Ferrari prefers aluminum over carbon fiber

  • 10-Nov-2011 09:49 EST
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The body-in-white is aptly named when the car is an aluminum Ferrari 458 Italia, seen here receiving the finishing touches to its metal work.

While carbon-fiber-reinforced plastic (CFRP) technology is understood to be the ideal combination of strength and weight, the difficulty of using it in automated production rather than for handbuilt racecars and extremely low-volume, high-price road vehicles creates an opportunity for aluminum.

That’s Ferrari’s conclusion, as the company builds all its current production models—the 458 Italia, 458 Spider, 599 GTB, California, and the FF—from aluminum. It is not for lack of acquaintance with carbon fiber that the company reached this conclusion. Not only do its Formula One racecars employ the material but so did its F50 and Enzo extreme performance models.

McLaren Automotive and BMW sell, or will sell, production carbon-fiber models built in Ferrari-like volumes. But the Maranello-based company insists that it can do better with aluminum at volumes of about 30 cars per day, because that production speed precludes the use of the time- and labor-intensive pre-impregnated carbon-fiber cloth in favor of a process that injects resin into the cloth after it is placed into a mold.

Because this causes excess material to be used, this high-volume carbon-fiber construction technique fails to realize the material’s weight-saving potential. “The potential that composites offer is still not well exploitable unless labor-intensive, low-rate technologies are used,” explained Ferrari Technical Director Franco Cimatti.

“We think the technology used to produce these cars in these volumes is not exploiting the material’s potential because of the high resin content, excess thickness, and fibers not used to their best advantage,” he said.

Aluminum, on the other hand, is better suited to higher production volumes despite Ferrari’s absolute optimization of its use of the material. That entails the use of numerous alloys tailored to the specific application within the car; the use of heat-treating as needed for added strength; and the use of sheet, cast, and extruded aluminum joined by welding, screws, rivets, and epoxy bonding.

For the 458, Ferrari employs five different alloys in the car’s extrusions, three different alloys for its castings, and three different sheet metal alloys. High-strength alloys of up to 320-MPa yield strength permit extrusions with a wall thickness as thin as 1.6 mm (0.063 in), while superplastic forming of high-strength sheet aluminum has whittled the thinnest body panels from 1.5 mm (0.059 in) in thickness on the F430 to 0.9 mm (0.035 in) on the 458, with 0.8 mm (0.031 in) as a near-term target.

One example of the application of technology is the torque box, the diagonal reinforcement across the front of the footwell, behind the front wheels. That is now a single part of heat-formed 6082 aluminum alloy using a wall thickness that varies between 3 and 6 mm (0.118 and 0.236 in). The finished product weighs 1.75 kg (3.86 lb), which is 25% lighter than the torque box on the F430.

Heat treatment is also important for the correct deformability of the front and rear crash boxes. In the past, these have been made of steel because of that material’s good energy absorption capacity. But aluminum is not only lighter; avoiding putting steel at the extreme ends of the car is better for its polar moment of inertia, contributing to better handling. Fortunately, heat-treating aluminum improves its deformability properties for crash absorption, Cimatti noted.

The engine cover is stamped using a superplastic forming process that permits deep, sharp-edged contours that project engineer Patrizio Moruzzi says wouldn’t be possible with traditional cold stamping. Superplastic forming is possible because 5000-series aluminum alloys have the characteristic to elongate several hundred percent when subjected to controlled strain while heated between 450 and 500°C (842 and 932°F).

The inner door frame on the 458 is a single piece of die-cast aluminum, replacing an assembly of six pieces of stamped sheets and extrusions for the F430’s inner door frame. This change produces a stiffer, stronger part, speeds and simplifies production, and reduces tolerances that lead to production variance. Additionally, the casting has machined surfaces where components attach, aiding assembly of the complete door because of these improved attachment points.

Just as a variety of materials contribute to Ferrari’s optimization of aluminum, so do a variety of fastening techniques. The 458 is constructed using 70 m (230 ft) of welds and 8 m (26 ft) of bonding adhesive, according to Moruzzi.

Ferrari employs cold metal transfer welding, a lower-temperature form of MIG (metal inert gas) welding that causes less heat distortion of the welded materials than conventional MIG welding, he said. In the recent past, all the welding was done by hand, but now it is 40% automatic, performed by robots.

Friction stir welding is also low-temperature, but there are physical challenges to using that technique, Moruzzi said. Friction welding is better for simple shapes and flat surfaces, because access is challenging. It also works with a more limited range of alloys, so the company instead uses its cold metal transfer technique.

In the future, the company anticipates that it will do more bonding than welding, especially as it increases its use of alloys that are unsuitable for welding, Moruzzi said.

Ferrari uses both tape and liquid epoxy bonding material, reported Carlo Muller, head of 3M’s Global Strategic Development group responsible for Fiat. Currently, many joints use both bonding and a mechanical fastener such as a rivet or screw. That is because while the bonding provides immense strength, the joint is susceptible to peeling forces that can pull it apart, Muller explained. Nevertheless, the potential for an aluminum car joined entirely by epoxy is possible. “We are going in that direction,” he said. “The aerospace industry is already there.”

Epoxy joining will be more critical as Ferrari moves toward its next aluminum material, metal matrix composite. This is an aluminum mesh soaked in epoxy, which yields stiffness that is comparable to steel, Moruzzi said. Within the next five years, Ferrari foresees the ability to trim 15-20% more weight from its aluminum cars, in part through use of aluminum metal matrix composites, he said.

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