Precipitation kinetics of secondary phases induced by heat accumulation in the deposit of Inconel 718

Abstract

The precipitation kinetics were investigated of the secondary phases including γ'', γ', and δ phases formed by the heat accumulation produced in Inconel 718 (IN718) deposition of cold metal transfer (CMT)-based wire arc additive manufacturing (WAAM) process. Heat accumulation from the layer-by-layer deposition causes a difference in the holding time within the aging temperature range for each layer, resulting in a difference in precipitation of secondary phases that enhances hardness. In order to investigate the precipitation kinetics, which lead to establish the WAAM-related heat treatment standard specification unlike that of the wrought or cast IN718, the thermal history was measured during the deposition process and microstructural analysis was performed according to the height of the deposit. In addition, continuous cooling transformation (CCT) and time-temperature transformation (TTT) diagrams were prepared for dendritic and inter-dendritic regions at each height of the deposit. Based on the thermal measurements, the holding time within each precipitation temperature bands for the δ phase and for the strengthening phases were correlated with the ratio of δ phase fraction and hardness depending on the height, respectively. In addition, using the thermal history, Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation was applied to calculate the hardness of the IN718 deposit which was compared with the measured hardness depending on the holding time.