| China is a country with abundant magnesium resources,but,the processing and quality of magnesium alloys still lags far behind developed countries at present.Nowadays,semi-continuous casting process is the mainly production method of magnesium alloy billet.The thermal properties of magnesium alloys lead to the problems of excessive temperature difference in the core and edge of the ingot,low density,composition segregation,unstable performance,cracking and deformation in the production of large-size billet in conventional semi-continuous casting method.This project is part of the national key research and development program "manufacture technology for large-size and high-quality magnesium alloy ingot billets".The numerical simulation techniques is used for studying the flow and heat transfer process in crystallizer of AZ80 magnesium alloy with a diameter of 500 mm during DC casting process.The low-frequency pulse magnetic field and DC process are combined to study the electromagnetic field distribution,flow and heat transfer behavior in pulsed electromagnetic semi-continuous casting process based on above the theories and results.At the same time,distribution of melt flow and temperature field in semi-continuous casting process with alternating magnetic field,pulsed magnetic field and out-phase pulse magnetic field are analysed and compared.Through the research on DC casting process,it shows that the use of shunt discs can cause the originally accelerating downward melt flows to the radial direction of the ingot and liquid curve becomes shallow.The smaller the shunt discs,the higher the core melt temperature in mold.Similarly,the larger shunt discs can lead to closer distance between shunt discs and solidification front.The simulation result shows that 210mm diameter shunt disc is more appropriate for DC casting about with a diameter of 500 mm of AZ80 alloy billet;the higher the casting temperature,the larger the melt flow area in mold and the deeper the liquid curve;The amount of secondary cooling water basically has no effect on melt flow.However,liquid curve becomes shallow with increasing secondary cooling water.The influence of secondary cooling water on temperature field is different in different regions,and the degree of influence on area below the solid phase line is much greater than that above the solid phase line.With increasing secondary cooling water,temperature contour shifts,but as cooling water continues to increase,contour shifting rate is significantly reduced.Casting speed has a significant effect on melt's flow and temperature.The faster casting speed can lead to significantly expanding area of melt flows and deeper liquid curve,the contour of temperature and thickness of ingot solidified shell have shifted significantly and become thinner respectively as casting speed increases.Comprehensive consideration of the influence of various parameters,simulation result shows that the reasonable parameters for DC casting of 500mm diameter AZ80 alloy are that the casting temperature is 953K,secondary cooling water is 600L/min,and casting speed is 35mm/min.During the DC casting process,application of a pulsed magnetic field around crystallizer shows that:a periodically varying magnetic flux density is generated in crystallizer,an electromagnetic force that is repeated tension&compression melt is generated and the maximum electromagnetic pressure is much higher than electromagnetic tension,the direction of induced current is different in different parts of the melt,the direction and the magnitude continuously change in one cycle.Melt macro-flow does not change significantly,but velocity appeared periodic oscillation.At the same time,temperature field is more evenly distributed.The study of different pulsed electromagnetic parameters shows,that the increasing pulse current could enhance the magnetic induction intensity,the Lorentz force and the induced current density,the faster melt velocity and the higher velocity amplitude,at the same time,temperature is more evenly distributed.The extreme value about Lorentz force and induced current density increases significantly with the enhanced pulse frequency;when magnetic induction intensity reaches the maximum value,the skin depth of magnetic field decreases,the melt at crystallizer side is strongly affected,and flows rate is faster,however,the amplitude is small.When the pulse frequency is at the low value,melt in the core of the crystallizer is subjected to a strong effect,the flow velocity is high and the amplitude is large.As frequency increases,temperature of the melt at the edge of mold rises,while temperature of the core does not change substantially,The skin depth at which magnetic induction intensity is at maximum value gets deeper with increasing duty cycle of pulsed magnetic field,the maximum value of Lorentz force and induced current density decrease,but the duration is significantly prolonged,and average flow velocity and amplitude of melt also enchanced.The inner high-temperature liquid region decreases,but the effect weakens as duty ratio continuous to increase.The simulation results show that the effect is better when select current is 250 A,and frequency is 20 Hz.When choosing duty ratio is 50%,it not only makes the stirring intensity powerful,but also makes the melt temperature field uniformity distributed.Low-frequency alternating magnetic field,pulse magnetic field and out-phase pulse magnetic field can all produce strong melt convection in the crystallizer,increase melt flow speed,make the temperature more uniform and the liquid curve shallow.Low-frequency alternating magnetic field makes the turbulence region in the melt larger and flow rate faster.However,the pulsed magnetic field and out-phase pulse magnetic field make the melt a greater velocity amplitude,and repeat tension&compression.The stretch and compression duration of melt under out-phase pulse magnetic field is longer than pulsed magnetic field.In alternating magnetic field,the utilization of magnetic energy is the lowest,while the magnetic energy utilization efficiency is highest under out-phase magnetic field.The melt temperature in crystallizer under alternating magnetic field is obviously lower than pulsed magnetic field and out-phase pulsed magnetic field,however pulsed magnetic field and out-phase pulsed magnetic field own same temperature distribution. |
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