CLADDING TECHNOLOGY SHANXI CO., LTDArc Molten Pool Characteristics of Transverse Alternating Magnetic Control DP-TIG Welding
Literature Overview
The research by Qu Huaiyu, Liu Fenxia, Liu Wenlan, Feng Xiaochun, Yu Jiangrui, and Fu Xiaoyue from Lanzhou University of Technology investigates a novel welding technique—Double-Pulse Transverse Magnetic Field Controlled TIG welding (DP-TIG with alternating magnetic control). Published in the Journal of Lanzhou University of Technology in 2021, this work explores how a transverse alternating magnetic field influences the arc behavior and molten pool dynamics during welding. The study was supported by the Gansu Provincial Higher Education Innovation Fund, reflecting its significance as an innovative welding technology advancement.
Core Technical Principles
Double-Pulse TIG Welding
Double-pulse (DP) TIG welding employs two distinct current levels within each welding cycle:
- High current pulse: Provides deep penetration and rapid heating
- Low current pulse: Allows surface tension-driven flow to flatten the pool surface, promoting solidification without excessive convexity
The pulse frequency, duty ratio, and current ratio between high and low pulses are critical parameters governing weld geometry and quality.
Transverse Alternating Magnetic Field Control
The application of a transverse alternating magnetic field introduces electromagnetic stirring to the molten pool. The Lorentz force generated by the interaction between the induced current in the pool and the external magnetic field creates additional convective flow patterns. This has several beneficial effects:
- Enhanced mixing: Promotes compositional homogeneity in the weld metal
- Reduced columnar grain growth: Refines the solidification microstructure
- Decreased porosity: Improved gas escape from the solidifying pool
- Modified pool geometry: Flatter, wider pools with reduced aspect ratio
Key Process Parameters
| Parameter | Typical Value | Function |
|---|---|---|
| High pulse current | 150–250 A | Deep penetration; rapid heating |
| Low pulse current | 50–100 A | Surface tension control; pool stabilization |
| Pulse frequency | 1–10 Hz | Controls solidification rhythm |
| Duty ratio | 30–70% | Balances penetration and surface quality |
| Magnetic field strength | 1–10 mT | Determines electromagnetic stirring intensity |
| Magnetic field frequency | 50–1000 Hz | Influences stirring pattern and effectiveness |
| Welding speed | 200–600 mm/min | Controls heat input and pool geometry |
Molten Pool Flow Analysis
The electromagnetic stirring effect creates a complex three-dimensional flow pattern within the molten pool. The transverse magnetic field induces currents perpendicular to both the magnetic field direction and the current density vector. This generates Lorentz forces that:
- Create a rotating flow component superimposed on the natural thermocapillary and buoyancy-driven flows
- Enhance heat transfer from the pool center to the pool boundaries
- Reduce the pool depth while increasing the width
- Promote equiaxed grain nucleation through increased undercooling at the pool boundaries
Engineering Applications and Significance
For cladding and overlay applications, the electromagnetic stirring effect of DP-TIG has several important implications:
- Improved dilution control: Enhanced mixing at the base metal-overlay interface can be tuned to achieve specific dilution ratios, critical when depositing corrosion-resistant alloys.
- Reduced cracking susceptibility: The refined microstructure and reduced columnar grain length decrease the probability of solidification cracking.
- Enhanced bond strength: More uniform composition and reduced segregation at the interface improve metallurgical bond quality.
- Applicability to thick overlays: The controlled pool geometry allows for more predictable multi-pass overlay builds.
Quality Assessment and Defect Analysis
| Quality Criterion | Effect of Magnetic Control | Verification Method |
|---|---|---|
| Weld width-to-depth ratio | Increased (flatter pool) | Cross-section metallography |
| Porosity content | Reduced | Radiographic testing (RT) |
| Grain size | Refined | Metallographic examination |
| Hardness distribution | More uniform | Microhardness traverse |
| Cracking tendency | Decreased | Dye penetrant testing (PT) |
Key Questions and Reflections
An important consideration for practical implementation is the integration of the magnetic field generation system into existing welding equipment. The transverse magnetic field must be precisely aligned with the weld axis and maintained at a consistent distance from the workpiece throughout the welding operation. For large-scale pressure vessel fabrication, this poses significant engineering challenges in terms of equipment design, operator accessibility, and process consistency. Additionally, the interaction between the external magnetic field and any ferromagnetic components in the workpiece (such as low-alloy steel base metals) may introduce unexpected effects that require careful characterization.
Study Insights and Conclusions
This research demonstrates the potential of combining double-pulse current control with electromagnetic stirring to achieve superior weld quality in TIG welding. For engineers involved in bimetal product manufacturing and overlay cladding, this technology offers a pathway to enhanced process control without requiring fundamental changes to the welding equipment architecture. The ability to refine microstructure, reduce defects, and control dilution through magnetic field parameters provides additional degrees of freedom in process optimization that complement traditional parameter adjustment. Future work should focus on scaling this technology for industrial applications on large-diameter pressure vessels and thick-section overlay operations where conventional TIG welding faces quality challenges.