Pushing scan speed limits in ultrashort pulse laser polishing of steel

Laser material processing with ultrashort pulsed (USP) lasers has become well-established in micromachining due to its high precision and flexibility. Its prominence has grown further in recent years, driven by advances in the average power output of USP laser beam sources. However, the surface roughness resulting from surface ablation is often insufficient for the target application. Thus, a subsequent polishing step is required. In contrast to electropolishing or conventional laser polishing using long pulsed or continuous-wave operation, USP laser polishing can be performed within the same setup as the micromachining process, eliminating lengthy setup times and enabling a fully integrated digital photonic process chain. Unlike polishing with longer laser pulses (ns-µs), material is not melted by a single USP pulse, but due to heat accumulation over several pulses. In this study, the approach of polishing with a laser seed frequency of 50 MHz is chosen. Previous studies in this regime have shown a shift of the process window toward higher fluences with increasing scan speed. However, these studies were constrained by the limited USP laser power available at the time. The power constraints of these earlier studies can be overcome by the advent of Trumpf’s new 1 kW USP laser TruMicro 9000, offering up to 20 µJ pulse energy at 50 MHz, enabling the exploration of significantly higher scan speeds, which require the application of increased pulse energies. Using this laser beam source in combination with an appropriate system technology, this study demonstrates that the shift toward higher fluences with increasing scan speeds continues even beyond the ablation threshold while maintaining comparable polishing results.