SOLIDIFICATION BEHAVIOR AND GRAIN SIZE OF SAND CASTING Mg-6Al-xZn ALLOYS

The solidification behavior and microstructure evolution of sand cast Mg-6Al-xZn alloy (named as AZ6x alloys, x=0, 2, 4, 6, mass fraction, %) were characterized by two-thermocouple thermal analysis technology and SEM. The grain sizes of the alloys were quantitatively determined by EBSD technology. Thermodynamic calculations were applied in Pandat software for phase diagram calculation, Scheil model solidification simulation and growth restriction factor values (GRF or Q values). The results show that solidification of AZ6x alloys follows non-equilibrium solidification paths. Besides the gamma-Mg17Al12 phase, which is the only secondary phase in AZ60 alloy, another Phi-Mg-21(Al, Zn)(17) phase appears in the as-cast microstructure of AZ62 to AZ66 alloys. With the increase of the Zn content, the amount of gamma-Mg17Al12 phase decreases and while increase the amount of Phi-Mg-21(Al, Zn)(17) phase. Calculated equilibrium phase diagram shows that in the AZ60 similar to AZ64 alloys both gamma-Mg17Al12 phase and Phi-Mg-21(Al, Zn)(17) phase can be dissolved into alpha-Mg under proper heat treatment conditions. However, Phi-Mg-21(Al, Zn)(17) phase in AZ66 alloy can not be completely dissolved into alpha-Mg for any temperature. The results also indicate that higher Zn content alloys have higher Q values and smaller grain size, and lower solid fraction at dendrite coherency point (f(s)(DCP)). The relationship of Q values, grain size and f(s)(DCP) has been also discussed.