Abstract
The flow behaviour of a powder is critical to its performance in many industrial applications and manufacturing processes. Operations such as powder transfer, die filling and powder spreading all rely on powder flowability. Multiple testing methods can help in assessing flowability, but it is not always clear which may better represent specific flow conditions or how different metrics correlate. This study compares 8 different flowability testing methods using 11 steel powders varying in chemistries and size fractions. Regression analysis was used to test the relationship between each flowability metric obtained. Some metrics, such as the conditioned bulk density, relate to many flowability indicators. Others, such as the basic flowability energy, show poor correlations to other variables, likely describing different aspects of the powder flow behaviour. When two metrics show a strong correlation, as between conditioned bulk density and Hausner ratio, a numerical relationship is derived: CBD = − (5.65 ± 0.86)HR g cm−3.
Highlights
The flow of a powder is a complex phenomenon to understand or model
Hall and Carney flow rate exhibits a strong correlation with several variables, while showing a weak dependence with most others
It is useful to note that, due to the cohesiveness of many of the powders used in this study, few could give quantitative results in flow rate testing: jamming or ratholing were observed on many samples, in agreement with what was stated from Marnani et al for fine and ultra-fine particles [71] and Muñiz-Lerma et al for fine metallic powders [73]
Summary
The flow of a powder is a complex phenomenon to understand or model. It is sensitive to properties of the powder itself and external factors such as humidity [1,2], geometrical impediments [3], or applied stresses [4]. Due to the complexity of the powder flow, there is no single technique to characterise flowability for a specific application [26] and studies do not always agree on the best one to use for any specific case [18,21,27]. Many common methods found in the literature are not standardised, such as the rotating drum angle of repose, powder rheometry and spreading devices [18,21,28] Others, such as tap density testing, static angle of repose and shear cell testing are standardised [29,30,31,32], but not included in the recommended standards when characterising the flowability for additive manufacturing metallic powders [26]. Multiple techniques would be needed to describe all aspects of the powder flow behaviour
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