Microstructural control during laser powder fusion to create graded microstructure Ni-superalloy components

This work explores the feasibility of additively manufacturing tailored microstructures through varying process parameters, to eventually control the mechanical properties and performance. The investigation focuses on controlling the heat input and thermal history during laser-powder bed fusion of IN718 through process parameter manipulation; notably the heat input parameters (power, scan speed, and hatch spacing) and island scanning parameters (island size, shift, and island overlap). The changes in preferred orientation, morphology and grain size were characterised in both the transverse and build cross-sections using scanning electron microscopy (SEM) and electron back scatter diffraction (EBSD), while the texture development was comparatively characterised using X-ray diffraction (XRD). The solidification cell size was quantified to estimate the influence of the process parameters on the cooling rates. This was also rationalised using a thermal model resolving the scan characteristics to provide the transient temperature distribution to a numerical grain growth model. Based on the obtained microstructures, graded microstructures were generated using the island strategy and identical laser parameters throughout but changing subtle features such as the island size and shift. A suitable post-process heat treatment was applied to retain the tailored microstructures, while obtaining the required hardness.