Suppression strategy for metal droplet overlapping fusion defects caused by droplet impact dynamics under coaxial shielding gas

High-precision droplet overlapping under coaxial shielding gas is a prerequisite for automated and lightweight metal micro-droplet deposition manufacturing. Unfortunately, the opening shielding environment exposes metal droplets directly to the atmosphere. Droplet overlapping fusion quality would be affected due to the coupling effects of impact dynamics, thermodynamics, and oxidation. In this study, based on experiments and theoretical modeling of molten droplet impact dynamics, a strategy to suppress droplet overlapping fusion defects under coaxial shielding gas was proposed for the first time. Results show that at lower shielding gas rates, molten droplet retraction, recoil, and oscillation would weaken or vanish due to the oxide film's self-limiting effect. This limits the improved model's accuracy in predicting the droplet spreading factor in lower shielding gas supply rates. The weakened droplet dynamic behaviors at low shielding gas supply rates would magnify the length and height defects of droplet overlapping, which is particularly evident at a small printing step distance. Finally, a quality mapping for different printing parameters is established, effectively suppressing overlapping defects and ensuring fusion quality through metallurgical bonding. This work could provide a solid evidence base and theoretical guidance for high-quality metal micro-droplet deposition manufacturing under an opening shielding environment.