Magnetic Control Characteristics of Low Current TIG Welding Arc

Literature Overview

This study, published in the Welding Journal (焊接学报) in 1990 by Zhang Jiuhai, Wang Qilong, and Wei Weiping from Harbin Institute of Technology, investigates the magnetic control behavior of the TIG welding arc under low current conditions. The research addresses a critical gap in understanding how electromagnetic forces interact with the arc plasma when welding current is reduced to levels typically below 50 A, a regime commonly encountered in thin-sheet welding, cladding of thin overlay layers, and repair welding of precision components.

Core Technical Content

The fundamental premise of this work is that the arc column in TIG welding is not a passive conduit of energy but rather a dynamic plasma channel whose geometry, stability, and heat input distribution are strongly influenced by external magnetic fields. At low currents, the arc exhibits greater susceptibility to magnetic deflection due to reduced arc pressure and lower self-generated magnetic fields, which has direct implications for weld bead geometry and process stability.

Key Findings on Arc Behavior

The researchers systematically examined how external magnetic fields—both axial and transverse orientations—affect the following arc parameters:

Parameter	Low Current (<50 A) Behavior	High Current (>200 A) Behavior	Practical Significance
Arc deflection magnitude	Large, easily controlled	Small, resistant to external fields	Enables precise heat input steering at low currents
Arc stability	Sensitive to magnetic perturbation	Inherently stable due to high arc pressure	Low-current applications require magnetic stabilization
Heat input distribution	Narrow, highly concentrated	Broad, diffused	Low-current magnetic control can create localized heating
Arc root position shift	Pronounced lateral movement	Minimal displacement	Useful for controlling weld bead width in overlay applications

Mechanism of Magnetic Arc Control

The electromagnetic force acting on the arc plasma is described by the Lorentz force equation, where the force density is proportional to the cross product of current density and magnetic field vector. At low currents, the self-generated magnetic field of the arc is weak, making the plasma column more responsive to externally applied fields. This creates an opportunity for active arc manipulation—steering the heat input to desired locations without mechanical torch movement.

The study identifies three distinct magnetic control regimes:

Weak field regime (B < 0.1 mT): Arc remains largely undisturbed; only subtle changes in arc shape are observed.
Moderate field regime (0.1–1.0 mT): Significant arc deflection occurs; weld bead width and penetration depth can be deliberately modified.
Strong field regime (B > 1.0 mT): Arc instability increases dramatically; arc wandering, spatter, and incomplete fusion become likely.

Engineering Practice Implications

From a cladding and weld overlay perspective, this research has several important applications:

Thin overlay layers: When applying thin cladding layers (1–3 mm) using low-current TIG, magnetic arc control can compensate for the inherently narrow heat input, ensuring adequate base metal fusion without excessive dilution.
Repair welding: For repairing thin-walled pressure vessels or heat exchanger tubes, magnetic arc guidance allows the welder to maintain the arc root at the correct position even when mechanical torch positioning is difficult.
Positional welding: In overhead or vertical-up welding positions where gravity affects arc stability, applied magnetic fields can counteract gravitational arc sag.

Defects and Countermeasures

The study also implicitly addresses common defects associated with low-current TIG welding:

Defect	Cause at Low Current	Magnetic Control Countermeasure
Lack of fusion	Insufficient arc pressure and heat input	Axial magnetic field to concentrate arc root
Excessive dilution	Arc wandering into base metal	Transverse field to maintain arc centering
Porosity	Poor arc stability causing gas entrapment	Stabilizing magnetic field to maintain consistent arc shape
Undercut	Arc deflection away from weld toe	Corrective transverse field to redirect arc energy

Study Insights and Reflections

This 1990 study represents a foundational contribution to the understanding of plasma physics in welding. Its enduring value lies in establishing quantitative relationships between magnetic field strength and arc behavior at low currents—relationships that remain applicable today. For engineers working with modern cladding technologies such as hot-wire TIG and GTAW overlay, the principles of magnetic arc control provide a framework for understanding and optimizing processes where arc stability is paramount. The work also foreshadows later developments in magnetic arc stabilization systems used in automated welding cells, where external coils are employed to maintain arc quality during multi-pass overlay welding of thick sections.