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  Tool and Part Design for Tube Hydroforming Part 1
Posted by: Arndt Birkert on Friday, February 01, 2002 - 06:05 PM
All Topics In designing the hydroforming process for the production of automotive structural components, a number of factors must be taken into account to ensure technically and economically viable production. The component design and the required mechanical component properties determine both the process flow and to a large extent the design of the necessary forming dies and required machines.
The following paper discusses key design and production aspects and illustrates them by reference to selected examples.

by Dipl.-Ing. Arndt Birkert, Dipl.-Ing. Jörg Neubert, Krupp Drauz GmbH - Germany

1. Introduction

A wide range of components can be hydroformed from tubular blanks for use in body, chassis and powertrain. Tube hydroforming provides many of the properties required above all for automotive structural components.

The high level of dimensional and design accuracy required for automated body-in-white manufacture is facilitated in particular by the naturally closed component cross sections and the capacity for a "minimum plasticization" of all cross-section areas and consequently low springback.

Completely closed cross sections over the full length of the component result in a high degree of torsional and bending stiffness. Similar stiffness can only be produced on shell designs by continuous welds (e.g. MIG or laser welding), which can lead to a considerable cost penalty versus largely conventional spot welds.

The parts count for complex-shaped components in particular can be reduced since the same components produced by conventional drawing techniques frequently have to be put together from several parts to avoid material waste. Moreover hydroforming permits the integration of certain additional shapes which would otherwise take the form of separate add-ons.

The process chain is shortened mainly due to a highly integrated hydroforming process which, in addition to the actual forming of the components with possible preforming and bending operations, can also include cutting functions such as piercing, partial flange trimming on extruded sections as well as cutting component ends. In addition, the naturally closed sections mean that the joining process otherwise required for half shells, with all its disadvantages as requires tooling expense and tolerance-related problems, can be dispensed with.

Requirements for minimized weight and packaging are met by the aforementioned improvements to the component properties and by the elimination of flanges.

Despite these and other advantages, the natural limits of the process must also be pointed out, since these can in part severely restrict design latitude, also in direct comparison with deep-drawn parts. Essentially this includes the variation of the component diameter over its length as well as completely different force and tension conditions for plasticizing and forming the components. The latter impact in particular on the equipment requirements and thus the cost-effectiveness of the entire process.
In the following, key aspects with regard to the design of components, processes and tools are described. This paper aims to improve understanding of the process and thus encourage engineers in particular to design components with the hydroforming process in mind.

2. Designing process-compatible component cross-section geometries

The required component cross sections are essentially produced by preforming, expanding and calibrating, with preforming being performed either on a separate tool or in the hydroforming die itself. In the following, all three process steps are discussed separately.

2.1 Expanding without axial force

When talking about expanding it is important to differentiate between expanding with and without the application of axial force. In both cases the blank must meet the conditions to permit error-free insertion in the hydroforming die. A major aspect in designing the diameter of tubular blanks is that the smallest component diameter should largely match the tube diameter of the blank. If the diameter of the tubular blank greatly exceeds the smallest component diameter, the cross section will be collapsed when the die closes, or blank material will be squeezed into the parting line of the die halves. If the diameter of the tubular blank is too small, it is possible that the component cannot be fully formed due to excessive circumferential elongation. This will cause premature component failure from necking or bursting.

When expanding under axial force, the expansion process is supported by axial compression. This permits considerably greater circumferential elongation than when expanding under internal pressure only. While expanding without axial force is shown almost as a plane-strain curve in the forming limit diagram (PHI 2=0) and failure thus occurs at low levels of PHI 1, expansion under axial force involves a distinct compression-tension forming process (PHI 1 greater 0, PHI 2 smaller 0).
However, on comparatively long automotive structural components which in addition can also be bent, the possibilities for introducing axial compressive stresses to the required component areas are very limited, Fig. 1.

Fig. 1: Characteristic stress distribution when expanding under axial force

As the pressure increases and with it the contact between die and workpiece, bringing with it a simultaneous increase in normal contact stresses, the compressive stresses introduced at the ends of the workpiece are increasingly weakened by frictional stresses between workpiece and die the greater the distance from the blank ends. Depending on component geometry, it is possible that the forming area will stop being supported by axial force at only a short distance from the ends of the workpiece. With regard to the maximum achievable circumferential elongation, these areas are governed by the parameters for virtually plane-strain.

The maximum circumferential elongation in percentage terms which can be achieved in the plane-strain condition for various materials with a virtually square cross section is shown in Fig. 2. The fact that the manufacture of the required cross-section geometry for the strain conditions observed is based - unlike expansion under axial stress or deep drawing - exclusively on reducing the wall thickness, makes it necessary to meet close material-specific design guidelines.

Fig. 2: Maximum circumferential elongation when expanding without axial force

When hydroforming tubes or sections of aluminum alloys in particular, the maximum expansion limits are very low. It is essential that this be taken into consideration when designing components. The problem is exacerbated if expansion is to take place in pre-bent blank zones. Due to non-linear strain curves, the inability to predict the beginning of necking by means of the forming limit diagram in conjunction with the otherwise extremely useful finite-element simulation is also particularly problematical.
Extensive practical experience and above all targeted practical model tests used to quantify the influence of pre-bending on different materials with different characteristic basic cross sections is particularly helpful here. If the test design is suitable, the results can be transferred to practical components with a sufficient degree of approximation. This problem is the subject of research work currently being performed by Thyssen Krupp Automotive in conjunction with Thyssen Krupp Stahl within the ThyssenKrupp organization and also in industrial cooperative research projects in which Krupp Drauz is involved.

The question as to whether lubricants need to be used when expanding without axial force can be answered by observing the required circumferential elongation and above all the wall thickness distribution after forming, Fig. 3.

Fig. 3: Wall thinning through expansion with / without lubrication

Even with compact cross-sectional geometries, comparatively homogeneous wall thickness distributions are only achieved under suitable tribological conditions. The higher the friction between workpiece and die, and the higher the percentage circumferential elongation in a given cross section, the more inhomogeneous the wall thickness distribution over the circumference after forming.
To allow cost-effective volume production, compact geometries with large corner radii and low circumferential elongation are desirable. This allows a reduction in lubricant requirements. Ideally it should be possible to dispense with any further treatment subsequent to initial greasing of the tubes in the rolling mill.

2.2 Preforming without internal pressure

Based on the above, preforming is illustrated in the following using box-type cross sections as an example.
As already described above, when expanding round tubes of common aluminum alloys in a die of e.g. square cross section, depending on the tribological conditions and material it is possible to achieve circumferential elongation of 8-12% in straight-sided component areas with a degree of reliability suitable for volume production. This is nothing like enough to fill a surrounding square with a corner radius of e.g. 3xs0.
Similar problems are encountered in pre-bent zones, where the remaining maximum circumferential elongation even of high-quality, extremely formable steels can be well below 10% depending on bending geometry.
Producing cross sections of this type requires the use of a larger tube blank which is preformed to an intermediate geometry close to the final cross section, Fig. 4.

Fig. 4: Preforming of cylindrical tubes without internal pressure

There are two main variants for preforming with or without internal pressure. In the simpler variant, the preforming area of the die is wider than the insertion diameter of the tube. In this case preforming can generally be effected simply by closing the hydroforming die, Fig. 4, top. In the second variant the die width is lower than the diameter of the tube being inserted. In this case preforming - if a compression method is used - is made possible by providing angled die faces, Fig. 4, bottom. A decision must be made as to whether preforming is to be effected in a separate die or integrated in the hydroforming die. If preforming takes place in the hydroforming die, the component radii in the upper half of the component cross section cannot be integrated in the upper die as negative contours due to the problem of how the die is split. The radii are formed freely during the hydroforming process, which has the disadvantage of larger radius tolerances than is the case when forming is controlled fully by the die shape.

The limits of this process - in particular without internal pressure - are reached when the wall of the workpiece starts to wrinkle. Depending on requirements for the surface quality of the component in specified areas, very slight wrinkling may be permissible. Tolerance limits are generally formed by a wrinkle height which would lead to irreversible wrinkling during subsequent expansion or calibration, Fig. 5.
Ideally cross sections should be dimensioned in such a way that the wall of the workpiece does not come away from the die during preforming, and expansion of approx. 5% can take place after forming so as to plasticize the overall cross section to a large extent.

Fig. 5: Irreversible wrinkling when preforming without internal pressure

In terms of the cost-effectiveness of the two variants w greater d0 and w smaller d0, it should be noted that an additional preforming stage - in particular for small components - can frequently be performed using the same press slide as for the hydroforming die. This involves only a slight increase in investment for tooling; cycle times (assuming suitable process design) and thus component price remain the same, disregarding the very slight addition to machine costs for the preforming die. Large-surface components such as subframes and similar are an exception.
To ensure the stability of the production process, preforming in a separate die is generally to be preferred in the case of w smaller d0.


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