Static recrystallization impact on grain structure and mechanical properties of heat-treated Hastelloy X produced via laser powder-bed fusion

Highlights: • The progress in static recrystallization of LPBF Hastelloy X samples at various heat-treatment times is investigated. • The effect of the as-built grain structure on the recrystallization process is studied. • The influence of introduced recrystallized microstructure on mechanical properties is discussed. • Anisotropic tensile behavior of the as-built samples significantly disappeares after heat treatment. Anisotropic behavior can be undesirable from a design point of view and may limit the widespread industrial use of the laser powder-bed fusion (LPBF) process in the future. In this case, the recrystallization of the columnar grain structure is beneficial to form an isotropic microstructure and restore the conventional properties of the material. However, the recrystallization of as-solidified single-phase material is difficult to achieve as it requires a driving force that rarely exists in this condition. In this study, it is shown that static recrystallization (SRX) of a single-phase Ni-based superalloy can take place after heat-treatment of LPBF-made parts. It is observed that up to ~52% of the columnar grain structure can be replaced by a recrystallized equiaxed grain structure after long solution treatment. Furthermore, oriented low-angle boundaries (with more than 50% relative frequency in the as-built structure) are disappeared in recrystallized regions where ∑3 boundaries (with up to ~26% relative frequency) take place and change the majority of boundaries to high-angle grain boundaries. It is also found that pre-existing closely spaced fine particles act as a barrier for grain boundary migration, along with the formation of immobile boundaries during SRX. With the achieved recrystallized fraction, isotropic yield stress and elongation have been improved. Moreover, with the applied heat-treatment, the difference in ultimate tensile strength (UTS) is decreased (by ~37% compared to as-built condition), when the remaining as-built columnar grain structure leads to a higher strain hardening rate during tensile test in the horizontal direction.