Influence of cooling rate on microsegregation behavior of magnesium alloys

The effect of cooling rate on microstructure and microsegregation of three commercially important magnesium alloys was investigated in the current research. Wedge (‘V’ shaped) castings of AZ91D, AM60B and AE44 alloys were made using a water-cooled permanent copper mold to obtain a range of cooling rates from a single casting. Variation of microstructure and microsegregation was studied using a combination of experiments. Chemical composition of alloying elements at the dendritic length scale and different cooling rates was examined using scanning electron microscopy. Solute redistribution profiles were drawn from the experimentally obtained data. Microstructural and morphological features such as dendrite arm spacing and secondary phase particle size were also analyzed using both optical and scanning electron microscopy. Dendrite arm spacing and secondary phase particle size have an increasing trend with decreasing cooling rate for the three alloys. Area percentage of secondary phase particles decreased with decreasing cooling rate for AE44 alloy. The trend was different for AZ91D and AM60B alloys, for both alloys, area percentage of β-Mg17Al12 increased with decreasing cooling rate up to location 4 and then decreased slightly. The tendency for microsegregation was more severe at slower cooling rates, possibly due to prolonged back diffusion. At slower cooling rate, the minimum concentration of aluminum at the dendritic core was lower compared to faster cooled locations. The segregation deviation parameter and the partition coefficient were calculated from the experimentally obtained data