Granular materials and fine powders are widely used in industrial applications. To control and optimize processing methods, these materials have to be precisely characterized. The characterization methods are related either to the properties of the grains (granulometry, morphology, chemical composition, ...) and to the behaviour of the bulk powder (flowability, density, blend stability, electrostatic properties, ...). The complex behaviours of granular and powder material has motivated the development of additional techniques to obtain reproducible and interpretable results. Many industries are concerned in different fields: additive manufacturing, food processing, pharmaceuticals, bulk material handling. The present technical report is focused on additive manufacturing. Metallic powders are widely used in Additive Manufacturing (AM) processes involving powder bed likepowder bed fusion (LBM, EBM, ...) or binder jetting. During such operations, successive thin layers of powderare created with a ruler or with a rotating cylinder. Each layer is then partially sintered or melted with an energy beam or glue with binder to build the parts. The layer thickness defines the vertical resolution of the printer; a thin layer leads to a better resolution. In order to obtain a thin layer, the powder is as fine as possible. However, as the grain size decreases, cohesiveness typically increases and spreadability, as defined within ASTM F42 / ISO/TC 261, is likely to decrease. The quality of the parts build with AM is thus directly influenced by powder flow properties.
Visual observation of layer homogeneity is usually the only way for operators to quantify the spreadability of powders during recoating. However, relating the powder characteristics to its spreadability during there coating process before hand should provide a more cost-effective way to classify and select the optimal powder and recoating speed combinations.
The aim of this technical report is to present an example of how the characterization of the macroscopic properties of metallic powders can be related to their spreadability inside LBM printers. A new technique combining measurements inside a LBM printer and image processing have been developed to quantify the homogeneity of the powder bed layers during recoating. Moreover, the flowability of four metal powders has been investigated with an automated rotating drum method, whose dynamic cohesive index measurement has been shown to correlate with the spreadability of the powder during the recoating process.Furthemore, the PSD and morphology of each powder was characterized for each batch before testing bystatic image analysis method (ISO_13322-1_2014). The general principle of the study is presented on Figure 1
dnaSRPS CEN ISO/ASTM TR 52952:2022
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May 12, 2023