Grain refinement process for Al-containing magnesium alloys

Grain refinement of cast magnesium alloys containing Al is successfully attained using MnCO₃ as a grain refiner. However, the direct addition of MnCO₃ in a magnesium melt causes the oxidation and ignition of the melt due to the gases that formed during the decomposition of MnCO₃. To improve the effectiveness and safety of the MnCO₃ addition process, inoculation was conducted by adding an Al master alloy with MnCO₃. The Al master alloy was prepared using cylindrical pressed pellets composed of a mixture of MnCO₃ powders and metal chips in the aluminum melt. The results revealed that the inoculation method using the Al master alloy is more effective and stable than previously used methods. The grain size of the as-cast AZ91 Mg alloy was significantly decreased from 420 to 70um by adding the Al master alloy of 3 wt%, which is mainly attributed to the formation of thermally stable Al₄C₃ particles as an effective grain refiner in the melt.