Laser increases fatigue life
How laser polishing influences the fatigue behavior of LPBF components made of Inconel 718, a well-known aerospace material, was investigated by Professor Yingchun Guan from Beihang University in Beijing. She pointed to an older study presented at LaP 2020 using turbine components in which laser polishing reduced Ra roughness from more than 10 to less than 0.1 µm. New analyses have now shown that the mechanical properties have also improved: Laser polishing increases the fatigue life at a mechanical stress of 840 MPa by 15 to 20 percent compared with the values for milled surfaces; at 500 to 600 MPa, it is just as high.
Special materials such as advanced high-strength steels are interesting for lightweight construction in the automotive sector since they have high strength (>1000 MPa). However, microdefects often occur at the edge during shear cutting or laser cutting, making the components susceptible to edge cracking. 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 smooths it. In the experiment, a 4 kW CW diode laser successfully processed 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 significantly improved performance after laser treatment. This process makes it possible to improve forming capacity by more than 200% 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 Fraunhofer ILT reported on German-Canadian cooperation in this field. In order to automate the setup of a polishing process, they integrated a white light interferometer (WLI), which detects surface structures with high precision, into a laser polishing machine. Also as part of the collaboration, a high-speed thermographic camera was integrated into another machine. The real-time data from the camera is used to close the control loop and adjust the parameters. Visibly fascinated, the LaP guests watched the video of a high-speed camera which, at 42,000 frames per second, visualized the solidification of liquid hot-work tool steel (1.2343) in the melt pool.
The thermal camera was used by Daniel Beyfuss, a scientist at the University of Western Ontario in 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. An important result is the fundamental role of the thermodynamic equilibrium between applied laser power and its conversion into remelting processes because it significantly influences the 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. They are focusing mainly on high-speed X-ray imaging in the synchrotron at the renowned Argonne National Laboratory (ANL), an approach that offered interesting insights into melt-pool dynamics during laser polishing. For example, the research team observed how melt-pool oscillations form and, thus, affect the keyhole.
Satisfied LaP initiator and moderator Dr. Willenborg from Fraunhofer ILT stated after the virtual LaP: “Sixteen presentations covered various aspects from classical glass polishing to melt-pool analysis in a synchrotron over two days. The mixture really makes the difference: This is probably why almost all 70 participants were present online throughout the entire conference. Despite the virtual success, however, I look forward to seeing the international laser polishing community again at the sixth LaP, which will then hopefully take place live in Aachen again in 2024!”