| With a series of advantages such as high cooling rate, perfect sphere degree and nearly free oxygen content, gas atomization, the dominant method of producing high quality metal and alloy powders, has been practiced widely in modern industry and national defense. With the rapid development of powder metallurgy and other related new high techniques, technical improvement even breakthrough on the atomization mechanism have to carry out so as to meet the urgent demands for producing uniform and fine powders massively at low cost.. However, gas atomization itself is an extremely complicated process with various inter-coupled factors and multi-phase fluid, which has been not understood thoroughly. A brief introduction on the history, progress and present situation of gas atomization was given, also, advantages and disadvantages of some main gas atomization technologies was analyzed. Based on those, we found that close-coupled gas atomization (CCGA) is the prevalent gas atomization technologies, but there are some fundamental questions remained in CCGA such as the basic feature of gas flow field of close-coupled atomizer, the influences of operation parameters on powder particle size, and the breakup mechanism and solidification behavior of melt during atomizing process as well. Those questions above were studied systematically in theory and experiment.(1) Using computational fluid dynamics software, the feature of gas flow field below the Close-coupled nozzle has been investigated. There exists a critical inlet gas pressure (P_c), over which the structure of gas recirculation zone will change from "Open-wake" to "Closed-wake". P_c is related to the nozzle design, asΔl is increased, P_c decreases,Δl=4.55mm, P_c=6.4MPa;Δl=0, P_c=2.0MPa.(2) The melt superheat (A7) has great influences on the atomization process and results. The non-linear feature of melt properties changing with temperature causes the obvious change of We number of melt droplets, resulting in the shift of atomization modes, the powder mean particle size is improved accordingly.(3) The "secondary breakup" of melt in CCGA experiences the rapid formation and breakup of elastic liquid films, which appears four main modes: (a) "Fingering breakup" of liquid film's front; (b) "Wrinkling breakup" of liquid film's back; (c) "Fractal breakup" of liquid film's edge; (d) Interactional disruption between films and droplets.(4) The preparation experiment results of given Al-based alloy amorphous powders indicate that the atomized powders appear a mixture of both crystalline and amorphous particles, with the diameter of amorphous particles basically being below 26μm. Further numerical calculation results based on the heat transmission reveal that the cooling rates of atomized powders range from 10~4 to 10~8K/s, and the critical cooling rate for glass formation of given Al-based alloy is about 10~6K/s.Due to the multimode of breakup, there are still many questions in CCGA, such as wide particle size distribution and melt droplets' cooling rates, as a result, present CCGA technology can only produce mixing powders of both crystalline and amorphous particles, moreover, there maybe different atomization mechanisms for crystalline and amorphous powders. Therefore, new atomization mechanisms to effectively produce fine crystalline and amorphous powder have been proposed. |
PDF Full Text Request