Femtosecond laser preparation and tensile behavior of taper/gully-free FCHs

The fabrication of film cooling holes (FCHs) in turbine blades constitutes a critical technology for resolving the inherent contradiction between thermal resistance requirements of hot-end components and turbine inlet temperature design in advanced aviation engines. The taper angle and wall quality of these holes directly determine the operational service life of blades. In comparison with alternative machining methodologies, femtosecond laser drilling demonstrates significant advantages, including minimal thermal impact zones, exceptional dimensional precision, and non-contact processing capabilities. To address the issue of insufficient hole wall quality associated with conventional machining approaches, this study proposes a coordinated control strategy for femtosecond laser beam overlap ratio and relative attitude angle. Initially, the influence of key process parameters, such as beam overlap ratio, on hole taper angle and wall quality was systematically investigated. Subsequently, to simultaneously achieve taper-free geometry and smooth inner walls, an optimization experiment was designed utilizing response surface methodology (RSM), and regression models for hole cone angle and inner wall roughness were developed and validated. The regression models exhibited robust predictive performance, with average prediction errors of 2.19 % for cone angle and 4.34 % for inner wall roughness, respectively. Ultimately, under the optimal parameter combinations determined by the model, taper-free and gully-free FCHs were fabricated, achieving a taper angle of <0.2° and an inner wall roughness of 0.34 μm in the central region. Based on these findings, tensile specimens were prepared and the tensile behavior of FCHs specimens was systematically characterized. Experimental results showed that gully-free holes consistently demonstrated superior toughness under varying temperature conditions. Notably, at 650 °C, the toughness and fracture elongation of gully-free specimens were 3.35 times and 6.67 times higher than those of gully specimens, respectively. Fractographic analysis indicated that gully specimens typically exhibited significant notch features and large-scale cracks on their fracture surfaces, while gully-free specimens displayed uniform fracture morphologies.