Effect of Refinement and Modification on Microstructure, Properties and Eutectic Silicon Growth Mechanism of Cast A356 Aluminum Alloy

The microstructures, mechanical properties and eutectic silicon growth mechanism of A356 aluminum alloy treated by novel Al-5Ti-1B-IRE and Al-10Sr master alloy with individual or composite refinement and modification were studied. The results show that under individual addition condition, the Al-5Ti-1B-1RE master alloy has a significant refinement effect on alpha-Al phase in A356 aluminum alloy and the tensile strength sigma(b), yield strength sigma(s) and Vickers hardness of the alloy are significantly improved. Al-10Sr master alloy has a strong modification effect on eutectic silicon, and the elongation delta of the alloy is obviously improved. Under composite addition condition, the shape and size of alpha-Al phase in A356 aluminum alloy become more uniform and finer, and the grain boundaries are clearer. The eutectic silicon phases are almost converted into dispersed, fine fibrous shape, and the lamellar eutectic silicon almost completely disappears. The length of the eutectic silicon is reduced from 40 similar to 60 mu m of the as-cast state to 1 similar to 2 mu m, achieving a complete modification effect. Its mechanical properties are much higher than those of the as-cast, any single refiner and modifier treated A356 aluminum alloy. The growth mode of eutectic Si in the unrefined and unmodified A356 aluminum alloy is typical small-plane step growth. The eutectic silicon with composite refinement and modification is mainly grown by the twin plane re-entrant edge mechanism, and the growth characteristics of the facet are gradually weakened until disappearance.