SpeedyV6

Technology
New Twists in Bodybuilding Put Curves in All the Right Places
By DON SHERMAN

November 21, 2005

THE styling features most likely to raise the pulse and lower the resistance of a car buyer - shapely fenders, a rakish roofline and curves the neighbors will envy - are easy for designers to sketch, but all too often, impossible for factories to produce.

Car and truck bodies are shaped by many factors, including the practical need to carry loads (a box is ideal) and the fuel-economy benefit of an aerodynamic form (which favors a rounded profile).

But another limit to a vehicle's shapeliness is the ability to manufacture its exterior panels with the contours and creases envisioned by designers. Metal will bend only so far before it tears or cracks; alternatives like fiberglass are more forgiving, but carry the stigma of being "plastic."

New technologies for bending sheets of steel and aluminum, and for lowering the cost of lightweight materials like carbon-fiber composites, are making it possible for automakers to produce vehicles more faithful to the lines originally drawn by the styling studio.

One example of how the new production methods are dressing up showroom models can be seen in the 2006 Pontiac Solstice, a stylish roadster that accurately reproduces the form of the design study that General Motors unveiled at the 2002 Detroit auto show. Building the car's complex body curves would not have been possible using conventional metal-forming methods.

The hood, trunk lid, doors and rear fenders of the Solstice - a total of six outer and four inner panels - are stamped by the Amino Corporation at a plant near St. Thomas, Ontario, using a process called sheet hydroforming. The technology is a variation of a method automakers have been using for a decade to shape the rectangular tubes used in vehicle frames.

Typically, body panels for mass-production vehicles are made by pressing a steel sheet, known as a blank, between two matched dies, the way a cook squeezes ground beef between cupped hands to mold a hamburger patty.

With hydroforming, only one die is used. The blank is clamped at its outer edge by a device called a binder. Beneath the blank is a large container of water.

To form the Solstice's panels, the die - called a punch because that's what it does - is pressed against the blank by a 3,400-ton press. As the blank is drawn through the grasp of the binder, the water below causes the metal to conform to the exact shape of the punch. The water pressure is kept at about 3,000 pounds per square inch by a relief valve that allows some water to escape as the punch and blank are lowered into the cavity. A computer controls the movement of the punch, the clamping force applied by the binder and the water pressure.

Hydroforming offers several advantages over traditional stamping methods. Because only one die is used, tooling costs are reduced by millions of dollars - 10 to 50 percent over all, General Motors says. Deep contours and complex details, which are difficult or impossible to achieve with other metal-forming processes, pose no problem. According Trent Maki, general manager of Amino North America, metal can be drawn 40 percent deeper with hydroforming than is possible using conventional presses.

The surface quality of the finished part is higher, too, because the die never contacts the outer surface of the panel; only water touches its display side. Because the water spreads the stamping pressure evenly across the entire panel, the panel is stiffer and more dent-resistant.

The main drawback of hydroforming is that each panel requires up to a minute to form; conventional stamping would require just a few seconds for each piece. That is not a concern for a car with an anticipated volume of 20,000 to 30,000 a year, but it does discourage its use on high-volume models.

As automakers work to improve fuel economy by reducing weight, they are using lighter materials, like aluminum and carbon-fiber composites, for bodies. The idea is not entirely new: the original Ford Model T had an aluminum hood.

Forming aluminum panels without tedious welding and hand-finishing is the challenge. One solution is a technique developed by Boeing called superplastic forming.

Any metal alloy capable of withstanding extreme shape changes without tearing or splitting is considered superplastic. Aerospace applications include titanium, steel and certain types of aluminum. To reach a superplastic state, aluminum is heated to 850-900 degrees Fahrenheit, enough to soften the metal but still hundreds of degrees less than its melting point. An inert gas like argon, compressed to several hundred pounds per square inch, then blows the aluminum over a die; the gas takes a role similar to the function of the punch in hydroforming.

The process takes up to 15 minutes to form each piece, so it is not suitable for high-volume production, but it does offer several desirable attributes for boutique models. As is the case in hydroforming, there is no contact between the die and the outer surface of the part, so perfect finishes can be achieved with minimal hand labor.

In contrast to the building of aluminum panels for Italian sports cars in the 1950's - which amounted to a quilt of hand-shaped pieces stitched together by welding - superplastic forming yields parts as large as a complete fender or hood. Models using the process include the Aston Martin Vanquish, Bentley Arnage, Ford GT, Morgan Aero 8 and Rolls-Royce Phantom.

Carbon-fiber body panels and chassis offer the potential for even greater weight savings. These molded composite plastics, which are built up from layers of high-strength fibers bonded with special resins, were developed for aircraft. In 1981, Lotus and McLaren began using composites to build lighter, stiffer Formula One racecars. The technology has since trickled down to a few road cars; today, the Bugatti Veyron 16.4, Mercedes-Benz SLR McLaren and Porsche Carrera GT are all built around chassis that use carbon fiber in place of steel or aluminum frames.

Carbon fiber is an attractive material for exotic applications because it is stronger than steel yet weighs a fourth as much. Unfortunately, it costs 100 times more and requires special manufacturing procedures, two factors that have so far restricted it to exotic sports cars costing upwards of a half-million dollars. But next year, the BMW M6 sports coupe will go on sale, at an expected price just over $100,000, with a roof of carbon fiber rather than steel.

BMW spent five years perfecting its own production method. The M6 roof panel is a sandwich - woven carbon fiber on the outside, randomly oriented mats of the same material inside - all held together with an epoxy resin. Six layers of carbon fiber (two woven, four mat) are placed into a die-molding machine, the dies are closed, resin is injected and heat is applied.

In less than a half hour, the roof panel is cured and ready for finishing. Although the carbon-fiber roof is twice as thick as a steel panel, it saves 12 pounds, said Stefan Behr, a BMW manager. The performance benefits are twofold: the power-to-weight ratio rises and the center of gravity is lowered.

To prepare the surface of the composite roof for its glossy finish, BMW uses the same light sanding procedures applied to other plastic exterior parts. While other makers hide their molded composite handiwork under paint, BMW proudly displays the woven carbon fiber material, protected by a clear coating, like a diamond necklace in a display case. The M6's special roof is not only a functional attribute, it could be next spring's hottest automotive fashion.
http://www.nytimes.com/2005/11/21/au...pagewanted=all