The morphology of grains produced during metal Additive Manufacturing (AM) can be very different from that given by conventional processing and is central to controlling the properties of the final build. In addition, the interfacial velocities and temperature gradients found during AM are much different than in traditional castings, with high solidification rates (10-3 – 1 m/s) and high thermal gradients (105-107 K/m). These high interfacial velocities and temperature gradients can lead to interfacial nonequilibrium. A thermodynamic description of moving non-equilibrium interfaces is developed that is applicable to concentrated multicomponent alloys. This theory has been integrated with a description of dendritic growth in multicomponent alloys that incorporates CALPHAD descriptions of the Gibbs free energies. The calculations show that in 316L the interface is likely planar or has low amplitude cells during AM. Using this insight, a phase field model is developed that follows the evolution of many thousands of grains in three dimensions. A comparison between the predicted grain shapes and those measured experimentally using an AM machine and a novel laser-SEM device (developed by the Upadhyay group at Ecole Polytechnique) will be given. The comparison shows the importance of weld pool shape, heat flow, and anisotropy of the interfacial mobility in the observed grain morphologies. The relationship between the grain structure, solidification conditions will be discussed.
(Speaker: Peter W. Voorhees, Department of Materials Science and Engineering, Northwestern University, Evanston IL)