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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)
and Tony Abbey of Siempelkamp Pahnke Engineering, Springfield - OH (U.S.A.)

Contents

  • Introduction
  • Design guide lines of starting parts for hydroforming
  • Planning of complex preforming processes
  • Quality of hydroformed workpieces
  • Quality of workpieces and tools
  • Achievable workpiece quality
  • Some characteristic features of hydroforming parts
  • Economy
  • References

Introduction

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the state of the art of the today's classic bending technology, the impression may arise that these technologies are outdated.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 interesting alternative.
Another topic which will be discussed is the component quality of the hydroformed part: process limits and achieved accuracy’s are explained with examples.

Production of starting parts for hydroforming

Figure 1 shows the different preforming variants starting from straight tubes

  • flattening
  • conical radial extrusion
  • conic radial reduction with synchronous bending of the tube center line

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 process variants
1) flattening
2) conic radial extrusion
3) conic radial extrusion with synchronously bending operation

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
The tool shown on picture 3 is used for conical radial extrusion with integral support. Eight ejectors in the lower tool enable the workpiece to be lifted out without deformation.

Figure 3: Lower tool for conical radial extrusion of tubes (CAD-view)
[preforming in a tapered die]

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 produced?
The known requirements are:

  • The starting part must be perfectly placed in the hydroforming tool.
  • After tool closing, a defined position of the workpiece ends must be ensured (so that no "sliding" occurs thus avoiding an inaccurate starting position for the hydroforming process)
  • The two following examples illustrate further design guide lines:
  • The center of gravity of the starting part cross section should be situated at the center of gravity of the hydroformed workpiece cross section.

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 persists.
In figure 4 cross sections preformed in a preforming tool are shown.
Picture 5 shows the result of the hydroforming operation for variants 1 and 5.2 from figure 4.FEA

Figure 4: Optimization of a preforming process by means of FEM: Example Results from Hydroforming

Figure 5: Defective hydroforming workpiece (fold)
Result of hydroforming operation Variant 1 from picture 4
Perfect hydroforming workpiece variants 5.1 from picture 4
A further design guideline for a starting part in the hydroforming process is:

  • All cross sectional radii of the starting part must be greater than or the same as the radii of the hydroforming workpiece.

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:
Tool optimization by means of FEM (three variants)

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:

  • Rigid tool design with net-shaped guiding of the upper/lower tools to avoid shearing at the part
  • Tool design: consideration of preforming/bending processes (tool parting line)
  • Hydraulic tool closing [possibly], depending on internal pressure
  • Highest material qualities with optimized coating according to the lubrication system (type, coat, etc.)
  • Water management in press and tool
  • Min. 5 % plastification in all workpiece cross sections to reduce springback effect (steel)
  • Defined materials/process inputs; "scrap in scrap out" also applies for hydroforming
  • Process supervision and specifications: tailored for the hydroforming process.

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 that:

  • absorb in ALL directions the shearing forces between upper and lower tool
  • are mounted as close as possible to the part contour
  • guide the upper and lower tool along the WHOLE length of the part contour.

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 tools
(example: workpiece similar to side member)

Workpiece quality

Workpieces that are produced by hydroforming present the following characteristic features:

  • No contact of the internal surface with the tool (excepted tube end area that can be calibrated by the axial forming punch during the hydroforming process)
  • Contact of the external surface with the tool
  • Marks caused by the tool parting line
  • Marks on the external surface caused by the use of inserts in the hydroforming tool, also possibly caused by ejector/positioner
  • Marks from processes (gripper/clamping jaw by bending)
  • Possibly burr formation at tube ends and mark of the axial forming/sealing punch

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 hydroforming tools
(example: workpiece similar to side member)

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:

  • Bending (12 bends)
  • Preforming (conical radial extrusion)
  • Hydroforming / Piercing / Punching

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.

Measuring method:

  • CMM
  • workpiece clamped (3-point bore holes)
  • 40 measuring points (x,y,z)

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
Result of part height measuring
Standard deviation s = 0.02 mm up to 0.05 mm / ? min./max. Approx. 0.1 mm
Result of wall thickness measuring
Standard deviation s = 0.05 mm up to 0.09 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.

Starting tube
approx. 1800 x 65 x 2.5 mm, steel
2 x CNC-bending, preforming, hydroforming
Prebending tolerances
length (total) = ± 3 mm
angle (bending) ± 1° per bend
"Spring back" - Measurement method
tube not clamped – distance measuring
100 workpieces, completely automatic operating mode
Max. = + 1.59 mm deviation from nominal value in mm
Min. = - 1.78 mm deviation from nominal value in m

Figure : 11 Achievable accuracy of hydroforming workpieces - Example II

Further investigations have been carried out and some relevant results are listed:

  • The correlation between calibrating pressure and part quality has been checked on the basis of examples I and II in order to only apply the necessary internal pressure on the tools as defined by the part quality and process stability.
  • The effects of tube bending tolerances on part quality have been checked so that tubes outside acceptable tolerances can be rejected from the production line prior to forming.
  • As part of an emergency strategy (tool damage) contour inserts are available as spare parts for the tools: dimensional differences between spare parts and original contour inserts have been checked.
  • The influences of semi-finished product tolerances have been checked by testing and examining examples with insufficient material.
  • The lubricating conditions have been intentionally modified and their effects analyzed to determine the limits.

Economy

Chart 1 /5/ compares the production variants for a finished fabricated engine cradle

  • Engine cradle steel, deep drawing variant
  • Engine cradle steel, hydroforming solution
  • Engine cradle aluminum, hydroforming solution
  • Engine cradle steel, hydroforming solution (so-called "low pressure" variant with low internal pressure and without calibrating operation)

considering

  • Number of parts
  • Weight
  • Tool costs

The results can be summarized as follows:

  • The number of single parts can be considerably reduced with internal high-pressure forming.
  • Weight can be reduced by 20 % with internal high-pressure forming.
  • Part costs can be lowered by 15 %.
  • Aluminum brings a further weight reduction of approx. 25 % at a simultaneously part costs increase of 30 %.
Variant Numper of single parts Weight / kg tool costs / US$ Part Costs / US$
Classic deep drawing 34 24.56 5,359,090.-- 51.00
Hydroforming - Steel 30 20.50 3,712,636.-- 42.83
Hydroforming - Aluminium 30 14.41 3,891,727.-- 73.17
Hydroforming - Steel - low pressure 30 22.77 3,512,398.-- 45.55

Chart 1: Comparison of costs: Production and material variants for a motor cradle /5/

References

/1/ Böhm, Alfons Part Cost Reduction in the Hydroforming Process
2nd International Conference on Innovations in Hydroforming Technology
September 15-17, 1997 Columbus, Ohio
Conference tape

/2/ Böhm, Alfons "Auf dem Weg in eine goldene Zukunft"
Elsinghorst, Detlev Part 1: Blech 2-99, page 40 ff
Part 2: Blech 3-99, page 72 ff

/3/ Böhm, Alfons Ausgewählte Probleme der Werkzeugentwicklung beim IHU und IHU-Lochen
Technische Akademie Esslingen, 1998: IHU von Rohr und Blech, Conference tape

/4/ Böhm, Alfons Werkzeugtechnologie des Innenhochdruckumformens - Ausgewählte Beispiele
Haus der Technik e.V., Essen, Seminar "IHU von Rohren und Blechen", Conference tape, September 30, 1997
(Continued with similar titles in 1998 and 1999)

/5/ Seifert, M. MAGNA Body and Chassis Systems:
Congress: IHU von Rohren, Universität Madgeburg,
Conference tape page 188 ff

Published with permission of Dr.-Ing. Alfons Boehm.

   

 

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Last update: März 18, 2000
Letzte Änderung: 18 März 2000

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