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Among half-Heusler alloys, MNiSn (M = Ti, Zr, Hf) compounds have been extensively investigated due to their unique crystal structure and promising thermoelectric properties. However, forming the MNiSn half-Heusler single phase directly during the solidification process is challenging, necessitating a prolonged annealing step for homogenization. Rapid Solidification Processing (RSP) was employed to synthesize materials with controlled microstructures and high thermoelectric performance within a short processing time. Optimized RSP processes were then used to produce ribbon-shaped samples, aiming to enhance thermoelectric performance through grain size reduction and the synthesis of half-Heusler alloys with minimal amounts of detrimental phases. In the alloy design step, Ti, Zr, and Hf were typically mixed at the M sites to reduce lattice thermal conductivity. To address cost and production efficiency concerns associated with incorporating Hf, alloy compositions with reduced Hf content were designed. A comparative analysis of the thermoelectric properties of Ti_0.5Zr_0.17Hf_0.33NiSn_(1-x)Sb_x (x = 0.02~0.08) was conducted by varying doping levels of Sb as the dopant element. The addition of Sb led to a gradual increase in electron concentration, resulting in a significant rise in electrical conductivity. However, this increase in electronic thermal conductivity had a detrimental effect on the overall dimensionless Fig. of merit ZT value. The maximum ZT value of 0.92 was achieved in Ti_0.5Zr_0.17Hf_0.33NiSn_0.98Sb_0.02 at 773K, demonstrating the potential of Hf-reduced half-Heusler alloys synthesized by RSP for thermoelectric applications.

keyword : Half-Heusler phase, Thermoelectric material, Rapid solidification

The irregular and highly rough surface of AlSi10Mg alloy parts fabricated by metal additive manufacturing technology can result in dimensional instability, poor surface quality and fluid turbulence. Therefore, post-treatments are needed to adjust the surface roughness and irregular surface morphology to satisfy industrial requirements. One of the most promising candidates is electropolishing, which dissolves the metal surface by electrochemical reactions. In this study, we investigated electropolishing processes to effectively reduce surface roughness while minimizing deformation of the shape and dimensions of additively manufactured AlSi10Mg parts. Sodium carbonate-trisodium phosphate (STP), perchloric acid-ethyl alcohol (PEA), choline chloride-ethylene glycol (CEG), and perchloric acid-ethylene glycol (PEG) solutions were employed. Then, electropolishing was conducted on AlSi10Mg specimens fabricated by Powder Bed Fusion (PBF). Changes in surface roughness and thickness with respect to the applied voltage and time were explored in the four types of electrolytes. The STP electrolyte was not suitable for electropolishing the AlSi10Mg alloy, while the other three types of electrolytes showed significant reduction in the surface roughness of the AlSi10Mg alloy. Surface roughness and the difference in roughness between up-skin and down-skin of additively manufactured AlSi10Mg were significantly reduced in the PEA electrolyte, but was accompanied by a serious reduction in thickness. In the CEG electrolyte, the dissolution rate during electropolishing was quite slow, taking more than 30 min. to achieve a surface roughness similar to PEA and PEG. However, the CEG solution had the advantage of minimizing dimensional change during electropolishing. The PEG solution was more chemically stable than PEA, since it contains less volatile ethylene glycol instead of highly volatile ethyl alcohol. In addition, the performance of the PEG solution was similar to that of the PEA solution for electropolishing additively manufactured AlSi10Mg alloy, so the PEG solution is considered more suitable for real applications for surface finishing.

keyword : Electropolishing, Metal additive manufacturing, Aluminum alloy, Surface roughness

In this study, a high-melting point element, flake type graphite (Gr.), was added to aluminum (Al) to improve the latter’s thermal properties. Al with different particle shapes, spherical, irregular and flake shape, was mixed with graphite. Al-50 vol.% Gr. powders were mixed by a shaking mixer and sintered under the following process conditions; a sintering temperature of 480℃ with a heating rate of 60℃/min, a sintering pressure of 60 MPa and a duration of 2 minutes at the 480℃. The relative density of the sintered body reached its highest value of 98.7% when the shape of the Al particle was irregular. The lowest value 92.8% was obtained with flaked-shaped Al particles. Pores were formed due to the wettability at the Al and Gr. interface, which varied with the shape of the Al powder, thereby affecting the relative density. The orientation of Gr. was measured as 86%, 94%, and 92% for Al with spherical, flake, and irregular powder shapes, respectively. The orientation significantly influenced the thermal conductivity, which was measured as 342.5, 373.8, and 392.2 W/m·K for Al with spherical, flake, and irregular powder shapes, respectively. The thermal expansion was measured as 19.1, 18,8, and 18.6 × 10^-6/K. The mechanical properties of the sintered bodies were studied to evaluate how the different Al shapes affect their characteristics based on the plane, considering the anisotropy of graphite.

keyword : Al-Gr, Composite, Particle shape, Spark plasma sintering method, Sintering behavior, Thermal properties

This study investigates the application of a unidirectional pulsating traveling magnetic field (UPTMF) for the semi-continuous casting of high-strength aluminum alloy 7075 slabs (350×150 mm). The experimental setup involved a comparative analysis of direct chill (DC) casting and electromagnetic stirring (EMS) with a 5 Hz amplitude modulated U-PTMF and 22.5 Hz carrier frequency, focusing on microstructural evolution, chemical composition homogeneity, and defect formation. Microstructural analysis demonstrated that slabs cast under conventional DC conditions exhibited columnar dendritic structures with a grain size heterogeneity ranging from 90 to 125 μm in the center and up to 160 μm near the surface. In contrast, the EMS-treated slabs showed a more uniform and refined microstructure with equiaxed grains of 80~90 μm in the center and 90~125 μm at the surface. The electromagnetic stirring facilitated grain refinement by inducing forced convection, the fragmentation of dendritic arms, and mechanical electromagnetic vibration effects. Chemical composition measurements of the DC slabs indicated severe macro segregation, with relative deviations (RD%) of Zn (5.27%~44%), Mg (7.2%~33.6%), and Cu (18%~51%). Conversely, the EMS-treated slabs displayed significantly reduced segregation, with deviations limited to 0.18%~2.7% for Zn, 0.4%~2.8% for Mg, and 0.67%~8.7% for Cu. These improvements resulted from enhanced melt mixing and homogenization induced by the traveling magnetic field. Additionally, the DC-cast slabs developed internal cracks due to post-casting thermal stress accumulation, while the EMS-treated slabs exhibited no visible defects, suggesting that the U-PTMF method effectively mitigates stress formation. The findings confirm that pulsed electromagnetic processing enhances microstructural uniformity and chemical homogeneity while preventing casting defects, offering a viable approach for improving aluminum alloy slab quality for aerospace and automotive applications.

keyword : Electromagnetic stirring, Semi-continuous casting, High-strength aluminum alloy, Microstructure, Segregation

With the recent introduction of various environmental policies focused on carbon neutrality has made recycling a crucial task in the electric vehicle industry. In particular, the recycling of lithium-ion batteries, a critical component of EVs, is becoming increasingly important. The process of recovering valuable metals such as lithium (Li), cobalt (Co), manganese (Mn), and nickel (Ni) from spent batteries typically involves leaching, precipitation, solvent extraction, and crystallization. However, these processes generate a significant amount of Na₂SO₄ wastewater. To address this issue, bipolar membrane electrodialysis (BMED) has been developed to recover H₂SO₄ and NaOH from Na₂SO₄. The conventional method uses a three-compartment configuration to simultaneously recover acid and base components, This approach, however has drawbacks such as high initial installation costs due to the expensive membranes and low current efficiency. To overcome these limitations, a two-compartment constant voltage method using cation exchange membranes and bipolar membranes was employed in this study. The effects of various process parameters on the recovery rate of NaOH, current efficiency, flux, and energy consumption were evaluated. Based on the optimal conditions from the experiment, a flow rate of 950 mL/min, a feed solution of 1.30 M Na₂SO₄ at 1.5 L, and an initial base solution of 0.1 M NaOH at 0.50 L resulted in an base recovery rate of 67.59%, with 0.91 L of 2.96 M NaOH recovered.

keyword : Bipolar membrane electrodialysis, BMED, Constant voltage, Two-compartment, NaOH, Ion flux

In hydrogen refueling stations, polymer O-rings are used for high-pressure hydrogen containers and transfer pipes to prevent gas leakage. A technology has been developed to measure the hydrogen uptake and diffusivity of the polymer materials used in these O-rings. The process involves charging the polymer material with hydrogen under a maximum pressure of 90 MPa, then depressurizing it. Afterward, the polymer is placed in a cylinder partially submerged in water. The pressure difference causes hydrogen gas to be released from the polymer material, resulting in a decrease in the water level in the cylinder, which corresponds to the increase in the volume of released hydrogen gas. To measure this volume change in real-time, a brightness analysis algorithm was developed to track the water level, which takes the form of a crescent shape inside the cylinder. This allows for precise tracking of the water level change. Using the real-time volume change data, the number of moles of released hydrogen gas is calculated based on the ideal gas law. This technology enables the evaluation of hydrogen uptake and diffusivity of polymer materials using a self-developed diffusivity analysis program. This technology was applied to evaluate hydrogen uptake and diffusivity in sulfur-crosslinked EPDM composites containing carbon black and silica fillers. The relationship between filler types and the hydrogen uptake/diffusivity of the EPDM composites was measured across a pressure range of 2 MPa to 90 MPa. The effects of fillers and pressure on hydrogen uptake and diffusivity were studied. Additionally, the correlation between the physical stability of the EPDM composites and their hydrogen uptake/diffusivity was investigated. A positive proportional relationship was found between the volume expansion of the EPDM composite and the hydrogen uptake/diffusivity, while a negative proportional relationship was observed between the polymer's density and its hydrogen uptake/diffusivity.Finally, the effectiveness of this technology was validated through uncertainty analysis of the charging amount and diffusion coefficient measurements, with all results falling within an uncertainty of 10%.

keyword : Image analysis algorithm, Polymer, Hydrogen uptake, Hydrogen diffusion, Uncertainty, Volumetric measurement

In this study, to investigate the evolution of microstructure and texture, as well as their effect on mechanical properties of AZMX1100 magnesium alloy, AZMX1100 plates were manufactured using the TRC (Twin roll casting) method, which is one of the continuous casting methods With TRC it is possible to reduce the segregation of magnesium alloys, improve the size and distribution of inclusions, and refine grain through rapid cooling. In the TRC-processed HR(0PASS) plate, coarse grains with an average size of 131.3 ± 80.68 μm were observed. Through hot rolling, the grain size was not only significantly refined to 8.58 ± 4.89 μm, primarily due to dynamic recrystallization (DRX) which not only reduced the average grain size, but also weakened the texture intensity. After either homogenization or aging heat treatment followed by hot rolling process, the grain size increased by over twofold. This occurred due to the combined effects of recrystallized grain growth and the high stored deformation energy during rolling. Although the mechanical strength improved progressively with hot rolling due to work hardening and grain refinement in the HR(8PASS) specimen, homogenization heat treatment led to grain growth, resulting in the lowest Vickers hardness and tensile strength in the HR(8PASS)-N specimen. The specimen HR(8PASS)-T6, processed using T6 heat treatment, demonstrated superior mechanical strength with the ultimate tensile strength of 268.3 MPa ± 3.7 and the yield strength of 184.3 MPa ± 2.2. This enhancement in ductility was attributed to the broad dispersion of texture along the transverse direction (TD) and rolling direction (RD) after heat treatment, which effectively reduced anisotropy during tensile deformation.

keyword : Magnesium alloy, Hot rolling, Heat treatment, Microstructure, Mechanical properties