Microstructure evolution and hardness enhancement of LMD Inconel 718 multilayer in as-deposited and post heat treated: A comparative study

This work aims to study the influence of heat treatment on the microstructure of laser metal deposited multi-layered Inconel 718 alloy. As-deposited samples were subjected to standard heat treatment, and the resulting microstructure and mechanical properties are studied. When observed under optical microscope and scanning electron microscope, the microstructure revealed microstructural homogenisation. In this case, the grain morphology shows that Nb-rich Laves phase in as-deposited sample is converted to small irregular granules in post heat treatment. The main finding of this study is that after heat treatment, the continuous bands of Nb-rich Laves phase are dissolved into small patches, as observed in elemental mapping through Energy Dispersive Spectroscopy. Also, Nb and Ti are distributed uniformly in the microstructure forming γ´ (Ni3Ti) and γ´´ (Ni3Nb) strengthening precipitates in Ni matrix. The precipitates developed has resulting in 50–60% higher hardness (419 HV) in the heat-treated sample compared to as-deposited sample.