Numerical Simulation of Plasma-MIG Hybrid Welding with Varying Plasma Current

Literature Overview

This 2023 study by Dong Junqiang, Chen Kexuan, and Chen Peng from Lanzhou University of Technology (including the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals) investigates the plasma-MIG hybrid welding process through numerical simulation, with particular focus on how different plasma current levels affect the welding process. Plasma-MIG hybrid welding combines the high-energy density of plasma arc with the high deposition rate of GMAW/MIG, offering a synergistic approach to welding and cladding.

Core Technical Content

Plasma-MIG hybrid welding is a composite process where a plasma arc and a GMAW arc are simultaneously applied to the workpiece. The plasma arc provides deep penetration and high energy density, while the GMAW arc supplies filler metal at a high rate. The numerical simulation examines the interaction between these two heat sources and the resulting weld pool behavior.

The study varies the plasma current while maintaining other parameters constant, allowing isolation of the plasma current effect on:

Weld pool geometry (penetration depth and width)
Temperature field distribution
Flow field and convection patterns
Solidification behavior and microstructure prediction

Key Process Parameters

Parameter	Range Studied	Effect
Plasma current	20–100 A	Primary variable; controls penetration depth
MIG current	150–300 A	Controls deposition rate
Wire feed speed	3–8 m/min	Controls filler metal input
Travel speed	100–400 mm/min	Controls heat input per unit length
Plasma arc power	5–25 kW	Controls energy density
MIG arc power	5–15 kW	Controls deposition energy
Shielding gas	Ar or Ar/He mixtures	Affects arc characteristics

Interpretation of Technical Points

The hybrid plasma-MIG process offers a unique combination of capabilities that is particularly relevant to cladding applications:

High deposition rate with good penetration: The MIG arc provides rapid filler metal deposition while the plasma arc ensures adequate bond strength through sufficient base metal melting.
Controlled dilution: The relative contribution of each arc can be adjusted to control the dilution ratio, which is critical for overlay applications.
Reduced heat input compared to conventional cladding: The high energy density of the plasma arc allows for faster travel speeds while maintaining adequate penetration, reducing overall heat input.

Weld Pool Behavior Under Different Plasma Currents

At low plasma current (20–40 A), the plasma arc acts primarily as a supplementary heat source, with the MIG arc dominating the weld pool geometry. The pool is relatively shallow and wide, with deposition-dominated characteristics.

At medium plasma current (40–70 A), both arcs contribute significantly to the heat input, creating a balanced pool with moderate penetration and good deposition rate. This range is often optimal for cladding applications.

At high plasma current (70–100 A), the plasma arc dominates the pool geometry, creating a deep, narrow pool with significant penetration. While this provides excellent bond strength, it may increase dilution beyond acceptable limits for overlay applications.

Connection with Engineering Practice

For bimetal pressure vessel fabrication, plasma-MIG hybrid welding offers several potential advantages over conventional single-process cladding:

Application Scenarios

Thick overlay layers: The high deposition rate of the MIG arc combined with the deep penetration of the plasma arc allows for rapid buildup of thick overlay layers with good metallurgical bonding.
Repair welding: For local repair of damaged overlay layers on pressure vessels, the hybrid process offers the flexibility to adjust both penetration and deposition independently.
Multi-material joining: The plasma arc can be used to melt dissimilar materials while the MIG arc deposits a compatible filler, facilitating the joining of dissimilar metals.

Process Optimization for Cladding

The numerical simulation results can guide the following process optimization strategies:

Dilution control: By adjusting the plasma-to-MIG current ratio, the dilution can be controlled within the required range for specific overlay applications.
Travel speed optimization: The simulation can identify the optimal travel speed for each plasma current level, balancing deposition rate with pool stability.
Multi-pass strategy: The results can inform the design of multi-pass cladding sequences, where each pass is optimized for the specific conditions of the previous pass.

Engineering Case Analysis

Consider a heat exchanger tube sheet requiring 2 mm of Inconel 625 overlay on SA-350 LF2 base material for high-temperature, high-pressure service. Using plasma-MIG hybrid welding:

The plasma arc at 60 A provides the necessary penetration to ensure metallurgical bonding
The MIG arc at 250 A deposits Inconel 625 wire at a high rate
The dilution is controlled at approximately 15–20%, well within the acceptable range for Inconel 625 overlay
The travel speed of 200 mm/min provides a deposition rate of approximately 1.5 kg/h

This compares favorably with conventional TIG overlay, which would require significantly longer fabrication time for the same overlay thickness.

Key Questions and Reflections

A significant question is the practical implementation of plasma-MIG hybrid welding for cladding. The equipment complexity is considerably higher than single-process welding, requiring two independent power sources, two wire feed systems, and careful coordination of the two arcs. This complexity raises concerns about:

Equipment cost and availability
Operator training requirements
Process reproducibility in production environments
Maintenance and troubleshooting complexity

Another important consideration is the interaction between the two arcs. The plasma arc and MIG arc must be positioned and synchronized to avoid interference, which requires precise robotic or mechanized positioning. For manual welding applications, this level of control may be difficult to achieve.

The simulation also raises the question of whether the predicted weld pool behavior accurately represents the actual process. Hybrid welding involves complex interactions between two plasma jets, two arc forces, and two heat sources, which may be difficult to capture accurately in numerical models.

Study Insights and Implications

This research demonstrates the potential of plasma-MIG hybrid welding as a versatile process for cladding and overlay applications. The ability to independently control penetration and deposition through the plasma and MIG current settings, respectively, offers a level of process flexibility that is not available with single-process welding.

For the bimetal pressure vessel industry, the key implication is that hybrid welding could significantly reduce fabrication time for thick overlay layers while maintaining or improving overlay quality. However, the path to industrial implementation requires addressing the practical challenges of equipment complexity, process control, and cost-effectiveness.

The study also highlights the value of numerical simulation in understanding complex welding processes. By providing a predictive tool for process parameter optimization, simulation can reduce the number of experimental trials required for process qualification, saving time and resources in the development of new cladding procedures.