Non-oxide precipitates in additively manufactured austenitic stainless steel

Abstract

Precipitates in an austenitic stainless steel fabricated via any Additive Manufacturing (AM), or 3D printing, technique have been widely reported to be only Mn-Si-rich oxides. However, via Transmission Electron Microscopy (TEM) studies on a 316L stainless steel, we show that non-oxide precipitates (intermetallics, sulfides, phosphides and carbides) can also form when the steel is fabricated via Laser Metal Deposition (LMD)—a directed energy deposition-type AM technique. An investigation into their origin is conducted with support from precipitation kinetics and finite element heat transfer simulations. It reveals that non-oxide precipitates form during solidification/cooling at temperatures ≥ 0.75Tm (melting point) and temperature rates ≤ 105 K/s, which is the upper end of the maximum rates encountered during LMD but lower than those encountered during Selective Laser Melting (SLM)—a powder-bed type AM technique. Consequently, non-oxide precipitates should form during LMD, as reported in this work, but not during SLM, in consistency with existing literature.

3D deep convolutional neural network segmentation model for precipitate and porosity identification in synchrotron X-ray tomograms

“…Previous investigations via TEM (Upadhyay et al, 2021;Ben Haj Slama et al, 2022) have revealed the presence of precipitates (oxide, non-oxide and mixed precipitates) at a length scale that is similar to the objects (precipitates or porosities) observed in TXM micrographs, although no distinction can be made between the objects in the TXM micrographs. Furthermore, while no porosities were observed in the TEM investigations (Upadhyay et al, 2021;Ben Haj Slama et al, 2022), it has to be reminded that TEM investigations only probe a very small volume within which porosities may not have been present. In comparison with TEM lamellae, micropillars have a volume that is four orders of magnitude higher.…”

“…A singletrack bidirectionally printed three-layer thin-wall of dimen-sions 100 mm  0.6 mm  0.6 mm (x, y, z) was manufactured using this machine; the process parameters were: laser power = 225 W, powder flow rate = 6.5 g min À1 , deposition speed = 2000 mm min À1 , vertical displacement of focusing head = 0.2 mm after depositing one layer. Additional details on the material, powder characteristics and specifications of the LMD machine have been given by Upadhyay et al (2021) andBen Haj Slama et al (2022).…”

Insight on precipitate evolution during additive manufacturing of stainless steels via in-situ heating-cooling experiments in a transmission electron microscope

“…The material and most of the experimental setup used in this work has been described in detail in Upadhyay et al [30]. In the following, only the essential details are recalled for the sake of completeness and self-sufficiency of this paper.…”

“…Until recently, precipitates occurring in as-built LMD 316LSS or SLM 316LSS were reported to be only Mn-Si-rich oxide precipitates [22,[24][25][26][27][28][29]. However, in a recent TEM study on five lamellae extracted from an LMD 316LSS thin-wall, Upadhyay et al [30] reported the presence of a large volume fraction of non-oxide precipitates including sulfides, carbides, phosphides and intermetallics, along with Mn-Si-rich oxide precipitates. The non-oxide precipitates were found to be smaller in their average size in comparison to the oxide precipitates.…”

“…The non-oxide precipitates were found to be smaller in their average size in comparison to the oxide precipitates. An investigation into the origin of these non-oxide precipitates [30], with the help of finite element and precipitation kinetics simulations, revealed that non-oxide precipitates can indeed occur in LMD 316LSS but not in SLM 316LSS. The simulation results also suggested that precipitates could form/disappear and evolve in the solid-state.…”