TIG Arc Behavior Under Applied Longitudinal Magnetic Field

Literature Overview

The study by Zhang Xiaohong, Si Zhongqi, Fan Yifei, Wen Yuanhua, and Zhang Wanchun, published in Aerospace Materials and Technology (2021, Vol. 51, No. 3, pp. 44-48), investigates the behavior of TIG arcs under externally applied longitudinal magnetic fields using magnetohydrodynamic (MHD) modeling. The work was conducted at Sichuan Aerospace Long March Equipment Manufacturing Co., Ltd., reflecting the practical relevance of magnetic field effects in aerospace manufacturing where magnetic fields may be present from adjacent equipment or magnetic forming processes.

Core Technical Findings

The researchers developed a mathematical model in rotating cylindrical coordinates for TIG arcs under longitudinal magnetic fields at different current densities. Using Fluent with user-defined functions for coupled flow field and electromagnetic field solving, they obtained the thermal field and flow field distributions. The key finding is that the longitudinal magnetic field fundamentally alters the arc temperature distribution, creating a hollow bell-shaped profile with a double-peak temperature distribution and a "low-temperature cavity" near the anode surface.

Thermal Field Distribution Characteristics

Condition	Temperature Profile	Peak Temperature Location	Anode Surface Pressure	Low-Temperature Cavity
No magnetic field	Solid bell-shaped	Single peak at arc axis	Uniform distribution	Absent
Longitudinal B-field, low current	Hollow bell-shaped	Double peak (off-axis)	Reduced by ~70%	Present, small
Longitudinal B-field, high current	Hollow bell-shaped	Double peak (off-axis)	Reduced by ~70%+	Present, larger

Magnetohydrodynamic Modeling Approach

The modeling methodology employed several key assumptions and equations:

Governing equations: Navier-Stokes equations for fluid flow, energy equation for thermal transport, Maxwell's equations for electromagnetic fields, and Ohm's law for current density.
Coordinate system: Rotating cylindrical coordinates (r, θ, z) appropriate for axisymmetric TIG arc geometry.
Coupling approach: The flow field, electromagnetic field, and thermal field are solved simultaneously through iterative coupling.
Boundary conditions: Tungsten cathode boundary with fixed temperature and current density; workpiece anode boundary with convective heat loss; open boundary at arc periphery.

Physical Mechanism of Magnetic Field Effects

The longitudinal magnetic field interacts with the arc current through the Lorentz force (J × B), producing several effects:

Radial force component: The azimuthal current density component interacts with the longitudinal B-field to produce a radial force that pushes arc plasma outward from the axis.
Temperature redistribution: The outward force displaces the high-temperature core radially, creating the characteristic hollow bell-shaped profile.
Pressure reduction: The modified flow pattern reduces the peak pressure at the anode surface, with the reduction magnitude increasing with current density.
Low-temperature cavity formation: The radial displacement of hot plasma creates a cooler region near the anode axis, which grows larger with increasing current density.

Engineering Relevance and Applications

Understanding TIG arc behavior under magnetic fields has direct implications for several manufacturing scenarios:

Aerospace Manufacturing Applications

Application Scenario	Magnetic Field Source	Potential Impact	Mitigation Strategy
Welding near MRI equipment	Static field from scanner	Arc deflection, penetration variation	Shielding or process modification
Magnetic forming operations	Forming coil fields	Arc instability during welding	Sequential operation scheduling
Induction heating stations	Time-varying fields	Arc oscillation, spatter	Field cancellation or distance maintenance
Magnetic particle inspection	MPI yoke fields	Minor arc perturbation	Field interruption during welding
Electric propulsion testing	Thruster magnetic fields	Significant arc distortion	Isolation of welding area

Quality Impact Assessment

The magnetic field-induced arc modification can affect weld quality through:

Penetration profile changes: The hollow temperature distribution may produce non-uniform penetration, with deeper penetration off-axis and shallower penetration at the weld center.
Weld width variation: Modified arc force distribution can alter weld bead width and profile geometry.
Porosity formation: The low-temperature cavity near the anode may promote gas entrapment and porosity formation.
Microstructural variation: Non-uniform thermal gradients can produce heterogeneous microstructures across the weld width.

Process Compensation Strategies

To mitigate adverse magnetic field effects, several compensation strategies can be employed:

Magnetic shielding: Use of mu-metal or ferromagnetic shields to reduce field penetration into the welding zone.
Current density adjustment: Reducing welding current to minimize the Lorentz force magnitude.
Arc oscillation: Deliberate oscillation of the arc to average out the asymmetric temperature distribution.
Travel speed modification: Adjusting travel speed to compensate for altered heat input distribution.
Multi-pass welding: Using multiple passes with different orientations to achieve uniform weld properties.

Key Questions and Reflections

The study provides valuable modeling insights but raises important questions about experimental validation:

Model accuracy: The MHD model assumes ideal magnetohydrodynamic behavior, but real arc plasmas exhibit complex non-equilibrium effects, particularly at lower temperatures near the anode.
Field strength range: The study examines specific current densities, but practical magnetic fields in manufacturing environments can vary over several orders of magnitude.
Dynamic effects: The model captures steady-state behavior, but transient magnetic fields from switching equipment may produce different arc responses.
Multi-physics coupling: The interaction between magnetic field effects and other process parameters (gas flow, electrode geometry, workpiece composition) requires further investigation.

Implications for Welding Procedure Specification

For aerospace and defense manufacturing environments where magnetic fields may be present, welding procedure specifications should include:

Magnetic field survey requirements: Pre-weld assessment of ambient magnetic field strength and direction.
Acceptable field limits: Maximum field strength below which arc behavior remains within normal parameter ranges.
Field compensation procedures: Documented methods for process adjustment when fields exceed acceptable limits.
Weld quality verification: Enhanced NDE requirements for welds produced in high-field environments.

Study Insights and Implications

This research contributes important fundamental understanding of how external magnetic fields interact with TIG arc plasmas, with the hollow bell-shaped temperature profile and anode low-temperature cavity representing novel findings with direct practical significance. For aerospace manufacturers operating in environments where magnetic fields are unavoidable, the modeling approach provides a quantitative basis for predicting arc behavior and implementing appropriate process compensations. The systematic analysis of current density effects offers a clear parameter for process optimization, and the identified pressure reduction at the anode surface suggests potential applications for magnetic field-assisted welding where controlled pressure profiles are beneficial. The work underscores the importance of considering electromagnetic environment as a process parameter in welding procedure development, particularly for critical aerospace applications where weld quality directly impacts structural integrity and mission success.