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1. Binder Jetting Additive Manufacturing of Ceramics: Comparison of Flowability and Sinterability between Raw and Granulated Powders

   Du, W. ( June 2019 , Proceedings of the ASME 2019 International Manufacturing Science and Engineering Conference (MSEC2019))

   The objective of this study is to compare three different feedstock powders for the binder jetting process by characterizing their flowability and sinterability. Binder jetting additive manufacturing is a promising technology for fabricating ceramic parts with complex or customized geometries. Granulation is a promising material preparation method due to the potential high sinterability and flowability of the produced powder. However, no study has been made to systematically compare raw and granulated powders in terms of their flowing and sintering behaviors. This paper aims at filling this knowledge gap. Two raw powders (i.e., fine raw powder of 300 nm and coarse raw powder of 70 μm) and one granulated powder from spray freeze drying were compared. Different flowability metrics, including volumetric flow rate, mass flow rate, Hausner ratio, Carr index, and repose angle were measured. Different sinterability metrics, including sintered bulk density, volume shrinkage, and densification ratio were compared for all three powders. Results show that granulated powder achieved comparably high flowability to that of the coarse raw powder and also comparably high sinterability to that of the fine raw powder. Moreover, suitable metrics for the characterization of the sinterability and flowability for these three powders are recommended. This study suggestsmore »spray freeze drying produces high-quality feedstock powder for binder jetting process.« less
2. Binder Jetting Additive Manufacturing of Ceramics: Feedstock Powder Preparation by Spray Freeze Granulation

   Du, W. ( June 2019 , Proceedings of the ASME 2019 International Manufacturing Science and Engineering Conference (MSEC2019))

   Objective of this study is to prepare the binder jetting feedstock powder by spray freeze drying and study the effects of its parameters on the powder properties. Binder jetting additive manufacturing is a promising technology for fabricating ceramic parts with complex or customized geometries. However, this process is limited by the relatively low density of the fabricated parts even after sintering. The main cause comes from the contradicting requirements of the particle size of the feedstock powder: a large particle size (>5 μm) is required for a high flowability while a small particle size (<1 μm) for a high sinterability. For the first time, a novel technology for the feedstock material preparation, called spray freeze drying, is investigated to address this contradiction. Using raw alumina nanopowder (100 nm), a full factorial design at two levels for two factors (spraying pressure and slurry feed rate) was formed to study their effects on the properties (i.e., granule size, flowability, and sinterability) of the obtained granulated powder. Results show that high pressure and small feed rate lead to small granule size. Compared with the raw powder, the flowability of the granulated powders was significantly increased, and the high sinterability was also maintained. Thismore »study proves that spray freeze granulation is a promising technology for the feedstock powder preparation of binder jetting additive manufacturing.« less
3. A Statistical Analysis of Powder Flowability in Metal Additive Manufacturing

   https://doi.org/10.1002/adem.202000022  

   Kiani, Parnian ; Scipioni Bertoli, Umberto ; Dupuy, Alexander D. ; Ma, Kaka ; Schoenung, Julie M. ( June 2020 , Advanced Engineering Materials)

   The quality of parts fabricated with additive manufacturing is influenced by the flowability of the feedstock particles, which is the result of many factors, including chemistry (e.g., true density, the presence of surface oxides, and impurities), morphology (e.g., particle shape and the presence of satellites), and particle size distribution. This work investigates the relationship between powder characteristics and flow behavior of different powders using three flowability testing methods. Six powders of two compositions (316L stainless steel and AlSi10Mg), made using two different methods (gas‐ and water‐atomization), are investigated to rationalize the effect of powder chemistry and morphology on flow behavior. The results show that the true density of the powders can influence several flowability metrics. In addition, aspect ratio strongly influences the initiation of flow from a static condition, whereas average particle size strongly dictates the ease of maintaining that flow.
4. Discrete-Element Simulation of Powder Spreading Process in Binder Jetting, and the Effects of Powder Size

   https://doi.org/10.1115/MSEC2021-63351  

   Clares, Ana Paula ; Manogharan, Guha ( June 2021 , Discrete-Element Simulation of Powder Spreading Process in Binder Jetting, and the Effects of Powder Size)

   Binder Jetting has gained particular interest amongst Additive Manufacturing (AM) techniques because of its wide range of applications, broader feasible material systems, and absence of rapid melting-solidification issues present in other AM processes. Understanding and optimizing printing parameters during the powder spreading process is essential to improve the quality of the final part. In this study, a Discrete Element Method (DEM) simulation is employed to evaluate the powder packing density, flowability, and porosity during powder spreading process utilizing three different powder groups. Two groups are formed with monoidal size distributions (75–84 μm and 100–109 μm), and the third one consisting of a bimodal distribution (50 μm + 100 μm).

   A thorough investigation into the effects of powder size distribution during the powder spreading step in a binder jetting process is conducted using ceramic foundry sand. It was observed that coarser particles result in higher flowability (62% decrease in repose angle) than finer ones due to the cohesion effect present in the latter. A bimodal size distribution yields the highest packing density (8% increase) and lowest porosity (∼12% reduction) in the powder bed, as the finer particles fill in the voids created between the coarser ones. Findings from this study aremore »directly applicable to binder-jetting AM process, and also offer new insights for AM powder manufacturers.

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5. Binder Jetting Additive Manufacturing: Effect of Particle Size Distribution on Density

   https://doi.org/10.1115/1.4050306  

   Du, Wenchao ; Roa, Jorge ; Hong, Jaehee ; Liu, Yanwen ; Pei, Zhijian ; Ma, Chao ( September 2021 , Journal of Manufacturing Science and Engineering)

   Abstract This paper reports a study on the effects of particle size distribution (tuned by mixing different-sized powders) on density of a densely packed powder, powder bed density, and sintered density in binder jetting additive manufacturing. An analytical model was used first to study the mixture packing density. Analytical results showed that multimodal (bimodal or trimodal) mixtures could achieve a higher packing density than their component powders and there existed an optimal mixing fraction to achieve the maximum mixture packing density. Both a lower component particle size ratio (fine to coarse) and a larger component packing density ratio (fine to coarse) led to a larger maximum mixture packing density. A threshold existed for the component packing density ratio, below which the mixing method was not effective for density improvement. Its relationship to the component particle size ratio was calculated and plotted. In addition, the dependence of the optimal mixing fraction and maximum mixture packing density on the component particle size ratio and component packing density ratio was calculated and plotted. These plots can be used as theoretical tools to select parameters for the mixing method. Experimental results of tap density were consistent with the above-mentioned analytical predictions. Also, experimental measurementsmore »showed that powders with multimodal particle size distributions achieved a higher tap density, powder bed density, and sintered density in most cases.« less