Porosity distribution in directionally solidified test bars sand cast from a controlled A356 melt

Heat transfer modelling of directionally cast A356 bars has been used to obtain values for all relevant thermal and solidification parameters throughout the freezing process. Porosity distributions of unmodified and Sr-modified bars were compared to previously proposed criteria functions for microporosity distributions. It was shown that porosity increases continuously from the chill to the feeder ends of the bars. Notwithstanding the obvious influence of other factors such as solidification shrinkage or mass feeding, this behaviour could be explained by the hydrogen build-up occurring in the remaining liquid as solidification proceeds.Experimental results confirm the higher microporosity usually associated with modified castings and the fundamentally different distribution between micro- and macro-shrinkage for the unmodified and modified conditions. From a practical standpoint, the average rate of solidification between solidus and liquidus temperatures, or the solidification time at a given point, were found to be adequate and simple criteria to which microporosity distribution may be related.