Characterizing Thermal History, Microstructure and Defect Interactions in Additively Manufactured Nickel Superalloys

Abstract

Additive manufacturing processes such as laser powder bed fusion produce material by localized melting of a powder feedstock layer by layer. The small melt pools and high energy density generate very different microstructures in nickel superalloys when compared to more traditional cast or wrought processing, including features such as cellular structures and epitaxial grain growth. The features of these microstructures vary depending on local thermal history, alloy chemistry, and processing parameters. There is a need to develop a systematic understanding of the influence the local thermal conditions during solidification have on the resulting microstructure. Such understanding will be useful in predicting and ultimately avoiding microstructural defects such as undesirable phases or non-optimal grain structures. In this work, in-situ Longwave Infrared imaging of a laser powder bed fusion process is used to characterize the local thermal conditions throughout additively manufactured builds for alloy IN718 and Haynes 282 processed using systematically varied process parameters. This information is then correlated to observations of the microstructural features of these alloys in the as-built condition. This correlation analysis shows clear influence of the local thermal conditions during solidification on the dimensions of the dendritic microstructures formed during the build process for IN718 and Haynes 282. These dendritic structures arise due to segregation of elements such as niobium during solidification, an observation which can be predicted using a Scheil modeling approach.