Exploring impact, spreading, and bonding dynamics in molten metal deposition for novel drop-on-demand printing

This study delves into the dynamics of molten metal deposition (MMD)-based drop-on-demand (DoD) printing, focusing on the interaction between aluminum droplets and substrates. Nozzle-to-substrate distances below 20 mm prevent droplet pinch-off, while distances exceeding 40 mm result in no droplet-substrate adhesion. Operating within the substrate temperature range of 350–550 °C is crucial, avoiding delamination, and shrinkage pits at lower temperatures, and substrate deformation and heightened oxidation at higher temperatures. Droplet adhesion is impeded at nozzle temperatures below 700 °C. Impact-driven and inviscid, DoD-MMD exhibits spreading outpacing overall solidification, particularly at higher temperatures. Surface tension forces dominate, influencing droplet spreading and leading to underdamped interfacial oscillations. Weak droplet-substrate adherence, facilitated by Van der Waals forces, allows easy droplet detachment, beneficial for successive drop-on-drop deposition. Unique to DoD-MMD, ridges along the droplet periphery act as solidification paths, influenced by thermal contraction and surface tension. The spherical pancake shape of the droplet, characterized by a solidification angle >90°, is elucidated through Weber and Freezing numbers. The final deposited droplet width to initial diameter ratio increases with the droplet temperature and deposition height. In contrast to other metal DoD studies, the spreading factor decreases with a rise in substrate temperature, attributed to intensified oxidation at higher substrate temperatures.