Real-time evolution of Selective Laser Melted (SLM) Inconel 718 with temperature through synchrotron X-rays

Abstract

Selective laser melting (SLM) is an additive manufacturing process that uses laser scanning to achieve melting and solidifcation of a metal powder bed. This process, when applied to develop high temperature material systems, holds great promise for more efficient manufacturing of turbine components that withstand extreme temperatures, heat fluxes, and high mechanical stresses associated with engine environments. These extreme operational conditions demand stringent tolerances and an understanding of the material evolution under thermal loading. This work presents a real-time approach to elucidating the evolution of precipitate phases in SLM Inconel 718 (IN718) under high temperatures using high-energy synchrotron x-ray diffraction. Four representative samples (taken along variable build height) were studied in room temperature conditions. Two samples were studied as-processed (samples 1 and 4) and two samples after different thermal treatments (samples 2 and 3). The as-processed samples were found to contain greater amounts of weakening phases. Precipitation hardening of Sample 2 reduced the detectable volume of weakening Delta phase, while also promoting growth of Gamma'' in the matrix. Inversely, solution treatment of Sample 3 produced an overall decrease in precipitate phases. High-temperature, in-situ synchrotron scans during ramp-up, hold, and cool down of two different thermal cycles show the development of precipitate phases. Sample 1 was held at 870C and subsequently ramped up to 1100C, during which the high temperature instability of strengthening precipitate, gamma'', was seen. Gamma'' dissolution occurred after 15 minutes at 870C and was followed by an increase of Delta-phase. Sample 4 was held at 800C and exhibited growth of gamma'' after 20 minutes at this temperature. These experiments use in-situ observations to understand the intrinsic thermal effect of the SLM process and the use of heat treatment to manipulate the phase composition of SLM IN718.