Effects of Sn addition on the microstructure and tensile properties of AX55 alloys

The microstructures and tensile properties at both room and elevated temperatures for both the as-cast and as-aged Mg-5Al-5Ca (AX55) alloy with 0-2 wt% Sn addition were studied. The results indicate that alpha-Mg dendrite is gradually refined and the interdendritic Al2Ca and Mg2Ca intermetallics become more connected with Sn addition. The as-cast AX55-1Sn alloy shows optimal ultimate tensile strength (UTS) at testing temperature from 25 to 225 degrees C. After T61 and T62 heat treatment, the eutectic-lamellar microstructure of the as-cast alloys tends to be spheroidized and distributed uniformly along the grain boundaries. While the alloys with higher Sn content show higher density of granulated and needle-shaped Al2Ca phases precipitated into a-Mg matrix, which results in the increase of UTS, yield strength (YS), elongation and microhardness with Sn addition. The morphology of CaMgSn phase can be improved by T62 treatment, which makes as-aged AX55-2.0Sn alloy exhibit a smaller decrease rate of the UTS at temperature up to 225 degrees C. The heat resistance of different heat-resistant magnesium alloys were compared and discussed by using the decrease rate of the UTS.