Surface features and their effects on parts produced by Laser Powder Bed Fusion (LPBF)

In the recent years, the application of Laser Powder Bed Fusion (LPBF) process has received a special attention due to its ability to produce geometrically complex and lightweight parts. Despite the disrupt benefits of LPBF, the full potential of the technology has a long journey to go. However, the field is rapidly evolving and an exhaustive understanding of the process is of paramount importance. LPBF suffers from a low surface quality which affects the stand-alone philosophy of this manufacturing process due to its need for several post manufacturing processes to improve the mechanical behaviour of such parts compared to their counter parts processed by conventional manufacturing processes. Moreover, the quality of LPBF surface and near surface regions, the governing physical phenomena and their effects on the mechanical behavior are not yet fully understood. In this regard, the current work starts with a particular attention to surface and sub-surface regions of parts produced by LPBF in term of their morphology, microstructure, chemistry and mechanical behaviour to improve the general understanding of defect genesis. Ballings, spatter particles and partially melted metal powders are distinguished by their morphology, size and microstructure. It is shown that these differences arise from different cooling rates during their generation. Ballings share the same microstructure with the bulk material, both experiencing cooling ruled by conduction with already consolidated substrate. Spatters and partially melted powders show coarser microstructure driven by solidification mainly governed by convection and radiation during their flight in the inert atmosphere of the process chamber. Spatters are identified as the most critical feature on LPBF surfaces and a study concentrated on the chemical, physical, microstructural and mechanical properties of the spatter particles is carried out. The study shed light on the criticality of these particles and their deteriorating effects on the generation of surface, sub-surface and bulk lack of fusion defects. Consequently, different surface finishing post manufacturing treatments are employed to improve these surfaces by removal of the typical as-built surface features and sub-surface defects, i.e. sandblasting, vibro-finishing, and machining followed by polishing. Through fatigue results it was demonstrated that the residual surface and sub-surface pores are the principal responsible for the pre-mature fatigue failure of LPBF parts. A significant improvement was achieved by machining followed by polishing due to the adequate material removal from surface regions. Last but not least, a comprehensive coupled investigation of metrological methods and cross-sectional analysis were performed to evaluate the effects of surface features and volumetric defects typical of additively manufactured materials. Fatigue tests and fractographic analyses were conducted to support the finite element simulations and a proposed fracture mechanics model. The results demonstrate that the standard metrological methods alone cannot provide all of the data needed to model the fatigue behaviour of additively manufactured materials robustly. Moreover, a statistical model describing the competition between volumetric defects and surface irregularities was developed and validated.