The effect of na and Sr modification on surface tension and volumetric shrinkage of A356 alloy and their influence on po
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INTRODUCTION
THE A1-Si alloys are widely used in different applications because of their good mechanical properties, light weight, and good castability. The mechanical properties of these alloys can be improved significantly by altering the shape of the eutectic Si from acicular to fibrous, a process called eutectic modification. The eutectic structure is usually modified by adding sodium or strontium to the melt at levels in the range of 0.01 to 0.02 wt pct. Unfortunately, Na or Sr modification significantly increases the pore volume fraction and the pore size in the castings, tl-41 These effects for Srmodified 319 alloy are shown in Figures 1 and 2, which show that the addition of 0.03 wt pct Sr increases the porosity from 2 to 3 pct and increases the pore radius from 55 to 70/xm. These effects can be so great that no significant increase in mechanical properties may be observed, even though the microstructure of the modified alloy is more favorable. 151 Various suggestions have been put forth by different authors to explain this behavior. Denton and Spittle trl have suggested that modification aggravates the porosity problem by increasing the hydrogen content or the regassing rate of the melt. Some authors have contended that the addition of strontium does not increase the hydrogen concentration or the rate of regassing.[7,81 Others have suggested that strontium helps pore formation by increasing both the inclusion content of the melt and the amount of hydrogen absorbed into oxides. 12,31Argo and Gruzleski 14j have indicated that the increased porosity is D. EMADI, Graduate Student, and J.E. GRUZLESK[, Professor and Chair, are with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2A7. J.M. TOGURI, INCO-NSERC Professor, is with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S IA4. Manuscript submitted December 10, 1992.
METALLURGICAL TRANSACTIONS B
due to problems associated with interdendritic feeding, while others have contended that sodium improves the feeding capability, and strontium has no effect on the feeding ability of the alloy. [31 It has also been suggested that Na and Sr reduce the surface tension and aid in the nucleation of the pores, t2'4'81 Despite this multitude of suggestions, the exact mechanism has not yet been established. The increase in porosity observed in Sr- and Natreated A1-Si alloys may be caused by one or more of the following factors: an increase in the H2 content of the melt or rate of regassing after an initial degassing; an increase in both the inclusion content of the melt and the amount of hydrogen absorbed into the oxides; a reduction of the surface tension of the liquid; a decrease in the hydrogen solubility in the solid state or an increase in the liquid state; an increase in the volumetric shrinkage; a reduction in interdendritic feeding by (a) changing the solid-liquid interface shape, (b) increasing the length of the mushy zone, and (c) increasing the viscosity of t
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