The origin and formation of oxygen inclusions in austenitic stainless steels manufactured by laser powder bed fusion

Abstract

The origins of nano-scale oxide inclusions in 316L austenitic stainless steel (SS) manufactured by laser powder bed fusion (L-PBF) was investigated by quantifying the possible intrusion pathways of oxygen contained in the precursor powder, extraneous oxygen from the process environment during laser processing, and moisture contamination during powder handling and storage. When processing the fresh, as-received powder in a well-controlled environment, the oxide inclusions contained in the precursor powder were the primary contributors to the formation of nano-scale oxides in the final additive manufactured (AM) product. These oxide inclusions were found to be enriched with oxygen getter elements like Si and Mn. By controlling the extraneous oxygen level in the process environment, the oxygen level in AM produced parts was found to increase with the extraneous oxygen level. The intrusion pathway of this extra oxygen was found to be dominated by the incorporation of spatter particles into the build during processing. Moisture induced oxidation during powder storage was also found to result in a higher oxide density in the AM produced parts. SS 316L powder free of Si and Mn oxygen getters was processed in a well-controlled environment and resulted in a similar level of oxygen intrusion. Microhardness testing indicated that the oxide volume fraction increase from extraneous oxygen did not influence hardness values. However, a marked decrease in hardness was found for the humidified and Si-Mn free AM processed parts.

Introduction

Metal-based additive manufacturing (AM) has made appreciable impacts in the metal fabrication industry due to the ability to fabricate complex geometry parts that cannot be easily achieved by conventional methods [1]. Selective laser melting (SLM), also known as laser powder bed fusion (L-PBF), has emerged as the optimum choice to offer high manufacturing resolution and excellent part quality among many metal AM technologies. With the assistance of computer-aided design (CAD), L-PBF creates complex structures in a layer-by-layer fashion through selective laser melting according to a sliced and pixelated, 3D computer model. The feedstock powder for L-PBF is generally manufactured by the inert gas atomization process [1], which usually results in the powder containing hundreds of ppm of oxygen [2]. Owing to its large anion radius, oxygen cannot be easily accommodated interstitially in austenitic stainless steels [3,4]. Thus, excess oxygen typically remains in the matrix where it can react with high oxygen affinity elements in steel to form oxide inclusions [5]. In the L-PBF process, there is greater opportunity for oxygen to form inclusion. As such, uniformly distributed nano-scale oxides rich in Si and Mn has been reported in 316L SS built by L-PBF [6,7]. Some studies show that nano-scale oxide inclusions can improve the mechanical properties of AM 316L SS compared to those devoid of them [[8], [9], [10], [11]], while others note that the strengthening effect of oxides is negligible at best [12]. Recent work has also suggested oxide inclusions in AM 316L SS suppresses MnS formation, which leads to superior pitting corrosion resistance [13]. However, when compared to their wrought counterparts, these inclusions were also shown to have detrimental effects on cracking behavior, resulting in lower toughness and higher stress corrosion cracking (SCC) susceptibility [14].

Inclusions in alloys are usually considered microstructural defects that typically deteriorate mechanical properties and/or corrosion resistance [15]. In metal casting, inclusions commonly consist of nitrides, carbides, sulfides and a mixture of compounds that can nucleate voids and microcracks that lead to premature failure under impact or fatigue loading [15]. Moreover, it has been demonstrated that inclusion-based defects can entrap hydrogen and increase the material’s susceptibility to hydrogen embrittlement [16]. Finally, although stainless steel is well known for its superior corrosion resistance, the existence of inclusions can compromise the protective chromium oxide layer to cause pitting corrosion [15]. Despite this significant list of detrimental effects, oxide inclusions have great potential to improve alloy strength by introducing uniformly dispersed, ultrafine particles into the metal matrix that impede dislocations during plastic deformation. A significant benefit of this oxide dispersion strengthening (ODS) is superior creep resistance at elevated temperatures [[17], [18], [19], [20], [21]]. Additionally, high-density nano-oxide particles can act as sinks to radiation-induced vacancies and interstitials towards significantly reducing radiation damage in nuclear reactor environments [18,22,23]. Recent efforts have also been made to produce effective nano-scale oxide dispersions in metals fabricated by AM [[24], [25], [26], [27]].

In L-PBF fabrication of SS, oxide formation can significantly impact the build quality and reproducibility, especially when printing materials with higher oxygen reactivity, such as aluminum and titanium. On the other hand, finding an effective way to control and optimize oxide dispersions in L-PBF fabrication can lead to the development of advanced ODS alloys with superior performance. Compared to the significant interests on the characteristics and properties that oxide inclusions can impart to AM parts, a fundamental understanding of where these oxides originated has not been well established.

Considering the entire AM processing chain, from powder fabrication to the end-use part, there are three possible oxygen sources/pathways of incorporation into the alloy. The first and most obvious is oxygen incorporation during the gas atomization fabrication of the powder. Another source would be in-situ oxidation during L-PBF from extraneous oxygen in the process environment. Another pathway involves moisture-induced oxidation from powder transportation and storage conditions. Studies have shown that oxygen contamination in precursor powders has a linear relationship with the density of oxide inclusions in parts produced by powder metallurgy with hot isostatic pressing (PM-HIP) [28,29]. Similar studies have also evaluated gas atomized 316L SS powder for AM applications [[30], [31], [32]]. However, the surface and interior oxidation of the powders during gas atomization and its impacts on the part quality has not been systematically evaluated.

It has also been a long-time interest of the AM community to understand the effects of moisture on the quality of AM parts, especially related to oxidation. Some studies have suggested that liquid bridging between particles and surface oxides caused by moisture contamination can retard the flowability of AM powders [33,34]. To our best knowledge, the effects of moisture exposure to oxide inclusion formation, including their effect on mechanical properties, have not been evaluated.

Compared to the potential oxygen contamination on the precursor powders, there are fewer concerns of oxygen intrusion during the L-PBF process, primarily due to the protective, closed-loop argon environment. However, a residual oxygen level in the process chamber is unavoidable and can fluctuate with build time and gas supply. Some efforts have been made to evaluate the effects of environmental oxygen in the process chamber on oxide inclusions in austenitic stainless steel. Eo et al. studied the effects of the laser metal deposition (LMD) process parameters on melt pool oxidation for 316L SS and suggested that yield stress was proportional to inclusion number density [35]. Song et al. examined the effects of environmental oxygen partial pressure on the morphology, composition, size and number density of oxide inclusions [36]. Metals printed by L-PBF generally experience much faster cooling rates as compared to those by LMD. Thus, different oxidation reaction dynamics is expected. The dominant path of oxygen transfer during L-PBF and its impacts on microstructure and mechanical properties is critical to understanding their effects and controlling their formation and evolution to improve mechanical performance. This work is aimed at providing a systematic understanding of the origin and formation of oxygen inclusions in austenitic stainless steels manufactured by L-PBF.