Influence of Double Aging Heat Treatment on Phase Transformation and Dimensional Accuracy of Inconel 718 Alloy made Through Laser-Based Additive Manufacturing

Abstract

Manufacturing defects, heterogeneous microstructure, and micro-segregation of secondary laves phases are the major issues to be resolved during the fabrication of alloy Inconel 718 (IN718) fabrication through metal additive manufacturing (AM). The present study delivers the importance of post AM double aging heat treatment (HT) of alloy IN718 fabricated through laser-based powder bed fusion process (L-PBF). Initially, the as-built IN718 sample was subjected to solution annealing at 980 °C for the duration of 1 hour followed by air cooling. Then, the L-PBF sample was subjected to double aging at 720 °C for 8 hours followed by furnace cooling to 620 °C in 2 hours. The specimen was kept at 620 °C for 8 hours and then cooled to room temperature. The beneficial influence of HT on the closure of AM-induced manufacturing defects such as pores and voids, microstructural changes, and microhardness have been reported. As-built microstructure showed the presence of heterogeneous columnar dendrites along the build direction with the presence of a large number of laves phases in the interdendritic regions. HT microstructure resulted in the formation of homogeneous austenite gamma (γ) matrix in which the strengthening phases such as gamma prime (γ′) (Ni3 (Al, Ti)) and gamma double prime (γ″) (Ni3Nb) were observed. As a result of HT, laves phases ((Ni, Fe, Cr)2·(Nb, Mo, Ti)) were dissolved in the grain interiors and precipitated in the grain boundaries as delta (δ) phase (NbNi3). The δ phase in the grain boundaries arrested the grain boundary dislocation, as well as grain growth, and revealed the relatively finer and homogenous grain structures. Further, a Micro-computed tomography (micro CT) examination revealed the absence of AM-induced pores after HT. The hardness of the heat-treated samples was increased to 35% and it showed an increase in hardness from 417 to 564 HV. Dimensional analysis was performed on the alloy IN718 to study the effect of thermal loading. The volumetric shrinkage of 3.084% was noted as a result of heat treatment at the micron-scale.