Microstructure evolution during solidification in a low superheat casting process of the Al-Mg2Si composites having excess Si: A phase field study

A two-dimensional phase field (PF) model of solidification during low superheat casting has been developed to grab insight into the microstructure formation mechanism of the Al-xMg 2 Si-ySi composite, having extra Si as the grain refining agent. The developed PF model employs a seed undercooling based nucleation model to simulate the formation of primary Mg 2 Si and alpha-Al grains, wherein the interfacial energy of the Al -melt interface is taken from literature, and a Molecular Dynamics (MD) model is employed to calculate the interfacial energy of the Mg 2 Si-melt interface. The simulation study predicts the microstructural parameters such as grain size, and sphericity of the primary Al and primary Mg 2 Si phases, as well identifies the appropriate numerical parameters such as mobility, anisotropy parameters to study the microstructure evolution in the Al-xMg 2 Si-ySi composites. The kinetics of grain growth of both alpha-Al and primary Mg 2 Si have been studied. Novelty of the study lies in developing a near -accurate PF model capable of optimising the weight fraction of Mg 2 Si and excess Si in the low superheat cast Al-xMg 2 Si-ySi composite system, in view of obtaining the lower grain size and higher sphericity of both primary Al and primary Mg 2 Si grains, which is in line with the experimental observations. The proposed PF model is capable of predicting the faceted growth of Mg 2 Si particles in the Al-xMg 2 Si-ySi composite, in presence of extra Si and during low superheat casting, and the dendritic growth of Mg 2 Si in the binary Al-Mg 2 Si composite having high Mg 2 Si weight fraction. Following the Jackson ' s model of interface growth, it has been demonstrated that the modification of kinetic coefficient leads to a transformation of morphology of Mg 2 Si particles from large dendritic to faceted ones.