On the development of powder spreadability metrics and feedstock requirements for powder bed fusion additive manufacturing

Existing powder feedstock metrics for powder bed fusion (PBF) additive manufacturing (AM) are related to packing efficiency and flowability, and newer techniques, such as powder rheometry and dynamic avalanche testing, have received recent attention in the literature. To date, however, no powder characterization technique is able to predict the spreadability of AM feedstock. In fact, no such spreadability metrics exist. This study endeavored to establish viable powder spreadability metrics through the development of a spreadability testing rig that emulates the recoating conditions present in commercial PBF AM systems. As no metrics for spreadability currently exist, four potential metrics were evaluated in a 3.2(3) split plot experimental design. These four metrics were: (1) the percentage of the build plate covered by spread powder, (2) the rate of powder deposition, (3) the average avalanching angle of the powder, and (4) the rate of change of the avalanching angle. Three samples of gas atomized, Al-10Si-0.5 Mg PBF powder representing differing degrees of quality were used as the levels of the powder quality input variable. As no powder quality metrics have been shown to be indicative of powder spreadability in PBF, various bulk powder characteristics were used as the powder quality indicator during ANOVA. Of the four metrics tested, the average avalanching angle, while statistically dependent of the powders angle of repose, showed poor correlation with experimental data. The remaining three metrics, however, were all found to have a statistically significant dependence on the angle of repose. Increasing the angle of repose resulted in significantly worse powder spreading, i.e. poor build plate coverage and powder clumping, as measured by the viable spreading metrics. Other processing parameters, such as the recoating speed and the recoater blade material were shown to also influence the spread quality.