摘要
焊接熔池的凝固过程直接决定着晶粒的形态、偏析程度、夹杂物的分布,影响着气孔及热裂纹等缺陷的形成,最终影响着焊缝的力学性能。然而,目前通过实验对焊缝凝固过程进行实时观察还难以实现,同时也无法准确地反映焊接熔池凝固过程中动态、瞬时等特点,因此通过数值模拟的方式对焊缝凝固过程中进行研究具有重要意义。本文中考虑了联生结晶、流体流动等焊接熔池中的凝固特点,构建了元胞自动机-有限差分-有限容积相互耦合的模型,再现了焊接熔池凝固过程的枝晶生长行为,分析了流体流动对枝晶形貌以及柱状晶一次枝晶间距的影响,并对熔池中的溶质再分配进行了深入的研究。
首先,本文在枝晶生长动力学的基础上,构建了枝晶生长速度模型。同时,为了模拟具有任意主轴角度晶粒生长,本文对之前学者所提出的“对角线模拟角度”法则进行修正,实现了在一套网格中进行多个偏角的计算,提高了计算效率,从而能够更好地描述焊接熔池中的联生结晶现象。在此基础上,开展了焊接熔池枝晶生长的模拟,再现了焊接熔池凝固过程的枝晶形貌演变。模拟结果表明,在凝固过程中,枝晶间的竞争生长激烈,主轴取向较好的晶粒可以获得持续的生长,而取向为其他方向的晶粒最终在竞争中被淘汰,最终形成错综复杂的熔池枝晶形貌。模拟结果与实验结果具有较好的一致性,说明本文所建立的模型能够反映熔池凝固过程的特点。
为了进一步研究焊接熔池中流体流动对枝晶生长的影响,本文构建了流场模型,并通过有限体积法对其进行求解。在求解过程中,分别应用交错网格技术和SIMPLER算法解决压力梯度项离散问题和缺少描述压力项独立方程的问题。在完成流场模型的建立后,将其与枝晶生长模型耦合,用溶质传输方程替代溶质扩散方程求解液相中的溶质分布。
在上述模型基础上,本文开展了流场作用下焊接熔池中等轴晶和柱状晶生长的模拟,并与纯扩散作用时的情况进行对比,再现了凝固过程中不同时刻枝晶前沿流体流动以及其对枝晶形貌和溶质分布的影响。模拟结果表明,相对于纯扩散作用,流场作用改变了枝晶前沿的溶质分布,进而改变了成分过冷,使枝晶的形貌发生了改变。来流方向不同时,枝晶生长形貌也不相同,但都有迎流侧枝晶生长速度较快,而背流侧生长速度较慢的特点。并且随着来流速度的增加,枝晶生长的不对称性更加明显。进一步研究了流场对柱状晶生长一次枝晶间距的影响。结果表明,流场改变了长势较弱枝晶尖端的溶质分布,使其在生长过程中更早地被淘汰,从而加快了枝晶间距的调节过程。
焊缝中的微观偏析可以引起非平衡第二相、气孔和裂纹等的形成,因此对其进行定量预测将为预测焊缝金属的力学性能提供依据。本文通过数值模拟研究了纯扩散和对流两种作用下柱状晶生长过程的溶质再分配和显微偏析的特点,并分析了焊接速度和形核密度对显微偏析的影响。研究结果表明,纯扩散作用下焊接熔池的柱状晶生长,沿柱状晶生长方向固相溶质浓度逐渐增加,但增加幅度逐渐减小,并最终趋近于一个稳定值;沿垂直于柱状晶生长方向,枝晶中心处溶质浓度较低,而二次枝晶部位溶质浓度较高。而对流作用下焊接熔池的柱状晶生长,其沿柱状晶生长方向的溶质分配与纯扩散情况类似;而在垂直于柱状晶生长方向,其溶质偏析程度要大于纯扩散时的溶质偏析程度;并且背流侧的溶质浓度要大于迎流侧的溶质浓度。在其他条件不变时,随着焊接速度的增加,溶质偏析程度更加严重;在其他条件不变时,随着形核密度的增加,熔池偏析程度减小。
在上述研究所建立的模型基础上,本文设计了一个焊接熔池枝晶生长模拟系统。该系统包括流场计算、温度场计算、枝晶生长计算和结果后处理四个模块,提供了简单易用的图形界面,实现了对于不同条件下焊接熔池枝晶生长的模拟,并可以对结果进行分析和后处理,以期达到对实际焊接过程提供参考的目的。
Solidification behavior during welding process controls the morphologies of microstructure, the extent of segregation, the distribution of inclusions, affects the formation of porosity and hot cracks, and ultimately influences the properties of the weld metal. To date, it’s still hard to observe the solidification process by experiment method, and the dynamic and instantaneous features of weld solidification process can’t be described accurately by experiment method either. So that it is of great importance to analyze solidification process of molten pool through numerical simulation method. In this dissertation, a coupled model of cellular automaton, finite difference and finite volume method is developed to reproduce dendrite growth behavior in molten pool. The characteristics of molten pool such as epitaxial growth and fluid flow are taken into consideration. The effects of fluid flow on dendrite morphology and primary dendrite spacing are studied, and solute redistribution in molten pool is also deeply researched.
First, the dendrite grain growth model is established based on grain growth kinetics theory. In addition, the method of“diagonal simulating angle”is modified and the computational efficiency is improved to simulate the grain growth with random crystallographic orientations, which could describe the epitaxial growth in molten pool better. Based on this model, the simulation of grain growth of full-sized molten pool is carried out, and the microstructure evolution during welding solidification process is reproduced. The simulated results indicate that the competitive growth among grains is severe during solidification process, and the grains with preferred orientations could grow constantly, while other grains are eliminated. Ultimately, a complicated morphology is formed in molten pool. The simulated results agree well with experiments, which confirm the reasonability and feasibility of the simulation model in this dissertation.
Flow field model is established to further study the effects of fluid flow in molten pool on grain growth, and the model is solved by finite volume method. Staggered-grid finite difference approach and SIMPLER algorithm are also employed to solve the problem of the discrete of pressure gradient term and the lacking of independent equation of pressure term, respectively. After the establishment of the flow field model, it is coupled with grain growth model, and the solute diffusion equation is replaced with solute transfer equation to solve the solute distribution in liquid.
The simulation of the equiaxed grain growth and columnar growth with fluid flow in molten pool is carried out based on the model mentioned above, and the results are also compared with the grain growth without fluid flow. The behavior of fluid flowing at the dendrite tips and its effects on dendritic morphologies and solute distribution is reproduced. The simulated results present that fluid flow alters solute distribution and constitutional undercooling tendency around the dendrite tips, accordingly, the morphologies of grains exhibit assymetrical. The dendritic morphologies are different with different inlet flow direction, but they all have the same characteristic that the dendrite arms on the upstream side grow fast, while the growth of dendrite arms on the downstream side is much delayed. With inlet velocity increasing, the asymmetry of dendrite growth becomes even severe. The effects of fluid flow on primary dendrite spacing are also examined. The results show that fluid flow changes the solute distribution at the dendrite tips of columnar grains that grow relatively slower and makes them eliminated in the competitive growth more quickly, accordingly, accelerates the primary dendrite spacing adjustment process.
Microsegregation in weld seam could induce the formation of second phase, pore and hot crack. Therefore, quantitative forecasting of the microsegregation could serve as basis for the prediction of the mechanical properties of weld metal. The characteristics of solute redistribution and microsegregation of columnar grain growth with and without flow are studied, and the effects of welding speed and nucleation density are also examined. The results indicate that for grain growth without flow, solute concentration increases from the bottom of the dendrite to the dendrite tips along the grain growth direction in the interior of a columnar grain, but increasing extent slows down. Eventually the solute concentration tends to a certain value. Along the direction normal to grain growth, the solid composition at the center of the main stem is low, while near the edge and at the secondary arms it is high. For grain growth with flow, the solute distribution along grain growth direction has the same tendency as that without flow, while the extent of solute segregation is greater along the direction normal to grain growth direction. With other conditions constant, with welding speed increasing, the extent of solute segregation becomes more severe, and with nucleation density increasing, the extent of solute segregation reduces.
A system for simulating grain growth in molten pool is designed based on this study, and it consists of 4 parts including fluid field computation, temperature field computation, grain growth simulation and data post-processing. The system provides a simple graphical user interface, by which the simulation of grain growth in molten pool could be completed easily and the results could also be post processed. The system is designed to achieve a goal of providing reference to actual welding process.
引文
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