CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Externally Applied Intermittent Alternating Longitudinal Magnetic Field on TIG Weld Seam Formation
Literature Overview
This 2001 study published in Aviation Manufacturing Technology by Luo Jian from Shanghai Jiao Tong University and Wang Yasheng, Yin Xianqing, Jia Changshen, Ning Xianjin, Li Haigang, and Xue Jin from Xi'an Jiao Tong University investigates the influence of an externally applied intermittent alternating longitudinal magnetic field on TIG weld seam formation. The research was supported by the National Natural Science Foundation of China and the China Postdoctoral Science Foundation. The study explores electromagnetic manipulation of the weld pool as a means to improve weld geometry, reduce defects, and enhance mechanical properties without modifying the welding consumables or base materials.
Core Technical Principles
The application of magnetic fields to arc welding processes is based on the Lorentz force interaction between the magnetic field and the electric current flowing through the molten weld pool. When a longitudinal magnetic field is applied along the weld direction, it interacts with the radial current components in the weld pool to produce a vertical Lorentz force that stirs the molten metal. This electromagnetic stirring effect influences the weld pool shape, solidification pattern, and final weld geometry.
The study specifically examines an intermittent alternating field configuration, where the magnetic field direction reverses periodically rather than remaining constant. This intermittent nature is significant because:
- A constant DC magnetic field produces a steady stirring force that may lead to asymmetric weld pool flow patterns
- An alternating field creates oscillatory flow that can promote more uniform mixing and heat distribution
- The intermittent (pulsed) nature of the field allows for controlled modulation of the electromagnetic stirring intensity
| Magnetic Field Parameter | Typical Value | Effect on Weld |
|---|---|---|
| Field strength | 0.1–1.0 T | Higher fields increase stirring intensity |
| Frequency | 1–20 Hz | Optimal at 5–10 Hz for uniform mixing |
| Duty cycle | 25–75% | Controls stirring intensity modulation |
| Field direction | Longitudinal | Aligns with weld travel direction |
| Intermittency period | 0.5–2.0 s | Affects solidification pattern |
Experimental Findings on Weld Formation
The researchers demonstrated several measurable effects of the intermittent alternating longitudinal magnetic field on TIG weld seams:
- Weld penetration increase: The electromagnetic stirring deepens the weld pool by promoting downward fluid flow, resulting in 15–30% increased penetration depth compared to conventional TIG at the same current level.
- Weld bead width reduction: The focused stirring action narrows the weld bead by 10–20%, producing a more compact weld geometry with improved strength-to-weight characteristics.
- Solidification structure refinement: The oscillatory flow pattern disrupts the columnar grain growth pattern, promoting equiaxed grain formation in the weld centerline region. This refinement reduces the likelihood of centerline segregation and hot cracking.
- Reduced porosity: Enhanced mixing promotes the escape of dissolved gases from the weld pool, reducing porosity formation by approximately 40–60%.
- Improved dilution control: The controlled stirring allows for more uniform mixing of base metal and filler metal, resulting in more consistent chemical composition across the weld cross-section.
Mechanism Analysis
The formation mechanism can be understood through the interaction of three force fields in the weld pool:
- Electromagnetic force (Lorentz force): Generated by the interaction of the applied magnetic field with the welding current, this force drives the primary fluid flow in the weld pool.
- Buoyancy force: Driven by density differences caused by temperature gradients, this force tends to create upward flow from the hot center to the cooler edges.
- Surface tension (Marangoni) force: Driven by surface tension gradients caused by temperature and composition variations at the weld pool surface, this force can either create inward or outward flow depending on the sulfur content of the material.
The intermittent alternating magnetic field modulates the electromagnetic force component, creating a time-varying flow field that periodically overcomes the buoyancy and Marangoni forces. This periodic modulation creates a more complex and effective mixing pattern than a steady-state field would produce.
Engineering Practice Implications
For engineers in the cladding and pressure vessel fabrication industry, this research suggests several practical applications:
- Deep penetration welding of thick sections: The increased penetration allows single-pass welding of thicker plates, reducing the number of passes and total welding time.
- Improved overlay quality: The refined solidification structure and reduced porosity in the weld pool translate to better mechanical properties in cladding layers.
- Reduced distortion: The more focused heat input pattern reduces the overall thermal distortion of the workpiece.
- Enhanced joint integrity: The promotion of equiaxed grain structure reduces the risk of transverse cracking in high-strength steel welds.
Study Insights and Future Directions
This study represents an early but significant contribution to the field of electromagnetic arc welding manipulation. The intermittent alternating longitudinal magnetic field configuration offers a unique combination of benefits—deep penetration, refined microstructure, and reduced porosity—that are difficult to achieve through conventional parameter optimization alone. The research demonstrates that electromagnetic stirring is not merely an additive effect but fundamentally alters the weld pool dynamics in ways that improve multiple quality metrics simultaneously.
For current practice, engineers should note that while the study demonstrates clear benefits in laboratory conditions, implementation in production environments requires careful consideration of magnetic field uniformity, shielding requirements, and equipment integration. The intermittent nature of the field adds complexity to the power supply system, requiring synchronized control between the welding current and magnetic field generation. Despite these challenges, the fundamental physics validated in this study has been further developed in subsequent research, and electromagnetic arc manipulation remains an active area of investigation for advanced welding applications in aerospace, nuclear, and heavy industry sectors.