Page 105 - IJB-8-3

Yang, et al.
Alloying treatment is commonly used to improve is expected to obtain Mg(Sc) parts with high content of
the degradation performance of Mg . Alloy elements are dissolved Sc. The microstructure feature and degradation
[11]
either dissolved in matrix or precipitated at grain boundary performance were studied. Besides, the biocompatibility
as second phase, which can improve the mechanical was also investigated using in vitro cell experiment.
properties by means of solution strengthening or
precipitation strengthening . However, the precipitates 2. Materials and methods
[12]
in Mg matrix usually induce galvanic corrosion due to the 2.1. Original materials and MA process
potential difference, thereby inevitably deteriorating the
degradation behavior . Assuming that alloy elements Spherical Mg powder (purity 99.9%, Shandong Weihao
[13]
can be completely dissolved in Mg matrix to form Magnesium Powder Ltd., China) and irregular Sc powder
supersaturated solid solution, it is expected to significantly (purity 99.99%, Hunan Rare Earth Metal Material
improve the degradation performance . However, the Research Institute, China) were used in this work.
[14]
solid solubility of most alloy elements in Mg matrix is MA processing was performed utilizing a high-energy
extremely small, which poses a huge challenge to prepare planetary mill (Pulverisette 6, Fritsch, Germany). In detail,
Mg supersaturated solid solution. the powder mixture containing a mass fraction of 25% Sc
Mechanical alloying (MA) is able to prepare was placed into a stainless steel grinding bowl, together
supersaturated solid solution through dislocation solute- with stainless steel grinding balls. The anhydrous alcohol
pumping mechanism . In detail, the particles are (4% mass fraction) was adopted as the process control
[15]
plastically deformed during ball milling due to the heavy agent. The ball to powder weight ratio was 20:1, and the
collision of the powders and milling balls, which generate rotation speed was fixed at 350 r/min. The whole milling
a large number lattice distortions and dislocations . It is process was carried out under argon atmosphere. To avert
[16]
known that the solute diffusivity along the dislocations is the temperature accumulation, an interval of 10 min was
relatively convenient as compared with that of the inner adopted after 10 min milling duration.
grain regions without lattice defect . Therefore, those The phase composition of the milled powders
[17]
crystal defects offer suitable diffusion paths for solute was identified by X-ray diffractometer (XRD, D8
atom . With continuously milling, the dislocations within Advance, Bruker, Germany) with Cu Kα radiation.
[18]
the particles will be continually forced to glide, which drags The scanning rate was determined at 4°/min. High-
the solute atoms existing in the dislocation region. This resolution transmission electron microscopy (HR-TEM,
allows the solute atoms to gradually diffuse into the crystal TecnaiG2-20, FEI Company, USA) was further used to
lattice of the solvent, obtaining solid solution structure. characterize Mg(Sc) powder. High-resolution imaging
To obtain final clinical application, the and selected area electron diffraction (SAED) were used
supersaturated solid solution particles should be to deeply reveal the atomic scale structure of the Mg(Sc)
shaped into parts with specific structure [19] . Selective powder. Meanwhile, energy-dispersive spectroscopy
laser melting (SLM) is a type of powder bed fusion (EDS, X-Max 20, Oxford instruments, UK) was adopted
technique [20-22] . Meanwhile, it is also a rapid melting/ to observe the distribution of Sc in Mg matrix.
solidification technology, which has great potential in 2.2. Samples preparation and characterization
preparing bulk supersaturated solid solution parts. It
is considered that laser beam has the characteristics of The consolidated samples of Mg, Mg-Sc, and mMg-
high-energy density and small action area [23] . SLM of Sc (as-milled 40 h) were prepared by a laser forming
solid solution powder exhibits a fast heating rate that system. This equipment contained an YLR-500-WC
can fuse the powder into supersaturated melt and then fiber laser, which had a spot size of ~ 70 μm. In addition,
the micro-molten rapidly cools by non-interface heat an inert gas protection system was adopted to prevent
conduction (>10 – 10 K/s), which is much higher than oxidation during processing. A series of preliminary
3
8
the critical cooling rate required to form supersaturated studies were carried out before determining the SLM
solid solution, so as to obtain the deposition layer of process parameters. Results indicated that a low laser
supersaturated solid solution [24-26] . At present, a large energy density resulted in the formation of pores and
number of researchers have reported SLM of Mg alloys consequent insufficient densification rate due to the high
and made some substantive progress [27] . liquid viscosity in the molten pool [29-31] . In contrast, a
Basing on above consideration, herein, rare earth high laser energy density led to the evaporation of
element Sc was used as alloying element to firstly Mg powder. The main processing parameters were
prepare Mg(Sc) solid solution powder by MA and then as follows: Laser scanning speed 600 mm/s, laser
developed into parts using SLM. Sc has a large solid power 135 W, layer thickness 50 μm, and hatch spacing
solubility of 24 wt% in α-Mg at 730°C, but only 1.5 wt% 50 μm. The previous study reported that the geometry
at room temperature . Thus, MA combined with SLM significantly affects the part properties [32-34] . To eliminate
[28]
International Journal of Bioprinting (2022)–Volume 8, Issue 3 97