May 15, 2001
Recent advances at the University of Stuttgart and acfross the industry have opened doors for hydroforming all kinds of materials and shapes.
Hydroforming tubes, extrusions, and sheet metal is a state-of-the-art enterprise and is just now becoming more popular in the industrial production of frames for light trucks and vans.
Hydroforming tubes with outer pressure, though not the norm in the industry, has several possible applications.
This article deals with this enterprise by showing the possibility of combining of conventional deep drawing and hydroforming. Furthermore, this article shows a new press concept for hydroforming tubes and extrusions a new press concept that can lead to shorter cycle times and fewer press costs.
Tubes can be hydroformed either by outer or inner hydraulic pressure, with and without axial forces.
In outer-pressure hydroforming (see Figure 1), 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 the joining of the mandrel and the tube. This process is under development.
In inner-pressure hydroforming (see Figure 2), the method that is usually used, a differentiation must be made between forming with and without axial forces. When using axial forces and inner hydraulic pressure, the stress situation enables very good forming rates.
In all cases of inner-pressure hydroforming, a system is needed to hold the upper and lower dies closed when forming with high pressure. Usually, a conventional single-action hydraulic press with high ram force is used for this purpose.
Inner-pressure hydroforming without (left) and with (right) axial forces uses an upper die and lower die with seals to create a closed system for forming a tube (Fs force for sealing, Fa axial force for influencing the forming process).
As an alternative, a German consortium of the Institute for Metal Forming Technology (IFU) at the University of Stuttgart, Germany, and German press builders Müller-Weingarten, SPS, and Hydrap, together with Mannesmann-Rexroth, has developed a press system that operates different cylinders for moving the ram with the upper die and for holding the upper and lower dies closed (see Figure 3).
To move the ram with the upper die, only small forces are necessary. The hydraulic cylinder on top of the press must provide a large stroke, but not a high force. When the die is closed, it must be held tightly closed. For this, cylinders with high forces but small strokes 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.
A new press concept for hydroforming processes developed by a German consortium uses different cylinders to move the ram with the upper die and to hold the dies closed.
This type of system minimizes hydraulic volume, which results in a shorter compression time. The cycle time is at least as short as with conventional presses for hydroforming.
An actual system with 3,500 tons capacity and a press table of 2,500 by 900 millimeters has been built at the IFU. The initial die was developed and produced by DaimlerChrysler. Further dies for investigating piercing holes under inner pressure were developed and produced by BMW. Additionally, Opel, Voest Alpine, VAW, OCAS, and Alusuisse Singen supported this project.
Tubes, in most of cases, are formed and welded from sheet metal. So hydroforming tubes is essentially hydroforming sheet metal by inner pressure.
Conventional sheet metal hydroforming has been known for years, but demand has increased recently for hydroformed parts for small volume sports cars and premium cars.
In hydraulic sheet metal forming (hydromechanical deep drawing), hydraulic counterpressure is used instead of a rigid die. This counterpressure builds as the fluid is compressed when the punch forces the blank downward. The counterpressure is controlled by a servo or proportional valve.
For deep drawing nonaxisymmetric parts, the metal flow between the binders can be directed by the following:
1. Draw beads
2. Lock beads
3. Blank shape
4. Friction between the blank and the binders
Using friction forces to direct the metal flow, the blankholder forces can be adjusted and directed over the stroke by hydraulic multipoint cushion systems. The cushion systems work with special dies that transfer the blankholder forces to defined blankholder surfaces.When hydroforming sheet metal, a multipoint cushion system with several hydraulic cylinders integrated in the draw die can be used (see Figure 4).
It is also possible to integrate this kind of cushion system in the ram of a single-action press. Each cylinder has its own proportional or servo valve, so that the pressure in the cylinders respective to the blankholder forces can be controlled over the stroke by adjusting the pressure in the cylinders.
Figure 4 also shows a specially designed blankholder with pyramidal (upside down) shaped ribs. In this design, there is a direct correspondence between the blankholder force acting on the pyramid and the pressure between the blank and the binders.
At the IFU-Stuttgart, a hydroforming process has been developed that combines conventional deep drawing and deep drawing with hydraulic counterpressure (see Figure 5). This process allows deep drawing with controlled metal flow into the cavity.
The IFU is presently working on hydroforming of double blanks (see Figure 6). For this, hydraulic fluid is pumped between the blanks after they have been formed by a conventional deep drawing process to a defined draw depth.
With 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. If needed, the punch can be withdrawn to a defined position when hydroforming.
Hydroforming has promising possibilities in both sheet and tube fabricating. Hydroforming of tubes can be performed with outer and/or inner pressure, with and without axial forces.
Although hydroforming tubes with outer pressure is not commonly used in production, it has potential for forming parts, especially those with complicated inner surface contours.
In addition, hydroforming of sheet metal can be made more effective by controlling the metal flow between the binders.For both of these hydroforming applications, new developments in presses and die designs can help to make the hydroforming process more productive and effective.
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