Papers by Author: Il Ho Kim

Abstract: Using commercial Ti powders and recycled TiH2 powders made from titanium turning chips, the effects of hydrogen on microstructures and oxygen reduction were investigated. Each powder was hot-pressed at 750oC and 1100oC, which are below and above β-transition temperature (880oC), respectively. The TiH2 and Ti samples of sintered at 1100oC showed equiaxed α phase. However, TiH2 sample sintered at 750oC consist of primary α phase, secondary α phase and titanium hydride (TiH2), though Ti sample represented single α phase. The formation of complex microstructure in TiH2 sample sintered at 750oC is due to hydrogen remained after sintering. The oxygen contents after sintering in TiH2 samples were relatively lower than Ti samples, although those before sintering were higher than Ti. This result can be explained that hydrogen gas generated from dehydrogenation of TiH2 leads to the reduction of oxygen contents and protection of re-oxidation during sintering.

Abstract: Some nano grained Al materials were produced by mechanical milling/alloying followed by vacuum hot pressing: nano grained pure Al, Al-1.5Mg and Al-0.7Mg-1.0Cu alloys in wt%. The nano bulk materials had average grain sizes of 90-150 nm and ball-on-disk wear equipment was used to investigate the effects of grain size comparing to coarse grained pure Al and T6-treated Al 6061 alloy. In comparison of coarse and nano grained pure Al materials, nano grained specimens showed much higher wear resistance and size of wear debris was very fine as much as 100 nm at applied load of 100g. Wear in nano grained materials proceeded by micro fracturing mechanism like abrasion in low applied and sliding velocity and the mechanism resulted in high wear resistance. Nano grained Al-1.5Mg and Al-0.7Mg-1.0Cu alloys showed much superior wear characteristics due to nano grains and high hardness. As a result, size of grains was a predominant factor for high resistance at low applied load and/or sliding velocity, while hardness at higher applied load or velocity.

Abstract: The effects of use of open cell Al foam and thermoelectric element in a hydrogen storage system were investigated. Extremely different heat conductivities were observed in two storage systems with or without open cell Al foam. By applying the open cell Al in this hydrogen storage system, the reaction sensitivity of the temperature and equilibrium pressure was far rapidly increased than that without the open cell Al foam. During increasing the temperature by heating element, heating rate of hydride powders was very fast in the storage system including Al foam, while temperature of powders was almost not changed in system without foam. Also, in case of using thermoelectric element, heating and cooling rate was very sensitive in the system with Al foam and heating-cooling cyclic behavior within the system controlled by thermoelectric element is seemed to be satisfied for some applications in industry.

Abstract: The open cell Al (or Al foam) was employed in the alloy hydride system due to the rapid heat conductivity of Al. The hydrogen storage alloy powders were packed in this storage system made of Al open cell, and the changes of temperature and equilibrium pressure of hydrogen desorbed were measured. By applying the open cell Al in this hydrogen storage system, the reaction sensitivity of the temperature and equilibrium pressure was far rapidly increased than that without the open cell Al.

Abstract: Manufacturing technology of bulk titanium by milling and sintering was studied for recycling of pure titanium tuning chip. Ti chips were milled at RT under H2 pressure of 0.5MPa. All titanium chips were pulverized and hydrided within several minutes, which is very short time comparing to other thermal processes. The hydrided powders were hot pressed at 750°C. After hot pressing, bulk sample was identified to α-phase Ti by XRD measurement and measured density(4.509g/cm3) was very close to theoretical density(4.512g/cm3) of pure Ti. Consequently, it is carefully suggested that hydriding and sintering process is efficient and practicable solution for recycling of pure titanium tuning chip.

Abstract: The microstructure and bonding strength of the wide-gap region brazed with different filler metal powder (BNi-3 and DF 4B) and various powder mixing ratios of additive powder to filler metal powder were investigated. The microstructure characterization showed that Cr borides with a blocky morphology were existed in the brazed region in both filler metal powder. The bonding strength of the wide-gap region brazed with 60 wt.% IN738 additive and 40 wt.% DF 4B powder exhibited 92% tensile strength of IN738 superalloy at room temperature. The Cracks in the wide-gap brazed region initiated at the intermetallic compound and eutectic structure, and then propagated through them.

Abstract: In order to enhance the corrosion and wear resistance of the STVAX alloy, the CrN film has been deposited by UBM(unbalanced magnetron) sputtering. In this study, the influence of process parameters, such as N2 gas concentration and Cr interlayer insertion, on the adhesive strength was investigated. By increasing the concentration of N2 gas from 30% to 33%, the microstructure of the coating layer was changed from hexagonal Cr2N to cubic CrN. By increasing N2 gas concentration to 38%, a CrN coating layer with a preferred orientation of <111> plane was formed. Nanoindentation testing showed that the mechanical properties of coatings varied according to the coating type and crystallographic orientation. Superior adhesive strength was obtained at the of N2 gas concentration of 38%, and higher hardness at 33% concentration. The adhesive strength of the coating film was higher with a Cr interlayer than that without. These findings showed that the Cr interlayer between the CrN coating and the STAVAX substrate significantly improved the adhesive strength of coating to the substrate.

Abstract: The phases formed in the wide-gap region brazed with BNi-3 filler metal powder in IN738 superalloy were investigated by electron backscatter diffraction (EBSD). The morphology and chemical composition of the phase were investigated by field emission scanning electron microscopy (FESEM) and Auger electron spectroscopy (AES). The wide-gap region brazed with BNi-3 filler metal had a microstructure consisting of primary Ni3B, binary eutectic of Ni3B-Ni solid solution and ternary eutectic of Ni3B-Ni-Ni3Si structure. EBSD pattern analysis revealed that the Ni3B had orthorhombic structure with lattice parameter of a=0.439, b=0.522 and c=0.662 nm, and the Ni3Si phase had cubic structure with lattice parameter of a=0.350 nm.

Abstract: The precipitation behaviors of γ″(Ni3Nb) in four Ni-base alloys were investigated. The four alloys were forged Ni20Cr20Fe5Nb alloy, mechanically alloyed Ni20Cr20Fe5Nb alloy, IN 718 alloy and ECAPed(equal channel angular pressing) IN 718 alloy. Aging treatment was employed at either 600°C or 720°C for 20 hrs. The TEM observation and hardness test were performed to identify the formation of γ″. The precipitation of γ″ was noticed after aging at 600°C for 20 hrs in the mechanically alloyed Ni20Cr20Fe5Nb alloy and ECAPed IN 718 alloy, while it was observed after aging at 720°C for 20 hrs in the forged Ni20Cr20Fe5Nb alloy and IN 718 alloy before ECAP. It seemed that the lower aging temperature for γ″ precipitation in the mechanically alloyed Ni20Cr20Fe5Nb alloy and ECAPed IN 718 alloy than in the forged Ni20Cr20Fe5Nb alloy and IN 718 alloy before ECAP appeared to be due to the severe plastic deformation which occurred during mechanical alloying or ECAP.

Abstract: The magnetic coercivity of martensitic 12Cr steel was measured in order to evaluate its degree of isothermal aging. As the aging time increased, the lath width increased and the dislocations were recovered. Aging resulted in the coarsening of the as-tempered carbides (M23C6 and MX) followed by additional precipitation of Fe2W. The magnetic coercivity rapidly decreased during the initial aging period of about 1,000 hours and then decreased slightly thereafter. The decrease in the coercivity with increasing aging time was related to the decrease in the number of pinning sites, those associated with the dislocations, fine precipitates and coarsening of the martensite lath/subgrain.