Papers by Author: Jeong Il Youn

Abstract: A356 alloy is one of the wide used casting aluminum alloys and intense research activities of worldwide on casting process or refinement agent and procedure. In this study, we attempted refinement on Al primary phase and eutectic phase of A356 alloy by ultrasonic injection into the melts. The effect of ultrasonic injection into the melts could be summarized as follows: the morphology of the Al primary phase was refined and changed shape from dendritic to nondendritic and the eutectic phase of A356 was also refined and changed morphology from dendritic to plate shape.

Abstract: A356 alloy is one of the wide used casting aluminum alloys and intense research activities of worldwide on casting process or refinement agent and procedure. In this study, we attempted refinement on Al primary phase and eutectic phase of A356 alloy by ultrasonic injection into the melts. The effect of ultrasonic injection into the melts could be summarized as follows: the morphology of the Al primary phase was refined and changed shape from dendritic to nondendritic and the eutectic phase of A356 was also refined and changed morphology from dendritic to plate shape.

Abstract: The main purpose of this research is to evaluate the wear resistance of titanium matrix composites (TMCs). Reinforcements, TiB and TiC, were formed by in-situ reaction between boron carbide and commercial pure titanium. The confirmation of the sound synthesis of TMCs was done by phase identification. And then, sliding wear test were carried out to verify the wear resistance of TMCs by means of the coefficient of friction, wear loss and morphology wear track. The results of wear test indicate that TMCs have superior resistance than AISI H13 tool steel at the condition of severe loads.

Abstract: Coarse primary Mg2Si was usually found in Al-Mg-Si alloys. In this experiment, to refine the coarse primary Mg2Si size, ultrasound was used as a new process. Ultrasound was injected to the molten Al-Mg-Si melts directly and its effect was significant. Average size of primary Mg2Si was reduced from 70㎛ to 15㎛, morphology was also changed to polygonal from irregular shape. Moreover tensile strength was improved by 30%, from 112 to 148 after ultrasonic injection.

Abstract: In this study, melting and casting of Mg-Ni alloys in ambient atmosphere for mass production, nickel content adjustment for enhancing hydrogen capacity and reducing alloy weight, and cyclic hydriding/dehydriding test for evaluation of degradation properties were investigated. Mg-Ni alloys were fabricated by Rotation-Cylinder Method (RCM) in ambient atmosphere. Hydriding/dehydriding reactions were repeated 350 times at 350°C to evaluate degradation property. SEM-EDS analysis revealed that the lamellar eutectic structure of Mg-Ni alloy consists of Mg-rich α-phase and β-Mg2Ni. It is supposed that the lamellar eutectic structure enhances hydrogenation properties of Mg-Ni alloys.

Abstract: This paper presents semi-solid processing of Cu-Ag alloy to produce squirrel cage for small and medium induction motors. Complete die filling could be achieved with the slug at 1065 oC, casting pressure of 9 MPa and ram speed of 0.08 m/s. There were no defects in squirrel cage and slug temperature and ram speed affected the slot filling of rotor importantly, however, casting pressure rarely has an effect of the filling in this experiment. As the same thixoforming condition, when the ram speed was 1.5 m/s, slurry could not fill the slot and solidified like a needle shape. The rapid flow like this would be the cause of non-filling defect of slot because the slurry injected and solidified in the slot firstly was became the obstacle not to flow the rest of the slurry.

Abstract: The candidate gases for the replacement of SF6 in magnesium melt protection were reviewed from the viewpoint of environmental effects, melt protection properties, and chemical properties. The surface films formed under fluorine bearing gases were analyzed by using XPS. The XPS results revealed that the surface films consist of MgO and MgF2. The fluorine content of the surface film increases with increasing gas concentration and exposure time. The Pilling- Bedworth ratio (PBR) of MgO was 0.73, and the porous magnesium oxide could not cover the whole melt surface. PBR of MgF2 was 1.29, and the surface film which contains MgF2 could cover a lager area of the melt surface. This higher PBR of composite surface film prevented further oxidation and evaporation of magnesium.

Abstract: A new process, melts dispersion technique, was developed to fabricate the solder powder in oil surroundings by forming molten metal’s droplets. The main parameters of the technique on shape and size of powders were investigated in the present paper. It is shown that a cooling rate of the melts and viscosity of oil for isolation of droplets affect the shape and the size. The cooling rate is a very important parameter, and there is a critical cooling rate at given conditions. Although the melts were dispersed perfectly in oil, fine droplets were merged and coalesced during solidification without rapid cooling. The shape of powder has an influence mainly on oil viscosity and was altered into a sphere from an irregular shape with increasing oil viscosity.

Abstract: Melts dispersion technique is a new process to produce solder powders in oil surroundings by the formation of molten metal’s droplets. The entire viscosity of the fluid after uniform mixture of melts and oil mainly affected the shape and size of the solder powder. The size of solder powder was increased and the irregular shape was altered into the spherical shape with increasing the fluid viscosity. The effect of alloying elements, such as Mg, Al and Ti, on Sn-Zn solder for the oxidation resistance was investigated using the solder paste mixed solder powder fabricated by melts dispersion technique at optimum conditions and RMA type flux. The oxygen concentration was reduced by the addition of alloying elements, especially, by Ti adding.