The Effect of Location and Post-treatment on the Microstructure of EBM-Built Alloy 718

Abstract

Additive manufacturing (AM) of Ni-based superalloys such as Alloy 718 may obviate the need for difficult machining and welding operations associated with geometrically intricate parts, thus potentially expanding design possibilities and facilitating cost-effective manufacture of complex components. However, processing AM builds completely free from defects, which may impair mechanical properties such as fatigue and ductility, is challenging. Anisotropic properties, microstructural heterogeneities and local formation of undesired phases are additional concerns that have motivated post-treatment of AM builds. This work investigates the microstructural changes associated with post-treatment of Alloy 718 specimens produced by Electron Beam Melting (EBM) for as-built microstructures at 3 build heights: near base plate, in the middle of build and near the top of the build. Two different post-treatment conditions, hot isostatic pressing (HIP) alone and a combined HIP with solutionising and two-step aging were examined and compared to the results for the as-built condition. The influence of various posttreatments on minor phase distributions (, γ", carbides), overall porosity, longitudinal grain widths and Vickers microhardness was considered. The HIP treatment led to significant reduction in overall porosity and dissolution of  phase, which led to appreciable grain growth for both post-treatment conditions. The variation in hardness noted as a function of build height for the as-built specimens was eliminated after post-treatment. Overall, the hardness was found to decrease after HIP and increase after the full HIP, solutionising and aging treatment, which was attributed to dissolution of γ" during HIP and its re-precipitation in subsequent heat treatment steps.

Effect of build location on microstructural characteristics and corrosion behavior of EB-PBF built Alloy 718

“…The average value was in the range of 380-410 HV 0.5 . This value was comparable to the values reported in the literature [1,8,10,41].…”

Encapsulation of Electron Beam Melting Produced Alloy 718 to Reduce Surface Connected Defects by Hot Isostatic Pressing

“…Three types of defects were observed, i.e., lack of fusion, shrinkage porosity, and gas porosity, and they are visualized in Figure 3. Such defects have also been previously observed by Goel et al [3], and the influence of the defects on mechanical properties has been elaborated elsewhere [16,17]. Round-shaped gas porosities (refer Figure 3a) were randomly distributed, and they are attributable to entrapped gas inside the virgin powder, which could have found its way into the EBM build [1].…”

“…This technology offers significant design freedom compared to conventional manufacturing methods such as forging, casting, etc. [3]. Electron beam melting (EBM) is one of the AM techniques; it utilizes an electron beam as the heat source [4].…”

“…However, EBM-manufactured Alloy 718 is typically characterized by the presence of inevitable defects such as lack of fusion, gas porosity, and shrinkage porosity, which can be detrimental to the mechanical properties of the material [6]. Therefore, thermal post-treatment involving hot isostatic pressing (HIPing) is typically employed to reduce such defects in the AM manufactured material [3,[7][8][9]. In this context, it is pertinent to state that micro-computed tomography is used to obtain detailed information about the location and size of defects present in the material in 3D [10][11][12].…”

On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718

“…For the EBM cases, the very different grain growth behaviours are observed in contour and hatch region after HIP at 1200 • C/120 MPa/4 hours: noticeable grain growth is present in the contour region but not in the hatch region [127,128]. In addition, even for the columnar grains of EBM IN718, the coarsening behaviours can be different from one case to another, due to the presence of grain boundary carbide and its impinging effect [98,129,130]. Comparatively, as mentioned, the as-built SLM IN718 usually associates with large residual stress than the as-built EBM counterpart, which can provide an additional driving force for grain growth.…”