液体流动和固相传输作用下枝晶生长行为的定量相场模拟研究

During solidification of steel castings, it is inescapably suffering from the influence of liquid flow which is due to the local density difference of melts. The liquid flow not only alters the transportation of heat and solute, but also leads to the movement of solid phase, such as the formed equiaxed dendrites and fragments of dendrite arms. The motion of solid phase results in the change of the original growth direction, growth environment and the growth behavior, meanwhile, it also redistributes the chemical elements in the castings. And thus the understanding of the dendrite growth behavior during solidification with consideration of interaction with flow of liquid and motion of solid is more significant to reveal the microstructure formation during complex solidification process, as well as the formation mechanism of macrosegregation in large scale of steel ingots. However, up to date, less reports can be found on the investigation on the dendrite growth with motion of solid phases both in theoretical analysis and numerical simulations. To model the dendrite growth under influence of liquid-solid flows is therefore the basic interest of this research topic. In this project, by in combination of the numerical simulation with in situ and real-time experimental observations, the fluid flow dynamic equations as well as the mechanics of solid motion are directly incorporated into the quantitative phase-field model of dendrite growth for pure heat or solute diffusion. Based on these methods, studies are focused on the evolution of morphology characteristics of dendrite and its growth dynamics under influence of liquid flow and solid transport. And then the simulation results will be quantitatively compared with in situ and real-time observed experiments by synchrotron radiation X-ray radiography. The achievements of these studies will deepen the insight on the dendrite growth fundaments, and eventually render theoretical guidelines to the understanding of the formation mechanism of macrosegregation in large scale ingots and solution to the reduction of such defect.

钢锭在凝固过程中，熔体内部不可避免地会存在自然对流。流动的液体不仅改变了凝固前沿的温度和成分分布，而且也致使已凝固的晶体（如等轴晶、脱落的枝晶臂等）发生迁移。固相的运动，不仅改变了晶体原有的生长环境、生长方向和生长趋势，也改变了钢锭的化学元素分布。因此研究液体流动和固相传输作用下晶体生长行为，对揭示复杂凝固过程中的组织演化特点和大型钢锭的宏观偏析机制有重要意义。然而，截至目前，考虑晶体运动的枝晶生长理论和模拟鲜有报道。本项目以此为出发点，采用数值模拟和原位观察实验紧密结合的研究方式，在计算纯扩散控制枝晶生长的定量相场模型基础之上，耦合流体动力学方程，一方面定量模拟热溶质自然对流作用下固定晶体的生长，另一方面同步耦合固相运动力学方程，模拟液固双相流作用下的枝晶生长，并将这些模拟结果与同步加速辐射X射线原位实时观察进行比较和验证。这一研究结果将会深化对枝晶生长过程和钢锭宏观偏析形成机制的认识。

在项目资助下，首先耦合固相颗粒运动方程，建立了考虑固相运动和液相流动的两相流晶体生长相场模型。其次，开发了求解复杂相场模型的分布式并行自适应有限元算法和程序，提高了计算效率和模拟尺度。再次，为进一步提高相场方法在模拟多晶凝固过程的效率，将定量相场模型与界面前沿追踪方法相结合，建立了高效率多晶相场模型。随后，为准确预测合金铸态组织与工艺参数的关系，模拟研究了定向凝固界面形貌演化过程，发现并揭示了海藻状组织的转变机制；结合晶体生长形核停止理论，建立了孕育处理铸态Al合金的晶粒尺寸预测解析模型。最后，为解决铸锭凝固偏析问题，在所建立的两相流模型基础上开发了多相流-宏观偏析模型，研究在夹杂物漂浮和枝晶间自然对流交互作用下，在钢中形成通道偏析的条件和演化机制。基于这些模拟和X射线原位实时观察，揭示了固液两相流作用下的晶体生长演化机制和生长动力学。根据固液两相流作用下的多晶生长模拟，阐明了铸锭中枝晶下落过程及铸锭沉积锥形成的基本机理。这些研究成果推动了凝固数值模拟研究的深入发展，深化了对枝晶生长过程和钢锭宏观偏析形成机制的认识，并为铸造工艺设计提供了基础理论指导。