The character of melt of A357 alloy and its influences on the solidification microstructures

The relativity between the alloy melt state and solidification microstructure has been received great interesting recently. Some previous studies proved that control the state of alloy melt can improve the microstructure and properties. In this paper, the character of melt of A357 alloy between 700°C-1150°C was studied by using DSC. The result indicates that there are exothermic reactions over liquidus which corresponds to dissolution of crystalline particle or cluster in the melt. In order to investigate the effects of character of melt on the microstructure, the alloys were first superheated at 720°C,820°C and 925°C respectively with different character of melt, held for 20min, then the melts cooled to 720°C before quenching in Ga-In-Sn melt. The microstructure of samples was analyzed. The results show that with the increase of superheating temperature, the secondary dendrite arm spacing (SDAS) of α-Al trends to decrease, and the eutectic silicon becomes fine and dispersion and its morphology becomes spherity instead of coarse platelike. These results can be explained by improving of uniformity of the melt and the changing of the solidification parameters.