Technical And Theoretical Research Of Annulus Electromagnetic Stirring

To improve metallurgical quality of metal materials by controlling solidification process is an important direction in materials science. The core of semisolid processing technology is to control the size, morphology and distribution of solid phase in the slurry by melt treatment. The most commonly used methods for melt treatment are mechanical stirring and electromagnetic stirring (EMS). Although mechanical stirring method is able to achieve uniform solidification even in a small volume of melt, there is pollution problem occurs due to the stirrer contacted with molten. It is difficult to dock with the follow-up process, and it is not conducive to bulk melt for uniform solidification. Electromagnetic stirring technology has been applied more broadly on a commercial scale due to its non-pollution, easy process control and continuous production. However, it has some shortcomings. For example, stirring force exerted in the slurry is larger in the external part but smaller in the inner one of the slurry because of the skin effect, which leads to inhomogeneous micro structures. In response to this key technical problem, this paper presents an annulus electromagnetic direct chill casting process to achieve bulk melt uniform solidification. The annulus electromagnetic stirring (AEMS) machine were self-designed and built. A series of experiments on casting A357aluminum alloy have been carried out to investigate the effects of AEMS parameters on the microstructure, and the effects of electromagnetic stirring on solidification. The main work and results are as follows:In this paper, the calculation models of the electromagnetic field,flow field and temperature field in semi-solid slurry preparation by AEMS were established, and effects of annulus gap width and centre pipe location on electromagnetic field,flow field and temperature field were analyzed. The simulation results showed that with annulus gap width decreases, the stirring intensity increases. As center pipe moves down the axial circulation flow is compressed, while the shear flow was expanded. Optimal match of shear flow and circulate flow is of key importance for realizing uniform temperature of bulk melt. The experiment results indicated that when annulus gap width is20mm, centre pipe location is0mm, stirring current is17A, stirring frequency is30Hz and stirring time is10s, semisolid slurry with high quality can be prepared. Based on above research, a comprehensive mathematic model was developed to describe the interaction of the multiple physics fields (electromagnetic field, fluid flow, heat transfer and solidification) during continuous casting with AEMS and optimize the design of continuous casting process with AEMS. Under enough stirring intensity, the location of center pipe can effectively reduce the liquid sump depth and temperature gradient. Technical prototype of continue casting process with AEMS was:three-phase with three pairs of poles (six coils) for stirrer; the annulus gap width of20-25mm; the centre pipe location is Omm and the annulus gap position is3/4-4/5.The experimental device was designed and built. The semi-solid ingots of A357aluminum alloy with diameter of200mm and length of1500mm were processed by continue casting with AEMS. The EDS spectrum analysis shows that the ingots with low macro-and micro-segregation were obtained. Comparison test show that average grain size of primary phase is122.2μm by AEMS. The average grain size of DC and EMS are407.2μm and257.9μm respectively. Spreading-Collecting-Whole controlling idea was achieved. The experimental results show that annulus electromagnetic stirring has significant advantages for industrial applications in bulk melt uniform solidification and preparation large diameter high-quality aluminum billet.Based on the second law of thermodynamics the effect of electromagnetic stirring on nucleation was investigated. A mathematical model between the magnetic induction intensity, rotation speed and the critical radius was developed. Analysis results show that under electromagnetic stirring, critical radius decrease, critical nucleation work decrease and nucleation rate increase. With rotation speed increase, critical radius decrease, critical nucleation work decrease and nucleation rate increase. With rotation speed increase, grain density increase, grain size decrease and shape factor increase.