Grain refinement of AZ31 magnesium alloy by titanium and low-frequency electromagnetic casting

The grain-refinement effects of titanium (Ti) additions and a low-frequency electromagnetic casting (LFEC) process on the AZ31 magnesium alloy have been investigated. It is shown that Ti has no effect on the formation and distribution of secondary phases in the AZ31 alloy. The results suggests that the grain size decreases with an increasing cooling rate for the AZ31 alloy; it decreases first, however, then increases slightly for Ti-containing AZ31 alloys, indicating that the grain-refinement effect of Ti decreases with an increasing cooling rate. However, the grain size decreases first, and then increases with increasing Ti for a given cooling rate. The lowest grain size is obtained at the addition of about 0.01% Ti and the cooling rate of about 6 K/s. The grain refinement effect is explained in terms of the growth restriction factor (GRF) due to the constitutional undercooling generated by the Ti solute elements, which produces grain refinement up to a critical GRF value corresponding to about 0.01% Ti addition. The yield strength of a conventional-cast AZ31 alloy can be improved by grain refinement and expressed as sigma(y) = 39.31 + 142.53 d(-1/2) according to the Hall-Petch relationship. The combination of the Ti grain refiner and the LFEC process during direct-chill (DQ casting of an AZ31 alloy can produce high-quality magnesium alloy billets with uniform and equiaxed grains of less than 40 mu m across the billet section up to at least a 100-mm diameter, which further improves the yield strength of the alloy, resulting in sigma(y) = 51.53 + 90.85 d(-1/2).