(38e) Understanding and Characterising the Spreading of Cohesive Powders in Powder Bed Fusion Additive Manufacturing Via Discrete Element Modelling

The quality of the bed of powder that is spread during a powder bed fusion additive manufacturing process is important to quality of the part produced and the reliability of the process. The spread layer properties are determined by the process conditions, the machine set-up and geometry and the powder properties. The impact of the powder properties on the spread powder layer are not well understood due to limitations in both the numerical approaches used and the characterisation methods. This is particularly so for fine cohesive powders which require more complex contact models within the commonly used discrete element method. This study uses an efficient GPU based discrete element method with a novel methodology for scaling particle stiffness to increase simulation speed and maintain accuracy [1].

The validity of the method was verified for both packing and spreading of fine powders before the effect of powder cohesivity was investigated. To characterise the spread particle layer a number of alternative metrics were introduced to give insight into the layer structure, these included pore size distribution in the bed and powder cluster size distriburtions. The analysis showed that, for the conditions used, the packing density was relatively insensitive to cohesivity with particle Bond number spanning 0 to 200, however for very cohesive particles with Bo > than 200, a drop off in the bed density was seen. However significant differences in the structure of the spread layer were noted and several regimes identified. In particular, significant clustering, and less uniform spreading, was observed with lower cohesivity, flowable, powders. The mechanisms behind the observations are discussed and a dimensionless inertial number proposed to help interpret the phenomena observed.

The method provides a valuable tool and metrics to quantitatively evaluate the quality of a spread powder layers, but also enables a better understanding of the physics underlying the spreading of cohesive fine powders.

[1] Y. He et al., 2021, Applied Mathematical Modelling 90, 817-844