Microstructure control of Hastelloy X by geometry-induced elevation of sample temperature during a laser powder bed fusion process

Abstract

In this study, the solidification behaviors of Hastelloy X during laser powder bed fusion (LPBF) were deliberately changed by fabricating a sample with constricted geometry. In-situ temperature measurement by a high-speed thermographic camera and part-scale thermal analysis by the finite element method revealed that the constricted geometry suppressed thermal diffusion in the building sample, thereby increasing the sample temperature to over 1000 °C and maintaining the temperature at 700–1000 °C throughout the LPBF process. Multi-track thermal analyses also revealed that both the temperature gradient and solidification rate decreased by more than one digit, leading to a three-digit decrease in the cooling rate. Compared to the parts with the usual thermal history, the parts built at such high temperature had larger crystal grains elongated along the building direction, thicker cellular structures, more carbide precipitates, and a reduction of microhardness. From these results, a solidification map was created based on the correlations between the temperature gradient and solidification rate and the microstructures, and the effects of the elevated temperature on the epitaxial grain growth were also discussed. Consequently, it was demonstrated that the solidification behaviors could be controlled directly during the LPBF process without any additional heating mechanism.