Simply using aluminum as a car’s primary body material is difficult enough. The lightweight metal is harder to stamp into shape than steel, requires different panel-joining techniques, and demands several cleaning processes that steel does not.
For General Motors engineers working on the upcoming Cadillac CT6 sedan, that challenge wasn’t enough.
Their design goal was a large sedan, such as the BMW 7-Series, with the weight of a mid-sized vehicle, such as Cadillac’s CTS. So engineers designed a car with 13 different alloys (mostly steel in the structure and aluminum in the body panels), but the challenge didn’t end there. Designers wanted seamless joints on the exterior – rejecting century old design tricks such as hiding ragged weld seams with trim and weather stripping.
“It’s a PQ – perceived quality – item where we’re really looking to improve the appearance of the joint,” says Greg Hagen, manufacturing executive chief engineer for GM’s Omega program, the vehicle group that includes the new CT6. “On much of this car, we don’t have to use any applique. It’s right there in front of the customer. In this luxury segment, customers have this expectation. They’re looking for these kinds of design cues on the car.”
The key to achieving those seamless joints is aluminum laser-welding. As GM engineers have gone through design and manufacturing prove-out on the new vehicle, they say they’ve learned a lot about laser processing and how to use such welding technologies on mass-market vehicles in the future.
Cadillac plants have used laser-joining techniques for several years to get nearly seamless joints on the CTS sedan, but Mike Poss, senior laser welding engineer for the Manufacturing Engineering Execution Group at GM, says that’s a brazing process. Lasers melt a copper-and-silicon fill material that attaches the steel roof to the steel body sides. Poss said that process is effective with steel, but it didn’t suit the CT6’s aluminum shell.
“As we go from steel to aluminum, the equipment set changes,” Poss says. “We looked at upgrading to newer versions of equipment. We went to a newer style laser. We went with newer equipment for laser optics. By adding the features of new equipment, we found that aluminum laser-welding the roof met all of our requirements. It wasn’t particularly more difficult, but you do have to buy the right equipment.”
Unlike the laser-brazing in the CTS, in which the steel sides remain solid and only the fill material melts, aluminum laser-welding requires melting both panels to be joined and the aluminum-wire fill. To accomplish this, Poss says GM needed a smaller process – a more tightly focused laser that imparted precise amounts of power.
German equipment company Scansonic, distributed in North America by Abicor Binzel, provided a key piece of equipment. Its ALO3 system tracks the weld seam by measuring input from the aluminum fill wire – a technology that GM calls tactile seam tracking. And the device auto-focuses the laser, supporting quick and continuous changes to the beam.
“As there are small variations in the up-down direction, the laser stays in focus,” Poss explains “With steel, it’s less important. With aluminum, it’s more important to have the focusing right.”
Tom Graham, key accounts manager at Abicor Binzel, says in brazing processes, the laser spot size is typically twice the wire size so accuracy of the process set up is less critical. With aluminum welding, the laser spot size is smaller than the wire, so set up and process control are much more important.
“Your spot-to-wire alignment is critical. Because of the propensity for cracking in aluminum, you have to have the wire and the right energy at the right place at the right time,” Graham says. He adds that the difficulty in getting a perfect seam is why many automakers use laser welding for structural parts that customers don’t see, not exterior metal. “When you get into using lasers in body-in-white, your tolerances go from 2mm, 3mm, or 4mm – which are OK for spot welding – to 10ths of a millimeter. You’ve got to have very low gaps to no gaps. You’ve got to know exactly where the edge is.”
Poss says the aluminum process is not radically different than the steel brazing process, but the precision levels demanded upgrades to laser-welding equipment.
“The joint between the two panels is located in space within a certain tolerance. We bring the laser down, and the wire that’s being fed into the laser-welding joint also tracks the joint. So if there are small variations in the cross-car direction, the wire follows the seam automatically,” Poss says. “It’s also used on steel, but the aluminum wires are softer than the copper wires, so we have a more-sensitive seam-tracking sensor that allows us to use the softer wire.”
Keeping it clean
Though the weld process for aluminum is more precise than steel-based brazing, it does create some mess. Poss explains that because both panels and the fill material melt during welding, aluminum laser processing can create some weld spatter – gobs of metal that could spoil the seamless look that designers wanted.
“At the beginning of the aluminum project, I recognized that there were some spatter issues, so we prepared for cleaning the spatter off post weld,” Poss says.
Working with robotics company Comau and nylon brush company Osborn, engineers developed a brushing step to gently remove any spatter from the seam.
Mike Akuszewski, technical engineer for Cleveland-based Osborn, says his company had provided spatter-cleaning brush lines throughout North America and Europe to automakers using laser-welding processes. The challenge with the CT6, he says, was the seamless appearance that GM wanted. Traditionally, automakers use two techniques to hide the typically rough seam between the body side and the roof of a vehicle. Body sides tend to be slightly taller than the roof, so when people look at the car from the side, they see the body side, not the joint where that panel merges with the roof. Then, most cars have a thick rubber strip over that seam to cover any imperfections.
“You could play in that area between the roof and side, and brushes can be very forgiving. And because of that trough (between the body side and the roof), certain things weren’t that important. If the metal gouges, it gouges. If it tore up the metal a little bit, that was OK. As long as you got a decent seam, you could cover it all up with trim,” Akuszewski says.
With the CT6 body leaving the weld shop with nearly perfect seams, Osborn engineers had to make sure its brushes removed only the weld spatter. The typical margin for error disappeared.
“Brushing’s still the most obvious choice for cleaning the spatter,” Akuszewski says. “With the right brush, it’s the only way you know you’re not going to gouge the metal.”
Poss says that while brushing works, he hopes to shut off that stage in the production process as plant workers and engineers get more comfortable with e laser-welding processes. As the automaker has developed the CT6 and its manufacturing steps, the amount of weld spatter has decreased dramatically.
The key was recognizing what caused the aluminum to spit out some waste as the lasers melted the metal. The culprit turned out to be lubricants used in stamping body panels into shape, Poss says. While steel bends easily and holds its shape after stamping, aluminum requires more effort and a lot more lubricant.
“We’ve seen it mostly as an appearance issue. If we have a lot of forming lubricant on the panel, the weld doesn’t look as good afterward,” Poss says. “So we’re just cleaning off the excess lubricant that might be on the panel. It makes a big difference in the appearance.”
Unlike steel panels, GM workers send the aluminum parts through a chemical wash process before welding them together to form the vehicle body. Since adding that step, Poss says the need for post-weld brushing has dropped.
“This is GM’s first use of laser welding aluminum on a roof, so we’re learning,” Poss says. “Sometimes, you prepare for the worst, then you can engineer yourself away from it.”
Akuszewski says Osborn understands that its role is potentially temporary. If GM stops brushing, it won’t be the first automaker to find ways of preventing weld spatter instead of cleaning it up later.
“Companies are going to have more chemical processes. They’re going to have their suppliers or their stamping plants have the metal prepped,” Akuszewski says. “We’re a necessary evil right now. Their goal is not to have that process.”
Still, he adds, Osborn is happy to provide the service, even temporarily. The company’s main automotive business is deburring engine components, but if engineers begin using brushes in other departments, they may gain a better understanding of other applications for Osborn’s systems in future projects.
“We’re always looking to tell our story, so the more engineers in the plants that see our product, the better our chances,” Akuszewski says.
Lasers support design
While much of the focus on using laser-welding aluminum was to get a perfect appearance, Hagen says the technology has other advantages. As teams went through the design and manufacturing processes, they found they had more design freedom.
“There were several design enablers from the laser-welding process. In the doors, for example, we’ve got shorter flange lengths, and that really improves the overall vision with larger window openings and lower costs for manufacturing,” Hagen says.
He adds that with premium vehicle programs such as the CT6, manufacturing cost savings tend to go back into new features for the car, but finding those savings should help bring the technology to more mass-market vehicles in the future.
Another laser-enabled design feature was a three-piece deck lid for the CT6’s trunk, rather than a flatter two-piece part.
“It’s enabled us to increase the license-plate pocket depth, so we can put two cameras back in that area of the vehicle,” Hagen says. “We can offer a 360° camera feature along with our dynamic inside rearview mirror camera, all within that same packaging space. We wouldn’t be able to do that if we were doing traditional spot-welding with steel.”
As with Ford’s F-150 pickup, the Cadillac CT6 is not all aluminum. Most of the body is, but the structural components are mostly made from high-strength steel. Hagen says it’s a design approach that’s going to become more prevalent in all vehicles in the future.
“The CT6 clearly demonstrates that we’ve got the technical ability to strategically mix high-strength steels and aluminum with various joining technologies. We know that we can produce a really lightweight vehicle that’s really optimized for mass,” Hagen says. “This is our current direction for Cadillac.”
At GM, Cadillac often serves as the test case for new technologies. Because of the higher price points and slower build times, systems that wouldn’t make sense for plants running 250,000 units per year work just fine in the luxury segment. So a vehicle exterior made almost entirely from laser-welded aluminum panels makes much more sense for Cadillac than for Chevrolet. But the technology is advancing and become more affordable and mainstream.
Poss adds, “We already have a part that’s in production on the current [GMC] Yukon, [Chevrolet] Tahoe, and [Cadillac] Escalade – an aluminum liftgate. And that’s also laser-welded together. That’s a higher-volume product, and we’re able to laser weld that without problems.”
Poss and Hagen say they can’t predict how quickly the technology will move into mass-market products, but they add that every vehicle needs to get lighter and more efficient to meet fuel-economy rules, and aluminum laser-welding is a promising technology to meet those goals.
“From a manufacturing perspective, we’re stretching with the CT6. These are things we haven’t done before,” Poss says. “Product engineering wanted to do something, so manufacturing engineering works on the stretch goal of getting this in production.
“We’ve done that on other vehicles,” he continues. “I’m always working on the stretch vehicle. The laser brazing with the CTS, that was a stretch goal.”
Hagen adds, “We call up Mike, bring him in, and tell him we need to get this done, and he figures it out for us. He’s a good guy to have on the team.”
General Motors Co. – Cadillac
About the author: Robert Schoenberger is the editor of TMV and can be reached at firstname.lastname@example.org or 216.393.0271.