Recent developments in sheet hydroforming technology.pdf

Journal of Materials Processing Technology 151 (2004) 237241 Recent developments in sheet hydroforming technology S.H. Zhanga, Z.R. Wangb, Y. Xua, Z.T. Wanga, L.X. Zhoua a Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China b School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Abstract In this paper, recent developments in sheet hydroforming technology are summarized, several key technical problems to be solved for the development of sheet hydroforming technology are analyzed, and varied sheet hydroforming technologies are discussed. Compound deformationbydrawingandbulgingisthemaindirectionforthedevelopmentofsheethydroformingtechnology,inwhichitisadvantageous to increase the feeding of materials, and the ratio of drawing deformation (drawing in of the blank fl ange) to bulging, enabling the forming limit of a sheet blank to be increased. It is also advantageous to increase the local deformation capacity for sheet hydroforming, to increase the range of application of the process. Press capacity is one of the important factors restraining the range of applications. As one of the fl exible forming technologies that is still under development, it has much potential for innovative applications. Its applications have been increasing remarkably, recently in automotive companies. A breakthrough in the technology will be obtained by the development of novel equipment. A new sheet hydroforming technology using a movable die is proposed in this paper, which has been developed recently by the authors. 2004 Elsevier B.V. All rights reserved. Keywords: Sheet hydroforming; Drawing in; Bulging; Flexible forming; Forming limit 1. Introduction Compared with conventional deep drawing, sheet hydro- forming technology possesses many remarkable advantages, such as a higher drawing ratio, better surface quality, less springback, better dimensional freezing and the capability of forming complicated-shaped sheet metal parts. For exam- ple a multi-pass forming process may be decreased to one pass for the forming parabolic parts. Sheet hydroforming technology has been applied to industries for the forming of automotive panels and aircraft skins 1. It is a soft-tool forming technology and as the development of this technol- ogy is imperfect compared with other rigid forming tech- nologies, there are more extensive demands and space for it to be improved with the development of modern industry. There are many demands for hydrofoming technology for use with some new materials, such as forming of magnesium alloy sheets, composite material sheets and sandwich sheets. Some new hydroforming processes have entered this area, such as viscous pressure forming technology, warm sheet hydroforming, the hydroforming of sheet metal pairs and the hydroforming of tailor-welded blanks. Through long-term Corresponding author. Tel.: +86-24-8397-8266/8721; fax: +86-24-2390-6831. E-mail address: (S.H. Zhang). investigation by the AP namely, the compound deformation of bulging and drawing due to the draw-in of blank fl ange area (blank feeding of the blank fl ange area), which compensates the materials for the stretch of the bulging area and avoids excessive thinning resulting from the increase of the blank area, thus assuring material strength and rigidity in the bulging area. It is very diffi - cult to realize the uniform distribution of thinning, the large local deformation of sheet the metal and the increasing of the forming limit of the blank without blank feeding and supplementation. Thus the advantages for the hydroforming of complicated-shaped parts from sheet cannot be revealed fully, although the breakthrough for tube hydroforming has been realized. A tubular component can be hydroformed if dealing with a high-pressure forming process with the simul- taneous feeding of the tube end 3, which increases the tube area and thus reduces little thinning. The requirements for the pressure of the tool in tube hydroforming are small. The internal pressure for the tube is closed and self-restrained, and the closing force involved is small. The material feeding of the tube end can be enforced without diffi culty for this technology, compared with the diffi culties of the feeding in of the material in hydroforming. As in tube hydroforming, a closing force is required for sheet hydroforming, but a diffi culty is that the closing force for sheet hydroforming is far greater than that in tube hydro- forming, and requires a high press tonnage: this is an impor- tant factor restraining the application of sheet hydroforming. The closing pressure can be supplied by a hydraulic press, but the pressure for sheet hydroforming is no limits and not self-restrained. 2.1. Hydroforming with a rubber diaphragm A rubber membrane was employed as the diaphragm of the hydraulic chamber and the blankholder in the early form of sheet hydroforming. This process has been applied to small batch production of automotive panels and aircraft skins (Fig. 1). There are many advantages for this process: better surface quality and the forming of more complicated workpieces. It is suitable for small batch production. How- ever, it also has some disadvantages, such as low process effi ciency and the requirement of heavy presses. In addition, it is easy to destroy the rubber membrane and diffi cult to control wrinkling. 2.2. The hydromechanical deep drawing process and the hydro-rim deep drawing process The hydromechanical deep drawing process has been de- veloped on the basis of rubber membrane hydroforming (Fig. 2(a). The pressure can be produced by the downwards movement of the punch into the fl uid chamber, or supplied by a hydraulic system, because a rubber membrane is not used. Thus, it is very easy to obtain hydraulic pressure. The tool device is similar to a conventional tool. All these param- eters lead to high effi ciency. The shape of the workpieces may be very complicated, and the drawing ratio may be in- creased, from 1.8 to 2.7, compared with that for conven- tional drawing processes. There are many applications for this process 1315. More local deformation and forming of complicated parts are realized by using this process. Forced feeding is diffi cult to practice in current sheet hy- droforming processes. To some extent, the radial hydrome- S.H. Zhang et al./Journal of Materials Processing Technology 151 (2004) 237241239 Fig. 1. Sheet hydroforming with a rubber membrane: (a) the process; (b) a hydroformed workpiece. chanical deep drawing (hydro-rim) process can realize some forced radial feeding (Fig. 2(b), which can signifi cantly in- crease the forming limit of the sheet metal. According to the research results in 2, the drawing ratio can be increased, from 2.6 to 3.2, compared with that for the common hy- dromechanical deep drawing process. 2.3. Hydroforming of sheet metal pairs A special case is the hydroforming of welded-closing sheet metal pairs (Fig. 3(a). The hydroforming technology of sheet metal pairs was developed by Kleiner et al at. Dort- mund University in the early 1990s 46. In the fi rst scheme the periphery of the sheet metal can be welded using laser welding. Then a liquid medium can be fi lled between the blanks,andpressurizationcanbeeffectedbyahydraulicsys- tem. Plastic deformation starts in the blank under the pres- sure and then further deformation occurs sequentially in the zone contacting with the die. However, it is very diffi cult to realize radial feeding using this method, as it is essentially a pure bulging deformation. The advantage is that the pres- sure is a kind of self-restraining pressure. There is a low re- quirement for the closing force. A stainless steel automotive model was formed with the new press of 100,000kN with hydroforming technology. To some extent, this technology is similar to tube hydroforming, however, it is very diffi cult to realize the radial feeding of the blank. Fig. 2. Showing: (a) hydromechanical deep drawing; (b) hydro-rim deep drawing. Another variation was proposed by Dortmund University (Fig. 3(b). The principle is that the tool system is made up of an upper and lower die and an intermediate plate. The intermediate plate can be applied on its own or together with the upper and lower blank, for hydroforming. The pressure pipeline may be connected or disconnected. Generally, the shape of the upper and lower workpieces is symmetrical when the pressure pipeline is connected, whilst the shapes of the upper and lower workpieces are independent when the pressure pipeline is not connected: infact, they may deform separately. This tool is for the realization of the compound deformation of drawing and bulging. 2.4. The compound deformation of drawing and bulging Sheet hydroforming with compound drawing and bulging has been investigated for many years. Since the early 1980s, the theory of hydroforming with draw-in has been studied by Shang at Singapore National University 7. He studied the reasonable match of draw-in and bulging, but it is still in the research stage and has not been applied. 2.5. The dieless integral hydro-bulge forming (IHBF) of spherical shells Another special case is the integral hydro-bulge forming (IHBF) of spherical shells. IHBF is a new dieless forming 240S.H. Zhang et al./Journal of Materials Processing Technology 151 (2004) 237241 Fig. 3. The hydroforming of sheet metal pairs with an intermediate plate. technology for sphere-inner-scribing polyhedral shell, that means, all the side inter-sections of the polyhedral shell sides are on the sphere; which was invented by Wang 8 at HarbinInstituteofTechnologyin1985.Itrealizedthedieless IHBF of fl at sheets. In fact, this technology is a pure bulging process as it is impossible to obtain the supplementation of materials. Moreover, it is a non-uniform bulging forming. The hydroforming of single curvature shells and the dieless IHBF of double spherical vessels, oblate spheroid shells, ellipsoidal shells and pairs of pressure vessel heads were developed later, which resulted in the full development of the dieless IHBF technology and secured wide applications. 3. A new sheet hydroforming technology: hydroforming with a movable die A sheet hydroforming technology with a movable female die was proposed by authors in 2001 (see Fig. 4) 11,12. Some hydroformed workpieces of stainless steel and magne- sium alloys are shown in Fig. 5. Forsheet hydroforming with a movable die, a combined die is used, which consists of a fi xed part and a movable part. As the technology can realize the compound deformation of drawing and bulging, it is suit- ableforformingcomplicated-shapedpartsandlow-plasticity diffi cult-to-form materials. That part of the blank in the fl ange area is drawn in during the process, which may real- ize the compound deformation of deep drawing and bulging. Fig. 5. Some hydroformed workpieces of stainless steel and magnesium alloy. Blankholder plate Movable die Combination die Bolster plate O-ring sealing Blank Dies Fig. 4. Schematic of the new set-up for sheet hydroforming using a movable die. The movable die component keeps in touch with the blank during the early stage. Plastic deformation and then defor- mation of the blank in the die-contacting area take place. The movable die remains in contact with the blank under the friction force, which makes the deformation area spread to the non-contacting area. Preliminary research shows that the thinning of the sheet metal can be alleviated remark- ably if this innovative process is adopted 12 (see Fig. 6). The forming limit of the sheet metal is increased. This pro- cess is suitable for the forming of complicated-shaped parts such as aluminum alloy sheets, as well as low-plasticity and light-weight materials such as aluminum lithium alloy and magnesium alloys. S.H. Zhang et al./Journal of Materials Processing Technology 151 (2004) 237241241 Fig. 6. Comparison of the thinning ratio between hydroforming with and without a movable die. It is diffi cult for the tool to be damaged or worn because of the use of hydraulic pressure, so the tool life is improved. Moreover, it is very easy to modify the product because the blankholder has versatility and the punch is not required to bechanged:itisonlyrequiredtochangethediefortheform- ing of different parts. It can be shown that this process has many advantages over conventional processes: it makes the dies contact well, which results in better shape, dimensional accuracy, less springback and higher precision, remarkably lower tools cost and obviously shorter production periods for small batch production. This process is especially suit- able for the production of large-scale sheet metal parts with complicated shape, varied size and of small batch. It makes the production of complicated shape parts simple and fl ex- ible and realizes the quick production of workpieces. It is especially suitable for the development of new products in the aerospace industry and prototypes in the automotive in- dustry. If the deformation methods of conventional tools are adopted, because the production batch is not great, the de- sign cycle is long and the manufacturing cost is high, whilst if the presently described process is adopted, the cost for the tool will be decreased and the production periods and development cycle will be shortened. It is expected to apply this technology to many other area of manufacture, such as the production of prototype workpieces, which may save the cost of development, shorten the development cycle for the development of new models and improve competitive power for the business. 4. Conclusions In this paper, recent developments of sheet hydroforming technologyarediscussedsystematically.Withtherealization of the compound deformation of drawing and bulging for further development of sheet hydroforming, more draw-in of blank fl ange (drawing deformation) and more capacity of local deformation, can be achieved. The forming limit of sheet metal can be signifi cantly increased, and a wider range of part shape can be formed. Moreover, the multi-pass form- ing process for conventional complicated sheet parts can be decreased to one or two passes. Thus higher effi ciency and lower costs can be achieved, which compensates for the low effi ciency of the single pass procedure of hydroforming. The pre-requisite to the application for this process is a large tonnage for the equipment and high automation. The com- pound deformation of drawing and bulging can be realized if hydroforming with movable dies is adopted. Moreover, the distribution of wall thickness can be controlled. Thin- ning can be decreased and the forming limit of sheet metal can be increased. There are wide prospects for this technol- ogy, and the process can meet the developing direction of production requirements. References 1 S.H. Zhang, Developments in hydroforming, J. Mater. Process. Tech- nol. 91 (1991) 236244. 2 S.H. Zhang, J. Danckert, Development of hydromechanical deep drawing, J. Mater. Process. Technol. 83 (1998) 1425. 3 F. Dohmann, Ch. Hartl, Hydroforminga method to manufacture light-weight parts, J. Mater. Process. Technol. 60 (1996) 669676. 4 M. Kleiner, A. Gartzke, R. Kolleck, J. Ramer, T. Weidner, Finite element simulation for high pressure sheet metal forming (H