Direct observation of the effect of strontium on porosity formation during the 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 eutectic. However, the addition of strontium also alters the amount and morphology of the porosity formed during solidification. The effect of strontium on porosity formation is not well understood and there is no consensus upon the cause of the changes observed. This paper investigates the effect of strontium, and its interaction with titanium-boride grain refiner, upon pore nucleation and growth using an experimental technique that allows the in situ observation of pore formation. The technique is a combination of a traditional temperature gradient stage¹ and real time micro-focus radiography, termed an x-ray temperature gradient stage or XTGS. The addition of strontium was found to depress the eutectic temperature causing an increased total time for pore growth; however, the initial growth rate of the pores was also reduced, producing little net change in final size. Strontium additions also altered the nucleation and stability of pores within the semi-solid region. Strontium in combination with titanium-boride grain refiner produced an even stronger change in the nucleation and stability of the pores, allowing pores that floated out of the semi-solid region to remain stable, rather than dissolving back into the liquid aluminium.