Plasma-MIG Dual Arc Welding Arc Ignition Process Analysis

Literature Overview

This paper published in The Welding Journal (2007, Vol. 28, No. 11) by Li Deyuan and colleagues from Shenyang University of Technology presents a fundamental study of the arc ignition process in plasma-MIG dual arc welding. The authors apply Paschen's law to analyze breakdown voltage along different paths in the arc space, providing theoretical and experimental insights into how the plasma arc ignites using the conductive channel provided by the MIG arc. This work represents an important contribution to understanding hybrid welding process physics.

Core Technical Concepts

Plasma-MIG dual arc welding combines the deep, narrow penetration of a transferred plasma arc with the high deposition rate of a MIG arc in a single welding operation. The key challenge in this process is the sequential ignition of two arcs—the plasma arc must ignite using the MIG arc as a pre-ionized conductive channel. Understanding this ignition mechanism is critical for reliable process operation and equipment design.

The fundamental physics governing arc ignition in this system involves:

Paschen's Law: The breakdown voltage between two electrodes in a gas depends on the product of gas pressure and electrode gap distance (p·d). The minimum breakdown voltage occurs at a specific p·d value characteristic of each gas species.
Thermal Ionization: At elevated temperatures, gas molecules undergo thermal ionization, creating free electrons and positive ions that facilitate electrical breakdown.
Conductive Channel Formation: The MIG arc creates a pre-ionized plasma channel that reduces the effective breakdown voltage required for plasma arc ignition.

Paschen's Law Application

Paschen's law states that the breakdown voltage V between two parallel plate electrodes is given by:

V = (B·p·d) / ln(A·p·d - ln(1/(γ_b + 1)))

Where:

A and B are gas-specific constants
p is the gas pressure
d is the electrode gap distance
γ_b is the secondary electron emission coefficient

For argon gas (the primary shielding gas in aluminum welding):

A ≈ 11.4 V/(cm·Torr)
B ≈ 138 V·cm/Torr
Minimum breakdown voltage ≈ 15 kV at p·d ≈ 10 Torr·cm

The application of Paschen's law to plasma-MIG ignition reveals that the breakdown voltage varies significantly along different paths in the arc space due to:

Non-uniform temperature distribution from the MIG arc
Variation in gas density due to thermal expansion
Different effective gap distances along radial and axial paths

Arc Ignition Process Analysis

The plasma-MIG ignition sequence proceeds as follows:

MIG arc establishment: The MIG arc ignites first using conventional high-frequency or contact start methods, establishing a stable conductive plasma channel between the MIG electrode and workpiece.
Thermal pre-ionization: The MIG arc heats the surrounding shielding gas, creating a region of elevated temperature and partial ionization that reduces the effective breakdown voltage.
Plasma arc ignition path selection: The plasma arc ignites along the path of minimum breakdown voltage, which is not necessarily the direct line between the plasma torch electrode and the workpiece. Instead, the arc follows a path through the pre-ionized region created by the MIG arc.
Stable plasma arc establishment: Once the initial breakdown occurs, the plasma arc stabilizes through constricted gas flow through the torch nozzle, achieving temperatures of 15,000-30,000 K.

The authors' high-speed camera observations confirmed the calculated ignition paths, demonstrating that the plasma arc consistently ignites through the MIG arc's conductive channel rather than directly to the workpiece.

Process Parameters and Equipment Considerations

The successful operation of plasma-MIG dual arc welding requires careful coordination of the two arc systems:

Parameter	Typical Value	Function
MIG current	100-200 A	Provides conductive channel
Plasma current	20-80 A	Deep penetration
Plasma gas flow	1.5-3.0 L/min	Arc constriction
Shielding gas flow	15-25 L/min	Protection
Arc length (MIG)	3-6 mm	Stability
Arc length (plasma)	1-3 mm	Penetration control
Ignition delay	50-200 ms	Sequential control
Torch offset	5-15 mm	Arc interaction zone

The ignition delay between MIG and plasma arcs is critical—too short a delay may result in failed plasma ignition due to insufficient pre-ionization, while too long a delay increases the risk of MIG arc instability before plasma establishment.

Engineering Practice Implications

For engineers considering plasma-MIG dual arc welding in production applications:

Equipment requirements: Specialized power sources with independent control of MIG and plasma circuits are required. The system must provide precise timing control for sequential arc ignition.
Torch design: The dual torch must maintain precise alignment between the MIG wire electrode and plasma torch nozzle. Typical designs incorporate the plasma torch concentrically around or offset from the MIG gun.
Process advantages: The dual arc provides:

Higher deposition rate than plasma alone (2-3× improvement)
Deeper penetration than MIG alone (50-100% improvement)
Reduced dilution of the base metal compared to conventional MIG
Lower heat input than equivalent single-process welds

Applications: Particularly suitable for thick-section aluminum welding, pipe welding where deep penetration is needed with minimal back deformation, and applications requiring both high productivity and deep weld profiles.

Key Questions and Reflections

The study provides valuable theoretical grounding for plasma-MIG ignition, but several practical questions remain:

How does the ignition behavior change with different shielding gas compositions (Ar, He, Ar+H2, Ar+CO2)?
What is the effect of welding position on the ignition reliability and arc stability?
Can the ignition process be further optimized through active gas flow modulation during the ignition transient?

The application of Paschen's law to this problem demonstrates the power of fundamental physics in solving practical engineering challenges. The authors' approach of considering both gap distance and temperature distribution provides a more complete picture than analyses that consider only geometric factors. This methodology could be extended to other hybrid welding processes such as laser-MIG, friction stir-MIG, and plasma-friction stir welding.

Study Insights and Implications

This research establishes a rigorous theoretical framework for understanding plasma-MIG dual arc ignition, validated by high-speed imaging. The key insight is that successful plasma ignition depends not only on the electrode gap but critically on the thermal and ionization state of the intervening gas medium. Engineers designing dual arc welding systems should account for the pre-ionization effect in their process parameter optimization and equipment design. The Paschen's law approach provides a predictive tool that can be adapted to other hybrid welding configurations, making this study broadly applicable beyond the specific plasma-MIG combination.