Solidification Conditions, Heat Treatment and Tensile Ductility of Al-7Si-0.4Mg Casting Alloys

摘要

An extraction technique has been used to study the size of eutectic silicon particles and flakes, after solution heat treatment, in samples solidified at different rates. It was found that, in samples solidified at 1℃/s or faster, the Si particles attain approximately the same size with solution treatment. For samples solidified at slower rates, the size of the Si particles and flakes increases rapidly with decreasing freezing rate. It is shown that the increase in the size of the Si flakes correlates with a decrease in tensile ductility observed for coarse microstructures. These results are used to discuss the relationship between the cooling rate during solidification, the dendrite cell size and interparticle spacing in the as-cast structure, and the Si particle size and tensile ductility after heat treatment. The analysis indicates that the relative susceptibility of the microstructure to the solution heat treatment and, hence, the relative improvement in tensile ductility with solution treatment depends on the eutectic interparticle spacing. Freezing rates faster than about 1℃/s result in eutectic spacings between 1 and 4 μm. These relatively fine eutectic structures break down and ripen to a uniform particle size of about 4-5 μm during solution heat treatment, resulting in a material with moderate-to-high ductility. Freezing rates of less than about 0.5℃/s result in large eutectic spacings and coarse flakes, which are not refined by solution treatment. Little or no improvement in the ductility can thus be expected, through solution treatment of these coarse microstructures.