Thermal Post-Treatment of Alloy 718 produced by Electron Beam Melting
SNEHA GOEL HÖGSKOLAN VÄST
AKADEMISK AVHANDLING
som med tillstånd av Forsknings- och forskarutbildningsnämnden vid Högskolan Väst, för avläggande av doktorsexamen i produktionsteknik,
framläggs för offentlig granskning.
Torsdag den 26 November 2020 klockan 13:15 i F104, Albertsalen, Högskolan Väst Opponent: Professor Moataz Attallah
University of Birmingham, United Kingdom
Abstract
Title: Thermal post-treatment of Alloy 718 produced by electron beam melting
Keywords: Additive Manufacturing, Electron Beam Melting; Alloy 718; Hot Isostatic Pressing;
Heat Treatment; Microstructure Evolution; Mechanical Properties ISBN 978-91-88847-76-8 (Electronic version)
ISBN 978-91-88847-77-5 (Printed version)
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 service requirements. 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’ hour) 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 each of the post-treatment steps to develop an improved understanding of how the 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 aging led 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-step aging, detailed investigation of microstructure evolution and hardness changes showed that, particularly the conventional ‘8+8’
hour long two-step aging treatment can be shortened to a ‘4+1’ hour 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.
