Papers by Author: Young Ok Yoon

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 main task of this study was to investigate the effect of Mg content on hardness of extruded Al-Si-Mg-Mn alloy after solution and artificial ageing heat-treatments. The Al-Si-Mg-Mn alloys with the variation of Mg content were fabricated through casting, homogenization and extrusion processes. The extruded samples were heat-treated – T6 temper. Thermal analysis with metallurgical examination was performed in order to optimize the condition of solution heat-treatment. Then, microstructure change was discussed by using optical microscopy and SEM-EDS. And the corresponding Vickers hardness was measured as a function of ageing time. It was found that the Vickers hardness slightly increased due to improved ageing efficiency and increased hard phase as the Mg content increased.

Abstract: In this study, the microstructures and mechanical properties of the recently developed Eco-2024-T3 alloy were examined. Eco-2024 is made using Eco-Mg (Mg-Al₂Ca) in place of element Mg during the manufacture of alloy 2024-T3. This is an alloy that has economic advantage and excellent properties. Alloy Eco-2024 showed smaller crystal grains that were distributed more evenly compared to the existing alloy 2024-T3. It consisted of Al matrices containing minute amounts of Al₂CuMg, Al₂Cu, and Ca phases and showed microstructures with reduced amounts of Fe phases or oxide. As a result of tensile tests, this alloy exhibited yield strength of 413 MPa, tensile strength of 527 MPa, and elongation of 15.4%. In other words, it showed higher strength than the existing alloy 2024 but was similar to the existing alloy 2024 in terms of elongation. In fatigue tests, alloy Eco-2024-T3 recorded fatigue limit of 330 MPa or around 80% of its yield strength; this is a much more excellent property compared to the existing alloy 2024-T3, which has fatigue limit of 250 MPa. Based on the aforementioned results, the correlation between the excellent mechanical properties of alloy Eco-2024-T3 and its microstructure was examined.

Abstract: This study was carried out to characterize thixoextruded AZ31 Mg alloy in terms of its symmetric behavior through the optical microscope, electron back scattering diffraction and mechanical test. The elongated grains to the extrusion direction were generally observed during hot extrusion, while the thixoextruded microstructures were isotropic. Texture of hot extruded bar showed the strong fiber component, while that of thixoextruded bar was random distribution. The aspect ratio of hot extruded bar was irregular at each position. On the contrary, the aspect ratio of thixoextruded bar was almost uniform below 2 at each position.

Abstract: The aim of this study was to improve extrudability limit, eliminate welding line and obtain optimum thixoextrusion conditions for manufacturing tubes of 7075 Al wrought alloy. By thixoextrusion, it was possible to improve deformability, control isotropy with extrusion direction, eliminate welding line (seamless) and save cost due to low energy consumption compared with conventional extrusion processes. The welded part was not observed at the welding line area. The grains of thixoextruded tube were homogeneously distributed and equiaxed grains were observed. Therefore, thixoextrusion is the most effective variable for the control of the magnitude of the welding line.

Abstract: Molten Mg alloys burn rapidly in air if not protected. In order to handle molten Mg safely, the molten metal should be protected from oxidation by covering the surface with flux or protective gases. Sulfur hexafluoride (SF6) gas is widely used for Mg alloy as a cover gas and has proved to be a successful inhibitor. However, the use of SF6 gas is limited because of its high cost and its significant impact on global warming potential (GWP). Therefore, SF6 gas is being replaced by alternative protection gases such as tetrafluoro ethane (HFC-134a) and 3M NovecTM 612. These gases are less expensive and have lower GWP, compared with SF6 gas. Recent studies show that there has been another attempt by adding CaO into Mg alloys to manufacture CaO added AZ31Mg alloy in terms of increasing ignition temperature and reducing protective cover gas amount during melting and casting. This paper discusses effect of CaO on ignition and oxidation behaviors of AZ31 Mg alloy as well as on surface roughness and mechanical properties of AZ31 Mg alloy extruded bars.

Abstract: The main emphasis of this study is to utilize thixoextrusion process for improving extrudability of 7075 Al wrought alloy. In the present study, the influences of thixoextrusion parameters, such as initial ram speed and die bearing length, on mechanical properties of thixoextruded 7075 Al wrought alloy were investigated. The results of thixoextrusion experiments about microstructures and mechanical properties were compared with conventional extrusion results. The maximum extrusion pressure of thixoextrusion processes was seven times lower than that of conventional hot extrusion processes. The hot extruded bars show a large amount of grains which are stretched along the extrusion direction and asymmetry of grains was formed to extrusion direction, while the microstructures of the thixoextruded bar did not show any asymmetry and grains were homogeneously distributed. The hardness values of thixoextruded bar were higher than that of conventional hot extruded bar.

Abstract: Thixoextrusion, one of the thixoforming processes, has advantages of high productivity, reduction of the extrusion pressure and cost saving because of the low energy consumption compared with the conventional extrusion processes. For the thixoextrusion, the low liquid fraction (fL<0.3) should be achieved and also the liquid fraction and average grain size should be uniform according to the reheating profile at the desired low liquid fraction. The main emphasis of this study is to investigate the feasibility of thixoextrusion for 2024 Al wrought alloy without additional pretreatment. The results show that the liquid fraction and average grain size were almost uniform with respect to isothermal holding temperature and time. It was very useful for thixoextrusion in terms of process control such as billet temperature control and actual extrusion time.

Abstract: The present study discussed extrudability improvement for 7000 series Al wrought alloys by thixoextrusion, with emphasis on controlling thixoextrusion parameters, such as initial ram speed, die bearing length and extrusion temperature of billet in semisolid state. The results of thixoextrusion experiments about microstructures and extrusion pressures were compared with conventional hot extrusion results. The maximum extrusion pressure for thixoextrusion was greatly decreased than that of conventional hot extrusion. This will contribute to extrudability in terms of extrusion pressure, which in turn means that shorter process time is required and smaller extrusion machine can be applied for the same operation. It is also possible to make complex shape components of 7000 series Al wrought alloys, which is not possible by conventional hot extrusion processes. The elongated grains to extrusion direction were generally observed during conventional hot extrusion, while the thixoextruded microstructures were isotropic.