Page 460 - IJB-9-4

International Journal of Bioprinting β-Ti21S auxetic FGPs produced by laser powder bed fusion

residual stresses . Furthermore, other manufacturing and a volume energy density between 40 and 90 J/mm .
3
[43]
imperfections can occur during the printing process, A 45° alternate scan strategy was used. Five samples for
namely, the variation of the cross-section and the strut each auxetic geometry were printed horizontally on the
waviness, which modify the final mechanical response of longer side to permit a better printability of the inclined
the lattice structure . struts (Figure 2).
[44]
Very few authors have investigated the manufacturability A pre-alloyed plasma atomized β-Ti21S alloy (GKN
and mechanical properties of cellular structures made Hoeganaes Corporation, Cinnaminson, NJ, USA) with a
by β-Ti. A recent study evaluated the printability of powder size distribution of 25 – 60 µm was selected. The
simple cubic cells in β-Ti21S alloy underlying a suitable chemical composition is shown in Table 2.
manufacturing quality for strut thickness above 0.5 mm .
[19]
2.2. Metrological and material characterizations
The aim of the present work is to evaluate the
manufacturability of two different auxetic FGPSs with aspect As-manufactured samples were characterized by means of
ratio equal to 1.5 and angle θ of 15° and 25° with a relative 2D and 3D metrological characterizations. In detail, SEM
density gradient of 0.34 – 0.49 – 0.66 and of 0.40 – 0.58 – inspections of the lateral and top sample surfaces were
0.75, respectively. 2D metrological characterization by used to conduct the 2D dimensional analysis of the strut
scanning electron microscopy (SEM) and 3D metrological and pore size. The size of 10 pores and 10 struts for each
characterization by X-ray micro-CT (µ-CT) imaging were level of density were measured using a 2D image analysis
carried out and compared. Preliminary investigation of the
mechanical properties and comparison with analytical and Table 1. Geometrical details of the designed auxetic FGPSs
numerical homogenization analyses was also conducted.
θ (°) ρ CAD (‑) Strut thickness Pore size
r
2. Materials and experimental procedures CAD (mm) CAD (mm)
15 0.34 1.17±0.02 1.12±0.47
2.1. Specimen design and preparation
0.49 1.47±0.03 0.98±0.39
Two different auxetic FGPSs were designed by means 0.66 1.78±0.10 0.78±0.32
of nTopology software, and the geometrical details are 25 0.40 1.20±0.02 1.00±0.40
summarized in Figure 1. Each relative density level was
characterized by a height of 3-unit cells and a solid base 0.58 1.51±0.06 0.81±0.31
with a thickness of 5 mm was added at the bottom of the 0.75 1.80±0.24 0.63±0.27
structure (Figure 1). The highest relative density level is CAD: Computer-aided design, FGPSs: Functionally graded porous
designed to improve osseointegration thanks to pore size structures
smaller than 800 µm, while the lowest density permits to
decrease the elastic modulus close to that of the cancellous Table 2. The chemical composition of β‑Ti21S (wt. %)
bone.
Element Mo Al Nb Si O Ni Fe
In the auxetic FGPS with θ = 25° the highest density level Weight % 14.6 2.8 2.8 0.3 0.11 0.004 Bal.
becomes too dense leading to the loss of auxetic geometry
(Figure 1C). All CAD parameters were characterized by
means of 3D image analysis software (ORS-Dragonfly) and
are summarized in Table 1.
Strut thickness and pore size were calculated by means
of the wall thickness analysis method which permits to
obtain the size distribution of the analyzed 3D elements.
This method evaluates the local thickness of the 3D
object, namely, strut or pore, by fitting its volume with the
maximum spheres at each location in the 3D structure [26,45] .
According to this method, a high pores size deviation is
expected, due to the small spheres fitting the pore near
the corners. The different FGPSs were printed by means
of a LPBF machine model MYSINT100 (SISMA SPA, Figure 2. As-manufactured functionally graded porous structures lying
Piovene Rocchette, Italy) on a platform of 100 mm in Ar down on the longer side with schematical representation to highlight the
atmosphere, with a laser spot of 55 µm, a power of 200W building direction.

Volume 9 Issue 4 (2023) 452 https://doi.org/10.18063/ijb.728