Page 54 - IJB-5-2

A continuous net-like eutectic structure enhances the corrosion resistance of Mg alloys
phase toward reducing their electrochemical nobility. For Cubic samples (10 × 10 × 10 mm ) were fabricated
3
example, Baek et al. investigated the effect of Y on the using a SLM system equipped with an YLR-500-WC
[11]
corrosion behavior of Mg-Al-Ca alloy. It was found that fiber laser (IPG Photonics Inc.). The process parameters
the Y-containing phase was less cathodic and drastically were determined at a laser power of 120 W, a scanning
weakened the galvanic corrosion tendency. Liu et al. speed of 10 mm/s, a layer thicknesses of 150 µm, a
[12]
reported that rare earth element enhanced the corrosion scanning spacing of 80 µm, and a spot size of 80 µm.
resistance of AM60, because the deposited phases All the experiments were performed under a high-purity
containing rare earth were less cathodic than β-Mg Al Ar atmosphere. A zigzag pattern was applied to scan the
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phase. Another strategy to increase the corrosion powder layer.
resistance is to ameliorate the distribution of the second
phase in Mg matrix. Wu et al. confirmed that adding 2.2 Microstructural Characterizations
[13]
proper Al into Mg-Ca alloy formed a continuous second The specimens were grounded, polished, and ultrasonically
phase, which provided barrier effect, resulting in better cleaned with ethanol. Then, the microstructures were
corrosion resistance. Shuai et al. also reported that investigated using SEM. Moreover, the crystalline
[14]
Nd-introduced continuous second phase enhanced the structure was observed using an optical microscope (Leica
corrosion resistance of Mg matrix, with the degradation DM200, Leica Microsystems, Germany) after etching
rate decreased from 5.25 to 1.56 mm/y. with a nitric acid alcohol solution (4%). Furthermore,
In this study, Ti was introduced into AZ61 to ameliorate the phase compositions were identified using X-ray
the characterizations of precipitates, with an aim to diffraction (XRD, D8 Advance diffractometer, Bruker
improve the corrosion resistance. In Al-Ti-Mg system, Ti Inc., German) at a scanning rate of 8 min .
−1
will combine with Al to form precipitates and increase the
diffusivity of Al in Mg solute, resulting in the increase of 2.3 Mechanical Characterizations
Al content near the eutectic point during the solidification
process . Thus, it was expected that Ti could promote The SLM fabricated samples were cut into 8 mm in
[15]
the formation of less cathode eutectic α phase, which can length and 4 mm in diameter for compressive tests. The
reduce the electrode potential differences of the matrix. compressive tests were carried out using a universal
On the other hand, the precipitation of eutectic α phase mechanical testing machine (Instron, USA) with a
will consume Al atoms, which is conducive to reducing compression speed of 0.5 mm/min. Microhardness
the formation of β-Mg Al phase, thus further alleviating was measured on a microhardness tester with a load of
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the galvanic corrosion in the matrix. The microstructures, 0.98 N and a holding time of 15 s. The microhardness
mechanical properties, and corrosion behaviors of measurements were taken 5 times for each specimen,
Ti contained AZ61 alloys fabricated with selective and the distance between each adjacent indentation was
laser melting (SLM) were systematically investigated. 500 µm.
Moreover, the biocompatibility was also studied through 2.4 Electrochemical Experiments
in vitro cell culture experiments.
Electrochemical experiments were performed three a
2. Experimental Methods three-electrode system in which the sample was used as the
working electrode, a platinum foil as a counter electrode,
2.1 Specimens Preparation and a saturated calomel electrode as a reference electrode.
The gas-atomized AZ61 powder (Weihao Magnesium All the electrodes were connected to an electrochemical
powder Ltd., China) had a particle size distribution of workstation (Interface 1000, Gamry Instrument, USA)
d =29.04 µm, d =52.68 µm, and d =84.71 µm. The and immersed in simulated body fluid (SBF) at 25±0.5°C.
50
90
10
[16]
Ti powder (Naiou Nano Science and Technology Ltd., The SBF was prepared according to reference . The open
China) had a particle size varying from 20 to 50 nm. The circuit potential was first monitored for 2400 s. Then, the
AZ61-xTi (x=0, 0.25, 0.5, 0.75, and 1.0 wt.%) mixed potentiodynamic polarization testing was conducted with
powders were prepared using a ball mill with a rotation a scanning rate of 0.333 mV/s.
speed of 200 rpm for 8 h under Ar atmosphere. The 2.5 Immersion Experiments
morphologies of mixed powders were observed using
a scanning electron microscope (SEM, Phenom proX, Immersion experiments were carried out to study the
Phenom-World BV, Netherlands) coupled with an energy corrosion behavior. Specimens were immersed in
dispersive spectroscopy (EDS), with results presented in SBF at 37°C, with a volume to exposure area ratio of
Figure 1. The element mapping results corresponding to 20 mL cm . The hydrogen release rate and the pH values
−2
AZ61-0.5Ti mixed powders indicated Ti nanoparticles during immersion were monitored. Meanwhile, the ion
homogeneously dispersed over the AZ61 particle surface. concentrations of the soaked media were measured

50 International Journal of Bioprinting (2019)–Volume 5, Issue 2