Improved solidification structures and mechanical properties of Al–20 wt% Sn alloys processed by an electromagnetic vibration technique

Abstract

Homogenous distribution of the Sn phase in the Al–Sn alloy is beneficial for improved properties. However, the alloy has a metastable immiscibility gap that results in macrosegregation of phases in a usual casting. In the present study, the alloy with an actual composition of Al–20 wt% Sn–1 wt% Cu (referred to Al–20 wt% Sn hereafter for simplicity) was solidified in a static magnetic field when alternating current flowed through the alloy, from which electromagnetic vibration (EMV) was imposed upon the solidifying liquid. Solidified microstructures were observed and characterized in terms of the low-melting-point Sn phase. It was found that when EMV was imposed at the frequency of f = 100 Hz, the Sn phase could be refined and dispersed homogenously in the primary Al matrix, which formed a contrast with the continuous Sn network in a usual casting solidified without EMV imposition. The specimen with fine Sn dispersions had an improved elongation in tensile tests. The solidification mechanism was discussed in terms of the structure formation and solute distribution when EMV was imposed during directional solidification. The uncoupled movement between the primary Al solid solution phase and the surrounding liquid may breakdown the Sn liquid film into discrete segments and thus form isolated particles at the final solidification stage. Meanwhile, this uncoupled movement can induce forced fluid flow that can stir the melt to homogenize the solute distribution in the remaining liquid pool. The composition transition was considered when EMV was imposed. To better support our analysis on the chemical composition variation during EMV processing, optical microstructures and differential scanning calorimetry (DSC) profiles were supplemented from a continuously cast Al–7 wt% Si alloy billet. The results of the Al–20 wt% Sn and Al–7 wt% Si alloys demonstrate that the EMV technique should be quite effective and applicable in improving solidification structures and mechanical properties provided that it is proceeded under appropriate conditions.