Laser-based powder bed fusion of stainless steels

Doctoral thesis, 2022

The aim of the present work has been to widen the knowledge of how variations within powder manufacturing affect laser-based powder bed fusion processing, and how this processing affects the microstructure and strength of stainless steels. The approach was to keep the processing parameters fixed while the powder feedstock was varied. This methodology enabled an isolation of the powder properties, which were correlated to the residual porosity in the printed samples. After establishing the relationship between printability and powder properties, a careful microstructural investigation was performed to understand what features are responsible for the relatively high strength of austenitic stainless steels. Two different alloying strategies were attempted to boost the strength further, introducing additional oxygen into the processing chamber for an in-situ synthesis of nanometric oxides, and designing a composition that produces strengthening precipitates upon aging.

The initial powder investigations revealed that 316L powder produced using vacuum induction melting inert gas atomization (VIGA) and conventional gas atomization (CGA) displayed similar oxidation states despite different atomization gases. The use of water in the atomization process however changed the oxidation state significantly, resulting in more extensive formation of oxide particulates on the powder surfaces. Analysis of the powder properties showed similar trends as the surface analysis, where the VIGA and CGA powder grades had similar flow properties. While water atomized (WA) powder had significantly lower flowability as compared to the other tested grades. The lower flowability caused a significant increase in residual porosity when printing with layer thicknesses above 20 µm.

Microstructural characterization of printed 316L specimens revealed a hierarchal structure consisting of elongated grains and within them a fine cellular structure. The cell structure was found to act as soft grain boundaries, hence strengthening the material without sacrificing ductility too much. This structure was found to be stable up to 800 °C.

Conceptually, the in-situ synthesis of finely distributed nanometric oxides using water atomized powder was shown to work. However, the size and number densities of the oxides must be further optimized to provide a strengthening effect. Another strategy for increasing the strength was by developing a heat-treatable composition using thermodynamic simulations. This resulted in the development of a novel stainless tool-steel composition. This new material had excellent printability with a fully martensitic structure in the as-printed condition and possessed a yield strength of nearly 1600 MPa after aging. The precipitates were found to have relatively slow coarsening rates and therefore the material retained much of its hardness despite long aging times.