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Laser Technology

Stiffening Corvette

By J. Neiland Pennington

August 2009 - One of the largest laser welding applications in a U.S.-produced vehicle joins the central tunnel components of Chevrolet’s Z06 and ZR1 Corvettes. Fourteen-point-six meters (47.9 ft.) of laser welding complete the lap-seam joints on the tunnel, which is the backbone structure for the two ultra-performance cars’ aluminum space frames.

First, a word on the need for a lightweight but stiff space frame: Overuse has soaked the starch out of the term "supercar," but in the case of the Z06 and ZR1, the designation is apropos. These two land missiles wring unprecedented horsepower from production pushrod V-8 engines: 505 hp from 7 l. (427 cu. in.) with normal aspiration for the Z06, and 638 hp from 6.2 l. (375 cu. in.) with supercharging for the ZR1. The resulting performance for the latter: 0 to 60 mph in a gut-checking 3.3 seconds and 1.1-g lateral acceleration on the skidpad, according to Road & Track’s February 2009 road test.

The aluminum space frame is exclusive to the two über ‘Vettes, both of which are hardtops for enhanced structural rigidity. The standard Corvette coupe and convertible--the term "standard" hardly seems appropriate--are built on steel space frames. The aluminum Z06 and ZR1 versions weigh 284 lbs., 32.4 percent lighter than the 420.4-lb. steel frame, but torsional stiffness and bending moment are comparable. Simultaneous engineering and design for manufacturing make both frames compatible with a single assembly line that produces all Corvette models interchangeably.

Outsourcing aluminum fabrication
The steel space frame is built at the Corvette assembly plant in Bowling Green, Ky., but the aluminum frame is the product of another Kentucky facility, the Structural Solutions Division USA of Dana Corp., 90 highway minutes west of Bowling Green in Hopkinsville. Although Dana engineers heard the argument that laser-welding the central tunnel would be impossible, they concluded that laser technology was the only process that could produce the required results. They’d simply have to make it work.

"Laser-welding is used because of the difficulty of spot-welding aluminum lap joints," says Larry Monacelli, laser engineer at Dana, a technician who has been involved with the process since industrial lasers were introduced. "With aluminum, your spot-welding tips don’t last long; the molten aluminum deposits itself on the tips."

"Spot-welding aluminum is costly," adds Don Fitch, operations manager at Dana. "You get perhaps five welds before you replace the tips."

Another problem is heat transfer. A sealant that prevents carbon monoxide from infiltrating the cockpit is applied to the steel frame, and the stamped components are spot-welded through the material. But sealant can’t be used with aluminum.

"You can’t spot-weld aluminum through sealant at all," Monacelli says. "The welding heat isn’t conducted through the material."

Lasers produce continuous fusion welds that close joints against CO without the need for sealant.

Not ideal for laser welding
The aluminum tunnel didn’t lend itself to laser welding either. The first problem, Monacelli notes, is the thermal conductivity of the metal.

"It’s high, and aluminum cools fast," he explains. "Laser welding vaporizes the material, forming a keyhole at the point of the beam. It’s the molten material around that hole that produces the weld. Cooling the material too fast keeps the keyhole from closing and causes voids and porosity in the weld. The metal must flow into the keyhole to form the weld."

Laser welding adds no filler metal, he emphasizes. It’s a pure fusion weld.

Several combinations of thicknesses are joined in the tunnel structure: 1 mm to 3 mm, 1 mm to 2.5 mm, 2 mm to 2.5 mm, and 2 mm to 3.5 mm.

"As long as you know where to set your [welding] speed, thicker material isn’t a problem," Monacelli notes. "The only issue that you have is, the thicker the material, the slower you’re going to weld." He says he prefers welding heavier gauges because there’s more metal to flow and close the weld.

What about continuous MIG welding? Too costly, Fitch reports. "The problem is the amount of heat you have to apply," he says. "The heat required would distort and warp the tunnel. We would have had to use a lot more heat to MIG-weld aluminum because of its thermal conductivity."

MIG would be inefficient, compared with laser. "The lap seams would take two to three times longer with MIG, even with robotic welding," Fitch continues. "And there’s the additional cost of filler wire."

YAG technology
Dana welds the 5000-series alloy sheet for the central tunnel with 4-kW Nd: YAG (neodymium: yttrium-aluminum-garnet) lasers, which are more compatible with aluminum than a CO2 laser. The wavelength of a YAG laser is an extremely short 1.06-microns (1,064 nm), which readily couples with a highly reflective metal like aluminum. CO2 lasers operate at 10.6 microns (10,600 nm), which couples less readily and tends to reflect from the surface.

Each laser head has two focus lenses arranged in tandem so that both beams strike the heat-affected zone. Four kW are beamed through the two lenses; it’s not a true split with 2 kW from each. "We use two beams so that the metal will remain molten longer and the weld will fill in more consistently," Monacelli notes. "The two beams overcome the tendency of aluminum to cool rapidly."

Highyag focus heads running argon shield gas are energized by Trumpf lasers. The heads are mounted onto six-axis Fanuc 2000i robots.

Once the stampings that form the central tunnel are loaded onto fixtures, manufacturing the central tunnel is entirely automated. It’s a four-stage process through two Comau-built laser cells with four welding stations per cell.

Each of the two cells has two revolving doors, and fixtures are mounted on both sides of the doors. Two cells and two doors times two fixtures per door equal eight fixtures total. One robot in each cell serves two fixture stations.

The operator loads the first subassembly onto an empty fixture, the door rotates and the subassembly is welded. When welding is complete, the fixture rotates out, then the subassembly is removed and loaded onto the fixture on the second door. The part on the second door rotates in for further welding. Following the weld cycle, it rotates out and is transferred via roller conveyor to the second cell, where the two-station process continues.

Four operators currently run the laser cells. One person can handle the task, but the four operators also do manual welding and other jobs concurrently.

Completing the space frame
Laser welding is used only for assembling the central tunnel. The remainder of the space frame is built with automated and manual MIG welding and self-piercing rivets. The 17-minute build cycle begins with left- and right-hand side rails that GM hydroforms from 14-ft.-long 6063 aluminum tubes at its Pontiac, Mich., fabricating facility. Dana adds the shock towers, which are MIG-welded by robots. Five-mm-thick material is attached to 4-mm material, so it requires a heavy bead.

The shock towers are aluminum alloy 356 tilt-pour permanent mold castings, which are made weldable by control of the mold temperature and cooling time to eliminate porosity. "This is a fillet weld, so we need filler wire," says Fitch. "Those are heavy castings, so MIG welding is the best possible process."

Front and rear bumper beams are joined by manual MIG welding to the completed side rail assembly, which creates the box section to which the central tunnel is attached. Roof bows and door hinge pillars are welded while the outer lock pillars are tack-welded for positioning, then attached with 240 self-piercing rivets.

The space frame is inspected for missing rivets or components, then it’s sent to an automated machining station where shock towers are machined to a true position of 0.5 nm. The cell also mills the location for the rear tub assembly to which the body panels are mounted and drills holes in the roof bow to mount a fascia trim.

Final inspection is conducted at the Perceptron station, a 3-D laser measuring system that checks 227 points on the frame. All of the frames pass through the scanning station, and the Perceptron correlates to Dana’s computer coordinate measuring machine at a rate of 95 percent.

"About three frames a day go to the CMM lab," Fitch reports. "That’s about the most we can measure per eight-hour shift because we have about 21/2 hours of CMM time per frame. The CMM measures 540 points, to an accuracy of ±0.025 nm." Specified tolerances for the frame average ±0.2 nm.

Not your average process
Monacelli hesitates to call Dana’s laser welding process unique, but he’s convinced that the extensive operation is still unusual in the auto industry. "Volkswagen is probably the No. 1 company in the world that uses laser welding technology. And they use it in an efficient way. They’re laser-welding both steel and aluminum, but it’s all thin sheet metal.

"Nobody to this day has achieved the thicknesses that we have," he adds. "We’re welding 2.5 mm to 3 mm, and everybody else is welding in the range of 1.7 mm. I don’t know of anyone who is lap-welding pre-formed parts like this. If they are, they aren’t doing the thicknesses that we are."

With the auto industry in shambles and GM filing for an unthinkable Chapter 11 bankruptcy, performance cars like the Corvette hardly seem relevant. Not so, according to Dana management. Until the Chapter 11 filing, Dana had been running one shift five days per week, shipping 24 frames daily. Delivery was on a just-in-time basis, with the assembly plant maintaining a 20-frame buffer in Bowling Green. Now Corvette assembly operates two weeks per month, and Dana continues the single shift on weeks when Bowling Green is running.

Customers at the upper end of the car market aren’t recession-proof, but they’re proving to be recession-resistant regarding the two top ’Vettes. Z06 and ZR1 production stands at about 30 percent of total Corvette output, and anyone wanting a ZR1 will have to queue up for the 2010 model. The 2009s have all been sold. FFJ

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