Optimization of laidback fan-shaped holes machined by femtosecond laser

Film cooling holes are crucial for reducing the surface temperature of aero-engine turbine blades. As the inlet temperature of aero-engine turbines increases, traditional circular holes fail to meet the required cooling performance. Consequently, non-circular film cooling holes, such as laidback fan-shaped holes (LFSHs), have been developed. However, the complex geometry of LFSHs and the machining difficulties associated with high-temperature superalloys pose significant challenges. Currently, there is limited research on fabricating LFSHs. In this paper, we systematically studied the processing method, shape evolution, hole wall quality and geometry profile optimization of LFSHs for the first time. Based on the characteristics of LFSHs, we proposed a segmented processing method: the diffuser section is fabricated using a layer-by-layer scanning method, while the cylindrical section is drilled using a concentric circle scanning method. We first investigated the shape evolution of the diffuser section under the layer-by-layer scanning method. Subsequently, we studied the effects of key process parameters on hole wall quality and morphology. To address the issue of poor hole wall quality of the diffuser section, we designed modification experiments using response surface methodology (RSM), which significantly improved the hole wall quality. Additionally, we established and verified regression models for both the forward expansion angle and hole wall roughness. To resolve the issues of "waist" and taper in the cylindrical section, we introduced an attitude adjusting method to fabricate circular holes without "waist" or taper. Finally, by combining the hole wall quality and geometry profile optimization schemes, we achieved high-quality LFSHs with smooth walls in the diffuser section and no ''waist'' or taper in the cylindrical section. The machining results indicated that there was no oxide layer on the hole wall, with the roughness of the diffuser section only 0.497 μm. No deformations occurred in γ and γ' phases, and no obvious dislocations were found in the hole wall of diffuser section. Furthermore, the regression models for the forward expansion angle and diffuser section roughness demonstrated high prediction accuracy, with average errors of 0.25 % and 5.22 %, respectively.