Hyun Kyu Lim

Effects of Alloying Elements and Cooling Rate on Morphology of Phases in CaO Added Mg-Al-Si Alloys

Mg2Si formed by the addition of Si in Mg-Al alloys is the very useful intermetallic compound. However, Mg2Si phases in the Mg-Al-Si alloys are prone to forming undesirable, coarse Chinese script shape, which will deteriorate the mechanical properties of Mg alloys. Therefore, in order to modify the Chinese script shaped Mg2Si phases in the Mg-Al-Si alloys, many methods have been studied such as alloying addition of Ca or P. This study was aimed at improving the mechanical properties of CaO added Mg-Al-Si alloys by the addition of alloying elements and varying cooling rate to change of morphology of Mg2Si and CaMgSi phases. The micro structure s of specimens were observed by OM and SEM, and the phase analysis was performed by XRD, TEM and EDS. To evaluate the mechanical properties of tested alloys, tensile tests were carried out at room and high temperatures.

Enhancement of Mechanical Properties of Extruded Mg–9Al–1Zn–1MM–0.7CaO–0.3Mn Alloy Through Pre-aging Treatment

The effect of pre-aging treatment before extrusion has been investigated in Mg–9.0Al–1.0Zn–1MM–0.7CaO–0.3Mn alloy. The as-cast microstructure consists of α-Mg dendrite with secondary solidification phase particles, (Mg, Al)2Ca, β-Mg17Al12 and Al11RE3 at the inter-dendritic region. After extrusion, β-Mg17Al12 precipitates are present, but higher density and more homogeneous distribution in pre-aged alloy. In addition, μm-scale banded bulk β-Mg17Al12 particles are generated during extrusion. Al11RE3 particles are broken into small particles, and are aligned along the extrusion direction. (Mg, Al)2Ca particles are only slightly elongated along the extrusion direction, providing stronger particle stimulated nucleation (PSN) effect by severe deformation during extrusion. The mechanical properties can be significantly enhanced by introducing pre-aging treatment, i.e. β-Mg17Al12 precipitates provide grain refining and strengthening effects and (Mg, Al)2Ca particles provide PSN effect.

Effect of CaO on Thermal Conductivities of Mg and Mg-Zn Alloys

The thermal conductivities of binary Mg-CaO and Mg-Zn, and ternary Mg-Zn-CaO alloys have been investigated by evaluating the effect of CaO on pure Mg and Mg-Zn alloys, with an emphasis to develop a new Mg alloy by compromising thermal conductivity, process-ability and mechanical property. The Mg alloys specimens were prepared by casting into a steel mold and then by machining. The thermal conductivities of the alloys were determined by evaluating the thermal properties of specific heat and diffusivity, from room temperature to 200 °C. OM, SEM, and EDS were used to analyze the microstructures and phases. The fluidity was also investigated by using a spiral fluidity mold for improved process-ability during actual die casting.

The Effect of Ca Addition on Microstructure and Mechanical Properties of Gravity Cast Mg–Zn–Y Alloy

Mg–Zn–Y alloy with I-phase (Mg3Zn6Y) is particularly attractive because of the coherent interface between the I-phase (Mg3Zn6Y) and matrix phase, which sustains a strong bond and has low interfacial energy improve plastic deformation in the process like rolling and extrusion. Therefore, these Mg–Zn–Y alloys with I-phase (Mg3Zn6Y) have been focused a lot recently for wrought Magnesium alloys. However, these alloys can be used as the cast alloys because the I-phase (Mg3Zn6Y) exhibits better thermal stability than many other phases, which could enhance creep resistance at elevated temperature.In this study, the effect of Ca addition on microstructure and mechanical properties of I-phase (Mg3Zn6Y) containing Mg–Zn–Y alloys were investigated to develop creep resistant Mg alloys.

Development of High-Performing Extruded Magnesium Alloy

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).

Effect of Misch Metal Addition on Thermal Conductivities and Mechanical Properties of Mg-4Zn-0.5Ca Alloys

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.

Effects of La Addition on Thermal Conductivity and Mechanical Properties of Mg‐4Zn‐0.5CaO Alloys

The thermal conductivities of quaternary Mg-Zn-CaO-La alloys have been investigated by evaluating the effect of La on Mg-4Zn-0.5CaO alloys, with an emphasis to develop a new Mg alloy without compromising thermal conductivity, process-ability and mechanical property. The thermal conductivity of these alloys had been determined by tests for thermal properties, such as specific heat and diffusivity, from room temperature to 200 °C. The microstructures of specimens were observed by OM and SEM, and the phase analysis was performed by XRD, EDS. The fluidity was also investigated by using a spiral fluidity mold for improved process-ability during actual die casting. To evaluate the mechanical properties of tested alloys, tensile tests were carried out at room temperatures.

Effects of Heat Treatment on Bio-Corrosion Properties of Mg-Zn-xMn (x= 0.5,1.0, and 1.5 wt.%) Alloys as Biodegradable Materials

Mg alloys have unique characteristics such as high specific strength, low density, high corrosion rate, etc., as functional as well as structural materials. Mg-Zn alloys have good biocompatibility because Mg and Zn are abundant nutritional elements in the human’s body. However, Mg alloys with multi-phase cause galvanic corrosion by corrosion potential differences among constituent phases. Therefore, the application of Mg alloys on bio-material parts are limited.

Phase Evaluation of Sr and CaO Added Mg‐Al‐Si Alloys

In the previous study, to modify the morphology of Mg2Si phase, we added CaO in Mg-Al-Si alloy. However, CaMgSi phase was formed by the addition of CaO in Mg-Al-Si alloy and the morphology of CaMgSi phase is needle type. Therefore, CaMgSi phase would deteriorate the mechanical properties. On the other hand, when adding Sr in CaO added Mg-Al-Si alloy, the modified CaMgSi phase was confirmed.

Effect of Plane Strain on the Microstructure and Texture of AZ31 Sheet with Various Reduction Ratios

In this study, the effect of plane strain on the microstructure and texture of AZ31 sheet was investigated with various reduction ratios. To achieve plane strain, that is, to prevent shear deformation during warm-rolling, AZ31 sheet was capsuled by pure Cu tube. The large strain induced by simple one-pass warm-rolling led to the significant grain refinement and texture randomization. The two-dimensional finite-element method showed that the warm-rolling of AZ31 sheet capsuled by Cu tube imposed a uniform plane strain in the whole sheet thickness. The high reduction ratio caused the dynamic recrystallization during annealing of the rolled sheet.