Three-Dimensional Molten Pool Free Surface Measurement Using Laser Dot Matrix for TIG Welding

Literature Overview

Published in the Chinese Journal of Mechanical Engineering in 2014, this study by Zhang Gang, Shi Yu, Huang Jiankang, Zhang Weijie, and Fan Ding from the State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals at Lanzhou University of Technology investigates a novel method for measuring the three-dimensional surface morphology of the TIG welding molten pool using a laser dot matrix projection technique. Funded by the National Natural Science Foundation of China (grant 51205179) and several provincial research programs, this work addresses a fundamental challenge in welding process monitoring: real-time measurement of the molten pool geometry.

Core Technical Points

The laser dot matrix method projects a grid of laser dots onto the molten pool surface and uses a camera to capture the deformed dot pattern. By analyzing the deformation of the dot grid, the three-dimensional surface profile of the molten pool can be reconstructed. Key findings include:

• The measurement system achieves a spatial resolution of approximately 0.1 mm and a vertical resolution of 0.05 mm.
• The molten pool surface profile is accurately captured, revealing a concave shape near the arc root that transitions to a convex meniscus at the pool boundary.
• The pool depth, width, and length are measured as functions of welding parameters, providing a database for process modeling.

Interpretation of Technical Significance

The ability to measure the molten pool surface in three dimensions is transformative for welding process understanding and control. Traditional methods for pool measurement rely on post-weld cross-sectional examination, which provides only a snapshot of the solidified weld geometry. The laser dot matrix method enables real-time or near-real-time observation of the pool dynamics during welding, which is critical for understanding transient phenomena such as pool oscillation, keyhole formation, and surface tension-driven flow.

For cladding and overlay welding, the molten pool geometry directly determines the dilution ratio and the bond line metallurgy. A deeper pool indicates greater penetration into the base metal, which increases dilution. The laser dot matrix measurement can therefore be used to predict dilution in real time, enabling adaptive control of welding parameters to maintain the desired dilution level.

The concave surface profile observed near the arc root is attributed to the electromagnetic force (Lorentz force) acting on the current-carrying molten pool. This force depresses the pool surface at the center and creates a depression that can influence the solidification pattern. Understanding this depression is important for predicting the formation of centerline defects such as hot tears and porosity.

Engineering Practice Implications

The laser dot matrix measurement technique has several practical applications in cladding and pressure vessel fabrication:

1. Process development: The technique can be used to systematically map the relationship between welding parameters and pool geometry, enabling the development of optimized welding procedures for specific overlay applications.
2. Process monitoring and control: In automated welding cells, the laser dot matrix system can be integrated with real-time feedback control to adjust welding parameters based on measured pool geometry. This is particularly valuable for maintaining consistent dilution in multi-pass overlay welding.
3. Weld quality prediction: By correlating pool geometry with post-weld properties such as dilution, grain structure, and mechanical properties, the technique can serve as a non-destructive quality prediction tool.

For pressure vessel fabrication, the technique could be used to verify that overlay welds meet the required dilution limits specified in codes such as ASME VIII Div.1 or GB/T 150. Excessive dilution can compromise the corrosion resistance of the overlay layer, leading to premature failure in aggressive service environments.

Key Questions and Reflections

The primary challenge in implementing the laser dot matrix technique in production environments is the extreme thermal and optical conditions at the welding arc. The intense radiation from the arc, spatter, and smoke can interfere with the laser projection and camera imaging. The researchers addressed these challenges through optical filtering and high-speed imaging, but practical implementation in a fabrication shop would require robust optical protection and possibly a shielded measurement configuration.

Another consideration is the measurement of the pool depth, which requires knowledge of the pool surface profile and the solidification boundary. The laser dot matrix method measures the free surface, but the actual pool depth depends on the solidification front position, which is not directly measurable. This limitation must be accounted for when using pool surface measurements to predict penetration and dilution.

Study Insights and Summary

This research represents a significant advancement in welding process measurement technology, providing a practical method for three-dimensional molten pool characterization. For the cladding and pressure vessel industry, the technique offers a pathway toward real-time process monitoring and control, which is essential for producing high-quality overlay welds with consistent dilution and metallurgical properties. The detailed pool geometry data presented in the study provides a valuable reference for process modeling and parameter optimization. While challenges remain in translating the laboratory technique to production environments, the fundamental capability demonstrated here has the potential to transform the way overlay welding processes are developed, monitored, and controlled. The work exemplifies the growing importance of advanced measurement technology in enabling precision welding for critical engineering applications.