PSI - Issue 34

Zoé Jardon et al. / Procedia Structural Integrity 34 (2021) 32–38 Zoé Jardon/ Structural Integrity Procedia 00 (2019) 000 – 000

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Low inner surface capillary roughness is required to avoid undesired fatigue crack initiation triggered by poor surface quality, near surface defects (pores, lack-of-fusion, scratches, voids, imperfections) and associated stress concentrations but also to minimize the influence on the propagation velocity of the waves through the channels, that needs to be well known for acceptable crack localization accuracy (Jardon, 2019). Therefore, a SEM and optical micrograph analysis are conducted on respectively the JEOL JSM-IT300 SEM-microscope and Leica DMI8-A microscope of the Electrochemical and Surface Engineering Research Group (SURF) of the VUB and on the Vision Engineering EVO Cam II microscope of the AM-Research Group of the VUB. A transition is visible on the micrographs (SEM and optical) and a color change is present at the interface between two sequences. No notches or important irregularities are observed on the capillary surface. For higher powers (sample 2), powder particles that flew through the drilled hole during the printing of sequence 1 tend to stick on the inner capillary surface of sequence 1 (see optical micrographs). These can negatively impact the surface roughness and eventually initiate crack nucleation. The use of an ultrasonic cleaner together with compressed air through the channel allowed to get rid of these particles and therefore to reach a better surface finish of the capillary (see SEM micrographs). For the second sequence of sample 3, the inner surface of the capillary exhibits a very poor finish due to unmolten particles. This strategy is therefore not recommended (for these settings). The drilling of the capillary through the fully printed sample (strategy 1) resulted in a substantial improvement of the capillary roughness compared to strategy 2. Therefore, this strategy is recommended for the build-up of a hybrid fatigue sample. Multi-focus depth images (1976x1475 px, 1 pixel = 1.1 µm) has been taken using the Leica microscope to extract the roughness profiles of the capillary surface (Fig. 6, top right) and of the transition between the milled surface of sequence 1 and the printed surface of sequence 2 (Fig. 6, down right). The printed surface exhibits a higher roughness. The corresponding arithmetical mean deviation and total roughness height of the assessed profiles equal , = 13.7µ , , = 52.1µ for the printed surface and , = 5.6µ , , = 22.9µ for the milled surface. The given values are averages over the three extracted profiles. The drilled capillary surface exhibits lower roughness parameters , = 3.7µ and , = 12.9µ . It is therefore unlikely that the fatigue crack will initiate at the capillary surface due to excessive roughness values. 3. Conclusions The final aim was to get insight in the effect of process parameters on the print geometry and quality and to identify the optimal parameter combination for the production of a DED hybrid fatigue sample. For smaller diameters the reached internal surface quality is too low, and the loss of material is negligible w.r.t. the full sample volume. Hence, without important laser power control, a practical recommendation is to omit the inclusion of features with diameters < 4 in the print trajectories and to integrate them using subtractive drilling operations. In case of very good laser power control, the inclusion of capillaries with diameter 3 − 4 mm might be achievable. For larger capillaries ( > 4 ), the second and third strategies are recommended, combined with a suitable surface post-processing such as chemical etching to reduce the surface roughness (Hinderdaël, 2018). References Mercelis, P., Kruth, J. P., 2006. 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