Structure, Texture and Phases in 3D Printed IN718 Alloy Subjected to Homogenization and HIP Treatments

Abstract

3D printing results in anisotropy in the microstructure and mechanical properties. The focus of this study is to investigate the structure, texture and phase evolution of the as-printed and heat treated IN718 superalloy. Cylindrical specimens, printed by powder-bed additive manufacturing technique, were subjected to two post-treatments: homogenization (1100 • C, 1 h, furnace cooling) and hot isostatic pressing (HIP) (1160 • C, 100 MPa, 4 h, furnace cooling). The Selective laser melting (SLM) printed microstructure exhibited a columnar architecture, parallel to the building direction, due to the heat flow towards negative z-direction. Whereas, a unique structural morphology was observed in the x-y plane due to different cooling rates resulting from laser beam overlapping. Post-processing treatments reorganized the columnar structure of a strong {002} texture into fine columnar and/or equiaxed grains of random orientations. Equiaxed structure of about 150 µm average grain size, was achieved after homogenization and HIP treatments. Both δ-phase and MC-type brittle carbides, having rough morphologies, were formed at the grain boundaries. Delta-phase formed due to γ -phase dissolution in the γ matrix, while MC-type carbides nucleates grew by diffusion of solute atoms. The presence of (Nb 0.78 Ti 0.22 )C carbide phase, with an fcc structure having a lattice parameter a = 4.43 Å, was revealed using Energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD) analysis. The solidification behavior of IN718 alloy was described to elucidate the evolution of different phases during selective laser melting and post-processing heat treatments of IN718.

Microstructural and Microhardness Evolution from Homogenization and Hot Isostatic Pressing on Selective Laser Melted Inconel 718: Structure, Texture, and Phases

“…found that post-manufacturing treatments reorganized the {002} dominant columnar grain structure into equiaxed grains with a {111} dominant orientation [13]. (Nb0.78Ti0.22)C carbides and δ-phase precipitates were also reported to form due to the diffusion of constituent elements and the high temperature treatment which led to dissolution of ′′ [13].…”

“…(Nb0.78Ti0.22)C carbides and δ-phase precipitates were also reported to form due to the diffusion of constituent elements and the high temperature treatment which led to dissolution of ′′ [13].…”

“…The representation of x, y, and z axes in the figure will be used as a reference in this paper. The laser scanning strategy consisted of bidirectional laser tracks and a hatch angle of 67 • in each consecutive layer as illustrated in Figure 1b [13]. The cylinders underwent homogenization (as-printed specimens heated to 1100 • C for an hour, then furnace cooled), and hot isostatic pressing (homogenized specimens were heated to 1160 • C under 100 MPa of pressure for 4 h, then furnace cooled) treatments.…”

“…The alloying elements of Inconel 718 include iron (Fe), chromium (Cr), niobium (Nb), and molybdenum (Mo), as well as small amounts of titanium (Ti), regions [11]. Mostafa et al [13] described the effects of homogenization and homogenization with HIP treatment on IN718 specimens SLM-printed perpendicular to the building direction. The study found that post-manufacturing treatments reorganized the {002} dominant columnar grain structure into equiaxed grains with a {111} dominant orientation [13].…”

“…Mostafa et al [13] described the effects of homogenization and homogenization with HIP treatment on IN718 specimens SLM-printed perpendicular to the building direction. The study found that post-manufacturing treatments reorganized the {002} dominant columnar grain structure into equiaxed grains with a {111} dominant orientation [13]. (Nb 0.78 Ti 0.22 )C carbides and δ-phase precipitates were also reported to form due to the diffusion of constituent elements and the high temperature treatment which led to dissolution of γ [13].…”