A novel method to prevent cracking in directed energy deposition of Inconel 738 by in-situ doping Inconel 718

Directed energy deposition (DED) of Ni-based superalloys has wide applications in the fields of aviation, energy and power. However, for the non-weldable superalloys like Inconel 738, cracking frequently occurs during DED and cannot be thoroughly controlled up to now. We propose a novel method to prevent the cracking during Inconel 738 DED, in which a small amount of Inconel 718 is in-situ doped between the deposited layers of Inconel 738. The obtained layered-gradient-material is found to be free of both macro- and micro-cracks. The microstructure shows that doping Inconel 718 cannot interrupt the epitaxial growth of grains, but can modify the precipitation of γ′. In Inconel 718 layers, nano γ′ particles are intensively precipitated only in the inter-dendrites, while in Inconel 738 layers, they are precipitated in both the inter- and inner-dendrites. This modification on γ′ precipitation can effectively decrease the inner stress and alleviate the stress concentration at the grain boundaries, thus the cracking is prevented. The tensile tests, which were conducted at room temperature, 600 °C and 800 °C respectively, demonstrate that the composite deposited workpieces possess promising strength and plasticity. The proposed method has great potential to improve the printability of un-weldable superalloys in additive manufacturing.