Phase formation kinetics during heat treatments of Inconel 718 deposits based on cold metal transfer-based wire arc additive manufacturing

Abstract

In this study, various heat treatments were conducted on Inconel 718 (IN718) products deposited by wire arc additive manufacturing (WAAM) to investigate the formation kinetics of phases at microstructural locations. The formation kinetics of the secondary phases precipitated by heat-treating of WAAM products differ from those of wrought or cast products because of the heat that accumulates during layer-by-layer deposition. Therefore, the secondary phases in the heat-treated WAAM products differ from those in conventional products according to the microstructural locations, e.g., the dendritic region, interdendritic region, and grain boundaries. To investigate such phase differences, a 100-layer IN718 wall was deposited using cold metal transfer (CMT)-based WAAM and then heat-treated under various conditions. In each heat-treated specimen, the δ, γ’, and γ” phases were identified by microstructural analyses. Based on the results, time–temperature–transformation diagrams were constructed for each microstructural location, which confirmed that the secondary phases were precipitated faster in the interdendritic region and at grain boundaries than those in the dendritic region. In addition, the applicability of single-aging, which can be used to satisfy the desired mechanical properties in less time with less processing, was evaluated by the hardness and tensile tests in the WAAM products.