Papers by Author: Won Ha

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: 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.

Abstract: In this study, melt protection property of SF6+synthetic air and HFC-134a+synthetic air cover gas mixtures were investigated for most commonly used magnesium alloys, AZ91D and AM60B. Each alloy was melted in mild steel crucible with different temperature, cover gas concentrations and sealing conditions. HFC-134a+synthetic air cover gas mixtures offered better melt protection property for all variables of temperatures, alloys, cover gas concentrations and sealing conditions. However, crucible (AISI 1020) surface that is exposed to HFC-134a cover gas mixtures was seriously damaged due to the corrosive characteristics of HFC-134a in molten magnesium temperature.