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https://doi.org/10.1007/s11837-020-04481-1  2020 The Minerals, Metals & Materials Society

IN SITU SYNCHROTRON AND NEUTRON CHARACTERIZATION OF ADDITIVELY MANUFACTURED ALLOYS

Microscale Observation via High-Speed X-ray Diffraction of Alloy 718 During In Situ Laser Melting SEUNGHEE A. OH ,1 RACHEL E. LIM ,1 JOSEPH W. AROH ANDREW C. CHUANG ,2 BENJAMIN J. GOULD ,3 JOEL V. BERNIER ,4 NIRANJAN PARAB ,2 TAO SUN ,5 ROBERT M. SUTER ,6 and ANTHONY D. ROLLETT 1,7

,1

1.—Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA. 2.—X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA. 3.—Applied Materials Division, Argonne National Laboratory, Lemont, IL 60439, USA. 4.—Lawrence Livermore National Laboratory, Livermore, CA 94550, USA. 5.—Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USA. 6.—Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA. 7.—e-mail: [email protected]

The laser melting process is accompanied by rapid evolution in temperature, phase, structure, and strain because of its high heating and cooling rates. In this study, the evolution of grains within a thin solid plate of Ni alloy 718 during laser processing was probed with in situ high-energy x-ray diffraction experiments. The high temporal and spatial resolution available in the measurement allowed us to study the rapid evolution of the melted region beneath the surface of the sample. The characterization of the evolution of secondary phases, i.e., Laves and carbide, was captured despite the weak diffracted peaks caused by small volume fractions. Thermal history was estimated based on changes in the lattice spacing from the thermal contraction upon cooling. The temporal variation in 2h with azimuthal direction revealed the evolution in anisotropy of lattice spacing and thus of the mechanical state during laser processing.

INTRODUCTION Laser melting is extensively used for welding and additive manufacturing (AM) of metal parts thanks to its high speed and accuracy. The highly focused laser light used during this process causes rapid evolution in temperature, phase, and structure.1 Because of the extreme heating and cooling rates, in the order of 106 K/s,2 various unexpected features occur, from unusual microstructures to non-equilibrium phase transitions.1 Understanding the associated dynamic phenomena that occur at short time and length scales is crucial to process optimization. Ni-based alloy 718 is a superalloy that has been extensively studied because of its unique combination of high strength and corrosion resistance which are maintained even at temperatures as high as (Received August 4, 2020; accepted October 27, 2020)

650C.3,4 In addition to the matrix phase, c, there are several secondary phases, including c¢¢, c¢, d, carbides, and Laves, which can significantly influence mechanical properties. In particular, the brittle nature of the Laves phase negatively affects the