Microstructure evolution and mechanical response-based shortening of thermal post-treatment for electron beam melting (EBM) produced Alloy 718

Sneha Goel,Enrico Zaninelli,Tejas Gundgire,Magnus Ahlfors,Olanrewaju Ojo,Uta Klement,Shrikant Joshi

doi:10.1016/j.msea.2021.141515

Abstract

Electron beam melting (EBM) produced Alloy 718 was subjected to thermal post-treatment involving hot isostatic pressing (HIPing) and heat treatment (HT). Subjecting the material to HIPing at 1120 °C led to significant densification. Study of microstructure evolution during HT (comprising of solution treatment and aging) showed possibility of significantly shortening the HT duration, particularly the time for two-step aging from the standard (8 h + 8 h) long cycle to possibly a shortened (4 h + 1 h) cycle. Another approach for shortening the post-treatment cycle by integrating the HIPing with HT inside the HIP vessel was also successfully implemented. The above observations were further substantiated by tensile response of the material subjected to the varied post-treatment cycles; out of all the post-treatments steps, tensile behaviour was observed to be mainly affected by the aging treatment. Further prospects for shortening the post-treatment protocol are also described, such as shortening of HIPing duration for the typical 4 h to 1 h cycle as well as possible elimination of solution treatment step from the entire post-treatment protocol specifically when prior HIPing is performed. Heat treatment with prior HIPing was found to be crucial for improving fatigue life, because subjecting EBM Alloy 718 to only HT, irrespective of the short or standard long protocol, rendered inferior fatigue response.

Highlights

Electron beam melting (EBM), which is one of the growing additive manufacturing (AM) techniques [1], is capable of producing complex geometries in layer-by-layer fashion
Consistent with the main goal of the present study, the effect of each steps associated with post-treatment, namely hot isostatic pressing (HIPing), solution treatment (ST) and two-step aging, on the microstructure and mechanical behaviour of EBM Alloy 718 was comprehensively investigated
These results demonstrate that the shortened aging treatment does not compromise in a notable way the tensile properties achieved following the post-treatment as specified in the ASTM standard

Summary

Introduction

Electron beam melting (EBM), which is one of the growing additive manufacturing (AM) techniques [1], is capable of producing complex geometries in layer-by-layer fashion. Electron beam melting has been used to produce geometries with different types of materials, such as Ti-based alloys, Co-based superalloys, Ni-based superalloys, etc. Due to the pro cessing conditions that prevail, defects are typically formed in EBM built Alloy 718. These defects have been known to be responsible for reducing fatigue life [5] and introducing anisotropy [6] in this material. More over, due to the segregation of the alloying elements, Nb, in the interdendritic region during solidification together with high pow der bed temperature, the as-built microstructure can exhibit large amounts of δ phase precipitates [7]. It is important to control the microstructure and the mechanical behaviour of EBM Alloy 718 through appropriate post-processing thermal treatments ( referred merely as post-treatments)

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Conclusion