Journal of Materials Processing Technology 151 2004 237–241Recent developments in sheet hydroforming technologyS.H. Zhanga,?, Z.R. Wangb,Y.Xua, Z.T. Wanga, L.X. ZhouaaInstitute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, ChinabSchool of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, ChinaAbstractIn this paper, recent developments in sheet hydroforming technology are summarized, several key technical problems to be solved forthe development of sheet hydroforming technology are analyzed, and varied sheet hydroforming technologies are discussed. Compounddeformation by drawing and bulging is the main direction for the development of sheet hydroforming technology, in which it is advantageousto increase the feeding of materials, and the ratio of drawing deformation drawing in of the blank flange to bulging, enabling the forminglimit of a sheet blank to be increased. It is also advantageous to increase the local deformation capacity for sheet hydroforming, to increasethe range of application of the process. Press capacity is one of the important factors restraining the range of applications. As one of theflexible forming technologies that is still under development, it has much potential for innovative applications. Its applications have beenincreasing remarkably, recently in automotive companies. A breakthrough in the technology will be obtained by the development of novelequipment. A new sheet hydroforming technology using a movable die is proposed in this paper, which has been developed recently bythe authors.? 2004 Elsevier B.V. All rights reserved.Keywords Sheet hydroforming; Drawing in; Bulging; Flexible forming; Forming limit1. IntroductionCompared with conventional deep drawing, sheet hydro-forming technology possesses many remarkable advantages,such as a higher drawing ratio, better surface quality, lessspringback, better dimensional freezing and the capabilityof forming complicated-shaped sheet metal parts. For exam-ple a multi-pass forming process may be decreased to onepass for the forming parabolic parts. Sheet hydroformingtechnology has been applied to industries for the formingof automotive panels and aircraft skins . It is a soft-toolforming 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 forit to be improved with the development of modern industry.There are many demands for hydrofoming technology foruse with some new materials, such as forming of magnesiumalloy sheets, composite material sheets and sandwich sheets.Some new hydroforming processes have entered this area,such as viscous pressure forming technology, warm sheethydroforming, the hydroforming of sheet metal pairs and thehydroforming of tailor-welded blanks. Through long-term?Corresponding author. Tel. 86-24-8397-8266/8721;fax 86-24-2390-6831.E-mail address shzhangimr.ac.cn S.H. Zhang.investigation by the AP namely, thecompound deformation of bulging and drawing due to thedraw-in of blank flange area blank feeding of the blankflange area, which compensates the materials for the stretchof the bulging area and avoids excessive thinning resultingfrom the increase of the blank area, thus assuring materialstrength and rigidity in the bulging area. It is very diffi-cult to realize the uniform distribution of thinning, the largelocal deformation of sheet the metal and the increasing ofthe forming limit of the blank without blank feeding andsupplementation. Thus the advantages for the hydroformingof complicated-shaped parts from sheet cannot be revealedfully, although the breakthrough for tube hydroforming hasbeen realized. A tubular component can be hydroformed ifdealing with a high-pressure forming process with the simul-taneous feeding of the tube end , which increases the tubearea and thus reduces little thinning. The requirements forthe pressure of the tool in tube hydroforming are small. Theinternal pressure for the tube is closed and self-restrained,and the closing force involved is small. The material feedingof the tube end can be enforced without difficulty for thistechnology, compared with the difficulties of the feeding inof the material in hydroforming.As in tube hydroforming, a closing force is required forsheet hydroforming, but a difficulty is that the closing forcefor 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 notself-restrained.2.1. Hydroforming with a rubber diaphragmA rubber membrane was employed as the diaphragm ofthe hydraulic chamber and the blankholder in the early formof sheet hydroforming. This process has been applied tosmall batch production of automotive panels and aircraftskins Fig. 1. There are many advantages for this processbetter surface quality and the forming of more complicatedworkpieces. It is suitable for small batch production. How-ever, it also has some disadvantages, such as low processefficiency and the requirement of heavy presses. In addition,it is easy to destroy the rubber membrane and difficult tocontrol wrinkling.2.2. The hydromechanical deep drawing process and thehydro-rim deep drawing processThe hydromechanical deep drawing process has been de-veloped on the basis of rubber membrane hydroformingFig. 2a. The pressure can be produced by the downwardsmovement of the punch into the fluid chamber, or suppliedby a hydraulic system, because a rubber membrane is notused. Thus, it is very easy to obtain hydraulic pressure. Thetool device is similar to a conventional tool. All these param-eters lead to high efficiency. The shape of the workpiecesmay 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 forthis process [13–15]. More local deformation and formingof complicated parts are realized by using this process.Forced feeding is difficult 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 237–241 239Fig. 1. Sheet hydroforming with a rubber membrane a the process; b a hydroformed workpiece.chanical deep drawing hydro-rim process can realize someforced radial feeding Fig. 2b, which can significantly in-crease the forming limit of the sheet metal. According to theresearch results in , 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 pairsA special case is the hydroforming of welded-closingsheet metal pairs Fig. 3a. The hydroforming technologyof sheet metal pairs was developed by Kleiner et al at. Dort-mund University in the early 1990s [4–6]. In the first schemethe periphery of the sheet metal can be welded using laserwelding. Then a liquid medium can be filled between theblanks, and pressurization can be effected by a hydraulic sys-tem. Plastic deformation starts in the blank under the pres-sure and then further deformation occurs sequentially in thezone contacting with the die. However, it is very difficult torealize radial feeding using this method, as it is essentiallya 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 automotivemodel was formed with the new press of 100,000 kN withhydroforming technology. To some extent, this technologyis similar to tube hydroforming, however, it is very difficultto 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 UniversityFig. 3b. The principle is that the tool system is made upof an upper and lower die and an intermediate plate. Theintermediate plate can be applied on its own or together withthe upper and lower blank, for hydroforming. The pressurepipeline may be connected or disconnected. Generally, theshape of the upper and lower workpieces is symmetricalwhen the pressure pipeline is connected, whilst the shapesof the upper and lower workpieces are independent when thepressure pipeline is not connected infact, they may deformseparately. This tool is for the realization of the compounddeformation of drawing and bulging.2.4. The compound deformation of drawing and bulgingSheet hydroforming with compound drawing and bulginghas been investigated for many years. Since the early 1980s,the theory of hydroforming with draw-in has been studiedby Shang at Singapore National University . He studiedthe reasonable match of draw-in and bulging, but it is stillin the research stage and has not been applied.2.5. The dieless integral hydro-bulge forming IHBF ofspherical shellsAnother special case is the integral hydro-bulge formingIHBF of spherical shells. IHBF is a new dieless forming240 S.H. Zhang et al. / Journal of Materials Processing Technology 151 2004 237–241Fig. 3. The hydroforming of sheet metal pairs with an intermediate plate.technology for sphere-inner-scribing polyhedral shell, thatmeans, all the side inter-sections of the polyhedral shellsides are on the sphere; which was invented by Wang  atHarbin Institute of Technology in 1985. It realized the dielessIHBF of flat sheets. In fact, this technology is a pure bulgingprocess as it is impossible to obtain the supplementation ofmaterials. Moreover, it is a non-uniform bulging forming.The hydroforming of single curvature shells and the dielessIHBF of double spherical vessels, oblate spheroid shells,ellipsoidal shells and pairs of pressure vessel heads weredeveloped later, which resulted in the full development ofthe dieless IHBF technology and secured wide applications.3. A new sheet hydroforming technology hydroformingwith a movable dieA sheet hydroforming technology with a movable femaledie 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. For sheet hydroforming witha movable die, a combined die is used, which consists of afixed part and a movable part. As the technology can realizethe compound deformation of drawing and bulging, it is suit-able for forming complicated-shaped parts and low-plasticitydifficult-to-form materials. That part of the blank in theflange 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 plateO-ring sealing Blank DiesFig. 4. Schematic of the new set-up for sheet hydroforming using amovable die.The movable die component keeps in touch with the blankduring 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 underthe friction force, which makes the deformation area spreadto the non-contacting area. Preliminary research shows thatthe thinning of the sheet metal can be alleviated remark-ably if this innovative process is adopted  see Fig. 6.The forming limit of the sheet metal is increased. This pro-cess is suitable for the forming of complicated-shaped partssuch as aluminum alloy sheets, as well as low-plasticity andlight-weight materials such as aluminum lithium alloy andmagnesium alloys.S.H. Zhang et al. / Journal of Materials Processing Technology 151 2004 237–241 241Fig. 6. Comparison of the thinning ratio between hydroforming with andwithout a movable die.It is difficult for the tool to be damaged or worn becauseof the use of hydraulic pressure, so the tool life is improved.Moreover, it is very easy to modify the product because theblankholder has versatility and the punch is not required tobe changed it is only required to change the die for the form-ing of different parts. It can be shown that this process hasmany advantages over conventional processes it makes thedies contact well, which results in better shape, dimensionalaccuracy, less springback and higher precision, remarkablylower tools cost and obviously shorter production periodsfor small batch production. This process is especially suit-able for the production of large-scale sheet metal parts withcomplicated shape, varied size and of small batch. It makesthe production of complicated shape parts simple and flex-ible and realizes the quick production of workpieces. It isespecially suitable for the development of new products inthe aerospace industry and prototypes in the automotive in-dustry. If the deformation methods of conventional tools areadopted, because the production batch is not great, the de-sign cycle is long and the manufacturing cost is high, whilstif the presently described process is adopted, the cost forthe tool will be decreased and the production periods anddevelopment cycle will be shortened. It is expected to applythis technology to many other area of manufacture, such asthe production of prototype workpieces, which may save thecost of development, shorten the development cycle for thedevelopment of new models and improve competitive powerfor the business.4. ConclusionsIn this paper, recent developments of sheet hydroformingtechnology are discussed systematically. With the realizationof the compound deformation of drawing and bulging forfurther development of sheet hydroforming, more draw-inof blank flange drawing deformation and more capacityof local deformation, can be achieved. The forming limit ofsheet metal can be significantly increased, and a wider rangeof part shape can be formed. Moreover, the multi-pass form-ing process for conventional complicated sheet parts can bedecreased to one or two passes. Thus higher efficiency andlower costs can be achieved, which compensates for the lowefficiency of the single pass procedure of hydroforming. Thepre-requisite to the application for this process is a largetonnage for the equipment and high automation. The com-pound deformation of drawing and bulging can be realizedif 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 metalcan be increased. There are wide prospects for this technol-ogy, and the process can meet the developing direction ofproduction requirements.References S.H. Zhang, Developments in hydroforming, J. Mater. Process. Tech-nol. 91 1991 236–244. S.H. Zhang, J. 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