Abstract
The post-processing on the additively manufactured component is of huge interest as the key to tailor the microstructure to obtain certain mechanical properties. In this present study, the effects of hot isostatic pressing, as well as heat treatment on the microstructure, phase configuration and mechanical properties of laser powder bed fused (LPBF) IN718 superalloy were systematically investigated. Three different post-processes were studied such as hot isostatic pressing (HIP), heat treatment (HT), and HIP followed by HT (HIP+HT). The HIP process effectively eliminated the Laves phase remained in the as-built microstructure and brought uniformly distributed super fine γ″ precipitates in nano-meter size. In the heat-treated microstructure, larger γ″ precipitates were promoted directly from the as-built material. In comparison the HIP+HT process caused a moderate growth of γ″. In the latter two cases, the developed γ″ significantly strengthened the material. Yield strength of IN718 was increased from 738 MPa in as-built condition to 1015 MPa and 1184 MPa after HT and HIP+HT, respectively. On the contrary the ductility in the as-built IN718 condition was reduced by more than 40% after HT and HIP+HT. This can be compared to an increase in the ductility by almost 30% when subjected the as-built specimens to only HIPping. Finally, the correlation between microstructure evolution and mechanical properties is discussed in detail.
Highlights
Owing to the excellent combination of high-temperature mechanical properties, manufacturability, and relatively low cost, the nickel-based superalloy Inconel 718 (IN718) has been widely implemented for the hot-section component application, for example, in the aerospace tur bine engine and industrial gas turbine [1]
The fine microstructures of as-built, hot isostatic pressing (HIP), heat treatment (HT), and HIP followed by HT (HIP+HT) samples were observed by transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning trans mission electron microscopy (TEM) (STEM) on thin foils prepared via twin-jet electropolishing
The cellular boundaries are featured by microsegregation of heavier alloying elements than those in the matrix, as it appears brighter in the backscatter electron (BSE) micrograph (Fig. 2(b))
Summary
Owing to the excellent combination of high-temperature mechanical properties, manufacturability, and relatively low cost, the nickel-based superalloy Inconel 718 (IN718) has been widely implemented for the hot-section component application, for example, in the aerospace tur bine engine and industrial gas turbine [1]. IN718 superalloy is extensively used in the form of casting [2] and wrought [3] parts In these conventional manufacturing processes, the macro-segregation of refractory elements is inevitably across the workpiece, leading to the significant variations in mechanical properties. For LPBF processed superalloys, hot isostatic pressing (HIP) is one of the most effective post-processes to improve the integrity of printed parts by eliminating the internal microcracks [14]. As the HIP temperature is as high as 1150 ◦C, it is sufficient to completely dissolve Laves phase as well as achieve homogeneous dis tribution of the alloying elements [15] Heat treatment is another important post-process to tailor the microstructure of IN718. The combined and detailed analyses provided an in-depth understanding of the relationship of process-microstructure-property regarding IN718 processed by LPBF
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