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Additive Manufactured Beta-Titanium Alloys

Figure 1. Powder bed fusion process.

mismatch of their modulus to that of natural bones. While effects on wear and corrosion resistance which have to be
the elastic modulus of human cortical bone ranges from avoided for biomedical applications. The complex PBF
17.6 to 28 GPa, that of CoCrMo and 316L stainless steel process involves multitude of physical phenomena, such
is more than 200 GPa and even Ti6Al4V has elastic as thermal energy absorption, reflection, and transfers.
[12]
modulus of approximate 130 GPa . These are many Phase transformations such as solid to liquid and then
[13]
times higher than that of human cortical bone even when back to solid also occur [17,18] .
they are commonly used . Osteolysis can be attributed For low modulus β-Ti alloys that are often
[14]
to this modulus mismatch as it lessens the loading on the metastable, a high cooling rate during the manufacturing
natural bones that are neighboring the implant, leading to process is required to retain the β-phase . As such, PBF is
[19]
bone resorption. Finally, implant loosening would occur. inherently designed for this due to the rapid solidification
Recently, due to their higher strength, lower modulus and cooling cycles that occur in the process . However,
[20]
and better corrosion resistance, beta-titanium (β-Ti) alloys many defects such as porosity, balling, oxide inclusions,
have been identified as potential materials to improve and cracking remain as metallurgical challenges for
implants quality [7,15,16] . β-Ti alloys are titanium alloys PBF. The details of these defects and their forming
where the β-phase is significantly retained in equilibrium mechanisms have been discussed in recent reviews [21,22] .
or at least on quenching from the β-phase without To minimize these defects in PBF parts, the effect of
transformation into martensite or α-phase . Furthermore, process parameters on the parts quality has been studied
[7]
β-Ti alloys usually consist of non-toxic elements such as extensively. The commonly investigated L-PBF process
tantalum, niobium, molybdenum, tin, and zirconium. To parameters include laser power (P), scanning speed (v),
obtain β-Ti alloys that are biocompatible, niobium and hatch spacing (h), and layer thickness (d) and they are
tantalum which are β-phase stabilizers for titanium are the often discussed using one equation:
common choices for alloying elements due to their high
biocompatibility. The other elements such as zirconium, P
molybdenum, and tin are added to further modify phases ε = v h d (1)
. .
and microstructures of the β-Ti alloys.
In this article, an overview of the processing,
where, ε is termed as the volumetric energy
microstructure, and properties of β-Ti alloys processed density [23,24] .
by PBF that can be used in biomedical applications is
discussed. The potential and limitations of using PBF For EB-PBF, the key process parameters include
for these materials in biomedical applications are also acceleration voltage (V), beam current (I), scanning speed
elucidated with focus on the perspectives from processes, (v), hatch spacing (h), and layer thickness (d). They are
materials, and designs. Finally, future trends and potential also often discussed using one equation:
research topics are highlighted. VI
ε = (2)
2. β-Titanium alloys by powder bed fusion v h d ⋅
⋅
2.1. Powder bed fusion
where, ε is also the volumetric energy density [25,26] .
It is important to understand the physical phenomena that Equation (ii) can be expressed as Equation (i) in
occur during the PBF process to obtain parts with good which P = VI where P is the beam power [27,28] .
quality, that is, parts that are defect free. For functional As an example to elucidate the process parameters
applications of PBF parts, defects can have detrimental effect on β-Ti alloys, the fabrication of Ti53Nb using

2 International Journal of Bioprinting (2022)–Volume 8, Issue 1