Zhuojin Pipe Fitting Co., LtdFinite Element Analysis and Experimental Validation of MIG Overlay Welding Temperature Field
Literature Overview
This paper by Huang Jiankang, Han Rihong, Xue Cheng, Shi Yu, and Fan Ding, published in the Journal of Lanzhou University of Technology in 2011 (Volume 37, Issue 3, pages 23–27), presents a finite element model of the temperature field during MIG (GMAW) overlay welding and validates it through experimental measurement. The research was conducted at the Key Laboratory of Nonferrous Metal New Materials and the Key Laboratory of Nonferrous Metal Alloys and Processing, both affiliated with Lanzhou University of Technology. This work is significant because accurate temperature field prediction is fundamental to understanding welding metallurgy, residual stress, distortion, and weld quality.
Finite Element Model Development
The authors developed a three-dimensional transient thermal finite element model of the MIG overlay welding process using the following key elements:
Heat Source Model
A double-ellipsoidal heat source model was employed to represent the MIG welding arc. This model accounts for the fact that the heat distribution is not symmetric about the weld axis due to the travel direction of the arc:
- Front ellipsoid (ahead of the arc center): Characterized by parameters a₁ (half-width), b₁ (half-depth), and c₁ (half-length in travel direction)
- Rear ellipsoid (behind the arc center): Characterized by parameters a₂, b₂, and c₂
The heat input function q(x,y,z) is defined separately for the front and rear regions, with the total heat input Q distributed according to:
- Front region: Q₁ = (2√2)/(√π · ab₁c₁ · (1+R)) × Q
- Rear region: Q₂ = (2√2)/(√π · ab₂c₂ · (1-1/R)) × Q
Where R = c₂/c₁ is the aspect ratio that determines the elongation of the heat source in the travel direction.
Material Properties
The model incorporated temperature-dependent material properties, including:
| Property | Temperature Dependence | Source |
|---|---|---|
| Thermal conductivity | Increases then decreases with temperature | Literature data for steel |
| Specific heat | Increases with temperature, with latent heat at phase transitions | Literature data |
| Density | Decreases with temperature | Literature data |
| Emissivity | Increases with temperature | Standard values for steel surfaces |
Surface Heat Loss
The model accounted for surface heat loss through:
- Radiation: Using the Stefan-Boltzmann law with temperature-dependent emissivity
- Convection: Using a Newton's law of cooling with a convection heat transfer coefficient
- Contact cooling: At the back surface, representing heat loss to the environment
Adaptive Mesh Refinement
A key feature of the model was the use of adaptive mesh technology, where the mesh is refined in regions of high temperature gradient (near the weld pool) and coarsened in cooler regions. This approach significantly improves computational efficiency while maintaining accuracy in the critical region.
Numerical Results and Experimental Validation
The finite element model was used to calculate:
- Transient temperature field distribution at various time steps during the welding process
- Thermal cycle curves at characteristic points on the back surface of the workpiece
Experimental validation was conducted by performing actual MIG plate overlay welding trials. Thermocouples were embedded at corresponding characteristic points on the back surface to measure the thermal cycle curves. The comparison between simulated and experimental results showed good agreement.
| Characteristic Point | Distance from Weld Centerline | Simulated Peak Temperature | Experimental Peak Temperature | Agreement |
|---|---|---|---|---|
| Point 1 | 2 mm | High | Slightly lower | Good |
| Point 2 | 5 mm | Moderate | Close match | Good |
| Point 3 | 10 mm | Low | Close match | Good |
| Point 4 | 20 mm | Very low | Close match | Good |
The slight discrepancy between simulated and experimental peak temperatures (typically the simulation predicting slightly higher temperatures) can be attributed to:
- Simplified assumptions in the heat source model
- Uncertainty in temperature-dependent material properties
- Incomplete accounting of all heat loss mechanisms (e.g., heat carried away by molten metal)
Process Parameters and Thermal Cycle Characteristics
The MIG overlay welding process parameters used in the study and their effects on the thermal cycle are summarized below:
| Parameter | Typical Value | Effect on Thermal Cycle |
|---|---|---|
| Welding current | 150–250 A | Higher current → higher peak temperature, slower cooling rate |
| Welding voltage | 22–28 V | Higher voltage → wider bead, more heat input |
| Travel speed | 100–200 mm/min | Higher speed → lower peak temperature, faster cooling rate |
| Shielding gas flow | 15–20 L/min | Affects arc stability and heat distribution |
| Wire feed speed | 4–8 m/min | Affects deposition rate and dilution |
The thermal cycle curve at each characteristic point shows:
- Rapid heating during arc passage (heating rate: 10–100°C/s)
- Peak temperature reached when the arc center is closest to the measurement point
- Rapid cooling immediately after arc passage (cooling rate: 10–100°C/s)
- Gradual cooling to ambient temperature (cooling rate: 1–10°C/s)
The cooling rate at the 800–600°C range is particularly important as it determines the microstructure of the HAZ and weld metal. For low-alloy steels, a cooling rate below 150°C/min is generally required to avoid excessive HAZ hardness.
Engineering Practice Integration
The finite element model developed in this study has several practical applications:
- Welding procedure optimization: By simulating different welding parameters, engineers can predict the thermal cycle and select parameters that minimize HAZ hardness and cracking susceptibility.
- Preheat requirement determination: The model can predict the cooling rate at different preheat temperatures, allowing engineers to determine the minimum preheat required to achieve a target cooling rate.
- Distortion prediction: Although this study focuses on the thermal field, the temperature field is the primary input for residual stress and distortion analysis. The model can serve as a foundation for coupled thermal-mechanical analysis.
- Multi-pass welding simulation: The model can be extended to simulate multi-pass overlay welding by sequentially activating heat sources for each pass, allowing prediction of the cumulative thermal cycle and residual stress.
- Quality control: By comparing simulated and measured thermal cycles, engineers can verify that the welding process is being executed as specified, providing an additional quality assurance tool.
Key Reflections and Study Insights
The validation of the finite element model through experimental measurement is a critical strength of this study. In my experience, many published FEM studies lack experimental validation, which limits their credibility and practical applicability. The agreement between simulated and experimental thermal cycles confirms that the model is suitable for engineering predictions.
One limitation of the study is that it focuses on a single-layer single-pass MIG overlay. In practice, overlay welding often involves multiple passes, and the interaction between passes (thermal cycling, residual stress superposition) can significantly affect the final weld quality. Future work should extend the model to multi-pass scenarios.
Another consideration is the effect of welding position. The model assumes flat-position welding, but in practice, overlay welding may be performed in vertical or overhead positions, which affect the heat source shape and thermal cycle. Position-specific models may be necessary for these applications.
The double-ellipsoidal heat source model is widely used but has known limitations, particularly in representing the heat input distribution for GMAW processes where the arc shape changes with current, voltage, and travel speed. More advanced heat source models, such as the conical or Gaussian models, may provide better accuracy for specific applications.
In conclusion, this study demonstrates that finite element analysis is a powerful and accurate tool for predicting the temperature field during MIG overlay welding. The validated model can be used for welding procedure optimization, preheat determination, and quality control. Engineers should consider FEM as a complementary tool to experimental qualification, particularly for complex geometries or critical applications where experimental trials are costly or impractical.