Effect of Shaped Film Cooling Hole Manufacturing Defects on the High-Cycle Fatigue Behavior of a Ni-Based Single-Crystal Superalloy

The complex geometrical configuration of shaped film cooling holes (FCHs) enhances the cooling efficiency of turbine blades, while the stress concentration at the shaped FCH edge and the manufacturing defects usually leads to high-cycle fatigue (HCF) failure under the service conditions. In this study, HCF tests at 900 degrees C were conducted on DD6 single-crystal superalloy containing dustpan and dovetail FCHs. The effect of shaped FCHs and manufacturing defects on the HCF strength was investigated, and the related HCF failure mechanism was analyzed using SEM, EDS, and EBSD. The results show that manufacturing defects including pores, recast layer, and polycrystalline microstructure region exist around the shaped FCH edge. The stress concentration of defective pores promotes the plastic deformation and oxidation at high temperatures, leading to crack initiation. The oxide layer thickness in the high stress area of the dovetail FCH specimen is 3.44 mu m thicker than that of the dustpan specimen, and correspondingly the fatigue strength of the dovetail FCH specimen is 13.96 MPa lower than that of the dustpan specimen (304.93 MPa vs. 318.89 MPa). The fatigue strength of these two kinds of FCH specimens is lower than that of the traditional cylindrical FCHs due to the existence of the manufacturing defects, indicating the necessity to consider the effect of manufacturing defects in the evaluation of the HCF strength of components with the shaped FCHs.