Hot cracking mechanism affecting a non-weldable Ni-based superalloy produced by selective electron Beam Melting

Chauvet, E. and Kontis, P. and Jägle, E.A. and Gault, B. and Raabe, D. and Tassin, C. and Blandin, J.-J. and Dendievel, R. and Vayre, B. and Abed, S. and Martin, G.

Volume: 142 Pages: 82-94
DOI: 10.1016/j.actamat.2017.09.047
Published: 2018

A non weldable nickel-based superalloy was fabricated by powder bed-based selective electron beam melting (S-EBM). The as-built samples exhibit a heterogeneous microstructure along the build direction. A gradient of columnar grain size as well as a significant gradient in the γ′ precipitate size were found along the build direction. Microstructural defects such as gas porosity inherited from the powders, shrinkage pores and cracks inherited from the S-EBM process were identified. The origins of those defects are discussed with a particular emphasis on crack formation. Cracks were consistently found to propagate intergranular and the effect of crystallographic misorientation on the cracking behavior was investigated. A clear correlation was identified between cracks and high angle grain boundaries (HAGB). The cracks were classified as hot cracks based on the observation of the fracture surface of micro-tensile specimens machined from as-built S-EBM samples. The conditions required to trigger hot cracking, namely, presence of a liquid film during the last stage of solidification and thermal stresses are discussed within the framework of additive manufacturing. Understanding the cracking mechanism enables to provide guidelines to obtain crack-free specimens of non-weldable Ni-based superalloys produced by S-EBM. © 2017 Acta Materialia Inc.

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