Papers by Author: Ki Tae Kim

Abstract: Small amount of Ag was added to high conductivity Cu-5%Cr alloy in order to increase the strength without sacrificing the conductivity. The typical microstructure of thermo-mechanically processed specimen mainly consists of Cu matrix and relatively large Cr phase. Although this typical microstructure was not changed with varied process variables, both the micro-hardness and conductivity were significantly affected by them. The electrical conductivity of alloy was slightly decreased by the Ag addition, but the micro-hardness could be enhanced by it. TEM analyses indicated that very fine Cr precipitates were formed in addition to the relatively large Cr phase in the Cu matrix. It is suggested that the properties of Cu-5Cr-xAg alloys can be optimized by carefully controlling the precipitation of the fine precipitates.

Abstract: Aluminum-Silicon based die-casting alloys have been extensively utilized in various industrial applications, but their relatively low electrical and thermal conductivities make them unsuitable as high conductivity parts. In this research, silicon content was restricted to a comparatively low level for higher conductivity and magnesium was added to enhance the castabilities. Al-1Fe-0.5Si-xMg alloys showed significantly higher electrical conductivity than conventional Al-Si based alloys. As the Mg content was increased, the mold filling ability measured using a fluidity serpentine test mold was a little decreased, however the hot cracking susceptibility was observed to be first increased and then decreased. The relationship between solidification characteristics and castabilities of Al-1Fe-0.5Si-xMg alloys was discussed based on the cooling curve analysis and microstructural characterization results.

Abstract: In this paper, it was aimed to develop a new Al-Zn-Mg base aluminum alloy having high electrical conductivity, strength, and formability simultaneously. As a result, Al-Zn-Mg base low aluminum alloy sheet can be effectively strengthened without significant thermal conductivity loss by multiply alloying precipitation hardening elements and properly controlling production process parameters.

Abstract: The effects of Sn addition on the cast microstructure, mechanical properties, and corrosion behavior were comprehensively investigated. Cast AM60 alloy mainly consisted of primary Mg matrix and Mg17Al12 phase, and the microstructure virtually remained unchanged by the Sn additions up to 2%. Most of added Sn solute atoms were observed to dissolve into the Mg17Al12 phase and only a few fine MgAlSn precipitates were found in the high Sn alloys. Tensile strength at both room and 175oC was rather deteriorated by increasing Sn content, however the creep strength was significantly enhanced by it. Polarization characteristics of cast AM60 based alloys were not remarkably affected by the Sn content.

Abstract: High strength high conductivity Cu-1%Cr-Mg-P alloy was selected as a base composition and Ag was added to the alloy in order to further increase the strength without sacrificing the conductivity. SEM and TEM analyses indicated that very fine MgP and Ag(Mg) precipitates were formed in addition to relatively large Cr phase in the Cu matrix. Significantly high strength could be obtained through the special cold rolling at an extremely low temperature using liquid nitrogen. The electrical conductivity of alloy was slightly decreased by the Ag addition, but the tensile strength could be further enhanced by it.

Abstract: Influences of Gd or Y addition on microstructures and tensile properties at room and elevated temperatures were investigated for T6-treated Mg-3%Nd-0.5%Zn-0.4%Zr casting alloys, on the basis of experimental results from X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, tensile and creep tests. Microstructures of the T6-treated alloys are characterized by recrystallized α-(Mg) grains containing various nano-sized precipitates such as Mg41Nd5, Zn2Zr3 and Zr particles. In T6 condition, most of added Gd and Y elements are dissolved in Mg41Nd5 precipitates rather than formation of new phases. Tensile properties and creep resistance of the Mg-3%Nd-0.5%Zn-0.4%Zr alloy are remarkably increased by the addition of Gd or Y, but their efficiencies are substantially the same.

Abstract: Influences of Ca addition on microstructures and mechanical properties at room and elevated temperatures have been investigated for Mg-1.5%Nd-1.0%RE-0.5%Zn-(0~1.0)%Ca casting alloys, on basis of experimental results from X-ray diffractometry (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), tensile and creep tests. Microstructures of the alloys are characterized by dendritic α-(Mg) grains surrounded by Mg12Nd-Zn-(Ca) eutectic network phase. The average size of α grains decreases gradually with an increase in Ca content. At room temperature, yield strength (YS) is enhanced with increasing Ca content with a decrease in ultimate tensile strength (UTS) and elongation to fracture, whereas the Ca addition leads to greater YS and UTS at 175oC. The tensile creep strain and secondary creep rate, measured at 150 and 200oC under 100MPa for 100hrs, become lower with the increase in Ca content. The obtained tensile properties at elevated temperature demonstrate that the addition of Ca plays a role in improving high temperature mechanical properties including creep resistance for the Mg-Nd-RE-Zn-(Ca) alloys. In view of microstructural evolution, this would be attributed to the refined primary α grains and higher thermal stability of the Mg12Nd-Zn-Ca eutectic strengthening phase.

Abstract: Mg-3%Zn-0.2%Zr based alloy sheets with various alloying elements additions were fabricated through thermo-mechanical process, and their microstructure and mechanical properties were investigated at room and elevated temperatures. CCV(conical cup value) and V-bend tests were also carried out to evaluate the formability of the fabricated alloy sheets. The experimental results showed that small amounts of Sn or Sr additions could improve the elongation at elevated temperatures, even though the room temperature tensile properties were slightly deteriorated by the Sr addition.

Abstract: Two-step isothermal heating and conventional one-step heating processes were used to produce the semi-solid slurry of Mg-5%Zn-0.5%Zr alloys with and without 1%RE. It was found that the slurry with better semi-solid characteristics could be fabricated by the two-step heating compared to the conventional process. Only the small amount of RE addition to the Mg-5%Zn- 0.5%Zr alloy was observed to reduce the solid particle size of slurry significantly and to improve the tensile strength at 150oC, probably owing to Mg-Zn-RE phase with a high thermal stability.

Abstract: Changes in microstructure and damping capacity with aging time for solutionized AZ91 (Mg-9%Al-1%Zn-0.2%Mn) alloy have been investigated based on experimental results from optical micrography, X-ray diffractometry, hardness test and damping capacity measurement vibrating in a flexural mode. Discontinuous β (Mg17Al12) precipitates form along the primary grain boundaries, the amount of which increases as the aging time increases. The hardness of α matrix with respect to aging shows a typical “S” shape, indicating a generation of fine continuous precipitation in the matrix during aging. The peak level of damping capacity for the AZ91 alloy is obtained after 1 hour of aging, over which the damping capacity becomes deteriorated continuously. The optimum density of continuous β precipitates with fine morphology, which may well act as pinning points for dislocation lines, might be responsible for the improvement of damping capacity.