Effect of calcium and boron addition on the microstructure and compression properties of AZ91 cast magnesium alloy

In this study, the effect of calcium and boron addition has been investigated on the microstructure and compression properties of AZ91 alloy. Initially, in order to determine the optimal calcium amount to reduce the grain size and the presence of secondary phases, 0.1, 0.5, 0.8 and 1 weight percent of calcium were added to the AZ91 alloy. The microstructure evaluation showed that an increase of calcium up to 0.5wt. % decreased the average grain size from 105 μm to 65 μm without the formation of Al2Ca phase. The similar tendency on grain boundary phases are also observed when increasing quantity of Ca added.  The microstructure and mechanical properties of the alloy was then evaluated by adding 0.03, 0.09 and 0.12wt. % B in the form of Al-3B master alloy. The results showed that the optimum concentration of boron is 0.09wt.% in order to achieve the lowest average grain size. In addition, the measured compression properties of samples containing 0.5wt. %   calcium with different amounts of boron showed that the alloy with 0.9wt. % B   presented the best mechanical properties.