CLADDING TECHNOLOGY SHANXI CO., LTDArc Morphology and Characteristic Temperature Evolution of Pulsed TIG Welding Based on Arc Image Analysis
Literature Overview
This research published in Journal of Tsinghua University (Science and Technology) in 2021 by Cheng Shijia, Zhu Zhiming, and Fu Pingpo from the Department of Mechanical Engineering, Tsinghua University, investigates the arc morphology and characteristic temperature evolution of pulsed TIG welding using arc image analysis techniques. The study was supported by the National Natural Science Foundation of China (Grant No. 51775301). This work represents a significant advancement in process monitoring and control for pulsed TIG welding, with direct implications for optimizing weld overlay and cladding processes.
Core Technical Points
The study employs high-speed imaging techniques to capture the arc morphology during each phase of the pulse cycle. The arc images are analyzed using image processing algorithms to extract quantitative information about arc shape, size, and brightness distribution. The characteristic temperature of the arc is determined from the spectral emission analysis of the arc radiation, providing direct measurement of the thermal conditions within the arc plasma.
The pulsed TIG welding process creates a dynamic arc environment where the morphology and temperature change rapidly between peak and valley current phases. Understanding these dynamic changes is essential for optimizing the welding process and ensuring consistent weld quality. The image-based analysis provides real-time information that can be used for process control and quality monitoring.
| Parameter | Continuous TIG | Pulsed TIG (Peak) | Pulsed TIG (Valley) |
|---|---|---|---|
| Arc length | Constant | Shorter | Longer |
| Arc radius | Constant | Smaller | Larger |
| Arc brightness | Constant | Higher | Lower |
| Arc temperature | ~10,000-15,000 K | ~12,000-18,000 K | ~8,000-12,000 K |
| Arc force | Constant | Stronger | Weaker |
| Arc stability | High | Moderate | Lower |
| Heat flux distribution | Uniform | Concentrated | Dispersed |
Arc Morphology Analysis
Peak Current Phase
During the peak current phase, the arc exhibits a constricted morphology with a narrow arc column and well-defined boundaries. The electromagnetic force compresses the plasma toward the center, creating a high-density core surrounded by a lower-density sheath. The arc length decreases as the increased arc force pushes the arc against the workpiece surface. The brightness distribution shows a concentrated hot spot at the arc root with gradual decrease toward the arc tip.
The arc root area on the workpiece surface is significantly smaller during peak current, concentrating the energy input into a smaller area. This results in higher heat flux density and deeper penetration. The arc root area on the tungsten electrode also contracts, which can lead to increased electrode erosion if the current density exceeds the electrode's carrying capacity.
Valley Current Phase
During the valley current phase, the arc expands significantly with a broader arc column and diffuse boundaries. The reduced electromagnetic confinement allows the plasma to expand radially, creating a more uniform temperature distribution with lower peak values. The arc length increases as the reduced arc force allows the arc to elongate. The brightness distribution shows a more uniform pattern with no distinct hot spot.
The arc root area on the workpiece expands during valley current, reducing the heat flux density and allowing partial solidification at the pool edges. This thermal cycling effect is beneficial for controlling the microstructure of the weld overlay, as it promotes fine grain formation through repeated nucleation and growth cycles.
Characteristic Temperature Evolution
The characteristic temperature of the arc is determined from the spectral emission analysis of the arc radiation. The temperature is calculated using the Boltzmann plot method, which relates the relative intensities of spectral lines to the excitation temperature of the plasma. The study reveals that the arc temperature varies significantly between peak and valley phases, with the peak temperature reaching 12,000-18,000 K and the valley temperature dropping to 8,000-12,000 K.
The temperature evolution follows a characteristic pattern during each pulse cycle. At the onset of the peak current, the temperature rises rapidly as the increased current density heats the plasma. The temperature reaches a maximum shortly after the peak current is established, then gradually decreases as the current transitions to the valley phase. During the valley phase, the temperature continues to decrease until the next pulse begins.
| Pulse Parameter | Effect on Arc Temperature | Effect on Arc Morphology | Weld Quality Impact |
|---|---|---|---|
| Peak current increase | Higher peak temperature | More constricted | Deeper penetration, higher dilution |
| Valley current decrease | Lower valley temperature | More expanded | Lower dilution, finer grains |
| Peak-valley ratio increase | Greater temperature swing | More dynamic | Better microstructure control |
| Pulse frequency increase | Reduced temperature swing | Less dynamic change | More uniform thermal cycling |
| Pulse duration increase | Longer high temperature phase | Sustained constriction | More heat input, higher dilution |
Engineering Practice Implications
The image-based analysis technique provides a powerful tool for real-time monitoring of pulsed TIG welding processes. By analyzing the arc images captured during welding, engineers can extract quantitative information about the process state and make adjustments to maintain optimal conditions. This capability is particularly valuable for cladding applications where consistent overlay quality is critical.
Process Monitoring Parameters
The following parameters can be extracted from arc image analysis for process monitoring:
- Arc length: Measured from the distance between the electrode tip and the arc root on the workpiece.
- Arc radius: Measured from the width of the arc column at its widest point.
- Arc brightness: Measured from the integrated intensity of the arc emission.
- Arc symmetry: Measured from the deviation of the arc profile from a symmetric shape.
- Arc root area: Measured from the area of the arc root on the workpiece surface.
These parameters provide real-time feedback on the welding process state and can be used to detect anomalies such as arc wandering, electrode erosion, or shielding gas insufficiency. For cladding operations, monitoring these parameters ensures consistent dilution control and overlay quality throughout the production run.
Quality Control Applications
The arc image analysis technique can be integrated into a process control system that automatically adjusts welding parameters to maintain optimal conditions. The system can detect deviations from the target arc morphology and temperature profile, and make adjustments to the current, voltage, or travel speed to restore the desired process state. This closed-loop control capability is particularly valuable for automated cladding operations where consistent quality is required over long production runs.
| Defect Type | Arc Image Indication | Corrective Action |
|---|---|---|
| Arc wandering | Asymmetric arc profile | Adjust torch position or gas flow |
| Electrode erosion | Changed arc root shape | Replace or dress electrode |
| Gas insufficiency | Increased arc brightness and noise | Increase gas flow rate |
| Current instability | Fluctuating arc brightness | Check power supply and cables |
| Workpiece contamination | Irregular arc root area | Clean workpiece surface |
Key Questions and Reflections
The image-based analysis technique, while powerful, has certain limitations that engineers should be aware of. The high-speed imaging system requires careful calibration to ensure accurate temperature measurements. The spectral analysis used to determine arc temperature assumes local thermodynamic equilibrium, which may not hold in all regions of the arc plasma. Additionally, the presence of filler wire or wire feeding can interfere with the arc image, making analysis more complex.
The transition from laboratory research to industrial application requires addressing several practical challenges. The imaging system must be protected from the harsh welding environment including intense optical radiation, fumes, and spatter. The data processing algorithms must be robust enough to handle the variability of real production conditions. The system must also be integrated with the existing welding equipment and control systems, which may require significant modification.
Study Insights and Implications
This research demonstrates the significant potential of arc image analysis for monitoring and controlling pulsed TIG welding processes. The technique provides real-time, non-contact measurement of critical process parameters that are difficult to obtain by other means. For engineers involved in cladding and overlay operations, this technology offers a pathway to improved process consistency and quality control. The ability to monitor arc morphology and temperature evolution in real time enables more rational process optimization and faster identification of quality issues. As imaging technology continues to advance in speed, resolution, and cost-effectiveness, arc image analysis will become an increasingly important tool for modern welding process control. The insights from this research support the development of intelligent welding systems that can maintain optimal process conditions throughout production, ensuring consistent overlay quality for critical applications such as bimetal pressure vessels and corrosion-resistant cladding.