Sheet hydroforming in automotive applications

By Dr. Taylan Altan

April 15, 2002

Sheet hydroforming can be used in the automotive industry for forming alloys with low formability and prototype applications, among other handy applications.

Figure 1
Object

Editor's Note: This column was prepared by the staff of the Engineering Research Center for Net Shape Manufacturing (ERC/ NSM), The Ohio State University, Professor Taylan Altan, director.

Various types of sheet hydroforming operations, specifically rubber forming and rubber diaphragm forming, have been used for years in the aircraft industry. Sheet hydroforming is well-suited for prototyping and low-volume production, which are predominant in the aerospace industry.

Recently, however, sheet hydroforming is garnering special attention in the automotive industry for the following applications:

• Forming alloys with relatively low formability, such as the aluminum alloys and high-strength steels
• Low-volume (niche applications) and prototype production

Basics of Sheet Hydroforming

The principle of sheet hydroforming is illustrated in Figure 1. The blank holder is provided with a seal (location 6 in Figure 1). A container (location 5 in Figure 1) maintains the pressure medium, usually water-oil emulsion. A hydraulic servo valve controls the counterpressure (PG) during the process. After the blank is placed on the die, the blank holder presses the sheet and the punch forms the sheet against the medium, creating a pressure that is controlled throughout the punch stroke.

Figure 2
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Hydroforming can be used for stretch forming (the flange material is not allowed to flow into the die cavity) or for deep drawing (flange material is allowed to flow into the die cavity).

Other variations of the process include:

• Active hydroforming—a process in which the medium presses the blank against a die contour
• Viscous pressure forming—a viscous material, instead of water, is used as a pressure medium
• Flex forming—water pressure acts on an elastic polymer membrane that wraps around the sheet and the punchAdapting the Process

Considerable hydroforming R&D is being conducted in Japan and Germany. Amino and Toyota (www.toyota.com) have used sheet hydroforming technology since the 1980s.¹Various German universities, including Stuttgart, Erlangen, and Dortmund, and press manufacturers, including Schuler, Schnupp, and SPS, are building vertical and horizontal hydraulic presses that have short cycle times for production and prototyping applications. Figure 2 illustrates this type of press.²

Figure 3
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The long-stroke, small-diameter ram cylinder provides a quick up-and-down stroke. The mechanical slide locking system; the blank holder cylinders; and the short-stroke, high-pressure cylinders located below the press table provide the press closure force.

Several blank holder cylinders apply forces that vary in location during the press stroke. This control system is expected to facilitate the forming of difficult-to-form alloys, such as aluminum and magnesium, and high-strength steels.

The forming of double blanks also is being explored (see Figure 3).³In this case, two blanks are welded around the periphery, except at a single location for entry of the pressure medium. In this technique, a containerlike part, for example a fuel tank, can be formed in one press cycle.

The application of sheet hydroforming is expected to increase during the next decade. While the process has a disadvantage, namely the low cycle times, it offers many advantages for automotive applications. Some include:

• Obtaining greater draw depths with better strain distribution.
• Drawing complex shapes in one press cycle.
• Reducing die costs because one die is used.
• Obtaining excellent finish of the sheet surface exposed to the medium.

References:

1. T. Nakagawa, K. Nakamura, and H. Amino, "Deep Drawing of Large Sheet Metal Parts With Hydraulic Counter Pressure," in K. Siegert, ed., Latest Developments in Sheet Metal Forming (DGM-Informationsgesellschaft Oberursel, 1988).

2. Schnupp GmbH & Co. Hydraulik KG, D-94327 Bogen, Germany.

3. P. Hein and F. Vollersten, "Hydroforming of Sheet Metal Pairs" Journal of Materials Processing Technology, Vol. 87 (1999), pp.154-164.

4. T. Altan, private communication.