In recent years, additive manufacturing (AM) of Alloy 718 has received increasing interest in the field of manufacturing engineering because of its attractive features compared with those of conventional manufacturing methods. Nevertheless, owing to the inherent nature of the process, the build material is exposed to complex thermal conditions that affect the microstructure. In addition, the post heattreatments applied to the built component further cause microstructural changes. Thus, obtaining the desired microstructure that gives the desired properties is still a challenging task. Therefore, understanding the microstructure formation during the build and subsequent post-heat treatment is important and is the objective of this thesis work.

To this end, a computational modelling approach was used that combines multiphase-field modelling with transformation kinetics modelling. Two different AM processes, laser metal powder directed energy deposition (LM-PDED) and electron beam powder bed fusion (EB-PBF), were considered in this study.Based on the modelling work, it was observed that solidification conditions (thermal gradients and cooling rates) that occur during the AM process have an impact on the as-solidified microstructure in Alloy 718 and the resultant Laves phase formation. With an increase in cooling rate, the Laves phase volume fraction becomes lower and the morphology tends to become discrete particles,which is important for resisting the formation of liquation cracks in Alloy 718. It was also found that the precipitates formed during the solidification process did not undergo any significant change during subsequent thermal cycles associated with the deposition of subsequent layers, given that the deposition of the subsequent layer does not increase the global temperature of the build to> 600 °C. If the global temperature increases above 600 °C, then phase changes are expected, depending on the temperature value. In the case of the EB-PBF process, the high build temperature maintained in the build chamber resulted in an ‘‘in situ’’ heat treatment, which had a homogenisation effect on the as-solidified microstructure because of the smaller dendrite spacing and relatively low Lavesphase size. In the case of the LM-PDED, the microsegregation of composition observed in the as-built microstructure was shown to change the equilibrium conditions and precipitation kinetics of Alloy 718. As a result, excess precipitationof γ'/γ″ and δ was observed in the interdendritic region compared with the dendrite core, depending on the type of heat treatment used.

In addition, modelling was performed to evaluate the elastic properties of EB-PBF Alloy 718. To this end, crystallographic orientation data gathered from EBSD data and single-crystal elastic constants were used. The prediction showed good agreement with published literature data. The hatch (bulk) region of the EB-PBF samples showed significant anisotropic elastic properties because of the strong crystallographic texture observed in the microstructure. The lowest Young’s modulus was observed along the build direction. Normal to the build direction, the elastic properties were shown to be isotropic. Overall, the elastic behaviour of the hatch region was similar to that of a transversely isotropic case