AZ80鎂合金低壓脈沖磁場半連續鑄造過程的數值模擬和實驗研究

AZ80鎂合金低壓脈沖磁場半連續鑄造過程的數值模擬和實驗研究Numerical simulation and experimental research of low voltage pulsed magnetic field DC casting of AZ80 magnesium alloy

采用有限元方法對AZ80鎂合金低壓脈沖磁場半連續鑄造過程的電磁場、流場和溫度場進行數值模擬，對鑄錠的組織進行觀察和分析，并與普通半連續鑄造鑄錠晶粒組織進行對比。模擬結果表明：在低壓脈沖磁場半連續鑄造過程中，結晶器中的熔體受脈沖電磁力作用產生強迫對流和電磁振蕩，使得熔體的徑向溫度梯度有所降低。組織觀察結果顯示，與普通半連續鑄錠相比，低壓脈沖磁場半連續鑄錠晶粒明顯細化，枝晶尖端發生鈍化。在低壓脈沖磁場半連續鑄造凝固過程中，結晶器壁附近形核的臨界形核半徑和臨界形核功有所降低，同時脈沖磁場形成的熔體對流使結晶器壁處形成的晶核隨著對流擴散到熔池內部，使形核率增加，另外脈沖磁場能夠通過尖端鈍化的方式抑制晶粒的生長，從而造成低壓脈沖磁場鑄錠的晶粒細化。

The electromagnetic field, flow field and temperature field during the low voltage pulsed magnetic field direct chill casting of AZ80 magnesium alloy were simulated with finite element method, and the microstructure of the billet was observed by comparing those of billet by conventional direct chill (DC) casting process. The simulation results indicate that, during the low voltage pulsed magnetic field DC casting process, affected by electromagnetic force, forced convection and velocity vibration generate in the melt, leading to the reduced temperature grade of melt along the radial direction. The results show that the grains of the billet cast with low voltage pulsed magnetic are significantly refined and the tip of dendrite is rounded, compared with those of the conventional one. During the low voltage pulsed magnetic field DC casting process, the critical nucleation radius and critical nucleation energy near the crystallizer decrease, at the same time, the nuclei formed ahead the inner wall of crystallizer can be dispersed into the centre of the molten pool with the convection that caused by the pulsed magnetic field, which causes the increased nucleation rate. In addition, the growth of dendrite tip is inhibited by the passivation effect of the pulsed magnetic field, which leads to the grain refinement in the low voltage pulsed magnetic field direct chill casting billet.

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