Effects of bismuth on the characteristics of eutectic Si particles in Na-modified A356 alloys

In this work, the effect of bismuth over Na-modified eutectic Si particles was investigated in an A356 alloy with different Bi and Na contents. Metallographic and thermal analysis techniques were used to examine the microstructural changes occurring at different Bi and Na levels. The cooling curves and the corresponding derivative curves were plotted to determine the nucleation T-N, minimum T-Min and growth T-G temperatures for the Al-Si eutectic reaction. The results indicate that the T-N, T-Min and T-G values decrease after Na modification. In the unmodified alloy, the eutectic Si shows a fully unmodified lamellar morphology. A fully modified structure is obtained with 55 ppm Na in the alloy containing 20 ppm Bi impurity, while the same level of Na can partially modify the alloy containing 200 ppm Bi. It was also determined that the Bi-Na-rich intermetallic in the microstructure causes counteraction and it can be a binary or triple eutectic system containing more than one compound.