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.

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As-built Ti-6Al-4V thin parts were manufactured in three different orientations using EBM and characterized by laboratory X-ray tomography. Fatigue tests were performed. The comparison with results for machined samples from the literature showed a large reduction of fatigue strength. SEM observations of the fracture surfaces showed that surface defects which were identified as notch-like defects on tomographic images caused the failure. Their impact on fatigue results was rationalized by Kitagawa-Takahashi diagrams. A build orientation impact on the fatigue properties was observed and linked to its effect on defects distributions and crack growth. The limits of roughness measurements were also discussed.

There are still debates regarding the mechanisms that lead to hot cracking in parts build by additive manufacturing (AM) of non-weldable nickel-based superalloys. This lack of in-depth understanding of the root causes of hot cracking is an impediment to designing engineering parts for safety-critical applications. Here, we deploy a near-atomic-scale approach to investigate the details of the compositional decoration of grain boundaries in the coarse-grained, columnar microstructure in parts built from a non-weldable nickel-based superalloy by selective electron-beam melting. The progressive enrichment in Cr, Mo and B at grain boundaries over the course of the AM-typical successive solidification and remelting events, accompanied by solid-state diffusion, causes grain boundary segregation induced liquation. This observation is consistent with thermodynamic calculations. We demonstrate that by adjusting build parameters to obtain a fine-grained equiaxed or a columnar microstructure with grain width smaller than 100 μm enables to avoid cracking, despite strong grain boundary segregation. We find that the spread of critical solutes to a higher total interfacial area, combined with lower thermal stresses, helps to suppress interfacial liquation.

The possibility to produce Ni-base superalloy single crystals by selective electron beam melting (S-EBM) is demonstrated. The production of single crystals specimens was achieved by a tight control of the processing conditions without requiring a grain selector or a crystal seed. The melting parameters are controlled so as to promote columnar grains and intensify the competitive grain growth.

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International audienceA major drawback of metal additive manufacturing is the surface roughness of the manufactured components. This is even more critical when complex lattice structures are considered. An octet-truss lattice structure was fabricated by Electron Beam Melting. A chemical post-treatment was applied in order to improve the surface quality. The morphology of individual struts was characterized experimentally by high resolution X-ray tomography after different chemical etching times. The chemical etching treatment was found to be beneficial as it decreases significantly the occurrence of surface defects. The evolution of the elastic mechanical properties with the etching time was determined by FFT computations directly applied to the 3D volume of the struts. A cellular automaton based model was also developed in order to predict the morphological evolution of the as-built strut during etching. The model enables to predict the kinetics of dissolution as well as the evolution of the surface defects and the elastic mechanical properties of the samples. It also enables to determine the required etching time to firstly remove powder particles stuck to the surface and secondly to reduce the plate-pile like defect occurrence. (C) 2016 Elsevier Ltd. All rights reserved