THE EFFECT OF NA AND SR MODIFICATION ON SURFACE-TENSION AND VOLUMETRIC SHRINKAGE OF A356 ALLOY AND THEIR INFLUENCE ON POROSITY FORMATION

Modification of the eutectic Si in Al-Si foundry alloys by adding stro ntium or sodium is, unfortunately, accompanied by an increase of poros ity in the casting. In an attempt to understand the nature of this pro blem, this study used a sessile-drop method to investigate the effect of Sr and Na on surface tension and volumetric shrinkage, two probable causes of porosity occurrence. The addition of 0.01 wt pct Sr and 0.0 05 wt pct Na to A356 alloy decreases the surface tension of the liquid by about 19 and 10 pct, respectively, and may increase the volume shr inkage by about 12 pct. These changes to surface tension and volumetri c shrinkage promote the early formation of the pores during solidifica tion and give the availability of a longer period of growth prior to c omplete solidification, resulting in a larger pore size. The effect of surface tension on the pores is more significant than volumetric shri nkage. Although the predicted pore diameter increases with lower surfa ce tension or higher volumetric shrinkage, these two effects alone do not seem able to completely account for the observed increase in poros ity that is associated with modification.