Effects of nucleation kinetics on the microstructure and surface quality of mono-sized spherical metal balls produced by capillary jet break-up

The uniform-droplet-spray (UDS) process, a controlled capillary jet breakup process, provides an ultimate means for the direct manufacturing of mono-size balls of metals and alloys in diameters ranging about 40--1000 microns. Commercialization of mono-size balls has already been realized in electronics packaging industry and other potential applications are in R&D. However, the direct use of solidified mono-size balls in manufacturing critically hinges on the ability to control the size uniformity, surface quality and sphericity of the balls.;In the present research, the nucleation kinetics of mono-sized droplets of Fe-5Si-2.7B-0.4C-0.8Mo alloy (JH201) and 99.99% pure copper generated by the UDS process was investigated with the primary objective of understanding how the solidification path affects the sphericity and surface quality of solidified droplets. For this purpose, a nucleation kinetics prediction model developed in the Advanced Materials Processing Laboratory was used in conjunction with a droplet splat quenching method adapted for JH201 and copper UDS droplets. The nucleation temperatures were calculated and presented in the form of continuous cooling transformation (CCT) diagrams for both volume and surface heterogeneous nucleation. The accuracy of the predicted CCT curves was cross-validated by additional UDS quenching experiments.;The conditions for multiple nucleation eventsper droplet wereinvestigatedby predicting the density of nucleation events. Also, the original nucleation kinetics simulation model was modified to address the transient effects, along with the effects of the temperature-dependent driving force and viscosity, which become significant at high undercoolings. Computed CCT diagrams indicate that the latter effects can be significant when the cooling curve intersects the CCT curves below the nose.;With the predicted nucleation kinetics, the post-recalascence plateau duration, i.e., the time available for the solidifying droplets to restore their smooth spherical surface by the surface tension of the remaining liquid, was calculated. The calculated plateau durations were compared with the time required for the dendrites to fragment into smaller crystals, an instability phenomenon that is considered to help liquid redistribution, hence minimizes shrinkage and promotes sphericity restoration. The predicted effects of plateau duration and dendrite fragmentation were compared with the microstructure and surface quality of the final products.