What Designer Should Know About Lasers

Gary S. Vasilash

No, we're not talking about spot sizes, wavelengths, and lasing mediums. Rather, how lasers can make better-looking vehicles (that are also lighter, stronger and safer).

While the design of the 2009 Cadillac CTS is remarkable and renowned, there is something that doesn't meet the eye about it.
Look carefully at the photo of the CTS (below). What do you see? And what don't you see? What you do see is a decklid that has a rather substantial angle. It is the sort of shape that would take considerable deep drawing to achieve-assuming it could be achieved at all. And deep drawing is a time-consuming operation that requires serious presswork and tooling.
Then consider the roof. More specifically, focus on the area where the roof meets the body side. If you look at most cars, you'll see something else there. You'll see a bit of trim on both sides, running along that mating edge. That's because there is a ditch where the roof meets the body side. An unsightly, long divot. It's formed to accommodate roller resistance welding that's performed to join the roof to body side. Then it is filled with a sealant so as not to leak. Then the trim is added on top. Yet there's nothing there on the roof of the CTS.

That is an example of how lasers can help improve vehicle design, points out David Havrilla, product manager, High Power Lasers, TRUMPF, Inc.
In the cases of both the decklid and the roof of the CTS, laser brazing is deployed. The trunk is produced with two pieces, with the seam invisible where the horizontal piece meets the vertical piece. The ditch joint is eliminated by simply laser joining the roof to the body sides. The seam is sealed so that there's no need for the sealant, no need for the trim.
Havrilla says that this is becoming more common on European automobiles (and realize that one of the things that Cadillac engineers set out to do was develop a car that could compete with the BMW 5-Series with a price-point of the 3-Series). For example, at Audi they actually have a term for the laser brazing process that is used even on vehicles as big as the Q7 SUV: the "zero-gap" look. (In all, Audi engineers utilize more than six meters of laser welding on the Q7.) And at the rear, some European cars have a laser brazed license plate inset, which would otherwise be another deep drawing challenge and that would require multiple sets of tooling to accommodate different plate sizes in Europe.
The point is, designers at places like General Motors and Audi are taking advantage of laser processing, as are those at Volvo (e.g., C-70) and Volkswagen (e.g., Passat). But there are some significant opportunities for those at other companies to do the same.
Flange elimination.
For example, Havrilla says that another area where there can be weight, cost and aesthetic advantages, and even improved safety through increased driver visibility, offered by lasers versus traditional resistance spot welding is in the elimination of flanges that are found in various areas on vehicles-such as below the rocker. The flanges, which are generally on the order of 16 to 18 mm in width, are sized to accommodate the tip of the spot welding gun. This, he claims, can be cut in half through the use of a laser. Which results in savings of time and money. Or because lasers make it possible to vary the length of the welds-from stitches that are analogous to spots all the way to continuous seams-it is possible to eliminate some structural reinforcements: Consider the door opening on a body-in-white; by running a continuous seam where the door will be attached, but stitches elsewhere, a reinforcement can be eliminated, thereby providing savings.

One of the problem areas that has existed in applying lasers in some sheet metal applications has been where two galvanized surfaces need to be joined. At issue here, Havrilla explains, is that because the zinc on the steel has a comparatively low boiling point, it melts and leads to porosity problems. However, this problem has been solved through the use of remote laser welding. In this case, a laser scans the surface of one of the materials to be joined so that there are melted protrusions of a repeatable size generated (known in the industry as "laser dimpling"). Then the second sheet is laser welded onto it. The little humps provide an escape route for the melted zinc, to the sides, which keeps the porosity blowouts from occurring.
Says Havrilla, "People need to rethink design in order to appropriately apply lasers to the manufacturing arena." He explains that it isn't a matter of just taking lasers and replacing spot welding equipment, because that's not particularly advantageous to realizing the benefits that lasers can provide. But by taking another look at what lasers can do-think only of the elimination of the ditch, which is still a "feature" on even some of the latest cars-aesthetic and economic benefits can be realized.
Audi is using this process in a processing cell in its Ingolstadt facility for producing the doors of the A4 and Q5. In the first step, a 7-kW laser fires pulses on the surface of the door inner to produce 0.15-mm high bumps. In the second, the second piece, the frame, is added, and a robot with an optical focusing system moves around such that 48 welds are produced. It is done so quickly that cycle times for each door are cut in half.
Creating Plastic-Metal Hybrid Components
Plastic-metal assemblies can be produced with a process developed by the Fraunhofer Institute of Technology (www.ilt.fraunhofer.de; Aachen, Germany). In the process, laser radiation is combined with mechanical pressure. The laser radiation passes through the plastic piece; the metal part to be attached is pressed onto the plastic and heated. It is then pushed into the plastic. Assuming there is appropriate geometry involved, and that the metal part has a higher melting point than the plastic, once there is cooling, there is a solid bond. (In addition to metal, the process, called LIFTEC, also works with ceramics and temperature-resistant plastics.)
Fast Stainless Marking
According to application engineers from Laser Photonics (www.laserphotonics.com; Lake Mary, FL), Q-switched fiber lasers are superior for marking on stainless steel than CO2 lasers. The reason: because stainless steel is reflective, it is necessary to coat the stainless steel surface with a “laser marking material” (LMM). This is sprayed on, allowed to dry, then the CO2 lasing commences. Afterward, the surface has to be cleaned. However, with a Q-switched fiber laser, such as the company’s 20-W FiberTower XP, no LMM is required, so the marking process is fast. In one application, they were able to mark at a rate of five inches per minute.
System for Welding Plastic Housings
Laser welding is an alternative that can offer higher weld quality and higher yields, according to LPKF Laser & Electronics (www.lpkfusa.com; Tualatin, OR). It has developed a diode-laser based off-the-shelf laser welding system for processing such things as electronic enclosures and sensor housings. The system is available with 30 to 600-W laser power; it adheres to Class 1 laser safety rating. The maximum part size that can be handled is 9 x 9-in. There is a rotary table that can be manually or automatically loaded. With standardized fixture adapters, retooling changeovers can be accomplished in 10 minutes or less.