A deep drawing method for double-curved bidirectional concave stainless steel butt skin

Abstract

The present invention discloses a deep drawing forming method for a hyperbolic bidirectional concave stainless steel butt skin, comprising the following steps: step-by-step design of the drawing die structure of the butt skin; construction of the butt skin drawing body; step-by-step determination of the size of the curved surface unfolded blank; initial drawing forming; secondary drawing correction; hydraulic correction. The present invention provides a closed drawing body design method with a nearly circular end and an inclined flange for the butt skin bidirectional concave structure, so that the open L-shaped cross-section hyperbolic part is bound by the edge pressure on all sides, increasing the plastic deformation of the hyperbolic bending edge and reducing the rebound jump of the butt skin; by designing a three-stage calendering rib structure and a reasonable process blank shape, the flow rate and feed amount of the closed drawing body material are balanced, the wrinkle risk of the main curved surface and the lateral curved surface of the butt skin is controlled, and the problem of surface wrinkle accumulation and poor forming quality of the bidirectional concave butt skin formed by drop press forming is solved.

Description

Technical Field

[0001] The invention relates to a sheet metal parts forming technology in the field of aircraft manufacturing, in particular to a deep drawing forming method and a forming die for double-curved, double-depressed, thicker stainless steel butt-jointed skins. Background Art

[0002] The connecting parts of the aircraft are required to have smooth streamlines to ensure a good aerodynamic shape. For this purpose, some connecting skins with complex surfaces are often needed to achieve the transition between different curvatures.

[0003] In order to improve the localization design capability and application function of the aircraft nacelle, a 1.5mm to 2.5mm thick stainless steel plate is used to complete the transitional special-shaped skin structure design in the middle section, rear section and docking point of the nacelle with the wing. This thickness is also required for the safety strength of the docking point between the nacelle and the wing. Due to the limitation of the installation space, the docking skin has a complex curvature and presents a two-way concave structure. The maximum edge height is close to 300mm. There are strict assembly relationships on all sides, and it is required to have a good aerodynamic shape.

[0004] The yield strength of austenitic stainless steel with a thickness of ≥1.5mm is ≥210MPa, which is 3 to 4 times that of conventional aluminum alloy, affecting the mold fit of the complex curved surface of the butt skin. In terms of appearance structure, the butt skin is a typical drop-pressed part. The rough manufacturing mode of the drop hammer can process aircraft sheet metal parts that cannot or are difficult to form with other process methods due to their complex appearance. However, the drop-press forming process cannot meet the assembly accuracy and use function of stainless steel butt skins with a certain aerodynamic shape and relatively large thickness (conventional stainless steel skin thickness ≤1.0mm) due to many defects such as unstable quality of processed products and difficult to control surface and internal damage after forming. Moreover, the drop-press forming process has gradually shrunk due to the high labor intensity of workers, low utilization rate of raw materials, high energy consumption, and high pollution. The innovation of the nacelle structure design concept poses a great challenge to the forming process of stainless steel skins with complex curvature.

[0005] The cross section of the butt skin is L-shaped, with an angle of about 87° at one end. It is swept along three curves in different directions to form a high-edge three-dimensional part with a distorted appearance. For such thick stainless steel, the best mechanized forming method according to existing processes is deep drawing. However, the butt skin structure is open, and the material is not constrained during forming. The free flow will cause the two concave surfaces to accumulate materials and form dense wrinkles due to the lack of longitudinal tensile stress. Dead wrinkles that cannot be extended will also appear at the steepest point of curvature. Increasing the blank holding force will cause the material around the maximum drawing high point to rupture. For hyperbolic, double-concave, and thicker stainless steel butt skins, it is urgent to innovate a deep drawing process profile design method to change the material stress deformation trend, transfer dangerous fracture surfaces, achieve balanced material flow, and control the occurrence of wrinkling and rupture during deep drawing. Summary of the invention

[0006] In order to solve the problem that double-curved, double-concave, thicker stainless steel butt skins have long been monopolized by the drop-press forming process and the product quality is uncontrollable, the purpose of the present invention is to provide a deep drawing method for L-section bidirectionally concave stainless steel butt skins.

[0007] A deep drawing forming method for a hyperbolic bidirectional concave stainless steel butt skin, wherein the drawing die consists of a female die, a male die, and a blank holder. A drawing die structure model is established in a CATIA environment through a digital model of the butt skin product. The butt skin drawing body is designed according to the drawing model surface. The forming method comprises the following steps:

[0008] Step 1: Determine the drawing die structure

[0009] Step 1-1: The surface with smaller curvature among the two concave surfaces in the digital model of the butt-jointed skin product is taken as the main surface, the concave surface with larger curvature in the cross section is taken as the lateral surface, the inner profile of the butt-jointed skin is taken as its inner surface, and t is the material thickness of the butt-jointed skin, in this example t=1.5mm;

[0010] Step 1-2: extract the boundary line of the inner surface of the butt-jointed skin, perform external extension, and obtain an inner extension profile, which is composed of a main extension profile and a side extension profile. The main extension profile corresponds to the main curved surface, and the side extension profile corresponds to the lateral curved surface. The main extension profile and the side extension profile intersect at the curved surface boundary line n;

[0011] Step 1-3: The three inner surface boundary lines of the butt-skin involved in the side extension profile are respectively recorded as a, b, and c, where a is the inner wall outline of the butt-skin, b is the curved edge outline of the butt-skin, and c is the tail outline of the butt-skin. The inner surface end outline of the butt-skin included in the main extension profile is recorded as s. The four curves s, a, b, and c are parallelly moved outward along the inner extension profile by d0=5mm~20mm to obtain the corresponding four curves s1, a1, b1, and c1. S1 is the effective boundary line of the end, a1 is the effective boundary line of the inner wall, b1 is the effective boundary line of the curved edge, and c1 is the effective boundary line of the tail. S, A, B, C, and D are s1, a1, b1, and c1 respectively. The endpoints of the four curves, the surface boundary line n and the tail effective boundary line c1 intersect at point E. The highest horizontal plane p is established through points A, D, and E. The distance from point B to the highest horizontal plane p is the drawing height h. The value of d0 affects the drawing height h. When determining the translation dimension d0, it should be noted that a higher point A will increase the drawing height h and increase the depth of the main extension surface depression. Similarly, a lower point B will also increase the drawing height h. The influence of points D and E on the drawing height h is weaker than that of points A and B. Therefore, in this example, the distance between s and s1 is d0 = 5mm, the distance between a and a1 is d0 = 8mm, the distance between b and b1 is d0 = 10mm, and the distance between c and c1 is d0 = 15mm;

[0012] Step 1-4: Make an initial edge pressing surface f through point B, parallel to the highest horizontal plane p, the initial edge pressing surface f intersects with the inner extension profile of the butt skin at a curve m, and the inner extension profile of the butt skin is cut with the curve m to obtain an effective inner extension profile of the butt skin;

[0013] Step 1-5: Use points D, E and C to establish plane g, make plane k parallel to plane g, and the distance between plane k and plane g is l1≤60mm, plane k intersects with the effective inner extension profile of the docking skin at curve d, plane k intersects with the main extension profile of the docking skin at points F and G, connect points F and G to obtain straight line segment FG, based on plane k and with straight line segment FG as the rotation axis, make the angle θ between plane j and plane k = 5° to 8°, plane j intersects with the effective inner extension profile of the docking skin at curve d1, on the side extension profile of the docking skin, the angle between curve d1 and curve d is θ, and curve d1 is the tail shape control line;

[0014] Step 1-6: Connect point D and point S to obtain a straight line segment DS, project the straight line segment DS to the main extension surface of the docking skin to obtain a curve e, establish plane q through the straight line segment DS and the curve e, make plane r parallel to plane q, and the distance between plane r and plane q is l2≤60mm, plane r intersects with the effective inner extension surface of the docking skin at points M and N, connect point M and point N to obtain a straight line segment MN, based on plane r and with straight line segment MN as the rotation axis, make the angle between plane u and plane r be θ1=3°~5°, plane u intersects with the effective inner extension surface at curve v, and curve v is the outer wall shape control line;

[0015] Step 1-7: Use the outer wall shape control line v, the tail shape control line d1, the initial pressure edge surface f, the plane j, and the plane u to close the effective inner extension surface of the butt skin through three-dimensional mapping, and obtain the tail termination surface, the outer wall termination surface, the intersection line z of the tail termination surface and the outer wall termination surface, the intersection line y of the tail termination surface and the initial pressure edge surface f, and the intersection line x of the outer wall termination surface and the initial pressure edge surface f. The three boundary lines of the intersection line z, the outer wall shape control line v, and the tail shape control line d1 are used to make the transition fillet R between the surfaces. It should be noted that the transition fillet R on the tail termination surface is ≤l1, and the transition fillet R on the outer wall termination surface is ≤(l2-5mm). The plane i is established through the effective boundary line a1 of the inner wall, and the end of the effective inner extension surface is cut by the intersection line of the effective inner extension surface and the plane i to obtain a semi-closed cavity tooling surface closed on three sides;

[0016] Step 1-8: Make a plane w nearly parallel to the end effective boundary line s1 and at a distance of l0, where R < l0 ≤ 100 mm. The intersection line x of the outer wall termination surface intersects the plane w at point B1. Make a circle Q1 with a radius of R1 in the initial blank-holding surface f and tangent to the intersection line x of the outer wall termination surface at point B1. The value range of the radius R1 is h ≤ R1 ≤ (h + 20 mm). The plane i passing through point B intersects the circle Q1 at point A1. In the plane i, along the inner wall effective boundary line a1, make a fair curve AA1 through point A to obtain a multi-section starting curve f1 with the starting point at B and the ending point at A1. In the initial blank-holding surface f, make a tangent to the semi-closed cavity tooling surface curve m through point B and smoothly transition to the circle Q1 to obtain a multi-section ending curve g1 with the starting point at B and the ending point at A1;

[0017] Step 1-9: Connect point B and point A1 to obtain a straight line segment BA1. After bisecting it into l segments, make a vertical plane of the straight line segment BA1 through the bisection point to obtain (l - 1) vertical planes i1, i2, i3... i l-1 , within the range enclosed by the multi-section starting curve f1 and the multi-section ending curve g1, respectively make longitudinal guiding lines of the multi-section enclosed surface in each vertical plane to obtain (l - 1) guiding lines m1, m2, m3... m l-1 , the starting point of each guiding line is on the multi-section starting curve f1, and the ending point is on the multi-section ending curve g1. The (l - 1) guiding lines are structurally similar to each other to ensure that the formed multi-section enclosed surface has a fair curvature. The curvature of the upper part of each guiding line is large and the lower part tends to be flat. The lower part of the guiding line has an inclination angle of not less than 70° with the initial blank-holding surface f. Using the multi-section curve command, input the multi-section starting curve f1, the multi-section ending curve g1, and the guiding lines m1, m2, m3... m l-1 , to form a multi-section enclosed surface. It should be noted that in order to better control the curvature of the multi-section enclosed surface, multiple transverse multi-section transition curves n1, n2, n3, n4... can be established between the multi-section starting curve f1 and the multi-section ending curve g1 to increase the limitation of the input conditions. In this way, the obtained multi-section enclosed surface better meets the design requirements. In this example, 6 longitudinal guiding lines m1, m2, m3, m4, m5, m6 and 4 transverse multi-section transition curves n1, n2, n3, n4 are established. The design principle of the multi-section enclosed surface is not to increase the drawing height h and ensure that the outer shape of the multi-section enclosed surface is fair;

[0018] Step 1-10: Denote the small end arc between point A1 and point B1 on the circle Q1 as the end side contour line t1. Use the curve between point A and point A1 on the multi-section starting curve f1 as the guiding curve, and use the sweeping function to obtain the end side blocking surface, which intersects the semi-closed cavity tooling surface at the end space cut-off line;

[0019] Step 1-11: Make a connecting fillet R2 between the end side baffle profile and the semi-closed cavity tooling profile at the end space cut-off line, combine it with the multi-section enclosing profile to form a closed tooling profile, extract the opening edge line of the closed tooling profile, stretch it along the closed tooling profile, and obtain a male mold tooling profile for the butt joint skin;

[0020] Step 1-12: Set the direction perpendicular to the initial blanking surface f downward as the drawing direction F0 of the butt skin, translate the initial blanking surface f downward along the drawing direction F0 by 7t to 9t, and intersect with the male mold tooling profile at the transitional blanking line. The projection point of point B on the transitional blanking line is recorded as point B0, and a blanking boundary line y0 is made through point B0 and along the direction of the intersection line y of the tail termination surface. In this example, the downward translation distance of the initial blanking surface f along the drawing direction F0 is 9t = 13.5 mm;

[0021] Step 1-13: Take the blank-holding boundary line y0 as the rotation axis and the initial blank-holding surface f as the basic plane, rotate clockwise by 10° to 15° to obtain the left blank-holding surface f z , on the left pressure edge surface f z The corresponding rectangular left edge-holding ring profile is designed on the top, and the edge-holding boundary line y0 is used as the rotation axis and the initial edge-holding surface f is used as the basic plane. The right edge-holding surface f is obtained by rotating counterclockwise by 10° to 15°. y , on the right pressure edge surface f y The corresponding rectangular right blank holder profile is designed above, and a fillet R0 is made at the blank holder boundary line y0 where the left blank holder profile intersects with the right blank holder profile, R0 ≥ 100mm, to obtain an inclined overall blank holder profile, which intersects with the male mold tooling profile at the blank holder inner hole line. The overall blank holder profile removes the area surrounded by the blank holder inner hole line, that is, the inclined blank holder tooling profile is obtained. The inclined blank holder tooling profile with a larger angle is designed to reduce the overall drawing height of the butt skin drawing body. In this example, the inclination angle of the overall blank holder profile is 150°, and the fillet R0 = 200mm. In this way, the inclined blank holder tooling profile changes more smoothly, which is conducive to deep drawing.

[0022] Step 1-14: Displace the inner hole line of the blank holder outward along the blank holder tooling profile by k f =35mm~40mm, get the overall rolling bead position line, cut off the parts where the material flow is not smooth and the parts with high drawing height, that is, remove the arc segments of the fillet area on both sides of the tail termination surface and the maximum drawing height area corresponding to the multi-section enclosed surface at the end, and get three sections of local drawing bead position lines, namely the tail drawing bead position line, the outer drawing bead position line and the inner drawing bead position line. The rolling bead height h y =[4.0+(t-1.5)×2]mm, in this example h y=4.0mm, discontinuous conformal calendering ribs are designed to balance the flow rate of the blank during the butt skinning process and eliminate the risk of wrinkling on the lateral curved surface and the concave part of the main curved surface;

[0023] Step 1-15: Design the female mold tooling profile according to the male mold tooling profile and the blank holder tooling profile, and make the transition radius R of the male mold tooling profile and the blank holder tooling profile at the inner hole line of the blank holder. t =7t~9t, the combination of the male die tooling surface and the blank holder tooling surface is obtained, and it is offset in the opposite direction of the drawing direction F0, and the offset distance d j =t~1.1t, and the female die tooling profile is obtained. The relatively straight area around the female die tooling profile is the female die flange surface, which corresponds to the blank holder profile. The female die tooling profile presents a deep concave closed area, namely the female die forming surface, which corresponds to the male die tooling profile. Due to the large change in material thickness of the butt-jointed skin during the deep drawing process, the distance between the female die forming surface and the male die tooling profile is increased accordingly, that is, the offset distance of this area is set to d j =1.1t, the female die flange surface and the female die forming surface are connected by a transition fillet surface, the radius of the transition fillet surface is R g =R t -t, the female mold tooling surface consists of three parts: female mold flange surface, female mold forming surface and transition fillet surface. In this example, R t =9t=13.5mm, R g =9t-t=12mm;

[0024] Step 1-16: Corresponding to the positions of the three sections of calendering ribs on the tooling surface of the blank holder, three sections of calendering rib concave cavities are set on the female die flange surface, namely, the tail calendering rib concave cavity, the outer calendering rib concave cavity, and the inner calendering rib concave cavity. The depth of the cavity is h a =2h y ;

[0025] Step 1-17: The working surface of the female mold is consistent with the surface of the female mold tooling, the working surface of the male mold is consistent with the surface of the male mold tooling, the working surface of the blank holder is consistent with the surface of the blank holder tooling, and the gap d between the non-working surface of the male mold periphery and the inner wall of the blank holder y =0.1mm~0.2mm, the drawing mold structure is obtained as designed above;

[0026] Step 2: Determine the butt skinning depth

[0027] The butt-jointed skin drawing body is designed according to the female mold tooling profile. The female mold forming surface and the transition fillet surface constitute the main components of the butt-jointed skin drawing body. The inner edge line of the flange is offset along the female mold flange surface by k. y =20mm~30mm, the flange surface of the drawing body and the outer edge line of the drawing body are obtained, and the butt-jointed skin drawing body is composed of three parts: the female die forming surface, the transition fillet surface and the flange surface of the drawing body;

[0028] Step 3: Create the surface of the drawn body and unfold the blank

[0029] Step 3-1: With the help of the blank back calculation function of the PAMSTAMP finite element analysis software, the butt-jointed skin-drawn body is unfolded along the drawing direction F0 to obtain the theoretical unfolded blank of the butt-jointed skin-drawn body;

[0030] Step 3-2: Use the theoretical unfolded blank of the butt-jointed skin drawing body to perform deep drawing forming finite element simulation. Since the existence of the calendering rib affects the feeding speed and feeding amount of the theoretical unfolded blank, the size of the theoretical unfolded blank is optimized according to the simulation results to avoid wrinkling and cracking defects. The blank holder is increased at the maximum concave curvature of the inner side of the theoretical unfolded blank. The maximum blank holder size Z n , smoothly transition to the theoretical unfolded blank outer edge line, and obtain the process unfolded blank and process unfolded blank outer edge line of the butt skin deep drawing body. In this example, Z n =15mm;

[0031] Step 3-3: First, in the finite element simulation of deep drawing, a curved blank is simulated for the unfolded blank of the butt-skinning process to form a curved unfolded blank that matches the blank holder tooling profile, and the calculation result of the curved unfolded blank line is derived and drawn to the corresponding position of the blank holder tooling profile for positioning of the curved unfolded blank before deep drawing;

[0032] Step 4: Deep drawing

[0033] Step 4-1: After assembling the female die, male die and blank holder, place them on the worktable of the drawing machine. The worktable enters the drawing machine. The male die and the blank holder are connected to the lower bed of the drawing machine. The female die is connected to the upper bed of the drawing machine. The female die forming surface matches the male die tooling profile. The female die flange surface matches the blank holder tooling profile. Guide plates are installed on both sides of the blank holder, corresponding to the guide grooves on both sides of the female die. The guide plates are inserted into the guide grooves to control the straightness of the drawing die in the drawing direction F0.

[0034] Step 4-2: Complete the laser cutting of the process unfolded blank according to the size of the process unfolded blank, and preliminarily make the inclination angle of the process unfolded blank according to the theoretical shape of the curved surface unfolded blank and the curved surface unfolded blank line on the blank holder tooling surface to obtain the curved surface unfolded blank;

[0035] Step 4-3: Cover the upper surface of the curved surface unfolded blank with a thin layer of plastic film and apply lubricating oil, place the curved surface unfolded blank on the blank holder tooling profile according to the curved surface unfolded blank line on the blank holder tooling profile, apply lubricating oil to the contact area between the lower surface of the curved surface unfolded blank and the blank holder tooling profile, and apply lubricating oil to the female mold forming surface, transition fillet surface, and female mold flange surface that will contact the upper surface of the curved surface unfolded blank;

[0036] Step 4-4: First deep drawing

[0037] Start the machine, the blank holder rises in the opposite direction along the drawing direction F0 to the highest point of the male die, the first drawing begins, the female die moves downward, the female die tooling profile gradually fits the upper surface of the curved surface unfolded blank and the blank holder tooling profile. As the female die and the blank holder move downward synchronously, the curved surface unfolded blank begins to deform according to the male die tooling profile under the combined effect of the blank holder force and the resistance of the calendering ribs. The tail calendering ribs and the outer calendering ribs control the wrinkling at the concave part of the main curved surface of the butt-skin drawn body, and the inner calendering ribs control the wrinkling at the concave part of the lateral curved surface of the butt-skin drawn body. The increased blank holder allowance Z n The wrinkles formed at the concave part of the main curved surface are limited, and the butt-jointed skin deep-drawn body is obtained after forming. The three-stage calendering ribs increase the plastic deformation of the stainless steel and reduce the springback of the butt-jointed skin deep-drawn body. If tiny wrinkles visible to the naked eye appear at the concave part of the main curved surface of the deep-drawn body during the forming process, a thin rubber plate is placed at the concave part for compensation deep-drawing. During the compensation deep-drawing, the rubber plate is squeezed between the female die and the male die, and the rubber plate transmits downward pressure to flatten the tiny wrinkles on the deep-drawn body.

[0038] Step 4-5: Solution heat treatment

[0039] The butt-jointed skin-drawn parts are subjected to solution heat treatment at a heating temperature of 1050-1100°C, followed by air cooling. Solution heat treatment can eliminate the cold work hardening of austenitic stainless steel and improve the plasticity of the material.

[0040] Step 4-6: Secondary deep drawing

[0041] The butt-jointed skin drawn body that has been solution heat treated is subjected to secondary deep drawing to eliminate the buckling springback of the drawn body. The unit blank holding force of the secondary deep drawing is increased by 0.1MPa to 0.2MPa compared with the unit blank holding force of the first deep drawing to improve the mold fit of the butt-jointed skin drawn body.

[0042] Step 5: Hydraulic shaping

[0043] The butt skin drawn body is initially cut according to the part outline on the butt skin inspection press die, and the hydraulic shaping of the butt skin is completed using a rubber pad hydraulic press and a press die. After another cutting, the butt skin part with streamlined appearance and dimensions that meet the requirements of the drawing is obtained.

[0044] The beneficial effects of the present invention are as follows: 1) The present invention provides a closed drawing body design method with nearly circular ends and inclined flanges for the bidirectional concave structure of the butt-jointed skin, so that the open L-section hyperbolic part is bound by the edge clamp on all sides, which increases the plastic deformation of the hyperbolic bending edge and reduces the rebound jump of the butt-jointed skin. 2) By designing a three-stage calendering rib structure and a reasonable process blank shape, the flow rate and feed amount of the closed drawing body material are balanced, the wrinkling risk of the main curved surface and the lateral curved surface is controlled, and the problem of wrinkle accumulation and poor forming quality of the surface of the bidirectional concave butt-jointed skin formed by drop pressing is solved. 3) The edge clamp ring structure with a large inclination angle and the precise curved blank outline are designed to reduce the drawing height of the end and tail of the closed drawing body, accurately control the outer dimensions of the drawing body, improve the success rate of the limit drawing, and avoid the problem of cracking of equal-height drawn parts. 4) By designing the solid solution heat treatment process of stainless steel, the processing difficulty of the closed drawing body is reduced, the buckling rebound of stainless steel parts with a thickness of ≥1.5mm is eliminated, and the mold fit of the parts is improved.

[0045] The present application is further described in detail below in conjunction with the accompanying drawings of the embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a schematic diagram of the aircraft's special-shaped butt-jointed skin assembly structure.

[0047] Figure 2 This is a schematic diagram of the butt-jointed skin structure.

[0048] Figure 3 This is an effective internal extension surface design method.

[0049] Figure 4 This is a schematic diagram of the design method of the tail shape control line and the outer wall shape control line

[0050] Figure 5 This is a schematic diagram of the design method of semi-closed cavity tooling surface

[0051] Figure 6 This is a schematic diagram of the multi-section enclosing surface design method

[0052] Figure 7 This is a schematic diagram of the design method of the end side baffle surface

[0053] Figure 8 This is a schematic diagram of the design structure of the male mold tooling surface

[0054] Fig. 9 This is a schematic diagram of the design structure of the blank holder tooling surface

[0055] Fig.10 This is a schematic diagram of the design structure of the female mold tooling surface

[0056] Fig.11 This is a schematic diagram of the design structure of the butt-skin deep body

[0057] Fig.12 This is a schematic diagram of the design structure of the butt-skin deep-drawn body surface unfolding blank

[0058] Fig.13 This is a schematic diagram of the assembly gap section of the three major components of the deep drawing mold

[0059] Fig.14 This is the assembly structure diagram of the three major components of the deep drawing mold

[0060] Explanation of numbers in the figure: 1 butt skin, 2 saddle skin, 3 lateral curved surface, 4 main curved surface, 5 inner surface, 6 inner extension profile, 7 main extension profile, 8 side extension profile, 9 effective inner extension profile, 10 tail end surface, 11 outer wall end surface, 12 semi-enclosed cavity tooling profile, 13 multi-section enclosing profile, 14 end side stop profile, 15 end space cut-off line, 16 closed tooling profile, 17 opening edge line, 18 male mold tooling profile, 19 transition pressure line, 20 pressure ring inner hole line, 21 left pressure ring profile, 22 right pressure ring profile, 23 pressure ring tooling Profile, 24 tail rib, 25 outer rib, 26 inner rib, 27 transition fillet, 28 female die flange, 29 female die forming surface, 30 female die tooling profile, 31 tail rib cavity, 32 outer rib cavity, 33 inner rib cavity, 34 flange inner edge line, 35 drawn body flange, 36 drawn body outer edge line, 37 drawn body, 38 process unfolded blank, 39 process unfolded blank outer edge line, 40 curved surface unfolded blank, 41 curved surface unfolded blank line, 42 theoretical unfolded blank, 43 female die, 44 male die, 45 pressure ring, 46 guide plate DETAILED DESCRIPTION

[0061] Referring to the accompanying drawings, the aircraft sheet metal parts provided in the embodiment are as follows Figure 1-2 As shown, the aircraft stainless steel butt skin 1 has a complex shape, a hyperbolic structure, and a bidirectional concave. The material thickness is in the range of 1.5mm to 2.5mm. The butt skin 1 has an L-shaped cross section with a large edge. The edge height is close to 300mm. It is a typical drop-pressed structural part. The surface quality of the drop-pressed forming is poor and the forming quality cannot be guaranteed. The assembly relationship around the butt skin 1 is complex, and one side is butt-jointed with the saddle-shaped skin 2.

[0062] like Figure 2 to Figure 14 As shown, a drawing forming method of a hyperbolic bidirectional concave stainless steel butt skin, wherein the drawing die is composed of a female die 43, a male die 44, and a blank holder 45. A drawing die structure model is established in a CATIA environment based on a digital model of a butt skin 1 product, and a drawing body 37 of the butt skin 1 is designed based on a drawing model surface. The forming method includes the following steps:

[0063] Step 1: Determine the drawing die structure

[0064] Step 1-1: The surface with smaller curvature among the two concave surfaces in the digital model of the butt-jointed skin 1 product is taken as the main curved surface 4, the concave curved surface with larger curvature in the cross section is taken as the lateral curved surface 3, the inner profile surface of the butt-jointed skin 1 is taken as the inner surface 5, and t is the material thickness of the butt-jointed skin 1, and t=1.5 mm in this example;

[0065] Step 1-2: extract the boundary line of the inner surface 5 of the butt-jointed skin 1, perform external extension, and obtain an inner extension profile 6, which is composed of a main extension profile 7 and a side extension profile 8. The main extension profile 7 corresponds to the main curved surface 4, and the side extension profile 8 corresponds to the side curved surface 3. The main extension profile 7 and the side extension profile 8 intersect at the curved surface boundary line n;

[0066] Step 1-3: The three boundary lines of the inner surface 5 of the butt-jointed skin 1 involved in the side extension profile 8 are respectively recorded as a, b, and c, where a is the outer shape line of the inner wall of the butt-jointed skin 1, b is the outer shape line of the bent edge of the butt-jointed skin 1, and c is the outer shape line of the tail of the butt-jointed skin 1. The outer shape line of the end of the inner surface 5 of the butt-jointed skin 1 included in the main extension profile 7 is recorded as s, and the four curves s, a, b, and c are moved outwardly along the inner extension profile 6 in parallel by d0=5mm~20mm to obtain the corresponding four curves s1, a1, b1, and c1, where s1 is the effective boundary line of the end, a1 is the effective boundary line of the inner wall, b1 is the effective boundary line of the bent edge, and c1 is the effective boundary line of the tail. S, A, B, C, and D are s1, a1, The endpoints of the four curves b1 and c1, the surface boundary line n and the tail effective boundary line c1 intersect at point E. The highest horizontal plane p is established through points A, D, and E. The distance from point B to the highest horizontal plane p is the drawing height h. The value of d0 affects the drawing height h. When determining the translation dimension d0, it should be noted that a higher point A will increase the drawing height h and increase the depth of the main extension surface depression. Similarly, a lower point B will also increase the drawing height h. The influence of points D and E on the drawing height h is weaker than that of points A and B. Therefore, in this example, the distance between s and s1 is d0 = 5mm, the distance between a and a1 is d0 = 8mm, the distance between b and b1 is d0 = 10mm, and the distance between c and c1 is d0 = 15mm.

[0067] Step 1-4: Make an initial edge pressing surface f through point B, parallel to the highest horizontal plane p, the initial edge pressing surface f intersects with the inner extension profile 6 of the butt skin 1 at a curve m, and the inner extension profile 6 of the butt skin 1 is cut with the curve m to obtain an effective inner extension profile 9 of the butt skin 1;

[0068] Step 1-5: Use points D, E and C to establish plane g, make plane k parallel to plane g, and the distance between plane k and plane g is l1≤60mm, plane k intersects with the effective inner extension profile 9 of the docking skin 1 at curve d, plane k intersects with the main extension profile 7 of the docking skin 1 at points F and G, connect points F and G to obtain straight line segment FG, based on plane k and with straight line segment FG as the rotation axis, make the angle θ between plane j and plane k = 5° to 8°, plane j intersects with the effective inner extension profile 9 of the docking skin 1 at curve d1, on the side extension profile 8 of the docking skin 1, the angle between curve d1 and curve d is θ, and curve d1 is the tail shape control line;

[0069] Step 1-6: Connect point D and point S to obtain line segment DS, project line segment DS to the main extension profile 7 of the docking skin 1 to obtain curve e, establish plane q through line segment DS and curve e, make plane r parallel to plane q, and the distance between plane r and plane q is l2≤60mm, plane r intersects with the effective inner extension profile 9 of the docking skin 1 at points M and N, connect point M and point N to obtain straight line segment MN, based on plane r, with straight line segment MN as the rotation axis, make the angle between plane u and plane r be θ1=3°~5°, plane u intersects with the effective inner extension profile 9 at curve v, and curve v is the outer wall shape control line;

[0070] Step 1-7: Use the outer wall shape control line v, the tail shape control line d1, the initial pressure edge surface f, the plane j, and the plane u to close the effective inner extension profile 9 of the docking skin 1 through three-dimensional mapping, and obtain the tail termination surface 10, the outer wall termination surface 11, the intersection line z of the tail termination surface 10 and the outer wall termination surface 11, the intersection line y of the tail termination surface 10 and the initial pressure edge surface f, and the intersection line x of the outer wall termination surface 11 and the initial pressure edge surface f. The three boundary lines of the intersection line z, the outer wall shape control line v, and the tail shape control line d1 are used to make the transition fillet R between the surfaces. It should be noted that the transition fillet R on the tail termination surface 10 is ≤ l1, and the transition fillet R on the outer wall termination surface 11 is ≤ (l2-5mm). The plane i is established through the inner wall effective boundary line a1, and the end of the effective inner extension profile 9 is cut by the intersection line of the effective inner extension profile 9 and the plane i to obtain a semi-closed cavity tooling profile 12 closed on three sides;

[0071] Step 1-8: Make a plane w parallel to the end effective boundary line s1 and at a distance of l0, where R < l0 ≤ 100 mm. The intersection line x of the outer wall termination surface 11 intersects the plane w at point B1. Make a circle Q1 with a radius of R1 in the initial blank-holding surface f and tangent to the intersection line x of the outer wall termination surface 11 at point B1. The value range of the radius R1 is h ≤ R1 ≤ (h + 20 mm). The plane i passing through point B intersects the circle Q1 at point A1. In the plane i, along the inner wall effective boundary line a1, make a fair curve AA1 through point A to obtain a multi-section starting curve f1 with the starting point at B and the ending point at A1. In the initial blank-holding surface f, make a tangent to the curve m of the semi-closed cavity tooling surface 12 through point B and smoothly transition to the circle Q1 to obtain a multi-section ending curve g1 with the starting point at B and the ending point at A1;

[0072] Step 1-9: Connect point B and point A1 to obtain a straight line segment BA1. After bisecting it into l segments, make a vertical plane of the straight line segment BA1 through the bisecting point to obtain (l - 1) vertical planes i1, i2, i3... i l-1 , within the range enclosed by the multi-section starting curve f1 and the multi-section ending curve g1, respectively make longitudinal guiding lines of the multi-section surrounding surface 13 in each vertical plane to obtain (l - 1) guiding lines m1, m2, m3... m l-1 , the starting point of each guiding line is on the multi-section starting curve f1, and the ending point is on the multi-section ending curve g1. The (l - 1) guiding lines are structurally similar to each other to ensure that the formed multi-section surrounding surface 13 has a fair curvature. The curvature of the upper part of each guiding line is large and the lower part tends to be flat. The lower part of the guiding line has an inclination angle of not less than 70° with the initial blank-holding surface f. Using the multi-section curve command, input the multi-section starting curve f1, the multi-section ending curve g1, and the guiding lines m1, m2, m3... m l-1 , to form the multi-section surrounding surface 13. It should be noted that in order to better control the curvature of the multi-section surrounding surface 13, multiple transverse multi-section transition curves n1, n2, n3, n4... can be established between the multi-section starting curve f1 and the multi-section ending curve g1 to increase the limitation of the input conditions. In this way, the obtained multi-section surrounding surface 13 better meets the design requirements. In this example, 6 longitudinal guiding lines m1, m2, m3, m4, m5, m6 and 4 transverse multi-section transition curves n1, n2, n3, n4 are established. The design principle of the multi-section surrounding surface 13 is not to increase the drawing height h and ensure the smoothness of the outer shape of the multi-section surrounding surface 13;

[0073] Step 1-10: Denote the small end arc between point A1 and point B1 on the circle Q1 as the end side contour line t1. Using the curve between point A and point A1 on the multi-section starting curve f1 as the guiding curve, obtain the end side blocking surface 14 by using the sweeping function, which intersects the semi-closed cavity tooling surface 12 at the end space cut-off line 15;

[0074] Step 1-11: a connecting fillet R2 of the end side baffle profile 14 and the semi-enclosed cavity tooling profile 12 is formed at the end space cut-off line 15, and the fillet R2 is combined with the multi-section surrounding profile 13 to form a closed tooling profile 16, an opening edge line 17 of the closed tooling profile 16 is extracted, and the closed tooling profile 16 is stretched along the closed tooling profile 16 to obtain a male mold tooling profile 18 for the butt-jointed skin 1;

[0075] Step 1-12: Set the direction perpendicular to the initial blanking surface f downward as the drawing direction F0 of the butt skin 1, translate the initial blanking surface f downward along the drawing direction F0 by 7t to 9t, and intersect with the male mold tooling profile 18 at the transitional blanking line 19. The projection point of point B on the transitional blanking line 19 is recorded as point B0. Make a blanking boundary line y0 through point B0 and along the direction of the intersection line y on the tail end surface 10. In this example, the downward translation distance of the initial blanking surface f along the drawing direction F0 is 9t = 13.5 mm;

[0076] Step 1-13: Take the blank-holding boundary line y0 as the rotation axis and the initial blank-holding surface f as the basic plane, rotate clockwise by 10° to 15° to obtain the left blank-holding surface f z , on the left pressure edge surface f z The corresponding rectangular left edge-holding ring profile 21 is designed on the top, and the edge-holding boundary line y0 is used as the rotation axis and the initial edge-holding surface f is used as the basic plane. The right edge-holding surface f is obtained by rotating counterclockwise by 10° to 15°. y , on the right pressure edge surface f y The corresponding rectangular right blank holder profile 22 is designed above, and a fillet R0 is made at the blanking boundary line y0 where the left blank holder profile 21 and the right blank holder profile 22 intersect, R0 ≥ 100mm, so as to obtain an overall inclined blank holder surface, which intersects with the male mold tooling profile 18 at the blank holder inner hole line 20, and the overall blank holder surface removes the area surrounded by the blank holder inner hole line 20, that is, the inclined blank holder tooling profile 23 is obtained, and the inclined blank holder tooling profile 23 with a larger angle is designed to reduce the overall drawing height of the deep drawing body 37 of the docking skin 1. In this example, the inclination angle of the overall blank holder surface is 150°, and the fillet R0 = 200mm, so that the inclined blank holder tooling profile 23 changes more smoothly, which is conducive to deep drawing;

[0077] Step 1-14: Displace the inner hole line 20 of the blank holder outward along the blank holder tooling profile 23 by k f=35mm~40mm, the overall rolling bead position line is obtained, the overall rolling bead position line is optimized, the parts where the material flow is not smooth and the parts with high drawing height are disconnected, that is, the arc segments of the fillet area on both sides of the tail termination surface 10 and the maximum drawing height area corresponding to the multi-section enclosing profile 13 at the end are removed, and three sections of local drawing bead position lines are obtained, namely, the tail drawing bead 24 position line, the outer drawing bead 25 position line and the inner drawing bead 26 position line. The rolling bead height h y =[4.0+(t-1.5)×2]mm, in this example h y =4.0mm, discontinuous conformal calendering ribs are designed to balance the flow rate of the blank 18 during the butt skin 1 process, eliminating the risk of wrinkling at the concave parts of the lateral curved surface 3 and the main curved surface 4;

[0078] Step 1-15: Design the female mold tooling profile 30 based on the male mold tooling profile 18 and the blank holder tooling profile 23, and make the transition fillet R of the male mold tooling profile 18 and the blank holder tooling profile 23 at the inner hole line 20 of the blank holder. t =7t~9t, a combination of the male mold tooling profile 18 and the blank holder tooling profile 23 is obtained, which is offset in the opposite direction of the drawing direction F0 by a distance d j =t~1.1t, the female mold tooling profile 30 is obtained. The relatively straight area around the female mold tooling profile 30 is the female mold flange surface 28, which corresponds to the blank holder tooling profile 23. The female mold tooling profile 30 has a deep concave closed area, namely the female mold forming surface 29, which corresponds to the male mold tooling profile 18. Due to the large change in material thickness of the butt-jointed skin 1 during the deep drawing process, the distance between the female mold forming surface 29 and the male mold tooling profile 18 is correspondingly increased, that is, the offset distance is set as d j =1.1t, the female die flange surface 28 and the female die forming surface 29 are connected by a transition fillet surface 27, and the radius of the transition fillet surface 27 is R g =R t -t, the female mold tooling profile 30 is composed of three parts: the female mold flange surface 28, the female mold forming surface 29 and the transition fillet surface 27. In this example, R t =9t=13.5mm, R g =9t-t=12mm;

[0079] Step 1-16: Corresponding to the positions of the three sections of calendering ribs on the blank holder tooling profile 23, three sections of calendering rib concave cavities are set on the female die flange surface 28, namely, the tail calendering rib concave cavities 31, the outer calendering rib concave cavities 32, and the inner calendering rib concave cavities 33. The depth of the concave cavities is h a =2h y ;

[0080] Step 1-17: The working surface of the female mold 43 is consistent with the female mold tooling profile 30, the working surface of the male mold 44 is consistent with the male mold tooling profile 18, the working surface of the blank holder 45 is consistent with the blank holder tooling profile 23, and the gap d between the outer peripheral non-working surface of the male mold 44 and the inner side wall of the blank holder 45 y =0.1mm~0.2mm, the drawing mold structure is obtained as designed above;

[0081] Step 2: Determine the butt skin 1 drawing body 37

[0082] The deep-drawn body 37 of the butt-jointed skin 1 is designed according to the female mold tooling profile 30. The female mold forming surface 29 and the transition fillet surface 27 constitute the main components of the deep-drawn body 37 of the butt-jointed skin 1. The inner edge line 34 of the flange is offset along the female mold flange surface 28. y = 20mm~30mm, the drawn body flange surface 35 and the drawn body outer edge line 36 are obtained, and the butt-jointed skin 1 drawn body 37 is composed of three parts: the female die forming surface 29, the transition fillet surface 27 and the drawn body flange surface 35;

[0083] Step 3: Create the curved surface of the drawn body 37 and unfold the blank 40

[0084] Step 3-1: With the help of the blank back calculation function of the PAMSTAMP finite element analysis software, the butt-jointed skin 1 drawn body 37 is unfolded along the drawing direction F0 to obtain a theoretical unfolded blank 42 of the butt-jointed skin 1 drawn body 37;

[0085] Step 3-2: The theoretical unfolded blank 42 of the drawn body 37 of the butt-jointed skin 1 is used for deep drawing forming finite element simulation. Since the existence of the calendering rib affects the feeding speed and feeding amount of the theoretical unfolded blank 42, the size of the theoretical unfolded blank 42 is optimized according to the simulation results to avoid wrinkling and cracking forming defects, and the blank holding margin Z is increased at the maximum concave curvature of the inner side of the theoretical unfolded blank 42. n , smoothly transition to the outer edge line of the theoretical unfolded blank 42, and obtain the process unfolded blank 38 and the process unfolded blank outer edge line 39 of the deep-drawn body 37 of the butt-jointed skin 1. In this example, Z n =15mm;

[0086] Step 3-3: First, in the finite element simulation of deep drawing, a curved blanking simulation of the unfolded blank 38 of the butt-jointed skin 1 process is performed to form a curved unfolded blank 40 that matches the blank holder tooling profile 23, and the calculation result of the curved unfolded blank line 41 is derived and drawn to the corresponding position of the blank holder tooling profile 23 for positioning the curved unfolded blank 40 before deep drawing;

[0087] Step 4: Deep drawing

[0088] Step 4-1: After the female mold 43, the male mold 44 and the blank holder 45 are assembled, they are placed on the work surface of the drawing machine. The work table enters the drawing machine. The male mold 44 and the blank holder 45 are connected to the lower bed surface of the drawing machine. The female mold 43 is connected to the upper bed surface of the drawing machine. The female mold forming surface 29 matches the male mold tooling profile 18. The female mold flange surface 28 matches the blank holder tooling profile 23. Guide plates 46 are installed on both sides of the blank holder 45, corresponding to the guide groove positions on both sides of the female mold 43. The guide plates 46 are inserted into the guide grooves to control the straightness of the drawing mold in the drawing direction F0;

[0089] Step 4-2: Complete laser cutting of the process unfolded blank 38 according to the size of the process unfolded blank 38, and preliminarily make the inclination angle of the process unfolded blank 38 according to the theoretical shape of the curved surface unfolded blank 40 and the curved surface unfolded blank line 41 on the blank holder tooling profile 23 to obtain the curved surface unfolded blank 40;

[0090] Step 4-3: Cover the upper surface of the curved surface unfolded blank 40 with a thin layer of plastic film and apply lubricating oil, place the curved surface unfolded blank 40 on the blank holder tooling profile 23 according to the curved surface unfolded blank line 41 on the blank holder tooling profile 23, apply lubricating oil to the contact area between the lower surface of the curved surface unfolded blank 40 and the blank holder tooling profile 23, and apply lubricating oil to the female mold forming surface 29, the transition fillet surface 27 and the female mold flange surface 28 that will contact the upper surface of the curved surface unfolded blank 40 of the female mold tooling profile 30;

[0091] Step 4-4: First deep drawing

[0092] The machine tool is turned on, and the blank holder 45 rises in the opposite direction along the drawing direction F0 to the highest point of the male die 44. The first drawing process begins, and the female die 43 moves downward. The female die tooling profile 30 gradually fits the upper surface of the curved surface unfolded blank 40 and the blank holder tooling profile 23. As the female die 43 and the blank holder 45 move downward synchronously, the curved surface unfolded blank 40 begins to deform according to the male die tooling profile 23 under the combined effect of the blank holder force and the resistance of the calendering ribs. The tail calendering ribs 24 and the outer calendering ribs 25 control the wrinkling at the concave part of the main curved surface 4 of the deep body 37 of the butt skin 1, and the inner calendering ribs 26 control the wrinkling at the concave part of the lateral curved surface 3 of the deep body 37 of the butt skin 1. The increased blank holder margin Z n The wrinkles formed at the concave part of the main curved surface 4 are limited, and the butt-jointed skin 1 deep-drawn body 37 is obtained after forming. The three-stage calendering ribs increase the plastic deformation of the stainless steel and reduce the springback of the butt-jointed skin 1 deep-drawn body 37. During the forming process, if the concave part of the main curved surface 4 of the deep-drawn body 1 has tiny wrinkles visible to the naked eye, a thin rubber plate is placed at the concave part to perform compensation drawing. During the compensation drawing, the rubber plate is squeezed between the female mold 43 and the male mold 44, and the rubber plate transmits downward pressure to flatten the tiny wrinkles on the deep-drawn body 1.

[0093] Step 4-5: Solution heat treatment

[0094] The butt skin 1 drawing part 37 is subjected to solution heat treatment at a heating temperature of 1050-1100°C, and then air-cooled. The solution heat treatment can eliminate the cold work hardening of austenitic stainless steel and improve the plasticity of the material.

[0095] Step 4-6: Secondary deep drawing

[0096] The solution heat treated butt joint skin 1 deep drawing body 37 is subjected to secondary deep drawing to eliminate the buckling springback of the deep drawing body 37. The unit blank holding force of the secondary forming is increased by 0.1MPa to 0.2MPa compared with the unit blank holding force of the first forming to improve the mold adhesion of the deep drawing body 37 of the butt joint skin 1.

[0097] Step 5: Hydraulic shaping

[0098] The butt skin 1 drawn body 37 is preliminarily cut according to the part outline on the butt skin 1 inspection press die, and the butt skin 1 is hydraulically corrected using a rubber pad hydraulic press and a press die. After cutting again, a butt skin 1 part with streamlined outline and composite dimensions that meets the requirements of the drawing is obtained.

[0099] It should be noted that the present invention is suitable for hyperbolic concave skins with a maximum edge height within the range of 250mm to 350mm and an elongation A of stainless steel material ≥ 25%; when designing a multi-section enveloping profile, if the drawing height h is increased, the area exceeding the highest horizontal surface p can be flattened and a smooth transition of the profile can be performed without affecting the appearance of the butt-jointed skin; when designing the drawing die structure, the PAMSTAMP finite element analysis software drawing forming simulation module is used to detect whether there is a closed angle in the design of the female mold tooling profile and the male mold tooling profile, and the profiles of the various components of the drawing die are optimized through CATIA drawing to ensure smooth material flow during the simulation process; when using a rubber plate to eliminate wrinkles, the thickness of the rubber plate is not more than 3mm, and the Shore hardness is about 75 degrees; the curvature of the butt-jointed skin is complex, and when supplementing the drawing process profile, try to use a larger circle Corner radius transition, such as the transition radius R on the tail termination surface, the transition radius R on the outer wall termination surface, and the connection radius R2 of the end side baffle profile and the semi-closed cavity tooling profile in this example, the value range is between 40mm and 50mm to ensure smooth and streamlined profile; the end profile of the butt-jointed skin shows a rapid upward trend, while the upward trend of the tail profile is relatively gentle. Therefore, when supplementing the process profile, the distance d0 between the end effective boundary line s1 and the end outer shape line s is as small as possible, otherwise the drawing height h will be increased, further increasing the difficulty of butt-jointed skin forming; the temperature of the solution heat treatment in this example is only applicable to austenitic stainless steels such as 0Cr18Ni9, while for precipitation hardening stainless steel A286, the intermediate solution heat treatment temperature range is 968-996°C; the butt-jointed skin drawing die and drawing body structure design method described in the present invention are also applicable to liquid filling forming.

Claims

1. A deep drawing method for a hyperbolic bidirectional concave stainless steel butt skin, characterized in that It includes a drawing die and a butt skin drawing body, the drawing die includes a female die, a male die, and a blank holder, the female die working surface is consistent with the female die tooling profile, the male die working surface is consistent with the male die tooling profile, the blank holder working surface is consistent with the blank holder tooling profile, and the gap d between the male die outer peripheral non-working surface and the inner side wall of the blank holder y =0.1mm~0.2mm, the butt-jointed skin drawing body is designed according to the drawing model surface, the curved surface of the drawing body is unfolded and placed on the blank holder, the butt-jointed skin drawing body is drawn by a machine tool, and then the butt-jointed skin is obtained by hydraulic correction, and the specific process includes the following steps: Step 1: determine the drawing die structure model; Step 2: Determine the butt-jointed skin-drawn body; Step 3: Create the curved surface of the drawn body and unfold the blank. The specific process is as follows: 3-1 With the help of the blank back calculation function of the PAMSTAMP finite element analysis software, the butt-jointed skin-drawn body is unfolded along the drawing direction F0 to obtain the theoretical unfolded blank of the butt-jointed skin-drawn body; 3-2 The theoretical unfolded blank of the butt-jointed skin drawing body is used for deep drawing forming finite element simulation. Since the existence of the calendering rib affects the feeding speed and feeding amount of the theoretical unfolded blank, the size of the theoretical unfolded blank is optimized according to the simulation results to avoid wrinkling and cracking defects. The edge holding margin Z is increased at the maximum concave curvature of the inner side of the theoretical unfolded blank. n , smoothly transition to the theoretical unfolded blank outer edge line, and obtain the process unfolded blank and the process unfolded blank outer edge line of the butt-jointed skin drawing body; 3-3 In the finite element simulation of deep drawing, the curved blank holding simulation of the unfolded blank of the butt skinning process is first performed to form a curved unfolded blank that matches the blank holding ring tooling profile, and the calculation result of the curved unfolded blank line is derived and drawn to the corresponding position of the blank holding ring tooling profile for positioning of the curved unfolded blank before deep drawing; Step 4: Deep drawing. The specific process is as follows: 4-1 After the female die, male die and blank holder are assembled, they are placed on the worktable of the drawing machine. The worktable enters the drawing machine. The male die and the blank holder are connected to the lower bed of the drawing machine. The female die is connected to the upper bed of the drawing machine. The female die forming surface matches the male die tooling profile. The female die flange surface matches the blank holder tooling profile. Guide plates are installed on both sides of the blank holder, corresponding to the guide grooves on both sides of the female die. The guide plates are inserted into the guide grooves to control the straightness of the drawing die in the drawing direction F0. 4-2 Complete the laser cutting of the process unfolded blank according to the size of the process unfolded blank, and preliminarily make the inclination angle of the process unfolded blank according to the theoretical shape of the curved surface unfolded blank and the curved surface unfolded blank line on the blank holder tooling surface to obtain the curved surface unfolded blank; 4-3 Cover the upper surface of the curved surface unfolded blank with a thin layer of plastic film and apply lubricating oil, place the curved surface unfolded blank on the blank holder tooling profile according to the curved surface unfolded blank line on the blank holder tooling profile, apply lubricating oil to the contact area between the lower surface of the curved surface unfolded blank and the blank holder tooling profile, and apply lubricating oil to the female mold forming surface, transition fillet surface and female mold flange surface that will contact the upper surface of the curved surface unfolded blank; 4-4 First deep drawing: Start the machine, the blank holder rises in the opposite direction along the drawing direction F0 to the highest point of the male die, the first deep drawing begins, the female die moves downward, the female die tooling profile gradually fits the upper surface of the curved surface unfolded blank and the blank holder tooling profile. As the female die and the blank holder move downward synchronously, the curved surface unfolded blank begins to deform according to the male die tooling profile under the combined effect of the blank holder force and the resistance of the calendering ribs. The tail calendering ribs and the outer calendering ribs control the wrinkling at the concave part of the main curved surface of the butt-skin drawn body, and the inner calendering ribs control the wrinkling at the concave part of the lateral curved surface of the butt-skin drawn body. The increased blank holder allowance Z n The wrinkles formed at the concave part of the main curved surface are limited, and the butt-jointed skin deep-drawn body is obtained after forming. The three-stage calendering ribs increase the plastic deformation of the stainless steel and reduce the springback of the butt-jointed skin deep-drawn body. If tiny wrinkles visible to the naked eye appear at the concave part of the main curved surface of the deep-drawn body during the forming process, a thin rubber plate is placed at the concave part for compensation deep-drawing. During the compensation deep-drawing, the rubber plate is squeezed between the female die and the male die, and the rubber plate transmits downward pressure to flatten the tiny wrinkles on the deep-drawn body. 4-5 Solution heat treatment: The butt-jointed skin drawing parts are subjected to solution heat treatment at a heating temperature of 1050-1100°C, followed by air cooling; 4-6 Secondary deep drawing: The butt-jointed skin drawing body that has been subjected to solution heat treatment is subjected to secondary deep drawing to eliminate the buckling springback of the drawing body. The unit blank holding force of the secondary deep drawing is increased by 0.1MPa to 0.2MPa compared with the unit blank holding force of the first deep drawing; Step 5: Hydraulic alignment.

2. The deep drawing method of a hyperbolic bidirectional concave stainless steel butt skin according to claim 1 is characterized in that The step 1 of determining the drawing die structure is to establish a drawing die structure model in the CATIA environment by docking the digital model of the skin product. The specific process is as follows: 2-1 The surface with smaller curvature among the two concave surfaces in the digital model of the butt-jointed skin product is taken as the main surface, the concave surface with larger curvature in the cross section is taken as the lateral surface, the inner profile of the butt-jointed skin is taken as its inner surface, and t is the material thickness of the butt-jointed skin; 2-2 Extract the boundary line of the inner surface of the butt-jointed skin, perform external extension, and obtain the inner extension profile, which is composed of a main extension profile and a side extension profile. The main extension profile corresponds to the main curved surface, and the side extension profile corresponds to the lateral curved surface. The main extension profile and the side extension profile intersect at the curved surface boundary line n; 2-3 The three inner surface boundary lines of the butt-skin involved in the side extension profile are respectively denoted as a, b, and c, where a is the inner side wall contour line of the butt-skin, b is the curved edge contour line of the butt-skin, and c is the tail contour line of the butt-skin. The inner surface end contour line of the butt-skin included in the main extension profile is denoted as s. The four curves s, a, b, and c are parallelly moved outward along the inner extension profile by d0 = 5mm to 20mm to obtain the corresponding four curves s1, a1, b1, and c1. S1 is the effective boundary line of the end, a1 is the effective boundary line of the inner side wall, b1 is the effective boundary line of the curved edge, and c1 is the effective boundary line of the curved edge. is the effective boundary line of the tail, S, A, B, C, D are the endpoints of the four curves s1, a1, b1, c1 respectively, the surface boundary line n intersects with the effective boundary line c1 of the tail at point E, the highest horizontal plane p is established through points A, D, and E, the distance from point B to the highest horizontal plane p is the drawing height h, the value of d0 affects the drawing height h, when determining the translation dimension d0, it should be noted that a higher point A will increase the drawing height h and increase the concave depth of the main extension profile, and a lower point B will also increase the drawing height h, while the influence of points D and E on the drawing height h is weaker than that of points A and B; 2-4 Make an initial edge pressing surface f through point B, which is parallel to the highest horizontal plane p. The initial edge pressing surface f intersects with the inner extension profile of the butt skin at a curve m. The inner extension profile of the butt skin is cut with the curve m to obtain an effective inner extension profile of the butt skin. 2-5 Use points D, E and C to establish plane g, make plane k parallel to plane g, and the distance between plane k and plane g is l1≤60mm, plane k intersects with the effective inner extension profile of the docking skin at curve d, plane k intersects with the main extension profile of the docking skin at points F and G, connect points F and G to obtain straight line segment FG, based on plane k and with straight line segment FG as the rotation axis, make the angle θ between plane j and plane k = 5°~8°, plane j intersects with the effective inner extension profile of the docking skin at curve d1, on the side extension profile of the docking skin, the angle between curve d1 and curve d is θ, and curve d1 is the tail shape control line; 2-6 Connect point D and point S to obtain straight line segment DS, project straight line segment DS to the main extension surface of the docking skin to obtain curve e, establish plane q through straight line segment DS and curve e, make plane r parallel to plane q, and the distance between plane r and plane q is l2≤60mm, plane r intersects with the effective inner extension surface of the docking skin at points M and N, connect point M and point N to obtain straight line segment MN, take plane r as the basis, take straight line segment MN as the rotation axis, make plane u and plane r have an angle of θ1=3°~5°, plane u intersects with the effective inner extension surface at curve v, curve v is the outer wall shape control line; 2-7 By using the outer wall contour control line v, the tail contour control line d1, the initial blank-holding surface f, the plane j, and the plane u, the effective inner extension surface of the butt skin is enclosed through 3D drawing to obtain the tail termination surface, the outer wall termination surface, the intersection line z of the tail termination surface and the outer wall termination surface, the intersection line y of the tail termination surface and the initial blank-holding surface f, and the intersection line x of the outer wall termination surface and the initial blank-holding surface f. Fillets R are made at the transitions between the surfaces for the three boundary lines of the intersection line z, the outer wall contour control line v, and the tail contour control line d1. It should be noted that the fillet R on the tail termination surface is ≤ l1, and the fillet R on the outer wall termination surface is ≤ (l2 - 5 mm). A plane i is established through the effective boundary line a1 of the inner wall, and the end of the effective inner extension surface is cut by the intersection line of the effective inner extension surface and the plane i to obtain a semi-closed cavity tooling surface that is enclosed on three sides; 2-8 Make a plane w that is nearly parallel to the end effective boundary line s1 and is at a distance of l0, where R < l0 ≤ 100 mm. The intersection line x of the outer wall termination surface intersects the plane w at point B1. Make a circle Q1 with a radius of R1 in the initial blank-holding surface f and tangent to the intersection line x of the outer wall termination surface at point B1. The value range of the radius R1 is h ≤ R1 ≤ (h + 20 mm). The plane i passing through point B intersects the circle Q1 at point A1. In the plane i, a fair curve AA1 is made along the effective boundary line a1 of the inner wall and passing through point A to obtain a multi-section starting curve f1 with the starting point at B and the ending point at A1. In the initial blank-holding surface f, a tangent to the semi-closed cavity tooling surface curve m is made through point B and smoothly transitioned to the circle Q1 to obtain a multi-section ending curve g1 with the starting point at B and the ending point at A1; 2-9 Connect point B and point A1 to obtain straight line segment BA1. After dividing it into l segments, make a perpendicular surface through the bisection point to obtain (l-1) perpendicular surfaces i1, i2, i3, etc. l-1 , within the range of the multi-section starting curve f1 and the multi-section ending curve g1, longitudinal guide lines of the multi-section enclosing profile are made in each vertical plane, and (l-1) guide lines m1, m2, m3...m l-1 , the starting point of each guide line is on the multi-section starting curve f1, and the ending point is on the multi-section ending curve g1. The (l-1) guide lines have structural similarities to each other to ensure that the curvature of the formed multi-section enclosing profile is smooth. The upper curvature of each guide line is large and the lower part tends to be straight. The lower part of the guide line has an inclination angle of not less than 70° with the initial blank holding surface f. Use the multi-section curve command to input the multi-section starting curve f1, the multi-section ending curve g1, and the guide lines m1, m2, m3...m l-1 , forming a multi-section enclosing surface. One thing that needs to be noted is that in order to better control the curvature of the multi-section enclosing surface, multiple transverse multi-section transition curves n1, n2, n3, n4, etc. can be established between the multi-section starting curve f1 and the multi-section ending curve g1 to increase the restrictions on the input conditions. In this way, the multi-section enclosing surface obtained better meets the design requirements; 2-10 Denote the small end arc between point A1 and point B1 on the circle Q1 as the end side contour line t1. Use the section of the curve between point A and point A1 on the multi-section starting curve f1 as the guiding curve, and use the sweeping function to obtain the end side blocking surface, which intersects the semi-closed cavity tooling surface at the end space cut-off line; 2-11 Make a connecting fillet R2 between the end side blocking surface and the semi-closed cavity tooling surface at the end space cut-off line, combine it with the multi-section enclosed surface to form a closed tooling surface, extract the opening edge line of the closed tooling surface, and stretch along the closed tooling surface to obtain the male die tooling surface of the butt skin; 2-12 Set the direction perpendicular to the initial blank-holding surface f downward as the drawing direction F0 of the butt skin. Translate the initial blank-holding surface f downward along the drawing direction F0 by 7t to 9t, which intersects the male die tooling surface at the transition blank-holding line. The projection point of point B on the transition blank-holding line is denoted as point B0. Make a blank-holding demarcation line y0 through point B0 and along the direction of the intersection line y on the tail termination surface; 2-13 Taking the edge pressure boundary line y0 as the rotation axis and the initial edge pressure surface f as the basic plane, rotate clockwise by 10°~15° to obtain the left edge pressure surface f z , on the left pressure edge surface f z The corresponding rectangular left edge-holding ring profile is designed on the top, and the edge-holding boundary line y0 is used as the rotation axis and the initial edge-holding surface f is used as the basic plane. The right edge-holding surface f is obtained by rotating counterclockwise by 10° to 15°. y , on the right pressure edge surface f y Design the corresponding rectangular right blank holder profile, make a fillet R0 at the blank holder boundary line y0 where the left blank holder profile intersects with the right blank holder profile, R0 ≥ 100mm, and obtain an inclined overall blank holder profile, which intersects with the male mold tooling profile at the blank holder inner hole line. Remove the area surrounded by the blank holder inner hole line from the overall blank holder profile, and obtain an inclined blank holder tooling profile; 2-14 The inner hole line of the blank holder is offset outward along the blank holder tooling profile k f =35mm~40mm, get the overall rolling bead position line, cut off the parts where the material flow is not smooth and the parts with high drawing height, that is, remove the arc segments of the fillet area on both sides of the tail termination surface and the maximum drawing height area corresponding to the multi-section enclosed surface at the end, and get three sections of local drawing bead position lines, namely the tail drawing bead position line, the outer drawing bead position line and the inner drawing bead position line. The rolling bead height h y =[4.0+(t-1.5)×2]mm; 2-15 Design the female mold tooling profile based on the male mold tooling profile and the blank holder tooling profile, and make the transition radius R of the male mold tooling profile and the blank holder tooling profile at the inner hole line of the blank holder t =7t~9t, the combination of the male die tooling surface and the blank holder tooling surface is obtained, and it is offset in the opposite direction of the drawing direction F0, and the offset distance d j =t~1.1t, the female mold tooling profile is obtained. The relatively straight area around the female mold tooling profile is the female mold flange surface, which corresponds to the blank holder tooling profile. The deep concave closed area inside the female mold tooling profile is the female mold forming surface, which corresponds to the male mold tooling profile. The offset distance of this area is set to d j =1.1t, the female die flange surface and the female die forming surface are connected by a transition fillet surface, the radius of the transition fillet surface is R g =R t -t, the female die tooling profile consists of three parts: female die flange surface, female die forming surface and transition fillet surface; 2-16 corresponds to the position of the three sections of calendering ribs on the tooling surface of the blank holder, and three sections of calendering rib concave cavities are set on the female die flange surface, namely the tail calendering rib concave cavity, the outer calendering rib concave cavity, and the inner calendering rib concave cavity. The depth of the cavity is h a =2h y .

3. The deep drawing method of a hyperbolic bidirectional concave stainless steel butt skin according to claim 1 is characterized in that The specific process of determining the butt-jointed skin deep-drawn body in step 2 is as follows: the butt-jointed skin deep-drawn body is designed according to the female mold tooling profile, the female mold forming surface and the transition fillet surface constitute the main components of the butt-jointed skin deep-drawn body, and the inner edge line of the flange is offset along the female mold flange surface by k y =20mm~30mm, and the flange surface of the drawn body and the outer edge line of the drawn body are obtained. The butt-jointed skin drawn body is composed of three parts: the female die forming surface, the transition fillet surface and the flange surface of the drawn body.

4. The deep drawing method of a hyperbolic bidirectional concave stainless steel butt skin according to claim 1 is characterized in that The hydraulic shaping in step 5 is to initially cut the drawn body of the butt skin according to the part contour line on the butt skin inspection and pressing die, complete the hydraulic shaping of the butt skin using a rubber pad hydraulic press and the pressing die, and obtain a butt skin part with the outer contour and dimensions meeting the drawing requirements after cutting again.

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