Insights into the impact and solidification of metal droplets in ground-based investigation of droplet deposition 3D printing under microgravity

Droplet deposition 3D printing is an additive manufacturing technique offering a great potential for metal parts fabrication in space, of which some preliminary testing is usually performed on ground in the early research. The previous work (Huang et al., 2020) mimicked the anti-gravity deposition of molten metal droplet through manipulating it into perpendicularly depositing on a vertical substrate. However, the ground-based simulation of droplet deposition 3D printing under microgravity remains an elusive goal, since the spreading and receding processes are still affected by gravity. To address this issue, the prevailing physical mechanisms of gravity effect on droplet impact and solidification are urgent to be defined. Here, we present the studies on the impact dynamics and transient solidification of the molten metal droplet deposited on vertical substrates through numerical modeling and experiments. It is observed that the spreading and retraction of the droplet are asymmetric, besides its solidification shape tilts to gravitational direction. The formation mechanisms of these undesired behaviors are further demonstrated. The results show that the asymmetrical spreading, retraction and solidification shape of the droplet originate from the interaction of gravity and solidification. Moreover, the tilt of the solidified droplet has a correlation with the critical process parameters, i.e. impact velocity, temperatures of droplet and substrate. With a larger impact inertia and a lower solidification rate, the undesired solidification shape can be effectively eliminated. This work provides a foundation for the further investigation of the ground-based physical simulation of outer space droplet deposition 3D printing.