Though performance versions of production vehicles regularly feature exotic lightweight components, switching frame materials is a bit extreme. But that's what GM did with the 2006 Corvette Z06. In place of the steel frame on the conventional C6, GM is substituting an almost identical aluminum spaceframe, thereby reducing mass by over 30%. The frame is produced for GM by the Structural Solutions Group, Dana Corp. (www.dana.com; Toledo, OH). Although it might seem like a simple substitution, according to William Kroppe, director, product engineering, at Dana's Structural Solutions, transforming a spot-welded steel frame into an aluminum structure held together by MIG and laser welds and self-piercing rivets while precisely maintaining the same dimensions as the original led Dana to a couple of industry firsts. Kroppe points out that while many automakers have produced aluminum-bodied vehicles that use panels on the order of 1-mm thick, few have worked with the 3-mm+ thicknesses needed for an aluminum frame. He says Dana's Z06 frame is the first to use laser welding and self-piercing rivets on these thicker aluminum panels in a mass production environment.
Laser welding is used in the driveline tunnel area. To prevent road noise from intruding into the cabin, GM insisted that no holes be cut in the folded aluminum sheet that forms the tunnel, which ruled out MIG welding it to the frame since that would require access holes for the guns. Although laser welding is performed without the need for access holes, Kroppe admits that adjusting both the head speed and angle and re-focusing the laser's optics to deal with the thick panels was laborious. But he notes that now Dana has the knowledge base—and ability—to cost-effectively laser weld aluminum.
SWISS WATCH WORK.
Dana has carved out a small cellular assembly area for the Z06 in its manufacturing facility in Hopkinsville, KY, that is otherwise dedicated to producing serious volumes of stamped and spot-welded steel frames. (Annual volume for the Z06 is projected at 7,000 units.) "Compared to the other frames we make in that plant, this is Swiss watch work," says Kroppe. To prove the point, he describes how the aluminum castings that provide the attachment points for the suspension components are CNC machined with a precision that exceeds steel frame fabrication techniques. To achieve the highest degree of dimensional accuracy, Dana found it best to assemble the body first and then machine it using a Tricept robot from ABB (Auburn Hills, MI; www.abb.com).
MITSUBISHI GETS CENTERED
For its 2005 Lancer Evolution MR Edition, Mitsubishi added an aluminum roof panel to what was already an aluminum-intensive vehicle featuring fenders, hood and suspension components made from the lightweight material. Why the roof? According to Bryan Arnett, manager of product strategy, Mitsubishi Motors North America, designers were looking for the best way to lower the center of gravity of the car to enhance its handling, and lightening its uppermost part gave them the most bang for the buck. By switching to aluminum from steel, the MR Edition shed 4 kg (8.8 lb.); the center of gravity is reduced by 3mm. Had the designers stuck with steel they would have had to lower the entire roof by 50 mm to achieve the same effect. To prevent a galvanic reaction between the roof panel and the steel frame on which it rests, Mitsubishi uses an adhesive as a separation layer, in addition to self-piercing rivets. Thermal warping resulting from aluminum's thermal expansion coefficient (twice that of steel) is countermeasured with a longitudinal design bead that essentially absorbs the differences.