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 aluminum when a Pulsed Magnetic Field (PMF) is applied during solidification. It is revealed that PMF of pure metal forms equiaxed grains by different nucleation and growth mechanisms depending on the casting conditions. One mechanism occurs when PMF is applied from above the melting point. On reaching the melting point copious nucleation occurs on the mold walls and these small grains are detached by pulses occurring every 100 milliseconds. Convection due the PMF generated Lorenz force distributes the grains in the melt resulting in a refined equiaxed grain structure throughout the casting. Another mechanism is nucleation of the columnar grains that form 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. In this case a bimodal grain structure is produced because the exposed wall generates fine grains as described above filling the gaps between the 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.