Femtosecond Laser Machining of Irregularly Shaped Film Cooling Holes: The Influence of Defocus Distance

Film cooling holes are critical cooling structures that enhance the temperature-bearing capacity of turbine blades. Irregularly shaped holes, such as laidback fan-shaped holes (LFSHs), have been demonstrated to exhibit superior cooling performance compared to traditional circular holes. However, the complex shapes and structures pose significant challenges for femtosecond laser processing. Due to the extremely limited reports available, the dependency of femtosecond laser processing on the formation of irregularly shaped holes has not yet been well understood. In this paper, the fabrication process of the LFSH expansion sections and the influence of defocus distance are discussed in detail. By systematically characterizing the surface micro- and nanostructures, three-dimensional topography, and roughness of the expansion sections, the processing differences under positive defocus, zero defocus, and negative defocus are compared. The formation mechanisms of the micro-hole structures on the expansion sections under positive defocus and zero defocus are elaborated by incrementally increasing the number of processing cycles. Under negative defocus, only a smooth honeycomb-like structure forms on the surface of the expansion section, yielding the highest surface quality. Under different processing times, the roughness under negative defocus can be reduced by up to 84.8% and 95.8% compared to zero defocus and positive defocus, respectively. Furthermore, a correlation between the surface micro-hole structure and the edge structure of the hole walls is established.