Characterization of the in-flight solidification of Sn-Pb uniform droplets

The effects of chamber atmosphere on the undercooling and microstructural evolution in traveling Sn-5wt.%Pb droplets were investigated using a non-adiabatic calorimetric method and a droplet solidification simulation model which accounted for large droplet undercoolings and subsequent rapid solidification. The droplets were produced by the controlled breakup of a laminar jet to have the same molten diameter of 185 mu m and solidified in an identical manner, Use of 99.9% pure industrial-grade nitrogen and a CaSO4-desiccated nitrogen - 2% hydrogen gas mixture as the chamber atmosphere produced droplet undercoolings of 74 and 146 K, respectively, in a consistent manner, The increase in droplet undercooling was achieved by control of oxide catalyst formation on the droplet surface. Droplets quenched from the fully molten state retained about 0.0003 J more enthalpy per droplet than those quenched after nucleation and recalescence. This retention of enthalpy was attributed to such structural metastability as supersaturation and size refinement, Formation of a non-equilibrium phase is thermodynamically possible though not confirmed in the present study, The results provide critical information for advanced microstructural control in both controlled spray forming and powder production.