【作者】 李述；

【导师】 吴萍；

【作者基本信息】 天津大学 ， 材料物理与化学， 2007， 硕士

【摘要】 采用自行研制的低温均匀液滴喷射(Uniform Droplet Spray–UDS)设备制备了Sn-37%Pb和Sn-3.0%Ag-0.5%Cu均匀合金颗粒,合金颗粒显示了较好的表面光洁度、球度和均匀度。通过五个对比实验清晰地展示了外加扰动、外加电场和环境氧浓度对射流断裂过程的显著影响。结果表明:氧浓度对射流断裂起着阻碍的作用;存在一临界氧浓度,当环境氧浓度超过这一临界值时射流将不发生断裂。提出了一种用于气雾化条件下雾化液滴凝固行为模拟和预测的理论方法,并将其用于雾化Sn-5%Pb合金液滴的凝固行为计算,定量的描述了不同初始雾化气体速度、不同氧浓度条件下不同直径液滴的运动、传热和传质行为。计算中,考虑了表面氧化催化引起的表面形核和内部杂质催化引起的内部形核这两种形核方式,得到了用于预测雾化液滴形核温度的连续冷却转变曲线。凝固计算得到了枝晶尖端曲率半径、固液界面迁移速率、分配系数和固相溶质浓度随着界面迁移距离的变化关系。结果表明:对于高雾化气体速度,表面氧化形核被抑制,进而发生内部形核;低雾化气体速度促进了表面氧化形核,进而使液滴过冷度降低;雾化气体速度越高、环境氧浓度越低,再辉过程所完成的凝固分数越高,最终颗粒中的亚稳相所占的比例就越大;相同条件下,液滴直径越小,其单位质量焓损失速率越高,冷却速率越快,最终颗粒中亚稳相的比例也越高。更多还原

【Abstract】 A uniform droplet spray (UDS) apparatus has been constructed which has been used to produce uniform droplets for low melting point alloy. Uniform Sn-37%Pb and Sn-3.0%Ag-0.5%Cu droplets were prepared by using the self-developed apparatus. The good surface smoothness, sphericity and uniformity of the droplets were shown under optical microscopy. Five experimental results revealed the important influence of disturbance, induction electric field and oxygen concentration in the container on the UDS process. Calculation results proved that the oxygen concentration had impeditive effect on the jet break up. Above a critical oxygen concentration, the jet breaking up would not occur.A method for predicting the nucleation kinetics and the solidification behavior of gas-atomized droplets has been developed by combining models predicting the nucleation temperature of cooling droplets with a model simulating the free dentritic grows. Application to a Sn-5%Pb alloy has yielded continuous cooling transformation (CCT) diagrams. Both internal nucleation caused by a catalyst present in the melt and surface nucleation caused by oxidation are considered. The modeling has also yielded the following relationship: tip radius of curvature, tip velocity, partition coefficient and solid composition as a function of the radial distance from nucleation point. Droplets atomized at a high atomizing gas velocity get around surface oxidation and nucleate internally at high supercoolings. Low atomization gas velocities promote oxidation-catalyzed nucleation which leads to lower supercoolings. The higher the atomization gas velocities, and the lower the oxygen concentration in the container, the higher the solid fraction during the recalescence process. Under same conditions, the smaller the droplet in diameter, the higher the mass-specific enthalpy loss rate, and in final, the higher the proportion of metastable phase.更多还原