Arc Stability in Coalesced Pool Double TIG Welding

Literature Overview

Published in Thermal Processing Technology (2008, Vol. 37, No. 17, pp. 95-98), this study by Wang Jianjun and Ma Wenxin from Shanghai Second Polytechnic University investigates the arc stability characteristics of coalesced pool double TIG welding. Funded by the Shanghai Municipal Education Commission Innovation Fund (08ZY81) and the University's internal research fund (DZ207005), the work addresses a specialized welding process that combines two TIG arcs to form a single coalesced weld pool, offering advantages in deposition rate and heat input control for thick-section welding.

Core Technical Findings

Coalesced Pool Double TIG Welding Characteristics

The coalesced pool double TIG process employs two tungsten electrodes positioned to create two arcs that interact and merge into a single weld pool. This configuration offers several advantages over conventional single-arc TIG welding, including higher deposition rates, deeper penetration, and improved arc stability through the interaction of two arc plasma columns. However, the stability of the dual-arc system is inherently more complex than a single arc, as the two arcs can interfere with each other through electromagnetic forces, plasma jet interaction, and thermal coupling.

Arc Stability Mechanisms

The study systematically examined arc stability under various current waveform combinations:

The research demonstrates that arc stability can be achieved through careful control of two parameters: (1) the waveform morphology of the welding current, and (2) the absolute difference between the two arc current values. This dual-parameter control strategy provides engineers with a practical framework for optimizing the process.

Critical Finding on Pulse Interleaving

A particularly important finding is that when using large dual pulsed currents, the pulse currents must exhibit interleaved delays to maintain dual-arc stability. Without interleaving, simultaneous pulse peaks from both arcs create excessive electromagnetic forces that destabilize the arc system. This insight has direct implications for the design of welding power sources for coalesced pool processes, requiring synchronized pulse control with programmable phase offsets.

Process Control Strategy Analysis

Current Waveform Selection

The choice of current waveform directly influences arc stability and weld quality:

• Dual DC arcs provide the simplest and most stable configuration, suitable for continuous welding of thick sections where high deposition rates are required. The absolute current difference between the two arcs should be kept within a narrow range to prevent arc wandering or detachment.
• Dual pulsed arcs offer superior control over heat input and weld bead geometry, but require careful management of pulse timing. The interleaved delay between pulses is critical at high current levels, as simultaneous peaks can cause arc instability.
• DC-Pulse hybrid configurations combine the stability of DC arcs with the controllability of pulsed arcs, representing a practical compromise for many engineering applications.

Arc Stability Control Methods

Based on the study's findings, the following control strategies can be implemented:

1. Current difference control: Maintaining the absolute difference between the two arc currents within a specified tolerance band prevents asymmetric arc behavior and promotes stable coalescence.
2. Pulse phase management: For pulsed configurations, implementing programmable phase offsets between the two pulse channels ensures that peak currents do not coincide, reducing electromagnetic interference.
3. Arc length monitoring: Real-time monitoring of arc voltage serves as an indicator of arc stability, enabling feedback control of current parameters to maintain optimal conditions.

Engineering Practice Implications

Application Scenarios

Coalesced pool double TIG welding is particularly advantageous for:

• Thick-section welding where single-arc TIG would require excessive passes
• Applications requiring high deposition rates with good weld quality
• Precision welding of reactive metals (titanium, nickel alloys) where inert gas shielding quality is critical
• Welding of pipe fittings where consistent weld bead geometry is essential for subsequent machining or inspection

Quality Control Considerations

When implementing coalesced pool double TIG welding in production, the following quality control measures should be established:

Common Defects and Countermeasures

• Arc wandering: Caused by excessive current difference; countermeasure is to reduce the current imbalance and verify electrode alignment.
• Porosity: Resulting from inadequate shielding gas coverage due to arc interference; countermeasure is to increase gas flow rate and optimize nozzle geometry.
• Undercut: Occurring at the weld toe due to excessive arc force; countermeasure is to reduce peak current and increase travel speed.

Study Insights and Outlook

This research provides a fundamental understanding of arc stability mechanisms in coalesced pool double TIG welding, which is essential for the reliable implementation of this process in industrial applications. The identification of current waveform and current difference as the two primary control parameters offers a clear and practical framework for process optimization. The finding regarding pulse interleaving at high currents is particularly valuable, as it addresses a critical limitation that could otherwise prevent the adoption of pulsed double TIG for thick-section welding. Future research should explore the application of advanced power sources with real-time arc sensing and adaptive control to further enhance process stability and weld quality. The principles established in this study can be extended to other multi-arc welding processes, including double-electrode submerged arc welding and multi-wire GMAW, providing a broader framework for multi-arc process development.