钛合金激光重熔熔池自由表面二维约束凝固行为与组织调控机理

液态金属自由表面的凝固是一种常见的物理现象却很少被关注。激光重熔熔池自由液面的凝固行为极具代表性，也是以激光等高能束为热源进行的熔覆、焊接、增材制造等加工技术面临的共性问题，会影响构件凝固组织、表面缺陷与表面性能。例如激光定向凝固杂晶组织、激光增材沉积层间冶金缺陷和激光表面渗氮高硬度渗层等的形成均与熔池表面的凝固行为有关，但一直缺乏针对性研究。申请人首次在激光重熔TC18钛合金中发现荷叶状枝晶与表面晶膜等新型凝固组织，其特征可反映熔池表面凝固过程。本项目将通过改变熔池凝固条件并结合定量多相场模拟，认识荷叶状枝晶与表面晶膜的成分、结构、形态与分布特征及其随凝固条件的演化规律，揭示熔池表面强过冷薄层熔体二维约束条件下的形核机理、生长取向选择机制和冶金缺陷形成规律，建立重熔熔池晶粒竞争生长模型并提出凝固组织调控方法及原理，为激光微小熔池凝固组织调控技术提供理论指导，为金属凝固理论研究提供新认识。

The solidification behavior at free surface of liquid metal is a common physical phenomenon, but little attention has been paid to it. The free surface solidification behavior of laser remelting molten pool is very representative. And it is also a common scientific issue in cladding, welding, additive manufacturing and other processing technologies using laser or other high energy beam as heat source, which can directly affect solidification structures, surface defects and surface properties of components. For example, the formation of stray grains in laser directional solidification, metallurgical defects between deposited layers during laser additive manufacturing and high hardness infiltrated layer of laser surface nitriding are all related to the solidification behavior at molten pool surface. However, there has been a lack of targeted research for a long time. The applicant obtained two kinds of new solidification structures, lotusleaf-like dendrite and crystal film, in laser remelting zone of TC18 titanium alloy for the first time. Their morphological characteristics can reflect the solidification process of the molten pool surface. In this study, different laser remelted molten pool solidification conditions will be realized and a quantitative multiphase field simulation will be used to understand composition, crystal structure, crystal orientation and distribution characteristics of the lotusleaf-like dendrite and crystal film. Their change rules with different solidification conditions will be known simultaneously. Based on these, non-equilibrium solidification nucleation mechanism and growth orientation selection mechanism of thin layer melt with strong undercooling at molten pool surface under two-dimensional constraints will be elucidated. The formation mechanism of surface metallurgical defects will then be revealed. Moreover, a competitive growth model of remelting molten pool grains will be established with grain structure control methods and principles proposed. This study can provide theoretical guidances for the solidification-structure-control technique of all the laser processing techniques with micro molten pools. Meanwhile, a new understanding for the solidification basic theoretical research will be presented.