July 29, 2014
Engineers at EWI have developed laser welding optics tailored for high-power applications.
High-powered solid-state lasers open up a world of possibility for the welding engineer. When fiber lasers began entering the metal manufacturing market, their relatively straightforward scalability piqued interest. Add another module and you get higher power: 2, 6, 10, 20 kW—the possibilities keep climbing. Scaling up requires much more than just a plug-and-play upgrade, but the concept behind it remains simple.
Certain laser beam welding applications can use a lot of laser power, and Stan Ream and his team at EWI came across one several years ago. The R&D organization based in Columbus, Ohio, had a client that needed to produce a single-pass, full-penetration, autogenous weld in a butt joint of stainless steel. Laser beam welding would have no problem with such a joint, but in this case, it happened to be 0.393 inch thick and 12 feet long, quite different from a test coupon. Such a large weld required 13 kW of laser power. Finding a laser with such power wasn’t a problem, but finding the right optics was.
“The focusing optic we were using had a transparent window on it, and we didn’t trust it,” said Ream, EWI’s laser technology leader, “because welding is a dirty process.”
Traditionally, laser light passes through a combination of reflective (mirrors) and transmissive (lenses) optics, then through a sacrificial window, or cover glass, to the work area. The cover glass protects the sensitive optics from weld fume particulate, spatter, and other unavoidable debris.
This works well at lower powers and for shorter weld lengths. But when welding at high power for relatively long, uninterrupted stretches, the total heat absorbed by transmissive optics can become problematic. A minute particle landing at the wrong place on the cover glass (that is, near or directly in the beam’s path) can absorb the laser’s 1-micron wavelength quite handily. The resulting heat buildup causes thermal lensing, or thermal shift, degrading the focal spot and changing the focus location. “This heat causes mechanical distortion of the optic as well as a change of the index of refraction,” said Ream, adding that eventually the beam fails to produce a weld and produces excessive spatter. Eventually the cover glass breaks altogether.
EWI researchers were left with a difficult equation: High laser power + Transmitting optics + Contamination = Focal shift. The full-penetration weld at the desired speed required high laser power, and such an extended beam-on period can produce a lot of contaminants. That left one conclusion: The transmitting optics should be eliminated.
A laser welding focusing optic could be built without them by using reflective optics—that’s all well and good, but what about the cover glass? Could it be removed? Could the job of that cover glass, as a protective barrier, be accomplished differently?
From all these questions (and more) came EWI’s recently patented High-Power Reflective Focusing Optics (HPRFO) system. It replaces the cover glass with what Ream and his team call the “aero window,” an arrangement that protects optics not with a cover glass but with a constant, high-velocity flow of an inert purge gas, such as nitrogen, sent through a 1.5-mm-diameter aperture. This helps deflect debris emanating from the welding process and eliminates the need for a cover glass. What’s left is a system consisting only of reflective optics, positioned in such a way to dissipate heat and continue reflecting a quality beam, even if weld debris fights its way past an air knife and lands on the final mirror’s surface.
Complexities abounded, of course, and researchers had to do more than just swap out a cover glass with a blast of nitrogen purge gas. First, the optics had to withstand the harshest laser beam welding environment. The mirrors’ geometry—off-axis asymmetric aspheres provided by II-VI Inc.—had to be extremely precise. Second, researchers had to find a way to control the gas flow precisely.
To this end, they found a way to control the gas flow so it protects the optics but doesn’t cause unwanted weld-pool turbulence. The aperture is set between two copper mirrors, the only optics the system requires. Light from the fiber is reflected off the first mirror to go through the aero window 5.9 in. away, and then travels another 9.8 in. to a second mirror. That extra distance allows the beam to expand; this, combined with the second mirror’s angled position (creating a large surface area), lowers the energy density. From there the light travels downward toward the workpiece.
“The second mirror, because it’s large and water-cooled, is really difficult to hurt,” Ream said. “Even if it has spatter balls stuck onto it, it keeps on welding.”
In subsequent tests, EWI welding engineers achieved some eye-opening results. For an application involving 12 panels of 12-ft.-long square butt joints—for a total of 144 linear ft. of welding—in 0.375-in.-thick INCONEL® 718, the optics handled 13 kW of fiber laser power for 108 seconds of continuous beam-on time for each panel. All panels passed applicable welding codes, and the optics performed well, despite the fact that the mirror itself had some spatter on it and was never cleaned until after the operation.
Ream emphasized that the HPRFO isn’t designed to replace conventional laser beam welding optics, which can work extremely well for lower-power applications. Instead, it is tailored for very high-power fiber or disk laser welding applications. In fact, EWI recently installed a new 20-kW IPG Photonics fiber laser to continue development of the HPRFO, which, like the laser, has the potential for some surprising scalability.
“This is a totally scalable concept,” Ream said. “We’re now configuring it to be able to take on power higher than 20 kW by changing angles of the mirrors and altering some cooling techniques. This concept could be adapted to become a 100-kW laser welding focusing optic.”
Images courtesy of EWI, 250 Arthur E. Adams Drive, Columbus, OH 43221, 614-688-5000, www.ewi.org.
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