Optimization of femtosecond laser processing technology for film cooling hole in nickel-based superalloy based on multi-step drilling and milling methods

This study investigates femtosecond laser helical drilling and milling of film cooling holes in nickel-based superalloy using a self-built optical platform. The multistep drilling method based on a single laser spot was proposed to fabricate small holes with a diameter of approximately 0.4 mm. Comparative experiments with conventional one-step drilling demonstrated the advantages of the multistep method: improved hole roundness, eliminated spatter and attachments, reduced ablation color zones, and yielded hole surfaces with continuous edges and no obvious recast layer, among other benefits. Key quality determinants include clearance hole diameter, single-step material removal thickness, and laser repetition frequency. When the laser repetition frequency is 100 kHz, the clearance hole diameter is 100 μm, and the material removal thickness of the final step is approximately 20 μm (equal to the laser spot diameter), the optimal microhole processing quality can be achieved. The multistep milling method based on a circular laser beam was proposed for milling large holes. The two-step method of layer-by-layer interpolation can achieve nearly the same processing quality as multistep drilling for a small hole. The interpolation trajectory and cutting width play a key role in the processing quality of the porthole and sidewall. The expansion allowance is set to slightly less than the diameter of the focus spot, approximately 0.02 mm, which can achieve dual improvements in processing efficiency and quality in the two-step method. The proposed methods and parameters for improving the quality of porthole and sidewall have important guiding significance for improving the service performance of turbine blades.