Effects of Strontium, Magnesium Addition, Temperature Gradient, and Growth Velocity on Al-Si Eutectic Growth in a Unidirectionally- solidified Al-13 wt% Si Alloy

Al-Si eutectic growth mechanism was investigated in a directionally-solidified Al-13 wt% Si alloy with different strontium (Sr) and magnesium (Mg) additions, growth velocities and temperature gradients. Macro-and microscale metallographic analyses revealed that addition level of Sr and Mg, temperature gradient and growth velocity are important factors affecting stability of solidifying Al-Si eutectic front and the final morphology of eutectic grains in the solidified Al-13 wt% Si alloys. By varying (tailoring) these factors, a variety of eutectic grain structures and morphologies such as planar front, cellular structure, a mix of cellular and columnar, or equiaxed dendrites, can be obtained. Increasing temperature gradient, reducing growth velocity, or decreasing Sr and Mg contents is beneficial to stabilizing planar growth front of eutectic grains, which is qualitatively in accordance with constitutional supercooling criterion for binary eutectic growth. In contrast, adding more Sr and Mg, increasing growth velocity, or decreasing temperature gradient produces large constitutional supercooling, leading to columnar-equiaxed transition (CET) of eutectic structure, which can be interpreted on the basis of Hunt's Model. It is also found that both solute concentration and solidification variables have significant impact not only on eutectic growth, but also on gas porosity formation.