SOLIDIFICATION BEHAVIOR OF PD-RH DROPLETS DURING SPRAY ATOMIZATION

Solidification microstructure in spray-atomized Pd-10 wt % Rh powders using high-pressure gas atomization was studied. The solidification co oling rate and the solidification front velocity were investigated usi ng a transient heat-transfer finite element method. Two different atom ization gases, nitrogen and helium, were considered in the modelling s tudies. On the basis of the results obtained, it was found that gas at omization using helium gas led to solidification cooling rates and sol idification front velocities which were two times higher than those ob tained using nitrogen gas. Moreover, the cooling rate and the solidifi cation front velocity increased with decreasing powder size for both t ypes of atomization gas. The numerically estimated solidification fron t velocity using finite element analysis for nitrogen gas atomization was found to be smaller than the analytically determined absolute stab ility velocity that is required to promote a segregation-free microstr ucture. This was noted to be consistent with the segregated microstruc ture that was experimentally observed in nitrogen gas atomized powders . In the case of helium gas atomization, however, the increased coolin g rate and solidification front velocity are anticipated to promote th e formation of a segregation-free microstructure in the gas-atomized p owders.