Microstructural Analyses of ATI 718Plus® Produced by Wire-ARC Additive Manufacturing Process

Abstract

A detailed microstructural study of ATI 718Plus superalloy produced by the wire-arc additive manufacturing (WAAM) process was performed through the use of scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe micro-analysis (EPMA), and electron backscatter diffraction (EBSD). Extensive formation of eutectic solidification microconstituents including Laves and MC-type carbide phases, induced by micro-segregation, are observed in the build of the alloy in the as-deposited condition. Notwithstanding the significant segregation of niobium (Nb), which has been reported to promote the formation of the δ-phase in ATI 718Plus, only η-phase particles are observed in the deposit. Excessive precipitation of η-phase particles is found to be linked to Laves phase particles that are partially dissolved in the deposit after post-deposition heat treatment (PDHT). The EBSD analysis shows a high textured build in the 〈100〉 directions with only a few misoriented grains at the substrate–deposit boundary and the top of the deposit. Investigation on the hardness of the build of the alloy, in the as-deposited condition, showed a softened zone about 2 mm wide at the deposited metal heat affected zone (DMHAZ), which has not been previously reported and potentially damaging to the mechanical properties. An extensive analysis with the use of both microstructural characterization tools and theoretical calculations shows that the DMHAZ has the lowest volume fraction of strengthening precipitates (γ′ and γ″) in terms of their number density, which therefore induces the observed softness. Delayed re-precipitation kinetics and the extent of the precipitation of γ′ and γ″ in the DMHAZ which is related to the diffusion of segregated solute elements from the interdendritic regions are attributed to this phenomenon. The microstructural analyses discussed in this work are vital to adequate understanding of properties of ATI 718Plus produced by the additive manufacturing process technique.