Microstructure Development in Electron Beam-Melted Inconel 718 and Associated Tensile Properties

Abstract

During the electron beam melting (EBM) process, builds occur at temperatures in excess of 800 C for nickel-base superalloys such as Inconel 718. When coupled with the temporal dierences between the start and end of a build, a top-to-bottom microstructure gradient forms. Characterized in this study is the microstructure gradient and associated tensile property gradient that are common to all EBM Inconel 718 builds. From the characteristic microstructure elements observed in EBM Inconel 718 material, the microstructure gradient can be classied into three distinct regions. Region 1 (top of a build) and is comprised of a cored dendritic structure that includes carbides and Laves phase within the interdendritic regions. Region 2 is an intermediate transition zone characterized by a diuse dendritic structure, dissolution of the Laves phase, and precipitation of needle networks within the interdendritic regions. The bulk structure (Region 3) is comprised of a columnar grain structure lacking dendritic characteristics with networks having precipitated within the grain interiors. Mechanically at both 20 and 650 C, the yield strength, ultimate tensile strength, and elongation at failure exhibit the general trend of increasing with increasing build height.