Papers by Author: Il Dong Choi

Abstract: Bake hardening steels have to resist strain aging to prevent the yield strength increment and stretcher strain during press process and to enhance the bake hardenability during baking process after painting. The bake hardening steels need to control the solute carbon and the solute nitrogen to improve the bake hardenability. Ti and/or Nb alloying for nitride and carbide precipitation and low carbon content below 0.003% are used to solve strain aging and formability problem for automotive materials. However, in the present study, the effect of micro-precipitation of copper sulfide on the bake hardenability and fatigue properties of extremely low carbon steel has been investigated. The bake hardenability of Cu-alloyed bake hardening (Cu-BH) steel was slightly higher (5MPa) than that of Nb-alloyed bake hardening (Nb-BH) steel, but the fatigue limit of Cu-BH steel was far higher (45MPa) than that of Nb-BH steel. All samples showed the ductile fracture behavior and some samples revealed distinct fatigue stages, such as crack initiation, stable crack growth and unstable crack growth.

Abstract: Mg alloy is the lightest material of structural materials and is noticed for lightweight automotive parts because of excellent castability and damping capacity compared with Al alloy. But Mg alloy has poor corrosion resistance and high temperature creep properties. In this study, Mg matrix composites were fabricated by squeeze casting method to improve high temperature creep properties and damping capacity. The creep properties and damping capacity of hybrid Mg composites reinforced with Alborex(Aluminum borate whisker: 9(Al2O3)•2(B2O3)), graphite particle, and carbon fiber were improved in comparison with Mg alloy. Hybrid Mg composites reinforced with carbon fiber and Alborex were better than those reinforced with graphite particle and Alborex in mechanical properties, creep characteristics, and damping capacity, etc.

Abstract: The influence of the texture development in Ti-added (0.03, 0.05 and 0.07wt%) IF (Interstitial Free) steels on physical properties, especially ř -value was investigated. It was intended to determine the proper Ti content for the promotion of deep drawability as well as mechanical properties of IF steels. The precipitation behavior in the IF steels was clarified with the aid of computer simulation. The quantitative X-ray analysis on the texture development of IF steels was performed using pole figure and orientation distribution function measurement. For the IF steel with the composition of 0.025C, 0.070Mn, 0.002N and 0.007S, the optimum Ti content was found to be 0.05wt%.

Abstract: Compositionally graded Al-SiC_p composites were fabricated using pressureless infiltration process. Microstructure was examined and thermal properties were characterized for Al-SiC_p composites. Al-SiC_p composites with fairly uniform distribution and compositional gradient of SiC reinforcement in the Al matrix though the thickness direction was successfully fabricated. The thermal conductivity of Al-SiC_p composites was measured at room temperature, 200°C and 400°C using laser flash method. Thermal conductivity of Al-SiC_p composites increases non-linearly with decreasing the volume fraction of SiC. Cyclic thermal shock fatigue tests were performed by immersing Al-SiCp functionally graded materials(FGM) into water from the various heating temperatures of 400°C , 300°C and 200°C , repeatedly. After cyclic thermal shock fatigue tests, micro-hardness was measured and formation of cracks was investigated.e fati

Abstract: Mg alloys have potential to use automotive parts because of their weight and castability. High temperature strength and damping capacity is important to the automotive power train parts. Mg alloy has lower creep and thermal fatique strength but has better damping capacity than Al alloy. It is known that short fiber reinforced Mg metal matrix composites(MMC) exhibits superior high temperature strength and graphite reinforced Mg MMC shows excellent damping capacity. Therefore, in this study, the effect of graphite particles(15-25%) and alborex (9Al₂O₃ּ2B₂O₃) whiskers(5-15%) on the damping behavior and mechanical properties of Mg MMC was studied. Graphite particles and alborex whiskers were chosen to increase damping capacity and high temperature strength, respectively. The Mg MMC was fabricated by squeeze casting and the total quantity of reinforcements(graphite + alborex) was maintained to 30 volume percent. The damping capacity of the metal matrix composites was increased and the flexural strength and hardness were decreased with increasing the volume fraction of graphite particles, that is, reducing the volume fraction of alborex whiskers.

Abstract: Hybrid AZ91 Mg matrix composites reinforced with ceramic short fiber (Al₂O₃) and particle (Si or SiC_p) were fabricated by squeeze casting method. Microstructure of hybrid Mg matrix was examined and mechanical and fracture properties were characterized. It was found that squeeze casting is effective method for the fabrication of hybrid Mg matrix composites with fairly well distributed reinforcement and near defect-free solidification microstructure. Strength and wear resistance were enhanced with hybridization of reinforcements. To investigate fracture properties of hybrid Mg matrix composites, in-situ micro-fracture experiment was performed in SEM. Crack was initiated and propagated mainly by decohesion of interfaces between Mg matrix and reinforcements. Large ceramic particles tend to fracture and thus seem to be undesirable for mechanical and fracture behavior of hybrid Mg matrix composites.

Abstract: The scroll compressor with high cooling efficiency and lower oscillating noise can be a candidate material for a part of car air conditioner, household air conditioner and so on. When Al alloy with high strength is used for scroll compressor instead of cast iron, rotation speed can be increased by weight reduction. To design Al alloy which has enough mechanical properties to replace cast iron, the effect of alloying elements, such as Si(13-15%), Cu(4-5%), and Mg(0.5-1%) and cooling rates on the mechanical properties of squeeze cast Al alloy was studied. The influence of Sr and P modification treatment on Si morphology was also studied. Aluminum alloy over 400MPa of tensile strength and 2% elongation was developed. Al scroll samples was fabricated by squeeze casting machine(UBE 800ton) with using 2 cavity mold. Flow-3D which is a three dimensional melt flow analysis program was used to find most proper mold design and optimum process conditions for squeeze casting before making samples. Relationship between secondary dendrite arm spacing and cooling rate was simulated by Flow-3D and compared with experimental results.