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LASER 2015: Better welding with silicon laser mirrors

03 Jun 2015

Durable and responsive micro-mirrors precisely direct laser power into metal workpieces.

Silicon micro-mirrors can guide laser beams at extremely high speeds, allowing operators to dose heat into workpieces with absolute precision. But to date these mirrors have not been robust enough to be used for laser cutting and welding. Now a team from two Fraunhofer institutes has developed durable mirrors that operate fast enough to perform challenging cutting and welding tasks.

The new mirrors will be demonstrated at the LASER World of Photonics trade show from June 22 to 25, 2015 in Munich (Hall B3, Booth 341 and Hall A3, Booth 121).

In the automotive and aircraft manufacturing industries, a variety of materials such as aluminum and high-strength steels are used in order to keep vehicle weight down. However, these materials often present manufacturers with laser cutting and welding challenges. Laser processing systems have needed to be specially adapted to individual materials. In many cases, this requires special optics that are installed specifically for a particular process.

The laser mirrors, which were jointly developed by engineers at the Fraunhofer Institute for Silicon Technology (ISIT) in Itzehoe and the Fraunhofer Institute for Material and Beam Technology IWS, in Dresden, are said to offer greater flexibility in laser processing. Micromirrors etched from silicon are at the heart of the new flexible laser system.

These scanning MEMS mirrors deflect the laser beam and guide it precisely over the workpiece. Up to now, it has been possible to use these small mirrors only with laser outputs of a few milliwatts. This was enough for head-up displays on car windshields, but not for laser cutting and welding. Higher outputs would have melted the mirrors. Thanks to a new protective coating developed in the cooperative project and a special mounting, the mirrors are now able to withstand even laser powers in the kilowatt range – enough to process aluminum or sheet steel.

Greater flexibility

A major advantage of the thin MEMS mirrors is that they can be swiveled back and forth at extremely high speeds, reaching frequencies of up to 100,000 hertz. This allows the laser energy to be distributed much more effectively than with conventional laser systems, whose mirrors swivel at 1,000Hz. Lasers have a specific energy profile in their focal point and always input the same amount of energy. If the mirror swivels slowly, the energy cannot be distributed and dosed as effectively at the weld. Dr.-Ing. Andreas Wetzig, specialist in the laser ablation and cutting department at Fraunhofer IWS said, “rapid oscillation of the laser beam allows us to distribute the heat and adjust it to the respective processing task much more effectively.”

“We can weld together aluminum and copper, for example, and use the heat input to precisely control which metal heats up more and thus makes up more of the melt,” said Wetzig. The mirrors also make it easier to weld aluminum alloys alone. At present, aluminum weld seams are often porous, because certain substances outgas from the alloys and form bubbles during the welding process. With the micromirror, heat input can be controlled so that the melt remains liquid until the substances have completely outgassed.

About the Author

Matthew Peach is a contributing editor to optics.org.

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