Page 272 - IJB-10-1

International Journal of Bioprinting Permeability of NiTi gyroid scaffolds

lower porosity of the fabricated NiTi gyroid structures. A typical cross-section of the as-built gyroid structure
The observation can be explained by the following two after mounting into the resin and polishing is shown in
reasons inherent to the LPBF consolidation process. First, Figure 4b. The absence of irregular shape pores caused by
it is related to the thickening of the struts in the x–y plane the lack-of-fusion defect and cracks inherent to the laser-
during the contour laser path. It is worth noting that in assisted consolidation of the intermetallic materials prove
this study, the hatching was followed by the contour scan. the correctness of the chosen process parameters listed in
Ren et al. demonstrated that a precontour scan leads Table 1. It is worth mentioning that a small amount of the
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to a local increase in laser absorptivity due to multiple gaseous pores with a size below 10 μm can be detected.
reflections between the powder particles. Consequently, These defects were caused by the local transition of the
in the opposite sequence, the geometry integrity can melt pool into the keyhole mode, which was influenced
be higher; however, the lack of fusion defect with near- by a change of the heat conduction regime due to the
surface porosity may appear. The second reason is related reduced heat sink in the case of thin wall structure
to the sintering of powder to the structure surface. During manufacturing. However, such defects are hard to avoid
the micro-CT analysis, measurements depended on the without new build processors with adaptive energy input
threshold value of the voxel counting as a boundary. The adjusting to the geometrical features. The struts of all
peak of the Gaussian distribution corresponds to 100 μm, samples were continuous, and no interlayer delamination
which is close to the d percentile of the used powder. or broken connections were observed; a typical strut under
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Thus, increasing the laser beam compensation parameter higher magnification is shown in Figure 4c. The surface
is not desirable, as the thickening caused by powder of the struts was extensively covered with fused particles
sintering is not considered load-bearing. A better solution that highly increased surface roughness. Some particles
is machining the critical parts with high requirements further from the surface were observed without visible
of size matching or chemical etching in the flow for the metallurgical contact that was slightly sintered during
porous parts. Furthermore, the relative discrepancy of the agglomeration formation.
pore size has a noticeable dependence on the unit cell size For the following discussion, gyroid samples obtained
due to an increase in the cumulative deviation per unit by partial consolidation are considered for analysis of the
volume with an increase in the specific surface area. On as-built inner structure. The topography of the sample top
the contrary, the dependence of the pore size deviation surface is demonstrated in Figure 4d. It shows that the
from the strut thickness of the gyroid structure was not contour scan following the hatching caused elevation of
observed taking into account the measurement error. the near-surface region.
In Figure 3e, the total statistical deviation by the whole
sample is presented as dependence from the surface Figure 4e shows the first layer of the strut connection,
area. Samples with different wall thicknesses and the which can be classified as an overhanging surface.
same unit cell size have Gaussian surface distribution Transition layers inevitably involve laser beads, partially
indicating good manufacturing accuracy and confirming formed on top of the powder without solid support, which
the absence of relative deviation dependence on the leads to the formation of an unstable melt pool due to higher
wall thickness. remelting depth due to local differences in density and
thermal conductivity of the substrate. Thus, such regions
Offsets from the actual boundary of the 3D model have high deviations, which are consistent with micro-CT
subjected to slicing operation are of great importance to findings. Another important aspect of LPBF consolidation
achieve high geometric accuracy. Laser beam compensation of curved or inclined surfaces is the approximation of the
(BC) and contour distance (CD) parameters are commonly CAD boundary with the accuracy of layer thickness value
used to describe the strategy of laser scanning near the in the slicing process. Such effect led to a staircase profile
border, where BC is the offset of the hatching region from of inclined surfaces of the structure, which is visible in
the border, and CD is the distance between the border and Figure 4f. Accordingly, the aforementioned morphological
the contour vector. Figure 4a shows the hatching pattern singularities of NiTi gyroid structures obtained via LPBF
and contour vector in a slice of sample 204. Optimal BC and manufacturing can directly affect the wetting conditions
CD were found to be 25 and 75 μm, respectively, consistent and momentum dissipation of the fluid flow.
with the findings reported by Tan et al. The study showed
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the combined effect of different combinations of laser BC 3.2. Mass-transport properties and
and CD on the morphology of the lattice struts and their numerical permeability
geometrical integrity. Li et al. demonstrated the change This section elaborates on and discusses fluid flow
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of the actual strut dimensions of NiTi stents achieved by characteristics studied utilizing the FE model. The effect of
variation of BC. working fluid viscosity and the pressure gradient applied

Volume 10 Issue 1 (2024) 264 https://doi.org/10.36922/ijb.0119