In situ X-ray and thermal imaging of refractory high entropy alloying during laser directed deposition

Abstract MoNbTiV high-entropy alloy was in situ alloyed with laser power-blown directed energy deposition additive manufacturing from a mixture of four elemental powders of Mo, Nb, Ti, and V. This study provides a fundamental understanding of the alloying process through in situ high-speed synchrotron X-ray imaging and infrared imaging. High-speed X-ray imaging was used to investigate the in situ alloying process through direct observation. The particle delivery velocities of four different elemental powders under the same processing conditions were studied to reveal the performance of the powders during the in situ alloying process. We found that the Ti and Nb powders showed the greatest and smallest averaged particle-delivery velocities among these four powders, respectively, and the particle delivery velocity would be affected by the particle characteristics, particle size, and density of powders. The velocities of the resulting melt pool flow were measured to show the melt flow dynamics in such a process. The residence time of each elemental powder was also obtained, and Mo powders showed the largest residence time followed by Nb, V, and Ti powders. The porosity induced by unmelted particles and entrapped gas occurred in the alloying process. The Mo powders resulted in the most unmelted particles, and the entrapped gas porosity was mainly induced by the keyhole fluctuation. With the assistance of an infrared camera, we reported the emissivity of the melt pool, the change of thermal properties, and melt pool morphology during alloying.