Abstract

Magnesium castings are widely used in the automotive sectordue to their high strength to weight ratio. Solidification of these alloys always results in inhomogeneous distribution of alloying elements (microsegregation), which can have adverse effects on the microstructure. To control the microstructure obtained from different solidification conditions, and thus attain desired properties, in-depth understanding of the microstructure evolution and the resulting microsegregationis of paramount importance.
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 and thermodynamic modeling. Chemical inhomogeneities of alloying elements at the dendritic length scale at different cooling rates were examined using scanning electron microscopy. Solute redistribution profiles were drawn from the experimentally obtained data and then compared with established models.Microstructural and morphological features such as dendrite arm spacing andsecondary phase particle size were also analyzed using both optical and scanning electron microscopy. Thermodynamic calculations were performed using FactSage to obtain data regarding two solidification extremities- equilibrium and non-equilibrium Scheil cooling. 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 rate, possibly due to prolonged back diffusion. At slower cooling rate, the minimum concentration ofaluminumat 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. These data would be valuable input for solidification modeling of microstructure of these alloys and can also be used to validate present models.