Think about a crystal the size of a pencil or pen. Now think of one that’s about the size of a dime. That’s the overall difference, more or less, between the heart of a conventional Nd:YAG laser and one that’s now aborning, the Yb:YAG disk-type diode pumped laser (the former uses neodymium and the latter ytterbium). Tim Morris, technical sales manager for TRUMPF Inc.’s Laser Technology Center (www.us.trumpf.com; Plymouth, MI), provides some precision with his analogy: the conventional YAG rod is about 6 mm in diameter and 150-mm long; the disk is about 14 mm in diameter and 0.2 mm-thick. And this small disk has a tremendous amount of potential for both cutting and welding applications, both in terms of standard approaches and scanning.
A brief bit about how a laser works. Essentially, when you see a box that looks like a chest freezer on steroids, a laser system, that box is full of devices that (a) pump energy into the rod (or gas if we’re talking about CO2 lasers, or other gaseous devices) and (b) cool the rod (because all of that energy makes the rod rather hot). This pumping takes the form of exciting the atoms within the rod by massive quantities of light (say from a series of arc lamps), which leads to the emission of radiation through the optics (emission of radiation are where the “er” in “laser” come from). Now one of the things about YAG lasers is that they’re not particularly energy efficient. That is, for all of the energy that’s put into the laser (for everything from the lamps, electronics, fans, cooling pumps, etc.), the output of energy (a.k.a., “wall-plug efficiency” is only about 3 to 4% for a lamp-pumped YAG laser. Granted, this is a highly concentrated output, which makes the laser useful for materials processing applications.
Although there are serious cooling mechanisms in place, such as water that circulates around the rod, Morris points out that there is a limitation to the amount of energy that can be put into a rod before there’s trouble. This trouble takes the form of what’s known as “thermal lensing.” Thermal lensing is simply the heat distortion of the crystal. It may be a small amount, but it is enough to affect the output of the laser. Beam quality suffers. Beam quality is important in laser operations for a variety of reasons but, most fundamentally, from the standpoint of how well—or whether—work can be done with the device. To avoid or to minimize thermal lensing, there is typically a reduction in the amount of pumping performed.
Enter the disk, which originally hails from the Institut für Stahlwerkzeuge at the University of Stuttgart (from which TRUMPF, and other companies, have secured a license). Given the fact that there is a comparatively large difference between the surface-to-volume ratio of the disk and the rod, the cooling of the disk can be performed far more efficiently. In fact, in the design of disk laser units produced by TRUMPF, one side of the disk is mounted directly to a cooling surface (which is mirrored to enhance excitation). A result of these two characteristics is that there is minimal thermal lensing even though there can be aggressive pumping of the disk. That means that there is high beam quality emergent from a disk. (And although for the sake of simplicity we’re talking about a disk, disks can be coupled in order to ramp up the power from the laser, just as is the case with rods.)
BENEFITS OF THE TECHNOLOGY. One more thing about pumping the disks: diodes* are used instead of lamps. One of the benefits of this is that the diode light is “tuned” for purposes of the lasing more specifically than can be the case with an arc lamp, which has a wider spectrum of output. Which is to say that better input helps result in better output, and this means better energy efficiency, in the 15% vicinity. Not great, but certainly much better than 3 to 4%.
Here’s the thing to know about disk lasers: “Improved beam quality at higher power—approaching CO2 beam qualities,” says Morris. This leads to two opportunities:
1. Given that there is a smaller, more powerful spot size than can be achieved with a rod-type YAG laser, it is possible to weld with a narrower bead, to have a reduced heat-affected zone, and to weld more rapidly. This could be beneficial for those who are welding such things as fuel injectors, airbag inflators, sensors, and the like at comparatively high volumes. (Morris cautions that people who might have this high-speed welding opportunity need to take material handling into account: can the workpieces be positioned fast enough to take advantage of the laser’s capability?) For those who have cutting applications, this laser can provide a narrower kerf, resulting in finer cut detail and/or faster cutting.
2. Scanning or remote laser welding. This is something that has been available with CO2 lasers for a few years because of the higher power levels and excellent beam quality provided by those lasers. The disk allows this to occur for YAG lasers. To understand what’s happening here, consider a flashlight. If you hold a flashlight close to a surface, the beam is small and well-defined. As you move the flashlight away from the surface, not only does the diameter of the beam grow, but it becomes more diffuse. This is exactly what happens with a focused laser beam. But because the disk laser allows more energy to be pumped in, and because there is the subsequent high energy density, there can be a distance between the laser optics and the workpiece yet still a concentration of energy such that work can be done.
WHY REMOTE LASER WELDING MATTERS. As Morris puts it, “The larger the standoff—the Z-axis—the larger the X-Y work area.” So, for example, TRUMPF offers a remote laser system that has a standoff of 0.5 m that provides a work area of 200 x 300 mm. This means that within that area, work can be done and done quickly, as all that is being moved are the mirrors that are within the optics. The light can be bounced to where needed on the order of within 2 to 20 milliseconds. The actual processing, say welding, doesn’t happen any faster than in a conventional, non-scanning setup. The amount of time required to weld is still the amount of time to weld. But what changes in a big way is the amount of time between welding, the time when the light is being repositioned. This is happening incredibly quickly compared with what can be done otherwise with, say, a robot.
One of the characteristics of the light from a YAG laser is that it has a wavelength (1.064 microns) that allows it to be transmitted through an optical fiber. (This isn’t the case with a CO2 laser, which has a wavelength of 10.64 microns.) This has permitted setups whereby robots have been able to wield YAG lasers for welding (or cutting) operations, taking advantage of the axes of motion provided by the robot rather than being restricted to fixed line of sight. (CO2 lasers can be manipulated by robots, but it is a far more complex setup than with the fiber.) What all of this means is that with a disk laser it is possible to put a scanning head right on the end of a robot arm (e.g., TRUMPF has one that measures 280 x 300 x 174 mm), so the beam manipulation can be a function of both the six axes of the robot and the two of the mirrors. This can be most beneficial in applications in places like body shops, where the beam can be more readily manipulated around the body. Morris points out, however, that unlike spot welding or even some laser welding, where there is clamping as a function of the robot end-of-arm tooling, with remote laser welding, the clamping is separate, so that is something that needs to be taken into account for deployment.
So is a disk laser going to obsolete the rod-type? Probably not. There are plenty of instances where the beam quality of the rod-type does the job well. But what the disk does do is provide an opportunity for a rethinking of part processing.
During a conversation with Stefan Heinemann, executive director, Center for Laser Technology, Fraunhofer USA (www.clt.fraunhofer.com; Plymouth, MI), it became increasingly curious to me that the implementation of lasers is so woefully limited in the U.S. auto industry vis-à-vis what’s going on elsewhere, in Europe as well as in Japan and Korea. The Fraunhofer facility was established in suburban Detroit in 1994 “solely focused on automotive applications,” Heinemann says. And the primary function of the operation is applied research*, working between what’s being developed in labs and elsewhere and then determining its usefulness for production operations and helping getting it deployed. Yet today, Heinemann admits, the Center for Laser Technology has more business with the medical industry than automotive. Even the commercial aircraft industry is showing interest in using lasers to replace rivets.
Heinemann notes that Germany, which has a population of about 82 million, has 11 universities that are producing laser engineers each and every year, who are finding jobs. The U.S., with a population of 296 million, has a couple of schools. Presumably, the students in both countries recognize that supply meets demand.
The use of lasers at places like Volkswagen—there are 70 meters of laser welding on the new Golf (up from five on the previous generation), a car that starts at about $16,000, not the Phaeton—and Mercedes is renown. Yet can the same be said for any U.S. production operation or vehicle? Yes, there are instances to be found, like at the Ford Chicago Assembly Plant, where the Five Hundred, Freestyle and Mercury Montego are having their roofs laser welded, but this is comparatively small.
Resistance welding holds the fore in most U.S. assembly plants. Spot welding. When asked to make a comparison from a purely technical point of view, Heinemann admits that if it is a comparison of welded spot to welded spot (I’m not letting him talk about the benefit of stitch welding that is easily accommodated by a laser, and which can provide a better joint), then they’re the same. A weld, in effect, is a weld. Maintenance is simpler for the conventional spot welding machine compared with a laser. And, he admits, that if it is a one-to-one comparison, then the laser costs a whole lot more. So this might seem like the story is over. The laser doesn’t (necessarily) provide a better weld, it is harder to maintain, and it costs more. But then Heinemann cites a study conducted of a hang-on body panel requiring from 30 to 50 welds. It would require 11 robot welding stations versus one laser welding station. Floor space. Flexibility. Efficiency. All those things that U.S. vehicle manufacturers talk about. Yet which they aren’t taking advantage of when it comes to the body shop. (One of the criticisms of lasers is that they are more difficult to maintain than a spot welder, which is undoubtedly true, especially since there are so many people who have a strong familiarity with working on spot welding equipment. This gets back, in part, to the lack of education and training. However, it should be pointed out that with developments like diode lasers, setup and maintenance are a whole lot easier than has been the case. But what is the extent to which this will be recognized by people who could benefit from the laser processing equipment, when lasers are not well represented in U.S. auto plants?)
There is another problem, however. It isn’t just a matter of manufacturing people when it comes to laser implementation. Heinemann points out, “Lasers can give you their full benefit when you design the product for the manufacturing process. If you just try to replace a resistance spot gun with a laser, it is very hard to make a cost case.” But these redesigns can be advantageous to the vehicle. For one thing, the flanges that are presently sized to accommodate spot welds can be downsized, with concomitant weight savings. Lasers can weld with one-sided access, rather than the two for a spot welding gun, so this means there are joining opportunities that can’t otherwise be realized.
It is probably difficult for people to want to change their approaches whether they’re designers, engineers, or manufacturers, especially now that the U.S. auto industry is under such tremendous competitive pressures. That said, there is probably no better time to change.
*“Applied” is the key word here inasmuch as in one case, a client that they worked with on selecting the proper laser, building the tooling, and proving out the process has actually left the system on site and a spin-off company from Fraunhofer is now running part production.