On the detailed morphological and chemical evolution of phases during laser powder bed fusion and common post-processing heat treatments of IN718

Abstract

IN718 is the most common Ni-based superalloy for manufacturing aircraft engine parts via thermo-mechanical treatments. The evolution of nanoscale strengthening phases is well researched, enabling optimization of strength, fatigue, and creep properties. Recently, IN718 has shown great viability for laser powder bed fusion (LPBF) additive manufacturing of aerospace parts. However, the detailed microstructure-property relationships during thermal profiles typical to LPBF are not yet well understood. Previous works reported interdendritic precipitation of Laves phase. These detrimental particles can be dissolved by heat treatments, however, the detailed nanoscale phase evolution remains unknown. Using atom probe microscopy, we report on the detailed morphological and chemical evolution of phases in IN718 after LPBF with chessboard versus meander scanning strategies, and direct ageing versus homogenization and ageing treatments. Due to differences in scanning vector length, up to 3.6 times larger dendritic structures, double volume fractions of Laves particles, and Al clusters are found in the chessboard strategy. Coarser matrix grains and a higher dislocation density are detected in the meander strategy. The precise chemical composition and morphology evolution of the matrix, Laves, MC, γ′, and γ′′ phases are obtained and correlated to hardness. Retained Laves phase after direct ageing causes precipitation of 4% volume fraction of γ′′, with additional coarsened precipitates formed along dislocations. Direct ageing leads to an increase in hardness corresponding to roughly 190 HV. Due to Laves phase dissolution, a volume fraction of 16% of compositionally stable, larger γ′′ precipitates is found after homogenization and ageing, also causing partial matrix recrystallization.