高亮度激光厚板焊接Z向多物理行为与气孔演化关联机制研究

During high-brightness laser welding on components with heavy section, keyhole-induced porosity become a key issue to determine weld quality. In this process, intensive and multi-angle evaporation of metal vapor leads to more complex deformation of keyhole, violent oscillation of molten pool and high cooling rate due to heavy sections, which tends to bring more pores. With the increase of thickness, the couple behavior between keyhole and laser beam, keyhole fluctuation, fluid pressure and solidification behavior of molten pool will play more important role in determining the formation of keyhole-induced pore in high-brightness laser welding. In this proposal, in-situ observation and a novel Nano-CT technique to represent three-dimensional 3D characteristics of porosity in space with high precision will be taken to study the defects. Meanwhile, based on the method of synchronous multi-curves absorption and reflection to laser beam, a multi-phase and multi-surface 3D numerical model will be established to reveal the scientific problems such as bubble formation, movement and evolution during laser welding. Based on experimental study and numerical model, the dynamic variation, solidification behavior in Z direction and the correlation regularity with porosity will be clarified at different radiation angles. To elucidate the temperature, pressure, velocity gradient in Z direction and fluid flow, evaporation and solidification and its correlation mechanism with the formation of porosity. The critical condition of porosity formation and controlling factors will be revealed, and then effective method to reduce pore and move out of molten pool will be put forward. The research results of this proposal can provide theory support for pushing further application of high-brightness laser welding on heavy sections.

高亮度激光厚板焊接中高强度、多角度蒸发致使匙孔界面形变更为复杂，熔池震荡加剧，凝固速率提高，从而易诱发形成气孔缺陷，成为影响焊接质量的关键问题之一。厚板激光焊时，匙孔形貌、光束与孔壁间能量耦合、流体压力、凝固速率等在Z向的特征变化对气孔缺陷的影响尤为突出。针对厚板激光焊接气孔形成过程这一科学问题，本项目拟采用原位观察、气孔Nano-CT三维空间精确表征等先进手段，并基于同步多曲面光束吸收与反射方法，构建多相、多自由界面三维瞬态气孔形成过程综合模型，研究不同激光入射角下熔池Z向的温度梯度、压力梯度、速度梯度及蒸发、流动、凝固等物理行为对气泡产生过程的影响，揭示气泡的生成、迁移及向气孔演化的时空路径，阐明匙孔波动、熔池流动、凝固速率梯度与气孔演化的内在关联机制，揭示气孔产生的临界条件及其控制因素，从而提出优化气泡逸出熔池的有效举措。本项目的研究可为推动高亮度激光在厚板焊接中的应用提供理论支撑。

大厚度构件高效率、高质量焊接是核电大型装备制造业面临的一个瓶颈，而高亮度激光焊接方法由于高能量密度、低变形及高焊速成为研究的热点。但是高亮度激光焊接大厚度构件时，小孔型气孔和裂纹等缺陷成为影响厚板高焊接质量的关键问题。本课题针对核电转子用钢，利用超窄间隙光纤激光焊接工艺，研究了焊接工艺参数对焊缝内部缺陷的影响规律，建立了大厚度构件光纤激光深熔焊三维热-流体焊接仿真数值模型，揭示了周期性凝固裂纹缺陷和周期性链状气孔缺陷的形成机理，阐明了超窄间隙光纤激光焊接工艺中坡口存在对气孔缺陷行为的影响机制，为核电厚板光纤激光深熔焊缺陷的形成及控制提供理论支持。研究表明随着热输入降低，焊缝深宽比逐渐增大，而凝固裂纹敏感性先增后减，并在线能量控制在900-1000 J/mm有效消除了裂纹缺陷。研究还发现焊缝中下部分布着与底部熔合线周期性波动一致的周期性链状气孔，这是由周期性产生的中部向内凹陷的熔池形状所造成的。该熔池形状在小孔后部熔池中两大涡流和位于顺时针涡流路径上气泡的共同作用下形成，会大大增加气泡逸出难度，导致气孔缺陷。同时发现深坡口条件下，熔池中上部的Z向导热使得熔池冷却速度加快，从而导致凝固前沿推进速度较快，易形成单个分散气孔，该气孔聚集分布在距离底部熔合线一定高度的区域内，线能量大于1000 J/mm时可有效降低气孔率甚至完全消除小孔型气孔缺陷。