Effect of post-heat treatment on microstructure and mechanical properties of wire arc additively manufactured Inconel 718: comparison with L-PBF

The present work intends to study static mechanical properties of the CMT+MIG based WAAMed Inconel 718 obtained at conditions as deposited and post-heat treated. During WAAM, evolution of location-specific microstructure takes place which is influenced by the phenomena of microsegregation, formation of brittle Laves phases, δ phase and various MC-type carbide precipitates. The WAAMed as built part thus requires post-heat treatment to overcome problems associated with its property anisotropy. The work investigates the effect of post-heat treatment (homogenization, solution annealing followed by double stage ageing) to obtain favorable microstructure and improved mechanical properties. It is experienced that undesirable Laves phase is completely dissolved after homogenization followed by solution treatment. After solution treatment, residues of δ phase are witnessed. Ageing promotes precipitation of strengthening phase which contributes towards imparting higher strength to the alloy when compared to the as built counterpart. Post-heat treatment causes ∼ 50% increase in average ultimate tensile strength and ∼ 85% increment in average microhardness. However, ductility is to somewhat reduced after heat treatment. The increased tensile strength as well as microhardness confirms the phenomenon of precipitation strengthening (age-hardening) during post-processing ageing treatments. Precipitation of fine γ″ (Ni3Nb) is thus witnessed through TEM. During dry sliding wear test (at room temperature), it is experienced that the post-heat treated specimen corresponds to better wear resistance than the as built counterpart. Finally, microstructural characteristics and tensile properties of the as built WAAMed Inconel 718 are compared to that obtained from the as fabricated Laser-Powder Bed Fused (L-PBFed) Inconel 718. It is observed that L-PBFed as built part corresponds to higher tensile strength (but low ductility) when compared with WAAMed as built Inconel 718 counterpart.