Direct observation of the effect of strontium on porosity formation duringthe solidification of aluminium-silicon alloys

Strontium is added to many aluminium-silicon casting alloys at the level of a few hundred parts per million in order to modify the structure of the eu tectic. However, the addition of strontium also alters the amount and morph ology of the porosity formed during solidification. The effect of strontium on porosity formation is not well understood and there is no consensus upo n the cause of the changes observed. This paper investigates the effect of strontium, and its interaction with titanium-boride grain refiner, upon por e nucleation and growth using an experimental technique that allows the in situ observation of pore formation. The technique is a combination of a tra ditional temperature gradient stage(1) and real time micro-focus radiograph y, termed an x-ray temperature gradient stage or XTGS. The addition of strontium was found to depress the eutectic temperature cau sing an increased total time for pore growth; however, the initial growth r ate of the pores was also reduced, producing little net change in final siz e. Strontium additions also altered the nucleation and stability of pores w ithin the semi-solid region. Strontium in combination with titanium-boride grain refiner produced an even stronger change in the nucleation and stabil ity of the pores, allowing pores that floated out of the semi-solid region to remain stable, rather than dissolving back into the liquid aluminium.