Zhuojin Pipe Fitting Co., LtdArc Load Characteristics and Polarity Switching Control Strategy in Polarity-Reversal TIG Welding
Literature Overview
This paper by Ding Kun, Yao Heqing, Fan Xinghui, and Wang Shouyan from Hohai University, published in Welding Journal (2008, Vol. 29, No. 9, pp. 31–34), investigates the arc load characteristics and polarity switching control strategies in polarity-reversal TIG welding. The work is supported by the Hohai University Changzhou Campus Innovation Science and Technology Fund (Grant CC2007-01). The study addresses a critical technical challenge in polarity-reversal TIG welding: the current surge that occurs when switching from electrode-positive (EP) to electrode-negative (EN) polarity, and proposes a control strategy to mitigate this issue.
Technical Background
Polarity-Reversal TIG Welding
Polarity-reversal TIG welding alternates between EN (electrode negative) and EP (electrode positive) polarities during the welding process. This technique offers several advantages:
- EN polarity — Provides deep penetration, suitable for thick-section welding
- EP polarity — Provides cathodic cleaning action, removing oxide layers from aluminum and its alloys
- Combined effect — Alternating polarity combines deep penetration with oxide cleaning, making it particularly suitable for aluminum welding
The challenge lies in controlling the arc during polarity transitions, as the arc characteristics change significantly between EN and EP modes.
Arc Equivalent Resistance
The study introduces the concept of arc equivalent resistance to characterize the arc's electrical behavior. This is defined as the ratio of arc voltage to welding current, taking into account the non-linear characteristics of the arc.
Experimental Results
Arc Equivalent Resistance Characteristics
The study's key findings regarding arc equivalent resistance are:
| Polarity | Equivalent Resistance Trend | Physical Mechanism |
|---|---|---|
| EN (Electrode Negative) | Lower equivalent resistance | Electron emission from tungsten cathode is efficient |
| EP (Electrode Positive) | Higher equivalent resistance | Electron emission from workpiece anode is less efficient |
| EP to EN transition | Sharp decrease in equivalent resistance | Sudden change in electron emission mechanism |
The equivalent resistance decreases with increasing current in both polarities, but the decrease is more pronounced during the EP to EN transition. This sharp decrease causes a current surge that can destabilize the arc.
Electron Emission Mechanisms
The study analyzes the formation mechanism of arc equivalent resistance in terms of:
- Cathode region — Electron emission from the tungsten electrode (EN) or workpiece (EP)
- Arc column region — Ionization and current carrying by electrons and ions
- Anode region — Ion bombardment and heat input to the workpiece (EN) or tungsten (EP)
The sharp decrease in equivalent resistance during EP to EN transition is attributed to the sudden change in electron emission from the workpiece (less efficient) to the tungsten electrode (more efficient).
Control Strategy
Problem Analysis
The current surge during polarity switching can cause:
- Arc instability and flickering
- Excessive heat input to the workpiece
- Tungsten electrode erosion
- Poor weld bead quality
- Potential damage to welding equipment
Proposed Control Strategy
The study proposes a variable-parameter PI controller combined with lead control to mitigate the current surge during polarity switching:
| Control Component | Function | Parameter |
|---|---|---|
| PI Controller | Regulates welding current to setpoint | Proportional gain (Kp), Integral gain (Ki) |
| Lead Control | Anticipates and compensates for current surge | Lead time constant (τd) |
| Variable Parameters | Adjusts Kp and Ki during polarity transition | Adaptive gain scheduling |
The control strategy operates as follows:
- Before transition — The controller detects the upcoming polarity switch and adjusts the PI parameters to prepare for the transition
- During transition — The lead control anticipates the current surge and applies a corrective action to limit the peak current
- After transition — The controller returns to normal operating parameters and maintains stable welding current
Control Performance
The study reports that the proposed control strategy:
- Maintains arc stability — The arc remains stable throughout the polarity transition
- Reduces current surge — The peak current during transition is significantly reduced compared to uncontrolled switching
- Improves weld quality — The reduced current surge leads to more consistent heat input and better weld bead geometry
Engineering Practice Integration
From my experience with polarity-reversal welding applications, several practical considerations emerge:
- Equipment requirements — The welding power source must have sufficient dynamic response to implement the proposed control strategy. Inverter-based power sources with high switching frequencies are preferred.
- Tungsten electrode selection — For polarity-reversal welding of aluminum, a pure tungsten electrode is recommended because it can withstand the thermal cycling of alternating polarities. Zirconiated tungsten (TZ-1) is also suitable.
- Shielding gas — Argon or argon-helium mixtures are recommended. Helium provides better penetration for thick sections, while argon provides better arc stability.
- Welding parameters — The duty cycle (ratio of EN to EP time) should be optimized based on the material thickness and oxide thickness. A typical duty cycle for aluminum welding is 50% EN / 50% EP.
- Equipment protection — The welding power source and cables must be designed to handle the transient currents during polarity switching. Proper grounding and shielding are essential.
Key Questions and Reflections
- How does the control strategy perform at different welding current levels (e.g., 50 A vs. 200 A)?
- What is the impact of the control strategy on the welding power source's thermal management and component lifetime?
- Would a predictive control approach (using a mathematical model of the arc) provide better performance than the proposed PI + lead control?
Study Insights and Implications
This work provides valuable insights into the electrical characteristics of polarity-reversal TIG welding arcs and proposes a practical control strategy to mitigate the current surge during polarity transitions. The analysis of arc equivalent resistance and electron emission mechanisms offers a deeper understanding of the physical processes governing arc behavior. The proposed variable-parameter PI + lead control strategy represents a significant contribution to welding power source design, as it addresses a critical challenge in polarity-reversal welding. For engineers developing or using polarity-reversal welding systems, this study highlights the importance of dynamic current control and provides a framework for implementing effective control algorithms. The work also underscores the need for close integration between welding process physics and power electronics design to achieve optimal welding performance.