液压成形技术最新发展@中英文翻译@外文翻译

Recent developments in hydroforming technologyKlaus Siegert*, Markus Haussermann, Bruno Losch, Ralf RiegerInstitute for Metal Forming Technology, University of Stuttgart, Holzgarten Str. 17, 70174 Stuttgart, GermanyAbstractThis paper shows an overview about possibilities of hydroforming sheet metal as well as hydroforming tubes and extrusions. Comingfrom the deep drawing process with rigid dies, specially designed dies for presses with multipoint cushion systems required forhydroforming sheet metal are discussed. Further special press systems for presses with high ram forces are shown.Keywords: Deep drawing with hydraulic counter pressure; Hydroforming process; Hydroforming of double blanks; Press systems for hydroforming1. IntroductionTo discuss the advantages and problems of forming metalwork pieces by hydraulic pressure it is useful to differentiatebetween hydroforming of sheet metal and hydroforming oftubes and extrusions.2. Hydroforming sheet metalThe conventional deep drawing process is shown inFig. 1. A sheet metal product like kitchenware, for examplepots, or automotive parts like hoods, fenders, etc. can bedrawn in single action presses as shown in Fig. 1 or indouble action presses as shown in Fig. 2. Drawing sheetmetal products is in most cases not deep drawing of axisym-metric parts like pots but drawing of non-axisymmetric partswith a combination of deep drawing and stretch forming.For this it is necessary to direct the metal flow between thebinders bydraw beads,lock beads,shape of the blank andfriction between the blank and the binders.For directing the metal flow between the binders byfriction forces, it is possible to adjust the blankholder forcesand to fit in the gap between the binders. The blankholderforces can be directed over the stroke by modern hydraulicmultipoint cushion systems. These systems make it possibleto build closed loop systems in order to avoid wrinkling andfracture when friction relevant input parameters like lubri-cant, lubrication amount and the microstructure of the sheetmetal surface change. This goes along with speciallydesigned dies that transfer the blankholder forces to definedblankholder surfaces 13.Fig. 3 shows a multipoint cushion system with severalhydraulic cylinders. Each cylinder has its own proportionalor servo valve, so that the pressure in the cylinders respectivethe blankholder forces can be controlled over the stroke byadjusting the pressure in the cylinders.Fig. 4 shows a different cushion system with four hydrau-lic cylinders supporting the cushion plate and a number ofcushion pins transferring the blankholder forces from thecushion plate to the blankholder.Fig. 5 shows a specially designed blankholder with pyr-amidal (upside down) shaped ribs. This design has theadvantage that there is a direct correspondence between theblankholder force acting on the pyramid and the pressurebetween the blank and the binders 4,5.In industrial production only a few presses have suchmodern hydraulic multipoint cushion systems. So it makessense to design dies with hydraulic cylinders between a baseplate and the blankholder. This can be seen in Fig. 6.When examining hydraulic sheet metal forming (hydro-mechanical deep drawing) we have to consider that instead arigid die there is a hydraulic counter pressure, Figs. 7 and 8.This counter pressure can be built up by compressing thefluid when the punch forces the blank downwards. Thecounter pressure is controlled by a servo or proportionalvalve.* Corresponding author.Journal of Materials Processing Technology 98 (2000) 251258Fig. 1. Conventional deep drawing in single action presses.Fig. 2. Conventional deep drawing in double action presses.Fig. 3. Hydraulic multipoint cushion.Fig. 4. Multipoint cushion system with 10 height adjustable pins.Fig. 5. Segmented elastic blankholder.Fig. 6. Hydraulic multipoint cushion included in the die.Fig. 7. Deep drawing with hydraulic counter pressure in single actionpresses.252 K. Siegert et al. / Journal of Materials Processing Technology 98 (2000) 251258It is also possible to produce pressure in the fluid at thebeginning of the process by a pump system. This makes itpossible to have a prebulging of the blank. The initialpressure has the advantage of preforming the work piecealong with inducing a work hardening in the middle of thepart. This work hardening can be useful in flat parts toproduce stiffer and more geometrically accurate productswith more hardness, which is useful for a better hail impact(dynamic denting). It can also be useful to have a prebulgingin order to get more sheet metal into the die cavity whendrawing deep parts. Further, it is possible to have a hydraulicforming of parts into the cavity of the die when using nopunch but only a rigid die cavity.This process and also the prebulging are in principalderived from superplastic sheet metal forming process,Fig. 9. One difference is that in superplastic forming thesheet metal is clamped totally between the binders so thatthere is no flow of sheet metal. Further, instead of a fluid, gasis used and it is a warm forming process with the use of ametal with special grain size at special strain rates.The forming of the sheet metal into a cavity can be doneby hydraulic pressure in principle like it is the case of thesuperplastic sheet metal forming process. The problem isthat there is no possibility to control the metal flow betweenthe binders. Further more there is a great danger to getplastic wrinkling and buckling when forming by hydraulicpressure. If plastic wrinkling or buckling occurs then theseparts cannot be used for parts which have to have anexcellent surface quality.At the Institute for Metal Forming Technology (Institutfur Umformtechnik, IFU) of the University of Stuttgart(Germany), a hydroforming process was developed whichis a combination of conventional deep drawing and deepdrawing with hydraulic counter pressure. This process isshown in Figs. 10 and 11. The advantage of this process isthe possibility of deep drawing with controlled metal flowinto the cavity. In the following process the use of thehydraulic pressure makes it possible to have a die withouta hard counter contour.The IFU is presently working on hydroforming of doubleblanks as it is shown in Fig. 12. For this the hydraulic fluid ispumped between the blanks after they have been formed by aconventional deep drawing process to a defined draw depth.Thereby the inner pressure one blank is formed into thecontoured cavity of the hard lower die and one blank isformed against the surface of the punch. It is possible ifneeded to withdraw the punch to a defined position whenhydroforming.In summary, for the industrial production of sheet metalparts it would be of interest to have a hydroforming processthat makes metal flow between the binders similar to theFig. 8. Deep drawing with hydraulic counter pressure in double actionpresses.Fig. 9. Superplastic sheet metal forming process.Fig. 10. Combination of conventional deep drawing and hydraulic counterpressure.Fig. 11. Combination of conventional deep drawing and hydraulic internalpressure.Fig. 12. Hydroforming of double blanks.K. Siegert et al. / Journal of Materials Processing Technology 98 (2000) 251258 253conventional deep drawing process, where new multipointcushions systems or multihydraulic cylinders for the blan-kholder forces are used.Further, it seems to be logical to have a combination ofconventional deep drawing and hydroforming for formingsingle and for forming double blanks as well.3. Hydroforming tubes and extrusionsHydroforming tubes can be differentiated in hydroform-ing by outer hydraulic pressure with and without axial forcesand in hydroforming by inner hydraulic pressure with andwithout axial forces.As shown in Fig. 13 when forming with outer pressure thetube is formed onto a mandrel by outer hydraulic pressurewhich acts between the outer surface of the tube and theinner surface of a container. On both ends the container issealed against the tube.This process enables accurate inner surface contours or tojoin two parts, the mandrel and the tube, Fig. 14. Thisprocess is under development at IFU-Stuttgart.As shown in Fig. 15 when forming with inner pressure,which is the usual case, we have to differentiate betweenforming with and without axial forces. When using axialforces and inner hydraulic pressure the stress situationenables great forming rates.Fig. 16 shows, for example, an exhaust system piece forthe engine of an auto which was formed by axial forces andinner hydraulic pressure.Fig. 17 shows, for example, a formed structural part of apersonal car which was formed with no axial forces only byinner hydraulic pressure.In all cases of hydroforming work pieces with innerhydraulic pressure, a system is necessary to hold the upperand the lower die closed when forming with high pressure.For that normally conventional single action hydraulicpresses with high ram forces are in use. These presses arevery expensive.Fig. 13. Hydroforming with outer pressure.Fig. 14. Joining two parts with outer pressure.Fig. 15. Hydroforming with inner pressure.Fig. 16. Hydroformed parts of the exhaust system (Schafer technology,Germany).Fig. 17. Hydroformed structural part (Schafer technology, Germany).254 K. Siegert et al. / Journal of Materials Processing Technology 98 (2000) 251258Therefore IFU-Stuttgart and the German press buildersMuller-Weingarten, SPS, SMG and Hydrap together withMannesmann-Rexroth (Hydraulic Components) developeda press system with reduced costs. This new system operatesthe base of different cylinders for moving the ram with theupper die and for holding the upper and the lower die closed.To move the ram with the upper die only small forces arenecessary. The hydraulic cylinder on top of the press for thathas to have a large stroke but not a high force. When the dieis closed it is only necessary to hold it tightly closed. For thiscylinders with high forces but only a small stroke arenecessary. These cylinders are placed between the frameof the press and the press table. Before these cylinders act,steel blocks (spacers) are pulled in between the frame of thepress and the ram so that the ram is mechanically locked.The process is shown in Figs. 1829.The advantage of this system is a minimized hydraulicvolume which result in a shorter compression time. Thecycle time is at least as short as with conventional pressesfor hydroforming. Further this press system is less expen-sive. This press system with 3500 t capacity and a press tableof 2500 mm 900 mm was built up at the IFU-Stuttgart(Fig. 30).The initial die was developed and produced byDaimler Benz. Further dies for investigating piercing holesunder inner pressure were developed and produced byBMW. Additionally Opel, Voest Alpine, VAW, OCAS andAlusuisse Singen support this project. So by the cooperationof eleven industrial companies and IFU it was possible tobuild this hydroforming system for about 1.2 millionDM. All the cost were paid by the project partners. Thedevelopment time from signing the contract to firstFig. 18. Process sequences of hydroforming (Step 1).Fig. 19. Process sequences of hydroforming (Step 2).Fig. 20. Process sequences of hydroforming (Step 3).Fig. 21. Process sequence of hydroforming (Step 4).K. Siegert et al. / Journal of Materials Processing Technology 98 (2000) 251258 255experimental parts formed in this new developed press wastwo years.In summary, with these developments in hydroformingtubes and extrusions a great variety of parts can be produced.Hydroforming tubes with outer pressure is not commonlyused in production but has the potential for forming partsespecially with complicated inner surface contour.The main interest is in hydroforming with innerhydraulic pressure with and without axial forces. For thisat IFU-Stuttgart a new cost optimized press systemwith 3500 t capacity was developed in cooperation witheleven industrial companies. This system shows excellentresults.4. SummaryThis paper has discussed the possibilities and advantagesof hdroforming in sheet metal forming as well as in formingtubes and extursions. For hydroforming sheet metal it isuseful to control the metal flow between the binders like inmodern single action presses with multipoint cushion sys-tems. In this context new die designs are presented.For hydroforming tubes it is possible to form with outerand/or inner pressure with and without axial forces. For thehydroforming process, presses with high forces are needed.This paper shows a new concept for a press which wasdeveloped as a cost minimized system.Fig. 22. Process sequence of hydroforming (Step 5).Fig. 23. Process sequence of hydroforming (Step 6).Fig. 24. Process sequences of hydroforming (Step 7).Fig. 25. Process sequences of hydroforming (Step 8).256 K. Siegert et al. / Journal of Materials Processing Technology 9