A new flexible sheet metal forming method and its stamping process

Transcription

1 The 14th IFToMM World Congress, Taipei, Taiwan, October 25-30, 2015 DOI Number: /IFToMM.14TH.WC.PS A new flexible sheet metal forming method and its stamping process W. W..Wang 1 B. B. Jia 2 J. B. Yu 3 Harbin Institute of Technology Harbin Institute of Technology Harbin Institute of Technology Weihai, China. P. R. Weihai, China. P. R Weihai, China. P. R Abstract: A reconfigurable flexible multi-point forming method was developed and its working principle was introduced. Arbitrarily three-dimensional curved surface can be formed just by adjusting the relative height of the upper punch matrixes and the forming load can be controlled easily through lower punch matrixes by hydraulic pressure. Different working modes can be obtained according to the punches movement sequence or their relative positions. The sheet metal surface is constrained by the rotating hemisphere hinge punches in the whole deforming process and cover the full surface which achieves an effectively control on the sheet metal forming defects especially on wrinkling. Numerical simulation and practical forming are performed to prove the advantages of the method and to expose the wrinkling elimination mechanism. The results indicate that the method can suppress the wrinkle efficiently and the forming surface can be constructed arbitrary. Keywords: sheet metal forming, FDSC multi-point forming, flexible method, wrinkle I. Introduction Using sheet metal as a rare material to manufacture varies curved surfaces is one of the most widely used manufacturing processes in automobile production and machine automobile production. Traditionally, sheet metal forming is realized by means of die stamping. The sheet metal is deformed to desired shape in stamping die activated by press. Influenced by the die cost and manufacturing period, this process is only suitable for the large volume production such as automobile production and household appliance industry. For the aerospace, shipbuilding and other industries, sheet metal formed covering parts are also used in great quantities. Due to the small amount of production, it might great increase the product cost and the product development cycle, if all the covering parts are formed by stamping die. In order to solve this problem, a series of sheet metal flexible manufacturing process and technology have been developed and have made applications in practical production. These flexible manufacturing process and technology can be divided into two groups. One is the die-less stamping technology such as shot peen forming[1], single-point incremental metal sheet forming[2] etc.. Another one is the reconfigurable die forming technology mainly composed of multi-point forming (MPF)[3, 4, 5]. But there are still many shortcomings and insufficiency existed in these new forming technologies and it is difficult to fully meet the needs of industrial production. 1 wangww@hitwh.edu.cn 2 joybee08@126.com 3 hityjb328@163.com A new flexible sheet metal forming method, named Force-Displacement-Separated-Control Multi-Point Forming (FDSC MPF)[6], is introduced in this paper. By the method, the normal constraints can be exerted to sheet metal surface in the whole deforming process and cover the full surface. This gets an effectively control on the sheet metal forming defects especially on wrinkling. At the same time it has the flexible forming features of stamping die rapid reconfiguration. II. The principles The basic component of FDSC multi-point forming device is shown in Figure 1. The device is made up of height-adjustable upper punch matrixes and lower punch matrixes supported by hydraulic cylinder. The upper punches could move reciprocating driven by the press machine. The blank is clamped by the upper punch matrix and the lower punch matrix. The normal constraining force of the blank controlled by hydraulic cylinder (lower punch matrix) is controlled by the overflow pressure of the pressure valve according to the thickness and material property. Each head of the matrix is installed with a hemisphere hinge indenter, which could swing according to the sheet metal contact state. The working process is as follows: Take Figure1 as follows, before forming, the top position of upper punch matrixes are adjusted according to the forming surface fitting to constitute the symmetry plane of the forming surface fitting. Fill the hydraulic cylinder with liquid, and then the blank is clamped by the upper punch matrix and the lower punch matrix. When the pressing begins, the upper punch matrixes run down successively driven by the press. The height of the punch matrix on the heart of the department is the largest, so the matrixes on the heart run down first, and the aforesaid position of the blank begin to deform. The corresponding position of the lower punch matrixes are pulled down. The liquid in the cylinder flow out through the overflow valve, resulting in flowing pressure, and all the matrixes clamp the blank tightly under the effect of fluid pressure, then the normal constraints of the blank is formed. As the press continues downward, the matrixes around begin to run down gradually, forcing much blank involved in the deformation. The forming process will complete, until all the matrixes running to the finish line. According to the aforesaid working principle, in this method, by adjusting the relative height of the upper punch matrixes and then controlling the height of each matrix, arbitrarily three-dimensional curved surface can be formed. The forming force acting on the blank is controlled by the hydraulic cylinder, and different forming force at different stage could be applied to adapt to forming requirement.

2 The rotating hemisphere hinge punch can share the forming force to the surface of the blank, and it can apply the normal constraints to the entire surface at the whole process of the forming. Therefore, this method makes the movement and force separate, and can control them respectively. The control method is simple and reliable, and the normal constraint at the whole process in the whole area is adjustable. Thus the constraint of the blank is strengthened drastically and the crystallized ability is improved. die reconfiguration is accomplished. Surface modeling FEM analysis Shape adjusting results Punch s height adjustment Practical forming Fig. 2 Reconfiguration process Fig. 1 Forming procedure of FDSC MPF Finite element analysis is of vital significance to forming quality control and precision control of curved plates. It is necessary to analyze and to predict the forming procedure, the possible defects and the prevention measures and the forming precision etc. by utilizing numerical simulation method before forming sheet metal, because there is no mature theory to guide for this new sheet metal forming method. The simulated results can be used to determine the adjustment of die surface and process parameters. Since each basic body head is with the hemisphere hinge structure and the relative height of its globes which are just on the sheet metal surface is not changed in this method, the surface accuracy can be always guaranteed. III. Reconfiguration of the die This method obtain a good flexible forming ability through the die reconfiguration, where the height of upper basic body can be adjusted arbitrarily to a random shaped forming surface with different curved surface as required. Figure 2 shows the forming surface reconfiguration process which mainly includes plates CAD three-dimensional modeling, finite element forming analysis, adjustment shaped result output, the basic body height adjustment, and sheet metal forming process in force-displacement separated control multi-point die system. The die reconfiguration is based on three dimensional models of the sheet metal parts. Firstly, the three dimensional model should be moved through the spatial rotation, so that the Y-axis and the vector sun at each point are in the same direction to eliminate the side thrust in the sheet metal forming process, or the height difference of each point on the surfa ce is minimum to reduce sheet deformation. Secondly, the data of three-dimensional model covering surface or neutral surface is extracted to generate the target surface. Thirdly, the resulting target surface was inputted into self-developed finite element simulation software for forming analysis, and adjustment shaped target surface is obtained by judging forming results. Then the height extension amount of each upper basic body should be calculated to adjust figuration in accordance with the symmetry surface of quasi-forming surface. Finally, the IV. Forming modes According to the movement sequence and relative position of the punches, various forming modes can be realized by force-displacement-separated-control multi-point forming (FDSC MPF). 4.1 Center priority forming and edge priority forming according to the punches movement sequence As is shown in Figure 1, when using the center priority forming mode, the center part of the upper punches are adjusted to the highest position while the edge part are adjusted to the lowest, so the center part of the upper punches moved downward firstly under pressure which force the deformation of the sheet s center part. At this time, as all the punches tightly pressed the sheet, deformation of the center is bound to drive the twitch of the perimeter of the sheet to the center to supplement the material flow. This process is similar to the deep drawing process, but the difference is that in the whole process of deformation, all areas of the sheet are in a state of blank holder and there are no vacant areas. When using the edge priority forming mode, the center part of the upper punches are adjusted to the lowest position while the edge part are adjusted to the highest, as is shown in Figure 3, by the driving of the upper punches, deformation occurs firstly in the edge part of the sheet which will lead to the increase of the stiffness of the sheet. Therefore, the subsequent deformation has a certain bulging. This kind of deformation also can ensure that the

3 sheet is under the constraints of the upper and the lower punches in the whole process forming and there are no vacant areas. V. Results and discussion The forming of saddle surface sheet part is taken as example to explore the characteristics and advantage of the FDSC MPF in this paper. Fig. 3 Edge priority forming 4.2 Point to point forming and dislocation forming due to punch s relative positions When the center line is overlap, this kind of forming is called point to point forming (Figure 1, Figure 3). At this point, each hemisphere hinge structure of the punch of both the upper and lower parts can clamp the sheet; two pairs of hemisphere hinge in local formed a sort of ball joint structure. So there is a small part vacant are as between the adjacent punches. Due to the different rotation angles of the punches, repeat bending will occur in the sheet between adjacent punches which greatly reduce the springback after forming. But when the sheet is thinner, the lower deformation rigidities of the sheet may lead to the adjacent hemispheres hinge rotates excessive or insufficient and produce crease on the sheet. Deformation which intertwine the upper and the lower punches called dislocation forming, as shown in Figure 4.Vacant areas on the sheet disappear completely when dislocated the upper and the lower punches, this is because the hemisphere hinge on either side of the plate are unable to form the ball hinge structure, hemisphere hinge s rotation are restricted by each other which prevent the adjacent hemispheres hinge rotates excessive or insufficient in dislocation forming, the quality of the forming sheet s surface is improved effectively. As shown in Figure 5, this experimental apparatus is designed and manufactured according to the forming principle. Main parameters is as follows: The biggest forming area is 200mm 200mm, number the punches is 10 10, the maximum height adjustment of the punch is 50 mm, maximum forming force of each hydraulic cylinder in the lower part is 2800N. Fig. 4 Dislocation forming Fig. 5 Experimental apparatus The saddle surface sheet part is one of the most difficult forming surfaces[7, 8]. Since the saddle surface curvatures oppositely in the two directions, the center region of the blank sheet completely dangling and inherits the tangential compressive stress in the early stages of the forming. At the same time, the surrounding line of saddle surface distributed in different heights, it is difficult to construct blank holder face to prevent wrinkles by blank holder. Therefore, it is basically impossible to forming saddle surface directly by the conventional stamping methods[9, 10]. Figure 6 shows the photo of a saddle surface sheet blank formed by conventional stamping die (sheet materials: Q235, thickness: 1mm). It is obviously that severe

4 wrinkling occurred in the early stages of the deformation and this interrupts the forming when using conventional stamping die forming. of deformation, whereas the instability occurs in the center part of sheet reverse movements occurred. Continue the deformation, the forming was unable to complete due to the development of wrinkle. Fig. 6 Blank formed by conventional stamping die Figure 7 shows the photo of a saddle surface sheet blank formed by FDSC MPF (sheet materials: Q235, thickness: 1mm). It can be seen that the surface of the saddle part obtained is bright and clean. This means that high forming quality has been got by FDSC MPF. Wrinkling in the core region of the blank occurred in the conventional die forming has been eliminated completely. This indicates that the application of a powerful and effective constrains on the blank surface greatly improve the mechanical condition of the sheet blank and the wrinkles is eliminated completely. (a) Conventional stamping (b) FDSC multi-point forming Fig. 8 Comparison of velocity distribution Fig. 7 Blank formed by FDSC multi-point forming Shown in Figure 8 is the numerical simulated velocity (along the stamping direction) distribution in the saddle surface during forming of the FDSC multi-point forming (MPF) contrasting with the conventional stamping. From Fig. 8, a) it can be seen that two pair of edges move in opposite directions (red is upward and blue is downward) in initial stage. Notice that the sheet is in a state of freedom without restraint except that the four edges middle contact with the upper or lower die. The core region of sheet is without any constraints and this result of an upward arch under the action of upper die. The constraint parts along sheet metal edges gradually increased with the proceedings For FDSC multi-point forming, form Fig. 8, b), it can be seen that the sheet metal has been constrained by the hemisphere hinges before the start of forming. In the upper punches move successively driven by press, the deformation of sheet metal is also started from the position of the first moved punch and gradually expand. When the deformation occurred in an area, the rest of the sheet metal doesn t participate due to the clamping of the hemisphere hinges. As the deformation, the deformed parts become already formed area, and the deformation moves to adjacent area to make it become the new deforming area. This creates the deformation characteristics of FDSC multi-point forming: (1) at any moment, the deformation always occurs in the strip area besides with already formed area and no deformation area, both are constrained by the

5 hemisphere hinge and without deformation. Therefore, under the condition of FDSC multi-point forming, freedom region without constraint is not existed and wrinkle, of course, also won't happen. (2) For saddle surface forming, the deformation occurs first from the edge center and then expands gradually to the both ends of the edge and to the sheet center. Since the other parts of sheet are constrained and can t deform, therefore, all the deformations occurred are in the state of drawing-bulging composite deformation. When the two deforming areas started from two edges extended to the interconnection, the deformation then develops from the sheet center to another two sides. Because the formed area is under the constraint and no longer participates in the deformation, the deformation characteristic of here is still drawing-bulging composite deformation. Therefore, at any time of forming, the deformation of sheet metal is in the state of draw-bulging composite deformation and the sheet metal is under the action of bidirectional tensile stress always. This means that the mechanical condition to cause wrinkling is fundamentally eliminated. [4] Z.Q. Hu, M.Z. Li, Z.Y. Cai, et al. Continuous flexible forming of three-dimensional surface parts using bendable rollers. Materials Science and Engineering A, 499(1-2), pp , [5] SONG Ai-ping, WU Wei-wei, ZHANG Jun. A Reconfigurable Stamping Die and Its Stamping Process[J]. Journal of Shanghai Jiaotong University. 15(3): , [6] Wang Weiwei, Yu Jingbo, Jia Binbin. A kind of device for multi-point forming. Patent of P. R.China, No , [7] FU Wen-zhi, LI M ing-zhe, DENG Yu-shan, LI Xiang- ji. A research on the flexible blank-holder in multi point forming press [J]. Journal of Plasticity Engineering. 13 (2): 32-35, 2006.(in Chinese) [8] Wang X, Jian C. On the prediction of side-wall wrinkling in sheet metal forming processes [J]. International Journal of Mechanical Science, 42(12): , [9] Lancaster E R, Calladine C R, Palmer S C. Paradoxical wrinkling behavior of a thin cylindrical shell under axial compression [J]. International Journal of Mechanical Science, 42(3): , [10] Liu Hong-wen. Shell theory [M]. Hangzhou: Zhejiang. University Press, 1988 (in Chinese). Ⅴ. Conclusions (1) The normal constraint of sheet metal is well realized by the method of FDSC MPF and is exerted large enough in all area of sheet metal during the whole process of forming. Since the normal constraint force can be adjusted and controlled precisely, it can prevent the wrinkling effectively. (2) FDSC MPF is a kind of new forming mode, which can realize drawing forming or drawing-bulging compound forming under the constrain condition of whole area and whole procedure. For saddle surface forming, the deformation state of sheet changes to bidirectional tensile stress from bidirectional compressive stress, eliminate the wrinkling mechanical condition fundamentally, therefore, wrinkle-less forming and smooth saddle surface can be obtained. (3) FDSC MPF belongs to the die reconfigurable flexible forming method. Practical experiments indicate that the device has good reconfigurable ability and the forming surface can be constructed arbitrary and efficiently, especially suitable for medium and small batch curved plate parts stamping processing. Acknowledgements This work was supported by the National Natural Science Foundation of China under grant No References [1] A. Gariépy a, S. Larose b, C. Perron b, M. Lévesque. Shot peening and peen forming finite element modeling-towards a quantitative method[j]. International Journal of Solids and Structures. 48 (20): , [2] Amar Kumar Behera, Bert Lauwers, Joost R. Duflou. Tool path generation framework for accurate manufacture of complex 3D sheet metal parts using single point incremental forming[j]. Computers in Industry, 65(4): , [3] Q. Zhang, T.A. Dean, Z.R. Wang. Numerical simulation of deformation in multi-point sandwich forming. International Journal of Machine Tools and Manufacture, 46(7), pp , 2006.