基于半解析VOF-DEM的激光直接沉积多尺度过程模拟

与传统铸造技术相比, 基于金属粉末的增材制造技术因其生产周期短、可操作性强而在航空航天、生物医学等领域具有很好的优越性. 尤其是激光直接沉积技术, 因其自由度高, 在复杂构件制造、部件修复中有着广泛的运用. 但是该激光直接沉积过程涉及多物理场、跨尺度、极端高温高压环境和相变问题, 仅靠实验不能很好地研究其中的机理. 已有数值模拟技术一般通过预设或者射入拉格朗日点作为颗粒输入, 不能做到同时考虑环境气体、颗粒碰撞和相变过程. 本文在近期发展的基于核函数近似背景流场的半解析CFD-DEM耦合方法中引入了流体体积分数法(VOF), 发展了可以同时模拟含热、刚体颗粒、相变和自由液面及相变界面的半解析VOF-DEM (或半解析CFD-DEM-VOF)方法, 从而首次实现了真实物理环境下激光直接沉积技术的数值模拟. 其中, VOF中的气相为环境气体, 液相为熔融和凝固的金属相, 界面通过iso-Advector重构, DEM为未熔化的金属粉末, 且流体网格可解析离散元颗粒形状. 这一模拟框架可以有效复现颗粒之间的碰撞、粘结、熔化、融合, 以及熔池熔道的形成, 为激光直接沉积技术的数值模拟提供了开拓性的范式, 并可以被应用到其他带相变的颗粒系统中. 

WHOLE-PROCESS CROSS-SCALE MODELLING OF LASER DIRECT DEPOSITION WITH SEMI-RESOLVED VOF-DEM COUPLING

Compared to casting and other traditional manufacturing techniques, metallic powder-based additive manufacturing is manifesting its superiority in many fields like aerospace engineering, bio-medical engineering due to its short product cycle and feasibility. Among them, laser direct deposition, which has higher degree of freedom, has been widely employed in manufacturing and repairing complicated components. However, during its process, cross-scale multi-physics phenomena and phase change simultaneously happen under the laser spot, with extremely high temperature and pressure gradient, which makes experiments per se incompetent in investigations. In previous simulation frameworks, powders are inserted as Lagrangian points without consideration of ambient fluid, particle-particle interactions and phase change. The proposed framework here introduces volume of fluid technique into the recently-developed kernel approximation-based semi-resolved CFD-DEM, leading to a new semi-resolved VOF-DEM (or semi-resolved CFD-DEM-VOF) method which takes both thermodynamics, solid particles, phase change and free surfaces into consideration. Therefore, for the first time, the developed semi-resolved VOF-DEM model realizes the simulation of real physics involved in direct laser deposition. In this framework, shielding gas and metal, either melted or solidified, are two phases in VOF, and the interface between them is reconstructed by iso-Advector. DEM represents the unmelted solid particle, and CFD cells herein can resolve the metal particles thanks to the kernel approximation. Hence, the collision, adhesion, melting and coalescence of metallic particles, formation and evolution of molten pool and tracks are all reproduced. It is believed that this semi-resolved VOF-DEM modeling framework can provide a paradigm for simulation of direct laser deposition, along with other fields where particle system evolves with phase change.