Zhuojin Pipe Fitting Co., LtdNumerical Simulation and Verification of Temperature Field in GTAW Plate Surfacing with External Longitudinal Magnetic Field
Literature Overview
This 2010 paper published in Hot Working Technology by Luo Jian, Zhao Guoji, Wang Xiangjie, and Qin Lingping from Chongqing University (State Key Laboratory of Mechanical Transmission and College of Materials Science and Engineering) presents a numerical simulation study of the temperature field during gas tungsten arc welding (GTAW) plate surfacing with an external longitudinal magnetic field applied. The research was supported by the Ministry of Education Doctoral Fund (Grant No. 20070611030) and the Chongqing Natural Science Foundation (Grant No. CSTC2008BB3303). The authors used the finite element analysis software ANSYS to simulate the temperature field and validated the simulation results against experimental measurements.
Core Technical Approach
The application of an external magnetic field to the welding arc is a technique known as magnetic arc control (MAC) or magnetic manipulation. The primary purpose is to modify the arc shape, heat distribution, and metal transfer characteristics without changing the electrical parameters of the welding process. In this study, a longitudinal magnetic field was applied along the welding direction, and the authors developed a simplified magnetic arc heat source model that accounts for the effect of the magnetic field on the arc geometry.
Heat Source Model Development
The key innovation in this paper is the development of a "bell-shaped" (or "bell-jar-shaped") arc heat source model that represents the arc morphology under the influence of the longitudinal magnetic field. Under normal GTAW conditions without a magnetic field, the arc is typically modeled as a double-ellipsoidal or single-ellipsoidal heat source. However, when a longitudinal magnetic field is applied, the Lorentz force acts on the charged particles in the arc plasma, causing the arc to elongate and shift in the direction of the magnetic field. This results in a characteristic "bell-shaped" heat distribution where the heat intensity is concentrated at the leading edge of the arc.
The heat source model is defined by the following parameters:
| Parameter | Symbol | Description |
|---|---|---|
| Arc current | I | Welding current |
| Arc voltage | U | Arc voltage |
| Travel speed | v | Welding speed |
| Magnetic field strength | B | Longitudinal magnetic field intensity |
| Arc radius | r | Effective arc radius |
| Heat efficiency | η | Fraction of electrical energy transferred as heat |
| Bell shape factor | f | Geometric factor describing the bell-shaped distribution |
The temperature field simulation was performed using the transient 3D finite element method in ANSYS. The governing heat conduction equation was solved with appropriate boundary conditions, including convective and radiative heat loss from the top surface and adiabatic conditions at the bottom surface. The material properties, including thermal conductivity and specific heat, were defined as functions of temperature to account for the nonlinear thermal behavior of the steel substrate.
Simulation vs. Experimental Validation
The authors compared the simulated temperature distributions with experimental measurements obtained from thermocouple readings during actual welding tests. The results showed good agreement between the simulation and experiment, validating the proposed magnetic arc heat source model. The key findings from the comparison are:
| Comparison Parameter | Simulation | Experiment | Deviation |
|---|---|---|---|
| Peak temperature | Consistent | Consistent | <5% |
| Cooling rate | Consistent | Consistent | <10% |
| Thermal affected zone width | Slightly larger | Measured | ~5–8% |
| Temperature distribution shape | Bell-shaped | Bell-shaped | Qualitative match |
The slight overestimation of the thermal affected zone width in the simulation is attributed to the simplifications inherent in the heat source model and the assumption of constant material properties. Despite this, the overall agreement confirms the validity of the approach for predicting thermal cycles and estimating the thermal affected zone (TAZ) geometry.
Engineering Practice Implications
The application of magnetic arc control in GTAW surfacing has several practical benefits for piping and pressure equipment manufacturing:
- Improved heat input control: By adjusting the magnetic field strength, the heat input distribution can be modified to achieve a more uniform or directional heat profile, which is beneficial for controlling the microstructure and residual stresses in the surfacing deposit.
- Reduced dilution: The bell-shaped heat source concentrates heat at the leading edge, which can reduce the amount of base metal melted and incorporated into the weld deposit. This is particularly important for surfacing applications where the overlay composition must be maintained with minimal dilution from the substrate.
- Improved weld geometry: Magnetic arc control can be used to optimize the weld bead profile, reducing the need for excessive travel speed or current adjustments that may compromise weld quality.
- Automation potential: Magnetic arc control can be integrated into automated welding systems to provide dynamic adjustment of the heat input profile in response to varying welding conditions.
Practical Implementation Guidelines
For engineers considering the implementation of magnetic arc control in GTAW surfacing operations, the following guidelines are recommended:
- Magnetic field strength: Typical longitudinal magnetic field strengths range from 10 mT to 100 mT. Higher fields produce more pronounced arc deflection but may cause arc instability.
- Electrode alignment: The electrode should be aligned with the magnetic field direction to maximize the effectiveness of arc manipulation.
- Shielding gas flow: The magnetic field may affect the shielding gas flow pattern, so the gas nozzle design and flow rate should be optimized for the specific magnetic field configuration.
- Welding parameters: The welding current, voltage, and travel speed should be optimized in conjunction with the magnetic field parameters to achieve the desired weld quality.
- Equipment considerations: The magnetic field generation system (electromagnet or permanent magnet) must be designed to provide a uniform field over the welding area and to withstand the thermal environment of the welding process.
This paper demonstrates the value of numerical simulation in understanding and optimizing advanced welding processes. The validated heat source model provides a foundation for further research into the effects of magnetic arc control on weld microstructure, residual stresses, and mechanical properties. For piping engineers, the technology offers a promising approach to improving the quality and consistency of surfacing operations on critical components.