May 8, 2007
Sheet hydroforming has fewer restrictions when forming complicated parts, which gives styling designers and manufacturing engineersmore flexibility during the design process. To provide a stylish body shape for the Pontiac Solstice®, GM chose sheet hydroforming to manufacture its hood, door, deck lid, and body side assemblies.
North American automakers are producing more specialty vehicles in limited quantities for niche markets. The vehicles are stylish and sporty and lure customers into showrooms.
The most economical way to put a variety of specialty cars on the road is with a common platform or chassis with different bodies and styling. North American OEMs are introducing single platforms for two or three different low-volume models. For example, a common GM platform frame is used in the Pontiac Solstice®. To provide a stylish body shape for the Solstice, GM chose sheet hydroforming to manufacture its steel hood, door, deck lid, and body side assemblies (see Figure 1).
Sheet hydroforming is a forming process that uses pressurized fluid, usually water, to form sheet metal to a die. A counterpressure deep-draw approach with a reverse toggle draw orientation, called the fluid forming process (FFP), is used to form the Solstice panels.
A punch with the shape of the part draws sheet metal into a pressure vessel. As the punch and sheet enter the sealed chamber, water displacement naturally builds a counterpressure. Outflow through a relief valve controls water pressure throughout the stroke.
The normal forming limitation of conventional deep drawing is localized thinning at the punch shoulder radius. In Figure 2a, the overhang area off of the punch shoulder stretches and breaks. However, in Figure 2b, water pressure pushes the sheet firmly to the walls of the punch during the forming process. The punch radius is fully supported, and an increase in friction along the wall ensures uniform thinning along the length of the wall instead of one local area.
Sheet hydroforming is slower than conventional stamping, because it takes time to control water pressure and refill the die with water. Because of this, sheet hydroforming is more suitable for small-lot production—from 5,000 to 40,000 vehicles per year.
Sheet hydroforming has fewer restrictions when forming complicated parts, which allows styling designers and manufacturing engineers more flexibility during the design process.
FFP can form parts in one hit that would normally take two to three operations in a stamping press. This reduces overall tooling costs because redraw and restrike operations are not needed, and part quality may improve because multiple after-forming operations can introduce defects.
The Solstice body side is a complicated deep-drawn component formed in three complete FFP operations compared to five in a stamping press. As water pressure pushes material firmly to the punch, strain is evenly distributed, reducing springback. As a result, inner body components have excellent dimensional stability. Additionally, tooling recuts to compensate for springback normally are not required.
Because of its complex shape, the Solstice's quarter panel cannot be produced conventionally, and even with sheet hydroforming, its many bends and stepped sill are challenging. However, with careful material flow control during the draw, the quarter panel's design is possible.
The door inner requires a large draw depth, perpendicular-to-draw piercings, and a tailored blank. The part is formed using FFP and servo link presses in three operations. While the hood inner can be stamped, sheet hydroforming allows the part to be thinner-gauge, with crisper radii and greater overall rigidity.
FFP can manufacture Class A body panels with good surface quality because water acts as the female die side, protecting material from tool marks, slip lines, and other surface markings. It's difficult to obtain good surface quality around pockets and other depressed features, but FFP water pressure acts to restrain the metal around the pocket as the depression is formed, reducing surface distortion and increasing panel accuracy. When deep-drawn outer shells are produced, slip lines tend to develop at major feature lines as material flows over these features in the draw direction. In FFP water pressure protects the sheet and can hold feature lines from washing out and creating a double line on the part.
An FFP die set consists of an upper binder ring and a lower binder ring that provide the blank holding force. A male punch contains the shape of the part, and the water chamber provides hydraulic counterpressure. The process begins with the slide descending to binder contact. The chamber is sealed, and blank-holding force is applied. The slide descends and the punch begins to draw material into the chamber. A water pressure relief valve activates, preventing water outflow and chamber pressure buildup.
The sheet is pushed against the punch as shown in Figure 3a. The slide continues drawing material to its bottom point, as shown in Figure 3b. Water pressure and blank-holding force may change during the cycle. The slide returns with the blank holder and punch. The part is then retrieved from the upper binder ring (see Figure 3c) and transferred to another press to trim and pierce and then to another press for additional trimming or flanging
As the automotive industry strives to reduce vehicle body weight, more difficult-to-stamp materials such as aluminium alloys and high-strength steel will be used for body panels. Because surface accuracy and quality are paramount, sheet hydroforming is being used more for low-volume, niche production. Currently GM uses sheet hydroformed body panels on the Pontiac Grand Prix® GXP and Saturn SKY®.
In the future more OEMs will use sheet hydroforming for specialty parts and vehicles.
Trent Maki is general manager and Cam Walter is project engineer at Amino North America Corporation (ANAC), 15 Highbury Ave., St. Thomas, ON Canada N5P 4M1, 519-637-2156, www.aminoac.ca.
Masaaki Amino, "Sheet Hydroforming of Automotive Body Panel Production," in proceedings from the 4th Annual North American Hydroforming Conference, London, Ontario, September 2006.
H. Amino, K. Makita, T. Maki, "Sheet Fluid Forming and Sheet Dieless NC Forming, New Developments In Sheet Metal Forming," in proceedings from IFU, Stuttgart, Germany, May 2000, p. 39.
J. Ocklund, N. Asnafi, H. Amino, T. Maki, "Hydromechanical Forming of Trunk Lid Outer," in proceedings from IDDRG Deep Draw Conference, Nagoya, Japan, May 2002.
T. Maki, "Fluid Forming and Dieless NC Forming," in proceedings from ATA's New Manufacturing Systems for Vehicle Production, Turin, Italy, November 2000.
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