Page 26 - MSAM-2-3

Materials Science in Additive Manufacturing Cast and 3D-printed fiber orientations

fibers are distributed along the extrusion direction due to methods is presented. In Section 4, the results of the
the impact of flow profile development [6-8] . In cast FRCs, tests are reported for cast and 3D-printed specimens.
the boundary constraint of molds determines the fiber In Section 5, the results are discussed on the impact of
orientation. Theoretical models, as presented by Lu and printing parameters on the boundary constraints, flow
[9]
Leung , address the 1D boundary constraint of molds. In fields, and fiber orientations. The mechanical performance
3D printable concrete, Hamhach and Volkmer proposed of the specimens has also been investigated, and the results
[10]
that fiber orientation exhibits a directional distribution, are reported here. In Section 6, a summary of this work is
which aligns with the printing direction. Figueiredo provided, and the future work is sketched.
et al. suggested that fibers mainly orientate themselves
[11]
diagonally within the cross-section plane of printed 2. Theory and modeling
filaments rather than parallel to the print path. 2.1. Impacts of boundary constraints on fiber
However, as discussed in detail in Section 2, the orientation
limitations of previous works lie in the insufficient Theoretically, with no flow in materials, the fiber dispersion
exploration and investigation of the impacts of should follow the idealized random distribution, in which
boundary constraints of molds and material flow field fibers have an equal probability of pointing to any direction
conditions on fiber orientation. Lu and Leung studied in three-dimensional space. However, this theoretical
[9]
the influence of boundary constraints of molds on fiber randomness is often not achieved in practice. This is due
orientation in a 1D case; however, they did not obtain an to:
analytical solution, and thus, they could not extend their (i) The casting process, which induces preferential flow of
[7]
findings to more practical 2D and 3D cases. Stahli et al. the paste in the horizontal direction, causing the fibers
noted the impact of flow profile on fiber orientation; to follow a distribution between 2D and 3D.
however, they considered fiber orientation in the plug (ii) The limited freedom of fibers near the surfaces to
flow region to remain unchanged, and the connection rotate in their corresponding dimension within the
between fiber orientation and flow field cannot be well specimen.
established.
Lu and Leung have stated three different scenarios
[9]
This work was motivated by the abovementioned based on 1D boundary constraints without providing
research gap. The impacts of boundary constraints and analytical solutions for fiber orientation. In the following
flow field on the fiber orientation and their corresponding sections, these three scenarios will be briefly reviewed, the
impacts on mechanical properties were investigated. analytical solutions for fiber orientation will be derived,
Analytical models were constructed to analyze and predict and the same approach will be extended to 2D and 3D
fiber orientation with respect to the boundary constraint cases.
and flow field, respectively. In the experimental part, the
cast and printed specimens were prepared for validation. 2.1.1. One-dimensional boundary constraint
Directional casting (DC) and random casting (RC) As shown in Figure 1, a 1D boundary constraint indicates
processes were adopted to fabricate casting specimens, that a restriction exists on one pair of opposite surfaces,
obtaining different types of flow fields while considering such as the top and bottom surfaces. z is defined as the
boundary constraints. Printed specimens were fabricated distance between the center of the fiber and the restricted
using different printing configurations to achieve various surface of the member, θ is the inclination angle, and l is
f
conditions of boundary constraints and flow fields. Fiber the fiber length. When the fiber is too close to the restricted
orientation was then analyzed through fluorescence image surface, it may intersect with the surface, and thus, the
processing and µ-computed tomography (µ-CT) scanning. orientation direction is limited depending on its distance
Finally, the impacts of fiber orientation on the mechanical from the surface. Otherwise, if the fiber is not in close
properties were evaluated. proximity to the restricted surface, it is free in the matrix.
The remainder of the article is organized as follows. Hence, to differentiate whether the fibers intersect with the
In Section 2, analytical models of the impacts of 1D, 2D, surface, two cases are discussed here.
and 3D boundary constraints on the fiber orientation are The first case (z < l /2) is schematically shown in Region
f
constructed, and the impacts of the boundary constraint 1 (Figure 2). Two different scenarios are discussed. When
and the flow field on fiber orientation are discussed and the fibers are at a small inclination angle θ, there is no
elaborated upon. In Section 3, the experimental design intersection between the fiber and the boundary. As shown
for casting, 3D printing process, and characterization by the bold dashed line in Figure 2A, a circular locus is

Volume 2 Issue 3 (2023) 2 https://doi.org/10.36922/msam.1603