Papers by Author: Hyung Ho Jo

Abstract: The rotor is a key determinant of the performance of a compressor and many attempts have been made to improve the efficiency of compressors by optimising rotor design. Rotors are usually made of several layers of steel sheets with thin cavities through the steel sheets, and aluminium alloys are used to fill the cavities by high pressure die casting process, and so bind the steel sheets together. Because of their high fluidity and good damping ability, magnesium alloys can be a good alternative for a high efficiency rotor. In this study, magnesium alloys were used for manufacturing rotors by high pressure die casting process using pin-point gate mold. By adopting a pin-point gate system, additional machining was eliminated and casting defects were reduced due to good castability of magnesium alloys.

Abstract: Energy requirement and yield for production of aluminum wirerods manufactured by continuous casting process were investigated to enhance yield of aluminum wiredrods. Also, it was compared with conventional process including extrusion process. The enhancement of yield more than 30% could be accomplished through the continuous casting process. The conventional process including extrusion process for production of wirerods consumes more energy compared to continuous casting. In addition, a number of intermediate annealing may be introduced during wiredrawing process of wirerods manufactured by conventional process including extrusion process. It can be mentioned that continuous casting process is suitable process for wiredrawing process because it can save energy requirement not only during manufacturing process of wirerods but also during following wiredrawing process.

Abstract: The anisotropy in mechanical strength and electrical conductivity in Cu-Ag alloy during cold rolling were investigated. The anisotropy in tensile strength and electrical conductivity was significant with increasing Ag content in Cu-Ag alloy. The cold rolled Cu-Ag alloys with higher Ag content have filamentary structure, which is composed of elongated eutectic phase and Cu matrix. In addition, the eutectic phase in Cu-Ag alloys with higher Ag content has a strong α-fiber texture. It can be mentioned that the eutectic phase in filamentary structure is promotes the anisotropy of electrical conductivity of Cu-Ag alloys. This strong a-fiber texture of eutectic phase was decreased by annealing process (300 OC for 1h), and the anisotropy in strength and conductivity was decreased.

Abstract: The influence of extrusion temperature and Ti content was investigated by observing the microstructure and determining the mechanical strength of A3003 alloy tube. The A3003 alloy was modified by inoculation in terms of addition of Ti and then extruded at the various extrusion temperatures. In case of A3003 alloy tube without addition of Ti, the yield strength was slightly improved with decreasing extrusion temperature. On the other hand, the yield strength was improved markedly in A3003 alloy tube with small addition of Ti, and also ductility was slightly reduced. Grain size distribution from observing the microstructure was different with addition of Ti. Coarse grains were formed on the outer and inner parts of the alloy tube without addition of Ti, whereas the finer grains were uniformly distributed in the alloy tube with addition of Ti. Thus the microstructure and mechanical strength of A3003 alloy tube could be controlled by addition of Ti. Threefore, optimum Ti content and extrusion temperature to fabricate high yield strength and ductility A3003 alloy tube (σy=60 MPa, ε=30%) for eco-friendly refrigerant application are above 0.05wt.Ti and below 480 °C, respectively. Introduction

Abstract: The influence of annealing atmosphere on mechanical and wear properties of free-cutting phosphor bronze alloy was investigated. After annealing in vacuum, the mechanical properties of the alloy decreases due to vaporization of alloying element. In addition, the softening of matrix induces decreasing the wear resistance dramatically. In contrast, high mechanical properties were observed in the alloy annealed in air and in nitrogen atmosphere. In particular, a large smearing of lead in the alloy annealed in air atmosphere was observed and was affected on the enhancement of wear resistance. Consequently, annealing in vacuum atmosphere is not suitable for high mechanical properties and wear resistance.

Abstract: The strength of aluminum alloys was improved by a severe deformation process while the decrease of elongation leads to the defect. Modification of A3003 alloy was attempted in order to develop a high strength and high formability Al alloy tube for the application of air-conditioning systems in transportation, such as passenger planes and automobiles. An A3003 alloy was modified by Ti addition, grain refiner. As a result of modification, the modified A3003 alloy was able to suitably satisfy the mechanical strength requirements, including elongation. Grain size and secondary dendrite arm spacing of A3003 alloy decreased by as much as 50 % by Ti addition. The finer grains were uniformly distributed in the modified A3003 alloy billet. The yield strength of the modified A3003 alloy was improved by 30 % without decreasing elongation.

Abstract: The proper control of total impurities and oxygen contents of oxygen-free high conductivity (OFHC) copper prepared by vacuum high-frequency melting technique was studied using Mahalanobis-Distance (MD) outlier detection method as functions of raw material purities, vacuum pressure, melting temperature and holding time. The properties of vacuum-melted OFHC copper was examined by thermo-gravimetric analysis, differential scanning calorimetry, hardness test, macro and optical microstructure analyses and ultimate tensile test. In multivariate systems, the existence of outlier makes it difficult to analyze the system and oultier detection belongs to the most important tasks in experimental data analysis. Mahalanobis Distance is most commonly used as a diagnosis of existance of outlier in multivariate system. The relationship between experiment conditions and total impurities and oxygen contents can be defined with the regression analysis results. At this research, our desirable manufacturing conditions is to obtain the total impurities under 40 ppm and oxygen contents under 5 ppm. After this statistical approach, the suggested minimum maufacturing conditions are the purity of raw material was 4N, vacuum pressure was 10-1 torr, melting temperature was 1150°C and melt holding time was 20 minutes.

Abstract: Thixoextrusion, one of the thixoforming processes, has advantages of high productivity, reduction of the extrusion pressure and cost saving because of the low energy consumption compared with the conventional extrusion processes. For the thixoextrusion, the low liquid fraction (fL<0.3) should be achieved and also the liquid fraction and average grain size should be uniform according to the reheating profile at the desired low liquid fraction. The main emphasis of this study is to investigate the feasibility of thixoextrusion for 2024 Al wrought alloy without additional pretreatment. The results show that the liquid fraction and average grain size were almost uniform with respect to isothermal holding temperature and time. It was very useful for thixoextrusion in terms of process control such as billet temperature control and actual extrusion time.

Abstract: The present study discussed extrudability improvement for 7000 series Al wrought alloys by thixoextrusion, with emphasis on controlling thixoextrusion parameters, such as initial ram speed, die bearing length and extrusion temperature of billet in semisolid state. The results of thixoextrusion experiments about microstructures and extrusion pressures were compared with conventional hot extrusion results. The maximum extrusion pressure for thixoextrusion was greatly decreased than that of conventional hot extrusion. This will contribute to extrudability in terms of extrusion pressure, which in turn means that shorter process time is required and smaller extrusion machine can be applied for the same operation. It is also possible to make complex shape components of 7000 series Al wrought alloys, which is not possible by conventional hot extrusion processes. The elongated grains to extrusion direction were generally observed during conventional hot extrusion, while the thixoextruded microstructures were isotropic.

Abstract: Free-cutting phosphor bronze alloy (ASTM C54400) was used as a material for automotive parts and industrial valve parts for its high strength, machinability and excellent corrosion resistance. To obtain desired mechanical strength of the alloy, the process parameters including continuous casting speed, reduction in area and annealing temperature have to be controlled. In this study, we investigated the effect of annealing on the mechanical property and machinability of free-cutting phosphor bronze alloy fabricated by continuous casting with different casting speeds of 80, 100, and 120 m/min, respectively. After annealing, the tensile strength of the alloy decreased with increasing annealing time owing to the occurrence of recrystallization and grain growth. In the cast and swaged alloy produced at casting speed of 120 m/min, the grain growth occurred rapidly. In contrast, in the alloy produced at the casting speed of 80 m/min, the grains growth rate was low. Furthermore, the effect of Pb particle in the alloy on machinability was also investigated by using a tool dynamometer installed force sensor.