Formation Mechanism of Three-Layer Solidified Structure in Commercial Purity Al During Solidification With a Forced Flow Field Induced by Pulsed Magnetic Field

In the present study, the effect of pulsed magnetic field (PMF) on the solidification structure of commercial purity Al during unidirectional solidification was investigated through a series of experiments. The results show that columnar grains are formed without PMF, whereas a refined equiaxed grain layer with a smooth interface inside the large columnar dendrites is formed by controlling application of PMF. The nucleation sites can be ascribed to the direct heterogeneous nucleation in the bulk liquid and the detachment of grain nucleus from the mold wall. With increasing nucleation sites, the blockage of the columnar front and columnar-to-equiaxed transition occurs based on a threshold value of the volumetric fraction of equiaxed grains, which can be proven by the delay of equiaxed grain formation after switching on the PMF. After the magnetic field is switched off, the nucleation sites are full of liquid metal and settle down along the direction of gravity. At this time, the temperature gradient of the liquid is rebuilt, and finally a three-layer solidified structure is formed. The study is beneficial to deepen the understanding of solidification structure evolution of metals with a forced melt flow and temperature field.