In-situ observation of dendrite morphology and growth in solidifying aluminum alloys using X-ray tomoscopy
Understanding solidification phenomena is crucial to be able to control the microstructure in many industrial casting processes. In-situ techniques such as X-ray radioscopy on thin samples or tomography of the bulk metal have become powerful tools to investigate solidification processes in real time. X-ray radioscopy is fast enough to resolve the formation of different phases in a wide range of cooling rates, but limited to thin (100-400 µm) 2D samples. However, 3D tomography has been so far limited by the temporal resolution to very slow cooling rates in the range of 0.1 K/s.
Therefore, to capture phenomena during solidification at realistic cooling rates, i.e. similar to those in industrial processing, we employed the newly developed tool for time-resolved, synchrotron tomography, called tomoscopy, to study in-situ the solidification in Al-based alloys. A new setup at the TOMCAT beamline at SLS allows us to investigate the microstructural evolution of the dendrite structure during solidification at high cooling rates, up to 5 K/s, with more than 50 tomograms per second.
Tomoscopy can provide a large number of quantitative results. This enables a three dimensional quantification of the evolving dendrite morphology and measurements of dendritic-tip growth velocities of columnar and equiaxed growing grains under aspects like convection and solute segregation. In addition, nucleation and growth of individual pores and the formation of interdendritic shrinkage, due to entrapped liquid is investigated to evaluate the effect of those defects on the final casting microstructure.