基于脉冲电流调控的水下湿法FCAW熔滴自由过渡机理研究

For the underwater wet flux-cored arc welding (FCAW), there exist complicated and coupled physicochemical reactions between the droplets, arc, bubbles, and the water environment. The current repelled droplets transfer and short-circuit transfer seriously restrict the weld formation and welding process stability. From the angle of forces control, a new approach of controlled free flight droplet transfer based on the pulse current is proposed to solve this problem in this project. The mechanisms of the rapid changing current influencing the thermal-force conditions of the droplets are analyzed. The scientific principle of lowering the repelling forces including gas drag force and arc forces in the same time via reducing the current rapidly is investigated. The synergism of inertial momentum and increased magnetic force is studied to push the droplets downward detaching from the wire tip. The repelling forces are reduced to maintain the droplet flight paths coaxial with the wire. Subsequently “One pulse one droplet” transfer mode with higher frequency, smaller size and coaxial flight paths is realized and the responding pulse parameters are acquired. Numerical models for direct and pulse current controlled wet FCAW are both developed. Afterwards, the physical processes including droplets formation and detachment, forces on droplets, arc behaviors, and bubbles evolution, etc. are all investigated quantitatively to examine the pulse current influences. In addition, the main characteristics and weld seam formation mechanism of this new approach are investigated. The preliminary theoretical structures of this new approach are expected to be established. It provides a novel active control method of the droplet transfer and is expected to improve the welding quality, process robustness and stability with practical application value and theoretical significance.

水下湿法FCAW气泡内部的熔滴、电弧与水环境持续发生互相耦合的、复杂的化学和物理作用，针对其特殊的熔滴过渡形式对焊缝质量和焊接过程稳定性形成严重制约的问题，本项目从控制熔滴受力的角度，提出一种基于脉冲电流调控的水下湿法FACW熔滴自由过渡新工艺。研究电流快速变化对熔滴形态和动态行为的热-力作用机制，揭示急速降低电流以减弱气体拖拽力和电弧力对熔滴排斥的科学依据，明确熔滴惯性动量与电磁力协同作用控制熔滴下落的机理，研究调节电流降低排斥力保证熔滴沿轴线下落的方法，获得较高频率和较小尺寸的“一脉一滴”过渡模式及脉冲参数设定规则，建立直流/脉冲电流条件下的熔滴过渡数值模型，定量分析脉冲电流作用下熔滴形成和脱落、熔滴受力、电弧形态以及气泡形态演变等物理过程，揭示该新工艺的特点及焊缝成形的规律，初步建立脉冲电流湿法FCAW熔滴可控自由过渡理论架构，为改善焊接过程稳定性和提高焊缝质量奠定基础。

人类对于海洋资源的需求日益迫切，海上石油平台、海底油气管线以及海上船舰等工程建设日益增多，而这类重大设施的紧急维修和日常维护等与水下焊接技术紧密联系。基于自保护药芯焊丝的水下湿法焊接技术（flux-cored arc welding, FCAW），具有方便操作、高效率、低成本和高质量等诸多优点，是水下自动化和智能化焊接的重要发展方向之一。因此，本项目提出基于脉冲电流调控的水下湿法FCAW熔滴自由过渡机理研究，从改变熔滴受力条件入手，通过调节焊接电流对气体拖拽力和电弧力等进行调控，实现了较高频率的、较小熔滴尺寸的“一脉一滴”自由过渡模式。主要研究内容包括以下几个方面：脉冲湿法FCAW熔滴过渡及气泡行为一体化调控机制、焊缝成形及焊接过程稳定性研究及水下湿法直流/脉冲FCAW熔滴过渡过程的数值模拟，并在此基础上发明了一种显著减少水环境干扰的新型水下埋弧焊接方法。. 利用同步检测视觉信号和电信号的手段，开展脉冲电流对熔滴过渡控制技术的研究，分析了脉冲电流对熔滴过渡及气泡行为的影响。设计了波形实现脉冲电流对熔滴过渡调控，提高了熔滴过渡频率，减小了熔滴直径大小。并提出了对熔滴过渡和气泡行为进行一体化调控，实现了“一脉一滴两泡”稳定的熔滴过渡过程。在此模式下，熔滴热量输入更加均匀，气泡对熔滴的干扰作用小，焊接过程更加稳定，水下焊缝的组织和性能得到改善。根据流体力学与电磁学基本理论，建立了水下药芯焊丝湿法焊接熔滴过渡过程的三维数值分析模型。考虑重力、表面张力、电磁力以及气泡上浮对熔滴过渡的影响，以及由药芯焊丝产生的气流和金属蒸发对熔滴的作用力，使熔滴会绕焊丝轴线旋转，分别进行了直流及脉冲条件下的熔滴过渡数值计算，分析了不同时刻的熔滴与电弧的形态、温度、电磁力及电流密度等物理量的分布及变化趋势。经过验证，数值计算结果在熔滴的运动行为以及电弧的飘移行为与实验结果基本一致。本课题优化了水下湿法焊接熔滴过渡过程，提高了焊接过程稳定性，改善了焊缝成型，丰富了现有的水下湿法焊接工艺，具有重要的学术理论意义和工程应用价值。