Page 141 - MSAM-4-2

Materials Science in Additive Manufacturing Bi-modal powder spreading behavior of ceramics

machine and impose design limitations when processed density. 15,27 Such segregation may result in anisotropy in
through conventional manufacturing processes. With mechanical properties, dimensional variation, or print
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the advancements in additive manufacturing (AM), failure. However, the nature and implications of spatial
producing intricate shapes with ceramics is feasible and segregation in bimodal ceramic powder beds remain
has garnered increased adoption. The binder jetting (BJT) inadequately characterized and are a central focus of this
process is a non-phase transformation AM process that is study.
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particularly advantageous for refractory materials such as The powder delivery system, including the feed
ceramics. BJT is a powder bed process where a binder is system, the geometry, and the dynamics of the spreading
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selectively jetted on a layer of deposited powder based on mechanism, further influences the homogeneity and
slice information of part geometry to selectively bind the compaction of the powder bed. BJT systems typically
powders layer-by-layer, followed by curing of the binder utilize either an “in-layer feed” approach, where powder is
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and post-AM sintering. Ceramic structures printed with moved laterally from a feedstock reservoir, or an “overhead
BJT can have complex internal and external shapes 10-12 that feed” design, in which powder is dispensed vertically and
are either infeasible or extremely difficult to manufacture leveled by a spreader. 28-32 In both configurations, the type
by other conventional manufacturing processes. of spreader, which is commonly either a blade or a roller,
In BJT manufacturing, the quality of the powder bed plays a pivotal role in shaping powder flow and deposition
directly influences dimensional accuracy, surface finish, behavior. 33,34 Experimental and numerical studies have
and final part density. Powder flowability is, therefore, an shown that roller spreaders outperform blades, promoting
important factor, governing how uniformly powder can powder bed uniformity and reducing surface defects. 35,36
be deposited into successive layers during the printing For instance, Lee et al. used discrete element method
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process. Flowability is dictated primarily by particle shape (DEM) simulations to show that spreader speed influences
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and size. Spherical particles offer reduced friction and particle segregation but lacked experimental validation.
interlocking during spreading, resulting in improved flow Moreover, many simulations predominantly focus on the
characteristics compared to irregularly shaped powders. 14-21 heap formation in front of the recoater, 35,36,38 rather than
Similarly, coarse powders exhibit better flowability than the characteristics of the deposited layer itself. Oh et al.
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fine powders due to weaker inter-particle cohesive forces. reported uniform packing density in printed green parts
However, this improved spreadability comes at the cost of but did not spatially resolve PSD within the bed. Other
lower powder bed density, as larger particles inherently approaches, including binder pre-wetting and layer-by-
create more void space between them. 22,23 Despite ongoing layer compaction, have been investigated to improve
efforts to balance the trade-off between flowability and homogeneity, 40,41 but these studies have primarily focused
packing efficiency, 24,25 robust strategies to consistently on unimodal powders. Thus, further investigation is
achieve both uniformity and high density in powder required regarding how spreading systems interact with
beds remain poorly understood, particularly for ceramic bimodal feedstocks to drive spatial variations in packing
feedstocks, where cohesion, brittleness, and sensitivity to and microstructure.
the environment exacerbates these challenges. While DEM simulations have advanced the
To address these competing requirements, bimodal understanding of powder spreading dynamics, 34,42,43
powder systems, comprising a mixture of coarse and experimental validation of segregation phenomena for
fine particles, have emerged as a promising approach. bimodal systems remains limited. Although counter-
By enabling fine particles to occupy the interstitial voids rotating roller configurations have been proposed to
between larger particles, bimodal distributions can reduce segregation, 44-46 their effectiveness in controlling
increase the packing density while preserving acceptable spatial heterogeneity in powder bed density has not
flowability. 7,15,26 Clares et al. reported a 20% improvement in been systematically demonstrated. Furthermore,
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green density and a 170% increase in flexural strength using the implications of such heterogeneity for printed
a bimodal distribution compared to the best-performing part characteristics, particularly concerning density
unimodal powders. Bai et al. further demonstrated that gradients and microstructural anisotropy, remain
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even when the median size remains constant, a broader inadequately understood. These challenges become even
particle size distribution (PSD) enhances bed density and more pronounced in the context of ceramic systems,
improves part quality. Despite these advantages, bimodal where powders are composed of finer particle sizes,
powders are inherently prone to particle size segregation exhibit irregular morphologies, and demonstrate high
during spreading, where differences in size, mass, and susceptibility to cohesive forces and environmental
cohesion lead to spatial variations in local packing moisture. 2,47-49 In contrast, metallic and polymeric powders

Volume 4 Issue 2 (2025) 2 doi: 10.36922/MSAM02510016