Jee-Hyun Kang

In the quest to produce high-purity alumina, bottom-up engineering via architecting the interior of ceramic with bimodal structures of alumina powders in the absence of any additives has gained considerable attention owing to the simplicity offered. The present work investigated the influence of bimodal structures containing micron (~35 μm) and submicron (~600 nm) Al₂O₃ powders on the formation of dense Al₂O₃ ceramic. To this end, ball-milling was conducted to prepare the desired sizes of powders, followed by two-step sintering in a vacuum at 1450 °C and 1650 °C with 6 h and 4 h holding times, consecutively. The bimodal structures induced the formation of Al₂O₃ ceramic with nearly full densification (>99%; ρ 3.95 g/cm³). Both the coarse and fine-grained moieties synergistically balanced the densification kinetics whilst suppressing abnormal grain growth. The uniform and homogeneous grain size minimized the plasma porosity down to <6.0%, limiting the penetration of plasma during the etching process.

In industrial applications, rolling is commonly performed with lubrication to prevent undesirable modification of the sheet. Although it is well established that lubrication influences the microstructure and texture of deformed sheets through its effect on shear deformation, the underlying mechanisms remain insufficiently understood. In this study, we investigated how lubrication affects slip system activity during asymmetrical rolling, using viscoplastic modeling of BCC ferritic steel. Two conditions—lubricated and non-lubricated samples—were examined under asymmetrical rolling. Slip system activity was inferred from the rotation axes between pairs of orientations separated by low-angle grain boundaries, based on the assumption that such boundaries represent the simplest form of orientation change. A Viscoplastic Self-Consistent (VPSC) model employing an affine linearization scheme was used. This proved sufficient for evaluating slip system activity in BCC polycrystalline metals undergoing early-stage plastic deformation involving either plane strain or combined plane strain and shear. The results demonstrated that lubrication had a limiting effect by reducing the penetration of shear deformation through the thickness of the sample. Understanding this effect could enable the optimization of lubrication strategies—not only to minimize defects such as bending, but also to achieve microstructural characteristics favorable for industrial applications.

Near-net-shaped metal products manufactured by high-pressure diecasting (HPD) encountered more or less critical failure during operation, owing to the development of micro-defects and structural inhomogeneity attributed to the complexity of geometrical die design. Because the associated work primarily relies on technical experience, it is necessary to perform the structural analysis of the HPDed component in comparison with simulation-based findings that forecast flow behavior, hence reducing trial and error for optimization. This study validated the fluidity and solidification behaviors of a commercial-grade Al-Si-Cu-Mg alloy (ALDC12) that is widely used in electric vehicle housing parts using the ProCAST tool. Both experimental and simulation results exhibited that defects at the interface of a compact mold filling were barely detected. However, internal micro-pores were seen in the bolt region, resulting in a 17.27% drop in micro-hardness compared to other parts, for which the average values from distinguished observation areas were 111.24 HV, 92.03 HV, and 103.87 HV. The simulation aligns with structural observations on defect formation due to insufficient fluidity in local geometry. However, it may underestimate the cooling rate under isothermal conditions. Thus, the simulation used in this work provides reliable predictions for optimizing HPD processing of the present alloy.

This study explores the combined potential of severe plastic deformation (SPD) via differential speed rolling (DSR) and plasma electrolytic oxidation (PEO) to enhance the material performance of 6061 Al alloys. To this end, DSR was carried out at a roll-speed-ratio of 1:4 to obtain ~75% total thickness reduction and a final microstructure of <1 µm. The rest of the samples were annealed to obtain various grain sizes of ~1, ~25, and ~55 μm. Through DSR, the hardness of the material increased from ~64 to ~102 HV. Different grain sizes altered the plasma behavior which further influence the growth of the coating layer, where the fine grain size produced a compact structure beneficial for corrosion protection. This synergy offers tailored materials ideal for high-performance applications across diverse industries, combining enhanced bulk properties from DSR with optimized surface attributes from PEO.