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

• Addition of Mg into alloys increases the modification level of silicon particles.
• A critical Mg content of 0.6% allows better modification level.
• The best modification level was achieved combining the cooling rate and Mg content.
• The silicon particle's area showed a good correlation with modification level.
• Eutectic undercooling is strongly affected with Mg contents above critical level.

This study investigates and clarifies the qualitative and quantitative effects of Mg content and cooling rate (ranging from 0.5 to 4 °C/s), on the modification of the silicon eutectic structure and on the undercooling of the silicon eutectic growth temperature (ΔTSi-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 0.6 wt.% Mg. A similar increase in the modification level was also observed when the cooling rate was increased to a maximum of 4 °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 (ΔTSi-eut) increased by up to ~ 23 °C at a relatively high Mg content and cooling rate and up to ~ 14 °C when the Mg content was increased from 0.4 to 0.6 wt.%.