EFFECT OF CHILLS OF DIFFERENT MATERIALS ON MECHANICAL PROPERTIES OF Al11.9%Si ALUMINIUM ALLOY

Abstract

This research work investigated the effect of chills on mechanical properties of aluminium alloy. Green sand gating system was designed with incorporation of external chills. Brass, mild steel and cast iron were selected as chill materials for investigation. Plate castings of Al11.9%Si alloy were produced and cooled using the chills, each of which were made into nine (9) blocks of same shapes but varying thicknesses of 10mm, 15mm and 20mm. A control sample was cast without a chill for the purpose of making comparison. For every casting chilled with a chill material, test samples were taken at three regular distances from the point of contact between castings and chills to the free ends. The samples were tested for Ultimate Tensile Strength (UTS) and Brinell Hardness Number (BHN). The results revealed that UTS and BHN decreased with increase in distances from the edges of contact between the chills and castings and increased with increase in chills thicknesses. The highest volume (20mm thick) chills gave the maximum values (173MPa & 113BHN, 164MPa & 99BHN, and 158MPa & 83BHN for Brass, Cast iron and Mild steel respectively) across all chill materials used, both for tensile strength and hardness. Compared to the results of UTS and BHN obtain from unchilled casting, brass chill was the most effective, having increased the UTS and BHN by 15% and 34% respectively. Second to brass was cast iron which improved UTS by 10% and hardness by 28%. Mild steel was the least effective, having raised the properties by 7% for UTS and 13% for hardness