Solidification Pattern Selection and In situ Imaging of Al-Ag Under Rapid Solidification Conditions Relevant for Additive Manufacturing
Brian Rodgers, Colorado School of Mines
Implementing additive manufacturing technology depends upon predicting and controlling microstructure during the build process. Currently, the majority of solidification theory and modelling is calibrated towards behavior at low solidification velocities. Solidification at high velocities differs from low velocities because the interface velocity becomes comparable to the rate of diffusion in the liquid. Non-equilibrium effects become significant when this phenomenon occurs. In general, rapid solidification effects are not well accounted for with current solidification models. To better understand solidification behavior at high velocities, novel in situ experiments are currently being pursued. Roughly millimeter thick cross-sections were observed with synchrotron X-rays at the Advanced Photon Source (APS) at Argonne National Laboratory and roughly 100 nanometer thick films were observed with electrons using Dynamic Transmission Electron Microscopy (DTEM) at Lawrence Livermore National Laboratory. Melt pools in Al-Ag alloy samples are created via laser radiation, followed by rapid solidification. In situ imaging is augmented by ex situ characterization to observe further details about the unique microstructures created. Microstructures in the APS melts pools consist of columnar dendrites preferentially aligned normal to the solid-liquid interface, with high velocity melt pools becoming planar by reaching absolute stability. Some of the melt pools transition from conduction mode to keyhole mode partway through a given laser raster melt, causing the solid-liquid interface to reverse direction, producing unique microstructures. Due to the nature of the technique, the DTEM melt pools experience relatively consistent velocities and thermal gradients. However, a transition from cellular/dendritic growth to planar growth is observed with increasing solute content. Similarly, the APS melt pools show <110> dendrite growth, indicating a dendrite orientation transition in this system.
Authors: B. Rodgers1, J.T. McKeown2, J. Roehling2, A. Karma3, A.J. Clarke1
1Colorado School of Mines, USA
2Lawrence Livermore National Laboratory, USA
3Northeastern University, USA
Abstract Author(s): (see above entries)