Zhuojin Pipe Fitting Co., LtdHigh-Frequency Composite Dual Tungsten Electrode TIG Welding and Digital Power Supply
Literature Overview
The 2018 paper by Wu Tongli, Wang Kehong, Yang Jiajia, and Zhou Xiaoxiao, published in the Transactions of the Welding Journal (Volume 39, Issue 9, pages 117-121), presents a novel non-consumable electrode composite welding process and the corresponding digital power supply development. The authors, from Nanjing University of Science and Technology's School of Materials Science and Engineering, proposed a high-frequency composite dual tungsten electrode TIG welding method that simultaneously outputs two distinct current waveforms through two separate tungsten electrodes. The research was funded by the National Defense Basic Research Project (JCKY2016208A001), indicating its relevance to defense and aerospace applications.
Process Concept and Power Supply Architecture
The core innovation of this work is the development of a digital power supply capable of simultaneously driving two tungsten electrodes with different current waveforms. One electrode carries a high-frequency square-wave current, while the other carries a polarity-reversing square-wave pulsed current. This dual-waveform approach combines the benefits of high-frequency welding (refined arc, reduced heat input, improved stability) with the cathodic cleaning effect of polarity reversal (oxide removal on aluminum and titanium alloys).
The power supply specifications are as follows:
| Current Channel | Waveform Type | Base Current | Peak Current | Frequency | Duty Cycle | Current Rise Rate |
|---|---|---|---|---|---|---|
| Channel 1 | High-frequency square wave | 0-300 A | 0-100 A | 0-80 kHz | 0-100% | 50 A/microsecond |
| Channel 2 | Polarity-reversing square wave pulse | 0-500 A (positive/negative half) | 0-500 A | 0-100 Hz | 0-100% | 300 A/ms |
The high-frequency channel operates at frequencies up to 80 kHz, which is significantly higher than conventional pulsed TIG welding frequencies (typically 1-100 Hz). At these frequencies, the arc behavior transitions from a quasi-steady state to a rapidly fluctuating regime, with the arc constricting and expanding thousands of times per second. The polarity-reversing channel operates at lower frequencies (up to 100 Hz), providing the cathodic cleaning effect during the negative half-cycle while maintaining penetration during the positive half-cycle.
Welding Performance on Aluminum Alloy
The authors validated the power supply through overlay welding experiments on aluminum alloy plates. The results demonstrated stable welding process characteristics and good weld bead geometry. The combination of the two current waveforms produced a synergistic effect: the high-frequency current provided a concentrated, stable arc with fine grain structure in the weld metal, while the polarity-reversing current ensured effective oxide removal from the weld pool surface.
The current rise rates are particularly noteworthy. The high-frequency channel achieves a rise rate of 50 A per microsecond, which is two orders of magnitude faster than conventional TIG power supplies. This extremely rapid current change enables precise control of the heat input on a microsecond timescale, allowing for rapid transitions between high and low current states within each frequency cycle. The polarity-reversing channel achieves 300 A per millisecond, which is also significantly faster than conventional polarity-reversing TIG supplies.
Engineering Significance and Process Advantages
This dual-waveform approach addresses several limitations of conventional TIG welding:
- Enhanced oxide removal: The polarity reversal provides cathodic cleaning without the need for excessive AC amplitude, reducing arc instability.
- Improved arc stability: The high-frequency component maintains a concentrated arc even at low average currents.
- Refined microstructure: The rapid current oscillation promotes fine grain formation in the weld metal.
- Flexible process control: The independent control of two current channels provides unprecedented flexibility in heat input management.
- Reduced heat input: The ability to rapidly modulate current allows for lower average heat input while maintaining adequate penetration.
For aluminum alloy welding, where oxide removal is critical and excessive heat input leads to porosity and distortion, this process offers a compelling solution. The high-frequency current can be used to maintain a narrow, stable arc during the positive half-cycle for penetration, while the negative half-cycle of the polarity-reversing current cleans the oxide layer. The result is a weld with reduced porosity, better surface finish, and improved mechanical properties.
Key Reflections
The development of digital power supplies with independent multi-channel control represents a significant advancement in welding technology. The ability to simultaneously apply two different current waveforms through separate electrodes opens up new possibilities for process optimization that were previously impossible with single-channel power supplies. The extremely high current rise rates (50 A/microsecond) demonstrate the capabilities of modern power electronics and suggest that even more sophisticated waveform designs may be feasible with further development.
The defense funding of this project suggests applications in welding of advanced aerospace alloys, where joint quality and reliability are paramount. The process could be particularly valuable for welding thick-section aluminum and titanium alloys, where conventional TIG welding struggles to achieve both deep penetration and good surface quality. Future development should focus on integrating real-time monitoring and adaptive control to optimize the dual-waveform parameters based on in-process measurements of arc voltage, current, and weld pool geometry.