Microstructural stress concentration: An important role in grain refinement of rheocasting structure

Though the mechanisms of dendrite fragmentation in agitated solidification are of great importance and have been investigated for many years, no accurate calculation of the stress fields of dendrites stirred in the metallic flow has been reported. In this study, the stress field and strain field of an aluminum dendrite model in moderate agitated fluid were calculated using the finite-element method. It was found that the stress concentrations at the roots of dendrite branches are so great that plastic deformation occurs when the fluid force is far from sufficient to bend the dendrite branches plastically. According to this calculation, a mechanism, detaching after fatigue erosion, was suggested. The new mechanism suggests that stress concentrations at the roots of dendrite branches create slip band intrusions and extrusions. These slip band intrusions and extrusions are thermally unstable and are soon remelted. If the erosion rate of the roots is sufficiently large, the roots will be thinned and dendrite branches will detach. Experiments on the effects of stirring intensity, cooling rate, and stirring time on the grain sizes of the A356 rheocasting structure, and reported high-temperature fatigue tests, corroborate the effectiveness of the new mechanism.