Zhuojin Pipe Fitting Co., LtdTIG Arc-Assisted Non-Contact Arc Striking Mechanism in MIG Hybrid Welding
Literature Overview
The research conducted by Tang Yingying, Zhu Zhiming, Yang Zhongyu, and Fu Pingpo from Tsinghua University's Department of Mechanical Engineering, published in Transactions of the China Welding Institution in 2018 (Volume 39, Issue 3, pages 21-25), investigates the mechanism by which a pre-ignited TIG arc assists the non-contact arc striking of MIG welding in a TIG-MIG hybrid welding configuration. The study was supported by the National Natural Science Foundation of China (Grant 51675303) and the National Science and Technology Major Project (2012ZX04012011). This work addresses a practical process challenge in hybrid welding: achieving reliable arc initiation without contact between the MIG wire and the workpiece.
Technical Background and Problem Statement
In conventional MIG welding, arc striking typically requires one of the following methods:
- Contact striking: The wire touches the workpiece, creating a short circuit that vaporizes metal and initiates the arc. This method produces spatter and wire tip contamination.
- High-frequency (HF) striking: A high-frequency voltage is applied to ionize the gas gap and initiate the arc. This requires additional equipment and can cause electromagnetic interference.
- DC lift-arc: The wire is lifted from the workpiece surface, creating an arc through metal vapor. This requires precise wire-to-workpiece distance control.
In TIG-MIG hybrid welding configurations, the TIG arc is typically ignited first and serves as a stable, spatter-free heat source. The research question is whether the pre-existing TIG arc can serve as an arc striking aid for the MIG process, eliminating the need for contact striking or HF equipment.
Experimental Methodology
The study employed a synchronized acquisition system combining electrical signal measurement with high-speed imaging to capture the arc striking process in real time. The measurement parameters included:
- Arc voltage: Measured across the MIG electrode to workpiece circuit.
- Welding current: Measured in the MIG welding circuit.
- Arc images: Captured using high-speed camera with frame rates sufficient to resolve the rapid arc initiation sequence.
The TIG arc was established first under controlled parameters, and then the MIG wire was presented to the TIG arc zone to observe the arc striking behavior. Multiple TIG parameter combinations were tested to determine the range of conditions under which successful non-contact MIG arc striking could be achieved.
Key Findings: Discharge Channel Formation
The critical finding of this research is that a thin, elongated discharge channel forms between the MIG wire tip and the TIG arc, and this channel is the key mechanism enabling non-contact arc striking. The formation process can be described as follows:
| Stage | Time Scale | Physical Process | Observable Feature |
|---|---|---|---|
| 1. Wire approach | ~ms | Wire tip enters TIG arc thermal zone | Wire heating begins |
| 2. Field enhancement | ~μs | Electric field between wire and arc intensifies | Pre-breakdown glow |
| 3. Streamer formation | ~μs | Thin discharge channel develops | Visible luminous thread |
| 4. Arc attachment | ~μs | Discharge channel connects wire to arc | Full arc established |
| 5. Arc stabilization | ~ms | MIG arc reaches steady state | Normal arc characteristics |
The discharge channel formation is explained using streamer theory of gas gap breakdown. When the MIG wire approaches the TIG arc, the local electric field intensity in the gas gap exceeds the breakdown threshold. The TIG arc provides both thermal ionization of the surrounding gas and a high electric potential reference point, creating favorable conditions for streamer propagation from the wire tip toward the arc.
Parameter Adaptability
A significant practical finding is that the TIG-assisted MIG arc striking mechanism demonstrates good adaptability to variations in TIG welding parameters. The study tested multiple TIG parameter combinations and found that successful non-contact striking was achievable across a wide range of:
- TIG current: From low to high current settings, the arc temperature and ionization density vary, but sufficient ionization is maintained for streamer initiation.
- TIG electrode polarity: Both AC and DC configurations allow successful MIG arc striking, though the arc attachment characteristics differ slightly.
- Shielding gas composition: Different gas mixtures produce varying ionization densities, but the TIG arc consistently provides adequate pre-ionization for MIG arc initiation.
This parameter adaptability is important for production applications where TIG parameters may be adjusted for different joint configurations or material thicknesses.
Mechanism Analysis Based on Streamer Theory
The study applies streamer theory to explain the detailed physics of the discharge channel formation. Streamer theory describes the development of ionization channels in gas gaps under high electric fields. The key physics include:
- Photoionization: Ultraviolet radiation from the TIG arc ionizes gas molecules in the gap between the wire and arc.
- Electron avalanche: Free electrons accelerated by the electric field collide with gas molecules, producing additional ionization events.
- Streamer propagation: The ionization channel grows from the wire tip (cathode) toward the TIG arc (anode), creating a conductive path.
- Arc establishment: Once the streamer bridges the gap, the current rises rapidly and the MIG arc transitions to a self-sustaining state.
The TIG arc serves as a pre-ionization source that dramatically reduces the voltage required for gap breakdown. Without the TIG arc, the gas gap would require significantly higher voltage to achieve breakdown, potentially exceeding the power supply capability.
Engineering Practice Applications
The TIG-assisted non-contact arc striking mechanism has direct applications in several industrial scenarios:
- Hybrid welding process development: Manufacturers of hybrid welding systems can use TIG-assisted striking to eliminate spatter during arc initiation, improving process reliability.
- Automated welding sequences: In multi-process welding sequences, the TIG arc can serve as a transition aid when switching between TIG and MIG processes.
- Spatter-sensitive applications: In welding applications where spatter is unacceptable (e.g., polished surfaces, thin sheet metal), TIG-assisted striking eliminates contact-induced spatter.
- Remote welding operations: The non-contact nature of the process reduces wear on wire feed mechanisms and contact tips.
Study Reflections and Implications
This research provides valuable fundamental understanding of arc physics in hybrid welding configurations. The demonstration that a pre-existing arc can serve as an arc striking aid opens new possibilities for process design in hybrid welding systems. The streamer theory analysis provides a rigorous physical framework for predicting the conditions under which successful arc striking can be achieved.
From a practical standpoint, this technology could simplify hybrid welding equipment design by eliminating the need for separate arc striking circuits. The parameter adaptability finding suggests that this approach can be integrated into existing hybrid welding systems with minimal modifications. For engineers developing next-generation hybrid welding processes, understanding the fundamental arc striking mechanism is essential for optimizing process reliability and minimizing spatter.