Aspects of building geometry and powder characteristics in powder bed fusion

Licentiate thesis, 2018

Additive manufacturing (AM) produces near-net-shaped parts directly from a 3D-CAD model in a layer-by-layer manner. One of the most common AM technique for fabricating metallic components is powder bed fusion (PBF). The PBF process has shown great potential in fabricating metallic parts with properties better or comparable to conventional methods. However, there are some challenges in reproducibility, process stability, robustness, etc. This thesis elaborates on several of these challenges and addresses the influences of feedstock material, build orientation and part design on the final outcome. The PBF process uses fine metal powder as feedstock material and in order to have an economically feasible process, powder recycling is a necessity. However, to ensure a robust process and consistent material properties, the feedstock material need to be handled with caution as powder properties will affect the part quality. The obtained results for 316L stainless steel from this study indicate that powder degradation in terms of surface product changes occurs when the powder is recycled. It was revealed that both recycled and virgin powder were covered by a heterogeneous oxide layer, composed by a homogeneous iron oxide layer with the presence of Cr-Mn-rich oxide particulates that were growing during PBF processing. The results showed that the powder degradation was more pronounced when used in the electron beam system compared to a laser based system due to the long exposure at high temperatures. The manufacturing capabilities of the PBF process has enabled the production of lattice structures without extensive tooling. The properties of such lattice will be influenced by the microstructure. Hence, it is of importance to understand how the part geometry would affect the microstructure. This study presents the effect of build geometry, as e.g. wall thickness and build angle on the 316L microstructure. The obtained results indicated that in the center of ribs over 0.6 mm in thickness, large elongated grains with preferential <101> orientation were created. Reducing the part thickness to below 0.6 mm reduced the predominant texture. The increased cooling rate close to the part surface inhibited grain growth and changed the preferential grain orientation. For the process parameters used, the critical part thickness to avoid large elongated grains was found to be about 0.4 mm. The obtained results could be used for further development of design rules and prediction of mechanical properties of AM parts with small wall thicknesses.