Design and Performance Evaluation of 777TD Hybrid Geometry for Additively Manufactured Meso-Scale Cooling Holes

To address overhang sagging in laser powder bed fusion (L-PBF) of meso-scale film-cooling holes, this study proposes a 777TD hybrid geometry combining the nominal 7° diffuser features of a 777 hole with a pointed, self-supporting roof. Ideal smooth-wall computational fluid dynamics (CFD) calculations compared cylindrical, teardrop, 777, and 777TD holes with diameters of 0.5–0.8 mm at a blowing ratio of 1.0 and a density ratio of 1.5. At D = 0.8 mm, the predicted area-averaged adiabatic film-cooling effectiveness was 0.3308 for 777TD and 0.3208 for 777, a nominal relative increase of 3.1%. Separately, Haynes 230 specimens were fabricated at build orientations of 30° and 60° and evaluated by X-ray computed tomography (CT). At D = 0.8 mm and a 30° build orientation, the in-tolerance proportion in the selected outlet region was 93% for 777TD and 76% for 777, an increase of 17 percentage points under a ±0.1D deviation criterion. These results support further evaluation of the hybrid geometry for meso-scale cooling hole manufacture. The CFD comparison and CT conformity assessment provide complementary evidence; the cooling performance of the as-built holes has not been experimentally validated.