March 2011- Two robotic laser brazing cells at General Motors’ Lansing (Mich.) Grand River Assembly plant produce Cadillac body shapes that would be impractical with any other process. Unlike spot welding, laser brazing forms narrow continuous weld beads that, in Cadillac’s application, range from barely noticeable to almost undetectable. The result: the appearance of seamless panels.
GM’s resurgent Cadillac division has embarked on a styling theme that the company is marketing as "Art and Science." The shapes are edgy in the literal sense, with sharp creases and deep folds; think of sheet metal origami.
That design direction appears on the latest generation of Cadillac’s performance-oriented CTS series: a sedan, coupe and station wagon directed at a younger population of auto enthusiasts. Producing the radically shaped CTS body panels strains the limits of conventional stamping and deep drawing.
A case in point is the rear deck lids for the sedan and coupe and the lift gate for the wagon. The shapes proved too radical for one-piece deep drawing, but the two brazed panels on the lids and gates appear as single stampings.
Behind the scenes
Michael Poss, senior manufacturing project engineer for advanced technology and welding at the GM Tech Center in Warren, Mich., and a 21-year GM veteran, explains how GM made the assemblies. He was Modern Metals’ guide through the usually off-limits pre-production operations facility - the new model prototype shop - at the Tech Center as well as Lansing Grand River Assembly to see firsthand how laser brazing is incorporated into GM manufacturing.
"We did a forming analysis," he says, "and to get the needed appearance, we wouldn’t be able to draw that deep. So we made a two-piece stamped assembly that is laser brazed. Laser brazing provided the appearance that styling wanted and still kept the cost down.
The alternative to laser brazing would be plastic panels, and they could increase the cost. Also, the appearance is different, and there are issues with finishing." The roof panels of all CTS vehicles are also laser brazed. The company formerly had spot-welded roof seams that extended the length of the roof and were covered by body-colored moldings. Brazing yields a cleaner look without the obvious appearance of a joint.
"There is a crease, which we call a feature line, and it aligns with the edge of the windshield and rear light," Poss says. "Laser brazing gives us greater styling flexibility without affecting structural efficiency. The car is still as strong as it needs to be, and we have the styling advantage."
Dealing with dollars
Cost-consciousness looms large at GM, even for its premier marque. The higher cost of laser welding hardware is weighed against downstream manufacturing costs. "It’s fair to say that the upfront investment for a laser is typically more than the initial investment for spot welding," notes Elaine M. Garcia, engineering group manager for advanced technology and welding. "But it may be cheaper in the long run - especially on a high-volume vehicle - because while you spend more for the laser, you save on every vehicle by reducing secondary operations. There is the tradeoff between initial investment and ongoing piece cost."
The piece-part cost for covering the spot-welded seam, which GM engineers call a ditch joint, is relatively high, running $15 to $20 per roof. In addition to the painted molding, the seam requires sealers and adhesives. Nevertheless, company executives emphasize laser brazing is primarily a styling tool, not a cost-containment process. "There are also reasons why we choose not to laser braze," Garcia points out. "For example, GM builds many full-size SUVs and pickup trucks. Having ditch joints on them provides solid attachment points for luggage racks. The ditch joints serve a functional purpose."
As with most new technologies, widening acceptance of laser brazing is lowering costs, making the process practical for less-expensive vehicles. Poss says Volkswagen, with manufacturing operations worldwide, attaches all of its vehicle roof panels with laser brazing. [See sidebar "Borrowing from Europe."]
Laser brazing was never intended to cut costs; it is employed to produce a particular appearance. "When we build a vehicle, we always optimize the cost to match what the people in the styling studios want," Poss adds. "They say ‘I want a car that looks like this.’ So as manufacturing engineers, our goal is to create that vehicle at the best possible cost."
What is laser brazing?
Brazing is a technique that does not melt the two surfaces to be joined. The laser beam melts a copper/silicon wire - called silicon bronze in the industry - that forms the joint by cohesion. There is no fusion of the parent metals.
The two edges of the joint don’t overlap, as in spot welding. The mating surfaces are downturned flanges, so the panels aren’t joined in an edge-to-edge butt weld. Poss calls the mating of two flanges a coach joint.
Poss was the primary author of the application for General Motors’ patent on the design of the coach joint. The configuration could be patented because it is uniquely shaped for brazing with mechanical seam tracking, a relatively low-cost technique compared to the intricacies of optical seam tracking.
Mechanical seam tracking, which eliminates a layer of computer technology, uses the brazing wire to follow the seam. Think back to the phonograph record, in which a stylus physically tracks the grooves, compared to a compact disc, which is scanned without contact.
The brazing head on the robot arm has a swivel axis with a servo motor. The servo motor moves the swivel axis to the zero position, and the robot arm places the brazing head in contact with the auto body. When the filler wire touches the seam, the servo motor turns off, allowing the swivel axis to move freely. The coach joint is V-shaped, and the filler wire follows the joint as the robot arm moves the brazing head down the length of the roof panel.
"Our laser brazing process will work without seam tracking if the car body is always at the same location," Poss says. "But it costs more to build tooling that locates the joint precisely. Seam tracking is a technique for reducing total manufacturing cost." The seam is not degraded by a small amount of brazing head tilt, and the process is tolerant of slight changes in position. A spring-loaded Z-axis pushes the filler wire into the seam, and if the path deflects in the cross-car direction, the axis swivels to follow the seam path.
The body panels are electrogalvanized low-carbon steel that is zinc-coated. Because it is Class A exposed metal, process engineers are understandably obsessed with cleanliness. Laser brazing is an inherently clean process. A properly adjusted operation produces zero spatter. The only residue is soot from burned forming lubricant, which robotic heads fitted with rotary brushes remove.
Laser brazing produces a very small heat-affected zone, insignificant distortion and no grain growth across the braze interface. "I have never measured the size of the heat-affected zone because there is no particular effect from it," says Poss. "And distortion doesn’t become a manufacturing issue."
Trapped stresses aren’t a problem, he adds, because of the shape of the joining surfaces. "We are brazing two sharply formed flanges. The flanges are rigid members that keep the metal in shape."
The structural strength of laser-brazed joints gives nothing away to spot welding. Garcia notes that "under all testing conditions, the tensile strength of the laser-brazed joint is at least three times the target strength. [Poss] varied a number of parameters and got results above acceptable limits every time." The silicon-bronze brazing wire has a minimum tensile strength of 50,000 psi.
Laser brazing is also a forgiving process. "Even if we have a problem with the filler wire feed, we still get good strength," Poss says. "If we accidentally reduce wire feed, we want to be sure that we are still above the required limit, that we stay within parameters. We want to make sure that if our manufacturing process varies, the joint stays strong under variation."
Direct comparison of laser brazing and spot welding speeds isn’t realistic, according to Garcia. "We look at what the line speed needs to be for the volume of the product," she notes, "and we put in the number of robots for spot welding or the number of laser stations we need. We can do 45 to 50 roof jobs per hour with one laser cell."
A collaborative process
There was a time in the auto industry when the various vehicle development departments worked in isolation. Stylists designed a vehicle’s shape, dispatched the design on a one-way trip to the production people, and never the twain did meet.
Not so today. "For example, when we were developing laser brazing for the Cadillac CTS, the product engineer and I worked together on the patent, and he is also named on the application," says Poss. "We also worked with the stamping people. There was a three-way collaboration with styling, product and manufacturing. We always try to give styling what it wants. They are the critical component. If the car looks right, it will sell." MM
Borrowing from Europe
Laser brazing made its North American debut at General Motors’ Lansing (Mich.) Grand River Assembly plant for the 2007 model year, but the process is nearly 11 years in production. European automakers, notably Volkswagen and Audi, have used it since 2000 to attach roofs and fabricate two-piece closure panels for deck lids and lift gates.
The initial vehicle sold in the United States that incorporated laser brazing was VW’s Touareg SUV, which shares body architecture with the Porsche Cayenne. A VW plant in the Czech Republic assembled the roof, where engineers for GM’s Opel brand in Germany first saw the process. The initial laser-brazed component for Opel was the lift gate for the Vectra station wagon, built in the Eisenach plant.
Laser brazing originated in the middle 1990s, the child of a technology initiative called IMPRO. Co-funded by the German government and the German auto industry, it was the genesis for many laser technologies that industries worldwide employ today. Several of the companies that currently supply GM with laser equipment were incubated by IMPRO.