Effect of conditions of unidirectional solidification on microstructure and pore morphology of Al-Mg-Si alloys

Aluminum and Al-Mg-Si alloy ingots with pores were fabricated by unidirectional solidification through thermal decomposition of Ca(OH) ₂ powders. The porosity of aluminum and Al-Mg-Si alloy were 10-17% and 0.1-2%, respectively. While the pores with 250-400 μm diameter were observed in a grain or across several grains in the aluminum ingots, smaller pores with 50-300 μm were observed in an eutectic region between primary α dendrites in the Al-Mg-Si ingots. In the alloys with Mg(0.25-0.5 mass%) and Si(0.2-0.4 mass%), the unidirectional pores were aligned between columnar dendrites grown in the unidirectional solidification. With higher Mg and Si contents, the equiaxed dendrite zones widi spherical pores were observed in a region with low temperature gradient. The results of thermal analysis showed that constitutional supercooling, which causes equiaxed dendrites, tends to occur with increase in Mg and Si contents and with low temperature gradient at the solid-liquid interface. Under this condition, spherical pores were evolved, because the surrounding or-dendrites solidified isotropicallv. Therefore, it is concluded that the pore growth direction is affected by morphology of dendrites.