2, Argonne National Laboratory, Argonne, Illinois, United States
Additive manufacturing (AM) of metals is based on a layer-by-layer additive process, in contrast to traditional manufacturing processes that often require labor-intensive and costly subtraction or forming.1 AM technologies provide great flexibility in manufacturing parts with complex geometrical shapes and can significantly reduce manufacturing lead times and associated cost. Thus, AM is fast becoming an attractive option for the fabrication of increasingly complex and high-valued metal components in the aerospace, oil & gas, automobile, electronics, and biomedical industries. Unfortunately, elemental segregation can present a ubiquitous problem for AM parts due to solute rejection and redistribution during the rapid solidification process.2 We have conducted a series of X-ray synchrotron in situ operando studies, based on simultaneous ultra-small-, small-, & wide-angle X-ray scattering & diffraction, USAXS/SAXS/WAXS (XRD), together with electron microscopy & thermodynamic modeling, to show that in one AM Ni-based superalloy, Inconel 625, deleterious δ-phase precipitates grow on much shorter time scales than in the corresponding wrought alloys (i.e., minutes versus tens of hours).3,4
The root cause of this δ-phase formation is localized elemental segregation that results in local compositions being outside the bounds of the allowable range set for wrought alloys. The in situ operando USAXS/SAXS experiments reveal that platelet-shaped δ phase precipitates grow continuously, preferentially along their lateral dimensions during stress-relief heat treatments, while their thickness dimension remains constant from very early heat treatment times.4 No significant nucleation barrier is observed in the WAXS/XRD measurements. The activation energy for δ-phase growth is ≈ 131 kJ mol-1. We further find that a subsequent homogenization heat treatment can be effective both in homogenizing the AM alloy and in removing the deleterious δ phase.3,4 We assert that the methodology established with these measurements could be extended to elucidate the phase evolution during heat treatments in a broad range of AM materials.
 I. Gibson, D.W. Rosen & B. Stucker. Additive manufacturing technologies, Springer, 2010.
 Y. Idell, L.E. Levine, A.J. Allen, F. Zhang, C.E. Campbell, G. Olson, J. Gong, D. Snyder & H. Deutchman; JOM, 68, 950-959, (2016).
 F. Zhang, L.E. Levine, A.J. Allen, C.E. Campbell, E.A. Lass, S. Cheruvathur, M.R. Stoudt, M.E. Williams & Y. Idell; Scripta Mater., 131, 98-102 (2017).
 F. Zhang, L.E. Levine, A.J. Allen, M.R. Stoudt, G. Lindwall, E.A. Lass, M.E. Williams, Y. Idell & C.E. Campbell; Acta Mater., submitted (2018).