Characterization of cooling rate and microstructure of CuSn melt droplet in drop on demand process

Different sized single droplets of Cu-6%Sn alloy were prepared by drop on demand (DOD) technique. The secondary dendrite arm spacing was measured and correlated with the droplet cooling rate by a semi-empirical formula. The microstructure of droplets was observed by optical microscopy (OM) and electro backscatter diffraction (EBSD). The dendrite feature of single droplets depends on solidification rate, cooling medium and flight distance. When droplets collide with each other at temperatures between solidus and liquidus, the dendrites and grains are refined obviously possibly because the collision enhances the heat transfer. The cooling rate of colliding droplets is estimated to be more than 4×104 K/s based on a Newton's cooling model. The dendrites grow along the colliding direction because of the temperature gradient induced by the internal flow inside the droplets.