Patent Publication Number: US-2021162482-A1

Title: Method for forming large-size curved thin-walled metal skin

Description:
TECHNICAL FIELD 
     The present invention relates to a method for forming a metal skin, in particular to a method for forming a large-size curved thin-walled skin. 
     BACKGROUND 
     In the fields of aerospace and weapon equipment, achieving a lightweight structure is one of the main goals pursued. In order to reduce the weight of the structure, it is necessary to optimize the design of components on the premise of meeting the requirements of use. For example, many lightweight hollow composite structures are used on a rocket or a missile, such as panels of a rocket propellant tank and fairings of a missile. This kind of lightweight structure usually consists of a curved panel and a thin-walled skin matched therewith. The curved panel is generally composed of thin-walled webs and stiffeners connected thereto, and the stiffeners are used to improve the overall stiffness of the panel. In order to further improve the stiffness of the curved panel and realize the continuity of the surface on the side with the stiffeners, it is necessary to provide the thin-walled skin matched therewith outside the discretely distributed stiffeners. 
     The above-mentioned lightweight hollow composite structures usually have relatively large sizes, and the lengthwise sizes can reach several meters or even dozens of meters. In order to further reduce weight, lightweight and high-strength aluminum alloy materials, such as 2024 and 2219, are mostly used as materials for the curved panel and the thin-walled skin, wherein the curved panel is usually made of a thick aluminum alloy plate with a wall thickness of 20-50 mm, and the thin-walled skin is usually made of a thin aluminum alloy plate with a wall thickness of 2-5 mm. As the thickness of the thick aluminum alloy plate used to manufacture the panel is very different from that of the thin aluminum alloy plate used to manufacture the skin, different forming methods and separate mould tooling are required. 
     At present, the forming methods of the curved panel include press bending, cold roll, stamping, etc. For a single-curvature cylindrical or conical panel, forming can be realized by the method of press bending or cold roll. However, for a curved panel with a small bending radius or a high dimensional accuracy requirement, dimensional accuracy requirement cannot be met often by press bending or cold roll. In this case, stamping need to be carried out by a rigidity punch and a rigidity female die. The rigidity punch and the rigidity female die need to be specially designed and manufactured according to the shape and size of a specific panel component. 
     For a thin-walled skin, forming is often realized by the methods such as stretching and stamping. During the stretch-forming of a large-size thin-walled skin, the edge of a rectangular metal blank need to be clamped with jaws, and then the middle area of the metal blank is deformed under the action of a stretching die to obtain a required profile shape. After the stretch-forming, only part of the middle area of the metal blank is fitted with the stretching die, and other surrounding material will be removed as a process remnant Therefore, the method of stretching is difficult to be used for forming a thin-walled skin with an obvious curved surface and especially with a double curvature. In this case, it is necessary to use mutually matched rigidity punch and rigidity female die to obtain a required thin-walled skin through stamping. 
     Since punches, female dies and corresponding tooling in one-to-one correspondence with parts to be formed are generally required to be processed for stamping, the cost of moulds and tooling is relatively high, and this is especially true for large-sized components. For example, for a curved panel and a thin-walled skin with a length of about 3 m and a width of 1 m, the cost of moulds and tooling used for stamping can reach hundreds of thousands yuan or even millions yuan. At the same time, large-scale special equipment is required for the manufacturing of large-size moulds and tooling, and it will take several weeks or an even longer time to complete the manufacturing. Therefore, it is very important to reduce the cost of moulds and tooling while ensuring the smooth forming of components and obtaining a high forming accuracy when the method of stamping must be used. 
     In order to solve the problems of high component manufacturing cost, long production cycle, high requirements for mould and tooling manufacturing equipment, and high requirements for the use of the equipment due to the need of separately manufacturing large-sized moulds and tooling when the existing method of stamping is used to manufacture a large-size curved panel and a thin-walled skin, it is necessary to develop a new method for forming a large-size curved thin-walled skin. 
     SUMMARY 
     In order to solve the problems of high component manufacturing cost, long production cycle, high requirements for mould and tooling manufacturing equipment, and high requirements for the use of the equipment due to the need of separately manufacturing large-sized moulds and tooling when the existing method of stamping is used to manufacture a large-size curved panel and a thin-walled skin, the present invention proposes a method for forming a large-size curved thin-walled metal skin. 
     The technical solution of the present invention is: 
     A method for forming a large-size curved thin-walled metal skin, comprising the following steps: 
     Step 1: extracting the position and size characteristics of stiffeners on a curved panel according to the assembly relationship between the curved panel and a thin-walled skin; 
     Step 2: assuming that the thickness of the curved panel and the thin-walled skin after assembly is m, determining the size of a forming punch based on the shape and size of the curved panel, and offsetting a forming surface of the punch outwards along a normal line by the thickness m based on the size of the forming surface to obtain the characteristics of the inner surface of a female die; 
     Step 3: determining the size of discrete support moulds to be the same as that of the curved panel according to the thickness of the curved panel, and determining forming surfaces of the discrete support moulds based on the inner surface of the forming thin-walled skin; 
     Step 4: setting the number of the discrete support moulds to be the same as the number n of the stiffeners based on the size and distribution of the stiffeners on the curved panel, and the positions of the discrete support moulds on the punch are consistent with the positions of the stiffeners on the curved panel; 
     Step 5: processing and manufacturing the punch, the female die and n discrete support moulds according to step 2 and step 3; 
     Step 6: fixing the n discrete support moulds on the punch according to step 4, and the relevant positions for connecting the discrete support moulds are corresponding to the positions of the stiffeners on the curved panel during assembly; and 
     Step 7: using the punch and the female die combined with the discrete support moulds to carry out stamping of a curved thin-walled slab to obtain a required thin-walled skin component. 
     The present invention has the following beneficial effects: 
     (1) In the method for forming a large-size curved thin-walled metal skin of the present invention, the n discrete support moulds used are combined with the curved panel forming punch, then the stamping of the thin-walled skin is carried out, and the positions of the discrete support moulds are consistent with the positions of the stiffeners on the curved panel, so that it can be ensured that the part of the thin-walled skin corresponding to the stiffeners obtains sufficiently high shape and dimensional accuracy, thereby realizing the subsequent reliable connection of the thin-walled skin and the panel. 
     (2) In the method for forming a large-size curved thin-walled metal skin of the present invention, the n discrete support moulds used are only in partial contact with the thin-walled slab, so that the areas on the thin-walled skin that do not need to be assembled with the stiffeners are in a relatively free state during stamping, which can avoid unreasonable, uncontrollable or redundant deformation in such areas when a solid punch is used for stamping. 
     (3) In the method for forming a large-size curved thin-walled metal skin of the present invention, the connecting positions of the discrete support moulds used on the punch can be easily adjusted, so that the problems such as wrinkling and warping which may occur during a forming process can be easily eliminated. 
     (4) In the method for forming a large-size curved thin-walled metal skin of the present invention, the discrete support moulds used can be assembled and connected with the punch, so that the discrete support moulds are easy to replace, and can be replaced by different discrete support moulds to be used for forming thin-walled skins of different thicknesses. 
     (5) In the method for forming a large-size curved thin-walled metal skin of the present invention, the discrete support moulds used only need to be in contact with the thin-walled slab in partial areas, so that the shape thereof is simple, the manufacturing cycle is short, and the manufacturing cost is greatly reduced. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1( a )  is a schematic diagram of a curved panel. 
         FIG. 1( b )  is a schematic diagram of a thin-walled skin. 
         FIG. 2  is a schematic diagram of stamping of a curved panel. 
         FIG. 3  is a schematic diagram of forming a large-size curved thin-walled skin by a discrete support mould; 
         FIG. 4  is a schematic diagram of assembly of a discrete support mould and a curved panel forming punch; 
         FIG. 5  is a schematic diagram of action of a discrete support mould on a thin-walled skin. 
     
    
    
     In the figures:  1  curved panel;  2  thin-walled skin;  3  punch;  4  female die;  5  connecting bolt;  6  discrete support mould. 
     DETAILED DESCRIPTION 
     The technical solution of the present invention is further described below in combination with the drawings and specific embodiments. 
     Embodiment 1 
     Referring to  FIG. 3  for description, a method for forming a large-size curved thin-walled metal skin, comprising the following steps: 
     Step 1: extracting the position and size characteristics of stiffeners on a curved panel according to the assembly relationship between the curved panel and a thin-walled skin; 
     Step 2: assuming that the thickness of the curved panel and the thin-walled skin after assembly is m, determining the size of a forming punch based on the shape and size of the curved panel, and offsetting a forming surface of the punch outwards along a normal line by the thickness m based on the size of the forming surface to obtain the characteristics of the inner surface of a female die; 
     Step 3: determining the size of discrete support moulds to be the same as that of the curved panel according to the thickness of the curved panel, and determining forming surfaces of the discrete support moulds based on the inner surface of the forming thin-walled skin; 
     Step 4: setting the number of the discrete support moulds to be the same as the number n of the stiffeners based on the size and distribution of the stiffeners on the curved panel, and the positions of the discrete support moulds on the punch are consistent with the positions of the stiffeners on the curved panel; 
     Step 5: processing and manufacturing the punch, the female die and n discrete support moulds according to step 2 and step 3; 
     Step 6: fixing the n discrete support moulds on the punch according to step 4, and the relevant positions for connecting the discrete support moulds are corresponding to the positions of the stiffeners on the curved panel during assembly; and 
     Step 7: using the punch and the female die combined with the discrete support moulds to carry out stamping of a curved thin-walled slab to obtain a required thin-walled skin component. 
     The n discrete support moulds used are combined with the curved panel forming punch, then the stamping of the thin-walled skin is carried out, and the positions of the discrete support moulds are consistent with the positions of the stiffeners on the curved panel, so that it can be ensured that the part of the thin-walled skin corresponding to the stiffeners obtains sufficiently high shape and dimensional accuracy, thereby realizing the subsequent reliable connection of the thin-walled skin and the curved panel. 
     Embodiment 2 
     Referring to  FIG. 4  for description, a method for forming a large-size curved thin-walled metal skin, comprising the following steps: 
     In step 6, the n discrete support moulds are dispersedly arranged on the punch, the dispersing positions of discrete support moulds can be determined according to the positions of the stiffeners on the curved panel, the replacement stiffeners of the discrete support moulds are distributed on the punch, and the dispersing positions of the discrete support moulds can also be adjusted according to the forming condition of the thin-walled slab after forming. Other steps are the same as those in embodiment 1. 
     When a sheet is formed by a solid die, due to the uneven load distribution and different contact states on various parts of a slab, the problems of wrinkling and warping are prone to occur during forming; whereas a certain distance exists among the discrete support moulds, so that the problems such as wrinkling and warping which may occur during a forming process can be easily eliminated by adjusting the positions of the discrete support moulds on the punch. The discrete support moulds are assembled and connected with the punch, so that the discrete support moulds are easy to replace, and can be replaced by different discrete support moulds to be used for forming thin-walled skins of different thicknesses, which saves the material and processing cost of the moulds. 
     Embodiment 3 
     Referring to  FIG. 5  for description, a method for forming a large-size curved thin-walled metal skin, comprising the following steps: 
     In step 6, the discrete support moulds and the punch can be effectively connected by bolts, and each discrete support mould is composed of a plurality of small moulds with surface shape and size requirements. Each discrete support mould used in other steps is composed of a plurality of small mould blocks, and a gap exists among adjacent small mould blocks. Other steps are the same as those in embodiment 1. 
     In a traditional stamping process, most of the moulds are in a solid form, which are prone to have excessive local contact stress and redundant deformation. The discrete support moulds composed of a plurality of small mould blocks are only in partial contact with the thin-walled slab, so that the areas on the thin-walled skin that do not need to be assembled with the stiffeners are all in a relatively free state, which can avoid unreasonable, uncontrollable or redundant deformation in such areas when a solid punch is used for forming.