Ivan CAZIC: Laser wire additive manufacturing of Inconel 718

Abstract:
The use of Inconel 718 alloy in additive manufacturing has been the subject of numerous studies in recent years. It has appeared early that the control of the morphology and size of the grains forming during solidification is a formidable challenge. Our work aims to address this challenge in the case of
coaxial laser wire technology by providing a better understanding of the formation of microstructures, including the conditions for the appearance of fine equiaxed grains observed at the bottom of the melt pools. First, the stability of the process as well as the thermal aspects have been investigated. Thanks to fast imaging and thermal imaging, we have been able to estimate the solidification conditions at the bottom of the bath. In a second step, we have analysed by EBSD the equiaxed zones and we have observed that they are very often clustered with twin relations compliant with some icosahedral symmetry. It is proposed that this particular organization is the signature of an "Icosahedral Short-Range Order mediated nucleation" (ISRO) mechanism, which would be favoured by the high cooling rates imposed by the process on the one hand, and by local enrichments of the liquid at the bottom of the melt pool during the melting of the previous layer on the other hand. We have therefore characterized the equiaxed zones by SEM and TEM. We have identified TiC carbides in the grain clusters and we have highlighted a strong local enrichment in Nb around the TiC by EDS. We have been able to identify Nb2Ni at the surface of a TiC carbide which requires a high local Nb composition to appear. From these observations we propose a possible scenario for the appearance of fine equiaxed grains in Inconel 718 deposited by the coaxial laser wire technology. TiC would play an important role by capturing minority elements during remelting, and by promoting local appearance of icosahedral clusters in the liquid that would favor the nucleation of grains whose relative orientations would reflect this icosahedral order.

Keywords: Solidification, Inconel 718, Laser - wire, Additive Manufacturing

Composition of the jury:
> Reporters:
- Maria Teresa PEREZ PRADO, Directrice de recherche, IMDEA Materials, Madrid, Espagne
- Patrice PEYRE, Directeur de recherche, PIMM, Paris, France
> Examiners:
- Güven KURTULDU, Chargée de recherche, ETH, Zürich, Suisse
- Florence PETTINARI STURMEL, Professeure, CEMES, Toulouse, France
> Direction of thesis:
- Benoît APPOLAIRE, Directeur de thèse, Professeur, IJL, Nancy, France
- Julien ZOLLINGER, Co-directeur, Maître de conférence, IJL, Nancy, France
- Maxime EL KANDAOUI, Encadrant industriel, Ingénieur de recherche, Institut de Soudure, Yutz, France
> Guests:
- Ulrike HECHT, Directrice de recherche, RWTH, Aachen, Allemagne
- Michel RAPPAZ, Professeur émérite, EPFL, Lausanne, Suisse