Structure and mechanical properties of AZ31 magnesium alloy billets by different hot-top semi-continuous casting technology

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.