Solid–liquid interface dynamics during solidification of Al 7075–Al2O3np based metal matrix composites

Abstract

Interactions between the solidification front and the remaining suspended particles take place during solidification of particles reinforced composites. The particles are either engulfed by the solidification front or pushed by the solid–liquid interface. In this study, the relation of distribution and clustering degree of nano-Al2O3 in Al7075 with the microstructure of alloy was investigated by conducting casting experiment. A hydrodynamic model for calculating the critical interface velocity required to push nano-particles by the interface was proposed. In addition, the influence of the nano-particles present ahead of solidification front on interface shape was discussed. Microstructural studies showed that the resulting microstructures of composites depended on the size and degree of nano-Al2O3 clustering. Based on the model, it was found that for particles below approximately 36% of the critical radius, r<0.36 r⁎=0.9973μm, the critical interface velocities were four orders of magnitude lower than those obtained by micron particles, suggesting that smaller particles are easily engulfed by the interface rather than pushed. The experimental results were validated by the prediction of the model. Result showed that the constitutional super-cooling of local melts and the accumulation of solute behind the particle caused formation of a cellular interface.