Zhuojin Pipe Fitting Co., LtdArc and Weld Pool Characteristics of DP-TIG Welding under Transverse Alternating Magnetic Field
Literature Overview
The paper by Qu Huaiyu and colleagues, published in the Journal of Lanzhou University of Technology in 2021 (Vol. 47, No. 4, pp. 22–25), investigates the effects of a transverse alternating magnetic field on the arc morphology and weld pool characteristics of Deep Penetration TIG (DP-TIG) welding. Experiments were conducted on 12 mm thick Q345B steel plate using a high-speed camera system for arc observation, the small hole method for arc pressure measurement, and metallographic etching for weld pool cross-section analysis. The research was supported by the Gansu Provincial University Innovation Fund (2020B-283).
Core Technical Findings
The study found that the application of a transverse alternating magnetic field significantly alters the DP-TIG arc morphology compared to welding without a magnetic field. The arc pressure peak first decreases and then increases with increasing alternating frequency. The weld pool cross-section transitions from a finger-shaped profile (characteristic of high-current DP-TIG without magnetic field) to a pot-bottom-shaped profile under the influence of the alternating magnetic field. Additionally, during nickel-based overlay welding, the iron content in the weld surface layer remains within 5%, indicating low dilution of the base metal by the weld metal.
DP-TIG Process Background and Magnetic Field Effects
DP-TIG welding is a high-current TIG variant that achieves deep penetration at high travel speeds by concentrating the arc energy. The process typically operates at currents exceeding 300 A and travel speeds above 1 m/min, producing narrow, deep weld beads with minimal heat input. The deep penetration is achieved through the Lorentz force acting on the electric current and the arc plasma, which drives the molten metal downward into the weld pool.
The application of an external transverse alternating magnetic field introduces additional Lorentz forces on the arc plasma and the molten metal. These forces modify the arc shape, the current density distribution, and the fluid flow within the weld pool. The alternating nature of the field means that the magnetic force direction reverses periodically, creating a dynamic interaction with the welding process.
| Parameter | Effect on Arc | Effect on Weld Pool |
|---|---|---|
| Transverse Alternating Magnetic Field | Arc morphology changes; arc pressure varies | Cross-section transitions from finger to pot-bottom shape |
| Increasing Frequency | Arc pressure peak first decreases then increases | Weld pool shape evolution |
| No Magnetic Field | Finger-shaped deep penetration | Narrow, deep weld bead |
| With Magnetic Field | Modified arc shape and pressure distribution | Wider, shallower weld bead with pot-bottom profile |
Arc Morphology and Pressure Analysis
The high-speed camera observations reveal that the transverse alternating magnetic field causes significant deflection and distortion of the DP-TIG arc. The arc, which is normally a relatively straight, narrow column in the absence of external fields, becomes curved and oscillating under the influence of the magnetic field. The arc pressure, measured using the small hole method, shows a non-monotonic relationship with frequency: at low frequencies, the arc pressure decreases as the magnetic field disrupts the natural Lorentz force concentration; at higher frequencies, the arc pressure increases as the rapidly alternating field creates a time-averaged compressive effect on the arc.
The transition of the weld pool cross-section from finger-shaped to pot-bottom-shaped is a direct consequence of the modified arc pressure distribution and electromagnetic stirring. The finger-shaped profile is characteristic of the deep penetration achieved by conventional DP-TIG, where the concentrated arc pressure drives the molten metal deep into the base metal. Under the alternating magnetic field, the arc pressure distribution becomes more uniform across the weld pool, resulting in a wider, shallower pot-bottom profile.
Engineering Practice and Application Considerations
The study mentions nickel-based overlay welding with iron content in the weld surface layer within 5%, which is relevant to engineering applications requiring corrosion-resistant or wear-resistant overlays. The low dilution rate indicates that the magnetic field-assisted DP-TIG process can produce overlay welds with high composition control. This is particularly valuable in applications such as hardfacing of pump impellers, valve components, or chemical processing equipment where the surface composition must be carefully controlled.
For pipe manufacturing and repair applications, the magnetic field-assisted DP-TIG process offers several advantages:
- The ability to control weld pool shape and penetration depth through magnetic field parameters
- Potential for improved weld bead uniformity through electromagnetic stirring
- Applicability to thick-section welding with controlled dilution in overlay applications
- The transition from finger-shaped to pot-bottom-shaped weld pool may reduce the risk of incomplete root fusion and enhance bead stability
Defect Analysis and Countermeasures
The finger-shaped weld pool profile of conventional DP-TIG can lead to defects such as undercut, excessive penetration, and lack of fusion at the root. The pot-bottom-shaped profile achieved under the alternating magnetic field may mitigate these defects by distributing the heat more evenly across the weld pool. However, the magnetic field introduces additional variables that must be controlled: the frequency, amplitude, and direction of the alternating field must be optimized for each specific welding application. Inconsistencies in magnetic field application can lead to asymmetric weld beads or irregular penetration.
Study Insights and Reflections
This study demonstrates the potential of external magnetic field control as a means of manipulating the welding arc and weld pool in advanced TIG processes. The DP-TIG process, already a high-performance variant of conventional TIG, benefits from magnetic field assistance by offering additional degrees of freedom in process parameter control. The finding that the weld pool shape transitions from finger to pot-bottom is particularly significant, as it suggests that the magnetic field can be used to tailor the weld geometry for specific applications. For engineering practice, the integration of magnetic field control into DP-TIG welding represents an emerging technology that could enhance welding quality, productivity, and flexibility. Future research should focus on optimizing the magnetic field parameters for specific materials and joint configurations, and on developing practical magnetic field application systems for industrial welding environments.