Effect of relative humidity on powder flowability and powder bed formation in additive manufacturing

Powder flowability is essential for powder-based additive manufacturing processes such as powder bed fusion, binder jetting, or directed energy deposition. Powder flow is the prerequisite for uniform powder distribution and delivery, which decisively influences the product's final material behavior and mechanical properties. Despite considerable advances in the field of additive manufacturing (AM), the identification of reliable methods for characterizing powder flow behavior that correlates with the powder spreading conditions in an AM machine remains an ongoing challenge. In this study, five conventional AM powders (H282, IN738, WC17, H13 and TiAl) were examined using standard and advanced flowability measurements under various storage conditions. The flowability results were correlated with powder spreadability using a self-built powder distribution machine and a process monitoring system for evaluating the powder bed. Given that moisture is a significant factor in the deterioration of flowability and that permanent control of humidity during storage can only be achieved at considerable expense and effort and is not feasible for all laboratories, this study also investigates the influence of humid storage conditions on flowability measurements and powder distribution. The findings of this study demonstrate that the powders exhibit disparate performance outcomes when subjected to the selected flowability testing methodologies. In general, the H282, WC17 and H13 powders demonstrate noticeably better flowability compared to the IN738 and TiAl in the majority of the evaluated measurements. Nevertheless, the distribution of powder in a manner that produces homogeneous powder layers with high packing density is achievable with all powders in the dry state, while the impact of elevated moisture storage conditions can be considerable and might result in agglomeration, which can markedly disrupt the distribution process. For the TiAl powder, with its specific low density, a significant influence of moisture on the powder flow can be reported.