The results of the English-language expert meeting of the Aachen-based Fraunhofer Institute for Laser Technology ILT are impressive. Once again, more than 70 experts from all over the world met at the "5th Conference on Laser Polishing LaP" in October 2022 to virtually discuss the latest developments in laser-based alternatives to the final polishing of glass, metal and plastic components, among others. The focus was on laser deburring and laser polishing as well as related topics such as metrology and process control.
The researchers from all over the world rely on similar recipes as the traditional polishing community, which is increasingly moving away from analogue grinding and is now also focusing on digitalization with Industry 4.0, digital twins and AI.
All of these topics are on the agenda of the international "Conference on Laser Polishing LaP", which Dr. Edgar Willenborg, Head of the Polishing Group at Fraunhofer ILT, has hosted every two years since 2014. Due to the uncertain travel situation for participants from Asia and America, LaP 2022 was once again held as an online conference.
12 October 2022: Glass and plastics in our sights
It's all in the mix: On the first day, LaP number 5 was primarily aimed at companies that produce optical surfaces and polish glass and plastics, for example.
The event kicked off with a presentation from Jena, one of the international centers for glass and optics production: Anett Jahn, Managing Director of ShapeFab, and Thomas Schmidt, Laboratory Engineer at the Institute of Joining Technology and Materials Testing (ifw), encouraged LaP participants to get started with laser polishing. ShapeFab and ifw use a combination of CNC milling and laser polishing to polish optical surfaces measuring up to 250 mm x 400 mm. Any contour and even complex free-form surfaces can be processed reliably, cost-effectively and quickly using this hybrid method.
A large, complex demonstrator component was produced in a total of around seven hours, of which only around 10 minutes were spent on laser polishing. Conventionally, the mechanical polishing process would have taken up to ten hours. In the next step, the company and the institute now want to further reduce the CNC processing time.
Process monitoring using pyrometry plays an important role in Jena. Manuel Jung from Fraunhofer ILT, who presented the development of a closed, low-noise control loop for laser polishing of optics at LaP, also relies on digital monitoring with this measurement technology. The research associate from Willenborg's team recommended the use of software with which laser polishing can be carried out reliably with a maximum temperature deviation of less than 0.5 percent. This enabled the Aachen team to reduce the waviness (MSFE) by a factor of 10. The scientist hopes to achieve further improvements by developing an even more thermally stable and low-noise control process, which no longer uses a pyrometer but a thermal camera and sophisticated statistical methods.
Producing free-form surfaces with the ultrashort pulse laser
Dr. Jie Qiao, Associate Professor at the Rochester Institute of Technology, USA, reported on how femtosecond lasers can be used to correct the shape of glass components with nm height resolution. The secret of success lies in a dynamic mathematical model that can be used to predict ablation and temperature development. The energy density serves as a measure for the further scaling of the process. The process demonstrated on flat samples now paves the way for the high-precision production of even more complex surfaces.
Emrah Uluz, a research associate from Fraunhofer ILT at Laser Beam Figuring LBF, also had his sights set on the sub-nano range. His presentation focused on reducing the waviness of polished quartz glass surfaces through nanometer-precise ablation. A key factor here is the use of a highly stable laser that operates at a constant laser power (standard deviation: ≈ 0.1 percent).
With conventional mechanical polishing of glass, there are always smaller and larger scratches that impair the surface quality. According to Dr. Kerstin Götze, group leader of laser polishing at the Ernst Abbe University of Applied Sciences in Jena, 99.9 percent of these scratches can be reliably removed with the CO2 laser - especially when it works at a low feed rate. The process has proven particularly effective in improving the quality of curved and structured surfaces.
Green laser removes "subsurface damage"
Professor Jiwang Yan from Keio University in Yokohama (Japan) observed thermal effects during conventional mechanical polishing of single-crystal silicon wafers, which led to damage directly under the surface. The Japanese experts succeeded in repairing the "subsurface damage" without material removal and environmental pollution by remelting with green laser light. The remelted layer grew as a single crystal onto the underlying material. However, this was only possible after simulating the molecular structure. Following the successful experiments, various laser systems were developed in Yokohama that are not only suitable for repairing surfaces, but also for creating functional surfaces.
An interesting phenomenon inspired scientist Dr. Bowei Luo from the Shenzhen Institute of Information Technology in China to combine cold and hot laser polishing of silicon carbide. Polishing this ceramic material with a UV laser reduces the surface roughness to 1.4 µm at a laser power of 15 watts. However, the polishing result can be reduced to 1.082 µm by preheating the ceramic to 1400 °C with an IR laser. However, this also requires the "cold" UV laser process to be adapted and optimized for the "hot" IR process. According to Dr. Luo, the shape of the laser beam plays a key role here: a uniform tophat laser beam with a diameter of 0.32 to 0.54 mm, for example, can effectively reduce thermal shock, particularly at the edges of the polishing area.
The shape of the laser beam also concerns scientist Karsten Braun from Fraunhofer ILT when developing process strategies for 3D-printed plastic parts, whose roughness varies from 14 µm (PA12) to 42 µm (PEEK) depending on the material. When polishing with a 120 WCO2 laser in the medium IR range (wavelength: 10,600 nm), Braun relies on fast quasi-tophat scanning (5-10 m/s) from a large distance (100 to 1000 mm). The scanning process is temperature-controlled up to 20 times. Depending on the material, the roughness Sa after laser polishing is 0.8 to 0.25 µm.
The second day of the conference was primarily aimed at companies that process conventionally manufactured or 3D-printed metal components. A very challenging task was presented by Dr. Safak Nelsi. The Assistant Professor from OSTİM Teknik Üniversitesi in Ankara (Turkey) reported on the laser polishing of an aerospace component (Ti48AL2Cr2Nb) produced using the electron beam melting process (Arcam A2X EBM). In this joint project with an industrial partner, it was possible to reduce the often very rough surfaces of an additively manufactured component by around 95 percent to up to 1.6 µm. A 600 W IPG fiber laser (wavelength: 1070 nm) was used, which polished the surface at a scanning speed of 220 mm/s. Challenges during the tests were surface cracks, waviness and oxidation.
Laser polishing in keyhole and conduction mode
Prof. Dr. Frank E. Pfefferkorn from the University of Wisconsin-Madison together with the Bremen Institute for Applied Beam Technology BIAS also dealt with the typical problems of components produced additively in the powder bed using the LPBF process: LPBF components often have poor surface quality due to partially adhering particles, layering effects and balling. In experiments with a component made of cobalt chrome (Stellite 21), the German-American team discovered that the laser polishing in conduction mode (CM) that is usually used has its limits here. In contrast, the best results in terms of roughness and waviness were achieved by laser polishing in keyhole mode (KM), with a CM process being used again for the final fine polishing step.
The roughness of metal components produced using laser-based 3D printing processes in a powder bed (Laser Powder Bed Fusion LPBF) can be reduced by laser polishing. Waviness is a problem, as Laura Kreinest, an employee at the Fraunhofer ILT, discovered when laser polishing an LPBF component made of 1.2343 tool steel. Even after 16-fold laser polishing, the waviness Wa was still around 1 µm. The scientist solved the problem with the "WaveShape" process developed at the institute, which creates the inverse structure of the undesired waviness on the metal surface by laser remelting and thus reduces the waviness.
Laser increases fatigue life
Professor Yingchun Guan from Beihang University in Beijing investigated the influence of laser polishing of LPBF components made of Inconel 718, a well-known material used in aviation, on fatigue behavior. She referred to an older study with turbine components presented at LaP 2020, in which the roughness Ra could be reduced from more than 10 to less than 0.1 µm by laser polishing. New analyses have now shown that the strength behavior has also improved: laser polishing increases the fatigue life at a mechanical stress of 840 MPa by 15 to 20 percent compared to the values for milled surfaces, and at 500 to 600 MPa it is just as high.
Special materials are AHSS steels (Advanced High Strength Steel), which are interesting for lightweight construction in the automotive sector due to their high strength (>1000 MPa). However, micro-defects occur at the edge during shear cutting or laser cutting, which makes the components susceptible to edge cracks. Scientist Dongsong Li from the Institute of Ferrous Metallurgy (RWTH Aachen University) presented a process for deburring and edge rounding using laser radiation, which was developed in collaboration with Fraunhofer ILT. The laser melts the edge, removes the micro-defects and smoothes them. In the test, a 4 KW CW diode laser was used to process a 1.5 mm thick sheet of high-strength dual-phase steel (strength: 1000 MPa) at 3.6 m/min. Hole expansion tests and diabolo tests show a significantly improved performance after laser treatment. This enables more than 200% greater forming before the first edge cracks appear.
Process monitoring is playing an increasingly important role in laser polishing: Dr. Evgueni V. Bordatchev, team leader at the National Research Council of Canada in London (Ontario) and Sven Linden from the Fraunhofer ILT reported on German-Canadian cooperation in this field, including with the Fraunhofer ILT. In order to automate the set-up of a polishing process, a white light interferometer (WLI) was integrated into a laser polishing machine, among other things, which records surface structures with high precision. A high-speed thermographic camera was also integrated into another machine as part of the collaboration. The real-time data from the camera is used to close the control loop and adjust the parameters. The LaP guests were visibly fascinated as they watched the video from a high-speed camera, which visualized the solidification of liquid hot-work tool steel (1.2343) in the molten bath at 42,000 frames per second.
The thermal camera was used by Daniel Beyfuss, a scientist at the University of Western Ontario, London (Canada), to monitor laser remelting (LRM) processes. The Canadian used coaxial optical measurement to analyze the effects of thermodynamic instabilities on the LRM process. One important result is the key role played by the thermodynamic equilibrium between the laser power supplied and its conversion into remelting processes, as this has a significant influence on process stability.
Melt pool analysis in the synchrotron
US scientist Patrick J. Faue from the University of Wisconsin-Madison reported on a research project with the Bremen Institute for Applied Beam Technology BIAS. The focus was on high-speed X-ray imaging in the synchrotron of the renowned Argonne National Laboratory (ANL), which led to interesting insights into melt pool dynamics during laser polishing. The research team observed, for example, how melt pool oscillations build up and thus also influence the keyhole.
After the virtual LaP, LaP initiator and moderator Dr. Willenborg from Fraunhofer ILT stated with satisfaction: "Over two days, 16 presentations covered a wide range of aspects from classic glass polishing to melt pool analysis in the synchrotron. It's the mix that makes the difference: That's probably why almost all 70 participants were present online throughout. Despite the virtual success, I look forward to seeing the international laser polishing community again at the sixth LaP, which will hopefully take place live in Aachen again in 2024!"
