Effect of cooling rate and Mg content on the Al-Si eutectic for Al-Si-Cu-Mg alloys

This study investigates and clarifies the qualitative and quantitative effects of Mg content and cooling rate (ranging from 0.5 to 4 degree C/s), on the modification of the silicon eutectic structure and on the undercooling of the silicon eutectic growth temperature ( Delta TSi-eut) in the series of Al-Si-Cu-Mg alloys. The critical Mg content to produce a notable improvement in the silicon eutectic by 1.5 modification levels (regardless of the cooling rate) is 0.6wt.% Mg. A similar increase in the modification level was also observed when the cooling rate was increased to a maximum of 4 degree C/s, regardless of the Mg content. Measurements of the area and roundness of the silicon particles showed a good correlation with the modification level. The undercooling ( Delta TSi-eut) increased by up to ~23 degree C at a relatively high Mg content and cooling rate and up to ~14 degree C when the Mg content was increased from 0.4 to 0.6wt.%.