Plastic inhomogeneity and crack initiation in hybrid wrought - additively manufactured Inconel 718

Directed energy deposition (DED) holds great promise for repair applications involving site-specific deposition that creates hybrid components. However, it has been reported that failure in hybrid components occurs in the additively manufactured (AM) side despite its higher strength and excellent consolidation. To unravel the underlying mechanisms responsible for the observed behaviour, we carried out both ex-situ and in-situ mechanical testing complemented with detailed microstructural characterisation of the wrought, AM and bond interface regions in hybrid Inconel 718. The role of the local microstructure in the spatial strain development and crack initiation in the hybrid wrought-AM Inconel 718 is revealed. Most importantly, it is shown that severe inhomogeneous plastic deformation quickly developed in the AM side of the hybrid sample, and this is primarily attributed to few, but very large columnar grains that were preferentially oriented for dislocation slip under external loading. Furthermore, the AM region of the hybrid Inconel 718 had a lower work-hardening rate that caused a higher thinning rate during deformation, promoting strain localisation in the AM side. The degradation in the ductility of AM Inconel 718 is shown to have occurred not because of defects as widely reported for AM components, but rather as a consequence of the interaction between the brittle Laves phase with the localised slip bands which were rapidly intensified by the plastic inhomogeneity. This interaction resulted in early crack initiation in the AM side, primarily leading to the final fracture of the hybrid wrought-AM Inconel 718. The study also demonstrates that the bond interface is the strongest region of the hybrid component under uniaxial loading, and reveals the origins of the strength of the bond interface. The insights revealed advance the understanding of the mechanical performance and the direct failure mechanism in hybrid wrought-AM Inconel 718 components, and opens new pathways to improve their mechanical integrity.