Page 53 - IJB-9-6

International Journal of Bioprinting Sub-regional design of the bionic bone scaffolds

Table 1. LPBF process parameters
Laser power (W) Scanning speed (mm/s) Hatch spacing (mm) Layer thickness (mm) Laser focus (mm) Atmosphere
Value 160 1250 0.08 0.03 0.08 Ar

values (denoted as a series) and a values (denoted as a Table 2. Design parameters of as-built specimens
2
1
2
series) were set up to evaluate the effect of dot pitch on
the mechanical properties and the porosity, where a = Series No. C (%) C (%) ε
2
1
2
2000 μm (for a series) and a = 4000 μm (for a series) Irregularity 01 50 90 0.06
1
2
1
were held constant. Other design variables were kept the series 02 0.12
same as those in randomness series.
03 0.18
To assess the expected mechanical behavior of the 04 0.25
as-designed models, the finite element analysis (FEA) 05 0.30
on ideal as-designed models was performed using the 06 0.39
commercial software Workbench (ANSYS, Inc., v.18.0).
The FEA models were assumed to be linear, elastic, and 07 0.47
homogeneous. The material properties were set to Ti- 08 0.50
6Al-4V, where Young’s modulus was equal to 113.8 GPa, Scale coefficient 09 50 90 0.47
Poisson’s ratio equal to 0.342, and the yield stress equal series 10 50 80
to 895 MPa. The loading conditions and the boundary 11 50 70
constraints were as follows: the top of the as-designed
model was loaded with a pressure of 80 MPa while the 12 50 60
bottom boundary was fixed. The stress distribution under 13 60 90
pressure was investigated through parameter settings and 14 70 90
model meshing and solving. The mechanical properties of 15 80 90
the as-designed models were evaluated by the maximum
Von-Mises stress value under the same loading condition.
compression test. Note that three identical specimens were
prepared for each model and the test results represent the
2.3 Fabrication and compressive testing of the average values of these specimens.
bionic bone scaffolds
The as-built specimens were fabricated using the LPBF According to the national standard GB/T 31930-
machine (NCLM2120, China). The process parameters, 2015, the static compression tests were performed using a
optimized by orthogonal tests, are shown in Table 1. The universal testing machine (UTM5305H) at a constant speed
commercial Ti-6Al-4V extra low interstitial (ELI) powder equal to 0.5 mm/min. The loading and displacement were
supplied by EOS GmbH was used in the experiment, recorded until the struts broke. Known as apparent elastic
meeting ISO 5832-3 and ASTM F1472 norms. modulus of cellular metals, the elastic modulus represents
the slope of the elastic straight lines, determined by the
The characteristic parameters that affect the mechanical
properties of the porous biomaterials mainly include elastic loading between 70% and 20% of plateau stress.
The ultimate strength was defined by the first maximum
porosity and aperture. Generally, the effect of porosity compressive strength, which determined the load-bearing
is dominant [39,40] . Irregularity is also one of the possible capacity of the as-built specimens.
influences due to the introduction of the probability sphere
model. In this study, two sets of uniaxial compression tests 3. Results and discussion
were set up to investigate the effects of irregularity and
of graded porosity on the compressive performance. As 3.1. Relationship between design variables and
shown in Table 2, the as-built specimens with different ε characteristic parameters
values were denoted as irregularity series and those with Different combinations of dot pitch should be discussed
different combinations of C and C values were denoted first, as this paper presented a combined probability sphere
2
1
as scale coefficient series. The other design variables were model. It is obvious from section 2.1 that adjusting the dot
as follows: a = 3333 μm and a = 1333 μm. Besides, a pitch essentially controls the upper limit of the aperture.
2
1
solid part of 20 × 20 × 0.5 mm was fixed at the top and at The first step was to determine the range of a in order
3
1
the bottom acting as the compensation area for the wire- to meet the optimal aperture (60–1200 μm) for bone
electrode cutting process and the boundary area for the implants [11,41] . The values of a for the as-designed models
1
Volume 9 Issue 6 (2023) 45 https://doi.org/10.36922/ijb.0222