⚡ electrical engineering

In-situ deformation monitoring during laser welding using single time-gated camera stereo-DIC

This study presents a single time-gated camera stereo-DIC method that effectively suppresses intense plasma and thermal radiation to enable full-field, in-situ deformation monitoring during laser welding of Ti6Al4V alloys, overcoming the overexposure limitations of conventional blue-light DIC systems and providing critical data on deformation evolution and boundary constraint effects.

Original authors: Yi Luo, Jianqiao Hu, Hengxu Song, Bing Pan

Published 2026-09-14

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CC BY 4.0

Original authors: Yi Luo, Jianqiao Hu, Hengxu Song, Bing Pan

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Titanium is a metal that engineers love for its strength and lightness, making it a favorite for building aircraft, submarines, and spacecraft. However, joining pieces of titanium together is a delicate business. When the metal is heated to extreme temperatures to melt and fuse, it expands and then shrinks as it cools. This cycle creates invisible internal forces called residual stress. If these forces are not understood and controlled, they can warp the final part or cause it to crack under pressure later in its life. To fix this, scientists need to watch the metal move in real time while it is being welded, capturing exactly how it stretches and contracts. The problem is that the welding process itself is blindingly bright. The laser used to melt the metal creates a cloud of super-hot gas and plasma that emits intense light, drowning out any camera trying to take a picture. It is like trying to read a book while someone shines a high-powered flashlight directly into your eyes; the details of the page simply vanish.

Researchers at Beihang University and the Chinese Academy of Sciences have developed a new way to see through this blinding glare. They created a system that uses a special camera capable of taking pictures in a tiny fraction of a second, fast enough to ignore the continuous, blinding light of the welding plasma. Instead of trying to block the light with physical shields, which would hide the very area they want to study, they use a trick of timing. They illuminate the metal with a blue laser that flashes for only 150 nanoseconds—a time so short it is almost instantaneous. The camera is synchronized to open its shutter for that exact same brief moment. Because the blinding plasma light is continuous and the laser flash is so short, the camera captures only the clear reflection of the blue laser from the metal surface, while the overwhelming glare of the plasma remains effectively invisible. This allows them to see the speckle pattern painted on the metal and track its movement with high precision, even when the temperature near the weld exceeds 2,000 degrees Celsius.

To test this method, the team welded two plates of titanium alloy together, a material known as Ti6Al4V, which is widely used in aerospace. They set up the experiment in two different ways: one where the metal plates were held rigidly in place by a clamp, and another where the plates were free to move as they cooled. Using their new single-camera system, they recorded thousands of images as the laser moved along the seam. In a direct comparison, a standard camera system using blue light failed immediately, with the images turning completely white and useless the moment the welding started. In contrast, the new system produced clear, sharp images throughout the entire process, from the moment the laser touched the metal to the long cooling period that followed. The researchers were able to map the full surface of the metal, watching how it expanded as it heated up and then shrank as it cooled down.

The results showed a clear story of how the metal behaves. As the laser moved, the area directly in front of it expanded due to heat, while the area behind it began to contract. The researchers observed that the metal stretched out along the direction of the weld and also pulled inward from the sides. When the metal was held in a clamp, the sideways movement was restricted, leading to less overall distortion. When the metal was free to move, it shrank more significantly, resulting in larger final deformations. This confirmed that holding the metal in place changes how the stress builds up and settles. The study proves that this time-gated camera technique can successfully capture the full history of deformation during laser welding, a task that was previously impossible due to the intense light. By providing clear data on how titanium moves and changes shape during the process, this method offers a reliable way to check computer models and improve the manufacturing of critical components, ensuring that the next generation of aircraft and deep-sea vessels remains safe and durable.

Technical Summary: In-situ Deformation Monitoring During Laser Welding Using Single Time-Gated Camera Stereo-DIC

Problem Statement
The structural integrity and service reliability of titanium alloy components, particularly in aerospace and defense applications, are heavily influenced by welding-induced residual stresses and distortions. Understanding the dynamic evolution of deformation during the welding process is critical for validating thermomechanical models and optimizing process parameters. However, acquiring accurate, full-field in-situ deformation data during laser welding of titanium alloys presents significant challenges. The process involves extreme temperatures (exceeding 2000°C) and intense plasma radiation generated by the laser-plume interaction. Conventional optical measurement techniques, such as blue-light Digital Image Correlation (DIC), suffer from severe image overexposure due to this radiation, leading to a loss of speckle pattern contrast and rendering DIC analysis unfeasible. While physical shielding or narrow-bandpass filtering with high-power pulsed lasers have been attempted, they often fail to capture genuine deformation fields near the weld or are insufficient for the extreme conditions of titanium alloy welding. Furthermore, traditional stereo-DIC systems require dual-camera synchronization, increasing complexity and cost.

Methodology
To address these challenges, the authors propose and validate a Single Time-Gated Camera Stereo-DIC (STG stereo-DIC) system. The methodology relies on a dual-domain filtering strategy to suppress intense plasma and thermal radiation:

Spectral Filtering: A narrow-bandpass optical filter (450 ± 10 nm) is matched to a high-power blue pulsed laser (450 nm, 150 ns pulse width) used as the active illumination source. This blocks broadband thermal radiation from the heated specimen.
Temporal Filtering: A gated camera with nanosecond-scale exposure capability (switching speeds of $10^{-8}$ to $10^{-9}$ s) is synchronized with the laser pulses. By limiting the exposure window to the duration of the reflected laser pulse, the continuous energy from plasma and thermal radiation is reduced by several orders of magnitude, becoming negligible compared to the active illumination signal.

To achieve 3D measurement without the complexity of dual-camera synchronization, the system employs a four-mirror adapter placed in front of a single gated camera. This adapter splits the light from the specimen into two optical paths, projecting left and right views onto the left and right halves of the same sensor. This emulates a stereo-vision system using a single camera.

The system was tested on Ti6Al4V titanium alloy plates (120 mm × 60 mm × 4 mm) using a 10 kW fiber laser. Experiments were conducted under two boundary conditions: clamped (rigidly fixed at ends and sides) and free (only taped to the worktable). A conventional blue-light stereo-DIC system served as a control for comparison.

Key Contributions

Novel Application: This study represents the first application of STG stereo-DIC to the laser welding of titanium alloys, a domain characterized by temperatures above 2000°C and severe plasma radiation.
Dual-Domain Suppression: The work demonstrates that combining spectral and temporal filtering effectively attenuates plasma radiation to negligible levels, enabling clear speckle imaging where conventional methods fail.
Single-Camera Stereo Configuration: The integration of a four-mirror adapter with a time-gated camera provides a cost-effective and synchronization-free solution for 3D in-situ deformation measurement.
Full-Field Data Acquisition: The method successfully captures full-field displacement and strain evolution throughout the entire welding and cooling cycles, including the transition from thermal expansion to contraction.

Results

Imaging Performance: The STG stereo-DIC system consistently captured clear, non-overexposed speckle images throughout the welding process. In contrast, the conventional blue-light DIC system suffered from immediate and severe overexposure upon welding initiation, with mean intensity values approaching the maximum grayscale (256) and mean intensity gradients dropping to near zero.
Measurement Accuracy: Static tests on a stationary specimen indicated sub-pixel measurement stability, with in-plane displacement fluctuations within ±0.2 μm and standard deviations of 0.5 μm.
Deformation Evolution: Full-field displacement maps revealed that in-plane deformation components (parallel and perpendicular to the weld) transitioned from thermal expansion during heating to contraction during cooling.
Boundary Condition Effects:
- Under clamped conditions, the perpendicular in-plane deformation and final residual deformation were significantly smaller than under free boundary conditions, consistent with mechanical constraint effects.
- Under free boundary conditions, the specimen exhibited larger overall deformation and residual distortion, particularly in the direction perpendicular to the weld line.
- The overall deformation patterns and evolution trends were similar for both conditions, indicating that clamping constrains magnitude but does not fundamentally alter the deformation mechanism.

Significance and Claims
The paper claims that the proposed STG stereo-DIC method provides a reliable experimental basis for understanding the generation and evolution of residual stress and distortion in titanium alloy welding. By resolving the critical issue of image overexposure, the method enables the acquisition of high-fidelity full-field data that was previously unattainable under such extreme conditions. The authors state that this capability is indispensable for validating thermomechanical models and optimizing welding parameters. The study concludes that the method offers a robust solution for in-situ monitoring, with future work planned to shift from global surface mapping to refined local measurements near the weld zone to better characterize local deformation gradients.

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