Papers by Author: Woon Jae Jung

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.

Abstract: A small amount of Zn or Sn was added to Mg-3%Al alloy to improve the formability of alloy. The elongation of as-rolled alloy was increased at both room temperature and 300oC by addition of Sn or Zn, even though the tensile strength at room temperature was remarkably increased only by Zn. When the as-rolled alloy was annealed to be fully recrystallized, the ductility at room temperature was significantly enhanced, however the elongation at 300oC was rather decreased probably attributed to the dynamic recrystallization occurred during the tensile test.

Abstract: The Effects of Li content and annealing treatment on microstructure and damping capacity for Mg-X%Li alloys have been investigated, based on experimental results from X-ray diffractometry (XRD), optical microscopy (OM), hardness tests and vibration damping tests in a flexural mode. The Mg-X%Li alloys containing Li of 3%, 8% and 13% consist of α (HCP) single phase, (α + β (BCC)) dual phases and β single phase, respectively. In as-rolled state, the damping capacity for Mg-Li alloys shows a similar level regardless of Li content. The annealing treatments at 200oC and 400οC give rise to an enhancement of damping capacity only for the Mg-3%Li and Mg-8%Li alloys containing α phase, and at the same annealing temperature, the Mg-3%Li alloy with fully α structure exhibits higher damping capacity. This result indicates that the damping capacity of Mg-Li alloys depends principally on α phase, and that the annealing treatment is necessary to improve its damping capacity.

Abstract: Microstructures and damping properties of semi-solid AM50 (Mg-5%Al-0.3%Mn) alloy were investigated and compared with those of die-cast AM50 alloy, based on experimental results of X-ray diffractometry (XRD), optical microscopy (OM), hardness tests and damping tests in a flexural mode. The semi-solid AM50 specimens show higher damping capacity than die-cast one in as-fabricated state, and the higher the fraction of solid α-(Mg), the greater the damping capacity. The annealing at 200oC deteriorates the damping properties of the semi-solid and die-cast specimens. This would be due to the segregation of solute atoms on dislocation lines, which eventually leads to lower internal friction by the restriction of dislocation movement. The peak damping values of the AM50 specimens are obtained after annealing at 400oC. The disappearance of segregates acting as pinning points of dislocations is thought to be responsible for the improvement in damping capacity. This result implies that the presence of solid α-(Mg) phase and annealing treatment at high temperature are beneficial to damping property of AM50 alloy.

Abstract: Various Mg-Al-Zn alloys with different Al and Zn contents were fabricated by conventional casting and semi-solid forming process. And the microstructure and mechanical properties of the alloys were investigated. In Mg-4%Al-(5~7)%Zn alloys most of the grain boundary phases were found to be Mg-Al-Zn while in Mg-6%Al or Mg-8%Al based alloys the Mg- Al-Zn phase coexisted with Mg17Al12 at grain boundaries. At room temperature the semi-solid formed alloys showed significantly higher tensile properties, especially elongation, than the conventionally cast alloys.

Abstract: Semi-Solid Al-5%Mg-3%Zn-1%Mn alloys with or without 0.2%Zr were prepared by the isothermal heating of either cast ingot or extruded billet. The microstructural and tensile properties variations during the isothermal heating, depending on alloy composition and slurry preparation method, were investigated. The grain (globule) size of semi-solid microstructure was not remarkably changed by the slurry preparation method, however it was significantly reduced by the Zr additions. The room temperature tensile elongation of the semi-solid formed alloys was generally higher in case of the extruded billet. T6 heat treatment was not found effective in improving the strength of the semi-solid alloys, unlike the casting alloys.

Abstract: Mg-8%Al-1%Ca was selected as a base alloy composition and small amounts of minor alloying elements, RE or Sr, were added. Microstructure of as-cast Mg-8%Al-1%Ca base alloys consists of dendritic primary Mg, Mg-Al-Ca and Mg17Al12 phases. Mg-Al-RE-(Ca) phase in the RE-added alloys and Mg-Al-Sr-(Ca) phase in the Sr-added alloys were additionally found. The creep resistance of Mg-8%Al based alloy was significantly improved by the minor alloying elements additions with 1%Ca. Comparatively globular primary phase could be obtained after just 30min. through an isothermal heating of as-cast Mg-Al-Ca alloys.