AZ31 alloy billets of 200 mm in diameter were produced by three different processes of conventional direct chill(DC) casting,low-frequency electromagnetic casting(LFEC) and low-frequency electromagnetic vibration casting(LFEVC),respectively.The effect of LFEC and LFEVC on the microstructures,macrosegregation and mechanical properties of AZ31 alloy billets was investigated.In conventional DC casting,the AZ31 alloy billets exhibited coarse grains(about 370 μm) and severe segregation of Al and Zn.In the presence of a solo low-frequency alternating magnetic field or a low-frequency electromagnetic vibration field applied during DC casting of φ200 mm AZ31 billets,grains in the AZ31 alloy billets were effectively refined(about 210 μm) and the macrosegregation of Al and Zn in the billets was greatly decreased.Furthermore,the tensile strength,fracture elongation and hardness of the as-cast AZ31 alloy billets were improved by the processes of LFEC and LFEVC relative to that cast by the process of conventional DC casting.
The effects of ultrasonic vibration on the grain size and morphology of Mg2Si in Mg-4 wt% Al-1 wt%Si(AS41) alloys designed were evaluated. The results show that the major constituents of the alloy include β-Mg17Al12 and Mg2Si phase, and no difference in the type of constituents between without ultrasonic vibration and with ultrasonic vibration. Without any ultrasonic vibration, the grain size and Mg2Si phase in AS41 alloy are coare structure. However, the microstructure with fine uniform grains and Mg2Si particles are achieved with ultrasonic vibration. The crystal grains and Mg2Si particles refine with increase in the ultrasonic vibration intensity. When the ultrasonic vibration intensity was too low or too high, coarse structures could be obtained. The analysis of refinement mechanism indicates that the acoustic cavitation and flows induced by ultrasonic vibration lead to the fine uniform microstructure.
