In Situ Investigation on the Roles of Microstructural Characteristics in Fatigue Anisotropy and Crack Propagation of LPBF–IN718 Alloy

ABSTRACT Laser powder bed fusion (LPBF) of Inconel 718 alloy inevitably introduces pronounced microstructural anisotropy, typified by columnar grains and robust crystallographic texture. This results in direction‐dependent fatigue behavior, raising critical reliability concerns for high‐stakes aerospace applications. To address this longstanding challenge, we establish an innovative liquid‐induced heat treatment combined with standard solution and double aging (LSA) process to achieve microstructural homogenization and systematically investigate its effects on fatigue anisotropy through integrated in situ scanning electron microscopy fatigue testing, macroscopic fatigue characterization, and comprehensive microstructural analysis. The results demonstrate that the LSA treatment induces a complete columnar to equiaxed grain transition, markedly elevates the fraction of high‐angle grain boundaries (HAGBs) to above 92%, and drastically weakens texture intensity while retaining the size and distribution of γ′ and γ″ strengthening precipitates. This microstructural homogenization effectively eliminates preferred paths for fatigue crack propagation, yielding comparable crack initiation cycles and growth rates in the building and transverse directions. Consequently, the scatter in fatigue life caused by microstructural heterogeneity is substantially reduced, and the uniformity of macroscopic fatigue performance across BD, TD, and 45° orientations is remarkably improved. The present work provides an efficient, feasible technological solution to resolve fatigue anisotropy in LPBF‐fabricated IN718 alloys, thereby enhancing their reliability for demanding engineering applications.