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Considering the greatly significance of high quality billet in industrial manufacture and subsequent deformation process, special casting technologies are employed usually. In this present work, traditional fixed-frequency ultrasonic field (FUF), low frequency electromagnetic field (LEF), variable-frequency ultrasonic field (VUF) and the combinational field of VUF+ LEF were introduced to refine the as-cast microstructure and promote the mechanical properties of AZ80 magnesium alloy during the semi-continuous casting. The results showed that all kinds of the external field treatments have their ability to refine the grain, while the combinational field represents the best efficiency in terms of grain refinement with the grain size of 116–141 μm decreased from 679–1454 μm (untreated billet) in the∅ 255 mm billet. The homogeneity of grains was improved dramatically as well. The main role of VUF was to promote …