Refinement of intermetallic compounds and aluminum matrix in 3xxx aluminum alloys solidified by an electromagnetic vibration technique

The industrial 3xxx aluminum alloy was solidified in a superconducting magnet, in which electromagnetic vibration (EMV) was imposed throughout the entire solidification process. The microstructure was examined as a function of vibration frequency. It is revealed that the microstructure contains intermetallic compounds and aluminum solid solution (Al(ss)) regardless of solidification condition. For both structures, quantitative characterization shows that their average grain sizes decrease to minima when vibration frequency, f, increases and further increases of f leads to an increase of average grain sizes. Moreover, the general uniformity of intermetallic compound becomes poor when f goes beyond 1000 Hz. The microstructure transition behavior can be elucidated when considering the electrical resistivities of phases, i.e., Al(ss), intermetallic compounds and remaining liquid that coexist in mushy zone. The different electrical resistivities of these phases produce different Lorentz forces and thus lead to different vibration amplitudes and eventually generate uncoupled movement between the leading phase and sluggish phase during vibration. The microstructure transition can be clarified when the distance covered by the leading phase is revealed as a function of frequency. The morphological appearance of intermetallic compounds is dependent on f in EMV processing; however, their phase species and volume proportion are independent of processing conditions.