Porosity formation in Sr-modified Al-Si alloys

Modification of the eutectic silicon in Al-Si foundry alloys by adding strontium is accompanied by an increase of porosity in the casting. This effect on porosity is due to an increase in both the pore size and the pore number density. In an attempt to understand the nature of this problem, the effect of strontium on the probable causes for porosity occurrence due to modification has been investigated.Experimental findings indicate that the addition of strontium to Al-Si alloys increases the volumetric shrinkage due to an increase in solid density, and at the same time reduces the surface tension and increases the viscosity of the liquid. Metallographic observations show that Sr addition slightly decreases the dendrite arm spacing and changes the solid-liquid interface to a more regular shape. Moreover, Sr-modification decreases the eutectic temperature, and therefore, increases the length of the mushy zone while the total solidification time remains constant.In addition, Sr addition increases the melt inclusion content, but these inclusions do not have a significant effect on pore nucleation. Hydrogen measurement in the liquid shows that Sr-modification has no effect on the rate of melt hydrogen pick-up and does not introduce hydrogen into the melt. Strontium also reduces the hydrogen solubility in the liquid state but has virtually no effect on the solid state solubility.A solidification model for pore formation has been developed to study the significance of the changes in these parameters on porosity formation. Based on the experimental results and the theoretical analysis, it is concluded that the decrease in the hydrogen solubility in the liquid, the eutectic temperature (or the length of the mushy zone) and the surface tension are the reasons for the observed increase in porosity in modified alloys. Among these parameters, the decrease in the hydrogen solubility in the liquid plays the main role in causing enhanced porosity formation