等离子弧穿孔熔池熔凝过程的绕流机制与变位置焊接稳定性控制

Addressing the keyhole molten pool broken problem appeared during the variable polarity plasma arc changing position welding, the mechanism linking flow characteristic parameters to solidification morphology is built based on the analysis of fluid flow in molten pool as Karman vortex street. The dynamic stability control strategy for changing position keyhole molten pool is proposed as: forced the keyhole expanding and contracting in a fixed frequency and magnitude to enhance the immunity of keyhole molten pool under the precondition of keeping the keyhole existing and wake closing. Use the pulse plasma gas variable polarity plasma arc to provide the pulse pressure to keyhole molten pool, to decrease the heat impact to the molten pool in mechanism. The study is carried out aiming at the melt flow behavior under variable force and state, the output characteristics of pulse plasma gas variable polarity plasma arc and its coupling mechanism to the keyhole molten pool. The interactive mechanism between molten pool vibration and interference of gravity is analyzed. The internal relations and regulatory mechanisms of molten pool vibration, stationary point location, wake closing condition and weld quality are revealed, to achieve the stability and quality control of pulse plasma gas variable polarity plasma arc in continuous changing position welding. All of these results can lay the foundation for large complex light alloy structure variable polarity plasma arc welding.

针对变极性等离子弧变位置穿孔焊接过程中出现的熔池失稳问题，从穿孔熔池绕流行为与卡门涡街现象的理论分析出发，建立熔池绕流特征参量与凝固形貌的关联机制；提出变位置穿孔熔池的动态稳定性控制策略，即在保证熔池小孔持续存在和绕流尾迹稳定闭合的前提下，令小孔以一定频率和幅度扩张与收缩，从而提高穿孔熔池抵抗外界扰动的能力；采用气脉冲复合变极性等离子弧为穿孔熔池提供脉动压力输出，从机理上减小对熔池的非预期热冲击。项目针对变力与变物态条件下金属熔体绕流行为、脉冲离子气作用下变极性等离子弧输出特性及其与穿孔熔池的耦合作用机理开展研究，分析熔池振荡与重力时变干扰之间的交互机制，揭示熔池受迫振动与绕流驻点位置、熔池尾迹闭合条件及焊缝成形质量的内在联系与调控机制，最终实现气脉冲复合变极性等离子弧变位置穿孔焊接的熔池稳定性与质量稳定性控制，为大型轻合金复杂结构的变极性等离子弧原位焊接制造与现场焊接装配奠定基础。

自2019年项目启动以来，本项目以等离子弧焊接过程中“穿孔熔池熔凝过程的绕流机制”和“变位置焊接稳定性控制”为重点研究方向开展研究。解决的关键问题和创新性研究成果概述如下:多位置连续穿孔施焊过程中，发现变极性电弧驱动下小孔-熔池体系内前壁面绕流和后壁面分流的三维流动行为，进而完善了穿孔焊小孔-熔池体系双驻点三维流动物理模型。提出了表征穿孔熔池稳定的行为特征向量，辨识任意位置等离子弧穿孔焊接熔池稳定性的新模型，最终解决了准稳态小孔熔池在复杂扰动下的状态表征和演变预测的基础科学难题。为了突破了焊接稳定性控制中，通过调整熔池状态以适应特定焊接条件的传统思路，提出了气-电联合复合脉冲增强了熔池自身的抗干扰能力和容错能力，解决因环境动态变化而导致的穿孔熔池失稳问题。针对VPPA穿孔焊接在焊缝路径空间位置发生连续变化时出现熔池失稳的问题，研究了焊接位姿变化时重力对穿孔熔池熔体形成与流动演化的影响规律，引入非对称施加热源来改善电弧熔池端的热力分布和增加传质应对因空间轨迹突变而引入的突发状况，保证了连续焊接的稳定性，取得了理想的焊缝成型。总结研究成果，在国内外学术期刊发表SCI论文19篇，参加会议并报告3 次；申请国家发明专利4项，其中授权3项。在课题研究过程中培养研究生7人，其中博士生2人；博士后1人。