In recent years, the industrial application of Inconel 625 has grown significantly. This material is a nickel-base alloy, which is well known for its chemical resistance and mechanical properties, especially in high-temperature environments. The fatigue performance of parts produced via Metallic Additive Manufacturing (MAM) heavily rely on their manufacturing parameters. Therefore, it is important to characterize the properties of alloys produced by a given set of parameters. The present work proposes a methodology for characterization of the mechanical properties of MAM parts, including the material production parametrization by Laser Directed Energy Deposition (DED). The methodology consists of the testing of miniaturized specimens, after their production in DED, supported by a numerical model developed and validated by experimental data for stress calculation. An extensive mechanical characterization, with emphasis on high-cycle fatigue, of Inconel 625 produced via DED is herein discussed. The results obtained using miniaturized specimens were in good agreement with standard-sized specimens, therefore validating the applied methodology even in the case of some plastic effects. Regarding the high-cycle fatigue properties, the samples produced via DED presented good fatigue performance, comparable with other competing Metallic Additive Manufactured (MAMed) and conventionally manufactured materials.

The application of maraging steels such as 18Ni300 alloy is noteworthy for mould industries, applying repair purposes through direct energy deposition process. This objective requires microstructural characterizations and the evaluation of mechanical behaviour such as hardness. The state of substrate material, including the heat-affected zone (HAZ) and the interface between the HAZ and deposited layer, is essential, the formation of hard phases and abrupt transitions. Thus, the influence of the number of deposited layers or the pre-heating condition appears noteworthy. In the current study, microscopy observations did not reveal the presence of any crack in the cross-sections of deposited 18Ni300 alloy powder on AISI 1045 sheet steel; however, pores were observed in deposited layers. Besides, microscopic analyses revealed the achievement of a smooth HAZ in the deposited layers composed of three-layered depositions or that received preheating, confirmed by hardness measurements as well. Dilution effect ensured a metallurgical bonding between depositions and substrate, strongly affected by preheating. The HAZ microstructure, mainly martensitic transformation, distribution of chemical composition, epitaxial growth at the interface, and the size of crystals and grains were affected by preheating or the number of layers. Moreover, the heat propagation and/or dissipation across the deposited layers influenced the dendrite morphology and the texture of grains. The preheating condition provoked the formation of cellular/equiaxed dendrites that was highlighted in the three-layered deposition, increase in dendrite interspace growth.

The mechanical behaviour of as-built DED-produced 18Ni300 Maraging steel was studied by manufacturing a wall-like structure from which three different specimen types were obtained: specimens in which the loading direction was the same as the printing direction (vertical), specimens in which these two directions were perpendicular (horizontal), and bimetallic specimens in which the interface between the AISI 1045 substrate and the 18Ni300 steel was tested. The yield strength of the produced samples was $987.9\pm 34.2$, $925.9\pm 89.7$ and $486.7\pm 47.2$$\mathrm{M}$$\mathrm{Pa}$ for the vertical, horizontal and bimetallic specimens, respectively, while the elongation to failure was $9.4\pm 1.9$, $18.3\pm 2.3$ and $14.06\pm 0.6\%$ in the same order. The latter specimen failed within the substrate-comprised portion of the specimen. Additionally, the fracture surfaces were analysed through scanning electron microscopy, concluding that while both surfaces consist of dimples, the horizontal specimen presented microporosities with a reduced diameter. A microhardness analysis in the printed wall-like structure following the printing direction yielded an average hardness of $392\pm 21$ HV0.3, with fluctuations along the build direction mostly within one standard deviation.

Direct Energy Deposition (DED) is a highly efficient technology for the manufacture and/or repair of different components for multiple industries, including aeronautical. However, a good understanding of the various process variables is essential for producing deposits with the desired properties and ensuring process efficiency. In this study, the optimisation of Inconel 718 single-track deposits was carried out by varying laser power, scanning speed, and powder feed rate to minimise dilution and maximise height, always ensuring the efficiency of the process. The influence of the DED process’ inputs (laser power, scanning speed, and feed rate) and outputs (deposit’s geometry and dilution) were evaluated in the construction of Response Surface Plots and regression analysis equations, allowing the selection of the most promising processing conditions that achieve high-quality single tracks, with predicted dilution percentage, and deposit height. Optimisation analysis selected process parameters, and a block with 300 × 50 × 3 mm³ was built. Mechanical and microstructural characterisation of the built block before and after heat treatment (homogenisation at 1080 °C, followed by solubilisation at 980 °C, and double ageing at 720 and 620 °C) was performed through Optical Microscopy, SEM/EDS analysis, Vickers hardness and tensile tests. It could be observed that the chosen process leads to the formation of a dendritic structure with considerable amounts of Laves phase. However, heat treatments make it possible to obtain a more homogeneous structure with higher amounts of hardening phases, such as γ’ and γ”, which promotes an increase in hardness and tensile strength, and partial elimination of the Laves phase. With proper optimisation of the DED process and understanding of the different parameters’ influence on the deposit’s microstructure, it is possible to manufacture or repair Inconel 718 components for several industries.

Performing fatigue characterisation is often an expensive task, being both time consuming and expensive. Taking that into account, ultrasonic fatigue testing is an interesting solution, since it can be thousands of times faster than traditional experiments. In ultrasonic fatigue testing, excitation frequencies are in the order of magnitude of 20 kHz, while common fatigue testing frequencies are typically approximately a few hundreds of Hz. Although promising, ultrasonic fatigue testing has some challenges, like high strain rates, heat generation and complex specimen design. Regarding the latter, since the working principle of ultrasonic fatigue tests relies on exciting the specimen in one of its natural frequencies, finding a specimen geometry to resonate at this given frequency might be challenging. Additionally, some materials often present challenges associated with high temperature during tests. The goal of this paper is to provide guidelines for specimen design, encompassing the effects of critical factors and their impact on important test parameters, like temperature and dimensions. The proposed methodology developed a parameter able to quantify the heat generation severity during ultrasonic fatigue testing for several materials based on their physical properties. Moreover, the effects of the geometry and material properties in the temperature during loading cycles, with special focus on thermal gradients were enumerated.