The Origins of Nanoscale Oxide Inclusion and its Evolution in Additively Manufactured Austenitic Stainless Steel During Laser Powder Bed Fusion and Post Heat Treatment

Abstract

The origins of nanoscale 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 nanoscale 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 and tensile testing indicated that the oxide volume fraction increase from extraneous oxygen did not influence mechanical properties. However, a marked decrease in hardness was found for the humidified and Si-Mn free AM processed parts. Spatter particle re-incorporation was found to be significantly affected by the laser scanning direction with respect to inert gas flow in the L-PBF process chamber. Laser scans parallel to the gas flow of the L-PBF process, more specifically, against the gas flow reduce the risk of spatter particle re-incorporation in as-printing parts. Laser scans perpendicular to the gas flow leads to more partially sintered spatter particles on the as-built sample surface. Except for the role of oxygen getters, spatter particles also result in higher surface roughness for as-built AM 316L SS owing to its larger average size than fresh powder. Other than spatter particles generated from the melt pool, spatter particles from adjacent parts on the build plate could also incorporate into the as-printing parts. Although spatter particles have been shown to introduce defects in AM parts, a decrease in tensile properties was not observed for as-built parts contain a higher amount of spatter particles, possibly due to the small build volume. The evolution of nanoscale oxide inclusions presented in 316L SS manufactured by L-PBF was explored from three aspects, size, chemistry and morphology, and distribution. The average size of oxide inclusion increased from 50 nm in as-built 316L SS to 392 nm in fully recrystallized 316L SS. The coarsening of oxides was found to be controlled by Ostwald ripening, and the recrystallization process considerably facilitated the coarsening. A fraction of MnSiO3 oxides converted to core-shell structure with Mn enriches at the shell and Si-rich core in the recrystallized grain, and some converted to CrMn2O4 in the unrecrystallized grain. The grain boundary migration has shown to capable of dragging oxides and move jointly. This led to a significant amount of oxides accumulated at the grain boundary. The fraction of grain boundary oxides decreases with increasing grain size.