Numerical Simulation of Plasma-MIG Hybrid Welding Based on Different Plasma Currents

Literature Overview

This research by Dong Junqiang, Chen Kexuan, and Chen Peng from Lanzhou University of Technology, published in 2023, investigates the numerical simulation of plasma-MIG hybrid welding with varying plasma current levels. The study was conducted at the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, which has significant relevance to the cladding and overlay welding of nickel-based alloys and other non-ferrous materials used in bimetal pressure vessel construction.

The plasma-MIG hybrid welding process combines the deep penetration characteristics of plasma arc welding with the high deposition rate of MIG welding, making it particularly attractive for multi-pass overlay welding applications where both penetration depth and deposit thickness per pass are important considerations.

Core Technical Content and Process Physics

Plasma-MIG Hybrid Welding Configuration

In the plasma-MIG hybrid welding process, two distinct heat sources are simultaneously applied to the workpiece:

• Plasma arc: Provides deep penetration and acts as the primary heat source for melting the base metal.
• MIG arc: Provides additional heat input and serves as the primary means of metal deposition.

The two arcs are arranged in a specific geometric configuration, typically with the plasma torch leading and the MIG torch trailing, or vice versa depending on the application requirements.

Effect of Plasma Current on Weld Pool Dynamics

The plasma current is the primary variable investigated in this study. Different plasma current levels produce fundamentally different weld pool morphologies, flow patterns, and solidification behaviors:

The simulation reveals that increasing the plasma current from 10 A to 40 A significantly increases the weld pool depth while maintaining a relatively constant deposition rate, since the metal deposition is primarily governed by the MIG wire feed rate.

Numerical Model Description

The numerical simulation employs a coupled multiphysics approach that solves for:

1. Electromagnetic field: Calculation of arc forces, electromagnetic stirring, and Lorentz forces in the weld pool.
2. Fluid dynamics: Weld pool flow patterns driven by electromagnetic forces, buoyancy, and surface tension gradients (Marangoni effect).
3. Heat transfer: Thermal field distribution accounting for both plasma and MIG arc heat sources.
4. Phase change: Solidification front tracking using the enthalpy-porosity method.

The plasma arc heat source is typically modeled using a double-ellipse or modified Gaussian distribution, while the MIG arc is represented by a conical or cylindrical heat source model. The interaction between the two arcs, including potential electromagnetic coupling and gas flow interference, must be carefully accounted for in the boundary conditions.

Application to Weld Overlay Cladding

Advantages for Cladding Applications

The plasma-MIG hybrid process offers several distinct advantages for weld overlay cladding in bimetal pressure vessel fabrication:

• Controlled dilution: By independently adjusting the plasma current and MIG parameters, the dilution ratio can be precisely controlled within a wide range, which is critical for achieving the required corrosion resistance in the overlay layer.
• High deposition rate: The MIG component ensures high metal deposition rates, making the process economical for thick cladding builds required in hydrogenation reactors and heat exchanger tubesheets.
• Deep penetration: The plasma arc provides excellent penetration into the base metal, promoting strong metallurgical bonding between the cladding layer and the substrate without excessive mechanical preparation.

Process Optimization for Overlay Welding

Defect Analysis and Quality Control

The simulation results help identify potential defect mechanisms in plasma-MIG hybrid overlay welding:

Study Insights and Practical Recommendations

The simulation results demonstrate that the plasma current is the most effective parameter for controlling dilution in plasma-MIG hybrid overlay welding. At plasma currents below 15 A, the process behaves predominantly as a MIG weld with limited penetration enhancement. Above 25 A, the penetration depth increases rapidly, but the risk of centerline cracking and excessive dilution also increases.

The optimal plasma current range for most overlay applications appears to be 15-25 A, where sufficient penetration is achieved without excessive dilution. This finding is consistent with practical experience in the fabrication of clad-plate pressure vessels, where dilution control is the primary quality parameter for weld overlay cladding.

For engineers involved in bimetal pressure vessel fabrication, this study provides a quantitative framework for selecting plasma-MIG hybrid welding parameters based on the specific requirements of the cladding application. The numerical simulation approach can be extended to predict weld pool geometry, solidification patterns, and residual stress distributions for different process parameter combinations, enabling rational process design before physical trial welding.

Future work should focus on validating the simulation predictions through systematic experimental studies, including metallographic analysis of the weld cross-sections, hardness mapping across the cladding interface, and corrosion testing of the overlay layer under simulated service conditions. Integration of the simulation results with finite element analysis of residual stresses would provide a complete picture of the cladding quality and long-term service performance.

In summary, this research on plasma-MIG hybrid welding simulation offers valuable insights for the rational design of weld overlay processes in bimetal pressure vessel fabrication, with the plasma current serving as the key lever for balancing penetration depth and dilution control in overlay welding applications.