Impact of tin molten droplets on ultrasonically vibrated copper and stainless-steel surfaces: Detachment, rebound, and receding

This study employs high-speed photography to monitor the dynamic spreading and receding processes of molten tin droplets impacting copper and stainless-steel substrates under ultrasonic excitation, with a focus on substrate temperature effects. Results reveal that without ultrasonic vibration, rapid solidification-induced thermal stress causes edge self-delamination during receding on high-thermal-conductivity copper substrates. In contrast, stainless steel (lower thermal conductivity) exhibits more uniform spreading and receding. Ultrasonic vibration significantly alters droplet spreading behavior, not only reducing the time to reach maximum spreading diameter but also decreasing the diameter itself compared with the non-vibrated case. The spreading process is predominantly governed by inertia, while receding is controlled by cavitation effects near the substrate interface. Receding kinetics depend critically on substrate temperature: elevated temperatures promote stable receding and improved surface contact. A critical temperature threshold further modifies the maximum spreading diameter, reflecting a transition in spreading dynamics and final solidification morphology. These findings elucidate the complex interplay among ultrasonic vibration, substrate temperature, and droplet dynamics. We propose a novel adhesion control strategy without surface treatment, minimizing residual deposition in non-target regions. This approach demonstrates substantial industrial relevance for additive manufacturing, metal deposition, and surface coating technologies.