Effects of an oscillation electromagnetic field on grain refinement and Al8Mn5 phase formation during direct-chill casting of AZ31B magnesium alloy

The effects of an oscillation electromagnetic field (EMF) generated by low frequency pulse current on the grain refinement and phase formation in direct-chill (DC) casting of AZ31B magnesium alloy have been investigated experimentally. The macrostructure evolution was quantitatively examined in terms of the grain size and its distribution, and fine equiaxed grains can be obtained. Given the Lorentz force and velocity variations, temperature distribution and heat extraction process during DC in different electromagnetic conditions, the grain refinement mechanism and the phase composition and formation of AZ31B magnesium alloy were discussed in detail. The grain sizes decrease from 549 ∼ 1094 μm (without EMF) to 402 ∼ 486 μm at the frequency of 15 Hz, and columnar grain region decreases significantly. Forced convection induced by EMF can strengthen the heat extraction along the diametrical direction of billet, weaken the contact heat transfer between melt and mold wall. For the formation of Mn-containing phases, eutectic transformations are dominant in DC casting process of AZ31B magnesium alloy, and the area fraction of eutectic Al8Mn5 phase decreases from center to edge of billet, and its distribution is more homogeneous in the presence of EMF.