Measurement of the nucleation and growth of microporosity during the solidification of an aluminum - 7% silicon casting alloy

A quench during solidification method was used to determine the nucleation and growth characteristics of microporosity in an aluminum - 7% silicon (A356) alloy. The process consisted of quenching samples of solidifying metal at different fractions of solid in order to track the change in pore density and percent porosity during freezing. Five hydrogen levels, and one local solidification time (215 seconds) were studied. Experimental results indicated that there were two modes of porosity formation; one at low hydrogen levels and a different one at high hydrogen levels. It was also shown that the fraction of solid at which pores begin to nucleate is independent of the hydrogen content while the fraction of solid at which pores begin to grow is strongly dependent on the amount of hydrogen present in the melt.