Abstract
The effect of cooling rate on microstructure and microsegregation of three commercially important magnesium alloys was investigated using Wedge (V-shaped) castings of AZ91D, AM60B, and AE44 alloys. Thermocouples were distributed to measure the cooling rate at six different locations of the wedge casts. Solute redistribution profiles were drawn based on the chemical composition analysis obtained by EDS/WDS analysis. Microstructural and morphological features such as dendrite arm spacing and secondary phase particle size were analyzed using both optical and scanning electron microscopes. Dendritic arm spacing and secondary phase particle size showed 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.
Highlights
Environmental concern was the key motivating factor behind development of Mg alloys
To understand the influence of cooling rate on microsegregation of magnesium alloys, this work aims to carry out an experimental investigation using wedge cast samples of AZ91D, AM60B, and AE44 alloys
In AE44 alloy, rare earth elements were added as mischmetal
Summary
Environmental concern was the key motivating factor behind development of Mg alloys. Better aerodynamic design of vehicles or engines with improved combustion efficiency can lessen fuel consumption, but weight reduction seems to be the most effective way to achieve a substantial fuel saving [1, 2]. With density of 1.74 g/cm, is the lightest of all the engineering structural metals [3]. Mg-based alloys have an excellent combination of properties which justifies their usage in transportation applications. These properties include excellent strength-to-weight ratio, good fatigue and impact strengths, and relatively large thermal and electrical conductivities [4]. All commercial magnesium alloys are multicomponent and form a variety of phases during solidification and subsequent processing stages. The wide range of operational conditions existing in foundry and casting processes generates, as a direct consequence, a diversity of solidification microstructures. Because microstructure determines the final properties of the material, proper understanding of the microstructure formation mechanisms is extremely important. Mechanical properties depend on the microstructural arrangement defined during
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