Study on crystallization and microstructure for new series of Al–Sn–Si alloys

The crystallization behaviors and microstructure for new series of Al–Sn–Si alloys have been investigated with the aids of OM, SEM, XRD and DTA in the paper. The phase transformations of the crystallization processes, solidification structure and its control method for the Al–Sn–Si alloys were determined. It was first proposed that the series of the Al–Sn–Si alloys belong to ternary eutectic system, and the equilibrium diagram of the ternary system was inferred. Influences of Sn and Si content, homogenization treatment on the microstructure were ascertained. The spheroidization model of β(Sn) and Si phases were first proposed. During the solidification of the alloys, at first, the primary grains separate from liquid phases (L→α(Al)); then binary eutectic forms (L→α(Al)+Si); finally, ternary eutectic reaction occurs at the temperature of about 225°C (L→α(Al)+Si+β(Sn)). The solidification structure of the alloys consists of α(Al), β(Sn) and Si phases. β(Sn) phases attach themselves to eutectic Si phases and distribute at matrix grain boundary as near-network structure. Modification can not refine only α(Al) and Si phases, but also make β(Sn) phases well distributed. β(Sn) phases gathered and were spheroidized around silicon particles which act as nuclei during homogenization. The ‘peritectic-type’ island structure of silicon surrounded with tin was formed. Tin thickness of the structure increases with increasing tin content or decreasing silicon contents. The ratio of tin to silicon content (Sn/Si) influences the volume fraction and distribution of ‘peritectic-type’ island structure. Only in the Sn/Si ratio range of 3∼4, the best microstructure was formed.