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In this study, A390 alloy was prepared using the combined process of a water-cooled copper serpentine pouring channel (SPC) and strontium (Sr) modifier, in order to simultaneously refine primary silicon (Si) and modify eutectic silicon (Si). The nucleation and growth mechanisms of the Si phase were discussed by morphology analysis and non-isothermal analytical kinetics. The results indicate that the size of primary Si is refined to 25.2–28.5 µm and the morphology of eutectic Si is modified from acicular into fibrous. The serpentine pouring channel process stimulates primary Si nucleation due to chilling effect and has no influence on eutectic Si nucleation. Impacts of Sr-modifier on primary and eutectic Si are similar, including three aspects: (1) poisoning of the nucleation site; (2) decreasing the interface energy between Si phase and liquid; (3) raising the activation energy for diffusion across solid-liquid interface. The content of Sr determines which one of the three aspects mentioned above is the dominant factor to promote or restrain the nucleation and growth of the primary and eutectic Si in hypereutectic Al-Si alloy.

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In order to refine both primary and eutectic Si in the A390 aluminum alloy to improve the mechanical properties, a combined process of the serpentine pouring channel (SPC) and Sb-modifier was developed to prepare a semi-solid slurry for the rheological high pressure die casting (Rheo-HPDC). The results indicate that the refinement of primary Si particles (PSPs) is attributed to the chilling and selfstirring induced by the SPC. Antimony has a negligible effect on the size of the PSPs and plays an important role in the refinement of eutectic Si. The Sb-rich intermetallic compounds (i.e., AlSb and Mg 3 Sb 2 ) cannot act as a heterogeneous nucleation site for the PSPs. The AlSb phase enclosed in the PSPs becomes the crack initiation of the PSPs. The refinement of primary and eutectic Si as well as the preparation temperature of semi-solid slurry (T PS ) are the crucial reasons for improving the mechanical properties of Rheo-HPDC A390-xSb alloy. The optimal ultimate tensile strength (UTS) and elongation (El) are 257 ± 6 MPa and 0.52 ± 0.04%, respectively, for the Rheo-HPDC A390-0.3 wt.% Sb at T PS of 680 °C.