Influence of reduced carbon content on microstructure and mechanical behavior of Inconel 718 prepared by laser powder bed fusion

Abstract

• Successful production of a reduced carbon Inconel 718 alloy via SLM. • Significant changes in the precipitation behavior of NbC and γ’’ are achieved. • Reductions in NbC content and finer γ’’ precipitation verified By Thermocalc. • Reduced γ’’ size results in improved tensile strengths at both 25 °C and 650 °C. Inconel 718 powder with reduced carbon content was used to manufacture coupons via laser powder bed fusion (LPBF) to study the influence of carbon content on microstructures and mechanical properties of HIP'ed and heat-treated material. The carbon concentration in this material was kept deliberately low (0.01 wt%) relative to a typical commercial powder composition (0.04 wt%) of IN718 in order to minimize the precipitation of primary carbides during post-processing heat treatments, which have been observed to be deleterious to elevated temperature mechanical properties such as elongation, stress rupture ductility, and notch sensitivity. The final microstructure of the lean C alloy is shown to have a significantly reduced NbC population relative to the baseline material. Furthermore, other microstructural variations such as an increased δ phase population and decreased γ’’ particle size were observed. Empirical observations agreed with microstructural simulations calculated by the ThermoCalc/TC-PRISMA precipitation module. These changes to the microstructure resulted in a 10% increase in the yield strength at both 25 and 650 °C in the lean C alloy, bringing the strength of LPBF-printed material closer to that of wrought IN718. Overall, the reduction in carbon content in the raw material has significant effects beyond limiting the pre-solution treatment precipitation of NbC, primarily by freeing Nb for formation of δ and γ’’ phases.