气体雾化制粉过程中金属熔体在导流管内的流动与传热机理研究

RESEARCH ON FLOW AND HEAT TRANSFER MECHANISM OF LIQUID METALS IN MELT DELIVERY NOZZLE DURING GAS ATOMIZATION PROCESS

刘畅

11849275

硕士

材料工程领域工程

朱强 黎兴刚

2020-06-02

2020-07-20

哈尔滨工业大学

深圳

金属增材制造（Metal Additive Manufacturing）是整个3D打印体系中最有发展潜力的领域。相对于传统的制造技术，金属3D打印在制备复杂形状构件和功能梯度材料方面具有独特的优势，在医疗、航空航天、汽车制造等领域有广阔的应用前景。粉末是金属增材制造工艺中最为广泛的原料形式，但粉末冶金、热喷涂等传统工艺使用的原料粉末不能直接应用于金属3D打印。一般而言，金属增材制造相对于传统工艺对原料粉末的粒径、形状、杂质含量等指标要求更高。例如，选区激光熔化（Selective Laser Melting，SLM）工艺要求原料粉末粒径为15-53 μm，形状为球形或近球形，纯净度高。因此，针对金属增材制造的粉末制备技术要相应改进。气雾化法是生产金属3D打印粉末的重要方法。目前，大部分研究集中在雾化气体流场和雾化喷射过程的数值模拟与实验观察，但对于气雾化制粉过程中金属熔体在导流管内的流动与传热过程关注较少，该过程作为整个雾化制粉过程中的前导部分，是后续稳定连续生产的前提。对上述过程的机理研究，对所有采用导流管导流的气雾化制粉工艺都有参考意义。围绕上述过程，本文的主要工作如下：1) 基于ANSYS Fluent计算流体软件，利用Volume of Fluid（VoF）两相流模型和Shear-Stress Transport k-ω（SST k-ω）湍流模型对金属熔体在导流管内的瞬态发展过程进行模拟，重点研究导流管参数对金属熔体（以熔融铝为例）流动的影响。分析和总结了接触角、粗糙度高度、导流管直径、导流管长度等参数对金属熔体在导流管轴线上的轴向速度分布和导流管长度1/2处轴向速度的径向分布的影响规律。2) 基于SST k-ω湍流模型、添加低雷诺数修正选项（Low Reynolds number correction）的SST k-ω湍流模型以及Spalart-Allmaras（SA）湍流模型，模拟计算了熔融铝、铁、镍三种金属熔体在不同直径导流管内的流动阻力。分析和讨论了不同湍流模型下的结果差异并将数值模拟结果与经验公式计算数值对比。以铝熔体为例，采用SST k-ω湍流模型，研究了在不同平均速度下熔融铝在相同直径导流管内的流动阻力，并对比了导流管直径和平均速度二者对流动阻力的影响差异。3) 使用SST k-ω湍流模型和凝固—熔化（Solidification / Melting）模型，系统地研究了熔融铝、铁、镍三种金属熔体在导流管内的传热过程。重点分析研究了熔融铝、铁、镍在不同直径导流管内的温度场云图分布，轴线上的温度分布以及导流管长度1/2处的径向温度分布。以铝熔体为例，分析并对比了导流管直径、入口速度、壁面温度等参数对金属熔体传热过程的影响。4) 搭建模拟验证装置，利用流体流动的相似性，选用常见的流体如水和酒精等作为实验材料，对金属熔体在导流管内的流动过程数值模拟结果进行验证。收集在导流管内已经凝固的金属熔体并进行金相组织分析，对金属熔体的传热数值模拟结果进行对比验证。5) 在数值模拟结果的基础上，优化导流管设计参数，制备三种典型的金属增材制造用铝、铁、镍合金粉末并进行标准化检测。本文中导流管内熔体流动阻力计算和传热过程分析对解决导流管堵塞和优化导流管设计具有重要的理论和实践意义，有利于提高金属增材制造用原料粉末适用粒度区间的粉末收得率。

Metal additive manufacturing gains a promising expectation in the entire 3D printing system. Compared with traditional manufacturing techniques, metal 3D printing has unique advantages in preparing complex shaped components and functionally gradient materials, and has broad application prospects in medical, aerospace, automobile manufacturing and other fields. Powder is the most commonly used feedstock form for metal 3D printing. However, the raw powders used in traditional processes, such as powder metallurgy, thermal spraying and other processes, cannot be directly applied to metal 3D printing. Generally speaking, metal additive manufacturing has higher requirements on the particle size, shape and impurity content of feedstock powders than traditional processes. For example, the selective laser melting (SLM) process prefers metal powders which are in a size range of 15-53 μm, spherical or nearly spherical, and of low impurity. Therefore, the production method of metal powders for metal additive manufacturing is supposed to be improved accordingly. Currently, gas atomization is an important method to produce metal powders for 3D printing. Most of the previous research has focused on the numerical simulation and experimental observation of atomization gas flow field, atomization and spray process. However, little attention has been paid to the process of metal melt flowing and heat transfer in melt delivery nozzle (MDN). This process is the fundamental part of the whole gas atomization production, and it is essential for the stability and continuity of the subsequent atomization process. The investigation on the mechanism of metal melt flowing and heat transfer in the MDN can provide theoretical support and technical reference for all the gas atomization technique using the MDN for melt delivering. Therefrom, the main work of this thesis is as follows:1) The Volume of Fluid (VoF) two-phase flow model and Shear-Stress Transport k-ω (SST k-ω) turbulence model which have been implemented in the ANSYS Fluent computational fluid dynamics software were employed to investigate the transient development process of the metal melt in the MDN, with an emphasis on the influence of the MDN parameters on the flow of metal melt (taking molten aluminum as testing material). The effects of the following parameters, including contact angle, roughness height, MDN diameter, MDN length, on the axial velocity distribution of the metal melt on the axis and the radial distribution of the axial velocity at the position of the 1/2 length of the MDN were analyzed and discussed.2) Three different turbulence models, i.e., the SST k-ω turbulence model, the modified SST k-ω turbulence model with the low Reynolds number correction option and the Spalart-Allmaras (SA) turbulence model, were employed to calculate the flow resistance of liquid metals (molten aluminum, iron, and nickel) in the MDN under different nozzle diameters. The numerical results based on different turbulence models were analyzed and compared with those derived from empirical formulas. Taking molten aluminum as an example, the SST k-ω turbulence model was used to study the flow resistance under different mean velocities, and the effects of the MDN diameter and mean velocity on the flow resistance were carefully compared.3) The SST k-ω turbulence model and the Solidification / Melting model were employed to systematically investigate the heat transfer process of molten aluminum, iron, and nickel in the MDN. The numerical results and analysis focused on the temperature field cloud distribution, the axial temperature distribution and the radial temperature distribution at the position of 1/2 length of the MDN under different MDN diameters. Taking molten aluminum as an example, the effects of the MDN diameter, inlet velocity and wall temperature on the heat transfer of liquid metal were analyzed and compared.4) Based on the similarity of fluid flow, water and alcohol were selected as experimental materials in the simulation verification device to validate the numerical simulation results on the flow process of the metal melt in the MDN. Additionally, the solidified metal melt in the MDN was collected and the metallographic structure was analyzed to verify the heat transfer process.5) Based on the numerical results, the experimental MDN parameters were optimized for gas atomization process to produce three typical powders of aluminum, iron, and nickel alloys for metal additive manufacturing. The standardized testing was subsequently conducted on these metal powders.The calculation of melt flow resistance and the analysis of melt heat transfer process in the MDN are of theoretical and practical significance for avoiding the clogging of the MDN and optimizing the design of the MDN. This is helpful to increase the yield of powders with suitable size range for metal additive manufacturing.

增材制造 金属粉末 气雾化 导流管 流动阻力 传热

additive manufacturing metal powder gas atomization melt delivery nozzle flow resistance heat transfer

中文

联合培养

南方科技大学

刘畅. 气体雾化制粉过程中金属熔体在导流管内的流动与传热机理研究[D]. 深圳. 哈尔滨工业大学,2020.