高功率光纤激光焊接中超细长小孔内部羽辉气体动力学行为的演变规律和调控机理

In high power (~10kW-100kW) fiber laser welding, evaporation of the irradiated material is very intense, the high temperature, high speed, and transient metallic vapor plume is difficult to flow out from the super deep keyhole. Hence, process defects are very easy to be produced. Quantitative understanding the evolutions and process controlling mechanisms of metallic vapor plume dynamics in the dynamic keyhole could be served as a scientific basis for optimizing the quality of the heavy thick single pass welding joints. However, current theoretical modelling methods usually neglect the macroscale discontinuous Knudsen layer between keyhole, weld pool and vapor plume, or usually assume the fluid flow of the vapor plume outside the Knudsen layer is steady that is, unfortunately, fully distinct from the real physical situations in which the fluid flow of vapor plume is highly transient and self-consistently related to the Knudsen layer. Therefore, they cannot be used to accurately simulate the plume behaviors. To solve the problems, this project proposes a new theoretical modelling framework for revealing the controlling and evolutions mechanisms of the gas dynamics of metallic vapor plume in keyhole. In the proposed method, a mesoscale Lattice Boltzmann model is proposed to accurately simulate the physical behaviors of the boundary layer between keyhole and vapor plume, since the boundary layer may violate the macroscale assumptions of continuum mechanics. Besides, a transient aerodynamics model of vapor plume inside keyhole, and a model of keyhole and weld pool dynamics are constructed based on the macroscale assumptions of continuum mechanics for efficient modelling.Two-way coupling boundary conditions are theoretically proposed to simulate the self-consistent behaviors between the boundary layer and the vapor plume, as well as the keyhole and weld pool. Controlling and evolutions mechanisms of metallic vapor plume dynamics behaviors in super deep keyhole are quantitatively revealed based on the proposed macro-meso scale coupling theoretical framework and experimental characterizations.

几十至上百千瓦的高功率光纤激光焊接中材料蒸发异常剧烈，超细长小孔内部的高温、高速、瞬态金属蒸汽羽辉难以逸出小孔，易产生严重缺陷。定量理解孔内羽辉气体动力学行为的演化规律和调控机理，可为厚壁构件激光单道焊接的工艺优化和缺陷抑制提供依据。现有理论常忽略小孔熔池与羽辉间存在的宏观不连续Knudsen边界层；或者假设Knudsen层外羽辉流动处于稳态，这与羽辉流动的高度瞬态、且与边界层自洽相关的物理事实不符，因而难以精确描述羽辉的演化行为。针对上述问题，考虑宏观假设在羽辉边界可能失效，根据分子动理论，提出可准确描述边界行为的介观尺度Lattice Boltzmann模型；同时耦合孔内羽辉空气动力学和小孔熔池动力学宏观理论模型；构建宏-介观模型间的双向耦合边界，描述羽辉、边界和小孔熔池间的自洽作用；通过宏-介观集成模拟和实验观测，定量揭示超细长小孔内部羽辉气体动力行为的演变规律和调控机理。

超高功率的激光导致材料的剧烈蒸发，焊接过程稳定性差，易产生缺陷，是超高功率激光焊接在制造业中广泛应用的突出问题。超细长小孔内高温高速的蒸汽羽辉与小孔的剧烈热力交互作用是影响焊接稳定性和最终焊接质量的关键。本项目通过建立超高光纤激光焊接小孔熔池-羽辉蒸汽动力学自洽耦合模型，并结合高速摄像和光谱测量等试验手段，揭示了高功率光纤激光焊接过程中小孔、熔池、蒸汽羽辉的瞬态动力学行为，阐明了孔内蒸汽羽辉对孔壁的力学作用及缺陷诱导机制，能够为光纤激光焊接的工艺优化和过程监测提供直接的理论依据。开发了首款专门面向激光焊接工艺过程CAE软件iLaser，可为激光焊接的机理研究、缺陷预测和工艺优化提供可靠的数值仿真工具。

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