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Recent developments in hydroforming technology Klaus Siegert*, Markus Ha ussermann, Bruno Lo sch, Ralf Rieger Institute for Metal Forming Technology, University of Stuttgart, Holzgarten Str. 17, 70174 Stuttgart, Germany Abstract This paper shows an overview about possibilities of hydroforming sheet metal as well as hydroforming tubes and extrusions. Coming from the deep drawing process with rigid dies, specially designed dies for presses with multipoint cushion systems required for hydroforming 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 hydroforming 1. Introduction To discuss the advantages and problems of forming metal work pieces by hydraulic pressure it is useful to differentiate between hydroforming of sheet metal and hydroforming of tubes and extrusions. 2. Hydroforming sheet metal The conventional deep drawing process is shown in Fig. 1. A sheet metal product like kitchenware, for example pots, or automotive parts like hoods, fenders, etc. can be drawn in single action presses as shown in Fig. 1 or in double action presses as shown in Fig. 2. Drawing sheet metal products is in most cases not deep drawing of axisym- metric parts like pots butdrawing of non-axisymmetric parts with a combination of deep drawing and stretch forming. Forthisitisnecessarytodirect themetal fl owbetweenthe binders by ? draw beads, ? lock beads, ? shape of the blank and ? friction between the blank and the binders. For directing the metal fl ow between the binders by friction forces, it is possible to adjust the blankholder forces and to fi t in the gap between the binders. The blankholder forces can be directed over the stroke by modern hydraulic multipoint cushion systems. These systems make it possible to build closed loop systems in order to avoid wrinkling and fracture when friction relevant input parameters like lubri- cant, lubrication amount and the microstructure of the sheet metal surface change. This goes along with specially designed dies that transfer the blankholder forces to defi ned blankholder surfaces 13. Fig. 3 shows a multipoint cushion system with several hydraulic cylinders. Each cylinder has its own proportional orservovalve,sothatthepressureinthecylindersrespective the blankholder forces can be controlled over the stroke by adjusting the pressure in the cylinders. Fig. 4 shows a different cushion system with four hydrau- lic cylinders supporting the cushion plate and a number of cushion pins transferring the blankholder forces from the cushion plate to the blankholder. Fig. 5 shows a specially designed blankholder with pyr- amidal (upside down) shaped ribs. This design has the advantage that there is a direct correspondence between the blankholder force acting on the pyramid and the pressure between the blank and the binders 4,5. In industrial production only a few presses have such modern hydraulic multipoint cushion systems. So it makes sense to design dies with hydraulic cylinders between a base plate 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 a rigid die there is a hydraulic counter pressure, Figs. 7 and 8. This counter pressure can be built up by compressing the fl uid when the punch forces the blank downwards. The counter pressure is controlled by a servo or proportional valve.*Corresponding author. Journal of Materials Processing Technology 98 (2000) 251258 Fig. 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 action presses. 252K. Siegert et al./Journal of Materials Processing Technology 98 (2000) 251258 It is also possible to produce pressure in the fl uid at the beginning of the process by a pump system. This makes it possible to have a prebulging of the blank. The initial pressure has the advantage of preforming the work piece along with inducing a work hardening in the middle of the part. This work hardening can be useful in fl at parts to produce stiffer and more geometrically accurate products with more hardness, which is useful for a better hail impact (dynamic denting). It can also be useful to have a prebulging in order to get more sheet metal into the die cavity when drawing deep parts. Further,it is possible to have a hydraulic forming of parts into the cavity of the die when using no punch but only a rigid die cavity. This process and also the prebulging are in principal derived from superplastic sheet metal forming process, Fig. 9. One difference is that in superplastic forming the sheet metal is clamped totally between the binders so that there is no fl ow of sheet metal.Further,insteadof a fl uid, gas is used and it is a warm forming process with the use of a metal with special grain size at special strain rates. The forming of the sheet metal into a cavity can be done by hydraulic pressure in principle like it is the case of the superplastic sheet metal forming process. The problem is that there is no possibility to control the metal fl ow between the binders. Further more there is a great danger to get plastic wrinkling and buckling when forming by hydraulic pressure. If plastic wrinkling or buckling occurs then these parts cannot be used for parts which have to have an excellent surface quality. At the Institute for Metal Forming Technology (Institut fu r Umformtechnik, IFU) of the University of Stuttgart (Germany), a hydroforming process was developed which is a combination of conventional deep drawing and deep drawing with hydraulic counter pressure. This process is shown in Figs. 10 and 11. The advantage of this process is the possibility of deep drawing with controlled metal fl ow into the cavity. In the following process the use of the hydraulic pressure makes it possible to have a die without a hard counter contour. The IFU is presently working on hydroforming of double blanks as it is shown inFig. 12. For this the hydraulicfl uidis pumpedbetweentheblanksaftertheyhavebeenformedbya conventional deep drawing process to a defi ned draw depth. Thereby the inner pressure one blank is formed into the contoured cavity of the hard lower die and one blank is formed against the surface of the punch. It is possible if needed to withdraw the punch to a defi ned position when hydroforming. In summary, for the industrial production of sheet metal parts it would be of interest to have a hydroforming process that makes metal fl ow between the binders similar to the Fig. 8. Deep drawing with hydraulic counter pressure in double action presses. Fig. 9. Superplastic sheet metal forming process. Fig. 10. Combination of conventional deep drawing and hydraulic counter pressure. Fig. 11. Combination of conventional deep drawing and hydraulic internal pressure. Fig. 12. Hydroforming of double blanks. K. Siegert et al./Journal of Materials Processing Technology 98 (2000) 251258253 conventional deep drawing process, where new multipoint cushions systems or multihydraulic cylinders for the blan- kholder forces are used. Further, it seems to be logical to have a combination of conventional deep drawing and hydroforming for forming single and for forming double blanks as well. 3. Hydroforming tubes and extrusions Hydroforming tubes can be differentiated in hydroform- ing by outer hydraulic pressurewith and withoutaxialforces and in hydroforming by inner hydraulic pressure with and without axial forces. As showninFig. 13whenformingwith outer pressure the tube is formed onto a mandrel by outer hydraulic pressure which acts between the outer surface of the tube and the inner surface of a container. On both ends the container is sealed against the tube. This process enables accurate inner surface contours or to join two parts, the mandrel and the tube, Fig. 14. This process 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 between forming with and without axial forces. When using axial forces and inner hydraulic pressure the stress situation enables great forming rates. Fig. 16 shows, for example, an exhaust system piece for the engine of an auto which was formed by axial forces and inner hydraulic pressure. Fig. 17 shows, for example, a formed structural part of a personal car which was formed with no axial forces only by inner hydraulic pressure. In all cases of hydroforming work pieces with inner hydraulic pressure, a system is necessary to hold the upper and the lower die closed when forming with high pressure. For that normally conventional single action hydraulic presses with high ram forces are in use. These presses are very 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 (Scha fer technology, Germany). Fig. 17. Hydroformed structural part (Scha fer technology, Germany). 254K. Siegert et al./Journal of Materials Processing Technology 98 (2000) 251258 Therefore IFU-Stuttgart and the German press builders Mu ller-Weingarten, SPS, SMG and Hydrap together with Mannesmann-Rexroth (Hydraulic Components) developed a press system with reduced costs. This new system operates the base of different cylinders for moving the ram with the upper die and for holding the upper and the lower die closed. To move the ram with the upper die only small forces are necessary. The hydraulic cylinder on top of the press for that has to have a large stroke but not a high force. When the die is closed it is only necessaryto hold it tightly closed. For this cylinders with high forces but only a small stroke are necessary. These cylinders are placed between the frame of the press and the press table. Before these cylinders act, steel blocks (spacers) are pulled in between the frame of the press 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 hydraulic volume which result in a shorter compression time. The cycle time is at least as short as with conventional presses for hydroforming. Further this press system is less expen- sive.Thispresssystemwith3500 tcapacity andapresstable of 2500 mm ? 900 mm was built up at the IFU-Stuttgart (Fig. 30). Theinitialdiewasdevelopedandproducedby Daimler Benz. Further dies for investigating piercing holes under inner pressure were developed and produced by BMW. Additionally Opel, Voest Alpine, VAW, OCAS and Alusuisse Singen support this project. So by the cooperation of eleven industrial companies and IFU it was possible to build this hydroforming system for about 1.2 million DM. All the cost were paid by the project partners. The development time from signing the contract to fi rst Fig. 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) 251258255 experimental parts formed in this new developed press was two years. In summary, with these developments in hydroforming tubesandextrusionsagreatvarietyofparts canbeproduced. Hydroforming tubes with outer pressure is not commonly used in production but has the potential for forming parts especially with complicated inner surface contour. Themaininterestis in hydroforming with inner hydraulic pressure with and without axial forces. For this at IFU-Stuttgart a new cost optimized press system with 3500 t capacity was developed in cooperation with eleven industrial companies. This system shows excellent results. 4. Summary This paper has discussed the possibilities and advantages of hdroforming in sheet metal forming as well as in forming tubes and extursions. For hydroforming sheet metal it is useful to control the metal fl ow between the binders like in modern 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 outer and/or inner pressure with and without axial forces. For the hydroforming process, presses with high forces are needed. This paper shows a new concept for a press which was developed 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). 256K. Siegert et al./Journal of Materials Processing Technology 98