Single-track investigation of rapid solidification conditions of Hastelloy-X Ni-based superalloy in laser powder-bed fusion and applicability of columnar equiaxed transition criteria

To understand the applicability of the columnar-equiaxed transition (CET) criteria to the rapid cooling conditions unique to the powder-bed fusion (PBF) type additive manufacturing (AM) process, we investigated relationships between the microstructure and the solidification conditions of Hastelloy-X (HX) Ni-based superalloy. Experimental observation revealed columnar crystals are formed in the single-track melt region, and the corresponding computational thermal-fluid dynamics simulations suggest the solidification conditions are mostly out of the range of the solidification conditions conventionally considered for the microstructure controlling: temperature gradient G = 1.0 × 105–2.0 × 108 K m−1 and solidification rate R = 1.0 × 10−3–4.0 × 100 m s−1. Combining the experimental and computational data, the microstructural morphology of HX was within the range predicted to be columnar crystals in terms of the CET criterion and the parameters obtained by the experiment under conventional solidification conditions. This study sheds light on the applicability of the CET criteria to the PBF process and facilitates the microstructure control of PBF-fabricated structural alloys.