Exploring dendrite coherency with the discrete element method

The particulate discrete element method (DEM) is used to simulate crystal rearrangement during equiaxed solidification of Al alloys for a range of morphologies across the globular to equiaxed-dendritic transition. It is shown that DEM is able to capture the key experimental results reported in past dendrite coherency studies, namely: the marked increase in resistance to shear at a critical solid fraction, the influence of crystal morphology on dendrite coherency, and that dendrite coherency marks the onset of dilatancy. Dendrite coherency is shown to be the lowest solid fraction at which long-range interconnectivity in the force chain network develops during shear. It is further found that dendrite coherency depends on both the internal solid fraction within dendrite envelopes and the mean shape of envelopes at coherency. The potential to extend DEM to the simulation of mushy-zone mechanics in casting problems is then discussed.