铝合金激光深熔焊在负压与磁场双重作用下的熔池行为及熔透控制机理

The light high-strength aluminum alloy is getting to be one of the key materials for manufacturing the low-temperature equipment due to its high cryogenic toughness, according to the requirements of the national medium- and long-term scientific and technology development strategy for the new materials of major equipment. The high power laser welding under sub-atmospheric pressure can be used to obtain a rather higher one-pass welding penetration, and it has obvious advantages in welding of medium-thick aluminum alloy components. However, the formation of its back weld bead is still hardly to be controlled, which is remained to be a bottleneck problem that restricts the manufacture of aluminum alloy components with laser welding. Based on the achievements in research on laser welding under sub-atmospheric pressures for several years, a novel weld formation control method with magnetic field for deep penetration laser welding of aluminum alloy under reduced ambient pressure is proposed in this project. The dynamic force balance of the molten pool can be altered by the additive magnetic field, and the flowing behavior of the molten pool can be affected to improve the weld formation. Firstly, based on the established laser welding experimental platform integrated with sub-atmospheric pressure and additive magnetic field, the molten pool fluid flow behavior and the penetrated weld bead formation characteristics of aluminum alloy laser welding will be systematically studied under reduced ambient pressure and magnetic field, and the influence law of the magnetic field parameters and the sub-atmospheric pressures on molten pool and weld bead formation will be clarified. Secondly, the interaction mechanism of both the reduced ambient pressure and magnetic field on the molten pool behavior will be deeply studied by numerical modeling method based on multi-field coupling to reveal the inherent relationship between the molten pool morphology and the weld formation. The critical factors to control the penetration of aluminum alloy laser welding will be clarified. The research achievements of this project can be used to provide a technological support and theoretical basis for the manufacturing of cryogenic aluminum alloy components.

根据国家中长期科技发展战略对新材料制备低温重大装备的要求，具有低温高韧性的轻质高强铝合金成为低温设备制造的关键材料之一。负压激光焊可一次性获得更大熔深，在中厚铝合金构件焊接制造方面具明显优势。然而其激光穿透焊背部成形难控制，成为制约铝合金构件焊接制备的瓶颈。本项目基于多年来负压激光焊的研究成果，提出一种通过外磁场控制铝合金负压激光焊缝成形新方法。外磁场的加入可改变熔池受力状态，影响熔池流动行为从而改善焊缝成形。基于构建的负压-外磁场激光焊熔透与焊缝成形控制一体化实验平台，系统研究负压与外磁场双重作用下，铝合金激光焊接熔池流动行为及穿透焊缝成形特征，阐明磁场及负压参量对熔池及焊缝成形的影响规律；并结合多场耦合数值建模方法，研究负压和外磁场双重作用对熔池流动行为的作用机理，揭示其与熔池形态及焊缝成形之间的内在关联机制，明确铝合金激光焊接熔透控制关键因素，为铝合金构件制备提供技术支持和理论依据。

铝合金作为一种高韧性的轻质高强结构材料在能源、航空航天和国防装备制造业中具有重要地位。传统的电弧焊连接方法存在焊接效率低、变形大、接头残余应力高、性能差等诸多问题。采用负压激光焊可获得更大的单道熔深，在铝合金中厚板焊接中具有明显优势。然液态铝合金黏度低、表面张力小，激光穿透焊接时，金属蒸汽的反作用力易导致熔池下淌，造成焊缝表面下凹和背部驼峰，焊缝成形难以控制，成为制约铝合金结构件焊接制造的瓶颈。本项目提出了一种通过外磁场控制铝合金负压激光焊缝成形的新方法。基于自主研发的磁场辅助负压激光焊接实验平台，系统开展了常压和负压环境下不同磁场强度和频率下铝合金激光穿透焊的焊缝成形研究，找出了影响焊缝成形的关键因素。采用不同方式分别建立了多重反射的单相模型和基于蒸发与冷凝模型的多相流模型，探究了金属蒸汽相对于熔池流动模式、匙孔波动特征和激光-匙孔能量耦合及传热传质行为的影响。基于常压环境下实验和数值模拟结果，阐明了不同的磁场强度下，激光与焊接熔池的相互作用机制，揭示了小孔及熔池动态行为在不同磁场下的演化规律，诠释了磁场对于熔透焊缝成形和气孔缺陷的影响规律，阐明了交变磁场关键参量对铝合金激光穿透焊熔池瞬态过程、匙孔动态行为、根部驼峰及小孔型气孔形成的影响规律。基于负压环境下的实验结果，并结合数值模拟方法，研究了小孔熔池行为与磁场及环境压力等关键参数之间的内在关联性，阐明了负压环境和交变磁场共同作用下，熔池流动对熔透行为的影响机制，关键参量对铝合金激光穿透焊焊缝成形的影响规律及临界条件。结果表明，随着环境压力的降低，焊缝熔宽逐渐减小，根部下漏的高度逐渐降低。随着磁感应强度和磁场频率的增加，根部下漏和上表面凹陷逐渐被改善。对于厚度为10mm的铝合金，当环境压力为3kPa，磁感应强度为80mT，磁场频率为300Hz，激光功率为6.9kW时，焊缝成形最佳。研究成果为铝合金中厚板激光穿透焊的焊缝成形控制提供了有效手段和理论支持。