The simultaneous application of variable frequency ultrasonic and low frequency electromagnetic fields in semi continuous casting of AZ80 magnesium alloy

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 heterogeneous nucleation and generate the large number of nuclei by enlarging cavitation area and acoustic pressure, while the main contribution of LEF was to transport these nuclei in the entire liquid cave. The same as grain size, the Mg17Al12 phases were also refined showing the increase of area fraction of tiny phase. The YS and UTS were remarkably increased after the combinational field treatment, showing the values of 96.8-105 MPa (YS) and 165.5-214.3 MPa (UTS) increased from 73.1 to 84.4 MPa (YS) and 138-153.7 MPa (UTS) of the untreated billet. Finally, the relation between UTS and grain size was established using Hall-Petch relation as a bridge.