Electron beam melting (EBM), a metal additive manufacturing (AM) process, has received considerable industrial attention for near net shape manufacture of complex geometries with traditionally difficult-to-machine materials. This has fuelled considerable academic interest in investigating EBM of Alloy 718, a nickel ironbased superalloy possessing an exciting combination of good mechanical behaviour and cost effectiveness. EBM production of Alloy 718 is particularly promising for aerospace and other sectors which value rapid production of components with large scope for design flexibility. The EBM builds are characterized by presence of inevitable defects and, anisotropy within a build is also a concern. Consequently, as-built Alloy 718 has to be subjected to post-build thermal-treatments (post-treatments) to ensure that the parts eventually meet the critical service requirements. Not withstanding the above, limited knowledge is available about optimal post-treatments for EBM-built Alloy 718. Therefore, the main focus of the work presented in this thesis was to systematically investigate the response of EBM-built material to post-treatments, which include hotisostatic pressing (HIPing), solution treatment (ST), and aging.

HIPing of EBM-built Alloy 718 led to more than an order of magnitude reduction in defect content, which was reduced from as high as 17% to < 0.2% in samples built with intentionally introduced porosity to investigate limits of defect closure achievable through HIPing. In addition, HIPing also caused complete dissolution of δ and γ" phases present in the as-built condition, with the latter causing dropin hardness of the material. HIPing had no effect on the carbides and inclusions such as TiN, Al2O3 present in the built material. The evolution of microstructure during ST and aging was systematically investigated. Growth of potentially beneficial grain boundary δ phase precipitates was found to cease after a certain duration of ST, with samples subjected to prior-HIPing exhibiting lesser precipitation of the δ phase during ST. While the specimen hardness increased onaging, it was observed to plateau after a duration significantly shorted than the specified ASTM 'standard' aging cycle. Therefore, prima facie there are promising prospects for shortening the overall heat treatment duration. A combination of HIPing, ST, and aging treatments in a single uninterrupted cycle was also explored. Future work involving incorporation of a shortened heat treatment schedule in a combined cycle can have significant industrial implications.

Additive manufacturing (AM) has emerged as a disruptive technology and it is a vital part in the present era of fourth industrial revolution, Industry 4.0.Electron beam melting (EBM), a metal AM process, has received considerable industrial attention for near net shape manufacture of complex geometries with traditionally difficult-to-machine materials. EBM production of Alloy 718, a nickel-iron based superalloy possessing good mechanical and corrosion properties at elevated temperatures, is particularly promising for aerospace and energy sectors. However, EBM Alloy 718 builds are typically characterized by presence of inevitable defects and anisotropy, warranting post-processing thermal-treatments (post-treatments) to ensure that the components eventually meet the critical servicerequirements. The existing post-treatment standards include hot isostatic pressing (HIPing) over the temperature range of 1120°C-1185°C, followed by solution treatment (ST) and a two-step (‘8+8’ hours) aging under conditions conventionally adopted for cast and wrought Alloy 718, and no effort has yet been invested in optimizing post-treatment schedules specifically for EBM Alloy 718. Consequently, the objective of this work was to systematically investigate the response of EBM-built material to eachof the post-treatment steps to develop an improved understanding of howthe microstructure evolves with time during each step, since such knowledge can lay the foundation for optimizing the post-treatment protocol.Through study of microstructure and mechanical property assessment it was found that the temperature during HIPing can be reduced to 1120°C compared to the common practice employing higher temperatures. In addition, HIPing also caused complete dissolution of δ and γ"/γ' phases, promoted homogenization and resulted in drop in hardness but had no evident effect on the carbides and inclusions such as TiN and Al2O3 present in the as-built material. Subjecting EBM Alloy 718 to ST and two-step agingled to precipitation of δ phase and γ"/γ' phases, respectively.

The evolution of microstructure during ST and two-step aging was also systematically investigated. Progressive precipitation and growth of grain boundary δ phase precipitates was observed during the entire 1 hour duration of ST, with samples not subjected to prior-HIPing exhibiting higher amount of the δ phase precipitation during ST. During the two-stepaging, detailed investigation of microstructure evolution and hardness changes showed that, particularly the conventional ‘8+8’ hour long two-stepaging treatment can be shortened to a ‘4+1’ hours treatment. Such shortened treatment was observed to be robust when applied to various kinds of EBM builds. Another approach for shortening post-treatment by integrating HIPing and HT inside the HIP vessel was also successfully implemented. These approaches with shortened post-treatment were also found to not compromise the mechanical response of EBM Alloy 718. Further shortening of the typical long thermal post-treatment cycle, through reduction in HIPing time from 4 hours to 1 hour and possible elimination of ST, also appears promising.