Effect of stirring speed and flow pattern on the microstructure of a rheocast Al-Mg alloy

Abstract

The objective of this study is to clarify the effect of flow pattern inside the processing crucible on the microstructure of a mechanically stirred Al-8%Mg alloy. By incorporating a set of baffles in the processing crucible the flow pattern changed from a so-called solid body rotation (tangential flow) to a mainly axial flow. Samples were cast with and without baffles being incorporated in the crucible at two different stirring speeds of 500 and 1000 rpm. Microstructure features of the samples were investigated using metallography and image analysis techniques. Results reveal that by increasing the stirring speed, morphology of primary particles become finer and more spherical. By using baffles, the flow pattern inside the crucible changes. This change causes the melt to accelerate along the crucible walls and that makes the structure finer, more spherical and non-dendritic. Although the model is named "nucleation and separation from the wall mechanism", this model is able to explain the evolution of the semi-solid microstructure. It must be mentioned that the other model named "fragmentation-agglomeration" cannot explain the changes in microstructure by using baffles. Copyright © 2013 MS&T'13®.