Lifei Wang

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.

Abrasive flow machining (AFM) is an effective finishing process for complex internal surfaces, particularly cavities, intersecting holes, and micro-channels that are difficult to access using conventional tools. However, when low-viscosity abrasive media is used (here defined, relative to conventional putty-like viscoelastic AFM carriers (with apparent viscosities of 10³–10⁵ mPa·s), as a water-based slurry with an apparent viscosity below 300 mPa·s over the operating shear-rate range), unfavorable flow conditions during the initial polishing stage can induce local over-polishing, erosion depressions, stepped patterns, and cavitation pits, resulting in non-uniform surface quality. The relationship between these flow behaviors and polishing defects remains insufficiently understood. To address this issue, this study investigates the AFM process applied to turbine blade film cooling holes through combined experimental and numerical approaches. The observed defects include erosion depressions, stepped surface patterns, and cavitation pits. The effects of abrasive injection pressure, flow velocity, hole geometry, abrasive viscosity, and particle size on defect formation are systematically examined. The results show that the initial abrasive filling level strongly affects defect distribution by altering the evolution of shear fields and void regions within the hole. Experimentally, at high Reynolds numbers (Re > 2 × 10⁴), intensified local shear and cavitation promote defect formation, while a moderate inclination angle (45–60°) and a higher aspect ratio (>8) are favorable for polishing uniformity. Complementary numerical simulations further indicate that smaller abrasive particles (<5 μm) and a moderate abrasive viscosity (~60 mPa·s) are predicted to improve polishing uniformity. This study clarifies the fluid-dynamic origin of polishing defects in film cooling holes and provides process guidance for suppressing local over-polishing, cavitation, and uneven material removal.