• Journal of Powder Materials
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• v.29 no.3
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• pp.247-251
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• 2022

This study demonstrates the effect of the compaction pressure on the microstructure and properties of pressureless-sintered W bodies. W powders are synthesized by ultrasonic spray pyrolysis and hydrogen reduction using ammonium metatungstate hydrate as a precursor. Microstructural investigation reveals that a spherical powder in the form of agglomerated nanosized W particles is successfully synthesized. The W powder synthesized by ultrasonic spray pyrolysis exhibits a relative density of approximately 94% regardless of the compaction pressure, whereas the commercial powder exhibits a relative density of 64% under the same sintering conditions. This change in the relative density of the sintered compact can be explained by the difference in the sizes of the raw powder and the densities of the compacted green body. The grain size increases as the compaction pressure increases, and the sintered compact uniaxially pressed to 50 MPa and then isostatically pressed to 300 MPa exhibits a size of 0.71 m. The Vickers hardness of the sintered W exhibits a high value of 4.7 GPa, mainly due to grain refinement.

• Journal of Powder Materials
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• v.29 no.1
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• pp.34-40
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• 2022

Recently, high-entropy carbides have attracted considerable attention owing to their excellent physical and chemical properties such as high hardness, fracture toughness, and conductivity. However, as an emerging class of novel materials, the synthesis methods, performance, and applications of high-entropy carbides have ample scope for further development. In this study, equiatomic (Hf-Ti-Ta-Zr-Nb)C high-entropy carbide powders have been prepared by an ultrahigh-energy ball-milling (UHEBM) process with different milling times (1, 5, 15, 30, and 60 min). Further, their refinement behavior and high-entropy synthesis potential have been investigated. With an increase in the milling time, the particle size rapidly reduces (under sub-micrometer size) and homogeneous mixing of the prepared powder is observed. The distortions in the crystal lattice, which occur as a result of the refinement process and the multicomponent effect, are found to improve the sintering, thereby notably enhancing the formation of a single-phase solid solution (high-entropy). Herein, we present a procedure for the bulk synthesis of highly pure, dense, and uniform FCC single-phase (Fm3m crystal structure) (Hf-Ti-Ta-Zr-Nb)C high-entropy carbide using a milling time of 60 min and a sintering temperature of 1,600℃.

• Journal of Powder Materials
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• v.29 no.1
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• pp.14-21
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• 2022

In the powder bed fusion (PBF) process, a 3D shape is formed by the continuous stacking of very fine powder layers using computer-aided design (CAD) modeling data, following which laser irradiation can be used to fuse the layers forming the desired product. In this method, the main process parameters for manufacturing the desired 3D products are laser power, laser speed, powder form, powder size, laminated thickness, and laser diameter. Stainless steel (STS) 316L exhibits excellent strength at high temperatures, and is also corrosion resistant. Due to this, it is widely used in various additive manufacturing processes, and in the production of corrosion-resistant components with complicated shapes. In this study, rectangular specimens have been manufactured using STS 316L powder via the PBF process. Further, the effect of heat treatment at 800 ℃ on the microstructure and hardness has been investigated.

• Journal of Powder Materials
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• v.29 no.1
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• pp.22-27
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• 2022

Conventionally, metal materials are produced by subtractive manufacturing followed by melting. However, there has been an increasing interest in additive manufacturing, especially metal 3D printing technology, which is relatively inexpensive because of the absence of complicated processing steps. In this study, we focus on the effect of varying powder size on the synthesis quality, and suggest optimum process conditions for the preparation of AlCrFeNi high-entropy alloy powder. The SEM image of the as-fabricated specimens show countless, fine, as-synthesized powders. Furthermore, we have examined the phase and microstructure before and after 3D printing, and found that there are no noticeable changes in the phase or microstructure. However, it was determined that the larger the powder size, the better the Vickers hardness of the material. This study sheds light on the optimization of process conditions in the metal 3D printing field.

• Journal of Powder Materials
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• v.29 no.2
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• pp.110-117
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• 2022

The grain growth behavior in the (1-x)K_0.5Na_0.5NbO₃-xCaZrO₃ (KNNCZ-x) system is studied as a function of the amount of CZ and grain shape. The (1-x)K_0.5Na_0.5NbO₃-xCaZrO₃ (KNNCZ-x) powders are synthesized using a conventional solid-state reaction method. A single orthorhombic phase is observed at x = 0 - 0.03. However, rhombohedral and orthorhombic phases are observed at x = 0.05. The grain growth behavior changes from abnormal grain growth to the suppression of grain growth as the amount of CaZrO₃ (CZ) increases. With increasing CZ content, grains become more faceted, and the step-free energy increases. Therefore, the critical growth driving force increases. The grain size distribution broadens with increasing sintering time in KNNCZ-0.05. As a result, some large grains with a driving force larger than the critical driving force for growth exhibit abnormal grain growth behavior during sintering. Therefore, CZ changes the grain growth behavior and microstructure of KNN. Grain growth at the faceted interface of the KNNCZ system occurs via two-dimensional nucleation and growth.

• Journal of Powder Materials
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• v.29 no.2
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• pp.159-165
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• 2022

Rare earth elements, which are important components of motors, are in high demand and thus constantly get more expensive. This tendency is driven by the growth of the electric vehicle market, as well as environmental issues associated with rare-earth metal manufacturing. TC 298 of the ISO manages standardization in the areas of rare-earth recycling, measurement, and sustainability. Korea, a resource-poor country, is working on international standardization projects that focus on recycling and encouraging the domestic adoption of international standards. ITU-T has previously issued recommendations regarding the recycling of rare-earth metals from e-waste. ISO TC 298 expands on the previous recommendations and standards for promoting the recycling industry. Recycling-related rare earth standards and drafts covered by ISO TC 298, as well as Korea's strategies, are reviewed and discussed in this article.

• Journal of Powder Materials
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• v.29 no.2
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• pp.99-109
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• 2022

Powder flowability is critical in additive manufacturing processes, especially for laser powder bed fusion. Many powder features, such as powder size distribution, particle shape, surface roughness, and chemical composition, simultaneously affect the flow properties of a powder; however, the individual effect of each factor on powder flowability has not been comprehensively evaluated. In this study, the impact of particle shape (sphericity) on the rheological properties of Ti-6Al-4V powder is quantified using an FT4 powder rheometer. Dynamic image analysis is conducted on plasma-atomized (PA) and gas-atomized (GA) powders to evaluate their particle sphericity. PA and GA powders exhibit negligible differences in compressibility and permeability tests, but GA powder shows more cohesive behavior, especially in a dynamic state, because lower particle sphericity facilitates interaction between particles during the powder flow. These results provide guidelines for the manufacturing of advanced metal powders with excellent powder flowability for laser powder bed fusion.

• Journal of Powder Materials
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• v.29 no.5
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• pp.424-431
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• 2022

3Y-TZP ceramics obtained by doping 3 mol.% of Y₂O₃ to ZrO₂ to stabilize the phase transition are widely used in the engineering ceramic industry due to their excellent mechanical properties such as high strength, fracture toughness, and wear resistance. An additional increase in mechanical properties is possible by manufacturing a composite in which a high-hardness material such as oxide or carbide is added to the 3Y-TZP matrix. In this study, composite powder was prepared by dispersing a designated percentage of WC in the 3Y-TZP matrix, and the results were compared after manufacturing the composite using the different processes of spark plasma sintering and HP. The difference between the densification behavior and porosity with the process mechanism was investigated. The correlation between the process conditions and phase formation was examined based on the crystalline phase formation behavior. Changes to the microstructure according to the process conditions were compared using field-emission scanning electron microscopy. The toughness-strengthening mechanism of the composite with densification and phase formation was also investigated.

• Journal of Powder Materials
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• v.29 no.4
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• pp.320-324
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• 2022

Lightweight steel is a crucial material that is being actively studied because of increased carbon emissions, tightening regulations regarding fuel efficiency, and the emergence of UAM, all of which have been recently labeled as global issues. Hence, new strategies concerning the thickness and size reduction of steel are required. In this study, we manufacture lightweight steel of the Fe-Mn-Al-C system, which has been recently studied using the DED process. By using 2.8 wt.% low-Mn lightweight steel, we attempt to solve the challenge of joining steel parts with a large amount of Mn. Among the various process variables, the laser scan power is set at 600 and 800 W, and the laser scan speed is fixed at 16.67 mm/s before the experiments. Several pores and cracks are observed under both conditions, and negligibly small pores of approximately 0.5 ㎛ are observed.

• Journal of Powder Materials
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• v.29 no.4
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• pp.283-290
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• 2022

A Cu-15Ag-5P filler metal (BCuP-5) is fabricated on a Ag substrate using a high-velocity oxygen fuel (HVOF) thermal spray process, followed by post-heat treatment (300℃ for 1 h and 400℃ for 1 h) of the HVOF coating layers to control its microstructure and mechanical properties. Additionally, the microstructure and mechanical properties are evaluated according to the post-heat treatment conditions. The porosity of the heat-treated coating layers are significantly reduced to less than half those of the as-sprayed coating layer, and the pore shape changes to a spherical shape. The constituent phases of the coating layers are Cu, Ag, and Cu-Ag-Cu3P eutectic, which is identical to the initial powder feedstock. A more uniform microstructure is obtained as the heat-treatment temperature increases. The hardness of the coating layer is 154.6 Hv (as-sprayed), 161.2 Hv (300℃ for 1 h), and 167.0 Hv (400℃ for 1 h), which increases with increasing heat-treatment temperature, and is 2.35 times higher than that of the conventional cast alloy. As a result of the pull-out test, loss or separation of the coating layer rarely occurs in the heat-treated coating layer.