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Process and Tool Technology for Hydroforming: Case Studies & Technical & Economical Considerations
by Dr.-Ing. Alfons Boehm of Siempelkamp Pressen
Systeme, Krefeld (Germany)
A change has been detected in the way of thinking about hydroforming in the automotive industry: The eighties were the period of fundamental research and initial applications. At the beginning of the nineties the first workpieces also in series production were publicly presented, and nowadays we have reached the point where users and customers question the economy or the reliability of hydroforming process.
If it was an advantage in the past to be one of the "first" to offer this technology, now, at the end of the nineties, hard competition rules prevail. The use of the hydroform process by itself is no longer sufficient and the presence of sample parts no longer creates a sensation as was once true. Customers now concentrate on optimum productivity and the lowest production engineering costs of the whole process sequence. Part quality is of course expected and taken for granted.
Thus, pre- and final machining of hydroforming workpieces and the associated processes are more and more the focus of attention. Bending and other tube forming processes and joining and cutting technologies have become of great interest too.
This article mainly deals with "preforming": How to produce starting shapes for the hydroforming process, e.g. starting from "simple round" tubes? What requirements with regard to the hydroforming process must be fulfilled?
When considering the state of the art of the today's classic bending technology, the impression may arise that these technologies are outdated. Stretch bending or CNC-bending processes have been around for some time and give cause for concern due to the unacceptable service life and changing time of dies, their high process cycle time together with high equipment investment costs. One has to ask if hydroforming technology took the bending experts and equipment suppliers by surprise and maybe gave rise to overcharging. The ambition "...to get a machine for preforming in compliance with the hydroforming plant cycle time" is increasing.
Sometimes, bending/preforming in a die can offer an
Production of starting parts for hydroforming
Figure 1 shows the different preforming variants
starting from straight tubes
If only a flattened tube is necessary as starting part for hydroforming it is possible to use elliptical tube material. In practice, a combination of these operations is often required: Picture 2 shows an example for a workpiece produced by means of a combination of all mentioned forming processes. With regard to the conical radial reduction, attention must be paid to the possibly considerable press forces and tool loads that may occur. Forming forces up to 5000 kN are to be expected with workpiece lengths from 1500 to 4000 mm and reductions from do = 60 mm to d = 55 mm having a wall thickness between 2 and 4 mm. Also tool loads are accordingly high: Material flow along the tool surface can easily give rise to contact stresses up to 1200 N/mm2. Reduction of friction between tube and tool by means of tribological measures (coating and lubrication optimization) has to be carefully carried out. This also applies to the conical radial reduction with superimposed bending (picture 3 variant 3).
Figure 1: Preforming in die: selected
Figure 2: Starting part for hydroforming produced by means of process combination according to Picture 1
left: hypothetical preform workpiece - right: real
component hydroforming workpiece
Figure 3: Lower tool for conical
radial extrusion of tubes (CAD-view)
Design guide lines of starting parts for hydroforming
How can a hydroform process oriented geometry be
obtained starting from a given part design that can be
manufactured from a (round) tube and economically
This leads to a homogeneous wall thickness reduction
during the hydroforming process with regard to the part
circumference and also minimizes the risk of a local tube
failure (crack). If the said requirement does not apply
the risk of cracking in areas with excess material
Figure 4: Optimization of a preforming process by means of FEM: Example Results from Hydroforming
Figure 5: Defective hydroforming
If this is not the case, the inner parts of these radii are subjected to tensile stress during hydroforming and are enlarged again. This can lead to cracks at the inside surface which may be impossible to detect (figure 6).
Guide Line for Preforming
Figure 6: Requirements on starting parts for hydroforming: All cross sectional radii greater than or equal to the radii of the hydroforming workpiece
Planning of complex preforming processes
It is taken for granted that the Finite Element Method (FEM) is the ideal tool for planning preforming processes to produce starting parts for hydroforming.
Figure 7 shows a bending operation taking place in a hydroform die as a preforming operation with tool optimization by means of FEM. The basic suitability of different tool variants with/without blank holder at the tube ends are first checked. The second step consists of optimizing more precisely the tool contour of the best variant (variant 2, figure 7). Today, such optimizations can be performed at single user workstations within relatively short periods and result in a high planning reliability but also in practice reduce prototyping costs by up to 70 %.
FEA for Preforming Tools
Figure 7: Bending in die as
preforming operation for hydroforming:
Quality of hydroformed workpieces
Workpiece and tool quality
The part quality depends on numerous influencing
variables but the decisive factor is the tool itself. In
previous publications the author has already illustrated
important aspects of the hydroforming tool design /1,2,3,4/
which are given hereafter in keywords:
The following is an examination of the aspect of tool guiding:
A displacement/misalignment of the lower to the upper
tool leads inevitably to a shearing of the workpiece wall.
Furthermore, the whole dimensional accuracy of the
workpiece is negatively affected. The shearing forces
exerted on the workpieces arise among others from the
axial forces and the action of the internal pressure. The
internal pressure continuously acting on the tool surface
via the workpiece requires a rigid guiding in ALL
directions. In practice, well-proven bar guides are those
Examples for such guiding systems are shown on Figure 8.
Hydroforming Tools: Guiding
Figure : 8 Guiding of hydroforming tools (examples: U-shaped workpieces)
Hydroforming Tool: Example
Figure : 9 Guiding of hydroforming
Workpieces that are produced by hydroforming present
the following characteristic features:
Figures 9.1 and 9.2 are showing examples of these characteristics.
Quality of hydroformed workpieces - visible marks of the tool parting lines - visible marks of the ejector
Figure : 9.1 & 9.2 Guiding of
Achievable workpiece quality Example I:
By means of an example in figure 10, selected quality
features are explained. The Rover 75 engine cradle
represented is manufactured in three production steps:
The resulting part quality is also summarized in figure 10. It is remarkable that defects occurring during prebending are mainly compensated for during the preforming operation. Defects intentionally caused during prebending, e.g. elongation of the straight tube parts between the bends, cannot be statistically traced back on the hydroformed final part.
40 points x-y-z (coordinate measuring machine)
Standard deviation s = 0.02 mm up to 0.17 mm / ? min./max.
Approx. 0.2 mm
Figure : 10 Achievable accuracy of hydroforming workpieces - Example I
Achievable workpiece quality - Example II
A similar workpiece is shown on figure 11 for comparison purposes. The piece is manufactured by means of similar production steps as mentioned at example I. Spring back data of the U-shaped motor cradle are given here.
Figure : 11 Achievable accuracy of hydroforming workpieces - Example II
Further investigations have been carried out and some
relevant results are listed:
Chart 1 /5/ compares the production variants for a
finished fabricated engine cradle
The results can be summarized as follows:
Chart 1: Comparison of costs: Production and material variants for a motor cradle /5/
/1/ Böhm, Alfons Part Cost Reduction in the
/2/ Böhm, Alfons "Auf dem Weg in eine
/3/ Böhm, Alfons Ausgewählte Probleme der
Werkzeugentwicklung beim IHU und IHU-Lochen
/4/ Böhm, Alfons Werkzeugtechnologie des
Innenhochdruckumformens - Ausgewählte Beispiele
/5/ Seifert, M. MAGNA Body and Chassis Systems:
Published with permission of Dr.-Ing. Alfons Boehm.
Last update: März 18, 2000
Letzte Änderung: 18 März 2000
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