Heat transfer and melt dynamics of millimetric ice particles impacting a heated water bath

In metallic additive manufacturing using direct energy deposition, particles and melt pool undergo complex interactions, including particle impact, penetration, and melting. The spatio-temporal evolution of these processes dictates the solidified material microstructure and final workpiece quality. However, due to the opaqueness of metallic melt pools, in-situ visualization is nearly impossible. To model this system, we use high-speed imaging to investigate the heat transfer and melting dynamics of spherical ice particles (D approximate to 2 mm) impacting heated water baths of varying temperatures (23-70 degrees C) with velocities ranging from 0.8 to 2.1 m/s. To visualize the outflow of molten ice, representative of mixing and material homogeneity, the particles were colored with food dye. We show that after impact, molten liquid forms an annular plume travelling downwards in the bath, until hitting the bottom of the enclosure and expanding radially. Due to positive buoyancy forces, unmolten ice particles rise to the top of the water bath, where they fully melt. As temperatures increase, we observe random particle movement, indicating the presence of convective currents. Through video analysis, we examine the relationships between bath temperature, impact velocity, and heat transfer. As expected, increasing the bath temperature decreases the total melt time of the ice particle. Interestingly, the impact velocity has only a minor effect on the melting time. Using non-dimensional analysis, we derive an expression for the correlation between Nusselt and Stefan numbers. Insights from this work can be used to match characteristic time scales during additive manufacturing to tailor material properties. (C) 2019 Elsevier Ltd. All rights reserved.