Spreading behavior of Sn droplets impacting Cu and stainless steel substrates

Abstract

With the development of droplet 3D printing, thermal spraying, spray molding, and other technologies, the collision diffusion process of metal droplets and metal substrates has become a key aspect in controlling these processes. This study used Sn droplets as the research object. Using a pneumatic pulse jet device, molten Sn droplets were impinged on the surface of copper and stainless steel substrates. The impact process was recorded by high-speed photography. The diffusion diameter and solid-liquid contact angle were measured from these images, revealing the difference in the diffusion of Sn droplets on different surfaces. This explains the difference in the droplet diffusion ability from the perspective of driving energy. The entire impact process and final morphology were considerably similar when the substrate temperature was lower than the melting point of Sn. When the substrate temperature was higher than the melting point of Sn, the entire diffusion behavior of the tin droplet from contacting the substrate to the steady state was investigated in stages. From the driving energy analysis, it was observed that the Sn droplet is mainly driven by its internal kinetic energy in the initial stage of diffusion. When the diffusion diameter approached the maximum value, the kinetic energy of Sn droplets gradually dissipated; the surface energy and viscous force work gradually increased, and the diffusion changed from a linear diffusion state to a slow diffusion state. Simultaneously, the surface energy and viscous force of the droplet played a key role. The adhesion of Sn droplets to the surface of the stainless steel substrate was very small, and the interface did not react, which caused the Sn droplets to shrink significantly on the stainless steel surface during the retraction stage.