DEVELOPMENT OF HIGH DUCTILITY ALUMINUM ALLOYS FOR DIE CASTING

Abstract

Aluminum-Silicon (Al-Si) alloys are often preferred in the die casting\nindustry due to excellent castability, high strength, corrosion resistance and\nlow cost. Commonly, iron (Fe) is alloyed with the alloys to prevent die\nsoldering. However, the addition of Fe in most of Al-Si alloys leads to\nformation of the intermetallic β-AlFeSi.\nThe β-AlFeSi is harmful\nto the alloy structural integrity due to its needle-like morphology that creates\nstress concentration at the microscopic level. The phase presence is\nunfavorable to the mechanical properties and significantly reduces the\nelongation of the alloys. This research attempted to find viable way to control\nthe morphology and formation of the β-AlFeSi\nphase.\n\nThermodynamic simulations were done to investigate the sequence of\nintermetallic formation and other phases at different alloy compositions. The analysis of solidification\npaths of different alloys provided the correlation between the phase formation\nsequence and the fraction of the β-AlFeSi phase. The analysis also identified the feasible region of alloy\ndesign for minimizing the β-AlFeSi formation. Based on the thermodynamics simulation analysis, five\nalloys of different compositions were designed to validate the finding of the\nsimulation. \n\nThe tensile test\nresults of the alloys indicated that lowering the Fe content increases the\nelongation of the alloy. The results also showed that elongation was reduced with\nthe increase of Si level due to the formation of eutectic Silicon. The change\nof both Fe and Mn did not significantly affect the mechanical property of the\nalloy when the ratio of Fe to Mn was constant. Microscopic analysis\nshowed that lowering the Fe level had effectively altered the morphology of the\nβ-AlFeSi needle\nlike structure. The β-AlFeSi\nwas found to be smaller in terms of size when Fe is lower, subsequently\nreducing the probability of β-AlFeSi\nphase to be stress riser and crack initiation. \n\nThe influence of heat treatment to the mechanical property of the alloys\nwas also studied. The mechanical result on the heat-treated samples indicated\nthat heat treatment is a viable method to improve the elongation property of\nthe alloy. Microscopic observations showed that the β-AlFeSi phase was broken into shorter structures\nover the solution heat treatment process, resulting in better elongation.