MODELING OF MICROPOROSITY FORMATION IN ALUMINUM-ALLOYS CASTINGS

The solidification of Al-7wt.%Si alloy in sand mold geometries has been simulated using a 2D-finite volumes model which takes full account of coupled heat, mass and momentum transport phenomena. The shrinkage boundary condition used in this model is detailed, and results of the simulation are presented in terms of pressure drops which are Viewed sometimes as one of the major causes of micropore formation in aluminium castings. Solidification of plates of different thicknesses in sand mold has been investigated. Calculations give a maximum absolute pressure drop of 27 000 Pa in the 7.5 mm thick casting whereas this maximum is only 300 Pa in the 30 mm thick casting. Considering the condition of microporosity formation, these results point out that pressure drops are far too low to account for the formation of micropores which are observed experimentally. Hydrogen segregation has to be considered and integrated in a model to give a good prediction of micropore appearance.