Mixed-Media Art: A Look at the 2016 Cadillac CT6 Sedan’s Structure
How Caddy joins steel with aluminum.
Steel saves weight. When Cadillac turned its body-structure computers loose and let them optimize each part for its mission, the CT6's unibody ended up 62 percent aluminum, 38 percent steel, and 220 pounds lighter than the smaller CTS. Here's how GM pulls off the sedan's mixed-material body.
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The front-door hinge pillars are two of 13 high-pressure aluminum die castings. Each pillar replaces 35 steel stampings, greatly simplifying assembly and dramatically reducing mass while improving strength and dimensional accuracy. The total part count is down 20 percent. Between these pillars is a steel firewall because a 1-mm-thick aluminum panel would have transmitted more noise, requiring 40-50 mm of acoustic damping material, adding considerable cost and weight. Steel is also used on the floor, the rear bulkhead, and the B-pillars.
Another GM breakthrough is its patented spot-welding technique for joining aluminum panels. A typical blunt, 8-mm spot-welding tip tends to burn through sheet aluminum, so GM devised a new tip with concentric rings that better distributes the current, fusing the panels without burning through them. Spot welds weigh less, they're faster and easier to execute than the more common rivets, and they permit flanges to shrink by 4 mm, enlarging door openings by that amount all around. The CT6 employs 3,073 such spot welds along with laser and arc welding. Where greater strength is required, the CT6 uses 879 feet of structural adhesives, and self-piercing rivets or flow-drilled screws join steel to aluminum.
I started critiquing cars at age 5 by bumming rides home from church in other parishioners’ new cars. At 16 I started running parts for an Oldsmobile dealership and got hooked on the car biz. Engineering seemed the best way to make a living in it, so with two mechanical engineering degrees I joined Chrysler to work on the Neon, LH cars, and 2nd-gen minivans. Then a friend mentioned an opening for a technical editor at another car magazine, and I did the car-biz equivalent of running off to join the circus. I loved that job too until the phone rang again with what turned out to be an even better opportunity with Motor Trend. It’s nearly impossible to imagine an even better job, but I still answer the phone…
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