CLADDING TECHNOLOGY SHANXI CO., LTDTransverse Alternating Magnetic Field Controlled DP-TIG Welding Arc Molten Pool Characteristics
Literature Overview
This 2021 study by Qu Huaiyu and colleagues at Lanzhou University of Technology investigates the effects of a transverse alternating magnetic field on the molten pool dynamics during Direct Pulse TIG (DP-TIG) welding. Supported by the Gansu Provincial College Innovation Fund (2020B-283), this research explores the application of external electromagnetic fields as a means of controlling weld pool geometry and improving weld quality without modifying conventional welding equipment parameters. The work represents an innovative approach to welding process optimization that has potential applications in advanced overlay and cladding operations.
Technical Framework and Experimental Configuration
The DP-TIG process employs a pulsed current waveform with a base current that maintains arc stability and a peak current that provides additional heat input for penetration. The transverse alternating magnetic field is applied perpendicular to the welding direction, generating Lorentz forces on the current-carrying molten metal that modify the flow patterns within the pool.
Process Parameters Investigated
| Parameter | Range | Optimal Value | Effect |
|---|---|---|---|
| Base current (I_b) | 30–60 A | 45 A | Arc stability |
| Peak current (I_p) | 100–200 A | 150 A | Penetration depth |
| Pulse frequency (f_p) | 1–20 Hz | 5 Hz | Pool dynamics control |
| Magnetic field strength (B) | 0–100 mT | 30–50 mT | Flow modification |
| Magnetic field frequency (f_B) | 1–50 Hz | 10 Hz | Force oscillation |
| Travel speed (v) | 3–10 mm/min | 5 mm/min | Heat input control |
Molten Pool Behavior Under Magnetic Field Influence
Flow Pattern Modification
The transverse alternating magnetic field generates time-varying Lorentz forces that superimpose on the existing Marangoni and buoyancy-driven flow patterns. The study identifies several distinct flow regimes depending on the magnetic field parameters:
- Low field strength (< 20 mT): The magnetic force is insufficient to overcome viscous damping, resulting in minimal flow modification. The weld pool geometry remains essentially unchanged from the no-field condition.
- Moderate field strength (20–50 mT): The magnetic force significantly modifies the flow pattern, creating additional circulation cells that enhance mixing and inclusion removal. The pool becomes slightly deeper and narrower due to the electromagnetic stirring effect.
- High field strength (> 50 mT): Excessive magnetic stirring destabilizes the pool surface, leading to spatter formation and irregular weld bead profiles. The pool becomes turbulent, which can entrain shielding gas and increase porosity.
Pool Geometry Changes
The application of a 30–50 mT transverse alternating magnetic field produces the following measurable changes in pool geometry:
- Depth increase: 15–25% relative to no-field condition
- Width decrease: 5–10% relative to no-field condition
- Pool aspect ratio increase: 0.3–0.5
- Solidification crack susceptibility: Reduced by 30–50%
These geometric changes are beneficial for weld quality because they promote a more favorable solidification pattern with reduced centerline segregation and improved inclusion removal.
Electromagnetic Force Analysis
The Lorentz force density is given by F_L = J × B, where J is the current density vector and B is the magnetic field vector. In the transverse alternating magnetic field configuration, the force oscillates in direction at the magnetic field frequency, creating a pumping effect on the molten metal.
Force Magnitude Estimation
For typical TIG welding parameters (150 A peak current, 30 mT magnetic field), the Lorentz force density within the molten pool is approximately 0.5–2.0 N/m³. This force is comparable to the Marangoni stress-driven body forces (0.1–1.0 N/m³) and buoyancy forces (0.01–0.1 N/m³), indicating that the electromagnetic stirring can significantly influence pool dynamics.
Application to Overlay Welding Processes
The electromagnetic stirring principle demonstrated in this study has direct applications to GTAW overlay welding, particularly for the following scenarios:
Multi-Pass Overlay Quality Enhancement
In multi-pass overlay welding, the electromagnetic stirring effect can be applied selectively to:
- Enhance mixing between the overlay material and substrate in the first pass
- Improve inclusion removal in subsequent passes
- Reduce hot cracking susceptibility in thick overlay deposits
Crack Suppression
The electromagnetic stirring effect reduces centerline segregation and promotes more uniform solidification, which can suppress both hot cracking and solidification cracking in overlay welds. For nickel-based alloy overlays on dissimilar substrates, where cracking susceptibility is elevated due to thermal mismatch, electromagnetic stirring can be a valuable process enhancement.
Parameter Optimization Strategy
| Overlay Scenario | Recommended Magnetic Field | Expected Benefit |
|---|---|---|
| Single-pass thin overlay | 20–30 mT, 10 Hz | Improved wetting and bonding |
| Multi-pass thick overlay | 30–50 mT, 5–10 Hz | Enhanced mixing and crack suppression |
| Dissimilar material overlay | 40–60 mT, 5 Hz | Reduced segregation and cracking |
| Thick-section overlay | 50–70 mT, 3–5 Hz | Improved penetration and mixing |
Defect Analysis and Prevention
The study identifies several defects that can be mitigated through electromagnetic stirring:
| Defect | Mechanism | Electromagnetic Stirring Effect |
|---|---|---|
| Centerline porosity | Gas entrapment in stagnant pool center | Enhanced convection removes gas bubbles |
| Hot cracking | High centerline segregation of low-melting phases | Promotes uniform solidification |
| Inclusion entrapment | Inadequate buoyancy-driven removal | Enhanced mixing lifts inclusions to surface |
| Lack of fusion | Insufficient heat at pool front | Electromagnetic pumping increases heat transfer |
Study Insights and Reflections
This research demonstrates that external electromagnetic fields represent a promising tool for enhancing TIG welding process quality without requiring modifications to the welding equipment itself. The transverse alternating magnetic field configuration is particularly attractive because it can be applied using simple coil arrangements positioned around the workpiece, making it relatively easy to implement in production environments.
For engineers involved in overlay welding qualification, the electromagnetic stirring concept offers a pathway to improving weld quality in challenging applications such as thick overlay deposits, dissimilar material overlays, and crack-sensitive alloy systems. The key consideration is that electromagnetic stirring parameters must be optimized for each specific application, as the force magnitude and flow modification effects depend strongly on the current density distribution, which varies with welding parameters and joint geometry.
The practical implementation of electromagnetic stirring in overlay welding requires careful consideration of coil positioning, magnetic field uniformity, and the interaction between the applied field and any existing electromagnetic forces from the welding current. Engineers should approach this technology with appropriate expectations, recognizing that while electromagnetic stirring can significantly improve weld quality, it does not eliminate the need for proper process parameter optimization and quality control. The technology is best regarded as an enhancement to conventional TIG welding rather than a replacement for sound process fundamentals.