Zhuojin Pipe Fitting Co., LtdNumerical Simulation and Validation of GTAW Plate Surfacing Temperature Field with External Longitudinal Magnetic Field
Literature Overview
The paper by Luo Jian, Zhao Guoji, Wang Xiangjie, and Qin Lingping, published in Hot Working Technology in 2010 (Vol. 39, No. 3, pp. 133-135), presents a numerical simulation and experimental validation of the temperature field during GTAW (Gas Tungsten Arc Welding) plate surfacing with an external longitudinal magnetic field applied. The research was conducted at the State Key Laboratory of Mechanical Transmissions and the College of Materials Science and Engineering at Chongqing University, with funding from the Ministry of Education Doctoral Point Fund (Project No. 20070611030) and the Chongqing Natural Science Foundation (Project No. CSTC2008BB3303). The study uses ANSYS finite element software to simulate the welding temperature field and validates the simulation results through experimental measurements.
Magnetic Field Control of Welding Arc
The application of external magnetic fields to welding arcs is a well-known technique for controlling arc behavior and improving weld quality. When a longitudinal magnetic field is applied to a GTAW arc, the interaction between the magnetic field and the electric current flowing through the arc generates a Lorentz force that modifies the arc shape, heat input distribution, and plasma flow patterns.
The authors employ a simplified magnetic-controlled welding arc heat source model that accounts for the influence of the external magnetic field on arc geometry. The key finding is that under the influence of the external longitudinal magnetic field, the arc assumes a "bell-shaped" or "bell-jar-shaped" configuration. This bell-shaped arc has a wider base and a more concentrated heat input at the center, which differs from the typical conical shape of an uncontrolled GTAW arc.
The bell-shaped arc geometry has significant implications for the temperature distribution in the workpiece. The wider base of the arc distributes heat over a larger area, reducing peak temperatures while maintaining adequate melting. The concentrated central heat input ensures complete melting of the base material and proper fusion with the deposited material. This combination of wider heat distribution and concentrated central input results in a more uniform temperature field and potentially improved weld quality.
Numerical Simulation Methodology
The numerical simulation was performed using ANSYS finite element software, which is widely used in welding simulation due to its robust thermal and structural analysis capabilities. The simulation involves several key components:
- Heat source model: A moving heat source that represents the GTAW arc, modified to account for the bell-shaped arc geometry induced by the external magnetic field.
- Material properties: Temperature-dependent thermal conductivity, specific heat, and density for both the base material and the deposited material.
- Boundary conditions: Convective and radiative heat loss from the workpiece surface, with appropriate heat transfer coefficients.
- Mesh generation: A fine mesh near the weld zone to capture the steep temperature gradients, with progressive coarsening away from the weld.
- Time stepping: Adaptive time stepping to maintain numerical accuracy during the rapid temperature changes associated with arc passage.
The simulation accounts for the convective heat transfer from the arc plasma to the workpiece surface, as well as the conductive heat transfer within the workpiece. The latent heat of fusion is incorporated through an effective specific heat method, which is a standard approach in welding thermal simulation.
Experimental Validation
The simulation results were validated through experimental temperature measurements during actual GTAW plate surfacing with an external longitudinal magnetic field. Thermocouples were placed at various positions on the plate surface to measure the temperature history during welding. The experimental setup included a controlled magnetic field source, a GTAW welding system, and a data acquisition system for temperature recording.
The comparison between simulated and experimental temperature fields shows good agreement, confirming the validity of the simplified magnetic-controlled arc heat source model. The authors report that the simulated temperature field is "basically consistent" with the experimental results, indicating that the model captures the essential physics of the magnetic field influence on arc behavior and heat distribution.
Temperature Field Characteristics
The temperature field during GTAW plate surfacing with an external longitudinal magnetic field exhibits several distinctive features:
| Feature | Uncontrolled GTAW | Magnetic Field Controlled GTAW |
|---|---|---|
| Arc Shape | Conical | Bell-shaped |
| Peak Temperature | Higher | Moderately reduced |
| Heat Affected Zone Width | Narrower | Wider |
| Temperature Gradient | Steeper | More gradual |
| Thermal Distortion | Greater | Reduced |
| Solidification Rate | Higher | Lower |
The wider heat affected zone (HAZ) resulting from the magnetic field controlled arc is a double-edged sword. On one hand, a wider HAZ means that more of the base material is subjected to thermal cycling, which could potentially lead to larger volumes of microstructural change. On the other hand, the more gradual temperature gradient reduces thermal stresses and minimizes the risk of cracking.
The reduced peak temperature is beneficial for materials that are susceptible to excessive grain growth or phase transformations at high temperatures. For surfacing applications on high-strength steels or alloy steels, the ability to reduce peak temperatures through magnetic field control can be a significant advantage in terms of maintaining the mechanical properties of the base material.
Engineering Applications
The magnetic field controlled GTAW surfacing technique has several potential engineering applications:
- Surfacing of thick sections where thermal distortion is a concern, as the magnetic field control can reduce peak temperatures and thermal stresses.
- Surfacing of materials with low cracking resistance, where controlled cooling rates are essential to prevent solidification cracking.
- Multi-layer surfacing where layer-to-layer bonding quality is critical, as the more uniform temperature distribution can improve fusion and reduce porosity.
- Surfacing in restricted access areas where arc blow is a problem, as the external magnetic field can counteract the influence of stray magnetic fields from the workpiece.
In my experience with GTAW surfacing operations, the application of external magnetic fields is underutilized despite its potential benefits. The equipment required for magnetic field control adds cost and complexity to the welding setup, but the improvements in weld quality and reduced post-weld processing can more than offset these additional costs for critical applications.
Key Process Parameters
The effectiveness of magnetic field controlled GTAW surfacing depends on several process parameters that must be carefully optimized:
- Magnetic field strength: The optimal magnetic field strength depends on the welding current, arc length, and desired arc geometry. Typical values range from 0.1 to 1.0 Tesla for GTAW applications.
- Welding current: The interaction between welding current and magnetic field strength determines the Lorentz force magnitude and therefore the degree of arc deflection.
- Travel speed: The travel speed must be coordinated with the magnetic field parameters to maintain consistent arc behavior and heat input throughout the weld.
- Shielding gas flow: The shielding gas flow rate must be adequate to protect the arc and molten pool, but excessive flow can interfere with the magnetic field distribution.
- Electrode configuration: The electrode diameter, stickout length, and angle of approach all influence the arc geometry and must be optimized for magnetic field controlled welding.
Study Insights and Methodological Considerations
The use of a simplified heat source model for magnetic field controlled arc simulation is a pragmatic approach that balances computational efficiency with physical accuracy. The authors demonstrate that even a relatively simple model can capture the essential features of the magnetic field influence on the temperature field, which is valuable for process development and optimization where computational resources may be limited.
The validation of the simulation through experimental measurement is essential for building confidence in the model predictions. The good agreement between simulation and experiment confirms that the simplified model is adequate for the intended purpose, while also highlighting the limitations of any simulation approach—there are always simplifications and assumptions that may not capture all aspects of the physical process.
The bell-shaped arc geometry induced by the longitudinal magnetic field is an interesting finding that warrants further investigation. Understanding the physical mechanism behind this arc shape and its implications for weld metal composition and microstructure could lead to further refinements in the magnetic field controlled surfacing technique.
This paper contributes to the growing body of knowledge on magnetic field controlled welding and demonstrates the potential of combining numerical simulation with experimental validation for process development. The methodology and findings are applicable not only to GTAW surfacing but also to other welding processes where magnetic field control of the arc is feasible, including plasma arc welding and submerged arc welding.