GM's learning curve on lightweighting

The plug-in hybrid version of the Cadillac CT6

What makes a new model jump from the drawing board to the showroom floor? Sometimes, it’s more than just the goals of generating higher sales or targeting a hot new segment.

The lightweight body of the new Cadillac CT6 luxury sedan is the most complex ever made by General Motors. It uses 11 materials and is held together by aluminum welding, rivets, screws and adhesives.

Question: Why has GM invested a fortune in developing a lightweight body and a new manufacturing system for a large sedan that probably few will buy?

Answer: Trucks, mostly. Bragging rights, perhaps a little, and credibility for Cadillac. GM’s luxury division needs a high-tech, rear-wheel-drive entry in its lineup, no matter how much the segment is shrinking. Acura has already proved that without one, it’s difficult to compete with BMW, Mercedes and Audi.

GM officials won’t say so, but I believe the primary reason the CT6 exists is to give the company a dry run at perfecting its manufacturing system for the next-generation Chevrolet Silverado and GMC Sierra pickups scheduled for late 2018 or early 2019.

A move such as this is not without precedent.

Ford’s big leap from steel to aluminum on the 2015 F-150 was more than a decade in the making and wasn’t really that big of an advance at all in terms of assembling the body panels.

When Ford owned Jaguar and Land Rover, it gave its riveting and bonding system a test run on lower-volume vehicles in England. The first aluminum-bodied Jaguar was produced in 2003. Last year, Ford began installing a nearly identical manufacturing system in its Dearborn, Mich., and Kansas City, Mo., truck plants for the F-150.

AutoPacific analyst Dave Sullivan also thinks the CT6 is more than just another entry in Cadillac’s lineup.

“I am very confident this is also a bit of a learning experience for them before they institute these new production techniques across other plants,” Sullivan says of the CT6.

The CT6 is built in pieces. Here is the rear module. Photo credit: RICHARD TRUETT

There’s been a lot of speculation in the media that GM will have to follow Ford and produce an all-aluminum truck. But if you pay attention to what GM is saying and doing, you know that is not likely to happen. GM doesn’t need to go to full aluminum to be weight competitive with the F-150. Using thinner-gauge, high-strength steel in select areas of the body reduces cost, and it can reduce weight, too, over thick-gauge aluminum.

Mark Reuss, GM’s global product development boss, has said the bodies of the next-generation trucks will be “mixed materials” -- the same language GM uses to describe the CT6. The new Cadillac is a rwd sedan that will compete with the BMW 7 series, Mercedes S class, Audi A8 and other large luxury sedans, whose sales have collapsed this year as the move to SUVs gathers momentum.

Reuss told me this week that the content and manufacturing system for the next-generation pickups will be similar, though not identical, to those of the CT6.

The mixed materials include cast aluminum structural components, such as the shock towers, sheet aluminum for the doors, fenders, hood and trunk, steel for the safety ring and parts of the floorpan, and magnesium. And in one place on the body, GM will weld steel to aluminum, an industry first.

“The CT6 is a very low-volume program,” Sullivan says. “In order to pay off the development for this mixed-material body structure, it will have to come to a higher-volume product. I have no doubt that we will see some of the ‘best practices,’ ‘lessons learned’ and other processes added to the pickups and SUVs.”

The way these parts are assembled is where the complexity comes in.

During a 90-minute tour of GM’s sprawling Detroit-Hamtramck plant last week, GM manufacturing officials showed me how the car is built. The assembly area is situated in an 180,000-square-foot area that has 205 robots that produce the CT6 body.

Audi, Ford and Jaguar Land Rover assemble aluminum-bodied vehicles with rivets, screws and adhesive. GM also uses these methods but adds some of its own patented manufacturing technologies to reduce costs and weight and add speed.

GM also welds aluminum to aluminum with a patented multiring dome process, which prevents the softer metal from warping. These welds are not only less expensive than self-piercing rivets, but they weight less. The CT6 body has 3,073 aluminum-to- aluminum spot welds.

Randy Kozicki, the CT6’s lead body-in-white manufacturing engineer, says GM has licensed its aluminum welding process to other automakers. GM’s welding system won’t replace rivets right away, but joining metal with welds is faster than riveting and looks to be the future.

Cadillac expects the CT6 to be not only the lightest vehicle in its class, but the lightest vehicle in the segment below it, Cadillac spokesman David Caldwell said. The car’s curb weight is expected to be around 3,700 pounds. An aluminum-bodied Audi A8, by comparison, weighs between 700 and 1,100 pounds more.

The CT6 is the first GM production car to have its entire outer skin made of aluminum. The engine compartment is aluminum, as is the trunk area. But GM uses advanced high-strength steel in many areas of the passenger compartment.

Several of the aluminum components are large cast items called extrusions that handle the vehicle’s heavy stresses and loads. They use some of the technology first pioneered on the frame of the current Corvette.

“We are leading the world in the ability to spot weld steel to steel, aluminum to aluminum and aluminum to steel,” Reuss told investors last month.

“This unrivaled capability and joining techniques allows us to use precisely the correct material in precisely the right location,” he added.

The CT6 is not that much different in size and shape than the current XTS, which like other large luxury cars, isn’t lighting up the U.S. sales scoreboard. U.S. deliveries of the XTS have slipped 11 percent this year.

But when the CT6 comes out, sales won’t be its major test. GM will have a few years to get its manufacturing system in order so that when the company’s next-generation, large trucks come out, the switchover from steel to mixed materials will go seamlessly.

And, GM hopes, in very high volume.

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