Study On Heat Transfer And Solidification Of Magnesium Alloy By LFEC Processing

Magnesium alloy, as the lightest structure materials, has been widely applied in military, aerospace, automobile and electronic communication industries due to its advantages of high specific strength, high stiffness, good damping capacity and electromagnetic shielding. However, magnesium alloys are applied mostly in form of cast magnesium instead of wrought magnesium. It is important to develop wrought magnesium billet which is the key point for developing wrought magnesium alloy products. Because of the low heat conductivity, low heat capacity and the high oxidizability characteristics, as the main method for magnesium billet preparation, semi-continuous direct chillling (DC) casting processing, poor quality metallurgy internal billet, great crack trend and low finished product rate by using this processing. Low frequency electromagnetic semicontinuous casting processing (LFEC) developed by Northeastern University, can provide high quality billet with characterists including refined grains, lower macrosegregation, lower crack tendenty and higher surface quality. But it is still not systematic profound study on how the electromagnetic field to change the temperature by forced convection, thus chang the solidification behavior of magnesium alloys. This paper, as an important research content of " the basis research of magnesium alloy by liquid forming " in National Key Fundamental Research Program(973), resaerched the effects of electromagnetic parameters on slurry shape and temperature field in liquid zone,etc. via continuous collecting temperature of the solidification process during semi-continuous casting Φ D160mm billet of AZ80-lmass%Y magnesium alloy. Meanwhile, the relationship between heat transfer behavior and solidification microstrctures was investigated, and the basic process control technology to prepare high-quality magnesium alloy billet was also investigated. The following are the main obtained results:(1) Casting speed has significant influence on heat transfer behaviors of Magnesium alloy semi-continuous casting. Low speed and large amount of water are available to lager the curvature radius at the bottom of sump and sluury zone, reduce the depth of sump. LFEC can significantly reduce the sump depth, and reduce temperature gradient difference at different parts of the billet. Meanwhile, the number of small temperature disturbance within liquid zone strengthened the energy fluctuations, which increase the nucleation rate, may result in microstructure refinement; (2) Temperature collecting results of mold sleeves show that the effect of second cooling water on sleeve temperature is of great influence. Soft contect of electromagnetic field reduce first colling indesity and the sleeve temperature significantly. Meanwhile, stainless steel sleeve can not be used because of low thermal conductivity;(3) The results indicate that the grain size is closely related to the temperature gradient of slurry zone. Improve the temperature gradient, grain refinement can be achieved, while dendritic growth is degressed and secondary dendrite arm spacing is reduced; columnar crystal direction is almost parallel to the direction of the temperature gradient of slurry zone. In experimental conditions, to improve casting speed or increase the amount of cooling water can reduce slurry temperature gradient difference, narrow columnar crystal area, reduce microstructure difference. LFEC casting process can reduce or even eliminate columnar grain in the center of billet, degress dendritic growth, reduce microstrcture differences and reduce the inverse segregation of Al, Zn alloy elements;(4) Hot deformation experiments show that the appropriate LFEC casting processing (under the experimental conditions, the best conditions was30Hz,60A) can reduce the hot deformation behavior difference between different part of magnesium alloy significantly. And this trend is more significant under high strain rate or low hot deformation temperature, that was, LFEC casting billet performanced better at high strain rate or low-temperature deformation. Meanwhile, LFEC cast billets had higher thermal activation energy of deformation, and the activation energy difference between cross section of billets reduced significantly, the difference of DC casting billets was25.654kJ/mol, while the LFEC is4.128kJ/mol, which shows that electromagnetic fields could significantly improve the billet uniformity of thermal deformation.