DC TIG Arc Current Density Distribution Characteristics and Measurement Methodology

Literature Overview

This paper, published in the Journal of Xi'an Jiaotong University (西安交通大学学报, Vol. 28, Issue 4, 1994, pp. 33-38) by Jia Changshen, Xiao Kemin, Liu Haixia, and Zhou Haobin, presents a fundamental study of current density distribution on the anode plate surface in DC TIG arcs. The work employs two measurement methods — the separated electrode plate method and a custom probe method — and provides mathematical expressions for the radial current density distribution under different current conditions.

Fundamental Significance

Understanding current density distribution in TIG arcs is essential for:

• Predicting heat input distribution on the workpiece
• Modeling weld pool geometry and penetration
• Optimizing welding parameters for specific joint configurations
• Developing welding process simulation tools
• Explaining arc behavior in special applications (e.g., micro-TIG, AC TIG)

Measurement Methodologies

Separated Electrode Plate Method

This method involves positioning a thin conductive plate near the arc to intercept current flow. The plate is electrically separated from the main circuit, and the current distribution across its surface is measured using a multi-point sensing array. Advantages include:

• Direct measurement of current density at multiple points simultaneously
• Relatively simple setup
• Suitable for steady-state conditions

Limitations include:

• The plate itself perturbs the arc field
• Limited spatial resolution
• Difficulty in capturing transient behavior

Custom Probe Method

The authors developed a specialized probe for point-by-point current density measurement. This approach offers:

• Higher spatial resolution at individual measurement points
• Reduced arc perturbation compared to plate methods
• Flexibility in measurement location selection

However, it requires:

• Sequential measurements at multiple points
• Precise positioning and alignment
• Longer measurement time per condition

Key Findings

Current Density Radial Distribution

This is a critical finding with direct implications for welding process modeling. At low currents (typically below 50-80 A for tungsten electrodes), the current density follows a Gaussian distribution, which simplifies thermal modeling significantly. At higher currents (common in industrial pipe welding, typically 100-300 A), the distribution becomes complex and cannot be captured by simple functions.

Effects of Arc Voltage and Current

• Arc voltage: Influences the spread of current density distribution; higher arc voltages generally correspond to broader distributions
• Current magnitude: Higher currents lead to flatter, more uniform distributions that deviate from Gaussian behavior
• Combined effect: The interaction between voltage and current creates complex distribution patterns that vary with electrode geometry, gas composition, and electrode-to-workpiece distance

Engineering Practice Implications

Weld Pool Modeling

For welding process simulation and FEA-based heat transfer modeling:

• Low-current applications (micro-welding, thin sheet TIG): Gaussian current density assumption is valid and simplifies calculations
• Industrial pipe welding (medium to high currents): More sophisticated current density models are required; simple Gaussian assumptions will underestimate heat input at the arc center and overestimate it at the periphery

Parameter Selection

Understanding current density distribution helps explain:

• Why penetration depth increases non-linearly with current
• The transition from shallow to deep weld profiles at certain current thresholds
• The role of arc length in modifying current density concentration

Quality Control

• Arc stability monitoring can be enhanced by understanding current density behavior
• Arc wander phenomena may be related to non-uniform current density distributions
• Electrode wear patterns correlate with current density at the electrode tip

Key Reflections

This 1994 paper, while technically focused on fundamental arc physics, provides the groundwork for modern welding process modeling. The finding that current density transitions from Gaussian to non-Gaussian behavior with increasing current is particularly important for contemporary computational welding science. Most commercial welding FEA software still employs simplified current density distributions, and this research highlights the limitations of such approaches for high-current industrial applications.

For pipe welding engineers, the practical implication is that welding procedure development at different current levels may require fundamentally different approaches to heat input estimation. A procedure qualified at 80 A cannot simply be scaled to 200 A by linear extrapolation of heat input, because the current density distribution — and therefore the thermal profile — changes character.

Study Insights and Outlook

The measurement techniques described, while dated by modern standards, represent rigorous experimental approaches that remain conceptually valid. Contemporary research could employ high-speed imaging, fiber-optic sensors, or electromagnetic measurements to achieve similar results with less arc perturbation. The non-Gaussian distribution finding at high currents has not been fully incorporated into standard welding codes or procedure qualification methodologies, representing an opportunity for improvement in engineering practice. Future work should develop empirical correlations for non-Gaussian current density distributions that can be directly applied in welding procedure design and qualification.