Manufacturing Process and Design Cars Lite
ALCOA engineers used supercomputers to design parts for the Plymouth Prowler

Aluminum Means Light and Quick

An engineering tour de force, a rare blend of "youthful exuberance, mature judgment and technical excellence" — that's what Chrysler Corp. is saying about the Plymouth Prowler. This update of a 1950s hot rod is a beauty to look at, and for the discerning its beauty is more than skin deep. The Prowler is the first U.S. car engineered from the ground up to exploit aluminum technology. At 2,800 pounds, 50% less than it would weigh with traditional steel design, the Prowler is zippety-quick, fuel efficient and impervious to rust — with no loss in crashworthiness and durability.

Aluminum space-frame for the Prowler

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As the world's largest aluminum company, Alcoa knows what aluminum can do, and it helped develop the Prowler as well as the Audi A8, a German-produced aluminum car making waves in the upscale market. Both cars have an aluminum skeleton, a "space frame," similar to those used in aircraft, with attached aluminum body panels for their sleek skin. Alcoa automotive engineers like Edmund Chu see these cars as harbingers of the future in automotive design and engineering.

"The auto industry tends to look at aluminum's cost per pound," says Chu, "which is substantially higher than steel, and they have years of experience with steel. We encourage them to look at the dollar per pound saved. Aluminum sheet weighs half as much as steel. Cost per pound isn't an appropriate measure of the economics. We emphasize overall cost, and we're working aggressively to make aluminum easier to use." The key, notes Chu, is computational modeling. Alcoa has sophisticated ability to do computer simulations that predict how aluminum structures and body sheets will perform, reducing costly prototyping and trial-and-error processes.

Alcoa has partnered with Pittsburgh Supercomputing Center since 1987, and Alcoa engineers used PSC resources in designing parts for the Prowler and Audi A8. "Automotive products are a key part of Alcoa's future," says Peter Bridenbaugh, Alcoa executive vice-president of automotive structures. "Scientific modeling on the supercomputer helps us design these parts and the manufacturing processes that make them. It allows us to solve time-critical problems in a competitive manner, which is particularly important in automotive design."

Tool & Die: The Inner Hood

A large part of the development cost for body sheets, such as hood and door panels, is designing the "dies" used for stamping sheet-metal parts in mass production. Chu leads Alcoa's effort in this area. Traditionally, sheet-metal forming relies on the ingenuity of tool-and-die craftsman, who have over many years built up sophisticated artisan's know-how for working with steel sheets.

This approach has its limits, however, with new materials like aluminum and with the complex geometries of modern automotive design, where it frequently involves many trial-and-error iterations. Advances in computing make it possible to use mathematical tools to predict the effectiveness of a design before casting the die and trying it out, potentially saving hundreds of thousands of dollars and weeks of time. And Alcoa's modeling ability with aluminum, says Chu, is more advanced than similar techniques with steel.

Hood Panel
This aluminum inner-hood panel shows the multi-cone design developed by Alcoa engineers.

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As an example, Chu points to recent work his engineering group did for a major car company on the underbody of a hood, a part known as an "inner-hood panel." Traditionally, these panels employ steel with a "beam" design, the beams giving strength and rigidity. Cut-outs in the flat part of the sheet reduce weight, but require an extra die and press step, adding production cost. To exploit the unique properties of aluminum, Alcoa developed a "multi-cone" design.

"To integrate product design with material design," says Chu, "you don't want to force aluminum to behave like steel. You want to build aluminum characteristics into your design." With the inner-hood panel, this meant using a lighter gauge than is possible with steel, leading to the multi-cone design, which gives structural integrity and rigidity equivalent to a steel beam panel at half the weight, and without cutouts, avoiding potentially millions of dollars in manufacturing cost.

Hood Panel Visual
Thickness Distribution of Inner-Hood Panel
By predicting high-stress areas prior to casting the sheet-metal die, simulations substantially reduce design costs.

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Initial modeling of this panel predicted several locations of high strain. As a validation check on the modeling, a die was cast and a Detroit "stamping house" stamped the panel. Analysis confirmed the predicted high-strain regions where problems occurred in the stamped part. To adjust, Alcoa modified the geometry of the cones and shifted to a more formable alloy — much easier to do, notes Chu, with computer simulation than the traditional trial-and-error process.

"By integrating material design with process design through computational modeling, we can select the optimum alloy to maximize formability of the part. When we run the model, we can do multiple iterations, trying a number of different alloys. "This is possible for two reasons, both of which depend on advanced scientific computing.

Supercomputing and Aluminum Product Design

Alcoa has developed a highly accurate "constitutive model" for aluminum — a mathematical description that relates the microstructure of the metal to how it behaves when formed into a manufactured product. Alcoa's partnership with PSC has allowed it to refine this model to its current high degree of accuracy. "This is one of our strengths," says Chu. "We have the ability with simulations to describe all complex loading conditions."

Supercomputing, furthermore, because it gives fast turnaround, makes it feasible to adjust parameters and look at multiple possibilities, providing design flexiblity that wouldn't otherwise be available. "With the Cray," says Chu, "we can look at five or six scenarios and all at once. The turnaround is six times faster than on our own workstations, and this is critical in the design stage, where you need to make changes quickly."

In the foreseeable future of design with aluminum, says Chu, engineers will create new alloys to meet product requirements. "For a particular product, I want to find out what governs the deformation. Is it strain hardening or elongation or something else? With the model, I can put in a design parameter and different material behavior, fictitiously — make up some new alloy, and then we could turn around and, based on the model predictions, say 'Hey, we can develop this alloy.'"

Researchers: Edmund Chu, Alcoa.
Hardware: CRAY C90
Software: User-developed Code.
Keywords: Alcoa, aluminum, cars, Plymouth Prowler, space frame, Edmund Chu, Audi A8, automotive design, tool-and-die, multi-cone, alloy, constitutive model, automotive engineering, sheet metal forming, metallurgy, materials.
Related Material on the Web:
Welcome to Plymouth — Prowler .
Projects in Scientific Computing , PSC's annual research report.

References, Acknowledgements & Credits
© Pittsburgh Supercomputing Center (PSC)
Revised: July 9, 1998