The Effects of Static- and Flowing-Water-Assisted Methods on the Quality of Femtosecond Laser Drilling of Thermal-Barrier-Coated Superalloys

Under high fluence and a high repetition rate, femtosecond laser drilling still produces defects due to heat accumulation. In order to suppress these defects, this study conducted research on water-assisted femtosecond laser drilling. This study focused on the impact of two different water-assisted methods, static-water-based and flowing-water-based approaches, on the quality of microholes made using layer-by-layer helical drilling with a femtosecond laser in thermal-barrier-coated superalloys. Furthermore, the effects of single-pulse laser energy on the hole entrance/exit diameter, taper angle, sidewall morphology, sidewall roughness, and sidewall oxygen content in the two water environments were compared and analyzed. Water-based-assisted laser drilling is an auxiliary method where the lower surface of the workpiece is placed in water while the upper surface remains in the air. On the other hand, the water flows horizontally in the flowing-water-based method. The experimental results demonstrate that both static- and flowing-water-based methods can significantly improve the quality of femtosecond laser drilling. Notably, the improvement effect was more pronounced with the flowing-water-based method. At a laser pulse energy of 50 μJ, the hole taper angle in the flowing-water environment was reduced by 38.80% compared with that in the air. With flowing-water-based assistance, the hole sidewall roughness was lower and the melt was less. Flowing water was better at carrying away the debris and heat generated by processing. The oxygen content of the hole sidewalls decreased significantly in both kinds of water-assisted environments. The experimental results provide a valuable reference for optimizing water-assisted femtosecond laser drilling.