Preparation of Mg-Nd-Zn-(Zr) alloys semisolid slurry by electromagnetic stirring

摘要

The effects of electromagnetic stirring (EMS) including stirring time, applied voltage and rotational frequency on the microstructure of semisolid slurries of the Mg-Nd-Zn-(Zr) alloys were investigated. The results indicate that all of the parameters have large effects on the microstructure of the slurries. After being treated by EMS, the morphology of primary a-Mg phases in the Mg-3Nd-0.2Zn (NZ30) alloy slurries evolves from dendrite to three types: rosette, dendrite, and spheroid, while a semisolid microstructure with small and spheroidal particles is obtained in the Mg-3Nd-0.2Zn-0.4Zr (NZ30K) alloy slurries. The data for solid fraction with stirring time can be fitted to linear equations. The increase of applied voltage and rotational frequency makes the primary a-Mg phases initially refined and then coarsened. The optimal processing parameters are stirring time 120-180 s, applied voltage 300-350 V, rotational frequency ～20 Hz for NZ30 alloy, and stirring time 30-180 s, applied voltage ～350 V, rotational frequency ～20 Hz for NZ30K alloy. The average particle size of NZ30 alloy can be refined from ～879 to ～457 μm. Furthermore, the effects of applied voltage and rotational frequency on undercooling and melt temperature were also discussed on the basis of thermodynamic theory.