Sustaining the open keyhole in slow-falling current edge during K-TIG process: Principle and parameters

• Published in: International journal of heat and mass transfer 2017-09, Vol.112, p.255-266
• Main Authors: Liu, ZuMing, Fang, YueXiao, Cui, ShuangLin, Yi, Song, Qiu, JiaYu, Jiang, Qu, Liu, Weidong, Luo, Zhen
• Format: Article
• Language: English
• Subjects: Analysis; Arc heating; CF-TIG; Current waveform; Energy efficiency; Energy managing; Evolution; Falling; Gas tungsten arc welding; GTAW; K-TIG; Keyhole; Metals; Process parameters; Rare gases; Thermal state; Welded joints; Welding; Welding current
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2017.04.092

•Keyhole dynamic evolution behaviuour during the current falling edge is imaged with a vision observation system. Keyhole is still fully open for a period even though the arc energy input is much lower than the threshold point.•Force field balance is analyzed in view of the saddle shape weld pool, Marangoni force is proposed to slow down the keyhole closing process during the current falling stage.•At a given situation, keyhole sustains for longer time with slower current falling speed or with the higher primary thermal state in weld pool.•Energy saving degree during the keyhole closing period increases with the keyhole sustaining period. Saving heat input from the arc is important to guarantee quality weld in keyhole mode Tungsten Inert Gas (K-TIG) Welding process. To this end, slowly falling edge was introduced in the current waveform. The fully-penetrated keyhole evolution behaviour during the current falling stage was visualized by a fast imaging system in real-time. It was found that, compared with the square current welding process, keyhole exit is closing at a slowly speed when the current is gradually decreasing and still opens even though the welding current is far below the threshold point. Based on the forces evolution analysis, Marangoni force in the weld pool should be the major factor to slow down the keyhole closing process during the current falling stage. Future experimental observation reflects that, the primary thermal state of the weld pool and the decreasing speed of the current can be used to control the keyhole closing behaviour. The degree of the saved energy input is related to the keyhole open period time in the slowly falling current edge. The research results lay foundation to deep understand the coupling behaviour between the “arc - weld pool - keyhole” system in K-TIG welding process and to further optimize the welding current waveform to stabilize the keyhole welding with low heat input.