| Various laminar and turbulent flows caused by the application of electromagnetic field during the solidification could change the processes of heat and mass transfer in melt metal and proceed to affect the solidification structure and compositional distribution of alloy.With the application of the New-type multi-functional electromagnetic stirrer, low melting alloys Pb-Sn and Sn-Sb were selected as research object and the effects of different magnetic fields on the solidification process, composition segregation and defects such as shrinkage cavity, porosity of the alloy ingot were investigated.The action mechanism of the spiral magnetic field (SMF) emphatically was discussed, and stirring technologic parameters for EEMS was optimized by the numerical simulation analysis and experimental study, which would provide a guide for the practical application of the SMF stirrer in the factory. In addition, the study of compound field(combination of different kinds of electromagnetic field and power ultrasonic field) on the solidification process and the effects of compound field on the surface quality of the ingot, microstructures, composition segregation and macro-defects were studied. Moreover, the action mechanism of compound field on the microstructures of the metal was also analyzed.(1) The magnetic induction distribution of three magnetic fields exhibits sinusoidal relationship with the circumferential angle, and decreases with the increase of the stirring frequency. The magnetic field distribution under rotating magnetic field(RMF) is seriously affected by the frequency and reaches its maximum value f=10Hz, while under spiral magnetic field(SMF) at the range of f=8-12Hz is very stable. The magnetic induction reaches its maximum value in90°and270°under RMF and TMF, and under SMF is in120°and180°. The magnetic induction increases with the exciting current as well as increases with the frequency within limits.(2) The magnetic induction decreased gradually from the centre to the upper or lower part of the stirrer along the axial direction and from the inwall to the centre in the same height section along the radial direction; the coverage area of SMF was wider along the axial direction than the others and the decrease speed of rotating magnetic field was lower along the radial direction, though SMF showed a more stationary distribution when R<60mm. Experiments and simulation results show that magnetic induction intensity under the SMF is greater than the RMF or TMF. (3) The simulation calculation results show that:with the increase of exciting current, the electromagnetic force value on the metal melt increases along all directions. The electromagnetic force distribution acting on the metal melt is uneven, the electromagnetic force value near the stirrer lateral is greater than the value far away from the stirrer. The distribution of Fz and Fθ playes important role in stirring strength. And its value also increases with the increase of the excitating current, which is benificial to the metal melt flow along the longitudinal and cross section. The electromagnetic force Fsum increases with the increase of stirring frequency and its distribution is stable at frequency range f=2-12Hz and increases along the radial direction(4) The induction heat under the SMF increases with the increase of the exciting current and uniformly distributes along the radial direction. However, the influence of induction heat on the melt became more obvious while the exciting current is large. The cooling rate decreases, the position of eutectic plateau changes and eutectic reaction time is prolonged.(5) At the fixed exciting current, the alloy exhibits anti-segregation at lower frequency while segregation has been gradually improved at higher frequency, and the frequency of f=10-13Hz is the best value in the improvement of macro-segregation. The appropriate stirring time is15-20min, reasonable stirring frequency is10-13Hz has been obtained. The grain size of Pb-Sn alloy is reduced from175μm to140μm and composition deviation decreases to1.5wt.%.(6) when the frequency is10Hz, in a range of the exciting currents with the increase of its value, the grain size is finer. The minimum grain size is133μm at I=125A. To further increase the current, the grain coarsenes. When I=115A the SMF can basically eliminate segregation. So, the SMF has the appropriate exciting current in improving segregation and solidification structures of115-125A. Moreover, the eutectic lamellar spacing coarsened and the transition of eutectic structures morphology from lamellar to rod-like eutectic when the stirring strength is high enough under the SMF.(7) The SMF, RMF and TMF stirring can effectively improve microstructure and segregation of the alloy. Comparison results of effects on the intrinsic quality for Pb-Sn and Sn-Sb alloy under different magnetic stirring show that in the refinement of grain SMF is better than RMF, which is better than TMF; in the improving segregation as:SMF is better than TMF, which is better than RMF.(8) Compound field has the appropriate ultrasonic power value and the stirring intensity in improving solidification structure and internal quality for the surpose of better metallurgical results. Compared to the compound field of rotating magnetic with ultrasonic field, under spiral magnetic with ultrasonic field the results show that, the tangential component of spiral magnetic field force expands the coverage of ultrasonic cavitation area and strengthenes the role of refinement of microstructures. The supersition stirring, which is composed of axial component of electromagnetic force and an axial-flow generating by ultrasonic field, promotes nucleation in the all metal molten. Compound field of spiral magnetic and ultrasonic field achieves better effects in improving macro-defects and structure refinement. |
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