Improved Lasers and Techniques Benefit Materials Processing

HANK HOGAN, CONTRIBUTING EDITOR

For laser materials processing systems, power levels are up, costs are down, and there are innovations in beam delivery and control. These developments are expanding the use of lasers in welding, cutting, surface treatment, and other applications. But there are still challenges to overcome, such as devising the best integrated solution or dealing with difficult materials.

Several recent examples illustrate these trends and offer solutions to problems. Some of the benefits of these laser system advancements are having the capabilities to handle new tasks, achieve faster processing, and work with new materials. What’s more, for further progress, fundamental research promises information needed about laser-matter interactions.

An ultrashort-pulse laser structures a semifinished steel product, a new materials processing technique made commercially practical by advancements in laser technology. Courtesy of Fraunhofer ILT, Aachen, Germany/Volker Lannert.

In general, throughput and cost per processed part are critical in laser materials processing. But they can’t override other considerations, according to Marco Mendes, director of applications in the materials processing systems division at IPG Photonics of Oxford, Mass.

“Improved throughput is important but only if good quality parts are produced, the assumption being that every part matters,” he said.

Mendes added that quality requirements show up in welding, cutting, and additive processes such as 3D printing and micromachining. Achieving higher quality often requires better control, which may demand enhanced process monitoring. During welding, for example, inline measurement of keyhole depth, joint position, surface quality, and other parameters provides the necessary data to fine-tune the system and process as needed, he said.

IPG makes fiber lasers, and its technology enables a range of advancements, according to Mendes. For instance, adjustable-mode lasers with a central core and outer ring fiber allow users to make on-the-fly changes to the combined beam. A laser with two side 500-W beams and one central 4000-W beam improves processing speed when brazing steel, he said. The offset side spots remove contaminants from the surface before the main beam melts the brazing wire used to solder a joint. Mendes said this approach allows for improved process stability and higher throughput.

A laser strikes a metal target, hardening the surface by inducing shockwaves in a process known as peening. Courtesy of Hamamatsu.

Another innovation that improves throughput and process control is dynamic beam control via high-speed galvanometric-based scanners and high-power sources. Called wobble heads, the technology allows fast beam oscillations with controllable shape, amplitude, and frequency of the beam path on the target. This technique can improve quality, repeatability, and throughput in welding, drilling, and cutting.

Post-laser structuring and selective polishing of a plastic molding produces an airbag cap with incorporated gloss. Courtesy of Fraunhofer ILT, Aachen, Germany.

In addition to beam configuration and control, there is another knob to turn: power. “Higher average power allows for increased throughput, but it also allows for new applications, such as allowing cutting thicker materials or welding deeper,” Mendes said. “Combining high average power with higher peak power achieves such benefits as faster and cleaner piercing.”

Record-breaking systems

Hitting higher power levels requires finding ways to mitigate the effects of the increased energy on the laser itself. In April, Hamamatsu Photonics of Hamamatsu City, Japan, announced it had developed a way to efficiently cool the gain medium in a laser diode-pumped system by using high-pressure cryogenic helium gas. The technology allowed Hamamatsu to produce what the company announced was a world-record 117-J laser diode-pumped system. Laser diodes optically pumped the gain medium, which was kept cool by a flow of −100 °C helium, resulting in 40-ns-wide pulses of 1030-nm wavelength output.

Hitting higher power levels requires finding ways to mitigate the effects of the increased energy on the laser itself.

Peening, the hardening of a metal target surface by laser-induced shockwaves, could be an application of the new laser. Another might be surface forming or paint removal.

“Many kinds of pulsed laser processing will see improved throughput by large-area treatments possible with a high-energy laser,” said Toshiyuki Kawashima, deputy general manager of the Central Research Laboratory. He noted that the laser processing project is supported by Japan’s NEDO, or New Energy and Industrial Technology Development organization.

Plans call for reaching a 250-J output, a nearly 50% increase, within two years, he said. This will be one stop on the way to demonstrating a kilojoule class of laser diode-pumped solid-state systems.

Hamamatsu expects to collaborate with machining system and integration companies to produce laser processing systems based on the technology, according to Kawashima. For this, the laser and the system need to be highly adjustable because process optimization often requires altering pulse width, wavelength, energy, or other parameters.

A promising laser-materials processing advancement comes from the other side of the world. Heriot-Watt University of Edinburgh is working with Oxford Lasers of Didcot, England, and other commercial partners to move from proof of principle to a commercially viable metal-glass laser welding system. Years of fundamental research shows the key is to create a melt region that is very small, said Duncan Hand, Heriot-Watt professor of applied photonics.

Picosecond IR laser pulses are used to generate a microplasma, leading to a small melt region and a minimal total heat input. “It is a balance of very fast processes and also some thermal accumulation in order to obtain a melt region of the right dimensions,” Hand said.

The partners have demonstrated welding of fused quartz, borosilicate glass, and sapphire to aluminum, titanium, and stainless steel. The challenge in the past has been that differences in thermal expansion shattered the glass. So adhesives are often used and these materials can be messy, or they can outgas or result in sagging.

Monitoring of key laser welding parameters during the process can improve results. Courtesy of IPG.

An initial application of the new glass-metal welding technology is mounting optical components inside laser systems, Hand said. There the requirements for surface finish are quite stringent, and there is a need to hold components in rigid alignment.

Looking to the future, Hand said average power ultimately limits processing rates. Ultrashort-pulse-width lasers — like the one in the Heriot-Watt system — are therefore more expensive on a per-watt basis than fiber lasers. However, he said, the cost for such lasers continues to fall, opening up more applications.

Surface texturing

Another example of increased power and precision in laser materials processing can be seen in a collaborative project headed by Andreas Brenner of the Fraunhofer Institute for Laser Technology of Aachen, Germany. The collaboration involved five industrial partners and three institutions and universities, with Volkswagen as the end user that worked with the others in the project. The automaker, which had the application targeted by the group and was a driving force behind the work, has already started to use the technology in its manufacturing.

A car’s dashboard and steering wheel often have decorative trim. The trim is created by structuring tools that emboss the plastic or other material. Making these tools takes time, in part because their surfaces must be finely textured.

Texturing this type of surface is well suited to ultrashort-pulsed lasers, those with a pulse duration from about 0.3 to <10 ps. The short pulse duration means the heat zone is small, which allows the minimum feature size to also be miniscule. But doing this laser texturing over a large area, such as would be needed on a structuring tool, is a challenge.

The project participants overcame this difficulty through the development of two key technologies, according to Brenner. The first was a beam steering system from industrial partner SCANLAB of Germany. This technology helped eliminate dead time that would otherwise be needed to move the laser or part from one point to another. The second technology was a high-power 150-W ultrashort-pulsed laser from Amphos, an Aachen-based member of the TRUMPF Group. The very short pulse duration allowed the system to texture the tool to a surface roughness of less than 0.5 µm, while the power level ensured a speedy ablation rate and fast processing.

The system cut tool-making time by two-thirds as compared to the traditional approach. While developed for one industry, the technology could benefit the aviation industry or any application where embossing with a textured tool surface is performed, said Brenner.

He noted that the laser can do more than texture a surface, with selective polishing or the cleaning of an oxide layer being just two additional applications. “We demonstrated that the ultrafast laser is a multifunctional tool,” Brenner said.

Finally, these developments build upon and could benefit from basic research about laser and matter interactions. So Brian Simonds, a physicist at NIST in Boulder, Colo., is working to do just that, thereby potentially handing industry a way to know just what laser to use — at what power, wavelength, and pulse width — to achieve a desired effect.

Simonds said laser welding, the area of investigation, is currently more art than science. “You have to empirically turn all these knobs to figure out the process because that’s all you can do at the moment.”

He is therefore taking measurements of what happens to NIST-standard stainless steel that is subjected to a 10-ms IR laser pulse. NIST has the best laser energy measurement capability in the world, he said. The effect on the metal is also measured, a nontrivial problem because the metal goes from highly reflective to near total absorption as it melts. Ultimately, this investigation will provide the data needed to rigorously validate laser welding models, leading perhaps to better, more accurate simulations and less tweaking to predict the processes needed to create successful welds.

Improved models and simulations could prove useful in other areas, such as metal-based additive manufacturing. Here, a laser moves over a bed of metal powder, sintering or fusing the powder as it does so. In this way, the laser builds up the desired part layer by layer. Simonds’ work could benefit models of this process.

“It’s like welding, but all you have left over is the weld bead,” he said.