Young Ok Yoon

Thixoextrusion for 7075 Al Wrought Alloy Tube

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.

Microstructural Control of 7075 Al Wrought Alloys through Thixoextrusion Route

Thixoextrusion, one of the thixoforming processes, has advantages of high productivity, reduction of the extrusion pressure, extension of the die life, and cost saving owing to its low energy consumption compared with the conventional extrusion processes. Especially, thixoextrusion process is expected to be very effective for hard-to-form materials with high strength. The present study focuses on 7075 Al wrought alloy to investigate the potential industrial applications of the thixoextrusion process. The microstructural evolution of 7075 Al wrought alloy for thixoextrusion was investigated with respect to isothermal holding temperature and time in the partially remelted semisolid state. The results showed that the liquid fraction increased with increasing isothermal holding temperature and time while the average grain size was inversely proportional to isothermal holding temperature and time up to 5min. However, there was no big change of liquid fraction and average grain size with respect to isothermal holding temperature and time. The important fact that the liquid fraction and average grain size were almost uniform after 5 min holding time is considered very useful for thixoextrusion in terms of process control.

Microstructure and Mechanical Properties of Eco-2024-T3 Aluminum Alloy

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.

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.

Development of Environment-Friendly CaO Added AZ31 Mg Alloy

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.

Development of Thixoextrusion Process for 7000 Series Al Wrought Alloys

Thixoextrusion for 7003 and 7075 Al wrought alloys with different content of Cu was carried out with respect to isothermal holding temperature in the partially remelted semisolid state. Especially, despite higher extrudability index of 7075 Al wrought alloy compared with the extrudability index of 7003 Al wrought alloy, the maximum extrusion pressures of 7075 Al wrought alloy were about the same as the maximum extrusion pressures of 7003 Al wrought alloy at temperature with the liquid fraction 10% (7003-624, 7075-609). The hardness values of thixoextruded specimens were higher than that of as-quenched specimens only after partial remelting. The microstructures of thixoextruded specimens did not show any dendrites or rosette-like structures. The average grain sizes of thixoextruded specimens were smaller than average grain sizes of homogenized specimens.

Effect of Partial Remelting Procedure on the Microstructural Evolution of 7075 Al Wrought Alloy for Thixoextrusion

The feasibility study for thixoextrusion of 7075 Al wrought alloys was carried out with respect to reheating profile, isothermal holding temperature and time during the partial remelting through simple partial remelting. The reheating rates were 40/min and 60/min. The isothermal holding temperatures were controlled at 609, 622, 628 and 632 for 0, 2, 5, 10 and 30 min. The interesting point of this study was that the thixotropic structures of 7075 Al wrought alloy without additional pretreatment could be obtained through simple partial remelting. The average grain size was significantly smaller in the quickly heated specimen. The liquid fraction was increased with increasing isothermal holding temperature and time while the average grain size was inversely proportional to isothermal holding temperature and time. However, there was no big change of liquid fraction and average grain size with respect to isothermal holding time. The important fact that the liquid fraction and average grain size were almost uniform after 5 min is considered very useful for thixoextrusion in terms of actual extrusion time.

Development of In-Ladle Direct Thermal Control (DTC) Rheocasting Process

In recent year, various rheocasting methods, other than stirring in the semisolid state, have been developed to produce a fine globular primary phase in cast alloys. These new methods, called as slurry-on-demand, involve the controlled nucleation and growth/ripening of primary phase during partial solidification. In-Ladle direct thermal control (DTC) rheocasting has been competitively and successfully developed. It requires no processing equipment outside of the casting machine, no grain refinement procedure and no additional cycling time except for cooling down to the desired casting temperature. Process concept of In-Ladle DTC rheocasting is simply based on the fact that there is slurry and mush transition in the range of liquid fraction of 0.1 to 0.6 and the fact that the transition could be controlled by controlling the shape of solid and the relative energy. This study is mainly concerned about the feasibility to achieve semisolid slurry during partial solidification only through thermal and time control, this is, by In-Ladle DTC rheocasting. Based on the optimum conditions obtained by thermal modeling, microstructural evolution of Al alloys was carried out by In-Ladle DTC rheocasting. The fluidity was also investigated by In-Ladle DTC rheocasting for Al alloys.

Process Development for Cost-Effective Ag-Ni Contact Materials by Simple Solidification Control

Ag-Ni is well known as contact materials with high workability and low contact resistance for medium to weak current contacts. However, Ag-Ni contact materials are difficult in controlling the Ni content, because Ag-Ni alloys exhibit no range of solid solubility of Ni in all temperature range. The purpose of this research is to develop a process for cost-effective Ag-Ni contact materials based on simple solidification control. The hardness of Ag-Ni contact materials produced by simple solidification control was higher than that of Ag-Ni contact materials produced by conventional powder metallurgy and coprecipitation method. The higher hardness value of contact materials produced by simple solidification control may be attributed to the uniform distribution and small size of Ni particles in Ag matrix. The properties of produced contact materials can be further improved by solidification process control.

Oxide Scale Behavior and its Effect on Oxidation Resistance in Al Alloys Containing Alkaline Earth Metal Elements

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.