Abstract
In the present study, the structural modification of sand cast Al-12wt%Si alloy with sulfur/sodium and its effect on mechanical properties were investigated. Different addition levels of sulfur and sodium were used to modify and produce castings of the same shape and size from the alloy. The results indicated that the addition of sodium or sulfur to eutectic Al-Si alloy can modify the Al-Si eutectic morphology from needle-like eutectic silicon structure to fine-scale eutectic silicon structure with significant improvement in mechanical properties of the alloy. The optimum levels of modification by sodium flux (60% NaF and 40% NaCl) and sulfur were found to be 0.6% - 1.0% and 0.02% - 0.05% of the weight of the alloy respectively. The alloy modified with 0.6% Na flux had the best mechanical properties closely followed by the one modified with 0.02% sulfur. Over modification of the alloy with sodium produced over modification band which consisted of aluminum dendrites and coarse silicon particles in the microstructure of the alloy. Increase in concentration of sulfur decreased the degree of fineness of the eutectic silicon structure with significant decrease in mechanical properties of the alloy and this is suggested to be as a result of the presence of a brittle sulfur compound at the grain interfaces of the alloy.
Highlights
Pacz [1] discovered modification of Al-Si alloys and since experimental studies on modification of Al-Si alloys are still going on till today
The results indicated that the addition of sodium or sulfur to eutectic Al-Si alloy can modify the Al-Si eutectic morphology from needle-like eutectic silicon structure to fine-scale eutectic silicon structure with significant improvement in mechanical properties of the alloy
Over modification of the alloy with sodium produced over modification band which consisted of aluminum dendrites and coarse silicon particles in the microstructure of the alloy
Summary
Pacz [1] discovered modification of Al-Si alloys and since experimental studies on modification of Al-Si alloys are still going on till today. According to restricted nucleation theory, modifier neutralizes the heterogeneous nuclei of AlP which is a nucleant for eutectic silicon or it reduces the diffusion coefficient of silicon in the melt This enhances undercooling of the melt before eutectic silicon solidification can take place, refining of the eutectic silicon structure occurs. According to restricted growth theories, the adsorption of modifier preferentially takes place on twin re-entrant grooves or growing surfaces of the silicon phase, restricting the silicon growth during eutectic silicon growth, and requiring eutectic solidification to occur at large undercooling This causes the eutectic silicon phase to grow isotropically with a fine fibrous structure rather than anisotropically with a faceted (plate-like) structure. In this work sodium in the form of sodium flux (60% NaF and 40% NaCl) and elemental sulfur were used to modify eutectic Al-Si alloys and their individual effectiveness as eutectic modifier analyzed and compared
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