| dc.description.abstract | The objective of this research is to investigate three approaches for improving the bulk density or mechanical strength of the parts from powder bed additive manufacturing technologies, with a focus on binder jetting additive manufacturing. At the beginning, a literature review was conducted to target the density issue. This review began with an overview of the process, material considerations, and process parameters. It then discussed different aspects of density. Afterward, it reviewed two categories of techniques to increase the part density: material preparation techniques and post-processing techniques. Finally, it presented the knowledge gaps in the literature.
Afterward, three approaches, i.e., powder mixing, powder coating, and powder granulation were studied. For the powder mixing approach, an analytical model was used first to study the mixture packing density. Both modeling and experimental results showed that powder mixtures (binary or ternary) with a multimodal particle size distribution could achieve a higher packing density than their component powders and there existed an optimal mixing fraction to achieve the maximum mixture packing density. Afterward, theoretical tools to select parameters for the powder mixing approach were established. Moreover, three linear packing models (de Larrard’s, Kwan’s, and Yu’s) were assessed for their prediction performances on micropowder mixtures in terms of predicted powder packing densities and optimal mixing fractions. Results showed that Kwan’s model achieved the best prediction performance on powder packing density. In terms of optimal mixing fraction, the prediction performance depends on the specific mixing system.
A new powder surface modification method, i.e., particle coating, was applied to increase the powder sinterability and the part strength. Specifically, coarse crystalline alumina particles were coated with amorphous alumina, in which the micron-sized core was designed to provide the high flowability and the amorphous shell to promote sintering due to its high activity. The sintered samples from the coated powders demonstrated enhanced necking, increased diameter shrinkage, and improved compressive strength, compared with those from the raw powders.
For powder granulation approach, powders were firstly prepared using spray freeze drying and the effects of preparation parameters on the powder properties were studied. Results showed that a low spraying pressure and a high feed rate led to a large granule size and consequently a higher flowability. Thereafter, the granulated powder from spray freeze drying was compared with nanopowder and micropowder in terms of flowability and sinterability. Results showed that the granulated powder had both a high flowability and a high sinterability. Finally, a granulation method called direct freeze drying was used to prepare the granulated powders from slurries with different solid loadings. Powder spreading and compaction tests were conducted on a commercially available binder jetting 3D printer equipped with a forward-rotating roller compaction system. Results showed that a low slurry solid loading could enhance the powder bed density in roller-compaction-assisted binder jetting. | |
