Papers by Author: Shae K. Kim

Abstract: Interface reaction between SKD61 and three Al melts at 973 K was investigated in this study. In pure Al and A380 alloy, soldering occurred on the samples. Pure Al showed two separated soldering areas consisting of Fe-Al based intermetallic phases, mainly FeAl₃. A380 alloy indicated the expanded soldering area consisting of Al₃Fe and two AlFeSi based intermetallic phases. Al-10mass%Mg alloy showed the absence of soldering, but a formation of a few Fe-Al intermetallic phases. This would be attributed to weakened interaction between Fe and Al caused by Mg enrichment. Microstructures near soldering of pure Al and A380 also showed the precipitations considered as Al₃Fe and AlFeSi intermetallic compounds, respectively. However, in Al-10mass%Mg alloy, there is only intermetallic compound including Mg, V and Ca without Al₃Fe.

Abstract: Oxide scale behaviors by surface segregation of Mg, Ca and Be in Al and their effects on oxidation resistance at melt temperature were investigated. With the addition of Ca and Be in Al-7.5mass%Mg alloy, the samples showed a suppressed weight gain. However, in the initial oxidation, Ca added samples exhibited improved oxidation resistance. As a result of oxide layer observation by microscopy, Ca added Al-7.5mass%Mg alloy exhibited the region overlapped by constituent elements, indicating multi-element oxide is formed on the surface. In the oxidation of Al-Mg-Be system, BeO is formed as primary oxide and mixed layer with MgO, while Ca addition in Al-Mg system causes no change in the primary and secondary oxides, but formation of CaMg₂Al₁₆O₂₇. BeO and BeAl₂O₄ may contribute to balanced layer by combination between constituent oxides in the Al-Mg-Be system. In the case of Ca addition, CaMg₂Al₁₆O₂₇ acts as a filler of the cracks in MgO layer.

Abstract: The microstructural evolution of AlMg4.5 and high-Mg containing AlMg10 alloys were investigated during homogenization by metallurgical analyses and micro-Vickers hardness test. It was concluded that the increased amount of Mg solute in AlMg10 alloy leads to formations of both super-saturated Al-Mg solid solution and increased eutectic Mg₂Al₃ phase in interdentrictic region at solidified state. The hardnesses of both AlMg4.5 and AlMg10 alloys increased after homogenization proportionally to the temperature. The resultant improved hardness of AlMg10 alloy after homogenization is higher than that of AlMg4.5 alloy and mainly contributed to additional solid-solution hardening by Mg solute dissolved from eutectic Mg₂Al₃ phase.

Abstract: The thermal conductivities of quaternary Mg-Zn-Ca-MM alloys have been investigated by evaluating the effect of MM on Mg-4Zn-0.5Ca alloys, with an emphasis to develop a new Mg alloy without compromising thermal conductivity, process-ability and mechanical property. As a result, the thermal conductivity of 0.5 wt.% MM-added Mg-Zn-Ca alloy was almost same as that of MM-free Mg-Zn-Ca alloy. However, with further increasing MM contents, thermal conductivities of MM-added alloys decreased. The tensile yield strength was improved with increasing MM contents. In addition, the average spiral flow lengths of Mg-Zn-Ca-xMM alloys were almost same levels with Mg-Zn-Ca alloy.

Abstract: To develop high-performing extruded magnesium alloys, we had investigated the effect of zinc into Mg-5Gd-2Y-xZn-0.7Ca (x = 1, 2, 3, and 4) alloys. With increasing of zinc content up to 3 wt.%, the volume fraction of LPSO phases and strength increased, while the volume fraction of LPSO and strength rapidly decreased when zinc content was 4 wt.% in Mg-5Gd-2Y-xZn-0.7Ca alloys. Ignition temperature and corrosion rate were directly proportional to the increase of zinc content in this study. The optimum zinc content was 2 wt.% in Mg-5Gd-2Y-xZn-0.7Ca alloys and VWZO52207 alloy exhibited high strengths (TYS: 407 MPa and UTS: 424 MPa), adequate elongation (6.9 %), and high ignition temperature (934 ^oC).

Abstract: The effects of Mg and Mn contents on hot-rolling temperature and resultant tensile properties of Al-Mg-Mn alloys were investigated. The Al-3.5Mg-0.3 alloy as a reference and Al-7.5Mg-0.3Mn, Al-3.5Mg-1.0Mn alloys were prepared by casting for hot-rolling experiment. The rolling temperatures of both Al-7.5Mg-0.3Mn and Al-3.5Mg-1.0Mn alloys had to be decreased due to surface cracking during hot-rolling, which is caused by increased fractions of Mg-containing phases like Mg₂Al₃. The tensile strength of the hot-rolled Al-7.5Mg-0.3Mn alloy was highly increased by the combined effects of enhanced solid-solution and work-hardening at lower rolling temperature. And the resultant tensile strengths of the hot-rolled Al-3.5Mg-1.0Mn alloy were also increased due to dispersoid hardening by increased Mn content and work hardening during hot-rolling.

Abstract: Grain growth behavior of AlMg5 alloy fabricated by using a new Mg mother alloy containing Al₂Ca (referred to as AlMg5-Al₂Ca hereinafter) was investigated during homogenization and subsequent hot compression test. Normal AlMg5 alloy using a commercial Mg mother alloy showed abnormally grown large grains in its microstructure after homogenization at 520 ̊C for 12hrs, while the grain growth in the AlMg5-Al₂Ca alloy was completely suppressed by formation of stable Al₄Ca during solidification on grain boundary. The compressive flow stress of normal AlMg5 alloy at 400 ̊C was significantly increased after homogenization because of lack of grains having proper slip directions to the applied load. But the flow stress of AlMg5-Al₂Ca alloy showing no grain growth during homogenization was slightly decreased implying lower energy needed for subsequent thermo-mechanical processing.

Abstract: Effects of Si addition on the microstructure, fluidity and tensile property of Al-5/7mass%Mg alloys with Mg+Al₂Ca master alloy as Mg raw material were investigated in this study. With 7 mass%Si addition, the fluidity was remarkably improved in both Al-5Mg and Al-7Mg alloys. With increasing the Si content, the amount of the eutectic phases was increased in the microstructures. As a result of the tensile test, there was a drastic reduction in elongation with increasing Si content. Al-5Mg-7Si alloy showed a significant improvement of strength after solution heat treatment and artificial aging.

Abstract: Supersaturation behavior of Al solid solutions and its effect on tensile property in Al-7Si-0.4Mg and Al-7Si-0.8Mg alloys during solution heat treatment were investigated in this study. As a result of Mg solubility measurement for Al solid solitions after solution heat treatment for 6 hrs by electron probe micro analyzer (EPMA)-wavelength-dispersive X-ray spectroscopy (WDS), Al-7Si-0.8Mg alloy had the higher Mg solubility, about 0.55 mass%, corresponding to that of Al-Si-Mg ternary phase diagram. From the tensile test, Al-7Si-0.8Mg alloy showed the improved strength and similar elongation to that of Al-7Si-0.4Mg alloy.

Abstract: Si modification behavior by twin probability approach with X-ray diffractometry (XRD) technique and microstructural evolution of the A356 alloys with Al₂Ca and Mg exceeded up to 1.5 mass% during the solution heat treatment were investigated. Al₂Ca added alloys showed the modified Si particles in the microstructure and higher twin density than those of Al₂Ca-free alloy. During the solution heat treatment, the Si particles of Al₂Ca-free alloy became coarser with the time at 540 °C, while there was no coarsening in the Al₂Ca added alloys throughout all the conditions.