Dock Young Lee

Effect of Electromagnetic Stirring during Solidification on the Morphological Evolution of Primary Solid Phase of Al Alloy

In this study, in order to develop the cast product of Al alloy having a globular microstructure by using the elctromagnetic (EM) stirrer, which was specially designed and manufactured to induce a various fluid flow type of melt during solidification, a morphlogy and size of primary solid phase of the solidifying slurry was investigated with respect to EM stirring condition such as an induced magnetic flux density (MFD) and a frequency of input current. The magnetic flux density of EM stirrer was measured by using a gaussmeter and its distribution and magnetic force within Al melt was simulated in ANSYS program. The induced MFD was increased with decreasing a frequency of input current at the same input voltage due to the increased penetrating depth of magnetic field. But, the magnetic force related directly with a stirring strength of melt was increased with the frequency. Both a roundness and size of primary α phase of Al alloy was decreased with increasing a frequency of input current and MFD within the experimental range. Therefore, the primary α phase was refined and globularized at the higher frequency and MFD.

Effect of Isothermal Holding Temperature on Globularization of Primary Solid Phase in Multiplex-Type Slurry Cup

In order to produce a high quality and cost effective part in the rheocasting process for an automobile industry it has been important to develop a slurry-on-demand process, which can manufacture the semi-solid slurry having a fine and globular structure of primary solid phase. The morphology of primary solid phase of semi-solid slurry is coarsened and globularized during an isothermal holding process in a solid-liquid region. Accordingly, in this paper, the microstructural evolution of semi-solid slurry during the isothermal holding at a various temperature in solid-liquid region was investigated to examine a coarsening and globularization behavior of primary solid phase. The semi-solid slurry of Al alloy was produced in a slurry maker using a multiplex-type slurry cup that we developed recently. A size of primary solid phase was the finer at the higher holding temperature, but under a constant holding time a roundness degree of primary solid phase was the lower at the lower holding temperature. Also, a coarsening rate of primary solid phase was not considered to be affected significantly by a holding temperature even though a little lower coarsening rate was obtained at the higher holding temperature.

Influence of the Electromagnetic Stirring on Globularization of Primary Solid Phase in Solid-Liquid Region

Forming of alloy and composite within a solid-liquid region, i.e. thixo-forming and rheo-forming, has been recognized as a technology offering several potential advantages over casting and forging such as a low forming pressure, reduction of macrosegregation, and reduction of porosity. In this paper, a globularization for primary solid phase by a forced fluid flow in melt due to electromagnetic stirring during solidification and isothermal stirring at solid-liquid region, was examined to produce Al alloy having a globular solid phase to get a thixotropic behavior. The electromagnetic stirrer was specially designed and manufactured to create all kind of fluid flow pattern such as a circumferential, vertical, helicoidally and contra-rotating flow pattern during the solidification of a melt. The size and roundness of primary solid phase of the quenched sample after isothermal stirring at solid-liquid region were measured with respect to a stirring time and frequency. The globularization of solid phase was enhanced with incresing the stirring time and electromagnetic flux density, i.e. stirring strength.

Hydrogen Permeation and Surface Characteristics of Pd-Coated Ni-Nb-Ta Amorphous Alloy Membrane

The hydrogen permeability of a palladium coated Ni 60 Nb 30 Ta 10 amorphous alloy was examined in the temperature range of 573-673 K and pressures up to 0.6 MPa. Commercialized Pd 60 Cu 40 alloy membranes were also evaluated in the same manner. The permeated hydrogen flux increased with increasing temperature and difference in the square-roots of hydrogen pressures on both sides of the membrane. It indicates that the diffusion of hydrogen atoms is the main factor for the hydrogen permeation in this alloy. The maximum hydrogen permeability of the Ni 60 Nb 30 Ta 10 glassy alloy was 4.1×10 -8 [mol/m s Pa 1/2 ] at 673 K, which was much higher than that of Pd 60 Cu 40 membranes measured under the same conditions, 1.9×10 -8 [mol/m s Pa 1/2 ] at 673 K. These permeation characteristics imply the possibility of future practical use of the amorphous alloys as the hydrogen permeable membrane. The thickness and compositional variations of sputtered overlayer were measured by using FE-SEM and AES, respectively, to examine the surface characteristics of Pd-coated membranes. The high-quality overlayer was achieved successfully by RF sputtering method.

Microstructural Evolution and Crystal Orientation of Electromagnetically Stirred Al Alloy in the Semi-Solid State

Through more than three decades of development, a semi-solid metal processing has been successfully established as a unique casting technique to produce a structural component for an automobile industry with high integrity and improved mechanical properties. A slurry-on-demand process to make the semi-solid slurry having a fine and globular microstructure has been very important in the semi-solid metal process. In the present study, the orientation distribution functions (ODFs) calculated from the pole figure data were examined and tried to characterize the bulky morphology of primary solid phase of the semi-solid slurry of Al-Cu alloy produced with various magnetic flux density of 100 to 300Gauss in the specially designed electromagnetic (EM) stirrer. Columnar dendritic structure of primary α phase was turned into a rosette and globular structure by EM stirring during solidification. The primary α phase was refined and globularized with increasing a magnetic flux density of EM stirring. Also, due to the EM stirring the tendency to random orientation was appeared. In the case of unstirred Al-Cu alloy the <110>//ND texture was developed strongly and <100>//ND and <111>//ND texture was weakly developed. But with the increase of the EM stirring strength, <100>//ND and <111>//ND texture were more strongly advanced. Due to EM stirring the texture was almost completely randomized.

Effect of crystal orientation on textures of electromagnetic stirred aluminum alloy

Recently, a rheocasting process has been interested to produce a structural part for an automobile industry and so the slurry-on-demand process to make the semi-solid slurry having a fine and globular microstructure has been very important to produce a high quality and cost effective part in the rheocasting process. An electromagnetic (EM) stirrer employing for a slurry making process was designed and prepared to induce simultaneously both a circumferential and vertical fluid flow of a melt in order to control a rotation angle of EM stirring of a poured melt. In the present study, the semi-solid slurry of Al-15%Cu alloy was produced in the EM stirrer and its microstructure and the orientation distribution function calculated from the pole figure data was examined in accordance with a various rotation angle of EM stirring between 0˚ (circumferential flow) and 90˚ (vertical flow). The size and morphology of primary α phase was affected with a rotation angle of EM stirring and the finest and the most globular primary α phase could be obtained at a rotation angle of EM stirring of 45˚ and 60˚. Also, due to the EM stirring the tendency to random orientation was appeared. Also, the tendency of random orientation was the most at a rotation angle of 45˚ and 60˚. Therefore, it was considered that the rotation angle of EM stirring of 45˚ and 60˚ was the most effective to induce the non-dendritic growth of primary solid phase of EM stirred Al-Cu alloy.

Microstructural Effects of Electromagnetic Stirring Strength and Casting Speed in Continuous Casting of Al Alloy

Semi-solid forming usually requires feed stock material in form of cylindrical billets with a uniform, globular and fine grain microstructure. These billets are commonly being produced by continuous casting involving agitation of the solidifying melt. The paper will present the development of a horizontal continuous casting machine using circumferential electromagnetic stirring to generate melt flow and shear rate in the continuous casting mould. The machine has been used to study the influence of various process parameters such as electromagnetic stirring strength and casting speed in the production of Al billet with a diameter of 76mm. In order to establish the quantitative relationship between microstructure and the process parameters, the morphology and primary crystal size of billet were observed according to the casting speed and electromagnetic stirring strength. A depth and distance of surface defect on the billet was decreased with increasing a casting speed and a very good smooth surface without any surface defect appeared on the billet produced at a casting speed of 600mm/min. A thickness of the solidifying shell was decreased with increasing the casting speed and the maximum casting speed without a break-out was 600mm/min in horizontal continuous caster designed in this study. Both the size and the aspect ratio of primary crystal were decreased with increasing the casting speed and electromagnetic stirring strength in the range of this study.

Surface Characteristic of Ni-Based Amorphous Alloy Membrane for Hydrogen Permeation

The production of high purity hydrogen gas is an important technical issue for future energy and environmental problem. The pure hydrogen is the ideal fuel in terms of fuel-cell performance. There have been many investigations on the hydrogen permeation of Pd-coated membrane of amorphous alloys. The purpose of this research is to characterize the surface such as roughness and morphology of surface layer of Ni-based amorphous alloy membrane by atomic force microscopy (AFM). The ribbon of Ni-Nb-Ta amorphous alloy was produced by a single-roller melt-spinning technique. The surface characteristic of the amorphous alloy membrane was discussed in view of the hydrogen permeation behavior. From the 3-dimensional AFM images, a lot of hole was observed on the surface of the amorphous alloy membrane produced in air atmosphere. But, a lot of protrusion were observed in the membrane produced in vacuum due to the selective growth on the high oxygen-affinity element of the amorphous alloy. The average roughness of the membrane produced in air atmosphere and vacuum was measured to be 0.359 and 0.263nm, respectively. Therefore, it was considered that it would be suitable to produce the amorphous alloy membrane in vacuum.

Dynamic mechanical properties of continuously cast Al alloy in electromagnetic stirrer

The mechanical properties of electromagnetically stirred billet of Al alloy during continuous casting such as a tensile, impact, and fatigue property was examined with respect to a size and roundness of primary α phase to examine the influence of the globularization and refining of primary α phase on the dynamic mechanical property. The billet was continuously cast in a casting speed of 100 to 600 mm/min during the electromagnetic stirring with a magnetic flux density of 700 Gauss and then was heat-treated. All tensile properties were enhanced with decreasing the size of primary α crystal and the ultimate tensile strength (UTS), yield strength, and elongation was obtained 320MPa, 235MPa, and 17.8%, respectively, at a size of primary α phase of 77 um. The fatigue life at fracture was largely improved by 37% from 1.25×105 to 1.7×105 cycles and the fracture toughness was obtained approximately 7.25 joules as a maximum value at a minimum size of primary α phase on the present experiment condition. Therefore, it indicates that all mechanical properties can be improved at the higher casting speed owing to the fine microstructure of a primary α phase and also eutectic phase due to the higher cooling rate.

Effect of Preheating Temperature of Multiplex-Type Slurry Cup on the Morphology of Primary Solid Phase of Semi-Solid Slurry

Semi-solid slurry for a rheocasting process requires a fine and globular structure of primary solid phase. In the present study, in order to find an optimum preheating temperature of a multiplex-type slurry cup in the slurry maker that we recently developed for the rheocasting process, the semi-solid slurry was held for a various time of up to 60 min. in a multiplex-type slurry cup preheated at a different temperature. A size and morphology of primary solid phase of the semisolid slurry was measured and analyzed with the difference of the recorded cooling curve during the solidification with respect to a preheating temperature of the slurry cup. Also, a microstructural evolution and globularization mechanism occurred during the isothermal holding were discussed. Finally, as considering the effect of a different casting condition on the morphology of primary solid phase the optimum manufacturing condition of the semi-solid slurry for the rheocasting process was proposed.