Preparation Of Mono-Sized Alloy Powder And Simulation Of Solidification Behavior Of Gas-Atomized Droplets

A uniform droplet spray (UDS) apparatus has been constructed which has been used to produce uniform droplets for low melting point alloy. Uniform Sn-37%Pb and Sn-3.0%Ag-0.5%Cu droplets were prepared by using the self-developed apparatus. The good surface smoothness, sphericity and uniformity of the droplets were shown under optical microscopy. Five experimental results revealed the important influence of disturbance, induction electric field and oxygen concentration in the container on the UDS process. Calculation results proved that the oxygen concentration had impeditive effect on the jet break up. Above a critical oxygen concentration, the jet breaking up would not occur.A method for predicting the nucleation kinetics and the solidification behavior of gas-atomized droplets has been developed by combining models predicting the nucleation temperature of cooling droplets with a model simulating the free dentritic grows. Application to a Sn-5%Pb alloy has yielded continuous cooling transformation (CCT) diagrams. Both internal nucleation caused by a catalyst present in the melt and surface nucleation caused by oxidation are considered. The modeling has also yielded the following relationship: tip radius of curvature, tip velocity, partition coefficient and solid composition as a function of the radial distance from nucleation point. Droplets atomized at a high atomizing gas velocity get around surface oxidation and nucleate internally at high supercoolings. Low atomization gas velocities promote oxidation-catalyzed nucleation which leads to lower supercoolings. The higher the atomization gas velocities, and the lower the oxygen concentration in the container, the higher the solid fraction during the recalescence process. Under same conditions, the smaller the droplet in diameter, the higher the mass-specific enthalpy loss rate, and in final, the higher the proportion of metastable phase.