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volumetric energy densities from 16 to 317 J/mm was 3. Potential research for β-titanium alloys by
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investigated . The porosity present in the samples is PBF
[24]
formed due to incomplete fusion and overmelting. As
observed for laser power of 100 W and 200 W, increasing 3.1. In situ alloying
scanning speed led to increasing lack of fusion pores . It In situ alloying using PBF, especially L-PBF, which
[24]
is common to have pores formation as a result of trapped uses powder mixtures instead of the typical pre-alloyed
gas or vapor during overmelting. In another study done, powders, has been popular to achieve β-Ti alloys. The past
also using Ti53Nb (wt%), volumetric energy density studies showed that this approach can be used to obtain
between 32 and 95 J/mm was used, relative density of β-Ti alloys with desirable properties [40,41] . For example,
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the samples ranged from 87% to above 99% . Due to Wang et al. used in situ alloying with L-PBF and found
[29]
the high niobium content, the samples maintained the that the niobium addition into titanium leads to better
β-phase with no phase transformation . in vitro apatite forming capability as compared to pure
[30]
titanium [42] . In a similar study done using in situ alloying
2.2. Properties of β-titanium alloys of Ti25Nb (wt%), better cell spread and proliferation are
Research on AM of β-Ti alloys confirmed the appearance observed when the alloy is benchmarked with titanium.
The β-Ti alloy also has superior in vitro appetite forming
of complex martensitic phases, the transformation of capability [43] . Using EB-PBF, in situ alloying of Ti10Nb
martensitic phases, and the improvement of mechanical (at%) was done. It was concluded that β-phase is dominant
properties . In a study on Ti45Nb (wt%), X-ray in the resulting alloy [44] . Comprehensive reviews on
[31]
diffraction patterns on the L-PBF part show β-Ti peaks in situ alloying have been conducted previously [21,45] .
with broadening characteristics possibly due to residual Comparison between typical PBF and in situ alloying is
stresses in the parts . The samples showed a compressive shown in Figure 2.
[32]
strength of 723 MPa. Another study was also done on
Ti42Nb (wt%) . In the work done on Ti53Nb (wt%), 3.2. Designed porosity for β-titanium alloys
[33]
metastable β-phase is also reported . A study done using
[29]
Ti15Mo3Nb3Al0.2Si (wt%) showed the as-built parts There is a limitation to the minimum elastic modulus
that can be achieved just by developing new materials.
exhibited columnar grains of β-phase oriented along the As such, an alternative solution to achieve modulus
build direction. The samples also have microhardness of matching between the implants and bone is by
278 HV, yield strength of 917 MPa, ultimate tensile strength introducing designed and controlled porosity to the
of 946 MPa with ductility of 25.3% . A study done on materials. Porous lattice structures can be fabricated by
[34]
Ti52Nb (wt%) shows that the scanning speed and laser L-PBF due to its capability for freeform fabrication [46-48] .
power, or potentially any process parameters, have effect on Coupling these structures with suitable alloys such as
the elastic modulus of the material. One of the parameters the β-Ti alloys, AM can potentially produce biomedical
set obtained Young’s modulus of 70.5 ± 1.5 GPa . implants that meet the stiffness and strength criteria
[30]
Schwab et al. studied the fabrication of Ti5Al5V5Mo3Mo while achieving excellent osseointegration. In their
(wt%) using L-PBF and were able to achieve relative work, Hafeez et al. fabricated porous structures
density of 99.5%. The samples also showed an ultimate with different pore dimensions using Ti35Nb2Ta3Zr
tensile strength of 800 MPa and maximum elongation of by L-PBF. They maintained the porosity for these
14% . In another study using Ti25Nb3Zr3Mo2Sn, the structures at around 50% and are able to obtain modulus
[35]
samples have ultimate tensile strength of 716 ± 14 MPa of approximately 3.1 GPa, which is quite close to the
and ductility of 37 ± 5% . Using Ti24Nb4Zr8Sn, L-PBF modulus of bones [49] . Porous lattice structures were
[36]
samples have Young’s modulus of 53 ± 1 GPa, ultimate
tensile strength of 665 ± 18 MPa, yield strength of 563 ± A
38 MPa, and elongation of 13.8 ± 4.1% . In a study using
[37]
Ti35Nb7Zr5Ta (wt%), relative density of more than 99.8%
was obtained. The alloy exhibits an ultimate tensile strength
of 631 MPa while having a low modulus of approximately
81 GPa . The same alloy was also fabricated using EB- B
[16]
PBF and has a modulus of 92 GPa . Wang et al. studied
[38]
the fabrication of Ti24Nb4Zr8Sn using EB-PBF, however,
no mechanical characterizations were conducted . Sun
[39]
et al. studied the fabrication of Ti15Mo5Zr3Al using both
L-PBF and EB-PBF. It is shown that the L-PBF samples
have higher ductility but lower strength when compared to Figure 2. Powder bed fusion process. (A) Typical approach.
the EB-PBF samples . (B) In situ alloying.
[28]
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