Characterization of the microstructure evolution of a semi-solid metal slurry during the early stages

A key success of semi-solid metal forming by the rheocasting route is having an efficient and effective technique to prepare semi-solid slurries. To achieve that it is necessary to be able to characterize the microstructure during the early stages at different solid fractions as the slurry is being cooled. This present work applied the rapid quenching method to study microstructure evolution during the early stages in a rheocasting process. Nine stages of microstructure evolution were characterized by applying rheocasting times of 5, 10, 12, 15, 20, 30, 35, 40 and 45 s during cooling. Analysis to evaluate the solid fraction, particle density, particle size, particle shape factor, and particle distribution was performed. The results show that the relationship between the solid fraction and the rheocasting time can be well fitted to a quadratic equation. For particle density, the relationship is better fitted to a linear equation. The change in particle size with time can be modeled using a ripening model with an upper bound value for the ripening coefficient. In addition, the particle distribution may be quantified by the dilation and counting technique to determine different levels of particle clustering. The results from this study suggest that this new characterization method can be used as a process and quality control tool in the rheocasting process. It can also be used to optimize the process and to study the mechanism of the rheocasting technique. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.