Formation mechanism of surface exudation in Cu-Sn alloy prepared by horizontal continuous unidirectional solidification

The Cu-5wt.%Sn alloy was prepared by using the horizontal continuous unidirectional so-lidification (HCUS) technique. The microstructure of HCUS Cu-5wt.%Sn alloy was analyzed by optical microscope and scanning electron microscope. Meanwhile, numerical simulation software ProCAST was used to simulate the Sn distribution of HCUS Cu 5wt.%Sn alloy. The results show that there exists a serious exudation layer on the lower surface of the HCUS Cu-5wt.%Sn alloy, while there is almost no exudation layer on the upper surface of the alloy. The simulation results show that the solid-liquid interface in the center protrudes to the liquid during the HCUS process. A narrow gap is formed between the mold and the alloy. During solidification, Sn solute is precipitated from solid to liquid. Therefore, the liquid between dendrites has a low melting point and does not solidify at high mold temperatures. These liquids flow down into the gap due to gravity and solidify at the last stage, forming an exudation layer on the lower surface enriched by Sn solute.