Copyright Society of Manufacturing Engineers Jun 1996

Convergent Energy's (Sturbridge, MA) president, Nathan Monty, believes automakers will extend their success with transmission components to other high-volume parts. "Laser processing, where the part rotates under a fixed-beam focus position, is ideal for parts that are axisymmetrical, that require minimization of thermal energy, and that require weld penetration of 6 mm or more," he says.

Cutting applications in the auto industry have been fueled by the money eliminating stamping dies can save and the ease with which lasers can handle customizing and short runs. After standard dies stamp underbodies at General Motors' Cadillac Luxury Car Div., lasers trim those bodies to fit specialty or niche market vehicles like its Allante. Cadillac laser-welds rear shelf assemblies and tailored blanks; we describe laser-welding the roof retainer on its Aurora in this article. GM Truck & Bus laser-cuts floorpans from basic stampings to customize orders. The company lasercuts all body side molding holes in its new Chevrolet Cavalier to customize the trim arrangement for each body style and laser-cuts standard stampings to make Cavaliers with right-hand-drive for Japan (also described here). European automakers like Volkswagen, BMW, and Volvo have been much more laser-friendly, using laser welding extensively in production, and we take a look at how Volvo's experiments culminated in production welding at its Ghent and Torslanda body plants.

Growing a Laser Culture

"We automakers tap only a tiny part of the potential of lasers," says David Roessler, staff research scientist at GM's R&D Center (Warren, MI). "It's mainly a cultural thing. The average auto engineer still sees them as an exotic toy rather than an economic tool."

Even if the troops were won over, what about the generals? Paul E. Ayres, director of advaced body-in-white at GM's Technical Center (Warren, MI), says "managers want fast model changes and low costs, but their tolerance for problems, given lasers' history on the floor, is very small. Laser builders, be careful: products must be reliable."

Many managers equate high tech with high risk, says Jerry N. Koons, staff project engineer at GM North American Operations. The laser champion can go down along with a sinking project, so it's rare for people on the manufacturing or design end to stick their necks out by proposing laser processing for parts traditionally stamped, punched, or spot welded.

When you reach the end of the pipeline, at vehicle assembly, managers are even more risk-averse than others. "Assembly plants are at the burning end of the fuse," says Robert Scheuerman, senior project engineer at GM's Cadillac Luxury Car Div. (Flint, MI). "Managers in those plants are paid to ship cars, not to take chances on a new technology." Plants building luxury cars, like the Orion plant's Aurora, are under even more pressure than others, he says, because each car generates so much profit. "When a line doesn't run in one of those plants," he says, "the amount of money lost to the bottom line is staggering."

High-Powered Applications

In power ranges to 3 kW, YAG and CO sub 2 systems drill, cut, and weld in a wide range of production applications. Dr. Reinhart Poprawe, managing director of Thyssen Laser-Technik GmbH (Aachen, Germany), told a University of Michigan lasear conference recently that for economical body welding in production, lasers over 5 kW are needed, and today that means CO sub 2 . Soon, though, he expects diode-pumped solid-state lasers in the 5-kW range to greatly increase laser use in body assembly.

YAG lasers are developing the power levels and track records that make them serious candidates for production, says Harald N. Bransch, applications manager, Hobart Lasers and Advanced Systems (Troy, MI). "Five or 10 years ago, CO, lasers were establishing themselves as the preferred method for making transmission gear components. The 3000-W continuous-wave Nd:YAG with fiberoptic beam delivery and solid. state power generator is about at that point today." Engineers will be designing parts around these highpower YAGs, but those designs won't appear until long after the auto industry's 2000 models are out in the marketplace.

CO sub 2 systems from 6 to 45 kW are here today, however, and laser and automotive manufacturers are collaborating on even higher power. Convergent, which specializes in high-power lasers, in April introduced a 45-kW CO sub 2 laser for welding and cutting heavy sections, highspeed welding, and surface treatment. Builders like Monty think a 100-kW continuous-power industrial laser will be available in a few years.

High Speed, High Energy at Kokomo

Chrysler's Kokomo, IN, transmissions plant is a believer. By 1992, Kokomo had eleven 6-kW Convergent lasers welding parts. By the end of 1995, three 14-kW systems were at work. Early this year, as Chrysler got ready to produce 45RFE four-speed automatic transmissions for minivans and Jeep Cherokees, it told suppliers that the capital and operating cost of the new systems must be lower and production uptime must increase. In the end, production experience convinced the planners to add eleven more 14-kW Convergent lasers.

The laser system makes three welds on the rear carriers of the 604 and 606 four-speed automatic transmission assesmblies: a 360deg continuous circular weld joining the hub to the plate; a circular interrupted weld joining the cup to the plate; and a circular interrupted weld joining the subassemblies produced in those two operations. Components are not heat-treated before welding. Hub and cup material is 4130 AK; plate material is 1008 AK. Cycle time for each weld, including loading and unloading, is about 13 sec.

As most of the eleven 6-kW lasers were upgraded to 9 kW, a second workstation could be added. A sliding mirror or beam switch replaced the fixed mirror at the first workstation, so the beam can pass through to the second workstation. Now, welding takes place at the second workstation while loading proceeds at the first.

Weldment components feed into the welding station automatically for positioning in the press station tooling by a pick-and-place unit. The pressed assembly is transported directly into the weld station tooling by another pick-and-place unit. A lift mates the spindle-mounted tooling carrying the workpiece with upper tooling to create a Class I safety enclosure. After the welding cycle, the lift retracts, and the pick-and-place unit puts the welded assembly on an output conveyor.

Laser Cutting Custom Parts

A classic short-run project GM began late last year is customizing a Cavalier for export to Japan with right-hand drive under a Toyota nameplate. Die cost was a heavy persuader in this project. "To do everything in dies would have been prohibitively expensive," says GM's Koons. To form the complex shape of the steering column lower panel completely, for example, would require several new dies. The cost-effective solution was to form the part with standard dies and customize it with the laser.

GM's Lordstown fabrication plant now ships sandard stampings to the Mansfield, OH, stamping plant, where two laser cells do the customizing of dash, dash inner, front dash extension, rear dash extension, shroud upper, and steering column lower panels. Three of the six panels are completely manufactured in one of two Robomatix (Wixom, MI) Modulaser five-axis trimming and cutting cells using Convergent's Everlase Arrow 1100-W CO, laser.

In the production system, the laser cuts the "nose" of the steering column lower panel. The part is loaded into a gap-type press next to the cell where three flanges are folded over (these will act as weld flanges in a later operation), then packed for shipping back to Lordstown.

Because presses were to be placed next to the laser cell, vibration testing produced a design for specific natural frequencies with isolation pads added that dampen 90% of the energy between floor and foundation.

Three parts are run in each cell, producing a week's worth of parts. All stamping, cutting, and material handling of the six panels is done in the plant's metal assembly area, which services and maintains the laser system. Batch requirements are based on weekly assembly plant build schedules, with each batch corresponding to one week's production. In this two-shift operation, changeovers, which take about 10 min, are done on the third shift.

The fixtures mount on pins on each turntable plate; the control cable plugs into the receptacle at the safety enclosure. The controller reads the part number and sends the appropriate program to the robot. Magnets locate and hold the panels while they are cut with no additional clamping.

Project planners had two constraints, says Koons. They had to laser-trim parts fast enough to meet the production schedule, and they had to produce these customized parts at a quality level the same as or higher than traditional dies would produce. Cycle times were hard to estimate because of the complex binder trimming and irregular cutouts in 3-D planes. The equation involved total time available for cutting, subsequent individual cycle time, total number of dies and tooling fixtures required, changeover time for dies and fixtures, storage space, and the flow of raw stock and in-process parts. Now that the system is up and running, cycle times are close to planners' original estimates, 1.5 to about 4 min/panel, depending on the relationship of small to large holes and the number of cutouts.

Koons has been heavily involved in laser welding and cutting projects for GM since 1988. He believes it helps a lot to have a laser culture in place. Mansfield did. The plant manager srted experimenting with laser processing in 1989, and employees were prepared to tackle preventive maintenance and process control programs for the laser and press equipment. Two suppliers trained all the skilled tradespeople involved in the Cavalier project on componens and the total system, including preventive maintenance.

"When plants don't educate the people working with these systems, health and safety concems will keep surfacing," Koons says. Standard safeguards and automated laser operation, however, make laser safety take a back seat to the mechanical movement of the robot. "That's the main safety issue," he says, "and that's where training is focused."

Welding Aurora's Roof Assembly

In January 1966, Cadillac Luxury Car Div.'s Orion, MI, Assembly Plant began using a 3-kW Nd:YAG Hobart laser in the body shop to weld roof retainer assemblies at a rate of 80 jobs/hr. Cost issues like intervals between scheduled maintenance, operating costs, ease of maintenance, and recovery times after breakdown dictated Cadillac's choice of YAG, according to Scheuerman. "Our engineering estimates showed a CO sub 2 laser would add $400,000." Fiberoptic beam delivery from the HLP3000 continuous-wave laser allowed Cadillac to use common welding componens, like an off-the-shelf Fanuc S-420 six-axis welding robot.

Because the roof retainer holds the molding that runs from the bottom of the windshield across the roof and around the backlite, attaching the roof panel and trim retainers, it is a critical part. In the Aurora design, it is also constantly exposed to trapped water that eventually drains down the A or C pillars of the car. Finish is all-important: the roof surface must not be discolored, deformed, or scratched because no secondary metal finishing can be done after attachment, and all corrosion-resistant properties of galvanized steel roof and retainers must be preserved during processing.

The trim retainer is welded on the inside of the roof on top of the roof assembly hem. Again, corrosion of this joint will degrade the vehicle. The power output of the laser and robot speed must be controlled so that the laser can weld the retainer to the hem without penetrating into the inner panel.

The Orion enclosure houses laser, Fanuc robot, and roof fixture. The 10-m-1ong fiberoptic cable is protected in a flexible conduit. Roof tooling mounted on a two-position ISI slide can extend through a door for manual load/unload of the roof outside the weld station. A special lift tool manipulates the roof assembly, and the enclosure exhausts to an air filter system. A partition between the laser and welding areas decreases the volume of the weld area and makes the exhaust system more effective.

The joining method chosen was a two-part epoxy dispensed by a robot, in which spots were induction-heated to cure the adhesive and locate the retainer assembly well enough to hold it while it traveled to cure ovens in the paint shop. Early in 1993 it was clear that spot curing wasn't strong enough to survive the trip. Noncontinuous joints that resulted could be corrosion sites.

Cadillac engineers decided to use a 2.4-kW Nd:YAG laser doing prototype work at the plant to make seam welds in the galvanized steel assembly. They developed a robotic end effector to provide clamp pressure to the retainer in a shape-conforming fixture containing the roof panel. Bids for a fiber-delivered YAG laser in late summer of 1993, to begin operating in January 1994, went out, and a 2.4-kW laser was chosen.

The system works well now, but startup was very slow because of joining, robot, enclosure, and cooling problems. The Hobart MM-2.4-kW developmental model chosen lacked features needed for a production environment. The robot end effector technology did not transfer well to the production system. The cable had to be redesigned and the robot path changed because welds were reflecting and re-radiating energy from workpiece to the fiber cable. The laser's closed-loop refrigerated water system became contaminated with abrasive particulate. The volume of water--over 450 1--and complexity of the system made diagnosis difficult. Meanwhile, water-cooled optical components like flow tubes and YAG rods failed; arc lamps failed after less than 200 hours of operation and had to be redesigned.

Then there was the smoke. In the original design, with the lasers in the welding area, smoke coated and contaminated all surfaces. Servicing optical and electrical areas was difficult and risky, and unscheduled downtime was a constant problem.

Separating the laser and weld activity by a partition increased the effectiveness of the exhaust system and eliminated the possibility that internal generator components would be contaminated during laser maintenance. A backup laser was installed to maintain production.

In January 1995, the 2.4-kW laser was replaced with Hobart's 3-kW HLP3000. Since the redesign, three arc lamps and two water flow sensors have been replaced, maintenance intervals have been as long as 1200 hrs, and uptime, including unscheduled maintenance, has been 98%.

"It was a rough start," says Scheuerman. "It took about two years to work out the problems we ran into early in 1993. Now, the YAG system is more reliable than our COz systems." The problem, he says, was that "expectations were inflated. Many people at the plant thought you bought a laser, plugged it in, and off you went. They found out that you have to play with systems like this, and assembly plants have little time to play."

Continuous Welriing at Vohro

"Continuous welding makes an auto body significantly more stable, yet it is little used in production technology," says Thyssen Laser's Poprawe. Although BMW has 12 m of continuous laser welds in the body of the newest model in its 5 series, he points out that most of these welds join the complete front inner structure between bumper and engine. There's little use of tailored blanks.

An exception to his rule is Volvo Car Corp. (Gothenburg, Sweden). Lutz Hanicke, Volvg manager, vehicle concept, says the company's 12-year R&D effort confirms Poprawe's statement. One million meters of continuous laser beam welds since 1991 produced joints with a fatigue strength 80-150% higher than a spot-welded joint. Other advantages included one-side access, reduced joint deformation, closer, more consistent body tolerances, and tighter seam-welded joints.

Volvo's 850 model combines a rugged body frame with energy-absorbent surfaces in the passenger compartment. Lateral reinforcing tubes in the floor, roof, and seats, B-post reinforcement, and something called a SIPS (side-impact protection system) box between the seats distribute stresses. The roof panel is welded directly to the body side, eliminating the drip molding.

Designing production equipment that uses lasers and identifying a precise high-performance robot was the easy part of the 850 project, Hanicke says. Figuring out how to fix the panels in place during welding was the hard part. Conventional fixtures designed for one application are cumbersome and expensive, he says. They often require two-sided access, and they take too long to change in the welding station. "We needed to design a system that followed the geometry of the component while pressing the panels together at the welding point."

The designers came up with a telescopic mechanism incorporated in the welding head that requires only coarse programming of the weld geometry in the Z direction, along the length of the car. The roller applies a constant pressure to the roof panel, enabling the welding head to follow its contours accurately despite any deviations between the programmed welding path and the actual position of the roof. The focusing mirror in the head, permanently attached to the pressure roller mounting, maintains the focal point at a fixed position in the material.

While the complete welding head with PRD (pressure roller device) moves with the mechanical system along the joint continuously, the system with PCD, or pressure-clamping device, developed from the PRD concept, works differently. The welding head traverses incrementally to selected welding positions. Welding takes place with the robot at rest: the necessary movement is incorporated in the head.

But production cells need more than welding heads. The head had to be adapted for use in a robot that could be integrated in the body-in-white flow along the assembly line. The pilot program chose a freely programmable five-axis gantry robot from Kuka with additional A1 axis for 3-D body welding, a working X-2 envelope of 3 x 2.5 X 1 m, and a maximum welding speed of 14 m/min.

This approach has been modified to produce the new 850 at Volvo's Ghent, Belgium, plant and its new flexible body plant at Torslanda, Sweden. The Torslanda and Ghent production systems use Rofin-Sinar's (Plymouth, MI) RS 6000 6-kW CO, lasers. At Torslanda, two 125-kg Kuka standard robots are placed on the left and right sides of the vehicle, with two telescopic arms for the laser beams.

At both plants, two welding heads use a modular system of A, B, Z1, and A1 axes that can use different focal lengths for welding and cutting; have interfaces for rapid replacement of preset system components in the head; and use the PRD system. A sensor system detects the exact position of the roof panel.

For redundancy in this production system, says Hanicke, two standard robots replace the gantry approach used in the pilot: if one fails, production can continue with the other. Hanicke points to another advantage of using the type of industrial robots used in nearby stations for spot welding: maintenance personnel know the equipment, spare parts are on hand, and the capital cost is about 25% lower than that of a conventional gantry robot.

Another fail-safe device was installing a second laser source, so that if one laser is down, the two robots can continue production using the remaining laser, though cycle time will go up. Because the two-laser system can weld both sides of a body simultaneously, it is also an inexpensive way of expanding capacity.

The production sequence in the laser welding cell at Ghent is simple. After the body enters and moves to the station where the desired roof type (sedan or station wagon, with or without sunroof) is placed in position, it moves on to the welding station, where the roof position is checked automatically and the robot movement corrected if necessary. Welding with the PRD may be 1310 or 2260 mm in length, depending on model. Weld quality is inspected visually.

Production in Ghent relie primarily not on hardware redundancies and backups but on a well-established Total Predictive Maintenance (TPM) system. All working groups take responsibility for meeting production targets, maintaining their production equipment, and quality assurance. "Success depended on the involvement of production and maintenance staff at a very early stage and on teamwork between project group, production staff, and maintenance staff from both the body plant and its components, tools, and parts suppliers," says Hanicke. At present, two shifts produce 400 cars/day.

"Laser beam welding is now established at Volvo as a production tool on an equal basis with welding," Hanicke says. The potential for body assembly, he thinks, is considerable. "Expansion of the existing system to include other roof welding applications promises major economic benefits to Volvo."

Tracking the Weld

"Choosing a laser or a punch to cut a hole is fairly easy," says Cadillac's Scheuerman. "Dies beat lasers every time in performance and cost as long as the volume is high enough, and laser cutting systems have good performance monitors." Then too, says Bransch, "tolerances aren't so tight when you're cutting a hole. It's the relative location of holes in sheetmetal parts that matters. In welding, joint location is all-important, and tolerances are tight. "Manufacturers can face major liability and safety issues when welded parts like ABS solenoids, transmissions, air bag detonators, or doors fail," he says.

That's why welding single-sided parts, like a hydroformed upper section on an automobile, requires "a machine smart enough to tell you it welded or it didn't," says Scheuerman, "and to tell you when it makes a mistake." Bransch agrees that users need to know more than that a weld is good--they also need to know it's in the right place. "The sensors may say the weld is great," he says, "but they don't say it missed the joint. Parts may move when hat is applied during the weld, but the sensor still says it's a good weld."

When users complain about the reliability of inspection systems, says Bransch, they should remember that this problem exists in many applications. "In resistance sheet metal welding of bodies in white, the inspection method may be tearing the part apart. If airbags or ABS components are joined with adhesives, the same problem applies. If the part is good, all the process tells you is that you should have shipped it. It doesn't tell you whether the ones you didn't test are good. Welded joints offer thermal profiles and other data that glued joints don't."

Where will reliable systems come from? The machine tool builder, says Schuerman. "They have to spend the money to develop the systems. Scientists have done so for cutting. It's up to them." Bransch isn't sure. Because sensor systems are part-specific, all interested parties--the builder, the system integrator, the parts manufacturer, the sensor manufacturer--face a basic issue: if we build it, will they come? If Hobart develops a system for one customer, and the customer switches to another sensor system or changes part designs, will the system work? Maybe not.

The Year of the Tailoned Blank

In Japan, the hot production application for lasers in auto companies is welding blanks for door and roof panels, trunk lids, floor pans, pillars, and other sheet-metal components. Unlike US auto manufacturers, who buy them from suppliers, the lapanese weld their own blanks. Toyota inscalled production lines to make blanks in 1987, and a few years ago Nissan, Daihatsu, and Isusu added these lines; Honda is following suit. Convergent's Monty expects the Korean manufacturer Daewoo to begin using a 6-kW laser system to weld blanks late this year.

Poprawe of Thyssen Laser calls 1995 a watershed year for tailored blanks in Germany. BMW began to use them on a large scale, and German production shot up to about 1 million blanks. This year, he says, the leading German manufacturer of tailored blanks is tripling production because of contracts signed by Porsche, Audi, Mercedes, and Volkswagen.

In the US, tailored blank manufacture for auto applications, half a million in 1992, by industry estimates, shot up to almost 6 million in 1995. They save weight as well as parts. For example, door hinges, which hold what assembly plants call the swing metal, are reinforced by doublers spot-welded into place. Tailor-welded laser blanks reinforce the door without the doublers.

Hobart's Bransch thinks the boom is only beginning. In shock tower assembly, a tailored blank could be formed by punching out a circle, replacing it with a thicker circle, and welding around it. The blank would allow the high-stress area at the top to be thicker. A YAG laser with improved shearing and clamping ana seam tracking technology could allow "tailoring" the tailored blanks, he says. Two pieces of metal with a wavy line, or a notched line, or an oval could be welded. Designers could make the part thicker in heavy loading areas, instead of calling for a gusset or a reinforcement. "You could then get rid of the spot welding station, the robot, and the extra part."

Do designers think about these possibilities? he wonders. Do manufacturing engineers think about how to get rid of a robot, or a spot welding station? "Ideas need champions," he says, "and in big companies they can be hard to find."

Products must not only be reliable but must be seen to be reliable, says GM's Roessler. When US automakers see major competitors like the Japanese embracing this technology, he says attitudes will change.