Cadillac pursues aluminum/steel mix for new CT6 luxury sedan

  • 01-Apr-2015 03:43 EDT
Cadillac-CT6-Structure.jpg

The new Cadillac CT6 body is about two-thirds high-strength aluminum and one-third steel and high-strength steel.

Next fall, Cadillac’s new top-of-the-line model, the CT6 sedan, rolls out of General Motors’ big Detroit-Hamtramck assembly plant and onto dealership sales floors nationwide to confront the likes of the Mercedes S-Class, Audi A8, and BMW 7 Series in the premium luxury large-car market.

To equip its flagship sedan for the challenge, Cadillac designers and engineers have built the CT6 on a unique vehicle architecture, an aluminum-intensive body structure that is fully one-third steel. Twenty-one patents are pending for the new full-size, rear-wheel-drive, transverse-engine configuration, a variation of GM’s Omega II platform.

“The mix of materials helps save almost 200 lb compared to all-steel construction, and it’s lighter than an aluminum design would be as well,” said Travis Hester, Executive Chief Engineer. “We got the curb weight under 3700 lb, which is approximately the same as a CTS, but what’s remarkable is that the CT6 is 8 in longer than the CTS,” the Australian engineer stated with Down Under brio. “So the CT6 has about the same mass as a vehicle that’s a full weight-class down in size.”

Cast alloy backbone

Shaving that weight helps maximize fuel economy, but it also enhances the sedan’s driving dynamics: “We’re lighter, quicker, yet we have great torsional rigidity to ensure world-class ride and handling performance, which was central to our goals. The structure delivers the chassis foundation we needed,” Hester said.

Thirteen complex high-pressure aluminum die-cast components comprise much of the lower structure of the CT6 body, he explained. “These linked node joints—the backbone—are huge castings, some of the biggest that our supplier is aware of. The large one in the tunnel area, for instance, gets us great powertrain response.”

The Cadillac engineers employed hot-stamped, high-strength steel (HSS) strategically to reinforce the body structure and in conjunction with high-strength aluminum to create a tight, resilient safety-cage cabin structure whose underbody is lined with steel close-out panels to isolate it from noise and vibration. Aluminum sheet and extrusion components make up the engine compartment, trunk, roof, and rocker panels. Engineers added a high-strength aluminum impact bar to the rear of the vehicle, and a combination of high-strength alloy and HSS parts in the front and side impact zones to boost safety.

Mixed materials allowed

“Four and a half years ago, when we began planning for a larger Cadillac, we did multiple investigations into materials selection,” Hester recalled. At the time, the standard approach taken by much of the luxury car competition was to use aluminum alloy as the predominant structural material. “And we started going down that road for a little bit, but then we got away from it for a couple of reasons.”

“Number one, our manufacturing guys had managed to come up with a series of technological enablers that allows us to construct joints between pretty much any material with any other material,” the chief engineer said, stressing the significant amount of work GM’s manufacturing research team had done to ready these new methods for the production line. “They did a lot of complicated R&D work that took a while.”

As a result, “alloy to steel, casting to extrusion..., the flexibility in joining and fastening opened up combinations which we normally could not have put together,” he continued. The free mixed-material design approach “let us use the best material in the best way in each location, integrating aluminum and steel where it makes sense.”

The body engineers used a multi-disciplinary design optimization model, a CAE-based parametric simulation that finds the “best” way to meet weight and performance objectives. The engineers specified property ranges and potential materials: “Alloy grades, x to y, steel grades, x to y, and ran the simulation multiple times,” he said. “The optimization model pretty much let every material select itself for every part. We continued to do that and it allowed us to really optimize every component on the car for its job.”

One example is the lower structure of the cabin, which is constructed of steel close-out panels. The computer analysis highlighted that steel alone would be lighter than aluminum with the added weight of extensive sound-deadening material that would be needed to compensate in the aluminum design. “The most common comment we get is how quiet the car is,” Hester noted.

“In another case, the design allowed us to combine what was over thirty parts into one part,” he said. “In fact, the entire body structure has only 412 parts; that’s 20% less than normal steel.” Fewer parts takes out a lot of manufacturing variability and provides a remarkable improvement in precision and production efficiency.

After the mass optimization process is "when things started to get pretty exciting,” he said. The large lightweight body made the car “be extremely wide and allowed us to add features that the competition does not offer such as the huge wheels—18-in to 20-in wheels.” The twin-turbo V6-powered CT6, which debuts at the New York International Auto Show, has a stretched wheelbase and long rear doors and hood.

Advanced assembly technology

To build the new body design, company management installed at the Detroit-Hamtramck plant a 138,000-ft² (12,820-m²) body shop fully automated with some $384-million worth of high-tech joining and fastening tools and equipment, including 203 new assembly robots.

“For the Cadillac CT6 we have developed additional new body-construction techniques and technologies allowing various types of advanced and lightweight materials to be combined within the manufacturing environment like never before,” said Cadillac President Johan de Nysschen when he announced the plant upgrade at the Washington Auto Show.

Several material-joining and fastening techniques are used to assemble the CT6, Hester said. “It starts off with 3000 alloy-to-alloy aluminum spot welds and includes conventional steel spot welds as well.”

When joining alloy to steel, for example, and the design allows two-side access to the assembly equipment, he said, self-piercing rivets are typically used, especially when a clean appearance is needed. These sharp-tipped fasteners are applied at high pressure, then the end is folded over with a tool to form a mechanical interlock.

When equipment access is limited to one side and the joint is to be between differing part forms or materials, they typically use flow drill screws in conjunction with adhesive. Flow drill screws are fasteners that “self-extrude” as they pierce the metal being joined as the screw is rotated at high-speed. “The alloy castings have a big draw in many areas,” he said, “so in that case we use flow drill screws.” They are also used when attaching HSS components.

The new sedan has about 180 m (590 ft) of structural adhesive in it. Aluminum arc welding and aluminum laser welding, which creates a seamless bond between exterior panels, are also employed in the body assembly process.

On the CT6 production line, 28 robots descend on the vehicle body in two separate framing stations to weld both the inner and outer vehicle frames, simultaneously joining the body-in-white together from all angles. As the choreographed robot arms move in, out, and around the vehicle in concert, they resemble a symphony orchestra.

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