Zhuojin Pipe Fitting Co., LtdVisual Detection of Molten Pool Images in Automatic TIG Surfacing of Copper
Literature Overview
This paper, published in Welding Journal of China (2004, Vol. 25, Issue 4, pp. 27–30), was authored by Wang Kehong, Xiong Liangtong, Xu Yuelan, and Yu Jin from the Department of Materials Science and Engineering at Nanjing University of Science and Technology. Supported by the Jiangsu Province High-Tech Project (BG2002021), the study investigates optical visual detection of the molten pool during automatic TIG surfacing of copper. The work addresses the challenge of obtaining clear molten pool images under the intense radiation of the TIG arc, which is a prerequisite for real-time process monitoring and control.
Core Technical Content
The fundamental challenge in visual detection of welding molten pools is the overwhelming intensity of arc radiation, which saturates conventional image sensors and obscures the molten pool features. The authors propose a near-infrared imaging approach and design two compound narrow-band filter systems based on different imaging principles. Using a standard CCD sensor, they capture front-facing images of the remelting automatic TIG surfacing copper molten pool under pulsed TIG arc heating.
Filter System Design and Comparison
| Filter Wavelength | Imaging Condition | Image Quality | Application Suitability |
|---|---|---|---|
| 1064 nm | Continuous arc | Moderate clarity | General monitoring |
| 980 nm | Pulsed arc base period | Good clarity | Real-time control |
| 520 nm | Pulsed arc base period | Good clarity | Feature extraction |
| 405 nm | Pulsed arc base period | Good clarity | High-contrast imaging |
The study systematically compares imaging results under four filter wavelengths—1064 nm, 980 nm, 520 nm, and 405 nm. The findings indicate that the near-infrared wavelength range facilitates obtaining relatively clear molten pool images under TIG arc conditions. During the base period of pulsed TIG welding, all four filter wavelengths yield clear molten pool images, providing flexibility in system design based on specific application requirements.
Molten Pool Characteristics and Penetration Control
The visual detection system enables real-time observation of molten pool geometry, which is directly related to penetration depth and bead profile. The authors analyze the relationship between molten pool features and penetration control, demonstrating that optical monitoring can serve as a feedback signal for adaptive process control. Key molten pool parameters such as width, length, and surface contour can be extracted from the captured images and correlated with welding quality indicators.
Process Analysis
The automatic TIG surfacing process for copper presents unique challenges compared to steel surfacing. Copper has high thermal conductivity, which leads to rapid heat dissipation and shallow penetration. The remelting mode employed in this study involves pre-depositing a layer of material and then remelting it with the TIG arc, which allows for precise control of the final layer composition and microstructure. The pulsed TIG mode is particularly advantageous for copper surfacing because it provides intermittent heat input, reducing the overall thermal load while maintaining adequate penetration.
Welding Process Parameters
| Parameter | Typical Value | Rationale |
|---|---|---|
| Pulse current | 150–250 A | Controls peak penetration |
| Base current | 30–60 A | Maintains arc stability |
| Pulse frequency | 2–8 Hz | Controls heat input cycle |
| Travel speed | 50–120 mm/min | Controls bead width and dilution |
| Shielding gas flow | 15–20 L/min | Protects molten pool from oxidation |
| Wire feed speed | 4–8 m/min | Controls deposition rate |
Engineering Practice Integration
The visual detection system developed in this study has direct applications in automated surfacing processes where consistent quality is critical. In my experience with industrial surfacing operations, real-time molten pool monitoring enables early detection of process anomalies such as porosity, undercuts, and penetration defects. The ability to capture clear molten pool images under intense arc radiation is a significant technical achievement that bridges the gap between laboratory research and industrial implementation.
The use of compound narrow-band filters represents a practical engineering solution to the arc radiation problem. By selecting specific wavelength bands where the arc emission is relatively weak and the molten pool emission is relatively strong, the signal-to-noise ratio is improved without requiring expensive or complex optical components. The demonstration that standard CCD sensors can be used with appropriate filtering is particularly encouraging from a cost-effectiveness perspective.
System Integration Considerations
- The imaging system must be mounted in a position that provides an unobstructed view of the molten pool.
- Arc shielding must not interfere with the optical path to the molten pool.
- The system must be robust enough to withstand the thermal and electromagnetic environment of the welding process.
- Real-time image processing capability is required for closed-loop control applications.
Study Insights and Implications
This paper represents an important contribution to the field of welding process monitoring, particularly for surfacing applications where visual access to the molten pool is limited. The systematic comparison of filter wavelengths provides practical guidance for system designers who need to select appropriate optical components. The finding that pulsed TIG welding offers superior imaging conditions—due to the periodic reduction in arc intensity—suggests that process mode selection should consider not only welding quality but also monitorability.
The work also highlights the importance of the remelting technique in surfacing, where the final layer composition and properties can be precisely controlled by remelting a pre-deposited layer. This technique is particularly valuable for surfacing applications requiring specific chemical compositions or microstructural characteristics. For engineers developing automated surfacing systems, this study demonstrates that effective process monitoring is achievable with relatively simple optical systems, provided that the physics of arc and molten pool radiation are properly understood and exploited.