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 <i>as deposited</i> and post-heat treated. During WAAM, evolution of location-specific microstructure takes place which is influenced by the phenomena of microsegregation, formation of brittle <i>Laves</i> phases, δ phase and various MC-type carbide precipitates. The WAAMed <i>as built</i> 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 <i>Laves</i> 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 <i>as built</i> 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 <i>as built</i> counterpart. Finally, microstructural characteristics and tensile properties of the <i>as built</i> WAAMed Inconel 718 are compared to that obtained from the <i>as fabricated</i> Laser-Powder Bed Fused (L-PBFed) Inconel 718. It is observed that L-PBFed <i>as built</i> part corresponds to higher tensile strength (but low ductility) when compared with WAAMed <i>as built</i> Inconel 718 counterpart.