Inclusion removal from a molten steel is essential problem to be solved for high quality steel production. Removal of micron-size inclusions from the molten steel by the conventional inclusion-removal process using density difference between the inclusions and the molten steel is difficult because their rising velocity is not enough for removal in the operating time. Collision and coagulation are useful phenomena to promote rising velocity of the inclusions by increasing their size. Imposition of oscillating electromagnetic field having a function of collision enhancement is one of the promising methods for the inclusion removal. The inclusion motion under imposition of the oscillating electromagnetic field is governed by inertial, viscous and Basset forces. However, effect of the Basset force on the inclusion motion is not clarified. In this study, an experimental model consisting of electrically insulating particles and a conductive and transparent aqueous solution was constructed to clarify the effect of the Basset force on the inclusion motion. Both amplitude of the inclusion motion and phase of inclusion position based on the oscillating electromagnetic force as a phase reference agreed with theoretical prediction under the consideration of the Basset force. Relative error of Al2O3 inclusion motion in the molten steel due to neglect of Basset force was theoretically estimated. Prediction without the Basset force induces serious errors of amplitude and phase of the inclusion motion, which lead to over estimation of collision frequency among inclusions.
Removal of non-metallic inclusions from a molten metal using buoyancy force acting on them has been carried out in steel industry. However, the productivity is restricted by rising velocity of the inclusions in the molten metal. Collision and coagulation of the inclusions promotes their rising velocity by increasing their apparent size because the rising velocity is proportional to the square value of inclusions diameter. Utilization of an oscillating electromagnetic field has a potential to enhance the collision among the inclusions and to promote their rising velocity. However, quantitative prediction of the collision enhancement effect of the oscillating electromagnetic field has not been investigated. In this study, the collision enhancement effect of the oscillating electromagnetic field on the collision among non-conductive particles in a conductive liquid has been theoretically investigated. Collision among the particles is enhanced over a critical electromagnetic volume force. In the case when the friction drag force becomes the dominant resistance force of the particle motion, the critical electromagnetic volume force is smaller than those in the case when the Basset force or inertial force becomes the dominant resistance force. And this case is desirable for the collision enhancement because the collision enhancement effect increases with decrease of the critical electromagnetic volume force. Imposition of the oscillating electromagnetic field enhances the collision among Al2O3 inclusions in the molten steel, especially among small inclusions, under the clam flow condition.
Concentration boundary layer formed in the vicinity of copper anodic electrode having a shape with triangles aligning in line located in a transparent electrolyte aqueous solution was observed under the imposition of a static magnetic field to clarify the effect of vibrating electromagnetic force on the boundary layer. Two current patterns of DC current and superimposition of AC and DC currents were adopted. The concentration boundary layer thickness at the right and left sides of a valley between the triangles of the anodic electrode were not the same under the imposition of the DC current because of difference of electromagnetic force direction between on the right and left sides of the valley. On the left side of the valley, decrease of the brightness in the concentration boundary layer under the imposition of the DC current was larger than that under the imposition of the AC and DC currents where decrease of the brightness corresponded to the amount of reaction products. This is because the AC and DC currents mixed the dense and dilute liquids of divalent copper ion around there. On the contrary, decrease of the brightness in the concentration boundary layer under the imposition of the AC and DC currents was larger than that under the imposition of the DC current on the right side of the valley. The reason can be explained as follows. Because drastic dissolution of divalent copper ion from the electrode and a strong electromagnetic force were simultaneously excited, dense liquid was transported far away.
Mass transfer is sometimes rate-determining step in a high temperature process. For enhancement of production efficiency, concentration boundary layer formed in the vicinity of interface between two phases should be controlled in these processes. Ultrasound is one of the candidates for this purpose because it can excite acoustic streaming, micro-jet and accompanying flow in a high temperature liquid from its outside. Thus a lot of researches on the effect of the ultrasound on the mass transfer have been done. However, researches on dynamic behavior of concentration boundary layer under the irradiation of the ultrasound have not yet been done until now. In this study, effect of the ultrasound irradiation on the boundary layer formed in an aqueous solution by anodic reaction of a copper electrode has been investigated. Because concentration of bivalent copper ions is related to brightness of the solution by Lambert-Beer law, the concentration boundary layer was evaluated using recorded data of a high speed camera. A constant voltage was imposed on the solution for excitation of the anodic reaction. The observed boundary layer thickness was roughly agreed with the theoretically calculated boundary layer thickness using Kármán-Pohlhausen methods when the ultrasound was not irradiated. As soon as the ultrasound was irradiated, brightness increased within 0.05 seconds in the region away from the anodic electrode more than 20 μm. This was caused by the micro-jet and/or flow excited by it. On the contrary, the current gradually increased and its relaxation time was 0.5seconds.
Imposition of oscillation electromagnetic force during the solidification process has been expected to be an excellent technique to produce finer solidified structures. An alternating current and a static magnetic field are imposed to liquid metal in mutually orthogonal directions to generate oscillation flow driven by an electromagnetic force near solidification interface. It was reported that dendritic structures were experimentally changed into fine equi-axed structures by imposition of the electromagnetic oscillation under some particular conditions, however necessary conditions for such refinement by the electromagnetic oscillation have not yet been specified. In order to apply this technique to a real industrial process, mechanism of refinement by the electromagnetic oscillation should be understood, and suitable conditions to achieve effective refinement should be clarified. In the present study, theoretical solutions of oscillation flows driven by the electromagnetic forces were analyzed. It was found that the flows through dendrite primary arms could be characterized by several dimensionless parameters such as oscillation Stuart number,Nω, shielding parameter, Rω, Womersley number, Wo1, Reynolds number, Re1, and Péclet number, Pe1. Required conditions for refinement by the electromagnetic oscillation were investigated and estimated as follows: Nω<<1, Rω>>1, Wo1≥1, Re1>>1, Pe1≥1. The result corresponds reasonably well with the experimental date in previous research.
Numerical simulations of oscillating flows of molten metal were carried out. These flows were driven by electromagnetic force under imposition of static magnetic field and alternating electric field. Solute transport around a dendrite was calculated simultaneously with the flow field. Results of simulations clarify effects of the electromagnetic oscillating flows on the solute transport when solute condensation occurs in liquid phase around the dendrite. Boundary layer of the oscillating flow is similar to Stokes layer. In the vicinity of the tip of the primary arm (or the tertiary arm), a vortex is generated from this shear layer every half period. These vortices blow up the solute and hence solute transport is enhanced. Simulation results of various conditions suggests the optimum Womersley number exists while the primary-arm Reynolds number is required to be much greater than one. Five times enhancement of solute transport is obtained in a typical case where Womersley number is 8.6 and the primary-arm Reynolds number is 14.
To evaluate the influence of local melt flow around solid-liquid interface on dendritic growth of alloy, we carried out phase-field simulations of dendritic growth coupled with the calculation of local melt flow. To simulate the local melt flow, we devised a model to generate the special melt flow around the solid-liquid interface. In the model, this special melt flow rotates clockwise and generates on the solid-liquid interface. Under the local melt flow, dendrite morphologies gradually varied from equiaxed to globular dendrites as the flow velocity increase. This is because the enriched solute in the diffusion layer was washed off due to advection of solute by the local melt flow. From these simulated results, it was found that the change of dendritic morphology will be occurred by the local movement of solute in the diffusion layer.
An analysis for solute redistribution and the macrosegregation during alloy solidification process under fluid flow were carried out by using a one dimensional cellular automaton method. The effect of fluid flow on the solute redistribution was incorporated as an eddy diffusion coefficient. Examination was carried out for the planar S/L interface solidification of a model alloy and the solidification of a Fe-0.6 mass%C alloy with mushy zone, respectivly. Steady state solidification was formed with no fluid flow condition for the planar S/L interface solidification. Macrosegregation was formed during the planar S/L interface solidification under the fluid flow, and the degree of the macrosegregation increased with increase in eddy diffusion coefficient. Two types of fluid flow, i.e. macroscale flow and micro scale flow were considered for the study of the solidification of the Fe-0.6 mass%C alloy with mushy zone. It was shown that the degree of the macrosegregation with micro scale flow was larger than that with macro scale flow. The effect of the limiting fraction of solid for flow in mushy zone was examined, and the degree of the macrosegregation in Fe-0.6 mass%C alloy was increased with increase in the limiting fraction solid for flow.
Control of segregation during the solidification of an alloy is important to improve the quality of product because it affects mechanical, physical and chemical properties. Convection is a tool of solute distribution control in the alloy though it is difficult to excite flow in the latter stage of the solidification using traditional method because of drastic increase in apparent viscosity. Thus, a controlling method of the solute distribution in the latter stage of the solidification is desired. In this study, electromagnetic vibration excited by a horizontal static magnetic field of 0.3 T and a vertical alternating current of 60 A, 1 kHz was applied to the Sn-10 mass%Pb during its solidification for clarification of its effect on the solute distribution, especially in the latter stage of the solidification. Both distributions of Pb concentration in the primary phase and the ratio of the eutectic phase area were explained by the gravity segregation when the electromagnetic vibration was not imposed. On the other hand, the Pb concentration in the primary phase increased if the electromagnetic vibration was imposed in the latter stage of the solidification, while it decreased if the imposing duration was the initial stage. The eutectic area ratio distribution became relatively uniform if the electromagnetic vibration was imposed both in the initial and the latter stages of the solidification. These distribution differences with and without the electromagnetic vibration suggest that the electromagnetic vibration induced flow in the solidifying alloy even though high solid fraction.
Time-resolved X-ray imaging has been adopted to directly observe the microstructural evolution under the influence of ultrasonic vibration in Sn-21 mass%Bi alloys. Simultaneously with the circulating convection ahead of dendrite tips (10 mm/s), the longitudinal oscillation of dendrites with low frequency (~20 Hz) occurred immediately after an imposition of ultrasonic vibration. The convection ahead of dendrites caused morphological change from dendritic to cellular at the dendrite tip, where the fragmentation rarely occurred. The tip radius of primary dendrite is approximately 3.5 times as large as that prior to the ultrasonic vibration. The growth velocity remarkably decreased due to the promotion of heat flow. In the mushy region, where the dendrite morphology remained mostly unchanged, the fragmentation of primary and secondary arms occurred. Some detached dendrite arms moved to the downstream side along the convection oscillating in the mushy region. The dendrite fragmentation frequently occurred in the downstream side. It is likely that the agitation of liquid induced by the dendrite oscillation as well as high accumulation of solute contributed to the frequent dendrite fragmentation. The solute concentration distribution indicated that the difference in the solute concentration of liquid flowing into the mushy region caused microstructural changes, such as dendrite fragmentation and change in the solid fraction.
Although the effect of ultrasonic vibrations on the structure of solidifying metals has been known for long time, the practical application of ultrasound to casting technology still remains a big challenge. Ultrasonic casting exploits cavitation in molten metal to disperse particles of grain refiners or to break dendrites during solidification. Therefore, care must be taken to control the passage of melt through the cavitation zone. There is still, however, a lack of data in this area.
The present study consisted of two parts. In the first one, intensity and spectral characteristics of cavitation noise generated during radiation of high-intense ultrasonic vibrations into water and molten aluminum alloys were investigated by using a high temperature cavitometer. Based on these data, a measure for evaluating the cavitation intensity was established and verified for relatively low and high vibration amplitudes. The second part presents results on the application of ultrasonic vibrations to a DC caster to refine the primary silicon grains of a model Al-17Si-0.01P alloy during the casting of 178-mm billets. High amplitude ultrasonic vibrations were radiated into a specially designed hot-top unit of a DC caster to allow a better control of the melt flow through the cavitation zone as compared, for example, to ultrasonic treatment in launder. It was shown that refinement effect of ultrasonic vibrations and structure uniformity can be significantly improved by optimizing the amplitudes of horn tip vibration and horn position in the unit.