紧耦合气雾化流场结构和雾化机理研究

【作者】 陈欣；

【导师】 欧阳鸿武；

【作者基本信息】 中南大学 ， 材料学， 2007， 硕士

【摘要】 气雾化制粉技术具有粉末球形度高、氧含量低以及冷却速率大等优点，是生产高性能球形金属及合金粉末的主要方法。随着粉末冶金工艺的不断发展，特别是近年来PM~2概念(Powder Metallurgy and Particulate Materials Technology,PM~2 TEC)的兴起，对高性能粉末的需求日益迫切。大批量、低成本制备微细高性能粉末成为当前气雾化制粉技术发展的主要趋势。然而，气雾化制粉是一个多相流相互耦合作用的复杂过程，人们对气雾化过程中气液间相互作用、熔体物性对雾化过程的影响，以及熔体破碎与冷却之间的耦合关系等基本问题至今仍未认识清楚。本文通过回顾和总结气雾化制粉技术的发展历程，比较分析各种气雾化技术的特点，消化吸收国际先进的紧耦合气雾化制粉技术(Close-coupled gas atomization，CCGA)，采用理论、实验和数值模拟方法，针对其气体流场结构特征、熔体的破碎模式以及冷却凝固特征开展研究。主要结论如下：(1)紧耦合气体流场的数值模拟探明了紧耦合气体流场的基本特征，发现流场由“开涡”转化为“闭涡”状态存在一个临界气压P_c，P_c与喷嘴结构相关，随导液管末端伸出长度△l减小而下降，△l为4.55mm时，P_c为6.4MPa；△l为0时，P_c为2.0MPa。(2)熔体过热度对雾化过程和结果有明显影响，其作用机制为：熔体物性随温度的非线性变化直接影响熔滴的We数，从而导致雾化模式的改变，引起粉末平均粒度的变化。(3)紧耦合气雾化“二次破碎”过程中液膜的快速形成及其破碎呈现出弹性特征，其破碎主要包括四种模式：a—液膜前端的“手指式”破裂；b—液膜末端“起皱”破碎；c—液膜边沿的分形破碎；d—液膜及液滴的相互干扰破碎。(4)气雾化(某一成分的)Al基合金非晶粉末的实验和理论分析结果表明，雾化粉末为晶态／非晶态颗粒的混合粉末，粉末的冷却速率为10~4～10~8K／s；其非晶化临界冷却速率大致为10~6K／s，非晶颗粒的粒径基本小于26μm。由于多种破碎模式共存，目前的紧耦合气雾化技术仍存在粒度分布较广、熔滴冷却速率跨度较大的问题。用于制备非晶合金粉末时，还只能制备出晶态／非晶态的混合粉末，并且，晶态与非晶粉末可能存在不同的雾化机理。为此，进一步探讨了紧耦合气雾化高效制备微细晶态以及非晶合金粉末的可行方案。更多还原

【Abstract】 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.更多还原