

[0124] Step 4-2: Expand the blank 15 and position it

[0125] Use a clean cotton cloth to clean the unrolled blank 15 to be formed, and lay a plastic film evenly on the surface of the unrolled blank 15. Clean the effective molding surface and the effective pressing surface of the mold again, and apply an appropriate amount of lubricant. Position the unrolled blank 15 on the blank holder 21.

[0126] Step 4-3: Deep drawing

[0127] The die 18 descends along the drawing direction F until it closes with the blank holder 21. It then continues downward, clamping the expanded blank 15 with the blank holder 21. Under the action of the punch 23, the expanded blank 15 gradually deforms and enters the cavity of the die 18. During the drawing process, the presence of the V-shaped ribs 19 and the arc-shaped ribs 20 increases resistance to material flow, preventing wrinkling and cracking during forming. Drawing is complete when the expanded blank 15 fully enters the gap between the die 18 and the punch 23. After removing the plastic film, the initial part is obtained.

[0128] Step 4-4: Heat treatment

[0129] According to the requirements of the digital model, the initial process parts are subjected to solution heat treatment to improve their strength and hardness. Taking advantage of the good plasticity of the material during the aging period, subsequent shape trimming and cutting are carried out.

[0130] Step 4-5: Reshaping after heat treatment

[0131] During the heat treatment process, the initial process part may be deformed due to thermal stress and structural transformation. The deformed initial process part is corrected by drawing equipment or manual shaping to make it fit completely with the die 18.

[0132] Step 5: Cutting the Outline

[0133] The initial process part is cut and sanded according to the variable curvature skin outline 24 on the outer contour surface 17 of the die, and finally a variable curvature skin 1 part that meets the design digital model requirements is obtained.

Claims

1. A method for controlling the instability of deep drawing of a skin with a small turning angle and narrow tapered curvature, characterized in that: The following steps are involved: Step 1: constructing a variable curvature skin drawing process profile; Step 2: Determine the size of the unfolded blank; Step 3: Design the drawing die structure of the variable curvature skin according to the variable curvature skin drawing process profile; Step 4: Deep drawing; Step 5: Cut the shape; The step 1 constructs a variable curvature skin drawing process profile, and the specific process is as follows: In the CATIA environment, the drawing process surface is designed based on the variable curvature skin surface; Step 1-1: Construct the auxiliary horizontal plane α An auxiliary horizontal plane α is established with the endpoints A, B, C, and D of the variable curvature skin; the drawing direction F of the variable curvature skin is perpendicular to the auxiliary horizontal plane α; Step 1-2: Create the initial work surface The gradual concave cavity and outer flange of the variable curvature skin are extracted separately, excluding the transition corner area; the gradual concave cavity is extended outward along the curvature to obtain the transition extension surface, and the outer flange is extended inward and outward to obtain the transition flange surface; the transition extension surface and the transition flange surface intersect at the conical curve L1, and the conical curve L1 is used to cut the transition extension surface and the transition flange surface to obtain the initial working surface; Step 1-3: Design the hemispherical end closure Extract the contour line of the opening of the variable curvature skin end, and extend it 15mm to 20mm outward along the curvature of the variable curvature skin to obtain the transition margin area; create a spline L2 with points E and N as control points and AN and BE as tangent directions; take the midpoint F of the spline L2 and the contour arc of the variable curvature skin end Midpoint G, establish guide line L3 with midpoints F and G as control points; through , spline L2 and guide line L3 create a multi-section surface, and finally obtain a hemispherical closed body; Step 1-4: Design the drawing process surface The gap between the outer flange and the hemispherical closed body is completed using the fill function to obtain an annular flange surface. The hemispherical closed body and the annular flange surface are spliced ​​into a whole, and a variable fillet transition is performed to obtain a closed extension body. The fillet radius of the initial working surface is transitioned according to the variable curvature skin corner radius R = 3.8mm, and then combined with the closed extension body to obtain the initial process surface. The flange outer contour line L4 of the initial process surface is extracted and extended outward 90mm along the flange curvature direction to obtain the initial flange surface. The initial process surface and the initial flange surface are combined to form the variable curvature skin deep drawing process surface. The step 3 is to design the drawing die structure of the variable curvature skin. The specific process is as follows: Step 3-1: Design the die structure The initial flange surface of the drawing process profile is extended outward along the curvature, and the extended drawing process profile is used as the concave model surface; The shape of the outer contour surface of the die is adapted to the concave model surface. The outer contour surface of the die can include the drawing process surface as a whole. The distance between the outer contour surface of the die and the outer contour line of the initial flange surface is relatively uniform, and the distance between the two is ≥50mm. The outer contour line of the variable curvature skin is drawn on the outer contour surface of the die. Step 3-2: Design the blank holder structure The outer contour surface of the die is extracted as the working surface of the blank holder. Three rolling ribs are set on the working surface of the blank holder: one V-shaped rolling rib and two arc-shaped rolling ribs to control the material flow and ensure uniform deformation of the raw material. The center line of the rolling rib matches the shape of the outer contour line of the flange of the initial process profile. The center line of the arc-shaped rolling rib is offset 15mm to 18mm outward from the outer contour line of the annular flange surface, and then evenly divided into three equal parts. The middle part is removed and the arcs at both ends are retained. A splitting horizontal plane β is established through the midpoint G, and the splitting horizontal plane β is perpendicular to the auxiliary horizontal plane α. The center line of the V-shaped rolled bead is offset outward by 15mm to 18mm from the outer edge flange contour line of the variable curvature skin, and the redundant line segments at both ends are removed using the splitting horizontal plane β. The cross-sectional dimensions of the V-shaped rolled bead and the arc-shaped rolled bead are both semicircular, but the dimensions are different. The V-shaped rolled bead is distributed around the gradually changing deep cavity of the variable curvature skin, where the external curvature changes significantly. The fillet radius R1 of the V-shaped rolled bead is 3.5mm, and the height H1 is 6mm to 7mm. Two arc-shaped rolled bead are distributed around the closed extension body, with a fillet radius R2 = 4.5mm and a height H2 = 5mm. Step 3-3: Design the calendering ribs of the die The die's calendering ribs are distributed on the initial flange surface of the die's outer contour. The die's calendering rib structure matches the blank holder's calendering ribs, and their shapes and sizes correspond to each other. Step 3-4: Design the punch structure The punch structure is designed based on the outer contour surface of the die, and the shape of the punch working surface is adapted to the inner shape of the drawing process surface.

2. The method for controlling instability of deep drawing of a skin with a small turning angle and narrow tapered curvature according to claim 1 is characterized in that: The step 2 above determines the size of the unfolded blank. The specific process is as follows: Step 2-1: Import the variable curvature skin surface into the finite element analysis software, and use the blank inverse calculation function to calculate the unfolded blank size and the unfolded outline of the variable curvature skin.

3. The method for controlling instability of deep drawing of a skin with a small turning angle and narrow tapered curvature according to claim 2 is characterized in that: The gap between the punch working surface and the outer contour surface of the die is i = (1.0 ~ 1.1) δ; δ is the material thickness of the variable curvature skin.

4. The method for controlling instability of deep drawing of a skin with a small turning angle and narrow tapered curvature according to claim 1 is characterized in that: The specific process of step 4 deep drawing is as follows: Step 4-1: Install the drawing die Install the die on the work surface of the drawing machine, and install the punch and blank holder on the lower work surface of the drawing machine, ensuring the matching accuracy between the blank holder and the die; ensure the parallelism and verticality of the die and the work surface, and install them firmly; use the adjustment device of the press to adjust the closing height of the die; Apply an appropriate amount of lubricant on the contact surface between the mold and the blank; Step 4-2: Expand the blank and position it Use a clean cotton cloth to clean the unrolled blank to be formed, and lay a plastic film evenly on the surface of the unrolled blank; clean the effective molding surface and the effective pressing surface of the mold again, and apply an appropriate amount of lubricant; position the unrolled blank on the blank holder; Step 4-3: Deep drawing The die descends along the drawing direction F until it closes with the blank holder, and then continues to descend with the blank held by the blank holder. The expanded blank is subjected to a certain blank holding force. Under the action of the punch, the expanded blank gradually deforms and enters the cavity of the die. During the drawing process, the presence of V-shaped and arc-shaped ribs increases the resistance to material flow, preventing wrinkles and cracks during forming. When the expanded blank completely enters the gap between the die and punch, the drawing is completed, and the initial process part is obtained after removing the plastic film. Step 4-4: Heat treatment Solution heat treatment is performed on the initial process parts according to the requirements of the digital model to improve their strength and hardness; Step 4-5: Reshaping after heat treatment During the heat treatment process, the deformed initial process parts are corrected by drawing equipment or manual shaping to make them fit perfectly with the die.

5. The method for controlling instability of deep drawing of a skin with a small turning angle and narrow tapered curvature according to claim 1 is characterized in that: The specific process of cutting the shape in step 5 is as follows: The initial process parts are cut and sanded according to the variable curvature skin outline on the outer contour surface of the die, and finally a variable curvature skin part that meets the design digital model requirements is obtained.

6. The method for controlling instability of deep drawing of a skin with a small turning angle and narrow tapered curvature according to claim 1 is characterized in that: The variable fillet radius Rf gradually increases from both ends to the middle, and the gradient value range is Rf = 3.8 mm to 25 mm.

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