Effect of scanning strategy on the formation of heterostructure and recrystallization behavior of additively manufactured IN718 alloy

The microstructural design of selective laser melting (SLM) processed Inconel 718 (IN718) superalloy is constrained by an incomplete understanding of the formation mechanisms and recrystallization kinetics of the special SLMed microstructure. This study is dedicated to elucidate the impact of laser scanning strategies on the as-built microstructural characteristics and static recrystallization behaviors of heterostructured IN718 by establishing a physical link between macroscopic laser scanning strategies and the microscale spatial distribution of stored deformation energy. Utilizing high-resolution electron backscatter diffraction (EBSD) and serial sectioning EBSD for three-dimensional characterization across interrupted isothermal annealing stages at 1080 °C, the investigation reveals that macroscopic scanning paths dictate a periodic topological network comprising melt pool centers (MPCs) with high stored energy and melt pool overlaps (MPOs) with low stored energy. Results demonstrate that static recrystallization is governed by strain-induced boundary migration rather than particle-stimulated nucleation. Static recrystallization preferentially initiates in the MPO-related regions with relatively low stored energy, which is attributed to the combined effects of the stored-energy difference across mobile boundaries, the high mobility of original HAGBs, and the presence of locally curved/roughened boundary segments. Subsequently, these highly mobile recrystallization fronts directionally consume the MPCs but experience appreciable kinetic suppression upon encountering adjacent straight columnar boundaries. Consequently, unidirectional scanning strategies yield profound microstructural heredity, preserving a highly anisotropic crystallographic lamellar microstructure after solution treatment. These fundamental mechanistic insights about the microstructure formation and evolution during SLM processing and subsequent solution treatment are of significance in tailoring the microstructure of SLM IN718 alloy for desired mechanical properties.