Chul Kyu Jin

Submarine mast covers are critical cantilever appendages subjected to extreme hydrostatic pressure during submerged transit. To ensure structural reliability and control internal defects inherent in heavy-section casting, an integrated design-to-manufacturing framework sequentially linking structural finite element analysis (FEA), fluid volume method (FVM)-based casting simulation, and full-scale experimental sand casting was established. FEA under a 600 m submergence depth (7.0 MPa hydrostatic pressure) identified stress concentrations on the inner surface along the major-axis section. Introducing an optimized fillet radius at a sharp step reduced peak equivalent stress from 216.0 MPa to below 194.0 MPa, securing a safety factor exceeding 2.0 against the yield strength of nickel–aluminum–bronze (390 MPa). MAGMA5 casting simulations verified an unpressurized bottom-gating system (S:R:G = 1.00:2.88:4.80), achieving smooth laminar filling with gate velocities under 1.25 m/s without cold shuts. To mitigate predicted shrinkage porosity, process modifications enlarging riser diameters from Ø30 mm to Ø60 mm and placing chills were implemented in the actual casting trial, while top porosity was removed via machining allowances. Specimens harvested from the full-scale prototype yielded 757.8 MPa UTS, 392.5 MPa yield strength, 17.9% elongation, and 197 HB hardness.

SUS 316L bipolar plates (BPPs) with rectangular micro‐channels for polymer electrolyte membrane fuel cell (PEMFC) were fabricated using powder bed fusion (PBF) technology, and the effects of flow‐field structure parameters (channel width, rib width, and channel depth) on fuel cell performance were analyzed. While PBF effectively realized complex micro‐geometries, surface roughness and minor deviations such as rib fillets and pores were observed. Fuel cell performance evaluation results revealed a clear positive correlation where reducing rib width expanded the plane area, increasing current density by approximately one‐third of the area expansion ratio. Notably, although the plane area of the BPP with 400 µm channel and 200 µm rib widths was 33.2% larger (1508 vs. 1132 cm ⁻² ) than the reference BPP (500/500 µm), the resulting current density was disproportionately higher at 48.2% (1.132 vs. 0.764 A cm ⁻² ). Regarding channel depth, optimal performance occurred at 600 µm, with a decline at 700 µm. These findings demonstrate that while channel dimensions are independent variables, their combined impact is effectively represented by the plane area. Consequently, prioritizing plane area expansion is suggested as a significant design consideration for enhancing the performance of micro‐channel BPPs.

A large-sized backward-curved fan with high shape difficulty was designed, and fan performance was roughly predicted from computational fluid dynamics. Three gating systems of aluminum sand casting were designed to fabricate the fan. The flow pattern and solidification process of molten metal were analyzed by casting simulation. Three types were applied: bottom-up with four gates, bottom-up with ten gates, and top-down with a feeder. The simulation results of the bottom-up with four gates show that a large temperature loss occurs while molten metal flows into thin blades, and there is a temperature range below the liquidus temperature. Due to nonuniform temperature distribution, the solidification pattern is also not uniform. The bottom-up with ten gates shows almost similar flow and solidification patterns but has the effect of slightly reducing the temperature loss of molten metal. The top-down type has a much smaller temperature loss, while molten metal flows into the mold cavity compared to the bottom-up type and has a directional solidification pattern. As the feeder also acts as a riser to compensate for the shrinkage of the thick part, the simulation results regarding porosities are also significantly reduced. The fan cast as a top-down type has soundness without any unfilled parts.

Finite element (FE) analysis of the tensile test of TWB-HPF 22MnB5 steel was performed and compared with the experimental results. To improve the accuracy of the simulation, the damage theory of FLD and ductile damage theory were used in 2D and 3D simulations. The tensile strength of 22MnB5 steel was determined under various welding heat inputs for FE simulation. Crack propagation of the welded region indicated that the fracture was observed in the base metal under normal welding conditions. Also, the crack propagated along the HAZ region due to higher heat input of the welding, and lead fractures have been highlighted as a potential complication.

Using a tailor-welded blank (TWB) and hot-press forming (HPF), a 22MnB5 blank was surface-treated under four conditions. The penetration rates of the Fe_xAl_y compounds under the four surface-treatment conditions were investigated, and the hardness values were measured. A finite element (FE) simulation was performed for the characteristics of the heat-affected zone (HAZ), using the hardness value and results of previous researchers. In particular, the mechanical property settings of the mesh were designed to realize the conditions for the Fe_xAl_y compounds in the HAZ. Fine meshing was performed by partitioning the HAZ sections. For the mechanical properties of the HAZ with the Fe_xAl_y compounds, the strength was predicted from the hardness value, and the elongation values investigated by other researchers were used. The forming limit diagram, which was proportional to the elongation, was predicted. Specific elements were defined as the areas with Fe_xAl_y compounds, which played the same role as impurities. Tensile TWB–HPF specimens with different HAZ characteristics under four surface-treatment conditions were fabricated. Experiments and FE simulations were performed and compared. Details are as follows: For loads, a minimum error rate of 3% and a maximum error rate of 6% were obtained. For displacement, a minimum error rate of 9% and a maximum error of 25% were obtained. The feasibility of the simulation was verified by comparing the simulation and experimental results. A match of more than 75% was obtained.

Bottom-up gated filling system was performed to produce worm wheel through a sand casting of zinc alloy. It was a symmetrical design in which two worm wheels were cast at once, the worm wheel was erected in the direction of gravity and had a gating system in which molten zinc alloy were poured from below. The recovery rate of the designed mold cavity was 60%. Casting analysis was performed to check that the worm wheel was sequentially filled without turbulent flow as the molten zinc alloy passed through the gate. Though the speed of the molten zinc alloy initially poured was unstable, it became stabilized while maintaining a constant speed due to the continuous inflow. The results of directional solidification were confirmed. An integrated mold cavity pattern was fabricated by 3D printer, and sand mold were manufactured accordingly. A sound worm wheel without defects could be fabricated through sand casting.

The thixo process has the benefit of producing a semi-solid material in which fine primary α-Al grains are uniformly distributed. However, it also has the disadvantage of a costly raw material billet. In this study, a semi-solid slurry was prepared from the rheo process by cooling and electromagnetic stirring, and then a billet for the thixo process was manufactured through an upsetting process with semi-solid slurry. Then, the billet was reheated (thixo process) to make a semi-solid state, and then a final sample was manufactured through a forming process. In both A356 and A6061 materials, the equivalent diameter of the primary α-Al grains became smaller and the roundness became close to one throughout rheo, upsetting, thixo, and forming processes. Due to the refinement and spheroidizing effect of the primary α-Al grains, the tensile strength was improved by each process, and the elongation was slightly decreased. However, after T6 heat treatment, the tensile strength of A356 was decreased, but the elongation was greatly improved. In the case of A6061, on the other hand, the tensile strength was significantly improved, and its elongation decreased after T6 heat treatment.

Various techniques can be applied to center pillars to enhance collision characteristics during side crashes. For instance, patchwork (PW) can be welded to the center pillar to increase its stiffness, and partial softening (PS) can be applied to provide ductility. Side crash tests are conducted by the Insurance Institute for Highway Safety (IIHS) to evaluate collision resistance. However, it is difficult to evaluate collision toughness and energy distribution flow for each automobile component. In this study, a side crash simulation was performed with IIHS instruction. We investigated the effect of hot press forming (HPF) a center pillar with a combination of PW and PS techniques on collision toughness and energy distribution flow. As a result, the role of PW and PS techniques were verified during side crashes. PW improved the strain energy and intrusion displacement by 10% and 7.5%, respectively, and PS improved the plastic deformation energy and intrusion displacement by 10%. When PW and PS were applied to the HPF center pillar simultaneously, a synergistic effect was achieved.

For the high performance of a fuel cell where a bipolar plate (BP) is applied, rectangular channel, microchannel width, micro-rib, enough channel quantity, adequate channel depth, and innovative flow field design should be realized from a configuration standpoint. In this study, a stainless-steel BP with a microchannel flow field is fabricated with a powder bed fusion (PBF) 3D printer to improve fuel cell performance. A BP with a triple serpentine flow field, rectangular channel, 300 μm channel width, 300 μm rib, and 500 μm channel depth is designed. The print is completed perfectly until the flow field. The bending phenomenon due to thermal deformation does not occur in the BP fabricated by designing the thickness at 2 mm. Performance tests are conducted using fabricated stainless-steel BPs. The current density value is 1.2052 A/cm² at 0.6 V. This value is higher by 52.8% than the BP with 940 μm channels (rectangle, 940 μm ribs, and 500 μm channel depth). In addition, the value is higher by 24.9% than a graphite BP with 940 μm channels (rectangle, 940 μm ribs, and 1000 μm channel depth). The current density values are measured at 0.6 V for 260 h.