Nickel-based Inconel 718 is a very good candidate for selective laser melting (SLM). During the SLM process, Inconel 718 develops a complex and heterogeneous microstructure. A deep understanding of the microstructural features of the as-built SLM material is essential for the design of a proper post-process heat treatment. In this study, the microstructure of as-built SLM Inconel 718 was investigated at different length scales using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Electron backscatter diffraction (EBSD) was also used to analyze the grain morphology and crystallographic texture. Grains elongated in the build direction and crossing several deposited layers were observed. The grains are not constrained by the laser tracks or by the melt pools, which indicates epitaxial growth controls the solidification. Each grain is composed of fine columnar dendrites that develop along one of their <100> axes oriented in the direction of the local thermal gradient. Consequently, prominent <100> crystallographic texture was observed and the dendrites tend to grow to the build direction or with occasional change of 90° at the edge of the melt pools. At the dendrite length scale, the microsegregation of the alloying elements, interdendritic precipitates, and dislocations was also detected.

The additive manufacturing concept for the production of complex near net shape metal parts is obtaining increasing attention due to the possibility of producing assembled and/or complex parts allowing optimal design and saving time and cost. The possibility to use the free design of Selective Laser Melting (SLM) techniques for the fabrication of complex 3D components using high performing, although difficult to work, materials such as Ni superalloys is really attractive. The particular process conditions that are established during additive manufacturing in SLM leads to microstructures different with respect to those observed in standard cast or wrought analogous material. Therefore, it is usually necessary to apply a post solution treatment, in order to reduce the segregation of heavier elements (in particular Nb) and dissolve the interdendritic precipitates. In this study, the influence of temperature and time of the solution treatment on the microstructure is investigated in order to find the best results in terms of the final oxidation resistance. Oxidation performances of solutioned Inconel 718 fabricated via SLM are reported and discussed. The growth rate of the superficial oxide at the temperature of 850 C was measured and the longterm stability of this passivating layer was tested until 908 h.

Additive manufacturing (AM) is one of the processes with the most potential for producing components used in internal combustion engines and features high efficiency due to the possibility of building very complex shapes. Several drawbacks of parts produced using AM are still unresolved, like poor surface quality, the presence of internal defects and anisotropic mechanical behaviour, which all contribute to decreasing the fatigue strength compared with the material produced using conventional processes. The effect of building direction on both the macroscopic mechanical behaviour and the crack propagation mechanism of Ni-base superalloy Inconel718 produced using AM was investigated under the combined effect of low cycle fatigue (LCF) and high temperature. The different crack growth mechanisms investigated using compact tension (CT) specimens, tested at high temperature, showed a significant difference between the two building directions. The LCF fatigue experiments also showed a significant difference in the ε-N curves from the two directions together with a high level of scatter due to the dispersion of the defect size at the fracture origin. The dimensions of the defects (as measured using the ffiffiffiffiffiffiffiffiffi area p parameter) were analysed by means of extreme value statistics and showed a significant difference between the two orientations investigated. The aim of this work is to propose a simplified approach (based on ΔJ eff concepts) to estimate the fatigue life of a component produced using AM that takes into account the material variability due to the combined effect of mechanical anisotropic behaviour and the presence of defects at high-temperature conditions.

The Ni-based superalloys are widely used in aerospace and aeronautic industries, but building complex shape components with conventional technologies is expensive. Nowadays, it is possible to use techniques such as direct metal laser sintering (DMLS) process to overcome this problem. In the present report are summarized the activities performed at the Department of Applied Science and Technology (DISAT) of Politecnico di Torino and Istituto Italiano di Tecnologia (IIT) di Torino on the development of Ni-based superalloys produced by DMLS. More precisely, we have studied the microstructure architectures of Inconel 625 and Inconel 718 produced by M270 Dual Mode version and the impact of different heat treatments on their microstructures in order to meet the industrial requirements. Additive manufacturingAdditive manufacturing (AM) represents a family of process that can fabricate components through layer by layer process, using a 3D computer aided design (CAD) data. These technologies allow the production of near net shape components, also with a very complex shape, without requesting long subtraction processing. Regarding the production of superalloys, the laser and electron beam melting are the most attractive AM technologies available. Laser technologies can be differentiated according to the deposition mode of powdery material, being direct laser deposition (DLD) and powder bed fusion-laser (PBF-L) the most important processes. In these cases, we have several producers of laser beam technologies. In the first family of manufacturing routes, one can include laser metal deposition (LMD) in which a high-energy laser beam is directed on a substrate to form a melt pool and at the same time powders or wires are delivered in the melt pool. Generally, in these machines the substrate is moved according to the shape of the components whilst the laser and feeding system is fixed. Instead, in the second family of processes, one can include direct metal laser sintering (DMLS), trade name of EOS for the selective laser melting (SLM) process, in which a laser selectively melt some areas of a layer of loose powder deposited on a substrate inside a chamber filled with inert gas. Finally, only one company (ARCAM) produces machines that use electron beam under high vacuum to melt the powder with the name of electron beam melting (EBM) [1][2][3]. This paper summarizes the main results on the study of Nibased superalloys produced by DMLS, developed at Department of Applied Science and Technology (DISAT) of Politecnico di Torino and Istituto Italiano di Tecnologia (IIT), in the Center for Sustainable Futures also settled in Torino. In particular, we have studied two Inconel alloys, Inconel 625 and 718, investigating their microstructures and the effects of heat treatments on the microstructure. The process parameters for these two alloys have been defined and components as well as prototypes production is currently available at our site. Inconel 625 and Inconel 718 produced by DMLSInconel 718 and 625 have been used in high-temperatu...

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