Enhanced strength of additively manufactured Inconel 718 by means of a simplified heat treatment strategy

Abstract

This study simplified the heat treatment route and reduced the post-processing burden for laser powder bed fusion IN718 (a nickel-based superalloy). The tailored route retained advantageous microstructures and improved tensile strength when compared to the conventional post-processing strategy. The implementation of a single pre-aging step (combining the stress relief, HIP, and solution steps into one) and a single-step aging treatment (as opposed to two-step aging) resulted in a total heat treatment time of 15 h, which is far shorter than the conventional five-step strategy (42 h). The grain structure, pore structure, and room-temperature tensile properties were measured for three material conditions: the as-built condition, the baseline condition (a conventional five-step heat treatment route), and the HIP1020RQSA condition (novel high pressure rapid quenching HIP treatment followed by a single-step age). In the HIP1020RQSA condition, the high pressure (200 MPa), short hold time at a carefully selected temperature (0.5 h at 1020 °C), and rapid quenching (2150 °C/min) were implemented. Unique findings in the microstructure include generation of localized micro-scale recrystallization throughout the retained columnar grain and sub-grain structures, internal pore closure with a shortened HIP soak time, incomplete dissolution of Laves phase, and the absence of δ phase. The single step aging treatment also generated γ′ and γ′′ precipitates. The HIP1020RQSA treatment improved the yield strength of the baseline condition from 1112 MPa to 1209 MPa, decreased total elongation from 24.9% to 23.2%, and still exceeds the minimum Z direction (build direction) tensile properties outlined in ASTM F3055–14a.