Microstructure and mechanical behavior of as-built and heat-treated Hastelloy-X alloy produced by Laser Powder Bed Fusion process

Abstract

In this study, microstructure and mechanical characterization of as-built Hastelloy X (HX) samples produced by laser powder bed fusion (LPBF) process and post-heat-treated samples were investigated. Two sets of samples, horizontal and vertical to build direction, were considered in as-built condition to understand the effect of build direction and two solution heat-treatment temperatures, 1177°C and 1220°C, followed by fast cooling were considered to study the effect of solution heat-treatment temperature on microstructure and mechanical properties. Microstructure characterization of as-built sample horizontal to build direction revealed a typical multi-layer molten pool boundaries and the sample vertical to build direction revealed multi-layered and multi-tracked molten pool boundaries. Electron backscatter diffraction results reveal a disrupted epitaxial grain growth for the as-built samples vertical to build direction whereas equiaxed grain structure with varying twin grain boundary fractions was observed for heat-treated HX samples. As-built LPBF HX samples exhibit higher mean hardness and yield strength than post-heat-treated samples. Higher elongation and lower yield strength were observed for the sample solution treated at 1220°C as compared to the sample solution annealed at 1177°C. Microstructural evolution at 20% engineering strain for the exact positions before the tensile test was presented for solution treated at 1220°C samples, which reveals distinct slip lines within each grain as well as increased dislocation density at grain boundaries.