Texture and Microstructural Features at Different Length Scales in Inconel 718 Produced by Selective Laser Melting

Abstract

Nickel-based Inconel 718 is a very good candidate for selective laser melting (SLM). During the SLM process, Inconel 718 develops a complex and heterogeneous microstructure. A deep understanding of the microstructural features of the as-built SLM material is essential for the design of a proper post-process heat treatment. In this study, the microstructure of as-built SLM Inconel 718 was investigated at different length scales using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Electron backscatter diffraction (EBSD) was also used to analyze the grain morphology and crystallographic texture. Grains elongated in the build direction and crossing several deposited layers were observed. The grains are not constrained by the laser tracks or by the melt pools, which indicates epitaxial growth controls the solidification. Each grain is composed of fine columnar dendrites that develop along one of their <100> axes oriented in the direction of the local thermal gradient. Consequently, prominent <100> crystallographic texture was observed and the dendrites tend to grow to the build direction or with occasional change of 90° at the edge of the melt pools. At the dendrite length scale, the microsegregation of the alloying elements, interdendritic precipitates, and dislocations was also detected.

Laser Powder Bed Fusion of Inconel 718: Residual Stress Analysis Before and After Heat Treatment

“…The e in the as-built condition usually exhibits epitaxial growth of columnar grains columnar or cellular dendrites along the built direction, owing to the high cooling rates process (10 6 K/s [19,26]) and the heat dissipation direction, i.e., vertically through the 21,27]. et al [25,28] studied the microstructure of IN718 parts fabricated by PBF-LB in as-built treated conditions, revealing carbides, Laves phases, and segregations; therefore, heat e crucial to homogenize the microstructure. Li et al [13] studied the microstructure of phase) rich in Ni, Cr, and Fe, which is strengthened by the precipitation of metastable Metals 2019, 9, x FOR PEER REVIEW semifinished or finished products in the aerospace, automo industries [6][7][8][9][10].…”

“…Calandri et al [25,28] semifinished or finished products in the aerospace, automotive, biomedical/medical, and energy industries [6][7][8][9][10]. Diverse metals AM techniques exist, and the most adopted in the industry are Powder Bed Fusion (PBF) and Directed Energy Deposition (DED), as for the classification by the ISO/ASTM 52900 (2015) international standard.…”

“…They are characterized by an arc-shaped or "fish-scale" patterns in the vertical plane, corresponding to the melt pool profile influenced by the Gaussian distribution of the laser energy. In the horizontal plane, the scanning tracks are usually observed as continuous lines [18,19,[22][23][24][25]. The microstructure in the as-built condition usually exhibits epitaxial growth of columnar grains including fine columnar or cellular dendrites along the built direction, owing to the high cooling rates of the PBF-LB process (10 6 K/s [19,26]) and the heat dissipation direction, i.e., vertically through the substrate [13,21,27].…”

“…Calandri et al [25,28] studied the microstructure of IN718 parts fabricated by PBF-LB in as-built and post-heat treated conditions, revealing carbides, Laves phases, and segregations; therefore, heat treatments are crucial to homogenize the microstructure. Li et al [13] studied the microstructure of an IN718 sample after SA plus DA heat treatments, varying the SA temperature from 940 • C to 1060 • C. In the as-built condition, performance components, for example, turbine blades, aerospace parts, and energy power plants.…”

“…. [25,28] studied the microstructure of IN718 parts fabricated by PBF-LB in as-built ed conditions, revealing carbides, Laves phases, and segregations; therefore, heat cial to homogenize the microstructure. Li et al [13] studied the microstructure of ', and δ precipitates in the heat-treated condition, and observed the presence of δ phase in heat-treated condition with SA at 1065 • C. Furthermore, and high cooling rates [12].…”