Microstructure evolution-based design of thermal post-treatments for EBM-built Alloy 718

Abstract

Alloy 718 samples were fabricated by electron beam melting (EBM) additive manufacturing process. The work focused on systematic investigation of response of the material to various thermal post-treatments, involving hot isostatic pressing (HIPing), solution treatment (ST) and two-step aging, to tailor post-treatment procedure for EBM-built Alloy 718. Results showed that HIPing at lowered temperature can be used for attaining desired defect closure while preserving grain size. Subjecting the material to ST, with or without prior HIPing, mainly caused precipitation of δ phase at the grain boundaries with prior HIPing decreasing the extent of δ phase precipitation. Moreover, results suggest that the utility of ST, with prior HIPing, could be dictated by the need to achieve a certain δ phase content, as the typically targeted homogenization after ST had already been achieved through HIPing. Detailed investigation of microstructural evolution during subsequent aging with and without prior HIPing showed that a significantly shortened aging treatment (‘4 + 1’ h), compared to the ‘standard’ long treatment (‘8 + 8’ h) traditionally developed for conventionally produced Alloy 718, might be realizable. These results can have significant techno-economic implications in designing tailored post-treatments for EBM-built Alloy 718. Graphical abstract

On the Thermomechanical Aging of Lpbf Alloy 718

“…Some studies observed that heat treatments (ageing at 650 • C for 168h [19], solution treating at 980 • C for 1h [45]) and HIP treatments [46] did not affect the formation or growth of carbides, probably because the temperatures were too low. Indeed, high homogenisation temperatures were shown to result in larger carbide size [47], which is said to be stable up to 1200 • C [48].…”