Revealing the Mechanisms for the Nucleation and Formation of Equiaxed Grains in Commercial Purity Aluminum by Fluid-Solid Coupling Induced by a Pulsed Magnetic Field

Abstract

This study considers the factors controlling the grain structure of commercially pure aluminium when a Pulsed Magnetic Field (PMF) is applied during solidification. It is revealed that PMF of pure metal forms equiaxed grains by two different nucleation and growth mechanisms depending on the casting conditions. One mechanism is nucleation of the columnar grains that grow into a shell on the walls of the mold before PMF is applied and once applied fluid-solid coupling develops due to the Lorenz force. Depending on the shell thickness, this fluid-solid coupling detaches the columnar shell or, for thicker shells, only the top few millimeters of the shell from the mold wall. The detached shell is then fragmented into large blocky grains due to a lower melting point iron-rich liquid phase on the grain boundaries. The other mechanism occurs on the mold wall when PMF is applied from above the melting point or on the exposed mold wall after detachment of the initially formed solidified shell as in the case above. In these conditions copious nucleation occurs on the mold wall and these fine grains are detached by pulses occurring every few milliseconds creating a bimodal structure of refined grains between the much fewer large blocky grains. The optimal condition for uniform refinement is when PMF is applied from above the melting point ensuring that a refined equiaxed grain structure forms throughout the casting.