Characterization of dendrite morphologies in rapidly solidified Al-4.5Â wt.%Cu droplets

The impulse atomization process developed at the University of Alberta (Canada) enables metallic powders to be solidified with well controlled process parameters. The inner microstructure of several droplets of Al-4.5 wt.%Cu alloy produced by this technique has been investigated by using both synchrotron X-ray micro-tomography and electron backscattered diffraction (EBSD). The 3D reconstructions obtained by micro-tomography are used to visualize cross sections of the droplets in any spatial directions. A wide range of dendrite morphologies are observed for different droplets of similar diameter and produced in the same batch. Microstructural features indicate that the development of the dendrite arms (primary and of higher orders) occurs in most droplets along 〈1 1 1〉 crystallographic axes. This was unexpected as 〈1 0 0〉 directions are observed in conventional casting technologies. EBSD measurements were carried out on a selection of representative droplets after their characterization by synchrotron X-ray micro-tomography. Results confirm that most of the droplets are single grains and the relationship between the dendrite growth directions and the crystal orientation is determined unambiguously. Moreover, the large number of droplets analyzed by micro-tomography enabled a statistical analysis of the dendrite morphology as a function of droplet size and cooling atmosphere. The impact of different processing parameters on the final distribution of dendrite morphologies is also discussed.