Comparative Study on Arc Stability of AC TIG Welding Power Sources

Literature Overview

The paper by Lai Zhongmin and Gao Fei (2006), published in the Journal of Jiangsu University (Natural Science Edition), Volume 20, Issue 5, addresses a fundamental challenge in AC TIG welding: maintaining arc stability during the current zero-crossing event. The study compares the arc stability mechanisms of several AC TIG power sources and proposes effective measures for improving arc stability. Funded by an industry research project (98.J56.2.2(1)), this work addresses a practical manufacturing concern for aluminum and magnesium alloy welding.

Fundamental Challenge of AC TIG Welding

AC TIG welding is the preferred process for welding aluminum and magnesium alloys because the AC waveform provides both arc cleaning (cathodic cleaning during the electrode-negative half cycle) and deep penetration (anodic heating during the electrode-positive half cycle). However, the periodic current zero-crossing creates a critical moment where the arc must be re-ignited, and failure to do so results in arc interruption, which causes:

Porosity from incomplete shielding during re-ignition
Irregular bead geometry from inconsistent heat input
Tungsten contamination from arc drift
Reduced welding speed due to frequent restarts

Arc Stability Mechanisms Analyzed

The authors identify three primary factors determining arc stability at the zero-crossing instant:

1. Arc Space Ionization Degree

At current zero-crossing, the plasma channel must maintain sufficient ionization to allow re-ignition. The ionization degree depends on:

Residual plasma temperature from the previous half-cycle
Shielding gas composition and flow rate
Electrode work function and surface condition
Gap distance between electrode and workpiece

2. Electrode Electron Emission Capability

The tungsten electrode must be capable of emitting electrons to initiate the arc. Key factors include:

Electrode tip geometry (truncated vs. conical)
Electrode material (pure tungsten, thoriated, ceriated)
Electrode surface temperature history
Pre-ignition current characteristics

3. Re-ignition Voltage Rise Rate

The rate at which the voltage rises above the breakdown threshold determines whether the arc re-ignites successfully. This is influenced by:

Power source voltage rise rate design
Capacitive energy storage in the power supply circuit
Arc impedance characteristics at low current levels

Comparison of Power Source Technologies

Power Source Type	Arc Stability Mechanism	Key Advantage	Limitation
Conventional AC	Natural zero-crossing	Simple design, low cost	Unstable at low currents
Voltage-pulsed AC	Controlled voltage pulse at zero-crossing	Improved re-ignition	Complex control circuitry
Current-pulsed AC	High-frequency current injection at zero-crossing	Reliable arc maintenance	Higher frequency interference
High-frequency AC	Superimposed HF signal	Excellent low-current stability	HF interference with nearby equipment

Technical Measures for Arc Stability Improvement

The study proposes several practical measures:

Pre-ignition voltage pulse: Applying a short high-voltage pulse just before the zero-crossing to maintain plasma channel ionization
Electrode-positive bias: Extending the electrode-positive half-cycle slightly beyond 50% duty cycle to improve electron emission from the tungsten
High-frequency superimposition: Adding a high-frequency (typically 100–500 kHz) signal to maintain ionization through the zero-crossing period
Optimized electrode preparation: Using truncated tungsten electrodes with proper diameter-to-current ratios
Shielding gas optimization: Maintaining adequate gas flow (15–20 L/min for AC welding) to support plasma stability

Engineering Practice Implications

For production welding of aluminum and magnesium alloys in pipe and fitting fabrication, arc stability directly impacts:

Weld quality: Arc interruption leads to porosity, which is particularly critical in pressure-containing applications
Productivity: Frequent arc restarts reduce effective welding speed by 10–30%
Equipment cost: Unstable arcs cause tungsten contamination and electrode wear, increasing consumable costs
Weld appearance: Arc instability produces irregular bead profiles requiring post-weld dressing

In the context of aluminum alloy piping systems (such as those used in aerospace fuel systems or cryogenic applications), the AC TIG process described here is often the only viable option for achieving the required weld quality. The stability improvements discussed in this paper are therefore directly relevant to meeting the stringent acceptance criteria of standards such as AMS 2750 and ASME Section IX.

Study Insights and Reflections

This paper, while published in 2006, remains highly relevant to current welding practice. The fundamental physics of arc stability at zero-crossing has not changed, and the mechanisms identified continue to guide power source design. Modern digital power sources have implemented many of the principles discussed here, including programmable zero-crossing voltage pulses and adaptive arc monitoring systems.

A particularly valuable insight is the recognition that arc stability is a multi-factor problem requiring simultaneous optimization of power source design, electrode preparation, and shielding gas delivery. In practice, engineers often focus on one aspect (typically the power source) while neglecting others (such as electrode condition or gas flow rate), leading to persistent quality issues. The holistic approach advocated in this study should be applied to any AC TIG welding operation.