Microstructural analysis of Inconel 718 manufactured via direct energy deposition: response surface methodology for process parameters optimisation and post-heat treatment

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 x 50 x 3 mm3 was built. Mechanical and microstructural characterisation of the built block before and after heat treatment (homogenisation at 1080 degrees C, followed by solubilisation at 980 degrees C, and double ageing at 720 and 620 degrees 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 gamma' and gamma", 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.