Zhuojin Pipe Fitting Co., LtdOxide Film Tearing Mechanism in DCEN Helium Arc Welding of Aluminum Alloys
Literature Overview
This paper, published in the Welding Journal (Chinese edition), Volume 42, Issue 12, 2021, by Fu Juan, Zhao Yong, Zou Jiasheng, Jia Zhanjun, and He Yuxiang from Jiangsu University's Provincial Key Laboratory of Advanced Welding Technology and Air Products China, investigates the mechanism of oxide film tearing during direct current electrode negative (DCEN) helium arc welding of aluminum alloys. The study employed high-speed photography to observe arc morphology and oxide film tearing behavior under different helium gas flow rates, and derived a differential equation for the molten pool surface profile to explain the observed phenomena. Supported by the National Key R&D Program of China (2018YFC0310400), this research contributes fundamental understanding to one of the most persistent challenges in aluminum alloy arc welding.
Core Observations and Mechanism
High-Speed Photography Results
The high-speed observations revealed several critical phenomena:
- Arc morphology: The arc shape and stability varied with helium gas flow rate, with higher flow rates producing a more constricted and stable arc.
- Oxide film tearing: The degree of oxide film tearing decreased with increasing helium gas flow rate within the tested range, despite the higher arc energy efficiency achieved.
- Weld penetration: The depth-to-width ratio of the weld increased with helium gas flow rate, indicating improved arc energy concentration and penetration efficiency.
Mechanism of Oxide Film Tearing
The authors proposed a multi-factor mechanism for oxide film tearing:
- Thermal weakening: The increased anode heating power under helium arc conditions weakens the chemical bonds within the aluminum oxide (Al2O3) film, reducing its mechanical strength.
- Surface tension gradient: The molten pool surface tension drives the oxide film toward the pool edge, creating tensile stress on the oxide film.
- Hydrodynamic forces: The electromagnetic and Marangoni forces acting on the molten pool surface exert shear and tensile stresses on the oxide film.
- Gas flow interaction: The helium gas flow creates aerodynamic forces on the oxide film and influences the molten pool surface profile.
Molten Pool Surface Profile Analysis
A key theoretical contribution of this study is the derivation of a differential equation for the intersection line between the molten pool liquid surface and the electrode plane. The analysis shows that:
| Gas Flow Rate | Pool Surface Concavity | Surface Tension (Edge) | Oxide Film Tearing Degree | Penetration Ratio |
|---|---|---|---|---|
| Low | Shallow | Higher | More severe | Lower |
| Medium | Moderate | Moderate | Moderate | Moderate |
| High | Deep | Lower | Less severe | Higher |
The deepening concavity of the molten pool surface with increasing gas flow rate is explained by the enhanced electromagnetic force and gas dynamic force acting on the pool surface. Simultaneously, the reduction in surface tension from the pool center toward the pool edge decreases the driving force for oxide film transport and tearing. These two factors act in opposition: the deeper pool increases hydrodynamic forces that could tear the oxide film, while the reduced surface tension gradient decreases the tensile stress on the film. The net result is a decrease in oxide film tearing severity with increasing gas flow rate.
Engineering Implications
Helium vs. Argon Shielding Gas for Aluminum Welding
| Property | Argon (Ar) | Helium (He) | Ar-He Mixtures |
|---|---|---|---|
| Ionization potential (eV) | 15.76 | 24.59 | Variable |
| Arc temperature (°C) | ~6000 | ~10000 | Variable |
| Penetration | Lower | Higher | Adjustable |
| Cost | Lower | Higher | Moderate |
| Arc stability | Good | Excellent | Good |
| Oxide film tearing | More severe | Less severe | Moderate |
| Typical use | Thin Al sheets | Thick Al plates | General purpose |
The higher ionization potential of helium results in a hotter, more concentrated arc with greater penetration efficiency. However, the cost of helium is significantly higher than argon, and the higher arc temperature can increase porosity susceptibility. The study's findings regarding oxide film tearing are particularly important because the integrity of the oxide film during welding affects weld porosity formation, surface quality, and mechanical properties.
Practical Recommendations
Based on the study's findings, the following practical recommendations emerge:
- For thick aluminum alloy sections: Helium or helium-enriched mixtures are preferred due to their superior penetration and reduced oxide film tearing.
- Gas flow rate optimization: While higher flow rates reduce oxide film tearing, excessive flow rates can cause turbulence and porosity. An optimal flow rate window must be identified for each welding configuration.
- DCEN polarity: The DCEN configuration places the majority of arc heat at the workpiece (anode), which is beneficial for aluminum welding where high penetration is desired. The study confirms that this polarity is appropriate for helium arc welding of aluminum alloys.
- Monitoring and control: High-speed observation techniques, while primarily research tools, highlight the need for in-process monitoring of arc stability and oxide film behavior in production welding.
Study Insights
This research makes a fundamental contribution to understanding the physical mechanisms governing oxide film behavior during aluminum alloy arc welding. The oxide film (Al2O3) is one of the most challenging aspects of aluminum welding because its melting point (approximately 2050°C) far exceeds that of the aluminum matrix (approximately 660°C), and it must be mechanically disrupted and removed during welding to achieve proper fusion. The derivation of a differential equation for the molten pool surface profile represents a significant theoretical advancement, providing a quantitative framework for predicting how process parameters affect pool geometry and, consequently, oxide film behavior. The finding that higher helium gas flow rates reduce oxide film tearing despite increasing arc energy is counterintuitive and demonstrates the value of fundamental research in challenging conventional assumptions. For welding engineers working with aluminum alloys, this study reinforces the importance of understanding the underlying physics rather than relying solely on empirical parameter optimization. The methodology of combining high-speed observation with theoretical analysis is a model for rigorous welding science research.