The developed model was validated by the checking of grain preferential growth orientation and the solidification experiment with low melting point alloy of Sn-21%Bi(mole fraction). It was also applied to predict the structure defects (e.g. stray grain) of unidirectionally solidified turbine blade. The results show that the developed model is reliable and has the following abilities: 1) reduce the misorientation caused by the orthogonal mesh used in simulation; 2) well reproduce the growth competition among the different-preferential-direction grains with less than 10% relative error; 3) predict the structure defect of stray grain with the accuracy over 80%; 4) optimize the grain selector to better obtain a single crystal avoiding the multigrain defect; 5) simulate the structure evolution (nucleation and growth) of the directional and single crystal turbine blade.
The results of experiments and simulations show that there is a turbulent flow in the molten aluminum and it is hard to be restrained in the thin tubule (diameter of 6 mm) when the electromagnetic body force is applied. The electromagnetic elimination experimental results show that the flow has serious effect on the elimination of 5 μm alumina inclusions, but has little effect on the 30 μm and 100 μm primary silicon. The effects of the electromagnetic field and the turbulent flow on the electromagnetic elimination are discussed.
In order to study the effect of the stirring flow on the grain diameter and solute concentration of hollow billet, the couple model of the two-phase solidification and electromagnetic field was built to simulate the solidification process of Sn-3.5%Pb hollow billet with the traveling magnetic field and rotating magnetic field. The effects of different kinds of flows on the temperature field, concentration field and grain diameter of molten metal during solidification were analysed. The results show that, there are different flow patterns in the molten metal induced by the traveling magnetic field and rotating magnetic field. Both flows can refine the grains in the hollow billet because of change of the temperature gradient and cooling rate of molten metal. The bigger the stirring velocity is,the smaller the grain diameter. Both flows can result in the macro-segregation in the hollow billet because of the non-homogeneous flows. The bigger the stirring velocity, the more serious the macro-segregation of the hollow billet. So, the stirring intensity should be controlled to acquire the high quality hollow billet.
A 3D dendrite envelope tracking model was developed for estimating the solidification structure of unidirectionally solidified turbine blade. The normal vector of dendrite envelope was estimated by the gradient of dendrite volume fraction, and growth velocity of the dendrite envelope (dendrite tips) was calculated with considering the anisotropy of grain growth. The solute redistribution at dendrite envelope was calculated by introducing an effective solute partition coefficient(ke). Simulation results show that the solute-build-up due to the rejection at envelope affects grain competition and consequently the solidification structure. The lower value of ke leads to more waved dendrite growth front and higher solute rejection. The model was applied to predict the structure of turbine-blade-shape samples showing good ability to reproduce the columnar and single grain structures.
In order to find the ways to improve the elimination efficiency with high frequency magnetic field, a mathematical model of electromagnetic elimination (EME) in the tubule with high frequency magnetic field was set up. The calculated results show that by ignoring the flow of molten metal, when the surface magnetic induction intensity of the metal (B0) is 0.03 T and the diameter of the tubule is 8 mm, the non-metallic inclusions with 30 μm diameter can be wiped off in 7 s from the center of the molten aluminum, whereas the elimination time of the 5 μm non-metallic inclusions is more than 240 s. When B0 is 0.03 T, the diameter of the tubule is 8 mm and elimination time is more than 30 s, the elimination efficiency of 5μm, 10 μm and 30 μm non-metallic inclusions is about 60%, 90% and 100%, respectively, the elimination efficiency increases with the decreasing diameter of the tubule. It can be concluded that increasing the magnetic induction intensity or decreasing the diameter of the tubule can decrease the elimination time and improve the elimination efficiency in EME with high frequency magnetic field.
The effects of separation time and magnetic induction intensity on the separation efficiency of alumina particles with diameters varying from 30 to 200 μm in aluminum melt were investigated. The experimental results show that the particle-accumulated layer is formed in the periphery of the solidified specimen when the diameter of the separated molten metal, the magnetic induction intensity and the separation time are 10 mm, 0.04 T and 1 s, respectively. When the separation time is 2 s, the particle-accumulated layer can be observed obviously and the separation efficiency is about 80%. There are few alumina particles in the inner of the solidified specimen when the separation time is 3 s. The separation efficiency higher than 85% can be achieved when the separation time is longer than 3 s. When the magnetic induction intensity is 0.06 T, the visible particle-accumulated layer can be formed in 1 s and the separation efficiency is higher than 95%. The experimental results were compared with the calculated results at last.
