A 3D microstructural model of freckle initiation from pre-existing imperfections

The quality and lifing of high performance turbine blades are severely hampered by presence of freckles, which are initiated by presence of enriched solute plumes during solidification [1]. Nickel based turbine blades can be cast as equiaxed, directionally solidified or single crystals. Several types of perturbations may occur during these casting techniques, which can eventually lead to freckles.
In this work, a 3D microstructural numerical model of freckle formation is presented [2]. A typical simulation of stabilized solute plume formation leading to a freckle channel is shown in Fig. 1. The model is validated with in situ x-ray radiographic measurements [3], comparing solute partitioning, convection and freckle channel width. This model was then used to investigate freckle formation under a range of solidification conditions. Using the dendrite tip growth and solute profiles, freckle onset was observed to occur in two distinct stages. The influence of imperfections that occur in primary arm spacing and grain boundary was investigated from the proposed model. It was found that the freckles initiate from these irregularities, with a higher propensity for converging grain boundaries. In addition, the effect of dendrite orientation angle on freckle formation is studied.