Effect of Bi-Sr interactions on Si morphology in a 319-type aluminum alloy

摘要

Samples of a 319-type aluminum alloy containing various concentrations of strontium and bismuth were prepared. A total of 85 cooling curves were recorded at varying Sr and Bi levels. The Sr level ranged from less than 0.001 to 0.020 wt%, and the Bi level varied between 0.0140 and 0.7067 wt%. As a result of varying Sr and Bi levels, the Al-Si eutectic temperature ranged from 556.0℃ to 568.1 ℃. Thermal, chemical and microstructural data were collected and analyzed to determine what factors influenced the changes in eutectic temperature and the modification level of the alloy. At Sr:Bi ratios (by weight) below 0.20, the Si structure was unmodified and the eutectic temperature remained between 564℃ and 568℃. At Sr:Bi ratios between 0.20 and 0.45, the eutectic temperature dropped sharply from 564℃ to 558℃, with the modification level increasing with increasing Sr:Bi ratios. At ratios above 0.45, the Si structure was modified and eutectic temperatures remained fairly consistent between 556℃ and 558℃. We conclude that, for Sr to modify the Si eutectic morphology, an Sr:Bi ratio of at least 0.45 must be maintained.