Page 458 - IJB-9-4

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

1. Introduction of a strong {100}<001> texture. A post solution heat
treatment promotes a significant increase in the intensity
Replacing and/or repairing the human bone with of the {100}<001> texture leading to an elastic modulus of
necessity to guarantee the same mechanical properties and around 75 GPa .
[15]
biocompatibility is of primary interest in the orthopedic
field. Ti-6Al-4V extra-low interstitial (ELI) is the most A novel metastable β-Ti21S alloy with the chemical
common biomaterial, as it combines high strength and composition of Ti-15Mo-3Nb-3Al-0.2Si (wt.%) has
corrosion resistance. However, this alloy, originally devised previously been investigated [17-19] . It is characterized by a
for aeronautical applications, turns out to be affected by fully β microstructure and good mechanical properties
some drawbacks when employed in biomedical applications. in as-built condition, potentially without the need for
[17]
Considering its chemical composition, elements such further heat treatment. In detail, Macias-Sifuentes et al.
as Al and V are deleterious for the patient’s health due demonstrated a β phase microstructure with a textured
to long-term harmful effects, namely, cytotoxicity and columnar structured oriented along the building direction
Alzheimer’s disease . For this reason, many researchers in the laser powder bed fusion (LPBF) sample and a
[1]
recently focused on new biomaterials with a reduction or precipitation of α phase into the grain and at the grain
a complete removal of these elements achieving similar boundaries after a solution treatment at 850°C for 30 min and
strength and corrosion resistance [2,3] . From a mechanical aging at 538°C for 8 h. The α precipitation leads to increased
point of view, Ti-6Al-4V ELI exhibit a high elastic modulus mechanical strength but a decreased ductility. A very low
(110 GPa), much higher compared to that of cortical Young’s modulus of 52 GPa and a good mechanical strength
(3 – 30 GPa) and trabecular bone (0.02 – 2 GPa). This of around 830 MPa and an extraordinarily elongation of
[18]
stiffness mismatch leads to the so-called “stress shielding 21% were demonstrated by Pellizzari et al. . A variation
effect” and consequently bone resorption . of <20% in Young’s modulus due to the texture and a lower
[4]
cytotoxicity compared to Ti6Al4V further confirmed the
It is not surprising that the scientific community is possibility to use it in as-built condition.
intensively researching novel biomedical titanium alloys However, the elastic modulus of around 52 GPa of the
with low amounts of harmful elements and lower elastic β-Ti21S alloy is still too high compared to the human
modulus. Beyond Ti-6Al-4V, in UNI EN ISO 5832, bone. Replacement of the full body prosthesis with a
unalloyed titanium and Ti-15Mo-5Zr-3Al have been cellular structure allows to decrease its stiffness. Instead,
[6]
[5]
reported for use as surgical implants. Unalloyed titanium the properties of cellular biomaterials are affected by the
is characterized by a Young’s modulus similar to Ti-6Al-4V base material and the specific architecture of the unit
but with about half the tensile strength, different from cell [20,21] . An exhaustive review of the mechanical properties
β-Ti-15Mo-5Zr-3Al, which shows an elastic modulus of of the different architectures present in the literature was
around 80 GPa and a tensile strength similar to Ti-6Al-4V conducted by Benedetti et al. . Different behaviors during
[22]
(900 MPa) . The other four wrought titanium grades compression tests are highlighted, that is, bending- and
[7]
standardized for biomedical application are Ti-6Al-7Nb , stretching-dominated depending on the structure response
[8]
Ti-3Al-2.5V , Ti-15Mo, and Ti-12Mo-6Zr-2Fe . to the load. Bending-dominated lattices are characterized
[9]
[10]
[11]
Considering these alloys obtained by AM techniques, the by too few struts to balance bending moments at nodes
first two alloys are characterized, after thermal treatment, when externally loaded leading to the bending of the struts,
by α + β microstructure with a Young’s modulus near and the stress-strain curve shows a uniform collapse after
Ti-6Al-4V but with a lower amount of dangerous the yielding point. On the contrary, a stretching-dominated
elements [12,13] . In contrast, metastable β-Ti alloys show structure is composed of enough struts to equilibrate the
lower elastic modulus thanks to the low intrinsic elastic applied external load and the struts result stressed mainly
modulus of the body-centered cubic structure of β phase, parallel to the load direction with the result of sequential
as well as good mechanical properties and extraordinary local collapse after the yielding. The elastic modulus E
corrosion resistance and biocompatibility. However, and the yield strength σ of the trabecular structures can
y
Ti-15Mo exhibits too low strength compared to Ti-6Al-4V be correlated to relative density using Gibson-Ashby
and evidences a strong tendency toward brittle ω phase model [20,21] , according to Equations I and II.
precipitation . The metastable β Ti-12Mo-6Zr-2Fe alloy n1
[14]
in as-built condition shows mechanical strength similar to E = C1     (I)
Ti-6Al-4V because of high density of α II[15,16] . A decrease E0  
in the elastic modulus from 107 to 85 GPa is observed
by changing the scanning strategy from a simple back- = C 2    n 2 (II)
and-forth to a chess scan strategy due to the formation 0   

Volume 9 Issue 4 (2023) 450 https://doi.org/10.18063/ijb.728