Thermal effects on microstructural heterogeneity of Inconel 718 materials fabricated by electron beam melting

Abstract

Additive manufacturing technologies, also known as 3D printing, have demonstrated the potential to fabricate complex geometrical components, but the resulting microstructures and mechanical properties of these materials are not well understood due to unique and complex thermal cycles observed during processing. The electron beam melting (EBM) process is unique because the powder bed temperature can be elevated and maintained at temperatures over 1000°C for the duration of the process. This results in three specific stages of microstructural phase evolution: (a) rapid cool down from the melting temperature to the process temperature, (b) extended hold at the process temperature, and (c) slow cool down to the room temperature. In this work, the mechanisms for reported microstructural differences in EBM are rationalized for Inconel 718 based on measured thermal cycles, preliminary thermal modeling, and computational thermodynamics models. The relationship between processing parameters, solidification microstructure, interdendritic segregation, and phase precipitation (d, c9, and c0) are discussed.

Additively Manufactured Inconel 718 : Microstructures and Mechanical Properties

“…The anisotropic tensile properties were further confirmed by Kirka et al [77,186]: both the tensile strength and elongation are higher tested along the building direction that tested perpendicular to the building direction in the as-manufactured sample, and these anisotropic properties almost disappear after post heat-treatment. The microstructure gradient along the building direction was first reported by Unocic et al [187], and was attributed to the height-dependant thermal exposure experienced by different parts of the component [61]. The strength is correspondingly gradient, namely progressively increased tensile strength and elongation with the distance from the bottom, which is associated with the decomposition of γ ′′ and precipitation of δ.…”

“…Therefore, the powder bed is maintained at relatively high temperature throughout the process. For instance, for Ti6Al4V it is typically between 550 to 700 • C [58][59][60], and for IN718 it can be over 900 • C [61,62]. This leads to mostly full in-situ stress relief and no residual stress in the as-manufactured microstructure [63].…”

“…The process-induced porosity, referred as lack of fusion, is formed due to the unoptimized process parameters, specifically inappropriate energy input [61]. This kind of porosity is mostly irregular in shape and size varies from micrometer to millimetre.…”

Additive manufacturing of materials: Opportunities and challenges

“…25 This understanding leads, in turn, to a fundamental question: Is it possible to control the crystallographic textures in site-specifi c regions within a component? S Conventional EBM scan strategies and process parameters result in coarse columnar grains aligned parallel to the build direction, similar to what is observed in a directionally solidifi ed microstructure.…”

Thermographic Microstructure Monitoring in Electron Beam Additive Manufacturing

“…EBM specifically faces issues with in situ aging of the material’s microstructure due to prolonged exposure to elevated temperatures during processing, overheating of the material which can lead to issues such as swelling and cracking, and a less favorable surface finish due to its use of a relatively large layer thickness and coarser powder145.…”