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Materials Science in Additive Manufacturing LPBF of Mg and its bio-applications

improved. Obviously, the introduction of bioactive to maintain consistently. Due to the complexity of the
ceramics can not only induce the formation of calcium and process chain, many potential fluctuations may occur
phosphorus layers, providing long-term stable protection, during manufacturing process, which leads to variable
but also greatly improve the biological performance. quality of LPBF parts. Recently, a large number of
Recently, the use of mesoporous bioglass as a reinforcing scholars have explored the use of machine learning
phase to prepare Mg-based composites for bone repair (ML) algorithms to overcome this obstacle using
has been proposed. Mesoporous bioglass has uniform datasets obtained at various stages of the LPBF process
[100]
and ordered mesoporous channels (2 – 50 nm) and high chain . Before LPBF, ML algorithms can be used
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specific surface area (500 – 800 m /g) . More importantly, for part design and document preparation. During
[99]
as a silicon-containing active ceramic, a large number of the LPBF process, ML can be applied for process
[101]
silanol groups will be formed on the interface at the initial parameter optimization and in situ monitoring .
stage of degradation, thereby forming a negatively charged In addition, ML can also be integrated into post-
silica gel layer. Under alkaline conditions, the silica gel processing. Therefore, in the future, it is promising
layer acts electrostatically to adsorb Ca and HPO in to attempt to integrate ML algorithms into different
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the solution, thereby inducing in situ deposition of apatite. stages of the LPBF process chain to better control the
In vitro degradation tests showed that this in situ deposited quality of LPBF Mg alloys.
calcium-phosphorus layer effectively enhanced the 4.2. Challenges of Mg alloys in biomedical applications
biological activity of Mg alloy substrates.
In the early stage, most of the medical Mg alloys were in
4. Challenges for the future the basic research stage, and the types of alloys that can
be clinically applied are rare. At present, there are only
4.1. Challenges of LPBF-processed Mg Alloys
high-purity Mg and WE43. Other Mg alloys still face great
Due to the inherent characteristics of Mg alloys such as challenges in clinical applications, including the following
low evaporation temperatures, high vapor pressures, and a problems.
high propensity to oxidize, the manufacture of degradable (i) The degradation rate is too fast. Since the electrode
Mg-based implants through AM presents a great number potential of magnesium is −2.37 V, it usually appears as
of challenges. an active anode. Corrosion reaction occurs in the body
(i) The preparation of Mg powder that can be used for fluid environment, and more Mg(OH) is generated
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AM processing and degradable Mg-based implants is on the Mg matrix. The corrosion layer has a loose
difficult. The preparation conditions of Mg powders are structure and poor corrosion resistance. In addition,
extremely demanding and the slightest inadvertence bodily fluids contain a large amount of Cl , which will
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can lead to explosive accidents. Moreover, in the further react with Mg(OH) to form the more soluble
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current market, the Mg alloy powders commonly MgCl , thereby accelerating the degradation rate.
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used in AM processing are pure Mg, AZ91D, and (ii) The mechanical strength and toughness are insufficient.
WE43 powders. Due to the biological toxicity of Al For materials used in bone fixation and support, high
element, AZ91D alloy contains 9% (mass fraction) of strength and moderate plasticity are required, such
Al; therefore, only pure Mg and WE43 powders are as yield strength ≥300 MPa and elongation ≥10%;
suitable for degradable Mg-based implants. and for materials used in coronary stents and balloon
(ii) The AM processing for Mg alloys always produces dilators, high plasticity and medium strength are
severe powder splashes due to low evaporation required, such as elongation ≥20% and yield strength
temperature and high vapor pressure of Mg alloys, ≥150 MPa. Mg alloys are difficult to enhance plasticity
and this phenomenon is very different from AM with increasing strength.
processing steel, Ti, or Al. Powder spattering can (iii) Biocompatibility verification is insufficient. Mg has
significantly reduce the stability of the Mg alloy good biocompatibility, but other alloying elements
during AM processing, as some Mg powder is are inevitably added in the smelting process, which is
removed by steam along the scan path, where defects potentially toxic to the human body. For example, Al
are likely to occur in subsequent scan passes. In this can cause chronic neurotoxicity and lead to Alzheimer’s
case, a strategy of powder replenishment is necessary disease; some rare earth elements (Y, Nd, Pr, etc.) are
for Mg alloys during AM processing. However, there potentially toxic after implantation. The corrosion
are no relevant studies on the interaction between Mg process will be accompanied by the production of a
powder evaporation, gas flow, and laser input. large amount of OH and H , which can easily trigger
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(iii) The quality of components prepared by LPBF is difficult an inflammatory response.
Volume 1 Issue 4 (2022) 12 https://doi.org/10.18063/msam.v1i4.24