气雾化制粉工艺中基于气流场调控的卫星粉控制技术研究

RESEARCH ON SATELLITE-PARTICLE CONTROL TECHNOLOGY BASED ON GAS FLOW FIELD REGULATION DURING GAS ATOMIZATION PROCESS

赫新宇

11930371

硕士

材料工程

黎兴刚、朱强

2021

2021年6月21日

南方科技大学

深圳

增材制造技术（又称 3D 打印）是当今最有发展潜力的制造技术之一，这种技术颠覆了传统的加工理念，为未来的加工技术提供了广阔的发展空间。而作为整个增材制造领域中的重要部分，金属增材制造（Metal Additive Manufacturing）一直以来有着较好的发展前景。区别于传统的减材制造以及等材制造，金属增材制造有着复杂程度高、材料利用率高、设计自由程度高等显著优点。金属粉末是最常用的金属增材制造原料，然而，相对于传统的基于粉体的加工技术，金属增材制造技术对金属粉末的粒径分布、纯度、球形度等指标要求更高，用于热喷涂、粉末冶金等工艺的金属粉末不能直接用于金属增材制造。因此，有必要对金属粉末制备技术进行改进以满足金属增材制造的要求。金属熔体气雾化法是制备增材制造专用金属粉末的重要方法。然而，气雾化工艺制得的粉末中通常混有大量“卫星粉”，即若干小颗粒粉末粘附在大颗粒粉末表面的现象。卫星粉的存在会降低粉末的流动性，对一些基于铺粉工艺的金属增材制造过程产生较大影响，如激光选区熔化（Selective Laser Melting , SLM）、电子束选区熔化（Electron Beam Melting,EBM）等。因此，需要对气雾化工艺进行改进以控制卫星粉的形成。尽管目前已经存在针对卫星粉控制的气雾化工艺改进措施，但是基于这些改进措施的卫星粉控制机理没有被系统地研究。本文基于上述背景，具体研究内容如下：设置不同的雾化压力（1.0 MPa~5.0 MPa），分析和总结雾化压力对宏观气流场结构以及涡流强度的影响；采用 SST k-ω 湍流模型，在雾化室顶端三个不同位置分别施加环缝辅助气流，并在各个位置分别改变辅助气流与雾化气流的流量比，模拟上述条件下雾化室内的宏观气流场。利用欧拉-拉格朗日法（Euler-Lagrange, E-L），在收敛的气流场中加入离散相颗粒，采用离散相数值模拟（Discrete Phase Model, DPM）对离散相的运动轨迹进行跟踪，并研究辅助气流的位置、流量对离散相运动轨迹的影响；采用阶梯状的雾化室结构，分别模拟阶梯的宽 D 以及阶梯的高 ΔH 对雾化室内宏观流场结构的影响。在已收敛的气流场中加入离散相，分析阶梯状雾化室的结构参数对离散相运动轨迹的影响，根据数值模拟结果确定优化的阶梯状雾化室结构；基于数值模拟结果，分别采用优化的辅助气流位置、流量以及优化的阶梯状雾化室结构，制备 TC4 钛合金粉末，并检测粉末的粒径分布、球形度、赘生物指数等。 I摘 要数值模拟与制粉试验结果表明：雾化压力的增加不会引起宏观涡流结构的变化，反而会加剧粉尘回旋；引入辅助气流和采用阶梯状的雾化室结构都能够有效地减少颗粒-熔滴碰撞；在距离雾化室中心 4/4R 处施加与雾化气流流量相同的辅助气流时最有效（R 为雾化室顶端半径）；阶梯宽为 300 mm、高为 575~600 mm 的雾化室结构最有效；采用优化的气流场调控参数制备 TC4 钛合金粉末，并检测粉末的粒径分布、球形度、赘生物指数等指标，发现气流场调控措施能够有效地减少气雾化工艺中的卫星粉含量。本文所研究的气流场调控技术对气雾化制粉过程抑制卫星粉的形成具有重要的理论和实践意义。减少金属粉末中卫星粉的含量可以改善粉体的表面质量，提高粉体的流动性，进而有利于提高金属增材制造工艺的稳定性以及金属增材制造构件的机械性能。

Additive Manufacturing (AM), also known as 3D Printing, is one of the most potential manufacturing technologies nowadays. It subverts the concept of traditional technology and provides extensive development space for future manufacturing methods. As an essential part of AM field, Metal Additive Manufacturing has bright prospect. Different from some traditional manufacturing techniques like Reduced Manufacturing(RM) or Equal Manufacturing (EM), AM has the advantages of high complexity, high material utilization and high degree of design freedom. Metal powder is the most commonraw material used by metal AM. However, in contrast to the traditional manufacturing techniques, metal AM requires specific particle size distribution, higher purity and sphericity. In other words, metal powders that are produced for traditional processing methods (such as thermal spraying or powder metallurgy) can’t be applied into metal AM directly. Therefore, in order to fulfill the requirements of metal AM, it is necessary to optimize the preparation techniques of metal powder.Gas Atomization (GA) is an essential method for the preparation of metal powders used for additive manufacturing. However, there exist quantities of “satellites” in the metal powders prepared by GA, that is, some particles with smaller size adhere to the surface of larger particles. The existence of those satellite-particles will reduce the flowability of the powder and significantly affect the spreading process of some metal AM methods based on powder bed fusion, such as Selective Laser Melting (SLM) or Electron Beam Melting (EBM). Therefore, GA process should be optimized to restrict the formation of satellites. Even though there already exist some improvement measures designed to reduce satellites during GA, the mechanism of such measures on satellites control has not been explored systematically yet. Based on the above background, themain research work of this dissertation is as follows: The development of macro-scale flow field pattern with the change of atomization pressure (1.0 MPa~5.0 MPa) was summarized; ancillary gas flow was introduced at three different locations by employing SST k-ω turbulence model. Meanwhile, the ratio of ancillary gas flow rate to atomization gas flow rate was changed at such locations,respectively. Discrete phase was added into the already convergent continuous phase field IIIAbstractand then was tracked by utilizing Euler-Lagrange method. The trajectory of particles was calculated by employing Discrete Phase Model (DPM). The influence of ancillary gas flow injection position and rate on trajectories of discrete phase was also researched; anew atomization chamber structure named step-shape chamber was designed to regulate the macro-scale flow field pattern. The influence of step width (D) and step height (ΔH) on macro flow field pattern was numerically investigated. Similarly, the discrete phase was added into the already convergent gas flow field. The influence of step-shape chamber parameters on the trajectories of discrete phase was analyzed. According to simulation results, the most optimized structure of step-shape chamber was summarized; based on the above simulation results, the optimized parameters were applied in the practical preparation of TC4 powders. The size distribution, sphericity and outgrowth rateof such powders were detected,respectively. The results showed that the increase of atomization pressure could not change the structure of macro eddy, instead exacerbating the swirl of dust; the introduction of ancillary flow as well as the application of step-shape chamber both can reduce the droplet-particle collision effectively; the most effective restriction happens when the ancillary gas flow is introduced at 4/4R from the chamber center with the ancillary gas flow rate equal to atomization gas flow (R is the radius of the top of chamber), or when the width and the height of the step-shape chamber are 300 mm and 575~600 mm, respectively; some TC4 powders were prepared under the above optimized parameters. The size distribution, sphericity and outgrowth rate of such powders were detected, respectively, which showed that the gas flow field regulation strategies can effectively reduce the formation of satellite-particles.Flow field regulation technology studied in this thesis has vast theoretical and practical significance of controlling the formation of satellites during GA. Lowerproportion of satellite-particles in metal powders will be beneficial to improve the powder surface quality and flowability, thus enhancing the stability of metal AM process and mechanical properties of the final products.

气雾化 卫星粉 计算流体力学 气流场调控 赘生物指数

gas atomization satellite-particle computational fluid dynamics gas flow field regulation outgrowth rate

中文

独立培养

南方科技大学

赫新宇. 气雾化制粉工艺中基于气流场调控的卫星粉控制技术研究[D]. 深圳. 南方科技大学,2021.