Effect of microstructure on pore morphology in Al-Mg-Si alloys fabricated by unidirectional solidification

Aluminum and Al-Mg-Si alloys ingots with pores were fabricated by unidirectional solidification through thermal decomposition of Ca(OH) ₂ powders. The porosity of aluminum and Al-Mg-Si alloys is 10-17% and 0.1-2%, respectively. While pores with 250-400 μm in diameter were observed in a grain or across several grains in the aluminum ingot, 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 (0.2-0.4mass%), the directional pores are aligned between columnar dendrites grown in the unidirectional solidification. With higher Mg and Si contents, the equiaxed dendrite zone with spherical pores were observed in a region with a low temperature gradient. The results of thermal analyses showed that constitutional supercooling tends to occur with increase in Mg and Si contents and with a low temperature gradient at the solid-liquid interface, and thus spherical pores are evolved. Thus, it is concluded that the pore growth direction has a strong relation with the direction of dendrites.