Research On Solidification Characteristics And Microstructure Of Micro-and Nano-sized Droplets

Solidification characteristic and microstructure could be changed with the decreasing droplet size. However, due to the coexistence of droplet size and cooling rate during rapid solidification by traditional atomization, the droplet solidification was generally considered to be attributed to the enhancement of cooling rate, neglecting the own influence caused by the reduction of droplet size. To investigate the effect of droplet size and cooling rate on rapid solidification separately, solution-encapsulation remelting and quenching technique (SERQT) was developed. In addition, rapid solidification of nano-sized droplets produced by solution-encapsulation self-consumable direct-current arc method (SEDAM) was investigated.Solidification characteristic was investigated firstly by theoretical calculation and DSC experiments. A simplified theoretical undercooling model was established, and therefore the effects of droplet size on the characteristic parameters for droplet solidification were calcaulted, showing that significant effects could be generated. In light of the calculated results of Al_4.5Cu_95.5 alloy, abrupt changes of the aforementioned parameters were found when the droplet size decreased to about 20μm, implying that size effect happened. The mathematic model of relative undercooling was established, and the influencing factors of undercooling under rapid solidification of droplets were analyzed. The impact factorζ=σ_SL³/(T_LΔH²) was defined to characterize the influencing factors of droplet solidification. Generally, the undercooling was changed with the variety of the solidification conditions, and then the cardinal reason was attributed to the impact factorζ. In the condition of droplet solidification, the characteristic parameters such as theoretical crystallizing temperature T_L, latent heat of solidificationΔH and liquid-solid interfacial energyσ_SL etc. were changed as the variety of solidification conditions.The microstructure evolution of droplet solidification for Sn₄₃Bi₅₇ alloy produced by high-pressure gas atomization was investigated, and the mechanism of its evolution was discussed. The results showed the microstructure changes from anomalous dendritic crystal to equiaxed crystal when the droplet size decreased from above 20μm to below15μm. By applying the simplified undercooling model and newton's model of cooling rate, undercooling and cooling rate were calcaultaed, respectively. The calculated results indicated the undercooling and cooling rate increased with the decreasing droplet size, and increased abrupt at the size of 20μm, which resulted in microstrucure evolution. In all, the combined effective of droplet size and cooling rate was the external factor of microstructure evolution, and the undercooling attributed to the combined effect of droplet size and cooling rate was the internal factor of microstructure evolution.As the mentioned above, the combined effect of droplet size and cooling rate resulted in not only microstructure eveolution but also grain refinement. However, the limited droplet size by traditional atomization method resulted in a limited cooling rate and a limited grain refinement. Therefore, the SEDAM was developed to increase the cooling rate even at the larger size of droplet, which bringed a new method to refine microstucture.Nevertheless, the decrease of droplet size and the increase of cooling rate by atomization and SEDAM coexisted, the SERQT was developed to separate the cooling rate and droplet size in droplet solidification. The effects of droplet size and cooling rate could be independently analyzed by the theoretical calculation coupled with the controlling of temperatures of the coolant. The results showed the cooling rate and droplet size did not directly impact on the solidification structure, but changed the solidification undercooling and subsequent variation of solidification structure.The SEDAM was also developed to produce nano-sized droplets, and nano-sized droplets of Sn-based alloys were prepared in this paper. It was found an oxide layer envolped the nano-sized powder of Sn-based alloy with easy oxidation, while there was no oxide appeared for the nano-sized pwoder of Sn-based alloy with strong resistense to oxidation. As a result, it could be concluded the SEDAM was an effective measure to produce nano-sized droplets. In addition, it was also found the phase separation of Sn-Bi eutectic happened when the vaporizing droplets cooled into fine powders resulting from infinitesimal size of nano-particles. The HRTEM results showed the nano-sized particles composed of Bi-rich were single crystal, while the particles composed of Sn-rich were multi-crystal.Compared with the above three methods to produce droplets and their microstrccture, it could be found the grain size of quehenced droplets and arc droplets was all less than that of atomized droplets not only with the same size but also with the less size. Furthermore, with the SERQT and SEDAM, the spheroidization of atomized powders has been achieved, which dramatically enhanced the spheroidization of the surface due to no strong collision. Additionally, the SEDAM not noly could produce micro-sized droplets, but also could produce nano-sized droplets.