Observation of porosity distribution in Al-4.4%Mg DC slab and porosity formation.

Porosity generation situation was observed from surface to center in a DC slab of Al–4.4%Mg alloy. The quantities and area fractions of the porosity were measured quantitatively by using an image analysis apparatus (LUZEX). An explanation of this kind of the porosity generation mechanisms and distribution of the porosity was attempted by using a local equivalent pressure in dendritic solidification. The next conclusion was obtained, (1) The quantities of the porosity increase with decreasing the local equivalent pressure. (2) Distributions of the quantities of porosity correspond to the distribution of the area fractions in the slab, both increase to 180 mm, after that they decrease until the center (203 mm). (3) In the surface layer from surface to 60 mm, the porosity sizes are very small and almost exist between secondary dendrite arm. In the middle layer from 80 mm to 140 mm, the porosity sizes become large and the quantities of the porosity become much too. The most of the porosity exist between secondary dendrite arm and a part distributes along grain boundaries. In the center layer from 160 mm to 203 mm, the porosity sizes become more large and they exist not only between the dendrite arm but also the grain boundaries. The shapes of porosity are irregular and a typical shrinkage porosity.