A drawing forming method for the skin of a large-curvature thin-walled spherical wedge-shaped tail cover

Abstract

The present invention discloses a drawing forming method for a drawn forming of a large curvature thin-walled spherical wedge-shaped tail cover skin, comprising the following steps: constructing a drawing process surface of the lower tail cover skin; determining the size of the developed blank; stepwise designing the structure of the drawing die for the lower tail cover skin; performing the first drawing forming; performing the second drawing and shape correction; trimming the outer shape. According to the hyperbolic structure of the cover skin, the present invention designs a wedge-shaped closed drawing workpiece with a springback closing surface, increasing the plastic deformation area of the material in the drawing forming of the part and solving the springback problem that cannot be controlled in the conventional drawing forming of the open curvature skin.

Description

Technical Field

[0001] The present invention relates to the sheet metal part forming technology in the field of aircraft manufacturing, and specifically to a drawing forming method and a forming die for a thin-walled sheet metal cover skin of a large curvature and spherical semi-wrapped shape. Background Art

[0002] The function of the aircraft skin is to maintain the aircraft's outer shape, making it have good aerodynamic characteristics and reducing flight resistance. Therefore, it is required that the skin material has high strength and good plasticity, and has a smooth outer surface after forming. Complex-shaped skins are mostly seen in parts such as aircraft wing tips, fairings, nose cones, and tail cones. The tail cone skin is located at the tail of the fuselage and has a large curvature and a thin-walled spherical structure. Four semi-wrapped irregular tail cone skins enclose the APU protection cover, just like opening an umbrella, protecting the aircraft auxiliary power unit APU, adjusting the airflow flowing over the surface during flight, and improving the flight ability of the aircraft.

[0003] The tail cone skin is made of hard aluminum alloy, and the material thickness does not exceed 1.2 mm to meet the requirements of aircraft lightweight design. Along the flight direction, the APU protection cover presents a transition from a front rear conical surface. The curvature of the upper and lower tail cone skins changes more rapidly than that of the side wall tail cone skins, and the curvature of the lower tail cone skin changes faster than that of the upper tail cone skin, thus realizing the designed outer shape of the upward contraction of the aircraft tail to prevent the tail from rubbing the ground during aircraft landing. In the past, the tail cone skin structural parts were manufactured by the skin stretching process. However, the spherical characteristics with such a large curvature have increased the drawing difficulty of the upper and lower tail cone skins, and the surface orange peel defects of the material are serious during forming.

[0004] The orange peel defect is a surface roughening phenomenon of metal materials, which not only affects the outer shape 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 the surface roughness of the upper and lower tail cone skins, the stretching amount is often reduced during forming, resulting in serious springback due to insufficient stretching deformation, not meeting the requirement of die fitting degree, reducing the forming accuracy of the parts, and affecting the outer shape and assembly of the aircraft. Due to the long-term bottleneck problems of difficult control of surface and internal damage and extremely unstable product quality in the stretching forming of the upper and lower tail cone skins, the hand trimming and repeated grinding are usually used for the cover skin of the body with orange peel defects to improve its surface morphology during production, and the rejection and scrapping phenomena often occur, bringing great economic losses to the enterprise.

[0005] The upper and lower tail fairing skins are about 1.2 m long, the maximum arch height of the contour is close to 400 mm, the maximum contour taper of the curved surface is about 53.7°, which has stronger wrapping than the 32.1° contour taper of the curved surfaces of the tail fairing skins on both sides. It is impossible to avoid orange peel phenomenon by stretch forming. When using the traditional deep drawing process to form the open upper and lower tail fairing skins, due to the conical contour of the skins, the deformation at the bottom of the curved surface is large, the side walls hardly deform, and material accumulation is likely to occur on the side walls to form wrinkles. Also, due to uneven plastic deformation, when the surplus is removed, the skins rebound severely and cannot reach the dimensions of the design drawings. For the thin-walled, large-arch-height, spherical wedge-shaped upper and lower tail fairing skins, a mechanized precision machining method is urgently needed to achieve non-orange peel, uniform deformation, and high-quality controllable forming of the parts. Summary of the Invention

[0006] In order to solve the problems of severe orange peel on the surface of the large-curvature thin-walled aluminum alloy tail fairing skin during stretch forming and high scrap rate after forming, the purpose of the present invention is to provide a deep drawing forming method for a large-curvature thin-walled spherical wedge-shaped closing tail fairing skin.

[0007] A deep drawing forming method for a large-curvature thin-walled spherical wedge-shaped tail fairing skin includes the following steps:

[0008] Step 1: Construct the deep drawing process surface of the lower tail fairing skin;

[0009] Under the CATIA environment, design the deep drawing process surface based on the outer surface of the lower tail fairing skin, and denote the material thickness of the lower tail fairing skin as δ

[0010] Step 1-1: Establish the design horizontal plane H

[0011] Establish the design horizontal plane H with the four intersection points A, B, C, and D of the lower tail fairing skin contour, so as to reduce the forming depth h of the lower tail fairing skin. The deep drawing direction F of the lower tail fairing skin is perpendicular to the design horizontal plane H;

[0012] Step 1-2: Design the ship-shaped extension

[0013] Extract the contour lines around the lower tail fairing skin, extend them around the curvature of the lower tail fairing skin to obtain the transition extension, establish the transition horizontal plane M based on the design horizontal plane H, translate the design horizontal plane H in the opposite direction of the deep drawing direction F to obtain the transition horizontal plane M. The translation distance d between the design horizontal plane H and the transition horizontal plane M is 15 mm - 20 mm. Cut the transition extension with the transition horizontal plane M, and retain the ship-shaped extension on the side with the lower tail fairing skin;

[0014] Step 1-3: Design the inclination angle θ of the tail inclined plane P

[0015] Establish a tail inclined plane P to form a certain inclination angle θ with the transition horizontal plane M. The value range of the inclination angle θ is 100° to 130°. The selection principle of the inclination angle θ is determined by the forming depth h of the lower tail cover skin. When the forming depth h ≤ 200mm, take the lower limit value 100° of the inclination angle θ. When the forming depth h is in the range of 200mm < h ≤ 500mm, the inclination angle θ is taken according to the formula Take and make;

[0016] Step 1-4: Design a closed extended body

[0017] The position of the tail inclined plane P is indirectly determined by the midpoint E of the bottom contour of the lower tail cover skin. The tail inclined plane P intersects with the ship-shaped extended body to obtain a U-shaped tail contour line l. The bottom midpoint of the tail contour line l is denoted as F, and the length of the line segment EF is denoted as n. Cut the ship-shaped extended body with the tail inclined plane P, retain the drawn extended body on the side with the tail cover skin, close the U-shaped tail contour line l to obtain a tail contour surface, suture the drawn extended body and the tail contour surface, and perform a fillet radius R transition on the tail contour line l to obtain a closed extended body. The value range of R is 80mm to 120mm to ensure a smooth streamline of the surface. The taking principle of the fillet radius R is affected by the proportion of the remaining plane area of the tail contour surface. The remaining plane area of the tail contour surface needs to account for more than two-thirds of the area of the original tail contour surface;

[0018] Step 1-5: Establish a drawing horizontal plane S

[0019] Establish a drawing horizontal plane S based on the transition horizontal plane M. Translate the transition horizontal plane M along the reverse direction of the drawing direction F by a translation distance f = 30mm to obtain the drawing horizontal plane S;

[0020] Step 1-6: Design the drawing closing line of the closed extended body

[0021] Extract the opening contour line of the closed extended body, project it onto the drawing horizontal plane S to obtain an opening projection line. G is any point on the opening contour line, and V is the projection point of G on the opening projection line. The length of the line segment GV is 30mm. Draw a vertical plane K of the drawing horizontal plane S through point G, which intersects with the closed extended body to obtain a side wall section line passing through point G and intersects with the opening contour line at point I. The line segment GI is called the section opening line. Draw a tangent t to the side wall section line through point G, and the tangent t intersects with the section opening line to form a section inclination angle α. Offset the opening projection line equidistantly around in the drawing horizontal plane S to obtain a drawing closing line. U is the projection point of G on the drawing closing line, and the included angle between the line segment GV and the line segment GU is denoted as the closing angle β. The length of the line segment VU is the equidistant offset distance m, and the offset distance m = (30 × tgβ)mm. The value range of the closing angle β is 90° to 98°. The size of the closing angle β is calculated according to the formula Calculation;

[0022] Step 1-7: Design the closing extension body

[0023] Smoothly close the opening contour line and the drawing closing line through the multi-section surface drawing function to obtain the springback closing surface. Make a fillet radius R1 transition between the closed extension body and the springback closing surface to obtain the closing extension body. The value range of R1 is R1 = 20mm to 25mm;

[0024] Step 1-8: Design the drawing process surface

[0025] The drawing closing line is equally offset 35mm outward along the periphery in the drawing horizontal plane S to obtain the process flange outer edge line. The planar ring enclosed by the drawing closing line and the process flange outer edge line is the initial flange surface. Make a fillet radius R f transition, and its appropriate value range is R f =(5 - 8)×δ, and the value-taking principle is related to the depth p of the closing extension body. Take the value according to the formula to obtain the drawing process surface of the lower tail cover skin;

[0026] Step 2 Determine the blank size for unfolding

[0027] Step 2-1: Design the drawing workpiece

[0028] Offset the drawing process surface of the lower tail cover skin in the opposite direction of the drawing direction F, and the offset distance is to obtain the drawing workpiece;

[0029] Step 2-2: Calculate the blank size for unfolding

[0030] Use the blank reverse calculation function of common finite element analysis software to calculate the unfolded shape of the drawing workpiece to obtain the wedge-shaped blank for unfolding and the outer edge line of the blank for unfolding;

[0031] Step 3 Design the structure of the drawing die for the lower tail cover skin

[0032] Design the structure of the drawing die based on the drawing process surface of the lower tail cover skin and the wedge-shaped blank for unfolding, including the punch, the die, and the blank holder

[0033] Step 3-1: Design the die structure

[0034] Extend the initial flange surface of the drawing process surface around in the drawing horizontal plane S, and use the enlarged drawing process surface as the working surface of the female die. Project the outer edge line of the wedge-shaped developed blank onto the corresponding position of the working surface of the female die, and determine whether the design of the outer contour dimensions of the working surface of the female die is reasonable according to it. The design principle is that the distance between the outer edge line of the developed blank and the outer contour of the working surface of the female die remains relatively uniform, and the distance between them is ≥ 80 mm to ensure the manufacturing stiffness of the blank holder and the application balance of the blank holding force. The working surface of the female die consists of three parts: the drawing flange surface, the transition fillet surface, and the drawing outer surface. Among them, the drawing flange surface corresponds to the initial flange surface, the drawing outer surface corresponds to the necking extension body, and the transition fillet surface is the fillet radius R f Transition region;

[0035] Step 3-2: Design the structure of the blank holder

[0036] The working surface of the blank holder matches the drawing flange surface. The outer edge line of the wedge-shaped developed blank is provided on the working surface of the blank holder. At the same time, there are also two drawing beads with U-shaped cross-sections and different sizes, namely the full-type drawing bead and the small-end drawing bead, to realize the uniform deformation of the wedge-shaped developed blank. The overall contour of the full-type drawing bead is similar to the drawing necking line and the outer shape of the developed blank. The overall position of the full-type drawing bead is obtained by offsetting 35 mm to 40 mm equidistantly from the drawing necking line to the periphery. The designed height y1 of the full-type drawing bead is 4 mm to 5 mm. The small-end drawing bead matches the tip shape of the full-type drawing bead and is obtained by offsetting 25 mm to 30 mm equidistantly from the tip area of the full-type drawing bead to the outside. It is only set for the shape of the small end of the developed blank to increase the deformation degree of the small-end developed blank. The designed height y2 of the small-end drawing bead is ≤ 3 mm;

[0037] Step 3-3: Design the drawing grooves of the female die

[0038] Matching the two drawing beads on the working surface of the blank holder, two drawing grooves are provided on the drawing flange surface of the working surface of the female die, corresponding to the U-shaped protrusions of the two drawing beads, namely the full-type drawing groove and the small-end drawing groove, which limit the flow velocity of the developed blank together with the drawing beads and increase the deformation resistance of the developed blank;

[0039] Step 3-4: Design the working clearance between the punch and the female die

[0040] Design the punch based on the drawing outer surface of the female die. The designed clearance between the working surface of the punch and the drawing outer surface of the female die is g, which is calculated according to the formula g = (0.95 - 1.0) × δ. Negative clearance drawing is adopted to improve the forming accuracy of the lower tail cover skin. There are exhaust holes with a diameter of 3 mm on the working surface of the punch;

[0041] Step 4 First drawing forming

[0042] Step 4-1: Install the drawing die

[0043] Install the punch and the blank holder on the workbench surface of the drawing machine tool. Fix the die to the upper bed surface of the drawing machine tool. The drawing outer surface of the die corresponds to the working surface of the punch, and the working surface of the blank holder corresponds to the drawing flange surface of the die. The blank holder can move up and down through the movable ejector rod. The die moves downward through the upper drawing bed surface to achieve drawing forming. Lubricating oil is brushed on the forming working surfaces of the die and the blank holder;

[0044] Step 4-2: Unfold the blank positioning

[0045] Cover the upper surface of the unfolded blank with a plastic film for protection, and place it on the working surface of the blank holder according to the outer contour line of the unfolded blank on the blank holder. Lubricating oil is brushed on the corresponding area between the lower surface of the unfolded blank and the working surface of the blank holder;

[0046] Step 4-3: First drawing forming

[0047] The die moves downward along the drawing direction F. After touching the blank holder, it clamps the unfolded blank with the blank holder and draws according to the shape of the punch. The unit blank holding force is set in the range of 8 MPa to 9 MPa. During forming, due to the material blocking effect of the full-type rolling rib, no wrinkles appear on the side wall of the wedge-shaped unfolded blank, and the existence of the small-end rolling rib also increases the compressive stress of the small-end blank, thereby increasing the tensile deformation amount of the material in this area. The height of the first drawing p1 = p - (15 - 20) mm. After the forming is completed, the initial workpiece is obtained;

[0048] Step 5 Second drawing forming

[0049] Step 5-1: Quenching

[0050] Quench the initial workpiece according to the requirements of the design digital model. The small end of the initial workpiece is fixed vertically downward in the quenching frame to reduce the deformation influence caused by the instantaneous impact of the hot and cold liquids on the initial workpiece during quenching;

[0051] Step 5-2: Second drawing forming

[0052] Utilize the best plastic period of the new quenched state of the hard aluminum alloy to complete the second drawing forming. First, place the quenched initial workpiece correspondingly on the blank holder. Brush lubricating oil between the contact surfaces of the die and the initial workpiece, and implement lubrication in the contact area between the initial workpiece and the blank holder. Start the drawing machine tool. The die and the blank holder clamp the initial workpiece and move downward together. After the final 15 mm - 20 mm necking drawing and rigid correction, a transition workpiece with uniform thinning of the material thickness is obtained. The unit blank holding force for the second forming is set to 9 MPa;

[0053] Step 6 Trim the outer shape

[0054] Cut the excess around and inside the transitional workpiece according to the outer contour line of the lower tail cover skin and the APU opening line on the drawing surface of the drawing die, and obtain the lower tail cover skin part with the outer contour accuracy and surface quality meeting the requirements of the drawing.

[0055] The beneficial effects of the present invention are as follows: 1) According to the hyperbolic structure of the cover skin, the present invention designs a wedge-shaped closed drawing workpiece with a springback closing surface, which increases the plastic deformation area of the material in the drawing forming of the part and solves the springback problem that cannot be controlled in the conventional drawing forming of the open curvature skin. 2) Taking the drawing depth of the drawing workpiece as the independent variable, the design formula of the inclination angle of the tail inclined surface and the springback closing surface is created, which solves the problems of excessive thinning and easy cracking of the bottom material of the large-depth closed drawing workpiece. 3) Using the similarity principle of the die opening structure, double-channel rolling beads with different structures are constructed. The full-type rolling bead solves the problem that the material flow at the die opening of the large-depth wedge-shaped skin is not restricted and is easy to stack and wrinkle. The small-end rolling bead increases the tensile stress of the blank in the small-end area of the wedge-shaped structure and increases the material stretching amount, realizing uniform thinning. 4) By designing the secondary closing drawing and sizing process after quenching, the overall drawing thinning amount of the wedge-shaped skin material is increased, the effective control of the springback error of the drawn part is realized, and the forming accuracy of the part is improved.

[0056] The following further describes the present application in detail with reference to the accompanying drawings of the embodiments. Description of the Drawings

[0057] Figure 1 is a schematic diagram of the assembly structure of the aircraft APU tail cover

[0058] Figure 2 is a schematic diagram of the structure of the lower tail cover skin

[0059] Figure 3 is a schematic diagram of the design method of the working surface of the drawing extension body

[0060] Figure 4 is a schematic diagram of the design structure of the closed extension body

[0061] Figure 5 is a schematic diagram of the design method of the working surface of the closing extension body

[0062] Figure 6 is a schematic diagram of the design structure of the drawing process surface

[0063] Figure 7 is a schematic diagram of the design structure of the drawing die

[0064] Figure 8 is a schematic diagram of the design structure of the blank holder

[0065] Figure 9 is a schematic diagram of the overall drawing die structure of the lower tail cover skin

[0066] Figure 10 Schematic diagram of the blank holder assembly position and the starting position of the first drawing operation

[0067] Figure 11 Schematic diagram of the relative position between the transitional workpiece after forming and the blank holder

[0068] Figure 12 Schematic diagram of the relative position between the transitional workpiece after forming and the die

[0069] Explanation of the numbers in the figure: 1. Aircraft tail cover; 2. Lower tail cover skin; 3. Upper tail cover skin; 4. Right tail cover skin; 5. Left tail cover skin; 6. Outer surface of the tail cover skin; 7. Transitional extension body; 8. Boat-shaped extension body; 9. Drawing extension body; 10. Closed extension body; 11. Tail contour surface; 12. Opening contour line; 13. Opening projection line; 14. Drawing necking line; 15. Section opening line; 16. Side wall section line; 17. Springback necking surface; 18. Necking extension body; 19. Outer edge line of the process flange; 20. Drawing process surface; 21. Die; 22. Working die surface; 23. Drawing flange surface; 24. Outer edge line of the developed blank; 25. Drawing outer surface; 26. Transitional fillet surface; 27. Full-type rolling groove; 28. Small-end rolling groove; 29. Blank holder; 30. Working blank holder surface; 31. Full-type rolling rib; 32. Small-end rolling rib; 33. Punch; 34. Working punch surface; 35. Outer contour line of the lower tail cover skin; 36. APU opening line; 37. Initial flange surface; 38. Transitional workpiece; 39. Developed blank; 40. Flange contour line Detailed implementation method

[0070] Referring to the attached drawings, the aircraft sheet metal part provided by the embodiment is as Figures 1 to 2 shown. The aircraft tail cover 1 is composed of the lower tail cover skin 2, the upper tail cover skin 3, the right tail cover skin 4 and the left tail cover skin 5. The lower tail cover skin 2 has a large curvature, a spherical wedge shape, a symmetrical structure. Point E is the midpoint of the large curvature contour of the lower tail cover skin 2. The material is hard aluminum alloy, the thickness ≤ 1.2 mm, the wall is thin. The ratio of the arc length of the wedge head of the lower tail cover skin 2 to the arc length of the tail is 1:7, the overall contour taper is large, and there is an APU opening inside. The lower tail cover skin 2 has long been formed by skin stretching. The main defects are serious surface orange peel, large amount of manual trimming and grinding, high scrap rate, and it is difficult to guarantee the assembly quality

[0071] As Figures 3 to 12 shown, a drawing forming method for a large curvature thin-walled spherical wedge-shaped tail cover skin includes the following steps

[0072] Step 1: Construct the drawing process surface 20 of the lower tail cover skin 2

[0073] Under the CATIA environment, design the drawing process surface 20 based on the outer surface 6 of the lower tail cover skin. The material thickness of the lower tail cover skin 2 is denoted as δ

[0074] Step 1-1: Establish the design horizontal plane H

[0075] Establish a design horizontal plane H based on the four intersection points A, B, C, and D of the contour of the following tail cover skin 2, so as to reduce the forming depth h of the lower tail cover skin 2. The drawing direction F of the lower tail cover skin 2 is perpendicular to the design horizontal plane H;

[0076] Step 1-2: Design the ship-shaped extension body 8

[0077] Extract the contour lines around the lower tail cover skin 2, extend them around along the curvature of the lower tail cover skin 2 to obtain the transitional extension body 7. Establish a transitional horizontal plane M based on the design horizontal plane H. Translate the design horizontal plane H in the opposite direction of the drawing direction F to obtain the transitional horizontal plane M. The translation distance d between the design horizontal plane H and the transitional horizontal plane M is 15 mm - 20 mm. Cut the transitional extension body 7 with the transitional horizontal plane M, and retain the ship-shaped extension body 8 on the side with the lower tail cover skin 2;

[0078] Step 1-3: Design the inclination angle θ of the tail inclined plane P

[0079] Establish the tail inclined plane P so that it forms a certain inclination angle θ with the transitional horizontal plane M. The value range of the inclination angle θ is 100° - 130°. On the one hand, it ensures that the created surface is conducive to material flow, and on the other hand, it restricts too much material from flowing to the bottom of the ship-shaped extension body 8. The selection principle of the inclination angle θ is determined by the forming depth h of the lower tail cover skin 2. When the forming depth h ≤ 200 mm, take the lower limit value 100° of the inclination angle θ. When the forming depth h is in the range of 200 mm < h ≤ 500 mm, the inclination angle θ is taken according to the formula Take and manufacture;

[0080] Step 1-4: Design the closed extension body 10

[0081] The position of the tail inclined plane P is indirectly determined by the midpoint E of the bottom contour of the lower tail cover skin 2. The tail inclined plane P intersects with the ship-shaped extension body 8 to obtain the U-shaped tail contour line l. The midpoint of the bottom of the tail contour line l is denoted as F, and the length of the line segment EF is denoted as n. Cut the ship-shaped extension body 8 with the tail inclined plane P, retain the drawn extension body 9 on the side with the lower tail cover skin 2, close the U-shaped tail contour line l to obtain the tail contour surface 11, sew the drawn extension body 9 and the tail contour surface 11, and perform a fillet radius R transition on the tail contour line l to obtain the closed extension body 10. The value range of R is 80 mm - 120 mm to ensure a smooth streamline of the surface and prevent material accumulation and wrinkling at the corner area during material drawing, which affects material flow. The taking and manufacturing principle of the fillet radius R is affected by the proportion of the remaining plane area of the tail contour surface 11. The remaining plane area of the tail contour surface 11 needs to account for more than two-thirds of the original area of the tail contour surface 11;

[0082] Step 1-5: Establish the drawing horizontal plane S

[0083] Establish the drawing horizontal plane S based on the transition horizontal plane M. Translate the transition horizontal plane M in the opposite direction of the drawing direction F by a translation distance f = 30 mm to obtain the drawing horizontal plane S;

[0084] Step 1-6: Design the drawing necking line 14 of the closed extension body 10

[0085] Extract the opening contour line 12 of the closed extension body 10 and project it onto the drawing horizontal plane S to obtain the opening projection line 13. G is any point on the opening contour line 12, and V is the projection point of G on the opening projection line 13. The length of the line segment GV is 30 mm. Draw a vertical plane K of the drawing horizontal plane S through point G, which intersects the closed extension body 10 to obtain the side wall section line 16 passing through point G, and intersects the opening contour line 12 at point I. The line segment GI is called the section opening line 15. Draw a tangent t to the side wall section line 16 through point G, and the tangent t intersects the section opening line 15 to form a section inclination angle α. Offset the opening projection line 13 equidistantly around in the drawing horizontal plane S to obtain the drawing necking line 14. U is the projection point of G on the drawing necking line 14. The included angle between the line segment GV and the line segment GU is denoted as the necking angle β, and the length of the line segment VU is the equidistant offset distance m. The offset distance m = (30×tgβ) mm, and the value range of the necking angle β is 90° - 98°. The size of the necking angle β is calculated according to the formula Calculate;

[0086] Step 1-7: Design the necking extension body 18

[0087] Smoothly close the opening contour line 12 and the drawing necking line 14 through the multi-section surface drawing function to obtain the springback necking surface 17. Perform a fillet radius R1 transition between the closed extension body 10 and the springback necking surface 17 to obtain the necking extension body 18. The value range of R1 is R1 = 20 mm - 25 mm;

[0088] Step 1-8: Design the drawing process surface 20

[0089] Offset the drawing necking line 14 equidistantly outward by 35 mm in the drawing horizontal plane S to obtain the process flange outer edge line 19. The planar ring enclosed by the drawing necking line 14 and the process flange outer edge line 19 is the initial flange surface 37. Perform a fillet radius R f transition, and its appropriate value range is R f =(5 - 8)×δ, and the value-taking principle is related to the depth p of the necking extension body 18. Take the value according to the formula to obtain the drawing process surface 20 of the lower tail cover skin 2.

[0090] Step 2 Determine the size of the developed blank 39

[0091] Step 2-1: Design the deep drawing workpiece

[0092] Offset the deep drawing process surface 20 of the lower tail cover skin 2 in the opposite direction of the deep drawing direction F, and the offset distance is to obtain the deep drawing workpiece;

[0093] Step 2-2: Calculate the size of the developed blank 39

[0094] Use the blank reverse calculation function of common finite element analysis software to calculate the developed shape of the deep drawing workpiece, and obtain the wedge-shaped developed blank 39 with a large difference in the sizes of both ends and the developed blank outer edge line 24;

[0095] Step 3 Design the deep drawing die structure of the lower tail cover skin 2

[0096] Design the deep drawing die structure based on the deep drawing process surface 20 of the lower tail cover skin 2 and the wedge-shaped developed blank 39, including the punch 33, the die 21 and the blank holder 29

[0097] Step 3-1: Design the structure of the die 21

[0098] Extend the initial flange surface 37 of the deep drawing process surface 20 around in the deep drawing horizontal plane S, and use the enlarged deep drawing process surface 20 as the die working surface 22. Project the wedge-shaped developed blank outer edge line 24 to the corresponding position of the die working surface 22, and judge whether the design of the outer contour size of the die working surface 22 is reasonable. The design principle is that the distance between the developed blank outer edge line 24 and the outer contour of the die working surface 22 remains relatively uniform, and the distance between them ≥ 80 mm to ensure the manufacturing stiffness of the blank holder 29 and the balance of the blank holding force application. The die working surface 22 consists of three parts: the deep drawing flange surface 23, the transition fillet surface 26 and the deep drawing outer surface 25. Among them, the corresponding surface of the die working surface 22 and the initial flange surface 37 is called the deep drawing flange surface 23, the corresponding surface of the die working surface 22 and the necking extension body 18 is called the deep drawing outer surface 25, and the fillet radius is R f The transition area is the transition fillet surface 26;

[0099] Step 3-2: Design the structure of the blank holder 29

[0100] The working surface 30 of the blank holder matches the drawing flange surface 23. The wedge-shaped developed blank outer edge line 24 is provided on the working surface 30 of the blank holder. At the same time, two drawing beads with U-shaped cross-sections and different sizes are also provided, namely the full-profile drawing bead 31 and the small-end drawing bead 32, to achieve uniform deformation of the wedge-shaped developed blank 39. The overall contour of the full-profile drawing bead 31 has similarity with the drawing necking line 14 and the outer shape of the developed blank 39. The overall position of the full-profile drawing bead 31 is obtained by offsetting 35 mm to 40 mm equidistantly from the drawing necking line 14 to the periphery. The designed height y1 of the full-profile drawing bead 31 is 4 mm to 5 mm. The small-end drawing bead 32 matches the tip shape of the full-profile drawing bead 31 and is obtained by offsetting 25 mm to 30 mm equidistantly from the tip area of the full-profile drawing bead 31. It is only set for the shape of the small end of the developed blank 39 to increase the deformation degree of the small-end developed blank 39. The designed height y2 of the small-end drawing bead 32 is ≤ 3 mm;

[0101] Step 3-3: Design the drawing groove of the female die 21

[0102] Matching the two drawing beads on the working surface 30 of the blank holder, two drawing grooves are provided on the drawing flange surface 23 of the working surface 22 of the female die, corresponding to the U-shaped protrusions of the two drawing beads, namely the full-profile drawing groove 27 and the small-end drawing groove 28. The function is to store and avoid the drawing beads, and together with the drawing beads, limit the flow speed of the developed blank 39 and increase the deformation resistance of the developed blank 39;

[0103] Step 3-4: Design the working clearance between the male die 33 and the female die 21

[0104] The male die is designed based on the drawing outer surface 25 of the female die 21. The designed clearance between the working surface 34 of the male die and the drawing outer surface 22 of the female die is g, which is calculated according to the formula g = (0.95 - 1.0) × δ. Negative clearance drawing is adopted to improve the forming accuracy of the lower tail cover skin 2 and reduce the springback after drawing of the lower tail cover skin 2. Exhaust holes with a diameter of 3 mm are provided on the working surface 34 of the male die;

[0105] Step 4 First drawing forming

[0106] Step 4-1: Install the drawing die

[0107] Install the male die 33 and the blank holder 29 on the working surface of the drawing machine table. The female die 21 is fixed to the upper bed surface of the drawing machine. The drawing outer surface 25 of the female die 21 corresponds to the working surface 34 of the male die. The working surface 30 of the blank holder corresponds to the drawing flange surface 23 of the female die 21. The blank holder 29 can move up and down through the movable ejector rod. The female die 21 moves downward through the upper bed surface of the drawing machine to achieve drawing forming. Lubricating oil is brushed on the forming working surfaces of the female die 21 and the blank holder 29;

[0108] Step 4-2: Position the developed blank

[0109] Protect the upper surface of the blank 39 by pasting a plastic film, and place it on the working surface 30 of the blank holder 29 according to the outer contour line 24 of the blank on the blank holder. Lubricating oil is brushed on the corresponding area between the lower surface of the blank 39 and the working surface 30 of the blank holder;

[0110] Step 4-3: First deep drawing forming

[0111] The female die 21 moves downward along the drawing direction F. After touching the blank holder 29, it holds the blank 39 with the blank holder 29 and performs deep drawing according to the shape of the male die 33. The unit blank holding force is set in the range of 8 MPa to 9 MPa. Due to the material blocking effect of the full-type rolling rib 31, no wrinkles appear on the side wall during the forming of the wedge-shaped blank 39. The existence of the small-end rolling rib 32 also solves the problem of large depth difference between the large and small ends of the deep drawing workpiece and no thinning of the small-end blank material. The first deep drawing height p1 = p - (15 - 20) mm, and the initial workpiece is obtained after the forming is completed;

[0112] Step 5 Second deep drawing forming

[0113] Step 5-1: Quenching

[0114] Quench the initial workpiece according to the requirements of the design digital model. The small end of the initial workpiece is vertically fixed downward in the quenching frame to reduce the deformation influence caused by the instantaneous impact of hot and cold liquids on the initial workpiece during quenching;

[0115] Step 5-2: Second deep drawing forming

[0116] Complete the second deep drawing forming by using the best plasticity period of the new quenched state of the hard aluminum alloy. First, place the quenched initial workpiece correspondingly on the blank holder 29. Lubricating oil is brushed on the contact surface between the female die 21 and the initial workpiece, and lubrication is implemented in the contact area between the initial workpiece and the blank holder 29. Start the drawing machine. The female die 21 and the blank holder 29 hold the initial workpiece and move downward together. After the final 15 mm - 20 mm necking drawing and rigid correction, the transition workpiece 38 with uniform thinning of the material thickness is obtained. The unit blank holding force for the second forming is set to 9 MPa. Comparing the relative positions of the flange contour line 40 of the transition workpiece 38 and the outer edge line 24 of the blank, it can be clearly seen that the material flow rate of the large-end blank 39 is much greater than that of the small-end blank 39. The two rolling ribs better restrict the material flow of the small-end blank 39, so that the blank 39 entering the transition fillet surface 26 of the small-end female die 21 is subjected to a large tensile stress, thereby realizing the transition of the small-end blank 39 from elastic deformation to plastic deformation, and the springback defect is controlled;

[0117] Step 6 Trim the outer shape

[0118] Cut the surplus around and inside the transition workpiece 38 according to the outer contour line 35 of the lower tail cover skin and the APU opening line 36 on the drawing die 21 drawing outer contour surface 25, and obtain the lower tail cover skin 2 part with the outer contour accuracy and surface quality meeting the requirements of the drawing.

[0119] It should be noted that the present invention is applicable to aluminum skins with symmetrical structures, wedge-shaped openings, and an initial forming depth h in the range of 150 mm to 500 mm; after completing the drawing die structure design and the calculation of the developed blank, the drawing effect can be predicted through the finite element simulation of the drawing forming. The ideal change range of the material thickness in the effective area of the transition workpiece, that is, the tail cover skin, is within 8%δ to 18%δ. When the material thickness change range is greater than 18%δ, the drawing die structure can be optimized by reducing the height of the full-type rolling rib and taking the lower limit value of y1 as 4 mm; when forming the tail cover skin by drawing, the unit blank holding force can be adjusted according to the actual machine tool situation.

Claims

1. A drawing forming method for the skin of a large-curvature thin-walled spherical wedge-shaped tail cover, characterized in that It includes the following steps: Step 1: Construct the drawing process surface of the lower tail cover skin. The specific process is as follows: In the CATIA environment, design the drawing process surface based on the outer surface of the lower tail cover skin. The material thickness of the lower tail cover skin is denoted as δ Step 1-1: Establish the design horizontal plane H Establish the design horizontal plane H based on the four intersection points A, B, C, and D of the lower tail cover skin contour, so as to reduce the forming depth h of the lower tail cover skin. The drawing direction F of the lower tail cover skin is perpendicular to the design horizontal plane H; Step 1-2: Design the ship-shaped extension body Extract the contour lines around the lower tail cover skin, extend them around along the curvature of the lower tail cover skin to obtain the transitional extension body. Establish the transitional horizontal plane M according to the design horizontal plane H, translate the design horizontal plane H in the opposite direction of the drawing direction F to obtain the transitional horizontal plane M. The translation distance d between the design horizontal plane H and the transitional horizontal plane M is 15 mm to 20 mm. Cut the transitional extension body with the transitional horizontal plane M, and retain the ship-shaped extension body on the side with the lower tail cover skin; Step 1-3: Design the inclination angle θ of the tail inclined plane P Establish a tail inclined surface P to form an inclination angle θ with the transition horizontal plane M. The value range of the inclination angle θ is 100° to 130°. The selection principle of the inclination angle θ is determined by the forming depth h of the lower tail cover skin. When the forming depth h ≤ 200 mm, take the lower limit value 100° of the inclination angle θ. When the forming depth h is in the range of 200 mm < h ≤ 500 mm, the inclination angle θ is taken according to the formula Take and manufacture; Step 1-4: Design the closed extension body The position of the tail inclined surface P is indirectly determined by the midpoint E of the bottom contour of the lower tail cover skin The tail inclined surface P intersects with the ship-shaped extension body to obtain the U-shaped tail contour line l. The midpoint of the bottom of the tail contour line l is denoted as F, and the length of the line segment EF is denoted as n. Cut the ship-shaped extension body with the tail inclined surface P, retain the drawn extension body on the side with the tail cover skin, close the U-shaped tail contour line l to obtain the tail contour surface, suture the drawn extension body and the tail contour surface, and perform a fillet radius R transition on the tail contour line l to obtain a closed extension body. The value range of R is 80 mm to 120 mm to ensure a smooth streamline of the surface. The principle of determining the fillet radius R is affected by the proportion of the remaining plane area of the tail contour surface. The remaining plane area of the tail contour surface needs to account for more than two-thirds of the area of the original tail contour surface; Step 1-5: Establish the drawing horizontal plane S Establish the drawing horizontal plane S according to the transitional horizontal plane M. Translate the transitional horizontal plane M in the opposite direction of the drawing direction F, and the translation distance f = 30 mm to obtain the drawing horizontal plane S; Step 1-6: Design the drawing closing line of the closed extension body Extract the opening contour line of the closed extension body, project it onto the drawing horizontal plane S to obtain the opening projection line. G is any point on the opening contour line, and V is the projection point of G on the opening projection line. The length of the line segment GV is 30 mm. Draw a vertical plane K of the drawing horizontal plane S through point G, intersect with the closed extension body to obtain the side wall section line passing through point G, and intersect with the opening contour line at point I. The line segment GI is called the section opening line. Draw a tangent t to the side wall section line through point G, and the tangent t intersects with the section opening line to form a section inclination angle α. Offset the opening projection line equidistantly around in the drawing horizontal plane S to obtain the drawing closing line. U is the projection point of G on the drawing closing line. The included angle between the line segment GV and the line segment GU is denoted as the closing angle β, and the length of the line segment VU is the equidistant offset distance m. The offset distance m = (30 × tgβ) mm. The value range of the closing angle β is 90° to 98°. The size of the closing angle β is calculated according to the formula Calculate; Step 1-7: Design the closing extension body Smoothly close the opening contour line and the drawing closing line through the multi-section surface drawing function to obtain the springback closing surface. Perform a fillet radius R1 transition between the closed extension body and the springback closing surface to obtain the closing extension body. The value range of R1 is R1 = 20 mm to 25 mm; Step 1-8: Design the drawing process surface The drawing necking line is offset 35 mm equidistantly outward to the periphery within the drawing horizontal plane S to obtain the outer edge line of the process flange. The planar ring enclosed by the drawing necking line and the outer edge line of the process flange is the initial flange surface. The fillet radius R f is used for transition, and its value range is R f =(5 - 8)×δ, and the value-taking principle is related to the depth p of the necking extension body. It is taken according to the formula to obtain the drawing process surface of the lower tail cover skin; Step 2: Determine the size of the developed blank. The specific process is as follows: Step 2-1: Design the drawing workpiece Offset the drawing process surface of the lower tail cover skin in the opposite direction of the drawing direction F by an offset distance of to obtain a drawn workpiece; Step 2-2: Calculate the size of the developed blank Use the blank reverse calculation function of common finite element analysis software to calculate the developed shape of the drawing workpiece, and obtain the wedge-shaped developed blank and the outer edge line of the developed blank; Step 3: Design the structure of the drawing die for the lower tail cover skin; Step 4: Perform the first drawing forming; Step 5: Perform the second drawing forming; Step 6: Trim the outer shape.

2. The drawing forming method for the skin of a large-curvature thin-walled spherical wedge-shaped tail cover according to claim 1, characterized in that The specific process of the above-mentioned Step 3 for designing the structure of the drawing die for the lower tail cover skin is as follows: Design the structure of the drawing die based on the drawing process surface of the lower tail cover skin and the wedge-shaped developed blank, including the punch, die, and blank holder Step 3-1: Design the die structure Extend the initial flange surface of the drawing process surface around in the drawing horizontal plane S, and use the expanded drawing process surface as the working surface of the female die. Project the outer edge line of the wedge-shaped developed blank to the corresponding position of the working surface of the female die, and judge whether the design of the outer contour dimensions of the working surface of the female die is reasonable. The design principle is that the distance between the outer edge line of the developed blank and the outer contour of the working surface of the female die is kept uniform, and the distance between them is ≥ 80 mm to ensure the manufacturing stiffness of the blank holder and the application balance of the blank holding force. The working surface of the female die consists of three parts: the drawing flange surface, the transition fillet surface, and the drawing outer surface. Among them, the drawing flange surface corresponds to the initial flange surface, the drawing outer surface corresponds to the necking extension body, and the transition fillet surface is the fillet radius R f Transition region; Step 3-2: Design the blank holder structure The working surface of the blank holder matches the drawing flange surface. A wedge-shaped developed blank outer edge line is provided on the working surface of the blank holder. At the same time, there are also two drawing beads with U-shaped cross-sections and different sizes, namely the full-type drawing bead and the small-end drawing bead, to achieve uniform deformation of the wedge-shaped developed blank. The overall contour of the full-type drawing bead is similar to the drawing necking line and the outer shape of the developed blank. The overall position of the full-type drawing bead is obtained by offsetting 35 mm to 40 mm equidistantly from the drawing necking line to the periphery. The designed height y1 of the full-type drawing bead is 4 mm to 5 mm. The small-end drawing bead matches the tip shape of the full-type drawing bead and is obtained by offsetting 25 mm to 30 mm equidistantly from the tip area of the full-type drawing bead to the outside. It is only set for the shape of the small end of the developed blank to increase the deformation degree of the small-end developed blank. The designed height y2 of the small-end drawing bead is ≤ 3 mm; Step 3-3: Design the drawing grooves of the female die Matching the two drawing beads on the working surface of the blank holder, two drawing grooves are provided on the drawing flange surface of the working surface of the female die, corresponding to the U-shaped protrusions of the two drawing beads, namely the full-type drawing groove and the small-end drawing groove, which, together with the drawing beads, limit the flow rate of the developed blank and increase the deformation resistance of the developed blank; Step 3-4: Design the working clearance between the male die and the female die The male die is designed based on the drawing outer surface of the female die. The designed clearance between the working surface of the male die and the drawing outer surface of the female die is g, which is calculated according to the formula g = (0.95 - 1.0) × δ. Negative clearance drawing is adopted to improve the forming accuracy of the lower tail cover skin. There are exhaust holes with a diameter of 3 mm on the working surface of the male die.

3. The drawing forming method of a large-curvature thin-walled spherical wedge-shaped tail cover skin according to claim 1, characterized in that The specific process of the first drawing forming in the said Step 4 is as follows: Step 4-1: Install the drawing die Install the male die and the blank holder on the working surface of the drawing machine tool. The female die is fixed to the upper bed surface of the drawing machine. The drawing outer surface of the female die corresponds to the working surface of the male die. The working surface of the blank holder corresponds to the drawing flange surface of the female die. The blank holder moves up and down through the movable ejector rod. The female die moves downward through the upper drawing bed surface to achieve drawing forming. Lubricating oil is brushed on the forming working surfaces of the female die and the blank holder; Step 4-2: Position the developed blank Protect the upper surface of the developed blank with plastic film and place it on the working surface of the blank holder according to the outer contour line of the developed blank on the blank holder. Lubricating oil is brushed on the corresponding area between the lower surface of the developed blank and the working surface of the blank holder; Step 4-3: First drawing forming The female die moves downward along the drawing direction F. After touching the blank holder, it clamps the developed blank with the blank holder and draws according to the shape of the male die. The unit blank holding force is set in the range of 8 MPa to 9 MPa. During forming, due to the material blocking effect of the full-type drawing bead, no wrinkles appear on the side wall of the wedge-shaped developed blank. The existence of the small-end drawing bead also increases the compressive stress of the small-end blank, thereby increasing the tensile deformation amount of the material in this area. The first drawing height p1 = p - (15 - 20) mm. The initial workpiece is obtained after the forming is completed.

4. The drawing forming method of a large-curvature thin-walled spherical wedge-shaped tail cover skin according to claim 1, characterized in that The specific process of the second drawing forming in the said Step 5 is as follows: Step 5-1: Quenching Quench the initial process part according to the requirements of the design digital model. The small end of the initial process part is vertically fixed in the quenching frame with the small end facing downwards to reduce the deformation influence caused by the instantaneous impact of hot and cold liquids on the initial process part during quenching. Step 5-2: Secondary drawing forming Utilize the best plasticity period of the new quenched state of the hard aluminum alloy to complete the secondary drawing forming. First, place the quenched initial process part correspondingly on the blank holder. Brush lubricating oil between the contact surfaces of the female die and the initial process part, and implement lubrication in the contact area between the initial process part and the blank holder. Start the drawing machine tool, and the female die and the blank holder clamp the initial process part and move downward together. After the final necking drawing and rigid correction of 15 mm to 20 mm, a transition process part with a uniformly thinned material thickness is obtained. The unit blank holding force for the secondary forming is set to 9 MPa.

5. The drawing forming method of a large-curvature thin-walled spherical wedge-shaped tail cover skin according to claim 1, characterized in that The specific process of the described Step 6: trimming the outer shape is as follows: Cut the margins around and inside the transition process part according to the outer skin contour line of the lower tail cover and the APU opening line on the outer surface of the female die drawing to obtain the lower tail cover skin part with the outer shape accuracy and surface quality meeting the requirements of the drawing.

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