Arc Load Characteristics and Polarity Reversal Control Strategy of Alternating Polarity TIG Welding

Literature Overview

This study, published in the Journal of Welding (2008) by Ding Kun, Yao Heqing, Fan Xinghui, and Wang Shouyan from Hohai University, investigates the arc load characteristics and polarity reversal control strategy of alternating polarity TIG (AC-TIG) welding. The research was supported by the Hohai University Changzhou Campus Innovation Science and Technology Fund (CC2007-01).

AC-TIG welding is essential for welding aluminum and its alloys, as well as magnesium alloys, because the AC cycle provides both cathodic cleaning (ACEN - cathode on workpiece) and anodic heating (ACWP - cathode on electrode) effects. The alternating polarity produces a self-cleaning action that removes the aluminum oxide (Al₂O₃) layer from the weld zone, which is critical for achieving sound welds in aluminum alloys.

Core Technical Content

The study examines the electrical characteristics of the AC-TIG arc during polarity reversal, including:

• Arc voltage waveform: The voltage characteristics during the positive and negative half-cycles, including the reversal transient.
• Arc current waveform: The current distribution and its relationship to the electrode and workpiece heating.
• Reversal transient: The brief period during which the arc transitions between polarities, characterized by voltage spikes and current fluctuations.

The polarity reversal control strategy is critical for maintaining arc stability and achieving consistent weld quality. The study proposes a control algorithm that manages the reversal transient by:

1. Rapid current ramping: Quickly transitioning the current through zero to minimize the reversal transient duration.
2. Voltage feedback control: Using real-time arc voltage measurement to detect and correct arc instability during reversal.
3. Duty cycle adjustment: Optimizing the ratio of positive to negative half-cycles to balance oxide cleaning and heat input.

Key Findings

The study reveals that the arc load characteristics during polarity reversal are significantly influenced by the electrode geometry, shielding gas composition, and power source design. A tungsten electrode with a blunt tip produces a more stable arc during reversal than a sharpened tip, due to the larger effective emission area.

The reversal transient typically lasts 0.1-0.5 ms, during which the arc voltage can spike to 2-3 times the normal operating voltage. This transient can cause arc instability, spatter, and weld porosity if not properly managed. The proposed control strategy reduces the transient duration by 40-60% and improves arc stability during reversal.

The duty cycle optimization is found to be critical for aluminum alloy welding. A duty cycle of 60-70% positive (ACEN) provides adequate oxide cleaning while maintaining sufficient heat input for penetration. However, for thick-section aluminum welding, a higher positive duty cycle (70-80%) may be required to ensure complete oxide removal.

Implications for Engineering Practice

For engineers working with aluminum alloy pressure vessels, heat exchangers, and clad plates, this research provides practical guidance on:

• Power source selection: AC-TIG power sources with advanced reversal control capabilities should be specified for aluminum alloy welding to ensure consistent weld quality.
• Process parameter optimization: The duty cycle should be carefully calibrated for the specific aluminum alloy system and thickness range, with consideration for the oxide thickness and composition.
• Electrode selection: Tungsten electrodes with appropriate geometry (blunt or truncated) should be used to minimize reversal transient effects and extend electrode life.
• Quality control: The arc voltage and current waveforms should be monitored during production welding to detect and correct process deviations in real time.

For bimetal applications involving aluminum-to-steel joints, the AC-TIG process is particularly relevant for the aluminum-side welding of transition pieces, where oxide removal is critical for achieving sound fusion.

Study Reflections

The control of polarity reversal in AC-TIG welding is a deceptively complex problem that directly impacts weld quality and process reliability. This study's systematic investigation of arc load characteristics and control strategies provides valuable insights for both power source designers and welding process engineers. The practical implementation of the proposed control strategy requires close integration between the power source electronics and the welding process parameters, highlighting the importance of system-level optimization rather than isolated parameter tuning. For production environments, the reliability and repeatability of the reversal control are paramount, and thorough process qualification under NB/T 47014 or ASME IX should include evaluation of the arc stability during polarity transitions. The continued development of AC-TIG technology, particularly with respect to advanced power source controls, will further expand the range of weldable aluminum alloy systems and improve the quality of aluminum alloy pressure vessel fabrication.