Structure evolution of AlSi6.5Cu2.8Mg alloy in semi-solid remelting processing

摘要

The transformation from solid to liquid and the structure evolution of primary α crystals of AlSi6.5Cu2.8Mg alloy were investigated in semi-solid remelting condition. Results show that the structure evolution of the alloy consists of three stages: liquid increasing, composition change of liquid from eutectic to hypoeutectic, and uniformity of hypoeutectic liquid. The α crystals grow in forms of the coalescence and the collision-amalgamation in the first and second stages and in the form of Ostwald ripening in the third stage. It was found from statistical analysis that 1) the power exponent in the parabolic function relation between α crystal diameter and remelting time changes with remelting temperatures, 2) the exponents with respect to remelting temperatures of 573℃, 578℃ and 584℃ are 4.004, 2.38 and 1.84, respectively, and 3) convection intensity in the semi-solid melt is a main factor to influence these exponents. It is found in this investigation that there is another mechanism of coalescence growth, which is responsible for the high exponent at low temperature. And at high temperature, the minimum value of the exponent is 1 (due to) Ostwald ripening growth.