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State of the art and perspectives of hydroforming of tubes and sheets

F. Vollertsen

Department for Metal Forming Technologies, University Paderborn, Paderborn, Germany; fv@luf.upb.de

Hollow parts of high accuracy and high strength can be produced by forming methods using liquid media. Hydroforming of tubes has reached a high standard for small parts (volume some 1000 cm3) and is further developed for larger parts (volume some 10.000 cm3). Processes for hydraulic sheet metal forming are sometimes used for small parts from single sheets. These processes are currently under intensive investigation, which is also true for the processing of double layered sheets. Single sheets can be formed using membranes which separate the workpiece and the liquid. This results in interesting possibilities for a part and process integration in one step. The forming performance of aluminum alloys can be enhanced by using a heated liquid media when forming without membranes.

1. Introduction

For the manufacture of thin walled parts there are two principle possibilities, casting and forming. Cast products have advantages concerning complex shaped geometries, but are less good concerning accuracy and mechanical properties. Therefore, a large amount of parts is produced by metal forming, starting from sheet metal or tubes, which in turn are often made from sheet metal. The most important method concerning the cycle time, realized number of variants in geometry, and number of parts is deep drawing.

In deep drawing a sheet is pushed into a deep drawing die using a punch, see Fig. 1. The process is well established, but has some limitations, if hollow parts or small lot sizes are to be produced. Undercuts can not be realized by conventional deep drawing, the manufacture of hollow parts is done by deep drawing of at least two shells and welding them after forming. This leads to undesirable long process chains with high efforts necessary for the aligning and clamping before welding [1]. Production of small lot sizes is costly as the stiff deep drawing tools must be thoroughly aligned in the press and time consuming try out is necessary before the first production run. In order to overcome these limitations, forming methods with hydraulic liquid media are established.

Tube hydroforming, as shown in Fig. 1, is well known for a long time. As no distortion by thermal welding after forming is introduced, products of high accuracy result from this process. On the other hand, the cycle times are significant larger than in deep drawing. These long cycle times reduce the cost saving effects which are attributed to hydroforming.

These cost savings are due to part and process integration. For example, 14 deep drawn parts for a car body can be substituted by one hydroformed part [2]. Processes like piercing can be integrated into the forming tool. This results in high cost savings for tools and labor costs for the machine operator. In the case of an engine cradle this effect saves 60% of tool costs. Due to the longer cycle times the cost savings for the part were only 20%, but the weight was also reduced by 30%, yielding an additional benefit of the hydroformed part [3].

From tubes, having – per definition – a constant cross section along the length axis, it is difficult to produce parts having large differences in the cross sectional contour along the length. The cross sectional contour can essentially only be increased by the hydroforming process. Due to the limited elongation of engineering materials, the increase in diameter and therefore the changes in cross section is limited to approx. 100%. Parts having larger differences can be manufactured from welded or unwelded sheets [4-7]. Sheet metal forming by hydraulic media is not restricted to closed hollow parts. It is also favorable used for production of small lot sizes, as the liquid adapts automatically to the stiff tool. This makes tool installation easier, resulting in remarkable cost savings at least for lot sizes less than 20.000 parts.

2. Definitions

In the current literature a large number of names exist, which refer to forming methods using liquid media. Hydroforming, low or high pressure forming, fluidform and so on are some of these words. While the German engineering association (VDI) has prepared standards for the definition of these expressions [8], there is a lack of such standards for the English language. In order to avoid confusion, a definition is given for the most important expressions, which is valid at least for this paper

hydroforming: forming method for thin walled parts using a liquid medium to introduce the forming stress in the surface of the part. Other words which are used for that are e. g. high internal pressure forming, high pressure forming, low pressure forming.

profile: raw part having an arbitrary cross section which is constant along the straight length axis of the part.

tube: profile having an annulus as cross section.

tube hydroforming: hydroforming process starting from a piece of a tube.

hydraulic sheet metal forming: Methods of sheet metal forming for manufacture of open or closed hollow parts, starting from sheet metal and using a medium (liquid, optional separated from the part by a thin rubber-like membrane) on one side and a stiff tool on the other side.

calibration: Hydroforming process without intended or considerable axial flow of the material. In tube hydroforming calibration leads to a local increase of the diameter of the part, often characterized by a plain strain forming mode.

3. Tube hydroforming

Figure 2 shows an approach to systematize the parts made by tube hydroforming. Tube hydroforming is intensively used for mass production in fitting industry. These are referred as tubular components, as the geometry is not very complex and the origin from a tube can be seen easily.

The parts for interior installation like water conduits are often made from copper, which has good forming capabilities. As the geometry is often a simple T-piece with short ends, forming is easy and can be done far away from the process limits. That is a precondition for the application of multiple dies, were 4, 8 or 16 parts are made simultaneously.

Sanitary appliances like hotheads are made from brass tubes by bending and calibration. Pipeline components, the T-piece in Fig. 2 has a diameter of approx. 500 mm, valve housings and so on are other examples for tubular components.

Structural parts, mostly applied in automotive industry, are of significant higher complexity. They often have a large number of openings, a multiple bend length axis and a wide variety of cross sections. They also may have flanges to enable welding of other components. State of the art is the manufacture of exhaust systems which comprise single walled T-pieces like shown in Fig. 2.

New developments in the field of exhaust systems are the manufacturing of double-walled parts. The principle patented in [9]. After preforming a two stage hydroforming is employed, which first forms the inner (and the outer) tube. The outer tube is formed in the second stage, while the shape of the inner tube remains unchanged as a hydrostatic pressure acts on it. A central feature of this technology is the sealing.

In the field of frame and chassis parts tubes up to length of 4 m, having a diameter of more than 200 mm are processed. Piercing of multiple holes in the hydroforming tool at the end of the process is standard. As hydroforming is used to realize light weight concepts the forming of aluminum alloy profiles is also investigated. Many applications restrict to calibration of bent profiles, e. g. profiles for a space frame car are calibrated by hydroforming. Due to the difficulties to form sharp edges it is also worthwhile to use extruded profiles as starting material. It was shown that forming of such profiles into T-pieces is possible, but has its specific problems [10].

4. Hydroforming of sheets

4.1. Membran method

A large amout of patents, especially in Germany, is focussed on the forming of single or double sheets using a liquid medium. Fig. 3 is an attempt to systematizise these methods.

Methods which basically push the sheet into a hollow stiff die by the action of the liquid are summarized as hydrostatic forming methods. These methods are suitable for the manufacture of flat parts. Deep hollow parts, having drawing ratios larger than 3, can be produced by the hydromechanic methods. The sheet is pushed using a stiff punch into the pressurized medium. The third class of methods are developed to form hollow parts, starting from welded or unwelded sheet pairs. The geometry which can be achieved by this methods is like to those of hydroformed tubes, but with the extension that larger differences of the diameter along the length axis are permitted

.

Figure 4 shows in the upper left corner one variant of the hydromechanic methods, which uses membranes to separate the liquid from the workpiece [11, 12]. It is called MULTIBRAN method, as it uses multiple membranes instead of a single membrane as known from the fluidform method. The advantage of using membranes is that sealing is significantly easier. There is no need for sealing the edge of the blank against the pressurized region. Therefore the closing force is easy to control and drawing of blanks having steps due to different thickness (tailored blanks) or even simultaneous drawing of multiple blanks is suitable.

This potential is demonstrated by deep drawing the cup and lid of a can as shown in Fig. 4. Both parts (cup and lid) are deep drawn, trimmed and calibrated in one step simultaneously. Despite the differences in sheet thickness of the two parts (see Fig. 4) it is a stable robust process, which is now repeated for some 100 times. Tool changes are made within some minutes. This shows the capability of the process with respect to flexible manufacturing.

In order to investigate the process stability, tests forming a wedge shaped part were run. Some series of steel parts schematically shown in Fig. 5 were drawn and the sources for inaccuracies were analyzed. The most significant source was the arbitrary shift of the asymmetric draw in of the blank, which can be characterized by the difference in the flange width after forming. As the part itself is asymmetric, the draw in is also. But the size of the asymmetry shows some significant scatter. Due to the scatter in length the radial stresses seem to vary, which in turn affects the spring back and the inner length dimension L of the part. The sources for the scatter in asymmetric draw in may be positioning errors of the raw part in the tool or changes of the draw in due to differences in friction at the beginning of the process.

4.2. Warm forming of aluminum sheets

There are many well known advantages of aluminum alloys concerning the application for structural parts. One of the disadvantages is the worse formability compared to steel. It was shown by a couple of researchers that the global or local heating of aluminum sheets can enhance the formability significantly. As the temperatures necessary are below 350°C it seams feasible to use heated liquid medium to do a warm forming. This was shown in [13] for welded blanks.

Significant enhancements in stretch forming can be seen from experiments documented in Fig. 6. An Al alloy sheet was rigidly clamped near the edge and stretch formed by hydraulic pressure. Due to the geometry and the friction effects a strain mode near plain strain develops at the longer side of the rectangular cup. Failure occurs early for room temperature experiments, while forming with heated medium (note that the tool was not directly heated) makes significant larger strains possible. From a comparison of the wall thickness distribution after forming, which can be seen in Fig. 7, the advantages become obvious.

The thickness distribution for the cold drawn sheet shows a nearly constant thickness of 1.3 mm in the center region of the part. The formability capacity of this region is not used for the forming process, failure occurs early.

The thickness distribution for the warm formed specimen was measured on a sample without failure. Therefore the wall thickness could be further reduced in the center, where it lies in the same order of magnitude of that of the (burst) room temperature sample in the edges. The thickness distribution can be understood from the mechanical and thermal history. Just before sealing by application of the closing force the sample is preheated for some seconds by heated liquid which flows from below the sample. As the tools are not heated actively, the sample cools rapidly down after contact especially with the cold upper tool under load (the lower tool is indirectly heated by the hot medium). The hottest area is in the center of the workpiece. Due to that, forming begins in this region. After the top gets in contact with the tool, the sheet cools down at this position and friction effects occur, which obstruct further thickness reduction. The forming zone is shifted to the neighborhood regions, but sheet thickness reduction becomes increasingly difficult as the temperature decreases due to the cooling effects from the flange and the top of the part. Finally, stretch forming of the remaining (nearly cold) edge regions sets in.

From these experiments it was concluded that warm forming using heated medium promises better formability of the parts. Additional active heating of the tools or passive heating in a production process with short cycle times would enhance the temperature field in the sample and therefore the overall thickness distribution.

Acknowledgements

The author gratefully acknowledges Dipl.-Ing. R. Breede and Dipl.-Ing. T. Prange for the preparation of the results.

References

[1] Hein, P.; Vollertsen, F.: Hydroforming of sheet metal pairs. J. of Mat. Proc. Tech. 87 (1999) 154 - 164

[2] Bruggemann, C.J.: Hydroumformung von Strukturteilen für Automobile. Hydroumformen von Rohren, Strangpreßprofilen und Blechen. Hrsg. K. Siegert, Matinfo Frankfurt 1 (1999) 421 - 439

[3] Giering, J.: Astra Vorderradträger - 1,5 Jahre Hydroforming-Erfahrung. Hydroumformung von Rohren, Strangpreßprofilen und Blechen. Hrsg. K. Siegert, Matinfo Frankfurt 1 (1999) 455 - 460

[4] Schaefer, A.W.: Internal high-pressure forming process and apparatus. US Patent 5,711,059 (1995)

[5] Geiger, M.; Vollertsen, F.: Verfahren zum Her­stellen von schalenförmigen Hohlstrukturen aus ge­doppelten Blechzuschnitten mittels Innenhoch­druck­umformen. Offenlegungsschrift 195 26 709.5 (1995)

[6] Schmoeckel, D.; Hielscher, C.; Prier, M.: Developments and perspectives of internal high-pressure forming of hollow sections. Advanced Technology of Plasticity, Proc. 6th ICTP. Ed.: M. Geiger, Springer Berlin 2 (1999) 1171 - 1182

[7] Geiger, M.; Müller, B.: New technologies in sheet metal forming for light-weight construction. Sheet Metal, eds. H.J.J. Kals, B. Shirvani, U.P. Singh, M. Geiger, University of Twente, Enschede I (1996) 3 - 14

[8] VDI: Innenhochdruck-Umformen: Grundlagen. VDI Richtlinie 3146, Blatt 1 (1999)

[9] Wells, G.L.; Dehlinger, J.R.; Rigsby, D.R.: Multi-stage dual wall hydroforming. US Patent 5,363,544 (1993)

[10] Vollertsen, F.: Umformen strukturierter Rohteile. Umformtechnik 2000 plus. Hsrg. M. Geiger, Meisenbach Bamberg (1999) 365 - 379

[11] Vollertsen, F.; Breede, R.; Lange, K.: A method for deep drawing with multiple elastomer mem­branes. Anals of the CIRP 48,1 (1999) 221 - 226

[12] Breede, R.; Vollertsen, F.: Friction in deep drawing using multiple elastomer membranes. Advanced Technology of Plasticity. Ed.: M. Geiger, Springer Berlin 3 (1999) 2163 - 2168

[13] Rösch, F.: Verfahren zum Umformen von flachen Werkstücken. Deutsches Patent DE 195 31 035 A1 (1995)

[1] This contribution was held at the ISMST 2000, June 4.-6. (2000) in Harbin/PR China

Published with permission of Prof. Dr. F. Vollertsen.

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Last update: Juli 02, 2000
Letzte Änderung: 02 Juli 2000

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