Tubular hydroforming: Riding the wave

Manufacturing Engineering, Oct 1999 by Hogarth, Sharon

A new twist on an old metalforming technology

Tubular Hydroforming: Riding the Wave

Many companies are reconsidering tubular hydroforming. Ed Opbroek, program director for the Ultralight Steel Automotive Body, Advanced Vehicle Concept project (ULSAB-AVC) (Middletown, OH), cites two major factors: mass and cost. Hydroforming "certainly does help to take weight-or mass-out of the vehicle," he says, "and lets you consolidate functions, and therefore take cost out." Compared to a stamping, welding, and assembly process, hydroforming offers advantages like part integration, lower tooling costs, reduced die wear, integration of piercing/punching operations, elimination of pinch weld flanges, and improved dimensional repeatability, as well as reduced mass.

Hydroformed structural components offer automakers additional advantages like improved rigidity-stiffness provides a better ride and improves handling-- along with greater accuracy and design flexibility.

General Motors Corp. made quite a splash with its completely redesigned frame for the 1999 Chevy Silverado and GMC Sierra trucks. Modular design allows the front, mid; and rear sections to be manufactured independently, with different technologies used for each section. The front-frame rails and cross members are hydroformed, the midsection is rollformed and drawbent, and the rear section is stamped. Formet Industries (St. Thomas, Ontario, Canada), part of Cosma Body and Chassis Systems Group, a division of Magna International Inc., produces the frames.

Magna pulled off what some consider a coup with the Chevy Silverado and GMC Sierra hydroformed front rails, but Magna and GM are not the only ones to see the feasibility of tubular hydroforming for high-volume applications. "In the late '?Os companies started looking at it for automotive applications," says Harry Singh, general manager, HydroDynamic Technologies Inc. (HDT) (Auburn Hills, MI). "In North America, General Motors did a lot of the early work. At the same time, a number of companies in Germany started doing work on hydroforming. The first high-volume hydroformed automotive part was made in 1990-an instrument panel beam on a Chrysler Minivan made by VariForm [Warren, MI]." The next highvolume part-an engine cradle used on the Ford Contour and Mercury Mystique-came into production in 1994, he says. "From that point onward, interest in hydroforming accelerated."

Terry Nardone, an engineer and manager at Vari-Form, says that at first the company attempted to keep the process under wraps, but soon realized it was futile. "You can't keep it secret and convince people that the process is good. We actually started out with a very high-volume application, cranking out a part in less than 20 seconds. Now we're getting down to 15 seconds. That part really established us as a viable production source and hydroforming as a viable process as well."

Vari-Form calls its process Pressure Sequence Hydroforming (PSR). It now produces over four million hydroformed parts each year. "The Vari-Form PSH process evolved from bulge forming," says Nardone. "Although we started with it as a base, our PSH process moves well beyond bulge forming and allows us to use different types of materials. It's also more economical for structural automotive applications with large-diameter tubes. Bulge forming is good for smalldiameter, short-length applications."

"The forming of steel and other metals with water pressure, or with fluid pressure, is not new," says Opbroek. "Hydraulic fittings and light reflectors have been hydroformed for many years. The key issues have been cycle times and high-volume automotive applications. That's where the real breakthrough has come. There's been a lot of progress toward making hydroforming `fast and affordable,' in the last few years." Steve Lotspaih, a project leader for hydroforming technology at Tower Automotive Technical Center (Milwaukee), concurs. "The plumb ing industry has hydroformed pipefittings out of copper and other materials for decades, and hydroforming has been used in vehicle exhaust systems for a fair number of years. Harley-Davidson uses hydroforming to create some of its exhaust components."

While Magna products range from traditional stampings and assemblies to roll forms, extrusions, and hydroformed components and assemblies, the company specializes in manufacturing structural parts for cars and trucks. Traditionally, parts like engine cradles, suspension components, and frame rails have been stamped and welded. In a quest for lighter, stronger and cheaper parts, Magna began looking for new manufacturing methods in the early '90s. The search led to hydroforming, which uses the force of water or hydraulic fluids to shape tubing or sheetmetal. Tubing is typically used when complex shapes are needed.

Magna acquired a license from GSA of Germany to use a low-pressure hydroforming process in North America, and within two years bought GSA. Drawing on its own knowledge and working with a German hydroforming press maker, Magna developed a patented, highpressure hydroforming process with cycle times as much as 75% faster than traditional methods. A steel tube is cut to length and pre-bent to a desired shape on a CNC bender. Bending allows the tube to fit properly in the die cavity. It's placed in the die, the press closes-holding the tube in position-and pressurized water is forced in the ends. The tube expands outward and takes the shape of the die cavity. Holes are hydro-pierced during the expansion process, eliminating the need for a secondary operation.