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

Abstract

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 microtensile 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.

Mechanism of heat affected zone cracking in Ni-based superalloy DZ125L fabricated by laser 3D printing technique

“…After a series of experiments, it is found that the grain boundary is likely to crack if the misorientation angle exceeded a critical value θ c , which is dependent on temperature and chemical composition. Wang et al [40] proposed that the θ c was 13°, while the values such as 15°and 16°were also reported by Chauvet et al [35] and Rong et al [41], respectively. The uncracked grain boundary misorientation (17°) is close to the reported values, thus it is not easy to tell if the uncracked grain boundary is a high angle grain boundary.…”

“…As seen from Fig. 7b, a typical dendritic morphology accompanied with spherical drops are observed, giving evidence of the complete wetting of liquid film along the grain boundary in the primary HAZ [35]. However, the fracture surface of the artificial crack is typically flat and some traces and shear bands can be clearly seen from the inset in Fig.…”

“…As a result, it is confirmed that the HAZ cracking initiated as liquation cracking; however, based on the present observations, it is difficult to identify whether the existing crack propagated as a liquation crack or a solidification crack in the layer-by-layer cladded materials due to the complex solidification path, including melting, re-melting, partial re-melting, cyclic annealing, etc. [35].…”

Atomic-scale grain boundary engineering to overcome hot-cracking in additively-manufactured superalloys

“…Cracks developing during S-EBM of non-weldable alloys are often qualified as hot cracks because cracking occurs in presence of liquid films, as is the case for the grade investigated in the present work [18]. The combination of liquid films and thermal stresses arising from shrinkage and cooling leads to the development of hot cracks.…”

Rafting‐Enabled Recovery Avoids Recrystallization in 3D‐Printing‐Repaired Single‐Crystal Superalloys

“…At some superalloy compositions and 3D‐printing parameters, detrimental Laves phase particles are reported 15. More importantly, a high density of dislocations build up in the heat‐affected zone (HAZ), due to unavoidable (often local) deformation under high thermal stresses during printing 16,17. Upon solution treatment at elevated temperatures, these regions riddled with dislocations readily undergo recrystallization (RX)18 that renders the microstructure polycrystalline, which would significantly degrade the high‐temperature creep performance of blades (Figure S1 in the Supporting Information) 2.…”