Journal of Materials Science & Technology  2020 , 43 (0): 189-196 https://doi.org/10.1016/j.jmst.2019.10.029

Research Article

Formation of spherical alloy microparticles in a porous salt medium

Hayk H. Nersisyana, Suk Cheol Kwonb, Vladislav E. Rib, Wan Bae Kimb, Woo Seok Choib, Jong Hyeon Leeab*

a RASOM, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea
b Graduate School of Materials Science and Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea

Corresponding authors:   ∗Corresponding author at: Graduate School of Materials Science and Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea.E-mail address: jonglee@cnu.ac.kr (J.H. Lee).

Received: 2019-08-2

Revised:  2019-10-1

Accepted:  2019-10-22

Online:  2020-04-15

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

This study describes the development of a one-pot strategy to produce spherical alloy microparticles for advanced near-net-shape manufacturing processes, including additive manufacturing and powder injection molding. The AlSi12 eutectic alloy (ca. 12 wt% Si) system was chosen as the model with which the main experiments were carried out. The proposed process synergistically integrates a few common, low-cost processing techniques including the mixing of Al micrometer size particles with silicon and sodium chloride, heat-treating the mixture at temperatures of 650-810 °C, and the dissolution of salt in water to produce spherical AlSi12 alloy particles without the need to rely on costly melting and atomizing techniques. This new process can use laow-cost source Al and Si powders as the raw materials to produce 10-200 μm-sized spherical particles of AlSi12. The Ansys-CFX computational fluid dynamics software was used to analyze the flow behavior of AlSi12 liquid droplets and particle size refinement in the narrow voids of the sample.

Keywords: AlSi12 alloy ; Morphology evaluation ; Particle distribution ; Simulation ; Solid-liquid transition ; Spheroidization

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Hayk H. Nersisyan, Suk Cheol Kwon, Vladislav E. Ri, Wan Bae Kim, Woo Seok Choi, Jong Hyeon Lee. Formation of spherical alloy microparticles in a porous salt medium[J]. Journal of Materials Science & Technology, 2020, 43(0): 189-196 https://doi.org/10.1016/j.jmst.2019.10.029

1. Introduction

Spherical particles have drawn considerable attention in recent years for their novel applications. Owing to the uniform size of spheres coupled with good flowability, applications of spherical particles include additive manufacturing for the medical, aerospace, jewelry, and automotive industries [[1], [2], [3]]. Therefore, the size and spherical shape of particles are critical factors in determining material properties, and the ability to control these properties in synthesis processes has become a major goal in the field of materials science [[4], [5], [6], [7], [8], [9], [10]]. As of this writing, atomization is the main synthesis technique that has been reported for the synthesis of spherical particles. The following types of atomization processes are known: water atomization [11,12], gas atomization [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]], soluble gas or vacuum atomization [19], centrifugal atomization [[20], [21], [22]], the ultra-rapid solidification process [23], and ultrasonic atomization [24,25]. In conventional (gas or water) atomization, liquid metal is produced by pouring molten metal through a tundish with a nozzle at its base. The stream of liquid metal is then broken into droplets by the impingement of high-pressure gas or water. The interaction between the liquid metal stream and the jets begins with the creation of small disturbances at the liquid surface, which grow into shearing forces that fragment the liquid into ligaments. The broken ligaments are further formed into spherical particles due to the high energy of the impacting jet. Like all powder processing applications, atomization methods are potentially sensitive to several powder characteristics that relate to the method in which the material was atomized. The atomization issues that affect key powder characteristics include alloy melting techniques, atomization media and collection, and process costs. This is why the atomization process does not always yield spherical and dense particles, and many irregularities (such as satellites) are formed by the interaction of molten particles during solidification.

A simple and widely applicable method of synthesizing a variety of spherical particles in large quantities would help in exploiting the potential of such particles and pave the way for a new field in spherical particle-based science. Recently, the synthesis of ultrafine metal particles from metal (powder)-graphite (graphene) mixtures was reported in Refs. [26,27]. After being heat-treated at 1000 °C, the reaction mass is quenched to room temperature and exposed to ultrasonic treatment to separate the spherical particles from the graphite. The shortcoming of the reported method is the contamination of metals by free and chemically bonded carbon. Skalon et al. [28] reported a new production method for spherical aluminum powder. Aluminum powder is mixed with silica nanoparticles and heated to a temperature above the melting point of aluminum. Due to an exothermic reaction between Al and SiO2, near-eutectic phase spherical AlSi12 droplets with Al2O3 particles are formed. However, the shortcoming of this synthesis method is the contamination of AlSi12 particles by oxygen, with concentrations ranging from 1.5 to 11 wt%.

Herein, we developed a highly efficient one-pot strategy to synthesize spherical particles of low-melting point metals and alloys. In our process, a metal powder was mixed with a large portion of inorganic salt and heated to a temperature at which the metal liquefies, but not the salt. Among all inorganic salts, alkali and alkali earth metal chlorides (NaCl, KCl, MgCl2, CaCl2) and fluorides (NaF and KF) are the best candidates as such salts are soluble in water, do not react with most metals, and are poorly wetted by molten metals. This technology can spheroidize any shapeless metal or metal alloy particles that have a melting point of ≤1000 °C and sizes ranging from 5 to 500 μm. The list of metals includes, but is not limited to, the following: Al (Tmelt = 660 °C), Cu (Tmelt = 1080 °C), Mg (Tmelt = 650 °C), Ag (Tmelt = 960 °C), and Zn (Tmelt = 450 °C). Compared to the metal-graphite system reported in Refs. [26,27], the proposed system has a number of advantages, such as the easy separation of metal particles from the halide salt, the high purity of the metal particles, a perfect spherical shape with narrow size distribution, and reduced particle size compared to the size of the precursor metal. In addition, this process is environmentally friendly as metal halides are easily recycled and returned within the process.

As a proof of our concept, spherical AlSi12 particles were fabricated in a solid medium of NaCl particles. The AlSi12 alloy has attracted increased attention in recent years as it is a near-eutectic alloy with a low melting point (573-585 °C), good corrosion resistance, high tensile strength with low specific gravity, and due to the fact that casting processes can be easily performed [29]. The aforementioned features of this alloy cause to use it for manufacturing lightweight components or prototypes in the field of automotive and aerospace industries.

2. Experimental

The following materials were used in the experiments: Al powder (purity 99 % particle size 50-100 μm), Si powder (purity 98 %, particle size 10 μm), and NaCl powder (purity 99.5 %, particle size ≥500, 150-250, ≤100 and ≤50 μm). All powders were purchased from Samchung Chemicals, Korea. The schematic of the process is shown in Fig. 1. The reaction mixture was prepared from the Al, Si, and NaCl powders, which were dried in air at 100 °C. The mixing was performed by hand using a ceramic mortar and pestle. Al and Si were mixed with a proportion of 88/12 wt% (or 1/0.13 mol) to produce the AlSi12 eutectic alloy. The amount of NaCl (k) was in the 0.15-1.2 mol range with respect to 1.0 mol of Al. Approximately 50 g of the as-prepared reaction mixture was loaded into an alumina boat, which was then placed in tube furnace and heated to the working temperature (660-810 °C) at a rate of 300 °C/h (Fig. 1(a)) in an argon gas atmosphere. The reaction mixture was maintained at the maximum temperature for 2 h and then allowed to cool naturally. After the heat treatment, the reaction mass was subjected to warm water purification to eliminate NaCl (Fig. 1(b)). The solid that was obtained after water washing was rinsed twice with ethanol and then dried in an oven at 100 °C. The dried powder had a metallic color and displayed good flowability (Fig. 1(c)).

Fig. 1.   Schematic for synthesizing Al/Si alloy spherical particles.

The crystal structure and morphology of the final powders were characterized using an X-ray diffractometer with Cu radiation (PANalytical X’Pert PRO X-Ray Diffractometer, Netherlands) and a field-emission scanning electron microscope (FESEM, JEOL JSM-6700 F, Japan). The oxygen concentration in the Al powder was determined using a Thermo Scientific FLASH 2000 Analyzer. Screenshots of the particle size distribution data were taken using a Microtrac S3500 laser diffraction particle size analyzer (LPSA). The flow characteristics of the liquid in the sample voids were simulated using the commercial fluid dynamics software Ansys 18.

3. Results and discussions

We conducted preliminary experiments to investigate the melting of the AlSi12 mixture in various porous salt media (NaCl, NaF, KCl, and KF). The experiments indicated that AlSi12 alloy particles become spherical in shape regardless of the type of salt used. Therefore, for the main experiments, a mixture of AlSi12 composition was selected as the model system, and the spheroidization process was conducted in the sodium halide (NaCl) medium. The effects of annealing temperature, NaCl concentration and particle size on the AlSi12 particle shape and size were researched.

3.1. Morphology of AlSi12 particles observed by using SEM

The effects of annealing temperature, NaCl particle size and concentration on the shape and size of AlSi12 particles are shown in Fig. 2. AlSi12 particles prepared at 660 °C mainly are spherical in shape and have diameters of less than 50 μm, but some particles have diameters between 50 and 100 μm (Fig. 2(A-a)). Due to the low annealing temperature, a proportion of the particles had not yet fully transformed into a spherical shape. A significant improvement to AlSi12 particle shape was achieved at 700 °C (Fig. 2(A-b). SEM images show that the alloy particles have a perfect spherical shape, better dispersion, and diameters between 20 and 80 μm. The spherical shape of the particles was sufficiently maintained at 750 °C, but the diameter increased by up to 100 μm (Fig. 2(A-c)). However, the spherical particle formation process is disturbed when the annealing temperature becomes equal to or higher than the melting point of NaCl (810 °C). In this situation, the molten alloy droplets move freely and merge with each other to create millimeter-scale globules (Fig. 2(A-d)).

Fig. 2.   Morphologies of AlSi12 particles. (A) Effect of annealing temperature: (a) 660 °C; (b) 700 °C; (c) 750 °C; (d) 810 °C; (k = 0.4, rNaCl<100 μm). (B) Effect of NaCl particle size: (a) rNaCl≥500 μm; (b) 150≤rNaCl ≤250 μm; (c) 50≤rNaCl ≤150 μm; (d) rNaCl ≤50 μm (k = 1.37, T = 750 °C). (C) Effect of NaCl concentration (k): (a) 0.15; (b) 0.4; (c) 0.8; (d) 1.37 (50≤rNaCl ≤150 μm, T = 750 °C).

The effects of NaCl particle size on the morphology and size of the AlSi12 particles at 750 °C are shown in Fig. 2B. The alloy particle size continuously decreases with decreasing NaCl particle size. For instance, alloy particles with diameters of 50-100 μm can be generated if the size of the NaCl particles is rNaCl≥500 μm (Fig. 2(B-a)). AlSi12 particles with diameters of 25-80 μm can be obtained when NaCl particle size is within the 150-250 μm range (Fig. 2(B-b)). Further reductions in AlSi12 particle size were achieved using NaCl particles with diameters of 50≤rNaCl ≤150 μm (Fig. 2(B-c)) and rNaCl ≤50 μm (Fig. 2(B-d)). In the latter case, the diameter of the alloy particles decreased to less than 40 μm. It is likely that the grouping of large NaCl particles forms wide voids that can fit several Al and Si particles. Consequently, the melting and merging of Al droplets produces alloy particles with large diameters. On the other hand, small NaCl particles generate small-scale voids that can only fit one or two particles. In this case, the size of the alloy particles tends to be closer to the size of the precursor aluminum (i.e. smaller particle size).

The effects of NaCl concentration (k, moles) on the morphology of AlSi12 particles are shown in Fig. 2(C). The micrographs show that the spherical shape of the AlSi12 particles were mainly maintained in the given range of k, but the diameter of the particles varies significantly. At k = 0.15 (Fig. 2(C-a)), the particles had diameters in the 50-150 μm range, followed by a continuous reduction in size to 20-60 μm with increasing values of k up to 0.8 (Fig. 2(C-b) and (C-c)). When k = 1.35 (Fig. 2(C-d)), most particles have diameters of less than 10 μm. At this size, the particles are not perfectly spherical in shape and some particles are agglomerated. This can be explained by the surface tension of the droplets: as surface tension decreases with decreasing Si droplet size [30], the small particles are not able to fully transform into spherical shapes.

3.2. Particle size determination by LPSA analysis

LPSA analysis was conducted to determine the mean size of the AlSi12 particles. Typically, two type of distributions were obtained, unimodal and bimodal. The most distributions were unimodal, but in some cases bimodal distributions were also recorded. Three randomly selected distributions are shown in Fig. 3A. Two of them are unimodal with mean size of approximately 37.7 and 165 μm (Fig. 3(A-a) and Fig. 3(A-c)), and one is bimodal with 88.8 μm mean size (Fig. 3(A-b)).

Fig. 3.   (A) Particle size distributions obtained by LPSA analysis; (B) Mean diameter of Al/Si particles according to process parameters. (a) Moles of NaCl; (b) Heat-treatment temperature; (c) Particle size of NaCl.

The mean sizes of the alloy particles according to the process parameters are shown in Fig. 3B. As observed from the graphs, a temperature increase from 660 to 750 °C generates an increase in particle diameter from 35 to 55 μm (Fig. 3(B-a)). However, as the NaCl particles melt (≥810 °C), the AlSi12 microdroplets coagulate and form millimeter size droplets. The diameter of the AlSi12 alloy particles shows a tendency to increase linearly with the size of the NaCl particles. As can be seen from Fig. 3(B-b), a tenfold increase in NaCl particle size (from 50 to 500 μm) leads to a threefold increase in the diameter of the AlSi12 alloy particles (from 35 to 90 μm). Taking into account the fact that the mean size of the starting Al particles was determined as 47 μm by LPSA, it became clear that after the heat-treatment, the diameter of the AlSi12 alloy particles may increase, remain stable, or decrease slightly. Lastly, the size of the particles decreases with increasing amounts of salt in the mixture (Fig. 3(B-c)). When the salt amount is 0.8 mol or greater, the change in particle diameter becomes negligible, which indicates that full separation of the molten AlSi12 microdroplets was achieved. In the given range of k, the mean diameter of the alloy particles was reduced almost threefold, from 95 (k = 0.15) to 35 μm (k = 0.8).

The proposed process was also tested on Al, Al/Mg, and Al/Si/Cr/Cu systems to demonstrate its versatility. The following salts were used to create a solid porous medium: NaCl, NaF, and KCl. In most cases, the aluminum metal and its alloy particles formed spherical shapes regardless of the type of salt used (Fig. 4). In these systems, size control was easily achieved by varying the process parameters, such as temperature, concentration, and the size of the metals and salts.

Fig. 4.   (a) XRD patterns of spherical particles: (1) Al powder; (2, 3) Al/Si alloy powder prepared with 0.43 mol NaCl having 150-250 μm and <50 μm particles size; (b) Surface polished AlSi12 particle; (c) EDS spectra of AlSi12 powder with elements concentration and map.

The XRD peaks of the spherical Al and AlSi12 powders are shown in Fig. 5. The XRD peaks captured from spherical Al particles matched face-centered-cubic structure (Fig. 5(a) (pattern 1). The peaks are sharp and narrow, signifying the particles possessed high crystallinity. The XRD patterns of the AlSi12 alloy prepared with two different sizes of NaCl (150-250 and <50 μm, respectively) show the presence of the two elements Al and Si, since AlSi12 is a eutectic alloy (Fig. 5(a), patterns 2, 3). The peaks had high intensities and no peaks of secondary phases (Al2O3, SiO2) were detected. Cross-sectional mapping was also conducted to show the distribution of Si within the Al sphere. To do this, the AlSi12 particles were mounted into a resin and polished until a mirror surface was achieved. The SEM micrograph captured under the COMPO mode shows the cross-sectional morphology of the AlSi12 particle (Fig. 5(b)). The elements detected by the EDS are Al, Si, and trace amounts of O (Fig. 5(c)). According to the mapping data, Si has a specific network distribution, whereas the oxygen is uniformly distributed in the sample volume. In addition, we analyzed the oxygen concentration using the Thermo Scientific FLASH 2000 Analyzer which was found to be in the range of 0.5-2.0 wt%.

Fig. 5.   SEM micrographs of Al and its alloy particles: (a) pure Al (T = 750 °C, salt-NaCl); (b) Al/Si particles (T = 700 °C, salt-KCl); (c) Al/Si/Cr/Cu alloy (T = 750 °C, salt-NaCl); (d) Al/Si (T = 850 °C, salt-NaF); (e) Al/Mg (T = 750 °C, salt-NaCl); (f) Al/Mg (T = 700 °C, salt-NaCl).

3.3. Mechanism of particle spheroidization

The mechanisms governing the spherical geometry of liquid AlSi12 droplets were also analyzed. Three main forces were selected to explain the spheroidization of AlSi12 particles: surface force (fs), solid-liquid interfacial force (fsi), and gravitational force (fg). Surface and interfacial tension forces tend to shape the microdroplets into a ball shape, whereas gravitational force tend towards a flat shape. The following equations were applied to calculate the values of the given forces [31,32]:

Gravitational force: fg=ρgD3 (1)

Surface force: fs=σD (2)

Solid-liquid interfacial force: fsi=σD (1+cosθ) (3)

where ρ is the density of liquid AlSi12 in g/cm3, D is characteristic length or droplet diameter in cm, g is gravitational acceleration in cm/s2, σ is liquid surface tension in N/cm, and θ is liquid contact angle with the NaCl material in degrees. The results of the calculations conducted for the AlSi12 droplets (50-100 μm) show that surface and interfacial forces (fs = 4 × 10-3 g, fsi = 3.2 × 10-3 g) are several orders of magnitude higher than the gravitational force (fg = 3 × 10-7 g). This leads to the formation of spherical particles during thermal annealing.

The apparent density and flowability of the AlSi12 spherical particles were measured and compared to particles prepared by the gas atomization technique [33]. The results are shown in Table 1. The apparent density of the AlSi12 particles were similar to the standard value (1.6-1.65 g/cc) regardless of the size of the particles. These values are 20 % higher compared to the density of precursor Al powders (1.35 g/cm3). Moreover, these values are higher than the density values of AlSi12 particles prepared using the gas atomization technique (1.05-1.28 g/cc) [33]. The measured flowability of the powders shows that 50 g of powder passes through a funnel over 48-57 s, which is lower compared to the passing time of AlSi12 particles prepared using the gas atomization technique (65-72 s). This comparison demonstrates the unique possibilities and future expectations of the proposed method.

Table 1   The mean diameter, bulk density and flow rate of spherical Al and Al-Si particles.

SystemCurrent MethodGas atomization [33]
d50 (μm)ρ (g/cc)f (s/50 g)ρ (g/cc)f (s/50 g)
Al precursor801.35No flow--
Al451.6548--
AlSi121651.657--
AlSi12951.650--
AlSi12581.65481.0572.2
AlSi12501.6481.2865.4

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From the SEM micrographs shown in Fig. 2(A-a), 2(B-d) and 2(C-d), it can be seen that a visible portion of the particles was smaller in size than the precursor Al particles. This indicates that, during the synthesis, a certain degree of refinement occurred with the Al particles. To estimate the size and portion of the refined particles, the powder shown in Fig. 2(B-d) was sieved and the fraction of particles with sizes smaller than 30 μm was collected and compared to the initial Al powder (Fig. 6). The precursor Al particles had globular shapes and sizes between 50 and 100 μm (Fig. 6(a)); however, it was difficult to observe particles smaller than 50 μm on the micrograph. On the other hand, the AlSi12 particles were smaller than 50 μm: more accurately between 3 and 30 μm (Fig. 6(b)). The fraction of small-sized particles (m, wt%) varied depending on NaCl particle size as follows: rNaCl>500 μm, m = 0 wt%; (b) 150≤rNaCl ≤250 μm, m = 5 wt%; (c) 50≤rNaCl ≤150 μm, m = 10 wt%, and (d) rNaCl ≤50 μm, m = 30 wt%. In other words, a reduction in the size of the salt particles leads to an increase in the number of fine alloy particles. EDS analysis was conducted on the selected area to provide semi-quantitative elemental composition information (Fig. 6(c)). The main elements in the selected area were Al and Si (Pt is a coating material) with ∼ 1.0 wt% oxygen.

Fig. 6.   SEM/EDS analysis of Al and AlSi12 particles: (a) precursor Al powder; (b) sieved AlSi12 powder; (c) EDS analysis of AlSi12 particles.

3.4. Size refining mechanism of AlSi12 particles

The size refining mechanism of AlSi12 particles was examined based on the density changes of pure Al and the AlSi12 alloy during the solid-liquid transition. According to Magnusson et al. [34], pure Al and AlSi12 alloys exhibit drastic changes in density at the melting temperatures of 660 °C for pure Al and 575 °C for the AlSi12 eutectic alloy. To describe this process, two linear regression equations were used to interpret the density changes. In this study, the density data were regressed with a sigmoidal function in Origin Pro 8.1. The equation and the parameters are depicted in Eq. (4) and Table 2.

$ρ=\bigg{\{}A1+\frac{ span×p }{1+10^{(L1-T) ×h1} } +\frac{ span×(1-p)}{ 1+10^{(L2-T)×h2} } \bigg{\}}$ (4)

Table 2   Coefficients for regression equations on the temperature-dependent density of pure Al and Al/Si eutectic alloy.

TypePure AlAl/Si eutectic alloy
A12.227491.01111
A22.672872.75639
L1645.726642106.55825
L2635.3015578.0147
h1-0.00167-0.000537
h2-0.02281-0.51692
p0.617660.93398
Span (A2-A1)0.445381.74528

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It was assumed that irregular aluminum particles are randomly distributed among the salt phase and the salt particles are fixed in their initial location throughout the melting and solidification processes (Fig. 7(a)). The temperature of the calculation domain was increased with sin((t×1[s-1]) ×850×π) ×800, and transient analysis was performed within 0.001 s with the final temperature maintained until 0.2 s, as shown in Fig. 7(b).

Fig. 7.   (a) Pseudo-2D initial mesh describing the distribution of salt and aluminum; (b) temperature increase vs. time; (c) calculated phase distribution of aluminum with increasing and decreasing temperature.

The volume of the aluminum phase increases with increasing temperature according to the temperature-dependent density model used in the calculation, and this expansion causes the melt to flow into the voids (cavities) between the salt particles. The volume expansion of aluminum ends at 0.0006 s, after which the aluminum begins to shrink due to a decrease in temperature. The aluminum phases are isolated in the voids between the salt particles during shrinking, which is followed by the formation of the spherical phase of aluminum (Fig. 7(c)). In this step, two different shapes of aluminum are observed: the spherical phase in the narrow voids and irregular shapes in the large voids. The formation of the spherical aluminum phase is attributed to the boundary conditions used in the simulation and explains the increase in the number of small particles with the use of smaller salt particles. The calculation also shows that, in the initial stages of heating, larger droplets are irregular in shape and additional time is required for such droplets to form into a spherical shape. Unfortunately, we were not able to determine the optimum time for droplet spheroidization due to the extensive period of time required for the calculation process.

4. Conclusion

In this study, an effective and one-pot strategy was developed to produce spherical particles of metals and alloys. As a proof of concept, spherical AlSi12 alloy particles were fabricated in a porous medium of NaCl. The effects of heating temperature and the size and amount of NaCl on the morphology of the AlSi12 particles were studied and established. The AlSi12 alloy particles synthesized through the developed approach possessed the following characteristics: diameters ranging from 10 to 200 μm, densities of ∼1.6 g/cm3 and flowability values in the 65.4-72.2 s/50 g range. In addition, the size refinement of alloy particles was determined according to the size of NaCl particles through experiments. According to computational fluid dynamic simulations of the Al-Si-NaCl system, the AlSi12 particle size refinement phenomenon in the narrow voids of the sample is caused by density changes during the solid-liquid transition.

Acknowledgments

This work was supported by the Technology Innovation Program (10063427, development of eco-friendly smelting technology for the production of rare metal production for lowering manufacturing costs using solid oxide membrane) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea) and the Competency Development Program for Industry Specialists of the Korean Ministry of Trade, Industry and Energy (MOTIE), operated by Korea Institute for Advancement of Technology (KIAT). (No. P0002019, HRD Program for High Value-Added Metallic Material Expert). This work was also supported by the Basic Research Laboratory Program through the Ministry of Education of the Republic of Korea (2019R1A4A1026125).


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