As-Built and Post-treated Microstructures of an Electron Beam Melting (EBM) Produced Nickel-Based Superalloy

Abstract

The microstructures of an electron beam melted (EBM) nickel-based superalloy (Alloy 718) were comprehensively investigated in as-built and post-treated conditions, with particular focus individually on the contour (outer periphery) and hatch (core) regions of the build. The hatch region exhibited columnar grains with strong 〈001〉 texture in the build direction, while the contour region had a mix of columnar and equiaxed grains, with no preferred crystallographic texture. Both regions exhibited nearly identical hardness and carbide content. However, the contour region showed a higher number density of fine carbides compared to the hatch. The as-built material was subjected to two distinct post-treatments: (1) hot isostatic pressing (HIP) and (2) HIP plus heat treatment (HIP + HT), with the latter carried out as a single cycle inside the HIP vessel. Both post-treatments resulted in nearly an order of magnitude decrease in defect content in hatch and contour regions. HIP + HT led to grain coarsening in the contour, but did not alter the microstructure in the hatch region. Different factors that may be responsible for grain growth, such as grain size, grain orientation, grain boundary curvature and secondary phase particles, are discussed. The differences in carbide sizes in the hatch and contour regions appeared to decrease after post-treatment. After HIP + HT, similar higher hardness was observed in both the hatch and contour regions compared to the as-built material.

A phase-field study of neck growth in electron beam powder bed fusion (EB-PBF) process of Ti6Al4V powders under different processing conditions

“…Thanks to the vacuum environment and the preheating step, the typical working temperature during an EB-PBF is high, and it ranges, e.g. between 700 °C [58,59] and 1050 °C [59][60][61][62] for titanium alloys.…”

Effect of post-processing on microstructure and mechanical properties of Alloy 718 fabricated using powder bed fusion additive manufacturing processes

“…Niobium segregation during high-temperature process leads to the formation of Laves phase known to be detrimental to the material strength and ductility, by depleting the principal elements needed for precipitation strengthening (Qi et al, 2009;Parimi et al, 2014). The microstructure of AM IN718 has been investigated in quite a few studies (Hosseini and Popovich, 2019;Deng, 2019;Qi et al, 2009;Deng et al, 2018;Goel et al, 2020;Murr et al, 2011;Parimi et al, 2014;Goel, 2020;Strondl et al, 2011;Lu et al, 2015). The presence of phases discussed above are confirmed in AM IN718 parts as well.…”

“…Goel et al showed that HIP and HIP 1 HT lead to significant reduction of defects including porosities and lack of fusion (Goel et al, 2018a). It is also reported that HIPing completely dissolve the d phase present in the as-built parts, leading to grain growth (Goel et al, 2020). HIP 1 HT increase the hardness for PBF-EB samples compared to the as built parts owing to precipitation of g 0 0 (Goel et al, 2018b).…”

Review of Powder Bed Fusion Additive Manufacturing for Metals

“…Precipitates in the as-built IN718 alloy included γ , γ", δ, TiN and NbC [112]. The alloy built using a contour scanning strategy showed a higher density of fine carbides than the one fabricated using the hatch scanning strategy; the difference is negligible after appropriate heat treatment [119]. An increase in grain width and precipitation coarsening was found as the distance from build table increased [109,116].…”