On Microstructural Heterogeneity in Additively Manufactured Ni-based Superalloys

Abstract

Recent discoveries have shown many parallels between metal additive manufacturing (AM) and physical metallurgy phenomena observed in casting, welding, powder metallurgy, and thermo-mechanical processes. However, it has been confirmed that the steady-state conditions assumed during traditional processes, are not valid in AM due to the formation of spatial and temporal transients. These are imposed by the abrupt, cyclical changes in energy delivery during the AM process. Hence, there is an intrinsic motivation to rationalise the effects of the new instabilities that arise with AM, which cause changes in local chemical bonding and associated physical properties. Ni-based superalloys, commonplace in high-temperature mission critical maritime, aerospace, and nuclear components have many traditional manufacturing complexities. This includes large swathes of reductive waste and difficulties in forming geometrically complex parts, both of which electron beam powder bed fusion (PBF-EB) serves as an excellent AM tool to overcome. However, due to a lack of full process-microstructure-property relationship understanding of PBF-EB superalloys, many challenges and opportunities remain that hamper their widespread use and implementation. This thesis present new insights on the metallurgical phenomenon from the atomic to macro length scales that arise with two characteristic commercial PBF-EB Ni-based superalloys, 'hard-to-weld' Inconel 738 and 'easy-to-fabricate' Haynes 282.