On the Effect of Strontium Additions over Macro-Shrinkage Porosity Defects in Hypo-eutectic Aluminum Alloys

摘要

Hypoeutectic aluminum-silicon alloys are nowadays widely used in many engineering sectors, such as automotive, aerospace or machinery construction owing to their favorable characteristics of good castability, reduced price and good mechanical properties. Their modification with strontium additions to achieve a fine eutectic structure has been long acknowledged in industry as a key factor to achieve optimum mechanical properties. Numerous researches, have shown, though, the negative effect of these additions on porosity (mainly micro-porosity) in well fed castings; however, very little works are available on the effect of strontium over macro-shrinkage defects in difficult to feed cast sections, as commonly found in industrial castings, such as bosses and other isolated thick areas. From these few works it would appear that strontium additions have the potential to reduce the extent of the defect in this kind of sections, although the exact mechanism of action remains unclear. Hence, this study aims to contribute to the knowledge on strontium additions in Al-Si alloys, with the focus in their effect and action mechanism over macro-shrinkage porosity defects. The research was driven to enlighten the influence of strontium over a critical factor for the feedability of these alloys: the dendrite coherency solid fraction, namely CSF. A series of thermal analysis experiments were performed to determine CSF for two alloys with different Sr contents: on one hand, the A356 alloy (Al-7%Si) of widespread use in industry, and on the other the binary Al-9%Si alloy. As main result, it was found that CSF increased with strontium content. The obtained CSFs were later used to perform numerical simulations of the sand casting of an Al-9%Si plate containing a boss region in its center, with and without strontium addition, following a separate experimental literature work. Consistently with this reference, it was found that the modified alloy yielded less macro-porosity in the boss region than the unmodified one. Furthermore, the modeled porosity values were found to be in good quantitative agreement with the experimental reference, indicating the soundness of the numerical simulation approach. Therefore, this study shows that, in spite up of their attributed draw backs in terms of generation of micro-porosity, strontium additions can be judiciously used to improve the quality of hypoeutectic aluminum-silicon castings, particularly in difficult to feed sections, on the grounds of their favorable effect on the CSF.