One-piece composite components are replacing multiple parts previously made both from composites and from metals or other materials. The motivations for the design and the challenges for manufacturing can differ as much as the end products do. But a common incentive for part consolidation is cost - composites frequently earn their way onto a project not just for their properties, but because reduction of part count makes it possible to mold and assemble what would otherwise be a much more expensive multi-part structure at or near the cost expected for parts made with competing materials. Yet a great deal of design and engineering innovation may be required before those benefits can be reaped.
Accommodating sleek contours
That was certainly the case when Ford Motor Co. sought a way to package the rear-mounted, 500-hp engine in its new GT sports car. Ford needed a tight, aerodynamic "deck lid" engine enclosure that would meet safety, weight and cost goals without compromising aesthetics. With that in mind, engineers looked for ways to reduce the cost and weight of the approximately 6-ft square deck lid assembly, which consists of a four-part exterior paintable aluminum skin (including left- and right-side quarter panels, a header, and an engine vent cover), with an inner structure to stiffen the assembly.
For the team at Ford's Research and Advanced Engineering group, a one-piece composite inner structure was the ultimate solution, despite some technological challenges - challenges which Ford and its composites fabricating partner, Sparta Composites (San Diego, Calif., U.S.A.) were able to resolve.
"With composites, we were able to do a more aggressive design," notes Eric Kleven, Ford's GT composites specialist. "We had to make far fewer manufacturing compromises" than would have been required for a multi-piece approach, he says. The one-piece inner assembly stiffens the entire deck lid by eliminating flex points where bonding and rivets would join multiple pieces.
A traditional stamped aluminum approach wasn't feasible: the part presented such complex curvatures, with reverse angles as well as angles of less than 90°, that stamping them would result in die lock. Manufacturing an aluminum inner structure would involve multiple parts that would require secondary assembly, as well - an option both cost- and weight-prohibitive. The remaining choice of materials, therefore, was between composites or superplastic-formed aluminum. (The superplastic forming process heats aluminum sheet stock to about 500°C/930°F, the point at which it can be plastically formed over a single-faced tool to create a part's shape.) Though lighter than stamped aluminum, superplastic-formed aluminum would cost more and still require four or more individual parts to make up the inner structure. However, the least expensive materials for a composite inner structure would cost nearly three times as much as the premium superplastic-formed aluminum product. A composite solution could go forward only if tooling and assembly costs could be reduced to make up the difference, says Kleven.
Ford briefly considered using composites only for the complex outer portions of the deck's inner structure assembly, which support the quarter panels. However, engineers quickly calculated that, in this case, the substantial difference in coefficient of thermal expansion (CTE) between the inner's aluminum and carbon/epoxy components would create unacceptable stresses at the joints.
Cost concerns notwithstanding, the Ford team requested a quote from Sparta Composites for a four-piece, all-composite design for the inner structure. However, Ford GT manufacturing manager Matthew Zaluzec reports, "Dimensionally, we could not effectively control the geometry of multiple pieces." With a producible manufacturing tolerance as variable as ±0.7 mm per piece, the overall dimensions of a multi-piece deck lid inner had the potential to vary from specification by as much as twice that amount. It was at this point that Kleven joined the GT team and suggested a one-piece composite component. "It seemed to me," Kleven recalls, "that instead of having to pay for four tools, this could be made in one piece, which would help us to meet our weight and cost targets, too."