PSI - Issue 53

Rainer Wagener et al. / Procedia Structural Integrity 53 (2024) 151–160 Author name / Structural Integrity Procedia 00 (2019) 000–000

153 3

2. Powder Bed Fusion – Laser Beam of metallic structures According to the name the Powder Bed Fusion – Laser Beam technology belongs to the Powder Bed Fusion methods using a laser source for the energy input. The build process is a sequence of powder deposition and selective melting by the usage of a laser beam. In this case a layer of powder is applied to the build platform and is selected, that means melted, by a laser beam in the regions of the part to build. After this step the platform is lowered, and a new layer of powder is applied. To increase the building process and to optimize the local structure properties different process parameters are used. Mainly it will be distinguished between the bulk and contour. The main purpose of the contour parameters is to ensure smooth surfaces, while the bulk parameters should decrease the building time and residual stresses. While the contour scan can be applied before or after the build of the core always on the same path and direction, the hatching direction rotates from layer to layer. The width of the contour is constant and therefore independent of the structure thickness. That means, with a change of the structure thickness the ratio of contour to bulk changes, too. Further and important to remember is, that the rim zone, even after the removal of the support structures has been influence by the different local cooling conditions. A seam of imperfections like pores and lack of fusions is present in the contour. Compared to the bulk material the probability to generate these defects in the rim zone is drastically increased. To attach the part on the build platform, as well as to enable overhanging component parts, support structures are used. Compared to the bulk material the density of these structures is lower, but they are more stable than the surrounding powder. After the building process the support structures must be removed. Additional to the mechanical and or chemical removing process, the microstructure of the rim zone is influenced by the different cooling conditions due to the different heat flow in the bulk, the support structures and surrounding powder. Just considering the different local cooling conditions it is understandable that the microstructure of the rim zone differs depending on the build orientation and conditions. Thus, the influence of up- and down-skin occurs, which is characterized by the presence of an increased probability of defects for the down-skin regions. At this point it must be emphasized that, besides the conventional features like stress concentrations due to geometrical notches and mean stresses, the influence of different rim zones caused by the exposure strategy must be analyzed to understand the component related material and structure behavior of additively manufactured metallic parts under cyclic loading conditions. 3. Geometrical notches The geometry of the notched specimens has been optimized to be built without support structures in case of a parallel build to loading orientation to eliminate the influence of the support structures on the fatigue. Typical sections of the resulting structure of each geometry including internal and surface-related inhomogeneities are depicted in Fig. 1. In case of the unnotched specimen, as expected, the seam of pores within the rim zone is visible along the whole section. In case of the notched ones the rim zone differs depending on the up- and down-skin conditions. Pores accumulate in the rim zone below the surface of the down-skin area. The surface on the notch flank under up-skin conditions is smooth with a tendency to show stairs due to the layer-by-layer building process. The density of surface related defects is low.