Formation mechanism of ring-like segregation and structure during directional solidification under axial static magnetic field

The effect of the axial static magnetic field on the macro-segregation and structure in the Al-Cu and Ni-Mn-Ga alloys during directional solidification is investigated experimentally and numerically. It is found that the ring-like segregation and structure in the above-mentioned two alloys form during directional solidification at a certain growth speed under a moderate magnetic field. For the Al-Cu and Ni-Mn-Ga alloys, the moderate values of the magnetic field under which the ring-like structure forms are about 0.5 T and 1.0 T at respective growth speed of 10 μm/s and 5 μm/s. Further, the distributions of the flow and solute in the Al-Cu alloy during directional solidification under the axial static magnetic field is numerically simulated. Numerical results reveal that the rotary thermoelectric (TE) magnetic convection forms in the mushy zone during directional solidification under an axial magnetic field. This flow will induce the formation of the ring-like macro-segregation and structure. Changes in structures under the magnetic field in the experimental results are in good agreement with the distributions of the TE magnetic convection and solute in the numerical results. Therefore, the formation of the ring-like structure and segregation under the magnetic field should be attributed to the solute redistribution induced by the TE magnetic convection.