Effect of a Transverse Magnetic Field on Solidification Structures in Unmodified and Sr-Modified Al-7wtpctSi Alloys During Directional Solidification

The influence of a transverse magnetic field on the microstructures in unmodified and Sr-modified Al-7wtpctSi alloys during directional solidification was investigated. Experimental results indicated that the magnetic field caused the channel and freckle macrosegregations during directional solidification. Comparison of the microstructures in unmodified and Sr-modified Al-7wtpctSi alloys showed that the Sr-addition enhanced the convection effects. Moreover, the EBSD analysis revealed that the magnetic field changed the alignment of the α-Al dendrite and modified the distribution of dendrite fragments in both unmodified and Sr-modified Al-7wtpctSi alloys. Indeed, the application of the magnetic field caused the 〈001〉-crystal direction of the α-Al dendrite to deflect from the solidification direction and induced the formation of dendrite fragments on one side of the sample. Further, the Seebeck signal (ES) at the liquid/solid interface was measured in situ during directional solidification of Al-7wtpct Si alloy and the results indicated that the value of the ES was of the order of 10 μV and decreased with the increase of the growth speed. The above results may be attributed to the thermoelectric magnetic convection and its effect on the distribution of the solute Si. It is proven that solute effects are primarily responsible for dendrite fragmentation.