A large-scale 3d computer tomography analysis of primary dendrite arm spacing response to withdrawal velocity change using dendrite centre tracking

Directional solidification of alloys is an extremely important process to understand due to the many high value products that are produced in this manner, e.g. turbine blades. Controlling how the columnar dendrites grow and pack together is an important aspect of increasing the strength and longevity of these components. As such 3D examination of a 200 mm in length and 6 mm diameter aluminium 10 wt % copper rod solidified under varying withdrawal rates (40 μm/s then a jump to 80 μm/s) has been undertaken in a lab based computerised tomography (CT) machine. Novel image analysis techniques involving active contours and skeletonisation have been used to track the dendrites through the sample itself. Sites of dendrite initiation and termination have been identified automatically within the dataset. These points of dendrite creation or deletion where found to be most prevalent after a step change in the withdrawal rate. Information on the array packing and primary dendrite arm spacing (PDAS, λ) for each grain within the sample has been obtained. The results show that there is a distance delay after the withdrawal rate change and the onset of a PDAS restructuring.