The Cadillac CT6 at the 2015 New York International Auto Show. (Image courtesy of Auto Week .)

In manufacturing engineering, the search for the “best way” to fasten components into subassemblies is a major part of the job. There are lots of options for fastening sheet metal, and in auto body assembly, spot welding has been the method of choice for decades.

Now that aluminum is becoming the preferred unibody construction material, General Motors is using a novel mixed approach to building the new Cadillac CT6 large sedan.

The Cadillac CT6 goes on sale in the fourth quarter of 2015. It will be built at the Detroit-Hamtramck Assembly plant using a unique combination of joining techniques, a first for any General Motors model. A new body shop with advanced technologies – including 205 robots – has been added to the plant. The fully automated, roughly 138,000-square-foot shop is the result of a $384 million investment from the firm.





The Cadillac CT6 will use 7 different ways to join body parts

The prominent material joining techniques GM will use in the body construction of the CT6 include:

Aluminum Spot Welding Technology

Aluminum Laser Welding, which creates a seamless joining of exterior panels

Self-Piercing Rivets, which are able to join different types of materials together with a clean appearance

Flow Drill Screws, which are able to join different types or materials and used in conjunction with adhesive

Aluminum arc welding and structural adhesive are also separately used for CT6 body assembly.

“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,” Cadillac President Johan de Nysschen said at the recent Washington Auto Show.





Self-piercing screws and rivets: A throwback to the early days of autobody assembly?

Engineers faced a new challenge in manufacturing the advanced mixed material vehicle structure for the CT6. Combining different types of joining methods, the team overcame previous manufacturing difficulties involving the joining of traditionally dissimilar materials, while still allowing the engineering team to optimize every panel for its desired purpose.

Conventional wisdom holds that welding offers the lowest cost in high-speed sheet fastening. In that light, Cadillac’s use of self-piercing screws and rivets may seem like a throwback to the era of low volume auto manufacturing. In fact, this technique reveals how robotics can make processes that were previously too slow efficient enough to compete with spot and seam welding.

The main reason for the near disappearance of rivets in auto manufacturing is the need to access the back of the panel, restricting the process to relatively open assemblies such as light truck frame cross-members and spring mounts. Self-piercing technologies combined with feedback systems that allow robots to sense torque and drive the fasteners through sheet without distortion, makes mechanical fastening competitive again.





Low-cost joining technologies could make lower volume sub-assemblies more economical

There may be implications down the supply chain. Spot welding is optimized for a very high production rates, while laser welding has high upfront costs, again favoring large unit volumes. Mechanical fastening however, is very adaptable to large or small production runs.

With GM pioneering the tooling and automation for advanced screw and rivet technology, it’s reasonable to assume that Tier Ones will access the new technology without the need to prequalify it with their OEM vendor. This could open the door to subassembly work that is profitable at low to medium production volumes.





Selecting the right joining technology for each application in the new CT6

Among the five techniques, the CT6’s engineers were able to select the best joining method depending on material combination and body location (for machine equipment access).

Robot laser welding.

To weld both the inner and outer vehicle frames, 28 robots descend on the vehicle body in two separate framing stations, joining the body-in-white together from all angles. The robots are mounted above and beside the vehicle and can also reach beneath it. The two framing processes were choreographed to compensate for different microscopic vibrations. As a result, CT6 body construction resembles an orchestra as the robotic arms move in and out around the vehicle.

“Never before has an automaker brought this combination of joining techniques together for a single vehicle,” said Travis Hester, CT6 executive chief engineer. “The manufacturing team has enabled body engineers to optimize the vehicle for mass, safety, stiffness and materials with more precision than ever.

“The result is a top-level large luxury sedan, with class-leading body stiffness that generates excellent driving characteristics and impressive fuel efficiency without compromises to safety, comfort or quality,” he said.

Once the body construction is complete, a large robotic arm lifts the entire vehicle from one part of the assembly line to an upper-level conveyor – unheard of for a vehicle the size of CT6 – to be transferred across the plant.

For more news from GM, check out Cadillac's Latest V-6 Engines Led by 3.0L Twin Turbo.